Curvature Rebuilt d-Orbital Energy Levels of TM@g-C6N6 Nanotubes for Water-Splitting Enhancement

Abstract Precisely constructing transition metal (TM) active sites in two-dimensional materials can enhance water splitting, whereas the role of geometric factors such as interfacial curvature remains poorly understood. Nanotubes (NTs) provide a well-defined curved architecture that can modulate electron interactions and reactivity. Here, we rationally design a series of TM@g-C6N6 NTs and use first-principles calculations assisted by machine learning to elucidate how dimensional reduction and curvature govern water-splitting activity. We find that reduced curvature radius systematically promotes water splitting. Notably, the Ir/Rh-decorated g-C6N6 NTs show remarkable electrocatalytic performance with calculated overpotentials of 0.05 eV for the hydrogen evolution reaction and 0.20 eV for the oxygen evolution reaction. Curvature-induced confinement drives reconstruction and reoccupation of the d-orbital states near the Fermi level upon transitioning from a flat plane to a tubular geometry. More importantly, for different reaction intermediates, we identify a curvature-dependent inversion of the dominant adsorption orbital transition from the dz2 to the dx2–y2, which directly correlates with enhanced catalytic activity. Our results establish interfacial curvature as a key manipulator for tuning electronic structure and water-splitting reactivity in TM@g-C6N6 NT single-atom catalysts and provide a new perspective on modulating electronic and catalytic properties in TM@g-C6N6 NT-like structures.