Metastable Ni(I)-TiO_2- Photocatalysts: Self-Amplifying H₂ Evolution from Plain Water without Noble Metal Co-Catalyst...

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO₂ nanoparticles in an aqueous Ni²⁺ solution. UV illumination creates in situ a Ni⁺/TiO₂/Ti³⁺ photocatalyst that self-activates and, over time, produces H₂ at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni⁺) surface states. Metallic nickel states, i.e., Ni⁰, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni⁺ surface states act as electron relay for electron transfer to form H₂ from water, in the absence of sacrificial species or noble metal cocatalysts.