Silicon Carbide Nanotubes Serving as a Highly Sensitive Gas

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Silicon Carbide Nanotubes Serving as a Highly Sensitive Gas Chemical Sensor for Formaldehyde Xiao Wang and K. M. Liew* Department of Building and Construction, City University of Hong Kong, Kowloon, Hong Kong SAR ABSTRACT: In order to search for a novel sensor to detect formaldehyde (HCOH), the interaction between HCOH and infinitely long zigzag single-walled silicon carbide nanotubes (SiCNTs) is investigated using density functional theory (DFT). Compared with the weak adsorption on carbon nanotubes, HCOH molecule tends to be chemisorbed to the Si
C bond of SiCNT with appreciable adsorption energy. The study of a second HCOH adsorption on SiCNT further confirms that the polarized Si
C pair is highly sensitive to the HCOH molecule. With the increase of the coverage of the adsorbed HCOH, the band gap of SiCNT is gradually decreased, thus leading to an enhancement of the conductivity property of SiCNT. It is expected that SiCNT could be a promising gas sensor for detecting the HCOH molecule.

1. INTRODUCTION Indoor air pollution is especially related with human health and has recently become a serious environmental issue that needs to be addressed with a sense of urgency.1
3 Among indoor volatile organic compounds, formaldehyde (HCOH) is the most abundant airborne carbonyl chemicals which are well-known to cause headache, nausea, coryza, childhood asthma, and even lung cancer.4,5 Therefore, it is necessary to develop an effective method to sense and remove HCOH. Many methods such as spectrophotometry, polarography, gas chromatography, and fluorometry6,7 have been used to detect HCOH. However, these methods require complicated and expensive instruments and are not sensitive enough as they either have high detection limits or require long sampling intervals. Therefore, gas sensors with high sensitivity to this gas are highly desired. Carbon nanotubes (CNTs) with high aspect ratios, large surface areas, and some unique thermal and electronic properties have been demonstrated to be effective gas chemical sensors for detecting many molecules, such as NH3,8 O2,9 NO2,10 and SO2.11 Such sensors have short response times and high sensitivities to gaseous molecules and are very convenient to use. However, although CNTs show extreme sensitivity to some gaseous molecules, they fail to detect the presence of HCOH due to the weak adsorption. In recent years, boron-doped CNTs have been proposed to detect HCOH molecules using density functional theory (DFT) calculation.12 The electron-rich oxygen atom of HCOH chemically interacts with the electron-scarce boron atom of the doped CNT and induces charge transfer between the nanotube and the adsorbed molecule. Moreover, adsorption of the HCOH molecule on the pristine and silicondoped boron nitride nanotube (BNNT) has also been investigated by the same group.13 Compared with the weak physisorption on the pristine BNNT, silicon-doped BNNT are proposed to be potential candidates for detecting HCOH molecules. In both kinds of doped nanotubes, however, only a very small portion of atoms (B in doped CNT and Si in doped BNNT) is r 2011 American Chemical Society

reactive to HCOH molecules. For B-doped CNT, the maximum concentration of B dopants is around 5%.14,15 A similar concentration limit has also been reported in Si-doped BNNT.16 Hence, for practical applications, nanotubes with large active regions are highly desired for detecting HCOH, as far as the sensitivity is concerned. Silicon carbide nanotubes (SiCNTs), which were first synthesized in 2001,17 have better reactivity than CNTs due to their polar nature.18 Additionally, unlike CNTs, SiCNTs are semiconducting, regardless of chirality, and thus are more suitable for gas sensor application.19 Theoretical calculations have already found that SiCNT can intrinsically be excellent sensors for detecting CO and HCN,17 NO and NNO,20 NO2,21 CO2,22 and molecule O2.23 In this study, we performed first-principles simulation to study the interaction between SiCNT and HCOH. The results show that SiCNTs are highly sensitive to HCOH and could be a potential candidate for serving as a sensor of this kind of gas molecules.

