Langmuir 2008, 24, 2695-2698
2695
Mobility Enhanced Photoactivity in Sol-Gel Grown Epitaxial Anatase TiO2 Films Hyun Suk Jung,†,‡ Jung-Kun Lee,*,‡,§ Jaegab Lee,† Bo Soo Kang,‡ Quanxi Jia,‡ Michael Nastasi,‡ Jun Hong Noh,|| Chin-Moo Cho,|| and Sung Hoon Yoon|| School of AdVanced Materials Engineering, Kookmin UniVersity, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea, Department of Mechanical Engineering and Materials Science, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, Materials Physics & Applications DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and School of Materials Science and Engineering, Seoul National UniVersity, Shillim-dong, Seoul, 151-744, Korea ReceiVed August 2, 2007. In Final Form: NoVember 15, 2007 Epitaxial anatase thin films were grown on single-crystal LaAlO3 substrates by a sol-gel process. The epitaxial relationship between TiO2 and LaAlO3 was found to be [100]TiO2||[100]LaAlO3 and (001)TiO2||(001)LaAlO3 based on X-ray diffraction and a high-resolution transmission electron microscopy. The epitaxial anatase films show significantly improved photocatalytic properties, compared with polycrystalline anatase film on fused silica substrate. The increase in the photocatalytic activity of epitaxial anatase films is explained by enhanced charge carrier mobility, which is traced to the decreased grain boundary density in the epitaxial anatase film.
The wide band gap semiconductor TiO2 has two important polymorphic phases: anatase, the metastable phase, and rutile, the stable phase.1 Anatase films have received a great deal of attention due to their unique photoactive properties, such as a higher photovoltaic conversion efficiency and photocatalytic activity.2,3 To maximize the photoactivity of anatase films, photoexcited electrons and holes should be effectively extracted to the surface of films without recombination and trapping. Therefore, highly photoactive TiO2 films need to reduce defective regions such as grain boundaries, which can scatter or trap carriers, leading to the recombination of electron-hole pairs. Since the density of defects strongly depends on the structural quality of films, there has been intensive research on the growth of singlecrystal quality anatase films.4-6 Epitaxially grown anatase films have been achieved by several techniques, such as molecularbeam epitaxy, pulsed laser deposition, and atomic layer deposition.4-6 However, the high cost of these processes has restricted the potential application of anatase films. Also, these techniques require a high processing temperature (>500 °C) to crystallize anatase film, making them unsuitable for thermally sensitive substrates such as plastics. Sol-gel techniques are a promising alternate process, because of their relatively inexpensive manufacturing costs and large area and low-temperature fabrication capability. Another advantage of sol-gel-grown epitaxial anatase films is the presence of a hydroxyl group on the surface of films. Lei * Corresponding author. Tel: 412-648-3395. E-mail:
[email protected]. † Kookmin University. ‡ Los Alamos National Laboratory. § University of Pittsburgh. || Seoul National University. (1) Jung, H. S.; Shin, H.; Kim, J. R.; Kim, J. Y.; Hong, K. S.; Lee, J. K. Langmuir 2004, 20, 11732. (2) O’regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (3) Fujishima, A.; Honda, K. Nature (London) 1972, 238, 37. (4) Murakami, M.; Matsumoto, Y.; Nakajima, K.; Makino, T.; Segawa, Y.; Chikyow, T.; Ahmet, P.; Kawasaki, M. Appl. Phys. Lett. 2001, 78, 2664. (5) Park, B. H.; Huang, J. Y.; Li, L. S.; Jia, Q. X. Appl. Phys. Lett. 2002, 80, 1174. (6) Gorbenko, O. Y.; Samoilenkov, S. V.; Graboy, I. E.; Kaul, A. R. Chem. Mater. 2002, 14, 4026.
