Phase transition-induced band edge engineering of BiVO₄ to split pure water under visible light

Significance

Hydrogen has been recognized as one of the most promising energy carriers for the future, because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO₄ by simultaneously substituting In³⁺ for Bi³⁺ and Mo⁶⁺ for V⁵⁺ in the host lattice of monoclinic BiVO₄, which induced partial phase transformation from pure monoclinic BiVO₄ to a mixture of monoclinic BiVO₄ and tetragonal BiVO₄.

Abstract

Through phase transition-induced band edge engineering by dual doping with In and Mo, a new greenish BiVO₄ (Bi_1-XIn_XV_1-XMo_XO₄) is developed that has a larger band gap energy than the usual yellow scheelite monoclinic BiVO₄ as well as a higher (more negative) conduction band than H⁺/H₂ potential [0 V_RHE (reversible hydrogen electrode) at pH 7]. Hence, it can extract H₂ from pure water by visible light-driven overall water splitting without using any sacrificial reagents. The density functional theory calculation indicates that In³⁺/Mo⁶⁺ dual doping triggers partial phase transformation from pure monoclinic BiVO₄ to a mixture of monoclinic BiVO₄ and tetragonal BiVO₄, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening.

Photocatalytic water splitting with a particulate semiconductor powered by sunlight is an ideal route to cost-effective, large-scale, and sustainable hydrogen production because of its extreme simplicity. However, it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability (1–5). Thus, reproducible photocatalytic systems for visible light-driven overall water splitting (OWS) by one-step photoexcitation are also rare, although there were several reports of such systems (4–6). In the best-known successful case, Domen and coworkers (5) reported in 2005 that a solid solution of GaN and ZnO [(Ga_1−xZn_x)(N_1−xO_x)] was a stable photocatalyst that could split water into H₂ and O₂ under visible light when modified with a cocatalyst. This system remains the most active and reproducible one-step OWS photocatalyst responsive to visible light so far (4).

Scheelite monoclinic (m-) BiVO₄ is a well-documented photocatalyst having suitable band gap energy (E_g ∼ 2.4 eV) for absorbing visible light (7–10). Also, it is chemically stable in aqueous solution under light irradiation. Thus, it functions as an excellent photocatalyst for O₂ evolution under visible light in the presence of an appropriate electron acceptor (e.g., AgNO₃). However, because the bottom of its conduction band is located at a more positive potential than the potential of water reduction [0 V_RHE (reversible hydrogen electrode) at pH 7], it is incapable of evolving H₂. In addition, it shows poor charge transport characteristics (11) and weak surface adsorption properties (12), causing low photocatalytic activity. To overcome these weaknesses, a variety of strategies, such as heterojunction structure formation (11, 13, 14), loading cocatalysts (8, 15–17), and impurity doping (1, 7, 12, 18–23), has been attempted. These strategies were successful in improving BiVO₄’s oxidation capability for photoelectrochemical water oxidation (1, 11, 13, 15, 19, 24–28) as well as the Z scheme (two-photon excitation) water splitting system (29). Also, cocatalyzed Bi_xY_1-xVO₄ (x ∼ 0.5) (7, 8) and cocatalyzed Bi_0.5La_0.5VO₄ (23) promoted OWS by raising the conduction band edge (CBE) position, but OWS under visible light irradiation over BiVO₄-based photocatalysts has not been fully shown.

To meet this challenge, we developed greenish BiVO₄ (GBVO_x; x = atom ratio of In and Mo), Bi_1-xIn_xV_1-xMo_xO₄, by simultaneously substituting In³⁺ for Bi³⁺ and Mo⁶⁺ for V⁵⁺ in the host lattice of m-BiVO₄. The new GBVO_x photocatalyst has a slightly larger band gap energy than the usual yellow scheelite m-BiVO₄ as supported by the unique color change to green and a higher (more negative) conduction band than H⁺/H₂ potential (0 V_RHE at pH 7). Consequently, as depicted in Fig. 1, GBVO_x is able to split water into H₂ and O₂ under visible light irradiation without using any sacrificial reagents (e.g., CH₃OH or AgNO₃). Herein, we report the dual-metal doping effects on the optical absorption behavior, crystal structure, and electronic band structure of BiVO₄, which led to one-photon OWS under visible light irradiation. We elucidate the physical origin of the augmented photoresponse behaviors of GBVO_x through density functional theory (DFT) calculation of electronic structure as well as a variety of physical and electrochemical characterizations.

