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Characteristics of N-doped titanium oxide prepared by the large scaled DC reactive magnetron sputtering technique
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Separation and Purification Technology 58 (2007) 200–205
Characteristics of N-doped titanium oxide prepared by the large scaled DC reactive magnetron sputtering technique Sang-Won Park ∗ , Jae-Eun Heo Department of Energy Environmental Science and Engineering, Keimyung University, 1000 Shindang-Dong, Dalseo-Gu, Daegu 704-701, Republic of Korea
Abstract N-doped TiO2 and TiO2 thin films were prepared by the DC reactive magnetron sputtering technique. Since sputtering system adopted in this experiment was specially designed to use the target with size of 200 mm × 500 mm, this system was made easy for practical application. It was observed that N2 addition reduced the hysteresis of the discharge voltage during sputtering of TiO2 onto substrate. Particle sizes doped on the glass ˚ while surface were found to be 50–60 nm with cylindrical structures. Average surface roughness of N-doped TiO2 film was measured to be 15.9 A, ˚ The binding energy of N 1s was measured at 398.9 and 394.9 eV by XPS. The XRD analysis represented that the films that of TiO2 to be 13.2 A. were mainly composed of anatase phase. Rutile phase was also observed in thin films annealed at above 400 ◦ C. Red shift was observed under the given conditions. It seemed that added nitrogen and mixed crystal structure improved the photoactivity of TiO2 films in the range of visible light. © 2007 Published by Elsevier B.V. Keywords: Photocatalysis; N-doped TiO2 thin film; Reactive magnetron sputtering; Glow discharge
1. Introduction Since Fujishima and Honda [1] studied the generation of hydrogen from the photocatalytic cleavage of water using TiO2 electrodes in 1972, TiO2 has attracted extensive interests for studies of semiconductor and catalyst. Especially, TiO2 has been used the standard material of photocatalyst because it is convenience of manufacture, long-term stability, non-toxicity, and easy reforming [2]. During last decade, more than 3000 papers were published on environmental photocatalysis. However, TiO2 is not only low photoefficiency under visible light irradiation but difficult commercial application due to the limit of powder type. Earlier studies suggested that the red shift was possible by the following techniques: various doped transition metals, added sensitizing dye, or doped typical elements, etc. [3–6]. In 2001, Asahi et al. [7] characterized an N-doped TiO2 film by the reactive sputtering method, and reported that the N-doped TiO2 film had a high photoefficiency at the visible range. TiO2 thin films can be prepared by several different methods such as reactive sputtering, chemical vapor deposition (CVD), sol–gel technique, etc. Among these, the reactive sputtering method is commercially preferred because of high deposition
∗
Corresponding author. Tel.: +82 535805212; fax: +82 535805164. E-mail address: [email protected] (S.-W. Park).
1383-5866/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.seppur.2007.07.033
rate at lower temperatures. This approach shows a good adhesion with substrate and easier control of atomic composition in thin films. Moreover, since mass production with large substrate is possible using the reactive sputtering technique, it is widely adopted to make a transparent conducting glass plate in a wideviewing display field, or in a protective coating of light reflection [8–10]. However, most of sputtering systems used in experiments for research purposes are limited to adopt the target size with lower than 100 mm of diameter. It is still necessary to use large scaled DC magnetron system for the practical application. The mechanism of the reactive sputtering can be explained by various factors such as deposition rates, target poisoning, pressure of the reactive gas, etc. It is important to investigate characteristics of the discharge voltage because they are greatly influenced by the pressure of the reactive gas. These characteristics provide valuable information in an attempt to prepare evenly thin film especially with large scale. Even though many studies are being actively made in characterizing thin film of visible photocatalysts, more efforts are still needed to elucidate the relationships between the band gap and the relevant factors such as the discharge voltage, the surface shape, crystal structure, etc. The size of the target used in this study was 200 mm × 500 mm which can be immediately applied to commercial scale. Efforts were made to investigate characteristics of TiO2 thin film as affected by the discharge voltage, surface shape, crystal
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Fig. 1. Schematic diagram of DC reactive magnetron sputtering equipment.
