Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping

Transcription

1 Reeived 5 Jul 2011 Aepted 14 De 2011 Pulished 24 Jn 2012 DI: /nomms1643 Chemil strutures of hydrzine-treted grphene oxide nd genertion of romti nitrogen doping Sungjin Prk 1, Yihen Hu 2, Jin k Hwng 3, Eui-Sup Lee 4, Leh B. Csin 2, Weiwei Ci 5, Jeffrey R. Potts 5, Hyung-Wook H 5, Shnshn Chen 5, Junghoon h 1, Sng uk Kim 3, Yong-Hyun Kim 4, Yoshitk Ishii 2,6 & Rodney S. Ruoff 5 Chemilly modified grphene pltelets, produed vi grphene oxide, show gret promise in vriety of pplitions due to their eletril, therml, rrier nd mehnil properties. Understnding the hemil strutures of hemilly modified grphene pltelets will id in the understnding of their physil properties nd filitte development of hemilly modified grphene pltelet hemistry. Here we use 13 C nd 15 N solid-stte nuler mgneti resonne spetrosopy nd X-ry photoeletron spetrosopy to study the hemil struture of 15 N- lelled hydrzine-treted 13 C-lelled grphite oxide nd unlelled hydrzine-treted grphene oxide, respetively. These experiments suggest tht hydrzine tretment of grphene oxide uses insertion of n romti N 2 moiety in five-memered ring t the pltelet edges nd lso restores grphiti networks on the sl plnes. Furthermore, density-funtionl theory lultions support the formtion of suh N 2 strutures t the edges nd help to eluidte the influene of the romti N 2 moieties on the eletroni struture of hemilly modified grphene pltelets. 1 Deprtment of Chemistry, Inh University, Inheon , Kore. 2 Deprtment of Chemistry, University of Illinois t Chigo, 845 W Tylor Street, Chigo, Illinois 60607, USA. 3 Deprtment of Mterils Siene nd Engineering, KAIST, Dejeon , Kore. 4 Grdute Shool of Nnosiene nd Tehnology (WCU) nd KAIST Institute for the NnoCentury, KAIST, Dejeon , Kore. 5 Deprtment of Mehnil Engineering nd the Mterils Siene nd Engineering Progrm, University of Texs t Austin, ne University Sttion C2200, Austin, Texs , USA. 6 Center for Struturl Biology, University of Illinois t Chigo, 1100 South Ashlnd Street, Chigo, Illinois 60607, USA. Correspondene nd requests for mterils should e ddressed to S.P. (emil: sungjinprk@inh..kr).

2 nture ommunitions DI: /nomms1643 In the 1960s, hydrzine (NH2NH2) ws used to hemilly redue thin grphite oxide (G) prtiles 1,2 ; Ruoff nd o-workers 3 developed the use of hydrzine to redue individul grphene oxide pltelets four dedes lter. For the lst hlf dede, mong severl hemil redutnts 4,5, hydrzine hs een the most ommonly used redutnt due to its ese of use (for exmple, vi one-pot synthesis in either liquid or gs phse) nd its ility to hieve high degree of redution of grphene oxide without the need for further tretment 3,6 8. Hydrzine-treted grphene oxide (hemilly redued grphene oxide, CReG ) 3 is one of the most promising grphene-sed mterils for severl pplitions, suh s polymer omposites, ultrpitors, rehrgele tteries, hemil/iosensors nd thin films A lrge frtion of the oxygen-sed funtionl groups of grphene oxide re removed y exposure to hydrzine; however, the resulting grphene mterils ontin smll mount of nd N toms (with pproximte C/ nd C/N rtios of 10 nd 22, respetively) 3,6. The presene of these heterotoms on the grphene pltelets nmely, eletron donting N toms nd eletron withdrwing toms my rete doping effet tht onsiderly ffets their eletroni struture 15. Also, hemil moieties ontining heterotoms or heteroyles n e utilized to open new pth to seletive hemil modifitions of grphene-sed systems. However, despite the use of CReG in vriety of different wys 9 11, the hemil identities of the funtionl groups ontining those heterotoms hve not yet een fully determined. 