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Nanocrystalline anatase TiO2 derived from a titanate-directed route for dye-sensitized solar cells
Journal of Photochemistry and Photobiology A: Chemistry 188 (2007) 19–24
Nanocrystalline anatase TiO2 derived from a titanate-directed route for dye-sensitized solar cells Po-Tsung Hsiao, Kai-Ping Wang, Chih-Wei Cheng, Hsisheng Teng ∗ Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received 24 May 2006; received in revised form 29 August 2006; accepted 16 November 2006 Available online 19 November 2006
Abstract TiO2 nanoparticles used in numerous applications are generally prepared from the sol–gel method. Because of the competitive, rather than exclusive, formation of the three TiO2 polymorphs, anatase, brookite and rutile, in the sol–gel synthesis, phase-pure nanoparticles can hardly be obtained. The present work demonstrates a unique route, alternative to the conventional sol–gel method, to prepare high-purity anatase TiO2 colloids, which can be deposited as electrodes for dye-sensitized solar cells (DSSCs) to facilitate electron transport and avoid charge recombination. In this developed route, a titanate with its TiO6 octahedra arranged in a zigzag configuration, which is also a characteristic feature of anatase TiO2 , is produced as an intermediate. Raman analysis shows that a phase-pure anatase TiO2 colloid is prepared from the developed route, while the TiO2 derived from the sol–gel at the same temperature is predominantly composed of anatase with the presence of a minute amount of rutile and brookite. Because of the high-purity in anatase phase, the TiO2 colloid derived from the titanate-directed route is shown to constitute a mesoporous film exhibiting high performance in a dye-sensitized solar cell. © 2006 Elsevier B.V. All rights reserved. Keywords: Phase-pure anatase TiO2 ; Titanate nanotube; Dye-sensitized solar cell; Sol–gel; Nanocrystalline TiO2 film
1. Introduction Dye-sensitized solar cells (DSSCs) that provide a costeffective alternative to conventional solar cells have attracted intensive studies [1–8]. The cells consist of a dye-adsorbed mesoporous metal-oxide film filled with iodide/triiodide redox electrolyte and a Pt counter electrode. The mesoporous metaloxide film with a large surface area plays a crucial role for the high performance of DSSCs [9–11]. It not only adsorbs a large number of dye molecules for efficient light-harvesting, but also serves as a semiconductor to provide a pathway for electron percolating through the film. The crystalline structure of the metal-oxide semiconductor significantly affects the transport efficiency of the injected electrons [12–18]. Anatase TiO2 has been considered so far the optimum choice of the semiconductor constituting the mesoporous film [19]. In this application phase-pure anatase TiO2 is expected to facilitate efficient electron transport and avoid charge recombination [12–19].
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Corresponding author. Tel.: +886 6 2385371; fax: +886 6 2344496. E-mail address: [email protected] (H. Teng).
1010-6030/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2006.11.013
To fabricate the mesoporous TiO2 film, hydrolysis of a titanium alkoxide through a sol–gel route was generally employed to prepare a colloidal suspension that was used for deposition on the substrate [1–8,20–22]. Anatase, rutile and brookite are the three TiO2 polymorphs [23,24]. Because in the sol–gel route TiO2 nanoparticles were derived from condensation of the hydrated Ti4+ ions, a TiO2 colloid of phase-pure anatase can hardly be obtained even if the hydrolysis and condensation kinetics are delicately controlled [25–28]. Previous studies using Raman spectroscopic analysis showed that brookite phase was generally present in the anatase TiO2 colloids derived from the sol–gel route [27,28]. The anatase polymorph, differing from the other two, owns a specific zigzag configuration of the edgeshared TiO6 octahedra along a particular crystalline direction. Taking advantage of the zigzag feature, the present work wishes to report a unique route, alternative to the sol–gel method, to prepare the anatase TiO2 film for DSSCs. In this developed route, a titanate with its TiO6 octahedra arranged in a zigzag configuration would be produced as an intermediate. It has been reported that the thermal treatment of TiO2 particles, either anatase or rutile, in alkali solutions resulted in formation of nanotubes [29–51]. The process of this treatment
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is rather unsophisticated. The nanotubes are constructed of a titanate [37–51], of which the structure has been assigned to titanates such as A2 Ti3 O7 [37–45], lepidocrocite-type species [47,49], or A2 Ti2 O5 ·H2 O [46,48,50], where A represents Na and/or H. Similar to anatase TiO2 , all these titanates own the zigzag configuration of the edge-shared TiO6 octahedra. Upon pH value adjustment, the titanate nanotubes have been shown to proceed with local shrinkage of the layers and transform into meta-stable anatase TiO2 [50]. We expect a hydrothermal treatment to transform this titanate into phase-pure TiO2 anatase because of their similarity in possessing the zigzag feature for the TiO6 arrangement [51]. 2. Experimental An anatase TiO2 colloid derived from the titanate described above was used to fabricate the mesoporous film for DSSCs in the present work. To give a suspension of the titanate intermediate, 3 g of a commercially-available TiO2 powder (P25, Degussa AG) was hydrothermally treated with 100 mL of 10 N NaOH in an autoclave at 130 ◦ C for 20 h, followed by repeated washing with 0.1 N HNO3 to reach a pH value of ca. 1.5. The titanate formed was in a tubular shape, of which the image has been shown elsewhere [50]. The anatase TiO2 colloid for DSSCs was obtained by autoclaving the low-pH titanate suspension at 240 ◦ C for 12 h. This titanate-derived TiO2 is designated as H240. It has to be noted that the preparation of H240 is rather unsophisticated, requiring no delicate optimization of the reaction condition. Auxiliary experiments revealed that the particle size could be easily tuned by adjusting the pH value and temperature for the final autoclaving. The P25 and the “Gr¨atzel-type” sol–gel TiO2 colloidal suspensions were used as well to fabricate the mesoporous films. In preparing the P25 colloid, 14 mL of deionized water and 0.2 mL of acetyl acetone was added to 6 g of the P25 powder in a mortar and pestle for grinding to mechanically separate aggregated TiO2 particles. An appropriate amount of Triton X100 surfactant was added to the mixture to give the P25 dispersion. The method for preparing the colloidal TiO2 dispersion from the sol–gel route was analogous to those reported by Gr¨atzel and co-workers [20–22]. In brief, 20 mL of titanium isopropoxide was added to 120 mL of 0.1 M nitric acid under vigorous stirring. Immediately after the hydrolysis, the slurry was heated to 80 ◦ C and stirred vigorously for 2 h, to achieve peptization. Then, the colloidal suspension was introduced into an autoclave that was heated at 240 ◦ C for 12 h. The resulting TiO2 colloid from the sol–gel synthesis is designated as S240. The phase identification of the samples was conducted with powder X-ray diffraction (XRD) using a Rigaku RINT2000 diffractometer equipped with Cu K␣ radiation. The phase was analyzed with Raman scattering using a Dilor XY-100 spectrometer. The microstructure was explored with a high-resolution transmission electron microscope (HRTEM; Hitachi FE-2000). The N2 surface area was determined with the Brunauer-EmmettTeller (BET) equation at −196 ◦ C, using an adsorption apparatus (Micromeritics, ASAP 2010).
