Bismuth vanadate (BiVO4) has, over the past decade, been the focus of intensive research as a promising photoanode material in photoelectrochemical (PEC) water splitting devices. To continue to improve overall material performance, stability, and efficiency, a fundamental understanding of the electronic structure of this material is desired.
In this work, a comprehensive approach to understanding both valence band (VB) and conduction band (CB) orbital character, as well as photoexcited carrier dynamics, has been undertaken using both experimental and theoretical means. Density functional theory calculations confirm the VB maximum and CB minimum to be comprised primarily of O 2p and V 3d orbitals, respectively. Triplet d-orbital splitting was calculated and ascribed to oxygen ligand anisotropy.
To confirm these theoretical findings, a range of optical and x-ray spectroscopies have been applied to study high quality monoclinic BiVO4 (m-BiVO4) thin films (~130 nm thickness) deposited by chemical vapor deposition (CVD) and sputtering. X-ray absorption spectroscopy (XAS) supports the predicted triplet splitting of the CB and the fundamental bandgap was measured to be 2.5 eV via combination of XAS with x-ray emission. Resonant inelastic x-ray scattering (RIXS) provides conclusive evidence that m-BiVO4is an indirect semiconductor.
Furthermore, inelastically scattered features (between 0.6 and 1.7 eV) were attributed to splitting of the V d states of the CB. A direct gap of 2.8 eV has also been measured by steady state and transient absorption spectroscopies in the optical range. The excited state carrier lifetimes were measured by transient absorption pump probe spectroscopy over probe wavelengths between 0.9 and 3.5 eV. A significant portion of the excited state species decayed with characteristic lifetimes of 5.2 and 45 ns; however, additional components were observed with lifetimes out to 0.3 ms.
Comparison of transient optical spectra with x-ray spectroscopies has allowed the assignment of specific transient features to both free and trapped minority holes, as well as photoexcited majority electrons. Therefore, this work provides a basis for determining carrier lifetimes in BiVO4photoanodes and, thus, for improving material quality and photoelectrochemical conversion efficiencies.