Mesoporous materials-

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Mesoporous materials

Outline:

Introduction to meso-porous materials Synthesis of mesoporous materials Formation mechanism, controlling properties


Naturally occuring porous materials

diatoms


Opals

Bone Lungs


Zeolites – naturally occurring crystalline aluminosilicates Axel Fredric Cronstedt, 1756, stilbite


Synthetic porous inorganic materials

- Zeolites - Mesoporous materials - MOFs (metalorganic framework)


IUPAC:

Microporous: < 2nm Mesoporous: 2-50nm Macroporous: > 50nm


Mesoporous materials Pores with diameters 20-500Å High surface area ~ 800-1000m2/g Ordered or disordered structures (but amorphous) Various chemical compositions Organic materials function as structure promoters


Huge surface area: 1000 m2/g



Structure Mesoscopic scale ca 50 Å pores organised long range Atomic scale ca 2Å Atoms - short range order, no long range order

By use of the present techniques, various mesoporoussilicates have been synthesized with different symmetries(Figure 13), or the same mesostructures with different cellparameters and pore sizes have been obtained, indicating thecontrollable synthesis on will.

4.1.2. 2D Mesostructures

2D mesostructured materials with hexagonal symmetry aremost easily produced, the classical products being MCM-41, FSM-16, SBA-3, SBA-15, etc. (Table 3). The idealmodels for these structures are hexagonally close packedcylindrical pore channels belonging to thep6mm space group.Typical TEM images can show two features: hexagonalstructures along the channel system and parallel stripes ifviewed perpendicular to the channel directions. Figure 14Ashows the hexagonally symmetric pore arrays.

MCM-41 is the simplest and most extensively investigatedmesoporous silica molecular sieve. It can be synthesized ina wide range of conditions, with the most popular synthesisusing CTAB as a SDA in a basic solution. The cell parameterof MCM-41 (!4.0 nm) can be easily obtained from XRDand TEM analysis. The pore channels in MCM-41 are oftensimply approximated as cylinders,230 although at least threekinds of shapes have been proposed. The other two modelsare the hexagonal prism and a so-called “cucurbit”, whichcan be envisaged as a string of connected spherical cagesalong the [001] direction.231,232 Typical isotherms of MCM-41 show no obvious hysteresis loop. The pore wall thicknessis estimated to be about 1 nm, and the Brunauer-Emmett-Teller (BET) surface area is generally higher than 1000 m2/g. In addition, micropores are not detected.

The second important 2D hexagonal mesostructure isSBA-15, which is normally synthesized using PEO-PPO-PEO triblock copolymer as a SDA under acidic conditions.The optimal template is triblock copolymer P123. Compared

with the synthetic system for MCM-41 involving a cationicsurfactant, the concentration of triblock copolymer is higher.SBA-15 materials prepared from P123 at 40-100 °C haveuniform pore sizes from!6.5 to 10 nm. The pore walls rangefrom 3.1 to 4.8 nm in thickness, much thicker than that ofMCM-41, which result in higher thermal stability andhydrothermal stability. SBA-15 with small pore sizes canbe templated by nonionic oligomeric surfactants, for example,Brij 56 (C16H33EO10), in acidic solutions after hydrothermaltreatment at 100 °C.31,78

Another feature of SBA-15 is the disordered microporesystem in the silicate walls. Micropores in SBA-15 meso-structure were noticed by Lukens et al.233 in 1999, althoughit was known from the pore expending phenomena by thehydrothermal treatment when the initial discovery of SBA-15 in 1998. However, the quantitative analysis and the reasonwere not given. Other experimental results reveal that thewall structure of SBA-15 is quite different from that ofMCM-41, although the two materials have the same spacegroup of p6mm (Figure 13A). Connections exist between themesopore channels of SBA-15.233,234 Evidence for this isobtained by the fact that metal oxides, metal sulfides,carbons, and even metal nanowire or nanorod arrays castfrom SBA-15 mesoporous silicas can retain the ordered 2Dhexagonal mesostructure.235 The final nanowire or nanorodarrangements are connected and supported by smaller nano-rod pillars. Only separated nanowires are obtained if MCM-41 is used as a hard template.236 Quantitative XRD analysisof the SBA-15 diffraction pattern supports the model ofhexagonal pore channels, surrounded by a corona of mi-cropores.237

It is found that the microporosity of SBA-15 can becontrolled by the treating method.238,239 Complementaryporosity of SBA-15 can be retained to a significant extenteven after calcination at 900 °C, but probably completely

Figure 13. Pore models of mesostructures with symmetries of (A) p6mm, (B) Ia3hd, (C) Pm3hn, (D) Im3hm, (E) Fd3hm, and (F) Fm3hm.Reprinted with permission from refs 73, 98, 218, and 219. Copyright 2000 Nature Publishing Group and Copyright 2002, 2004, and 2006American Chemical Society.

