Preparation And Photocatalytic Properties Of G-C₃N₄ Nanocomposites

Hydrogen,a clean and renewable energy source,is considered as one of the suitable alternatives to fossil fuels,and has received attention in the development of new energy and semiconductor photocatalytic technology.Semiconductor photocatalysis,which allows hydrogen to be released directly from water,is considered a promising strategy to solve the energy crisis and environmental pollution.Since g-C₃N₄ was first reported as a photocatalyst for hydrogen production,metal-free polymer photocatalyst g-C₃N₄ has been widely used due to its suitable band gap（～2.7e V）and band position.However,the photocatalytic activity of g-C₃N₄ still has much room to be improved.The construction of heterojunction and supported cocatalyst has become a very effective method to improve the photocatalytic hydrogen production activity of g-C₃N₄.In this paper,two-dimensional porous nanosheet g-C₃N₄（PCN）was prepared by using gas template agent NH₄Cl assisted calcination.Zn_xNi_1-xSe（Ni Se/Zn Se）bimetal selenide(Z_xN_1-xSe)and metal-organic frame MOF material NH₂-MIL-101（Al）（Al-MOF）were prepared by hydrothermal method.The PCN nanosheets were compounded with Z_xN_1-xSe and Al-MOF by solvothermal method to construct the heterojunction.The different ratios of Zn and Ni in Z_xN_1-xSe and the effects of Z_xN_1-xSe and Al-MOF on the photocatalytic hydrogen production performance of PCN were investigated.The structure,optical properties,separation and transfer of photocarriers and photocatalytic activity of synthetic photocatalyst were analyzed.Combined with the analysis results,the possible mechanism of photocatalytic decomposition of water under visible light was given.First,using urea as precursor and gas template agent NH₄Cl assisted calcination to synthesize PCN two-dimensional porous nanosheets,Z_xN_1-xSe and Al-MOF nanoparticles were synthesized by solvothermal method.XRD analysis showed that the peak position of the prepared catalyst was basically consistent with that of the pure phase characteristic diffraction peak,indicating that the catalyst was successfully prepared.The characteristic peaks of Ni Se and Zn Se appear in XRD of Z_xN_1-xSe,indicating that Z_xN_1-xSe exists in the form of Ni Se/Zn Se heterojunction.DRS analysis of Z_xN_1-xSe shows that with the increase of Ni/Zn ratio,the visible light absorption range of Z_xN_1-xSe expands and the light absorption intensity increases gradually.Second,The Z_xN_1-xSe/PCN heterojunction was synthesized from PCN and Z_xN_1-xSe by solvothermal method.SEM characterization showed that many nanoparticles were deposited on PCN nanosheets,indicating that Z_xN_1-xSe was successfully loaded on PCN.DRS characterization analysis showed that with the loading of Z_xN_1-xSe,Z_xN_1-xSe/PCN absorption edge had an obvious red shift phenomenon,and the visible light absorption intensity gradually increased.Moreover,characterization analysis of PL,TRPL and EIS showed that the recombination efficiency of photoelectron-hole pair was effectively reduced,and the construction of Z_xN_1-xSe/PCN heterojunction could inhibit the recombination of photoelectron-hole pair,reduce the internal resistance and improve the photocatalytic activity.The results of photocatalytic hydrogen production test showed that the 7 wt%Z_0.7N_0.3Se/PCN with the ratio of Zn to Ni 3:7 and the Z_0.7N_0.3Se loading of 7wt%Z_0.7N_0.3Se had the highest hydrogen production activity of 410.16μmol·h^-1·g^-1,34 times that of pure PCN.These results indicate that the compact heterojunction formed by Z_xN_1-xSe and PCN composite improves the visible light catalytic hydrogen production performance of PCN.Third,Al-MOF/PCN（PCNM-x）photocatalytic materials were synthesized by in-situ solvothermal method,and a series of PCNM-x composites with different load ratios were prepared by changing the mass percentage of Al-MOF.The photocatalytic hydrogen production of PCNM-x was analyzed.The results showed that the best photocatalytic rate of Al-MOF supported 10 wt%photocatalyst was 572.86μmol·h^-1·g^-1.SEM and TEM analysis showed that Al-MOF was nanoparticles and was successfully loaded on the surface of PCN material.Al-MOF load can broaden the material’s absorption of visible light,and through PL,TRPL,EIS and other characterization analysis,the results show that the composite efficiency of photoelectron-hole pair is effectively reduced,which is conducive to improve the photocatalytic activity,and effectively improve the visible light photocatalytic activity of water decomposition to produce hydrogen.