Daisuke Noguchi

The Response of TiO2 Photocatalysts Codoped with Nitrogen and Carbon to Visible Light

We successfully achieved deposition of Ti(O,C,N) 2 films codoped with nitrogen (N) and carbon (C) using dc reactive sputtering utilizing a Ti target in an Ar/N 2 /CO 2 ambient. The crystallinity and surface morphology of the films were observed using X-ray diffraction (XRD) and field-effect scanning electron microscopy. Interstitial N and C in the films were also measured using X-ray photoelectron spectroscopy (XPS), from which the doping level was calculated. The photocatalytic activity was evaluated using methylene blue decomposition under visible and ultraviolet irradiation. All the films were transparent yellow, and it was confirmed using XRD that the crystal form was polycrystalline anatase. Two peaks resulting from Ti-N bonding (397 eV) and Ti-C bonding (282 eV) were observed using XPS. Thus, N and C were considered to be at substitutional O sites. It was verified that the Ti(O,C,N) 2 films deposited in this study undergo a photocatalystic reaction when exposed to visible light in the wavelength range of 400-500 nm.

Relationship between the photocatalytic characteristics and the oxygen partial pressure of TiO2 thin films prepared by a DC reactive sputtering method

It was confirmed that the oxygen partial pressure that is used during DC reactive sputtering can influence the generation of oxygen defects on TiO2 thin films and/or the surface structure, which then affects the photocatalytic characteristics. TiO2 films were deposited by DC reactive magnetron sputtering on a Si wafer at 250°C under oxygen partial pressures of 0.1 Pa, 0.5 Pa and 0.8 Pa. Structural analysis of the TiO2 thin films was conducted using X-ray diffraction (XRD), field-effect scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), the photo luminescence method (PL). Characterization of the photocatalytic properties was performed by measuring the concentration change of methylene blue (wet methylene blue method) and the change of the contact angle with pure water. All films showed a polycrystalline anatase structure. TiO2 thin films deposited under reducing conditions (an oxygen partial pressure of 0.1 Pa) exhibited peaks that are characteristic of Ti3+ and can also contain a number of oxygen defects. The domain size was about 150–300 nm, and the domain boundaries were unclear. On the other hand, for TiO2 thin films deposited in oxidizing mode (oxygen partial pressures of 0.5 Pa and 0.8 Pa), the peak corresponding to Ti3+ was not observed. The domain size was between 300–1000 nm and the boundaries were clear. TiO2 thin films exhibit better photocatalytic properties when the domain size is large and there are fewer oxygen defects in the film. It has been suggested that the decreasing proportion of domain boundaries, which act as trapping centers for electrons, and the lower concentration of oxygen defects, which act as recombination centers for electron-hole pairs, improved the photocatalytic characteristics.

Effects of Sputtered High-Energy Particles on the Structure and Photocatalytic Performance of TiO2 Films Prepared by a DC Reactive Sputtering Method

By using the Kevin–Meyer formula, we calculated the energy of the high-energy particles (recoil argon ions and negative oxygen ions) that are unique to sputtered thin-film formation when they arrived at a substrate during the sputtering process. We found that the energy of the high-energy particles arriving at the substrate decrease if total gas pressure increases, whereas the photocatalytic performance of a TiO2 film increases if the total gas pressure increases. We also found that as total gas pressure increases, the surface morphology changed from a gap-free structure to a porous structure in which subgrains were observed. Accordingly, the average surface roughness (Ra) and surface area (S) of the thin film increased. The number of defects, mainly oxygen defects, in the forbidden band also changed depending on the energy of the high-energy particles. Therefore, it was found that the high-energy particles affect the surface area of the TiO2 thin films and the formation of defects in the thin films, which consequently influences the photocatalytic performance.

Photocatalytic Characteristics of TiO2 Thin Films Prepared by Dc Reactive Magnetron Sputtering with added H2O

A high-performance photocatalytic TiO2 thin film was successfully obtained by dc reactive magnetron sputtering with added H2O. The film was deposited onto SiO2-coated glass at a substrate temperature of 300°C using a titanium metal target in a combined Ar, O2 and H2O atmosphere. Photocatalytic performance was evaluated by measuring the change in concentration of methylene blue (MB) under UV irradiation. The photocatalytic performance of the film increased as the quantity of added H2O increased: when 70 sccm of H2O was added, the decomposition capability was about 1.6 times greater than a reference sample (sol–gel-derived TiO2 thin film). The film was characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), UV-VIS-NIR spectrophotometry, attenuated total reflection Fourier transform infrared spectrometry (ATR-FTIR), and photoluminescence spectrometry. Included in the film were a large number of OH groups. The surface morphology of the film had a domain structure, with each domain consisting of many sputter grains with the same orientation. The number of oxygen vacancies level in the forbidden band decreased as the quantity of H2O added during deposition increased. In addition, the Ti–OH dependent peak intensity increased when H2O was added. It was observed that this process would enable us to hydrogenate TiO2 more easily and thereby improve photocatalytic performance.

