We successfully achieved deposition of Ti͑O,C,N͒ 2 films codoped with nitrogen ͑N͒ and carbon ͑C͒ 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 ͑XRD͒ and field-effect scanning electron microscopy. Interstitial N and C in the films were also measured using X-ray photoelectron spectroscopy ͑XPS͒, 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 eV͒ and Ti-C bonding ͑282 eV͒ were observed using XPS. Thus, N and C were considered to be at substitutional O sites. It was verified that the Ti͑O,C,N͒ 2 films deposited in this study undergo a photocatalystic reaction when exposed to visible light in the wavelength range of 400-500 nm.
It was confirmed that the oxygen partial pressure that is used during DC reactive sputtering can influence the generation of oxygen defects on TiO 2 thin films and/or the surface structure, which then affects the photocatalytic characteristics. TiO 2 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 TiO 2 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. TiO 2 thin films deposited under reducing conditions (an oxygen partial pressure of 0.1 Pa) exhibited peaks that are characteristic of Ti 3þ 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 TiO 2 thin films deposited in oxidizing mode (oxygen partial pressures of 0.5 Pa and 0.8 Pa), the peak corresponding to Ti 3þ was not observed. The domain size was between 300-1000 nm and the boundaries were clear. TiO 2 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 electronhole pairs, improved the photocatalytic characteristics.
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 (R
a) 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.
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