Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Abstract

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V–O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

1. Introduction

1.1. Vanadium

Vanadium was first discovered by Andrés Manuel del Rio in Mexico City from Pb₅(VO₄)₃Cl in 1801.¹ However, it was wrongly identified as a form of chromium by Hippolyte Victor Collet-Descotils in 1805.² Until 1831, Swedish chemist Nil Gabriel Self-ström in Stockholm named the element vanadium, which is from the Norse Goddess Vanadis and means beauty and fertility.³ In the Earth’s crust, vanadium is the 20th most abundant element and the sixth most abundant element among the transition metals.³⁻⁵ However, some literature indicates that vanadium is the fourth most abundant transition metal after iron, titanium, and manganese.⁵⁻⁸ High purity vanadium (about 99.7%) was first produced in 1925 by reducing vanadium pentoxide (V₂O₅) with calcium metal.¹ Pure vanadium exhibits a transition metal feature, which shows a high melting point and good corrosion resistance at low temperatures. Vanadium can be dissolved in nitric and sulfuric acids but is insoluble in hydrochloric acid.⁹ In nature, vanadium is difficult to exist in metal form because it easily reacts with oxygen, even nitrogen and carbon at elevated temperatures.^2,10 Vanadium is an important component of specific steel alloys, which provides additional tensile strength and extra protection against rust and corrosion of these materials.

1.2. Vanadium Oxides

Vanadium has the electronic configuration [Ar]4s²3d³. Therefore, the oxidation state of vanadium can range from +5 to −3, and the valences of +5, +4, +3, and +2 are most commonly observed.^11,12 Four vanadium oxides feature single oxidation states (+2 for VO, +3 for V₂O₃, +4 for VO₂, and +5 for V₂O₅), and others have mixed oxidation states. Different oxidation states exhibit various colors: +5 (orange to yellow), +4 (blue), and +3 (green).⁹ The vanadium oxides exhibit crystalline structures with different oxygen coordinations, which result in the formation of octahedral, pentagonal bipyramids, square pyramids, and tetrahedral sharing corners, edges, or faces.¹² The oxidation state of the vanadium cations dramatically affects the physicochemical properties of the vanadium oxides with different phases.

Due to the multioxidation states and various crystalline structures, the vanadium oxides exhibit excellent intercalation properties to host–guest molecules or ions,⁵ giving excellent catalytic activities,⁴ strong electron–electron correlations,¹³ outstanding phase transitions (metal–insulator transition),¹⁴ and high electrical conductivity.⁵ Furthermore, the abundant nanostructures of vanadium oxides can be achieved by different preparation methods, which not only shorten the transportation distance of ions or electrons and yield a faster solid-state diffusion in electrochemical energy conversion systems,^15,16 but also provide more active positions for the interaction with other molecules or ions and more exposed active crystal facets for catalysis applications.^17,18 Thus, the vanadium oxides provide promising applications in energy conversion/saving fields,^19,20 such as ion batteries,²¹⁻²⁵ water splitting,²⁶ smart windows,^27,28 supercapacitors,^29,30 sensors,³¹ and so on.

A series of vanadium oxides with strong electron–electron correlations exhibit metal–insulator transition (MIT). The V₂O₃, VO₂, and V₂O₅ with single oxidation undergo MIT at 160 K,³² 340 K,³³ and 530 K,³⁴ respectively. These phase transitions are reversible and accompanied by a change of crystallographic, magnetic, optical, and electrical properties. The mixed-valence vanadium oxides belong to either Magnéli series (V_nO_2n-1) or Wadsley series (V_nO_2n+1). For the Wadsley series, V₃O₇ and V₆O₁₃ exhibit the phase transition at 5.2 and 155 K, respectively.^35,36 Except for V₇O₁₃ (metallic), all the Magnéli series show a transition from a paramagnetic to an antiferromagnetic state and consequently exhibit an antiferromagnetic ground state at low temperatures, including V₃O₅ (430 K), V₄O₇ (250 K), V₅O₉ (135 K), V₆O₁₁ (170 K), and V₈O₁₅ (70 K) with different phase transition temperatures, respectively.³⁷

1.3. Scope of the Review

Vanadium oxides have a long history and rapid development in recent years as they are one of the most promising candidates in versatile applications in batteries, energy-saving smart windows, sensors, catalysts, optoelectronic devices, and so on. Therefore, many research papers and reviews have been published. Pioneering reviews on the chemistry of oxovanadium were published in 1965.³⁸ The synthesis of vanadium oxides through hydrothermal and gas phase was reviewed by Whittingham,³⁹ Livage,⁴⁰ and Bahlawane.⁴¹ The atomic layer deposition of vanadium oxides was summarized by Papakonstantinou.¹² The synthesis, properties, and applications of vanadium oxide nanotube were described by Kianfar.⁶ The catalytic applications of vanadium oxides have been described by Delferro,³ Hess,⁴² Carrero,⁴³ and Granozzi.⁴⁴ The Raman spectroscopy of vanadium oxides was recently reviewed by Shvets.⁴⁵ The sensing properties of vanadium oxide nanostructures were described by Sheikhi⁴⁶ and Madanagurusamy.³¹ The energy-related applications of vanadium oxides were reviewed by Xie,^5,20 Jiang,⁴ and Streb.⁴⁷ A large number of reviews described the progress in metal ions batteries, including those by Chen,²⁵ O’Dwyer,²² Mai,^21,24,48 Lowe,²³ Whittingham,⁴⁹ Rashada,⁵⁰ Lee,⁵¹ Yang,⁵² Zheng,⁵³ Kim,⁵⁴ Liang,⁵⁵ and Cao.⁵⁶ The vanadium oxides in supercapacitors were reviewed by Chen,³⁰ Dutta,⁵⁷ and Li.²⁹ Furthermore, our group also reviewed the multistimuli responsive properties and energy applications of some vanadium oxides.^19,27,58,59

The great potential of vanadium oxides for new applications and accelerated industrialization led to the dramatically increase of the importance of vanadium oxides over the last 5–10 years. Furthermore, due to the complexity of various oxidation states of vanadium, vanadium oxides show a large variety of stable and metastable structures, which pose an inevitable challenge to synthesize vanadium oxides with high purity, well controlled stoichiometry, and meticulously designed nanostructures, a must for high performance devices. Even though lots of reviews have been published, most of them focus on a specific kind or application of vanadium oxides, such as batteries, spectra, supercapacitors, and so on. No comprehensive reviews illustrate the different vanadium oxides with different applications. In this review, we focus on the most recent progress on the structure, synthesis, and applications of five thermodynamically stable vanadium oxides (V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅) and some metastable vanadium oxides (V₂O₂, V₆O₁₃, V₄O₉, etc.), which can provide a better understanding of a specific vanadium oxide phase and their process-structure–property interrelationships. Figure 1 summarizes the main contents of this review. This review begins with the phase diagram of the V–O system to show the different vanadium oxide phases, followed by the vanadium oxides with different stoichiometries. For each vanadium oxide, the structures, synthesis methods, and applications in several fields will be covered. The last section presents the future prospects and a summary of this review.

Figure 1.

Schematic illustration of the thermodynamically stable vanadium oxides and their applications.

2. Phase Diagram of the V–O System

The V–O binary phase diagram was compiled according to previously reported experimental data in 1989.⁶⁰ The oxygen-rich phases are well-defined, which reveal more than 20 compounds.⁴¹ However, the vanadium-rich phases exhibit broad homogeneity ranges and high nonstoichiometry.^61,62

Several groups have calculated the V–O binary phase diagram.⁶¹⁻⁶⁴Figure 2a shows a calculated phase diagram of the V–O system in the entire composition range at 1 atm. In the V-rich range, four types of solid solutions exist. The α and β solid solutions are formed by a certain amount of oxygen dissolved in the vanadium. The maximum solubilities of oxygen in α-V and β-V phase are up to 17.9 atom % and 27.4 atom %, respectively. The β-phase exhibits a wide range of homogeneities. With the increase of oxygen content, the γ and δ solid solutions phase can be formed. The γ-phase is monoclinic and δ-phase has the stoichiometry of VO with NaCl-type structure.

Figure 2.

(a) Calculated phase diagram of the V–O system in the entire composition range at 1 atm. (b) Enlarged phase diagram of the V₂O₃–V₂O₅ system at 1 atm. Reproduced with permission from ref (61). Copyright 2015 Elsevier.

For the stoichiometric phases, only five phases are thermodynamically stable compositions as pure compounds, which include divanadium trioxide (V₂O₃, cubic, Ia3), trivanadium pentoxide (V₃O₅, monoclinic, P2/c), vanadium dioxide (VO₂, tetragonal, P4₂/mnm), trivanadium heptaoxide (V₃O₇, monoclinic, C2/c), and divanadium pentoxide (V₂O₅, orthorhombic, Pmnm). Other metastable phases or unstable phases are likely to decompose to stable phases. All the stable and metastable phases are listed in the enlarged phase diagram of the V₂O₃–V₂O₅ system, which is shown in Figure 2b.

There are two types of vanadium oxides with a mixed valence of vanadium. One is the Magnéli series, which is defined by the general stoichiometric formula:

This type of homologous series has been reported for molybdenum oxides for the first time by Magnéli.⁶⁵ The other homologous series is the Wadsley series, which has the general formula of V_nO_2n+1 (n ≥ 3). All the Magnéli phases maintain a triclinic symmetry (P1) and are metastable. They are expected to yield VO₂ and V₃O₅ after lowering the system entropy. For the Wadsley series, V₃O₇ exhibits lower formation entropy compared with other Wadsley phases, which indicates the phase is more stable. Other Wadsley phases, such as V₄O₉ and V₆O₁₃, can be decomposed into a mixture of VO₂ and V₃O₇.⁴¹ V₄O₉ and V₆O₁₃ show multiple metastable crystalline structures due to the close formation energies. Therefore, these compounds are candidates for polymorphism, where three crystalline structures were identified for V₆O₁₃ and two for V₄O₉.⁶⁰

3. V₂O₃

3.1. Structures and Synthesis

V₂O₃ has a typical corundum-type hexagonal structure (space group: R3̅c) with lattice parameters of a = b = 4.9492(2) Å, c = 13.988(1) Å at room temperature.^66,67 The crystal structures of V₂O₃ viewed in different directions are illustrated in Figure 3. It is interesting that after being calcined at 600 °C for several hours, vanadium vacancies (red circle in Figure 3a) formed, which is suitable for aqueous zinc metal batteries.⁶⁷ From another viewing direction, V₂O₃ possesses an open tunnel structure consisting of a 3D V–V framework (Figure 3b).⁶⁸ Such tunnel structures could efficiently facilitate the insertion of alkali metal ions, which provide potential application in metal ion batteries.

Figure 3.

Perspective view of V₂O₃: (a) along the (001) direction; (b) along the (110) direction.

V₂O₃ with different morphologies can be obtained by several methods, including reduction, oxidation, and hydrothermal approaches. For the reduction pathway, V₂O₅ was employed as initial materials, and hydrogen or ammonia gas was used as reducing agents, whereas vanadium metal was used as a starting material for the oxidation method. In the hydrothermal route, vanadium alkoxides or sulfides with some small organic molecules (e.g., thiourea, benzyl alcohol, etc.) were added together to the autoclave to grow V₂O₃.

Li et al.⁶⁹ designed a plasma hydrogen reduction system to synthesize V₂O₃ nanocrystals via a single precursor of V₂O₅ powders. The coarse-grained V₂O₅ powders are injected into the hydrogen plasma by a powder feeder, reducing the V₂O₅ powders into V₂O₃ by hydrogen. Such single crystalline V₂O₃ nanocrystals have a spherical shape with average sizes in the range of 35–50 nm (Figure 4a). The morphology of the raw materials determines the V₂O₃ morphology. Seshadri et al.⁷⁰ first synthesized the V₂O₅ nanorods via a hydrothermal reaction followed by reducing in 5% H₂:95% N₂ (reduction time = 3 h and reduction temperature = 600 °C) to obtain V₂O₃ nanorods (Figure 4b). The ammonia gas (NH₃) is another agent to reduce V₂O₅ to V₂O₃.⁷¹ V₂O₃ shows a much larger size with several micrometers, and the morphologies of the V₂O₃ particles were micrometer layered structures that were assembled by nanometer or micrometer sheets. Tao et al.⁷² reported that the V₂O₃ nanoparticles had been synthesized by supercritical ethanol fluid reduction of VOC₂O₄ with an average size of 50 μm. Madanagurusamy et al. employed the oxidation way to obtain the V₂O₃ nanosheets.⁷³ High-density vertically aligned V₂O₃ nanosheets on glass substrates were obtained via a simple one-step sputtering technique. Vanadium metal was first deposited on the well-cleaned glass substrates followed by oxidation in the argon and oxygen mixture gas with a ratio of 3:1. Well-ordered, ultrathin, vertically aligned V₂O₃ nanosheets with voids were obtained (Figure 4c).

Figure 4.

(a) Typical TEM image of V₂O₃ nanocrystals. Reproduced with permission from ref (69). Copyright 2012 Elsevier. (b) SEM image of V₂O₃ nanorods after reduction (time = 3 h and temperature = 600 °C). Reproduced with permission from ref (70). Copyright 2008 American Chemical Society. (c) SEM images of V₂O₃ nanosheets. Reproduced with permission from ref (73). Copyright 2018 Royal Society of Chemistry. (d) TEM of V₂O₃ nanocrystals. Reproduced with permission from ref (74). Copyright 2004 Elsevier.

Hydrothermal is one of the most popular methods for crystal growth, which is conducted under moderate temperature and a high vapor pressure environment in sealed containers with the unique advantages of being easy to handle and environmentally friendly. Compared with the reduction and oxidation methods, the hydrothermal reaction is more convenient for synthesis the V₂O₃ with various morphologies. Niederberger et al.⁷⁴ adopted vanadium alkoxides and benzyl alcohol as precursors to synthesize V₂O₃ nanocrystals through hydrothermal reaction sizes ranging from 20 to 50 nm with good yields (Figure 4d). Without using any surfactant and template, Su et al.⁷⁵ successfully synthesized dandelion-like V₂O₃ microspheres with core–shell structures. With increasing reaction time, the morphology of V₂O₃ can be tuned from a solid sphere to dandelion-like. If the reaction time is further increased, some broken V₂O₃ microspheres with core–shell structures could be observed with an average diameter of the core–shell microspheres of 2 μm.

3.2. Applications

3.2.1. Batteries

As the low valence of V³⁺ in vanadium oxide, corundum-type V₂O₃ with a metallic behavior shows that the electrons in the V-3d orbital travel along the V–V chains. The 3D V–V framework provides an intrinsic tunnel structure, which is suitable for ion transport and intercalation. Mai et al. reported uniform nitrogen-doped carbon-confined V₂O₃ (V₂O₃@NC) hollow spheres (Figure 5a). The in situ carbonization hollow structure can provide high ion/electron conductivity, short diffusion distance, and excellent structure adaptability, which is beneficial for lithium-ion storage (Figure 5b). The V₂O₃@NC delivers an average discharge capacity of 785, 599, 528, and 361 mAh g^–1 at a current density of 100, 500, 1000, and 5000 mA g^–1, respectively (Figure 5c). Furthermore, 811 mAh g^–1 can be maintained after 120 cycles at a current density of 200 mA g^–1 (Figure 5d).⁷⁶ In addition, V₂O₃ can also be used as other ions (Na⁺/K⁺/Zn²⁺) storage material. Jiao’s group⁶⁸ fabricated a flexible and self-standing electrode of V₂O₃ nanoparticles embedded in porous N-doped carbon nanofibers (V₂O₃@PNCNFs) through electrospinning assisted high-temperature sintering method, which can directly be used as a KIB anode material (Figure 5e). V₂O₃@PNCNFs deliver a capacity of 240 and 134 mAh g^–1 at a current density of 50 and 1000 mA g^–1, respectively. High-capacity retention of 94.5% can be maintained after 500 cycles (Figure 5f). The density functional theory (DFT) results demonstrate that 1 mol K⁺ can insert into the V₂O₃ crystal forming KV₂O₃, with the K⁺ occupying the 6e sites of the KV₂O₃ crystal. The main capacity of V₂O₃ in KIB is main contribution from capacitance (surface or near-surface redox reactions), which is very different from the conversion reaction of conventional transition metal oxides for ion storage. When K⁺ inserts into the tunnels of V₂O₃@PNCNFs, the material exhibits no structural phase transitions, which indicates that V₂O₃@PNCNFs can exhibit excellent K⁺ rate/cycling performance. This work suggests that the pseudocapacitive electrode materials could be suitable for a large-scale energy storage system. Generally, the conventional low valent V₂O₃ cannot effectively accommodate Zn²⁺ intercalation during the discharging process due to its inherently unsuitable structure and inferior physicochemical properties. Luo et al.⁷⁷ attempted to utilize V₂O₃ in aqueous ZIBs by the in situ anodic oxidation strategy, and the hierarchical microcuboid structure V₂O₃ can accommodate nearly 2 electrons intercalation. Moreover, they found H₂O is a reactant that participates in the first charge oxidation process of V₂O₃. Furthermore, the porous structure with a higher specific surface area of the V₂O₃ leads to more reaction sites and a faster phase transition from V₂O₃ to V₂O_5–x·nH₂O (Figure 5h). Meanwhile, the high surface and the small size of V₂O₃ nanoparticles benefit the first charge oxidation reaction process. The V₂O₃ delivers a Zn²⁺ discharging capacity of 625 mAh g^–1 at 0.1 A g^–1, corresponding to 1.75-electron intercalation (Figure 5g). Specifically, the capacities can maintain 87% and 78% when the current increases to 10 and 20 A g^–1, respectively. The V₂O₃ can maintain 100% after 10000 cycles at 10 A g^–1, which is better than some previously reported ZIB cathode materials.

Figure 5.

XRD pattern (a), SEM image (b), charge/discharge profiles (c), and cycling performance (d) of V₂O₃@NC hollow spheres. Reproduced with permission from ref (76). Copyright 2018 Royal Society of Chemistry. The digital photos of the as-spun membrane and the self-standing (e), and rate performance (f) of V₂O₃@PNCNFs. Reproduced with permission from ref (68). Copyright 2018 Elsevier. The charge/discharge profiles at different voltage window (g) and schematic illustration of the oxidation at full charge state (h) of V₂O₃. Reproduced with permission from ref (77). Copyright 2020 American Chemical Society.

3.2.2. Catalysts

V₂O₃ and its composites have been widely used as catalysts for chemical looping dry reforming of methane,⁷⁸ ammonium perchlorate decomposition,⁷⁹ the hydrogen evolution reaction (HER),^30,80 the oxygen evolution reaction (OER),⁸⁰ water splitting,⁸⁰ etc.

3.2.2.1. Propane Dehydrogenation and Ammonium Perchlorate Decomposition

Very recently, Zhu et al.⁸¹ examined the catalytic properties of propane dehydrogenation through single transition metal atom doping of a V₂O₃ (0001) surface by self-consistent DFT calculation. The results indicated that the single atoms act as promoters and active sites, and Mn–V₂O₃ is a good candidate as a catalyst for propane dehydrogenation. Huang et al.⁷⁹ synthesized V₂O₃ and V₂O₃/carbon composites by a facile hydrothermal route, which exhibited excellent performance for ammonium perchlorate decomposition. The decomposition temperature decreased by 49 and 73 K for V₂O₃ and V₂O₃/carbon composites, respectively.

3.2.2.2. HER, OER, and Water Splitting

Electrocatalytic water splitting consisted of two half-reactions: HER and OER. Electrocatalysis is used to accelerate the rate of a chemical reaction through lowering the activation energy to reduce the electrochemical overpotentials.

V₂O₃ composited with other functional materials can exhibit efficient electrocatalysis performance. Zhang et al.⁸² reported a self-supported MoS_x/V₂O₃ heterostructure for the HER. Two steps were involved in the synthesis process (Figure 6a). V₂O₃/carbon cloth (CC) was first obtained by a hydrothermal method, and the dense V₂O₃ was uniformly distributed on CC fibers. The as-prepared V₂O₃/CC was immersed in an ammonium thiomolybdate solution and dried under a vacuum. Then, followed by a thermal decomposition process, the final MoS_x/V₂O₃/CC was successfully achieved with the same morphology as V₂O₃/CC. The XRD pattern (Figure 6b) shows the composite contains the MoS_x and hexagonal V₂O₃. MoS_x/V₂O₃/CC displayed an overpotential of 146 mV to achieve a 10 mA cm^–2 HER current density (Figure 6c), which is lower than that of MoSx/CC (221 mV). The Tafel slope of MoS_x/V₂O₃/CC is around 45 mV dec^–1, suggesting the HER process obeys the Volmer-Heyrovsky mechanism. Meanwhile, the MoS_x/V₂O₃/CC presents a higher double-layer capacitance (C_dl: 85 mF cm^–2) than MoS_x/CC (4 mF cm^–2), indicating V₂O₃ can create more active sites for the HER (Figure 6d). Furthermore, the MoS_x/V₂O₃/CC electrocatalyst has great stability in the acid electrolyte for the HER. Two reasons for the MoS_x/V₂O₃/CC electrocatalyst were given to understand the improved HER activity: (1) V₂O₃ enhanced the electrochemically active surface area with more active sites for the HER; (2) better electron transfer between MoS_x and V₂O₃. Qiu et al.⁸⁰ synthesized V₂O₃ nanosheets anchored with NiFe nanoparticles as a bifunctional electrode for overall water splitting. A self-templated strategy was employed to synthesize the NiFe@V₂O₃ (Figure 6e). By a calcined reduction process, ZnO@NiFe@V₂O₃ nanosheets can be obtained. In-situ alkaline media corrosion was performed to dissolve the ZnO NPs and produce the porous NiFe@V₂O₃ nanosheets, which exhibit a clear V₂O₃ porous matrix and NiFe nanoparticles (Figure 6f). The porous NiFe@V₂O₃ exhibits good OER performance in an alkaline medium with an overpotential of 255 mV at 10 mA cm^–2 and good stability (Figure 6g,h). Meanwhile, the NiFe@V₂O₃ also gave good HER performance in the same alkaline medium. It shows an overpotential of 84 mV at 10 mA cm^–2 and good stability (Figure 6i,j). Furthermore, the porous NiFe@V₂O₃ delivers a small Tafel slope of 51 mV dec^–1 for OER and a Tafel slope of 85.4 mV dec^–1 for the HER, respectively. Considering the excellent HER and OER performance, two identical NiFe@V₂O₃ electrodes are integrated into a two-electrode cell to investigate the water splitting performance. The catalyst shows a cell potential of 1.56 V to reach 10 mA cm^–2 with an ignorable cell voltage increase during 20 h measurements (Figure 6k).

Figure 6.

(a) Schematic diagram of the main steps in the synthesis of the MoS_x/V₂O₃/CC. (b) XRD patterns of MoS_x/V₂O₃/CC, V₂O₃/CC, and MoS_x/CC. (c) Polarization curves for MoS_x/V₂O₃/CC. (d) Double-layer capacitance for MoS_x/V₂O₃/CC. Reproduced with permission from ref (82). Copyright 2020 Elsevier. (e) Synthetic strategy for preparing 3D hierarchical nanoporous NiFe@V₂O₃. (f) High-magnification TEM image of NiFe@V₂O₃. (g) OER polarization curves of NiFe@V₂O₃. (h) Long-term durability test conducted at 10 mA cm^–2 of NiFe@V₂O₃. (i) HER polarization curves of NiFe@V₂O₃. (j) Long-term durability test conducted at 10 mA cm^–2 of NiFe@V₂O₃. (k) Long-term potential-time curve set at 10 mA cm^–2 of NiFe@V₂O₃||NiFe@V₂O₃ in 1.0 M KOH. Reproduced with permission from ref (80). Copyright 2019 American Chemical Society.

The stability of the catalyst is a key parameter for practical applications, which is crucial to determine whether the catalyst can be commercialized.⁸³ Several factors limit the catalytic stability, such as poor chemical/electrochemical stability of the catalysts under operation, abandoned gas evolution leading to the physical detachment of catalysts, dissolution of the catalysts in the electrolytes with different pHs, poor mechanical stability, and so on.⁸⁴⁻⁸⁶ For example, Schmidt et al.⁸⁷ proved that the metal atoms are thermodynamic unstable during the OER process due to metal oxide released lattice oxygen, which leads to the low stability of the catalysts. From the theoretical perspective, there are three critical criterion of the catalysts that should be considered: excellent water dissociation performance, suitable Gibbs adsorption energy of H* (ΔG_H*), and faster H₂ desorption.⁸³ Such properties enable the fast release of the active site without destroying the active site and further improve the stability of the catalysts.

