Controlled morphologies and growth direction of WO3 nanostructures hydrothermally synthesized with citric acid

Hexagonal tungsten oxide (h-WO3) nanorods have been synthesized by a hydrothermal process at 180 °C using ammonium metatungstate and citric acid as starting materials. The phase constituent, morphology and growth direction of the h-WO3 nanocrystals were characterized by using XRD, SEM, HRTEM techniques. We found that the morphology, size and preferred orientation of the h-WO3 nanocrystals can be controlled by simply adjusting the amount of citric acid. In the absence of citric acid, 2-dimensional (2D) nanoplates or nanosheets 10-20 nm thick were formed. With increasing the amount of citric acid, 1-dimensional (1D) nanorods or nanoneedles grew along [0 0 2] crystallographic orientation from some certain nanoplate crystallographic planes. The as-synthesized WO3 nanorods were up to 1 μm in length and 40-50 nm in diameter. It is suggested that the adsorption of carboxyl group on certain crystallographic faces leads to an anisotropic crystal growth and the formation of 1D morphology.