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A novel synthesis route for ethylenediamine-protected ruthenium nanoparticles
Journal of Colloid and Interface Science 268 (2003) 77–80 www.elsevier.com/locate/jcis
A novel synthesis route for ethylenediamine-protected ruthenium nanoparticles Jim Yang Lee,a,b,∗ Jun Yang,a T.C. Deivaraj,b and Heng-Phon Too b,c a Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore b Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, 117576 Singapore c Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore
Received 25 February 2003; accepted 30 July 2003
Abstract A novel method has been developed to prepare water-dispersible ethylenediamine (en)-stabilized ruthenium nanoparticles. The procedure involves the reduction of an en–RuCl3 complex by sodium borohydride. The Ru nanoparticles so prepared are fairly stable in water. TEM imaging shows a mean diameter of about 2.1 nm for the particles and a narrow particle size distribution. 2003 Elsevier Inc. All rights reserved. Keywords: Nanoparticles; Ethylenediamine; Ruthenium nanoparticles
1. Introduction The properties of metal nanoparticles are dominated by size and surface effects not present in the bulk [1,2] but which may enhance material performance in certain application areas [3–9]. For instance, ruthenium metal nanoparticles are renowned for their catalytic activity [5,6,10]. One of our current research interests is the self-assembly of Ru and Pt nanoparticles using biological molecules as templates and molecular design tools. The approach necessitates the use of metal nanoparticles dispersible in water. Unfortunately, most current methods of Ru nanoparticle preparation are carried out in organic solvents [11–13] and the products are not water-dispersible. The nonaqueous route to nanoparticles is therefore of limited use for our purpose. However, reports on Ru nanoparticle preparation in water are relatively scarce [14,15]. In our search for simple procedures for preparing water-based Ru nanoparticle dispersions, we discovered ethylenediamine (en) as a capable capping agent suitable for deployment in aqueous media. The complete procedure leading to the preparation of water-dispersible enprotected ruthenium nanoparticles is reported in this article.
* Corresponding author.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.07.036
2. Materials and methods Ruthenium (III) chloride hydrate from Aldrich, ethylenediamine (99%) from Avocado Research Chemicals, ethanol from Merck, and sodium borohydride from Fluka were used as received. Deionized water was purified by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. Transmission electron microscopy (TEM) using a Philips CM 300 FEG system operating at 200 kV and a magnification of 1 500 000× was used to size the particles. For TEM measurements a drop of the metal nanoparticle solution was placed onto a 3-mm copper grid covered with a continuous carbon film. Excess solution was removed by an adsorbent paper. The mean particle size and particle size distribution were obtained from a few randomly chosen areas in the digitized image containing approximately 200 nanoparticles each. X-ray photoelectron spectroscopic analysis (XPS) of the samples was carried out on a VG ESCALAB MKII spectrometer. Narrow scan photoelectron spectra were recorded for the Ru3p region. Fourier transform infrared (FT-IR) spectra of KBr pellets of samples were obtained from a BIO-RAD FTS 135 FT-IR spectrophotometer. Vacuum-dried samples were also characterized by X-ray powder diffraction (XRD) measurements on a Rigaku D/Max-3B diffractometer employing CuKα radiations.
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J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
In a typical experiment, 0.1 ml of ethylenediamine was added dropwise to 20 ml of 2 mM stirred aqueous RuCl3 solution. Stirring was continued for five more minutes after the end of addition. A black precipitate was formed, which is presumably a ruthenium chloride–ethylenediamine complex [1]. The black solid was spun down in an ultracentrifuge and the supernatant liquid was discarded. The black precipitate thus obtained was dissolved in 30 ml ethanol. To this ethanolic solution, 1 ml of freshly prepared 0.1 M aqueous solution of NaBH4 was added dropwise under constant stirring. Stirring was continued for three more minutes at the end of addition, thereafter the ruthenium nanoparticles salted out as a black precipitate. The ruthenium nanoparticles were centrifuged and rinsed thrice with a 30% (v/v) water/ethanol mixture to remove any unbound ethylenediamine and inorganic (Na, Cl, B) impurities. The cleansed precipitates were then dried under vacuum. Ruthenium nanoparticles thus obtained were readily dispersible in water.
