Abstract
The interaction of ozone (O3) with B12N12 nano-cage is analyzed using ab initio methods. This provided information about the geometric, structural and electronic properties of the two interacting molecules. It is found that some of the adsorption processes are endothermic while some are exothermic with adsorption energy in the range of –0.63 to –1.80 eV. The chemisorption of the O3 molecule on B12N12 induces dramatic changes in the electronic properties of the cluster that includes a reduction of the HOMO-LUMO gap from 7.01 to 1.53 eV. This phenomenon increases the electrical conductivity of B12N12 nano-cage. The details of this study will be useful for the application of BN nano-cages as chemical sensors.
Introduction
In the last two decades great development in the area of small particles and nanostructured materials have been achieved [1–12]. Nanostructures such as nanotubes and nanoclusters have been established as promising candidates for building new generation of molecular sensors [13–20]. The adsorption of molecules on the surface of these materials induces changes in their electrical conductance as a consequence of charge transfer between the adsorbed chemical species and the nanostructures [21–26]. Recently, the sensitivity of the electronic properties of single-walled carbon nanotubes (SWCNT) to oxygen has sparked a wide ranging discussion. Collins et al. [27] found that oxygen dramatically influences the electrical conductivity of SWCNT. Ozone (O3) interaction with CNTs has also been studied experimentally and theoretically [28–31]. Experimental studies have shown that ozone-reacted CNTs have surface-bounded functional groups such as C=O, C=C, and C-O [29]. Recently, Picozzi et al. [28] have investigated ozone adsorption on CNTs theoretically and found that the adsorption energy of ozone on CNT is 0.2∼0.3 eV, depending on the diameter and chirality. Since the physisorption energy was too small to explain ozone exposure effect on the CNT resistance, some defect sites were speculated to be responsible for its adsorption and the corresponding change in resistance.
Boron nitride (BN) pair is isoelectronic with a pair of carbon atoms, while their bulk electronic properties differ considerably. C-based nanomaterials show metallic or semiconducting properties, and BN-based nanostructures form wide band gap materials [32–35]. The BN systems are also more stable than carbon materials, in terms of thermal and chemical stability [36, 37]. For example, according to the experimental studies, boron-nitride nanotubes (BNNTs) with well-crystallized structures have been reported to be more chemical stable than CNTs toward oxygen [38].
BN nanostructured materials have different forms such as nanotubes, nanosheets, nanocages, etc [39–44]. In spite of several theoretical and experimental works on chemical adsorptions on nanotubes [45–50], there are only a few reports about BN nanocages. The geometries and stability of (BN) n (n = 4–30) have been already studied by many research groups [51–53]. Jensen and Toftlund [54] carried out ab initio quantum chemical calculations on B12N12 with various geometries. They found that B12N12 fullerenes are characterized by higher stability than C24, provided that they have four- and six-membered rings, but no so-called “erroneous” BB and NN bonds. Oku et al. [55] have synthesized the B12N12 detected by laser desorption time-of-flight mass spectrometry.
In this work, we report a first-principles simulation of the interactions between O3 and B12N12 nanocage. The model systems are carefully chosen to cover several basic issues. The O3 molecule is of great practical interest in industrial and environmental applications. The motivation of the present work is to gain fundamental insights into the influence of adsorbed molecule on the structural and electronic properties of BN nanocage, as well as finding how these effects could be used to design more sensitive gas sensing devices.
Computational details
Geometry optimizations, and density of states (DOS) analysis were performed on a BN nanocluster of B12N12 and different O3/B12N12 complexes at the spin unrestricted X3LYP/6-31G* level of theory as implemented in GAMMES suite of program [56]. Vibration frequencies were also calculated at the same level to confirm that all the stationary points correspond to true minima on the potential energy surface. All frequency calculations were performed using numerical second derivatives and verified that all structures are true minima by frequency analysis and obtained all positive Hessian eigenvalue.
We define the adsorption energy (Ead) of O3 as follows: Ead = E(O3/ B12N12) – E(B12N12) – E(O3),
where E(O3/B12N12) is the total energy of an adsorbed O3 molecule on the pristine B12N12 surface, and E(B12N12) and E(O3) are the total energies of the pristine B12N12 and an O3 molecule, respectively. By the definition, negative values of Ead correspond to exothermic adsorption processes.
Results and discussion
At first, the accuracy of the method used in this work has been evaluated for describing the properties of the O3 molecule. The calculated bond length of O-O and angle of free O3 are 1.26 Å and 117.92° through our approach, which are in good agreement with the previous experimental values of 1.28 Å and 118° [57], respectively. The optimized B12N12 cluster includes eight six-membered (6MR) and six four-membered rings (4MR) with T h symmetry. The angles in tetragons and hexagons vary from 80° to 98° and from 111° to 125° (Fig. 1a), respectively. Two topologically nonequivalent B–N bonds are present in B12N12 cluster.