Fig. 1.

OWS reaction mechanism by GBVO_x.

Results and Discussion

Physical Properties of GBVO_x.

The magnified X-ray diffraction (XRD) patterns of pristine BiVO₄ and four GBVO_x samples with different target atom ratios (x = 0.02, 0.05, 0.10, and 0.15) are shown in Fig. 2A (more details are in SI Appendix, Fig. S1 A–D) to identify the evolution of the crystal structure by doping. Split peaks around 35° are merged into one peak as the dopant ratio (x) increases, indicating phase change of BiVO₄ from m to tetragonal (t) (11). Furthermore, Rietveld analysis was performed using X'pert Plus 3.0 software (PANalytical) to determine the fraction of m- and t-phases as illustrated in Fig. 2B and SI Appendix, Fig. S2. As the dopant ratio (x in GBVO_x) increases, the crystal structure evolves from almost pure m-BiVO₄ (clinobisvanite; space group: 12/a/(15); JCPDS card no. 014–0688) to a mixture of m- and t-BiVO₄, where t-BiVO₄ fraction reaches a maximum of 60% at x = 0.10. This doping-induced crystal structure evolution results in unit cell volume growth (Fig. 2B) that generates compressive lattice strain, which is discussed in SI Appendix, section S4 and visualized in SI Appendix, Fig. S1 E and F. In particular, the compressive lattice strain is the key physical driving force to lift the conduction band of GBVO_x over H⁺/H₂ potential (0 V_RHE at pH 7), taking collectively both experimental (SI Appendix, Fig. S1 E and F) and theoretical findings (as discussed later).

Fig. 2.

Photophysical characterization of GBVO_x. (A) Magnified view of powder XRD patterns of pristine BiVO₄ and all GBVO_x samples. (B) Crystal-phase diagram with calculated unit cell volume for different x values of GBVO_x. (C) UV/Vis absorption spectra of pristine BiVO₄ and all GBVO_x samples. Inset shows color change of the samples. (D) Mott–Schottky plot of pristine BiVO₄, GBVO_0.05, GBVO_0.10, and GBVO_0.15.

The UV/Vis (visible) absorption spectra of all samples are shown in Fig. 2C. With higher x values of GBVO_x, the greater blue shift is observed, indicating that the optical band gap energy of GBVO_x increases with dopant level, whereas the blue shift is not caused by the single Mo doping into BiVO₄ (SI Appendix, Fig. S4F). Actually, the blue shift trend is consistent with the band gap energy transition estimated by extrapolating the linear part of the (αhυ)²- vs. hυ-plot to the energy axis (1) as shown in SI Appendix, Fig. S3. According to the Tauc plot, the band gap energy increases from 2.43 eV of pristine BiVO₄ to 2.50 eV of GBVO_0.10. Furthermore, the alteration in the optical band gap energy corresponds to the color evolution of GBVO_x powder samples from yellow to greenish as shown in Fig. 2C, Inset and SI Appendix, Fig. S4. Thus, the ultimate physical driving force for the apparent color evolution is also the compressive lattice strain that widens the band gap by lifting the conduction band. The Mott–Schottky plot in Fig. 2D that displays the conduction band of GBVO_0.10 is elevated to higher than 0 V (vs. RHE at pH 7) by 97.2 mV or ∼151 mV from that of pristine BiVO₄ (a more clear visualization of the contrast between schematic band structures of pristine BiVO₄ and GBVO_0.10 is in SI Appendix, Fig. S5). This finding indicates that photocatalytic water reduction, impossible with pristine BiVO₄, becomes possible with GBVO_0.10, because the dual doping lifts the CBE above the proton reduction potential of 0 V_RHE (at pH 7). For the valance band of GBVO_0.10, it is 81 mV higher than that of pristine BiVO₄, but it is still far below the water oxidation potential of 1.23 V_RHE (at pH 7) as shown in SI Appendix, Fig. S5.

