Cooperative catalytic behavior of SnO₂ and NiWO₄ over BiVo₄ photoanodes for enhanced photoelectrochemical water splitting performance

n-BiVO₄ is a favorable photoelectrode candidate for a photoelectrochemical (PEC) water splitting reaction owing to its suitable energy level edge locations for an oxygen evolution reaction. On the other hand, the sluggish water oxidation kinetics of BiVO₄ photoanodes when used individually make it necessary to use a hole blocking layer as well as water oxidation catalysts to overcome the high kinetic barrier for the PEC water oxidation reaction. Here, we describe a very simple synthetic strategy to fabricate nanocomposite photoanodes that synergistically address both of these critical limitations. In particular, we examine the effect of a SnO₂ buffer layer over BiVO₄ films and further modify the photoanode surface with a crystalline nickel tungstate (NiWO₄) nanoparticle film to boost PEC water oxidation. When NiWO₄ is incorporated over BiVO₄ /SnO₂ films, the PEC performance of the resultant triple-layer NiWO₄ /BiVO₄/SnO₂ films for the oxygen evolution reaction (OER) is further improved. The enhanced performance for the PEC OER is credited to the synergetic effect of the individual layers and the introduction of a SnO₂ buffer layer over the BiVO₄ film. The optimized NiWO₄/BiVO₄/SnO₂ electrode demonstrated both enriched visible light absorption and achieves charge separation and transfer efficiencies of 23% and 30%, respectively. The photoanodic current density for the OER on optimized NiWO₄ /BiVO₄ /SnO₂ photoanode shows a maximum photocurrent of 0.93 mA/cm² at 1.23 V vs. RHE in a phosphate buffer solution (pH~7.5) under an AM1.5G solar simulator, which is an incredible five-fold and two-fold enhancement compared to its parent BiVO₄ photoanode and BiVO₄/SnO₂ photoanodes, respectively. Further, the incorporation of the NiWO₄ co-catalyst over the BiVO₄ /SnO₂ film increases the interfacial electron transfer rate across the composite/solution interface.