选择性还原氧化石墨烯

文章编号:　1007⁃8827(2014)01⁃0061⁃06选择性还原氧化石墨烯徐　超1,　员汝胜1,　汪　信2(1.福州大学光催化研究所福建省重点实验室⁃国家重点实验室培育基地,福建福州350002;2.南京理工大学教育部软化学与功能材料重点实验室,江苏南京210094)摘　要:　还原氧化石墨烯已被广泛用于制备基于石墨烯的材料㊂目前,还原处理方法均是尽可能地将氧化石墨烯中的功能团去除,恢复石墨烯的电子结构㊂由于氧化石墨烯中氧基功能团(如羟基㊁羧基及环氧基)不同的反应活性,氧化石墨烯是可能通过分步的方法进行还原㊂利用醇溶剂如乙醇㊁乙二醇㊁丙三醇还原氧化石墨烯,并采用不同分析手段对样品进行表征㊂结果发现,在一定条件下这些醇可选择性地还原氧化石墨烯㊂经这些醇的处理后,氧化石墨烯中环氧功能团被大部分去除,而其他的功能团如羟基和羧基仍被保留㊂这种选择性去除氧化石墨烯表面功能团的方法可利于有效地控制氧化石墨烯的还原程度㊁获得具有特定功能团的石墨烯衍生物,从而扩大这类材料的使用范围㊂关键词:　氧化石墨烯;氧化功能团;醇;选择性还原基金项目:国家自然科学基金(21201036,21077023);福建省自然科学基金(2010J01035,2012J01039).作者简介:徐　超,博士,讲师.E⁃mail:cxu@Selective reduction of graphene oxideXU Chao1,　YUAN Ru⁃sheng1,　WANG Xin2(1.Research Institute of Photocatalysis,Fujian Provincial Key Laboratoryof Photocatalysis⁃⁃State Key Laboratory Breeding Base,Fuzhou University,Fuzhou350002,China;2.Key Laboratory for Soft Chemistry and FunctionalMaterials of Ministry Education,Nanjing University of Science and Technology,Nanjing210094,China)Abstract:　The reduction of graphene oxide has been widely used to control the properties of graphene⁃based materials.Traditional methods thoroughly remove oxygenated functional groups in graphene oxides.We show that ethanol,ethylene glycol and glycerol can se⁃lectively reduce epoxy groups in graphene oxide while hydroxyl and carboxyl groups remain unchanged.Hydrazine hydrate can reduce ox⁃ygen functional groups except carboxyl groups.These selective removals can be used to control the reduction degree of graphene oxides and their properties.The electrical conductivity of the reduced graphene oxides with different types of oxygen functional groups varied sig⁃nificantly and increased with the degree of reduction.Keywords:　Graphene oxide;Oxygenated functional groups;Alcohols;Selective reductionCLC number:　TQ127.1+1Document code:　AReceived date:2013⁃07⁃10;　Revised date:2013⁃12⁃22Corresponding author:XU Chao,Ph.D,Lecturer.E⁃mail:cxu@Foundation items:National Natural Science Foundation of China(21201036,21077023);Natural Science Foundation of Fujian Province (2010J01035,2012J01039).English edition available online ScienceDirect(http:∕∕∕science∕journal∕18725805).DOI:10.1016/S1872⁃5805(14)60126⁃81　IntroductionGraphene oxide(GO),utilized as precursor for a large⁃scale production of graphene⁃based materials,has attracted a great deal of attention in recent years[1⁃5]. GO sheets are electrically insulating,owing to their oxygenated functional groups(hydroxyl,carboxyl and epoxy groups)on surface,which usually need further treatments to restore the electrical conductivity for spe⁃cific applications[6].A lot of methods,such as chemi⁃cal reduction[7⁃9],laser irradiation[10,11],microwave ir⁃radiation[12,13],photocatalysis[14,15],solvothermal re⁃duction[16,17],have been explored to remove these atta⁃ched groups thoroughly and to recover graphene net⁃works of sp2bonds.Actually,researchers recently have found that the reduction degree of graphene oxide or oxidation degree of graphene has certain influences on their properties,such as electrical conductivity,catalysis activity and semi⁃conductive band positions[18⁃20]. Among these research work,the reduction degree of　第29卷　第1期2014年2月新　型　炭　材　料NEW CARBON MATERIALS Vol.29　No.1 Feb.2014　graphene oxide sheets(or oxidation degree of gra⁃phene)is controlled by adjusting the reactive temper⁃ature or time[19⁃21].It is very difficult for these meth⁃ods to control the reduction degree precisely and the repeatability is usually far from satisfactory,which limit further studies on their inherent characteristics.It is well known that the oxygenated functional groups on GO sheets mainly consist of hydroxyl,carboxyl and epoxy groups[22].In theory,these functional groups should have different reaction activities.Ac⁃cordingly,it is possible to utilize such diversified re⁃activities to remove these functional groups selectively through