Metastable Ni(I)-TiO2-X Photocatalyst: Self-Amplifying H2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent

Abstract

This contribution describes the self-assembling and self-activating nature of a TiO2 photocatalyst that generates hydrogen from water in the absence of sacrificial species or noble metal co-catalysts [1]. Such system forms when nanoparticles of reduced anatase TiO2 are illuminated in an aqueous Ni2+ solution [2,3]. UV illumination creates in-situ a Ni+/TiO2/Ti3+ photocatalyst that over time produces H2 at a higher rate. Thus, UV light is both the synthesis tool that forms this active metastable photocatalytic entity and the energy provider for this entity to enable H2 evolution from water.Operando X-ray absorption (XAS) and electron paramagnetic (EPR) spectroscopies show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor [4], which enables the formation of monovalent Ni+ surface states. Metallic nickel states, i.e., Ni0, do not form, neither in the dark (resting state) nor 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.Self-amplifying reaction schemes of this type entail considerable potential for developing a new generation of photocatalysts via simple one-pot synthesis routes.