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Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin films
Applied Surface Science 200 (2002) 21±26
Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin ®lms Taishi Sumita*, Tetsuya Yamaki, Shunya Yamamoto, Atsumi Miyashita Department of Materials Development, Japan Atomic Energy Research Institute, Watanuki 1233, Takasaki, Gunma 370-1292, Japan Received 10 September 2001; received in revised form 7 May 2002; accepted 21 May 2002
Abstract Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile ®lms was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied ®eld. According to the measurements, the quantum ef®ciency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap. # 2002 Published by Elsevier Science B.V. PACS: 81.15.F; 78.20.C; 73.50.P; 73.25; 78.66; 82.65.F Keywords: Titanium oxide; Laser ablation; Photocatalysis; Photoconductivity; Charge separation
1. Introduction Titanium dioxide (TiO2) has been widely investigated by many research groups, because its photoactive potential is applicable to new intelligent materials, such as photocatalytic decomposition of organics and contaminants [1±4]. Photo-irradiated TiO2, if the photon energies are larger than the bandgap, generate electron±hole pairs. The generated holes are transported toward the surface because the charged carriers (electron and hole) are separated by the surface band bending [5,6] and produce oxidation on the surface as a photocatalyst. Therefore, investigation of the charge separation behavior of photo-generated *
Corresponding author. Tel.: 81-27-346-9422; fax: 81-27-346-9687. E-mail address: [email protected] (T. Sumita).
carriers on the TiO2 surface is most fundamental approach for characterization of photocatalysts. Use of metal contacts and an applied bias for the measurements, however, disturbs the existing surface band bending of TiO2 due to carrier injection. For semiconductor photocatalysts, a unique measurement technique must be used to observe photo-carrier behavior. TiO2 exists in three different crystalline phases: anatase, rutile and brookite [7,8]. Although both rutile and anatase have been studied for their photocatalytic activities, anatase is generally more active in photocatalysis than rutile [9]. This enhanced photoactivity is attributed to the larger bandgap of the anatase than that of rutile. Photocatalytic activity strongly depends on the surface redox potential and the lifetime of photo-generated electron±hole pairs. Anatase, which has a larger bandgap, tends to increase the surface redox potentials and to prolong the carrier lifetime in comparison with
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 1 4 - 1
22
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
rutile. However, a photocatalytic activity of TiO2 is very sensitive not only to the carrier characteristics but also to the surface stabilities attributed to the difference of crystalline orientation [10,11]. Consequently, highly oriented ®lms should be used to investigate the correlation between the photocatalytic activity and the inherent carrier behavior for anatase and rutile. We recently reported a new technique for characterizing photocatalysis of semiconductor photocatalyst ®lms using a pulse laser as the excitation source without metal contacts and an applied bias [12]. The total number of photo-carriers transported toward the surface was observed as a function of the irradiated photon energy due to the surface charge separation effect of transient photo-generated carriers. Thus, the performance of the photocatalysis can be directly estimated under various light sources (e.g. sunlight and various types of lamps). In this letter, we apply the photo-induced transient charge separation (PITCS) measurement to highly oriented anatase and rutile ®lms and examine the photocatalytic activity for both with respect to the inherent dynamics of the photoinduced charge separation at the ®lm surface. 2. Experimental TiO2 anatase and rutile ®lms were deposited by pulsed laser deposition (PLD) on LaAlO3 (1 0 0) for
the anatase and a-A12O3 (0 0 0 1) for the rutile. The depositions were performed with the second harmonic Q-switched Nd:YAG laser operating at a wavelength of 532 nm, frequency of 10 Hz and pulse width of 145 ms. The average laser energy density was 50 mJ/ cm2 for anatase and 100 mJ/cm2 for rutile deposition. The focused laser ablated a titanium (purity: 99.99%) target for anatase and a sintered titanium dioxide (purity: 99.99%) target for rutile, and the target substrate distance was 5 cm. The substrates were held at 500 8C and under 35 mTorr O2 gas pressure during the deposition. Both ®lms had a thickness of 250 nm. The crystalline structures of the ®lms were determined by X-ray diffraction measurements using a high-resolution diffractometer (Philips X'PertMRD). To observe the surface morphology, atomic force microscopy (AFM; JEOL JSPM-4200) was applied. The optical transmission and re¯ection were measured with a CARL ZEISS MPM/UV instrument. The photoconductivity spectra were measured at room temperature using a Xe lamp as the excitation source and corrected for the emission spectrum of the lump. Indium electrodes were deposited on the surface of the ®lms with 100 mm separation using a planer type mask. Photo-generated carrier dynamics for photocatalytic activity were characterized by PITCS measurements. Fig. 1 shows a schematic diagram of the experimental apparatus. An N2 pulse dye laser (wavelength
Fig. 1. Schematic diagram of the photo-induced transient charge separation (PITCS) measurements.
