Photoelectrocatalytic Activity of ZnO/RuO2 Composites Toward HER and OER Reactions: The Importance of Surface and Bulk Oxygen Vacancies

With the aim of reducing catalysts’ cost while maintaining high performance in water splitting, ZnO and RuO2 were combined into composites with ZnO to RuO2 mass ratios of 1:1, 2:1, and 10:1. The ZnO/RuO2 composites were prepared by microwave processing of a suspension containing Zn(OH)2 in situ precipitated onto RuO2 powder, and subsequently thermally modified at 600 °C to promote heterojunction formation and alter the defect chemistry. Phase composition, crystal structure, morphology, and optical properties were analyzed in detail employing XRD, TEM/HRTEM, HAADF-STEM with EDS, PL and XPS spectroscopy. The photoelectrocatalytic (PEC) activity of the composites toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was evaluated by linear sweep voltammetry in alkaline electrolyte (0.1 M NaOH, pH 13), before and after one hour of electrochemical system illumination. The analysis focused on surface and bulk oxygen vacancies, which may have a crucial impact in PEC activity, by (1) promoting charge separation and increasing the number of active sites thus enhancing PEC activity, or (2) acting as electron–hole traps and recombination centers, reducing the lifetime of photo-induced charge carriers and thus deteriorating PEC activity. The presented results demonstrate that the combination of ZnO with RuO2 in a specific mass ratio, along with controlled defect structure, offers a worthwhile route for developing bifunctional, noble-metal-reduced catalysts for green hydrogen and oxygen production.