Kinetics of the Formation of 2D-Hexagonal Silica

ARTICLE pubs.acs.org/JPCB

Kinetics of the Formation of 2D-Hexagonal Silica Nanostructured Materials by Nonionic Block Copolymer Templating in Solution Sabine Manet,† Julien Schmitt,† Marianne Imperor-Clerc,*,† Vladimir Zholobenko,‡ Dominique Durand,§ Cristiano L. P. Oliveira,||,z Jan Skov Pedersen,|| Christel Gervais,^ Niki Baccile,^ Florence Babonneau,^ Isabelle Grillo,# Florian Meneau,r and Cyrille RochasO †

Laboratoire de Physique de Solides, UMR 8502, Bât. 510, Universite Paris-Sud, F-91405 Orsay, France Chemistry Department, Keele University, Staffordshire, ST5 5BG, United Kingdom § Institut de Biochimie et de Biophysique Moleculaire et Cellulaire, Bât. 430, Universite Paris-Sud, F-91405 Orsay, France Department of Chemistry and iNANO Interdisciplinary Nanoscience Center, Århus University, DK-8000 Århus, Denmark ^ Laboratoire de Chimie de la Matiere Condensee, College de France, F-75231 Paris Cedex 05, France # Institut Laue Langevin, BP 156, F-38042 Grenoble, France r SWING, Synchrotron Soleil, BP 48, F-91192 Gif-sur-Yvette, France O D2AM, ESRF, 6 rue Jules Horowitz, F-38000 Grenoble, France

)

‡

bS Supporting Information ABSTRACT: The different steps of the self-assembly in solution of several 2D-hexagonal silica nanostructured SBA-15 materials have been investigated by SAXS and SANS in situ experiments. Unique quantitative information about the shape and size evolution upon time of the micellar aggregates throughout the self-assembly process is obtained using a complete model that describes well the scattering data for the various synthesis conditions. In all cases, before the precipitation of the material, the micelles shape changes from spherical to rod-like, where the structure of the rod-like micelles is linked to the structure of the 2D-hexagonal precipitated material. In addition, the kinetics of hydrolysis of the inorganic precursor (TEOS) has been determined by in situ Raman spectroscopy. More specifically, by comparing synthesis made with different acids (HNO3, HBr, HCl, H2SO4, and H3PO4), it is found that materials prepared using the “salting-out” anions (SO42- and H2PO4) are much better ordered than with the “salting-in” anions (NO3 and Br).

’ INTRODUCTION Surfactant-templated inorganic materials have raised a very large interest in the material science community during the last ten years, and have opened the way to many applications in various fields. These materials are nowadays developed in many directions, like porous media in catalysis, nanomolds to synthesize nanoparticles of controlled size, or biomaterials for drug delivery or bone tissue regeneration. On the fundamental aspect, it is still important to determine how surfactants, thanks to their self-assembly properties, are governing the structure of a hybrid organic/inorganic material, in order to optimize a material for a required specific application. One way to answer this question is to perform kinetics studies during the synthesis of a hybrid material, in order to follow in real time how surfactant and the inorganic species are interacting. Following this approach, several recent publications have been devoted to the detailed understanding of the self-assembly mechanism in solution of SBA-15 and related materials. Both TEM observations upon time14 and r 2011 American Chemical Society

in situ experiments using SAXS511 and SANS12,13 allowed us to identify the main steps involved during the formation of these materials. These main steps are the formation of hybrid organic/ inorganic micelles in the solution, followed by the nucleation and growth of the ordered 2D-hexagonal hybrid material. In this paper, we present new SAXS and SANS in situ experiments on the kinetics of formation of different silica based materials related to SBA-15. Unique quantitative data about the evolution of the micelles upon time and the nucleation/growth step of the organized material are obtained, in the framework of a complete model developed for the SAXS/SANS data analysis.8 Additional in situ Raman experiments allow to relate the evolution of the micelles upon time to the hydrolysis rate of the inorganic precursor.14 On the experimental point of view, various materials Received: January 8, 2011 Revised: July 1, 2011 Published: August 24, 2011 11330

dx.doi.org/10.1021/jp200213k | J. Phys. Chem. B 2011, 115, 11330–11344

The Journal of Physical Chemistry B

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Table 1. Summary of the Different Kinetics Experimentsa

composition temperature (C) 20 40 65 40 65 HCl concentration 2.5 M 1.6 M 0.4 M 0.2 M P123/TEOS molar ratio 0.017/0.5 TEOS 0.5 0.017/1 TEOS 1 0.017/2 TEOS 2 different acids HNO3 HBr HCl H2SO4 H3PO4, pH = 0.9

TEOS in situ precipitation hydrolysis data time (min) time (min)

HCl 1.6 M in D2O HCl 1.6 M in D2O HCl 1.6 M in D2O HCl 0.2 M in D2O HCl 0.2 M in D2O

SANS SANS SANS SANS SANS

40 23 9 >180 120

HCl 2.5 M in D2O HCl 1.6 M in D2O HCl 0.4 M in H2O HCl 0.2 M in D2O

SANS SANS SAXS SANS

12 23 172 >180

2.8 6.2

HCl 1.6 M in H2O HCl 1.6 M in H2O HCl 1.6 M in H2O

SAXS SAXS SAXS

>180* 23; 39* 18*

2.8

HNO3 1.6 M in H2O HBr 1.6 M in H2O HCl 1.6 M in H2O H2SO4 0.8 M in H2O H3PO4 1.6 M in H2O

SAXS SAXS SAXS SAXS SAXS

16 ;36* 19 ;37* 23; 39* 17; 50* 192; 114*

1.5 2 2.8 6 10

2.8

a

The effect of temperature, HCl concentration, P123/TEOS molar ratio, and acidic source has been investigated. The synthesis temperature is 40 C if not mentioned. For SAXS, measurements made at the SOLEIL synchrotron are indicated by an asterisk (*). The TEOS hydrolysis time is determined using in situ Raman (see Table 2).

made using Pluronic P123 triblock copolymer (EO20PO70EO20) have been investigated, with a special focus on the influence of the nature of the acidic source, in relation with the Hofmeister series of anions.15 The paper is organized as follows. After the description of the experimental methods, the models used for the interpretation of the scattering experiments are detailed introducing the different model parameters. The results section is divided in three parts. The first one is devoted to the in situ Raman experiments and the second one gives a first broad overview of the in situ SAXS results. In the third part, the influence of several synthesis parameters is detailed. The influence of the synthesis temperature, the pH and the quantity of added TEOS (in situ SANS/ SAXS) is discussed comparing different synthesis performed with the same acid, HCl, and the influence of the acidic source is investigated comparing five different acids (in situ SAXS): HNO3, HBr, HCl, H2SO4, and H3PO4.