The field emission SEM images in SI Appendix, Fig. S6 reveal that all samples consist of irregular polyhedrons of unsystematic size distribution, typical of powder samples synthesized by solid-state reactions. The real atomic ratios in all GBVO_x samples determined by the inductively coupled plasma analysis in SI Appendix, Table S1 are close enough to the target values. The high-resolution transmission EM (HR-TEM) and energy-dispersive X-ray spectroscopy elemental mapping in SI Appendix, Figs. S6–S10 show that Bi, V, In, and Mo are homogeneously distributed within the GBVO_x particle. All of these characterization results univocally prove that In³⁺ and Mo⁶⁺ have been effectively inserted into the BiVO₄ lattice. The HR-TEM image in SI Appendix, Fig. S9F illustrates the lattice spacing of 0.450 nm corresponding to the interplanar spacing of the (040) plane of GBVO_0.10. XPS binding energies in SI Appendix, Fig. S11 indicate that the involved elements are all in their stable oxidation states of Bi³⁺, V⁵⁺, Mo⁶⁺, and In³⁺.

OWS Under Visible Light.

The OWS reaction was carried out under visible light irradiation (λ ≥ 420 nm; 500-W Hg-arc lamp) with 0.3 g photocatalyst powder dispersed in 100 mL distilled water at pH 7. In phase I shown in Fig. 3, GBVO_0.10 (the most active GBVO_x) showed stoichiometric H₂/O₂ evolution even without modification by any cocatalyst. Hence, the GBVO_x can split pure water by visible light-driven OWS without any sacrificial reagents or additives. In particular, this phenomenon represents the first example, to our knowledge, of a pure water-splitting photocatalyst responding to visible light without any metal cocatalyst. Next, we purged the system with N₂ and tested the OWS reaction after photodepositing 3 wt% RuO₂ on GBVO_0.10 as a cocatalyst. The photocatalytic activity was significantly improved by adding an RuO₂ cocatalyst that collects electrons/holes and provides active sites for catalytic water reduction and/or oxidation. The other common cocatalysts, such as Pt, Rh, Rh_xCr_yO₃, and Pt_xCr_yO₃ (4–6), were much less effective on GBVO_0.10. Phases II–IV present repeated runs of the same catalysts to show the stability of GBVO_0.10. In phase V, heat treatment (623 K in air for 1 h) further intensified the photocatalytic activity of RuO₂-cocatalyzed GBVO_0.10. This treatment was aimed at converting the photodeposited RuO₂•xH₂O on GBVO_0.10 into the more stable and active form of cocatalyst (i.e., RuO₂) (30).

Fig. 3.

OWS by GBVO_0.10 under the visible light (λ ≥ 420 nm) irradiation. Solid and dashed lines indicate evolved H₂ and O₂, respectively. Baseline represents a control experiment by pristine BiVO₄. Phase I, unmodified GBVO_0.10; phases II–IV, 3 wt% RuO₂/GBVO_0.10; phase V, heat treated 3 wt% RuO₂/GBVO_0.10.

