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

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 UV sensitivity of Vis-responsive anatase films.

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

#### 302 Heat Treatment – Conventional and Novel Applications

photoreactivities under UV irradiation are significant from the viewpoint of solar energy efficiency.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 303

a nitrogen-free anatase thin film through heat-treatment for 30 min in air at a temperature of 450°C or higher. Based on these results, the heat-treatment of molecular precursor films spin-coated with **SED** on ITO glass substrates was examined in an Ar gas flow of 0.1 L min−<sup>1</sup> at 500°C for 30 min. The XRD pattern indicated that the spin-coated precursor film crystallized to anatase through heat-treatment at a temperature of 500°C or higher under atmospheric conditions. Thus, the anatase form was created even if oxygen was not supplied externally to remove organic residues in the metal complex. Furthermore, the chemical bond toward the Ti4+ ion from both oxygen and nitrogen atoms can be observed in the XPS spectra of the resultant thin films. The binding energies of Ti 2p3/2 attributed to Ti–O and Ti–N had typical values of 459.1 and 455.3 eV, respectively (Figure 4) [5, 70-72]. Importantly, the binding energy of N 1s was 396.7 eV, and the existing nitrogen was only in the oxygen-substituted form, not in the chemisorbed form [73, 74]. Thus, the heat-treatment of the precursor films in an Ar gas flow was effective in preserving nitrogen atoms in the complex. However, the photoreactivity of the thin film was not observed after Vis irradiation with a weak fluorescent lamp. Because the partially nitrided film obtained by the MPM alone did not respond to irradiation with a fluorescent lamp, a precursor solution, **SOX**, with no Ti–N bonds, in complexes whose ligand was oxalic acid (OX), was freshly prepared and the layering of the anatase films was achieved using the two precursor

**Figure 4.** XPS spectra of (A) Ti 2p3/2 and (B) N 1s of the nitrogen-substituted TiO2 thin film obtained by heat treatment of the precursor film containing the Ti complex with EDTA ligand on a Na-free glass in an Ar gas flow. The thin solid lines are original data of XPS. The thick solid curves are theoretical Gaussian distribution curves. The dashed curves are theoretically fitted curves by assuming Gaussian

solutions.

distribution [5].

**Figure 3.** An ORTEP view of the precursor complex having the EDTA and peroxo ligands linked to the central Ti4+ ion. The molecular structure was determined by an X-ray single crystal structure analysis of the diethylammonium salt of the complex, although the dibutylammonium salt was employed for the coating solution in order to prevent from the crystallization of the precursor. The single crystals of the identical orange-yellow color could be obtained from a reacted solution of the complex with the diethylamine instead of dibutylamine. The single crystal was {(C2H5)2NH2}[Ti(O2)(Hedta)]·1.5H2O; in a monoclinic crystal system, P21/c with a = 8.583(1), b = 6.886(1), c = 36.117(2) Å, and β = 92.780(3)°. The full-matrix least-squares refinement on *F*2 was based on 3206 observed reflections that were measured at 250 K by using an imaging plate as a detector, and converged with unweighted and weighted agreement factors of R = 0.054 and Rw = 0.061 respectively, and GOF = 1.63. Two Ti–N(edta) bond lengths of 2.307 and 2.285 Å, are slightly longer than the bond length of 2.12 Å in the TiN single crystal. Results indicated that EDTA acts as a pentadentate ligand in the complex, and the peroxo ligand linked to the Ti4+ ion has a side-on coordination structure.

The MPM forms transparent titania thin films using the ethanol solution obtained as a coating solution (**SED**) by the reaction of alkylamine with a titanium complex of EDTA as the ligand [1, 2, 4-9]. According to single-crystal structural analysis, this Ti complex contains Ti– N bonds (Figure 3). If the Ti–N bonds in this complex can be preserved in the anatase thin film obtained by heat-treatment after coating, a partially nitrided anatase film can be directly formed. However, XRD and X-ray photoelectron spectroscopy (XPS) confirmed that the precursor film formed on a glass substrate using the ethanol solution is transformed into a nitrogen-free anatase thin film through heat-treatment for 30 min in air at a temperature of 450°C or higher. Based on these results, the heat-treatment of molecular precursor films spin-coated with **SED** on ITO glass substrates was examined in an Ar gas flow of 0.1 L min−<sup>1</sup> at 500°C for 30 min. The XRD pattern indicated that the spin-coated precursor film crystallized to anatase through heat-treatment at a temperature of 500°C or higher under atmospheric conditions. Thus, the anatase form was created even if oxygen was not supplied externally to remove organic residues in the metal complex. Furthermore, the chemical bond toward the Ti4+ ion from both oxygen and nitrogen atoms can be observed in the XPS spectra of the resultant thin films. The binding energies of Ti 2p3/2 attributed to Ti–O and Ti–N had typical values of 459.1 and 455.3 eV, respectively (Figure 4) [5, 70-72]. Importantly, the binding energy of N 1s was 396.7 eV, and the existing nitrogen was only in the oxygen-substituted form, not in the chemisorbed form [73, 74]. Thus, the heat-treatment of the precursor films in an Ar gas flow was effective in preserving nitrogen atoms in the complex. However, the photoreactivity of the thin film was not observed after Vis irradiation with a weak fluorescent lamp. Because the partially nitrided film obtained by the MPM alone did not respond to irradiation with a fluorescent lamp, a precursor solution, **SOX**, with no Ti–N bonds, in complexes whose ligand was oxalic acid (OX), was freshly prepared and the layering of the anatase films was achieved using the two precursor solutions.

