TiO₂ Based Nanostructures for Photocatalytic CO₂ Conversion to Valuable Chemicals

Abstract

Photocatalytic conversion of CO₂ to useful products is an alluring approach for acquiring the two-fold benefits of normalizing excess atmospheric CO₂ levels and the production of solar chemicals/fuels. Therefore, photocatalytic materials are continuously being developed with enhanced performance in accordance with their respective domains. In recent years, nanostructured photocatalysts such as one dimensional (1-D), two dimensional (2-D) and three dimensional (3-D)/hierarchical have been a subject of great importance because of their explicit advantages over 0-D photocatalysts, including high surface areas, effective charge separation, directional charge transport, and light trapping/scattering effects. Furthermore, the strategy of doping (metals and non-metals), as well as coupling with a secondary material (noble metals, another semiconductor material, graphene, etc.), of nanostructured photocatalysts has resulted in an amplified photocatalytic performance. In the present review article, various titanium dioxide (TiO₂)-based nanostructured photocatalysts are briefly overviewed with respect to their application in photocatalytic CO₂ conversion to value-added chemicals. This review primarily focuses on the latest developments in TiO₂-based nanostructures, specifically 1-D (TiO₂ nanotubes, nanorods, nanowires, nanobelts etc.) and 2-D (TiO₂ nanosheets, nanolayers), and the reaction conditions and analysis of key parameters and their role in the up-grading and augmentation of photocatalytic performance. Moreover, TiO₂-based 3-D and/or hierarchical nanostructures for CO₂ conversions are also briefly scrutinized, as they exhibit excellent performance based on the special nanostructure framework, and can be an exemplary photocatalyst architecture demonstrating an admirable performance in the near future.

1. Introduction

Enormous amounts of CO₂ emissions, mainly due to industrialization and burning of fossil fuels, are considered to be a primary source of global warming [1]. Hence, fossil fuel consumption for fulfilling energy demands has led to increased atmospheric CO₂ levels, along with depletion of respective resources. To deal with such a critical energy and environmental issue, developments in the field of renewable energy such as wind, hydel, biomass, nuclear and solar energy, are being carried out by global scientists and researchers. Among these, solar energy, in terms of its utilization in converting anthropogenic CO₂ into value-added chemicals on a photocatalyst surface in the presence of a reducing agent, is an alluring and auspicious research area to counter environmental pollution with the possibility of matching the renewable energy infrastructure [2]. The photocatalytic CO₂ conversion (PCC) to value-added chemicals like CO, CH₄, C₂H₆, C₂H₅OH, C₂H₄, CH₃OH, HCOOH, etc., in general mimics the concept of natural photosynthesis and is considered a subordinate of the “Artificial Photosynthesis” research domain [2,3,4,5].

Since the invention of water photocatalysis by Fujishima and Honda in 1972 [6], TiO₂ photocatalysts have emerged as the premier and champion material with splendid properties including favorable surface area, non-toxicity, abundant availability, high stability, and cost effectiveness [7,8,9]. On the contrary, TiO₂ nanoparticles (0-D), with a disadvantage of only UV light absorption, also exhibit drawbacks of fast electron–hole recombination, slow charge transfer, limited light trapping and difficulty in reuse/recycling [10]. Despite the development of many alternative photocatalysts to TiO₂, TiO₂ still remains a distinguished and premier choice because of the specific above-mentioned attributes.

In the last few years, TiO₂-based nanostructured photocatalysts such as one-dimensional (1-D) [11,12,13], two-dimensional (2-D) [14,15,16], and three-dimensional (3-D) or hierarchical structures [17,18], have received massive attention in a variety of photocatalysis domains [19]. The distinct properties of large surface area, high aspect ratio, directional flow of photogenerated charges resulting in decreased charge recombination, light scattering, stability and improved reusability/recyclability—especially for 1-D arrays and hierarchical nanostructures—has made them valuable and worthwhile to employ in the photocatalysis research domain. Moreover, the synthesis strategies for the 1-D nanostructures are simple and facile; however, for 2-D and hierarchical nanostructured photocatalysts, fabrication is generally limited to following/adopting complicated procedures. Hence, due to their specific geometry configurations [20,21], 1-D (nanotubes, nanowires, nanorods, etc.) and 2-D (nanosheets, nanolayers, etc.) nanostructured photocatalysts provide an attractive and exemplary opportunity to overcome the limitations of 0-D nanoparticles restricting the performance of photocatalysts. Furthermore, 3-D and hierarchical nanostructures offer superb light trapping properties within specific nanostructures, resulting in a slow photon effect and improved photocatalytic performance. Despite the eminent benefits of TiO₂ nanostructured photocatalysts, to some extent, they possess limitations with respect to the ultra violet region (a small portion of terrestrial solar spectrum) in terms of light absorption due to their wider band gap (~3.0–3.2 eV). Therefore, to overcome such limitations, similar strategies are commonly adopted as for 0-D nanoparticles, including metal and non-metal doping [18,22,23,24,25], noble metal loading [16,26,27,28,29], graphene derivative coupling [30,31,32,33], and hetero-junctioning TiO₂ nanostructures through the coupling of low band gap materials [11,17,34,35,36,37].

In this review, the recent progress of TiO₂-based nanostructures employed for photocatalytic CO₂ conversion to useful/value added chemicals is briefly overviewed. The key parameters promoting the improvement of photocatalytic performance, as well as the potential benefits offered by important frameworks, i.e., 1-D, 2-D and hierarchical nanostructures, are discussed. The photocatalytic performance of the TiO₂-based nanostructured photocatalysts, along with their reaction conditions and procedures, is summarized. In short, this review is focused on the architectural engineering of TiO₂-based nanostructured photocatalysts, such that they offer excellent properties with enhanced photocatalytic performance.

2. Photocatalytic CO₂ Conversion: Fundamentals and Mechanism

As is well established, CO₂ in gaseous form is a thermodynamically stable molecule with a Gibbs free energy of ΔG° = −394.4 KJ∙mole⁻¹ [38]. Thus, a suitable amount of energy is required to transform gaseous CO₂ into value-added products. Equation (1) (Gibbs free energy) and Equation (2) (overpotential) demonstrate that more negative potential than E° is needed to compensate the overpotential and make the ΔG° negative enough for spontaneous conversion of CO₂ gaseous molecule to proceed.

ΔG° = nFE° (1)

where ΔG° = Standard Gibbs free energy, n= number of electrons involved in reaction, F = Faraday constant (96485 Cmole⁻¹) and E° = standard potential of the respective reaction.

η= E − E° (2)

where η = overpotential, E = required potential and E° = standard potential.

Hence, the required overpotential can be supplied to the reaction mixture in one of several ways, including as thermal energy, electrical energy, chemical energy and solar energy. Among these sources, utilization of solar energy in the presence of a photocatalyst is considered to be one of the most sustainable and cost-effective approaches. Moreover, for the reason of high stability, it is quite difficult for a photocatalyst alone to reduce CO₂ into products; therefore, the reduction to CO₂ proceeds with the support of reducing agents such as H₂O, H₂, etc. [38,39], which can regenerate the photocatalyst by readily providing the electrons in order to fill the holes. In turn, protons are released, which react with the surface-adsorbed CO₂ and electrons to give the desired product: a relatively feasible and less energetic pathway. Such a process is commonly known as a “proton-assisted multi-electron photoreduction process”, and it is a commonly accepted mechanism for photocatalytic CO₂ conversion. Table 1 shows the redox potentials for various CO₂ reactions (at pH = 7.0 and E vs. NHE) with H₂O in vapor or liquid phase [40]. It can be noted that the selectivity of the product, another important parameter, can also be manipulated by aligning the conduction band and valence band edge of the photocatalyst with respect to the redox potentials of the reactions.