et al.7 reported the photocatalytic properties of epitaxial and polycrystalline anatase films prepared by rf-sputtering. Both films showed low photocatalytic activity with little difference; the low photocatalytic property was ascribed to the vacuum growth process. The surface of films grown in vacuum lacks hydroxyl groups, which produce highly reactive hydroxyl radicals. This indicates that the photocatalytic properties of sol-gel-derived TiO2 films containing hydroxyl groups should be improved relative to those synthesized by rf-sputtering and that sol-gel grown films provide an ideal venue to study the effect of the epitaxy on the photocatalytic activity. In this paper, we report epitaxial anatase thin films prepared by a sol-gel process and compare their photocatalytic activities with polycrystalline TiO2 films. The superior photocatalytic activities of epitaxial films are discussed in terms of the enhanced charge transport associated with the elimination of grain boundaries. Experimental Section A TiO2 sol containing titanium alkoxide, H2O, HNO3, and ethanol was prepared as follows. Titanium isopropoxide (TTIP, Aldrich, 97%) was dissolved in dry ethanol (Aldrich, 99.9%). The resultant solution was partially hydrolyzed with a mixture of distilled water, nitric acid, and ethanol. The molar ratio of TTIP:H2O:HNO3 was 1:4:0.04. The resultant sol was spun on LaAlO3 (LAO) (001) and quartz substrates at 3000 rpm for 30 s. Though LaAlO3 has a rhombohedral structure, the crystallographic notation of LAO in this paper uses that of a pseudocubic structure whose lattice constant is 3.789 Å. Then, the films were annealed at 300 °C for 5 min to decompose residual organics in the film. The spinning and pyrolysis steps were repeated until the thickness of TiO2 films was 120 nm. Subsequently, the TiO2 films were ramped up to 500 °C at the rate of 1 °C/min and were annealed in O2 atmosphere for 1 h. X-ray diffraction (XRD) measurements were performed to investigate the crystal structure of the TiO2 films. The morphology and crystallographic information of TiO2 films were investigated by transmission electron microscopy (TEM). Atomic force microscopy measurement was carried out to observe the surface morphology. (7) Lei, M.; Sakae, T.; Yoichi, K.; Misao, I.; Shoichi, T.; Kenji, K. Appl. Surf. Sci. 2004, 238, 125.
10.1021/la702379y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008
2696 Langmuir, Vol. 24, No. 6, 2008
Jung et al.
Figure 2. (a) A low-magnification bright-field micrograph of 500 °C-annealed epitaxial anatase film on LaAlO3 and (b) a HRTEM image and selective area diffraction of 500 °C-annealed epitaxial anatase film.
Figure 1. (a) XRD θ-2θ scan results of 500 °C-annealed TiO2 film on LaAlO3 and (b) XRD Φ-scan results for (101) TiO2 and (101) LaAlO3. UV-visible absorption spectra of TiO2 films were measured using a UV/vis spectrophotometer. The frequency dependence of complex impedance was measured using an impedance analyzer from 103 to 107 Hz. This impedance analysis of TiO2 film was performed using a patterned coplanar structure8(see the Supporting Information). The photocatalytic activity of the anatase films was characterized by monitoring the decomposition rate of stearic acid [CH3(CH2)16COOH], which was casted on top of the epitaxial film, following the method reported elsewhere.9-12 The amount of stearic acid was 100 µL and its concentration was 0.07 M in ethanol. The decomposition behavior of the stearic acid was determined by measuring the reduction in the integrated IR absorbance of the C-H stretching vibrations between 2800 and 3000 cm-1. The integrated absorbance of these modes was approximately 3.0 cm-1, corresponding to 9.4 × 1015 molecules/cm2 on a TiO2 film.9 Two 8 W lamps (365 m, Spectronics Corp.) were used as the UV light source. The intensity of incident light on TiO2 films was 2.0 mW/cm2.
