MoS_(2)/CQDs/g-C_(3)N_(4)三相复合材料的合成及其光催化性能

Preparation and Photocatalytic Properties of MoS_2/CQDs/g-C_3N_(4) Composites

为了使石墨相氮化碳(g-C_(3)N_(4))高效地应用于光催化反应,以硫脲、柠檬汁和四水合钼酸铵为前驱体制备了MoS_(2)/CQDs/g-C_(3)N_(4)三相复合光催化剂,成功构建n-n型异质结。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)、荧光光谱、氮气吸附-脱附和紫外可见光漫反射分析(UV-Vis DRS)等方法对复合材料的晶体结构、微观形貌、光吸收特性和孔隙结构等进行了详细表征。结果表明,MoS_(2)/CQDs/g-C_(3)N_(4)复合材料的比表面积大、反应活性位点多。在可见光照射下(波长(λ)>420 nm),MoS_(2)的含量为10%时所合成MoS_(2)/CQDs/g-C_(3)N_(4)复合材料对罗丹明B(RhB)的光催化降解效率达到其降解效率最高的75.8%,降解速率最快且为g-C_(3)N_(4)的4倍。经过4次降解循环后,复合材料的降解效率仍为70.2%。

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 technology needs to design an ideal photocatalyst that is eco-friendly,low-cost and high-efficient in utilizing sunlight.At present,graphitic carbon nitride (g-C_3N_4),which is an n-type semiconductor photocatalyst,has attracted much attention due to its low cost,facile fabrication,nontoxicity,appropriate band gap,and excellent photostability.However,g-C_3N_(4) has still a variety of defects such as insufficient 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 sustainable and economical method to prepare CQDs/g-C_3N_(4) was developed.In addition,sulfides of transition metal had attracted much attention due to their acceptable band gaps,high stability,and easily tunable structures,which could be considered to introduce into CQDs/g-C_3N_(4) to improve the photocatalytic activity of g-C_3N_4.Based on the above considerations,g-C_3N_(4) was synthesized by secondary heat treatment using thiourea as a precursor in this paper.Green and harmless lemon juice was selected to prepare uniformly distributed small size of CQDs.Two-dimensional laminar MoS_(2) was synthesized by using (NH_4)_6Mo_7O_(24)·4H_(2O) and CH_4N_2S,and then heated in an oven at 210℃for 8 h.MoS_2/CQDs/g-C_3N_(4) composites with different MoS_(2) mass ratios were successfully prepared by ultrasonic composite 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 MoS_2/CQDs/g-C_3N_(4) composites,and the microscopic 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 MoS_2/CQDs/g-C_3N_(4) 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%MoS_2/CQDs/g-C_3N_(4) 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 MoS_(2) and g-C_3N_(4) and encouraged the photocatalysis of the materials.Nitrogen adsorption-desorption test (BET) results showed that 10%MoS_2/CQDs/g-C_3N_(4) 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 MoS_(2) changing the pore structure of g-C_3N_(4) and the uneven pore size distribution of MoS_2.The absorbance of 10%MoS_2/CQDs/g-C_3N_(4) composite was further improved by ultravioletray-visible diffuse reflectance (UV-Vis DRS) measurement.According to the optical absorption theory of the material,the band gap of the material could be obtained by using KubelkaMunk formula.The predicted band gap of 10%MoS_2/CQDs/g-C_3N_(4) composite was lowered to 2.02 eV,which meant that electrons needed 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 disodium salt (EDTA-2Na),tert-butanol (t-BuOH) and benzoquinone (BQ) were employed to capture h~+,·OH and·O~(2-)free radicals.The experimental results showed that·O~(2-)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 MoS_2/CQDs/g-C_3N_(4) composites were better than g-C_3N_(4) and4%CQDs/g-C_3N_4,and 10%MoS_2/CQDs/g-C_3N_(4) composite material had the best photocatalytic degradation performance.About 75.8%of RhB could be degraded with 10%MoS_2/CQDs/g-C_3N_(4) composite and the degradation rate was 0.0108 min~(-1),which was about 4 times that of g-C_3N_4.Besides,taking into account the actual application value of the material,reusability and recyclability of catalyst played an important 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 MoS_2/CQDs/g-C_3N_(4) composites with good photocatalytic degradation effect and stability had been successfully constructed,which were potential candidates for pollutant purification.