Sol−Gel Transition of Niobium Oxide Nanosheet Colloids:  Hierarchical Aspect of a Novel Macroscopic Property Appearing in Colloidally Dispersed States of Layered Niobate K<INF>4</INF>Nb<INF>6</INF>O<INF>17</INF>

Colloidal suspension of exfoliated layered niobate K<INF>4</INF>Nb<INF>6</INF>O<INF>17</INF> in water exhibited a pH-induced sol−gel transition. Viscosity of the sols drastically increased on acidification, by which thick colloids were gelated. The viscosity change depended not on the acid species (H<INF>2</INF>SO<INF>4</INF> or HCl) added to the sols but on the pH of the colloid. Basification of the acidified colloids caused a decrease in the viscosity to the initial value, indicating reversibility of the sol−gel transition. On the other hand, optical microscope observations indicated that orientationally ordered niobate nanosheets in the sols are gathered to form domains with the size of submillimeters, indicating that the gels were hierarchically constructed. The results suggested that the niobate nanosheets were stabilized with electrical repulsion among the protonated nanosheets, which were weakly aggregated in part, to form the hydrogels. Sedimentation of the nanosheets was observed in both the sols and gels by standing them for a long time, indicating the metastable nature of the colloids, whereas the gels did not exhibit the deposition as rapid as that observed in the sols.