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Microwave-assisted synthesis of silver nanoparticles using ethanol as a reducing agent
Materials Chemistry and Physics 114 (2009) 530–532
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Microwave-assisted synthesis of silver nanoparticles using ethanol as a reducing agent Angshuman Pal, Sunil Shah, Surekha Devi ∗ Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 11 November 2008 Accepted 17 November 2008 Keywords: Nanostructures Electron microscopy Optical properties
a b s t r a c t Silver nanoparticles were prepared by microwave irradiation of silver nitrate (AgNO3 ) solution in ethanolic medium using polyvinylpyrrolidone (PVP) as a stabilizing agent. Ethanol was observed to act as a reducing agent in the presence of microwave. Appearance of surface plasmon band at 416 nm indicated the formation of silver nanoparticles. Highly monodispersed stable polycrystalline silver nanoparticles were obtained within 5 s of microwave irradiation. Through transmission electron microscopy silver nanoparticles were observed to be spherical with 10 ± 5 nm diameter. Silver nanoparticles exhibited fluorescence band at 491 nm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanosize metal particles have attracted significant attention because of their unusual size-dependent optical and electronic properties. Among the metals, silver nanoparticles show potential applications in various fields such as the environment, biomedicinal, catalysis, optics and electronics [1]. In particular, Ag nanoparticles are used in diagnostic biomedical optical imaging [2]. The surface plasmon resonance and large effective scattering cross-section of individual silver nanoparticles make them ideal candidates for molecular labelling, where phenomenon such as surface enhanced Raman scattering can be exploited [3]. Since the anisotropic nanoparticles have greater surface area than spherical nanoparticles, their metal enhanced fluorescence is higher than that of spherical nanoparticles [4]. Silver nanoparticles can be synthesized using various methods such as chemical reduction [5], electrochemical [6], ␥-radiation [7], laser ablation [8], photochemical [9], sonochemical [10] and sputtering [11]. Among these the most popular method for preparation of silver colloid is chemical reduction of silver salt in the presence of any stabilizing agent. Most commonly used stabilizing agents are polymers [12] and surfactants [13]. Recently for the preparation of silver nanoparticles, microwave heating (MW) has been reported to have better promise over thermal heating. In the microwave frequency range 300 MHz to 300 GHz, polar molecules such as H2 O try to orient with the electric field. When the dipolar molecules try to re-orient with respect to an alternating electric field, they lose energy in the form of heat
∗ Corresponding author. Tel.: +91 2652795552. E-mail address: surekha [email protected] (S. Devi). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.056
by molecular friction. The MW power dissipation per unit volume in a material (P) is given by Eq. (1) P = cE 2 fε = cE 2 fε tan ı
(1)
where c is a velocity of radiation, E is an electric field in the material, f is frequency of radiation, and ε and ε are the dielectric and dielectric loss constants, respectively. ε represents the relative permittivity, which is a measure of the ability of a molecule to be polarized by an electric field and tan ı = ε /ε is the energy dissipation factor or loss tangent. In the above equation ε is most important physical parameter that describes the ability of a material to heat in the MW field. Water and alcohols have high dielectric losses and a high reducing ability. Therefore they are the ideal solvents for microwave heating. The boiling point of ethanol is 78 ◦ C and ε and ε values are 6.08 and 24.3 respectively. Wang et al. [14] have synthesized shorter than 50 nm Ag nanoparticles by dissolving PVP, glucose and sodium hydroxide in water at 60 ◦ C through thermal heating. Gao et al. [15] have reported the synthesis of silver nanoparticles by PVP-assisted N,N-dimethylformamide (DMF) reduction at 140 ◦ C for 3 h. They could get Ag decahedrons with edge size 80 nm. Tusji and co-workers [16] have reviewed the preparation of Ag nanostructures in PVP using microwave. Polymer–metal nanoparticle composites seem to receive special attention because of their application potential in sensors [17], actuators [18], electrodes for fuel cells [19], optical and optoelectronic devices [20]. Silver nanoparticles have characteristic fluorescence emission property [21]. Zheng et al. [22] observed fluorescence emission for dendrimer-encapsulated silver nanodots. For all the above applications, especially for sensors and optical devices highly monodispersed particles are required.
