Síntese e caracterização de materiais modificados: titânio:nióbio:rutênio

Abstract

The vast majority of commercial semiconductor oxides are limited by their band gap value, photoactivated by ultraviolet light, representing only 5% of the entire solar electromagnetic spectrum. To provide improvement in the photoactivation capacity with visible sunlight radiation of some semiconductor metallic oxides, this work aimed at the development and morphological/chemical, spectroscopic and structural characterization of niobium pentoxide (Nb2O5), titanium dioxide (TiO2) and mixed TiO2: Nb2O5, as well as modified with ruthenium (Ru). In the synthesis process, the sol-gel method was used, which offers the advantage of promoting high crystallinity at low temperatures and allowing control of factors such as moderate temperature and pressure. The new materials were characterized by the following techniques: scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS); photoacoustic spectroscopy (PAS); and X-ray diffraction (XRD). According to the SEM/EDS analyses, the synthesis methodology employed provided bases with high purity at both levels of calcination temperature, containing only the expected chemical elements of titanium and niobium. On the other hand, the modified samples showed ruthenium adsorption on the surface with the formation of crystals and agglomerates visible more clearly after higher calcination temperature. The non-destructive characterization of PAS in the ultraviolet-visible region suggested that increasing calcination temperature promoted changes in chemical structures and an apparent decrease in gap energy. The separation of superimposed absorption bands referring to charge transfers from the ligand to the metal and the nanodomains of the transition metals suggested the possible absorption centers present at the absorption threshold of the analyzed oxides. Through XRD analysis, stable phases such as T-Nb16.8O42 and rutile were formed at a lower temperature level, suggesting pore induction and increased surface area, for the oxides studied, at a calcination temperature lower than expected by the related literature. In addition, the synthesis with a higher temperature level altered the previously existing morphologies of the mixed oxides of Ti:Nb, base and modified with Ru, forming the new mixed crystallographic phases Ti2Nb10O29 and TiNb2O7, respectively. In general, as several semiconductor oxides applications aim to reduce costs with photoexcitation in visible light, the modified Ti:Ru oxide calcined at a temperature of 800 °C and synthesized according to the sol-gel methodology used in this work is suggested as the optimum preparation point since this material presented: the formation of a stable crystallographic phase (rutile); a significant decrease in gap energy (2.01 eV); and a visible absorption threshold (620 nm).