Mechanistic Insight into the Light Absorption and Charge Carrier Separation in Photoelectrochemical Performance of Oxygen-Doped g-C ₃ N ₄ and Oxygen-Vacancy-Enriched Mn ₃ O ₄ 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 Mn₃O_4-<i>x</i> 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.