- Email: [email protected]
(polyvinyl acetate) nanocomposites with controllable morphology
5 May 2000
Chemical Physics Letters 321 Ž2000. 504–507 www.elsevier.nlrlocatercplett
Synthesis of lead sulfider žpolyvinyl acetate / nanocomposites with controllable morphology Zhengping Qiao, Yi Xie ) , Meng Chen, Jianguang Xu, Yingjie Zhu, Yitai Qian Structure Research Lab and Department of Chemistry, UniÕersity of Science and Technology of China, Hefei, Anhui 230026, China Received 8 December 1999; in final form 20 January 2000
Abstract PbSrŽpolyvinyl acetate. nanocomposites with different morphology that inorganic particles embedded in polymer matrix and inorganic nanowires in polymer nanotubes, respectively, were first synthesized upon g-irradiation at room temperature and under ambient pressure in a simple system. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction During the past few years, synthetic techniques have been developed to provide feasible ways to prepare nanomaterials having the desired morphology w1,2x. To exploit novel nanodevices such as electronic or optical devices, it is necessary to prepare nanoparticles without their mutual aggregation. Moreover, morphology control is an alternative tool to adjust the optical or catalytic properties of the nanomaterials. Thus a number of strategies on nanostructure fabrication are currently being developed to control not only the size but also the dispersion and shape of the products. Among these strategies, the template self-assembly process has been demonstrated to be effective since the pre-organized tem-
) Corresponding author. Fax: q86-551-363-1760; e-mail: [email protected]
plate regulates the nucleation, growth, morphology and orientation of nanocrystals w3,4x. To date, many kinds of template have been used: colloidal suspensions, zeolites, liquid crystalline, porous anodic aluminum oxide, carbon nanotubes, etc. Recently, various types of polymers have been applied to the synthesis of metallic and semiconductor particles w5,6x. Block copolymers, consisting of different blocks with variant solubility, have long been known to self-assemble in selective solvents to form stable aggregates, such as micelles or micelle-like aggregates, offering a template for further synthesis of nanocomposites. The use of block copolymers has also attracted considerable interest because of their potential for processability and hence ease of application. Nevertheless, there are few reports on simple polymers such as polyŽvinyl acetate., which has both an hydrophilic group ŽC5O. and an hydrophobic part in its monomer molecule, acting as matrix to control the morphology of semiconductors by the self-assembly route. In this Letter, we develop a
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 3 7 7 - 8
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
template method to synthesize nanocomposites with different morphology in a polyŽvinyl acetate. matrix under mild conditions. In heterogeneous systems, such as in an aqueous solution, the limited mutual solubility between vinyl acetate and water make it possible to form a micelle-like structure that the hydrophilic groups shrink together to form cylinder supramolecular to minimize the surface free energy in the upper monomer layer w7x. The driving force for self-assembly of the supramolecular structure primarily comes from the enthalpic contribution of hydrogen-bonding interactions w8–10x. Then the following slow polymerization of vinyl acetate under conditions mild enough to keep the structure of the as-formed supramolecular will produce a tubular polyŽvinyl acetate. template. In such a system, particle growth of the inorganic components will be restricted in the inner hydrophilic cavity of the as-formed supramolecular and polymer tubes, providing a good nanoreactor for both size control and epitaxial growth. Thus, through this simple monomer–supramolecular–polymer transformation process, an inorganicrpolymer nanocable can be synthesized. On the contrary, in a homogeneous system, such as in an ethanol solution, the supramolecular could not form; meanwhile, reactants mixed at the molecular level in the solution will produce the homogeneous composites in which the inorganic component displays homogenous dispersion w11x. To date, studies on PbS, which is one of the most important semiconductors, are fewer than those made using CdS although PbS nanocrystalline or PbSrpolymer nanocomposites have displayed good properties w12–14x. To our knowledge, this is the first time that the PbSrŽpolyvinyl acetate. nanocomposites with controllable morphology have been synthesized. The polymerization of vinyl acetate and preparation of PbS are carried out using a g-irradiation method – an attractive method with the advantage of simplicity, convenience and easy control w7,11,15–19x. In addition, g-irradiation offers an ideal means by which the supramolecules with tubular structure can be solidified to a polymer tube with the desired diameter at room temperature under ambient pressure. The structure of supramolecules will not collapse as in heat polymerization under higher temperature. Also the use of the toxic gas H 2 S and
505
organometallic complex can be avoided in this process.
