Exploration of the g-C3N4 Heterostructure with Ag–In Sulfide Quantum Dots for Enhanced Photocatalytic Activity

Photocatalysis is an effective technology to convert solar energy into chemical energy, which has attracted great attention for the degradation of water pollutants and the hydrogen production by water splitting. The nonmetallic polymer g-C3N4 (GCN) can meet the thermodynamic conditions of photocatalytic water splitting, but its performances are not satisfying due to its narrow light absorption range and high recombination rate of photogenerated charge carriers. Among metal sulfide semiconductors, Ag–In sulfide quantum dots (AIS QDs), such as AgInS2, show excellent visible light absorption and promising photoactivity. In this work, AIS QDs-modified GCN is synthesized by an in situ growth method in mild conditions. The photocatalytic activity of the AIS-QDs/GCN nanocomposite is notably higher than that of the pure phase g-C3N4. Especially, the sample containing 10 wt % AIS QDs has the best activity in both tetracycline degradation and hydrogen generation, reaching 48.5% degradation efficiency in 1 h of visible light exposure (3.2 times that of GCN) and a hydrogen evolution rate of 62.3 μmol·g–1·h–1 (that of bare GCN being negligible). The optical and photoelectrochemical characterization highlights the interplay between the two components, suggesting that the enhanced photocatalytic activity of AIS-QDs/GCN is mainly due to the broadening of the light absorption range, the acceleration of charge transfer, and the reduction of the carrier pair recombination rate due to the formation of a type-II heterojunction inside the composite catalyst. This work is among the first attempts to modify g-C3N4 with polysulfide quantum dots to improve its catalytic performance, and the results provide an important step for advances in the application of these systems.