The chemistry of metallic cations in aqueous solution is a relevant method to the elaboration of nanometric scale metallic oxides. This method is extremely versatile and allows one to control many characteristics of materials like iron, aluminium and titanium oxides, in particular, the crystalline structure for polymorphic compounds, the particles size and shape. The main parameters being more relevant are the acidity of solution, the presence of specific anions, and the temperature. The synthesis process has been optimised in order to study each parameter separately. The acidity of the medium and the temperature govern the first steps of oxide formation namely the hydroxylation of metal complexes in solution involving the nucleation. Species complexing the cations modify the nature of coordination sphere of molecular complexes and play an important role on the size and the shape of solids, and sometime on the crystalline structure for polymorph. We have shown in the case of magnetite and anatase particles that the particles size decreases as the pH of precipitation departs more and more from the point of zero charge, PZC. For others materials such as brucite (Mg(OH)₂) and boehmite (g-AlOOH), the variation of morphology occurs. These examples bring decisive experimental arguments allowing to connect the size and shape of nanoparticles to the acidity of synthesis medium. Results have been interpreted using a thermodynamic approach of equilibrium state taking account the variation of the solubility and the surface energy as a function of acidity of the medium, through a simple semi-quantitative model. In order to improve this approach, we have investigated some experiments using weak complexing molecules and we have shown that the use of small quantity of polyols allowed also to tune surface energy for each crystallite faces involving some change in aspect ratio for a given particle shape.

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