- Email: [email protected]
channeling
Nuclear Instruments and Methods in Physics Research B 206 (2003) 268–271 www.elsevier.com/locate/nimb
Characterization of metal-doped TiO2 films by RBS/channeling S. Yamamoto a
a,*
, T. Yamaki a, H. Naramoto b, S. Tanaka
a
Department of Materials Development, JAERI Takasaki, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Advanced Science Research Center, JAERI Takasaki, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
Abstract Epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W are successfully prepared by pulsed laser deposition. Anatase TiO2 (0 0 1) films are grown on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates. Rutile TiO2 (1 0 0) films are also obtained on a-Al2 O3 (0 0 0 1) substrates. The typical temperature of substrates during the deposition is 500 °C and the pressure of O2 gas is 4.7 Pa. The crystal quality of films, crystallographic relationships, and the concentration of doped metals were assessed by X-ray diffraction and Rutherford backscattering spectroscopy with channeling. The high quality metal-doped epitaxial TiO2 films are obtainable through the post-deposition annealing at 800 °C in air for 5 h. The concentration of doped metals is in the range from 0.2 to 1.5 at.%. RBS/channeling analysis reveals that doped Cr, Nb, Ta and W atoms are incorporated into the substitutional lattice sites of the anatase and rutile TiO2 . Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 81.15.Fg Keywords: X-ray diffraction; Laser epitaxy; Titanium compounds
1. Introduction TiO2 doped with several kinds of transition metals have been studied for improving photocatalysis or modification of optical, electrical and magnetic properties using ion implantation [1–5] and several deposition techniques [6,7]. Especially in anatase TiO2 films, the influence of metal doping on the photocatalytic properties has not
* Corresponding author. Tel.: +81-27-346-9444/9422; fax: +81-27-346-9687. E-mail address: [email protected] (S. Yamamoto).
been clarified. Therefore, the synthesis of high quality metal-doped epitaxial TiO2 films has been required for the reliable characterization of photocatalytic properties. Recently, we have grown high quality epitaxial TiO2 films with anatase and rutile structures on a LaAlO3 and a a-Al2 O3 substrate by pulsed laser deposition (PLD) [8,9]. Following the success, in the present study, we explore the preparation conditions of epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W by PLD. The annealing effect on the crystal quality, concentration of doped metal and orientation relationships have been studied using ion beam channeling and X-ray diffraction (XRD).
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00742-0
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
2. Experimental Epitaxial TiO2 films were grown on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) for anatase and a-Al2 O3 (0 0 0 1) for rutile by PLD using a KrF excimer laser (wavelength: 248 nm, repetition rate: 10 Hz). The average laser energy density was 150 mJ/cm2 . The laser beam was incident on a single crystal TiO2 (rutile) and a metal target, alternately. Oxygen gas was flowed into a growth chamber through a mass flow meter to achieve the pressure about 4.7 Pa. The temperature of substrates during the deposition was 500 °C. The typical thickness of TiO2 films was about 150 nm after deposition for 4 h. RBS/channeling analysis using a 3 MV single-stage-accelerator at JAERI/ Takasaki was employed to characterize the epitaxial films. The analyzing 2.0 MeV 4 Heþ ions were incident and backscattered particles were detected at 165° scattering angle. The film thickness and composition were evaluated from RBS spectra using the RUMP simulation program [10]. The crystallographic relationships between TiO2 films and substrates were determined by XRD.
