Nanocrystalline ceria is a promising oxide material, which become widely used as an abradant for mechanochemical polishing of silicon wafers, protective anti-UV radiation coating and catalyst.

Here we have proposed two methods of ceria synthesis based on precipitating CeO₂ by ammonia from cerium nitrate aqueous alcoholic solutions of different concentrations (50, 65, 75, 85, 95 vol.%) and homogeneous hydrolysis of cerium nitrate in the presence of hexamethylenetetramine (HMTA).

According to heat flux calorimetry data, CeO₂ formation process is accompanied by a considerable heat effect (-73.2 kJ/mol); kinetic analysis indicates that the process runs by the mechanism of autocatalysis – the hydroxocomplexes decomposition proceeds on the surface of the already formed CeO₂ particles, leading to their growth. This was confirmed by the UV-visible spectroscopy: the absorption band edge drifts towards the longer wavelength region, so that the band gap energy changes from 3.35 to 3.15-3.20eV. We can conclude that in the aqueous alcoholic CeO₂ suspensions a gradual particle growth takes place, which is accompanied by the decrease of the band gap energy and the oxidation of the residuary Ce(III) ions. The micromorphology of the samples was studied by XRD, TEM, SANS and low temperature nitrogen adsorption. It was found that the surface of nanopowders possesses fractal properties, and the value of fractal dimension depends both on concentration of initial solutions and conditions of subsequent thermal treatment.

It was also established that synthesis of ceria via hydrolysis of cerium nitrate in the presence of HMTA proceeds through formation of unstable cerium hydroxocarbonate. It was shown that CeO₂ mean particle size and the band gap value can be finely tuned by variation of synthesis duration, temperature, concentration of reactants and their molar ratio.

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