Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2026 Sep 6;11(37):56645–56650. doi: 10.1021/acsomega.6c08706

Nature of the Localized V 3d Electrons in Mo- and W‑Doped BiVO4 Revealed by Soft X‑ray Photoemission and Absorption Spectroscopy

Hiroki Honda †, Manato Yamaguchi †, Koto Fujinuma †, Daisuke Takegami ‡, Daisuke Shiga §, Hiroshi Kumigashira §, Oksana Yastrubchak ∥, Naurang L Saini ⊥, Takashi Mizokawa †,*
PMCID: PMC13613902  PMID: 42799078

Abstract

We have investigated the effects of Mo and W doping on BiVO4 by means of X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The V 2p XPS spectra indicate that the substitution of W6+ and Mo6+ for V5+ induces the formation of V4+ species. In addition, the Bi 4f XPS spectra exhibit a minor Bi5+ component, which further contributes to an increase in the V4+ concentration. A V 3d-derived feature emerges at approximately 1.5 eV below the Fermi level and is associated with the enhanced catalytic activity of Mo- and W-doped BiVO4. The center of mass of the V 3d spectral distribution is shifted toward the Fermi level with increasing dopant concentration, suggesting the importance of the long-range d-d Coulomb interaction in determining the electronic structure of the localized V 3d states.

graphic file with name ao6c08706_0008.webp

graphic file with name ao6c08706_0006.webp

Introduction

Since the discovery of the Honda–Fujishima effect on photocatalytic water splitting, numerous inorganic and organic semiconductors have been reported to exhibit photocatalytic activity for hydrogen and oxygen production from water under light illumination. − Transition-metal compounds with either d 0 (such as Ti4+, V5+, and Nb5+) or d 10 (such as Zn2+) configurations often demonstrate excellent photocatalytic activities. Among them, BiVO4 is one of the most extensively studied photocatalytic materials and is generally considered to contain V5+ (3d 0) and Bi3+ (6s 2). The crystal structure of BiVO4 is illustrated in Figure a. The V ions are tetrahedrally coordinated by four oxygen ions, whereas the Bi ions are surrounded by eight oxygen ions. The structural difference between the tetragonal and monoclinic phases is relatively small. BiVO4 acts as an extremely efficient oxygen evolution catalyst in various Z scheme systems. , Cooper et al. investigated the electronic structure of the high-quality BiVO4 thin film by means of X-ray absorption, emission, and photoemission spectroscopy, and confirmed that the valence band top and the conduction band bottom are derived from the O 2p and V 3d states, respectively.

1.

1

Open in a new tab

(a) Crystal structure of tetragonal and monoclinic BiVO4 created by VESTA. (b) Powder X-ray diffraction results of W-doped, Mo-doped, and codoped BiVO4 compared with the calculated results for tetragonal and monoclinic BiVO4.

The valence state of Bi in BiVO4 remains a subject of discussion. Liu et al. reported that BiVO4 synthesized by the photoassisted sol–gel method, which exhibited enhanced photocatalytic activity, showed two Bi 4f 7/2 peaks at 159.4 and 162.6 eV, assigned to Bi3+ and Bi5+, respectively. This assignment was based on the assumption that a higher oxidation state, possessing fewer 6s electrons, provides weaker screening of the core hole and therefore results in a higher binding energy. However, because the screening effect associated with the Bi 6s electrons is complicated by lone-pair formation, this interpretation is not straightforward. Indeed, more recent studies on Bi1–x Pb x NiO3 have shown that the Bi 4f core level of Bi3+ is located around 159 eV, which is higher than the Bi 4f binding energy of Bi5+, about 158 eV. , Apart from the possible charging effect, the assignment of Bi3+ and Bi5+ by Liu et al. should be reconsidered.

In the Bi 4f photoemission spectra of Bi2MoO6, Bi3+ and Bi0 species are respectively observed around 159 and 157 eV in the reduced surface. , The position of the Bi0 peak is close to that of Bi5+, adding further complexity to the interpretation of the Bi 4f spectra. For example, in the recent studies on MA­(Pb,Bi)­I3 and MA3Bi2I9, the Bi 4f peaks of Bi3+ and Bi0/5+ are located around 159 and 158 eV, respectively.

