multiwalled carbon nanotubes and nanofibers ...

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Functional Materials Letters Vol. 3, No. 4 (2010) 289–294 © World Scientific Publishing Company DOI: 10.1142/S1793604710001421

MULTIWALLED CARBON NANOTUBES AND NANOFIBERS: SIMILARITIES AND DIFFERENCES FROM STRUCTURAL, ELECTRONIC AND CHEMICAL CONCEPTS; CHEMICAL MODIFICATION FOR NEW MATERIALS DESIGN

Funct. Mater. Lett. 2010.03:289-294. Downloaded from www.worldscientific.com by STELLENBOSCH UNIVERSITY LIBRARY SERVICE on 11/23/12. For personal use only.

SERGUEI SAVILOV∗ , NIKOLAI CHERKASOV, MARINA KIRIKOVA, ANTON IVANOV and VALERY LUNIN Chemistry Department, M. V. Lomonosov Moscow State University Leninskie gory, 1-3, 119991 Moscow, Russia ∗[email protected] Received 8 September 2010; Revised 22 November 2010

Present work points out the differences between possible tubular carbon structures: nanotubes and nanofibers, as well as describes ways of their modification for utilization for new materials design. For material characterization, XRD, XPS, Raman spectroscopy, thermal analysis, HRTEM and SEM, pore size distribution, EELS, elemental analysis and adiabatic bomb calorimetry were used. Heats of formation for nanotubes and nanofibers and their dependence on carboxylation extent as well as properties of the modified materials are also discussed. The perspectives of applications of modified carbon nanotubes in catalysis and polymers chemistry are given. Keywords: Multiwalled carbon nanotubes; carbon nanofibers; surface modification; nanomaterials.

a lower temperature (650◦C for conic CNTs and 450◦ C for CNFs). All materials obtained were treated at 350◦C in air and washed by HCl.2 All the materials were characterized by thermal gravimetric and mass-spectral analysis (Netzsch STA 449C +QMS 403C), liquid NMR spectroscopy (Bruker Avance 400), solid state NMR spectroscopy (Varian Inova 500), IR (Bruker Vertex 70) and Raman spectroscopy (Jobin Yvon Labram 800HR), HRTEM (JEOL JEM 2100 F), SEM (JEOL JSM 6490LV) XPS (Mustang Phoibos 150), bomb calorimetry (DDS E2K), elemental (CHNSO) analysis (Elementar Vario MicroCube), EDXRF (Respect), XRD (STOE Stadi-P) and porometry (Quantachrome Autosorb 1C/MS/TPR). Different reactants were used for functionalization of CNTs by carboxyl groups: mineral acids (HNO3 , H2 SO4 ), hydrogen peroxide, liquid ozone, electrical discharge in oxygen. Modification of carboxyl to alkyl groups was performed using LiAlH4 (THF, 66◦ C, 4 h); for allyl-fragments, pristine MWCNTs were treated with allylbromide and AlCl3 (50◦C, 2 h). Composite materials with polymethylmetacrylate (PMMA) were obtained by radical polymerization of modified nanotubes suspension in monomers and 0.03% benzoyl peroxide at 70◦ C, with polycarbonate — via it dissolving in

The multitude of applications of materials based on multiwalled carbon nanotubes (MWCNTs) causes the increasing interest of researchers in these objects. Three structures can be realized for tubular geometrically anisotropic carbon forms. They are almost disordered nanofibers with diameter more than 80 nm, and two ordered structures of size 10–60 nm with graphene layers parallel to the central axis, and forming an angle ca. 33◦ with it.1 The second structure is similar to carbon nanofibers (CNFs). Differences between these conic carbon nanotubes (CNTs) and CNFs as well as their analogy with classic CNTs can be discovered from structural (XRD, electron diffraction, pore size), electronic (Raman spectrum, gas adsorption) and chemical (typical reactions) viewpoints independent of the way of their synthesis. Unique properties of these materials make them suitable for utilization in many spheres of science. In the present work, catalytic pyrolysis of the injected solutions of organometallic precursors (ferrocene and nickel acetylacetonate) in toluene and benzene–ethanol mixtures were used to obtain classic and conic MWCNTs respectively. CNFs were synthesized in the same way as conic CNTs from benzene–ethanol solutions of nickel acetylacetonate, but at ∗ Corresponding Author.

