ARTICLE pubs.acs.org/JPCC
Pressure-Induced Collapse in Double-Walled Carbon Nanotubes: Chemical and Mechanical Screening Effects A. L. Aguiar,†,‡ E. B. Barros,† R. B. Capaz,§ A. G. Souza Filho,*,† P. T. C. Freire,† J. Mendes Filho,† D. Machon,‡ Ch. Caillier,‡ Y. A. Kim,^ H. Muramatsu,^ M. Endo,^ and A. San-Miguel*,‡ †
Departamento de Física, Universidade Federal do Ceara, 60455-900 Fortaleza, Ceara, Brazil Universite de Lyon, F-69000, France; Universite Lyon 1, Laboratoire PMCN, CNRS, UMR 5586, F-69622 Villeurbanne Cedex, France § Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21941-972, Brazil ^ Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan ‡
ABSTRACT: The vibrational properties of double-walled carbon nanotubes (DWNTs) is investigated by high-pressure resonance Raman scattering up to 30 GPa in two different pressure-transmitting media (PTM): paraffin oil and NaCl. The protection effect on the outer tube during compression is verified .The collapse of DWNTs is experimentally observed for the first time, showing to be two-step: the onset of the outer 1.56 nm diameter tube collapse at ∼21 GPa is followed by the collapse of the inner 0.86 nm diameter tube at a higher pressure of ∼25 GPa. This observation is supported by calculations. We show that filling a tube with another tube leads to a pressure stabilization against collapse, in strong opposition to what is observed when filling a tube with fullerenes or iodine. The collapse pressure in DWNTs appears to follow a 1/dtav3 law, where dtav is the average diameter from the inner and outer tubes, in agreement with predictions [Yang, X.; et al. Appl. Phys. Lett. 2006, 89, 113101]. Contrary to SWNTs and peapods, for DWNTs, the observed collapse pressure is independent of the PTM nature. Those differences are discussed in terms of tube filling homogeneity and of the separate roles of inner and outer tubes: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. This leads to high collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs ideal fillers for composite nanomaterials for high load mechanical support.
’ INTRODUCTION Carbon nanotubes exhibit striking properties regarding their geometrical and electronic structure. Their electronic structures can be tuned by external variables, such as doping, strain, and hydrostatic pressure.1-4 Most of the high-pressure Raman scattering studies of carbon nanotube systems have been performed on single-walled carbon nanotubes (SWNTs). However, there has recently been a large increase of interest in double-walled carbon nanotubes (DWNTs). The DWNT system is interesting because it is an intermediate structure between SWNTs and multi-walled carbon nanotubes (MWNTs). Because DWNTs have only two tubes and the diameters of the outer tubes are often similar to those of SWNTs, the quantum confinement effects are almost as prominent as in SWNTs. Theoretical calculations and experimental studies based on Raman spectroscopy showed that strong structural changes take place in SWNTs under hydrostatic pressure. These changes have been indentified to be the ovalization and collapse of the SWNTs' cross section.5-7 In particular, the collapse pressure at which the cross section exhibits curvature with different signs is predicted to exhibit a 1/dt3 dependence on the tube diameter, dt. These ideas were extended for DWNTs,8-11 and calculations suggest that the DWNTs can support higher pressures than the corresponding individual SWNTs before any strong structural change or collapse.8,12 On the other side, experiments on fullerene-filled r 2011 American Chemical Society
SWNTs (peapods)6 and iodine-filled SWNTs13 clearly show that, despite the support effect of the filler, the system collapses at lower pressures than the corresponding pristine SWNT. This appears to be opposite to what has been predicted for a tube filler, that is, a DWNT. Up to now, no experiments have allowed one to observe DWNT collapse. Tube collapse is now well stablished to have a characteristic Raman signature.6,7 The objective of our study is to experimentally and theoretically investigate DWNT collapse and confront our results to previous predictions and experiments. We should also cite that hydrostatic pressure application in DWNTs has been used as well as a way of separating the spectral contribution of the inner and outer tubes in the Raman G-band profile.14,15 In this paper, we report a study of pristine, high-purity bundled DWNTs (essentially free of catalyst particles and SWNTs) using resonance high-pressure Raman scattering with different pressuretransmitting media (paraffin oil and NaCl) up to a maximum pressure of 30 GPa. The purity of the DWNT samples is fundamental for investigating the high-pressure effects and to further compare and establish differences between DWNTs and Received: November 8, 2010 Revised: February 3, 2011 Published: March 04, 2011 5378
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Figure 1. (a) Transmission electron microscope (TEM) images of double-walled carbon nanotubes.17 (b) Raman spectra in the RBM region for the studied DWNTs. (c) The Kataura plot can be used to relate inner and outer tubes. Open red and solid blue circles denote, respectively, metallic and semiconducting nanotubes. The horizontal dashed line is the laser energy used in the experiments.
