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Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors
Carbon 45 (2007) 2105–2110 www.elsevier.com/locate/carbon
Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors Lu Bai a, Zhen Zhou b
b,*
a Department of Chemistry, Nankai University, Tianjin 300071, PR China Institute of New Energy Material Chemistry, Institute of Scientific Computing, Nankai University, Tianjin 300071, PR China
Received 21 March 2007; accepted 17 May 2007 Available online 2 June 2007
Abstract The adsorption of NH3 and NO2 in B- or N-doped (10, 0) single-walled carbon nanotubes (SWCNTs) was investigated by using density functional computations to exploit their potential applications as gas sensors. NH3 can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, so B-doped SWCNTs can be used as NH3 sensors. Both B- and N-doping make NO2 chemisorption feasible in SWCNTs, but the binding of NO2 with B is too strong, indicating an impractical recovery time as gas sensors. Due to the medium (optimal) adsorption energy and the conductance reduction accompanied with the charge transfer between SWCNTs and gas molecules, N-doped SWCNTs are potentially good NO2 sensors. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Based on pioneering explorations [1,2], carbon nanotubes (CNTs) are proposed as chemical sensors to detect toxic gases and other species, such as NH3, NO2, and O2 [3–7]. Even at low concentration, gas chemisorption can change the conductance of CNTs due to the charge transfer between gases and tubes. The recent exciting progress on separating metallic and semiconducting CNTs generally coexisting in as-grown materials [8–15] will further promote the applications of CNTs to field emitting display, gas sensor, etc. However, it was recently demonstrated that the reported conductance sensitivity of CNTs to O2 [2] is attributed to the contact between CNTs and metal electrodes in electrical measurements [16], or residual contaminants such as Na and catalyst particles [17]. In the case of NH3, CNTs are sensitive to ammonia gas only when water vapor is present, indicating that the ammonia–water solution (instead of NH3 alone) changes the conductance of CNTs
*
Corresponding author. Fax: +86 22 23498941. E-mail address: [email protected] (Z. Zhou).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.05.019
[16,18]. This is supported by other experimental and computational studies: pure NH3 is only weekly adsorbed on pristine CNTs by van der Waals (vdW) interaction; such physisorption leads to little charge transfer and band modification, and does not change the conductance of pristine CNTs apparently [19,20]. Similarly, NO2 is also weakly physisorbed on single-walled CNTs (SWCNTs) larger than (10, 0) tube (0.79 nm in diameter), and charge transfer between tubes and NO2 can be neglected [21–24], according to recent density functional theory (DFT) investigations. Thus, it is difficult to detect NH3 and NO2 through variations in the conductance of pristine CNTs, and other strategies are necessary. For example, the capacitance of SWCNTs is highly sensitive to chemical vapors and can be used to prepare sorption-based chemical sensors [25]. Doping (heteroatom substitution) is also a promising approach to enable SWCNTs to detect gas molecules as well as organic chemicals and biological substances, since the adsorption capability of SWCNTs can be improved through introducing heteroatom impurities (such as boron and nitrogen) and forming active sites in tube walls. Even CO and H2O molecules can be detected by B- or N-doped SWCNTs [26], and B-doping makes SWCNTs highly sensitive to HCN and HCHO molecules [27,28]. CNx nanotubes
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were also found to be more efficient for monitoring toxic species than pristine CNTs, probably due to the presence of highly reactive pyridine-like sites in tube walls [29]. B- or N-doped CNTs are well studied doped SWCNTs: experimentally such tubes were synthesized via B and N substitution reactions [30,31], pyrolysis of C2H2–B2H6 mixtures [32], laser ablation, chemical vapor deposition (CVD), and plasma-assisted CVD [33]; theoretically the doping effects of B and N on SWCNTs were widely investigated through DFT computations [34–41]. The NH3 and NO2 adsorptions in B- or N-doped SWCNTs, the focus of this paper, have not been investigated systematically. In this work, computations based on DFT within periodic boundary condition (PBC) were performed to elucidate the relationship between the electronic structures of doped SWCNTs and the characteristics of adsorbed molecules, in order to reveal some clues for chemical sensor design. 2. Computational methods Our DFT calculations employed the plane-wave pseudopotential technique implemented in the Vienna ab initio simulation package (VASP) [42–44]. The generalized gradient approximation (GGA) with the PW91 functional [45], and a 360 eV cut-off for the plane-wave basis set were used in all computations. The electron–ion interactions were modeled by ultrasoft pseudopotentials [46]. To simulate infinitely long (rather than truncated) nanotube systems, one-dimensional (1-D) PBC was applied along the tube axis. Interactions between adsorbed gas molecules and their 1-D periodic images were avoided in our computational supercell models, which include two unit cells of the zigzag tube (for example, supercell ˚ for (10, 0) SWCNT). The positions of all the atoms in length c = 8.53 A the supercell were fully relaxed during geometry optimizations. Five k points were used for sampling the 1-D Brillouin zone, and the convergence ˚ in force. threshold was set as 104 eV in energy and 103 eV/A To further clarify the interaction between adsorbed molecules and SWCNTs, we also performed spin-unrestricted all-electron DFT/GGA computations with a double numerical plus polarization (DNP) basis set [47] and the PW91 exchange-correlation functional [45], as implemented in the DMol program [47,48]. Based on the above equilibrium structures obtained through VASP, further geometry optimization was performed using 3 k points along the 1-D Brillouin zone, and band structures and electronic density distribution were computed by using 21 k points. The adsorption energy (Eads) is defined as, Eads = [E(NTgas) ENT Egas], where E(NTgas), ENT, and Egas stand for the energies of the gas adsorbed nanotube, the pristine nanotube, and the gas molecule, respectively. By this definition, Eads < 0 corresponds to exothermic adsorption leading to local minima stable toward dissociation into nanotubes and gas molecules.
