- Email: [email protected]
First principles study of titanium-coated carbon nanotubes as sensors for carbon monoxide molecules
Surface Science 601 (2007) 4102–4104 www.elsevier.com/locate/susc
First principles study of titanium-coated carbon nanotubes as sensors for carbon monoxide molecules R. Mota b
a,1
, Solange B. Fagan b, A. Fazzio
c,*
a International Center for Condensed Matter Physics, Universidade de Brası´lia, 70919-970 Brası´lia-DF, Brazil A´rea de Cieˆncias Naturais e Tecnolo´gicas, Centro Universita´rio Franciscano, 97010-032 Santa Maria-RS, Brazil c Instituto de Fı´sica, Universidade de Sa˜o Paulo, 05315-970 Sa˜o Paulo-SP, Brazil
Available online 4 May 2007
Abstract In this work, the electronic properties of the system composed by the CO molecules adsorbed on Ti-coated single-wall carbon nanotubes (SWNTs) are studied through first principles calculations. The changes in the electronic properties of the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT are analyzed for different CO concentrations. A strong interaction between CO molecules and the SWCT/Ti system is observed, which decreases when the concentration of CO molecules increases. The resulting system are shown to be very sensitive to the CO concentration adsorbed on the tube/Ti system, making that the SWNT, which is originally semiconductor and becomes metallic after Ti covering, to recover the semiconductor behavior again when enough high concentrations of CO molecules is adsorbed on the SWNT/Ti system. These three distinct steps (semiconductor/metallic/semiconductor) constitute the basis for a feasible, flexible and efficient sensor device for CO molecule recognition. Ó 2007 Elsevier B.V. All rights reserved.
Carbon nanotubes have demonstrated to be very promising materials with unusual physical, chemical and mechanical properties [1]. Their extraordinary properties and practical possible industrial applications, such as nanoelectronics and nanodevices, nanotube-based gears and bearings, have made the realm of nanotubes an interdisciplinary place in the intersection of many sciences. SWNTs have been studied for use as gas sensors as hydrogen storage [2–4] or to remove toxic gas [4–6] for example. SWNTs interact with transition-metal (TM) atoms and it has been shown that they can lead to an even more diverse range of applications in nanodevices [7,8]. Although TM atoms have been used in the nanotubes during the initialization growth processes, the role played by them is not yet well understood. In fact, the bonding of TM atoms to the SWNT surface depends on the detailed contact conditions [9–11]. The coating of SWNT with various TM atoms *
Corresponding author. Tel.: +55 11 30917039; fax: +55 11 30916831. E-mail address: [email protected] (A. Fazzio). 1 On leave from Departamento de Fı´sica, Universidade Federal de Santa Maria, 97105-900 Santa Maria-RS, Brazil. 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.165
has been experimentally demonstrated to depend on the specific coating element, forming alternatively wires or clusters [7,8]. In particular, Ti atoms have been shown to form continuous wires. Theoretical works have demonstrated that the Ti atoms present stronger interaction with the C atoms of the surface than among themselves [12–16]. However, atoms like Au [12] tend to form cluster when adsorbed on the tube surface while Fe [17,18] and Mn [19] show moderate tendency to form clusters or wires depending on the metal deposition on the tube. One interesting application of Ti-coated SWNT is to adsorb atoms or molecules and Yildirim and Ciraci in a recent work [2] showed the high-capacity hydrogen storage of Ti-decorated SWNTs. In this work, we theoretically analyze the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT. We analyze the changes in the structural and electronic properties of the resulting systems with different CO concentrations. The theoretical calculations of CO molecules adsorbed on the Ti-wire covering an (8, 0) SWNT are based on first principles spin-polarized density-functional theory using
R. Mota et al. / Surface Science 601 (2007) 4102–4104
