Nitrogen Vacancy Modified Carbon Nitride:preparation And Improvement Of Photocatalytic Performance For Hydrogen Production

The continuous and large consumption of fossil energy has brought energy crisis and environmental pressure around the world.Clean and renewable have become important standards of new energy.As one of the ideal new energy sources,the research of solar energy is developing rapidly.Converting solar energy into hydrogen energy facilitates the storage of solar energy and provides an ideal choice for the product of hydrogen energy.The key to this technology is to develop efficient,environment friendly and stable photocatalysts.Graphitic carbon nitride（g-C₃N₄）is a visible-light-response mental-free material,that can be synthesized from nitrogen-rich precursors in a simple way.It also has the multiple advantages of non-toxicity,thermal stability and chemical stability,and has been used in many fields of photocatalysis.In this thesis,the g-C₃N₄bandgap and photogenerated carriers transport efficiency are modified by changing synthesis conditions to introduce nitrogen defects,loading co-catalyst and constructing heterojunctions.Ultimately,efficiency photocatalytic water splitting activity was achieved.Specific research contents are as follows:（1）Two kinds of high performance of hydrogen evolution activity g-C₃N₄are prepared by modifying thermal polymerization conditions and cold-drying N-rich precursors.The results show that nitrogen defects were introduced in wrapped g-C₃N₄（CN-UC）,which effectively improve the photogenerated carriers separation efficiency.The obtained sample are curved nanosheets exhibits a photocatalytic hydrogen evolution rate of 1876.7μmol g^-1h^-1,which is 9.1 times of the g-C₃N₄prepared by conventional thermal polymerization（CN-U）.The cold-drying treatment increases the crystal spacing of the precursors,which affects the electrons band structure and morphology of the samples.The cold-drying-assisted preparation of g-C₃N₄（CN-UF）introduces more nitrogen defects,the absorption edge further redshifts,and the separation efficiency of electron and hole pairs is also significantly improved.Under visible light irradiation,the average hydrogen production efficiency of the CN-UF is 2557.4μmol g^-1h^-1,which is 13.7 times of CN-U.（2）Pt anchored on g-C₃N₄（Pt-CN）is prepared by ultrasound,oil bath and calcination phase assisted methods.After low-temperature calcination,intimate bonds are formed between Pt and g-C₃N₄,the band gap of g-C₃N₄is narrowed to 2.65 e V,corresponding to a wider visible light absorption.In addition,Pt are uniformly distributed on the surface of the g-C₃N₄nanosheets,which inhibits the recombination of photogenerated electrons and holes and provides more active sites for the proton reduction reaction,making it possible for the higher hydrogen production activity of g-C₃N₄.With triethanolamine as the hole sacrificial agent,the optimised photocatalytic hydrogen production efficiency of Pt-CN can reach 4730.2μmol g^-1h^-1,which is more efficient than the g-C₃N₄loaded Pt by photodeposition and far higher than the pristine g-C₃N₄.（3）Oxygen-doped boron phosphide(O-doped BP/g-C₃N₄)heterojunction is prepared by calcination.Oxygen doping makes the valence band of boron phosphide move up,and the band gap is narrowed to 1.17 e V,enabling efficient capture of solar light with wavelengths up to NIR region O-doped BP nanoparticles distribute on g-C₃N₄nanosheets,a mental-freeⅠ-type heterojunction has been built by intimate contact,which promote the efficient transfer of photoproduction electronics.Under visible light irradiation,the photocatalytic hydrogen evolution rate of O-doped BP/g-C₃N₄hybrid is 2004.5μmol g^-1h^-1,which is 54.9 times and2.3 times of O-doped BP and g-C₃N₄,respectively.The photocatalytic hydrogen production efficiency of the heterojunction was 10.9μmol g^-1h^-1under near-infrared light irradiation.