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Synthesis and characterization of highly ordered titania-alumina mixed oxide mesoporous films with high alumina content
Microporous and Mesoporous Materials 134 (2010) 150–156
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis and characterization of highly ordered titania-alumina mixed oxide mesoporous films with high alumina content Hamid Oveisi a,b, Ali Beitollahi b, Masataka Imura a, Chia-Wen Wu c, Yusuke Yamauchi a,d,* a
World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan Center of Excellence in Advanced Materials and Processing, Department of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran 16844, Iran c Department of Chemical Engineering, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan d Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
a r t i c l e
i n f o
Article history: Received 23 February 2010 Received in revised form 20 May 2010 Accepted 20 May 2010 Available online 2 June 2010 Keywords: Mesoporous film Mixed oxides Titania Anatase Alumina
a b s t r a c t Highly ordered mesoporous titania-alumina mixed oxide thin films are synthesized by using spin-coating method based on solvent evaporation process. The mole ratios of doped alumina are controlled in the range from 0 to 30 mol%. In order to achieve a highly ordered mesostructure, a low-temperature and low-humidity aging condition after the spin-coating is applied by means of controlling the hydrolysis and condensation reactions of metal species. During thermal treatment, the Im-3m mesostructure is unidirectionally contracted along the direction perpendicular to the substrates. The mesochannel walls are composed of periodically arranged cages with connecting necks between the neighboring cages. On the top-surface of the films, the uniformly-sized mesopores are observable. By doping alumina species in the pore walls, the thermal stability of the films is enhanced up to 450 °C. By finely tuning the amount of the doped alumina species, we can realize the successful anatase crystallization in the frameworks with the retention of the ordered mesoporous structures. The obtained mesoporous titania-alumina mixed oxide films are fully characterized by 2D-grazing-incidence small-angle X-ray scattering (2D-GISAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since the discovery of ordered mesoporous silica materials with high surface area [1–5], mesoporous TiO2 has attracted particular attention because of the expectation of potential applications such as highly sensitive chemical sensors, advanced detectors, photocatalysts, lithium batteries, gas sensors, inorganic membranes, electrochromic devices and solar cells [6–28]. Therefore, extensive efforts have been made to synthesize highly ordered mesoporous titania [6–28]. The synthetic procedures that appeared to be successful for mesoporous silica usually fail for TiO2. The reason is that the crystallization of titania takes place at 350–400 °C temperature and is accompanied by high shrinkage of the mesostructure framework and is followed by its total collapsing at higher temperatures due to extensive crystal grain growth. In comparison with mesoporous silica, mesoporous titania materials show less-ordered mesostructures, lower thermal stability, and lower surface area, which is a major drawback for many applications. Therefore,
* Corresponding author. Address: World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. E-mail address: [email protected] (Y. Yamauchi). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.05.020
extensive efforts have recently been directed toward the synthesis of ordered mesoporous titania-based mixed oxides with crystalline frameworks [29–32]. Among other metal oxides combined with titania, mesoporous alumina is a highly attractive material. Mesoporous alumina currently is expected as many applications such as adsorbents, heterogeneous catalysis supports for hydrodechlorination, and in petroleum refinement, due to its specific properties such as hydrolytic stability, amphoteric character, thermal stability, and ability to alter catalytic activity by interacting with active phases [33–35]. Therefore, it becomes of our interest to study the titaniaalumina composites as new materials that can be used in many applications. From the viewpoint of synthetic field, it is a significant challenge to obtain titania-alumina frameworks with highly ordered mesostructures via a one-step, convenient, and economic approach. The phase segregations have sometimes been occurred because of the differences in hydrolysis-condensation behavior of two kinds of metal alkoxides (Ti and Al) (When especially in the case of high aluminum amount). The critical point in the synthesis of Ti-Al mixed oxide mesoporous thin films is thus the atomic-level homogeneity of the inorganic framework, which was achieved by tuning the kinetics of hydrolysis and condensation reactions of
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both the metal alkoxides. The kinetics of hydrolysis and condensation reactions of titanium-aluminum can be slowed down by prehydrolysis step of titanium alkoxide before mixing with the aluminum metal precursors, chelating or decreasing the pH of the solution, or tuning the water content in the film [9,33,34,36]. From the viewpoint of application sides, morphological controls of mesoporous titania-alumina materials are necessary. For example, in the case of photocatalytic application using titaniabased systems, it is not facile that mesoporous powders are removed from solution after reaction, which can be simplified by the use of mesoporous films with high surface area [26]. As another application, it will be expected that the thin dielectric films based on titania can grow significantly due to their application in microelectronics, optoelectronics and micro-optical devices. In this way, the control of compositional ratios is a very important point to achieve desirable films with controlled optical and dielectric properties. In this paper, we report the preparation and characterization of ordered mesoporous titania-alumina mixed oxide films with various compositional ratios (up to 30 mol% alumina). The films were synthesized through evaporation-induced self-assembly (EISA) using titanium isopropoxide, aluminium isopropoxide, and F127 block copolymers. After spin-coating, the as-prepared films were aged under low-humidity and low-temperature condition in order to slow down the hydrolysis and condensation reactions of titanium and aluminum species. Although mesoporous titania films with low content of alumina (up to 8 mol % alumina) was reported recently [37], here we demonstrated the control of the framework compositions with much wider ranges and carefully investigated the mesostructural ordering and the thermal stability of the films, by using 2D-grazing-incidence small-angle X-ray scattering (2DGISAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. 2. Experimental 2.1. Preparation of precursor solutions for mesoporous titania-alumina films Titanium tetra-isopropoxide (TTIP) and aluminum tetra-isopropoxide (AIP) were used as a titanium and an aluminum source, respectively. A triblock copolymer, Pluronic F127, as a surfactant was purchased from Aldrich. For the preparation of precursor solutions, AIP was dissolved in a concentrated HCl solution under vigorous stirring at 50 °C for 15 min. Then, TTIP was added into the solution under vigorous stirring. After 15 min, the highly transparent hydrolyzed solution was added into an ethanol solution containing F127. Then, the mixture was stirred for 3 h under mild stirring at room temperature. The final molar ratio was as follows: TTIP:AIP:F127:HCl:EtOH = 0.7–0.95:0.3–0.0.05:0.005:3.80:25. The mole ratios of Al to total Al + Ti species in the precursor solutions were varied from 5 to 30 mol%. 2.2. Preparation of mesoporous titania-alumina films The precursor solution was used for making titania-alumina mesoporous thin films by spin coating. The precursor solution was spin-coated on clean glass substrates at 23 °C and 60% relative humidity. The spinning speed and time were fixed at 3000 rpm and 30 s, respectively, to form thin films with less than 350 nm in thickness. As-prepared thin films were aged under low temperature and low humidity condition ( 20 °C and 20% relative humidity) for 72 h. After the aging, the films were calcined at different temperatures. The calcination temperatures were varied from 350 to 450 °C.
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2.3. Characterizations 2D-GISAXS studies were performed with NANO VIEWER (Rigaku) equipped with Micro Max-007HF high intensity micro focus rotating anode X-ray generator. The data were collected during 1 h using an X-ray wavelength (k) of 1.540 Å on a two-dimensional position sensitive vacuum chamber detector at 68 cm from the sample. Three beam collimation set were used to control the beam size and divergence. The covered scattering vector (q) ranges from 0.0163 Å 1 to 0.256 Å 1. All the images were taken by 1 h measurement. The highly ordered mesoporous structure was confirmed by a JEOL-JEM 2100F TEM. The accelerating voltage of the electron beam was 200 kV. Raman spectra were obtained by exciting the samples by Kr-Ar ion laser at 514 nm with Spectra-Physics Beamlok 2060-RS laser and exposed time of 64 s. Spectra were collected at room temperature in the wavelength range from 100 to 2000 cm 1 on Symphony CCD-1LS detection system. The spectral resolution is estimated to be 0.7 cm 1. The surface morphology of the thin film was observed with Hitachi S-4800 SEM by using an acceleration voltage of 5 kV.
3. Results and discussion Fig. 1 shows 2D-GISAXS patterns of mesoporous titania and titania-alumina films with varying calcination temperatures and alumina content in the range from 0 to 30 mol%. Each 2D-GISAXS pattern labels meso_x_y where x and y denote the molar percents of the doped alumina and the calcination temperatures, respectively. The mesoporous films calcined below 350 °C exhibited several intense peaks corresponding to (110), (101) and (1–10) reflections (Fig. 1a–c), indicating that the film had highly ordered porous structure with a Im-3m symmetry. The (1–10) spot in 2DGISAXS patterns indicated the plane that arranged normal to the substrate, while the (110) spot indicated the plane that arranged parallel to the substrate. After the amount of the alumina content was increased, no significant loss of the intensities of the spots was confirmed. The titania-alumina mesoporous films (meso_5%_350 °C and meso_20%_350 °C) with 5 mol% and 20 mol% alumina contents were observed by TEM, as illustrated in Fig. 2a and b. A well-ordered mesoporous structure with a linear arrangement of mesopores with regular intervals was confirmed. Fig. 2a and b are cross-sections perpendicular to [100] and [111] directions, respectively, which are typical of an Im-3m mesostructure. Other the titania-alumina films calcined below 350 °C also possessed the typical mesopore arrangements of an Im-3m mesostructure. From TEM images, it was revealed that mesoscopically ordered single domains were extended up to hundreds of nanometers in size. It has been reported that two possible orientations of the Im3m mesostructure on a flat substrate, i.e. the [110] direction or the [001] direction of the structure is oriented perpendicular to the substrate, as shown in Fig. 3a and b, respectively. On one hand, Alberious et al. reported the synthesis of mesoporous titania thin films with the Im-3m mesostructure by using titanium tetraethoxide (TEOT; Ti(OC2H5)4) as titanium source and the triblock copolymer P123 as surfactants. In their case, the [001] direction of the structure was oriented perpendicular to the substrate [12]. On the other hand, Grosso et al. employed TiCl4 as titanium source and F127 as surfactants to successfully generate mesoporous titania thin films with the [110] direction of the Im-3m mesostructure perpendicular to the substrate [17]. In addition, recently Wu et al. used TTIP as titanium source and P123 as surfactants to synthesize mesoporous titania thin films, but the mesostructure was determined to be P63/mmc [20]. These examples indicated that slight difference of titanium sources or
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Fig. 1. 2D SAXS patterns of mesoporous titania and titania-alumina films calcined at various temperatures. Each pattern labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively.
surfactants would affect not only the final mesostructure but also the orientation of the structure. On the basis of 2D-GISAXS and TEM results, we conclude that the mesostructure of our synthesized thin films was 3D cubic Im3m structure, and the orientation of the structure includes a major orientation where the [110] direction is perpendicular to the substrate and a minor orientation where the [001] direction is perpendicular to the substrate. When the loading of Al was less than 30 mol%, all the films exhibited extra 2D-GISAXS spots next to the (1–10) spots, as the positions that arrows point out in Fig. 1. These basal spacings of the in-plane spots were slightly smaller values, which resulted from the (100) spot in minor Im-3m domain where its [001] direction was perpendicular to the substrate. Although the reasons for different orientations in the same structure are not clear yet, we speculate distortion or defect of a 3D structure in films would cause such phenomenon. These spots became clear upon calcination that creates more structural disordering. It is worthy to note that this different orientation of Im-3m structure was disappeared (in other words, the structure only exhibited the orientation where the [110] direction is perpendicular to the substrate) when 30 mol% Al was added into mesoporous titania films. To the best of our knowledge, this is the first report about the enhancement of the orientation of a 3D mesostructure
in the mesoporous thin films by adding another metal specie. The results obtained here may inspire the synthesis of mesoporous mixed oxide thin films with a fully oriented 3D orientation. Nanoscale elemental mapping of Al and Ti contents was carried out for the mesoporous titania-alumina films with various amount of the Al content. Both elements were homogeneously distributed without phase separations. As one of the examples, Fig. 4 shows elemental mapping of Al and Ti contents in the meso_20%_350 °C film. Such a uniform distribution of both elements in the map indicated that the two inorganic species were homogeneously consolidated around the F127 micelles. From the EDS spectrum (Fig. 4f), the composition was measured to be around Ti:Al = 80:20. The initial stoichiometry in the precursor solution was strictly retained in the final product. Complete indexation of a F127-templated Im-3m mesostructure can be done for all the mesoporous films calcined below 350 °C. However, when mesoporous titania calcined over 400 °C, the ordered mesostructures collapsed. No out-of-plane and in-plane spots were observable (Figs. 1d and e). This phenomenon is due to the distortion of the pore walls by the titania crystallization, which have been already reported [17]. Actually, as the crystallization proceeded, the out-of-plane spots of the (110) spots moved to higher angle range, that is, their basal spacing were gradually decreased (Fig. 5). The d110 value of the mesoporous titania film
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Fig. 2. TEM images of mesoporous titania-alumina films calcined at various temperatures. Each image labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively. The images are cross-sections perpendicular to (a) [100], (b) [111], (c and d) [110] directions, respectively.
Fig. 3. Possible orientation of Im-3m mesostructure on substrate surface in film. (a) shows the [110] direction is oriented perpendicular to substrate, while (b) shows the [001] direction is oriented perpendicular to substrate.
calcined at 350 °C were reduced up to 30% compared to those of the calcined films at 200 °C (Fig. 5). Mesoporous mixed oxide titania-alumina films showed enhanced thermal stability. In the cases of mesoporous mixed oxide titania-alumina films, the degree of the out-of-plane contraction was not so larger compared to the mesoporous titania films (Fig. 5). On the other hand, the in-plane spacings of the (1–10) spots were almost constant, as is seen in 2D-GISAXS patterns (Fig. 1 and Table 1). These results indicate, with the thermal treatment, the strong unidirectional contraction of the Im-3m mesostructure was occurred along the direction perpendicular to the substrates. Even after the calcination over 400 °C, the in-plane spots were well retained and their positions were not changed, although the out-of-plane spots were disappeared. Although the disappearance of the (110) spots can be partially explained by the exponential delay of the reflection intensity in higher Bragg angles, the main reason is that the mesostructural ordering along the direction parallel to the substrate was reduced [17]. In other words, the initial periodicity was preserved only in the direction normal to the substrate. In order to understand how an Im-3m mesostructure was contracted along the perpendicular direction, the mesoporous mixed
oxide titania-alumina films with only in-plane spots (calcined at 450 °C) were observed by TEM. The mesoporous structures of meso_5%_450 °C and meso_30%_450 °C were shown in Fig. 2c and d. These images showed cross-sectional images perpendicular to the [110] direction of the Im-3m. Importantly, the mesochannel walls were composed of continuous connection of periodically arranged mesocages, which is creased by connection of the neighbor pores during thermal shrinkage (Fig. 6a). At the initial stage of the thermal shrinkage (calcination temperature was below 350 °C), by unidirectional contraction of an Im-3m mesostructure, the cagetyped mesopores were transformed to ellipsoid-typed mesopores. At this stage, both in-plane and out-of-plane spots were observable (Fig. 1a–c). With further increase of the calcination temperatures (over 400 °C), the ellipsoid-typed mesopores were merged each other along the [111] and [11–1] directions, which broke the three dimensional mesostructures but keep a constant correlation in only the in-plane direction (Fig. 1d and e). In order to investigate the crystal structure in the calcined titania-alumina mesoporous films, selected-area electron diffraction patterns were measured (Fig. 6b, inset image). The pore walls in titania-alumina mesoporous films calcined below 400 °C showed amorphous phases. But, after the calcination at 450 °C, only the
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Fig. 4. (a–d) High resolution elemental mapping of mesoporous titania-alumina film (meso_20 _350) and (f) EDS spectrum.
Table 1 In-plane contraction as a function of temperature calculated from the 2D-GISAXS patterns. The d1-10 values in the films calcined at 250 °C, 300 °C, 350 °C, 400 °C, and 450 °C are divided by those in the films calcined at 200 °C, respectively.
Fig. 5. Out-of-plane contraction as a function of calcination temperature calculated from the 2D-GI-SAXS patterns in Fig. 1. The d110 values in the films calcined at 250 °C, 300 °C, and 350 °C are divided by those in the films calcined at 200 °C, respectively.
meso_5%_450 °C showed ring-like patterns that can be assigned to a TiO2 anatase crystal structure (Not assigned to c-alumina phase) (Fig. 6b, inset image). Since the intensity of the ED patterns was quite low, the crystal domain size was thought to be very small.
Samples
200 [°C]
250 [°C]
300 [°C]
350 [°C]
400 [°C]
450 [°C]
meso_0 meso_5 meso_10 meso_20 meso_30
— — — — —
1.01 1.00 1.00 1.02 0.99
1.00 0.96 0.98 1.02 1.00
1.00 0.98 1.00 1.04 1.15
collapse 0.97 1.00 1.02 1.01
collapse 0.98 1.00 1.07 1.16
Actually, highly magnified TEM image could not clearly detect the extending atomic fringes, although several lattice fringes were locally observed. To make this point to be clearer, a Raman spectroscopic analysis, which is very sensitive to crystal structure of anatase, was also carried out (Fig. 7). It have been already reported that the TiO2 anatase phase showed characteristic four bands located at 146 cm 1, 200 cm 1, 400 cm 1, and 643 cm 1 in Raman spectra [38]. Only meso_5%_450 °C showed the primary Raman peak at 146 cm 1. Considering big different crystallization temperatures between alumina (around 900 °C) and titania (around 350 °C), it can be reasonably understood that only the titania crystallization proceeded preferentially in the pore walls. Then, the amorphous alumina phase is thought to cover around titania nanocrystals with small sizes. With the increase of the amount of the doped aluminum, the anatase peak was disappeared (Fig. 7). The doping of other metal
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Fig. 6. TEM images of mesoporous titania-alumina film (meso_5_450). Both images are cross-sections perpendicular to [110] directions. The inset image is selected-area ED patterns.
Fig. 7. Raman spectra of mesoporous titania-alumina films calcined at various temperatures. Each spectrum labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively. The intense peaks (*) are derived from glass substrates.
oxides inhibits crystallite growth of titania nanocrystalline phases and retards the crystallization of mesoporous films. Because of the relatively mismatch between Ti4+ (0.605 Å) and Al+3 (0.535 Å) cations size, Al+3 are not expected to occupy the titanium sites in the lattice of anatase. It is well understood that the alumina species have the ability to limit crystal growth of the titania nanocrystal-
lites, thus delaying or preventing widespread crystallization of the meso_10%_450 °C, meso_20%_450 °C, and meso_30%_450 °C. In this study, by doping a very small amount of alumina species, we could realize the successful anatase crystallization in the frameworks with the retention of the ordered mesoporous structure. Because the titania nanocrystallites can be stabilized by alumina amorphous phase whose crystallization temperature is very high, the thermal stability of the films was increased compared to mesoporous titania system. The mesoporous structure could be retained up to 500 °C without further crystallization of anatase. Surface morphology of the films was investigated by SEM observation. Fig. 8 shows the top surface SEM images of titania-alumina mesoporous films prepared on glass slide with varying the doping amount of aluminum and calcination temperatures. The uniform mesopores of around 10 nm were well periodically arranged over a large domain. The quality of the film surface is depending on sol composition, aging procedure, and calcination process. The presenting titania-alumina system has a very low condensation rate that enables the system to form a very flat and uniform surface. No cracks were appeared across the surface for the films. Such cracks are caused by fast hydrolysis and condensation reactions and rapid film densification, resulting in mesostructural collapse.
4. Conclusion Highly ordered mesoporous titania-alumina films with high thermal stability were prepared by using a triblock copolymer
Fig. 8. Top-surface SEM images of mesoporous titania-alumina films calcined at various temperatures. Each image labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively.
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Pluronic F127 as a template through evaporation-induced selfassembly (EISA) process. The creation of highly ordered mesoporous titania-alumina thin films was achieved by adjusting the synthetic parameters such as precursor compositions, aging conditions, and calcination procedures. From 2D GISAXS patterns, it was proved that thermal shrinkage of the mesostructure was unidirectional (normal direction of the substrate surface), while no contraction parallel to the substrate was observed. By tuning the doping amount of alumina and the calcination temperatures, we can realize the formation of the anatase crystalline phase with retention of highly ordered mesostructure. Such a film is considered as new material candidates for applications such as catalytic supports, ceramics, electrode materials and many more applications due to their optical and electronic properties. Our synthetic concept reported here should be important for exploring novel titania-based mixed oxides films with novel functions. References [1] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988. [2] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 1535. [3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.W.T. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J. Am. Chem. Soc. 114 (1992) 10834. [5] S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc. Chem. Commun. (1993) 680. [6] C.J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (1999) 579. [7] E.L. Crepaldi, G.J.A.A. Soler-Illia, D. Grosso, F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770. [8] L. Nicole, C. BoissiWre, D. Grosso, A. Quach, C. Sanchez, J. Mater. Chem. 15 (2005) 3598. [9] G.J.A.A. Soler-Illia, P. Innocenzi, Chem. Eur. J. 12 (2006) 4478. [10] M.A. Carreon, S.Y. Choi, M. Mamak, N. Copra, G. Ozin, J. Mater. Chem. 17 (2007) 82. [11] C.W. Koh, U.H. Lee, J.K. Song, H.R. Lee, M.H. Kim, M. Suh, Y.U. Kwon, Chem. Asian J. 3 (2008) 862. [12] P.C.A. Alberious, K.L. Frindell, R.C. Hayward, E.J. Karmer, G.D. Stucky, B.F. Chmelka, Chem. Mater. 14 (2002) 3284.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis and characterization of highly ordered titania-alumina mixed oxide mesoporous films with high alumina content Hamid Oveisi a,b, Ali Beitollahi b, Masataka Imura a, Chia-Wen Wu c, Yusuke Yamauchi a,d,* a
World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan Center of Excellence in Advanced Materials and Processing, Department of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran 16844, Iran c Department of Chemical Engineering, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan d Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
a r t i c l e
i n f o
Article history: Received 23 February 2010 Received in revised form 20 May 2010 Accepted 20 May 2010 Available online 2 June 2010 Keywords: Mesoporous film Mixed oxides Titania Anatase Alumina
a b s t r a c t Highly ordered mesoporous titania-alumina mixed oxide thin films are synthesized by using spin-coating method based on solvent evaporation process. The mole ratios of doped alumina are controlled in the range from 0 to 30 mol%. In order to achieve a highly ordered mesostructure, a low-temperature and low-humidity aging condition after the spin-coating is applied by means of controlling the hydrolysis and condensation reactions of metal species. During thermal treatment, the Im-3m mesostructure is unidirectionally contracted along the direction perpendicular to the substrates. The mesochannel walls are composed of periodically arranged cages with connecting necks between the neighboring cages. On the top-surface of the films, the uniformly-sized mesopores are observable. By doping alumina species in the pore walls, the thermal stability of the films is enhanced up to 450 °C. By finely tuning the amount of the doped alumina species, we can realize the successful anatase crystallization in the frameworks with the retention of the ordered mesoporous structures. The obtained mesoporous titania-alumina mixed oxide films are fully characterized by 2D-grazing-incidence small-angle X-ray scattering (2D-GISAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since the discovery of ordered mesoporous silica materials with high surface area [1–5], mesoporous TiO2 has attracted particular attention because of the expectation of potential applications such as highly sensitive chemical sensors, advanced detectors, photocatalysts, lithium batteries, gas sensors, inorganic membranes, electrochromic devices and solar cells [6–28]. Therefore, extensive efforts have been made to synthesize highly ordered mesoporous titania [6–28]. The synthetic procedures that appeared to be successful for mesoporous silica usually fail for TiO2. The reason is that the crystallization of titania takes place at 350–400 °C temperature and is accompanied by high shrinkage of the mesostructure framework and is followed by its total collapsing at higher temperatures due to extensive crystal grain growth. In comparison with mesoporous silica, mesoporous titania materials show less-ordered mesostructures, lower thermal stability, and lower surface area, which is a major drawback for many applications. Therefore,
* Corresponding author. Address: World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. E-mail address: [email protected] (Y. Yamauchi). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.05.020
extensive efforts have recently been directed toward the synthesis of ordered mesoporous titania-based mixed oxides with crystalline frameworks [29–32]. Among other metal oxides combined with titania, mesoporous alumina is a highly attractive material. Mesoporous alumina currently is expected as many applications such as adsorbents, heterogeneous catalysis supports for hydrodechlorination, and in petroleum refinement, due to its specific properties such as hydrolytic stability, amphoteric character, thermal stability, and ability to alter catalytic activity by interacting with active phases [33–35]. Therefore, it becomes of our interest to study the titaniaalumina composites as new materials that can be used in many applications. From the viewpoint of synthetic field, it is a significant challenge to obtain titania-alumina frameworks with highly ordered mesostructures via a one-step, convenient, and economic approach. The phase segregations have sometimes been occurred because of the differences in hydrolysis-condensation behavior of two kinds of metal alkoxides (Ti and Al) (When especially in the case of high aluminum amount). The critical point in the synthesis of Ti-Al mixed oxide mesoporous thin films is thus the atomic-level homogeneity of the inorganic framework, which was achieved by tuning the kinetics of hydrolysis and condensation reactions of
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both the metal alkoxides. The kinetics of hydrolysis and condensation reactions of titanium-aluminum can be slowed down by prehydrolysis step of titanium alkoxide before mixing with the aluminum metal precursors, chelating or decreasing the pH of the solution, or tuning the water content in the film [9,33,34,36]. From the viewpoint of application sides, morphological controls of mesoporous titania-alumina materials are necessary. For example, in the case of photocatalytic application using titaniabased systems, it is not facile that mesoporous powders are removed from solution after reaction, which can be simplified by the use of mesoporous films with high surface area [26]. As another application, it will be expected that the thin dielectric films based on titania can grow significantly due to their application in microelectronics, optoelectronics and micro-optical devices. In this way, the control of compositional ratios is a very important point to achieve desirable films with controlled optical and dielectric properties. In this paper, we report the preparation and characterization of ordered mesoporous titania-alumina mixed oxide films with various compositional ratios (up to 30 mol% alumina). The films were synthesized through evaporation-induced self-assembly (EISA) using titanium isopropoxide, aluminium isopropoxide, and F127 block copolymers. After spin-coating, the as-prepared films were aged under low-humidity and low-temperature condition in order to slow down the hydrolysis and condensation reactions of titanium and aluminum species. Although mesoporous titania films with low content of alumina (up to 8 mol % alumina) was reported recently [37], here we demonstrated the control of the framework compositions with much wider ranges and carefully investigated the mesostructural ordering and the thermal stability of the films, by using 2D-grazing-incidence small-angle X-ray scattering (2DGISAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. 2. Experimental 2.1. Preparation of precursor solutions for mesoporous titania-alumina films Titanium tetra-isopropoxide (TTIP) and aluminum tetra-isopropoxide (AIP) were used as a titanium and an aluminum source, respectively. A triblock copolymer, Pluronic F127, as a surfactant was purchased from Aldrich. For the preparation of precursor solutions, AIP was dissolved in a concentrated HCl solution under vigorous stirring at 50 °C for 15 min. Then, TTIP was added into the solution under vigorous stirring. After 15 min, the highly transparent hydrolyzed solution was added into an ethanol solution containing F127. Then, the mixture was stirred for 3 h under mild stirring at room temperature. The final molar ratio was as follows: TTIP:AIP:F127:HCl:EtOH = 0.7–0.95:0.3–0.0.05:0.005:3.80:25. The mole ratios of Al to total Al + Ti species in the precursor solutions were varied from 5 to 30 mol%. 2.2. Preparation of mesoporous titania-alumina films The precursor solution was used for making titania-alumina mesoporous thin films by spin coating. The precursor solution was spin-coated on clean glass substrates at 23 °C and 60% relative humidity. The spinning speed and time were fixed at 3000 rpm and 30 s, respectively, to form thin films with less than 350 nm in thickness. As-prepared thin films were aged under low temperature and low humidity condition ( 20 °C and 20% relative humidity) for 72 h. After the aging, the films were calcined at different temperatures. The calcination temperatures were varied from 350 to 450 °C.
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2.3. Characterizations 2D-GISAXS studies were performed with NANO VIEWER (Rigaku) equipped with Micro Max-007HF high intensity micro focus rotating anode X-ray generator. The data were collected during 1 h using an X-ray wavelength (k) of 1.540 Å on a two-dimensional position sensitive vacuum chamber detector at 68 cm from the sample. Three beam collimation set were used to control the beam size and divergence. The covered scattering vector (q) ranges from 0.0163 Å 1 to 0.256 Å 1. All the images were taken by 1 h measurement. The highly ordered mesoporous structure was confirmed by a JEOL-JEM 2100F TEM. The accelerating voltage of the electron beam was 200 kV. Raman spectra were obtained by exciting the samples by Kr-Ar ion laser at 514 nm with Spectra-Physics Beamlok 2060-RS laser and exposed time of 64 s. Spectra were collected at room temperature in the wavelength range from 100 to 2000 cm 1 on Symphony CCD-1LS detection system. The spectral resolution is estimated to be 0.7 cm 1. The surface morphology of the thin film was observed with Hitachi S-4800 SEM by using an acceleration voltage of 5 kV.
3. Results and discussion Fig. 1 shows 2D-GISAXS patterns of mesoporous titania and titania-alumina films with varying calcination temperatures and alumina content in the range from 0 to 30 mol%. Each 2D-GISAXS pattern labels meso_x_y where x and y denote the molar percents of the doped alumina and the calcination temperatures, respectively. The mesoporous films calcined below 350 °C exhibited several intense peaks corresponding to (110), (101) and (1–10) reflections (Fig. 1a–c), indicating that the film had highly ordered porous structure with a Im-3m symmetry. The (1–10) spot in 2DGISAXS patterns indicated the plane that arranged normal to the substrate, while the (110) spot indicated the plane that arranged parallel to the substrate. After the amount of the alumina content was increased, no significant loss of the intensities of the spots was confirmed. The titania-alumina mesoporous films (meso_5%_350 °C and meso_20%_350 °C) with 5 mol% and 20 mol% alumina contents were observed by TEM, as illustrated in Fig. 2a and b. A well-ordered mesoporous structure with a linear arrangement of mesopores with regular intervals was confirmed. Fig. 2a and b are cross-sections perpendicular to [100] and [111] directions, respectively, which are typical of an Im-3m mesostructure. Other the titania-alumina films calcined below 350 °C also possessed the typical mesopore arrangements of an Im-3m mesostructure. From TEM images, it was revealed that mesoscopically ordered single domains were extended up to hundreds of nanometers in size. It has been reported that two possible orientations of the Im3m mesostructure on a flat substrate, i.e. the [110] direction or the [001] direction of the structure is oriented perpendicular to the substrate, as shown in Fig. 3a and b, respectively. On one hand, Alberious et al. reported the synthesis of mesoporous titania thin films with the Im-3m mesostructure by using titanium tetraethoxide (TEOT; Ti(OC2H5)4) as titanium source and the triblock copolymer P123 as surfactants. In their case, the [001] direction of the structure was oriented perpendicular to the substrate [12]. On the other hand, Grosso et al. employed TiCl4 as titanium source and F127 as surfactants to successfully generate mesoporous titania thin films with the [110] direction of the Im-3m mesostructure perpendicular to the substrate [17]. In addition, recently Wu et al. used TTIP as titanium source and P123 as surfactants to synthesize mesoporous titania thin films, but the mesostructure was determined to be P63/mmc [20]. These examples indicated that slight difference of titanium sources or
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Fig. 1. 2D SAXS patterns of mesoporous titania and titania-alumina films calcined at various temperatures. Each pattern labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively.
surfactants would affect not only the final mesostructure but also the orientation of the structure. On the basis of 2D-GISAXS and TEM results, we conclude that the mesostructure of our synthesized thin films was 3D cubic Im3m structure, and the orientation of the structure includes a major orientation where the [110] direction is perpendicular to the substrate and a minor orientation where the [001] direction is perpendicular to the substrate. When the loading of Al was less than 30 mol%, all the films exhibited extra 2D-GISAXS spots next to the (1–10) spots, as the positions that arrows point out in Fig. 1. These basal spacings of the in-plane spots were slightly smaller values, which resulted from the (100) spot in minor Im-3m domain where its [001] direction was perpendicular to the substrate. Although the reasons for different orientations in the same structure are not clear yet, we speculate distortion or defect of a 3D structure in films would cause such phenomenon. These spots became clear upon calcination that creates more structural disordering. It is worthy to note that this different orientation of Im-3m structure was disappeared (in other words, the structure only exhibited the orientation where the [110] direction is perpendicular to the substrate) when 30 mol% Al was added into mesoporous titania films. To the best of our knowledge, this is the first report about the enhancement of the orientation of a 3D mesostructure
in the mesoporous thin films by adding another metal specie. The results obtained here may inspire the synthesis of mesoporous mixed oxide thin films with a fully oriented 3D orientation. Nanoscale elemental mapping of Al and Ti contents was carried out for the mesoporous titania-alumina films with various amount of the Al content. Both elements were homogeneously distributed without phase separations. As one of the examples, Fig. 4 shows elemental mapping of Al and Ti contents in the meso_20%_350 °C film. Such a uniform distribution of both elements in the map indicated that the two inorganic species were homogeneously consolidated around the F127 micelles. From the EDS spectrum (Fig. 4f), the composition was measured to be around Ti:Al = 80:20. The initial stoichiometry in the precursor solution was strictly retained in the final product. Complete indexation of a F127-templated Im-3m mesostructure can be done for all the mesoporous films calcined below 350 °C. However, when mesoporous titania calcined over 400 °C, the ordered mesostructures collapsed. No out-of-plane and in-plane spots were observable (Figs. 1d and e). This phenomenon is due to the distortion of the pore walls by the titania crystallization, which have been already reported [17]. Actually, as the crystallization proceeded, the out-of-plane spots of the (110) spots moved to higher angle range, that is, their basal spacing were gradually decreased (Fig. 5). The d110 value of the mesoporous titania film
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Fig. 2. TEM images of mesoporous titania-alumina films calcined at various temperatures. Each image labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively. The images are cross-sections perpendicular to (a) [100], (b) [111], (c and d) [110] directions, respectively.
Fig. 3. Possible orientation of Im-3m mesostructure on substrate surface in film. (a) shows the [110] direction is oriented perpendicular to substrate, while (b) shows the [001] direction is oriented perpendicular to substrate.
calcined at 350 °C were reduced up to 30% compared to those of the calcined films at 200 °C (Fig. 5). Mesoporous mixed oxide titania-alumina films showed enhanced thermal stability. In the cases of mesoporous mixed oxide titania-alumina films, the degree of the out-of-plane contraction was not so larger compared to the mesoporous titania films (Fig. 5). On the other hand, the in-plane spacings of the (1–10) spots were almost constant, as is seen in 2D-GISAXS patterns (Fig. 1 and Table 1). These results indicate, with the thermal treatment, the strong unidirectional contraction of the Im-3m mesostructure was occurred along the direction perpendicular to the substrates. Even after the calcination over 400 °C, the in-plane spots were well retained and their positions were not changed, although the out-of-plane spots were disappeared. Although the disappearance of the (110) spots can be partially explained by the exponential delay of the reflection intensity in higher Bragg angles, the main reason is that the mesostructural ordering along the direction parallel to the substrate was reduced [17]. In other words, the initial periodicity was preserved only in the direction normal to the substrate. In order to understand how an Im-3m mesostructure was contracted along the perpendicular direction, the mesoporous mixed
oxide titania-alumina films with only in-plane spots (calcined at 450 °C) were observed by TEM. The mesoporous structures of meso_5%_450 °C and meso_30%_450 °C were shown in Fig. 2c and d. These images showed cross-sectional images perpendicular to the [110] direction of the Im-3m. Importantly, the mesochannel walls were composed of continuous connection of periodically arranged mesocages, which is creased by connection of the neighbor pores during thermal shrinkage (Fig. 6a). At the initial stage of the thermal shrinkage (calcination temperature was below 350 °C), by unidirectional contraction of an Im-3m mesostructure, the cagetyped mesopores were transformed to ellipsoid-typed mesopores. At this stage, both in-plane and out-of-plane spots were observable (Fig. 1a–c). With further increase of the calcination temperatures (over 400 °C), the ellipsoid-typed mesopores were merged each other along the [111] and [11–1] directions, which broke the three dimensional mesostructures but keep a constant correlation in only the in-plane direction (Fig. 1d and e). In order to investigate the crystal structure in the calcined titania-alumina mesoporous films, selected-area electron diffraction patterns were measured (Fig. 6b, inset image). The pore walls in titania-alumina mesoporous films calcined below 400 °C showed amorphous phases. But, after the calcination at 450 °C, only the
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Fig. 4. (a–d) High resolution elemental mapping of mesoporous titania-alumina film (meso_20 _350) and (f) EDS spectrum.
