Non-Carbon Nanotubes: Fabrication & Application

Transcription

1 Non-Carbon Nanotubes: Fabrication & Application Benjamen P. Reed DST CDT, University of Warwick This report contains 2026 words December 13, 2014 Carbon has demonstrated on several occasions, to be an extremely versatile element capable of a vast number of structural configurations. Arguably, the most well-known arrangements of carbon are sp 2 bonded graphitic sheets, and the sp 3 bonded diamond structure. However, other interesting structures are also possible, such as graphene, buckminster fullerenes ( buckyballs ), and nanoonions. C 60 and other fullerenes were soon able to be mass-produced using arc-discharge plasma methods in an inert atmosphere, with the preferred method using chemical vapour deposition (CVD). It was discovered by Sumio Iljima in 1991, that the same arc-discharge method could also produce sp 2 -bonded carbon rolled into cylindrical seamless tubes, nanometres in diameter and micrometres in length. 1 These carbon nanotubes (CNTs) exhibit unusual properties, from remarkable axial tensile strength, to electrical conductivity sensitive to tube diameter and chirality. These characteristics have made CNTs suitable for a number of applications. 2 It wasn t long after the discovery of CNTs, that the possibility of nanotubes (NTs) being synthesised from other compounds for other applications was considered. In 1992, transition metal dichalcogenide compounds (MX 2 M=transition metal; X=chalcogen) were investigated by Tenne et al. as potential candidates for non-carbon tubular nanostructures, notably for their quasi-two-dimensional structures, as shown in figure 1. Specifically, they synthesised the first non-carbon NTs using lamellar films of tungsten disulphide WS 2 and then molybdenum disulphide MoS 2. 3 Focusing on the former, WS 2 in nanostructured form has a range of uses, such as inorganic-organic nanocomposites for batteries and other electrochemical devices, solid lubricants, and as a catalyst in hydrodesulphurisation of crude oil. 4 In the original study, Tenne et al. produced Figure 1: A ball-and-stick model of a transition metal dichalcogenide, specifically tungsten disulphide. This structure typically arranges itself into layers. (Source: Wikimedia) the WS 2 NTs by annealing films of tungsten on quartz substrates at 1,000 C in a reduction atmosphere gas flow of H 2 S/H 2. Similar to graphitic films, the WS 2 films initiated bending and closure into polyhedral or tubular forms under intense thermal stress. Once the crystals closed B.P.Reed@warwick.ac.uk; S.I.D Post-submission version, corrected spelling 1

2 using strong inter-layer covalent bonding, the structures formed had reached a stable state. The closing mechanism of these films was the main subject of inquiry. From an energetic standpoint, the elimination of immobilised radicals at the films edges would encourage the mechanism, or the contamination of oxygen species from the quartz substrate. Geometrically, complete closure of the structures would necessitate the substitution of hexagons with polygonal units, or the formation of structural defects. Indeed, experimental evidence obtained by electron microscopy suggested irregularities in the crystal structure, or the generation of amorphous phase material at the closures. 3 Continuing the focus on WS 2 NTs, Tenne was co-authored on Zak et al. s 2009 paper which discussed the growth mechanism involved with NT synthesis in a fluidised-bed reactor (FBR), with emphasis on reproducible mass production. 5 In this study, micrometre long tungsten pentoxide nanowhiskers with a stoichiometry of WO 2.72 (W 18 O 49 ) were grown, and then underwent a sulphurisation process to convert them into WS 2 nanotubes. These nanowhiskers were synthesised by using a precursor consisting of different WO x (2.83 x 3) phases and morphologies. This precursor was then reacted with H 2 S and H 2 /N 2 gas at C. The nanowhiskers grow quickly in the initial oxide environment, with WO 2.83 being the most reduced oxide phase. The growth subsides as soon as the entire oxide structure is reduced to the non-volatile WO 2.72 phase. With all of the volatile oxide phases depleted, the sulphurisation process takes over, slowly converting the oxide in the outer nanowhisker rings to sulphide. Once the sulphurisation on the outer layers in complete, the outer diameter of the NT is fixed. Inner layers are converted as the reactant gases diffuse into the nanowhiskers. The empty core of the NT opens up as the sulphurisation continues due to the difference in the specific densities of the oxide (7.15 g cm 3 ) and the sulphide (7.5 g cm 3 ); the small difference between these densities give rise to a narrow inner diameter. In the decade after the initial success of Tenne s 1992 investigation, a variety of non-carbon NTs from other compounds began to emerge. Hexagonal boron nitride (α-bn) was next on the NT menu, with theoretical studies predicting its tubular energetic stability in 1994, 6 and then the first successful synthesis of crystalline BN NTs was achieved by Chopra et. al one year later; the method used to grow the BN NTs was a plasma arc discharge very similar to the method used to grow CNTs. The compound anode in Chopra et al. s apparatus was made by inserting a 3.17 mm diameter rod of α-bn into a hollow tungsten electrode with an outer diameter of 6.30 mm. A pure BN electrode could not be used due to the insulating properties of bulk BN. A rapidly cooled pure copper electrode was used as the cathode. Arc discharge occurred between the tungsten and copper electrodes, with a constant potential drop of 30 V maintained by ramping the D.C. current from 50 to 140 A. Chopra et al. reported that a dark grey soot was found on the copper cathode, alongside pieces of solidified tungsten which had been deposited around the inside of the experimental chamber. The presence of this tungsten inferred that the temperature of the anode during arc discharge had exceeded the melting point of tungsten (3695 K). Transmission electron microscopy (TEM) was used to characterise the cathode deposits and the presence of multi-walled BN NTs was confirmed. These NTs had inner diameters on the order of 1 to 3 nm, and outer diameters of 6 to 8 nm, with lengths surpassing 200 nm.the walls of the BN NTs had an interlayer distance of approximately 3.3 Å, which agreed with the typical interplanar distance between layers in bulk α-bn of 3.33 Å. 7 BN NTs have a variety of interesting properties and potential applications. Similar to CNTs, BN NTs exhibit a notable Young s modulus of 1.18 TPa, making them suitable for the reinforcement of ceramics and polymer composites to make light ultra-strong materials. Such materials could have many applications in the transportation industry, making lighter, stronger, and more economically competitive vehicles. 8 BN NTs demonstrate partial ionic bonding which gives rise to an electronic 2

