Scientific Papers in SCI

Tb-doped TiO2 hollow spheres (HSs) in the range 0.0-2.0 at.% have been synthesized by the first time to the best of our knowledge. The HSs are compared with nanoparticles (NPs) to evaluate the impact of morphology on their physicochemical and photoluminescence (PL) behavior upon increasing calcination temperature. After calcination at 550 degrees C, the particles are anatase with a primary average size of 10.0 +/- 0.2 nm for the NPs and 12.0 +/- 0.2 nm for those that form the micron sized hollow spheres of 1.8 +/- 0.2 mu m diameter and ca. 64 nm shell thickness. The temperature of the anataseerutile transition is found to be strongly dependent on the presence of Tb as well as on morphology. Contrarily to the usual stabilization of anatase when doping with trivalent rare-earth ions, the transition temperature is reduced when doping with Tb. The rutile phase is further favored for the HSs compared to the NPs probably related to the low density of the HSs and/or a more efficient packing density and/or a bigger crystal size of the nanoparticles that form those spheres with respect to the packing and the size of the NPs and/or the crystal size of the nanoparticles of the HSs with respect to the size of the NPs. Only a slight unit-cell volume increase for the anatase structure is observed upon Tb doping, in both the NPs and in the HSs, contrary to the expected increment due to the larger ionic radius of Tb3+ compared to Ti4+. In addition, the intensity of the characteristic f-f Tb3+ emission bands is extremely weak both in the anatase and rutile phases. The transition is accompanied with the emergence of an infrared emission band centered at 810 nm related to the formation of defects during the structural transformation providing deep levels in the gap that partly quench the f-f emissions in the rutile phase. The results are consistent with the presence of Tb in both +3 and +4 valence states. XPS measurements confirmed the presence of Tb3+ as well as of Tb4+ in both HSs and NPs. The large fraction of Tb4+ present in the samples originates the weak f-f emission intensity, an only slight increase of the cell parameters and the destabilization of the anatase phase. 

The optimization of the catalysts incorporated to the electrodes for anion exchange membrane water electmlysers is a key issue to maximize their performance through the improvement of the oxygen evolution reaction (OER) yield. In this work, we show that the modification of the microstructure and the chemical properties of a mixed copper-cobalt oxide anode may contribute to increase the activity of this reaction. For this purpose, the OER has been systematically studied, either in a half cell or in a membrane electrode assembly configuration, as a function of the load and agglomeration degree of the catalysts used as electrodes, as prepared on a carbon paper support by magnetron sputtering deposition in an oblique angle configuration. Chemical analysis by X-ray absorption spectroscopy and electrochemical analysis by cyclic voltammetry and impedance spectroscopy have shown that cobalt-copper mixed oxide catalysts with a 1.8 Co/Cu atomic ratio and about one micron equivalent thickness maximizes the cell performance. The chemical, structural and microstructural factors controlling the final behaviour of these anodes and accounting for this maximization of the reaction yield are discussed on the basis of these characterization results and as a function of preparation variables of the electrodes and operating conditions of the cell.