’ EXPERIMENTAL METHODS Sample Preparation. Pluronic P123 (EO20PO70EO20) (Aldrich) was used as received to make acidic aqueous solutions prior to the in situ experiments. TEOS (tetraetoxysilane) was used as inorganic precursor. Deuterated water (D2O) instead of H2O was used as a solvent for the SANS measurements. During the Raman, SAXS, or SANS in situ experiments, the synthesis was performed using about 80 mL of solution placed on a hot-plate to control temperature and under permanent stirring. Using an already described setup7,12,13 with a peristaltic pump, part of the synthesis solution was continuously circulating (flow rate of about 10 mL/min) in a 2 mm quartz capillary (SAXS) or a 1 mm flat quartz flow-cell (SANS). Because of the continuous stirring, the composition of the sample, in front of the

beam, is the same as for the solution inside the reaction beaker, even after the precipitation of the material. All of the different kinetics experiments are summarized in Table 1. The standard conditions for the SBA-15 synthesis are at 40 C in HCl 1.6 M (pH < 0.1) with the following composition in molar ratio: HCl/H2O + D2O/P123/TEOS = 6/200/0.017/1. The molar ratio water:P123 was kept close to 200:0.017 for all of the samples corresponding to a molar concentration close to 4.5 106 mol/cm3 before TEOS addition and close to 4.0 106 mol/cm3 after TEOS addition. In HCl, the effects of temperature (20, 40, and 65 C), HCl concentration (2.5, 1.6, 0.4, and 0.2 M), and P123/TEOS molar ratio (0.017/0.5 (TEOS 0.5), 0.017/1 (TEOS 1), and 0.017/2 (TEOS 2)) have been investigated. Synthesis with different acids (HNO3, HBr, HCl, H2SO4, and H3PO4)20 have been made at 40 C by in situ SAXS. Precipitation times were determined by the appearance of Bragg peaks during the in situ SANS/SAXS experiments. For SAXS, measurements made at the SOLEIL synchrotron are indicated by an asterisk (*). For some compositions, the experiments have been made twice and noticeable variations of the precipitation times are observed (Table 1). For example, the precipitation time for HCl 1.6 M at 40 C measured during the experiment at SOLEIL is of 39 min instead of 23 min for the other experiments (ESRF and ILL). However, the overall evolution upon time is always the same for a given composition. This variation of the precipitation time is attributed to small changes of the experimental conditions (temperature control, stirring rate, etc.) between the setups used at the three different experimental places. Especially, a small change of the speed of the magnetic stirrer is a parameter that is known to affect the kinetics rate. Raman. Time-resolved in situ Raman spectroscopy was used to follow the hydrolysis kinetics of TEOS. Raman spectra were recorded using a Hololab 5000 R apparatus (Kaiser Optical Systems, Inc.) equipped with a 750 nm laser (power: 400 mW) connected to an optical fiber and also with a CCD camera. The Raman emission was collected at 180 with respect to the laser source. An optical lens system was used to focus the light at the center of the reaction beaker containing the aqueous solution of P123. The solution was continuously stirred, and the experiment was performed at 40 C. The time interval between data points was set to 5 s, and the start of the experiment (i.e., t = 0) was when TEOS was added to the solution. Data were acquired and treated with the HoloGRAMS software package. Scanning Electron Microscopy. The materials have been taken out from the solution by filtration typically 4 h after the precipitation. After drying at room temperature for 2 days, the materials have been observed using scanning electron microscopy SEM. A Zeiss SUPRA55-VP type scanning electron microscope (SEM) was used for microstructure observation. This field effect gun microscope operates at 0.5 to 30 kV. High resolution observations are obtained by two secondary electron detectors: A lens SE detector and a Everhart-Thornley SE detector. SANS (Small Angle Neutron Scattering). SANS experiments were performed on the D22 spectrometer of the Institute LaueLangevin, Grenoble, France. The neutron wavelength was 0.6 nm with a wavelength spread (fwhm) of Δλ = 10%, and the beam size was 0.7 cm2 at the sample position. The data were recorded at two different sampledetector distances of, respectively, 5 m (beam intensity = 5.6 106 neutrons/s for a range of scattering vector moduli q from 1 102 to 2 101 Å1) and 17 m (beam 11331

dx.doi.org/10.1021/jp200213k |J. Phys. Chem. B 2011, 115, 11330–11344


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intensity = 1.5 106 neutrons/s for a q range from 3 103 to 6 102 Å1). Note that the scattering vector modulus is q = 4π sin θ/λ, where 2θ is the scattering angle and λ is the wavelength. Absolute scale intensities (cm1) were obtained by calibration using the incoherent scattering signal from a 1-mm thick water sample. SAXS (Small Angle X-ray Scattering). SAXS measurements were carried out at the D2AM ESRF beamline (energy 11 keV) and at the SWING SOLEIL beamline (energy 12 keV). At D2AM, the sample-CCD camera distance was 1.623 m, and the q range was 1.0 1031.6 101 Å1. At SWING, the sample-CCD camera distance was 3.28 m, and the q range was 3.3 1041.6 101 Å1. At both beamlines, the q-range calibration was made using a silver behenate standard sample (dref = 58.38 Å). The measured intensity is always divided by the sample transmission. For the absolute intensity calibration, scattering patterns of the empty capillary and the capillary filled with deionized water were always recorded first. The value of the constant intensity contribution of water is equal to 0.016 cm1 on absolute scale.16 Then, the signal of the same capillary filled with the solvent solution was recorded for subtraction purposes before the introduction of the synthesis solution. During each kinetics experiment, the capillary was kept fixed in the beam using a capillary holder, so that all patterns were recorded at the same position. For SAXS, the scattering length density is the product of the classical radius of the electron re = 2.81794 1015 m and the electron density (number of electrons per unit volume). In the tables, we give the electron density values in e/nm3 rather than the scattering length density, but the coefficient re is taken into account for the absolute scaling (in cm1) in the intensity expression. Values of the SLD and ED. The values of the coherent neutron scattering lengths were taken from the NIST database.17 For the different solvents (H2O, D2O, acidic solutions), the electron densities (ED) and scattering length densities (SLD) were determined from the density values as a function of temperature using literature data.18 For PPO and PEO, the SLD and ED values were derived from the experimental values of their apparent specific volume in the micellar state.19 More details can be found in the previous related paper about the structure of the micelles before TEOS addition.20 The value of the ED of TEOS in the liquid state at room temperature is 310 e/nm3.

’ MODELS FOR SAXS/SANS The theoretical basis for the modeling of the scattering by micelles of nonionic surfactants are well-known and have been already reported in the literature.21,22 Based on the works by F€orster et al.23 and Freiberger et al.,24 a quantitative expression for the scattering curves by an ordered 2D hexagonal phase has been recently successfully applied to describe the formation of SBA-15 hybrid materials.8,9 Following these works, we give in this paper a general expression to model the scattered intensity Itot(q) as a function of time during the formation of a 2Dhexagonal hybrid material. This expression contains three different contributions Itot ðqÞ ¼ IðqÞ þ Ibg ðqÞ þ IPorod ðqÞ