SWNTs with the same average diameter. Our work has benefited from advances in synthesis having allowed the production of very high quality DWNTs with a negligible amount of SWNT constituents,16 as is described in the Experimental Section and Sample Characterization. In the next section, we describe our observations in the low-pressure domain (up to 10 GPa) where essentially our results confirm previous studies. The main experimental results of our work are found in the high-pressure domain section in which we describe the collapse of DWNTs, which is experimentaly observed for the first time. The theoretical model in good agreement with observations is then introduced before concluding with the Discussion and Conclusions sections.
’ EXPERIMENTAL SECTION AND SAMPLE CHARACTERIZATION High-purity DWNTs were produced by an adapted CVD method, followed by purification oxidation, leading to a buckypaper material. The detailed description of the synthesis method is described elsewhere.18 Figure 1a shows high-resolution TEM images of pristine DWNT samples used in the highpressure studies. It appears from the TEM images and scanning electron microscope18 images that the samples are of very high quality (to quantify the purity, we estimate our samples to be composed of 99% of DWNTs and 1% of SWNTs þ catalyst particles). Images did not reveal the presence of any metal
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particles or amorphous carbon. The DWNT bundles are also well-ordered as the disorder-induced mode is not observed in the Raman spectra.17 The diameter distributions of both the inner (dt = 0.86 ( 0.25 nm) and the outer (dt = 1.56 ( 0.54 nm) tubes were determined from TEM images. The average difference between inner and outer tubes corresponds to approximately the double of the turbostratic interlayer of graphite at ambient conditions (0.344 nm). Resonance Raman spectroscopy is a very well-suited technique for studying DWNTs as the laser excitation energy allows the resonance of both outer and inner tubes. By combining the sample tube diameter distribution and the energy excitation, it is possible to distinguish among four outer/inner configurations, that is, S/M, M/S, S/S, and M/M (S and M mean semiconductor and metallic, respectively). The electronic transition energies for the inner and outer tubes are well separated from each other, which facilitates the correlation between the Raman radial breathing mode (RBM) and the structural (n,m) indexes. High-pressure Raman scattering experiments were carried out using a membrane diamond-anvil cell with low-fluorescence diamonds having a culet size of 325 μm. Samples were loaded in a 125 μm hole drilled in a stainless steel gasket. Several ruby chips were distributed throughout the sample chamber, and the pressure was determined using the ruby fluorescence method.19 In our experiments, we used NaCl powder or paraffin oil as the pressure-transmitting medium (PTM). Raman spectra were obtained using a home-built high-throughput optical system based on Kaiser optical notch filters and an Acton 300i spectrograph with sensitive CCD detection. Spectra were excited using 514.5 nm (2.41 eV) radiation from an air-cooled argon-ion laser. The beam was focused on to the sample using a Mitutoyo 50 objective, with a beam diameter