Anomalous electro-migration of oxygen vacancies in reduced TiO2

We have found that the reduced rutile TiO2 shows anomalous electrical property under a DC electric field of several tens of V/cm. When a DC electric field was applied to the sample of reduced TiO2, a single pulse of electric current was observed together with a constant background current. This pulsed electric current occurs only once when the polarity of the electric field was kept unchanged. However, when we changed the polarity of the electric field we again observed a pulsed electric current. This pulsed current is attributed to migration of oxygen vacancies in reduced TiO2. We have also found that the migration of oxygen vacancies is visually observable under a DC electric field. Namely, when a DC electric field was applied to the sample after repeated change of its polarity, the color of the sample around the negative electrode changed into dark blue due to higher density of oxygen vacancies, while the region around the positive electrode lost the color.