One-step Double In-situ Approach For The Preparation Of Carbon Dots Modified G-C₃N₄ Photocatalyst For Photocatalytic Hydrogen Evolution

As a new type of photocatalyst,graphitic carbon nitride（g-C₃N₄）has good application prospect.However,the narrow light absorption range and high electron-hole recombination rate of g-C₃N₄ hinder its widely used in the photocatalytic field.Many studies have shown that carbon dots（CDs）,as a new zero-dimensional carbon material,can improve the light absorption of g-C₃N₄,and inhibit its electron-hole recombination,thereby improve the photocatalytic performance.In this thesis,some types of CDs modified g-C₃N₄ photocatalysts were prepared by using different compounds as the precursors of CDs or g-C₃N₄ via a one-step double in-situ approach.The chemical structures and physical properties of the prepared photocatalysts were characterized by modern instruments such as TEM,XRD,and their photocatalytic performance was evaluated under visible light irradiation.The effects of the precursor types and catalyst morphology on the performance of CDs/g-C₃N₄ photocatalyst were studied.The details are as follows:（1）Three types of CDs/g-C₃N₄ photocatalysts were prepared by one-step double in-situ approach using three sugar molecules as CDs precursors and urea as g-C₃N₄precursor.When the CDs precursors were glucose,maltose,and glycogen,respectively,the corresponding photocatalysts were named as CNM_x,CND_x,and CNP_x.The results showed that the smaller the molecular weight of the CDs precursor,the higher the photocatalytic activity of the CDs/g-C₃N₄ photocatalyst.When glucose and urea were used as precursors,and the mass ratio of glucose to urea was 0.017 wt.%,the CNM_1.0had the most excellent photocatalytic hydrogen evolution rate（HER）,which was about4.17 times that of bulk g-C₃N₄（BCN）prepared by pure urea.（2）Using three kinds of cyanamide molecules as g-C₃N₄ precursors and glucose as CDs precursor,three types of CDs/g-C₃N₄ photocatalysts were prepared by a one-step double in-situ approach.When the g-C₃N₄precursors were cyanamide,dicyandiamide and melamine,respectively,the corresponding photocatalysts were named as CN-CY_x,CN-DC_x and CN-ML_x.The results indicated that the hydrogen evolution performance of photocatalyst was related to the type of g-C₃N₄ precursor and the amount of CDs precursor added.As cyanamide has the smallest molecular weight and the best water solubility,the series of CN-CY_x photocatalysts based on cyanamide had the highest hydrogen evolution performance.Among them,CN-CY_1.0 with the best CDs content had the highest HER,which was about 3.36 times that of bulk g-C₃N₄（CN-CY）prepared by pure cyanamide.（3）Preparation of CDs modified hollow g-C₃N₄ spheres by using cyanamide and glucose as precursors,SiO₂ as hard template.Firstly,the influence of shell thickness on the photocatalytic performance of hollow g-C₃N₄ spheres（HCNS）was studied,and the optimal shell thickness（about 79 nm）of HCNS photocatalyst was determined.Then,a series of CDs modified HCNS photocatalysts were prepared by using the optimum SiO₂spheres as template and named as HCNS-C_x.Due to the introduction of CDs and the existence of hollow structures,the specific surface area,light absorption capacity,and electron-hole mobility rate of HCNS-C_x have been significantly improved,resulting in the significant improvement of their hydrogen evolution performance.Among them,HCNS-C_1.0 photocatalyst with the best shell thickness and the best CDs content had the highest hydrogen evolution rate(2322μmol g^-1 h^-1),which was about 19.03 times that of CN-CY.In summary,in this thesis,some different CDs and g-C₃N₄ precursors were used to prepare CDs/g-C₃N₄ photocatalysts via a one-step double in-situ approach.Then the influence of precursor types on the hydrogen evolution performance of photocatalysts was studied.Finally,using the optimum precursors of CDs and g-C₃N_4,and combining a one-step double in-situ approach with a morphology control method,a series of CDs modified HCNS photocatalysts with better photocatalytic performance were prepared.