Simultaneous in situ and Time-Resolved Study of Hierarchical Porous

ARTICLE pubs.acs.org/JPCC

Simultaneous in situ and Time-Resolved Study of Hierarchical Porous Films Templated by Salt Nanocrystals and Self-Assembled Micelles Luca Malfatti,† Daniela Marongiu,† Heinz Amenitsch,‡ and Plinio Innocenzi†,* †

Laboratorio di Scienza dei Materiali e Nanotecnologie, CR-INSTM, D.A.D.U., Universita di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (SS), Italy ‡ Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria ABSTRACT: Time-resolved simultaneous experiments using synchrotron radiation have been realized to study in situ the concurrent formation of different templates in hierarchical porous thin films. Silica hybrid organic
inorganic films have been dip-coated and processed to obtain two porous nanostructures: spherical ordered mesopores templated by surfactant micelle, and cubic pores formed by crystallization of sodium chloride salts. The experimental setup has allowed following in real time the chemical
physical processes behind the formation of the two different templating agents. During deposition, the self-assembly and organization of the micelles have been studied by grazing incidence small-angle X-ray scattering while the crystallization of the salt template has been monitored by wide-angle X-ray scattering. After processing, the films have been further characterized by transmission electron microscopy, two-dimensional grazing incidence small-angle X-ray scattering and X-ray diffraction. The in situ analysis has revealed a short time delay between micelles and salt crystals formation; the micelles self-assemble and organize at the end of the first evaporation stage, when ethanol evaporates, while salt starts nucleating and growth only during the second stage, when water evaporation begins.

’ INTRODUCTION Self-assembly of nanomaterials through a liquid phase is a chemical
physical phenomenon that is directly related to evaporation processes;1 self-organization is triggered by solvent evaporation into a time-dependent assembly of nanostructures. Such phenomena are generally indicated as evaporation-induced self-assembly (EISA) and used for nanofabrication of several types of structures such as mesostructured porous materials2 and photonic crystals.3 In general, the self-assembly is governed by the processing parameters and is more critical in the case of thin films when evaporation is fast and the kinetic of the process more difficult to control.4,5 During the formation of organized mesoporous films, the micelles self-assemble and organize into a well-defined long-range mesostructure according to solvent evaporation. EISA processes can be also exploited for designing more complex materials, such as hierarchical porous films, by using different templates6 or controlled phase separation7 and a self-assembly route. Hierarchical structures can be obtained only if the templates do not interfere with each other during self-assembly and can be easily removed at the end of the process.8 Several examples have been reported in literature, and different templating strategies have been suggested; more commonly some preformed structures are employed, such as sacrificial templates9 and porous micro- and nanoparticles.10,11 A more challenging approach can be envisaged in evaporation phenomena trigger the formation of different templates on a similar time scale. In a previous work,12 we have used this strategy to produce hierarchical r 2011 American Chemical Society

porous films with an ordered mesoporous structure through self-assembly of micelles and macropores in the 100
280 nm range. The macropores have been templated by crystalline salts that form during film processing; cubic pores that are obtained after removal of the surfactant by a simple washing step with water. The advantage of this route is the possible selective functionalization of the two porosities, in fact, the surfactant organic template is removed by a thermal treatment that does not affect the salt which is then washed out in a second step. Another interesting feature is the possibility of obtaining pores of different dimensions and shapes, spherical-ellipsoidal pores from the micelles self-assembly and cubic nanoboxes from the salt crystal template. The formation of the salt templates during EISA is an intriguing feature, which opens interesting perspective to simultaneous self-assembly on different length scale. Despite the widespread use of NaCl in sol
gel synthesis as templating agent, only few works have reported the formation of regular NaCl crystal formation inside the materials; in most part of the papers NaCl caused phase separations or irregular macropore formation.13,14 Nanocubes formation in thin films, in fact, appears as a tricky combination of concurrent phenomena; as we have already observed, for istance, the addition of Na2HPO4 to the solution of precursors is crucial to trigger the nucleation of the nanocrystals.15 Received: December 10, 2010 Revised: May 19, 2011 Published: May 31, 2011 12702

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Figure 1. (a) Schematic top view of the experimental setup used for the simultaneous GISAXS-WAXS time-resolved experiments. Top view pictures of the experimental setup mounted at the SAXS Austrian beamline at Elettra; detail of the PSD detector position for GISAXS (b) and WAXS analysis (c).

