(20d) Single-Walled Carbon Nanotube (SWNT) Diameter Tuning Using C10 Co-Mcm-41
Selectivity of SWNT diameter has become crucial to its applications. Small (<0.7 nm) and uniform diameter SWNT of diameter exhibits very special electronic properties because of its high radius of curvature; SWNT with a large (>1.6nm) diameter tends to be very de-bundled and even exists individually which makes its application much easier without any further functionalization. Cobalt incorporated MCM-41 by carbon monoxide (CO) disproportionation has been proven to be an efficient catalyst for the synthesis of SWNT with narrow diameter distribution at a wide range of mean diameters (0.6-1.7nm). We have shown using in-situ X-ray Absorption Spectroscopy (XAS), CO significantly enhances the mobility of the cobalt particles on the MCM-41 surface. In this study, we have found that by operating at low/high reaction temperature compared with those normally used in CO disproportionation (750oC), the mean diameter of SWNT varies greatly while all maintaining a narrow distribution. Extended X-Ray Absorption Fine Structure (EXAFS) studies together with Transmission Electron Microscopy (TEM) images indicate that cobalt particles which act as catalyst are growing larger along with the increase of reaction temperature from 450oC to 950oC, resulting in the systematic shift of mean SWNT diameter from 0.6nm to 1.7nm confirmed by Raman spectroscopy, TEM and fluorescence spectroscopy. As we reported earlier, metal content of produced SWNT after purification is 0.7 wt% and the surface area of the purified SWNT without any further functionalization is as high as 1200 m2/g. The total carbon yield in this study varies from 10wt% to 50wt% (weight per catalyst weight) depending on the SWNT diameter, and the SWNT selectivity is all above 95%. All these features and the choice of SWNT diameter suggest many promising applications of SWNT in electronics, bio-medical engineering, environmental engineering, new catalytic support, etc.