Controllable Preparation And Electrochemical Properties Of Molybdenum-Based Sulfide And Phosphide Nanostructures

Nowadays,the rapid consumption of fossil fuels has brought about a series of serious environmental issues.Therefore,it is strongly desired to find an alternative clean energy and develop an effective energy storage technology.As a clean and reproducible energy carrier,hydrogen(H2)energy is thought as a promising candidate for replacing fossil fuels,which can reduce our dependence on fossil fuels and benefit the environment by reducing the emissions of greenhouse and other toxic gases.The preparation of hydrogen energy has become particularly important,toward this end,an effective and promising approach is based on the electrolysis of water for hydrogen production.On the other hand,lithium-ion batteries(LIBs)are another approach to store and carry energy.LIBs exhibit numerous merits as typical secondary batteries,including small volume,light weight,high energy density and long cycling life.LIBs play a significant role in clean energy storage and in electric vehicles industry.The development of new electrode materials for electrolysis of water to produce hydrogen and lithium ion batteries has become a research hotspot.In recent years,molybdenum-based compounds have quickly attracted the attention of experts and scholars in many fields at home and abroad due to its unique microstructure and physicochemical properties.As the most representative material in molybdenum-based compounds,molybdenum disulfide(MoS2)and molybdenum phosphide(MoP)have been widely used in electrolysis of water to produce hydrogen and lithium ion batteries.In this thesis,nanostructured molybdenum-based compounds(MoS2 and MoP)are studied as electrode materials and reaction mechanisms has been explored.MoS2 nanoflowers,monolayers,pyramid-like pyramids and multilayer MoS2 nanostructures,ultrathin MoS2 nanosheet arrays,MoS2/FeS2 nanocomposites and MoP nanosheet arrays were designed and constructed.The influence of process parameters on the morphological regulation of these nanostructured materials,and the application of these materials in lithium storage performance and electrocatalytic hydrogen evolution performance were explored.The primary contents and results of the study are listed as follows:(1)MoS2 nanoflowers with ultrathin nanosheets(petals)were prepared on silicon substrates by chemical vapor deposition(CVD)using MoO3 and S powders as raw materials in a vacuum tube furnace.The three-dimensional nanoflower structure was grown on the substrate surface independent,and the lateral dimension and the thickness of nanosheets is 3-4 ?m and 10-30 nm,respectively.And the growth of MoS2 nanoflowers is caused by the formation of edge dislocations and curve.(2)MoS2 nanostructures from monolayer flakes,few-layer pyramids to multilayer blocks were successfully fabricated by a catalyst-assisted thermal-evaporation-based chemical vapor deposition method(using MoO3 and S powders as raw materials)via simply adjusting the carrier Ar gas flow rate.The research indicates that the nucleation and growth of samples were proposed with specifically highlighting the influence of the carrier gas flow rate:a lower flow rate of Ar gas may result in the stable growth of MoS2 nanostructures to form a homogeneous monolayer nanoflakes,while a higher flow rate would lead to the fast and continuous nucleation,and even unstable and disorder growth of MoS2 nanostructures,forming the few-layer MoS2 pyramids or irregular shaped multilayer MoS2 blocks.(3)Vertical ultrathin MoS2 nanosheet arrays(NSAs)were prepared directly on conductive molybdenum foil by a simple CVD method.Various dense and vertical MoS2 nanosheets formed a large number of micro-mesoporous structures,each of which was composed of MoS2 nanosheets with 10 nm thickness.Using the as-synthesized MoS2 NSAs sample as the anode,LIBs were directly assembled without using any binder and carbon black.The as-fabricated MoS2 electrode exhibits high specific capacity(1061 mAh g-1 at a current density of 0.1 A g-1),good cycling stability(maintained 82%of the initial capacity after 40 cycles)and outstanding rate performance.The excellent electrochemical performance for LIBs could be explained as follows:MoS2 nanosheet arrays adhere well to conductive current collectors(Mo foils),greatly facilitating the transport of electrons and lithium ions;ultrathin MoS2 nanosheets can greatly reduce lithium ion diffusion and electron transport paths;MoS2 nanosheets can provide a large number of nano-and micro-scale mesopores,which facilitates the full penetration of electrolytes and effectively avoids the expansion of material volume and the destruction of original structures during cycling.(4)The MoS2/FeS2 nanocomposites were successfully prepared on the flexible carbon cloth by one-step CVD by using MoO3,S and FeCl2·4H2O.Systematical characterization indicates that the vertically ultrathin MoS2 nanosheets uniformly coated on the surface of the FeS2 nanoparticles.We investigated the effects of different reaction temperatures and the amount of iron source on the composition and morphology of the samples.The results show that the reaction temperature at 650 ? and 100 mg of FeCl2·4H2O are the optimal reaction conditions.The self-supported MoS2/FeS2 nanocomposite electrode exhibits small overpotentials of 134 mV at 10 mAcm-2,with a low Tafel slope of 76.8 mV dec-1and excellent stability in 0.5 MH2SO4.Such remarkable hydrogen evolution performance is attributed to good electrical conductivity,large specific surface area and harmonious synergistic effect between MoS2 nanosheets and FeS2 nanoparticles.(5)Vertically standing MoP nanosheet arrays on molybdenum substrate(MoP NSAs/Mo)were obtained via the phosphorization by MoS2 NSAs precursor.The MoP nanosheet array structure has large yield,high crystallinity and controllable morphology;and has a large specific surface area,which is easy to expose more active sites and can improve the reactivity.As a binder-free,integrated and self-supported hydrogen evolution reaction electrode,the MoP nanosheet arrays exhibits outstanding catalytic activity,which needs only an overpotential of 95 and 106 mV to drive 10 mA cm-2 of current in 0.5 M H2SO4 and 1 M KOH,respectively,and presents correspondingly a relatively small Tafel slope of 50.0 and 56.0 mV dec-1.Moreover,the MoP nanosheet arrays possesses superior hydrogen evolution reaction durability and strong tolerance to the corresponding working environments.Therefore,this unique self-supporting MoP nanosheet array structure has a wide application prospect as a catalyst for hydrogen production.