A. Pal et al. / Materials Chemistry and Physics 114 (2009) 530–532
531
Here in this paper we are describing the preparation of spherical, monodispersed, PVP stabilize silver nanoparticles under MW heating using ethanol as a reducing agent. 2. Experimental 2.1. Materials All chemicals and materials were used as received: AgNO3 and polyvinylpyrrolidone (PVP) (Mw = 10,000) were purchased from Sigma–Aldrich, UK and ethanol from Fluka. 2.2. Preparation of PVP-silver nanoparticle composites For the synthesis of Ag nanoparticles SHARP make microwave oven (model: R259) was used. In a typical procedure, 10 ml of 1% (w/v) ethanolic solution of PVP and 0.2 ml of 0.1 M AgNO3 were taken in a 25 ml closed conical flask and placed in a microwave oven that was operated at the 100% power of 800 W and frequency 2450 MHz for 5 s. The colourless solution instantaneously turned to the characteristic pale yellow colour, indicating the formation of silver nanoparticles. The advantage of microwave-mediated synthesis over the conventional heating is the improved kinetics of the reaction generally by one or two order of magnitude, due to rapid initial heating and the generation of localized high-temperature zones at reaction sites [23]. 2.3. Characterization Characteristic optical properties of Ag nanoparticles were recorded using PerkinElmer Lambda 35 UV–vis spectrophotometer. Spectra were recorded using 1 cm3 quartz cell. Emission spectrum of the solution was recorded by using spectrofluorometer from JASCO. Size, shape and particle size distributions were determined using a JEOL JEM-2011 transmission electron microscope operated at an accelerating voltage of 200 kV. Images were recorded using a Gatan DualVision 600t CCD camera attached to the microscope and were analyzed using Gatan Digital Micrograph Version 3.11.1. The TEM was calibrated for diffraction and imaging mode using standard samples. The resolution of the system was calibrated with manganese (Mn). Samples were prepared for TEM analysis by placing a drop of the solution on a carbon coated copper grid and drying in air. The energy dispersive X-ray analysis was undertaken with a Princeton Gamma Tech Prism 1G system with a 10 mm2 silicon detector attached to the TEM and the peaks were analysed with Imix 10.594 software.
3. Results and discussion A typical TEM image of the Ag nanoparticles in Fig. 1 shows the spherical particles of 10 ± 5 nm diameter. The particles were observed to be highly monodispersed and uniformly distributed. As
Fig. 1. TEM image of the silver nanoparticles. Inset shows the digital photograph of PVP stabilized silver nanoparticles in ethanol medium.
the TEM analysis was carried out on 100 times dilution of colloidal suspension only few particles were observed in the small section of high-resolution image. Use of microwave irradiation in the synthesis is showing promise not only due to faster heating but it also gives internal uniform heating resulting into uniformly distributed monodispersed particles. The formation of silver nanoparticles was confirmed by change in colour of the solution. The colourless silver nitrate solution turns yellow within 5 s of microwave irradiation. Inset in Fig. 1 shows the digital photograph of Ag nanoparticles in PVP ethanol solution. The visible spectrum (Fig. 2) of silver nanoparticles shows the characteristic surface plasmon band at 416 nm that is slightly higher than our previous report [5,13]. The surface plasmon band not only depends on the particle size but also on the refractive index of the surrounding medium and that is the reason for slightly red shift on surface plasmon resonance peak for these silver nanoparticles. To examine the fluorescence property of these
Fig. 2. UV–vis and fluorescence spectroscopy of silver nanoparticles.
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A. Pal et al. / Materials Chemistry and Physics 114 (2009) 530–532
dispersive X-ray spectroscopy (EDX) (Fig. 4) confirms the formation of silver particles. The EDX spectrum was taken from a random assembly of Ag nanoparticles. The signal of Cu originated from the carbon coated Cu grid. 4. Conclusions Spherical, monodispersed silver nanoparticles were synthesizes under microwave oven by using PVP as a stabilizing agent and ethanol as reducing agent. This rapid synthesis technique can be a promising method for the preparation of highly monodispersed spherical silver nanoparticles and that can be applicable to the other noble metals also. Acknowledgements Fig. 3. High-resolution TEM image of silver nanoparticles.
