III Solid-State Processing
Evaluation Feature of Nano Grain Growth of TiO2 Thin Film via Sol-Gel Route
Habibollah Aminirastabi, Zhangzhao Weng, Zhaoxian Xiong, Goli Ji and Hao Xue
Abstract TiO2 films were successfully coated via a spin coating sol-gel route, using tetrabutyl titanate as the main raw reagent. One batch of samples was sintered at different temperatures, ranges from 300 to 800 °C, for 2 h, while other ones were treated at 700 °C for different soaking times (1-600 min). The phase compositions of the films were identified with X-ray diffraction (XRD). The microstructure of the coating were observed by a scanning electron microscope (SEM). Experimental results indicated that well-developed crystallinity and good morphology of TiO2 could be synthesized above 500 °C for 2 h. The crystallinity and morphology of grains in films were affected by the reaction temperature and time. However, image analysis shows that sintering temperature had a great impact on the crystallinity and particles size of the samples above 700 °C. Kinetic exponents of grain growth were also obtained with image processing software.
Keywords TiO2 Grain growth Fractal dimension Sol-gel route
Anatase, brookite and rutile are three polymorphs of TiO2 found in nature. Brookite is a naturally occurring phase, and is extremely difficult to synthesize, but anatase and rutile can be synthesized in laboratory without difficulty [1, 2].
Titanium oxide (TiO2) is used for various applications, i.e. photocatalysis, optoelectronics, dye solar cells, gas sensing, luminescent materials, water photolysis, bactericidal use, dielectric materials, cosmetics and pigments, etc. [3-8].
For miniaturization of most of modern systems that use TiO2, and achieving desirable physical properties, small grains in submicron or nanometer scales are used. In recent years, because of the increased dependence of modern technologies on miniaturization, size effect has found a focal importance in scientific societies
© The Minerals, Metals & Materials Society 2017 33
A. Allanore et al. (eds.), Materials Processing Fundamentals 2017,
The Minerals, Metals & Materials Series, DOI 10.1007/978-3-319-51580-9_4
and research centers. By reducing the size, radical changes appear in the physical properties of ceramics [9, 10].
Depositing of thin films through sol gel processing in wet chemistry is a method that is used in the synthesis of colloidal dispersions of inorganic and organic- inorganic hybrid materials. Sol gel route is capable of producing of nanoparticles, nanorods and thin films. Sol gel process has many advantages, including low processing temperature and homogeneity in molecular levels. In this method, the morphology of microstructure and size of nanocluster can be controlled by hydrolysis and condensation reaction. During sintering processes of sol gel films, final morphology is depended on the kinetics of different fundamental formation processes, e.g., nucleation, coagulation and sintering [11, 12].
Titanium oxide that prepare with sol-gel method usually present an amorphous structure, which must be heated in order to achieve the crystal structure [13-15].
It has been shown that the activity of thin film TiO2 is influenced by crystal structure, surface area, size distribution, porosity, band gap, and density [16-19]. Retaining nanoscale grain sizes and the rapid grain growth during sintering are the main challenging factors in the sintering process of nanoparticles . Usually, the density increases with increasing sintering time or temperature [21, 22].
The process of grain growth is considerably interconnected with the development and morphology of the pores in grain aggregates. Dimension change of pores during the process of sintering can be an index of grain growth.
Fractal theory is used in many fields, such as physics, chemistry, electro-ceramics, biology and hydrology [23, 24]. There are a lot of papers indicating that the aggregates pores in solids have fractal structures [25-30]. Therefore, the fractal dimension of pores and grain boundaries can be used as an index of grain growth, which can represent a phenomenological description of grain growth in ceramics. Another advantage of the fractal description of grain growth lies on its usage in the aggregations of nano and molecular systems that are too complicated for modeling and understanding their mechanisms.
In this work, we prepared TiO2 films using sol-gel route, and investigated the influence of sintering conditions, temperature and soaking time, on the grain parameters that can significantly alter the microstructure of the sample. Fractal Dimension, as a microstructure index, describes the relations between grain growth kinetics and grain microstructure, and traces any changes in grain microstructure during sintering process.