of 2 μm at the sample. The backscattered light was collected using the same lens. In Figure 1b, we show the low frequency Raman spectrum of pristine DWNTs obtained at ambient conditions. We can identify the contribution of outer and inner tubes and make a correspondence of the vibrational frequency with nanotube families, as shown in Figure 1c. In this panel, we show the so-called Kataura plot, which is a plot of electronic transition energies, ES/M ii , as a function of the RBM frequency ωRBM, where ωRBM is related to the tube diameter by ωRBM = A/dt þ B þ (C þ D cos 3θ)/d2t , with ωRBM and dt in units of cm-1 and nm, respectively. Indeed, the Kataura plot shows branches called families where the relation 2n þ m is constant for the nanotube indices n and m.20 The A, B, C, and D values used for constructing this plot were, respectively, 223 (228), 73 (14), -1.1 (-2.7), and -0.9 (-2.7) for semiconductor (metallic) nanotubes. The horizontal line is the laser excitation energy used in our experiments. The bands at 250, 262, and 315 cm-1 are from the inner tubes corresponding to metallic tubes probably originating from families 24, 21, and 18, respectively. The band at about 210 cm-1 could come from the family 29, and the outer tubes observed at about 160 cm-1 come from resonances of Elaser with the ES44 semiconductor conditions. The high-pressure behavior of inner and outer tubes can be tracked in the Raman experiments, which are discussed in the next section. Experimental Results. The pressure evolution of DWNTs for pressures below 10 GPa has been well-studied, and our results basically agree with previous reports. The positions of low RBM frequencies are linear with pressure and qualitatively in agreement with a previous study15 on DWNTs in a 4:1 methanolethanol PTM, and they are described in Table 1. Nevertheless, 5379
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Table 1. Phonon Frequencies Intercepts (ω0) and Pressure Slopes (DωRBM/DP) of RBM Evolution of DWNTs Using Paraffin and NaCl as PTMa linear fitting
linear fitting
linear fitting
paraffin oil
NaCl
4:1 methanol-ethanol15
mode
ω0 (cm-1)
∂ω/∂P (cm-1/GPa)
ω0 (cm-1)
R1
162.6
4.8 ( 0.4
160.3
R2 R3
175.1 206.7
4.1 ( 0.2 0.5 ( 0.2
172.4 209.9
∂ω/∂P (cm-1/GPa)
ω0 (cm-1)
∂ω/∂P (cm-1/GPa)
7.0 ( 0.3
175
5.8
6.7 ( 0.3 0.2 ( 0.2
186 267
5.8 2.2
R4
215.0
0.3 ( 0.2
R5
249.5
1.8 ( 0.1
250.1
1.4 ( 0.1
323
1.5
R6
259.8
2.3 ( 0.2
260.8
2.1 ( 0.3
384
1.1
R7
267.6
1.6 ( 0.2
270.1
1.4 ( 0.2
R8
316.7
-0.8 ( 0.3
315.3
-0.2 ( 0.1
a
Independent of the used PTM, the inner tubes (Ri, i = 5-7) pressure slopes are much lower than the outer tube ones (R1, R2). In analogy with what was already observed for the G band,10 we observe that the pressure slope of the outer RBM frequencies is strongly dependent on the nature of the PTM, whereas the inner tube slopes are much less affected. The physical origin of R3, R4, and R8 is less clear (see text). The zero-pressure phonon frequencies ω0 that come from the linear fitting have an uncertainty of 0.5 cm-1.
Figure 2. Radial breathing mode frequency vs pressure for pristine DWNTs using paraffin oil (a) and NaCl (b) as pressure-transmitting media during compression (filled symbols) and decompression (open symbols). The solid lines are fit to the experimental data using a linear function.
