The following section is a short introduction to my most recent research (2007 to 2009) at the University of Geneva in the laboratory for colloids and surface chemistry.

The adsorption of macromolecules at the solid-liquid interface is a very common yet complicated phenomenon. One has simply to think of what happens when a drop of blood falls on a surface. Blood is a complex colloidal system. The fluid contains several macromolecules (proteins, biopolymers) as well as ions (Fe, K, Cl, Na etc.) and other cells all interacting with each other and with the surface. Preventing or promoting the adsorption of blood (or its specific constituents) to a surface may be critical in some biomedical applications. This example illustrates how macromolecular adsorption is ubiquitous, yet can be of great importance for both industry and fundamental science.

In terms of nano-technological applications, charged macromolecules from solutions can be used to pattern mineral surfaces. Typically, an interface will acquire a charge when immersed in a solution, water in this case. Similarly, the macromolecule’s ionizable functional groups can take or release protons, thus have a pH-dependent charge. When the charges of both molecule and surface have opposite signs and are of sufficient magnitude, adsorption via electrostatic interactions occurs.

Adsorption of PAMAM dendrimers

A fair deal of research in our lab deals with cationic poly(amidoamine) dendrimers (Fig.1). These hyperbranched synthetic polymers are ideal molecules for surface patterning. They are globular in shape, adsorb rapidly either reversibly or irreversibly at interfaces^[1] and are available in a wide range of sizes depending on the generation.

Figure 1: Structure of poly(amido amine) dendrimers (PANAM)

Adsorption of PAMAM dendrimers on silica is the result of strong electrostatic interactions between the charged amine groups (NH3+) on the dendrimer and the dissociated silanol (Si-O-) sites on the surface. Hydrogen bonding is also believed to contribute to the adsorption (Fig.2). The dendrimers tend to flatten once at the interface so as to maximize surface-segment binding.

Figure 2: Schematic representation of PAMAM dendrimer adsorption on silica

The spontaneously organization of dendrimers at the silica-water interface is not random but rather liquid-like as can be seen from AFM imaging (Fig.3).

Figure 3: AFM image of G10 PAMAM dendrimer on silica

The inter-dendrimer spacing depends on the electrostatic repulsions, which can be modulated via the pH and ionic strength of the solution^[3]. Some degree of control over the monolayer’s structure is achievable as feature dimensions (height, width) depend on the selected dendrimer generation. Therefore solid substrates can patterned at the nanometer scale using adsorbed dendrimers^[4]. The reader should consult the cited articles for more details.

Literature

[1] R. Longtin, P. Maroni, M. Borkovec, Langmuir 2009, 25, (5), 2928-2934

[2] I. Popa, R. Longtin, P. Maroni, G. Papastavrou, M. Borkovec, CHIMIA International Journal for Chemistry 2009, 63, 279

[3] B. P. Cahill, G. Papastavrou, G. J. M. Koper, M. Borkovec, Langmuir 2008, 24, (2), 465-473

[4] R. Pericet-Camara, B. P. Cahill, G. Papastavrou, M. Borkovec, Chemical Communications 2007, 266-268.

Author: Remi Longtin

Nanotechnology and nanoscience have certainly been one of the most popular fields of research in the last decade. Manufacturing processes are now able to carry out the deterministic synthesis of nanostructures with properties radically different from their macroscopic forms, enabling the realization of previously unthinkable devices. Despite these advances, few nanofabrication techniques feature the required characteristics for the commercial manufacturing of these new products in an effective fashion since they are either too slow, or too expensive and complex.

This page is a summary of my doctoral thesis project, accomplished at the Laboratory for multiscale mechanics at the École Polytechnique de Montréal (LM2, mechanical engineering). The main objective of this project was to develop a new manufacturing technique allowing the local synthesis of nanostructures on a surface for their eventual integration into nanodevices. The desired process has to be selective, reproducible, versatile, simple, fast and inexpensive for potential industrial utilization. Moreover, the manufacturing process must have a minimal environmental impact for sustainable development.

To implement the required specifications, a laser process combining the characteristics of laser-induced chemical liquid deposition (LCLD) and of sol-gel synthesis was proposed. The technique is very simple and consists of three steps. A precursor solution is first prepared. Next, a droplet of a controlled volume is transferred on a substrate by means of a micropipette. The droplet is then irradiated using a laser emitting in the infrared to induce the fast synthesis of nanostructures.

ZnO nanostructure synthesis by LCLD

In order to prove the feasibility of the manufacturing concept for the synthesis of nanostructures, the new process was used for the synthesis of zinc oxide (ZnO) nanostructures. ZnO has become one of most studied nanomaterials in the five last years as it presents very interesting properties for optoelectronics and sensing applications, while being synthesizable in a plethora of nanoscale morphologies.

Using laser processing, the deposition of coatings of several square millimetres of various nanostructures (nanorods, nanowires, porous films of nanoparticles) was carried out. In particular, nanorods with an average width of 300 nm and a length of two micrometers with hexagonal cross-sections and almost atomically flat surfaces were synthesized (see figure 1). Nanowires with diameters of approximately 50 nm and lengths exceeding four micrometers were also grown. This constituted an innovation among the laser processing techniques, as only laser-induced chemical vapour deposition (LCVD) and pulsed laser deposition (PLD) had been used to produce ZnO, and just in the form of thin films and nanoparticles.

