Article pubs.acs.org/JPCC
DFT Studies on the Interaction of an Open-Ended Single-Walled Aluminum Nitride Nanotube (AlNNT) with Gas Molecules Wen-guang Liu,† Guang-hui Chen,*,† Xiao-chun Huang,† Di Wu,§ and Yun-peng Yu‡ †
Department of Chemistry and ‡Department of Physics, Shantou University, Guangdong 515063, People’s Republic of China § State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *
ABSTRACT: The interaction of various gas molecules including H2, N2, O2 (in triplet and singlet), and H2O on the geometrical structures, energies, and electronic structures with open edges of zigzag (10, 0) and armchair (6, 6) aluminum nitride nanotubes (AlNNTs) has been systematically studied using density functional theory. It is shown that the σ-type species including H2 and H2O will dissociate when chemisorbed on open AlNNTs, whereas the π-type species including N2 and O2 prefer to form cyclic structures on open AlNNTs; for example, a [2 + 2] cycloaddition configuration is formed when singlet O2 is adsorbed on the Al−N at the armchair edge. The edge of the open Nrich-ended (10, 0) AlNNT generally shows higher reactivity toward gas molecules than that of the open Al-rich-ended (10, 0), open-ended (6, 6) except for O2 (in triplet and singlet) on the Al-rich-ended (10, 0) AlNNT. However, unlike the largest adsorption energy from O2 on B-rich-ended (8, 0) BNNT, that of AlNNT comes from the H2 on the N-rich-ended (10, 0) AlNNT with the Ead of −12.17 eV. Moreover, we note that the adsorbates at the edges of the open AlNNT can modify their electronic properties in various ways. A considerable amount of charge transferred through the adsorption of gas molecules on the open-ended AlNNTs may account for the changes of the electronic properties. Interestingly, the adsorption of open-ended (10, 0) AlNNT will narrow band-gaps, for example, N2 and triplet O2 adsorbed on the Al-rich-ended (10, 0) AlNNT decrease gaps from 1.58 to 0.62 and 0.83 eV, respectively, whereas both triplet and singlet O2 adsorbed on the N-rich-ended (10, 0) AlNNT decrease gaps from 2.71 to 0.30 and 0.31 eV, respectively, which should result from the influence of introduced energy-levels of N2 and O2 on the energy bands of the AlNNT. Accompanied by the change of HOMO−LUMO gaps, the adsorption of H2, N2, and H2O on the N-rich-ended (10, 0) AlNNT lift the Fermi level toward conduction band largely at 15.9, 14.7, and 15.9%, respectively, thus significantly decreasing the work function. The present results may be useful for the design of AlNNT-based nanomaterials devices such as chemical sensor and field emitter.
1. INTRODUCTION Since the work of Iijima in 1991,1 carbon nanotubes have opened up new fields in science and technology and have been shown greatly useful in nanoscale materials due to their unique electronic,2,3 optical,4,5 and mechanical6,7 properties. Significant efforts have been taken to study nanoscale materials both experimentally and theoretically. Among these materials, 1-D III−V semiconductor nanostructures of boron nitride nanotubes (BNNTs) have attracted much attention due to their direct band gaps. As an important member of semiconductor, BNNTs have a wide band gap of ∼5.5 eV independently of the tube diameter, helicity, and wall number,8 have been synthesized by several routes such as arc-discharge,9 metalboride-catalyzed chemical vapor deposition,10 continuous laser heating at super high,11 or ambient pressure,12,13 and so on. Recently, experimental and theoretical studies also focus on the tips of BNNTs; for example, Celik-Aktas et al. reported the use of high-energy electron irradiation of multiwalled BNNT by controlling the electron beam size.14 Theoretically, Hao et al. reported that the intrinsic magnetism of BNNT can be induced by their open ends,15 whereas Ding et al. evaluated the geometrical, electronic structure, and effects by adsorption of gas molecules, including H2, N2, O2, and H2O.16,17 It was © 2012 American Chemical Society
shown that H2, O2, and H2O dissociate and chemisorb on BNNT edges, and the adsorption of these molecules induces a charge transfer and thus results in changes of the electronic properties.16,17 Similar to BNNT, another 1-D III−V semiconductor nanostructure, aluminum nitride nanotube (AlNNT), has been an important research target. It is well known for its unique properties such as direct wide band gap (∼6.2 eV), high thermal conductivity, superior mechanical strength, high piezoelectric response, small or even negative electron affinity, and so on. So far, many research efforts have contributed to the preparation of various AlN nanostructures such as nanowires,18 nanotubes,19 nanobelts,20 and nanocones21 with various methods. Although AlNNT has not yet been synthesized, Zhao et al. have testified the strain energy and stability of single-walled AlNNT using DFT calculations.22 Recently, AlNNTs have received many theoretical studies with regard to stability,22−24 defect properties,25 and functionalization of the AlNNT’s wall;26,27 for example, Lim et al. investigated the Received: August 12, 2011 Revised: February 2, 2012 Published: February 6, 2012 4957
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chemisorption of H2 on AlNNTs,26 and Jiao et al. reported the interactions of CO2 with the AlNNTs using DFT calculations.27 However, all of these studies just paid attention to the functionalization of the wall of the ANNTs. To the best of our knowledge, no study on the tip functionalization and the interaction of open-ended AlNNTs with adsorbates was reported, although it is of significance to understand deeply the properties of the open-ended AlNNTs and even to design AlNNT-based nanoscale devices. Therefore, in this Article, we will evaluate the geometrical structures, adsorption energies, and electronic structures of gas molecules including σ-species of H2 and H2O as well as πspecies of N2 and O2 adsorbed on the edges of open-ended zigzag (10, 0) and armchair (6, 6) AlNNTs using DFT calculations. Note that both the adsorption of ground triplet O2(X3Σg−) and excited singlet O2(a1Δg) with electron configurations as (1σg)2(1σu)2(2σg)2(2σu)2(3σg)2(1πu)4(1πg)2 and (1σg)2(1σu)2(2σg)2(2σu)2(3σg)2(1πu)4(3σu)2 are considered, respectively.