302 Heat Treatment – Conventional and Novel Applications

efficiency.

photoreactivities under UV irradiation are significant from the viewpoint of solar energy

**Figure 3.** An ORTEP view of the precursor complex having the EDTA and peroxo ligands linked to the central Ti4+ ion. The molecular structure was determined by an X-ray single crystal structure analysis of the diethylammonium salt of the complex, although the dibutylammonium salt was employed for the coating solution in order to prevent from the crystallization of the precursor. The single crystals of the identical orange-yellow color could be obtained from a reacted solution of the complex with the diethylamine instead of dibutylamine. The single crystal was {(C2H5)2NH2}[Ti(O2)(Hedta)]·1.5H2O; in a monoclinic crystal system, P21/c with a = 8.583(1), b = 6.886(1), c = 36.117(2) Å, and β = 92.780(3)°. The full-matrix least-squares refinement on *F*2 was based on 3206 observed reflections that were measured at

250 K by using an imaging plate as a detector, and converged with unweighted and weighted agreement factors of R = 0.054 and Rw = 0.061 respectively, and GOF = 1.63. Two Ti–N(edta) bond lengths of 2.307 and 2.285 Å, are slightly longer than the bond length of 2.12 Å in the TiN single crystal. Results indicated that EDTA acts as a pentadentate ligand in the complex, and the peroxo ligand linked

The MPM forms transparent titania thin films using the ethanol solution obtained as a coating solution (**SED**) by the reaction of alkylamine with a titanium complex of EDTA as the ligand [1, 2, 4-9]. According to single-crystal structural analysis, this Ti complex contains Ti– N bonds (Figure 3). If the Ti–N bonds in this complex can be preserved in the anatase thin film obtained by heat-treatment after coating, a partially nitrided anatase film can be directly formed. However, XRD and X-ray photoelectron spectroscopy (XPS) confirmed that the precursor film formed on a glass substrate using the ethanol solution is transformed into

to the Ti4+ ion has a side-on coordination structure.

**Figure 4.** XPS spectra of (A) Ti 2p3/2 and (B) N 1s of the nitrogen-substituted TiO2 thin film obtained by heat treatment of the precursor film containing the Ti complex with EDTA ligand on a Na-free glass in an Ar gas flow. The thin solid lines are original data of XPS. The thick solid curves are theoretical Gaussian distribution curves. The dashed curves are theoretically fitted curves by assuming Gaussian distribution [5].

Two types of three-layer film, **OX–OX–OX** and **OX–ED–OX**, were formed on an ITO precoated glass substrate by coating and heat-treating the precursor films under an Ar gas flow. A schematic diagram of these structures is shown in Figure 5. The first layer, of thickness 100 nm, was formed by applying **SOX**. The second and third layers were 50 nm thick. The third layer of **OX–OX–OX** was fabricated as a reference by applying only **SOX**, although the corresponding layer of **OX–ED–OX** was fabricated on the second layer using **SED**, and then the third layer was formed with **SOX** (Figure 6). The overall amount of the heat-treated molecular precursor, energy consumed, film area, and film thickness were kept constant.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 305

The XRD patterns of both films indicated that anatase was formed. The field-emission scanning electron microscopy (FE-SEM) data of both films show even surfaces without cracks or pinholes. These surfaces were too smooth to detect the roughness by measuring with a stylus profilometer, whose detective limit is ca. 10 nm. The XPS depth profiles of these films are shown in Figure 7. The depth profile for **OX–ED–OX** revealed that nitrogen and carbon were locally distributed in the deep portion corresponding to the second layer. A relative decrease in the amount of oxide ions in the corresponding layers was observed. This suggests that other anions, *e.g*., nitride ions, compensate for the charge balance. This confirmed that significant amounts of carbon and nitrogen atoms were still present in the second layer, and the substitutional nitrogen atoms were locally distributed in the deep portion corresponding to the second layer. Most nitrogen atoms did not diffuse to other layers, although the amounts of nitrogen and carbon atoms in the other layers could not be neglected. The absorption spectra of **OX–ED–OX** indicate characteristic absorption bands near 480 nm. The

