Highly selective photocatalytic production of H2O2 on sulfur and nitrogen for co-doped graphene quantum dots tuned TiO2

Photocatalytic production of hydrogen peroxide (H2O2) using water and molecular oxygen as the sole material source is a promising and sustainable solar fuel approach. Herein, we developed an efficient photocatalyst (SN-GQD/TiO2) for H2O2 syntheses by tuning TiO2 with sulfur and nitrogen co-doped graphene quantum dots (SN-GQDs). The high luminescent SN-GQDs homogeneously dispersed on TiO2 surface, which induces the extended visible light absorption and enhanced electron migration. The SN-GQD/TiO2 exhibited 3.2 times H2O2 yield (451 mol L-1) as that of bare TiO2 under simulated sunlight irradiation, which was also significantly higher than that over GQD/TiO2 and N-GQD/TiO2. Kinetic evaluations suggested that the formation of H2O2 on SNGQD/TiO2 was dramatically accelerated by 2.4 times compared with that on TiO2, while the decomposition of H2O2 was moderately suppressed (only 25% reduction). The increased H2O2 formation on SN-GQD/TiO2 was attributed to the boosted two-electron reduction of oxygen, which was confirmed by the electron transfer numbers (n = 2.2) obtained from Koutecky-Levuch plots, the less sensitivity of H2O2 production to pH, and the insignificant signals for DMPO-O-2 center dot(-) in ESR measurements. According to theoretical calculations and free energy diagrams of the ORR pathway, a mechanism of proton-coupled electron transfer (PCET) to produce H2O2 was proposed to understand the highly selective two-electron H2O2 production on SN-GQD/TiO2. This study brings an insight to modulate highly selective two-electron photocatalytic reduction of oxygen by introduction of dual doped GQDs that can provide active sites for *OOH formation and proton relays.