2. Experimental Solutions were prepared by dissolving analytically pure carbon disulfide Ž0.1% by vrv., lead acetate Ž0.02 mol dmy3 ., vinyl acetate Ž5%, vrv. and isopropyl alcohol Ž23%, vrv. in solventrwater Žsystem A. or ethanolrwater Žsystem B.. The solutions were irradiated in the field of a 5 = 10 4 Ci 60 Co g-ray source. After being irradiated, for system A, the as-prepared nanocomposites ŽSample A. precipitated in the lower water layer immediately after being irradiated since the density of polyŽvinyl acetate. is larger than that of water. System B is a black homogeneous suspension, and a large amount of water was added to separate the nanocomposites ŽSample B.. Both samples were black gel-like masses and were washed with dilute acetic acid and distilled water in sequence to remove the by-products. The final products were dried in vacuum at room temperature for 4 h.
3. Results and discussion The samples were characterized by X-ray powder diffraction ŽXRD. patterns. XRD patterns were obtained on a Japan Rigaku DrMax gA X-ray diffractometer equipped with graphite monochromatized ˚ .. The uneven baseCuK a radiation Ž l s 1.54178 A line in XRD patterns ŽFig. 1. is due to the larger
Fig. 1. XRD patterns of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
506
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
Fig. 2. IR Spectra of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
amount of amorphous polymer component. All of the diffraction peaks can be indexed as PbS with cubic phase ŽJCPDS Card File No. 5-592.. The infrared ŽIR. spectra were made using a Hitachi 850-fluorescence spectrophotometer with a Xe lamp at room temperature. IR spectra were used to characterize the polymer component shown in Fig. 2. From it one can see that the IR spectra of the products are the same and that both are similar to the standard IR spectrum of polyŽvinyl acetate.. In Fig. 2, the strongest peak is at 1736 cmy1 Ž n C 5 O . and the characteristic peaks are at 1237 and 1018 cmy1 Ž n C – O . and at 1375 cmy1 Ž d CH3 . of polyŽvinyl acetate., which confirms the successful polymerization
of vinyl acetate monomer in water and ethanol upon g-irradiation. The peak of low component PbS could not be observed because the peak of PbS is extremely weaker than that of polyŽvinyl acetate. w16x. The morphology and particle sizes were determined by transmission electron microscopy ŽTEM.. TEM images were taken with a Hitachi Model H-800 transmission electron microscope with an accelerating voltage of 200 kV. As predicted, Sample A is shown having a tubular morphology. From the lower TEM magnification of the micrograph of Sample A, one can see the tubular structure being ca. 100 nm in diameter and 10 mm in length ŽFig. 3a.. Careful observation of the TEM micrograph with a higher magnification clearly indicates that Sample A is a nanocable 30 nm in diameter of a PbS core in a 80 nm polyŽvinyl acetate. sheath ŽFig. 3b.. For Sample B, its TEM images are shown in Fig. 3c, from which one can see that the nanocomposites consist of fine quasi-spherical lead-sulfide particles homogeneously dispersed and well separated in the polyŽvinyl acetate. matrix. The diameter of lead-sulfide is in the range of 13 nm, similar to the XRD result Ž15 nm. which was calculated from the half-width of diffraction peaks of Ž111. using the Scherrer formula. In our experiment, the key to the formation of PbSrŽpolyvinyl acetate. nanocable is slow polymerization under mild conditions which ensures that there is sufficient time for self-organization of the monomer into the supramolecular, maintenance of the tubular structure throughout the polymerization
Fig. 3. TEM images of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
process and the growth of inorganic crystallites in good orientation. The irradiation process carried out at a rather low dose rate Ž20.3 Gyrmin. for a relatively long time was designed in our approach. Another tick of slow polymerization is the adding of ŽCH 3 . 2 CHOH, which is present in a much higher concentration than metal salts. OH and H radicals, which are generated in the radiolysis of water and induce the polymerization of vinyl acetate, were scavenged by abstracting an H atom from ŽCH 3 . 2 CHOH, leading to the rate of chain inducement and thus the rate of whole polymerization is decreased.
