Effects of Al Doping and Double-Antisite Defect on the Adsorption of

Article pubs.acs.org/JPCC

Effects of Al Doping and Double-Antisite Defect on the Adsorption of HCN on a BC2N Nanotube: Density Functional Theory Studies Ali Ahmadi Peyghan,† Nasser L. Hadipour,*,† and Zargham Bagheri‡ †

Department of Physical Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran Physics Group, Science Department, Islamic Azad University, Islamshahr Branch, P.O. Box 33135-369, Islamshahr, Tehran, Iran

‡

ABSTRACT: By using density functional theory, we investigated the reactivity and electronic sensitivity of pristine and structurally manipulated BC2N nanotubes (BC2NNT) to a HCN molecule. It was mainly found that (i) the pristine BC2NNT can weakly adsorb the HCN with adsorption energy of −1.1 kcal/mol, and its electronic properties are not sensitive to HCN; (ii) doping the tube by an Al atom can largely improve its reactivity to HCN, but it does not have a significant effect on its sensitivity; (iii) B−B antisite defect on the tube wall can improve both reactivity and sensitivity of the tube to HCN; (iv) N−N antisite could improve neither the reactivity nor the sensitivity. Upon the adsorption of HCN on the B−B antisite defect, the HOMO−LUMO energy gap of the tube is significantly reduced from 2.23 to 1.82 eV and energy of 6.3 kcal/mol is released.

1. INTRODUCTION Carbon nanotube (CNT) as a novel nanomaterial has been the center of attention during the past decade.1 Owing to their unique quasi-one-dimensional atomic structure and superb mechanical and electronic properties, the CNT has been playing a significant role in emerging nanotechnology.2−4 Purification or controlled synthesis of CNTs with selected helicity has not been achieved so far which has made electronic device manufacturing with CNTs difficult. Therefore, it is interesting and important to further find or design other new low-dimensional tube-like nanostructured materials suitable for miniaturization of electronic devices.5−8 For example, the BC3, BC2N, and B2C nanotubes were theoretically proposed, among which the BC3 nanotubes as well as composite Cx(BN)y nanotubes,9−11 including signature of BC2N, have been experimentally realized. Considerable experimental efforts have been devoted to the synthesis of Cx(BN)y nanotubes, and they have been successfully obtained by electrical pyrolysis, laser ablation, hot-filament chemical vapor deposition, and the template route.12−15 Nanostructure materials are not defectfree, and some experimental and theoretical reports have demonstrated the existence of various defects such as Stone− Wales, vacancies, and antisite in their walls.16−18 Gas molecular adsorption in nanotubes is an important issue for both fundamental research and technical application. For example, the adsorption of gas molecules on nanotubes has the considerable potential for applications in fuel cells, gas sensors, and hydrogen storage.19−23 Nanotubes are porous materials with a high reactivity exterior surface, which makes them sensitive to molecular adsorption. The reactivity of these nanotubes is often adjusted by doping with other elements. For example, Hamadanian et al. found, by performing density functional theory (DFT) calculations, that the introduction of © 2013 American Chemical Society

substitutional aluminum impurity increases the reactivity of the CNT to the CO molecule.24 Recently, theoretical calculations on the pristine BC2N nanotubes (BC2NNTs) have shown that these nanostructures are useful for energy-storage applications.25 Hydrogen cyanide (HCN) gas has high toxicity and is highly lethal to humans and animals, so its monitoring and control of exposure, in both industrial and residential environments, are of special interest.26 Zhang et al. proposed that the boron-doped CNT is more sensitive to the detection of HCN molecules than that of perfect CNT, using DFT calculations.27 Here, we are interested in the following: (1) whether there is a potential possibility of BC2NNTs serving as a chemical sensor for HCN; (2) if not, can we find a method of improving the sensitivity of BC2NNTs to HCN?

