**3. Principle of MPM**

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 the coated substrate.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 301

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

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.

**5. Vis-responsive anatase thin film fabricated using the MPM** 

UV sensitivity of Vis-responsive anatase films.

Many researchers study the fabrication and photoreactivity of Vis-responsive thin films by physical and/or chemical modification of anatase films because of the importance of Visphotoreactive materials [53-61]. However, there is little information on the enhancement of

The implantation of various transition-metal ions such as V5+, Cr3+, and Cu2+ into the lattice of Ti4+ in anatase thin films was investigated by Anpo *et al.* [62-67]. The photoreactivities of chemically modified anatase thin films decreased under UV irradiation, although those anatase thin films modified with transition-metal ions can behave as photocatalysts under Vis irradiation. Since Asahi *et al.* reported that non-metallic ions such as a substitutional nitride ion at the oxygen sites of anatase are also effective at enabling the thin film to be responsive to Vis light, methods for modifying anatase with tetravalent carbon or hexavalent sulfur cations have also been investigated [68]. Miyauchi *et al.* achieved another chemical fabrication of Vis-responsive anatase thin films, which were modified under NH3 gas by heat-treating the resulting films formed using a sol–gel method [69]. However, the photoreactivities of those films are lower under UV irradiation than those before modification in all cases. Nevertheless, they can work as photocatalysts under Vis light. Thus, studies on the chemical formation of Vis-responsive anatase thin films with enhanced

photoreactivity [37-52].

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 obtained using the sol–gel method.
