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Computational study of Al- or P-doped single-walled carbon nanotubes as NH3 and NO2 sensors
Applied Surface Science 285P (2013) 102–109
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Computational study of Al- or P-doped single-walled carbon nanotubes as NH3 and NO2 sensors Khaled Azizi ∗ , Mohammad Karimpanah Department of Chemistry, University of Kurdistan, Sanandaj, Iran
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 22 July 2013 Accepted 27 July 2013 Available online 20 August 2013 Keywords: Ammonia Nitrogen dioxide Sensor SWCNT DFT
a b s t r a c t Density functional theory (DFT) calculations were carried out to analyze the electronic and structural properties of pristine and aluminum or phosphorus doped (8,0) single walled carbon nanotube (SWCNT) as a sensor for the detection of nitrogen dioxide (NO2 ) and ammonia (NH3 ). The binding energies, equilibrium gas-nanotube distances, the amounts of charge transfer and molecular orbital schemes as well as the density of states have been calculated and used to interpret the mechanism of gas adsorption on the surface of nanotubes. In agreement with the experimental data, our results show considerable binding energy and energy gap alteration due to the adsorption of NO2 on pristine SWCNT. The results reveal that the doping of both Al and P atoms increase the capability of the nanotube for the adsorption of NO2 , and the effect is more significant for the Al-doped nanotube. The Al-doped nanotube can also be considered as a good sensor for NH3 due to its high binding energy, considerable amount of charge transfer and energy band gap alteration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since 2000, Kong et al. [1] have demonstrated experimentally the dramatic effect of very small amounts of NO2 and NH3 gases on the current–voltage (I–V) curves of a SWCNT. Afterwards, many efforts have been devoted to the study of the relationship between gas sensing behavior, on the one hand, and structural and electronic properties of pristine and deformed SWCNTs on the other [2–11]. These studies have focused on two main subjects. First, they have endeavored to provide a detailed molecular-level understanding of the interaction between SWCNTs and gaseous molecules, and second, to improve the gas sensing behavior of single walled nanotubes by mechanical deformation or doping heteroatoms [10,12]. Moderate values of binding energy, considerable amount of charge transfer and sensible energy band gap variation have been mentioned as desirable prerequisites for the selection of adsorbents as electrical gas sensors [13–15]. Based on the recovery time, Peng et al. estimated the binding energy of NO2 and NH3 on SWCNT to be 23 and 2.2 kcal mol−1 , respectively [16]. These estimations are consistent with the values of 27.2 kcal mol−1 for NO2 and 1.8 kcal mol−1 for NH3 , deduced by Ellison et al. using temperature-programmed desorption (TPD) [17]. The low amount of charge transfer between SWCNT and either adsorbed NO2 or NH3 molecules and negligible amount of binding energy of NH3 to SWCNTs were
∗ Corresponding author. Tel.: +98 871 6624133; fax: +98 871 6660075. E-mail address: [email protected] (K. Azizi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.146
currently confirmed by the results of theoretical investigations [2,5,18]. However, a unique perspective on the calculated values of the binding energy of NO2 on carbon nanotubes, could not be understood due to very different reported results (0.9 to 18.6 kcal mol−1 ) [2,16,18–22]. According to Yim et al., the energy of two chemisorbed NO2 on an (8,0) SWCNT is about 20 kcal mol−1 less than the total energy of the nanotube and two free NO2 molecules [20]. They found that the chemisorbed NO2 reduces the nanotube energy band gap, which probably explains the experimentally observed change in I–V curve with NO2 exposure. Using different ab initio approaches, Ricca and Bauschlicher further studied this subject and found that chemisorption energy intensively decreases with the diameter of the nanotube, and therefore chemisorption does not seem to be a reasonable explanation for the change in current with NO2 exposure [23]. Consequently, to date we still do not have a complete understanding of how carbon nanotube sensors work. When an intrinsic carbon nanotube is doped with heteroatoms through a replacement of carbon atoms, the local physical properties around the impurity atoms undergo a significant change, resulting in the change of the local chemical reactivity. The change in the local chemical reactivity alters the binding energy of gas molecules on the doped nanotube [24]. For example, it was revealed that the electronic properties of boron-nitride nanotubes (BNNTs) can be affected by the adsorption of NH3 [12] and there is a considerable different between B and N positions for the adsorption of NH3 [25]. Also, Bai and Zhou have investigated the adsorption of NH3 and NO2 in B- or N-doped (10,0) SWCNTs by
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using DFT computations. Their study showed that NH3 can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, but both B- and N-doping make NO2 chemisorption feasible in SWCNTs [15]. More recently, Cruz-Silva et al. have confirmed that the substitution of phosphorus and phosphorus–nitrogen codopants within SWCNTs strongly modifies the chemical properties of the surface, thus creating highly localized sites with specific affinity toward acceptor and donor molecules, respectively [26]. It turned out that the adsorption of both NO2 and NH3 induce the suppression of a conductance dip. NH3 and NO2 gases are the subject of many theoretical and experimental studies related to designing and construction of gas sensors [27–33]. This is due to their importance in industry and air pollution, on one hand, and being good examples of electron donor and acceptor molecules in theoretical investigations, on the other. In this study, the applicability of Al-doped (8,0) SWCNT (SWCNT/Al) as a sensor of NO2 and NH3 gases is investigated and compared with that of pristine and P-doped (8,0) SWCNT (SWCNT/P). Two main goals of this research focus on two major issues. First, we briefly discuss the adsorption of NO2 and NH3 on pristine (8,0) SWCNT. The main focus is on the estimated values of binding energies and the variation of the electronic properties of SWCNT due to gas adsorption. A new interpretation for the considerable alteration of the conductance of pristine SWCNT, due to the adsorption of NO2 , is also put forward. Second, a detailed discussion will be followed in which the effect of Al and P heteroatoms doping on the gas sensing behavior of (8,0) SWCNT with respect to NO2 and NH3 molecules will be presented. In particular, it will be shown that SWCNT/Al adsorbs both NO2 and NH3 molecules with considerable amounts of binding energies, and from this point of view, it is more preferable compared with both pristine and P-doped SWCNT.
2. Computational methods All calculations were performed with the Gaussian 98 quantum chemical package [34]. The geometries of the isolated SWCNTs, NH3 and NO2 moieties and their complexes were fully optimized without any symmetry constraints at the DFT level of computation [35] in conjunction with the B3LYP exchange–correlation functionals [36] and 6-31G(d,p) basis set. In this work we used 6-311G(d,p) basis set along with exchange–correlation functional of Perdew and Wang (PW91) [37], which has been distinguished to be superior to the B3LYP for the estimation of nonbonding interactions [38,39], for the calculation of the energy and electronic properties. Fig. 1 shows a schematic side view of (8,0) SWCNT, which specifies the chemical bonds and angles including heteroatom and pre-optimization locations of the gas molecule on the surface of the nanotube. Different initial configurations of the gas molecule were considered for each nanotube–gas couple. In the first configuration the gas molecule is on the heteroatom (X position in Fig. 1), in the second one the gas molecule is placed on one of the two six-member rings (6MR) specified in Fig. 1 (over A and B positions). It should be noted that in the above mentioned configurations we have the option of approaching the gas molecule on both sides, N atom being either near to or far from the surface of SWCNT. Total density of states (DOS) [40] of pristine and doped (8,0) SWCNTs was visualized by the GaussSum software [41]. The binding energy of gases, Eads , is defined as Eads = [ESWCNT-gas − ESWCNT − Egas ], where ESWCNT-gas , ESWCNT and Egas stand for the energies of the gas adsorbed nanotube, the nanotube, and the gas molecule, respectively. By this definition, Eads < 0 corresponds to exothermic adsorption, which leads to local minima toward dissociation into the nanotube and the gas molecule. The calculated binding energies have been corrected for the basis set superposition error (BSSE). The energy gap variation,
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Fig. 1. A schematic side view of doped (8,0) zigzag SWCNT. X, A and B specify positions on which the gas molecule is placed at the beginning of optimization.