4. Conclusions In conclusion, morphology controllable PbSr Žpolyvinyl acetate. nanocomposites were first synthesized upon g-irradiation at room temperature and under ambient in a simple system. In an homogeneous ethanol system, the inorganic PbS nanoparticles were homogeneously dispersed in a polyŽvinyl acetate. matrix. In a heterogeneous water system, the nanocomposites are nanocable with a 30 nm diameter PbS core in a 80 nm polyŽvinyl acetate. sheath up to 10 mm in length. The ease, reproducibility and versatility of the synthetic approach will facilitate development of new materials and the examination of their structure– property relationship w20x. Now we are focusing our attention on developping this method in order to synthesis other semiconductorrŽpolyvinyl acetate. nanocables with a desired diameter. Most recently, according to the design strategy discussed above, a CdSrŽpolyvinyl acetate. nanocable with thinner inorganic nanowires has been prepared by control the reaction condition. In addition, long straight CdS nanorods have also been obtained by improvement of this monomer–supramolecular–polymer route. With the development of techniques for diameter control, it is predicted that semiconductor nanowires will be obtained. In addition, as many kinds of metal or metal oxide nanocrystals can be easily obtained
507
under g-irradiation, it is reasonable to expand this method to synthesize metalrpolymer and metal oxiderpolymer nanocables. Acknowledgements Financial support from the Chinese National Foundation of Natural Science Research and Huo Yingdong Foundation for Young Teachers is gratefully acknowledgement. References w1x S. Lijima, Nature 354 Ž1991. 56. w2x C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 Ž1993. 8706. w3x A. Sellinger, P.M. Weiss, A. Nguyen, Y. Lu, R.A. Assink, W. Gong, C.J. Brinker, Nature 394 Ž1998. 256. w4x A. Blaaderen Van, R. Ruel, P. Wiltzius, Nature 385 Ž1997. 321. w5x M. Antonietti, E. Wenz, L. Bronstein, M. Seregina, Adv. Mater. 7 Ž1995. 1000. w6x M. Moffitt, L. McMahon, V. Pessel, A. Eisenberg, Chem. Mater. 7 Ž1995. 1185. w7x Y. Xie, Z. Qiao, M. Chen, X. Liu, Y. Qian, Adv. Mater. 11 Ž1999. 1512. w8x S. Mann, Nature 265 Ž1993. 499. w9x M. Whitesides, J.P. Mathias, C.T. Seto, Science 254 Ž1991. 1312. w10x M.R. Ghadiri, J.R. Granja, L.K. Buchler, Nature 369 Ž1994. 301. w11x Y. Zhu, Y. Qian, X. Li, M. Zhang, Chem. Commun. Ž1997. 1081. w12x M. Mukherjee, A. Datta, D. Chakravorty, Appl. Phys. Lett. 64 Ž1994. 1159. w13x V. Sankaran, C.C. Cummins, R.R. Schrock, R.E. Cohen, R.J. Silbey, J. Am. Chem. Soc. 112 Ž1990. 6858. w14x Z. Zeng, S. Wang, S. Yang, Chem. Mater. 11 Ž1999. 3365. w15x Z. Qiao, Y. Xie, M. Chen, Y. Zhu, Y. Qian, J. Mater. Chem. 9 Ž1999. 1001. w16x Y. Yin, X. Xu, X. Ge, C. Xia, Z. Zhang, Chem. Commum. Ž1998. 1641. w17x Y. Yin, X. Xu, X. Ge, C. Xia, Z. Zhang, Chem. Commum. Ž1998. 941. w18x A. Henglein, Chem. Rev. 89 Ž1989. 1861. w19x N.N. Parvathy, G.M. Pajonk, A.V. Rao, J. Crystal Growth 166 Ž1996. 769. w20x B.T. Holland, C.F. Blanford, A. Stein, Science 281 Ž1998. 538.
