Controllable Preparation Of Molybdenum-based Mox₂（X=S,Se）Piezocatalysts And Their Environmental Catalyitc Performance

Fossil energy crisis and environmental pollution have become two major problems restricting the sustainable development of human society,especially for water environment pollution.The existence and elimination of organic pollutants in the water environment have become a research hotspot in the field of environmental catalysis.Piezocatalysis is an emerging advanced oxidation technology,which uses piezoelectric materials to convert mechanical energy into chemical energy,showing great application potential in degradation of organic pollutants.In recent years,two-dimensional transition metal dichalcogenides(TMDs)have become a kind of promising energy harvesting and conversion materials due to their large specific surface area,good flexibility,and excellent mechanical,electronic and piezoelectric properties.Among them,semiconducting molybdenum-based TMDs(MoX2(X=S,Se))exhibit high piezoelectric properties comparable to those of traditional piezoelectric materials,and have high carrier concentration and mobility,which make them ideal candidates for catalytic applications.However,molybdenum-based piezocatalysts still face the problems of only odd number of layers exhibiting piezocatalytic activity,difficulty of precisely control the number of layer,limited active edge sites,low electrical conductivity,poor structural stability,and difficulty in recycling.How to optimize the activity and stability of piezocatalysts and improve energy conversion efficiency through structure and property modulation has always been a research hotspot and difficulty in this field.This dissertation mainly focuses on the design and microstructures construction of high-performance molybdenum-based piezocatalysts.Four efficient and stable molybdenum-based piezocatalysts have been successfully prepared through defect engineering,construction of hierarchical hollow structures,chemical exfoliation and design of piezocatalytic active menbranes.Moreover,the underlying mechanisms of piezocatalytic activity enhancement have been deeply analyzed to provide novel strategies for high-performance piezocatalysts design and construction,which will promote the practical application of piezocatalytic technology in the field of organic pollutant removal.The main research contents are as follows:1.Multi-flaw and flaw-free MoS2 nanosheets(NSs)(MoS2 NSs)piezocatalysts were prepared by hydrothermal method,and the effect of flaws on piezocatalytic performance was investigated.SEM and TEM characterization results demonstrated that multi-flaw MoS2 NSs had abundant odd numbers of atomic layers and pore flaws,which not only increased the asymmetry of layered structure to improve piezoelectric properties,but also provided more catalytic active edge sites.The larger specific surface area of multi-flaw MoS2 NSs also facilitated the adsorption of reactant molecules.The piezocatalytic degradation experiments showed that compared with flaw-free MoS2 NSs,multi-flaw MoS2 NSs exhibited the best degradation performance for ciprofloxacin(CIP),decomposing 99.1%of CIP in 30 s.Meanwhile,multi-flaw MoS2 NSs also showed good degradation performance for sulfamethazine(SMZ),tetracycline(TC)and oxytetracycline(OTC),and the degradation rates were 40.0%,87.9%and 91.5%,respectively.Radical trapping experiments demonstrated that ·OH was the main active species in multi-flaw MoS2 NSs reaction system.The intermediates of CIP degradation were identified by HPLC-MS,and three possible degradation pathways were proposed,which were mainly based on hydroxylation leading to removal of the piperazinyl ring and oxidative cleavage of quinolone moiety.2.Hierarchical MoS2 nanotubes(NTs)(MoS2 NTs)piezocatalysts with different aspect ratios were prepared by solvothermal method.The characterization results confirmed that the nanotubes possessed hierarchical porous structures self-assembled from single/few-layer MoS2 nanosheets,which could effectively suppress the stacking and agglomeration of nanosheets while fully demonstrate their piezoelectric properties.MoS2 NTS-13 with large aspect ratio had a diameter of 200～250 nm,a length greater than 2 μm,a wall thickness of about 38 nm,and a larger specific surface area(85.83 m2g-1),which could expose more catalytic active edge sites.Due to the poor degradation performance of SMZ by multi-flaw MoS2 NSs,the degradation performance of multi-flaw MoS2 NSs,MoS2 NTs-10 and MoS2 NTs-13 towards SMZ was comparatively studied.The experimental results showed that MoS2 NTs-13 exhibited the best SMZ degradation performance,which could degrade 93.3%of SMZ within 10 min.The main active species in this reaction system were ·OH and ·O2-.Six intermediates of SMZ degradation were detected by HPLC-MS,and the main decomposition mechanisms included the removal of SO2,the cleavage of pyrimidine and aniline rings,and the oxidation of methyl and amino groups.3.AB-stacked ultrathin MoSe2 nanosheets(MoSe2 NSs)piezocatalysts were prepared via a HNO3-assisted exfoliation strategy.The results of SEM,TEM,N2 adsorption-desorption experiment,contact angle test and electrochemical impedance test showed that MoSe2 NSs-10 had an ultrathin layered structure,large specific surface area,good hydrophilicity and electrical conductivity.The results of SEM-EDS,XPS and EPR characterization confirmed that MoSe2 NSs-10 contained abundant Se vacancies.PFM measurements found that MoSe2 NSs-10 showed typical butterfly-shaped amplitude curves and 150° polar domain switching,which confirmed the out-of-plane ferroelectricity induced by interlayer sliding motion.Degradation experiments showed that MoSe2 NSs-10 exhibited the highest degradation efficiency for TC,the degradation efficiencies of TC,OTC,SMZ and CIP were 93.9%,89.9%,19.8%and 88.9%,respectively,·OH and ·O2-were the main active free radicals in this catalytic system.Based on the intermediates identified by HPLC-MS,the degradation pathway of TC in the MoSe2 NSs-10 piezocatalytic system was proposed.4.A series of flexible E-MoS2/PVDF electrospun nanofiber membranes(EFMs)(EMoS2/PVDF EFMs)were prepared by electrospinning technology to further promote the practical application of piezocatalysts.The results of SEM,TEM characterization and electrical conductivity test showed that,compared with pure PVDF EFMs,10.0 wt%E-MoS2/PVDF EFMs possessed non-uniform fiber diameters,many ultrathin E-MoS2 nanosheets were exposed on the surface of fiber,which provided abundant active edge sites for catalytic reactions and also significantly enhanced the electrical conductivity of the fiber membranes.In addition,the nucleation of E-MoS2 nanosheets increased the content of piezoelectric β phase in PVDF to 94.1%.Inorganic and organic piezoelectric materials showed excellent synergistic catalytic effect,10.0 wt%E-MoS2/PVDF EFMs showed the best OTC degradation performance,the degradation rate of OTC reached 93.1%within 24 min,and the TOC mineralization rate was 68.5%,·OH and ·O2-are the main active species in this reaction system.The degradation rates of TC,SMZ and CIP were 91.6%,19.9%and 84.4%,respectively.Nine intermediates of OTC degradation were detected by HPLC-MS,and the main decomposition mechanisms included stepwise hydroxylation,demethylation,decarbonylation and the ring-opening reactions.