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Visible light induced photobleaching of methylene blue over melamine-doped TiO2 nanocatalyst
Citation:
Virkutyte, J., B. Baruwati, AND R. S. VARMA. Visible light induced photobleaching of methylene blue over melamine-doped TiO2 nanocatalyst. Nanoscale. RSC Publishing, Cambridge, Uk, 2(7):1109 - 1111, (2010).
Impact/Purpose:
To inform the public.
Description:
TiO2 doping with N-rich melamine produced a stable, active and visible light sentisized nanocatalyst that showed a remarkable efficiency towards the photobleaching of a model compound – methylene blue (MB) in aqueous solution. The photobleaching followed a mixed reaction order kinetics with the distinctive induction and acceleration periods. TiO2 is one of the most promising photocatalyst for various applications including degradation of organic pollutants in both, the liquid and gas phase environments, photocatalytic dissociation of water, partial oxidation of aromatic hydrocarbons under room temperature and atmospheric pressure, air purification, antibacterial and self-cleaning coatings, sterilization, anti-fouling and demister.1-5 However, the main drawback of photocatalysis is that titanium dioxide is only active in the UV range (< 390 nm) due to the very large band gap of 3.2 eV,6,7 therefore it must be modified in order to take advantage of abundant solar light or be applicable for the use in even poorly illuminated indoor areas.8 Thus, it is important to expand the range of TiO2 photocatalytic activity for practical purposes, i.e. to narrow the band gap of the catalyst to allow electron excitation in the visible (400 – 800 nm) light range.9 Various approaches are pursued with cationic10 and anionic5,6,11 doping of TiO2. Doping with nitrogen results in substitutional ((NO)s, Ns etc) and interstitial ((NO)x, Nx, etc) nitrogen species.12 It has been determined that increased photocatalytic activity of N-doped TiO2 under the visible light is attributed to the narrowing of the band gap by mixing the O 2p and substitutional N 2p states as well as interstitial N species doped into the TiO2 lattice.6,13 However, many of N-doping methods require high temperature and usage of expensive precursors or preparation instruments.5 Herein, we present a facile and simple visible light sensitization of nano TiO2 with melamine as N-doping agent and its applicability for the photobleaching of a model pollutant methylene blue in aqueous solution. N-doped TiO2 nano-powders were prepared by a simple sol-gel method: i) 20 g of TiCl4 (digested to 50 % in HCl) was dispersed in 1000 ml of distilled water. Then 80 g of melamine was added to the solution and magnetically stirred for 30 min. The amount of dopant was calculated based on the assumption that in order to assure an increased photocatalytic activity, the content of nitrogen in the catalyst after hydrolysis should not be less than 0.5 wt % of the catalyst dry mass.14 Therefore, the ratio between the molar concentration of Ti4+ and melamine was maintained 1:4.
