Mesoporous Aluminophosphate Thin Films with Cubic Pore ...

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Mesoporous Aluminophosphate Thin Films with Cubic Pore Arrangement M. Mazaj,† S. Costacurta,‡ N. Zabukovec Logar,† G. Mali,† N. Novak Tušar,† P. Innocenzi,‡ L. Malfatti,‡ F. Thibault-Starzyk,§ H. Amenitsch,| V. Kaucˇicˇ,† and G. J. A. A. Soler-Illia*,⊥,# National Institute of Chemistry, HajdrihoVa 19, 1000 Ljubljana, SloVenia, Laboratorio di Scienza dei Materiali e Nanotecnologie, Dipartimento di Architettura e Pianificazione, UniVersità di Sassari, INSTM, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (SS), Italy, Laboratoire Catalyse et Spectrochimie, ISMRA-CNRS, 14050 Caen Cedex, France, Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Schmiedelstrasse 6, A-8042 Graz, Austria, Gerencia de Quimica, Comisión Nacional de Energía Atómica, AV. Gral Paz 1499 B1650KNA, San Martín, Buenos Aires, Argentina, and CONICET, AV. RiVadaVia 1917, C1033AAV Buenos Aires, Argentina ReceiVed NoVember 15, 2007. ReVised Manuscript ReceiVed January 25, 2008 Mesoporous aluminophosphate thin films with 3D cubic (Im3jm) pore arrangement were synthesized for the first time. Thin films were templated with block copolymer nonionic templates Pluronic F127 and F108 and deposited on a glass substrate by dip-coating. In situ SAXS investigations show the formation of a highly ordered mesostructure upon the dip-coating process, which remains stable up to at least 670 K. A cubic mesostructure was observed also by TEM. Template removal process was monitored by TG and FT-IR. A transition from an amorphous aluminophosphate gel to a well-defined aluminophosphate framework was observed by MAS NMR.

1. Introduction The discovery of M41S ordered mesoporous silicates and aluminosilicates1,2 mesoporous materials with high specific areas (∼1000 m2/g) and large pore sizes (2–15 nm) opened a whole new field of research of mesoscale-templated materials. For the purposes of catalytic processes where larger molecules are involved or where molecular diffusion should be improved, mesoporous materials with large, interconnected pore systems are becoming increasingly important.3–9 These materials are synthesized by a surfactant templating process which has been extended also to other systems such as mesoporous aluminophosphates (AlPO). In spite of the relatively large number of published mesoporous aluminophosphate synthesis strategies,10 the difficulty in keeping an ordered mesostructure after surfactant removal still represents the main drawback in applications of * To whom correspondence should be addressed. † National Institute of Chemistry. ‡ Università di Sassari. § ISMRA-CNRS. | Austrian Academy of Sciences. ⊥ Comisión Nacional de Energía Atómica. # CONICET. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Corma, A. Chem. ReV. 1997, 97, 2373. (4) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 57. (5) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. AdV. Mater. 1999, 11, 579. (6) Antonietti, M.; Ozin, G. A. Chem. Eur. J. 2004, 10, 28. (7) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (8) Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opin. Colloid Interface Sci. 2003, 8, 109. (9) Grosso, D.; Cagnol, F.; Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. AdV. Funct. Mater. 2004, 14, 309. (10) Kimura, T. Microporous Mesoporous Mater. 2005, 77, 97.