We have tried to get a direct understanding of the phenomenon by an in situ time-resolved experiment which combines different analytical techniques, grazing incidence small-angle X-ray scattering (GISAXS) and wide-angle X-ray scattering (WAXS). We used a special experimental setup and employed synchrotron radiation as light source for performing the experiments; we have therefore been able to follow in situ the two phenomena, i.e., self-assembly of the micelles and salt crystallization, and correlate them with specific stages of evaporation during film processing. The simultaneous SAXS-WAXS in situ experiment using synchrotron light allows reaching a time resolution and an accuracy that could not be obtained performing two separate experiments.

’ EXPERIMENTAL SECTION Tetraethylorthosilicate (TEOS), methyltriethoxysilane (MTES), ethanol (EtOH), hydrochloric acid (1N), triblock copolymer surfactant, Pluronic F127 (EO106
PO70
EO106), NaCl (99.5%), and Na2HPO4 (99%) were purchased from Aldrich and used without further purification. P-type/boron-doped, (100)-oriented, 400-μm-thick silicon wafers (Si-Mat) were used as the substrates. The precursor solution was prepared in three different steps: a silica solution was obtained by mixing 2.84 cm3 of TEOS and 1.42 cm3 of MTES in 3.08 cm3 of EtOH with 0.36 cm3 of aqueous HCl solution 0.77 M. The sol was stirred for 45 min at 25 °C and then added to a second solution prepared by dissolving 1.3 g of Pluronic F127 in 15 cm3 of EtOH and aqueous HCl solution 0.06 M. After 15 min of stirring, 20 cm3 of the silica
surfactant sol were mixed to 3.6 cm3 of an aqueous solution containing 0.15 g of NaCl and 0.024 g of Na2HPO4. The sol was stored under stirring in a closed vessel at 25 °C for 1 h and then used for dip-coating. A customized dip-coating system was used to deposit thin films on silicon substrates, previously cleaned with water, EtOH, and rinsed with acetone. This equipment, which has been especially made for in situ experiments, was built to fit the sample stage dimensions in the beamline experimental hutch.

The silicon substrates were mounted on a fixed sample holder, while the beaker containing the precursor solution was moved vertically using a remote-controlled linear module. With this layout, the sample was maintained at a fixed position and orientation with respect to the X-ray beam while the solution beaker was raised and lowered. During the deposition the experimental setup was enclosed into a plastic box to maintain the relative humidity (RH) around 30 ( 5%. The experiments were realized at the Austrian-SAXS beamline of the ELETTRA sinchrotron radiation facility (Trieste, Italy).16 The design of the experimental hutch in the beamline allows the insertion of different types of devices and detectors close to the sample position (e.g., dip-coater, position-sensitive linear detector (PSD), hygrometer, etc.). GISAXS and WAXS measurements were performed by using an incident wavelength of 1.54 Å (8 KeV); the angle of incidence was set slightly above the typical critical angle of a hybrid silica film. Two position sensitive detectors (PSDs) “Bruker Vantec” were used to collect simultaneously the Bragg-diffraction peaks from the mesostructure and NaCl nanocrystals. The distances between the sample and the detectors were calculated using the diffraction pattern of standard samples having known lattice constants; silver behenate, H(CH2)21COOAg, and p-bromo benzoic acid for GISAXS and WAXS, respectively. For GISAXS and WAXS time-resolved experiments, the film formation and drying was followed up to 512 s from the deposition with a time of 1 s. The integrated area of SAXS and WAXS peaks were calculated from data fitting using Gaussian peaks. In the case of SAXS patterns a quadratic background was subtracted. For 2D GISAXS measurements, the films, previously measured by time-resolved experiments, were fired immediately after deposition at 350 °C for 30 min. A 2D-CCD detector (Photonic Science) was used to collect the pattern; each measure is the average of 10 acquisition with 2 s of integration time. FIT2D program (A. P. Hammersley/ESRF)17 was used for correcting images for flat field space distortion, GISAXS pattern spot indexation, and d-spacing calculation. X-ray diffraction (XRD) measurements were recorded by a Bruker D8 diffractometer equipped with a scintillator counter. 12703