3.2.3. Supercapacitors and Electromagnetic Wave Absorber

Supercapacitors (SCs) have attracted attention in recent years to bridge between a classic electrolytic capacitor and rechargeable battery, which are characterized by high power density and good cyclic stability. SCs are capable of storing electrical energy via two mechanisms: the electrochemical double layer capacitors having a nonfaradaic charge character and the pseudocapacitors based on faradaic electrochemical redox reactions.⁸⁸ The theoretical specific capacitance of a pseudocapacitive electrode is proportional to the number of electrons involved in a specific redox reaction, and vanadium oxides possess four readily accessible valence states, making vanadium oxides especially promising for high pseudocapacitance.⁸⁹ The conductivity of V₂O₃ (∼10³ Ω^–1 cm^–1) is higher than monoclinic VO₂ (∼4 Ω^–1 cm^–1) and comparable with Ru (∼10⁴ Ω^–1 cm^–1).⁹⁰ Meanwhile, V₂O₃ is stable in both acid and base mediums.⁹¹ Thus, V₂O₃ is a suitable material for energy storage, especially for SCs. However, the reported specific capacitances of V₂O₃-based materials are not good enough, suggesting that design and synthesis of new structured V₂O₃-based materials with high performance are required.⁹² Cao et al.⁹³ synthesized V₂O₃ nanoparticles highly dispersed in amorphous carbon composites through the calcination of the (NH₄)₂V₃O₈/C precursor, which was fabricated through the hydrothermal reaction by using commercial NH₄VO₃ and glucose (Figure 7a). The as-prepared V₂O₃-based composites exhibit a specific capacitance of the electrode of 458.6 F g^–1 at a current density of 0.5 A g^–1 (Figure 7b), which is higher than other reported V₂O₃-based composites.⁹⁴ Meanwhile, the asymmetric supercapacitors assembled by the as-prepared V₂O₃-based composites display good flexibility properties (Figure 7c). The capacitances are almost constant with the bending from 180° to 30°. The V₂O₃-based composites are one kind of high-efficiency electromagnetic wave absorber. Yan et al.⁹⁵ produced the hierarchical Co/C@V₂O₃ hollow spheres by hydrothermal, aging, and annealing methods (Figure 7d). The VO₂ spheres were first fabricated through a hydrothermal method followed by coating with ZIF-67 by precipitation. After that, the ZIF-67@VO₂ composite was calcined in H₂/Ar to form the hierarchical Co/C@V₂O₃ hollow spheres. The Co/C@V₂O₃ hollow spheres exhibited excellent electromagnetic wave absorption performance with a reflection loss of @40.1 dB and a broad bandwidth of 4.64 GHz at a small thickness of only 1.5 mm (Figure 7e). The good performance is mainly due to the impedance matching and low density, which come from the combination of hollow V₂O₃ spheres and porous Co/C.

Figure 7.

(a) A schematic illustration of the synthesis of highly dispersed VO-C. (b) GCD curves of VO-C collected at different current densities. (c) Capacitance retention of the VO-C||AC device measured under different bend conditions. Reproduced with permission from ref (93). Copyright 2019 Elsevier. (d) The synthesis process of hierarchical Co/C@V₂O₃ hollow spheres. (e) Reflection loss as a function of the frequency. Reproduced with permission from ref (95). Copyright 2019 John Wiley and Sons.

4. V₃O₅

4.1. Structures and Synthesis

V₃O₅ crystal structure was first reported in 1954,⁹⁶ which possesses a monoclinic symmetry (space group P2/c).⁹⁷ The crystal structure is shown in Figure 8. The oxygen atoms are occupied in the octahedral sites, and vanadium atoms are located in the center of octahedron. There are two types of octahedra chains along the c axis: face-shared octahedra via edge-sharing and corner-sharing octahedra. Such chains formed a framework with many large open spaces, which is capable of accommodating lithium ions.⁹⁸

Figure 8.

Crystal structure of V₃O₅.

It is difficult to stabilize the V₃O₅ phase by using solid-state chemistry and to control the stoichiometry between oxygen and vanadium by using a solution method, which makes V₃O₅ an uncommon phase used for electrochemical and other applications.^98,99 Therefore, the reported synthetic method for V₃O₅ is very limited. Reduction of V₂O₅ by different reductants is commonly used to obtain the V₃O₅ polycrystalline powders. Reisner et al.¹⁰⁰ selected vanadium metal as a reductant to reduce V₂O₅, and the V₃O₅ polycrystalline powders were synthesized according to the following chemical reaction: V + V₂O₅ → V₃O₅. Vanadium powder and V₂O₅ powder were mixed together and pressed into bars, which were sealed in a quartz ampule and heated for 24 h at 870 K and for 100 h at 1220 K. The V₃O₅ polycrystalline powders exhibit a size around 10 μm. Alternatively, Yu et al.⁹⁸ used sulfur powders as a reductant. The mixture of sulfur and V₂O₅ powders were vigorously grounded and calcined at 1023 K in a tube furnace under a vacuum for 2 h. The V₃O₅ microcrystals can be obtained by washing the calcined powders with nitrogen tetrasulfide several times. The obtained V₃O₅ microcrystals range from 1 to 3 μm. The large size single crystal of V₃O₅ was grown by chemical vapor transport in the 1970s.^101,102 V₃O₅ powders or V₂O₃–VO₂ mixed powders were used as starting materials, and TeCl₄ was used as a transport agent. Both starting materials and transport agent were sealed in a quartz tube with low pressure (∼1 × 10^–5 mbar), and then the tube was put into a two-zone furnace. Generally, the source zone has higher temperature, while the crystallization zone has a lower temperature, and the temperature gradient is around 100 to 150 K. The size of final V₃O₅ single crystal is as large as 1 cm.

4.2. Applications

4.2.1. Batteries

V₃O₅ is relatively less-studied though it exhibits a three-dimensional open-framework structure, which is due to the strict synthesis condition. Yu’s group⁹⁸ successfully synthesized V₃O₅ microcrystals via vacuum calcination and first employed it as an LIB anode material. The oxygen atoms are closely arranged in a hexagonal shape, and the vanadium atoms take up three-fifths of the octahedral interstices (Figure 9a). The 3D open-framework structure of V₃O₅ is formed with chain connections, which contain distorted, sharing corners, edges, and faces of VO₆ octahedral, respectively (Figure 9b). The 3D framework of V₃O₅ with much large open space endows the capacity of Li⁺ intercalation/deintercalation. No impurity peaks from the XRD pattern are observed (Figure 9c), which indicates that the V₃O₅ powder shows a single-phase nature with a monoclinic structure within the P2/c space group (JCPDS card 72-0977). It delivers a high capacity of 628 mAh g^–1 at 100 mA g^–1, a good rate (125 mAh g^–1 at 50 A g^–1), and a long stable cycling performance (117 mAh g^–1 after 2000 cycles) (Figure 9d,e). There is no obvious crystal structure change of the V₃O₅ with Li⁺ intercalation, which is the main reason for the good rate and cycling performance (Figure 9f).

Figure 9.

[101] projection illustrating the hexagonal close packing of oxygen atoms (a), the connection of VO₆ octahedral (b), the XRD pattern (c), charge/discharge profiles (inset: corresponding cycling performance at 100 mA g^–1) (d), rate performance (e) and ex-situ XRD patterns (f) of V₃O₅. Reproduced with permission from ref (98). Copyright 2019 under CC BY license.

4.2.2. Other Applications

V₃O₅ thin films exhibit a photoinduced insulator-to-metal phase transition, which results in a strong nonlinear optical response.^103−105 Fernández et al.¹⁰⁴ deposited the V₃O₅ directly on the SiO₂ substrates by DC magnetron sputtering to form thin films. The ultrafast nonlinear optical response was probed by using a pump–probe scattering technique. A reduction in the transient relative scattered light signal was observed, which showed an ∼10% decrease within 800 fs. Such a response is due to the changes in the material’s optical constants and very likely related to the photoinduced insulator-to-metal phase transition.¹⁰⁶ The photoinduced screening of electron correlations followed by melting of polaronic Wigner crystal and coalescence of V–O octahedra is the main reason for the order–disorder structural transition.¹⁰³

5. V₃O₇

5.1. Structures and Synthesis

Figure 10a shows the crystal structure of V₃O₇. The unit cell contains 36 vanadium atoms (12 vanadium atoms are inside the octahedra and 24 vanadium atoms are five-coordinated).¹⁰⁷ The V₃O₇ consists of VO₆ octahedra and VO₅ polyhedra, which are linked by corners and edges to form a three-dimensional framework. The crystal structure of V₃O₇·H₂O shows a two-dimensional structure compared with V₃O₇ (Figure 10b). Each V₃O₈ layer consists of corner- or edge-shared VO₆ octahedra and VO₅ polyhedra.¹⁰⁸ The water molecules are located at the sides of the V₃O₈ layer, where the hydrogen atoms are directly bonded to the vanadium atom of a VO₅ polyhedron and hydrogen bonds connect two neighboring V₃O₈ layers.^7,109

Figure 10.

(a) Crystal structure of V₃O₇ with VO₆ octahedra (yellow) and VO₅ (blue). (b) Crystal structure of V₃O₇·H₂O (H₂V₃O₈), with VO₆ octahedra (yellow) and VO₅ (green) and VO₅ (blue) polyhedra. Hydrogen atoms are bonded to the oxygen atoms (purple), which are not shown. Reproduced with permission from ref (108). Copyright 2018 American Chemical Society.

Bulk V₃O₇ single crystal can be grown by a typical chemical vapor transport method using V₃O₇ polycrystalline powders as starting materials and NH₄Cl as a transport agent.¹¹⁰ The mixed V₃O₇ polycrystalline powders and NH₄Cl were pressed into a pellet and heated at 823 K in an evacuated silica tube for 7 days. The black needle-like single crystals of V₃O₇ were formed with a length of 2 mm (Figure 11a). Nanostructured V₃O₇ was generally obtained by a typical hydrothermal method. The precursor solution dramatically affects the morphology of the V₃O₇ nanostructures. Wen et al. combined a soft chemical topotactic synthesis and hydrothermal process to prepare V₃O₇ nanobelts.¹¹¹ In the beginning, layered structured KV₃O₈ plate-like particles were first prepared as the precursor by a hydrothermal method of V₂O₅ and KOH. The H⁺-form vanadate (HVO) nanobelt colloidal solution was subsequently obtained by reacting KV₃O₈ plate-like particles with HNO₃. Finally, the one-dimensional (1D) pure single crystal V₃O₇ nanobelts were successfully prepared after hydrothermally treating the colloidal solution at 180 °C for 12 h (Figure 11b). By using NH₄VO₃ and HCl as starting materials, the nest-like V₃O₇ self-assembled by porous V₃O₇ nanowires on Ti foil was also synthesized through a hydrothermal method.¹¹² In the beginning, a layer of V₃O₇ nanosheets was deposited on the Ti substrate, which was subsequently placed in the NH₄VO₃–HCl solution and kept at 160 °C for 10 h for a hydrothermal method. With the increase of hydrothermal time, the nest-like V₃O₇ is self-assembled by nanowires. The schematic synthesis process and SEM images for nest-like V₃O₇ are shown in Figure 11c–f. The V₃O₇ fibers can be obtained by thermal treatment of the electrospun NH₄VO₃/PVP nanofibers in the presence of reductant.¹¹³ The thermal treatment condition dramatically affects the final products, and V₂O₅ may be obtained together with V₃O₇.

Figure 11.

(a) Optical images of bulk V₃O₇ single crystals. Reproduced with permission from ref (110). Copyright 2009 Elsevier. (b) The SEM image of V₃O₇ nanobelts obtained through a soft chemical topotactic synthesis and hydrothermal process. Reproduced with permission from ref (111). Copyright 2016 Royal Society of Chemistry. (c) Schematic synthesis process for nest-like V₃O₇. (d–f) SEM images of V₃O₇ prepared with the hydrothermal times of 1, 5, and 10 h. Reproduced with permission from ref (112). Copyright 2018 Royal Society of Chemistry.

The hydrothermal reaction is also a facile way to synthesize V₃O₇·H₂O nanostructures. The V₃O₇·H₂O nanobelts can be achieved by using V₂O₅, phenolphthalein and distilled water as starting materials through a hydrothermal method for 4 days of reaction at 180 °C.¹¹⁴ By changing phenolphthalein to ethanol or glucose, V₃O₇·H₂O nanobelts were also successfully synthesized at 180 °C, while the reaction time was shortened to 12 h.^115,116 In above-mentioned the methods, phenolphthalein, ethanol, or glucose is used as the reductant. Without the reductant, the V₃O₇·H₂O nanobelts or nanowires can be obtained by a hydrothermal reaction of only V₂O₅ or NH₄VO₃.^117,118 The key point is the pH of the precursor solution. After adjusting the pH to 3 by adding concentrated HCl, V₃O₇·H₂O nanobelts and nanowires were obtained by a reaction at 190 °C for 24 h and 160 °C for 3 h, respectively. Furthermore, ultrafine V₃O₇·H₂O nanogrids can be obtained through electrochemical oxidation.¹¹⁹ In the beginning, the nsutite-type VO₂ black powder was synthesized by a hydrothermal method. Then, a three electrode system was employed to electrochemically transform the VO₂ precursor to V₃O₇·H₂O. The slurry, consisting of VO₂ nanoplates, was coated on the working electrode. With a constant current density of 50 mA cm^–2 and a cutoff potential of 1.7 V, V₃O₇·H₂O nanogrids were obtained (Figure 12).

Figure 12.

Schematic illustration of the preparation of the nanogrid-shaped V₃O₇·H₂O. Reproduced with permission from ref (119). Copyright 2019 Royal Society of Chemistry.

5.2. Applications

5.2.1. Batteries

V₃O₇ is a mixed-valence vanadium oxide for metal-ion storage. Yan et al.¹²⁰ designed and synthesized a V₃O₇ nanowire templated graphene scroll (VGS) via an “oriented assembly” and “self-scroll” strategy. They used joint experimental-MD simulation to investigate the construction and formation mechanisms of VGS. The systemic energy, the curvature of nanowires, and the reaction time determined the length and formation process of the semihollow bicontinuous structure. Through this strategy, the VGS with a length up to 30 μm has interior cavities between the nanowire and scroll (Figure 13a). The unique structure of VGS with the nanowire templated graphene scroll offers a continuous Li⁺/ion transfer channel and free volume expansion space during Li⁺ de/intercalation (Figure 13b). The VGSs exhibit a high capacity of 321 mAh g^–1 and good cycle stability (87.3% after 400 cycles), which is better than the pure V₃O₇ nanowire and V₃O₇ nanowire/graphene structure (Figure 13c).

Figure 13.

TEM image of VGS (the inset gives an HRTEM image of a V₃O₇ nanowire in graphene scrolls) (a), schematic of the VGS nanoarchitecture with continuous electron and Li ion transfer channels (b), and cycling performances (c) of VGS, V₃O₇ nanowire/graphene, and V₃O₇ nanowire. Reproduced with permission from ref (120). Copyright 2013 American Chemical Society. The schematic illustration of reaction mechanism (d), charge/discharge profiles in aqueous (e), and nonaqueous (f) electrolytes of Zn//V₃O₇·H₂O batteries. Reproduced with permission from ref (121). Copyright 2018 Royal Society of Chemistry. The powder X-ray Rietveld refinement profile and crystal structure (g), charge/discharge profiles (h), and cycling performance (i) of V₃O₇·H₂O nanowires in MIBs. Reproduced with permission from ref (108). Copyright 2018, American Chemical Society.

In addition, Nazar et al.¹²¹ synthesized layered V₃O₇·H₂O nanobelts with single crystalline via a microwave solvothermal method and applied it to a ZIB cathode in nonaqueous and aqueous electrolytes. The electrochemical studies and in situ XRD results demonstrate the different electrochemical behaviors of layered V₃O₇·H₂O in nonaqueous and aqueous electrolytes. Combining the DFT calculations, ∼2 mol Zn²⁺ can insert into V₃O₇·H₂O per the formula in the ZnSO₄/H₂O aqueous electrolyte (Figure 13d). The V₃O₇·H₂O delivers a capacity of 400 mAh g^–1 (>2 mol Zn²⁺ insertion) with an average voltage of ∼0.65 V at the first discharge process, and maintains 375 mAh g^–1 at the subsequent charge process (Figure 13e), while in the Zn(CF₃SO₃)₂/acetonitrile nonaqueous electrolyte, the V₃O₇·H₂O exhibits a poor Zn²⁺ storage performance (59 and 175 mAh g^–1 for the first and 50th cycles at 5 mA g^–1, respectively) (Figure 13f).

Magnesium-ion battery (MIB) as another multivalent ion battery has been attracting more attention due to its high abundance in the Earth and low redox potential (−2.37 V vs. SHE). V₃O₇·H₂O with high electronic conductivity (V^+4.67) has been widely used as a cathode material in LIB/NIB and hybrid Li⁺/Mg²⁺ batteries. Hong et al.¹⁰⁸ synthesized V₃O₇·H₂O nanowires via a one-step hydrothermal method and applied them to a high-energy MIB cathode (Figure 13g). The electrochemical tests and structural characterization results demonstrate that the structured water in V₃O₇·H₂O will remain stable during the cycling. 0.97 mol Mg²⁺ inserts into V₃O₇·H₂O, accompanying the formation of Mg_0.97H₂V₃O₈ at the first discharged state. V₃O₇·H₂O exhibits an initial discharge capacity of 231 mAh g^–1 at 10 mA g^–1 with an average discharge voltage of ∼1.9 V, and the energy density can reach 440 Wh kg^–1 (Figure 13h). Meanwhile, V₃O₇·H₂O delivers a 171 mAh g^–1 and maintains 132 mAh g^–1 (77%) after 100 cycles at 40 mA g^–1 (Figure 13i). The excellent Mg²⁺ storage performance is attributed to the unique crystal structure with direct bonding. This strategy of applying water-metal bonding and hydrogen bonding provides a new idea to search for new oxide-based MIB materials with stable and high energy density.

5.2.2. Ammonium Perchlorate Decomposition

Ammonium perchlorate, a common oxidizer, plays a key role in the combustion of composite solid propellants. Furthermore, the catalyst greatly affected the performance of composite solid propellants by the thermal decomposition of ammonium perchlorate.^122,123 Huang et al.¹²⁴ found that the thermal decomposition temperatures of ammonium perchlorate in the presence of V₃O₇·H₂O nanobelts and V₃O₇·H₂O@C core–shell structures can be dramatically reduced. Both V₃O₇·H₂O nanobelts and V₃O₇·H₂O@C core–shell structures were synthesized by a hydrothermal method. Especially, the core–shell structures are synthesized by using V₃O₇·H₂O nanobelts as the cores and glucose as the source of carbon. A well-defined nanobelt morphology with a length up to several micrometers can be observed (Figure 14a), which consists of a V₃O₇·H₂O core and carbon shell (Figure 14b). The thermogravimetric analysis (TGA) indicated that the addition of V₃O₇·H₂O or V₃O₇·H₂O@C in ammonium perchlorate exhibited a significant reduction in the decomposition temperature of ammonium perchlorate (Figure 14c). The thermal decomposition temperature was lowered by 70 and 89 °C by adding V₃O₇·H₂O or V₃O₇·H₂O@C, respectively. The V₃O₇·H₂O@C core–shell structures exhibited a higher catalytic activity than V₃O₇·H₂O, with two possible mechanisms proposed. First, the partially filled 3d orbit in the vanadium atom promoted the electrotransfer process by accepting electrons from ammonium perchlorate and further accelerated the thermal decomposition of ammonium perchlorate. Second, the amorphous carbon shell possessed lots of active groups (such as C=C, C=O), which could facilitate the thermal decomposition of ammonium perchlorate.

Figure 14.

(a) and (b) TEM images of V₃O₇·H₂O@C core–shell structures, (c) TGA curves of pure ammonium perchlorate, V₃O₇·H₂O with ammonium perchlorate and V₃O₇·H₂O@C with ammonium perchlorate. Reproduced with permission from ref (124). Copyright 2011 Elsevier.

5.2.3. Supercapacitors

V₃O₇ and V₃O₇·H₂O are promising supercapacitor materials due to their layered structure and mixed oxidation states of +4 and +5. V₃O₇ can be converted to V₆O₁₃ at the lowest potential of −0.6 V and V₂O₅ at the highest potential of 0.2 V. Thus, the working potential window is in the range of −0.6 and 0.2 V. Huang et al.¹¹² fabricated porous V₃O₇ nanowire self-assembled nest-like V₃O₇ and investigated the supercapacitor properties. The pure nest-like V₃O₇ exhibits worse supercapacitor performance compared with N-doped carbon coated nest-like V₃O₇ composites. The N-doped carbon coated nest-like V₃O₇ electrode showed a higher specific capacity of 660.63 F g^–1 at 0.5 A g^–1 compared to V₃O₇ (362.63 F g^–1). Even at a higher current density of 50 A g^–1, the N-doped carbon - V₃O₇ electrode still exhibits a better performance (187.72 F g^–1) than V₃O₇ (33.18 F g^–1), as shown in Figure 15a. Furthermore, the N-doped carbon coated nest-like V₃O₇ electrode also delivers better stability (80.47% capacitance retention after 4000 cycles) compared with pure V₃O₇ (23.16% capacitance retention after 4000 cycles)_. The superior performance of N-doped carbon coated nest-like V₃O₇ is mainly due to the unique three-layer structure: V₃O₇ core/carbon/nitrogen doped carbon (Figure 15b). Such a unique three-layer structure can not only stabilize V₃O₇, but also provide high-speed ionic and electronic transmission channels, which is responsible for the good supercapacitor performance of N-doped carbon coated nest-like V₃O₇. Yu et al.¹²⁵ reported the growth of V₃O₇ nanowires on a carbon fiber cloth through a hydrothermal method. The obtained V₃O₇/carbon fiber cloth composites show a spider web-like morphology, which exhibits robust adhesion. The composite electrode gives a maximum specific capacitance of 151 F g^–1 at a current density of 1 A g^–1 with ultrahigh cycling stability of 97% (after 100000 cycles) in a full cell configuration (Figure 15c). Meanwhile, the V₃O₇/carbon fiber cloth composites reveal maximum power and energy densities of 5.128 kW kg^–1 and 24.7 Wh kg^–1, respectively by using 1-ethyl-3-methylimidazolium trifluoromethanesulfonate as the electrolyte. Furthermore, coin cell-type configuration with the V₃O₇- carbon fiber cloth composites electrode was assembled. The symmetric supercapacitors successfully and effectively power light-emitting diodes to produce blue light (Figure 15d). Huang et al.¹¹⁵ reported that V₃O₇·H₂O nanobelts exhibited a capacitance of 447.6 F g^–1. However, the cycling performance is limited by the poor conductivity and high solubility in an aqueous electrolyte. Therefore, composing with another conductive phase could be an alternative way to fabricate high-performance V₃O₇·H₂O based materials. When the V₃O₇·H₂O nanobelt is incorporated with carbon nanotube and reduced graphene, the formed 3D hierarchical porous composites exhibit outstanding electrochemical performance with a high specific capacitance (685 F g^–1 at 0.5 A g^–1) and excellent cycle stability (99.7% after 10,000 cycles) (Figure 15e,f).¹²⁶ Meanwhile, the composites also give relatively high energy densities and power densities of 34.3 Wh kg^–1 and 150 W kg^–1, respectively. The better electrochemical performance can be attributed not only to the highly conductive carbon materials, but also to the 3D hierarchical porous structure. The carbon materials offer the transport pathway bridges, leading to the rapid transfer of charges. Meanwhile, the 3D porous structure minimizes the diffusion distance and supplies a large surface area with abundant active sites.

Figure 15.

(a) Rate capability of V₃O₇ and N-doped carbon coated nest-like V₃O₇ calculated from the charge/discharge curves as a function of current density, (b) schematic diagram of the crystal structure and bonding in N-doped carbon coated nest-like V₃O₇. Reproduced with permission from ref (112). Copyright 2018 Royal Society of Chemistry. (c) Cycling stability of V₃O₇- carbon fiber cloth symmetric supercapacitors devices at a constant current density of 10 A g^–1 for 100 000 cycles, (d) series connection of three-coin cells for practical applications, charging process of serially connected coin cells, and a demonstration of LEDs lit with charged coin cells. Reproduced with permission from ref (125). Copyright 2018 Royal Society of Chemistry. (e) Comparison of specific capacitance at different current densities from GCD curves, (f) cycling performance at 100 mV s^–1 of V₂O₅ nanosheets, VO_x/CNT, and V₃O₇·H₂O/CNT/rGO. Reproduced with permission from ref (126). Copyright 2018 Elsevier.

