J. Phys. Chem. C 2007, 111, 6263-6267
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Electron Spin Resonance Studies of Hydrogen Adsorption on Defect-Induced Carbon Nanotubes C. F. M. Clewett,† Peng Li,‡ and T. Pietrass*,§ Department of Chemistry and Department of Physics, New Mexico Tech, Socorro, New Mexico 87801 and Department of Earth and Planetary Sciences, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed: NoVember 6, 2006; In Final Form: February 2, 2007
Multi-walled carbon nanotubes were subjected to harsh acid treatment for subsequently increasing periods of time. Transmission electron micrographs show that metallic particles are largely removed in this process and that nanofibers are preferentially destroyed. Acid treatment induces defects through segmentation of the tube walls. These changes were studied using electron spin resonance spectroscopy, and the response of the defects to hydrogen adsorption was monitored over a temperature range from ambient to 5 K. The interaction of hydrogen mainly with defects is manifested in the largest increase in signal amplitude for the sample with the most defects and in the increase of the spin lattice relaxation time at temperature below 10 K.
1. Introduction In 1997, carbon nanotubes (CNTs) were reported to store large volumes of hydrogen gas with uptake capacities in the range of 5-10% by weight (wt %).1 This result sparked intense research activity due to the potential of exceeding the benchmark set by the Department of Energy (6.5 wt % uptake capacity) for an economically viable hydrogen storage medium.2 To date, this result has not been conclusively reproduced, and the interest in CNTs as hydrogen storage materials is waning.3,4 In 2000, the potential of CNTs as gas sensors was first reported based on an increase in the conductivity by several orders of magnitude upon exposure of CNTs to oxygen.6 This result was subsequently questioned because other gases present in air (mainly nitrogen and water vapor) were also shown to interact weakly with single-walled nanotubes (SWNT) bundles through dispersion forces.5 Gases such as sulfur dioxide, ammonia, and nitrogen dioxide induced reversible changes in the CNTs’ electronic structure.5 It remains a challenge to measure the conductivity of an individual tube and to exclude the possibility that the resistance of the contact affects the results. Moreover, transport properties of CNTs are still under investigation, and the exposure to gases further complicates the picture. To avoid these limitations, we use electron spin resonance (ESR) spectroscopy to study the electronic properties of CNTs before and after gas exposure. It should be noted, however, that our technique has limitations of its own: the need for macroscopically sized samples providing information on properties of the bulk material and that sample impurities are averaged in. In the following, we discuss the effect of defects in multi-walled carbon nanotubes (MWNTs) on hydrogen adsorption in light of their potential applications as sensors. 2. Experimental Procedures ESR experiments were conducted on a Bruker EMX spectrometer equipped with a 10 kG magnet, operating in the X-band * Corresponding author. E-mail:
[email protected]. † Department of Physics, New Mexico Tech. Current address: Department of Physics, Fort Hays State University, Hays, Kansas. ‡ University of New Mexico. § Department of Chemistry, New Mexico Tech.
(9.4 GHz). The temperature was controlled with an Oxford continuous flow cryostat, model 900. Experimental details for spectra acquisition are provided in the figure captions. Defects were induced in MWNTs by acid digestion in a mixture of nitric and sulfuric acid. The CNTs were produced by chemical vapor deposition using an Fe catalyst.7 For acid digestion, about 10 mg of sample was dispersed in 10 mL of a mixture of concentrated sulfuric and nitric acid (70/30 v/v). The dispersions were sonicated for 0, 1, 8, and 12 h, diluted with deionized water, and filtered.7 The samples are referred to as “parent” (no acid digestion), “1 h”, “8 h”, and “12 h”, respectively. Multiple high-resolution transmission electron microscopy (TEM) images were recorded for each sample on a Jeol 2010F STEM/TEM with 0.23 nm point resolution operated at 200 kV. 3. Results and Discussion Representative TEM micrographs are shown in Figure 1. The parent material consists of CNTs and nanofibers. Metal particles adhering to the outer walls as well as included in the interior of the tubes are also evident. In addition, carbonaceous material seems to coat some of the tubes as suggested by the irregular wall structure. With subsequent acid digestion, the exterior particles are largely removed. After 1 and 8 h of acid treatment, metal particles with darker contrast in the interior of the tubes are still visible. These metal particles are iron-rich, as confirmed by nanoprobe energy dispersive spectroscopy (EDS) analysis. After 12 h, the metal particle number is reduced while the number of the big particles with similar contrast as the nanotubes and nanofibers is increased. Nanoprobe EDS analyses of these species indicate that they are carbon particles. In addition, after 1 h defects are introduced into the tube walls that lead to disruption of the continuous wall structure, giving rise to bamboolike stacking. The carbonaceous material on the outside of the tube walls has been largely destroyed. Defects in the tube walls also lead to kinks that become pronounced after 8 h of acid digestion. Apparently, after 12 h these defects lead to the complete destruction of these nanotubes, as all the remaining tubes are straight after 12 h of acid treatment. The carbon particles adhering to the tube surface after 12 h have well-
10.1021/jp067314e CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007
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Figure 1. High-resolution TEM images of the parent sample and after various times of acid digestion.