The electron doping in the transition-metal-based d 0 semiconductors introduces correlated d electrons that exhibit intriguing chemical activities and physical properties. It is well established that the catalytic activity of BiVO4 can be enhanced by W or Mo substitution for Bi, which may correspond to electron injection into the V 3d states. − Jovic et al. performed X-ray photoemission spectroscopy on the W- and Mo-substituted systems and successfully observed the V 3d states around 1.5 eV below the Fermi level. This pioneering work was followed by an angle-resolved photoemission study by Mohamed et al., which revealed the polaronic nature of the V 3d states. More recently, Chi et al. further investigated the polaronic V 3d states in W-doped BiVO4 by means of X-ray absorption spectroscopy (XAS). Oktariza et al. studied the electronic structure of Mo-doped BiVO4 by means of XAS and X-ray photoemission spectroscopy (XPS). The doping dependence of V 2p XAS observed by Oktariza et al. is different from that reported by Chi et al. In the valence band XPS, Oktariza et al. assigned the in-gap states to the defect states rather than the polaronic states. Besides the Mo and/or W doping, oxygen vacancies are expected to introduce electrons in the V 3d conduction bands or the V 3d-derived in-gap states. The effect of oxygen vacancies on BiVO4 has been investigated by Li et al., and the Bi 4f and V 2p core levels are shifted to the lower binding energy side in their samples.

Although the polaronic or defect-induced V 3d states have been observed in the valence band photoemission spectra, their impact on the core-level spectra also includes important information about the degree of localization, screening effect on the core hole, and orbital character of the V 3d electrons. In this context, it is useful to investigate the valence band and core-level spectra of Mo- and W-doped BiVO4 in a systematic manner. In particular, the V and Bi valence changes evaluated from the core-level spectra should be compared with the V 3d spectral weight evolution near the Fermi level.

In the present work, we report XPS and XAS results of Mo- and W-substituted BiVO4 to investigate its fundamental electronic properties. The V 3d states induced by the doping are observed around 1.5 eV below the Fermi level, consistent with previous reports. − A small fraction of Bi5+ species is detected in the Bi 4f XPS spectra. The increase in the Bi5+ component is accompanied by the extra V 3d peak, indicating charge transfer from Bi to V. Furthermore, the V 3d spectral distribution changes with the doping level, suggesting the effect of the long-range Coulomb interaction between V 3d electrons.

Methods

Polycrystalline samples of W-doped, Mo-doped, and codoped BiVO4 were grown by solid-state reaction. The mixed powders were sintered at 750 °C for 5 h and were pressed into pellets, which were sintered further at 700 °C for 3 h. X-ray diffraction (XRD) experiments were performed by a RINT-Ultima III (Rigaku) at the Materials Characterization Central Laboratory at Waseda University with a Cu Kα (λ = 1.541 Å) X-ray source. The angular resolution is 0.01°.

XPS and XAS measurements were performed at the MUSASHI end-station of the BL-2A beamline in Photon Factory, KEK. The sintered samples were fractured under ultrahigh vacuum of 10–6 Pa at 300 K in order to obtain clean surfaces. The V 2p and O 1s XAS measurements were conducted in total electron yield (TEY) mode. The resolving power of the beamline is ∼10,000, and the energy resolution is about 50 meV at the V 2p and O 1s absorption edges. The XPS measurements were performed at 300 K with a photon energy of 1200 eV. The photoelectrons were collected with a hemispherical electron analyzer. The total energy resolution, including the photons and photoelectrons, was about 250 meV. The binding energy was calibrated using the Fermi edge of Au.

Results and Discussion

Figure b shows the X-ray diffraction patterns of the W-doped, Mo-doped, and codoped samples as well as the model X-ray diffraction patterns of the monoclinic and tetragonal BiVO4. For the W-doped sample, when Bi2O3, NH4VO3, and WO3 powders were mixed at a molar ratio of 10:9:1, the Bi2O3 impurity signal was observed (sample #1 in the second row of Figure b). In order to avoid the Bi2O3 impurity, the amount of Bi2O3 was reduced by about 10%, and Bi2O3, NH4VO3, and WO3 powders were mixed at a molar ratio of 9:9:1. The obtained W-doped sample (sample #2 in the first row of Figure b) has no impurity signal and exhibits the monoclinic distortion. The splitting of (200) and (020) peaks is clearly observed in the X-ray diffraction pattern of the W-doped sample #2. Bi2O3, NH4VO3, and MoO3 powders were mixed at a molar ratio of 10:9:1 for the Mo-doped sample, which exhibits the X-ray diffraction pattern of the tetragonal phase without an impurity signal, as shown in the third row of Figure b. As for the codoped sample, Bi2O3, NH4VO3, WO3, and MoO3 were mixed at a molar ratio of 9:9:0.4:1.2. The X-ray diffraction pattern displayed in the fourth row of Figure b has no impurity signal and is close to the tetragonal case.

