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Microporous and Mesoporous Materials 101 (2007) 224–230

Formation of ZMM-n: The composite materials having both naturesof zeolites and mesoporous silica materials

Masaru Ogura a,*, Yiwen Zhang b, S.P. Elangovan b, Tatsuya Okubo b

a Institute of Industrial Science, Department of Materials and Environmental Science, The University of Tokyo, Komaba 4-6-1,

Meguro-ku, Tokyo 153-8505, Japanb Department of Chemical System Engineering, The University of Tokyo, Japan

Received 27 July 2006; received in revised form 21 October 2006; accepted 23 October 2006Available online 6 December 2006

Abstract

The formation mechanism of ZMM-n, composite mesoporous aluminosilicate materials, which are obtained by use of vapor phasetransport of structure-directing agent for zeolites, is clarified. Filling the mesopores by carbon is essential to preserve the mesoporousstructure of the source, and addition of aluminum on mesoporous silica critically determines the formation rate and stability of zeoliticbuilding units on the surface of mesopores.� 2006 Elsevier Inc. All rights reserved.

Keywords: Zeolite; Mesoporous silica; Composite; Carbon filling; Aluminosilicate; Vapor-phase transport

1. Introduction

For industrial applications, zeolites are still known andexpected as a promising component in acid catalysts. Masstransfer to the active acid site of zeolite inside micropores isquite important from the industrial aspects, however, theefficiency of zeolite is sometimes limited because of itsmicroporosity, leading to the deficiency in diffusion limita-tion throughout the crystal, resulting in saturation of cata-lyst efficiency and performances [1]. Even though zeoliteshows us a promising feature in a laboratory scale, combi-nation with binders or something to avoid pressure dropand to relax interfacial pressure into micropores of zeoliteis inevitably required for use in industrial applications.

Now two approaches are acquired for many researchersto overcome one of the most serious drawbacks mentionedabove. One is fabrication of new type of microporous andsuper-microporous materials including new type of zeoliteframework with a wider pore mouth close to 1 nm, such

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.10.032

* Corresponding author. Tel.: +81 3 5452 6321; fax: +81 3 5452 6322.E-mail address: oguram@iis.u-tokyo.ac.jp (M. Ogura).

as VPI-5 [2] and ECR-34 [3]. Up to now, more than 160kinds of zeolite framework are recognized by InternationalZeolite Association, and in another way, more than100,000 types of zeolite can be simulated on computer the-oretically [4]. On the contrary, only 11 types, which aredivided into nine analogues, of framework have been prac-tically utilized. There might exist a new type of frameworkthat can overcome the current limitation, but anotheraspect is also required simultaneously to obtain efficientzeolite framework.

Another promising route to improve the efficiency ofzeolite framework is to create composite materials of zeo-lite with wider porous materials. Among them, mesoporouszeolite is of wide interest in academic and industrial pointsof view. Numerous efforts have been performed to fabri-cate mesoporous zeolites not only by bottom–up approachsuch as utilization of nano-crystalline or precursor solutionto organize mesostructure with surfactant micelles [5–9] ormesostructured carbon [10–14], but also top–down synthe-sis by partially dissolving zeolite crystal [15], and so on. Inthe former case, two-stage fabrication are the commonmethod; preparation of zeolitic precursor solution at thefirst stage, followed by organization of the precursor

M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230 225

through intermolecular interaction and self-assembly usingamphiphilic surfactant-derived supermolecular micelles.Double templating approach, such as attempting by a mix-ture of hexadecyltrimethylammonium cation as an SDAfor MCM-41 and tetrapropylammonium cation as anSDA for ZSM-5, is also found in the literature [16].

Utilization of confined space such as that observed inactivated carbon, mesoporous carbon, and carbon fiber isperformed, and therein nanosized crystalline zeolites arefabricated. As the opposite method, addition of carbona-ceous materials in the medium of zeolite synthesis was firstreported to synthesize mixture of zeolite particles and car-bon [10], resulting in mesoporous zeolites after burning offthe carbonaceous compounds.

