DOI: 10.1002/chem.200801688

Insight Into the Defects of Cage-Type Silica Mesoporous Crystals with Fd3̄mSymmetry: TEM Observations and a New Proposal of “Polyhedron Packing”

for the Crystals

Lu Han,[a, b] Yasuhiro Sakamoto,*[b] Shunai Che,*[a] and Osamu Terasaki[b]

Introduction

It is well known that crystal defects in a solid affect its prop-erties, such as crystal nucleation and growth, bulk and sur-face catalytic reactivity, and diffusion. Defects in crystalsprovide information about the stability between neighboringstructures and also tell us about the unit and manner ofcrystal growth.

Silica mesoporous crystals (SMCs) can be formed uponsurfactant micelles as templates for the self-assembly andsubsequent condensation of inorganic precursors.[1] Thepacking of the organic surfactant and the charge densitymatching between the surfactant and the inorganic precur-sor are essential for the formation of an ordered mesostruc-ture.[2] It is natural to imagine that the defects of SMCs aremore diverse than those of ordinary inorganic crystallinesolids because a part of mesoporous silica is so flexiblebefore completion of silica polymerization that the struc-tures of SMCs respond sensitively to a change in the synthe-sis conditions by introducing various defects into the crystal.Planar defects such as intergrowth, and twin and stackingfaults, are typical examples observed in SMCs, especiallycage-type structures with Fm3̄m or P63/mmc symmetry, andcertainly affect the diffusion property inside crystals.[3–8]

Two-dimensional (2D) hexagonal, cubic (including micel-lar cubic and bicontinuous cubic), and lamellar mesoporoussilicas (M41S family and SBA-1, SBA-2, SBA-3, SBA-7,etc.) have been prepared by using a cationic quaternary am-monium surfactant by means of S+I� or S+X�I+ synthesisroutes under basic or acidic conditions.[1,2,9–13] Nonionic sur-factants were used as templates to prepare mesoporous ma-terials with large pores (HMS, MSU, SBA-11, SBA-12,

Abstract: Silica mesoporous crystalswere synthesized by using a gemini cat-ionic surfactant (C18-3-1) as the directingagent, carboxyethylsilanetriol sodiumsalt as the co-structure directing agent(CSDA), and varying amounts of HCl.By using transmission electron micro-scopy (TEM) we observed 1) a struc-tural change from the close-packedstructures of spherical micelles—face-centered cubic (Fm3̄m) and hexagonalclose-packed (P63/mmc)—to Fd3̄mstructures with an increase of HCl and2) a few structural defects in the crys-

tals with Fd3̄m symmetry. The structureof a crystal with Fd3̄m symmetry is de-scribed as one of the tetrahedrallyclose-packed (tcp) structures consistingof 512 and 51264 polyhedra. The ob-served TEM images of the structuraldefects were explained well throughuse of simulated TEM images by intro-ducing new 13–15 polyhedra compris-

ing 51262, 51263, 4151062, 425865, and4151064, which have been observed inbubbles by Matzke. The mesostructuralchanges and defect formation are dis-cussed in terms of the hardness of mi-celles composed of surfactant/CSDA/silica species that have formed througha change of the interaction betweenthe surfactant and CSDA, which causesthe micelles to change from a regimeof close-packing to one of minimum-area packing.

Keywords: crystal growth · defects ·electron microscopy · mesoporousmaterials · template synthesis

[a] L. Han, Prof. S. CheSchool of Chemistry and Chemical TechnologyState Key Laboratory of Metal Matrix CompositesShanghai Jiao Tong University800 Dongchuan Road, Shanghai 200240 (P.R. China)Fax: (+86) 21-5474-1297E-mail : chesa@sjtu.edu.cn

[b] L. Han, Dr. Y. Sakamoto, Prof. O. TerasakiStructural ChemistryArrhenius Laboratory and Berzelii Center EXSELENTStockholm University10691 Stockholm (Sweden)Fax: (+46) 816-3118E-mail : yasuhiro@struc.su.se

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.200801688.