Results and Discussion Figure 1a shows the XRD θ-2θ scan results of a TiO2 film on an LAO substrate, which was annealed at 500 °C. Only the (004) peak of the anatase phase was observed, indicating the formation of a pure anatase film with strongly preferred c-axis orientation. Figure 1b is the XRD Φ-scan results for (101) TiO2 (8) Lee, J. S.; Kang, B. S.; Lin, Y.; Li, Y.; Jia, Q. X. Appl. Phys. Lett. 2004, 85, 2586. (9) Paz, Y.; Luo, Z.; Rabenberg, R.; Heller, A. J. Mater. Res. 1995, 10, 2842. (10) Shibata, T.; Irie, H.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 10696. (11) Remillard, J. T.; McBride, J. R.; Nietering, K. E.; Drews, A. R.; Zhang, X. J. Phys. Chem. B 2000, 104, 4440. (12) Jung, H. S.; Lee, J. K.; Nastasi, M.; Kim, J. R.; Lee, S. W.; Kim, J. Y.; Park, J. S.; Hong, K. S.; Shin, H. Appl. Phys. Lett. 2006, 88, 013107.
and (101) LaAlO3. Four-fold symmetry reveals the epitaxial growth of an anatase TiO2 film on the LAO substrate. The minimum temperature for the epitaxial growth in this study was 350 °C. In the Supporting Information, we present XRD data of the epitaxial anatase film, which was grown at 350 °C. This data demonstrates that the sol-gel process is a good way to facilitate the synthesis of the epitaxial anatase films at low temperature. Cross-sectional transmission electron microscope (TEM) images of a 500 °C-annealed film on LAO is shown in Figure 2. A low-magnification bright-field image in Figure 2a demonstrates that the interface between TiO2 and LAO is flat and that the film does not have either microscopic defects or secondary phase such as compounds of Ti and LAO. The high-resolution TEM (HRTEM) image in Figure 2b shows that anatase films were grown epitaxially on LAO. The epitaxial orientation relationship between the anatase TiO2 and the LaAlO3 can be described as (001)TiO2||(001)LaAlO3 and [100]TiO2||[100]LaAlO3.13 As shown in Figure 2b, selective area diffraction (SAD) of the same film is in good agreement with the epitaxial relationship between TiO2 and LaAlO3 suggested by the X-ray diffraction and the HRTEM data. The distinctive diffraction spots marked with a subscript L represent the diffraction pattern from the LAO substrate. The second set of diffraction spots, indicated by a subscript A, is indexed as anatase TiO2. The diffraction spots from anatase are sharp and distinguished, which is consistent with a highly crystalline anatase film. The photodegradation of the stearic acid on anatase films as a function of UV irradiation time is shown in Figure 3a. The decrease in the integrated IR intensity with UV irradiation time is plotted for epitaxial and polycrystalline anatase films in Figure 3b (see the Supporting Information). The stearic acid decomposes faster on epitaxial anatase films than on polycrystalline anatase films. Approximately, 80% of the stearic acid on epitaxially grown anatase films is decomposed in 15 min, while only 23% of the stearic acid is decomposed on polycrystalline films under the same amount of UV irradiation. This demonstrates that the epitaxial anatase film enables the photocatalytic decomposition to happen more efficiently than the polycrystalline anatase film. The photocatalytic activity of TiO2 is determined by several factors, including the surface morphology, light absorption, band gap, preferred crystal orientation, the degree of hydroxylation, and charge transport. In the discussion below, the effects of the twin boundary of LAO substrates are ignored. Given that the grain size of polycrystalline anatase films is smaller than 100 (13) Huang, J. Y.; Park, B. H.; Jan, D.; Pan, X. Q.; Zhu, Y. T.; Jia, Q. X. Philos. Mag. A 2002, 82, 735.
Sol-Gel Grown Epitaxial Anatase TiO2 Films
Langmuir, Vol. 24, No. 6, 2008 2697
Figure 4. UV-visible absorption spectra of 500 °C-annealed anatase films (black line, polycrystalline anatase; red line, epitaxial anatase).
