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Preparation of high quality nitrogen doped TiO2 thin film as a photocatalyst using a pulsed laser deposition method
Thin Solid Films 453 – 454 (2004) 162–166
Preparation of high quality nitrogen doped TiO2 thin film as a photocatalyst using a pulsed laser deposition method Yoshiaki Suda*, Hiroharu Kawasaki, Tsuyoshi Ueda, Tamiko Ohshima Department of Electrical Engineering, Sasebo National College of Technology, 1-1 Okishin, Sasebo, Nagasaki 857-1193, Japan
Abstract Nitrogen doped titanium oxide (TiO2yxNx ) photocatalysts, which were reported to be activated by visible light irradiation as well as ultraviolet irradiation, have been prepared by pulsed laser deposition (PLD) method using TiN target in nitrogenyoxygen gas mixture. Crystalline structure, nitrogen states in the lattice, composition and surface morphology were analyzed by using Xray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. As the results, it is found that film structure and properties strongly depend on target material and nitrogen concentration ratio in the gas mixture. The materials show anatase structure with nitrogen doped into TiO2 oxygen sites, which leads to band gap narrowing. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; Plasma; Titanium oxide; Nitrogen doped; Photocatalyst
1. Introduction Titanium dioxide (TiO2) is a widely studied material and has numerous industrial applications including photocatalysis, optical coatings and opto-electronic devices, because of its favorable physical, chemical and optoelectrical properties and high stability. Therefore, extensive efforts have been made in the development of TiO2 photocatalysts that can efficiently utilize solar or indoor light. As the results of these efforts, several types of visible-light-active TiO2 photocatalysts have been proposed w1–4x. In the past years, it has been studied about the appearance of the activity using substitution of several metal ions, such as Fe, Cr, Pt, Ta, etc. for the Ti state in the lattice of TiO2 w1,2x. It is also discussed the method to form the oxygen deficient state between the valence and the conduction bands in the TiO2 band structure w3,4x. However, several serious problems have been pointed out about the chemical stability for photocatalysts prepared using those methods. Recently, it is found that nitrogen doped titanium oxide photocatalyst (TiO2yxNx) is also activated by visible light irradiation as well as ultraviolet irradiation w5–7x. Therefore, many techniques had been used to produce the visible-lightactive TiO2yxNx films, such as spray pyrolysis w8x, sol– *Corresponding author. Tel.yfax: q81-956-34-8478. E-mail address: [email protected] (Y. Suda).
gel method w9x, dip-coating w10x, ion-implantation w11x, plasma-treatment w12x, etc. However, the performance of the TiO2yxNx films as visible-light-active photocatalyst and the reproducibility to prepare high quality films was not high enough. However, pulsed laser deposition (PLD) method has become a widely used technique for the deposition of thin films during the past few years due to the advantages of a simple system setup, a wide range of deposition conditions, wider choice of materials and higher instantaneous deposition rates. Especially, it is well known that this method shows a high reproducibility to prepare crystalline thin films. Because of this versatility, we have developed several kinds of functional thin films, such as tungsten carbide, silicon carbide, chromium carbide, titanium carbide, cubic boron nitride, carbon nitride and silicon nitride, etc. using the PLD method w13–23x. In this paper, we prepared TiO2yxNx thin films by PLD method using TiN and TiO2 targets in oxygeny nitrogen gas mixture, and evaluated the crystalline structure, chemical states of nitrogen in the lattice, composition and surface morphology using X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. From the results, we found the conditions required to fabricate high quality TiO2yxNx photocatalyst thin films.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.185
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the decomposition of methylene blue (C16H18N3S-Cl3H2O) on the TiO2 and TiO2yxNx surface under UV light and visible light irradiation. TiO2 and TiO2yxNx films were dipped into an aqueous methylene blue solution with a concentration of 50 mmolyl and irradiated with a black light lamp (362 nm, 50 mWycm2) and fluorescent light (540–620 nm, 120 mWycm2). The transmittance of the solution was measured every 2 h for 8 h by means of a Hitachi UV-3300 spectrometer at a wavelength of 664 nm. 3. Results and discussion Fig. 1. Schematic diagram of the pulsed Nd:YAG laser deposition apparatus.
