The crystal growth process in colloidal nanocrystal systems is usually associated with the Ostwald-ripening mechanism. Here, we report on experimental evidence indicating that another crystal growth process took place in a colloidal nanocrystal system at room temperature. This crystal growth process is based on grain rotation among neighboring grains, resulting in a coherent grain–grain interface, which, by eliminating common boundaries, causes neighboring grains to coalesce, thereby forming a single larger nanocrystal. This phenomenon was observed in SnO2 nanocrystals (particle size ranging from 10 to 30 Å).
Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2. The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process.
Tin dioxide nanoparticle suspensions were synthesized at room temperature by the hydrolysis reaction of tin
chloride (II) dissolved in ethanol. The effect of the initial tin (II) ion concentration, in the ethanolic solution,
on the mean particle size of the nanoparticles was studied. The Sn2+ concentration was varied from 0.0025 to
0.1 M, and all other synthesis parameters were kept fixed. Moreover, an investigation of the effect of agglomeration on the nanoparticle characteristics (i.e., size and morphology) was also done by modifying the pH of
the SnO2 suspensions. The different samples were characterized by transmission electron microscopy, optical
absorption spectroscopy in the ultraviolet range, and photoluminescence measurements. The results show that
higher initial ion concentrations and agglomeration lead to larger nanoparticles. The concentration effect is
explained by enhanced growth due to a higher supersaturation of the liquid medium. However, it was observed
that the agglomeration of the nanoparticles in suspension induce coarsening by the oriented-attachment
mechanism.
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