Effect of structure and morphology on photocatalytic properties of TiO2 layers

TiO2 layers were deposited by reactive pulse magnetron sputtering at a substrate temperature of 400 deg C on float glass. The total pressure during the deposition was varied between 0.3 and 3Pa for modification of the surface morphology. The morphology of 500nm thick layers was analyzed by AFM and SEM investigations. X-ray diffraction investigations reveal that the structure consists mainly of anatase and small contents of rutile phase. With increasing total pressure from 0.3 to 1.2Pa the average roughness value (Ra) is increased from 4.5 to 8.0nm. The lateral grain size on layer surface determined by AFM amounts to between 75 and 100nm. By a further increase of total pressure to 2Pa and 3Pa the formation of a nanocrystalline microstructure with grain size less than 50nm can be recognized. Simultaneously the Ra value is reduced to 2.4nm. The effect of structure and morphology on photocatalytic activity of the layers was determined by the decomposition of methylene blue solution under UV-A radiation. For total pressures between 0.3 and 1.2Pa the photocatalytic activity is drastically enhanced by increasing roughness and therefore higher free surface area for the photocatalytic reaction. In contrast the formation of nanocrystalline structure at higher total pressures of 2 and 3Pa is linked with a deterioration of photocatalytic properties. This effect can be explained mainly by the high defect density of these layers and therefore high recombination rates for electron hole pairs.