Synthesis, morphology and phase transition of the zinc molybdates ZnMoO4·0.8H2O/α-ZnMoO4/ZnMoO4 by hydrothermal method

ZnMoO4 with a rhombus sheet or flower-like structure, α-ZnMoO4 and needle-like ZnMoO4·0.8 H2O were successfully synthesized by simple hydrothermal crystallization processes with citric acid. ZnMoO4·0.8 H2O was easily synthesized in a shorter reaction time (2 h) at a higher reactant concentration. It gradually transformed into ZnMoO4 with a monoclinic wolframite tungstate structure with an increased reaction time, and pure ZnMoO4 was obtained with a longer reaction time (8 h). Citric acid (CA) played an important role in controlling the morphology of the as-obtained molybdates. The α-ZnMoO4 and ZnMoO4 were synthesized by heating ZnMoO4·0.8 H2O at 130 °C for 4 h and 8 h, respectively, under hydrothermal conditions. With transforming of ZnMoO4·0.8 H2O to α-ZnMoO4 and further to ZnMoO4, the needle-like crystals gradually disappeared and were transformed into crystals with rhombus sheet morphology and then further to pentacle or flower-like crystals that can be ascribed to continuous splitting and growing of the rhombus sheets.