numerical atomic orbitals as basis set and implemented in the SIESTA code [20]. The calculations are done using the generalized gradient approximation for the exchangecorrelation term, as proposed by Perdew et al. [21]. The standard norm-conserving Troullier–Martins pseudopotentials [22] are used to replace the core electrons. Due to the large overlap between the semicore and the valence states, the 3s and 3p electrons of Ti were explicitly included in the calculation [23]. In all procedures we have used a split-valence double-zeta basis set with a polarization function. A cutoff of 150 Ry for the grid integration was utilized to represent the charge density. Along the tube axis, 15 Monkhorst-Pack k-points for the Brillouin zone integration were used in the studied systems. The relaxed atomic structures of the tubes were obtained by minimization of the total energy using Hellmann–Feynman forces including Pullay-like corrections. Structural optimizations were performed using the conjugate gradient algorithm until the ˚. residual forces were smaller than 0.05 eV/A The calculations were performed using an (8, 0) semiconducting SWNT; 32 carbon atoms and two Ti atoms compose the adopted supercell with the CO molecules being adsorbed on the Ti sites in different concentrations. In the linear chain of Ti atoms it is included (i) 1 CO, (ii) 2 CO, or (iii) 4 CO molecules in the unit cell, with structural positions shown in Fig. 1a–c, respectively. In terms of electronic properties it is interesting to observe, for comparison, that the electronic band structure for pristine (8, 0) SWNT is semiconductor, nevertheless, when the Ti-wire is covering the tube a metallic behavior is observed, as can be seen in Fig. 2a and b, respectively. In order to study the modifications on the electronic structures due to the gas adsorption on the Ti/SWNT system, we plot the electronic band structures for 1, 2 and 4 CO concentrations, as can be observed in Fig. 3a–c, respectively. Several modifications on the electronic band structures after the CO adsorption are observed. The Ticovered SWNT is metallic with many electronic levels around the Fermi energy region (Fig. 2a), as long the concentration of CO molecule is very low the system remains
4103
Fig. 2. Electronic band structures for: (a) pristine (8, 0) SWNT, (b) Ti wire covering the tube with high spin configuration. In the figure (b) the filled and pointed lines correspond, respectively, to the majority and minority spins [16]. The dashed horizontal lines correspond to the Fermi energy.
metallic, presenting few electronic levels near the Fermi energy; although, by increasing the CO concentration, the system becomes semiconductor. This electronic behavior is very similar of previous results with CO adsorbed on deformed (8, 0) SWNTs [5]. In summary, through ab initio simulations, the electronic properties of the CO adsorption on Ti-covered (8, 0) SWNT are studied. It is observed that the main reason for the transition of the Ti-coated SWNT, which is originally metallic, to gradually become semiconducting is associated with the strength of the interactions of the Ti atoms with the CO molecules and with the C atoms of the tube. As long as the interaction with the CO molecules becomes stronger, the interaction of the Ti atoms with the tube will result weaker, allowing to the tube to become closer to the original structure of the semiconducting (8, 0) SWNT. Then, considering the final electronic configurations of the semiconducting pristine SWNT (see Fig. 2a), the metallic Ti-coated SWNT without CO (Fig. 2b) or for the CO low-concentration regime (see Fig. 3a and b)
Fig. 1. Final fully-relaxed configurations for different concentrations of CO molecules adsorbed on the Ti covered SWNT. In (a), (b) and (c) it is observed 1, 2 and 4 CO molecules per unit cell, respectively.
4104
R. Mota et al. / Surface Science 601 (2007) 4102–4104
Fig. 3. Electronic band structures for CO adsorption in Ti-covered SWNT with (a) 1, (b) 2 and (c) 4 CO molecules per unit cells. The dashed horizontal lines correspond to the Fermi energy.
