Kinetic Insight into Zn Vacancy-Mediated Deactivation of GaN:ZnO Photocatalysts

The GaN:ZnO solid solution, a typical oxynitride semiconductor with narrow bandgaps, is one of the most promising catalysts for photocatalytic and photoelectrochemical (PEC) water splitting. However, the photostability of this material has been a major drawback. Herein, we identify the primary cause of the deactivation of GaN:ZnO as the dissolution of the ZnO component. Based on in situ characterizations combining time-resolved and steady-state PEC and spectroscopic techniques, a defect model is proposed where zinc vacancies (VZn) exhibit dual charge states: an electron-filled state that traps holes and an oxidized state that traps electrons. The excessive VZn arising from ZnO dissolution increases the population of vacancies in the oxidized state, making recombination by electron trapping more dominant. It is further revealed that both the initial and oxidized states of VZn directly influence the recombination dynamics of photo-generated carriers on μs–ms timescales. Based on these mechanistic insights, the NiO cocatalyst serving as a surface hole reservoir is modified to suppress the photodissolution of ZnO, prolonging carrier lifetime from μs to over 1 s and enhancing the PEC stability of GaN:ZnO from minutes to 40 h. These findings not only provide pioneering mechanistic insights and a robust strategy for enhancing the PEC performance of GaN:ZnO, but also establish a holistic approach to probing the prevalent stability issue of the oxynitride photocatalysts.