Formation Mechanism of 0.4,nm Single-Walled Carbon Nanotubes in CoAPO-5 Single Crystals

Abstract

Carbon nanotubes (Iijima, 1991) have been extensively studied in the past decade due to their promising applications, which largely derives from their particular structural and electronic properties. Especially, single-walled carbon nanotubes (SWNTs) can behave as metal or semiconductor depending on the tube diameter and chirality. Therefore, it is very important to control the purity in diameter, chirality and alignment of SWNTs. In our previous work, we reported fabrication of mono-dispersed SWNTs of diameter as small as 0.4 nm inside the channels of AlPO4-5 (structure code AFI) zeolite crystals (Tang et al., 1998; Wang et al., 2000). They constitute the best example of near one-dimensional (1D) quantum wires, and show extremely interesting physical properties (Tang et al., 2001; Li et al., 2001; Mei et al., 2005). Although the 0.4 nm SWNTs have been systematically studied, some of their basic physical and chemical properties are not yet well understood. For instance, the understanding of the formation mechanism is still not precisely known. The 0.4 nm SNTs are synthesized by pyrolyzing tripropylamine (TPA) hydrocarbon molecules which are incorporated inside the channels during hydrothermal growth of the AlPO4-5 crystal (Tang et al., 1998). However, the mechanisms by which the TPA molecules transform to the SWNTs inside the channels are poorly understood. This problem arises from the fact that the channel wall is relatively inertial and has very weak local dipole moment because of the regular alteration of tetrahedral (AlO4)− and (PO4)+ in AlPO4-5 framework. Thus, the adsorption force of the channel wall to guest molecules is relatively weak. As a result, a significant amount of TPA organic molecules can escape from the channels before they are thermally decomposed. The negatively charged framework and Bronsted acid sites are generated, when Al3+ is replaced by divalent cation Co2+ in the AlPO4-5 frameworks, which can play an important catalytic role and increase the adsorption force by improving the adsorption potential on the channel walls, the resulting