Solution-processed TiO₂/BiVO₄/SnO₂ triple-layer photoanode with enhanced photoelectrochemical performance

The design of heterostructured multilayer oxide films for photoanodes enables the control of interfacial, charge transport/transfer and optical properties as well as stability, thus resulting in efficient photoelectrochemical (PEC) water splitting. Here, we report a triple-layered TiO₂/BiVO₄/SnO₂ (T/B/S) photoanode that shows improved PEC water-oxidation performance and high visible transmittance at the wavelengths above 510 nm. The T/B/S photoanode was fabricated by a solution spin-coating method via a sequential deposition of the three layers. First, a bottom SnO₂ layer with thickness ∼200 nm was deposited on a F:SnO₂ (FTO) substrate. Subsequently, a BiVO₄ middle layer (∼130 nm) and a TiO₂ nanoparticle top layer (∼100 nm) were deposited. The three distinct layers of TiO₂, BiVO₄, and SnO₂ deposited on the FTO substrates were free of voids and cracks. Importantly, the bottom SnO₂ layer caused an increase in the lateral grain size (up to ∼600 nm) of the BiVO₄ layer and formed a type-II heterojunction with the layer (similar to the BiVO₄/WO₃ case), thus efficiently improving charge separation and electron transport properties. Furthermore, the top TiO₂ (anatase phase) layer formed a staggered conduction band structure with the BiVO₄ layer and also protected the underlying layers against photocorrosion. The resultant T/B/S photoanode, which was devoid of any electrocatalyst, showed a higher photocurrent density of ∼2.3 mA/cm² and ∼3.7 mA/cm² at 1.23 V versus reversible hydrogen electrode for water oxidation and H₂O₂ oxidation, respectively, and a higher stability compared to those of BiVO₄/SnO₂ and pristine BiVO₄ photoanodes.