Figure 1. Optimized geometrical structures of pristine open-ended AlNNTs: (a) Al-rich-ended, (b) N-rich-ended (10, 0) AlNNT, and (c) (6, 6) AlNNT. The bond lengths are in angstroms. Pink balls represent aluminum atoms, blue balls represent nitrogen atoms, and white balls represent hydrogen atoms.
3. RESULTS AND DISCUSSION For conciseness, we mainly organize the discussion as follows: in Section 3.1, the structures of the optimized pristine openended (10, 0) and (6, 6) AlNNTs are given; in Section 3.2, the optimized structures and calculated Ead of adsorbates on openended AlNNTs are discussed; in Section 3.3, the effects of adsorbates on the electronic structures of open-ended AlNNTs are analyzed. Where possible, the comparisons with the analogous BNNTs16 are made. 3.1. Optimized Geometries of Pristine Open-Ended (10, 0) and (6, 6) AlNNTs. In principle, both the Al-richended and N-rich-ended (10, 0) AlNNTs are generated from an H-saturated tube through the respective breaking of the Al− H and N−H bonds. The initial structures have unpaired electrons at the terminal Al and N atoms with the shortest Al− Al and N−N distances of 3.18 and 3.25 Å for Al-rich-ended and N-rich-ended (10, 0) AlNNTs, respectively. The eventual optimization of the open Al-rich-ended (10, 0) AlNNT leads to a closing mouth just similar to open B-rich-ended BNNT,16 with the mouth diameter contracting from 10.02 to 9.06 Å due to the effective coupling of the neighbored Al atoms, as shown in Figure 1a. The dimerized Al−Al bonding of Al-rich-ended (10, 0) AlNNT is very weak with the length of 2.46 Å. As a result, five Al2N three-membered rings are formed at the open edge. In contrast with Al-rich-ended (10, 0) AlNNT, the open N-rich-ended one has a broader mouth with the diameter of 10.67 Å, which may result from the strong repulsion induced by the lone pairs of the neighbored N atoms, as shown in Figure 1b. This is also similar to the case of analogous open N-richended (8, 0) BNNT.16 Breaking of N−H and Al−H bonds of one terminal of Hsaturated (6, 6) AlNNT generates an optimized capped structure with the open mouth contracting from 10.51 to 6.94 Å of the diameter, as shown in Figure 1c. This is different from the analogous armchair BNNT16 with an uncapped mouth, whereas it is just consistent with the prediction by Hou et al.; that is, the diameter less than (8, 8) AlNNT occurs a selfclosure behavior.34 The capped structure might be caused by the spontaneous interaction of Al−N pairs at the open edge so that the dangling bonds of all atoms are saturated. At the same time, a hexagon and quadrangle alternating network is formed at the armchair edge with two types of Al−N pairs, namely, Al− Nh and Al−Nq, located at hexagon and quadrangle at the tips of the AlNNT, respectively, as shown in Figure 1c. It should be noted that the Al-rich-ended (10, 0) AlNNT is intrinsic antimagnetic with the singlet ground state due to the fact that the weak Al2 dimerization effectively “saturates” the original dangling bonds of the neighbored Al atoms with spin
2. THEORETICAL AND COMPUTATIONAL DETAILS The spin-polarized DFT calculations on gas molecules adsorbed on open-ended AlNNTs are performed by using DMol3 code28 to derive equilibrium geometries, total energies, and electronic structures. Perdew−Wang generalized-gradient approximation (PW91)29 is adopted to treat the electron exchange-correlation energy of interaction. All-electron calculations are employed with the double numerical plus polarization (DNP)28a basis set, which is comparable to the valence double-ζ polarized basis set 6-31G(d).30 The convergence criteria for energy change between optimization cycles are chosen to be 10−5 Ha, 0.001 Ha·Å−1 for force field, 0.005 Å for displacement, and 4.8 Å for global orbital cutoff. To get the charge and bond order at the tips of the nanotubes accurately, we have performed the natural bond orbital (NBO) calculations as implemented in Gaussian 0931 using the exchange-correlation function of PW91PW91, which is the same as PW91 of DMol3 with the basis set of 6-31G(d).30 For the consideration of computational cost, the geometrical optimization is performed with the DMol3 code. Transitionstate geometries are searched by using the complete LST (linear synchronous transit)/QST (quadratic synchronous transit) method,32 which is confirmed further by minimumenergy pathway (MEP) calculations using the nudged elastic band (NEB) method.33 Two models of finite-sized AlNNTs with open edges, that is, zigzag (10, 0) Al60N60H10 and armchair (6, 6) Al60N60H12, are selected in this work, where only one terminal is saturated by H atom to model a long AlNNT. There are two types of terminal atoms at the open-ended (10, 0) AlNNT, that is, Al and N atoms. Therefore, two forms of (10, 0) AlNNTs with open edge including Al-rich-ended and N-rich-ended (10, 0) AlNNTs together with open-ended (6, 6) AlNNT are considered, as shown in Figure 1a−c, respectively. The adsorption energy (Ead) of adsorbate is defined as Ead = E(molecule + tube) − E tube − Emolecule
