Theoretical Study of O2 Molecular Adsorption and Dissociation on

Dec 16, 2009 - E-mail: [email protected] (W.R.)., †. Sun Yat-sen University. , ‡. Hong Kong University of Science and Technology. , §. University o...
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J. Phys. Chem. C 2010, 114, 970–976

Theoretical Study of O2 Molecular Adsorption and Dissociation on Silicon Carbide Nanotubes Fenglei Cao,† Xianyan Xu,† Wei Ren,*,‡,§ and Cunyuan Zhao*,† MOE Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, P. R. China, Physics Department, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong, and Physics Department, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed: October 23, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

The adsorption/dissociation of the O2 molecule on the surface of silicon carbide nanotubes (SiCNTs) was investigated by density functional theory. We found several adsorption configurations, including chemisorption and cycloaddition configurations, for triplet and singlet O2. Unlike the case for carbon nanotubes, the chemisorption of triplet O2 on SiCNTs is exothermic with remarkable charge transfer from nanotubes to the O2 molecule. Singlet O2 adsorption on the surface of SiCNTs can yield cycloaddition structures with large binding energies and sizable charge transfer. The reaction mechanism studies show that for triplet O2, the chemisorption configuration is favorable, but the cycloaddition configuration is preferred for singlet O2. For singlet O2, we also studied the dissociation of the O2 molecule, and a two-step mechanism was presented. The dissociation of molecular O2 results in formation of two three-membered rings with large binding energies. The key to the dissociation process of singlet O2 on the SiCNT surface is the first step with a barrier energy of 0.40 eV. Finally, the electronic properties of SiCNTs with adsorbed triplet and singlet O2 are shown to be dramatically influenced. Introduction The potential applications of carbon nanotubes (CNTs) are numerous.1,2 Some widely investigated examples include the applicability to nanoscale devices3-6 and gas sensors.8-15 Detection of gas molecules, such as O2, H2, H2O, CO2, NH3, and CO, is of critical importance in industrial and environmental monitoring. It is well-known that the electronic properties of semiconducting single-wall carbon nanotubes (SWCNTs) can be altered by the presence of gaseous molecules such as O2,7,16,17 NO2,8,18,19 NH3,8,10,19 and SO2.11 Therefore, CNTs can be applied as chemical gas sensors by measuring the conductance changes when the CNTs are exposed to the target gas. In general, the sensing mechanism has been theoretically explained in terms of charge transfer between the adsorbed molecules and CNTs. O2 is an important oxidative reagent, simply because it is present in air. Collins et al.7 first reported that exposure to air or oxygen dramatically influences electrical resistance and the thermoelectric power of SWCNTs. The sensitivity of CNTs to oxygen exposure has also been demonstrated by means of nuclear magnetic resonance (NMR) measurements, suggesting that adsorption of triplet O2 affects the magnetic property.20 Subsequently, a number of theoretical calculations were reported on the interaction between O2 and carbon nanotubes.16,17,21-23 On the basis of density functional theory (DFT), ground-state triplet O2 can only physisorb on the surface of SWCNTs, thus affecting the electronic properties. However, singlet O2 adsorption on the sidewall forms a metastable cycloaddition structure with a reaction barrier around 1.5 eV for (8, 0) SWCNT. * To whom correspondence should be addressed. Fax: 86-20-8411-0523. Phone: 86-20-8411-0523. E-mail: [email protected] (C.Z.). E-mail: [email protected] (W.R.). † Sun Yat-sen University. ‡ Hong Kong University of Science and Technology. § University of Arkansas.

