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Nanocrystalline lithium manganese oxide (LiMn2O4), synthesized by the sol-gel method, crystallizes in the spinel structure. The physical, electrical, and photoelectrochemical properties were studied for the photocatalytic degradation of methyl orange, a toxic compound, under solar irradiation. The diffuse reflectance spectrum allowed the direct band gap (1.99 eV) to be calculated. The Fourier transform IR spectrum contained all the characteristic peaks of the spinel LiMn2O4, which exhibited n-type behavior. The thermal evolution of the electrical conductivity exhibited an Arrhenius-type behavior with an activation energy of 0.27 eV. The Mott-Schottky curve allowed the determination of the flat band potential (−0.24 V vs. the saturated calomel electrode) as well as the carrier density (1.78 × 1021 cm−3). The detoxification of water containing methyl orange with LiMn2O4 as a photocatalyst was successfully completed. Total degradation for an initial concentration of 10−4 M methyl orange at pH ∼3 and 25 °C was obtained in less than 5 h under solar irradiation. No adsorption was obtained in the dark in the presence of LiMn2O4.

The CuBi 2 O 4 /TiO 2 heterojunction was tested with success for the photo-catalytic reduction of chromate ions under sunlight. CuBi 2 O 4 , prepared by nitrate process, was characterised photo-electrochemically. The oxide is stable against photo corrosion by consumption of holes in presence of oxalic acid. The light absorption promotes electrons in the conduction band of the sensitizer (CuBi 2 O 4 ) with a very negative potential (−1.74 V SCE ) to participate in the exchange of the electron with HCrO 4 − . The enhanced activity is due to electron injection of activated CuBi 2 O 4 into TiO 2 -CB (−0.97 V SCE ). The band gap of the semiconductor CuBi 2 O 4 is 1.50 eV with a direct optical transition. This compound is a p -type semiconductor with a flat band potential of −0.39 V SCE and activation energy of 0.18 eV. The electrochemical impedance spectroscopy was undertaken to study the semiconductor/electrolyte interfacial phenomena. The photoactivity on the heterojunction is strongly enhanced. A remarkable performance is obtained in less than 4 h for a concentration of 30 mg in (Cr (VI)) at pH ∼ 4 and a dose of 1 mg/mL; a 98% reduction has been obtained. The kinetic of chromate photoreduction is well described by the Langmuir-Hinshelwood model. The chromate elimination obeys to a pseudo-first order kinetic with an apparent rate constant of 0.014 min −1 .