Purification of multiwall carbon nanotubes obtained by AC arc discharge method

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1 OPTOELECTRONICS AN AVANCE MATERIALS RAPI COMMUNICATIONS Vol. 5, No. 8, August 2011, p Purifiction of multiwll cron nnotues otined y AC rc dischrge method M. STANCU,*,. RUXANA,. CIUPARU, A. INESCU Petroleum-s University of Ploiesti, 39 Bucuresti Blv., , Ploiesti, Romni Ntionl Institute for Reserch nd evelopment in Microtechnologies, Buchrest, Romni In this pper we report severl different methods tht cn e used for the purifiction of multiwll cron nnotues (MWCNT) otined y the AC rc dischrge method. The purifiction of multiwll cron nnotues is required in order to eliminte the metl ctlyst nd morphous cron. The removl of the metl ctlyst ws crried out y tretment with different cids. Susequently, we hve pplied two different methods for removing lso the morphous cron. The first pproch for the purifying of the MWCNT ws the wek oxidtion performed y refluxing the MWCNTs in strong cid for 6 h nd the second method ws the oxidtion with ir for 10 minutes. The smples were nlyzed using FE-SEM (Field Emission Scnning Electron Microscope), powder X-Ry diffrction (XR) nd Rmn spectroscopy nd the results were discussed. (Received June 7, 2011; ccepted August 10, 2011) Keywords: Multiwll cron nnotues, Purifiction, Refluxing tretment 1. Introduction It is well known from long time tht the elementl cron exists only in three forms - morphous, grphite nd dimond. ifferent types of cron in the sp 2 hyridiztion cn form vriety of mzing structures. One of these mzing structures ws discovered y Kroto et l. nd ws clled fullerene [1]. Fullerenes re similr in structure with grphite nd re composed y sheet of linked hexgonl rings, ut they contin pentgonl (or sometimes heptgonl) rings tht prevent the sheet from eing plnr. In very short period of time fter the discovery of fullerenes, new forms of cron were identified. In 1991 Iijim [2] oserved for the first time tuulr cron structures which consisted of up to severl tens of grphitic shells (so clled multi-wlled cron nnotues (MWCNT)). Two yers lter, Iijim nd Ichihshi [3] nd Bethune et l. synthesized single-wlled cron nnotues (SWNT). No mtter of the method used for the synthesis of multiwll cron nnotues (MWCNT), the product otined presents in its composition, cron nnostructures, grphitic sheets, morphous cron, metl ctlyst nd smll mount of fullerenes. Before the use of cron nnotues in different pplictions [4-9] it is necessry to e purified. Purifiction of cron nnotues must e performed for the impurities nd unrected regents removl. The most common purifiction methods re the oxidtion y different techniques, tretment with different cids, clcintions etc. In this pper we present the reserches crried out to get new purifiction methods of cron nnotues which consist in wek oxidtion y refluxing the smples in cid nd the oxidtion in ir. 2. Experimentl 2.1 Multiwll cron nnotues synthesis Multiwll cron nnotues were synthesized y AC rc dischrge method in rgon mient with the pressure of 250 mr using pure grphite electrodes with different dimeter 12 mm nd 3 mm, nd iron ctlyst [10]. 2.2 Purifiction procedure Multiwll cron nnotues were purified pplying two different methods. One method ws used in order to eliminte the metl ctlyst from multiwll cron nnotues nd the other one for the removing of morphous cron. The removl of the metl ctlyst ws crried out y refluxing the cron nnotues in nitric cid (63%) for 6 h t 118 o C nd in hydrochloric cid (37%) for 6 h t 105 o C. The purifiction procedure is presented in Fig. 1, (). The method for removing morphous cron from multiwll cron nnotues ws ir oxidtion for 10 minutes t 550 o C. The heting regime is presented in Fig. 1, (). The smple with MWCNT purified y tretment with nitric cid ws noted s MWNT_HNO 3 nd the smple with MWCNT purified y tretment with hydrochloric cid ws noted s.