The authors are thankful to GUJCOST (Gadhinagar, Gujarat) for the financial support. References
Fig. 4. Energy dispersive X-ray for silver nanoparticles.
particles the solution was excited at 416 nm and emission of fluorescence was observed at 491 nm (Fig. 2). For Ag nanoparticles stabilized by [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] in solutions Gao et al. [24] have reported emission band at 486 nm upon excitation at 408 nm. The origin of the fluorescence can be attributed to the promotion of d band electrons of the silver metal nanoparticles on absorption of the incident photon energy, to higher electronic states in the sp-band. The high-resolution TEM image of these silver nanoparticles (Fig. 3) reveals crystalline character and ordered orientations of the lattice fringes. The energy
[1] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Soc. Rev. 29 (2000) 27. [2] T. Vo-Dinh, F. Yan, M.B. Wabuyele, J. Raman Spectrosc. 36 (2005) 640. [3] G. Wei, H. Zhou, Z. Liu, Z. Li, Appl. Surface Sci. 240 (2005) 260. [4] K. Aslan, J.R. Lakowicz, C.D. Geddes, Anal. Bioanal. Chem. 382 (2005) 926. [5] A. Pal, S. Shah, S. Devi, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 51. [6] M.T. Reetz, W. Helbig, J. Am. Chem. Soc. 116 (1994) 7401. [7] S.H. Choi, Y.P. Zhang, A. Gopalan, K.P. Lee, H.D. Kang, Colloids Surf. A: Physicochem. Eng. Aspects 256 (2005) 165. [8] H. Willwohl, J. Wolfrum, V. Zumbach, P. Albers, K. Seibold, J. Phys. Chem. 98 (1994) 2242. [9] Z. Li, Y. Li, X.F. Qian, J. Yin, Z.K. Zhu, Appl. Surface Sci. 250 (2005) 109. [10] K.S. Suslick, S.B. Choe, A.A. Cichowlas, Nature 353 (1991) 414. [11] K. Hayakawa, S. Iwama, J. Cryst. Growth 99 (1990) 188. [12] A. Murugadoss, A. Chattopadhyay, Nanotechnology 19 (2008) 015603. [13] (a) A. Pal, S. Shah, S. Devi, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 483; (b) A. Pal, S. Shah, S. Devi, Aust. J. Chem. 61 (2008) 66. [14] H. Wang, X. Qiao, J. Chen, X. Wang, S. Ding, Mater. Chem. Phys. 94 (2005) 449. [15] Y. Gao, P. Jian, L. Song, J.X. Wang, L.F. Liu, Y.J. Xiang, Z.X. Zhang, X.W. Zhao, X.Y. Dou, S.D. Luo, W.Y. Zhou, S.S. Xie, J. Crystal Growth 289 (2006) 376. [16] M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tusji, Chem. Euro. J. 11 (2005) 440. [17] H. He, J. Zhu, N.J. Tao, L.A. Nagahara, I. Amlani, R. Tsui, J. Am. Chem. Soc. 123 (2001) 7730. [18] E. Jager, O. Ingalis, L. Lundstrom, Science 288 (2000) 2335. [19] R. Bashyam, P. Zelenay, Nature 443 (63) (2006) 13. [20] B.S. Sih, O.M. Wolf, Chem. Commun. (2005) 3375. [21] D. Ievlev, I. Rabin, W. Schulze, G. Ertl, Chem. Phys. Lett. 328 (2000) 142. [22] J. Zheng, R.M. Dickson, J. Am. Chem. Soc. 124 (2002) 13982. [23] L. Fu-Ken, H. Pei-Wen, C. Yu-Cheng, K. Chu-Jung, K. Fu-Hsiang, C. Tieh, J. Cryst. Growth 273 (2005) 439. [24] J. Gao, F. Jun, L. Cuikun, L. Jun, H. Yanchun, Y. Xiang, P. Caiyuan, Langmuir 20 (2004) 9775.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Microwave-assisted synthesis of silver nanoparticles using ethanol as a reducing agent Angshuman Pal, Sunil Shah, Surekha Devi ∗ Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 11 November 2008 Accepted 17 November 2008 Keywords: Nanostructures Electron microscopy Optical properties
