There are large variations in the properties of WO3 sputtered under di
fferent conditions and two samples sputtered from an oxide target and
reactively sputtered from a metal target were compared in detail. The
thin film sputtered from an oxide target was found to color and bleach
rapidly in 1 M LiClO4 in propylene carbonate, while the thin Blm reac
tively sputtered from a metal target could be colored deeply, but blea
ched only slowly. By calculating the rate of change of optical density
during cyclic voltammetry, it was possible to directly compare the co
loration response with the current/voltage behavior of the electrodes.
In both cases at least two lithium insertion reactions appear to occu
r. The distinction between the two reactions was especially clear in t
he sample sputtered from a metal target, in which an insertion of high
electrochromic efficiency occurred up to Li,,WO, and then an insertio
n of considerably lower electrochromic efficiency up to Li,,WO,. Altho
ugh a small amount of coloration and bleaching continued to occur afte
r switching the reactively sputtered sample to open circuit during the
coloration and bleaching cycles, transmission change was largely halt
ed by disconnecting the external current supply. The slow end to the b
leach of the reactively sputtered sample corresponded to a reaction of
high electrochromic efficiency.