Figure a shows the O 1s XPS spectra of W-doped (sample #2), Mo-doped, and codoped BiVO4 samples taken at 300 K with hν = 1200 eV. The main peak of the W-doped sample is slightly shifted to the lower binding energy side compared to the Mo-doped sample. The main peak of the codoped sample is slightly broader than those of the W- and Mo-doped samples and is shifted further to the lower binding energy side. The O 1s spectra of the W-doped and Mo-doped samples exhibit a higher binding energy peak around 533 eV, which can be assigned to contamination at the grain boundaries.

2.

2

Open in a new tab

(a) O 1s and (b) Bi 4f spectra of W-doped, Mo-doped, and codoped BiVO4 taken at 300 K with hν = 1200 eV. The thin solid curves in (b) represent a curve fit to the Bi 4f 7/2 branch with four Gaussian functions.

Figure b shows Bi 4f XPS spectra. The main peaks of the 4f 7/2 and 4f 5/2 branches are slightly shifted to the lower binding energy side in the codoped sample. The Mo-doped sample and the codoped sample gain spectral weight around 158 eV, which can be assigned to minor Bi5+ species. In the study by Liu et al., BiVO4 synthesized by the photoassisted sol–gel method exhibits excellent photocatalytic activity. In their study, the Bi 4f 7/2 peak at 159.4 eV was assigned to Bi3+, and the sample grown by the photoassisted method revealed an additional Bi 4f 7/2 peak at 162.6 eV assigned to Bi5+. This assignment is based on the assumption that two Bi 6s electrons of Bi3+ screen the core hole charge and lower the binding energy relative to Bi5+. However, the screening effect due to the 6s electrons in Bi3+ is complicated due to the lone-pair formation as discussed in the context of Bi1–x Pb x NiO3. Following the studies on the bismuth nickelates in which Bi 4f 7/2 peaks at 159.4 and 157.9 eV are assigned to Bi3+ and Bi5+, respectively, , the small spectral weight around 158 eV can be assigned to Bi5+, as indicated in Figure b.

The Bi 4f 7/2 spectra can be fitted to four Gaussian functions with the common full width at half-maximum (0.40 eV for the W- and Mo-doped samples and 0.41 eV for the codoped sample). The obtained parameters are listed in Table . The two Gaussian functions around 159.5 eV are assigned to Bi3+, while those around 158 eV are assigned to Bi5+. Since the Bi3+ and Bi5+ components have asymmetric shapes due to various core hole screening effects, they are, respectively, fitted to two Gaussian functions. The area fraction of Bi5+ is about 4% in the W-doped sample and 6% in the Mo- and codoped samples, as shown in the last column of Table . Although the W-doped sample apparently has no spectral weight around 158 eV, the low-energy tail of the main peak can be assigned to Bi5+.

1. Peak Position and Area of the Four Gaussian Functions (the First Two for Bi3+ and the Latter Two for Bi5+) Fitted to the Bi 4f 7/2 Spectra and the Area Fraction of Bi5+ for the W-Doped, Mo-Doped, and Codoped Samples.

  Bi3+ peak (eV) Bi5+ peak (eV) Bi3+ area Bi5+ area fraction of Bi5+
W-doped 159.44, 160.18 158.54, 157.66 0.481, 0.044 0.018, 0.004 0.040
Mo-doped 159.43, 160.15 158.56, 157.70 0.483, 0.052 0.023, 0.011 0.060
codoped 159.33, 160.11 158.41, 157.57 0.484, 0.040 0.023, 0.008 0.056
Open in a new tab

Figure shows the V 2p XPS spectra of W-doped, Mo-doped, and codoped BiVO4 samples taken at 300 K with hν = 1200 eV. The main peak around 517 eV corresponds to the V5+ state, while the shoulders around 516 eV can be assigned to the V4+ species introduced by the doping. In the codoped sample, the V 2p peak is shifted toward the lower binding energy. The amount of the energy shift is about 0.1 eV all in the O 1s, Bi 4f, and V 2p spectra, indicating that the energy shift is derived from the chemical potential shift.