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SBA-15, SBA-16, FDU-1, FDU-5, FDU-12, KIT-5, KIT-6,etc.) with the S0I0 or [S0H+]ACHTUNGTRENNUNG[X�I+] synthesis route.[14–22]

However, only lamellar or disordered mesophases could beobtained by the use of anionic surfactants with a similar syn-thesis route to those of the cationic and nonionic surfactants,which indicates that the interaction between the inorganicprecursor and the surfactant is too weak.

In 2003, Che et al. reported a new anionic surfactant tem-plating route to silica mesoporous crystals by introducing anadditional co-structure directing agent (CSDA), which re-sults in an electrostatic interaction between the negativelycharged head group of the anionic surfactant and the posi-tively charged organo groups of the CSDA.[5–8, 23–25] Anionic-surfactant-templated mesoporous silicas (AMS-n), which un-dergo a systematic structural change from the cubic close-packed (ccp) with Fm3̄m or hexagonal close-packed (hcp)with P63/mmc, to the Pm3̄n, P42/mnm, Fd3̄m, 2d-p6mmstructures followed by Pn3̄m have been obtained by control-ling the ionization degree of the surfactant and CSDA, whichresults in changes of the surfactant packing parameter g.[24,25]

Several defects have been observed from these AMSs.[5]

To expand the anionic CSDA synthesis method to cationicsurfactant systems as well, the carboxyethylsilanetriolsodium salt (CES) with negatively charged carboxylategroups has been introduced as the CSDA.[26] This cationicsurfactant and CSDA system also revealed a structuralchange from ccp/hcp to the Fd3̄m-type structure by increas-ing the amount of HCl present. By slightly adjusting the syn-thesis conditions, Fd3̄m-type structures with defects havebeen produced.

Herein, we have introduced new structural descriptionsfor cage-type Fd3̄m structures that have soft spheres. Itgives us a clear direction to describe these kinds of mesopo-rous materials and defect structures. A crystal structure withFd3̄m symmetry can be described as one that is tetrahedral-ly close-packed (tcp) and consists of 512 and 51264 polyhedra.The observed TEM images of defects were explained wellthrough use of simulated images by introducing new 13–15polyhedra of 51262, 51263, 4151062, 425865, and 4151064, whichwere observed in Matzke�s[27] experiment of bubble shapes.The observations are discussed in terms of changing from aregime of the close-packing of spheres to one of the mini-mum-area packing of soft micelles through a change of theionization degree of the CSDA.

Results and Discussion

The mesostructural change from ccp/hcp to an Fd3̄m-typestructure : Mesoporous silicas were synthesized by usinggemini surfactant [C18H37N ACHTUNGTRENNUNG(CH3)2ACHTUNGTRENNUNG(CH2)3N ACHTUNGTRENNUNG(CH3)3]Br2 (C18-3-1)as the template and CES as the CSDA.[26] Unlike the anion-ic surfactant templating route, in the cationic surfactant syn-thesis system, ordered mesostructures can be formed with-out a CSDA. However, we found that the CSDA played avery important role in the control of the formation of thecationic-templated mesoporous materials.[26,27]

The XRD patterns of the calcined mesoporous silicas syn-thesized using different HCl/CES molar ratios are shown inFigure 1. Samples synthesized with HCl/CES molar ratios of

0 and 0.60 show an intense peak in the region of 2q=1.5–2.08, which was indexed to a 111 reflection of Fd3̄m symme-try, and two additional weak peaks in the range of 2.0–3.58were indexed to 220 and 311 reflections. The unit-cell pa-rameter (a) of the samples shown in (a) and (b) were 85 and104 �, respectively. Samples synthesized with higher HCl/CES molar ratios of 0.775 and 0.80 display several peaks inthe 2q range of 1.4–48, which can be indexed to 220, 311,222, 422, 333, and 440 reflections on the basis of cubic Fd3̄msymmetry with a unit-cell parameter (a) of 173 and 167 �,respectively. The determination of crystal systems and spacegroups was carried out with the help of TEM observations.

