Energy-level dependent H2O2 production on metal-free, carbon-content tunable carbon nitride photocatalysts

Abstract

Light-driven production of H2O2 from water and molecular oxygen could be a promising way for obtaining both solar fuels and fundamental chemicals. During that process, the H2O2 yield is strongly dependent on the reaction pathway associated with two-electron reduction of dioxygen by the photo-generated electrons. Herein, we synthesized a series of metal-free, carbon-content tunable carbon nitride photocatalysts (named C3N4Carbon) by a facile hydrothermal reaction and subsequent thermal treatment at appropriate temperatures. The energy levels of the C3N4Carbon catalysts vary with the carbon doping level, which is conveniently tuned by changing the initial glucose concentration during the hydrothermal reaction. The surface carbon species evolve with the carbon content and the nitrogen atoms in the structure of carbon nitride are partially substituted by foreign carbon atoms based on XPS measurements. The optimal catalyst leads to the highest H2O2 yield of 1271 µmol L−1 in an acidic aqueous solution (pH 3) after a reaction period of 4 h, twice higher than the pristine C3N4. In addition, the largest formation rate constant and the smallest decomposition rate constant of H2O2 are obtained on the optimal one according to the kinetics analyses. The decomposition tests of H2O2 indicate that the formation rate could be a dominant factor impacting the H2O2 yield. The conduction band position of the optimal catalyst is positively shifted to 0.06 V versus RHE, which is more favorable to the reduction of dioxygen to H2O2 (O2/H2O2 at 0.69 V versus RHE). The positive shift of valence band also improves hole collection and leads to enhanced formation of H2O2.