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J. Phys. Chem. C 2007, 111, 2514-2519
Ferrocene Catalyzed Carbon Nanotube Formation in Carbonaceous Solid Teddy M. Keller,*,† Matthew Laskoski,† and Syed B. Qadri‡ Chemistry DiVision, Code 6127, and Materials Science and Technology DiVision, Code 6364, NaVal Research Laboratory, Washington, DC 20375 ReceiVed: October 5, 2006; In Final Form: NoVember 21, 2006
Carbon nanotubes are formed in a carbonaceous solid from thermal decomposition of various amounts of 1,4-diferrocenylbutadiyne in the presence of an excess amount of a multi(ethynyl)aromatic compound. Only a small amount of the ferrocenyl compound is needed to achieve the formation of carbon nanotubes in high yield. The method described here permits the large-scale production of carbon nanotubes in a shaped, solid configuration. The carbon nanotubes form under atmospheric pressure during the carbonization process above 500 °C in the carbonaceous solid. The Fe atoms, nanoclusters, and/or nanoparticles formed from the decomposition of the 1,4-diferrocenylbutadiyne are the key to the formation of the carbon nanotubes in the developing carbonaceous solid by reacting with the developing polycondensed aromatic ring system. The carbonaceous solids were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and scanning electron microscopy studies.
Introduction Carbon nanotubes (CNTs) have attracted considerable attention in both the scientific and technological communities and is driving research in several important directions due to their unique magnetic,1,2 electrical,3,4 optical,5 and mechanical6 properties. Many applications have been proposed ranging from polymeric structural composites7-10 to nanoelectronic devices,11,12 which would benefit from the availability of CNTs of varying diameters and helicities and in large quantities. CNTs with a variety of diameters are advantageous for testing many theoretical predictions and for practical applications needing a range of sizes. These potential applications depend on the development of a low-cost and readily scalable route to CNTs. CNTs are currently synthesized in high yield but in limited quantities by chemical vapor deposition.13-19 Metal nanoparticles are essential for the growth of CNTs and act as catalysts for their formation. The size and shape of the metal particle play a key role in the CNT diameter.20-22 The use of an organometallic compound as a source of metal atoms and nanoparticles is very attractive as precursors to the formation of MWNTs in the solid state.23 Our efforts have focused on developing polymeric precursors containing transition metals and the thermal conversion at elevated temperatures under atmospheric pressure into a shaped CNT-containing carbonaceous solid during the carbonization process. Organometallic compounds that decompose at fairly low temperatures to produce metal atoms and clusters have been found to be a good source of metal in the synthesis of MWNTs in a bulk solid.24,25 Only a small or catalytic amount of metal source is needed for the synthesis and production of CNTs by this chemical method. The interaction of the metal atoms early in the carbonization process with the developing fused aromatic ring systems of the precursor carbon source appears to be important in the formation of the CNTs. The size and concen* Corresponding author. E-mail:
[email protected]. † Chemistry Division. ‡ Materials Science and Technology Division.
