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Fabrication of Discrete Nanosized Cobalt Particles Encapsulated Inside Single-Walled Carbon Nanotubes Codruta Zoican Loebick, Magdalena Majewska, Fang Ren, Gary L. Haller, and Lisa D. Pfefferle* Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06511 ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: May 12, 2010
Single-walled carbon nanotubes (SWNT) with encapsulated nanosized cobalt particles have been synthesized by a facile and scalable method. In this approach, SWNT were filled with a cobalt acetylacetonate solution in dichloromethane by ultrasonication. In a second step, exposure to hydrogen at different temperatures released discrete cobalt particles of controllable size inside the SWNT cavity. The SWNT-Co particles systems were characterized by transmission electron microscopy, X-ray absorption spectroscopy, Raman spectroscopy, and thermal gravimetric analysis. 1. Introduction The unique electronic and structural properties of singlewalled carbon nanotubes (SWNT) make them attractive for a wide range of applications in fields ranging from catalysis to bioengineering.1-3 In particular, due to their one-dimensional hollow structure, nanotubes (both single and multiwall) are excellent candidates for the confinement of various other atoms, molecules, or compounds.4-6 Multiple applications of carbon nanotubes encapsulating metal particles or rods have been proposed. Carbon nanotubes encapsulating small catalytic particles were shown to act as nanosized reactors for enhanced ethanol production from syngas.7,8 The same types of materials were studied for lowtemperature fuel cells9 or room temperature methane detection.10 Other authors have shown iron-filled nanotubes to be excellent candidates for bio applications such as cancer treatment.11 Typically carbon nanotubes encapsulating metals or oxides can be synthesized by one of three methods: heating a mixture of carbon nanotubes and metal or metal salts above the melting temperature of the filling material under high vacuum,12,13 simultaneous opening and filling by refluxing nanotubes in a solution of metal precursor in concentrated acid (typically nitric),14 or incipient wetness impregnation.15 All these methods have proved to be very successful in depositing materials inside the cavity of multiwalled carbon nanotubes (MWNT) which have an inner diameter of around 5-20 nm. Single-walled carbon nanotubes (SWNT) have a smaller diameter of around 0.5-2 nm. This can lead to difficulties in introducing various filling materials inside their cavity due to size limitations.16-19 Several authors have shown nanowire or particle formation inside SWNT by heating metal precursor and nanotube mixtures under high vacuum.16,18 However, no precise control over the size of wires and particles size distribution was achieved. For example, Govindraj and co-workers have shown that a large portion of the filling material forms large particles on the outer surface of the nanotubes.18 Moreover, as shown by Sloan and co-workers,16 only wide bore SWNT (around 2 nm) were successfully filled, suggesting that there exists a limit of SWNT diameter under which confinement or surface tension effects * Corresponding author. E-mail:
[email protected].
prevent materials from filling the inside of the cavity of the nanotube. Direct growth of SWNT with encapsulated metal particles was also shown to be possible by CO decomposition over catalytic iron particles.19 However, a high density of large particles (over 10 nm) encapsulated in layers of graphite were obtained as well. Li and co-workers have successfully encapsulated iron molecules into SWNT by a two-step method consisting of heating a SWNT ferrocene mixture under vacuum followed by flash annealing.20 Both discrete and chainlike particles were imaged by TEM inside the SWNT with no apparent control over their distribution. Furthermore, as in the case of other studies, only the largest SWNT in the sample appeared to be susceptible to filling. Semiconducting SWNT show an inverse proportionality of band gap to their diameter. Because of this dependence, the synthesis of small diameter nanotubes is desired for many electronic applications such as field effect transistors.21,22 As opposed to MWNT, SWNT have all their constituent atoms on the surface such that any interaction which takes place (for example with a metal particle) can be sensitively reflected in the electronic properties of the carbon nanotube. Furthermore, it was shown that SWNT decorated with metal particles can be exploited to tailor either the transfer characteristics of nanoscale devices or to tune the sensitivity of nanotube-based chemical and biological nanosensors.23 It is therefore important to establish routes for decorating the inner cavity of smaller diameter SWNT with metallic particles. We present a simple and scalable method by which small, narrow size distributed Co nanoparticles can be deposited inside the pores of SWNT by ultrasonication in a metal precursor solution followed by exposure to hydrogen. Moreover, by varying the temperature at which the hydrogen exposure takes place, a certain degree of control over the size and shape of these particles can be achieved. Cobalt was chosen for this study; however, this method can be easily extended to other materials such as Ni or Fe. 2. Experimental Section Pristine SWNT were purchased from Cheap Tubes Inc. The SWNT had a purity of 97%, a diameter distribution of 0.9-2
10.1021/jp1026515 2010 American Chemical Society Published on Web 06/08/2010
