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J. Phys. Chem. C 2007, 111, 8414-8417
ARTICLES Rhenium-Catalyzed Growth Carbon Nanotubes Manfred Ritschel, Albrecht Leonhardt,* Dieter Elefant, Steffen Oswald, and Bernd Bu1 chner Leibniz-Institute for Solid State and Materials Research Dresden, P.O. Box 27 00 16, D-01171 Dresden, Germany ReceiVed: January 19, 2007; In Final Form: April 18, 2007
Using a modified “fixed-bed” chemical vapor deposition method, diamagnetic single-, double-, and multiwalled carbon nanotubes were synthesized. For the first time, we show that rhenium is a suitable catalyst for the synthesis of such diamagnetic nanotubes with uniform diameter and a defined numbers of shells. Scanning and transmission electron microscopy investigations, Raman spectroscopy, and magnetic measurements show the tubular structure, the high crystallinity, and the diamagnetic character of the grown nanotubes. We compare the growth mechanism of the Re-catalyzed nanotubes with the growth behavior supported by the conventional catalyst metals ferromagnetic iron, cobalt, and nickel.
Introduction For a chemical vapor deposition (CVD) process for synthesizing carbon nanotubes, the selected catalyst material has an essential influence on the properties of the deposited products. Various transition metals have been investigated, but only the ferromagnetic metals iron, cobalt, nickel, or their alloys show the highest activity for the catalytical growth of carbon nanotubes. As a precursor for the synthesis process, metal organic compounds, which by their thermal decomposition deliver both the catalyst material and the carbon feedstock for the tube growth, are frequently used. However, solid catalyst particles that are deposited onto different supports also can be used for the growth of carbon nanotubes. These catalysts are mainly prepared by the impregnation of materials such as silica, alumina, or zeolites with solutions of metal salts followed by their transformation into oxides or metals by subsequent oxidation and reduction processes. Another preparation method for solid catalyst particles is the deposition of thin metal films onto a substrate by the evaporation or sputter techniques. In detail, the catalysts are described in the literature as Fe,1 Mo,2,3 Co,4 and Pdp;5 the binary catalysts are Co/Ni,2 Fe/Mo,3,6 Fe/ Co,4 and Fe/Ru6 on alumina; Fe,7 Co,8,9 Ni,10 Ru,11 and Co/ Mo12 on silica; and Fe, Co, Fe/Co on CaO.13 Further catalyst/ support systems are Fe/Co, Co/Mo, Co/V, Rh/Pd on zeolites14,15 and Fe, Co, Ni, and their alloys (and partially Mo as an additional promoter) on MgAl2O416 and MgO.17-20 The results of carbon nanotube growth on thin metal films are performed by Fe and Co,21 Ni,22 Pd,23 and Fe/Ru and Fe/Pt24 on SiO2/Si, Co, Fe/Mo, Ni on Al,25,26 and Pd on Ti/Si.27 Among other difficulties relating to these metals, the existence of residual ferromagnetic catalyst particles in carbon nanotubes is the main obstacle for research on the magnetic properties of carbon nanotubes in the as-grown state. * To whom correspondence should be addressed. E-mail:
[email protected].
In the present work, we demonstrate for the first time that rhenium is an effective catalyst for the synthesis of tubular carbon nanotubes using the CVD method with methane (CH4) as the carbon feedstock and with the fixed-bed technology. The obtained carbon nanotubes were characterized using scanning and transmission electron microscopy, (SEM, TEM) Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), measurement of the specific surface area (using the BrunauerEmmett-Teller (BET) isotherm), and magnetometry. Experimental Procedures The Re-supported MgO catalyst was prepared by wet mechanical mixing followed by a gas-producing combustion reaction28 with citric acid as a foaming and combustion additive.19 Ammoniumperrhenate (NH4ReO4) and magnesium nitrate ((Mg(NO3)2 × 6H2O), which were mixed in different molar ratios (between 1 and 10), were dissolved in deionized water that contained a small amount of citric acid. This mixture was transferred directly into the preheated zone of a furnace (560 °C) where it was ignited and spontaneously burned. The reaction was accompanied by a large and strong release of different gases, which varied with the citric acid content. The total combustion process was finished after 10 min of exposure to result in a uniform, foamy material with a relatively high specific surface area (∼80 m2/g). The synthesis of carbon nanotubes was carried out in a socalled fixed-bed reactor consisting of a furnace with a quartz tube inside (diameter 40 mm). For the synthesis, a quartz boat containing the prepared catalyst material was placed in the hot zone of the horizontal reactor tube. At first, the reactor was exposed to a flow of Ar (∼250 standard cubic centimeters per minute (sccm)) to remove the oxidizing atmosphere; afterward, the catalyst reduction was performed at 650 °C for 30 min in a hydrogen medium (∼150 sccm). The temperature was increased by 6 °C/min, up to the desired growth temperature between 950 and 1100 °C, during the injection of CH4 into the reactor. The
10.1021/jp070467x CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
Rhenium-Catalyzed Growth Carbon Nanotubes
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Figure 1. SEM image of Re-catalyzed carbon nanotubes.