ð1Þ

The intensity I(q) is the main contribution and corresponds to the scattering by the micelles with precipitated silica either as

single objects or as a 2D lattice of objects. Ibg(q) is a background term detailed at the end of this section and IPorod(q) is a Porod term contributing only in the low q-range (q < 0.01 Å1) and comes from the scattering by the interfaces when the reaction medium is multiphasic. Different expressions are used for I(q) (micelles contribution) upon time as detailed hereafter. Indeed, during the kinetics, the micelles (spherical or cylindrical ones) are first present in a solution state and becomes ordered (cylindrical micelles only) inside the precipitated 2D-hexagonal material. The expression of I(q) corresponds always to a “powder” average over the spatial orientation of the scattering objects, and it can be decomposed into the product of a form factor term P(q) and a structure factor term S(q). A coreshell model is always assumed for P(q) both for spherical and cylindrical micelles. IðqÞ ¼ APðqÞSðqÞ ¼ nðFcore F0 Þ2 PðqÞSðqÞ 2 PðqÞ ¼ nðFshell F0 Þ SðqÞ α2 A ¼ nðFcore F0 Þ2 Fshell F0 2 ¼n α

and

α¼

Fshell F0 Fcore F0

ð2Þ

ð3Þ

PðqÞ ¼ FðqÞ2

ð4Þ

Itot ðqÞ ¼ APðqÞSðqÞ þ Ibg ðqÞ þ IPorod ðqÞ

ð5Þ

where n, is the number of micelles per unit volume and the scattering length densities (cm2) of the core, shell, and solvent are respectively Fcore, Fshell, and F0. F(q) is the amplitude of the form factor of the scattering objects. Brackets notation (Ææ) stands for the averaging over polydispersity in size over the two dimensions (Rcore, Rtot). Spherical Micelles (Parameters: Rcore, Rtot, α, σ, RHS, ϕHS, and A). During the first step of the kinetics (steps ac in Figure 1), spherical micelles are present. They are modeled using the previously introduced expression for core/shell spheres20 IðqÞ ¼ APðq, Rcore , Rtot , α, σÞSðq, RHS , ϕHS Þ

ð6Þ

It includes the polydispersity in size of the micelles (parameter σ) and the structure factor term using the PercusYevick expression for interacting hard spheres, governed by two parameters, the hard-sphere interaction radius RHS and volume fraction ϕHS. Cylindrical Micelles (Parameters: Rcore, Rtot, α, σ, σcorona, L, νRPA, and A). Upon time, a shape transformation of the micelles toward cylindrical micelles is often observed (step d in Figure 1). During the transition period from spherical to cylindrical micelles, a mixture of shape can be involved, and it is then difficult to model the scattered intensity. However, after this transition period, the scattered intensity becomes well modeled by the contribution of only cylindrical micelles of average length L. The expression of I(q) used in this case is a simplified version, valid only for long cylindrical micelles with a sufficient aspect ratio (typically for a ratio L/2R greater than five) (see the SI for more details). The form factor contribution P(q) can then be factorized as the product of a term Prod(q) 11332



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the Bragg peaks (term (1 β(q)G(q))). And accordingly, the intensities of the Bragg peaks are decreased by a factor β(q)G(q), compared to the intensity without disorder (term Z0(q)) IðqÞ ¼ nðFcore F0 Þ2 Prod ðqÞ FCS 2 ðqÞ ½1 βðqÞGðqÞ þ βðqÞGðqÞZ0 ðqÞ ð10Þ

Figure 1. Sketch of the different steps involved during the selfassembly: (a) Initial micellar solution. (b) Addition of the inorganic precursor (TEOS). (c) Beginning of hydrolysis. (d) Shape evolution of the micelles. (e) Nucleation of the 2D-hexagonal phase. (f) Growth and precipitation of the 2D-hexagonal phase.

depending on the length L and a term ÆFCS2(q)æ depending on the cross section ð7Þ PðqÞ ¼ Prod ðqÞ FCS 2 ðqÞ 2J1 ðqRcore Þ qRcore 2J1 ðqRtot Þ 1 þ απRtot 2 exp q2 σcorona 2 qRtot 2

þ βðqÞGðqÞZ0 ðqÞ

FCS ðqÞ ¼ ð1 αÞπRcore 2

ð8Þ

No polydispersity dependence on the length L is included and the polydispersity over the diameter of the micelles (Ææ in the cross-section term) is taken into account in the same function way as for spherical micelles (SchulzZimm distribution).20 σcorona is the width of the smeared outer interface at Rtot.8 In addition, a structure factor contribution S(q) is included to model a repulsive excluded-volume interaction term between the cylindrical micelles, using the random phase approximation (RPA)22 SðqÞ ¼ SRPA ðqÞ ¼

1 1 þ vRPA PðqÞ=Pð0Þ

The value of the numerical coefficient in the Z0(q) term has been entirely determined via a new calculation of the integrated intensity of the Bragg peaks in the case of a perfect 2D-hexagonal lattice without disorder (see section D in SI). Then, in contrast with previous publications,23,24 the numerical coefficient does not contain a constant c, expected to be close to one, and related to the Porod invariant. Nucleation of the 2D Hexagonal Phase (Rcore, Rtot, α, σ, σcorona, L, a, σDW, D, p, νRPA, and A). During the nucleation step of the 2D-hexagonal material (step e in Figure 1), a coexistence is observed between “free” cylindrical micelles in solution and micelles packed on a 2D-hexagonal lattice (Bragg peaks). Introducing the proportion p (0 < p < 1) of the “free” micelles in solution, the expression of I(q) becomes IðqÞ ¼ nðFcore F0 Þ2 Prod ðqÞ½p FCS 2 ðqÞ mic SRPAmic ðqÞ !2 Fcore FΜ 0 þ ð1 pÞ hFCS 2 ðqÞihex ½1 βðqÞGðqÞ Fcore F0

ð9Þ

where vRPA is a positive coefficient which is an increasing function of the concentration of the micelles. Cylindrical Micelles on a 2D-Hexagonal Lattice (Rcore, Rtot, α, σ, σcorona, L, a, σDW, D, and A). When the 2D-hexagonal material is formed (steps e and f in Figure 1), Bragg peaks are recorded at discrete positions qhk. The Bragg peaks profiles are modeled using peak-shaped functions Lhk(q,qhk) and their width is inversely proportional to the average domain size D of the grains.8 The intensity is computed including two types of disorder in the 2D-hexagonal lattice: A distribution of the size of the cross sections of the cylinder (polydispersity parameter σ), described by the term β(q), and a positional disorder, described by a DebyeWaller term G(q) (see the SI for more details).8,23 σDW is the relative mean-square displacement for this positional disorder. The inclusion of these disorder terms in the model has two consequences. First, it gives raise to some intensity in-between

ð11Þ

as p is the proportion of the micelles in solution and (1 p) the proportion inside the material. n is always the total number of micelles per unit volume. Inside the 2D-hexagonal material, it is assumed that the core density is the same as for the free micelles, but the density in-between the micelles FM 0 may be different from the solvent density F0. However, in the case of the SBA-15 synthesis, a good description is obtained using the same form factor for the “free” micelles and for the micelles inside the ordered material, and the expression simplifies as follows: 2 ðqÞ ½pSRPAmic ðqÞ IðqÞ ¼ nðFcore F0 Þ2 Prod ðqÞ FCS þ ð1 pÞ½1 βðqÞGðqÞ þ βðqÞGðqÞZ0 ðqÞ ð12Þ Background Expression (Parameters: Rg, B, and C). Ibg(q) is a background term attributed to the scattering by polymer chains or silica oligomers. It appears to be proportional to the quantity of silica precursor (TEOS) and this term is increasing upon time following the condensation rate of silica (Figures 4 and 6)

Ibg ðqÞ ¼ BIchain ðqÞ þ C Ichain ðqÞ ¼ 2

expð q2 Rg 2 Þ 1 þ q2 Rg 2 ðq2 Rg 2 Þ2

ð13Þ ð14Þ

The expression used for Ichain(q) corresponds to the scattering by polymer chains of radius of gyration Rg, where B is the constant scaling factor of this contribution. Typical values used for Rg are between 0.3 and 0.8 nanometres and, upon time, this value is always increasing. This value is attributed to the size of the silica oligomers resulting from the silica condensation. 11333



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C in an additional constant background contribution that may be added in the model, that is usually fixed to zero for the results presented here. Porod Term (Parameter: K). The last term IPorod(q) is contributing only in the low q-range (q < 0.01 Å1) and comes from the scattering by the interfaces when the reaction medium is biphasic IPorod ðqÞ ¼

K q4

Σ K ¼ 2π ðhFi F0 Þ2 V

ð15Þ

ð16Þ

where Σ/V is the specific area of the subphase and ÆFæ is the average density of the subphase with respect to the solvent F0. At the beginning of the kinetics, the interface between nonhydrolyzed TEOS droplets and the solvent will contribute to the Porod term (see step b in Figure 1). After precipitation of the material, the IPorod(q) term comes from the interface of the grains surrounded by the solvent (see step f in Figure 1). As a consequence of the presence of the Porod term, the contribution of the micelles in the low q-range is more difficult to determine.