Figure 1: Example of nanorods grown by LCLD

Caracterization and optimization of the process

One of the secondary objectives of this thesis was to improve the properties of the deposits for one of the target applications of ZnO, photoluminescent devices. For this reason, a parametric study was carried out during which the influence of the laser-related parameters (irradiation time, intensity) and of the solution-related parameters (precursor, additives, concentration) on morphology and crystallinity was studied. The use of scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that an increase in laser intensity and irradiation time increased nanostructure length and crystallite size (see figure 2A). Transmission electron microscopy (TEM) demonstrated that the nanorods grew along the c axis of the crystal lattice, at the apex of randomly oriented ZnO crystals forming a seed layer on the substrate. Additionally, an increase in precursor concentration was found to increase the thickness of this seed layer and the introduction of additives in the solution had the effect of promoting the vertically aligned growth of nanorods. Raman and photoluminescence (PL) spectroscopy also showed that the deposits were of high quality with few crystalline defects. In particular, the PL spectroscopy results gave evidence that the ZnO nanostructured deposits produced by the laser process were good for ultraviolet emission applications with the presence of an intense peak at 390 nm (see figure 2B). The extensive characterization of the samples also allowed the development of a qualitative growth model for the laser-grown ZnO nanostructures inspired by the growth and nucleation models used for conventional chemical synthesis in a liquid medium.

Figure 2: Microstructure of samples A)XRD, B)PL

This project led to the development of a new technique allowing the synthesis of high quality ZnO nanostructures in a few seconds whereas the traditional techniques of chemical synthesis need several hours. Once the reproducibility and selectivity problems are resolved, this very promising technique could easily be upgraded for the production of nanodevices such as UV nanolasers or light emitting diodes. Three scientific articles were published during this project:

[1] Fauteux, C., Smirani, R., Pegna, J., El Khakani, M. A., Therriault, D., Fast synthesis of ZnO nanostructures by laser-induced chemical liquid deposition, Applied Surface Science, vol. 225, 2009, pp 5359-5362;

[2] Fauteux, C., El Khakani, M. A. , Pegna, J., Therriault, D., Influence of solution parameters for the fast growth of ZnO nanostructures by laser-induced chemical liquid deposition, Applied physics A: Materials Science and Processing, vol.94, 2009, pp. 819-829;

[3] Fauteux, C., Longtin, R, Pegna, J., Therriault, D., Fast Synthesis of ZnO Nanostructures by Laser-Induced Decomposition of Zinc Acetylacetonate, Inorganic Chemistry, v. 46, 2007, pp. 11036-11047;

Author: Christian Fauteux

Freeform fabrication is a type of manufacturing process that allows the fabrication of 3D objects with a large variety of shapes, structure and composition. Laser-assisted chemical vapor deposition (LCVD) is a freeform fabrication technique that allows the deposition of micro-scale structures at a high growth rate with the flexibility of conventional CVD.

The LCVD process

During chemical vapour deposition (CVD), energy is given to a gas (the precursor gas) which contains the atom that we want to deposit. This energy dissociated the gaseous molecules by a series of chemical reactions in order to obtain a solid product (the deposit) on a surface where we want the deposition to occur (the substrate). This can be conceptualized by the following equation which describes the deposition of the gaseous species AB_u :
AB_u →  A(↓) + u B(↑)

The energy can be given to the gas by many different means: heating of the substrate, plasma, lamp, laser, etc. Chemical reactions can occur in the gas itself (homogeneous reactions) and at the surface of the substrate (heterogeneous reactions). The two most important parameters in a CVD process are temperature and pressure since they strongly influence the growth rate. Two possible cases can occur: the growth can be controlled by surface reactions (kinetically controlled regime, the growth rate increases with temperature) or by diffusion (mass-transport controlled regime, the growth rate increases with pressure).

LCVD has been used to produce a large variety of products (fibers, thin films, microcoils, band-gap structures, etc.) made also from a large variety of materials (C, B, BN, W, WC, Si, SiO₂, Ti, TiN, Mo, etc.).^[1,2,3,4] As for conventional CVD, the advantage of this method over the other microfabrication processes is its ability to use many precursor gases without the need to modify heavily the deposition system. It is also possible to obtain very high growth rates compared to other processes. Indeed, while the deposition surface is small, linear growth rates up to 0.3 mm/s have been reported for the growth of carbon fibers by LCVD.^[5] LCVD is also a freeform fabrication process which allows the direct generation of 2D or 3D structures without any additional patterning steps. The two most used LCVD techniques are direct writing, where the laser beam is scanned in 2D on the surface of the substrate, and 3D-LCVD, where the laser focus is moved in 3D above the substrate after the start of growth on its surface. It is also possible to focus the laser slightly above the surface of the substrate in order to induce homogeneous deposition in the gas.

