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2008, 112, 3157-3163 Published on Web 02/13/2008
Sorption Properties of Mesoporous Multilayer Thin Films Marı´a Cecilia Fuertes,† Silvia Colodrero,‡ Gabriel Lozano,‡ Agustı´n R. Gonza´ lez-Elipe,‡ David Grosso,§ Ce´ dric Boissie` re,§ Cle´ ment Sa´ nchez,§ Galo J. de A. A. Soler-Illia,*,†,| and Herna´ n Mı´guez*,‡ Gerencia de Quı´mica, Comisio´ n Nacional de Energı´a Ato´ mica, AVenida Gral Paz 1499, B1650KNA, San Martı´n, Buenos Aires, Argentina, Instituto de Ciencia de Materiales de SeVilla, Consejo Superior de InVestigaciones Cientı´ficas (CSIC), Ame´ rico Vespucio 49, 41092 SeVilla, Spain, Laboratoire de Chimie de la Matie` re Condense´ e, UniVersite´ de Paris VI, 4, Place Jussieu, 75252 Paris Cedex 05, and CONICET, AVenida RiVadaVia 1917, C1033AAV, Buenos Aires, Argentina ReceiVed: NoVember 6, 2007; In Final Form: January 15, 2008
The vapor sorption properties of multilayers made of ordered mesoporous thin films with tailored composition and mesostructure are herein investigated. Optical reflectance measurements versus partial pressure of several vapors are performed to analyze the interplay between the affinity to and the accessibility of the different types of layers in the structure. We find that the behavior of a mesoporous oxide layer within the multilayer largely differs from that of the isolated thin film, its sorption properties being determined by the interaction with neighboring films. An explanation of the phenomena observed in these complex systems is provided in terms of the pore size, the affinity of each type of layer to specific compounds, and the effect of neighboring layers in the sorption properties of bilayers by an independent environmental ellipsometric study.
Introduction Mesoporous thin films (MTF) constitute a very active research area in advanced materials, with potential applications in sensors, selective membranes, and ultralow-k devices.1-6 At present, it is possible to produce a great variety of oxide, carbon, or metal MTF using supramolecular or polymeric templates. Recently, these MTF with selected organic functions attached to the pores surface have been stacked up in film bilayers or multilayers, leading to complex multifunctional multilayer, multiscale systems with accessible and selectively functionalized pore surfaces.7 Moreover, periodic multilayers made up of different MTF have been proven to behave as one-dimensional photonic crystals whose optical response depends on the type and quantity of molecules present in the environment (in solution or gas phase).8,9 For certain applications such as environmentalresponsive coatings or optical switches, it is crucial to understand the sorption properties of molecules present in the gas phase into these complex porous systems. Several studies have addressed the sorption properties of single-layer MTF, either by environmental ellipsometric porosimetry (EEP),10 X-ray reflectivity (XRR),11,12 or small-angle X-ray spectroscopy (SAXS) coupled to N2 adsorption.13 However, no analysis of more complex structures has been reported to date. In this work, we make use of the variation of the onedimensional photonic crystal (or Bragg reflector) properties of multilayer structures made up of MTF components under * To whom correspondence should be addressed. E-mail: (H.M.)
[email protected]; (G.J.A.A.S.I.)
[email protected]. † Comisio ´ n Nacional de Energı´a Ato´mica. ‡ Consejo Superior de Investigaciones Cientı´ficas (CSIC). § Universite ´ de Paris VI. | CONICET.
