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.