Zinc nitrate precipitated with NaOH solution at room temperature under double-jet conditions produced micrometric zinc oxide particles with ellipsoid or starlike morphology separated by only a slight variation of pH (9.5 and 10.5, respectively). The formation kinetics of starlike particles was followed by SEM observations, suggesting that they result from a solid-state transformation of the hydroxide matrix. This mechanism involved the nucleation of 30-nm ZnO particles inside the matrix, followed by their aggregation into starlike particles. The introduction of sodium sulfate or sodium dodecyl sulfate in the bath before precipitation led to a drastic size reduction and to a diversity of particle shapes (from half-ellipsoids to full ellipsoids). The presence of these additives provided important hints on particle formation and confirmed that submicronic particles resulted from nanocrystals oriented aggregation.

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