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 produce hydrogen utilizing light. Among the different materials used for photoelectrocatalysis, graphitic carbon nitride (g-C3N4) is a promising photoelectrode Material. 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, decorated with oxygen-vacancy-containing Mn3O4 nanoparticles with a lateral size range of 20-30 nm and thicknesses less than 2 nm, was synthesized through an industrially straightforward method, and its photocathodic hydrogen generation performance was investigated. 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 -0.39 V vs RHE at pH = 7. A suppressed recombination rate and a reduced electrode/electrolyte charge transfer resistance were observed in the nanocomposite, as shown by PL and EIS results. Oxygen doping and Mn3O4-x nanoparticle decoration resulted in a significant decrease of the bandgap from 2.9 eV for g-C3N4 to 1.6 eV for the resulting nanocomposite, which clearly expanded the light absorption ranges for the materials 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 2.5-fold 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.