Through the cycloaddition, the broken O-O bond of singlet O2 and the formation of epoxy structures have been found in subsequent theoretical studies. Furthermore, the adsorption and oxidation of O2 on non-carbon (e.g., boron nitride or zinc oxide) nanotubes also have been investigated in several recent studies.24-26 The silicon carbide nanotube (SiCNT), an analogue of the carbon nanotube, has been experimentally synthesized from the reaction of SiO and multiwalled CNTs.27 On the basis of DFT computations, the graphitic SiC in tubular form was also predicted to exist (similar to BN nanotubes), although this geometric configuration is not favorable from total-energy calculations. SiCNTs with the ionicity of silicon and carbon elements are semiconducting regardless of their chirality or diameter,28,29 in contrast to carbon nanotubes which exhibit diversified metallic and semiconducting characteristics. This has been confirmed by our and others previous DFT calculations specifically on the energy band gaps. Because of the sp3 hybridization and polar nature of silicon, SiCNTs are expected to have better reactivity than CNTs. In fact, theoretical studies show that H2,30 CO,31 HCN,31 NO,32 and N2O32 can be chemisorbed on the surface of SiCNTs with larger binding energies but only physisorbed on the surface of CNTs. Therefore, SiCNTs can be potentially applied as chemical gas sensors for these gaseous molecules. However, the interaction between O2 and SiCNTs, with important implications for the stability of SiCNTS upon air exposure and applications in chemical sensors for oxygen, has so far not been studied. We focused our investigations on the adsorption and dissociation of O2 on the surface of SiCNTs using spin-polarized density functional methods. As mentioned above, SiCNTs exhibit better activity than CNTs for reaction with the O2 molecule. Groundstate triplet O2 can chemisorb on the sidewall of SiCNTs, while the more reactive singlet O2 reacts with SiCNTs to form a stable

10.1021/jp910025y  2010 American Chemical Society Published on Web 12/16/2009

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[2 + 2] cycloaddition product. We then consider the O2 dissociation reaction from the [2 + 2] cycloaddition products, and we also briefly examine the electronic properties of the oxygen-functionalized SiCNTs. Method and Model We carried out the all-electron ab initio DFT calculations using the spin-polarized generalized-gradient approximation with the Perdew-Burke-Ernzerhof (PBE) functional33 and the double numerical basis set including polarization function (DNP basis set) implemented in the DMol3 package.34 (8, 0) Zigzag single-wall SiCNT was chosen as the benchmark model. The periodic boundary condition was used with a tetragonal supercell of 40 Å × 40 Å × 10.70 Å, and the length of c equals twice the period of a single-wall zigzag SiCNT. The Brillouin zone was sampled by 1 × 1 × 3 special k-points using the Monkhorst-Pack scheme.35 The binding energy (Eb) of functionalization for SiCNT is defined as Eb ) E[SiCNT+O2] - E[SiCNT] - [O2], where E[SiCNT+O2] is the energy of the O2-adsorbed system, E[SiCNT] is the total energy of pristine SiCNT, and the E[O2] is the energy of an isolated O2 molecule. In such a definition, negative binding energy means exothermic absorption on SiCNT sidewalls. The charge transfer between SiCNT and the O2 molecule is analyzed based on the Mulliken method. In order to predict the electronic properties, we also performed DFT calculations on the band structures and density of states (DOS) using denser 1 × 1 × 20 special k-points. Transition state geometries were first searched by the linear/quadratic synchronous transit (LST/QST) method36 and then fully optimized, confirmed by harmonic vibrational frequency calculations. Minmum-energy pathway (MEP) calculations using the nudged elastic band (NEB) method37 were also performed to confirm the transition states connecting the relevant reactants and products. Limited by our computation formalism, currently we do not have enough data to predict Henry’s constant of oxygen sorption on various nanotube systems, and a comparison between experimental measurements and theoretical calculations is still lacking. In future investigations, we would like to study the equilibrium constant with newly available modeling methods. Results and Discussion 1. Adsorption of Triplet and Singlet O2 Molecules on the Surface of SiCNTs. The cylindrical structure of SiCNTs results in two types of C-Si bonds, labeled as “axial” (A) and “circumferential” (C) (Figure 1a). The lengths of C-Si bonds are about 1.79 Å and 1.80 Å for A and C, respectively, in accordance with the theoretical studies reported in the latest literature.28,29 The charge analysis using the Mulliken method indicates that about 0.33 e charges are transferred from the silicon atom to the vicinity of the carbon atoms, indicating that the C-Si bonds of the sidewall are partially ionic. First, a number of initial configurations for O2 adsorption have been considered in our investigation, including on the top of C C-Si bonds, on the top of A C-Si bonds, on the top of silicon or carbon atom, and above the center of the hexagonal siliconcarbon rings. The triplet and singlet states were considered in the spin-polarized calculations. Each initial configuration is fully relaxed. Figure 1 shows different O2 adsorption configurations. The calculated structural parameters and binding energies of O2 adsorbed on the surface of SiCNT are summarized in Table 1. Ground-state triplet O2 on (8, 0) SiCNT yields one physisorption and two chemisorption stable configurations. For the

Figure 1. Optimized configurations for O2 adsorbed on (8, 0) SiCNT.