structure, etc. Efforts were also made to study the feasibility of photodegradation of dyes in the aqueous phase. 2. Experimental Thin films of TiO2 and N-doped TiO2 were deposited on substrates with the DC magnetron sputtering method at the room temperature. Fig. 1 shows the schematic diagram of large scaled equipments. The sputtering was carried out for 2 h using 1.2–5.8 kW power range with 99.995% pure Ti as a target. Gases were 99.99% pure Ar, O2 and N2 , and flow rates of these gases were separately controlled by mass flow controllers. 100 mm × 300 mm glass substrate and 20 mm × 20 mm quartz substrate were cleaned with isopropyl alcohol prior to use, and then they were positioned in a substrate holder of the sputtering machine. After an initial vacuum was below than ca. 2 × 10−5 Torr, the thin films were prepared at ca. 0.7–1.0 mTorr with 90 sccm of total gas flow during a rotation of substrate. The flow rates of reactive gases, O2 and N2 , were 30 and 60 sccm each, and that of N-dope TiO2 was Ar/N2 = 1. To observe the discharge voltage characteristics for thin films, the flow rate of O2 gas have been varied between 0 and 90 sccm. Thicknesses of prepared TiO2 and N-doped TiO2 films were measured as ca. 200 and 300 nm, respectively. The surface of the target material was cleaned using 5 min pre-sputtering. The prepared film was then annealed in nitrogen condition for 4 h on 300–500 ◦ C. Physical properties of thin films were measured using various instruments, i.e., ␣-step for film thickness, FESEM for the surface structure, atomic force microscope (AFM) for the surface roughness, X-ray diffraction (XRD) for crystallinity, X-ray photoelectron spectroscopy (XPS) for the surface composition and Varian Cary 500 scanning UV–vis spectrometer for light absorption. Quartz substrate, having transmittance above than 90% in the spectrum range of 250–800 nm, was used especially for the measurement of light absorption. XRD data of the thin films was determined by a MAC Science Mx-
Labo diffractometer using a Cu K␣ radiation at a scanning speed of 0.5◦ /s. XPS data was corrected with was corrected with C 1s bonding energy at 284.6 eV by a VG Microtech ESCA2000. The photocatalytic behaviour under UV and visible was performed by degradation of the dye: Suncion Yellow (SY) with a batch reactor. The initial concentration of SY was 10 mg/L and the remnant concentration of SY was then analyzed using a UV–vis spectrophotometer. Two 50 W interior light sources, which were connected in series, were used to provide visible light source. A film was fixed horizontally in the middle of the solution and the solution was stirred for cooling during the experiment. 3. Results and discussion 3.1. Discharge characteristics The characteristics of glow discharge can give a direct indication of the effect of oxygen content in the system [11]. Changes of discharge voltage as a function of O2 flow rate at several different powers of 1.2, 2.9, and 5.8 kW were shown in Fig. 2 for TiO2 thin films. Total amount of gas flow, the sum of Ar and O2 , was 90 sccm and the pressure under the Ar gas environment was about 1 mTorr. For the forward process of increasing O2 flow at 1.2 kW, the discharge voltage showed the highest peak at 20 sccm of O2 flow (the critical oxygen pressure) and, then, slowly decreased to 40 sccm. Above 40 sccm, it increased again gradually. The critical oxygen pressure was shown due to formation of TiO2 film on the target surface by the sputtered O2 particle. As the power increased to 2.9 and 5.8 kW, each critical pressure each increased at 30 and 40 sccm. Characteristics of discharge voltage were explained well by model of Mohan Rao and Mohan [11]. Fig. 2 shows the hysteresis phenomenon. That is, for the backward process of decreasing O2 flow, the peaks were shown at 10 sccm of O2 flow with
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Fig. 2. Discharge voltage characteristics for TiO2 thin film prepared with several different powers (total flow of Ar and O2 : 90 sccm): (a) 1.2 kW, (b) 2.9 kW and (c) 5.8 kW.