In general, vanadium oxides have received massive interest as supercapacitor electrodes that exhibit high theoretical specific capacity than most of the other transition metal oxides due to their variable valence state from V²⁺ to V⁵⁺. In addition, the layered structure of vanadium oxides facilitates the intercalation/deintercalation of electrolyte ions during the charging/discharging process. However, vanadium oxides-based supercapacitor electrode materials still suffer from poor long-term cycling stability, which is usually caused by the collapse of a layered crystal structure, severe agglomeration of particles, and low electrical conductivity. The electrochemical stability of vanadium oxide-based supercapacitors can be improved by material modification, optimization of the structure, or combining with other materials with excellent electrical conductivity. Developing vanadium oxide nanomaterials with suitable micro-/nanostructures is an important factor to improve the cycling stability. 3D vanadium oxides with micro-/nanostructures are often employed, including microspheres and hollow spheres, which can inherit the superior high surface area characteristics of nanobuilding blocks, and simultaneously possess a decent structural stability.^127−130 Furthermore, the 3D structure can effectively reduce the agglomeration of particles, which is beneficial for the cycling and rate performance.¹³¹ Integrating vanadium oxides with carbon materials has been demonstrated to be effective to suppress the structural degradation upon cycling. The carbon materials, such as porous carbons and graphene, can serve as elastic buffering layers to release the strain within metal oxides during cycling.¹³² To some extent, carbon materials can avoid loose attachment between the electrode material and current collector, which helps improve both the conductivity and the stability of the supercapacitor.

5.2.4. Electrochromism

Nanostructured V₃O₇ thin films showed electrochromic properties by using lithium perchlorate as the electrolyte, which was prepared by a nebulizer spray pyrolysis technique.¹³³ The color of the films is changed from yellow to pale blue by applying an external potential of 1.5 V for intercalation of Li⁺ ions, while the color is reversed by applying an external potential of −1.5 V for deintercalation of Li⁺ ions. Such results indicate that the nanostructured V₃O₇ thin films could be effectively used for smart window applications. In general, several models are proposed to explain the electrochromic phenomenon, such as the color center model (Deb model), electrochemical redox model, and so on.¹³⁴ The “Deb’s color center model” is in nature related to the defects (such as oxygen vacancies) induced change of the visible light absorption, which is generally independent of the external electric field,¹³⁵ while in the “electrochemical redox model” it is believed that the injection and trapping of a large density of electron and hole lead to coloration.¹³⁶ The mechanism of electrochromism for V₃O₇ thin films is the variation of the band structure caused by intercalation/deintercalation of Li⁺ ions, which belongs to the electrochemical redox model. During the application of negative bias, Li⁺ ions are absorbed onto the surface of V₃O₇ and diffuse into the lattice of V₃O₇. The intercalated Li⁺ ions react with O^2– ions to introduce oxygen vacancies in the lattice and reduce the V⁵⁺ in the mixed state V₃O₇ to V⁴⁺. As the result, the optical transparency of the film changes.¹³⁷ Moreover, the intercalation of Li⁺ ions upshifts the Fermi level close to the conduction band, which leads to an increased transmittance of V₃O₇. The process can be reversed by deintercalating of Li⁺ ions through applying positive bias. Furthermore, the V₃O₇·H₂O thick films also show good electrochromic properties (good reversibility and good color switching between reduced and oxidized states).¹³⁸ By using lithium bis-trifluoromethanesulfonimide (LiTFSI) as the electrolyte, the color of the V₃O₇·H₂O films changes from green to orange by applying a positive potential of 1.9 V, while it changes from green to blue by applying a negative potential of −1.2 V (Figure 16a). Such film presented a maximum optical reflectance modulation of 29% at 590 nm (Figure 16b). The V₃O₇·H₂O films still exhibited color changes by using a Na-based electrolyte (Figure 16c) and a maximum optical reflectance modulation of 10% at 590 nm (Figure 16d). The maximum optical reflectance modulation of Na-based electrolyte is lower than that in the Li-based electrolytes, which is mainly due to a larger faradaic contribution resulting from the larger cation size of Na⁺ ions.

Figure 16.

(a) The visual appearance of V₃O₇·H₂O films in the reduced and oxidized states in LiTFSI, (b) diffuse reflectance spectra for the V₃O₇·H₂O films cycled in Li-electrolyte/Pt vs SCE, in reduction at −1.2 V and reoxidized at +1.9 V, (c) the visual appearance of V₃O₇·H₂O films in the reduced and oxidized states in NaTFSI, (d) diffuse reflectance spectra for the V₃O₇·H₂O films cycled in Na-electrolyte/Pt vs SCE, in reduction at −1 V and reoxidized at +1.9 V. Reproduced with permission from ref (138). Copyright 2020 Royal Society of Chemistry.

6. VO₂

6.1. Structures and Synthesis

Vanadium dioxide (VO₂) can exist in various polymorphic phases, including but not limited to VO₂ (B), VO₂ (A), VO₂ (M), VO₂ (R), VO₂ (D), and VO₂ (P).¹³⁹ Some of these phases and their corresponding lattice parameters are shown in Figure 17. The discussion will focus more on VO₂ (R) and its reversible MIT to VO₂ (M) as well as on VO₂ (B) due to its application as cathode materials in electrochemical devices.

Figure 17.

Different phases of VO₂ with respective lattice parameters. Reproduced with permission from ref (147). Copyright 2016 under CC BY 4.0 license.

VO₂ was first demonstrated to undergo MIT in 1959 by Morin. At the critical temperature (τ_C) of about 340 K, VO₂ transforms from high-temperature, conducting rutile VO₂ (R) to low-temperature, insulating monoclinic VO₂ (M). There are two mechanisms that have been used to describe this ultrafast phase transition phenomenon in VO₂, namely the Peierls model and Mott-Hubbard model. The Peierls model describes the nature of MIT as the change from a shared d-orbital between all vanadium atoms to localized d-orbital in the V–V dimer, which is a result of the change in the V–V distance from 2.88 Å in VO₂ (R) to 2.65 and 3.12 Å in VO₂ (M).²⁰ Therefore, the Peierls model stated that the structural distortion is the cause of MIT. Wentzcovitch et al. applied the local-density approximation to study the electronic and structural change of VO₂ during the MIT.¹⁴⁰ From the view of band theory, a monoclinic distorted state is in good agreement with the experiment result. Meanwhile, the structural distortion enhances the bonding between neighboring V atoms, which is expected in the Peierls model. Moreover, Baum et al. utilized four-dimensional (4D) femtosecond electron diffraction to visualize the phase transformation process of VO₂.¹⁴¹ They pointed out that during the MIT, the displacement of atoms happened within picoseconds, and followed by the sound wave shear motion of the crystal in the time scale of nanoseconds. The observation indicates the occurrence of fast structural distortion during the MIT.

On the other hand, the Mott-Hubbard model states that MIT would occur when the electron density (n_e) and Bohr radius (a_H) satisfy n_e^1/3a_H ≈ 0.2.¹⁴² Compared with the Peierls model, the Mott-Hubbard model has the advantage in explaining the phenomenon such as the anomalously low conductivity in the metallic phase.¹⁴³ Whittaker et al. summarized multiple experiment cases and pointed out that the metallic phase of VO₂ might be introduced without the structural phase transformation if the excitation of carriers reaches a threshold density.¹⁴⁴ Their observation provides key evidence in revealing the nature of MIT. While there remains a debate on which mechanism best describes the MIT, the usage of both models is strongly encouraged due to the transition kinetics¹⁴⁵ as well as the stimuli involved during the transition. Shao et al.¹⁴⁶ reported recent progress in understanding the mechanism and kinetics of MIT, including the lattice distortion and electron correlations (Peierls phase transition, Mott phase transition) and modulation methods (elemental doping, external electric field, light irradiation, and strain engineering).

Upon application of suitable external stimuli (i.e., photons, heat, electric, magnetic, electrochemical, and stress) to initiate the MIT, physical properties of VO₂, such as electrical resistance, optical transmittance, and thermal conductivity, are reversibly and drastically changed. Long et al.²⁷ have summarized the connections between the stimuli and responses of VO₂-based devices in detail. This flexibility in external stimuli and corresponding responses is the main reason why VO₂ is a material of interest in multiple novel devices spanning from thermally,^139,148 electrically,^149−151 to optically^152,153 activated devices. The functional performance of these devices is thus highly dependent on, but not limited to, the physical attributes, such as dimension, morphology, doping level, and crystallinity, of the fabricated VO₂. Multiple techniques have been used to fabricate functional VO₂ devices, each with their own strengths and weaknesses. Some of these fabrication processes are reviewed with polymer-assisted deposition methods,¹⁵⁴ hydrothermal,¹³⁹ sol–gel,¹⁵⁵ chemical vapor deposition (CVD),¹⁹ and physical vapor deposition (PVD)¹⁵⁶ methods discussed in detail regarding the controlled synthesis of VO₂ for thermochromic application.

6.2. Applications

In recent years, multiple omnibus reviews^157−159 have been done in attempts to give the most encompassed view of VO₂ research progress, often including a combination of the following topics: MIT mechanism, kinetics, fabrication techniques, and applications. While these reviews could offer wide coverage of VO₂ research progress in multiple topics, in-depth discussion of each topic, applications in this case, is in much needed demand. In subsequent sections, reported applications of VO₂ in functional devices both based and not based on MIT in recent years, especially in the last three years, are classified and discussed in different categories: optical, electrical, and mechanical applications.

6.2.1. Optical Applications

The operations of these VO₂-based optical devices are often based on the changes in refractive index, n, and extinction coefficient, k of VO₂ upon crossing the MIT. The changes to n and k at different temperatures are shown in Figure 18. These optical functional devices are further divided into two main groups: infrared regulators and optical switches.

Figure 18.

Optical constants n and k of VO₂ film with respect to the wavelength and the temperature changes. Reproduced with permission from ref (160). Copyright 2019 John Wiley and Sons.

6.2.1.1. Infrared Regulators

Since its first introduction in 1985,¹⁶¹ the smart window has attracted much attention due to rising energy consumption in the commercial building sector, which contributes up to 40% of total consumption globally and leads to 30% of global greenhouse gas emissions today.¹⁶² VO₂ is the prime candidate for smart window materials due to its ability to seamlessly and rapidly regulate the amount of infrared (IR) across MIT with miniscule side effects to the visible transmission. However, intrinsic limitations (high τ_C ≈ 68 °C, low luminous transmittance (T_lum) < 40%, poor solar modulation (ΔT_sol) < 10%, and poor durability) prevent pristine VO₂ from meeting the requirements for commercial smart window applications. The most common way to reduce τ_C is by elemental (i.e., W, Mo, Ti, F, Mg, etc.) doping as summarized by Cui et al.¹⁶³ Different approaches exist to enhance T_lum and ΔT_sol via advanced device morphological engineering such as multilayered VO₂,¹⁶⁴ biomimetic structure,^165−168 nanothermochromism,¹⁶⁹ porous,¹⁷⁰ and gridded structure.^171−173 Zhou et al. recently reported a new customized VO₂ composite structure in which a new factor, the incident angle, was considered in the development of smart window devices. Figure 19a shows a schematic of how different incident angles in the summer and winter can be taken advantage of with the reported customizable composite structure. The VO₂ composite structure was fabricated through a 3D printing procedure, enabling the flexibility of device dimensions. Several samples of different thicknesses, widths, and spacing are demonstrated in Figure 19b. The incident-angle dependency properties of this structure are shown in Figure 19c,d in which modulating performance of the 3D printed device improves significantly, and ΔT_sol improves from 9.7% to 25.8% with respect to an increase in the incident angle from 0° to 45°.¹⁷³

Figure 19.

(a) Schematic design of the 3D printed smart windows design. (b) Optical microscopic pictures of the printed composite structures with identical 0° tilted angles with a range of thickness, spacing, and width. (c) UV–vis–NIR transmittance spectrum of the printed composite structure with respect to different incident angles and temperature. (d) T_lum, ΔT_sol, and ΔT_IR diagram of the printed composite structures with different incident angles. Reproduced with permission from ref (173). Copyright 2020 John Wiley and Sons.

Aside from NIR transmittance regulation in smart windows, VO₂ also has an unusual ability to change its long-wave infrared emissivity (ε_LWIR) upon crossing the MIT. An ideal smart window should have high transparency in the visible region (380–780 nm), while having a transparent state in the winter and an opaque state in the summer (Figure 20a). Moreover, the ideal smart window should have a high ε_LWIR at a high temperature to promote radiative cooling (RC) and a low ε_LWIR at a low temperature to suppress RC. Based on this concept, Long et al.¹⁷⁴ fabricated a VO₂-based multilayer structure which was able to regulate NIR transmittance and ε_LWIR spontaneously (Figure 20b). Through forming a Fabry–Perot resonator, the passive RC regulating thermochromic (RCRT) smart window possessed an ε_LWIR of 0.21 at 20 °C, while the ε_LWIR increased to 0.61 above τ_c. In addition, the RCRT window kept a promising T_lum of 27.8% and a ΔT_sol of 9.3% (Figure 20c). With the actual building energy consumption simulation conducted with a 12-story building, the RCRT window yielded a higher energy savings compared with a commercial low-E window across different climate zones (Figure 20d). Meantime, Wu et al.¹⁷⁵ designed a flexible temperature-adaptive radiative coating (TARC) through embedding lithographically patterned W-doped VO₂ in a dielectric BaF₂ layer on top of a reflective gold layer (Figure 20e). The TARC film had a low ε_LWIR (∼0.2) in the insulation state and a high ε_LWIR (∼0.9) at the metallic state (Figure 20f), and the observation agreed with the simulation (Figure 20g). Long et al.¹⁷⁶ further expanded the concept of RC regulation from window to wall by preparing a switchable interwoven structure. As shown in Figure 20h, through pulling the block of interwoven structure, the original exposed block on the top side becomes concealed, and the underneath block becomes exposed. As a result, the structure switches its phase from phase 0 to phase 1. Taking into account different requirements of windows and walls, Long’s group designed on-demand interwoven structures. Figure 20i shows the interwoven structure for window and wall applications. As discussed in Figure 20a, a window requires high visible transmittance and dual-band regulation for NIR and LWIR ranges. An ITO/VO₂/PVC combination was employed for windows. In this structure, VO₂ was used to regulate NIR, while the ε_LWIR was regulated by alternatively exposing low-E ITO and high-E PVC. On the other hand, an ideal wall has high solar absorption and low ε_LWIR in the winter, and low solar absorption and high ε_LWIR in the summer. An ITO/black paint/PVDF-HFP combination was utilized to cater to this demand. On cold days, visible transparent ITO is exposed, and sunlight will be absorbed by black paint underneath, while on hot days, the highly solar reflective high-E PVDF-HFP is exposed to prompt RC. Hence, compared with a conventional performance index T_lum and ΔT_sol, the newly proposed ε_LWIR needs to be included to gauge the real energy-saving performance.¹⁷⁷ Moreover, VO₂ is notoriously known for poor durability, which is the bottleneck for the applications in smart windows. There are two recent reports to embed VO₂ in the V₂O₅ matrix, and such a strategy could increase the lifetime up to 33 years.^178,179

Figure 20.

(a) Concept of the ideal energy-saving smart window. The red and blue lines represent the spectra for an ideal energy-saving smart window in the summer and winter. (b) Schematic structure of the RCRT window. (c) Spectra of the sample with RC regulation in the visible-NIR and LWIR range at 20 °C (blue line) and 90 °C (red line) against a normalized AM1.5 global solar spectrum (yellow shadow) and LWIR atmospheric transmittance window (blue shadow). (d) Estimated world heating and cooling energy-savings of a W-doped max Δε sample against a commercial low-E glass as the baseline. Reproduced with permission from ref (174). Copyright 2021 American Association for the Advancement of Science. (e) Schematics of materials composition and working mechanism of the TARC. (f) IR images of TARC compared with those of two conventional materials (references) with constantly low or high thermal emittance showing the temperature-adaptive switching in the thermal emittance of TARC. (g) Solar spectral absorptance and part of the thermal spectral emittance of TARC at a low temperature and a high temperature. Measurements (solid curves) show consistency with theoretical predictions (dashed curves). Reproduced with permission from ref (175). Copyright 2021 American Association for the Advancement of Science. (h) Demonstration of the surface transition in a meter-scale Al-paper sample. (i) Photographs and corresponding thermal images of the ITO/VO₂/PVC sample (“window” in the figure) and the ITO/black paint/PVDF-HFP sample (“wall” in the figure) on the two phases. Effective areas are marked by the dashed lines. Reproduced with permission from ref (176). Copyright 2022 American Chemical Society.

Besides the application in building, the unique property of emissivity switching makes VO₂ the material of interest for IR camouflage and passive radiator for military and aerospace applications because VO₂-based devices can function entirely on the thermal trigger with no additional sources, electrical or otherwise, required. VO₂-based camouflage devices work by reducing the amount of IR emitted into the environment, shrouding the user from being detected with an IR detector such as most night-vision technologies. Examples include VO₂/graphene/CNT heterostructure by Xiao et al.,¹⁸⁰ VO₂/carbon hybrid by Wang et al.,¹⁸¹ and VO₂/ZnS core–shell structure (Figure 21a) by Ji et al.¹⁸² As seen in Figure 21b, under a similar IR detector and temperature, the VO₂/ZnS core–shell pallets exhibit the ability to control their IR radiation intensity and lower their detected temperature as compared to V₂O₅ pallets with constant emissivity.

Figure 21.

(a) Schematic of the VO₂/ZnS core–shell nanopowder design. (b) The infrared thermal images of same actual temperature for VO₂/ZnS core–shell nanopowder and reference V₂O₅ pellets at 45 °C (upper) and 90 °C (lower). Reproduced with permission from ref (182). Copyright 2018 Elsevier.

Different from a camouflage device, a VO₂-based passive radiator requires modification to the device structure to counter the lower emissivity at the higher temperature. This intrinsic problem could be overcome by depositing VO₂ on a highly reflective metal substrate with^183,184 or without¹⁸⁵ a spacer layer. Figure 22a is a multilayer Si/VO₂/BaF₂/Au structure reported by Kim et al.,¹⁸⁴ which was designed and tested specifically for simulated space (vacuum) applications. Figure 22b demonstrates the radiated thermal power of the multilayer device. Experimental data were compared with simulated ones for both high and low-temperature operations. The measured radiated power at 300 and 373 K was 72 W/m² and 552 W/m², respectively, showing a massive jump in emitted radiation upon crossing the MIT threshold.¹⁸⁴ Due to the typical multilayer design of a VO₂-based passive radiator, factors such as functional emissivity difference between low and high temperature as well as the wavelength of emitted light can be further fine-tuned by adjusting the substrate/spacer/film combination. The electrochromic setup has also been shown to also result in IR regulating behavior in VO₂-based devices.¹⁸⁶Figure 22c is the schematic of a three-terminal thin-film-transistor-type electrochromic device by Katase et al.¹⁸⁶ Upon application of external voltage (+12 V according to literature), the VO₂ channel undergoes protonation, and the device becomes IR opaque, similar to smart window applications. When a reversed voltage is applied, deprotonation happens, and the device becomes IR transparent once again. Figure 22d shows optical transmittance spectra measured during this transition with +12 V stimulus. The optical transmittance modulation ratio at λ of 3000 nm was 49%.¹⁸⁶ Electrical input into a VO₂-based device can also be utilized as a Joule heating source for MIT. To realize this, VO₂ can be combined with transparent conductive electrode materials such as ITO, Ag NWs, CNT, etc.^187,188 to become an electro-optic modulator. An example of such a modulator is a VO₂+Au/GaN/Al₂O₃ device by Fan et al.,¹⁸⁹ which has the ability to change the transmission step-by-step according to the applied voltage.

Figure 22.

(a) Schematic of the Si/VO₂/BaF₂/Au passive radiator design. (b) Radiated thermal power of the Si/VO₂/BaF₂/Au device, with heating and cooling indicated with arrows; the dotted line is the simulation data for VO₂ (M) and VO₂ (R) respectively. Reproduced with permission from ref (184). Copyright 2019 under CC BY 4.0 license. (c) Schematic of the three-terminal transistor design. (d) Optical transmittance spectra measured before (red line) and after (blue line) applying V = +12 V. Reproduced with permission from ref (186). Copyright 2017 under CC BY license.

6.2.1.2. Optical Switches

Based on the sudden change in the optical constants n and k of VO₂ across MIT, radio frequency (RF) switches or waveguides can be fabricated to control the flow of electromagnetic waves (i.e., microwave and radiowave).¹⁹⁰ Even though the design of each device is largely dependent on whether it is thermally¹⁹¹ or electrically activated,¹²⁰ the mechanism for turning from the ON to OFF state is still entirely based on the transition from VO₂(M) to VO₂(R) respectively. Examples of RF-switches are the thermally configurable hybrid Al nanoholes/VO₂ photonic switch by Sun et al.,¹⁹² metamaterial design by Ding et al. which can act as an absorber from 0.562 to 1.232 THz at room temperature and a high-efficiency halfwave plate at high temperature,¹⁹³ and temperature controlled asymmetric optical switch by Liu et al.¹⁹⁴Figure 23a shows the working principal of this design in which different output of the same polarized electromagnetic wave input can be achieved at low temperature by physically reverting the device by 180° while achieving similar output at high temperature. This asymmetrical mechanism is demonstrated in Figure 23b,c. A large asymmetry exceeding 20% was detected at 23 °C, while it disappeared almost entirely at 87 °C (Figure 23b). With incident x-polarized waves, the device gave y-polarized waves output at 23 °C, and this can be considered the ON state. Upon heating to 87 °C, output waves returned to approximately x-polarized, similar to input waves, turning the switch OFF (Figure 23c).¹⁹⁴ While the thermal- and electrical-activated optical switches mainly depend on the change of optical constant upon MIT, the optical-activated switches of VO₂ focus on the speed of the transition as the defining factor. However, the details of the ultrafast induced phase transition of VO₂ are not the focus of this review; it can be found in a summary and discussion by Wegkamp et al.¹⁹⁵ VO₂ ability to switch between the insulator and metal state within picoseconds is promising for the field of nanophotonics as well as all-optical integrated circuits (i.e., switches, modulators, and data-storage devices). VO₂ integrated metamaterials have been reported to exhibit nonlinear transmittance by Liu et al.¹⁵³ in the THz range as well as broadband responses spanning from the visible to mid-infrared range by Guo et al.¹⁹⁶ VO₂/Au nanoplate memory device was also reported by Lei et al.,¹⁹⁷ giving a stepwise tuning ability with the use of successive laser pulses.

Figure 23.

(a) Schematic of the temperature-controlled VO₂ metamaterial asymmetric optical switch. (b) Frequency dependence of the asymmetric transmission parameter for linearly polarized waves, and (c) transmitted polarization state at 1.15 THz for illumination with x-polarized waves. Reproduced with permission from ref (194). Copyright 2019 under CC BY 4.0 license.

