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Decorating Titanate Nanotubes with CeO2 Nanoparticles Bartolomeu C. Viana,†,| Odair P. Ferreira,‡ Antonio G. Souza Filho,*,†,‡ Carolina M. Rodrigues,‡ Sandra G. Moraes,§ Josue Mendes Filho,† and Oswaldo L. Alves*,‡ Departamento de Fı´sica, UniVersidade Federal do Ceara´ s UFC, P.O. Box 6030, 60455-900, Fortaleza-CE, Brazil, Laborato´rio de Quı´mica do Estado So´lido - LQES, Instituto de Quı´mica, UniVersidade Estadual de Campinas s UNICAMP, P.O. Box 6154, 13083-970, Campinas-SP, Brazil, Laborato´rio de Energia e Meio Ambiente, UniVersidade Metodista de Piracicaba s UNIMEP, RodoVia SP 306 - km 24, 13450-000, Santa Ba´rbara; d’Oeste-SP, Brazil, and Departamento de Fı´sica, UniVersidade Federal do Piauı´ s UFPI, 64049-550, Teresina-PI, Brazil ReceiVed: July 18, 2009; ReVised Manuscript ReceiVed: October 1, 2009
In this work, we report the synthesis, characterization, and application of Ce ion-exchanged titanate nanotubes (Ce-TiNTs). The physicochemical properties of these Ce-TiNTs are discussed in comparison with their pure titanate nanotube counterparts. The transmission electron microscope images showed that the Ce-TiNTs have the same morphology as that of pristine nanotubes and their external walls are decorated with cerium oxide nanoparticles. The mechanism of nanoparticle formation is based on the precipitation of Ce ions at the nanotube surface. We observed a red shift of the absorption band edge toward the visible region whose main contribution comes from the Ce ion intercalation. A red shift of vibrational modes associated with metal ion-oxygen interaction was observed and identified as being due to the effect of Ce addition to the lattice as well as the anchoring of CeO2 nanoparticles to the nanotube wall. We show that this hybrid system is promising for applications in photocatalysis using the blue region of the electromagnetic spectrum. This was demonstrated for photodegradation of Reactive Blue 19 textile dye. 1. Introduction Many technological applications that make use of the energy available in the sunlight have been gaining importance because this energy is free and also environmentally friendly. The correct choice of materials with suitable properties for using this clean energy is a key point for transforming the promising potential into real world technology, and band gap engineering is an approach used for achieving this goal. Combining the sizedependent properties of nanomaterials, such as the tunability of the band gap toward the visible range and their large surfaceto-volume ratio, opens up the opportunity for developing a new generation of very efficient materials for photocatalysis and related applications. TiO2-based materials have been intensively investigated because their physical and chemical properties are suitable for important applications in solar cells, gas sensors, photocatalysis, and other environmentally related applications.1-7 In particular, one-dimensional or elongated TiO2-based nanostructures, such as nanotubes and nanoribbons, exhibit improved properties different from their bulk counterparts, and therefore, they have a great potential for many applications, especially for photocatalysis.2,8 Their elongated morphology allows the lifetime of photogenerated carriers to be extended relative to other TiO2based materials.9 Titanate nanotubes have a large surface area responsible for increasing the active surface.10,11 They have also been tested * To whom correspondence should be addressed. Phone: +55 35213147. Fax: +55 35213023. E-mail:
[email protected] (A.G.S.F.),
[email protected] (O.L.A.). † Universidade Federal do Ceara´ s UFC. ‡ Universidade Estadual de Campinas s UNICAMP. § Universidade Metodista de Piracicaba s UNIMEP. | Universidade Federal do Piauı´ s UFPI.
