Preparation Of Carbon Quantum Dots And Their Photocatalytic Activity For Hydrogen Production By Composite G-C₃N₄

Nowadays,the energy crisis and environmental problems are becoming more and more serious.Developing photocatalytic technology which can convert solar energy into chemical energy and control environmental pollution is one of the effective ways to solve this problem.g-C₃N₄,as a new type of visible semiconductor material,has potential application value.As a new type of carbon nanomaterials,carbon quantum dots have attracted many researchers'attention due to their unique excellent properties.Therefore,the introduction of carbon quantum dots to modify g-C₃N₄ can expand the optical absorption range,enhance the electronic transmission ability,and inhibit the recombination of photogenerated carriers,which has important research significance.The results of the study are as follows:1.The carbon dots containing only C,H and O elements were prepared by hydrothermal method using citric acid as precursor at different temperatures.Characterization results show that the carbon dots prepared at different temperatures have obvious lattice fringes,and the degree of carbonization of carbon dots increases with the increase of reaction temperature.Carbon dot-doped g-C₃N₄ was prepared by calcining the prepared carbon quantum dots mixed with urea at high temperature.The addition of carbon dots inhibited the recombination of photogenerated carriers and enhanced the electron transport ability,which significantly enhanced the photocatalytic hydrogen production activity of g-C₃N₄.The optimum hydrogen production efficiency of carbon quantum dots doped with g-C₃N₄ was 14347?mol h^-1 g^-1 at 160°C,which was 12 times higher than that of pure g-C₃N₄.2.The amorphous N-doped carbon dots were prepared by hydrothermal method at different temperatures,with citric acid as carbon source and ethylenediamine as nitrogen source.The carbon dots were mixed with urea and calcined at high temperature to prepare carbon dot-doped g-C₃N₄.Among three composite materials,the photocatalytic hydrogen production efficiency of CN₁₈₀ can reach 6528.1?mol h^-1g^-1,which is 18 times higher than that of pure g-C₃N₄.3.Different types of N-doped carbon dots were prepared by hydrothermal method using citric acid as carbon source and three different small molecule N-containing:ethylenediamine,urea and L-tryptophan as N source.The various types of N contained in three N-doped carbon quantum dots were analyzed by XPS.The carbon dot doped g-C₃N₄ was prepared by calcining the prepared carbon quantum dots with urea at high temperature.The investigated results showed that the carbon dots showed the highest photocatalytic activity which using urea as source and doped with graphite N and pyrrolidine N.The efficiency of hydrogen production was 7646.9?mol h^-1 g^-1,which was 23 times higher than that of the pure g-C₃N₄.The conclusions drawn in this paper have good theoretical significance and application value for the improvement of carbon dots implanted g-C₃N₄ modification technology.