Chemical Physics Letters 321 Ž2000. 504–507 www.elsevier.nlrlocatercplett
Synthesis of lead sulfider žpolyvinyl acetate / nanocomposites with controllable morphology Zhengping Qiao, Yi Xie ) , Meng Chen, Jianguang Xu, Yingjie Zhu, Yitai Qian Structure Research Lab and Department of Chemistry, UniÕersity of Science and Technology of China, Hefei, Anhui 230026, China Received 8 December 1999; in final form 20 January 2000
Abstract PbSrŽpolyvinyl acetate. nanocomposites with different morphology that inorganic particles embedded in polymer matrix and inorganic nanowires in polymer nanotubes, respectively, were first synthesized upon g-irradiation at room temperature and under ambient pressure in a simple system. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction During the past few years, synthetic techniques have been developed to provide feasible ways to prepare nanomaterials having the desired morphology w1,2x. To exploit novel nanodevices such as electronic or optical devices, it is necessary to prepare nanoparticles without their mutual aggregation. Moreover, morphology control is an alternative tool to adjust the optical or catalytic properties of the nanomaterials. Thus a number of strategies on nanostructure fabrication are currently being developed to control not only the size but also the dispersion and shape of the products. Among these strategies, the template self-assembly process has been demonstrated to be effective since the pre-organized tem-
) Corresponding author. Fax: q86-551-363-1760; e-mail: [email protected]
plate regulates the nucleation, growth, morphology and orientation of nanocrystals w3,4x. To date, many kinds of template have been used: colloidal suspensions, zeolites, liquid crystalline, porous anodic aluminum oxide, carbon nanotubes, etc. Recently, various types of polymers have been applied to the synthesis of metallic and semiconductor particles w5,6x. Block copolymers, consisting of different blocks with variant solubility, have long been known to self-assemble in selective solvents to form stable aggregates, such as micelles or micelle-like aggregates, offering a template for further synthesis of nanocomposites. The use of block copolymers has also attracted considerable interest because of their potential for processability and hence ease of application. Nevertheless, there are few reports on simple polymers such as polyŽvinyl acetate., which has both an hydrophilic group ŽC5O. and an hydrophobic part in its monomer molecule, acting as matrix to control the morphology of semiconductors by the self-assembly route. In this Letter, we develop a
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 3 7 7 - 8
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
template method to synthesize nanocomposites with different morphology in a polyŽvinyl acetate. matrix under mild conditions. In heterogeneous systems, such as in an aqueous solution, the limited mutual solubility between vinyl acetate and water make it possible to form a micelle-like structure that the hydrophilic groups shrink together to form cylinder supramolecular to minimize the surface free energy in the upper monomer layer w7x. The driving force for self-assembly of the supramolecular structure primarily comes from the enthalpic contribution of hydrogen-bonding interactions w8–10x. Then the following slow polymerization of vinyl acetate under conditions mild enough to keep the structure of the as-formed supramolecular will produce a tubular polyŽvinyl acetate. template. In such a system, particle growth of the inorganic components will be restricted in the inner hydrophilic cavity of the as-formed supramolecular and polymer tubes, providing a good nanoreactor for both size control and epitaxial growth. Thus, through this simple monomer–supramolecular–polymer transformation process, an inorganicrpolymer nanocable can be synthesized. On the contrary, in a homogeneous system, such as in an ethanol solution, the supramolecular could not form; meanwhile, reactants mixed at the molecular level in the solution will produce the homogeneous composites in which the inorganic component displays homogenous dispersion w11x. To date, studies on PbS, which is one of the most important semiconductors, are fewer than those made using CdS although PbS nanocrystalline or PbSrpolymer nanocomposites have displayed good properties w12–14x. To our knowledge, this is the first time that the PbSrŽpolyvinyl acetate. nanocomposites with controllable morphology have been synthesized. The polymerization of vinyl acetate and preparation of PbS are carried out using a g-irradiation method – an attractive method with the advantage of simplicity, convenience and easy control w7,11,15–19x. In addition, g-irradiation offers an ideal means by which the supramolecules with tubular structure can be solidified to a polymer tube with the desired diameter at room temperature under ambient pressure. The structure of supramolecules will not collapse as in heat polymerization under higher temperature. Also the use of the toxic gas H 2 S and
505
organometallic complex can be avoided in this process.
