Ana Rovisco

Visible-light-driven Ca1-xLnxMnO3 (Ln=Sm, Ho; 0.1≤x≤0.4) films were grown by RF- magnetron sputtering onto fused silica substrates. The effects of Ca2+ substitution for Ho3+ or Sm3+ in Ca1-xLnxMnO3 on the structural, morphological and photocatalytic properties for Rhodamine 6G dye degradation under visible light irradiation were investigated. XRD showed a pure typical perovskite phase for all the prepared films, except for Ca0.9Ho0.1MnO3 and a decrease of the crystallite size with the increase of the amount of ion substituted. SEM and AFM revealed that the films surface is dense, with low roughness. UV-vis spectroscopy indicated for the two series band gaps in the range of 1.6 - 2.8eV, being lower for the films containing holmium. The results showed that some Ca1-xHoxMnO3 and Ca1-xSmxMnO3 films present higher photocatalytic activity for Rh6G degradation in comparison with TiO2 films and for the same x value the Ho-films exhibited higher photocatalytic activity. For both films series the maximal degradation rate was obtained for x=0.2; above this content the degradation percentage exhibits a decreasing trend with the increase of Ho or Sm substitution, except for x=0.4 in the case of Ho system, which is observed again an increase in the degradation rate. The Rh6G photocatalytic degradation followed a pseudo first-order reaction kinetics. XRD and SEM of the used photocatalysts evidenced high photochemical stability.

Ca1-xSmxMnO3 (0≤x≤0.4) films were successfully fabricated on Indium Tin Oxide (ITO) coated quartz glass substrates by radio frequency magnetron sputtering technique (RF- magnetron sputtering) from compacted nanosized powder targets, and subsequent annealing at 800°C in air, for 6h. X-ray diffraction shows a pure typical perovskite phase for x≥0.1. Scanning electron microscopy and atomic force microscopy revealed that the film surface is dense, with low roughness, depending on the Sm content, even though a few cracks were observed. Crystallite size was found to decrease with the Sm content. The electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).The oxide electrode's capacitance was estimated using both techniques and the corresponding roughness factors evaluated. The values obtained from the two methods show a good agreement. A comparison between the voltammetric data and those referred in the literature allowed finding out that the redox reaction occurring at the electrode surface involves the pair Mn4+/Mn3+. EIS measurements confirm the voltammetric data and they also give additional information about the film porosity and the charge transfer resistance. This last parameter is associated with the oxidation and reduction of the pair Mn3+/Mn4+and after normalized by the roughness factor shows an increase with samarium content.