a stepwise manner,resulting in a controllable reduction of GO.Herein,we will show that under manipulative conditions,alcohols such as ethanol,ethylene glycol and glycerol can act as mild reducing agents to reduce GO selectively.Although some of these alcohols have been utilized to reduce GO,few attention is paid to their special reducing abilities[17,23,24].Our experi⁃mental results indicate that when GO was treated with these alcohols under appropriate conditions,only the signals from epoxy groups have been attenuated,but no obvious reactions are observed for the hydroxyl and carboxyl groups on GO.Such selective elimina⁃tion of functional groups could enable us to manipu⁃late the reduction degree of GO in a well⁃controlled manner and to obtain certain functional groups⁃in⁃volved graphene consequently.2　Experimental2.1　Chemical reduction of GOGO was prepared from purified natural graphite with a mean particle size of325mesh according to the method reported by Hummers and Offeman[25].All other reagents were purchased from Shanghai Ling⁃feng Chemical Reagents Co.Ltd.,China.Three types of alcohols(ethanol,ethylene glycol and glycerol)were used to react with GO.Typically, GO powder was dispersed in50mL of ethylene glycol under sonication for30min at room temperature. Then the as⁃obtained suspension was heated in an oil bath at160℃for6h under vigorous stirring.Subse⁃quently,the mixture was centrifuged while still hot, which was further washed with anhydrous alcohol and deionized water completely and dried at60℃.The as⁃prepared products are labeled as RGO⁃EG.Sam⁃ples reduced by glycerol were prepared using the same procedure,and the products are labeled as RGO⁃GL. Reduction of GO by ethanol was carried out in a sealed autoclave and heated at160℃for6h,and the as⁃prepared samples are labeled as RGO⁃ET.The hy⁃drazine hydrate,a typically strong reducing agent for GO,was also used here as a comparison[8].200mg of GO was dispersed in100mL of water by sonication for30min to form a suspension.2mL of hydrazine hydrate(50%w/v)was then added to the suspen⁃sion that was refluxed(100℃)for24h.After the re⁃action,the solid product(RGO⁃Hy)was isolated by filtration and washed with water and anhydrous alco⁃hol,and finally dried at60℃in vacuum.2.2　CharacterizationPowder X⁃ray diffraction(XRD)　were per⁃formed on a Bruker D8Advance diffractometer with Cu Kαradiation.The diffraction data were recorded for2θangles between5°and60°.Scanning electron microscopy(SEM)were carried out on a JEOL JSM⁃6380LV scanning electron microscope.Fourier⁃trans⁃formed infrared spectroscopy(FT⁃IR)were carried out on a Bruker Vector⁃22,for which samples were prepared in potassium bromide pellets.The X⁃ray photoelectron spectra(XPS)were recorded on a Per⁃kin⁃Elmer PHI5300X⁃ray photoelectron spectrome⁃ter,using Mg Kα(hυ=1253.6eV)X⁃ray as the excitation source.The elemental analysis were carried out on a Vario ELⅢ,and before test all samples were dried at100℃in vacuum for two days.Raman spectra were recorded from200to2000cm⁃1on a Renishaw Invia Raman Microprobe using a514.5nm argon ion laser.The electrical conductivity of each sample was was analyzed by a SDY⁃4four⁃point probe instrument.3　Results and discussionHydrazine hydrate is a strong reducing agent which was employed here to reduce GO as a compari⁃son for other methods in this study[8].Fig.1shows the XRD patterns of GO and its derivatives treated by ethylene glycol and hydrazine hydrate(the resulting materials are labeled as RGO⁃EG and RGO⁃Hy,re⁃spectively).It can be clearly seen that the XRD pat⁃terns of RGO⁃EG are quite different from that of GO .Fig.1　XRD patterns of GO,RGO⁃EG and RGO⁃Hy.