Electronic Structures of Passive Film Formed on Ti and Cr in a Phosphate Buffer Solution of pH 7

The electronic structures, including the semiconductive properties, of the passive films formed on Cr and Ti in a phosphate buffer solution of pH 7 were evaluated using photoelectrochemical response, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). Based on the results, electronic band structure models of the passive films were proposed: the passive films on Cr consisted of an inner p-type oxide layer with a band gap energy, E _g, of 3.6 eV and an outer p-type hydroxide layer with E _g of 2.5 eV whereas those on Ti consisted of an inner n-type oxide layer with E _g of 3.2 eV and an outer n-type hydroxide layer with E _g of 2.5 eV. The type of semiconductor evaluated for the oxide and hydroxide layers was strongly supported by valence-band XPS, which can provide the energy difference between the Fermi energy of the underlying metal and the highest energy of the valence bands of the oxide and hydroxide layers in the passive films.