Preparation Of Redispersible Reduced Graphene Oxid Under Synergistic Control

Graphene is a two-dimensional carbon nanomaterial with a hexagonal honeycomb structure of a single layer of carbon atoms.This single-atom layer structure shows excellent electrical,thermal,optical,and mechanical properties with important applications in optoelectronic devices,energy storage devices,gas adsorption and separation,seawater desalination,catalysis,and biomedicine.However,its application to practical industrial production still faces great challenges,making the large-scale production of high-quality,highly dispersible and even redispersible graphene to be a hot research topic.The current preparation strategies focus on physical exfoliation methods and chemical redox methods.Physically exfoliated graphene is generally of better quality and structural integrity,which greatly exceeds that of the usual chemically reduced graphene,but the physical exfoliation method generates higher energy consumption and the complexity of the process is not conducive to large-scale continuous production.The chemical method obtains high quality graphene by oxidation of natural graphite,after chemical reduction in mild conditions that are suitable for high volume production.To apply the inherent properties of graphene to macroscopic graphene-based materials,the preparation of uniform and stable graphene dispersions is one of the key issues.The chemical reduction oxidation method meets this point well.However,the reduced graphene oxide(RGO)prepared by the current solution method is not ideal,with conductivity not exceeding tens of thousands of levels,insufficient reduction,and the powder products obtained from preparation in a single solvent are usually not redispersed.Based on the difference in the hydrophilicity of graphene oxide(GO)and reduced graphene oxide,the current chemical redox preparation suffers from a mismatch of solvent systems before and after reduction,with obvious aggregation problems,resulting in incomplete reduction.Moreover,the surface adsorption caused by the great specific surface area of graphene severely affects the conductivity expression of the final reduced graphene oxide.In this paper,we analyzed three challenges of graphene oxide reduction by solution method as:aggregation,incomplete reduction and surface adsorption,for which we conducted a study.Accordingly,the reduction processes of gradient solvent system,high dose and high reaction temperature,and the corresponding surface adsorption experiments were designed to investigate the preparation of redispersible reduced graphene oxide in gradient organic solvent in detail,and a reduced graphene oxide with high dispersibility and high electrical conductivity was obtained.First,graphene oxide with different degrees of oxidation was synthesized to control the defect density of the reduced graphene crystal surface.To address the hydrophilic differences between graphene oxide and reduced graphene oxide,a gradient gradient solvent system was designed for the reduction of graphene oxide using water and N-methylpyrrolidone(NMP)as the initial solvent system,different surfactants as dispersants,and ascorbic acid was selected as an efficient green reducing agent.As the reduction reaction proceeded,the system gradually shifted to a nonpolar state,and a gradient solvent gradient was performed using the strategy of continuous dropwise addition of xylene to match the continuous dispersion process of the reduction process,which prevented the agglomeration of the reduction process.The effects of different solvent systems,surfactant types and ratios,reducing agent amounts and reduction times on the conductivity of RGO were investigated.The most suitable gradient gradient solvent system and preparation process were determined.It is shown that the reduced graphene oxide in gradient solvent system is without aggregation and precipitation,and the obtained reduced graphene oxide dispersion can be maintained for up to one month with no flocculation or precipitation.Furthermore,the separated reduced graphene oxide solid powder can still be redispersed in NMP with particle size as low as 243 nm after direct sonication.Secondly,based on gradient solvent system,to ensure uniform dispersion of reactants both before and after,we increased the amount of reducing agent and used higher reduction temperature.The results showed that the conductivity and C/O ratio were not significantly enhanced because the large doses of reducing agents,high boiling point solvents and strongly interacting dispersants produced different degrees of surface adsorption after reduction.For the surface adsorption caused by the huge specific surface area of graphene in the reduction process.Further desorption studies were carried out using different dispersants,solvents as representatives of different types of adsorption,and continuous solvent washing,solvent thermal extraction and high temperature heat treatment.It showed that the method of continuous solvent washing of NMP had good film formation performance,but it was difficult to achieve the removal of large amount of adsorbate.Compared with the AEO series,the RGO produced has a lower electrical conductivity owing to the presence of a benzene ring structure on the molecular structure of the OP series,which has a stronger interaction with reduced graphene oxide.After the first thermal extraction of ethylene glycol methyl ether(EGM)/cyclohexane,the conductivity doubled,indicating the presence of partial removal of sorbents;after the secondary reflux replaced with ethanol,the conductivity remained unchanged,indicating the limited removal capacity of the thermal extraction.Further heat treatment of RGO at different drying temperatures showed that the conductivity was 5236 S/m increased from 9024 S/m to 18000 S/m after drying at 300℃ and 500℃,respectively,while XPS tests confirmed that some adsorbed material was still present,thus proving that the surface adsorption removal of reduced graphene oxide is subject to further selection of better methods.For the preparation of reduced graphene with high conductivity and corresponding properties,the reduction process needs to avoid the extensive use of high-boiling solvents and strongly interacting dispersants as much as possible.