Chirally selective synthesis of single walled carbon nanotubes on novel catalysts.
Date of Issue2013
School of Chemical and Biomedical Engineering
Single-walled carbon nanotubes (SWCNTs) have excellent electronic, thermal, optical, and magnetic properties, which depend on their specific (n,m) structure; therefore precise control of SWCNT (n,m) structure is crucial to realize their potential applications. So far, a few catalysts can achieve selective synthesis of SWCNTs with a narrow (n,m) distribution; however, most of the chiral-selective growth is restricted to one or two small diameter SWCNTs, such as (6,5) and (7,5) nanotubes (d<0.9nm). The development of novel catalysts is suggested as the premier target to realize chirally selective synthesis of SWCNTs, especially large diameter SWCNTs. Co incorporated mesoporous TUD-1 catalyst (Co-TUD-1) and CoSO4/SiO2 catalyst were developed for SWCNT synthesis. Both catalysts are highly selective towards a large diameter (9,8) SWCNTs (d=1.17nm) which account for more than 50% of semiconducting nanotubes under optimal growth conditions. The uniqueness of Co-TUD-1 catalyst lies in its low reduction temperature (483°C), large surface area (740 m2/g), and strong metal-support interaction, which stabilizes Co clusters responsible for the growth of (9,8) nanotubes. While the specificity of the CoSO4/SiO2 catalyst is the presence of chelating bidentate SO4 2- bonded with cobalt on the silica surface. The coexistence of sulfur atoms near Co atoms may limit the aggregation of Co atoms and/or form various Co–S compounds during the SWCNT growth, which contributes to the formation of Co clusters for (9,8) nanotube growth. Both XAS results and theoretical study confirm the close matching between the size of stable Co clusters and the diameter of (9,8) nanotubes. Compared to Co-TUD-1 catalyst, CoSO4/SiO2 catalyst has moreadvantages: lower cost, shorter synthesis time, easier preparation, and higher carbon yields (3.8wt% vs. 1.5wt%). Therefore, CoSO4/SiO2 catalyst is more promising for scaling up the production of (9,8) SWCNTs. The CoSO4/SiO2 catalyst also shows tunable nanotube chiral selectivity from large diameter (9,8) nanotubes to small diameter nanotubes by changing catalyst calcination temperature from 400°C to 800°C. At low calcination temperature (≤400°C), it has excellent single chiral selectivity towards (9,8) nanotubes, while the removal of sulfur at higher calcination temperatures forms surface cobalt oxides and silicates that shift the chiral selectivity towards small diameter nanotubes. Since the presence of sulfur affects the formation of stable Co clusters with an appropriate size for (9,8) SWCNT growth, a sulfur doping method was developed to prepare catalysts. Three types of Co/SiO2 catalysts, which are either inactive for the SWCNT growth or only selective towards small diameter nanotubes, can be converted into chirally selective catalysts to grow SWCNTs enriched with large diameter (9,8) tubes. Last, SWCNT thin film field effect transistors were fabricated using (9,8) nanotubes from the synthesis process, which demonstrated better electronic performances than those using (6,5) nanotubes. Overall, work in this thesis has made progresses on the development of novel catalysts for chirally selective synthesis of SWCNTs: (1) growth of a large diameter (9,8) SWCNTs with a narrow (n,m) distribution using Co-TUD-1 catalyst and CoSO4/SiO2 catalyst; (2) shifting the (n,m) distribution by sulfur removal (calcination) in catalysts; and (3) converting non-selective Co/SiO2 catalysts for the (9,8) SWCNT growth by sulfur doping.