’ RESULTS In a related paper,20 we have determined the structure of the micelles before the addition of the silica source. It appears that for P123, the temperature range to form micelles in solution is between 20 and 65 C, even in strong acidic conditions. Most interestingly, one observes that it is always necessary to perform the synthesis of the SBA-15 material in the same temperature range, meaning that the presence of the micelles in the starting solution is mandatory to be able to form the surfactant-templating material. Moreover, the structure of the micelles is always spherical before the addition of the silica source and is significantly modified by the nature of the anions coming from the acidic source. Following the Hofmeister series,15 the micelles are smaller and more polydisperse with salting-in anions (NO3 and Br) than with salting-out anions (SO42- and H2PO4). In addition, the contrast between the shell and the solvent is higher with the salting-in anions than with the salting-out ones, indicating that the salting-in anion concentration may be larger within the shell of the micelles than in the bulk solvent.20 In Figure 1, the main steps of the self-assembly discussed in this paper are depicted. At time t = 0, the inorganic precursor (TEOS) is added to the micellar solution. The first step is the hydrolysis of the precursor. 1. In Situ Raman: Kinetics of the TEOS Hydrolysis. Timeresolved in situ Raman spectroscopy was used to follow the hydrolysis kinetics of TEOS for the different acidic aqueous solutions of P123. This method is well-suited for time-resolved in situ studies of nanostructured silica materials synthesis in aqueous medium because of the fast response of the Raman signal, and because it does not detect the HOH angulardeformation vibration band of water.14 After the introduction of TEOS in the acidic P123 solution, hydrolysis starts, and the peaks of TEOS disappear and are replaced by the characteristic peaks of Ethanol (Figure 2a). The corresponding vibrational-mode assignments of the RamanStokes bands are given in Figure 2. Ethanol gives rise to three very strong peaks, located at 885, 1055, and 1095 cm1 and assigned to the CCO stretch [ν(CCO)], CO symmetric stretch [νs(CO)], and CO

asymmetric stretch [νas(CO)] modes, respectively. The integrated intensity of the most intense EtOH Raman peak at 885 cm1 as a function of time (Figure 2b) enables us to follow the kinetics of the hydrolysis of TEOS. Raman bands of P123 are not detected between 600 and 1200 cm1.25,26 Only in the case of H3PO4 and H2SO4, the presence of additional peaks, respectively, at 890 cm1 (P(OH)2 asymmetric stretching vibration)25 and 893 cm1 (asymmetric stretching vibration26 (νSOH) of HSO4) may slightly perturb the analysis. To check whether the hydrolysis was completed, a calibration measurement with an ethanol solution was performed under the same experimental conditions (laser intensity, probe position, temperature, stirring speed, etc.) for each solution. The ethanol concentration of this solution was taken to be four times the TEOS concentration, corresponding to the amount of ethanol produced by the complete hydrolysis of TEOS, producing four ethanol molecules per one TEOS. In Table 2, the different hydrolysis times are summarized. In all cases, the hydrolysis was completed within a few minutes. For each solution, measurements have been performed with and without P123 (Figure 2b) and the presence of P123 systematically reduces the hydrolysis time (Table 2). Without P123, the TEOS/water emulsion is only mechanically stabilized by the strong stirring of the mixture. In presence of P123, a part of the P123 molecules plays the role of a surfactant and helps to stabilize the TEOS emulsion, increasing the specific TEOS/water interface. As the hydrolysis reaction takes place at the TEOS/water interface, the increase of the specific surface of the TEOS droplets explains the increase of the hydrolysis rate upon P123 addition. Moreover, this specific interface has been estimated using the in situ SAXS measurements, as explained in the next section. 2. In Situ SAXS. Using in situ SAXS, the evolution of the scattered intensity upon time has been recorded and can be visualized either in a 2D-color map (Figure 3) or in a multiframe representation (Figure 4). Typical conditions of data recording were an acquisition time of 2 s for each curve and a time resolution of 30 s between two subsequent acquisitions. For experimental reasons, there is a dead time of one minute after TEOS addition before starting the SAXS data recording. A 2Dcolor map provides a very compact visualization of one kinetics experiment. The scattered intensity is plotted upon time (vertical axis) in the available q-range (horizontal axis). Increasing intensities are represented in the following order: blue, green, yellow and red. In such a map the different contributions to the SAXS signals are easily identified. For example, in the case of HCl TEOS 2 (Figure 3, bottom right), during the first 18 min, the signal is typical of a micellar solution with broad oscillations as a function of the scattering vector q in contrast to the sharp Bragg peaks that are observed afterward when a 2D-hexagonal material is formed. For instance, from the shift of the Bragg peaks in the map toward higher q-values, it can be directly observed that the lattice parameter is decreasing upon time. The multiframe representation for the same experiment is given in Figure 4. Selected individual intensity frames in absolute units (cm1) are given in Figure 4a using different shade of colors upon time. And the curves recorded at t = 0, 5, 15, and 30 min after TEOS addition with the corresponding fits (black dotted lines) are given in Figure 4b. It should be emphasized that all of the data from the different in situ SAXS experiments have been successfully modeled using the previously introduced intensity expressions as summarized for instance in Figure 5. Based on this modeling, we can analyze 11334



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Figure 2. Time-resolved Raman spectra at 40 C in HCl 1.6 M. (a) Time evolution of the spectra in the 6001220 cm1 wavenumber’s range. The corresponding vibrational-mode assignments of the RamanStokes bands of ethanol and TEOS are given on the left. (b) Comparison of intensities of the EtOH Raman peak at 885 cm1 as a function of time with and without P123. On the left, the intensity recorded for a calibration ethanol solution of concentration corresponding to a complete hydrolysis of TEOS (4 EtOH for 1 TEOS) is given.

