Photoelectrochemical analysis on the passive film formed on Ni in pH 8.5 buffer solution

The structure and composition of passive film formed on Ni in pH 8.5 buffer solution were examined by comparing the photocurrent and Mott–Schottky responses for both the passive film and the thermally grown oxide on Ni in air at 400 °C.The photocurrent spectra for the passive film on Ni are resolved into two spectral components, each of which originated from inner NiO and outer Ni(OH)2 layers, respectively. The band gap energy (Eg) for the inner NiO and the outer Ni(OH)2 were measured to be 4.80 ± 0.1 and 3.05 ± 0.1 eV, respectively. Eg (4.80 ± 0.1 eV) of the inner NiO of the passive film on Ni was close to that (4.97 eV) of the thermally grown oxide of Ni within an error range, suggesting that the inner NiO in the passive film is crystalline. The Mott–Schottky plots for both the passive film and the thermal oxide film on Ni demonstrated that the two films exhibited p-type semiconductivity with different values of flat band potential (Ufb); 0.3 VSCE for the passive film and 0 VSCE for the thermally grown NiO. An electronic energy band model of P–p heterojunction composed of inner NiO and outer Ni(OH)2 layers was proposed to explain the photocurrent and Mott–Schottky responses for the passive film on Ni.