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Materials Chemistry and Physics 123 (2010) 417–420

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ffect of pre-calcination process on thermal stability of mesoporous titaniaynthesized by nanocasting of nonsurfactant-templated mesoporous silica

ong-Wook Lee, Chang-Yeol Yu, Kew-Ho Lee ∗

ational Research Laboratory for Functional Membranes, Environment and Resources, Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107,useong, Daejeon 305-606, Republic of Korea

r t i c l e i n f o

rticle history:eceived 7 August 2009eceived in revised form 24 March 2010ccepted 26 April 2010

eywords:esoporous titaniaanocastinghermal stability

a b s t r a c t

In this study, we observed effect of pre-calcination on the thermal stability of mesoporous titania repli-cated from citric acid-templated mesoporous silica by changing the post-calcination temperature aftersilica template removal. The pre-calcination before the elimination of silica templates was conductedto crystallize and consolidate amorphous titania framework. After the silica template removal, the post-calcination was carried out to observe thermal stability of the mesoporous titania replica in the absence ofthe silica template. For comparison with the replicated mesoporous titania, sol–gel derived mesoporoustitania was also synthesized through simple drying and calcination of colloidal titania sol. As a result,mesostructure of the sol–gel derived mesoporous titania almost collapsed above 400 ◦C, whereas thereplicated mesoporous titania did not show a significant falloff in its high pore properties up to 700 ◦C ofthe post-calcination temperature. Consequently, it was revealed that the pre-calcination process before

the elimination of silica templates was effective for improvement in thermal stability of the mesoporoustitania replica after the template removal. Since the anatase crystal formation and growth during thepre-calcination process occurred in a state of silica-supported titania nanocomposites with nonporousstructure, mesostructure deformation through the anatase crystal growth was suppressed. Moreover, thepreformed anatase crystals during the pre-calcination process led to a little crystal growth during thepost-calcination process, resulting in higher thermal stability of mesoporous titania replica during the post-calcination process.

. Introduction

Mesoporous titania has been prepared by employing vari-us templates, such as phosphate [1,2], amine [3–5], ionic [6–8],lock polymer [9,10], nonionic surfactants [11], and nonsurfactants12–15]. However, it is not easy to synthesize thermally stable

esoporous titania. The relatively low thermal stability of theitania-based mesoporous materials is often attributed to defor-

ation of their mesostructures mainly derived from anatase–rutilehase transformation [16,17]. Mesoporous titania with higher ther-al stability was obtained only by poly(ethylene oxide)-based

emplating [16,18–21]. Recently, Yoshitake et al. reported themprovement in thermal stability of template(primary amine)-xtracted titania through CVD treatment of titanium isopropoxide

5]. Cassiers et al. synthesized NH3-treated titania, which wastable up to 600 ◦C [17]. More recently, we reported synthesisf mesoporous titania (KRICT-MT) with high thermal stabilityy nanocasting of nonsurfactant-templated mesoporous silica

∗ Corresponding author. Tel.: +82 42 860 7240, fax: +82 42 860 7237.E-mail address: khlee@krict.re.kr (K.-H. Lee).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.04.033

© 2010 Elsevier B.V. All rights reserved.

[22]. The anatase phase is generally transformed into the rutilein the temperature range of 600–800 ◦C along with significantdeformation of mesostructure, whereas the anatase–rutile phasetransformation of the KRICT-MT was not observed up to 900 ◦C, giv-ing rise to high pore properties at high temperature. In our previouspaper [22], we investigated the thermal stability of the KRICT-MTby varying the calcination temperature before the removal of meso-porous silica template.

In this study, we observed its thermal stability by changingthe post-calcination temperature after silica template removal. Thepre-calcination before the silica elimination was carried out to crys-tallize and consolidate amorphous titania framework, followed byconducting the post-calcination after the silica removal to observethermal stability of the KRICT-MT in the absence of the silica tem-plate.