Egap , for any species is defined as Egap = Egap,after − Egap,before , where Egap is the difference between the energies of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO. Finally, the natural bond orbital (NBO) calculations and population analysis were performed to investigate charge transfer and chemical bond formation processes between SWCNTs and gases with or without doped heteroatoms. 3. Results and discussion 3.1. Structural and electronic parameters of pristine and Al- or P-doped (8,0) SWCNTs The structural parameters of Al- and P-doped SWCNTs, which include bond lengths, bond angles, and the protrusion of heteroatom, in good agreement with the previous reported data [42–44], are summarized in Table 1. Protrusion on the surface of doped SWCNT was attributed to the larger atomic radius of heteroatom relative to that of carbon atom [42,45]. Considering the fact that the radius of Al atom is greater than that of P atoms, we expect that the protrusion for SWCNT/Al to be greater than that of SWCNT/P (unlike the data given in Table 1). This can be attributed to the ionic nature of Al C bonds which are stronger than that of their counterpart P C bonds in SWCNT/P. As it can be seen from the data given in Table 2, in both SWCNT/Al and SWCNT/P the hetero atom and ortho-carbon atoms have considerable amounts of positive and negative electrical charge, respectively, while the meta-carbon atoms are negligibly charged. In addition, the negative charge of the para-carbon of SWCNT/P is considerably larger than that of SWCNT/Al. The charge density redistributions depicted in Fig. 2, for both doped nanotubes, confirm an increase in the electron density at ortho and para carbon atoms, as well as an electron density reduction at meta carbon atom. This result is consistent with the electrical
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Table 1 The structural parameters of (8,0) X-SWCNT (X = Al, P). System
SWCNT/Al SWCNT/P a b c
Bond length (Å)
Bond angle (deg)
Protrusion
X-C1
X-C2
X-C3
C1-X-C2
C1-X-C3
C2-X-C3
1.847 (1.822)a 1.787 (1.79)b,c
1.880 (1.886)a 1.836 (1.79)b,c
1.880 (1.887)a 1.836
106.09 98.5 (100.34)b,c
106.07 98.49 (99.81)b,c
106.50 93.68 (94.04)b,c
0.812 0.818
From Ref. [42]. From Ref. [43]. From Ref. [44].
charges of atoms given in Table 2. As we shall see in the next sections, the electrical charges mentioned above, play an important role in the mechanism of the adsorption of NO2 and NH3 molecules on the surface of SWCNTs. 3.2. NO2 adsorption in pristine and doped (8,0) SWCNTs The most stable geometries of NO2 adsorbed on pristine and doped SWCNTs which were optimized at the B3LYP level are shown in Fig. 3. For better vision, optimized structures have been shown from two different sides. A remarkable point in Fig. 3 is the geometry by which NO2 adsorbs on the surface of SWCNT/Al; this is different from that of pristine and P-doped SWCNTs. This is reasonable regarding the repulsion between positively charged Al and N atoms in SWCNT/Al and NO2 moieties, respectively. The values of binding energies, Eads , equilibrium distance between SWCNTs and NO2 , Rmin , the amount of charge transferred between nanotube and gas molecule, CT, and the variation of HOMO–LUMO gap, Egap are listed in Table 3. The binding energy of NO2 on the pristine SWCNT is calculated to be −20.97 kcal mol−1 ˚ Our results are in good and the molecule–tube distance is 2.91 A. agreement with previously estimated values [1,16,18] and are consistent with the available experimental data [17]. The results of the NBO analysis do not confirm the formation of any chemical bonds between SWCNT and NO2 . Consequently, NO2 is almost
physisorbed on the pristine (8,0) SWCNT. As it can be seen in Table 3, the amount of charge transfer between NO2 and the pristine nanotube is negligible, which is in agreement with previous reports. Therefore, the change in the electrical properties of SWCNT during gas adsorption can not be attributed to charge transfer [46]. According to the data given in Table 3, due to the adsorption of NO2 , the HOMO–LUMO gap of SWCNT increases by 0.635 eV, which is insufficient for exerting dramatic changes in electronic properties of SWCNT. The DOS diagrams of SWCNTs before and after the gas adsorption are indicated in Fig. 4. As shown in Fig. 4a, the adsorption of NO2 has considerably changed the shape of the DOS diagram. After the adsorption, the two peaks with moderate intensity (in DOS diagram of pristine SWCNT) around the Fermi level at −4.2 eV and −3.7 eV, respectively, have been disappeared, and instead a single relatively low-intensity peak appears at −3.9 eV. Furthermore, the intensity of states in both valance and conductance bands increased considerably. A close look over the electronic energy states of the system, around the Fermi level, indicates that the LUMO is a single beta state at −3.90 eV and the HOMO, HOMO − 1, LUMO + 1 and LUMO + 2 states are located at −4.96, −4.95, −2.88 and −2.87 eV, respectively. The single beta state corresponds to the little peak appeared at the right side of the Fermi level in Fig. 4a. Hence, while the energy gap for transition (from valance to conduction band) of the first electron is 1.05 eV, the other electrons should traverse a band gap of 2.07 eV energy. Accordingly, it makes sense that the effective energy gap for SWCNT/NO2 system should be considered 2.07 eV rather than 1.05 eV. This drastic variation of the energy gap can be responsible for the dramatic changes in the electronic properties of the nanotube. As it can be seen from Fig. 3b, being in agreement with the previously reported data for N-doped SWCNTs [15], NO2 molecule prefers the top of para carbon atom in the same 6MR. The main axis of SWCNT, C2 symmetry axial of NO2 , P atom and para C atoms of the tube will be placed in the same plane. The results of NBO analysis confirm the formation of a chemical bond between para C atom of SWCNT and N atom of NO2 . It should be noted, that two stable configurations were found for the NO2 adsorbed on SWCNT/P system: either N atom bonded to an ortho C atom or it bonded to a para C atom. The binding energy for adsorption on para C atom is considerably larger than that of ortho C atom, hence the later configuration not being discussed here. The interesting point is that the geometry obtained in our calculation differs from that previously reported results in which NO2 molecule chemically bonded to P atom [26]. According to the electrical charge values given in Table 2, both ortho and para C atoms Table 2 The electrical charge of heteroatom and the carbon atoms placed at the ortho, meta and para positions with respect to the doped atom. System
Fig. 2. Charge density distributions for (a) SWCNT/P and (b) SWCNT/Al nanotubes.