2. Experimental Solutions were prepared by dissolving analytically pure carbon disulfide Ž0.1% by vrv., lead acetate Ž0.02 mol dmy3 ., vinyl acetate Ž5%, vrv. and isopropyl alcohol Ž23%, vrv. in solventrwater Žsystem A. or ethanolrwater Žsystem B.. The solutions were irradiated in the field of a 5 = 10 4 Ci 60 Co g-ray source. After being irradiated, for system A, the as-prepared nanocomposites ŽSample A. precipitated in the lower water layer immediately after being irradiated since the density of polyŽvinyl acetate. is larger than that of water. System B is a black homogeneous suspension, and a large amount of water was added to separate the nanocomposites ŽSample B.. Both samples were black gel-like masses and were washed with dilute acetic acid and distilled water in sequence to remove the by-products. The final products were dried in vacuum at room temperature for 4 h.
3. Results and discussion The samples were characterized by X-ray powder diffraction ŽXRD. patterns. XRD patterns were obtained on a Japan Rigaku DrMax gA X-ray diffractometer equipped with graphite monochromatized ˚ .. The uneven baseCuK a radiation Ž l s 1.54178 A line in XRD patterns ŽFig. 1. is due to the larger
Fig. 1. XRD patterns of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
506
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
Fig. 2. IR Spectra of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
amount of amorphous polymer component. All of the diffraction peaks can be indexed as PbS with cubic phase ŽJCPDS Card File No. 5-592.. The infrared ŽIR. spectra were made using a Hitachi 850-fluorescence spectrophotometer with a Xe lamp at room temperature. IR spectra were used to characterize the polymer component shown in Fig. 2. From it one can see that the IR spectra of the products are the same and that both are similar to the standard IR spectrum of polyŽvinyl acetate.. In Fig. 2, the strongest peak is at 1736 cmy1 Ž n C 5 O . and the characteristic peaks are at 1237 and 1018 cmy1 Ž n C – O . and at 1375 cmy1 Ž d CH3 . of polyŽvinyl acetate., which confirms the successful polymerization
of vinyl acetate monomer in water and ethanol upon g-irradiation. The peak of low component PbS could not be observed because the peak of PbS is extremely weaker than that of polyŽvinyl acetate. w16x. The morphology and particle sizes were determined by transmission electron microscopy ŽTEM.. TEM images were taken with a Hitachi Model H-800 transmission electron microscope with an accelerating voltage of 200 kV. As predicted, Sample A is shown having a tubular morphology. From the lower TEM magnification of the micrograph of Sample A, one can see the tubular structure being ca. 100 nm in diameter and 10 mm in length ŽFig. 3a.. Careful observation of the TEM micrograph with a higher magnification clearly indicates that Sample A is a nanocable 30 nm in diameter of a PbS core in a 80 nm polyŽvinyl acetate. sheath ŽFig. 3b.. For Sample B, its TEM images are shown in Fig. 3c, from which one can see that the nanocomposites consist of fine quasi-spherical lead-sulfide particles homogeneously dispersed and well separated in the polyŽvinyl acetate. matrix. The diameter of lead-sulfide is in the range of 13 nm, similar to the XRD result Ž15 nm. which was calculated from the half-width of diffraction peaks of Ž111. using the Scherrer formula. In our experiment, the key to the formation of PbSrŽpolyvinyl acetate. nanocable is slow polymerization under mild conditions which ensures that there is sufficient time for self-organization of the monomer into the supramolecular, maintenance of the tubular structure throughout the polymerization
Fig. 3. TEM images of the as-prepared nanocomposites. Sample A Ža., Sample B Žb..
Z. Qiao et al.r Chemical Physics Letters 321 (2000) 504–507
process and the growth of inorganic crystallites in good orientation. The irradiation process carried out at a rather low dose rate Ž20.3 Gyrmin. for a relatively long time was designed in our approach. Another tick of slow polymerization is the adding of ŽCH 3 . 2 CHOH, which is present in a much higher concentration than metal salts. OH and H radicals, which are generated in the radiolysis of water and induce the polymerization of vinyl acetate, were scavenged by abstracting an H atom from ŽCH 3 . 2 CHOH, leading to the rate of chain inducement and thus the rate of whole polymerization is decreased.