3 bandgap of 5.5 ev (bulk α-bn has a bandgap of 5.8 ev), making them a good electrical insulator at room temperature. Interestingly and unlike CNTs, this bandgap is not affected by changes in the NTs diameter, chirality, or number of inner tubes, although molecular structure orientation can dictate whether it is direct or indirect. 6 Carbon substitution has also been shown theoretically to adjust the magnitude of the BN NT bandgap. 9 The insulating nature of BN NTs have made them ideal as shielding for lots of different nanowires, and the aforementioned ionic bonding structure is particular strong with hydrogen, making BN NTs preferable to CNTs for hydrogen storage. 10 Whilst BN NTs are an electrical insulator at room temperature, they have a very high thermal conductivity; approximately 350 W mk 1 for a NT with an outer diameter of 30 to 40 nm. This makes them suitable for use in nanoelectronics where operations dictate an increased rate of heat dissipation. Furthermore, BN NTs demonstrate a increased chemical stability at high temperatures; BN NTs in air can survive temperatures up to 1100 C. 11 Since Chopra et al. s investigation in 1995, methods of mass producing BN NTs have been emerging frequently as well as other principle techniques. Chemical vapour deposition (CVD) remains the most popular method of synthesising BN NTs due to the high yields produced. 12,13 Recently however, a new pressurised vapour/condenser (PVC) method was proposed by Smith et al. to grow exceptionally long and few-walled BN NTs in very large amounts. Their technique utilises the forced condensation of seed particles in a rising plume of pure boron vapour at elevated pressure. Figure 2 shows a simplified version of the apparatus, with the resulting BN NT fibril mass shown in figure 3. Figure 2: A simplified schematic of the PVC apparatus used to produce the BN NT fibrils. (Source: Smith et al., 2009) Figure 3: BN NT fibril mass after a 200 mg PVC run. (Source: Smith et al., 2009) The boron vapour was produced by heating a target in the centre of a long vertical chamber filled with room temperature N 2 gas. This boron vapour is released at an extremely high temperature ( 4000 C) and thus exhibits a large density difference relative to the ambient N 2 gas. This produced a buoyancy force which drove the boron vapour up through the N 2 gas (A). A grid of cooled metal wires crossed the path of the plume, which acted as condensers and homogeneous nucleation sites for boron droplets (B). These boron droplets continued to rise (C) and N 2 diffused through the shear layer surrounding the plume. Now that both boron and N 2 were present, the growth of 3

