Thermal Behavior of Raw and Purified SWNT Samples: XRD Studies

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1 Iowa State University From the SelectedWorks of Iris V. Rivero 2010 Thermal Behavior of Raw and Purified SWNT Samples: XRD Studies Rula Allaf Shayla E. Swain, Texas Tech University Iris V. Rivero, Iowa State University Available at:

2 Proceedings of the 2010 Industrial Engineering Research Conference A. Johnson and J. Miller, eds. Thermal Behavior of Raw and Purified SWNT Samples: XRD Studies Rula Allaf, Shayla Swain, and Iris V. Rivero Department of Industrial Engineering Texas Tech University, Lubbock, Texas , USA Abstract The thermal behavior of as-produced and purified single-wall carbon nanotube (SWNT) samples was investigated using powder x-ray diffraction (XRD). XRD profiles were collected with increasing and decreasing temperatures in air at the following steps: 25, 100, 300, and 450ºC. At room temperature, the raw SWNTs sample profile showed the 2D triangular lattice peaks associated with the SWNT bundles. However, the purified SWNTs sample profile had a less distinct (10) peak, assumed to be due to adsorption of air into the SWNTs. The rope s 2D lattice constant and the SWNTs average diameter were estimated at 18.0 Å and 14.8 Å, respectively. Keywords SWNTs, XRD, adsorption, heat treatment 1. Introduction Single-walled carbon nanotubes (SWNTs) with diameters ranging from 0.4 to 3 nm, and lengths of the micrometer order, were first synthesized in 1993 [1, 2]. They can be viewed as sheets of graphene rolled into seamless hollow cylinders in a range of diameters, chiralities, and lengths, with half-fullerene caps (for closed tubes), which often aggregate to form finite-size crystalline-like ropes (bundles) due to the van der Waals interactions between the nanotubes [2,3]. They can also be considered as the building block of multi wall carbon nanotubes (MWNTs), which are most commonly described as a number of concentric SWNTs with successively larger diameters, and interplanar spacings of ~ 0.34 nm [3,4]. Since 1993, significant research has been going on worldwide to control, optimize and scale up their production techniques; study their formation mechanism and properties; and explore potential applications. Several characterization techniques are used to investigate SWNTs properties, including analytical techniques, mechanical, thermal, and electrical tests. Today, there is plenty of experimental evidence that they have exceptional mechanical, thermal, chemical, and electrical properties. However, the lack of standard carbon nanotube (CNT) characterization techniques makes it difficult to compare measurements of different samples (both unpurified and purified) [5]. Arepalli et al. [5] reported the development of a NASA-JSC protocol for characterization of SWNTs which standardizes measurements in TEM, SEM, thermogravimetry (TGA), Raman and UV VIS spectrometry and establishes measures for homogeneity, dispersability, metal content and thermal stability so that comparison of various samples is simplified. X ray diffraction (XRD) characterization technique is a nondestructive and successful characterization technique, which has long been used to provide information on crystalline samples. TEM and XRD measurements of the SWNT soot produced by arc-discharge or laser ablation indicated that SWNTs self assemble into bundles [6, 7], where the nanotubes crystallize in a triangular (hexagonal close-packed) 2D lattice, thus allowing XRD utilization to characterize SWNT samples. Nonetheless, SWNT samples present many challenges for the use of XRD due to their inherent characteristics. In particular, the general profile of the XRD spectrum including peak positions and widths depend on the tube symmetries, distribution of tubes diameters and bundles diameters [8]; as previously mentioned, SWNTs crystallize in a 2D lattice structure with SWNTs of different diameter, length, chirality, and bundle size distributions, leading to peak broadening and overlapping; besides the effect of small angle scattering at the low diffraction angles associated with SWNT bundles. In addition to that, SWNT samples contain many impurities such as amorphous carbon, catalysts, MWNTs, and graphitic particles, as well as defects that contribute to the XRD