6.2.1.3. Plasmonic Applications

The reversible crystal phase transition makes VO₂ very unique among plasmonic materials. It undergoes a crystal phase transition from the monoclinic semiconductor state to rutile metallic state with significantly promoted conductivity and free carrier density,²⁷ leading to a significant difference in its plasmonic property. Recently, VO₂ nanoparticles (NPs) have been reported with thermal-responsive localized surface plasmonic resonance (LSPR) in the NIR region.^168,198 Based on the colloidal lithography method,⁵⁹ Long’s group successfully produced the hexagonally patterned VO₂ NPs on quartz with controllable average diameters from ∼70 to ∼280 nm.¹⁹⁸ It was observed that the LSPR position of metallic VO₂ shifts to the longer wavelength on the larger NPs (∼1120 to ∼1220 nm) or under the increasing reflective index of the surrounding medium (∼1120 to ∼1360 nm) (Figure 24a). Besides, the NIR LSPR is temperature-responsive that is quenched on a low-temperature semiconductor state and can be gradually switched on from 20 to 100 °C (Figure 24b). They further investigated the LSPR-induced absorbance and scattering effects of VO₂ plasmonics through a finite-difference time-domain method.¹⁶⁸ On a single VO₂ NP, it is revealed that both the absorbance and scattering are low at the semiconductor state (monoclinic, M), while a strong absorbance emerges in the metallic state (rutile, R) (Figure 24c). This result suggests that the LSPR in metallic VO₂ is characterized as a strong absorbance enhancement and a relatively weak scattering effect. Moreover, they reported the dispersity- and strain-induced LSPR response on VO₂ NPs in the polydimethylsiloxane (PDMS) elastomer matrix (Figure 24d,e).²⁸ The dispersity-induced LSPR position can be attributed to the changes in average gaps among VO₂ NPs in the matrix, which is consistent with the simulation result (Figure 24d), while the strain-dependent LSPR position change can be explained by the local reflective index change induced by the delamination between the NP and matrix under applied strains as being demonstrated by the finite element method (Figure 24e,f). A more recent report used a similar approach to tailor the VO₂ surface plasmon by manipulating its atomic defects and establishing a universal quantitative understanding.¹⁹⁹ Record high tunability is achieved for LSPR energy from 0.66 to 1.16 eV and a transition temperature range from 40 to 100 °C. The Drude model and DFT calculation reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. It is believed the investigation of VO₂ LSPR is still in its early stage. The reversible crystal transition makes VO₂ an intrinsic active plasmonic material, which is unique among the plasmonic field. It is expected for researchers to further understand the VO₂ plasmonic and to explore its potential applications.

Figure 24.

(a) Effects of the particle diameter and medium reflective index to the LSPR position. Insets are the SEM images of VO₂ NPs with corresponding diameters, and the array structures are highlighted as yellow hexagons. (b) Extinction spectrum of VO₂ NP under different temperatures from 20 to 100 °C. Reproduced with permission from ref (198). Copyright 2017 American Chemical Society. (c) Simulated absorbance and scattering intensity crossing a VO₂ NP embedded in PDMS matrix. Reproduced with permission from ref (168). Copyright 2020 Elsevier. (d) Experimental LSPR position of VO₂ NPs with different dispersion degrees in PDMS, and the simulation result of two adjacent VO₂ NPs with a different gap. (e) Experimental and simulation results of the LSPR position of VO₂–PDMS composites under applied strains from 0% to 100%. (f) Simulated stress contours of the representative VO₂–PDMS composite under applied strain from 0% to 100%. Reproduced with permission from ref (28). Copyright 2019, Cell Press.

6.2.2. Electrical Applications

Not only optical constants, but the electrical conductivity of VO₂ is also altered dramatically upon transitioning from insulating VO₂ (M) to metallic VO₂ (R). This measurable electrical response to various external stimuli makes VO₂ the prime candidate for electrical applications such as sensors or transistors. The following section discusses the electrical applications of VO₂ and their corresponding devices.

6.2.2.1. Sensors

A sensor is defined by its ability to measure physical input and translate these measurements into interpretable data. Based on the significant conductivity changing of VO₂ across the MIT, it is possible to convert physical environmental input into a readable electrical signal. Some examples of VO₂-based sensors include temperature sensors, photodetectors, flexible strain sensors, and gas sensors. Intrinsically, VO₂ is not suitable for temperature sensing applications because the change in its electrical conductivity only happens at 68 °C, even though Kim et al.²⁰⁰ managed to fabricate a programmable VO₂ critical temperature sensor. VO₂ was deposited on an Al₂O₃ (1010) substrate and between two nickel (Ni) electrodes. A voltage can be applied across these electrodes to cause the τ_C of VO₂ to decrease, causing the VO₂ (M) film to go into an intermediate phase before fully transitioning into VO₂ (R). During this intermediate phase, the measured current through the device was found to be linear with the change in temperature. At a voltage of 20 V, the τ_C is found to be ∼20 °C, enabling full range sensing capabilities from 20 to 68 °C. Another approach, which is based on the sensing ability resulting from an abrupt change of the dielectrically constant of VO₂ during MIT instead of conductivity, was done by Yang et al.²⁰¹ As mentioned in the previous section, optically stimulated applications of VO₂ are promising due to the ultrafast transition mechanics, stability, as well as the broadband optical response of VO₂-based devices. Hou et al.²⁰² demonstrated the device stability and speed of response using a VO₂ (M) nanowire on Au electrode setup (Figure 25a). It was reported that the device needs less than 1.6 s to detect IR (980 nm) and <1.0 s to recover (Figure 25b). The device was also reported to maintain responsivity for more than 500 cycles. Another design from Takeya et al.²⁰³ combined the photoresponsivity of VO₂ film and the localized surface plasmon resonance of silver nanorods. The results indicated a correlation between the incident light transmission and resistivity within a wavelength of 400–900 nm. While the VO₂ acted as a photosensitive component, the nanorod array introduced a wavelength and polarization sensitivity to the photodetector. Because the MIT of VO₂ results in the change in the lattice structure and constant, it is also possible to induce MIT by causing changes to the lattice through mechanical force, which serves as the basis for VO₂ flexible strain sensor application. Hu et al.²⁰⁴ showed that a VO₂ strain sensor device could be fabricated by bonding one single VO₂ nanobeam to a polystyrene (PS) substrate with silver paste and measuring the resistivity of the nanobeam as tensile and compressive stress is applied along the length of the nanobeam. In this study, VO₂ (R) was not formed, and only the VO₂ (M₂) phase was formed due to the constraint of force applied (only 0.25% as compared to the required 2% at room temperature). However, the device showed remarkable potential when exhibiting a stepwise response to as small as 0.05% tensile or compressive strain.

Figure 25.

(a) Schematic of the VO₂ nanowire photodetector. (b) Time-dependent photodetection capabilities of the VO₂ nanowire device. Reproduced with permission from ref (202). Copyright 2018 Elsevier. (c) Schematic of the VO₂/CNT gas sensor. (d) Time-dependent humidity sensing capabilities of the VO₂/SWCNT and n-doped variant at different humidity levels. Reproduced with permission from ref (206). Copyright 2018 Elsevier.

A typical gas sensor design is demonstrated in Figure 25c. For gas sensor application, the semimetallic VO₂ (B) phase is more commonly used than the insulating VO₂ (M) phase to maintain sensing capability at room temperature. VO₂, regardless of the phase, responds to humidity, ammonia (NH₃), and nitrogen dioxide (NO₂).^205,206 Compositing VO₂ with carbon species such as single- or multiwalled carbon nanotubes (SWCNTs or MWCNTs) was reported by Evans et al.²⁰⁶ The setup was effective in creating a stable, responsive VO₂–CNT gas sensor. Figure 25d shows excellent response and good recovery of VO₂–SWCNT to different humidity levels. The resistive response was increased dramatically from 0.5 for pure VO₂ (B) to 2.7 for VO₂–SWCNT and 7.1 for VO₂-MWCNT at 50% humidity. This p-type gas sensing response was also reported for NH₃ in the same study despite the longer and lower recovery level recorded. Depending on the applications, property change across the MIT is not the only viable option to use VO₂ in a functional device.

6.2.2.2. Electrical Switches, FETs, Oscillators, and Memristors

Different from sensing applications, the MIT of VO₂ can be deliberately triggered with programmable duration and patterns to great advantage in electrical switching, FET, oscillator, and memory devices. Similar to optical switches in the previous section, by toggling VO₂ across the MIT, it is possible to create an ON/OFF switching mechanism based on the difference in electrical resistance of VO₂ (M) and VO₂ (R). It has been demonstrated by Zhou et al.²⁰⁷ that a two-terminal VO₂-based switching device can have ultrafast, reliable 2 orders of magnitude ON/OFF toggling ability within 2 ns. While the MIT in this report was induced by an applied current, an electrical switch activated by Joule heating was also reported in a separate study by Li et al.²⁰⁸ Mott FET is a gated FET device in which the conventional semiconductor channel is swapped with a Mott insulator, a material with the ability to switch from insulator to metal through external voltage to the gate. VO₂, as a Mott insulator, is the prime material for novel Mott FET studies. An example of a typical Mott FET setup was reported by Yajima et al.²⁰⁹ in which a large current modulation can be observed at 315 K, indicating a positive-bias gate-controlled MIT near τ_C of VO₂. Another novel Mott FET design was also fabricated by Shukla et al.²¹⁰ with a VO₂ as a source terminal. This design functioned well at room temperature (300 K) with reversible MIT triggered by the critical applied current. Taking advantage of the VO₂-based Mott FET designs and combining it with a ferroelectric material, Zhang et al.²¹¹ fabricated a nonvolatile ferroelectric FET (FeFET) device with VO₂ nanowires as the channel and Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin film as the dielectric gate (Figure 26a). The novel FeFET device was reported to achieve up to 85% resistance change under a gate voltage of 18 V (Figure 26b). Interestingly, the presence of the ferroelectric materials created a polarization effect after the applied voltage was removed, in which the channel resistance could attain up to 50%. Through this mechanism, it is possible to achieve multiple resistive states by the sweeping suitable gate voltage. To overcome the disadvantage of solid-gate oxide dielectric FET, such as current leakage, which might interfere with the MIT of VO₂, ionic liquid (IL) and solid-state electrolyte gating have been the research interest for VO₂ FET devices in recent years.²¹² However, the mechanism in which IL drives the MIT of VO₂ is still a debate between different studies. Nanako et al.¹⁵¹ suggested that the underlying mechanism is the bulk carrier delocalization caused by the electrostatic effect. On a different train of thought, Jeong et al.²¹³ attributed the transition to the field-induced creation of oxygen vacancies, rather than the purely electrostatic effect. Ji et al.²¹⁴ and Shibuya et al.,²¹⁵ however, suggested that electrochemical protonation was the origin of the modulation of electrical property in VO₂, similar to what was observed in the electrochromic setup in the previous section. An electronic oscillator is a common component in modern electronic circuitry which can produce periodic signals such as a square wave or a sine wave. Due to the periodicity of the output, it is often used to convert a direct current (DC) input into an alternate current output. The two main types of electronic oscillators are the linear (harmonic) and nonlinear (relaxation) oscillator. Because of the ability to undergo a nonlinear MIT, VO₂ can be used as the basis for a nonlinear oscillator with a relaxation behavior stimulated by external electricity input. A VO₂-based oscillator design by Leroy et al.²¹⁶ and its I–V characteristic curve is shown in Figure 26c. The inset shows the schematic of the oscillator circuit in which a resistor (R_s) is connected to the VO₂ device to produce a current-controlled negative differential resistance (NDR). The NDR portion happens when VO₂ enters the transitive state between VO₂ (M) and VO₂ (R). It was reported that by controlling R_s, the VO₂-based oscillator circuit can become self-sustaining, and the frequency can range from kHz to 1 MHz.¹⁵² Aside from the standard setup as shown in Figure 26c, studies have also been done in which two oscillators are coupled with a resistor, a capacitor, or FET in between.²¹⁷ A memristor is a nonvolatile electronic memory device which has a programmable resistance. The resistance of a memristor is retained even after removal of the power and is dependent on the original applied voltage. It is crucial that the resistance can be reversed or reprogrammed. Thus, a two-terminal VO₂ electrical device with the nonvolatile switching of resistance across the MIT can also be adapted into memristors.¹⁴⁹ Bae et al.¹⁵⁰ demonstrated a two-terminal memristor based on a single VO₂ nanobeam. The nanobeam undergoes MIT when a bias of 3 and 5 V was applied for 0.25 s; the resistance of the device goes from an initial 10¹¹ Ω to 10⁹ and 10⁸ Ω respectively. The resistance change can be reset with a zero-voltage bias. VO₂ has also been utilized in other memory devices including a multistate free-standing VO₂/TiO₂ cantilever,²¹⁸ resistive random-access memory (ReRAM) devices.²¹⁹ and 3D memory array.¹⁸⁹ Not included in the above discussion is the minor application of VO₂ in field emitter and spintronic devices which are based on the abrupt drop in resistance across thermal- and magnetical-activated MIT, respectively. Studies on VO₂/ZnO core–shell nanotetrapod thermal-activated field emitters were reported by Yin et al. in 2014.²²⁰ On the other hand, VO₂-based spintronic devices and the behavior of the magnetoresistance of VO₂ were reported in detail by Li et al.,²²¹ Choi et al.,²²² and Singh et al.²²³

Figure 26.

(a) Schematic of the VO₂–NW-FeFET design. (b) Resistance change of the VO₂–NW-FeFET ranging from 0 to 18 V. Reproduced with permission from ref (211). Copyright 2020 Royal Society of Chemistry. (c) I–V characteristic in voltage- and current-mode of a device incorporating a VO₂ pattern; inset is the schematic test circuit. Reproduced with permission from ref (216). Copyright 2012 Cambridge University Press and the European Microwave Association.

6.2.3. Mechanical Applications

The actuator is typically a component in a machine or a system which converts provided energy into mechanical motion. The concept of the actuator has been widely adapted into novel scientific research, especially in the field of microrobotics or micro-/nanoelectromechanics.²²⁴ VO₂, which has a high theoretical work density (≈ 7 J cm^–3) and fast response rate to external stimuli, is suitable for actuator applications. It offers the ability to offset disadvantageous low work density and the slow response rate of common actuator materials such as piezoelectric ceramics or polymers and CNT, respectively.²²⁵ In device fabrication, single crystal VO₂ or composite bimorph of VO₂ can be designed to respond to specific external stimuli such as light, heat, or electrical current. An example of a photodriven VO₂ bimorph design was reported by Ma et al.²²⁶ in 2018. The VO₂/CNC device was conceived by combining the carbon nanocoil (nanosprings twisted by hollow carbon nanofibers) core with a VO₂ shell. When exposed to 980 nm radiation, the temperature of the spring increases unevenly, forming a temperature gradient from tip to end. This results in a transition gradient in which the tip becomes VO₂(R) first and shrinks, creating the curvature. The VO₂/CNC actuator delivers a large displacement-to-length ratio (∼0.4), fast response rate (9400 Hz), and long durability (>10⁷ cycles). More recently, Shi et al.²²⁷ fabricated thermal-activated single-crystalline VO₂ actuators (SCVAs) which were designed so that the τ_C of a single VO₂ nanobeam is a gradient along the radial direction. When exposed to heat, one side of the fabricated W-doped VO₂ nanobeam with lower τ_C would undergo MIT first and shrink, creating a bending as seen in Figure 27a. It was reported that this SCVAs design performed competitively with other reported VO₂ bimorph actuator designs with an extremely high displacement-to-length ratio (∼1), high energy efficiency (∼0.83%), fast response rate in the order of kHz, and long durability (>10⁷ cycles) (Figure 27b). VO₂ electrothermal devices with joule heating activation for oscillator²²⁸ and microelectromechanical systems (MEMS)²²⁹ have also been fabricated with variable degrees of success. A resonator is a device that exhibits resonance at its eigenfrequency. With VO₂, the eigenfrequency of a resonator can be dynamically controlled by thermally triggering the MIT. This specific frequency is positively related to Young’s modulus of VO₂, which is widely different between the monoclinic phase (151 ± 2 GPa) and the rutile phase (218 ± 3 GPa).²³⁰ Studies have been made to determine the effects of elemental doping on the frequency modulating ability of VO₂-based resonators. Rúa et al.²³¹ compared the Cr-doped VO₂ resonator with the undoped one and concluded that the doped sample had a higher frequency change due to a lower Young’s modulus. Other strategies to improve performance such as changing the shape from a simple cantilever have also been done by Manca et al.²³²

Figure 27.

(a) Optical images of the VO₂ nanobeam undergoing MIT. (b) Amplitude versus cycle number plots of the nanobeam subjected to a chopped laser (100 Hz in a vacuum and 2000 Hz in air). Reproduced with permission from ref (227). Copyright 2019 John Wiley and Sons.

6.2.4. Supercapacitors

VO₂ (B) with its layered structure and multioxidation states is ideal as an electrode material with charge storage through insertion and fast surface Faradaic reaction.²³³ However, being in common with all metastable VO₂ phases, VO₂ (B) structural instability is not suitable for cyclic stability of SC application. Multiple studies have been done to combine VO₂ (B) with various carbon composites to create stable electrode materials.²³⁴ An example of a VO₂ and reduced graphene oxide (VO₂ (B)/rGO) device by Liu et al.²³⁵ is shown in Figure 28. The schematic diagram of the all-solid-state sandwich-structured supercapacitor design is shown in Figure 28a, where symmetrical VO₂ (B)/rGO and PVA/LiCl gel are used as electrodes and electrolytes, respectively. The performance of this design was reported to have a superior specific capacitance of 353 F g^–1 at 1 A g^–1 and a maximum power density of 7152 W kg^–1 at an energy density of 3.13 Wh kg^–1. By compositing VO₂ (B) with rGO, 78% capacitance was retained after 10000 cycles, improving the stability of the device immensely (Figure 28b).

Figure 28.

(a) Schematic illustration of the all-solid-state supercapacitor. (b) Cyclic stability test of the VO₂/rGO supercapacitor device. Reproduced with permission from ref (235). Copyright 2019 under CC BY 4.0 license.

6.2.5. Magnetic Refrigeration

Magnetic refrigeration is a cooling technique that is based on the magnetocaloric effect (MCE). The MCE describes the phenomenon in which a suitable material can be heated up or cooled down when exposed to a changing magnetic field. Due to the changing magnetization when crossing the MIT, VO₂ was first shown to be suitable for magnetic refrigeration application by Wu et al.²³⁶ in 2011 with a single crystalline nanorod fabrication technique. Although the potential was shown for VO₂ in magnetic refrigeration applications, studies to further improve this are still limited.

6.2.6. Batteries

VO₂ formed by edge-sharing VO₆ octahedra with a unique bilayer structure exhibits a large lattice spacing that can accommodate Li⁺ (0.76 Å), Na⁺ (1.02 Å), K⁺ (1.38 Å), and Zn²⁺ (0.74 Å) insertion/extraction. Fan et al.²³⁷ designed and synthesized a binder-free VO₂ cathode via biface VO₂ arrays directly growing on a graphene foam (GF) network (Figure 29a). They constructed a geometric model of bilayered VO₂ nanobelts through the growth direction and lattice spacings (Figure 29b). The relatively high stacking rate of the “steplike” VO₆ octahedra along the [010] direction determines the preferred growth direction. As a result, the (001) facet of the VO₂ nanobelt is the thinnest, and the interlayers between the (200) crystal planes provide a facile channel for Li⁺ and Na⁺ diffusion. Meanwhile, the graphene quantum dots (GQDs) coating on the VO₂ surfaces can act as highly efficient surface protection to further enhance the Li⁺/Na⁺ storage. When as-prepared GF-supported GQD-anchored VO₂ arrays (GVGs) are directly used as a LIBs/NIBs cathode, it exhibits two advantages: high ion diffusion sensitization and charge transport kinetics are beneficial to obtain high-rate capacities, and the homogeneous GQDs suppress VO₂ dissolution which is in favor of retaining long-term cycles. The GVG electrode delivers a high specific capacity of 421 mAh g^–1 at 1/3 C for Li⁺, which is much higher than that of the uncoated GF@VO₂ electrode (391 mAh g^–1). It can maintain 151 mAh g^–1 even at 120 C,and 94% of the initial capacity can be retained after 1500 cycles (Figure 29c). When it is used as an NIB cathode, it exhibits a specific capacity of 306 mAh g^–1 at 1/3 C and good capacity retention (Figure 29d). These results demonstrate that the GVG is an excellent electrode material for Li⁺/Na⁺ storage.

Figure 29.

SEM image (a), geometrical model (b), LIB rate performance (c), and SIB charge/discharge profiles (d) of bilayered VO₂ nanobelt. Reproduced with permission from ref (237). Copyright 2015 American Chemical Society. The GITT curves (e), rate performances (f) of SA-VO₂ and VO₂, and the schematic of the enhanced K ion storage ability (g) of SA-VO₂. Reproduced with permission from ref (238). Copyright 2020 John Wiley & Sons. The Rietveld refinement result from the XRD data and crystallographic structure (h), GITT curve (i), and most significant changes of lattice parameter in each stage (j) of VO₂. Reproduced with permission from ref (239). Copyright 2019 American Chemical Society.

Zhang et al.²³⁸ first designed and synthesized a surface amorphized VO₂ (B) nanorod (SA-VO₂) with a crystalline core and a surface-amorphized shell heterostructure by an interfacial engineering strategy. The crystalline/amorphous heterointerface in SA-VO₂ substantially narrows the bandgap, lowers the surface energy, and reduces the K⁺ diffusion barrier of VO₂ (B) via DFT calculations. Therefore, the as-obtained SA-VO₂ electrode exhibits a higher reversible capacity of 288.3 mAh g^–1 (at 50 mA g^–1), superior rate capacity (141.4 mAh g^–1), and long-term cyclability (86% after 500 cycles at 500 mA g^–1) (Figure 29e), while the VO₂ only delivers a specific capacity of 147.2 mAh g^–1 at 50 mA g^–1 and maintains 16.5% capacity after 200 cycles at 500 mA g^–1 (Figure 29f). Compared with oxygen-rich defect amorphous shell VO₂, the crystalline/amorphous heterointerface SA-VO₂ enhances the K⁺ storage capacity and enables rapid K⁺/electron transfer, which results in large capacity and outstanding rate capability (Figure 29g).

Mai et al.²⁴⁰ reported VO₂ hollow microspheres with a high surface area and excellent structural stability via a facile and controllable ion-modulating approach. VO₂ hollow microspheres deliver the best Li⁺ storage performance compared to six-armed microspindles and random nanowires. The highest surface area of VO₂ hollow microspheres can provide efficient self-expansion, self-shrinkage buffering, and self-aggregation during lithiation/delithiation, which delivers 3 times higher capacity than that of random nanowires. In addition, they also synthesized highly homogeneous VO₂ nanorods by a rapid and simple hydrothermal method for aqueous ZIBs cathode material (Figure 29h).²³⁹ The in situ XRD and ex-situ XPS/TEM results demonstrate that the VO₂ undergoes a single-phase reaction during the discharge process, accompanying a phase transition process of VO₂–Zn_0.07VO₂–Zn_0.29VO₂–Zn_0.54VO₂ with a unit cell volume expansion of 6.69%. On the contrary, the evolution of Zn_0.54VO₂–Zn_0.25VO₂–Zn_0.09VO₂–Zn_0.04VO₂ occurs during the Zn²⁺ deintercalation from the Zn_0.54VO₂. Meanwhile, detailed qualitative analysis verified that the VO₂ unit cell expands in the a, b, and c directions sequentially during the discharge/charge processes. Satisfactorily, the VO₂ nanorods deliver a high specific capacity of 325.6 mAh g^–1 and excellent long cycle performance (86% after 3000 cycles), which is outstanding performance among the reported cathode materials of the aqueous ZIBs (Figure 29i,j).

6.2.7. HER, OER, and Water Splitting

VO₂ is a well-known semiconductor material with a band gap of 0.7 eV, which is seldom considered as a candidate material as a catalyst or photocatalyst for the production of hydrogen.²⁴¹ VO₂ can be used as a photocatalyst for hydrogen evolution through phase engineering. Ajayan et al.²⁴² synthesized the body-centered-cubic nanostructured VO₂, which shows excellent photocatalytic activity with a hydrogen production rate up to 800 mmol m^–2 h^–1 from a mixture of water and ethanol under UV light at a power density of ∼27 mW cm^–2. Furthermore, vanadium oxide composites have the great potential to accelerate water dissociation kinetics and reduce charge-transfer resistance.^243,244 Tao et al.²⁴⁵ synthesized MoS₂/VO₂ hybrids by using a two-step hydrothermal method. The phase transition of VO₂ exhibits a significant effect on hydrogen evolution properties of the heterostructures (an onset potential of 99 mV and a Tafel slope of 85 mV dec^–1). The enhanced performance is mainly due to the faster electron transport as well as the strain effect on MoS₂. Tu et al.²⁴⁶ fabricated Co₃O₄/VO₂ heterogeneous nanosheet structures on carbon cloth (Co₃O₄/VO₂/CC) by the combination of hydrothermal and electrodeposition methods. The Co₃O₄/VO₂/CC composites gave good HER performance with a low overpotential of 108 mV at 10 mA cm^–2 and a Tafel slope of 98 mV dec^–1, which results from the abundant active sites, effective electron transport, and improved hydrogen binding energy. Najafi et al.²⁴⁷ prepared room temperature-stable metallic rutile VO₂ nanosheets by the topochemical transformation of two-dimensional VSe₂. By an O₂ plasma pretreatment of the VSe₂ nanosheets, the obtained VO₂ nanosheets show a porous structure, which shows good HER and OER performances in either acidic or alkaline media. The symmetric two-electrode water splitting cell based on the porous VO₂ nanosheets as both the anode and cathode delivers a current density of 10 mA cm^–2 at cell voltages of 1.710 and 1.660 V in 0.5 M H₂SO₄ and 1 M KOH, respectively.

