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 ZnWO4/WO3 composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO3 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 % ZnWO4/WO3 resulted in a 2.5-fold increase in the photocurrent compared to pristine WO3 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 WO3 and ZnWO4/WO3 electrodes. X-ray diffraction showed the composite nature of the WO3 and ZnWO4 mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO3 has a smaller band gap than ZnWO4, while Mott–Schottky analysis determined that ZnWO4 has a more negative flat-band potential than WO3. 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 ZnWO4/WO3 composite was formed in a single annealing step.