Optimizing the use of light in supported TiO2 photocatalysts: Relevance of the shell thickness

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Heterogeneous photocatalysts have attracted significant interest over the last few years. In this sense, improved photocatalytic properties for SiO2@TiO2 materials have been correlated with higher semiconductor's specific surface area and adsorption capability. However, the reported TiO2 shell optimizations did not consider the percentage of the photo-excited electron-hole (e−-h+) pairs occurring in the inaccessible surface of the TiO2 shell. The present study aimed to find the best TiO2 shell thickness for the optimal harvest of light. For this purpose, the sol–gel method was modified by applying a precise humid airflow to get a uniform deposition of titania on the surface of the SiO2 particles. As a result, photocatalysts with a robust and homogeneous TiO2 shell of different thicknesses (from 14 to 42 nm) with well-defined particle size for TiO2 crystals (ca. 12 nm) were obtained. The SiO2@TiO2 materials were extensively characterized. Finally, their photocatalytic activity was evaluated against methylene blue under irradiation by UVA light. Analysis of the results revealed that the most efficient photocatalyst has a TiO2 shell thickness of ca. 30 nm. Emission and photocurrent studies also agreed with the photocatalytic results suggesting a reduced e−-h+ recombination rate. Thereby, for the first time, the relevance of shell thickness in supported photocatalysts has been demonstrated.