Infrared response in photocatalytic polymeric carbon nitride for water splitting via an upconversion mechanism

Broad-spectrum light activation in photocatalytic materials is considered vital for effective solar-to-hydrogen energy conversion. Here, we propose an upconversion process in oxygen-doped polymeric carbon nitride (C3N4) nanosheets, resulting in high photocatalytic activity for hydrogen generation from water splitting under infrared light irradiation. Due to the upconversion, the photoexcited electrons are transferred to the conduction band, and a σ* carbon-oxygen orbital localized on the triazine matrix ring resulting from oxygen doping extends the lifetime of photogenerated electrons, which increases the photocatalytic activity under both infrared and visible light. Our material exhibits high apparent quantum efficiency of 0.014% and 23% at 850 nm and 420 nm, respectively. Under infrared light (λ ≥ 800 nm, 24 mW cm−2), the hydrogen production rate of our material is 22.84 μmol h−1 g−1, which can reach a yield similar to that of pristine C3N4 under visible light. Photocatalysts with broad-spectrum light activation are needed for high efficiency water splitting. Here, an upconversion process in oxygen-doped carbon nitride nanosheets contributes to enhanced photocatalytic activity under both visible and infrared light.