7. V₂O₅

7.1. Structures and Synthesis

V₂O₅ has the highest oxygen state in vanadium–oxygen systems and is the most stable member of the series of vanadium oxides. V₂O₅ has multiple distinctive polymorphs, including α-V₂O₅ (orthorhombic), β-V₂O₅ (monoclinic or tetragonal), and γ-V₂O₅ (orthorhombic). Among them, the most common α-V₂O₅ is the thermodynamically stable phase (the unit cell structure belonging to the Pmnm space group with lattice parameters of a = 11.150 Å, b = 3.563 Å, and c = 4.370 Å), and the other two phases (β-V₂O₅ and γ-V₂O₅) can be transformed from the α-V₂O₅ phase under high pressure and high-temperature conditions.²⁴⁸ The orthorhombic structure of α-V₂O₅ is shown in Figure 30, in which each single layer of V₂O₅ consists of edge- and corner-sharing square pyramids, and the adjacent layers are bonded together along the c-axis by weak van der Waals bonds between the vanadium and oxygen of neighboring pyramids. Additionally, three different oxygens atoms, O₁, O₂, O₃, have different coordinations depending on the position in each layer. The terminal/apical and bridging (corner-sharing) coordinated vanadyl oxygen atoms O₁ and O₂ have V–O bond lengths about of 1.54 and 1.77 Å, respectively; the triply coordinated O₃ links three vanadium atoms via edge-sharing VO₅ square pyramids, and the three corresponding V–O bond lengths are 1.88, 1.88, and 2.02 Å.²⁴⁹

Figure 30.

Perspective view (a) along the b-axis and (b) along the c-axis of two layers of V₂O₅, V atoms are gray balls, O atoms are red balls, and weak van der Waals bonds are omitted for clarity. Reproduced with permission from ref (249). Copyright 2018 Elsevier.

The outstanding characteristics of V₂O_5, such as a layered structure, a direct band gap in the visible-light region, high chemical and thermal stability, electrochemical safety, low cost, and easy preparation, make V₂O₅ a suitable material for electrochemical energy conversion and storage,²⁵⁰ catalysis,^251−253 solar cells,²⁵⁴ gas sensors,³¹ electrochromic devices,²⁵⁵ and optoelectronic devices.²⁵⁶ Compared with bulk V₂O₅, the nanostructured ones have higher surface to volume ratios, which is beneficial to improve various performances. Over the past few years, a variety of methods, such as sol–gel, hydrothermal, chemical vapor deposition, magnetron sputtering, and atomic layer deposition, have been developed to prepare V₂O₅ nanostructures.

The sol–gel method has been used to fabricate V₂O₅ thin films and nanopowders through V₂O₅ sols, which were prepared by ion exchange, alkoxide hydrolysis, peroxide-assisted hydrolysis, and melt-quenching. The disadvantage of using an ion exchange is the difficulty to control the vanadium concentration as it varies throughout the whole process. In addition, some foreign ions such as Na⁺ may remain in the gel after ion exchange.²⁵⁷ The alkoxide hydrolysis always involves some expensive raw materials, and the molten V₂O₅ quenching process will produce toxic gas.

Hence the synthesis of V₂O₅ sol by peroxide-assisted hydrolysis stands out due to its advantages of being environmental friendly, inexpensive, and requiring simple fabrication. Vanadium metal or commercial V₂O₅ powders are commonly used as a vanadium source that can be dissolved vigorously in a solution of hydrogen peroxide. According to Alonso et al. the dissolution of V₂O₅ into H₂O₂ produces unstable diperoxo [VO(O₂)₂]⁻ and then is dissociated to the aqueous solution of [VO₂]⁺ and [H₂V₁₀O₂₈]^4–.²⁵⁸ Etman et al. also found [H₂V₁₀O₂₈]^4– is the main species via real-time nuclear magnetic resonance.²⁵⁹ It is noted that, not related to the preparation method, the V₂O₅ sols are comprised of a fibrous structure, dissimilar from many inorganic sols that are typified by a random aggregate of particle structure.²⁶⁰ These fibrous structures can self-assemble into V₂O₅ nanofibers upon long-term aging through a coagulation mechanism.²⁶¹ The obtained V₂O₅ sol was applied onto substrates via coating techniques, including spin coating, dip-coating, and spray process, and the subsequent drying and heat treatment are necessary to obtain V₂O₅ films.^262−264Figure 31a highlight the evaporation, hydrolysis, and subsequent solidification procedure during the formation of V₂O₅ thin films by dip-coating.²⁶⁵Figure 31b,c shows TEM diffraction patterns and corresponding FFT of V₂O₅ thin films formed from low concentration dilution (LCP) and high concentration dilution (HCP) using PEG-400 as an additive (HCP-PEG), respectively. V₂O₅ thin films formed from an LCP precursor show the formation of grains of orthorhombic V₂O₅ with defined grain boundaries (Figure 31b), while the HCP-PEG samples have a polycrystalline structure without uniformed grain (Figure 31c). Meanwhile, as shown in AFM images in Figure 31d–g, thin films formed from higher concentration precursors have a larger surface roughness (R_S). Thermal treatment can further increase the R_S due to the formation of crystallites on the thin film surface. The thin film formed from LCP-PEG precursor has the lowest R_S of 0.2 nm. Liu et al. studied the substrate effect on the structure and electrical properties of nanocrystalline V₂O₅ thin films prepared by the sol–gel method.²⁶⁶ They found the annealed V₂O₅ film on the Si substrate exhibited more uniform rod-like morphology, and electrical measurements indicated the typical n-type semiconducting behavior. Senapati et al. prepared nanoscale V₂O₅ films having thicknesses ranging from 92 to 137 nm by spin coating V₂O₅ sol at different stages of aging.²⁶⁷ They reported the decrease of strain in the films with aging, and the electrical conductivity increased with aging due to the improved crystallinity of the films.

Figure 31.

(a) The formation process of V₂O₅ thin film through a combination of evaporation, hydrolysis, and solidification is shown optically and schematically; TEM diffraction patterns and corresponding FFT of V₂O₅ thin films formed from (b) low concentration dilution (LCP) and (c) high concentration dilution using PEG-400 as an additive (HCP-PEG); AFM surface images of as-deposited and post-thermally treated orthorhombic V₂O₅ films that were dip-coated from (d) LCP, (e) LCP-PEG, (f) HCP, and (g) HCP-PEG solutions. Reproduced with permission from ref (265). Copyright 2015 under CC BY 4.0 license.

Besides thin films, the sol–gel method also provides good control over the size, morphology, doping, and chemical composition of V₂O₅ powders.^268−271 Li et al. reported the flower-like V₂O₅ powders prepared by coagulating V₂O₅ sol and subsequent annealing crystallization.²⁷² V₂O₅/graphene hybrid aerogel was prepared by Wu et al. at ambient pressure through a simple one-pot sol–gel method from commercial V₂O₅ powder.²⁷³Figure 32a illustrates the fabrication process and images of VO_x nanofibers and graphene oxide sheets. First, graphene oxide (GO) aqueous solution was added to the V₂O₅ sol under vigorous stirring to induce hydrolysis and in situ recombination of the GO sheets and VO_x oligomers. Then, a dark red VO_x/GO hybrid gel was obtained after about 5 min because of the rapid formation of intermediate vanadium phases and the growth of nanofibers. After aging for 2 days, the gel gradually changed to deep green, and the V₂O₅ nanofibers are anchored and in situ grown on the graphene surfaces. The V₂O₅/graphene hybrid aerogel is so light that it can be lifted by a feather, but it is strong enough to support the weight of 100 g (Figure 32b).

Figure 32.

(a) The fabrication process and structure of the V₂O₅/graphene hybrid aerogel and the corresponding digital images of different formation stages: (i) hydrolysis of VO_x oligomers and self-assembled coordination of VO_x nanofibers and graphene oxide sheets; (ii) aging of VO_x/graphene oxide gel and growth of VO_x nanofibers along graphene oxide sheets; (iii) solvent replacement and drying, and (iv) thermal reduction of graphene oxide, oxidation, and partly crystallization of V₂O₅. (b) Lightweight V₂O₅/graphene hybrid aerogel standing on a feather; it can support the weight of 100 g. Reproduced with permission from ref (273). Copyright 2015 Royal Society of Chemistry.

The hydrothermal method has been widely used for the synthesis of a vast range of V₂O₅ nanostructures with a desired size and morphology, such as nanoparticles,²⁷⁴ nanowires,²⁷⁵ nanotube,²⁷⁶ nanosheets,²⁷⁷ and micro-/nanostructures.^278,279 The importance of solubility of precursors, the pH value, the surfactant, as well as the hydrothermal temperature, reaction time, and solution filling factor are highlighted in many references. Li’s research group conducted extensive research on the hydrothermal treatment of V₂O₅ sol.²⁸⁰ They prepared V₂O₅ nanoparticles and ultralong nanobelts with the usage of an inorganic V₂O₅ sol precursor (Figure 33a,b).^261,274 The obtained single-crystalline V₂O₅ nanobelts have a large specific surface area, with width and thickness of 30–200 nm and length in millimeters or even longer (Figure 33b). Strong evidence suggested that the oriented attachment growth mechanism was responsible for the formation of V₂O₅ nanobelts. Pan et al. reported a one-step solvothermal method to form V₂O₅ hollow spheres without adding surfactants.^281,282 As shown in Figure 33c, the SEM image of the V₂O₅ hollow spheres has a uniform size of around 1 μm in diameter. They investigated the time-dependent interior structural evolution by TEM and gave the possible growth mechanism of the V₂O₅ microspheres (Figure 33d): vanadium oxide nanoparticles are first generated by the hydrolysis of VOC₂O₄ and then aggregation to form solid microspheres in stage I. The solid spheres undergo the initial inside-out Ostwald ripening process and transform to the yolk–shelled structure (stage II). With extended solvothermal reaction, secondary Ostwald-ripening takes place on the preformed solid cores, resulting in the formation of a multishelled structure (stage III). Finally, completely hollow microspheres are obtained as a result of the thorough dissolution and recrystallization of the less stable interior architectures (stage IV). Hence, the interior structure of the VO₂ microspheres could be effectively tailored by simply controlling the reaction duration and concentration of the precursor.

Figure 33.

V₂O₅ with different morphologies: (a) Nanoparticles. Reproduced with permission from ref (274). Copyright 2015 Elsevier. (b) Ultralong nanobelts. Reproduced with permission from ref (261). Copyright 2011 Royal Society of Chemistry. (c) Hollows spheres and (d) growth mechanism. Reproduced with permission from ref (281). Copyright 2013 Wiley. (e) 2D nanosheets. Reproduced with permission from ref (283). Copyright 2015 Elsevier. (f) Urchin-like microflowers. Reproduced with permission from ref (284). Copyright 2012 American Chemical Society. (g) Nanoring. Reproduced with permission from ref (285). Copyright 2010 under CC BY 2.0 license.

The fabrication of two-dimensional (2D) V₂O₅ nanosheets has been studied by Cao et al.²⁸³ As cathode materials for lithium-ion batteries, the resulting 2D V₂O₅ nanosheets (Figure 33e) exhibit remarkable electrochemical performances, including high reversible capacity, good cyclic stability, and great rate capability. 3D hierarchical vanadium oxide microstructures, including urchin-like microflowers (Figure 33f), have been successfully synthesized by Lou et al. through a solvothermal method.²⁸⁴ The morphologies of the microstructures can be easily tailored by varying the concentration of the vanadium oxalate solution, and the obtained V₂O₅ microflowers are highly porous with a surface area of 33.64 m² g^–1 giving high lithium storage capacity, and enhanced cycling stability and rate capability. V₂O₅ nanorings and microloops are rarely reported, but they are very interesting morphologies (Figure 33g). The cation-induced asymmetric strain is the main driving force in making a layered V₂O₅ coil into a ring structure.²⁸⁵

CVD is widely used for depositing high-quality and high-performance solid materials. As shown in Figure 34a, CVD involves the transfer of precursor molecules, which are either liquid or gaseous to a reaction chamber by a carrier gas (step 1), and then it is followed by the reaction and/or decomposing on the surface of the substrate to produce the desired films (steps 2a, 3, and 4) or powders (step 2b), and the byproducts and unreacted precursor are transported out from the chamber at the end of the process (step 5a, 5b).¹⁹ There are several types of CVD systems such as atmospheric pressure CVD, aerosol-assisted chemical vapor deposition (AACVD), atomic layer CVD, plasma enhanced CVD, metal–organic CVD, and so forth.²⁸⁶ The morphology, size, crystal phase, and specific surface area of V₂O₅ can be affected by various parameters, namely the reaction time, substrate temperature, pressure, precursor properties, and reaction position during the CVD method. SEM images in Figure 34b display the effect of growth temperature on the morphological characteristics of V₂O₅ coatings. V₂O₅ grown at 350 and 375 °C has rod-like structures of nonuniform thickness and width, while at 400 °C pellet-like features of V₂O₅ are observed, and the morphology evolution could be due to the coexistence of both α-V₂O₅ and β-V₂O₅.²⁸⁷ Chun et al. prepared V₂O₅ nanosheets via the reaction of VCl₃ vapor with oxygen in the CVD system without a vacuum system.²⁸⁸Figure 34c shows representative optical images and their corresponding SEM images of the obtained V₂O₅ nanosheets and three distinguished shapes: hexagons, triangles, and truncated triangles. Wang et al. used the CVD method to control the morphologies of V₂O₅ by changing the reaction distance from the source position using vanadyl acetylacetonate (VO(acac)₂) as the vanadium precursor.²⁸⁹ They found that VO_x vapor and VO(acac)₂ vapor existed simultaneously during the growth process, and the different supersaturation distributions of these two vapors led to three main growth areas. 1. V₂O₅ thin-films were formed at a high concentration and supersaturation of VO_x in the region near the source material (Figure 34d); 2. nanowires with a length of about 10 μm and width about 200 nm were formed at a distance of 18 cm from the source due to the low vapor concentration of VO_x (Figure 34e); 3. nanospheres with a diameter of about 200–500 nm were obtained when the source material was far away (about 30 cm) due to the high concentration and supersaturation of VO(acac)₂ that was oxidized to V₂O₅ nanospheres (Figure 34f).²⁸⁹

Figure 34.

(a) Schematic diagram of the CVD process. Reproduced with permission from ref (19). Copyright 2018 Elsevier. (b) SEM images of AACVD grown V₂O₅ films at 350 °C, 375 °C, 400 °C. Reproduced with permission from ref (287). Copyright 2016 Elsevier. (c) Representative optical and SEM images of the as-synthesized V₂O₅ nanosheets with well-defined shapes, such as hexagon, triangle, and truncated triangle. All scale bars are 3 μm. Reproduced with permission from ref (288). Copyright 2018 American Chemical Society. (d) The growth schematic diagram of the V₂O₅ nanomaterials prepared by chemical vapor deposition using VO(acac)₂ powder as the precursor, and the formed (e) nanowire and (f) nanospheres at a distance of 18 and 30 cm away from the source, respectively. Reproduced with permission from ref (289). Copyright 2010 Institute of Physics.

ALD is considered a specific type of CVD, which was first introduced in the 1960s and is currently receiving ever-growing attention as a method of choice for the growth of conformal coatings on nanostructures with high aspect ratios.¹²Figure 35a depicts the typical ALD process, in which the precursor gases sequentially react with a surface to form an ultrathin film through a self-limiting process, and all byproducts and unreacted precursor molecules are purged out of the reactor.²⁹⁰ The primary advantages of ALD lie in subnanometer film thickness and conformality control that profit from the cyclic, self-saturating nature of ALD processes. Moreover, the ALD is unique as it is able to coat complex 3D structures with a high degree of uniformity and smoothness.^291,292 Chen et al. successfully fabricated a multiwall carbon nanotube (MWCNT)/V₂O₅ core/shell sponge by ALD.²⁹¹Figure 35b shows the experimental flow schematically: the MWCNT sponge structure exhibits a very low density (∼7 mg/cm³) and high porosity (>99%), allowing for a high amount of active material loading; the V₂O₅ layer of about 16 nm is subsequently deposited on the MWCNT sponge by 1000 cycles of H₂O-based ALD; finally, the MWCNT/V₂O₅ sponge is compressed and assembled in a coin cell battery, which enables de/lithiation in active material within a very short time. Figure 35c,d shows SEM images of the MWCNT sponge before and after ALD V₂O₅ coating, giving uniform and smooth V₂O₅ coating. Two different oxidants, O₃ and H₂O, have been studied during the ALD process, and it was found that as the ALD cycle numbers increased from 100 to 2500, the roughness for the O₃-based films kept increasing from 0.7 to 10.4 nm while that for H₂O-based films only increased from 0.4 to 1.9 nm (Figure 35e).²⁹³ Østreng et al. prepared V₂O₅ films by ALD using the β-diketonate VO(thd)₂ and ozone as precursors.²⁹⁴ They found that the crystallographic orientation, optical properties, band gap, and surface roughness of the V₂O₅ films were correlated and could be varied by controlling the deposition temperature and film thickness.

Figure 35.

(a) Schematic of a single ALD cycle consisting of half-cycles of Precursor A and Precursor B separated by purge steps to remove excess precursor and byproducts. Reproduced with permission from ref (290). Copyright 2020 Royal Society of Chemistry. (b) Schematic of experimental flow to fabricate MWCNT/V₂O₅ sponge, and SEM images of MWCNT sponge (c) before and (d) after ALD V₂O₅ coating. Reproduced with permission from ref (291). Copyright 2012 American Chemical Society; (e) Compares the RMS roughness of the O₃-based and the H₂O-based films prepared by ALD as a function of cycle number. Reproduced with permission from ref (293). Copyright 2013 Royal Society of Chemistry.

PVD techniques (Figure 36a) involve evaporation and many different modes of physical sputter deposition, in which the primary source of the depositing species is a solid or liquid, as opposed to generally gaseous precursors in CVD. However, chemical reactions can and do occur in PVD systems, such as in the reactive sputtering deposition. The presence of the reactive gas (oxygen or nitrogen) in the chamber can significantly alter the PVD source.²⁹⁵ PVD possesses some unique advantages for the creation of uniform and dense solid thin films that strongly adhere to the substrates. Meanwhile, the thickness, composition, crystallinity, and crystal orientation of the thin film can be well controlled by changing the growth conditions with minimal risk of contamination due to the absence of organic reactants. Another advantage is to sequentially deposit several materials to form well-defined multilayer systems as well as special alloy compositions and structures.²⁹⁶ Magnetron sputtering is one of the most used PVD methods to fabricate a large range of materials, including metal oxides. The application of a negative voltage to the cathode will generate positively charged argon ions that can bombard the target ions to be ejected toward the substrate to form a film. Magnets are used in order to increase ion bombardment. This technique has been developed on an industrial scale to make large surface deposits with a wide variety of materials. V₂O₅ film consisted of fine long strip particles deposited by radio frequency magnetron sputtering, and the thickness of the V₂O₅ film was determined to be approximately 150 nm according to the cross-sectional SEM image (Figure 36b). V₂O₅ films underwent four different thermal transition behaviors to other vanadium oxides that were closely related to the oxygen proportion of the annealing ambient.²⁹⁷ Amorphous V₂O₅ film can be used as a hole injection layer in quantum dot light-emitting diodes, which exhibited a maximum luminance of 198.5 cd/m², a turn-on voltage of 1.7 V, and a max external quantum efficiency of about 8.3%.²⁹⁸

Figure 36.

(a) Schematic illustration of different physical vapor deposition techniques: sputter deposition, pulsed laser deposition, electron beam evaporation, and thermal evaporation. In evaporation, atoms are removed from the source by thermal means, while in sputtering they are dislodged from a solid target via bombardment by gaseous ions. Reproduced with permission from ref (296). Copyright 2021 John Wiley and Sons. SEM images of V₂O₅ prepared by (b) megnetron sputtering. Reproduced with permission from ref (297). Copyright 2021 Elsevier. (c) PLD. Reproduced with permission from ref (299). Copyright 2021 Elsevier. (d) Electron beam evaporation. Reproduced with permission from ref (301). Copyright 2017 Elsevier. (e) Thermal evaporation. Reproduced with permission from ref (306). Copyright 2019 American Chemical Society.

Pulsed laser deposition (PLD) is another PVD method, which has been preferred to grow different structures ranging from high-quality epitaxial thin films to various nanostructured layers. During PLD, a high-power pulsed laser beam is focused on a target of the desired composition. Material vaporized from the target is deposited as a thin film on a substrate that faces the target in an ultrahigh vacuum (UHV) environment. Polycrystalline V₂O₅ thin film in the desired orientation can be prepared by PLD, which has aligned nanorod morphology on a flexible stainless steel substrate (Figure 36c).²⁹⁹ Huotari et al. found the film surface morphology varied largely according to oxygen partial pressure: lower O₂ partial pressures resulted in a denser and thinner film, while higher O₂ partial pressures gave a film surfaces formed with randomly agglomerated nanoparticles or agglomerates with pillar-like morphology.³⁰⁰

Electron-beam deposition (EBD) is another form of PVD where a target anode is bombarded with a high-energy electron beam that is given off from a charged tungsten filament under a high vacuum. The electron beam leads to joule heating and converts the target into the gaseous phase, which subsequently precipitates into the solid form on the desired substrate. Han et al.³⁰¹ reported the growth of nanocolumnar V₂O₅ molecules that were aggregated with each other and collapsed after annealing treatment (Figure 36d). Most of the nanosized V₂O₅ columns’ structure could retain its original shape during the annealing process by changing the source from V₂O₅ to VO₂. Highly oriented V₂O₅ thin films with nanosized grains were grown by EBD, and the film thickness was found to be in the range of 800–1200 nm that varied by adjusting the substrate position.³⁰² Meanwhile, the mobility and carrier concentration of the oriented V₂O₅ thin films increased with the increase of V₂O₅ film thickness. Thermal evaporation is the vaporization of a material by heating to a temperature that the vapor pressure becomes appreciable, and the materials are sublimated from the target surface in a vacuum. By this method, heterojunctions,³⁰³ nanorods,³⁰⁴ nanoparticles,³⁰⁵ or highly crystalline V₂O₅ films³⁰⁶ have been studied in several reports. Wang et al. synthesized Ga-doped V₂O₅ nanorods by thermal evaporation at 850 °C and found interstitial Ga and Ga–O phases influence the photoluminescence properties of V₂O₅ nanorods.³⁰⁴ Berouaken et al. used thermal evaporation to prepare V₂O₅ nanoplatelets on the quartz crystal microbalance, followed by rapid thermal annealing.³⁰⁷ The obtained V₂O₅ nanoplatelets exhibited good sensing performance toward NH₃ vapor at room temperature: a fast response time, a short recovery time, good stability, reproducibility, reversibility, and linearity. Velmurugan et al. prepared highly crystalline V₂O₅ films with a controlled thickness of about 530 nm and an average particle size of around 560 nm using a thermal evaporation process (Figure 36e).³⁰⁶ The films were fabricated in electrochemical microcapacitors and subjected to various electrochemical characterizations, which display improved reliability and excellent capacitance retention.