AlPO-based mesostructured materials.11 Thermally unstable lamellar mesostructured aluminophosphates were first synthesized using diaminoalkanes (H2N(CH2)nNH2) as the structure directing agents.12 Hexagonal mesoporous AlPO-based materials were prepared by self-assembly between cationic surfactant (CTACl) and aluminophosphate precursor in the presence of alkylammonium cations.13 The use of triblock copolymer Pluronic F127 as the structure-directing agent has extended the pore size and improved the thermal stability of the hexagonal aluminophosphate mesostructure.14 Most of the synthesis strategies led to lamellar, hexagonal, or disordered AlPO-based mesostructures.10 Threedimensional cubic AlPO mesoporous films, where accessibility and diffusivity for the host molecules are enhanced,15–17 comparing to the two-dimensional hexagonal systems, have not been yet prepared, to our knowledge. With the preparation of thin films, the use of AlPO-based materials can be widely extended from catalysis to various other applications such as electronic, chemical or optical sensing, and chemical separations.18–21 A general route toward mesoporous metal phosphates has been recently developed by Tian et al., based on the “acid-base pair” concept, where the production of mesoporous AlPO thin films with a 2D hexagonal mesostructure (11) Nishiyama, Y.; Tanaka, S.; Hillhouse, H. W.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Langmuir 2006, 22, 9469. (12) Kraushaar-Czarnetzki, B.; Stork, W. H. J.; Dogterom, R. J. Inorg. Chem. 1993, 93, 5029. (13) Zhao, D.; Luan, Z.; Kevan, L. Chem. Commun. 1997, 11, 1009. (14) Wang, L.; Tian, B.; Fan, J.; Liu, X.; Yang, H.; Yu, C.; Tu, B.; Zhao, D. Y. Microporous Mesoporous Mater. 2004, 67, 123. (15) Soler-Illia, G. J. A. A.; Innocenzi, P. Chem. Eur. J. 2006, 12, 4478. (16) Angelomé, P. C.; Soler-Illia, G. J. A. A. Chem. Mater. 2005, 17, 322. (17) Otal, E. H.; Angelomé, P. C.; Aldabe-Bilmes, S.; Soler-Illia, G. J. A. A. AdV. Mater. 2006, 18, 934. (18) Quach, A.; Escax, V.; Nicole, L.; Goldner, P.; Guillot-Noël, O.; Aschehoug, P.; Hesemann, P.; Moreau, J.; Gourier, D.; Sanchez, C. J. Mater. Chem. 2007, 17, 2552. (19) Mori, K.; Imaoka, S.; Nishio, S; Nishiyama, Y.; Nishiyama, N.; Yamashita, H. Microporous Mesoporous Mater. 2007, 101, 288. (20) Fryxell, G. E. Inorg. Chem. Commun. 2006, 9, 1141. (21) Coluccia, S.; Gianotti, E.; Marchese, L. Mater. Sci. Eng., C 2001, 15, 219.

10.1021/la7035746 CCC: $40.75  2008 American Chemical Society Published on Web 05/15/2008

Mesoporous Aluminophosphate Thin Films

has been demonstrated.22 More recently, Nishiyama et al. developed Zr phosphate mesoporous films with a 2D hexagonal mesostructure, aiming at proton conductive membranes.11 The same EISA-based (evaporation induced self-assembly) procedure can also be used for the production of cubic mesoporous films. In this work, we report on the detailed preparation and structural characterization of thermally stable large-pore cubic aluminophosphate thin films for the first time. The large pore dimensions and three-dimensional interconnection of these mesoporous AlPO thin films templated with Pluronic F127 and F108 nonionic polymers can enable hosting for metal oxides or pure metal nanoclusters. Such a transparent, mesoporous material with acidic properties can lead to novel catalytic, electronic, optical and chemical sensing properties.