13 C solid-stte nuler mgneti resonne (SSNMR) spetrosopy my e the most powerful method to study the detiled hemil strutures of grphene-sed mterils suh s grphene oxide, G, redued grphene oxide nd other hemilly modified grphenes (CMGs) 16,17. However, the low nturl undne (1.1%) of the 13 C isotope results in low signl-to-noise rtio, whih, long with the rod peks typilly oserved from CMGs, preludes the use of multidimensionl SSNMR. This ws the motivtion for our reent work, whih used 99% 13 C-lelled grphite mde y hemil vpour deposition (CVD) s the preursor to 13 C-lelled G, whih ws studied using 13 C SSNMR 18,19. This investigtion reveled tht hydroxyl, 1,2-epoxide nd sp 2 rons on the sl plnes 20 re diretly onded to eh other in G. It hs een proposed tht sp 2 networks in G re terminted with roxyl nd ketone groups, predomintely t the edges of the struture 21. Seprtely, the existene of five- or six-memered ltol rings t the edges of G hs een suggested y Go et l 22. While onsiderle struturl informtion hs een otined for G, few struturl detils, espeilly t the pltelet edges, hve een otined for CReG or unexfolited G redued y hydrzine, the ltter of whih we define s redued grphite oxide or RG. For SSNMR mesurements, protool to produe 13 C- nd 15 N- lelled RG y reduing 13 C-lelled G with 15 N-hydrzine monohydrte ws developed, nd 1 H/ 13 C/ 15 N triple-resonne SSNMR experiments were performed on the smples. SSNMR spetrosopy of N typilly requires the use of isotopilly lelled smples euse of the low nturl undne of 15 N (~0.4%) nd lso lrge qudrupolr intertions of nturlly undnt 14 N nulei. Herein, we present new struturl insight into redued grphene oxide mterils, reveled y 13 C- nd 15 N rottionl-eho douleresonne (REDR) SSNMR experiments 23,24 on 13 C- nd 15 N- lelled RG s well s y X-ry photoeletron spetrosopy (XPS) mesurements on unlelled, hydrzine-treted grphene oxide. This work helps to provide deeper understnding of the hemil strutures nd eletroni properties of redued grphene oxides. Results NMR study of isotope-lelled RG smples. A 13 C-lelled grphite film ws grown y CVD of 13 C-enrihed methne on Ni foil s desried previously 18. Rmn spetrosopy onfirmed tht the s-grown grphite film ws of high qulity nd highly enrihed (99%) in the 13 C isotope (Supplementry Fig. S1) 18. As expeted, the first-order G-nd ws oserved t 1,518 m 1, while the D-nd typilly deteted t 1,297 m 1 ws ompletely sent. This result indites high-qulity films nd n enrihment of pproximtely 99% 13 C. 13 C-lelled grphite film ws removed from the Ni sustrte y soth tpe, then rinsed nd sonited in etone. As shown in Supplementry Figure S1, the grphite film ws ut into smll piees (10 50 µm). This 13 C-lelled grphite ws then oxidized to 13 C-lelled G, nd susequently redued using 98% 15 N-lelled hydrzine monohydrte to produe 13 C-, 15 N-lelled RG (see the Methods setion for experimentl detils). Figure 1 shows high-resolution mgi ngle spinning (MAS) 13 C-SSNMR spetr of () the lelled G nd () the lelled RG 193 sp sp 2 C-H C--C H H Hydrzine H H H H H H H H H H H H H H H H C hemil shift (p.p.m.) 50 HN N Pyrzole formtion t the edges Figure 1 Chemil strutures of lelled smples efore nd fter hydrzine tretment. 13 C MAS SSNMR spetr of () 13 C-lelled G nd () 13 C- nd 15 N-lelled RG. The spetrum in ws reprodued from Figure 1 in our previous work y Ci et l. 18 Reprinted with permission from AAAS. All the spetr were otined with diret 13 C exittion y π/2 pulse t 13 C NMR frequeny of MHz nd with 20 khz MAS. () A shemti illustrting pyrzole formtion t the edges of the pltelets during hydrzine tretment of grphene oxide ( CH groups t the pltelet edges hve een omitted for lrity).

3 nture ommunitions DI: /nomms1643 ARTICLE smples. High-resolution SSNMR spetrosopy using MAS hs een used to study the hemil strutures of G 3,18,22. A spetrum of the lelled RG smple (Fig. 1) shows n exellent signl-tonoise rtio, despite the smll smple quntities of pproximtely 2 mg nd reltively short quisition time of 21 min. Notly, otining spetrum of similr qulity from n unlelled smple would require signl umultion for nerly 140 dys. There re severl ovious spetrl hnges from the smples efore nd fter hydrzine tretment. In Figure 1, there is weker pek t 193 p. p.m., whih ws previously ssigned to ketone groups (RR C = ) t the edges of the pltelets 22. The edge strutures ould e present round hole defets or torn edges in the interior of the pltelets 25,26. The high sensitivity resulting from our novel isotope lelling method is espeilly useful for SSNMR nlysis of importnt speies with lower popultions. Interestingly, the pek t 193 p.p.m. disppered in Figure 1, suggesting tht the hnges ourred in the hemil struture t the edges. As G is thermlly unstle, therml tretment of G under our retion onditions might led to hnges in the hemil struture of G. However, stirring G in wter t 80 C without hydrzine resulted in little hnge