Research On Enhanced Liquid Phase Exfoliation To Prepare Two-dimensional Molybdenum Disulfide And Its Composites

The rise of two-dimensional(2D)nanomaterials has promoted the development of energy storage,conversion,environmental remediation,and other fields.Among many 2D nanomaterials,two-dimensional molybdenum disulfide(2D MoS2)has attracted much attention due to its outstanding physicochemical properties.To achieve low-cost and large-scale preparation of 2D MoS2 is the current research hotspot.The liquid phase exfoliation is considered to be the most promising method to realize the large-scale preparation of 2D nanomaterials,but its further development and application are limited by the low exfoliation efficiency.In this paper,based on the in-depth analysis of the mechanism and influencing factors of liquid phase exfoliation,the technology,mechanism,and application of enhanced liquid phase exfoliating MoS2 are studied through surface modification of raw materials,pretreatment,improvement of conductivity,and adjustment of the system viscosity.The main contents are as follows:1.Surface modification enhances liquid phase shear exfoliation to prepare 2D MoS2.In the liquid phase exfoliation process,the surface energy of MoS2 is an important factor affecting the exfoliation effect.Bulk MoS2 was shear-exfoliated by an easy-to-scale high-speed dispersion homogenizer and using sodium citrate as a modifier.The influence of process parameters on the 2D MoS2 concentration was systematically studied.Compared with the 2D MoS2 concentration in pure NMP,the surface modification of MoS2 can increase the 2D MoS2 concentration by 1.5 times.The lateral size distribution of 2D MoS2 is 50-200 nm,of which about 65%is less than 4 layers.Sodium citrate can increase the MoS2 surface energy,increase its polarity,and increase the electrostatic repulsion between the lamellae,which makes the polarity of MoS2 closer to that of NMP,thereby increasing the solubility of MoS2 in NMP.At the same time,the insoluble sodium citrate particles enhance the impact and shear forces on the MoS2 base and edges,which promotes the increase of 2D MoS2 concentration and the further reduction in size and thickness.2.Changing the viscosity enhances liquid phase shear exfoliation to prepare 2D MoS2.In the liquid phase shear exfoliation process,the hydrodynamic effect can promote the liquid phase exfoliation,in which the viscous shear force of the fluid plays an important role.Sodium carboxymethyl cellulose(CMC),a common thickening agent,is selected as the exfoliation assistant to achieve the shear-exfoliation of bulk MoS2 in a relatively green and environmentally ethanol/water mixed solvent.By optimizing the operating conditions,the average lateral size and the average layer number of 2D MoS2 are about 100 nm and 4 layers,respectively.The addition of an appropriate amount of CMC can increase the viscous shear force of the fluid to strengthen the shear exfoliation.In addition,CMC contains a large number of hydroxyl and carboxyl groups,which can be closely combined with 2D MoS2 through hydrogen bonding and coordination interactions to effectively prevent 2D MoS2 reagglomeration.2D MoS2 and carbon composites with mesoporous structure(2DMoS2@C)are obtained by directly annealing the prepared 2D MoS2 and CMC composites.The adsorption behavior and mechanism of 2DMoS2@C on rhodamine B(RhB)and methylene blue(MB)were studied in detail.Due to the influence of pore structure and dye molecule size,the adsorption effect of 2DMoS2@C on MB(272 mg/g)is better than that on RhB(65 mg/g).3.Ball-milling enhances liquid phase shear exfoliation to prepare 2D MoS2.In the liquid phase exfoliation process,the structure and surface characteristics of the raw materials have a great influence on the exfoliation effect.Easily decomposable carbonate as the exfoliation assistant to assist ball-milling can not only change the structure and surface characteristics of MoS2,but also strengthen the subsequent sonication exfoliation(S-MoS2NS)and shear exfoliation(H-MoS2NS)to prepare 2D MoS2.MoS2 quantum dots(S-MoS2Qs)can also be obtained by prolonging the sonication time.Compared with the absence of an assistant,the 2D MoS2 concentration is increased by more than 30 times.The lateral size of most S-MoS2NS and H-MoS2NS is less than 150 nm,and the size of S-MoS2QS is concentrated around 5 nm.The easily decomposable carbonate-assisted ball-milling can enhance the shear force and normal stress on MoS2 to promote the exposure of more hydrophilic edges of MoS2,which is conducive to the stable dispersion of MoS2 in polar solvents.At the same time,the assistant can insert the edge defect to weaken the interlayer van der Waals force,thereby strengthening the liquid phase exfoliation.The H-MoS2NS and aminated multi-walled carbon nanotubes(MWCNT)can be physically mixed to form H-MoS2NS/MWCNT composites through the electrostatic interaction.The H-MoS2NS/MWCNT composites have a good conductive network,stable structure,and fully exposed edge active sites(282 mV,10 mA/cm2).First-principles calculations prove that the introduction of MWCNT is beneficial to electron transport and effectively improves the hydrogen evolution activity of the 2D MoS2 inert surface.The graphene oxide(GO),H-MoS2NS,and MWCNT are vacuum filtered and reduced to obtain a high-conductivity self-supporting flexible thin-film electrode,which exhibits high specific capacitance and excellent cycle stability.After assembling the self-supporting film into a solid flexible supercapacitor,the capacitance retention rate of the supercapacitor is still higher than 90%after bending at different angles.4.Copper doping enhances the electrochemical liquid phase exfoliation of MoS2.In the electrochemical exfoliation process,the electrode conductivity significantly affects the exfoliation effect.In order to realize the electrochemical exfoliation of semiconducting MoS2,copper powder with good conductivity and MoS2 are mixed in a certain proportion and then annealed as a cathode to achieve electrochemical exfoliation in a low-cost sodium sulfate solution.The effect of conductive metal doping ratio,working voltage,and exfoliation time on the morphology,structure,and performance of the product are systematically explored.Under the optimized process,the yield of highly conductive Cu0.5Mo3S4(?3060 S/m)is as high as 59%.The lateral size distribution is wide(50 nm-1?m)and the base surface is multi-cracked,thereby exposing more edge sites.Therefore,Cu0.5Mo3S4 shows excellent hydrogen evolution activity(268 mV,10 mA/cm2)and good catalytic stability.The introduction of conductive metal promotes the rapid insertion and extraction of cations and hydrated cations,and accelerates the reduction of water to produce hydrogen bubbles.When the bubbles burst instantaneously,it is accompanied by a strong jet force to strengthen the electrochemical exfoliation.