2 Purifiction of multiwll cron nnotues otined y AC rc dischrge method 847 Refluxing the rw mteril in nitric cid (63%) for 6 h t 118 o C Rw mteril Wshing with distilled wter followed y filtrtion rying for 10 h t 100 o C Refluxing the rw mteril in hydrochloric cid (37%) for 6 h t 105 o C 3. Results nd discussion Fig. 2 shows the X-ry diffrctogrms of the MWNT_HNO 3, nd rw mteril smples for 2θ from 1 to 60. Intrinsic pek of rw mteril is oserved t c. 26 nd ll spectr hve tht pek t the sme position. This pek corresponds for the typicl structure for MWCNT [11]. Air oxidtion for 10 minutes t 550 o C Smple nme Smple nme Chrcteriztion y X-Ry diffrction, y Rmn spectroscopy, y FE-SEM nd y TEM Signl XR (r. units) (1) (2) (3) rw mteril t( C) 20 C 50 min 550 C 550 C ir 10 min ir cooling t ( min) Fig. 1. () Schemtic of the purifiction procedure; () Heting regime Structurl nlysis The mterils otined efore nd fter purifiction were chrcterized y powder X-Ry diffrction (XR), y FE-SEM (Field Emission Scnning Electron Microscope), y Rmn spectroscopy nd y TEM (Trnsmission Electron Microscopy). iffrctogrms were recorded on 8 AVANCE Nov diffrctometer using the chrcteristic rdition Kα of copper, t voltge of 40 kv nd 5 ma intensity. The qulity, ut lso the mjor modifictions of the multiwll cron nnotues cused y this purifiction procedure were investigted y Field Emission Scnning Electron Microscopy (FE-SEM). The equipment used ws Nov NnoSEM 630. Rmn spectr of the multiwll cron nnotues produced were recorded using one excittion wvelength (532 nm) on NRS-3000 Rmn equipment from Jsco. The TEM investigtions were performed using TECNAI/FEI F20 instrument equipped with 200 kv electron gun θ( ) Fig. 2. X-ry diffrctogrms of the MWNT_HNO 3, nd rw mteril smples The concentrtions of nitric cid nd hydrochloric cid used for purifiction of cron nnotues don t ffect the position of this pek. Crystllite size ws clculted y Scherrer's eqution using the dt of integrl redth of the peks (Tle 1). In conclusion, the reflux with nitric cid nd hydrochloric cid mke no chnge of crystllinity in cron nnotues, ut mke chnge of the crystllite size of the nnopowders. The diffrctogrm of the rw mteril smple shows pek diffrction of Fe ctlysts t c o. After purifiction tretment, the diffrction intensity of the Fe (110) pek is reduced. We concluded tht reduction of Fe content due to the chemicl tretment. Smple nme Tle 1. Indexing of diffrction lines.. lines 2 θ( ) d ( Å) FWHM ( ) Fe (110) (nm) MWNT_HNO Rw mteril Fig. 3 presents SEM imge of the unpurified smple, hereinfter clled rw mteril nd the Figs. 3 nd c present SEM imges of the purified smples. In SEM imge of the rw mteril smple re oserved lrge undles of cron nnotues nd cronceous impurities. The rw mteril smple hs three components: microcrystlline grphitic component, nnotuulr component nd grnulr component with the size estimted t 20 nm. In Fig. 3c the grnulr component

848 M. Stncu,. Ruxnd,. Ciupru, A. inescu hs spheroidl shpe ut lso hs n ellipsoid shpe [12] nd cn e found locted in prticulr ggregtes. s is pprent in Fig.

The use of Rmn spectroscopy llows the oservtion of mny spects of cron mterils, especilly cron nnotues. The Fig. 4 shows the positions for the most intense components corresponding to the,, nd ' nds.

4, we cn oserved, the nd round 1335 cm -1 ( pek is ssigned to disordered or defective structures of the mteril) nd the second order of the nd nds.

