Study On The Preparation Of MS_x(M=Co,Mo) Ultrathin Nanosheets And Their Degradation Performances For Rhodamine B Dye

Two-dimensional?2D?transition metal sulfides possess excellent physical and chemical properties,such as small layer space,large specific surface area,and abundant edge sites,etc.In addition,few-layered MoS₂ has attracted extensive attention in environmental catalysis due to its excellent piezoelectric potential.In this dissertation,2D ultrathin MS_x?M = Co,Mo?nanosheets with defects were prepared by using a bottom-up liquid phase synthesis strategy.With a series of studies on the doping modification,photo-piezoelectric catalytic properties,and catalytic mechanism,the high-performance MS_x catalysts were obtained.It provides a fundamental basis for the development of high-efficiency catalysts.By using the inorganic-organic hybrid CoS-TEA lamellar compound generated in situ by chemical transformation,CoS ultrathin nanosheets with thickness of ?4.6 nm composed of surface Co vacancy defects were fabricated.SEM,XRD,and electrochemical tests were used to study the effects of both raw material ratios and solvent types on the composition,structure,and photocatalytic degradation of rhodamine B?Rh B?of Co-S materials.The results show that CoS ultrathin nanosheets have 1.77 times higher photocatalytic performance than CoS nanoplates,i.e.,97.34% degradation of Rh B in 80 min for CoS nanosheets as compared to only 55.10% degradation for CoS nanoplates.Under the excitation of visible light,CoS ultrathin nanosheets and nanoplates exhibit different fluorescence emission intensities and carrier dynamic relaxation processes.CoS ultrathin nanosheets have the faster electron transport to effectively avoid the electron-hole recombination,and show the good photostability properties under five cycles.For the utilization of the piezoelectric effect in MoS₂,MoS₂ ultrathin nanosheets with a sheet thickness of ?3.4 nm was successfully synthesized via acid etching hydrothermal method.The intrinsic relationship between the material structure and its piezoelectric catalytic performance was studied.The results show that synthetic MoS₂-3 with pH value of 3 exhibits the best piezocatalytic activity under ultrasonic vibration in dark with the degradation efficiency of 99.8% in 5 min.In comparison,the degradation efficiencies of defect-free MoS₂?MoS₂-6.3?synthesized with pH value of 6.3 and commercial MoS₂ are 65.85% and 2.00%,respectively.The electrochemical analysis including EIS,LSV,and Mott-Schottky indicated that the enhanced degradation performances by the increased ultrasonic power is attributed to the increased carrier density?the electron density of MoS₂-3 is 15 times than that of MoS₂-6.3?and efficient charge carrier separation induced by piezoelectric polarization.Catalase and isopropanol active species trapping experiments combined with theoretical calculations showed that hydrogen peroxide?H2O2?and hydroxyl radical?·OH?are the main active oxygen species which play key role in the piezoelectric catalytic process.Combining the results of the binary system CoS photocatalysis and MoS₂ piezoelectric catalysis,Co doped MoS₂ piezoelectric photocatalysts were fabricated through a facile one-pot hydrothermal method by mixing Mo,S,and Co precursor.The effects of Co doping concentration on the composition,structure,and catalytic performance of the catalyst were studied by Raman,TEM,Mott-Schottky,and PL measurements.The results showed that ultrathin nanosheets?Co-MoS₂-0.10?using 0.10 mole Co precursor possesses the richest sulfur vacancies and active edge sites and the best catalytic performance,i.e.,99.90% degradation of Rh B within 30 s under piezoelectric assisted photocatalytic process.In comparison,only 68.90% and 29.12% of Rh B can be degraded under only ultrasonic vibration or solar light irradiation.This is due to that the piezoelectric electric field of ultrathin nanosheets effectively reduces the electron-hole recombination probability and elongates the carrier lifetime.The reactive oxygen species of catalytic reaction are mainly superoxide radicals?·O₂^–?,hydrogen peroxide?H₂O₂?,and hydroxyl radicals?·OH?generated by the one-electron-reaction pathway of Co²⁺/Co³⁺ and the two-electron-reaction pathway of Mo⁴⁺/Mo⁶⁺.Theoretical calculations showed that oxygen molecule interacts with Co active site,and the adsorption energy between them is 10 times higher than that of oxygen molecule on the surface of pristine MoS₂,promoting the generation of superoxide radical?·O₂^–?and further improving the catalytic activity.