Oxometalate-Glass Composites and Thin Films
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OXOMETALATE-GLASS COMPOSITES AND THIN FILMS Karin Moiler%, Thomas Bein* and C. Jeffrey Brinker"* *Department of Chemistry and Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, NM 87131 "*Sandia National Laboratories, P. 0. Box 5800, Albuquerque, NM 87185
ABSTRACT New glass-composites with ion exchange properties have been developed. Ammonium 12-molybdophosphate (AMP) (NH4 )3 PM012040, and ammonium 12tungstophosphate (AWP) (NH4)3PW 12 04 0 , known for their ion exchange capabilities, are included either in preformed aerogels with defined pore size, or are added to sol-gel mixtures during the process of gel formation. Characterization is carried out by FTIR, Raman and EXAFS spectroscopy. Ion exchange capacities for the oxometalate precursors are determined for silver and rubidium and are compared to those of the glass composites. Glass composites show high ion exchange capacity, but some portion of the metalate complexes leaches from the glass during the procedure. This is in contrast to thin composite films, which have almost no porosity and do not show loss of metalate. EXAFS spectroscopy demostrates that the oxometalate microstructure is maintained in glass composites and that rubidium ions after ion exchange in glasses occupy similar cation positions as in the precursor compounds.
INTRODUCTION The combination of different functional properties in glass-based composites offers great potential for the design of tailored materials. Examples include sol-gel derived glass-organic composites, glass-included laser dyes, catalysts, and nonlinear optical materials. In view of the increasing demand for stable, ion-selective sensors we have initiated a program aimed at the design of tailored thin films with selective ion exchange capabilities. In combination with highly sensitive acoustic devices, it is envisioned that inexpensive, rugged ion sensors for process and environmental monitoring can be designed. The concept is based upon the encapsulation of oxometalate clusters in porous, sol-gel derived glasses. The resulting materials combine selective ion exchange sites with tailored porosity, such that ultimately large fractions of unwanted species (e.g., organics, biological matter) are screened from the ion exchange sites. The following benefits are expected from this approach: The design of ion exchange composites allows independent adjustment of both ion selectivity through the choice of different oxometalates, and porosity through the choice of sol-gel chemistry. Furthermore, the resulting inorganic films are temperature stable up to at least 300 0C and not subject to fouling. Heteropoly oxometalates such as anions with the Keggin structure are known since the last century, but their crystal structure was first solved by Keggin in 1934. Figure 1 depicts the Keggin anion structure and a defect structure with one MO6 octahedron missing. These materials find numerous applications in catalysis, analytical chemistry and biochemical or medical areas. Their ability to exchange their counter cation
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