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Russian Chemical Bulletin, International Edition, Vol. 62, No. 3, pp. 640—645, March, 2013

Carboxylated and decarboxylated nanotubes studied by Xray photoelectron spectroscopy* T. M. Ivanova,a K. I. Maslakov,b S. V. Savilov,a,b A. S. Ivanov,b A. V. Egorov,b R. V. Linko,c and V. V. Lunina,b aN.

S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky prosp., 119991 Moscow, Russian Federation. Fax: +7 (495) 954 1279 bDepartment of Chemistry, M. V. Lomonosov Moscow State University, Building 3, 1 Leninskie Gory, 119991 Moscow, Russian Federation. Fax: +7 (495) 939 4575. Email: [email protected] cRussian University of Peoples´ Friendship, 6 ul. MiklukhoMaklaya, 117198 Moscow, Russian Federation. Fax: +7 (495) 433 1511

Carbon nanotubes (CNTs) of the conic and cylindrical structure were studied by Xray photoelectron spectroscopy in the initial state and after carboxylation and decarboxylation reactions. The O=C—O and С—О groups were revealed on the surface of the chemically modified samples. It was found that both the carboxylated and decarboxylated cylindrical CNTs contain a smaller amount of oxygen than the corresponding conic CNTs apparently due to differences in their structures. Key words: carbon nanotubes, carboxylation, decarboxylation, Xray photoelectron spectroscopy.

Quasimolecular solids, such as carbon nanotubes (CNTs) attract attention of researchers. Due to a unique set of physicochemical and mechanical characteristics, CNTs are promising objects of practical use, in particular, for the production of composite materials.1 The synthesis of multiwalled CNTs is technologically most simple, and the CNTs synthesized, depending on the conditions, can represent a system of either concentrated embedded cylin ders (cyCNTs) or cones (coCNTs). The surface of cy CNTs consists of interconnected sp2hybridized carbon atoms and is fairly inert, whereas the nearsurface layer of coCNTs contains both sp2 and sp3hybridized atoms, which can be responsible for various mechanisms of the processes involving nanotubes. Among them, chemical functionalization, that changes the chemical properties and adhesion characteristics of composites, is of tremendous prac tical significance.2 Carboxylation is usually the first stage of functionalization.2 Being simple, it plays an important role in the covalent modification3 of nanotubes. Many works are devoted to the carboxylation of cyCNTs,2,4—8 whereas the data are few for coCNTs. Xray photo electron spectroscopy (XPS) can efficiently be used to an alyze distinctions in the chemical behavior of these mate * Dedicated to the Academician of the Russian Academy of Sciences I. P. Beletskaya on the occasion of her anniversary.

rials in carboxylation. The purpose of this work is to ap ply XPS for the investigation of the surface of cyCNTs and coCNTs modified by carboxylation and decarb oxylation. Experimental CNTs used in the work were synthesized in a quartz reactor placed in a TZF 12/100/900 threezone tubular furnace (Carbo lite, Great Britain).9 The temperature was maintained constant during the synthesis using an internal heater placed at the center of the tube. Aerosol of a benzene solution of ferrocene (for cy CNTs) or a benzene—ethanol solution of nickel(II) acetylaceto nate (for coCNTs) at 650 С were introduced in the reactor with a carrier gas flow controlled by a mass flow controller (Meta khrom, Russia). Unreacted benzene vapor was condensed at the reactor outlet. After the precursor mixture was consumed, the aerosol generator (Al´bedo, Russia), furnace, and inner heater were switchedoff, and the reactor was cooled in a nitrogen flow. The synthesis products were collected from the reactor walls and from the surface of the internal heater. The material was purified by annealing in air (3 h) at 330 С followed by washing from metal in an HCl solution by ultrasonic activation in a bath (Sapfir, Russia) and drying in a vacuum box. Purity of the tubes was monitored by thermal analysis on a 409 PC Luxx analyzer (Netzsch, Federal Republic of Germany) and Xray fluorescence spectroscopy on a Respekt instrument (Tolokonnikov, Russia). Morphology of the samples was studied

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0639—0644, March, 2013. 10665285/13/6203640 © 2013 Springer Science+Business Media, Inc.