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
337±520 nm, pulse width 5 ns) was used as the excitation source. Deposited TiO2 ®lm on substrate was set between two transparent indium tin oxide (ITO) electrodes, where the TiO2 ®lms were blocked against the electrodes by a Mylar sheet to prevent carrier injection. A standard resistor R was also connected to the photo-irradiated side. The laser pulses were aimed at the TiO2 surface through the ITO and Mylar sheet. When the photo-carriers are generated by pulse laser irradiation, the charge separation produces transient electric charge attributed to the existing surface band bending in TiO2. After the laser irradiation, the electric charge subsequently discharges through the R where the applied voltage drop was observed using a digital oscilloscope. The total number of carriers transported toward the surface can be calculated from the time resolved voltage drop. The actual photocatalytic performances of the anatase and rutile ®lms were also investigated by photocatalytic decomposition of methylene blue dye [13]. The ®lms were placed in an aqueous solution of the dye with a concentration of 1 10 5 mol/l and irradiated with a UV lamp (l < 400 nm, 20 W). The decomposition rate of the dye was determined using a UV±VIS spectrophotometer (Hitachi U-3310). 3. Results and discussion 3.1. Structure and morphology Anatase and rutile ®lms were obtained under the deposition conditions described in the previous section. Fig. 2 shows the typical X-ray diffraction patterns from the anatase ®lms on LaAlO3 (1 0 0) substrate and rutile ®lms on a-A12O3 (0 0 0 1) substrate. In Fig. 2a, only the re¯ection of the anatase (0 0 4) was observed without any re¯ection from the substrate, which indicates that the anatase TiO2 (0 0 1) ®lms were highly oriented deposition on the (1 0 0) plane of the LaAlO3 substrate. The rutile ®lms also displayed only the re¯ection of the rutile (2 0 0) as shown in Fig. 2b. Thus, we con®rmed that the obtained anatase and rutile ®lms are highly oriented in a single phase. Surface morphology was observed to obtain the average grain size of the ®lms, because the photocatalytic activity, of course, strongly depends on not
23
Fig. 2. The X-ray diffraction pattern from the highly oriented TiO2 (a) anatase ®lm on the LaAlO3 (1 0 0), (b) rutile ®lm on the aAl2O3 (0 0 0 1).
only the charge separation ef®ciency, but also the net surface area associated with the grain size [14±16]. The aim of the present study was to analyze the photocatalytic activity for the anatase and rutile through observation of inherent photo-generated carrier behavior. Consequently, it is desirable that there is not a great difference in grain size between the deposited anatase and rutile ®lms. From the AFM images, the grain size of both of the ®lms was in the range of 20±30 nm. On the basis of these results, we assumed that the anatase and rutile ®lms have nearly the same net surface area. 3.2. Optical properties Because TiO2 is a semiconductor with a large bandgap, the optical bandgap Eg can be determined from the spectra of absorption coef®cient a. The a near the absorption edge was derived from transmittance T and re¯ectivity R, measured on the anatase and rutile ®lms. The a which depends on the wavelength l of incident light, can be obtained by using the following relationship, T
1 R2 e ad 1 R2 e 2 ad
where d is the thickness of the sample [17]. When scattering effects are neglected, the a above the
24
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
Fig. 3. Optical absortion spectra of anatase and rutile ®lms plotted a1/2 vs. photon energy (E).
threshold of fundamental absorption follows the (E Eg)2 energy dependence characteristic of indirect allowed transitions, as shown by the a1/2 versus photon energy plot in Fig. 3. The optical bandgap Eg, derived from extrapolation of a, of anatase and rutile ®lms are 3.23 and 3.02 eV, respectively. Fig. 4 shows the photoconductivity spectra of anatase and rutile ®lms. The arrows in Fig. 4 indicate the photoconductivity threshold energies of anatase and rutile, respectively. The threshold energies approximately agree with the optical bandgap Eg derived from absorption spectra (Fig. 3).
