Structural Changes on SnO 2 Nanoparticles for Gas Sensor Applications induced by Calcination Treatments and Grinding
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INTRODUCTION Gas sensors based on SnO 2 have become the predominant solid state devices to monitor a wide variety of gases'. Its high surface sensitivity to gas adsorption, simplicity and fast response time- are the main advantages for these sensors. On the other hand, in the last few years, some investigations have revealed that nanometric materials show excellent characteristics that make them very useful for gas detection, such as higher sensitivities and shorter response times, which increase with decreasing the grain size3. Nevertheless, in contrast with their widespread application, the present control of gas detection is still immature and fundamental research will be required. In solid state gas sensors, the sensing mechanism arises primarily from effects on the surface of the solid. The mechanism of operation is complex, through interactions between gaseous molecules and surfaces. Thus, through variations of the fabrication parameters, nanoparticles can be obtained with the desired sensing properties, but in most cases nothing can assure their permanence with time and operation conditions. Therefore, previous to the fabrication of the paste used to make thick film gas sensors, it is very important to structurally characterize the precursor powders to test their properties, mainly stability. In this work, the structural properties of the nanoparticles underwent to two basic processes, i.e., calcination and grinding, have been analyzed by a wide set of techniques such as Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman, Fourier transform infrared, and X-ray photoelectron (XPS) spectroscopies. The obtained information about size, faceting, agglomeration, crystalline quality, and stoichiometry will provide a limit beyond which tin dioxide particles obtained by a wet chemical method are useful for gas sensing. Special emphasis will be stressed on surface or intra-grain disorder and humidity content, which modifies surface reactions and sensor response. 53 Mat. Res. Soc. Symp. Proc. Vol. 501 01998 Materials Research Society
EXPERIMENTAL The polycrystalline powders have been prepared by a wet chemical method. First, Sn(OH) 4 was precipitated by adding ammonia to an aqueous SnCl 4 solution. The precipitated gel was subsequently dried and repeatedly redispersed in distilled water to remove chlorine contamination. After that, a white powder of hydrated tin dioxide was obtained. According to this preparation, two groups of samples were fabricated. In the first group, tin dioxide powders with different grain sizes were obtained by calcinating the hydrate at temperatures between 250 and 1000°C for 8h. In the second group, a grinding on a centrifugal mill for 2h at 200rpm was performed in the starting powder. Then the powder was calcinated at 1000°C for 8 or 24h. Finally, some part of the powder was retired and the rest was grinded again. The Transmission electron microscopy (TEM) observations of these precursor powders were carried out on a Philips CM30 SuperTwin electron microscope, operati
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