**7. O deficiency in rutile thin film**

308 Heat Treatment – Conventional and Novel Applications

because the stoichiometric Ti2O3 did not appear at all.

peak of anatase was also too weak to determine the crystallite size.

**Annealing Time (min) Crystallite size (nm)**

0 - 10 13 5 - 10 13 10 5 11 13 15 4 10 13 20 7 11 13 30 7 12 13

**Table 3.** The crystallite size of anatase in **ED**, **EDair**, **SG** and post-annealed thin films [6]. The crystallite size of anatase was measured with a typical Scherrer-Hall method by employing a peak assignable to only (1 0 1) of anatase, because other peak intensities due to anatase were too low to measure accurately. The crystallite size of anatase in **ED** and **ED-PA5** could not be obtained because the (1 0 1)

**Figure 8.** Plausible route of the O-deficient anatase lattice formation from the precursor complex skeleton through the heat treatment in an Ar gas flow and the sequential post-anneal [6].

**ED EDair SG** 

A coordination skeleton of (TiO4N2) or (TiO5N2) can be assumed in the EDTA complex as a precursor molecule from the structural study of a Ti complex [Ti(H2O)(EDTA)]·1.5H2O reported by Fackler *et al.* [80]. In the precursor films, two N and at least four O atoms link to one Ti ion. As a result of heat-treating the precursor complex in an Ar gas flow, neighboring complexes reacted with each other. In this process, several O atoms linked to one Ti ion could be covalently bonded by other Ti ions, and the anatase lattice was gradually created. By eliminating large amounts of C, H, and N atoms with O atoms, oxide ion sites of the anatase lattice were partially occupied by a rather stable nitride ion derived from the coordinated N atom originally belonging to the ligand. As a result, the total negative charge of the N-substituted anatase in the **ED** thin film is ca. 3.6 toward one Ti ion. This value is the summation of 2.8 from the oxide ions and 0.8 from the nitride ions. This charge toward one Ti ion is larger than that of ca. 3.3 by the oxide ions in the **SG** thin film. The substitutional N atoms could be removed from the anatase lattice by post-annealing the **ED** thin film. Consequently, the total negative charge of the **ED-PA15** thin film, whose photoreactivity is the highest, decreased to ca. 3.0. Longer annealing treatment replenished oxide ions in the anatase thin films from their surfaces and the photoreactivity decreased (Figure 8) [6]. Thus, it was elucidated that O deficiency is an important factor to consider when designing anatase photoreactivity. It is also notable that the O-deficient anatase lattice is rather robust

Rutile is the most stable crystal form of titania. Since Nishimoto *et al.* showed that anatase is more sensitive to UV light than rutile in photoreactions, rutile was believed to be inferior to anatase in terms of photoreactivity [81]. Anatase is important for photocatalysis in pollutant degradation and in the development of photofunctional materials such as films with hydrophilic surfaces under UV-light irradiation. The poor photoreactivity and photosensitivity of rutile is generally believed to be due to its crystal structure. Rutile is primarily known as a useful pigment for white paint, due to its chemical stability [82, 83].

Because the band edge of a rutile single-crystal is 3.0 eV, rutile has the potential to respond to Vis light. Using this knowledge and the results of previous experiments on anatase responses to Vis light, this section describes an attempt to achieve direct fabrication of Odeficient rutile thin films with high photoreactivity using a MPM. The first Vis-lightresponsive thin film created from O-deficient rutile is discussed here. This material works without application of an electric potential, due to its unprecedentedly high photosensitivity under UV-light irradiation. The present findings should facilitate widespread practical use of rutile in light-related applications.

The thin films were formed by heat-treating the precursor films after spin-coating onto a quartz glass substrate. **SED** and **SSG** were applied in an Ar gas flow. The transparent precursor films formed by spin-coating the solutions and pre-heating in a drying oven at 70 °C for 10 min were heat-treated at 700 °C for 30 min in a furnace made from a quartz tube with an Ar gas flow rate of 0.1 L min–1. When **SED** was used, a transparent rutile thin film **R** was formed. When **SSG** was used, a transparent anatase thin film **A** was formed. The film thickness was 100 nm in both cases.

Each structure was characterized using XRD, Raman spectroscopy, and transmission electron microscopy (TEM). The selectivity was due to the O-vacant sites in the oxide thin films formed at different levels due to the differences between the amounts of oxygen in the two precursors. In this case, the oxygen source required to structure titania was available only in the precursor films when these thin films were fabricated. Therefore, crystallization into rutile, which has many O-vacant sites, and the accompanying rapid elimination of organic residues from the **R** precursor film, occurred because of the heat-treatment.

In contrast, the amount of oxygen available to Ti4+ in titanoxane polymers, though significant, was insufficient to develop stoichiometric TiO2 from **A**. The oxygen defects in an anatase lattice generally lower the temperature of the phase transformation from anatase to rutile [84, 85]. Thus, selective formation occurred according to the differing degrees of O deficiency.