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by scanning electron microscopy on a JEOL JSM 6490 LV micro scope equipped with an energy dispersion Xray spectral analyz er (Jeol Co. Ltd., Japan). Carboxylation was carried out using reflux of CNTs in a 70% solution of HNO3 (3 h) by ultrasonic activation followed by washing with deionized water until washing waters are neutral. For decarboxylation, CNTs were subjected to thermal treatment in purified argon at 350 С, which should result, according to the TG data, in the removal of the most part of the surface oxygen containing groups. Photoelectron spectra were recorded on a Kratos Axis Ultra spectrometer using monochromatic AlK radiation with a power of the Xray gun of 150 W. Samples were mounted as a powder on a doublesided adhesion tape. The spectra were decomposed into components using the CasaXPS software. The line shape described by the sum of the Gaussian (70%) and Loretzian (30%) functions was used to decompose the spectra into components. Measurements were performed at least two times at a pressure of 10–9 Torr. The spectra were recorded at ambient temperature and charge referenced to the С1s line accepting that the binding energy of the component corresponding to the C—C bonds for the sp2hybridized carbon atoms is equal to 284.55 eV. Highresolution images of the carbon materials were ob tained at an accelerating voltage of 200 kV on a JEM 2100F transmission electron microscope (Jeol Co. Ltd., Japan) equipped with a corrector of spherical and chromatic aberrations, an en ergy dispersion Xray spectral analyzer, and an electron energy loss spectrometer (EELS) (Gatan, USA).

Results and Discussion Depending on the synthesis temperature and the used precursor of the catalyst for carbon nanostructure growth, cyCNTs and coCNTs were obtained as confirmed by the highresolution images detected with a transmission electron microscope (Fig. 1).

According to the scanning electron microscopy data, particles of the metal catalyst and partially amorphized carbon impurities are observed on the ends of all nano tubes (Fig. 2) as indirectly evidenced by the thermal anal ysis results (Fig. 3). So, the metal content in the coCNT sample is established as 2.3 wt.%, and that in cyCNTs is 5.1 wt.%. The fraction of amorphized impurities can be estimated as 1.5 and 5.5 wt.%, respectively. To remove these impurities and metallic particles of the catalyst, the obtained samples were purified according to a standard procedure, viz., annealing in oxygen fol lowed by the nonoxidative acidic treatment. The anneal ing temperature was chosen on the basis of the results of thermal analysis conducted for the samples (see Fig. 3). The study under the same conditions showed that the pu rified samples contained no amorphized carbon forms and the metal concentration decreased from 1.5 to 0.1 wt.%. It is known that carboxylation with acids used as oxi dizers leads to the formation of predominantly carboxyl, hydroxyl, ketone, and lactone groups on the surface of the carbon materials. A consequence of the reverse process (decarboxylation) is the disappearance of the most part of these groups. The XPS study of the obtained sam ples showed that the observed content of oxygen in the coCNTs is higher than that in the cyCNT samples after both carboxylation and decarboxylation (Table 1). Ac cording to the XPS data, the metal content does not ex ceed 0.1 at.%, which agrees well with the thermal analysis results. A comparison of the C1s spectra of the CNT samples before and after carboxylation (Fig. 4) shows that the main distinction in the spectra is related to the carboxylation induced increase in the signal at a binding energy range of 288.7 eV corresponding to the carbon atom in the

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Fig. 1. Highresolution TEM images of the initial samples of cyCNT (а) and coCNT (b).

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Fig. 2. SEM images of the initial samples of cyCNT (а) and coCNT (b); size is indicated in nm.

TG (%) 100

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Fig. 3. Results of thermal analysis in an oxygen atmosphere for the initial samples cyCNT (а) and coCNT (b).