Fig. 1. The most stable configurations for NH3 adsorption in the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. B, N and H atoms are grey, black and white, respectively.
result is consistent with the previous report [20]. At this configuration the adsorption energy is 0.14 eV, and the mol˚ . We also investigated NH3 ecule-tube distance is 3.080 A adsorption in SWCNTs at LDA/PWC (the Perdew and Wang correlation functional [45]) level. NH3 is not covalently bound to SWCNTs, either, and only shows a little higher adsorption energy of 0.19 eV. It is known that DFT does not describe long range interactions very well: GGA normally underestimates the interaction, while LDA usually overestimates the strength. Therefore, NH3 can only be adsorbed weakly on pristine SWCNTs and induce little effect to the electronic structures of SWCNTs. In the case of B-doped SWCNTs, NH3 is attached to B with the N atom pointing to tube walls, and the B–N ˚ (Fig. 1b). The NH3 adsorption distance is 1.694 A on N-doped SWCNTs is similar to that in pristine SWCNTs, i.e., weak physisorption on top of the N atom ˚ (Fig. 1c). The adsorption energy, with a distance of 3.195 A equilibrium tube-molecule distance, and charge transfer at Dmol/GGA/PW91 level are summarized in Table 1 for the three configurations in Fig. 1. The NH3 adsorption energy in the B-doped SWCNT (0.70 eV, Table 1) is much larger than those in the pristine SWCNT (0.14 eV) and in the N-doped SWCNT (0.24 eV). The highly exothermic adsorption of NH3 in the B-doped SWCNT is attributed to the strong interaction
3. Results and discussion 3.1. NH3 adsorption in pristine and doped SWCNTs An NH3 molecule was initially placed on top of a carbon atom or the center of a six-membered ring (6MR) with N or H atoms pointing to tube walls, respectively. However, after full relaxation, NH3 adopts the orientation of H atoms pointing to tube walls, and the configuration of NH3 located on top of 6MR with C3 axial perpendicular to the tube surface is the most stable, as shown in Fig. 1a. This
Table 1 NH3 adsorption energy (Eads), equilibrium tube-molecule distance (d), and charge transfer (Q) in different (10, 0) SWCNTs ˚) SWCNT Eads (eV) d (A Q (jej)a Pristine B-doped N-doped
0.14 0.70 0.24
3.080 1.694b 3.195
0.00 0.31 0.00
a Q is defined as the total Mu¨lliken charge on the molecules, and positive numbers mean charge transfer from molecules to tubes. b The N atom points toward the tube wall.
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Fig. 2. Electronic density difference after NH3 adsorption in the B-doped SWCNT. Dark means increase in electronic density, while grey means decrease.