3.

3

Open in a new tab

V 2p spectra of W-doped, Mo-doped, and codoped BiVO4. The thin solid curves represent a curve fit to the V 2p 3/2 branch with four Gaussian functions. The dotted and dashed curves represent V4+ to V5+ components, respectively.

As shown by the thin solid curves, the V 2p 3/2 branch is fitted by four Gaussian functions with a common full width at half-maximum (0.42 eV for the W-doped sample, and 0.44 eV for the Mo- and codoped sample). The obtained parameters are listed in Table . The two Gaussian functions around 517 eV are assigned to V5+, while those around 516 eV are assigned to V4+, as indicated by the dotted and dashed curves in the figure. While the V5+ peak exhibits a small tail on the higher binding energy side, the V4+ peak is accompanied by a spectral feature on the lower binding energy side. This feature would be derived from the multiplet splitting of V4+ with 2p 53d 1 final states, which is absent in V5+ with 2p 53d 0. The area fraction of V4+ is estimated to be 19%, 23%, and 30% in the W-doped, Mo-doped, and codoped samples, as shown in the last column of Table .

2. Peak Position and Area of the Four Gaussian Functions (the First Two for V5+ and the Latter Two for V4+) Fitted to the V 2p 3/2 Spectra and the Area Fraction of V4+ for the W-Doped, Mo-Doped, and Codoped Samples.

  V5+ peak (eV) V4+ peak (eV) V5+ area V4+ area fraction of V4+
W-doped 517.09, 517.95 516.12, 515.27 0.472, 0.048 0.094, 0.025 0.19
Mo-doped 517.08, 518.04 516.05, 515.05 0.483, 0.043 0.124, 0.032 0.23
codoped 516.98, 517.94 516.00, 515.12 0.480, 0.048 0.176, 0.051 0.30
Open in a new tab

The nominal doping concentration per V is 0.1/0.9 for the W-doped and Mo-doped samples and is 0.16/0.84 for the codoped sample. In addition, the amount of charge transfer from the Bi to V sites, as indicated by the Bi5+ component in the Bi 4f spectra, is 0.08/0.9, 0.12/0.9, and 0.11/0.84 for the W-, Mo-, and codoped samples, respectively. In total, the fraction of V4+ is estimated to be 0.2, 0.24, and 0.32 for the W-, Mo-, and codoped samples. This estimation is consistent with the area fraction of V4+ obtained by curve fitting to the V 2p 3/2 spectra (19%, 23%, and 30% in the W-, Mo-, and codoped samples, respectively). Although the Bi5+ component seen in the Bi 4f spectra is apparently small, the Bi–V charge transfer has a significant impact on the creation of the V4+ states. The systematic evaluation of the V and Bi valences from the core-level spectra is consistent with the evolution of the V 3d spectral weight near the Fermi level, supporting the assignment of Bi5+ to the lower binding energy component in the Bi 4f core-level spectra.

Figure a shows the V 2p XAS spectra of W-doped, Mo-doped, and codoped BiVO4 samples taken at 300 K. The multiplet structures observed in the W- and Mo-doped samples are consistent with the previous reports. ,,,, The distortion of the VO4 tetrahedra is responsible for the complicated spectral shapes. The main peak at 518.8 eV corresponds to the transition to the V 3d t 2g states, while the spectral features around 516–517 eV are partly derived from the transition to the low-lying V 3d e g states. The spectral weight around 516–517 eV relative to the main peak at 518.8 eV substantially increases in the codoped sample, in which the fraction of V4+ increases from about 20% to 30% due to codoping. The V4+ state with d 1 contributes to the spectral weight around 516–517 eV in the codoped sample. In addition, the pre-edge region around 515 eV gains spectral weight in the codoped BiVO4, consistent with the observation by Chi et al.

4.

4

Open in a new tab

(a) V 2p and (b) O 1s XAS spectra of W-doped, Mo-doped, and codoped BiVO4. The O 1s XAS spectra are compared with the calculated O 2p PDOS in the conduction band of monoclinic BiVO4.

