J. Phys. Chem. B 2006, 110, 4671-4674
4671
Novel Chemical Sensor for Cyanides: Boron-Doped Carbon Nanotubes Yuemei Zhang, Dongju Zhang, and Chengbu Liu* School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: January 12, 2006
Boron-doped (B-doped) single-walled (8, 0) carbon nanotubes (SWCNTs) are investigated using density functional theory (DFT) calculations as sensor models to detect the presence of cyanides, such as hydrogen cyanide (HCN) and cyanogen chloride (CNCl). Comparing the results of the intrinsic SWCNTs with HCN and CNCl, we discover that B-doped SWCNTs present a high sensitivity to the gaseous cyanide molecules, which is indicated by optimized geometry and electronic properties of these systems. On the basis of calculated results, we call attention to the fact that B-doped SWCNTs would be potential candidates for the detection of gaseous cyanide molecules. The present results provide guidance to experimental scientists in developing CNT-based chemical sensors.
The viability of single-walled carbon nanotubes (SWCNTs) as sensors has been demonstrated for detecting many molecules, such as NH3,1 NO2,1 and O2,2 by determining the sensitive conductance changes of nanotubes before and after the absorption of those molecules. Such sensors are very convenient for usage3 due to their short response times and high sensitivities to gaseous molecules. However, these devices cannot detect molecules which do not bind to or adsorb weakly on the surface of intrinsic SWCNTs,4 such as toxic molecules (e.g., carbon monoxide and cyanide), water molecules, and biomolecules.4 To overcome this problem, recent interesting propositions have been presented that are based on functionalized SWCNTs using radial deformation,5,6 covalent or noncovalent functionalization on the surface of carbon nanotubes,7-9 and substitutional doping of impurity atoms into SWCNTs.4,10,11 In particular, Peng et al.4 have substantiated a high sensitivity of boron-doped (Bdoped) SWCNTs for carbon monoxide and water. Their results revealed that B-doped SWCNTs can act as either electron acceptors or electron donors upon exposure of different molecules. Cyanides, which are rich in electrons, are highly lethal to man and animals, since they inhibit the consumption of oxygen by the bodily tissue.12 Consequently, the importance of developing sensitive responding cyanide sensors is apparent. Existing sensors are usually used for the detection of cyanide anions in solution.12-14 It is known that cyanides, such as hydrogen cyanide (HCN) and cyanogen chloride (CNCl), are low boiling point compounds and are volatile. Thus, it is important to develop sensitive sensors to detect the presence of gaseous cyanides. To this end, we here propose a novel and potential chemical sensor based on B-doped SWCNTs. It was reported that the boron concentrations of individual doped nanotubes were of the order of 1-5%, as determined from analytical transmission electron microscopy,15 and those of bulk B-doped-SWCNT materials were between 10 and 25%.16-20 Here, we model B-doped SWCNTs with different doping levels by substituting 1, 2, 4, 8, and 16 carbon atoms in the intrinsic CNTs, respectively. * Corresponding author. E-mail:
[email protected].
Figure 1. A semiconducting (8, 0) carbon nanotube, where the carbon atoms in sites 1 and 2 are replaced by a boron atom to model two kinds of symmetry for B-doped SWCNTs, respectively.
We perform density functional theory (DFT) calculations on (8, 0) intrinsic- and B-doped-SWCNT systems with and without a cyanide molecule, using Perdew and Wang’s local density approximation (LDA)21 with a double numerical basis set including the d polarization function (DND). The periodic boundary condition is used with a super cell of 20 × 20 × 8.439 Å3, which includes 64 atoms in the nanotube (64 carbon atoms for the intrinsic SWCNT and 63 carbon atoms and 1 boron atom for the B-doped SWCNT) with and without a cyanide molecule, respectively. The Brillouin zone sampling is approximated by two k-points along the tube axis, which is shown to be a good approximation for (8, 0) carbon nanotubes.22 We consider two kinds of symmetries for the one-boron-atomdoped (8, 0) SWCNT, which is marked by numerals 1 and 2 in Figure 1. However, from calculated results, we find that no obvious difference of either the geometry or electronic properties is observed for cyanide absorption on two kinds of the B-doped SWCNTs. We first calculate the binding energy (Eb) and charge transfer (QT) between a cyanide molecule and a SWCNT. To evaluate the changes of the electronic properties of these intrinsic- and doped-SWCNT systems, we also determine their electronic densities of states (DOSs). Eb is defined as
Eb ) E(cyanide+SWCNT) - E(SWCNT) - E(cyanide) where E(cyanide+SWCNT) is the total energy of a cyanide molecule adsorbed on the intrinsic- or B-doped-SWCNT surface and E(SWCNT) and E(molecule) are the total energies of the
10.1021/jp0602272 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/14/2006
4672 J. Phys. Chem. B, Vol. 110, No. 10, 2006
Zhang et al.