a b s t r a c t Silver nanoparticles were prepared by microwave irradiation of silver nitrate (AgNO3 ) solution in ethanolic medium using polyvinylpyrrolidone (PVP) as a stabilizing agent. Ethanol was observed to act as a reducing agent in the presence of microwave. Appearance of surface plasmon band at 416 nm indicated the formation of silver nanoparticles. Highly monodispersed stable polycrystalline silver nanoparticles were obtained within 5 s of microwave irradiation. Through transmission electron microscopy silver nanoparticles were observed to be spherical with 10 ± 5 nm diameter. Silver nanoparticles exhibited fluorescence band at 491 nm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanosize metal particles have attracted significant attention because of their unusual size-dependent optical and electronic properties. Among the metals, silver nanoparticles show potential applications in various fields such as the environment, biomedicinal, catalysis, optics and electronics [1]. In particular, Ag nanoparticles are used in diagnostic biomedical optical imaging [2]. The surface plasmon resonance and large effective scattering cross-section of individual silver nanoparticles make them ideal candidates for molecular labelling, where phenomenon such as surface enhanced Raman scattering can be exploited [3]. Since the anisotropic nanoparticles have greater surface area than spherical nanoparticles, their metal enhanced fluorescence is higher than that of spherical nanoparticles [4]. Silver nanoparticles can be synthesized using various methods such as chemical reduction [5], electrochemical [6], ␥-radiation [7], laser ablation [8], photochemical [9], sonochemical [10] and sputtering [11]. Among these the most popular method for preparation of silver colloid is chemical reduction of silver salt in the presence of any stabilizing agent. Most commonly used stabilizing agents are polymers [12] and surfactants [13]. Recently for the preparation of silver nanoparticles, microwave heating (MW) has been reported to have better promise over thermal heating. In the microwave frequency range 300 MHz to 300 GHz, polar molecules such as H2 O try to orient with the electric field. When the dipolar molecules try to re-orient with respect to an alternating electric field, they lose energy in the form of heat
∗ Corresponding author. Tel.: +91 2652795552. E-mail address: surekha [email protected] (S. Devi). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.056
by molecular friction. The MW power dissipation per unit volume in a material (P) is given by Eq. (1) P = cE 2 fε = cE 2 fε tan ı
(1)
where c is a velocity of radiation, E is an electric field in the material, f is frequency of radiation, and ε and ε are the dielectric and dielectric loss constants, respectively. ε represents the relative permittivity, which is a measure of the ability of a molecule to be polarized by an electric field and tan ı = ε /ε is the energy dissipation factor or loss tangent. In the above equation ε is most important physical parameter that describes the ability of a material to heat in the MW field. Water and alcohols have high dielectric losses and a high reducing ability. Therefore they are the ideal solvents for microwave heating. The boiling point of ethanol is 78 ◦ C and ε and ε values are 6.08 and 24.3 respectively. Wang et al. [14] have synthesized shorter than 50 nm Ag nanoparticles by dissolving PVP, glucose and sodium hydroxide in water at 60 ◦ C through thermal heating. Gao et al. [15] have reported the synthesis of silver nanoparticles by PVP-assisted N,N-dimethylformamide (DMF) reduction at 140 ◦ C for 3 h. They could get Ag decahedrons with edge size 80 nm. Tusji and co-workers [16] have reviewed the preparation of Ag nanostructures in PVP using microwave. Polymer–metal nanoparticle composites seem to receive special attention because of their application potential in sensors [17], actuators [18], electrodes for fuel cells [19], optical and optoelectronic devices [20]. Silver nanoparticles have characteristic fluorescence emission property [21]. Zheng et al. [22] observed fluorescence emission for dendrimer-encapsulated silver nanodots. For all the above applications, especially for sensors and optical devices highly monodispersed particles are required.