Figure b shows the O 1s XAS spectra of W-doped, Mo-doped, and codoped BiVO4 samples. Since the O 2p states are hybridized with the unoccupied V 3d states, the O 1s spectra are compared with the O 2p partial density of states (PDOS) calculated for the monoclinic BiVO4 by means of generalized gradient approximation using Quantum ESPRESSO v6.7. The double-peak structure is roughly consistent with the O 2p PDOS. The lowest-energy part corresponds to the V 3d e g states, which are partially occupied by the Mo/W doping and the Bi–V charge transfer. The effect of polaron formation suggested in the previous photoemission studies − is not clearly seen in the spectral distribution of the unoccupied states.

Figure shows the valence band XPS spectra of W-doped, Mo-doped, and codoped BiVO4 samples taken at 300 K with hν = 1200 eV. The spectra are compared with the V 3d and O 2p PDOS calculated for monoclinic BiVO4. The main band ranging from 2 to 9 eV is derived from the O 2p states. The V 3d electrons of the V4+ states are observed around 1.5 eV below the Fermi level. The suppressed spectral weight at the Fermi level and the V4+(d 1)/V5+(d 0) mixed valence are similar to those in the Hollandite-type Ba x Ti8O16+δ (mixed valence of Ti3+(d 1)/Ti4+(d 0)), in which small polaron formation in the bad metallic state is established. In Ba x Ti8O16+δ, the Ti 3d t 2g electrons are localized due to the strong local lattice distortion and the long-range d-d Coulomb interaction in the lattice of edge-sharing TiO6 octahedra. On the other hand, in BiVO4, the isolated VO4 tetrahedra are connected through the oxygen and Bi sites. Since the V 3d e g orbitals are directed to the neighboring Bi sites, the Bi 6s electrons are expected to play an important role. Here, one can speculate that the minor Bi5+ species induce strong lattice distortion and support polaronic localization of the neighboring V 3d e g electrons.

5.

5

Open in a new tab

Valence band XPS spectra of W-doped, Mo-doped, and codoped BiVO4. The dashed and solid curves represent the V 3d and O 2p PDOS calculated for monoclinic BiVO4.

The area of the V 3d peak increases with the doping level and the Bi–V charge transfer, as indicated by the increase of the V4+ component in Figure . As indicated by the arrows in the figure, the V 3d binding energy gradually decreases with the increase of the peak area. The amount of the energy shift is about 0.3 eV, which is much larger than the chemical potential shift ∼0.1 eV estimated from the core-level binding energy shift. Since each polaron is well localized, one can assume that the energy gain due to the local lattice distortion does not depend on the concentration. If only the lattice distortions are responsible for the localization of the V 3d e g electrons, the decrease of the V 3d electron binding energy cannot be explained. In transition-metal oxides, when a polaron is transferred to neighbors of another polaron, they lose the long-range d-d Coulomb energy. Thus, the long-range d-d Coulomb interaction is involved in the energy gap of polaronic systems. It is expected that the effect of long-range d-d Coulomb interaction is reduced with the concentration in a similar manner to the long-range Coulomb interaction in an electron gas. The decrease of the V 3d binding energy suggests screening of the Coulomb interaction between V 3d electrons. Therefore, one can speculate that the localization of the V 3d electrons in the doped BiVO4 is mediated by the long-range d-d Coulomb interaction in addition to the local lattice distortion.

Conclusion

In conclusion, the electronic structure of the W- and Mo-doped BiVO4 has been investigated by means of X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The V 2p XPS spectra show the substitution effect of W6+ and Mo6+, which introduces the V4+ species. In addition, the effect of Bi–V charge transfer is shown by the minor Bi5+ component in the Bi 4f XPS. The V 3d peak evolves at around 1.5 eV below the Fermi level. The V 3d binding energy is shifted toward the Fermi level with increasing doping level, suggesting a screening effect on the long-range d-d Coulomb interaction. The localized V 3d electrons in the doped BiVO4 are sustained by the combination of the local lattice distortion and the long-range d-d Coulomb interaction. The effect of d-d Coulomb interaction and the Bi–V charge transfer can be controlled in the well-defined surface, as suggested in MgTi2O4, to enhance the catalytic activity of the localized V 3d electron.

