Electronic-reconstruction-enhanced hydrogen evolution catalysis in oxide polymorphs

Abstract

Transition metal oxides exhibit strong structure-property correlations, which has been extensively investigated and utilized for achieving efficient oxygen electrocatalysts. However, high-performance oxide-based electrocatalysts for hydrogen evolution are quite limited, and the mechanism still remains elusive. Here we demonstrate the strong correlations between the electronic structure and hydrogen electrocatalytic activity within a single oxide system Ti₂O₃. Taking advantage of the epitaxial stabilization, the polymorphism of Ti₂O₃ is extended by stabilizing bulk-absent polymorphs in the film-form. Electronic reconstructions are realized in the bulk-absent Ti₂O₃ polymorphs, which are further correlated to their electrocatalytic activity. We identify that smaller charge-transfer energy leads to a substantial enhancement in the electrocatalytic efficiency with stronger hybridization of Ti 3d and O 2p orbitals. Our study highlights the importance of the electronic structures on the hydrogen evolution activity of oxide electrocatalysts, and also provides a strategy to achieve efficient oxide-based hydrogen electrocatalysts by epitaxial stabilization of bulk-absent polymorphs.

Converting solar energy to hydrogen fuel requires light-absorbers that well-match the wavelengths of incoming sunlight. Here, authors prepare a broadband visible-light-absorbing molecular complex that efficiently produces hydrogen from water.

Introduction

With the growing concerns about the environmental pollution, global warming and the rapid depletion of petroleum resources, exploiting alternatively sustainable, clean, and renewable energy sources is becoming the most urgent scientific challenge for us in the modern society^1,2. Supplying energy without toxic emissions, hydrogen, as a promising energy carrier, is believed to have a crucial role in the future scenario of energy applications³. In the past several decades, numerous efforts have been devoted to developing more sustainable hydrogen-production routes from renewable energy sources^3,4. Among them, electrochemical water splitting using the hydrogen evolution reaction (HER, 2H⁺ + 2e⁻ → H₂) is considered as a promising method for hydrogen production^3–5, owing to its unparalleled capacity and carbon-free nature. Thus, the efficiency of hydrogen production through electrolysis mainly depends on the catalytic performance of the HER electrocatalysts. Currently, the state-of-the-art electrocatalyst for HER is Pt/C, but it suffers from the prohibitive cost and scarcity². Hence, it is highly desirable to explore new efficient HER electrocatalysts with low-cost and earth-abundant elements.

Transition metal oxides (TMOs) have attracted great interest in both condensed matter physics and materials science due to their fascinating tunable physical and chemical properties with high stability, low cost, and environmental friendliness^6–8. Titanium dioxide (TiO₂) is one of the most studied TMOs, and has been extensively explored as a photocatalyst for water splitting. Its efficiency can be dramatically enhanced by increasing its light absorption via fabricating black hydrogenated TiO₂ nanocrystals^9,10. The perovskite oxides with a general formula of ABO₃ have been well studied as electrocatalysts for water splitting, and high-performance oxygen evolution reaction (OER) was realized in them^11–14. Then, substantial efforts were devoted to understanding its mechanism and investigate the fundamental parameters that govern the catalytic activity^15–20. Consequently, tuning the electronic structures of the electrocatalysts has been considered as an efficient method to enhance the OER activity of TMOs with stronger metal–oxygen (M–O) hybridization or higher covalency of the M–O bonds^12,20,21. For HER, pure oxides were usually inactive, because of the extremely strong hydrogen adsorption on oxygen atoms. Nevertheless, most recently, several oxides (such as Pr_0.5(Ba_0.5Sr_0.5)_0.5Co_0.8Fe_0.2O_3-δ², SrNb_0.1Co_0.7Fe_0.2O_3-δ (nanorods)²², and CoO (nanorods)²³) were reported to exhibit efficient HER activities. However, the origin of TMO-based HER mechanism with oxides’ characteristics, e.g., metal–oxygen hybridization, and the fundamental parameters that dominate the HER activity still remains unclear.

In this work, we demonstrate the close correlation between the electronic structures and HER catalytic activities in a single-oxide system (Ti₂O₃) with strongly correlated electrons. Generally, Ti₂O₃ has a corundum structure^24,25 in bulk with an ultra-narrow bandgap (E_g ≈ 0.1 eV), exhibiting excellent photothermal effect²⁶ and mid-infrared photodetection²⁷. Interestingly, a new orthorhombic Ti₂O₃ polymorph that is absent in bulk was stabilized on Al₂O₃ single-crystal substrates via epitaxial stabilization, with intriguing emergent properties^28,29. Here, extension of the polymorphism of Ti₂O₃ is further achieved by epitaxially stabilizing a cubic phase, which has not been reported before and is also bulk-absent. Taking advantages of the excellent structural flexibility of Ti₂O₃, we are able to explore the correlation between the physical electronic structures and HER activities within a single-material system, which provides a more precise understanding on the correlation without disturbance of varied elements. Systematical investigations are performed on the electronic structures, transport properties, and HER activities of three Ti₂O₃ polymorphs. Evident electronic reconstructions are observed in the epitaxially stabilized orthorhombic and cubic phases with modulated electron–electron interactions and charge-transfer energy. Importantly, smaller charge-transfer energy leads to the stronger hybridization strength of the Ti 3d –O 2p orbitals, which lower the d-band center of Ti and weakens the H adsorption, further resulting in the enhanced HER activity in those epitaxially stabilized polymorphs. This electronic-reconstruction enhanced HER activity, achieved in the structure-tailored Ti₂O₃, introduces a previously unrecognized route to explore more efficient TMO-based HER electrocatalysts by enhancing the metal–oxygen hybridization via selective stabilization of polymorph phases.