4. Conclusions In conclusion, morphology controllable PbSr Žpolyvinyl acetate. nanocomposites were first synthesized upon g-irradiation at room temperature and under ambient in a simple system. In an homogeneous ethanol system, the inorganic PbS nanoparticles were homogeneously dispersed in a polyŽvinyl acetate. matrix. In a heterogeneous water system, the nanocomposites are nanocable with a 30 nm diameter PbS core in a 80 nm polyŽvinyl acetate. sheath up to 10 mm in length. The ease, reproducibility and versatility of the synthetic approach will facilitate development of new materials and the examination of their structure– property relationship w20x. Now we are focusing our attention on developping this method in order to synthesis other semiconductorrŽpolyvinyl acetate. nanocables with a desired diameter. Most recently, according to the design strategy discussed above, a CdSrŽpolyvinyl acetate. nanocable with thinner inorganic nanowires has been prepared by control the reaction condition. In addition, long straight CdS nanorods have also been obtained by improvement of this monomer–supramolecular–polymer route. With the development of techniques for diameter control, it is predicted that semiconductor nanowires will be obtained. In addition, as many kinds of metal or metal oxide nanocrystals can be easily obtained
507
under g-irradiation, it is reasonable to expand this method to synthesize metalrpolymer and metal oxiderpolymer nanocables. Acknowledgements Financial support from the Chinese National Foundation of Natural Science Research and Huo Yingdong Foundation for Young Teachers is gratefully acknowledgement. References w1x S. Lijima, Nature 354 Ž1991. 56. w2x C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 Ž1993. 8706. w3x A. Sellinger, P.M. Weiss, A. Nguyen, Y. Lu, R.A. Assink, W. Gong, C.J. Brinker, Nature 394 Ž1998. 256. w4x A. Blaaderen Van, R. Ruel, P. Wiltzius, Nature 385 Ž1997. 321. w5x M. Antonietti, E. Wenz, L. Bronstein, M. Seregina, Adv. Mater. 7 Ž1995. 1000. w6x M. Moffitt, L. McMahon, V. Pessel, A. Eisenberg, Chem. Mater. 7 Ž1995. 1185. w7x Y. Xie, Z. Qiao, M. Chen, X. Liu, Y. Qian, Adv. Mater. 11 Ž1999. 1512. w8x S. Mann, Nature 265 Ž1993. 499. w9x M. Whitesides, J.P. Mathias, C.T. Seto, Science 254 Ž1991. 1312. w10x M.R. Ghadiri, J.R. Granja, L.K. Buchler, Nature 369 Ž1994. 301. w11x Y. Zhu, Y. Qian, X. Li, M. Zhang, Chem. Commun. Ž1997. 1081. w12x M. Mukherjee, A. Datta, D. Chakravorty, Appl. Phys. Lett. 64 Ž1994. 1159. w13x V. Sankaran, C.C. Cummins, R.R. Schrock, R.E. Cohen, R.J. Silbey, J. Am. Chem. Soc. 112 Ž1990. 6858. w14x Z. Zeng, S. Wang, S. Yang, Chem. Mater. 11 Ž1999. 3365. w15x Z. Qiao, Y. Xie, M. Chen, Y. Zhu, Y. Qian, J. Mater. Chem. 9 Ž1999. 1001. w16x Y. Yin, X. Xu, X. Ge, C. Xia, Z. Zhang, Chem. Commum. Ž1998. 1641. w17x Y. Yin, X. Xu, X. Ge, C. Xia, Z. Zhang, Chem. Commum. Ž1998. 941. w18x A. Henglein, Chem. Rev. 89 Ž1989. 1861. w19x N.N. Parvathy, G.M. Pajonk, A.V. Rao, J. Crystal Growth 166 Ž1996. 769. w20x B.T. Holland, C.F. Blanford, A. Stein, Science 281 Ž1998. 538.