Synthesis of Copper Nanocatalysts with Tunable Size Using Diblock Copolymer Solution Micelles Yang Liu, † Chai Lor, † Qiang Fu, † David Pan, † Lei Ding, ‡ Jie Liu, ‡ and Jennifer Lu* ,† School of Engineering, UniVersity of California, Merced, Merced, California 95348, and Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708 ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: February 18, 2010 Self-assembled solution micelles prepared from polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP), have been employed as templates to synthesize copper nanocatalysts which are regarded as an excellent catalyst system for 1D nanomaterial synthesis. We have demonstrated that uniform-sized nanoparticles with diameters ranging from 1 to 15 nm have been generated. We have revealed that nanocatalyst size can be rationally tailored by adjusting the interaction between copper precursors and ligands and metal sequestration time. Ordered arrays of copper nanocatalysts derived from depositing a monolayer of solution micelles exhibit excellent thermal stability and do not agglomerate during the thermal treatment at 850 °C, typical growth temperature for 1D nanomaterial using the chemical vapor deposition technique. High-density and aligned single-walled carbon nanotubes with uniform diameter have been synthesized using the chemical vapor deposition technique. The average diameter is 1.4 nm, which is on the same order of catalyst size, around 2.0 nm. The combination of tunable size and spacing with superb thermal stability and outstanding catalytic activity offered by this new copper nanocatalyst system will enable growth of high-yield 1D nanomaterials with controllable diameter and spacing consistently and reproducible properties. It also paves a new path to study the effect of nanocatalyst size on 1D nanomaterial synthesis and their properties. Introduction Copper nanoparticles are of great interest in a broad tech- nological arena including catalysis and energy conversion. 1-5 In particular, copper nanoparticles have shown excellent catalytic capability to synthesize a variety of 1D nanomaterials using chemical vapor based techniques. Copper nanoparticles can catalyze single-walled carbon nanotube (SWNT) growth. 3 It has been shown that high-density aligned SWNT arrays with over 95% semiconducting nanotubes have been synthesized using copper nanoparticles. 3a Unlike VIIIA transition metals such as iron, cobalt, and nickel, copper has weak interaction with silicon oxide substrate thus facilitates to the formation of aligned tubes. 3b In addition, the use of copper enables the study of the intrinsic magnetic property of carbon nanotubes. Copper nano- particles have been proven to be an excellent catalyst system to catalyze ZnO nanowire growth. 4 Well-aligned and vertically grown ZnO nanowires have been generated using a copper- based catalyst system. It is believed that copper can promote the epitaxial growth of ZnO nanowires. 4a,b Copper nanoparticles can catalyze silicon nanowire growth as well. High-purity silicon nanowires with enhanced electron transport property have been synthesized recently. 5a Gold has been widely used for nanowire growth. However gold, a deep-level trap 6a with gold-silicon eutectic temperature of 370 °C, can easily diffuse into the silicon-based active device region during device fabrication. Thus gold is not compatible with Si-based device fabrication. 6b,c Unlike gold, copper, which will not form any compounds with silicon below 800 °C, 6d is being extensively used in semicon- ductor device fabrication. 5c In the chemical vapor deposition (CVD) techniques, catalyst species enable selective adsorption of precursor molecules, facilitate precursor molecule decomposition, and initiate and maintain the growth of nanotubes and nanowires. 7-9 Therefore, the properties of a 1D nanomaterial dictated by diameter is controlled largely by nanocatalyst size. 10,11 Two critical growth parameters, local precursor vapor concentration per catalyst species and catalyst activity, are very sensitive to catalyst size. When the growth condition is reached and if all nanocatalysts are similar in size, they all can initiate the growth at the same time leading to high growth yield. Needless to say, the prerequisite for achieving controllable synthesis in high yield is to establish a methodology of synthesizing nanocatalysts with uniform and tunable size. Several methods have been developed to make copper nanoparticles, including laser ablation 12 and wet chemical reactions. 1,2 These methods can produce nanoparticles with controlled size in solution. However, there is no means to generate ordered arrays on surfaces, which is essential for substrate-based synthesis of 1D nanomaterials. Ordered nano- particle arrays can be prepared by either top-down or bottom- up approaches. Conventional top-down lithography has suc- cessfully pushed the feature size down to the sub-100-nm regime. Alternative methods such as e-beam lithography and nanoimprinting have been able to create features in the sub- 20-nm region, 14,15 but they are limited by high cost and low throughput. On the contrary, the bottom-up approaches can generate sub-20-nm features at low cost and high throughput by building up nanoscale units through controlled assembling and organization of molecules and atoms. 16 Self-assembled block copolymers have been widely used for nanostructure fabrication. Block copolymers can self-assemble into a wide range of nanoscale morphologies such as spheres, * To whom correspondence should be addressed, jlu5@ucmerced.edu. † University of California, Merced. ‡ Duke University. J. Phys. Chem. C 2010, 114, 5767–5772 5767 10.1021/jp9099545  2010 American Chemical Society Published on Web 03/10/2010