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Photocatalytic water splitting is a promising way to produce hydrogen, which could serve as an eco-friendly fuel produced from renewable energy sources like the sun. TiO2 is one of themost widely used photocatalysts, but it can only utilize UV light, which constitutes only a very small part of the whole spectrum of natural sunlight. To produce photocatalysts that utilize the solar spectrum more efficiently, it is necessary to modify TiO2 and make it visible light active. In this project, vanadium-modified TiO2 photocatalysts with 1 at%-, 5 at%- and 25 at%V were produced to investigate the role of V-species as a visible light absorber as well as their possible role as a co-catalyst. For V-concentrations of 5 at%- and 25 at%, the resulting catalystsconsisted of composites between TiO2 and V2O5 while no individual VOx compounds could be detected for the low-concentration 1 at%-samples. All of the samples showed an improved visible light absorption.The catalysts were tested for their photocatalytic activity towards OER and HER under UV light in order to investigate the ability of the V-species to act as a co-catalyst. Their role as a light absorber was analysed with HER experiments under visible light with the addition of Pt. All experiments were performed using a sacrificial agent for the respective complementary reduction and oxidation reaction. While none of the synthesized catalysts was active under UV light, the sample with 5 at% was able to produce hydrogen under visible light illumination, contrary to unmodified anatase. Due to the formation of a heterojunction between the two oxides, the V2O5 in the composite structure was able to act as a light absorber, transferring excited electrons into TiO2 where they could then react with protons to form H2.

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&ndash, 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.