Nanocrystalline anatase TiO2 thin films:: preparation and crystallite size-dependent properties

Nanocrystalline anatase TiO2 films with crystallites of either of three different sizes (nominally 6, 12, and 20 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These suspensions were painted onto the respective substrates, dried in air and then calcined at 720 K in air for I h; the resulting films have a whitish appearance and are several gm thick. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. X-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) data demonstrated that two of the films are solely composed of the anatase phase whereas in the third also the brookite phase of TiO2 might be present. The average crystallite size was derived from the width of the XRD diffraction peaks and was found to agree within similar to 5% with the nominal size. The elemental composition and the amount of impurities in the different films was investigated by secondary-ion mass spectrometry. Whereas the Ti/O concentration is constant throughout all films, the presence of contaminants is substrate-dependent: while in films deposited on silicon or sapphire the atomic fraction of impurities (most prominently Na and K) is in the range of similar to 10(-4), for glass substrates an about 100 times higher level of those species is observed, probably the result of outdiffusion during the calcination step or during the reduction of the samples. A detailed study of the influence of the sample temperature T and the ambient oxygen pressure P(O-2) on the DC electrical conductivity a of the films was performed. For all films, a power-law dependence of a on P(O-2), sigma alphap(O-2)(-n), was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T= 470 K, n amounts to 1.31, 1.15 and 0.56 for the 6-, 12-, and 20-nm films, respectively. For reduced films, an exponential dependence of sigma on T-1 was determined, yielding activation energies E-A with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes. Impedance spectroscopy was utilized to determine the frequency-dependent complex resistance of the various films. In all cases, a pure RC behavior was found: whereas the magnitude of R strongly depends on the crystallite size, the sample temperature and the ambient oxygen pressure, C exhibits little variations and falls in the range similar to 18 to similar to 25 pF. Using thermal desorption mass spectrometry, distinct desorption peaks for O-2 species were observed which shift to higher temperature values with increasing heating rate. An activation energy for that desorption process of similar to 0.9 eV was obtained thereof for a 6-nm nanocrystalline film. (C) 2004 Elsevier B.V. All rights reserved.