E. Flahaut's Homepage on Double-walled
Carbon Nanotubes (DWNTs)

  • Brief introduction
  • A step forward: MgO-based catalysts
  • Filling CNTs with 1D-crystals
  • Carbon-encapsulated Co nanoparticles
  • Localised growth of CNTs
  • Nanocomposites
  • High yield synthesis of Double-Walled CNTs "new"
  • Toxicity and Ecotoxicity of CNTs "new"

  • Introduction:
    I am working at Toulouse University (France) in the CIRIMAT (Inter-University Research Center on Materials Engineering). Most of my research activity is actually dealing with the synthesis of Carbon Nanotubes (CNTs) by a Catalytic Chemical Vapour Deposition (CCVD) method, including the localized growth of CNTs on different patterns but it also deals with the synthesis of bulk composite materials containing CNTs. This part of my work is well described in the "Nanocomposites and Carbon Nanotubes Group" website so I will not duplicate it here. French readers will find an introduction to carbon nanotubes here (extracted from my D. Phil. Thesis). The science of CNTs is still rather new and it is amazing to realize how little we still know about these fascinating objects!

    A step forward: MgO-based catalysts
    One of the most interesting result we have obtained is the use of an easy-to dissolve catalyst for the synthesis of Single-walled CNTs (SWNTs) [E. Flahaut et al., J. Mater. Chem., (2000), 10, 249-452]. MgO was chosen because it can be dissolved in mild acids even at room temperature (avoiding dangerous treatments using for example HF or refluxing HNO3), thus allowing for the easy extraction of the CNTs by dissolution of Co metal (formed during the CNTs synthesis step) and the remaining unused catalyst. This result has been published some time after we first used this method during my Ph. D (1996-1999) and has now been re-used by many authors to prepare SWNTs (see for example Chem. Phys. Lett., 317, (2000), 83). We were the first to use a Mg1-xCoxO solid solution , prepared by the ureic-combustion method. Other solid solution such as Mg 1-xFexO and Mg1-xNix O were also prepared at that time but were found less or not interesting for the synthesis of SWNTs, especially in the case of Ni (as Chen et al. published in 1997, we also found, using our specific method, that Ni led only to multi-walled CNTS in very low yield). We found that if high loadings of transition metal were to be used, the ratio of urea had to be increased to maintain the transition metal oxide in solid solution in MgO (D. Phil. Thesis). We have recently modified the combustion step, replacing urea by citric acid (according to the litterature). We found that this lowers the reaction temperature compared to the ureic process, and allows to keep higher specific surface areas for the oxides.

    The images below illustrate the combustion process, the formation of a CNTs-Co-MgO composite powder from the reduction of the Mg 1-xCoxO solid solution and then the extraction of the CNTs using HCl (chosen for being a non-oxidising acid).

    Filling CNTs with 1D-crystals
    I am also interested in the use of CNTs as templates for the synthesis of confined 1D nanocrystals ; I have learned a lot in this area, from Malcolm Green's group (and especially from Jeremy Sloan) during my post-doctoral stay in Oxford. I will describe below some of the nice results we got there together. I am focusing here on the filling of low diameter CNTs (such as SWNT and DWNT), which is far more difficult than the filling of wide-diameter MWNTs.

    Metal halides may be introduced (in the melted state) into SWNTs by capillarity. These experiments permit the study of low dimensional crystal growth whereby the incorporated material is constrained by the encapsulating van der Waals surface of the carbon capillaries, to a few atomic layers in thickness. The preparation of highly anisotropic 1D structures confined into SWNTs is one of the key objectives in carbon research, as the introduction of these materials into the hollow nanotubes cavities could result in interesting effects on their physical and electronic properties.

