Synthesis and atmospheric instability of well crystallized rod-shaped V2O4 2H2O powders prepared in an aqueous solution

Synthesis of well crystallized rod-shaped V2O4•2H2O powders of 100 nm in width in an aqueous solution containing NH4VO3 and N2H4•H2O under atmospheric pressure had been investigated. The optimal conditions for atmospherically hot-water treatment at 95°C were found to be as follows: pH 2.5-3.5 and an appropriate amount of N2H4•H2O. How the solution conditions affect the crystal structures and morphologies of the products is discussed. At pH<2.5, no precipitates formed. At pH 4.0-4.5, V10O24•12H2O would generally be obtained if N2H4•H2O was excessive. It was independent of reduction of NH4VO3 solution to tetravalent solids on the dosage of reducing agent N2H4•H2O. The morphologies of the V2O4•2H2O had no obvious relationship with the pH and the dosage of N2H4•H2O. It seems that before treated at 95°C the products were sheet-shaped, and then cleaved along specific crystalline directions after treated, forming rod bunches. The stability of V2O4•2H2O in acid solution and in air was also investigated. The results showed that V2O4•2H2O was unstable in air and could undergo oxidization to be a quasi-amorphous rod-shaped V10O24•12H2O, especially at high pH, with excessive N2H4•H2O, drying at 150°C in flowing air, and/or being exposed to air for several days.