2. Experimental

2.1. Synthesis of mesoporous titania (KRICT-MT) through nanocasting method

The mesoporous titania replicated from citric acid-templated mesoporous sil-ica was synthesized by the method reported in our previous publication [22].In a typical synthesis, transparent colloidal silica sol with about 5 nm in particle

418 D.-W. Lee et al. / Materials Chemistry a

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Fig. 1. EDS results of the KRICT-MT (a) before and (b) after the NaOH treatment.

iameter was synthesized from base-catalyzed condition via hydrolysis of tetraethylrthosilicate (TEOS) and condensation reaction. A TEOS:NH3:H2O:EtOH molar ratioas 1:0.086:53.6:40.7. The reflux of the final mixture for 3 h resulted in the sta-

le colloidal silica sol. Subsequently, the silica nanosphere-CA nanocomposite solas obtained by dissolving 8 g of citric acid (CA) in 100 mL of the prepared sil-

ca sol at room temperature. CA-templated mesoporous silica (CMS) was preparedy drying and calcination of the nanocomposite at 70 ◦C for 12 h, at 500 ◦C for 2 h,espectively.

The CMS was used as silica template for synthesis of mesoporous titania replica.titanium(IV) isopropoxide (TIP)/ethanol mixture (a volume ratio = 1) was incor-

orated into the CMS. After decantation of the solution, the TIP-CMS compositeas transferred into a water/ethanol mixture (a volume ratio = 1) for the sol–gel

eaction of TIP, followed by vigorous stirring at room temperature for 20 h. Therocess above was repeated once more, and the nanocomposite, which was fil-ered and dried, was then calcined at 500 ◦C in air for 2 h, resulting in formationf the nanocrystalline anatase-CMS nanocomposite. After elimination of the sil-ca template by NaOH treatment, we successfully obtained the mesoporous titaniaeplica, designated KRICT-MT-x (Korea Research Institute of Chemical Technology-esoporous Titania), where x is pre-calcination temperature before the removal of

ilica template. As shown in Fig. 1, it was confirmed that the silica template waslmost eliminated after NaOH treatment.

For observation of thermal stability of KRICT-MT after the elimination of sil-ca template, the KRICT-MT-x was calcined again in the temperature range of00–700 ◦C after the silica removal. The materials calcined again were denotedRICT-MT-x-y, where x and y are pre-calcination and post-calcination temperaturesefore and after the removal of silica template, respectively.

able 1ore properties of KRICT-MTs obtained from nitrogen sorption and XRD analysis.

Sample code Calcination temperaturebefore silica removal [◦C]

Calcination temperatureafter silica removal [◦C]

KRICT-MT-500 [22] 500 –b

KRICT-MT-700 [22] 700 –KRICT-MT-500-500a 500 500KRICT-MT-700-500 700 500KRICT-MT-700-700 700 700

a KRICT-MT-x-y, where x and y are pre-calcination and post-calcination temperatures bb After silica removal, post-calcination was not conducted.c BET surface area.d Total pore volume taken from the volume of N2 adsorbed at P/Po = 0.995.e BJH desorption average pore diameter.f Calculated from the (1 0 1) peak of anatase phase using the Scherrer equation.

nd Physics 123 (2010) 417–420

2.2. Synthesis of sol–gel derived mesoporous titania (SG-MT)

For comparison with thermal stability of the KRICT-MT, sol–gel derived meso-porous titania was also synthesized through simple drying and calcination ofcolloidal titania sol. Typically, 0.45 mL of distilled water was slowly added into themixture of 14.8 mL TIP (Aldrich, 99.999%) and 50 mL ethanol (Merck, 99.5%) withvigorous stirring at room temperature, resulting in titania white precipitate. Forpeptization of the titania precipitate, the mixture of 50 mL ethanol and 0.59 mLnitric acid (Junsei, 62%) was added into the solution including titania precipitatewith vigorous stirring at 90 ◦C. After the reflux of final solution at 90 ◦C for 12 h,light milky titania sol was obtained.

The titania sol was dried at 70 ◦C, and then calcined in the temperature rangebetween 300 ◦C and 600 ◦C. After the drying and calcination, sol–gel derived meso-porous titania was prepared, which was designated SG-MT-x, where x is calcinationtemperature.

2.3. Characterization

Pore properties of the KRICT-MT and SG-MT such as surface area, pore volumeand pore size were taken by nitrogen sorption experiments with a micromerit-ics ASAP 2020 instrument. X-ray diffraction (XRD) patterns were collected usinga Rigaku D/MAX-2200 V instrument (operated at 1.6 kW).