Figure 1: illustration of the LCVD process with the laser beam focused on the substrate surface

The use of LCVD to deposit cylindrical carbon rods from methane and ethane was reported in 1972 by Nelson and Richardson.^[6] Leyendecker and Bäuerle then improved the process by using an Ar+ laser and additional gases, such as ethylene and acetylene.^[7] It became clear that the strong temperature gradient induced at the tip of the rod influenced the shape and structure of the deposits. Indeed, the normal growth rate is maximized at the center of the tip and decreases as we go further from this point.^[8] This creates radially different growth conditions. On the basis of observations of the surface of the rods and of polished longitudinal sections with an electronic microscope, it was then suggested that the rods were made of pyrolitic graphite with a columnar layered structure.^[8] Investigation of LCVD-drawn carbon lines by Raman spectroscopy showed that the surface of the carbon lines (covered by nodules from homogeneous nucleation) consisted of glassy carbon and the interior of ordered pyrolitic graphite.^[3]

In the case of LCVD, the laser beam is focused (beam diameter of w) on the substrate and oriented orthogonally to the surface of the substrate. It induces a temperature rise ΔT(r):
ΔT(r) = ΔT₀ exp(r² / w²)

where ΔT₀ is a constant function of the parameters of the laser beam. Since the pressures used for this project where atmospheric or sub-atmospheric, we were theoretically in the kinetic regime and the linear growth rate R can be written like this:
R(r) ~ exp(-E_a / k T(r))

where E_a is the global activation energy of the deposition reaction and k the Boltzmann constant. When the pressure or temperature increases, homogeneous reactions are starting to happen in the gas and this approximation is no longer valid.

According to previously published experimental and theoretical results on LCVD grown carbon fibers,^[2,7,8] the growth rate increases with laser power and pressure while the diameter of the fibers increases with laser power but is independent of pressure. These relations are valid for atmospheric and sub-atmospheric pressures. When the pressure is higher (HP-LCVD), it was reported that the diameter of the fibers decreases with increasing pressure.^[5] This is happening because of the cooling effect of the fast movement of the gas near the fiber. Indeed, since the growth rate is very high at high pressure, a convection phenomenon is going on near the deposition area: the gas must quickly fill the space that has been emptied by the deposition. A good part of the heat produced by the laser is then evacuated in the gas before the completion of the deposition, which reduces the size of the reaction zone.

Few theoretical models for the growth of LCVD fibers are available. Most models are very complex, are limited to specific deposition conditions and don’t allow clear results that resemble the experimental observations.^[2,10] However, from Arnold’s model,^[10] we can gather that the deposition of the fibers is occurring by the accumulation of layers at the tip of the fiber and that once the reaction is started, it no longer depends on the substrate’s nature. Figure 1 illustrates a deposition model for LCVD fibers. The layers have a elliptic or parabolic shape that can be observed with an electronic microscope (SEM) when a fiber is fractured (see Figure 2 and Figure 3).

Figure 2: tip of a LCVD fiber

Characterization of the microstructure of LCVD deposits

Research on LCVD and its possibilities has been pursued for more than 20 years and it had become important to characterize the internal structure of the deposits in order to achieve a better understanding of the process. Investigation of the cross-sectional microstructure of carbon fibers grown by LCVD using Raman spectroscopy has been done. During the first part of the project,^[11] a radial Raman analysis was done on a single fiber. Spectra were taken at regular intervals to cover the whole diameter of a fiber. Key parameters of the Raman spectra were studied (G, D, D’ peaks and D/G intensity ratios) and provided insight on the microstructure of the fibers. Indeed, it was discovered that the microstructure changed radially with a well-graphitized center, a nanocrystalline graphite edge and an amorphous carbon surface.

Figure 3: longitudinal view of a LCVD fiber

The second part of the project,^[12] involved the same kind a radial Raman analysis but several fibers, grown at different laser powers and precursor gas pressures, were studied. This deepened our understanding of the fiber’s microstructure and allowed the discovery of experimental parameters dependencies:

• An increase in precursor gas pressure, increasing the linear growth rate, was accompanied by a decrease in graphitization of the fiber;
• An increase in laser power, increasing the growth temperature and linear growth rate, first induced an increase in graphitization followed by a decrease in graphitization as the growth rate becomes too high.

A turning point in the graphitization level of the carbon fibers was thus observed when the laser power is increased. The knowledge gathered during this research project allows the growth of carbon fibers with desired microstructural properties and further comprehension of the growth process.

Mechanical properties of LCVD deposits

A second project was begun to investigate the local and bulk mechanical properties of LCVD fibers. The goal was to directly relate the mechanical properties to the fibre microstructure.

The investigation of the local mechanical properties was achieved by radial nanoindentation^[13] of the fiber cross-sections. The bulk mechanical properties were determined through three-point microbending^[14] experiments carried out with the use of a micromanipulator. A clear mechanical properties dependence on experimental parameters was found. The mechanical properties investigated (Young’s modulus, the hardness, the tensile and fracture strengths as well as the bending resistance) will help us determine the type and the extent of applications.