10.1021/jp710612y CCC: $40.75
varying partial vapor pressure conditions for different molecules to understand the sorption-condensation properties of these systems. The refractive indexes of the different films that are present in the multilayer in contact with such vapors are obtained by fitting its optical reflectance spectra. The pore volume fraction filled can be extracted from the index and thickness values in all the pressure range and independently for each kind of mesoporous film present in the multilayer. Our results indicate that a strong interaction between neighboring layers occurs. The rationalization of these results is done in terms of the affinity of each type of layer to a specific compound and by comparison with the sorption properties of mono- and bilayer structures analyzed by environmental ellipsometric porosimetry analysis using variable angle spectrometric ellipsometry (VASE-EEP). A description of the processes involved, coherent with all independent observations, is provided in terms of abrupt variations of the local pressure induced by the selective vapor condensation in the pore systems of a specific type of layer. This selective condensation is a consequence of pore size and the pore surface nature but also depends on the relative position of the different layers. Experimental Section Precursor solutions were prepared as previously described, using tetraethoxysilane (TEOS) and TiCl4 as inorganic precursors, and surfactant porogens: cetyltrimethylammonium bromide (CTAB) and (EO)106(PO)70(EO)106, (Pluronics F127); EO and PO are ethylene oxide and propylene oxide monomers.7 A solution with TEOS/H2O/EtOH/HCl/CTAB of 1:5:20:0.004:0.1 was used to produce CTAB-templated silica (SC) films with three-dimensional (3D) hexagonal (P63/mmc) mesostructure. © 2008 American Chemical Society
3158 J. Phys. Chem. C, Vol. 112, No. 9, 2008 Large pore cubic titania (TF) were produced from a solution with TiCl4/H2O/EtOH/F127 proportions of 1:10:40:0.005, according to ref 14. MTF multilayers were deposited by a sequence of dip-coating and stabilization processes. Dip-coating was performed under controlled withdrawal speed and relative humidity. Film thickness can be tuned between 30 and 200 nm by changing the withdrawal speed and solution composition. Freshly deposited films were submitted to 50% relative humidity (RH) chambers for 24 h and a stabilizing thermal treatment consisting in two successive 24 h steps at 60 and 130 °C and a final 2 h step at 200 °C. After this process, another MTF can be deposited on top of a stabilized film; crack-free multilayers of up to 20 films can be produced in this way. Template is eliminated by heating up a multilayer system to 350-400 °C under still air atmosphere in tubular ovens. Bilayered and eightlayered structures using SC and TF as building blocks were studied; Multilayers are named as following: a substrate-SCTF bilayer is named SC-TF. For periodic multilayers, 4×(SCTF) indicates four-period multilayers composed of a sequence of SC-TF pairs, respectively. Pore size and network geometry of each MTF can be designed by an adequate choice of the inorganic precursor, solvent, and organic template employed. Film mesostructure was characterized by 2D SAXS at the D11A-SAXS2 line at the Laborato´rio Nacional de Luz Sı´ncrotron, Campinas, SP, Brazil, using λ ) 1.608Å, a sampledetector distance of 650 mm, and a charge-coupled device detector (3 or 90° incidence). Field emission scanning electron microscopy (FESEM) images were taken with an electron microscope ZEISS LEO 982 GEMINI; microscopy images were obtained in the secondary-electron mode, using an in-lens detector to improve resolution (CMA, FCEyN-UBA). Transmission electron microscopy (TEM) images were collected using a Philips CM 200 high-resolution transmission electron microscope equipped with an ultratwin objective lens and an acceleration voltage of 200 kV (CAB, CNEA). TEM specimens prepared in cross section were obtained by sequential thinning processes that include mechanical dimpling and ion-milling to obtain larger transparent areas in the sample. The sorption properties of the mesoporous multilayer against three different solvents (water, isopropyl alcohol, and toluene) were experimentally analyzed through the variation of the optical reflectance response that took place as the corresponding vapor pressure in the environment changed. The multilayers were introduced in a closed chamber in which the partial pressure of a volatile liquid could be varied from P/Ps ) 0 to 1 (Ps being the saturation vapor pressure of the liquid at room temperature). The chamber possesses a flat quartz window through which the reflectance spectra of multilayer structures at normal incidence were measured in situ using a Bruker IFS-66 FTIR spectrophotometer (ICMSE, CSIC) attached to a microscope with a 4× objective with 0.1 numerical aperture (light cone angle ( 5.7°). The specular reflectance spectra at a