Results

Fabrication and structural characterizations of Ti₂O₃ polymorphs

Three Ti₂O₃ polymorphs were successfully fabricated in the epitaxial film form by using the pulsed laser deposition (PLD) technique (Fig. 1a). During the film growth, the same corundum (trigonal) Ti₂O₃ target was used for stabilizing all Ti₂O₃ polymorphs. The phase separation was controlled by carefully varying deposition temperature and substrates’ symmetry, which are widely used for the oxides epitaxial stabilization^30,31. With the advent of thin-film epitaxial stabilization, new polymorphs that do not exist in bulk could be stabilized on single-crystal substrates^28,29,31, resulting in the extension of the oxides’ polymorphism. Polymorphism describes the occurrence of different lattice structures and symmetries in a crystalline material with identical composition, which is a critical and attractive phenomenon in material science and condensed matter physics^32,33. Polymorphs would exhibit different physical, chemical, and mechanical properties due to their varied structural characteristics^34–37, e.g., the polymorph-dependent metal–semiconductor transitions in Ti₃O₅^38,39.

Fig. 1.

Fabrication and structural characterizations of the Ti₂O₃ polymorphs. a Schematic of the PLD chamber where Ti₂O₃ polymorphs were fabricated using the same (α-Ti₂O₃) target. b Unit cells for γ-Ti₂O₃, o-Ti₂O₃, and α-Ti₂O₃ polymorphs from top to bottom, respectively. c–e In-plane epitaxial relationships for γ-Ti₂O₃ on STO, o-Ti₂O₃, and α-Ti₂O₃ on Al₂O₃, respectively. TiO₂–STO (001) view refers to view the TiO₂-terminated STO (Supplementary Fig. 5) from the < 001 > direction. f HR-XRD patterns of Ti₂O₃ polymorphs epitaxially stabilized on the single-crystal substrates. Source data are provided as a Source Data file

By recrystallization on the substrates during epitaxial growth (Fig. 1a), three different Ti₂O₃ polymorphs (Fig. 1b), trigonal (denoted as α-Ti₂O₃), orthorhombic (denoted as o-Ti₂O₃), and cubic (denoted as γ-Ti₂O₃), are obtained on single-crystalline α-Al₂O₃ and SrTiO₃ (STO) substrates. More details of the unit cell parameters for Ti₂O₃ polymorphs can be found in Supplementary Fig. 1. Both α- and o-Ti₂O₃ are stabilized on α-Al₂O₃ (0001) substrates, selected by increasing the deposition temperature from 600 to 900 °C, while cubic γ-Ti₂O₃ is stabilized on STO substrates at 600 °C. More details for the stabilization of Ti₂O₃ polymorphs can be found in the Supplementary Fig. 2 and the Supplementary Note 1. Atomic force microscopy (AFM) was used to characterize the surface microstructure and roughness of the Ti₂O₃ films. As shown in Supplementary Fig. 3, the root mean square (RMS) roughness for all Ti₂O₃ samples is quite similar and close to 1 nm, which is very small with respect to their thickness (~300 nm), indicating the flat surfaces that are further confirmed by the scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) images (Supplementary Fig. 4). Interestingly, unexpected fascinating properties (such as ferromagnetism²⁸ and superconductivity²⁹) were discovered in the o-Ti₂O₃, demonstrating strong structure–property correlations in Ti₂O₃, which motivated us to further explore the polymorphism of Ti₂O₃.

Figure 1c–e shows the in-plane epitaxial relationships for γ-, o-, and α-Ti₂O₃ film/substrate heterostructures, respectively. The corresponding high-resolution X-ray diffraction (XRD) patterns are shown in Fig. 1f. The epitaxial relations for α- and o-Ti₂O₃ have been determined to be (0001) α-Ti₂O₃ || (0001) α-Al₂O₃ and (011) o-Ti₂O₃ || (0001) α-Al₂O₃ by high-resolution STEM and selected area electron diffraction (SAED)²⁹. Since γ-Ti₂O₃ is a newly stabilized polymorph, we utilized HR-XRD and HR-STEM (Fig. 2) to investigate its epitaxial growth on STO. As shown in Fig. 1f, (004) γ-Ti₂O₃ was directly grown on (002) STO (Supplementary Fig. 5), indicating a cubic-on-cubic growth. Figure 2a schematically illustrates the epitaxial growth of γ-Ti₂O₃ on STO, which is further confirmed by in-plane synchrotron-based X-ray φ-scan (Supplementary Fig. 6 and Supplementary Note 2). As isomorphs of γ-Ti₂O₃, γ-Al₂O₃^40,41, and γ-Fe₂O₃⁴² were also stabilized on STO with the same epitaxial relationship. Moreover, γ-Ti₂O₃ is also successfully stabilized on LaAlO₃ (LAO) substrates, which is identical with that on STO (Supplementary Fig. 6).

Fig. 2.

Microstructural characterization of the cubic γ-Ti₂O₃ polymorph. a Sketch of the γ-Ti₂O₃/STO heterostructure. b Cross-sectional low-resolution STEM image and (c) high-resolution HAADF-STEM image of the γ-Ti₂O₃/STO interface. d HAADF-STEM image and the corresponding HAADF signal profile obtained along (100) and (001) directions of γ-Ti₂O₃ from (c)

In-depth STEM measurements were performed to investigate the cubic γ-Ti₂O₃ on STO. Figure 2b shows the cross-section low-resolution STEM image of the spinel/perovskite γ-Ti₂O₃/STO heterointerface. The thickness of the γ-Ti₂O₃ film is about ~300 nm. It needs to be pointed out that the thickness for all Ti₂O₃ films is fixed at around ~300 nm in order to get the bulk intrinsic properties of these polymorphs. High-resolution high-angle annular dark-field (HAADF) STEM imaging (Fig. 2c) presents the highly crystalline γ-Ti₂O₃ and the cubic-on-cubic epitaxial growth on STO. The lattice parameters of γ-Ti₂O₃ measured from the HAADF-STEM signal profiles (Fig. 2d) along (100) and (001) are a = c ≈ 8.5 Å, consistent with the HR-XRD result ((004): 42.35˚ corresponds to c = 8.53 Å). To shed light on the evolution of Ti valence states at the interface, electron energy loss spectroscopy (EELS) line scan was collected across the γ-Ti₂O₃/STO interface. As shown in Supplementary Fig. 7, the valence state of Ti evolves from 4 + to 3 + crossing the interface from STO to γ-Ti₂O₃ with obvious variation at the Ti L_2,3-edge and O K-edge (Supplementary Note 3). Moreover, the Ti L_2,3-edge EELS spectrum collected from γ-Ti₂O₃ is same as those obtained from α- and o-Ti₂O₃^28,43, indicating the same Ti³⁺ chemical environment. Three Ti₂O₃ polymorphs with same Ti³⁺ valence states but different lattice symmetries are identified and confirmed. Subsequently, the electronic band structures of these Ti₂O₃ polymorphs are carefully investigated by spectroscopic techniques. In order to eliminate the influence of STO substrates with possible oxygen vacancies, formed during the deposition, that would affect that light absorption and conductivity of STO⁴⁴, the following measurements of γ-Ti₂O₃ were mainly performed on the γ-Ti₂O₃/LAO samples.

