Electrochromic nanostructured tungsten oxide films by sol-gel: Structure and intercalation properties

As-deposited sol-gel derived amorphous tungsten oxide films transform into nanostructured films with an interconnected framework of grains and pores and a dominant triclinic crystalline phase upon annealing at 250 degrees C. Transmission electron microscopy and scanning electron microscopy images clearly reveal the annealing-induced microstructural evolution for the film. Subsequent to lithium intercalation, the film annealed at 250 degrees C shows quasi-reversible structural changes, as ascertained by X-ray diffraction and Fourier transform infrared spectral data. Dynamic transmission modulation for film revealed a high optical modulation of 72% (gimel=650nm) and a coloration efficiency maximum of 132 cm(2) C-1 at 800 nm under a lithium intercalation level of x = 0.20. X-ray photoelectron spectroscopy of the W 4f core levels demonstrated a progressive increase in the W5+ content at the expense of W6+ proportion as the insertion coefficient was raised from 0 to 0.25, with 0.20 as the threshold value above which the W5+ content exceeds the W6+ proportion. A new W4+ state also appears which acts to lower the coloration efficiency for x >= 0.22. The presence of charged oxygen interstitials in the vicinity of electrochemically active tungsten sites is also responsible for the coloration efficiency decline at high ion insertion levels. (c) 2006 The Electrochemical Society.