Facile methods to synthesize nanaosized iron oxide colloidal dispersion and its characterization

In this study, we develop two experimental procedures to synthesize a nanosized α-Fe2O3 colloidal suspension within a relatively short reaction time. In the first method, we use ferric chloride as the precursor, both NaOH and urea as the precipitant, and polyvinyl pyrrolidone (PVP) as the protecting agent; we find that by adding ferrous ion (Fe2+), nanosized α-iron oxide colloids are formed in about 40 min. The ferrous ion accelerates the conversion of Fe(OH)3 to α-Fe2O3, probably by helping with the dissolution of iron hydroxide and then its re-crystallization into iron oxide. In the second method, two thirds of the stoichiometric amount of NaOH is first used to precipitate iron hydroxide. It is then heated at boiling temperature in the presence of urea or sodium bicarbonate (NaHCO3) to complete the transformation into hematite in less than an hour. The solution pH increases with time and the transformation from iron hydroxide to α-Fe2O3 probably takes place by the solid state transformation mechanism. The common point between urea and sodium bicarbonate is the generation of CO2 during the reaction, which might play some role in helping with the solid state dehydration reaction.