2. COMPUTATIONAL DETAILS The interactions between HCOH and single-walled zigzag SiCNTs were studied by unrestricted spin-polarized density functional theory (DFT) calculations which were performed using the Dmol3 package.24
26 Structure optimizations and the corresponding total energy calculations of the most stable geometries are based on the generalized-gradient approximation (GGA) with the Perdew
Burke
Ernzerhof (PBE) correction,27 and the all-electron calculations and a double numerical basis set plus polarization functional (DNP) are adopted.24 The k-point is set to 1  1  5 for all models, which bring out the convergence criterion of 2  10
6 a.u. on energy and electron density, and that of maximum force criterion of 0.001 Ha/Å. A self-consistent field procedure is carried Received: January 20, 2011 Revised: May 5, 2011 Published: May 12, 2011 10388

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Table 1. Calculated Data for the HCOH
SiCNT Systems configuration

Eb (eV)

D (Å)

qT (e)

1

1.474

1.707


0.150

2

1.422

1.704


0.148

3

0.027

4.497

0.002

4

0.037

2.950

0.009

5

0.026

2.886


0.001

the SiCNT is defined as Eb ¼ EðSiCNTÞ þ EðHCOHÞ
EðSiCNT þ HCOHÞ Figure 1. Structure model and various adsorption sites of HCOH on (8, 0) SiCNT. C, carbon atom; S, silicon atom; BA, axial Si
C bond; and BZ, zigzag Si
C bond.

where E(HCOH þ SiCNT) is the total energy of a HCOH molecule adsorbed on the SiCNT surface and E(SiCNT) and E(HCOH) are the total energies of the SiCNT and a HCOH molecule, respectively. It should be noted that the correction for basis set superposition error (BSSE) is not considered within the calculation of the binding energy because Inada et al. have proven that the numerical basis sets implemented in Dmol3 can minimize or even eliminate BSSE.28 To investigate electronic charge changes through the SiCNT, the charge transfer (qT) between HCOH molecule and the nanotube is calculated using Hirshfeld charge29 analysis, which is defined as the charge difference between the HCOH molecule adsorbed on the nanotube and an isolated HCOH molecule.

3. RESULTS AND DISCUSSION

Figure 2. Optimized structures for the HCOH
SiCNT systems labeled by panels 1
5 with O and H atoms of HCOH molecule close to either Si atom or C atom of SiCNT, respectively.

out with a convergence criterion of 10
6 a.u. on energy and electron density. In this work, a series of zigzag (n, 0) SiCNTs are calculated in a periodically repeating tetragonal supercell with lattice constants of a = b = 40 Å and c taken to be twice the one-dimensional lattice parameter as the studied systems. For example, the structural model of the (8, 0) SiCNT (Figure 1) includes 32 carbon and 32 silicon atoms. The binding energy Eb of a HCOH molecule on

3.1. Adsorption of a Single HCOH Molecule on SiCNT. The stable adsorption geometry of a single HCOH molecule on SiCNTs is first studied. Taking (8, 0) SiCNT as an example, various possible adsorption geometries are investigated, including when O, C, and H atoms of HCOH molecule are close to either the silicon atom (S site) or the carbon atom (C site) of the SiCNT, with the HCOH molecular axis being vertical to the surface of the tube and parallel to the axial Si
C bond (BA) or zigzag Si
C bond (BZ), respectively. After careful structural optimization, five stable adsorption configurations in which the oxygen and hydrogen of HCOH are close to either the S or C site are shown in Figure 2 as panels 1
5. The calculated adsorption energy, charge transfer, and the binding distance (D) (defined as the length between the adsorbed C or Si atom of the tube and the atom of HCOH molecule attacking the tube) are summarized in Table 1. The most stable configurations are panels 1 and 2, the C
O bond of HCOH attacks the Si
C bond of either BA or BZ sites of SiCNT, and C, O atoms of the molecule chemically interact with C, Si atoms of the tube, respectively, forming a four-membered ring (named as cycloaddition type). Configuration panel 1 has an Eb of 1.474 eV, which increased 1.173 eV compared with the pristine CNT system, indicating that the interaction of the HCOH with the SiCNT is much stronger than with the CNT. The interaction distances of C
C and Si
O are 1.591 and 1.707 Å, respectively. Moreover, HCOH adsorption induces a local structural deformation to both the HCOH molecule and the SiCNT. The bond angle of H
C
H and two H
C
O of HCOH are significantly decreased from 116°, 122°, and 122° in free HCOH to 108.5°, 110.4°, and 110.5°, respectively, in the adsorbed form. The HCOHadsorbed Si
C bond is pulled outward from the tube wall with the bond length increasing from 1.771 Å of pristine tube to 1.905 Å. Such structural deformation is attributed to the change from sp2 to 10389