3. Results and discussion

Surfactant-templated mesoporous titania is usually preparedthrough surfactant removal from titania-surfactant supramolecu-lar assemblies by calcination. In that case, the titania mesostructureeasily collapses during a calcination process at high temperature,because sintering and crystal growth of titania framework occursimultaneously with thermal decomposition of template. Like-wise, in the case of sol–gel derived mesoporous titania preparedby simple drying and calcination of colloidal titania sols, thermalstability is not good due to a similar reason to the surfactant-templated mesoporous titania. In contrast, for the mesoporoustitania replicated from mesoporous silica template, a calcina-tion process for crystallization of amorphous titania frameworkis generally conducted prior to the elimination of silica template.Therefore, compared with surfactant-templated or sol–gel derivedmesoporous titania, the titania mesostructure is more stable duringa calcination process at higher temperature, because calcination isconducted in a state of nonporous titania–silica nanocomposites.In other words, the silica template plays an important role as abarrier against anatase crystal growth through coordination of pri-mary anatase clusters, which is a main reason for the mesostructuredestruction during high temperature calcination.

In this study, we investigated the thermal stability of replicatedmesoporous titania after the elimination of silica template by con-ducting post-calcination procedures of the replicated mesoporous

titania KRICT-MT. Table 1 shows the pore properties of KRICT-MT-x-y, where x and y are pre-calcination and post-calcinationtemperatures before and after silica removal via NaOH treatment.The KRICT-MT-500 synthesized without post-calcination gave highBET surface area and pore volume of 300 m2 g−1 and 0.45 cm3 g−1,

BET surfacearea [m2 g−1]c

Total pore volume[cm3 g−1]d

Average poresize [nm]e

Anatase crystalsize [nm]f

300 0.45 5.8 10.3230 0.37 6.3 12.9250 0.37 6.5 9.6170 0.26 6.2 13.9100 0.24 13.0 18.0

efore and after silica removal, respectively.

D.-W. Lee et al. / Materials Chemistry and Physics 123 (2010) 417–420 419

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Fig. 2. XRD patterns of the KRICT-MT-x and KRICT-MT-x-y.

nd its pore size was 5.8 nm. The KRICT-MT-500-500, preparedy conducting post-calcination of KRICT-MT-500 at 500 ◦C afterhe silica removal, showed a slight decrease in pore properties,ut gave still high surface area and pore volume of 250 m2 g−1

nd 0.37 cm3 g−1. Its pore size was a little larger compared withhat of KRICT-MT-500. In the case of KRICT-MT-700, the surfacerea, pore volume and pore size were 230 m2 g−1, 0.37 cm3 g−1, and.8 nm, respectively. After once more calcination of KRICT-MT-700t 500 ◦C following the silica template removal, the KRICT-MT-700-00 showed nearly constant pore diameter with a slight decrease

n surface area and pore volume. However, the surface area andore volume of KRICT-MT-700-700 decreased to 100 m2 g−1 and.24 cm3 g−1, and its pore size increased to 13 nm, implying that theollapse of titania mesostructure started. Fig. 2 shows XRD patternsf KRICT-MT-x-y. All of the KRICT-MT samples gave anatase crys-al peaks with different intensity and FWHM. The KRICT-MT-500as low intensity and large FWHM of the anatase peaks, indicatinghat the pore wall of KRICT-MT-500 was composed of the nanocrys-alline anatase. Despite the post-calcination of the KRICT-MT-500nd KRICT-MT-700 at high temperature, only a slight change in thentensity and FWHM of anatase crystal peaks was observed excepthe KRICT-MT-700-700. Table 1 exhibits the anatase crystal sizef KRICT-MTs calculated from the (1 0 1) peak of anatase phasesing the Scherrer equation. Compared with the KRICT-MT-500ith anatase crystal size of 10.3 nm, the KRICT-MT-500-500 had

lmost constant anatase crystal size. The KRICT-MT-700-500 alsohowed a slight increase in the crystal size in comparison with that

f the KRICT-MT-700. However, in the case of the KRICT- MT-700-00, the anatase crystal size increased from 12.9 nm to 18.0 nm,ut not a significant change. The results shown in Fig. 2 and Table 1orroborate the conclusion that there was no substantial growth of

able 2ore properties of SG-MTs obtained from nitrogen sorption.