The solution of each TiO2 colloid, H240, S240 or P25, was mixed with polyethylene glycol (PEG; Fluka, 20,000 in molecular weight) to form a viscous TiO2 dispersion at a PEG/TiO2 ratio of 0.4. The dispersion was then spread onto a conducting glass substrate (F-doped SnO2 overlayer TCO glass; TEC 8, Hartford Glass Co.). The apparent area of the film was 0.5 × 0.5 cm2 . The TiO2 -coated (0.5 × 0.5 cm2 ) substrate was subsequently calcined at 450 ◦ C in air for 30 min to form a mesoporous electrode. Transmission spectra of the different TiO2 films supported on the conducting glass were obtained by using a UV-Vis spectrometer (Varian CARI-100) and corrected for conducting glass background. The ruthenium complex dye, N3 (cis-di(thiocyanate)bis(2,2 bipyridyl-4,4 -dicarboxylate) ruthenium (II); ruthenium 535, Solaronix), was used as the sensitizer for DSSCs. The dyesensitized TiO2 electrode and a Pt-coated conducting glass were assembled to form a cell by sandwiching a redox (I− /I3 − ) electrolyte solution, which was composed of 0.1 M LiI, 0.05 M I2 , 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile. In the cell performance test, an Oriel 300-W Xe lamp served as a light source in conjunction with an IR filter (Oriel 59044). The AM 1.5 Globle filter (Oriel 81094) was placed in the light beam to simulate the AM 1.5-type solar emission, with an intensity on the cell fixed at 100 mW cm−2 . 3. Results and discussion It has been reported that treating TiO2 nanoparticles in NaOH resulted in formation of lamellar sheets due to the rupture of Ti O Ti bonds [29,30]. Washing the treated sample with water and/or acid to partially remove the electrostatic charge led to formation of nanotubes. Fig. 1 shows the XRD patterns of the materials obtained from hydrothermal treatment of P25 in NaOH and those with a subsequent washing to pH values of 6.9 and
Fig. 1. XRD patterns of the titanate specimen obtained from hydrothermal NaOH treatment on TiO2 at 130 ◦ C, those with subsequent washing to pH values of 6.9 and 1.5, and the TiO2 colloid derived from hydrothermally treating the pH 1.5 titanate (H240) and that from the sol–gel route (S240). The standard diffraction pattern of H2 Ti2 O5 ·H2 O from JCPDS is provided at the bottom of this figure.
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Fig. 3. Raman spectra of TiO2 colloids prepared from the sol–gel route (S240) and from the hydrothermal-treatment on a titanate (H240).
Fig. 2. The scheme for the formation of anatase TiO2 from a titanate (H2 Ti2 O5 ·H2 O) and a tyical TEM image of the TiO2 colloid (H240) derived from this scheme. The image shows the lattice fringes of the anatase phase. In the scheme the projection along the [0 0 1] of the titanate exhibits layers of the TiO6 octahedra edge-shared in a zigzag co configuration, which can be correlated with the principal unit layer of the anatase TiO2 projected along [1 0 1].
1.5. By comparing the XRD data of the present specimens with those of the H2 Ti2 O5 ·H2 O titanate documented in the Powder Diffraction files of JCPDS (bottom of Fig. 1) [52,53], the structure of the specimen obtained right from the alkali treatment and that washed to pH 6.9 should be assigned to that of A2 Ti2 O5 ·H2 O, which comprises of two-dimensional layers in which TiO6 octahedra are combined through edge sharing in a zigzag configuration (Fig. 2) [52,53]. The inter-layer ions, H+ , Na+ and OH− , are not shown in the figure [50]. With further decrease of the pH to 1.5, there shows the appearance of anatase TiO2 phase (Fig. 1) with the fading of the titanate characteristic peaks, especially the (2 0 0). Fig. 2 shows the principal unit layer of anatase TiO2 projected along [1 0 1], exhibiting a zigzag configuration of the edge-shared TiO6 octahedra. This observation reflects the intimate connection between the structures of H2 Ti2 O5 ·H2 O and anatase TiO2 . Thus, using titanate as an intermediate for high-purity anatase synthesis, as shown in the scheme in Fig. 2, would provide a feasible route, alternate to the sol–gel method, for fabricating TiO2 film electrodes used in applications such as DSSCs. The pH 1.5 suspension was subjected to hydrothermal treatment at 240 ◦ C to give the H240 colloid. The TEM image of this TiO2 colloid is shown in Fig. 2. The lattice fringe of the crystals in Fig. 2 reflects that anatase is the constituting phase of this titanate-derived TiO2 colloid. Auxiliary experiments showed that the size of these nanocrystals is an increasing function of the treatment temperature. The powder X-ray diffraction analysis of
H240 was conducted and the data are compared with those of the sol–gel derived S240 in Fig. 1. Only the anatase phase can be obviously observed in the patterns for H240 and S240. On the other hand, P25 is known to contain both anatase (70%) and rutile (30%) [2]. The particle sizes, examined with TEM images, are of ca. 24, 16 and 12 nm for P25, H240 and S240, respectively, with corresponding BET surface areas of 50, 97 and 110 m2 g−1 for these colloids. The TiO2 colloids were further subjected to analysis using Raman spectroscopy. The presence of the brookite polymorph has been reported for the anatase TiO2 nanoparticles prepared from the sol–gel route [27,28]. Fig. 3 shows the Raman spectra of the H240 and S240 colloids. The most distinctive peaks at 638, 517, 398 and 145 cm−1 are assigned to the anatase phase, while those at 240 cm−1 the rutile and 362 and 320 cm−1 the brookite [54–57]. The results obtained from the Raman analysis clearly reflects that S240 is predominantly composed of anatase while with the presence of a minute amount of rutile and brookite. The appearance of rutile, which is the most stable polymorph of TiO2 , should result from the thermal effect [9]. In an auxiliary experiment for the sol–gel synthesis of TiO2 colloids at lower autoclaving temperatures (e.g., 230 ◦ C), we found only the presence of the predominant anatase and minute brookite while no rutile was detectable. On the other hand, H240 is of nearly phase-pure anatase TiO2 as shown in the Raman spectra. This difference in the crystalline phase has to be attributed to the different mechanisms involved in nucleation or crystal growth. Sol–gel chemistry is very commonly involved for preparing nanosized TiO2 colloids. In order to obtain monodispersed particles of desired size and material morphology, the precursor chemistry and precipitation environment are critical mechanistic factors that have to be taken into account during hydrolysis and precipitate formation [9,10]. Additional peptization has been designed to segregate the precipitates to produce a sol, which is subsequently subjected to hydrothermal Ostward ripening, a mechanism describing crystal growth in terms of growth of large particles at the cost of smaller ones. In the hydrolysis and precipitation at elevated temperatures, a rapid nucleation and crystallization of TiO2 are expected and the crystal growth is
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Fig. 4. Relationship between film thickness of the TiO2 electrodes and photocurrent density Jsc .