Cobalt Particles Encapsulated Inside SWNT nm, and an average length of 5-10 µm (this information was provided by the vendor and is consistent with our Raman and TEM measurements). We have performed TEM measurements of more than 500 tubes and found the average diameter to be between 0.6 and 2.5 nm with most tubes having diameters between 1 and 1.6 nm. Subsequent treatment with a 5 M solution of nitric acid at 60 °C for 20 h was performed. This procedure was employed for opening any closed ends of the SWNT. After nitric acid treatment, the SWNT were annealed in Ar at 900 °C for 5 h in order to remove any oxygen-containing groups from the surface. The oxygen-containing groups which can result from the nitric acid treatment can anchor metal particles on the tube external surface. The SWNT after annealing were imaged using a Phillips Tecnai 12 transmission electron microscope. Prior to imaging, the SWNT were dispersed in ethanol using a sonicating bath and added dropwise to a holey carbon grid. This procedure has been employed for all TEM analysis presented in this study. One hundred mg of the as-annealed SWNT was added to a solution of 10 g/L cobalt(II) acetylacetonate in dichloromethane. As shown by Panpranot and co-workers,24 use of organic precursors such as acetate and acetylacetonate as precursor for metallic cobalt particles on various substrates by reduction under hydrogen leads to smaller, better dispersed particles than inorganic precursors. The solvent was chosen because it has a small molecule and more importantly has a low surface tension (around 28 mN/m). The capillarity of nanotubes is directly related to the surface energies of interaction between the liquid and the solid surface of the nanotube and is therefore a problem of wetting. Low surface tension solvents such as the one used in this study are ideal for wetting the inner surface of the nanotubes.25 The nitric acid-treated and Ar-annealed SWNT-containing solution was placed in an ice bath and continuously sonicated for 6 h using a probe sonicator at 40 W power. After sonication, the sample was filtered, washed with dichloromethane, and dried for 12 h at 70 °C. Around 20 mg of the sample was loaded in a quartz reactor and placed in a ceramic fiber radiant heater connected to gas lines. The temperature was brought to the desired reduction temperature under flowing argon (200 cm3/min), and then the flow was switched to pure hydrogen at the same flow rate. The sample was maintained at the reduction temperature under hydrogen for 20 min. Three different experiments were carried out at hydrogen exposure temperatures of 300, 400, and 500 °C. Each sample was imaged by TEM. Extended X-ray absorption fine structure (EXAFS)-X-ray absorption measurements were performed at the Co-K edge. The experiments were carried out on line X18B at the NSLS, 2.5 GeV storage ring, Brookhaven National Laboratory. Samples were pressed into self-supporting wafers and placed in a stainless steel cell equipped with Be windows, a gas inlet and outlet, and a heating unit allowing in situ gas treatments. Pellets of SWNT-Co acetylacetonate were heated to the desired reduction temperature under flowing helium and then reduced isothermally for 20 min with pure hydrogen while continuously monitoring by XAS. Details of the experimental procedure are given elsewhere.26 Raman spectra of these samples as well as the pristine SWNT were collected on a JASCO Raman spectrophotometer NRS3000 Series at a 532 nm laser excitation wavelength. Each spectrum represents an average of five measurements taken on different points of the sample.
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Figure 1. TEM images of the pristine SWNT after nitric acid treatment and annealing in argon at 900 °C.
3. Results and Discussion TEM images of the pristine SWNT after treatment with nitric acid and annealing in Ar at 900 °C are shown in Figure 1. Figure 2a-c shows images collected for the same sample after sonication in the cobalt acetylacetonate solution and exposure to hydrogen at 300 (a), 400 (b), and 500 °C (c). The TEM images of the as-annealed SWNT show tubes of several micrometers in length and 0.9-2 nm in diameter with no indication of metallic particles on either the inside or the outside of the nanotubes. This conclusion was reached after multiple TEM observations on different portions of the sample were made. In contrast, the images taken after exposure to hydrogen of the samples sonicated in the cobalt acetylacetonate solution show metal particles of elongated shape inside the SWNT for the samples synthesized at 300 and 400 °C and larger spherical particles at 500 °C. These particles can be easily identified in all images as they contrast with the nanotubes. In the images collected for the samples exposed to hydrogen at 300 and 400 °C, small elongated particles are seen mainly inside the pores of the SWNT. The shape of these particles can be considered proof that they are confined inside the SWNT since if they would have been on the SWNT surface; they would have had a more spherical shape consistent with minimal free energy. At 500 °C, control over the particles and their confinement inside the SWNT is lost. Large spherical particles are imaged on the tube surface. At this temperature, the Co atoms become more mobile, and their rate of sintering is increased. Most particles likely migrate from inside the SWNT onto the tube surface and sinter into the larger particles, such as those that are visible in Figure 2c. Statistical measurements were performed on the particles confined inside the SWNT pores obtained at 300 and 400 °C. Since the particles had an elongated shape, two types of measurements were performed. First, the width of the particles was measured and compared with the distribution of the SWNT diameters which was found to be between 0.9 and 2 nm from Raman spectroscopy measurements, TEM images, and information provided by the vendor. Second, the particle length was measured for the two samples reduced at 300 and 400 °C to assess the influence of the temperature of hydrogen exposure on the particle size. The results are shown in Figure 3a,b. Each analysis is the result of 500 individual measurements.
