Brown, NMD

McLaughlin, JAD

McNally, T

Murphy, H

Okpalugo, TIT

Papakonstantinou, P

English

Surface-to-depth analysis of functionalized multi-wall carbon nanotubes, FMWCNT was achieved by the use of high resolution x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometer (SIMS). The pristine nanotubes were functionalized through various steps. These included i) treatment in nitric acid followed by dispersion in dimethylformimide (DMF) and ii) further modification by the linking molecule, 1-ethyl-3-(3-dimethylaminopropyl carbodiimide), (EDC) through covalent attachment. Pristine nanotubes dispersed in DMF, possessed an atomic percentage of oxygen up to 3%, which more than doubled after treatment in nitric acid. After the second functionalization step, the atomic percent of oxygen was lowered relative to nitrogen as some oxygen was possibly displaced by the anchored molecules of EDC. In addition positive ions of CHNH, N2, CO, CH3NH, C2H3O, and negative ions of O, OH, and CN were detected by SIMS. On SIMS depth profiling, the intensity levels of both OH- and CN- ions were found to be steady throughout the 180 sec etch duration, indicating good in depth chemical functionalization of the carbon nanotube structure. Preliminary studies on functionalization of carbon nanotubes by atmospheric pressure dielectric barrier discharges (APDBD) are also reported. Copyright Taylor and Francis 200