4 BN NTs was possible. The BN NTs quickly grew, interlocked, and were shaped by shear stress in the rising plume, finally consolidating into large fibrils and then fibril masses as shown in figure Clearly, this method has potential to be scaled up for industrial purposes given the amount of NTs it produces and its efficiency (80% of boron converted to NTs). Synthesising non-carbon NTs with two or three element compounds has become commonplace in the nanotechnology field. In the early nineties, research was also conducted into single element NTs, or specifically, metal NTs (MNTs). Due to the typical high electrical conductivity of these structures, many MNTs are used for electrical components, such as heterojunctions and transducers. 15 They exhibit a particular capacity in electrochemistry, being used for the determination of ultratrace concentrations of ions and molecules. 16 The first metal NTs to be synthesised experimentally were made of gold, and were used in conjunction with a polycarbonate filtration membrane for the purpose of selectively rejecting or transporting ions depending on their charge. Nishizaea et al. used a commercially available membrane containing cylindrical nanopores of uniform radius (25 nm, pores per square centimetre) that extended the full width of the membrane (6 µm). Gold atoms were deposited onto the membrane surface via an electroless plating technique, which ensured that deposition also occurred on the inner walls of the nanopores, hence producing the gold NTs (AuNTs). Nishizaea et al. reported inner wall diameters of 0.8 nm, although the thickness of the AuNTs could be varied by adjusting the plating time. 17 A similar polycarbonate filtration membrane was used by Tourillon et al. to produce MNTs and wires of cobalt and iron. In this case, a thin layer of gold was evaporated onto the bottom side of the membrane to serve as a working electrode for electrochemical deposition. The electrolyte used to deposit the atoms of cobalt/iron consisted of cobalt/iron (II) sulphate (CoSO 4 or FeSO 4 ) and the electrodeposition took place in purified water with a resistivity exceeding 18 MΩ. Tourillon et al. were hence able to produce MNTs of cobalt and iron with an inner diameter of 30 nm and a length of 6 µm. 18 It is typical of metals such as gold, iron, and cobalt to be formed by electrochemical deposition, because it is undesirable to have other elements and compounds, like oxides and sulphides, being deposited on the template surface as well. The template method can also be applied to semiconducting materials such as silicon and gallium nitride (GaN), where contamination is unlikely; doping of these semiconductor materials can be performed post-synthesis. In 2004, Wang et al. synthesised copper nanotubes using an alumina membrane instead, and sputtered gold atoms onto the bottom surface to act as an electrode. Electrochemical deposition was then used to cover the membrane in copper atoms. In this study, the membrane was treated with a mixture of cyanosilane and methyl-γ-diethylenetriaminopropyldimethoxysilane to provide molecular anchors for the copper to stick to. 19 The NTs explored in this review are only a fraction of the possible NT variants being researched and successfully synthesised. It is clear that NTs of certain compounds appear to have very interesting properties and some extremely useful applications, and as such continue to be at the forefront of the nanotechnology field. NTs have found themselves being used in polymer chemistry, hydrogen storage, nano-electronics (e.g. heterojunctions, electrochemical sensors, etc), nano-biology (e.g. precise delivery systems for medicine), and reinforcing materials. As with any new technology or material, the ability to mass produce it will determine its success commercially. Where a particular NT compound has shown promise, such as BN NTs, focus has turned toward scaling the synthesis operation up, as with the case of Smith et al.. However, progress toward mass production is slow, and many compounds are still in their characterisation stage, and of course competing with CNTs. Despite this, the future of non-carbon NTs seems promising and exciting, as well as the future technology they will hopefully assist in producing. 4

5 References [1] S. Iljima, Nature, 1991, 354, [2] A. Zettl, Advanced Materials, 1996, 8, [3] R. Tenne, L. Margulis, G. M. and G. Hodes, Nature (London), 1992, 360, 444. [4] P. K. Panigrahi and A. Pathak, Sci. Technol. Adv. Mater., 2008, 9, [5] A. Zak, L. Sallacan-Ecker, A. Margolin, M. Genut and R. Tenne, NANO: Brief Report and Reviews, 2009, 4, [6] X. Blase, A. Rubio, S. G. Louie and M. L. Cohen, Europhys. Lett., 1994, 28, 335. [7] N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louie and A. Zettl, Science, 1995, 269, [8] Q. Huang, Y. Bando, X. Xu, T. Nishimura, C. Zhi, C. Tang, F. Xu, L. Gao, and D. Golberg, Nanotechnol., 2007, 18, [9] Y. Miyamoto, A. Rubio, M. L. Cohen and S. G. Louie, Phys. Rev. B: Condes. Matter, 1994, 50, [10] G. Mpourmpakis and G. E. Froudakis, Catal. Today, 2007, 120, [11] J. Wang, C. H. Lee and Y. K. Yap, Nanoscale, 2010, 2, [12] C. Zhi, Y. Bando, C. Tan and D. Golberg, Solid State Comm., 2005, 135, [13] C. Sealy, Nano Today, 2010, 5, [14] M. W. Smith, K. C. Jordan, C. Park, J. W. Kim, P. T. Lillehei, R. Crooks and J. S. Harrison, Nanotech., 2009, 20, [15] D. Yang, G. Meng, S. Zhang, Y. Hao, X. An, Q. Wei, M. Ye and L. Zhang, Chem. Comm., 2007, [16] X. Zhang, H. Wang, L. Bourgeois, R. Pan, D. Zhao and P. A. Webley, J. Mater. Chem., 2008, 18, [17] M. Nishizawa, V. P. Menon and C. R. Martin, Science, 1995, 268, [18] G. Tourillon, L. Pontonnier, J. P. Levy and V. Langlais, Electrochem. and Solid-State Lett., 2000, 3, [19] Y. Wang, C. Ye, X. Fang and L. Zhang, Chem. Lett., 2004, 33,