Hydrogen permeation through PECVD (plasma-enhanced chemical vapor deposition) TiO2 film on Pd by the time lag method

Abstract

Hydrogen transport through a PECVD (plasma-enhanced chemical vapour deposition) TiO2 film on a Pd substrate was investigated using the time lag method and a.c. impedance spectroscopy. The permeation experiment was carried out on a Pd/TiO2 film bilayer specimen by applying a constant cathodic current density of 80 mu A cm(-2) to the Pd side and anodic potentials from -300 to 300 mV vs. SCE (saturated calomel electrode) to the TiO2 side. A.c. impedance measurements were made at the TiO2 side before and after permeation. The diffusivity of hydrogen in the TiO2 film was determined from the time lag to be (3.9-5.2) x 10(-14) cm(2) s(-1) in the applied anodic potential range from -300 to 300 mV (SCE). The relatively low value of hydrogen diffusivity in the oxide film was explained in terms of hydrogen trapping in the film. Hydrogen injection into the TiO2 film greatly raised the capacitance of the oxide film estimated from the measured impedance in the lower measuring frequency range. The result suggested that trapped hydrogen forms deep donor levels in the band gap of the oxide film.