Fig. 4. Photoconductivity spectra of the highly oriented TiO2 anatase (0 0 1) and rutile(1 0 0) ®lms.
Fig. 5. Photocatalyti destruction of methylene blue dye by anatase and rutile ®lms. Co indicates the methylene blue concentration at the same irradiation time without the ®lms.
3.3. Photocatalytic activity Fig. 5 shows the photocatalytic activities of anatase and rutile ®lms investigated by photocatalytic decomposition of methylene blue. The anatase ®lm has higher photocatalytic activity than rutile. Because there was no difference in the net surface area between the ®lms, which was con®rmed by AFM analysis, the photocatalytic activity must re¯ect differences in inherent photo-generated carrier behavior for photocatalysis in the highly oriented anatase and rutile ®lms. Photo-induced charge separation ef®ciency was measured on the anatase and rutile ®lms by PITCS with changing the incident laser energy (wavelength 337±520 nm). Fig. 6a shows the typical transient discharge at 337 and 370 nm of laser irradiation for anatase ®lm, where the discharge values are normalized by the irradiated laser power. Because the pulse width of the laser was 5 ns, the irradiation was completely terminated at t 0 in Fig. 6a. The positive discharge indicates that the holes diffused toward the surface and the photocatalytic reaction on the anatase surface is an oxidation reaction, which is consistent with previous reports [18]. Rutile ®lm was examined using the same method. The total number of holes transported toward the surface can be calculated analytically by the procedure described previously [12].
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
25
Fig. 6. (a) Typical disharge waveforms v(t) for anatase ®lm. (b) The photoactivity (h per photon) as a function of the incident laser wavelength (photon energy) for anatae and rutile ®lms by PITCS measurements, and spectral irradiance of sunlight (AM 1.0).
Fig. 6b shows the incident photon energy dependence of the surface charge separation ef®ciency of anatase and rutile. The values of the vertical axis are the ratios of the total number of surface transported holes h to induced photons, namely, quantum ef®ciency of photo-generated holes transported toward the surface. The charge separation ef®ciency of anatase ®lm is more remarkable than that of rutile ®lm in this energy region. The bandgap and absorption coef®cients of both ®lms were determined based on absorption and photoconductivity measurements. For rutile, the surface-transported holes disappeared at 345 nm (3.59 eV) in spite of much higher photon energy irradiation than its bandgap. The PITCS measurements indicate
that the charge separation at the rutile surface occurred only for the electrons and holes that were generated close to the surface due to high-energy irradiation. Photo-generated holes at the anatase surface, however, were ef®ciently transported to the surface up to an irradiation of 380 nm (3.26 eV) associated with its bandgap, though absorption coef®cients of anatase are lower than that of rutile in the photon energy range of the measurement. Those phenomena can be explained by surface band bending of anatase, which is inherently formed in a deeper region in the ®lm with a steeper potential slope than in rutile. Thus, the prolonged carrier diffusion length with the lifetime displayed a high performance of surface charge
26
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
separation in this measurement. This analysis is consistent with actual photocatalytic activities due to decomposition of methylene blue (Fig. 5). Furthermore, anatase ®lm exhibits the transportation of photo-generated holes even for photon energy irradiation less than the bandgap (460 nm), though there were only a few. The photocurrent for irradiation at the photon energy below the bandgap was not obtained on the photoconductivity measurement (Fig. 4). The photoconductivity measurement can reveal the photo-generated carriers behavior in the plane of the ®lms between the planer type electrodes with 100 mm separation on the surface, while PITCS measurement can detect the photo-generated carrier diffusion toward the surface. These experimental results indicate that photo-carriers can be generated close to the surface by irradiation below the anatase bandgap (<3.23 eV) and the generated carrier can diffuse only toward the ®lm surface. Although the origin of precise behavior is not clear at present, anatase has an advantage over rutile for photocatalysis, particularly under sunlight. If both samples are irradiated under the condition of AM 1.0 sunlight (the sunlight at the earth's surface when the sun is at zenith), the spectral irradiance remarkably increases in the region of wavelength (337±460 nm) as shown in Fig. 6b, the total number of photo-generated holes transported toward surface for anatase is approximately 16 times that of rutile by estimation. 