**OX–OX–OX** films do not show such absorption bands in the Vis-light region.

**Figure 7.** Depth profile of the amount of components in the 3-layer thin film (A) **OX-OX-OX** and (B) **OX-ED-OX**. Notations indicate the energy levels of five atoms in parentheses, ―●― (Ti 2p), ―○― (O 1s), ― ♦ ― (N 1s), ― ◊ ― (C 1s), ―☐― (In 3d). Net multiplication part is corresponding to the second

**0**

**20**

**40**

**Area (%)**

**60**

**80**

**100**

**0 50 100 150 200**

**Film thickness (nm)**

The photoreactivities of the films were tested using the decolorizing reaction of methylene blue (MB) aqueous solution [5-8, 75-77]. The decoloration rate of 0.01 mol L–1 MB solution by the photoreaction with both multilayered thin films under UV or Vis irradiation are summarized in Table 1. The **OX–ED–OX** film has an effective photoreactivity under Vis irradiation. The **OX–OX–OX** film only responded to UV light. It is important that the photoreactivities of the Vis-responsive films are also extremely

layer formed by applying the solution **SED**.

**0**

**20**

**40**

**60**

**Area (%)**

**80**

**100**

**0 50 100 150 200**

(A) (B)

**Film thickness (nm)**

high under UV irradiation.

**Figure 5.** Schematic diagram structures of triple layer thin films. Each precursor film for the first layers was formed by applying **SOX** and heat-treated at 475°C for 30 min. The precursor film of the second layer for **OX-OX-OX** formed by employing the half-diluted solution of **SOX**, and **OX-ED-OX** formed by using the half-diluted solution of **SED** were heat-treated at 500°C for 30 min. The precursor films of the third layers for **OX-OX-OX** and **OX-ED-OX** formed by employing the half-diluted solution of **SOX** were heat-treated at 475°C for 30 min.

**Figure 6.** Photographs of the 3-layer **OX-OX-OX** and **OX-ED-OX** films on the ITO glass substrate.

The XRD patterns of both films indicated that anatase was formed. The field-emission scanning electron microscopy (FE-SEM) data of both films show even surfaces without cracks or pinholes. These surfaces were too smooth to detect the roughness by measuring with a stylus profilometer, whose detective limit is ca. 10 nm. The XPS depth profiles of these films are shown in Figure 7. The depth profile for **OX–ED–OX** revealed that nitrogen and carbon were locally distributed in the deep portion corresponding to the second layer. A relative decrease in the amount of oxide ions in the corresponding layers was observed. This suggests that other anions, *e.g*., nitride ions, compensate for the charge balance. This confirmed that significant amounts of carbon and nitrogen atoms were still present in the second layer, and the substitutional nitrogen atoms were locally distributed in the deep portion corresponding to the second layer. Most nitrogen atoms did not diffuse to other layers, although the amounts of nitrogen and carbon atoms in the other layers could not be neglected. The absorption spectra of **OX–ED–OX** indicate characteristic absorption bands near 480 nm. The **OX–OX–OX** films do not show such absorption bands in the Vis-light region.

304 Heat Treatment – Conventional and Novel Applications

heat-treated at 475°C for 30 min.

Two types of three-layer film, **OX–OX–OX** and **OX–ED–OX**, were formed on an ITO precoated glass substrate by coating and heat-treating the precursor films under an Ar gas flow. A schematic diagram of these structures is shown in Figure 5. The first layer, of thickness 100 nm, was formed by applying **SOX**. The second and third layers were 50 nm thick. The third layer of **OX–OX–OX** was fabricated as a reference by applying only **SOX**, although the corresponding layer of **OX–ED–OX** was fabricated on the second layer using **SED**, and then the third layer was formed with **SOX** (Figure 6). The overall amount of the heat-treated molecular precursor, energy consumed, film area, and film thickness were kept constant.

**Figure 5.** Schematic diagram structures of triple layer thin films. Each precursor film for the first layers was formed by applying **SOX** and heat-treated at 475°C for 30 min. The precursor film of the second layer for **OX-OX-OX** formed by employing the half-diluted solution of **SOX**, and **OX-ED-OX** formed by using the half-diluted solution of **SED** were heat-treated at 500°C for 30 min. The precursor films of the third layers for **OX-OX-OX** and **OX-ED-OX** formed by employing the half-diluted solution of **SOX** were

**Figure 6.** Photographs of the 3-layer **OX-OX-OX** and **OX-ED-OX** films on the ITO glass substrate.

**Figure 7.** Depth profile of the amount of components in the 3-layer thin film (A) **OX-OX-OX** and (B) **OX-ED-OX**. Notations indicate the energy levels of five atoms in parentheses, ―●― (Ti 2p), ―○― (O 1s), ― ♦ ― (N 1s), ― ◊ ― (C 1s), ―☐― (In 3d). Net multiplication part is corresponding to the second layer formed by applying the solution **SED**.