Controllable Soft-Templating Approach Chemical Reviews, 2007, Vol. 107, No. 7 2841

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

100

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TEM

SAXS/SAXD

N2-sorption


X-ray diffraction gives fingerprint of the structure Small angle region!


Transmission electron microscopy gives direct images – structures can be determined


Nitrogen sorption gives information on pore geometry, pore size and surface area.


Carrier for catalysts



Nanocasting: using the silica as a hard template


STRATEGY TO SYNTHESIS POROUS MATERIALS


How are porous materials “constructed”? Complex synthesis:

Aqueous solution with silica precursor, acid or base catalysed Strategy:

Use a “template” that can later be removed



Building materials using templates

Put stones around the template


Building completed - remove the template


Segovia, Spain


Molecular building Stones –

“silica molecules” Templates –

molecules that can be removed


“Cement” – covalent bonds


Pores templated by single molecules

Zeolites microporous (10Å) crystalline (order on atomic scale)


Zeolites Crystalline (order on atomic scale) Synthesis in aqueous solution

template silica precursor

Removal of template - porous materials


Single molecule template

Template


Amphiphiles

Hydrophobic tail Hydrophilic head


I)  Attraction – induces association of molecules Decreases interfacial area per molecule in contact with water

II) Repulsion of head-group

Increases interfacial area per molecule in contact with water

TWO OPPOSING FORCES


throughout the whole systemwhich, of course, must be even larger. And “time” also playsa role here: the properties at any locationmust be averaged over a sufficiently long time toavoid any bias from fluctuations.

Genuine two-phase and three-phase systems can also occur, where monomers,micelles, vesicles, or liposomes separate out into distinct phases in equilibrium with eachother while separated by a single meniscus or phase boundary. However, such phaseseparations can take a long time to reach equilibrium, so that it is often difficult toexperimentally identify the true thermodynamic state of an amphiphilic system.

20.2 Optimal Headgroup AreaThe major forces that govern the self-assembly of amphiphiles into well-defined struc-tures such as micelles and bilayers, as well as three-dimensional networks, derive fromthe hydrophobic attraction at the hydrocarbon-water interface, which induces themolecules to associate, and the hydrophilic, ionic, or steric repulsion of the headgroups,which imposes the opposite requirement that they remain in contact with water. Thesetwo interactions compete to give rise to the idea of two “opposing forces” (Tanford, 1980)acting mainly in the interfacial region: the one tending to decrease and the other tendingto increase the interfacial area a permolecule exposed to the aqueous phase (Figure 20.1).

The attractive interaction arises mainly from the hydrophobic or interfacial tensionforces which act at the fluid hydrocarbon-water interface. This interaction may be

Headgrouparea,

Volume,v

Headgroup (hydrophilic)repulsion

Interfacial(hydrophobic)

attraction

Interchainrepulsion

D

Packing parameteror factor v c

cRadius R

FIGURE 20.1 The hydrocarbon interiors in both micelles and bilayers are normally in the fluid state at roomtemperature (see Table 20.1). Repulsive headgroup forces and attractive hydrophobic interfacial forces determine theoptimum headgroup area a0 at which mo

N is a minimum (see Figure 20.2). The chain volume v and chain length ‘c setlimits on how the fluid chains can pack together, on average, inside an aggregate. Thus, the preferred molecularconformation depends on a0, v, and ‘c. In stressed micelles or bilayers, the headgroup area a is larger or smaller thana0. Such stresses can come from compressing a monolayer or bilayer either normally or laterally, stretching it, orbending it.