the very low pressure coefficients of R3, R4, and R8 call for considering a possible different physical origin either as coupling modes of intertube interaction or localized modes from endtubes. In particular, the R8 mode at about 315 cm-1 shows a quite singular behavior exhibiting a slightly negative pressure coefficient and which does not show pressure attenuation, contrary to all other low-frequency modes (see Figure 2). This behavior was already observed, even if not quantified, in ref 21. Further investigations using different excitation energies or including calculations would be needed to clarify both the origin of those three modes (RBM or other) as well as their singular pressure behavior. The evolution of the Raman G-band spectra during compression using paraffin oil or NaCl as the pressure-transmitting medium is shown in Figure 3a,b, respectively. We concentrate our analysis and main conclusions on the evolution of Gþ components of both inner and outer tubes as they could be followed for all pressures. Global attenuation of the G band with þ pressure or intensity inversion between Gþ inner and Gouter peaks at about 2 GPa are between the observed features, in good
Figure 3. High-pressure Raman spectra of the G band taken during compression (excitation energy, Elaser = 2.41 eV) for pristine DWNTs using (a) paraffin oil and (b) NaCl as pressure-transmitting media. We identify this band as being composed of four peaks corresponding to the overlap of Gþ and G- components of the inner and outer tubes.10,15 The four-peak fitting aproach is stable up to 13 GPa. The lowest-frequency peak consists of an asymmetric tail (centered at about 1530 cm-1), which is assigned as the Breit-Wigner-Fano profile related to the Gof the metallic inner tubes.17
agreement with previous reports.10,15,22 Contrary to previous experimental studies, which were limited to maximum pressures of ∼10 GPa, we do not find a linear behavior of the G-band frequency pressure dependence. We can, nevertheless, make a linear fit of the low-pressure domain and compare our results with previous ones (see Table 2). As for the RBM, the mechanical inner tube protection effect by the outer tubes is verified10,15 as G-band pressure slopes are smaller for inner than for outer tubes. All those observations have been already explained evoking (i) pressure screening from the outer to inner tube, (ii) strong coupling between the inner and the outer tubes, and (iii) mechanical support of the outer tube by the inner one. For pressures higher than 10 GPa, the RBM modes progressively attenuate and their intensities are not visible at pressures 5380
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Table 2. Phonon Frequencies Intercepts (ω0) and Pressure Slopes (DωG/DP) of G-Band Evolution of DWNTs with Pressure Using Paraffin or NaCl as PTMd linear fitting (0-3 GPa)
a
-1
-1
linear fitting (0-10 GPa) ω0 (cm )
∂ω/∂P (cm-1/GPa)
∂2ω/∂P2 (cm-1/GPa2)
1595.8
8.3 ( 0.2
1593.4
10.3 ( 0.1
-0.48 ( 0.01
1580.2
5.1 ( 0.2
1579.8
5.8 ( 0.1
-0.23 ( 0.01
8.4 ( 0.2 5.5 ( 0.4
1596.0 1579.2
8.0 ( 0.1 4.6 ( 0.1
1592.9 1577.6
10.4 ( 0.2 5.8 ( 0.1
-0.51 ( 0.02 -0.23 ( 0.01
Gþ o /Met-Et
6.8b
1592a/1594c
Gþ i /Met-Et
b
ω0 (cm )
∂ω/∂P (cm /GPa)
Gþ o /paraffin oil
1594.1
9.6 ( 0.3
Gþ i /paraffin oil
1578.6
6.4 ( 0.3
Gþ o /NaCl Gþ i /NaCl
1595.7 1578.6
8.5
-1
parabolic fitting (all ranges)
∂ω/∂P (cm /GPa)
mode/PTM
-1
ω0 (cm )
a
c
1579 /1582
6.1a/5.8c a
c
3.3 /3.3
-1
1592a
7.50a
-0.14a
a
a
þ0.07a
1579
2.65
15 b
0-10 GPa pressure range in a 4:1 Met-Et proportion. 0-3 GPa pressure range in a slightly hydrated Met-Et mixture (16:3:1).8 c 0-10 GPa pressure range in a 4:1 Met-Et proportion.10,22 d The zero-pressure phonon frequencies ω0 that come from linear fitting have an uncertainty of 0.7 cm-1. The experiments carried out in a methanol-ethanol mixture were included for comparasion.