Determination of electronic reconstructions in Ti₂O₃ polymorphs

With an unpaired 3d¹ election, α-Ti₂O₃ is regarded as an antiferromagnetic Mott insulator with a unique and broad metal–insulator transition (MIT)^45–47. In strongly correlated TMO systems, the electron–electron interaction, represented by the on-site Coulomb repulsion U, is significant, and opens the Mott–Hubbard gap inside the transition metal 3d band between the lower Hubbard band (LHB) and upper Hubbard band (UHB). Charge-transfer energy Δ, the energy difference between the oxygen 2p band and transition metal 3d band, is another crucial parameter in determining the physical and chemical properties of TMOs^20,48. As illustrated in Fig. 3a, depending on the relative magnitudes of U and Δ, strongly correlated TMOs could be classified into Mott insulators (U < Δ) and charge-transfer insulators (U > Δ). With tunable U and Δ, modulations of the electronic properties that impact on electrochemical activities could be achieved^49–51.

Fig. 3.

Electronic reconstructions in Ti₂O₃ polymorphs. a Schematic energy band diagram for the Mott insulator and charge-transfer insulator. b Proposed electronic structure of Ti₂O₃ by Goodenough et al.⁵² c–e Optical conductivity spectra of the α-, o-, and γ-Ti₂O₃ polymorphs, taken by the ellipsometry at room temperature. Insets are the corresponding absorption coefficient (α) of the Ti₂O₃ polymorphs, collected by the UV–Vis spectroscopy at room temperature. f Evolution of the U and Δ in Ti₂O₃ polymorphs. Source data are provided as a Source Data file

Proposed by Goodenough et al.⁵², the electronic band structure of α-Ti₂O₃ could be well described using the molecular orbital theory (Fig. 3b). The trigonal crystal field splits Ti 3d t_2g orbitals into a pair of $e_g^{\pi }$ orbitals and an a_1g orbital near the Fermi level. Furthermore, the a_1g band splits into the bonding a_1g and the antibonding $a_{1g}^{*}$ bands, while the $e_g^{\pi }$ band splits into the bonding $e_g^{\pi }$ and the antibonding $e_g^{\pi *}$ bands, mainly because of the hybridization of the Ti–Ti orbitals. (The $e_g^{\pi }$ and $e_g^{\pi *}$ bands are usually located at the same energy level, since the $e_g^{\pi }$ − $e_g^{\pi *}$ splitting is very small.) The MIT of Ti₂O₃ could be explained by the band-crossing scenario^53,54. That is, the metallic state emerges when the $e_g^{\pi }$ and a_1g bands overlap with increased temperature, while the insulating state occurs when a gap arises between the $e_g^{\pi }$ and a_1g bands with decreased temperature⁵⁴. At room temperature, the bandgap of insulating α-Ti₂O₃ is around 0.1 eV, which was verified by both electrical and optical measurements^24,28. Based on the above proposed electronic structure, the U and Δ in Ti₂O₃ could be assigned specifically to the a_1g − $e_g^{\pi }$ and $O_{2p}$ − $e_g^{\pi }$ transitions, respectively.

To quantify the U and Δ in different Ti₂O₃ polymorphs, spectroscopic ellipsometry and light absorption measurements were performed at room temperature. Figure 3c–e shows the optical conductivity (σ) of α-, o-, and γ-Ti₂O₃, respectively, demonstrating the a_1g − $e_g^{\pi }$ and a_1g − $a_{1g}^{*}$ interband transitions are located at around 1 and 3 eV⁵⁵. Concomitant with the different lattice symmetries, substantial electronic reconstructions were observed among the three Ti₂O₃ polymorphs. Specifically, the U $\left(a_{1g}-e_g^{\pi }\right)$ in α-, o-, and γ-Ti₂O₃ polymorphs increase from 0.85 eV to 1.01 eV, then to 1.10 eV, respectively, revealing stronger electron–electron interactions. Moreover, typical Drude absorptions are observed in o- and γ-Ti₂O₃ at low photon energies (ħω), indicating high free electron concentrations in these two polymorphs. This point will be discussed later, combined with the electronic transport results. The Δ $\left(O_{2p}-e_g^{\pi }\right)$ in the Ti₂O₃ polymorphs were obtained from the UV–Vis light absorption data, which are correspondingly shown as insets of Fig. 3c–e. The absorption coefficients (α) of Ti₂O₃ polymorphs are in the range of ~0.6 to 2.1 × 10⁵ cm⁻¹ at the UV–Vis range, demonstrating a strong light absorption. In order to extract the energy positions for each optical transitions, Gaussian-fitting analysis was performed. Similarly, the $a_{1g}-a_{1g}^{*}$ transitions are still located at around 3 eV, which is consistent with the ellipsometry results. Importantly, the Δ $\left(O_{2p}-e_g^{\pi }\right)$ for α-, o-, and γ-Ti₂O₃ polymorphs are verified to be 4.78, 4.48, and 4.37 eV, respectively. As a result, the Δ decreases in the Ti₂O₃ polymorphs with increasing U (Fig. 3f). According to these electronic reconstructions, strong polymorph-dependent properties could be expected in Ti₂O₃. It is well known that Δ is directly related to the hybridization of M–O orbitals, which would further impact on the conductivity^48–50 and electrochemical activity^20,51.