Figure 2. Reaction path from the chemisorption structure of config.SiA to cycloaddition structure of config.-A for triplet O2.

Figure 3. Reaction path from the chemisorption structure of config.SiA to cycloaddition structure of config.-A for singlet O2.

physisorption configuration (labeled as config.-PA, Figure 1a), the equilibrium distance of the O2-tube is larger than 3.0 Å, with small adsorption energy (0.04 eV). For this weak adsorption configuration, the charge transfer between the nanotube and the O2 molecule is very small (about 0.06 e). There exists a wellknown inaccuracy in the van der Waals interaction due to current DFT methods, though this does not affect our whole investigation of oxygen chemisorptions presented in this paper. Unlike

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TABLE 1: Calculated Structural Parameters and Charge Transfer and Binding Energies of Triplet and Singlet O2 Adsorbed on the Surface of (8, 0) SiCNT configuration PA SiA SiC A C Si-Si

spin state

d(Si-O)

d(C-O)

d(C-Si)

d(O-O)

Eb(eV)

q(e)

triplet triplet singlet triplet singlet triplet singlet triplet singlet singlet

4.18 1.86 1.81 1.85 1.81 1.74 1.73 1.73 1.73 1.77

3.62 3.07 3.08 3.12 3.06 1.50 1.48 1.49 1.47 -

1.83 1.83 1.84 1.83 1.91 1.91 1.97 1.97 -

1.23 1.32 1.33 1.32 1.34 1.50 1.52 1.53 1.52 1.51

-0.04 -0.28 -0.13 -0.34 -0.19 0.12 -1.00 0.29 -1.02 -0.62

-0.06 -0.38 -0.44 -0.39 -0.46 -0.58 -0.65 -0.63 -0.66 -0.81

TABLE 2: Calculated Structural Parameters of Transition States, Intermediates, and Dissociation Products from O2 Adsorbed/ Dissociated on the Surface of (8, 0) SiCNT TS1 TS2 TS3 IM1 TS4 TS5 zigzag meta a

d(Si-O1)a

d(C-O1)a

d(C-Si)1b

d(Si-O2)a

d(C-O2)a

d(C-Si)2b

d(O-O)

1.83 2.32 2.15 1.69 1.69 1.68 1.69 1.68

1.48 1.49 1.50 1.49 1.52 1.50

1.89 1.86 1.85 1.96 1.94 1.95 1.92 1.95

1.70 1.60 1.63 1.62 1.69 1.68

1.77 1.77

1.89 1.86

2.28 2.36 1.51 1.51

1.88 1.90 1.93 1.94

1.46 1.41 1.80 3.22 3.34 3.06 3.51 2.90

The oxygen atoms O1 and O2 are labeled in Figure 2-4. b The distance of C-Si bonded with corresponding oxygen atom.

SWCNTs,16,17 triplet O2 can also chemisorb on the surface of SiCNTs besides physisorption. In two chemisorption configurations, the oxygen atom attaches to the silicon atom on the nanotube. However, the orientation of the O2 is slightly different: one is along the A C-Si bond (labeled as config.-SiA, Figure 1b) and the other is along the C C-Si bond (labeled as config.SiC, Figure 1c). The distances between oxygen atoms and silicon atoms are about 1.86 Å and 1.85 Å for config.-SiA and config.SiC, slightly bigger than the length of the singlet O-Si bond (about 1.83 Å). The binding energies are -0.28 and -0.34 eV for config.-SiA and config.-SiC, indicating that the adsorption processes are chemical in nature. For each chemisorption configuration, the silicon atom bonded to the oxygen atom is slightly pulled out of the sidewall and the three corresponding C-Si bonds are thus slightly elongated (e.g., for config.-SiA bond lengths are about 1.81, 1.81, and 1.83 Å, respectively). Consequently, the local hybridization of the silicon atom is transformed from sp2 to sp3,38 due to the chemically adsorbed O2. In each chemisorption configuration, the strong chemical interaction also increases the O-O length from 1.23 Å to 1.32