2. COMPUTATIONAL METHODS Geometry optimizations and natural bond orbital (NBO) and density of states (DOS) analyses were performed on a (8, 0) zigzag BC2NNT (constructed of 36 B, 36 N, and 72 C atoms) and different HCN-adsorbed BC2NNT complexes at the B3LYP level of theory with 6-31G(d) basis set as implemented in the GAMESS suite of program.28 This level of theory is a popular approach which has been commonly used for structures.29−33 The length and the diameter of the optimized pure BC2NNT were computed to be about 17.13 and 6.22 Å, respectively. To avoid boundary effects, atoms at the open ends of the tube were saturated with hydrogen atoms. The adsorption energy (Ead) of a HCN molecule on the pristine, Received: December 19, 2012 Revised: January 9, 2013 Published: January 11, 2013 2427

dx.doi.org/10.1021/jp312503h | J. Phys. Chem. C 2013, 117, 2427−2432

The Journal of Physical Chemistry C

Article

where four types of bonds, namely, B−N, B−C, N−C, and C− C, can be identified, with corresponding lengths of 1.46, 1.52, 1.44, and 1.36 Å, respectively. There are two types of carbon atoms in BC2NNT: CI is a carbon atom that is bonded to two B atoms and one C atom, while CII is a carbon atom that is bonded to two N atoms and one C atom. Buckling of B−N and C−C bonds was found in the structures. After optimization, this buckling moves N atoms inward and B atoms outward of the nanotube surface in the B−N bonds. On the other hand, for C−C bonds, the CI atoms are relaxed outward, while the CII atom is relaxed inward of the nanotube surface. Buckling of atoms from a perfect cylindrical model is according to the previous results for BNNTs, and it is a solution to minimize the total energy and strain energy.34 To find minimum adsorption configurations, the HCN molecule was initially placed at different positions above a BC2NNT, including on top of a sidewall B, CI, CII, or N atom from its H or N head. Also, the linear HCl molecule was located on the tube at different positions, being parallel or diagonal to the tube axis. Full geometrical optimization was then performed with several different orientations of the HCN molecule. However, only one stable structure was obtained upon the relaxation process (Figure 2). It is worth saying that other initial configurations were reoriented to stable configuration. More detailed information from the simulation of the HCN−BC2NNT system including values of Ead and Eg (HOMO−LUMO gap) for this configuration is listed in Table 1.

Al-doped, and double-antisite defective (this defect includes a B−B and an N−N homonuclear bonds) BC2NNTs was obtained using the following equation Ead = E(HCN/tube) − E(tube) − E(HCN) + E(BSSE) (1)

where E(tube/HCN) is the total energy of the HCN molecule adsorbed on the tube surface, and E(tube) and E(HCN) are the total energies of the tube and a molecule, respectively. E(BSSE) is the basis set superposition error (BSSE) corrected for all the interaction energies. By the definition, a negative value of Ead corresponds to exothermic functionalization.

3. RESULTS AND DISCUSSION 3.1. Adsorption of HCN on Pristine BC2NNT. In Figure 1, we have shown a partial structure of the optimized BC2NNT,

Table 1. Adsorption Energy (Ead in kcal/mol), HOMO Energies (EHOMO), LUMO Energies (ELUMO), HOMO− LUMO Energy Gap (Eg), and Fermi Level (EF) of Systems (Figures 1−4) in eV configuration

Ead

EHOMO

EF

ELUMO

Eg

ΔEg (%)

BC2NNT BC2NNT/HCN Al-doped BC2NNT Al-doped BC2NNT/HCN

−1.1 −24.8

−5.41 −5.11 −5.22 −4.90

−4.12 −3.96 −4.01 −3.72

−2.84 −2.82 −2.80 −2.55

2.57 2.29 2.42 2.35

−10.1 −2.8

a

Figure 1. Partial structure of optimized BC2NNT and its density of states (DOS). Bonds are in Å.

a

Change of Eg of tube after HCN adsorption.

Figure 2. Model for stable adsorption of HCN on the BC2NNT and its density of states (DOS). Bonds are in Å. 2428



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Figure 3. Partial structure of optimized Al-doped BC2NNT and its density of states (DOS) plot. Bonds are in Å.

Figure 4. Model for stable adsorption of HCN on the Al-doped BC2NNT and its density of states (DOS) plot. Bonds are in Å.

BC2NNT is dramatically distorted (Figure 3). In the optimized Al-doped tube, the Al atom impurity is projected out of the tube surface to reduce stress due to its larger size compared to the B atom. The calculated bond lengths are 1.87 and 1.78 Å for the Al−CI and Al−N bonds, respectively, in the doped tube, being much longer than the corresponding CI−CII bonds in the pristine tube (Figure 3a). Also, the CI−Al−CI angle in the doped tube is 110° which is smaller than CI−B−CI in the pristine one (116°), which the NBO analysis suggests can be attributed to the change of doped site hybridization from sp2 to nearly sp3. Calculated DOS of the Al-doped tube is shown in Figure 3, indicating that its Eg value is reduced to 2.24 eV compared to the pristine tube. The DOS plot clearly shows that the Al-doped BC2NNT is still a semiconductor. Subsequently, we have explored HCN adsorption on the Aldoped tube by locating the molecule above the Al atom with different initial orientations including N or the H atom of the molecule close to Al. After careful relax optimization of initial structures similar to pristine BC2NNT, only one final stable structure was obtained which is shown in Figure 4. Interestingly, during the optimization, the HCN was reoriented in such a way that its N atom has gotten closer to the Al site with Ead of −24.8 kcal/mol. Also, the corresponding interaction distance between the Al atom of the doped tube and the HCN is 2.00 Å. A more negative Ead between the Al-doped BC2NNT and HCN indicates that the doping of Al in BC2NNT can