tration of metal species can be easily varied as a function of the quantity of organometallic compound relative to the carbon precursor material. The overall physical properties of the turbostratic carbonaceous solid will depend on the size and concentration of the CNTs and the metal species. In this paper, a potentially inexpensive and efficient chemical method is described for producing large quantities of MWNTs in high yield from heat treatment of various mixtures of 1,4diferrocenylbutadiyne 1 and 1,2,4,5-tetrakis(phenylethynyl)benzene 2. Large quantities of CNTs can be readily produced in the carbonaceous solid under ambient pressure at elevated temperatures. Experimental Details 1,4-Diferrocenylbutadiyne 1 and 1,2,4,5-tetrakis(phenylethynyl)benzene 2 were prepared according to published procedures.26,27 Thermogravimetric (TGA)-differential thermal (DTA) analyses were performed on a TA SDT 2960 Simultaneous DTA-TGA module equipped with a TA 3100 thermal analyzer. All thermal analyses were performed using heating rates of 10 °C min-1 under a nitrogen atmosphere with flow rates of 100 cm3 min-1. X-ray diffraction analyses were performed using a Rigaku 18 kW X-ray generator and a high-resolution powder diffractometer. X-ray diffraction scans of the samples were measured using Cu KR radiation from a rotating anode X-ray source. Raman spectroscopic studies were carried out at roomtemperature using a Renishaw Raman Spectrometer equipped with a 514.5 nm argon laser. Transmission electron microscopy (TEM) was performed on a Hitachi H-8100 electron microscope at 200 kV. Scanning electron microscopy (SEM) was performed using a LEO 1550 electron microscope. Preparation of Various Mixtures of 1 and 2. Mixtures of 1 and 2 were mixed using a mortar and pestle resulting in an orange composition. Various mixtures (1:99, 10:90, 25:75, and 50:50 molar percent) of 1 and 2 were prepared and used in the studies. Upon heating above 225 °C, the various mixtures would melt and were thoroughly mixed by stirring. Further heating of
10.1021/jp0665527 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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Figure 1. Structures of 1 and 2. Figure 3. TGA (top) and DTA (bottom) thermograms of two precursor mixtures containing 10:90 and 50:50 molar concentrations of 1 and 2 heated to 1000 °C in a flow of nitrogen.
Figure 2. TGA (top) and DTA (bottom) thermograms of 1 and 2 heated to 1000 °C in a flow of nitrogen.
the mixtures for an extended period resulted in solidification or conversion to a networked system. Conversion of 1 and 2 to CNT/Fe Nanoparticle-Containing Carbonaceous Solid. Various mixtures of 1 and 2 were melted and heated at temperatures up to 1400 °C under an inert atmosphere resulting in the formation of CNT-Fe nanoparticle carbonaceous solids. The various carbonaceous solid residues were used for our studies. At any given temperature, the yield of CNTs within the carbonaceous solid was dependent on the pyrolytic exposure time. Preparation of Samples for TEM studies. Samples of 25: 75 molar amounts of 1 and 2 were dissolved in a minimal amount of CH2Cl2 and dropped onto a clean silicon wafer to form a thin film before heating at 10 °C min-1 to 1000 °C and holding for 1h. After cooling, the thin films, which were present on the silicon surface, were suspended in ethanol and dropped onto holey carbon grids for TEM studies. Thicker solid samples (30-50 mg) of the various molar precursor compositions were placed on a silicon wafer, heated to 1000 and 1300 °C, and held for 1 h at each temperature yielding a small carbonaceous solid. After the solid sample was ground in a mortar and pestle, the powder was transferred to a beaker, 5 mL of acetone was added, and the mixture was sonicated in an ultrasonic processor (Sonics and Materials, 40% amplitude) for 1 h at ambient temperature. The sample was solution deposited onto a holey carbon grid for TEM studies. Results and Discussion Metal nanoparticles are essential for the growth of CNTs and their size and chemical composition determine the diameter of the nanotubes.28,29 In this study, 1,4-diferrocenylbutadiyne 1 and 1,2,4,5-tetrakis(phenylethynyl)benzene 2 (Figure 1) were used as the source of Fe atoms/nanospecies and carbon, respectively.
Figure 4. X-ray diffraction scans of 2 and a 50:50 molar sample of 1 and 2 heated to 1000 °C.