CH4 flow was stopped after the temperature had been maintained for 10 min. Finally, the furnace was cooled to 300 °C in a flow of hydrogen, and further cooling to room temperature was done under a flow of Ar. For the purification, the as-grown products were sonicated in nitric acid (HNO3) for 2 h at room temperature. They were then filtered and washed with deionized water and dried at 110 °C for several hours. SEM was performed on a FEI NOVA NANOSEM 200 with an accelerating voltage of 10 kV. TEM was carried out on a FEI Tecnei F30 that was operated with an accelerating voltage of 300 kV. Both microscopes were equipped with an energydispersive X-ray (EDX) detector for chemical element analysis. The Raman spectra were recorded with a Bruker FT-Raman spectrometer for the 1.16 eV (1064 nm) excitation. The magnetic properties were measured with a SQUID magnetometer (MPSM XL, Quantum Design). The XPS measurements were done on a PHI 5600Cl system (Physical Electronics) using monochromatized Al-KR X-rays.
Figure 2. TEM image of Re-catalyzed carbon nanotubes. MWNTs are dominantwith an outer diameter of 5-8 nm.
Results and Discussion Carbon nanotubes were successfully synthesized on the surface of Re-catalyzed MgO-catalyst material. Figure 1 shows a representative SEM image of such carbon nanotubes after the purification process. Only a wool-like network of nanotubes of high purity are visible, and nontube material is absent. EDX spectra also confirm that the MgO is totally dissolved by the wash in HNO3 (no Mg is detectable). Furthermore, because of the acid treatment, free Re-catalyst particles are also not visible in Figure 1. BET measurements resulted in an average value of 700 m2/g. This relatively high value indicates that this network of nanotubes is a mixture of single-, double-, and multiwalled structures. The TEM image at low magnification (Figure 2) shows multiwalled carbon nanotubes (MWNTs) with an average diameter of 5-8 nm, and the number of graphene shells is 4-7. These geometric dimensions are relatively independent of the synthesis temperature. As compared to Fe-, Co-, or Ni-supported MWNTs, the interior diameters are unusually small, and they vary between 0.7 and 1.5 nm (Figure 3). Moreover, individual double-walled carbon nanotubes (DWNTs) and bundles of single-walled carbon nanotubes (SWNTs) can be often observed. Their presence in the synthesized material is also clearly confirmed by Raman spectroscopy (Figure 4). The radial
Figure 3. The high-resolution TEM image of Re-catalyzed carbon nanotubes shows (left side of image) a bundle mixture of SWNTs and DWNTs as well as (right side of image) a MWNT with an interior diameter of 1.1 nm and an outer diameter of 3.8 nm.
breathing modes (RBM) region is given in the inset of Figure 4. The tube diameters were calculated using the equation d ) 234/ωRBM,29 where d is the tube diameter and ωRBM is the RBM frequency. The peak marked “D-band” in the resonance spectrum corresponds to defects in the shell structure and to a residue of amorphous carbon in the as-grown, acid-washed material. The high-resolution TEM image (Figure 3) shows traces of such shell defects and amorphous deposits on the surface of the MWNT that were formed after tube growth by further thermal decomposition of the hydrocarbon. Figure 5 illustrates an typical EDX spectrum, with elemental analysis according to the area of the corresponding HRTEM image, of an individual carbon nanotube with a Re-particle at
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Figure 4. The Raman spectrum of Re-catalyzed carbon nanotubes shows a mixture of SWNTs, DWNTs, and MWNTs; the width and hight of the D-peak represents the presence of shell structure defects and of traces of amorphous carbon in the as-grown, acid-washed material. The inset shows the calculated tube diameters using the equation d ) 234/ωRBM, where d is the tube diameter (nm) and ωRBM is the RBM frequency (cm-1).