㊃26㊃　新　型　炭　材　料第29卷The characteristic diffraction peak (001)of GO al⁃most disappears ,while a new broad peak at around 24°is formed ,which is one characteristic diffraction peak observed in the traditional reduced GO [8,17].Furthermore ,the XRD patterns of RGO⁃ET and RGO⁃GL are similar to that of RGO⁃EG and RGO⁃Hy ,indicating structure of GO could be changed by reducing with these alcohols [17,26,27].Fig .2displays SEM images of GO ,RGO⁃EG and RGO⁃Hy .By comparison ,it can be found that GO sheets possess a plat lamellar structure (Fig .2a ,b ),whereas RGO⁃EG and RGO⁃Hy possess a floccu⁃lent morphology ,forming a disordered solid (Fig .2b ),which is similar to that of reduced GOs reported in previous literatures [8,17].Fig .2　SEM images of (a ,b )GO ,(c )RGO⁃EG and (d )RGO⁃Hy .Fig .3　FT⁃IR spectra of GO ,RGO⁃EG and RGO⁃Hy . It is worth noting that there are some differences between these reduced GO using alcohols and hydra⁃zine hydrate .Fig .3shows the FT⁃IR spectra of GO ,RGO⁃EG and RGO⁃Hy .As shown in Fig.3,the characteristic features in the FT⁃IR spectra of GO are =the absorption bands corresponding to the C Ocarbonyl stretching at 1720cm ⁃1,the C OH stretc⁃hing at 1224cm ⁃1,and the C O stretching at 1050cm ⁃1[28,29].=The spectra also show a C C peak at 1620cm ⁃1corresponding to the remaining sp 2charac⁃ter [30].These absorption bands in the RGO⁃Hy are not pronounced ,indicative of an apparent removal of the oxygen⁃containing groups [31].Several absorption bands at around 1721,1568and 1210cm ⁃1are still ob⁃servable in the spectrum of RGO⁃EG .It has been =demonstrated that C O carbonyl stretching at a⁃round 1720cm ⁃1is hard to be reduced ,even with hy⁃drazine hydrate .The same phenomenon is observed in our samples [7].The bands at around 1568and 1210cm ⁃1may be attributed to the vibrations of skeleton and C OH bonds of RGO⁃EG ,respectively [17].It is worth noting that the absorption of epoxy groups (around 1050cm ⁃1)almost disappear as the arrow ,which may be ascribe to the reactions between the ep⁃㊃36㊃第1期XU Chao et al :Selective reduction of graphene oxideoxy groups and alcohols .Analogously ,the RGO⁃ET and RGO⁃GL possess almost the same absorption bands .The functional groups present in these samples are further analyzed by XPS .The C 1s XPS spectrum of GO (Fig .4a )clearly indicates that there are four kinds of carbon :the sp 2⁃hybridized C C ,the C in C OH bonds ,the epoxy C ,and the carboxylate car⁃bon [8,32].Strong reductants such as hydrazine hydrate can usually remove oxygen in GO considerably (Fig .4c ).However ,by comparison ,it can be clearly seen that only epoxy C 1s peak intensity of RGO⁃EG re⁃duced significantly after reduction with ethylene gly⁃col ,and the change of the C 1s peak intensities of hy⁃droxyl and carboxyl groups in RGO⁃EG is not notice⁃able (as shown in Fig .4b ),which is consistent with the results of FT⁃IR spectra .On the other hand ,the increased area percentage of the peak associated with sp 2=⁃hybridized C indicates the formation of C C bond .Therefore ,it is assumed that in our system ,the oxygen in epoxy groups are mostly removed ,while hydroxyl and carboxyl groups left un⁃attacked .Fig .4　C 1s XPS spectra of (a )GO ,(b )RGO⁃EG and (c )RGO⁃Hy . Theoretically ,the C /O atomic ratio will increase as a result of the de⁃oxygenation of the functional groups of GO .Indeed ,there is a significant change of the C /O atomic ratios by chemical reductions from the starting GO (2.9)to the reduced GO [8].Howev⁃er ,the C /O atomic ratio of RGO⁃EG (5.4)is much lower than that of RGO⁃Hy (10.8)whose oxygen functional groups are almost removed .Such feature ofselective reduction for GO with alcohols can be also supported by Raman spectroscopy .In Fig .5,two prominent peaks of GO appear at around 1355and 1598cm ⁃1,which are attributed to D and G band ,re⁃spectively [33,34].After reduction with alcohols or hy⁃drazine hydrate ,the D and G peaks still exist but with an increased D /G intensity ratio compared to that of GO (0.85).Generally ,D /G