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Figure 2. (a) TEM image of SWNT after ultrasonication in cobalt acetylacetonate solution and exposure to hydrogen at 300 °C. The positions of some of the metal particles are marked by arrows. (b) TEM image of SWNT after ultrasonication in cobalt acetylacetonate solution and exposure to hydrogen at 400 °C. The positions of some of the metal particles are marked by arrows. (c) TEM image of SWNT after ultrasonication in cobalt acetylacetonate solution and exposure to hydrogen at 500 °C.
The particle width distribution was found to be close to the values of the SWNT diameter distribution. At 300 °C, most particles have a width of less than 2 nm corresponding very well to the SWNT diameter distribution. This is further evidence that at this temperature most of the particles formed are confined inside the nanotube pores. At 400 °C, the resultant particle size is shifted to somewhat higher values due to the faster rate of sintering of Co atoms. Some particles as large as 4 nm are observed on the outside surface of the SWNT. However, most particles imaged were still under the maximum value of the SWNT diameter (2 nm) and were clearly visible inside the SWNT. As noted before, at 500 °C, most particles are observed on the outer surface of the nanotube. These particles are spherical and have a diameter distribution between 2 and 30 nm.
Loebick et al. The length of the particles also varies with temperature. At 300 °C, most particles have a length between 1 and 6 nm with the majority of particles around 3 nm in length, while at 400 °C, the particle length is distributed between 1 and 8 nm with an average around 4.5 nm. The statistical analysis of the TEM data shows that by controlling the temperature at which the SWNT-cobalt acetylacetonate system is exposed to hydrogen, a certain degree of control over the particle size can be achieved. This result is important especially for some of the catalytic applications of these systems as mentioned in the Introduction. Next, it is important to probe the state of the Co particles as well as the transformations that take place during hydrogen exposure at the different temperatures. Figure 4 shows the XANES (X-ray absorption near edge structure) region of the EXAFS (extended X-ray absorption fine structure) spectrum collected around the Co K-edge for the samples before and after in situ exposure to hydrogen at 300 and 400 °C. Two features are of interest when analyzing the in situ reduction at the Co-edge; first, the pre-edge peak assigned to the dipole-forbidden transition, a strong function of the local environment of the Co atoms, and the white line feature, a measure of the density of empty states at the Fermi level. The pre-edge feature at around 7710 eV (Figure 3a) and the lowintensity white line are characteristic of metallic cobalt as shown in the spectra of the Co foil reference. In the sample before reduction, the Co atoms present in the sample are clearly oxidized as shown by the high intensity of the white-line feature and the absence of the pre-edge peak. This is not an unexpected result as Co is in the acetylacetonate form. After exposure to hydrogen at 300 °C, a pre-edge peak develops and the intensity of the white line is reduced. We can conclude that after hydrogen exposure at 300 °C the Co in the sample is in a mixture of metallic and oxidized phases. After reduction at 400 °C, the Co in the sample is completely reduced as visible from the similarity with the Co foil reference. The same trend is reflected in the Fourier transform of the EXAFS data which gives a radial distribution function. The major peak in the data collected after hydrogen exposure is situated at around 2.45 Å, characteristic of Co-Co bonds. This peak is absent in the data collected for the sample before reduction where the dominant bond in situated at around 2 Å, characteristic of a Co-O bond.27 A fit of the EXAFS data collected after the in situ reduction of the SWNT-Co acetylacetonate system at 300 and 400 °C was performed using the IFEFFIT program in a K-range from 2 to 12.5 Å-1. Paths from both oxidized and metallic Co calculated from FEFF 6.0 were used in the fit. Data collected for the metallic Co foil and the Co oxides references were used to determine the value of the amplitude functions used for each path. The background was subtracted by means of cubic spline function. Fourier transforms were performed using a parametrized Kaiser Window between 2 and 10 Å-1.28 Results are shown in Table 1. The fitting of the EXAFS data is in good agreement with the qualitative estimation from the data plots. At 300 °C, around 60% of all Co in the sample is in metallic form. This proportion increases to more than 90% when the hydrogen exposure takes place at 400 °C. EXAFS has also been previously employed to determine an average size of nanoparticles due to the strong nonlinear correlation between the particle diameter and the coordination number of atoms in small clusters. We have employed a (111)-truncated hemispherical cuboctahedron model to correlate the Co cluster size with the average first shell Co-Co coordination number. However, a note has to be made
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Figure 3. (a) Particle width distribution for particles formed by exposure to hydrogen at 300 and 400 °C. (b) Particle length distribution for particles formed by exposure to hydrogen at 300 and 400 °C.