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(a)

(b)

Fig. 1. HRTEM images of (a) CNF and (b) classic CNT as obtained.

CH2 Cl2 with further ultrasonic dispergation and evaporation of solvent at 25◦ C for 24 h and 120◦C for 2 h. The pyrolysis, performed under different conditions, yielded conic MWCNTs and CNFs in the case of nickel, classic MWCNTs in the case of iron. According to HRTEM data (Fig. 1), metals were localized on top of the tubes and in the layered structure. Before conducting the investigations, all materials obtained were heated in air to remove amorphous carbon admixtures. X-ray diffraction shows that the typical line for most of the carbon structures (002) is shifted to greater angles in the following sequence: graphite, CNFs, conic and classic CNTs (Fig. 2). Atomic distance decreases in this sequence and this fact is in good agreement with electron diffraction data. The burning temperatures increases from 430◦ C for CNFs and 475◦ C for conic CNTs to 485◦C for classic CNTs (Fig. 3). The pore sizes distribution is quite narrow in the case of classic and conic CNTs (3–8 nm), while for CNFs, this parameter is in the range 3–17 nm. In Raman spectrum (Fig. 4), conic CNTs are more similar to CNFs due to the high intensity of the D-line and D/G line ratio. This fact can be easily explained by their structures, namely the passage of graphite layers to the surface with following damage of sp2 hybridization of the terminal carbon atoms. This fact is supported by C1s XPS spectroscopy data (Fig. 5(a)): major components correspond to the sp2 - and sp3 hybridized carbon atoms. The development of new effective selective adsorbents for organic compounds and ions, materials for physiologically active compounds delivery, filled polymers, new catalysts,

etc. are impossible without preliminary functionalization of carbon nanotubes by carboxyl groups, which also opens up ways to their subsequent modification with different functional fragments: fluorescent, biologically active, biopolymer and others.3 The behavior of CNFs after acidic treatment is different: thermal analysis with mass-spectral gas identification of products after functionalization with HNO3 and H2 SO4 show nitration and sulfating processes in the case of CNFs. Thus, the experimental data show that conic CNTs are more similar to classic CNTs than CNFs. For further practical use, determination of the degree of functionalization is important. It can be done using thermogravimetry with mass-spectral analysis of outgoing gases, NMR 1 H and 13 C spectroscopy, titrimetric analysis or other methods.4−6 Usually conic CNTs yields ca. 6–10% of carboxyl groups while for classic CNTs — 3–5%. TG-MS and NMR methods give the total value of carboxyl groups in the sample, while titrimetric technique enables us to determine only surface groups. Compared to other oxidizing agents (hydrogen peroxide, liquid ozone and electrical discharge in oxygen), HNO3 is the most suitable for functionalization of all types of nanotubes. Dependence of functionalization degree on time of the treatment and concentration of nitric acid is shown in Fig. 6. XPS spectra (Figs. 5(b) and 5(c)) show the presence of hydroxyl groups on the surface. Acidic oxidation leads to the increase in their amount, compared to pristine material. Spectra obtained at different photon energies show that oxidation affects not only the upper atomic layer but also several layers nearer to the center of tube, and supports the presence of two types of oxygen-containing groups in the material. Spectral parameters are given in Table 1.

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Multiwalled Carbon Nanotubes and Nanofibers 291


(a)

(b)

(c) Fig. 2. X-ray diffraction data for CNF, classic CNT and conic CNT.

The heat of formation, calculated from the results of bomb calorimetry according to Eqs. (1)–(3), changes as shown at Fig. 7. c Hm = c Um + ( pV ) = c Um + nRT

(1)

c Hm = c Um + 0.005RT = c Um + 12.4 J

(2)

f Hm = c Um − c Ug , where c Ug = −32762 ± 11 J/g

7

and it makes approximately 0.022–0.025% of total enthalpy. Also, the contributions of metal oxidation processes to total enthalpy were revised according to Eq. (4), but it is close to error limits and has no great significance. o o (CNT) = c H298.15 (CNT-Me) c H298.15

−r H (Ni → NiO) × ν(Ni)/r U (Fe → Fe2 O3 ) (3)

The values obtained were calculated at high-pressure conditions (30 atm of O2 ). To calculate them under standard conditions, Washburn correction was applied,8

× ν(Fe)

(4)

It was shown that oxidized MWCNTs have f H < 0. In opposite, non-oxidized MWCNTs and CNFs have f H > 0. Heats of combustion and formation are given in Table 2.