3 848 M. Stncu,. Ruxnd,. Ciupru, A. inescu hs spheroidl shpe ut lso hs n ellipsoid shpe [12] nd cn e found locted in prticulr ggregtes. s is pprent in Fig. 3 nd c, is improved, though some grnulr components remined in the smple. Fig. 4 shows the Rmn spectr recorded using the 532 nm lser wvelength for MWNT_HNO 3, nd rw mteril smples. The use of Rmn spectroscopy llows the oservtion of mny spects of cron mterils, especilly cron nnotues. The Fig. 4 shows the positions for the most intense components corresponding to the,, nd ' nds. The first nd second order Rmn spectr of the smples produced re compred. In Fig. 4, we cn oserve the presence of the so-clled -nd t round 1580 cm -1 for ll the smples. Also, in Fig. 4, we cn oserved, the nd round 1335 cm -1 ( pek is ssigned to disordered or defective structures of the mteril) nd the second order of the nd nds. The most prominent secondorder Rmn pek historiclly nmed ' [13], is locted t ~2700 cm -1. The presence of the second order of Rmn spectr is n dditionl proof of the high order degree of the smples [13]. The nd shift nd the nd shift of the rw mteril smple re strongly shifted fter the tretment with cids. Therefore, in the smples treted with cids, the nd shifts from 1357 to 1350 cm -1 nd the nd shifts from 1585 to 1580 cm -1. These shifts re relted to direct electron chrge trnsfer process from the cron nnotues to the cceptor molecule [14, 15]. The frequencies of the vrious -nd peks exhiit cler nd well-estlished dependence on dimeter, s well s dependence on whether the nnotue is metllic or semiconducting [16]. The intensity rtio of the -nd to the -nd cn e employed to ssess the qulity of the MWCNT smples. For the investigted smples the following vlues of / pek intensity rtios were found: I / I = for rw mteril smple, I / I = for MWNT_HNO 3 smple nd I I for smple. / = Signl Rmn (r. units) nd nd ` nd rw mteril c Fig. 3. SEM imges of rw mteril smple (), MWCNT smple treted with HNO 3 () nd MWCNT smple treted with HCl (c). Fig. 3 shows tht the dimeters of the multiwll cron nnotues re from rnge 6 to 20 nm. As result of purifiction, the purity of the multiwll cron nnotues, Fig. 4. Rmn spectr for MWNT_HNO 3, nd rw mteril smples. (cm 1 ) From tht it vlues we conclude tht the rw mteril smple shows hs grphitic component concentrtion more pronounced thn morphous component. The result

Purifiction of multiwll cron nnotues otined y AC rc dischrge method 849 is tht fter ech chemicl ttck, the grphitic component in the ech smple ws reduced compred with the grphitic component from the

The grphitic component from ech smples re found s tiny crystls nd s the form of grphitic nnostructured mteril (multiwll cron nnotues or smll nnostructured forms considered s evolutionry phses of

This ws due to presence of the guest molecule cn either interclte the wlls [17] or rect with the defects in the surfce creting ttched functionl groups [18].

4 Purifiction of multiwll cron nnotues otined y AC rc dischrge method 849 is tht fter ech chemicl ttck, the grphitic component in the ech smple ws reduced compred with the grphitic component from the rw mteril smple. The grphitic component from ech smples re found s tiny crystls nd s the form of grphitic nnostructured mteril (multiwll cron nnotues or smll nnostructured forms considered s evolutionry phses of multiwll cron nnotues). The tretment with cids shows decrese of the I / I rtio. This ws due to presence of the guest molecule cn either interclte the wlls [17] or rect with the defects in the surfce creting ttched functionl groups [18]. We cn conclude tht the cid tretment strongly ffects the numer of defects in the MWCNT surfce. In Fig. 4 shows diverse ` nd peks for the smples purified with different cids nd cn e explined y the different moleculr composition of the guest molecules. The reltive purity of the treted MWCNT ws lso influenced y the introduction of the guest molecules. Fig. 5 presents TEM imges of the rw mteril MWNT_HNO 3, nd smples. This figure shows the presence of MWCNT. In Fig. 5 cn e oserved tht the morphous ggregtes pper distriuted on the xis of MWCNT more thn on its wlls. The multiwll cron nnotue from Fig. 5, hs outer dimeter 7.1 nm. The multiwll cron nnotue from Fig. 5 hs outer dimeter ~13 nm nd the multiwll cron nnotue from Fig. 5 c hs outer dimeter ~7 nm. In Fig. 5 nd c cn e oserved tht the MWCNT which ws treted with cids were purified.therefore, the metl ctlyst nd the morphous cron were removed. According to the TEM oservtions, the est qulity of MWCNT (neither contining mny defects nor ny morphous cron deposit) ws otined fter tretment with cids. c Fig. 5. TEM imges of rw mteril smple (), MWCNT smple treted with HNO 3 () nd MWCNT smple treted with HCl (c). 4. Conclusions The purifiction procedure is dependent on the method of synthesis of cron nnotues. In this pper we report simple procedure to purify multiwll cron nnotues otined y the AC rc dischrge method. Metl ctlyst nd lso morphous cron were oxidized y refluxing in cids including hydrochloric cid nd nitric cid. Oxidtion in ir t 550 o C removes disordered cron, ut does not eliminte the -nd. The purifiction procedure consisted in refluxing in different cids followed y ir oxidtion to otin highly pure MWCNT smples. Acknowledgements Authors cknowledge the finncil support from the Europen Socil Fund through POSRU/89/1.5/S/54785 project: Postdoctorl Progrm for Advnced Reserch in the field of nnomterils.