Nanoindentation

Nanoindentation is a near surface characterisation technique used mainly to determine the mechanical properties of thin films. A sharp indenting tip moves deeper in a material as the applied force is increases and the apparatus measures the degree of response of the material. Load-displacement curves are used to determine the materials stiffness and hardness.

In the case of LCVD produced fibers, a radial change in mechanical properties was observed. The fibers produced at atmospheric and sub-atmospheric pressures have low hardness and elastic modulus. The elastic modulus decreases, from edge to center, as the microstructure changes radially from nanocrystalline to more graphitic structure. The local mechanical properties are directly related to the microstructure induced by the Gaussian laser intensity distribution. Varying the deposition parameters showed that the local elastic modulus and hardness increases with amorphitization. This is achieved by increasing the precursor pressure and by increasing the laser power above a specific value.

Three-point microbending

Bulk mechanical properties investigation was done by micromechanical testing using a high-precision micromanipulator. During three-point bend testing (Figure 4), the fibers showed an elastic response with no residual strain upon unloading. The low pressure LCVD fibers have poor mechanical properties when compared to other forms of carbon and when compared to HP-LCVD or other commercially produced carbon fibers. It is believed that the fiber’s structural weakness is due to their layered graphitic nature. Again, the mechanical properties improve with amorphization. Internal structural defects due to difficult process control occur at higher pressures and laser powers. Furthermore, a rough surface morphology, due to the presence of surface nodules could lead stress concentrations. This in turn would reduce the overall strength of the fibers.

Figure 4: typical fiber bending during three-point bend testing

Laser assisted surface-bound growth of carbon nanofibers

This poster reviews the recent advances in laser assisted surface-bound growth of carbon nanofibers:
Recent advances in laser assisted surface-bound growth of carbon nanofibers: from vertical nanofibers arrays to thin films and nanocomposite coatings (pdf)

Carbon nanofibers and nanotubes are grown by selectively heating catalytic metal nanoparticles in the presence of a gaseous hydrocarbon precursor with a low power (1-5 W) argon ion laser beam (488 nm). The nanofibers are the result of atomic carbon self-assembly initially into graphene sheets, which are essentially large planar 2D molecules, and subsequently into various arrangements of graphene sheets.

The prime innovation reported herein deals with the improved control over nanofiber characteristics during their synthesis and their assembly into various surface coatings. Assemblies in the form of thin nanofiber films, vertically aligned nanofiber arrays, horizontally aligned nanofiber mats and carbon-carbon nanocomposite coatings were synthesized locally on the surface of a porous alumina substrate using a nickel catalyst. Scanning electron microscopy and Raman spectroscopy were used to characterize the different assemblies whereas individual nanofiber texture and nanotexture were assessed by transmission electron microscopy.

Irradiation time ranges leading to the formation of specific assemblies have been identified and are presented. The different pre- or post-synthesis strategies used to control diameter, location and alignment are described. Also, the growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure by choosing the appropriate process parameters are provided.

Literature

[1] F. T. Wallenberger, P. C. Nordine, M. Boman, Inorganic fibers and microstructures directly from the vapor phase, Comp. Sci. Technol. 51 (1994), 193-212

[2] D. Bäuerle, Laser Processing and Chemistry, 2nd ed. Berlin (1996), Springer, 649 pages

[3] H. Westberg, M. Boman, A.-S. Norekrans, J.-O. Carlsson, Carbon growth by thermal laser-assisted chemical vapour deposition, Thin Solid Films 215 (1992), 126-133

[4] M. C. Wanke, O. Lehmann, K. Müller, Q. Wen, M. Stuke, Laser Rapid Prototyping of Photonic Band-Gap Microstructures, Science 275 (1997), 1284-1286

[5] F. T. Wallenberger, P. C. Nordine, Strong, pure and uniform carbon fibers obtained directly from the vapor phase, Science 260 (1993), 66-68

[6] L. S. Nelson, N. L. Richardson, Formation of thin rods of pyrolytic carbon by heating with a focused carbon dioxide laser, Mater. Res. Bull. 7 (1972), 971-976

[7] G. Leyendecker, D. Bäuerle, P. Geittner, H. Lydtin, Laser induced chemical vapor deposition of carbon, Appl. Phys. Lett. 39 : 11 (1981), 921-923

[8] G. Leyendecker, H. Noll, D. Bäuerle, P. Geittner, H. Lydtin, Rapid determination of apparent activation energies in chemical vapor deposition, J. Electrochem. Soc. 130:1 (1983), 157-160

[9] M. Lax, Temperature rise induced by a laser beam, J. Appl. Phys. 48 (1977), 3919-3924

[10] N. Arnold, E. Thor, N. Kirichenko, D. Bäuerle, Pyrolitic LCVD of fibers: A theoretical description, Appl. Phys. A. 62 (1996), 503-508

[11] C. Fauteux, J. Pegna, Radial characterization of 3D-LCVD carbon fibers by Raman spectroscopy, Applied physics A: Materials science and processing 78 (2004), 883-888 (abstract) (pdf)

[12] C. Fauteux, R. Longtin, J. Pegna, M. Boman, Microstructure and growth mechanism of laser grown carbon microrods as a function of experimental parameters, Journal of applied physics 95 (2004), 2737-2743 (abstract) (pdf)