269
RBS measurements, the minimum yields in the anantase TiO2 (0 0 1) film on SrTiO3 (0 0 1) and the rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate under each axial channeling condition are 21% and 2%, respectively. In order to study the stability of anatase TiO2 (0 0 1) films for post-deposition annealing treatment, the transformation temperature from anatase to rutile structure was evaluated. Deposited anatase TiO2 (0 0 1) films on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates were annealed between 800 and 1100 °C in air for 1 h. Fig. 1 shows the XRD patterns from the anatase TiO2 (0 0 1) films on the SrTiO3 (0 0 1) substrate after annealing at (a) 1000 °C and (b) 1100 °C. XRD analyses show no transformation to rutile structure up to 1000 °C. After annealing at 1100 °C, the transformation
3. Results and discussion The epitaxial anatase TiO2 (0 0 1) films were obtained on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates. Also the epitaxial rutile TiO2 (0 0 1) films were grown epitaxially on a-Al2 O3 (0 0 0 1) substrates. The crystal quality of anantase and rutile TiO2 films was determined by X-ray h rocking curves. For the TiO2 films deposited at 500 °C, the full width at half maximum (FWHM) of anatase TiO2 (0 0 4) rocking curves on LaAlO3 (0 0 1) and SrTiO3 (0 0 1) were 0.054° and 0.615°, respectively. This result indicates that the higher crystalline anatase TiO2 (0 0 1) films were obtained on LaAlO3 (0 0 1) substrates. On the other hand, the best quality rutile TiO2 (1 0 0) film was obtained on the a-Al2 O3 (0 0 0 1) where the FWHM in rutile TiO2 (2 0 0) rocking curve was 0.019°. In this study, in order to separate the doped-metal and the heavy element component of the substrate in RBS spectra, metal-doped anatase TiO2 (0 0 1) films were prepared on SrTiO3 (0 0 1) substrates. From the
Fig. 1. XRD patterns for the epitaxial anatase TiO2 films on the SrTiO3 (0 0 1) substrate annealed at different temperatures (a) 1000 °C and (b) 1100 °C in air for 1 h.
270
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
of the anatase TiO2 (0 0 1) to the (1 1 0)-oriented rutile is observed. Similar results were obtained in the anatase TiO2 (0 0 1) film on LaAlO3 (0 0 1) substrate. In general, the anatase-rutile transformation takes place between 700 °C and 900 °C for polycrystalline films mixed with anatase and rutile phases [11]. In this experiment, the higher transformation temperature from anatase to rutile structure was observed, suggesting that the thermal stability of high quality anatase TiO2 (0 0 1) films is affected the substrates with small lattice mismatch. Fig. 2 illustrates the 2.0 MeV 4 Heþ RBS spectra for the W-doped anatase and rutile TiO2 films on the SrTiO3 (0 0 1) and those on the a-Al2 O3 (0 0 0 1) under the axial channeling and the random condition. These films were simultaneously deposited at 500 °C up to 150 nm thick. After the deposition,
Fig. 2. 2.0 MeV 4 Heþ RBS/channeling spectra for (a) W doped anatase TiO2 (0 0 1) film on SrTiO3 (0 0 1) substrate and (b) W-doped rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate.
films were annealed at 800 °C in air for 5 h. The concentration of W is estimated to be 0.4 at.% from RBS measurements. In Fig. 2(a), the huge reduction of Ti (1.37–1.45 MeV) and W components of the TiO2 film in the aligned spectrum compared with the random one is observed. It indicates that the doped W atoms incorporate into the substitutional sites in the anantase TiO2 lattice. In Fig. 2(b), the peaks from Ti and W components in rutile TiO2 film are clearly separated, and thus it is possible to evaluate the crystal quality independently. The respective minimum yields in the TiO2 h1 0 0i aligned spectrum is 2.9% for Ti and 2.8% for W components, which suggests that the crystal quality of the rutile film is high as that in a bulk single crystal. It is also derived from the result
Fig. 3. XRD patterns of W-doped anatase and rutile TiO2 films. (a) Anatase TiO2 (0 0 1) film on SrTiO3 (0 0 1) substrate and (b) rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate. Both films were annealed at 800 °C in air for 1 h.
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
that doped W atoms are incorporated into the substitutional sites in the rutile TiO2 lattice almost completely. The similar results were obtained in Cr-, Nb- and Ta-doped anantase TiO2 (0 0 1) and rutile TiO2 (1 0 0) films with a doping level up to 1.5 at.%. RBS/channeling analysis reveals that Cr-, Nb- and Ta-doped atoms are incorporated substitutionally into the anatae and rutile TiO2 lattice sites. Fig. 3 shows the XRD patterns for the same samples in Fig. 2. As can be seen in Fig. 3(a), only the reflections from the anatase TiO2 (0 0 4) and (0 0 8) are observed without any reflection from the substrate. Also in Fig. 3(b), only the reflections from the rutile TiO2 (2 0 0) and (4 0 0) are observed. Thus we confirm that W-doped anatase TiO2 (0 0 1) and rutile TiO2 (1 0 0) are epitaxially grown on the SrTiO2 (0 0 1) and a-Al2 O3 (0 0 0 1) substrates, respectively. In addition, the epitaxial growth of anatase TiO2 (0 0 1) and rutile TiO2 (1 0 0) films doped with Cr, Nb and Ta were confirmed by XRD analysis.
