Rare-earth doped TiO2 films for the visible light photocatalytic oxidation of ethanol in air

In the last decades, TiO2 has emerged for its excellent photocatalytic properties applicable to pollutant abatement. In particular, it is a promising material to remove small amounts of volatile organic compounds (VOCs) from contaminated indoor atmosphere. Most applications are limited to UV light irradiation because TiO2 only absorbs light with wavelengths shorter than 380nm. Much research has been prompted on modification procedures to extend its spectral sensitivity to visible light, which forms the largest part of solar radiation and is the main radiation emitted by lamps used in indoor illumination. In recent years, different approaches, such as coupling with visible-light activated semiconductors, the use of smaller band gap materials and doping of TiO2, have been proposed to achieve this goal. In this work thin films were doped with rare-earth metal ions showing transitions in the visible region. La, Ce, Nd, Sm, Eu, Gd, Dy and Ho doped TiO2 sols were prepared through a sol-gel method which allows to control the stoichiometry, work in mild and ambient atmospheric conditions and achieve high purity and homogeneity of the final product. The aim of the present study was to investigate the influence of these rare earth-metal dopants in different contents (1, 2 and 3% in weight, respectively) on the photocatalytic activity of TiO2 using ethanol as VOC molecule, under both UV and visible illumination. The films were deposited by spin coating on sodium free glass [1] and calcined at 450 C for 2 hours. The microstructure, optical and morphological properties of the films were investigated using SEM, UV-Vis spectroscopy, ellipsometry and photoluminescence (PL). The nature of the crystalline phases was ascertained by X-ray diffraction and the effective composition of the samples determined by X-ray photoelectron spectroscopy (XPS). The photocatalytic measurements were carried out in a stainless steel batch reactor in a controlled Ar/O2 atmosphere (ratio 80:20 to simulate air). The ethanol concentration was monitored by means of an atmospheric gas analyzer containing a mass spectrometer and a mercury high-pressure short arc lamp was used as light source. The initial reaction rate was determined from the slope of the first linear data points measured during the breakdown reaction of ethanol into CO2 and H2O. The doped TiO2 films show good photocatalytic activity in degradation of the tested VOC molecule, compared to titania films prepared using P25 Degussa. In particular, the addition of rare earth elements with 4f electron configuration, enhances the photoresponse in the visible region of the sol-gel derived catalyst. Among the developed catalysts, the Eu doped titania has the largest effect on the photocatalytic activity of TiO2 under visible light. (1) H. Tomaszewski, K. Eufinger, H. Poelman, D. Poelman, R. De Gryse, P.F.Smet, G.B. Marin, Int. J. Photoenergy 8, 1 (2007