Hybridization strength of Ti–O orbitals in Ti₂O₃ polymorphs

In order to estimate the hybridization of Ti–O orbitals in Ti₂O₃, synchrotron-based X-ray absorption spectroscopy (XAS) was performed to examine the electronic structures of Ti₂O₃ polymorphs. The experimental XAS configuration is shown as inset in Fig. 4a. Ti L₃ and L₂-edge transitions (Fig. 4a) in all Ti₂O₃ polymorphs are located at ~458.6 eV and ~463.7 eV, respectively, consistent with the bulk Ti₂O₃⁵⁶. The similar Ti L-edge XAS spectra obtained from the different Ti₂O₃ polymorphs reveal that they are sharing the same Ti³⁺ chemical environment as expected. However, distinct variations are observed at the O K-edge XAS spectra. The O K-edge XAS spectrum has been widely utilized to study the hybridization of the M–O orbitals in oxides^21,57,58. Notably, the O K-edge pre-edge region near the threshold of the O K-edge XAS demonstrates the hybridization between O 2p and transition metal 3d states^21,57. As shown in Fig. 4b, two pre-peaks, located at ~531.7 and ~533.7 eV, are used to evaluate the extent of the hybridization of O 2p with Ti 3d e_g and t_2g orbitals. Besides the pre-edge, another region of the O K-edge at higher energy range corresponds to O 2p orbitals hybridized with Ti 4 s,4p orbitals. Obviously, hybridization of O 2p with Ti 3d and Ti 4 s,4p orbitals are changed in the Ti₂O₃ polymorphs duo to their distinct crystal structures.

Fig. 4.

Hybridization strength of the Ti–O orbitals in different Ti₂O₃ polymorphs. a Ti L_2,3-edge and (b) O K-edge XAS spectra of the Ti₂O₃ polymorphs, collected in the TEY mode at room temperature. The experimental configuration is shown as inset in (a). The intensity of the O-K edge XAS spectra in (b) are normalized at 547.7 eV. c Fitting and (d) Integrated intensities of the O K-edge pre-edge region (from 528 to 537.2 eV) with subtraction of the linear backgrounds for Ti₂O₃ polymorphs. The hybridization factor (H.F.) for O_2p-e_g and O_2p-t_2g is obtained by calculating the integrated intensities of the fitted e_g (blue lines in c) and t_2g (red lines in c) curves. Source data are provided as a Source Data file

Integrated intensity of the O K-edge pre-edge region with the subtraction of linear backgrounds is commonly used to quantify the hybridization strength of the M–O orbitals in oxides^11,21. Here, we define a hybridization factor (H.F.) as the integrated intensities of the XAS O K-edge pre-edge region (from 528 to 537.2 eV) with the subtraction of the linear backgrounds (shaded area in Fig. 4c). As a consequence, Ti–O hybridization is the strongest in γ-Ti₂O₃ with the smallest Δ, while that is the weakest in α-Ti₂O₃ with the largest Δ (Fig. 4d). The same conclusion about the Ti–O hybridization can be obtained by the X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 8, Supplementary Table 1 and Supplementary Note 4). Stronger Fe 3d –O 2p hybridization was observed in the γ-Fe₂O₃ than that in the α-Fe₂O₃⁵⁹, indicating that the stronger M–O hybridization can be induced by the crystal structure and electronic structure of the γ-phase. As we know, smaller Δ with stronger hybridization leads to the delocalization of electrons, and thus increases the conductivity of oxides^48–50.

Electronic transport properties of Ti₂O₃ polymorphs

As expected, various electronic transport behaviors (Fig. 5) are observed in the Ti₂O₃ polymorphs with their electronic reconstructions, revealing strong lattice–charge coupling. As shown in Fig. 5a, all Ti₂O₃ polymorphs show very low resistivity, and semiconducting behavior is dominant for all below 400 K (except a metal–insulator transition is observed at 360 K for o-Ti₂O₃). The kink feature in the resistivity of the newly stabilized γ-Ti₂O₃ at around 140 K reveals a semiconductor–semiconductor transition (SST), which is similar to that of o-Ti₂O₃²⁹ and Mg[Ti₂]O₄⁶⁰ caused by the instability of Ti–Ti dimerization. Interestingly, α-Ti₂O₃ shows a nearly flat resistivity below 20 K, while a sudden drop (superconductivity)²⁹ and continuous increase were observed in that of o-Ti₂O₃ and γ-Ti₂O₃, respectively (inset of Fig. 5a), indicating the diversity and complexity of Ti₂O₃. In addition to the electron–electron interaction, the electron–phonon interaction is also an important factor in determining the electronic behaviors of the Ti₂O₃ polymorphs^29,54, which makes Ti₂O₃ more complicated and interesting. The change of the resistivity from 400 to 2 K in γ-Ti₂O₃ is more than one order of magnitude, which is larger than that of α-Ti₂O₃ and o-Ti₂O₃. Moreover, the transport behavior of γ-Ti₂O₃ is consistent even when its thickness decreases to 20 nm (Supplementary Fig. 9).

Fig. 5.