Figure 2a and b show high-resolution transmission elec-tron microscopy (HRTEM) images of the samples synthe-sized with HCl/CES=0 and 0.60, respectively, both of whichwere taken along the [110] direction. White contrast in theimages corresponds to areas of low electron-scattering den-sity, namely, the mesopores. Uniform hard spheres arrangein a plane to form a hexagonal close-packed layer, whichwill be called the “layer” hereafter. The layer can be A or Bor C depending on the origin of the spheres. Both ABC andAB stacking sequences of the layer can be observed in thetwo samples. The ABC stacking corresponds to a ccp struc-

Figure 1. XRD patterns of the calcined mesoporous materials. The reac-tant molar composition of C18-3-1/HCl/CES/TEOS/H2O was 1/x/2/15/2000(TEOS= tetraethoxysilane). HCl/CES molar ratios were a) 0, b) 0.60,c) 0.775, and d) 0.80. The XRD patterns were recorded on a PhillipsPANalytical instrument equipped with CuKa radiation.

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ture, in which the normal to the layer is one of the <111>directions of the cubic system. The AB stacking correspondsto a hcp structure, and the normal of the layers is parallel tothe [001] axis of the hexagonal system. Both ccp and hcpstructures have the same layers and the difference betweenthem lies only in their stacking sequence, therefore inter-growths of the two structures are commonly observed. InFigure 2, the layers are marked by A, B, or C and their se-quences are marked by ccp or hcp. The occurrence of cubicand hexagonal intergrowth produces streaks perpendicularto the layer in the Fourier diffractogram (FD) as shown inthe insets of Figure 2. In these two systems, spheres are con-sidered to be hard and each sphere has 12 nearest neigh-bors.

Unlike the ccp/hcp structure, the structure of SMCs withFd3̄m symmetry can be described either by a packing of bi-modal spherical cages, which are interconnected by smallwindows,[7] or by a partition of space through a polyhedronwith a tcp structure, in which the micelle is not consideredto be a perfect sphere but is soft and changeable in its shapelike foam. Four polyhedra, 512, 51262, 51263, and 51264 (shownin Figure 3), are useful to construct the tcp structures pro-posed by Frank and Kasper.[28,29]

In a previous study, the structure of SBA-1 or SBA-6(Pm3̄n) was described as clathrate type I (A3B) with 512 and51262 building units.[30] The Fd3̄m-type structure is the struc-tural analogue of clathrate type II, which comprises sixteensmall 512 polyhedra and eight large 51264 polyhedra in a unit

cell. The 51264 polyhedra are arranged in a diamond struc-ture 8b site (Wyckoff letter with site symmetry 4̄3m) andthe 512 polyhedra are in between the 51264 polyhedra, that is,in a 16c site (Wyckoff letter with site symmetry 3̄m). The51264 polyhedra connect to four 51264 polyhedra and twelve512 polyhedra, and the 512 polyhedra connect to six 51264

polyhedra and six 512 polyhedra. This structure can also bedescribed as a stacking of two kinds of layers made of thesetwo polyhedra along the <111> direction. One layer ismade up of only 512 polyhedra arranged in a Kagom� net,which are typical in Laves phases (we will call the layer A,B, or C). The other has 512 and 51264 polyhedra (we will callthe layer a, b, or g and their mirrors are denoted by a’, b’,or g’).

