Preparation Of Graphene Quantum Dots Nanocomposite Photocatalyst And Study On Hydrogen Production By Decomposing Water

In recent years,graphene quantum dots（GQDs）of small size（<20 nm）have attracted considerable attention,widely used in biological imaging,cell imaging,drug delivery,biosensing,optoelectronic devices,photocatalysis,etc.aspect.Due to the obvious quantum confinement effect and edge effect,GQDs have very good ability to accept and transfer photogenerated electrons,and the band gap energy is less than 2.0 eV,which can absorb visible light well.In photocatalytic systems,it is generally believed that GQDs mainly act as photosensitizers,electron libraries and solid electron transfer agents.This thesis is mainly aimed at the problems of easy photo-electron-hole recombination and low photon yield in semiconductor photocatalysts.By in-situ growing graphene quantum dots on semiconductor materials,new nanocomposite photocatalysts are constructed to enhance the absorption of visible light by improve the photogenerated carrier fractional efficiency and photon quantum yield,thereby improving the efficiency of visible light splitting water to produce hydrogen.The main research contents are as follows:1.Preparation of nitrogen-doped titanate nanotube composite graphene quantum dot photocatalyst and its performance in hydrogen production from photolysis water.The titanate nanotubes（TNTs）were N-doped using a simple solvothermal method to prepare N-TNTs,then using citric acid as the raw material,the GQDs were grown in situ on N-TNTs by solvent heat treatment in dimethylformamide（DMF）to prepare nitrogen-doped titanate nanotube composite graphene quantum dots（N-TNTs/GQDs）photocatalysts.N-doping can replace the lattice oxygen sites and introduce impurity levels in the forbidden band above the valence band to reduce the band gap energy,thereby improving the response to visible light.GQDs can be used as electron reservoirs to capture photo-generated electrons from the conduction band of TNTs,thereby promoting the separation of electrons and holes,while GQDs can act as photosensitizers,electron libraries and solid electron transfer agents.Under the synergy between N-doping and GQDs coupling,the N-TNTs/GQDs photocatalyst can efficiently produce H₂ under simulated sunlight,the total output in 8 hours reached 2901.78μmolg^-1,and its hydrogen production efficiency was 210%higher than TNTs.2.Preparation of Nano-P-N semiconductor graphene quantum dots/Cu₂ZnSnS₄（GQDs/Cu₂ZnSnS₄）and study on the performance of hydrogen production by photolysis water.Cu₂ZnSnS₄（CZTS）was prepared by solvothermal method for solvent heat treatment of the mixed precursor solution of copper nitrate,zinc acetate,tin chloride,and thiourea.Then,using citric acid as raw material,GQDs were grown on CZTS by solvent heat treatment in dimethylformamide（DMF）,and a new graphene quantum dot/Cu₂ZnSnS₄（GQDs/CZTS）composite photocatalyst was prepared.The coupling of GQDs and CZTS further enhances the absorption of visible light,at the same time,GQDs in the GQDs/CZTS heterojunction improves the efficiency of photo-generated carrier separation and photon yield,thereby increasing the efficiency of GQDs/CZTS producing H₂ under simulated sunlight.The total output in 105 minutes reached1540.18μmolg^-1,and the hydrogen production efficiency is increased by 493%compared to CZTS.3.Preparation of g-C₃N₄ quantum dots,titanium dioxide nanorods,graphene quantum dots ternary composite photocatalysts and their performance in hydrogen production from photolysis water.Using melamine and titanate nanotubes as raw materials for mixing and milling,g-C₃N₄ quantum dots-titanium dioxide nanorods（CNQDs-TiO₂NRs）were prepared by a mixed calcination method,and then citric acid（CA）was used as the carbon source and DMF was used as the solvent.GQDs were grown in situ on CNQDs-TiO₂NRs by solvothermal method to synthesize g-C₃N₄ quantum dots-titanium dioxide nanorods/graphene quantum dots（CNQDs-TiO₂NRs/GQDs）ternary composite photocatalyst.It shows better hydrogen evolution efficiency under simulated sunlight,the total output in 6 hours reached 2321.43μmolg^-1,and its hydrogen production efficiency is 176%higher than that of titanate nanorods calcined at 500℃.