4. Summary Epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W are obtained on SrTiO3 and a-Al2 O3 substrates by PLD. XRD analysis shows that the anatase TiO2 (0 0 1) transforms to
271
the (1 1 0)-oriented rutile structure after annealing at 1100 °C. High quality metal-doped anatase and ritule TiO2 films are obtained by annealing at 800 °C in air for 5 h. It is found from RBS/channeling and XRD measurements that Cr-, Nb-, Ta- and W-doped atoms are incorporated into the substitutional sites in the anatase and rutile TiO2 lattice sites. References [1] Y. Aoki, S. Yamamoto, H. Takeshita, H. Naramto, Nucl. Instr. and Meth. B 136–138 (1998) 400. [2] R.C. da Silva, E. Alves, L.M. Redondo, R. Fromknecht, O. Meyer, Nucl. Instr. and Meth. B 136–138 (1998) 442. [3] R. Fromknecht, I. Khubeis, S. Massing, O. Meyer, Nucl. Instr. and Meth. B 147 (1999) 191. [4] S. Nakano, T. Nonami, P. Jin, Y. Miyagawa, S. Miyagawa, Surf. Coat. Technol. 128 (2000) 446. [5] R.C. da Silva, E. Alves, M.M. Cruz, Nucl. Instr. and Meth. B 191 (2002) 158. [6] Y. Gao, S. Thevuthasan, D.E. McCready, M. Engelhard, J. Cryst. Growth 212 (2000) 178. [7] Y. Matsumoto, M. Murakami, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, K. Nakajima, T. Chikyow, H. Koinuma, Appl. Surf. Sci. 189 (2002) 344. [8] S. Yamamoto, T. Sumita, Sugiharuto, A. Miyashita, H. Naramoto, Thin Solid Films 401 (2001) 88. [9] S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, H. Naramoto, J. Cryst. Growth 237–239 (2002) 569. [10] L.R. Doolittle, Nucl. Instr. and Meth. 15 (1986) 227. [11] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Thin Solid Films 300 (1997) 113.
Characterization of metal-doped TiO2 films by RBS/channeling S. Yamamoto a
a,*
, T. Yamaki a, H. Naramoto b, S. Tanaka
a
Department of Materials Development, JAERI Takasaki, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Advanced Science Research Center, JAERI Takasaki, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
Abstract Epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W are successfully prepared by pulsed laser deposition. Anatase TiO2 (0 0 1) films are grown on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates. Rutile TiO2 (1 0 0) films are also obtained on a-Al2 O3 (0 0 0 1) substrates. The typical temperature of substrates during the deposition is 500 °C and the pressure of O2 gas is 4.7 Pa. The crystal quality of films, crystallographic relationships, and the concentration of doped metals were assessed by X-ray diffraction and Rutherford backscattering spectroscopy with channeling. The high quality metal-doped epitaxial TiO2 films are obtainable through the post-deposition annealing at 800 °C in air for 5 h. The concentration of doped metals is in the range from 0.2 to 1.5 at.%. RBS/channeling analysis reveals that doped Cr, Nb, Ta and W atoms are incorporated into the substitutional lattice sites of the anatase and rutile TiO2 . Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 81.15.Fg Keywords: X-ray diffraction; Laser epitaxy; Titanium compounds
1. Introduction TiO2 doped with several kinds of transition metals have been studied for improving photocatalysis or modification of optical, electrical and magnetic properties using ion implantation [1–5] and several deposition techniques [6,7]. Especially in anatase TiO2 films, the influence of metal doping on the photocatalytic properties has not
* Corresponding author. Tel.: +81-27-346-9444/9422; fax: +81-27-346-9687. E-mail address: [email protected] (S. Yamamoto).
been clarified. Therefore, the synthesis of high quality metal-doped epitaxial TiO2 films has been required for the reliable characterization of photocatalytic properties. Recently, we have grown high quality epitaxial TiO2 films with anatase and rutile structures on a LaAlO3 and a a-Al2 O3 substrate by pulsed laser deposition (PLD) [8,9]. Following the success, in the present study, we explore the preparation conditions of epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W by PLD. The annealing effect on the crystal quality, concentration of doped metal and orientation relationships have been studied using ion beam channeling and X-ray diffraction (XRD).