Table 2. Comparison of the Hydrolysis Times Measured by in Situ Raman with and without P123 TEOS hydrolysis composition

TEOS hydrolysis

time without

time with

P123 (min)

P123 (min)

HNO3

3.4

1.5

HBr

5.3

2

HCl

6.5

2.8

H2SO4

11

6

H3PO4

4554

10

HCl 0.4 M

14

6.2

the influence of the different synthesis parameters on the kinetics (part 3 of the Results section). We will first describe the common features of all of the kinetics experiments obtained in the framework of this modeling step. First, before TEOS addition, the SAXS curves can always be modeled by spherical micelles formed by the P123 molecules.20 Just after TEOS addition, an additional signal with a 1/q4 slope in the low q-range (q < 0.01 Å1) is systematically detected as illustrated for example in Figure 4 for HCl 1.6 M TEOS 2 at 40 C. It is attributed to the Porod scattering by the sharp interface of the TEOS droplets. As derived from the Raman results, the complete hydrolysis of TEOS is performed only after a few minutes. Then, within this time, there is a contribution of the TEOS droplets interface to the scattering. The specific interface of the droplets Σ/V can be estimated from the value of the Porod constant K (see eq 16), and the electron density contrast between TEOS (ÆFæ) and the solvent (F0), which is between 30 and 40 e/nm3 for all of the different acidic solvents.20 A value of Σ/V = 100300 cm1 can be derived from K equals 1023 cm5, which is a typical value obtained after one minute of experiment. As the volume fraction of added TEOS is known, this corresponds to an average diameter of a few micrometers of the TEOS droplets (assuming a spherical shape), which is a reasonable order of magnitude. When the precipitation of the 2D-hexagonal material occurs, K is strongly increasing (see Figures 4 and 6a), just before the Bragg peaks at large angles are detected. In this later case, this increase reflects the growing of the precipitated grains and K is proportional to the specific area of the grains. The contrast contribution (ÆFæ F0) to K is more difficult to estimate for the material than for the TEOS droplets at

the beginning of the kinetics, because the exact amount of incorporated silica inside the hexagonal phase is difficult to quantify. The dependence of K upon time for different kinetics is shown in Figure 6a. The strong increase of K corresponds to the precipitation time in all cases. For the well-organized materials (HCl, H2SO4, and H3PO4), the specific surface of the grain is much higher than with HNO3 and HBr. It is only for H2SO4 and H3PO4 that the value of K decreases completely to zero after the end of the hydrolysis and before the precipitation of the material. In the other cases (including all synthesis in HCl), it may indicate the presence inside the synthesis mixture of some “flocs” without internal order,10,11 contributing only to the Porod term. Only for HNO3, a very weak isolated peak is transiently detected, as detailed in the following. Second, the background term is systematically increasing upon time before precipitation as shown in Figure 6b. This background is attributed mainly to the scattering by the silica oligomers that are forming during the silica condensation. Indeed, in Figure 6b, the comparison of the evolution upon time of the background term for several kinetics experiments is given. Synthesis performed with different strong acids but with the same quantity of TEOS (HNO3, HBr, HCl (= TEOS 1), and H2SO4) exhibit the same evolution of the background term (Figure 6b). For the three kinetics experiments performed in HCl with increasing quantities of TEOS in the ratio 0.5:1:2 (labeled as TEOS 0.5, TEOS 1, and TEOS 2 in Figure 6b), the “plateau” value of the background term is proportional to the quantity of added TEOS: B = 0.05 cm1 (TEOS 0.5 after one hour), B = 0.08 cm1 (TEOS 1 after 20 min), and B = 0.15 cm1 (TEOS 2 after 10 min; see eq 13 for the definition of B). As expected, in the case of the weak acid H3PO4, the background contribution is weaker and evolves more slowly upon time than in the other cases. In conclusion, the evolution of the background term appears to reflect quite accurately the condensation rate of silica upon time. Lastly, a similar global evolution upon time of the scattered intensity is observed for all experimental conditions. This evolution can be decomposed in four successive steps (spherical micelles, spheres-to-rods, rod-like micelles, and 2D-hexagonal material), and, as summarized in Figure 5, the duration of these steps has been established thanks to the modeling of the scattered intensity upon time. During the first step, the signal of the micelles is well modeled by polydisperse core/shell spherical micelles. Within this period, the TEOS hydrolysis is completed as determined from the in situ Raman experiments. It is only in the 11335



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Figure 3. In situ SAXS: Comparison of four different kinetics using 2D-color maps of the experimental intensities upon time: HNO3 (ESRF), HBr (SOLEIL), H2SO4 (ESRF), and HCl TEOS 2 (SOLEIL). In each 2D-plot the scattered intensity is plotted upon time (vertical axis) in the available qrange (horizontal axis). Increasing intensities are represented in the following order: blue, green, yellow, and red.

case of H3PO4 that the end of the TEOS hydrolysis is reached only a few minutes after the beginning of the spheres-to-rods step (Figure 5). Then, during the second step, a shape evolution toward rod-like micelles is taking place. This transition period is detected by the fact that the intensity curve can not be modeled satisfactorily neither by a spherical shape or a rod-like one. After this transition period, the third step is defined by the fact that the intensity curve is well described by the rod-like micelles model. For the last step, Bragg peaks are recorded as the 2D-hexagonal material is formed. As detailed below, two different behaviors are possible. As observed in Figure 3, when HNO3 and HBr are used, the intensity of the Bragg peaks is quite low, and they are observed in coexistence with a still strong signal of free micelles in solution until the end of the experiment. On the contrary, materials prepared with HCl, H2SO4 and H3PO4 exhibit after a short nucleation step only signal from the hybrid material with several well-defined resolutionlimited sharp peaks, as can be seen in Figures 3 and 4. For example, in the case of H2SO4 (Figure 3, bottom left), four well-defined Bragg peaks appear simultaneously after 25 min, and the signal of the hybrid micelles is vanishing at the same time. In agreement, the SEM observation of the grains (H2SO4, Figure 7) reveals typical elongated shapes with a well-defined internal 2D-hexagonal lattice. For these later materials, a detailed modeling linking upon time the structure of the free rod-like micelles to the ones inside the 2Dhexagonal material has been done (Figure 8), as detailed below.

3. Effect of the Different Synthesis Conditions. Materials Prepared in HCl: Effect of Temperature, HCl Concentration, and TEOS Quantity. HCl is the acid used for the standard synthesis

conditions of SBA-15 (see the Experimental Section for more details). Our original purpose was to understand why these wellknown standard synthesis conditions are optimum with respect to the formation of a well-ordered 2D-hexagonal material. As no systematic studies based on the modeling of in situ experiments are available in the literature, we investigated in this work various different synthesis conditions as summarized in Table 1. In HCl, the effects of the temperature and of the pH have been investigated using in situ SANS experiments. The influence of these two parameters on the precipitation time is shown in Table 1. As expected,8,12,13 the kinetics is faster at higher temperatures and higher HCl concentrations. As the global evolution is always similar to the one already determined for the standard synthesis conditions, no detailed modeling of these in situ SANS data is done. About the amount of added inorganic precursor (TEOS), a quite narrow range of P123:TEOS ratio is always used in the literature,3,5,7,8,10,11 because only these ratios lead to a wellordered 2D-hexagonal SBA-15 material. It is then interesting to investigate what happens for larger or smaller amounts of added TEOS and we compared three different ratio P123:TEOS by in situ SAXS (Table 1) in HCl 1.6 M at 40 C. Again, TEOS 1 corresponds to the standard synthesis conditions as TEOS 0.5 11336



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Figure 4. In situ SAXS for HCl TEOS 2 (same experiment as in Figure 3 on the bottom right). Precipitation of the 2D-hexagonal phase happens after 18 min (see Table 1). (a) Evolution upon time of the intensity in cm1 (loglog plot). Different shades of color are used upon time in the following order: red, green, yellow, blue, and magenta. (b) Comparison of the experimental curves with the fitted curves (black dotted lines) at four different reaction times: spherical micelles before TEOS addition (t = 0), spherical micelles after 5 min, cylindrical micelles after 15 min, and 2D-hexagonal material after 30 min. At 30 min, the black arrow indicates a small additional bump attributed to a small amount of disordered “wormlike type” material. In panel b, the curves are shifted vertically to avoid overlaps.

