Epitaxial Strain Modulation of Photoelectrochemical Properties of Epitaxial Bismuth Vanadate Photoanodes

Abstract

Solar fuels derived from photoelectrochemical water splitting have the potential to significantly contribute to achieving sustainability in our global energy infrastructure. Transition metal oxide (TMO) photoabsorbers are interesting candidates as photoelectrodes due to their excellent bandgap tunability, relative stability and cost-effectiveness. Further improvements in the photoelectrochemical performance of such photoelectrodes would entail strategies beyond the discovery of new compositions and structures [1]. Strain engineering in particular is a powerful tool to alter the material’s electronic structure and the resulting properties. For instance, optoelectronic properties (i.e., light absorption characteristics) [2] and surface catalytic properties [3] could be modulated by lattice-mismatch imposed epitaxial strain. Much less understood, however, is how epitaxial strain modulation affects the band bending and surface electronic structure; these properties dictate the photogenerated carrier separation at the photoelectrode/electrolyte interface, and thus the photoelectrochemical performance.To gain further insight into the impact of epitaxial strain on the photoelectrochemical performance of TMO photoelectrodes, monoclinic bismuth vanadate (BiVO4) was chosen as a material platform for study given its desirable (bandgap ~2.4 eV, favorable band edge positions and relatively long minority diffusion lengths) and undesirable (localized carrier transport, photocorrosion) characteristics [1]. High crystalline quality epitaxial BiVO4 thin films were used as a model system in order to minimize complications arising from grain boundaries and different crystalline orientations inherent in polycrystalline systems. BiVO4 films with an epitaxial indium tin oxide (ITO) conducting layer were deposited on (001)-oriented yttrium-stabilized zirconia (YSZ) single crystalline substrates. YSZ substrates with different dopant concentrations were used to impose different epitaxial strain states on the BiVO4 films. This strategy allowed us to conduct extensive photoelectrochemical measurements (voltammograms, incident photon-to-current efficiency, impedance spectroscopy) and x-ray photoelectron spectroscopic (XPS) measurements as a function of the strain state. Further investigation with varying BiVO4 film thickness was also performed to decouple any thickness-dependent contributions from the epitaxial strain effects. Through strain tensor decomposition analysis, Tauc bandgaps correlates well with deviatoric strains, which in turn seem to suggest the role of volume-preserving lattice distortions. Finally, we will show that the observed trends in photocurrents, incident photon-to-current efficiency spectra, flatband potentials obtained from Mott-Schottky analysis, and surface electronic structure obtained from XPS can be rationalized in terms of lattice-induced distortions on the BiO8 dodecahedra.