Preparation And Properties Of Hydrophilic Graphene By Liquid Phase Exfoliating

Since Novoselov and Geim discovered graphene in 2004,graphene has been widely used in mobile devices,aerospace,electronic devices,composite materials,energy storage,sensors and other fields due to its excellent conduc-tivity,thermal conductivity,mechanical properties and large specific surface area.With the in-depth study of graphene from laboratory to industrial applica-tion,it is found that the liquid phase exfoliating method can produce high-qual-ity graphene at low cost and on a large scale,but the water-insoluble nature of graphene greatly limits its preparation process and application field.How to prepare hydrophilic graphene quickly,efficiently and on a large scale has be-come a research hotspot.In this paper,the hydrophilic graphene was prepared by liquid phase shear exfoliating and electrochemical exfoliating with green water as solvent,and several potential applications of hydrophilic graphene were studied on the basis of it.The main contents of this paper are as follows:1.Preparation of hydrophilic graphene from graphite by liquid phase shear exfoliating.The hydrophilic graphene/graphene oxide nanocomposites with two-dimensional layered structure were successfully fabricated by high-speed homogenizer shear exfoliating in water with graphite oxide as additive,which is called hydrophilic graphene(2/2).The effects of exfoliating time,shear speed,liquid volume and other process parameters on the concentration of hydrophilic graphene(2/2)were studied.Graphene oxide and graphene prepared under shear force are assembled into two-dimensional layered structure through the interaction of aromatic rings on the parallel plane.This unique structure can not only maintain the inherent properties of graphene,but also increase the hydro-philicity of graphene.As graphene oxide has strong polar carboxyl and hydroxyl groups,it can form strong electrostatic repulsion force,so that hydrophilic gra-phene(2/2)can exist stably in pH=3-10.The synergistic effect of graphene and graphene oxide is conducive to the preparation of homogeneous composite ma-terials and the formation of 3D network structure materials between hydrophilic graphene(2/2)and polymer or inorganic materials in water phase.The unique structure and properties lay the foundation for the application of hydrophilic graphene(2/2).Based on this research,a pilot production line of hydrophilic graphene with an annual output of 1-2 tons was established.2.Application of hydrophilic graphene(2/2)in supercapacitor.The syner-gistic effect of graphene with double electric layer capacitance and pseudo-capacitor materials is beneficial to improve the specific capacity and cycle sta-bility of the composite.(1)hydrophilic graphene(2/2)and polyaniline were self-assembled by one-step hydrothermal method to form 3D porous gra-phene/polyaniline composite.The results show that the hydrophilic graphene(2/2)is helpful to the uniform distribution of poly aniline on its surface and the formation of 3D porous structure.The 3D graphene/polyaniline composite can be directly used as electrode material without adding conductive agent and binder,thus simplifying operation and reducing cost.The electrode materials exhibit high specific capacity(483 F g-1,1 A g-1)and excellent cycle stability due to its 3D structure,high conductivity and large specific surface area.The graphene/polyaniline composite was assembled into a symmetrical supercapac-itor device,and its energy density was as high as 17.9 Wh kg-1 at a power density of 500 W kg-1.(2)the graphene/nickel hydroxide composite was prepared by in-situ growth with nickel sulfate as the precursor in the aqueous suspension of hydrophilic graphene(2/2).The synergism of graphene and graphene oxide in hydrophilic graphene(2/2)is beneficial to the anchoring of nickel sulfate and the growth of larger size Ni(OH)2.The specific capacity of the composite is up to 1510 F g-1(2 A g-1).Graphene/nickel hydroxide composite and reduced gra-phene oxide were assembled into an asymmetric supercapacitor device.The en-ergy density of the device was as high as 44.9 Wh kg-1 at a power density of 400 W kg-1.It is proved that the hydrophilic graphene(2/2)has a good application prospect in the electrode of supercapacitor.3.The application of hydrophilic graphene(2/2)in solar evaporator.Gra-phene/PVA composite with 3D porous structure was synthesized by one-step hydrothermal method from hydrophilic graphene(2/2)and polyvinyl alcohol(PVA).The introduction of PVA improved the hydrophilicity and mechanical properties of the material.The composite has high and wide absorbance(94.5%,250-2500 nm),high porosity(97.2%),low thermal conductivity(0.02 W m-1 k-1)and low density(0.011 g cm-3).The excellent properties make the composite material can be directly used as solar evaporator without additional thermal in-sulation and support layer,and can effectively prevent heat conduction to bulk water.Graphene/PVA composite exhibited high water evaporation rate(1.44 kg m-2 h-1)and photothermal conversion efficiency(86%)under 1 kW m-2 light.It is proved that the hydrophilic graphene(2/2)is an ideal raw material for low-cost preparation of solar evaporator.4.Preparation and application of hydrophilic graphene from graphite by alternating electrochemical exfoliating.With 1 M Na2SO4 as electrolyte,alter-nating voltage is applied between two graphite plates.At a certain voltage,the water of anode is electrolyzed into hydroxyl radical,which acts on the edge and grain boundary of graphite,thus opening the graphite layer is conducive to the embedding of SO42-and H2O.When the voltage changes from positive potential to negative potential,SO42-and H2O are reduced to SO2 and H2.At the same time,the complex electrochemical reaction on the cathode makes the oxygen-containing functional groups on the graphite layer be reduced to O2 and CO2.These gases contribute to the exfoliation of the graphite layer.Then,the hydro-philic graphene/graphene quantum dots were prepared under the assistance of ultrasound,which was called for hydrophilic graphene(2/0).The effects of pro-cess parameters such as voltage,alternating frequency and plate spacing on the exfoliating effect,the quality and size of hydrophilic graphene(2/0)were inves-tigated.The results show that low voltage,low alternating frequency and large plate spacing are favorable for the preparation of high quality and small size hydrophilic graphene(2/0).The obtained hydrophilic graphene(2/0)can dis-perse stably in water,NMP,isopropanol and ethanol for several months without precipitation.The formation of graphene quantum dots(1-4 nm)can reduce the aggregation of graphene and help graphene to disperse stably in water.The syn-ergistic effect of graphene and graphene quantum dots makes the hydrophilic graphene(2/0)show good detection performance for dopamine,and its detec-tion limit is as low as 3 × 10-8 M(S/N=3).This method is firstly used to prepare graphene and graphene quantum dots composite materials,which provides a new way for low cost and green preparation of hydrophilic graphene.