thus, governed by kinetics, rather than thermodynamics. Under this circumstance, the simultaneous formation of anatase and brookite polymorphs, both are metastable, in the sol–gel synthesis is favored, as reflected by the results of the present work. In the preparation of TiO2 colloids from the titanate, on the other hand, small particulates should have been produced at the initial stage of hydrothermal treatment on the titanate solution at a pH of 1.5 [51]. It is hypothesized that, upon hydrothermal treatment, the titanate framework would shrink locally, by reducing the interlayer distance, and transform into the anatase TiO2 structure, as indicated in the scheme of Fig. 2. We have found in an auxiliary experiment that the small particulates (obtained with 1 h treatment) are of highly crystalline anatase TiO2 . Extended hydrothermal treatment (i.e., 24 h) of the particulates in the autoclave resulted in crystal growth to form the H240 colloids through the Ostwald ripening mechanism. Through this titanatedirected route, the formation of the metastable brookite can be suppressed to a minimum level. The three TiO2 colloids were coated on conducting glass to form mesoporous film electrodes. The I–V characteristics of the cells equipped with the TiO2 films of different thicknesses were obtained. For each type of TiO2 the short-circuit photocurrent density (Jsc ) increases with the film thickness to reach a maximum and then decreases, as shown in Fig. 4. Because the extinction coefficient of the N3 dye is small in the red, increasing the film thickness is required to improve the harvesting of red light and thus, the value of Jsc [1–5]. The maximum Jsc values for these films were 18, 16 and 11 mA cm−2 for H240, S240 and P25, respectively, obtained at a film thickness of 14–16 m. The decrease of Jsc with further increase in thickness should be ascribed to the intrusion of mass-transfer limitation for electrolyte motion in the film. The large Jsc of the H240 film shown in Fig. 4 should relate to the crystalline framework of the TiO2 colloids. It is known that lattice imperfections in the film are generally the trap states or centers of charge recombination [27]. Improved crystallinity would increase the mobility
Fig. 5. Photocurrent–voltage and darkcurrent–voltage characteristics of cells based on different colloidal TiO2 films sensitized by N3 dye under a light intensity of 100 mW cm−2 . The curves for H240, S240 and P25 were obtained at the TiO2 film thicknesses rendering the maximum Jsc for the cells (Fig. 4), i.e. 14–16 m. The curves designated as double-layered were ob obtained with a film of 12 m H240 covered by 6 m P25.
of electron in the film and thus, the resulting photocurrent. The high-crystallinity anatase of the H240 film must have improved the electron transport and led to the larger Jsc of the resulting cell. The I–V curves obtained from the TiO2 films exhibiting the maximum Jsc are shown in Fig. 5. Although the H240 cell exhibits the largest Jsc , it shows a smaller open-circuit voltage (Voc ). The value of Voc depends on the electron injection current (Jinj ) and the number of recombination centers on the TiO2 film surface [2,19], i.e. Jinj kT Voc = ln ncb ket [I3 − ] e where k is the Boltzman constant, T is the cell temperature, e is the charge on the electron, ncb is the concentration of electrons at the TiO2 surface, and ket is the rate constant for I3 − reduction. Due to the occurrence of the interaction at the film surface, the distance required for the photo-induced electrons to diffuse thus, affects the degree of electron loss into recombination. Transmission spectra for such TiO2 films, as shown in Fig. 6, are obtained to estimate the degree of photon energy penetration into the film. The H240 film exhibits a transparent feature. This feature should result from a low-degree light scattering of the film, even though the constituting H240 colloid has a larger particle size than S240. The presence of rutile phase, which was reported to be more efficient in scattering white light than anatase [19], can be one of the causes for the lower transmittance of the S240 and P25 films. Due to the lack of light-scattering in the H240 film, charge carriers are generated uniformly over the whole film thickness [1–5]. This would lead to a long distance for the photo-injected electrons to diffuse and thus, a large number of recombination centers. This may explain, in part, the smaller Voc for H240 in comparison with those for the S240 and P25. On the other hand, the H240 cell shows the largest dark current under a voltage bias. This may reflect a higher mobility
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as well as the corresponding crystal-size growth. In contrast, due to the lack of the rutile seeds, H240 showed the absence of rutile even if the size of H240 was larger than that of S240. It is obvious that the high-purity in anatase has resulted in suppression of the anatase–rutile transition during crystal growth. Thus, the method presented here provides a route producing anatase nanoparticles with a larger flexibility in size selection, which would be considered advantageous in the design of the mesoporous network for DSSCs [9]. 4. Conclusions
Fig. 6. Transmission spectra of the different TiO2 films supported on conducting glass. The spectra were obtained by using a UV-Vis spectrometer and corrected for conducting glass background. All the TiO2 films had a thickness of ca. 10 m.
of electrons in the H240 film, in accordance with the argument drawn from the large Jsc exhibited by H240. The large Jsc of the H240 film has led to the highest light-to-electric energy conversion efficiency (η) for the H240. The efficiency shown in Fig. 5 is calculated according to η=
Jopt Vopt Pin
where Jopt and Vopt are respectively, the current and voltage for maximum power output and Pin the power of incident light. Even though the photocurrent of the H240 film is the highest among the three types of films, a significant loss of red light in the H240 cell is anticipated due to the transparent feature of the film. The photocurrent response of DSSCs to red light has been shown to increase due to the enhancement of the light scattering in the TiO2 films [9,58–60]. Thus, the scattering capability of the film strongly influences the global values of both Jsc and η. Again, we have to emphasize that the high-purity in anatase have led to the high performance of the H240 cell, although the H240 film shows the deficiency in light scattering. A light-scattering layer made of P25 was added to the top of the H240 film to assist the harvesting in red [58–60]. The I–V curve of this specially-designed cell is also shown in Fig. 5 (designated as double-layered, with 12 m H240 covered by 6 m P25), showing that the Jsc and η were significantly enhanced for the H240. Of course, the enhancement should be partially contributed by the photovoltaic effect of the P25 layer. The effect of this light-management strategy is not as prominent for the S240 due to the less transparent feature of the film. Apart from the high-purity feature presented by the anatase colloids synthesized from the titanate-directed route, the transition of metastable anatase to stable rutile for such colloids was seen to be significantly hindered. It has been reported that once there is the presence of rutile seeds the growth of rutile from anatase transformation during thermal treatment would be accelerated [61]. Also, the anatase–rutile transition can be promoted by an increase in the size of anatase particles [24]. On the basis of the above arguments, the appearance of rutile in S240 can be attributed to the high-temperature (i.e., 240 ◦ C) treatment
We developed a route for the synthesis of high-purity anatase colloids from a titanate that owns a zigzag TiO6 -octahedra arrangement, which is characteristic of anatase TiO2 . In the conventional sol–gel route for preparing anatase nanoparticles, the other phases, especially the metastable brookite, are generally present because of the competitive, rather than exclusive, formation of the three different, polymorphs. Owing to the highcrystallinity feature that facilitates electron at transport, the mesoporous TiO2 films composed of the high-purity anatase nanoparticles exhibit a high performance in dye-sensitized solar cells. By covering the transparent films of the high-purity anatase with a scattering layer to improve red-light harvesting, the resulting photocurrent and energy-conversion efficiency of the cells can be significantly improved. Because of the high-purity feature of the products, the titanate-directed route also allows a larger flexibility for the growth of anatase nanocrystals while remaining the contents of the other phases to a minimum level. Acknowledgments This research was supported by the National Science Council of Taiwan, through Projects NSC 94-2214-E-006-007 and NSC 95-2120-M-006-001. References [1] B. O’Regan, M. Gr¨atzel, Nature 353 (1991) 737–740. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. M¨uller, P. Liska, N. Vlachopoulos, M. Gr¨atzel, J. Am. Chem. Soc. 115 (1993) 6382–6390. [3] S. Pelet, J.-E. Moser, M. Gr¨atzel, J. Phys. Chem. B 104 (2000) 1791–1795. [4] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, M. Gr¨atzel, J. Phys. Chem. B 107 (2003) 14336–14341. [5] M. Gr¨atzel, J. Photochem. Photobiol. C 4 (2003) 145–153. [6] G. Schlichth¨orl, S.Y. Huan, J. Sprague, A.J. Frank, J. Phys. Chem. B 101 (1997) 8141–8155. [7] L. Dloczik, O. Ileperuma, I. Lauermann, L.M. Peter, E.A. Ponomarev, G. Redmond, N.J. Shaw, I. Uhlendor, J. Phys. Chem. B 101 (1997) 10281–10289. [8] B.A. Gregg, F. Pichot, S. Ferrere, C.L. Fields, J. Phys. Chem. B 105 (2001) 1422–1429. [9] C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Gr¨atzel, J. Am. Ceram. Soc. 80 (1997) 3157–3171. [10] K. Kalyanasundaram, M. Gr¨atzel, Coord. Chem. Rev. 177 (1998) 347–414. [11] M. Zukalova, A. Zukal, L. Kavan, M.K. Nazeeruddin, P. Liska, M. Gr¨atzel, Nano Lett. 5 (2005) 1789–1792. [12] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044–2052.