of the hemil struture of G, s shown in Supplementry Figure S2. These results onfirm tht hydrzine is responsile for the hnges in the hemil struture of grphene oxide. Advned SSNMR experiments orrelting the onnetivity of 13 C nd 15 N hve een used for 13 C- nd 15 N-lelled iomoleules ; however, for grphene-sed mterils, suh SSNMR experiments hve not een performed to dte, prtly euse of the diffiulties in the preprtion of pproprite smples. In this work, we quired the 13 C MAS SSNMR spetr without nd with 13 C 15 N dipolr dephsing using REDR pulse sequene, s shown in Figure 2,, respetively. Detils on the pulse sequene re provided in Figure 2d (refs 23, 24). The 13 C toms ssoited with 15 N toms lose their signl in the 13 C MAS spetrum with 13 C 15 N dephsing. Sutrtion of Figure 2 from Figure 2 shows NMR signls of the 13 C toms tht re one or t most two onds wy from 15 N toms 24. The resulting differene spetrum, shown in Figure 2, revels new spetrl feture round 150 p.p.m. (lelled with n rrow in Fig. 2), whih hs not een reported previously. The rod signl with two mxim t 140 nd 157 p.p.m. nnot e oserved s seprte pek in the spetrum shown in Figure 2 without 13 C 15 N dephsing, s they re overlpped with mjor pek entred t 118 p.p.m. In n ttempt to eliminte the effet of ny ssoited 15 N-lelled ontminnts resulting from the 15 N-lelled hydrzine tretment, the RG smple ws thoroughly wshed with etone nd susequently dried under high vuum; then the REDR experiment ws repeted. The REDR experiment on the wshed RG smple yielded n lmost identil result to tht shown in Figure 2 (dt not shown). Beuse the nturl undne of 15 N is extremely low, this oservtion suggests tht the RG ontins 15 N speies tht originte from 15 N-lelled hydrzine nd tht the 15 N speies re likely to e hemilly onded to 13 C speies of RG. We found this pek t pproximtely 150 p.p.m. for nother lelled smple, whih ws prepred from seprte th (Supplementry Fig. S3). Given these oservtions, we suggest tht the 13 C NMR pek round 150 p.p.m. n e ttriuted to pyrzole groups romti five-memered rings with two djent N toms s shown in Figure 1 whih usully show 13 C NMR signls etween 150 nd 160 p.p.m. 30 Pyrzole groups hve een produed y the retion of diketone groups with primry mine moieties 31. In ddition, the formtion of pyrzole groups from the retion etween hydrzine (one of the primry mine moieties) nd diketone groups hs een well estlished; for exmple, see the retion shown in Supplementry Figure S4 (ref. 31). Alterntively, the new 13 C NMR peks round 150 p.p.m. my e lso ssigned to C = N onds. Hydrzone groups, whih ontin non-romti C = N doule onds, hve een d 1 H 13 C 15 N 200 (π/2) φ1 π φ3 1 H Deoupling 2n π φ2 π X C hemil shift (p.p.m.) τ R /2 2τ R τ R /2 Figure 2 13 C REDR SSNMR mesurement of lelled RG. 13 C MAS SSNMR spetr with π/2-pulse exittion of 13 C- nd 15 N-lelled RG, () without nd () with dipolr dephsing y 13 C 15 N REDR during period of 1.6 ms. () The differene spetrum of nd. Note tht the vertil sle in is mgnified five times, ompred with tht of nd. The spetr in nd were olleted with the pulse sequene shown (d), () without nd () with 15 N π-pulse trins. Although the 13 C signl intensities for oth ontrol nd dephsed spetr re mildly modulted y 13 C T 2 relxtion nd 13 C 13 C J-oupling, the differene spetrum in highlights spetrum of 13 C speies within few onds from 15 N toms. The phse yles in d re s follows: φ 1 = x, x, y, y; φ 2 = y, y, x, x. The reeiver phse ws yled s x, x, y, y. The phses for φ 3 nd φ 4 in the rotor-synhronous π-pulse trins were inremented fter every π-pulse, following the XY-16 phse yle (x, y, x, y, y, x, y, x, x, y, x, y, y, x, y, x). τ R denotes one rottion period of mgi ngle spinning. produed y the retion etween hydrzines nd ketones or ldehydes s shown in Supplementry Figure S4 (refs 32, 33). In either se, the retions my tke ple t the ketone groups of G vi nuleophili ttk y the hydrzine moleules. Consequently, oth retions n explin the omplete removl of the ketone pek in the 13 C NMR spetrum. However, it is diffiult to distinguish etween pyrzole nd hydrzone groups sed on the hemil shifts in the 13 C NMR spetrum. The 15 N MAS SSNMR spetrum of the 13 C- nd 15 N-lelled mteril ws otined with ross-polriztion (CP) from 1 H spins nd is shown in Figure 3. To the est of our knowledge, this is the first report of 15 N NMR study on grphene-sed mteril. The spetrum in Figure 3 shows rod pek entred t pproximtely 190 p.p.m., whih n e ssigned to pyrzole moieties 34. Beuse of π φ4 2n 5 0