Figure 4. Frequency vs pressure plots of pristine DWNTs using (a) NaCl and (b) paraffin oil as pressure-transmitting media during compression (filled symbols) and decompression (open symbols). The solid lines are fit to the experimental data using a second-order polynomial function.
higher than 8 GPa (except for the R8 line), so our discussion for the high-pressure domain will focus on the G band. The Gþ pressure slopes start to decrease, and a nonlinear dependence can be clearly identified. A change of sign of the pressure derivative of the G band appears for both the inner and the outer Gþ mode. This change of sign has been experimentally and theoretically assigned to the onset of the nanotube collapse.6,7 Accordingly, our experiments show that the outer tubes collapse at 21.4 (20.4) GPa for paraffin oil (NaCl), whereas inner tubes experience the collapse at about 25.0 GPa both for paraffin oil and NaCl (see Figure 4). Surprisingly and in contrast with the observed pressure dependence of the tubes' Raman modes (see Tables 1 and 2), we note that the DWNT collapse pressure is independent of the pressure-transmiting medium. This equally contrasts with the strong PTM dependence of the collapse pressure observed in SWNTs and peapods.6 That different behavior and the structural stabilitiy of DWNTs will be discussed further on in the next sections. We also note that the observed collapse is not a single event but takes place in two steps: first initiated in the outer tube and then followed by the inner-tube collapse. During decompression, we observe a large hysteresis in the G-band frequency evolution with pressure, which has been experimentally6 and theoretically8,12 shown to be an additional signature of the pressure collapse. The initial spectra at zero pressure are recovered for both PTMs after the pressure cycle,
Figure 5. (a) Loading curves for SWNT bundles based on circularoval and oval-peanut transitions for the (10,0) SWNT (black symbols) and circular-peanut for the (18,0) SWNT (red symbols). (b) The loading curve for a DWNT (10,0)@(18,0) (blue symbols) where we observe discontinuities on the volume bundle as the pressure is increased. The vertical red arrows indicate the two-step process of the collapse of DWNTs.
pointing out the reversibility of the mechanical deformations introduced in the DWNTs.
’ MODELING THE DWNT COLLAPSE We performed zero-temperature structural minimization of carbon DWNT bundles under compression, and we compared the obtained results to the corresponding calculations for SWNTs having the inner and outer tube diameters of the DWNT system. The carbon-carbon bonding (elastic term) in the CNTs is modeled by a reactive empirical bond order (REBO) potential proposed by Brenner23 and successfully applied to graphite, diamond, carbon nanotubes, and carbon clusters.24-26 Additionally, a pairwise Lennard-Jones potential model for the nonbonding van der Waals interactions is used (ε/kb = 44 K, σ = 3.39). These terms are essential for describing the intertube interactions. Our method consists of performing gradual and controllable reductions of the bundle volume (in steps of ΔV/V0 = -0.01%, where V0 is the zero-pressure volume) for each fixed nanotube phase studied (circular, oval, and peanut-like, i.e., collapsed), while minimizing the energy by variation of cell parameters. We obtain an univocal correspondence between minimum energy 5381
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The Journal of Physical Chemistry C and volume for each studied phase. In this way, the pressure at zero temperature is well defined through the thermodynamic relation p = -dU/dV. We can then construct p-V diagrams that allow us to study phase transitions. The enthalpy H = U þ pV is also calculated to determine the most stable structures. For our simulations, we selected zigzag (10,0) and (18,0) SWNTs that have diameters of 0.78 and 1.41 nm, respectively, which are close to the mean values of the experimentally studied system. In Figure 5a, we compare