Creation of mesopores inside zeolite crystals is one ofthe well-known processes to improve catalytic perfor-mance. USY zeolite is the one that mesopores are formedwhen dealumination process is carried out by steaming orusing hydrochloric acid. Another method is to createmesopores inside zeolite crystal by desilicification [15].Desilicification occurs when highly concentrated sodiumhydroxide aqueous solution is used to treat high silica zeo-lites, and crystallinity is hardly affected while the morphol-ogy is drastically changed to increase mesoporosity.

Among them, our approach for new type of composite isto use vapor phase transport (VPT) of the structure-direct-ing agent for zeolites to the parent mesoporous silica as asource of zeolitic framework [17,18]. Nucleation and crys-tal growth occur only on the mesopore wall, resulting inavoidance of physical mixture of each grain of zeoliteand mesoporous silica. By the method offered here, onecan select any zeotype and any mesoporous silica, whilemainly two-dimensional hexagonal mesostructure, such asMCM-41, SBA-15, or wormhole-like mesoporous materialhas been thermodynamically fabricated in the conventionalmethods reported previously. In this study, we will reporton the role of Al and carbon filled inside the mesoporesin the formation mechanism of ZMM-n, the composite ofzeolite and mesoporous material.

2. Experimental

The synthesis procedure of ZMM-1 involves the follow-ing steps: (1) synthesis of mesoporous material; (2) intro-duction of Al on the mesopore surface; (3) filling ofcarbon inside the mesopores; (4) recrystallization of por-tion of amorphous wall of the mesoporous materials byVPT for zeolite crystallization; and (5) removal of carbonby calcination.

2.1. Preparation of SBA-15

Ten grams of triblock copolymer P123 (EO20PO70EO20,BASF) as a surfactant and 62.6 g of 35 wt% HCl werepoured into 319 g of distilled water. After the complete dis-solution of P123, 20.8 g of tetraethylorthosilicate (TEOS)were added and stirred vigorously for about 24 h at

308 K. Afterward the solution was heated at 363 K forabout 24 h. The white precipitate was filtered and driedat the temperature for 24 h. Finally the surfactant wasremoved by calcination in air at 813 K for 6 h.

2.2. Impregnation of aluminum

One gram of SBA-15 were slurried in distilled water con-taining 0.074 g of AlCl3. The Si/Al molar ratio in the start-ing gel was adjusted at 30. The solution was then heated at333 K under stirring until being dried. The dried samplewas then calcined at 813 K for 5 h in air.

2.3. Carbon filling in accordance with the procedure of

preparation of mesoporous carbon

In order to support the mesostructure of SBA-15 whenVPT was carried out, carbonization in the mesopores withsucrose or furfuryl alcohol was performed. Sucrose or fur-furyl alcohol was filled in the mesopore channels usingincipient wetness according to the procedure as reportedfor the preparation of mesoporous carbon, CMK-3 [19].The Al-SBA-15/carbon source composite was then heatedin Ar flow at 1073 K for 6 h to carbonize the sourcethoroughly.

2.4. Recrystallization of pore-wall of Al-SBA-15 by VPT

method

The Al-SBA-15/CMK-3 composite was placed in anautoclave, at the bottom of which SDA solution (1.0 ethy-lenediamine: 7.7 triethylamine: 10H2O) for MFI crystalli-zation was separated away from the composite accordingto the procedure reported in the literature [20]. The pore-wall of Al-SBA-15 was crystallized at 448 K for severaldays. Then carbonaceous materials and SDA wereremoved by heating the material at 813 K for 6 h in a flowof air to regenerate mesopores. The resultant composite ofzeolitic mesoporous aluminosilicate is named ZMM-1.When using SBA-16, the composite named ZMM-2 wasobtained.

2.5. Characterization

Calcined samples were characterized by powder X-raydiffraction (XRD) with Cu Ka (k = 0.15418 nm) radia-tion. Nitrogen adsorption–desorption measurementswere carried out at 77 K on Autosorb 1 instrument. Thetotal surface area was calculated according to Brunaer–Emmett–Teller (BET) isotherm equation, while the micro-pore volume and the external surface area were evaluatedby the t-plot method. The mesopore and micropore sizewas determined from a pore size distribution curve by theBJH method and HK method, respectively. FT-IR spectraof samples were recorded by using a Jasco FT-IR 230 spec-trometer. A field-emission scanning electron microscopy(S-5200, Hitachi Corp.) was used for the investigation of

226 M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230

particle morphology. Transmission electron microscopy(TEM) images were recorded on a Philips CM200FEGwith an acceleration voltage of 200 kV.

