The crystal structure and the grain size of thin TiO2 films grown by atomic layer deposition (ALD) were characterized by scanning electron microscopy, grazing incidence X-ray diffraction, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, atomic force microscopy, and near-edge X-ray absorption fine structure spectroscopy. The films of different thicknesses between 50 and 150 nm were grown at temperatures between 200 and 250 degrees C with a TiCl4-H2O ALD process on two different substrates, Si and NiTi. The grain size of the anatase TiO2 was dramatically increased if a thin buffer layer of Al2O3 was deposited on substrates in the same ALD sequence prior to the TiO2 deposition. The largest TiO2 plate-like grains of more than one micrometer in diameter were observed on 150 nm thick films grown at 250 degrees C. The present work demonstrates that the grain size of an anatase TiO2 film can be tailored and controlled on different substrates not only by the processing temperature and film thickness, but, more dramatically, by the nanometric intermediate Al2O3 layers deposited on substrates in the same ALD sequences. The large lateral grain size is explained in terms of low density of the initial nucleation grains created in TiO2 films grown on Al2O3 layers. (C) 2017 Elsevier B.V. All rights reserved.