TABLE 1: Fitting Results of the EXAFS Data Collected after Isothermal Reduction at 300 and 400 °C
reduction temp, °C
av Co-O first shell coordination number
Co-O bond distance, Å
av Co-Co first shell coordination number
Co-Co bond distance, Å
300 400
3.0 ( 0.9 0.5 ( 0.2
1.85 1.85
5.6 ( 1.2 8.6 ( 0.4
2.45 2.45
TABLE 2: Estimated Co Particle Size from TEM and EXAFS Measurements
Figure 4. XANES spectrum of the SWNT-Co acetylaccetonate system before and after exposure to hydrogen at 300 and 400 °C. Inset showing the radial distribution function (0 < R > 6 Å) after K2 weighed Fourier transformation of the EXAFS data.
that the values obtained from EXAFS are approximate, especially since the elongated shape of the particles as seen in TEM cannot be fully taken into account by our model. Nevertheless a comparison between the size estimate from TEM and EXAFS is made in Table 2. The particle size estimated at 300 °C was corrected to account for the fraction of metallic Co in the sample since EXAFS is a volume average technique. From the data comparison in Table 2, the estimated particle sizes from the two methods (TEM and EXAFS) appear to be in good agreement.
reduction temp,°C
av particle size from TEM, nm
av particle size from EXAFS, nm
300 400
1.35 ( 0.8 1.95 ( 1
1.55 ( 0.5 2.2 ( 0.4
Thermal gravimetric analysis (TGA) data were collected for the sample exposed to hydrogen at 400 °C in a Setaram Setsys 1750 instrument under air flow. The sample was held at 150 °C in pure argon for 1 h to dehydrate before initiating the temperature program. The initial weight change in the sample was monitored between 100 and 1000 °C at 10 °C/min under flowing air. The assumption was that within this temperature interval all carbon in the sample will be oxidized, such that the remaining residue can be attributed to the cobalt that was originally loaded in the carbon tubes. Figure 5 shows the TG/DTG data obtained for our SWNT sample loaded with metal particles. The major weight loss between 400 and 700 °C can be attributed to oxidation of the carbon nanotubes.29 We expect a small weight gain due to oxidation of the metal particles to be a factor in assesing the
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Figure 5. Thermal gravimetric (TG) and derivative of the thermal gravimetric (DTG) weight loss variation with temperature of SWNT sample loaded with Co metal particles.
