Preparation and microstructure evolution of both freestanding and supported TiO 2 thin films
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Thin films of TiO2 were fabricated by spin-coating silicon wafers and cover glass with a titanium citrate complex precursor. The grain growth and phase development of both freestanding and supported films were studied using a combination of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. Freestanding films prepared at 400 °C possess only the anatase phase, while supported films treated under the same conditions formed a small amount of the rutile phase. After heat treatment at various temperatures, results indicated that porosity was introduced into the films when the grain size grew close to the film thickness. Grain growth studies show that the grain size of the freestanding film underwent a drastic increase during the transformation from anatase to rutile. The grain size of the supported films did not show an abrupt change upon heat treatment. The grain size of the freestanding films treated at 900 °C was approximately three times larger than that of the supported films.
I. INTRODUCTION
Titanium dioxide (TiO2) has long been known as a useful and inexpensive material for many applications.1–6 Because of its chemical durability, TiO2 can be utilized as an inorganic membrane material for ultrafiltration.5 Compared with organic membranes, inorganic membranes offer several advantages: (i) increased pressure resistance; (ii) increased stability at high temperatures; (iii) insensitivity to bacterial action; (iv) ability to sterilize with steam. TiO2 is a good semiconductor whose electrical resistivity changes when exposed to reduced oxygen partial pressures. This property is used in a variety of gas detectors such as oxygen, hydrogen, carbon monoxide, and water vapor.1– 4 The refractive index of TiO2 ranges from 2.6 to 2.9 in visible light wavelengths. This is greater than that of soda lime glass thus allowing TiO2 to be used for many thin-film optical applications.7–10 For instance, multiple coatings of TiO2 thin films in combination with other low-index films, such as SiO2, function as antireflective coatings for optical lenses. When the quality of thin films is evaluated, several parameters must be considered to get the desired mechanical, optical, or electrical effect. Some important parameters are porosity, grain size, film thickness, surface roughness, and crystal structure. It is difficult to change these material parameters through the coating process which is generally designed to get even coatings of specific initial thickness. To vary parameters such as enumerated above, heat treatment (annealing) is usually 1520
J. Mater. Res., Vol. 17, No. 6, Jun 2002
used. Consequently, information regarding the influence of annealing temperature on the thin-film characteristics is very useful for the designer of thin films. Various techniques for the preparation of TiO2 thin films have been developed and well studied. Common methods are sol-gel process, thermal decomposition of organometallic precursors, chemical vapor deposition, magnetron sputtering, and electron beam evaporation.6,7
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