Figure 2. Optimized configurations for cyanide on the intrinsic and B-doped SWCNTs. Panels A1, A2, and A3, where the SWCNT is close to the N, C, and H atom of a HCN molecule, respectively, describe the HCN-SWCNT system; panels B1, B2, and B3 describe the corresponding HCN-B-doped-SWCNT system; and panels A4, A5, and A6 and panels B4, B5, and B6 describe the CNCl-SWCNT and CNCl-B-dopedSWCNT systems, respectively.
TABLE 1: Calculated Data for the Absorption of Cyanide on the Intrinsic and B-Doped SWCNTs
HCN-SWCNT
CNCl-SWCNT
HCN-B-doped SWCNT
CNCl-B-doped SWCNT
configna
Egb (eV)
Eb (eV)
D (Å)
QTe (el)
A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6
0.58c 0.57 0.58 0.57 0.58c 0.59 0.58 0.58 0.39d 0.23 0.09 0.40 0.39d 0.12 0.09 0.09
-0.17 -0.16 -0.16 -0.35 -0.40 -0.37 -0.78 -0.26 -0.23 -0.74 -0.32 -0.23
2.98 2.95 2.24 2.97 3.02 2.90 1.53 2.75 2.24 1.53 2.84 2.86
0.01 -0.02 -0.05 0.02 0.01 -0.01 0.34 -0.06 0.005 0.37 -0.004 0.03
a All of the configurations are given in Figure 1. b HOMO-LUMO band gap. c HOMO-LUMO band gap for intrinsic SWCNTs. d HOMOLUMO band gap for B-doped SWCNTs. e Electron charge transfer from cyanide molecules to nanotubes.
intrinsic or B-doped SWCNT and a cyanide molecule, respectively. QT is defined as the charge difference between the cyanide molecule adsorbed on the nanotube and an isolated cyanide molecule.4 For both the HCN- and CNCl-SWCNT systems, we obtain three stable configurations, in which the nanotube is close to the nitrogen, carbon, and hydrogen (or chlorine) atom, as shown by panels A1, A2, and A3 and A4, A5, and A6 in Figure 2, respectively. The calculated Eb, QT, and interaction distance (D) between the SWCNT and the cyanide molecule are shown in Table 1, where we also give the energy gap (Eg) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In good agreement with previous calculations,4 the intrinsic (8, 0) SWCNT is found to be semiconducting with a gap of 0.58 eV, which is comparable to the value in the literature 0.56 eV.4 This fact indicates that the method used in the present calculations is accurate enough for describing the behavior of SWCNTs. From Table 1, we see
that the Eb values for cyanide-SWCNT systems are very small while the D values are large, indicating that the cyanides undergo physical absorption on intrinsic SWCNTs. This observation is consistent with the corresponding small QT values, suggesting that the interaction between the cyanides and nanotubes is intrinsically electrostatic. As a carbon atom in the SWCNT is substituted by a boron atom in a super cell, we discover that the properties of the nanotubes have dramatic changes. As seen in Table 1, compared with the cyanide-SWCNT systems, the Eg and D values of the cyanide-B-doped-SWCNT systems remarkably decrease, while the corresponding Eb and QT values considerably increase. All of these changes conformably recommend that the interaction between the cyanide molecules and B-doped SWCNT is chemisorption, in contrast with the physisorption on the intrinsic SWCNT. This fact reveals that B-doped SWCNTs are sensitive to the presence of HCN and CNCl molecules. For both the HCN- and CNCl-B-doped-SWCNT systems, we obtain three stable configurations: for the former, the boron atom of the nanotube is close to the nitrogen, carbon, and hydrogen atom, respectively, as shown by panels B1, B2, and B3 in Figure 2, while, for the latter, the boron atom of the nanotube is close to the nitrogen, carbon, and chlorine atom, respectively, as shown by panels B4, B5, and B6 in Figure 2. From Table 1, we find that each of the Eb values for the cyanide-B-doped-SWCNT systems is much higher than those of the corresponding cyanide-SWCNT systems, indicating that B-doped SWCNTs are energetically much more favored for cyanide absorption than intrinsic SWCNTs. Note that the most stable configurations for HCN- and CNCl-B-doped-SWCNT systems are B1 and B4, respectively, where the boron atom of the nanotube is close to the nitrogen atom of a cyanide molecule. This is attributed to strong interaction between the electron-rich nitrogen atom in cyanide and the electron-scarce boron atom in the doped SWCNT. Figure 3 shows the calculated DOSs for the intrinsic-SWCNT systems and B-doped-SWCNT systems with and without one
Novel Chemical Sensor for Cyanides
J. Phys. Chem. B, Vol. 110, No. 10, 2006 4673
Figure 3. Calculated electronic densities of states (DOSs) for the intrinsic SWCNT (panel I), HCN-SWCNT (panel II), CNCl-SWCNT (panel III), B-doped SWCNT (panel IV), HCN-B-doped-SWCNT (panel V), and CNCl-B-doped-SWCNT (panel VI).