SWCNT/Al SWCNT/P
Type of atom Heteroatom
C (ortho)
C (meta)
C (para)
0.91 0.67
−0.31 −0.35
−0.04 0.02
−0.13 −0.21
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Fig. 3. The most stable configurations for adsorption of NO2 on (a) pristine, (b) P- and (c) Al-doped (8,0) SWCNT.
Fig. 4. Density of states plots for different systems (a) pristine SWCNT, SWCNT-NO2 and SWCNT-NH3 (b) SWCNT/P, SWCNT/P-NO2 and SWCNT/P-NH3 and (c) SWCNT/Al, SWCNT/Al-NO2 and SWCNT/Al-NH3 . The vertical dotted lines indicate the Fermi level.
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Table 3 The structural and electronic parameters of adsorption of NO2 on pristine and Al- or P-doped (8,0) SWCNTs. Type of nanotube
Eads a (kcal mol−1 )
SWCNT SWCNT/P SWCNT/Al
−20.97 −37.09 −50.64
a b c d e
f g
BSSEb (kcal mol−1 ) 0.94 3.96 4.47
Rmin c (Å)
CTd |e|
2.91 1.59 1.82
−0.012 −0.173 −0.530
Egap,before e (eV) 0.429 0.596 0.973
Egap f (eV)
Egap,after (eV) g
1.064 (2.069) 0.401 0.438
0.635 −0.195 −0.535
The binding energy between nanotube and gas molecule. The basis set superposition error. The equilibrium distance between nanotube and gas molecule. The amount of charge transferred between nanotube and gas molecule. The difference between the energies of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, before the adsorption of gas molecule on the surface of nanotube. Egap,after − Egap,before . The effective band gap, the difference between the energies of the LUMO + 1 and HOMO.
are more favorable for NO2 adsorption than meta C atom. Due to the positive electrical charge on P atom, there is a strong repulsive interaction between the N atom of NO2 and the P atom of SWCNT/P. Therefore, contrary to the previous report, our results show that P atom is not preferable for NO2 adsorption. The binding energy of NO2 on SWCNT/P is quite exothermic (−37.09 kcal mol−1 ), with the ˚ It should be noted that, the applicability C N bond length of 1.59 A. of SWCNT/P as NO2 sensor may be limited by both the obtained relatively large amount of binding energy and the chemical bond formation between the gas molecule and the nanotube. However, there is convincing evidence that the amount of binding energy per adsorbed molecule, when gas molecules are absorbed in bulk, is significantly less than that of individually adsorbed molecules [23,25,28,45,47]. Therefore, further studying of SWCNT/P as NO2 sensor is plausible. Since NO2 is an electron acceptor, the amount of (−0.17 |e|) charge is transferred from nanotube (SWCNT/P) to NO2 . A comparison between the electrical charge of the P atom before (0.67 |e|) and after (0.71 |e|) the adsorption of NO2 indicates that the charge transfer is mainly done by the nanotube rather than by the P atom. Therefore, we expect the electronic properties of the nanotube to be seriously affected by the adsorption of NO2 . According to the obtained results, the binding energy of NO2 on SWCNT/P is about twice as much as that of the pristine (8,0) SWCNT and the amount of charge transfer is considerably larger for SWCNT/P. As a result, we conclude that the P-doping makes the para C atom preferable for NO2 chemisorption. According to the data given in Table 3, the adsorption of NO2 slightly reduces the energy gap of SWCNT/P, which cannot be considered as a serious electronic change. A comparison between the DOS diagram of SWCNT/P before and after NO2 adsorption, depicted in Fig. 4b, shows that the number of states in both valance and conduction bands decrease; in particular, a strong band at −4.9 eV disappears. Therefore, we expect the electronic properties of SWCNT/P to be significantly affected by the adsorption of NO2 . Hence the nanotube can be considered for further studying as an NO2 sensor. In the case of Al-doped SWCNTs, NO2 shows strong interaction with Al when O atoms of NO2 point toward the Al atom. The equilibrium distance between NO2 and SWCNT/Al is 1.82 A˚ and there is no chemical bond between Al and O atoms. The distance between the ˚ respecAl atom and the near and far O atoms are 1.92 A˚ and 2.26 A, tively. The C2 symmetry axis of NO2 is perpendicular to the main axis of SWCNT/Al, and all three atoms of NO2 and the main axis of tube are placed in the same plane. According to the electrical charge values given in Table 2, due to the positive charge accumulated on the Al atom and the negative charge on O atoms of NO2 , we expect some strong attractive interactions between the nanotube and the gas molecule. The strong interaction between Al and NO2 leads to highly exothermic adsorption energy (−50.64 kcal mol−1 ). Furthermore, the adsorption of NO2 is accompanied by a considerable
amount of charge transfer (−0.53 |e|) from the tube to NO2 . A comparison between the electrical charge of the Al atom before (0.91 |e|) and after (1.15 |e|) adsorption, indicates that the main contribution to charge transfer is made by the nanotube rather than by the Al atom. Therefore, as in the case of SWCNT/P, the electronic properties of the nanotube are seriously affected by the adsorption of NO2 . The data given in Table 3 indicate that due to the adsorption of NO2 , the energy gap of SWCNT/Al is considerably decreased, and its total variation is remarkably larger than that of SWCNT/P. This amount of variation of energy gap is expected to have a significant impact on the electronic properties of the nanotube. A comparison between the DOS diagrams of SWCNT/Al before and after NO2 adsorption, depicted in Fig. 4, indicates that, near the Fermi level, two new peaks appear close together with sufficient intensity. Furthermore, some of the prominent peaks in the conduction band were disappeared and, a uniform distribution of the peaks in the range of both conduction and valance bands appeared. Overall, we expected that these changes would increase the electrical conductivity of the nanotube. Considering the binding energy, charge transfer, energy gap variation and DOS redistributions, we conclude that SWCNT/Al can be considered as a good candidate for NO2 detection and is superior to both pristine SWCNT and SWCNT/P. As mentioned above, the relatively large amount of binding energy alone cannot be considered as a reason for excluding SWCNT/Al as NO2 sensor. 3.3. NH3 adsorption in pristine and doped (8,0) SWCNTs The most stable optimized geometries of NH3 adsorbed on pristine and doped (8,0) SWCNTs are shown in Fig. 5. The interesting point is that NH3 selects three different orientations with respect to three nanotubes. While it selects two opposite orientations relative to pristine and SWCNT/Al, i.e. either N atom or H atoms pointed to the surface of the tube, respectively, it selects a tilted orientation in which the C3 symmetry axis of NH3 is perpendicular to the plane containing Al and two ortho C atoms, as shown in Fig. 5a. The structural and electronic properties of NH3 adsorbed on pristine and P- or Al-doped (8,0) SWCNTs are given in Table 4. As we can see from Table 4 and Fig. 5, in agreement with previous reports [15,48] the binding energy, charge transfer, energy gap variation as well as DOS diagram rearrangement for the adsorption of NH3 on the pristine SWCNT are too low to be used for any discussion on the sensing properties of this nanotube with respect to NH3 . Except for the slight change in DOS diagram shape, because of band splitting, we find that the DOS of SWCNT-NH3 is very close to that of the pristine nanotube. From Table 4 we can see that, as in the case of pristine SWCNT, due to the weak physisorption, the binding energy of NH3 in SWCNT/P is also considerably small. The charge transfer, energy gap variation, and the modification of DOS shape are also negligible. This is in a good agreement with the previously reported data for the
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Fig. 5. The most stable configurations for adsorption of NH3 on (a) pristine, (b) P- and (c) Al-doped (8,0) SWCNT.