The HRTEM image (taken from the [1̄10] direction) ofthe sample synthesized by using HCl/CES= 0.80 is shown inFigure 4a, clearly showing the typical contrast of the Fd3̄m-type structure. From the HRTEM image, an AaBbCg stack-ing sequence with several stacking faults can be observed.TEM image simulations of the idealized mesoporous struc-tures were performed by using the dedicated software Meso-PoreImage.[31] MesoPoreImage provides images calculatedfrom a 3D continuum model of mesoporous crystal struc-tures. To adjust the image contrast of the simulated to thatof the observed, a parameter representing surface roughnesson the pore surface was introduced. The simulated TEMimage for a twin, which is shown in a white rectangle in Fig-ure 4a, agrees very well with the observed HRTEM image.The position and size of each cage used for image simula-tion is listed in the Supporting Information, in which a cagecorresponds to a mesopore that is occupied by the surfactantbefore calcination. For simplification, all cages were as-sumed to be perfect spheres instead of polyhedra, and eachsphere can overlap neighboring spheres if necessary. Themodel of the Fd3̄m-type structure made up of polyhedra isoverwritten onto the image. The contrast of the model dove-tailed nicely with the observed HRTEM image.[32] Fig-ure 4b–d show the structure models of the two types oflayers from the [1̄10] (Figure 4b) and [111] directions (Fig-ure 4c and d), respectively. If the voids (hexagonal part) inlayers A, B, and C are marked by letters A, B, and C, fromthe [111] direction (Figure 4e), the voids clearly have theABC stacking; the same contrast was found from all the<110> directions and all the <211> directions.

Structural study of the defects in the Fd3̄m-type structure :Figure 5 shows HRTEM images of two successive tilt series

Figure 2. HRTEM images and Fourier diffractograms (insets) of thesample synthesized by using a) HCl/CES=0 and b) HCl/CES=0.60.Both samples have ccp/hcp structures.

Figure 3. The four types of polyhedron that can form the tcp structures.

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along the [111] direction (308 between them) of the samplesynthesized by using HCl/CES= 0.775. From the imagestaken along the [1̄10] (Figure 5a) and [1̄21̄] (Figure 5b) di-rections, several twin structures and three kinds of defect(marked with white, black, and gray arrows) can be ob-served.

Figure 6a and b show the HRTEM images taken from the[1̄10]c and [1̄21̄]c directions including the defect marked with

white arrows in Figure 5, which consists of an ABBA stack-ing sequence of 512 polyhedron layers in an Fd3̄m-type struc-ture. The stacking layer between the same two 512 poly-hedron layers (layer B) is named layer z, so the defect layerhas an AaBzBa’A stacking sequence, which can be ex-plained well by introducing the other two types of poly-hedron suggested by Frank and Kasper.[33] One is the 51263

polyhedron arranged in a triangular net. The other is the51262 polyhedron lying in the center of the triangular net;the schematic drawing of the stacking layers is shown in Fig-ure 6c–f. The space group of the defect layer is P6/mmmand the dimensions of the unit cell are a=c=115 �. Theschematic figures of the layers and simulated TEM imagesof layer z have been inserted into the HRTEM images (Fig-ure 6a and b) and they match the contrast of the image verywell (see the Supporting Information for the position andsize of each cage).

The other two kinds of defect (marked with black andgray arrows in Figure 5) have a different contrast from thetypical Fd3̄m-type structure, and have not been previouslyfound in mesoporous silica structures. However, after tiltingthe crystal to the [1̄21̄]c direction by 308, the two defectsshow the same contrast. Therefore, it is possible that thetwo HRTEM images are taken from the same defect.Images showing the contrast from the [21̄1̄]c, [11̄0]c, [12̄1]c,[01̄1]c, [1̄1̄2]c, and [1̄01]c incidences are shown in Figure 7a,which were obtained by tilting the crystal along the [111]c

direction (308 between each of them; see the Supporting In-formation); these results indicate that the defect does not

Figure 4. For the sample synthesized by using HCl/CES=0.80: a) HRTEM image taken from the [1̄10] direction; b)–e) the structural description of theFd3̄m-type structure with polyhedra.

Figure 5. HRTEM images of the sample synthesized with a HCl/CESmolar ratio of 0.775: a) image taken along the [1̄10] direction; b) imagetaken of the crystal tilted to the [1̄21̄] direction by 308.

