Journal of Colloid and Interface Science 391 (2013) 36–44

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Journal of Colloid and Interface Science

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Controlling pore size and uniformity of mesoporous titania by early stagelow temperature stabilization

Sebastiaan Johan Frans Herregods a,b,⇑, Myrjam Mertens a, Kristof Van Havenbergh c, Gustaaf Van Tendeloo c,Pegie Cool b, Anita Buekenhoudt a, Vera Meynen b

a VITO NV, Flemish Institute for Technological Research NV, Boeretang 200, B-2400 Mol, Belgiumb Laboratory of Adsorption and Catalysis (LADCA), Department of Chemistry, University of Antwerp, CDE, Universiteitsplein 1, B-2610 Wilrijk, Belgiumc EMAT, Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

a r t i c l e i n f o

Article history:Received 2 July 2012Accepted 23 July 2012Available online 12 October 2012

Keywords:MesoporousTitaniaEvaporation induced self-assemblyThermal treatment

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.07.098

⇑ Corresponding author at: VITO NV, Flemish InstituNV, Boeretang 200, B-2400 Mol, Belgium. Fax: +32 14

E-mail address: sebastiaan.herregods@vito.be (S.J.F

a b s t r a c t

The control of the formation process during and after self-assembly is of utmost importance to achievewell structured, controlled template-assisted mesoporous titania materials with the desired propertiesfor various applications via the evaporation induced self-assembly method (EISA). The present paperreports on the large influence of the thermal stabilization and successive template removal on the porestructure of a mesostructured TiO2 material using the diblock copolymer Brij 58 as surfactant. A con-trolled thermal stabilization (temperature and duration) allows one to tailor the final pore size and uni-formity much more precise by influencing the self-assembly of the template. Moreover, also thesuccessive thermal template removal needs to be controlled in order to avoid a structural collapse. N2-sorption, TGA, TEM, FT-Raman spectroscopy, and small angle & wide angle XRD have been used to followthe crystal growth and mesostructure organization after thermal stabilization and after thermal templateremoval, revealing its effect on the final pore structure.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Since the discovery of the template-assisted approach by Beckand coworkers [1,2] the research on directed mesostructuredmaterials experienced a major breakthrough. Nowadays, a varietyof ionic and neutral templates [3,4] are applied in order to obtaindifferent porous structures. The majority of these structures aresilica-based materials [5], although mesoporous titania-basedand other metaloxide materials gain growing interest due to theirsemiconducting properties and high stability (hydrothermal andchemical). Mesoporous titania materials are widely reported forapplications in photocatalysis, DSSC, smart coatings, separation,etc. [6,7].

To meet the demands of the various applications, insight andcontrol of the formation process are needed to direct the structuralproperties of these porous materials (e.g., pore size, pore size dis-tribution, etc.). In the case of titania, an evaporation induced self-assembly method (EISA) [8–10] is often applied to control the highreactivity of the Ti-source. The method is specifically designed toharmonize the hydrolysis and condensation of the inorganic sourcewith the self-assembly of the template in order to avoid the forma-

ll rights reserved.

te for Technological Research321186.

. Herregods).

tion of a nonporous solid. Therefore, the reactivity of the inorganicsource is tempered by working in an acidic solvent environmentwith a controlled water content [11,12]. During aging, the solventevaporates resulting in the formation of a modulable steady state(MSS) where the inorganic network is still flexible, but the chemi-cal composition is constant [10,13]. During the MSS, it is possibleto direct the template self-assembly by adjusting, for example,the surrounding atmosphere [6,10,13–15]. In addition, the estab-lished structural organization needs to be maintained during thesuccessive process steps, like thermal stabilization and templateremoval. Before the thermal stabilization, the inorganic networkis only weakly condensed due to the tempered reactivity of theinorganic source. The thermal stabilization of the weakly con-densed inorganic structure is applied in order to form a stronglycondensed and stable inorganic network before template removalin order to avoid a structural collapse during the template removal.The effect of the high temperature template removal (calcination)is well-known in literature [16–20]. Also, the evaporation and con-densation during low temperature thermal stabilization have beenstudied through in situ FTIR [21–23]. However, a detailed study onthe effect of the thermal stabilization (temperature and duration)on the final pore structure, specifically of a small pore material,is missing in literature. Although a detailed study on the thermalstabilization during the EISA method has not been reported, differ-ent stabilizations methods that are commonly used in practice to

Fig. 1. Schematic representation of the synthesis of the titania mesoporousmaterial.

S.J.F. Herregods et al. / Journal of Colloid and Interface Science 391 (2013) 36–44 37

avoid structural collapse of materials such as pre-crystallization,posttreatment with various additives etc. have been described indetail [15,24]. However, some applications, like membranes, re-quire small uniform pores, while an ordered structure is not re-quired. In that case, the use of pre-crystallization [25] is not anoption as the crystal nanoparticles are typically larger than thepore dimensions. Furthermore, the crystal growth will have amuch larger influence on the structure of the small pores com-pared to larger pore systems like pluronics (e.g., P123 or F127).In addition, the pluronic templated materials typically result inthicker pore walls, which can accommodate larger crystals com-pared to the Brij template materials described in this paper. Never-theless, the thermal stabilization is recognized as a crucial step toavoid uncontrolled crystallization and structural collapse duringcalcinations [16,17,26,27].

In the present paper, we report on the optimization of thethermal stabilization and its influence on the pore structure ofsmall mesoporous titania materials (£ < 5 nm) formed by meansof EISA with Brij 58 as a template with an in situ nanocrystal for-mation. We show that the temperature and duration of the ther-mal stabilization have a pronounced effect on the pore structureof the obtained material. Therefore, the thermal stabilization isnot only necessary for the consolidation of the inorganic frame-work but also for the uniformity and control of the pore size,especially when small pores are desired. Also, the influence ofthe successive template removal by means of calcination isinvestigated.