2. Experimental The schematic of the experimental apparatus is shown in Fig. 1. In this experiment, TiO2yxNx thin films are prepared by pulsed Nd:YAG laser deposition method. Deposition chamber was fabricated of stainless steel with a diameter of 400 mm and a length of 370 mm. The chamber was evacuated to a base pressure (below 4=10y4 Pa) using a turbo molecular pump and a rotary pump. The gas pressure was kept at 10 Pa by feeding nitrogenyoxygen gas mixture into the chamber with mass flow controllers. A pulsed Nd:YAG laser (Continuum SureliteIII; wavelength of 532 nm, pulse duration of 3.5 ns, maximum output energy of 340 mJ) was used to irradiate TiN (purity 99.9%) target and TiO2 (99.99%) target. Their radiated area was kept at 2.8 mm2. The laser energy density (Ed) was fixed at 3.8 Jycm2. The Si (100) and SiO2 (Corning 7059) substrates were located at 6.0 cm from the target. The targets were rotated at 20 rev.ymin to avoid pitting during deposition. The substrates were cleaned using an ultrasonic agitator by repeated bathing in ethanol and then rinsed in highpurity deionized water prior to loading into the deposition chamber. The substrates were heated to 400 8C by an IR lamp. After 36 000 laser shots at a 10 Hz repetition rate, the deposition process was completed. The crystalline structure and crystallographic orientation of the thin films were characterized by X-ray diffraction (XRD; RIGAKU RINT2100V) using a CuKa radiation. The composition of the films was measured by X-ray photoelectron spectroscopy (XPS; JEOL JPS9010). The surface morphology of the films on Si (100) substrates was observed by an atomic force microscope (AFM; JOEL JSPM4210). The UV–VIS absorption spectra of the films were measured using a spectrophotometer (UV–VIS; Hitachi U-3300). Film thickness was measured by a-step (Shimadsu; Dektak). The photocatalytic properties of the prepared TiO2 and TiO2yxNx films were tested by the measurement of
Fig. 2 shows the photograph of the films prepared by PLD method as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. Fig. 2a shows the TiO2 film prepared by PLD method using TiO2 target. As shown in this figure, the film looks highly transparent. Fig. 2b–f shows the TiO2yxNx thin films prepared using a TiN target in nitrogenyoxygen gas mixture. As shown in Fig. 2b–d, the color of the film prepared in low nitrogen concentration ratio in nitrogenyoxygen gas mixture (N2 yO2F9) is achromatous glaucous-yellow. With increasing the nitrogen concentration ratio in nitrogenyoxygen gas mixture, the film color changes from transparent to dark brown as shown in Fig. 2e–f. This tendency may be due to the concentration rate of the nitrogen atom in the TiO2yxNx films, and the films change from TiO2 (transparent) to TiN (dark brown). Fig. 3 shows the surface morphology of the TiO2yxNx thin film measured by using AFM. The TiO2yxNx film prepared on Si (100) substrate in nitrogenyoxygen gas mixture (N2:O2s1:1) was uniform and the mean roughness of the film was lower than 10 nm. The micrograph of the TiO2yxNx thin film reveals that the nanocrystalline nanoporous structure of the film has a wide surface area. Spherical particles, with average
Fig. 2. Photograph of the films prepared by PLD method as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. (a) N2:O2s0:1, TiO2 target, (b) N2:O2 s0:1, TiN target, (c) N2:O2s 1:1, TiN target, (d) N2:O2s9:1, TiN target, (e) N2:O2s19:1, TiN target, (f) N2:O2s1:0, TiN target (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
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Fig. 4. XRD patterns of the films prepared using TiN target as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s19:1, (d) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s10 Pa, Tss400 8C).
Fig. 3. Surface morphology of the TiO2yx Nx thin film using TiN target. (N2:O2s1:1, Eds3.8 Jycm2, PsPO2qPN2s10 Pa, Tss400 8C).