3. Results and discussion

3.1. Effect of carbon filling on the preservation of

mesostructure of SBA-15 during VPT to ZMM-n

Fig. 1 summarizes X-ray diffraction patterns of theproducts from SBA-15 after VPT for various days. As pre-viously reported [17,18], on Al-containing SBA-15, that is

Inte

nsity

/ cp

s

5 10 15 20 25 30 35 402theta (CuKα) / degree

a

b

b

c c

a

2theta (CuKα) / degree

Inte

nsity

/ cp

s

0.5 1 1.5 2 2.5 3 3.5 4 4.5

aa

b

b

c

c

d

d

e

e

a

Inte

nsity

/ cp

s

a a

b

b

c

c

d

d

ee

5 5 10 15 20 25 30 35 402theta (CuKα) / degree

55 10 15 20 25 30 4035

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Fig. 1. Synthesis of microporous/mesoporous composite by VPT method:(A) without carbon filler in mesopores – (a) parent Al-SBA-15; (b) VPTfor 5 days; and (c) VPT for 12 days, (B) mesopores were filled with carbonmade from sucrose. VPT was carried out for – (a) 12 h; (b) 1 day; (c) 3days; (d) 5 days; and (e) 12 days, and (C) mesopores were filled withcarbon made from furfuryl alcohol. VPT was carried out for – (a) 12 h; (b)1 day; (c) 3 days; (d) 5 days; and (e) 12 days.

the aluminosilicate source for zeolite, without a carbon fil-ler (Fig. 1A), its high intensity of the diffraction could notbe maintained at a low angle, which can be assigned as[10 0] of SBA-15, upon VPT for 5 days. Even 12 h ofVPT synthesis was confirmed enough long to diminishthe mesostructure of SBA-15. The collapse of the orderedstructure in the mesophase range, in turn, led to the forma-tion of crystalline ZSM-5 after 12 days of VPT. The prod-uct did not hold at all the mesostructural ordering observedat the lower angle of XRD pattern.

On the other hand, carbon filling enhanced the stabilityof mesostructure during VPT. Fig. 1B shows the time-on-stream change of micro-/meso range diffraction on carbon/SBA-15 composite prepared using sucrose as the carbonsource. The diffraction at the lower angle was preservedto 5 days during VPT synthesis without any diffractionat a higher angle of 2h. The peaks corresponding toZSM-5 started to appear at 12 days of VPT synthesis,while mesostructure wholly disappeared. Compared tothe products without filling carbonaceous compound, crys-tallization seems to be retarded by the carbon inside themesopores. This indicates that the crystallization takesplace not only on external surface of SBA-15, but mainlyalong mesochannels, which means inside the mesopores.

When the filling was conducted by use of furfuryl alco-hol, the kinetics is furthermore affected. Even after 12 daysof VPT synthesis, mesostructure was observed by XRD(Fig. 1C), while ZSM-5 was hardly crystallized. Crystalliza-tion kinetic is apparently affected by the carbon filled in themesopores of SBA-15. ZMM-1, which has been named onthe sample prepared by this method [17], holds a micro-scale building unit of zeolite, which was confirmed by infra-red spectroscopy as shown in Fig. 2. Jacobs et al. havesummarized the structural vibration at the wavenumberbelow 1000 cm�1, and suggested that the absorption at550 cm�1 could be assigned as 5-membered ring structurein pentasil zeolites such as ZSM-5, ZSM-11 [21]. The zeo-litic building unit was observed before whole destruction

400500600700800900100011001200wavenumber / cm-1

Tra

nsm

ittan

ce /

%

a

b

e

c

f

d

Fig. 2. Infrared spectra of microporous/mesoporous composite withcarbon from sucrose. VPT was carried out to Al-SBA-15 (a) for: (b) 12 h;(c) 1 day; (d) 3 days; (e) 5 days; and (f) 12 days.

Fig. 3. Images of ZMM-1 by TEM and SEM.