and, the semiconducting Ti-covered tube with CO highconcentration (see Fig. 3c), we can observe, that, depending on the Ti-coating and the CO-concentration, three distinct steps (semiconductor/metallic/semiconductor) are possible to result. More specifically, based only on variation of CO-concentration on the Ti-covered SWNT, the variation from metallic to semiconducting behavior can be obtained through appropriate variation of the CO molecules concentration, which could constitute the basis for a flexible sensor device for CO molecule recognition. Acknowledgements The authors thank the CENAPAD-SP for the computational time. This research is supported by Brazilian agencies: CNPq, CAPES, FAPESP and FAPERGS. S.B. Fagan thanks L’oreal for the Woman in Science Grant 2006. References [1] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes, Springer, Berlin, 2001. [2] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. [3] M. Paulose, O.K. Varghese, G.P. Mor, C.A. Grimes, K.G. Ong, Nanotechnology 17 (2006) 398.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
C. Matranga, B. Bockrath, J. Phys. Chem. B 109 (2005) 4853. L.B. da Silva, S.B. Fagan, R. Mota, Nano Letters 4 (2004) 65. S.J. Kim, J. Phys. D: Appl. Phys. 39 (2006) 3026. H. Dai, Surf. Sci. 500 (2002) 218. Y. Zhang, N.W. Franklin, R.J. Chen, H. Dai, Chem. Phys. Lett. 331 (2000) 35. A.N. Andriotis, M. Menon, G. Froudakis, Phys. Rev. Lett. 85 (2000) 3193. Y. Liu, Phys. Rev. B 68 (2003) 193409. S. Dag, O. Gu¨lseren, S. Ciraci, T. Yildirim, Appl. Phys. Lett. 83 (2003) 3180. C.-K. Yang, J. Zhao, J.P. Lu, Phys. Rev. B 66 (2003) 041403 (R). S. Dag, S. Ciraci, Phys. Rev. B 71 (2005) 165414. E. Durgun, S. Dag, V.M.K. Bagci, O. Gu¨lseren, T. Yildirim, S. Ciraci, Phys. Rev. B 67 (2003) 201401 (R). H. Oymak, S. Erkoc¸, Chem. Phys. 300 (2004) 277. S.B. Fagan, A. Fazzio, R. Mota, Nanotechnology 17 (2006) 1154. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Phys. Rev. B 67 (2003) 2055414. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Microelectron. J. 34 (2003) 481. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, J. Phys.: Condens. Matter 16 (2004) 3647. P. Ordejo´n, E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) 10441; D. Sa´nchez-Portal, E. Artacho, J.M. Soler, Int. J. Quantum Chem. 65 (1997) 453. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. J. Junquera, M. Zimmer, P. Ordejo´n, P. Ghosez, Phys. Rev. B 67 (2003) 155327.
First principles study of titanium-coated carbon nanotubes as sensors for carbon monoxide molecules R. Mota b
a,1
, Solange B. Fagan b, A. Fazzio
c,*
a International Center for Condensed Matter Physics, Universidade de Brası´lia, 70919-970 Brası´lia-DF, Brazil A´rea de Cieˆncias Naturais e Tecnolo´gicas, Centro Universita´rio Franciscano, 97010-032 Santa Maria-RS, Brazil c Instituto de Fı´sica, Universidade de Sa˜o Paulo, 05315-970 Sa˜o Paulo-SP, Brazil
Available online 4 May 2007
Abstract In this work, the electronic properties of the system composed by the CO molecules adsorbed on Ti-coated single-wall carbon nanotubes (SWNTs) are studied through first principles calculations. The changes in the electronic properties of the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT are analyzed for different CO concentrations. A strong interaction between CO molecules and the SWCT/Ti system is observed, which decreases when the concentration of CO molecules increases. The resulting system are shown to be very sensitive to the CO concentration adsorbed on the tube/Ti system, making that the SWNT, which is originally semiconductor and becomes metallic after Ti covering, to recover the semiconductor behavior again when enough high concentrations of CO molecules is adsorbed on the SWNT/Ti system. These three distinct steps (semiconductor/metallic/semiconductor) constitute the basis for a feasible, flexible and efficient sensor device for CO molecule recognition. Ó 2007 Elsevier B.V. All rights reserved.