1.2 kW and at 20 sccm with both 2.9 and 5.8 kW. It seemed to occur due to the residual O2 on the target surface. During the forward process, part of sputtered O2 poisoned the surface [11]. On the other hand, changes of discharge voltage as a function of O2 flow rate at several different powers of 1.2, 2.9, and 5.8 kW were shown in Fig. 3 for N-doped TiO2 thin films. In addition to above described conditions for TiO2 , N2 /Ar = 1 ratio was set
Fig. 3. Discharge voltage characteristics for N-doped TiO2 film prepared with several different powers (total flow of Ar, O2 and N2 : 90 sccm): (a) 1.2 kW, (b) 2.9 kW and (c) 5.8 kW.
to be constant. In contrast to TiO2 , any hysteresis phenomenon was not observed up to 2.9 kW of applied power. For N-doped TiO2 , it appeared that the hysteresis was still observed at the high power of 5.9 kW. Added N2 inhibited poisoning the target surface, and it made discharge voltages stable. It has advantages to be able to control the electronic and/or optical characteristics of thin films.
Fig. 4. Surface and cross-sectional FE-SEM images of TiO2 and TiO–N films annealed with 5.9 kW power: (a) TiO2 and (b) N-doped TiO2 .
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Fig. 5. AFM images of TiO2 and N-doped TiO2 thin films prepared with 2.9 kW power: (a) TiO2 and (b) N-doped TiO2 .
3.2. Surface characteristics Cross-sections and surfaces of TiO2 and N-doped TiO2 thin films prepared with 5.8 kW at 300 ◦ C, of which thickness was about 300 nm, were also observed by FE-SEM as shown in Fig. 4. Columnar structures for both thin films were observed, which were tiny, dense, and vertical. Particle sizes of thin film deposited on glass were about 50–60 nm. Moreover, as shown in Fig. 5 of AFM data, average surface roughness of N-doped TiO2 film ˚ while that of TiO2 to be 13.2 A. ˚ was measured to be 15.9 A, The hysteresis could be reduced by the suppression of the target poisoning, inducing the sputtered particle to carry the high kinetic energies. This interpretation could partially explain the difference of the surface roughness of TiO2 with and without N. The N composition of N-doped TiO2 was identified with XPS. Fig. 6 shows XPS spectra for N 1s and Ti 2p core levels of N-doped TiO2 thin film at 500 ◦ C. As shown in Fig. 6(a), two peaks of N 1s were observed at 398.9 and 394.9 eV. The peak at 398.9 eV (approximately 400 eV) indicates N–O bonding, and the peak at 394.9 eV (approximately 396 eV) assigns -N [12–16]. It was induced by the substitution of N instead of O in the TiO2 structure. Fig. 6(b) shows the spin-orbit doublet of Ti 2p at 457.2 (2p3/2 ) and 463.1 eV (2p1/2 ) [16–18]. Fig. 7 shows the XRD patterns of TiO2 and N-doped TiO2 thin films prepared with power of 5.8 kW at different temperatures. The XRD analysis represents that the films were mainly composed of anatase phase. Rutile phase was observed in thin films annealed at above 400 ◦ C. Observed peaks were more distinct in the case of TiO2 than N-doped TiO2 , while any metallic Ti and TiN phase were not observed in N-doped TiO2 film.
Fig. 6. XPS spectra of N-doped TiO2 film prepared at 500 ◦ C: (a) N 1s and (b) Ti 2p spectrum.
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Fig. 9. Degradation of SY with TiO2 and N-doped TiO2 thin films under visible light irradiation. Fig. 7. XRD patterns of N-doped TiO2 and TiO2 thin films prepared with 5.8 kW power and various annealing temperature: (a) N-doped TiO2 300 ◦ C, (b) N-doped TiO2 400 ◦ C, (c) N-doped TiO2 500 ◦ C and (d) TiO2 500 ◦ C.
