Structure and proton conductivity of 12-tungstophosphoric acid doped silica

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Abstract

Sol–gel syntheses offer a wide range of possibilities for introduction of materials with specific properties (electronic, optic and ion-conductive, etc.) into optically clear matrices. Preparation of silica gel bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as its structural and conduction characteristics are reported. Characteristics of the obtained doped gels depend on the gel preparation, gelation process and the WPA content. According to the obtained results, especially for the high conductivity (σ∼0.1 S/cm), WPA doped silica gel is a promising material for solid electrolytes.

Introduction

It is well-known that heteropolyacids (HPA) are superionic protonic conductors at room temperature, and, therefore, they are promising materials for potential applications as solid electrolytes in sensors, electrochromic displays, and so on. But, it is also well-known that these compounds are sensitive to surrounding conditions: relative humidity, partial pressure of hydrogen and temperature 1, 2.

The sol–gel syntheses offer new possibilities in the field of solid state ionics by increasing the stability and conductivity of various inorganic material films on gel matrices. Attempts to enhance the conductivity of α-layered zirconium phosphate (α-ZrP) have been made by mixing colloidal dispersions of delaminated α-ZrP with silica 3, 4or colloidal silica [5]. 12-Tungstophosphoric acid (WPA) has also been incorporated in optically clear gel films to get chromogenic film displays 6, 7, 8. This material, however, has a relatively long bleaching time and the effect of polymeric host matrices has not been well understood. The aim of this paper, therefore, is to give a better understanding of the process of gelation of WPA-doped silica sol, its surface, pores structure, and its influence on proton conductivity of these new materials.

Section snippets

Synthesis

Silica gels containing different amounts of WPA were prepared by the sol–gel method. Colloidal silicas obtained from sodium silicate solution by ion exchange method contained spherical SiO2 particles in an aqueous medium, stabilized at pH∼10. The mean diameters of SiO2 particles were 11, 22 and 41 nm, and the specific surface areas of solid phase were 247, 124 and 66 m2/g, respectively.

All samples were prepared by dissolving WPA in the starting sol in a quantity adjusted to obtain the samples

Keggin's ion entrapping and homogeneity

In this work, special attention was focused on the influence of sol particle size, WPA concentration and conditions of the gelation process (temperature) on the structure and features (characteristics) of gels doped with WPA. This is necessary for standardization of gel characteristics, which were controlled by means of the above mentioned experimental techniques.

Changing the SiO2 particle size we wanted to see its influence on the mesopores size, WPA doping and the gel characteristics.

The

Conclusion

This paper contributes to the clarification of the nature of WPA soaked in silica gel in order to ensure reproducible synthesis of these gels as good protonic conductors. The sol–gel process is a promising way for obtaining materials with new characteristics used for solid state ionics. The knowledge of the molecular precursors, their structure and morphology, as well as gelation process, allow a better control of characteristics of doped silica gels. The experimental data show that WPA is

Acknowledgements

This work was partially supported by the Ministry of Science and Technology, Belgrade, Serbia.

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