Characterization of N, F Co-Doped Mixed-Anion TiO2 Thin Films Prepared By Reactive Pulsed Laser Deposition for Visible-Light Responsive Photocatalyst
Authors: Kawakami Nozomi , Uchiyama Tomoki , Yamamoto Kentaro , Maeda Kazuhiko , Uchimoto Yoshiharu 
Published: February 2020

Introduction
The study of water splitting by photocatalyst as the method of directly producing hydrogen from water has been reported in 1972, well-known as “Honda-Fuzishima effect”. Conventional TiO2 material can produce hydrogen under only ultraviolet light and the its production rate is still low. In order to use TiO2 under visible light, the anion doping is one of the effective solution. In this study, photocatalytic N, F-doped TiO2 thin film was synthesized by the responsible pulsed laser deposition(PLD) technique. Nitridation and fluoridation of TiO2 were simultaneously conducted with PLD under O2/N2/NF3 gas.
Experimental
TiO2 thin film was prepared by pulsed laser deposition (PLD). A Nd:YAG laser operating at a wavelength of 532 nm was focused on a sintered TiO2 target. We used a chamber which can be evacuated to a base pressure of 5.0 x 10-4 Pa by means of a turbo-molecular pump. When depositing TiO2 thin film, the total pressure of the mixed gas was fixed to 5 Pa. FTO (Fluorine doped tin oxide) was used as the glass substrate for film deposition. The substrate temperature was changed from room temperature to 400℃. The target (φ10) was produced by sintering the Rutile TiO2 powder at 1000℃ for 12h in air. Structural analysis of TiO2 is carried out by XRD (X-ray diffiraction) and XAFS (X-ray Absorption Spectroscopy). Especially, the product identification was observed by XRD with parallel beam method at SPring-8 BL02B2. An incident angle of X-ray is 0.1o and wavelength of X-ray is 0.7Å. Measurement of the absorption edge of Ti K was carried out by XAFS with electron yield method. In addition, for defining of the flatband level and bandgap carried out by UV-vis and AC impedance measurement. Photoelectrochemical measurements were conducted in an aqueous Na2SO4 solution (0.1 M, pH=9) using a potentiostat (Hokuto Denko HSV-110) connected to the photocatalyst-deposited FTO glass as a working electrode an Ag/AgCl reference electrode (in saturated KCl aqueous solution), and Pt wire as a counter electrode. The solution was purged with Ar gas for 30 min. To measure photocurrent responses, intermittent visible light (420 < λ < 800 nm) was applied to the working electrode with a Xe lamp (300 W, output current: 20 A).
Result & Discussion
Fig. 1 shows that XRD results under various gas atmosphere. In the case of under the O2 atmosphere, Anatase/Rutile TiO2 composition was observed. Under the N2 atmosphere, TiN was observed and under the NF3 atmosphere, Anatase TiO2 was observed. Under the N2 and N2/NF3 atmosphere, FTO substrate was decomposed. Under the O2/N2/NF3 atmosphere, the decomposition of the substrate was suppressed while the compounds derived from Ti couldn’t be observed. These results indicated that O2 gas was necessary for depositing TiO2 by suppressing of substrate as well as supplementation of the oxygen defects. Fig. 2 shows that the visible light response as a function of the photocurrent and voltage. By switching Xe lamp on/off per 3 second, the increase and decrease of current density was observed over 0.4 µA cm-2. The photocurrent response under visible light was observed for the atmosphere of O2/N2/NF3 and O2/N2 while that of O2/NF3 was not. In addition, the photocurrent was larger for the materials prepared under O2/N2/NF3 than O2/N2. UV-vis diffuse reflectance spectrum indicated that N, F-doped TiO2 showed a wide absorption band in visible region in contrast to a narrow one of N-doped TiO2. This extended absorption band of N, F-doped TiO2 in the visible region allowed the materials to improve the efficiency of the catalytic reactions such as water oxidation.
Conclusion
It is necessary that O2 gas to suppress substrate decomposition and compensate oxygen defect. PLD performed under O2/N2/NF3 mixed gas could successfully incorporated N3- and F- into TiO2 matrix. The visible light response of co-doping of N3- and F- was better than N3- single doping. It was subjected to the UV-vis measurement, the N3-, F- co-doped TiO2 thin film has wide absorption band of a visible light, which was considered to have led to a visible light absorption.
Reference
1) A. Nakada et al., J.Mater. Chem. A,
2017, 5, 11710-11719.
2) K. Maeda et al., J. Phys. Chem. C, 2007, 111 (49), 18264–18270.

Figure 1