adsorption of NH3 on both N- and P-doped SWCNTs [15,44]. Therefore, we conclude that SWCNT/P is not suitable for NH3 sensing since it does not fulfill any of the previously mentioned criteria. As in the case of pristine SWCNT, the DOS diagrams of NH3 adsorbed SWCNT/P before and after of NH3 adsorption are very close to each other. As a result, the interaction between the SWCNT/P and NH3 molecules is weak and does not have a significant influence on the electronic properties of the nanotube. Based on the above results, the application of SWCNTs as a sensor for NH3 detection is not suggested. In the case of Al-doped SWCNTs, NH3 is attached to Al with the N atom pointing to the tube surface (Fig. 5a). As indicated in Table 4, the binding energy for the adsorption of NH3 in
SWCNT/Al is −37.41 kcal mol−1 . To our knowledge this is one of the largest amounts of binding energy which has been found until now for the physisorption of NH3 on pristine or deformed carbon nanotubes. The highly exothermic adsorption of NH3 on the SWCNT/Al may be attributed to two effects; first, the strong electrostatic interaction between positively charged Al atom and the negatively charged N atom and the second, the strong interaction between the electron-deficient hole on the Al atom and the lone pair of N atom. In addition, the distance between the N ˚ whereas atom of NH3 and the Al atom in SWCNT/Al is 2.04 A, the tube–molecule distances in both pristine and SWCNT/P are ˚ This indicates that NH3 is strongly adsorbed in the about 3.0 A. SWCNT/Al.
Table 4 The structural and electronic parameters of adsorption of NH3 on pristine and Al- or P-doped (8,0) SWCNTs. Type of nanotube
Eads a (kcal mol−1 )
BSSEb (kcal mol−1 )
Rmin c (Å)
CTd (|e|)
Egap,before e (eV)
Egap,after (eV)
Egap f (eV)
SWCNT SWCNT/P SWCNT/Al
−0.33 −0.74 −37.41
0.13 2.12 3.41
3.02 2.95 2.04
0.009 0.048 0.196
0.429 0.596 0.973
0.431 0.653 1.301
−0.001 0.057 0.540
a b c d e
f
The binding energy between nanotube and gas molecule. The basis set superposition error. The equilibrium distance between nanotube and gas molecule. The amount of charge transferred between nanotube and gas molecule. The difference between the energies of the high occupied molecular orbital, HOMO, and the low unoccupied molecular orbital, LUMO, before the adsorption of gas molecule on the surface of nanotube. Egap,after − Egap,before .
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Finally, we can see from Fig. 4 that, unlike pristine and p-doped SWCNTs, the change of DOS diagram near the Fermi level is evident for SWCNT/Al; in particular, the energy gap increases and two peaks with sufficient intensity appear in valance and conduction bands. As it can be seen, the amount of charge transfer between NH3 and nanotube, the energy gap variation and DOS diagram deformation are more evident in the case of SWCNT/Al in comparison with that of the pristine and p-doped SWCNTs. Therefore, we expect that the electronic properties of SWCNT/Al will change considerably upon NH3 exposure and the SWCNT/Al can be considered as a good sensor for NH3 . 4. Conclusion The adsorption of NO2 and NH3 on the pristine and Al or Pdoped SWCNTs have been theoretically studied at the DFT level of calculation. The energies of adsorption, Ead , equilibrium distances, Rmin , charge transfer values and energy gap variation as well as DOS diagrams have been calculated and used to interpret the sensing behavior of pristine and Al- or P-doped (8,0) SWCNTs. The results reveal that both doped nanotubes can strongly adsorb NO2 , and that the obtained binding energy for pristine SWCNT is in a good agreement with available experimental data. Due to the adsorption of NO2 , the variations of the electronic properties of SWCNT/Al and SWCNT/P are clearer than that of the pristine nanotube. According to the obtained results, NH3 molecule is weakly adsorbed on both pristine and P-doped SWCNTs. The energy gap variation, the DOS diagram alteration and the amount of charge transfer are also negligible. In contrast, NH3 is strongly adsorbed in the SWCNT/Al and the electronic parameters of the nanotube are considerably changed by gas adsorption. Therefore, SWCNT/Al can be considered and further studied as a sensor for both NO2 and NH3 molecules. References [1] J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [2] S. Peng, K. Cho, Chemical control of nanotube electronics, Nanotechnology 11 (2000) 57–60. [3] S.P. Chen, G. Chen, X.G. Gong, Z.F. Liu, Oxidation of carbon nanotubes by singlet O2 , Physical Review Letters 90 (2003) 086403-1–086403-4. [4] M. Arab, F. Picaud, M. Devel, C. Ramseyer, C. Girardet, Molecular selectivity due to adsorption properties in nanotubes, Physical Review Letters 69 (2004) 165401-1–165401-11. [5] X. Feng, S. Irle, H. Witek, K. Morokuma, R. Vidic, E. Borguet, Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities, Journal of American Chemical Society 127 (2005) 10533–10538. [6] H.J. Liu, C.T. Chan, Z.Y. Liu, J. Shi, Density functional study of oxygen adsorption on 4 A˚ carbon nanotubes, Physical Review B 72 (2005) 075437-1–075437-6. [7] Y. Chen, Y. Li, H. Wang, M. Yang, Gas sensitivity of a composite of multi-walled carbon nanotubes and polypyrrole prepared by vapor phase polymerization, Carbon 45 (2007) 357–363. [8] Y. Zhang, C. Suc, Z. Liu, J.Q. Li, Carbon nanotubes functionalized by NO2 : coexistence of charge transfer and radical transfer, Journal of Physical Chemistry B 110 (2006) 22462–22470. [9] X.J. Huang, Y.K. Choi, Chemical sensors based on nanostructured materials, Sensors and Actuators B 122 (2007) 659–671. [10] M. Penza, P. Aversa, G. Cassano, W. Wlodarski, K. Kalantar-Zadeh, Layered SAW gas sensor with single-walled carbon nanotube-based nanocomposite coating, Sensors and Actuators B 127 (2007) 168–178. [11] A. Sadrzadeh, A.A. Farajian, B.I. Yakobson, Electron transport of nanotube-based gas sensors: an ab initio study, Applied Physics Letters 92 (2008) 0221031–022103-3. [12] W. An, X. Wu, J.L. Yang, X.C. Zeng, Adsorption and surface reactivity on singlewalled boron nitride nanotubes containing stone−wales defects, Journal of Physical Chemistry C 111 (2007) 14105–14112. [13] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science 287 (2000) 1801–1804. [14] S. Peng, K. Cho, Ab initio study of doped carbon nanotube sensors, Nano Letters 3 (2003) 513–517. [15] L. Bai, Z. Zhou, Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors, Carbon 45 (2007) 2105–2110. [16] S. Peng, K. Cho, P. Qi, H. Dai, Ab initio study of CNT NO2 gas sensor, Chemical Physics Letters 387 (2004) 271–276.
