Unexpected Formation of ansp2-Carbon Needle from Dimethyl Ether Vapor Under Mild Conditions on Exposure to Radiation from a Low-Power Ar+Laser

Doklady Chemistry, Vol. 386, Nos. 4–6, 2002, pp. 251–254. Translated from Doklady Akademii Nauk, Vol. 386, No. 4, 2002, pp. 492–495. Original Russian Text Copyright © 2002 by Bukalov, Mikhalitsyn, Leites. CHEMISTRY Unexpected Formation of an sp2-Carbon Needle from Dimethyl Ether Vapor Under Mild Conditions on Exposure to Radiation from a Low-Power Ar+ Laser S. S. Bukalov, L. A. Mikhalitsyn, and L. A. Leites Presented by Academician Yu.N. Bubnov June 13, 2002 Received June 13, 2002 The formation of carbon modifications from carbon-containing light molecules in the gas phase has attracted considerable attention in recent years. In particular, processes of this type giving rise to diamond or nondiamond carbon and proceeding at atmospheric pressure but at high temperatures (1000–1500°ë) and often in the presence of a catalyst are described in a review [1]. We discovered the formation of a carbon needle from dimethyl ether vapor, which took place at a pressure of ~1.5 atm and a temperature of 150°ë upon irradiation with a ~250-mW Ar+ laser at a wavelength of 514.5 nm during recording of the Raman spectrum. Dimethyl ether was placed into a molybdate-glass tube (wall thickness ~1 mm, diameter 25 mm) by freezing in a vacuum. The vapor pressure was about 1 atm. Nitrogen, which was meant for measurement of the real gas temperature in the laser beam (from the intensity ratio of the Stokes and anti-Stokes lines in the rotational Raman spectrum of nitrogen) was admitted into the tube (at a pressure of 0.4 atm). The tube with this gas mixture was sealed and placed in the chamber of the Raman spectrometer. The tube with the gas was heated by an electric furnace. The temperature was controlled by a heating controller and measured by a thermocouple. The diameter of the laser beam at the inlet of the tube was ~2.5 mm. During recording of the Raman spectrum, periodic bright flashes (approximately one flash per second) started unexpectedly in the gas bulk exposed to the laser beam. This process lasted for several minutes and then the experiment was stopped to avoid the possible explosion of the tube. Subsequent examination showed that a thin black needle had formed inside the tube along the laser beam. The needle was welded to the tube glass at the point of laser beam entrance and was arranged strictly at a right angle to the tube wall. It was several mm shorter than the tube internal diameter, and its end fell short of the opposite wall and remained free (see Fig. 1). The needle had a circular cross section and gradually thinned along the length (from 0.20 to 0.12 mm in diameter). Unfortunately, when the tube was taken out of the spectrometer chamber, the needle broke off from the tube wall and fell to the bottom, having split into several pieces. The photograph of the free tip of the needle taken using an electron microscope is shown in Fig. 2 with a magnification degree of 1500. It can be seen that the tip surface is smooth and rounded. A fragment of the needle with the round tip (with a length of 6 mm and an average diameter of 0.15 mm) was studied by X-ray powder diffraction using a Bruker Smart 1K CCD diffractometer with an area detector (MoKα radiation, λ = 0.71073 Å). The X-ray diffraction pattern exhibits three clear-cut maxima with different intensities (Fig. 3), which correspond to interplanar spacings d ≈ 3.35, 2.08, and 1.70 Å. These distances are characteristic of the crystal lattice of hexagonal graphite. The Vapors of (CH3)2O and N2 Laser beam Lense Needle Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, GSP-1, 117813 Russia Fig. 1. Experimental setup (the tube with dimethyl ether vapor in the chamber of the Raman spectrometer). 