Diameter Tuning of Single-Walled Carbon Nanotubes with Reaction Temperature Using a Co Monometallic Catalyst Nan Li, Xiaoming Wang, Fang Ren, Gary L. Haller, and Lisa D. Pfefferle* Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06520 ReceiVed: April 5, 2009 Metal incorporated MCM-41 has proven to be a valuable template for the growth of narrow distributions of single-walled carbon nanotubes (SWNT), producing samples with a wide range of different mean diameters. The ability to obtain narrow diameter distributions at different mean diameters is important for applications that require particular (n,m) nanotubes. Another advantage of this system is the ease of cleaning and low metal content as compared to bimetallic systems. In this Article, we show that Co-MCM-41 allows diameter tuning of SWNT produced over a broad diameter range (from 0.6-0.8 to 1.8-2.0 nm) by changing reaction temperature. The lower temperature reaction provides a robust means to obtain very small diameter SWNT. X-ray absorption experiments show that the change in SWNT diameter correlates with the change in metal particle size. 1. Introduction The unique electronic and mechanical properties of single- walled carbon nanotubes (SWNT) have made them attractive for a large number of applications. Electronic properties of SWNT depend on their diameter and chirality. 1 For the typical diameters at which SWNT are grown, there are about 100 different chiralities. 2 Progress on applications has been hindered by quality issues in commercial SWNT and/or lack of a commercially viable separation process. Small, <0.7 nm, diameter SWNT have very special properties including high temperature superconductivity, they are one of the strongest 1-D materials, 3 and they exhibit unusual, unpredicted electronic properties. 4-6 High curvature is associated with properties that vary dramatically with applied force enabling switches and nonvolatile memories. To date, however, no economically feasible methods exist for reliably preparing SWNT of a predetermined tube identity, either by selective synthesis or through postsynthesis separation. Significant work has been directed toward obtaining a narrow distribution synthesis of SWNT. One area that we and others have worked on is the use of bimetallic systems 7-11 either to anchor ultrasmall particles of the SWNT growth catalyst 10 or to perturb its properties (e.g., providing a physical constraint, 7 acting as a carbon sink, 11 etc.). A drawback of these catalysts, however, is that purification of SWNT requires removal of two metals and can lead to increased impurities in the products, and the second metal is usually difficult to remove using routine chemical reagents (e.g., molybdenum carbide is highly refrac- tory). The uncleaned metal catalyst residue can greatly affect the performance of SWNT for a large variety of applications, including, but not limited to, biotechnology, 12 microelectronics, 13 and catalysis. 14 Thus, a simple yet effective way to tune the SWNT diameter using easily removable monometallic catalysts is needed and not yet reported. We have reported SWNT formed on subnanometer metal particles controlled by the framework of an amorphous meso- porous silica template, MCM-41. 15-22 The fact that the silica is amorphous is important because the chemistry of the wall can be altered independent of the pore structure. Our previous work used templates with 10-18 carbon atoms in the alkane portion of the surfactant used to template the MCM-41 corresponding to pore sizes from 1.8 to 3.1 nm. The pore diameter affects the reducibility of the metal in the silica framework with smaller pores resulting in higher reduction temperatures. 19 The 10-carbon alkane surfactant, C10, is different from other templates we have used because the pore size is smaller and the silica may be more mobile as evidenced by slightly lower stability of the silica framework. In a previous paper, 22 we characterized the C10 Co- MCM-41 catalyst and growth of carbon nanotubes over this catalyst. An interesting finding was that the yield based on carbon per catalyst weight (grams of carbon per gram catalyst) was extremely high (over 50%) as compared to other CVD processes using carbon monoxide (CO) as the carbon source and compared to the 10-20% yield observed for our larger pore diameter catalysts. The reaction also has very high selectivity toward SWNT (96%, as reported in ref 22), and the amorphous carbon species can be easily removed by burning in air at 300 °C. 25 Another observation of note was that the highest yield was obtained for catalysts synthesized at a lower pH (10.5 or 11) than for larger pore MCM-41. We theorized that the occlusion of the Co clusters by the silica of the MCM-41 framework (a physical constraint on particle growth) may be responsible for more smaller particles or domains. We have previously observed that the reaction temperature causes a change in SWNT diameter distribution on Co-MCM- 41 catalysts, 16 and similar results have been reported for Co-Mo/SiO 2 catalysts. 8 While this has not been generally reported for monometallic catalysts, 23 what the Co-MCM-41 and bimetallic systems such as Co-Mo/SiO 2 likely have in common is an anchoring mechanism between the active metal and the other catalyst components. The anchoring mechanism of Co metal particles to Co ions in the MCM-41 silica matrix is discussed in ref 18, and the anchoring of Co metal to MoC in the Co-Mo/SiO 2 catalyst is presented in ref 24. In this Article, we report a detailed study of “tuning” of the SWNT mean diameter distribution using a monometallic catalyst, by CO disproportionantion over C10 Co-MCM-41 under different reaction temperatures. Tube identity can be varied greatly by * Corresponding author. E-mail: lisa.pfefferle@yale.edu. J. Phys. Chem. C 2009, 113, 10070–10078 10070 10.1021/jp903129h CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009 Downloaded by YALE UNIV on October 6, 2009 | http://pubs.acs.org Publication Date (Web): May 14, 2009 | doi: 10.1021/jp903129h