P/Ps range of 0-1 measured for water, isopropyl alcohol, and toluene vapors were analyzed using a scalar wave approximation that allowed us to attain precise curves of the variation of the refractive index of the different mesoporous layers in the multilayer as the amount of adsorbed liquid increased. This type of analysis was only possible due to the extreme sensitivity of the photonic crystal properties of the multilayer to the refractive index of the layers, which allowed us to unambiguously correlate the observed changes observed in the specular reflectance spectra with the variation of refractive index in a particular type of layer in the structure. It should be mentioned that this approach to porosimetry of complex periodic architectures, which is herein
Letters presented for the first time, cannot be easily made through any alternative micropore or surface analysis technique. In addition to the above-mentioned technique, and to independently analyze the basic building units of the multilayers, water adsorption curves (at 298 K) for monolayer and bilayer MTF were measured by EEP, according to the protocols developed in ref 10a. A continuous flux of air containing a fixed partial water pressure was directly in contact with the mesoporous film to analyze. Equilibration times were in the order of seconds to minutes. Only water was used in EEP measurements due to restrictions imposed by the nature of the components of the cell, because they could be affected by other solvents. Film thickness and refractive index values for single layer thin films were obtained from the ellipsometric parameters Ψ and ∆ at each P/Ps value. Film porosity was evaluated by adjusting a three-medium (air, water, oxide) Bruggeman effective medium approximation (BEMA). Silica and titania refractive index values were obtained from nonporous thin films treated in the same way as the mesoporous samples. Pore sizes calculated from the adsorption branches of the EEP curves yield 2.6 and 10 nm for SC and TF films, respectively. In the case of bilayer films, refractive indexes were separately obtained for each layer, adjusting point by point along P/Ps, and considering constant thickness for each film in a first approximation. Thickness values of the bilayer components were obtained by evaluating ellipsometry measurements at P/Ps ) 0 and 1 and crossing with FESEM measurements. Results and Discussion Periodic ordered mesoporous multilayers were prepared by a sequence of deposition-stabilization steps, as indicated in refs 7 and 9. MTF with different composition and porosity were deposited by dip-coating a glass or silicon substrate with a precursor solution; each MTF layer behaves as a building block of a more complex 1D-photonic structure with periodicity in the submicron scale. Pore size and network geometry of each MTF can be designed by an adequate choice of the inorganic precursor, solvent, and organic template employed. In this work, we studied two and eight-layered structures using SC and TF films as the building blocks: SC-TF, TF-SC, and 4×(SC-TF). The water sorption properties of substrate-SC-TF and substrateTF-SC bilayers, as well as single layers deposited onto silicon were studied by EEP, which makes use of VASE. Figure 1 shows high-resolution FESEM and TEM images of a 4×(SC-TF) multilayer system. The low-magnification FESEM image of a cross section of a mesoporous thin-film multilayer shown in Figure 1a demonstrates the uniformity of the thickness of both types of layers over long distances, which ensures the good optical quality of the ensemble. Figure 1b shows a cross-section TEM image of the same system, obtained after sample thinning by ion-milling. The corresponding SAXS with 2D detection (2D-SAXS) data confirming the mesoscale order are shown in Figure 1c,d: the films present highly ordered mesopores (p63/mmc for SC and distorted Im3m for TF) and orientation along the z-axis (i.e., perpendicular to the surface); the pore domains are polyoriented in the xy plane, as previously observed.15 The alternated multilayered structures present a periodic variation of the refractive index that gives rise to strong colored reflections due to the interference of light beams coherently scattered at each interface. To analyze the variation of the optical response with the vapor pressure in the environment, specular reflectance spectra were measured for water, isopropyl alcohol, and toluene vapors, at a P/Ps range of 0-1. As an example, a
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Figure 1. Structural characterization of mesoporous Bragg multilayers. (a) FESEM image of an eight-layered Bragg reflector, 4×(SC-TF): dark gray, TiO2; light gray, SiO2; substrate, silicon. (b) TEM image of the same system, showing two mesophases with different pore size. SAXS patterns obtained at (c) 3 and (d) 90° of 4×(SC-TF) multilayer calcined at 350 °C. Arrows indicate the spots corresponding to the silica mesostructure.
