Role of Catalyst Transformations, Defect Reconstruction, and Seawater Splitting under a Broad Solar Spectrum using MOF-derived (Cu)TiO2 Model System

Abstract

Semiconductor defect engineering is a well-established strategy to enhance photocatalytic efficiency. However, these defects often evolve during the reaction, accompanied by structural transformations of the catalyst itself—an aspect that has been rarely explored. Here, we employ a benchmark seawater-relevant condition to investigate the structural and defect reconstruction in a model (Cu)TiO₂ photocatalyst. In this study, a metal-organic framework (MOF)-derived, Cu-incorporated TiO2 (M-CuTiO2) photocatalyst exhibits a defect-rich structure and undergoes a significant local structural change during photocatalytic hydrogen evolution from a brine sacrificial donor solution. The copper site in the catalyst exhibits transformative effects on activity through its valency and associated phase transitions, while the prevalence of defects in TiO2 is further substantiated through real-time, quasi-in-situ, and ex-situ analysis. The study also reveals that catalytic stability is less influenced by the catalyst phase change and more by external factors such as the presence of electron donors and light intensity. We also find that in natural seawater photolysis, the activity is primarily governed by cations other than the primary constituent, Na+, which accompanies the disruption of the solid-liquid interface. These findings offer valuable insights into the design of post-synthetic MOF-derived photocatalysts for noble metal-free, low-cost, and scalable next-generation seawater hydrogen technology with industrial relevance.