the collapse of the two studied SWNTs within their respective p-V diagrams. Upon increasing pressure, the (10,0) SWNT bundles (black symbols) reach the oval phase at about 1.55 GPa, and for larger pressures, we observe the bundle collapse to peanut-section phase at 9.6 GPa. For the larger diameter (18,0) SWNT (red symbols), the collapse to the peanut-section phase occurs at much lower pressures (1.48 GPa) and without the appearance of the intermediate oval phase, in agreement with Tangney et al.27 Our results for the collapse pressure values (pc) show a strong dependence with tube diameter, 5,27 In fact, we have tested in relative agreement with the d-3 t law. several tube diameters, and as previously observed, the d-3 t law is well verified for diameters above 0.9 nm, a value from which some deviations start to be detected due to the strong curvature effects. The sequence of phase transitions in DWNTs is more complex. In Figure 5b, we show the p-V curve for (10,0)@ (18,0) DWNT bundles under pressure. At around 1.0 GPa, we observe a first phase transition where the outer tube reaches the hexagon-like polygonalized phase while the inner tube keeps its circular cross section. Interestingly, the noncollapsed DWNT (with hexagon-like polygonalization for the outer tube and with an inner circular cross section) remains a metastable phase up to 13 GPa (not shown), but after 5.8 GPa, the flatness state corresponds to the global minimum of enthalpy. Around 5.8 GPa, the inner tube then becomes oval and the outer tube changes from hexagon-like to a rectangle-like polygonalized shape, a modification probably driven by the symmetry breaking of the inner tube. The inner tube then continuously reduces its volume up to 9.0 GPa, where the DWNT is completely collapsed. Indeed, it is interesting to note that the flattening of the DWNT is observed in a two-step process, as indicated by arrows in Figure 5b, where the first step (5.8 GPa) is a discontinuous change of bundle volume due to the outer tube transition and then the second (around 9.0 GPa) is the continuous flattening of the inner tube. Other tube combinations were also tested as (20,0)@(12,0) and (21,0)@(13,0) (not shown), in which a similar scheme of phase transitions was obtained with lower transition pressures corresponding to the higher tube diameters.
’ DISCUSSION In resonance Raman experiments, below any collapse, we find the known manifestation of the structural support of the inner tubes: (i) reduced pressure slopes of both the radial breathing mode (RBM) and the G-band frequency of the internal tubes in DWNTs10,28 and (ii) PTM dependence of the outer tube Raman signature evolution with pressure. Our results support these findings, which will not be further discussed here. We shall then concentrate our discussion on the effect of nanotube filling on its structural stability toward collapse. As already discussed, it is now well stablished for SWNT the d-3 t dependence of the pressure collapse pc for dt > 0.9 nm. The effect of including an additional inner tube on the stability toward collapse of SWNTs has been discussed in some theoretical studies. Yang et al12 obtained that
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Table 3. Collapse Transition Pressures for SWNTs as a Function of the Filler Naturea pc
Δpc = pfilled - pSWNT c c
(GPa)
(GPa)
dt (ext. tube) filler
(nm) 6
1.35 1.35
none C706
14 10
1.35
iodine13
7-9
1.35
SWNT dt = 0.65
34
þ20
1.35
argon30
>40
>þ26
1.55
none
9
1.55
SWNT dt = 0.85
21
0 -4 -6
0 þ12
a
Two tube diameters are considered (1.35 and 1.55 nm). DWNTs are considered as SWNTs having an SWNT as filler. For the 1.35 nm tube, all values are experimental except for the tube filler, which was extrapolated from our experimental values on the basis of the dtav-3 law. Our experimental value is shown for the 1.55 nm tube with a tube filler and compared with an unfilled tube of the same diameter, for which the collapse pressure is estimated from the unfilled 1.35 nm tube and the dt-3 law.