where E(molecule+tube) is the total energy of the system after gas adsorption, Etube is the energy of pristine AlNNT, and Emolecule is the total energy of adsorbate. The positive or negative value of Ead represents endothermic or exothermic adsorption, respectively. 4958
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antiparallel electrons. However, the N-rich-ended (10, 0) AlNNT has a magnetic moment of 10 μB, due to the fact that the spin-parallel configuration with unpaired electron in terminal N atom is energetically favorable. The (6, 6) AlNNT is antimagnetic, which may be attributed to the large overlap between the electron clouds of the Al and N atoms, leading to the antiparallel spin configuration of the neighbored electrons. Therefore, the magnetism of AlNNTs is similar to the study of BNNTs by Hao et al.15 in that the magnetic moment is sensitive to the chirality. 3.2. Optimized Geometries and Calculated Ead of Adsorbates at the Edges of AlNNTs. To make an explicit discussion, the optimized geometries and calculated Ead of adsorbates including H2, H2O, O2 (in triplet and singlet), and N2 at the edge of (10, 0) AlNNT are discussed in Section 3.2.1, whereas those at the edge of (6, 6) AlNNT are analyzed in Section 3.2.2 3.2.1. Optimized Geometries and Calculated Ead of Adsorbates at the Edges of (10, 0) AlNNTs. After optimization at the GGA-PW91 level, the most stable configurations of the gas molecules including H2, H2O, O2 (in triplet and singlet), and N2 adsorbed at the edges of the open Al-rich-ended and Nrich-ended (10, 0) AlNNTs are shown in Figures 2 and 3,
Figure 3. Fully optimized geometrical structures of various adsorbates at the edge of the N-rich-ended (10, 0) AlNNT, including (a) H2, (b) H2O, (c) triplet O2, (d) singlet O2, and (e) N2. The bond lengths are in angstroms.
forms for triplet and singlet O2 on the Al-rich-ended (10, 0) AlNNTs. Initially, when triplet and singlet O2 is adsorbed on the open Al-rich-ended (10, 0) AlNNT, a structure with a fivemembered ring at the terminal with cleavage of Al2 dimer is obtained. At the same time, the bond lengths of O2 are elongated from 1.21 to 1.52 Å for both triplet and singlet O2 with the Ead of −4.11 and −5.80 eV, as shown in Figure 2c,d, respectively. However, the further both O−O bonds (1.52 Å) elongate, the more stable configurations with two fourmembered rings are observed with the separated O−O distance at 4.40 and 4.36 Å with larger Ead of −9.23 and −11.40 eV, as shown in Figure 2e,f and Table 1, respectively. Note that there exists each transition state (TS) between structures of Figure 2c and 2e (in triplet), as well as Figure 2d and 2f (in singlet), respectively, which are energetically 0.36 and 0.37 eV higher than 2c and 2d structures, respectively, implying that 2e and 2f structures can exist as both thermodynamically and kinetically stable ones. The Cartesian coordinates of the TSs can be found in the Supporting Information. For the adsorption of O2 (in triplet and singlet) on the N-rich-ended (10, 0) AlNNT, it is found that O−O dissociates accompanied by the formation of two strong N−O bonds of 1.29 Å with Ead at −3.03 eV for triplet one, and 1.28 Å with Ead at −5.78 eV for singlet one, as shown in Figure 3c, 3d and Table 1, respectively; (iv) different from the adsorption of H2, O2 and H2O, no dissociation takes place for N2. A five-membered ring is formed when N2 adsorbed at the edges of Al-rich-ended and N-rich-ended (10, 0) AlNNT as shown in Figure 2g and 3e, respectively. The bond length of N−N is elongated to 1.24 and 1.28 Å from initial 1.10 Å with Ead at −0.24 eV and −7.81 eV, respectively. Interestingly, for the adsorption of H2 and H2O as σ-type species, they will dissociate directly when interacting with the open-ended (10, 0) AlNNT. Yet, the situation is different for
Figure 2. Fully optimized geometrical structures of various adsorbates at the edge of the Al-rich-ended (10, 0) AlNNT, including (a) H2, (b) H2O, (c) triplet O2, (d) singlet O2, (e) triplet O2 (dissociated), (f) singlet O2 (dissociated), and (g) N2. The bond lengths are in angstroms.