particle size ranging ;10 nm can be observed from this figure. The film thickness measured by a-step was approximately 550–750 nm. The deposition rate was approximately 9.2–12.5 nmymin. XRD measurements were carried out to study the crystalline properties of the prepared TiO2yxNx thin films. Fig. 4 shows XRD patterns of the films as a function of nitrogen concentration ratio in nitrogeny oxygen gas mixture. In this experiment, the substrate temperature (Ts) was 400 8C and the total gas pressure was 10 Pa. The distinct reflex of TiO2 (101) can be observed on the films prepared in the oxygen gas (N2:O2s0:1) (Fig. 4a) and nitrogenyoxygen gas mixture (N2:O2s1:1) (Fig. 4b). Besides the reflex of TiO2 (101), the weak reflexes of TiN (200) and TiN (220) appeared in the high nitrogen concentration gas mixture (N2:O2s19:1) (Fig. 4c). The TiO2 (101) reflex belongs to the anatase phase, and the reflex intensity is independent of the nitrogen concentration ratio in nitrogeny oxygen gas mixture at Tss400 8C. As shown in Fig. 4d, the TiO2 (101) reflex disappeared and two distinct reflexs of TiN (200) and TiN (220) can be observed on the film synthesized in the nitrogen gas (N2:O2s1:0). These results suggest that crystalline structure in the film prepared by PLD method using TiN target strongly depends on the nitrogen concentration ratio in nitrogeny oxygen gas mixture. The XPS spectra of Ti 2p peaks of the film after 60 s sputtering are shown in Fig. 5. The films were etched by the sputtering with Arq ion bombardment accelerated
by 600 V. The XPS spectrum of the film prepared in the oxygen gas (N2:O2s0:1) is shown in Fig. 5a. The Ti 2p spectrum shows a doublet structure with slight asymmetry. The binding energies of Ti 2p transition corresponding to valence states obtained from the deconvoluted spectra w9,24–27x. The spectrum shows the presence of one large peak of Ti 2p incorporated with TiO2(460 eV), and small peaks of TiN(457 eV) and TiN(463 eV) w26,27x. The binding energy of approximately 460 eV is 1.7 eV higher than that of Ti state in TiO2(458.3 eV), which may be due to the incorporation of N into TiO2. With increasing the nitrogen concentration ratio (spectra (b)–(d)), the Ti 2p spectra became broad and more asymmetric. Fig. 5e shows the XPS spectrum of the film prepared using TiN target in nitrogen gas (N2:O2s1:0). The spectrum shows the
Fig. 5. XPS spectra of Ti 2p peaks of the TiO2yxNx films as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s9:1, (d) N2:O2s19:1, (e) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
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Fig. 7. UV–VIS absorption spectra of the TiO2 and TiO2yxNx thin films. (a) N2:O2s0:1, TiO2 target, (b) N2:O2s0:1, TiN target, (c) N2:O2s1:1, TiN target, (d) N2:O2 s9:1, TiN target, (e) N2:O2s19:1, TiN target, (f) N2:O2s1:0, TiN target (Eds3.8 Jycm2, PsPO2q PN2s10 Pa, Tss400 8C).
Fig. 6. XPS spectra of N 1s peaks of the TiO2yx Nx films as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s9:1, (d) N2:O2s19:1, (e) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
presence of two Ti 2p peaks of TiN(457 eV) and TiN(463 eV) w24–27x. The XPS spectra of N 1s peaks of the film after 60 s Arq ion sputtering are shown in Fig. 6. The XPS spectrum of the film prepared in the oxygen gas (N2:O2s0:1) is shown in Fig. 6a. The spectrum shows the presence of two peaks of N 1s. The binding energy of approximately 400 eV may be due to incorporation of N–O into TiO2yxNx. However, the binding energy of approximately 396 eV may be due to incorporation of Ti–N w25,26x. The peak of approximately 400 eV is slightly stronger than that of approximately 396 eV on the film prepared in the oxygen gas (N2:O2s0:1). With increasing the nitrogen concentration ratio in the gas mixture (spectra (b)–(d)), the N 1s peak approximately 396 eV increased. The spectrum of the film prepared in the nitrogen gas (N2:O2s1:0) shows the presence of strong peak at approximately 396 eV (Fig. 6e). These results also suggest that the nitrogen composition rate of the film depends on the nitrogen gas concentration ratio during the deposition. Based on XRD and XPS results, it was found that the TiO2yxNx films prepared using TiN target in nitrogenyoxygen gas mixture have anatase crystalline structure with nitrogen doped into substitutional sites of oxygen in TiO2. Fig. 7 shows the UV–VIS absorption spectra of the TiO2 and TiO2yxNx thin films prepared using PLD method. The absorption of the TiO2 film prepared using a TiO2 target in oxygen gas (N2:O2s0:1) shows a shoulder near the 400 nm and approaches zero at approximately 320 nm. For the nitrogen doped films, the absorption edge shifts toward the visible light regions
depending on the amount of nitrogen concentration ratio in the gas mixture. The absorption of the TiO2yxNx film prepared in nitrogenyoxygen gas mixture (N2:O2s9:1) shows that an edge of absorption approaches near zero at approximately 360 nm. However, the absorption of the film prepared in the high nitrogen concentration gas mixture (N2:O2s19:1) and nitrogen gas (N2:O2s1:0) drastically decreased. Fig. 8 shows the time-dependence of the transmittance of an aqueous methylene blue solution (0.05 mmolyl) for the TiO2 and TiO2yxNx films (1.0 cm2). The TiO2 film was deposited using a TiO2 target at Tss400 8C in the oxygen gas at 10 Pa and the TiO2yxNx film was deposited using a TiN target at Tss400 8C in nitrogeny oxygen gas mixture (N2:O2s1:1). It is clearly seen that these films under UV light irradiation (362 nm, 50 mWycm2) exhibit almost the same decomposition ability of methylene blue. The photocatalytic activity of TiO2yxNx film is higher than that of TiO2 film under normal fluorescent light (540–620 nm, 120 mWycm2) irradiation. 4. Conclusions TiO2yxNx thin films have been prepared by PLD method using a TiN target in nitrogenyoxygen gas
Fig. 8. The temporal evolutions of the transmittance of an aqueous methylene blue solution (0.05 mmolyl) for the TiO2 and TiO2yxNx films (1.0 cm2).