This configuration stands for a weak interaction between the HCN molecule and the tube surface. In this configuration, the HCN molecule from its nitrogen head was put atop of a boron site of the tube, and its corresponding calculated Ead value is about −1.1 kcal/mol. Computing molecular electrostatic potentials on the BCN nanotube models, Politzer et al.35have previously shown that above the borons are positive regions. It corroborates that these atoms are reactive to the electron-rich N atom of HCN. The less negative Ead of HCN on pristine BC2NNT in this structure reveals the physical nature of the interaction. The interaction distance of B···N is about 1.71 Å. To further understand the electronic properties of pristine BC2NNT, the DOS plots for tube, before and after adsorption of HCN, were calculated. For the bare BC2NNT (Figure 1), it can be concluded that it is a semiconductor material with an Eg of 2.57 eV. By referring to Figure 2, the valence level slightly moves to higher energies (−5.11 eV) compared to the bare tube (−5.41 eV), while the conduction level remains constant so that Eg of the tube slightly decreased from −4.12 eV in the bare tube to −3.96 eV for the HCN/BC2NNT complex. Thus, we conjecture that the pristine BC2N is insensitive to the HCN molecule. 3.2. Adsorption of HCN on Al-Doped BC2NNT. To overcome the insensitivity of the BC2NNT to the HCN, the adsorbing B atom was replaced by an Al one. Substituting the B atom by the impurity of Al, the geometric structure of the 2429



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Figure 5. Partial structure of optimized double-antisite defective BC2NNT and its density of states (DOS) plot. Bonds are in Å.

Figure 6. Models for stable adsorption of HCN on the double-antisite defective BC2NNTs and their density of states (DOS) plot. Bonds are in Å.

increased from −4.01 eV in the Al-doped tube to −3.72 eV for the HCN/Al-doped BC2NNT complex. The improved effect with the Al atom may also be a result of its greater polarizability (compared to the B atom) and hence greater charge capacity.36 As a result, although the physisorption to chemisorption transition was induced by Al doping in the nanotube, electronic properties of the mentioned tube are not sensitive to the toxic HCN and cannot detect it. 3.3. Adsorption of HCN on the Double-Antisite Defective BC2NNT. Afterward, we studied the interaction of the defective BC2NNT with an HCN molecule. Among various native defects, we focused on the double-antisite defect (dBC2NNT), in which simultaneously boron and nitrogen atoms sit at the original nitrogen and boron sites, respectively, as shown in Figure 5. Substituting these atoms by the impurity of

improve the reactivity of the tube toward the HCN molecule, which is in agreement with the usual mechanism that is proposed for such binding: the HCN binds at the exposed Al atom which is electron deficient and can receive electrons from the lone pair orbitals of nitrogen. Why is the reaction of HCN with the Al-doped tube more favorable than that of the pristine one? To answer this question we have investigated NBO analysis of Al-doped BC2NNT. NBO analysis indicates that, unlike boron atoms, the hybridization of the Al atom is sp3, and it can have a coordination number of 4. This site which can accept electrons is referred to as ‘‘Lewis acid sites” (and, conversely, the N atom of the molecule is termed as a “Lewis base”). By referring to Figure 4, both the HOMO and LUMO levels move to higher energies, and the Fermi level (EF, middle of the Eg) of the tube 2430