536 INTERMOLECULAR AND SURFACE FORCES

Supramolecular (Micellar aggregates) template


As-synthesised, composite Template removed, porous


Kresge et al, Nature 1992


Publications - mesoporous

0

500

1000

1500

2000

2500

2010200019901980197019601950

Number of publications per year


1. Kresge, Leonowicz, Roth, Vartuli, Beck. Nature 359 (1992) 710, 2. Inagaki, Fukushima, Kuroda J. Chem. Soc. Chem. Commun. (1993) 680

Pioneering work (1992-93):

• FSM - materials2 2 D hexagonal

• M41S - materials1 MCM-41 - 2D hexagonal MCM-48 - cubic(Ia3d) MCM-50 - lamellar

MCM-41

MCM-50

MCM-48


Examples of synthesis systems.

1)  Cationic 2)  Non-‐ionic triblock copolymer 3)  Anionic+costructure directing 4)  Organoalkoxysilanes


”Classic” MCM-‐41, MCM-‐48, MCM-‐50, FSM-‐structures

strongly anisotropic structures are formed, mostly inthe shape of fibers, but also as ribbons, platelets, orcylinders.115-118

Several families of organogelators exist; however,the physical-chemical phenomena leading to thegelation of organic liquid phases is not yet wellunderstood, and most organogelators have been ser-endipitously discovered. These molecules are mostlyclassified according to the main forces present in thegel formation step, although different interactionscan be responsible for the organization at the su-pramolecular level: H-bonding, van der Waals forces,dipole-dipole interactions, charge transfer, electro-static interactions, coordination bonds, etc.

Figure 11 presents some typical examples of orga-nogelators belonging to the different families: ca-pable of performing H-bonding, based on a steroidalor organometallic skeleton, or other molecules suchas phthalocyanines or 2,3-bis-n-decyloxyanthracene(DDOA).

Recent AFM studies demonstrated the role of thesolvent in the formation of fibrous organogels, basedin cholesterol derivatives. These gels are composedby fibers imprisoning !30% of the solvent molecules;every organogelator molecule is thus able to “fix”

!103 solvent molecules. The rest of the solvent isplaced between these fibers, bearing a weak interac-tion with the organogelators; this “weakly bonded”solvent may also be a cosolvent, not capable offorming gels.118

Organogels are already being used in the photo-graphic, cosmetic, oil, and food industries. Being ableto reversibly form fibrous networks, with well-definedgeometry and shape, they have been recently usedas templates for the synthesis of nano- and micro-structured materials, as will be shown below.Polymeric Templates. Dendrimers.119-121 Den-

drimers are macromolecules composed of monomersthat are associated in a fractal-like manner arounda multifunctional central core. Two synthesis ap-proaches (convergent or divergent) have been de-scribed. After successive reactions, an nth-generationpolymer (Figure 12) is obtained, resulting in a highlybranched arrangement of functionalized chains ofoverall spherical shape. The terminal functions in theperiphery can be adequately tailored, as well as thenature of the inner cavities, closer to the dendrimercore.

Dendrimers are polymers of very well-definedstructure, isomolecular and multifunctional, present-ing characteristic solubility, viscosity, and thermalstability. This high structural definition, associatedwith their flexibility in size and functions, makesdendrimers a very promising template for the syn-thesis of novel materials. The most interesting stud-ies should aim to the synthesis of new hybrid

Figure 10. Schematic representation of the different typesof silica-surfactant interfaces. S represents the surfactantmolecule and I, the inorganic framework. M+ and X-

represent the corresponding counterions. Solvent moleculesare not shown, except for the I0S0 case (triangles); dashedlines correspond to H-bonding interactions. For a detailedexplanation, refer to the text.

