Review

1.6.1 XRD patterns of N-doped TiO2

X-ray diffraction patterns of the N-doped TiO2 films deposited with NH3 flow rate of 150 sccm before and after annealing are shown in Fig. 1-8. The as-grown film does not show any diffraction peaks. After annealing, the crystalline phase of anatase is found in the 400°C annealed film, both anatase and rutile phases in the 600°C annealed film, and the rutile phase in the 900°C annealed film, respectively. Similar results are obtained in the films deposited by changing NH3 flow rate. In the undoped TiO2 films, only anatase phase is observed after 600°C annealing. Because the intermingled structure of anatase and rutile phases is formed in the N-doped TiO2

films annealed at 600°C, it is found that the transition temperature from anatase to rutile becomes lower by nitrogen doping ^[11]. To evaluate the effect of UV-visible absorption performance on photocatalytic activity of the nanoparticles, UV-visible absorption spectra of anatase and rutile TiO2 nanoparticles in water were measured in Fig. 1-9. Clearly, absorption and scattering of anatase TiO2 nanoparticles are stronger than those of rutile TiO2, which guarantees the relatively high photocatalytic activity of the former ^[17].

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Figure 1-8 XRD patterns of N-doped TiO2 films deposited with NH3 flow rate of 150 sccm before and after annealing.

Figure 1-9 UV-visible absorption spectra of anatase and rutile TiO2 nanoparticles in water.

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1.6.2 XPS patterns of N-doped TiO2

Fig. 1-10 shows the N 1s XPS spectra of TiO2-xNx and TiO2 powders. Since peaks at 396 eV, which have previously been found to result from Ti-N bonds, are observed for the powders annealed under NH3, it was determined that the oxygen sites were substituted by nitrogen atoms. Since the XRD did not indicate the formation of TiN bonds, it was determined that O-Ti-N bonds formed. Therefore, these powders were described as TiO2-xNx. In contrast, the air-annealed samples did not display a peak at 396 eV and is TiO2. The peak around 400 eV is the chemisorbed N2 molecule, which absorbs onto the surface. While the N 1 s X-ray photoelectron spectrum of TiO2-xNx

shown in Fig. 1-11 features a peak at 399.95 eV, known to be attributable to adsorbed NO or N in Ti-O-N, no weak peak attributable to Ti-N bonding can be seen at 396 eV due to the noise, as shown in the upper trace in Fig. 1-11. In the case of undoped TiO2

powder, neither peak is observed, as shown in the lower trace in Fig. 1-11^[13].

Figure 1-10 N 1s XPS spectra of TiO2-xNx and TiO2 powders.

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Figure 1-11 N 1 s X-ray photoelectron spectra of TiO2-xNx (upper trace) and TiO2

powders (lower trace).

XPS spectra of the N-doped TiO2 film deposited with NH3 flow rate of 150 sccm are shown in Fig. 1-12 for Ti 2p, b for O 1s, and c for N 1s electrons, respectively. The binding energy of Ti 2p at 459.1 eV shifting from 454 eV of metallic Ti is the signal of Ti in TiO2, and that of O 1s at 531 eV is assigned to metallic oxide. For N 1s electrons, two bonding states of nitrogen atoms are observed whose binding energies are 396.1 and 399.3 eV, and these are assigned to the nitrogen atoms substituting for the oxygen atoms (Nsub) and those existing interstitially in the TiO2 matrices (Nint), respectively.

Nsub:Nint ratio estimated from the spectrum intensities becomes about 80:20 ^[11].

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Figure 1-12 XPS spectra of N-doped TiO2 films deposited with NH3 flow rate of 150 sccm: N 1s respectively.

1.6.3 Absorption and energy band gap

In the case of Si- TiO2-xNx films, the crystalline forms before and after doping with nitrogen were characterized as being of anatase-type, with a crystalline size of 11 nm. Fig. 1-13(A) shows a comparison of the UV/Vis diffuse reflectance spectra of TiO2-xNx and undoped TiO2. The prepared TiO2-xNx photocatalysts, in the form of powders or thin films, were very vivid yellow in color, and showed a shift to longer wavelengths in accordance with color. Fig. 1-13(B) shows plots of the modified Kubelka–Munk function versus the photon energy, from which the band-gap energies can be obtained. The band-gap energy for TiO2-xNx can be seen to be 2.95 eV,

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corresponding to the visible-light region, whereas that for undoped TiO2 is 3.2 eV.

Figure 1-13. A): UV/Vis diffuse reflectance spectra of the samples. B): Plot of the modified Kubelka–Munk function versus the photon energy of the samples. (a) After doping with nitrogen. (b) Commercial UV photocatalyst. Doping conditions: 10 min at 873 K under a stream of ammonia gas.

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Fig. 1-14(a) shows the band-gap energy structure of the anatase-type UV-driven photocatalyst, while Fig. 1-14(b), reported by Asahi et al., shows the narrowed band-gap structure obtained by mixing of N 2p and O 2p orbit. Fig. 1-14(c), reported by Nakamura et al., shows the midgap energy level formed slightly above the valence band. The schemes represented in Figure 1-14(b) and (c) are both used in engineering for visible-light-driven photocatalyst materials. Mechanisms to achieve responsiveness to visible light are still under investigation, and no complete interpretation has yet been formulated. Thus, the mechanisms of reactions of specific compounds with TiO2-xNx

may help in delineating the mechanistic aspects of reactivity in the visible-light region

[13].

Figure 1-14. Models of the energy band gap structure of TiO2-xNx for response in the visible-light region.

1.6.4 Water contact angle

Photo-induced hydrophilicity after the visible-light irradiation was evaluated by measuring the contact angle of water on the film surface. The contact angle of

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as-grown and annealed N-doped TiO2 films deposited using NH3 flow rates of 150 sccm as a function of visible-light irradiation time is shown in Fig. 1-15. It is found that weak photo-induced hydrophilicity for visible-light irradiation is observed even in the as-grown film. The photo-induced hydrophilicity enhances remarkably when the film is annealed. Higher photo-induced hydrophilicity is also observed in the film with intermingled structure of anatase and rutile ^[11]. All of these experiments heat the substrate for a high temperature.

Figure 1-15. Water contact angle of N-doped TiO2 films as a function of visible- or UV-light irradiation time.

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