7.2. Applications

7.2.1. Batteries

V₂O₅ is a feasible cathode material for metal-ion storage due to its high output voltage and unique crystal structure with large interlayer spacing (4.4 Å). Mai et al.³⁰⁸ designed and synthesized a V₂O₅ hollow microclew (V₂O₅–HM) through a facile solvothermal assisted calcination method. The amorphous V₂O₅–HM precursor can convert into crystalline V₂O₅ through calcination (Figure 37a). Compared with crystalline V₂O₅ (V₂O₅-Ms) and V₂O₅ nanowires (V₂O₅–NWs), the V₂O₅–HMs exhibit the best Li⁺ storage performance (145.3 and 94.8 mAh g^–1 at 0.67 and 65 C, respectively), which is due to the 3D hierarchical microstructure with intertangled nanowires (Figure 37b). This 3D hierarchical microstructure not only inherits fast electrolyte penetration, and short ionic and electronic transport pathways, but also significantly alleviates the strain during the Li⁺ intercalation/deintercalation. Meanwhile, compared to disordered V₂O₅ nanowires, a unique V₂O₅–HM structure effectively helps improve the tap density, which is more suitable for commercial applications (Figure 37c). V₂O₅ can be used as other metal ions (Na⁺/K⁺/Zn²⁺) electrode material except for LIB storage material. Chung group synthesized a nanosized V₂O₅/C composite cathode by ball milling the nanosized V₂O₅ with acetylene black and investigating the reaction mechanism in the NIB system.³⁰⁹ Generally, compared with other vanadium oxides (VO₂, V₂O₃, et al.), V₂O₅ consists of the square-based pyramid with the highly distorted environment and exhibits the highest pre-edge intensity from the X-ray absorption near edge structure (XANES) result. Thus, the average vanadium oxidation state of the calcinated V₂O₅/C sample is +4.83 (Figure 37d), which may be due to the calcined carbon reduction (a slight difference with standard V₂O₅ in the pre-edge peak position). They used ex-situ XRD to demonstrate the major NaV₂O₅ with a minor Na₂V₂O₅ phase formed at the first discharge state, accompanying a c lattice parameter increase by 9.09% and unit cell volume increase by 9.2%. At the subsequent charge, the NaV₂O₅ + Na₂V₂O₅ will transform into NaV₂O₅ + V₂O₅ along with a V⁴⁺ → V⁵⁺ change (Figure 37e). The V₂O₅/C delivers an initial discharge capacity of 195 mAh g^–1, and increases to 255 mAh g^–1 at the 10th cycle corresponding to 1.7 Na⁺ inserts into per unit formula (Figure 37f). Besides LIB/NIB cathodes, V₂O₅ can be investigated as an aqueous zinc-ion battery (ZIB) cathode material, and the intercalation of water into the vanadium oxide can increase the interlayer distance, which is in favor of expanding the gallery for Zn²⁺ intercalation. Yang et al.³¹⁰ reported V₂O₅·H₂O/graphene (VOG) synthesized via a freeze-drying method. They investigated the critical role (“lubricating” effect) of structural H₂O on the Zn²⁺ intercalation into bilayer V₂O₅·nH₂O, and H₂O-solvated Zn²⁺ possesses a largely reduced effective charge and improves electrochemical performance. VOG delivers a high capacity of 381 mAh g^–1 at 60 mA g^–1 and maintains 248 mAh g^–1 at a high current density of 30 A g^–1, which are much higher than those of most aqueous ZIB cathode materials (Figure 37g,h). The interlayer distances of VOG are 12.6, 13.5, and 10.4 Å at initial, discharge, and charge states by ex-situ XRD and MAS NMR (Figure 37i). These results demonstrate that water in vanadium oxide layers plays an important role in the performance of an aqueous ion battery.

Figure 37.

XRD patterns (a), rate performances (b) of V₂O₅–HMs, V₂O₅-Ms, and V₂O₅–NWs. Schematic illustration of the V₂O₅–HMs during charge/discharge process (c). Reproduced with permission from ref (308). Copyright 2016 John Wiley & Sons. The XANES spectra of as prepared and after calcination at 400 °C are plotted with solid lines (d), the Na⁺ de/intercalation channels crystal structure (e), charge/discharge profiles and cycling performance (f) of orthorhombic V₂O₅/C. Reproduced with permission from ref (309). Copyright 2016 American Chemical Society. The CV curves (g), rate performances (h), and the proposed crystal structures at different states (i) of V₂O₅·H₂O/graphene. Reproduced with permission from ref (310). Copyright 2018 John Wiley & Sons.

To investigate the electrical conductivity and structural mechanism during lithium insertion/deinsertion of V₂O₅, Yoon et al.³¹¹ developed a 3D V₂O₅/rGO/CNT with short Li⁺ diffusion, and high continuous 3D conductive network, and investigated its structural mechanism during Li⁺ intercalation/deintercalation by in situ XRD/XANES analysis (Figure 38a). The 3D V₂O₅/rGO/CNT delivers a high discharge capacity of 100 mAh g^–1 at 20 C, which is much higher than 2D V₂O₅/rGO (68 mAh g^–1). There are numerous metastable phases of Li_xV₂O₅ during Li⁺ intercalation into V₂O₅. The α-Li_0.26V₂O₅, ε-Li_0.93V₂O₅, δ-Li_1.27V₂O₅, γ-Li_1.93V₂O₅, and ω-Li_2.65V₂O₅ phase form in turn during the first discharge to 3.4, 3.3, 3.19, 2.28, and 2.01 V, respectively. In addition, the most reflection will return to the same position at a pristine state during the subsequent charge process, which can confirm the high structural reversibility of V₂O₅ in the ternary composite upon Li⁺ intercalation/deintercalation (Figure 38b). The 3D V₂O₅/rGO/CNT delivers a high discharge capacity of 100 mAh g^–1 at 20 C, which is higher than 2D V₂O₅/rGO (68 mAh g^–1) (Figure 38c). The preintercalation interlayer metal ions can act as pillars to increase the electronic conductivity, ion diffusion rate, and stability of layered vanadium oxides. Mai et al.³¹² designed and assembled Na_0.33V₂O₅ (NVO) and V₂O₅ single nanowire devices to investigate the effect on the intrinsic electrical conductivity of Na⁺ intercalation. The conductivity of NVO is 5.9 × 10⁴ S m^–1, while the conductivity of V₂O₅ is 7.3 S m^–1, which indicates that the electronic conductivity of V₂O₅ is greatly improved by the Na⁺ intercalation (Figure 38d). Stucky et al.³¹³ fabricated a series of nanostructured Mn-doped V₂O₅ cathode materials and found that the larger Mn doping in the modified V₂O₅ structure can increase the cell volume, which facilitates high Li⁺ diffusion and improves the electronic conductivity (Figure 38e). The Mn_0.01V_1.99O₅ delivers a high discharge capacity (251 mAh g^–1 at 1 C) and excellent cycling stability (80% after 50 cycles), which is much higher than V₂O₅ (215 mAh g^–1 vs. 70%) (Figure 38f,g). Fan et al.³¹⁴ reported lightweight, freestanding V₂O₅ nanoarray-based positive electrodes (UGF-V₂O₅/PEDOT), which were prepared by growing a V₂O₅ nanobelt array directly on 3D ultrathin graphite foam (UGF), followed by coating the V₂O₅ with a mesoporous thin layer of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) (Figure 38h). In addition, the PEDOT coating constructs an integrated conductive network for the V₂O₅, providing decreased electrode polarization, improved charge transfer kinetics, and a prolonged discharge plateau of V₂O₅ (Figure 38i,j), and therefore it can lead to an increased proportion of high-voltage capacity and energy density than that without PEDOT.

Figure 38.

FESEM image (a), in situ XRD (b), and rate performance (c) of 3D V₂O₅/rGO/CNT composite. Reproduced with permission from ref (311). Copyright 2016 under CC BY 4.0 license. The I–V curves of Na_0.33V₂O₅ (NVO) and V₂O₅ (d). Reproduced with permission from ref (312). Copyright 2018 John Wiley and Sons. Rietveld refined XRD patterns of the Mn_xV_2–xO₅ compounds (e), rate performance (f), and cycle performance (g) at 300 mA g^–1 of Mn_0.01V_1.99O₅ and V₂O₅. Reproduced with permission from ref (313). Copyright 2015 American Chemical Society. The SEM image of UGF-V₂O₅/PEDOT (h), the charge/discharge profiles (i), Nyquist plots at fully charged stage (j) of UGF-V₂O₅/PEDOT and UGF-V₂O₅. Reproduced with permission from ref (314). Copyright 2014 John Wiley and Sons.

7.2.2. Supercapacitors

Among all types of vanadium oxides, V₂O₅ has attracted attention for the application of SCs due to its broad oxidation states, high specific capacitance, and low acquisition cost. Palani et al. fabricated RuO₂ nanoparticle-decorated V₂O₅ nanoflakes by a solvothermal method.³¹⁵Figure 39a illustrates the corresponding schematic of the fabricated asymmetric cell that exhibited a high specific capacitance of 421 F g^–1 at a current density of 1 A g^–1 with excellent cyclic retention of 94.6% over 10000 cycles (Figure 39b). The symmetric device of V₂O₅||PVA-KOH||V₂O₅ was fabricated using thin flexible substrate by Velmurugan et al. (Figure 39c), where both annealed (A-500) and as-prepared (RT) V₂O₅ films were used as electrode material separately.³⁰⁶ A schematic representation of the structural image of the V₂O₅ is provided in Figure 39d. The CV curves in Figure 39e denote that the symmetric A-500 gives a larger area under the curve than the symmetric-RT, suggesting the improved performance of the annealed sample. The A-500 devices display a maximum energy density of 0.68 μWh cm^–2, which is obviously much higher than that of the RT electrodes (0.05 μWh cm^–2, Figure 39f). Moreover, the symmetric A-500 shows excellent cycle life up to 30000 cycles with a Coulombic efficiency of 99%. As shown in Figure 39g, the practical feasibility of the as-fabricated devices was demonstrated by lighting blue light-emitting diodes. Several groups reported the hybrid structure of V₂O₅ with carbon materials fabricated by different strategies for enhancing the SC property. For example, Sahu et al.³¹⁶ synthesized graphene nanoribbon @V₂O₅ nanostrip composites to improve the conductive property of V₂O₅, which displays a high energy density of 42.09 Wh kg^–1 and power density of 475 W kg^–1. W. Sun et al.³¹⁷ prepared a 3D monolithic aerogel composed of uniform carbon nanofibers/V₂O₅ core/shell nanostructures. The composite aerogel exhibits high specific capacitance (595.1 F g^–1), excellent energy density (82.65 Wh kg^–1), and good cycling behavior (>12000). Zhu et al. proposed a simple “liquid phase impregnation template” strategy to successfully synthesize hierarchically porous V₂O₅/C nanocomposites that exhibits a specific capacitance of 492.1 F g^–1, as well as an energy density of 87.6 Wh kg^–1.³¹⁸ Yao et al. successfully synthesized SCs based on 3D networks hybrids of reduced graphene oxide and V₂O₅ nanobelts through a simple hydrothermal method.³¹⁹ The rGO/V₂O₅ hybrid aerogel electrodes showed a high energy density of 249.7 W kg^–1 and excellent long-term cycle stability (remaining 90.2% after 5000 cycles).

Figure 39.

(a) Schematic illustration of the 3 wt % RuO₂ in V₂O₅ asymmetric supercapacitor device and (b) cyclic stability 3 A g^–1. Reproduced with permission from ref (315). Copyright 2021 Royal Society of Chemistry. (c) As-fabricated V₂O₅ flexible thin film, and the inset view is the FESEM image of the film. (d) Schematic representation of the symmetric V₂O₅ capacitor. (e) Comparison of CV curves of the symmetric SCs with V₂O₅ electrode prepared at room temperature (RT) and 50 °C (A-500) at a scan rate of 50 mV s^–1. (f) Ragone plot comparison of both symmetric-RT and symmetric A-500 capacitors. (g) Lighting of LED using symmetric A-500 connected in series (inset: various bent position of the symmetric A-500). Reproduced with permission from ref (306). Copyright 2019 American Chemical Society.

7.2.3. Catalysts

7.2.3.1. OER

V₂O₅ exhibited good OER performance due to the multivalent states of the V element, which can enrich active intermediates (*OH, *O, and *OOH) by regulating the valence electron structure of the V element.^275,320 The OER performance could be enhanced by fabricating the composites with other materials. Lan et al.³²⁰ synthesized CoV₂O₆-V₂O₅/nitrogen-doped reduced graphene oxide composites (CoV₂O₆-V₂O₅/NRGO) by a one-pot hydrothermal method integrating polyoxovanadate, ethylenediamine (EN), and graphene oxide (GO) for the precursor and postcalcined process (Figure 40a). Without V₂O₅, the CoV₂O₆/NRGO delivered a relatively acceptable OER performance with an overpotential of 379 mV at a current density of 10 mA cm^–2, which is comparable to that of IrO₂ (337 mV). By adding V₂O₅, the OER performance could be enhanced with an overpotential of 239 mV at a current density of 10 mA cm^–2 (Figure 40b). Furthermore, the CoV₂O₆–V₂O₅/NRGO exhibits a higher current density (47.08 mA cm^–2) at an overpotential of 300 mV compared with CoV₂O₆/NRGO (0.45 mA cm^–2) and IrO₂ (3.95 mA cm^–2) (Figure 40c). Meanwhile, the CoV₂O₆–V₂O₅/NRGO shows the fastest reaction kinetics with a Tafel slope of 49.7 mV dec^–1, which could be due to the enhanced charge transport (Figure 40d). The CoV₂O₆–V₂O₅/NRGO gives good stability from the polarization curves, which are almost overlapping before and after 1000 cycles (Figure 40e). The theoretical calculation found that the existence of the hydrogen bond between V₂O₅ and intermediate HOO* of OER decreases the adsorption energy, which may be responsible for the low overpotential.

Figure 40.

(a) Schematic illustration of the synthetic process of CoV₂O₆–V₂O₅/NRGO composite, (b) polarization curves, (c) comparison of catalysts’ overpotential at a current density of 10 mA cm^–2, corresponding current density at an overpotential of 300 mV, and (d) corresponding Tafel plots. (e) Initial polarization curves of CoV₂O₆–V₂O₅/NRGO and after 1000 CV cycles. Inset: time-dependent current density curve of CoV₂O₆–V₂O₅/NRGO under a potential of 239 mV for 24 h. Reproduced with permission from ref (320). Copyright 2017 American Chemical Society.

7.2.3.2. HER

In general, V₂O₅ is generally considered as an HER-inactive material due to the weak H* adsorption on V sites, which limits the formation of H* at the active site.^321,322 Three main methods can be adopted to promote the HER performance of V₂O₅: defects creation, interfacial engineering, and forming composites. First of all, creating defects, especially vacancies, is regarded as an efficient way to enhance the HER performance.^323−325 Oxygen vacancy (V_O) in transition metal oxides can accelerate the adsorption of the H* intermediates by activating the delocalized electrons of the metal center, which could lead to a better HER performance.^326,327 Li et al.³²⁸ synthesized the V₂O₅ nanosheet arrays with V_O on Co foam through a hydrothermal reaction. The highest V_O concentration of 34.2% of the V₂O₅ nanosheet arrays on the Co foam (Co–V₂O₅–H) is easily obtained by controlling the pH = 1 of the NH₄VO₃ precursor solution. The Co–V₂O₅–H exhibits a low overpotential of 51 mV at a current density of 10 mA cm^–2, which shows better performance compared to the V₂O₅ powder and low V_O concentration samples (Figure 41a). Meanwhile, the catalyst shows a negligible potential drop at different current densities, indicating long-time stability (Figure 41b). The Co–V₂O₅–H is employed as both a cathode and anode to establish a two-electrode alkaline electrolyzer for overall water splitting, which can maintain a steady output current at a cell voltage of 1.6 V for 24 h (Figure 41c). Second, the interfacial engineering between V₂O₅ and transition metal also provides a route to improve the HER performance. Kim et al.³²⁹ directly grew the V₂O₅ particles on Ni foam via a one-step hydrothermal method. The as-prepared V₂O₅/Ni(OH)₂@NF catalyst shows a low overpotential of 39 mV at a current density of 10 mA cm^–2, which is comparable to Pt (35 mV @ 10 mA cm^–2). The Ni(OH)₂@NF samples show an overpotential of 188 mV at 10 mA cm^–2, which indicates that the V₂O₅ plays an important role in the HER performance (Figure 41d). DFT calculation was performed to investigate the active sites of the V₂O₅/Ni(OH)₂@NF catalyst. The exposed facet of V₂O₅ is the (010), (001), and (310) planes, which is confirmed by TEM (Figure 41e). The H adsorption energy of the V₂O₅ (001) and (310) surfaces is larger than that of V₂O₅ (010), which indicates that V₂O₅ (010) is the active surface. Furthermore, the ΔG_H* values of the Ni-sites at the edges of interfaces in Ni(OH)₂@Ni and V₂O₅@Ni and the O-site of V₂O₅ (corresponding to 2, 4, and 5, respectively, as shown in Figure 41f) are close to zero, which is nearly equal to that of the Pt(111)-surface. The calculation indicated the edges of the interfaces in V₂O₅/Ni(OH)₂@NF play a significant role in the HER process. Moreover, the hierarchical V₂O₅@Ni₃S₂ hybrid nanoarray also exhibited a good overpotential of 95 mV at 10 mA cm^–2,³³⁰ which is also due to the interfaces between V₂O₅ and Ni₃S₂. Third, V₂O₅ is also used as an additive with other materials to form a composite and further enhance the HER performance. By doping phosphorus and adding V₂O₅ into Pt/graphene, the prepared catalysts exhibit a good HER performance of the initial potential of 32 mV and a Tafel slope of 23 mV dec^–1.³³¹

Figure 41.

(a) HER polarization curves of the Co–V₂O₅–H sample, (b) stability of the Co–V₂O₅–H at 10–100 mA cm^–2, (c) stability of the Co–V₂O₅–H // Co–V₂O₅–H couple at 1.6 V for overall water splitting. Reproduced with permission from ref (328). Copyright 2020 Elsevier. (d) Polarization curves of NF, 20% Pt/C@NF, V₂O₅@NF, Ni(OH)₂@NF, and V₂O₅/Ni(OH)₂@NF at a scan rate of 2 mV s^–1, (e) HRTEM image of V₂O₅/Ni(OH)₂@NF, (f) calculated free energies at sites 1 to 6 (1: the reconstructed Ni-surface, 2: the Ni-site of Ni(OH)₂@Ni, 3: the Ni-site between V₂O₅@Ni and Ni(OH)₂@Ni, 4: the Ni-site of V₂O₅@Ni, 5: the O-site on the V₂O₅ surface, 6: the O-site inside the V₂O₅ channel) and various individual pristine materials. Reproduced with permission from ref (329). Copyright 2019 Royal Society of Chemistry.

7.2.3.3. Photocatalysis

It is well documented that V₂O₅ has a typical narrow band gap (∼2.3 eV) and wide optical absorption range and is a high electron mobility semiconductor, which exhibits good photoresponsive properties by capturing visible light and is widely used in the electro-photocatalytic field, such as for hydrogen production, environmental pollutant degradation, etc.^332−334 Garcia et al.³³⁵ found that the morphology of V₂O₅ nano-/microparticles dramatically affected the hydrogen production by photocatalysis. The V₂O₅ with microbelts, nanoplates, and nanopillars morphology can be obtained by using sunflowers’ petals and the center of the sunflower as biodegradable templates during the synthesis.³³⁵ The nanopillar V₂O₅ delivers a maximum hydrogen generation rate in the presence of Na₂SO₃ as a sacrificial agent with 65.5 μmol g^–1 h^–1, which is higher than microbelts (26.3 μmol g^–1 h^–1) and nanoplates V₂O₅ (19.4 μmol g^–1 h^–1) (Figure 42a). The high hydrogen generation rate is attributed to the large surface area, high absorbance in the UV–vis range, high photocurrent, and high content of defects of the V₂O₅ nanopillars. By fabricating the nanocomposite with V₂O₅ containing 1 or 2 heterojunctions, the hydrogen generation rate will be further improved, which is due to the delayed electron–hole recombination.³³⁵ For example, graphitic carbon nitride nanosheets/V₂O₅ composites (578 μmol g^–1 h^–1),³³⁶ Na₂Ti₃O₇/V₂O₅/g-C₃N₄ composites (11000 μmol g^–1 h^–1),³³⁷ Na₂TiO₃/V₂O₅/g-C₃N₄ composites (567 μmol g^–1 h^–1),³³⁷ and Nb-doped SnO₂/V₂O₅ (1346 μmol g^–1 h^–1).³³⁸ Transition metal oxides (TiO₂, ZnO, etc.) have been applied in the removal of environmental pollutants in water, which could completely decompose organic pollutants into CO₂ and H₂O by photocatalytic oxidation.^339,340 Hollow V₂O₅ microspheres consisting of randomly packed platelets showed an enhanced UV light absorption compared to the commercial V₂O₅ powder, which led to the highest activity for degrading rhodamine B under UV light.³⁴¹ Composites, consisting of 1D V₂O₅ nanorod and 2D carbon-based materials, are promising photocatalysts for environmental pollutant degradation. The V₂O₅ nanorods/graphene oxide and V₂O₅ nanorods/graphene nanocomposites showed good degradation performance of Victoria blue dye and methylene blue dye (>95% degradation within 90 min), respectively.^342,343 Especially, the nanocomposites exhibited the best degradation performance under direct sunlight irradiation compared to UV and visible light. The g-C₃N₄ is also used to construct the heterojunctions with V₂O₅ for a high-performance degradation catalyst.³⁴⁴ The photoexcited electron in the conduction band of g-C₃N₄ shows a strong reducing ability, while the photoexcited hole on the valence band of V₂O₅ exhibits a strong oxidizing ability. Thus, the g-C₃N₄ /V₂O₅ heterojunctions exhibited efficient degradation performance of rhodamine B, methyl orange (MO), and methylene blue (MB) dyes under visible light (Figure 42b). Besides the graphene or g-C₃N₄, the V₂O₅ composites with other inorganic photocatalytic oxides also showed enhanced photocatalytic properties for degradation of organic pollutants, such as V₂O₅/BiVO₄,^332,345 V₂O₅/CeO₂,³⁴⁶ V₂O₅/TiO₂,^347,348 and so on. The V₂O₅-based composite can degrade not only the dye molecules in solution, but also some small solvent molecules in the gas phase. The ternary V₂O₅/BiVO₄/TiO₂ nanocomposites exhibited a well-aligned band structure and increasing photoinduced charge carriers through the charges separation across their multiple interfaces, which resulted in good light absorption from the UV to the visible region and better photocatalytic activity for the decomposition of gaseous toluene compared to pure TiO₂ and V₂O₅/BiVO₄ under visible light irradiation (Figure 42c).³⁴⁹

Figure 42.

(a) The hydrogen generation curves for VO-microbelts, VO-nanoplates, and VO-nanopillars with a sacrificial agent. Reproduced with permission from ref (335). Copyright 2021 Elsevier. (b) The photodegradation performance of g-C₃N₄/V₂O₅ photocatalyst for RhB, MO, and MB degradation under visible light irradiation. Reproduced with permission from ref (344). Copyright 2016 Elsevier. (c) Schematic diagram of lectron–hole pairs separation of the V₂O₅/BiVO₄/TiO₂ nanocomposites under visible-light irradiation. Reproduced with permission from ref (349). Copyright 2014 American Chemical Society.

7.2.4. Electrochromism

Since Colton et al. did the pioneering work on the electrochromism of V₂O₅ in 1976–1977,^350,351 V₂O₅ has attracted increasing attention among transition metal oxides because it can exhibit both anodic and cathodic coloration.³⁵² From 1975 to 1999, studies focused on the absorption-transmission spectra modulation in visible and infrared light, while, after 2000, V₂O₅ with various micro- and nanostructures were emerging, and researchers paid attention to improve some important key figures of merit to evaluate the electrochromic performance, such as the coloration efficiency, cycling life, switching time, and so on.^353−358 Recently, several reviews were published on the electrochromic application of V₂O₅ film, which provide more specific and detailed information on this topic.^{256,352,359,360} For V₂O₅, the discoloration mechanism is explained as the result of injection/extraction of electrons and electrolyte cations and variation of valence change of vanadium ions, which is widely accepted.^361,362

In order to improve the performance of V₂O₅ as an electrochromic device, many strategies have been designed. Especially, V₂O₅ nanostructures with small sizes and large specific surface areas are expected to facilitate the ion intercalation/deintercalation process, thereby enhancing the electrochromic properties. Panagopoulou et al.³⁶³ successfully prepared Mg-doped V₂O₅ thin films using RF sputtering, and found the 15 atom % Mg-doped films displayed optimal electrochromic properties with the fastest switching time of t_c = 10/4 s (intercalation/deintercalation), the best coloration efficiency of 71.3 cm² C^1– at 560 nm, higher visible transmittance of 85%, and the highest contrast value between the coloration states of ΔT (34.4% @ 560 nm). Qi et al.³⁶⁴ fabricated flexible V₂O₅ nanosheets/graphene oxide films, which exhibit ultrafast coloring response time (1.6 s) and bleaching time (2 s) attributed to the reduced charge transport distances of the ultrathin nanosheet structure (4–40 nm). They also display an excellent transmittance contrast of 57.5% at 425 nm and reversible yellow/green/blue-gray multicolor changes. Tong et al.³⁶⁵ fabricated a 3D crystalline V₂O₅ nanorod architecture on ITO substrates by a colloidal crystal-assisted electrodeposition method. Such architecture exhibits a highly reversible Li-ion insertion/extraction process (Columbic efficiency up to 96.9%), five distinct color change, good transmittance modulation ΔT (38.48% @ 460 nm), and acceptable response times (8.8 s for coloration and 9.3 s for bleaching), making it a promising film electrode for electrochromic devices. Kim et al.³⁶⁶ prepared highly crystalline 2D V₂O₅ nanosheets by using single-layer V₂CT_x film as a sacrifice template (Figure 43a). The mean thickness and lateral size of V₂CT_x are 2.38 nm and 0.78 μm, respectively (Figure 43b). Figure 43c shows the SEM image of V₂O₅ nanosheets film after annealing of V₂CT_x at 350 °C. The 2D V₂O₅ nanosheets based electrochromic device has sharp multicolor transformations with a robust optical contrast from yellow to green to blue (Figure 43d). The corresponding optimal electrochromic performance shows a high optical contrast (53.98% @ 700 nm) and a fast response time (6.5 s for coloration and 5.0 s for bleaching).

