g-C3N4/ZnS Type-II Heterojunction for Photocatalytic Water Splitting

One of the top priorities is to reduce global warming by replacing conventional fuels with hydrogen. A promising solution is artificial photosynthesis, which utilizes solar energy for water splitting. However, the biggest challenge lies in creating a catalytic system that optimizes efficiency while minimizing electron and hole recombination. To achieve this, various strategies such as doping, dye sensitizing, and coupling are being explored. Metal sulfides with a negative CB are found to have a high reduction capacity for H2 production or CO2 reduction and are being studied as potential options. The aim of this study is to investigate the use of ZnS and g-C3N4 heterojunction system in the photocatalytic process of water splitting. The first step involved synthesis of g-C3N4 through one-step thermal polymerization method using pure urea precursor. In the second stage, zinc nitrate, thiourea, and ethylene diamine were used as the precursors for the growth of ZnS rods through the hydrothermal method. FTIR, XRD, SEM, PL, and UV-DRS analyses, as well as electrochemical methods were utilized to investigate the physical and chemical properties of the heterostructure. In order to assess its H2 production capabilities, a TCD detector was utilized along with a laboratory-made photocatalytic reactor. The results revealed that the g-C3N4 underwent morphological changes in the alkaline medium of ZnS rod synthesis, forming spheres with a diameter of about 5 μm, upon which ZnS rods were grown. DRS analyses indicated a shift in the band gap from 3.1 to 2.9 eV and revealed the presence of C=N, C-N, and N-H functional groups related to g-C3N4 in the final structure. The H2 production results demonstrated a 58, 65 % improvement in the H2 production rate in both visible and UV light respectively. The reduced band gap of the ZnS and g-C3N4 heterostructure facilitated a favorable path for electrons, allowing the use of visible light to produce hydrogen. When ZnS was paired with a reductant photocatalyst such as g-C3N4, the electrons generated through light excitation in the ZnS conduction band combine with photo holes in the valence band of g-C3N4. This leads to the transfer of electrons in the valence band of the photocatalyst and holes in the conduction band of ZnS. This transfer of electrons improves the oxidation and reduction capacity of the remaining carriers corresponded with heterostructure type II. In the presented system, the band gap has been reduced, allowing for visible light to be used in the water splitting reaction. Additionally, the rapid recombination of electrons and holes, which was mostly observed in g-C3N4, has been corrected