4 nture ommunitions DI: /nomms1643 d Binding energy (ev) Binding energy (ev) 15 N hemil shift (p.p.m.) e π/2 φ1 R R 1 H CP y HN N Pyrzoline-like group t the edge HN N Pyrzole group t the edge 2n π φ2 15 N CP φ2 π φ3 π X π φ4 13 C 2n Figure 4 Chemil strutures of unlelled smples efore nd fter hydrzine tretment. N1s nrrow sns in XPS spetr of redued grphene oxide smples; in, the redued grphene oxide is pperlike mteril produed y filtrtion of suspension of hydrzine-treted grphene oxide pltelets; (), smple () following therml nneling tretment; (), hydrzine vpour-treted grphene oxide film; nd (d), smple () following therml nneling tretment. (e) A shemti digrm for pyrzoline-to-pyrzole onversion t the edges. τ R /2 2τ R τ R /2 Figure 3 15 N REDR SSNMR mesurement of lelled RG. 15 N CPMAS SSNMR spetr of 13 C- nd 15 N-lelled RG () without nd () with 13 C 15 N dipolr dephsing y () REDR pulse sequene with dephsing period of 1.6 ms, () without nd () with 15 N π-pulse trins for the 15 N hnnel. Although the 15 N-signl intensities for oth ontrol nd dephsed spetr re mildly modulted y 15 N T 2 relxtion, residul 1 H 15 N dipolr ouplings under fst MAS, nd 15 N 15 N J-oupling, the signl of 15 N speies within few onds from 13 C re sustntilly suppressed y 15 N 13 C dipolr ouplings when 13 C π-pulse trin is pplied s shown in. The phse yles in re s follows: φ 1 = x, x; φ 2 = y, y, x, x. The reeiver phse ws yled s x, x, y, y. The phses for φ 3 nd φ 4 in the rotor-synhronous π-pulse trins were inremented fter every π-pulse, following the XY-16 phse yle. τ R denotes one rottion period during mgi ngle spinning. the short ontt time of 0.35 ms used in the CP period, the signl is likely due to protonted 15 N toms. From the dt, n N tom ( NH2 moiety) in hydrzone group ( C = N NH 2 ) n e ruled out euse NMR signls orresponding to NH 2 groups re usully oserved etween 0 nd 100 p.p.m. in 15 N NMR (ref. 30). Figure 3 shows the 15 N MAS spetrum with 13 C 15 N dipolr dephsing, quired using REDR pulse sequene (Fig. 3). It n e seen tht the pek t 190 p.p.m. is onsiderly deresed y 13 C 15 N dephsing, suggesting ovlent tthment of the 15 N speies on 13 C toms. n the sis of the SSNMR experiments with lelled RG smples, we elieve this suggests the formtion of romti pyrzole groups t the edges of pltelets fter hydrzine tretment. Although the defet strutures in the interior of the pltelets re not well understood, these pyrzole rings ould e generted t the edges round the interior defets. XPS mesurements nd lultions on XPS spetr. As the SSNMR spetr of the unlelled hydrzine-treted grphene oxide showed limited signl-to-noise rtio nd rod signls, XPS ws mesured on two groups of unlelled hydrzine-treted grphene oxide smples: grphene oxide pltelets in homogeneous olloidl suspensions tht were treted with hydrzine nd then vuum filtered to rete free-stnding pper-like mterils 35,36, nd film omposed of grphene oxide pltelets spin-oted on glss nd then treted with hydrzine vpour. In oth ses we oserved similr N1s spetr inditing pyrzole moieties, s shown in Figure 4. The N1s spetr of the s-prepred smples in Figure 4, show one symmetril pek round 400 ev, whih n e ssigned to pyrzoline groups (non-romti five-memered rings) rther thn pyrzole groups, s disussed elow. Although the two N toms in pyrzoline groups re not identil, the two N1s peks in XPS pper t lmost the sme position, resulting in the genertion of one overlpped pek 37. As shown in Figure 4,d, therml nneling (s desried in the Methods setion) splits the N1s pek into two well-resolved lines t nd ev, suggesting tht two N toms hve onsiderly different hemil environments s oserved in pyrzole groups. While the twofold-oordinted N tom in the pyrzole group hs lolized lone pir of eletrons, the threefold-oordinted N ssoites with delolized eletrons in the romti ring. Density-funtionl theory (DFT) lultions 37 were performed on pyrzole groups t the edges of grphene pltelets for understnding their hemil struture, prtiulrly ssoited with the experimentl XPS pek seprtion of 1.9 ev (ref. 38).