corresponds to a quantity of TEOS divided by 2 and TEOS 2 to a double amount of TEOS. The evolution of TEOS 1 is given in Figure 5, in comparison with the other acidic sources (see below). For TEOS 0.5, no precipitation was obtained after three hours and, most interestingly, instead of the usual precipitate of hybrid material, a macroscopic gel was formed in the reaction beaker after about 10 h, when stored at 40 C without stirring after the kinetics experiment. This preliminary visual observation (no further SAXS/SANS characterization of this gel could be performed) suggests that the precipitation phenomena takes place only for a sufficient quantity of added TEOS, between TEOS 0.5 and TEOS 1. And that in the case of TEOS 0.5, a different association between the micelles and the inorganic gel is taking place, leading to the formation of a macroscopic gel, instead of a phase separation (precipitation phenomena). Possibly, in this

case, the concentration of the inorganic species is too small to produce a strong enough attractive interaction between the hybrid micelles leading to the phase separation. For this reason, the hybrid micelles are not linked together via silica species and no hybrid material is obtained. Instead, upon condensation, a low density silica gel may form throughout the sample and the micelles probably do not act as a template. However, additional experiments should be performed to investigate this point. The results obtained with TEOS 2 are given in Figure 3 (bottom right) and Figure 4. Compared to the standard synthesis conditions (TEOS 1), the overall Bragg peak intensity is stronger, as more silica species are probably incorporated inside the hybrid material. But in addition, a small bump near to the left of the (10) Bragg peak can be seen in Figure 4 and is indicated by an arrow. This bump is interpreted by the presence of a small fraction of disordered “wormlike type” 11337



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Figure 5. Time evolution for the different acids obtained by modeling the in situ SAXS data using four successive steps: (1) Spherical micelles. (2) Spheres-to-rods transition. (3) Rod-like micelles. (4) 2D-hexagonal material in coexistence with rod-like micelles. Hydrolysis times derived from the in situ Raman experiments (Tables 1 and 2) are reported for comparison.

material. However, all the scattering curves are well modeled (black dotted curves in Figure 4), excepting this additional small bump. More precisely, for TEOS 2, the micelles are described using the spherical model during the first 10 min, and with the cylindrical model between 14 min and the precipitation time after 18 min. For the cylindrical model, the value of the interaction parameter νRPA derived from the modeling of the experimental data (like in Figure 4 after 15 min) is about 8, a value larger than in the case of the saltingin anions NO3 and Br (see below). In conclusion, the addition of a double amount of TEOS with respect to the standard synthesis conditions gives essentially a well-ordered 2D-hexagonal material, in coexistence with a small amount of disordered ‘worm-like’ material. It would also be interesting to determine what happens for even higher amounts of added TEOS. Materials Prepared with Different Acids: Hofmeister Series. As already mentioned, two groups of acids can be identified, HNO3 and HBr on one side (with the more salting-in anions), and H2SO4 and H3PO4 on the other side (with the more saltingout anions). The anion Cl is an intermediate case in the Hofmeister series and the results obtained in HCl have been already summarized in the previous section. Essentially, using HCl, well-ordered materials can be formed. We will now detail how the in situ SAXS results help to understand why the salting-in anions do not give well-ordered materials as the salting-out anions do. HNO3 and HBr. With these two acids, the SEM observations of the grains of the materials show large micrometer-size blocs (pictures not shown) and no well-defined shapes like with the salting-out anions (Figure 7). This can be understood to some extent from the in situ SAXS results. First, for both acids, the

contribution of the micelles to the scattering is always preponderant, even after Bragg peaks are observed (see Figure 3). Again, a complete modeling of these in situ SAXS allows the derivation of the following quantitative information. A few minutes after TEOS addition, the intensity is high enough to be fitted accurately by the form factor of the spherical model. Within the first 15 min, the form factor does not evolve much. The dimensions are significantly larger in HBr (Rcore = 4.62 nm, Rtot = 7.43 nm) than in HNO3 (Rcore = 3.9 nm, Rtot = 7.03 nm) with a relatively high polydispersity σ, which decreases upon time (from 25 to 15% in the case of HNO3). The TEOS addition induces a slight shrinkage of the core sizes: from Rcore = 4.5 to 3.9 nm for HNO3 and from Rcore = 5.3 to 4.62 nm for HBr. The contrast term α has similar values for both acids: α = 1.6 ( 0.1 for HNO3 and α = 2 ( 0.1 for HBr. Only in the case of HBr, this corresponds to a significant increase of the shell electron density levels, from 368 ( 4 to 396 ( 6 e/nm3. Then, discrimination between spherical and cylindrical models is not successful until the reaction proceeds to about 5 min before the appearance of the 2D-hexagonal Bragg peaks. At that moment, it becomes clear that the cylindrical model describes the form factor better than the spherical one. The cylindrical model used in this region is characterized by a strong influence of the interaction parameter νRPA (values between 2 and 5), as no 1/q behavior is observed on the experimental data in this region. The value of νRPA is slightly decreasing upon time (from 3.8 to 2.8 for HBr; from 5.3 to 3.8 for HNO3). The rod length value L is estimated between 60 and 90 nm. When the 2D-hexagonal material is formed, the main difference in the form factor is in the core radius, which is again 11338



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Figure 6. Comparison of the evolution upon time of two fit parameters (SAXS modeling results) for different kinetics: (a) The Porod constant K. (b) The background constant B.

significantly larger in HBr (Rcore = 3.45 nm) than in HNO3 (Rcore = 2.7 nm). For both acids, the Porod constant K is rather high before the appearance of the Bragg peaks (K = 0.09 cm5 for HNO3 after 2 min, K = 0.12 cm5 for HBr after 7 min, see Figure 6a). This indicates that some precipitate may be already in solution. In fact, in the case of HNO3, a very weak isolated diffraction peak at q = 0.053 Å1 appears already one minute and an half after TEOS addition, in coexistence with the dominating signal of the spherical micelles. Indeed, this acid has the shortest TEOS hydrolysis time (1.5 min; Table 2). This very weak peak moves to higher q values upon time (up to 0.06 Å1), but does not get stronger in intensity and disappears after 58 min. Because no higher-order peaks are associated, it is not possible to get more information about the type of order that is involved. However, after 36 min, in coexistence with this very weak peak, a set of two stronger Bragg peaks (0.056 and 0.096 Å1) in the ratio 1: 31/2 is growing upon time, which is typical of the precipitation of a 2Dhexagonal phase. Interestingly, upon time, as can be observed on zooming on the (10) Bragg peak region (figure not shown), the

hexagonal (10) peak splits into two separated peaks (59 min) and then into three (72 min), revealing the coexistence of different lattice spacing in the material at q10 = 0.053, 0.056, and 0.057 Å1. For HBr, no weak peak is observed just after the addition of TEOS, and only two Bragg peaks are observed after 36 min at 0.053 and 0.093 Å1, again in the ratio 1:31/2. These Bragg peaks are in coexistence with a still strong contribution to the scattering of free cylindrical micelles in solution and the proportion of these micelles is estimated to be as high as p = 0.6 at the end of the experiment. Upon time, as for HNO3, several domains with different lattice spacing contribute to the (10) peak. The specific behavior of HNO3 and HBr with respect to the other acids is dominated by a greater inhomogeneity inside the synthesis solution: High polydispersity of the spherical micelles (1525%), nucleation of grains of the 2D-hexagonal material with different lattice spacing, coexistence of these grains with a large proportion of free cylindrical micelles. Lastly, a small amount of an undefined phase may be formed, as indicated by the presence of a transient additional weak peak for HNO3 and by the plateau of the Porod constant for both acids. 11339



Figure 7. SEM observations at different magnifications of the as-made material prepared with H2SO4. The high degree of order of this material is revealed both by the strongly anisotropic morphology of the grains (a and b) and by the well-defined order inside the grains (c and d). The 2D-hexagonal packing of the cylindrical micelles surrounded by silica is observed both in a longitudinal view (c) and in a transverse view (d).