The rate of hydrogen evolution (∼17 μmol h⁻¹) from this 3 wt% RuO₂ on GBVO_0.10 could be compared with that of the best-known example [∼56 μmol h⁻¹; i.e., 5 wt% RuO₂-loaded (Ga_1−xZn_x)(N_1−xO_x) under visible light irradiation (λ ≥ 400 nm; 450-W Hg-arc lamp)] (5). The activity of our photocatalyst corresponds to an apparent quantum yield of 3.2% under overall visible light (420–800 nm) irradiation, which was calculated from the amount of evolved hydrogen and photon flux measured by chemical actinometry with ferrioxalate (SI Appendix, section S3). The value is also compared with 5.9% at 420–440 nm for (Ga_1−xZn_x)(N_1−xO_x) photocatalysts (31). Considering that we used a longer wavelength filter (λ ≥ 420 nm), we can state that the photocatalytic activity of GBVO_0.10 is comparable with that of the best-known (Ga_1−xZn_x)(N_1−xO_x) photocatalyst. However, this work represents the first report, to our knowledge, of visible light-driven OWS over modified BiVO₄ in pure water of pH 7 without cocatalyst. This work also represents the first, to our knowledge, OWS photocatalyst based on d(0) electron configuration (V⁵⁺), whereas (Ga_1−xZn_x)(N_1−xO_x) is made of d(10) configuration (Zn²⁺ and Ga³⁺). The incident photon to current conversion efficiency was measured for unmodified GBVO_0.10 film (details are in SI Appendix, section S2). As shown in SI Appendix, Fig. S12, incident photon to current conversion efficiency of 0.5–2% is observed in 350–480 nm, with the shape of its curve well reflecting the absorption spectrum in Fig. 2C. It confirms that the photoresponse of the material corresponds to light absorption by the band to band transition (Fig. 3).

To understand the nature of the photocatalytic water splitting with GBVO_x in more detail, its half reactions were studied in the presence of sacrificial reagents. Namely, Ag⁺ ions from AgNO₃ act as electron scavengers, and CH₃OH acts as a hole scavenger:

$$2{\text{H}}_2\text{O}\left(l\right)+\hspace{0.17em}h\nu +4{\text{Ag}}^{+}\hspace{0.17em}\to \hspace{0.17em}{\text{O}}_2\left(g\right)+4{\text{H}}^{+}+4{\text{Ag}}^0\hspace{0.17em}\text{and}$$

$${\text{H}}_2\text{O}\left(l\right)+\hspace{0.17em}h\nu +{\text{CH}}_3\text{OH}\hspace{0.17em}\to \hspace{0.17em}3{\text{H}}_2\left(g\right)+{\text{CO}}_2\left(g\right).$$

In SI Appendix, Fig. S13A, O₂ evolution rates increased greatly with the single Mo doping (BiV_0.98Mo_0.02O₄). This increase is because of the improved charge transfer properties of BiVO₄ as reported earlier (21). However, the dual-metal doping improved the water oxidation activity much further, showing a maximum at the low doping level of GBVO_0.02. According to the numerical data from SI Appendix, Fig. S13A, GBVO_0.02 has dramatically increased photocatalytic water oxidation activity by factors of about 20 and 5 relative to pristine BiVO₄ and single-doped BiV_0.98Mo_0.02O₄, respectively. SI Appendix, Fig. S14 shows that the single In doping did not induce any t-phase formation. However, it increased the rate of O₂ evolution in Bi_0.99In_0.01VO₄ and reduced the rate at the higher In doping level of Bi_0.98In_0.02VO₄.

As SI Appendix, Fig. S13B exhibits, the effects of doping on H₂ evolution were quite different from those of doping on O₂ evolution. Neither the single doping of Mo⁶⁺ or In³⁺ nor the low level of dual doping (x = 0.02) produced H₂. The optimum doping level for H₂ evolution was found to be x = 0.10. This trend indicates that, when 10% of In³⁺/Mo⁶⁺ is inserted into the BiVO₄ lattice, the conduction band gains more negative (higher) potential than 0 V_RHE (at pH 7) to achieve photocatalytic water reduction. However, even with the appropriate In³⁺/Mo⁶⁺ dopant concentration (x = 0.10), the H₂ evolution rate was two orders of magnitude smaller than the O₂ evolution rate. This comparison reveals that water reduction using photogenerated electrons is the rate-determining step in the OWS reaction by GBVO_0.10. In this sense, the main role of RuO₂ is most likely to provide augmented reaction sites for H₂ evolution (water reduction).