5 nture ommunitions DI: /nomms1643 ARTICLE The N N moiety of the pyrzole ring in grphene nnorions with zigzg edges (zgnr) onsists of two different N toms; one is twofold oordinted with lone pir of eletrons, nd the other is threefold oordinted with one hydrogen tom in the sp 2 plnr onfigurtion, s shown in Supplementry Figure S5. The former is pyridini-like nd the ltter is quternry-like. The two N toms should e in different ioniztion sttes with different N1s orelevel inding energies s in the pyrzole moleule. ur first-priniples DFT lultions give ore-level seprtion of 1.52 ev for pyrzolted zgnr, onsistent with the experimentl XPS pek seprtion of 1.9 ev. These vlues re lso very lose to 1.59 nd 1.8 ev for pyrzole moleules otined from DFT lultion nd XPS mesurements, respetively. Unlike the zgnr model, G my not e perfetly flt euse there is often onsiderle mount of residul oxygen fter typil hydrzine tretment. If this is indeed the se, the tertiry N should undergo out-of-plne relxtion s in n mine group with lone pir, s shown in Supplementry Figure S5. Then, the ore level of the mine-like N should nerly oinide with tht of the pyridini-like N, t round 400 ev. ur lultions for zgnr distorted y two extr hydroxyls results in nerly oinident ore levels. The ore-level seprtion is only 0.22 ev, whih n e viewed s single pek in the XPS mesurement. The theoretil ore-level seprtion for pyrzoline moleules is 0.17 ev. Therefore, we n explin the single-to-doule trnsition of N1s XPS peks on therml tretment s follows. The hydrzine tretment of G would inorporte mny N N moieties into the RG smple. Beuse of inomplete redution of RG, the inorported N N moieties hve n sp 3 onfigurtion s in pyrzoline groups. After further redution of RG y therml tretment, the N N moieties in RG re more kin to sp 2 pyrzole groups. Clultion on retion pthwys nd eletroni strutures. To understnd the formtion mehnism of the pyrzole groups, we onsidered severl possile retion routes using grphene nnorions with zgnr s shown in equtions (1 4). DFT lultions lso support the formtion of five-memered rings t the edges fter hydrzine tretment. zgnr( =, = ) + NH NH H zgnr( NNH ) H, E = 1. 96eV 2 zgnr( =, H) + NH NH zgnr( NNH ) H2 + H2, E = 0. 14eV zgnr( H, H) + NH NH zgnr( NNH ) H + 2H, E = 0. 67eV 2 2 zgnr( =, H) + NH NH zgnr( = NNH, H) H2, E = 0. 38eV (1) (2) (3) (4) Two zigzg edge sites re funtionlized s represented y dshes inside the prentheses. E is the retion energy from the DFT lultions, with negtive retion energy inditing n exothermi retion. When diketone is presented on the zigzg edge, the formtion of pyrzole group is very likely s the retion energy is out 2 ev, s shown in eqution (1). ther types of edge strutures ontining ketone or hydroxyl groups n lso e onverted into pyrzole rings nd show non-negligile exothermi retion energies, s shown in equtions (2) nd (3). n the ontrry, the formtion of hydrzone group is not energetilly fvourle with n endothermi retion energy of 0.38 ev, s shown in eqution (4). This is onsistent with our experimentl oservtions. It is possile tht hydrzine-ssisted hemil redution produed N tom-free speies t the edges, whih, in prt, might ontriute to the dispperne of the NMR pek t 190 p.p.m. tht ws previously ssigned to C = groups t the pltelet edges. However, this mehnism is not suffiient to explin the presene of pyrzole/pyrzoline groups s suggested y our SSNMR nd XPS dt. In the field of grphene-sed mterils, the term redution typilly refers to the removl of oxygen toms from grphene oxide/g. However, pyrzole formtion t the edges is more properly referred to s sustitution rther thn redution (Fig. 1). The romtiity of the pyrzole moiety should enhne eletron ondution euse of the redued nd gp nd the delolized eletroni sttes ssoited with the N in the sp 2 onfigurtion. Figure 5 illustrtes the totl nd lol densities of sttes of zgnr with pyrzole groups t the edges, nd severl eletroni sttes tht re ssoited with the N 2 pyrzole moieties. We n lerly identify the lolized lone pir eletroni stte t 3 ev (elow the Fermi level), whih orresponds to the pyridini N (N P ). The eletroni stte of the sp 2 or quternry N (N Q ) n e found t severl energies, for exmple, 7.5, 2.4 nd 2.0 ev. The stte t 7.5 ev is pentgonl, s in the romti pyrzole. The N Q eletroni sttes ner the Fermi level re well hyridized with the grphene ulk sttes. Consequently, romti N 2 doping (prtiulrly, N Q ) strongly influenes the eletroni strutures of hydrzine-treted grphene oxide mterils. Disussion The SSNMR mesurements reveled the presene of other hemil groups on the lelled RG smple. A pek orresponding to sp 2 - ound 13 C toms in G t roughly 130 p.p.m. hd shifted upfield to 118 p.p.m. s shown in Figure 1. This is onsistent with previous reports on the SSNMR spetr of unlelled, redued grphene oxides 3,22. The pek shift indites