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mixture. The color of the film changed from TiO2(transparent) to TiN(dark brown) with increasing the nitrogen concentration ratio. The crystalline structure of the films prepared strongly depends on the nitrogen concentration ratio in the gas mixture and the target material. The anatase type TiO2 crystalline structure can be observed independent of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. Also, XPS analyses for Ti 2p and N 1s peaks of the films suggest that the composition of the film depends on the nitrogen concentration ratio in the gas mixture. When the nitrogen concentration ratio increased, the film structure changed from TiO2 to TiN. Based on these results, it was found that the TiO2yxNx films have anatase crystalline structure with nitrogen doped into TiO2 oxygen sites, which leads to band gap narrowing. It is also confirmed that the edges of the absorption of the films shifted from 320 nm for TiO2 film to 360 nm for the TiO2yxNx film. In photocatalytic tests using methylene blue solution, the TiO2yxNx film is activated by normal fluorescent light (visible light) irradiation as well as UV light irradiation. Acknowledgments This work was supported in part by the Grant-in-Aid for Young Scientists (A), Grant-in-Aid for Scientific Research on Priority Areas (417), Grant-in-Aid for Scientific Research (B) and a Research Fund from Nagasaki Super Technology Development Association. The authors wish to thank Drs K. Ebihara and T. Ikegami of Kumamoto University for their helpful discussions. The authors also wish to thank Dr H. Abe and Mr H. Yoshida of the Ceramic Research Center of Nagasaki for their technical assistance with the experimental data. References w1x E. Borgarello, J. Kiwi, M. Gratzel, B. Pelizzetti, M. Visca, J. Am. Chem. Soc. 104 (1982) 2996.
w2x H. Yamashita, Y. Ichihashi, M. Takeuchi, S. Kishiguchi, M. Anpo, J. Synchrotron Radiat. 6 (1999) 451. w3x I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuchi, J. Mol. Catal. A: Chem. 161 (2000) 205. w4x K. Ando, M. Ando, M. Fujii, Photocatalysis 6 (2001) 20. w5x R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. w6x Y. Sakatani, H. Koike, Japan Patent (2001) P2001–72419A. w7x T. Ihara, M. Ando, S. Sugihara, Photocatalysis, Applied Catalysis A: General 5 (2001) 19. w8x L. Fan, N. Ichikuni, S. Shimazu, T. Uematsu, Applied Catalysis A: General, 246 (2003) 87. w9x M.M. Rahman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, J. Phys. Chem. Solids 60 (1999) 201. w10x Y.M. Qiu, Y.C. Guan, J. Yao, Chemosphere 44 (2001) 1087. w11x S.K. Zheng, T.M. Wang, W.C. Hao, R. Shen, Vacuum 65 (2002) 155. w12x I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuchi, J. Mol. Catal. A: Chem. 161 (2000) 205. w13x H. Kawasaki, K. Doi, S. Hiraishi, Y. Suda, Jpn. J. Appl. Phys. 39 (2000) 4525. w14x Y. Suda, H. Kawasaki, K. Doi, S. Hiraishi, Grain Boundary Eng. Ceram. 118 (2001) 535. w15x Y. Suda, K. Doi, J. Namba, F. Imura, H. Kawasaki, Jpn. J. Appl. Phys. 40 (2001) 1061. w16x K. Doi, S. Hiraishi, H. Kawasaki, Y. Suda, Mater. Res. Soc. Symp. Proc. 617 (2001) J7.8.1. w17x H. Kawasaki, K. Doi, J. Namba, Y. Suda, Mater. Res. Soc. Symp. Proc. 617 (2001) J3.22.1. w18x H. Kawasaki, K. Doi, J. Namba, Y. Suda, T. Ohshima, K. Ebihara, Jpn. J. Appl. Phys. 40 (2001) 2391. w19x Y. Suda, H. Kawasaki, K. Doi, J. Namba, T. Nakazono, T. Ohshima, IEEE Trans. (2001) 785, IIT2000. w20x Y. Suda, H. Kawasaki, K. Doi, J. Namba, T. Ohshima, Mater. Charact. 