which are described in the following parts (i)−(iv), respectively. (i) The H2 molecule dissociates to 3.36 and 3.25 Å on Al−Al and N−N bond at the terminal of Al-rich-ended and N-rich-ended (10, 0) AlNNT, as shown in Figures 2a and 3a, respectively. When H2 is adsorbed on the Al-rich-ended (10, 0) AlNNT, one Al2N three-membered ring is broken with a slight elongation of the Al−Al from 2.46 to 3.14 Å, whereas the other four rings still keep with the exothermicity of −2.18 eV, as listed in Table 1. At the same time, the distance of the adjacent N−N at the terminal of the N-rich-ended (10, 0) AlNNT decreases from 3.30 to 3.22 Å with a very large exothermicity of −12.17 eV. (ii) Similar to the separation of H2, H2O dissociates into H atom and OH group with the Ead of −3.31 and −9.66 eV on the terminals of the Al-rich-ended and N-rich-ended (10, 0) AlNNT, respectively, as shown in Figures 2b and 3b. (iii) Note that there are two types of chemisorption 4959
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Table 1. Structural Parameters and Adsorption Energies (Ead) of Adsorbates at the Edges of Open-Ended (10, 0) and (6, 6) AlNNTs (Distance in angstroms; Ead in electronvolts) H2 (10, 0) Al-rich-ended (10, 0) N-rich-ended (6, 6) top-Al (6, 6) top-N (6, 6) Al−Nh (6, 6) Al−Nq
H2O
O2
N2
dH−H
Ead
dO−H1
dO−H2
Ead
dO−Ob
Eadb
dO−Oc
Eadc
dN−N
Ead
3.36 3.25 0.76 0.75 2.74 2.13
−1.09a −6.09a −0.15 −0.10 −0.42a −0.28a
0.97 0.99 0.98
3.36 3.24 0.98
−3.31 −9.66 −1.00
4.40 3.32 1.27
−4.62a −1.52a 0.01a
4.36 3.32
−5.70a −2.89a
1.24 1.28 1.13 1.17
−0.24 −7.81 0.36 2.55
3.58
−0.32a
3.66 1.49
−1.47a −0.87a
0.97
2.17
−1.66
a
Ead for dissociated O2 (in triplet and singlet) and H2 are given by isolated atom. bParameters structural and Ead for the adsorption of triplet O2. c Parameters structural and Ead for the adsorption of singlet O2.
the adsorption of π-type species of N2 and O2, that is, N2 will not dissociate, and O2 (both triplet and singlet) have to pass barriers to separate on the Al-rich-ended (10, 0) AlNNT. 3.2.2. Optimized Geometries and Calculated Ead of Adsorbates at the Edge of (6, 6) AlNNT. There are various adsorption sites for the gas molecules at the (6, 6) AlNNT edge, and we mainly focus on the adsorption at the terminal atoms (including Al and N atoms) as well as bridge sites (including Al−Nh and Al−Nq). From Figure 4, it is shown that all of the molecules can be adsorbed at the terminal atoms of (6, 6) AlNNT except for
H at 2.35 Å and N−H at 2.85 Å, as shown in Figure 4a and 4b, respectively. The H−H bond is slightly elongated to 0.76 and 0.75 Å from 0.74 Å with small Ead of −0.15 eV and −0.10 eV, respectively; (ii) similar to the adsorption of H2, when H2O is adsorbed on the terminal Al atom, the O−H bond is elongated from 0.96 to 0.98 Å with the Ead of −1.00 eV as shown in Figure 4c and Table 1, respectively. While stable configuration of the adsorption of H2O on the terminal N atom cannot be found; (iii) the triplet O2 on the terminal Al atom is shown in Figure 4d, where a weak Al−O bond of 2.05 Å has been formed, and the O−O is slightly elongated to 1.27 Å with a small Ead of 0.01 eV, indicating a physisorption. After numerous attempts, we can not find any stable configuration of triplet O2 adsorption on the terminal N atom; (iv) the configurations of N2 on the terminal Al and N atoms are shown in Figure 4e and 4f, respectively. It is found that the N−N bonds are slightly elongated from 1.10 to 1.13 and 1.17 Å with the Ead of 0.36 and 2.55 eV, respectively, indicating endothermic reactions. It is found that all of the molecules can be adsorbed at the bridge sites including Al−Nh and Al−Nq except for N2, which is shown in Figure 5 and discussed in the following parts (i)-(iii),
Figure 4. Fully optimized geometrical structures of open-ended (6, 6) AlNNT with various adsorbates, including (a,b) H2 on terminal Al and N atoms, (c) H2O, and (d) triplet O2 on terminal Al atom as well as (e,f) N2 on terminal Al and N atoms, respectively. The bond lengths are in angstroms.