greter mount of sp 2 onjugtion in RG thn in G, thus suggesting tht the sp 2 network ws restored y hydrzine tretment. A pek t pproximtely 70 p. p.m. orresponding to hydroxyl ( CH) groups 18,19 ws nerly removed, while pek t 60 p.p.m., previously ssigned to 1,2-epoxide funtionlities 18,19, ws ompletely suppressed fter hydrzine tretment. This suggests tht hydroxyl groups on the sl plne were mostly removed nd 1,2-epoxide groups on the sl plnes were ompletely removed y the hydrzine tretment (within the detetion limit of these NMR mesurements). In other words, the sl plnes of G hve lrgely een redued y the hydrzine tretment. Although the mehnism is not well understood, the redution of G is generlly elieved to restore the sp 2 networks on the sl plnes while reduing the onentrtion of hydroxyl nd 1,2-epoxide groups. The region round 170 p.p.m. in Figure 1, whih ws previously ssigned to roxyl groups ( CH) t the edges of pltelets of RG 18,19, overlps with til of the pek t 150 p.p.m. Consequently, it is diffiult to onlusively determine whether roxyl groups remin in the RG smple sed on the pek ssigned to them. It is well known tht roxyl groups n e redued to hydroxyl groups y retion with strong redutnts suh s lithium luminium hydride 32,33. However, in the se of hydrzine tretment under the retion onditions used here, the redution of roxyl groups to hydroxyl groups ould e ruled out due to the sene of pek orresponding to hydroxyl groups in the 13 C NMR spetrum of lelled RG. In ddition, it my e expeted tht the roxyl groups n e onverted to mide groups ( C NH NH 2 ) y retion with hydrzine. However, the 15 N SSNMR spetr did not indite the presene of NH 2 groups, thus suggesting tht no mide groups were generted. In onlusion, we investigted the hemil strutures of hydrzine-treted grphene oxide/g, whih is one of the most widely

6 nture ommunitions DI: /nomms Totl 4 N P 4 N Q DS Energy (ev) Lolized stte Delolized stte 7.46 ev 3.02 ev 2.40 ev 1.98 ev Figure 5 Theoretil lultion on eletroni strutures of RG mterils with N 2 doping t the edges. () Density of sttes (DS) of zigzg grphene nnorion with pyrzoles t oth the edges, nd (,) set of hrge density plots of N 2 -relted eletroni sttes t severl energies for () lolized sttes nd () delolized sttes. used grphene-sed mterils, using SSNMR spetrosopy nd XPS. We first produed 13 C- nd 15 N-lelled RG mterils y retion of 13 C-lelled G with 15 N-lelled hydrzine monohydrte. The isotope-lelled mterils showed tremendously improved SSNMR signls reltive to unlelled mteril nd we found tht the hydrzine tretment used the formtion of pyrzole groups romti five-memered rings with two djent N toms t the edges of resulting pltelets. Furthermore, XPS experiments on unlelled, hydrzine-treted grphene oxide reveled the formtion of five-memered pyrzoline or pyrzole rings (fter het tretment) t the edges. The evidene of hemil sustitution y hydrzine my offer novel pth to seletive hemil modifitions t the edges of grphene pltelets for grphene-sed systems. For exmple, n idi proton in the pyrzole rings ould e tivted y tretment with strong se for further modifition 31. The methods nd results presented here show tht isotopi lelling with NMR-tive nulei suh s 13 C nd 15 N, prtiulrly when omined with other spetrosopi methods suh s XPS, n provide detiled insight into the hemil struture of CMGs. The SSNMR spetr of 13 C- nd 15 N-lelled RG lso showed lmost omplete removl of epoxy nd hydroxyl groups, nd signifint restortion of the sp 2 network on the sl plnes. n the sis of our nlysis, we suggested tht the term hydrzine-redued/sustituted grphene oxide is more orret nd desriptive thn hydrzineredued grphene oxide whih, to dte, hs generlly een used in the peer-reviewed literture to refer to this mteril. Methods Preprtion of 13 C-lelled grphite. Uniformly 13 C-lelled grphite films were prepred using old-wll CVD system, s desried previously 18. The grphite ws otined y preipittion from the Ni foil during ooling, nd formed on oth sides of the Ni foil. The 13 C-lelled grphite film ws removed from the Ni sustrte using soth tpe. The grphite films on the soth tpe were immersed in etone for 1 dy to dissolve the tpe, then sonited for 30 min. The superntnt ws dented nd this dissolving nd denting proess ws repeted two more times. After filtrtion followed y wshing with dimethylformmide, ethnol nd etone, three times eh, the produt ws dried in the oven (60 C) for 2 dys, nd 13 C-lelled grphite flke ws otined. Preprtion of 13 C- nd 15 N-lelled RG. 13 C-lelled G ws produed y the modified Hummers method 39. Control experiments were used to determine the proper mounts of hydrzine monohydrte to dd for the retion of RG smples. We tested the effet of the mount of hydrzine monohydrte reltive to G in liquid-phse retion without isotope lelling. It ws found tht 1 µl of hydrzine monohydrte