for using as catalysts in heterogeneous photocatalysis and have shown excellent performance for degrading textile dyes, which makes these titanate nanomaterials very important ecomaterials.12,13 However, the absorption band edge of titanate nanotubes is below 400 nm,14 which makes them not very efficient for converting the free available photon energy from the sun in the visible range. Therefore, band gap engineering of titanate nanotubes, which consists of performing chemical modifications for shifting the absorption band edge of titanate nanotubes toward the visible range, is an important issue to be addressed in the search for the nanomaterials that would make efficient photocatalysts. The optical properties of sodium titanate nanotubes may be effectively modified and controlled via ionexchange reactions or intercalation of other metals. Furthermore, the presence of Na+ in the lattice decreases the photocatalytic activity of TiO2 because they are efficient recombination centers.15 Therefore, submitting the titanate nanotubes to ionexchange reactions involving some transition metals is important, and the literature has reported that the absorption band edge experiences a red shift toward the visible range.16-18 These exciting results make the intercalated/modified titanate nanotubes promising systems for photocatalysis. To the best of our knowledge, there are no investigations on Ce ion-intercalated titanate nanotubes. On the other hand, ceria has attracted much attention due to its technological applications as an active catalyst, as a polishing agent, for sunscreens, for use in solid oxide fuel cells,19,20 as an electrode material for gas sensors, silicon-on-insulator structures,21 and high-Tc superconductors,20 among others. The catalytic properties of ceria can also be increased when prepared as nanoparticles with high surface-to-volume ratios.22,23 In this paper, we present the synthesis and characterization of titanate nanotubes with Ce ions incorporated in the nanotube
10.1021/jp9068043 CCC: $40.75 2009 American Chemical Society Published on Web 10/26/2009
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wall through an exchange process, as described by Ferreira et al.24 The ion-exchange process leads to the formation of a new hybrid system that consists of Ce ion-intercalated titanate nanotubes along with the presence of 4 nm sized CeO2 nanoparticles decorating the titanate nanotube surfaces (CeTiNT). The absorption band edge was red shifted from the nearultraviolet to the visible-light range. The photocatalytic activity of this novel nanotube/nanoparticle hybrid system was then evaluated by studying the photodegradation of Reactive Blue 19 textile dye. 2. Experimental Section All chemicals (reagent grade, Aldrich, Merck, or Baker’s Analyzed) were used as received, without further purification process. All solutions were prepared with deionized water. 2.1. Titanate Nanotubes Preparation. Titanate nanotubes were prepared by the hydrothermal method. In a typical synthesis, 2.00 g (25.0 mmol) of TiO2 (anatase) was suspended in 60 mL of 10 mol L-1 aqueous NaOH solution during 30 min. The white suspension formed was transferred to a 90 mL Teflon-lined stainless steel autoclave and kept at 165 ( 5 °C for 170 h. After cooling to room temperature, the resulting white solid was separated by centrifugation and washed several times with deionized water until pH 11-12. After drying under vacuum, titanate nanotubes rich in sodium were obtained (NaTiNT). 2.2. Ion-Exchange Reactions and Decoration of Titanate Nanotubes with CeO2 Nanoparticles. It has been reported in the literature that, in protonated titanate nanotubes (H-TiNT), the Mn+-exchange reactions occurs faster than in Na-TiNTs.25 However, in this work, we used Na-TiNTs for exchanging reactions because the methodology we used in the synthesis generates Na-TiNTs and they are also suitable for exchanging reactions without going through the intermediate step of protonation. Metal (Ce4+) ion-exchange reactions were carried out by suspending 100 mg of Na-TiNTs in 100 mL of 0.05 mol L-1 aqueous solutions of (NH4)2Ce(NO3)6. The suspension was left under magnetic stirring for 24 h at room temperature (at about 25 °C). The solid product was isolated by centrifugation under 3000 rpm, and it was washed several times with deionized water, aiming to remove remaining soluble ions (Ce4+, NH4+, NO3-) from the precursor. The solid product was dried under vacuum conditions during 6 h to obtain the Ce-intercalated titanate nanotubes (Ce-TiNTs) decorated with CeO2 nanoparticles. 2.3. Photocatalysis Experiments. All photocatalytic experiments were performed in a 450 mL cylindrical photoreactor apparatus, equipped with a Phillips lamp that simulates solarlight activity. This photoreactor is composed of five lamps positioned parallel to the reactor top. Batch experiments were performed at 25 °C. In a typical reaction, 400 mL of 50 mg L-1 Reactive Blue 19 textile dye and 200 mg of photocatalysts were added to the reactor under magnetic stirring, which proved to supply enough oxygen for photodegradation under an oxidative atmosphere. The dispersion was sampled and analyzed after certain irradiation time intervals. The first sample was taken at the end of the dark adsorption period, just before the light was turned on. 2.4. Characterization Techniques. Fourier transform infrared (FTIR) spectroscopy was recorded using the KBr pellet technique on a Bomen FTLA 2000 spectrometer. A total of 32 scans and a resolution of 4 cm-1 were used for getting spectra with good signal-to-noise ratios. The Raman spectroscopy
Figure 1. Fourier Transform Infrared spectra of Na-TiNT (lower trace) and Ce-TiNT (upper trace) samples. (inset) High wavenumber region.