Table 1.

Electrochemical redox potentials (E° vs. NHE, pH = 7.0) for CO₂ reduction into a variety of useful chemical products [4,40,41].

No.	Reactions	E° vs. NHE (V)
1	CO2 + 2H+ + 2e− → HCOOH	−0.61 V
2	CO2 + 2H+ + 2e− → CO + H2O	−0.53 V
3	CO2 + 4H+ + 4e− → HCHO + H2O	−0.48 V
4	CO2 + 6H+ + 6e− → CH3OH + H2O	−0.38 V
5	CO2 + 8H+ + 8e− → CH4 + 2H2O	−0.24 V
6	2H+ + 2e− → H2	−0.41 V
Water Oxidation Reactio
7	2H2O → O2 + 4H+ + 4e−	+0.81V

As mentioned above, the conversion or reduction of CO₂ is more feasible in the presence of reducing agents such as H₂O, H₂, etc. H₂O is commonly chosen as a reducing agent because it has benefits such as being inexpensive, less dangerous than gaseous reducing agents (H₂, H₂S, etc.), and it only requires simple handling. Thus, a reaction mixture of gaseous CO₂ and H₂O (vapors or liquid) or H₂ gas is commonly used for the photocatalytic conversion of CO₂ into the desired products. Upon light irradiation on the photocatalyst with a CO_2(g)/H₂O_(g/l) or CO_2(g)/H_2(g) mixture, the photogenerated electrons are rapidly transferred to the adsorbed CO₂ on the photocatalyst surface, and the presence of protons (H⁺, which are provided by the reducing agents) yields the desired products. The definitive reaction mechanism for photocatalytic CO₂ conversion is still in need of extensive research; however, based on the binding and bridging mode of CO₂ with a photocatalyst surface, two commonly accepted pathways reported in the literature [42,43,44,45] include (i) the carbene pathway, and (ii) the formaldehyde pathway.

The possible reactions reported in the literature for the carbene pathway are presented in Equations (3)–(11). It is expected that CO₂^●− radicals are formed by the quick reaction of a single electron to the surface-adsorbed CO₂. These radicals can then react with protons/hydrogen radicals and electrons to form CO as an intermediate product, which is believed to be adsorbed onto the photocatalyst surface, further reacting with electrons and protons in a multi-step process to produce CH^● radical, carbene, methyl radical and finally methanol or methane as valuable products.

CO₂ + e⁻ → CO₂^●− (3)

CO₂^●− + e⁻ + H⁺ → CO + OH⁻ (4)

CO + e⁻ → ^●CO⁻ (5)

^●CO + e⁻ + H⁺ → C + OH⁻ (6)

C + e⁻ + H⁺ → CH^● (7)

CH^● + e⁻ + H⁺ → CH₂ (8)

CH₂ + e⁻ + H⁺ → CH₃^● (9)

CH₃^● + e⁻ + H⁺ → CH₄ (10)

CH₃ + OH⁻ → CH₃OH (11)

In the formaldehyde pathway, the monodentate configuration via binding of one oxygen atom to a metal (titanium) atom, or binding of materials surface oxygen atom to the carbon atom of CO₂, favors the formation of carboxyl radical (COOH). Carboxyl radical (COOH) can react with the protons/hydrogen radicals, forming formic acid, which proceeds through the multi-step process of electron accepting and dehydration reactions to produce formaldehyde, methanol and methane. The proposed reactions are presented in Equations (12)–(21).

CO₂ + e⁻ → CO₂^●− (12)

CO₂^●− + H⁺ → ^●COOH (13)

^●COOH + e⁻ + H⁺ → HCOOH (14)

HCOOH + e⁻ + H⁺ → H₂OOC^● (15)

HC^●OOH + e⁻ + H⁺ → HCOH + H₂O (16)

HCOH + e⁻ → H₂C^●O⁻ (17)

H₂C^●O⁻ + H⁺ → H₂OHC^● (18)

H₂OHC^●+ e⁻ + H⁺ → CH₃OH (19)

CH₃OH + e⁻ + H⁺ → ^●CH₃ + H₂O (20)

^●CH₃ + e⁻ + H⁺→ CH₄ (21)

3. One-Dimensional (1-D) Nanostructured Photocatalysts

One dimensional (1-D) TiO₂ nanostructures, such as nanotubes, nanowires, and nanorods, have been a topic of great interest within the photocatalysis research domain, and they have a variety of applications. The key advantages offered by 1-D TiO₂ nanostructures include large surface areas, better charge transfer, improved adsorption capacity, and extended light absorption due to the light trapping and scattering effect [46]. Until now, a moderate amount of research has been done on the development of one-dimensional (1-D) TiO₂ photocatalysts with a key focus being on photocatalytic CO₂ conversion to hydrocarbon fuel/useful chemicals. Several strategies encompass heterojunction formation with other visible light active photocatalysts, doping, noble metals loading and graphene coupling for attaining the key aim of improved photocatalytic CO₂ conversion.

Xin et al. proposed CdS and Bi₂S₃ heterostructured TiO₂ nanotubes (CdS-TNT and Bi₂S₃-TNT) photocatalysts prepared by a simple two-step synthesis approach [47]. TNT was first prepared using an alkaline hydrothermal method, followed by deposition of CdS and Bi₂S₃ in a fixed concentration using a simple precipitation approach from their respective precursors. The prepared photocatalysts were tested under visible light irradiation for photocatalytic CO₂ conversion. The Bi₂S₃-TNT photocatalyst showed the maximum efficiency, yielding 224.6 µmol/L of CH₃OH after 5 h of irradiation, as compared to CdS-TNT (159.5 µmol/L) and pure TNT (102.59 µmol/L). The increased production rate is mainly attributed to the extended light absorption of heterostructured photocatalysts and their efficient photogenerated charge separation. The conduction band edges of both CdS and Bi₂S₃ lie well above the conduction band edge of TiO₂; therefore, upon light irradiation, the excited electron can easily flow to the conduction band of TiO₂ and might react with the adsorbed CO₂ species. A depiction of the band gap alignment and CO₂ conversion to CH₃OH is displayed in Figure 1a. Moreover, it was found that CdS and Bi₂S₃ deposition decreased the surface area and CO₂ adsorption, in comparison to pure TNT, but did not significantly affect the photocatalytic performance.

Figure 1.

(a) Proposed mechanism of photocatalytic CO₂ conversion to CH₃OH employing CdS and Bi₂S₃ TNT photocatalysts (taken with permission from [47]). (b) Production yield of CO and CH₄ from Pt-MgO-covered TNN films (taken with permission from [48]).

Another effective approach is coating the 1-D photocatalyst surface with a good CO₂ adsorbent, such as MgO-covered TiO₂ nanotube networks (TNN) [48]. The TNN were fabricated by subjecting Ti foil to alkaline hydrothermal reaction, which resulted in well-aligned TNN. The TNN was then dipped in magnesium salt solutions of various concentrations, followed by calcination in air to finally obtain MgO-covered TNN. It was observed that MgO coverage with optimum concentration displays higher CO and CH₄ yields when compared with bare TNN. Moreover, the deposition of platinum nanoparticles (Pt NPs) onto MgO-TNN greatly enhances the CO and CH₄ yield. The increased performance of MgO-TNN is attributed to the chemisorption of CO₂ molecules and their conversion to MgCO₃ species, which are more reactive with atomic H than linear CO₂ molecules in terms of giving the respective products. In addition, Pt NPs act as an electron trap for the efficient separation of photogenerated electron–hole pairs. The photocatalytic conversion of CO₂ into CO and CH₄ when employing various MgO-TNN and Pt-MgO-TNN films is displayed in Figure 1b.

