Mechanistic Consideration of pH Effect on the Enrichment of Semiconducting SWCNTs by Conjugated Polymer Extraction

• Published in: Journal of physical chemistry. C 2016-09, Vol.120 (38), p.21946-21954
• Main Authors: Ding, Jianfu, Li, Zhao, Lefebvre, Jacques, Du, Xiaomei, Malenfant, Patrick R. L
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b05965

Enrichment schemes providing high-purity semiconducting single-walled carbon nanotubes (sc-SWCNTs) will enable their implementation into high end and printed electronics. Conjugated polymer extraction (CPE) has been shown to be a very effective and scalable method to isolate sc-SWCNTs with purities >99.9%. However, this method is often plagued with variability, and the mechanism is not thoroughly understood. Herein, we probe the origins of selectivity in polyfluorene-assisted enrichment using poly­(9,9-di-n-dodecylfluorene) (PFDD) and find that the affinity of the wrapping polymer to bind metallic (m-) and semiconducting SWCNTs is similar and may not contribute to the selectivity, but rather that oxygen-driven p-doping of the nanotubes and its dependence on the surface acidity may play a vital role for the selective dispersion of sc-SWCNT. The latter hypothesis is tested by titration experiments in which sodium hydroxide (NaOH) is used to neutralize the SWCNT surface, thus mitigating oxygen p-doping of SWCNTs that arises via the oxygen/water (O2/H2O) redox couple. The selectivity of the enrichment is completely lost when 1 equiv of NaOH per 70 SWCNT carbons is used. Oxygen doping is believed to be the driving force that triggers the aggregation of highly polarizable m-tubes. These novel observations stress that care must be taken during purification schemes prior to enrichment using CPE, which is strongly influenced by acid/base treatments, likely due to the pH dependence of p-doping via the O2/H2O redox process.