whole series of reflectance spectra attained at different partial pressure of toluene vapor are displayed in Figure 2a (spectra obtained for water and isopropyl alcohol vapors are displayed in the Supporting Information). The maximum corresponds to the Bragg reflection of the structure, while the secondary maxima are Fabry-Perot resonances due to the finite size of the multilayer. The intensity and width of such Bragg reflections depend on the contrast between the refractive index of the layers (i.e., ∆n), while its position depends on both the lattice parameter and, in a first approximation, the average refractive index of the composite (i.e., ). These are the relevant parameters to analyze the variations of the refractive index of the film as liquid condenses onto the pore surface (sorption) or pore volume (capillary condensation) of the mesoporous network. In particular, changes in the reflected intensity provide a mean to identify the preferential adsorption in one specific type of layer: A decrease of the intensity must be due to preferential solvent condensation within the pore system of the low refractive index film (CTAB-templated SiO2 in our case) because that reduces the dielectric contrast between both types of films. On the contrary, preferential adsorption onto the high refractive index inorganic TiO2 framework would increase the dielectric contrast, hence raising the intensity of the Bragg reflection. The evolution of both the maximum spectral position and its intensity are plotted for all three solvents used in the sorption experiments in Figure 2, panels b and c, respectively. Figure 2b shows that in all cases vapors condense into the mesopore walls, increasing the average refractive index and gradually shifting the Bragg maximum toward higher wavelengths.16 Figure 2c reveals that in all the explored systems sorptioncondensation takes place preferentially onto the smaller silica pore walls at lower pressures, according to the Kelvin-Laplace equation.10a Sorption-condensation on silica small pores leads to a decrease in the dielectric contrast of the photonic crystals and hence its reflectance. At higher pressures, the same process
takes place onto the larger titania pore walls with the opposite effect on the optical response. In addition, the adsorption properties of the multilayer strongly depend on the type of compound vaporized in the chamber. Interestingly, water seems to adsorb indistinctly and at the same time onto SiO2 and TiO2 walls, while isopropyl alcohol and toluene vapors adsorb preferentially onto SiO2 during the first stages of the process and onto TiO2 at higher partial pressures. Quantitative information on the variation of the refractive index versus vapor pressure can also be extracted from the analysis of the optical reflectance spectra. All experimental spectra were fitted using a model based on the scalar wave approximation,17 in which the thickness of each mesoporous film in the multilayer is permitted to vary up to 10% around the value obtained from the FESEM images. The refractive indexes of both types of films are left as adjustable variables. The model assumes that all films in the multilayer with equal composition or porosity will present the same refraction index in equilibrium conditions with a given vapor pressure. We assume that small molecules such as those analyzed in this work can evenly access all the layers, even if some pore plugging might occur at the interfaces. Previous work has demonstrated that even bulky organic molecules with strong interactions with the surface can access a great fraction of a multilayer system.9 The optimum fit is chosen by comparison with the experiment using the method of squared minima. Figure 3a shows two examples of the fits attained through this method for in vacuo (10-2 Torr.) and high toluene vapor partial pressure (P/Ps ) 0.93) conditions. These fits allow us to estimate the variation of the refractive index with pressure for each layer independently and from this the evolution of the free volume fraction of the pore network through a Bruggeman expression for a three (oxide-liquid-air) dielectric component medium (intermediate results of modeled n vs P/Ps for all solvents and oxides studied in this work are displayed in the Supporting Information).18
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Figure 2. (a) Specular reflectance spectra at different partial pressures of toluene vapors. The arrow indicates the increase in the toluene vapor partial pressure P/Ps from 0 to 1. Evolution of both the maximum spectral position (b) and its intensity (c) for all three solvents used in the sorption experiments.