Table 4. Examples of Mesostructured InorganicMaterials Showing Different Interactions betweenthe Surfactant and the Inorganic Framework

surfactanttype

interactiontype

example materials(structure)a ref

cationic S+ S+I- silica: MCM-41 (hex) 37MCM-48 (cub) 37MCM-50 37tungsten oxide (lam, hex) 33, 103Sb oxide (V) (lam, hex, cub) 33tin sulfur (lam) 33, 104aluminum phosphate

(lam, hex)105, 106

S+X-I+ silica: SBA-1 (cub Pm3a) 33SBA-2 (hex 3D) 33, 94SBA-3 (hex) 33zinc phosphate (lam) 33zirconium oxide (lam, hex) 108titanium dioxide (hex) 283

S+F-I0 silica (hex) 102

anionic S- S-I+ Mg, Al, Ga, Mn, Fe, Co, Ni,Zn (lam) oxides

33

lead oxide (lam, hex) 33aluminum oxide (hex) 109tin oxide (hex) 110titanium oxide (hex) 111

S-M+I- zinc oxide (lam) 33alumina (lam) 33

neutral S0 S0I0 silica: HMS (hex) 99or N0 N0I0 MSU-X (hex) 100

silica (lam, cub, hex) 112Ti, Al, Zr, Sn (hex) oxides 100, 110

N0X-I+ silica: SBA-15 (hex) 107N0F-I+ silica (Hex) 102(N0Mn+)I0 silica (Hex) 148S-Me(OEt) Nb, Ta (hex) oxide 113, 114

a hex, hexagonal; lam, lamellar; cub, cubic.

4104 Chemical Reviews, 2002, Vol. 102, No. 11 Soler-Illia et al.

Hydrophilic

Hydrophobic

Cethyltrimethyl ammouniumchloride + TEOS (kanemite) – basic conditions

1) Cationic (Mobil, Kuroda)


hydrophobic

hydrophilic hydrophilic

2.) Non-ionic block copolymer - Pluronic polymer SBA-15

(EO)x-‐(PO)y-‐(EO)x

SiO2


SBA-15 SiO2 2D hexagonal

By use of the present techniques, various mesoporoussilicates have been synthesized with different symmetries(Figure 13), or the same mesostructures with different cellparameters and pore sizes have been obtained, indicating thecontrollable synthesis on will.

4.1.2. 2D Mesostructures

2D mesostructured materials with hexagonal symmetry aremost easily produced, the classical products being MCM-41, FSM-16, SBA-3, SBA-15, etc. (Table 3). The idealmodels for these structures are hexagonally close packedcylindrical pore channels belonging to thep6mm space group.Typical TEM images can show two features: hexagonalstructures along the channel system and parallel stripes ifviewed perpendicular to the channel directions. Figure 14Ashows the hexagonally symmetric pore arrays.

MCM-41 is the simplest and most extensively investigatedmesoporous silica molecular sieve. It can be synthesized ina wide range of conditions, with the most popular synthesisusing CTAB as a SDA in a basic solution. The cell parameterof MCM-41 (!4.0 nm) can be easily obtained from XRDand TEM analysis. The pore channels in MCM-41 are oftensimply approximated as cylinders,230 although at least threekinds of shapes have been proposed. The other two modelsare the hexagonal prism and a so-called “cucurbit”, whichcan be envisaged as a string of connected spherical cagesalong the [001] direction.231,232 Typical isotherms of MCM-41 show no obvious hysteresis loop. The pore wall thicknessis estimated to be about 1 nm, and the Brunauer-Emmett-Teller (BET) surface area is generally higher than 1000 m2/g. In addition, micropores are not detected.

The second important 2D hexagonal mesostructure isSBA-15, which is normally synthesized using PEO-PPO-PEO triblock copolymer as a SDA under acidic conditions.The optimal template is triblock copolymer P123. Compared

with the synthetic system for MCM-41 involving a cationicsurfactant, the concentration of triblock copolymer is higher.SBA-15 materials prepared from P123 at 40-100 °C haveuniform pore sizes from!6.5 to 10 nm. The pore walls rangefrom 3.1 to 4.8 nm in thickness, much thicker than that ofMCM-41, which result in higher thermal stability andhydrothermal stability. SBA-15 with small pore sizes canbe templated by nonionic oligomeric surfactants, for example,Brij 56 (C16H33EO10), in acidic solutions after hydrothermaltreatment at 100 °C.31,78