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Figure 3. Electronic density difference isosurfaces for panel 1. The blue region shows electron accumulation, while the yellow region shows electron loss. 3

sp hybridization of Si and C atoms. Similar deformation can also be observed in covalently functionalized CNT,30 BNNT,31 or CO2 chemisorbed SiCNT.22 The deformation of the system structure and the appreciable binding energy suggest that the interaction in this configuration belongs to chemisorptions. The same chemisorption is also found when the HCOH molecule is adsorbed parallel to the BZ site of the SiCNT shown in panel 2, with deformation caused to both the molecule and the tube. The adsorption energy is 1.422 eV, a little weaker than that on the BA site. Moreover, compared with this strong chemical interaction between SiCNT and HCOH, the adsorption energy of CO2 or triplet O2 chemically interacted with the (8, 0) SiCNT seems relatively week that are about 0.57 and 0.34 eV, respectively.22,23 In addition, the Eb values for configurations shown in panels 3, 4, and 5 with O atom close to C atom of SiCNT and H atom close to either C or Si atom of SiCNT are 0.027, 0.037, and 0.026 eV, respectively. These are much smaller than Eb values for chemisorptions in configurations shown in panels 1 and 2, and the adsorption of HCOH in these three configurations does not result in any significant structural distortion, neither in the tube nor in the molecule. The interaction distances between molecule and SiCNT, in turn, are 4.497, 2.950, and 2.886 Å, much larger than in panels 1 and 2. These small Eb values and large interaction distances indicate that the HCOH molecule adsorbs weakly in these configurations due to weaker van der Waals interaction between SiCNT and HCOH. To investigate the changes of electronic structures in SiCNT caused by adsorption of HCOH molecule, electron density difference ΔF, based on panels 1 and 4, is calculated. This illustrates how the charge density changes during this adsorption process. ΔF is defined as ΔF ¼ Ftotal
ðFSiCNT þ FHCOH Þ where Ftotal, FSiCNT, and FHCOH denote electron density of SiCNT adsorbed system, SiCNT, and a HCOH molecule for the adsorbed system, respectively. Figure 3 is the electronic density difference isosurfaces for the most stable configuration panel 1; loss of electrons is indicated in yellow, while electron enrichment is indicated in blue. In this case, the nearest Si and C atoms of SiCNT lose electrons while O and C atoms of the molecule gain electrons. Notably, some charges accumulate around the bond of C
C and Si
O, which confirm the binding between HCOH and SiCNT induced by adsorption. The above

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Figure 4. Electronic density difference isosurfaces for panel 4. The blue region shows electron accumulation, while the yellow region shows electron loss.

Table 2. Calculated Results of a Single HCOH Molecule Adsorbed on Various Zigzag (n, 0) SiCNTs for Model of Panel 1 SiCNT

diameter (Å)

Eb (eV)

D (Å)

(10, 0)

10.088

1.301

1.710

(12, 0)

12.061

1.192

1.711

(16, 0)

16.004

1.068

1.714

(18, 0)

18.013

1.025

1.715

(24, 0)

23.966

0.949

1.716

results are supported by data from the Hirshfeld charge analysis, where charge of O atom increases from
0.195 to
0.242 e and that of C atom accumulates from 0.102 to 0.010 e. As listed in Table 1, about 0.150 electrons are transferred from the tube to the HCOH molecule, which is 7 times more than that in HCOH adsorbed CNTs system12 and, furthermore, 11 times more than that in HCOH adsorbed BNNTs system.13 Also, as an example, we investigate the electron structure of physisorption panel 4; its electron density difference isosurfaces are shown in Figure 4. No charge accumulation could be seen between SiCNT and HCOH. This indicates that the interaction between the adsorbed molecule and the tube is mainly electrostatic in nature. The same distribution of electron density has been found in panels 3 and 5. The Hirshfeld charge analysis of these three cases shows that there are almost no charge transfers between SiCNT and HCOH molecule. All results confirm that physisorption in this adsorbed system is an energetically less favorable adsorption configuration, and the HCOH molecule prefers to bond with Si and C atom as cycloaddition type. In addition to the (8, 0) SiCNT, we also explore adsorption of a single HCOH on other zigzag (n, 0) (n = 10, 12, 16, 18, and 24) SiCNTs. For simplification, only the most stable adsorption configuration (cycloaddition style interacted on BA site of the tubes) is considered. Table 2 summarizes results of adsorption energy and optimized Si
O distance for adsorption of a single HCOH molecule on various zigzag SiCNTs with increasing diameters. For all of the studied tubes, HCOH molecule is stably adsorbed to the tube and structural deformation of both molecule and 10390

