Single-atom cobalt catalysts engineered on tunable CNNO nanosheets for hydrogen generation

Abstract

Single Atom Catalysts (SAC), through stable coordination with the substrate, effectively improve the physicochemical and catalytic characteristics of the 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 reactions for hydrogen generation. However, SAC embedded layered substrates demand cumbersome fabrication approaches; here, we report microwave-assisted (MA) anchoring of a cobalt (Co) SAC on an exfoliated Ca₂Nb_n−3Nb_nO_3n+1⁻ (CNNO) nanosheet substrate and investigate its photoelectrocatalytic 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—contributes to 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 to be 0.442% for CNNO nanosheets (n = 6 layers) at a rate of 3.07 × 10⁻³ mol s⁻¹m⁻². On the other hand, the STH efficiency significantly improved by ∼ two-fold for MA Co-anchored photoelectrodes, reaching 0.867%, with a hydrogen generation rate of 3.30 × 10⁻³ mol s⁻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.