24
P.-T. Hsiao et al. / Journal of Photochemistry and Photobiology A: Chemistry 188 (2007) 19–24
[13] K.D. Benkstein, N. Kopidakis, J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 107 (2003) 7759–7767. [14] Z.S. Wang, C.H. Huang, Y.Y. Huang, Y.J. Hou, P.H. Xie, B.W. Zhang, H.M. Cheng, Chem. Mater. 13 (2001) 678–682. [15] L.M. Peter, N.W. Duffy, R.L. Wang, K.G.U. Wijayantha, J. Electroanal. Chem. 524 (2002) 127–136. [16] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 108 (2004) 4818–4822. [17] M. Adachi, Y. Murata, J. Takao, J.T. Jiu, M. Sakamoto, F.M. Wang, J. Am. Chem. Soc. 126 (2004) 14943–14949. [18] J.T. Jiu, F.M. Wang, S. Isoda, M. Adachi, Chem. Lett. 34 (2005) 1506–1507. [19] N.G. Park, J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 104 (2000) 8989–8994. [20] S.Y. Huang, G. Schlichth¨orl, A.J. Nozik, M. Gr¨atzel, A.J. Frank, J. Phys. Chem. B 101 (1997) 2576–2582. [21] M. Gr¨atzel, J. Sol–gel Sci. Technol. 22 (2001) 7–13. [22] M. Gr¨atzel, J. Photochem. Photobiol. A 164 (2004) 3–14. [23] M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev. B 63 (2001) 155409. [24] A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261–1266. [25] B.L. Bischoff, M.A. Anderson, Chem. Mater. 7 (1995) 1772–1778. [26] A. Zaban, S.T. Aruna, S. Tirosh, B.A. Gregg, Y. Mastai, J. Phys. Chem. B 104 (2000) 4130–4133. [27] G.J. Wilson, G.D. Will, R.L. Frost, S.A. Montgomery, J. Mater. Chem. 12 (2002) 1787–1791. [28] S. Musi´c, M. Got´c, M. Ivanda, S. Popovi´c, A. Turkovi´c, R. Trojko, A. Sekuli´c, K. Furi´c, Mater. Sci. Eng. B 47 (1997) 33–40. [29] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160–3163. [30] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307–1311. [31] D.S. Seo, J.-K. Lee, H. Kim, J. Cryst. Growth 229 (2001) 428–432. [32] C.C. Tsai, H.S. Teng, Chem. Mater. 16 (2004) 4352–4358. [33] D.V. Bavykin, V.N. Parmon, A.A. Lapkin, F.C. Walsh, J. Mater. Chem. 14 (2004) 3370–3377. [34] D.V. Bavykin, S.N. Gordeev, A.V. Moskalenko, A.A. Lapkin, F.C. Walsh, J. Phys. Chem. B 109 (2005) 8565–8569. [35] Z.V. Saponjic, N.M. Dimitrijevic, D.M. Tiede, A.J. Goshe, X. Zuo, L.X. Chen, A.S. Barnard, P. Zapol, L. Curtiss, T. Rajh, Adv. Mater. 17 (2005) 965–971. [36] J. Zhao, X. Wang, T. Sun, L. Li, Nanotechnology 16 (2005) 2450–2454. [37] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.M. Peng, Appl. Phys. Lett. 79 (2001) 3702–3704.
[38] Q. Chen, W. Zhou, G. Du, L.-M. Peng, Adv. Mater. 14 (2002) 1208–1211. [39] X. Sun, Y. Li, Chem. Eur. J. 9 (2003) 2229–2238. [40] S. Zhang, L.M. Peng, Q. Chen, G.H. Du, G. Dawson, W.Z. Zhou, Phys. Rev. Lett. 91 (2003) 256103. [41] H.Y. Zhu, Y. Lan, X.P. Gao, S.P. Ringer, Z.F. Zheng, D.Y. Song, J.C. Zhao, J. Am. Chem. Soc. 127 (2005) 6730–6736. [42] D.V. Bavykin, A.A. Lapkin, P.K. Plucinski, J.M. Friedrich, F.C. Walsh, J. Catal. 235 (2005) 10–17. [43] S. Pavasupree, Y. Suzuki, S. Yoshikawa, R.J. Kawahata, J. Solid State Chem. 178 (2005) 3110–3116. [44] H. Yu, J. Yu, B. Cheng, M. Zhou, J. Solid State Chem. 179 (2006) 349– 354. [45] D.V. Bavykin, J.M. Friedrich, A. Alexei, A.A. Lapkin, F.C. Walsh, Chem. Mater. 18 (2006) 1124–1129. [46] J. Yang, Z. Jin, X. Wang, W. Li, J. Zhang, S. Zhang, X. Guo, Z. Zhang, Dalton Trans. (2003) 3898–3901. [47] R. Ma, Y. Bando, T. Sasaki, Chem. Phys. Lett. 380 (2003) 577–582. [48] M. Zhang, Z. Jin, X.G. Zhang, J. Yang, W. Li, X. Wang, Z. Zhang, J. Mol. Catal. A 217 (2004) 203–210. [49] R. Ma, K. Fukuda, T. Sasaki, M. Osada, Y. Bando, J. Phys. Chem. B 109 (2005) 6210–6214. [50] C.C. Tsai, H.S. Teng, Chem. Mater. 18 (2006) 367–373. [51] J.N. Nian, H.S. Teng, J. Phys. Chem. B 110 (2006) 4193–4198. [52] Powder Diffraction Files of the Joint Committee on Powder Diffraction Standards, Card No. 47-0124, International Center for Diffraction Data. [53] M. Sugita, M. Tsuji, M. Abe, Bull. Chem. Soc. Jpn. 63 (1990) 1978–1984. [54] C.A. Melendres, A. Narayanasamy, V.A. Maroni, R.W. Seigel, J. Mater. Res. 4 (1989) 1246–1250. [55] J.C. Parker, R.W. Seigel, J. Mater. Res. 5 (1990) 1246–1252. [56] M. Goti´c, M. Ivanda, S. Popovi´c, S. Musi´c, A. Sekuli´c, A. Turkovi´c, K.J. Furi´c, J. Raman Spectrosc. 28 (1997) 555–558. [57] S. Musi´c, M. Goti´c, M. Ivanda, S. Popovi´c, A. Turkovi´c, R. Trojko, A. Sekuli´c, K.J. Furi´c, Mater. Sci. Eng. B 47 (1997) 33–40. [58] N.G. Park, G. Schlichth¨orl, J. van de Lagematt, H.M. Cheong, A. Mascarenhas, A.J. Frank, J. Phys. Chem. B 103 (1999) 3308– 3314. [59] S. Ngamsinlapasathian, S. Sakulkhaemaruethai, S. Pavasupree, A. Kitiyanan, T. Sreethawong, Y. Suzuki, S. Yoshikawa, J. Photochem. Photobiol. A 164 (2004) 145–151. [60] S. Ngamsinlapasathian, T. Sreethawong, Y. Suzuki, S. Yoshikawa, Sol. Energy Mater. Sol. Cells 86 (2005) 269–282. [61] P.I. Gouma, M.J. Mills, J. Am. Ceram. Soc. 84 (2001) 619–622.