4. Conclusion The charge separation ef®ciency of photo-generated carrier behavior at the ®lm surface was evaluated on highly oriented single-phase anatase and rutile by means of PITCS analysis and the correlation with actual photocatalytic activity under conditions in which there was no difference in the ®lm net surface area. Irradiation-induced transported holes of the rutile surface are generated near the surface and
the charge separation that occurred due to the irradiation has much larger photon energy than its bandgap. The photo-generated holes in anatase can be transported toward the surface until photon energy irradiation reaches its optical bandgap. These phenomena indicate that anatase has inherent surface band bending that is spontaneously formed in a deeper region with steeper potential slope in comparison with rutile. The advantages of anatase for photocatalysis were revealed with the photocatalytic decomposition measurements. References [1] J.E. Parcheco, M.R. Prairie, L. Yellowhorse, J. Sol. Energy Eng. 115 (1993) 123. [2] S.M. RodriÂguez, C. Richter, J.B. GaÂlvez, M. Vincent, Sol. Energy 56 (1996) 401. [3] K. Tanaka, W. Luesaiwong, T. Hisanaga, J. Mol. Catal. A 122 (1997) 67. [4] C. DomiÂngues, J. GarciÂa, M.A. Pedraz, A. Torres, M.A. GalaÂn, Catal. Today 40 (1998) 85. [5] K. Itaya, E. Tomita, Chem. Lett. 2 (1989) 285. [6] F.-R.F. Fan, A.J. Bard, J. Phys. Chem. 94 (1990) 3761. [7] S.D. Mo, W.Y. Ching, Phys. Rev. B 51 (1995) 13023. [8] D.-W. Kim, N. Enomoto, Z. Nakagawa, J. Am. Ceram. Soc. 79 (1996) 1095. [9] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. [10] S. Ichikawa, R. Doi, Thin Solid Films 292 (1997) 130. [11] A. Fahmi, C. Minot, Surf. Sci. 304 (1994) 343. [12] T. Sumita, T. Yamaki, S. Yamamoto, A. Miyashita, Jpn. J. Appl. Phys. 40 (2001) 4007. [13] T. Tatsuma, S. Tachibana, T. Miwa, D.A. Tryk, A. Fujishima, J. Phys. Chem. B 103 (1999) 8033. [14] T. Wang, H. Wang, P. Xu, X. Zhao, Y. Liu, S. Chao, Thin Solid Films 334 (1998) 103. [15] A.P. Xagas, E. Androulaki, A. Hiskia, P. Falaras, Thin Solid Films 357 (1999) 173. [16] C.K. Chan, J.F. Porter, Y. Li, W. Guo, C.-M. Chan, J. Am. Ceram. Soc. 82 (1999) 566. [17] H. Tang, K. Prasad, R. SanjineÂs, P.E. Schmid, E. LeÂvy, J. Appl. Phys. 75 (1994) 2042. [18] D.W. Bahnemann, M. Hilgendorff, R. Memming, J. Phys. Chem. B 101 (1997) 4265.
Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin ®lms Taishi Sumita*, Tetsuya Yamaki, Shunya Yamamoto, Atsumi Miyashita Department of Materials Development, Japan Atomic Energy Research Institute, Watanuki 1233, Takasaki, Gunma 370-1292, Japan Received 10 September 2001; received in revised form 7 May 2002; accepted 21 May 2002
Abstract Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile ®lms was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied ®eld. According to the measurements, the quantum ef®ciency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap. # 2002 Published by Elsevier Science B.V. PACS: 81.15.F; 78.20.C; 73.50.P; 73.25; 78.66; 82.65.F Keywords: Titanium oxide; Laser ablation; Photocatalysis; Photoconductivity; Charge separation
1. Introduction Titanium dioxide (TiO2) has been widely investigated by many research groups, because its photoactive potential is applicable to new intelligent materials, such as photocatalytic decomposition of organics and contaminants [1±4]. Photo-irradiated TiO2, if the photon energies are larger than the bandgap, generate electron±hole pairs. The generated holes are transported toward the surface because the charged carriers (electron and hole) are separated by the surface band bending [5,6] and produce oxidation on the surface as a photocatalyst. Therefore, investigation of the charge separation behavior of photo-generated *
Corresponding author. Tel.: 81-27-346-9422; fax: 81-27-346-9687. E-mail address: [email protected] (T. Sumita).