between the electron-deficient B atom in the tube wall and the electron-donating N atom of NH3. Also, the NH3 molecule-tube distances are quite different in the three SWCNTs. The distance between the N atom of NH3 and ˚ , whereas the B atom in the B-doped SWCNT is 1.694 A the tube-molecule distances in the pristine and N-doped ˚ , indicating that SWCNTs are both longer than 3.0 A NH3 is chemisorbed in the B-doped SWCNT, but physisorbed in the pristine and N-doped SWCNTs. As shown in Fig. 1 the B-doped SWCNT undergoes an obvious distortion upon NH3 adsorption, indicating that the B site transforms from sp2 hybridization to sp3 hybridization. Since NH3 is an electron donor, some charge (0.31 jej) is transferred from NH3 to B-doped SWCNTs. As shown in Fig. 2, after NH3 adsorption, the electronic density decreases around the lone pair of electrons of N, and increases in the region between the B and N atoms, indicating that charge is transferred from the N atom of NH3 to ˚) the B atom in the tube wall. The B–N distance (1.694 A is only a little longer than the measured and computed ˚ ) [49]. Donor-acceptor B–N distance in BH3NH3 (1.6576 A r bonding may also form between NH3 and B in tube walls, as in BH3NH3, but the bonding is much weaker compared with the B–N bond energy of 1.12 eV (0 K) in BH3NH3 [49,50]. Due to large charge transfer, the conductance of B-doped SWCNTs will change accordingly upon NH3 adsorption, which will be discussed later. The binding energies of NH3 in the pristine and the N-doped SWCNTs are much lower due to weak physisorption, with little charge transfer, so both the pristine and N-doped SWCNTs are not suitable for NH3 sensing. 3.2. NO2 adsorption in pristine and doped SWCNTs Similarly, NO2 was initially placed on top of C, doped B or N atom with N or O close to tube walls, and other possible sites for NO2 were also explored, e.g., NO2 was initially located in the meta, ortho and para sites of the doped atoms in the same 6MR. The most stable structures and the related binding energies are summarized in Fig. 3 and Table 2. When adsorbed on the pristine and N-doped
Fig. 3. The most stable configurations for NO2 adsorption in the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. B, N and O atoms are grey, black and white, respectively.
Table 2 NO2 adsorption energy (Eads), equilibrium tube-molecule distance (d), and charge transfer (Q) in different SWCNTs ˚) SWCNT Eads (eV) d (A Q (jej)a Pristine B-doped N-doped
0.12 1.05 0.59
3.167 1.582b 1.741
0.00 0.30 0.17
a Q is defined as the total Mu¨lliken charge on the molecules, and negative number means charge transfer from tube to molecule. b The O atom points to the tube wall.
SWCNTs, NO2 is located above the tube wall with C2 axial perpendicular to the tube surface. In the N-doped SWCNT, NO2 is not preferably attached to N, but to C in the para site of N in the same 6MR. In the B-doped SWCNT, NO2 shows strong interaction with B when the O atom of NO2 points to B. The O–B distance is ˚ , and the bond length of one of N–O bonds in 1.582 A ˚ to 1.342 A ˚. NO2 elongates from 1.209 A The adsorption of NO2 in the pristine SWCNT is the least exothermic (0.12 eV), and the molecule-tube dis˚ ; therefore, NO2 is physisorbed on the pristance is 3.167 A tine SWCNT. This is in good agreement with the recent report that the chemisorption of a single NO2 is only feasible for the (n, 0) zigzag tube with a small diameter (n < 10) [24]. In the B-doped SWCNT, the strong interaction between B and NO2 leads to highly exothermic adsorption energy (1.05 eV) and the formation of tight B–O bond (bond dis˚ ), accompanied with apparent charge transfer tance 1.582 A (0.30 jej) from tube to NO2. The charge transfer is in stark contrast to the NH3 adsorption where electrons are transferred from NH3 to tubes. This is understandable since NH3 is an electron-donor, while NO2 is an electronwithdrawing molecule.