Electronic transport properties of Ti₂O₃ polymorphs. a Resistivity, b carrier concentration, and c carrier mobility of α-, o-, and γ-Ti₂O₃ polymorphs. The inset in (a) shows the resistivity at temperatures below 20 K. Source data are provided as a Source Data file

Furthermore, the carrier information of the Ti₂O₃ polymorphs was investigated by Hall-effect measurement. Interestingly, o-Ti₂O₃ and γ-Ti₂O₃ are confirmed to be n-type semiconducting, whereas α-Ti₂O₃ is a p-type semiconductor. As shown in Fig. 5b, high electron concentrations at the scale of ~10²² cm⁻³ are observed in o-Ti₂O₃ and γ-Ti₂O₃, which could contribute to the strong Drude absorptions in Fig. 3d, e. Surprisingly, an unexpected increase is observed in the carrier concentration of o-Ti₂O₃ at T < 250 K, while that of γ-Ti₂O₃ decreases continuously below 400 K. Increase of the carrier concentration in o-Ti₂O₃ may be correlated to its superconductivity at lower temperatures, which needs further investigations. Lower carrier concentration (~10²¹ cm⁻³) is observed in α-Ti₂O₃, which is about one order of magnitude lower than in o-Ti₂O₃ and γ-Ti₂O₃. However, the carrier mobility (>10 cm² V⁻¹ s⁻¹) in α-Ti₂O₃ is about one order of magnitude higher than those in o-Ti₂O₃ and γ-Ti₂O₃ (Fig. 5c). It should be noted that higher electron concentration leads to more carrier scattering, which results in the lower electron mobility in o-Ti₂O₃ and γ-Ti₂O₃. Besides, the more significant change of the resistivity (400–2 K) in γ-Ti₂O₃ results from the constructive effect between the decreased carrier concentration and mobility, while destructive effects between the carrier concentration and mobility are observed in the α-Ti₂O₃ and o-Ti₂O₃ that results in the smaller change of the resistivity (400–2 K). More details are shown in Supplementary Note 5. Notably, the carrier concentrations of Ti₂O₃ polymorphs at 300 K increases with smaller Δ (Fig. 3f) that result in more delocalized electrons, consistent with the previous scenario^48–50.

Correlation between HER activity and electronic reconstructions

In order to elucidate the relationship between the electronic reconstructions and HER catalytic activity, Ti₂O₃ polymorphs were used directly as the working electrodes for HER measurements in 0.5 M H₂SO₄. The configuration of the HER electrochemical cell is schematically illustrated as the inset of Fig. 6a. Obviously, the electronic reconstructions in Ti₂O₃ influenced the HER activities significantly, representing strong polymorph dependence. The influence of the surface microstructure or defect on the observed polymorph-dependent HER performance in Ti₂O₃ is discussed and ruled out in Supplementary Note 6. As shown in Fig. 6a, b, the newly stabilized γ-Ti₂O₃ has the best activity with the smallest Tafel slope (199 mV dec⁻¹), while the bulk-phase α-Ti₂O₃ is the least active HER catalyst with the largest Tafel slope (241 mV dec⁻¹). Noticeably, the γ-Ti₂O₃ displays the smallest overpotential of 271 mV (Fig. 6c) to produce a current density of 10 mA cm⁻², which is a ~45% reduction compared with that of the α-Ti₂O₃ (495 mV). The Tafel plots observed here are a little bit larger than those of the nanostructured oxide samples^22,23, which would be caused by the limited surface area of our film samples. Thus, further improvements could be expected by nanostructure-array fabrications on the films with increased surface areas.

Fig. 6.

Electrocatalytic hydrogen-evolution activities of Ti₂O₃ polymorphs. a Linear sweep voltammetry (LSV) and (b) corresponding Tafel plots of the LSV curves for α-, o-, and γ-Ti₂O₃ polymorphs (electrolyte: 0.5 M H₂SO₄, scan rate: 5 mV s⁻¹). c Overpotential of HER for Ti₂O₃ polymorphs (@ −10 mA cm⁻²), deduced from (a). d Correlations between the physical parameters and HER activity in Ti₂O₃ polymorphs. Source data are provided as a Source Data file

Impressively, the enhanced HER activities are observed in those epitaxially stabilized phases (o- and γ-Ti₂O₃) that do not exist in bulk form, which may be applicable to other oxide materials. Hence, efficient TMO-based HER catalysts could be achieved by the selective stabilization of bulk-absent polymorphic phases. Most importantly, the HER activity of the Ti₂O₃ polymorphs are strongly correlated to their electronic structures. As shown in Fig. 6d, the overpotentials of HER decreases with smaller Δ accompanied with stronger Ti–O hybridization, unambiguously demonstrating the HER activity is highly enhanced by the electronic reconstructions. Noteworthily, the Faradaic resistances of Ti₂O₃ polymorphs, obtained from the electrochemical impendence spectra (Supplementary Fig. 10), do not follow the trend of their HER performances, indicating the enhanced activity is not simply derived from their electrical conductivity, but indeed from the electronic reconstructions. The overall HER catalytic activity of γ-Ti₂O₃ may not be comparable with those of the state-of-the-art noble metals (e.g., Pt/C), but considering the tiny weight of the γ-Ti₂O₃ film (~100 μg for 300 nm on 1  × 1 cm substrates), the activity of γ-Ti₂O₃ is still remarkable as an oxide material. (More discussion is presented in Supplementary Note 7). Moreover, the correlation between the electronic reconstructions and HER activities in the Ti₂O₃ polymorphs are robust.

Subsequently, the mechanism behind the electronic-reconstruction enhanced HER activity in Ti₂O₃ is explored. In general, a multi-step electrochemical process would take place, during the HER, on the surface of the electrocatalyst, where gaseous hydrogen (H₂) is generated. In acid solution, the HER would proceed via either Volmer–Heyrovsky or Volmer–Tafel pathways⁶¹ (Volmer: ${\mathrm{H}}^{+}+\mathrm{M}+{\mathrm{e}}^{-}\leftrightharpoons \mathrm{M}-{\mathrm{H}}^{*};$ Heyrovsky: $\mathrm{M}-{\mathrm{H}}^{*}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\leftrightharpoons \mathrm{M}+{\mathrm{H}}_2;$ Tafel: 2 $\mathrm{M}-{\mathrm{H}}^{*}\leftrightharpoons 2\mathrm{M}+{\mathrm{H}}_2$, where H^* designates a hydrogen atom chemically adsorbed on the active sites of the electrocatalyst surface (M)). The hydrogen adsorption free energy (ΔG_H*) is a well-known descriptor for the HER activity, and its optimal value |ΔG_H*| should be zero, indicating the H^* adsorption is neither too strong nor too weak⁶¹. Because the H^* adsorption is excessively strong on the oxygen atoms, TMOs are usually considered to be inactive HER electrocatalysts. However, the H^* adsorption on oxides could be weakened by increasing the covalency of the M–O bonds, which results in an activated or enhanced HER efficiency²³.

