Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition
Introduction
Titanium dioxide (TiO2) is a well-known photocatalytic material and has received great attention as a photocatalyst since it is a wide-gap semiconductor and exhibits strong oxidation activity and hydrophilicity under UV irradiation [1]. However, TiO2 can only be activated by irradiating with ultraviolet (UV) light due to its high band-gap energies (for example, 3.0 eV for rutile phase or 3.2 eV for anatase phase) [2]. Therefore, only a small fraction (∼5%) of the solar energy (<390 nm) can be utilized in practical application. Therefore, the modification of TiO2 to render its sensitivity to visible-light became one of the most important goals to increase the utility of TiO2. Many techniques have been proposed to achieve this purpose. Earlier investigations investigated the doping of transition metals of Fe, Ni, and Cr into TiO2, which can increase the absorption of visible-light, but these doped materials suffer from thermal instability and an increased number of carrier recombination centers [3], [4], [5], [6]. Recently, the doping of nonmetal atoms such as N [7], [8], [9], [10], [11], S [12], [13], and C [14], [15] into the TiO2 lattice were reported, which can shift the absorption edge or band-gap to lower energies and hence increase the visible absorption. Particularly, the substitution doping of nitrogen was found to be effective in narrowing the band-gap through mixing of N with O2p states [7].
Many techniques have been explored to fabricate TiO2 thin films, such as chemical vapor deposition (CVD) [16], sol–gel process [17], [18], radio-frequency sputtering [19], pulse laser deposition (PLD) [20]. Among them, pulse laser deposition method has become a widely used technique for the deposition of thin films due to its advantages including simple system setup, wide range of deposition conditions, wider choice of materials and high laser energy or instantaneous deposition rates.
In the present work, we prepared TiO2−xNx thin films by PLD method using TiO2 targets under nitrogen and oxygen atmosphere. The structure, surface morphology and nitrogen-doping state of the TiO2−xNx films were investigated with X-ray diffraction (XRD), field emission scan electronic microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS), respectively. The visible-light photocatalytic activity was estimated by the degradation of methylene blue and methylene orange with the TiO2−xNx films. Relationship between the nitrogen-doping state and visible-light photocatalytic activity was discussed.
Section snippets
Experiment
TiO2−xNx films were prepared on quartz glass substrate by pulse laser deposition and TiO2 films without doping was also prepared for comparison. Pulsed Nd:YAG laser with a wavelength of 1064 nm was used. The repetition rate is 20 Hz and the fluency on target was set at 60 J/cm2 for all samples. A target of TiO2 with rutile phase was used. The distance between the target and the quartz substrate was kept at 2.5 cm. The chamber was evacuated first to a base pressure (below 5 × 10−4 Pa) using a turbo
Result and discussion
Fig. 1 shows XRD patterns of the TiO2−xNx and TiO2 film. Diffraction peaks of TiO2 film at 25.28°, 37.9° and 53.89° are corresponding, respectively, to the (1 0 1), (0 0 4) and (1 0 5) planes of the anatase TiO2 phase. Compared with the standard anatase structure XRD spectrum (from JADE card) it could be clearly seen that the TiO2 film exhibits the preferred orientation of (0 0 4). The diffraction peaks of TiO2−xNx film at 25.28° and 48.05° are corresponding, respectively, to the (1 0 1) and (2 0 0) planes
Conclusion
- 1,
TiO2−xNx film was deposited on quartz glass substrates by pulsed laser deposition. The TiO2−xNx film shown single anatase crystal phase. The x value (N-doping concentration) was 0.567 determined from X-ray photoelectron spectroscopy measurements.
- 2,
The TiO2−xNx film deposited by PLD is a broad-spectrum UV–vis-light absorption film (about 200–700 nm) characterized by its optical absorption spectrum. The film shows two deep levels located at 1.0 eV and 2.5 eV. The 1.0 eV level is attributed to O vacancy
Acknowledgments
This work was supported by Foundation of National Key Basic Research and Development Program (No.2004CB619301) and Project 985-automotive engineering of Jilin University.
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