The Composite Modification,Microstructure Regulation Of Molybdenum Disulfide And Their Electrocatalytic Activity For Hydrogen Evolution

Two-dimensional transition metal chalcogenides(TMDs)have been widely studied and applied in catalysis,electronics,biosensor,energy conversion and storage owing to their attractive physical and chemical characteristics,such as unique electronic structure,diverse chemical compositions,and excellent material properties.However,in electrocatalysis,TMDs still face the problems of high catalytic overpotential and poor stability,which seriously lead to low catalytic activity and limit their practical applications.The electrocatalytic performance of TMDs can be effectively improved by increasing the active sites,conductivity or enhancing the intrinsic catalytic activity.Therefore,in this thesis,a series of MoS₂-based nanocomposites were constructed and effectively applied in electrocatalytic hydrogen evolution reaction(HER).Moreover,the crystal structure and intrinsic properties of MoS₂ were designed and regulated in order to achieve more efficient catalytic activity and expand their applications in energy storage and conversion.The specific research results are as follows:(1)Graphitic carbon nitride(g-C₃N₄)with a graphene-like framework can be effectively used as a hydrogen evolution catalyst due to its ultra-high nitrogen content and adjustable two-dimensional layered structure.The MoS₂/C₃N₄ nanosheets were synthesized by combining MoS₂ sheets with acidified C₃N₄ via a one-step hydrothermal method.The results showed that the ultrathin MoS₂/C₃N₄-3(the concentration of acidified C₃N₄ was 3 mg/m L)nanocomposite was composed of flower-like structure,and the thickness of the nanosheets was 4.6 nm.The introduction of acidified C₃N₄ can effectively restrain the agglomeration of MoS₂ sheets and enlarge the interlayer spacing of MoS₂/C₃N₄ nanocomposite.Untrathin layer thickness with enlarged interlayer spacing and high surface area make MoS₂/C₃N₄-3 nanosheets exposed more active sites,thus effectively improving its catalytic property.MoS₂/C₃N₄ nanocomposite exhibited low onset overpotential of 153 m V(vs.RHE),small Tafel slope of 43 m V/dec,large electrocatalytic active surface area of 5.4 m F/cm² and excellent long-term stability after 3000cycling voltammetry(CV)cycles under alkaline conditions.Moreover,density functional theory(DFT)calculations demonstrated that the C₃N₄ nanosheets strongly interacted with the MoS₂ layers,which was valuable for increasing the reactivity of active nitrogen atoms.The DFT results further verfied that MoS₂/C₃N₄-3 nanocomposite processed abundant active sites,which significantly improved its HER catalytic activity.(2)Transition metal nickle,as a comparatively powerful electrocatalytic activity electrode material,exhibits good HER activity under alkaline conditions.The Ni-doped MoS₂nanocomposite was in-suit growth on carbon paper(Ni-MoS₂/CP)by a simple hydrothermal approach.When the Ni doping amount was 0.5 mmol,the prepared Ni-MoS₂/CP-0.5nanocomposite had a uniform and continuous nanostructure.X-ray diffraction(XRD)and Raman results showed that the crystal structure of MoS₂ was hexagonal 2H phase,and the introduction of Ni did not significantly change its original crystal structure.In alkaline electrolyte,the onset overpotential of Ni-MoS₂/CP-0.5 nanosheets was 82 m V(vs.RHE),and the Tafel slope was 127.1 m V/dec.The catalytic performances of Ni-MoS₂/CP were apparently better than that of MoS₂/CP.Moreover,Ni-MoS₂/CP-0.5 could retain?95%of its initial current density after 20000 s cycling test,indicating the excellent stability of Ni-MoS₂/CP-0.5.Meanwhile,the incorporation of Ni could bond to the free sulfur and retard the rapid growth of MoS₂ nanosheets,which effectively reduced the stacking and agglomeration between MoS₂nanosheets and enhanced the intrinsic catalytic properties of MoS₂ in alkaline electrolyte.(3)The metallic phase of MoS₂(1T-MoS₂)can effectively catalyse hydrogen evolution due to its abundant active sites and high electronic conductivity.1T-MoS₂ nanosheets were prepared through chemical lithium intercalation and exfoliation.The thickness of the nanosheets was about 1.4 nm.The self-supported MoS₂ membrane was then obtained by vacuum filtration.The interlayer spacing of the membrane is 0.615 nm.Raman spectrum and X-ray photoelectron spectroscopy(XPS)results showed that the as-prepared MoS₂ membrane was metallic phase with a 1T concentration of?61%.High 1T-phase concentration of MoS₂membrane endowed it with high conductivity and excellent catalytic activity.The resultant 1T-MoS₂ membrane exhibited favourable HER performance with onset overpotential of 85 m V(vs.RHE),the overpotential of 220 m V(vs.RHE)at current density of 10 m A/cm²,the Tafel slope of 83.9 m V/dec and superior durability after 3000 CV cycles in acidic electrolyte.Meanwhile,compared to powdery catalyst,the self-supported 1T-MoS₂ membrane processed enhanced HER activity,indicating that the ordered assembly can generate interconnected transport channels,which was conducive to hydrogen generation and significant improvement of catalytic activity.This work can greatly expand the possibility of electrocatalysts in practical applications.(4)The interlayer spacing of MoS₂ is an important property that determines the electronic structure in MoS₂ multilayered structures.The theoretical simulation suggests in such a MoS₂membrane,the firmly stacked MoS₂ with an interlayer spacing of 0.62 nm holded a more near-optimal hydrogen adsorption free energy than that of interlayer expanded MoS₂(IE-MoS₂).Meanwhile,the p-orbital levels of 0.97 nm had larger HOMO-LUMO(highest occupied molecular orbital-lowest unoccupied molecular orbital)splitting than that of 0.62 nm,showing more charge transfer for a proton-electron exchange for H⁺to bind with S in the IE-MoS₂ system.The firmly stacked MoS₂ had a slightly higher H*adsorption free energy than that of IE-MoS₂,that is,a lower reaction barrier,indicating a favorable property toward HER performance for the interlace stacked MoS₂ nanosheets with an interlayer spacing of 0.62 nm.However,there lacks systematic experimental technique for studying the impact of interlayer spacing in those systems.The MoS₂ membrane with different interlayer spacing was fabricated using intercalation of ionic liquids with different cationic chain lengths.The results showed that the hydrated MoS₂ membrane treated by intercalation of ionic liquids achieved an interlayer spacing in the range of 0.62 to 0.98 nm,indicating that the ionic liquid modulating the restacking of MoS₂ nanosheets can induce interlayer expansion.High resolution transmission electron microscope(HRTEM)images demonstrated that the restacking of MoS₂ nanosheets exhibited an interlaced feature at the edge sites.Attenuated total reflectance-fourier transform infrared(ATR-FTIR)and XPS results indicated that the ionic liquid in the membrane has been completely removed after electrolyte exchange.Meanwhile,except for interlayer spacing,the IE-MoS₂ and dry-MoS₂ membranes are highly consistent in terms of morphology,crystallinity/defectiveness,chemical composition,polymorph distribution and electron conductivity.The dry-MoS₂ showed the lowest initial overpotential of 78 m V(vs.RHE)and smallest Tafel slope of 81.2 m V/dec in acid electorolyte.In contrast,the onset overpotential and tafel slope of IE-MoS₂ were higher than that of the dry one,indicating a decay of the MoS₂HER activity as the expansion of the interlayer spacing in the interlaced stacking nanosheets.This result agrees with the above-mentioned simulation.In future,this MoS₂ membrane can serve as a platform in the studies where the interlayer spacing is important.