Å, suggesting some charge transfer between the nanotube and adsorbed O2. Calculations show that the charge transfers are about 0.38 and 0.39 e from nanotube to O2, for config.-SiA and config.-SiC, respectively. In addition, the triplet state O2 at the underlying A (labeled as config.-A, shown in Figure 1d) or C (config.-C, Figure 1e) C-Si bond can form a [2 + 2] four-membered ring, with distances d(Si-O) of 1.74 Å (1.73 Å) and d(C-O) of 1.50 Å (1.49 Å) for the config.-A (config.-C) site. The bond between the oxygen atoms is longer (about 1.50 Å and 1.53 Å for config.-A and config.-C), possibly a single bond, compared with the O2 ion (about 1.50 Å). The Mulliken population analysis indicates a sizable (0.58 and 0.63 e for config.-A and config.-C, respectively) charge transfer from the nanotube to the chemisorbed O2. Likewise, the lengths of the C-Si bond underlying the O2 have also increased by about 0.12 Å for config.-A and 0.17 Å for config.-C. It appears clear that the local hybridization of the silicon and carbon atoms bonded to the oxygen atoms are transformed from sp2 to sp3. However, the reactions of triplet

Figure 4. Reaction path of singlet O2 dissociation from the structure of config.-A for singlet O2.

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Figure 5. Band structure of (a) pristine (8,0) SiCNT, (b) config.-SiA for triplet O2, (c) config.-A for singlet O2, (d) config.-Si-Si for singlet O2, and (e) config.-Zigzag for singlet O2. The spin-up and spin-down electronic structures are distinguished with “+” and “-”. The Fermi level is indicated with a black dotted line.

O2 on (8, 0) SiCNT to form the four-membered ring are calculated to be endothermic by 0.12 eV for A site and 0.29 eV for C site. Singlet O2 (as the excitation state of oxygen) adsorption on SiCNTs can also yield chemosorption and a [2 + 2] addition product. Inspection of Table 1 and Figure 1 shows that the structures of singlet O2 adsorption on SiCNTs are similar to those in the case of triplet O2. However, for singlet O2, the binding energies (-0.13 and -0.19 eV for config.-SiA and config.-SiC, respectively) are higher than those in the corresponding cases for triplet O2, with relatively larger charge transfers (0.44 e for config.-SiA and 0.46 for config.-SiC). It is remarkable that the reactions of singlet O2 on (8, 0) SiCNT to form the four-membered rings are strongly exothermic by -1.00 eV for A site and -1.02 eV for C site. This suggests that, contrary to the case for triplet O2, configurations for [2 + 2] cycloaddition can exist for singlet O2 from the viewpoint of thermodynamics. In this case, the formation of a four-numbered ring induces a large charge transfer from nanotube to O2 molecule. Surprisingly, we also found another stable configuration, in which O2 bonds with two neighboring silicon atoms, for singlet O2 adsorption on the surface of (8, 0) SiCNT, shown in Figure 1f (labeled as config.-Si-Si). For this configuration, the optimized lengths of two new Si-O bonds are 1.77 Å, and the corresponding C-Si bonds on the surface are elongated to 1.85,