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⎛ −Eg ⎞ σ ∝ exp⎜ ⎟ ⎝ 2kT ⎠

B and N (namely, double-antisite defect), the geometric structure of the BC2NNT is distorted, and two unusual B−B and N−N bonds were formed. The calculated bond lengths are 1.62 and 1.46 Å for the B−B and N−N bonds in the defective tube, respectively, being much longer than the corresponding B−N bonds in the pristine tube. Also, the B−CII−B and N− CI−N angles in the d-BC2NNT are 111° and 121° which are different from B−CI−B and N−CII−N angles in the pristine tube (115° and 116°, respectively). Figure 6 displays a side view of stable configurations of the HCN molecule on the defective tubes. It was found that the HCN molecule can be adsorbed with two orientations with respect to the sidewall surface: (A) the nitrogen atom of HCN is nearest to a B impurity of the surface or (B) the hydrogen atom of the molecule is nearest to an N atom of the tube surface. Other initial configurations have reoriented to one of these stable structures during relax optimization. The configuration A gives rise to an Ead of −6.3 kcal/mol (Table 2), which is more negative than the Ead values for the B

where σ is the electric conductivity and k is the Boltzmann constant.37 According to the equation, smaller values of Eg at a given temperature lead to larger electric conductivity. Therefore, the predicted substantial decrement of Eg in d-BC2NNT upon the adsorption process induces a change in the conductivity of the defective tube. According to the obtained results, it can be seen that double B−B antisite-defective BC2NNT can effectively interact with toxic HCN molecule, and their electronic and transport properties are dramatically changed upon exposure to this molecule. Thus, we believe that generating a double-antisite defect may be a good strategy for improving the sensitivity of BC2NNT to HCN, which cannot be trapped and detected by the pristine BC2NNT.

4. CONCLUSION The adsorption of the HCN molecule on the pristine, Aldoped, defective (double-antisite) BC2NNTs was investigated by using DFT calculations. It was found that the HCN molecule is weakly adsorbed on the pristine BC2NNT, and its electronic properties have slightly changed upon the adsorption of the HCN molecule. In contrast, the HCN molecule shows strong interactions with the Al- and d-BC2NNTs. We found that especially the B−B antisite defect on the tube wall can significantly improve the reactivity and sensitivity of the tube to HCN.

Table 2. Adsorption Energy (Ead in kcal/mol), HOMO Energies (EHOMO), LUMO Energies (ELUMO), HOMO− LUMO Energy Gap (Eg), and Fermi Level (EF) of Systems (Figures 5 and 6) in eV Ead

EHOMO

EF

ELUMO

Eg

ΔEg (%)

−6.3

−5.24 −4.63

−4.12 −3.71

−3.01 −2.80

2.23 1.82

−18.0

−0.8

−5.44

−4.28

−3.12

2.32

+4.0

a

configuration d-BC2NNT (A) d-BC2NNT/ HCN (B) d-BC2NNT/ HCN a

(2)

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AUTHOR INFORMATION

Corresponding Author

Change of Eg of the tube after HCN adsorption.

*Phone: (+98) 218288-3495. Fax: (+98) 218288-9730. E-mail: [email protected].

configuration (−0.8 kcal/mol). On comparison, HCN adsorption on a defective tube (in the case of configuration A) energetically is more favorable than pristine BC2NNT. The higher favorability of adsorption of the HCN in d-BC2NNT through configuration A can be explained by the fact that the double-acceptor antisite (B−B bond) can interact with a HCN molecule more favorably than the single-acceptor B atom in a pristine tube. In the most stable configuration (A, Figure 6), the nitrogen atom of the HCN molecule is close to the B atom with bond length of 1.58 Å, confirming strong adsorption of HCN in d-BC2NNT through configuration A. The adsorption induces an apparent local structural deformation on both the HCN molecule and the d-BC2NNT. In this configuration, the length of the B−CII bond (B is adsorbing atom) is increased from 1.51 to 1.61 Å after adsorption. The nature of the nanotube’s DOS near the Fermi level is critical to the understanding of electrical transport through this material. The calculated DOS of d-BC2NNT was shown in Figure 5, indicating that HOMO and LUMO levels changed slightly, and its Eg value is reduced to 2.23 eV compared to pristine BC2NNT. The DOS plot of the HCN/d-BC2NNT in configuration A shows a considerable change, indicating that the electronic properties of the d-BC2NNT are sensitive to the toxic HCN adsorption. The conduction and valence levels shift upward in configuration A compared to nonadsorbed dBC2NNT. As can be seen in Figure 6A, the Eg value of the defective tube is obviously decreased from 2.23 to 1.82 eV (by about 18% change) in adsorbed form which would result in an electrical conductivity change of the defective tube according to the following equation

Notes

The authors declare no competing financial interest.

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REFERENCES

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Effects of Al Doping and Double-Antisite Defect on the Adsorption of

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