Nanocrystalline anatase TiO2 derived from a titanate-directed route for dye-sensitized solar cells Po-Tsung Hsiao, Kai-Ping Wang, Chih-Wei Cheng, Hsisheng Teng ∗ Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received 24 May 2006; received in revised form 29 August 2006; accepted 16 November 2006 Available online 19 November 2006
Abstract TiO2 nanoparticles used in numerous applications are generally prepared from the sol–gel method. Because of the competitive, rather than exclusive, formation of the three TiO2 polymorphs, anatase, brookite and rutile, in the sol–gel synthesis, phase-pure nanoparticles can hardly be obtained. The present work demonstrates a unique route, alternative to the conventional sol–gel method, to prepare high-purity anatase TiO2 colloids, which can be deposited as electrodes for dye-sensitized solar cells (DSSCs) to facilitate electron transport and avoid charge recombination. In this developed route, a titanate with its TiO6 octahedra arranged in a zigzag configuration, which is also a characteristic feature of anatase TiO2 , is produced as an intermediate. Raman analysis shows that a phase-pure anatase TiO2 colloid is prepared from the developed route, while the TiO2 derived from the sol–gel at the same temperature is predominantly composed of anatase with the presence of a minute amount of rutile and brookite. Because of the high-purity in anatase phase, the TiO2 colloid derived from the titanate-directed route is shown to constitute a mesoporous film exhibiting high performance in a dye-sensitized solar cell. © 2006 Elsevier B.V. All rights reserved. Keywords: Phase-pure anatase TiO2 ; Titanate nanotube; Dye-sensitized solar cell; Sol–gel; Nanocrystalline TiO2 film
1. Introduction Dye-sensitized solar cells (DSSCs) that provide a costeffective alternative to conventional solar cells have attracted intensive studies [1–8]. The cells consist of a dye-adsorbed mesoporous metal-oxide film filled with iodide/triiodide redox electrolyte and a Pt counter electrode. The mesoporous metaloxide film with a large surface area plays a crucial role for the high performance of DSSCs [9–11]. It not only adsorbs a large number of dye molecules for efficient light-harvesting, but also serves as a semiconductor to provide a pathway for electron percolating through the film. The crystalline structure of the metal-oxide semiconductor significantly affects the transport efficiency of the injected electrons [12–18]. Anatase TiO2 has been considered so far the optimum choice of the semiconductor constituting the mesoporous film [19]. In this application phase-pure anatase TiO2 is expected to facilitate efficient electron transport and avoid charge recombination [12–19].
∗
Corresponding author. Tel.: +886 6 2385371; fax: +886 6 2344496. E-mail address: [email protected] (H. Teng).
1010-6030/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2006.11.013
To fabricate the mesoporous TiO2 film, hydrolysis of a titanium alkoxide through a sol–gel route was generally employed to prepare a colloidal suspension that was used for deposition on the substrate [1–8,20–22]. Anatase, rutile and brookite are the three TiO2 polymorphs [23,24]. Because in the sol–gel route TiO2 nanoparticles were derived from condensation of the hydrated Ti4+ ions, a TiO2 colloid of phase-pure anatase can hardly be obtained even if the hydrolysis and condensation kinetics are delicately controlled [25–28]. Previous studies using Raman spectroscopic analysis showed that brookite phase was generally present in the anatase TiO2 colloids derived from the sol–gel route [27,28]. The anatase polymorph, differing from the other two, owns a specific zigzag configuration of the edgeshared TiO6 octahedra along a particular crystalline direction. Taking advantage of the zigzag feature, the present work wishes to report a unique route, alternative to the sol–gel method, to prepare the anatase TiO2 film for DSSCs. In this developed route, a titanate with its TiO6 octahedra arranged in a zigzag configuration would be produced as an intermediate. It has been reported that the thermal treatment of TiO2 particles, either anatase or rutile, in alkali solutions resulted in formation of nanotubes [29–51]. The process of this treatment
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is rather unsophisticated. The nanotubes are constructed of a titanate [37–51], of which the structure has been assigned to titanates such as A2 Ti3 O7 [37–45], lepidocrocite-type species [47,49], or A2 Ti2 O5 ·H2 O [46,48,50], where A represents Na and/or H. Similar to anatase TiO2 , all these titanates own the zigzag configuration of the edge-shared TiO6 octahedra. Upon pH value adjustment, the titanate nanotubes have been shown to proceed with local shrinkage of the layers and transform into meta-stable anatase TiO2 [50]. We expect a hydrothermal treatment to transform this titanate into phase-pure TiO2 anatase because of their similarity in possessing the zigzag feature for the TiO6 arrangement [51]. 2. Experimental An anatase TiO2 colloid derived from the titanate described above was used to fabricate the mesoporous film for DSSCs in the present work. To give a suspension of the titanate intermediate, 3 g of a commercially-available TiO2 powder (P25, Degussa AG) was hydrothermally treated with 100 mL of 10 N NaOH in an autoclave at 130 ◦ C for 20 h, followed by repeated washing with 0.1 N HNO3 to reach a pH value of ca. 1.5. The titanate formed was in a tubular shape, of which the image has been shown elsewhere [50]. The anatase TiO2 colloid for DSSCs was obtained by autoclaving the low-pH titanate suspension at 240 ◦ C for 12 h. This titanate-derived TiO2 is designated as H240. It has to be noted that the preparation of H240 is rather unsophisticated, requiring no delicate optimization of the reaction condition. Auxiliary experiments revealed that the particle size could be easily tuned by adjusting the pH value and temperature for the final autoclaving. The P25 and the “Gr¨atzel-type” sol–gel TiO2 colloidal suspensions were used as well to fabricate the mesoporous films. In preparing the P25 colloid, 14 mL of deionized water and 0.2 mL of acetyl acetone was added to 6 g of the P25 powder in a mortar and pestle for grinding to mechanically separate aggregated TiO2 particles. An appropriate amount of Triton X100 surfactant was added to the mixture to give the P25 dispersion. The method for preparing the colloidal TiO2 dispersion from the sol–gel route was analogous to those reported by Gr¨atzel and co-workers [20–22]. In brief, 20 mL of titanium isopropoxide was added to 120 mL of 0.1 M nitric acid under vigorous stirring. Immediately after the hydrolysis, the slurry was heated to 80 ◦ C and stirred vigorously for 2 h, to achieve peptization. Then, the colloidal suspension was introduced into an autoclave that was heated at 240 ◦ C for 12 h. The resulting TiO2 colloid from the sol–gel synthesis is designated as S240. The phase identification of the samples was conducted with powder X-ray diffraction (XRD) using a Rigaku RINT2000 diffractometer equipped with Cu K␣ radiation. The phase was analyzed with Raman scattering using a Dilor XY-100 spectrometer. The microstructure was explored with a high-resolution transmission electron microscope (HRTEM; Hitachi FE-2000). The N2 surface area was determined with the Brunauer-EmmettTeller (BET) equation at −196 ◦ C, using an adsorption apparatus (Micromeritics, ASAP 2010).
The solution of each TiO2 colloid, H240, S240 or P25, was mixed with polyethylene glycol (PEG; Fluka, 20,000 in molecular weight) to form a viscous TiO2 dispersion at a PEG/TiO2 ratio of 0.4. The dispersion was then spread onto a conducting glass substrate (F-doped SnO2 overlayer TCO glass; TEC 8, Hartford Glass Co.). The apparent area of the film was 0.5 × 0.5 cm2 . The TiO2 -coated (0.5 × 0.5 cm2 ) substrate was subsequently calcined at 450 ◦ C in air for 30 min to form a mesoporous electrode. Transmission spectra of the different TiO2 films supported on the conducting glass were obtained by using a UV-Vis spectrometer (Varian CARI-100) and corrected for conducting glass background. The ruthenium complex dye, N3 (cis-di(thiocyanate)bis(2,2 bipyridyl-4,4 -dicarboxylate) ruthenium (II); ruthenium 535, Solaronix), was used as the sensitizer for DSSCs. The dyesensitized TiO2 electrode and a Pt-coated conducting glass were assembled to form a cell by sandwiching a redox (I− /I3 − ) electrolyte solution, which was composed of 0.1 M LiI, 0.05 M I2 , 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile. In the cell performance test, an Oriel 300-W Xe lamp served as a light source in conjunction with an IR filter (Oriel 59044). The AM 1.5 Globle filter (Oriel 81094) was placed in the light beam to simulate the AM 1.5-type solar emission, with an intensity on the cell fixed at 100 mW cm−2 . 3. Results and discussion It has been reported that treating TiO2 nanoparticles in NaOH resulted in formation of lamellar sheets due to the rupture of Ti O Ti bonds [29,30]. Washing the treated sample with water and/or acid to partially remove the electrostatic charge led to formation of nanotubes. Fig. 1 shows the XRD patterns of the materials obtained from hydrothermal treatment of P25 in NaOH and those with a subsequent washing to pH values of 6.9 and
Fig. 1. XRD patterns of the titanate specimen obtained from hydrothermal NaOH treatment on TiO2 at 130 ◦ C, those with subsequent washing to pH values of 6.9 and 1.5, and the TiO2 colloid derived from hydrothermally treating the pH 1.5 titanate (H240) and that from the sol–gel route (S240). The standard diffraction pattern of H2 Ti2 O5 ·H2 O from JCPDS is provided at the bottom of this figure.
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Fig. 3. Raman spectra of TiO2 colloids prepared from the sol–gel route (S240) and from the hydrothermal-treatment on a titanate (H240).
Fig. 2. The scheme for the formation of anatase TiO2 from a titanate (H2 Ti2 O5 ·H2 O) and a tyical TEM image of the TiO2 colloid (H240) derived from this scheme. The image shows the lattice fringes of the anatase phase. In the scheme the projection along the [0 0 1] of the titanate exhibits layers of the TiO6 octahedra edge-shared in a zigzag co configuration, which can be correlated with the principal unit layer of the anatase TiO2 projected along [1 0 1].
1.5. By comparing the XRD data of the present specimens with those of the H2 Ti2 O5 ·H2 O titanate documented in the Powder Diffraction files of JCPDS (bottom of Fig. 1) [52,53], the structure of the specimen obtained right from the alkali treatment and that washed to pH 6.9 should be assigned to that of A2 Ti2 O5 ·H2 O, which comprises of two-dimensional layers in which TiO6 octahedra are combined through edge sharing in a zigzag configuration (Fig. 2) [52,53]. The inter-layer ions, H+ , Na+ and OH− , are not shown in the figure [50]. With further decrease of the pH to 1.5, there shows the appearance of anatase TiO2 phase (Fig. 1) with the fading of the titanate characteristic peaks, especially the (2 0 0). Fig. 2 shows the principal unit layer of anatase TiO2 projected along [1 0 1], exhibiting a zigzag configuration of the edge-shared TiO6 octahedra. This observation reflects the intimate connection between the structures of H2 Ti2 O5 ·H2 O and anatase TiO2 . Thus, using titanate as an intermediate for high-purity anatase synthesis, as shown in the scheme in Fig. 2, would provide a feasible route, alternate to the sol–gel method, for fabricating TiO2 film electrodes used in applications such as DSSCs. The pH 1.5 suspension was subjected to hydrothermal treatment at 240 ◦ C to give the H240 colloid. The TEM image of this TiO2 colloid is shown in Fig. 2. The lattice fringe of the crystals in Fig. 2 reflects that anatase is the constituting phase of this titanate-derived TiO2 colloid. Auxiliary experiments showed that the size of these nanocrystals is an increasing function of the treatment temperature. The powder X-ray diffraction analysis of
H240 was conducted and the data are compared with those of the sol–gel derived S240 in Fig. 1. Only the anatase phase can be obviously observed in the patterns for H240 and S240. On the other hand, P25 is known to contain both anatase (70%) and rutile (30%) [2]. The particle sizes, examined with TEM images, are of ca. 24, 16 and 12 nm for P25, H240 and S240, respectively, with corresponding BET surface areas of 50, 97 and 110 m2 g−1 for these colloids. The TiO2 colloids were further subjected to analysis using Raman spectroscopy. The presence of the brookite polymorph has been reported for the anatase TiO2 nanoparticles prepared from the sol–gel route [27,28]. Fig. 3 shows the Raman spectra of the H240 and S240 colloids. The most distinctive peaks at 638, 517, 398 and 145 cm−1 are assigned to the anatase phase, while those at 240 cm−1 the rutile and 362 and 320 cm−1 the brookite [54–57]. The results obtained from the Raman analysis clearly reflects that S240 is predominantly composed of anatase while with the presence of a minute amount of rutile and brookite. The appearance of rutile, which is the most stable polymorph of TiO2 , should result from the thermal effect [9]. In an auxiliary experiment for the sol–gel synthesis of TiO2 colloids at lower autoclaving temperatures (e.g., 230 ◦ C), we found only the presence of the predominant anatase and minute brookite while no rutile was detectable. On the other hand, H240 is of nearly phase-pure anatase TiO2 as shown in the Raman spectra. This difference in the crystalline phase has to be attributed to the different mechanisms involved in nucleation or crystal growth. Sol–gel chemistry is very commonly involved for preparing nanosized TiO2 colloids. In order to obtain monodispersed particles of desired size and material morphology, the precursor chemistry and precipitation environment are critical mechanistic factors that have to be taken into account during hydrolysis and precipitate formation [9,10]. Additional peptization has been designed to segregate the precipitates to produce a sol, which is subsequently subjected to hydrothermal Ostward ripening, a mechanism describing crystal growth in terms of growth of large particles at the cost of smaller ones. In the hydrolysis and precipitation at elevated temperatures, a rapid nucleation and crystallization of TiO2 are expected and the crystal growth is
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Fig. 4. Relationship between film thickness of the TiO2 electrodes and photocurrent density Jsc .