These results were well supported by early studies [19–21]. Suhail et al. [19] reported that thin films made by DC magnetron sputtering with the pressure of 3 × 10−2 Torr (O2 partial pressure: 4 × 10−4 ∼ 2 × 10−5 Torr) at 300 ◦ C showed the anatase structure, while those films at 400 ◦ C exhibited a rutile crystal structure in addition to the anatase phase. Schiller et al. [20] found the mixed structures of anatase and rutile phase in the thin films made at the substrate temperature of 25–500 ◦ C. Pawlewicz and Busch [21] also found the mixed structures at temperature range of 200–500 ◦ C. There are many conditions which can affect these structures of anatase and rutile phases, which are two main crystal structures of TiO2 . These are the process pressure, the ratio of O2 partial pressure, the applied power, and the substrate temperature. Among them, the most important one is the substrate temperature which is a key factor influencing the crystal structure. In most cases, anatase crystal phase starts to form at about 300 ◦ C. Almost all the rutile phase exists above
than 600 ◦ C, while the mixed anatase with rutile phases exist in temperature range of 300–600 ◦ C [19]. Besides, the presence of nitrogen neither influences on the transformation of anatase to rutile, nor creates new crystal phases [22]. 3.3. Photocatalytic characteristics The optical characteristic of thin films, prepared on quartz substrate with power of 5.8 kW at 500 ◦ C, were represented by UV–vis spectra, as shown in Fig. 8. It shows that the absorption edge of the N-doped or undoped TiO2 shifted continuously toward the visible range. Earlier studies [23–27] reported the red shift of absorption edge for N-doped TiO2 , and the maximum shift was about 550 nm. Fig. 9 shows the remnant concentration of SY with N-doped TiO2 is lower than that with TiO2 film. In Fig. 9, the amount of SY removal in the presence of N-doped TiO2 was more significant than that of TiO2 . Although the red shift in Fig. 8 was not pronounced as much as those reported by others, the results shown in Fig. 9 still indicated that the addition of N2 could enhance the photoactivity in the visible range. Fig. 9 also shows the effect of the annealing temperature on the photoactivity as related to the crystal structure. That is, the removal efficiency with N-doped at 300 ◦ C is similar to that with nonedoped at 500 ◦ C. However, the photoactivity increased at 400 ◦ C which is the temperature starting to form the rutile structure. The assumption that the added N2 could suppress recombination of electron-hole of rutile may not exclusively explain the increase of the photoactivity [28]. Data shown in Fig. 9 agree with those reported by Asahi et al. [7]. 4. Conclusions
Fig. 8. Optical absorption spectra of TiO2 and N-doped TiO2 thin films annealed at 500 ◦ C.
N-doped TiO2 and TiO2 thin films were prepared by the reactive sputtering method on the quartz and the soda lime glass plates. Especially glass plate used in this study was larger than other researcher’s substrates. As the power increased up to 1.2 and 5.8 kW, maximum discharge voltage was also appeared at increased O2 flow in the forward cycle, while the maximum was
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shown at lowered O2 flow in the backward cycle. This hysteresis was considered as the results of the residual O2 on the target surface. Cylindrical structures to the surface were observed as tiny and dense particles in TiO2 and N-doped TiO2 films. Average surface roughness of TiO–N film was bigger than that of TiO2 due to the kinetic energy of sputtering particles. The XPS spectra for N 1s represented that N composition substituted for O in the TiO2 structure. The TiO–N film was also observed both anatase and rutile phases at above 400 ◦ C from XRD. N-doped TiO2 thin film showed photocatalytic activity under visible range. Farther studies should be made to enhance the photoactivity in the visible range if cheaper and larger scaled substrates are needed for commercial application. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] A.K. Ghosh, H.P. Maruska, J. Electrochem. Soc. 124 (1997) 1516. [4] W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669. [5] G.A. Epling, C. Lin, Chemosphere 46 (2002) 561. [6] T. Ohno, M. Akiyoshi, T. Umebayahi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A: Gen. 265 (2004) 115. [7] R. Asahi, T. Mohikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269.