is suffiient to redue 3 mg of G (dt not shown). Consequently, we used the sme rtio of hydrzine monohydrte to G with the lelled hemils in this work. The 13 C-lelled grphite (60 mg) ws dded to flsk filled with H 2 S 4 (30 ml), then the mixture ws sonited for 2 h. Solid KMn 4 (300 mg, 99.2%, Fisher Sientifi) ws then slowly dded to the flsk in n ie th. After ddition of the KMn 4, the flsk ws immersed in wter th t 35 C, nd stirred with mgneti r for 24 h. Deionized wter (150 ml) ws dded to the flsk in the ie th, followed y stirring in this ie th for 1 h. The flsk ws then tken off the ie th nd stirred t room temperture for 2 h. Hydrogen peroxide (30 wt% in wter, Sigm-Aldrih) ws then dded to the mixture until no further gs evolution ws oserved, followed y stirring for 2 h. The mixture ws then filtered using n Anodis memrne filter (47 mm in dimeter, 0.2-µm pore size, Whtmn, Middlesex, UK), followed y ir drying for 1 week. Then, two thes of ir-dried, 13 C-lelled G were pled in vil with wter (15 ml), followed y ddition of 15 N-lelled hydrzine monohydrte (98 tom% 15 N, Sigm-Aldrih; 1 µl for 3 mg of G). The mixture ws stirred t 80 C for 1 dy. After ooling to room temperture, the mixture ws filtered using the Anodis memrne filter, followed y ir drying for 1 dy, nd finlly dried under vuum for 1 dy. ne th of the smple (roughly 2 mg) ws used to quire the dt s in Figures 1, 2. The seond th (10 mg), whih hd lower degree of redution, ws used to quire the dt s in Figure 3 nd Supplementry Figure S3. Het tretment of grphene oxide in wter without hydrzine. An queous suspension of unlelled grphene oxide (3 mg of G per 1 ml of purified wter) ws prepred y sonition of G, prepred vi the modified Hummers method nd derived from nturl grphite (SP-1, By Cron) s desried previously. This suspension ws stirred t 80 C for 12 h without hydrzine. The suspension ws stle nd the olour of the suspension ws unhnged y the tretment. A pperlike mteril ws prepred y filtrtion of the resulting suspension s previously reported 35,36, nd this mteril ws used for XPS mesurements. SSNMR mesurements. SSNMR experiments were onduted t 9.4 T using Vrin InfinityPlus 400 NMR spetrometer nd home-uilt 2.5-mm tripleresonne MAS proe t 1 H NMR frequeny of MHz. All 13 C hemil shifts were referened to tetrmethylsilne using dmntne s n externl referene. The 15 N hemil shifts were referened to liquid 15 NH 3 t 0 p.p.m. using the indiret referening of 13 C signl for dmntne 40,41. In ll the experiments, the externl ir temperture ws set to 10 C using Vrin VT ontroller, nd the MAS spinning speed ws 20,000 ± 10 Hz.

7 nture ommunitions DI: /nomms1643 ARTICLE For the mesurement shown in Figure 1, 13 C single-pulse eho sequene ws used with π/2 pulse of 2.5-µs width nd no 1 H deoupling. We onfirmed tht removing 1 H deoupling did not lter the resolution. A seprte 13 C 1 H REDR experiment for the RG smple onfirmed tht the peks t 120 nd 150 p.p.m. in Figure 1 ould likely e ssoited with non-protonted rons. Thus, fst spinning t 20 khz suffiiently removed line rodening due to 1 H 13 C dipolr ouplings. For Figure 2,, totl of 5,400 sns were olleted with reyle delys of 5 s nd 15 N nd 13 C π-pulse widths of 15 nd 10 µs, respetively. A 13 C 15 N REDR pulse sequene (Fig. 2d) ws used with totl of 32 rotor periods (1.6 ms) for dipolr dephsing. 1 H deoupling during the REDR period ws 55 khz; we onfirmed tht there ws little hnge in the spetr without 1 H deoupling. No 1 H deoupling ws pplied during the quisition period. The spinning speed of 20,000 ± 10 Hz ws used to remove reltively strong 13 C 13 C dipolr ouplings. The π-pulse widths were 10 nd 15 µs for 13 C nd 15 N, respetively. In these REDR experiments, the 13 C nulei in the viinity of 15 N nulei lose their signls in the 13 C MAS spetrum with 13 C 15 N dipolr dephsing. In ontrol REDR experiment for uniformly 13 C- nd 15 N-lelled l-lnine 24, signl for 13 C α tht is diretly onded to 15 NH 3 + deresed y 78%, ompred with the ontrol spetrum without 13 C 15 N dipolr dephsing. In the sme study, signls for C 2 nd CH 3, whih re two onds wy from 15 N, deresed y only 6 nd 8%, respetively. The integrted intensity of the differene spetrum in Figure 2 is out 7.5 ± 0.5% with respet to tht of Figure 2. Although the signl intensities in the experiment re modulted y 13 C 13 C J-oupling nd 13 C trnsverse spin relxtion, the result indites tht onsiderle frtion of 13 C toms oserved here re likely to e onded to 15 N speies. Gussin line rodening of 200 Hz ws pplied. For Figure 3,, pulse sequene in Figure 3 ws used. First, 15 N spin polriztion ws prepred y rmped CP 42. Beuse of the limited popultion of the trget 15 N speies nd rod line width, the signl-to-noise rtio in the diret-deteted 15 N CPMAS spetr ws low even with 15 N enrihment. Thus, we used 1 H deteted 15 N 1 H doule CP for the djustment