Self-doping, such as the introduction of oxygen vacancies into the TiO₂, generally referred to as reduced TiO₂, is another attractive strategy for improving the photocatalytic performance. A recently published article reported the synthesis of black TiO₂ films comprised of unique porous grid-like structures using a simple and safe hydrothermal method [49]. The elemental analysis of the synthesized TiO₂ films displayed oxygen deficiency, indicating the presence of oxygen vacancies. When employed for photocatalytic CO₂ conversion, the black TiO₂ films exhibited improved CO and CH₄ yields as compared to pure TiO₂ film. The increased product yield was mainly attributed to the extended light absorption and the efficient charge separation resulting from the degree of defects caused by the oxygen vacancies. Another study proposed a novel heterostructure consisting of octahedral Cu₂O nanoparticle-loaded TiO₂ nanotube (TNT) arrays [50]. TNT arrays were prepared by a conventional electrochemical anodization method, while Cu₂O nanoparticles were deposited from Cu salt solution using an electrodeposition approach. The Cu₂O-TNT arrays were prepared with varied Cu₂O deposition times. It was observed that the Cu₂O-TNT arrays prepared with an electrodeposition time of 30 min showed the maximum CH₄ yield under visible light irradiation. However, under simulated solar light illumination, the Cu₂O-TNT arrays prepared with an electrodeposition time of 15 min exhibited the maximum CH₄ yield. The key factors to which the enhanced performance was attributed include: (i) TNT arrays provide better charge transportation and light absorption, (ii) optimum loading of Cu₂O nanoparticles leading to improved visible light performance, and (iii) well-aligned band edges of Cu₂O and TNT arrays leading to better charge separation. The proposed mechanism involved is displayed in Figure 2a.

Figure 2.

Schematic of the mechanism involved in the photocatalytic conversion of CO₂ into: (a) CH₄ employing Cu₂O NP-incorporating TNT (taken with permission from [50]), and (b) CH₃OH using Cu-modified TNF films (taken with permission from [51]).

Another investigation reported a simple strategy for synthesizing Cu-modified TiO₂ nanoflower films (TNF) for enhanced photocatalytic conversion of CO₂ into CH₃OH [51]. The synthesis methodology includes the growth of TNF on Ti foil using a hydrothermal method, followed by a microwave-assisted reduction process for Cu modification. The Cu-TNF with optimum loading of Cu (0.5 millimol concentration of Cu²⁺) exhibited enhanced CH₃OH yield under UV-visible and UV light irradiation conditions, at a factor of 6 and 3.6 times higher than pure TNF.. The increased CH₃OH yield was mainly attributed to good charge separation and local surface Plasmon resonance (LSPR), which were induced by Cu NPs providing hot electrons to contribute to the photocatalytic reactions. The possible mechanism involved in the photocatalytic conversion of CO₂ into CH₃OH is depicted in Figure 2b.

Recently, Cheng et al. reported CdS and Cu²⁺ ion deposition onto TiO₂ nanorod (TNR) array film, and investigated its performance under visible light irradiation [52]. The TNR film was synthesized using a hydrothermal approach, while Cu²⁺ ions and CdS deposition were obtained using the cation adsorption and successive ionic layer reaction (SILAR) methods. Various samples were prepared by varying CdS deposition by SILAR cycles. The CdS-Cu^2+/TNR film prepared with 2 SILAR cycles showed the maximum ethanol yield under visible light irradiation, thus suggesting this to be the optimum sample. Moreover, the influence of the CO₂ flow rate and reaction temperature was also analyzed, and the optimal conditions for yielding maximum ethanol yield were found to be 4 mL/L and 80 °C.

Li et al. recently proposed a heterostructure of TiO₂ nanotubes with CoO_x, fabricated with a specially designed synthesis strategy for grafting CoO_x nanoparticles onto TNT and inducing defects through hydrogenation within the heterostructure by N₂/H₂ annealing [53]. The TNT employed in the investigation consisted of TiO₂ (B) and anatase (A) phases. Two different samples were prepared by selecting the hydrogenation step before and after the deposition of CoO_x NPs, denoted AB-H-CoO_x and AB-CoO_x-H, respectively. Moreover, a new concept of photothermal catalytic conversion (PTC) was employed for conversion of CO₂ into useful products. It was observed that the TNT-CoO_x sample prepared using the AB-H-CoO_x sequence at a temperature of 393 K resulted in a greater CO and CH₄ yield as compared to the other samples. The key parameters associated with this performance enhancement includes: (i) improved surface area, (ii) oxygen vacancies, which increase the CO₂ adsorption on the surface sites, and (iii) upon light irradiation, the photogenerated electrons are trapped by oxygen defects, whereas the holes are rapidly transported by CoO_x clusters, thereby resulting in efficient charge separation. The synthesis methodology and CO₂ conversion rates are displayed in Figure 3.

Figure 3.

(a) Synthesis procedure for hydrogenated TiO₂ nanotubes CoOx photocatalyst. (b) Photocatalytic CO and CH₄ yield employing the AB-H-CoOx sample using photocatalytic and photothermal catalytic approaches (taken with permission from [53]).

Noble metal loading, such as with silver (Ag) and gold (Au) nanoparticles, is also an effective strategy for improving the photocatalytic performance of the material. A report presented a size-controlled study of 1-D TiO₂ nanowires (TNW) loaded with Au NPs for photocatalytic conversion CO₂ into value-added chemicals [54]. The TNW were synthesized using an alkaline hydrothermal approach followed by deposition of Au NPs using a well-established chemical reduction method. The loading with the Au NPs was optimized by varying the concentration of the respective salts. The 0.5% Au-TNW exhibited the highest yields of CO, CH₄ and CH₃OH (1237, 13 and 12.6 µmol/g h, respectively) under visible light irradiation as compared to pure TNW (9, 3 and 0 µmol/g h). Moreover, the effect of TNW size was also investigated, and it was found that TNW synthesized with a reaction time of 2 h offered the best performance due to its uniform and smooth surfaces and increased surface areas. Upon loading with Au NPs, the surface area was reduced a bit, but the performance was improved due to the synergetic effect of Au NPs, with smaller-sized NPs acting as electron extractors, while larger NPs injected hot electrons into the TNW conduction band as a result of the LSPR effect. The proposed mechanism for the Au-TNW in photocatalytic CO₂ conversion is well displayed in Figure 4a. Similarly, TNW loaded with Ag nanoparticles [55] or a combination of both Au and Ag nanoparticles [56] exhibited an analogous performance attitude under UV and visible light irradiation.. Another similar strategy involves the electrodeposition of Ag nanoparticles on the inner side of TiO₂ nanotube (TNT) arrays [57]. The photocatalytic performance of Ag-TNT with respect to the conversion of CO₂ was evaluated against pure TNT and Ag-TNT fabricated using a chemical bath deposition approach. The electrodeposited Ag NPs at a fixed voltage and time resulted in a uniform size distribution and good junctioning to the inner side of the TNT arrays. This heterostructure resulted in an enhanced CH₄ yield with a small amount of CH₃OH under light irradiation, as compared to pure TNT arrays and those of Ag-TNT arrays fabricated using conventional chemical bath deposition. The performance enhancement of the prepared photocatalyst was attributed to the light scattering inside the TNT arrays, which led to a greater degree of light absorption by Ag NPs, thus intensifying the LSPR effect. A more intensified LSPR effect leads to the injection of hot electrons into the conduction band of TNT, and thus to efficient charge separation and reaction with the adsorbed CO₂ to yield CH₄ or CH₃OH. Su et al. proposed the deposition of palladium (Pd) nanoparticles onto TiO₂ nanowires (Pd-TNW) and evaluated photocatalytic performance by CO₂ conversion to CO and CH₄ [58]. TNW were synthesized using a hydrothermal approach, while Pd nanoparticles were deposited using a chemical reduction method. The Pd deposition was optimized by varying the Pd salt concentration, and it was found that 0.5% Pd-TNW exhibited the maximum CO and CH₄ yield, which was mainly attributed to the efficient electron separation of Pd from the TNW conduction band, resulting in rapid reaction with the adsorbed CO₂.