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When adsorbed in the N-doped SWCNT, NO2 prefers to the top of C at the para site of N in the same 6MR (Fig. 3b). The adsorption energy is quite exothermic ˚ . In this case, (0.59 eV), with the C–N length of 1.741 A the charge transfer is 0.17 jej, indicating an apparent charge transfer from tube to NO2. As discussed above, NO2 cannot be bound to pristine (10, 0) SWCNT, but the N-doping makes the para C preferable for NO2 chemisorption. Since there is one extra electron in the N site, the radical-like state will be transferred to meta, ortho and para sites in the 6MR. The para C site is more preferable for NO2 chemisorption than the N site because there is repulsive interaction between the N atom of NO2 and the nitrogen in the N-doped SWCNT. It was found previously that the chemisorption of a single NO2 molecule on SWCNTs leads to radical transfer from NO2 to C atoms at the ortho and para sites, and the second NO2 can be chemisorbed at the para site more easily [24]. Chemisorptions of NO2 in B- and N-doped SWCNTs both modify the conductance of SWCNTs, which will be discussed in the following section. 3.3. Band structures of pristine and doped SWCNTs The electronic band structures were computed for the pristine, B-doped, and N-doped (10, 0) SWCNTs, and are shown in Fig. 4 for comparison. The pristine (10, 0) SWCNT is a semiconductor with a small band gap of 0.75 eV. Since B contains one electron less than C, replacing C by B in the (10, 0) pristine SWCNT induces electron holes in the band structures. As shown in Fig. 4b, an acceptor level appears above the valence band maximum (VBM), and the tube is changed into a p-type semiconductor. The B-doping also narrows the band gap to 0.50 eV. When N is substituted for C, the Fermi level is shifted upward into conduction band, leading to the formation of an ntype semiconductor, and the band gap is also narrowed
to 0.39 eV. All the above results are consistent with previous reports [34,35,39]. 3.4. Changes in band structures of doped SWCNTs after gas adsorption As discussed in the above sections, NH3 is chemisorbed only in the B-doped SWCNT, and NO2 is chemisorbed both in the B- and N-doped SWCNTs, accompanied with apparent charge transfer. Accordingly, the band structures in these SWCNTs are expected to change apparently upon gas adsorption, as confirmed by our computations (Fig. 5). The gas physisorption does not change the band structures of SWCNTs. After NH3 is chemisorbed on the B-doped SWCNT (a p-type semiconductor), the band structure is modified apparently (Fig. 5a). A localized half-filled level appears 0.27 eV above the VBM. As an electron donor with a lone pair of electrons, NH3 fills the empty level in the VBM caused by the electron-deficient B atom, and forms an acceptor level above the VBM. The NH3-attached B-doped SWCNT is still a p-type semiconductor, but the acceptor level is higher above the VBM than in the B-doped SWCNT without NH3 attachment. Hence, the conductance of B-doped nanotubes decreases upon NH3 chemisorption, which can be used to detect NH3 gas. When the electron-withdrawing NO2 is chemisorbed on the B-doped SWCNT, the strong bonding leads to an empty level 0.22 eV above the VBM (Fig. 5b). Thus the NO2-attached B-doped SWCNT is also a p-type semiconductor, but the conductance will decrease. Unfortunately, the interaction is too strong, which precludes the application of B-doped SWCNTs as NO2 sensors. The chemisorption of NO2 in the N-doped SWCNT (an n-type semiconductor) changes the band structure greatly (Fig. 5c). When attached to the tube, NO2 withdraws electrons from the N-doped SWCNT, and pulls down the
Fig. 4. Band structures of the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. The dashed lines indicate the position of Fermi level.
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Fig. 5. Band structures of the NH3-attached B-doped (10, 0) SWCNT (a), the NO2-attached B-doped SWCNT (b), and the NO2-attached N-doped SWCNT (c). The dashed lines indicate the position of Fermi level.
Fermi level to the valence band edge, so the NO2-attached N-doped SWCNT is recovered to a semiconductor with a band gap of 0.76 eV. Accordingly, the conductance of the N-doped SWCNT will decrease sharply upon NO2 chemisorption, so N-doped SWCNTs will be highly sensitive to NO2 adsorption. The recovery time of N-doped SWCNT sensors for NO2 at room temperature is roughly estimated to be 9 ms at the adsorption energy of 0.59 eV, according to the formula, ðEads =K B T Þ s ¼ t1 , where T is temperature, KB the Boltz0 e mann’s Constant (8.62 · 105 eV K1), and m0 the attempt frequency (m0 = 1012 s1 for NO2) [3]. Therefore, N-doped SWCNTs should be good NO2 sensors with quick response as well as short recovery time. Due to the strong binding of NO2 in B-doped SWCNTs, the gas desorption is difficult for B-doped SWCNTs as reversible NO2 sensors. 4. Conclusion The adsorption of NH3 and NO2 was compared in the pristine and B- or N-doped SWCNTs through DFT computations. NH3 and NO2 are physisorbed in the (10, 0) pristine SWCNTs with weak binding and little charge transfer. The N-doping does not change NH3 adsorption in SWCNTs, but the B-doping makes NH3 chemisorption in SWCNTs feasible with the adsorption energy of 0.70 eV and the charge transfer of 0.31 jej. The conductance change of B-doped SWCNTs due to charge transfer can be used to detect NH3. NO2 can be chemisorbed on the B- or N-doped SWCNTs. However, the very strong binding of NO2 in Bdoped SWCNTs makes desorption difficult, which precludes its applications to NO2 sensors. In N-doped SWCNTs, NO2 prefers the attachment to C at the para site of N. The charge transfer (0.17 jej) from tube to NO2 changes the conductance significantly. Moreover, the computed adsorption energy (0.59 eV) corresponds to a recover time of 9 ms, thus N-doped SWCNTs should be
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Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors Lu Bai a, Zhen Zhou b
b,*
a Department of Chemistry, Nankai University, Tianjin 300071, PR China Institute of New Energy Material Chemistry, Institute of Scientific Computing, Nankai University, Tianjin 300071, PR China
