Well-defined colloidal tin(IV) oxide particles
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I. INTRODUCTION
Because of the transparency and conductivity, stannic oxide has become technologically very important. Some relevant applications include gas sensors,1"3 transistors,4'5 electrode materials,6"8 catalysts,9"11 opticalconductive films for solar cells,1213 etc. In many of these uses, it is desirable to have SnO2 powders consisting of particles of different shapes of narrow size distribution. Such uniformity should improve the processing of the materials and the reproducibility of their properties. The well-defined powders can also be employed to relate their various characteristics to particle morphology and size. The classical method to obtain metal oxides by the addition of a base to an electrolyte solution resulted, as a rule, in ill-defined precipitates. Several procedures have now been developed for the preparation of "monodispersed" inorganic colloids.14"16 The essential aspect of these processes is the control of the kinetics of the generation of solutes, which are precursors to the solid phase formation. In the case of metal (hydrous) oxides, this aim can be achieved either by "forced hydrolysis" of the hydrated cations at elevated temperatures, or by a release of the hydroxide ion on decomposition of organic molecules (e.g., urea or formamide).14"16 Furthermore, it has been amply demonstrated that anions other than OH may play an essential role in determining the composition and the morphology of the resulting particles.14"16 This work describes the preparation of spherical SnO2 particles in acidified tin(iv) chloride solutions in the absence and in the presence of urea or formamide. Rod-like SnO2 particles were obtained by slow aging of highly acidic tin(iv) chloride solutions to which formamide or urea was added. The influence of the factors controlling the kinetics of these reactions (cona)On
leave from Institute) de Ciencia de Materiales (C.S.I.C), Madrid, Spain. J. Mater. Res., Vol. 5, No. 5, May 1990
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centration of the electrolyte, pH, time of aging, and temperature) on the nature of the precipitated particles was investigated in detail. II. EXPERIMENTAL A. Materials
Tin(iv) chloride pentahydrate (98%, Alfa), certified A.C.S. urea (Fisher), formamide (practical, Eastman), and hydrochloric acid (Dilute-it concentrate) were used without further purifications. Stock solutions of the tin salt (0.020 mol dm"3) were prepared in concentrated hydrochloric acid (2,1, or 0.5 mol dm"3) and found to be very stable toward hydrolysis; no change could be detected at room temperature even after several weeks of storage. These solutions were further diluted before carrying out the experiments. All solutions were passed through 0.22 fim millipore membranes in order to remove any possible particulate contaminants. B. Preparation of the particles
Powders, identified as SnO2, were obtained by aging acidic solutions of tin(iv) chloride in the presence of urea or formamide at elevated temperatures (2 particles (Fig. 8). Calculated IR spectrum of SnO2 f
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