[13] R. Longtin, C. Fauteux, E. Coronel, U. Wiklund, J. Pegna, M. Boman, Nanoindentation of carbon microfibers deposited by laser-assisted chemical vapor deposition, Applied physics A: Materials science and processing 79 (2004), 573-577 (abstract) (pdf)

[14] R. Longtin, C. Fauteux, J. Pegna, M. Boman, Micromechanical testing of carbon fibers deposited by low pressure laser-assisted chemical vapor deposition, Carbon 42 (2004), 2905-2913 (abstract) (pdf)

[15] R. Longtin, Synthèse et caractérisation de recouvrements organisés de nanofibres de carbone par dépôt chimique en phase vapeur assiste par laser [Ph.D. dissertation]. Canada: Ecole Polytechnique, Montréal (Canada); 2007. Available from: Dissertations & Theses: A&I. Accessed November 14, 2008, Publication Number: AAT NR29203.

[16] R. Longtin, L.-P. Carignan, C. Fauteux, D. Therriault, J. Pegna, Laser-assisted synthesis of carbon nanofibers: from arrays to thin films and coatings, Surface and Coatings Technology, vol. 202, pp. 2661–2669, 2008.

[17] R. Longtin, L.-P. Carignan, C. Fauteux, D. Therriault, J. Pegna, Selective area synthesis of aligned carbon nanofibers by laser-assisted catalytic chemical vapor deposition, Diamond & Related Materials, vol. 16, pp. 1541–1549, 2007.

[18] R. Longtin, C. Fauteux, R. Goduguchinta, J. Pegna, Synthesis of carbon nanofiber films and nanofiber composite coatings by laser-assisted catalytic chemical vapor deposition, Thin Solid Films, vol. 515, pp. 2958-2964, 2007.

Authors: Christian Fauteux and Rémi Longtin

Figure 1: Geometric structure of nanotubes

The discovery of carbon nanotubes in 1991, by the japanese researcher Sumio Iijima, during the study of fullerenes (C60) synthesis reveals a new cristalline form of carbon. These cylindric structures of carbon consist of a graphitic plane rolled into a tube having a nanometric diameter (10^-9 meter = 1 nanometer) and can be divided in two groups: single wall carbon nanotubes (C-SWNT) and multi-walls carbon nanotubes (C-MWNT).

Figure 2: Schematic of C-SWNT and C-MWNT

The amazing properties of carbon nanotubes have motivated an intense scientific research effort. Indeed, this material presents outstanding mechanical, electrical, thermal and chemical properties: 100 times stronger than steel, best field emission emitter, can maintain current density of more than 10⁹ A/cm², thermal conductivity comparable to that of diamond. Such properties could lead to the development of many applications as field emission device (flat screen displays), composite materials, conductive polymers, sensors,etc.

Figure 3: Carbon nanotubes pictures (left) C-SWNT deposit obtained with plasma torch, bundles diameter of 10-15 nm (right) C-MWNT on carbon paper, diameter of 20-100nm.

Present state of production techniques

The major obstacle to the development of these technological applications to a commercial scale is the poor yield of conventionnal production processes, (electrical arc, CVD, laser vaporization), the high production cost and also the lack of understanding of the nucleation of the C-SWNT and C-MWNT. Keeping in mind this problematic, Olivier Smiljanìc and professor Barry L. Stansfield, of the INRS-EMT (Energy, Materials and Telecommunication), developed a new synthesis process based on an atmospheric plasma torch.^[1,2] This process has the advantage to be continuous and also to be scalable to a commercial production.

Frédéric Larouche contribute to the process improvement by developing a recuperation system of the C-SWNT.

A new growth mechanism: the BMI model

We believe the comprehension of the carbon nanotubes growth mecanism is an unavoidable question for a fair control of the C-SWNT production. Several models have been proposed in the last few years to explain the growth of single-walled carbon nanotubes. They have succeeded in clarifying the role of the catalyst in the growth of nanotubes and the general scenario leading to their growth.

However, the nucleation process of the nanotubes at the surface of the catalyst is still a problem. The last models^[3,4] have suggested that an instability present at the surface of the catalyst (similar to those involved in solidification processes in crystal growth) could be responsible for this phenomenon. Since this hypothesis has been presenting some problems, we recently suggested a new idea, the BMI model.^[5]

In this new model, it is proposed that an hydrodynamical instability, -the Bénard-Marangoni instability- could be generated at the surface of the catalyst, explaining both the nucleation process and the bundle structure of single-walled carbon nanotubes observed in experiments. The segregation process of carbon towards the surface of the catalyst, invoked in the previous models to explain the growth in a general way, would be responsible for the formation of a nanometric liquid layer, supersaturated in carbon, at the surface of the catalyst. Then, the conditions present in the synthesis of the nanotubes could allow the instability to be generated in this layer in order to form a pattern of hexagonal convection cells, which would be responsible for the collective growth of the nanotubes, one nanotube per cell.

