J. Phys. Chem. B 2001, 105, 4853-4859
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Comparative Study on the Electronic Structure of Arc-Discharge and Catalytic Carbon Nanotubes L. G. Bulusheva,*,† A. V. Okotrub,† I. P. Asanov,† A. Fonseca,‡ and J. B. Nagy‡ Institute of Inorganic Chemistry SB RAS, pr. Ak. LaVrentieVa 3, NoVosibirsk 630090, Russia, and Laboratoire de Re´ sonance Magne´ tique Nucle´ aire, Facultes UniVersitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium ReceiVed: January 5, 2001
Electronic structure of multiwall carbon nanotubes has been studied by X-ray emission and photoelectron spectroscopy. The measurements were performed for two samples synthesized by different methods such as arc-discharge graphite evaporation and catalytic decomposition of hydrocarbons on supported catalysts. X-ray spectra of the former sample were shown to be similar to those of the highly oriented pyrolytic graphite, while the C KR spectrum of catalytic nanotubes exhibited an enhanced density of π occupied states and the C 1s line of sample expanded toward both lower and higher binding energy regions. The models incorporating pentagon-heptagon pair and sp3-hybridized atoms have been calculated by AM1 quantum-chemical method to reveal the influence of defects on the electronic structure of carbon nanotube. Carbon KR and 1s lines were found to be practically unaffected by inserting pentagon and heptagon into the hexagonal network on diametrically opposite sides of tube. However, the changes observed in the X-ray spectra of catalytic carbon nanotubes may result from adjacent pentagon-heptagon pairs and interlayer linkages with more noticeable effect from the latter defect kind.
Introduction Revealing a connection between the nanotube properties and the structural peculiarities is the important point in the carbon nanotubes investigation. A perfect nanotube may be thought of as a cylindrical graphene sheet composed only of hexagons with a few pentagonal rings needed to close the tips.1 The typical carbon nanotubes formed under high temperature conditions, which can be induced by an electric arc for example, are predominantly straight having the largest numbers of topological defects at their ends.2 From theoretical calculations it has been suggested that the electronic properties of a thin graphite cylinder would be varied from semiconducting to metallic depending on the tube diameter and chirality.3-5 Later the electrical transport measurements confirmed these assumptions.6-8 During a catalytic decomposition of hydrocarbons on metal nanoparticles a large amount of curved and coiled nanotubes is obtained9,10 due to a steric hindrance blocking the end of a growing straight tube.11,12 Such nanotubes were demonstrated to have resistivity an order of magnitude larger than that of straight nanotubes that is caused by the structural imperfections of the former ones.13 The curvature of carbon nanotubes is usually attributed to the pentagon-heptagon pairs incorporated in the honeycomb lattice.14-16 A maximal separation of pentagon and heptagon along the tube circumference creates the largest angle bend of tube and the conductance of such type of junction was measured to be strongly suppressed relative to that of the straight tube segments.17 Electrical conductivity results from the density of states in a vicinity of the Fermi level, at the same time the evaluation of defects influence on the electronic structure of carbon nanotubes in larger energy interval is also of interest. * Author to whom correspondence should be addressed. † Institute of Inorganic Chemistry SB RAS. ‡ Facultes Universitaires Notre-Dame de la Paix.
One of the experimental methods for directly probing the valence electronic structure of a substance is X-ray emission spectroscopy. X-ray emission arisen as a result of electron transitions from occupied valence states to a previously created core hole is governed by the dipole selection rules. Therefore, the C KR spectrum measures the C 2p partial density of occupied states of carbon compound. A study of two samples synthesized by arc-discharge graphite evaporation had indicated significant difference between the C KR spectrum of multiwall nanoparticles from the inner part of the cathode deposit and that of the carbon soot collected on the cold walls of the reactor chamber.18 Quantum-chemical calculations on the tube fragments showed that this difference may be attributed to high density of structural defects in the short nanotubes composing the latter sample. At the same time the C KR spectra of arcdischarge multiwall nanotubes18 and the ropes of single-wall nanotubes prepared by a laser ablation19 are quite similar to the spectrum of nontextured graphite, that points to about the same issue of defects in these carbon materials. The valence band structure of catalytic multiwall carbon nanotubes has been detected by photoelectron spectroscopy to be different from that of graphite.20 In the present work X-ray emission spectroscopy was applied in the comparative study of carbon nanotubes synthesized by catalytic process and arc-discharge graphite evaporation. Both kinds of nanotubes are multilayered with the outer diameter values of about the same magnitude. An interpretation of the spectral features was performed using quantum-chemical calculations of various models. The goal of this paper is to reveal the structural defects in the catalytic carbon nanotubes, which cause the density of π-valence electrons to increase. The measurement of C1s core-level binding energies of the samples was additionally involved to detect the carbon atoms being
10.1021/jp010056v CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001
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Figure 1. Transmission electron micrographs of carbon tubular and polyhedral particles from the inner part of the cathode deposit (a) and purified carbon nanotubes produced by catalytic decomposition of acetylene on a supported catalyst (b).
