Solar-Driven H_2O_2 Generation From H_2O and O_2 Using Earth-Abundant Mixed-Metal Oxide@Carbon Nitride Photocatalysts

Abstract

Light-driven generation of H_2O_2 only from water and molecular oxygen could be an ideal pathway for clean production of solar fuels. In this work, a mixed metal oxide/graphitic-C_3N_4 (MMO@C_3N_4) composite was synthesized as a dual-functional photocatalyst for both water oxidation and oxygen reduction to generate H_2O_2. The MMO was derived from a NiFe-layered double hydroxide (LDH) precursor for obtaining a high dispersion of metal oxides on the surface of the C_3N_4 matrix. The C_3N_4 is in the graphitic phase and the main crystalline phase in MMO is cubic NiO. The XPS analyses revealed the doping of Fe^(3+) in the dominant NiO phase and the existence of surface defects in the C3N4 matrix. The formation and decomposition kinetics of H_2O_2 on the MMO@C_3N_4 and the control samples, including bare MMO, C_3N_4 matrix, Ni- or Fe-loaded C_3N_4 and a simple mixture of MMO and C_3N_4, were investigated. The MMO@C_3N_4 composite produced 63 μmol L^(−1) of H_2O_2 in 90 min in acidic solution (pH 3) and exhibited a significantly higher rate of production for H_2O_2 relative to the control samples. The positive shift of the valence band in the composite and the enhanced water oxidation catalysis by incorporating the MMO improved the light-induced hole collection relative to the bare C_3N_4 and resulted in the enhanced H_2O_2 formation. The positively shifted conduction band in the composite also improved the selectivity of the two-electron reduction of molecular oxygen to H_2O_2.