Characterization and application of the hetero-system LaCoO3/ZnO for degradation of Orange II under solar light

LaCoO3 elaborated by nitrate route was characterized physically and electrochemically to be applied in the Orange II (OII) oxidation upon solar light. The X-ray diffraction (XRD), SEM analysis and electrical conductivity were undertaken. The diffractogram showed that the single phase is completed at 850 degrees C and the oxide crystallizes in a distorted perovskite structure. The peak at 643 cm(-1) in the attenuated total reflection (ATR) spectrum confirmed the purity of the phase. The forbidden band (1.35 eV), determined from the diffuse reflectance, is assigned to Co3+: d-d transition in a low spin configuration (3d(6), E-5/2) in conformity with the black color of the oxide. The field-dependent magnetization was measured at 300 K in the range +/- 25 kOe, and the perovskite exhibits a low magnetism with a saturation magnetization (0.28 emu/g) confirming the low spin state Co3+. The thermal variation of the conductivity is characteristic of non-degenerate behavior and a hole mobility of 10(-2) cm(2) V-1 s(-1) and an activation energy of 0.11 eV. The latter is different from that obtained from the thermo-power, a signature of a conduction mechanism by small polaron hopping. LaCoO3 is chemically stable above pH 5; an exchange current density of 40 mu A cm(-2), a polarization resistance of 1.92 k Omega cm(2) and a corrosion potential of 0.99 V-SCE were obtained at pH similar to 7 (Na2SO4, 0.1 M). Curiously and unlike the perovskites, the variation of the inverse of the square of the capacitance as a function of potential (C-2 - E) indicates p type behavior with a flat band potential of 0.30 V-SCE and holes density of 1.97 x 10(17) cm(-3). The conduction band (- 0.94 V-SCE), made up of Co3+: 3d orbital, enables the formation of the radical O-2(center dot-), and as application, the oxide was successfully experimented for the oxidation of OII upon solar irradiation. The hetero-junction p-LaCoO3/n-ZnO enhances considerably the photo-activity, due to the facile transfer of electrons with an abatement of 86% at neutral pH. The oxidation follows a first-order kinetic with a half photocatalytic life of 23 min.