Controllable Synthesis Of Carbon Nitride-based Photocatalysts By Supramolecular Pre-assembly Method And Study On Photocatalytic Water Splitting Performance

Graphic carbon nitride(g-C₃N₄),a new non-metallic semiconductor organic polymer,has many advantages,such as good chemical stability,suitable energy band structure,response to visible light,low price and simple preparation method,etc.It is regarded as an ideal photocatalytic material.However,graphic carbon nitride materials prepared by traditional methods have some defects,such as small specific surface area,narrow visible light absorption range,high recombination rate of photo-generated carriers and slow charge migration rate,which lead to low photocatalytic efficiency and limit its application prospect in photocatalysis.In view of the above problems,this paper optimizes the structure of graphic carbon nitride by means of element doping,morphology control and energy band control,etc.,to promote the absorption of visible light by carbon nitride,increase the specific surface area,optimize the oxidation-reduction potential,and improve the photocatalytic decomposition.Hydrogen production performance of water;through surface kinetic control,it is to create active sites,reduce the overpotential of photolysis water reaction,improve the half reaction of oxygen production of oxidized water,and realize photocatalytic water decomposition.The research work includes the following three aspects:(1)Porous banded sulfur-doped carbon nitride by supramolecular preassembly method to improve photolysis of aquatic hydrogen:using melamine and cyanuric acid as raw materials,sulfur-containing supramolecular polymer MTA was preassembled in dimethyl sulfoxide as carbon nitride precursor,and carbon nitride(S-GCN-X)was prepared by high temperature thermal polymerization.EA,BET,UV-vis DRS and other test results show that the successful doping of S element into carbon nitride makes the sample have larger specific surface area,narrower band gap and faster photo-generated carrier migration rate,thus improving its absorption efficiency of visible light and separation efficiency of photo-generated electrons and holes.Thanks to this,S-GCN-X has better photolysis performance of producing hydrogen under irradiation of light.Especially when the thermal polymerization temperature is 650?,the hydrogen production rate can reach 196.6?mol·h-1,which is 35 times that of BCN(?5.6?mol·h^-1).(2)Preparation of Co doped graphic carbon nitride by supramolecular preassembly for enhanced photocatalytic oxygen evolution:Using melamine and cyanuric acid as raw materials,cobalt nitrate hexahydrate as cobalt source,pre-assembled in ethylene glycol solvent to prepare Co doped with supramolecular precursor(MCA-Co),thermally polymerized at high temperature to prepare Co doped graphite phase carbon nitride(GCN-Co).EA,XPS,ICP and other test results show that the use of cobalt nitrate hexahydrate coordinated with cyanuric acid and then self-assembled with melamine can successfully dope cobalt into the graphite phase carbon nitride large?conjugate structure.It has strong oxidizing ability,low photo-generated electron-hole recombination rate,and fast photo-generated charge transfer rate,so the photolysis water has better oxygen production performance.Especially when the amount of cobalt nitrate hexahydrate is 3%,the oxygen production rate can reach 7.7?mol·h^-1,which is about 27 times that of pure carbon nitride(?0.3?mol·h^-1).(3)The morphology and energy band of graphic carbon nitride are controlled for photocatalytic overall water spiliting:melamine and cyanuric acid are used as raw materials,and supramolecular polymer MCA is preassembled in the mixed solvent of water and dimethyl sulfoxide,and carbon nitride(GCN-M)is prepared by thermal polymerization of MCA at high temperature.The test results of BET,Mott-Schotty,PL and EIS show that the graphic carbon nitride prepared by the supramolecular preassembly method in a mixed solvent has a larger specific surface area,better redox ability,and a low photogenerated electron-hole recombination rate.The photo-generated charge has a fast migration rate,so it has good water decomposition performance.By changing the concentration of reactants,the energy band of carbon nitride can be adjusted,its redox capacity can be increased,and the total water solution performance can be improved.In particular,the graphite carbon nitride prepared when the reactant concentration is 40 m M has the best total hydrolysis performance,with a hydrogen production rate of 4.9?mol·h^-1and an oxygen production rate of 2.6?mol·h^-1.