2. Experimental Section 2.1. Preparation of Thin Films. Aluminophosphate solution was prepared by a procedure derived from the one described in the literature,14 using block copolymer surfactants Pluronic F127 or F108 as supramolecular templates, with a large hydrophilic head volume, which is a favorable condition to obtain high curvature mesophases.23 Lower s ([surfactant]/[Al] molar ratio) conditions were used, in order to direct toward three-dimensional interconnected pore arrays. In a typical synthesis, 0.67 g of aluminum chloride (99% AlCl3, Merck) was dissolved in 30 g of ethanol prior to the addition of 0.5 g of phosphoric acid (85% H3PO4, Merck). Subsequently, 0.38 g of Pluronic F127 or 0.44 g of Pluronic F108 block copolymers (Sigma-Aldrich) was added, under stirring. The obtained clear solution with Al:P:H2O:template:ethanol molar ratios 1:1:1:0.005:130 was stirred for 30 min at ambient temperature. Thin films were deposited by dip-coating the precursor solution on a glass substrate at ambient temperature, at a constant relative humidity of 40% and 14 cm min–1 withdrawal speed. AlPO bulk xerogels were obtained by transferring the precursor solution into Petri dishes, followed by drying at 323 K. The films or solid products were thermally treated at different temperatures. Surfactant was completely removed after calcination at 673 K for 6 h in air flow. 2.2. Characterization Methods. Film thickness was assessed by scanning electron microscopy (SEM) on a Zeiss Supra 3VP SEM microscope. Line scan elemental analysis was performed by energy dispersive X-ray spectroscopy (EDX) using an INCA Energy system attached to the same microscope. For grazing incidence small-angle X-ray scattering (GISAXS) measurements, homemade dip-coating equipment was employed at the Austrian SAXS beamline at the Elettra synchrotron facility (Trieste, Italy). The dip-coater was inserted in the beam path so in situ diffraction patterns of the freshly formed AlPO film could be acquired. The instrumental grazing angle was set slightly above the critical angle of the film (wavelength of incident X-ray beam 1.54 Å). Indexing of the SAXS diffraction patterns and calculation of the cell parameters was performed using Fit2D.24 The sample-to-detector distance was calculated from the diffraction pattern of a silver behenate (CH3(CH2)21COOAg) calibration standard powder in a capillary glass. The beam center position was calculated from the least-squares fit of a circle of 20 coordinates in the silver behenate diffraction ring. Transmission electron microscopy (TEM) was performed on a 200-kV field-emission gun (FEG) microscope JEOL JEM 2100. For TEM studies, thin film samples were scratched from a substrate to a drop of ethanol placed on a copper holey carbon grid and dried at room temperature. Thermogravimetric analysis (TG-DTG) was performed on aluminophosphate materials scratched from the asprepared films on an SDT 2960 thermal analysis system (TA Instruments, Inc.). The measurements were carried out in static air (22) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Y. Nat. Mater. 2003, 2, 159. (23) Israelachvili, J. N. Intermolecular and Surface forces, Academic Press: Amsterdam, 1992. (24) http://www.esrf.eu/computing/scientific/FIT2D/FIT2D_REF/ fit2d_r.html; accessed September, 2007.

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Figure 1. FEG-SEM images of the (a) surface and (b) cross-section of the F127-templated AlPO based thin film.

with a heating rate of 10 K/min. Infrared spectra were recorded on a Bruker 66 spectrometer, with MCT detector, on a homemade stainless steel in situ cell. The sample (thin film on both sides of a silicon wafer) was heated under air up to 573 at 2 K/min. Spectra were recorded during the thermal treatment. The spectrum of a silicon wafer without thin film was used as the background. 31P and 27Al MAS NMR spectra of as-deposited and at 673 K treated (calcined) samples scratched from films (50 two-faced films deposited on glass slides) were recorded on a 600 MHz Varian NMR system equipped with a Varian 3.2 mm MAS probe. Larmor frequencies for phosphorus and aluminum at 14.1 T are 242.89 and 156.35 MHz, respectively. Chemical shifts are reported relative to the signal of 31P in 85% H3PO4 and 27Al in 1 M solution of Al(NO3)3.