48 (2002) 221. w21x H. Kawasaki, J. Namba, K. Iwatsuji, Y. Suda, K. Wada, K. Ebihara, T. Ohshima, Appl. Surf. Sci. 197 (2002) 547–551. w22x Y. Suda, H. Kawasaki, K. Iwatsuji, T. Ohshima, Trans. Institute Electr. Engineers Jpn. 123 (2003) 222. w23x Y. Suda, J. Namba, H. Kawasaki, K. Iwatsuji, T. Ohshima, K. Wada, K. Ebihara, Surf. Coat. Technol. (in press). w24x N.C. Saha, H.G. Tompking, J. Appl. Phys. 72 (1992) 3072. w25x M. Odelius, P. Persson, S. Lunell, Surf. Sci. 529 (2003) 47. w26x J.P. Contour, A. Salesse, M. Froment, M. Garteau, J. Thevenin, D. Warin, J. Microsc. Spect. Electron. 4 (1979) 127. w27x W.E. Slinkard, P.B. Gegroot, J. Catalyst 68 (1981) 423.
Preparation of high quality nitrogen doped TiO2 thin film as a photocatalyst using a pulsed laser deposition method Yoshiaki Suda*, Hiroharu Kawasaki, Tsuyoshi Ueda, Tamiko Ohshima Department of Electrical Engineering, Sasebo National College of Technology, 1-1 Okishin, Sasebo, Nagasaki 857-1193, Japan
Abstract Nitrogen doped titanium oxide (TiO2yxNx ) photocatalysts, which were reported to be activated by visible light irradiation as well as ultraviolet irradiation, have been prepared by pulsed laser deposition (PLD) method using TiN target in nitrogenyoxygen gas mixture. Crystalline structure, nitrogen states in the lattice, composition and surface morphology were analyzed by using Xray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. As the results, it is found that film structure and properties strongly depend on target material and nitrogen concentration ratio in the gas mixture. The materials show anatase structure with nitrogen doped into TiO2 oxygen sites, which leads to band gap narrowing. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; Plasma; Titanium oxide; Nitrogen doped; Photocatalyst
1. Introduction Titanium dioxide (TiO2) is a widely studied material and has numerous industrial applications including photocatalysis, optical coatings and opto-electronic devices, because of its favorable physical, chemical and optoelectrical properties and high stability. Therefore, extensive efforts have been made in the development of TiO2 photocatalysts that can efficiently utilize solar or indoor light. As the results of these efforts, several types of visible-light-active TiO2 photocatalysts have been proposed w1–4x. In the past years, it has been studied about the appearance of the activity using substitution of several metal ions, such as Fe, Cr, Pt, Ta, etc. for the Ti state in the lattice of TiO2 w1,2x. It is also discussed the method to form the oxygen deficient state between the valence and the conduction bands in the TiO2 band structure w3,4x. However, several serious problems have been pointed out about the chemical stability for photocatalysts prepared using those methods. Recently, it is found that nitrogen doped titanium oxide photocatalyst (TiO2yxNx) is also activated by visible light irradiation as well as ultraviolet irradiation w5–7x. Therefore, many techniques had been used to produce the visible-lightactive TiO2yxNx films, such as spray pyrolysis w8x, sol– *Corresponding author. Tel.yfax: q81-956-34-8478. E-mail address: [email protected] (Y. Suda).