Compound 2 was chosen as the primary carbon source due to the inherent symmetry within the structure, high char yield on exposure to elevated temperatures, and potentially easy pathway to the formation of CNTs. Heat treatment of pure 2 to 1000 °C did not result in the formation of CNTs. Ferrocene-based compound 1 was used in our studies because of its stability under ambient conditions and nonvolatility at 300 °C. When heated between 300 and 500 °C, ferrocene is known to decompose leading initially to the formation of Fe atoms.30,31 The ability to control the amount of Fe incorporated into the precursor mixture, 1 and 2, will result in the formation of the Fe atoms uniformly dispersed in the resulting crosslinked polymeric solid domain. Shaped components can be readily formulated from the melt or liquid state of the nanotube precursor mixtures, 1 and 2, above 200 °C. The mixtures are thoroughly mixed and heated for an extended period resulting in gelation and solidification to a shaped configuration. Upon gelation, the diffusive properties and mobility of the individual Fe atoms, clusters, and/or nanoparticles in the polymerizing matrix will be reduced. The Fe atoms that initially form from the degradation of the ferrocene units interact with the π-electrons of the aromatic groups within the polymeric compositions. As the polymeric mixture is heated above 500 °C, conversion to CNTs commences to form during the carbonization process within the carbonaceous solid. During the pyrolysis, a progressive increase in the formation of polycondensed ring structures occurs, resulting in the formation
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Figure 5. X-ray diffraction scans of various molar samples of 1 and 2 heated to 1000 °C.
TABLE 1: Fe Nanoparticle Sizes in Various Molar Concentrations of 1 and 2 Heated at 10 °C min-1 to 1000 °C and Cooled molar conc. 50:50 25:75 10:90 1:99
Fe nanoparticle size (nm) 13.8 13.0 11.0 _
of carbon nanoparticles from pairing of free radicals.32,33 The developing polycondensed ring structures and/or carbon nanoparticles interact with the Fe species to initiate the CNT growth process23 in the carbonaceous solid. The CNTs are formed in a non-ordered array within the carbonaceous solid domain. Only a small concentration of 1, which is the source of the Fe nanospecies, is needed to achieve conversion of the carbonaceous solid to MWNTs. Different combinations of 1 and 2, changes in the pyrolytic temperature, and time of thermal exposure at elevated temperatures produce CNT-Fe turbostratic carbonaceous solids with different and potentially tailored physical properties. The size and concentration of the Fe nanospecies in the developing CNT carbonaceous solid depend on the initial concentration of 1 relative to 2 and the pyrolytic temperature. Thermogravimetric (TGA)-differential thermal (DTA) analyses were used to study the thermal transitions (exothermic and endothermic) of 1, 2, and two mixtures containing 10:90 and 50:50 molar concentrations of 1 and 2 upon heat treatment at 10 °C min-1 to 1000 °C under a flow of nitrogen (Figures 2 and 3). The DTA thermogram for pure 1 displayed a broad exotherm from approximately 200 to 400 °C attributed to the reaction of the acetylenic unit to a thermoset and decomposition of the ferrocene unit. An exotherm commencing above 600 °C and peaking at about 750 °C was assigned to the formation of CNTs.25 Similarly for 2, endothermic and exothermic transitions were observed peaking at about 196 and 295 °C, respectively, for the melting point and reaction of the phenylethynyl units to a networked polymeric system. The 10:90 and 50:50 molar mixtures (Figure 3) showed the previously observed endothermic and exothermic transitions between 150 and 350 °C. The exothermic peak responsible for the formation of CNTs during the carbonization process above 600 °C was more intense and occurred at a lower temperature for the 50:50 molar mixture relative to the 10:90 molar mixture. The exothermic transition peaked at about 815 and 750 °C for the 10:90 and 50:50 mixtures of 1 and 2, respectively. These results demonstrated
Figure 6. Raman spectrum of a 50:50 molar sample of 1 and 2 heated at 10 °C min-1 to (A) 1000 and (B) 1400 °C and held isothermally for 30 min.
