Tetragonal alkali metal tungsten bronze and hexagonal tungstate nanorods synthesized by alkalide reduction

Abstract

Tetragonal alkali metal tungsten bronze and hexagonal alkali metaltungstate nanorods have been synthesized by sub-ambient reduction by alkalide solutions and subsequent oxidation. Tetragonal potassium tungsten bronze, K_0.4WO₃, nanorods result after post-reduction annealing at 250 °C for 3 h followed by 600 °C for 4 h in vacuum if the alkalide reduction is performed using K⁺(15-crown-5)₂Na⁻ and a mixed sodium/potassium tungsten bronze, Na_0.3K_0.2WO₃, if K⁺(18-crown-6)₂Na⁻ is used. The rods are generally ∼40 nm wide and 400–500 nm long single crystals, their principal axis being in the [001] crystallographic direction. Annealing in air at 500 °C changes the material to a bronze-like hexagonal alkali metal tungstate, without substantial effect on its nanorod morphology. The hexagonal alkali metal tungstate, K_0.3W_0.95O₃, nanorods can be made directly by heating to 400 °C rather than 600 °C in the 2-step annealing process, consistent with the tungstate being the first phase formed, transforming to the bronze at higher temperature.