different from the ones composing the graphite network by local arrangement. Experimental Details The apparatus for arc-discharge graphite evaporation is described elsewhere.21,22 The cathode deposit was produced by a simultaneous evaporation of seven graphite rods in helium gas of 800 Torr. The dc arc current was typically 1000 A at 35-40 V. The inner part of the deposit had a laminated texture and was estimated by transmission electron microscopy (TEM) to consist of 80% carbon nanotubes.23 Figure 1a shows a micrograph of multiwall tubular and polyhedral structures from the inner part of carbon deposit. The catalytic tubes were produced by decomposition of acetylene at 700 °C over 2.5 wt % Co-2.5 wt % Fe/NaY zeolite catalyst. After reaction, the catalyst was removed by repeated dissolution in hydrofluoric acid. Since the nanotubes contained some pyrolytic carbon, it was eliminated by oxidation using KMnO4 in diluted sulfuric acid solution at room temperature.
Figure 1b is a typical low magnification TEM image of purified carbon nanotubes where curved nanotubes with open tips can be seen. Note that the tips, opened during the oxidation step, were closed on the as-made carbon nanotubes. C KR spectra of highly oriented pyrolytic graphite and carbon nanotubes were recorded with a laboratory X-ray spectrometer “Stearat” using ammonium biphthalate (NH4AP) single crystal as a crystal-analyzer. This crystal has not uniform reflection efficiency, which is corrected by the procedure described elsewhere.24 Due to the highest reflection efficiency of NH4AP crystal-analyzer in the region near the absorption edge, only the short-wave region (272-285 eV) of carbon emission spectrum was measured. The samples were deposited on copper supports and cooled to liquid nitrogen temperature in the vacuum chamber of the X-ray tube with copper anode (U ) 6 kV, I ) 0.5 A). As the carbon nanotubes prepared by the catalytic process are randomly arranged in the sample, the arc-discharge nanotubes have the predominant orientation in the flake layers.23 To average an angular dependence of X-ray fluorescence,25 the
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samples of graphite and arc-produced nanotubes were located at 45° to the takeoff direction of the emitted radiation. Determination accuracy of X-ray band energy was (0.15 eV with spectral resolution of 0.5 eV. The spectra were normalized at the maximal intensity. X-ray photoelectron C1s spectra of graphite and carbon nanotube samples were recorded using a VG Microtech spectrometer. Electron excitation was performed by characteristic Mg KR -radiation, and the transmission energy of the analyzer was 20 eV. Full width at half-maximum (fwhm) of Ag 3d5/2 line is 0.9 eV. The spectra were normalized at the maximal intensity. Calculations The geometry of structural models was optimized by the molecular mechanic MM+ force field.26 The dangling bonds at the fragment edges were saturated by hydrogen atoms. The quantum-chemical calculation on the fragments were carried out using semiempirical AM1 method27 within GAMESS package.28 X-ray transition intensity was calculated by summing the squared coefficients with which carbon 2p atomic orbitals (AOs) involved in the concrete occupied molecular orbital (MO). The energy location of intensity corresponded to the MO eigenvalue. Calculated intensities were normalized by maximal value and broadened by convolution of Lorenzian functions with a fwhm of 0.6 eV. The charge qµ on the atom µ was calculated by
qµ ) Nµ -
∑i Cµi2
where Nµ is the number of valence electrons of the atom and Cµi is a coefficient describing the amount of AOs to be found in the ith occupied MO. Results and Discussion Figure 2 compares C KR spectra of graphite (a) and multiwall carbon nanotubes produced in electric arc (b) and by catalytic decomposition of acetylene (c). All spectra exhibit the dominant maximum D around 276 eV and the maximum B at 280.5 eV characterized by detectable high-energy shoulders C and A, respectively. Due to the peculiarities of used crystal-analyzer the intensity of X-ray fluorescence is reduced in the low-energy region that makes certain separation of the spectral features below 275 eV difficult. The relative intensities of the features in the C KR spectra of graphite and arc-produced carbon nanotubes are almost the same being indicative of about identical electronic states of carbon atoms in both materials. The maximum B and shoulder A correspond to the density of π occupied states, the feature C is formed by X-ray transitions of π- and σ-electrons, and the maximum D is assigned to the σ-sates only.29 C KR spectrum of catalytic carbon nanotubes is significantly different from other two spectra by increase of intensities of the features C, B, and A. As π-electrons participate in origin of all these features the valence band of catalytic tubes is characterized by rising density of π states that may be connected with high portion of defects breaking the uniformity of hexagonal carbon network. The observed difference in the electronic structure of two kinds of carbon nanotubes is most likely caused by a distinction in their construction. Really, as the arc-discharge carbon nanotubes are predominantly straight, the catalytic ones are often bent and curved. The tube bend is realized with insertion of pentagonal and heptagonal rings in the otherwise perfect
Figure 2. C KR spectra of graphite (a), arc-discharge (b), and catalytic (c) carbon nanotubes.