DFT simulations

To gain an in-depth understanding of the observed polymorph-dependent HER in Ti₂O₃, we conducted the density functional theory (DFT) simulations to calculate the ΔG_H* of the Ti sites on the Ti₂O₃ polymorphs’ surfaces. Since the oxygen atoms cannot provide extra electrons to H, the HER-active sites of Ti₂O₃ are the Ti sites (Ti³⁺) with the unpaired 3d¹ electrons. The optimized models of H adsorbed on the surfaces of Ti₂O₃ polymorphs are shown in the Supplementary Fig. 11. H prefers to adsorb at the top site of the Ti atom on the α-Ti₂O₃ (0001) surface with a bond length of d_H−Ti = 1.718 Å, while it prefers to adsorb at the bridge sites of the Ti atoms on the o-Ti₂O₃ (011) and γ-Ti₂O₃ (001) surfaces resulting in bond lengths of d_H−Ti = 1.908 Å and d_H−Ti = 1.915 Å, respectively. The shortest Ti–H bond length on the α-Ti₂O₃ (0001) surface implies the strongest bond strength among those adsorption surfaces.

We further analyzed the bond formation between the adsorbed H and Ti atoms by calculating the charge accumulation and depletion around H. The charge density difference is determined by using the formula $\Delta \rho ={\rho }_{\left({\mathrm{H+Ti}}_2{\mathrm{O}}_3\right)}-\left({\rho }_{\mathrm{H}}+{\rho }_{{\mathrm{Ti}}_2{\mathrm{O}}_3}\right)$, where ${\rho }_{\left({\mathrm{H+Ti}}_2{\mathrm{O}}_3\right)}$, ρ_H, and ${\rho }_{{\mathrm{Ti}}_2{\mathrm{O}}_3}$ represent the charge density of the H adsorbed on the Ti₂O₃ surface, isolated H, and the clean Ti₂O₃ surface, respectively. The calculated charge density difference for H adsorbed on different Ti₂O₃ polymorphs’ surfaces are shown in Fig. 7a–c, where the magenta regions show the electron accumulation (bonding states) while the yellow regions show electron depletion (antibonding states). As shown in Fig. 7a–c, there is depletion of electron density on the Ti atoms and accumulation of electron density around the H atoms on all Ti₂O₃ surfaces. That is, there is electron charge transfer from Ti to H. The adsorbed H gains electrons from its bonded Ti atoms, leading to the electron accumulation at H and depletion at Ti atoms. Apparently, more yellow regions (electron depletion) accumulate near the adsorbed H on the γ-Ti₂O₃ (001) surface (Fig. 7c) than those on the α-Ti₂O₃ (0001) and o-Ti₂O₃ (011) surfaces, demonstrating the most antibonding states are generated when H adsorbs on the γ-Ti₂O₃ (001) surface, which can weaken the adsorption energy of H.

Fig. 7.

DFT simulations. a–c Electron charge density differences for H adsorbed on the α-Ti₂O₃ (0001), o-Ti₂O₃ (011), and γ-Ti₂O₃ (001) surfaces, respectively. The surface planes for calculations are chosen based on the experimental orientations of the Ti₂O₃ epitaxial films (Fig. 1f). The red, blue, and green spheres represent oxygen, titanium, and hydrogen atoms, respectively. The isosurface is taken as 0.002 e Å⁻³. The electron charge accumulation (bonding states) and depletion (antibonding states) are represented by the magenta and yellow regions, respectively. d Calculated Gibbs free energy of H on the surfaces of Ti₂O₃ polymorphs. Source data are provided as a Source Data file

The top valence band of Ti₂O₃ (Fig. 3b), just below the Fermi level, is derived from the Ti 3d orbitals (a_1g band), whose center is the so called d-band center in the d-band theory⁶². Based on Hammer et al.’s work⁶², the energy of the d-band center (E_d) with respect to the Fermi level is the critical factor which determines the strength of the interaction between the metal and the adsorbate. Impressively, the E_d of the H-adsorbed α-Ti₂O₃ (0001), o-Ti₂O₃ (011), and γ-Ti₂O₃ (001) surfaces are calculated to be −1.39, −1.53, and −1.97 eV (Supplementary Fig. 12a), respectively. The difference of the d-band center in Ti₂O₃ polymorphs can be attributed to their distinct hybridization strength between the Ti 3d and O 2p orbitals. Since the O 2p band is further away from the Fermi level than the Ti 3d (a_1g) band (Fig. 3b), stronger Ti–O hybridization could make the d-band center downshift away from the Fermi level. Thus, a lower d-band center (E_d) is achieved with stronger Ti–O hybridization (Supplementary Fig. 12b). Meanwhile, the lowered d-band center will result in an increased filling of the antibonding states⁶³, which weakens the strength of the Ti–H bond and thus decreases the adsorption energy of H.

Finally, the calculated chemisorption energy of H on the γ-Ti₂O₃ (001) surface is −0.593 eV, while those on the o-Ti₂O₃ (011) and α-Ti₂O₃ (0001) surfaces are −0.809 and −0.979 eV, respectively, which is consistent with the results of the optimized Ti–H bond lengths and the d-band center. Considering the correction value of 0.216 eV (ΔE_ZPE − TΔS_H), ΔG_H* on the surfaces of Ti₂O₃ polymorphs are obtained. As shown in Fig. 7d, ΔG_H* is −0.376, −0.593, and −0.763 eV on the surfaces of γ-Ti₂O₃ (001), o-Ti₂O₃ (011), and α-Ti₂O₃ (0001), respectively, indicating that H has the strongest chemical bonding on the α-Ti₂O₃ (0001) surface and the weakest chemical bonding on the γ-Ti₂O₃ (001) surface. Our theoretical simulations are consistent with the experimental results (Fig. 6c), where it can be found that the overpotential is the smallest (0.271 V) on the γ-Ti₂O₃ (001) surface and the largest (0.495 V) on the α-Ti₂O₃ (0001) surface. It should be noted that the difference between theoretical and experimental values may arise from the theoretical perfect surface models without considering any intrinsic defects, whereas experimental samples may contain some intrinsic defects on the surfaces. Nevertheless, our calculated ΔG_H* can act as a descriptor for the experimental HER performance of the Ti₂O₃ polymorphs as they show the same tendency (Table 1).