the importance of the concentration of Fe toward the formation of the CNTs within the developing carbonaceous solid. Figure 4 shows the XRD scan of the base material 2 and a 50:50 molar mixture of 1 and 2 that had been heated to 1000 °C. For the base material 2, there were two broad peaks characteristic of amorphous material. For the 50:50 molar sample, peaks from MWNTs, Fe3C, and Fe nanoparticles were observed. The grain size of Fe nanoparticles was estimated to be about 14 nm as determined from the full width at halfmaximum (fwhm) of Fe (110) Bragg peak (2θ ) 44.67) using Scherrer’s formula34 to calculate the particle size. Room-temperature X-ray diffraction (XRD) studies were initiated in an attempt to understand the growth process of the CNTs within the carbonaceous solid. An XRD scan of a 50:50 molar mixture that had been heated at 500 °C for 4 h showed the presence of peaks from the Fe3C phase. Samples that had been heated to 700 °C for various exposure times showed both CNTs and Fe3C in the carbonaceous solid. Since the strong peak from the bcc Fe at 44.43 overlaps with the peak of Fe3C, the presence of Fe nanoparticles cannot be ruled out in the carbonaceous sample. As the exposure time of the molar mixture at 700 °C was increased, the crystalline CNT peak (002) at 25.95 grew in intensity while the peaks attributed to the Fe3C at 37.66 diminished. Other carbonaceous samples that had been heated from 700 to 1000 °C for various lengths of time showed various concentrations of Fe3C, CNTs, and Fe nanoparticles. A 50:50 molar sample that was slowly heated to 1000 °C at a rate of 1 °C min-1 showed an abundant amount of nanotubes and a small quantity of Fe3C. When a sample was heated to 1400 °C and held for 1 h, the Fe3C phase was absence in the XRD plot leaving only peaks attributed to CNTs and bcc Fe nanoparticles.
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Figure 7. TEM images for a 25:75 molar sample of 1 and 2 heated to 1000 °C for 1 h: (A) low-resolution images showing MWNTs and (B-D) high-resolution images showing the turbostratic carbon pattern.
A comparison of the XRD spectra for different molar mixtures of 1 and 2 heated at 10 °C min-1 to 1000 °C is shown in Figure 5. A greater amount of Fe3C and Fe nanoparticles was present in the carbonaceous solid with increasing concentration of 1 in the precursor mixtures. A 1:99 molar sample showed basically the pattern for MWNT formation. The peaks at 25.95, 43.01, 53.34 and 78.80 degrees (2θ) were attributed to the structure of MWNTs.35 The d-spacing or intertubular spacing between the concentrically stacked sp2 carbon layers is 3.43 Å compared to the interlayer spacing of 3.35 Å for graphite.36,37 In essence, a 1 molar percent of 1 was sufficient as a source of Fe nanoparticles for the large scale formation of CNTs in the carbonaceous solid. The average size of the Fe nanoparticles found in the turbostratic carbonaceous solids from heating various molar mixture of 1 and 2 to 1000 °C is shown in Table 1. Raman studies were performed using a 514.5 nm argon-ion laser on the surface of the CNT-containing carbonaceous solids. The temperature induced growth of MWNTs in the bulk carbonaceous solid samples was analyzed in terms of the width and relative intensity of the D and G peaks (first-order Raman), which lie at about 1347 and 1575 cm-1, respectively, and the second-order scattering peaks between 2450 and 3250 cm-1.38-42 The rate of temperature increase and the exposure time at a given temperature appeared to influence the carbon order within the solid carbonaceous residue. It was observed that the D, G, and second-order peaks became sharper as the sample exposure time at a given temperature was increased indicating lower levels of carbon impurities and an enhancement in the graphitic crystallinity. As the thermal treatment was increased to higher temperatures, a sharper, more intense G peak and a reduction
in the intensity of the D peak were observed. Figure 6 shows the Raman spectrum of a 50:50 molar concentration of 1 and 2 heated to 1000 and at 1400 °C (30 min), respectively. The sample heated at 1400 °C showed a weak, sharp D-band and sharp intense peaks for the G-band and second-order peaks. TEM studies were performed on carbonaceous solids obtained from pyrolysis of 25:75 molar samples of 1 and 2 heated at 1000 °C on a silicon wafer (Figure 7). MWNTs were found to exist in abundance throughout a thin carbonaceous film. At low magnification (Figure 7A), the MWNTs were visible as a tangled mass of tube-shaped structures within the thin film. The TEM image also showed Fe nanoparticles as dark spots embedded within the nanotube aggregates. Similar images were obtained on thin films formulated from 10:90 and 50:50 molar mixtures of 1 and 2 heated at 1000 °C. At higher magnification (7B-D) on a MWNT within the turbostratic carbon film, graphitic layered structures with a spacing attributed to the MWNTs were observed. Figure 7B shows a MWNT with the Fe nanoparticle situated inside of the tubular structure. The graphite-like spacing of the MWNT walls is clearly discernible and shows the underlying texture formed by the concentric layers of the MWNT walls. The interlayer spacing was measured to be 0.34 nm, which was consistent with the d-spacing found from XRD analysis. Scanning electron microscopy (SEM) studies were performed on films deposited on a silicon wafer (Figure 8). A 1:99 molar concentration of 1 and 2 was thoroughly mixed by heating to the melting point (∼200 °C) and degassing under reduced pressure for 30 min to remove any volatile residue. A portion of the uncured solid material was placed on a silicon wafer, melted to a film, and slowly heated under inert conditions
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Figure 8. Low-resolution SEM images of film deposited on silicon wafer by heating 1:99 molar sample of 1 and 2 at 0.4 °C min-1 to 1000 °C and holding isothermally for 1 h followed by slowly cooling to ambient temperature.
