UV photo-chlorination and -bromination of single-walled carbon nanotubes
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Michael Mehan and Thomas Debies Xerox Analytical Services, Xerox Corporation, Webster, New York 14580 (Received 20 August 2013; accepted 13 December 2013)
Electron-withdrawing halogen atoms are often bonded to the surface of carbon nanotubes to assist in the conversion from metallic to semiconducting properties. Single-walled carbon nanotubes (SWCNTs) were surface modified using UV photolysis with: (i) a broad band of wave lengths from approximately 250 to 400 nm having a maximum intensity at approximately 300 nm for photolysis of Cl2, (ii) low-pressure Hg lamps emitting 253.7 nm photons for photo-decomposition of HBr, and (iii) low-pressure Hg lamps emitting both 253.7 and 184.9 nm for photo-dissociation of HCl and HBr, respectively, and analyzed by x-ray photoelectron spectroscopy. Chlorine atoms adhered more readily than bromine atoms with the p-conjugation of the SWCNTs. The dominant increase with treatment was observed in the singly bonded chlorine moiety. Chlorine atoms, generated by UV photolysis of Cl2, produced a higher Cl saturation level of approximately 36 at.% than previously observed for multi-walled carbon nanotubes (13 at.%).The degree of chlorination depended on the amount of oxygen on the surface of the SWCNTs. Photo-dissociation of gaseous HCl and HBr showed lower amounts of halogenation on SWCNTs (approximately 5.8 at.% Cl and 2.5 at.% Br, respectively) than photolysis of Cl2.
I. INTRODUCTION
To help control the electronic behavior of carbon nanotubes (CNTs), electron-withdrawing halogen atoms are often covalently bonded to the surface to assist in the conversion from metallic to semiconducting properties. Theoretical calculations predict that side wall fluorination yields either metallic or semiconducting nanotubes depending on the type of fluorination and composition.1–3 The reaction of diatomic molecular chlorine with CNTs has been achieved using mechano-chemical ball milling4,5 and sonicating with chlorinated solvents.6 Chlorine atoms, generated by UV photolysis of Cl2, react more readily than chlorine molecules with the p-conjugation of multi-walled carbon nanotubes (MWCNTs) to produce a saturation level of approximately 13 at.% as observed by x-ray photoelectron spectroscopy (XPS).7 In contrast to covalent functionalization, as observed for fluorine and chlorine, the reaction of molecular bromine with single-walled carbon nanotubes (SWCNTs) leads to doping of the CNTs, resulting in charge transfer from the p-conjugation to the electron acceptor.8–12 In this study, SWCNTs were treated with UV photodissociation of gaseous Cl2, HCl, and HBr, where the UV
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.382 J. Mater. Res., Vol. 29, No. 2, Jan 28, 2014
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radiation may interact with the p-conjugation and H atoms may add across the p-conjugation of SWCNTs to assist with bromination. Surface modification was investigated using XPS. II. EXPERIMENTAL DETAILS A. SWCNT powder
The SWCNT powder, purchased f
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