Preparation Of MS（M=Cu,Mo） Doped SnO₂ Cathode Materials And Photoelectrochemical Reduction Of CO₂

The environmental damage caused by excessive exploitation of fossil energy has aroused wide attention in the world.The application of photoelectric technology transforming carbon dioxide（CO₂）into high value-added chemical products has been a research topic.Owing to its advantages of good conductivity,high catalytic activity,fast electron migration rate and low price,SnO₂ has the superiority in carbon dioxide conversion.However,the band gap of SnO₂ is 3.6eV.So,the output energy is still in low level.It is reported that the semiconductor oxide band gap can be regulated by element doping and recombination of semiconductors.Therefore,the SnO₂ can be modified according to the above two methods to improve its photoelectric properties and broaden application in CO₂conversion.In this paper,Cu doped SnO₂,Cu,S co-doped SnO₂ and MoS₂ loaded on SnO₂ particles,the above three kinds of modified materials were synthesized by hydrothermal method with SnO₂ as the substrate for the photoelectric transforming of CO₂.The physical and chemical properties of various composite materials were investigated,and the optimal doping ratio was optimized.The mechanism of photoelectrocatalytic CO₂ reduction was discussed.The specific research content is as follows:（1）SnCl₄·5H₂O was used as precursor materials and CuCl₂ was used as dopant for synthesizing Cu doped SnO₂ through one-step hydrothermal method.The surface morphology,crystal structure and element composition as well as valence state were characterized by SEM,XRD and XPS.The properties of the material and the reduction of CO₂ were investigated by cyclic voltammetry（CV）,tafel plot and electrochemical impedance spectroscopy（EIS）under normal temperature and pressure in 0.5 mol L^-1 NaHCO₃.The results show that SnO₂ is rutile structure and Cu²⁺ions replace the Sn⁴⁺ions of SnO₂.Besides,the particles size decreased slightly after doping Cu²⁺.Through electrochemical performance evaluation,the best catalytic performance achieved as the doping content is 1.5%.The current density reached 3.5 mA cm^-2 and tafel slope was 55.1 mV dec^-1 as well as the maximum faradic efficiency reached 23%when the catalyst loading content was 1 mg cm^-2,which is 12 times higher than pure SnO₂.Therefore,the Cu doped SnO₂ can be used as cathode material for electrocatalytic reduction of CO₂.（2）Copper（Cu）and sulfur（S）co-doped SnO₂ material prepared by a facile hydrothermal method was demonstrated as an efficient electrocatalyst for reduction of CO₂ to HCOOH.The results of XRD,SEM,XPS,FT-IR and electrochemical characterization shows that the SCS₁₀ is outstanding.The as-prepared SCS₁₀ holds rutile structure,meanwhile,both of the Cu and S are doped well in SnO₂,in which S^2-and Cu²⁺replace O^2-and Sn⁴⁺,respectively.In CO₂-saturated 0.5 mol L^-1 NaHCO₃ solution the maximum current density increaseed to 5.5 mA cm^-2 at-1.2 V,the overpotential calculated is as low as 200 mV（vs RHE）,the current density is 7 times as high as pure SnO₂.It can electrolyze more than 30 h and the Faradic efficiency of HCOOH is 44.5%,10 times higher than pure SnO₂.So,this is an efficient catalytic material.（3）SnCl₄·5H₂O,Na₂MoO₄ and thiourea were used as raw materials to synthesize MoS₂ loaded on SnO₂ nanoparticles（labeled as MS/SON）for photoassisted electrocatalytic conversion of CO₂ to HCOOH.Through physical structure characterization and photoelectric performance test,the results indicated that the 5%MS/SON exhibits exclusive HCOOH selectivity,the maximum FE is43.8%,9 times higher than pure SnO₂.The current density reached to 9 mA cm^-2at-1.4 V and the photocurrent value is 12.5μA cm^-2 under 0.5 V bias.The overpotential is as low as 245 mV（vs.RHE）.Besides,after the introduction of MoS₂,there is a red shift in the adsorption band edge from 380 nm to 500 nm,indicating an enhancement of light response;the peak intensity of PL（fluorescence spectrum）decreased,showing that the electron and hole are effectively separated;the values of EIS and Tafel plots(43.2 mV dec^-1)were smaller than other ratios,demonstrating that the faster charge transfer rate;the proper introduction of MoS₂ increased the specific surface area from 47.64 m² g^-1 to 55.99 m² g^-1,which was helpful to expand the number of active sites.It is demonstrated that MS/SON is an excellent CO₂ reduction material.All the above three materials showed catalytic activity for CO₂ reduction to HCOOH,and through the comparison,the SCSx had better performance than the other two materials,and the Faraday efficiency of producing HCOOH reached44.5%,indicating that SCS_x was a better CO₂ reduction cathode material.