The stability of GBVO_0.10 is supported by SI Appendix, Fig. S15C, revealing no difference in the XRD patterns of GBVO_0.10 before and after the 40-h OWS reaction. The stability is also confirmed by the turnover number of RuO₂ loaded on GBVO_0.10 (number of hydrogen atoms produced/number of Ru atoms in RuO₂ as discussed in SI Appendix, section S4). The turnover number is estimated to be 11 during the OWS reaction phases II–IV shown in Fig. 3, guaranteeing sustainable catalytic cycles on GBVO_0.10. However, at 15% In³⁺/Mo⁶⁺ dopant concentration (GBVO_0.15), the crystal structure is damaged after a 40-h OWS reaction, which is shown in SI Appendix, Fig. S15C. This inferior crystal structure stability of GBVO_0.15 is most likely caused by the too high In³⁺/Mo⁶⁺ dopant concentration of 15%. According to the volumetric strain method and the Williamson–Hall method discussed in SI Appendix, Figs. S1 E and F and S5, the CBE position is proportional to the compressive (minus) lattice stain up to 10% dopant concentration. However, the proportional relation is not valid anymore at 15% dopant concentration because of the inferior crystal structure stability at the too high In³⁺/Mo⁶⁺ doping ratio.

First Principle DFT Calculations.

Collectively taking into account all of the experimental results from a wide array of instruments and methods, this study has found that there is a significant positive correlation between the compressive (minus) lattice strain and the CBE at least up to 10% In³⁺/Mo⁶⁺ dopant concentration. This positive correlation provides the physical driving force to transform (only) O₂-evolving yellow BiVO₄ into (both) H₂/O₂-evolving GBVO_x by lifting the CBE, widening the band gap, and entailing the apparent color change. This finding based on a variety of experimental outcomes is also corroborated by the theoretical study using DFT calculations as follows.

The effect of In³⁺/Mo⁶⁺ dual doping was investigated by first principle DFT electronic structure calculation on scheelite type m-BiVO₄ (space group: I2/a) and t-BiVO₄ (space group: I41/amd). The detailed calculation procedures were based on those described in previous studies (1, 32). The lattice constants as well as atomic coordinates were simultaneously optimized, and the obtained lattice parameters for a conventional cell were a = 7.306 Å, b = 11.747 Å, c = 5.167 Å, and β = 135.003° for m-BiVO₄ and a = b = 7.307 Å and c = 6.587 Å for t-BiVO₄. To study the effect of In³⁺/Mo⁶⁺ substitution into Bi³⁺/V⁵⁺ sites, we used 96-atom BiVO₄ host supercell of 2a × b × 2c and conventional cell of √2a × √2b × 2c for m-BiVO₄ and t-BiVO₄, respectively. We examined a single pair of In and Mo atoms’ substitution (i.e., the doping concentration x = 0.0625 in BiVO₄). The calculated electronic density of states is shown in Fig. 4, where the origin of energy is set to be the valence band edge (VBE) of pristine m-BiVO₄. The energy levels of m-BiVO₄ and t-BiVO₄ with and without doping are effectively compared by aligning the deep-lying O 2s orbital levels.

Fig. 4.

Density of states of the t-BiVO₄ and m-BiVO₄ system with and without In³⁺/Mo⁶⁺ dual doping. The energy levels in different structures are aligned comparing the deep-lying oxygen 2s orbital. The Fermi level of In³⁺/Mo⁶⁺ doped structure is near CBE, which is shown with vertical dotted lines. The VBE in t-structure is −0.32 eV lower than in m-BiVO₄. In each crystal structure, doping causes band gap narrowing.

For m-BiVO₄, the calculated band gap energy is 2.02 eV without doping and 1.84 eV with doping. This calculation means that the band gap is slightly narrowed on doping. The VBE of doped m-BiVO₄ is at the same energy level as that of pristine m-BiVO₄, and the CBE is 0.18 eV lower than pristine m-BiVO₄. Both of these calculations are contradictory to the experimental findings of increased band gap energy and higher CBE induced by In³⁺/Mo⁶⁺ doping. In the case of t-BiVO₄, the band gap in the equilibrium cell volume is 2.47 eV without doping and 2.31 eV with dual doping. Although the band gap of t-BiVO₄ is larger (∼0.45 eV) than that of m-BiVO₄, the CBE is slightly higher (∼0.13 eV) than its counterpart, because the average electronic potential is deeper (work function is higher) in t-BiVO₄ compared with in m-BiVO₄. Like m-BiVO₄ mentioned above, the band gap of t-BiVO₄ narrows upon doping, and the CBE of doped t-BiVO₄ is slightly lower than that of pristine m-BiVO₄, which is also contradictory to the experimental results.