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Fig. 3. DSC curves for CNF (1), conic CNT (2), classic CNT (3).

Fig. 4. Raman spectra of CNF (1), conic CNT (2), classic CNT (3).

Ozonation is also effective but only for classic CNTs, since O3 reacts with multiple bonds on their surface and allow –COOH groups to be obtained not only at the top of the tubes, occurring under acid treatment (Fig. 8). A striking example of application of carboxylated CNTs — in catalysis: under heavy sonication with easily decomposing compounds of catalytically active metals, small sized nanoparticles (3–7 nm) can be separately stabilized on the tubes surface with loading up to 20 wt.%. They demonstrate high efficiency in catalytic hydrodechlorination of 4chloroacetophenone.9 Transformation of carboxyl groups to hydroxymethyl, allyl, alkyl leads to easy implantation of

MWCNTs to different polymer matrixes such as PMMA or polycarbonate. Hardened by ca. 1 wt.% of MWCNTs, they increased the stability to cresoformation. The effect in all these cases depends on the amount of carboxyl groups. XRD, XPS and Raman spectroscopy, as well as bomb calorimetry, show that CNTs with parallel and conic arrangement of graphene layers appreciably differ from CNFs. Chemical reactivity of CNFs is also distinguished from CNTs, e.g., nitration and sulfation processes preferably takes place in the case of CNFs compared to carboxylation. Functionalization of CNTs offers challenges to their utilization in catalyst preparation and implantation to polymers that results

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Multiwalled Carbon Nanotubes and Nanofibers 293 Table 1. Spectral parameters and relative intensities for C1s spectra obtained at photon energy 355 eV. Carbon atom C (sp2 ) C (sp2 ) C−OH C−OOH

BE (eV)

Pristine conic CNT

Oxydized conic CNT

Pristine classic CNT

Oxydized classic CNT

284.5 284.95 286.0–286.3 288.6–288.8

71.35 16.03 10.04 3.63

54.71 30.27 8.91 4.36

49.25 35.85 6.5 1.56

51.43 24.13 12.26 7.57


(a)

(b)

(a)

(c) Fig. 5. Photoemission spectra C1s (hv = 355 eV) for (a) pristine conic MWCNT, (b) photoemission spectra C1s (hv = 355 eV) and (c) O1s (hv = 600 eV) for HNO3 -treated conic MWCNT.

(b) Fig. 7. Dependence of (a) heat of combustion and (b) formation on amount of oxygen content ( — experimental, — theoretical value).

Table 2. Thermodynamic parameters of conic MWCNT, classic MWCNT and CNF.

Fig. 6. Dependence of amount of –COOH groups on the time of HNO3 treatment for different acid concentrations.

Conic MWCNT Classic MWCNT CNF

c U (J/g)

f H (kJ/g)

34050.46 ± 123.38 32975.61 ± 127.85 32380.61 ± 72.44

1.288 ± 0.123 0.2136 ± 0.128 −0.381 ± 0.0724

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Germany) and A. Egorov (MSU, Moscow, Russia) for HRTEM images, E. A. Nesterova (MSU, Moscow, Russia) for SEM data, RF President’s Grants Council (MK-1426.2010.3), Russian Academy of Sciences and Russian Foundation for Basic Research for financial support. This paper was presented at 3rd International Conference on Functional Materials and Devices (ICFMD-2010).


References

Fig. 8. Oxygen distribution map from the EELS data.

in effective metal nanoparticles stabilization and decrease of cresoformation with improvement of mechanical properties respectively.

Acknowledgment Authors thank Dr. Lada V. Yashina (Moscow University, Russia) for XPS measurements, Dr. P. Simon (MPS, Dresden,

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