cyanide molecule. Comparing with the intrinsic SWCNT, we find out that the DOSs of cyanide-SWCNT systems near the Fermi level show rare changes (cf. panel I and panels II and III), which would not lead to a conductance change of the nanotubes upon absorption of the cyanide molecules; hence, we conclude that intrinsic SWCNTs cannot serve as sensors for cyanide molecules. For the B-doped SWCNT, we learn that the band gaps near the Fermi level disappear after absorption of a HCN or CNCl molecule (cf. panel IV and panels V and VI), indicating the nanotube has become conductive. From an electronic view, the electronic structure of the SWCNT contains electronic holes after doping boron atoms, which is responsible for generating a p-type semiconductor. When it interacts with an electron-rich HCN or CNCl molecule, a large charge transfer from the cyanide molecules to the B-doped SWCNT occurs, which dramatically changes the conductance of the nanotube and makes the semiconducting B-doped SWCNT a conductor. Thus, by detecting the conductivity change of the B-dopedSWCNT systems before and after the absorption of cyanides, the presence of these toxic molecules can be detected. Thus, we suggest that B-doped SWCNTs would be promising candidates for serving as effective sensors to detect the presence of cyanide molecules. To determine the effect of boron doping level on the sensitivity of CNTs to cyanides, we investigate HCN-B-dopedSWCNT systems with different doping levels. The results in Figure 4 clearly show that Eb increases with the boron
Figure 4. Variety of bonding energies for HCN-B-doped-SWCNT systems with different doping levels. The two values in parentheses near the data points denote the boron concentration and binding energy, respectively.
concentration thickening, indicating that CNTs would become more sensitive to cyanides at high doping levels. In summary, we have performed a DFT study on the sensitivity of B-doped (8, 0) SWCNTs to the presence of gaseous cyanide molecules. From the calculated results, we conclude that the properties of the B-doped SWCNTs present dramatic changes after the absorption of cyanide molecules; hence, we propose that B-doped SWCNTs can serve as good sensors for detecting the presence of cyanide molecules. We
4674 J. Phys. Chem. B, Vol. 110, No. 10, 2006 believe that the present results will provide useful guidance to develop novel CNT-based sensors for the detection of cyanide molecules. Acknowledgment. We acknowledge the Major State Basic Research Development Programs (no. 2004CB719902) and the National Natural Science Foundation of China (no. 20473047 and no. 20373033) for financial support. We thank the High Performance Computational Center of Shandong University for providing computer resources. References and Notes (1) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (2) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (3) Ciraci, S.; Dag, S.; Yildirim, T.; Gu¨lseren, G.; Senger, R. T. J. Phys.: Condens. Matter 2004, 16, R901. (4) Peng, S.; Cho, K. Nano Lett. 2003, 3, 513. (5) Fagan, S. B.; Silva, L. B.; Mota, R. Nano Lett. 2003, 3, 289. (6) Silva, L. B.; Fagan, S. B.; Mota, R. Nano Lett. 2004, 4, 65. (7) Pan, H.; Feng, Y. P.; Lin, J. Y. Phys. ReV. B 2004, 70, 245425. (8) Bekyarova, E.; Davis, M.; Burch, T.; Zhao, B.; Sunshine S.; Haddon, R. C. J. Phys. Chem. B 2004, 108, 19717.
Zhang et al. (9) Zhao, J.; Lu, J. Appl. Phys. Lett. 2003, 82, 3746. (10) Zhou, Z.; Gao, X.; Yan, J.; Song, D.; Morinaga, M. J. Phys. Chem. B 2004, 108, 9023. (11) Villalpando-Paez, F.; Romero, A. H.; Munoz-Sandoval, E.; Martinez, L. M.; Terrones, H.; Terrones, M. Chem. Phys. Lett. 2004, 386, 137. (12) Mirmohseni, A.; Alipour, A. Sens. Actuators, B 2002, 84, 245. (13) Pavel, A.; Daniel, S. T.; Karolina, J.; Felix, N. C. J. Am. Chem. Soc. 2002, 124, 6232. (14) Massimiliano, T.; Francisco, M. R. Org. Lett. 2005, 7, 4633. (15) Carroll, D. L.; Redlich, Ph.; Blase, X.; Charlier, J.-C.; Curran, S.; Ajayan, P. M.; Roth, S.; Ru¨hle, M. Phys. ReV. Lett. 1998, 81, 2332. (16) Borowiak-Palen, E.; Pichler, T.; Fuentes, G. G.; Graff, A.; Kalenczuk, R. J.; Knupfer, M.; Fink, J. Chem. Phys. Lett. 2003, 378, 516. (17) Fuentes, G. G.; Borowiak-Palen, E.; Knupfer, M.; Pichler, T.; Fink, J. Phys. ReV. B 2004, 69, 245403. (18) Borowiak-Palen, E.; Pichler, T.; Graff, A.; Kalenczuk, R. J.; Knupfer, M.; Fink, J. Carbon 2004, 42, 1123. (19) Lammert, P. E.; Crespi, V. H.; Rubio, A. Phys. ReV. Lett. 2001, 87, 136402. (20) Golberg, D.; Bando, Y.; Han, W.; Kurashima, K.; Sato, T. Chem. Phys. Lett. 1999, 308, 337. (21) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (22) Srivastva, D.; Menon, M.; Cho, K. Phys. ReV. Lett. 1999, 83, 2973.