The photoreactivities of the thin films were evaluated by the decoloration rates of MB solutions, which served as a model for organic pollutants in water. The results measured under Vis- and UV-light irradiation are summarized in Table 4, along with those measured under dark conditions (reference values). The data show the effects of adsorption on the samples, vessels, and self-decoloration of MB under each condition. Moreover, the


photoreactivity of **R** was extremely high under UV irradiation and higher than the photoreactivity of **A.** This is without precedent.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 311

**Surface Deeper portion** 

semiconductor particles to understand the photoreactivities of the particles [87]. In this section, we report the changes in the PL and photoreactive properties of the Vis-responsive rutile thin film fabricated by the MPM, which are effected by annealing in air at 700°C. The relationship between O deficiency and PL emission was examined to understand the incredibly high photoreactivity of the rutile thin film. Furthermore, the level of crystal orientation of the rutile thin film was quantitatively evaluated on the basis of data from XRD analyses. The amount of oxygen supplied during the annealing process was analyzed by XPS measurements. The growth of crystals and particles was also investigated by crystallitesize measurements and SEM observations. The heat-treatment of the fabricated O-deficient rutile **R** thin film was carried out in air at 700°C for 15, 30, and 60 min. The number in the notation of the post-annealed films indicates the annealing time (min). For example, **R-PA15** indicates that post-annealing treatment of the **R** thin film was carried out for 15 min. The extent orientation factor (*f*) for the (110) plane of the **R-PA***n* thin films increased with

**Film Crystallite size**a) **/ nm Orientation factor**; *<sup>f</sup>* **O/Ti ratios** 

**R** 15(2) 0.35 1.74 1.75 **R-PA15** 21(3) 0.67 1.84 1.73 **R-PA30** 21(3) 0.69 1.89 1.79 **R-PA60** 20(4) 0.75 1.94 1.85

**Table 5.** The crystallite size, orientation factor and O/Ti ratio of rutile crystals in the **R** thin film and in the post-annealed **R-PAn** thin films [8]. The crystallite size of rutile was measured with a typical Scherrer-Hall

The extent orientation could be estimated from the XRD peak intensity by using the Lotgering method [8]. The terms *I*(*hkl*)ideal and Σ*I*(*hkl*)ideal are defined as the intensity of the peak attributable to the specific plane (*hkl*) and the sum of each intensity obtained for the

 *P*0 = *I*(*hkl*)ideal/Σ*I*(*hkl*)ideal (1)

In this study, each *I*(*hkl*)ideal value was cited from the standard data of the corresponding

method by employing the peaks assignable to (1 1 0), (1 0 1) and (2 1 1) of rutile. The extent of the orientation was estimated in terms of Logtering orientation factor, *f*, from the XRD peak intensities (*I*). For calculating the orientation factor, the intensity data of non-oriented rutile was cited from the JCPDS card 21-1276. The O/Ti ratios determined by the XPS peak areas of O 1s and Ti 2p3/2 peaks observed from the surfaces of **R** and post-annealed thin films. The XPS peaks of the thin film surface were measured without bombarding Ar+ ion beam. The peak area of O 1s and Ti 2p was calculated by FWHM and peak height at the positions 531.0 and 459.0 eV, respectively. The averaged O/Ti ratios determined by the XPS peak areas of O 1s and Ti 2p3/2 peaks of **R** and post-annealed thin films. The XPS peaks of thin films were measured after bombarding Ar+ ion beam with 2 kV and 18 μA cm–2 for 3 min, in order to remove surface oxides. The peak area of O 1s and Ti 2p was calculated by FWHM and peak height at the positions 531.0 and 459.0 eV,

respectively, obtained from each depth profile in Ar+ ion etching mode. a) The estimated standard deviations are presented in parentheses

non-oriented rutile crystals; thus, *P*0 can then be expressed as

rutile phase.

annealing time in air (Table 5).

**Table 4.** The reaction rate *ν* of the decoloration reaction in an aqueous solution containing 0.01 mol L–1 of methylene blue under visible- and UV-light irradiation and under dark conditions [7]. Calculated standard deviations are presented in parentheses.

The photosensitivities of **R** and **A** were also examined by measuring the effects of Vis and UV irradiation on the water contact angle for the surfaces of the thin films. The results are shown in Figure 9 [7]. The rutile thin film **R** exhibited Vis-light-induced hydrophilicity with a fluorescent light, even though high-energy light below 400 nm was eliminated. In contrast, Vis light alone did not reduce the contact angle on **A** under the same conditions. Furthermore, a rapid decrease in the water contact angle for **R** was observed with weak UVlight irradiation. The super-hydrophilic property of **R** appeared after only 1 h. When fluorescent light with a UV component was employed, the contact angle on **R** rapidly reduced and the values reached 38° and 10° after irradiation for 1 and 24 h respectively.

**Figure 9.** Comparison of the contact angles of a 1.0-μL water droplet on the thin films **R** and **A**. Before the measurement, the thin films were exposed to UV irradiation of 1.3 mW cm–2 at 356 nm obtained by a black light (each on the left side), and to visible light without UV light was obtained from a fluorescent light by removing light of wavelengths shorter than 400 nm using a cut-off filter. The Vis light intensity after removing UV components from the fluorescent light, which was estimated by an illuminometer was 0.8 mW cm-2 [7].

It is noteworthy that the simple fabrication of a Vis-responsive rutile film with high photoreactivity could be attained. Thus, the O defects in titania are also effective at providing photoreactivity of rutile, which is usually insensitive to both UV and Vis light.