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Figure 2. (a) 3D representation of GISAXS time-resolved measures on hierarchical porous films as a function of time and scattering vector, Sz. The intensity variation of the peaks is reported in false color scale. (b) In situ GISAXS spectra collected after 80, 150, 300, and 450 s from the dip-coating of hierarchical porous films.

Figure 3. (a) 3D representation of WAXS time-resolved measures on hierarchical porous films as a function of time and 2θ values. The intensity variation of the peaks is reported in false color scale. (b) In situ WAXS spectra collected after 80, 150, 300, and 450 s from the dip-coating of hierarchical porous films. The peak is attributed to NaCl nucleation and growth as depicted by the sketch of the crystal.

The Cu KR radiation was used to perform an ω/2θ scan from 23 to 70° with a resolution of 0.02°. Transmission electron microscopy (TEM) of the films after thermal treatment was obtained by using a JEOL 200C microscope equipped with a tungsten cathode operating at 200 kV. Finely ground fragments of the films scratched from the silicon substrate were deposited on a carbon-coated copper grid for TEM observations.

’ RESULTS AND DISCUSSION Two different phenomena are driven by the evaporation of the solvent during the preparation of hierarchical porous thin films; the self-assembly of micelles into an ordered mesostructure and the nucleation and growth of salt nanocrystals. The dimensions of the two structures, which are both in the nano range, are in the mesoscale (2
50 nm, the micelles) and macroscale (100
280 nm, the salt crystals); the two processes are time dependent and can be well monitored with a time resolution of seconds.18,19 To follow the overall time-dependent phenomena, we have realized a specific experimental setup using a synchrotron light source, as shown in Figure 1. The X-ray beam is illuminating the film in grazing incidence, and the scattered light at low angles is recorded at the end of the vacuum chamber by the GISAXS PSD detector and CCD camera; at the same time the WAXS PSD detects the diffracted signal at high angles from the sample. We have chosen the WAXS analysis

range to follow the most intense diffraction signal produced by crystalline fcc sodium chloride (200) at 31.8°.15 This setup allows the simultaneous detection of signals produced by structures of few nanometers (GISAXS, mesostructures) and 100
280 nm (WAXS, salt crystals). In the first case the signal is generated by the electronic contrast between the micelles and the silica pore walls20 while in the second one the signal is from classic Bragg diffraction of atomic planes in crystalline structures. The results of the time-resolved GISAXS analysis are reported in parts a and b of Figure 2; Figure 2a shows the 3D representation of GISAXS signal intensity as a function of time (in seconds) and scattering vector, Sz, (nm
1). The intensity variation of the scattered signal is shown in false color scale. The first step of EISA process is characterized by the evaporation of the solvent, in this case ethanol, which produces a strong diffuse scattering at low Sz values; this first initial evaporation stage21,22 generally lasts few tens of seconds. At this stage the micelles are not yet formed, as the absence of signals at higher scattering vector values reveals. After around 70 s, we observed the rise of an intense signal at 0.077 nm
1, which is attributed to the organization of the micelles into a long-range periodic mesostructure and it corresponds to a d-spacing of 12.9 nm. The maximum intensity of this signal is reached at 80 s from the beginning of EISA process, after this time the signal decreases in intensity and shifts to lower Sz values. In Figure 2b the GISAXS spectra after 80, 150, 300, and 450 s are reported; in the figure only some selected spectra at specific times which well describe the overall trend are shown. 12704

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Figure 4. Integrated intensity of the GISAXS (a) and WAXS (b) signals at 0.077 nm
1 and 31.8° as a function of time during simultaneous and timeresolved in situ measures on hierarchical porous films.

