VIDROS DE FOSFATOS DE METAIS DE TRANSIÇÃO CONTENDO NANOPARTÍCULAS DE METAIS NOBRES

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1 VIDROS DE FOSFATOS DE METAIS DE TRANSIÇÃO CONTENDO NANOPARTÍCULAS DE METAIS NOBRES J.V. Lino 1, B. Pastena 1, R. P. R. D. Nardi 1, J. T. G. Junior 1, J.L. Ferrari 2, F. C. Cassanjes 1 and G. Poirier 1* 1 Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas, UNIFAL-MG, Campus de Poços de Caldas, Rodovia José Aurélio Vilela 11999, Cidade Universitária, Poços de Caldas-MG, CEP Departamento de Ciências Naturais, Universidade Federal de São João Del Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, São João Del Rei-MG, CEP * gael.poirier@unifal-mg.edu.br ABSTRACT The glass composition 50NaPO 3-50WO 3 was prepared by melt-quenching method and presented a strong dark coloration due to tungsten reduction in the glass matrix. Addition of 4% of Sb 2 O 3 to the starting compounds was helpful for oxidation of reduced species in the melt and transparent glass samples. This ternary composition fixed as 48NaPO 3-48WO 3-4Sb 2 O 3 was doped with AgNO 3, Au 2 O 3 and PtCl 4 and the final transparency of all glasses was checked by UV-visible absorption. It has been observed that undoped and doped samples exhibit similar absorption spectra, suggesting that the metallic ions stay in the ionic state in the glass network. After 1406

2 heat-treatment of these samples above the glass transition temperature, a strong specific coloration is observed for each doped sample. UV-visible absorption spectra of these heat-treated samples exhibit intense absorption bands at specific wavelengths for each doped sample attributed to plasmon resonance of metallic nanoparticles precipitated in the glass during heat-treatment. Key-words: glasses, phosphate, tungsten, metallic nanoparticles. INTRODUCTION Tungsten phosphate glasses were largely investigated in the past few years because of interesting specific properties obtained from the insertion of WO 6 octahedra inside the phosphate chains, resulting in cross-linking bonds and higher network connectivity (1-3). This highly connected vitreous network promotes high glass transition temperatures, high viscosities and high thermal stability against devitrification. Other interesting optical properties related with WO 6 clusters in the glass network include non linear optical absorption and photochromic properties under UV, visible or NIR exposure (4-7). On the other hand, precipitation of metallic nanoparticles (MNP) of noble metals such as silver or gold are of great interest because of resulting new optical or thermal properties due to these MNP in the glass matrix. Several works reported on non linear properties, energy transfer between rare earth ions and MNP or crystalline phase growth from these MNP nuclei (8-9). For these reasons, the glass composition 50NaPO 3-50WO 3, already known for its high thermal stability, was selected in this work for precipitation of silver, gold and platinum obtained by heat-treatment above the glass transition temperature. The optical absorption of these glass samples was investigated before and after heat-treatment for identification of MNP. EXPERIMENTAL PART 1407

3 Glass compositions were prepared from the starting compounds NH 4 H 2 PO 4 48% in P 2 O 5, Na 2 CO 3 98%, WO 3 99,9%, Sb 2 O 3 99,98%, AgNO 3 99%, Au 2 O 3 99,99% and PtCl 4 99,99%, all from Aldrich.. The starting powders were weighted using an analytical balance and grinded in an agate mortar. The resulting powder was transferred to a platinum crucible, heated at 400ºC for 1 hour to remove residual moisture and adsorbed gases, at 600ºC for 1 hour to promote NH 4 H 2 PO 4 and Na 2 CO 3 decomposition and at 1100ºC for for 1 hour for melting of these starting powders. Finally, the melt was poured in a steel mold preheated 20ºC below Tg and annealed at this temperature for 4 hour before cooling to room temperature inside the furnace. The glass compositions prepared in this work are presented in Tab. 1. Tabela 1: Molar compositions of glass samples. Sample Molar composition (%) NaPO 3 WO 3 Sb 2 O 3 AgNO 3 Au 2 O 3 PtCl 4 NW NW50Sb NW50SbAg 47, ,1 0 0 NW50SbAu 47, ,05 0 NW50SbPt 47, ,1 UV-visible absorption spectra were obtained between 400nm and 1100nm using a spectrophotometer DR3900 Benchtop. Each absorption spectrum was normalized by the thickness and is presented as a function of coefficient absorption. RESULTS AND DISCUSSION The glass composition 50NaPO 3-5WO 3 was selected for this study for its high thermal stability and ability to dissolve other compounds. The visual aspect of this sample is presented in Fig. 1 and exhibits a strong dark coloration and low transparency. This strong absorption in the visible is related to electronic d-d 1408

