**1. Introduction**

296 Heat Treatment – Conventional and Novel Applications

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Industrial processes must minimize heat-treatment to reduce energy consumption and CO2 emissions during the manufacture of products. In particular, fossil fuels should be conserved for the next generation. However, the use of heat-treatment is essential in the manufacturing processes of many highly functional materials.

In many cases, the materials' functions depend on their surfaces. From this point of view, modification by thin film fabrication on various substrates, as opposed to manufacturing the entire body with the functional material, can generally save resources. The molecular precursor method (MPM) that was developed in our study is a wet chemical process for fabricating metal oxide and phosphate thin films [1-11]. This method requires heattreatment to eliminate organic ligands from metal complexes involved in spin-coated precursor films and to fabricate thin films of crystallized metal oxides or phosphates. We emphasize the importance of heat-treatment by describing recent results obtained using the MPM, which show great potential for the development of nanoscience and nanotechnology tools and materials.

This chapter focuses on the transparent thin film fabrication of both a visible (Vis)-lightresponsive anatase thin film having enhanced UV sensitivity and an unprecedented Visresponsive rutile thin film on glass substrates. These photoreactive thin films were easily fabricated using the MPM. Heat-treatment under controlled conditions produced these attractive thin films. Thin film fabrication of a highly conductive Ag nanoparticles/titania composite and several metal oxides, including Cu2O, will be also discussed, illustrating the broad utility of the MPM and the importance of heat-treatment in this novel wet process.

© 2012 Nagai and Sato, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Nagai and Sato, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **2. Solution-based thin film formation**

Novel thin films are an active area of research and are widely used in industry. Most of the thin films have thicknesses ranging from monolayer to nanometer levels up to several micrometers. Due to their relatively high hardness and inertness, ceramic coatings are of particular interest for the protection of substrate materials against corrosion, oxidation, and wear resistance [12-17]. The electronic and optical properties of thin films are used in many electronic and optical devices [18-20]. The wide range of materials, techniques for preparation, and range of applications make this an interdisciplinary field. Many different methods are used to fabricate thin films, including physical techniques and chemical processes. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are the two most common types of thin film formation methods. PVD methods such as thermal evaporation and sputtering involve atom by atom, molecule by molecule growth, or ion deposition on various materials in a vacuum system [21-23]. CVD and sol–gel methods are less expensive than PVD [24, 25]. The heat-treatment of thin films in these methods is generally important for the formation of crystallized metal oxides.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 299

solvent, and an acid or base as a catalyst. Metal compounds undergo hydrolysis and polycondensation near room temperature, giving rise to a sol in which polymers or colloidal particles are dispersed without precipitation. Further reaction connects the fine particles, solidifying the sol into a wet gel, which still contains water and solvents. Vaporization of solvents and water produces a dry gel. Heating the gel to higher temperatures, where the organic constituents and residues are removed, gives rise to microstructures of inorganic– inorganic composites or hybrids, and glasses and ceramics. In Figure 2, the structural changes to metal oxides from the corresponding hydrolysed polymers by heat treatment of the as-deposited gel. The processes accompanied with dehydration can be categorized into

**Figure 2.** Plausible schematic diagrams for the oxide nucleation process of (A) intra-chain and (B) interchain condensation of the metalloxane polymers formed at the early stage in the sol-gel method.

two reactions; (A) the intrachain and (B) the interchain condensation.

**Figure 1.** Typical sol-gel process for SiO2 formation from silicone alkoxide.

The sol–gel method is a versatile technology in which metal/organic polymers are used to produce ceramics and glasses [26-34]. This technology can be used to manufacture thin films in a relatively cheap way compared with PVD. In a typical sol–gel protocol (Figure 1), the process starts with a solution consisting of metal compounds, such as a metal alkoxide, acetylacetonate, carboxylate, and soluble inorganic species as the source of cations in the target oxide. Additional reactants include water as the hydrolysis agent, alcohols as the solvent, and an acid or base as a catalyst. Metal compounds undergo hydrolysis and polycondensation near room temperature, giving rise to a sol in which polymers or colloidal particles are dispersed without precipitation. Further reaction connects the fine particles, solidifying the sol into a wet gel, which still contains water and solvents. Vaporization of solvents and water produces a dry gel. Heating the gel to higher temperatures, where the organic constituents and residues are removed, gives rise to microstructures of inorganic– inorganic composites or hybrids, and glasses and ceramics. In Figure 2, the structural changes to metal oxides from the corresponding hydrolysed polymers by heat treatment of the as-deposited gel. The processes accompanied with dehydration can be categorized into two reactions; (A) the intrachain and (B) the interchain condensation.

298 Heat Treatment – Conventional and Novel Applications

**2. Solution-based thin film formation** 

generally important for the formation of crystallized metal oxides.

**Figure 1.** Typical sol-gel process for SiO2 formation from silicone alkoxide.

The sol–gel method is a versatile technology in which metal/organic polymers are used to produce ceramics and glasses [26-34]. This technology can be used to manufacture thin films in a relatively cheap way compared with PVD. In a typical sol–gel protocol (Figure 1), the process starts with a solution consisting of metal compounds, such as a metal alkoxide, acetylacetonate, carboxylate, and soluble inorganic species as the source of cations in the target oxide. Additional reactants include water as the hydrolysis agent, alcohols as the

Novel thin films are an active area of research and are widely used in industry. Most of the thin films have thicknesses ranging from monolayer to nanometer levels up to several micrometers. Due to their relatively high hardness and inertness, ceramic coatings are of particular interest for the protection of substrate materials against corrosion, oxidation, and wear resistance [12-17]. The electronic and optical properties of thin films are used in many electronic and optical devices [18-20]. The wide range of materials, techniques for preparation, and range of applications make this an interdisciplinary field. Many different methods are used to fabricate thin films, including physical techniques and chemical processes. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are the two most common types of thin film formation methods. PVD methods such as thermal evaporation and sputtering involve atom by atom, molecule by molecule growth, or ion deposition on various materials in a vacuum system [21-23]. CVD and sol–gel methods are less expensive than PVD [24, 25]. The heat-treatment of thin films in these methods is

**Figure 2.** Plausible schematic diagrams for the oxide nucleation process of (A) intra-chain and (B) interchain condensation of the metalloxane polymers formed at the early stage in the sol-gel method.