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keep its threefold symmetry. In addition, the layers next tothe defect still keep the same contrast as layer A (or B orC ; 512 polyhedra) from both the <110> c and <211> c direc-tion, which suggests that the defect does not affect the struc-ture of these two layers, so we call them layer A and A’. Fig-ure 7b shows the relative position of layer A and A’; thehexagonal voids in layer A and A’ are marked the same wayas in Figure 4e.

From the cage positions andthe contrast from the HRTEMimages, a polyhedra model wasbuilt to interpret the defectstructure. However, the defectcannot be built by using onlyfour polyhedra, 512, 51262, 51263,and 51264. For this reason, threenew types of polyhedra (4151062,425865, and 4151064) were em-ployed (Figure 8). It is impor-tant to note that all of the poly-hedra were discovered inMatzke�s experiment onrandom foam structure.[27, 34]

The HRTEM images takenfrom the [11̄0]c, [12̄1]c, [01̄1]c,and [1̄1̄2]c directions of theFd3̄m-type structure includingthe defect structure are shownin Figure 9a–d. Figure 9e–hshow the schematic drawings ofthe defect structure, which hasa monoclinic unit cell. Thespace group of this layer is C2/m and the dimensions of theunit cell are as follows: a= 200,b= 115, c=180 �; b=1238.

Thus, the defects in the HRTEM images (Figure 9a–d)become [010]m (Figure 9a), [130]m (Figure 9b), [110]m (Fig-ure 9c), and [100]m (Figure 9d) directions of the monoclinicunit cell. Simulated TEM images and the structure modelsare also inserted (see the Supporting Information for the po-sition and size of each cage). They have a good agreementwith the observed HRTEM images, which support the faith-fulness of the structure model.

In addition, strange contrast had been observed from the[110] direction of the Fd3̄m structure (Figure 10a). The con-trast changes from the typical [110] direction to the [114] di-rection through an intermediate part. This can be explainedby the existence of the twin plain, which is not parallel tothe zone axis [110]. Figure 10b shows the structure modelfrom the [1̄10] direction with a twin plane. The stacking se-quence is ABCBA, in which C is the mirror plane. The[110], [111], and [114] directions are in the same plane per-

Figure 6. HRTEM images with simulated TEM images and the schematic drawing of the defect from thea) [1̄10] and b) [1̄21̄] directions; schematic drawings of the AaBzBa’A stacking defect from the c) [1̄10],d) [1̄21̄], and e) [111] directions of the layer z and the f) [111] direction of layer B. The red dots marked in (e)and (f) overlap in the [111] direction.

Figure 7. a) The TEM images taken from the [21̄1̄]c, [11̄0]c, [12̄1]c, [01̄1]c,[1̄1̄2]c, and [1̄01]c incidences and b) the schematic model of the defectalong the [111]c direction.

Figure 8. Three new polyhedra for building the defect structure presentedin this paper.

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pendicular to the [1̄10] direction, and they differ by 35.38from each other. From the figure, it is clear that the [110] di-rection of the blue part is parallel to the [114] direction ofthe red part. The mirror plane C is 54.78 from the [110] di-rection, so the image contrast gradually changes. Figure 10cand d show the schematic drawing and the simulated TEMimages, which have a good agreement with observedHRTEM images.

Formation mechanism of the defects : The formation mecha-nism of the observed defects can be understood in terms ofthe synthesis conditions and the packing arrangement of themicelles. We have recently assessed the curvature of thestructures of SMCs obtained by electron crystallography andfound that the Gaussian curvature of the silica wall—exceptin proximity to pore openings where the curvature is alwaysnegative—is positive (parabolic surface) for Fm3̄m- and P63/mmc-type structures, and negative (hyperbolic surface) forFd3̄m- and Pm3̄n-type structures.[35,36] The formation of theFd3̄m-type structure can be explained by a principle pro-posed by Ziherl and Kamien, that is, changing from a close-packing rule associated with hard-core interactions to a min-imum-area rule associated with soft-tail potential.[37,38]

