Comparison of the stability of multiwalled carbon nanotube dispersions in waterw Dan H. Marsh, a Graham A. Rance, a Mujtaba H. Zaka, a Richard J. Whitby b and Andrei N. Khlobystov* a Received 5th June 2007, Accepted 2nd August 2007 First published as an Advance Article on the web 22nd August 2007 DOI: 10.1039/b708460a Continuous real-time monitoring of the nanotube concentration in aqueous solution using UV-Vis spectroscopy allows quantitative comparison of the stability of different types of nanotube dispersions. Systematic investigation of the effects of nanotube length and functionalisation for thin multiwalled carbon nanotubes (MWNT) has revealed that shorter MWNT form more stable dispersions than longer nanotubes of the same diameter. MWNT shortened to an average length of approximately 1 mm form stable dispersions in water with concentrations up to 0.013 mg ml 1 in the absence of surfactants or solubilising functional groups. The introduction of carboxylic or thiol groups on the surface of shortened nanotubes further increases the stability of MWNT dispersions (up to 0.24 mg ml 1 ). The introduction of surfactant or surface charge on MWNT has contrasting effects on functionalised and non- functionalised nanotubes, destabilising and stabilising their dispersions, respectively. Introduction Carbon nanotubes are extended, cylindrical assemblies of aromatic graphene with a number of unique electronic and mechanical properties dependent on the chirality and form of an individual tube. 1 The ability to handle carbon nanotubes as individual components and the understanding of their solution properties are essential for characterisation of their intrinsic properties and for further development of their applications. Chemical functionalisation of nanotube sidewalls 2 and surfac- tant adsorption on nanotube surfaces 3 are two of the most widely-used methods for solubilisation of nanotubes. Unfor- tunately, both methods have significant drawbacks, as cova- lent attachment of chemical groups on the nanotube surface alters a nanotube’s intrinsic properties and surfactants ad- sorbed on nanotubes create a physical barrier between the nanotubes and the environment. For many applications, the ideal case would be to have a stable suspension of nanotubes that is inherently soluble, stabilised not by the addition of secondary components, but by aspects of the chemical nature of the nanotube itself. We present herein an examination of both the absolute solubility of thin multiwalled carbon nano- tubes (MWNT), in terms of suspendable mass, and their time- domain stability, in terms of resistance of the suspension to aggregation over time. The behaviour of thermally-shortened and functionalised nanotubes is compared to conventional methods of solubilisation using surfactants or charge doping. Experimental All reagents and solvents were purchased from Sigma-Aldrich and used without further purification. Catalytic chemical vapour deposition (CCVD) synthesised thin multiwall carbon nanotubes with an average diameter of 10 nm were purchased from Nanocyl. Vis spectra were determined using a Unicam UV-1 spectrophotometer at a fixed wavelength of 530 nm, within glass cuvettes with a path length of 10 mm. Transmis- sion electron microscopy (TEM) imaging was performed on a JEOL JEM-4000EX TEM (LaB 6 source) at room tempera- ture. Samples were deposited onto Cu-grid mounted lacy carbon films (Agar Scientific). All MWNT products were synthesised from a single batch of nanotubes to ensure com- parability. Synthetic methods were adapted from those re- ported for SWNT or MWNT and full synthetic procedures for each, based on these literature methods, 4–8 together with spectroscopic confirmation of their structure are included in the ESI.w Results and discussion Preparation of nanotube samples In order to assess the impact of nanotube length and nanotube sidewall functionality on the stability of nanotube dispersions in water, we have prepared a number of related samples whose properties can be directly compared (Scheme 1). Gas-phase oxidation, via high temperature, furnace-assisted thermal annealing in air, efficiently reduces the overall length distribution of carbon nanotubes and is dependent on initial end-cap removal and subsequent ‘burning’ from the ends towards the nanotube centre without introducing new defects (Fig. 1a). a School of Chemistry, University of Nottingham, Nottingham, UK NG7 2RD. E-mail: andrei.khlobystov@nottingham.ac.uk; Fax: +44 (0)115 951 3563 b School of Chemistry, University of Southampton, Southampton, UK SO17 1BJ w Electronic supplementary information (ESI) available: Full experi- mental details, spectroscopic characterisation and additional support- ing data. See DOI: 10.1039/b708460a. 5490 | Phys. Chem. Chem. Phys., 2007, 9, 5490–5496 This journal is  c the Owner Societies 2007 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics