CTAB facilitated spherical rutile TiO2 particles and their advantage in a dye-sensitized solar cell

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Solar Energy 82 (2008) 1042–1048 www.elsevier.com/locate/solener

CTAB facilitated spherical rutile TiO2 particles and their advantage in a dye-sensitized solar cell Dae-Un Lee a, Song-Rim Jang a, R. Vittal a, Jiwon Lee b, Kang-Jin Kim a,* a

Department of Chemistry, Korea University, Seoul, 136-713, Republic of Korea b Samsung SDI Co. Ltd., Gyeonggi-Do, 449-577, Republic of Korea

Received 27 November 2007; received in revised form 29 February 2008; accepted 27 April 2008 Available online 28 May 2008 Communicated by: Associate Editor Sam-Shajing Sun

Abstract Spherical rutile TiO2 particles (14–20 nm) and their corresponding well-defined round clusters (500–600 nm) were obtained by using a cationic surfactant cetyltrimethylammonium bromide (CTAB). The surfactant was employed in two stages, i.e., in the hydrolysis of TiCl4 and then in the precipitation of the corresponding Ti(IV) polymers at approximately 46 °C. On the other hand, without CTAB in the hydrolyzing solution, irregular clusters consisting of typical ellipsoidal TiO2 particles were produced. The advantage of such spherical rutile TiO2 particles and clusters was examined in terms of photovoltaic characteristics of a dye-sensitized solar cell (DSSC). Significantly higher overall solar energy conversion efficiency was obtained for a DSSC using the film of these spherical rutile TiO2 particles, compared with that of a cell using a TiO2 film of ellipsoidal particles. A mechanism for the formation of these spherical rutile particles and clusters is proposed. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Spherical rutile TiO2; CTAB; Dye-sensitized solar cell

1. Introduction Altering the morphology and structure of a TiO2 film and thereby the TiO2 film electrode is one of the strategies adopted for improving the efficiency of a dye-sensitized solar cell (DSSC, Kavan et al., 1996; Jung et al., 2002; Papageorgiou et al., 1998; Barbe´ et al., 1997), because the optical, electrical, conductive and other characteristics of a TiO2 film vary greatly with the size and shape of the particles and the clusters comprising the film. A variety of TiO2 films and materials have been produced using different types of surfactants as templates, e.g., alkylphosphate (Antonelli and Ying, 1995), amphiphilic triblock copolymer (Yun et al., 2001), laurylamine hydrochloride/tetraisopropylorthotitanate (Adachi et al., 2003), self-designed *

Corresponding author. Tel.: +82 2 3290 3127; fax: +82 2 3290 3121. E-mail address: [email protected] (K.-J. Kim).

0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.04.006

amphiphilic compounds containing cationic charge moieties (Kobayashi et al., 2000), poly(alkylene oxide) block copolymer (Kavan et al., 2000b), cationic surfactant cetyltrimethylammonium chloride (Kavan et al., 2000a), and the non-ionic surfactant Triton X-100 (Kluson et al., 2001). Hirashima et al. examined the effects of immersing TiO2 gels in an ethanolic solution of cetyl/benzyl trimethylammonium chloride, and reported that this treatment increases the BET surface area, pore volume, and size of the TiO2 material (Hirashima et al., 2001). Reverse micelles routes have also been used to prepare films of TiO2 nanoparticles (Stathatos et al., 1997). Hollow microspheres of mesoporous titania with thin anatase shells were obtained by a simple procedure of surfactant [poly(ethylene oxide)]-assisted nanoparticle assembly in a nonaqueous system (Ren et al., 2003). Very recently nanocrystalline rutile TiO2 powder having ultrafine and narrowdistributed diameters was prepared via an improved liquid

D.-U. Lee et al. / Solar Energy 82 (2008) 1042–1048

one-step method involving sodium dioctyl sulfosuccinate (Liu et al., 2007). A modified-templated-hydrothermal technique was used to prepare mesoporous titania powders through the interaction of tiny anatase seeds with block copolymer Pluronic P123 in the presence and absence of HCl. The acid-free route produced a full anatase crystalline domain and showed higher DSSC performance (Kartini et al., 2004). As can be seen above, primary surfactants were rarely used to improve the morphology, crystallinity or particle size of TiO2, thereby to improve the photovoltaics of a DSSC. Surfactants have been used by others (Antonelli and Ying, 1995; Yun et al., 2001; Adachi et al., 2003; Kobayashi et al., 2000; Kavan et al., 2000a,b; Kluson et al., 2001; Li et al., 2004) as templates in non-electrochemical methods to obtain organized TiO2 film structures after the removal of the surfactant, and not to examine their beneficial influences in terms of improved TiO2 film properties for a DSSC. This paper essentially deals with the formation of spherical rutile TiO2 particles and clusters by means of the introduction of CTAB at two stages of the preparation of TiO2 film, i.e., at hydrolysis of TiCl4 and then at the precipitation of the corresponding Ti(IV) polymers. The advantages of such spherical rutile TiO2 particles are also examined for a DSSC. The spherical shape of rutile is abnormal and surprising, because rutile particles have typically prolate or rod-like shape and no report has ever demonstrated such spherical rutile particles. Though there have been reports on spherical TiO2 particles (Chen and Chen, 2003; Murakami et al., 1999; Cho and Kim, 2003; Kozuka et al., 2000; Wei et al., 1999; Gerischer, 1995; Nedeljkovic´ et al., 1997), none of them claimed a restricted formation of spherical rutile TiO2, i.e., to say either anatase was obtained or a mixture of anatase and rutile phases. It has been reported that the photovoltages of anatase and rutile DSSCs are comparable at one-sun intensity (100 mW cm2) (Park et al., 2000, 1999). Han et al. have reported that a DSSC with 71% anatase (and remaining rutile) in its TiO2 film has shown a larger conversion efficiency of 6.8%, compared to 5.3% of a cell with pure anatase TiO2 film (Han et al., 2005). Compared with anatase, rutile TiO2 has superior light scattering properties because of its higher refractive index and is chemically more stable and potentially cheaper to produce (Kim et al., 2002). Higher light scattering properties are beneficial from the perspectives of effective light harvesting. In view of these interesting findings about rutile TiO2 from the perspectives of DSSCs, we believe that obtainment of spherical rutile TiO2 particles is a newly added dimension to the research in the area. 2. Experimental Titanium tetrachloride was used as the starting material to prepare the TiO2 films. All the chemicals were of analytical grade and used without further purification. The influence of CTAB was examined in two steps: (1) the

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preparation of an aqueous solution of TiCl4 (hydrolysis step), and (2) the precipitation of Ti(IV) polymers from the hydrolyzed TiCl4 solution onto a conducting substrate (precipitation step). In the hydrolysis step, 22 mL of TiCl4 was added dropwise to 78 mL of chilled water (at ca. 4 °C) containing 1.3 mM CTAB to produce a 2.0 M Ti(IV) solution. This 2.0 M Ti(IV) solution was stored in a refrigerator at 4 °C and used as stock solution for further experiments. Before the precipitation process, fluorine-doped tin oxide (FTO) conducting glass plates (1.5  1.5 cm; Libbey-Owens-Ford, TEC 8, 75% visible light transmittance) were first cleaned with water and ethanol, and kept in a slanting position. Two to three drops of an ethanolic solution of Ti(IV) butoxide (7% v/v) were allowed to flow freely on the surfaces. This treatment produces a thin layer of TiO2, which isolates the conducting glass surface from the redox electrolyte (Smestad, 1994). The plates were then annealed at 450 °C for 30 min, cooled and then placed in different plastic petri dishes with the conducting side facing upwards. In the precipitation step, 1.0 ml aliquots of the hydrolyzed 2.0 M Ti(IV) solution were pipetted into petri dishes containing the conducting glass plates, and 9.0 mL of aqueous solution was added to each aliquot to give the concentration range of 0.001 to 0.1 M CTAB. All the solutions were prepared using milli Q (18.2 MX) H2O. The closed dishes were set aside at different temperatures in order to allow the Ti(IV) polymers to deposit onto the conducting glass plates. Subsequently, the glass plates were rinsed with distilled water, dried for 10 min in an oven at 100 °C, and annealed at 450 °C for 1 h. The TiO2 film thickness was then measured using a Tencor alpha-step 250 profiler. The TiO2 films thus obtained were coated with 0.3 mM N3 ([RuL2(NCS)2]2H2O, where L = 2,20 -bipyridine-4,40 dicarboxylic acid) in absolute ethanol for 12 h at room temperature. In order to minimize the rate of TiO2 hydration through moisture from the ambient air, the films were immersed in the dye solution at ca. 100–120 °C after the annealing step. The preparation of the counter electrode and the fabrication of the DSSC were described elsewhere (Byun et al., 2004). The resulting cell had an active area of 0.4  0.4 cm2. The photocurrent–voltage (J–V) curves were obtained using a Keithley M 236 source/measuring unit. A 300 W Xe arc lamp (Oriel), with an AM 1.5 solar simulating filter for spectral correction, was used to illuminate the working electrode. Its light intensity was adjusted to 100 mW cm2 using a Si solar cell. An HP 8453A diode array spectrophotometer was used to obtain the absorption spectra. The Raman spectra were obtained using a Jasco NR1100 spectrophotometer. The surface morphology and film thickness were determined using a Hitachi S-4300 field emissionscanning electron microscope (FE-SEM). The XRD measurements were carried out with a MAC Science Co. MO3XHF X-ray diffractometer using Cu Ka radiation. The surface areas were measured using a Micromeritics ASAP 2010 BET apparatus. The morphology and size of

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the TiO2 particles were obtained using a JEOL 1200EX transmission electron microscope (TEM). 3. Results and discussion The spherical rutile TiO2 particles were obtained from TiCl4 using CTAB in both the hydrolyzing and precipitating solutions at precipitation temperatures of approximately 46 °C. The concentrations of CTAB in the hydrolyzing and precipitating solutions were 1 and 7 mM, respectively, which both exceeded the CMC of 0.92 mM. Fig. 1a and b show, respectively, the TEM images of the annealed TiO2 films obtained with the presence or absence of 1 mM CTAB in their hydrolyzing TiCl4 solutions. The films in both the cases were obtained by precipitation from 0.2 M hydrolyzed TiCl4 solutions in the presence of 7 mM CTAB at 46 °C. Fig. 1c shows the top view SEM image of clusters of the film shown in Fig. 1a, i.e., of the film formed with CTAB both in the hydrolysis stage and precipitation stage. Fig. 1d on the other hand depicts the clusters of the film formed in the absence of CTAB at the hydrolysis stage. The CTAB-influenced clusters (Fig. 1a and c) have a uniform diameter (ca. 500–600 nm) and consist of welldefined spherical TiO2 particles, of 14–20 nm in diameter, whereas the clusters formed without CTAB in the hydrolysis step (Fig. 1b and d) were irregular in size and consisted of ellipsoidal TiO2 particles. The clusters composed of these ellipsoidal particles tended to fuse together to form large aggregates.

It is surprising that distinct spherical TiO2 particles and clusters were obtained over a narrow range of CTAB concentrations and precipitating temperatures: ca. 1 mM of CTAB in the hydrolyzing solution and ca. 7 mM in the precipitating solution near 46 °C. When the CTAB concentration in the hydrolyzing solution was increased to 2 mM, the TiO2 particles were not as round as those obtained at 1 mM of CTAB, regardless of the CTAB concentration (up to 10 mM) in the precipitating solution. At precipitating solution temperatures either below 40 °C or between 50 and 60 °C, irregular clusters containing ellipsoidal particles were obtained, regardless of the presence of CTAB in the hydrolysis or precipitation steps. Moreover, at above 60 °C, featureless large lumps were formed instead of round clusters. Therefore, it is concluded that the morphologies of the particles and clusters depend on the CTAB concentration in the hydrolyzing solution and the temperature of the precipitating solution. Wang et al. obtained rod shaped rutile TiO2 particles through thermohydrolysis of Ti(IV) chloride in hydrochloric acid–alcohol aqueous solution at 40–90 °C, despite the addition of SDS or CTAB (Wang et al., 2004). Obtainment of spherical TiO2 particles in our case with the addition CTAB indicates the importance of specific experimental conditions for the formation of such particles. Unlike our case, anatase TiO2 was obtained by Lee et al., using hydrolysis of titanium tetraisopropoxide in the presence of non-ionic surfactants (Lee et al., 2005). Fig. 2 shows the influence of CTAB on the XRD patterns of the annealed TiO2 films, deposited on FTO for

a 8000

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Fig. 1. TEM images of the clusters of annealed TiO2 films prepared: (a) with and (b) without 1 mM of CTAB in TiCl4 hydrolyzing solution. (c) The top view of SEM image of clusters of the film shown in (a), and (d) depicts the clusters of the film formed in the absence of CTAB.

25

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A 40

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2θ Fig. 2. XRD patterns of the annealed TiO2 films prepared: (a) with and (b) without CTAB in both the hydrolyzing and precipitating solutions. ‘‘A” denotes anatase.

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W r ¼ 1=½1 þ 0:8ðI a =I r Þ;

ð1Þ

44 h

41 h

38 h

Intensity (a.u.)

2 days at three different precipitation temperatures, i.e., 40, 50 and 60 °C. Fig. 2a shows the XRD patterns obtained when 1 and 7 mM of CTAB were used in the hydrolysis and precipitation steps, respectively, while Fig. 2b shows those obtained without CTAB in any steps. It can be seen that the TiO2 film formed with CTAB completely crystallize as rutile at 40 °C (Fig. 2a). However, in the absence of CTAB an additional small but discernible peak appeared at 2h = 25.3° at this temperature (Fig. 2b). This peak was assigned to anatase from the excellent match of this peak and other minor anatase peaks at 37.8° and 48.0° in Fig. 2b, corresponding to (1 0 1), (0 0 4) and (2 0 0) planes, respectively, with the reference data (JCPDS files 21-1272, 21-1276). At 50 °C and above, this anatase peak at 25.3° appeared in all cases. At 50 and 60 °C, the rutile: anatase molar ratio was 0.89 and 0.90 in the presence of CTAB, but 0.88 and 0.74 in the absence of CTAB, respectively. These were calculated using the Spurr and Myers’ method (Spurr and Myers, 1957),

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A 35 h

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200

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where Wr is the mole fraction of rutile, and Ia and Ir are the peak intensities of the anatase (1 0 1) and rutile (1 1 0) reflections, respectively. The validity of Eq. (1) was checked by applying it to the XRD data obtained for P25 (Degussa, TiO2), and the rutile content was found to be close to 21%, which is in agreement with the manufacturer’s specified value of 20%. These molar ratio values and XRD peaks at 40 °C suggest that CTAB generally suppresses anatase formation. Furthermore, an analysis of the peak widths of the rutile peak at 27.4° indicates that the size of the TiO2 particles was approximately 7% smaller when they were prepared in the presence of CTAB. Fig. 3 shows the deposition-time dependent Raman spectra of the annealed TiO2 films prepared from the 0.2 M hydrolyzed TiCl4 solutions, which each contained 10 mM CTAB in the precipitation stage at room temperature (no CTAB was added at the hydrolysis stage). When the deposition time was >36 h, the TiO2 particles were found to be mainly rutile, as shown by the bands at 612, 447 and 232 cm1 (Porto et al., 1967; Felske and Plieth, 1989). At deposition times <38 h, at which the film thickness was approximately 12 lm (Byun et al., 2004), a band at 142 cm1, corresponding to the Eg mode of anatase was observed. This suggests that anatase is present with rutile in the case of relatively thin TiO2 films, even those produced at room temperature in the presence of CTAB. An XRD pattern (not shown) also showed a small anatase peak at 25.3° when the TiO2 powder was prepared in petri dishes without the FTO conducting glass at 46 °C. This suggests that the conducting glass substrate does not play a role in producing anatase. The formation of rutile from TiCl4 using CTAB assumes importance considering the fact that anatase was obtained from TiCl4 using other surfactants (Kavan et al., 2000a,b; Yang et al., 1998). This may be attributed to the fact that surfactants were used in these

Raman Shift (cm ) Fig. 3. Raman spectra of the annealed TiO2 films formed with CTAB in the precipitating solution, as a function of deposition time (no CTAB was added at the hydrolysis stage). ‘‘A” denotes anatase.

other studies as templating or structure directing agents at relatively high concentrations. The BET surface areas were 37 and 40 m2 g1 for the TiO2 powders formed without and with CTAB, respectively; in the latter case 1 and 7 mM of CTAB were used in the hydrolysis and precipitation stages, respectively. This increase in surface area was further evidenced by the observation that the amount of dye desorbed from the CTABinfluenced TiO2 film surface was approximately 45% higher than that from the film formed without the CTAB, as shown by the absorption spectra in Fig. 4. The absorption spectra in the figure were obtained for the respective dyes, desorbed from TiO2 films formed in the presence and absence of CTAB into 1.0  103 M KOH solution over a 24-h period. This observation is consistent with the observation derived from XRD pattern that CTAB facilitates smaller TiO2 particles. As the optical, electrical, conductive and other characteristics of TiO2 film depend strongly on the size and shape of the TiO2 particles as well as the clusters comprising the film, we expected an enhanced performance of a DSSC fabricated with a CTAB-influenced TiO2 film. The rationale behind this expectation lies in the regular structure of the film, composed of spherical particles and clusters. This is indeed verified, as can be seen in Fig. 5. Fig. 5 shows the J–V characteristics of the DSSCs with the TiO2 film electrodes that had been prepared in the presence or absence of CTAB. In the former case, 1 and 7 mM of CTAB were used in the hydrolysis and precipitation steps, respectively. The DSSC fabricated using the CTAB-influenced TiO2 film

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Absorbance

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Fig. 4. Absorption spectra of the N3 dye from the TiO2 films prepared using CTAB (dashed line) in both the hydrolyzing and precipitating solutions and without CTAB in either solution (solid line).

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shows a higher short-circuit photocurrent (Jsc), open-circuit voltage (Voc) and solar conversion efficiency than that prepared without CTAB. The increase in Jsc may be primarily related to the increased surface area. Furthermore, this increase in Jsc can be related to an increase in the number of pathways due to the increased packing of round particles. A more densely packed film improves inter-particle electrical contact. The increase in Voc is in agreement with a decrease in the dark current (Fig. 5). This dark current is a manifestation of the back electron transfer from the conduction band to I 3 ions. The observed increase in Voc might also be due to an increased packing density of the TiO2 particles in the film. Increased packing of small and round TiO2 particles produce small pores. The effective concentration of I 3 ions in the TiO2 film with smaller pores would be lesser than that in the film with larger pores,

because the rate of I 3 ion replenishment in the TiO2 film with smaller pores becomes slower. This slow replenishment of I 3 ions leads to the accumulation of photo-injected electrons in the TiO2 conduction band in the open-circuit condition, which in turn leads to an increase in the Voc. A mechanism for the formation of spherical TiO2 particles is suggested. Polymeric Ti(IV) hydroxide has terminal and bridged OH groups in hydrolyzed TiCl4 solution (Boehm, 1971). These bridged OH groups are expected to be acidic in character (Ragai and Selim, 1987). Therefore, the hydrogen atoms of the bridged OH groups can be exchanged with the cationic head groups of CTAB. Because of this exchange process, Ti(IV) polymers reside on the exterior head groups of the CTAB micelle formed in 1 mM CTAB, which results in a spherical composite consisting of a micelle and the peripheral layer of Ti(IV) polymers. In the precipitation step in the solution containing 7 mM CTAB, the surface of the spherical composite formed in the hydrolysis step can further attach to cationic CTAB groups, producing reverse micelle-like species, in which the core consists of a micelle and Ti(IV) polymers layer, as shown schematically in Fig. 6a. The tails of these reverse micelle-like species can combine with the tails of other similar species to form an aggregate, which is shown schematically in Fig. 6b. A hydrophobic interaction is expected to favor this type of aggregate formation (Ratajczak and Orville-Thomas, 1982). The entire process can lead to the formation of a critical aggregate mass that is heavy enough to precipitate onto the conducting glass substrate. After annealing the CTAB-TiO2 deposits at 450 °C, the CTAB micelles burn away, leaving spherical particles, which can aggregate to form round clusters. On the other hand, the Ti(IV) polymers produced in the absence of CTAB in the hydrolysis stage can form bundles, arising from an interaction between the chains of the Ti(IV) polymers due to inter-chain hydrogen bonding, which can result in the formation of non-spherical TiO2 particles. Regarding TiO2 precipitation at temperatures P60 °C, rapid particle aggregation can result in the formation of featureless large lumps. At temperatures 640 °C, the formation of non-uniform clusters comprised of ellipsoidal

Fig. 6. Schematic representation of (a) reverse micelle-like species and (b) an aggregate formed from the species shown in (a).

D.-U. Lee et al. / Solar Energy 82 (2008) 1042–1048

particles was observed, which were possibly the result of strong interactions between the aggregates in the longitudinal direction. The focus of the present study was to control the TiO2 film morphology in such a way that it promotes the performance of the pertinent DSSC. This has been achieved in terms of Jsc, Voc and thereby over all solar-to-electricity conversion efficiency. In the research process, interestingly, spherical rutile TiO2 particles and clusters were abnormally obtained. The approach is different from that where surfactant is used at the film-deposition stage as templating agent at relatively higher concentration than that in our case. Organized morphology of particles and clusters such as that achieved in this study can allow the film structure to be optimized for a DSSC. The mechanism, we believe, can be a model for the formation of spherical or cylindrical particles in the presence of surfactants, in view of the characteristic feature of surfactants to form spherical or cylindrical micelles. 4. Conclusions This paper reports for the first time the formation of a TiO2 film consisting of spherical nanocrystalline rutile TiO2 particles and clusters, and includes a discussion of the merits and production of such films. Spherical rutile TiO2 particles were obtained using CTAB in a hydrolyzing TiCl4 solution, and in the corresponding Ti(IV) precipitating solution at approximately 46 °C. When hydrolysis was performed in the absence of CTAB or when the precipitation temperature was less than 40 °C or between 50 to 70 °C, irrespective of the presence of CTAB in the hydrolysis or precipitation step, ellipsoidal rutile TiO2 particles were produced. Above 60 °C, featureless large lumps were formed. XRD results reveal that CTAB generally suppresses the formation of anatase phase of TiO2. The DSSC prepared with rutile TiO2 film, consisting of spherical particles and clusters, showed improved Jsc, and Voc as well as substantially improved solar energy conversion efficiency, compared with those of a cell fabricated with TiO2 film of ellipsoidal TiO2 particles. We proposed the following mechanism for the formation of spherical rutile particles and clusters. During hydrolysis, a spherical composite consisting of a micelle and a surrounding polymeric Ti(IV) hydroxide shell is formed, which in the precipitation stage is attached to CTA+ molecules to produce a reverse micelle-like species. Such species aggregate tail-to-tail due to interactions between their non-polar groups in aqueous medium. This process leads to the formation of spherical aggregates, which grow to form round particles. The formation of the spherical composite in the hydrolysis solution is believed to be the result of cation exchange between the exterior head groups of the CTAB micelle and the bridging OH groups of the polymeric Ti(IV) hydroxides. Spherical TiO2 particles combine to form round clusters. The mechanism, we believe, can be a model for the formation of spherical or cylindrical parti-

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cles in the presence of a surfactant, in view of the characteristic feature of surfactants to form spherical or cylindrical micelles. Acknowledgments This work was supported by the Sol–Gel Innovation Project and the MOCIE new and renewable energy R&D project under contract 2006-N-PV12-P-05. References Adachi, M., Murata, Y., Okada, I., Yoshikawa, S., 2003. Formation of titania nanotubes and applications for dye-sensitized solar cells. J. Electrochem. Soc. 150, G488–G493. Antonelli, D.M., Ying, J.Y., 1995. Synthesis of hexagonally packed mesoporous TiO2 by a modified sol–gel method. Angew. Chem. Int. Ed. Engl. 34, 2014–2017. Barbe´, C.J., Arendse, F., Comte, P., Jirousek, M., Lenzmann, F., Shklover, V., Gra¨tzel, M., 1997. Nanocrystalline titanium oxide electrodes for photovoltaic applications. J. Am. Ceram. Soc. 80, 3157–3171. Boehm, H.P., 1971. Acidic and basic properties of hydroxylated metal oxide surfaces. Discuss. Faraday Soc. 52, 264–275. Byun, H.-Y., Vittal, R., Kim, D.Y., Kim, K.-J., 2004. Beneficial role of cetyltrimethylammonium bromide in the enhancement of photovoltaic properties of dye-sensitized rutile TiO2 solar cells. Langmuir 20, 6853– 6857. Chen, K.-U., Chen, Y.-W., 2003. Synthesis of spherical titanium dioxide particles by homogeneous precipitation in acetone solution. J. Sol–Gel Sci. Technol. 27, 111–117. Cho, C.H., Kim, D.K., 2003. Photocatalytic activity of monodispersed spherical TiO2 particles with different crystallization routes. J. Am. Ceram. Soc. 86, 1138–1145. Felske, A., Plieth, W.J., 1989. Raman-spectroscopy of titanium-dioxide layers. Electrochim. Acta 34, 75–77. Gerischer, H., 1995. Photocatalysis in aqueous solution with small TiO2 particles and the dependence of the quantum yield on particle size and light intensity. Electrochim. Acta 40, 1277–1281. Han, H., Zan, L., Zhong, J., Zhao, X., 2005. A novel hybrid nanocrystalline TiO2 electrode for the dye-sensitized nanocrystalline solar cells. J. Mater. Sci. 40, 4921–4923. Hirashima, H., Imai, H., Balek, V., 2001. Preparation of meso-porous TiO2 gels and their characterization. J. Non-Cryst. Solids 285, 96–100. Jung, K.-H., Hong, J.S., Vittal, R., Kim, K.-J., 2002. Enhanced photocurrent of dye-sensitized solar cells by modification of TiO2 with carbon nanotubes. Chem. Lett. 8, 864–865. Kartini, I., Menzies, D., Blake, D., da Costa, J.C.D., Meredith, P., Riches, J.D., Lu, G.Q., 2004. Hydrothermal seeded synthesis of mesoporous titania for application in dye-sensitized solar cells (DSSCs). J. Mater. Chem. 14, 2917–2921. Kavan, L., Gra¨tzel, M., Rathousky´, J., Zukal, A., 1996. Nanocrystalline TiO2 (anatase) electrodes: surface morphology, adsorption, and electrochemical properties. J. Electrochem. Soc. 143, 394–400. Kavan, L., Attia, A., Lenzmann, F., Elder, S.H., Gra¨tzel, M., 2000a. Lithium insertion into zirconia-stabilized mesoscopic TiO2 (anatase). J. Electrochem. Soc. 147, 2897–2902. Kavan, L., Rathousky´, J., Gra¨tzel, M., Shklover, V., Zukal, A., 2000b. Surfactant-templated TiO2 (anatase): characteristic features of lithium insertion electrochemistry in organized nanostructures. J. Phys. Chem. B 104, 12012–12020. Kim, K.-J., Benkstein, K.D., van de Lagemaat, J., Frank, A.J., 2002. Characteristics of low-temperature annealed TiO2 films deposited by precipitation from hydrolyzed TiCl4 solutions. Chem. Mater. 14, 1042– 1047.

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