experiments were performed on a JOBIN-YVON T64000 spectrometer with an Olympus microscope. A 532 nm laser from Verdi V-5 was used for exciting the Raman spectra. Low laser power density was used in order to avoid sample overheating. A spectral resolution of 1 cm-1 was used, and measurements were performed using a backscattering geometry. X-ray powder diffraction (XRD) patterns were obtained with a Shimadzu XRD7000 diffractometer, using Cu KR (λ ) 1.5406 Å) radiation operating with 30 mA and 40 kV. A scan rate of 1° min-1 was used. Energy-dispersive X-ray spectroscopy (EDS) data were collected using a Noran system S1X (Thermo Electron Corporation, model 6714A01SUS-SN) probe attached to a scanning electron microscope. Transmission electron microscopy (TEM) images were obtained using a JEOL-JEM 2100 ARP and FEITECNAI G2S-Twin setup, both operating with 200 kV. The TEM samples were prepared by dropping an aqueous suspension of sample powder on a holey carbon-coated copper grid and letting the water evaporate at room temperature. UV-vis absorption spectroscopy measurements of titanate samples were performed over the wavelength range of λ ) 200-800 nm using a Varian Cary 5G equipped with a diffuse reflectance integration sphere attachment. A Shimadzu Total Organic Carbon (TOC5000A) analyzer was used for determination of the carbon content in solution. Absorption spectra of solutions were obtained using a UV-visible spectrophotometer, Shimadzu model UV-1650 PC. 3. Results and Discussion 3.1. Structure, Composition, and Morphology. In Figure 1, we show the FTIR spectra of Na-TiNTs (lower trace) and Ce-TiNTs (upper trace). The FTIR spectra of Na-TiNTs and Ce-TiNTs are similar, thus suggesting that the structure of the nanotube is not affected very much by Ce4+ intercalation because the structure of the nanotube wall is composed primarily of Ti-O octahedrons. However, the peak located about 890 cm-1 (lower trace) was affected by Ce4+ intercalation (upper trace), leading to a decrease in the vibrational energy of the Ti-O octahedrons and resulting in a “softening” of the internal Ti-O vibrations. The changes in the stretching modes for terminal Ti-O bonds (890 cm-1) can be understood in terms of the interaction of the Ce ion with the terminal bonds. The large width of the infrared bands prevents a detailed analysis of changes in the structure of the nanotubes due to the ionexchange reactions by looking at their vibrations modes, but further understanding can be obtained from Raman spectroscopy.17,26,27 In the inset to Figure 1, we show the high wavenumber region of samples and we identify the vibrations
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Figure 2. Raman spectra of Na-TiNTs (lower trace) and Ce-TiNTs (upper trace) samples.
Figure 3. X-ray diffraction patterns of Na-TiNTs (lower trace) and Ce-TiNTs (upper trace). The JCPDS standard for bulk CeO2 is also shown as vertical lines at the bottom of the Ce-TiNT pattern.