Received 21 March 2007; accepted 17 May 2007 Available online 2 June 2007
Abstract The adsorption of NH3 and NO2 in B- or N-doped (10, 0) single-walled carbon nanotubes (SWCNTs) was investigated by using density functional computations to exploit their potential applications as gas sensors. NH3 can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, so B-doped SWCNTs can be used as NH3 sensors. Both B- and N-doping make NO2 chemisorption feasible in SWCNTs, but the binding of NO2 with B is too strong, indicating an impractical recovery time as gas sensors. Due to the medium (optimal) adsorption energy and the conductance reduction accompanied with the charge transfer between SWCNTs and gas molecules, N-doped SWCNTs are potentially good NO2 sensors. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Based on pioneering explorations [1,2], carbon nanotubes (CNTs) are proposed as chemical sensors to detect toxic gases and other species, such as NH3, NO2, and O2 [3–7]. Even at low concentration, gas chemisorption can change the conductance of CNTs due to the charge transfer between gases and tubes. The recent exciting progress on separating metallic and semiconducting CNTs generally coexisting in as-grown materials [8–15] will further promote the applications of CNTs to field emitting display, gas sensor, etc. However, it was recently demonstrated that the reported conductance sensitivity of CNTs to O2 [2] is attributed to the contact between CNTs and metal electrodes in electrical measurements [16], or residual contaminants such as Na and catalyst particles [17]. In the case of NH3, CNTs are sensitive to ammonia gas only when water vapor is present, indicating that the ammonia–water solution (instead of NH3 alone) changes the conductance of CNTs
*
Corresponding author. Fax: +86 22 23498941. E-mail address: [email protected] (Z. Zhou).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.05.019
[16,18]. This is supported by other experimental and computational studies: pure NH3 is only weekly adsorbed on pristine CNTs by van der Waals (vdW) interaction; such physisorption leads to little charge transfer and band modification, and does not change the conductance of pristine CNTs apparently [19,20]. Similarly, NO2 is also weakly physisorbed on single-walled CNTs (SWCNTs) larger than (10, 0) tube (0.79 nm in diameter), and charge transfer between tubes and NO2 can be neglected [21–24], according to recent density functional theory (DFT) investigations. Thus, it is difficult to detect NH3 and NO2 through variations in the conductance of pristine CNTs, and other strategies are necessary. For example, the capacitance of SWCNTs is highly sensitive to chemical vapors and can be used to prepare sorption-based chemical sensors [25]. Doping (heteroatom substitution) is also a promising approach to enable SWCNTs to detect gas molecules as well as organic chemicals and biological substances, since the adsorption capability of SWCNTs can be improved through introducing heteroatom impurities (such as boron and nitrogen) and forming active sites in tube walls. Even CO and H2O molecules can be detected by B- or N-doped SWCNTs [26], and B-doping makes SWCNTs highly sensitive to HCN and HCHO molecules [27,28]. CNx nanotubes
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were also found to be more efficient for monitoring toxic species than pristine CNTs, probably due to the presence of highly reactive pyridine-like sites in tube walls [29]. B- or N-doped CNTs are well studied doped SWCNTs: experimentally such tubes were synthesized via B and N substitution reactions [30,31], pyrolysis of C2H2–B2H6 mixtures [32], laser ablation, chemical vapor deposition (CVD), and plasma-assisted CVD [33]; theoretically the doping effects of B and N on SWCNTs were widely investigated through DFT computations [34–41]. The NH3 and NO2 adsorptions in B- or N-doped SWCNTs, the focus of this paper, have not been investigated systematically. In this work, computations based on DFT within periodic boundary condition (PBC) were performed to elucidate the relationship between the electronic structures of doped SWCNTs and the characteristics of adsorbed molecules, in order to reveal some clues for chemical sensor design. 2. Computational methods Our DFT calculations employed the plane-wave pseudopotential technique implemented in the Vienna ab initio simulation package (VASP) [42–44]. The generalized gradient approximation (GGA) with the PW91 functional [45], and a 360 eV cut-off for the plane-wave basis set were used in all computations. The electron–ion interactions were modeled by ultrasoft pseudopotentials [46]. To simulate infinitely long (rather than truncated) nanotube systems, one-dimensional (1-D) PBC was applied along the tube axis. Interactions between adsorbed gas molecules and their 1-D periodic images were avoided in our computational supercell models, which include two unit cells of the zigzag tube (for example, supercell ˚ for (10, 0) SWCNT). The positions of all the atoms in length c = 8.53 A the supercell were fully relaxed during geometry optimizations. Five k points were used for sampling the 1-D Brillouin zone, and the convergence ˚ in force. threshold was set as 104 eV in energy and 103 eV/A To further clarify the interaction between adsorbed molecules and SWCNTs, we also performed spin-unrestricted all-electron DFT/GGA computations with a double numerical plus polarization (DNP) basis set [47] and the PW91 exchange-correlation functional [45], as implemented in the DMol program [47,48]. Based on the above equilibrium structures obtained through VASP, further geometry optimization was performed using 3 k points along the 1-D Brillouin zone, and band structures and electronic density distribution were computed by using 21 k points. The adsorption energy (Eads) is defined as, Eads = [E(NTgas) ENT Egas], where E(NTgas), ENT, and Egas stand for the energies of the gas adsorbed nanotube, the pristine nanotube, and the gas molecule, respectively. By this definition, Eads < 0 corresponds to exothermic adsorption leading to local minima stable toward dissociation into nanotubes and gas molecules.