Another feature of SBA-15 is the disordered microporesystem in the silicate walls. Micropores in SBA-15 meso-structure were noticed by Lukens et al.233 in 1999, althoughit was known from the pore expending phenomena by thehydrothermal treatment when the initial discovery of SBA-15 in 1998. However, the quantitative analysis and the reasonwere not given. Other experimental results reveal that thewall structure of SBA-15 is quite different from that ofMCM-41, although the two materials have the same spacegroup of p6mm (Figure 13A). Connections exist between themesopore channels of SBA-15.233,234 Evidence for this isobtained by the fact that metal oxides, metal sulfides,carbons, and even metal nanowire or nanorod arrays castfrom SBA-15 mesoporous silicas can retain the ordered 2Dhexagonal mesostructure.235 The final nanowire or nanorodarrangements are connected and supported by smaller nano-rod pillars. Only separated nanowires are obtained if MCM-41 is used as a hard template.236 Quantitative XRD analysisof the SBA-15 diffraction pattern supports the model ofhexagonal pore channels, surrounded by a corona of mi-cropores.237

It is found that the microporosity of SBA-15 can becontrolled by the treating method.238,239 Complementaryporosity of SBA-15 can be retained to a significant extenteven after calcination at 900 °C, but probably completely

Figure 13. Pore models of mesostructures with symmetries of (A) p6mm, (B) Ia3hd, (C) Pm3hn, (D) Im3hm, (E) Fd3hm, and (F) Fm3hm.Reprinted with permission from refs 73, 98, 218, and 219. Copyright 2000 Nature Publishing Group and Copyright 2002, 2004, and 2006American Chemical Society.

Controllable Soft-Templating Approach Chemical Reviews, 2007, Vol. 107, No. 7 2841

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Structure Repeat 10nm

Particle morphology Hexagonal prisms Size 300nm-1μm


SEM TEM


Research on porous materials Applications – Materials – Fundamental Novel applications, improved performance Improving properties – compositions, structures, morphology, functionalization, tuning of pore size. Relating function to properties. Calculation of structures, synthesizing structures, synthesizing compositions, tuning properties for applications Understanding formation, driving forces, molecular interactions, characterization techniques (new models).


”Classic” SBA-15

mesoporous silica with large pore volumes, high surfaceareas, and, most important of all, abundant silanols serve asideal hosts for nanocasting replica mesostructures. Highlyordered metal oxide nanowire arrays can be replicated withvarious compositions including Cr2O3, MnxOy, Fe2O3, Co3O4,NiO, CeO2, and In2O3.164 This method can be widely appliedin porous materials, for example, zeolites and macroporoussilicates templated by polystyrene (PS) nanospheres, butexcept for those that are either easily oxidized or sensitiveto acids, such as organic-containing frameworks and meso-porous titania.

3.2. Nonaqueous Synthesis

Nonaqueous synthesis is a very convenient method toprepare ordered mesoporous materials especially for meso-porous thin films, membranes, monoliths, and spheres. Thismethod has become more and more powerful. Most of thesyntheses conducted in the nonaqueous media adopt the well-known EISA process.33 For the preparation of mesostructuredsilica films, TEOS is dissolved in the organic solvent(normally ethanol, THF, and acetonitrile) and prehydrolyzedwith stoichiometric quantities of water (catalyzed by acids,such as HCl) at a temperature of 25-70 °C. Then low-polymerized silicate species can randomly assemble withsurfactants. Upon solvent evaporation, the silicate speciesfurther polymerize and condense around the surfactants. Thepolymerization rate is gradually increased due to the increas-

ing acid concentration during the solvent evaporation.Simultaneously, templating assembly in the concentratedsurfactant solution occurs, resulting in the formation ofordered mesostructures. The process is very fast and needsonly several seconds.120

Mostly solvents with weak polarity are used. Surfactantslose the hydrophilic/hydrophobic properties in the weak-polarity solvents because both hydrophilic and hydrophobicsegments can interact with these solvents. The surfactant self-assembly would be inhibited. However, the assembly canbe induced upon the solvent evaporation. Nonpolar and oilysolvents are seldom adopted. In toluene or xylene solution,silica nanowires with adjustable diameters were synthesizedwith P123 and F127 (EO106PO70EO106) by the EISA ap-proach.165 The formation of this kind of arrays correspondsto the reversed mesophases of surfactants in oily solvents.Hollow sphere silicates can also be obtained by tuning theratios of oil/water.166 The synthetic conditions are, however,quite strict. In addition, the possible products include silicamesostructures, reversed mesostructures, and their mixturesbecause a little water (sometimes from wet air) incorporatesin the process upon the evaporation of the oily solvent andthe reversed micelles turn back.

