Heat treatment as a key factor for enhancing the photodegradation performance of hydrothermally-treated sol–gel TiO2–SiO2 nanocomposites

Most studies related to the synthesis of TiO2–SiO2 composites for photocatalytic applications did not systematically evaluate the textural and photocatalytic properties obtained from different integration levels between these two phases. Thus, this work investigates the influence of the heat treatment step on the structural properties and photodegradation activity of TiO2–SiO2 composites synthesized by different sol–gel and hydrothermal routes. The as-prepared samples showed significant textural and photocatalytic differences, whereas the heat-treated samples showed similar structures and properties. However, some critical differences could be observed, such as different resistances to crystal growth due to the distinct location of the silica phase. Moreover, it was possible to identify a great photocatalytic enhancement by comparing the heat-treated and as-synthesized samples, which was mainly attributed to the segregation of SiO2 toward surface sites. Here, it was observed that the crystallization of titania after the heat-treatment step had a minor effect on the composites photoactivity since even the as-prepared samples were crystalline. The heat treatment was shown to be fundamental for a controlled segregation of the two phases, promoting a synchronous effect between the adsorption of MB molecules on the silica-rich portion and photocatalysis in the vicinity of such sites, where the titania phase is present. Such behavior was more pronounced in samples prepared with an early addition of TEOS during the synthesis.