碳纳米管

FTIR studies of nitrogen doped carbon nanotubesAbha Misra,Pawan K.Tyagi,M.K.Singh,D.S.Misra*Department of Physics,Indian Institute of Technology,Bombay,Mumbai-400076,IndiaAbstractPurified and defect free carbon nanotubes have great potential for applications in electronic,polymer composites and biological sciences.The removal of impurities(carbon nanoparticles and amorphous carbon)is an important step before the CNT applications can be realized.We report the results of FTIR and TGA/DTA studies of the impurities present in the carbon nanotubes.The multiwalled CNTs were grown using Microwave Plasma Chemical Vapor Deposition(MPCVD)technique.Fourier transform infrared(FTIR)spectroscopy was carried out in the range of400–4000cmÀ1to study the attachment of the impurities on carbon nanotubes.FTIR spectra of the as-grown MWCNTs show dominant peaks at1026,1250,1372,1445,1736,2362,2851,2925cmÀ1that are identified as Si–O,C–N,N–CH3,CNT,C–O,and C–H x respectively.The peaks are sharp and intense showing the chemisorption nature of the dipole bond.The intensity of the peaks due to N–CH3,C–N and C–H reduces after annealing and the peaks vanish on annealing at high temperature(900 -C).The presence of C–N peak may imply the doping of the CNTs with N in substitution mode.TGA/DTA measurements,carried out under argon flow,show that the dominant weight loss of the sample occurs in the temperature range400–600-C corresponding to the removal of the impurities and amorphous carbon.Keywords:MWCNTs;FTIR spectroscopy;TGA/DTA1.IntroductionCarbon nanotubes[1](CNTs)are found attractive on account of their potential for applications in hydrogen storage,nanoscale devices and sensor[2–6].High quality and well-aligned carbon nanotubes are essential for the applications in the field of nanoelectronics and many of these applications are dependent upon the chirality and diameter of CNTs[7].The nanotubes can be either metallic or semiconducting depending upon their diameter and chirality.The presence of defects and impurities that are electronically and chemically active can change these properties.Therefore,the control of the defects and impurities has become important for many applications of CNTs.Doping is a practical way to tailor the electrical properties of the CNTs.The nitrogen doped CNT shows n-type semiconducting behavior regardless of tube chirality [8].A great deal of attention has been devoted to the N-doped CNTs.Several groups have produced carbon nanotubes containing nitrogen or nitrogen and boron.The specific interest in CN x tubes is due to the fact that substituted N makes exclusively semiconducting tubes.Many groups have proposed that the electronic structure of N doped pyridine-like,pyrolic,and graphite like structure(Carbon atom has been replaced by N atom in graphite layers)is similar to triple-bonded CN[9–11].Another group has proposed that N atoms might exist mainly inside the inner core of CNTs[12]. In this paper we investigate the nitrogen doped multiwalled carbon nanotubes using Fourier transform infrared(FTIR) spectroscopy and thermo gravimetric analysis(TGA).2.ExperimentalCNTs were grown onto nickel electroplated copper foil substrates.The commercial grade copper foils(purity,98%), polycrystalline in nature,were used without any special pre-treatment to improve their smoothness.The electroplating ofNi on copper foil was performed in an electroplating bath. The copper substrate(1cmÂ1cmÂ0.1mm)was used as cathode and a thin platinum wire was used as the anode. Commercial grade nickel sulfate(NiSO4I7H2O)(275g/l) and nickel chloride(NiCl2I6H2O)(60g/l)were used as electrolytes in a dc electroplating process.The bath temperature and current were maintained at60-C and80 mA,respectively.The nickel layers having a thickness of 5–10A m were treated in the plasma of ammonia(NH3)gas for10s.Ammonia plasma treatment was performed for10s in the temperature range of300–750-C at10Torr for the formation of nano-sized nickel catalyst,which was the precursor for subsequent CNTs growth using chemical vapor deposition(CVD).The flow rate of ammonia was kept at180sccm.Methane(CH4)and hydrogen(H2)were used with respective flow rates of6and40sccm.The temperature of the substrate was maintained at820T20-C at deposition pressure;40Torr.The deposition time was5 min.Nitrogen doping was achieved by adding ammonia gas with flow rate of180sccm in the gas mixture at the time of deposition.Thermal Gravimetric Analysis(TGA)were carried out on Mettler–Toledo(TGA/SDTA851)thermal analyzer under argon flow at the heating rate of5-/min to obtain information on the decomposition and the burning proper-ties of carbon nanotubes and impurities present in it.The temperature of the sample was varied from room temper-ature to900-C.The Fourier transform infrared(FTIR) spectrometer were recorded on Nicolet Magna550FT-IR Spectrum.The samples for FTIR studies were prepared by suspending approximately6mg of MWCNT materials in ¨15ml isopropyl alcohol by sonication with an ultra sonic probe for several minutes.One drop of this solution was sprayed onto silicon wafer and a uniform thin MWNT film on the IR transparent silicon substrate was thus obtained. FTIR studies were carried out in the range of400–4000 cmÀ1in the absorbance mode.The FTIR results with the support of TGA results give a reasonably good picture of the attachment on the carbon nanotubes.3.Results and discussionFig.1(a)shows the SEM micrograph of high density MWCNTs grown on nickel electroplated copper substrate after ammonia plasma treatment for2min.The average length and diameter of the tubes are100A m and20nm respectively.Fig.1(b–c)show TEM images of the multiwalled carbon nanotubes free of encapsulated and with encapsulated catalytic nickel particles,respectively.The inset in Fig.1(c)shows that the inner and outer diameter of the CNT is12.84and18.43nm respectively and the tips of the CNTs are closed.Fig.1(d)is the High-Resolution Transmission Electron Microscopy(HRTEM)imageof Fig.1.