Metastable Ni(I)-TiO 2- x Photocatalysts: Self-Amplifying H 2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent.

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 TiO 2 nanoparticles in an aqueous Ni 2+ solution. UV illumination creates in situ a Ni + /TiO 2 /Ti 3+ photocatalyst that self-activates and, over time, produces H 2 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., Ni 0 , 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 H 2 from water, in the absence of sacrificial species or noble metal cocatalysts.