Síntese e caracterização de nanopartículas de óxido de estanho (SnO2) obtidas a partir de suspensões coloidais.
Lee, Eduardo Jian Hua
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A novel chemical route was use to obtain colloidal suspensions of SnO2 nanoparticles. The synthesis is based on the hydrolysis reaction of tin chloride (II), and allows the preparation of nanocrystals at room temperature. The growth behavior of the dispersed nanoparticles was studied in order to elucidate the mechanisms involved. For this purpose, the superficial properties of the nanoparticles were modified by adding the surfactant tetra-butyl ammonium hydroxide and by modifying the pH of the suspensions, which promotes variations in the surface charge and consequently in the agglomeration state. These samples were studied by transmission electron microscopy, and their optical properties were analyzed. The results indicate that the oriented attachment mechanism is significant even at room temperature. Hydrothermal treatments were applied over the suspensions, for further investigations in the growth behavior. The suspensions were also dilluted in alcohol and submitted to hydrothermal treatments, in order to verify if growth mechanism is inhbited. The results show that the Ostwald ripening process is not significant for tin oxide, due to its low solubility in the solvent. Therefore, it can be concluded that the oriented attachment mechanism is the dominant growth process for this material. The dilluted samples and those that were hydrothermally-annealed at lower temperatures presented lower growth rates. This result can be ascibed to the fact that oriented attachments occur when particles collide with a similar crystallographic orientation and, considering that the Brownian motion determines the particle velocity in suspension, the collision frequency increases with temperature and particle concentration. Finally, the photoluminescent properties of the SnO2 nanocrystals were characterized. The effective mass model was used to investigate the quantum confinement effects in the nanoparticles. The results are strong indicatives that the photoluminescence in the SnO2 nanocrystals is due to the radiative decay of free excitons.