g-C3N4-based homojunction induced by Ag and P selective doping for improved photocatalytic H2 evolution coupled with tetracycline degradation: Type-II versus S-scheme mechanism

The reasonable design of g-C3N4(CN)-based homojunctions is a promising method to improve the photocatalytic performance. Controlling the interfacial charge-transport process is challenging. Herein, Ag and P-doped CN (ACN and PCN) were selectively synthesized. The band edges of CN move downward continuously with increasing Ag or P doping concentration. CN/4ACN (type-II), CN/8PCN (S-scheme) and 4ACN/8PCN (S-scheme) were formed by suitable energy level matching, and their charge transfer mechanisms were confirmed via VB-XPS, UPS and ESR analysis. In the presence of the hole sacrificant triethanolamine (TEOA), both CN/4ACN and 4ACN/8PCN exhibited better H2 evolution reaction (HER) rate than CN/8PCN due to the improvement in conductivity caused by Ag doping. However, only CN/8PCN and 4ACN/8PCN exhibited HER coupled with tetracycline (TC) degradation performance owing to the high redox potential of the S-scheme. Benefiting from a good carrier transport property and high redox potential, 4ACN/8PCN exhibited the best HER rate (2305.6 μmol·g−1·h−1) with TEOA as the hole sacrificant, which was 11.7 times than that of pure CN, respectively, and achieved a high HER rate (117.8 μmol·g−1·h−1) coupled with simultaneous TC degradation. This study provides a basic understanding of CN homojunctions, which shows potential for expanded applications in photocatalytic H2 evolution and pollutant degradation.