XPS Spectra Analysis of Ti2+, Ti3+ Ions and Dye Photodegradation Evaluation of Titania-Silica Mixed Oxide Nanoparticles

Abstract

TiO2-SiO2 mixed oxides have been prepared by the sol–gel technique from tetrabutyl orthotitanate and tetraethyl orthosilicate. The prepared materials were characterized by x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, nitrogen physisorption, Fourier-transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS). The results indicate that the TiO2-SiO2 mixed oxides have a large surface area and a nanoscale size. FT-IR spectra show that Ti atoms are bonded to silica by oxygen bridging atoms in Ti-O-Si bonds. The titanium valence states in TiO2-SiO2 mixed oxides were investigated by XPS, and their spectra report the presence of Ti2+ and Ti3+ cations for high silica concentration, suggesting the formation of oxygen vacancies. The photocatalytic activity of the prepared materials has been evaluated for the photodegradation of methylene blue (MB). The mixed oxides were activated by means of a UV light source, and the concentration of MB was monitored by UV–Vis spectroscopy. The synthesized TiO2-SiO2 shows significantly higher MB removal efficiency in comparison with that of the commercial TiO2 Degussa, P25. In this paper, we observed three valence states of titanium: Ti4+, Ti3+ and Ti2+ in TiO2-SiO2 40%. This issue has not yet been reported. XPS analysis show that the content of Ti2+ and Ti3+ amounts to 25.26 at.% and 13.08 at.%, respectively, while the concentration of Ti4+ is 61.72 at.%, much lower than in the TiO2-SiO2 9% sample. This behavior is explained observing that in TiO2-SiO2 40%, Ti4+ is reduced to Ti3+ and Ti2+ to a larger extent with respect to TiO2-SiO2 9%.