Figure 4: Growth of a nanotube bundle within an hexagonal pattern of convection cells generated by a Bénard-Marangoni instability

Our aim is to reinforce this hypothesis by developping the mathematical background behind the model. First, we proceeded to a linear stability analysis of the flow at the surface of the catalyst to determine if the Bénard-Marangoni instability (solutal type) could be generated in the conditions of synthesis, this for a plane and spherical geometry. Then, we have done a nonlinear stability analysis of the flow in order to determine if a pattern of hexagonal convection cells was favored in these conditions (in the same geometries). These analyses have revealed themselves to be clearly positive from a theoritical point of view.^[6] Numerical simulations could be done to investigate further this hypothesis.

Literature

[1] O. Smiljanìc, T. Dellero, A. Serventi, G. Lebrun, B.L. Stansfield, J.-P. Dodelet, M. Trudeau and S. Désilets, Growth of carbon nanotubes on Ohmically heated carbon paper, Chem. Phys. Lett. 342 (2001), 503-509. (pdf)

[2] O. Smiljanìc, B.L. Stansfield, J.-P. Dodelet, A. Serventi and S. Désilets, Gas-phase synthesis of SWNT by an atmospheric pressure plasma jet, Chem. Phys. Lett. 356 (2002), 189-193. (pdf)

[3] J. Gavillet et al., Microscopic mechanisms for the catalyst assisted growth of single-wall carbon nanotubes, Carbon 40 (2002), 1649-1663.

[4] J. Gavillet et al., Nucleation and growth of single-walled nanotubes: the role of metallic catalysts, J. Nanosci. Nanotech. 4 (2004), 346-359.

[5] F. Larouche, O. Smiljanìc, X. Sun and B.L. Stansfield, Solutal Bénard-Marangoni instability as a growth mechanism for single-walled carbon nanotubes, Carbon 43 (2005), 986-993. (abstract) (pdf)

[6] F. Larouche, J. Duquette, L. Cortelezzi, B.L. Stansfield and N. Nigam, Nucleation and growth of bundles of single-wall carbon nanotubes (C-SWNTs): the Bénard-Marangoni Instability (BMI) model, Preprint at LANL. (pdf)

Authors: Frédéric Larouche et Jonathan Duquette

The automobile fleet is contributing significantly to air quality degradation in large cities. The use of hydrogen as a fuel is interesting since combustion delivers a lot of energy without polluting. However, some obstacles, like the efficient storage of this gas, remain before it can be implemented in the transportation industry.

Figure 1: Hydrogen powered car built by BMW

Hydrogen storage solutions

The United States Department of energy (DOE) has fixed two targets for hydrogen storage solutions applied to automotive transportation. The first target requires a ratio of hydrogen weight / tank weight that is superior to 0,065 (6,5% weight). This target limits the weight of the tank. The second target requires a hydrogen volumetric density higher than 62 kg/m in order to limit the volume of the tank.

Four main solutions were proposed to solve this problem:

• Compression or liquefaction of hydrogen
• Metal hydrides
• Chemical tanks
• Adsorbent materials

Figure 2 shows the commercially available solutions. Each one is compared to the DOE targets. To satisfy both targets, a solution must be located in the grey region of the graph. Carbon nanostructures are not displayed on the graph since they are not readily available in the market at the moment.

Figure 2: Comparison of storage solutions available on the market

Carbon nanotubes

Solid state carbon may exist under three different crystaline forms named allotropes. Diamond and graphene, are the two well known allotropes. The third allotrope is called fullerene. It is a new class of material which is formed by a spherical or cylindrical wrapping of graphene sheets.

Nanotubes are fullerenes formed by graphene sheets wrapped as shown in Figure 3. The diameter of those tubes can be as small as 1 nm.

Figure 3: Representation of the carbon nanotube structures

The porosity of nanotubes is large. This property enables the adsorption of various gases including hydrogen.

Hydrogen storage in carbon nanostructures

Experimental results obtained in the evaluation of hydrogen storage capacity of carbon nanotubes vary significantly from one research team to another. Some groups (Dillon et al., Chambers et al.) obtained results indicating this material allows to reach DOE targets. Others (Ahn et al., Hirscher et al.), are indicating the contrary.

We chose to evaluate the storage capacity of nanotubes with Monte-Carlo numerical simulations. Our results indicate that carbon single-walled nanotubes can store 0.22% to 0.79% weight (3.95 to 7.94 kg/m) of hydrogen at room temperature and under a pressure of 10 MPa.

Figure 4: Simulation of the interaction between nanotubes (20,0) and hydrogen

Conclusion

Our results indicate that carbon nanostructures are falling short of the DOE targets. Such structure as thus more likely to be inappropriate for hydrogen storage in transport applications. However, we obtained interresting results by including the effect of impurities dispersed in nanotubes. We invite you to consult those papers to learn more about this subject:

P. Guay, Modélisation Monte-Carlo de l’adsorption de l’hydrogène dans les nanostructures de carbone, INRS-EMT, Montréal, 2003 (pdf)

P. Guay, B. L. Stansfield and A. Rochefort, On the Control of Carbon Nanostructures for Hydrogen Storage Applications, Carbon 42 (2004), 2187-2193 (abstract) (pdf)

A review of the literature is also available for consultation.