Figure 4.

(a) Schematic presentation of the proposed photocatalytic conversion of CO₂ into CO, CH₄, CH₃OH and other value-added chemicals employing Au-TNW (taken with permission from [54]). (b) Z-scheme proposed for ZnFe₂O₄-TNB for CO₂ conversion to its respective products (taken with permission from [59]).

Another research work proposed the formation of heterostructures with a visible light active photocatalyst: ZnFe₂O₄ nanoparticles with TiO₂ nanobelts (TNB) [59]. The ZnFe₂O₄ nanoparticles were allowed to grow onto the TNB and tested for photocatalytic conversion of CO₂ in cyclohexanol to cyclohexanone (CH) and cyclohexyl formate (CF). With the optimum concentration of ZnFe₂O₄ nanoparticles, i.e., 9.78%, the CH and CF yields approached their maximum; beyond this concentration, the yields decreased due to the flight shielding effect of heavy loading. The enhanced performance was attributed mainly to the efficient charge separation by a Z-scheme mechanism; Figure 4b.

Another research work proposed replacing the noble metal loading with rGO sheets deposited onto TiO₂ nanotube arrays (TNT). Interestingly, using the designed synthesis strategy, the resulting photocatalyst consisted of TNT arrays covered with rGO sheets with embedded TiO₂ nanoparticles [30]. When employed for photocatalytic CO₂ conversion, the resulting photocatalyst provided a CH₄ yield that was 4.4 times higher than that achieved with pure TNT arrays, which was mainly attributed to the improved charge separation and extended light absorption. This proposed scheme for photocatalytic CO₂ conversion is displayed in Figure 5a.

Figure 5.

(a) Schematic view of photocatalytic conversion of CO₂ into CH₄ on rGO-TNT (taken with permission from [30]). (b) Production rate of CO and CH₄ from GR-TNR with varied concentrations of graphene (taken with permission from [60]).

Similarly, various TiO₂ nanostructures, i.e., nanoparticles, nanotubes and nanosheets combined with graphene, were have been synthesized and studied for the purposes of photocatalytic conversion of CO₂ into useful products [60]. Different synthesis procedures were adopted for each nanostructure; however, TiO₂ nanorods (TNR) were synthesized using a hydrothermal method and graphene compositing was also performed using the hydrothermal approach. The 1% graphene-TNR (GR-TNR) exhibited the highest yields of CO and CH₄, which was mainly attributed to the increased surface area and the better interaction of graphene and TNR leading to improved charge separation under UV and visible light irradiation. The CO and CH₄ yields from the various GR-TNR investigated are shown in Figure 5b.

Recently, Kar et al. proposed a novel strategy using flame-annealed TNT, resulting in mixed-phase (rutile and anatase) square-shaped nanotubes with defect-induced oxygen vacancies [61]. The TiO₂ nanotubes were fabricated using a conventional electrochemical anodization method, employing a water base and an ethylene glycol base as an electrolyte. The synthesized TNT were then flame-annealed using a propane torch at a temperature of 750 °C for about 2 min. The flame-annealed TNT (FANT) using a water-based electrolyte exhibited the highest CH₄ yield under simulated solar light and visible light irradiation of a 50 W LED lamp, and this was around 1.7 and 1.56 times higher than TNT. The improved performance of FANT (TNT prepared with a water-based electrolyte) was mainly attributed to the improved light absorption achieved by using a mixed phase of FANT, i.e., anatase and rutile, along with the well-aligned band edges of the rutile phase, which were 0.2 eV lower than in the anatase phase, resulting in efficient charge separation. The presence of Ti³⁺ states was also observed, which might have been generated due to improper oxidation of TNT; hence, extending light absorption as a result of the presence of shallow defects. Moreover, the most important aspect of the square shape of FANT is its interaction with light, resulting in a higher density of electromagnetic hotspots, and thus also contributing to improved photocatalytic performance. The synthesis procedure for FANT and the proposed mechanism involved in photocatalytic CO₂ conversion are shown in Figure 6.

Figure 6.

(a) Schematic overview of the synthesis procedure for FANT, and (b) the proposed mechanism for photocatalytic conversion of CO₂ into CH₄ employing the FANT photocatalyst (taken with permission from [61]).

A summarized overview of the numerous TiO₂-based 1-D nanostructures reviewed, including product type and production rate, reaction conditions, and key process parameters influencing photocatalytic performance, is presented in Table 2.

Table 2.

Summary of various 1-D nanostructured photocatalysts, including reaction conditions, production of value-added chemicals by photocatalytic CO₂ conversion, and key parameters for performance improvement.