For most of the face-centered cubic (fcc) structures in-cluding the Fd3̄m-type structure, the crystal growth occursby means of a layer-by-layer growth at the {111} plane.[39] In

Figure 11a, the particle growth layers, that is, the stacking-fault planes, are highlighted in different colors. Domainsthat contain a stacking fault can be observed, and oftenshare one or many stacking-fault layers with other domains.This means that the nucleation rate and the growth rate arecomparable, thus the crystal growth occurs at many differentplaces, and a domain structure is created. Because the do-mains cannot fit each other well, some voids are created(white arrows) and defects like layer z (blue arrow) areformed at the interface between different domains. Fig-ure 11b shows how the defect layer c is formed. The defectis formed as a result of a mismatch of the Fd3̄m stacking,such as ACABCACBACABCABCA and ACBCACBCACB-CABCAB. A similar defect caused by a mismatch of thestacking sequence was observed in the ETS-10 structure.[40]

In addition, extra dots can be found in the electron diffrac-tion pattern and Fourier diffractogram (FD) from theHRTEM image taken from the [111] direction. An inversedfast Fourier transform (FFT) image obtained from the extradots in the FD also indicates that the structure containsmany domain structures (Figure 12).

Table 1 lists the relative volume and surface area of eachpolyhedron. The 512 polyhedron has the smallest volumeand surface area. The 51264 polyhedron, which also forms theFd3̄m-type structure, has twice the volume of the 512 poly-hedron, indicating that it is probably formed by combiningtwo 512 micelles together. The volumes of the other poly-

Figure 9. HRTEM images of the defect and schematic drawings of thestacking layers c in the Fd3̄m-type structure.

Figure 10. a) HRTEM image taken from the [110] direction of the Fd3̄m-type structure, which shows the contrast changing from the typical [110]direction to the [114] direction. b),c) The schematic drawing of the twinplane and d) simulated TEM image, in which the overlap part consists of50% of the [114] domain and 50% of the [110] domain.

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hedra fall in between. Apparently, the Matzke cell, 4151062

polyhedron, has the same volume as the 512 polyhedron,which means that they are the same type of micelle/CSDA/silica species. However, a different shape is produced whenthey stack in a different position. Moreover, 51262 and 51263

polyhedra also have the same volume as 425865 and 4151064

polyhedra, respectively, indicating that these micelles arequite similar before condensation.

In the synthesis system, after CSDA and tetraethoxysilane(TEOS) have been added into the solution, the spherical mi-celle/CSDA/silica species composite is formed, followed byself-assembly and further condensation. Interaction and con-densation of different micelle/CSDA/silica species surfacesprovides nucleation sites and then introduces furthergrowth. When the micelles/CSDA/silica species is “soft”, itmay form polyhedra rather than spheres by following theminimum-area rule. The micelles/CSDA/silica species com-posite with a certain volume (for example, the same volumeas the 512 polyhedra) will form a stable structure that con-tains layer A and layer a. At the same time some micelle/CSDA/silica species change their volume and become thesame volume as 51264 polyhedra, which can fit in the voids ofthe a layer and then become an Fd3̄m-type structure. Whendefects occur by either the slightly different conditions ordomain structures, some micelle/CSDA/silica species canchange their shape and size to fit the defect structure with acertain volume, for example, the 4151064 polyhedra, to form astable structure because of their softness.