The photoreactivities of the films were tested using the decolorizing reaction of methylene blue (MB) aqueous solution [5-8, 75-77]. The decoloration rate of 0.01 mol L–1 MB solution by the photoreaction with both multilayered thin films under UV or Vis irradiation are summarized in Table 1. The **OX–ED–OX** film has an effective photoreactivity under Vis irradiation. The **OX–OX–OX** film only responded to UV light. It is important that the photoreactivities of the Vis-responsive films are also extremely high under UV irradiation.

The Vis-responsive property of the **OX–ED–OX** film was mainly due to the colored materials that were formed spontaneously during heat-treatment by chemical reactions between the reductant derived from the precursor complex containing OX in the upper layer and the organic residues derived from EDTA ligands in the lower one. Thus, the thermal reactions between the residues derived from the ligand of the precursor complex can afford novel functions such as the Vis-responsive nature of the resultant thin films through heat-treatment of the thin films fabricated by the MPM. The design of metal complexes for the precursor and of the heating program are crucial.

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 307

*ν* under UV-light irradiation **ED EDair SG**

The maximum photoreactivity of **ED-PA15** produced by the MPM is twice that of **SG-PA10**

0 2(1) 6(1) 5(1) 5 5(1) 12(1) 6(1) 10 9(1) 12(1) 8(1) 15 16(1) 9(1) 7(1) 20 11(1) 9(1) 7(1) 30 8(1) 5(2) 5(1)

**Table 2.** The rate *ν* [nmol L–1 min–1] of decoloration rate of 0.01 mol L–1 MB solution by the

are also indicated. Calculated standard deviations are presented in parentheses.

photoreaction with each thin film under UV-light irradiation [6]. The rate was measured by the decrease of absorption value at 664 nm of each test solution. Those obtained from the data measured under dark

It is generally accepted that the main factors to consider when designing enhanced photoreactivity of anatase are (1) higher crystallinity, (2) larger surface area, and (3) decreased impurities. The crystallite size is an indicator of crystallinity [78, 79]. Among the crystallite sizes of the three anatase thin films, **ED**, **EDair** and **SG**, the **SG** thin film had the largest value and the **ED** film had the smallest (Table 3). These values for the anatase crystallites in **EDair** and **SG** thin films were not affected by post-annealing treatment in air. The thin film **ED-PA15** (whose crystallite size was the smallest) showed the highest photoreactivity in the decoloration of an MB aqueous solution among the various thin films formed in this study. The specific surface areas of the thin films were not measured quantitatively because of the difficulties involved. However, the degrees of adsorption of MB molecules in aqueous solution were nearly equal among the thin films, including those formed by the sol–gel method. Therefore, the differences in the photoreactivity among these thin films should be due to other factors than the specific surface area. The XPS spectra suggested that the thin films **SG** and **SG-PA***n* have higher purities than the other thin films. Therefore, the highest photoreactivity, of **ED-PA15** thin film, cannot be due to its purity. The O/Ti peak area ratio determined from the XPS of the anatase film **ED-PA15** with the highest photoreactivity was extremely small, 1.5. The refractive indices of the thin films **EDair** and **SG** increased gradually, depending on the post-annealing time. On the other hand, the refractive index of the **ED** thin film decreased with post-annealing treatment time up to 15 min and then increased with further annealing. The largest index (2.17; **ED** thin film) may be related to the strong and wide absorbance by the above-mentioned impurities. Furthermore, the smallest value (1.99; **ED-PA15)** could be affected by the largest O deficiency in the anatase thin film after purification. The decrease in permittivity of the thin film arose from the lower charge density derived from the O deficiency because the structure of the anatase lattice is rigid. Thus, the O deficiency formed by this method was one of the most important factors for fabricating highly UV-sensitive anatase. This O deficiency was formed during

prepared by a conventional sol–gel procedure.

Annealing Time (min)

heat-treatment of the precursor metal complexes.


**Table 1.** The rate *ν* [nmol L–1 min–1] of decoloration rate of 0.01 mol L–1 MB solution by the photoreaction with both thin films under UV and VIS irradiation [5]. The rate was measured by the decrease of absorption value at 664 nm of each test solution. Those obtained from the data measured under dark are also indicated. Calculated standard deviations are presented in parentheses.
