Surface Decorated Photocatalytic TiO2 by High Energy Ultrasound

The industrial society produced a wide variety of pollutants[1]: among many advanced oxidation processes (AOPs), TiO2 photocatalysis has received huge attention as one of the most viable environmental cleanup technologies; indeed, alternative photocatalytic materials that are as “versatile, economical, stable, abundant, and non-toxic” as TiO2 are hard to be found[2][3]. The wide band gap of TiO2 is a problem in particular when the material must be activated by solar light and not by UV light. One of the most viable and practical approaches in developing better photocatalysts is to modify TiO2[4]: although the traditional doping proceedings require the insertion of either non-metal or metal atoms in the TiO2 structure, it is possible to enhance its photocatalytic activity acting on its surface. The most frequently employed modification method is to load nano-sized compounds on TiO2 surface via impregnation[1]. The surface nanoparticles can affect the photochemical properties of TiO2 support in different ways: for example, because of the presence of a metal, the Fermi level of TiO2 can shift to the metal level and in this way the absorption shifts to longer wavelengths. Secondly, metals or metal-oxides can act as electron-traps and they can inhibit the electron-hole recombination, improving the photocatalytic activity and they can also promote the charge separation[1][6]. In this work, sonochemical routes of molybdenum, rhenium, copper and tungsten mixed oxides syntheses are used in order to decorate the surface of commercially available, micrometer TiO2 catalysts. This is a new approach to decorate titanium oxide by using sonochemistry in order to obtain metal nanoparticles or metal oxides nanoparticles on the surface of the substrate. The sonochemical method described in this paper is usually used e.g. for the SOFC decoration of SOFC anode materials[5]. The solution of the nanoparticles is co-sonicated with the commercial TiO2 powder used as support, to obtain particles of titanium dioxide decorated with nanoparticles of a metal or metal oxide. To get this result, a combined process of deposition and ultrasonication is used in aqueous or organic solution, where the TiO2 powder is dispersed. The metals used are the following: molybdenum, tungsten, copper and rhenium; the related precursor molecules are: Mo(CO)6, W(CO)6, CuCl2*2H2O, and Re2(CO)10. The experimental procedure with the different molecules listed before is now described: for each preparation, the defined quantity of TiO2 and of the precursor is weighted into a glass flask and the settled solvent is added; the organic or aqueous solution obtained is sonicated at a constant temperature, amplitude and intensity. Then, it is washed, centrifuged and calcinated to completely remove the organic scents. All the samples were characterized by XRD, SEM, TEM and FT-IR (OH rating) analysis: surface decoration with metal particles is confirmed for all samples. It is shown that with this method it is possible to synthesize some metal or metal oxide nanoparticles using high-energy ultrasound and make sure that they are deposited on a definite support, in this case a micro-sized TiO2. Furthermore, the photocatalytic properties of titanium dioxide can be exploited for the degradation of pollutants; the TiO2 photocatalyst (Kronos 1077), which were decorated with the metals previously described, was tested on the degradation of VOC’s (acetone and toluene) and NOx, using both UV light and LED light. The results obtained show that some of the decorated samples have an interesting photocatalytic activity. Although the surface decoration of TiO2 can not be defined as a doping and it does not change the TiO2 structure lowering the band-gap, certainly it affects the photocatalytic activity of TiO2. For this reason, the possibility to obtain a controlled surface decoration of the TiO2 is very interesting and the sonochemical method can be considered a novelty.