carriers on the TiO2 surface is most fundamental approach for characterization of photocatalysts. Use of metal contacts and an applied bias for the measurements, however, disturbs the existing surface band bending of TiO2 due to carrier injection. For semiconductor photocatalysts, a unique measurement technique must be used to observe photo-carrier behavior. TiO2 exists in three different crystalline phases: anatase, rutile and brookite [7,8]. Although both rutile and anatase have been studied for their photocatalytic activities, anatase is generally more active in photocatalysis than rutile [9]. This enhanced photoactivity is attributed to the larger bandgap of the anatase than that of rutile. Photocatalytic activity strongly depends on the surface redox potential and the lifetime of photo-generated electron±hole pairs. Anatase, which has a larger bandgap, tends to increase the surface redox potentials and to prolong the carrier lifetime in comparison with
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 1 4 - 1
22
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
rutile. However, a photocatalytic activity of TiO2 is very sensitive not only to the carrier characteristics but also to the surface stabilities attributed to the difference of crystalline orientation [10,11]. Consequently, highly oriented ®lms should be used to investigate the correlation between the photocatalytic activity and the inherent carrier behavior for anatase and rutile. We recently reported a new technique for characterizing photocatalysis of semiconductor photocatalyst ®lms using a pulse laser as the excitation source without metal contacts and an applied bias [12]. The total number of photo-carriers transported toward the surface was observed as a function of the irradiated photon energy due to the surface charge separation effect of transient photo-generated carriers. Thus, the performance of the photocatalysis can be directly estimated under various light sources (e.g. sunlight and various types of lamps). In this letter, we apply the photo-induced transient charge separation (PITCS) measurement to highly oriented anatase and rutile ®lms and examine the photocatalytic activity for both with respect to the inherent dynamics of the photoinduced charge separation at the ®lm surface. 2. Experimental TiO2 anatase and rutile ®lms were deposited by pulsed laser deposition (PLD) on LaAlO3 (1 0 0) for
the anatase and a-A12O3 (0 0 0 1) for the rutile. The depositions were performed with the second harmonic Q-switched Nd:YAG laser operating at a wavelength of 532 nm, frequency of 10 Hz and pulse width of 145 ms. The average laser energy density was 50 mJ/ cm2 for anatase and 100 mJ/cm2 for rutile deposition. The focused laser ablated a titanium (purity: 99.99%) target for anatase and a sintered titanium dioxide (purity: 99.99%) target for rutile, and the target substrate distance was 5 cm. The substrates were held at 500 8C and under 35 mTorr O2 gas pressure during the deposition. Both ®lms had a thickness of 250 nm. The crystalline structures of the ®lms were determined by X-ray diffraction measurements using a high-resolution diffractometer (Philips X'PertMRD). To observe the surface morphology, atomic force microscopy (AFM; JEOL JSPM-4200) was applied. The optical transmission and re¯ection were measured with a CARL ZEISS MPM/UV instrument. The photoconductivity spectra were measured at room temperature using a Xe lamp as the excitation source and corrected for the emission spectrum of the lump. Indium electrodes were deposited on the surface of the ®lms with 100 mm separation using a planer type mask. Photo-generated carrier dynamics for photocatalytic activity were characterized by PITCS measurements. Fig. 1 shows a schematic diagram of the experimental apparatus. An N2 pulse dye laser (wavelength
Fig. 1. Schematic diagram of the photo-induced transient charge separation (PITCS) measurements.