M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230 227

of mesostructures and before the appearance of zeoliticphase which were confirmed by XRD. Fig. 3 shows theimages by microscopes of typical ZMM-1, obtained by fur-furyl alcohol-based carbon/Al-SBA-15 composite followedby VPT for 5 days. ZMM-1 was found mesostructuredthrough TEM, while a minor morphological change wasobserved on the external surface as shown in SEM. Manyviews in the TEM analysis suggested that the original mes-ostructured periodicity of SBA-15 is wholly maintainedand that zeolite crystal cannot be found. It is, however, dif-ficult to estimate the yield of the zeolitized part. Therefore,it is concluded that ZMM-1 has both natures of zeolitesand mesoporous materials; mesoporous structure is main-tained and a zeolitic building unit is created on the meso-pore walls.

It is interesting to note again that the compositeobtained along with carbon filled from furfuryl alcohol,rather than that from sucrose, kept a better mesostructuralordering for a longer period without a longer-range order-ing of zeolite crystal. Table 1 summarizes the analyses ofporous structures of the products by nitrogen adsorption.SBET and SBJH mean that the surface areas were calculatedby the theory of BET and BJH, respectively. SBJH indicatesthe surface along mesopores. CMK-3, known as the carbo-naceous product obtained by use of SBA-15 as an inor-ganic template [19], was prepared by dissolving silicate

Table 1Surface area and porosity of carbon/SBA-15 composites and mesoporouscarbon CMK-3

Sample SBET

(m2/g)SBJH

(m2/g)Smicropore

(m2/g)Vmicropore

(cc/g)

Al-SBA-15/carbon fromfurfuryl alcohol

490 131 240 0.12

CMK-3 from furfurylalcohol

1050 870 53 0.020

Al-SBA-15/carbon fromsucrose

1125 310 650 0.32

CMK-3 from sucrose 1500 1200 0.0 0.00

species from carbon/SBA-15 composites by NaOH concen-trated solution. It is interesting to note that the carbon/SBA-15 composite showed a large SBET even though mes-opores are filled, and that of the composite prepared bysucrose was more than 1000 m2/g. The large area is mainlydue to the microporous structure of the composite. Micro-porosity was hardly found in CMK-3 prepared by bothcarbon sources. SBA-15 is well known to have microporesin the mesopore wall [22], and on the contrary it is alsoreported that the micropores disappeared by treating at ahigher temperature of 1123 K [23]. Almost no microporeson Al-SBA-15 after carbonization at higher temperaturessuch as 1073 K was confirmed; therefore, the microporositycomes from loosely packed structural space between car-bon and silicate wall of SBA-15. The carbonaceous mate-rial from furfuryl alcohol was filled with the mesoporesof Al-SBA-15 more tightly and less micropores wereremained. This microporous structure helps the SDA mol-ecules for zeolite to diffuse throughout the particles of com-posite. Therefore, the filling of mesopores has a critical rolenot only to maintain mesostructure but also to control therate of zeolitization of the mesopore walls as resources ofzeolite nuclei and the secondary building unit by diffusioncontrol of SDA molecules.

3.2. Effect of alumination of mesopores to form ZMM-n

Infrared spectroscopy revealed the secondary buildingunit of zeolites on ZMM-1, as shown in Fig. 2. Even beforethe generation of long-range ordering of microstructure, 1day of VPT synthesis, the absorption was observed onthe sample. On the sample of 12 days of VPT synthesis,the absorption was clearly observed. The absorption by5-membered ring structure in pentasil zeolites [21] wasobserved at around 550 cm�1 on ZMM-1 and ZMM-2,indicating that they have similar building unit of zeolite.As a result along with XRD, the building unit without along-range order is generated at the surface of meso-pore walls of ZMM-n by using the silicate of the wall.

Table 2Catalytic activities for cracking of triisopropylbenzene

Sample Temp. (K) Conv./% Selectivity/%

Diisopropylbenzene Cumene

ZSM-5 593 15 >99 0.0623 22 >99 0.0673 41 >99 0.0

ZSM-1 593 73 93 7.0623 91 86 14673 >99 55 45

Experimental conditions: catalyst weight, 100 mg and TIPB pulse, 1 lL.

228 M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230

The zeolitic building unit was observed after 8 h of VPTsynthesis when without carbon filling, so that it can be con-sidered that carbon decelerates formation of the buildingunit on the mesopore wall.

As previously demonstrated, the catalytic activity, theactivation energy, of ZMM-1 for cumene cracking was sim-ilar to that of ZSM-5, while the conversion on ZMM-1 wasmuch less than that on ZSM-5 [17]. These indicate that thesimilar active, strong acidic site as ZSM-5 is formed onZMM-1, even though the amount is still much less thanthat in ZSM-5. The building unit of zeolite generated onZMM-1 functions as the strong acid site as ZSM-5, sinceamorphous silica–alumina did not show so high activityas crystalline aluminosilicate, zeolites, under the experi-mental conditions used in this work.

Alumination of SBA-15 is necessary when ZMM-n is tobe supplied as an acid catalyst. Aluminum site is alsorequired to crystallize zeolite by the method of VPT. Puresiliceous Si-SBA-15 after VPT of 9 days hardly showed theabsorption of the building unit. Mesoporous structure ofSi-SBA-15 was hardly preserved and, on the other hand,ZSM-5 was not confirmed crystallized for a prolonged per-iod up to 14 days, indicating that the aluminum has impor-tant roles in preserving mesostructures and in generatingzeolitic building unit and further nucleation and growthof zeolite. The SDAs used to crystallize ZSM-5 are ethy-lenediamine and triethylamine, and both showed strongLewis basicity. Silica is easy to dissolve in basic solution,but the solubility of aluminosilicate is rather less than thatof silicate [15]. Aluminum species attached to the surface ofSBA-15 creates acidic site, which plays important roles,first to catalyze hydrolysis and oligomerization of carbona-ceous species inside the mesopores, and second to give anadsorption site for basic SDA molecules. Formation ofzeolitic building unit is accelerated by the electrostaticinteraction of SDA to a negatively-charged aluminum siteon mesoporous aluminosilicate.

Sodium aluminate was revealed to make SBA-15 crys-tallize to ZSM-5 faster than aluminum chloride [18], butmesostructure was hardly remained. Less sodium isrequired to maintain mesostructure of aluminosilicate.

3.3. Catalytic performance for acid-catalyzed cracking of

triisopropylbenzene

As mentioned, ZMM-1 has been reported to show sim-ilar catalytic activity to that of ZSM-5, and cracking ofcumene into benzene occurred but at a much less conver-sion than ZSM-5 [17]. From the result, ZMM-1 has strongacid site whose nature is zeolitic, but the acid amount ismuch less than Al sites on Al-SBA-15.

Table 2 summarizes the catalytic performance of ZMM-1 along with ZSM-5 for cracking of triisopropylebenzene.The cracking is known as a unique shape selective cracking;the molecule cannot enter into micropores whose size cor-responds to 10-membered ring such as ZSM-5. On ZSM-5,the product observed was only diisopropylbenzene, indicat-

ing that only one branch of isopropyl groups is removedby the cracking because of the molecular size. On thecontrary, ZMM-1 cracked triisopropylbenzene intocumene, mono-isopropylbenzene. That means that formeddiisopropylbenzene subsequently reacts and decomposes toform cumene. From these results along with the catalyticactivities for cumene cracking, ZMM-1 holds acid sites asstrong as ZSM-5 but microporosity of ZMM-1 is notdeveloped which gives us diffusion limitation of bulkymolecules.

It is noted that these results obtained under the sameconditions of catalytic contact time, and that the conver-sions obtained on each catalyst were significantly different.This might be due to the diffusion kinetics inside mesop-ores, and resident time in the mesopores results in thedifference.

