Hydrogen transport through rf-magnetron sputtered amorphous and crystalline WO3 films

Abstract

The present work involves hydrogen ion transport through amorphous WO3 (a-WO3) and crystalline WO3 (c-WO3) films. From the comparisons of cyclic voltammograms and electrochemical impedance spectra for the a- and c-WO3 films, it was found that hydrogen ion absorption resistance for the c-WO3 film was much larger than that for the a-WO3 film. It is suggested that the different behaviours in cyclic voltammograms and electrochemical impedance spectra between a- and c-WO3 films result from an open-structured nature of a-WO3 film containing H2O, which does not exist in the c-WO3 film. The chemical diffusivity of hydrogen ions in the a-WO3 film was determined as a function of applied cathodic potential by analysing the electrochemical impedance spectra obtained from WO3 film r.f.-magnetron sputtered onto indium tin oxide glass ('impermeable modified electrode') in 0.1 M H2SO4 solution. As the applied cathodic potential decreased from -0.45 to -0.8 V(SCE), the chemical diffusivity of hydrogen ion in the a-WO3 film increased from about 10(-10) to 10(-9) cm(2) s(-1). The applied potential dependence of the chemical diffusivity of hydrogen ions is discussed in terms of the interaction between hydrogen ions and lattice oxygen ions, hydroxide ions or water. The two-stage variation of potentiostatic current decay transients suggested the existence of trap sites for hydrogen ion with relatively strong binding energy.