Nickel Formate Route to the Growth of Carbon Nanotubes Junfeng Geng, Hongwei Li, Vladimir B. Golovko, Douglas S. Shephard, David A. Jefferson, and Brian F. G. Johnson* Department of Chemistry, Cambridge UniVersity, Lensfield Road, Cambridge CB2 1EW, U.K. Stephan Hofmann, Britta Kleinsorge, and John Robertson Department of Engineering, Cambridge UniVersity, Trumpington Street, Cambridge CB2 1PZ, U.K. Caterina Ducati Department of Materials Science and Metallergy, Cambridge UniVersity, Pembroke Street, CB4 3QZ, U.K. ReceiVed: May 17, 2004 Nickel formate serves as an ideal catalyst precursor for surface-bound thermal and plasma-enhanced CVD growth of carbon nanotubes. As the method of deposition of the Ni catalyst is solution-based, it benefits from a low-cost, large-area growth, ease of operation and suitability for coating substrates with complex shapes and structures. Introduction Carbon nanotubes (CNTs) have been considered as ideal candidates for many applications due to their unique atomic structure, an inherent small tip radius, high surface area and aspect ratio, excellent chemical inertness and mechanical strength. 1,2 Among the many growth techniques, chemical vapor deposi- tion (CVD) or plasma-enhanced chemical vapor deposition (PECVD) offer controlled surface-bound growth on flat sub- strates. 3,4 In a CVD-based process, catalyst and catalyst prepara- tions are essential because the growth occurs at the interface of the catalyst and the hydrocarbon vapor. 5 Hence, formation of an active, uniform catalyst layer is a critical step in a successful surface-bound CVD process. Deposition of catalyst may be realized by a physical method, as for example, using thermal evaporation or magnetron sputtering to generate a homogeneous thin metal film, which in a subsequent growth step, breaks into small metal islands as catalyst. 6,7 Although physical methods have proven very effec- tive, they require a vacuum facility to process catalyst deposi- tion, and in general, are not suitable for coating a substrate with complex shapes. Alternatively, a chemical method may be used, as for example, introducing metal colloidal nanoparticles (Fe, Co, and Ni) to the catalyst. 8,9 Metal colloidal nanoparticles may be pre-controlled in a narrow size distribution in either the synthesis process or using an after-process technique for size selection. However, the colloids are frequently air-sensitive, making their manipulation difficult and, to some extent, limiting the large-scale production of carbon nanotubes. Another com- monly used method is through a catalyst precursor, such as an inorganic or organometallic salt, leading to the desired metal nanoparticles by the thermal decomposition of the precursor at high temperatures. 10,11 The precursor method takes advantages of low-cost, good stability in air, ease of operation and commercially available starting materials. It is also suitable for coating 3D substrates with complex shapes and structures. We have found that nickel formate may serve as an ideal catalyst precursor for formation of the desired Ni nanoparticles for growth of carbon nanotubes. In our previous report, we demonstrated that using this compound, high-purity single-wall carbon nanotubes were produced in high yield. 12 In fact, nickel formate may be used not only for single-wall nanotubes but also for versatile growth of carbon nanotubes with varied structures. In this paper, we wish to address the issue of why nickel formate is such an ideal catalyst precursor on one hand, and to demonstrate surface-bound growth of multiwalled carbon nanotubes on the other. The choice of the surface growth on the flat SiO 2 /Si substrate is based on a common recognition that such grown nanotubes are potentially significant in micro- electronic applications, such as for electron field emission, flat panel display and supercapacitors. 13-15 Experimental Section Deposition of the Catalyst. For deposition of the nickel catalyst, silicon wafer was first cleaned by ethanol and acetone in an ultrasonic bath and rinsed with distilled water. Nickel formate dihydrate was purchased from Degussa, as a fine green powder. Deposition of the catalyst was carried out by simply casting a drop of the aqueous or methanol solution of nickel formate onto the substrate. The samples were then left to dry in air. The concentration of the solution may be varied, according to the growth purpose and catalyst loading. A typical concentration falls in the range 5 × 10 -3 to 2.2 × 10 -2 M. The thickness of the formate film depends on the amount of solution on the surface. To keep the film uniform, a trace amount of anionic surfactant, such as lauric acid, may be mixed into the solution. The Ni nanoparticles were formed in situ by thermal decomposition of the formate precursor in the CNTs growth process. Growth of the Carbon Nanotubes. The growth methods are basically the same as described previously. 7,15 In brief, a stainless steel vacuum chamber was used with a base pressure below 10 -6 mbar. A 20 nm thick SiO 2 layer was grown by thermal oxidation or low-temperature electron cyclotron reso- nance (ECR) onto a polished n-type Si(100) substrate. The samples were normally heated for 15 min to reach the desired temperatures using a resistively heated graphite stage. For the plasma-enhanced growth, samples with freshly deposited catalyst * Corresponding author. E-mail: bfgj1@cam.ac.uk. 18446 J. Phys. Chem. B 2004, 108, 18446-18450 10.1021/jp047898p CCC: $27.50 © 2004 American Chemical Society Published on Web 11/06/2004