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[8] J.M. Bennett, et al., Appl. Opt. 28 (1989) 3303. [9] H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 (1994) 2042. [10] A.J. Perry, H.K. Pulker, Thin Solid Films 124 (1985) 323. [11] G. Mohan Rao, S. Mohan, J. Appl. Phys. 69 (1991) 6652. [12] N.C. Saha, H.G. Tompkins, J. Appl. Phys. 72 (1992) 3072. [13] Y. Suda, H. Kawasaki, T. Ueda, T. Ohshima, Thin Solid Films 453 (2004) 162. [14] L. Miao, S. Tanemura, H. Watanabe, Y. Mori, K. Kaneko, S. Toh, J. Cryst. Growth 260 (2004) 118. [15] P. Xu, L. Mi, P.N. Wang, J. Cryst. Growth 289 (2006) 118. [16] L. Wan, J.F. Li, J.Y. Feng, W. Sun, Z.Q. Mao, Appl. Surf. Sci. 253 (2007) 4764. [17] S. Schiller, G. Beister, S. Schneider, W. Sieber, Thin Solid Films 72 (1980) 475. [18] P.W. Shum, Z.F. Zhou, K.Y. Li, Y.G. Shen, Mater. Sci. Eng. B 100 (2003) 204. [19] M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 (1992) 1421. [20] S. Schiller, G. Beister, W. Sieber, Thin Solid Films 38 (1976) 271. [21] W.T. Pawlewicz, R. Busch, Thin Solid Films 63 (1979) 251. [22] S.F. Chen, X.Q. Liu, Y.Z. Liu, G.Y. Cao, Appl. Surf. Sci. 253 (2007) 3077. [23] M. Masahiko, W. Teruyoshi, J. Electrochem. Soc. 153 (2006) 186. [24] P. Xu, L. Mi, P.N. Wang, J. Cryst. Growth 289 (2006) 433. [25] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483. [26] T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, Jpn. J. Appl. Phys. 40 (2001) L561. [27] H.M. Yates, M.G. Nolan, D.W. Sheel, M.E. Pemble, J. Photochem. Photobiol. A 179 (2006) 213. [28] A.R. Gandhe, J.B. Fernandes, J. Solid State Chem. 178 (2005) 2953.
Separation and Purification Technology 58 (2007) 200–205
Characteristics of N-doped titanium oxide prepared by the large scaled DC reactive magnetron sputtering technique Sang-Won Park ∗ , Jae-Eun Heo Department of Energy Environmental Science and Engineering, Keimyung University, 1000 Shindang-Dong, Dalseo-Gu, Daegu 704-701, Republic of Korea
Abstract N-doped TiO2 and TiO2 thin films were prepared by the DC reactive magnetron sputtering technique. Since sputtering system adopted in this experiment was specially designed to use the target with size of 200 mm × 500 mm, this system was made easy for practical application. It was observed that N2 addition reduced the hysteresis of the discharge voltage during sputtering of TiO2 onto substrate. Particle sizes doped on the glass ˚ while surface were found to be 50–60 nm with cylindrical structures. Average surface roughness of N-doped TiO2 film was measured to be 15.9 A, ˚ The binding energy of N 1s was measured at 398.9 and 394.9 eV by XPS. The XRD analysis represented that the films that of TiO2 to be 13.2 A. were mainly composed of anatase phase. Rutile phase was also observed in thin films annealed at above 400 ◦ C. Red shift was observed under the given conditions. It seemed that added nitrogen and mixed crystal structure improved the photoactivity of TiO2 films in the range of visible light. © 2007 Published by Elsevier B.V. Keywords: Photocatalysis; N-doped TiO2 thin film; Reactive magnetron sputtering; Glow discharge
1. Introduction Since Fujishima and Honda [1] studied the generation of hydrogen from the photocatalytic cleavage of water using TiO2 electrodes in 1972, TiO2 has attracted extensive interests for studies of semiconductor and catalyst. Especially, TiO2 has been used the standard material of photocatalyst because it is convenience of manufacture, long-term stability, non-toxicity, and easy reforming [2]. During last decade, more than 3000 papers were published on environmental photocatalysis. However, TiO2 is not only low photoefficiency under visible light irradiation but difficult commercial application due to the limit of powder type. Earlier studies suggested that the red shift was possible by the following techniques: various doped transition metals, added sensitizing dye, or doped typical elements, etc. [3–6]. In 2001, Asahi et al. [7] characterized an N-doped TiO2 film by the reactive sputtering method, and reported that the N-doped TiO2 film had a high photoefficiency at the visible range. TiO2 thin films can be prepared by several different methods such as reactive sputtering, chemical vapor deposition (CVD), sol–gel technique, etc. Among these, the reactive sputtering method is commercially preferred because of high deposition
∗
Corresponding author. Tel.: +82 535805212; fax: +82 535805164. E-mail address: [email protected] (S.-W. Park).
