Structural Changes in Electrochromic WO₃ Thin Films Induced by the first Electrochemical Cycles

International audienceWe have investigated the effect of the very first electrochemical cycles on the local structure and bonding for two different amorphous transparent electrochromic WO3 thin films, namely, a sputtered film and an electrodeposited film. The cycles are performed at a constant potential scan rate between 0.65 and –0.55 V vs. NHE in a 0.3 M LiClO4/propylene carbonate solution. We show that this treatment has a weak influence on the short range order in sputtered films. On the contrary, dramatic changes are found in electrodeposited ones. Both Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) data show that the initial electrodeposited oxide films are highly disordered with a highly asymmetric structure. The Fourier transforms of the EXAFS spectra of these films show four different W-O distances in the first oxygen shell with values ranging between 1.5 and 2.4 Å (after phase correction). After the electrochemical treatment, the radial distribution of the oxygen atoms is changed. The structure becomes similar to that classically reported for amorphous WO3 films and can be fitted assuming two different main distances. Raman spectroscopy and EXAFS reveal a larger W-O distance distribution in the two different amorphous films compared to a monoclinic WO3 reference sample. They show a higher dispersion of the W-O distances in cycled electrodeposited films, compared to the cycled sputtered ones