intensity ratio is in⁃versely proportional to the average crystallite size in graphite materials ,so the increase in D /G intensity ratio after reduction indicated a decrease in the aver⁃age size of the graphitic domains ,which is caused by the small re⁃graphitized sp 2domains [8,33,34].Due to the selective reduction with alcohols ,the small gra⁃phitic domains in RGO⁃EG are created merely by graphitization of the epoxy groups ,while that of RGO⁃Hy consisted of the graphitization of hydroxyl and carboxyl groups in addition to epoxy groups .As a result ,the D /G intensity ratio of RGO⁃EG (0.97)is lower than that of RGO⁃Hy (1.1).The similar phenomena are also found in Raman spectra of RGO⁃ET and RGO⁃GL .Thus ,the Raman analysis provides further evidences for the selective reduction of GO with these alcohols .Fig.5　Raman spectra of GO ,RGO⁃EG and RGO⁃Hy . Since RGO⁃EG is only partially reduced ,some of its physical properties ,for example electrical con⁃ductivity ,should be between that of GO and RGO⁃Hy .Earlier researches have suggested that the GO is electrically insulating ,and chemical reduction can re⁃store the electrical conductivity of such materials closed to that of pure graphite [8].We find that after reduction with EG ,the electrical conductivity of RGO⁃EG indeed increase from 0.02to 6.7S ㊃m ⁃1.Nevertheless ,compared with that of RGO⁃Hy (2100S ㊃m ⁃1),the extent of restoration of electrical conductivity is much lower .Though the re⁃established graphite domains increase the conductivity of RGO⁃EG ,the presence of functional groups especially the hydroxyl groups are still decorated sporadically on the surface of RGO⁃EG ,makes the electrically conduc⁃㊃46㊃　新　型　炭　材　料第29卷tive graphite domains discontinuous.Consequently, the conductivity of RGO⁃EG is much lower than that of RGO⁃Hy(Table1).Table1　Electrical conductivity of the graphite,GO,RGO⁃EG and RGO⁃HyGraphite GO RGO⁃EG RGO⁃Hy Electricalconductivityk/S㊃m⁃12800±100.02±0.0026.5±0.52100±150 Furthermore,the as⁃prepared RGO⁃EG can be further de⁃oxygenated by hydrazine hydrate,resulting in the formation of a well⁃reduced GO.Accordingly, GO sheets can be reduced controllably by a stepwise removal of functional groups,which may be benefi⁃cial for studying properties and potential applications of graphene derivatives relating to the type of oxygen groups in the future.4　ConclusionsWe have demonstrated that GO can be selectively reduced by alcohols such as ethanol,ethylene glycol and glycerol under thermal treatments.One typical feature of this reduction is that the epoxy groups are mainly reduced and re⁃graphitized,while hydroxyl and carboxyl groups are maintained.Our research in⁃dicates that it is feasible to reduce GO selectively ac⁃cording to the activity differences between these atta⁃ched functional groups.And such selective removal of functional groups could be utilized to control the reduction degree of graphene oxide and prepare gra⁃phene derivatives with specific functional groups, which is useful to relate functional groups and proper⁃ties of graphene⁃based materials. 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[32]　Jeong H K,Lee Y P,Lahaye R J W E,et al.Evidence of gra⁃phitic stacking order of graphite oxides[J].J Am Chem Soc,2008,130(4):1362⁃1366.[33]　Tuinstra F,Koenig J L.Raman spectra of graphite[J].JChem Phys,1970,53(3):1126⁃1130.[34]　Kudin KN,Ozbas B,Schniepp H C,et al.Raman spectra ofgraphite oxide and functionalized graphene sheets[J].NanoLett,2008,8(1):36⁃41.Instructions to Authors New Carbon Materials is a bimonthly journal published with the permission of the Ministry of Science and Technology and of the State News and Publication Agency.The journal is sponsored by the Institute of Coal Chemistry,Chinese Academy of Sciences,and is published by Science Press. 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