    Amongst many examples, here is the of TlCl. This salt, which can crystallize both with the Pm3m CsCl-type structure and with the Fm3m rocksalt structure, crystallizes predominantly in the latter form inside SWNTs. In the examples in Figures 1(a) and (b), two examples of (2x2) TlCl crystals formed inside 1.4nm diameter (i.e. conforming to (10,10)) SWNTs are shown. The bilayer crystals image as a continuous array of dark spot pairs forming along the SWNT capillaries, as shown in the lattice images in Figs. 1(a) and (b). The detail in Figure 1(c) shows clearly the microstructure of the incorporated crystal. An image simulation in Figure 1(d) shows that each of the dark spots images with identical contrast which is to be expected because each atom column contains the same pair of atoms in projection (i.e. Tl-Cl or Cl-Tl). The crystallisation behaviour of this material is therefore similar to that of KI, which forms both (2x2) and (3x3) crystals inside SWNTs. As with (2x2) KI crystals observed in 1.4nm (ca. (10,10) SWNTs), lattice distortions were observed in the incorporated crystals. Along the capillaries a d-spacing of ca. 0.32nm was obtained (which corresponds to {200} for bulk TlCl) while across the capillaries, this spacing increased to nearly 0.4nm. Wider SWNTs were also observed incorporated with TlCl, with the rocksalt form predominating although the lattice distortions were less pronounced.

    Figure 1.

    This cubic structure is the simplest one, so the modelization is not so tricky but with a more complex structure, the results become more difficult to interpret. The case of PbI2 is a lot more complex than that observed for TlCl, presumably because this material forms a relatively more complex structure in the bulk. The P-3m1 layered form of PbI2 is essentially similar to CdCl2 and consists of stacked layers of edge sharing PbI6 octahedra (Figure 2). This structure was found to arrange in different ways inside SWNTs and DWNTs according to the diameter of the confining tubules, as well as the obtained direction of crystal growth inside their capillaries (Figures 3 to 5).

    Figure 2. Bulk structure of PbI 2 consisting of stacked layers of PbI6 edge sharing octahedron (cf. CdCl2).

    Figure 3. (a) HRTEM image of a SWNT bundle completely filled with PbI2. (b) Detail obtained from the boxed region in (a) ; 0.36nm (or {110}) lattice planes of PbI 2 are clearly visible in two adjacent tubules on the periphery of the bundle. (c) FFT obtained from a slightly larger region of bundle than (b). The indicated maxima correspond to lattice planes apparently related by a mirror plane (however, the planes occur in adjacent tubules !). (d) HRTEM image of a discrete SWNT filled with a 1D crystal of PbI2 . (e) Detail from boxed region in (d), showing arrangement of PbI2 polyhedra, which appear, as dark spots. (f) and (g) 'best fit' image simulation and corresponding structural model conforming to a three polyhedral thick slab of PbI2. Edge terminating PbI5 square pyramids are indicated in red. (h) End on view of SWNT/PbI2 composite showing a 1D chain of PbI6 octahedra bounded by two 1D chains of reduced coordination PbI5 square pyramids.

    In the case of SWNTs, most of the obtained encapsulated 1D PbI2 crystals show a strong preferred orientation with their {110} planes aligning at an angle of ca. 60° to the SWNT axes as shown in Figures 3(a) and (b). Due to the extremely small size of the nanotubule capillaries, individual crystallites were often only a few polyhedral layers think, as outlined in Figures 3(d) to (h). As a result of lattice terminations enforced by capillary confinement, the edge polyhedra must be of reduced coordination, as indicated in Figures 3(g) and (h) (terminating square pyramidal polyhedra are represented in red). This crystal growth behaviour resembles that recently reported for lanthanide halides incorporated within SWNTs.

    In some cases, capillary confinement gives rise to some interesting folding effects in the incorporated PbI2 crystals, such as that shown in Figure 4. In this case a five-layer thick polyhedral slab of PbI2 is folded about its centre to form a highly strained crystal within the tip of a 2nm SWNT (Figures 4(a)-(f)). As with the example shown in Figure 3, this crystal terminates with square pyramidal polyhedra (Figure 4(g)).

    Figure 4. (a) and (b) two HRTEM images obtained at different defocus of a PbI2 1D crystal formed in the tip of a 2nm diameter (ca. (15,15) conformation) SWNT. In enlargement ((c) and (d)) an apparent bend in the crystal is observed. The bend forms down the middle of the crystallite, as indicated by the arrows in (e) which, assuming the same general microstructure as in Figure 2, corresponds to a 'folded' slab of PbI2 polyhedra, as indicated in side on projection in (f) and in end on projection in (g). As with Figures 3(g) and (h), terminating PbI2 square pyramids are indicated in red.

