Unique Properties of RhCrOx Cocatalyst Regulating Reactive Oxygen Species Formation in Photocatalytic Overall Water Splitting

Cocatalysts play important roles in the photocatalytic overall water splitting (POWS) reaction. Electron paramagnetic resonance (EPR) spin trap experiments were carried out to study the effects of the RhCrOx cocatalyst on GaN-ZnO in reactive oxygen species (ROS) formation during the POWS reaction. DMPO spin trap studies in aqueous solution suggest that ROS formed during POWS could oxidize DMPO to DMPOX confirmed by H217O isotope experiments. Further spin trap studies by DMPO in DMSO, TEMPO, and 4-oxo-TEMP in aqueous solutions indicate that RhOx could facilitate both the desired hydrogen evolution reaction (HER) and undesired reactions of superoxide radicals and singlet oxygen formation. On the contrary, CrOx inhibits formation of these two ROS although it could not promote the POWS reaction. RhCrOx compromises the above two effects to be an outstanding cocatalyst for POWS. This work suggests that regulation of ROS also should be considered for the design and synthesis of a more efficient cocatalyst for POWS.