The reduction of the ruthenium–ethylenediamine complex by sodium borohydride can be categorically represented as follows [17]: − + 8Ru3+ + 3BH− 4 + 12H2 O → 8Ru + 3B(OH)4 + 24H . (1)
Figure 1b shows the FT-IR spectrum of the Ru nanoparticles. The IR fingerprints of ethylenediamine remain evident, indicating that ethylenediamine was present on the surface of the Ru nanoparticles and serving as a capping agent for the latter. The peaks due to ethylenediamine were shifted to slightly higher wavenumbers suggesting amine coordination to the Ru nanoparticle surface. The ruthenium nanoparticles obtained by the current procedure could be easily dispersed in water, giving rise to a transparent dark brown colloidal solution of ruthenium. The colloidal solution displayed excellent stability. No precipitation was visible even after storage for three weeks. Figure 2 is a representative TEM image of ruthenium nanoparticles that had been redispersed in water. The particles were well
3. Results and discussion When ethylenediamine was introduced to the aqueous solution of RuCl3 with stirring, a black precipitate appeared within a very short time. Figure 1a shows the FT-IR spectrum of solid 1, suggesting the possibility of coordinating complex formation between ethylenediamine and ruthenium (III) chloride. The exact structure of solid 1 is not known at present, although a structure similar to Scheme 1 may be proposed based on the report of Broomhead and KaneMaguire in 1967 [16]. A detailed characterization of compound 1 is in progress and will be reported once the work is complete.
Fig. 1. FT-IR spectra of (a) RuCl3 –en complex [1] and (b) en-capped Ru nanoparticles.
Scheme 1. Proposed structure for the RuCl3 –ethylenediamine complex.
Fig. 2. TEM image of Ru nanoparticles and the electron diffraction pattern of a selected region in the TEM image.
J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
Fig. 3. Histogram showing the narrow size distribution of Ru nanoparticles.
Fig. 4. X-ray photoelectron spectra of Ru nanoparticles: the peak centering at 461.8 eV is the line for Ru0 ; the peak centering at 463.42 eV is the line for RuIV .
separated with an average diameter of about 2.1 nm. The particle size distribution, obtained from counting 200 particles imaged by TEM, was fairly narrow, as shown in the histogram of Fig. 3. The electron diffraction pattern of representative Ru nanoparticles is also shown in Fig. 2. The nanoparticles were examined by XPS to confirm their compositions. Unfortunately, the Ru3d3/2 peak overlaps with the C1s peak, preventing an unambiguous analysis of the nanoparticle surface. However, the Ru3p3/2 signal could be deconvoluted into two peaks of different intensities at 461.8 and 463.4 eV, respectively (Fig. 4). The peak at 461.8 eV corresponds to the zerovalent state of Ru, while the peak at 463.4 eV may be assigned to RuIV (e.g., RuO2 ) [18–21]. In addition, the XPS signal due to nitrogen was also detected, which further confirms the presence of ethylenediamine on the surface of the Ru nanoparticles. A wide-scan XPS analysis was also carried out for the sample. The results showed no evidence for the presence of Na (∼1070 eV) and B (∼190 eV), therefore eliminating the possibility of metal borohydride or metal boride formation
79
Fig. 5. Powder X-ray diffraction pattern of ethylenediamine-stabilized Ru metal particles.
that is sometime found in the reduction of group VIII metal coordinating complexes with NaBH4 [22]. The powder X-ray diffraction pattern of a representative sample is shown in Fig. 5. The two broad peaks around 2θ = 37◦ and 58◦ correlate with the presence of RuO2 [23– 26] and amorphous ruthenium nanoparticles, respectively [27–30]. The diffused electron diffraction pattern in Fig. 2 also indicates the predominance of the amorphous phase. The interaction between ethylenediamine and the surface of ruthenium nanoparticles was not very strong, and the solubility of the nanoparticles in water was found to diminish with time, indicating slow but progressive loss of ethylenediamine. On the other hand, this is an encouraging indication of the suitability of ethylenediamine-protected ruthenium nanoparticles as precursors in more elaborate functionalization schemes, where it is necessary to displace ethylenediamine sequentially using other ligands.