A. Pal et al. / Materials Chemistry and Physics 114 (2009) 530–532
531
Here in this paper we are describing the preparation of spherical, monodispersed, PVP stabilize silver nanoparticles under MW heating using ethanol as a reducing agent. 2. Experimental 2.1. Materials All chemicals and materials were used as received: AgNO3 and polyvinylpyrrolidone (PVP) (Mw = 10,000) were purchased from Sigma–Aldrich, UK and ethanol from Fluka. 2.2. Preparation of PVP-silver nanoparticle composites For the synthesis of Ag nanoparticles SHARP make microwave oven (model: R259) was used. In a typical procedure, 10 ml of 1% (w/v) ethanolic solution of PVP and 0.2 ml of 0.1 M AgNO3 were taken in a 25 ml closed conical flask and placed in a microwave oven that was operated at the 100% power of 800 W and frequency 2450 MHz for 5 s. The colourless solution instantaneously turned to the characteristic pale yellow colour, indicating the formation of silver nanoparticles. The advantage of microwave-mediated synthesis over the conventional heating is the improved kinetics of the reaction generally by one or two order of magnitude, due to rapid initial heating and the generation of localized high-temperature zones at reaction sites [23]. 2.3. Characterization Characteristic optical properties of Ag nanoparticles were recorded using PerkinElmer Lambda 35 UV–vis spectrophotometer. Spectra were recorded using 1 cm3 quartz cell. Emission spectrum of the solution was recorded by using spectrofluorometer from JASCO. Size, shape and particle size distributions were determined using a JEOL JEM-2011 transmission electron microscope operated at an accelerating voltage of 200 kV. Images were recorded using a Gatan DualVision 600t CCD camera attached to the microscope and were analyzed using Gatan Digital Micrograph Version 3.11.1. The TEM was calibrated for diffraction and imaging mode using standard samples. The resolution of the system was calibrated with manganese (Mn). Samples were prepared for TEM analysis by placing a drop of the solution on a carbon coated copper grid and drying in air. The energy dispersive X-ray analysis was undertaken with a Princeton Gamma Tech Prism 1G system with a 10 mm2 silicon detector attached to the TEM and the peaks were analysed with Imix 10.594 software.
3. Results and discussion A typical TEM image of the Ag nanoparticles in Fig. 1 shows the spherical particles of 10 ± 5 nm diameter. The particles were observed to be highly monodispersed and uniformly distributed. As
Fig. 1. TEM image of the silver nanoparticles. Inset shows the digital photograph of PVP stabilized silver nanoparticles in ethanol medium.
the TEM analysis was carried out on 100 times dilution of colloidal suspension only few particles were observed in the small section of high-resolution image. Use of microwave irradiation in the synthesis is showing promise not only due to faster heating but it also gives internal uniform heating resulting into uniformly distributed monodispersed particles. The formation of silver nanoparticles was confirmed by change in colour of the solution. The colourless silver nitrate solution turns yellow within 5 s of microwave irradiation. Inset in Fig. 1 shows the digital photograph of Ag nanoparticles in PVP ethanol solution. The visible spectrum (Fig. 2) of silver nanoparticles shows the characteristic surface plasmon band at 416 nm that is slightly higher than our previous report [5,13]. The surface plasmon band not only depends on the particle size but also on the refractive index of the surrounding medium and that is the reason for slightly red shift on surface plasmon resonance peak for these silver nanoparticles. To examine the fluorescence property of these
Fig. 2. UV–vis and fluorescence spectroscopy of silver nanoparticles.
532
A. Pal et al. / Materials Chemistry and Physics 114 (2009) 530–532
dispersive X-ray spectroscopy (EDX) (Fig. 4) confirms the formation of silver particles. The EDX spectrum was taken from a random assembly of Ag nanoparticles. The signal of Cu originated from the carbon coated Cu grid. 4. Conclusions Spherical, monodispersed silver nanoparticles were synthesizes under microwave oven by using PVP as a stabilizing agent and ethanol as reducing agent. This rapid synthesis technique can be a promising method for the preparation of highly monodispersed spherical silver nanoparticles and that can be applicable to the other noble metals also. Acknowledgements Fig. 3. High-resolution TEM image of silver nanoparticles.