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Table 1. Concentrations of carbon and oxygen (at.%) on the surface of the studied CNT samples based on the O1s and C1s XPS spectra recorded at 20 eV pass energy Sample coCNTs initial coCNTs carboxylate coCNTs decarboxylated cyCNTs initial cyCNTs carboxylated cyCNTs decarboxylated

C

O

98.9 87.2 92.2 99.4 94.6 98.0

1.1 12.8 7.8 0.6 5.4 2.0

O=C—O groups.8 At the same time, the signal from the carboxylated coCNT in this energy range increases more strongly than that for the carboxylated cyCNT. This in dicates that the carboxylation of coCNTs results in the formation of additional O=C—O groups compared to that of cyCNTs. Moreover, for cyCNT decarboxylation, the signal intensity of the carboxyl groups decreases almost to the level typical of the initial cyCNT sample, whereas for the decarboxylated coCNT sample the contribution to the spectrum from the carbon atoms of the carboxyl groups is still significant. For the quantitative estimation of the amount of functional groups in the CNT samples, the C1s spectra were decomposed to the components correspond ing to different states of carbon and oxygen atoms. One of the used components corresponded to the shape of the spectrum of the initial nanotubes, whereas three others corresponded to possible bonds of the carbon atoms in the carboxylated and decarboxylated samples. This decompo sition makes it possible to distinguish changes in the chem ical state of carbon and oxygen atoms in the initial sam ples after carboxylation and decarboxylation. The bonds of carbon with oxygen that can be in the initial sample are ignored. Among the components used for decomposition, the component with Eb 288.8 eV corresponds to the O=C—O groups and that with Eb = 286.3—286.5 eV can be attributed to single bonds of the carbon atom with oxygen of the C—O type. The component observed at Eb = 285.3 eV can be ascribed to the sp3hybridized carbon atoms. The used binding energies of the components and

their assignment are consistent with the values presented by other authors.6,8 The results of decomposition are given in Table 2. It is seen that after both the carboxylation and decarboxylation of the nanotubes the contribution to the spectrum from the components corresponding to the sp3hybridized carbon atom and the carbon atoms bonded with oxygen is higher for coCNTs than for cyCNTs. In addition, for the decarboxylated CNT samples of both types, the component corresponding to the carboxyl groups decreases and that corresponding to the C—O, probably, hydroxyl groups, somewhat increases and that can be due to higher temperatures needed for their destruction. The O1s spectra of the studied CNTs were decom posed to three components corresponding to different states of oxygen atoms (Table 3); two of them refer to the oxygen atoms double (O=C) and single (O—C) bonded to carbon atom.8 The third component is probably related to the satellite structure of the spectrum or to adsorbed oxy gencontaining molecules. The results of decomposition of the O1s spectra are shown in Fig. 5. The spectra were normalized according to the intensity of the C1s spectra and plotted in the same scale for coCNTs and CyCNTs. Therefore, the relative intensities of the O1s spectra reflect the relative oxygen content. All oxygen spectra consist mainly of two components of approximately equal inten sity and correspond to bonds of the O—C and O=C types. The component corresponding to the O=C bonds is relat ed to the carboxyl groups, whereas the component corre sponding to the O—C bonds can be ascribed to the oxygen atom in both carboxyl and hydroxyl, or other groups. An analysis of the spectra suggests that both initial and carboxylated coCNTs contain a larger amount of oxygen than the cyCNT samples. This is caused by the fact that the arrangement of graphene layers as stacked cones in the coCNTs results in the presence of a large number of edge carbon atoms on the surface, which are more accessible for oxidation. In cyCNTs, these atoms are observed only at the ends of nanotubes or in defects. The CNTs are predo minantly oxidized due to the formation of carboxyl (O=С—O) groups. The С—O (hydroxyl, ester, etc.) groups and the C=O and O—C—O groups are formed in smaller amounts. In addition, upon carboxylation the sp3hybrid

