Rapid Screening of Photoanode Materials Using Scanning Photoelectrochemical Microscopy Technique and Formation of Z‑Scheme Solar Water Splitting System by Coupling p- and n‑type Heterojunction Photoelectrodes

A scanning photoelectrochemical microscopy (SPECM) technique is applied to rapidly screening the photoelectrochemical (PEC) activities of cobalt (Co)-incorporated bismuth vanadate (BiVO₄) photocatalyst arrays with varying Co concentrations on conducting FTO and Ti substrates. The SPECM screening study is successfully utilized to determine an optimal Co concentration of 6% to improve the photocatalytic performance of BiVO₄. Subsequently, pristine and Co-doped BiVO₄ thin film photoanodes are fabricated by spin-coating/drop-casting methods with optimal precursor concentrations of Co, Bi, and V to validate the results of SPECM. Structural characterization by X-ray diffraction shows that 6% Co-BiVO₄ contains a photocatalytically active scheelite-monoclinic phase of BiVO₄. Scanning electron microscopy images and EDAX show that 6% Co is partly incorporated into the BiVO₄ lattice and the remaining accumulates on the surface in the form of cobalt oxide, which is further evidenced by X-ray photoelectron spectroscopy (XPS) and Raman studies. Co-doped BiVO₄ thin film photoanode prepared by spin-coating method exhibits similar remarkable PEC response with ∼150% increase in photocurrent density at 1.0 V vs RHE with respect to the pristine BiVO₄ photoanode. Additionally, such photoanode exhibits a cathodic shift of ∼200 mV in the onset of water oxidation photocurrent. The Mott–Schottky analysis confirms an increase in charge carrier density of Co-doped BiVO₄ photoanode. Thus, the enhanced water splitting performance by Co is attributed to largely due to (1) enhancement in water oxidation kinetics via formation of cobalt oxide (Co₃O₄) on the surface of BiVO₄, and partially due to (2) enrichment in electronic conductivity of BiVO₄ in the presence of Co. An unbiased Z-scheme solar water splitting system is demonstrated at the end of this work by combining an optimized Co-doped BiVO₄/WO₃ photoanode with a CuO/CuBi₂O₄ photocathode in a two-electrode configuration