This apparent inconsistency between calculation and experiments can be solved by considering the partial phase change of m-BiVO₄ to t-BiVO₄ on dual doping. Indeed, the best-performance water splitter, GBVO_0.10, contains ∼60% of t-phase (Fig. 2B). Because the synthesized GBVO_x crystal has the mixed structure of varying fraction m- and t-phases with respect to the doping concentration, each crystal structure would not have its equilibrium lattice constants. The conventional cell volume of t-BiVO₄ (351.68 ų) containing four BiVO₄ units is quite larger than that of m-BiVO₄ (313.54 ų). The energy-level alignment and the total energy variation of m-BiVO₄ and t-BiVO₄ are illustrated with respect to the cell volume in Fig. 5. For doped m-BiVO₄ with larger cell volume, the band gap is smaller than that of pristine m-BiVO₄ with the equilibrium lattice constants. Thus, only the energy-level alignment of t-BiVO₄ is compared with the CBE and VBE of pristine m-BiVO₄, as shown in Fig. 5A. According to the comparison, compressed t-BiVO₄ with In³⁺/Mo⁶⁺ substitution has larger band gap energy as well as higher CBE position than pristine m-BiVO₄.

Fig. 5.

(A) Cell volume dependency of CBE and VBE positions in t-BiVO₄ systems. The horizontal dashed lines indicate corresponding energy positions in pristine m-BiVO₄. (B) Total energy vs. cell volume plot of m-BiVO₄ and t-BiVO₄. The points are calculation results, and the lines stand for the Birch fit to the equation of state.

This DFT calculation outcome supports the experimental finding of higher CBE. To be more specific, In³⁺/Mo⁶⁺ substitution causes partial phase transformation from pure m-BiVO₄ to a mixture of m-BiVO₄ and t-BiVO₄, and the phase transformation results in unit cell volume growth and compressive lattice strain increase. As a result of the compressive lattice strain increase, GBVO_0.10 (having the optimum mixed crystal structure of ∼60% t-BiVO₄ and ∼40% m-BiVO₄ to maximize the compressive lattice strain) can take advantage of uplifted conduction band edge to achieve OWS under visible light without any additives. The physical mechanism on how In³⁺/Mo⁶⁺ dual doping triggers the partial phase transformation from pure m-BiVO₄ to a mixture of m-BiVO₄ and t-BiVO₄ is investigated in the following discussion.

To figure out the physical reason why t-phase expands as doping concentration grows, we estimated the total energy of each case as shown in Fig. 5B. Based on the total energy values, we could also estimate the formation energy of In³⁺/Mo⁶⁺ dual dopants within m-BiVO₄ and t-BiVO₄ according to the cell volume by calculating the total energy differences between pristine BiVO₄ and doped BiVO₄. There is no ambiguity to determine the chemical potentials, because the contributions of cation chemical potentials are compensated. The formation energy of In³⁺/Mo⁶⁺ dopants in m-BiVO₄ is about 1.31 eV and independent of lattice strain, but that in t-BiVO₄ strongly depends on cell volume variation. The formation energy of In³⁺/Mo⁶⁺ dopants in t-BiVO₄ is 1.79 eV in its equilibrium cell volume (1,407 ų), which is about 0.48 eV higher than that in m-BiVO₄ at 1,253 ų. However, as the cell volume goes down, the formation energy of In³⁺/Mo⁶⁺ dopants in t-BiVO₄ decreases and eventually gets smaller than that in m-BiVO₄ when the cell volume is smaller than ∼1,320 ų. In addition, the total energy of doped t-BiVO₄ gets smaller than that of undoped t-BiVO₄ when the cell volume is smaller than 1,254 ų. This tendency suggests that, as the cell volume becomes smaller and smaller, the formation of In³⁺/Mo⁶⁺ dopants is more promoted within t-BiVO₄ rather than within m-BiVO₄, and ultimately, the compressed t-crystal structure can be more stabilized with doping than without doping. This doping-induced phase change and the subsequent rise of the CBE represent a new concept to create a visible light-active OWS photocatalyst by the band structure engineering.