3. Results and Discussion 3.1. Structure Characterization. Continuous and smooth transparent thin films obtained by dip-coating under controlled humidity conditions (40–60% RH) were observed by FEG-SEM as shown in Figure 1a. In all studied AlPO-based samples, a film thickness of 0.5 µm was estimated by cross-section FEG-SEM images shown in Figure 1b. The [Al]/[P] molar ratio determined by EDX was 1 for as-deposited and calcined (i.e., template-free) thin film products, respectively, throughout the samples. Mesostructure symmetry was investigated by GISAXS. In situ GISAXS experiments point out that the mesostructures are formed upon solvent evaporation at 30–60% relative humidity, as observed for silica- and titania-based block copolymertemplated mesoporous thin films.25 Figure 2, panels a and c, shows the GISAXS patterns of the as-deposited, whereas Figure 2, panels b and c, shows thermally treated F127- and F108templated aluminophosphate thin films at 673 K. The patterns were assigned to a cubic symmetry mesostructure (body-centered cubic, space group Im3jm) with the (110) plane oriented perpendicular to the substrate, uniaxially distorted in the z axis.26,27 The cell parameter a, calculated according to each single reflection from their relative d spacings dhkl using the formula ahkl ) dhkl(h2 + k2 + l2)1/2, for as-deposited films is 17.5 nm for F127 and 18.9 nm for F108-templated samples. Strictly speaking, the mesophase symmetry in the as-deposited films is lower than a cubic symmetry due to uniaxial contraction; a values are better calculated from the in-plane spacings.28 However, for practical purposes we can consider the structure at room temperature to be a slightly z distorted Im3jm rather than a lower symmetry structure such as orthorhombic or tetragonal, and calculate lattice spacings rather than cell constants. The distortion is a contraction of the unit cell in the out-of-plane direction ([110] direction, normal to the (25) Crepaldi, E. L.; Soler-Illia, G.J.A.A.; Grosso, D.; Ribot, F.; Cagnol, F.; Sanchez, C J. Am. Chem. Soc. 2003, 125, 9770. (26) Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Grosso, D.; Durand, D.; Sanchez, C. Chem. Commun. 2002, 2298. (27) Innocenzi, P.; Falcaro, P.; Grosso, D; Babonneau, F J. Phys. Chem. B 2003, 107, 4711. (28) Urade, V. N.; Hillhouse, H. J. Phys. Chem. B 2005, 109, 10538.

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Figure 3. TEM images of calcined aluminophosphate thin film samples: F-127 templated along (a) [111] and (b) [110] pore directions; F108templated along (c) [111] and (d) [110] pore directions.

Figure 2. GISAXS patterns of the as-deposited mesoporous aluminophosphate thin films templated with Pluronic F127 (a) and F108 (c). Thermally treated F127-templated (b) and F108-templated (d); templated films at 673 K. The indexation is shown in panel a only.

substrate, hereafter d110z) and it is observed even in as-deposited samples and in samples treated at low temperature, becoming more marked upon thermal treatment at higher temperatures. The out-of-plane lattice spacings calculated for as-deposited F127and F108-templated films are d110z(F127) ) 8.9 nm and d110z(F108) ) 9.5 nm, respectively. Shrinkage takes place even after deposition and drying at ambient temperature. This effect has already been observed in titania mesoporous films, and is a consequence of having a loosely bound inorganic framework, presenting small hydrophilic inorganic clusters surrounding the micelles.25 The cubic mesophases obtained in this work are a consequence of using templates with larger hydrophilic heads, and smaller s ratios (s ) [template]/[Al]) in the precursor solutions. Pluronic F127 and F108 surfactants permitted to obtain 3D interconnected AlPO powders due to their larger hydrophilic head/hydrophobic tail volume ratio, compared to P123 or Brij 56.21 The use of P123 normally leads to a 2D-hexagonal mesostructure.22 In our case, when using Brij 56 with s ) 0.016, 2D hexagonal frameworks with smaller pore diameters result, due to the smaller template head/tail ratio, and dimensions compared to the Pluronics (see details in SI). The template/metal molar ratio (s) is also an important variable. Wang et al.14 reported the preparation of mesoporous aluminophosphates with F127 template using a chloride route with s ) 0.02. These conditions lead to hexagonal mesostructures, due to the high proportion of surfactant in the final template-inorganic mesophase. In this work we used lower s conditions (typically between 0.005 and 0.01), which direct toward higher curvature mesophases, and thus to threedimensional interconnected pore arrays. Similar trends in the

