ZnWO4/WO3 Composite for Improving Photoelectrochemical Water Oxidation

A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO 4 /WO 3 composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO 3 with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO 4 /WO 3 resulted in a 2.5-fold increase in the photocurrent compared to pristine WO 3 for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO 3 and ZnWO 4 /WO 3 electrodes. X-ray diffraction showed the composite nature of the WO 3 and ZnWO 4 mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO 3 has a smaller band gap than ZnWO 4, while Mott–Schottky analysis determined that ZnWO 4 has a more negative flat-band potential than WO 3 . A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO 4 /WO 3 composite was formed in a single annealing step.