Photoelectrochemical evaluation of SILAR-deposited nanoporous BiVO4 photoanodes for solar-driven water splitting

We report a photoelectrochemical investigation of BiVO4 photoanodes prepared by successive ionic layer adsorption and reaction (SILAR), a facile method that yields uniform nanoporous films. After characterization of the phase, morphology, composition, and optical properties of the prepared films, the efficiencies of charge separation (eta(sep)) and water oxidation (eta(o)(x)) in solar water splitting cells employing these photoanodes were estimated following a previously reported procedure. Unexpected wavelength and illumination direction dependencies were discovered in the derived efficiencies, casting doubt on the validity of the analysis. An alternative approach using a diffusion-reaction model that explicitly considers the efficiency of electron collection resolved the discrepancies and explained the illumination direction dependence of the photocurrent. Electron diffusion lengths (L-n) of 0.45 mu m and 0.55 mu m were derived for pristine and cobalt phosphate (Co-Pi) modified BiVO4, respectively, which are much shorter than the film thickness of similar to 2.1 mu m. The Co-Pi treatment also increased eta(ox )from 0.86 to similar to 1, which is the main reason for the overall performance enhancement caused by adding Co-Pi. These findings suggest that there is little scope for improving the performance of SILAR-deposited BiVO4 photoanodes by further catalyzing water oxidation, but enhanced performance is achievable if electron transport can be improved.