Sample code Calcination temperature [◦C] BET surface area[m2 g−1]b

SG-MT-300a 300 110SG-MT-400 400 18SG-MT-500 500 16SG-MT-600 600 7

a SG-MT-x, where x is calcination temperature.b BET surface area.c Total pore volume taken from the volume of N2 adsorbed at P/Po = 0.995.d BJH desorption average pore diameter.e Calculated from the (1 0 1) peak of anatase phase using the Scherrer equation.

Fig. 3. XRD patterns of the SG-MT-x.

the anatase crystallites, leading to the high thermal stability of theKRICT-MT with high pore properties retained.

For comparison with thermal stability of KRICT-MT, SG-MTswere also synthesized through simple drying and calcination ofcolloidal titania sol. Table 2 shows the pore properties of SG-MT-x, where x is the calcination temperature of dried colloidal titaniasol. The SG-MT-300 showed relatively high BET surface area andpore volume of 110 m2 g−1 and 0.1 cm3 g−1, and its pore size was2.6 nm. However, in the case of the SG-MT-400, the surface area andpore volume significantly decreased to 18 m2 g−1 and 0.03 cm3 g−1,which indicates that the mesostructure of the SG-MT considerablycollapsed. The SG-MT-500 and SG-MT-600 also showed negligi-ble surface area and pore volume. Fig. 3 exhibits XRD patterns ofSG-MT-x. The SG-MTs showed anatase crystal peaks with differ-ent intensity and FWHM. Compared to the KRICT-MT, the intensityand FWHM of anatase crystal peaks dramatically increased anddecreased with an increase in calcination temperature from 300 ◦Cto 600 ◦C. It implies that the growth rate of anatase nanocrystal forthe SG-MT was faster than that for the KRICT-MT, resulting in the

mesostructure collapse of the SG-MT at much lower temperature.

As shown in Fig. 3 and Table 2, the SG-MT synthesized by cal-cination under hard template-free condition showed considerablecollapse of the titania mesostructure above 400 ◦C, because there

Total pore volume[cm3 g−1]c

Average pore size [nm]d Anatase crystalsize [nm]e

0.10 2.6 11.90.03 3.4 17.80.02 4.1 23.10.02 4.8 25.7

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[19] D. Grosso, G.J. de, A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.A. Albouy, A.Bruneau, A.R. Balkenade, Adv. Mater. 13 (2001) 1085.

20 D.-W. Lee et al. / Materials Chem

as not a barrier against crystal growth through coordination ofnatase nanoclusters. In other words, calcination of the SG-MT withmpty pores facilitates the anatase crystal growth and mesostruc-ure collapse. Contrarily, the intensity and FWHM of anatase crystaleaks for the KRICT-MT were slightly changed with an increase inost-calcination temperature, and its surface area and pore vol-me were not significantly changed despite post-calcination upo 700 ◦C under silica template-free condition. Therefore, it cane concluded that the pre-calcination process before the elimina-ion of silica template was effective for improvement in thermaltability of the KRICT-MT after the template removal. Since thenatase crystal growth during the pre-calcination process occurredn a state of silica-supported titania nanocomposites, mesostruc-ure deformation through the anatase crystal formation and growthas suppressed. In addition, the preformed anatase crystals during

he pre-calcination process resulted in a little crystal growth duringhe post-calcination process, giving rise to higher thermal stabilityuring the post-calcination process.

. Conclusions

In this study, we investigated the thermal stability of theRICT-MT after the elimination of silica template by conductingost-calcination procedures. The SG-MT synthesized by calcinationnder hard template-free condition showed significant collapsef the mesostructure above 400 ◦C, and its pore properties suchs surface area and pore volume dramatically decreased, which

as attributed to the fast growth of anatase crystals. Contrar-

ly, in the case of the KRICT-MT, a significant change in anataserystal peaks was not observed despite the post-calcination up to00 ◦C under silica template-free condition, and its surface area andore volume slightly decreased. Therefore, it was revealed that the

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nd Physics 123 (2010) 417–420

pre-calcination process in the presence of silica template led tothermally stable anatase nanocrystal framework and mesostruc-ture of the titania replica under silica template-free condition.

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