Grain size effect on photocatalytic activity of TiO2 thin films grown by atomic layer deposition

We have studied the effect of grain size on the photocatalytic activity of TiO2 thin films grown by atomic layer deposition technique. A number of thin films of different crystallinity (anatase, rutile, amorphous) and grain sizes (55 nm to 715 nm) were grown on Si substrates under different processing conditions, by varying the reactor temperature, deposition time or buffer layers. The films are characterized by scanning electron microscopy, grazing incidence x-ray diffraction, x-ray photoelectron spectroscopy, secondary ion mass spectrometry and atomic force microscopy. The photocatalytic activity was examined by measuring the degradation rates of methylene blue in aqueous solutions under the ultraviolet light irradiation. The photocatalytic activity exhibits a maximum for grains of average size of 200 nm, indicating existence of two competing effects, which occur when the average grain size decreases: increase of active surface area (which promotes a high photocatalytic activity) and increase of grain boundaries (which degrades the photocatalytic activity).