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Figure 5. Optimized structures for adsorption of a second HCOH molecule on a (8, 0) SiCNT with bond distances. The adsorption energies are shown at the bottom.

nanotube, induced by HCOH adsorption, is also observed in the optimized system. Additionally, because of the curvature effects, adsorption energies become weaker with increasing diameters of the tubes, though this change is very smooth. For example, adsorption energy on the (8, 0) tube is 1.474 eV, while it decreases a little to 1.301 eV when adsorbed on the (10, 0) tube and when diameter of the tube continually increases to 23.966 Å, Eb slowly decreases to 0.949 eV. On the other hand, the change in the Si
O distance increases gradually from 1.710 to 1.716 Å, following the trend of changes in tube diameters, further testifies that the HCOH
tube interaction becomes much weaker. 3.2. Adsorption of a Second HCOH Molecule on SiCNT. From the calculation results of adsorption of a single HCOH molecule onto SiCNT, it is found that the polarized Si
C pair is highly sensitive to the HCOH molecule and each pair could be a potential chemisorptions site of HCOH. To confirm our conclusion, we further studied the (8, 0) SiCNT adsorbed with two HCOH molecules. Here, on the basis of results of a single HCOH molecule adsorption, we focused only on the most stable configurations (cycloaddition type) for adsorption of a second HCOH molecule on an (8, 0) SiCNT. Although the unpaired electron on the nanotube wall introduced by adsorption of HCOH should be delocalized, it is nonetheless reasonable to assume that it is near the first adsorption by the Si
C pair. Hence, we only consider Si
C pairs in the same six-member ring with the first one. Also, as the nanotube is rolled up, the two zigzag bonds next to the axis bond are not equivalent to each other.32 Figure 5 shows the structural optimization results and the calculated Eb values for the three structures when another HCOH molecule is added to configuration panel 1. It is found that energy of the associated adsorption is strongly site-dependent. The most favorable configurations (labeled as BA-BZ1 and BA-BZ2) have the second molecule located parallel to Si
C bond of BZ site nearest to the first molecule, which is placed above the BA site. The Eb of configuration BA-BZ2 is 1.575 eV, and the Si
C bond of SiCNT was weakened severely by interaction with the second HCOH as bond lengths increased from 1.788 to 1.957 Å. On the other hand, adsorption energy of configuration BA-BA is 1.206 eV, which is much lower than that of configuration BA-BZ2. The energy difference between BA-BA and BA-BZ models is an interesting illustration of the special bonding environment on the SiCNT surface. Part of it could be attributed to the orientation of the HCOH molecule. As described previously for the single HCOH chemisorptions

structure panel 1, the HCOH molecule prefers an orientation with the chemisorptions of the CdO bond parallel to the C
Si bond of SiCNT which bonds with Si and C of the tube using O and C atoms, respectively. Because of the existence of H atom in the molecule, the steric repulsion between two HCOH molecules in the same hexagon may be responsible for the adsorption energy of BA-BA configuration being lower than that of BA-BZ. The other important factor is related to distortion of the hexagonal ring upon chemisorption of two HCOH molecules. One of the Si
C bonds in the hexagonal ring is pushed out due to chemisorption of the first HCOH molecule. Hence, addition of the second HCOH on Si and C atoms near the first one is favorable. Both factors contribute to the higher energy of BA-BZ relative to BA-BA. No doubt 0.152 e is transferred from SiCNT to the second molecule in configuration BA-BZ2, while similar charge transfer (0.147 e) in configuration BA-BA is also observed by Hirshfeld charge analysis. Overall, adsorption of a second HCOH molecule is energetically favored, especially for BA-BZ configurations. 3.3. Electronic Density of State for the Adsorbed Systems. For HCOH detection, HCOH molecule should be absorbed on SiCNTs and electrical conductivity of the tube should induce some change by charge transfer as gas molecules adsorbed on the tube surface. The calculated results have already suggested that HCOH can be chemically absorbed at Si
C bond forming fourmembered ring and draw significant charge from the tube. Now, to better understand the electronic properties of SiCNT systems, electronic densities of states (DOS) for the pure SiCNT and HCOH adsorbed SiCNT systems panels 1 and 4 are calculated and shown in parts a, b, and c of Figure 6, respectively. Compared with DOS of the pure tube, adsorption of the HCOH molecule alters the electronic structure of SiCNT in panel 1, as shown in Figure 6b. For the bare (8, 0) tube, which is a semiconductor, our GGA calculation obtains a band gap of 1.415 eV, compared to 1.21 eV reported by Gali33 using a local spin density approximation. When one HCOH molecule is chemically adsorbed on the tube, the value of the local energy levels occurs near the Fermi levels is enhanced due to the chemical interaction between molecule and the tube and the band gap of the system slightly decreases by 0.082 eV. Since Si atoms of SiCNTs prefer sp3 hybridization, and considering the cooperation effect on Si
C bond, when the electron-rich HCOH molecule interacts with SiCNT, electron densities of both are largely overlapped. Meanwhile, C and Si atoms of the tube hybridize to sp3 hybrid orbital. 10391