1383-5866/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.seppur.2007.07.033
rate at lower temperatures. This approach shows a good adhesion with substrate and easier control of atomic composition in thin films. Moreover, since mass production with large substrate is possible using the reactive sputtering technique, it is widely adopted to make a transparent conducting glass plate in a wideviewing display field, or in a protective coating of light reflection [8–10]. However, most of sputtering systems used in experiments for research purposes are limited to adopt the target size with lower than 100 mm of diameter. It is still necessary to use large scaled DC magnetron system for the practical application. The mechanism of the reactive sputtering can be explained by various factors such as deposition rates, target poisoning, pressure of the reactive gas, etc. It is important to investigate characteristics of the discharge voltage because they are greatly influenced by the pressure of the reactive gas. These characteristics provide valuable information in an attempt to prepare evenly thin film especially with large scale. Even though many studies are being actively made in characterizing thin film of visible photocatalysts, more efforts are still needed to elucidate the relationships between the band gap and the relevant factors such as the discharge voltage, the surface shape, crystal structure, etc. The size of the target used in this study was 200 mm × 500 mm which can be immediately applied to commercial scale. Efforts were made to investigate characteristics of TiO2 thin film as affected by the discharge voltage, surface shape, crystal
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Fig. 1. Schematic diagram of DC reactive magnetron sputtering equipment.
structure, etc. Efforts were also made to study the feasibility of photodegradation of dyes in the aqueous phase. 2. Experimental Thin films of TiO2 and N-doped TiO2 were deposited on substrates with the DC magnetron sputtering method at the room temperature. Fig. 1 shows the schematic diagram of large scaled equipments. The sputtering was carried out for 2 h using 1.2–5.8 kW power range with 99.995% pure Ti as a target. Gases were 99.99% pure Ar, O2 and N2 , and flow rates of these gases were separately controlled by mass flow controllers. 100 mm × 300 mm glass substrate and 20 mm × 20 mm quartz substrate were cleaned with isopropyl alcohol prior to use, and then they were positioned in a substrate holder of the sputtering machine. After an initial vacuum was below than ca. 2 × 10−5 Torr, the thin films were prepared at ca. 0.7–1.0 mTorr with 90 sccm of total gas flow during a rotation of substrate. The flow rates of reactive gases, O2 and N2 , were 30 and 60 sccm each, and that of N-dope TiO2 was Ar/N2 = 1. To observe the discharge voltage characteristics for thin films, the flow rate of O2 gas have been varied between 0 and 90 sccm. Thicknesses of prepared TiO2 and N-doped TiO2 films were measured as ca. 200 and 300 nm, respectively. The surface of the target material was cleaned using 5 min pre-sputtering. The prepared film was then annealed in nitrogen condition for 4 h on 300–500 ◦ C. Physical properties of thin films were measured using various instruments, i.e., ␣-step for film thickness, FESEM for the surface structure, atomic force microscope (AFM) for the surface roughness, X-ray diffraction (XRD) for crystallinity, X-ray photoelectron spectroscopy (XPS) for the surface composition and Varian Cary 500 scanning UV–vis spectrometer for light absorption. Quartz substrate, having transmittance above than 90% in the spectrum range of 250–800 nm, was used especially for the measurement of light absorption. XRD data of the thin films was determined by a MAC Science Mx-
Labo diffractometer using a Cu K␣ radiation at a scanning speed of 0.5◦ /s. XPS data was corrected with was corrected with C 1s bonding energy at 284.6 eV by a VG Microtech ESCA2000. The photocatalytic behaviour under UV and visible was performed by degradation of the dye: Suncion Yellow (SY) with a batch reactor. The initial concentration of SY was 10 mg/L and the remnant concentration of SY was then analyzed using a UV–vis spectrophotometer. Two 50 W interior light sources, which were connected in series, were used to provide visible light source. A film was fixed horizontally in the middle of the solution and the solution was stirred for cooling during the experiment. 3. Results and discussion 3.1. Discharge characteristics The characteristics of glow discharge can give a direct indication of the effect of oxygen content in the system [11]. Changes of discharge voltage as a function of O2 flow rate at several different powers of 1.2, 2.9, and 5.8 kW were shown in Fig. 2 for TiO2 thin films. Total amount of gas flow, the sum of Ar and O2 , was 90 sccm and the pressure under the Ar gas environment was about 1 mTorr. For the forward process of increasing O2 flow at 1.2 kW, the discharge voltage showed the highest peak at 20 sccm of O2 flow (the critical oxygen pressure) and, then, slowly decreased to 40 sccm. Above 40 sccm, it increased again gradually. The critical oxygen pressure was shown due to formation of TiO2 film on the target surface by the sputtered O2 particle. As the power increased to 2.9 and 5.8 kW, each critical pressure each increased at 30 and 40 sccm. Characteristics of discharge voltage were explained well by model of Mohan Rao and Mohan [11]. Fig. 2 shows the hysteresis phenomenon. That is, for the backward process of decreasing O2 flow, the peaks were shown at 10 sccm of O2 flow with
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Fig. 2. Discharge voltage characteristics for TiO2 thin film prepared with several different powers (total flow of Ar and O2 : 90 sccm): (a) 1.2 kW, (b) 2.9 kW and (c) 5.8 kW.
