Nature of the Active Ni State for Photocatalytic Hydrogen Generation

Abstract Thermal treatments can have detrimental effects on the photocatalytic hydrogen (H₂) evolution performance and impact the formation mechanism of the active state of surface‐supported co‐catalysts. In this work, a range of Ni‐based co‐catalysts is investigated immobilized on TiO₂, evaluated their H₂ evolution rates in situ over 21 h, and analyzed the samples at various stages with a comprehensive set of spectroscopic and microscopy techniques. It is found that achieving the optimal hydrogen evolution (HER) performance requires the right Ni⁰:Ni²⁺ ratio, rather than only Ni⁰, and that Ni needs to be weakly adsorbed on the TiO₂ surface to create a dynamic state. Under these conditions, Ni can undergo an efficient redox shuttle, involving the transformation of Ni²⁺ to Ni⁰ and back after releasing the accumulated electrons for H⁺ reduction (i.e., Ni²⁺ ↔ Ni⁰). Yet, when the calcination temperature of the Ni/TiO₂ photocatalysts increases, resulting in stronger coordination/adsorption of Ni on TiO₂, this process is gradually inhibited, which ultimately leads to decreased HER performances. This work emphasizes the significance and influence of thermal treatments on the Ni active state formation – a process that can be relevant to other HER co‐catalysts.