Improved High-Temperature Stability and Sun-Light-Driven Photocatalytic Activity of Sulfur-Doped Anatase TiO₂

Abstract

Of the various forms of titania (anatase, rutile and brookite) anatase is found to be the best photocatalyst. Without any chemical additives, the anatase to rutile transformation in pure synthetic titania usually occurs at a temperature range of 600 to 700 °C. High temperature (≥800 °C) stable photoactive anatase titania is required for antibacterial, application in building materials. A simple methodology to extend the anatase phase stability by modifying the titanium isopropoxide precursor by sulphur modification using sulphuric acid is presented. Chemical synthesis by sol-gel method involved the reaction of titanium tetraisopropoxide (TTIP) with sulphuric acid (H 2 SO 4 ), followed by hydrolysis and condensation. The xerogel formed after drying was subjected to further calcination at different temperatures. Various TTIP:H 2 SO 4 molar ratios such as, 1:1, 1:2, 1:4, 1:8 and 1:16 were prepared and these samples characterized by XRD, DSC, Raman spectroscopy, XPS and BET surface area analysis.Sulphur modified samples showed extended anatase phase stability up to 900 ºC, while the control sample prepared under similar conditions completely converted to rutile phase at 800 ºC. Stoichiometric modification up to 1:4 TTIP: H 2 SO 4 composition (TS4) was found to be most effective in extending the anatase to rutile phase transformation by 200 °C compared to the control sample and it shows 100 % anatase at 800 °C and 20 % anatase at 900 ºC. Samples of 1:4 TTIP:H 2 SO 4 composition calcined at various temperatures such as 700, 800, 850 and 900 ºC showed significantly higher photocatalytic activity compared to the control sample. The 1:4 TTIP:H 2 SO 4 composition calcined at 850 ºC showed the highest photoactivity and it decolorized the rhodamine 6G dye within 12 minutes (rate constant 0.27 min -1 ) whereas the control sample prepared under identical condition decolorized the dye after 80 minutes (rate constant 0.02 min -1 ). It was also observed that the optimal size for highly photoactive anatase crystal is ca. 15 nm. XPS studies indicated that the retention of the anatase phase at high temperatures is due to the existence of small amounts of sulphur up to 900 °C.

A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern

“…It was also suggested that despite the decrease in band gap energy, metals were not always successfully introduced into the TiO 2 framework, and that the remaining metals on the outside surface sheltered the photoreaction sites [44]. Significant attention has been directed at reducing the band gap and in turn increasing the visible light activity of TiO 2 by doping with a range of non-metal elements including nitrogen [34,[45][46][47][48][49], carbon [43,[50][51][52][53], sulphur [54][55][56], phosphorous [57], boron [58,59], oxygen [60], and fluorine [61,62], along with heterojunctions [63][64][65][66][67] and combinations of these elements [68][69][70][71][72][73].…”

Improved high temperature stability of anatase TiO2 photocatalysts by N, F, P co-doping

“…The importance of this is its applications in several industries, most notably in the production of ceramic materials (e.g. wall tiles) which retain an antibacterial property [39]. Additional areas which have been identified as benefitting from a high temperature stable form of anatase are sanitary wares and self-cleaning glass.…”

A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties, and Mechanism

“…1,2,3,4,5,6 Of the semiconducting materials, ZnO offers significant opportunity in providing electronic, photonic, and spin-based functionality (spintronics) because of its direct wide band gap (3.37 ev) and large exciton binding energy of ~60 meV. This makes it worthy in the potential and established hi-tech applications such as ceramics, piezoelectric transducers, optical coatings, high speed and display devices, 7,8,9,10 gas sensors, 11 varistors, 12, 13 photocatalysts 14 and photovoltaics.…”