Figure 43.

Fabrication of a 2D V₂O₅ nanosheet based electrochromic device. (a) Schematic for fabrication of an electrochromic device based on a V₂CT_x-derived 2D V₂O₅ nanosheet, resulting in minimized optical scattering and enhanced ion diffusion. (b) Representative AFM image of single layer V₂CT_x with a submicron lateral size. (c) SEM image of cross-section of 2D V₂O₅ nanosheet based electrochromic layer and corresponding EDS analysis in terms of vanadium, carbon, and oxygen. (d) Digital image of electrochromic device operating from +1.9 V to −1.2 V. Reproduced with permission from ref (366). Copyright 2023 Elsevier.

8. Other Vanadium Oxides

8.1. V₂O₂

V₂O₂ is generally used as a tool to investigate the nature of metal–oxygen bonds, which is crucial to provide a proper rationalization of the relationship between structure and properties at an atomic scale.³⁶⁷ Stable V₂O₂ could not be synthesized by a traditional solid-state reaction or solution methods. However, it can be observed by IR spectroscopy on V, Ne, and O₂ codeposited matrices.³⁶⁸ The calculation results show two possible vanadium bonding situations: 1) no bonds between the two vanadium atoms; 2) a short distance between the two vanadium atoms, which indicated multiple bonding (Figure 44a).^368−370 However, Himmel et al.³⁶⁸ found that there is a multiple vanadium–vanadium bond in V₂O₂ molecules. There are three V–V bonding and antibonding orbitals, which are occupied by 1.70, 1.58, 1.50 electrons and 0.42, 0.40, 0.49 electrons, respectively. Thus, 4.78 electrons are in V–V bonding orbitals, and 1.31 electrons are in V–V antibonding orbitals. Furthermore, five bond critical points (four points between the oxygen and the vanadium atoms and one point in the center between the vanadium atoms) and two ring critical points (between the center of the cluster and the oxygen atoms) can be identified in V₂O₂ molecules (Figure 44b), which confirms the presence of a V–V bond. The presence of the strong V–V bond may lead to unique optical and magnetic properties.

Figure 44.

(a) Two possible Lewis representations highlighting the unclear bonding situation in V₂O₂. (b) Plot of the electron density within the molecular plane. Rectangles: bond critical points; triangles: ring critical points. Reproduced with permission from ref (368). Copyright 2017 John Wiley and Sons.

8.2. V₄O₉

V₄O₉ has an orthorhombic structure with a space group of Cmcm, which processes three types of VO polyhedra (Figure 45a). The VO₅ pyramids and VO₆ octahedra make pairs, which are connected by corner oxygen atoms from the VO₄ tetrahedra.³⁷¹ The valence of the vanadium ion in the octahedron and pyramid is +4, while that in the tetrahedron is +5. V₄O₉ is difficult to synthesize by a solid-state reaction from the mixture of binary V₂O₅ and V₂O₃ (or VO₂), but it can be synthesized by the reduction of V₂O₅ using reducing agents of carbon, SO₂, and sulfur.^371,372 The amount of reducing agents dramatically affects the final products, which may consist of other vanadium oxides, such as V₆O₁₃ and VO₂. Consequently, reducing V₂O₅ by the solvothermal method is a facile way to obtain V₄O₉. Different solvents (tetraethylene glycol, 2-propanol, and tetrahydrofuran) are used to synthesize V₄O₉ with different morphologies, such as nanoflakes, nanosheets, and so on.^373−375 Liang et al.³⁷⁵ investigated the aqueous zinc ion batteries of V₄O₉. It is found that the V₄O₉ exhibits fast zinc ion and electron diffusion, which is due to the unique tunnel structure and the V⁵⁺/V⁴⁺ mixed-valences induced metallic behavior. The V₄O₉ cathode shows a high reversible discharge capacity (420 mA h g^–1 at 0.5 C). Even at a high current density of 50 C, it also exhibits an impressive discharge capacity of 234.4 mA h g^–1, suggesting a fast Zn²⁺ storage ability of V₄O₉ (Figure 45b). Furthermore, V₄O₉ delivers an energy density of 175.8 W h kg^–1 at a high power of 17625 W kg^–1, which gives a high power density compared with other vanadium-based cathode materials in aqueous zinc ion batteries, such as V₂O₅,³⁷⁶ V₆O₁₃,³⁷⁷ LiV₃O₈,³⁷⁸ Na₃V₂(PO₄)₃,³⁷⁹ VO₂,³⁸⁰ and VS₂.³⁸¹

Figure 45.

(a) Crystal structure of V₄O₉ with VO₆ octahedron (yellow), VO₅ pyramid (green), and VO₄ tetrahedron (blue). (b) Rate performance at various currents ranging from 0.5 to 50 C of the aqueous Zn//V₄O₉ second battery. Reproduced with permission from ref (375). Copyright 2021 Royal Society of Chemistry. (c) The cycling stability of the specific capacitance of flower V₄O₉ at 2 A g^–1. The inset shows the charge–discharge curves at different cycle numbers. (d) UV–vis absorbance curve (with polynomial fitting) of 75 mg flower V₄O₉ (blue) at varying H₂O₂ concentrations. Reproduced with permission from ref (373). Copyright 2013 Royal Society of Chemistry.

The 2D single layered V₄O₉ nanosheet assembled 3D microflowers exhibit good supercapacitor performance with a specific capacitance of 392 F g^–1 at a current density of 0.5 A g^–1 and 75% retained capacitance after 2000 cycles (Figure 45c).³⁷³ The flower-like structure assembled from ultrathin and well-separated nanosheets was unchanged during the charge–discharge cycles, which is responsible for the high capacitance and stability. Meanwhile, the V₄O₉ flower demonstrates a good ability to sense H₂O₂ and methanol with a detection limit of ∼0.1 μM and ∼60 μM, respectively (Figure 45d).

8.3. V₆O₁₃

The mixed-valence V₆O₁₃ attracts extensive attention because it can be used in a variety of ion batteries, such as Li⁺, Na⁺, Mg²⁺, Zn²⁺, and so on. As shown in Figure 46a, V₆O₁₃ is composed of alternating single and double vanadium oxide layers. There are two types of VO₆ octahedra: V⁴⁺ occupied the yellow octahedra, and V⁵⁺ occupied the blue octahedra. All the octahedra are connected with corner O atoms, which form a tunnel-like structure.^49,382 The solvothermal reaction is widely used to obtain V₆O₁₃ nanostructures, particles, and related composites, such as V₆O₁₃ nanogrooves,³⁸³ V₆O₁₃@hollow carbon microspheres,³⁸⁴ nest-like V₆O₁₃,³⁸⁵ V₆O₁₃ nanosheets,³⁸⁶ V₆O₁₃ nanowires,³⁸⁷ V₆O₁₃ nanorods,³⁸⁸ and so on (Figure 46b). Cao et al.³⁸⁹ recently developed a new strategy to synthesize V₆O₁₃ nanosheets by microwaves, which could reduce the thickness of V₆O₁₃ nanosheets greatly compared to that prepared by a hydrothermal method (Figure 46c,d).

Figure 46.

(a) Crystal structure of V₆O₁₃ with V two types of VO₆ octahedra. (b) Different morphologies of V₆O₁₃ synthesized by a solvothermal method. Reproduced with permission from ref (385) (Copyright 2020 Royal Society of Chemistry), ref (384) (Copyright 2021 Elsevier), ref (383) (Copyright 2015 American Chemical Society), and ref (390) (Copyright 2019 Elsevier). (c, d) V₆O₁₃ nanosheet obtained by hydrothermal and microwave-assisted synthesis. Reproduced with permission from ref (386) (Copyright 2021 Elsevier) and ref (389) (Copyright 2022 Elsevier).

Due to the tunnel-like structure and mixed-valence, V₆O₁₃ exhibits a metallic character at room temperature, which is beneficial for high-rate charge and discharge. When it is used as an LIB electrode material, 8 mol Li⁺ intercalated per formula unit endowing a high theoretical specific capacity of 417 mAh g^–1 and energy density of 900 Wh kg^–1. The Yu group³⁸³ synthesized a 3D V₆O₁₃ nanotextile with interconnected 1D nanogrooves via a facile solution-redox-based self-assembly route at room temperature (Figure 47a). They confirmed that the precursor concentration affected the mesh size in the textile structure. The 3D V₆O₁₃ delivers a high capacity of 326 mAh g^–1 at 20 mA g^–1 and maintains 80% capacity after 100 cycles at 500 mA g^–1. The energy density can reach 780 Wh kg^–1, which is much higher than those of commercialization cathodes (LiFePO₄ and LiCoO₂) (Figure 47b). The excellent electrochemical performance is due to the unique structure of 3D textiles, which can be maintained upon cycling and are beneficial for ion transport and cycle stability (Figure 47c). In addition, Mai et al.³⁹¹ synthesized a novel ultrathin prelithiated V₆O₁₃ nanosheet by a secondary hydrothermal prelithiation process (Figure 47d). A single-nanosheet device was employed to in situ probe the intrinsic advantages of prelithiated nanosheets. Compared with nonlithiated V₆O₁₃ nanosheets, the ultrathin prelithiated V₆O₁₃ nanosheets exhibit a higher electrical conductivity and maintain the same conductance level after the Li⁺ intercalation (Figure 47e). Meanwhile, the specific capacity of the ultrathin prelithiated V₆O₁₃ nanosheets can be maintained at 98% after 150 cycles at a current density of 1000 mA g^–1, which is much higher than 46% capacity of nonlithiated V₆O₁₃ nanosheets (Figure 47f). These results demonstrate that prelithiation is a strategy to obtain high-energy and long-cycling energy storage cathode materials.

Figure 47.

XRD pattern and SEM image (a), different CV curves (b), and the schematic diagram of the Li⁺ intercalation process (c) of 3D V₆O₁₃ nanotextile electrodes in LIB. Reproduced with permission from ref (383). Copyright 2015 American Chemical Society. The TEM image of ultrathin lithiated V₆O₁₃ nanosheets (d), the transport properties (e), and cycling performance at 1000 mA g^–1 (f) of nonlithiated ultrathin and ultrathin lithiated V₆O₁₃ nanosheets. Reproduced with permission from ref (391). Copyright 2014 Elsevier. Optimized geometry of Zn intercalated V₆O₁₃ with water (g) and without water (h), the galvanostatic voltage-capacity profiles for V₆O₁₃ cycled in electrolytes with different water contents in 1 M Zn(CF₃SO₃)₂ acetonitrile (i). Reproduced with permission from ref (392). Copyright 2019 John Wiley & Sons. (j) Charge/discharge curves of K–V₆O₁₃ at different rates. (k) The migration energy barriers of K–V₆O₁₃ and V₆O₁₃. Reproduced with permission from ref (394). Copyright 2022 Royal Society of Chemistry. (l) Rate capability under various currents of V₆O₁₃ on carbon cloth. Reproduced with permission from ref (385). Copyright 2020 Royal Society of Chemistry.

Doping of various metal ions can improve the electrochemical performance and achieve a good capacity of the battery. The V₆O₁₃ with Al/Ga, Al/Fe, and Al/Na doping delivered an initial discharge specific capacity of 411.5 mA h g^–1, 426.9 mA h g^–1, and 514 mA h g^–1 at 0.1 C, respectively.³⁸⁸ However, the electrochemical performances were poor at a high current density. V₆O₁₃ was also employed in multivalent ion (Zn²⁺, Mg²⁺) batteries.^{384,385,389,392,393} Choi et al.³⁹² applied V₆O₁₃ as the ZIB cathode material and investigated its electrochemical behavior for Zn²⁺. In particular, they analyzed the effect of water in the electrolyte on the Zn²⁺ storage to investigate the physicochemical characteristics of V₆O₁₃. DFT calculation results demonstrate that the coordination environments of Zn show a big difference with/without water (Figure 47g,h). It will form octahedral coordination with water, but undercoordination without water. The Zn²⁺ storage of V₆O₁₃ increases with increasing water content in the electrolyte, and it can deliver a high capacity of 360 mAh g^–1 and be maintained at 92% after 2000 cycles in an aqueous Zn(CF₃SO₃)₂ (∼1 M) electrolyte (Figure 47i). Even at a high current density of 24 A g^–1, it maintains a relatively high capacity of 145 mAh g^–1.¹⁶ This work highlights that cointercalating water molecules play a vital role in enhancing the electrochemical performance of the aqueous ion storage system. Zhao et al.³⁹⁴ found that the K⁺ intercalated V₆O₁₃ exhibited a specific capacity of 367 mAh g^–1 at 0.5 A g^–1 and 198.8 mAh g^–1 at 10 A g^–1 (Figure 47j). Meanwhile, the capacity could be maintained at 90% after 2000 cycles at 10 A g^–1. The DFT calculation suggested that K⁺ intercalation could significantly contribute to the reduction of the Zn²⁺ diffusion energy barrier (from 0.94 to 0.33 eV), which enables Zn ions to migrate away from the intercalation sites more easily (Figure 47k). Thus, K⁺ intercalated V₆O₁₃ showed good battery performance. Furthermore, by growing the V₆O₁₃ on carbon cloth and using the ZnSO₄ as the electrolyte, the V₆O₁₃ cathode showed a capacity of 520 mAh g^–1 (at a current density of 0.5 A g^–1) and good cycle life (a stable capacity of 335 mAh g^–1 over 1000 cycles) (Figure 47l).³⁸⁵

8.4. Other Vanadium Oxides for Energy-Related Application

V₅O₁₂·6H₂O is another layered monoclinic vanadium oxide separated by water pillars with a large interlayer spacing of 1.179 nm. Wang et al.³⁹⁵ added a small amount of platinum (Pt, 1.5 wt %) into the interlayer of V₅O₁₂·6H₂O. The obtained V₅O₁₂·6H₂O–Pt electrode delivers a specific capacity of 440 mAh g^–1 at 500 mA g^–1, and increases to 489 mAh g^–1 for the third cycle, much higher than V₅O₁₂·6H₂O electrodes (270 mAh g^–1 at 500 mA g^–1) (Figure 48a,b). They also demonstrated that the Pt additive makes no contribution, and is even counterproductive to conductivity, but facilitates a significant enhancement of pseudocapacitance. Therefore, it is clear that there is a strong relationship between Pt and the new phase Zn₄SO₄(OH)₆·5H₂O. The in situ XRD patterns show that obvious characteristic peaks shifted to lower angles during the discharge process and returned to the pristine state during charging, indicating the interlayer spacing gradual enlargement and recovery upon Zn²⁺ intercalation/deintercalation. Meanwhile, the formation/disappearance of the zinc hydroxyl complex is accompanied by Zn²⁺ insertion/extraction (Figure 48c).

Figure 48.

XRD pattern (a), charge/discharge profiles (b), and in situ XRD patterns and 2D contour map of peak intensities of V₅O₁₂·6H₂O–Pt (c). Reproduced with permission from ref (395). Copyright 2021 John Wiley & Sons.

Mai et al.³⁹⁶ artificially constructed a VO_x cluster/reduced graphene oxide (rGO) cathode material with interfacially inserted Zn²⁺ repelling the pristine-bonded C atom into the plane of the rGO and constructing interfacial V–O–C bonds (Figure 49a). Meanwhile, the VO_x consists of subnanoclusters (less than 1 nm per dimension) and partial nanoclusters (slightly larger than 1 nm). In combination with the electrons transferred to the rGO during the discharge process (Figure 49b–d), the reduced degree of defect is additional proof of the interfacial Zn²⁺ storage. As a result, they have discovered a new mechanism in which Zn²⁺ ions are stored mainly at the interface between VO_x and rGO, which leads to anomalous valence changes compared to conventional mechanisms and exploits the storage capacity of the nonenergy storing active but highly conductive rGO. The obtained VO_x-G heterostructure delivers a superior rate performance with a capacity of 174.4 mAh g^–1 at an ultrahigh current density of 100 A g^–1, with capacity retention of 39.4% for a 1000-fold increase in current density (Figure 49e). To the best of our knowledge, the rate performance is one of the best among ZIBs.

Figure 49.

Characterization of the interfacial configuration in the VO_x-G heterostructure and its anomalous Zn²⁺ storage mechanism (a), DFT simulations in pristine (b) and fully discharged (c) states, formation energy evolution during Zn²⁺ ion insertion into the interface (d), galvanostatic discharge curves varying from 0.1 to 100 A g^–1 (e) of VO_x-G heterostructure. Reproduced with permission from ref (396). Copyright 2021 John Wiley & Sons.

Besides the battery application, the nanostructured VO_x materials/composites have been considered as a good catalyst for the HER due to the creation of the more active site and modification of the adsorption and desorption of H atoms.³⁹⁷ Guan et al.³⁹⁸ prepared 2D Zn-VO_x-Co ultrathin nanosheets on carbon fiber paper by an electrodeposition method. The Zn-VO_x-Co electrocatalysts contain the amorphous Co metal phase and crystalline Zn–Co alloy phase, which gives the materials a good HER performance (an overpotential of 46 mV at 10 mA cm^–2 and a Tafel slope of 75 mV dec^–1). Furthermore, Zhao et al.³⁹⁷ adopted the same method to fabricate a Ni(Cu)VO_x catalyst by changing Zn and Co to Ni and Cu (Figure 50a). The Ni(Cu)VO_x electrode displays a small overpotential of 21 mV at a current density of 10 mA cm^–2 and a Tafel slope of 28 mV dec^–1 (Figure 50b), which is comparable to the commercial 20% Pt/C catalyst (15 mV @ j = 10 mA cm^–2 and 25 mV dec^–1). Meanwhile, the Ni(Cu)VO_x electrode also remains relatively stable for more than 100 h HER at 100 mA cm^–2 (Figure 50c). The good HER performance is due to the existence of Ni–O–VO_x sites, which promote the formation of highly disordered metallic Ni structures and further induce electron transfer from Ni to VO_x.

Figure 50.

(a) Schematic to prepare the Ni(Cu)VO_x electrode for HER electrolysis. (b) LSV curves for Ni(Cu)VO_x and control samples without iR compensation. The black dotted curve is the HER activity on the Ni(Cu)VO_x electrode with iR correction. (c) Chronopotentiometric curve of Ni(Cu)VO_x in comparison with Ni(Cu). Reproduced with permission from ref (397). Copyright 2020 under CC BY license.

The VO_x based composites also exhibit good OER performance. Dong et al.³⁹⁹ synthesized 3D nanoflower-like VO_x nanosheets (VO_x/NiS/NF) by a hydrothermal method, which showed the OER performance with a low overpotential (330 mV at 50 mA cm^–2) and a small Tafel slope (121 mV dec^–1). Yang et al.⁴⁰⁰ synthesized a ternary Co-VO_x-P catalyst with a nanoflower structure directly onto the Ni foam. The as-prepared materials demonstrated excellent catalytic performance for both the HER (an overpotential of 98 mV at 10 mA cm^–2 and a Tafel slope of 59 mV dec^–1) and OER (an overpotential of 230 mV at 100 mA cm^–2 and a Tafel slope of 64 mV dec^–1) under an alkaline environment.

9. Conclusions and Future Outlook

In the past two decades, there has been accelerated development of vanadium oxides due to the fact that they are the most promising candidates in versatile applications, such as batteries, energy saving smart windows, sensing, catalysts, optoelectronic devices, etc. In this review, we have discussed the V–O binary phase diagram, the structure and synthesis methods of five thermodynamically stable vanadium oxides (V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅) and some metastable vanadium oxides (V₂O₂, V₄O₉, V₆O₁₃) with selected applications on hydrogen evolution catalysis, supercapacitors, batteries, smart windows, and some other aspects. The battery, supercapacitor, and HER/OER performances of these vanadium oxides are summarized for comparison to provide an overview of the research in the field (Tables 1, 2, and 3).

