Chirally selective synthesis of single walled carbon nanotubes on novel catalysts

Abstract

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.