J. Phys. Chem. C, Vol. 114, No. 2, 2010 973 1.84, and 1.80 Å, respectively. It can be observed that the two silicon atoms bonded to O2 are pulled out of the surface, while the carbon atom bridged in between the two silicon atoms is pushed into the surface. This local structural deformation results in a near tetrahedral structure of the silicon atom with three surrounding carbon atoms and one oxygen atom on the top. Remarkably, this configuration also gives a large binding energy of -0.62 eV, with the largest charge transfer of 0.81 e. 2. Reaction Mechanisms of O2 Adsorption and Dissociation on the Surface of SiCNTs. The reaction path of the O2 chemisorption [2 + 2] cycloaddition configuration was studied. For each state of O2, only the configuration of O2 on the A site was considered. The calculation results are depicted in Figure 2 and Figure 3, and the structural parameters of the transition states are summarized in Table 2. As shown in the Figure 2, for triplet O2, the [2 + 2] cycloaddition product is formed through TS1 (TS1 has only one imaginary frequency of 502i cm-1). In TS1 the distance of C-O is 1.83 Å, indicating that the C-O bond was formed. Meanwhile, the distance of O-O in TS1 is 0.14 Å longer than that in the config.-SiA chemisorption configuration. To form the [2 + 2] four-membered ring, an energy barrier of about 0.60 eV must be overcome, and the reaction is endothermic with binding energy of 0.40 eV. Conversely, the reaction from the configuration of [2 + 2] cycloaddition structure to the config.-SiA configuration only has to overcome a small energy barrier of about 0.20 eV with moderate binding energy of -0.40 eV. This reaction path clearly indicates that for triplet O2 adsorption on the surface of SiCNTs, the [2 + 2] cycloaddition config.-A (config.-C) is less favorable than the config.-SiA (config.-SiC) configuration. Figure 3 shows the reaction path for singlet O2 chemisorption to the [2 + 2] cycloaddition configuration. Note that there is a transition state of TS2 (TS2 has only one imaginary frequency of 192i cm-1) when the [2 + 2] four-membered ring is formed. In TS2, the length of C-O is 2.32 Å, larger than that for triplet O2 by 0.49 Å, and the O-O length is 1.41 Å, slightly shorter than that of triplet O2. Compared with TS1, TS2 is a transition state with much lower transition-state energy. Precisely, it is a small energy barrier of 0.2 eV to be overcome before the formation of the [2 + 2] four-membered ring, with a significant binding energy of -0.87 eV. Because of the small barrier and large binding energy, it is expected that the reaction from chemisoption to [2 + 2] cycloaddition can be achieved quite easily. Conversely, to break the [2 + 2] four-membered ring, a huge energy barrier of 1.07 eV must be overcome with an endothermicity binding energy of 0.87 eV. Clearly, for singlet O2 adsorption on the SiCNT sidewall, the [2 + 2] cycloaddition product config.-A (config.-C) is preferred over that of the chemisoption product config.-SiA (config.-SiC), in contrast to the case for triplet O2. As mentioned above, singlet O2 adsorption on the surface of SiCNTs will mainly yield [2 + 2] cycloaddition products. In config.-A or config.-C configurations, the O-O double bond is elongated to over 1.50 Å, possibly that of a O-O single bond. Obviously, these results prompted us to speculate whether the O2 molecule could be dissociated on the surface of SiCNTs. Previously, O2 molecule dissociation on the surface of defect CNTs was investigated by Govind et al.23 Having gained a detailed understanding of the adsorption process, we then considered the O2 dissociation reaction in the config.-A of singlet O2. The reaction path is depicted in Figure 4, and structure parameters of transition states, intermediates, and dissociation products are listed in Table 2.

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Figure 6. Density of states (DOS) of (a) pristine (8,0) SiCNT, (b) config.-SiA for triplet O2, (c) config.-A for singlet O2, (d) config.-Si-Si for singlet O2, and (e) config.-Zigzag for singlet O2. The spin-up and spin-down electronic DOS are distinguished with “+” and “-”. The Fermi level is indicated with a black dotted line. The projected DOS of O2 is plotted with red lines.

In Figure 4, we show two possible dissociation products: (1) two oxygen atoms diffuse along the zigzag C-Si bond of two adjacent hexagonal silicon-carbon rings (labeled as config.Zigzag), and (2) two oxygen atoms dissociate in one hexagonal silicon-carbon ring located at meta C-Si bonds (labeled as config.-Meta). In both products, the dissociation of molecular O2 results in formation of two three-membered rings. Notably, an intermediate state (IM1) exists before the O2 is completely dissociated. In IM1, the O1 atom moves to the circumferential C-Si bond and results in the formation of a three-membered ring (the corresponding Si-O, C-O, and C-Si bond lengths are 1.69, 1.49, and 1.96 Å, respectively), while the distance between the O2 atom and the silicon atom is about 1.60 Å and slightly lower than the Si-O bond length in the quartz crystal (about 1.61 Å), suggesting the formation of a Si-O double bond. To approach IM1, the transition state TS3 (TS3 has only one imaginary frequency of 473i cm-1) is encountered. The reaction from config.-A to IM1 is calculated to have a barrier of 0.40 eV and a large binding energy of -1.09 eV relative to the

