Photo-electrochemical properties of nanostructured metal-semiconductorAl/TiO2 thin film. Application to Rhodamine B oxidation under sunlight

Abstract

The physical and photo-electrochemical properties of the hetero-junction Al/TiO2 layers grown on quartz substrate by Spray Ultrasonic Method (SUM) were studied. The Grazing Incidence X-Ray Diffraction (GIXRD) exhibits narrow lines belonging to the anatase structure with a quadratic symmetry and a good crystallinity, a result confirmed by the Raman spectroscopy. The direct optical transition (3.34 eV) permits to exploit photo-electrochemically the solar UVA radiation; another transition indirectly allowed is observed at 2.92 eV. The capacitance-potential graph, plotted in Na2SO4 electrolyte, shows that TiO2 acts as n-type semiconductor with a flat band potential (Efb) of −0.27 V and a donor concentration (ND) of 1.2 × 1021 cm−3 due to oxygen vacancies. The conduction band (−0.66 VSCE) deriving from Ti4+: 3d orbital, is more negative than the level O2/O2•- (−0.4 VSCE). Therefore, the excited electrons are channelled to Al auxiliary electrode, leading to reduce oxygen into O2•- radicals, for the mineralization of the Rhodamine B (Rh B) at free pH (~ 7). The electrons transfer is facilitated through the bi-functional hetero-junction, due to the low over-voltage onto Al. An abatement of 30% (Rh B, 10 ppm) within 4 h of solar irradiation is obtained.