**4. Heat-treatment for titania thin film fabrication**

## **4.1. Thin film fabrication and basic properties of TiO2**

Titania is a popular industrial material that has been used as a white pigment for paints, cosmetics, and foodstuffs [35]. The photoreactivity of titania is known from observations of phenomenon such as the chalking of white paints containing titania after long-term outdoor exposure. The photoreactivity of titania was first observed by Honda and Fujishima in 1967, and reported in the literature in 1972 [36]. When the surface of a titania electrode (under an appropriate bias between a Pt counter electrode) was irradiated with light of energy shorter than its band gap, 3.0 eV, a photocurrent flowed from the Pt electrode to the titania electrode through the external circuit. The oxidation reaction occurs at the titania electrode, and the reduction reaction occurs at the Pt electrode. This observation showed that water molecules could be split into oxygen and hydrogen using UV light in a sulfuric acid electrolyte. The energy-conversion process that occurs on the titania surface is termed the Honda–Fujishima effect. When titania particles absorb UV radiation, they produce pairs of electrons and holes inside the particles. Because the photoinduced electrons and holes can be incorporated into redox reactions on the titania surface before spontaneous recombination, the surface states of the titania particle are quite important for photoreactivity [37-52].

300 Heat Treatment – Conventional and Novel Applications

The MPM is a wet process for the formation of thin films of various metal oxides, including titania or calcium phosphate compounds [1-11]. This method is based on the design of metal complexes in coating solutions with excellent stability, homogeneity, miscibility, coatability, *etc*., which have many practical advantages. This is because metal complex anions with high stability can be dissolved in volatile solvents by combining them with the appropriate alkylamines. Furthermore, the resultant solutions can form excellent precursor films through various coating procedures. The precursor films involving metal complexes should be amorphous, just as with the metal/organic polymers in the sol–gel processes. If not, it is impossible to obtain the resulting metal oxide thin films spread homogeneously on substrates by subsequent heat-treatment. For this purpose, the alkyl groups in the amines play an important role. Single-crystals of the metal complex can be obtained from the precursor solution in several cases when the alkyl groups in the alkylamines are sufficiently small, *e.g.*, an ethyl group. The model structure of the amorphous precursor films formed on substrates can be examined by means of crystal engineering and based on the crystal structures. Heat-treatment is necessary to fabricate the desired metal oxide films by eliminating the ligand in the metal complex and alkylamine as the counter cation. It is important that densification during heat-treatment occurs only in the vertical direction of

To the best of our knowledge, the crystallite size of the oxide particles in the resultant thin films fabricated by the MPM is generally smaller than those prepared by the conventional sol–gel method. The smaller size of the crystallites obtained using the MPM may be related to the nucleation process of the crystallized metal oxides. In the nucleation process in the sol–gel method, the polymer chains themselves are rearranged by heat-treatment (Figure 2). The polymer chains should move to produce the core structure of the metal oxide, especially during interchain condensation. In contrast, the nucleation of metal oxides occurs more easily during the MPM. When coupled with elimination of the organic ligands via heattreatment, a vast number of crystallites can be rapidly formed. It is consequently feasible that the crystallite sizes of metal oxides fabricated using the MPM are smaller than those

Titania is a popular industrial material that has been used as a white pigment for paints, cosmetics, and foodstuffs [35]. The photoreactivity of titania is known from observations of phenomenon such as the chalking of white paints containing titania after long-term outdoor exposure. The photoreactivity of titania was first observed by Honda and Fujishima in 1967, and reported in the literature in 1972 [36]. When the surface of a titania electrode (under an appropriate bias between a Pt counter electrode) was irradiated with light of energy shorter than its band gap, 3.0 eV, a photocurrent flowed from the Pt electrode to the titania

**3. Principle of MPM** 

the coated substrate.

obtained using the sol–gel method.

**4. Heat-treatment for titania thin film fabrication** 

**4.1. Thin film fabrication and basic properties of TiO2**

Titania has three polymorphs: anatase, rutile, and brookite. Anatase thin film deposition on a glass substrate has been achieved using the MPM. A water-resistant coating solution was prepared by the reaction of a neutral [Ti(H2O)(EDTA)] complex (EDTA is ethylenediamine-*N*,*N*,*N*ʹ,*N*ʹ-tetraacetic acid) with dipropylamine in ethanol [1, 2, 4-9]. The anatase phase appears during the heat-treatment of the precursor film at a temperature between 400 and 500°C and is transformed to the rutile one between 500 and 700°C. X-ray diffraction (XRD) analysis of the films prepared by a conventional sol–gel process showed that the anatase phase appeared between 400 and 500°C and was not transformed to the rutile one, even when heat-treated at 900°C. The irreversible phase transformation from anatase to rutile requires heat-treatment. During heat-treatment of anatase, the atoms in the original tetragonal lattice can be rearranged into the rutile tetragonal lattice. The temperature difference between the phase transformation from anatase to rutile in the sol–gel method and the MPM will be discussed in the section **O deficiency in rutile thin film**s.