In order to find minimum adsorption configurations, the O3 molecule was initially placed at different positions above a B12N12 nano-cage with different orientations, including: (P) both of the terminal oxygen atoms of O3 close to B66 bond (the B-N bond which is shared between two 6MRs), (Q) both of the terminal oxygen atoms of O3 close to B64 bond (the B-N bond which is shared between a 4MR and a 6MR), or (T) near to two B atoms of 4MR, (R) one of the O-O bond horizontally on the B64, and (S) one terminal oxygen atom of O3 near to B atom of the cluster. After geometrical full optimization without any constraints, the most stable adsorption configurations of the O3/B12N12 complex are shown in Figs. 2 and 3.
The calculated Ead values for different configurations are in the range of –1.80 to +0.32 eV (Table 1). We have divided the interactions into three categories: (I) endothermic adsorption; (II) chemical functionalization, and (III) chemisorption. Among the five considered configurations, R and T thermodynamically are unstable with positive Ead of 0.12 and 0.32 eV, respectively, called endothermic adsorption (Fig. 2). In the next step, we have considered two chemical functionalized configurations which are shown in Fig. 3 (panel P and Q). As it is depicted, the O3 molecule strongly adsorbed on the B64 (P configuration) or B66 (Q configuration). Based on the NBO analysis, both the B64 and B66 bonds are broken after the adsorption process and two new bonds are formed, namely, O-N and O-B.
The P and Q configurations have Ead values of –1.80 and –1.54 eV, with rather a significant Mulliken charge transfer of 0.50 and 0.48 e from the B12N12 to the O3 molecule, respectively. Geometric parameters, such as the increased O-O bond length of the O3 (see the configurations P and Q in Fig. 3) as compared to 1.20 Å in isolated O3, reduced O-O-O bond angle (≈ 102.8° as compared to 117.9° in isolated O3), clearly indicate a high sp3 character on the central O atom. Moreover, the shortest distances between the ozone and the cluster in P and Q configurations are 1.38 and 1.42 Å, respectively.
Any change in electronic properties is an important factor in evaluating the potential application of a substance in gas sensing. We found that B12N12 has a large HOMO-LUMO gap (Eg) equals to 7.01 eV (Fig. 1b). It is well known that the change of Eg during the adsorption process is related to the sensitivity of the sorbent for a particular adsorbate. Upon the chemical functionalization of the cluster by ozone, the Eg value slightly decreases from 7.01 to 5.92 and 5.33 eV in R and T configurations, respectively (Fig. 2b). Although these processes are thermodynamically favorable, but they are not suitable for O3 detection because of low change in the Eg value and the fact of likely being irreversible.
The interesting case is the chemisorption of the O3 molecule on the pristine cluster surface (Fig. 4, configuration S). In this configuration, the Ead value is about –0.65 eV, somewhat weaker than that of the cases R and T. Mulliken charge, transferred from the cluster to the O3 molecule, is about 0.22 electrons. As shown in Fig. 4, (I) the O (central) –O (non - adsorbed) bond length of the O3 is about 1.24 Å, which is almost equal to the O-O length of the free O3 molecule (II) the distance between adsorbed O atom of the O3 and the adsorbing B atom of the cluster is nearly 1.47 Å, indicating a strong interaction, (III) the bond distance of the adsorbed and central O atoms of O3 has been increased from 1.26 to 1.35 Å during the chemisorption process. It suggests that this bond cleavage may be activated by using substrate of B12N12 nancage as a catalyst helping the conversion of the O3 to O2. In order to investigate the influence of chemisorption and chemical functionalization phenomena on the electronic properties of B12N12, the DOSs were calculated for the configurations P, Q, and S, compared to that of pristine cluster (Fig. 1). It was found that the DOS of cluster is not significantly changed near the conduction or valence level upon the chemical functionalization processes (Fig. 3).
The influence of the O3 chemisorption on the electronic structure of the host nanocluster was interesting. As shown in Fig. 4, once the O3 chemisorption occurred (configuration S), two impurity states have been appeared above and beneath of the valance and conduction level, respectively, so that the Eg value of cluster reduced from 7.01 to 1.53 eV (78 % change in Eg). It suggests that the cluster transits from a semi-insulator to a semiconductor substance with high electrical conductivity in comparison, because [58] where σ is the electric conductivity, and k is the Boltzmann’s constant. Regarding this equation, decreased Eg at a given temperature will lead to an increase of electrical conductivity. However, we think that B12N12 nano-cluster may be used in fabrication of a sensitive gas sensor for the O3 detection. When this material is placed in an electric circuit, its electrical conductivity significantly will be increased, upon exposure to the O3 molecules, which may be detected by turning on an alarm.
Conclusion
We have studied ozone (O3) adsorption on B12N12 nanocage using DFT method. It was found that this process can be endothermic or exothermic, depending on the adsorption configuration. The maximum adsorption energy of – 1.80 eV is released when a chemical functionalization is occurred. Based on the density of state analysis, we have showed that a great change of electrical conductivity may be seen by chemisorptions of the O3 on the B12N12. It suggests that the B12N12 may be used as appropriate promising sensor device for O3 molecule detection. Moreover, it was shown that the cluster may catalyze the conversion of the O3 to O2.