Sulfur-doped g-C3N4 photocatalyst for significantly steered visible light photocatalytic H2 evolution from water splitting

Semiconductor polymeric graphitic carbon nitride (g-C3N4) is considered a favorable candidate for converting solar energy into chemical energy driven by visible light. Nevertheless, its practical applications are largely impeded by the low specific surface area, fast recombination of photoexcited electron–hole pairs, and limited visible-light response range. Therefore, it is highly desirable to improve the photocatalytic performance of metal-free g-C3N4 photocatalysts at the molecular level by elemental doping. This study developed a rational cross-linking copolymerization strategy to synthesize sulfur-doped g-C3N4 (S-CN) using urea and 4-mercaptobenzoic acid (4-MBA) as precursors. The results indicate that the introduction of S atoms onto the g-C3N4 framework through the formation of C–S bonds leads to the reduction of the interlayer stacking distances, narrowing of the band gap, expansion of visible-light absorption regions, and increased separation and transfer of photoinduced electron–hole pairs. The optimal S-CN-7 sample exhibits outstanding photocatalytic performance under visible light with the highest hydrogen production rate of 133.12 μmol h−1, which was nearly 8-fold higher than that of pure g-C3N4 (16.83 μmol h−1). In addition, the S-CN sample shows high photocatalytic stability after recycling experiments of up to 20 hours. Theoretical calculations show that doping with S leaves the sample with impurity states, making it easy for photogenerated electrons to jump from the valence band to the impurity state or from the impurity state to the conduction band.