Figure 3b shows the calculated adsorbed volume fraction versus toluene in all the pressure range for each kind of the mesoporous films forming the multilayer; analogous curves for isopropyl alcohol and water are included as Supporting Information. A comparative analysis between the total volume adsorbed of each of the three compounds employed in both types of films is shown in Figure 3c. The total adsorbed volume on SiO2 walls decreases as we move from toluene to isopropyl alcohol and to water, whereas the total volume adsorbed on TiO2 is very similar in the case of isopropyl alcohol and toluene, which are in turn much larger than in the case of water. This comparison indicates that the multilayer as a whole presents a rather hydrophobic character, which is in good agreement with recent analysis of porous SiO2-TiO2 multilayers obtained by a different approach.19 Apart from these general considerations, it is clear from the adsorbed volume versus P/Ps curves that the multilayer presents a complex behavior. In all cases, the largest filling of
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Figure 3. (a) Spectra fitting for the reflectance of a multilayer in vacuum and the same multilayer after being exposed to a toluene vapor partial pressure of P/Ps ) 0.93 (red-shifted). Experimental: solid line. Fitting: dashed line. (b) Variation of the adsorbed toluene volume fraction versus toluene vapor partial pressure for the SiO2 and TiO2 mesoporous films forming the multilayer. (c) Comparative analysis between the total volume adsorbed of each of the three compounds employed in both types of films.
the CTAB-templated SiO2 mesopores occurs for P/Ps 0.5 in the cases of water and isopropyl alcohol. This reflects the variability in solvent sorption with the solvent and the film nature (i.e., pore size, pore connectivity, surface hydrophilicity/phobicity), which was evidenced in the qualitative analysis. Moreover, in the case of isopropyl alcohol, the results
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Figure 4. Adsorption-desorption isotherms obtained by VASE-EEP of single and bilayer mesoporous oxide thin films: (a) single layer SC; (b) single layer TF; (c) SC layer in a TF-SC bilayer; (d) TF layer in a TF-SC bilayer; (e) SC layer in an SC-TF bilayer; (f) TF layer in an SC-TF bilayer. Closed circles: adsorption isotherms. Open circles: desorption isotherms.
presented here indicate that an extra degree of complexity arises. It is well known that small alcohols can react upon sorption on silica, forming silicate esters, leading to a more hydrophobic surface.20 Therefore, chemisorption of isopropyl alcohol results in a variation of the surface hydrophobic character with P/Ps. Experiments are in due course to understand these features. The observed phenomena also suggest that the molecule sorption-condensation behavior in one kind of mesoporous layer could be influenced by the porosity characteristics of an adjacent porous layer. In short, in addition to the usual curvature and surface tension effects present in classical theories, there seems
to be a synergic behavior in the vapor sorption-condensation features in multilayer mesoporous thin films. To rationalize the results observed above, the recently developed VASE-EEP technique10,21 was performed on single and bilayer films. This in situ vapor sorption measurement permits an independent thickness and porosity analysis of each layer in a bilayer. This is central to understand the sorption properties of a complex multilayer based in a comparison with the single components. Typical EEP results on silicon-supported SC and TF monolayers and SC-TF and TF-SC bilayers are displayed in Figure 4. A clear increase of the adsorbed volume
