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Fabrication and characterization of anatase TiO2 thin film on glass substrate grown by pulsed laser deposition
Solid State Ionics 172 (2004) 105 – 108 www.elsevier.com/locate/ssi
Fabrication and characterization of anatase TiO2 thin film on glass substrate grown by pulsed laser deposition Y. Choi a,*, S. Yamamoto a, T. Umebayashi b, M. Yoshikawa a a b
Department of Materials Development, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan Received 9 November 2003; received in revised form 10 March 2004; accepted 12 March 2004
Abstract Titanium dioxide (TiO2) films were prepared on glass substrates by pulsed laser deposition using a titanium carbide (TiC) target. The effects of substrate temperature on the crystal structures, surface morphologies and chemical states of the thin films were examined by the Xray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. The results of XRD and XPS indicated that TiC film was grown at substrate temperature of 293 K, whereas the anatase TiO2 film was formed at that of 773 K, which had round grains with 11 – 28 nm and surface roughness was 0.5 – 1.1 nm. D 2004 Elsevier B.V. All rights reserved. PACS: 81.15 Fg; 68.55 Jk; 82.80 Pv Keywords: Titanium dioxide; Thin film; Titanium carbide; Pulsed laser deposition
1. Introduction During the recent decade, a photocatalytic application using Titanium dioxide (TiO2) has been received much attention to solve the environmental problems [1– 3]. This oxide has three different crystalline modifications: anatase, rutile and brookite. Among these crystals, anatase TiO2 has attracted a great deal of interests because of its excellent photocatalytic behavior. It is, however, difficult to synthesize the anatase TiO2 thin film because this phase is thermodynamically more unstable than the rutile phase. Therefore, the improvement of the production technique of the anatase TiO2 thin film is of great importance. The anatase TiO2 thin films have been prepared by a number of chemical methods such as spin coating and dip coating of a sol –gel solution [4,5]. In comparison with these methods, the pulsed laser deposition (PLD) has a lot of advantages because the growth conditions can be controlled easily by adjusting the laser wavelength, energy, fluence, ambient pressure of the gas, substrate, target-substrate
* Corresponding author. Tel.: +81-27-346-9444; fax: +81-27-3469687. E-mail address: [email protected] (Y. Choi). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.03.014
distance and the chemical property of the target material [6,7]. The anatase TiO2 powder could be synthesized by oxidative heating of the TiC [8,9] or TiS2 powders [10, 11]. Based on these studies, the authors expect the PLD using the titanium carbide (TiC) target with in-situ heating to provide the anatase TiO2 thin film. In the present study, the anatase TiO2 thin film on the glass substrate was synthesized by the PLD using a TiC target. This paper reports the effects of substrate temperature on the crystal structures, surface morphologies and chemical states of the thin films.
2. Experimental A schematic diagram of the experimental apparatus is shown in Fig. 1. A deposition chamber was evacuated to a base pressure (below 2 10 3 Pa) using a turbo molecular pump with a rotary pump. The gas pressure was varied from the base pressure to 4 Pa by feeding oxygen (O2) gas. Three type films were deposited by a KrF excimer laser (wavelength: 248 nm and pulse length: 22 ns) at TS of 293, 473 and 773 K. The laser energy was 150 mJ/pulse with a repetition of 10 Hz. From the focal spot size of
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Y. Choi et al. / Solid State Ionics 172 (2004) 105–108
Fig. 1. A schematic diagram of pulsed laser deposition (PLD) system for thin film preparation.
1 2 mm2, the laser power density was estimated to be f 3.4 108 W/cm2. Laser beam was incident 45j to the target surface. A sintered TiC disk (99%, B = 5 cm) was used as a target and it was rotated by a motor to prevent degradation of target surface and to achieve smooth homogeneous films during deposition. The target-to-substrate distance was f 7 cm. Film thickness was measured by a surface profiler (Dektak3ST). Thicknesses of three films deposited for 120 min at TS of 293, 473 and 773 K were 130, 112 and 105 nm, respectively. Therefore, the deposition rates of three films were evaluated as f 1.08 (for 293 K), 0.93 (for 473 K ) and f 0.88 (for 773 K) nm/min. The crystalline structures of the films were confirmed by the X-ray diffraction (XRD). The chemical states of the films were examined by X-ray photoelectron spectroscopy (XPS). Surface morphology was observed by atomic force microscopy (AFM, Nanoscope III) at tapping mode with a silicon probe.