Photoinduced hydrophilicity and structural evaluation of SiOx:OH/TiO2 multilayer films by DC reactive magnetron sputtering

We have examined the photoinduced hydrophilicity of SiOx:OH/TiO2 multilayer films containing a high degree of Si–OH bonding. The ultrahydrophilic properties were observed under UV irradiation of 10 µW/cm2, and a technique for preserving these changes in the dark has been successfully developed. Different thicknesses of SiOx:OH films were deposited onto photocatalytic TiO2 layers in an Ar/H2O ambient by DC reactive magnetron sputtering. The deposited structures were analyzed by secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), and their hydrophilic properties were evaluated by contact-angle measurement with respect to purified wafer. Cross sections of the structure of the SiOx:OH/TiO2 multilayer films show that SiOx:OH is deposited on the columnar-structured TiO2 film in a discontinuous manner. The amount of hydrogen in the SiOx:OH film depends on the H2O partial pressure, and the concentration depends heavily on the presence of Si–OH. The SiOx:OH/TiO2 multilayer films exhibit hydrophilic properties, even without any UV irradiation, and the hydrophilic properties under photoexcitation are also improved. The presence of –OH groups in the SiOx:OH films is considered to have an effect on this improvement in hydrophilicity.

Technique for High-Rate, Low-Temperature Deposition of TiO2 Photocatalytic Thin Film Using Radical-Assisted Sputtering

We have developed a two-step deposition technique. In the first step, a TiO2 nucleus is formed on a SiO2 thin film by radical-assisted sputtering using reactive-mode sputtering and O2 radicals. The second step is a growth step where TiO2 is deposited at a high rate and at a low temperature on the nucleation layer using metallic-mode sputtering and O2 radicals. The results were evaluated by transmission electron microscopy. They demonstrate that this deposition method can form TiO2 thin films with high crystallinities from the growth boundary. The effectiveness of this method was also demonstrated by measuring the photocatalytic properties.

Structural control and photocatalytic properties of photocatalytic TiO2 thin films prepared using two-step deposition including radical-assisted sputtering

TiO2 films were deposited using a two-step growth process, in which an ultra-thin nucleation layer was first produced by reactive-mode sputtering, followed by a thicker growth layer by radical-assisted sputtering. The effect of the structure of the nucleation layer on the density, crystallinity and photocatalytic properties of the final films was investigated. The kinetic energy of sputtered particles reaching the substrate during the first growth step was also considered. This was found to affect the amount of three-dimensional island growth that occurred, and thus the number of seed particles and gaps between them. The optimum structure is found to be one in which few seed particles have undergone island growth, so that there are a large number of gaps. This not only produces a dense final structure, but the crystallinity is improved due to chemical annealing by radicals during the second growth step, leading to a film with excellent photocatalytic properties. This indicates the importance of an initial structure that facilitates absorption and diffusion of radicals.

Ion Conductivity of Ta2O5 Solid Electrolyte Thin Film Prepared by Combination Sputtering with Radio Frequency Oxygen Plasma Irradiation

In light of the need to simplify combination sputtering systems and to use them to fabricate thin-film electronic materials, we fabricated a Ta2O5 solid electrolyte thin-film that exhibits superior ion conductivity by using combination sputtering with RF oxygen plasma irradiation. The formation of Ta2O5 thin films using this technique was confirmed by Rutherford backscattering spectroscopy and the effectiveness of the resulting thin film as a solid electrolyte was demonstrated by its ion conductivity, which was determined by AC impedance measurement.

Effect of Nucleation Layer of Photocatalytic TiO2 Thin Film Prepared by Two-Step Deposition Using Radical-Assisted Sputtering

A nucleation layer forms in the nucleation step (the first step) of a two-step deposition method that we have developed for forming TiO2 thin films. We found that the nucleation layer affects the crystallinity of TiO2 thin films during the growth step (the second step). The crystallinity of the growth layer can be controlled by varying the kinetic energy of the sputter particles when they arrive at the substrate in the nucleation step. Since the nucleation density varies with the kinetic energy, the crystallinity is thought to improve, owing to the improvement in the nucleation density.

Photocatalytic Characteristics and Structural Evaluation of TiO2 Thin Films Reactively DC Sputtered with H 2 O

TiO 2 thin films containing Ti-OH bonds have been fabricated using dc reactive sputtering of Ti in a H 2 O ambient. The structures of these films and their photocatalytic characteristics were analyzed. The structures of the TiO 2 thin films were analyzed using Fourier transform infrared spectroscopic analysis, X-ray diffraction, scanning electron microscope inspection, nuclear magnetic resonance, and photoluminescence. To evaluate the photocatalytic characteristics of the TiO 2 films, we used a wet methylene blue method and pure-water contact angle measurement. We found the following: (i) The total quantity of OH groups decreases as the substrate temperature is increased. (ii) OH groups can be either densely or sparsely distributed in the TiO 2 film. (iii) The abundance ratios of the concentrated and sparse OH groups to the total quantity of OH groups change as the substrate temperature is increased. Measurements made with nuclear magnetic resonance and photoluminescence show that the sparse OH groups ([0H] n ) are effective in removing oxygen deficiency and other deficiencies, while the localized, concentrated OH groups ([OH] w ) are ineffective. We concluded that the distribution of OH groups in the TiO 2 thin film is an important factor that must be considered for improvement of photocatalytic characteristics.