3162 J. Phys. Chem. C, Vol. 112, No. 9, 2008 occurs for single SC films at P/Ps ≈ 0.4 (Figure 4a), whereas for single TF films, this abrupt rise of the adsorbed volume takes place above P/Ps ≈ 0.8 (Figure 4b). This reflects (a) the sharpness of the pore distribution (steep condensation step) and (b) the significant difference in pore size due to the template selection (2.6 and 10 nm, respectively, obtained from the adsorption curve).10 The pronounced hysteresis loop in the TF film can be attributed to the typical pore entrance sizes of a cubic mesostructure.22 Bilayers present a very different behavior that depends on the sequence in which the mesoporous films are deposited. For TF-SC bilayers (i.e., TF deposited on the substrate, SC on top), condensation is attained at P/Ps ≈ 0.35 in the SC layer (Figure 4c); water sorption on the TF layer is dramatically shifted to lower vapor pressures, around P/Ps ≈ 0.5 (Figure 4d). In addition, although the shape of the adsorption curve for the SC layer does not significantly differ from the single SC system, the adsorption curves on titania are significantly altered. Apart from the pronounced adsorption-condensation shift and the larger adsorption pressure range, desorption takes place at pressures comparable to those needed to extract all water from the silica film. In this case, water desorption from a TF-SC bilayer seems to be limited by the smaller silica pore size: the SC top film acts like a bottleneck of the whole pore system. When the order of the bilayer is reversed (SC-TF), the sorption curves for both SC and TF films are altered but not as superimposed as in the TF-SC bilayer (Figure 4d,e). Condensation in the SC layer takes place at P/Ps ≈ 0.4 as in the single silica film; condensation in the FT layer takes place at P/Ps ≈ 0.6, which is lower than the P/Ps at which condensation takes place in the single TF film P/Ps ≈ 0.8. In this case, both layers present sorption behavior closer to that of individually deposited layers. However, the presence of a water-filled bottom silica layer modifies the pore-filling dynamics of the larger pore titania layer deposited on top. Similar features have been observed in combined SC and TB (i.e., Brij 58-templated titania) mesoporous bilayers, where both components present 2.6 and 3.9 nm pore diameter, respectively (see Supporting Information). The observed behavior reveals that the response to the chamber pressure of the larger pore layer is mediated and to some extent determined by the smaller pore layer. If the film that is on top undergoes condensation first, as it is the case for the TF-SC bilayer, the titania film equilibrates with an effective local vapor pressure, which is higher than the one in the chamber due to the fact that the topmost pores are already filled with solvent. Thus the volume adsorbed in the TF layer rises steadily at relatively lower vapor pressures. On the other hand, when condensation first occurs in the lower film, as it is the case for the SC-TF bilayer, the upper film will experience a gradient of vapor pressure, which decreases in the direction perpendicular to the substrate. In this case, condensation in the TF film occurs at higher pressures than for the substrate-TF-SC case but still lower than for the single TF film because the local pressure is not uniform and higher in average. Capillary effects, which imply liquid solvent transport through the multilayers, can also influence the formation of the first water surface layer and therefore the wettability and condensation behavior. In this framework, the porosity at the interface between both films should play an important role in the solvent transport dynamics.23 These factors should be also taken into account for a complete explanation. Efforts are being devoted to shed light on this problem and will be addressed in future work. The conclusions of this analysis can be extrapolated to partially explain the sorption properties of the multilayer and
Letters SCHEME 1: Water Sorption and Release in TF and SC Bilayersa
a (A) SC-TF system where the synergic effect of bilayers in water sorption is less important and pore systems can be considered independent. (B) TF-SC system where the equilibrium of the smaller pore silica layer with the ambient controls the bilayer water uptake and desorption (SC behaves as a bottleneck).
