Sol-gel Auto Combustion Synthesis Of Zn_xCd_1-xS Nanoparticles And Their Photocatalytic Degradation Mechanism Of Rhodamine B

Over the past few decades,photocatalytic materials have attracted great attention due to their enormous potential in solving environmental pollution and utilizing solar energy.Among many photocatalytic materials,ZnS,as an important II-IV semiconductor material,has been studied by many scholars because of its unique optical and electrical properties.Moreover,owing to its high theoretical efficiency of photocarrier generation,which is much higher than the most famous photocatalyst TiO₂.However,the band gap of ZnS is about 3.6 ev,which makes it impossible to use visible light that accounts for a large part of solar energy,and this greatly limits the application of ZnS.In order to overcome the weakness of ZnS,people have tried many ways,such as:the introduction of S vacancies,doping transition metal elements,synthetic solid solution.Among them,synthesis of solid solution by combining with narrow band gap semiconductor is regarded as a good method to utilize visible light.Since the band gap of solid solution Zn_xCd_1-xS can be adjusted by simply adjusting the molar ratio of Zn and Cd,it has attracted a lot of attention.In recent years,although there have been some articles reported Zn_xCd_1-xS in photocatalytic applications,there are still some shortcomings in the study of Zn_xCd_1-xS:long experimental period,unable to mass production,the raw materials with a certain toxicity and lack of photocatalytic degradation mechanism of Zn_xCd_1-xS under different proportion.In order to overcome the above problems,this paper has done related research work.Here,we successfully synthesized Zn_xCd_1-xS nanoparticles with high photocatalytic activity under visible light by sol-gel auto combustion method.Among them,in the study of the mechanism of degradation of RhB under the visible light with Zn_xCd_1-xS samples?x=0-0.8?,we found that the behavior of photogenerated holes on the surface of Zn_xCd_1-xS is closely related to its surface polarity.Its main content is as follows:?1?Zinc nitrate,cadmium nitrate and thiourea,ethylene glycol,glycine were used as raw materials to prepare Zn_xCd_1-xS nanomaterials by sol-gel auto-combustion method?x=0,0.2,0.4,0.6,0.8,1?.The ICP,SEM,TEM and nitrogen adsorption desorption?BET?were used to study the chemical compontent,the particle size and specific surface area of the samples in detail.The crystal structure of the sample was characterized by XRD.The shift of the main diffraction peaks in the XRD patterns and the linear relationship between the lattice constant c and the content x of Zn confirms the successful synthesis of the solid solution ZnxCd1-xS.The light absorption characteristics of the samples?x=0-1?were studied by UV-vis.It was found that as the x decreases,the absorption edge of the sample gradually moves toward to the visible light region.This shows that the prepared ZnxCd1-xS has the ability to absorb visible light,and further confirms the successful synthesis of ZnxCd1-xS solid solution.And this also confirmes that it is feasible to prepare ZnxCd1-xS by sol-gel self-combustion method.In addition,using Rhodamine-B?RhB?as an organic pollution source,xenon lamp as the light source,we studied the photodegradation activity of Zn_xCd_1-xS?x=0-1?under visible light??>420 nm?.It was found that the sample with x=0.2 has the highest ability to degrade RhB.?2?There are three main species that play a role in the degradation of organic pollutants by photocatalytic materials:h⁺,·O₂^-and·OH.By respectively to add Edta-Na₂,p-benzoquinone,isopropanol as the scavenger for h⁺,·O₂^-and·OH in the process of degradation,we have studied the mechanism of degradation RhB under visible light with different proportions of Zn_xCd_1-xS?x=0-0.8?.It was found that:when isopropanol was added,there was almost no effect on the degradation of RhB for all Zn_xCd_1-xS?x=0-0.8?,and no more than 5%;when p-benzoquinone was added,Zn_xCd_1-xS with x=0-0.8 showed almost no degradation of RhB.This indicates that during the degradation process,·O₂^-is the main degradation group,and the hydroxyl radical·OH play a very weak role in the degradation of RhB.After the addition of Edta-Na₂,with the decrease of x,its effect on the degradation of RhB molecules by Zn_xCd_1-xS showed a transformation from the promotion of degradation to the inhibition of degradation.Because of in the solution,the photogenerated holes on the surface of the material not only participate in the photocatalytic reaction,but also recombination with the photogenerated electrons.Therefore,this is due to the difference in the degree of recombination of the photogenerated electrons and holes at the sample surface during the degradation of RhB.?3?The reason why the Zn_xCd_1-xS samples?x=0-0.8?have a difference in the degree of recombination of photogenerated holes and electrons at the sample surface is studied.Since RhB is a cationic dye in solution,we think that one possible explanation is that:as x decreases from 0.8 to 0,the surface charge of Zn_xCd_1-xS shifts to a more negative direction and this has led to this result.At low Zn content,due to Coulomb interaction,more RhB molecules are adsorbed on the surface of the sample,which making more photogenerated holes in touch with RhB molecules and participating in the degradation process.and thereby inhibiting the recombination of photogenerated electrons and holes.The zeta potential was used to characterize the surface polarity of Zn_xCd_1-xS.It was found that as the x decreases,the zeta potential of Zn_xCd_1-xS shifts to a more negative direction,which is consistent with our prediction.In addition,the experimental results of the addition of surfactant DBS in photodegradation also confirm our prediction.