Preparation Of G-C₃N₄-based Semiconductor Heterojunction And Its Performance In Visible Light Hydrogen Evolution

With the rapid development of global society and economy,fossil energy consumption has caused serious environment pollution.It has been found through research that semiconductor photocatalysis technology can realize the conversion of low-density solar energy to high-density chemical energy,which is considered to be the most promising and the best way to solve the problem of energy shortage and environmental pollution at the source.The ability of photocatalyst to capture light determines the conversion efficiency of solar energy.Based on this,seeking new types of semiconductor materials with suitable structures will become long-term goals and challenges in the field of photocatalysis research.Graphite phase carbon nitride（g-C₃N₄）is favored by many researchers because of its unique electronic band structure,special optical properties,excellent physical and chemical stability,as well as its simple synthesis method,low cost and green pollution-free.Since g-C₃N₄ was found to be applicable to photocatalytic hydrogen evolution,the research on the properties of g-C₃N₄ and its modification methods have been gradually deepened.However,the defects of g-C₃N₄,such as its low response to visible light,poor electrical conductivity,easy recombination of photogenerated electrons and holes,are still the main problems hindering its development in the field of photocatalysis.Here in,heterojunction with other semiconductors was constructed using as a matrix,to improve the photocatalytic activity of g-C₃N₄.The light-capturing ability,the separation of photo-induced electron-hole pairs,and the enhancement mechanism of visible light hydrogen evolution were investigated.In the first part,the Z-Scheme Ag₃PO₄/g-C₃N₄ heterojunction composite photocatalysts were successfully prepared by calcing the mixture of urea and trace amount of Ag₃PO₄.The results of hydrogen evolution under visible light（λ>400 nm）show that Ag₃PO₄/g-C₃N₄ can produce hydrogen continuously and has good stability under the condition of no cocatalyst.Among them,the photocatalytic hydrogen evolution rate of 0.04AP-CN is 8.93μmol/h,which is 3.3 times higher than that of pure g-C₃N₄.After the addition of 3 wt%Pt cocatalyst,the hydrogen evolution rate of 0.04AP-CN is27.36μmol/h,which is 5.6 times higher than that of pure g-C₃N₄.Based on various experimental results,it is reasonably speculated that the enhanced visible light hydrogen evolution of the Ag₃PO₄/g-C₃N₄ photocatalyst is operated by a Z-Scheme heterojunction structure,which can enhance visible light hydrogen evolution.This experiment provides references for the synthesis of Z-Scheme heterojunction composite photocatalysts and the construction of photocatalytic hydrogen production systems without cocatalyst.In the second part,the BiPO₄/S-CN composite photocatalysts were successfully prepared by calcing the mixture of urea with 2-thiobarbituric acid and BiPO₄ nanorods at elevated temperature.BiPO4 nanorods were prepared by hydrothermal method.The heterojunction between g-C₃N₄ and BiPO₄ nanorods was formed on the basis of S doping by using the solubility of urea and 2-thiobarbituric acid in water.The surface morphology,crystal structure,specific surface area,chemical composition and photoelectric properties of the composite photocatalyst were characterized by FESEM,XRD,FTIR,BET,XPS,UV-Vis,PL and electrochemical tests.The photocatalytic activity was identified by measuring the photocatalytic hydrogen evolution performance under visible light（λ>420nm）.The experimental results show that the specific surface area of the BiPO₄/S-CN composite sample is larger than that of the pure g-C₃N₄,which can provide more active sites,and the response range of BiPO₄/S-CN composite photocatalyst to visible light becomes wider.Among all components,the hydrogen evolution rate of 0.1%BiPO₄/S-CN was the highest,which was 7.3 times higher than that of pure g-C₃N₄ and 3.6 times higher than that of S-doped g-C₃N₄.Through the analysis of experimental data,the superior conductivity of BiPO₄ makes more electrons tend to concentrate on its surface,which improves the separation of electron hole pairs in g-C₃N₄.In addition,S-doping makes the conduction band position of g-C₃N₄ more negative,making water easier to be reduced to hydrogen.It provides a new basis for the application of BiPO₄ in the field of visible light catalytic hydrogen production.In the third part,the C₆₀（nanowire）/g-C₃N₄ nano-heterojunction photocatalysts were still synthesized by calcing urea and C₆₀ nanowires at elevated temperature.While the C₆₀nanowires were obtained by the liquid-liquid interface precipitation method.The C₆₀（nanowire）/g-C₃N₄ composites have better solar light utilization,high efficiency transfer and separation performance of photogenerated carriers.Under visible light（λ>420 nm）,C₆₀/g-C₃N₄-0.03wt%has the best photocatalytic hydrogen evolution rate of 8.73μmol/h,about 4.7 times higher than that of pure g-C₃N₄.The significant improvement in the photocatalytic performance of C₆₀/g-C₃N₄-0.03 wt%is mainly due to the interfacial synergy between g-C₃N₄ and C₆₀ nanowires.The morphology and structure of C₆₀ have great influence on g-C₃N₄.In this paper,the photocatalytic hydrogen evolution rate of C₆₀（nanowire）/g-C₃N₄ composite material is explored,which is 30%higher than that of C₆₀（bulk）/g-C₃N₄.Meanwhile,C₆₀/g-C₃N₄ was deeply explored using first-principles.The introduction of C₆₀ can stably exist in the triangle vacancy center of g-C₃N₄,optimizing its energy band structure.The results of HOMO and LUMO verified that C₆₀can effectively promote the separation and transfer of photo-generated carriers,thereby enhancing the photocatalytic activity.Experimental and theoretical calculation results show that the C₆₀（nanowire）/g-C₃N₄ heterojunction photocatalyst enhances visible light hydrogen evolution due to the synergistic effect of the interface between C₆₀ and g-C₃N₄,which provides a new way to construct an efficient composite photocatalyst.