Deconvoluting Photoelectrochemical\nActivity in Monoclinic–Scheelite\nBiVO₄ Facet Selected Thin Films

Abstract

Crystal facet engineering is one of the promising strategies\nto\ntune the band edge positions and surface electrochemistry of a material,\nwhich are essential to improve photoelectrochemical (PEC) water splitting.\nMaterials with low-crystal symmetry structures demonstrate facet-dependent\nproperties due to asymmetric coordination, and facet engineering can\nmodulate PEC properties. In this regard, different facets [e.g., (002),\n(121), and (040)] of the monoclinic–scheelite polymorph of\nBiVO₄ (low-crystal symmetry structure) have been grown\nby controlling the thickness (deposition time) of thin films [22 nm\n(10 s) to 265 nm (50 s)] by the electron beam deposition technique.\nX-ray diffraction and high-resolution transmission electron microscopy\nanalysis suggest the presence of different exposed planes that display\ndifferent EC and PEC activity. PEC water splitting measurements suggest\nthat the 110 nm/30 s thin-film sample, that is, the (040) facet, has\nthe highest current density, that is, 0.29 mA/cm² (under\nlight) and 0.068 mA/cm² (under dark) at 1.8 V versus RHE.\nHowever, the applied bias photon to current efficiencies (ABPEs) of\nboth the thin films, that is, (040)/30 s and (121)/40 s facets, are\nnearly equal, whereas (121)/40 s has enhanced electrical and solar\npower-to-hydrogen (ESPH) conversion efficiencies compared to the (040)/30\ns facet sample. Band edge positions computed via density functional\nsimulations of exposed surfaces suggest that different facets have\ndifferent band edge positions, thereby offering different driving\nforces to perform oxygen evolution reaction (OER). The relation between\nefficiency (ABPE and ESPH) and driving force suggests that the enhanced\nPEC performance of the (040) facet of BiVO₄ is due to the\nincreased driving force to perform OER, whereas the improved efficiency\nof (121)/40 s can be due to enhanced surface catalytic activity. The\nhighest catalytic activity of the (040)/30 s sample is due to low\nelectron–hole recombination in the bulk and maximum charge\ninjection at the surface.