Figure 5. Fully optimized geometrical structures of open-ended (6, 6) AlNNT with various adsorbates including (a,b) H2 on Al−Nh and Al− Nq bond, (c) H2O on Al−Nq bond, (d) triplet O2, (e) singlet O2, (f) singlet O2 (dissociated) on Al−Nh bond, as well as (g) singlet O2 on Al−Nq bond, respectively. The bond lengths are in angstroms.
singlet O2, which is discussed in the following parts (i)-(iii), respectively. (i) There exists only the physisorption form for H2 on the top of terminal Al and N atom with the distances of Al−
respectively. (i) H 2 will undergo the adsorption and dissociation at the top of Al−Nh or Al−Nq with the H2 of 0.74 Å separated into 2.74 and 2.13 Å, respectively, as shown in 4960
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Table 2. Calculated Wiberg Bond Indexes of Various Gas Molecules Adsorbed at the Edges of Open-Ended (10, 0) and (6, 6) AlNNTs at the PW91PW91/6-31G(d) Calculated Level with Gaussian 09 Program Package, which is Equivalent to PW91/DNP Calculated Level with DMol3 Code Wiberg bond index O2 H2
species
H2O
adsorbate (10, 0) Al-rich-ended
H−H Al−H
1.00 0.80
(10, 0) N-rich-ended
N−H
0.80
(6, 6) top-Al (6, 6) top-N (6, 6) Al−Nh
Al−H1 N−H Al−H1 N−H2
0.05 0.01 0.80 0.78
(6, 6) Al−Nq
Al−H1 N−H2
0.77 0.78
H−OH Al−OH Al−H N−OH N−H Al−OH
Al−OH N−H
triplet 0.79 0.64 0.80 1.64 0.79 0.23
0.57 0.75
N2
singlet
O−O Al−O
0.75 0.15
O−O Al−O
2.02 0.61
N−N Al−N
3.02 0.56
N−O
0.26
N−O
1.70
N−N
1.15
Al−O
0.06
Al−N1 N−N2
0.29 0.59
Al−O1 Al−O2 N−O2
0.17 0.11 0.39
Al−O1 N−O1
0.58 0.98
Al−O1 N−O2
0.52 0.10
It is of interest to explore the effect of the nanotube’s size on the adsorption of gas molecules. Taking H2O as an example, we checked its adsorption on the open-ended (9, 0) and (11, 0) AlNNTs, and it is shown that the configurations and Ead are close to those of on the (10, 0) AlNNTs. In a word, the adsorption of gas molecules on open-ended AlNNT is sizeindependent just like that of open-ended BNNT,16 which is as expected because the tube edge with dangling bonds has a high reactivity toward guest adsorbates. From the structural parameters, Wiberg bond index that is approximately the same value as the traditional bond order and Ead summarized in Tables 1 and 2, it is shown that the N-richended (10, 0) AlNNT has generally larger reactivity toward adsorbates than that of the other two types of the open-ended AlNNTs, except for O2 (in triplet and singlet) on the Al-richended (10, 0) AlNNT. Moreover, the Ead of the singlet O2 is generally larger than that of triplet O2, just like the theoretical results of the interaction between O2 and CNT by Zhang35 and Hu et al.36 Understandably, the Ead depends on not only the bond dissociation energy but also the binding energy of various adsorbates on the open AlNNTs. For example, the O2 molecule has a moderate dissociation energy (119.1 kcal/mol), whereas the binding energy of 114.96 kcal/mol for doubly bonded N O is slightly lower than that of 122.37 kcal/mol for the singly bonded Al−O.37 So, the Ead values (−3.03 eV for triplet, −5.78 eV for singlet) of O2 on the N-rich-ended (10, 0) AlNNT are less than that of (−9.23 eV for triplet, −11.40 eV for singlet) Al-rich-ended (10, 0) AlNNT, respectively. Moreover, the Ead of all adsorbates on the N-rich-ended (10, 0) AlNNT are generally larger than those on the N-rich-ended (8, 0) BNNT,16 which may due to the fact that the Wiberg bond order of B−N is larger than that of Al−N, and thus the N atom of Al−N bond possesses larger reactivity than that of B−N bond, as listed in Table 3. However, the Ead for the Al-richended (10, 0) and open-ended (6, 6) AlNNTs are generally less than those of the boron-analogs, that is, B-rich-ended (8, 0) and open-ended (5, 5) BNNTs,16 respectively, which should be attributed to the fact that the binding energies for the singly bonded Al−O (122.37 kcal/mol), Al−N (70.98 kcal/mol), and Al−H (68.12 kcal/mol) are less than that of the singly bonded boron-analogs, that is, B−O (192.63 kcal/mol), B−N (92.97 kcal/mol), and B−H (78.87 kcal/mol),37 respectively.
