Improved high-temperature stability and sun-light-driven photocatalytic activity of sulfur-doped anatase TiO₂

Of the various forms of titania (anatase, rutile, and brookite), anatase is reported to be the best photocatalyst. The anatase-to-rutile transformation in pure synthetic titania usually occurs at a temperature range of 600-700 °C. High-temperature (≥800°C) stable anatase titania photocatalyst is required for antibacterial applications in building materials. A simple methodology to extend the anatase phase stability by modifying the titanium isopropoxide precursor with sulfur modification using sulfuric acid is presented. Various TTIP/H₂SO₄ molar ratios such as 1:1, 1:2, 1:4, 1:8, and 1:16 were prepared, and these samples were characterized by XRD, DSC, Raman spectroscopy, XPS, and BET surface area analysis. Sulfur-modified samples showed extended anatase phase stability up to 900°C, while the control sample prepared under similar conditions completely converted to rutile at 800°C. Stoichiometric modification up to 1:4 TTIP/H ₂SO₄ composition (TS4) was found to be most effective in extending the anatase-to-rutile phase transformation by 200°C compared to that of the control sample and showed 100% anatase at 800°C and 20% anatase at 900°C. The TS4 composition calcined at various temperatures such as 700, 800, 850 and 900°C showed significantly higher photocatalytic activity compared to the control sample. The TS4 composition calcined at 850°C showed visible light (sunlight) photocatalytic activity, and it decolorized the rhodamine 6G dye within 35 min (rate constant, 0.069 min^-1), whereas the control sample prepared under identical conditions decolorized the dye only after 3.5 h (rate constant, 0.007 min-¹). It was also observed that the optimal size for the highest photocatalytically active anatase crystal is ∼15 nm. XPS studies indicated that the retention of the anatase phase at high temperatures is due to the existence of small amounts of sulfur up to 900°C.