Table 2. Parameters of decomposition of the C1s spectra of the studied samples into the components corresponding to different chemical states of the carbon atoms Sample

coCNTs initial coCNTs carboxylated coCNTs decarboxylated cyCNTs initial cyCNTs carboxylated cyCNTs decarboxylated

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Intensity of component (%) (Eb/eV) С—С (sp2)

С—С (sp3)

C—O

100 (284.6) 87.1 (284.6) 90.7 (284.6) 100 (284.6) 93.4 (284.6) 95.8 (284.6)

— 6.2 (285.3) 4.9 (285.3) — 3.6 (285.4) 2.1 (285.4)

— 1.8 (286.5) 2.1 (286.5) — 0.8 (286.5) 1.3 (286.5)

O=C—O — 4.9 (288.7) 2.3 (288.6) — 2.2 (288.8) 0.8 (288.7)

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Fig. 4. Xray photoelectron C1s spectra of the coCNT (a—c) and cyCNT (d—f) samples in the initial state (a, d) and after carboxylation (b, e) and decarboxylation (c, f) (normalized by intensity); Eb is binding energy.

ized carbon atoms forming no bonds with oxygen atoms appear on the surface, but their presence indicates an in crease in the deficiency of the carbon layers. After decarb oxylation, the number of carboxyl groups decreases no ticeably, but the number of C—O groups does not de crease, most likely, because they are more resistant to decarboxylation at a specified temperature. The fraction of the sp3hybridized carbon atoms decreases upon decarb oxylation but not to the level of the initial samples, indi

cating that the deficiency in the structure of the sp2hybrid ized carbon atoms retains after decarboxylation. Thus, samples of conic and cylindrical CNTs in the initial state and after carboxylation and decarboxylation were studied by the XPS method. It was shown that the carboxylation of the samples results in a substantial in crease in the oxygen content due to the formation of pre dominantly carboxyl groups. After decarboxylation, their content decreases substantially, but the CNT state is not

Table 3. Oxygen content (number of atoms per 100 carbon atoms) in various groups according to the results of decomposition of the O1s spectrum of the studied samples into components Sample

coCNTs initial coCNTs carboxylated coCNTs decarboxylated cyCNTs initial cyCNTs carboxylated cyCNTs decarboxylated

Oxygen content (Eb/eV) O (total)

O=С—O

O=С—O, С—O

Satellite

1.2 14.6 8.5 0.7 5.7 2.2

0.5 (531.4) 7.0 (531.4) 3.7 (531.4) 0.4 (531.9) 2.9 (531.9) 1.1 (531.9)

0.6 (533.2) 7.2 (533.1) 4.5 (533.4) 0.3 (533.2) 2.5 (533.4) 1.0 (533.6)

0.1 (535.6) 0.4 (535.3) 0.3 (535.9) — 0.3 (535.8) 0.1 (536.0)

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Fig. 5. Xray photoelectron O1s spectra of the coCNT (a—c) and cyCNT (d—f) samples in the initial state (a, d) and after carboxylation (b, e) and decarboxylation (c, f) (normalized by intensity according to the normalization by intensity of the С1s spectra).

identical to the initial one. The cyCNT sample after both carboxylation and decarboxylation was found to contain a noticeably smaller amount of oxygen and carboxyl groups that the corresponding coCNT samples. The studies were carried out using the instrumental base at the Research Center for Collective Use of Chemi cal Department of the M. V. Lomonosov Moscow State University in the framework of the program "Nanochem istry and Nanomaterials. Chemistry of Atmosphere" and the Center for Collective Use of the M. V. Lomonosov Moscow State University. The authors are grateful to E. V. Raitman and N. E. Strokova for help in studying the samples by thermal anal ysis methods. The authors acknowledge support from the M. V. Lo monosov Moscow State University Program of Develop ment and the Center for Collective Use of the M. V. Lo monosov Moscow State University (state contract No. 16.552.11.7081). This work was financially supported by the Russian Foundation for Basic Research (Project Nos 120300971a and 120301115a) and the Presidium of the Russian Academy of Sciences.

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Received February 27, 2013