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00742-0
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
2. Experimental Epitaxial TiO2 films were grown on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) for anatase and a-Al2 O3 (0 0 0 1) for rutile by PLD using a KrF excimer laser (wavelength: 248 nm, repetition rate: 10 Hz). The average laser energy density was 150 mJ/cm2 . The laser beam was incident on a single crystal TiO2 (rutile) and a metal target, alternately. Oxygen gas was flowed into a growth chamber through a mass flow meter to achieve the pressure about 4.7 Pa. The temperature of substrates during the deposition was 500 °C. The typical thickness of TiO2 films was about 150 nm after deposition for 4 h. RBS/channeling analysis using a 3 MV single-stage-accelerator at JAERI/ Takasaki was employed to characterize the epitaxial films. The analyzing 2.0 MeV 4 Heþ ions were incident and backscattered particles were detected at 165° scattering angle. The film thickness and composition were evaluated from RBS spectra using the RUMP simulation program [10]. The crystallographic relationships between TiO2 films and substrates were determined by XRD.
269
RBS measurements, the minimum yields in the anantase TiO2 (0 0 1) film on SrTiO3 (0 0 1) and the rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate under each axial channeling condition are 21% and 2%, respectively. In order to study the stability of anatase TiO2 (0 0 1) films for post-deposition annealing treatment, the transformation temperature from anatase to rutile structure was evaluated. Deposited anatase TiO2 (0 0 1) films on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates were annealed between 800 and 1100 °C in air for 1 h. Fig. 1 shows the XRD patterns from the anatase TiO2 (0 0 1) films on the SrTiO3 (0 0 1) substrate after annealing at (a) 1000 °C and (b) 1100 °C. XRD analyses show no transformation to rutile structure up to 1000 °C. After annealing at 1100 °C, the transformation
3. Results and discussion The epitaxial anatase TiO2 (0 0 1) films were obtained on SrTiO3 (0 0 1) and LaAlO3 (0 0 1) substrates. Also the epitaxial rutile TiO2 (0 0 1) films were grown epitaxially on a-Al2 O3 (0 0 0 1) substrates. The crystal quality of anantase and rutile TiO2 films was determined by X-ray h rocking curves. For the TiO2 films deposited at 500 °C, the full width at half maximum (FWHM) of anatase TiO2 (0 0 4) rocking curves on LaAlO3 (0 0 1) and SrTiO3 (0 0 1) were 0.054° and 0.615°, respectively. This result indicates that the higher crystalline anatase TiO2 (0 0 1) films were obtained on LaAlO3 (0 0 1) substrates. On the other hand, the best quality rutile TiO2 (1 0 0) film was obtained on the a-Al2 O3 (0 0 0 1) where the FWHM in rutile TiO2 (2 0 0) rocking curve was 0.019°. In this study, in order to separate the doped-metal and the heavy element component of the substrate in RBS spectra, metal-doped anatase TiO2 (0 0 1) films were prepared on SrTiO3 (0 0 1) substrates. From the
Fig. 1. XRD patterns for the epitaxial anatase TiO2 films on the SrTiO3 (0 0 1) substrate annealed at different temperatures (a) 1000 °C and (b) 1100 °C in air for 1 h.
270
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
of the anatase TiO2 (0 0 1) to the (1 1 0)-oriented rutile is observed. Similar results were obtained in the anatase TiO2 (0 0 1) film on LaAlO3 (0 0 1) substrate. In general, the anatase-rutile transformation takes place between 700 °C and 900 °C for polycrystalline films mixed with anatase and rutile phases [11]. In this experiment, the higher transformation temperature from anatase to rutile structure was observed, suggesting that the thermal stability of high quality anatase TiO2 (0 0 1) films is affected the substrates with small lattice mismatch. Fig. 2 illustrates the 2.0 MeV 4 Heþ RBS spectra for the W-doped anatase and rutile TiO2 films on the SrTiO3 (0 0 1) and those on the a-Al2 O3 (0 0 0 1) under the axial channeling and the random condition. These films were simultaneously deposited at 500 °C up to 150 nm thick. After the deposition,
Fig. 2. 2.0 MeV 4 Heþ RBS/channeling spectra for (a) W doped anatase TiO2 (0 0 1) film on SrTiO3 (0 0 1) substrate and (b) W-doped rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate.