3 spectrum. Finally, XRD patterns are not responsive to small SWNT bundles and are sensitive to the presence of adsorbates on the SWNTs, which also affects peak positions, widths, and intensities. Still, XRD has a great potential, it is a global (averaging) characterization technique which provides statistical characterization of SWNTs compared to other techniques, such as TEM where SWNTs can be characterized at the individual level [4]; it provides more objective measures than the subjective ones provided by other techniques; it is easier and simpler than TEM/SEM techniques; it is more comprehensive in the sense that it detects the SWNTs and all the other impurities to some extent; and it can be used to measure anything that causes intertube spacing changes, such as stress and temperature. In an initial effort to establish an XRD standard characterization protocol for consistent and reliable characterization (diameter distribution and purity) of SWNT samples to be used in SWNT production process optimization and control, the present work aims at comparing the characteristics of as-produced and purified arc-discharge SWNT samples using high temperature XRD and to investigate the effect of adsorbed gases and temperature on their XRD profiles. 2. Experimental Details 2.1 SWNT Sample Purification The SWNT material was purchased from Carbon Solutions, Inc. One sample was purified according to a modified version of the purification method developed by Zujin et al. [9]. The procedure is described as follows: A 200mg sample of as-produced SWNTs was heated in an oven at 350 C for 2 hours then soaked in 37% (w/w) HCl for 24 hours. The resulting solution was centrifuged in an Allegra X-12 Centrifuge for 3 hours at 3270 rpm. The sediment was washed with de-ionized water three times and subsequently sonicated in 200mL of 0.2% benzalkonium chloride for 1 hour using Vibra-cell ultrasonic tip (20 khz, 130W). After that, the dispersion was filtered under vacuum using a 1µm porous polytetrafluoroethylene membrane disc filter paper. The sonication and filtration steps were then repeated twice. 2.2 Sample Characterization Morphological observations on as-produced and purified specimens were made using Hitachi S-4300 SE/N highresolution field emission scanning electron microscope (FESEM) operated at an accelerating voltage of 2-3 kv. The Rigaku Ultima III x-ray powder diffractometer was used for the XRD experiments, using the standard stage with a zero background sample holder (ZBH) for the room temperature powder diffraction runs and the high-temperature stage with a platinum sample holder for the high temperature runs. The diffractometer was used in the parallel beam geometry on flat-plate samples, using a Cu Kα x-ray tube with a wavelength of = Å, running at 40 kv and 44 ma. Room temperature runs were done in fixed time scanning mode, with a step width of 0.02º and a count time of 12 sec/step; whereas the high temperature runs were done in fixed time scanning mode, with a step width of 0.1º and a count time of 7 sec/step. K filters were employed in both the source and the detector sides for the room temperature runs. High temperature XRD data were collected at 25, 100, 300, and 450 ºC in increasing, decreasing, and then increasing temperature modes. Final XRD runs were carried on the same samples 3-5 days after the high temperature runs using the same high-temperature XRD conditions but with the standard stage and the ZBH. 3. Results and Discussion 3.1 SEM The SEM images show large amount of impurities in the raw sample (Figure 1a); whereas the purified sample micrographs clearly show the SWNT bundles with much higher purity (Figure 1b). SEM images were obtained after the high temperature XRD runs which clearly assured the persistence of the SWNT bundles (results not shown).

4 Figure 1: SEM images for (a) raw and (b) purified arc-discharge SWNT samples. 2.2 Room Temperature XRD The raw SWNT sample profile (Figure 2a) shows the (10) peak (~ 5.74º 2 ) and the weaker peaks identified in the literature [10-13]: (11) (~ 9.3º 2 ), (20) (~ 11.7º 2 ), (21) (~ 14.7º 2 ), (22) and (31) (~20.3º 2 ). However, the purified SWNT sample profile (Figure 2b) shows much less distinct peaks: the (10) peak is obvious, but with a much lower intensity, Maniwa et al. [14] explained the loss in the (10) peak intensity due to adsorption of air into the SWNTs, whose ends get opened during the purification process. The (22) and (31) peaks are obvious in the profile, as well as a broad peak appearing at ~20.6º 2, which is assumed to arise from some adsorbed or intercalated molecules during the purification routine. Bendiab et al. [15] showed that Iodine intercalation in SWNT bundles causes the appearance of several broad peaks at high Q and an increase in the background level. Both profiles show the presence of turbostratic graphite or MWNTs (~ 26º 2 peak) and ordered graphite (~ 26.5º 2 ) in the samples. Moreover, the peaks at ~ 44.4º 2 and ~ 52º 2 reveal the presence of some amount of catalyst in both samples. However, the purified material profile shows sharper peaks in contrast to the broad peak at ~ 26º 2 in the raw sample indicating less amorphous carbon agreeing with the SEM images. XRD profile from the ZBH (result not shown) revealed that the ZBH did not contribute to the SWNT samples XRD profiles. Using the (10) peak position from the raw sample profile: 2 = 5.74º, the d 10 spacing can be calculated as Å. Thus the rope s 2D lattice constant can be estimated as Å [16]. Using an intertube spacing, d tt = 3.2 Å, the SWNT average diameter can be estimated as Å [16]. Figure 2: Room temperature XRD profiles for (a) raw and (b) purified SWNT samples.

