Preferential Growth of Single-Walled Carbon Nanotubes on Silica Spheres by Chemical Vapor Deposition Weiwei Zhou, † Yan Zhang, † Xuemei Li, † Shiling Yuan, ‡ Zhong Jin, † Junjian Xu, † and Yan Li* ,† Key Laboratory for the Physics and Chemistry of NanodeVices, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China, and Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Jinan, Shandong 250100, China ReceiVed: February 4, 2005; In Final Form: March 16, 2005 The preferential growth of single-walled carbon nanotubes (SWNTs) on silica spheres with various diameters was realized for the first time by chemical vapor deposition (CVD) of methane. SWNTs tend to wrap the silica spheres to form a new superstructure of uniform SWNT nanoclaws when the diameters of the silica spheres are larger than 400 nm. The SWNTs obtained on silica spheres have highly graphitic tubular walls as characterized by Raman spectroscopy and HRTEM. This is a new method to obtain tunable uniform elastic deformation of SWNTs, which may act as the model for the study about the effect of delocalized bending on the properties of SWNTs. In addition, the combination of SWNTs with monodispersed silica spheres could conveniently integrate SWNTs into photonic crystals. I. Introduction Carbon nanotubes (CNTs) have attracted extensive attention due to their extraordinary electrical, chemical, optical, and mechanical properties 1-3 since they were discovered by Iijima in 1991. 4 Single-walled nanotubes (SWNTs), as an atomically well-defined one-dimensional system, have been a strong focus due to their promising applications in nanometer-scaled elec- tronic devices such as transistors, 5-7 logic gates, 8 and sensors. 9,10 Recently, significant progress has been made in controlling the growth site, 11,12 the orientation, 13-15 and the diameter of straight SWNTs 16-20 on flat silicon substrates by the CVD method. Comparing with straight SWNTs whose electrical properties are determined by the diameter and the chirality, many theoretical calculations and experiments have demonstrated that the electri- cal properties of the curved SWNTs are strongly influenced by the bending degree. 21-25 For example, Dai et al. detected that the electrical behavior of the curved SWNTs was related to the local defects created by bending the SWNTs using an AFM tip. 25 In the meantime, various curved structures of SWNTs such as toris and coils were obtained on the flat substrate and in bulk. Liu et al. first observed a small amount of carbon nanorings in bulk CNTs synthesized by laser ablation. 26 A similar result was also reported in CNTs produced catalytically by thermal decomposition of gaseous hydrocarbons. 27 Furthermore, Martel et al. reported that nanotube rings were fabricated from oxidated straight SWNTs with the yields of over 50% by using ultrasonic irradiation. 28,29 Though closed rings of CNTs with a narrow size distribution could be synthesized in solution by covalent ring- closure reaction, 30 the low concentration of CNTs in solution limited the amount of rings obtained, and the size of the rings could not be controlled. Until now there was still no effective method to obtain a large amount of curved SWNTs with perfect graphitic structures in a controlled way. In this paper, we prepared an interesting new structure, nanoclaws of SWNTs, by the preferential growth of SWNTs on monodispersed silica spheres using CVD of methane. The stability of the curved superstructure is attributed to the strong van der Waals interaction between the nanotubes and the substrate, which is estimated from the results of molecular dynamics (MD) and molecular mechanics (MM) simulation. Though there were some reports of synthesis of CNTs on the curved substrates in the past years, 31-33 such curved super- structures of CNTs have not been observed before. II. Experimental Section Preparation of Silica Spheres with Catalysts. The uniform silica spheres of different sizes were prepared according to the Sto ¨ber method. 34 The spheres were soaked in a fresh 0.01 M FeCl 3 aqueous solution for 30 min to deposit the Fe(III) species (such as Fe(OH) 3 ) on the surface of spheres. Then, the silica spheres were separated by centrifuging, washed three times with deionized water, and re-dispersed in ethanol for the following usage. Chemical Vapor Deposition. The silicon wafer was cleaned by Piranha solution (a mixture of 98% H 2 SO 4 and 30% H 2 O 2 with the volume ratio of 7:3) for 30 min at 90 °C to make sure that the surface of the silicon wafer is hydrophilic. 1∼2 wt % ethanol solution of silica spheres was dropped onto the silicon wafer and then dried in ambient atmosphere to form a flat layer of silica spheres. The wafer was put into a horizontal quartz tube furnace and calcined in air at 700 °C for 5 min to remove the solvent and obtain iron oxide nanoparticles on the surface * To whom correspondence should be addressed, Tel: +86-10-62756773, E-mail: yanli@ pku.edu.cn. † Peking University. ‡ Shandong University. 6963 2005, 109, 6963-6967 Published on Web 03/25/2005 10.1021/jp050629u CCC: $30.25 © 2005 American Chemical Society