gel method w9x, dip-coating w10x, ion-implantation w11x, plasma-treatment w12x, etc. However, the performance of the TiO2yxNx films as visible-light-active photocatalyst and the reproducibility to prepare high quality films was not high enough. However, pulsed laser deposition (PLD) method has become a widely used technique for the deposition of thin films during the past few years due to the advantages of a simple system setup, a wide range of deposition conditions, wider choice of materials and higher instantaneous deposition rates. Especially, it is well known that this method shows a high reproducibility to prepare crystalline thin films. Because of this versatility, we have developed several kinds of functional thin films, such as tungsten carbide, silicon carbide, chromium carbide, titanium carbide, cubic boron nitride, carbon nitride and silicon nitride, etc. using the PLD method w13–23x. In this paper, we prepared TiO2yxNx thin films by PLD method using TiN and TiO2 targets in oxygeny nitrogen gas mixture, and evaluated the crystalline structure, chemical states of nitrogen in the lattice, composition and surface morphology using X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. From the results, we found the conditions required to fabricate high quality TiO2yxNx photocatalyst thin films.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.185
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the decomposition of methylene blue (C16H18N3S-Cl3H2O) on the TiO2 and TiO2yxNx surface under UV light and visible light irradiation. TiO2 and TiO2yxNx films were dipped into an aqueous methylene blue solution with a concentration of 50 mmolyl and irradiated with a black light lamp (362 nm, 50 mWycm2) and fluorescent light (540–620 nm, 120 mWycm2). The transmittance of the solution was measured every 2 h for 8 h by means of a Hitachi UV-3300 spectrometer at a wavelength of 664 nm. 3. Results and discussion Fig. 1. Schematic diagram of the pulsed Nd:YAG laser deposition apparatus.
2. Experimental The schematic of the experimental apparatus is shown in Fig. 1. In this experiment, TiO2yxNx thin films are prepared by pulsed Nd:YAG laser deposition method. Deposition chamber was fabricated of stainless steel with a diameter of 400 mm and a length of 370 mm. The chamber was evacuated to a base pressure (below 4=10y4 Pa) using a turbo molecular pump and a rotary pump. The gas pressure was kept at 10 Pa by feeding nitrogenyoxygen gas mixture into the chamber with mass flow controllers. A pulsed Nd:YAG laser (Continuum SureliteIII; wavelength of 532 nm, pulse duration of 3.5 ns, maximum output energy of 340 mJ) was used to irradiate TiN (purity 99.9%) target and TiO2 (99.99%) target. Their radiated area was kept at 2.8 mm2. The laser energy density (Ed) was fixed at 3.8 Jycm2. The Si (100) and SiO2 (Corning 7059) substrates were located at 6.0 cm from the target. The targets were rotated at 20 rev.ymin to avoid pitting during deposition. The substrates were cleaned using an ultrasonic agitator by repeated bathing in ethanol and then rinsed in highpurity deionized water prior to loading into the deposition chamber. The substrates were heated to 400 8C by an IR lamp. After 36 000 laser shots at a 10 Hz repetition rate, the deposition process was completed. The crystalline structure and crystallographic orientation of the thin films were characterized by X-ray diffraction (XRD; RIGAKU RINT2100V) using a CuKa radiation. The composition of the films was measured by X-ray photoelectron spectroscopy (XPS; JEOL JPS9010). The surface morphology of the films on Si (100) substrates was observed by an atomic force microscope (AFM; JOEL JSPM4210). The UV–VIS absorption spectra of the films were measured using a spectrophotometer (UV–VIS; Hitachi U-3300). Film thickness was measured by a-step (Shimadsu; Dektak). The photocatalytic properties of the prepared TiO2 and TiO2yxNx films were tested by the measurement of
Fig. 2 shows the photograph of the films prepared by PLD method as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. Fig. 2a shows the TiO2 film prepared by PLD method using TiO2 target. As shown in this figure, the film looks highly transparent. Fig. 2b–f shows the TiO2yxNx thin films prepared using a TiN target in nitrogenyoxygen gas mixture. As shown in Fig. 2b–d, the color of the film prepared in low nitrogen concentration ratio in nitrogenyoxygen gas mixture (N2 yO2F9) is achromatous glaucous-yellow. With increasing the nitrogen concentration ratio in nitrogenyoxygen gas mixture, the film color changes from transparent to dark brown as shown in Fig. 2e–f. This tendency may be due to the concentration rate of the nitrogen atom in the TiO2yxNx films, and the films change from TiO2 (transparent) to TiN (dark brown). Fig. 3 shows the surface morphology of the TiO2yxNx thin film measured by using AFM. The TiO2yxNx film prepared on Si (100) substrate in nitrogenyoxygen gas mixture (N2:O2s1:1) was uniform and the mean roughness of the film was lower than 10 nm. The micrograph of the TiO2yxNx thin film reveals that the nanocrystalline nanoporous structure of the film has a wide surface area. Spherical particles, with average
Fig. 2. Photograph of the films prepared by PLD method as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. (a) N2:O2s0:1, TiO2 target, (b) N2:O2 s0:1, TiN target, (c) N2:O2s 1:1, TiN target, (d) N2:O2s9:1, TiN target, (e) N2:O2s19:1, TiN target, (f) N2:O2s1:0, TiN target (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
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Fig. 4. XRD patterns of the films prepared using TiN target as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s19:1, (d) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s10 Pa, Tss400 8C).
