Transition metal Dual–Atom Ni2/TiO2 catalysts for photoelectrocatalytic hydrogen Evolution: A density functional theory study

Dual-atom catalysts (DACs), compared with single-atom catalysts (SACs) that have been widely studied, are now in their infancy, although they are expected to have unique catalytic properties. Here, we propose and investigate the Ni2 dimer on TiO2 system, Ni2/TiO2, that is taken as a representative of metal oxide-supported transition metal DACs, for electrocatalytic and photocatalytic hydrogen evolution using density functional theory calculations. A bottom-up self-assembly approach is developed, which can be generalized to transition metal DACs. The localization and hybridization of electronic states of the Ni2 dimer are illustrated from the coordination environments. As compared with Ni1/TiO2 SACs, the delocalized states of the Ni2 dimer of Ni2/TiO2 not only increase the reaction sites and the reduction capacities, but also positively change the H adsorption kinetics and thermodynamics, facilitating hydrogen production. The band gap of Ni2/TiO2 is narrowed into the visible region, and deep trap levels in the gap are smeared, improving the photoactivity efficiency. Our study presents a facile adsorption-deposition method to design and fabricate oxide-supported transition metal DACs for photo(electro)catalytic H2 production.