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
337±520 nm, pulse width 5 ns) was used as the excitation source. Deposited TiO2 ®lm on substrate was set between two transparent indium tin oxide (ITO) electrodes, where the TiO2 ®lms were blocked against the electrodes by a Mylar sheet to prevent carrier injection. A standard resistor R was also connected to the photo-irradiated side. The laser pulses were aimed at the TiO2 surface through the ITO and Mylar sheet. When the photo-carriers are generated by pulse laser irradiation, the charge separation produces transient electric charge attributed to the existing surface band bending in TiO2. After the laser irradiation, the electric charge subsequently discharges through the R where the applied voltage drop was observed using a digital oscilloscope. The total number of carriers transported toward the surface can be calculated from the time resolved voltage drop. The actual photocatalytic performances of the anatase and rutile ®lms were also investigated by photocatalytic decomposition of methylene blue dye [13]. The ®lms were placed in an aqueous solution of the dye with a concentration of 1 10 5 mol/l and irradiated with a UV lamp (l < 400 nm, 20 W). The decomposition rate of the dye was determined using a UV±VIS spectrophotometer (Hitachi U-3310). 3. Results and discussion 3.1. Structure and morphology Anatase and rutile ®lms were obtained under the deposition conditions described in the previous section. Fig. 2 shows the typical X-ray diffraction patterns from the anatase ®lms on LaAlO3 (1 0 0) substrate and rutile ®lms on a-A12O3 (0 0 0 1) substrate. In Fig. 2a, only the re¯ection of the anatase (0 0 4) was observed without any re¯ection from the substrate, which indicates that the anatase TiO2 (0 0 1) ®lms were highly oriented deposition on the (1 0 0) plane of the LaAlO3 substrate. The rutile ®lms also displayed only the re¯ection of the rutile (2 0 0) as shown in Fig. 2b. Thus, we con®rmed that the obtained anatase and rutile ®lms are highly oriented in a single phase. Surface morphology was observed to obtain the average grain size of the ®lms, because the photocatalytic activity, of course, strongly depends on not
23
Fig. 2. The X-ray diffraction pattern from the highly oriented TiO2 (a) anatase ®lm on the LaAlO3 (1 0 0), (b) rutile ®lm on the aAl2O3 (0 0 0 1).
only the charge separation ef®ciency, but also the net surface area associated with the grain size [14±16]. The aim of the present study was to analyze the photocatalytic activity for the anatase and rutile through observation of inherent photo-generated carrier behavior. Consequently, it is desirable that there is not a great difference in grain size between the deposited anatase and rutile ®lms. From the AFM images, the grain size of both of the ®lms was in the range of 20±30 nm. On the basis of these results, we assumed that the anatase and rutile ®lms have nearly the same net surface area. 3.2. Optical properties Because TiO2 is a semiconductor with a large bandgap, the optical bandgap Eg can be determined from the spectra of absorption coef®cient a. The a near the absorption edge was derived from transmittance T and re¯ectivity R, measured on the anatase and rutile ®lms. The a which depends on the wavelength l of incident light, can be obtained by using the following relationship, T
1 R2 e ad 1 R2 e 2 ad
where d is the thickness of the sample [17]. When scattering effects are neglected, the a above the
24
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
Fig. 3. Optical absortion spectra of anatase and rutile ®lms plotted a1/2 vs. photon energy (E).
threshold of fundamental absorption follows the (E Eg)2 energy dependence characteristic of indirect allowed transitions, as shown by the a1/2 versus photon energy plot in Fig. 3. The optical bandgap Eg, derived from extrapolation of a, of anatase and rutile ®lms are 3.23 and 3.02 eV, respectively. Fig. 4 shows the photoconductivity spectra of anatase and rutile ®lms. The arrows in Fig. 4 indicate the photoconductivity threshold energies of anatase and rutile, respectively. The threshold energies approximately agree with the optical bandgap Eg derived from absorption spectra (Fig. 3).
Fig. 4. Photoconductivity spectra of the highly oriented TiO2 anatase (0 0 1) and rutile(1 0 0) ®lms.
Fig. 5. Photocatalyti destruction of methylene blue dye by anatase and rutile ®lms. Co indicates the methylene blue concentration at the same irradiation time without the ®lms.
