Mechanism of soft solution processing formation of alkaline earth metal tungstates: an electrochemical and in situ AFM study

摘要

The mechanism of alkaline earth metal tungstate formation during soft solution processing was studied by cyclic voltammeter, electrochemical impedance spectroscopy and b direct in situ observation of he surface changes using atomic force microscopy. The electrochemical oxidation of W to WO_3 was followed by dissolution of WO_3 and, with some delay, by precipitation of tungstates at he metal surface. The same Tafel slopes observed in Li~+, Ba~(2+), Sr~(2+) and Ca~(2+) containing solutions indicate that the course of the oxidation process is independent of the cation present in solution. The observed differences in the current-voltage curves outside the Tafel region are accounted for by the different film-forming tendencies of the various alkaline earth metal cations. The growth of tungstate layers at the W substrate decreases the electrochemically active area and limits the production of WQO_4~(2-) at later stages of deposition. At low potentials (E<0.2 V) the oxidation of W is the rate-controlling step. At higher potentials, however, the dissolution process slows down due to a relative decrease of the pH in the electrode vicinity and dissolution becomes the rate-limiting step.