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 degrees C. High-temperature (>= 800 degrees 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 degrees C, while the control sample prepared under similar conditions completely converted to rutile at 800 degrees C. Stoichiometric modification up to 1:4 TTIP/H2SO4 composition (TS4) was found to be most effective in extending the anatase-to-rutile phase transformation by 200 degrees C compared to that of the control sample and showed 100% anatase at 800 degrees C and 20% anatase at 900 degrees C. The TS4 composition calcined at various temperatures such as 700, 800, 850 and 900 degrees C showed significantly higher photocatalytic activity compared to the control sample. The TS4 composition calcined at 850 degrees 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 similar to 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 degrees C.