films were annealed at 800 °C in air for 5 h. The concentration of W is estimated to be 0.4 at.% from RBS measurements. In Fig. 2(a), the huge reduction of Ti (1.37–1.45 MeV) and W components of the TiO2 film in the aligned spectrum compared with the random one is observed. It indicates that the doped W atoms incorporate into the substitutional sites in the anantase TiO2 lattice. In Fig. 2(b), the peaks from Ti and W components in rutile TiO2 film are clearly separated, and thus it is possible to evaluate the crystal quality independently. The respective minimum yields in the TiO2 h1 0 0i aligned spectrum is 2.9% for Ti and 2.8% for W components, which suggests that the crystal quality of the rutile film is high as that in a bulk single crystal. It is also derived from the result
Fig. 3. XRD patterns of W-doped anatase and rutile TiO2 films. (a) Anatase TiO2 (0 0 1) film on SrTiO3 (0 0 1) substrate and (b) rutile TiO2 (1 0 0) film on a-Al2 O3 (0 0 0 1) substrate. Both films were annealed at 800 °C in air for 1 h.
S. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 268–271
that doped W atoms are incorporated into the substitutional sites in the rutile TiO2 lattice almost completely. The similar results were obtained in Cr-, Nb- and Ta-doped anantase TiO2 (0 0 1) and rutile TiO2 (1 0 0) films with a doping level up to 1.5 at.%. RBS/channeling analysis reveals that Cr-, Nb- and Ta-doped atoms are incorporated substitutionally into the anatae and rutile TiO2 lattice sites. Fig. 3 shows the XRD patterns for the same samples in Fig. 2. As can be seen in Fig. 3(a), only the reflections from the anatase TiO2 (0 0 4) and (0 0 8) are observed without any reflection from the substrate. Also in Fig. 3(b), only the reflections from the rutile TiO2 (2 0 0) and (4 0 0) are observed. Thus we confirm that W-doped anatase TiO2 (0 0 1) and rutile TiO2 (1 0 0) are epitaxially grown on the SrTiO2 (0 0 1) and a-Al2 O3 (0 0 0 1) substrates, respectively. In addition, the epitaxial growth of anatase TiO2 (0 0 1) and rutile TiO2 (1 0 0) films doped with Cr, Nb and Ta were confirmed by XRD analysis.
4. Summary Epitaxial anatase and rutile TiO2 films doped with Cr, Nb, Ta and W are obtained on SrTiO3 and a-Al2 O3 substrates by PLD. XRD analysis shows that the anatase TiO2 (0 0 1) transforms to
271
the (1 1 0)-oriented rutile structure after annealing at 1100 °C. High quality metal-doped anatase and ritule TiO2 films are obtained by annealing at 800 °C in air for 5 h. It is found from RBS/channeling and XRD measurements that Cr-, Nb-, Ta- and W-doped atoms are incorporated into the substitutional sites in the anatase and rutile TiO2 lattice sites. References [1] Y. Aoki, S. Yamamoto, H. Takeshita, H. Naramto, Nucl. Instr. and Meth. B 136–138 (1998) 400. [2] R.C. da Silva, E. Alves, L.M. Redondo, R. Fromknecht, O. Meyer, Nucl. Instr. and Meth. B 136–138 (1998) 442. [3] R. Fromknecht, I. Khubeis, S. Massing, O. Meyer, Nucl. Instr. and Meth. B 147 (1999) 191. [4] S. Nakano, T. Nonami, P. Jin, Y. Miyagawa, S. Miyagawa, Surf. Coat. Technol. 128 (2000) 446. [5] R.C. da Silva, E. Alves, M.M. Cruz, Nucl. Instr. and Meth. B 191 (2002) 158. [6] Y. Gao, S. Thevuthasan, D.E. McCready, M. Engelhard, J. Cryst. Growth 212 (2000) 178. [7] Y. Matsumoto, M. Murakami, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, K. Nakajima, T. Chikyow, H. Koinuma, Appl. Surf. Sci. 189 (2002) 344. [8] S. Yamamoto, T. Sumita, Sugiharuto, A. Miyashita, H. Naramoto, Thin Solid Films 401 (2001) 88. [9] S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, H. Naramoto, J. Cryst. Growth 237–239 (2002) 569. [10] L.R. Doolittle, Nucl. Instr. and Meth. 15 (1986) 227. [11] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Thin Solid Films 300 (1997) 113.