Bo Wang

Selective synthesis of (9,8) single walled carbon nanotubes on cobalt incorporated TUD-1 catalysts.

Selective synthesis of single walled carbon nanotubes (SWCNTs) with specific (n,m) structures is desired for many potential applications. Current chirality control growth has only achieved at small diameter (6,5) and (7,5) nanotubes. Each (n,m) species is a distinct molecule with structure-dependent properties; therefore it is essential to extend chirality control to various (n,m) species. In this communication, we demonstrate the highly selective synthesis of (9,8) nanotubes on a cobalt incorporated TUD-1 catalyst are (Co-TUD-1). When catalysts were prereduced in H(2) at the optimized temperature of 500 °C, 59.1% of semiconducting nanotubes have the (9,8) structure. The uniqueness of Co-TUD-1 relies on its low reduction temperature (483 °C), large surface area, and strong metal-support interaction, which stabilizes Co clusters responsible for the growth of (9,8) nanotubes. SWCNT thin film field effect transistors fabricated using (9,8) nanotubes from our synthesis process have higher average device mobility and a higher fraction of semiconducting devices than those using (6,5) nanotubes. Combining with further postsynthetic sorting techniques, our selective synthesis method brings us closer to the ultimate goal of producing (n,m) specific nanotube materials.

Selective Enrichment of (6,5) and (8,3) Single-Walled Carbon Nanotubes via Cosurfactant Extraction from Narrow (n,m) Distribution Samples

Highly selective enrichment of (6,5) and (8,3) SWCNTs (above 85% of the semiconducting tubes) was achieved through multistep extraction by sodium dodecyl sulfate (SDS) and sodium cholate (SC) cosurfactant solution from narrowly (n,m) distributed SWCNTs produced by the catalyst Co-MCM-41. A systematic change in the chirality selectivity was observed when the weight ratio between SDS and SC varied in cosurfactant solutions, with maximum enrichment selectivity for (6,5) tubes yielded at 1:4. Furthermore, surfactants were washed away easily to produce clean SWCNTs. This observation sheds light on the possibility of obtaining SWCNTs with the desired (n,m) structure via an easily scalable approach. No selectivity was detected when using sodium dodecyl benzene sulfonate (SDBS)/SC cosurfactants, hence suggesting the need for further exploration of various cosurfactant combinations for more effective extraction of different (n,m) species.

Low-defect, purified, narrowly (n,m)-dispersed single-walled carbon nanotubes grown from cobalt-incorporated MCM-41.

A mild, four-step purification procedure using NaOH reflux, HCl wash, and oxidation by 4 mol % molecular oxygen at 500 degrees C was developed to purify single-walled carbon nanotubes (SWCNTs) with narrow semiconducting (n,m) distribution produced from cobalt-incorporated MCM-41 (Co-MCM-41) in order to obtain bulk low-defect-density nanotubes. Three key features of Co-MCM-41 allow this mild purification technique: (1) ultrathin silica walls versus dense silica or other crystalline oxide supports are soluble in dilute NaOH aqueous solution, which avoids the damage to SWCNTs usually caused by using HF treatment to remove catalytic supports; (2) the small metallic particles are easily dissolved in HCl, a significantly milder chemical treatment compared to HF or HNO(3); (3) the high selectivity to SWCNTs with negligible multiwalled carbon nanotubes or graphite, which facilitates the removal of undesired carbon species by selective oxidation. The effectiveness of this purification procedure was evaluated by high-resolution transmission electron microscopy, scanning electron microscopy, Raman, UV-vis-NIR, and fluorescence spectroscopy, solution redox chemistry on fractionated (6,5) tubes, and SWCNT-based field effect transistor device performance. The results demonstrate that Co-MCM-41 catalyst not only provides tubes with narrow semiconducting (n,m) distribution but also allows a mild purification procedure and, therefore, produces SWCNTs with fewer defects.

IN SITU FORMATION OF COBALT NANOCLUSTERS IN SOL–GEL SILICA FILMS FOR SINGLE-WALLED CARBON NANOTUBE GROWTH

A facile method was developed for in situ formation of Co nanoclusters in sol–gel silica thin films spin-coated on Si wafers. The size and density of Co nanoclusters can be controlled by spin-coating speeds, annealing methods, reduction temperatures under H2, and metal precursor concentrations in tetraethylorthosilicate solutions. The optimized preparation condition, spin-coating speed of 9000 rpm, annealing at 500°C in air followed by reduction at 800°C in H2, resulted in silica films as thin as 60 nm and Co nanoclusters with a mean diameter of 1.5 nm. Morphological and chemical characteristics of thin films and nanoclusters were studied by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Subsequently, these Co nanoclusters were successfully used to grow SWCNTs by CO decomposition. Film containing Co monometallic clusters produced SWCNTs of 1.3 nm in diameter, whereas film having Co/Mo bimetallic clusters produced SWCNTs of 0.9 nm. This sol–gel approach allowed not only easy catalyst patterning on a thin film but also a fine-tuning of SWCNT properties, e.g., diameter.