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INVESTIGATION OF TUBULES RAMAN SPECTRA
A.P. KUZNETSOV, N.V. KHOKHRIAKOV, V.N. STERKALOVSKY*, V.I. KODOLOV
Basic research-High educational center of chemical physics and mesoscopy, Udmurt Scientific center, UrD RAS, 426001, Izhevsk, 222 Gorky St. e-mail: [email protected]
♦Institute of high-temperature electrical chemistry UrD RAS, Ekaterinburg
ABSTRACT. The paper is dedicated to experimental and quantum-chemical investigation of Raman spectra of carbon-metal containing tubules. The investigations of Raman spectra are carried out using parametric method of strong bond. The spectra of carbon nanotubes with different radii and spiralities are numerically calculated. When the tube diameter increases, the calculated parameters approximate to experimental for carbon-metal containing tubules obtained by low-temperature synthesis from aromatic hydrocarbons in lamellar melts of Lewis acids.
INTRODUCTION
Earlier [1-5] the synthesis and investigation of carbon-metal containing tubules from such aromatic hydrocarbons as anthracene and phenathrene in lamellar mineral media containing chlorides of transition metals of iron class were carried out. Quantum-chemical and experimental investigations show the possibility of dehydropolycondensation proceeding in interface layers of Lewis acid melts with the following rolling of an aromatic polymer being formed into "a scroll". The application of such approach, namely the combination of quantum-chemical and experimental investigations to study Raman spectra of tubules obtained by low-temperature synthesis method, is of interest.
CALCULATING AND EXPERIMENTAL METHODS
Quantum-chemical investigation is carried out using a strong bond method, which obviously considers the change of wave functions of electrons at electron subsystem shifting. Numerical calculations are done with pre-optimized end fragments of tubes by total energy with periodic boundary conditions along atom screw lines in designations given in [6, 7].
Samples for Raman spectra investigations are prepared by low-temperature synthesis method from anthracene in eutectic melts containing A1C13, NaCl, NiCl2 with ratio 10:10:3. The ratio of anthracene mass to medium mass is 1:21. Synthesis and the products extraction from reaction mass are carried out in accordance with the methods described in [8]. Raman spectra are registered at spectrometer of REN1SHAW company, model 1000, with agitation by argon laser (agitation wavelength - 514,5 nm).
RESULTS AND DISCUSSION
The oscillating spectra of nanotubes of different radii and chilarities are numerically calculated. In accordance with common agreements [6, 9] the tubes with indexes (5,0), (6,0), (10,0), (5,5), (4,3) are considered. The clusters investigated are 50 A, corresponding to 400 carbon atoms. The second energy derivatives by atoms shifting obtained for die final fragment are rather small. In Fig. 1 and 2 there are numerical data for the calculation of dispersion curves and density of phonon conditions in nanotubes. The marks on phonon spectra correspond with the values of oscillation frequencies of final clusters. Optically active oscillation frequencies of nanotubes obtained from symmetric analysis of phonon modes are shown in Fig. 2. It is seen from their analysis that when the tube radius becomes greater, the frequency values increase in the upper part of the spectrum and decrease in the bottom part. The lines corresponding to high frequencies group in the frequency area arrogated to Raman frequency of graphite plane calculated in the frameworks of strong bond model being used, when the tubule radius increases.
Experimental data (Fig. 3, 4) by Raman spectra are close to those shown in Fig. 2. The lines groups in the regions 1522-1600 cm'1 and 1355-1377 cm'1 are observed in the spectra. The lines in the region 1355-1377 cm'1 are caused by two factors: structure defectiveness and graphite lattice distortion, this is characteristic for carbon tubules. It is necessary to note that the calculated spectra contain optically active lines in the region mentioned. In literature the estimation of defectiveness of carbon substances is carried out by the ratio of lines intensity Q=Ii36(/Ii595 [11]- In our investigation this ratio approximately equals 0,80 - 0,85. According to the value this ratio is in the limits for analogous intensity ratios of Raman spectra obtained by graphite spraying in electric arc without metals or with metal [10]. Apparently, the increase of half-width of the lines and intensities ratio are determined by the size and defectiveness of tubules being formed. The comparison of calculated and experimental data indicates the tendency of intensities increase with frequency growth and decrease when the frequency goes down. There are only a few lines in low-frequency region, and there are practically no lines in region 860-1100 cm'1.
The comparison of experimental, literature data and data of quantum-chemical calculations indicates, together with the results given in [4, 5], the possibility to form tubules in the conditions of synthesis method assumed.
ACKNOWLEDGEMENT
Authors do appreciate the assistance of E.G. Vovkotrub, the employee of High-temperature Electrical Chemistry Institute, for the preparation of Raman spectra of the samples.
A.P. KUZNETSOV, N.V. KHOKHRIAKOV, V.N. STERKALOVSKY, V.I. KODOLOV
Fig. 1. Phonon spectra (a), relative densities of phonon conditions (b) and optically active frequencies of nanotubes (c) (5,0), (6, 0)
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Fig. 2. Phonon spectra (a), relative densities of phonon conditions (b) and optically active frequencies of nanotubes (c) (10, 0), (7,3)
A.P. KUZNETSOV, N.V. KHOKHRIAK.OV, V.N. STERKALOVSKY, V.I. KODOLOV
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Fig. 3. Raman spectrum of the material containing nanotubes obtained during the catalysis of 1 mol of NiCh per 1 mol of anthracene
Fig. 4. Raman spectrum of the material containing nanotubes obtained during the catalysis of 1 mol of NiCl2 per 2 mol of anthracene
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