hexagonal network and relative position of these rings determines angle of bending.30 As indicated by the quantum-chemical calculations on bent tubes31,32 and scanning tunneling microscopy (STM) study on closed nanotube tip,33 such type defects may result in an appearance of localized states in the vicinity of the Fermi level. To clarify the effect of pentagon-heptagon pair defect on C KR spectral profile of carbon nanotubes three fragments of composition C370H38, C382H40, and C432H36 (Figure 3) were calculated. We started with the (10,10)-(18,0) junction having pentagon and heptagon in diametrically opposite position that was governed by two main reasons. First, carbon nanotubes with this diameter value are really synthesized. Second, (10,10)(18,0) tube is involved in the series (5n,5n)-(9n,0) being “perfectly graphitizable” because the diameter difference between the straight segments within the same shell is small enough, the interlayer spacing is constant through the series and close to that in graphite, and the middles of all pentagons and heptagons are aligned.16 According to theoretical viewpoint the (10,10)-(18,0) system matches the metallic and semimetallic tubes. In addition the fragments of metal-semiconductor (10,10)-(11,9) and semimetal-semiconductor (18,0)-(17,1) junctions with an adjacent pentagon-heptagon pair were considered. Theoretical C KR spectra of straight sections were plotted for the atoms composing the tube circumference and maximum distant from the fragment edges (selected atoms in Figure 3) to reduce the boundary effect. In the C370H38 fragment a few atoms of selected chains are located too close to the heptagonal defect due to the shorter concave sides of the (10,10)-(18,0) junction. However, we believe the total calculated density of C 2p electron states would only slightly be changed by these atoms. A
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Figure 3. (1) C370H38 fragment of the (10,10)-(18,0) junction and theoretical C KR spectra calculated for the marked atoms of (10,10) part (a), (18,0) part (b), and pentagon-heptagon pair (c). (2) C382H40 fragment of the (10,10)-(11,9) junction and theoretical C KR spectra calculated for the marked atoms of (10,10) part (a), (11,9) part (b), and pentagon-heptagon pair (c). (3) C432H36 fragment of the (18,0)-(17,1) junction and theoretical C KR spectra calculated for the marked atoms of (18,0) part (a), (17,1) part (b), and pentagon-heptagon pair (c).
comparison of C KR spectra for the straight sections from various junctions (Figure 3a,b) detected their similarity. The spectra show the dominant maximum D at -15 eV and the less intense maximum B around -10.6 eV whose energetic separation is in excellent agreement with the corresponding value for C KR spectrum of arc-discharge carbon nanotubes (Figure 2(b)). The relative intensity of the maximum B is practically kept constant within all theoretical spectra but it is somewhat higher than that in the experimental spectrum. This discrepancy may be explained as the restricted size of calculated system and the complex morphological composition of carbon material under study. Like the experimental data for arc-produced nanotubes and graphite, the theoretical C KR spectra exhibit also the features A and C positioned at the lower ionization potential sides of the maxima B and D, respectively. As Figure 3a,b indicates the arrangement of bent tube has the more significant effect on the feature C varying as its relative intensity and structure. Calculated density of 2p electron states for carbon atoms composing pentagon-heptagon pair defect in the considered
systems is presented in Figure 3c. The maximum separation of a pentagon and a heptagon in the (10,10)-(18,0) junction tends to some increase of relative intensity of the features A, B, and C that is more noticeable for the latter feature located around -13 eV. When pentagon and heptagon are adjacent the electron density in this energetic range rises so that the maximum C becomes dominant one. Relative to the theoretical spectra for straight segments the spectra for adjacent defects exhibit also an enhancement of intensity of the shoulder A being more significant in the (10,10)-(11,9) junction. For all considered bent tubes the insertion of pentagon-heptagon pair defect will increase greatly the intensity of feature C and much less the intensities of maximum B and shoulder A. However, these latter features increase the most as the experimental C KR spectrum of catalytic nanotube compared to that of arc-produced nanotubes (Figure 2b,c). It is believed that occurrence of pentagonheptagon topological defects in the carbon nanotube structure cannot provide the experimentally observed enhancement of density of weekly bonding π states. Another type of the possible structural defects, which can
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Figure 4. The top (a) and side (b) views of the layered model consisting of two fragments C84H24. The atoms linking the layers are cross-marked.