Author : Patrice Guay

Reference books

R. Saito, G. Dresselhaus and M. S. Dresselhaus, Physical Properties of carbon nanotubes, Imperial College Press, Londres, 1998

G. Gao, T. Cagin and W. A. Goddard III, Energetics, Structure, Mechanical and Vibrational Properties of Single Walled Carbon Nanotubes (SWNT), Foresight Institute, 1997 (pdf©)

D. Frenkel and B. Smit, Understanding molecular simulation: from algorithms to applications, Academic Press, San Diego, 1996

C. Ngô and H. Ngô, Physique statistique, Masson, Paris, 1995

R. A. Oriani, The physical and metallurgical aspects of hydrogen in metals, Fourth International Conference on Cold Fusion, 1993 (pdf)

Nanotube fabrication methods

S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991), 56-58 (pdf©)

S. Iijima and T. Ichihashi, Single-shell carbon nanotubes of 1-nm diameter, Nature 363 (1993), 603-605 (pdf©)

A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tománek, J. E. Fischer and R. E. Smalley, Crystalline Ropes of Metallic Carbon Nanotubes, Science 273 (1996), 483-487 (pdf©)

P. Nikolaev, M. J. Bronikowski, R. K. Bradley, F. Rohmund, D. T. Colbert, K. A. Smith and R. E. Smalley, Gas-phase Catalytic Growth of Single-Walled Carbon Nanotubes from Carbon Monoxide, Chem. Phys. Lett. 313 (1999), 91-97 (pdf©)

O. Smiljanìc, B. L. Stansfield, J.-P. Dodelet, A. Serventi and S. Désilets, Gas-phase synthesis of SWNT by an atmospheric pressure plasma jet, Chem. Phys. Lett. 356 (2002), 189-193 (pdf©)

P. E. Anderson and N. M. Rodriguez, Growth of graphite nanofibers from the decomposition of CO/H2 over silica-supported iron-nickel particles, J. Mater. Res. 14 (1999), 2912-2921 (pdf©)

C. Singh, T. Quested, C. B. Boothroyd, P. Thomas, I. A. Kinloch, A. I. Abou-Kandil and A. H. Windle, Synthesis and Characterization of Carbon Nanofibers Produced by the Floating Catalyst Method, J. Phys. Chem. B 106 (2002), 10915-10922 (pdf©)

Review of hydrogen storage

P. Harcouët and J. Demoment, Comment stocker l’hydrogène sûrement et efficacement?, Clefs CEA 44 (Hiver 2000-2001), 68-72 (pdf)

K. Atkinson, S. Roth, M. Hirscher and W. Grünwald, Carbon nanostructures : An efficient hydrogen storage medium for fuel cells?, Fuel Cells Bulletin 38 (2001), 9-12 (pdf©)

R. Dagani, Tempest in a tiny tube, Chem. Eng. News 25 vol. 2 (2002), 25-28 (pdf©)

F. L. Darkrim, P. Malbrunot and G. P. Tartaglia, Review of hydrogen storage by adsorption in carbon nanotubes, Intl. J. Hydrogen Energy 27 (2002), 193-202 (pdf©)

Experimental measure of hydrogen adsorption in carbon nanostructures

A. C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H. Kiang, D. S. Bethune and M. J. Heben, Storage of hydrogen in single-walled carbon nanotubes, Nature 386 (1997), 377-379 (pdf©)

A. Chambers, C. Park, R. T. K. Baker and N. M. Rodriguez, Hydrogen Storage in Graphite Nanofibers, J. Phys. Chem. B 102 (1998), 4253-4256 (pdf©)

C. C. Ahn, Y. Ye, B. V. Ratnakumar, C. Witham, R. C. Bowman and B. Fultz, Hydrogen desorption and adsorption measurements on graphite nanofibers, Appl. Phys. Lett. 73 (1998), 3378-3380 (pdf©)

Y. Ye, C. C. Ahn, C. Witham, B. Fultz, J. Liu, A. G. Rinzler, D. Colbert, K. A. Smith and R. E. Smalley, Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes, Appl. Phys. Lett. 74 (1999), 2307-2309 (pdf©)

C. Liu, Y. Y. Fan, M. Liu, H. T. Cong, H. M. Cheng and M. S. Dresselhaus, Hydrogen storage in Single-Walled Carbon Nanotubes at Room Temperature, Science 286 (1999), 1127-1129 (pdf©)

X. B. Wu, P. Chen, J. Lin and K. L. Tan, Hydrogen uptake by carbon nanotubes, Intl. J. Hydrogen Energy 25 (2000), 261-265 (pdf©)

F. E. Pinkerton, B. G. Wicke, C. H. Olk, G. G. Tibbetts, G. P. Meisner, M. S. Meyer and J. F. Herbst, Thermogravimetric Measurement of Hydrogen Absorption in Alkali-Modified Carbon Materials, J. Phys. Chem. B 104 (2000), 9460-9467 (pdf©)