Figure 5. Scanning TEM image of an individual carbon nanotube with an Re particle at its end; the Cu signal results from the TEM grid.
its end. The tubes consist only of carbon (primarily in the shell) and rhenium (as catalyst particle). The Cu signal results from the TEM-grid, and no other element is detectable. A comparison of Re- and Fe-catalyzed carbon nanotubes analyzed by XPS reveals an equivalent peak shape of the C1s-spectra, which points to similar C-C binding states in both samples (Figure 6). Therefore, it follows that both of the differently grown carbon nanotubes have essentially the same surface constitution. Additionally, XPS measurements have shown that, besides carbon, no other elements or compounds are significantly adsorbed onto the tube surface. Magnetization measurements showed ferromagnetic impurities to be lower than the detection limit. An orientation-averaged diamagnetic susceptibility of χ ) -(3.5 ( 0.3) × 10-6 emu/g was estimated for T ) 300 K in fields up to µ0H ) 1 T on a sample with mass m ) 0.6 mg and consisting of multi-, double-, and single-walled carbon nanotubes. This susceptibility value is within the published range χ ) (2.5-10) × 10-6 emu/g at T ) 300 K for mixtures of carbon nanotubes with different diameters,30-33 which have been synthesized by the arc-
Ritschel et al.
Figure 6. XPS spectra of Re- (solid line) and Fe-supported (dashed line) carbon nanotubes; the C1s-spectra show the same geometry.
discharge method and with an unverifiable amount of Fe inside the nanotubes. These properties of the Re-supported carbon nanotubes, especially the diamagnetism in the as-grown state, open new fields of application. For instance, such nanotubes can be filled with magnetic sensitive materials (e.g., molecular magnets). They are also suitable and accessible for applications as nanocontainers in such cases where very weak ferromagnetism, caused by the enclosed Fe-catalyst, is disturbing. The growth of carbon nanotubes supported by a Re-catalyst is surprising because the solubility of carbon in rhenium is extremely low and because, in general, the growth mechanism of carbon nanotubes is explained by a dissociation-diffusionprecipitation process where elemental carbon is formed on the surface of a metal particle followed by diffusion and crystallization as cylindrical graphite carbon. For such a vapor-liquidsolid mechanism with molten or liquid-like metal catalysts, a finite solubility is necessary. Therefore, the transition metals Fe, Co, or Ni are the most commonly used catalysts for carbon nanotube growth, because the phase diagrams of carbon-(Fe, Co, or Ni) display such finite solubilities of carbon in these metals at higher temperatures. The solubility of carbon in Re is lower that 1% w/w, and the formation of rhenium carbide is not known.34 Therefore, for the growth of Re-supported carbon nanotubes, this classical growth mechanism is not valid and other approaches are required. Perhaps dissolution of the carbon into the catalyst nanoparticle is not needed, but the carbon is transported on the particle only via a surface diffusion to the growth front of the carbon shell. Summary We have synthesized single-, double-, and multiwalled carbon nanotubes without ferromagnetic catalysts. These Re-supported carbon nanotubes show a typical diamagnetic behavior. To the best of our knowledge, this work is the first to report on the bulk production of carbon nanotubes using rhenium as a catalyst. On the basis of the extremely small solubility of carbon in rhenium and on the nonexistence of rhenium carbide, a new growth mechanism of carbon nanotubes has to be discussed. Such studies are ongoing in our group. Acknowledgment. Special thanks to G. Kreutzer and S. Pichl for carrying out electron microscopy and to J. Liebich for the Raman measurements.
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