Fig. 3. Surface morphology of the TiO2yx Nx thin film using TiN target. (N2:O2s1:1, Eds3.8 Jycm2, PsPO2qPN2s10 Pa, Tss400 8C).
particle size ranging ;10 nm can be observed from this figure. The film thickness measured by a-step was approximately 550–750 nm. The deposition rate was approximately 9.2–12.5 nmymin. XRD measurements were carried out to study the crystalline properties of the prepared TiO2yxNx thin films. Fig. 4 shows XRD patterns of the films as a function of nitrogen concentration ratio in nitrogeny oxygen gas mixture. In this experiment, the substrate temperature (Ts) was 400 8C and the total gas pressure was 10 Pa. The distinct reflex of TiO2 (101) can be observed on the films prepared in the oxygen gas (N2:O2s0:1) (Fig. 4a) and nitrogenyoxygen gas mixture (N2:O2s1:1) (Fig. 4b). Besides the reflex of TiO2 (101), the weak reflexes of TiN (200) and TiN (220) appeared in the high nitrogen concentration gas mixture (N2:O2s19:1) (Fig. 4c). The TiO2 (101) reflex belongs to the anatase phase, and the reflex intensity is independent of the nitrogen concentration ratio in nitrogeny oxygen gas mixture at Tss400 8C. As shown in Fig. 4d, the TiO2 (101) reflex disappeared and two distinct reflexs of TiN (200) and TiN (220) can be observed on the film synthesized in the nitrogen gas (N2:O2s1:0). These results suggest that crystalline structure in the film prepared by PLD method using TiN target strongly depends on the nitrogen concentration ratio in nitrogeny oxygen gas mixture. The XPS spectra of Ti 2p peaks of the film after 60 s sputtering are shown in Fig. 5. The films were etched by the sputtering with Arq ion bombardment accelerated
by 600 V. The XPS spectrum of the film prepared in the oxygen gas (N2:O2s0:1) is shown in Fig. 5a. The Ti 2p spectrum shows a doublet structure with slight asymmetry. The binding energies of Ti 2p transition corresponding to valence states obtained from the deconvoluted spectra w9,24–27x. The spectrum shows the presence of one large peak of Ti 2p incorporated with TiO2(460 eV), and small peaks of TiN(457 eV) and TiN(463 eV) w26,27x. The binding energy of approximately 460 eV is 1.7 eV higher than that of Ti state in TiO2(458.3 eV), which may be due to the incorporation of N into TiO2. With increasing the nitrogen concentration ratio (spectra (b)–(d)), the Ti 2p spectra became broad and more asymmetric. Fig. 5e shows the XPS spectrum of the film prepared using TiN target in nitrogen gas (N2:O2s1:0). The spectrum shows the
Fig. 5. XPS spectra of Ti 2p peaks of the TiO2yxNx films as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s9:1, (d) N2:O2s19:1, (e) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
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Fig. 7. UV–VIS absorption spectra of the TiO2 and TiO2yxNx thin films. (a) N2:O2s0:1, TiO2 target, (b) N2:O2s0:1, TiN target, (c) N2:O2s1:1, TiN target, (d) N2:O2 s9:1, TiN target, (e) N2:O2s19:1, TiN target, (f) N2:O2s1:0, TiN target (Eds3.8 Jycm2, PsPO2q PN2s10 Pa, Tss400 8C).
Fig. 6. XPS spectra of N 1s peaks of the TiO2yx Nx films as a function of the nitrogen concentration ratio in nitrogenyoxygen gas mixture using TiN target. (a) N2:O2s0:1, (b) N2:O2s1:1, (c) N2:O2s9:1, (d) N2:O2s19:1, (e) N2:O2s1:0 (Eds3.8 Jycm2, PsPO2qPN2s 10 Pa, Tss400 8C).