The authors are thankful to GUJCOST (Gadhinagar, Gujarat) for the financial support. References
Fig. 4. Energy dispersive X-ray for silver nanoparticles.
particles the solution was excited at 416 nm and emission of fluorescence was observed at 491 nm (Fig. 2). For Ag nanoparticles stabilized by [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] in solutions Gao et al. [24] have reported emission band at 486 nm upon excitation at 408 nm. The origin of the fluorescence can be attributed to the promotion of d band electrons of the silver metal nanoparticles on absorption of the incident photon energy, to higher electronic states in the sp-band. The high-resolution TEM image of these silver nanoparticles (Fig. 3) reveals crystalline character and ordered orientations of the lattice fringes. The energy
[1] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Soc. Rev. 29 (2000) 27. [2] T. Vo-Dinh, F. Yan, M.B. Wabuyele, J. Raman Spectrosc. 36 (2005) 640. [3] G. Wei, H. Zhou, Z. Liu, Z. Li, Appl. Surface Sci. 240 (2005) 260. [4] K. Aslan, J.R. Lakowicz, C.D. Geddes, Anal. Bioanal. Chem. 382 (2005) 926. [5] A. Pal, S. Shah, S. Devi, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 51. [6] M.T. Reetz, W. Helbig, J. Am. Chem. Soc. 116 (1994) 7401. [7] S.H. Choi, Y.P. Zhang, A. Gopalan, K.P. Lee, H.D. Kang, Colloids Surf. A: Physicochem. Eng. Aspects 256 (2005) 165. [8] H. Willwohl, J. Wolfrum, V. Zumbach, P. Albers, K. Seibold, J. Phys. Chem. 98 (1994) 2242. [9] Z. Li, Y. Li, X.F. Qian, J. Yin, Z.K. Zhu, Appl. Surface Sci. 250 (2005) 109. [10] K.S. Suslick, S.B. Choe, A.A. Cichowlas, Nature 353 (1991) 414. [11] K. Hayakawa, S. Iwama, J. Cryst. Growth 99 (1990) 188. [12] A. Murugadoss, A. Chattopadhyay, Nanotechnology 19 (2008) 015603. [13] (a) A. Pal, S. Shah, S. Devi, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 483; (b) A. Pal, S. Shah, S. Devi, Aust. J. Chem. 61 (2008) 66. [14] H. Wang, X. Qiao, J. Chen, X. Wang, S. Ding, Mater. Chem. Phys. 94 (2005) 449. [15] Y. Gao, P. Jian, L. Song, J.X. Wang, L.F. Liu, Y.J. Xiang, Z.X. Zhang, X.W. Zhao, X.Y. Dou, S.D. Luo, W.Y. Zhou, S.S. Xie, J. Crystal Growth 289 (2006) 376. [16] M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tusji, Chem. Euro. J. 11 (2005) 440. [17] H. He, J. Zhu, N.J. Tao, L.A. Nagahara, I. Amlani, R. Tsui, J. Am. Chem. Soc. 123 (2001) 7730. [18] E. Jager, O. Ingalis, L. Lundstrom, Science 288 (2000) 2335. [19] R. Bashyam, P. Zelenay, Nature 443 (63) (2006) 13. [20] B.S. Sih, O.M. Wolf, Chem. Commun. (2005) 3375. [21] D. Ievlev, I. Rabin, W. Schulze, G. Ertl, Chem. Phys. Lett. 328 (2000) 142. [22] J. Zheng, R.M. Dickson, J. Am. Chem. Soc. 124 (2002) 13982. [23] L. Fu-Ken, H. Pei-Wen, C. Yu-Cheng, K. Chu-Jung, K. Fu-Hsiang, C. Tieh, J. Cryst. Growth 273 (2005) 439. [24] J. Gao, F. Jun, L. Cuikun, L. Jun, H. Yanchun, Y. Xiang, P. Caiyuan, Langmuir 20 (2004) 9775.