Fig. 1. The most stable configurations for NH3 adsorption in the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. B, N and H atoms are grey, black and white, respectively.
result is consistent with the previous report [20]. At this configuration the adsorption energy is 0.14 eV, and the mol˚ . We also investigated NH3 ecule-tube distance is 3.080 A adsorption in SWCNTs at LDA/PWC (the Perdew and Wang correlation functional [45]) level. NH3 is not covalently bound to SWCNTs, either, and only shows a little higher adsorption energy of 0.19 eV. It is known that DFT does not describe long range interactions very well: GGA normally underestimates the interaction, while LDA usually overestimates the strength. Therefore, NH3 can only be adsorbed weakly on pristine SWCNTs and induce little effect to the electronic structures of SWCNTs. In the case of B-doped SWCNTs, NH3 is attached to B with the N atom pointing to tube walls, and the B–N ˚ (Fig. 1b). The NH3 adsorption distance is 1.694 A on N-doped SWCNTs is similar to that in pristine SWCNTs, i.e., weak physisorption on top of the N atom ˚ (Fig. 1c). The adsorption energy, with a distance of 3.195 A equilibrium tube-molecule distance, and charge transfer at Dmol/GGA/PW91 level are summarized in Table 1 for the three configurations in Fig. 1. The NH3 adsorption energy in the B-doped SWCNT (0.70 eV, Table 1) is much larger than those in the pristine SWCNT (0.14 eV) and in the N-doped SWCNT (0.24 eV). The highly exothermic adsorption of NH3 in the B-doped SWCNT is attributed to the strong interaction
3. Results and discussion 3.1. NH3 adsorption in pristine and doped SWCNTs An NH3 molecule was initially placed on top of a carbon atom or the center of a six-membered ring (6MR) with N or H atoms pointing to tube walls, respectively. However, after full relaxation, NH3 adopts the orientation of H atoms pointing to tube walls, and the configuration of NH3 located on top of 6MR with C3 axial perpendicular to the tube surface is the most stable, as shown in Fig. 1a. This
Table 1 NH3 adsorption energy (Eads), equilibrium tube-molecule distance (d), and charge transfer (Q) in different (10, 0) SWCNTs ˚) SWCNT Eads (eV) d (A Q (jej)a Pristine B-doped N-doped
0.14 0.70 0.24
3.080 1.694b 3.195
0.00 0.31 0.00
a Q is defined as the total Mu¨lliken charge on the molecules, and positive numbers mean charge transfer from molecules to tubes. b The N atom points toward the tube wall.
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Fig. 2. Electronic density difference after NH3 adsorption in the B-doped SWCNT. Dark means increase in electronic density, while grey means decrease.