presence of two Ti 2p peaks of TiN(457 eV) and TiN(463 eV) w24–27x. The XPS spectra of N 1s peaks of the film after 60 s Arq ion sputtering are shown in Fig. 6. The XPS spectrum of the film prepared in the oxygen gas (N2:O2s0:1) is shown in Fig. 6a. The spectrum shows the presence of two peaks of N 1s. The binding energy of approximately 400 eV may be due to incorporation of N–O into TiO2yxNx. However, the binding energy of approximately 396 eV may be due to incorporation of Ti–N w25,26x. The peak of approximately 400 eV is slightly stronger than that of approximately 396 eV on the film prepared in the oxygen gas (N2:O2s0:1). With increasing the nitrogen concentration ratio in the gas mixture (spectra (b)–(d)), the N 1s peak approximately 396 eV increased. The spectrum of the film prepared in the nitrogen gas (N2:O2s1:0) shows the presence of strong peak at approximately 396 eV (Fig. 6e). These results also suggest that the nitrogen composition rate of the film depends on the nitrogen gas concentration ratio during the deposition. Based on XRD and XPS results, it was found that the TiO2yxNx films prepared using TiN target in nitrogenyoxygen gas mixture have anatase crystalline structure with nitrogen doped into substitutional sites of oxygen in TiO2. Fig. 7 shows the UV–VIS absorption spectra of the TiO2 and TiO2yxNx thin films prepared using PLD method. The absorption of the TiO2 film prepared using a TiO2 target in oxygen gas (N2:O2s0:1) shows a shoulder near the 400 nm and approaches zero at approximately 320 nm. For the nitrogen doped films, the absorption edge shifts toward the visible light regions
depending on the amount of nitrogen concentration ratio in the gas mixture. The absorption of the TiO2yxNx film prepared in nitrogenyoxygen gas mixture (N2:O2s9:1) shows that an edge of absorption approaches near zero at approximately 360 nm. However, the absorption of the film prepared in the high nitrogen concentration gas mixture (N2:O2s19:1) and nitrogen gas (N2:O2s1:0) drastically decreased. Fig. 8 shows the time-dependence of the transmittance of an aqueous methylene blue solution (0.05 mmolyl) for the TiO2 and TiO2yxNx films (1.0 cm2). The TiO2 film was deposited using a TiO2 target at Tss400 8C in the oxygen gas at 10 Pa and the TiO2yxNx film was deposited using a TiN target at Tss400 8C in nitrogeny oxygen gas mixture (N2:O2s1:1). It is clearly seen that these films under UV light irradiation (362 nm, 50 mWycm2) exhibit almost the same decomposition ability of methylene blue. The photocatalytic activity of TiO2yxNx film is higher than that of TiO2 film under normal fluorescent light (540–620 nm, 120 mWycm2) irradiation. 4. Conclusions TiO2yxNx thin films have been prepared by PLD method using a TiN target in nitrogenyoxygen gas
Fig. 8. The temporal evolutions of the transmittance of an aqueous methylene blue solution (0.05 mmolyl) for the TiO2 and TiO2yxNx films (1.0 cm2).
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mixture. The color of the film changed from TiO2(transparent) to TiN(dark brown) with increasing the nitrogen concentration ratio. The crystalline structure of the films prepared strongly depends on the nitrogen concentration ratio in the gas mixture and the target material. The anatase type TiO2 crystalline structure can be observed independent of the nitrogen concentration ratio in nitrogenyoxygen gas mixture. Also, XPS analyses for Ti 2p and N 1s peaks of the films suggest that the composition of the film depends on the nitrogen concentration ratio in the gas mixture. When the nitrogen concentration ratio increased, the film structure changed from TiO2 to TiN. Based on these results, it was found that the TiO2yxNx films have anatase crystalline structure with nitrogen doped into TiO2 oxygen sites, which leads to band gap narrowing. It is also confirmed that the edges of the absorption of the films shifted from 320 nm for TiO2 film to 360 nm for the TiO2yxNx film. In photocatalytic tests using methylene blue solution, the TiO2yxNx film is activated by normal fluorescent light (visible light) irradiation as well as UV light irradiation. Acknowledgments This work was supported in part by the Grant-in-Aid for Young Scientists (A), Grant-in-Aid for Scientific Research on Priority Areas (417), Grant-in-Aid for Scientific Research (B) and a Research Fund from Nagasaki Super Technology Development Association. The authors wish to thank Drs K. Ebihara and T. Ikegami of Kumamoto University for their helpful discussions. The authors also wish to thank Dr H. Abe and Mr H. Yoshida of the Ceramic Research Center of Nagasaki for their technical assistance with the experimental data. References w1x E. Borgarello, J. Kiwi, M. Gratzel, B. Pelizzetti, M. Visca, J. Am. Chem. Soc. 104 (1982) 2996.
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