related to water molecules and hydroxyl groups at 1640 and 3400 cm-1, respectively. We did not observe the vibrational band at 1400 cm-1, which indicates that the NH4+ were not intercalated in the titanate nanotubes lattice. The NH4+ intercalation in the Na-titanate nanotubes is possible, but under our experimental conditions, we did not observe its intercalation, as indicated by the FT-IR spectrum. The Raman spectra for Na-TiNTs (lower trace) and for CeTiNTs (upper trace) are shown in Figure 2, and they show some differences regarding the peak positions. The Raman spectrum of Na-TiNTs exhibits vibrational modes at about 160 and 196 cm-1, which can be assigned to lattice modes and Na-O-Ti modes. Although a considerable amount of data are reported in the literature, it is not yet clear the atomic structure of the NaTiNTs and a definite assignment of the various Raman modes is not yet established as well. Recent papers by Gao and co-workers25,28 assigned the various Raman modes to the lepidocrocite-type layered structure based on the spectroscopic data of unrolled and restacked nanotubes. The modes at about 280, 454, and 663 cm-1 were assigned to 2D lepidocrocitetype TiO6 layers. Regardless, the structure of these modes are related to vibrations from TiO6 octahedrons and the band at about 908 cm-1 is related to terminal Ti-O bonds (stretching) directed either toward the interlayer space or to the outer surface of the nanotubes.29-37 We observed that Na-TiNTs and CeTiNTs have the same Raman spectral features of intermediate energy (TiO6 octahedron modes), thus showing that the intermediate energy modes are not much affected by Ce4+ intercalation except the mode at 280 cm-1, which is red shifted after Ce4+ intercalation. The mode at 908 cm-1 that is related to terminal Ti-O bonds was highly affected by the Ce4+ intercalation, experiencing a red shift of about 70 cm-1. The modes at about 160 and 195 cm-1 were also red shifted when Ce4+ replaces Na+. These results may be understood due to Ce4+ interaction that softens some vibrations. We did not observe the presence of vibrational modes related with other crystalline phases. In Figure 3, we show the X-ray diffraction patterns of NaTiNT and Ce-TiNT samples. The XRD pattern of Na-TiNTs (lower trace) resembles that observed by Chen et al., having some small discrepancies that suggest a scroll-like crystalline structure closer to that of Na2-xHxTi3O7.24,38 The scroll-like structure of the titanate nanotubes does not allow an exact correlation of the nanotubes’ symmetry with the Na2-xHxTi3O7 crystalline bulk phase. There is no symmetry along the nanotubes’ radial direction, and the concept of crystal structure does only apply along the nanotubes’ axis for each layer. Therefore, the diffraction planes assigned in the lower panel of Figure 3
are associated with what would be expected for a trititanatelike phase. The XRD pattern of Ce-TiNTs (upper trace) resembles that of the Na-TiNT sample, given some assumptions. Variations in the relative intensity of the different diffraction peaks for Na-TiNTs and Ce-TiNTs are observed and can be related to intercalation of Ce4+, replacing Na+. The diffraction peaks close to 10° in the XRD pattern of Ce-TiNTs are slightly shifted toward lower 2θ values, when compared with Na-TiNTs, thus indicating an increase in the interlayer distance, which is caused by exchanging Na+ by Ce4+ in the nanotube layers. The profile of this peak is broad and asymmetric, suggesting that the chemical environment between the layers is disordered. These results allow us to conclude that the Ce ion-exchanged titanate nanotubes have the same tubular morphology as that of pristine samples. The broad diffraction peak observed at about 58° cannot be attributed to the pristine titanate nanotube structure. This peak is identified as the (311) plane of crystalline CeO2. Furthermore, shoulders are observed at about 33° and 48°, which are assigned to the (200) and (220) planes of CeO2, thus suggesting the presence of a CeO2 phase in the Ce-TiNT samples. The angular position 2θ and the relative intensity of the diffraction peaks of bulk CeO2 are indicated by the vertical lines in Figure 3 (JCPDS 78-0694). The CeO2 nanoparticle average size could be estimated from X-ray line broadening analysis using the Scherrer equation.21 Because of the interference of the titanate nanotube diffraction peaks, we have used only the CeO2 (311) diffraction peak for estimating the nanoparticle size using X-ray diffraction data. The average size of CeO2 nanoparticles was roughly estimated as being about 4 nm. The CeO2 band cannot be observed in infrared and Raman spectra (Figures 1 and 2) because of the TiO6 octahedron vibrations.29 Moreover, the absence of a CeO2 vibrational signature could be related to the fact that CeO2 nanoparticles are very small, which leads to a very small vibrational signal, and the amount of the scattering centers is much larger for titanate nanotubes.21 The chemical compositions of Na-TiNTs and Ce-TiNTs samples were investigated by energy-dispersive spectroscopy. The EDS spectra of the samples were obtained, and the results are shown in Figure 4. In the case of the Ce-TiNTs sample, we have not observed the presence of a Na signal in the EDS data. By examining the spectrum shown in Figure 4, we can conclude that ion-exchange reactions (Ce4+ replacing Na+) were very efficient. However, the Ce/Ti ratio (0.23) in Ce-TiNTs is smaller than the Na/Ti ratio (0.53) in Na-TiNTs because the Ce signal also comes from the nanoparticles decorating the nanotubes. To confirm the presence of CeO2 nanoparticles and their morphology, we performed transmission electron microscope
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Figure 4. Energy-dispersive spectra (EDS) of Na-TiNTs (lower trace) and Ce-TiNTs (upper trace) samples.