obtained mesostructures have been observed in titania-based mesoporous materials, and related systems, where the surfactant nature and concentration controls the mesostructure obtained and the mesoscale order, provided that extended condensation of metal centers is hindered.15,26 When higher s values (s > 0.01) were used for F127- and F108-templated systems, the formation of a disordered mesophase was observed in our synthesis conditions. This can be explained by taking into account the formation kinetics of these films. For such high template concentrations, a viscous initial gel is formed upon solvent evaporation. This slows down the disorder to order transition, which leads from an initial local order to highly ordered mesostructures.15 Thus, the system is kinetically quenched in a local order mesostructure. These results suggest that the formation mechanism of these AlPO phases is similar to the one observed for transition metal oxides such as titania:9,15 (a) small hydrophilic cluster-like nanobuilding blocks (NBB) are formed in the initial solutions, (b) these clusters coassemble with template micelles or liquid crystalline mesophases upon evaporation; the fact of the NBB being hydrophilic directs them to interact with the hydrophilic block of the template molecule; (c) an initial local order mesophase is formed, which evolves to a highly ordered mesophase provided that the initially formed template-NBB-solvent gel is mobile enough to permit rearrangements; (d) the mesostructure is “locked” upon thermal treatment. 3.2. Thermally Treated Films: Structure and Functional Evolution. As shown in Figure 2, panels b and d, thermal treatment leads to further shrinkage along the [110] direction, whereas the in-plane lattice spacings are not affected. In thermally treated (calcined) films, out-of- plane distances are d110z(F127) ) 6.1 nm and d110z(F108) ) 5.8 nm, which corresponds to a ≈30% contraction in the F127-templated film and a ≈40% contraction in the F108-templated film. The mesostructure is completely stable within this range. Diffraction spots become more intense for samples thermally treated at 673 K, indicating a higher contrast between the inorganic wall and the empty pores. This is due to two phenomena, which contribute to enhance the electronic density differences: (a) an extended condensation of the inorganic framework upon heating, which increases the density


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Figure 4. (a) TG and (b) DTG curves of the as-deposited aluminophosphate thin film samples.

of the walls, and (b) elimination of the template, which lowers the pore density. Theta-two theta XRD measurements of assynthesized and calcined aluminophosphate thin films yield d110z values, which are in agreement with the 2D SAXS experiments (see the Supporting Information). XRD patterns of as-deposited samples show the single (110) reflection peak at d ) 11 nm, corresponding to a slightly contracted cell, due to drying. The X-ray pattern of an F127-templated sample calcined at 673 K shows shifting of (110) reflection peak to d ) 10.3 nm. These values are slightly higher than those observed in SAXS but do not represent significant differences, probably being due to alignment failures in the θ-2θ experiment. Samples show higher order reflections for the sample thermally treated at 673 K in the range between 1.5 and 4.5 ° 2θ; the diffraction peak at 3.9 nm corresponds to the (211) plane. Analogous results are obtained for as-deposited and thermally treated F108-templated mesoporous aluminophosphate thin films: reflections at d ) 11.6 and 8.3 nm correspond to (110) and (200) reflection planes of Im3jm cubic mesopore structure, respectively. XRD patterns of the thermally treated samples (673 K), show similar shifting of the (110) reflection peak of 1.7 nm toward shorter distances, as well as another intense reflection peak at d ) 6.4 nm, corresponding to the (211) reflection plane (SI). XRD and SAXS 2D investigations showed that cubic mesoporous aluminonophosphate thin films remain thermally stable at least up to 673 K. TEM micrographs of F127- and F108-templated AlPO-based thin films thermally treated at 673 K reveal highly ordered thermally stable large-pore mesostructures, as shown in Figure 3a-d. In the projection along [110] the spherical pores seem to merge into cylindrical structures: this is a well-known effect due to the superposition of (110) planes (i.e., plane view of the film), where the pores are most densely packed.25 The estimated distance between the channels along the (110) direction is 9 and 10 nm for F127 and F108, respectively, which is in a good agreement with the SAXS measurements. The estimated pore sizes are 11 and 10 nm for F127- and F108-templated aluminophosphate thin films, respectively. Thermogravimetric analysis of mesoporous aluminophosphates (TG and DTG curves are shown in Figure 4, panels a and b, respectively) indicates a total weight loss of about 50 wt% up to 650 K for F127- and F108- templated samples. Both curves are very similar, irrespectively of the template employed, and reveal two major weight losses. The first weight loss, in the range between 20 and 30 wt% up to 400 K is assigned to ethanol and water desorption. The second weight loss up to 650 K, spanning about 30 wt%, corresponds to the decomposition of surfactant and desorption of its fragments. These values are in