defined edges, suggesting that these carbon particles possess a graphitic structure. The images indicate that carbonaceous material present from the destruction of nanofibers and defected tubes, as well as amorphous carbon, has recrystallized into graphitic particles. These results are confirmed by previous Raman analyses of these samples.7 The ID/IG ratio, which correlates with the number of defects, is smallest for the parent material, increases up to an acid digestion time of 8 h, and decreases again for the 12 h sample. Similarly, after 1 h of acid digestion, a D′ band appears in the Raman spectra that gains in intensity for the 8 h sample and that is not observed for the parent or 12 h sample. This resonance has been linked to the formation of functional groups; in our case, probably carboxylic acid functionalities.7 Sun et al.8 however, report that extensive functionalization leads to overwhelming luminescence interference in the Raman spectra, which we did not observe. Figure 2 shows ESR spectra at various stages of acid digestion before and after exposure to a hydrogen pressure of about 100 kPa. Note that the spectra before hydrogen adsorption were recorded in the presence of helium gas at a pressure of 27 kPa, to facilitate temperature equilibration. When evacuating the samples after hydrogen exposure, the original ESR spectra prior to exposure were reproduced, indicating physisorption only.
Before acid digestion, the line shape of the dominant resonance is asymmetric indicating the detection of conduction electrons.9 Also, the temperature dependence of the signal intensity does not follow the Curie law (filled circles in Figure 4) as is expected for conduction electrons. With acid digestion, the line shape of this resonance becomes more symmetric, and the signal intensity follows the Curie law, indicative of localized spins. This implies that acid digestion removes structures from the sample that give rise to conduction electron spin resonance. Small diameter CNTs are known to be chemically more reactive and may be destroyed by acid treatment. Their removal alone, however, would not explain the disappearance of the Dysonian line shape. In addition, larger nanofibers are removed in the acid purification process (Figure 1). These have graphitic structures that should also give rise to Dysonian line shapes. Acid treatment introduces defects into the tubes that causes the signal from localized spins to increase. The line width decreases from 20 G for the 0 h sample to 10 G for the 12 h sample. The same effect was observed for MWNTs in which defects had been introduced through irradiation.12 Beuneu et al.12 report that the g-factor and line width diminish with increasing irradiation time. At low temperature, these parameters show the same trend for our samples, indicating that isolated spins dominate the contribution to the magnetic susceptibility, whereas at high temperatures, conduction electrons dominate.
ESR Studies of Hydrogen Adsorption on CNTs
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Figure 2. ESR spectra before and after hydrogen adsorption at various stages of acid digestion. Spectra were recorded at 5 K. Note the different vertical scales.
Figure 3. Decomposition into two components of the ESR spectrum of the 12 h sample after hydrogen exposure at 5 K. (a) Gray ) fit, narrow component; black ) fit, broad component. (b) Gray ) sum of the two fitted components shown in (a). Black dots ) experimental data points.
After 1 h of acid digestion, a broad resonance appears in the spectra that decreases in intensity after 8 h of acid digestion and disappears after 12 h. This resonance is tentatively assigned to amorphous/graphitic carbon debris from the destruction of nanotubes and may correlate with the D′ band observed in Raman spectroscopy.7 From the TEM images in Figure 1, it is reasonable to suggest that this “debris” is composed of the highly kinked tubes whose walls have been segmented. After 12 h, this material has been oxidized and disappeared as CO2. The graphitic particles evident in Figure 1 after 12 h of acid digestion do not seem to give rise to a distinctive ESR signal as only one ESR resonance is observed for this sample (Figure 2). However, this resonance cannot be fit with a single Lorentzian, while a fit with two Lorentzians is nearly perfect (Figure 3). The two components are a broad resonance of low intensity (line width 58 G at 5 K) and a narrow resonance (line width 8 G at 5 K). The line width of the former suggests it be assigned to the graphitic particles, while the narrow resonance most likely
Figure 4. Temperature dependence of the ESR signal intensity. Filled (unfilled) symbols correspond to the sample before (after) hydrogen adsorption. The straight line shows the Curie law dependence.
