A metal oxide sol can be prepared very simply by conventional precipitation-peptization procedures. Is it possible to process the oxide species contained in the sol in order to obtain nanocrystals? A positive answer will be given to this question, with the condition that the metal oxide sol contains “small oxide species”. It will be shown that the expression “small oxide species” implies a sol processing from simple inorganic precursors in order to avoid the usual precipitation of amorphous, hydrous oxides. When the corresponding sols are injected at 160°C in a coordinating environment, generally constituted by a solution of an amine in tetradecene, competition takes place between the two roles displayed by the amine: catalysis of condensation reactions between metal oxide species and growth hindering by bonding to the surface metal atoms. The result of such competition is the formation of oxide nanoparticles, whose crystalline nature depends on the synthesis temperature and on the particular system under investigation. The corresponding formation mechanism has been determined as a spatially confined, self-arresting sol-to-gel transition, where the metal oxide nanocrystals are formed during the first few minutes after the injection step. It will be shown a careful account of the investigation leading to such conclusion, that was carried out by FTIR, NMR, XRD and HRTEM analysis of the synthesis products obtained in a series of experiments on various systems. The process is extremely general, providing nanocrystals of ZnO, SnO₂, CeO₂, NiO, TiO₂, Ga₂O₃, In₂O₃, magnetic ferrites etc. By a suitable control of the injection environment, completely soluble, stable and processable nanocrystals can also be obtained. As an example of application, it will be discussed the processing of chemoresistive sensors based on SnO₂ and In₂O₃ nanocrystals and their outstanding gas-sensing properties towards a broad range of gases.

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