between the electron-deficient B atom in the tube wall and the electron-donating N atom of NH3. Also, the NH3 molecule-tube distances are quite different in the three SWCNTs. The distance between the N atom of NH3 and ˚ , whereas the B atom in the B-doped SWCNT is 1.694 A the tube-molecule distances in the pristine and N-doped ˚ , indicating that SWCNTs are both longer than 3.0 A NH3 is chemisorbed in the B-doped SWCNT, but physisorbed in the pristine and N-doped SWCNTs. As shown in Fig. 1 the B-doped SWCNT undergoes an obvious distortion upon NH3 adsorption, indicating that the B site transforms from sp2 hybridization to sp3 hybridization. Since NH3 is an electron donor, some charge (0.31 jej) is transferred from NH3 to B-doped SWCNTs. As shown in Fig. 2, after NH3 adsorption, the electronic density decreases around the lone pair of electrons of N, and increases in the region between the B and N atoms, indicating that charge is transferred from the N atom of NH3 to ˚) the B atom in the tube wall. The B–N distance (1.694 A is only a little longer than the measured and computed ˚ ) [49]. Donor-acceptor B–N distance in BH3NH3 (1.6576 A r bonding may also form between NH3 and B in tube walls, as in BH3NH3, but the bonding is much weaker compared with the B–N bond energy of 1.12 eV (0 K) in BH3NH3 [49,50]. Due to large charge transfer, the conductance of B-doped SWCNTs will change accordingly upon NH3 adsorption, which will be discussed later. The binding energies of NH3 in the pristine and the N-doped SWCNTs are much lower due to weak physisorption, with little charge transfer, so both the pristine and N-doped SWCNTs are not suitable for NH3 sensing. 3.2. NO2 adsorption in pristine and doped SWCNTs Similarly, NO2 was initially placed on top of C, doped B or N atom with N or O close to tube walls, and other possible sites for NO2 were also explored, e.g., NO2 was initially located in the meta, ortho and para sites of the doped atoms in the same 6MR. The most stable structures and the related binding energies are summarized in Fig. 3 and Table 2. When adsorbed on the pristine and N-doped
Fig. 3. The most stable configurations for NO2 adsorption in the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. B, N and O atoms are grey, black and white, respectively.
Table 2 NO2 adsorption energy (Eads), equilibrium tube-molecule distance (d), and charge transfer (Q) in different SWCNTs ˚) SWCNT Eads (eV) d (A Q (jej)a Pristine B-doped N-doped
0.12 1.05 0.59
3.167 1.582b 1.741
0.00 0.30 0.17
a Q is defined as the total Mu¨lliken charge on the molecules, and negative number means charge transfer from tube to molecule. b The O atom points to the tube wall.
SWCNTs, NO2 is located above the tube wall with C2 axial perpendicular to the tube surface. In the N-doped SWCNT, NO2 is not preferably attached to N, but to C in the para site of N in the same 6MR. In the B-doped SWCNT, NO2 shows strong interaction with B when the O atom of NO2 points to B. The O–B distance is ˚ , and the bond length of one of N–O bonds in 1.582 A ˚ to 1.342 A ˚. NO2 elongates from 1.209 A The adsorption of NO2 in the pristine SWCNT is the least exothermic (0.12 eV), and the molecule-tube dis˚ ; therefore, NO2 is physisorbed on the pristance is 3.167 A tine SWCNT. This is in good agreement with the recent report that the chemisorption of a single NO2 is only feasible for the (n, 0) zigzag tube with a small diameter (n < 10) [24]. In the B-doped SWCNT, the strong interaction between B and NO2 leads to highly exothermic adsorption energy (1.05 eV) and the formation of tight B–O bond (bond dis˚ ), accompanied with apparent charge transfer tance 1.582 A (0.30 jej) from tube to NO2. The charge transfer is in stark contrast to the NH3 adsorption where electrons are transferred from NH3 to tubes. This is understandable since NH3 is an electron-donor, while NO2 is an electronwithdrawing molecule.
