Abstract: Fabricating homojunction photocatalyst is a promising approach to accelerate the separation and transfer of photogenerated charge carriers, and to boost photocatalytic performance. Herein, a novel poly (heptazine imide) (PHI)-based S-scheme homojunction photocatalyst (U/T-LHPHI) is fabricated through an ionothermal synthesis route, which exhibits particular high and low crystallinity property, intimate interface combination, and locally N self-doping. The regulation of crystallinity contributes to the differentiated electronic structure in PHI, which leads to the establishment of internal electric field (IEF). The intense IEF and N doping level with electron extracting capacity synergistically promote the charge transfer from the high crystalline PHI (HPHI) to the low crystalline PHI (LPHI) following the S-scheme mechanism. Additionally, the strong interfacial interaction improves the interfacial charge transfer dynamics. As a consequence, photogenerated electrons with powerful reducing ability are maintained effectively. Upon light irradiation, the optimized U/T-LHPHI performs an H₂ evolution rate of 4880, 2416, and 2375 µmol g^-1 h^-1 in deionized water, simulated seawater, and real seawater, respectively, which exceed that of many carefully designed noble metal Pt containing photocatalyst. This work provides an important verification that the rational design and construction of homojunction photocatalysts could effectively enhance photocatalytic activity.

Key words: PHI, Homojunction construction, Photocatalyst, H₂ production, S-scheme