final loading of Co; therefore, the calculated loadings are given as approximate values. We assumed for our calculations that after exposure to air at up to 1000 °C all of the Co in the sample is in a Co3O4 form. From this data it is therfore possible to determine an approximate loading of Co in the carbon nanotubes. The total residue weight was around 14.5% of the original amount used in the experiment. As baseline for this experiment, a TGA of pristine SWNT was performed, resulting in a total residue weight of about 3%, consistent with the vendor specification. The TGA data places the Co loading in the SWNT samples at ∼7%. In order to assess the impact of the metal precursor concentration over the loading of nanoparticles in our systems, we have performed an extra set of experiments, similar to the one described above, except that the concentration of the cobalt acetyl accetonate solution used in the experiment was reduced to 5 g/L. By TGA we have found the approximate loading of Co in the SWNT to be around 4%. This is evidence that we can have a certain degree of control over the amount of Co loaded in the tubes as well. The electron-photon coupling in the one-dimensional structured SWNT results in distinctive Raman spectra. Carbon
Loebick et al. nanotubes exhibit sharp van Hove singularities in the electronic density of states. The fundamental vibrational modes of SWNT in Raman spectroscopy provide information on their structure. Two spectral features are particularly of interest. The radial breathing mode (RBM) typically appears between 100 and 400 cm-1. Their frequency depends linearly on the reciprocal tube diameter. The G-band in carbon nanotubes occurs between 1500 and 1600 cm-1 as a consequence of the folding of the 2D graphene Brillouin zone into the 1D nanotube Brillouin zone.30-32 Figure 6 shows the RBM and G-band regions of the Raman spectra collected using a 532 nm laser excitation wavelength. The spectra were normalized to the G-band height. The RBM region of pristine and particle containing SWNT are somewhat different. The three main peaks in the RBM region appearing at 222, 276, and 324 nm diameter correspond to tubes with diameter of 0.95, 1.2, and 1.4 nm, respectively.30-32 A dampening of the RBM peaks intensity (RBM/G ratio) in the particle containing tubes is noted. This intensity difference can be attributed to the presence of the particles inside the SWNT cavity20,33 which affect the electron-phonon coupling responsible for the Raman signal enhancements in SWNT that are resonating with the excitation wavelength. This results in a slight intensity loss of the Raman resonance since discrete particles are formed inside the SWNT. The dampening effect would be more pronounced if the SWNT were filled to a larger extent with the Co particles. It is also important to note that the signal originating from the smallest nanotube identified in the sample (0.95 nm) is also decreased in intensity. Previously, Li et al.20 have shown that the Raman signal originating from nanotubes with small particles deposited inside their cavity only decreases in a range characteristic for tubes with diameters over 1.3 nm. In our case, however, even the signal coming from the thinnest SWNT is decreased. Both the Raman measurements and the TEM statistical analysis provide evidence that we were able to successfully add metal particles inside the pores of subnanometer SWNT as well as larger SWNT. To the best of our knowledge this is the first time such a result was reported for small diameter SWNT. Another significant difference appears in the G-band region. Notably, after metal nanoparticles have been introduced inside the SWNT, an upshift of the G-band by about 10 cm-1 is measured. Yang and co-workers have shown this to be an effect
Figure 6. Raman spectra of pristine SWNT and SWNT after treatment with cobalt acetyl accetonate solution and exposure to hydrogen at 300 and 400 °C: (a) Raman breathing mode region; (b) G-band region.
Cobalt Particles Encapsulated Inside SWNT of strain induced in the carbon nanotube by pore occlusions such as metal particles.34 4. Conclusions Discrete nanosized cobalt particles were loaded inside the cavity of SWNT by a two-step method. In the first step, pristine SWNT heat treated to remove oxygen containing groups from the surface were filled with a cobalt acetylacetonate solution in an organic compund with low surface tension by ultrasonication. In the second step, the samples were exposed to hydrogen at different temperatures to form nanosized cobalt particles. At temperatures below 500 °C, control of the particle size by varying the temperature of hydrogen exposure was achieved as determined by TEM measurements. Furthermore, metallic Co particles were present even in the smallest (subnanometer) SWNT in our sample. We have also shown that by varying the concentration of the Co precursor solution the loading of nanoparticles inside the SWNT can be modified. The method can be easily extended to other metals such as Fe or Ni, and these SWNT-metal particles systems are expected to extend the performance limit for many catalytic and electronic applications of SWNT. Acknowledgment. The authors gratefully acknowledge financial support from NSF CBET-0828771 and from AFOSRMURI 1492161. We acknowledge Dr. Nebojsa Marinkovic from the National Synchrotron Light Source, Beamline X18B, for support with collecting the EXAFS data. References and Notes (1) Saito, R.; Dresselahus, G.; Dresselahus, M. S. Phys. ReV. B 2000, 61, 2981–2990. (2) Zheng, B.; Lu, C.; Gu, G.; Makarovski, A.; Finkelstein, G.; Liu, J. Nano Lett. 2002, 2, 895–898. (3) Baughman, R. H.; Zakhidov, A. A.; de Heer, w. A. Science 2002, 297, 787–792. (4) Hatakeyama, R.; Jeong, G. H.; Hirata, T. IEEE Trans. Nanotechnol. 2004, 3, 333–337. (5) Philip, E.; Sloan, J.; Kirkland, A.; Meyer, R.; Friedrichs, S.; Hutchinson, J.; Green, M. H. Nat. Mater. 2003, 2, 788–792. (6) Okada, T.; Kaneko, T.; Hatakeyama, R.; Tohiji, K. Chem. Phys. Lett. 2006, 417, 288–292. (7) Pan, X.; Fan, Z.; Chen, W.; Ding, Y.; Luo, H.; Bao, X. Nat. Mater. 2007, 6, 507–511. (8) Guan, J.; Pan, X.; Liu, X.; Bao, X. J. Phys. Chem. C 2009, 113, 21687–21692.
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