1-D Nanostructure	Light Source and Reactants	Production Rate of Value-Added Chemicals	Key Parameters for Improved Performance	Ref.
CdS/TiO2 nanotubes and Bi2S3/TiO2 nanotubes	500 W Xenon lamp CO2 bubbled through a solution of Sodium hydroxide and sodium nitrite	After 5 h of irradiation CH3OH: 102.5 µmol/L for TNT CH3OH: 159.5 µmol/L for CdS/TNT CH3OH and 224.6 µmol/L for Bi2S3/TNT	Improved surface area Light absorption enhancement Improved CO2 adsorption Efficient electron–hole separation	[47]
MgO amorphous layers on Pt loaded TiO2 nanotubes networks	300 W Hg lamp CO2 bubbled through water	CO: 10.4 ppm/h cm−2, and CH4: 100.2 ppm/h cm−2 for Pt loaded on 0.01 M MgO coated TiO2 Nanotube Networks	Improved CO2 adsorption Enhanced electron–hole separation	[48]
Black TiO2 films with grid-like structures	300 W Xenon lamp CO2 bubbled through water	CO: 115 µmol/g h, and CH4: 12 µmol/g h	Improved light absorption Improved charge separation due to extended charge lifetime	[49]
Cu2O nanoparticles modified TiO2 nanotube arrays	350 W Xenon lamp with and without UV cutoff filter CO2 bubbled through water	After 4 h of irradiation using simulated solar light CH4: 400 ppm/g for sample with 15 min of Cu2O electrodeposition After 4 h of visible light irradiation CH4: 8 ppm/g for sample with 30 min of Cu2O electrodeposition	Visible light absorption by Cu2O TNT providing a pathway for efficient electron–hole charge separation	[50]
Cu deposited on TiO2 nanoflowers films	500 W Xenon lamp with UV cutoff filter CO2 bubbled through water	CH3OH: 1.8 µmol/cm2 h under UV and Visible light irradiation For 0.5 Cu/TiO2 film	LSPR effect due to Cu nanoparticles and efficient charge transfer property	[51]
Au nanoparticles deposited on TiO2 nanowires	HID 35 W Car lamp CO2 and H2 gaseous mixture	CO: 1237 µmol/g h, CH4: 13 µmol/g h, and CH3OH:12.65 µmol/g h For 0.5 Au TiO2 NW	LSPR effect Efficient photogenerated charge extraction Improved Surface area	[54]
ZnFe2O4 nanoparticles on TiO2 nanobelts	250 W high pressure Hg lamp CO2 and cyclohexanol	After 8 h of UV illumination Cyclohexanone (CH):170.2 µmol/g and Cyclohexyl formate (CF): 178.1 µmol/g With sample containing 9.78 wt. % loading of ZnFe2O4	Improved Charge separation by a Z-scheme mechanism Enhanced Surface area	[59]
TNT arrays covered with rGO TiO2 nanoparticles	100 W Xenon solar simulator CO2 and Water vapors	CH4: 5.67 ppm/cm2 h	Efficient Charge separation Light absorption enhancement	[30]
Ag NPs TiO2 nanowires	35 W HID car lamp as visible light source 200 W Hg Reflector lamp as a UV light source CO2 and H2 at feed ratio of 1.0, temperature 100 °C and Pressure 1 atm.	CO: 983 µmol/g h CH4: 9.73 µmol/g h CH3OH: 13 µmol/g h From 3% Ag deposited TiO2 NWs	Moderate surface areas LSPR effects and efficient charge separation	[55]
Au-Ag NPs TiO2 nanowires	35 W HID car lamp as visible light source 200 W Hg Reflector lamp as a UV light source CO2 and H2 at feed ratio of 1.0, temperature 100 °C and Pressure 1 atm.	CO: 1813 µmol/g h CH4: 35 µmol/g h C2H4: 0.95 µmol/g h C2H6: 2.52 µmol/g h C3H6: 3.94 µmol/g h C3H8: 3.52 µmol/g h CH3OH: 18.76 µmol/g h From, 2% Ag-0.5% Au deposited TiO2 NWs	Improved surface area LSPR effects and efficient charge separation	[56]
Ag loaded TiO2 nanotube arrays (TNT)	300 W Xenon arc lamp with a 400 nm cutoff filter CO2 and water vapors	CH4: 48 mmol/h m2 Using the sample with electrodeposited Ag NPs at 3 V and 1 min	Schottkey Junction formation and SPR effect leading to injection of hot electrons to TiO2 CB	[57]
CdS QDs-Cu2+-TiO2 nanorods (NR)	300 W solar simulated Xenon lamp CO2 and water vapors mixture Temperature of 80 °C	C2H5OH: 109.12 µmol/g h Using sample with 2 SILAR cycles CdS QDs-0.02 M Cu2+ ion onto TiO2 NRs	Improved surface area Extended light absorption of the photocatalysts	[52]
Graphene QDs deposited TiO2 nanotube arrays (TNT)	100 W Xenon Solar simulator CO2 and water vapors	CH4: 1.98 ppm/h cm2 Employing G-TNT 3 sample prepared with 3 sec electrophoretic deposition time of GQDs	Enhanced light absorption with efficient extraction of photogenerated electron–hole pairs	[31]
Graphene TiO2 nanostructures including nanoparticles, nanotubes and nanosheets	300 W Xenon lamp CO2 and water vapors	CO: 75.8 µmol/g h CH4: 12.3 µmol/g h By 1% graphene TiO2 nanotubes	Increased surface area Increased interaction between the photogenerated electrons Improved CO2 adsorption	[60]
Flame annealed TiO2 nanotubes (FANT)	100 W Xenon solar simulator Also 50 W LED lamps employed as a visible light source CO2 and water vapors	CH4: 156 µmol/g h From FANT-aq, synthesized using water as an electrolyte medium	Visible light activity due to defect mediated performance enhancement Higher density of electromagnetic hotspots for visible light and stronger absorption of UV light	[61]
Pd-TiO2 nanowires (NW)	400 W Hg Lamp CO2 and Water vapors	After 8 h of irradiation CO: 50.4 µmol/g CH4:26.7 µmol/g Using 0.5% Pd-TiO2 NW	Pd NPs acts as an electron transfer mediator leading to improved charge transfer	[58]
TiO2 nanotubes (NT) coated with CoOx nanoclusters	150 W UV lamp with temperature increased to 393 K CO2 and water vapors	After 8 h of irradiation CO: 16.403 µmol/g CH4: 10.051 µmol/g-s	Improved surface areas Efficient photogenerated electrons-holes separation Oxygen vacancies improved CO2 adsorption	[53]

4. Two Dimensional (2-D) Nanostructured Photocatalysts

Fabrication of two-dimensional (2-D) nanostructured photocatalysts is another interesting and effective strategy for achieving improved photocatalytic activity. The relevant research domains have been widely investigated due to the intriguing properties they offer, which include large surface area, improved surface adsorption, improved interfacial charge transfer, and band edge alignment leading to product selectivity. With regard to photocatalytic CO₂ conversion, a moderate amount of work has been done on the development of TiO₂-based 2-D nanostructured photocatalysts with improved performance resulting from combinations of the factors mentioned above.

Loading TiO₂ nanoparticles onto graphitic carbon nitride (g-C₃N₄) layered nanosheets was investigated by Zhou et al. for the purposes of photocatalytic CO₂ conversion [62]. Various photocatalysts were prepared with varied amount of urea as a precursor of g-C₃N₄. Interestingly, it was observed that when the amount urea was lower than a certain limit, it acted as an N-doping source of TiO₂, leading to the formation of N-doped TiO₂; however, upon increasing the urea concentration, a composite of g-C₃N₄ and N-TiO₂ was obtained. Such variation resulted in the selectivity of the product, yielding CH₄ for N-TiO₂ and CO for the g-C₃N₄/N-TiO₂ composite. The product selectivity is attributed to the alignment of the band edges with respect to the redox potentials of prospective products. The g-C₃N₄/N-TiO₂ with 70:30 mole ratio resulted in the maximum yield of CO only, and no CH₄. However, the sample prepared with 50:50 mole ratio exhibited production of both CO and CH₄. The increased photocatalytic yield of CO and CH₄ was mainly attributed to the visible light absorption, moderate surface areas, enhanced interfacial charge transfer, and the alignment of band edges in accordance with product redox potentials. The transmission electron microscopy (TEM) image of the layered structure of N-TiO₂ nanoparticles loaded onto g-C₃N₄ nanosheets is displayed in Figure 7a. The proposed mechanism for the g-C₃N₄/N-TiO₂ is shown in Figure 7b.

Figure 7.

(a) TEM image showing the loading of N-TiO₂ nanoparticles onto g-C₃N₄ nanosheets, and (b) proposed scheme involved in photocatalytic CO₂ conversion (taken with permission from [62]). (c) TEM image showing the 2-D nanostructure of TiO₂ nanosheets coupled with g-C₃N₄ nanosheets, and (d) schematic view of the interfacial charge transfer within TNS-CNN with the proposed photocatalytic CO₂ conversion mechanism (taken with permission from [63]).

