A. Giesriegl, J. Blaschke, S. Naghdi, D. Eder:
"Rate-Limiting Steps of Dye Degradation over Titania-Silica Core-Shell Photocatalysts";
In this work, we employed a step-by-step sol-gel process to controllably deposit ultra-thin
layers of SiO2 on anatase nanoparticles in the range between 0 and 1 nm. The deposition was
confirmed by TEM, EDX, and ATR-FTIR (e.g., Ti-O-Si band at 960 cm-1). Zeta potential measurements
unravelled a continuous change in surface charge density with increasing silica shell thickness.
The photocatalysts were evaluated towards adsorption and degradation of positively-charged and
negatively-charged dyes (methylene blue, methyl orange) under UV illumination. The growth
mechanism follows the Stranski-Krastanov model with three thickness regimes: (a) Flat islands (first
step), (b) mono/bilayers (second/third step), and (c) regular thick films (fourth/fifth step). The
results suggest different rate limiting processes for these regimes: (a) For the thinnest scenario, acidic
triple-phase boundaries (TPBs) increase the activity for both dyes with their accessibility being the rate
limiting step; (b) for continuous mono/bilayers, dye adsorption on the negatively-charged SiO2 shells
becomes the rate liming step, which leads to a stark increase in activity for the positively-charged MB
and a decrease for MO; (c) for thicker shells, the activity decreases for both dyes and is limited by the
charge transport through the isolating shells.
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