Preparation And Electrochemical Properties Of WS 2 Nanosheets And Their Nanocomposites

Layered-like tungsten disulfide?WS₂?is one of the first discovered transition metal dichalcogenides?TMDs?and has raised a lot of concern.WS₂,as well as other TMDs,belongs to a class of weak van der Waals layered compounds and can be easily exfoliated into mono-layered or few-layered nanosheets by several methods.Furthermore,due to the good stability and exotic electronic properties,WS₂ has more possibilities for applications in energy devices.In this paper,liquid exfoliation method has been applied for preparing mono-layered WS₂ nanosheets,which were further loaded on metal and metal oxide nanoparticles,respectively,to form uniform zero dimensional/two dimensional structures,leading to enhanced electrochemical properties.These synthesis methods may provide new ways and have prospects for WS₂-based nanocomposites as supercapacitor electrode.The main research works in this paper are as following:The commercial WS₂ used as raw powders were mixed up with 200 m L 0.05 wt.% sodium dodecyl sulfate?SDS?-water.The mixture were exfoliated at different ball milling speeds and followed with sonication,leading to homogeneous and stable WS₂ dispersion.The liquid exfoliation method is very simple and green,which could be applied to prepare high quality nanosheets in mass production.The results show that the thickness of WS₂ nanosheets is 0.5-1.0 nm when the ball-milling speed is 200 rpm,which is consistent with mono-layered WS₂ nanosheets.The exfoliated WS₂ nanosheets with good crystalline provide larger specific surface area and specific capacitance.The WS₂ nanosheets exfoliated at 200 rpm ball milling speed has the best properties,thus they are further used for preparing WS₂-based nanocomposites.The cyclic voltammetry,galvanostatic charge-discharge and electrochemical impedance spectroscopy were carried out on WS₂ nanosheets?200 rpm?to be a comparison with the following nanocomposites.The sodium citrate reduction method was applied by using AgNO3 to prepare stable Ag sol at room temperature.Then the Ag nanoparticles were loaded on as-prepared 1L-WS₂ nanosheets through Cu?II?ion coordination method,preparing AgNPs@WS₂ nanocomposites with different weight ratios of 5%,10% and 20% of Ag.The Ag nanoparticles are at Ag0 state without oxidation and the particle size is 10-20 nm.With the increasing content of Ag nanoparticles,the Raman affect is obviously enhanced and the specific surface area becomes larger.In the charge-discharge period,the contact surface between electrode and electrolyte becomes larger,which effectively improve the utilization of active substances,leading to larger specific capacitance.At the current density of 0.5 A g-1,the specific capacitance of 20% AgNPs@WS₂ nanocomposite is 42.67 F g-1 and remain 72% after 2000 cycles.Redox peaks are obviously appeared in the CV curves due to the pseudo-capacitance behavior of Ag nanoparticles.In addition,the current density through Ag nanoparticles is quite small,which effectively shortens the path of solid phase diffusion,leading to better electronic conductivity.FeCl3·6H2O was used as the reactant in hydrothermal method to prepare ultra-small Fe₃O₄ nanoparticle and form Fe₃O₄NPs@WS₂ nanocomposites at one step with magnetic property.The Fe₃O₄ nanoparticles with diameter of 3 nm were formed the heterojunctions with WS₂ nanosheets,and remained stable after sonication.The heterojunction between Fe₃O₄ nanoparticles and WS₂ nanosheets validly reduces the volume change during charge-discharge period.Fe₃O₄ with large theoretical specific capacitance is contributed to the enhanced specific capacitance of Fe₃O₄NPs@WS₂ nanocomposites.The specific capacitance of Fe₃O₄NPs@WS₂-S3 is 149.25 F g-1,which is 7 times of that of 1L-WS₂ nanosheets.MoO₂NPs@WS₂ nanocomposite was prepared for the first time by using solvent thermal method and was further annealed in Ar atmosphere to improve the crystallinity of nanoparticles.The results indicate that,in MoO₂NPs@WS₂ nanocomposite,Mo4+ was bonded with dangling bond of S atom,forming a sandwich-like structure which remains stable at lower bonding energy.With the increasing content of MoO₂ nanoparticles,the specific capacitance of nanocomposite is improved.At the scan rate of 20 mV s-1,the specific capacitance of 50% MoO₂NPs@WS₂ nanocomposite is 148.40 F g-1,showing weak pseudo-capacitance phenomena.What's more,the MoO₂NPs@WS₂ nanocomposites with different ratio have mesoporous of two sizes,which provide paths for electrolyte transmission,reducing series resistance.The capacitances of the materials are strongly enhanced and the resistances are also reduced by loading metal or metal oxide nanoparticles,forming 0D/2D nanocomposites.Furthermore,the heterojunction between nanoparticles and WS₂ nanosheets validly reduces the volume change during charge-discharge period,leading to better stability of the materials.