Table 1.

Summary of the parameters of Ti₂O₃ polymorphs

Polymorphism	α-Ti2O3	o-Ti2O3	γ-Ti2O3
Structure	Trigonal	Orthorhombic	Cubic
Space group	R $\overline{3}$ c	Immm	Fd $\overline{3}$ m
Z	6	2	8
Lattice parameters	a = 5.15 Å	a = 9.39 Å	a = 8.53 Å
	b = 5.15 Å	b = 4.42 Å	b = 8.53 Å
	c = 13.64 Å	c = 2.81 Å	c = 8.53 Å
	α = 90˚	α = 90˚	α = 90˚
	β = 90˚	β = 90˚	β = 90˚
	γ = 120˚	γ = 90˚	γ = 90˚
V (Å3)	313.22	116.63	620.65
U (eV)	0.85	1.01	1.10
Δ (eV)	4.78	4.48	4.37
H.F.	9.03	9.18	9.51
S.T.	p	n	n
ρ (Ω·cm)	2.22 × 10−4	3.01 × 10−4	1.31 × 10−4
n (cm−3)	1.15 × 1021	2.06 × 1022	3.23 × 1022
μ (cm2 V−1·s−1)	24.44	1.01	1.48
Ed (eV)	−1.39	−1.53	−1.97
ΔGH* (eV)	−0.763	−0.593	−0.376
Overpotential (V)	0.495	0.442	0.271

S.T. semiconducting type, H.F. hybridization factor

The electronic transport parameters are obtained from Fig. 5 at 300 K

Discussion

In summary, we systematically studied the strong correlations between the polymorphism (lattice symmetry), electronic structure, and HER activity in the epitaxial Ti₂O₃ films with three different phases. A bulk-absent cubic Ti₂O₃ polymorph is successfully grown via epitaxial stabilization using PLD, which further extends the polymorphism of Ti₂O₃. Distinct polymorph-dependent electronic structures and properties are observed in Ti₂O₃, indicating strong structure–property correlations. More importantly, the electronic reconstructions (varied U and Δ) observed in the epitaxially stabilized Ti₂O₃ polymorphs (orthorhombic and cubic) impact significantly on their HER catalytic activities. Decreased Δ leads to a substantial enhancement in the HER performance of γ-Ti₂O₃, reducing the overpotential by ~45%, with strongest Ti–O hybridization (lowest d-band center). Thus, epitaxial stabilization of bulk-absent polymorphs is demonstrated to be an effective way to discover more efficient HER electrocatalysts in oxides. Moreover, we envision that greater enhancement of the HER activity could be realized by further decreasing the Δ via doping, or increasing the surface area via nanostructure-array fabrications. Our work provides an unambiguous descriptor for the HER activities of Ti₂O₃ polymorphs, which could be applied to other strongly correlated TMO systems.

Methods

Fabrication of Ti₂O₃ epitaxial films

All Ti₂O₃ films were deposited on single-crystal substrates using pulsed laser deposition (PLD), with a 248 -nm Laser (KrF, Coherent). The thickness of the films is ~300 nm. α- and o-Ti₂O₃ films were deposited on (0001) Al₂O₃ substrates at 600 °C and 900 °C, respectively²⁹. The phase separation between α- and o-Ti₂O₃ was controlled by deposition temperature. γ-Ti₂O₃ films were deposited on (001) SrTiO₃ and (001) LAO substrates at 600 °C. The pressure of the PLD chamber was lower than 3.0 × 10⁻⁶ Torr. The energy density of the laser on the target was fixed to ~2 J cm⁻². The same corundum α-Ti₂O₃ target (Sigma-Aldrich 99.99%) was used for all films’ deposition.

Structural characterizations

X-ray diffraction patterns were recorded using a Bruker D8 DISCONVER high-resolution diffractometer, which is equipped with Cu Kα radiation source and LynxEye detector. During the measurements, the diffractometer was operated at 35 kV and 50 mA. In-plane φ scans were performed using the synchrotron-based XRD in the Singapore Synchrotron Light Source (SSLS) with a step size of 0.02°. STEM-HAADF images and EELS spectra were collected using the JEOL-ARM200F microscope equipped with an ASCOR aberration corrector, operated at 200 kV. The cross-section TEM samples were prepared by the focused ion beam technique. The EELS line scans were collected with an energy resolution of ~0.1 eV and a spatial resolution of ~0.4 nm.

Synchrotron-based XAS measurements

All the XAS data were taken in an ultra-high-vacuum chamber with a base pressure of ~1 × 10⁻¹⁰ mbar at the Surface, Interface and Nanostructure Science (SINS) beam-line⁶⁴ of SSLS. The XAS data were obtained by using linear polarized X-rays impinging at an incidence angle of Ѳ = 90° from the sample surface (Fig. 4a) at the Ti L_2,3-edge and O K-edge regions. The data were obtained by averaging 20 XAS spectra. All spectra were recorded at room temperature (300 K) using the total electron yield (TEY) mode. The photon energy was calibrated using a standard gold sample in the chamber. In order to record the intrinsic bulk electronic structures of Ti₂O₃ polymorphs, samples measured by the synchrotron-based XAS were sputter-cleaned before the measurements.