4. Conclusion A self-consistent rationalization for these experimental observations is the persistence of ethylenediamine attachment to Ru even after chemical reduction of the Ru precursor salt to Ru0 . The presence of ethylenediamine is believed to contribute to the inhibition of Ru nanoparticle agglomeration in the aqueous environment.
References [1] G. Schmid, Chem. Rev. 92 (1992) 1709. [2] P. Mulvaney, Langmuir 12 (1996) 788. [3] J.S. Bradley, in: G. Schmid (Ed.), In Cluster and Colloids, From Theory to Applications, VCH, Weinheim, 1994, p. 459, Ch. 6. [4] S. Chen, U. Nickel, J. Chem. Soc. Chem. Commun. 133 (1996). [5] G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 101. [6] G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 202. [7] J. Osuna, D. de Caro, C. Amiens, B. Chaudret, E. Snoeck, M. Respaud, J. Broto, A. Tert, J. Phys. Chem. 100 (1996) 14571.
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J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
[8] D. de Caro, T.O. Ely, A. Mari, B. Chaudret, E. Snoeck, M. Respaud, J. Broto, A. Fert, Chem. Mater. 8 (1996) 1987. [9] M.P. Andres, G.A. Ozin, Chem. Mater. 1 (1989) 174. [10] L.N. Lewis, L. Lewis, Chem. Mater. 1 (1989) 106. [11] C.-H. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante, M.-J. Casanove, J. Am. Chem. Soc. 123 (2001) 7584. [12] Y. Wang, J.-W. Ren, K. Deng, L.-L. Gui, Y.-Q. Tang, Chem. Mater. 12 (2000) 1622. [13] W.-X. Tu, H.-F. Liu, J. Mater. Chem. (2000) 2207. [14] J. Kiwi, M. Graetzel, Chimia 33 (1979) 289. [15] N. Toshima, K. Hirakawa, Polym. J. 31 (1999) 1127. [16] J.A. Broomhead, L.A.P. Kane-Maguire, J. Chem. Soc. A 4 (1967) 546. [17] S. Chen, K. Kimura, Langmuir 15 (1999) 1075. [18] Z. Liu, J.Y. Lee, M. Han, W.X. Chen, L.M. Gan, J. Mater. Chem. (2002) 2453. [19] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST Standard Reference Database 20, Version 3.2 (Web version).
[20] X. Zhang, K.-Y. Chan, Chem. Mater. 15 (2003) 451. [21] A.S. Arico, P. Creti, H. Kim, R. Mantegna, N. Giordano, V. Antonucci, J. Electrochem. Soc. 143 (12) (1996) 3950. [22] I. Jardine, F.J. McQuillin, Chem. Commun. (1969) 502. [23] S. Music, S. Popovic, M. Maljkovic, K. Furic, A. Gajovic, Mater. Lett. 56 (2002) 806. [24] W.E. van Zyl, L. Winnubst, T.P. Raming, R. Schmuhl, H. Verweij, J. Mater. Chem. 12 (2002) 708. [25] W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, J. Phys. Chem. B 106 (2002) 12677. [26] I. Zhitomirsky, L. Gal-Or, Mater. Lett. 31 (1997) 155. [27] D.-S. Lee, T.-K. Liu, J. Non-Cryst. Solids 311 (2002) 323. [28] H. Yamashita, H. Yoshikawa, T. Funabiki, S. Yoshida, J. Chem. Soc. Faraday Trans. I 82 (1986) 1771. [29] J. Deng, J. Yang, S. Sheng, H. Chen, G. Xiong, J. Catal. 150 (1994) 434. [30] Y. Okamoto, Y. Nitta, T. Imanaka, S. Teranishi, J. Chem. Soc. Faraday Trans. I 76 (1980) 998.