5 2.3 High Temperature XRD The raw material high temperature XRD profiles (Figure 3a) show a drop in the background intensity with increasing temperature, which got more pronounced after 300ºC implying loss of amorphous material upon heating in air as suggested by the findings of Maniwa et al. [14]. The intensity kept dropping throughout the three sets of runs, which may be attributed to continued amorphous carbon removal during the runs. The second run at increasing temperature shows more similar profiles at the different temperatures which can be attributed to reaching stable conditions due to the complete loss of the amorphous carbon from the sample. No obvious adsorption effect has been recognized in those profiles. Figure 3: Temperature dependence of XRD profiles for raw SWNTs sample: (a) increasing temperature; (b) decreasing temperature; and (c) re-increasing temperature. The purified sample high temperature XRD profiles (Figure 4a) show desorption occurring upon heating [14]; where, at 450 ºC, the profile evolved into the typical profile referenced in the literature, clearly showing the (10) peak in addition to some of the weaker peaks associated with the SWNT bundles 2D lattice. The decreasing and second increasing temperature runs show more similar profiles, as in the raw sample results, emphasizing stable conditions. It should be mentioned that the low resolution of the high temperature runs impeded the investigation of the thermal expansion properties of the raw and purified samples. Figure 4: Temperature dependence of XRD profiles for purified SWNTs sample: (a) increasing temperature; (b) decreasing temperature; and (c) re-increasing temperature. Figure 5 reveals the disappearance of (10) peak in the raw SWNT sample profile obtained approximately 3-5 days after the high temperature runs; whereas, the purified sample shows a more clearly representative XRD profile, which can be attributed to the use of the ZBH instead of the platinum holder. The loss of the SWNT bundle peaks in the raw sample is attributed to the adsorption of air into the SWNTs after heat treatment as explained by Maniwa et al. [14]. However, the behavior of the purified sample is harder to explain. Further tests are required to provide additional insight into this phenomenon. Finally, Figure 5 also shows a pronounced unknown peak at ~ 37º 2 in both raw and purified samples profiles.

6 Figure 5: XRD profiles for (a) raw SWNT sample and (b) purified SWNT sample ~ 3-5 days after high temperature runs. 4. Conclusions This study investigated the characteristics and thermal behavior of two SWNT samples, namely raw and purified samples, in air. XRD patterns were collected with increasing and decreasing temperatures at the following steps: 25, 100, 300, and 450ºC. The room temperature raw SWNTs sample XRD profile showed the 2D triangular lattice peaks associated with the SWNT bundles as identified in the literature. However, the purified SWNTs sample profile had a less distinct (10) peak, which is attributed to the adsorption of molecules or atoms into the SWNTs. Both SWNT samples contained turbostratic graphite or MWNTs and ordered graphite as well as catalyst particles as indicated in the XRD results. The purified material had less amorphous carbon. The rope s 2D lattice constant and the SWNT average diameter were estimated at Å and Å, respectively. The raw material XRD profiles showed a drop in the background intensity with increasing temperature implying loss of amorphous material upon heating in air. The purified material XRD profiles indicated desorption of molecules/atoms upon heating; where the profile evolved into the typically referenced profile in the literature. Both loss of amorphous carbon and desorption were more pronounced above 300ºC in the first heating run. The decreasing and second increasing temperature runs showed additional similar profiles for both samples. The (10) peak disappeared in the raw SWNT sample profile obtained approximately 3-5 days after the high temperature runs, which is associated with the adsorption of air into the SWNTs after heat treatment. However, the purified sample did not show similar behavior. Both raw and purified samples profiles showed a more pronounced unknown peak at ~ 37º 2. Future research will concentrate on four areas: (1) validating the XRD results using TEM and/or Raman spectroscopy for diameter and purity measurements, and using TGA and BET surface area analyzer for adsorption/desorption investigation; (2) running high temperature XRD profiles at 600 ºC in air to investigate higher temperature effect on the XRD profile and to establish complete desorption temperature; (3) running hightemperature higher resolution XRD at 25, 100, 300, 450, 700 ºC under a vacuum controlled environment to investigate the effect of adsorbed gases and temperature on the XRD profiles in vacuum and to investigate the thermal expansion properties of the samples; and finally (4) establishing a protocol for the consistent XRD characterization of the SWNT product quality, which includes sample preparation, XRD standard method(s), and XRD-derived measures describing parameters such as purity and SWNT diameter uniformity. Acknowledgements We acknowledge the Texas Tech University Imaging Center, Department of Biological Sciences and the Department of Chemistry & Biochemistry at Texas Tech University for using their Hitachi S-4300SE/N (NSF MRI ) and Rigaku Ultima III X-ray powder diffractometer, respectively.

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