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When adsorbed in the N-doped SWCNT, NO2 prefers to the top of C at the para site of N in the same 6MR (Fig. 3b). The adsorption energy is quite exothermic ˚ . In this case, (0.59 eV), with the C–N length of 1.741 A the charge transfer is 0.17 jej, indicating an apparent charge transfer from tube to NO2. As discussed above, NO2 cannot be bound to pristine (10, 0) SWCNT, but the N-doping makes the para C preferable for NO2 chemisorption. Since there is one extra electron in the N site, the radical-like state will be transferred to meta, ortho and para sites in the 6MR. The para C site is more preferable for NO2 chemisorption than the N site because there is repulsive interaction between the N atom of NO2 and the nitrogen in the N-doped SWCNT. It was found previously that the chemisorption of a single NO2 molecule on SWCNTs leads to radical transfer from NO2 to C atoms at the ortho and para sites, and the second NO2 can be chemisorbed at the para site more easily [24]. Chemisorptions of NO2 in B- and N-doped SWCNTs both modify the conductance of SWCNTs, which will be discussed in the following section. 3.3. Band structures of pristine and doped SWCNTs The electronic band structures were computed for the pristine, B-doped, and N-doped (10, 0) SWCNTs, and are shown in Fig. 4 for comparison. The pristine (10, 0) SWCNT is a semiconductor with a small band gap of 0.75 eV. Since B contains one electron less than C, replacing C by B in the (10, 0) pristine SWCNT induces electron holes in the band structures. As shown in Fig. 4b, an acceptor level appears above the valence band maximum (VBM), and the tube is changed into a p-type semiconductor. The B-doping also narrows the band gap to 0.50 eV. When N is substituted for C, the Fermi level is shifted upward into conduction band, leading to the formation of an ntype semiconductor, and the band gap is also narrowed
to 0.39 eV. All the above results are consistent with previous reports [34,35,39]. 3.4. Changes in band structures of doped SWCNTs after gas adsorption As discussed in the above sections, NH3 is chemisorbed only in the B-doped SWCNT, and NO2 is chemisorbed both in the B- and N-doped SWCNTs, accompanied with apparent charge transfer. Accordingly, the band structures in these SWCNTs are expected to change apparently upon gas adsorption, as confirmed by our computations (Fig. 5). The gas physisorption does not change the band structures of SWCNTs. After NH3 is chemisorbed on the B-doped SWCNT (a p-type semiconductor), the band structure is modified apparently (Fig. 5a). A localized half-filled level appears 0.27 eV above the VBM. As an electron donor with a lone pair of electrons, NH3 fills the empty level in the VBM caused by the electron-deficient B atom, and forms an acceptor level above the VBM. The NH3-attached B-doped SWCNT is still a p-type semiconductor, but the acceptor level is higher above the VBM than in the B-doped SWCNT without NH3 attachment. Hence, the conductance of B-doped nanotubes decreases upon NH3 chemisorption, which can be used to detect NH3 gas. When the electron-withdrawing NO2 is chemisorbed on the B-doped SWCNT, the strong bonding leads to an empty level 0.22 eV above the VBM (Fig. 5b). Thus the NO2-attached B-doped SWCNT is also a p-type semiconductor, but the conductance will decrease. Unfortunately, the interaction is too strong, which precludes the application of B-doped SWCNTs as NO2 sensors. The chemisorption of NO2 in the N-doped SWCNT (an n-type semiconductor) changes the band structure greatly (Fig. 5c). When attached to the tube, NO2 withdraws electrons from the N-doped SWCNT, and pulls down the
Fig. 4. Band structures of the pristine (a), B-doped (b), and N-doped (c) (10, 0) SWCNTs. The dashed lines indicate the position of Fermi level.
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Fig. 5. Band structures of the NH3-attached B-doped (10, 0) SWCNT (a), the NO2-attached B-doped SWCNT (b), and the NO2-attached N-doped SWCNT (c). The dashed lines indicate the position of Fermi level.
Fermi level to the valence band edge, so the NO2-attached N-doped SWCNT is recovered to a semiconductor with a band gap of 0.76 eV. Accordingly, the conductance of the N-doped SWCNT will decrease sharply upon NO2 chemisorption, so N-doped SWCNTs will be highly sensitive to NO2 adsorption. The recovery time of N-doped SWCNT sensors for NO2 at room temperature is roughly estimated to be 9 ms at the adsorption energy of 0.59 eV, according to the formula, ðEads =K B T Þ s ¼ t1 , where T is temperature, KB the Boltz0 e mann’s Constant (8.62 · 105 eV K1), and m0 the attempt frequency (m0 = 1012 s1 for NO2) [3]. Therefore, N-doped SWCNTs should be good NO2 sensors with quick response as well as short recovery time. Due to the strong binding of NO2 in B-doped SWCNTs, the gas desorption is difficult for B-doped SWCNTs as reversible NO2 sensors. 4. Conclusion The adsorption of NH3 and NO2 was compared in the pristine and B- or N-doped SWCNTs through DFT computations. NH3 and NO2 are physisorbed in the (10, 0) pristine SWCNTs with weak binding and little charge transfer. The N-doping does not change NH3 adsorption in SWCNTs, but the B-doping makes NH3 chemisorption in SWCNTs feasible with the adsorption energy of 0.70 eV and the charge transfer of 0.31 jej. The conductance change of B-doped SWCNTs due to charge transfer can be used to detect NH3. NO2 can be chemisorbed on the B- or N-doped SWCNTs. However, the very strong binding of NO2 in Bdoped SWCNTs makes desorption difficult, which precludes its applications to NO2 sensors. In N-doped SWCNTs, NO2 prefers the attachment to C at the para site of N. The charge transfer (0.17 jej) from tube to NO2 changes the conductance significantly. Moreover, the computed adsorption energy (0.59 eV) corresponds to a recover time of 9 ms, thus N-doped SWCNTs should be
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