thus, governed by kinetics, rather than thermodynamics. Under this circumstance, the simultaneous formation of anatase and brookite polymorphs, both are metastable, in the sol–gel synthesis is favored, as reflected by the results of the present work. In the preparation of TiO2 colloids from the titanate, on the other hand, small particulates should have been produced at the initial stage of hydrothermal treatment on the titanate solution at a pH of 1.5 [51]. It is hypothesized that, upon hydrothermal treatment, the titanate framework would shrink locally, by reducing the interlayer distance, and transform into the anatase TiO2 structure, as indicated in the scheme of Fig. 2. We have found in an auxiliary experiment that the small particulates (obtained with 1 h treatment) are of highly crystalline anatase TiO2 . Extended hydrothermal treatment (i.e., 24 h) of the particulates in the autoclave resulted in crystal growth to form the H240 colloids through the Ostwald ripening mechanism. Through this titanatedirected route, the formation of the metastable brookite can be suppressed to a minimum level. The three TiO2 colloids were coated on conducting glass to form mesoporous film electrodes. The I–V characteristics of the cells equipped with the TiO2 films of different thicknesses were obtained. For each type of TiO2 the short-circuit photocurrent density (Jsc ) increases with the film thickness to reach a maximum and then decreases, as shown in Fig. 4. Because the extinction coefficient of the N3 dye is small in the red, increasing the film thickness is required to improve the harvesting of red light and thus, the value of Jsc [1–5]. The maximum Jsc values for these films were 18, 16 and 11 mA cm−2 for H240, S240 and P25, respectively, obtained at a film thickness of 14–16 m. The decrease of Jsc with further increase in thickness should be ascribed to the intrusion of mass-transfer limitation for electrolyte motion in the film. The large Jsc of the H240 film shown in Fig. 4 should relate to the crystalline framework of the TiO2 colloids. It is known that lattice imperfections in the film are generally the trap states or centers of charge recombination [27]. Improved crystallinity would increase the mobility
Fig. 5. Photocurrent–voltage and darkcurrent–voltage characteristics of cells based on different colloidal TiO2 films sensitized by N3 dye under a light intensity of 100 mW cm−2 . The curves for H240, S240 and P25 were obtained at the TiO2 film thicknesses rendering the maximum Jsc for the cells (Fig. 4), i.e. 14–16 m. The curves designated as double-layered were ob obtained with a film of 12 m H240 covered by 6 m P25.
of electron in the film and thus, the resulting photocurrent. The high-crystallinity anatase of the H240 film must have improved the electron transport and led to the larger Jsc of the resulting cell. The I–V curves obtained from the TiO2 films exhibiting the maximum Jsc are shown in Fig. 5. Although the H240 cell exhibits the largest Jsc , it shows a smaller open-circuit voltage (Voc ). The value of Voc depends on the electron injection current (Jinj ) and the number of recombination centers on the TiO2 film surface [2,19], i.e. Jinj kT Voc = ln ncb ket [I3 − ] e where k is the Boltzman constant, T is the cell temperature, e is the charge on the electron, ncb is the concentration of electrons at the TiO2 surface, and ket is the rate constant for I3 − reduction. Due to the occurrence of the interaction at the film surface, the distance required for the photo-induced electrons to diffuse thus, affects the degree of electron loss into recombination. Transmission spectra for such TiO2 films, as shown in Fig. 6, are obtained to estimate the degree of photon energy penetration into the film. The H240 film exhibits a transparent feature. This feature should result from a low-degree light scattering of the film, even though the constituting H240 colloid has a larger particle size than S240. The presence of rutile phase, which was reported to be more efficient in scattering white light than anatase [19], can be one of the causes for the lower transmittance of the S240 and P25 films. Due to the lack of light-scattering in the H240 film, charge carriers are generated uniformly over the whole film thickness [1–5]. This would lead to a long distance for the photo-injected electrons to diffuse and thus, a large number of recombination centers. This may explain, in part, the smaller Voc for H240 in comparison with those for the S240 and P25. On the other hand, the H240 cell shows the largest dark current under a voltage bias. This may reflect a higher mobility
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as well as the corresponding crystal-size growth. In contrast, due to the lack of the rutile seeds, H240 showed the absence of rutile even if the size of H240 was larger than that of S240. It is obvious that the high-purity in anatase has resulted in suppression of the anatase–rutile transition during crystal growth. Thus, the method presented here provides a route producing anatase nanoparticles with a larger flexibility in size selection, which would be considered advantageous in the design of the mesoporous network for DSSCs [9]. 4. Conclusions
Fig. 6. Transmission spectra of the different TiO2 films supported on conducting glass. The spectra were obtained by using a UV-Vis spectrometer and corrected for conducting glass background. All the TiO2 films had a thickness of ca. 10 m.
of electrons in the H240 film, in accordance with the argument drawn from the large Jsc exhibited by H240. The large Jsc of the H240 film has led to the highest light-to-electric energy conversion efficiency (η) for the H240. The efficiency shown in Fig. 5 is calculated according to η=
Jopt Vopt Pin
where Jopt and Vopt are respectively, the current and voltage for maximum power output and Pin the power of incident light. Even though the photocurrent of the H240 film is the highest among the three types of films, a significant loss of red light in the H240 cell is anticipated due to the transparent feature of the film. The photocurrent response of DSSCs to red light has been shown to increase due to the enhancement of the light scattering in the TiO2 films [9,58–60]. Thus, the scattering capability of the film strongly influences the global values of both Jsc and η. Again, we have to emphasize that the high-purity in anatase have led to the high performance of the H240 cell, although the H240 film shows the deficiency in light scattering. A light-scattering layer made of P25 was added to the top of the H240 film to assist the harvesting in red [58–60]. The I–V curve of this specially-designed cell is also shown in Fig. 5 (designated as double-layered, with 12 m H240 covered by 6 m P25), showing that the Jsc and η were significantly enhanced for the H240. Of course, the enhancement should be partially contributed by the photovoltaic effect of the P25 layer. The effect of this light-management strategy is not as prominent for the S240 due to the less transparent feature of the film. Apart from the high-purity feature presented by the anatase colloids synthesized from the titanate-directed route, the transition of metastable anatase to stable rutile for such colloids was seen to be significantly hindered. It has been reported that once there is the presence of rutile seeds the growth of rutile from anatase transformation during thermal treatment would be accelerated [61]. Also, the anatase–rutile transition can be promoted by an increase in the size of anatase particles [24]. On the basis of the above arguments, the appearance of rutile in S240 can be attributed to the high-temperature (i.e., 240 ◦ C) treatment
We developed a route for the synthesis of high-purity anatase colloids from a titanate that owns a zigzag TiO6 -octahedra arrangement, which is characteristic of anatase TiO2 . In the conventional sol–gel route for preparing anatase nanoparticles, the other phases, especially the metastable brookite, are generally present because of the competitive, rather than exclusive, formation of the three different, polymorphs. Owing to the highcrystallinity feature that facilitates electron at transport, the mesoporous TiO2 films composed of the high-purity anatase nanoparticles exhibit a high performance in dye-sensitized solar cells. By covering the transparent films of the high-purity anatase with a scattering layer to improve red-light harvesting, the resulting photocurrent and energy-conversion efficiency of the cells can be significantly improved. Because of the high-purity feature of the products, the titanate-directed route also allows a larger flexibility for the growth of anatase nanocrystals while remaining the contents of the other phases to a minimum level. Acknowledgments This research was supported by the National Science Council of Taiwan, through Projects NSC 94-2214-E-006-007 and NSC 95-2120-M-006-001. References [1] B. O’Regan, M. Gr¨atzel, Nature 353 (1991) 737–740. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. M¨uller, P. Liska, N. Vlachopoulos, M. Gr¨atzel, J. Am. Chem. Soc. 115 (1993) 6382–6390. [3] S. Pelet, J.-E. Moser, M. Gr¨atzel, J. Phys. Chem. B 104 (2000) 1791–1795. [4] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, M. Gr¨atzel, J. Phys. Chem. B 107 (2003) 14336–14341. [5] M. Gr¨atzel, J. Photochem. Photobiol. C 4 (2003) 145–153. [6] G. Schlichth¨orl, S.Y. Huan, J. Sprague, A.J. Frank, J. Phys. Chem. B 101 (1997) 8141–8155. [7] L. Dloczik, O. Ileperuma, I. Lauermann, L.M. Peter, E.A. Ponomarev, G. Redmond, N.J. Shaw, I. Uhlendor, J. Phys. Chem. B 101 (1997) 10281–10289. [8] B.A. Gregg, F. Pichot, S. Ferrere, C.L. Fields, J. Phys. Chem. B 105 (2001) 1422–1429. [9] C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Gr¨atzel, J. Am. Ceram. Soc. 80 (1997) 3157–3171. [10] K. Kalyanasundaram, M. Gr¨atzel, Coord. Chem. Rev. 177 (1998) 347–414. [11] M. Zukalova, A. Zukal, L. Kavan, M.K. Nazeeruddin, P. Liska, M. Gr¨atzel, Nano Lett. 5 (2005) 1789–1792. [12] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044–2052.