Relatively wide diffraction peaks at 2! of 3-5° aredetected in the XRD patterns of the SBA-15 samplesprepared by using P123 as a template from the EISA method.Apparently, the mesostructure regularity is quite low. TEM

Figure 9. (A) Schematic representation of the stepwise generation of mesopores and micropores in SBA-15 by treatment with concentratedH2SO4 at 95 °C (i) and subsequent calcination at 200 °C (ii). (B) 13C CP/MAS NMR spectra of as-synthesized SBA-15 (a) and of the samesample after treatment with 60 wt % H2SO4 (b) and subsequent calcination at 200 °C (c). (C) N2 sorption isotherms and (D) t-plots of Arsorption at 87.29 K for calcined SBA-15 (a), 48 wt % H2SO4-treated SBA-15 (b), and the same sample after calcination at 200 °C (c).Reprinted with permission from ref 156. Copyright 2003 American Chemical Society.

2834 Chemical Reviews, 2007, Vol. 107, No. 7 Wan and Zhao

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Primary mesospores + extra intrawall pores (micro-meso)

Non-ionic polymer Pluronic triblock co-polymer

2D hexagonal structure


Understand the formation (sequence of events, intermolecular Interactions). Control properties – rational design.


Triblock-copolymer serves as structure directors (templates) (Pluronic)

Alexandridis, Olsson, Lindman. Langmuir 1998, 2627

Hydrophobic

Hydrophilic

Hydrophilic

(EO)x-‐(PO)y-‐(EO)x


Synthesis - 24h (T = RT - 100°C)

Calcination (T ≈ 500°C) –removal of the structure director


H2O

TEOS

TMOS

EtOH/MeOH

”Meso-‐soup”

SixOyOHz

Driving force (EO)x-‐(PO)y-‐(EO)x


Reactants SYNTHESIS Product

Chemical reactions Phase separation Self-assembly Micellar properties Molecular interactions Nucleation Growth Structure formation Aggregation


European Synchrotron Radiation Facility ESRF, Grenoble, France

SAXS/SAXD Small angle x-ray scattering/diffraction USAXS Ultra small angle x-ray scattering Cryo-TEM

In-situ – experiments to follow the synthesis




What happens?

Si(OH)4+MeOH

Si(OH)4 SiO2+H2O

Chemical reactions - Hydrolysis - Polymerisation

Self-assembly Aggregation


!

USAXS+SAXS/SAXD

Mesoporous materials Formation

+ Pluronic micelles Siliceous species

+ Phase separation


Particles form in a stepwise manner

Linton, P.; Rennie, A. R.; Zackrisson, M.; Alfredsson, V. Langmuir, Published on the web. March 05, 2009 2009. Linton, P.; Alfredsson, V. Chemistry of Materials 2008, 20, 2878.



Chemical reactions Phase separation Self-assembly Micellar properties Molecular interactions Nucleation Growth Structure formation Aggregation



UNDERSTANDING of the formation events.

into loose aggregate (seeUSAXS sectionwhere the data havebeen fitted with fractals). We believe, however, based on theelectron microscopy results, that the particles retain theiridentity in this process.

Discussion

In brief, the syntheses can be divided into two groupsreflecting the formation (and hence scattering) behavior. InTable 1, the characteristics for each synthesis are tabulated. Inall cases, the structure is the same (2D hexagonal, plane groupp6m) and only small differences in the cell parameters aredetected.The largedifferencebetween the products from thesesmall differences in synthesis conditions lies in the size andcrystal habit of the particles which is a result of the growthpattern.

The initial SAXS data support the results of our previousinvestigations.3,20 The siliceous species attach to the PEO partof the micelles, which is also consistent with results fromothers,9,21 and this eventually leads to phase separation offlocs,2 consisting of siliceous species and Pluronic molecules,in a dilute aqueous solution. The initial micelles rearrange inthis concentratedphase into cylindricalmicelles.2,3We suggestthat the cylindrical micelles in the floc form with nematicorder, providing an oriented structure, and that this nematic-like phase rapidly transforms to the ordered hexagonal phase.Hence, either anisotropic flocs with nematic-like behavior orparticles with hexagonal order are present, and these twospecies coexist as long as the micellar scattering is observedtogether with the Bragg reflections. Gradually the number ofhexagonally ordered particles increases at the expense ofnematic-like particles. During this process, silica connectivitymust be low to allow substantial rearrangement. In otherwords, the mesoscopic arrangement must be liquidlike in thesame sense as lyotropic liquid crystals. The liquidlike naturehas been demonstrated by Galarneau et al.15 who havemanaged to alter the pore size of SBA-15 after 24 h ofsynthesis. Electron paramagnetic resonance (EPR) spectro-scopyhas also been used to demonstrate that SBA-15 is highlyfluid prior to drying.14