are summarized in Scheme 1. Condensation in the F127templated TiO2 films forming the multilayer at low pressures, observed in all three cases under study, can be understood now as the consequence of variations of the local pressure due to condensation within the SiO2 mesopores. The smaller SiO2 pores determine thus the response of the whole multilayer in a first approximation. However, these local pressure effects do not provide an explanation for the different qualitative response versus the different solvents. The comparative analysis presented in Figure 3c when contrasted to the adsorbed volume curves of Figures 3 and 4 suggests that the larger the difference in affinity of a film to a specific compound, the more independent the response of the layers is. This affinity is determined by the combination of the ease of diffusion and wetting of a solvent within a specific type of layer. In the case of toluene, to which both layers seems to present a similar affinity after the results of Figure 3c, adsorption in the TF film is practically controlled by the condensation in the SC one. In the case of water, the affinity is less similar and therefore water adsorption in TF films increases for P/Ps > 0.5. Finally, in the case of isopropyl alcohol, to which the affinity of both layers differs the most, an abrupt solvent uptake is observed for P/Ps > 0.5, indicating that the adsorption within TiO2 mesopores is only partially determined by prior condensation in the SiO2 film. These features can be advantageously used to create arrays of
Letters nanorreactors ordered in space, which can be selectively filled under an external solicitation, such as vapor pressure increase of a given solvent. Conclusions This work presents the first description and experimental study of the solvent sorption properties of ordered mesoporous multilayers. From the in situ analysis of their optical response, we obtain the changes in solvent uptake that take place in each type of layer in the ensemble as ambient pressure is varied. We find that the optical response of a multilayer system depends on the pore size, wall nature, and surface features of the multilayer components in a first approximation. However, for the ultimate tailoring of the optical response, the collective as well as the individual behavior of each monolayer must be taken into account. We find that the solvent sorption behavior of the mesoporous layer components cannot be described with the classical sorption-chemisorption approach of a single, isolated mesoporous film (i.e., only taking into account pore size and shape, wall refraction index, and surface features). Indeed, the sorption properties of complex mesoporous multilayers are also determined by the interaction between neighboring layers. The relative location and probably the pore structure of the interface between adjacent layers lead to complex solvent confinement and transport effects, which reflect in the vapor sorption and thus the collective optical behavior observed. The analysis of the comparative response to different solvents, combined with the results of the study of the sorption properties of monolayers and bilayers by an independent environmental ellipsometric study, provides a first rationalization of the phenomena observed. Work in progress is devoted to a thorough characterization of the structure of the interface between layers, in particular to the interconnectivity between different pore size systems, to better understand possible pore plugging effects. Our work demonstrates that preferential condensation of a particular compound within a specific type of layer occurs for well-defined pressure ranges and can be indeed tailored by adequately choosing the pore size, surface features, and the spatial location of neighboring layers. This opens the possibility of performing selective spatial adsorption, chemical reactivity, and directed mass transport in these complex multiscale architectures. Acknowledgment. This work has been realized under the framework of collaborative projects between CSIC and CONICET (ref. 2005AR0070), and between CNRS and CONICET (PICS 3175). Research has been funded by the Spanish Ministry of Science and Education under Grant MAT2005-03028, the Ramo´n Areces Foundation under the “Colloidal Photovoltaic Materials” project, by CONICET (PIP 5191) and ANPCyT (PICT 34518), LNLS (SAXS Projects Nos. 5867/06 and 6721/ 07) and Gabbos (GXNG 017). S.C. and G.L. thank CSIC for funding their scholarships; M.C.F. thanks CONICET for a graduate scholarship. H. E. Troiani (CAB, CNEA) and M. C. Marchi (CMA, FCEN, UBA) are gratefully acknowledged for the TEM and SEM images and sample preparation. G.J.A.A.S.I. is grateful to the Universite´ de Paris VI for a Visiting Professor position that facilitated the VASE-EEP experiments.