1.2 kW and at 20 sccm with both 2.9 and 5.8 kW. It seemed to occur due to the residual O2 on the target surface. During the forward process, part of sputtered O2 poisoned the surface [11]. On the other hand, changes of discharge voltage as a function of O2 flow rate at several different powers of 1.2, 2.9, and 5.8 kW were shown in Fig. 3 for N-doped TiO2 thin films. In addition to above described conditions for TiO2 , N2 /Ar = 1 ratio was set
Fig. 3. Discharge voltage characteristics for N-doped TiO2 film prepared with several different powers (total flow of Ar, O2 and N2 : 90 sccm): (a) 1.2 kW, (b) 2.9 kW and (c) 5.8 kW.
to be constant. In contrast to TiO2 , any hysteresis phenomenon was not observed up to 2.9 kW of applied power. For N-doped TiO2 , it appeared that the hysteresis was still observed at the high power of 5.9 kW. Added N2 inhibited poisoning the target surface, and it made discharge voltages stable. It has advantages to be able to control the electronic and/or optical characteristics of thin films.
Fig. 4. Surface and cross-sectional FE-SEM images of TiO2 and TiO–N films annealed with 5.9 kW power: (a) TiO2 and (b) N-doped TiO2 .
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Fig. 5. AFM images of TiO2 and N-doped TiO2 thin films prepared with 2.9 kW power: (a) TiO2 and (b) N-doped TiO2 .
3.2. Surface characteristics Cross-sections and surfaces of TiO2 and N-doped TiO2 thin films prepared with 5.8 kW at 300 ◦ C, of which thickness was about 300 nm, were also observed by FE-SEM as shown in Fig. 4. Columnar structures for both thin films were observed, which were tiny, dense, and vertical. Particle sizes of thin film deposited on glass were about 50–60 nm. Moreover, as shown in Fig. 5 of AFM data, average surface roughness of N-doped TiO2 film ˚ while that of TiO2 to be 13.2 A. ˚ was measured to be 15.9 A, The hysteresis could be reduced by the suppression of the target poisoning, inducing the sputtered particle to carry the high kinetic energies. This interpretation could partially explain the difference of the surface roughness of TiO2 with and without N. The N composition of N-doped TiO2 was identified with XPS. Fig. 6 shows XPS spectra for N 1s and Ti 2p core levels of N-doped TiO2 thin film at 500 ◦ C. As shown in Fig. 6(a), two peaks of N 1s were observed at 398.9 and 394.9 eV. The peak at 398.9 eV (approximately 400 eV) indicates N–O bonding, and the peak at 394.9 eV (approximately 396 eV) assigns -N [12–16]. It was induced by the substitution of N instead of O in the TiO2 structure. Fig. 6(b) shows the spin-orbit doublet of Ti 2p at 457.2 (2p3/2 ) and 463.1 eV (2p1/2 ) [16–18]. Fig. 7 shows the XRD patterns of TiO2 and N-doped TiO2 thin films prepared with power of 5.8 kW at different temperatures. The XRD analysis represents that the films were mainly composed of anatase phase. Rutile phase was observed in thin films annealed at above 400 ◦ C. Observed peaks were more distinct in the case of TiO2 than N-doped TiO2 , while any metallic Ti and TiN phase were not observed in N-doped TiO2 film.
Fig. 6. XPS spectra of N-doped TiO2 film prepared at 500 ◦ C: (a) N 1s and (b) Ti 2p spectrum.
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Fig. 9. Degradation of SY with TiO2 and N-doped TiO2 thin films under visible light irradiation. Fig. 7. XRD patterns of N-doped TiO2 and TiO2 thin films prepared with 5.8 kW power and various annealing temperature: (a) N-doped TiO2 300 ◦ C, (b) N-doped TiO2 400 ◦ C, (c) N-doped TiO2 500 ◦ C and (d) TiO2 500 ◦ C.