Carbon nanotubes have demonstrated to be very promising materials with unusual physical, chemical and mechanical properties [1]. Their extraordinary properties and practical possible industrial applications, such as nanoelectronics and nanodevices, nanotube-based gears and bearings, have made the realm of nanotubes an interdisciplinary place in the intersection of many sciences. SWNTs have been studied for use as gas sensors as hydrogen storage [2–4] or to remove toxic gas [4–6] for example. SWNTs interact with transition-metal (TM) atoms and it has been shown that they can lead to an even more diverse range of applications in nanodevices [7,8]. Although TM atoms have been used in the nanotubes during the initialization growth processes, the role played by them is not yet well understood. In fact, the bonding of TM atoms to the SWNT surface depends on the detailed contact conditions [9–11]. The coating of SWNT with various TM atoms *
Corresponding author. Tel.: +55 11 30917039; fax: +55 11 30916831. E-mail address: [email protected] (A. Fazzio). 1 On leave from Departamento de Fı´sica, Universidade Federal de Santa Maria, 97105-900 Santa Maria-RS, Brazil. 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.165
has been experimentally demonstrated to depend on the specific coating element, forming alternatively wires or clusters [7,8]. In particular, Ti atoms have been shown to form continuous wires. Theoretical works have demonstrated that the Ti atoms present stronger interaction with the C atoms of the surface than among themselves [12–16]. However, atoms like Au [12] tend to form cluster when adsorbed on the tube surface while Fe [17,18] and Mn [19] show moderate tendency to form clusters or wires depending on the metal deposition on the tube. One interesting application of Ti-coated SWNT is to adsorb atoms or molecules and Yildirim and Ciraci in a recent work [2] showed the high-capacity hydrogen storage of Ti-decorated SWNTs. In this work, we theoretically analyze the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT. We analyze the changes in the structural and electronic properties of the resulting systems with different CO concentrations. The theoretical calculations of CO molecules adsorbed on the Ti-wire covering an (8, 0) SWNT are based on first principles spin-polarized density-functional theory using
R. Mota et al. / Surface Science 601 (2007) 4102–4104
numerical atomic orbitals as basis set and implemented in the SIESTA code [20]. The calculations are done using the generalized gradient approximation for the exchangecorrelation term, as proposed by Perdew et al. [21]. The standard norm-conserving Troullier–Martins pseudopotentials [22] are used to replace the core electrons. Due to the large overlap between the semicore and the valence states, the 3s and 3p electrons of Ti were explicitly included in the calculation [23]. In all procedures we have used a split-valence double-zeta basis set with a polarization function. A cutoff of 150 Ry for the grid integration was utilized to represent the charge density. Along the tube axis, 15 Monkhorst-Pack k-points for the Brillouin zone integration were used in the studied systems. The relaxed atomic structures of the tubes were obtained by minimization of the total energy using Hellmann–Feynman forces including Pullay-like corrections. Structural optimizations were performed using the conjugate gradient algorithm until the ˚. residual forces were smaller than 0.05 eV/A The calculations were performed using an (8, 0) semiconducting SWNT; 32 carbon atoms and two Ti atoms compose the adopted supercell with the CO molecules being adsorbed on the Ti sites in different concentrations. In the linear chain of Ti atoms it is included (i) 1 CO, (ii) 2 CO, or (iii) 4 CO molecules in the unit cell, with structural positions shown in Fig. 1a–c, respectively. In terms of electronic properties it is interesting to observe, for comparison, that the electronic band structure for pristine (8, 0) SWNT is semiconductor, nevertheless, when the Ti-wire is covering the tube a metallic behavior is observed, as can be seen in Fig. 2a and b, respectively. In order to study the modifications on the electronic structures due to the gas adsorption on the Ti/SWNT system, we plot the electronic band structures for 1, 2 and 4 CO concentrations, as can be observed in Fig. 3a–c, respectively. Several modifications on the electronic band structures after the CO adsorption are observed. The Ticovered SWNT is metallic with many electronic levels around the Fermi energy region (Fig. 2a), as long the concentration of CO molecule is very low the system remains
4103
Fig. 2. Electronic band structures for: (a) pristine (8, 0) SWNT, (b) Ti wire covering the tube with high spin configuration. In the figure (b) the filled and pointed lines correspond, respectively, to the majority and minority spins [16]. The dashed horizontal lines correspond to the Fermi energy.