In summary, through In³⁺/Mo⁶⁺ dual doping into the host lattice of m-BiVO₄, we have created a new visible light-responsive OWS photocatalyst. The resulting GBVO_x splits water into H₂ and O₂ by one-step photoexcitation, because it has an upshifted CBE transforming (only) O₂-evolving yellow BiVO₄ into (both) H₂/O₂-evolving GBVO_x, a one-photon OWS photocatalyst. The DFT calculation indicates that In³⁺/Mo⁶⁺ dual doping triggers partial phase transformation from pure m-BiVO₄ to a mixture of m-BiVO₄ and t-BiVO₄, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening. This domino effect is also corroborated experimentally. The GBVO_x is an active and stable one-photon OWS photocatalyst made of earth-abundant elements and works in pure water without any additives.

Materials and Methods

Sample Preparation.

Powder samples of GBVO_x were prepared by solid-state reaction. Each stoichiometric amount of bismuth oxide (Aldrich; 99.999%), vanadium oxide (Aldrich; 98%), indium oxide (Alfa Aeser; 99.9%), and molybdenum oxide (Aldrich; 99.99%) was mixed well by grinding in an agate mortar and manually pelletized under 2,500 psi. All pellets were preheated at 873 K for 5 h, and then, they were vigorously reground and manually repelletized as mentioned earlier. These pellets were heated again at 1,073 K for 3 h. After this main heating, they were thoroughly reground to get powder samples.

Modification of GBVO_0.10 with RuO₂.

GBVO_0.10 was modified by photodepositing 3 wt% RuO₂ (the loading had been optimized). Specifically, the photodeposition was carried out at room temperature under atmospheric pressure in a closed Pyrex glass vessel containing nitrogen-purged suspension of GBVO_0.10 powder and RuCl₃6H₂O in 50 mL methanol and 50 mL distilled water under visible light irradiation for 8 h. The light source was an Hg-arc lamp (500 W; Oriel) equipped with a UV cutoff filter (λ ≥ 420 nm). After this photodeposition, RuO₂-loaded GBVO_0.10 was vacuum-filtered, washed with distilled water, and dried in an air oven at 70 °C for 12 h. Finally, the dried powder sample was heated at 623 K for 1 h.

Physicochemical Characterization.

The structural properties of GBVO_x were investigated by using powder XRD (X’pert Pro; Phillips). Rietveld analysis was performed using X'pert Plus 3.0 software (PANalytical) to calculate the phase ratios of m-structure to t-structure in GBVO_x. The UV-visible diffuse reflectance spectra were measured with a UV-visible spectrometer (UV-2401PC; Shimadzu Co.) equipped with integrated sphere method. The particle size and morphology were examined by field emission SEM (JEOL JSM-6330F). The HR-TEM images, high-angle annular dark field images, and corresponding elemental mapping images were obtained by using Cs-corrected high-resolution scanning transmission EM (JEM 2200FS; 200 kV; JEOL) at the National Research Center for Nanomaterial Technology at Pohang University of Science and Technology in Korea. The atomic ratios of bismuth to indium and vanadium to molybdenum in GBVO_x were analyzed by using inductively coupled plasma.

Photocatalytic Activity Measurement Under Visible Light Irradiation.

The structural properties of GBVO_x were investigated by using powder XRD. The photocatalytic activities under visible light were investigated by measuring H₂ and O₂ evolution at room temperature under atmospheric pressure in a closed Pyrex glass vessel (193.5, 187, and 189 mL) containing nitrogen-purged suspension of 0.3 g photocatalyst powder in 100 mL distilled water. The light source was an Hg-arc lamp (450 W; Oriel) equipped with a UV cutoff filter (λ ≥ 420 nm). The evolved amounts of H₂ and O₂ were analyzed by a gas chromatograph (HP5890) with a thermal conductivity detector and a molecular sieve 5-A column.