Effect Of Halogen-doped And Homojunction Construction On The Photocatalytic Performance Of Carbon Nitride

Semiconductor photocatalytic technology is considered to be a promising method of controlling environmental pollution because it can effectively utilize the low-density solar energy to generate reactive oxygen radicals to gradually decompose pollutants in the environment.As a kind of non-metallic semiconductor photocatalyst,graphitic carbon nitride（g-C₃N₄）has some advantages such as easily synthesis by low price raw materials,tunable band structure and excellent chemical stability.However,the pristine g-C₃N₄ suffers from several shortcomings of limited visible light harvesting capability,fast recombination of photo-generated electron-hole pairs and small specific surface area,resulting in its low photocatalytic efficiency,which cannot meet the practical needs.In order to overcome the inherent defects of pristine g-C₃N₄,we used morphology optimization,elemental doping and homojunction construction to modify this material.On the one hand,supramolecular self-assembly and halogen doping were used to modify the morphology and tune the band structure of the material.On the other hand,the low crystallinity carbon nitride prepared by supramolecular self-assembly process was compounded with poly triazine imide（PTI）type carbon nitride with high crystallinity to form crystalline/amorphous g-C₃N₄ homojunction（PTI/g-C₃N₄ homojunction）.As a result,the photocatalytic performance of g-C₃N₄ for organic pollutants degradation in simulated wastewater was enhanced by the above two approaches,which provided a new idea for the construction of g-C₃N₄ photocatalytic system based on supramolecular self-assembly method.The main contents and conclusions of this paper are as follows:（1）Halogen-doped carbon nitride nanosheets with high photocatalytic activity were synthesized by a supramolecular self-assembly process using melamine-cyanuric acid（MCA）aggregates as the precursor and NH₄Cl（NH₄F/NH₄Br）as the dopant source.The results showed that the supramolecular self-assembly process significantly affected the morphology of g-C₃N₄,resulting in the bottom-up assembly of g-C₃N₄ into a petal-like nanosheet structure,which effectively enlarged the specific surface area,and the BET specific surface area increased up to 98.330 m²·g^-1,which was 10.2-folds of unmodified carbon nitride(9.660 m²·g^-1).The halogen ions（X^-）doping in the gaps and layers of the triazine ring broadened the light response range and reduced the negative effect of the"quantum confinement effect"caused by the reduction of particle size.The synergistic effect of two modification strategies,morphology control and elemental doping,led to a significant photocatalytic activity enhancement of carbon nitride,with the removal efficiency of Rh B and TC-HCl reaching 94.3%and 65.7%,respectively,which were higher than those of the control samples modified by only one approach.（2）The difference of the doping effects of fluorine,chlorine,and bromine were investigated.Due to the different atomic characteristic of halogen elements,the structure and properties of the materials doped with F,Cl,and Br are also different.Because of the moderate electronegativity and atomic radius,chlorine-doping enhanced the photocatalytic performance of g-C₃N₄ more significantly than fluorine and bromine.The introduction of a proper amount of halogen ions into the g-C₃N₄ molecular structure can cause the increase of local negative charge density,so the holes can be attracted,while electrons can be repelled,thus inhibiting the recombination of electron-hole pairs.However,as the doping amount increased or the electronegativity was enhanced,holes would be trapped by excess halogen ions and the photocatalytic activity decreased.（3）The molten salt method was used to construct the PTI/g-C₃N₄homojunction hierarchical structure,which effectively inhibited the recombination of photogenerated carriers.The PTI/g-C₃N₄ homojunction was synthesized using guanidine hydrochloride and melamine-cyanuric acid（MCA）aggregates as the precursors,Li Cl-KCl mixture as the medium by molten salt method.When the mass ratio of MCA to guanidine hydrochloride was 2:1,the photocatalytic activity of the prepared homojunction was the best,and the removal efficiency of tetracycline hydrochloride was 68.0%,which was higher than the single component before compounding.Due to the different crystallinity of the two carbon nitride components,the energy band potentials were staggered,and the transfer of photogenerated electrons and holes followed the"type II heterojunction"path,which effectively inhibited their rapid recombination.