A PHOTOELECTROCHEMICAL IMPEDANCE SPECTROSCOPIC STUDY OF PASSIVE TUNGSTEN

In order to obtain some insight into the interaction between the electronic structure and ionic defects in passivating oxide films, photoelectrochemical impedance spectroscopy was employed to explore passive films formed on tungsten in phosphoric acid. The photoelectrochemical impedance could be measured at low frequencies and at photon energies of up to 5.5 eV using either square-wave-modulated irradiation or step-function excitation. Three different procedures were employed to demonstrate that the system's photocurrent response fulfilled the requirements (linearity, causality, stability and finity) necessary for impedance spectroscopy. Along with photoelectrochemical impedance spectra of passive tungsten in phosphoric acid, photocurrent transients, the electrochemical impedance and transient capacitance and reflectance responses were measured. Photocurrent transients show a fast exponential decay during the first 2 s and a linear increase for the following 10-20 s. Equivalent circuits could be derived to model this behavior. The properties of the first atomic layer of the oxide next to the solution seem to be responsible for the observed effects, which we attribute to the kinetics of electron-hole recombination via surface states: initially, surface recombination reduces the photocurrent and, subsequently, the number of surface states diminishes gradually by dissolution of the first atomic layer. This dissolution could actually be monitored by transient capacitance and transient reflectance measurements.