Similarly, another recently published report proposed an in situ pyrolysis approach for the synthesis of a TiO₂ nanosheets-g-C₃N₄ nanosheets nanostructure (TNS-CNN) [63]. Figure 7c shows the TEM image for the TNS-CNN, clearly displaying the fractions of TNS on the CNN. TNS-CNN has been employed for photocatalytic CO₂ conversion using both H₂O and H₂ as reducing agents. It was observed that TNS-CNN provided a CO yield almost 12 times higher than pristine TNS and 37% higher than TiO₂-P25 (with H₂ as a reducing agent). When H₂O was employed as a reducing agent, the CO yield decreased to one third that obtained when using H₂ as a reducing agent. This is mainly attributed to the competition for H₂ produced as a side reaction of water splitting, along with its reducing capability. The overall performance might be attributed to the increased surface area, the enhanced interfacial charge transfer between the nanosheets, the effectiveness of H₂ as a reducing agent, and the visible light absorption of the CNN. A schematic representation of the CO₂ conversion mechanism is shown in Figure 7d.

Recently, Shi et al. reported defect-rich TiO₂ quantum dots (QDs) embedded within g-C₃N₄ nanosheets (TiO_2−x/g-C₃N₄) for efficient photocatalytic conversion of CO₂ into CO under solar light irradiation [64]. The fabricated TiO_2−x/g-C₃N₄ nanostructure exhibited a superior photocatalytic performance with a CO yield 5 times higher than that of pristine g-C₃N₄. The TiO_2−x/g-C₃N₄ was synthesized by a novel and facile strategy of in situ pyrolysis of melamine with MIL-125-NH₂ (Ti). Various samples were prepared with varying the mass ratios. The improved photocatalytic performance was mainly attributed to the improved light absorption, which extended towards the red region, the enhanced CO₂ adsorption due to the defective TiO₂ QDs, the efficient charge separation, as confirmed by transient photocurrent and photoluminescence spectroscopy, and the large surface area due to the nanostructured architecture of the material. Figure 8a displays the SEM image of a representative sample of sheet-type g-C₃N₄ embedded with 0-D TiO_2−x nanoparticles. The proposed photocatalytic mechanism is depicted in Figure 8b, which provides a clear demonstration of the alignment of the band edges to the redox potentials. It can be seen that upon light irradiation TiO_2−x/g-C₃N₄ generates electron–hole pairs, and the electrons from the conduction band of g-C₃N₄ flow towards the conduction band of TiO_2−x, where in the presence of Co(bpy)₃²⁺, the co-catalyst reacts sharply with the adsorbed CO₂ to provide CO. In contrast, the holes within the TiO_2−x and g-C₃N₄ are regenerated by the Triethanolamine (TEOA) hole scavenger.

Figure 8.

(a) SEM image of a representative TiO_2−x/g-C₃N₄ sample displaying sheet-type g-C₃N₄ embedded with TiO_2−x nanoparticles, and (b) schematic of the electronic structure of TiO_2−x/g-C₃N₄ with the proposed mechanism of photocatalytic CO₂ conversion (taken with permission from [64]).

Ultrathin TiO₂ nanosheets, prepared from a TiO₂-Octylamine lamella structure, resulted in an efficient photocatalyst for CO₂ conversion [65]. The extravagant increase in the surface area of the TiO₂ nanosheets led to greater light absorption and an increased number of CO₂ adsorption active sites. It was observed that, in order to obtain ultrathin nanosheets, decreasing the bulk thickness towards atomic-scale thickness might provide the surface atoms with efficient surface active sites for photocatalytic CO₂ conversion. Moreover, the fluorescence lifetime of the photogenerated charge within the ultrathin TiO₂ nanosheets was observed to be higher when compared to their bulk counterparts; therefore, suggesting that the ultrathin nanosheets provide an efficient charge separation pathway along its 2-D channels. As a result of the contributions of such parameters, the formate formation from ultrathin TiO₂ nanosheets was around 450 times higher than that of bulk TiO₂. Figure 9a shows a SEM image of ultrathin TiO₂ nanosheets, while Figure 9b displays a schematic view of the CO₂ conversion mechanism.

Figure 9.

(a) SEM image of the TiO₂-Octylamine hybrid nanostructure, and (b) schematic view of photocatalytic conversion of CO₂ into formate product using ultrathin TiO₂ nanosheets (taken with permission from [65]). Schematic view of (c) unsaturation of ultrathin nanosheets with the appearance of oxygen vacancies, and (d) photocatalytic CO₂ conversion to CH₄ with water vapors employing ultrathin TiO₂ nanosheets with highly dispersed Pt nanoparticles (taken with permission from [66]).

Recently, Liu et al. reported the fabrication of TiO₂ ultrathin nanosheets (TiO₂-U) using a simple hydrothermal approach followed by Pt nanoparticle deposition using a photochemical deposition method [66]. The Pt-TiO₂-U, when employed for photocatalytic CO₂ conversion, resulted in increased CO (54.2 µmol/g h) and CH₄ (66.4 µmol/g h) yields when compared to the pristine and reference samples. It was observed that TiO₂-U exhibited visible light absorption, indicating the presence of oxygen vacancies, which were confirmed by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analysis. The appearance of such self-defects (oxygen vacancies or Ti³⁺ states) was attributed to the narrowing of the thickness, leading to the formation of uncoordinated surface sites, as shown in Figure 9c. Such defects with respect to Ti³⁺ promote the adsorption of CO₂ and the uniform deposition of Pt nanoparticles. The improved photocatalytic performance was mainly attributed to the increased surface area, extended light absorption and CO₂ adsorption sites, and enhanced electron–hole separation by Pt nanoparticles. A schematic view of the photocatalytic mechanism is shown in Figure 9d.

Surface modification of the photocatalyst is another effective approach, leading to improved surface characteristics for photocatalytic CO₂ conversion [67]. One research work reported that the acidification of TiO₂ nanosheets using sulfuric acid led to enhanced CH₄ yield after 4 h of light irradiation. Acidification did not affect the sheet structure or the exposed facet of the material. However, the surface area decreased a bit due to acid molecules plugging the pores. Moreover, the acid treatment extended the light absorption of the material towards the red region; this was mainly ascribed to the generation of oxygen vacancies during the process. Such vacancy formation also enhanced the electron–hole separation under light irradiation, leading to improved photocatalytic performance.

Lamellar structures also promote the photocatalytic activity by adsorbing photoactive species within the layers. Such layered structures with improved surface area and loading with visible active materials can lead to efficient photocatalysts, as proposed by Junior et al. [68]. The authors reported a TiO₂ pillared K₂Ti₄O₉ 2-D structure loaded with visible light-active Cu₂O nanoparticles. The introduction of TiO₂ pillars into layered K₂Ti₄O₉ drastically increased the surface area, which was further increased after loading with Cu₂O nanoparticles. Also, the loading with Cu₂O nanoparticles shifted the light absorption toward the red region, along with efficient charge separation at the interface of TiO₂, K₂Ti₄O₉ and Cu₂O. All the mentioned parameters improved the photocatalytic yield of CH₃OH from moist CO₂, with a yield 2 times as high as that obtained from the pristine sample. The schematic depiction of the mechanism involved is presented in Figure 10a.

Figure 10.

(a) Schematic view of the band energy diagram for Cu₂O-loaded TiO₂ pillared photocatalysts for conversion of CO₂ into value-added chemicals (taken with permission from [68]). (b) FESEM image for TT550 sample displaying the layered structure of MXenes loaded with TiO₂ nanoparticles, and (c) the proposed mechanism of photogenerated charge transfer within the TiO₂-Ti₃C₂ 2-D nanostructure (taken with permission from [69]).