Table 1 In-plane contraction as a function of temperature calculated from the 2D-GISAXS patterns. The d1-10 values in the films calcined at 250 °C, 300 °C, 350 °C, 400 °C, and 450 °C are divided by those in the films calcined at 200 °C, respectively.
Fig. 5. Out-of-plane contraction as a function of calcination temperature calculated from the 2D-GI-SAXS patterns in Fig. 1. The d110 values in the films calcined at 250 °C, 300 °C, and 350 °C are divided by those in the films calcined at 200 °C, respectively.
meso_5%_450 °C showed ring-like patterns that can be assigned to a TiO2 anatase crystal structure (Not assigned to c-alumina phase) (Fig. 6b, inset image). Since the intensity of the ED patterns was quite low, the crystal domain size was thought to be very small.
Samples
200 [°C]
250 [°C]
300 [°C]
350 [°C]
400 [°C]
450 [°C]
meso_0 meso_5 meso_10 meso_20 meso_30
— — — — —
1.01 1.00 1.00 1.02 0.99
1.00 0.96 0.98 1.02 1.00
1.00 0.98 1.00 1.04 1.15
collapse 0.97 1.00 1.02 1.01
collapse 0.98 1.00 1.07 1.16
Actually, highly magnified TEM image could not clearly detect the extending atomic fringes, although several lattice fringes were locally observed. To make this point to be clearer, a Raman spectroscopic analysis, which is very sensitive to crystal structure of anatase, was also carried out (Fig. 7). It have been already reported that the TiO2 anatase phase showed characteristic four bands located at 146 cm 1, 200 cm 1, 400 cm 1, and 643 cm 1 in Raman spectra [38]. Only meso_5%_450 °C showed the primary Raman peak at 146 cm 1. Considering big different crystallization temperatures between alumina (around 900 °C) and titania (around 350 °C), it can be reasonably understood that only the titania crystallization proceeded preferentially in the pore walls. Then, the amorphous alumina phase is thought to cover around titania nanocrystals with small sizes. With the increase of the amount of the doped aluminum, the anatase peak was disappeared (Fig. 7). The doping of other metal
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Fig. 6. TEM images of mesoporous titania-alumina film (meso_5_450). Both images are cross-sections perpendicular to [110] directions. The inset image is selected-area ED patterns.
Fig. 7. Raman spectra of mesoporous titania-alumina films calcined at various temperatures. Each spectrum labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively. The intense peaks (*) are derived from glass substrates.
oxides inhibits crystallite growth of titania nanocrystalline phases and retards the crystallization of mesoporous films. Because of the relatively mismatch between Ti4+ (0.605 Å) and Al+3 (0.535 Å) cations size, Al+3 are not expected to occupy the titanium sites in the lattice of anatase. It is well understood that the alumina species have the ability to limit crystal growth of the titania nanocrystal-
lites, thus delaying or preventing widespread crystallization of the meso_10%_450 °C, meso_20%_450 °C, and meso_30%_450 °C. In this study, by doping a very small amount of alumina species, we could realize the successful anatase crystallization in the frameworks with the retention of the ordered mesoporous structure. Because the titania nanocrystallites can be stabilized by alumina amorphous phase whose crystallization temperature is very high, the thermal stability of the films was increased compared to mesoporous titania system. The mesoporous structure could be retained up to 500 °C without further crystallization of anatase. Surface morphology of the films was investigated by SEM observation. Fig. 8 shows the top surface SEM images of titania-alumina mesoporous films prepared on glass slide with varying the doping amount of aluminum and calcination temperatures. The uniform mesopores of around 10 nm were well periodically arranged over a large domain. The quality of the film surface is depending on sol composition, aging procedure, and calcination process. The presenting titania-alumina system has a very low condensation rate that enables the system to form a very flat and uniform surface. No cracks were appeared across the surface for the films. Such cracks are caused by fast hydrolysis and condensation reactions and rapid film densification, resulting in mesostructural collapse.
4. Conclusion Highly ordered mesoporous titania-alumina films with high thermal stability were prepared by using a triblock copolymer
Fig. 8. Top-surface SEM images of mesoporous titania-alumina films calcined at various temperatures. Each image labels meso_x _y where x and y denote the mol percents of the doped alumina and the calcination temperatures, respectively.
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