Preparation And Property Study Of Nanomaterials Based On Molybdenum Disulfide

In recent years,two-dimensional transition metal chalcogenides represented by molybdenum disulfide(MoS2)have attracted extensive attention due to their unique electronic structures and physical properties.It is found that MoS2,as a two-dimensional inorganic semiconductor material,has stable structure and adjustable band gap.It has been widely used in basic research and device applications,such as field effect transistor,photoelectric detector,gas sensors and hydrogen production catalysts.Recently,noble-metal-free MoS2 has attracted extensive attention as a hydrogen production catalyst.It can be used not only as an electrocatalyst,but also as a co-catalyst for photoelectrochemical hydrogen production integrated with other semiconductor materials.The catalytic active sites of MoS2 are along its edges.However,MoS2 is a layered 2D structure,and the edge sites on basal plane are limited.Since the catalytic performance is related to active site density and electrical transport to these sites,it is still a great challenge to prepare MoS2 with high active site density and conductivity.In addition,as the most popular 2D semiconductor material,magnetic properties of MoS2 have also received much attention due to the prospective application of spintronic devices.Theoretical calculations show that the intrinsic magnetism of MoS2 is generally derived from dislocations,grain boundaries and zigzag edges,which are a small portion of the material.Few experiment directly measures the intrinsic magnetism of MoS2.In this thesis,several MoS2 nanostructures and composites are prepared by chemical vapor deposition(CVD)on different substrates and nano-templates.The electrocatalytic and photoelectrocatalytic hydrogen production properties and their magnetic properties are studied.The details are as follows:(1)Monolayer MoS2 is grown by a CVD method,and then monolayer porous MoS2 is prepared by anodized aluminum oxide(AAO)template-assisted ion bombardment to activate the catalytic activity of the MoS2 basal plane.We directly visually label the position of the active sites on the microscopic scale by Cu electrochemical deposition,demonstrating that these edges introduced by ion bombardment also have catalytic hydrogen production activity.Moreover,we test the catalytic hydrogen production performance of the monolayer porous MoS2.Compared to the monolayer MoS2,the catalytic hydrogen production performance of the monolayer porous MoS2 is greatly improved due to the significant increase in the edge density.The exchange current density increases from 1.1 μA/cm2 to 2.8 μA/cm2.The potential corresponding to a current density of-10 mA/cm2 is reduced from-509 mV to-385 mV;the Tafel slope is also reduced from 187 mV/dec to 109 mV/dec.(2)The vertically conductive multi-layered MoS2 pyramids with a high density of active edge sites are synthesized via a CVD method.Conductive Atomic force microscopy(c-AFM)shows the thickness-independent vertical conductivity of the MoS2 pyramids.The high-density active edge sites on the surface of the MoS2 pyramids are confirmed through Cu electrochemical deposition,and the catalytic hydrogen production performance of the MoS2 pyramids is further evaluated by electrochemical characterization.Due to the thickness-independent vertical conductivity and high density of active edge sites,the MoS2 pyramids demonstrate a much higher hydrogen production performance than the monolayer MoS2 flakes.The MoS2 pyramids exhibit a current density of-10 mA/cm2 at-393 mV,which is 100 mV lower compared to that of the monolayer MoS2 flakes.And the corresponding Tafel slop deceases from 237 mV/dec to 93 mV/dec.(3)The intrinsic magnetism of MoS2 pyramids has been investigated using vibrating sample magnetometry(VSM)and magnetic force microscopy(MFM).The magnetic measurements reveal the robust ferromagnetism of MoS2 pyramids.The existence of ferromagnetism is mostly attributed to the presence of abundant zigzag-edges in the layered pyramids,confirmed by transmission electron microscopy(TEM),VSM and MFM.Moreover,a clearly identified remnant and switchable magnetic moment is revealed for the first time in the MoS2 pyramid.This study provides sound evidence with the zigzag-edge induced ferromagnetism of the MoS2 materials.(4)Ultrathin MoS2 nanoflakes are directly synthesized to coat arrays of Ag-core Si-shell nanospheres(Ag@Si NSs)using a CVD method.Due to the high surface area ratio and large curvature of these NSs,the as-grown MoS2 nanoflakes can accommodate more active sites compared to those grown on planar substrates.In addition,the high-quality coating of MoS2 nanoflakes on the Ag@Si NSs protects the photocathode from damage during the photoelectrochemical reaction.Through photoelectrochemical testing,high efficiency and good stability of the photoelectrocatalytic hydrogen production are obtained for the Ag@Si/MoS2 photocathode:the photocurrent density reaches-33.4 mA/cm2 at-0.4 V relative to the reversible hydrogen potential,and the stability of the photoelectrochemical reaction is over 12 h.