Published: April 29, 2011 r2011 American Chemical Society 7192 dx.doi.org/10.1021/la200730k | Langmuir 2011, 27, 7192–7198 ARTICLE pubs.acs.org/Langmuir Influence of Air Oxidation on the Surfactant-Assisted Purification of Single-Walled Carbon Nanotubes Alejandro Ans on-Casaos,* M onica Gonz  alez, Jos  e M. Gonz  alez-Domínguez, and M. Teresa Martínez Instituto de Carboquímica ICB-CSIC, Miguel Luesma Cast  an 4, 50018 Zaragoza, Spain b S Supporting Information 1. INTRODUCTION Optical spectroscopy has been often applied to the study of single-walled carbon nanotubes (SWCNTs), which show char- acteristic absorption bands in the near-infrared (NIR)Àvisible region. The wavelength of the absorption peaks depends on the SWCNT diameter, chirality, and conducting properties. 1,2 Cur- rently available SWCNT samples contain nanotubes with het- erogeneous diameter distributions, in which the most intense absorption bands correspond to the S 11 ,S 22 , and M 11 transitions of semiconducting (S) and metallic (M) nanotubes. Additionally, pristine SWCNT samples contain impurities, including metallic catalyst particles, amorphous carbon, and graphitic materials. Samples with the highest SWCNT purity demonstrate the most prominent characteristic absorption features. It was proved that the relative intensity of the S 22 band transition can be applied to the reliable evaluation of SWCNT purity, 3À5 giving an estimate of the amount of carbonaceous impurities present in the sample. SWCNT optical spectra are usually measured on samples previously dispersed in liquid media, either an organic solvent or water containing a surfactant. 6À8 The ability of the surfactant to disperse SWCNTs can be evaluated in terms of the total mass suspended in the liquid, the proportion of individual SWCNTs, and the intensity of absorption/fluorescence spectral features. 7,8 Sodium dodecylbenzene sulfonate (SDBS) is an anionic surfac- tant that has been widely utilized to disperse and debundle SWCNTs. 8,9 Ultracentrifugation in a sodium cholate surfactant can be utilized to obtain SWCNT samples free from graphitic impurities. 10 Alternatively, the separation of SWCNTs from graphitic materials can be effected by moderate-speed centrifu- gation in SDBS or in Pluronic F68 solutions. 11 Pluronic F68 is a block copolymer based on polyethylene oxide and polypropylene oxide that works as a nonionic surfactant. In the present article, we demonstrate that classical oxidation treatments in air strongly increase the relative intensity of SWCNT NIRÀvisible features for samples subsequently centrifuged in SDBS or Pluronic F68 solutions; however, other oxidation or gasi fication processes do not substantially change the SWCNT relative absorbance. The surface properties of carbon materials have been studied for years, particularly those regarding the effects of covalent oxygen functionalities such as carbonyl, quinone, phenol, ether, anhydride, lactone, and carboxylic groups. 12,13 It is also known that acidic or basic groups on SWCNT surfaces modify the energy of nanotubeÀsurfactant interactions. 14 We hypothesize that surface chemistry could strongly influence the final relative intensity of SWCNT features in the centri- fuged dispersions. The significance of the present work is to easily achieve ultra- high-purity SWCNT suspensions in water with relatively high yields and mild processing conditions. The method will be particularly useful when the presence of the surfactant is bene- ficial for the subsequent application of the SWCNTs. For example, suspensions of carbon nanotubes in SDBS can be utilized for the preparation of TiO 2 /carbon nanotube composites, 15 while Pluronic F68 works as a compatibilizing agent in the synthesis of high-toughness SWCNT/epoxy com- posite materials. 16 Received: February 24, 2011 Revised: April 12, 2011 ABSTRACT: Arc discharge single-walled carbon nanotube (SWCNT) soot was treated under different experimental conditions including gas- and liquid-phase oxidation, heat treatment in an inert gas, and hydrogen gasification. Afterward, the samples were dispersed in a surfactant and centrifuged at a moderately high speed. Near-infrared spectra of all the dispersions were compared with that of raw SWCNT soot. The relative intensity of SWCNT characteristic spectral bands strongly increased for air-oxidized samples after centrifugation, while it did not substantially change for samples oxidized with nitric acid or reduced with hydrogen. The relative SWCNT spectral intensity was associated to the sample purity through the so-called purity index, which was calculated from the S 22 band transition of semiconducting SWCNTs. Air-oxidized samples experienced a 7-fold increase in the purity index during centrifugation, while it increased by only 2À3 times for nonoxidized samples. Air oxidation specifically improves the preferential stability of SWCNTs over carbonaceous impurities in the dispersions, leading to the highest purity index values reported so far.