Spectroscopic ellipsometry and light absorption measurements

The ellipsometry parameters Ψ (the ratio between the amplitude of p- and s-polarized reflected light) and Δ (the phase difference between of p- and s-polarized reflected light) were measured using spectroscopic ellipsometer with a photon range of 0.5‒4 eV at incident angles 60°, 65°, and 70° at room temperature. The optical conductivity of Ti₂O₃ films was extracted from the parameters Ψ and Δ utilizing an air/Ti₂O₃/Al₂O₃ (or air/Ti₂O₃/LaAlO₃) multilayer model, where the Ti₂O₃ films were considered as average homogeneous and uniform mediums. The light absorption of the Ti₂O₃ films was measured by a UV–Vis spectrophotometer (Shimadzu SolidSpec-3700) in the transmission mode. Backgrounds from the Al₂O₃ and LaAlO₃ substrates were all subtracted.

Electronic transport and electrochemical measurements

The electronic transport properties of the Ti₂O₃ films were measured in a Quantum Design physical property measurement system (PPMS). The resistivity and Hall effect were collected using the Van der Pauw geometry. The sample size was 5  × 5 mm. Al wires were used to connect the samples and the PPMS puck. The electrochemical measurements were performed in a three-electrode electrochemical cell with a standard VMP3 electrochemical workstation (Bio-logic Inc) at room temperature. During the HER measurements, Hg/HgO and graphite plate were used as the reference and counter electrodes, respectively, while the Ti₂O₃ polymorphs films on 10 × 10 mm² single-crystal substrates were acting as the working electrodes directly. The polarization curves were recorded in 0.5 M H₂SO₄ with a scan rate of 5 mV s⁻¹. All potentials were calibrated with respect to the reversible hydrogen electrode (RHE) using the equation⁶⁵:

$$E_{\mathrm{vsRHE}}=E_{\mathrm{vsHg∕HgO}}+0.059\times \mathrm{pH}+0.098$$ 1

where E_vsHg/HgO was the potential measured against the Hg/HgO reference electrode.

First-principles calculations

All the calculations were carried out using DFT + U (U_eff = U − J = 1.9 − 2.3 eV)⁶⁶ with the generalized Perdew–Burke–Ernzerhof (PBE)⁶⁷ and the projector augmented-wave (PAW) pseudopotential plane-wave method⁶⁸ as implemented in the VASP code⁶⁹. For the PAW pseudopotentials, we considered 1s¹ for H, 3d³4s¹ for Ti, and 2s²sp⁴ for O. The Monkhorst–Pack (MP) k-point grids of 6 × 6 × 2, 2 × 6 × 8, and 2 × 2 × 1 were used for α-Ti₂O₃, o-Ti₂O₃, and γ-Ti₂O₃ unit cell geometry optimization calculations with a plane-wave basis set with an energy cutoff of 500 eV, respectively. Good convergence was obtained with these parameters, and the total energy was converged to 1.0 × 10⁻⁶ eV per atom, as well as the stress exerted on the cell is less than 0.1 kbar and the forces exerted on the atoms are less than 0.01 eV/Å. Optimized unit cells were used to build surface models for H Gibbs free energy calculations (Supplementary Fig. 11). For H adsorption calculations, we cleaved a (0001) slab to build the surface model and expand to 2 × 2 × 1 supercell for α-Ti₂O₃, a (011) slab to build the surface model and expand to 2 × 2 × 1 supercell for o-Ti₂O₃, and a (001) slab to build the surface model for γ-Ti₂O₃ according to the experimental results. All the surface models have a vacuum separation of ~15 Å along the z-direction. We carried out calculations with the van der Waals (vdW) correction by employing optPBE-vdW functional⁷⁰ using a 2 × 2 × 1 MP k-point grid. The Gibbs free energy of H was calculated by using $\Delta {\mathrm{G}}_{{\mathrm{H}}^{*}}=\Delta E_{\mathrm{H}}+\Delta E_{\mathrm{ZPE}}-T\Delta S_{\mathrm{H}}$. Hydrogen chemisorption energy ΔE_H is computed using $\Delta E_{\mathrm{H}}=E_{\left(\mathrm{surface}+{\mathrm{H}}^{*}\right)}-E_{\left(\mathrm{surface}\right)}-\frac{1}{2}{E}_{{\mathrm{H}}_2}$, where $E_{\left(\mathrm{surface}+{\mathrm{H}}^{*}\right)}$ and $E_{\left(\mathrm{surface}\right)}$ are the total energies of the surface with one adsorbed hydrogen atom and the clean surface, respectively, $E_{{\mathrm{H}}_2}$ is the energy of hydrogen gas phase. $\Delta E_{\mathrm{ZPE}}$ and $\Delta S_{\mathrm{H}}$ are the differences in zero-point energy (ZPE) and entropy between the adsorbed ${\mathrm{H}}^{*}$ and gas phase H₂. The calculated correction value of $\Delta E_{\mathrm{ZPE}}-T\Delta S_{\mathrm{H}}$ at the temperature (T) of 300 K is ~0.216 eV in this study.

Author contributions

Y.L. and J.S.C. conceived the project. Y.L. designed the experiments, and performed the samples fabrication, characterization, data analysis, and interpretation. L.W. performed the electrochemical measurements. L.M.W., S.W., J.W.C., D.W., and T.V. assisted with the optical measurements. C.S.T., X.M.Y., A.T.S.W., J.W., K.H., and X.R.W. assisted with the XRD measurements. X.J.Y. and M.B.H.B. carried out the XAS measurements. H.W. and S.J.P. assisted with the TEM measurements. Z.G.Y. and Y.W.Z. performed the theoretical calculations. Y.W. and S.D. assisted with the theoretical calculations. Y.L., J.M.X., and J.S.C. discussed the data interpretation. Y.L. and J.S.C. wrote the paper, and all authors commented on the results and the paper.

Data availability

All relevant data presented in this paper are available from the authors upon reasonable request. The source data underlying Figs 1f, 3c–f, 4–6, 7d and Supplementary Figs 6, 8-10, 12b are provided as a Source Data file.

Competing interests

The authors declare no competing interests.

Supplementary information

Supplementary Information accompanies this paper at 10.1038/s41467-019-11124-w.