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.

Tetragonal-zircon BiVO4: a better polymorph for the formation of coherent type-II heterostructures for water splitting applications

“…In this study, In addition to the above computed band edge positions of low index surfaces of the t-z polymorph of BiVO 4 , the computed band edge positions of m-s polymorph and t-s polymorph of BiVO 4 (our previous work) 19,22 have also been taken in order to give full combinations of heterostructures with the ZnO, TiO 2 , CdSe and ZnS. It is to be noted that the band alignment data of ZnO, TiO 2 , CdSe and ZnS are taken from our previous study 19 and also provided in the ESI† (Table S1).…”

“…19 Furthermore, it is highly desirable to do facet engineering to construct efficient type-II heterostructures, because different facets have different atomic arrangements and coordination, which can have different band edge positions and it is essential to control facets during the construction of type-II interfaces. [20][21][22][23][24] In this regard, Li et al 25 1 for more BiVO 4 -based isomaterial and heteromaterial heterostructures). Furthermore, it is worth mentioning that a lack of control over the interfacial contact facets would lead to the presence of heterojunctions with various energy band alignments, which can inhibit the separation of charge carriers.…”

“…19 Furthermore, it is highly desirable to do facet engineering to construct efficient type-II heterostructures, because different facets have different atomic arrangements and coordination, which can have different band edge positions and it is essential to control facets during the construction of type-II interfaces. 20–24 In this regard, Li et al 25 selectively grew TiO 2 on the (110) facet and (010) facet of BiVO 4 and found that the BiVO 4 -(110)/TiO 2 heterostructure has the highest photocurrent density due to more electron transfer ( i.e. smaller interfacial energy barrier due to type-II band alignment) than the BiVO 4 -(010)/TiO 2 heterostructure.…”

A large size BiVO4photoanode with high stability for efficient water oxidation and wastewater treatment coupled with H2evolution

“…5,6 Among various photoanode materials, N-type semiconductor BiVO 4 is identied as a desirable candidate because the appropriate bandgap (2.4-2.5 eV) can absorb a high portion of visible light and the favorable conduction band (CB) edge position is very close to the thermodynamic potential of hydrogen (H 2 ) evolution. [7][8][9] Nevertheless, poor carrier mobility and sluggish surface water oxidation kinetics severely restrict the solar to hydrogen efficiency. [10][11][12] Specically, there are fewer active sites on the surface of BiVO 4 , so the bulk of photogenerated electron-hole pairs recombine rather than participating in the water splitting reaction at the photoanode/ electrolyte interfaces.…”