Figure 5. Room temperature infrared spectrum of the F127-templated sample after calcinations at 575 K, in the spectral range of structure vibrations (1400–600 cm-1). Inset: Spectra of the same sample in the spectral range of water (νOH) and template vibrations (νCH), recorded in situ during thermal treatment.

good agreement with block-copolymer templated titania or silica thin films reported in the literature.29,30 No further weight losses are observed at higher temperatures. Results of thermal analysis indicate that AlPO-based thin films thermally treated at 673 K, where high order of mesostructure was confirmed by SAXS and TEM, are template-free. The detailed thermal behavior of the mesostructured AlPObased thin film samples was studied also by in situ infrared spectroscopy. Results for F-127 templated film are shown in Figure 5. Removal of water (broad OH bands at 3470 cm-1; sorbed water vibrations at 1630 cm-1) takes place between 300 and 400 K, confirming the TG results. Decomposition of the organic template can be monitored by the decrease of the νCH vibration bands of the template methyl and methylene groups at 2880 and 2930 cm-1. Organics removal mostly takes place between 475 and 575 K in these conditions; taking into account the thermal treatment conditions (different temperature ramps in both cases), the temperature ranges found for organic decomposition are in good agreement with TG data. The structure vibrations of the AlPO framework are located around 1110 cm-1. Before calcination, a very broad set of bands is observed in this region, due to superposition of the phosphate vibrations with the C-C-O stretching bands of the organic template. After thermal treatment, (29) Alberius, P. C. A.; Frindell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 2002, 14, 3284.

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Figure 6. 31P and 27Al MAS NMR spectra of F127-templated AlPO materials (A) initial sol and powders treated to different temperatures; (B) as-deposited (RT) and thermally treated (573K) aluminophosphate thin film.

the framework structural bands emerge, and the underlying fine structure is clearer. A main framework band is located at 1100 cm-1, and two shoulders at 1133 and 1245 cm-1, indicating phosphates or aluminophosphates. At higher temperature, the most intense band shifts to 1185 cm-1. These observations are in agreement with previous data on powdered AlPO samples. The microporous material has been reported at 1140 cm-1,31 whereas mesoporous AlPO was reported at 1100 cm-1,32 at a frequency close to the ones observed in our samples. It is interesting to observe that an important change in the framework connection occurs between 323 and 423 K, while the template is still present; this is in agreement with MAS NMR measurements (see below). The separation of the framework rearrangementconsolidation and the template elimination might be responsible of the thermal stability of the mesoporous phases. 3.3. NMR and MAS NMR Analysis of Precursor Solutions and Obtained Mesophases. NMR and MAS NMR spectra were performed in order to gain insight into the processes taking place in the framework, in the close environment of the Al and P centers.33 Figure 6 shows 31P and 27Al NMR spectra of the (30) Hwang, Y. K.; Patil, K. R.; Jhung, S. H.; Chang, J.-S.; Ko, Y. J.; Park, S.-E. Microporous Mesoporous Mater. 2005, 78, 245. (31) Jacobs, W. P. J. H.; de Man, A. J. M.; van Wolput, J. H. M. C.; van Santen, R. A. Proceedings of 9th International Zeolite Conference; Montreal, 1992; Vol. 2, 529. (32) Tiemann, M.; Schulz, M.; Jager, C.; Fröba, M. Chem. Mater. 2001, 13, 2885. (33) Epping, J. D.; Chmelka, B. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 81.

