Structure–property correlation of nanostructured WO3 thin films produced by electrodeposition

Abstract

Nanocrystalline tungsten oxide (WO3) films were electrodeposited potentiostatically at room temperature on transparent conducting substrates from an ethanolic solution of acetylated peroxotungstic acid prepared from a wet chemistry process. The changes that occur in the microstructure and the grain size of the as-deposited WO3 films as a function of annealing temperature are simultaneously accompanied by a continually varying electrochromic performance. X-ray diffraction studies revealed the transformation of a nanocrystalline as-deposited WO3 film into a highly crystalline triclinic WO3 as the annealing temperature was raised from room temperature to 500 °C. The microstructural evolution with the increasing annealing temperature of the as-deposited film was further exemplified by transmission electron microscopy (TEM) studies. While the as-deposited film was composed of uniformly distributed ultra fine nanograins, the most noticeable feature seen in these films annealed at 250 °C was the presence of open channels which are believed to promote lithium ion motion. Films annealed at 400 °C exhibited coarse grains with prominent grain boundaries that hinder lithium ion movement, which in turn reduces the film’s ion insertion capacity. In concordance with the TEM results, the 250 °C film had the highest ion storage capacity as it exhibited a charge density of 67.4 mC cm−2 μm−1. The effect of microstructure was also reflected in the high transmission modulation (64%) and coloration efficiency (118 cm2 C−1) of the 250 °C film at 632.8 nm. Contrary to the superior electrochromic performance of the 250 °C film, the optical switching speeds between the colored and bleached states of the as-deposited WO3 film declined considerably as a function of annealing temperature. Also, the diffusion coefficient for lithium ions was greater by at least an order of magnitude for the as-deposited film as compared to the 250 and 500 °C films. In this report, the influence of microstructural changes that are brought about by the annealing of the as-deposited WO3 films on their coloration-bleaching dynamics is evaluated in terms of their structural, electrochromic, and electrochemical properties.