A novel synthesis route for ethylenediamine-protected ruthenium nanoparticles Jim Yang Lee,a,b,∗ Jun Yang,a T.C. Deivaraj,b and Heng-Phon Too b,c a Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore b Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, 117576 Singapore c Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore
Received 25 February 2003; accepted 30 July 2003
Abstract A novel method has been developed to prepare water-dispersible ethylenediamine (en)-stabilized ruthenium nanoparticles. The procedure involves the reduction of an en–RuCl3 complex by sodium borohydride. The Ru nanoparticles so prepared are fairly stable in water. TEM imaging shows a mean diameter of about 2.1 nm for the particles and a narrow particle size distribution. 2003 Elsevier Inc. All rights reserved. Keywords: Nanoparticles; Ethylenediamine; Ruthenium nanoparticles
1. Introduction The properties of metal nanoparticles are dominated by size and surface effects not present in the bulk [1,2] but which may enhance material performance in certain application areas [3–9]. For instance, ruthenium metal nanoparticles are renowned for their catalytic activity [5,6,10]. One of our current research interests is the self-assembly of Ru and Pt nanoparticles using biological molecules as templates and molecular design tools. The approach necessitates the use of metal nanoparticles dispersible in water. Unfortunately, most current methods of Ru nanoparticle preparation are carried out in organic solvents [11–13] and the products are not water-dispersible. The nonaqueous route to nanoparticles is therefore of limited use for our purpose. However, reports on Ru nanoparticle preparation in water are relatively scarce [14,15]. In our search for simple procedures for preparing water-based Ru nanoparticle dispersions, we discovered ethylenediamine (en) as a capable capping agent suitable for deployment in aqueous media. The complete procedure leading to the preparation of water-dispersible enprotected ruthenium nanoparticles is reported in this article.
* Corresponding author.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.07.036
2. Materials and methods Ruthenium (III) chloride hydrate from Aldrich, ethylenediamine (99%) from Avocado Research Chemicals, ethanol from Merck, and sodium borohydride from Fluka were used as received. Deionized water was purified by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. Transmission electron microscopy (TEM) using a Philips CM 300 FEG system operating at 200 kV and a magnification of 1 500 000× was used to size the particles. For TEM measurements a drop of the metal nanoparticle solution was placed onto a 3-mm copper grid covered with a continuous carbon film. Excess solution was removed by an adsorbent paper. The mean particle size and particle size distribution were obtained from a few randomly chosen areas in the digitized image containing approximately 200 nanoparticles each. X-ray photoelectron spectroscopic analysis (XPS) of the samples was carried out on a VG ESCALAB MKII spectrometer. Narrow scan photoelectron spectra were recorded for the Ru3p region. Fourier transform infrared (FT-IR) spectra of KBr pellets of samples were obtained from a BIO-RAD FTS 135 FT-IR spectrophotometer. Vacuum-dried samples were also characterized by X-ray powder diffraction (XRD) measurements on a Rigaku D/Max-3B diffractometer employing CuKα radiations.
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J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
In a typical experiment, 0.1 ml of ethylenediamine was added dropwise to 20 ml of 2 mM stirred aqueous RuCl3 solution. Stirring was continued for five more minutes after the end of addition. A black precipitate was formed, which is presumably a ruthenium chloride–ethylenediamine complex [1]. The black solid was spun down in an ultracentrifuge and the supernatant liquid was discarded. The black precipitate thus obtained was dissolved in 30 ml ethanol. To this ethanolic solution, 1 ml of freshly prepared 0.1 M aqueous solution of NaBH4 was added dropwise under constant stirring. Stirring was continued for three more minutes at the end of addition, thereafter the ruthenium nanoparticles salted out as a black precipitate. The ruthenium nanoparticles were centrifuged and rinsed thrice with a 30% (v/v) water/ethanol mixture to remove any unbound ethylenediamine and inorganic (Na, Cl, B) impurities. The cleansed precipitates were then dried under vacuum. Ruthenium nanoparticles thus obtained were readily dispersible in water.