the DWNT collapse pressure values should be proportional to d-3 tav , where dtav is the average value of inner and outer tube diameters. In that work, a two-step collapse was found to be possible for certain tube symmetries. On the other hand, Gadagkar et al8,29 find that the DWNT collapse pressure is unique and equal to the sum of individual collapse pressure values of corresponding SWNTs. As discussed, the G-band maximum in the pressure shift of the G-band frequency has been identified by experiments and calculations6,7 as being associated with the collapse of nanotubes. Our experiments clearly show that these maxima are separated for inner and outer tubes, with the inner tubes having a collapse pressure 4-5 GPa higher than the outer tube. The collapse transition is then two-step. The two-step collapse scenario is also supported by our calculations for DWNT zig-zag tubes and was also obtained in calculations by Yang et al12 under certain symmetry conditions. We note, nevertheless, that all calculations appear to underestimate the collapse pressure observed here . We shall now consider the DWNT collapse pressure observed here and compare it with that of the SWNT. The experimental observed critical pressure of volume collapse for the outer tube having a diameter of 1.56 nm (22.5 GPa) (Table 3) is significantly higher than the 14 GPa value found by Caillier et al.6 for tubes of a smaller diameter of 1.35 ( 0.1 nm with the same PTM. If we now consider the expected collapse pressure calculated following the d-3 t law taking as reference the SWNT of 1.35 nm (14 GPa) and apply it for both the external (1.55 nm) and the internal tubes (0.85 nm), we obtain transition pressures of 9 and 54 GPa, respectively. These values strongly differ from the ones observed in our G-band analysis (∼21 and ∼25 GPa). The outer tube in the DWNT is destabilized then at a pressure 12 GPa higher than for the corresponding SWNT, confirming predictions of inner tube support against collapse. On the other hand, the inner tube, with a potential of structural stability up to 54 GPa, is destabilized by the outer tube collapse at a pressure of 25 GPa. Those observations put into evidence a mechanism of cascade in the two-step collapse. Our results give then a DWNT collapse pressure of 63 GPa in the model of Gadagkar,8,29 whereas if we consider an scaled d-3 tav law proposed by Yang,12 and using the same proportionaly constant as for SWNT, we obtain a collapse transition of 20 GPa. The pc ∼ d-3 tav compares quite well with the observed pressure collapse for the external tube (∼21 GPa). 5382
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Figure 6. Filling effect on the collapse pressure. The pressure evolution of the Gþ frequency is shown for an SWNT (no filler) and for tubes filled with C70 or with other tubes (DWNT). In all cases, silicone oil was used as the pressure-transmitting medium. The collapse pressure is evidenced by the maximum of the Gþ(P) curve. The stabilizing effect of tube filling toward fullerene filling is evident despite the 0.2 nm larger diameter of the DWNT external tube. After collapse, all frequencies tend to approach the graphite extrapolated values.
Let us then compare the effect of different SWNT fillers on the tube mechanical stability, which we define as Δpc = pfilled c . In Table 3, we compare the SWNT collapse pressures for pSWNT c different fillers: C70, argon, and iodine6,13,30 for tubes having a narrow diameter distribution centered at 1.35 nm. Whereas argon filling leads to an important mechanical stabilization, the effect is opposite for iodine or C70 filling. It is also seen in Table 3 that, in our DWNT sample, the external tube diameter of 1.55 nm already leads to a stabilization toward collapse of Δpc = þ12, which can be computed also for a DWNT having a diameter of 1.35 nm dependence of pressure collapse for considering the d-3 t DWNTs, as supported by all calculations. The obtained value for a DWNT having a 1.35 nm external tube diameter is then Δpc = þ20. The case of argon filling or tube filling (DWNT) leads to positive and large Δpc values, that is, a strong stability of the tube toward collapse, whereas iodine or C70 lead to small, but negative, values of Δpc. Figure 6 shows the pressure evolution of the Gþ frequency for SWNTs compressed in silicone oil pressure-transmitting medium having either no filler, C70 fillers (peapods), or SWNT fillers (DWNT) from our work. The Gþ evolution before collapse is perfectly analogous in the three cases even if the tube diameter is larger for the DWNT. We clearly observe the destabilization effect toward collapse of C70 filling, whereas SWNT filling leads to a stability of Δpc ∼ 5 GPa, even if the external tube of the DWNT has a 0.2 nm larger diameter. We also note that, in all measurements, after collapse, the Gþ frequency tends to approach the extrapolated evolution of the graphite G-band frequency, as a characteristic signature of collapse. In the case of iodine filling.13 the collapse pressure has been seen to vary from 7 to 9 GPa in different samples depending on the tube filling degree. For C70 peapods, the fullerene distribution leads to an inhomogeneous interaction between the tube and the inner molecules at the molecular level, which can lead to the tube mechanical instability even at ambient pressure.31 In general, strain inhomogeneities are know to lead to structural changes at lower pressures than for fully hydrostatic conditions. Iodine- or C70-filled tubes for different reasons present an inhomogenous filling at the molecular scale, which should introduce strain inhomogeneity and, consequently, instability toward collapse. On the other hand, tube filling and likely filling with argon provide a homogeneous tube filling consolidating its mechanical stability.