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  PLEASE SCROLL DOWN FOR ARTICLEThis article was downloaded by: [University of Ulster]On: 23 November 2009Access details: Access Details: [subscription number 909444082]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UKFullerenes, Nanotubes and Carbon NanostructuresPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597253Surface-to-Depth Analysis of Functionalized Multi-Wall CarbonNanotubes (FMWCNTS)T. I. T. Okpalugo a; P. Papakonstantinou a; H. Murphy a; J. McLaughlin a; N. M. D. Brown a; T. McNallyba NIBEC, University of Ulster, Northern Ireland, UK b School of Mechanical Engineering, Queen'sUniversity Belfast, Belfast, UKTo cite this Article Okpalugo, T. I. T., Papakonstantinou, P., Murphy, H., McLaughlin, J., Brown, N. M. D. and McNally,T.'Surface-to-Depth Analysis of Functionalized Multi-Wall Carbon Nanotubes (FMWCNTS)', Fullerenes, Nanotubes andCarbon Nanostructures, 13: 1, 477 — 484To link to this Article: DOI: 10.1081/FST-200039448URL: http://dx.doi.org/10.1081/FST-200039448Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdfThis article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.Surface-to-Depth Analysis of FunctionalizedMulti-Wall Carbon Nanotubes(FMWCNTS)T. I. T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin,and N. M. D. BrownNIBEC, University of Ulster, Northern Ireland, UKT. McNallySchool of Mechanical Engineering, Queen’s University Belfast,Belfast, UKAbstract: Surface-to-depth analysis of functionalized multi-wall carbon nanotubes,FMWCNT was achieved by the use of high resolution x-ray photoelectron spectroscopy(XPS) and secondary ion mass spectrometer (SIMS). The pristine nanotubes were func-tionalized through various steps. These included i) treatment in nitric acid followed bydispersion in dimethylformimide (DMF) and ii) further modification by the linkingmolecule, 1-ethyl-3-(3-dimethylaminopropyl carbodiimide), (EDC) through covalentattachment. Pristine nanotubes dispersed in DMF, possessed an atomic percentage ofoxygen up to 3%, which more than doubled after treatment in nitric acid. After thesecond functionalization step, the atomic percent of oxygen was lowered relative tonitrogen as some oxygen was possibly displaced by the anchored molecules of EDC.In addition positive ions of CHNH, N2, CO, CH3NH, C2H3O, and negative ions of O,OH, and CN were detected by SIMS. On SIMS depth profiling, the intensity levels ofboth OH2 and CN2 ions were found to be steady throughout the 180 sec etchduration, indicating good in depth chemical functionalization of the carbon nanotubestructure. Preliminary studies on functionalization of carbon nanotubes by atmosphericpressure dielectric barrier discharges (APDBD) are also reported.Keywords: Functionalized carbon nanotubes, SIMS, XPS, biomedical sensors, dielec-tric barrier dischargeAddress correspondence to T. I. T. Okpalugo, NIBEC, University of Ulster,Shore Road, BT37 0QB, Northern Ireland, UK. E-mail: tit_okpalugo@yahoo.comand thomas@nibec-sl.nibec.ulst.ac.ukFullerenes, Nanotubes, and Carbon Nanostructures, 13: 477–484, 2005Copyright# Taylor & Francis, Inc.ISSN 1536-383X print/1536-4046 onlineDOI: 10.1081/FST-200039448Downloaded By: [University of Ulster] At: 14:29 23 November 2009INTRODUCTIONCarbon nanotubes (CNT) (1) have been a subject of great interest and researchfor many applications such as composite materials, battery electrodematerials, microfluidics, biosensors, field emission, interconnects, and so on.CNTs are of nano-dimensions and are important for nanofabrication and min-iaturization. This material has an unusual ring-type structure and can formself-assembling nano-structures. It also exhibits unusual properties such ashigh Young’s modulus, mechanical hardness, and flexibility; an electronicconductivity that can vary from metallic to insulating with exceptionalcurrent-carrying capacity, ballistic transport, extremely high thermal conduc-tivity, and high optical polarizability. Its nano-dimensions with graphiticsurface chemistry makes it ideal candidate for biochemical sensor application(2). In developing CNTs for biomolecular detection, they are usually function-alised in stages by various methods such as chemical covalent, noncovalent,defect, endohedral, and exohedral functionalization in order to increasetheir solubility, processibility, electron transport, and immobilization ofbiomolecules.In recent years, nonequilibrium plasmas under atmospheric pressure havebeen developed as an effective means for surface modification of materials.Atmospheric pressure plasma/discharge (APP/D) processing of solid surfacesoffers many advantages compared to vacuum plasma and wet chemicalmethods, including the absence of solvents, and expensive vacuum components,speed, room temperature treatment, and the capability to coat large sizesubstrate. In spite of the industrial use of APP/D for treating polymers thereare no reports on the APP treatment of carbon nanotubes. APP/D is particularlysuitable for functionalization