Figure 9. CNT-containing fiber formulated from 10:90 molar concentration of 1 and 2 heated to 1000 °C.
(argon) to 1000 °C. The low-resolution SEM images of the film showed MWNTs that had propagated in all directions in an isotropic array on the surface. Copious amounts of CNT bundles were apparent throughout the film. It appeared that some of the bundles are interconnected through the growing CNTs (Figure 8A). MWNTs of various shapes and form are readily apparent in Figure 8B with the majority of tubes being between 20 and 200 nm in diameter. In an effort to develop a method for the fabrication of CNT carbonaceous fibers, studies were initiated to draw fibers from the melt of 1 and 2. There is a great deal of interest in the dispersion of MWNTs into polymeric fibers, which show improvements in their mechanical properties.7-10 Carbon nanofibers have been recently produced from the pyrolysis of electrospun nanofibers from PAN43 and from pitch.44 Thus, it would be desirable to spin polymeric fibers that could be directly converted into MWNT-containing carbon fibers during the carbonization process. In our preliminary studies, fibers were drawn from the melt of various concentrations of 1 and 2 using a glass rod. The melt of the precursor mixture was heated to increase the viscosity before drawing the fibers. The fibers were pulled from the melt at 350 °C before gelation had occurred. Upon cooling, the solid fibers were then heated at 225 °C for 12 h under an air atmosphere so the fibers would retain their shape upon carbonization. The cured fibers were then placed in an open small quartz tube and heated at 1 °C min-1 to 1000 °C and held for 2 h. The CNT fibers were slowly cooled at 1 °C min-1 to room temperature. Thus, by controlling the viscosity and with the proper spinning equipment, fibers can be obtained from the melt of various compositions of 1 and 2 and converted during the carbonization process into MWNT-containing carbonaceous fibers (Figure 9).
varying amounts of Fe3C and Fe nanoparticles can be readily fabricated from the precursor mixtures. Considerable amount of Fe3C were present in the charred solid residue especially on thermal exposures for short times. Fe3C was found to decompose yielding CNTs and Fe nanoparticles. At any given time, the amount of Fe3C and Fe nanoparticles within the carbonaceous solid depends on the carbonization temperature and exposure time. The carbonaceous solid can be tailored to have mainly CNTs or varying amounts of CNTs and Fe-containing nanoparticles. Within the carbonaceous samples, the CNTs appear to be more uniformed as a function of higher processing temperatures and/or longer dwell times at a given temperature. The ability to produce large amounts of MWNTs and to control the size and concentration of Fe nanospecies in the carbonaceous solid composition is an important achievement for the formation and production of CNTs. The limited availability of pure CNTs has restricted the development of new technologies. Large quantities of MWNTs can be prepared in any shape or form by the described method. The CNT carbonaceous solid residue can be tailored to have only a small amount of Fe nanoparticles (
