Visible light-active non-metal doped titanium dioxide materials for photocatalytic oxidation

Abstract

One of the most important technologies for a better human life is environmental purification which has drawn attention and gained importance over the past years. Titanium dioxide has been the apple of the eye of both air and water purification systems for its strong ability of oxidation, low cost, nontoxicity, inertness and availability. However, being a wide band gap semiconductor, titanium dioxide can mostly absorb UV photons (nm) in the sun light, which is only about 3% of the total solar radiation. In this regard, sensitizing titanium dioxide based materials capable of visible light absorption via doping methods is a challenging but yet a rewarding effort. In the current work, a variety of doping protocols have been employed in conjunction with sol-gel titanium dioxide synthesis protocols in an attempt to prepare visible-active photocatalytic powders. This study has been a preliminary work to propose a simple sol-gel synthesis route for the preparation of visible-active titanium dioxide in order to combine with previously studied UV active titanium dioxide powders to create tandem systems that will harvest both visible and UV light for water and air purification. Along these lines, two different sets of samples were prepared and investigated. The first set of samples was prepared by a sol-gel route with the addition of non-metallic compounds of Ti which are TiN, TiC and TiS2. Non-metal atom to titanium mol ratio was kept at 0.1:1 and the syntesized powders were characterized by XRD, Raman Spectroscopy, BET, UV-VIS Diffuse Reflectance Spectroscopy in order to investigate the effect of calcination temperature, surface area and band gap on photocatalytic activity. Besides, these commercial TiN, TiC and TiS2 powders that were used as dopants, were also annealed in open air to prepare partially oxidized titanium materials. Secondly, inexpensive sources of non-metal compounds such as boric acid, diethanolamine (DEA), triethylamine (TEA), thiourea, urea and cyclohexanol were added in an alternative sol gel synthesis route. Dopant compound to titanium dioxide mol ratio was also kept at 0.5:1. Structural and electronic characterization of this family of materials were also carried out in addition to photocatalytic activity tests. Photocatalytic activity measurements were done in liquid phase via the degradation of an organic contaminant, Rhodamine B, in a custom-designed VISilluminated photocatalytic reaction cells. Photocatalytic performance of all samples were compared with that of a commercially available Degussa P25 TiO2 benchmark catalyst. Photocatalytic preformance tests revealed improved photocatalytic activity for non-metal compound added titanium dioxide compared to unmodified titanium dioxide prepared with the same method. Also, several samples presented even higher photocatalytic activity compared to Degussa P25. Characterization experiments showed hinderance in anatase to rutile transformation due to foreign atoms. It was also observed that although a small band gap is important for the photocatalytic activity, there are other critical parameters such as particle size, surface area, crystallinity, active facets, oxygen vacancies which have to be fine tuned for photocatalytic performance optimization.