Ab initio investigation of Al- and Ga-doped single-walled boron nitride nanotubes as ammonia sensor

Abstract

We performed first-principles calculations on the ammonia (NH₃) adsorption properties with zigzag and armchair single-walled BN nanotubes (SWBNNTs) using B3LYP/6-31G* basis set implemented in Gaussian 98 program. We considered the ammonia adsorption on structural and electronic properties of Al- and Ga-doped (8, 0), (5, 5) BNNTs. The adsorption energy for the most stable configuration of NH₃ on Al-doped (8, 0) BNNT is about −0.182 eV, which is typical for the chemisorptions. We determined that both aluminum and gallium doping can significantly enhance the adsorption energy of NH₃/BNNTs complexes. Our electronic results reveal that there is a significant orbital hybridization between two species in adsorption process being an evidence of covalent interaction.

Introduction

In the recent years, the nanotubes and nanowires materials have received special attentions because of their potential applications in nanodevices. Among them, carbon nanotubes (CNTs) have been essentially studied because of important applications such as gas sensors and for hydrogen storage [1], [2], [3], [4]. Between CNTs and boron nitride nanotubes (BNNTs) are many similarities that one of them is BNNT that is always semiconductors with almost constant band gap (5.5 eV), nearly independent of the tube diameter and helicity, but CNTs can present metallic or semiconductor behavior. In addition, studies have shown that CNTs, BNNTs are the most important nanotubes because of the alternative properties to carbon nanotubes. At the present time, there has been new investigation in the field of nanotube materials for gas sensor applications that BNNTs can be the best candidate because of their similarities to CNTs [5], [6], [7]. BNNTs were found to be wide band gap semiconductors and independent of the diameters and chirality of the tube [8]. BNNTs were first theoretically investigated in 1994 and then experimentally synthesized in 1995 as being mainly semiconductor materials with wide band gaps [9], [10]. Due to some essential properties such as their unusual mechanical properties and excellent chemical and thermal stabilities, the BNNTs have been favorable potential for applications in nanoelectronic devices for instance sensor and hydrogen storage media. Hence, recently, systematic studies have been performed to explore the application of various gas molecules adsorbed BNNTs [11], [12], [13], [14]. One of the materials that is known for its common environmental pollutant and serious toxicity is ammonia (NH₃). It is known that NH₃ is a low boiling point compound and is volatile. It is therefore very important to develop sensitive sensors to detect the gaseous NH₃ molecules. Other important properties of NH₃ are colorless and corrosive with normally encountered in the gaseous form with a characteristic pungent odor. It can be smelled at a level as low as 50 ppm in the air and is a common nitrogen-containing compound in the atmosphere [15], [16]. Recently, Zhou and Seif have reported the electronic properties and NMR parameters of the NH₃ molecule on pure BNNT, showing that these interactions are physisorption (0.16 eV) on the BNNT [16]. For example, Hadipour et al. have shown that substitution of an N atom by C-, O-, and Si-doped on AlN nanotubes is energetically more notable than perfect AlNNT for ammonia detection [17]. The electronic and structural properties of SCN⁻ molecules adsorbed on Al- and Ga-doped BNNT that have been investigated by first-principles calculations [8] showed pure BNNT that presents a high sensitivity to SCN⁻, but Al- and Ga-doped BNNT are energetically the most notable and can improve the interaction between gas molecules and BNNT. In this research, we expect that our studies can provide utilizable information in the application of dopant metals for detecting as NH₃ sensor.