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It is noticiable that, in SWNT- or in C70-filled tubes, the collapse transition was found to be strongly dependent on the nature of the pressure-transmitting medium, with values close to 3-fold differences.6 In the case of DWNTs, we have not observed any dependence of the collapse transition on the nature of the PTM. This effect is even more relevant when we consider that, before the pressure collapse, external tubes' Raman modes showed a pressure slope strongly dependent on the PTM nature. This chemical sensitivity of the external tube has then no impact on the mechanical stability: the outer tube offers both a mechanical and a chemical screening to the inner tube, while the latter acts as a mechanical support for the system. The overall interpretation of experimental results is in perfect agreement (despite scale factors) with our theoretical results. The reason for the theoretical values of collapse to be quite different from experimental observation could be the nonperfect bundle arrangement of real samples, the presence of nonbundled DWNT nanotubes, or the influence of ordering or shell formation of pressure-transmiting media around the individual nanotubes, which could lead to screening effects on outer nanotubes and, consequently, on inner tubes. In fact, as already discussed, both PTM used are solid at the collapse transition and no PTM was used in the calculations in which the transmission of pressure effects are due to intertube interactions.
’ CONCLUSIONS In summary, the vibrational and structural properties of pristine double-walled carbon nanotubes (DWNTs) were investigated by high-pressure resonance Raman scattering for two different pressure-transmitting media (paraffin oil and NaCl). We find that, in all cases, the outer tube is mechanically supported through its interaction with the inner tube, leading to significantly higher collapse pressures than in the corresponding SWNT with similar diameters. The collapse transition takes place in two-steps, in good agreement with our calculations and scales as d-3 tav , where dtav is the average tube diameter of the DWNT, as proposed by Yang.12 We have compared the effect on the mechanical stability of SWNT filled with another tube or other fillers. A significant stabilization is observed for SWNT or argon fillers, whereas iodine or C70 fillers leads to lower collapse pressures. Filling homogeneity at a molecular level appears to be a key factor to explain such differences. Finally, we have shown that the collapse transition of DWNTs is independent of the chemical effects of the pressure-transmitting media. This observation contrasts with the chemical sensitivity of the external tube during the compression process. Consequently, a conjugated and complementary role of inner and outer tubes can be advanced: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. The overall result is a higher collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs an ideal filler for composite nanomaterials for high load mechanical support. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: agsf@fisica.ufc.br (A.G.S.F.), alfonso.san.miguel@ univ-lyon1.fr (A.S.-M.). 5383
dx.doi.org/10.1021/jp110675e |J. Phys. Chem. C 2011, 115, 5378–5384
The Journal of Physical Chemistry C
’ ACKNOWLEDGMENT The authors acknowledge the CAPES-COFECUB (Grant 608) grant for the partial support of this research. The Brazilian authors acknowledge Rede Nacional de Pesquisa em Nanotubos de Carbono (MCT-CNPq) and INCT NanoBioSimes. A.G.S.F. acknowledges CNPq for grants 556927/2008-7 and 307317/2010-2. M.E. acknowledges the support from the the Regional Innovation Cluster Program of Nagano and MEXT grants (No. 19002007), Japan. ’ REFERENCES
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