3.4. On creation of ZMM-n: mechanism and requirement tobe formed

It is already suggested that, by the method of VPT alongwith carbon filling, any composite material of zeolite andmesoporous materials can be created by selecting SDA spe-cies which can be vaporized, and by selecting mesostruc-ture. When using ethylenediamine/triethylamine andSBA-16, ZMM-2 was produced which showed the naturesof ZSM-5 and cubic mesostructure. However, when usingMCM-41 and MCM-48, the composite was not success-fully developed up to now. Fig. 4 shows the typical XRDpatterns of the composite of Al-MCM-48 and furfurylalcohol-derived carbon after VPT treatment for severaldays. VPT synthesis for 1 day led Al-MCM-48 to poorly-ordered structure, and 3 days of VPT are enough to dimin-ish the mesostructure even by the method along with fillingof carbon in the mesopores of Al-MCM-48. Normally,MCM-48 has a thin mesopore wall of less than 2 nm, whichseems too small to create a zeolitic building unit whose sizeis around 2 nm in the case of ZSM-5. Mesopore wallshould be also enough thick to hold the building unit andto bind the units with each other by amorphous silicate.Therefore, the thickness of mesopore wall should be takeninto account to create ZMM-n composite.

When using uncalcined SBA-15 and aluminum isimpregnated, and then supplied as the aluminosilicate

1 2 3 4 5 6 7 8 9 102theta (CuKα) / degree

Inte

nsity

/ cp

s

abc

Fig. 4. Synthesis of composite from MCM-48 with carbon filling byfurfuryl alcohol: (a) parent Al-MCM-48; (b) VPT for 1 day; and (c) VPTfor 3 days.

Scheme 1. Synthesis of microporous/mesoporous composite by VPTmethod: (A) without carbon filling and (B) with carbon filling.

M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230 229

source, ZMM-1 was never obtained and a dense (alu-mino)silicate phase such as crystoballite was formedinstead. This means that surfactant micelles cannot playthe key role to stabilize mesoporous structure duringVPT synthesis, and that silicate which is reactive, more sil-anol, rather than that after calcination at higher tempera-tures is not adequately used for this type of synthesis.

Proposed formation mechanism of ZMM-n is illustratedin Scheme 1. Al is deposited along mesopores. Without car-bon filling (Scheme 1A), molecules of SDA, ethylenedia-mine and triethylamine in this study, easily diffuse andreach at the Al sites to react and nucleate into ZSM-5,and at last nano-crystalline ZSM-5 is formed. ZSM-5obtained here is the same as that synthesized by a conven-tional synthesis method, and shows the catalytic perfor-mances originated from the active sites inside micropores.Shape selective diffusion limitation is possibly observed,and triisopropylbenzene is reacted to form diisopropylben-zene as the only product under the experimental conditionsapplied in this study.

According to the method of synthesis of mesoporouscarbon, mesopores of parent mesoporous silica are filledwith carbon, but a gap between the carbonaceous com-pound and silica mesopore wall because of the differenceof thermal expansion as shown in Scheme 1B. The gap isenough wide and microporous for the SDA to diffusethrough and reach to Al site. Then, nucleation occursrather slowly than that without carbon filling. The meso-structure is maintained by the carbon owing to its support-ing effect of mesostructure, and maybe also due to limiteddiffusion for SDA molecules, resulting in slow nucleationand crystal growth of zeolite. On ZMM-n, a building unitof zeolite is grown on the internal surface of mesoporesand the microporous structure is not fully developed; there-fore the catalytic performance is not limited by the microp-ores originated from the zeolite to be formed. The key forobtaining microporous/mesoporous composite is kineticcrystallization and preservation of mesostructures. The car-bon synthesized inside mesopores shows the two important

roles, along with addition of Al to serve a crystallizationsite.

4. Conclusions

The composites ZMM-n; the combined nature of zeo-lites and mesoporous materials, are successfully obtainedby use of VPT method and carbon filling inside the mesop-ores of the source. ZMM-n has a zeolitic building unitwithout a long-range order on the mesopore walls. The car-bon performs not only as a filler to maintain mesoporousstructure, but also as a diffusion controller for SDA toadjust the rate of formation of zeolitic building unit. Analuminum site has a critical role to generate the buildingunit, and aluminosilicate, of which mesopore walls arecomposed, is to be a nucleus of zeolite, leading to the for-mation of composite material without consideration of theformation of physical mixture of each phase.

230 M. Ogura et al. / Microporous and Mesoporous Materials 101 (2007) 224–230

Acknowledgment

MO shows appreciation to Professor Takashi Tatsumiand Dr. Toshiyuki Yokoi for their kind help of SEMobservation.

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