Nanocrystalline WO3 films were grown by reactive magnetron sputter-deposition in a wide range of oxygen gas flow rates while keeping the deposition temperature fixed at 400 degrees C. The physical characteristics of WO3 films were evaluated using grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), and transmission electron microscopy (TEM) measurements. Physical characterization indicates that the thickness, grain size, and density of WO3 films are sensitive to the oxygen gas flow rate during deposition. XRD data indicate the formation of tetragonal WO3 films. The grain size increases from 21 to 25 nm with increasing oxygen gas flow rate to 65%, at which point the grain size exhibits a decreasing trend to attain the lowest value of 15 nm at 100% oxygen. TEM analysis provides a model consisting of isotropic WO3 film (nanocrystalline)-SiO2 interface (amorphous)-Si(1 0 0) substrate. XRR simulations, which are based on this model, provide excellent agreement to the experimental data indicating that the normalized thickness of WO3 films decreases with the increasing oxygen gas flow rate. The density of WO3 films increases with increasing oxygen gas flow rate. (C) 2012 Elsevier B.V. All rights reserved.