    In the case of PbI2 formed inside DWNTs, in general, similar crystal growth behaviour was observed to occur in narrow tubules with comparable diameters to those of SWNTs. However, as the diameter of the incorporating capillaries increased, frequently different preferred orientation was observed, as shown by the example in Figure 5. In this example, the crystal has grown and is viewed with the [121] direction arranged parallel to the direction of the electron beam (Figures 5(a)-(d)). If the SWNT/PbI 2 composite is viewed 'side on' (as indicated by the arrow in Figure 5(e)), it can be seen that polyhedral slabs (cf. Figure 2) are arranged along the capillary oriented at an angle of ca. 45° to the tubule axis.

    Figure 5. (a) HRTEM image of a DWNT continuously filled with PbI2. The inset FFT indicates that this crystal is being viewed in a [121] projection. (b) Detail from the boxed region in (a). (c) shows an image produced by applying an adaptive filter followed by an inverse Fourier transform to an FFT produced from (b). (d) 'Best fit' image simulation obtained from the structural model in (e) with the incorporated PbI2 crystallite arranged in a [121] orientation with respect to the beam direction. (f) If we now look at the DWNT/PbI 2 composite in a 'side on' projection (i.e. in the direction of the large arrow in (e)), we see that the PbI2 layers (cf. Fig. 2) are arranged at ca. 45º to the tubule axis.

    The capillary method can be used to grow 1D crystals of different p-block halides in single-walled carbon nanotubes and we have found that the diameter of the host SWNT profoundly influences the obtained structure of the filling material. The reduction of the coordination of the ions in the periphery of the nanotubes, due to the confinement into the capillaries, has been demonstrated and must lead to physical properties different from those of the bulk crystals. This could be especially interesting in the case of PbI2, which exhibits both semiconducting, X-ray imaging and optical properties. Experiments are also under way in our laboratory to investigate the physical properties of these novel composites (coll. LNCMP, Pr J.M. Broto, Dr B. Raquet, Dr M. Sagnes, Dr B. Lassagne, Dr W. Escoffier).

    Carbon-encapsulated Co nanoparticles:

    The precise control of the CCVD parameters also allows the preparation of carbon-encapsulated metal nanoparticles, mainly by adjusting the carbon supply. Here is an example of carbon-encapsulated cobalt nanoparticles (Chem. Mater., 14, (2002), 2553-2558) showing a very high resistance to air oxidation at room temperature (evidenced by magnetic measurements (SQUID))

    HRTEM Image

    Energy-filtered imaging

    Localised growth of Carbon nanotubes:

    We are developing different approaches to achieve the localised growth of carbon nanotubes on different substrates. We have especially focused our research work on the stamping technique (we developed a new sol-gel ink to be stamped on silicon substrates using a PDMS stamps, see Microelectronic Engineering, 73-74, (2004), 564-569) and more recently on electron-lithography. The main interest of localised-growth is to allow the precise positioning of CNTs on a substrate, avoiding for example the tedious nano-manipulation required for CNTs randomly deposited from a suspension. This is very important in the case of nano-devices and nanoelectronics in particular.

    We have recently started working on the localized growth of CNTs on different substrates, such as AFM tips (see picture on the right). The controled growth of such nanometric wires could represent an important step forward in nanoelectronics. For this project, we collaborate with
    Dr Christophe Vieu (LAAS).

    Ceramic-based nanocomposites
    Information on the work dealing with composite materials containing CNTs can be found on the "Nanocomposites and Carbon Nanotubes Group" website so I will not duplicate it here. We do not limit our research to ceramic-based nanocomposites but also work on polymer-based composites. In the case of the ceramic-matrix composites, we have evidenced that our in situ synthesis technique leads to very homogeneous distributions of the CNTs within the composites, which would not be possible to obtain by simple mixing of oxyde grains and CNTs. Although we did not evidence any improvement of the mechanical properties due to the presence of the CNTs (however, the presence of the CNTs usually limits the densification to ca. 90%), we have shown that CNTs bring electrical conductivity to the composites. The percolation threshold was found around 1 vol.%, and even lower in the case of polylmer-matrix composites.

