Mechanical Degradation of Carbon Nanotubes: ESR investigations
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Mechanical Degradation of Carbon Nanotubes: ESR investigations M. D. Chipara, A. Kukovecz1), Z. Konya1), J. M. Zaleski2), M. Chipara2)
PartTec Ltd, Bloomington, Indiana. 1)
Department of Applied and Environmental Chemistry, University of Szeged, Hungary
2)
Indiana University, Department of Chemistry, Bloomington, Indiana.
ABSTRACT ESR investigations on milled multiwalled carbon nanotubes are reported. The ESR spectra of pristine and milled carbon nanotubes consists of two resonances, a wide line located at g > 3.0 assigned to magnetic particles (catalysts residues) and a broad line with a peak to peak linewidth of about 10-2 T located at g = 2.05 and assigned to the interaction between the conducting electrons delocalized over carbon nanotubes and magnetic ions. It was observed that the parameters of this line (line position, linewidth, and line amplitude) depend on the milling time. For short milling times (up to 125 minutes) both the resonance linewidth and the g-factor increase as the milling time is increased. Longer milling times resulted in a decrease of both the g-factor and resonance linewidth (as the milling time was increased). This behavior was assigned to the removal of magnetic nanoparticles from carbon nanotubes. No resonance line due to the destruction of carbon nanotubes was observed.
INTRODUCTION The outstanding physical (Young modulus, tensile strength, thermal conductivity, and electrical conductivity) and chemical properties of carbon nanotubes (CNTs), triggered much theoretical and experimental research. For certain applications such as gas storage and absorption, short opened end CNTs are required. Mechanical milling and sonication are the most frequently utilized techniques to cut long CNTs into short ones [1-4]. Sonication in external magnetic fields has been used to purify CNTs by removing magnetic catalyst residues [1].
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TEM investigation of CNTs revealed the increase of carbon defects concentration and buckling after their sonication in solution (CH2Cl2) at 0 oC [3]. The sonication of CNTs produced ripple like damages (the breakage of small fraction of graphene layers) and the thinning of carbon nanotubes (by stripping the outer layers) [2]. The effect of high power sonication on freshly prepared CNTs characterized by an electron spin resonance (ESR) spectrum located at g=2.000 and characterized by a linewidth of about 30 Gauss (assigned to conducting electrons) has been reported in [2]. The resonance line of the pristine sample was reported to exhibit a reduced asymmetry [2]. The sonication of these CNTs broadened the resonance line up to about 40-50 Gauss [2]. The appearance of a new resonance line, located at g=1.991and with a linewidth of about 6 Gauss was observed [2]. The intensity of this line increases as the sonication time is increased [2]. Accordingly, this line was assigned to local defects like dangling bonds produced by sonication within the carbon nanotubes.
EXPERIMENTAL METHODS Multiwalled carbon nanotubes were prepared by using acetyle
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