Figure 5. (a) GISAXS pattern of a hierarchical porous films film after thermal treatment at 350° for 30 min. Indexation of the body centered cubic organized porous structure is shown in the figure. (b) XRD pattern of a of a hierarchical porous films containing the salt crystals; the peaks are indexed according to a crystalline phase of sodium chloride.

The results of the time-resolved WAXS analysis are reported in parts a and b of Figure 3; we have used the same graphical representation of GISAXS data (Figure 2) with 2θ/° in the x scale. Also in this case the first stage of the evaporation process produces a diffuse scattered broad signal that occupies almost all the spectral range in the Figure 3a. The crystallization of NaCl is revealed by the rise of a diffraction peak at 31.8° after around 150 s from the beginning of the process; this peak appears quite suddenly and exhibits a sharpening and smooth increase in intensity with time. We have integrated the intensity of the GISAXS and WAXS signals at 0.077 nm
1 and 31.8° and plot the data as a function of processing time; this analysis allows a direct cross correlation between micelle organization (Figure 4a) and crystal nucleation and growth (Figure 4b). The GISAXS curve shows a fast decrease in intensity of the signal, which is time-correlated with the simultaneous increase of the WAXS peak. The comparison of the two curves indicates also that there is a small time shift between the two phenomena; the salt crystals appear after 80 s when the micelles are already well formed. In correspondence of the rise of the NaCl diffraction peak, the GISAXS signal on the contrary begins to decrease. We have then analyzed the final structure of the film after thermal treatment at 350 °C by 2D-GISAXS and XRD analysis and the data are shown in parts a and b of Figure 5; at this stage of film processing the surfactant template has been fully removed as shown by FTIR spectra (not shown in figures), while the NaCl crystals remain in the material. The 2D-GISAXS pattern of the

film (Figure 5a) shows the typical signature of a body-centered cubic porous organized structure, Im3m in the space group, oriented with the [110] direction normal to the substrate (z direction), uniaxially distorted. On the basis of this attribution, the GISAXS measures allow calculating the d-spacing of the diffraction spots, which are d110 = 10.3 ( 1.4 nm, d101 = 12.3 ( 0.7 nm, d
110 = 13.4 ( 0.8 nm. The NaCl crystals are in the face-centered cubic, as revealed by the XRD diffractogram, which shows the (111), (200), (220), and (222) diffraction planes.23 The simultaneous presence of the crystals and mesopores is confirmed by TEM analysis which is reported in Figure 6; the salt nanoboxes appear randomly but homogeneously distributed within the matrix and the mesopores very well organized into the cubic structural order. The average size of the nanoboxes ranges between 100 and 280 nm while the mesopores size has been estimated to be 6.4 ( 0.6 nm. The possibility of following in situ the hierarchical film formation by simultaneous time-resolved analytical techniques has allowed to get a better insight of the evaporation driven processes with different templates. The use of two templates during film processing gives pores of different dimensions, in the meso and macro range, and different shapes, spherical and cubic. The data set has shown that the formation of spherical micelles and sodium chloride crystalline templates is a time-dependent process that is simultaneously realized during evaporation. We have, however, to underline that the solvent evaporation is a quite complex phenomenon because the presence of a water
ethanol 12705

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Figure 6. (a) Bright-field TEM image of a hierarchical porous films after thermal treatment at 350°. (b) TEM image of a nanobox templated by NaCl crystal embedded into the organized mesoporous film matrix. (c) Sketch of the mesopore organization into a Im3m body-centered cubic structure.

