Investigating the reactive sites and the anomalously large changes in surface pKa values of chemically modified carbon nanotubes of different morphologies

Adam T. Masheter; Poobalasingam Abiman; Gregory G. Wildgoose; Elicia Wong; Lei Xiao; Neil V. Rees; Robert Taylor; Gary A. Attard; Ronan Baron; Alison Crossley; John H. Jones; Richard G. Compton

doi:10.1039/B702492D

Investigating the reactive sites and the anomalously large changes in surface pK_a values of chemically modified carbon nanotubes of different morphologies

Adam T. Masheter,^a Poobalasingam Abiman,^a Gregory G. Wildgoose,^a Elicia Wong,^a Lei Xiao,^a Neil V. Rees,^a Robert Taylor,^b Gary A. Attard,^b Ronan Baron,^a Alison Crossley,^c John H. Jones^d and Richard G. Compton*^a

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* Corresponding authors

^a Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK
E-mail: richard.compton@chem.ox.ac.uk
Fax: +44 (0)1865 275410
Tel: +44 (0)1865 275413

^b Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff, CF10 3TB, UK

^c Materials Department, University of Oxford, Parks Road, Oxford, OX1 3PH, UK

^d Chemical Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom

Abstract

“Bamboo-like” multiwalled (b-MWCNT), “hollow-tube” multiwalled (h-MWCNT) and single-walled carbon nanotubes (SWCNT), chemically modified with 1-anthraquinonyl (AQ) or 4-nitrophenyl (NP) groups, are characterized using voltammetric, electron microscopic and Raman spectroscopic techniques. The pK_a values of the AQ-modified CNTs are found to be shifted by greater than three units when compared to the pK_a values of anthrahydroquinone (AHQ, the reduced form of AQ) in aqueous solution to beyond pH 14. These large changes in the surface pK_a values of the modified CNTs are explored further by comparing the pK_a values of CNTs modified with an anthraquinonyl-2-carboxylic acid group. These groups are attached to the CNT surface via the formation of an amide bond with an aminophenyl “spacer” unit derived from the chemical reduction of NP modified CNTs. The location of reactive sites on the CNT surface is investigated and their influence on the pK_a of the modified materials is discussed. Comparison with modified pyrolytic graphite electrodes exposing pure edge-plane or pure basal-plane crystal faces indicates that the modifying aryl groups are predominantly located on edge-plane like defects at the tube ends of MWCNTs. The effect of polymer formation on electron transfer kinetics of b-MWCNTs and h-MWCNTs is also discussed. In contrast SWCNTs show both significant side-wall functionalisation and fast electron transfer kinetics which is attributed to their different electronic structure.

Journal of Materials Chemistry

Investigating the reactive sites and the anomalously large changes in surface pK_a values of chemically modified carbon nanotubes of different morphologies

Abstract

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Investigating the reactive sites and the anomalously large changes in surface pK_a values of chemically modified carbon nanotubes of different morphologies

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