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Computational study of Al- or P-doped single-walled carbon nanotubes as NH3 and NO2 sensors Khaled Azizi ∗ , Mohammad Karimpanah Department of Chemistry, University of Kurdistan, Sanandaj, Iran
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 22 July 2013 Accepted 27 July 2013 Available online 20 August 2013 Keywords: Ammonia Nitrogen dioxide Sensor SWCNT DFT
a b s t r a c t Density functional theory (DFT) calculations were carried out to analyze the electronic and structural properties of pristine and aluminum or phosphorus doped (8,0) single walled carbon nanotube (SWCNT) as a sensor for the detection of nitrogen dioxide (NO2 ) and ammonia (NH3 ). The binding energies, equilibrium gas-nanotube distances, the amounts of charge transfer and molecular orbital schemes as well as the density of states have been calculated and used to interpret the mechanism of gas adsorption on the surface of nanotubes. In agreement with the experimental data, our results show considerable binding energy and energy gap alteration due to the adsorption of NO2 on pristine SWCNT. The results reveal that the doping of both Al and P atoms increase the capability of the nanotube for the adsorption of NO2 , and the effect is more significant for the Al-doped nanotube. The Al-doped nanotube can also be considered as a good sensor for NH3 due to its high binding energy, considerable amount of charge transfer and energy band gap alteration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since 2000, Kong et al. [1] have demonstrated experimentally the dramatic effect of very small amounts of NO2 and NH3 gases on the current–voltage (I–V) curves of a SWCNT. Afterwards, many efforts have been devoted to the study of the relationship between gas sensing behavior, on the one hand, and structural and electronic properties of pristine and deformed SWCNTs on the other [2–11]. These studies have focused on two main subjects. First, they have endeavored to provide a detailed molecular-level understanding of the interaction between SWCNTs and gaseous molecules, and second, to improve the gas sensing behavior of single walled nanotubes by mechanical deformation or doping heteroatoms [10,12]. Moderate values of binding energy, considerable amount of charge transfer and sensible energy band gap variation have been mentioned as desirable prerequisites for the selection of adsorbents as electrical gas sensors [13–15]. Based on the recovery time, Peng et al. estimated the binding energy of NO2 and NH3 on SWCNT to be 23 and 2.2 kcal mol−1 , respectively [16]. These estimations are consistent with the values of 27.2 kcal mol−1 for NO2 and 1.8 kcal mol−1 for NH3 , deduced by Ellison et al. using temperature-programmed desorption (TPD) [17]. The low amount of charge transfer between SWCNT and either adsorbed NO2 or NH3 molecules and negligible amount of binding energy of NH3 to SWCNTs were
∗ Corresponding author. Tel.: +98 871 6624133; fax: +98 871 6660075. E-mail address: [email protected] (K. Azizi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.146
currently confirmed by the results of theoretical investigations [2,5,18]. However, a unique perspective on the calculated values of the binding energy of NO2 on carbon nanotubes, could not be understood due to very different reported results (0.9 to 18.6 kcal mol−1 ) [2,16,18–22]. According to Yim et al., the energy of two chemisorbed NO2 on an (8,0) SWCNT is about 20 kcal mol−1 less than the total energy of the nanotube and two free NO2 molecules [20]. They found that the chemisorbed NO2 reduces the nanotube energy band gap, which probably explains the experimentally observed change in I–V curve with NO2 exposure. Using different ab initio approaches, Ricca and Bauschlicher further studied this subject and found that chemisorption energy intensively decreases with the diameter of the nanotube, and therefore chemisorption does not seem to be a reasonable explanation for the change in current with NO2 exposure [23]. Consequently, to date we still do not have a complete understanding of how carbon nanotube sensors work. When an intrinsic carbon nanotube is doped with heteroatoms through a replacement of carbon atoms, the local physical properties around the impurity atoms undergo a significant change, resulting in the change of the local chemical reactivity. The change in the local chemical reactivity alters the binding energy of gas molecules on the doped nanotube [24]. For example, it was revealed that the electronic properties of boron-nitride nanotubes (BNNTs) can be affected by the adsorption of NH3 [12] and there is a considerable different between B and N positions for the adsorption of NH3 [25]. Also, Bai and Zhou have investigated the adsorption of NH3 and NO2 in B- or N-doped (10,0) SWCNTs by
K. Azizi, M. Karimpanah / Applied Surface Science 285P (2013) 102–109
using DFT computations. Their study showed that NH3 can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, but both B- and N-doping make NO2 chemisorption feasible in SWCNTs [15]. More recently, Cruz-Silva et al. have confirmed that the substitution of phosphorus and phosphorus–nitrogen codopants within SWCNTs strongly modifies the chemical properties of the surface, thus creating highly localized sites with specific affinity toward acceptor and donor molecules, respectively [26]. It turned out that the adsorption of both NO2 and NH3 induce the suppression of a conductance dip. NH3 and NO2 gases are the subject of many theoretical and experimental studies related to designing and construction of gas sensors [27–33]. This is due to their importance in industry and air pollution, on one hand, and being good examples of electron donor and acceptor molecules in theoretical investigations, on the other. In this study, the applicability of Al-doped (8,0) SWCNT (SWCNT/Al) as a sensor of NO2 and NH3 gases is investigated and compared with that of pristine and P-doped (8,0) SWCNT (SWCNT/P). Two main goals of this research focus on two major issues. First, we briefly discuss the adsorption of NO2 and NH3 on pristine (8,0) SWCNT. The main focus is on the estimated values of binding energies and the variation of the electronic properties of SWCNT due to gas adsorption. A new interpretation for the considerable alteration of the conductance of pristine SWCNT, due to the adsorption of NO2 , is also put forward. Second, a detailed discussion will be followed in which the effect of Al and P heteroatoms doping on the gas sensing behavior of (8,0) SWCNT with respect to NO2 and NH3 molecules will be presented. In particular, it will be shown that SWCNT/Al adsorbs both NO2 and NH3 molecules with considerable amounts of binding energies, and from this point of view, it is more preferable compared with both pristine and P-doped SWCNT.
2. Computational methods All calculations were performed with the Gaussian 98 quantum chemical package [34]. The geometries of the isolated SWCNTs, NH3 and NO2 moieties and their complexes were fully optimized without any symmetry constraints at the DFT level of computation [35] in conjunction with the B3LYP exchange–correlation functionals [36] and 6-31G(d,p) basis set. In this work we used 6-311G(d,p) basis set along with exchange–correlation functional of Perdew and Wang (PW91) [37], which has been distinguished to be superior to the B3LYP for the estimation of nonbonding interactions [38,39], for the calculation of the energy and electronic properties. Fig. 1 shows a schematic side view of (8,0) SWCNT, which specifies the chemical bonds and angles including heteroatom and pre-optimization locations of the gas molecule on the surface of the nanotube. Different initial configurations of the gas molecule were considered for each nanotube–gas couple. In the first configuration the gas molecule is on the heteroatom (X position in Fig. 1), in the second one the gas molecule is placed on one of the two six-member rings (6MR) specified in Fig. 1 (over A and B positions). It should be noted that in the above mentioned configurations we have the option of approaching the gas molecule on both sides, N atom being either near to or far from the surface of SWCNT. Total density of states (DOS) [40] of pristine and doped (8,0) SWCNTs was visualized by the GaussSum software [41]. The binding energy of gases, Eads , is defined as Eads = [ESWCNT-gas − ESWCNT − Egas ], where ESWCNT-gas , ESWCNT and Egas stand for the energies of the gas adsorbed nanotube, the nanotube, and the gas molecule, respectively. By this definition, Eads < 0 corresponds to exothermic adsorption, which leads to local minima toward dissociation into the nanotube and the gas molecule. The calculated binding energies have been corrected for the basis set superposition error (BSSE). The energy gap variation,
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Fig. 1. A schematic side view of doped (8,0) zigzag SWCNT. X, A and B specify positions on which the gas molecule is placed at the beginning of optimization.