originates from defects. Closer inspection of Figure 1 reveals that many of the graphitic particles seem to preferentially encoat residual metal particles that may even serve as catalysts in the recrystallization process. We postulate that the presence of these particles gives rise to rapid relaxation, rendering many of these structures unobservable to ESR spectroscopy. Hydrogen adsorption causes an increase in signal intensity (Figures 2 and 4) while retaining the original line shape. Neither the g-factor nor the line width are affected. We observed an increase in signal intensity upon hydrogen adsorption previously for MWNTs10 and hypothesized a change in band gap upon hydrogen adsorption. For SWNTs, however, a reduction in signal intensity due to quenching of defect sites by hydrogen occurred. For the spectra shown in Figure 2, hydrogen adsorption causes an increase in signal intensity of all spectral components. This fact might suggest that the signal enhancement is due to a change in experimental parameter and not in electronic structure. Inadequate temperature equilibration would lead to this effect. However, the samples were exposed to helium prior to hydrogen adsorption, facilititating heat transfer. Hence, we conclude that the increase in signal intensity is truly due to
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TABLE 1: Ratio of the Signal Amplitude of the Narrow Resonance before and after Hydrogen Exposure at 5 and 30 Ka sample
5K
30 K
parent 1h 8h 12 h
1.6 1.4 1.5 2.0
1.5 1.2 1.5 2.4
a
The data reported for the 8 h sample in the 5 K column were actually recorded at 8 K. Error estimated at ( 0.1.
TABLE 2: Different Enhancement of the Two Signal Components of the 12 h Sample from Fits to Two Lorentzians (Figure 3)a ratio
5K
30 K
narrow broad narrow/broad
1.7 1.4 1.2
1.4 0.5 2.8
The values are the ratios of the integrated signal intensity for the hydrogen exposed sample divided by that of the helium exposed sample for the narrow and broad signal component, respectively.
Figure 5. Temperature dependence of the ESR line width. Note that for the 12 h sample, the signal was observable up to ambient temperature. No change in the line width was observed above 80 K (not shown).
a change in the electronic structure of the material. Because the line width neither changes appreciatively nor consistently during the experiment for each sample (( 1 G), the change in signal intensity may be due to a relaxation effect. From a close inspection of Figure 4, it is evident that the extent of signal increase varies from sample to sample. It becomes most pronounced after 12 h of acid digestion. Data for two temperatures are summarized in Table 1. The data show that signal enhancement is not greatly affected by temperature. At a pressure of about 100 kPa and a temperature of 5 K, pure hydrogen is solid, while it is gaseous at 30 K. Thus, a lesser effect on the spectra would be expected at 5 K. The fact that this is not observed is an indication that hydrogen does not form a bulk solid at this temperature. The signal enhancement in the presence of hydrogen is much more pronounced for the 12 h sample, suggesting a strong interaction of hydrogen with defects, that is more prominent when hydrogen is present in the gaseous phase. For this sample, the two signal components (Figure 3) are enhanced at different degrees (Table 2) with the component assigned to defects experiencing the greater enhancement. This supports the stronger interaction of hydrogen with defects than with other structures giving rise to an ESR signal (graphitic particles). To probe the dynamics of the system, we conducted a power saturation study to obtain relaxation data of the 12 h sample. Using the method by Poole,16 the analysis yields spin-spin lattice relaxation times T2 on the order of (660 ( 50) ns at all temperatures, while T1 is clearly a function of temperature. T1 is ≈ 150 µs below 10 K, and ≈ 7 µs above 125 K, following roughly an exponential decrease. Despite the error associated with this technique, it must be concluded that we are outside the extreme narrowing limit as expected for a rigid solid, and that the electronic spin-lattice relaxation time is a motionally activated process with an activation energy on the order of 200 J/mol. The greatest increase in relaxation time is observed below 10 K at which hydrogen gas is expected to solidify, given some uncertainty in our pressure measurement. This suggests that the mobility of hydrogen is responsible for the relaxation process. Indeed, the extent of signal enhancement upon hydrogen exposure is much greater for the narrow component of the resonance for the 12 h sample, where the signal is due to defects (Table 2). An analysis of the g-factors reveals that with temperature, the g-factor is constant, within error (( 0.0005), for the 8 h