    High yield synthesis of DWNTs
    We have recently (Chem. Commun., (2003), 1442-1443)), developped the high yield CCVD synthesis of DWNTs. The catalyst used is MgO-based and contains only 1% of active catalyst (Co and Mo oxides), the rest being the support (MgO). One gram of this catalyst leads to at least 130 mg of very pure DWNTs (no contamination by amorphous carbon deposits) which do not require any further purification, apart of course from the dissolution of the remaining catalyst at the end of the CCVD (HCl washing). The selectivity towards DWNTs is close to 80%, which is unique amongst all the results currently published in the literature. We generally work with around 10g of catalyst in one batch, which means that we can produce almost 1.4 g of clean DWNTs in one run. Of course, this is limited by the size of our lab-scale reactors and could be scaled-up. These DWNTs have been characterised by electron diffraction and it has been shown that the samples contains some very nicely organised bundles of similar-diameter DWNTs (Nano Letters, 3 (5),(2003), 685-689). The composition of this catalyst has been found after an in-depth study of the ratio between Co and Mo (J. Mater. Chem., 14, (2004), 646-653) which indicated that only a small ratio of Mo (Co:Mo ratio) must be used if the formation of clean DWNTs is wanted. This is in opposition with the results published by Resasco et al. but it must be noted that they used CO instead of CH4, which makes an important difference in terms of operating temperature, and thus of reactivity. Other studies (Raman spectroscopy, photoluminescence, electrical characterisation, etc.) are also available (see list of publications).

    Bundle of DWNTs (HRTEM Image)

    Through focal series

    Toxicity and Ecotoxicity of CNTs
    The CIRIMAT is also involved in the study of the toxicity of CNTs for many years now. The cytotoxicity of different samples of CNTs (double-walled CNTs, DWNTs [a] or a sample containing a mixture of DWNTs and triple-walled CNTs) prepared from different catalysts was investigated in collaboration with the INSERM U577 in Bordeaux (Pr Baquey) towards human umbilical vein endothelial cells (in vitro tests) [b]. These specific cells were chosen because CNTs are often thought to be used for drug-delivery and would be injected in the human body to be carried to targeted places by the blood. No sign of cytotoxicity was found for any of the tested samples. The inflammatory response after exposition to DWNTs was investigated in collaboration with the University of Oxford (Pr M.L.H. Green and Dr B. Sim) and compared to other CNTs samples [c]. It was found that all tested samples activate in vitro the human complement system by the classical pathway, and that DWNTs also activate the alternative pathway. Experimental results clearly show the selective adsorption of a very limited number of human serum and human plasma proteins onto the CNTs. This may open the door to drug delivery because the adsorption of theses proteins may facilitate the translocation of the CNTs within the target cells. The CIRIMAT is also involved in the investigation of the ecotoxicity of CNTs (PhD student starting Oct. 2005) and especially towards the aquatic environment, in collaboration with the LEH (Dr L. Gauthier, Toulouse). As a conclusion, the huge difference between available CNTs samples (even between different batches made using the same synthesis technique in the case of industrial/commercial sources) makes difficult any comparison and shows that extrapolation is dangerous. This makes the determination of the toxicity threshold of CNTs difficult. More work is needed to understand the mechanisms of toxicity of CNTs and to prevent the toxicity if possible. An important work of characterisation of the tested samples (chemical purity, morphology and structure, composition) is essential if we want to achieve this goal.

    We have recently been awarded a grant by the French "National Research Agency" (ANR) to suuport our work in this field (2007-2010).

    The CIRIMAT is also the French partner of the Marie-Curie Research Training Network (RTN) CARBIO (2006-2010), involving 8 partners from 6 European coutries (Germany, Austria, France, UK, Poland, Netherlands). The aim of this project is tu use functionalised CNTs filled with magnetic materials for selective thermotherapy of cancer cells.

    [a] E. Flahaut et al., Chem. Commun., (2003), 1442-1443

    [b] E. Flahaut et al., Carbon, 44, (6), (2006), 1093-1099

    [c] C. Salvador-Morales et al., Molec. Immun., 43, (3), (2006), 193-201

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    Last update: Jan. 2007