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Figure 6. Electronic density of state (DOS) for (a) pure SiCNT; (b) HCOH adsorbed SiCNT system, panel 1; and (c) HCOH adsorbed SiCNT system, panel 4. The labels “HCOH” and “Tube” stand for the projected DOS (PDOS) of the molecule and tube in the adsorbed system, respectively. The Fermi level is set to zero, as indicated by the vertical dotted line.

Figure 7. Schematic illustration for more HCOH molecules adsorption on an (8, 0) SiCNT.

However, unlike configuration panel 1, there is no evidence of hybridization between the HCOH molecule and the tube in panel 4 (see in Figure 6c); molecular orbitals of adsorbates are recognizable as sharp peaks, and the band gap of this configuration remains the same as that of the pure tube. The result shows that SiCNTs remain nearly unaltered by physisorption of the HCOH molecule. To further understand the changes of electronic properties of SiCNT, we also calculate the electronic properties of SiCNT after more HCOH molecules adsorption. For simplicity, from two to eight HCOH molecules are attached to the tube based on configuration BA-BA as shown in Figure 7, and their calculated average adsorption energy, charge transfer per molecule, and band gap are summarized in Table 3. The adsorption energy per HCOH molecule slowly decreases when increasing the number of adsorbed HCOH molecules. For example, in the case of eight molecules adsorbed on the surface of the tube, Eb decreases to 0.886 eV per HCOH compared to that of three molecules

Figure 8. PDOS for (a) eight HCOH adsorbed SiCNT system; (b) the tube in the adsorbed system; and (c) HCOH in the adsorbed system. The Fermi level is set to zero, as indicated by the vertical dotted line.

adsorbed structure (1.090 eV). The diameter of the eight HCOH adsorbed tube is increased from 7.974 to 8.585 Å because of the large structural deformation upon the molecules adsorption. Meanwhile, the band gap of the HCOH-SiCNT system also gradually reduces with the increase of the adsorbed number of HCOH. As showed previously, when one molecule is adsorbed on the tube, the band gap slightly decreases by 0.082 eV. However, when eight HCOH molecules are adsorbed on the tube, some new local energy levels occur within the DOS of SiCNT (see Figure 8) which make the band gap of the tube 10392

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The Journal of Physical Chemistry C dropped to 0.218 eV correlated to the charge transfers (0.137 e per HCOH) from the tube to the adsorbates. The Si
C bonds near the molecule are weakened due to this charge transfer that induces an increased local charge fluctuation around the nanotube. Such fluctuation may have a pronounced effect on the electronic conductivity of SiCNT.34 Thus, by detecting the conductivity change of the SiCNT systems before and after adsorption of HCOH, the presence of this toxic molecule can be detected.