Figure 3. (a) FT-IR spectral changes (in the C-H stretching region) for the photodegradation of stearic acid on a 500 °C-annealed epitaxial anatase film. (b) Normalized absorbance for stearic acid on polycrystalline (black square) and epitaxial (red circle) anatase films as a function of UV irradiation time.
nm, the density of low-angle boundary of anatase films produced by the twins of LAO is much less than that of the grain boundary of the polycrystalline films. Hence, the effect of the low-angle boundary on the carrier scattering is assumed to be negligible compared with that of the grain boundary. The surface morphology of TiO2 film can influence the photoactivity of anatase films. Lee et al.14 observed that a rough TiO2 film contained more Ti3+ on the film surface than a smooth TiO2 film, leading to enhanced photocatalytic activity. The root-mean square surface roughness of epitaxial and polycrystalline films in our study is 4.4 and 5.1 nm, respectively (see the Supporting Information). This indicates that the surface morphology cannot explain the difference in the photocatalytic activity of our anatase films. Another possible source for the enhanced photoactivity is a decrease in the band gap, which would promote the utilization of visible light. Figure 4 shows the UV/vis absorption spectra of both epitaxial and polycrystalline films. The absorption spectra for both the films are almost identical, indicating that the films have the same band gap and that the number of photogenerated carriers affecting the photocatalytic activity was also identical. The preferred orientation (14) Lee, H. Y.; Park, Y. H.; Ko, K. H. Langmuir 2000, 16, 7289.
of TiO2 films may also affect photocatalytic properties of thin films. In our experiments, the (001) orientation in anatase film might contribute to enhancing photocatalytic properties. However, according to the previous reports, (001)-orientated anatase films have not shown better photocatalytic properties than other crystal orientations, such as (101). This indicate that (001)-oriented anatase film in this study should have shown a low photocatalytic activity, since other extrinsic factors such as accessible surface are similar in both epitaxial and polycrystalline films. Our results contradict the predictions of previous studies, clearly showing that the decrease in the grain boundary density of the epitaxial films with (001) orientation compensates the crystallographic disadvantage of (001)-orientated films in terms of the photocatalytic property.15-17 A large amount of hydroxyl groups on TiO2 film may improve the photocatalytic activity. Since the epitaxial and polycrystalline anatase films were synthesized by using the same source materials and heat treatment conditions, both films experienced the same hydroxylation history and therefore the amount of the residual hydroxyl groups on the surface could not be so different for the two kinds of TiO2 films. In addition to the source material and thermal treatment method, the oxygen vacancy affects the concentration of hydroxyl groups. An oxygen vacancy can be a seed to produce the hydroxyl groups on the surface by adsorption of water from air. Therefore, the amount of hydroxyl group is related to the amount of oxygen vacancy. It is reported that the anatase (101) surface contains more oxygen vacancies than the (001) surface.17 This indicates that (001)-oriented films may have a smaller amount of surface hydroxyl group than the polycrystalline anatase film. However, the photocatalytic activity of (001)-oriented epitaxial anatase film is superior to that of polycrystalline film. This indicates that there should be another factor besides the degree of hydroxylation that affects the photocatalytic activities of both the epitaxial and polycrystalline anatase films. One more parameter to be considered is the specific surface area. The difference in the surface areas can change the photocatalytic behavior of the films by changing the area for the photocatalytic reaction.18,19 Instead of directly measuring the specific areas of thin films by the BET method, we used a qualitative method to compare the difference in the surface area of two films. Ru-dye molecules [ruthenium (2,2′-bipyridyl-4,4′-dicarboxylate)2(NCS)2] were adsorbed on TiO2 films and then dissolved in an alkaline alcoholic solution. (15) Kim, B; Byun, D; Lee, J. K.; Park, D. Jpn. J. Appl. Phys. 2002, 41, 222. (16) Tokita, S.; Tanaka, N.; Saitoh, H. Jpn. J. Appl. Phys. 2000, 39, L169. (17) Thomas, A. G.; Flavell, W. R.; Mallick, A. R.; Kumarasinghe, A. R.; Tsoutsou, D.; Khan, N.; Chatwin, C.; Raner, S.; Smith, G. C.; Stockbauer, R. L.; Warren, S.; Johal, T. K.; Patel, S.; Holland, D.; Taleb, T. K.; Patel, S.; Wiame, F. Phys. ReV. B: Condens. Matter 2007, 75, 035105. (18) Baiju, K. V.; Shukla, S.; Sandhya, K. S.; James, J.; Warrier, K. G. K. J. Phys. Chem. C 2007, 111, 7612. (19) Ma, B.; Goh, G. K. L.; Ma, J. J. Electroceram. 2006, 16, 441.