Conclusions

In this study, we have determined the different processes thatcontrol the final structure and morphology of a mesoporoussilicamaterial structuredwith a block copolymer. The combina-tion of several experimental methods and investigations atdifferent length scales allows a comprehensive picture of thegrowth processes to be established. The in situ studies providethe means to temporally relate these processes (Figure 8). How-ever, the system investigated needs to be well-defined and have a

relative homogeneous temporal evolution as is the case with theSBA-15 material investigated in this work. The bulleted listbelow summarizes the processes involved in the formation. Priorto the addition of the silica source, the amphiphilic polymersform spherical micelles.

• After addition of the silica source (time zero), thespherical micelles remain even though there is acontinuous increase of density in the EOx layer dueto silicate enrichment.

• At about the same time as the appearance and growthof flocs is observed in the VIS data, and shortlythereafter in the USAXS region (note that theUSAXS data are less accurate regarding timing, aseach curve takes several minutes to obtain), the SAXSdata cannot be satisfactorily fitted (marked as mixedmicellar system in the SAXS line in Figure 8). Thisprobably reflects a mixed system with, just afternucleation, spherical micelles in solution and spheri-calmicelles in the floc and, later, amixture of sphericaland differently sized elongated micelles in the floc.

• As a consequence of the higher local concentration ofpolymer and silicate in the flocs, the micelles coalesceto cylinders and the SAXS data are consistent withpolydisperse cylinders. The cylindrical micellesarrange in what we suggest is a nematic-like phase.During this period, when the particles are anisotropic,the secondary aggregation (at the lower synthesistemperatures) occurs.

• Eventually, the hexagonal phase appears.• The USAXS data are still changing after the appear-

ance of the hexagonal phase, indicating the formationof aggregates.

Table 1. Summary of the Unit Cell Sizes, Particle Sizes, and Final Morphology (Secondary or Primary Particle) of the Four Syntheses

particle size (μm) obtained from TEM

synthesis temperature (!C) unit cell size (A) of the calcined material diameter height final particle (secondary or primary)

50 101.0 1.8 0.4 secondary55 102.3 0.97 0.43 secondary60 103.4 0.55 0.53 primary65 105.8 0.38 0.68 primary

Figure 8. Various events taking place in the formation of thesyntheses performed at 50, 55, 60, and 65 !C. The different processesare related to each other in time. It is clear that the temperature has alarge effect on the kinetics of the synthesis. The syntheses follow thesame pattern, and only the onset and duration for the differentprocesses vary. The problems with fitting the SAXS data arises atthe same time as larger objects are detected with VIS measurements.

(20) Flodstr::om,K.; Alfredsson, V.MicroporousMesoporousMater. 2003,

59, 167–176.(21) Boissi!ere, C.; Larbot, A.; Bourgaux, C.; Prouzet, E.; Bunton, C. A.

Chem. Mater. 2001, 13, 3580–3586.

DOI: 10.1021/la803543z Langmuir 2009, 25(8),4685–46914690

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Controlling the formation – tuning material properties

into loose aggregate (seeUSAXS sectionwhere the data havebeen fitted with fractals). We believe, however, based on theelectron microscopy results, that the particles retain theiridentity in this process.

Discussion

In brief, the syntheses can be divided into two groupsreflecting the formation (and hence scattering) behavior. InTable 1, the characteristics for each synthesis are tabulated. Inall cases, the structure is the same (2D hexagonal, plane groupp6m) and only small differences in the cell parameters aredetected.The largedifferencebetween the products from thesesmall differences in synthesis conditions lies in the size andcrystal habit of the particles which is a result of the growthpattern.