3.3. Photocatalytic activity Fig. 5 shows the photocatalytic activities of anatase and rutile ®lms investigated by photocatalytic decomposition of methylene blue. The anatase ®lm has higher photocatalytic activity than rutile. Because there was no difference in the net surface area between the ®lms, which was con®rmed by AFM analysis, the photocatalytic activity must re¯ect differences in inherent photo-generated carrier behavior for photocatalysis in the highly oriented anatase and rutile ®lms. Photo-induced charge separation ef®ciency was measured on the anatase and rutile ®lms by PITCS with changing the incident laser energy (wavelength 337±520 nm). Fig. 6a shows the typical transient discharge at 337 and 370 nm of laser irradiation for anatase ®lm, where the discharge values are normalized by the irradiated laser power. Because the pulse width of the laser was 5 ns, the irradiation was completely terminated at t 0 in Fig. 6a. The positive discharge indicates that the holes diffused toward the surface and the photocatalytic reaction on the anatase surface is an oxidation reaction, which is consistent with previous reports [18]. Rutile ®lm was examined using the same method. The total number of holes transported toward the surface can be calculated analytically by the procedure described previously [12].
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
25
Fig. 6. (a) Typical disharge waveforms v(t) for anatase ®lm. (b) The photoactivity (h per photon) as a function of the incident laser wavelength (photon energy) for anatae and rutile ®lms by PITCS measurements, and spectral irradiance of sunlight (AM 1.0).
Fig. 6b shows the incident photon energy dependence of the surface charge separation ef®ciency of anatase and rutile. The values of the vertical axis are the ratios of the total number of surface transported holes h to induced photons, namely, quantum ef®ciency of photo-generated holes transported toward the surface. The charge separation ef®ciency of anatase ®lm is more remarkable than that of rutile ®lm in this energy region. The bandgap and absorption coef®cients of both ®lms were determined based on absorption and photoconductivity measurements. For rutile, the surface-transported holes disappeared at 345 nm (3.59 eV) in spite of much higher photon energy irradiation than its bandgap. The PITCS measurements indicate
that the charge separation at the rutile surface occurred only for the electrons and holes that were generated close to the surface due to high-energy irradiation. Photo-generated holes at the anatase surface, however, were ef®ciently transported to the surface up to an irradiation of 380 nm (3.26 eV) associated with its bandgap, though absorption coef®cients of anatase are lower than that of rutile in the photon energy range of the measurement. Those phenomena can be explained by surface band bending of anatase, which is inherently formed in a deeper region in the ®lm with a steeper potential slope than in rutile. Thus, the prolonged carrier diffusion length with the lifetime displayed a high performance of surface charge
26
T. Sumita et al. / Applied Surface Science 200 (2002) 21±26
separation in this measurement. This analysis is consistent with actual photocatalytic activities due to decomposition of methylene blue (Fig. 5). Furthermore, anatase ®lm exhibits the transportation of photo-generated holes even for photon energy irradiation less than the bandgap (460 nm), though there were only a few. The photocurrent for irradiation at the photon energy below the bandgap was not obtained on the photoconductivity measurement (Fig. 4). The photoconductivity measurement can reveal the photo-generated carriers behavior in the plane of the ®lms between the planer type electrodes with 100 mm separation on the surface, while PITCS measurement can detect the photo-generated carrier diffusion toward the surface. These experimental results indicate that photo-carriers can be generated close to the surface by irradiation below the anatase bandgap (<3.23 eV) and the generated carrier can diffuse only toward the ®lm surface. Although the origin of precise behavior is not clear at present, anatase has an advantage over rutile for photocatalysis, particularly under sunlight. If both samples are irradiated under the condition of AM 1.0 sunlight (the sunlight at the earth's surface when the sun is at zenith), the spectral irradiance remarkably increases in the region of wavelength (337±460 nm) as shown in Fig. 6b, the total number of photo-generated holes transported toward surface for anatase is approximately 16 times that of rutile by estimation. 4. Conclusion The charge separation ef®ciency of photo-generated carrier behavior at the ®lm surface was evaluated on highly oriented single-phase anatase and rutile by means of PITCS analysis and the correlation with actual photocatalytic activity under conditions in which there was no difference in the ®lm net surface area. Irradiation-induced transported holes of the rutile surface are generated near the surface and
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