Graphite Carbon-Confined Transition Metal Nanoparticle Cocatalysts on TiO ₂ Nanosheets for Enhancing Photocatalytic Activity of Hydrogen Evolution

The development of noble-metal-free cocatalysts with high activity and durability remains a critical challenge for photocatalytic hydrogen production. Herein, we constructed a family of 0D/2D metal@graphitic-carbon/TiO₂ (M@C/TiO₂) nanocomposite photocatalysts, composed of graphitic carbon-confined transition metal nanoparticle (M@C, M = Ni, Co, Cu, and Fe) cocatalysts and TiO₂ nanosheet support. Ni@C/TiO₂ exhibited exceptional photocatalytic hydrogen evolution activity, comparable to that of Pt/TiO₂, which was significantly superior to pristine TiO₂, Ni/TiO₂ and C/TiO₂ by factors of 47, 28, and 6, respectively. The improved photocatalytic activity is attributed to not only the extended light absorption, altered electronic properties, and enhanced charge carrier separation mediated by graphitic carbon-confined transition-metal nanoparticles but also the improved charge carrier transfer through the Ti-O-C-Ni heteroatomic channel between the metal core and the TiO₂ support. Results confirmed the formation of an intimate 0D/2D heterointerface with an optimal band structure, preserved crystallinity, and enriched electron density at Ni active sites. It was also revealed that the graphitic-carbon shell not only protected the metallic nanoparticle from oxidative corrosion but also synergistically enhanced charge separation via interfacial electron transfer mediation. Importantly, the mechanism study indicated that the shallowly trapped electrons that resided at the heterointerface of M@C and TiO₂ contributed to the photocatalytic hydrogen evolution activity, rather than the deeply captured electrons on metal nanoparticles. This work establishes a universal strategy for designing stable, cost-effective transition metal-based cocatalysts through interfacial engineering and protective carbon-shell encapsulation, offering insights into sustainable solar-to-hydrogen conversion systems.