2698 Langmuir, Vol. 24, No. 6, 2008
Figure 5. Optical absorption spectra of desorbed dye solution from each TiO2 thin film (black line, polycrystalline anatase; red line, epitaxial anatase).
Figure 6. Cole-Cole plots for polycrystalline (black square) and epitaxial (red circle) anatase films.
Then, the absorption of the desorbed dye in UV/vis range was measured to compare the relative surface area of two films.1,20 UV/vis absorbance in Figure 5 shows that the amount of dye that was originally absorbed on the films is slightly larger for the polycrystalline film than for the epitaxial film. This result demonstrates that the surface area of the polycrystalline film is slightly larger than that of the epitaxial film and that enhanced photocatalytic ability is not attributed to the specific surface area. The effect of grain boundaries on the conductivity of the anatase films was evaluated using ac impedance spectroscopy; Figure 6 shows the Cole-Cole plots of TiO2 thin films. Only one semiarc was observed in the complex plane for both films. Among several components contributing to the impedance of TiO2 films, the (20) Wang, Z-S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912.
Jung et al.
grain boundary is dominant and the contribution from the bulk and electrode is negligible in the polycrystalline film.21 The large semiarc observed in Figure 5 for polycrystalline TiO2 is attributed to the impedance of the grain boundary. The radius of the semiarc for the epitaxial anatase film is approximately 40 times smaller than that for the polycrystalline anatase film (inset). This remarkably reduced semicircle is due to the significant elimination of grain boundaries in epitaxial anatase and originates from the impedance of the grain component22 (see the Supporting Information). In semiconductors, the grain size and alignment have been primarily emphasized as the factors that strongly influence the conductivity by changing the mobility of carriers. Choi et al. reported that the well-aligned and large Si grains exhibited improved charge mobility.23 Su et al. also studied the effect of grain shape on the electron transport of TiO2 films.24 Films consisting of aligned columnar grains showed much better transport behavior of photogenerated carriers than films composed of randomly oriented grains. The enhanced transport behavior was attributed to the decrease in the number of the grain boundaries that the photogenerated carriers passed. We attribute the improved carrier transport of the epitaxial anatase film of this study to the elimination of grain boundaries, which in turn promotes the extraction of photogenerated carriers and leads to the superior photocatalytic activity. In conclusion, the epitaxial growth of anatase TiO2 films was achieved by a sol-gel process on LaAlO3 substrates. The epitaxial anatase films showed much better photocatalytic activity than polycrystalline anatase films. With the aid of impedance analysis, it was found that the impedance of the epitaxial anatase was significantly reduced, leading to enhanced carrier transport behavior. These results indicate that improved photoactivity observed in sol-gel grown anatase epitaxial films results from improving the carrier extraction behavior. Acknowledgment. The Los Alamos portion of this work has been supported by Laboratory Directed Research and Development at LANL. This work was also supported by the ERC (CMPS, Center for Materials and Processes of Self -Assembly) program of MOST/KOSEF (R11-2005-048-00000-0). Supporting Information Available: Coplanar structured TiO2 film, XRD data, AFM topographies, and FESEM images of epitaxial and polycrystalline TiO2 films. This information is available free of charge via the Internet at http://pubs.acs.org. LA702379Y (21) Kajihara, K.; Yao, T. Phys. Chem. Chem. Phys. 1999, 1, 1979. (22) Chen, L.; Chen, C. L.; Lin, Y.; Chen, Y. B.; Chen, X. H.; Bontchev, R. P.; Park, C. Y.; Jacobson, A. J. Appl. Phys. Lett. 2003, 82, 2317. (23) Choi, W.; Matias, V.; Lee, J. K.; Findikoglu, A. T. Appl. Phys. Lett. 2005, 87, 152104. (24) Su, Y. F.; Chou, T. C.; Ling, T. R.; Sun, C. C. J. Electrochem. Soc. 2004, 151, A1375.