Figure 5. Comparison of C KR spectra calculated for the flat fragment C84H24 (a) and the layered model (b). The σ- and π-components are separated in the C 2p density of occupied states.
perturb the density of π electronic states in the multiwall nanotubes, is an sp3-hybridization of carbon atoms. Highresolution transmission electron microscopy shows that the layers of the catalytic nanotubes are often wavy and segmented that might be explained by chemical interactions between the neighboring shells. We constructed a model consisting of two graphite fragments C84H24 being aligned to each other (Figure 4). Six carbon atoms from each layer are bound so that the central part of the graphite fragments is a coronene-like. The optimized length of the bonds between sp3-hybridized atoms is equal to 1.6 Å, that is somewhat longer than the characteristic value for aliphatic carbon compounds. The coronene-like regions are separated by 3.41 Å, in close correspondence with the interlayer spacing in multiwall carbon nanotubes. Figure 5 compares the C KR spectra calculated for the atoms of central hexagon in the flat fragment C84H24 and for those in the layered model. The former spectrum might simulate the C KR spectrum of graphite crystallites or defect-free carbon nanotubes having enough large diameter that is confirmed by its very well agreement to the spectra depicted in Figure 2a,b. An increase in the size of the calculated graphite fragment closer to that of the real crystallite is believed to improve this correspondence in the framework of used quantum-chemical approach. Again the main features A, B, C, and D, being similar in origin to those in C KR spectra analyzed above, can be selected. Relative to the flat fragment spectrum, the spectrum of the layered model exhibits an enhancement of the features A and B intensity in correspondence with the observed tendency in the experimental spectra of the samples of different nanotube kinds (Figure 2b,c). The calculated densities of occupied states were divided onto σ- and π-components (Figure 5). A distribution of σ-electrons in both structures is similar while the density
of π-states is considerably changed in the energy interval from -8.5 to -11.5 eV. Within this interval three orbitals formed the features B′, B, and C′ of C84H24 spectrum stand out. The MOs of the layered structure, which are analogous to the selected orbitals by character of the electron interactions, occupy the energy interval from -9.0 to -10.8 eV. The energetic approach of the MOs is obviously caused by the covalent linking the graphite sheets that changes the π-electron interaction of the central atoms of graphite fragment with all other atoms. The disruption of the π-system on the sp3-hybridized atoms decreases the efficiency of such interaction. As a result, the π-electrons of central atoms are localized forming an intense peak. In the theoretical C KR spectral profile this manifests as an integration of the features B′, B, and C′ of the flat fragment C84H24 (Figure 5a) into single maximum B in the spectrum of the layered structure (Figure 5b). The similar change of the density of π-electron states might believe to be observed for partially hydrogenated tube walls, that is possible in the experiment condition, when the acetylene is cracked with hydrogen generation. The carbon sp3-hybridized states in this case originate from the formation of C-H bonds. Broken carbon network uniformity could result in changing the binding energy of inner electrons. Figure 6 represents the X-ray photoelectron spectra of graphite (a), arc-discharge (b), and catalytic (c) carbon nanotubes. The C1s core peaks of graphite and arc-produced multiwall carbon nanotubes are essentially the same, while the X-ray photoelectron spectrum of catalytic carbon nanotubes shows a broadening toward both lower and higher binding energy regions. The broadening value was estimated from the difference in the spectra of catalytic carbon nanotubes and graphite to be equal to about 1 eV at the lower energy side and about 1.5 eV at the higher energy side
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Bulusheva et al. the charge on such atoms is equal to 0.22e. The negative charges with value of -0.16e arise on the neighboring sp2-hybridezed atoms due to the breaking of conjugated π-system.34
Figure 6. C1s spectra of graphite (a), arc-discharge (b), and catalytic (c) carbon nanotubes. The profile (d) is obtained by subtraction of spectrum (a) from spectrum (c).
