Photocatalytic Water Oxidation of WO3 Nanoparticles with Regenerative Redox Couple

Photocatalytic water splitting is one of the promising ways to produce green hydrogen with zero CO₂ emission. However, current stage, solar to hydrogen conversion efficiency (~1%) is much lower than industrial meets. for visible light utilization, the band gap of semiconductor photocatalyst is limited to less than ~3 eV, however, water splitting reaction requires a large driving force due to its high overpotential for each hydrogen evolution and oxygen evolution reaction and also due to fast electron-hole recombination at the semiconductor photocatalyst surface. Therefore, sacrificial agents such as Ag+, persulfate, and O₂ were used to facilitate the photoelectrochemical reaction and fast carrier extraction from the semiconductor. However, only a half-reaction of water splitting occurs in presence of the sacrificial agent. Moreover, the sacrificial agent is also an energy-contained chemical resource, therefore consuming the sacrificial agent is not the regenerative pathway for water splitting or green hydrogen production. Lastly, some sacrificial agents can lead to the poisoning of photocatalysts. Therefore it is better to use regenerative redox couple as electron or hole donner for photocatalytic reaction. with redox coupled in a water medium, the photocatalysts successfully store solar energy via half-reaction, then the redox couple can be regenerated by the counterpart reaction. In this study water oxidation of WO₃ was carried out with an electron-accepting sacrificial agent, Ag⁺, and redox couples such as Io₃^-/I^-, Co(phen)₃^2+/3+, Fe²⁺/³⁺. To improve the kinetics of both the electron acceptor reduction reaction and water oxidation reaction, the PtRu alloying nanoparticle co-catalyst was introduced. The ratio of Pt and Ru was controlled for the best activity, and also confirmed the PtRu co-catalyst is active for both redox couple reduction and water oxidation reaction. This study shows that the PtRu cocatalyst is useful for tandem photocatalyst systems with a redox mediator or other tandem system for photocatalytic full water splitting systems.