(g ) 2.0032) and 12 h (g ) 2.0020) samples, and before and after hydrogen exposure. The g-factor is close to the free electron value for the 12 h sample, consistent with localized spins. A value of g ) 2.0028 is considered characteristic for defects in carbon materials. For the parent material and the 1 h sample, the g-factor increased with temperature from 2.002 to about 2.004. This behavior is similar to that of graphite, where a decrease of the g-factor is observed at low-temperature due to exchange coupling.13 The temperature dependence of the line width after hydrogen exposure is shown in Figure 5. The data for the sample before hydrogen exposure are identical to those shown within error and are not included in the graph. At the lowest temperature (5 K) in which the signal for all samples is clearly observable, the line width is reduced from (20 ( 1) to (9 ( 1) G with increasing acid digestion time. This is in agreement with Beuneu’s results in which defects were introduced through irradiation.12 Consistent with the observation for the g-factor, the line width is largely independent of temperature for the 8 and 12 h samples, while it decreases for the 1 h sample, and increases for the parent material with increasing temperature. The behavior of the parent material agrees with that of graphite, and the temperature independence of the line width of the 8 and 12 h samples is consistent with localized spins. However, the 1 h sample shows an unexpected behavior: although the g-factor decreases at low temperature, the line width increases. The spectrum for this sample also shows the greatest Dysonian asymmetry in the narrow line, and a broad spectral component that was tentatively assigned to carbon debris. In addition, this sample had the lowest normalized signal intensity. An increase in line width from room temperature to 30 K has been observed for graphite and has been explained with motional narrowing and spin-lattice relaxation through interaction with the spin-orbit coupling of impurity atoms.14,15 We postulate that a similar mechanism could be responsible due to the segmentation of the tubes into smaller fragments, giving rise to a loss in the quasi one-dimensionality of the system and rendering the CNTs more graphite-like. In conclusion, harsh acid digestion seems to consume amorphous carbon, carbon fibers, and possibly small diameter CNTs. In the process, defects are introduced that behave like localized spins and follow a Curie law temperature dependence. Hydrogen adsorption caused the ESR signal intensity to increase, regardless of the digestion time, although the effect was more
a
ESR Studies of Hydrogen Adsorption on CNTs pronounced after longer periods of acid consumption, which confirms a preferential interaction of hydrogen with defect sites. The extent of signal enhancement did not depend on temperature unless there were many defects present. Signal enhancement even at 5 K in which pure hydrogen is solid at the applied pressure suggests that hydrogen never formed a bulklike phase. Relaxation data support that it is most strongly immobilized on defects. Acknowledgment. We would like to thank Dr. S. Curran and his co-workers for providing the samples. This material is based upon work supported by the National Science Foundation under Grant Numbers EPS-0312632 and 0420366. Helpful discussions with Dr. A. Angerhofer are gratefully acknowledged. References and Notes (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377-379. (2) Hynek, S.; Fuller, W.; Bentley, J. Int. J. Hydrogen Energy 1997, 22, 601-610.
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6267 (3) Panella, B.; Hirscher, M.; Roth, S. Carbon 2005, 43, 2209-2214. (4) Shen, K.; Pietrass, T. J. Phys. Chem. B 2004, 108, 9937-9942. (5) Goldoni, A.; Larciprete, R.; Petaccia, L.; Lizzit, S. J. Am. Chem. Soc. 2003, 125, 11329-11333. (6) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801-1804. (7) Pietrass, T.; Dewald, J. L.; Clewett, C. F. M.; Tierney, D.; Ellis, A. V.; Dias, S.; Alvarado, A.; Sandoval, L.; Tai, S.; Curran, S. A. J. Nanosci. Nanotechnol. 2006, 6, 135-140. (8) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096-1104. (9) Dyson, F. J. Phys. ReV. 1955, 98, 349-359. (10) Shen, K.; Tierney, D. L.; Pietrass, T. Phys. ReV. B. 2003, 68, 165418. (11) Shen, K.; Xu, H.; Jiang, Y. B.; Pietrass, T. Carbon 2004, 42, 23152322. (12) Beuneu, F.; L’Huillier, C.; Salvetat, J.-P.; Bonard, J.-M.; Forro´-, L. Phys. ReV. B 1999, 59, 5945-5949. (13) Salvetat, J.-P.; Fehe´r, T.; L’Huillier, C.; Beuneu, F.; Forro´, L. Phys. ReV. B 2005, 72, 075440. (14) Kosaka, M.; Ebbesen, T. W.; Hiura, H.; Tanigaki, K. Chem. Phys. Lett. 1994, 225, 161-164. (15) Wagoner, G. Phys. ReV. B 1960, 118, 647-653. (16) Poole, C. P. Electron Spin Resonance; Dover Publications: Mineola, NY, 1996.