of aligned nanotubes, where wet chemicalmanipulation could damage and even destroy the highly desirable verticalalignment.A preliminary study on the oxidation of CNTs using atmospheric pressuredielectric barrier discharge (APDBD) will be presented in this paper. Asurface-to-depth analysis is performed based on XPS and SIMS techniquesto probe various stages in the chemical and APP/D functionalizationprocess of CNTs.EXPERIMENTAL DETAILSMWNTs with 10–30 nm diameter were synthesized by CVD process usingacetylene as carbon source and Fe-Ni as catalyst. The growth temperaturewas approximately 7508C. The as grown MWNTs were rinsed with HCland distilled water. The HCl treated product was composed of 85%, 10%amorphous carbon, and 5% NiO, FeO, and Al2O3. These nanotubes aredenoted as pristine nanotubes. MWNTs were further purified by refluxingT. I. T. Okpalugo et al.478Downloaded By: [University of Ulster] At: 14:29 23 November 2009the pristine nanotubes at concentrated HNO3 for 4 hr. The solid product wascollected on a PTFE membrane (Millipore porous filter, 0.2mm) afterdiluting with deionized water until the filtrate pH became nearly neutral.These purified nanotubes are deemed as functionalized.MWNT suspension was prepared by dispersing 10 milligrams in 10 mL oforganic solvent N,N-dimethylformamide (DMF) with the aid of ultrasonicagitation. Moreover, 23 mg of EDC were added to 50 mg of functionalizedmulti-walled nanotubes and then dispersed in 10 ml of dimethylformamide(DMF). The solution was sonicated for 1–2 hr. Films were deposited on Si/Au substrate by the drop-dry technique.The ESCA300 XPS machine equipped with the high resolution Scienta-ESCA300 spectrometer and Kratos XSAM 800 equipped with an energyanalyser; Mg Ka x-ray source at 15 KeV and 20 mA was used for this study.The SIMS analysis was done with Mini SIMS, MC300 MkII, MillbrookInstruments having 1 amu resolution. An ISA Labram system was used forcon focal micro Raman analysis with a wavelength of 514.53 nm. DoubleGaussian product function was used for the curve deconvolution. TheBruker D8 x-ray diffractometer was used for powder XRD analysis.RESULTS AND DISCUSSIONChemical FunctionalizationNitric acid is the standard reagent for purification of nanotubes and has tra-ditionally constituted the first step in many different purification schemes. Ithas been demonstrated that nitric acid removes the transition metal catalystthat is used in the production of carbon nanotubes together with amorphouscarbon. However, it produces a significant destruction in the structure ofcarbon nanotubes and introduces –COOH and –OH groups at the defectsites, present at the nanotube ends and sidewalls.The degree and depth of functionalization of multiwall carbon nanotubesare shown in SIMS spectra (Fig. 1), XPS spectra (Fig. 2), and Table 1.Following the acid purification/functionalization procedure, an EDCbifunctional linking molecule was used to activate certain sites for further bio-molecular attachment. SIMS analysis detected the presence of positive ions ofCHNH, N2, CO, CH3NH, C2H3O, and negative ions of O, OH, CN, and N. TheSIMS relative intensity ratio of CN2:O2(OH2) ¼ 1 : 2, is comparable withthe XPS intensity ratio of N1s : O1s ¼ 3.45% : 6%. The O2, OH2, CN2,and N2 ions from the surface to depth analysis were found to be steadythrough the 180 sec etch time duration. These detection levels of ions overrelatively large depths of profiling are indicative of true surface-to-depth func-tionalization. The O2 and OH2 are most likely to be present due to the acidFunctionalized Multi-Wall Carbon Nanotubes 479Downloaded By: [University of Ulster] At: 14:29 23 November 2009treatment, while the N2 and CN2 are due to the second stage of the function-alization procedure used in the study.XPS results presented in Table 1 indicate more than 100% increase (3.3–8.16%) in the oxidative group (O atomic % concentration) when pristineCNTs are compared to the acid treated ones. A very small amount of N(0.81%) was traced on the acid-treated CNTs dispersed in DMF solution.Figure 1. FMWCNT-EDC_DMF: (a) Intensity (a.u) of SIMS depth profile of O2,OH2, N2 and CN2 ions over 200 s scan duration, (b) Spectral intensity (a.u) versusmass-to-charge ratio (m/z) of the negative (-ve) ions.T. I. T. Okpalugo et al.480Downloaded By: [University of Ulster] At: 14:29 23 November 2009Figure 2. XPS analysis of (a) NFMWCNT in DMF before and (b) after atm. DBDtreatment.Functionalized Multi-Wall Carbon Nanotubes 481Downloaded By: [University of Ulster] At: 14:29 23 November 2009However, the N content rose to 3.45% upon subsequent treatment with theEDC bi-functional molecular linkage. These N atoms are likely from theEDC bi functional molecule.The results from the described SIMS depth profiling suggest that the typeof bonding between the N atoms of the EDC bifunctional molecules and the Catoms of CNT is a covalent bond, but the exact configuration/hybridization isnot definite. Nitrogen and carbon atoms are both multivalent, each with apossibility of