Table 1. Electrochemical Properties of the Typical Vanadium Oxides for Metal-Ion Batteries.

materials	battery types	Ccapacity (mAh g–1)/current density (A g–1)	retention (%)/Ccycle numbers/current density (A g–1)	rate capacity (mAh g–1)/current density (A g–1)	ref
VOx–rGO heterostructure	ZIBs	443/0.1	∼92%/1000/20	174.4/100	(396)
V2O3 hollow spheres	LIBs	785/0.1	Over 100%/700/2	361/2	(76)
V2O3 porous nanofibers	KIBs	240/0.05	94.5%/500/0.05	134/1	(68)
V2O3	ZIBs	625/0.1	97.7%/1000/5	486/20	(77)
V2O3 with CNTs	SIBs	612/0.1	100%/6000/2	207/10	(361)
			70%/10000/10
Carbon-confined V2O3	ZIBs	633.1/0.2	90.3%/10000/12	271.4/24	(401)
V3O5	LIBs	412/0.2	78.5%/2000/50	125/50	(98)
Graphene quantum dots coated onto the VO2 surfaces	LIBs	421/0.1	94%/1500/18	151/36	(237)
Graphene quantum dots coated onto the VO2 surfaces	NIBs	306/0.1	88%/1500/18	93/36	(237)
VO2 (B) nanorods	KIBs	209.2/0.05	86%/500/0.5	141.4/2	(238)
VO2 (B) nanorods	MIBs	391/0.025	41.9%/60/0.85	341/0.1	(402)
VO2 hollow microspheres	LIBs	203/0.1	80%/1000/2	134/2	(240)
VO2 interwoven nanowires coated with Carbon quantum dots	LIBs	427/0.1	112%/500/19.2	168/19.2	(403)
VO2 with graphene ribbons	LIBs	415/0.4	93%/1000/37.2	204/37.2	(404)
VO2 (B) nanobelt forest	LIBs	475/0.1	63%/47/0.1	100/27	(405)
VO2 nanorods	ZIBs	325.6/0.05	86%/5000/3	72/5	(239)
VO2 nanofibers	ZIBs	357/0.1	N.A.	171/51.2	(406)
V3O7 nanowire templated graphene scrolls	LIBs	321/0.1	87.3%/400/2	162/3	(120)
V3O7·H2O	ZIBs	370/0.375	80%/200/3	270/3	(121)
H2V3O8	MIBs	231/0.01	77%/100/0.04	97/0.08	(108)
H2V3O8 nanowires	ZIBs	423.8/0.1	94.3%/1000/5	113.9/5	(407)
H2V3O8 nanowires with graphene	ZIBs	394/0.1	87%/2000/6	270/6	(408)
V4O7	LIBs	291/0.05	∼100%/100/3	159/3	(409)
V4O9	ZIBs	420/0.235	78.8%/1000/9.4	233.4/23.5	(375)
3D V6O13 nanotextiles	LIBs	326/0.02	80%/100/0.5	134/0.5	(383)
V6O13 nanosheet	LIBs	331/0.1	98%/150/1	150/2	(391)
V6O13	ZIBs	360/0.2	92%/2000/4	145/24	(392)
V6O13 microflowers	SIBs	159.8/0.16	73.5%/30/0.16	N.A.	(410)
Oxygen-deficient V6O13	ZIBs	401/0.2	95%/200/0.2	223/5	(411)
V5O12·6H2O	ZIBs	440/0.5	∼95%/400/10	158/15	(395)
V5O12·6H2O nanobelt	ZIBs	354.8/0.5	∼94%/1000/2	228/5	(412)
V10O24·12H2O	ZIBs	164.5/0.2	90.1%/3000/10	80/10	(413)
V10O24·12H2O	ZIBs	365.3/0.2	83.2%/3000/5	127.2/80	(414)
V7O16 nanotube	ZIBs	314.6/0.1	80.5%/950/2.4	87.8/9.6	(415)
V2O5 hollow microclew	LIBs	145.3/0.1	94.4%/50/0.1	94.8/10	(308)
V2O5 hollow nanosphere	SIBs	159.3/0.04	72.6%/100/0.16	112.4/0.64	(416)
V2O5 nanowires	AIBs	305/0.125	89.5%/20/0.125	N.A.	(417)
V2O5	ZIBs	470/0.2	91.1%/4000/5	386/10	(418)
V2O5 nanosheet	ZIBs	224/0.1	81.3%/30/0.1	N.A.	(419)
V2O5 nanofibers	ZIBs	319/0.02	81%/500/0.6	104/3	(376)
Porous V2O5	LIBs	142/0.075	∼90%/100/0.075	86.7/8.2	(420)
V2O5/C	SIBs	255/0.015	95%/30/0.015	170/0.294	(309)
V2O5-polyaniline superlattice	MIBs	270/0.1	61.5%/500/4	130/4	(421)
Polyaniline intercalated V2O5	NIBs	307/0.5	42%/100/5	69/20	(422)
V2O5·nH2O	SIBs	338/0.05	73%/50/0.5	96/1	(423)
V2O5·nH2O	ZIBs	372/0.3	71%/900/6	248/30	(310)
V2O5·nH2O	CIBs	204/0.14	86%/350/0.7	28/2.8	(424)
3D V2O5/RGO/CNT	LIBs	304/0.0294	90%/80/0.294	100/5.88	(311)
Na0.33V2O5	ZIBs	367.1/0.1	93%/1000/1	96.4/2	(312)
Mn0.01V1.99O5	LIBs	251/0.294	80%/50/0.294	171/1.47	(313)
UGF-V2O5/PEDOT	LIBs	297/0.294	98%/1000/17.64	85/23.52	(314)

Table 2. Electrochemical Properties of the Typical Vanadium Oxides for Supercapacitors.

materials	electrolyte	Specific capacitance (F g–1)/Current density (A g–1)	Retention (%)/Cycle numbers	ref
VOOH nanosheets	1 M LiClO4/PPC	323/0.2	70%/2000	(425)
V2O3 nanoflakes@C core–shell composites	1 M NaNO3	205/0.05	76%/500	(94)
V2O3/C nanocomposites	5 M LiCl	458.6/0.5	86%/1000	(93)
V2O3@C core–shell nanorods	5 M LiCl	228/0.5	81%/1000	(426)
V2O3@C core–shell nanorods	1 M Na2SO4	192/1	66%/1000	(427)
V2O3@C core–shell composites	1 M Na2SO4	223/0.1	39.7%/100	(428)
N-doped carbon coated nest-like V3O7	1 M Na2SO4	660.63/0.5	80.47%/4000	(112)
		187.72/50
V3O7 nanowires on carbon fiber cloth	1 M Na2SO4	198/1	97%/100000	(125)
V3O7·H2O nanobelts/CNT/rGO composites	5 M LiCl/PVA	685/0.5	99.7%/10000	(126)
V3O7-rGO-polyaniline composites	1 M H2SO4	579/0.2	95%/2500	(429)
VO2(B) nanobelts/rGO composites	0.5 M K2SO4	353/1	78%/10000	(235)
VO2 nanosheet	1 M LiClO4/PPC	405/1	82%/6000	(430)
VO2 nanosheet	6 M KOH	663/10	99.4%/9000	(431)
VO2(B)/C core–shell composites	1 M Na2SO4	203/0.2	10.4%/100	(432)
Graphene foam/VO2 nanoflakes/hydrogen molybdenum bronze composites	1 M K2SO4	485/2	97.5%/5000	(433)
		306/32
VO2 microarrays	1 M Na2SO4	265/1	100%/3000	(434)
VO2@polyaniline coaxial nanobelts	0.5 M Na2SO4	246/0.5	28.6%/1000	(435)
V4O9 nanosheets	1.5 M KOH	392/0.5	75%/2000	(373)
V6O13@C	1 M Na2SO4	545/0.5	88.3%/2000	(436)
V6O13 sheets	1 M NaNO3	285/0.05	96.7%/300	(437)
V6O13	1 M LiNO3	456/0.6	65%/2000	(438)
Sulfur-doped V6O13–x@C	5 M LiCl	1353/1.9	92.3%/10000	(439)
RuO2 nanoparticle decorated V2O5 nanoflakes	1 M KCl	421/1	94.6%/10000	(315)
Graphene nanoribbons @ V2O5 nanostrips	0.5 M Na2SO4	335.8/1	∼98.5%/10000	(316)
N-doped carbon nanofibers/V2O5 core/shell	1 M Na2SO4	595.1/0.5	97%/12000	(317)
Interconnected V2O5 Nanoporous Network	0.5 M K2SO4	304/0.1	24%/600	(440)
V2O5/rGO nanocomposites	8 M LiCl	537/1	84%/1000	(441)
V2O5/rGO hybrids	1 M Na2SO4	468.5/1	91.5%/10000	(442)
V2O5/graphene hybrid aerogels	1 M Na2SO4	486/0.5	90%/20000	(273)
V2O5/rGO composite hydrogel	0.5 M Na2SO4	320/1	70%/1000	(443)
Carbon coated V2O5 nanorods	0.5 M K2SO4	417/0.5	76%/1000	(444)
		341/10
Hollow spherical V2O5	5 M LiNO3	559/3	70%/100	(445)
V2O5 nanorods	1 M LiClO4	347/1	94.3%/10000	(446)
V2O5 nanorods/rGO	0.75 M NaPF6	289/0.01	85%/1000	(447)
V2O5 nanosheets/rGO	1 M KCl	635/1	94%/3000	(448)
V2O5 nanobelts	1 M LiClO4/ PPC	132.5/1	95%/500	(449)
V2O5 nanobelts/rGO	0.5 M K2SO4	310.1/1	90.2%/5000	(319)
V2O5 microtubules	1 M LiNO3	680/1	70%/10000	(271)
V2O5 nanoparticles	1 M LiClO4/ PPC	545/1	70%/500	(450)
V2O5/graphene hybrid	1 M Na2SO4	484/0.6	80%/10000	(451)
V2O5/graphene hybrid aerogel composite	1 M LiClO4/ PPC	384/0.1	82.2%/10000	(452)
		197/2
V2O5/MWCNT core/shell hybrid aerogels	1 M Na2SO4	625/0.5	120%/20000	(453)
Carbon coated flowery V2O5	1 M K2SO4	417/0.5	100%/2000	(454)
V2O5/mesoporous carbon microspheres	1 M Al2(SO4)3	290/0.5	88%/10000	(455)
Graphene-wrapped V2O5 nanospheres	1 M Na2SO4	612.5/1	89.6%/10000	(456)
V2O5/polypyrrole	5 M LiNO3	448/0.8	81%/1000	(457)
V2O5/polyaniline	0.5 M LiClO4/ PPC	1115/1	90%/4000	(458)
V2O5@Ni3S2	1 M KOH	854/1	60%/1000	(330)
V2O5/Na0.33V2O5	1 M LiClO4	334/1	96%/1000	(459)
V2O5/TiO2	1 M LiNO3	587/0.5	92%/5000	(460)
V2O5 nanowire arrays/N-doped graphene aerogel	8 M LiCl	710/0.5	95%/20000	(461)
V2O5 nanofibers/conductive polymer	1 M Na2SO4	614/0.5	111%/15000	(462)
V2O5/nanoporous carbon network	0.5 M K2SO4	314.6/0.2	89.5%/5000	(463)
V2O5/Ni foam	1 M KOH	399.7/0.01	96.1%/2000	(464)
V2O5/WO3	1 M H2SO4	386/0.1	104%/5000	(465)
V2O5/g-C3N4	1 M KOH	192.3/0.5	85.7%/5000	(466)
V2O5@Ti	1 M LiCl	1520/1.5	99%/12000	(467)
V2O5/vertically aligned CNT	1 M Na2SO4	284/2	76%/5000	(468)
V2O5 nanosheets/carbon fiber felt	5 M LiCl	475.5/1	89.7%/6000	(469)

Table 3. A Brief Survey of Typical Vanadium Oxides Electrocatalysts.

catalyst	catalyst type	electrolyte	η @ j = 10 mA cm–2 (mV)	Tafel slope (mV dec–1)	stability (cycle number or time @ current density)	ref
V2O3/ MoSx /CC	HER	0.5 M H2SO4	146	45	1000 cycles	(82)
NiFe@V2O3	HER	1 M KOH	84	85	24 h @ 10 mA cm–2	(80)
NiFe@V2O3	OER	1 M KOH	255	51	1000 cycles	(80)
					24 h @ 10 mA cm–2
Ni0.8/V2O3	HER	1 M KOH	44	38	24 h @ 10 mA cm–2	(470)
V2O3–Ni3N	HER	1 M KOH	57	50	24 h @ 10 mA cm–2	(471)
V2O3@Ni	HER	1 M KOH	47	74	1000 cycles	(472)
					10 h @ 10 mA cm–2
V2O3–CoFe2O4	HER	1 M KOH	61	58	80 h @ 500 mA cm–2	(473)
V2O3–CoFe2O4	OER	1 M KOH	226	56	80 h @ 500 mA cm–2	(473)
Ni4Mo–V2O3	HER	1 M PBS	40	66	1000 cycles	(474)
					5.5 h @ 50 mA cm–2
MoS2/VO2	HER	0.5 M H2SO4	99 @ 1 mA cm–2	85	N. A.	(245)
Ni3S2/VO2	HER	1 M KOH	100	114	15 h @ 10 mA cm–2	(244)
Ni3S2/VO2	OER	1 M KOH	150	47	15 h @ 10 mA cm–2	(244)
Co3O4/VO2/CC	HER	1 M KOH	108	98	10 h @ 10 mA cm–2	(246)
porous VO2 nanosheets	HER	0.5 M H2SO4	184	70	120 h @ 90 mA cm–2	(247)
porous VO2 nanosheets	OER	1 M KOH	209	92	120 h @ 70 mA cm–2	(247)
CoV2O6 - V2O5/NRGO	OER	1 M KOH	239	50	1000 cycles	(320)
Co–V2O5	HER	1 M KOH	51	42	24 h @ 10 mA cm–2	(328)
V2O5/Ni(OH)2@NF	HER	1 M KOH	39	44	10000 cycles	(329)
Pt (P)–V2O5/graphene	HER	0.5 M H2SO4	32	23	1000 cycles	(331)
V2O5@Ni3S2	HER	1 M KOH	95	108	9000 cycles	(330)
Ni–Co–P/V2O5–TiO2/GO	HER	1 M KOH	101 @ 100 mA cm–2	36	N. A.	(475)
Zn-VOx-Co	HER	1 M KOH	46	75	36 h @ 100 mA cm–2	(398)
Ni(Cu)VOx	HER	1 M KOH	21	28	125 h @ 100 mA cm–2	(397)
VOx/NiS/NF	OER	1 M KOH	330 @ 50 mA cm–2	121	1000 cycles	(399)
Co-VOx-P	HER	1 M KOH	98	59	1000 cycles	(400)
					24 h @ 30 mA cm–2
Co-VOx-P	OER	1 M KOH	230 @ 100 mA cm–2	64	1000 cycles	(400)
					25 h @ 100 mA cm–2
VOx@NiFe/NiCoP/TM	HER	1 M KOH	45	34	1000 cycles	(476)
					16 h @ 100 mA cm–2
VOx@NiFe/NiCoP/TM	OER	1 M KOH	215	37	20 h @ 500 mA cm–2	(476)
Co(VOx)	HER	1 M KOH	178 @ 100 mA cm–2	40	60 h @ 100 mA cm–2	(477)

While vanadium oxides are important materials for many applications, more detailed, mechanistic and systematical studies are needed to fully explore their potential as the bottleneck is increasingly related to the materials’ quality and device fabrications. We propose a few future works in this field which could be further developed in the following aspects (Figure 51):

• (1)Searching the applications of vanadium oxides in underexplored areas. One example is biological thermal imaging by leveraging its thermal phase transition characteristics as the insulator-to-metal phase transition of some vanadium oxides gives sharply enhanced optical absorption above their critical temperature. It has potential applications in biochemistry, especially in the fields of bioimaging. Another example is that some nanostructured vanadium oxides also possess optimal physicochemical properties (e.g., optical, thermal, magnetic properties), which give the opportunity to be applied in intelligent medicine based on micro-/nanorobots.
• (2)Development of high-level theoretical calculations to guide the rational design of vanadium oxides and their composites and to better understand the fundamentals of high-performance devices. The use of advanced computational tools and theories enables researchers to understand the origin of complex and interacting phenomena at multiple scales, which could accelerate our understanding of the fundamentals of high-performance devices and improve the operation and design of new materials systems. Additionally, machine learning is a useful toolkit for designing and exploring new vanadium oxides with desired properties. It can also aid the understanding of the complex correlations between structures and properties in vanadium oxides.
• (3)Fabrication of ultrathin or two-dimensional vanadium oxides, which could be applied in some new electronic devices. Owing to the atomic-scale thickness of single layers, the 2D materials exhibit tunable electrical properties and bandgaps. Therefore, the 2D vanadium oxides may hold promise for a wide range of applications in low-power electronics, flexible electronics, optoelectronics, catalysis, batteries, and so on. Furthermore, the 2D vanadium oxides may also form novel 2D heterostructures with other ultrathin 2D nanomaterials, which would be of certain interest.
• (4)Exploration of new approaches to stabilize the vanadium oxides related devices, especially thermal, light, moisture, and oxygen environmental stability. Long-term device stability is one of the most important challenges for all devices. Most vanadium oxides are sensitive to oxygen and moisture, which may lead to the degradation of the device performances. Furthermore, most stability measurements of the devices are performed under ambient conditions, which limits their application in high temperature, high humidity, and high light intensity conditions. Meanwhile, it is also important to understand the degradation mechanisms in the different types of devices based on vanadium oxides, which could be the key to improving the device stabilities.
• (5)Exploring new green chemistry synthesis methods to eliminate the toxicity of vanadium oxides. Vanadium oxides cause a variety of toxic effects such as biochemical changes, neurobehavioral injury, and functional lesions in the liver and bones. Especially, vanadium oxides in breathing air can cause pulmonary problems and DNA damage in leukocytes. The toxicity is more related to the phase structure, stoichiometric ratio, concentration, particle size, and crystalline degree, which could be considered in all the processes of application of vanadium oxides. Therefore, exploring new green synthesis methods and avoiding risks to humans and the environment during vanadium oxides’ production, use, and disposal processes deserve more systematic and comprehensive studies. First, it is important to develop new synthesis protocols with minimum steps to prepare vanadium oxides, such as one-pot synthesis, completely enclosed-system synthesis, and so on. Second, the encapsulation of the devices to confine the toxicity needs to be considered when designing a new device. Third, the release of vanadium oxides to the atmosphere should be controlled during normal operation. For example, the marked decrease in toxicity is confirmed via silica coating on vanadium oxides due to the perception that the toxicity of vanadium oxide is closely related to the solubility and the robust silica barrier can isolate air and water to reduce the solubility.
• (6)Realization of the bifunctional, trifunctional, or even multifunctional vanadium oxides to achieve integrated functionality. Different device integrations based on the same materials is expected to greatly reduce the cost and the incompatibility of different materials. Meanwhile, multifunctional materials would reduce the complexity of designing devices and promote the application in designated situations.

Figure 51.

Future development and research directions for vanadium oxides.

Overall, we believe that vanadium oxides are great candidates for future applications in related fields and help solve key challenges in the global warming crisis. Moreover, the integration of vanadium oxides with multidisciplinary fields such as material science, device physics, civil engineering, mechanical design, and bioscience would continue to attract the interest of many scientists from different disciplines for new fascinating fields.

Glossary

Abbreviations

1D

one-dimensional

2D

two-dimensional

3D

three-dimensional

4D

four-dimensional

AACVD

aerosol-assisted chemical vapor deposition

AC

activated carbon

AFM

atomic force microscope

AIB

aluminum-ion battery

ALD

atomic layer deposition

AM 1.5

air mass 1.5

BP

black paint

CC

carbon cloth

C_dl

double-layer capacitance

CIB

calcium-ion battery

CNC

carbon nanocoils

CNT

carbon nanotube

CV

cyclic voltammetry

CVD

chemical vapor deposition

DC

direct current

DFT

density functional theory

DNA

DNA

EBD

electron-beam deposition

EN

ethylenediamine

GCD

galvanostatic charge/discharge

GF

graphene foam

GITT

galvanostatic intermittent titration technique

GQD

graphene quantum dot

GVG

graphene foam supported graphene quantum dot anchored VO₂ arrays

FeFET

ferroelectric field-effect transistor

FET

field-effect transistor

GO

graphene oxide

HCP

high concentration dilution

HER

hydrogen evolution reaction

High-E

high emissivity

HM

hollow microclew

IL

ionic liquid

IR

infrared

ITO

indium tin oxide

KIB

potassium-ion battery

LCP

low concentration dilution

LED

light emitting diode

LIB

lithium-ion battery

LiTFSI

lithium bis-trifluoromethanesulfonimide

Low-E

low emissivity

LWIR

longwave infrared

LSPR

localized surface plasmonic resonance

MAS NMR

magic-angle-spinning nuclear magnetic resonance

MB

methylene blue

MCE

magnetocaloric effect

MD

molecular dynamics

MEMS

microelectromechanical systems

MIB

magnesium-ion battery

MIT

metal–insulator transition

MO

methyl orange

MWCNT

multiwalled carbon nanotube

N.A.

not applicable

NaTFSI

sodium bis-trifluoromethanesulfonimide

NDR

negative differential resistance

NF

Ni foam

NIB

ammonium-ion battery

NIR

near-infrared

NMR

nuclear magnetic resonance

NP

nanoparticle

NRGO

nitrogen-doped reduced graphene oxide

NW

nanowire

OER

oxygen evolution reaction

PBS

phosphate-buffered saline

PDMS

polydimethylsiloxane

PEDOT

poly(3,4-ethylenedioxythiophene)

PEG

polyethylene glycol

PLD

pulsed laser deposition

PNCNF

porous N-doped carbon nanofiber

PPC

propylene carbonate

PS

polystyrene

PVA

poly(vinyl alcohol)

PVC

polyvinyl chloride

PVD

physical vapor deposition

PVDF-HFP

polyvinylidene fluoride-hexafluoropropylene

PVP

polyvinylpyrrolidone

PZT

Pb(Zr_0.52Ti_0.48)O₃

RC

radiative cooling

RCRT

radiative cooling regulating thermochromic

ReRAM

resistive random-access memory

rGO

reduced graphene oxide

RhB

rhodamine B

RF

radio frequency

RT

room temperature

SA-VO₂

surface amorphized VO₂

SC

supercapacitor

SCE

saturated calomel electrode

SCVA

single-crystalline VO₂ actuator

SEM

scanning electron microscope

SHE

standard hydrogen electrode

SIB

sodium-ion battery

SWCNT

single- or multiwalled carbon nanotube

TARC

temperature-adaptive radiative coating

TEM

transmission electron microscope

TGA

thermogravimetric analysis

TM

titanium mesh

UGF

ultrathin graphite foam

UHV

ultrahigh vacuum

UV–vis–NIR

ultraviolet–visible-near-infrared

VOG

V₂O₅·H₂O/graphene

VGS

V₃O₇ nanowire templated graphene scroll

V₂O₃@NC

nitrogen-doped carbon-confined V₂O₃

XANES

X-ray absorption near edge structure

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffraction

ZIB

zinc-ion battery

Biographies

Peng Hu received his PhD degree (under the guidance of Prof. Christian Kloc) in materials science at Nanyang Technological University, Singapore, in 2016. Then, he was a research fellow at Nanyang Technological University, Singapore, in Prof. Christian Kloc’s group (2016–2017) and Dr. Yi Long’s group (2017–2018). He is currently a professor in the School of Physics at Northwest University, China. His research interests focus on the single crystal growth of organics, low-dimensional materials and organic–inorganic hybrids, and their electronic, optoelectronic, and energy applications.

Ping Hu received his PhD degree from Wuhan University of Technology in 2020. He is currently a postdoc at Wuhan University of Technology. His research interests focus on advanced electrode materials for electrochemical energy storage devices.

Tuan Duc Vu is a PhD candidate from Nanyang Technological University (NTU) in Dr. Long Yi and Dr. Xianting Zeng’s group, School of Materials Science and Engineering. He previously obtained his B.Eng. (Materials) from the same school. In 2018, he received the prestigious A*STAR Graduate Scholarship AGS(S), which funded his PhD candidacy in NTU in collaboration with Singapore Institute of Manufacturing Technology (SIMTech). He specializes in thin film processing using a physical vapor deposition process such as thermal evaporation and magnetron sputtering. His current research focuses on the fabrication of high performance and high durability vanadium dioxide smart window glazing.

Ming Li is an Associate Professor in the Laboratory of Nanomaterials & Nanostructures at the Institute of Solid State Physics, Chinese Academy of Science (ISSP, CAS). He received his PhD in condensed matter physics from the ISSP, CAS. His research focuses on developing thermochromic materials of vanadium oxides for energy-efficient windows. His research topics cover the synthesis of nanomaterials, the regulation of photoelectric properties, and the design of thermochromic devices.

Shancheng Wang is a research fellow at Nanyang Technological University under the supervision of Dr. Yi Long. He obtained his PhD degree from NTU in 2021. His research interests include the synthesis of stimuli-response materials and design of stimuli-response smart structures for optical and thermal applications. In 2021, Shancheng received the MSE Doctorate Technopreneur Award from the School of Materials Science and Engineering, NTU.

Yujie Ke is a scientist at the Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR) Singapore. He received his Ph.D. from NTU under the supervision of Dr. Long Yi in 2019 and his master’s degree from University at Buffalo (SUNY-Buffalo) in 2015. He is interested in phase-change materials, mechano-/thermochromics, plasmonics and photonic crystals, mechanical metamaterials, smart window, and building energy.

Xianting Zeng is Director of the Knowledge Transfer Office (KTO) of the Singapore Institute of Manufacturing Technology (SIMTech), of the Agency for Science, Technology and Research (A*STAR). He obtained his PhD degree in Thin Film Physics from the Chinese University of Hong Kong in 1995, and MSc and BSc degrees from the Huazhong University of Science and Technology, China in 1987 and 1982, respectively. He has been an active research scientist in the fields of nanocomposite coating materials and plasma enhanced PVD thin film processes. After joining SIMTech in 1995, he established the KTO in 2009 to transfer knowledge and capability to industry through case-studies and hands-on practical training to meet the technology and skills gap in productivity improvement, skills, and capabilities upgrading and business transformation.

Liqiang Mai is a Chair Professor of Materials Science and Engineering at the Wuhan University of Technology. He received his PhD degree from Wuhan University of Technology in 2004 and carried out his postdoctoral research at the Georgia Institute of Technology in 2006–2007. He worked as an advanced research scholar at Harvard University in 2008–2011 and the University of California, Berkeley in 2017. His current research interests are focused on new nanomaterials for electrochemical energy storage and micro-/nanoenergy devices.

Yi Long is a Fellow of the Royal Society of Chemistry. She obtained her PhD from the University of Cambridge, UK. She is currently an professor in the Department of Electronic Engineering, The Chinese University of Hong Kong. Her research focuses on nanostructured functional materials for different applications. She has successfully implemented technology transfer from lab to industry for a hard-disk company in her early career. Her recent research is to develop various smart materials by manipulation of the structure at the nanoscale to achieve unusual properties. She is the recipient of GreenAwards Top 3 London and Falling Walls 10 Breakthrough in Engineering and Technology Berlin in 2022.

Author Contributions

^# Peng Hu and Ping Hu contributed equally to this work. CRediT: Tuan Duc Vu writing-original draft; Ming Li writing-original draft; Shancheng Wang writing-original draft; Yujie Ke writing-original draft; Xianting Zeng supervision; Liqiang Mai funding acquisition, supervision, writing-review & editing; Yi Long conceptualization, funding acquisition, supervision, writing-review & editing.

The authors declare no competing financial interest.