measurements. Figure 5a,b shows high-magnification TEM images of the pristine titanate nanotubes and the nanotubes submitted to Ce4+ ion-exchange reactions, respectively. Pristine titanate nanotubes have a tubular morphology, and they are multiwalled with an average outer (inner) diameter of 10 nm (5 nm) and a length of several tens of nanometers. From Figure 5b, we can observe that, after the Ce4+ ion-exchange reaction, the modified titanate nanotube samples preserve the tubular morphology and average diameters. However, the Ce-TiNTs is decorated with crystalline nanoparticles, as is shown in Figure 5c. When the TEM images were analyzed, the average nanocrystal size was found to be about 3.9 nm (inset of Figure 5b), which agrees very well with X-ray diffraction estimates. We used about 50 particles to construct the histogram shown in the inset to Figure 5b. Therefore, we conclude that the Ce4+ ionexchange reactions do not affect the morphology of the pristine titanate nanotubes and that process leads not only to the substitution of Na+ by Ce4+ ions but also to the formation of crystalline CeO2 nanoparticles on the external walls of the nanotubes, thus forming a new hybrid system. The formation of CeO2 nanoparticles on the nanotube surface occurs when the nanotube is added to the (NH4)2Ce(NO3)6 aqueous solution via the precipitation mechanism. The precipitation on the tube surface is due to the highly alkaline pH of the nanotube surface that contains OH- adsorbed on the walls since the samples of pristine nanotubes were washed only until a pH of 11-12 was reached. In Figure 6, we show the high-resolution TEM (HRTEM) image of a typical titanate nanotube decorated with crystalline CeO2 nanoparticles. The inset to Figure 6 shows the Fourier transform pattern of the HRTEM image. The estimated atomic interplane distance (at about 0.31 and 0.32 nm) for the nanocrystals is close to the interplanar distance between the (111) planes of CeO2. The small differences between nanocrystals’ atomic interplane distances can be caused by different concentrations of oxygen vacancies in the nanocrystals.39 3.2. UV-vis Absorption. In Figure 7, we show the UV-vis absorption spectra for Na-TiNTs and Ce-TiNTs. We can observe that Na-TiNTs have a strong band edge absorption in the ultraviolet region (below 350 nm), which limits their use as a heterogeneous catalyst operating with excitation in the visible range. The addition of Ce4+ to the titanate nanotube structure shifts the band edge absorption to about 450 nm, thus making it possible to use these CeO2 decorated titanate nanotubes as photocatalysts operating with excitation in the violet-blue region of the electromagnetic spectrum. This red shift in the band edge absorption can be understood as being due to the effects of Ce4+ addition (intercalation) and/or the presence of CeO2 nanoparticles, which change the band gap energy of the system. The band gap shift toward the visible range has also been observed for titanate nanotubes intercalated with other metals, such as
Figure 5. TEM images of (a) Na-TiNTs and (b) Ce-TiNTs. (c) HRTEM image of Ce-TiNTs. The normal distribution of nanocrystal size is also shown in the inset of (b). In (c), we show a selected area for illustrating that the nanoparticles decorating the titanate nanotubes are crystalline.
Fe, Bi, Cd, Ni, and Co.14,16,19,40,41 It is seems that the red shift observed in Ce-TiNTs has a main contribution from the Ce ion intercalation because CeO2 nanoparticles with a diameter distribution close to that of the nanoparticles that are decorating the titanate nanotubes have a 3.44 eV band gap.42 By using standard Kubelka-Munk methodology, we estimated the band gap for our Ce-TiNTs as being 2.70 eV, which lower than the 3.45 eV bang gap of Na-TiNTs. This band gap is comparable with what has been reported for Co-TiNTs14 and Ni-TiNTs16 and lower than for Bi- and Cd-TiNTs.16 The exact contribution of intercalation and nanoparticles to the band gap change needs further studies.
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Figure 8. Relative absorbance peak values (solid circles) and total organic carbon (TOC) (open circles), measured as a function of exposure time of Reactive Blue 19 textile dye to Ce-exchanged titanate nanotubes when illuminated with a lamp that simulates a sunlight spectrum.
