The invention relates to a process for preparing nanocrystals of hematite - alpha FeO, in a liquid medium, by the hydrothermal decomposition of the Fe - EDTA (ethylenediaminetetraacetic acid) complex, said nanocrystals with semiconductor properties being employed in photo-electrochemistry with multiple applications in the solar energy conversion, photo-catalysis, sensors, environment medicine and water decomposition process. According to the invention, the process consists, in a first stage, in mixing an aqueous solution of double iron and ammonium sulphate - FeNH(SO)12HO with NaEDTA until a reddish liquid is obtained, followed by the preparation of another solution obtained by completely dissolving urea into oxygenated water, the two solutions being then mixed together and poured into a steel-jacketed teflon autoclave, up to a filling degree of 80% at the most, the autoclave being heated to a process temperature ranging between 150...250°C and pressures ranging between 20...40 barr, depending on the filling degree thereof, the said parameters being maintained for a period of time depending on the desired nanoparticle size, where, during heating about 140°C, there takes place the decomposition of the Fe-EDTA complex as well as the release of Fe which subsequently oxidates due to the oxygenated water, the nucleation centers formation and nanoparticles growth taking place at temperatures ranging between 150...250°C, and finally, the nanoparticles are separated by filtration, washed with deionized water and dried.
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