How isopolyanions self-assemble and condense into a 2D tungsten oxide crystal: HRTEM imaging of atomic arrangement in an intermediate new hexagonal phase

The Structure and structural evolution of tungstic acid solutions, sols and gels are investigated by high-resolution electron microscopy (HRTEM). Acidification Of sodium tungstate Solutions, through a proton exchange resin, is achieved in a way that ensures homogeneity in size and shape of intermediate polytungstic species. Gelation is shown to involve polycondensation followed by a self-assembling Process Of polytungstic building blocks leading to sheets with a layered hexagonal structure. Single layers of this new metastable phase are composed of three-, four- and six-membered rings of WO6 octahedra located in the same plane. This is the first time that a 2D oxide crystal is isolated and observed by direct atomic resolution. Further ageing and structural evolution leading to single sheets of 2D ReO3-type Structure is directly observed by HRTEM. Based on this atomic level imaging, a model for the formation of the oxide network structure involving a self assembling process Of tritungstic based polymeric chain is proposed. The presence Of tritungstic groups and their packing in electrochromic WO3 films made by different techniques is discussed. (C) 2008 Elsevier Inc. All rights reserved,