Egap , for any species is defined as Egap = Egap,after − Egap,before , where Egap is the difference between the energies of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO. Finally, the natural bond orbital (NBO) calculations and population analysis were performed to investigate charge transfer and chemical bond formation processes between SWCNTs and gases with or without doped heteroatoms. 3. Results and discussion 3.1. Structural and electronic parameters of pristine and Al- or P-doped (8,0) SWCNTs The structural parameters of Al- and P-doped SWCNTs, which include bond lengths, bond angles, and the protrusion of heteroatom, in good agreement with the previous reported data [42–44], are summarized in Table 1. Protrusion on the surface of doped SWCNT was attributed to the larger atomic radius of heteroatom relative to that of carbon atom [42,45]. Considering the fact that the radius of Al atom is greater than that of P atoms, we expect that the protrusion for SWCNT/Al to be greater than that of SWCNT/P (unlike the data given in Table 1). This can be attributed to the ionic nature of Al C bonds which are stronger than that of their counterpart P C bonds in SWCNT/P. As it can be seen from the data given in Table 2, in both SWCNT/Al and SWCNT/P the hetero atom and ortho-carbon atoms have considerable amounts of positive and negative electrical charge, respectively, while the meta-carbon atoms are negligibly charged. In addition, the negative charge of the para-carbon of SWCNT/P is considerably larger than that of SWCNT/Al. The charge density redistributions depicted in Fig. 2, for both doped nanotubes, confirm an increase in the electron density at ortho and para carbon atoms, as well as an electron density reduction at meta carbon atom. This result is consistent with the electrical
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Table 1 The structural parameters of (8,0) X-SWCNT (X = Al, P). System
SWCNT/Al SWCNT/P a b c
Bond length (Å)
Bond angle (deg)
Protrusion
X-C1
X-C2
X-C3
C1-X-C2
C1-X-C3
C2-X-C3
1.847 (1.822)a 1.787 (1.79)b,c
1.880 (1.886)a 1.836 (1.79)b,c
1.880 (1.887)a 1.836
106.09 98.5 (100.34)b,c
106.07 98.49 (99.81)b,c
106.50 93.68 (94.04)b,c
0.812 0.818
From Ref. [42]. From Ref. [43]. From Ref. [44].
charges of atoms given in Table 2. As we shall see in the next sections, the electrical charges mentioned above, play an important role in the mechanism of the adsorption of NO2 and NH3 molecules on the surface of SWCNTs. 3.2. NO2 adsorption in pristine and doped (8,0) SWCNTs The most stable geometries of NO2 adsorbed on pristine and doped SWCNTs which were optimized at the B3LYP level are shown in Fig. 3. For better vision, optimized structures have been shown from two different sides. A remarkable point in Fig. 3 is the geometry by which NO2 adsorbs on the surface of SWCNT/Al; this is different from that of pristine and P-doped SWCNTs. This is reasonable regarding the repulsion between positively charged Al and N atoms in SWCNT/Al and NO2 moieties, respectively. The values of binding energies, Eads , equilibrium distance between SWCNTs and NO2 , Rmin , the amount of charge transferred between nanotube and gas molecule, CT, and the variation of HOMO–LUMO gap, Egap are listed in Table 3. The binding energy of NO2 on the pristine SWCNT is calculated to be −20.97 kcal mol−1 ˚ Our results are in good and the molecule–tube distance is 2.91 A. agreement with previously estimated values [1,16,18] and are consistent with the available experimental data [17]. The results of the NBO analysis do not confirm the formation of any chemical bonds between SWCNT and NO2 . Consequently, NO2 is almost
physisorbed on the pristine (8,0) SWCNT. As it can be seen in Table 3, the amount of charge transfer between NO2 and the pristine nanotube is negligible, which is in agreement with previous reports. Therefore, the change in the electrical properties of SWCNT during gas adsorption can not be attributed to charge transfer [46]. According to the data given in Table 3, due to the adsorption of NO2 , the HOMO–LUMO gap of SWCNT increases by 0.635 eV, which is insufficient for exerting dramatic changes in electronic properties of SWCNT. The DOS diagrams of SWCNTs before and after the gas adsorption are indicated in Fig. 4. As shown in Fig. 4a, the adsorption of NO2 has considerably changed the shape of the DOS diagram. After the adsorption, the two peaks with moderate intensity (in DOS diagram of pristine SWCNT) around the Fermi level at −4.2 eV and −3.7 eV, respectively, have been disappeared, and instead a single relatively low-intensity peak appears at −3.9 eV. Furthermore, the intensity of states in both valance and conductance bands increased considerably. A close look over the electronic energy states of the system, around the Fermi level, indicates that the LUMO is a single beta state at −3.90 eV and the HOMO, HOMO − 1, LUMO + 1 and LUMO + 2 states are located at −4.96, −4.95, −2.88 and −2.87 eV, respectively. The single beta state corresponds to the little peak appeared at the right side of the Fermi level in Fig. 4a. Hence, while the energy gap for transition (from valance to conduction band) of the first electron is 1.05 eV, the other electrons should traverse a band gap of 2.07 eV energy. Accordingly, it makes sense that the effective energy gap for SWCNT/NO2 system should be considered 2.07 eV rather than 1.05 eV. This drastic variation of the energy gap can be responsible for the dramatic changes in the electronic properties of the nanotube. As it can be seen from Fig. 3b, being in agreement with the previously reported data for N-doped SWCNTs [15], NO2 molecule prefers the top of para carbon atom in the same 6MR. The main axis of SWCNT, C2 symmetry axial of NO2 , P atom and para C atoms of the tube will be placed in the same plane. The results of NBO analysis confirm the formation of a chemical bond between para C atom of SWCNT and N atom of NO2 . It should be noted, that two stable configurations were found for the NO2 adsorbed on SWCNT/P system: either N atom bonded to an ortho C atom or it bonded to a para C atom. The binding energy for adsorption on para C atom is considerably larger than that of ortho C atom, hence the later configuration not being discussed here. The interesting point is that the geometry obtained in our calculation differs from that previously reported results in which NO2 molecule chemically bonded to P atom [26]. According to the electrical charge values given in Table 2, both ortho and para C atoms Table 2 The electrical charge of heteroatom and the carbon atoms placed at the ortho, meta and para positions with respect to the doped atom. System
Fig. 2. Charge density distributions for (a) SWCNT/P and (b) SWCNT/Al nanotubes.
SWCNT/Al SWCNT/P
Type of atom Heteroatom
C (ortho)
C (meta)
C (para)
0.91 0.67
−0.31 −0.35
−0.04 0.02
−0.13 −0.21
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Fig. 3. The most stable configurations for adsorption of NO2 on (a) pristine, (b) P- and (c) Al-doped (8,0) SWCNT.
Fig. 4. Density of states plots for different systems (a) pristine SWCNT, SWCNT-NO2 and SWCNT-NH3 (b) SWCNT/P, SWCNT/P-NO2 and SWCNT/P-NH3 and (c) SWCNT/Al, SWCNT/Al-NO2 and SWCNT/Al-NH3 . The vertical dotted lines indicate the Fermi level.
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Table 3 The structural and electronic parameters of adsorption of NO2 on pristine and Al- or P-doped (8,0) SWCNTs. Type of nanotube
Eads a (kcal mol−1 )
SWCNT SWCNT/P SWCNT/Al
−20.97 −37.09 −50.64
a b c d e
f g
BSSEb (kcal mol−1 ) 0.94 3.96 4.47
Rmin c (Å)
CTd |e|
2.91 1.59 1.82
−0.012 −0.173 −0.530
Egap,before e (eV) 0.429 0.596 0.973
Egap f (eV)
Egap,after (eV) g
1.064 (2.069) 0.401 0.438
0.635 −0.195 −0.535
The binding energy between nanotube and gas molecule. The basis set superposition error. The equilibrium distance between nanotube and gas molecule. The amount of charge transferred between nanotube and gas molecule. The difference between the energies of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, before the adsorption of gas molecule on the surface of nanotube. Egap,after − Egap,before . The effective band gap, the difference between the energies of the LUMO + 1 and HOMO.
are more favorable for NO2 adsorption than meta C atom. Due to the positive electrical charge on P atom, there is a strong repulsive interaction between the N atom of NO2 and the P atom of SWCNT/P. Therefore, contrary to the previous report, our results show that P atom is not preferable for NO2 adsorption. The binding energy of NO2 on SWCNT/P is quite exothermic (−37.09 kcal mol−1 ), with the ˚ It should be noted that, the applicability C N bond length of 1.59 A. of SWCNT/P as NO2 sensor may be limited by both the obtained relatively large amount of binding energy and the chemical bond formation between the gas molecule and the nanotube. However, there is convincing evidence that the amount of binding energy per adsorbed molecule, when gas molecules are absorbed in bulk, is significantly less than that of individually adsorbed molecules [23,25,28,45,47]. Therefore, further studying of SWCNT/P as NO2 sensor is plausible. Since NO2 is an electron acceptor, the amount of (−0.17 |e|) charge is transferred from nanotube (SWCNT/P) to NO2 . A comparison between the electrical charge of the P atom before (0.67 |e|) and after (0.71 |e|) the adsorption of NO2 indicates that the charge transfer is mainly done by the nanotube rather than by the P atom. Therefore, we expect the electronic properties of the nanotube to be seriously affected by the adsorption of NO2 . According to the obtained results, the binding energy of NO2 on SWCNT/P is about twice as much as that of the pristine (8,0) SWCNT and the amount of charge transfer is considerably larger for SWCNT/P. As a result, we conclude that the P-doping makes the para C atom preferable for NO2 chemisorption. According to the data given in Table 3, the adsorption of NO2 slightly reduces the energy gap of SWCNT/P, which cannot be considered as a serious electronic change. A comparison between the DOS diagram of SWCNT/P before and after NO2 adsorption, depicted in Fig. 4b, shows that the number of states in both valance and conduction bands decrease; in particular, a strong band at −4.9 eV disappears. Therefore, we expect the electronic properties of SWCNT/P to be significantly affected by the adsorption of NO2 . Hence the nanotube can be considered for further studying as an NO2 sensor. In the case of Al-doped SWCNTs, NO2 shows strong interaction with Al when O atoms of NO2 point toward the Al atom. The equilibrium distance between NO2 and SWCNT/Al is 1.82 A˚ and there is no chemical bond between Al and O atoms. The distance between the ˚ respecAl atom and the near and far O atoms are 1.92 A˚ and 2.26 A, tively. The C2 symmetry axis of NO2 is perpendicular to the main axis of SWCNT/Al, and all three atoms of NO2 and the main axis of tube are placed in the same plane. According to the electrical charge values given in Table 2, due to the positive charge accumulated on the Al atom and the negative charge on O atoms of NO2 , we expect some strong attractive interactions between the nanotube and the gas molecule. The strong interaction between Al and NO2 leads to highly exothermic adsorption energy (−50.64 kcal mol−1 ). Furthermore, the adsorption of NO2 is accompanied by a considerable
amount of charge transfer (−0.53 |e|) from the tube to NO2 . A comparison between the electrical charge of the Al atom before (0.91 |e|) and after (1.15 |e|) adsorption, indicates that the main contribution to charge transfer is made by the nanotube rather than by the Al atom. Therefore, as in the case of SWCNT/P, the electronic properties of the nanotube are seriously affected by the adsorption of NO2 . The data given in Table 3 indicate that due to the adsorption of NO2 , the energy gap of SWCNT/Al is considerably decreased, and its total variation is remarkably larger than that of SWCNT/P. This amount of variation of energy gap is expected to have a significant impact on the electronic properties of the nanotube. A comparison between the DOS diagrams of SWCNT/Al before and after NO2 adsorption, depicted in Fig. 4, indicates that, near the Fermi level, two new peaks appear close together with sufficient intensity. Furthermore, some of the prominent peaks in the conduction band were disappeared and, a uniform distribution of the peaks in the range of both conduction and valance bands appeared. Overall, we expected that these changes would increase the electrical conductivity of the nanotube. Considering the binding energy, charge transfer, energy gap variation and DOS redistributions, we conclude that SWCNT/Al can be considered as a good candidate for NO2 detection and is superior to both pristine SWCNT and SWCNT/P. As mentioned above, the relatively large amount of binding energy alone cannot be considered as a reason for excluding SWCNT/Al as NO2 sensor. 3.3. NH3 adsorption in pristine and doped (8,0) SWCNTs The most stable optimized geometries of NH3 adsorbed on pristine and doped (8,0) SWCNTs are shown in Fig. 5. The interesting point is that NH3 selects three different orientations with respect to three nanotubes. While it selects two opposite orientations relative to pristine and SWCNT/Al, i.e. either N atom or H atoms pointed to the surface of the tube, respectively, it selects a tilted orientation in which the C3 symmetry axis of NH3 is perpendicular to the plane containing Al and two ortho C atoms, as shown in Fig. 5a. The structural and electronic properties of NH3 adsorbed on pristine and P- or Al-doped (8,0) SWCNTs are given in Table 4. As we can see from Table 4 and Fig. 5, in agreement with previous reports [15,48] the binding energy, charge transfer, energy gap variation as well as DOS diagram rearrangement for the adsorption of NH3 on the pristine SWCNT are too low to be used for any discussion on the sensing properties of this nanotube with respect to NH3 . Except for the slight change in DOS diagram shape, because of band splitting, we find that the DOS of SWCNT-NH3 is very close to that of the pristine nanotube. From Table 4 we can see that, as in the case of pristine SWCNT, due to the weak physisorption, the binding energy of NH3 in SWCNT/P is also considerably small. The charge transfer, energy gap variation, and the modification of DOS shape are also negligible. This is in a good agreement with the previously reported data for the
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Fig. 5. The most stable configurations for adsorption of NH3 on (a) pristine, (b) P- and (c) Al-doped (8,0) SWCNT.
adsorption of NH3 on both N- and P-doped SWCNTs [15,44]. Therefore, we conclude that SWCNT/P is not suitable for NH3 sensing since it does not fulfill any of the previously mentioned criteria. As in the case of pristine SWCNT, the DOS diagrams of NH3 adsorbed SWCNT/P before and after of NH3 adsorption are very close to each other. As a result, the interaction between the SWCNT/P and NH3 molecules is weak and does not have a significant influence on the electronic properties of the nanotube. Based on the above results, the application of SWCNTs as a sensor for NH3 detection is not suggested. In the case of Al-doped SWCNTs, NH3 is attached to Al with the N atom pointing to the tube surface (Fig. 5a). As indicated in Table 4, the binding energy for the adsorption of NH3 in
SWCNT/Al is −37.41 kcal mol−1 . To our knowledge this is one of the largest amounts of binding energy which has been found until now for the physisorption of NH3 on pristine or deformed carbon nanotubes. The highly exothermic adsorption of NH3 on the SWCNT/Al may be attributed to two effects; first, the strong electrostatic interaction between positively charged Al atom and the negatively charged N atom and the second, the strong interaction between the electron-deficient hole on the Al atom and the lone pair of N atom. In addition, the distance between the N ˚ whereas atom of NH3 and the Al atom in SWCNT/Al is 2.04 A, the tube–molecule distances in both pristine and SWCNT/P are ˚ This indicates that NH3 is strongly adsorbed in the about 3.0 A. SWCNT/Al.
Table 4 The structural and electronic parameters of adsorption of NH3 on pristine and Al- or P-doped (8,0) SWCNTs. Type of nanotube
Eads a (kcal mol−1 )
BSSEb (kcal mol−1 )
Rmin c (Å)
CTd (|e|)
Egap,before e (eV)
Egap,after (eV)
Egap f (eV)
SWCNT SWCNT/P SWCNT/Al
−0.33 −0.74 −37.41
0.13 2.12 3.41
3.02 2.95 2.04
0.009 0.048 0.196
0.429 0.596 0.973
0.431 0.653 1.301
−0.001 0.057 0.540
a b c d e
f
The binding energy between nanotube and gas molecule. The basis set superposition error. The equilibrium distance between nanotube and gas molecule. The amount of charge transferred between nanotube and gas molecule. The difference between the energies of the high occupied molecular orbital, HOMO, and the low unoccupied molecular orbital, LUMO, before the adsorption of gas molecule on the surface of nanotube. Egap,after − Egap,before .
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Finally, we can see from Fig. 4 that, unlike pristine and p-doped SWCNTs, the change of DOS diagram near the Fermi level is evident for SWCNT/Al; in particular, the energy gap increases and two peaks with sufficient intensity appear in valance and conduction bands. As it can be seen, the amount of charge transfer between NH3 and nanotube, the energy gap variation and DOS diagram deformation are more evident in the case of SWCNT/Al in comparison with that of the pristine and p-doped SWCNTs. Therefore, we expect that the electronic properties of SWCNT/Al will change considerably upon NH3 exposure and the SWCNT/Al can be considered as a good sensor for NH3 . 4. Conclusion The adsorption of NO2 and NH3 on the pristine and Al or Pdoped SWCNTs have been theoretically studied at the DFT level of calculation. The energies of adsorption, Ead , equilibrium distances, Rmin , charge transfer values and energy gap variation as well as DOS diagrams have been calculated and used to interpret the sensing behavior of pristine and Al- or P-doped (8,0) SWCNTs. The results reveal that both doped nanotubes can strongly adsorb NO2 , and that the obtained binding energy for pristine SWCNT is in a good agreement with available experimental data. Due to the adsorption of NO2 , the variations of the electronic properties of SWCNT/Al and SWCNT/P are clearer than that of the pristine nanotube. According to the obtained results, NH3 molecule is weakly adsorbed on both pristine and P-doped SWCNTs. The energy gap variation, the DOS diagram alteration and the amount of charge transfer are also negligible. In contrast, NH3 is strongly adsorbed in the SWCNT/Al and the electronic parameters of the nanotube are considerably changed by gas adsorption. Therefore, SWCNT/Al can be considered and further studied as a sensor for both NO2 and NH3 molecules. References [1] J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [2] S. Peng, K. Cho, Chemical control of nanotube electronics, Nanotechnology 11 (2000) 57–60. [3] S.P. Chen, G. Chen, X.G. Gong, Z.F. Liu, Oxidation of carbon nanotubes by singlet O2 , Physical Review Letters 90 (2003) 086403-1–086403-4. [4] M. Arab, F. Picaud, M. Devel, C. Ramseyer, C. Girardet, Molecular selectivity due to adsorption properties in nanotubes, Physical Review Letters 69 (2004) 165401-1–165401-11. [5] X. Feng, S. Irle, H. Witek, K. Morokuma, R. Vidic, E. Borguet, Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities, Journal of American Chemical Society 127 (2005) 10533–10538. [6] H.J. Liu, C.T. Chan, Z.Y. Liu, J. Shi, Density functional study of oxygen adsorption on 4 A˚ carbon nanotubes, Physical Review B 72 (2005) 075437-1–075437-6. [7] Y. Chen, Y. Li, H. Wang, M. Yang, Gas sensitivity of a composite of multi-walled carbon nanotubes and polypyrrole prepared by vapor phase polymerization, Carbon 45 (2007) 357–363. [8] Y. Zhang, C. Suc, Z. Liu, J.Q. Li, Carbon nanotubes functionalized by NO2 : coexistence of charge transfer and radical transfer, Journal of Physical Chemistry B 110 (2006) 22462–22470. [9] X.J. Huang, Y.K. Choi, Chemical sensors based on nanostructured materials, Sensors and Actuators B 122 (2007) 659–671. [10] M. Penza, P. Aversa, G. Cassano, W. Wlodarski, K. Kalantar-Zadeh, Layered SAW gas sensor with single-walled carbon nanotube-based nanocomposite coating, Sensors and Actuators B 127 (2007) 168–178. [11] A. Sadrzadeh, A.A. Farajian, B.I. Yakobson, Electron transport of nanotube-based gas sensors: an ab initio study, Applied Physics Letters 92 (2008) 0221031–022103-3. [12] W. An, X. Wu, J.L. Yang, X.C. Zeng, Adsorption and surface reactivity on singlewalled boron nitride nanotubes containing stone−wales defects, Journal of Physical Chemistry C 111 (2007) 14105–14112. [13] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science 287 (2000) 1801–1804. [14] S. Peng, K. Cho, Ab initio study of doped carbon nanotube sensors, Nano Letters 3 (2003) 513–517. [15] L. Bai, Z. Zhou, Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors, Carbon 45 (2007) 2105–2110. [16] S. Peng, K. Cho, P. Qi, H. Dai, Ab initio study of CNT NO2 gas sensor, Chemical Physics Letters 387 (2004) 271–276.
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