5 850 M. Stncu,. Ruxnd,. Ciupru, A. inescu References [1] H. W. Kroto, J. R. Heth, S. C. O Brien, R. F. Curl, R. E. Smlley, Nture 318, 162 (1985). [2] S. Iijim, Nture 354, 56 (1991). [3] S. Iijim, T. Ichihshi, Nture 363, 603 (1993). [4] I.. Simndn, M. Popescu, A. Lorinczi, A. Vele, E. Fgdr-Cosm, igest Journl of Biomterils nd Biostructures 5(4), 1029 (2010). [5] I.. Simndn, F. Sv, A. Vele, A. Lorinczi, M. Popescu, Optoelectron. Adv. Mter. Rpid Commun. 4(8), 1178 (2010). [6]. Nechifor, S. I. Voicu, A. C. Nechifor, S. re, eslintion 241, 342 (2009). [7] S. I. Voicu, A. C. Nechifor, O. les,. Nechifor, Bioelectronics, Biomedicl, nd Bioinspired Systems V; nd Nnotechnology V, Proceedings of SPIE Vol (SPIE, Bellinghm, WA 2011) 80680Y (2011), doi: / [8] S. I. Voicu, F. Alde, A. C. Nechifor, Revist de Chimie 61(9), 817 (2010). [9] F.. Blcinu, A. C. Nechifor, R. Brtos, S. I. Voicu,. Nechifor, Optoelectron. Adv. Mter. Rpid Commun. 3(3), 219 (2009). [10]. Ruxnd, M. Stncu, S. Vizirenu,. inescu,. Ciupru, J. Optoelectron. Adv. Mter. 10(8), 2047 (2008). [11] L. Wei-Xue, C. Young-Fu, H. Yun, C. Ti-Jun,. Jin-Feng, W. Qing, W. Zhong-Li, L. Yng, Trns Nonferrous Met. Soc. Chin 17, s696 (2007). [12] Colert, R. E. Smlley, J. Phys. Chem. 99, 27 (1995). [13] M. S. resselhus,. resselhus, R. Sito, A. Jorio, Physics Reports 409, 47 (2005). [14] A.. Souz Filho, V. Meunier, M. Terrones, B.. Sumpter, E. B. Brros, F. Villlpndo-Pez, J. M. Filho, Y. A. Kim, H. Murmtsu, T. Hyshi, M. Endo, M. S. resselhus, Nno Lett. 7, 2383 (2007). [15] A. M. Ro, S. Bndow, E. Ritchter, P. C. Eklund, Thin Solid Films 331, 141 (1998). [16] M. S. resselhus,. resselhus, A. Jorio, A.. Souz Filho, R. Sito, Cron 40, 2043 (2002), nd references therein. [17] J. Fisher, Acc. Chem. Res. 35, 1079 (2002). [18] S.. Kim, J. W. Kim, J. S. Im, Y. H. Kim, Y. S. Lee, J. Fluorine Chem. 128, 60 (2007). * Corresponding uthor: elel11@yhoo.com

Electronic Supplementary Information

Electronic Supplementary Information Electronic Supplementry Informtion Assemling cron-coted hollow α-fe 2 nnohorns on the CNT ckone for superior lithium storge cpility Corresponding uthor: Xiong Wen (Dvid) Lou* School of Chemicl nd Biomedicl