Photo-catalytic activity of BiVO4 thin film electrodes for solar-driven water splitting

There is an ever-increasing attention directed to the development of solar fuels by photo-electrochemical water splitting, given the inexhaustible availability of solar energy. The water oxidation half-reaction is a critical step for the overall water splitting reaction, and the development of suitable photoanodes is therefore required. The present research work focuses on bismuth vanadate thin films’ deposition on FTO glass electrodes, through the dip coating technique, and discusses the influence of the film preparation technique on the electrode's photo-electrochemical performance. The bismuth vanadate thin films were synthesized with thicknesses ranging from 60 to 210 nm, depending on a number of dip coatings from 2 to 15. The structural and optical characterization of the films showed that monoclinic scheelite-type phase was obtained in all samples, with crystal sizes ranging from 24 to 65 nm, at increasing film thicknesses, and corresponding band gaps between 2.55 and 2.35 eV. A maximum photo-current density of about 0.57 mA cm−2 at 1.23 V vs. RHE under sunlight illumination was obtained for an electrode thickness of 160 nm. The electrochemical impedance spectroscopy elucidated the transport mechanisms occurring at the electrolyte–electrode interface, as well as inside the film. The estimation of the equivalent circuit parameters showed that an increasing film thickness decreased the resistance associated to the charge transfer between the electrolyte and the electrode (from 1100 to 450 Ω, from 60 to 160 nm layer thickness, respectively), given the higher number of active sites involved in the reaction. However, excessive film thicknesses increase the probability of charge recombination within the film and, in the specific case here investigated, can also be associated to film imperfections arising from several deposition-calcination cycles, which further act as traps. These concurring phenomena are of high relevance to isolate the rate-determining step of the water oxidation half-reaction, in the perspective of an optimization of bismuth vanadate film coating on FTO to obtain photo anodes.