Figure 7. Schematic representation of the two different templates formation; during dip-coating the ethanol evaporates first inducing the self-assembly of the micelle. Afterward, the water evaporation pushes the Cl
ions to swell inhomogeneously the micelles and drives the NaCl formation. Finally, the thermal dehydration produces a migration from the micelles to the nanocubes restoring a narrow micelle size distribution.

mixture implies different stages of evaporation; ethanol evaporates faster than water and leaves behind a water enriched phase. We have already observed by in situ and time-resolved simultaneous experiments (SAXS and FTIR) that the micelles organize at the end of the ethanol evaporation; at this point, in fact, the first SAXS spots are formed.18 On the other hand the solubility of NaCl is different in ethanol or water rich mixtures; sodium chloride in ethanol at 25 °C has a solubility that is 481 times lower than in water.24 A water-rich film remains after the full evaporation of ethanol, and the salt formation is driven by the water content, which indeed affects the solubility of the salt. The nanocubes nucleation is triggered by Na2HPO4, and the crystals growth is so fast, which is not affected by the proceeding of the polycondensation reactions of the silica network.15 This mechanism of formation of the two different templates explains the time shift between the appearance of the SAXS and WAXS signals.

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The SAXS-WAXS data also show that the formation of nanocrystals apparently disrupts the organization of the mesophase, but this is in contradiction with the TEM and GISAXS measures done after film processing, which instead show the presence of a well organized mesophase. A tentative explanation of this phenomenon can be formulated on the basis of some recent works. During in situ experiment on SBA-15 formation, Teixeira et al. have reported that the addition of an inorganic salt, especially NaCl, affects the size of the micelles leading to larger cell parameters of the mesoporous materials.25 In that case, selfassembly occurred in solution with a negligible contribution of the solvent evaporation. Our experiments, on the contrary, concern the formation of a mesoporous film, which is mainly triggered by the ethanol evaporation. In this case, therefore, we can deduce that, as long as the Naþ and Cl
increase their relative concentrations, a minor part of the chlorine anions interacts with the hydrophilic PEO tails of the block-copolymer templates inducing an inhomogeneous swelling of the micelles and a broadening of the micelle size distribution. At this point, since the inorganic silica network is still not completely dense some structural modifications are still allowed; the swelling of the micelles also affects the mesophase organization leading to a lowering and broadening of the Bragg peak in the SAXS patterns. This process occurs simultaneously and concurrently to the growth of NaCl nanocubes which is likely nucleated by the presence of Na2HPO4, as we have shown in previously studied.15 Finally, during thermal treatment the Naþ and Cl
ions migrate from the micelles toward the NaCl crystals as a consequence of film dehydration and thus restore a narrow distribution of the micelle dimensions. This leads to a better organized mesophase and a well-defined GISAXS pattern. The overall process is illustrated in Figure 7.

’ CONCLUSIONS Simultaneous in situ and time-resolved GISAXS-WAXS analysis has allowed to reveal the kinetics of formation of micelles and salt templates in hierarchical porous films. The templates have different shape and length scale, from meso to macro. Salt crystals and micelles form and organize in situ in a time scale of seconds but with a small time shift between the appearance of the organized micelles mesophase and the salt crystallization. The cross correlation of the two data set has shown that the ethanol evaporation mainly drives the micelles organization, while the salt nanocrystals nucleation and growth depends on water evaporation. The analysis has also shown that the formation of the two templating agents is an evaporation-triggered phenomenon, which is time-dependent. The role of salt is complex because the two templates, the nanoboxes, and the micelles form in a sequential stage but salt ions mobility during evaporation and salt crystallization are concurrent phenomena which allow getting multishape and hierarchical porous films. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by the Italian Ministero dell’Universita e della Ricerca (MiUR) through the PRIN (contract n. 2008AFRTFW_003). Paolo Falcaro and Stefano 12706

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Costacurta are gratefully acknowledged for the support during experiments. Maria F. Casula is acknowledged for TEM measurements.

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