Composite Modification Of Graphite Phase Carbon Nitride And Its Photocatalytic Performance

Semiconductor photocatalytic technology can effectively degrade pollutants and reduce CO₂,which has great advantages in solving environmental pollution and energy scarcity in natural ecosystems.Graphitic carbon nitride（g-C₃N₄）is highly regarded for its low toxicity,stable properties and low band gap energy,which facilitates the use of visible light.However,g-C₃N₄has some drawbacks,such as easy recombination of photogenerated carriers,narrow spectral response range,and low migration rate,leading to low utilization of solar energy.To address these issues,this study used citric acid and urea in different molar ratios as carbon and nitrogen sources to synthesize N-doped high-fluorescence CQDs with upconversion luminescence.Furthermore,CQDs/g-C₃N₄was prepared,and when irradiated by light greater than700 nm,it exhibited near-infrared light catalytic activity,with a degradation rate of organic dyes of 33.1%after 6 hours.CQDs/g-C₃N₄:Yb³⁺,Tm³⁺was then prepared,and when irradiated by light greater than 700 nm,it showed further enhancement of near-infrared light photocatalytic ability compared to CQDs/g-C₃N₄and g-C₃N₄:Yb³⁺,Tm³⁺.Through a series of characterization tests,the reason for the enhancement of photocatalytic ability and energy transfer among rare earth ions,CQDs and g-C₃N₄was explored.The main research content of this paper is as follows:（1）N-doped highly fluorescent CQDs were synthesized using lemon acid and urea with different molar ratios as carbon and nitrogen sources through a hydrothermal method,followed by filtration and dialysis for further purification.The CQDs surface contains carboxyl（-COOH）and amino（-NH₂）functional groups,which give the carbon quantum dots excellent fluorescence properties and good water solubility.The emission peak of the CQDs red-shifts with increasing excitation wavelength（280 nm-400 nm）,and the highest fluorescence quantum efficiency is68.21%.Under the excitation of long-wavelength light（600 nm-800 nm）,the CQDs exhibit up-conversion emission that red-shifts in the short-wavelength range（400nm-500 nm）,and when excited by 800 nm light,the emission wavelength is greater than 466 nm,exceeding the bandgap energy of g-C₃N₄.（2）CQDs were adsorbed onto g-C₃N₄using a stirring adsorption method to prepare CQDs/g-C₃N₄composite.The visible light catalytic efficiency of g-C₃N₄was increased from 61%to 83.4%by the addition of CQDs,enhancing the visible light catalytic ability of g-C₃N₄.In near-infrared light catalytic experiments,using light sources greater than 700 nm（xenon lamp loaded with a filter greater than 700 nm）,CQDs/g-C₃N₄showed a degradation rate of 33.1%for organic dyes over 6 hours.XRD,PL,FT-IR and other characterization tests were carried out,and it was found that the CQDs coated the g-C₃N₄lattice.The up-conversion luminescence properties of CQDs were used to broaden the spectral response range of g-C₃N₄,effectively improving the photocatalytic performance of g-C3N4.（3）Further modification of g-C₃N₄was carried out to enhance its solar energy utilization.The precursor was doped using a liquid-phase method to prepare g-C₃N₄:Yb³⁺,Tm³⁺,and then CQDs were compounded with it through constant temperature stirring to prepare CQDs/g-C₃N₄:Yb³⁺,Tm³⁺.Characterization tests such as XRD,UV-vis,PL were carried out on the samples,and it was found that the band structure of g-C₃N₄was changed due to the compounding of CQDs and the doping of rare earth ions,resulting in a smaller bandgap energy and an expanded spectral absorption range.In the photocatalytic experiment,after 60 minutes of light exposure,the visible light catalytic efficiency of g-C₃N₄:Yb³⁺,Tm³⁺increased to 75%,and the visible light catalytic efficiency of CQDs/g-C₃N₄:Yb³⁺,Tm³⁺increased to 85%.The compounding of CQDs is beneficial for charge separation,but as the amount of CQDs increases,the photocatalytic efficiency first increases and then decreases.Too many CQDs dispersed on the surface of g-C₃N₄:Yb³⁺,Tm³⁺will shield the photocatalytic active sites of g-C₃N₄:Yb³⁺,Tm³⁺,thereby affecting subsequent redox reactions.When near-infrared light was used to carry out photocatalytic experiments on the samples,it was found that compared with CQDs/g-C₃N₄and g-C₃N₄:Yb³⁺,Tm³⁺,the degradation rate of Rhodamine B by CQDs/g-C₃N₄:Yb³⁺,Tm³⁺was increased by 11.1%and 21.8%,respectively,and CQDs/g-C₃N₄:Yb³⁺,Tm³⁺can further enhance the photocatalytic ability of near-infrared and visible light.Through characterization tests such as XRD,PL,and UV-vis,it was found that CQDs/g-C₃N₄:Yb³⁺,Tm³⁺has the following advantages:（1）using upconversion materials to modify g-C₃N₄effectively reduces the bandgap energy of g-C₃N₄and broadens the spectral response range;（2）promotes the effective separation of photogenerated carriers.These advantages make the performance of CQDs/g-C₃N₄:Yb³⁺,Tm³⁺far superior to that of g-C₃N₄.