Figure 5a and 5b. The hexagon at the terminal has been changed to a quadrangle when H2 adsorbed on the Al−Nh which results from the breaking of the interaction of the saturated dangling bond between the neighbored Al−N pairs with the Ead of −0.83 eV, as shown in Figure 5a and Table 1. When H2 is adsorbed at the top of Al−Nq, the corresponding quadrangle is broken with an elongated Al−N bond at 3.25 Å with the Ead of −0.55 eV, as shown in Figure 5b and Table 1, respectively; (ii) similar to the adsorption on the open-ended (10, 0) AlNNTs, H2O dissociates into the H atom and OH group at the top of Al−Nq with the slightly elongated Al−N bond at 2.17 Å with the Ead of −1.66 eV as shown in Figure 5c; (iii) similar to H2, the adsorbed triplet O2 will dissociate from 1.21 Å to 3.58 Å with the Ead of −0.63 eV at the top of Al−Nh, as shown in Figure 5d and Table 1. By contrast, for the adsorption of the singlet O2 on Al−Nh, an interesting [2 + 2] cycloaddition configuration is initially formed with O−O at 1.49 Å, as shown in Figure 5e. Subsequently, the O−O bond undergoes further dissociation to reach a more stable configuration of the O−O at 3.66 Å with the exothermicity of −2.94 eV, as shown in Figure 5f and Table 1. There also exists a TS between the structures in Figure 5e,f, which is energetically 0.38 eV above the 5e structure. Therefore, the 5f structure with dissociated O−O is thermodynamically and kinetically stable. The Cartesian coordinates of the TS can be found in the Supporting Information. For the adsorption of singlet O2 on Al−Nq, similar to the adsorption on Al−Nh, a [2 + 2] cycloaddition structure is also observed with the O−O elongated to 1.49 Å, as shown in Figure 5g. After numerous attempts, we can not find any stable configuration for H2O on Al−Nh and triplet O2 on Al−Nq. It is clear that the Ead of H2 and H2O at the bridge sites of (6, 6) AlNNT are generally much larger than that of at the terminal atoms, which should be attributed to the formation of two bonds at the bridge sites, whereas only one bond is formed at the terminal atoms. Additionally, similar to the adsorption on the open-ended (10, 0) AlNNT, the σ-type species, such as H2 and H2O on Al−N bonding of the open-ended (6, 6) AlNNT, will also dissociate. By contrast, the situation is different for the π-type species including N2 and O2; for example, N2 will not dissociate, and singlet O2 prefers to form cycloaddition configuration on Al−N bonding of the open-ended (6, 6) AlNNT. The reason may be that the breaking of the σ-type species is energetically favorable. 4961
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electrons on the terminal N and bridge sites of Al−N bonds. The calculated results indicate that the charge transfer is related to the atomic configuration at the mouth of the open-ended AlNNT and different adsorbates. In general, the charge transfer may modify the HOMO− LUMO gaps, Fermi levels, and density of states. As shown in Table 5, the band gaps of the open-ended (10, 0) AlNNT generally decrease except for singlet O2 adsorbed on the Alrich-ended (10, 0) AlNNT. For example, when N2 and triplet O2 are adsorbed on the Al-rich-ended (10, 0) AlNNT, two narrow gaps of 0.62 and 0.83 eV are observed, which are decreased from 1.58 eV, respectively, whereas the adsorption of O2 (in triplet and singlet) on the N-rich-ended (10, 0) AlNNT even narrow gaps from 2.71 to 0.30 and 0.31 eV, respectively. Upon the (6, 6) AlNNT, the adsorption also generally decreases the band gap except for H2, which may originate from the changes of HOMO and LUMO positions of these systems. As shown in Figure 6, the HOMO and LUMO
Table 3. Calculated Wiberg Bond Indexes and Natural Bond Orbital (NBO) Atomic Charges of the N-Rich-Ended (10, 0) AlNNT and N-Rich-Ended (8, 0) BNNT at the PW91PW91/ 6-31G(d) Calculated Level with Gaussian 09 Program Package, which is Equivalent to PW91/DNP Calculated Level of DMol3 Code species
Wiberg bond index
AlNNTa
Al1−N2 0.72 B1−N2 1.09
BNNTb
NBO atomic charge (e) Al1 1.77 B1 0.91
N2 −2.08 N2 −0.73
a
Sites of the Al and N atoms of (10, 0) AlNNT are shown in Figure 1b. bSites of B and N atoms of the N-rich-ended (8, 0) BNNT.16
3.3. Effects of Adsorption on the Electronic Structures of the Open-Ended AlNNT. The chemical functionalization of adsorbates on the open-ended AlNNTs might lead to charge transfer. The calculated charge transfer between adsorbates and nanotubes by using Hirshfeld method38 is listed in Table 4, Table 4. Charge Transfer between Various Adsorbates and Open-Ended (10, 0) or (6, 6) AlNNTs charge transfer (e)a (10, 0) tube
(6, 6) tube
adsorbates
Al-rich
N-rich
top-Al
top-N
Al−Nh
Al−Nq
H2 H2O triplet O2 singlet O2 N2
−0.30 −0.36 −1.48 −1.48 −0.35
0.19 0.25 −0.40 −0.38 −0.09
0.06 0.24 −0.18
−0.02
−0.08
−0.07 −0.20
0.13
−0.18
−0.89 −0.58
Figure 6. Side views of HOMO and LUMO for (a) open-ended (6, 6) AlNNT, (b) N2 adsorbed on the terminal Al and (c) the terminal N at the edge of (6, 6) AlNNT.