Acknowledgments

D.T. acknowledges the support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the Walter Benjamin Programme, Projektnummer 521584902. O.Y. acknowledges the National Research Foundation of Ukraine for financial support of this research (Project No. 2025.07/0271). This work was supported by Grants-in-Aid from the Japan Society of the Promotion of Science (JSPS) (No. JP22H01172). This work was supported by a Joint research program from ZAIKEN, Waseda University (Project No. 31010). This work was the result of using research equipment (RINT-Ultima III: Material Characterization Central Laboratory) shared in the MEXT Project for promoting public utilization of advanced research infrastructure (Program for supporting construction of core facilities), Grant Number JPMXS0440500023.

The authors declare no competing financial interest.

References

  1. Fujishima A., Honda K.. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature. 1972;238:37–38. doi: 10.1038/238037a0. [DOI] [PubMed] [Google Scholar]
  2. Ghosh A. K., Maruska H. P.. Photoelectrolysis of Water in Sunlight with Sensitized Semiconductor Electrodes. J. Electrochem. Soc. 1977;124:1516–1522. doi: 10.1149/1.2133104. [DOI] [Google Scholar]
  3. Nozik A. J.. Photochemical diodes. Appl. Phys. Lett. 1977;30:567–569. doi: 10.1063/1.89262. [DOI] [Google Scholar]
  4. Kraeutler B., Bard A. J.. Heterogeneous photocatalytic synthesis of methane from acetic acid - new Kolbe reaction pathwayClick to copy article link. J. Am. Chem. Soc. 1978;100:2239–2240. doi: 10.1021/ja00475a049. [DOI] [Google Scholar]
  5. Sato S., White J. M.. Photodecomposition of water over Pt/TiO2 catalysts. Chem. Phys. Lett. 1980;72:83–86. doi: 10.1016/0009-2614(80)80246-6. [DOI] [Google Scholar]
  6. Kudo A., Kato H.. Effect of lanthanide-doping into NaTaO3 photocatalysts for efficient water splitting. Chem. Phys. Lett. 2000;331:373–377. doi: 10.1016/S0009-2614(00)01220-3. [DOI] [Google Scholar]
  7. Asahi R., Morikawa T., Ohwaki T., Aoki K., Taga Y.. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science. 2001;293:269–271. doi: 10.1126/science.1061051. [DOI] [PubMed] [Google Scholar]
  8. Irie H., Watanabe Y., Hashimoto K.. Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2–x N x Powders. J. Phys. Chem. B. 2003;107:5483. doi: 10.1021/jp030133h. [DOI] [Google Scholar]
  9. Maeda K., Xiong A., Yoshinaga T., Ikeda T., Sakamoto N., Hisatomi T., Takashima M., Lu D., Kanehara M., Setoyama T., Teranishi T., Domen K.. Photocatalytic Overall Water Splitting Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution under Visible Light. Angew. Chem., Int. Ed. 2010;49:4096–4099. doi: 10.1002/anie.201001259. [DOI] [PubMed] [Google Scholar]
  10. A M., Madhavan J., Ashokkumar M., Arunachalam P.. A review on BiVO4 photocatalyst: Activity enhancement methods for solar photocatalytic applications. Appl. Catal., A. 2018;555:47–74. doi: 10.1016/j.apcata.2018.02.010. [DOI] [Google Scholar]
  11. Sleight A. W., Chen H.-y., Ferretti A., Cox D. E.. Crystal growth and structure of BiVO4 . Mater. Res. Bull. 1979;14:1571–1581. doi: 10.1016/0025-5408(72)90227-9. [DOI] [Google Scholar]
  12. Momma K., Izumi F.. VESTA3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011;44:1272. doi: 10.1107/S0021889811038970. [DOI] [Google Scholar]
