External Chemical Reactivity of Fullerenes and Nanotubes
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External Chemical Reactivity of Fullerenes and Nanotubes 1
Seongjun Park, 2Deepak Srivastava, and 3Kyeongjae Cho* Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025 2 Computational Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035-1000 3 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-4040 *[email protected] 1
ABSTRACT The external chemical reactivity of graphene sheet, fullerenes and carbon nanotubes has been investigated. The total reaction energy is analyzed with several contributing terms and formulated as a function of the pyramidal angles of C atoms. We have determined the parameters for the formulae from ab initio simulation of graphene. We have applied them to predict hydrogenation energy of several nanotubes and C60, and demonstrated that the predicted total reaction energies are very close to the results of total energy pseudo-potential density functional theory calculations. This analysis can be used to predict the reaction energy and local bonding configuration of a reactant with diverse fullerenes and nanotubes within 0.1 eV accuracy.
INTRODUCTION There has been much research interest in carbon nanotubes and fullerenes since the discovery of C60 [1]. They have been considered as promising materials for nanotechnology applications, such as biochemical and gas sensors [2] and molecular transistor [3]. From the recent studies on possible nanodevice applications, it has been recognized that the surface functionalization of nanotubes and fullerenes would play an important role for nanodevice development. In order to functionalize nanotubes and fullerenes, the chemical reactivity of carbon atoms need to be understood with a quantitative accuracy. Generally, the chemical reactivity on the external surface of a fullerene or a nanotube is characterized by local bonding configuration of carbon atoms, more specifically, pyramidalization (θp) of C atoms as illustrated in Fig. 1 [4, 5]. Since the surface of a fullerene or a nanotube is curved, it is natural to have pyramidalized C atoms as shown in Fig. 1. Pyramidalization changes the hybridization of atomic orbitals at the C atom so that the π orbital contains different portion of s and p orbitals leading to different chemical reactivity. For example, graphite has planar structure (θp = 0) corresponding to sp2 for σ bonds and p for π bond. However, fullerenes and nanotubes have the hybrid bonding orbitals between sp2 to sp3. Because of this hybridization, fullerenes and nanotubes are known to be more reactive than graphite. The degree of pyramidalization (θp) is defined by the angle between σ bond and π orbital and named as pyramidal angle (Fig. 1). When a C atom interacts with an external chemical reactant, several processes occur simultaneously: orbital hybrid changing toward sp3, π bond breaking, and reaction between free π orbital and external reactant. In order to quantify the analysis of the chemical reactivity, we divide a reaction into several contributing parts: (a) straining
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