Tracking Polaron Dynamics in Epitaxial BiVO₄ Thin Films by Pump–Probe X-ray Diffraction

Guan-Ruei Chen (陳冠睿)

Department of Chemistry, College of Science, National Taiwan University (國立台灣大學理學院化學系)

Abstract:

Bismuth vanadate (BiVO₄) with the monoclinic Scheelite-type structure (space group I2/b, No. 15) has been widely studied as a photocatalyst due to its relatively narrow band gap (~2.4 eV) and favorable band-edge positions. However, the strong electron–phonon coupling in this material leads to the formation of small polarons, resulting in poor charge mobilities and rapid recombination of photogenerated electron–hole pairs. Understanding how small polarons form upon illumination and developing strategies to mitigate them are therefore essential for improving the photocatalytic performance of BiVO₄. Although several experimental methods have been employed for investigating small polarons in bulk single-crystals of BiVO₄, including transient absorption spectroscopy (TAS) [1] and angle-resolved photoemission spectroscopy (ARPES) [2] in conjunction with DFT+U calculations, obtaining direct structural signatures of small polarons with temporal resolution remains challenging. In this work, we first grow epitaxial thin films of pristine and Mo-doped BiVO₄ via a scalable chemical solution process [3], then use pump–probe X-ray diffraction to track changes in long-range lattice order under photoexcitation, which correspond to the formation of small polarons. Femtosecond pulses provided by a Ti:Sapphire regenerative amplifier, frequency-doubled to 400 nm, serve as the optical pump, whereas synchrotron hard X-ray pulses probe both the (004) and (112) diffractions of BiVO₄, representing symmetric and asymmetric reflection relative to the surface normal, respectively. Variations in lattice parameters and diffraction intensities are observed upon photoexcitation, and their temporal evolutions inform the relaxation/diffusion of small hole polarons in BiVO₄. In addition, the influence of Mo incorporation into V-sites on the excited-state lifetime is revealed. These results highlight the capability of the beamline station TPS09A in NSRRC, enabled by its high temporal coherence, to capture photo-induced structural distortion in highly polarized functional materials.

References
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[3] G.-Z. Tu, J.-Y. Chen, Z.-X. Zhen, Y. Li., C.-W. Chang, W.-J. Chang, H. M. Chen, and C.-M. Jiang, “Elucidating the Epitaxial Growth Mechanisms of Solution-Derived BiVO₄ Thin Films Utilizing Rapid Thermal Annealing” ACS Appl. Electron. Mater., vol. 6, no. 3, pp. 1872-1885, February 2024.