Mechanistic insight into the light absorption and charge carrier separation in photoelectrochemical performance of oxygen-doped g-C3N4 and oxygen-vacancy-enriched Mn3O4 nanocomposites

With the increasing awareness of the universal need to access clean and renewable energy resources, semiconductor photoelectrocatalysis has emerged as an efficient way to utilize light photons and produce hydrogen. Among different materials used for photoelectrocatalysis, graphitic carbon nitride (g-C3N4) is a promising photoelectrode; but possesses some drawbacks. Also, most researchers are focused on the anodic application of g-C3N4-based materials; while its cathodic performance has remained less investigated. In the present study, a g-C3N4-based nanocomposite made of oxygen-doped g-C3N4 nanosheets, and oxygen-vacancy-containing Mn3O4 nanoparticles with a size range of 20-30 nm was synthesized as photocathode through a facile method and carefully characterized. The internal electric field in the interface enhanced the charge carrier separation compared to both g-C3N4 and oxygen-doped g-C3N4, with an impressive cathodic photocurrent density of the resulting nanocomposite (-5.28 mA.cm-²) higher than those of g-C3N4 (-2.51 mA.cm-²) and oxygen-doped g-C3N4 (-5.05 mA.cm-²) at -1 V vs. Ag/AgCl at pH=7. The bandgap of the oxygen-doped g-C3N4, oxygen-vacancy-enriched Mn3O4, and the resulting nanocomposite were approximately 1.5 eV, 1.4 eV, and 1.6 eV, respectively, and the light absorption ranges for the materials were clearly expanded to the visible region. The specific surface area of the nanocomposite was obtained by BET analysis to be 224.65 m2.g-1, representing a remarkable rise in comparison with oxygen-doped graphitic carbon nitride, contributing to the high photoelectrochemical performance of the nanocomposite electrode. In this platform, the oxygen-vacancy-enriched Mn3O4 also serves as an effective co-catalyst, eliminating the need to use precious materials such as platinum particles.