Oxidized Single-Walled Carbon Nanotubes: Removal of Carbonaceous Functionalized Material by Washing with Solvents or Bas
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Oxidized Single-Walled Carbon Nanotubes: Removal of Carbonaceous Functionalized Material by Washing with Solvents or Base Elisa Del Canto, Kevin Flavin, Dania Movia and Silvia Giordani* School of Chemistry / Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), University of Dublin, Trinity College, Dublin 2, Ireland *[email protected] ABSTRACT Oxidized single-walled carbon nanotubes (SWNTs) have been prepared following a widely reported two-step purification/oxidation procedure in the presence or absence of a treatment with base (NaOH). The oxidized nanotube samples washed with solvents or base appear close to identical with respect to both appearance and properties. Efficient removal of both metal and carbonaceous impurities and introduction of –COOH groups on the nanotube surface have been demonstrated by AFM, Raman and FTIR spectroscopy. Furthermore, persistence of optical properties was confirmed using UV-vis/NIR absorption and NIR photoluminescence spectroscopies. INTRODUCTION Single-walled carbon nanotubes (SWNTs) are regarded as ideal scaffolds for various chemical payloads due to their remarkable mechanical, thermal, electronic, and optical properties, as well as their biocompatibility and ease in decoration [1- 4]. However, some aspects still impede the widespread exploitation of this promising material: the presence of impurities, the poor dispersibility, the difficult processibility, the possible loss of electronic properties during chemical treatments and the presence of carbonaceous material in the sample are only some noteworthy examples. Raw SWNTs need to be purified from secondary unwanted products coming from the synthetic production, and the use of acid-oxidizing conditions has been demonstrated as a valid purification technique [5, 6]. A widely reported two-step purification/oxidation procedure [7-9] consisting of a nitric acid purification followed by an oxidative treatment with sulfuric acid and hydrogen peroxide, yields oxidized nanotubes that can be further functionalized via amidation or esterification [10, 11]. However, during this process, small polycyclic aromatic sheets edge-terminated with -COOH groups, named carboxylated carbonaceous fragments (CCFs), may be generated besides the desired carbonyl functionalities. The CCFs create a reactive coating on the surface of nitric acid treated SWNTs and NaOH aqueous washings have revealed to be efficient in their removal [12, 13]. We have recently demonstrated that the clearing of the undesired CCFs, via solvent or base washing, is a crucial step to ensure that the further chemical functionalization is performed on –COOH groups effectively attached to the nanotube surface and not on fragments physisorbed onto their exterior [14]. On the light of these results, the study reported here critically compares oxidized SWNTs obtained by following two different protocols. In the first the tubes were subjected to exhaustive solvent washing following the two-step purification/oxidation protocol whereas in the second approach the nitric ac
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