Fig. 3 presents XPS spectra for the C 1s and Ti 2p core levels of the films prepared at TS of 293 and 773 K. In the case of the film prepared at TS of 293 K, the C 1s spectrum was composed of two peaks at 279.4 and 285.3 eV. Based on previous studies [12,13], these signals at 279.4 and 285.3 eV were assigned to the C 1s peaks due to the Ti– C and C –C bonds, respectively. The Ti 2p peaks were situated at 457.3, 458.8 and 464.2 eV. The former signal agreed with that of pure TiC [12]. The latter signals at 458.8 and 464.2 eV implied the oxidation of the surface [8,12]. On the other hand, in the case of the film prepared at TS of 773 K, the C 1s spectrum was composed of two peaks at 284.6 and 289.4 eV. These signals were ascribed to the C 1s peak due to the C –C bond (for the 284.6 eV peak) and the carbonate compounds (for the 289.4 eV peak) [14]. The Ti 2p peaks were observed around 459.2 and 464.9 eV. This spectrum corresponded to that of pure TiO2 [12].
3. Results and discussion Fig. 2 shows XRD patterns of the three films prepared by PLD under an oxygen atmosphere of 4 Pa at substrate temperatures (TS) of 293, 473 and 773 K. When the film was grown at TS of 293 K, several peaks appeared around 2h = 35.9j, 41.7j and 60.6j as shown in Fig. 2a. This pattern was in good agreement with that of pure TiC powder. The film fabricated at TS of 473 K did not show any peaks (Fig. 2b), whereas two peaks appeared around 2h = 25.2j and 48.3j in the film grown at TS of 773 K (Fig. 2c). The latter signals corresponded to the one due to the (101) and (200) reflections of anatase TiO2.
Fig. 2. XRD analysis of thin films prepared by PLD under an oxygen pressure of 4 Pa at substrate temperatures of 293 K (a), 473 K (b) and 773 K (c). o: anatase, : TiC, D: sample holder.
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Y. Choi et al. / Solid State Ionics 172 (2004) 105–108
Fig. 3. XPS spectra for the (a) C 1s and (b) Ti 2p of thin films prepared by PLD under an oxygen pressure of 4 Pa at substrate temperatures of 293 and 773 K.
These XRD and XPS results indicated that TiC and anatase TiO2 thin films were formed at TS of 293 and 773 K, respectively. The differences in the chemical properties and crystal structures of thin films are probably caused by the in-situ heating treatment. Because the only TiC crystal was grown at TS of 293 K, the Ti, C and Ti– C ions, molecules, particles, and clusters should be included in the plume during the ablation without the oxide (Ti and O) species (Fig. 4a). This means that the in-situ heating induced the decomposition and oxidation of TiC, resulting in the formation of the oxides. Consequently, the anatase TiO2 thin
107
Fig. 5. AFM images (500 500 nm2, height mode) of (a) TiC and (b) anatase TiO2 films fabricated by PLD. These thin films were prepared at substrate temperatures of (a) 293 and (b) 773 K under an oxygen pressure of 4 Pa.
film would be formed at TS of 773 K by the oxidative heating of TiC (Fig. 4b). This formation mechanism is similar to that of the anatase TiO2 powder, which was prepared by the annealing of the TiC powder in air at 623 K [8]. Fig. 5 shows the AFM images of the (a) TiC and (b) anatase TiO2 thin films. The surface morphologies were different in each film. In the case of the TiC thin film prepared at TS of 293 K, the size of the round grains and the factor of surface roughness, which represented by a root mean square (RMS), were estimated to be 17 –45 nm and 1.4 –2.3 nm over a 500 500 nm2 area, respectively. In the case of the anatase TiO2 film grown at TS of 773 K, the size of the round grains was 11 –28 nm and the surface roughness factor was 0.5 –1.1 nm. The difference in the grain size between the TiC and TiO2 thin films would be due to different agglutinability in each compound. The average surface roughness was decreased with increasing the substrate temperature. The small grain size and restructuring of the particle formation due to the heating would cause smooth surface of the anatase TiO2 thin film.
4. Conclusion Fig. 4. A formation process of (a) TiC and (b) anatase TiO2 thin films prepared by PLD using a TiC target under an oxygen atmosphere of 4 Pa.
Anatase TiO2 thin film on the glass substrate was successfully grown by PLD using a sintered TiC target.