−0.58
densities of the open-ended (6, 6) AlNNT are mainly positioned at the tips, as shown in Figure 6a, whereas those of the N2 at the terminal Al (Figure 6b) and N atoms of the (6, 6) AlNNT (Figure 6c) are mainly localized within the Al−N or N−N bonds. Accompanied by the modification of HOMO−LUMO gaps, the positions of Fermi levels of the adsorption configurations are also changed, as listed in Table 5. For both Al and N-richended (10, 0) AlNNT, all the adsorbates induce the Fermi level of the nanotube lift toward conduction band (decreasing the work function) to different degrees except for the adsorption of O2 and H2O on the Al-rich-ended (10, 0) AlNNT. For example, a large lift of Fermi level toward conduction band is observed with 15.9, 14.7, and 15.9% for H2, N2, and H2O adsorbed on the N-rich-ended (10, 0) AlNNT, respectively. In addition, the adsorbates on the (6, 6) AlNNT edge also induce the Fermi level lift toward conduction bands to different
a
Positive value indicates charge transferred from the adsorbate to the open AlNNT.
which is based on the deformation density and is more stable than Mulliken charge analysis with respect to the basis set. It is shown that the charge transfer takes place from the Al-richended (10, 0) AlNNT to all adsorbates including H2, N2, O2 (in triplet and singlet), and H2O with 0.30, 0.35, 1.48, 1.48, and 0.36 e, respectively, due to the fact that the terminal Al owns smaller electronegativity than H, N, and O. Upon the N-richended (10, 0) AlNNT, it is found that the charge transfer takes place from adsorbates including H2 and H2O to nanotube with 0.19 and 0.25 e, respectively. On the contrary, the charge transfer takes place from nanotube to adsorbates including N2 and O2 (both triplet and singlet) with 0.09, 0.40, and 0.38 e, respectively. As to (6, 6) AlNNT, H2, N2, and H2O will lose electrons on the terminal Al atom, whereas they will gain
Table 5. Calculated HOMO-LUMO Gaps and Position of Fermi levels for Adsorbates at the Edges of Open-Ended (10, 0) and (6, 6) AlNNTs HOMO−LUMO gaps (eV) (10, 0) tube
position of Fermi (eV)
(6, 6) tube
species
Al-rich
N-rich
top-Al
AlNNT H2 H2O triplet O2 singlet O2 N2
1.58 1.58 1.56 0.83 1.62 0.62
2.71 2.18 2.16 0.30 0.31 2.13
3.03 3.04 2.95 2.35 1.99
(10, 0) tube
top-N
Al−Nh
Al−Nq
3.03
3.05
3.06 2.73
1.64 2.17
2.39
2.13 4962
(6, 6) tube
Al-rich
N-rich
top-Al
−3.66 −3.65 −3.67 −3.37 −3.68 −3.34
−5.41 −4.55 −4.55 −5.32 −5.18 −4.61
−4.07 −3.95 −3.87 −4.08 −4.40
top-N
Al−Nh
Al−Nq
−3.98
−3.98
−3.97 −3.62
−2.92 −3.79
−3.84
−3.64
dx.doi.org/10.1021/jp2124857 | J. Phys. Chem. C 2012, 116, 4957−4964
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Figure 7. Density of states (DOS) of (a) pristine Al-rich-ended (10, 0) AlNNT, (b) triplet O2 and (c) N2 adsorbed on the Al-rich-ended (10, 0) AlNNT, (d) pristine N-rich-ended (10, 0) AlNNT, (e) triplet O2, and (f) singlet O2 adsorbed on the N-rich-ended (10, 0) AlNNT. The spin-up and spin-down electronic DOS are distinguished with “+” and “−”. The Fermi level is set to zero, which is indicated by black dotted line, whereas the local DOSs of adsorbates are plotted with red lines.
