A comparison of electrochromic properties of sol–gel derived amorphous and nanocrystalline tungsten oxide films

Abstract

A pristine acetylated peroxotungstate sol with and without 4 wt% of oxalic acid dihydrate (OAD) yielded nanocrystalline and amorphous tungsten oxide (WO3) films respectively by dip coating technique. Contrary to the expected trend, whereby, the nanostructured 4% OAD film with a triclinic modification should have shown superior electrochromic efficiency, its amorphous 0% OAD counterpart exhibits higher optical modulation and coloration efficiency at photopic wavelengths. This anomalous behavior of the amorphous 0% OAD film was correlated to a higher W5+ content in its colored state. The colored nanocrystalline 4% OAD film contained a lower proportion of the W5+ color centers. Under the same level of lithiation, while the 4% OAD film was also constituted by W4+ states, its 0% OAD counterpart did not contain any 4+ states of tungsten. X-ray photoelectron spectroscopic (XPS) investigations also confirmed that the single peak at ∼1.4 eV in the absorption coefficient–wavelength spectrum of the colored 0% OAD film arises from the small polaronic transitions between the W6+ and W5+ states of tungsten whereas the W6+–W5+ and the W5+–W4+ charge transitions produce two distinct peaks at 1.2 and 1.6 eV in the α–λ spectrum of the colored 4% OAD film. The microstructure of the 4% OAD film, characterized by an interconnected network of nanocrystallites and pores promotes rapid ion insertion and extraction. Therefore, the larger magnitudes of NIR reflectance modulation, diffusion coefficient for lithium, electrochemical activity and faster color–bleach kinetics observed for the 4% OAD film, are a direct consequence of its structure. Band gap widening upon lithium insertion observed for both films, is a repercussion of Burstein–Moss effect and structural changes that occur upon coloration.