The reduction of the ruthenium–ethylenediamine complex by sodium borohydride can be categorically represented as follows [17]: − + 8Ru3+ + 3BH− 4 + 12H2 O → 8Ru + 3B(OH)4 + 24H . (1)
Figure 1b shows the FT-IR spectrum of the Ru nanoparticles. The IR fingerprints of ethylenediamine remain evident, indicating that ethylenediamine was present on the surface of the Ru nanoparticles and serving as a capping agent for the latter. The peaks due to ethylenediamine were shifted to slightly higher wavenumbers suggesting amine coordination to the Ru nanoparticle surface. The ruthenium nanoparticles obtained by the current procedure could be easily dispersed in water, giving rise to a transparent dark brown colloidal solution of ruthenium. The colloidal solution displayed excellent stability. No precipitation was visible even after storage for three weeks. Figure 2 is a representative TEM image of ruthenium nanoparticles that had been redispersed in water. The particles were well
3. Results and discussion When ethylenediamine was introduced to the aqueous solution of RuCl3 with stirring, a black precipitate appeared within a very short time. Figure 1a shows the FT-IR spectrum of solid 1, suggesting the possibility of coordinating complex formation between ethylenediamine and ruthenium (III) chloride. The exact structure of solid 1 is not known at present, although a structure similar to Scheme 1 may be proposed based on the report of Broomhead and KaneMaguire in 1967 [16]. A detailed characterization of compound 1 is in progress and will be reported once the work is complete.
Fig. 1. FT-IR spectra of (a) RuCl3 –en complex [1] and (b) en-capped Ru nanoparticles.
Scheme 1. Proposed structure for the RuCl3 –ethylenediamine complex.
Fig. 2. TEM image of Ru nanoparticles and the electron diffraction pattern of a selected region in the TEM image.
J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
Fig. 3. Histogram showing the narrow size distribution of Ru nanoparticles.
Fig. 4. X-ray photoelectron spectra of Ru nanoparticles: the peak centering at 461.8 eV is the line for Ru0 ; the peak centering at 463.42 eV is the line for RuIV .
separated with an average diameter of about 2.1 nm. The particle size distribution, obtained from counting 200 particles imaged by TEM, was fairly narrow, as shown in the histogram of Fig. 3. The electron diffraction pattern of representative Ru nanoparticles is also shown in Fig. 2. The nanoparticles were examined by XPS to confirm their compositions. Unfortunately, the Ru3d3/2 peak overlaps with the C1s peak, preventing an unambiguous analysis of the nanoparticle surface. However, the Ru3p3/2 signal could be deconvoluted into two peaks of different intensities at 461.8 and 463.4 eV, respectively (Fig. 4). The peak at 461.8 eV corresponds to the zerovalent state of Ru, while the peak at 463.4 eV may be assigned to RuIV (e.g., RuO2 ) [18–21]. In addition, the XPS signal due to nitrogen was also detected, which further confirms the presence of ethylenediamine on the surface of the Ru nanoparticles. A wide-scan XPS analysis was also carried out for the sample. The results showed no evidence for the presence of Na (∼1070 eV) and B (∼190 eV), therefore eliminating the possibility of metal borohydride or metal boride formation
79
Fig. 5. Powder X-ray diffraction pattern of ethylenediamine-stabilized Ru metal particles.
that is sometime found in the reduction of group VIII metal coordinating complexes with NaBH4 [22]. The powder X-ray diffraction pattern of a representative sample is shown in Fig. 5. The two broad peaks around 2θ = 37◦ and 58◦ correlate with the presence of RuO2 [23– 26] and amorphous ruthenium nanoparticles, respectively [27–30]. The diffused electron diffraction pattern in Fig. 2 also indicates the predominance of the amorphous phase. The interaction between ethylenediamine and the surface of ruthenium nanoparticles was not very strong, and the solubility of the nanoparticles in water was found to diminish with time, indicating slow but progressive loss of ethylenediamine. On the other hand, this is an encouraging indication of the suitability of ethylenediamine-protected ruthenium nanoparticles as precursors in more elaborate functionalization schemes, where it is necessary to displace ethylenediamine sequentially using other ligands.