  13. Wang S., Li C., Qi Y., Zhang J., Wang N., Liu M., Zhang B., Cai X., Zhang H., Wei S., Ma G., Yang J., Chen S., Zhang F.. Etched BiVO4 photocatalyst with charge separation efficiency exceeding 90% Nat. Commun. 2025;16:3776. doi: 10.1038/s41467-025-59076-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Cooper J. K., Gul S., Toma F. M., Chen L., Glans P.-A., Guo J., Ager J. W., Yano J., Sharp I. D.. Electronic Structure of Monoclinic BiVO4 . Chem. Mater. 2014;26:5365–5373. doi: 10.1021/cm5025074. [DOI] [Google Scholar]
  15. Liu H., Nakamura R., Nakato Y.. Promoted Photo-oxidation Reactivity of Particulate BiVO4 Photocatalyst Prepared by a Photoassisted Sol-gel Method. J. Electrochem. Soc. 2005;152:G856–G861. doi: 10.1149/1.2051868. [DOI] [Google Scholar]
  16. Nakano K., Oka K., Watanuki T., Mizumaki M., Machida A., Agui A., Kim H., Komiyama J., Mizokawa T., Nishikubo T., Hattori Y., Ueda S., Sakai Y., Azuma M.. Glassy Distribution of Bi 3+ /Bi5+ in Bi1–x Pb x NiO3 and Negative Thermal Expansion Induced by Intermetallic Charge Transfer. Chem. Mater. 2016;28:6062–6067. doi: 10.1021/acs.chemmater.6b01160. [DOI] [Google Scholar]
  17. Sakai Y., Nishikubo T., Ogata T., Ishizaki H., Imai T., Mizumaki M., Mizokawa T., Machida A., Watanuki T., Yokoyama K., Okimoto Y., Koshihara S.-y., Das H., Azuma M.. Polar–Nonpolar Phase Transition Accompanied by Negative Thermal Expansion in Perovskite-Type Bi1–x Pb x NiO3 . Chem. Mater. 2019;31:4748–4758. doi: 10.1021/acs.chemmater.9b00929. [DOI] [Google Scholar]
  18. Uchida K., Ayame A.. Dynamic XPS measurements on bismuth molybdate surfaces. Surf. Sci. 1996;357–358:170–175. doi: 10.1016/0039-6028(96)00083-0. [DOI] [Google Scholar]
  19. Wildberger M. D., Grunwaldt J. D., Maciejewski M., Mallat T., Baiker A.. Sol-gel bismuth-molybdenum-titanium mixed oxides. I. Preparation and structural properties. Appl. Catal., A. 1998;175:11–19. doi: 10.1016/S0926-860X(98)00200-2. [DOI] [Google Scholar]
  20. Saito K., Iguchi R., Muneda R., Saini N. L., Mizokawa T.. Pb and Bi Mixed Valence States Induced by X-ray Irradiation in MAPbI3 and MAPb0.9Bi0.1I3 . ACS Appl. Energy Mater. 2025;8:9544–9550. doi: 10.1021/acsaem.5c01190. [DOI] [Google Scholar]
  21. Saito K., Tsunoda T., Saini N. L., Mizokawa T.. Impact of x-ray irradiation on the Bi valence of MA3Bi2I9 and Cs3Bi2I9 . J. Phys. Chem. C. 2025;129:17998–18003. doi: 10.1021/acs.jpcc.5c05694. [DOI] [Google Scholar]
  22. Park H. S., Kweon K. E., Ye H., Paek E., Hwang G. S., Bard A. J.. Factors in the Metal Doping of BiVO4 for Improved Photoelectrocatalytic Activity as Studied by Scanning Electrochemical Microscopy and First-Principles Density-Functional Calculation. J. Phys. Chem. C. 2011;115:17870–17879. doi: 10.1021/jp204492r. [DOI] [Google Scholar]
  23. Liang Y., Tsubota T., Mooij L. P. A., Krol R. V. D.. Highly improved quantum efficiencies for thin film BiVO4 photoanodes. J. Phys. Chem. C. 2011;115:17594–17598. doi: 10.1021/jp203004v. [DOI] [Google Scholar]
  24. Rettie A. J. E., Lee H. C., Marshall L. G., Lin J.-F., Capan C., Lindemuth J., McCloy J. S., Zhou J., Bard A. J., Mullins C. B.. Combined charge carrier transport and photoelectrochemical characterization of BiVO4 Single Crystals: Intrinsic behavior of a complex metal oxide. J. Am. Chem. Soc. 2013;135:11389–11396. doi: 10.1021/ja405550k. [DOI] [PubMed] [Google Scholar]