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The XRD and XPS results demonstrated that the TiC and anatase TiO2 thin films were formed at TS of 293 and 773 K, respectively. The anatase TiO2 crystal was grown by the oxidation and decomposition of the deposits composed of the Ti and C atoms. This formation mechanism is similar to that of the anatase TiO2 powder synthesized by the oxidative heating of the TiC powder in air. The anatase TiO2 thin film prepared by PLD using the TiC target had nano-grains with 11 –28 nm, leading to the large surface area. We, therefore, expect this film to show a high photocatalytic activity.
References [1] A. Fujishima, T.N. Rao, D.A. Tryk, Electrochim. Acta 45 (2000) 4683. [2] T. Ibusuki, K. Takeuchi, J. Mol. Catal. 88 (1994) 93.
[3] D. Robert, S. Malato, Sci. Total Environ. 291 (2002) 85. [4] K.R. Patil, S.D. Sathaye, Y.B. Khollam, S.B. Deshpande, N.R. Pawaskar, A.B. Mandale, Mater. Lett. 57 (2003) 1775. [5] N. Negishi, K. Takeuchi, Mater. Lett. 38 (1999) 150. [6] S. Yamamoto, T. Sumita, Sugiharuto, A. Miyashita, H. Naramoto, Thin Solid Films 401 (2001) 88. [7] Y. Choi, S. Yamamoto, H. Abe, H. Itoh, Surf. Sci. 499 (2002) 203. [8] Y. Choi, T. Umebayashi, S. Yamamoto, S. Tanaka, J. Mater. Sci. Lett. 22 (2003) 1209. [9] Y. Choi, T. Umebayashi, M. Yoshikawa, J. Mater. Sci. 39 (2004) 1837. [10] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. [11] T. Umebayashi, T. Yamaki, S. Tanaka, K. Asai, Chem. Lett. 32 (2003) 330. [12] F. Santerre, M.A.E. Khakani, M. Chaker, J.P. Dodelet, Appl. Surf. Sci. 148 (1999) 24. [13] K. Baba, R. Hatada, Surf. Coat. Technol. 136 (2001) 241. [14] A.B. Christie, I. Sutherland, J.M. Walls, Vacuum 31 (1981) 513.
Fabrication and characterization of anatase TiO2 thin film on glass substrate grown by pulsed laser deposition Y. Choi a,*, S. Yamamoto a, T. Umebayashi b, M. Yoshikawa a a b
Department of Materials Development, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan Received 9 November 2003; received in revised form 10 March 2004; accepted 12 March 2004
Abstract Titanium dioxide (TiO2) films were prepared on glass substrates by pulsed laser deposition using a titanium carbide (TiC) target. The effects of substrate temperature on the crystal structures, surface morphologies and chemical states of the thin films were examined by the Xray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. The results of XRD and XPS indicated that TiC film was grown at substrate temperature of 293 K, whereas the anatase TiO2 film was formed at that of 773 K, which had round grains with 11 – 28 nm and surface roughness was 0.5 – 1.1 nm. D 2004 Elsevier B.V. All rights reserved. PACS: 81.15 Fg; 68.55 Jk; 82.80 Pv Keywords: Titanium dioxide; Thin film; Titanium carbide; Pulsed laser deposition
1. Introduction During the recent decade, a photocatalytic application using Titanium dioxide (TiO2) has been received much attention to solve the environmental problems [1– 3]. This oxide has three different crystalline modifications: anatase, rutile and brookite. Among these crystals, anatase TiO2 has attracted a great deal of interests because of its excellent photocatalytic behavior. It is, however, difficult to synthesize the anatase TiO2 thin film because this phase is thermodynamically more unstable than the rutile phase. Therefore, the improvement of the production technique of the anatase TiO2 thin film is of great importance. The anatase TiO2 thin films have been prepared by a number of chemical methods such as spin coating and dip coating of a sol –gel solution [4,5]. In comparison with these methods, the pulsed laser deposition (PLD) has a lot of advantages because the growth conditions can be controlled easily by adjusting the laser wavelength, energy, fluence, ambient pressure of the gas, substrate, target-substrate
* Corresponding author. Tel.: +81-27-346-9444; fax: +81-27-3469687. E-mail address: [email protected] (Y. Choi). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.03.014
distance and the chemical property of the target material [6,7]. The anatase TiO2 powder could be synthesized by oxidative heating of the TiC [8,9] or TiS2 powders [10, 11]. Based on these studies, the authors expect the PLD using the titanium carbide (TiC) target with in-situ heating to provide the anatase TiO2 thin film. In the present study, the anatase TiO2 thin film on the glass substrate was synthesized by the PLD using a TiC target. This paper reports the effects of substrate temperature on the crystal structures, surface morphologies and chemical states of the thin films.