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precursor solution and MAS NMR spectra of the as-deposited and at 673 K thermally treated mesoporous xerogels and thin films prepared using F127 as a template. Spectra look identical for both templates employed. Liquid NMR spectra of the initial precursor solution indicate that the AlCl3/H3PO4 acid–base pair efficiently forms Al-O-P bonds (Figure 6a). In 31P spectrum, a strong signal at -12.6 and a weak signal at -24.5 ppm suggest that most P atoms are bonded to 2 Al atoms, whereas only a small amount of P atoms are bonded to 4 Al atoms.14 The 27Al spectrum exhibits two broad and intense signals at 0 and -7 ppm that can both be assigned to octahedral Al complexes with either water, alkoxy groups or Cl atoms, and probably less than four -O-P links. MAS NMR spectra of the as-deposited mesoporous AlPObased thin films show relatively broad signals in 31P spectra in the region between -3 and -28 ppm with the peak value of –15 ppm in both cases. The P shifts are in agreement with those reported previously.22 Broad signals reflect a rigid and not very well ordered aluminophosphate structure which is not yet entirely condensed. 27Al spectra again show that all aluminum in asdeposited films is octahedrally coordinated and bonded to three or four phosphorus atoms. After thermal treatment of films at 573 K, condensation of aluminophosphate structure is increased, which is clearly reflected in the 27Al MAS NMR spectra. The new peaks at approximately 45 and 17 ppm belong to four- and five-coordinated aluminum, respectively. Both types of aluminum are most probably bonded to four phosphorus atoms. The contributions of six-coordinated Al in the NMR spectra of thermally treated films still represent however about 55% of the total Al signal. Such a large fraction of octahedrally coordinated aluminum species is not usual for powdered microporous and mesoporous aluminophosphate materials, where conversion from octahedrally coordinated aluminum to tetrahedral one is expected in much higher extend.34,35 Indeed, Figure 6 shows a clear difference between the thermal evolution of Al and P environment for films or gels, the latter presenting a more complete conversion to a condensed framework at equivalent temperatures. Such behavior of aluminum might be due to features unique to thin films: (a) 6-coordinated pore surface sites might represent a much larger fraction of the material in thin films than in powders; (b) a substrate effect could retard the formation of an aluminophosphate framework with tetrahedral Al coordination; (c) surfactant elimination proceeds faster in thin films due to shorter diffusion distances; in this case, the large and sudden pore shrinkage prevents a rearrangement of the AlPO framework in a more stable configuration. Hypothesis (a) cannot explain the large differences observed, as it has been proven through the literature that surface area is essentially the same in mesoporous films or powders. The higher shrinkage of the mesoporous films due to uniaxial contraction (hypothesis (c)) could lead to a reversed trend, where Al(III) centers assume a more compact tetrahedral coordination after the depart of water upon thermal evolution. On the contrary, it has been well documented that the evolution of P and Al local environment of AlPO glasses is very sensitive to processing as bulk pieces or thin films. Moreover, the role of a glass substrate in hindering structural evolution has been well documented for example in the crystallization process of titania dense36 and mesoporous thin films, migration of alkaline cations being one of the most common causes of the modification of this (34) Zabukovec Logar, N.; Novak Tusˇar, N.; Mali, G.; Mazaj, M.; Arcˇon, I.; Arcˇon, D.; Recˇnik, A.; Ristic´, A.; Kaucˇicˇ, V. Microporous Mesoporous Mater. 2006, 96, 386. (35) Novak Tusˇar, N.; Zabukovec Logar, N.; Arcˇon, I.; Mali, G.; Mazaj, M.; Ristic´, A.; Lázár, K.; Kaucˇicˇ, V. Microporous Mesoporous Mater. 2005, 87, 52. (36) Paz, Y.; Heller, A. J. Mater. Res. 1997, 12, 2759.


