May 27, 2009
Boron nitride nanotube study considers amine bonding
Boron nitride nanotubes (BNNTs), a class of non-carbon nanotubes, are evoking a lot of research interest. Compared with their carbon counterparts, BNNTs show many unique electronic properties. For instance, BNNTs are uniformly semiconductors with a wide bandgap regardless of their chirality or diameter, in contrast to carbon nanotubes (CNTs), which exhibit diversified metallic and semiconducting characteristics. In addition, BNNTs possess excellent mechanical properties, high thermal conductivity and superb resistance to oxidation. These properties make BNNTs one of most promising materials for nanotechnology applications, especially under oxidative, hazardous and high-temperature environments. Unfortunately, similar to other nanotube materials, the poor solubility and difficulties of purifying and processing have hampered the future application of BNNTs.
Now, chemically modified (functionalized) nanotubes are being extensively promoted as one of the solutions to overcome these problems. However, the low chemical reactivity of the surface of BNNTs means that the chemical modification of BNNTs could be much more difficult than that of CNTs. Fortunately, it turns out that amine functional groups could provide a convenient and highly efficient approach to functionalizing BNNTs.
Scientists in China are using density functional theory (DFT) and frontier orbital theory (FMO) to examine the reaction behaviour in detail. The calculated band structures and density of states (DOS) indicate that modification by ammonia plasma is an effective method to modulate the electronic properties of BNNTs. Defects involved in the reactions are also considered, where the reactivity of BNNTs functionalization with NH2* is found to be enhanced.
Previous investigations have been carried out on cases where either NH3 is physically absorbed on the BNNT surface or only amine–boron ionic bonds are formed. Distinct from this kind of reversible and unstable functionalization with NH3 and amino groups, the researchers found that NH2* can form a strong B–N covalent bond on the BNNT sidewall. Correspondingly, the local hybridization of the boron atom is transformed from sp2 to sp3. The reaction energy shows a significant energetic preference to be functionalized by NH2* radicals over H radicals on the BNNT. Then the authors performed the NH2* radical reaction on BNNTs with various diameters and BN sheet, which is considered as a BNNT with an infinitely large diameter. It is seen that an increase in diameter results in a dramatic decrease of the reaction energies. The BN sheet gives the lowest reaction energies due to the decrease of the curvature of the BNNTs. Once the nanotube is disrupted by the modification of the NH2 radical, adjacent atoms increase a lot in chemical reactivity. The H atom absorption on the sidewall of BNNT was then studied. This reaction behaviour can be understood in terms of the FMO theory.
BNNT is an insulator with a wide bandgap weakly dependent on chirality or diameter. Now we suggest that the new functionalization method with the ammonia plasma can modulate the electronic properties of the BNNT. The system displays a degenerate p-type semiconducting behaviour for the functionalization with NH2* radicals, and the BNNTs bandgap is reduced for the most stable structure of H radicals on the N atom in NH2-functionalized BNNT.
In practice, the collision of N2+ ions would result in the generation of defects on the surface of BNNTs. Among four typical defects we considered in detail, the reactivity of the functionalization of BNNTs with NH2* are all enhanced compared with the pristine BNNTs except for the NB defect.
The researchers presented their work in Nanotechnology.
About the author
This work was done at Sun Yat-Sen University, in Guangzhou, China, and The Hong Kong University of Science and Technology (HKUST). Fenglei Cao is a PhD student in the theoretical and computational chemistry group at the Sun-Yat Sen University. Dr Wei Ren has been working in the physics department of HKUST since 2006. Dr Yue-Meng Ji is a postdoctoral researcher also in the theoretical and computational chemistry group, led by Prof. Cunyuan Zhao. Prof. Zhao is the director of the Institute of Physical Chemistry at Sun Yat-Sen University. He now focuses his research interests on the structures, reactivities and catalytic properties of inorganic and organometallic compounds; development and application in the design of novel catalysts; molecular design of advanced functionalized nanomaterials.