Therefore, the adsorption of gas molecules can modify HOMO−LUMO gaps, Fermi levels, and density of states of the nanotube in various ways. The increase or decrease in band gap may extend the application of the open-ended AlNNT; for example, the existence of small band-gaps suggests that AlNNTs can be converted into narrow band gap semiconductor material. In addition, the lift toward conduction band of the Fermi levels will efficiently decrease the work function of the nanotube, which might be useful to modify the field emission property of AlNNT.
degrees except for N2 on the terminal Al atom with a downshift of 8% toward valence band, as listed in Table 5. Therefore, the change of Fermi level strongly depends on the types of nanotube and adsorbates. From Table 5, it is shown that the adsorption of triplet O2 and N2 on Al-rich-ended (10, 0) AlNNT as well as O2 (in triplet and singlet) on the N-rich-ended (10, 0) AlNNT modify DOS and local DOS (LDOS) largely, as plotted in Figure 7. From the comparisons of those before and after adsorption of triplet O2 and N2 on the Al-rich-ended (10, 0) AlNNT, as shown in Figure 7a−c, it is found that the DOS shifts to lower energy region after adsorption. Nevertheless, new states around Fermi level are observed for the N2 adsorption, as shown in Figure 7c, which should be induced by the hybridization of nitrogen and nanotube. For pristine N-rich-ended (10, 0) AlNNT, it is found that the occupied spin-up states are more than that of spin-down, indicating the intrinsic magnetism of the N-rich-ended (10, 0) AlNNT, as shown in Figure 7d. When adsorbed with triplet O2, it is found that emerged new states are around the Fermi level, which is mainly contributed by triplet oxygen. Besides, from the plotted LDOS in Figure 7e, we find that the occupied spin-down states of triplet oxygen are more than that of spin-up, which is contrary to the pristine N-richended (10, 0) AlNNT and may thus cause the decrease in the magnetic moment of nanotube. After the adsorption of the singlet O2, as shown in Figure 7f, we can find that there exists impurity energy levels near the Fermi level accompanied by the dramatically decreased magnetic moment of the N-rich-ended (10, 0) AlNNT.
4. CONCLUSIONS The interaction of various gas molecules including H2, N2, O2 (in triplet and singlet), and H2O on the geometrical structures, energies, and electronic structures with open edges of (10, 0) and (6, 6) AlNNTs have been studied using density functional theory. It is shown that the σ-type species including H2 and H2O will dissociate when chemisorbed on the open AlNNTs, whereas the π-type species including N2 and O2 prefer to form cyclic structures on the open AlNNTs; for example, a [2 + 2] cycloaddition configuration is formed when singlet O2 is adsorbed on the Al−N at the armchair edge. The edge of the open N-rich-ended (10, 0) AlNNT shows generally higher reactivity toward small molecules than the open Al-rich-ended (10, 0) and (6, 6) AlNNTs except for O2 (in triplet and singlet) on the Al-rich-ended (10, 0) AlNNT. However, different from the largest Ead associated with the O2-adsorption on the B-richended (8, 0) BNNT, that of AlNNT comes from the H2adsorption on the N-rich-ended (10, 0) AlNNT. Moreover, the adsorbates induce charge transfer between adsorbates and 4963
dx.doi.org/10.1021/jp2124857 | J. Phys. Chem. C 2012, 116, 4957−4964
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nanotube and thus significantly modify the electronic structures. The adsorbates mainly narrow band gaps, with the exception of singlet O2 on the Al-rich-ended (10, 0) and H2 on (6, 6) AlNNTs, which should result from the influence of introduced energy-levels of N2 and triplet O2 on the energy bands of the AlNNT, as shown in the plotted density of states (DOS) and local DOS. Accompanied by the changes of HOMO−LUMO gaps, the adsorption of H2, N2, and H2O on the N-rich-ended (10, 0) AlNNT lifts the Fermi level 15.9, 14.7, and 15.9% toward conduction band, which may significantly decrease the work function and modify the field emission property. Although AlNNT has not yet been synthesized, Zhao et al.22 have testified the strain energy and stability of single-walled AlNNT using DFT calculations. The calculated results might not only be useful for us to understand deeply the gas adsorption property of open-ended AlNNTs but also can help us to develop AlNNT-based nanomaterial devices such as field emitters and chemical sensor.
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ASSOCIATED CONTENT
* Supporting Information S
Cartesian coordinates of the TSs of O2 (in triplet and singlet) adsorbed on the edge of an Al-rich-ended (10, 0) AlNNT as well as that of singlet O2 on open-ended (6, 6) AlNNT. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by project 211 foundation of Guangdong Province and project of academic innovation group of Shantou University. The financial support from the Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, South China Normal University, is also acknowledged. In addition, we owe gratitude to Miss Min-min Ma for her help in polishing the English.
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