2. Experimental A schematic diagram of the experimental apparatus is shown in Fig. 1. A deposition chamber was evacuated to a base pressure (below 2 10 3 Pa) using a turbo molecular pump with a rotary pump. The gas pressure was varied from the base pressure to 4 Pa by feeding oxygen (O2) gas. Three type films were deposited by a KrF excimer laser (wavelength: 248 nm and pulse length: 22 ns) at TS of 293, 473 and 773 K. The laser energy was 150 mJ/pulse with a repetition of 10 Hz. From the focal spot size of
106
Y. Choi et al. / Solid State Ionics 172 (2004) 105–108
Fig. 1. A schematic diagram of pulsed laser deposition (PLD) system for thin film preparation.
1 2 mm2, the laser power density was estimated to be f 3.4 108 W/cm2. Laser beam was incident 45j to the target surface. A sintered TiC disk (99%, B = 5 cm) was used as a target and it was rotated by a motor to prevent degradation of target surface and to achieve smooth homogeneous films during deposition. The target-to-substrate distance was f 7 cm. Film thickness was measured by a surface profiler (Dektak3ST). Thicknesses of three films deposited for 120 min at TS of 293, 473 and 773 K were 130, 112 and 105 nm, respectively. Therefore, the deposition rates of three films were evaluated as f 1.08 (for 293 K), 0.93 (for 473 K ) and f 0.88 (for 773 K) nm/min. The crystalline structures of the films were confirmed by the X-ray diffraction (XRD). The chemical states of the films were examined by X-ray photoelectron spectroscopy (XPS). Surface morphology was observed by atomic force microscopy (AFM, Nanoscope III) at tapping mode with a silicon probe.
Fig. 3 presents XPS spectra for the C 1s and Ti 2p core levels of the films prepared at TS of 293 and 773 K. In the case of the film prepared at TS of 293 K, the C 1s spectrum was composed of two peaks at 279.4 and 285.3 eV. Based on previous studies [12,13], these signals at 279.4 and 285.3 eV were assigned to the C 1s peaks due to the Ti– C and C –C bonds, respectively. The Ti 2p peaks were situated at 457.3, 458.8 and 464.2 eV. The former signal agreed with that of pure TiC [12]. The latter signals at 458.8 and 464.2 eV implied the oxidation of the surface [8,12]. On the other hand, in the case of the film prepared at TS of 773 K, the C 1s spectrum was composed of two peaks at 284.6 and 289.4 eV. These signals were ascribed to the C 1s peak due to the C –C bond (for the 284.6 eV peak) and the carbonate compounds (for the 289.4 eV peak) [14]. The Ti 2p peaks were observed around 459.2 and 464.9 eV. This spectrum corresponded to that of pure TiO2 [12].
3. Results and discussion Fig. 2 shows XRD patterns of the three films prepared by PLD under an oxygen atmosphere of 4 Pa at substrate temperatures (TS) of 293, 473 and 773 K. When the film was grown at TS of 293 K, several peaks appeared around 2h = 35.9j, 41.7j and 60.6j as shown in Fig. 2a. This pattern was in good agreement with that of pure TiC powder. The film fabricated at TS of 473 K did not show any peaks (Fig. 2b), whereas two peaks appeared around 2h = 25.2j and 48.3j in the film grown at TS of 773 K (Fig. 2c). The latter signals corresponded to the one due to the (101) and (200) reflections of anatase TiO2.
Fig. 2. XRD analysis of thin films prepared by PLD under an oxygen pressure of 4 Pa at substrate temperatures of 293 K (a), 473 K (b) and 773 K (c). o: anatase, : TiC, D: sample holder.
.
Y. Choi et al. / Solid State Ionics 172 (2004) 105–108
Fig. 3. XPS spectra for the (a) C 1s and (b) Ti 2p of thin films prepared by PLD under an oxygen pressure of 4 Pa at substrate temperatures of 293 and 773 K.
