Mechanistic insight into the improved photocatalytic degradation of dyes for an ultrathin coating of SiO2 on TiO2 (P25) nanoparticles

Abstract

The photocatalytic mechanism of TiO2 (P25) nanoparticles coated with SiO2 (SiO2:TiO2) by atomic layer deposition was investigated. The deposition of SiO2 on TiO2 not only gives a photocatalytic improvement for the degradation of both Rhodamine B (3.6–fold) and Acid Blue 9 (3–fold). SiO2 deposition also changes the mechanism from direct oxidation of the pollutant at the surface of TiO2 to a predominantly OH⁎ radical based degradation of the pollutants originating from SiO2:TiO2. Low SiO2 loadings on TiO2, where the coating is incomplete, improve the OH⁎ radicals generation due to the higher number of acidic Si–OH groups combined with the facilitated charge separation at the TiO2–SiO2 interface. As a consequence of incomplete coverage, the TiO2 surface remains accessible, which allows both the oxidation and reduction reactions at the SiO2:TiO2 surface. On the other hand, high loading of SiO2 (>3 wt.% Si) results in photocatalytic suppression due to the coverage of TiO2 surface by SiO2. The degradation of differently charged dyes on the SiO2:TiO2 surface demonstrates the independence of the adsorption properties on the photocatalytic improvement. Simultaneous degradation of two dyes demonstrated the advantage of SiO2:TiO2 being less selective and, therefore, better suited for general water purification.

Keywords

• Photocatalysis
• Atomic layer deposition
• Dye degradation
• Mechanism
• SiO2 on TiO2