Synthesis Of Graphitic Carbon Nitride （g-C₃N₄）-based Nanophotocatalysts For Efficient Activity

Semiconductor photocatalysis technology is one of the effective means to solve energy shortage and environmental pollution.Graphitic carbon nitride（g-C₃N₄）is a two-dimensional layered metal-free material with good visible light response and good stability.As a new type of semiconductor photocatalyst,it has received extensive attention from researchers.However,g-C₃N₄ has a narrow visible light absorption range due to its band gap energy（Eg≈2.7 e V）,and it has a high photogenerated carrier recombination rate,these characteristics inhibit its visible light catalytic activity.Our work starts from two aspects which are reducing the recombination rate of photogenerated electron-hole pairs and broadening the light response range of g-C₃N₄ to solve the above-mentioned shortcomings of g-C₃N₄.Two g-C₃N₄-based nanophotocatalytic materials were designed and synthesized by combining it with upconversion nanoparticles,metal deposition,semiconductor coupling,etc.The specific research content is as follows:（1）Preparation of Ag/Na YF₄:Yb,Tm@Na YF₄:Yb/g-C₃N₄ composite material and its photocatalytic performanceUsing melamine as the precursor,g-C₃N₄ was prepared by high-temperature thermal polycondensation method,and the influence of the change of thermal polycondensation temperature on the morphology,phase and optical properties of the obtained product was explored.Na YF₄:Yb,Tm upconversion nanoparticles were synthesized by high-temperature pyrolysis,and used it as the core to prepare homogeneous active core-shell structure Na YF₄:Yb,Tm@Na YF₄:Yb upconversion nanoparticles（UCNPs）by epitaxial growth method.By acid treatment,UCNPs transform from hydrophobic to hydrophilic,and the surface is positively charged.The g-C₃N₄ nanosheets obtained after ultrasonication are mixed with hydrophilic UCNPs,and Na YF₄:Yb,Tm@Na YF₄:Yb/g-C₃N₄（UCNPs/g-C₃N₄）binary composite material were obtained by electrostatic attraction.By adjusting the content of UCNPs to explore the binary composite materials about the phase morphology,optical properties and photocatalytic activity.According to the results,the optimal mass fraction was determined to be 35%.On this basis,the Ag/UCNPs/g-C₃N₄ ternary composite material was obtained by in-situ reduction of Ag⁺.By adjusting the content of Ag nanoparticles to explore the ternary composites about phase morphology,optical properties and photocatalytic activity,the optimal mass fraction of Ag is determined to be 4%.The experimental results show that the introduction of UCNPs can make g-C₃N₄ indirectly use near-infrared light to improve the utilization of sunlight.Under the irradiation of 980 nm near-infrared light,g-C₃N₄ nanosheets absorb the ultraviolet/visible light emitted by Tm³⁺and Yb³⁺co-doped UCNPs and then generate photogenerated electron-hole pairs,finally the electrons migrate to the Ag nanoparticles to realize the effective separation of the photogenerated electron-hole pairs.The photogenerated electrons further react with the oxygen in the environment to produce strong oxidizing·O₂^-,thus exhibiting excellent photocatalytic activity.（2）Preparation of Ag₂CrO₄/g-C₃N₄ composite material and its photocatalytic performanceUsing porous g-C₃N₄ nanosheets as the substrate,Ag₂CrO₄ nanoparticles are deposited on the surface by in-situ growth method to synthesize a highly efficient visible light driven Ag₂CrO₄/g-C₃N₄ photocatalytic composite material,and its morphology and phase,optical properties,photocatalytic activity and mechanism of action have been systematically studied.The photocatalysis of the Ag₂CrO₄/g-C₃N₄ composite material follows the Z-scheme mechanism.Under visible light irradiation,the photogenerated electrons of Ag₂CrO₄ migrate and recombine with the photogenerated holes on the g-C₃N₄ valence band.The Ag₂CrO₄/g-C₃N₄ composite material retain photogenerated holes with the valence band with more positive conduction potential and photogenerated electrons with more negative conduction band potential.So that this feature endows the composite material with stronger oxidation and reduction capabilities.The introduction of Ag₂CrO₄ broadens the visible light response range of g-C₃N₄ nanosheets.At the same time,g-C₃N₄ with excellent chemical stability also inhibits the photocorrosion of Ag₂CrO₄,so that the Ag₂CrO₄/g-C₃N₄ composite material can maintain excellent stability during the reaction.The composite material exhibits excellent photocatalytic activity in the degradation experiment of rhodamine B（Rh B）.The composite material containing 30 wt%Ag₂CrO₄ has the best photocatalytic performance.After it reacts under visible light（≥400 nm）for 40 min,the degradation rate of Rh B reached100%,while the degradation of Rh B by pure g-C₃N₄ and Ag₂CrO₄ was only 64.3%and 69.4%,respectively.