4. Conclusion A self-consistent rationalization for these experimental observations is the persistence of ethylenediamine attachment to Ru even after chemical reduction of the Ru precursor salt to Ru0 . The presence of ethylenediamine is believed to contribute to the inhibition of Ru nanoparticle agglomeration in the aqueous environment.
References [1] G. Schmid, Chem. Rev. 92 (1992) 1709. [2] P. Mulvaney, Langmuir 12 (1996) 788. [3] J.S. Bradley, in: G. Schmid (Ed.), In Cluster and Colloids, From Theory to Applications, VCH, Weinheim, 1994, p. 459, Ch. 6. [4] S. Chen, U. Nickel, J. Chem. Soc. Chem. Commun. 133 (1996). [5] G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 101. [6] G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 202. [7] J. Osuna, D. de Caro, C. Amiens, B. Chaudret, E. Snoeck, M. Respaud, J. Broto, A. Tert, J. Phys. Chem. 100 (1996) 14571.
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J.Y. Lee et al. / Journal of Colloid and Interface Science 268 (2003) 77–80
[8] D. de Caro, T.O. Ely, A. Mari, B. Chaudret, E. Snoeck, M. Respaud, J. Broto, A. Fert, Chem. Mater. 8 (1996) 1987. [9] M.P. Andres, G.A. Ozin, Chem. Mater. 1 (1989) 174. [10] L.N. Lewis, L. Lewis, Chem. Mater. 1 (1989) 106. [11] C.-H. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante, M.-J. Casanove, J. Am. Chem. Soc. 123 (2001) 7584. [12] Y. Wang, J.-W. Ren, K. Deng, L.-L. Gui, Y.-Q. Tang, Chem. Mater. 12 (2000) 1622. [13] W.-X. Tu, H.-F. Liu, J. Mater. Chem. (2000) 2207. [14] J. Kiwi, M. Graetzel, Chimia 33 (1979) 289. [15] N. Toshima, K. Hirakawa, Polym. J. 31 (1999) 1127. [16] J.A. Broomhead, L.A.P. Kane-Maguire, J. Chem. Soc. A 4 (1967) 546. [17] S. Chen, K. Kimura, Langmuir 15 (1999) 1075. [18] Z. Liu, J.Y. Lee, M. Han, W.X. Chen, L.M. Gan, J. Mater. Chem. (2002) 2453. [19] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST Standard Reference Database 20, Version 3.2 (Web version).
[20] X. Zhang, K.-Y. Chan, Chem. Mater. 15 (2003) 451. [21] A.S. Arico, P. Creti, H. Kim, R. Mantegna, N. Giordano, V. Antonucci, J. Electrochem. Soc. 143 (12) (1996) 3950. [22] I. Jardine, F.J. McQuillin, Chem. Commun. (1969) 502. [23] S. Music, S. Popovic, M. Maljkovic, K. Furic, A. Gajovic, Mater. Lett. 56 (2002) 806. [24] W.E. van Zyl, L. Winnubst, T.P. Raming, R. Schmuhl, H. Verweij, J. Mater. Chem. 12 (2002) 708. [25] W. Dmowski, T. Egami, K.E. Swider-Lyons, C.T. Love, D.R. Rolison, J. Phys. Chem. B 106 (2002) 12677. [26] I. Zhitomirsky, L. Gal-Or, Mater. Lett. 31 (1997) 155. [27] D.-S. Lee, T.-K. Liu, J. Non-Cryst. Solids 311 (2002) 323. [28] H. Yamashita, H. Yoshikawa, T. Funabiki, S. Yoshida, J. Chem. Soc. Faraday Trans. I 82 (1986) 1771. [29] J. Deng, J. Yang, S. Sheng, H. Chen, G. Xiong, J. Catal. 150 (1994) 434. [30] Y. Okamoto, Y. Nitta, T. Imanaka, S. Teranishi, J. Chem. Soc. Faraday Trans. I 76 (1980) 998.