Another interesting and excellent approach to developing 2-D nanostructures is the growth of photocatalytic material onto a 2-D conductive substrate. Recently, Low et al. reported a simple approach for synthesizing TiO₂ nanoparticles coated onto a conductive Ti₃C₂ MXenes (TT) [69]. The TiO₂ nanoparticles appeared as the Ti₃C₂ MXenes were calcined at different temperatures and tested for photocatalytic CO₂ conversion. When employed, the photocatalytic yield of CH₄ was found to be 3.7 times higher than that of commercial TiO₂-P25. Figure 10b displays the SEM image of the TT sample calcined at 550 °C, which was the optimum sample. It can be seen that, upon calcination, TiO₂ nanoparticles appear on the surface and edges of the layered Ti₃C₂ MXenes, resulting in a rougher surface and an improved surface area. Upon testing for photocatalytic CO₂ conversion, the optimum sample, TT550, yielded the maximum CH₄ production, with minor production of CH₃OH and C₂H₅OH. Increasing the temperature beyond this point, the photocatalytic performance decreased due to the lower content of conductive Ti₃C₂ MXenes. Therefore, it can be concluded that the presence of Ti₃C₂ MXenes as a conductive pathway offered efficient separation of electron–hole pairs, leading to improved performance. In addition, the improved surface area contributed significantly by providing more reactive sites for CO₂ adsorption and conversion. The proposed scheme for photocatalytic CO₂ conversion is displayed in Figure 10c.

Another study reported Bi₂WO₆-TiO₂ binanosheets (BT) as a 2-D nanostructure for photocatalytic conversion of CO₂ into CO and CH₄ [70]. The study mainly aimed to provide an enlightened view regarding carbonaceous intermediates or surface species in the generation of value-added chemicals. However, when tested under CO₂ gas, it was observed that the BT sample exhibited enhanced yields of CO and CH₄ when compared to the pristine samples. The key causes were ascribed to the improved interfacial charge transfer and the Z-scheme mechanism, which were deemed to be solely responsible for the enhanced photocatalytic performance. Table 3 provides a summarized overview of the numerous TiO₂-based 2-D nanostructures employed for photocatalytic conversion of CO₂ into various products, with their production rate, reaction conditions and the key process parameters promoting photocatalytic performance.

Table 3.

Summary of various 2-D nanostructured photocatalysts with reaction conditions, value-added chemicals produced as a result of photocatalytic CO₂ conversion, and key parameters for improved performance.

2-D Nanostructure	Light Source and Reactants	Production Rate of Value-Added Chemicals	Key Parameters for Improved Performance	Ref.
g-C3N4/N-TiO2 nanosheets	300 W Xenon arc lamp CO2 bubbled through Deionized water	After 12 h of irradiation CO: 14.73 µmole/g, employing CT7 sample CO: 5.71 µmole/g CH4: 3.94 µmole/g, employing sample CT5	Moderate surface area Extended light absorption Efficient charge separation at the heterojunction Product selectivity due to band regulation	[62]
TiO2 nanosheets modified with sulfuric acid	500 W Xe Arc lamp CO2 and water vapors	After 4 h of irradiation CH4: 7.63 µmole/g	Acidification facilitates oxidation of water by Ti-OH Ti3+ active sites i.e., oxygen vacancies enhanced adsorption of CO2 Efficient charge separation	[67]
Ultrathin TiO2 nanosheets	300 W Hg Lamp CO2 bubbled through Solution of photocatalyst powder in water	Formate formation 1.9 µmole/g h 450 times higher than counterpart 9 time higher than commercially available anatase TiO2	Surface area increased Promoted life time of electron Efficient charge separation across the 2-D path	[65]
Cu2O nanoparticles loaded on TiO2 pillared K2Ti4O7 layers	Polychromatic light AM 1.5 from solar simulator CO2 and water vapors	After 5 h of irradiation CH3OH: 2.93 µmole/g 2 times more as compared to pristine sample	Increased surface area Visible light absorption Efficient charge separation	[68]
Cu modified g-C3N4 sheets with TiO2 nanoparticles	254 nm UV Lamp as a UV light source 500 W Xe arc lamp as a visible light source CO2 bubbled through the water solution containing photocatalyst sample	After 8 h of irradiation under UV light CH3OH: 2574 µmol/g, HCOOH: 5069 µmol/g Under Visible light CH3OH: 614 µmol/g, HCOOH:6709 µmol/g Optimum sample: 3 wt.% Cu and 30:70 ratio of g-C3N4 and TiO2	Extended light absorption and efficient charge separation by copper doping Band edges alignment reflects the selectivity for CH3OH and HCOOH	[71]
TiO2 nanoparticles on Ti3C2 nanosheets	UV LED 3 W 365 nm CO2 and water vapor generated in situ by reaction of NaHCO3 and HCl	CH4: 0.22 µmole/ h for 50 mg sample with small amounts of CH3OH and C2H5OH	Improved surface area Nanosheets providing active sites Efficient electron hole separation	[69]
Pt nanoparticles loaded ultrathin TiO2 nanosheets	300 W Xenon lamp CO2 and Water vapors	CH4: 66.4 µmole/h g CO: 54.2 µmole/h	26 times higher surface area Efficient electron hole separation Improved CO2 adsorption due to defective surface	[66]
2-D g-C3N4 with 0-D TiO2−x nanoparticles	300 W Xenon lamp CO2 bubbled through solution containing 5 mg photocatalyst dispersed in 5 mL of solution of MeCN/TEOA with cocatalyst of Co(bpy)32+	After 5 h of irradiation CO: 388.9 μmol/g 5 times higher than pristine g-C3N4	Promoted charge transfer due to electron channel formed between g-C3N4 and TiO2	[64]

5. Hierarchical Nanostructures: A Dynamic and Potent Approach

In recent years, hierarchical nanostructures have received wider attention for the purpose of heterogeneous photocatalysis, as they possess admirable and exemplary properties at the micro/nanometer scale. Since the invention of 3-D/hierarchical nanostructures, intensive research has been carried out with the aim of developing competent and efficient hierarchical nanostructures for a variety of photocatalysis applications [21,72]. Based on nano-sized building blocks such as nanotubes, nanorods, nanosheets, etc., these nanostructures possess superb properties, including porous and interconnected networks, high surface areas, multi-dimensional domains for efficient charge transfer, improved light harvesting, and increased reactant adsorption, which together lead to enhanced photocatalytic performance. Therefore, these captivating aspects of hierarchical nanostructures provide a potentially dynamic opportunity to pursue the development of nanostructured photocatalysts to be employed for the photocatalytic conversion of CO₂ into useful chemicals/fuels.

Recently, Wang et al. proposed a 3-D hierarchical TiO₂ microsphere (MS) with tunable pore and chamber size to facilitate the diffusion of the gas and its subsequent photocatalytic conversion under simulated solar light irradiation [73]. Three different types of TiO₂ microspheres were prepared, i.e., solid MS, yolk/shell MS and hollow MS. Photocatalytic activities were tested under both UV and simulated solar light irradiation. It was observed that under UV light, solid MS exhibted the maximum CO yield (17.7 µmol/g h), which was 1.2 and 1.6 times higher than that obtained with yolk/shell MS and hollow MS, respectively. This increase in yield was attributed to the intense light absorption of solid MS. However, under simulated solar light irradiation, hollow MS showed the maximum CO yield (34 µmol/g, after 3 h of irradiation), which was 1.6 and 1.4 times higher than the yields obtained with solid and yolk/shell MS, respectively. The increased photocatalytic performance under simulated solar light was ascribed to the increased pore size of the hollow MS, leading to rapid diffusion of CO₂ molecules towards active sites, which is a result of its unique hierarchical nanostructure. Figure 11 shows SEM images of all of the MS samples and their CO production rate under simulated solar light irradiation.