config.-A for singlet O2. From IM1, two transition states TS4 (TS4 has only one imaginary frequency of 208i cm-1) and TS5 (TS5 has only one imaginary frequency of 162i cm-1) were identified for two pathways. The transition structures TS4 and TS5 are similar in geometry to each other, and both states involve ring-closure reactions. The pathway through TS4 is calculated to have a barrier of 0.19 eV, and the resulting config.Zigzag product is calculated to lie at 0.82 eV relative to the IM1. On the other hand, the pathway involving TS5 needs to overcome the same energy barrier and yields config.-Meta product with binding energy of -0.90 eV with respect to IM1. The computation results thus reveal a two-step mechanism for dissociation of singlet O2 on the SiCNT sidewall, in contrast to the case of the O2 molecule on the surface of CNTs.23 The key reaction in the dissociation of singlet O2 on the surface of SiCNTs is the first step, with a barrier energy of 0.40 eV. The binding energies from config.-A to config.-Zigzag and config.Meta are -1.91 and -1.99 eV, respectively. Thus, it is expected

Silicon Carbide Nanotubes that the dissociation of singlet O2 on the surface of SiCNTs can occur easily. 3. Electronic Properties of Triplet and Singlet O2 Chemisorption on the (8, 0) SiCNT Surface. Because of the large charge transfer between the tube and O2 molecule, we expected that the chemisorption of O2 onto silicon carbide nanotubes would lead to significant effects on the electronic properties of SiCNTs. The band structures and density of states (DOS) of pristine (8, 0) SiCNT, as well as several stable configurations after oxygen adsorption/dissociations (included config.-SiA for triplet O2, and config.-A, config.-Si-Si, and config.-Zigzag for singlet O2), are calculated and plotted in Figure 5 and Figure 6. (8, 0) SiCNT is semiconducting with a band gap of 1.3 eV, as shown in Figures 5a and 6a. This result agrees with the theoretical studies reported in the latest literature,28,29 indicating that the model and the method used in our paper are appropriate for the following analysis. Calculations based on the spin-polarized DFT, as displayed in Figures 5b and 6b, reveal that chemisorption of triplet O2 on (8, 0) SiCNT strongly influences the electronic structures of pristine (8, 0) SiCNT. The conduction band edge and the valence band edge of pristine (8, 0) SiCNT are not modified significantly, as shown in the spin-up band structure. In comparison, for spin-down, two energy levels appear within the band gap: one level is close to the valence band edge, while the other is just above the Fermi energy by approximately 0.2 eV. The energy separation between these two levels is 0.8 eV. This results in a net magnetic moment for chemisorption of triplet O2 on the SiCNT surface. The local DOS shown in Figure 6b illustrates that the new spin-down levels are mainly contributed by the O2 molecule, indicating that the magnetic moment is due to the triplet O2 molecule. In addition, the presence of the new spin-down levels causes a band gap reduction from 1.3 to 0.8 eV, suggesting that the chemisorption of triplet O2 on SiCNTs would possibly result in an increase in the electronic conductivity of SiCNTs. In Figure 5c-e and Figure 6c-e, we show the band structures and DOSs calculated for the O2 molecule on the (8, 0) SiCNT in the singlet state. The reaction that forms the four-membered ring on top of the A site results in the creation of new states below the conduction band and reduces the band gap by only 0.1 eV (shown in Figures 5c and 6c for config.-A); the O2 dissociation also slightly influences the band gap (shown in Figures 5e and 6e for config.-Zigzag). However, as shown in Figure 5d and 6d, the adsorption of config.-Si-Si downshifts the Fermi energy and gives rise to two levels near the Fermi level. One of the two new levels is about 0.05 eV above the Fermi level and the other is just below the Fermi level, indicating that the system displays a degenerated p-type semiconducting behavior. The results of local DOS show that these levels should be induced by the hybridization of oxygen and the nanotube, as indicated by the red curve in Figure 6d. On the basis of the results discussed above, it is reasonable to postulate that the chemisorption of O2 on SiCNTs could dramatically influence the electronic properties of SiCNTs. For the ground-state triplet O2 molecule, the chemisorption significantly reduces the band gap from 1.3 to 0.8 eV, and the config.Si-Si of singlet O2 displays a degenerated p-type semiconducting behavior. Consequently, SiCNTs can be expected to be an excellent gas sensor for O2 detection in nanoscale electronic devices. On the other hand, it is also interesting that the triplet O2 molecule adsorption would create a magnetic moment in the O2/SiCNTs system. As a result, the ground-state