  25. Talasila G., Sachdev S., Srivastva U., Saxena D., Ramakumar S. S. V.. Modified synthesis of BiVO4 and effect of doping (Mo or W) on its photoelectrochemical performance for water splitting. Energy Rep. 2020;6:1963–1972. doi: 10.1016/j.egyr.2020.07.024. [DOI] [Google Scholar]
  26. Jovic V., Laverock J., Rettie A. J. E., Zhou J.-S., Mullins C. B., Singh V. R., Lamoureux B., Wilson D., Su T.-Y., Jovic B., Bluhm H., Sohnelab T., Smith K. E.. Soft X-ray spectroscopic studies of the electronic structure of M:BiVO4 (M = Mo, W) single crystals. J. Mater. Chem. A. 2015;3:23743–23753. doi: 10.1039/c5ta07898a. [DOI] [Google Scholar]
  27. Mohamed M., May M. M., Uecker R., van de Krol R., Kanis M., Brützam M., Janowitz C., Mulazzi M.. The electronic structure and the formation of polarons in Mo-doped BiVO4 measured by angle resolved photoemission spectroscopy. RSC Adv. 2019;9:15606–15614. doi: 10.1039/C9RA01092K. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Chi X., Mandal L., Liu C., Fauzi A. D., Chaudhuri A., Whitcher T. J., Jani H. K., Chen Z., Xi S., Diao C., Naradipa M. A., Yu X., Yang P., Castro-Neto A. H., Breese M. B. H., Loh K. P., Venkatesan T. V., Rusydi A.. Unravelling a new many-body large-hole polaron in a transition metal oxide that promotes high photocatalytic activity. NPG Asia Mater. 2022;14:19. doi: 10.1038/s41427-022-00364-w. [DOI] [Google Scholar]
  29. Oktariza L. G., Sato Y., Gofurov S., Ozawa K., Islam M. M., Ikeda S., Sakurai T.. Characterization of the electronic structure of sputter-deposited Mo-doped BiVO4 thin-film photoanodes. Jpn. J. Appl. Phys. 2024;63:02SP41. doi: 10.35848/1347-4065/ad12ac. [DOI] [Google Scholar]
  30. Li X., Wang Z., Sasani A., Baktash A., Wang K., Lu H., You J., Chen P., Chen P., Bao Y., Zhang S., Liu G., Wang L.. Oxygen vacancy induced defect dipoles in BiVO4 for photoelectrocatalytic partial oxidation of methane. Nat. Commun. 2024;15:9127. doi: 10.1038/s41467-024-53426-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Wang S., Shi Z., Du K., Ren Z., Feng H., Wang J., Wang L., Cui D., Du Y., Hao W.. Synergistic Surface Engineering of BiVO4 Photoanodes for Improved Photoelectrochemical Water Oxidation. Small Methods. 2025;9:2401443. doi: 10.1002/smtd.202401443. [DOI] [PubMed] [Google Scholar]
  32. Giannozzi P., Andreussi O., Brumme T., Bunau O., Buongiorno Nardelli M., Calandra M., Car R., Cavazzoni C., Ceresoli D., Cococcioni M.. et al. Advanced capabilities for materials modelling with Quantum ESPRESSO. J. Phys.: Condens. Matter. 2017;29:465901. doi: 10.1088/1361-648X/aa8f79. [DOI] [PubMed] [Google Scholar]
  33. Dash S., Kajita T., Okawa M., Saitoh T., Ikenaga E., Saini N. L., Katsufuji T., Mizokawa T.. Anomalous metallic state with strong charge fluctuations in Ba x Ti8O16+δ revealed by hard x-ray photoemission spectroscopy. Phys. Rev. B. 2018;97:165116. doi: 10.1103/PhysRevB.97.165116. [DOI] [Google Scholar]
  34. Murata R., Sato T., Okuda T., Horibe Y., Tsukasaki H., Mori S., Yamaguchi N., Sugimoto K., Kawaguchi S., Takata M., Katsufuji T.. Electronic phase transition in hollandite Ba x Ti8O16+δ . Phys. Rev. B. 2015;92:220408(R) doi: 10.1103/PhysRevB.92.220408. [DOI] [Google Scholar]
  35. Fujiwara H., Ishige Y., Mizokawa T., Sasaki T., Isobe M., Ueda Y.. Valence instability and photochemical reaction at surface of strongly correlated MgTi2O4 . Appl. Phys. Lett. Mater. 2013;1:022110. doi: 10.1063/1.4818355. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES