J. Phys. Chem. C 2008, 112, 15985–15988
15985
Silicon Carbide Nanotubes As Potential Gas Sensors for CO and HCN Detection R. Q. Wu, M. Yang, Y. H. Lu, and Y. P. Feng* Department of Physics, Faculty of Science, National UniVersity of Singapore, Singapore 117542
Z. G. Huang and Q. Y. Wu Department of Physics, Fujian Normal UniVersity, Fuzhou 350007, China ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: August 9, 2008
Semiconducting carbon nanotubes (CNTs) have demonstrated extreme sensitivity to molecules such as NH3, NO, NO2, ans so forth. Yet, intrinsic CNTs cannot be used to detect some highly toxic molecules such as CO and HCN. In this article, we examine the possibility of silicon carbide nanotubes (SiCNTs) as a potential gas sensor for CO and HCN detection by first-principles calculations based on density functional theory (DFT). It is found that CO and HCN molecules can be absorbed to Si atoms on the wall of SiCNTs with binding energies as high as 0.70 eV and can attract finite charge from SiCNTs. By comparison to oxygen absorption on CNTs, we infer that molecular CO and HCN absorbed on SiCNTs can induce significant change in the conductivity of SiCNTs. In view of the high portion of the reactive area, SiCNTs can be potential efficient gas sensors for CO and HCN detection. I. Introduction Due to a wide variety of potential applications, carbon nanotubes (CNTs) have been widely studied since their discovery.1 CNTs as chemical gas sensors have attracted much interest due to their superior sensitivity in chemical gas detection. Experimentally, it was demonstrated that CNTs have a faster response and lower detection limit than existing solidstate compartments to gaseous molecules such as NO2, NH3, and O2.2,3 This superior sensitivity has been theoretically explained in terms of charge transfer between adsorbed molecules and CNTs, which can dramatically influence the electrical conductivity of the latter (by modifying the electronic structure of CNTs).4,5 Another important feature of CNT gas sensors is that a much larger portion of atoms of CNTs are exposed to and have charge transfer with gaseous molecules compared to their solid-state counterparts. Chemical gases such as CO and HCN are highly toxic to human beings and animals as they inhibit the consumption of oxygen by body tissues. They are colorless, odorless, and tasteless, and thus, human beings do not have timely alertness to their presence. Therefore, gas sensors with high sensitivity to these two gases are highly desired. While CNT gas sensors show extreme sensitivity to some gaseous molecules, they fail to detect the presence of these two toxic gases. The reason is due to very weak adsorption of CO and HCN molecules on CNTs and subsequent insufficient charge transfer to affect the electrical conductivity of CNTs. To overcome the insensitivity of CNTs to CO and HCN molecules, boron-doped CNTs have been proposed for both CO and HCN detections.6,7 The boron (B) dopants act as absorption sites for CO and HCN molecules and induce charge transfer between the nanotube and the absorbed molecules, making the doped CNTs capable of sensing CO and HCN. Highly deformed CNTs have also been proposed for CO detection as CO molecules can be absorbed to regions of CNTs with large curvatures.8 These two proposals represent * To whom correspondence should be addressed. E-mail: phyfyp@ nus.edu.sg.
significant improvements in detecting CO and HCN gases using CNTs. However, in both proposed methods, the sensitivity that CNTs can provide is not fully achieved as only a very small portion of atoms on the CNTs is reactive to CO and HCN molecules since the maximum concentration of B dopants that can be reached in CNTs is around 5% in the first approach and molecules can only be absorbed to highly deformed regions of the CNTs in the second method. For practical applications, nanotubes with a large active region are highly desired for detecting CO and HCN as far as the sensitivity is concerned. Silicon carbide nanotubes (SiCNTs) have been experimentally synthesized.9 Theoretical studies have shown that SiCNTs have better reactivity than CNTs due to their polar nature. For example, SiCNTs show better hydrogen storage performance than CNTs10 in that hydrogen molecules can bind to the side walls of SiCNTs with larger binding energies than those of CNTs. Unlike CNTs, SiCNTs are semiconducting regardless of chirality and thus are intrinsically suitable for gas sensor application. It would be interesting and important to find out whether CO and HCN molecules can be absorbed on intrinsic SiCNTs and whether there would be sufficient charge transfer between the nanotube and the molecule to make SiCNTs excellent sensors for detecting CO and HCN gases. Therefore, the objective of this article is to examine, by first-principles calculations, the possibility of SiCNTs as gas sensors to detect CO and HCN gases. II. Methodology All of the calculations are performed using VASP code11,12 based on density functional theory (DFT). A supercell consisting of two basic units of a zigzag (8,0) SiCNT in the direction of tube axis and lateral separation of 20 Å between tube centers is used. The projector augmented wave (PAW) potentials are used to represent the interactions between valence electrons and cores,13 and the local density approximation (LDA) is adopted for the exchange-correlation functional. The electron wave functions are expanded in plane waves up to a cutoff energy of
10.1021/jp804727c CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
15986 J. Phys. Chem. C, Vol. 112, No. 41, 2008
Wu et al.
Figure 1. Absorption sites (C, h, and Si) of CO/HCN molecule on the (8,0) SiCNT. The molecular axis is initially normal to the tube wall, and the distance between the binding site and the closest atom of the molecule is set to 3.0 Å.
TABLE 1: Calculated Binding Energy (Eb), Distance between the Molecule and the Tube Wall (D, defined as the distance between the binding site and the closest atom of the molecule), and Charge Transfer from the Nanotube to the Molecule (ET) for Molecular CO Adsorption on The (8,0)SiCNT site
configuration
Eb (eV)
D (Å)
ET (e)
C
CO OC COa OC CO OC
-0.14 -0.08 -0.69 -0.13 -0.74 -0.13
3.60 3.21 1.94 2.74 1.93 2.70
0.01 0.01 0.20 0.02 0.20 0.02
h Si
TABLE 2: Calculated Binding Energy (Eb), Distance between the Molecule and the Tube Wall (D), and Charge Transfer from the Nanotube to the Molecule (ET) for Molecular HCN Adsorbed on SiCNT site
configuration
Eb (eV)
D (Å)
ET (e)
C
HCN NCHa HCN NCH HCN NCH
-0.24 -0.75 -0.17 -0.76 -0.19* -0.77*
2.22 1.90 2.34 1.92 2.85 1.90
0.04 0.09 0.04 0.08 0.03 0.08
h Si
a The optimized configuration is shown in Figure 2a, and D is the distance from the O atom to the Si atom.
450 eV, and a 1 × 1 × 6 K mesh within the Monkhost-Pack scheme14 is used for Brilloiun Zone sampling. Ionic relaxation is performed using the conjugate gradient method.15 These parameters ensure a total energy convergence better than 1 meV. The bind energy Eb of a molecule on the SiCNT is defined as
Eb ) E(SiCNT + Mol) - E(SiCNT) - E(Mol)
Figure 2. Bond-and-stick models of the two stable configurations of the CO molecule adsorbed at the atop Si site on the (8,0) SiCNT. Balls in gray, yellow, and red are for carbon, silicon, and oxygen atoms, respectively.
(1)
where E(SiCNT + Mol) is the total energy of the supercell containing the adsorbed molecule, E(SiCNT) is the total energy of the SiCNT, and E(Mol) is the total energy of the free molecule. All energies are calculated from relaxed structures with the maximum force, being less than 0.04 eV/Å. The charge transfer between SiCNT and absorbed molecules is analyzed based on Bader decomposition of the charge density of the system.16 It should be noted that calculation of the charge on atoms is quite difficult. However, the charge transfer between the molecule and the nanotube is calculated from the charge difference of each atom before and after molecule absorption, which should be independent of the method used, although, in practice, some minor deviations may exist. III. Results and Discussion Three absorption sites, that is, atop a carbon atom (C), at the center of the hexagon (h), and atop a silicon atom (Si) on the (8,0) SiCNT, as depicted in Figure 1, are considered. The CO molecule is placed at the respective site initially with its axis normal to the tube surface. For each of the above absorption sites, two configurations are considered, with either C (referred as the CO configuration) or O (referred as the OC configuration) closer to the tube wall. Each structure is fully relaxed. The calculated adsorption energy, charge transfer, and distance between the adsorped molecule and the tube wall are summarized in Table 1. We can see that the CO molecule cannot be or will only be weakly adsorbed on the atop C site with Eb
a The optimized configuration is shown in Figure 3a, and D is the distance from the N atom to the Si atom.
e 0.14 eV. The site-molecule distance is larger than 3.20 Å, and there is no apparent charge transfer between the molecule and the tube. This indicates that the absorption is physically electrostatic and it is not strong enough to prevent desorption at room temperature. For the h site, the absorption in the OC configuration is also very weak, with little charge transfer. Upon structural optimization, the CO configuration at the h site eventually converges to the atop Si site with the C atom of CO binding to a Si atom in the nanotube, as shown in Figure 2a. This absorption configuration has an Eb of -0.69 eV and an unambiguous charge transfer of 0.20 e from the tube to the molecule. For the atop Si site, molecular CO can be absorbed in the CO configuration, while the OC configuration has a very weak absorption. The Eb for the CO configuration is -0.74 eV, with a charge transfer of 0.20 e. The two most stable configurations for CO adsorption on SiCNT are shown in Figure 2. It is noted that the orientation of the CO molecule is slightly different in the two configurations. Both configurations are characterized by large binding energies and significant charge transfer between the SiCNT and CO. The same three absorption sites are considered for the HCN molecule. Similarly, in each case, we consider two configurations, the H atom in the HCN molecule (denoted by HCN) or the N atom (denoted by NCH) being closer to the binding sites. Other absorption configurations are possible, but they will not compromise our conclusions. Results of our calculations are summarized in Table 2, and the final optimized configurations are shown in Figure 3. We find that the HCN configuration is more energetically unfavored than the NCH configuration for each adsorption site. The binding energies Eb are less than 0.25 eV, and there is little charge transfer from the tube to the molecule. The NCH configuration at the atop C site is unstable and eventually converges to the atop Si site (Figure 3a), with Eb ) -0.75 eV. Similarly, for adsorption at the hole site in the
Silicon Carbide Nanotubes As Potential Gas Sensors
J. Phys. Chem. C, Vol. 112, No. 41, 2008 15987 tubes to influence the electrical conductivity of the SiCNTs. Our results suggest that both CO and HCN molecules can be absorbed at the Si sites (Eb g 0.7 eV) on the wall of SiCNTs and draw significant charges from the SiCNTs (Q g 0.08 e). That CO and HCN molecules can be absorbed on SiCNTs while not on CNTs might originate from the great polarity of SiCNTs as charge analysis indicates that in SiCNTs, each Si atom donates 2.4 e to a C atom. These binding energies are large enough to prevent spontaneous desorption at room temperature. The CO molecule is absorbed on SiCNTs with a charge transfer of around 0.2 e from the nanotube to the molecule, while the HCN molecule is absorbed on SiCNTs with a charge transfer of around 0.08 e. Since the inverse charge transfer is not observed, the conductivity change does not suffer from compensation. These two factors make SiCNTs suitable for CO and HCN detection. It is difficult to give a direct evaluation on the conductivity change of SiCNTs induced by CO and HCN molecule absorption from first-principles calculations. However, we can compare our results to those of well-established cases. The experimental study has shown that exposure of CNTs to dried air or oxygen causes dramatic change in their electrical conductivity.3 Calculations based on DFT in the LDA scheme suggest that the oxygen molecule can bind to the (8,0) CNT with Eb ) 0.25 eV and a charge transfer of ∼0.1 e from CNTs to oxygen, which is responsible for the conductivity change of CNTs.17 In the absorption of CO and HCN molecules at the Si site of SiCNTs, the calculated binding energies and charge transfers are significantly larger than or comparable to those of oxygen adsorption on CNTs. Thus, in principle, exposure of SiCNTs to CO and HCN gases should also induce a similar effect on the electrical conductivity of SiCNTs. In our calculations, the same exchangecorrelation functional LDA is used as that in ref 17, and the radius of the (8,0) SiCNT is larger than that of the (8,0) CNT; thus, the calculated larger binding energies and charge transfer do not originate from the difference in the exchange-correlation functional and the spurious curvature effect. The comparison to the interaction between oxygen and (8,0) CNTs is valid, and the conclusions should hold. In combination with the fact that half-lattice sites on the wall of SiCNTs are reactive to CO and HCN molecules, SiCNTs can be expected to be an excellent gas sensor for CO and HCN detection.
Figure 3. Ball-and-stick models for the three stable configurations for HCN absorbed on the (8,0) SiCNT. The ball in white is the hydrogen atom, and the blue is the nitrogen atom. Balls in gray, yellow, and red are for carbon, silicon, and oxygen atoms, respectively.
NCH configuration, the HCN molecule relaxes toward the atop Si site (Figure 3b). The final structure has an Eb of -0.76 eV. Finally, adsorption at the atop Si site also favors the NCH configuration, with an Eb of -0.77 eV. For the three most stable configurations, the charge transfer is around 0.08 e, and the molecule-tube distance is less than 2.0 Å. There are only very minor differences such as the tilt of the molecule axis between the three configurations. For each stable configuration, the tube donates electrons to the molecule, while the inverse is not observed for all configurations considered. Now, we discuss the potential application of SiCNTs for CO and HCN detection based on our results. We find that Si atoms (occuping half of the lattice sites on the wall of SiCNTs) are reactive to CO and HCN. For CO and HCN detection, CO and HCN molecules should be absorbed on SiCNTs and have sufficient charge transfer with the nano-
IV. Conclusions To conclude, we have performed DFT-LDA calculations to study the adsorption of CO and HCN molecules on SiCNTs. Our results suggest that CO and HCN can be absorbed on SiCNTs at Si lattice sites, both with significant binding energies and charge transfer, which could induce significant change in the electrical conductivity of SiCNTs. Thus, SiCNTs are a promising candidate for CO and HCN detection. Acknowledgment. Z. G. Huang and Q. Y. Wu acknowledge NSF support of Grant No. 60676055 and the National Key Project for Basic Research of China under Grant No. 2005CB623605. References and Notes (1) Lijima, S. Nature 1991, 354, 56. (2) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622.
15988 J. Phys. Chem. C, Vol. 112, No. 41, 2008 (3) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettle, A. Science 2000, 287, 1801. (4) Jhi, S. H.; Louie, S. G.; Cohen, M. L. Phys. ReV. Lett. 2000, 85, 1710. (5) Zhao, J. J.; Buldam, A.; Han, J.; Lu, J. P. Nanotechnology 2002, 13, 195. (6) Peng, S.; Cho, K. Nano Lett. 2003, 3, 513. (7) Zhang, Y. M.; Zhang, D. J.; Liu, C. B. J. Phys. Chem. B 2006, 110, 4671. (8) da Silva, L. B.; Fagan, S. B.; Mota, R. Nano Lett. 2004, 4, 65. (9) Sun, X. H.; Li, C. P.; Wong, W. K.; Wong, N. B.; Lee, C. S.; Lee, S. T.; Teo, B. T. J. Am. Chem. Soc. 2002, 124, 14464. (10) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. P.; Samios, J. Nano Lett. 2006, 6, 1581.
Wu et al. (11) (a) Kresse, G.; Hafner, H. Phys. ReV. B 1993, 47, 558. (b) Kresse, G.; Hafner, H. Phys. ReV. B 1994, 48, 13115. (12) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (13) Blo¨chl, P. E. Phys. ReV. B 1994, 24, 17953. (14) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (15) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. New Numerical Recipes; Cambridge University Press: New York, 1986. (16) Henkenman, G.; Arnaldsson, A.; Jo´nsson, H. Comput. Mater. Sci. 2006, 36, 354. (17) Jhi, S. H.; Louie, S. G.; Cohen, M. L. Phy. ReV. Lett. 2000, 85, 1710.
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