The initial SAXS data support the results of our previousinvestigations.3,20 The siliceous species attach to the PEO partof the micelles, which is also consistent with results fromothers,9,21 and this eventually leads to phase separation offlocs,2 consisting of siliceous species and Pluronic molecules,in a dilute aqueous solution. The initial micelles rearrange inthis concentratedphase into cylindricalmicelles.2,3We suggestthat the cylindrical micelles in the floc form with nematicorder, providing an oriented structure, and that this nematic-like phase rapidly transforms to the ordered hexagonal phase.Hence, either anisotropic flocs with nematic-like behavior orparticles with hexagonal order are present, and these twospecies coexist as long as the micellar scattering is observedtogether with the Bragg reflections. Gradually the number ofhexagonally ordered particles increases at the expense ofnematic-like particles. During this process, silica connectivitymust be low to allow substantial rearrangement. In otherwords, the mesoscopic arrangement must be liquidlike in thesame sense as lyotropic liquid crystals. The liquidlike naturehas been demonstrated by Galarneau et al.15 who havemanaged to alter the pore size of SBA-15 after 24 h ofsynthesis. Electron paramagnetic resonance (EPR) spectro-scopyhas also been used to demonstrate that SBA-15 is highlyfluid prior to drying.14

Conclusions

In this study, we have determined the different processes thatcontrol the final structure and morphology of a mesoporoussilicamaterial structuredwith a block copolymer. The combina-tion of several experimental methods and investigations atdifferent length scales allows a comprehensive picture of thegrowth processes to be established. The in situ studies providethe means to temporally relate these processes (Figure 8). How-ever, the system investigated needs to be well-defined and have a

relative homogeneous temporal evolution as is the case with theSBA-15 material investigated in this work. The bulleted listbelow summarizes the processes involved in the formation. Priorto the addition of the silica source, the amphiphilic polymersform spherical micelles.

• After addition of the silica source (time zero), thespherical micelles remain even though there is acontinuous increase of density in the EOx layer dueto silicate enrichment.

• At about the same time as the appearance and growthof flocs is observed in the VIS data, and shortlythereafter in the USAXS region (note that theUSAXS data are less accurate regarding timing, aseach curve takes several minutes to obtain), the SAXSdata cannot be satisfactorily fitted (marked as mixedmicellar system in the SAXS line in Figure 8). Thisprobably reflects a mixed system with, just afternucleation, spherical micelles in solution and spheri-calmicelles in the floc and, later, amixture of sphericaland differently sized elongated micelles in the floc.

• As a consequence of the higher local concentration ofpolymer and silicate in the flocs, the micelles coalesceto cylinders and the SAXS data are consistent withpolydisperse cylinders. The cylindrical micellesarrange in what we suggest is a nematic-like phase.During this period, when the particles are anisotropic,the secondary aggregation (at the lower synthesistemperatures) occurs.

• Eventually, the hexagonal phase appears.• The USAXS data are still changing after the appear-

ance of the hexagonal phase, indicating the formationof aggregates.

Table 1. Summary of the Unit Cell Sizes, Particle Sizes, and Final Morphology (Secondary or Primary Particle) of the Four Syntheses

particle size (μm) obtained from TEM

synthesis temperature (!C) unit cell size (A) of the calcined material diameter height final particle (secondary or primary)

50 101.0 1.8 0.4 secondary55 102.3 0.97 0.43 secondary60 103.4 0.55 0.53 primary65 105.8 0.38 0.68 primary

Figure 8. Various events taking place in the formation of thesyntheses performed at 50, 55, 60, and 65 !C. The different processesare related to each other in time. It is clear that the temperature has alarge effect on the kinetics of the synthesis. The syntheses follow thesame pattern, and only the onset and duration for the differentprocesses vary. The problems with fitting the SAXS data arises atthe same time as larger objects are detected with VIS measurements.

(20) Flodstr::om,K.; Alfredsson, V.MicroporousMesoporousMater. 2003,

59, 167–176.(21) Boissi!ere, C.; Larbot, A.; Bourgaux, C.; Prouzet, E.; Bunton, C. A.

Chem. Mater. 2001, 13, 3580–3586.

DOI: 10.1021/la803543z Langmuir 2009, 25(8),4685–46914690

Article Linton et al.

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