of the Hrtmnn Hhn ondition 43,44. The 1 H polriztion ws exited y 1 H π/2 pulse of 3.5-µs width. During ontt period of 0.35 ms for polriztion trnsfer y CP, the 15 N rdiofrequeny (RF) field mplitude ws linerly swept from 21 to 36 khz, while the 1 H RF mplitude ws kept onstnt t 48 khz. 15 N nd 13 C π-pulse widths in the REDR sequene were 15 µs. The signls of 21,504 sns were umulted with reyle delys of 3 s. Gussin line rodening of 400 Hz ws pplied. The totl experimentl time ws ~19 h for eh smple. XPS mesurements. An queous suspension of grphene oxide ws prepred y exfolition of G. The queous suspension (3.1 mg ml 1 ) ws diluted with ethnol for file spin sting onto glss sustrtes (Corning 1737). Grphene oxide thin films were prepred y spin sting the solution on glss sustrte lened y ultrviolet oxygen rdition. The films were susequently redued y 1-h exposure to hydrzine vpour generted t 100 C in seled ontiner. ne suset of the hydrzine vpour-treted smples ws set side for testing fter the hydrzine vpour tretment. The remining vpour-treted smples were redued with therml nneling t 750 C in CVD hmer under H 2 flow (100 sm) for 5 min. Pper-like mterils of redued grphene oxide were prepred y simple filtrtion s previously reported 35,36. Therml nneling of the pper smple ws done under Ar flow for 12 h t 500 C. After the redution proess, surfe hemil nlysis of eh redued grphene oxide smple ws performed y using XPS (Sigm Proe, Thermo VG Sientifi). DFT lultions on the pyrzole formtion. For DFT lultions, we used the projetor-ugmented wve potentils 45 with plne-wve sis set nd the Perdew Burke Ernzernhof 46 exhnge-orreltion funtionl s implnted in the Vienn A-Initio Simultion Pkge 38. A kineti energy utoff of 400 ev nd Gussin smering of 0.05 ev ws used. A (6 6) zigzg grphene nnorion superell with vuum seprtion of 10 Å nd (8 1 1) k-points smpling were used in lultions. All tomi fores were minimized t < ev Å 1, nd the in-plne stresses were fully relxed. Referenes 1. Doelle, W. H. & Beer, M. Chemilly leved grphite support films for eletron mirosopy. J. Cell Biol. 39, (1968). 2. Boehm, V. H. P., Cluss, A., Fisher, G.. & Hofmnn, U. Dunnste Kohlenstoff-folien. Z. Nturforshg. 17, (1962). 3. Stnkovih, S. et l. Synthesis of grphene-sed nnosheets vi hemil redution of exfolited grphite oxide. Cron 45, (2007). 4. Si, Y. & Smulski, E. T. Synthesis of wter solule grphene. Nno Lett. 8, (2008). 5. Wng, G. et l. File synthesis nd hrteriztion of grphene nnosheets. J. Phys. Chem. 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8 nture ommunitions DI: /nomms Ishii, Y. & Tyko, R. Sensitivity enhnement in solid stte 15 N NMR y indiret detetion with high-speed mgi ngle spinning. J. Mgn. Reson. 142, (2000). 44. Ishii, Y., Yesinowski, J. P. & Tyko, R. Sensitivity enhnement in solid-stte C-13 NMR of syntheti polymers nd iopolymers y H-1 NMR detetion with high-speed mgi ngle spinning. J. Am. Chem. So. 123, (2001). 45. Blohl, P. E. Projetor ugmented-wve method. Phys. Rev. B 50, (1994). 46. Perdew, J. P., Burke, K. & Ernzerhof, M. Generlized grdient pproximtion mde simple. Phys. Rev. Lett. 77, (1996). Aknowledgements S.P. nd J.. were supported y Glol Frontier Reserh Center for Advned Soft Eletronis. This study ws supported in prt y the U.S. Deprtment of Energy, ffie of Bsi Energy Sienes, Division of Mterils Sienes nd Engineering under Awrd DE- SC (R.S.R nd Y.I.). The SSNMR methodology development in this work ws lso supported y the NSF (CHE , CHE ) nd the Dreyfus Foundtion Teher- Sholr Awrd progrm for Y.I. L.B.C. held n Amerin Fellowship from the Amerin Assoition of University Women during the ourse of this work. J..H nd S..K. were supported y the NRL (R0A ) nd WCU (R ) progrms funded y the Koren government. E.-S.L nd Y.-H.K were supported y the WCU (R ) nd Bsi Siene Reserh ( ) progrms through the NRF of Kore. Author ontriutions S.P. orgnized experiments, synthesized mterils nd wrote the mnusript. Y.H. nd L.B.C. mesured NMR spetr nd J..H. mesured XPS spetr. E.-S.L. ontriuted theoretil lultions. W.C., S.C., H.-W.H. nd J.. ssisted with the synthesis of mterils. J.R.P. onduted dt nlysis. S..K. supervised XPS mesurements nd gve intelletul input. Y.-H.K. supervised XPS mesurements nd theoretil lultions nd wrote the mnusript. Y.I. performed nd supervised NMR mesurements, nlysed the NMR dt for the struturl nlysis nd wrote the mnusript. R.S.R. supervised synthesis of mterils nd wrote the mnusript. Additionl informtion Supplementry Informtion ompnies this pper t ntureommunitions Competing finnil interests: The uthors delre no ompeting finnil interests. Reprints nd permission informtion is ville online t reprintsndpermissions/ How to ite this rtile: Prk, S. et l. Chemil strutures of hydrzine-treted grphene oxide nd genertion of romti nitrogen doping. Nt. Commun. 3:638 doi: /nomms1643 (2012).