metallic, presenting few electronic levels near the Fermi energy; although, by increasing the CO concentration, the system becomes semiconductor. This electronic behavior is very similar of previous results with CO adsorbed on deformed (8, 0) SWNTs [5]. In summary, through ab initio simulations, the electronic properties of the CO adsorption on Ti-covered (8, 0) SWNT are studied. It is observed that the main reason for the transition of the Ti-coated SWNT, which is originally metallic, to gradually become semiconducting is associated with the strength of the interactions of the Ti atoms with the CO molecules and with the C atoms of the tube. As long as the interaction with the CO molecules becomes stronger, the interaction of the Ti atoms with the tube will result weaker, allowing to the tube to become closer to the original structure of the semiconducting (8, 0) SWNT. Then, considering the final electronic configurations of the semiconducting pristine SWNT (see Fig. 2a), the metallic Ti-coated SWNT without CO (Fig. 2b) or for the CO low-concentration regime (see Fig. 3a and b)
Fig. 1. Final fully-relaxed configurations for different concentrations of CO molecules adsorbed on the Ti covered SWNT. In (a), (b) and (c) it is observed 1, 2 and 4 CO molecules per unit cell, respectively.
4104
R. Mota et al. / Surface Science 601 (2007) 4102–4104
Fig. 3. Electronic band structures for CO adsorption in Ti-covered SWNT with (a) 1, (b) 2 and (c) 4 CO molecules per unit cells. The dashed horizontal lines correspond to the Fermi energy.
and, the semiconducting Ti-covered tube with CO highconcentration (see Fig. 3c), we can observe, that, depending on the Ti-coating and the CO-concentration, three distinct steps (semiconductor/metallic/semiconductor) are possible to result. More specifically, based only on variation of CO-concentration on the Ti-covered SWNT, the variation from metallic to semiconducting behavior can be obtained through appropriate variation of the CO molecules concentration, which could constitute the basis for a flexible sensor device for CO molecule recognition. Acknowledgements The authors thank the CENAPAD-SP for the computational time. This research is supported by Brazilian agencies: CNPq, CAPES, FAPESP and FAPERGS. S.B. Fagan thanks L’oreal for the Woman in Science Grant 2006. References [1] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes, Springer, Berlin, 2001. [2] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. [3] M. Paulose, O.K. Varghese, G.P. Mor, C.A. Grimes, K.G. Ong, Nanotechnology 17 (2006) 398.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
C. Matranga, B. Bockrath, J. Phys. Chem. B 109 (2005) 4853. L.B. da Silva, S.B. Fagan, R. Mota, Nano Letters 4 (2004) 65. S.J. Kim, J. Phys. D: Appl. Phys. 39 (2006) 3026. H. Dai, Surf. Sci. 500 (2002) 218. Y. Zhang, N.W. Franklin, R.J. Chen, H. Dai, Chem. Phys. Lett. 331 (2000) 35. A.N. Andriotis, M. Menon, G. Froudakis, Phys. Rev. Lett. 85 (2000) 3193. Y. Liu, Phys. Rev. B 68 (2003) 193409. S. Dag, O. Gu¨lseren, S. Ciraci, T. Yildirim, Appl. Phys. Lett. 83 (2003) 3180. C.-K. Yang, J. Zhao, J.P. Lu, Phys. Rev. B 66 (2003) 041403 (R). S. Dag, S. Ciraci, Phys. Rev. B 71 (2005) 165414. E. Durgun, S. Dag, V.M.K. Bagci, O. Gu¨lseren, T. Yildirim, S. Ciraci, Phys. Rev. B 67 (2003) 201401 (R). H. Oymak, S. Erkoc¸, Chem. Phys. 300 (2004) 277. S.B. Fagan, A. Fazzio, R. Mota, Nanotechnology 17 (2006) 1154. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Phys. Rev. B 67 (2003) 2055414. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Microelectron. J. 34 (2003) 481. S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, J. Phys.: Condens. Matter 16 (2004) 3647. P. Ordejo´n, E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) 10441; D. Sa´nchez-Portal, E. Artacho, J.M. Soler, Int. J. Quantum Chem. 65 (1997) 453. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. J. Junquera, M. Zimmer, P. Ordejo´n, P. Ghosez, Phys. Rev. B 67 (2003) 155327.