(a)SEM micrograph of a sample of multiwalled carbon nanotubes deposited on a copper substrate.The high density of tubes is noteworthy.(b)–(c) TEM images of the multiwalled carbon nanotubes free of encapsulated and with encapsulated catalytic nickel particles,respectively.(d)High-Resolution Transmission Electron Microscopy(HRTEM)image of graphitic walls of a typical MWCNT.386graphitic walls of a MWCNT.The separation between twoconsecutive walls is 3.22A˚.FTIR is used to characterize the functional elements absorbed by carbon nanotubes.Fig.2(a–d)shows the FTIR spectra in the range of 400–4000cm À1a)of as-grown tubes,b)of the tubes collected after TGA at 400-C,c)of the tubes annealed at 600-C,and d)of the tubes collected after TGA at 900-C.FTIR of a)shows dominant peaks at 1026,1250,1372,1445,1736,2362,2851,2925cm À1which corresponds to Si–O,C–N,N–CH 3,CNT,C–O,C–H x respectively.The infra-red absorbance at 1026cm À1is consistent with Si–O stretching vibrations due to slightly different concentrations in the native oxide layer of the silicon before and after coating with MWCNTs film [13].Out of the above identified bonds,the presence of C–N and N–CH 3bonds at 1250and 1372cm À1is most interesting.The strong peaks at 1250and 1372cm À1are consistent with C–N and N–CH 3stretching vibrations attributed by Choi et al.[14]to the presence of intercalated N atoms between the graphite layers at the inner part of the nanotube walls.However,in our opinion the intercalated nitrogen atoms in between the graphite walls may not be strongly IR active.The halfwidths of the C–N and N–CH 3peaks are 36.72and 18.53cm À1respectively,and suggest that the chemisorption process may be dominant implying chemical bonding between carbon and nitrogen atoms.It has been pointed out [15–18]that substituting a N atom in place of a C atom in a sp 2bonded carbon network will induce strong IR activity;consequently the absorption in the 1200–1600cm À1region is expected if the N atoms are bonded into the carbon network.We therefore strongly believe that N doping of graphene sheets may be taking place and a C–N bond identical to the sp3bonded carbon nitride sample may be forming.Features at 1445and 1736cm À1attributed to MWCNT vibrational modes are also apparent [19].The peak at 2362cm À1corresponds to the C–O bonds and the features between 2851and 2925cm À1are consistent withC–H x stretching vibrations of chemisorbed hydrogen of various types presents in all carbon films [20,21].The intensity of the peaks due to N–CH 3,C–N and C–H are gradually suppressed,after annealing at 400-C.The suppression of the peaks would be related to the gradual removal of nitrogen and hydrogen bonded species as impurity.The bonding of N to carbon atom in graphene sheet will give rise to a defect in structure,which would get annealed,at high temperature.This results in the reduction of the intensity of the IR peaks with annealing.Further annealing of the sample at 600-C does not induce any appreciable change.It is interesting to note that these peaks vanish on annealing at higher temperature (900-C)implying that at high temperature the defects inside the tubes are getting strongly mobile leading to the destruction of the attachments along with the tubes.TGA/DTA curve in Fig.3reveals a small weight loss due to water removal around 80-C.The dominant weight loss steps are due to the removal of carbon materials and also due to the decomposition of nanotubes [22–24]taking place in the temperature range between 400and 600-C.The weight loss starts near 400-C and the process completes by 600-C.This result agrees with the FTIR spectra in Fig.1(b–c)discussed above where we have shown a significant decrease in the intensity of IR peaks upon annealing to 400-C.The MWCNTs are completely destroyed at 900-C,suggesting that at this temperature amorphous carbon as well as MWCNTs converts to gaseous form.The complete removal of all the dominant peaks from the FTIR spectra in Fig.1(d)confirms the burning of MWCNTs at 900-C resulting in the change of the color of the sample.4.ConclusionFTIR technique gives information about gaseous ele-ments attached to the tube.Nitrogen attachment is evident from strong and intense IR peaks at 1250and 1372cm À1.From the intensity and halfwidth of these peaks we conclude that the N atom is incorporated at substitutional500100015002000250030003500A b s o r b a n c e (a r b . u n i t s )Wave number (cm -1)Fig.2.FTIR spectra of (a)as-grown MWNTs,(b)after TGA at 400-C,(c)after annealing at 600-C,and (d)after TGA at 900-C.-20020040060080010001200-20020406080100120DTATGATemperature (0C)W e i g h t P e r c e n t0Fig.3.TG-DTA results for MWNTs.387sites in the carbon network.TGA/DTA has been used to study the weight loss of carbon nanotube as a function of temperature.We found that the dominant weight loss occurs the between400and600-C range and agrees well with disappearance of IR peaks corresponding to N-attachment.TGA can be used to get the information about the weight loss of the sample.AcknowledgementsThe authors would like to thank Dr.P.V.Satyam of Institute of Physics Sachivalaya Marg,Bhubaneshawar for high-resolution 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