Preparation And Photocatalytic Properties Of G-C₃N₄-based Double Z-Scheme Photocatalysts

Graphitic carbon nitride（g-C₃N₄）is a non-metallic semiconductor photocatalyst with a two-dimensional layered structure,which has the advantages of tunable band structure,high chemical stability and green environment protection.Bismuth-based compound is a metal photocatalyst with narrow band gap（2.0-2.9 e V）,which has better absorption and higher visible photocatalytic performance in the visible region.The disadvantage of single photocatalysts is the high photogenerated electron-hole recombination rate,which affects their photocatalytic performance.In recent years,multivariate heterojunction photocatalysts have gained more and more attention,which can significantly reduce the photogenerated electron-hole recombination rate and improve the photocatalytic performance by utilizing the conduction band and valence band differences between multivariate material heterojunctions to form Z-scheme or double Z-scheme structures.In this work,g-C₃N₄/Bi VO₄/Co Fe₂O₄,g-C₃N₄/Bi₂WO₆/Co₃O₄and g-C₃N₄/Bi₂WO₆/Bi₂O₂CO₃heterojunction composites were synthesized by hydrothermal method combined with high-temperature solid-phase method,and their visible photocatalytic degradation performance of common organic dyes and tetracyclines was investigated by free radical scavenging experiments.The photocatalytic mechanism was investigated.The samples were characterized by X-ray diffraction,field emission scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,UV-vis diffuse reflectance,N₂adsorption-desorption analysis,and photoluminescence spectroscopy.The main contents and results of the study are as follows.（1）A double Z-scheme g-C₃N₄/Bi VO₄/Co Fe₂O₄composite photocatalyst was synthesized,its visible photocatalytic activity and stability against target pollutants such as rhodamine B（Rh B）,methylene blue（MB）,methyl orange（MO）and tetracycline（TC）for degradation were investigated.The results showed that the synthesized g-C₃N₄were nanosheets with stacked and curled structures and lengths of about 6-8μm,Bi VO₄were polyhedral bulk particles of about 0.5-2μm,and Co Fe₂O₄were spherical particles with particle size of about10 nm.After 100 min of visible light irradiation,the pure Co Fe₂O₄catalyst had almost no degradation effect on Rhodamine B.The removal rates of Rh B by pure Bi VO₄,pure g-C₃N₄,g-C₃N₄/Bi VO₄and g-C₃N₄/Bi VO₄/2.5%Co Fe₂O₄were 24.6%,68.5%,81.8%and 94.5%,respectively,indicating that the heterojunction photocatalyst g-C₃N₄/Bi VO₄/2.5%Co Fe₂O₄has the highest photocatalytic activity,which is attributed to the double Z-scheme heterojunction structure that facilitates the rapid separation of carriers.The scavenger test results confirmed that the hydroxyl radical·OH plays the most important role,followed by the superoxide anion radical·O₂^-.It was found that the cavity scavengers EDTA,citric acid,propanol and oxalic acid all contributed to the photocatalytic degradation of Rh B by this composite photocatalyst,and the order of promotion was EDTA>citric acid>propanol>oxalic acid.Among them,when the EDTA concentration was 0.2 mmol·L^-1,the degradation rate of Rh B was increased nearly 3.5 times.Within the degradation time of 100 min,the g-C₃N₄/Bi VO₄/2.5%Co Fe₂O₄composite photocatalysts TC,MB and MO degraded well,and the degradation efficiency reached 66.7%,95.0%and 56.3%,respectively.（2）g-C₃N₄/Bi₂WO₆/Co₃O₄composite photocatalysts with different doping ratios were synthesized,in which g-C₃N₄was a nanosheet with about 200 nm in diameter,Bi₂WO₆was a sphere-like particle with about 500 nm in diameter,and Co₃O₄was a spherical particle with a diameter of about 10 nm,which synthesized by using polystyrene emulsion as a template.The photocatalytic experimental results showed that the catalysts degraded Rh B,MB,MO and TC by 97.5%,64.3%,73.7%and 89.9%respectively after visible light irradiation for 60 min.The radical scavenging experiments showed that h⁺and·OH are the main reactive radicals for the degradation of TC,and·O₂^-plays a minor role.（3）g-C₃N₄/Bi₂WO₆/Bi₂O₂CO₃composite photocatalysts with different doping ratios were constructed and the degradation activity for Rh B,MB,MO and TC Under visible light irradiation was investigated.For the degradation of TC,with the increase of doping rate of Bi₂O₂CO₃,the degradation percentage shows a trend of first increase and then decrease,the highest degradation rate of94.76%was achieved when the doping rate of Bi₂O₂CO₃was 50%.The radicals of h⁺and·OH play the primary role and·O₂^-plays a minor role in the degradation of TC,which showing the same degradation mechanism as the g-C₃N₄/Bi₂WO₆/Co₃O₄photocatalyst.The degradation efficiency of this photocatalyst was 89.2%,81.1%and 68.8%for Rh B,MO and MB,respectively.The absorption spectroscopy tests showed that the constructed three double Z-scheme heterojunction photocatalysts broadened the absorption range of light and increased the photogenerated carrier yield.The fluorescence spectroscopy tests showed that the fluorescence intensity of three photocatalysts decreased significantly compare to pure g-C₃N₄and Bi₂WO₆,manifesting that a higher separation efficiency for electron-hole carrier has been obtained through double-Z electron transfer mechanism,which prolongs the carrier lifetime and finally improves the visible photocatalytic efficiency.