Modification And Performance Of Graphitic Carbon Nitride For Photocatalytic Hydrogen Production

With the rapid development of modern technology,the demand for energy is increasing.Traditional fossil energy sources are seriously damaging to the environment and have limited resources,and there is an urgent need to find sustainable green energy.The emerging developed photocatalytic decomposition of water to produce hydrogen is one of the effective ways to solve environmental pollution and energy shortage.In recent years,graphitic carbon nitride（g-C₃N₄,g-C₃N₅,etc.）,as conjugated polymers,has advantages such as attractive band structure,excellent physical and chemical stability,thermal stability,and low toxicity.As a metal-free and visible light responsive photocatalyst,it has attracted wide attention in the field of photocatalytic decomposition of water to produce hydrogen.However,the graphite-phase carbon nitride photocatalyst suffer from low utilization rate of sunlight,small specific surface area,and high photogenerated electron-hole complexation rate,which lead to its unsatisfactory photocatalytic performance.In view of the shortcomings of graphite-phase carbon nitride,it is of great significance to modify graphitic carbon nitride by morphology regulation and doping to reduce the photogenic carrier recombination rate and enhance electronic conductivity,and optimize its photocatalytic performance.Therefore,in this paper,supramolecular self-assembly and thermal polymerization techniques are used to prepare morphologically controllable g-C₃N₄and carbon quantum dots-doped g-C₃N₅,and the photocatalytic performance and catalytic mechanism of the two catalysts are explored.The details are as follows:（1）g-C₃N₄（GCN_x）with porous ultra-thin scale-like structure is prepared by supramolecular preassembly thermal polymerization.Firstly,2-amino-1-butanol,DL-aminopropanol,and ethanolamine are used as morphologically guiding agents to form preassembles with dicyandiamide through hydrogen bonding,and then modified g-C₃N₄ is obtained by thermal polymerization.It was found that 2-amino-1-butanol had obvious morphology guiding effect on g-C₃N₄.By changing the volume of 2-amino-1-butanol from 0-1.0 Vol%,GCN_0.7 prepared with an optimal volume percentage of 0.7 Vol%has a porous ultra-thin scaly structure,with a specific surface area of 91 m²/g,about 8.3 times that of the unmodified bulk g-C₃N₄.According to Kubelka-Munk function calculations and Mott-Schottky test curve,GCN_0.7 has a band gap of 2.68 e V and a more negative conduction value than g-C₃N₄.The photoelectric response test shows that GCN_0.7 has faster electron transport efficiency and lower electron-hole recombination rate.The hydrogen production rate of GCN_0.7 was up to 18.12 mmol/g/h,which was 75.5 times that of unmodified g-C₃N₄.The ultra-thin porous structure not only provides more active sites for the reaction but also reduces the length of photocharge transmission,thus improving the photocatalytic activity of the catalyst.（2）g-C₃N₅ is prepared by hot polymerization using 3-amino-1,2,4-triazole as raw material.The thin layer of g-C₃N₅ is obtained by KOH treatment.At the same time,the g-C₃N₅ is further optimized by doping carbon quantum dots withπ-conjugated structure and excellent optical and electronic properties.The best photocatalytic activity of g-C₃N₅（g-C₃N₅-1.0%）prepared with 1.0%doped carbon quantum dots is selected by spectroscopic analysis and photoelectrical performance test.According to Kubelka-Munk function calculation and Mott-Schottky curve analysis,g-C₃N₅-1.0%has a band gap of2.12 e V and a more negative conduction band value.g-C₃N₅-1.0%showed excellent carrier mobility and low electron-hole recombination rate,and the decomposition rate of aqua hydrogen as photocatalyst is 2.9 mmol/g/h,about 9.7 times that of unmodified g-C₃N₅.This work provides a feasible scheme for the subsequent modification of graphite phase carbon nitride.