M. Hirscher, M. Becher, M. Haluska, U. Dettlaff-Weglikowska, A. Quintel, G. S. Duesberg, Y.-M. Choi, P. Downes, M. Hulman, S. Roth, I. Stepanek and P. Bernier, Hydrogen storage in sonicated carbon materials, Appl. Phys. A 72 (2001), 129-132 (pdf©)

M. Hirscher, M. Becher, M. Haluska, A. Quintel, V. Skakalova, Y.-M. Choi, U. Dettlaff-Weglikowska, S. Roth, I. Stepanek, P. Bernier, A. Leonhardt and J. Fink, Hydrogen storage in carbon nanostructures, J. Alloys & Compounds 330-332 (2002), 654-658 (pdf©)

P. Bénard and R. Chahine, Determination of the Adsorption Isotherms of Hydrogen on Activated Carbons above the Critical Temperature of the Adsorbate over Wide Temperature and Pressure Ranges, Langmuir 17 (2001), 1950-1955 (pdf©)

Theoretical measure of hydrogen adsorption in carbon nanostructures

P. Guay, B. L. Stansfield and A. Rochefort On the control of carbon nanostructures for hydrogen storage applications, Carbon 42 (2004), 2187-2193 (pdf)

P. Guay, Introduction à l’étude par modélisation numérique de l’adsorption de l’hydrogène dans les nanostructures de carbone, 2001 (pdf)

P. Guay, Validation du modèle utilisé pour l’étude par simulations moléculaires de l’adsorption d’hydrogène par les nanotubes de carbone, 2001 (pdf)

P. Guay, Modélisation Monte-Carlo de l’adsorption de l’hydrogène dans les nanostructures de carbone, INRS-EMT, Montréal, 2003 (pdf)

P. Guay, Improved adsorption capacity of doped carbon nanotube bundles, NT’02 Conference, 2002 (pdf)

H. Cheng, G. P. Pez and A. C. Cooper, Mechanism of Hydrogen Sorption in Single-Waled Carbon Nanotubes, J. Am. Chem. Soc. 123 (2001), 5845-5846 (pdf©)

H. Dodziuk and G. Dolgonos, Molecular modeling study of hydrogen storage in carbon nanotubes, Chem. Phys. Lett. 356 (2002), 79-83 (pdf©)

Q. Wang and J. K. Johnson, Molecular simulation of hydrogen adsorption in single-walled carbon nanotubes and idealized carbon slit pores, J. Chem. Phys. 110 (1999), 577-586 (pdf©)

K. A. Williams and P. C. Eklund, Monte Carlo simulations of H2 physisorption in finite-diameter carbon nanotube ropes, Chem. Phys. Lett. 320 (2000), 352-358 (pdf©)

F. Darkrim and D. Levesque, High Adsorptive Property of Opened Carbon Nanotubes at 77 K, J. Phys. Chem. B 104 (2000), 6773-6776 (pdf©)

F. Darkrim and D. Levesque, Monte Carlo simulations of hydrogen adsorption in single-walled carbon nanotubes, J. Chem. Phys. 109 (1998), 4981-4984 (pdf©)

V. V. Simonyan, P. Diep and J. K. Johnson, Molecular simulation of hydrogen adsorption in charged single-walled carbon nanotubes, J. Chem. Phys. 111 (1999), 9778-9783 (pdf©)

P. A. Gordon and R. B. Saeger, Molecular Modeling of Adsorptive Energy Storage: Hydrogen Storage in Single-Walled Carbon Nanotubes, Ind. Eng. Chem. Res. 38 (1999), 4647-4655 (pdf©)

M. Rzepka, R. Lamp and M. A. de la Casa-Lillo, Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes, J. Phys. Chem. B 102 (1998) 10894-10898 (pdf©)

Q. Wang and J. K. Johnson, Computer Simulations of Hydrogen Adsorption on Graphite Nanofibers, J. Phys. Chem. B 103 (1999), 277-281 (pdf©)

Software

B. Smit and J. I. Siepmann, Computer simulations of the energetics and siting of n-alkanes in zeolites, J. Phys. Chem. 98 (1994), 8442-8452 (pdf©)

B. Smit, S. Karaborni and J. I. Siepmann, Computer simulations of vapour-liquid phase equilibria on n-alkanes, J. Chem. Phys. 102 (1995), 2126-2140

B. Smit, S. Karaborni and J. I. Siepmann, Erratum : Computer simulations of vapour-liquid phase equilibria on n-alkanes, J. Chem. Phys. 109 (1999), 352 (pdf©)

T. J. H. Vlugt, R. Krishna and B. Smit, Molecular simulation of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite, J. Phys. Chem. B 103 (1999), 1102-1118 (pdf©)

Other interaction potentials

D. W. Brenner, Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films, Phys. Rev. B 42 (1990), 9458-9471 (pdf©)

D. W. Brenner, Erratum : Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films, Phys. Rev. B 46 (1992), 1948 (pdf©)

S. J. Stuart, A. B. Tutein and J. A. Harrison, A reactive potential for hydrocarbons with intermolecular interactions, J. Chem. Phys. 112 (2000), 6472-6486 (pdf©)