Deconvoluting Photoelectrochemical Activity in Monoclinic–Scheelite BiVO₄ Facet Selected Thin Films

Abstract

Crystal facet engineering is one of the promising strategies to tune the band edge positions and surface electrochemistry of a material, which are essential to improve photoelectrochemical (PEC) water splitting. Materials with low-crystal symmetry structures demonstrate facet-dependent properties due to asymmetric coordination, and facet engineering can modulate PEC properties. In this regard, different facets [e.g., (002), (121), and (040)] of the monoclinic–scheelite polymorph of BiVO4 (low-crystal symmetry structure) have been grown by controlling the thickness (deposition time) of thin films [22 nm (10 s) to 265 nm (50 s)] by the electron beam deposition technique. X-ray diffraction and high-resolution transmission electron microscopy analysis suggest the presence of different exposed planes that display different EC and PEC activity. PEC water splitting measurements suggest that the 110 nm/30 s thin-film sample, that is, the (040) facet, has the highest current density, that is, 0.29 mA/cm2 (under light) and 0.068 mA/cm2 (under dark) at 1.8 V versus RHE. However, the applied bias photon to current efficiencies (ABPEs) of both the thin films, that is, (040)/30 s and (121)/40 s facets, are nearly equal, whereas (121)/40 s has enhanced electrical and solar power-to-hydrogen (ESPH) conversion efficiencies compared to the (040)/30 s facet sample. Band edge positions computed via density functional simulations of exposed surfaces suggest that different facets have different band edge positions, thereby offering different driving forces to perform oxygen evolution reaction (OER). The relation between efficiency (ABPE and ESPH) and driving force suggests that the enhanced PEC performance of the (040) facet of BiVO4 is due to the increased driving force to perform OER, whereas the improved efficiency of (121)/40 s can be due to enhanced surface catalytic activity. The highest catalytic activity of the (040)/30 s sample is due to low electron–hole recombination in the bulk and maximum charge injection at the surface.