The formation of cage hardness can be explained in termsof the interaction between the surfactant and the CSDA.Here, the anionic carboxylate part of the CSDA interactselectrostatically with the cationic ammonium groups of thesurfactants and those attached to the surface of the micelle.The strong interaction between the cationic surfactant andthe anionic carboxylate of CSDA drives the silica close tothe micelle surface and results in the dense packing of silica,which leads to the composite being “hard”. On the otherhand, the carboxylic acids are weak acids, with a pKa ofabout 1–5. Therefore, an equilibrium can usually be reachedbetween the uncharged and the negatively charged carboxyl-ate molecules in the solution. With the addition of HCl, theionization degree of the CSDA decreases, and the interac-tion between the carboxylate groups of the CSDA and thesurfactant weakens, resulting in the long distance (i.e., thelarge vacancy) between the micelle and silica wall, which

Figure 11. Formation of the defect structure: a) the different nucleationsites that make the domain structure, b) the stacking mismatch that cre-ates the voids that form the defect.

Figure 12. a) HRTEM image, b) electron diffraction pattern, andc) Fourier diffractogram of the [111] direction of the Fd3̄m-type struc-ture, and d) an inversed FFT image from the extra dots marked in theFourier diffractogram.

Table 1. Relative volume and surface area of different polyhedra.

Polyhedron type Relative volume Surface area[a]

512 1.0 20.651262 1.5 25.851263 1.7 28.451264 2.0 31.04151062 1.0 23.4425865 1.5 28.84151064 1.7 28.6

[a] The length of each edge of the polyhedra was kept at 1.

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Y. Sakamoto, S. Che et al.

causes the formation of loosely packed, transformable“soft” cages.

Conclusion

To conclude, we have investigated the detailed structure ofthe defects in the Fd3̄m-type mesostructure by usingHRTEM and proposed a polyhedra-packing mechanism,which provided us with information about the relationshipbetween different structures and the mechanism of crystalgrowth. This work shows the possibility of identifying thestructure type or defect structure of mesoporous crystals byusing TEM image simulation. For the cage-type structures,the size, shape, and surface area of each cage (includingpolyhedra and spheres) are important to understand notonly the structural changes but also the crystal growth. Thissubject will be of interest to researchers in diverse areas ofchemistry, particularly those in inorganic, colloid, physical,and materials chemistry.

Experimental Section

Synthesis : The gemini surfactant [C18H37N ACHTUNGTRENNUNG(CH3)2 ACHTUNGTRENNUNG(CH2)3N ACHTUNGTRENNUNG(CH3)3]Br2

(C18-3-1) and the mesoporous silicas were synthesized according toref. [26]. The surfactant-free materials for HRTEM analyses were ob-tained by calcination at 550 8C in air for 6 h.

Characterization : Powder XRD patterns were recorded on a PhillipsPANalytical instrument equipped with CuKa radiation (40 kV, 20 mA) ata rate of 0.1 deg min�1 over a range of 1–68 (2q). For TEM observations,the sample was crushed in an agate mortar, dispersed in ethanol, anddropped onto a thin carbon film on a Cu grid. HRTEM analyses wereperformed with a JEOL JEM-3010 microscope operating at 300 kV (Cs =

0.6 mm, point resolution 1.7 �) and a JEOL JEM-2100 microscope oper-ating at 200 kV (Cs=1.4 mm, point resolution 2.5 �). Images were re-corded with a charge-coupled device (CCD) camera (Gatan MultiScanCCD camera model 794, 1024 � 1024 pixels, pixel size 24 mm) at 50000–80000 times magnification under low-dose conditions. TEM image simu-lations of the idealized mesoporous structures were performed by usingthe dedicated software MesoPoreImage.[31]

Acknowledgements

The authors thank T. Ohsuna for helpful discussions and for the Meso-PoreImage software for TEM image simulations. Y.S. and O.T. thank theSwedish Research Council (VR), Japan Science and Technology Agency(JST), and Berzelii EXSELENT for financial support. S.C. thanks theNational Natural Science Foundation of China (grant nos. 20425102C,20501015, and 20521140450). TEM studies were performed at the Elec-tron Microscopy Center (EMC) at Stockholm University, which is sup-ported by the Knut and Alice Wallenberg Foundation.

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Received: August 14, 2008Published online: February 3, 2009

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FULL PAPERDefects of Silica Mesoporous Crystals