AlN nanotube as a potential electronic sensor for nitrogen dioxide
Highlights
► Electron transport through an AlN nanotube is significantly increased by the presence of NO2 molecules. ► AlN nanotubes could be a promising gas sensor for detecting the NO2 molecule. ► Recovery time and sensitivity of AlN nanotubes are smaller and larger than those of SiC nanotubes, respectively.
Introduction
Monitoring and controlling the amount of pollutants in the atmosphere are of great importance for modern society [1]. Nitrogen dioxide (NO2), as an environmental pollutant, is highly harmful which can cause photochemical smog and acid rain [2], [3]. This toxic gas is mainly emitted from combustion facilities, including automobiles, industrial plants, and domestic heaters [4]. Therefore, developing gas sensors with the ability of NO2 detection is of great importance in environmental issues. Up to now, several kinds of NO2 sensors have been developed by some research groups. Among various NOx gas sensing methods which have been proposed, semiconductors [5], solid electrolytes [6], and organic compounds [2] are the most commonly used materials.
Carbon nanotubes (CNTs), as quasi-one-dimensional structures, have raised great interest since their discovery by Iijima in 1991 [7]. Theoretical calculations and experimental measurements on CNTs have shown many exceptional properties which have made them promising candidates for several proposed applications, such as solid state sensors [8], [9], [10], [11]. So far, adsorption of NO2 on CNTs has been theoretically and experimentally investigated by different research groups [12], [13]. Valentini et al. [12] showed that CNTs could detect NO2 concentrations as small as 10 ppb. However, since the electronic properties of CNTs are mainly dependent on tabular chirality and diameter [13], separation of nanotubes with the desired electronic properties from other kinds is very difficult.
In recent years, semiconductor nanostructures of the type III–V (nanotubes, nanowires, nanoclusters, and nanotips) have attracted enhanced scientific interest [14], [15], [16], [17], [18] due to their numerous technological applications in nanoengineering, such as optoelectronic devices in the ultraviolet (UV) and visible regions. Among these nanostructures, aluminum nitride (AlN) with a large band gap presents high temperature stability, considerable thermal conductivity, low thermal expansion and high resistance towards chemicals and gases, and has been used in semiconductor processing due to its reliable dielectric properties [19], [20], [21]. Recently, several studies have been focused on AlN nanostructures such as nanotubes, nanosheets, nanocages, and nanowires [22], [23], [24].
AlN nanotubes (AlNNTs) with diameters of 30–80 nm have been experimentally synthesized via direct nitriding of aluminum powder [25], and the highly non-equilibrium dc–ac plasma technique [26]. As early pioneers of theoretical study of AlNNTs, Zhang et al. proposed a smooth surface and uniform diameter for possible structures of these tubes in 2003 [27]. We have shown that unlike semiconductive CNTs, the HOMO–LUMO energy gap (Eg) of AlNNTs increases with increasing tube diameter [28].
Despite extensive studies which have been reported on CNTs and BN nanotubes [29], [30], [31], [32], there are a few studies about AlNNTs [33], [34], [35], [36]. AlNNTs are wide band gap semiconductors, exhibiting good dielectric properties, high thermal conductivity, and low thermal expansion coefficient [34]. Herein, we have investigated the NO2 adsorption on the exterior surface of AlNNT using DFT calculations for the first time, and have explored the effects of adsorption on the electronic properties of AlNNTs. Finally, we have tried to find out how these effects can be employed in designing more sensitive gas sensing devices.
Section snippets
Computational methods
A zigzag (5,0) AlNNT consisting of 30 aluminum and 30 nitrogen atoms was considered, with its end atoms saturated by 10 hydrogen atoms to avoid the boundary effects. Geometry optimizations, natural bond orbital (NBO), and density of states (DOS) analyses were performed on the pristine AlNNT and different NO2–AlNNT complexes at spin-unrestricted B3LYP/6-31G (d) level of theory. The length and diameter of the optimized tube have been obtained to be about 13.98 and 5.25 Å, respectively. The
Results and discussion
At first, accuracy of the used method in this work was examined to describe the properties of NO2 molecule in gas phase. Based on the obtained results, bond length of the individual NO and angle of the free NO2 were found to be 1.20 Å and 133.8°, respectively, which are in good agreement with the previously reported experimental values of 1.19 Å and 134.1° [39]. In the optimized AlNNT, two types of Al–N bonds can be found: one with a bond length of 1.81 Å and in parallel with the tube axis, and
Conclusion
The interaction between a single-walled AlNNT and NO2 molecule was explored by DFT calculations to examine the possibility of using these tubes in potential gas sensor applications for NO2 detection, for the first time. The results indicate that NO2 molecules can be adsorbed on the exterior surface of AlNNT with appreciable adsorption energies and charge transfer, which would lead to significant changes of AlNNTs conductance. Thus, AlNNT can be a promising candidate for NO2 detection. We
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