| In the coexistence of the global energy crisis and the environmental pollution,the use of semiconductors as a photocatalyst to spilt water into hydrogen is considered as an important way to convert turn solar energy into clean energy.In this process,the research and development of semiconductors with high catalytic efficiency is the key.Since 2009,when Wang et al.used semiconductor materials to decompose water by carbon nitride,carbon nitride has been widely used as a new type of inorganic semiconductor material.The research,until now is still high,because it has the advantages of non-toxic,cheap raw materials,good chemical stability and thermal stability,and suitable energy band structure.However,due to its high carrier recombination rate,its photocatalytic activity is relatively low,which is not only due to its own structure,but also has a great relationship with its photocatalytic reaction system.In this paper,different structures of graphite phase carbon nitride were prepared by simple thermal polymerization method.Different systems were selected for photocatalytic activity test.The hydrogen production mechanism of different structures of catalysts in different systems was studied,including the following three parts:（I）Melamine photocatalysts with different polymerization degrees were synthesized by thermal depolymerization of melamine diverse at different temperatures（520°C,620°C,650°C）,and the differences between their composition and structure were investigated.The sample obtained under 520°C has a low degree of polymerization,and contains small molecules such as melem,which has poor absorption of light and a small specific surface area.The sample obtained at high temperature（620°C,650°C）has a relatively high degree of polymerization.The absorption of light is strong,and has a large specific surface area and has a porous structure,the degree of polymerization obtained at 650°C is higher than that obtained at 620°C.（Ⅱ）Photocatalytic tests were carried out on the above three photocatalysts with four organic acids（formic acid,oxalic acid,lactic acid,acetic acid）as electron donors.The experiments showed that the sample obtained at 520°C use the formic acid as the electron donor,which has the highest photocatalyst activity,and it’s significantly higher than that of 620°C and 650°C.When the Pt loading was 0.5 wt%,the H2evolution reached 160μmol/h.The quantum efficiency at the 420 nm monochromatic wavelength also reached the maximum 39.4%.（Ⅲ）The above results indicate that the sample who has the perfect catalytic efficiency with the low degree of polymerization and small surface area.Under the research,we have investigated the mechanism of high hydrogen production efficiency of the low polymerization sample.It is found that this is due to the formation of intermolecular hydrogen bonds between-NH2/-NH-groups in the carbon nitride chain,which facilitates the entry of organic acid molecules into the interior of the catalyst,which is due to the special planar structure of the formic acid molecule and the smaller molecular size,and the layer spacing of the sample obtained at 520°C is relatively large,the degree of polymerization is relatively low,so the formic acid molecule can effectively enter the interior to quickly eliminate photogenerated holes,so the electrons reach the catalyst surface fastly,thereby significantly increasing the efficiency of photocatalytic hydrogen production. |