These results were well supported by early studies [19–21]. Suhail et al. [19] reported that thin films made by DC magnetron sputtering with the pressure of 3 × 10−2 Torr (O2 partial pressure: 4 × 10−4 ∼ 2 × 10−5 Torr) at 300 ◦ C showed the anatase structure, while those films at 400 ◦ C exhibited a rutile crystal structure in addition to the anatase phase. Schiller et al. [20] found the mixed structures of anatase and rutile phase in the thin films made at the substrate temperature of 25–500 ◦ C. Pawlewicz and Busch [21] also found the mixed structures at temperature range of 200–500 ◦ C. There are many conditions which can affect these structures of anatase and rutile phases, which are two main crystal structures of TiO2 . These are the process pressure, the ratio of O2 partial pressure, the applied power, and the substrate temperature. Among them, the most important one is the substrate temperature which is a key factor influencing the crystal structure. In most cases, anatase crystal phase starts to form at about 300 ◦ C. Almost all the rutile phase exists above
than 600 ◦ C, while the mixed anatase with rutile phases exist in temperature range of 300–600 ◦ C [19]. Besides, the presence of nitrogen neither influences on the transformation of anatase to rutile, nor creates new crystal phases [22]. 3.3. Photocatalytic characteristics The optical characteristic of thin films, prepared on quartz substrate with power of 5.8 kW at 500 ◦ C, were represented by UV–vis spectra, as shown in Fig. 8. It shows that the absorption edge of the N-doped or undoped TiO2 shifted continuously toward the visible range. Earlier studies [23–27] reported the red shift of absorption edge for N-doped TiO2 , and the maximum shift was about 550 nm. Fig. 9 shows the remnant concentration of SY with N-doped TiO2 is lower than that with TiO2 film. In Fig. 9, the amount of SY removal in the presence of N-doped TiO2 was more significant than that of TiO2 . Although the red shift in Fig. 8 was not pronounced as much as those reported by others, the results shown in Fig. 9 still indicated that the addition of N2 could enhance the photoactivity in the visible range. Fig. 9 also shows the effect of the annealing temperature on the photoactivity as related to the crystal structure. That is, the removal efficiency with N-doped at 300 ◦ C is similar to that with nonedoped at 500 ◦ C. However, the photoactivity increased at 400 ◦ C which is the temperature starting to form the rutile structure. The assumption that the added N2 could suppress recombination of electron-hole of rutile may not exclusively explain the increase of the photoactivity [28]. Data shown in Fig. 9 agree with those reported by Asahi et al. [7]. 4. Conclusions
Fig. 8. Optical absorption spectra of TiO2 and N-doped TiO2 thin films annealed at 500 ◦ C.
N-doped TiO2 and TiO2 thin films were prepared by the reactive sputtering method on the quartz and the soda lime glass plates. Especially glass plate used in this study was larger than other researcher’s substrates. As the power increased up to 1.2 and 5.8 kW, maximum discharge voltage was also appeared at increased O2 flow in the forward cycle, while the maximum was
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shown at lowered O2 flow in the backward cycle. This hysteresis was considered as the results of the residual O2 on the target surface. Cylindrical structures to the surface were observed as tiny and dense particles in TiO2 and N-doped TiO2 films. Average surface roughness of TiO–N film was bigger than that of TiO2 due to the kinetic energy of sputtering particles. The XPS spectra for N 1s represented that N composition substituted for O in the TiO2 structure. The TiO–N film was also observed both anatase and rutile phases at above 400 ◦ C from XRD. N-doped TiO2 thin film showed photocatalytic activity under visible range. Farther studies should be made to enhance the photoactivity in the visible range if cheaper and larger scaled substrates are needed for commercial application. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [3] A.K. Ghosh, H.P. Maruska, J. Electrochem. Soc. 124 (1997) 1516. [4] W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669. [5] G.A. Epling, C. Lin, Chemosphere 46 (2002) 561. [6] T. Ohno, M. Akiyoshi, T. Umebayahi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A: Gen. 265 (2004) 115. [7] R. Asahi, T. Mohikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269.
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