Single-Atom Cobalt Catalysts Engineered on Tunable CNNO Nanosheets for Hydrogen Generation

Abstract

Single Atom Catalysts (SAC), through formation of stable coordination bonds, effectively improve the physicochemical and catalytic characteristics of its substrate. Additionally, layered metal oxide substrates offer strong anchoring sites and enhance the charge transfer dynamics between SACs—a desirable trait for photoelectrochemical (PEC) catalytic reaction for hydrogen generation. However, SAC embedded layered substrates demand cumbersome fabrication approaches; here we report a microwave-assisted (MA) anchoring of cobalt (Co) SAC on exfoliated Ca2Nbn-3NbnO3n+1- (CNNO) nanosheets substrate and investigate its photo electrocatalytic activity towards hydrogen generation. The two-dimensional Dion Jacobson (DJ) CNNO nanosheets exhibit oxygen vacancies induced by distortions in the NbO6 octahedra. The elevated Nb4+/Nb5+ ratio—attributed to the tunable layers of the nanosheet—contribute to the local charge carrier densities and their transfer by trapping electrons or holes. Upon anchoring Co onto CNNO nanosheets, the Co SACs introduce localized states in the CNNO nanosheets. Notably, the bivalent and trivalent oxidation states of Co act as active catalytic sites for hydrogen generation. The solar-to-hydrogen (STH) efficiency of CNNO photoelectrodes was calculated as 0.442% for CNNO nanosheets (n = 6 layers) at the rate of 3.07 x 10-3 mols-1m-2. On the other hand, the STH efficiency significantly improved by ~two-fold for MA Co-anchored photoelectrodes, 0.867%, and hydrogen generation rate of 3.30 x 10-3 mols-1m-2. These findings address the need for a facile approach towards SAC embedded metal oxide substrates and provide insights for future hydrogen generation devices.