Anisotropic Crystal Growth of Layered Vanadates with Bulky Interlayer Cations

Abstract

The crystallization behavior of layered tetramethylammonium octavanadate [N(CH3)4]V8O20 including both V4+ and V5+ during hydrothermal synthesis is examined. According to the Bravais–Friedel–Donnay–Harker law, layered metalate crystals with bulky interlayer cations tend to have shapes with large surfaces parallel to the metalate layers; however, small rodlike crystals elongated along the direction normal to the vanadate layers are produced at the early stage of crystallization. As the crystallization proceeds, the crystal shapes are changed to large belt-like and platelike ones with the largest surfaces parallel to the vanadate layers. If sols containing only the small rodlike crystals formed at the early stage of crystallization are prepared, they will be useful for the fabrication of thin films with the vanadate layers vertically aligned normal to the substrate surface. Furthermore, ultrasonic treatment of the aqueous suspensions of the [N(CH3)4]V8O20 crystals leads to their exfoliation to a thickness of several tens of nanometers. For the relation between crystal shapes and crystallographic directions, the small crystals obtained using ultrasonic treatment are different from the ones formed at the early stage of crystallization. Probably, the morphology of the small rodlike crystals formed at the early stage of crystallization is attributed to the metastable growth form.