The study of polarons—the interaction of charge carriers with a surrounding lattice—has been a longstanding area of interest in condensed matter physics. Polarons are believed to influence properties like superconductivity, metal-insulator transitions, and ferroelectricity. Their role in photocatalysis, particularly water splitting, is also gaining attention. Oxide semiconductors like BiVO₄, TiO₂, SrTiO₃, and WO₃ are promising photocatalytic materials due to their ability to absorb photons and generate electron-hole pairs. However, the mechanisms behind their high photocatalytic activity remain unclear. While it's been theoretically suggested that large polarons can aid in charge separation and prevent recombination, transition metal oxides typically exhibit low conductivity due to charge carrier localization, forming small polarons. BiVO₄, with its tunable bandgap and favorable conduction band edge, shows promise but suffers from low photocatalytic performance in its pure form, potentially due to small-electron polaron hopping. Doping with Mo or W improves conductivity and photocatalytic activity, suggesting a different mechanism at play. Theoretical studies have proposed a large-hole polaron in BiVO₄ as the key to enhanced photocatalysis, but experimental evidence has been lacking. This study aims to experimentally verify the existence of this large-hole polaron and explore its connection to the enhanced photocatalytic performance of W-doped BiVO₄. A smaller bandgap is crucial for increased visible light absorption, improving photocurrent. Moreover, an indirect bandgap may assist in charge separation by reducing electron-hole recombination. This work focuses on W-doped BiVO₄ because it's a strongly correlated electron system conducive to many-body polaron formation, W's multivalency helps create hole states, and W modifies the crystal structure, facilitating the creation of vanadium vacancies and subsequent hole polaron formation. A combination of spectroscopic ellipsometry (SE), X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy, and X-ray diffraction, along with theoretical calculations, will be used to characterize the material and understand the observed phenomena.

The literature extensively documents the importance of polarons in various material properties and their potential role in photocatalysis. Early theoretical work laid the foundation for understanding polaron formation and behavior. More recent studies have explored the role of polarons in enhancing photocatalytic water splitting efficiency in transition metal oxides, with a focus on mitigating electron-hole recombination. BiVO₄, in particular, has been studied extensively due to its favorable bandgap and redox potentials. Previous work highlights the limitations of pure BiVO₄, often attributing poor performance to small-electron polaron hopping. The effect of doping with elements like Mo and W has been explored, demonstrating improved photocatalytic activity. However, the underlying mechanism responsible for the enhancement remains a subject of ongoing research, with theoretical predictions suggesting the involvement of large-hole polarons. This study directly addresses the lack of experimental confirmation for the existence and role of large-hole polarons in enhancing BiVO₄'s photocatalytic performance.

W-doped BiVO₄ thin films (0%, 0.5%, 1%, 2%, and 5% W doping) were epitaxially grown on YSZ (001) substrates using pulsed laser deposition (PLD). The crystal structures were characterized using synchrotron-based high-resolution X-ray diffraction, with reciprocal space mapping near the (−204) diffraction used to analyze strain and texture. Photocatalytic activity was evaluated by measuring H₂ evolution during water splitting under visible light irradiation in a solution containing Na₂S and Na₂SO₃. X-ray absorption spectroscopy (XAS) at the V L- and O K-edges was employed to probe the electronic structure and unoccupied states of BiVO₄. Spectroscopic ellipsometry (SE) provided information on the complex dielectric function, enabling the study of spectral weight transfer (SWT). First-principles density functional theory (DFT) calculations, including the Hubbard U parameter to account for strong electron correlation, were performed to simulate the electronic structure of BiVO₄.

High-resolution synchrotron X-ray diffraction revealed a change in crystal structure from orthorhombic (pure BiVO₄) to monoclinic (low W doping) and then to tetragonal (high W doping). The 1% W-doped monoclinic BiVO₄ exhibited the highest photocatalytic H₂ evolution activity. XAS at the V L-edge showed a hole prepeak at ~514.8 eV, attributed to a new many-body large-hole polaron, with maximum intensity at 1% W doping. This prepeak was polarization-dependent, confirming its nature as a hole state. Analysis of orbital energy splitting, obtained by fitting XAS data with Voigt functions, showed significant lattice distortion induced by W doping, with a notable jump in splitting between 1% and 2% doping. XAS at the O K-edge confirmed the presence of the large-hole polaron, demonstrating O 2p hybridization with V 3d and Bi 6sp orbitals. Spectroscopic ellipsometry revealed anomalous spectral weight transfer (SWT) with W doping, indicating strong electronic correlations and screening. SWT from higher energy bands to the midgap state (associated with the large-hole polaron) was most pronounced in the 1% W-doped sample. The DFT calculations supported these experimental findings, explaining the observed spectral weight transfer and confirming the formation of the large-hole polaron as a midgap state.

The results demonstrate the crucial role of the many-body large-hole polaron in enhancing the photocatalytic activity of BiVO₄. The formation of this polaron, facilitated by W doping and associated crystal lattice distortions, introduces a midgap state that acts as a trap for holes, effectively separating charges and reducing electron-hole recombination. The transition to an indirect bandgap further enhances charge separation. The observed SWT provides strong evidence for significant electronic correlations and screening effects, which play a key role in the formation and stability of the large-hole polaron. The optimum photocatalytic performance at 1% W doping likely reflects the optimal balance between polaron formation and charge carrier mobility. High doping levels may lead to excessive scattering and reduced conductivity, while low doping levels may not provide enough vacancies and distortion for efficient polaron formation.

This study provides experimental evidence for the existence of a new many-body large-hole polaron in W-doped BiVO₄ and demonstrates its crucial role in enhancing the material's photocatalytic activity. The interplay of electronic correlations, lattice distortions, and indirect bandgap transitions are all critical factors. Future research could explore other dopants or strategies to further optimize polaron formation and improve photocatalytic efficiency. Investigations into the dynamics of polaron formation and their impact on charge transport would also provide valuable insights.

The study primarily focuses on thin films grown by PLD, which may not fully represent the behavior of bulk BiVO₄. The DFT calculations utilize approximations, such as the Hubbard U correction, which may introduce uncertainties. Further investigations using other characterization techniques and theoretical approaches could provide a more comprehensive understanding. The photocatalytic activity measurements were performed under specific experimental conditions, and the generalizability to other conditions needs further exploration.