H2SO4 and H3PO4. These two salting-out anions give the most ordered materials. The SEM observations of the grains are similar for both acids. SEM pictures of the as-made material prepared with H2SO4 (Figure 7) reveal typical elongated shapes with a well-defined internal 2D-hexagonal lattice. Moreover, the grains often show curved shapes (see Figure 7b,c for example). This nicely illustrates the fact that the nanostructured material keeps the memory of the first state of the precipitated material inside the solution, which is rather a 2D-hexagonal liquid-crystalline state than an already rigid form. The curved shapes are then explained by a low bending elasticity constant of the liquidcrystalline phase.27 Both acids show a very similar behavior during the kinetics, except that, because the pH equals 0.9 in the case of H3PO4, as it is a weak acid, the kinetics is slower than for H2SO4 (see Figures 5 and 6). Otherwise, all the results obtained are quantitatively very close. This comes with the fact that, before TEOS addition, the micelles have almost the same structure.20 Compared to the salting-in anions (see before), the overall dimensions (Rcore = 5.6 nm, Rtot = 9.4 nm, H2SO4) are larger, the polydispersity (σ = 9%, H2SO4) is lower and the contrast between the shell and the solvent much less pronounced. Upon time, the different parameters used in the model exhibit very similar evolutions for H3PO4 and H2SO4, in terms of micelles dimensions, contrast, nucleation/growth of the material. For H2SO4 (SOLEIL experiment), the evolution of several parameters obtained from the modeling is reported in Figure 8. Within the first ten minutes, the micelles are still spherical and the only evolution is the decrease of the contrast parameter α. Making the assumption that both the core and the solvent electron density levels are still constant upon time (Figure 8b), the decrease in α corresponds to an increase of the shell level, that is attributed to the presence of some silica oligomers inside the shell. Recalling that the hydrolysis time is 6 min for this kinetics, one can then conclude that, already after a few minutes, some silica species migrate to the surface of the micelles and interact with the EO groups. Then, the

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shape of the micelles evolves toward a rod-like shape. During this transition period, no fits of the data are performed, as probably a mixture of shapes (spherical and cylindrical ones) is involved. Nevertheless after 18 min, the results are well interpreted using the model of long coreshell cylindrical micelles (see theoretical part for more details), with an L value of more than 100 nm. Moreover, the anisotropy of the micelles is directly evidenced by the anisotropy of the 2D scattering pattern obtained on the 2D CCD detector (Figure 9). It is because the cylindrical micelles are long enough to get aligned by the flow inside the thin capillary (2 mm in diameter) during the experiment. Then, the 2D-pattern observed in Figure 9 reveals the preferred orientation of the cylindrical micelles with their long axis along the flow. Assuming that the interaction term between the micelles can be neglected (S(q)=1), the scattered intensity results in the averaging of the form factor contribution over the different orientations in the flow (see the SI, Section B for more details). Between 18 and 50 min, the main evolution of the form factor of the cylindrical micelles is observed for the contrast parameter α, implying a significant increase of the electron density inside the shell. Again, this variation of the shell density upon time is interpreted by an increasing quantity of silica species linked to the micelles. An estimation of the amount of silica within the shell is given in the Discussion. Concomitantly, the interaction parameter νRPA is decreasing from 2 to about 0. The value obtained for the length L is estimated to be about 100 to 300 nm, however, because of the cutoff at small angles in the accessible scattering vector range, it is in fact difficult to get a precise value for this parameter, and its value is fixed at 100 nm when the Bragg peaks are present. The Bragg peaks are first observed after 50 min and are in coexistence with the signal of free micelles until 65 min. After this, the signal is satisfactorily described only by the contribution of the 2Dhexagonal phase. Within the transition period (50 to 65 min), the same form factor describes both the free micelles still in solution and the micelles inside the 2D-hexagonal lattice and the parameter p (proportion of the free micelles in solution) evolves from 0.8 to 0 after 65 min. The increase of the Porod constant K associated to the grain formation starts a few minutes earlier, after already 40 min (Figure 6), indicating that the nucleation of the grains may have already started at that time, before the first detection of the Bragg peaks. In fact, a sufficient volume fraction of material is needed to get a measurable signal for these peaks. And, during the nucleation step, the average grain size D (lateral dimension derived from the width of the Bragg peaks) may be too small to allow the peak detection. After 50 min, the value of D is increasing upon time, as the grains are growing in size. Note that the values of D are not corrected for the experimental resolution, since it is expected that the effects are small due to the low divergence of the X-ray beam and the high resolution of the CCD detector. At the same time, the DebyeWaller disorder term σDW and the lattice parameter a are decreasing, indicating a consolidation of the packing inside the material, in agreement with previous results.5,7,8 Lastly, a/2 and Rtot have very close values (taking into account σcorona, the spread of the outer interface), which means that, at this early stage, the 2D-hexagonal mesophase consists in the close packing of the hybrid micelles. Earlier results5,7 have shown that it is in fact only after one hour or more after precipitation that a continuous inorganic wall is really formed in the material in-between the micelles. In conclusion, the modeling of the in situ SAXS allows describing in a very precise and quantitative way the formation of the well-organized materials. First, before the nucleation of the 11340



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Figure 8. Evolution upon time of the main fit parameters for the kinetics experiment using H2SO4 (SOLEIL). The global time evolution of this experiment is also given in Figure 5. (a) Rcore, Rtot, and σcorona compared with a/2, where a is the lattice parameter. (b and c) The contrast parameter α and the electron density levels. (d and e) the length L and the interaction parameter νRPA of the cylindrical micelles. (f) The proportion p (0 < p < 1) of the “free” cylindrical micelles in solution. (g and h) The average domain size of the grains D and the DebyeWaller coefficient σDW of the 2D-hexagonal phase.

material, elongated hybrid micelles are forming inside the solution and are characterized by a small polydispersity and an

increasing quantity of silica species interacting with these micelles upon time. Then, the nucleation of a 2D-hexagonal phase is 11341



Figure 9. 2D SAXS pattern for the kinetics experiment using H3PO4 (ESRF) observed one hour before the precipitation of the 2D-hexagonal phase (same experiment as in Figure 3 on the bottom left). The anisotropy of this pattern comes from the alignment of the cylindrical micelles inside the flow cell.

well described by the packing of these cylindrical hybrid micelles (same form factor) in a 2D-hexagonal lattice. After a short transition period (parameter p decreasing to 0), the signal of the free micelles is no more detectable, showing that almost all the micelles are used to nucleate the material. The lattice parameter a of the 2D-hexagonal phase has one well-defined value, this fact being probably related to the small polydipersity of the micelles initially formed in the solution. Lastly, during the beginning of the growth period, both the DebyeWaller term σDW and the lattice parameter a are decreasing, in agreement with a consolidation of the packing of the micelles within the grains.

