Preparation Of G-C₃N₄-based Photocatalytic Materials And Study On Their Photocatalytic Performance

The utilizing of solar energy as the driving force to split water for photocatalytic hydrogen evolution has been regarded as one of the most promising means of clean energy production（hydrogen energy）.The key point of current research is to develop efficient and stable visible light photocatalysts.g-C₃N₄ comprises only carbon and nitrogen atoms,making it an environmentally friendly and metal-free semiconductor-based photocatalyst.It is suitable for H₂-evolution from water splitting under visible light owing to its appropriate bandgap,visible-light harvesting capacity,and thermal/chemical stability.However,the application of g-C₃N₄ is impeded because of its poor absorption beyond 470nm,low specific surface area,significant internal resistance,and rapid recombination of photoexcited charge carriers.Accordingly,we prepared various g-C₃N₄ composites photocatalysts to address the above weakness.First,Ag nanoparticles（NPs）were introduced in g-C₃N₄ by an in-situ calcination method,the photocatalytic performance of g-C₃N₄ was improved by controlling the position of Ag NPs deposition.Then the co-catalyst（NiS）was introduced on the surface of g-C₃N₄,which efficiently promoted the photogenerated charge carrier separation,increased the reactive sites and intensified photocatalytic H₂ production kinetics.Finally,we studied the synergistic effect of Ag₂Se/（Ag）/g-C₃N₄ heterogeneous junction.The main research conclusions were listed as follows:1.Ag nanoparticles（NPs）were introduced in g-C₃N₄ by an in-situ calcination method.Ag NPs intercalated g-C₃N₄ and Ag NPs loadded g-C₃N₄ composites were synthesized by controlling the position of Ag NPs deposition.The photocatalytic performance was characterized through photocatalytic water splitting for H₂ generation.The results showed that the 1.0 wt%-（Ag）/g-C₃N₄ composite exhibits excellent photocatalytic H₂ generation performance.An H₂ production rate of 3.524 mmol·g^?1·h^?1 was achieved,which is 4.62-times higher than pure g-C₃N₄.The mechanism analysis revealed that the enhancement of photocatalytic activity could be ascribed to the efficiently reduced recombination of photogenerated electron–hole pair,intensified light harvesting capacity,and excellent electrical conductivity in the interior of composites.2.NiS was successfully loaded on the surface of g-C₃N₄ by a hydrothermal method.The results of XRD,FT-IR,XPS and TEM indicates that the Ag NPs and NiS could be homogeneously distributed in the interior of g-C₃N₄ and surface.The crystalline structure of g-C₃N₄ was not affected by the introduction of Ag and NiS.The results of UV-Vis,EIS and PL indicates that the harvesting capacity and photogenerated charge carrier separation were all improved,which was favorable for photocatalytic H₂ evolution.The H₂ generation results of the different photocatalysts showed that the 10 wt.%-NiS/1.0 wt.%-（Ag）/g-C₃N₄sample exhibits the highest activity of 9.728 mmol·g^–1·h^–1,which is 10.82-,3.45-,and2.77-times higher than those of g-C₃N₄,10 wt.%-NiS/g-C₃N₄,and 1.0 wt.%-（Ag）/g-C₃N₄.In order to investigate the photocatalytic stability of the 10 wt.%-NiS/1.0wt%-（Ag）/g-C₃N₄ photocatalyst,cycling tests were performed and the XRD pattern of the catalyst was obtained after four cycles.The XRD pattern and photocatalytic H₂ production rate hardly change,indicating that the as-prepared sample is stable.3.Ag₂Se is an efficient photocatalyst.The intimate interface contact was formed between the Ag₂Se/（Ag）/g-C₃N₄ heterojunction using thermal reflow.In order to make the separation and transfer of carrier more prompt between g-C₃N₄ and Ag₂Se semiconductor,the Ag NPs were intercalated into the interface between g-C₃N₄ and Ag₂Se.The Ag NPs could act as diversion channel,which was favourable for the transfer of photogenerated eletrons from the g-C₃N₄ to Ag₂Se.The results of photocatalytic H₂ generation showed that the 5 wt.%-Ag₂Se/1.0 wt.%-（Ag）/g-C₃N₄ sample exhibits the highest activity of 7.937mmol·g^–1·h^–1,which is 9.69-times higher than g-C₃N₄.The cycling tests were performed and showed that the photocatalytic H₂ production rate hardly change,indicating that the as-prepared sample is stable.The mechanism analysis revealed that once the the heterostructure of semiconductor/Ag NPs/semiconductor was constructed,the synergistic effect among g-C₃N₄,Ag NPs and Ag₂Se could promote the transfer of carriers between different semiconductor interfaces and enhance the photocatalytic activity of composite materials.