Phase transition between layered tungstates and polyoxotungstates in aqueous solutions.

Aqueous sols of colloidal layered tetramethylammonium (TMA) tungstate nanocrystals were obtained by diluting an aqueous suspension of TMA polyoxotungstate precipitates. The slow evaporation of the colloidal-layered tungstate sols led to the deposition of TMA polyoxotungstate and significantly decreased the amount of layered tungstate nanocrystals. Moreover, the increase in the amount of TMA(+) in the sols also facilitated precipitation of the polyoxotungstates because of a common ion effect. Thus, the dissolution and formation of polyoxotungstate were reversible phenomena. The reaction between the dissolved species, which were formed by the dissolution of TMA polyoxotungstate, likely provided the highly water-dispersible layered tungstate nanocrystals. Furthermore, transmission electron microscopy observation suggested that the layered tungstate nanocrystals had a layer structure similar to those of H2WO4 and H2WO4·H2O, which is a layered tungstic acid.