Preparation and Photocatalytic Properties of MoS2/CQDs/g-C3N4 Composites
In recent years,the massive discharge of industrial wastewater has seriously polluted environment.As a low-cost,green,and sustainable technology,photocatalysis technology shows great potential and has become a research hotspot.However,the current photocatalytic technology is limited in the industry and practical application.Therefore,the key factor of efficient photocatalysis tech-nology needs to design an ideal photocatalyst that is eco-friendly,low-cost and high-efficient in utilizing sunlight.At present,graphitic carbon nitride(g-C3N4),which is an n-type semiconductor photocatalyst,has attracted much attention due to its low cost,facile fabri-cation,nontoxicity,appropriate band gap,and excellent photostability.However,g-C3N4 has still a variety of defects such as insuffi-cient visible light absorption,difficult separation of photogenerated electron-hole pairs,small specific surface area,and few active sites for interfacial reaction,which prevents achieving efficient photocatalytic reactions on its own.The carbon-based zero-dimensional material known as carbon quantum dots(CQDs)is frequently employed because of its superior electrical and optical characteristics.A series of strategy were utilized to produce CQDs,but very few of them were environmentally friendly or cost-effective.Therefore,a sus-tainable and economical method to prepare CQDs/g-C3N4 was developed.In addition,sulfides of transition metal had attracted much at-tention due to their acceptable band gaps,high stability,and easily tunable structures,which could be considered to introduce into CQDs/g-C3N4 to improve the photocatalytic activity of g-C3N4.Based on the above considerations,g-C3N4 was synthesized by secondary heat treatment using thiourea as a precursor in this paper.Green and harmless lemon juice was selected to prepare uniformly distribut-ed small size of CQDs.Two-dimensional laminar MoS2 was synthesized by using(NH4)6Mo7O24·4H2O and CH4N2S,and then heated in an oven at 210 ℃ for 8 h.MoS2/CQDs/g-C3N4 composites with different MoS2 mass ratios were successfully prepared by ultrasonic com-posite and evaporation solvent method,which could construct n-n heterojunction.X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)were used to detect the crystal structure and chemical composition of MoS2/CQDs/g-C3N4 composites,and the mi-croscopic morphology of the composites was observed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results of XRD,XPS,SEM and TEM showed that MoS2/CQDs/g-C3N4 composites had been successfully synthesized,and n-n heterojunction had been constructed with 2D0D2D special structure.The results of the fluorescence spectrum test(PL)showed that 10％MoS2/CQDs/g-C3N4 composite had the longest life of a photogenerated carrier and had efficient separation of photogenerated electron holes.This was because that the n-n heterojunction was created between MoS2 and g-C3N4 and encouraged the photocatalysis of the materials.Nitrogen adsorption-desorption test(BET)results showed that 10％MoS2/CQDs/g-C3N4 composite had a higher specific surface area and the pore size distribution corresponding to the desorption data produced by Barrett-Joyner-Halenda(BJH)method was poor.This might be caused by the load of MoS2 changing the pore structure of g-C3N4 and the uneven pore size distribution of MoS2.The absorbance of 10％MoS2/CQDs/g-C3N4 composite was further improved by ultravioletray-visible diffuse reflectance(UV-Vis DRS)mea-surement.According to the optical absorption theory of the material,the band gap of the material could be obtained by using Kubelka-Munk formula.The predicted band gap of 10％MoS2/CQDs/g-C3N4 composite was lowered to 2.02 eV,which meant that electrons need-ed less energy to jump from the valence band to the conduction band,resulting in enhancing the material's capacity to absorb visible light.In order to explore the primary active species and the possible photocatalytic mechanism,ethylenediaminetetraacetic acid disodi-um salt(EDTA-2Na),tert-butanol(t-BuOH)and benzoquinone(BQ)were employed to capture h+,·OH and·O2-free radicals.The experimental results showed that·O2-free radicals were the main factors of photocatalytic degradation,followed by h+free radicals.Based on the band gap and the position of the conduction band,the possible photocatalytic mechanism of the material was proposed.Rhodamine B(RhB)was selected as the degradation product to evaluate the photocatalytic activity of the materials.The photocatalytic degradation experiment of 30 mg·L-1 RhB showed that the performance of MoS2/CQDs/g-C3N4 composites were better than g-C,N4 and 4％CQDs/g-C3N4,and 10％MoS2/CQDs/g-C,N4 composite material had the best photocatalytic degradation performance.About 75.8％of RhB could be degraded with 10％MoS2/CQDs/g-C,N4 composite and the degradation rate was 0.0108 min-1,which was about 4 times that of g-C3N4.Besides,taking into account the actual application value of the material,reusability and recyclability of catalyst played an impor-tant role for degradation experiments.The deterioration efficiency of the composite material remained at 70.2％after four cycles,nearly not degrading noticeably.The results demonstrated that 2D0D2D n-n heterojunction structure of MoS2/CQDs/g-C3N4 composites with good pho-tocatalytic degradation effect and stability had been successfully constructed,which were potential candidates for pollutant purification.
graphitic carbon nitridecarbon quantum dotstransition metal sulfidesheterojunctionsphotocatalytic property
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