forming sp1, sp2, and or sp3 hybridization, making a total ofnine different configurations. The N atom of EDC has a probability ofp-bonding configuration such as in a pyridine and pyridine-like bonding con-figuration with the C atoms of the CNT rings. The bonding configurationsdepend on the site (side-wall, defect sites or end termini) of the linkagebond formation of the N atom of EDC with the CNT atoms. The unterminatedatoms of the CNT end termini have a possibility of bonding with N in itsfourfold coordinate substitution sites as in N4þ-C32, N4þ, and N30 bonding con-figuration and or its twofold coordinate sites (double bond unit having eithertwo electrons in s-bond, one p-bond electron and a non bonding pair electron,or a double bond unit with three s-bond electrons, one p-bond electron and ananti-bonding p state). Thus it may be valuable to monitor the specific sitesand the binding configurations for specific molecular immobilisation.Atmospheric Pressure Dielectric Barrier Discharge ProcessingOne of the alternatives to chemical functionalization is the use of atmosphericpressure dielectric barrier discharges (APDBD). XPS analysis of DBD(setting ¼ 120 cycles, 0.42 W, 20% speed) functionalized CNTs, showedincorporation of oxygen up to 20% atomic concentration (Table 1)compared to the pristine one (Sample II). The XPS deconvoluted peak ofpristine MWCNT and DBD functionalized MWCNT are presented inTable 1. The XPS atomic % composition of MWCNTs (F ¼ functionalized, NF ¼nonfunctionalized)C1s(at. %)O1s(at. %)N1s(at. %)Sample I-chemical (ESCA300 used)NF, MWCNT, DMF 96.7 3.3 —F, MWCNT, DMF 90.3 8.16 0.81F, MWCNT, EDC, DMF 90.55 6 3.45Sample II-DBD (XSAM 800 used)NF, MWCNT 91.8 8.2DBD-F, MWCNT 71.8 28.2T. I. T. Okpalugo et al.482Downloaded By: [University of Ulster] At: 14:29 23 November 2009Fig. 2. In addition to functionalization it could be possible to use APDBD toremove surface impurities. The above XPS peak deconvolution of DBDtreated sample suggests that the sp3C-C seems to increase, however thisrequires further investigation. The observed chemical shift following theDBD treatment in air atmosphere is about 1 eV. This could be due to the elec-tronegative oxygen electron pull on the C atom of the CNT. The results fromthe Raman analysis indicated no significant change in the radial breathingmode peaks following DBD treatment. This suggests that (at the DBDsettings used) there was no damage to the nanostructure of the CNT.A complementary XRD pattern of the pristine MWCNT is shown in Fig 3.This profile is typical of a well-crystallized carbon material with a possiblecontribution from the catalytic metallic element following CNT synthesis.The detected pattern assigned to the CNT crystal planes of [0 0 2], [1 0 0],and (101) could also be assigned to graphene crystal (4).CONCLUSIONSSurface-to-depth analysis of both chemical functionalized and DBD function-alized multi-wall CNT is shown to be useful for characterization of nanotubesat various stages in the functionalization procedure to ascertain the nature anddegree of modification and the exact bonding configuration. The atomicpercent of oxygen was lowered relative to nitrogen as some oxygen waspossibly displaced by the anchored molecules of EDC. Positive ions ofCHNH, N2, CO, CH3NH, C2H3O, and negative ions of O, OH, CN, and Nwere detected. On SIMS depth profiling, the OH2 as well as CN2 wereFigure 3. XRD pattern of pristine nonfunctionalized-MWCNT.Functionalized Multi-Wall Carbon Nanotubes 483Downloaded By: [University of Ulster] At: 14:29 23 November 2009found to be steady all through the 180 sec scan time duration. The results fromthe Raman analysis indicated no significant change in the radial breathingmode peaks following DBD treatment. This suggests that (at the DBDsettings used) there was no damage to the nanostucture of the CNT. Finally,the XRD diffraction profile is typical of a well-crystallized carbon material.ACKNOWLEDGMENTWe wish to acknowledge CCLRC for permission to use ESCA300 for some ofthe analysis and Dr A. Boyd for XRD measurement.REFERENCES1. Iijima, S. (1991) Helical microtubles of graphitic carbon. Nature, 354: 56–58.2. Xue, H.G., S.W., He, B.J., and Shen, Z.Q. (2002) A new application of carbonnanotubes constructing biosensors. Chinese Chem. Lett., 13 (8): 799–800.3. Borcia, G., Anderson, C.A., and Brown, N.M.D. (2003) Dielectric barrier dischargefor surface treatment: application to selected polymers in film and fibre form.Plasma Sources Sci. Technol., 12: 335–344.4. Gao, X.P., Zhang, Y., Chen, X., Pan, G.L., Yan, J., Wu, F., Yuan, H.T., andSong, D.Y. (2004) Carbon nanotubes filled with metallic nanowires. Carbon, 42:47–52.T. I. T. Okpalugo et al.484Downloaded By: [University of Ulster] At: 14:29 23 November 2009

Surface-to-Depth Analysis of Functionalized Multi-Wall Carbon Nanotubes (FMWCNTS)

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Surface-to-Depth Analysis of Functionalized Multi-Wall Carbon Nanotubes (FMWCNTS)

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