4. CONCLUSION In this paper, we performed the DFT calculations to study the interaction between SiCNT and HCOH molecules; it is found that HCOH can be chemically adsorbed on SiCNTs with appreciable adsorption energies. The HCOH molecule uses its O and C atoms to bond with Si and C atoms, respectively, of the tube, forming a four-membered ring. Significant charges are transferred from SiCNT to HCOH molecules, which lead to changes of conductance of SiCNTs and render this kind of nanotubes suitable for HCOH detection. ’ AUTHOR INFORMATION

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(18) Wu, R. Q.; Yang, M.; Lu, Y. H.; Feng, Y. P.; Huang, Z. G.; Wu, Q. Y. J. Phys. Chem. C 2008, 112, 15985. (19) Zhao, M. W.; Xia, Y. Y.; Li, F.; Zhang, R. Q.; Lee, S.-T. Phys. Rev. B 2005, 71, 085312. (20) Gao, G. H.; Kang, H. S. J. Chem. Theory Comput. 2008, 4, 1690. (21) Gao, G. H.; Park, S. H.; Kang, H. S. Chem. Phys. 2009, 355, 50. (22) Zhao, J. X.; Ding, Y. H. J. Chem. Theory Comput. 2009, 5, 1099. (23) Cao, F. L.; Xu, X. Y.; Ren, W.; Zao, C. Y. J. Phys. Chem. C 2010, 114, 970. (24) Delley, B. J. Chem. Phys. 1990, 92, 508. (25) Delley, B. J. Chem. Phys. 1991, 94, 7245. (26) Delley, B. Int. J. Quantum Chem. 1998, 69, 423. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. B 1996, 77, 3865. (28) Inada, Y.; Orita, H. J. Comput. Chem. 2008, 29, 225. (29) Hirshfeld, F. L. Theor. Chim. Acta 1977, 44, 129. (30) Zhao, J. J.; Park, H.; Han, J.; Lu, J. P. J. Phys. Chem. B 2004, 108, 4227. (31) Wu, X. J.; An, W.; Zeng, X. C. J. Am. Chem. Soc. 2006, 128, 12001. (32) Zhang, Y. F.; Suc, C.; Liu, Z. F.; Li, J. Q. J. Phys. Chem. B 2006, 110, 22462. (33) Gali, A. Phys. Rev. B 2006, 73, 245415. (34) Zhao, J. J.; Buldum, A.; Han, J.; Lu, J. P. Nanotechnology 2002, 13, 195.

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the computing facilities provided by the ACIM of CityU. ’ REFERENCES (1) Ashmorea, M. R.; Dimitroulopouloub, C. Atmos. Environ. 2009, 43, 128. (2) Fiedler, N.; Laumbach, R.; Kelly-McNeil, K.; Lioy, P.; Fan, Z.-H.; Zhang, J.; Ottenweller, J.; Ohman-Strickland, P.; Kipen, H. Environ. Health Perspect. 2005, 113, 1542. (3) Lee, K. J.; Shiratori, N.; Lee, G. H.; Miyawaki, J.; Mochida, I.; Yoon, S.-H.; Jang, J. Carbon 2010, 48, 4248. (4) Cogliano, V. J.; Grossem, Y.; Baan, R. A.; Straif, K.; Secretan, M. B.; El Ghissassi, F. Environ. Health Perspect. 2005, 113, 205. (5) Hauptmann, M.; Lubin, J. H.; Stewart, P. A.; Hayes, R. B.; Blair, A. Am. J. Epidemiol. 2004, 15, 1117. (6) Magada, B.; El-Yazbi, H. Anal. Lett. 1991, 24, 857. (7) Kennedy, E. R.; Hill, R. H., Jr. Anal. Chem. 1982, 54, 1739. (8) Kong, J.; Franklin, N.; Zhou, C.; Chapline, M.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (9) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (10) Li, J.; Lu, Y. J.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 929. (11) Goldoni, A.; Larciprete, R.; Petaccia, L.; Lizzit, S. J. Am. Chem. Soc. 2003, 125, 11329. (12) Wang, R. X.; Zhang, D. J.; Zhang, Y. M.; Liu, C. B. J. Phys. Chem. B 2006, 110, 18267. (13) Wang, R. X.; Zhang, R. X.; Zhang, D. J. Chem. Phys. Lett. 2008, 467, 131. (14) Peng, S.; Cho, K. Nano Lett. 2003, 3, 513. (15) Zhang, Y. M.; Zhang, D. J.; Liu, C. B. J. Phys. Chem. B 2006, 110, 4671. (16) Cho, Y. J.; Kim, C. H.; Kim, H. S.; Park, J. H.; Choi, H. C.; Shin, H. J.; Gao, G. H.; Kang, H. S. Chem. Mater. 2009, 21, 136. (17) Pham, H. C.; Keller, N.; Ehret, G.; Ledouxi, M. J. J. Catal. 2001, 200, 400. 10393

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