Direct Electron Transfer from Upconversion Graphene Quantum Dots to TiO₂ Enabling Infrared Light-Driven Overall Water Splitting

Utilization of infrared light in photocatalytic water splitting is highly important yet challenging given its large proportion in sunlight. Although upconversion material may photogenerate electrons with sufficient energy, the electron transfer between upconversion material and semiconductor is inefficient limiting overall photocatalytic performance. In this work, a TiO₂/graphene quantum dot (GQD) hybrid system has been designed with intimate interface, which enables highly efficient transfer of photogenerated electrons from GQDs to TiO₂. The designed hybrid material with high photogenerated electron density displays photocatalytic activity under infrared light (20 mW cm^-2) for overall water splitting (H₂: 60.4 μmol g_cat. ^-1 h^-1 and O₂: 30.0 μmol g_cat. ^-1 h^-1). With infrared light well harnessed, the system offers a solar-to-hydrogen (STH) efficiency of 0.80% in full solar spectrum. This work provides new insight into harnessing charge transfer between upconversion materials and semiconductor photocatalysts and opens a new avenue for designing photocatalysts toward working under infrared light.