4 transitions of reduced tungsten atoms created from oxygen losses during melting and reduction of W 6+ to W 5+ to ensure the charge neutrality. This oxygen loss is also wellknown in crystalline tungsten oxides and results in non-stochiometric compounds. Figure 1. Glass samples prepared in this work. Since one of the main objective of this work is obtaining glass samples with potential optical applications, transparent samples are required. For this reason, 4 mole% of Sb 2 O 3 was added to the starting compounds and the glass was prepared under the same conditions. As can be seen in Fig. 1, this antimony addition results in transparent glasses by oxidation of tungsten reduced species by Sb 5+ formed during heating. Then, this NW50Sb sample has been doped with 0,1 mole% of Ag + (using 0,1% of AgNO 3 ), 0,1 mole% of Au 3+ (using 0,05% of Au 2 O 3 ) and 0,1% of Pt 4+ (using 0,1% of PtCl 4 ) and the resulting glasses are also presented in Fig. 1 and exhibit a good transparency in the visible. UV-visible absorption spectra of these antimony containing glasses were also obtained in order to verify the absorption behavior in the UV-visible and near infrared range as shown in Fig. 2a. All glasses exhibit a high transparency between 480nm and 1100nm, indicating that noble metal ions don t influence significantly the absorption in this spectral range. A more detailed observation of these spectra between 500nm and 610nm (Fig. 2b) points out that the gold doped glass presents a very weak abosprtion band centered around 560nm and can be due to reduction and precipitation of some gold nanoparticles in these glasses. In order to promote metal reduction and precipitation, heat-treatments were performed in these samples at 550ºC between Tg and Tx for 30min. The resulting glasses are shown in Fig. 3 under back illumination and present specific colors for each metal doping with a dominant purple color for the silver-doped glass, blue color for the gold-doped glass and green color for the platinum-doped glass. The undoped glass appears yellow-colored as for the untreated sample but with a cloudy aspect. 1409

5 Figure 2. UV-visible absorption spectra of glasses NW50Sb, NW50SbAg, NW50SbAu and NW50SbPt. Figure 3. Back light pictures of glass samples befor and after heat-treatment at 550 C for 3 hours. UV-visible absorption spectra of these heat-treated samples were also obtained and are presented in Fig

6 Figure 4. UV-visible absorption spectra of glasses NW50Sb, NW50SbAg, NW50SbAu and NW50SbPt heat-treated for 30min at 550 C. The first unexpected result is a very large absorption band between 600nm and 1100nm for the undoped sample NW50Sb. This absorption is attributed to partial reduction of W 6+ to W 5+ under heat-treatment, resulting in both d-d electronic transitions of W 5+ and polaron transitions between W 6+ and W 5+. Intense absorption bands centered at 577nm, 641nm and 746nm are identified for the Ag-doped, Audoped and Pt-doped glass respectively. These absorption bands are attributed to plasmon resonance of metallic nanoparticles in the glass host.in addition, some absorption bands present shoulders at 548nm for the Au-doped sample and 565nm and 678nm for the Pt-doped sample. It is suggested that this absorption behavior is related with several size distributions and non-spherical morphology for these nanoparticles. These results are promising for further optical characterizations since these MNP- containing glasses could exhibit non linear optical properties or interesting energy transfer mechanisms with rare-eart ions. CONCLUSION Transparent glass samples could be obtained in the ternary composition 48NaPO 3-48WO 3-4Sb 2 O 3. These glasses were doped with small contents of silver, 1411

7 gold and platinum without decrease of the transparency in the UV, visible and near infrared. Heat-treatment above Tg produce strongly colored glasses with intense absorption bands attributed to Plasmon ressoance of metallic nanoparticles in the glass. The gold and platinum doped samples also exhibits non symmetric absorption bands with shoulders, suggesting that these MNP are not monodisperse or not spherical in these vitreous materials. These results indicate that the tungsten phosphate glass is a suitable host for silver, gold and platinum nanoparticle precipitation and their optical properties such as non linear optical absorption or energy transfer with rare earth ions must be investigated. ACKNOWLEDGMENTS The authors would like to thank brazilian funding agencies FAPEMIG, FINEP, CNPq and CAPES for financial support and UNIFAL-MG for the laboratory structure. REFERENCES [1] G. Poirier, Y. Messaddeq, S. J. L. Ribeiro, M. Poulain, Journal of Non Crystalline Solids, 2005; 351-4: 293. [2] G. Poirier, Y. Messaddeq, S. J. L. Ribeiro, M. Poulain, Journal of Solid State Chemistry, 2005; 178: [3] S. Santagneli, C. C. de Araujo, W. Strojek, H. Eckert, G. Poirier, S. J. L. Ribeiro, Y. Messaddeq, Journal of Physical Chemistry B, 2007; 111: [4] G. Poirier, M. Nalin, L. Cescato, S. J. L. Ribeiro, Y. Messaddeq, Journal of Chemistry and Physics, 2006; 125: [5] G. Poirier, M. Nalin, S. J. L. Ribeiro, Y. Messaddeq, Solid State Ionics, 2007; 178: [6] G. Poirier, M. Nalin, S. J. L. Ribeiro, Y. Messaddeq, Brazilian Pending Patent, 2005; PI

8 [7] G. Poirier, C. B. de Araujo, Y. Messaddeq, S. J. L. Ribeiro, Journal of Applied Physics, 2002; 91-12: [8] P. Chakraborty, Journal of Materials Science, 1998; 33: [9] F. Gonella, P. Mazzoldi, Handbook of Nanostructured Materials and Nanotechnology, 2000; Vol. 4: 81, Academic Press, San Diego. 1413