24
P.-T. Hsiao et al. / Journal of Photochemistry and Photobiology A: Chemistry 188 (2007) 19–24
[13] K.D. Benkstein, N. Kopidakis, J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 107 (2003) 7759–7767. [14] Z.S. Wang, C.H. Huang, Y.Y. Huang, Y.J. Hou, P.H. Xie, B.W. Zhang, H.M. Cheng, Chem. Mater. 13 (2001) 678–682. [15] L.M. Peter, N.W. Duffy, R.L. Wang, K.G.U. Wijayantha, J. Electroanal. Chem. 524 (2002) 127–136. [16] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 108 (2004) 4818–4822. [17] M. Adachi, Y. Murata, J. Takao, J.T. Jiu, M. Sakamoto, F.M. Wang, J. Am. Chem. Soc. 126 (2004) 14943–14949. [18] J.T. Jiu, F.M. Wang, S. Isoda, M. Adachi, Chem. Lett. 34 (2005) 1506–1507. [19] N.G. Park, J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 104 (2000) 8989–8994. [20] S.Y. Huang, G. Schlichth¨orl, A.J. Nozik, M. Gr¨atzel, A.J. Frank, J. Phys. Chem. B 101 (1997) 2576–2582. [21] M. Gr¨atzel, J. Sol–gel Sci. Technol. 22 (2001) 7–13. [22] M. Gr¨atzel, J. Photochem. Photobiol. A 164 (2004) 3–14. [23] M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev. B 63 (2001) 155409. [24] A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261–1266. [25] B.L. Bischoff, M.A. Anderson, Chem. Mater. 7 (1995) 1772–1778. [26] A. Zaban, S.T. Aruna, S. Tirosh, B.A. Gregg, Y. Mastai, J. Phys. Chem. B 104 (2000) 4130–4133. [27] G.J. Wilson, G.D. Will, R.L. Frost, S.A. Montgomery, J. Mater. Chem. 12 (2002) 1787–1791. [28] S. Musi´c, M. Got´c, M. Ivanda, S. Popovi´c, A. Turkovi´c, R. Trojko, A. Sekuli´c, K. Furi´c, Mater. Sci. Eng. B 47 (1997) 33–40. [29] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160–3163. [30] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307–1311. [31] D.S. Seo, J.-K. Lee, H. Kim, J. Cryst. Growth 229 (2001) 428–432. [32] C.C. Tsai, H.S. Teng, Chem. Mater. 16 (2004) 4352–4358. [33] D.V. Bavykin, V.N. Parmon, A.A. Lapkin, F.C. Walsh, J. Mater. Chem. 14 (2004) 3370–3377. [34] D.V. Bavykin, S.N. Gordeev, A.V. Moskalenko, A.A. Lapkin, F.C. Walsh, J. Phys. Chem. B 109 (2005) 8565–8569. [35] Z.V. Saponjic, N.M. Dimitrijevic, D.M. Tiede, A.J. Goshe, X. Zuo, L.X. Chen, A.S. Barnard, P. Zapol, L. Curtiss, T. Rajh, Adv. Mater. 17 (2005) 965–971. [36] J. Zhao, X. Wang, T. Sun, L. Li, Nanotechnology 16 (2005) 2450–2454. [37] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.M. Peng, Appl. Phys. Lett. 79 (2001) 3702–3704.
[38] Q. Chen, W. Zhou, G. Du, L.-M. Peng, Adv. Mater. 14 (2002) 1208–1211. [39] X. Sun, Y. Li, Chem. Eur. J. 9 (2003) 2229–2238. [40] S. Zhang, L.M. Peng, Q. Chen, G.H. Du, G. Dawson, W.Z. Zhou, Phys. Rev. Lett. 91 (2003) 256103. [41] H.Y. Zhu, Y. Lan, X.P. Gao, S.P. Ringer, Z.F. Zheng, D.Y. Song, J.C. Zhao, J. Am. Chem. Soc. 127 (2005) 6730–6736. [42] D.V. Bavykin, A.A. Lapkin, P.K. Plucinski, J.M. Friedrich, F.C. Walsh, J. Catal. 235 (2005) 10–17. [43] S. Pavasupree, Y. Suzuki, S. Yoshikawa, R.J. Kawahata, J. Solid State Chem. 178 (2005) 3110–3116. [44] H. Yu, J. Yu, B. Cheng, M. Zhou, J. Solid State Chem. 179 (2006) 349– 354. [45] D.V. Bavykin, J.M. Friedrich, A. Alexei, A.A. Lapkin, F.C. Walsh, Chem. Mater. 18 (2006) 1124–1129. [46] J. Yang, Z. Jin, X. Wang, W. Li, J. Zhang, S. Zhang, X. Guo, Z. Zhang, Dalton Trans. (2003) 3898–3901. [47] R. Ma, Y. Bando, T. Sasaki, Chem. Phys. Lett. 380 (2003) 577–582. [48] M. Zhang, Z. Jin, X.G. Zhang, J. Yang, W. Li, X. Wang, Z. Zhang, J. Mol. Catal. A 217 (2004) 203–210. [49] R. Ma, K. Fukuda, T. Sasaki, M. Osada, Y. Bando, J. Phys. Chem. B 109 (2005) 6210–6214. [50] C.C. Tsai, H.S. Teng, Chem. Mater. 18 (2006) 367–373. [51] J.N. Nian, H.S. Teng, J. Phys. Chem. B 110 (2006) 4193–4198. [52] Powder Diffraction Files of the Joint Committee on Powder Diffraction Standards, Card No. 47-0124, International Center for Diffraction Data. [53] M. Sugita, M. Tsuji, M. Abe, Bull. Chem. Soc. Jpn. 63 (1990) 1978–1984. [54] C.A. Melendres, A. Narayanasamy, V.A. Maroni, R.W. Seigel, J. Mater. Res. 4 (1989) 1246–1250. [55] J.C. Parker, R.W. Seigel, J. Mater. Res. 5 (1990) 1246–1252. [56] M. Goti´c, M. Ivanda, S. Popovi´c, S. Musi´c, A. Sekuli´c, A. Turkovi´c, K.J. Furi´c, J. Raman Spectrosc. 28 (1997) 555–558. [57] S. Musi´c, M. Goti´c, M. Ivanda, S. Popovi´c, A. Turkovi´c, R. Trojko, A. Sekuli´c, K.J. Furi´c, Mater. Sci. Eng. B 47 (1997) 33–40. [58] N.G. Park, G. Schlichth¨orl, J. van de Lagematt, H.M. Cheong, A. Mascarenhas, A.J. Frank, J. Phys. Chem. B 103 (1999) 3308– 3314. [59] S. Ngamsinlapasathian, S. Sakulkhaemaruethai, S. Pavasupree, A. Kitiyanan, T. Sreethawong, Y. Suzuki, S. Yoshikawa, J. Photochem. Photobiol. A 164 (2004) 145–151. [60] S. Ngamsinlapasathian, T. Sreethawong, Y. Suzuki, S. Yoshikawa, Sol. Energy Mater. Sol. Cells 86 (2005) 269–282. [61] P.I. Gouma, M.J. Mills, J. Am. Ceram. Soc. 84 (2001) 619–622.