(Figure 6d). Assuming the direct connection between the energy of 1s electrons and the atomic charge, the catalytic tubes can be concluded to have detectable portions of atoms, which are more negatively or more positively charged than the carbon atoms in graphite and arc-produced nanotubes. The ratio of difference line integral (Figure 6d) to C1s core line one (Figure 6c) is about 20% that might correspond to a percent content of carbon atoms in catalytic nanotubes different from those in graphite by electronic state. We inspected the charges on carbon atoms of the calculated fragments. The absolute value of charge on the atoms composing selected chains in straight segments is not over 0.03e. The charges on the defective atoms in the (10,10)-(18,0) junction have about the same values being negative or positive depending on whether pentagon or heptagon is viewed. In the junctions incorporating adjacent pentagon-heptagon pair, the atoms of defect and those of straight parts differ significantly by charge values. So, the pentagon (heptagon) atoms in the (18,0)-(17,1) junction are characterized by the charges about -0.06e (0.07e) and in the (10,10)-(19,10) junctions the charge may reach a value of -0.11e (0.09e). From the result obtained it may be deduced that C1s binding energy data for catalytic nanotubes cannot be successfully attributed to the bent tube arrangement with maximum separation of pentagon and heptagon. The adjacent position of defect pair gives the better while still not adequate fit to the experiment. Actually, as the C1s core peak of catalytic nanotubes is more expended to the higher binding energy region, the absolute charge on positively charged atoms may reach the greater value than that on negatively charged atoms. However, the pentagon-heptagon pair defects exhibit similar variation in the values of opposite charges. Considerable positive charge can be generated on sp3-hybridized carbon atoms incorporated in the graphite-like network. In the layered model,
Conclusion X-ray emission and photoelectron spectroscopy were applied to study the electronic structure of multiwall carbon nanotubes prepared by different synthetic techniques. The methods used detected notable distinctions in the distribution of π valence electrons and the binding energy of core electrons for two kinds of carbon nanotubes. These distinctions were attributed to the peculiarities in the structure of catalytic nanotubes. From the similarity between X-ray spectra of arc-produced nanotubes and graphite may be assumed that these carbon materials are characterized by about the same portion of defects, which mainly concentrated close to the tips of nanotubes and at the boundary of graphite crystallites. The alternative production method of multiwall carbon nanotubes based on the catalytic decomposition of hydrocarbons resulted in the appearance of a large amount of curved, bent and coiled structures. The bending and coiling of the tubes is suspected to be caused by the occurrence of pentagon-heptagon pairs in the honeycomb structure. As it is a more pronounced structural distinction between the two kinds of the measured nanotubes, the influence of pentagon-heptagon pair defects on C KR profile and charge distribution was investigated by quantum-chemical calculation on the (10,10)(18,0), (10,10)-(11,9), and (18,0)-(17,1) tube junctions. The theoretical data were found to be in better correspondence with the experimental ones when the pentagon and heptagon are adjacent than when they are separated at the most. Nevertheless, an incorporation of this topological defect into the tube construction cannot satisfactorily explain the experimental features, especially in the C1s core line. Other structural defect type, which might occur in the multiwall carbon nanotubes and perturbs their density of occupied states, is a linkage between neighboring layers more likely appeared in the low-temperature conditions. The calculation of a model constructed from two graphite layers connected by the covalence bonds indicated the strong change of density of π-electron states relative to that in graphene sheet. The sp3hybridized atoms in the hexagonal carbon honeycomb are the traps for π-electrons decreasing the efficiency of their interaction. As a result, the π-electron states are localized and cause an increase in the intensity of the X-ray emission spectrum. The disruption of conjugated π-system originates the negative charges on atoms being the closest neighbors with positively charged sp3-defects. The presence of portion atoms having positive and negative charges corresponds to the observed broadening of C1s core peak of catalytic multiwall carbon nanotubes relative to that of arc-discharge tubes and graphite. Acknowledgment. The authors thank Dr. A. L. Chuvilin for TEM measurements on carbon nanotube samples. This work was financially supported by INTAS (Project No. 97-1700), the Russian scientific and technical program ,Actual directions in physics of condensed states. on the ,Fullerenes and atomic clusters. (Project No. 98055) and the Russian Foundation for Basic Research (Project No. 00-03-32463a). A.F. acknowledge financial support through the Belgian Program on Inter University Poles of Attraction initiated by the Belgian State, Prime Minister’s Office for Scientific, Technical and Cultural Affairs (OSTC-PAI-IUAP No. 4/10 on Reduced Dimensionality Systems). References and Notes (1) Ebbesen, T. W.; Takada, T. Carbon 1995, 33, 973.
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