J. Phys. Chem. C, Vol. 114, No. 2, 2010 975 triplet O2 molecule has the potential for use in a magnetic switch for spintronics applications. Conclusion In summary, we have theoretically investigated the structural, energetic, and electronic properties for the adsorption of O2 molecule on the zigzag (8, 0) SiCNTs using density functional theory. Then, the reaction mechanisms of O2 molecular adsorption/dissociation on the surface of (8, 0) SiCNTs was studied. In particular, triplet and singlet O2 were considered in the present study. We expect that further experimental work will be performed and compared with the molecular functionalizations of the SiCNT surface in our DFT study, as summarized below. (1) For both states of O2, several adsorption configurations can be obtained. For triplet O2, the chemisorption configurations are exothermic, but the [2 + 2] addition configurations are endothermic. However, for singlet O2, both types and config.Si-Si are exothermic, and the [2 + 2] addition configurations give larger binding energies of -1.00 and -1.02 eV. On the basis of the results of reaction mechanisms considered, the chemisorption product is favorable for triplet O2, while the [2 + 2] addition product is preferred for singlet O2. (2) The dissociation of O2 on the surface of SiCNTs was considered, based on the structure of config.-A for singlet O2. We have found that the reaction mechanism of dissociation is a two-step mechanism. Two oxygen atoms take turns in diffusing to adjacent C-Si bonds, leading to two threemembered rings formed in the dissociation products. The first step is the key to the reactions with a barrier energy of 0.40 eV. In addition, the binding energies from config.-A to final dissociation products are strongly exothermic. Thus, dissociation of singlet O2 on the surface of SiCNTs can take place easily. (3) The electronic properties of SiCNTs adsorbed with triplet and singlet O2 are dramatically influential. For the ground-state triple O2 molecule, the chemisorption significantly reduces the band gap from 2.3 to 1.2 eV and creates a magnetic moment in O2/SiCNTs system. For singlet O2, the system of config.-Si-Si displays a degenerated p-type semiconducting behavior. Therefore, SiCNTs can be expected to be an excellent gas sensor for O2 detection in nanoscale electronic devices. Acknowledgment. This research has been supported by the National Natural Science Foundation of China (Grant No. 20673149, 20973204, and J0730420) and Natural Science Foundation of GuangDong (Grant No. 7003709) to C.Y.Z. W.R. gratefully acknowledges financial support of HKUST through RPC06/07.SC21. References and Notes (1) Dresselhaus, M. S.; Dressehaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: New York, 1996. (2) Saito, R.; Dressehaus, G.; Dresselhaus, M. S. Physics Properties of Carbon Nanotubes; World Scientific: New York, 1998. (3) Feldman, A. K.; Steigerwald, M. L.; Guo, X. F.; Nuckolls, C. Acc. Chem. Res. 2008, 41, 1731. (4) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (5) Chiu, P.-W.; Kaempgen, M.; Roth, S. Phys. ReV. Lett. 2004, 92, 246802. (6) Guo, X.; Small, J. P.; Klare, J. E.; Wang, Y.; Purewal, M. S.; Tam, I. W.; Hong, R. H.; Caldwell, R.; Huang, L.; O’Brien, S.; Yan, J.; Bresiow, R.; Wind, S. J.; Hone, J.; Kim, P.; Nuckolls, C. Science 2006, 311, 356. (7) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (8) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; peng, S.; Cho, K.; Dai, H. J. Science 2000, 287, 622. (9) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384.

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