Synthesis and characterization of mesoporous H4SiW12O40/TiO2 composites

Abstract

Mesoporous H4SiW12O40 /TiO2 composites were successfully synthesized by sol-gel reactions using urea as a low-cost template, and adding tungstosilicic acid (TSA) at the same time as the template. The prepared materials were characterized by different physicochemical techniques. The samples presented mesopores with a diameter higher than 3.1 nm. The specific surface area of the solids decreased with both the increase of the TSA content and the calcination temperature. From FT-IR studies it was observed that the main heteropolyoxometallate species present in the composites is the [SiW12O40]4- anion, which was partially transformed into the [SiW11O39]8- lacunar phase when samples were calcined at 600 ºC. The XRD patterns of the modified samples only exhibited the characteristic peaks of anatase phase of titanium oxide. The presence of TSA retarded the crystallization of the anatase phase and its transformation into the rutile phase. The point of zero charge decreased with both the increase of TSA content in the solids and the calcination temperature. The band gap energy decreased as a result of the introduction of TSA into the titania matrix, though it remained almost constant with the calcination temperature increase. The activity of the materials in the photodegradation of methyl orange was higher when the TSA content and the calcination temperature of the samples were raised. A lower recombination degree of the photogenerated electrons and holes, lower band gap values, and a higher crystallinity degree of the samples are the main effects responsible for the activity increase.