Deoxygenation Of Graphene Oxide By Solid-state Microwave Heating Method

Graphene,as a new type of two-dimensional carbon monoatomic layer,with unique properties such as ultra-high specific surface area,high conductivity,thermal conductivity and chemical stability,has attracted tremendous attention from researchers in recent years.The exfoliation and reduction of graphite oxide?GO?is one of the established procedures for the bulk production of graphene to satisfy its extensive application.Many established procedures such as chemical reduction or high-temperature thermal reduction have been adopted to remove the surface oxygen-containing groups of GO to recover the intrinsic properties of graphene.In comparison with traditional heating method,microwave heating has attracted great attentions being a more rapid and energy efficient approach.However,the reduction quality of GO based on the solvent medium reaction is low,attributed to the limited reaction temperature since most of irradiated microwave is absorbed by solvent.And when GO is exposed directly to microwave in solid-state,nearly no response of pure GO to microwave irradiation because of its poor absorption capacity attributed to large amounts of oxygen-containing functional groups on it.In order to improve the reduction efficiency of GO via the solid-state microwave heating method,the effective approaches including pre-reduction using chemical reducing agents and hybrid microwave heating using external susceptor were adopted to enhance the microwave absorbing ability of GO,and the corresponding deoxygenation mechanisms were investigated.The specific contents and main conclusions are as follows.I.The deoxygenation efficiency of GO under solid-state microwave irradiation was improved via chemical pre-reduction using different reductants?Vitamin C,hydrazine hydrate and sodium borohydride?.The influence of concentrations and types of oxygen-containing groups on the reduction efficiency during microwave heating process was investigated.It is shown that the R_C/O of GO is 1.62,which increases with the increasing of the mass ratio of Vitamin C to GO(R_VC/GO),which is 2.36,3.55 and 4.74 for sample VRGO-0.33,VRGO-1.5and VRGO-5,respectively.The R_C/O of GO rises to 5.7 and 2.89 after reduction by hydrazine and sodium borohydride,respectively.Although the VRGO-0.33,VRGO-1.5 and VRGO-5samples have different initial oxygen concentrations,almost same proportions of oxygen-containing groups is left under the microwave irradiation at 2000 W for 30s,achieving a ultimate value of 10¹1%for residual oxygen concentrations.It is interesting to observe that the deoxygenation efficiency of RGOs at the similar initial oxygen concentrations via further microwave irradiation is different.For instance,the oxygen concentration of VRGO-0.33 and SRGO decreases to 10.75%and 19.19%,and the oxygen concentration of VRGO-5 and HRGO decreases to 10.66%and 13.78%,respectively.Compared with VRGO-0.33,the reduction efficiency is lower for SRGO that with lower content of carbonyls and higher content of hydroxyls,carboxyls and epoxides.This can be explained by that the lower energy required to remove carbonyls relative to that for the other functional groups.Moreover,because HRGO contains fewer content of low stable carbonyls and some amount of nitrogen-containing groups,which are relatively stable at high temperature,especially for pyrrolic N and graphitic N,the reduction efficiency of HRGO is lower than that of VRGO-5 and some doped nitrogen atoms are remaining on MHRGO.II.GO was reduced effectively via a hybrid microwave heating?HWH?method by using mildly reduced graphene oxide?MG?membrane as a external susceptor,in which MG was obtained by annealing of GO at 300 ^oC for 1 h under nitrogen by a conventional heating method.The real part of complex permittivity????of GO increases from 2.25 to 2.69-2.87 for MG,demonstrating the absorption ability of MG is definitely improved in comparison with GO.When the mass ratio of MG to GO is 0.045,the atomic ratio of carbon to oxygen(R_C/O)of GO increases from 1.8 to 17.84 by microwave irradiation under argon at 2000 W for 30 s,meanwhile,the BET surface area increases from 151 to 313 m²/g.However,when GO was irradiated solely under the same conditions,the R_C/O/O was unchanged?1.88?.The MG was further irradiated by microwave at 2000 W for 30 s,its R_C/O is enhanced from 4.17 to 8.46,demonstrating the higher reduction efficiency due to a mixed heating mode in the HWH process.The conventional heating of GO occurs during the initial stages and a two-way heating mode during the later stages,in which MG heat the GO from the surface and the microwave heating from the GO itself.Therefore,more amount of carboxyls and epoxides can be removed in the HWH process to achieve higher R_C/O compared with that of the product obtained by direct microwave irradiation of MG.III.The influence of oxidation degree of GO on the deoxygenation efficiency via HWH method at 600 W by using cheap graphite powder as the external susceptor was investigated.The GO samples with R_C/O/O of 1.66,1.95 and 2.24 were obtained by the modified Hummers method when the dosage of potassium permanganate is 3 g,5 g and 9 g,which was denoted as3-GO,5-GO and 9-GO,respectively.It is found that no response of GO to microwave irradiation when the pure GO samples are exposed directly to microwave.While the R_C/O/O increases to 6.66,5.46,5.18 for sample 3-GO,5-GO and 9-GO,respectively,indicating the reduction happens under under assistance of graphite susceptor,and the reduction efficiency is closely related with the oxidation degree of GO.In this process,graphite susceptor converts radiation energy into heat energy,part of which makes GO remove some oxygen-containing functional groups and produces stable carbonyl groups?such as anhydride?and epoxy?ether?.The partial reduction of GO can improve its own microwave absorption ability,which can benefit itself converting microwave into heat energy to further remove a large number of in-plane carbonyl groups and hydroxyl groups.