behavior.37–39 Experiments are in due course to understand these findings. 31P MAS spectra of thermally treated films at first sight seem broader, but this can be due to appearance of new signals, which are manifested as shoulders at approximately -22 ppm. These new contributions can be assigned to phosphorus atoms bonded to four- and five-coordinated aluminum species, which arise when water coordinated to Al centers is eliminated upon thermal treatment.40 Chemical shifts of such phosphorus atoms are, namely, expected to be more negative than chemical shifts of phosphorus atoms that are bonded to six-coordinated aluminum. Overall, the NMR experiments suggest that as-deposited thin films present aluminophosphate species similar to those present in the initial sols (based on hexacoordinated Al(III)). In the synthesis conditions, we propose that fresh films are composed of small size AlPO clusters, which are well disposed around the template hydrophilic regions. Clusters are hydrophilic due to the presence of hexacoordinated Al centers with coordination positions available for water: phosphates bound to one or two Al centers, still bearing hydroxyl groups enhance the hydrophilic character. These conditions (low condensation, small size, and hydrophilic character) are ideal to form a well-defined mesostructured material with long-range ordering. On the other hand, it is interesting to observe that both in films and in powders, an important improvement in the framework connection and Al-O-P condensation (likely, a framework consolidation process) occurs upon thermal treatment. The ability to keep Al(III) relatively uncondensed in an octahedral environment in freshly formed films helps to efficiently coassemble the template and inorganic nanobuilding blocks in a highly organized mesophase, in agreement with the general trend observed.9,15 Even in the absence of excess water ([H2O]:[Al] ) 1 in the initial solutions, moderate humidity), small condensed aluminophosphate units seem to be a flexible and hydrophilic enough wall building block that can lead to highly ordered mesostructures.7,31 The kind of mesostructure can thus be controlled by the surfactant nature or the s ratio. Subsequent thermal treatment smoothly improves the (37) Zhang, Y.; Lin, J.; Wang, J. Chem. Mater. 2006, 18, 2917. (38) Štangar, U. L.; Cˇernigoj, U.; Trebše, P.; Maver, K.; Gross, S. Monatsh. Chem. 2006, 137, 647. (39) Angelomé, P. C.; Andrini, L.; Calvo, M. E.; Requejo, F. G.; Bilmes, S. A.; Soler-Illia, G. J. A. A. J. Phys. Chem. C 2007, 111, 10886. (40) van Wüllen, L.; Wegner, S.; Tricot, G. J. Phys. Chem. B 2007, 111, 7529. (41) Soler-Illia, G. J. A. A.; Scolan, E.; Louis, A.; Albouy, P. A.; Sanchez, C. New J. Chem. 2001, 25, 156.

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connectivity between these nanobuilding units, resulting in a robust material.

4. Conclusions Large-pore mesoporous aluminophosphates with cubic Im3jm symmetry were successfully deposited on glass substrates by dip-coating. The symmetry of pore arrangements in thin films was determined by SAXS and TEM. The chemical composition corresponds to an aluminophosphate-based framework, as confirmed by EDX and FTIR. For the first time, it has been shown that these cubic thin films are stable to thermal treatment, and that there is an evolution in the inorganic framework toward condensed glass-like walls, according to 27Al and 31P NMR spectra. This work demonstrates that the mesostructure of stable AlPO materials can be designed by adequately choosing a template with the right hydrophilic/hydrophobic relative volume and an adequate template:metal ratio, provided that extended condensation is hindered, in a similar way to what can be obtained in mesoporous transition metal oxides. A better accessibility of the 3D pore openings, compared to the similar 2D hexagonal mesostructures, can improve diffusivity which is an important factor in heterogeneous catalysis and chemical separations. The large pore dimensions attained with these templates enable hosting for metal oxide or pure metal nanoclusters. The ability of the aluminophosphate framework to incorporate various transition metals and the thermal stability of this 3D mesostructure can lead to a variety of possible applications (optical, electrochemical, sensing, etc.). Acknowledgment. Authors are indebted to ANPCyT PICT 34518 and to the following joint projects: Argentina-Slovenia PA05E03 “Mixed Mesoporous Oxides” (SECyT-MHEST) and MAE project “Nano and Meso-structured Materials for Functional Applications”. Paolo M. P. Falcaro is acknowledged for the use of the in situ dip-coater. Nina Daneu from Institute Jozeˇf Stefan, Slovenia is acknowledged for the help with TEM measurements. Supporting Information Available: θ-2θ patterns of cubic and hexagonal mesophases and SAXS of 2D hexagonal Brij 56 mesostructured AlPO. This material is available free of charge via the Internet at http://pubs.acs.org. LA7035746