Figure 11.

SEM images of (a) solid MS, (b) yolk/shell MS, and (c) hollow MS. (d) CO production rate for various samples via photocatalytic CO₂ conversion (taken with permission from [73]).

Similarly, a recent report presented the replication of Camella tree leaves for synthesis of a unique porous TiO₂ architecture with enhanced photocatalytic conversion of CO₂ into CH₄ and CO [74]. The TiO₂ artificial leaves (AL) were synthesized using a bio-template approach. The unique architecture of the AL consists of interconnected nanosheets, leading to improved porosity and a surface area greater than that of TiO₂-P25. The photocatalytic CO₂ conversion was tested under both UV and visible light irradiation. Under UV light irradiation, the AL yielded mostly CH₄, whereas reference nonporous P25 yielded CO. This difference was attributed to the increased residence time and contact between reactant and catalyst within the porous network of the AL. Similar results were obtained when irradiated with visible green light; however, upon loading the Ru₂O nanoparticles, the CO and CH₄ yields with AL were drastically increased, which can be attributed to the efficient charge separation at the metal–semiconductor junction. Thus, the factors of porous networks with surface defects, increased surface area and efficient charge separation lead to improved photocatalytic performance in the AL hierarchical nanostructure. Figure 12 shows an image of the porous hierarchical TiO₂ AL obtained using the bio-template approach, and the rate of photocatalytic conversion of CO₂ into CH₄ and CO under UV and visible light sources.

Figure 12.

Image of camellia leaf, SEM image of artificial leaf displaying a porous structure, and the production rate of CO and CH₄ via the photocatalytic conversion of CO₂ and water vapors (taken with permission from [74]).

Another work presented the fabrication of a 3-D Z-scheme nanostructured photocatalyst consisting of ZnIn₂S₄ nanosheets assembled onto TiO₂ nanobelts [75]. This hierarchical nanostructure, with an optimum ratio of ZnIn₂S₄ (0.33:1 mole ratio), exhibited an enhanced CH₄ yield (1.135 µmol/g h) from the photocatalytic conversion of CO₂ and water vapors under UV-visible light irradiation, which is around 39 times higher than the yield obtained from bare ZnIn₂S₄ nanosheets. The increased CH₄ yield was attributed to the improved surface area, light absorption and effective charge separation due to the Z-scheme mechanism. Figure 13a,b shows the CH₄ production rate from photocatalytic CO₂ conversion.

Figure 13.

(a) Photocatalytic CH₄ evolution and (b) production rate employing various samples with varied ratios of ZnIn₂S₄ to TiO₂ (taken with permission from [75]). (c) SEM image of Cu₂O/S-TiO₂/CuO p-n-p nanostructure, and (d) band gap diagram with the mechanism involved in the conversion of CO₂ to CH₄ (taken with permission from [76]).

Kim et al. developed a unique architecture composed of Cu₂O dendrites covered with S-TiO₂ micro-blocks and CuO nanowires, resulting in a p-n-p heterojunction formation [76]. When employed in photocatalytic CO₂ conversion, this nanostructure yielded a high CH₄ production rate, at 2.31 µmole/m² h, which is ten times higher than with TiO₂ nanotubes. The increased CH₄ yield was attributed to the improved light absorption and efficient charge separation at the interfaces of the Cu₂O dendrites, CuO nanowires and S-TiO₂ micro-blocks. Figure 13b shows a SEM image of the nanostructured photocatalyst, while Figure 13d displays the proposed band gap diagram, depicting the charge transfer mechanisms within the nanostructure.

The development of 3-D ordered macroporous TiO₂ (3DOM-TiO₂) is also an interesting strategy with the aim of improving photocatalytic CO₂ conversion. Furthermore, the loading of noble metals like Au, Ag and Pd onto 3DOM-TiO₂ results in an efficient photocatalyst nanostructure for improved conversion of CO₂ into useful chemicals. As previously reported, AuPd has an optimum ratio of 3:1 weight percent. This was loaded onto 3DOM-TiO₂, resulting in an enhanced CH₄ yield of 18.5 µmol/g h, whereas AuPd 1:3 exhibited an increased CO yield, 14 µmol/g h, when employing CO₂ and water vapors as reactants under UV visible light irradiation [77]. The key parameters responsible for the improved performance were deemed to be the increased light harvesting due to the slow photon effects of 3DOM-TiO₂ and the LSPR effect induced by the noble metals, and the efficient charge extraction at the interface of the noble metals and the semiconductors. Figure 14a represents the proposed mechanism for the photocatalytic conversion of CO₂ into CH₄ and CO when employing 3DOM-TiO₂ hierarchical nanostructures loaded with noble metals. Another study presented 3DOM-TiO₂ hierarchical nanostructures loaded with Au nanoparticles as an efficient photocatalyst for the conversion of CO₂ and water vapors into CH₄ under visible light irradiation [78]. The sample with the optimum Au loading exhibited an increased CH₄ yield (23.90 µmol/g h) as compared to the pristine sample (11.39 µmol/g h). This increase was attributed to the increased light harvesting due to the slow photon effect, the LSPR due to the uniformly loaded Au nanoparticles, and the effective charge separation at the interface between the Au nanoparticles and 3DOM-TiO₂. A representation of the proposed mechanism for the photocatalytic CO₂ conversion is depicted in Figure 14b.

Figure 14.

Proposed mechanism of photocatalytic CO₂ conversion with water vapors employing (a) AuPd-3DOM TiO₂ photocatalyst (taken with permission from [77]), and (b) Au-3DOM TiO₂ photocatalyst (taken with permission from [78]).

6. Conclusions

The present review briefly reveals the influential and imperative aspects of TiO₂-based photocatalyst nanostructures, when applied for the purposes of photocatalytic CO₂ conversion. The recent developments in TiO₂-based nanostructures, i.e., 1-D, 2-D, and hierarchical nanostructures, have made it clear that, based on the geometry and configuration of the nanostructured photocatalyst, improved photocatalytic performance can be ascribed to a combination of (i) large surface areas, (ii) efficient separation of photogenerated charges, (iii) directional charge transport, (iv) improved light harvesting due to light trapping/scattering, and (v) the slow photon effect. Furthermore, the fabrication of hetero-junctioned nanostructured photocatalysts, coupled with noble metals, graphene derivatives, or another semiconductor, multiplies the photocatalytic performance, thus reaping the benefits of both, i.e., the aspects of both nanostructures and hetero-junction formation. Hence, it can be established that the unique and peculiar properties of nanostructured photocatalysts are accompanied by an improvement in photocatalytic performance, and their study is a potent research domain offering excellent prospects for photocatalytic conversion of CO₂ into value-added chemicals.

Author Contributions

A.R. collected the data, analyzed it, and wrote the review. S.I. analyzed, modified and rewrote the manuscript.

Funding

The authors gratefully acknowledge the support of the Ministry of Science and ICT (2017R1E1A1A01074890 and 2017M2A2A6A01070912). This research was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (2015M1A2A2074670) as well as by the DGIST R&D Program of the Ministry of Science and ICT (19-BD-0404). Abdul Razzaq would also like to acknowledge the support of Higher Education Commission, Pakistan, under the Start-Up Research Grant for Fresh PhD Holders, Project No. 21-1952/SRGP/R&D/HEC/2018.

Conflicts of Interest

The authors declare no conflict of interest.