ANODIC-OXIDATION OF TUNGSTEN IN NONAQUEOUS ELECTROLYTE .2. ELECTROCHEMICAL COLORING AND BLEACHING OF THE OXIDE FILM

Ellipsometric and electrochemical measurements are used to determine the mechanisms by which anodic oxide films on tungsten are reduced (colored) and reoxidized (bleached) in acetic acid electrolyte. In the reduction process, a layer of HWO3 forms on the outer surface of the oxide, and, as reduction proceeds, a phase boundary between HWO3 and WO3 moves inward toward the substrate. Competition from hydrogen evolution limits the depth to which the boundary penetrates, but films up to 50 nm in thickness can be reduced completely to HWO3 with an appropriate sequence of cycles. The highly absorbing HWO3 layer (refractive index 1.642 and extinction coefficient 0.811) is readily distinguished from the anisotropic transparent oxide (refractive index 2.170 transverse and 2.078 parallel to the 5.02 MV/cm anodizing field). The reoxidation process begins with the formation of a layer of WO3 on the outer surface of the HWO3, and again a phase boundary moves inward across the film. A field on the order of the anodizing field is required to move hydrogen to the electrolyte through the outer reoxidized layer, and this field also moves oxygen into the outer layer, where it combines with mobile hydrogen cations to form H2O groups within the WO3. The buildup of H2O enables hydrogen to penetrate more deeply into the film on subsequent cycles. When the phase boundary between WO3 and HWO3 reaches the substrate, tungsten begins to enter the film, and an interface between oxide and hydrated oxide sweeps outward across the film as tungsten replaces hydrogen. When this interface reaches the electrolyte, anodizing conditions are re-established. Open-circuit transients applied during the reoxidation process are sensitive to the nature of the mobile ionic species, and detect the changes in the ionic transport process during the reoxidation.