’ DISCUSSION In this paper, we show that in situ experiments are powerful experimental tools to follow both the kinetics of the TEOS hydrolysis (in situ Raman) and the kinetics of the silica condensation (in situ SAXS, background contribution). By combining these two types of experiments, new information is obtained. First we establish that the TEOS hydrolysis is completed within a few minutes and during the first step when the micelles are still spherical. The silica condensation begins as soon as some TEOS is hydrolyzed in the solution. In fact, an evolution of the form factor of the spherical micelles is already observed within the first minutes and can be interpreted by the presence of a very small amount of silica species in the shell region of the micelles already during this early step. Another new result is that in situ SAXS allows following in a simple and direct way the silica condensation rate via the time evolution of the background contribution. Indeed, the experimental results (Figure 6) show that the value of this background term appears to be proportional to the overall amount of condensed silica species inside the reaction medium. The sphere-to-rod shape transformation of the micelles is always observed after the TEOS hydrolysis is completed, and probably after the silica condensation has reached some critical value. Our study confirms that, as reported previously,79,12,13 the sphereto-rod transition is attributed to the formation of hybrid micelles driven by the condensation of the silica. An open question is to determine the amount of silica within the hybrid micelles. Thanks to our approach based on a simple modeling by a coreshell form factor, we can provide an estimation of the amount of silica oligomers within the shell based on the value of the contrast parameter α. The regular increase of this parameter

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α upon time is interpreted by an increasing amount of silica oligomers within the shell (Figure 8c). For example, in the case of H2SO4, we can estimate the composition of the shell for the hybrid cylindrical micelles in solution just before precipitation, assuming that the volume fraction of EO is not varying (EO volume fraction of 10%). As the electron density of the shell equals 375 ( 10 e/nm3, it corresponds to a composition of the shell of about 8% vol. fraction of silica, 10% vol. fraction of EO groups and 82% of solvent (assuming a value of 660 e/nm3 for the silica species, 387 e/nm3 for EO and 346 e/nm3 for the solvent). In other units, it corresponds to a molar ratio Si:EO = 1.1 ( 0.4 of Si atoms per EO group. This value compares well to the overall molar ratio in the solution Si:EO = 1.5, meaning that in this case, most of the silica atoms are inside the hybrid micelles. This quantity is important because it corresponds to the critical value of silica within the hybrid micelles above which precipitation occurs. To our knowledge, only in situ SAXS experiments are able to provide this type of information. The comparison of the materials prepared with different acids reveals how counterions strongly influence the cooperative selfassembly process. We are not aware of comparable data based on in situ scattering experiments in the literature. We have already shown that the counterion has a significant effect on the structure of the micelles themselves before TEOS addition,20 as salting-in anions are expected to interact more strongly with the micelles than the salting-out ones.15 Qualitatively, the salting-in anions (NO3 and Br) are more strongly bond to the micelle interface and are competing or hindering the interaction of the silica species with the micelles. In contrast, the other anions (Cl, SO42-, and H2PO4) do not bind specifically to the micelles, allowing the silica species to interact with the micelles. What the in situ experiments mainly reveal is the very high homogeneity of the materials prepared in the presence of salting-out anions (Cl, SO42- and H2PO4), as compared with the ones prepared in the presence of salting-in anions (NO3 and Br). In this latter case, the polydispersity of the starting spherical micelles is quite large (1525%), grains nucleate with several distinct values of the 2Dhexagonal lattice spacing a, and these grains coexist with a large proportion of free cylindrical micelles. These features indicate that the polydispersity of the micelles could directly influence the order quality inside the final material. Apparently, when the polydispersity of the micelles is too large (more than 15%), as with HNO3 and HBr, it induces in the material a distribution of the lattice parameter value a that prevents a well-developed longrange order inside the grains. The fact that an excess of micelles is observed in coexistence with the hybrid material at the end of the kinetics could mean that only a fraction of the micelles is used to nucleate the hybrid material. But, because of the limited time window of the in situ SAXS experiments, it may be rather due to the fact that the nucleation proceeds during a longer period of time (more than one hour). And this again will favor a lot of inhomogeneity inside the hybrid material. In contrast, in the presence of salting-out anions, highly homogeneous hybrid materials are formed, because they nucleate within a short period of time, using almost all the hybrid micelles present in the solution, which are characterized by a small polydispersity in size (less than 5%). Consequently, the hybrid material presents only one well-defined lattice spacing a, in accordance with the small polydispersity of the free hybrid micelles. In conclusion, the important role of the nature of the counterion in the selfassembly is confirmed. Interestingly, a similar trend following the Hofmeister series has been recently reported on the morphology of 11342


The Journal of Physical Chemistry B related SBA-15 materials prepared at much higher pH values than in this study.28

’ CONCLUSION In this paper, we use a complete model to quantitatively analyze the formation of hybrid nanostructured materials by in situ SAXS/SANS experiments during the early stages of the precipitation in solution. Focusing on the SBA-15 material, this model allows successfully establishing the relations between the micelles and the 2D-hexagonal material throughout the selfassembly process. This model could be generalized to many other materials, varying either the nature of the surfactant used as structure-directing agent or the inorganic source. Then, it will be interesting to investigate, for example, whether the mechanism determined for SBA-15 extends to other 2D-hexagonal materials or not. Another less explored field is the case of materials with other types of architecture, like 3D cubic structures. Indeed, for these later materials, an additional mechanism is expected to play a role, involving internal rearrangements upon time of the structural units inside the hybrid material. Ultimately, a deeper comprehension of the different self-assembly mechanisms will help to finely tune the properties of the materials to a desired specific application. ’ ASSOCIATED CONTENT

bS

Supporting Information. Models for SAXS/SANS. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: + 33 1 69 15 60 59. Fax: +33 1 69 15 60 86. E-mail: marianne. [email protected]. Present Addresses z

Instituto de Física, Universidade de S~ao Paulo, Caixa Postal 66318, 05314-970 S~ao Paulo, Brasil.

’ ACKNOWLEDGMENT The authors thank the C0 Nano IDF for the postdoc support of S.M., and the ILL, the CRG-D2AM beamline of the ESRF and the SOLEIL synchrotron for beam-time allocation. The support from the Danish Research Foundation and the Danish Council for Independent Research, Natural Sciences is gratefully acknowledged. ’ REFERENCES (1) Omer, L.; Ruthstein, S.; Goldfarb, D.; Talmon, Y. High-Resolution Cryogenic-Electron Microscopy Reveals Details of a Hexagonal-toBicontinuous Cubic Phase Transition in Mesoporous Silica Synthesis. J. Am. Chem. Soc. 2009, 131 (34), 12466–12473. (2) Gov, N.; Borukhov, I.; Goldfarb, D. Morphological transitions during the formation of templated mesoporous materials: Theoretical modeling. Langmuir 2006, 22 (2), 605–614. Langmuir 2006, 22 (9), 4456. (3) Ruthstein, S.; Schmidt, J.; Kesselman, E.; Talmon, Y.; Goldfarb, D. Resolving intermediate solution structures during the formation of mesoporous SBA-15. J. Am. Chem. Soc. 2006, 128 (10), 3366–3374.

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Kinetics of the Formation of 2D-Hexagonal Silica

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