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3163 Supporting Information Available: Spectra obtained for water and isopropyl alcohol vapors, intermediate results of modeled n versus P/Ps for all solvents and oxides studied in this work, and curves for isopropyl alcohol and water analogous to toluene in all the pressure range for each kind of the mesoporous films forming the multilayer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Soler-Illia, G. J. A. A.; Innocenzi, P. Chem.sEur. J. 2006, 12, 4478. (2) Nicole, L.; Boissie`re, C.; Grosso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 35, 3598. (3) 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. (4) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (5) Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opin. Colloid Interface Sci. 2003, 8, 109. (6) Sanchez, C.; Boissie`re, C.; Grosso, D.; Laberty, C.; Nicole, L. Chem. Mater. 2008, DOI: 10.1021/cm702100t. (7) Angelome´, P. C.; Fuertes, M. C.; Soler-Illia, G. J. A. A. AdV. Mater. 2006, 18, 2397. (8) Choi, S. Y.; Mamak, M.; von Freymann, G.; Chopra, N.; Ozin, G. A. Nano Lett. 2006, 6, 2456. (9) Fuertes, M. C.; Lo´pez-Alcaraz, F. J.; Marchi, M. C.; Troiani, H. E.; Mı´guez, H.; Soler Illia, G. J. A. A. AdV. Funct. Mater. 2007, 17, 1247. (10) (a) Boissie`re, C.; Grosso, D.; Lepoutre, S.; Nicole, L.; BrunetBruneau, A.; Sanchez, C. Langmuir 2005, 21, 12362. (b) Bourgeois, A.; Bruneau, A. B.; Fisson, S.; Demarets, B.; Grosso, D.; Cagnol, F.; Sanchez, C.; Rivory, J. Thin Solid Films 2004, 447, 46. (c) Baklanov, M. R.; Mogilnikov, K. P.; Polovinkin, V. G.; Dultsev, F. N. J. Vac. Sci. Technol. B 2000, 18, 1385. (11) Gibaud, A.; Dourdain, S.; Vignaud, G. Appl. Surf. Sci. 2006, 253, 3. (12) Klotz, M.; Rouessac, V.; Re´biscoul, D.; Ayral, A.; van der Lee, A. Thin Solid Films 2006, 495, 214. (13) Albouy, P.-A.; Ayral, A. Chem. Mater. 2002, 14, 3391. (14) Crepaldi, E. L.; Soler-Illia, G. J. A. A.; Grosso, D.; Ribot, F.; Cagnol, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (15) (a) Klotz, M.; Albouy, P. A.; Ayral, A.; Menager, C.; Grosso, D.; Van der Lee, A.; Cabuil, V.; Babonneau, F.; Guizard, C. Chem. Mater. 2000, 12, 1721. (b) Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Grosso, D.; Durand, D.; Sanchez, C. Chem. Commun. 2002, 2298. (16) Please notice that peak shift is plotted in energy units rather than wavelengths to provide an absolute scale independent of the spectral range in which the effect takes place. (17) Shung, K. W. K.; Tsai, Y. C. Phys. ReV. B 1993, 48, 11265. (18) Van de Hulst, H. C. Light Scattering by Small Particles; Dover Publications: New York, 1981. (19) Wu, Z.; Lee, D.; Rubner, M. F.; Cohen, R. E. Small 2007, 3, 1445. (20) Brinker, C. J.; Scherer, G. W. Sol-Gel Science - The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (21) Bourgeois, A.; Brunet-Bruneau, A.; Fisson, S.; Demarets, B.; Grosso, D.; Cagnol, F.; Sanchez, C.; Rivory, J. Thin Solid Films 2004, 447, 46. (22) Kim, T. W.; Ryoo, R.; Kruk, M.; Gierszal, K. P.; Jaroniec, M.; Kamiya, S.; Teresaki, O. J. Phys. Chem. 2004, 108, 11480. (23) Please note that the relative vapor volumes for each film in Figure 4 (reflecting sorption-condensation processes) are in the same order of magnitude in single and bilayer systems. This suggests that the whole pore volume is accessible to vapors. Pore occlusion at the interface between two layers, if present, should be minor and should not have a pronounced effect in the sorption equilibrium. Differences in adsorbed volumes are related to slight variations in film thickness and porosity due to the deposition of mesoporous films on porous substrates. Experiments are in due course to thoroughly assess these important aspects in particular regarding sorption/condensation kinetics.
