Synthesis of Nanosized Titania

Chapter Outline

Introduction
Experimental
Characterization
Results & discussion
Conclusion

Introduction

Nanosized titania particles have been the subject of a great deal of research because of its unique physicochemical properties and applications in the areas of pigments, catalysts and supports, fine ceramics, cosmetics, gas sensors, inorganic membranes, environmental purification, and dielectric materials.[1–9] Much interest has been shown in photochemical reactions on nanosized titania particles due to its potential application in the conversion of solar energy into chemical energy [10–15] and electric energy [14, 15]. When titania powder is irradiated with photon energy larger than the band-gap energy, electrons (e−) and holes (h+) are generated in the conduction band and the valence band, respectively. These electrons and holes are thought to have the respective abilities to reduce and oxidize chemical species adsorbed on the surfaces of titania particles [16]. The use and performance for given applications are, however, strongly influenced by the crystalline structure, morphology, and the size of the particles. It is well known that titania exists in three kinds of crystal structures namely anatase, rutile and brookite. Anatase and brookite phases are thermodynamically metastable and can be transformed exothermally and irreversibly to the rutile phase at higher temperatures. The transition temperatures reported in the literature ranges from 450 to 1200 ◦C. The transformation temperature depends on the nature and structure of the precursor and the preparation conditions [17, 18]. Among the three kinds of crystal structures of titania, commercially available anatase titania fine particles are the most effective for photocatalytic degradation of organic compounds. Therefore, it is very important to develop methods for the synthesis of nanosized titania particles in which the particle size and the crystal structure of the products can be controlled. Various synthesis methods including CVD method [19], colloidal template [20], hydrolysis [21, 22], sol–gel [23–25], microemulsion (or reverse micelle systems) [17, 18, 26, 27] and hydrothermal synthesis [28, 29], have been used to prepare nanosized titania particles. The sol–gel method [30] requires costly organic solvents. The direct hydrolysis of titanium salts and chemical vapor deposition procedure, in which TiCl4 vapor is oxidized at very high temperature (∼500 0C) can be used to prepare nanosized titania particles [31–33]. In the last few years reverse micelle method was successfully applied to synthesize nanosized titania particles in reverse micelles or water/oil (W/O) microemulsion systems using titanium alkoxides as starting materials[18–20]. Reverse micelles are small aggregates (60-800 Å) formed by surfactant molecules that surround a well defined nanometer-sized water core [34]. This unique formation of water droplets in a microemulsion may be considered as a small reactor used for the synthesis of nanoparticles. The reactants are confined within such dispersed droplets when water-soluble precursors are used. It has been shown that this structure is the most suitable for the preparation of fine inorganic colloidal particles, since the aggregates have very small size and are monodispersed. Additionally, the fact that most metal precursors are water-soluble that enhances the particle synthesis procedure, which takes place inside the water core of the reverse micelles. Even though the microemulsions have been considered as being stable systems, it was demonstrated by Agrell, Li and Park [35, 36] that they are dynamic systems, wherein the droplets collide continuously with each other, resulting sometimes in formation of coalesced drops that tends to break up, since as they lose their thermodynamic stability. As the particle formation takes place inside the droplets, the nature of the formed colloidal particles will be influenced by the droplet structure and its ability to exchange micellar-containing material [37]. Additionally, the size of the water droplets will determine the size of the catalyst nanoparticles. Generally, a low water to surfactant ratio (w0) is required to form reverse micelles, depending also on the type of the surfactant, i.e. number and length of hydrophobic chains. For a given surfactants, w0 will give aggregates of different size and shape (spherical micelles, rod-like micelles and others) [38]. The synthesis of the metal nanoparticles may be carried out in two different manners [39, 40]. The first manner includes the addition of a reducing agent, such as hydrazine directly into the microemulsion containing the metal precursor. The second manner involves the mixing of two reverse-micelle microemulsion solutions, one containing the metal precursor and the other one containing the reducing (or precipitating) agent [41]. In the present work we prepared nanosized titania particles using the single microemulsion system in which mixed reverse microemulsion of water, surfactant, and oil phase was used. titanium tetraisopropoxide(TTIP) diluted by isopropyl alcohol(IPA) was directly added to the above microemulsion system. To study the effect of water to surfactant ratio (w0) on the size of titania particles, a mixed reverse microemulsion solution containing water droplets which was precipitated by TTIP diluted in IPA. The synthesis involves hydrolysis of TTIP in a reverse micelle system leading to the formation of phase pure tetragonal nanosized titania particles at room temperature.