These XRD and XPS results indicated that TiC and anatase TiO2 thin films were formed at TS of 293 and 773 K, respectively. The differences in the chemical properties and crystal structures of thin films are probably caused by the in-situ heating treatment. Because the only TiC crystal was grown at TS of 293 K, the Ti, C and Ti– C ions, molecules, particles, and clusters should be included in the plume during the ablation without the oxide (Ti and O) species (Fig. 4a). This means that the in-situ heating induced the decomposition and oxidation of TiC, resulting in the formation of the oxides. Consequently, the anatase TiO2 thin
107
Fig. 5. AFM images (500 500 nm2, height mode) of (a) TiC and (b) anatase TiO2 films fabricated by PLD. These thin films were prepared at substrate temperatures of (a) 293 and (b) 773 K under an oxygen pressure of 4 Pa.
film would be formed at TS of 773 K by the oxidative heating of TiC (Fig. 4b). This formation mechanism is similar to that of the anatase TiO2 powder, which was prepared by the annealing of the TiC powder in air at 623 K [8]. Fig. 5 shows the AFM images of the (a) TiC and (b) anatase TiO2 thin films. The surface morphologies were different in each film. In the case of the TiC thin film prepared at TS of 293 K, the size of the round grains and the factor of surface roughness, which represented by a root mean square (RMS), were estimated to be 17 –45 nm and 1.4 –2.3 nm over a 500 500 nm2 area, respectively. In the case of the anatase TiO2 film grown at TS of 773 K, the size of the round grains was 11 –28 nm and the surface roughness factor was 0.5 –1.1 nm. The difference in the grain size between the TiC and TiO2 thin films would be due to different agglutinability in each compound. The average surface roughness was decreased with increasing the substrate temperature. The small grain size and restructuring of the particle formation due to the heating would cause smooth surface of the anatase TiO2 thin film.
4. Conclusion Fig. 4. A formation process of (a) TiC and (b) anatase TiO2 thin films prepared by PLD using a TiC target under an oxygen atmosphere of 4 Pa.
Anatase TiO2 thin film on the glass substrate was successfully grown by PLD using a sintered TiC target.
108
Y. Choi et al. / Solid State Ionics 172 (2004) 105–108
The XRD and XPS results demonstrated that the TiC and anatase TiO2 thin films were formed at TS of 293 and 773 K, respectively. The anatase TiO2 crystal was grown by the oxidation and decomposition of the deposits composed of the Ti and C atoms. This formation mechanism is similar to that of the anatase TiO2 powder synthesized by the oxidative heating of the TiC powder in air. The anatase TiO2 thin film prepared by PLD using the TiC target had nano-grains with 11 –28 nm, leading to the large surface area. We, therefore, expect this film to show a high photocatalytic activity.
References [1] A. Fujishima, T.N. Rao, D.A. Tryk, Electrochim. Acta 45 (2000) 4683. [2] T. Ibusuki, K. Takeuchi, J. Mol. Catal. 88 (1994) 93.
[3] D. Robert, S. Malato, Sci. Total Environ. 291 (2002) 85. [4] K.R. Patil, S.D. Sathaye, Y.B. Khollam, S.B. Deshpande, N.R. Pawaskar, A.B. Mandale, Mater. Lett. 57 (2003) 1775. [5] N. Negishi, K. Takeuchi, Mater. Lett. 38 (1999) 150. [6] S. Yamamoto, T. Sumita, Sugiharuto, A. Miyashita, H. Naramoto, Thin Solid Films 401 (2001) 88. [7] Y. Choi, S. Yamamoto, H. Abe, H. Itoh, Surf. Sci. 499 (2002) 203. [8] Y. Choi, T. Umebayashi, S. Yamamoto, S. Tanaka, J. Mater. Sci. Lett. 22 (2003) 1209. [9] Y. Choi, T. Umebayashi, M. Yoshikawa, J. Mater. Sci. 39 (2004) 1837. [10] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. [11] T. Umebayashi, T. Yamaki, S. Tanaka, K. Asai, Chem. Lett. 32 (2003) 330. [12] F. Santerre, M.A.E. Khakani, M. Chaker, J.P. Dodelet, Appl. Surf. Sci. 148 (1999) 24. [13] K. Baba, R. Hatada, Surf. Coat. Technol. 136 (2001) 241. [14] A.B. Christie, I. Sutherland, J.M. Walls, Vacuum 31 (1981) 513.