0012-5008/02/0010-0251$27.00 © 2002 åÄIä “Nauka /Interperiodica” 252 BUKALOV et al. ~ 20 µm Fig. 3. X-ray diffraction pattern of the needle. Fig. 2. Photograph of the free tip of the needle (magnification 1500×). value of 3.35 Å, equal to the distance between graphite layers in the unit cell, is especially typical [2, 3]. Raman spectroscopy is known to be a diagnostic method for identification of various carbon polymorphs [4, 5]. We recorded the Raman spectra of the needle using a T64000 Jobin Yvon Raman spectrometer equipped with a highly sensitive CCD detector cooled by liquid nitrogen and a microscope with a TV camera. The 514.5-nm line of a 1-mW Är+ laser was used for excitation. The spectra were recorded both for the lateral surface of the needle and for the chip butt-end; neither of these contained a narrow line at 1332 cm–1 typical of diamond [4, 5]. The Raman spectrum of the needle butt-end (Fig. 4a) exhibits a single first-order line at 1581 cm–1 (G line), which is typical of the spectrum of highly ordered crystalline graphite [4–7]. This line corresponds to the Raman-active ν2 mode of the E2g class [6]. The Raman spectrum of the lateral surface (Fig. 4b) is markedly different and contains two broadened lines, a line at 1588 cm–1 (G line) with a shoulder at 1619 cm–1 and a line at 1354 cm–1 (D line). The opinions concerning the origin of the latter line vary [5, 9]; however, it is always attributed to defects or disorder in the crystal structure of graphite. A Raman spectrum similar to that shown in Fig. 4b is typical of synthetic diamond-like carbon (DLC) films and of so-called glassy carbon [4, 6, 8, 9]. In particular, the spectrum of the needle surface is very similar in line positions, intensities, and halfwidths to the Raman spectrum of the carbon part of natural schungite (glassy carbon), reported recently [8], and also resembles the spectrum that we recorded for the DLC films covering the crystalline germanium whiskers formed from tetraalkylgermanes upon a MOCVD procedure [10]. The micromap making performed by Raman spectroscopy indicates homogeneity of the needle surface. These results provide the unambiguous conclusion that the needle consists of sp2 carbon. It is ordered crystalline graphite coated by a film of so-called glassy carbon. Unfortunately, we did not analyze the composition of the gas formed due to a microcrack in the tube wall. However, by analogy with [1], one can suggest that the reaction proceeded as follows: (CH3)2O hν, 150°C Cg + H2O + 2H2. The nature of the periodic flashes in the laser beam resulting in cleavage of strong CH and CO bonds in the dimethyl ether molecule to give sp2 carbon (at a pressure not exceeding 1.5 atm, a temperature of 150°ë, and relatively low power of laser radiation) has not yet been elucidated and requires further investigation. However, the results (confirmed by Raman spectroscopy and X-ray diffraction) demonstrate that this decomposition can, in principle, take place under mild conditions. It is also worth noting that we repeatedly observed decomposition of various organic or organometallic compounds to give sp2 carbon (with appearance of intense Raman lines at ~1580 and ~1350 cm–1) induced by radiation of an Ar+ or He–Ne laser during DOKLADY CHEMISTRY Vol. 386 Nos. 4–6 2002 UNEXPECTED FORMATION 253 1581 ‡ 1354 1588 b 1300 1400 1500 1600 ∆ν, cm–1 Fig. 4. Raman spectra of (a) the butt-end and (b) the surface of the needle. our extensive experience in recording Raman spectra at the Science and Engineering Center for Raman Spectroscopy at the Russian Academy of Sciences. This work was supported by the Russian Foundation for Basic Research, project nos. 01–03–33057, 02–03– 06307, and 00–15–97307. ACKNOWLEDGMENTS REFERENCES The authors are grateful to M.Yu. Antipin and I.I. Vorontsov for performing X-ray diffraction analysis. DOKLADY CHEMISTRY Vol. 386 Nos. 4–6 2002 1. Rudenko, A.P., Kulakova, I.I., and Skvortsova, V.L., Usp. Khim., 1993, vol. 62, no. 2, pp. 99–102. 2. Kurdyumov, A.V. and Pilyankevich, A.N., Fazovye prevrashcheniya v uglerode i nitride bora (Phase Transi- 254 3. 4. 5. 6. BUKALOV et al. tions in Carbon and in Boron Nitride), Kiev: Naukova Dumka, 1979, p. 235. Ubbelode, A.R. and L’yuis, F.A., Grafit i ego kristallicheskie soedineniya (TRaNSl), Moscow: Mir, 1965, p. 356. Raman Microscopy. 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