Figure 6. HRTEM image of selected CeO2 nanoparticles anchored onto titanate nanotubes. The inset indicates the Fourier transform pattern of the nanoparticles.
Figure 7. Diffuse reflectance UV-vis spectra, plotted as absorbance, of Na-TiNTs (left trace) and Ce-TiNTs (right trace) samples and BaSO4, used as blank reference (at the bottom).
3.3. Photocatalytic Activity. To check the photocatalytic performance of the Ce-TiNTs, we have performed heterogeneous photocatalysis experiments using Reactive Blue 19 textile dye as the chemical specimen to be oxidized. This chemical dye is widely used in the textile industry for coloring cotton fibers. The as-prepared Ce-TiNTs exhibited enhanced photocatalytic activity for degradation of Reactive Blue 19 dye under simulated solar-light irradiation as compared with pristine titanate nanotubes. The photocatalytic properties of the powdered sample were evaluated through the photodegradation of aqueous Reactive Blue 19 dye solution under solar-light irradiation. Color reduction and decrease of total organic carbon (TOC) after solarlight irradiation for 120 min were observed, as shown in Figure 8. Under the same experimental conditions the pristine titanate nanotubes did not present photocatalytic activity. The photocatalyitc activity of the Ce-TiNTs was also tested under natural sunlight, and the results (not shown here) showed a color reduction of about 30% in 50 min. 4. Conclusions Summarizing, we have reported the synthesis and characterization of Ce-TiNTs. The ion-exchange reactions performed with titanate nanotubes lead to the intercalation of Ce4+ in the nanotube wall as well as the decoration of their outer surface
with CeO2 nanocrystals. Results from several techniques showed that the ion-exchange reaction preserved the atomic structure of the titanate nanotubes but introduces changes in their optical properties. The synergistic effect of titanate nanotubes and CeO2 nanoparticles leads to a hybrid system whose absorption band edge is shifted toward the visible range. The results point out that the red shift observed in Ce-TiNTs has a main contribution from the Ce ion intercalation, but the origin of this red shift needs to be further investigated in order to separate the contribution of Ce4+ intercalation from the presence of CeO2 nanoparticles. The photocatalytic properties of the Ce-modified titanate nanotube samples were evaluated through the photodegradation of Reactive Blue 19 textile dye that showed a 70% reduction of TOC along with a 65% color reduction under the described experimental conditions. Color reduction was also obtained when using Ce-TiNTs under natural sunlight conditions. Thus, the Ce-TiNTs obtained in this work open up the opportunity of using these novel hybrid nanostructures in photocatalysis with improved efficiency, as compared with other TiO2-based photocatalysts. Acknowledgment. A.G.S.F. acknowledges the Visiting Research grant 08/58194-7 from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and CNPq (Grant 306335/ 2007-7). Financial support from the Brazilian funding agencies CNPq, CAPES, FUNCAP, and FAPESP is gratefully acknowledged. The authors are indebted to MSc. Eduardo Padro´n Herna´ndez from CETENE, Recife-Brazil, for his assistance with the TEM images and to Prof. C. H. Collins (IQ-UNICAMP, Brazil) for a critical reading of the manuscript. B.C.V. thanks the Laborato´rio Nacional de Luz Sincrotron (LNLS, Brazil) for training and facilities in TEM. This is a contribution of INCT of functional complex materials (INOMAT). References and Notes (1) Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428. (2) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (3) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (4) Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1993, 97, 12651. (5) Idakiev, V.; Yuan, Z. Y.; Tabakova, T.; Su, B. L. Appl. Catal., A 2005, 281, 149. (6) Li, J. R.; Tang, Z. L.; Zhang, Z. T. Electrochem. Commun. 2005, 7, 62. (7) Bavykin, D. V.; Walsh, F. C. Eur. J. Inorg. Chem. 2009, 2009, 977. (8) Tenne, R. Nat. Nanotechnol. 2006, 1, 103. (9) Yamashita, H.; Ichihashi, Y.; Takeuchi, M.; Kishiguchi, S.; Anpo, M. J. Synchrotron Radiat. 1999, 6, 451. (10) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gratzel, M. ACS Nano 2008, 2, 1113.
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JP9068043