2. Experimental

Brij 58 was selected as surfactant as it typically forms uniformpores of approximately 4 nm, which is envisaged in this work.Templates with other alkyl and polyethyleneoxide chain lengthswill result in different pore sizes. Initial solutions were preparedby dissolving, 3.44 g surfactant [11,15,24] (Brij 58, C16H33(OCH2-

CH2)20OH, purchased from Aldrich), hydrochloric acid (37%, pur-chased from Merck) and then the inorganic source (titaniumtetraisopropoxide 98+%, TTIP, purchased from Acros Organics)into isopropanol (IPA, 99.8+%, purchased from Merck). After15 min of stirring, an IPA solution with controlled water contentwas added dropwise. The final molar ratio of a typical solutionwas TTIP:1; Brij 58:0.05; IPA:40; H2O:10; and HCl:2. After30 min stirring, the solution was poured in a 20 cm petri dishand aged for a period of 24 h under controlled atmosphere(RH = 75% + IPA vapor). Then, the samples were thermally stabi-lized under different conditions by transferring them to an ovenand heating them at 1 �C/min up to 60, 80, 100 and 120 �C. Thistemperature was maintained for a duration of 4–6 h (if necessary,a longer thermal stabilization than 4 h was applied in order toobtain a dry powder) or 24 h. The slow heating rate was appliedto allow a more gradual evaporation of the solvent and thereforea more controlled structural formation. The formed powderswere calcined at 360, 400 or 440 �C (2 h, 30 �C/h, ambient atmo-sphere). Fig. 1 displays a schematic overview of the synthesisprocedure.

The N2-sorption measurements at �196 �C were determinedusing a Quantachrome Quadrasorb SI automated gas sorption sys-tem. Prior to the measurements, the samples were outgassed un-der vacuum during 16 h at a temperature of 120 �C. TheBrunauer-Emmet-Teller (BET) method was applied to calculatethe apparent specific surface area. The total pore volume wasdetermined at P/P0 0.98. The Barret-Joyner-Halenda (BJH) methodapplied on the adsorption branch of the isotherm was used todetermine the pore size distribution (PSD). It is worth mentioningthat the adsorption branch of the isotherm was used for the calcu-

lation of the pore size although the desorption branch is often fa-vored from both thermodynamical and historical point of view.However, the desorption branch could exhibit a sudden drop in ad-sorbed volume around P/P0 �0.42 (in case of N2 at �196 �C), whichis not a result of the presence of physical pores. This is specificallythe case in porous structures with pore network effects (e.g., ink-bottle pores or cavities). The sudden drop in adsorbed volume re-sults from a forced closure of the hysteresis loop around P/P0

�0.42 (lower closure point of the hysteresis loop in case of N2 at�196 �C) arising from the tensile strength effect (TSE) of the ad-sorbed phase [28–31]. This means, that the desorption pore sizedistribution shows a sharp peak around 3.8 nm, which is ofteninaccurately assigned to uniform pores of 3.8 nm. Therefore, oneshould always compare the adsorption and desorption branch ofthe isotherm to confirm that the peak in the desorption PSD is a re-sult of the presence of real pores. Consequently, the adsorptionbranch is highly favored for pore size calculations if the sampleindicates pore network effects as the adsorption branch of the iso-therm is hardly affected by the tensile strength effect. The support-ing information (Fig. S1) demonstrates the tensile strength effect inour samples. Thermogravimetric analysis (TGA) results were re-corded on a NETZSCH STA 449C jupiter. The analyses were per-formed in air, whereby the samples were heated from 30 �C to600 �C with a heating rate of 25 �C/min. Small and wide angle X-ray diffraction (XRD) measurements were carried out on a PANa-lytical X’PERT PRO MPD diffractometer with Cu-radiation. Also, abracket stage, a monochromator, and soller slits were used. Themeasurements were performed in the 2h-mode using a monocrys-tal with a scanning speed of 0.04�/4 s and measuring in continuousmode. The crystal size was determined by means of the Scherrerequation. FT-Raman spectroscopy was carried out on a NicoletNexus 670 bench equipped with a InGaAs detector in a 180� reflec-tive sampling configuration using a 1064 nm Nd:YAG laser. Thetitania materials were further characterized by transmission elec-tron microscopy (TEM) analysis using a FEI FEG CM30 microscopeoperating at 300 kV. The TEM samples were prepared by crushingthe material in ethanol in an agate mortar and dropping this dis-persion of finely ground material onto a holey carbon film sup-ported on a Cu grid.

Table 1Adsorption pore size, apparent specific surface area and total pore volume of thecalcined samples thermally stabilized at different temperature and for differentduration obtained from N2-sorption.

60 �C 80 �C 100 �C 120 �C

Øads (24 h) (nm) 4.3 4.3 5.7 6.8mBET (24 h) (m2/g) 94 121 157 145Vtot (24 h) (cc/g) 0.093 0.12 0.21 0.23Øads (4–6 h) (nm) 5.7 5.2 4.2 6.8mBET (4–6 h) (m2/g) 109 159 111 184Vtot (4–6 h) (cc/g) 0.14 0.19 0.11 0.34

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3. Results and discussion

3.1. Influence of temperature of the thermal stabilization

Fig. 2A shows the N2-sorption isotherms of the calcined samplesstabilized at different temperatures during the thermal stabiliza-tion (24 h). The calcination was done at 360 �C for 2 h in all cases.Clearly, all the materials are mesoporous structures since they ex-hibit the characteristic type IV isotherm (IUPAC classification [32]).All the isotherms show a sudden increase in adsorbed volume as aresult of capillary condensation of the adsorptive in the mesopores.The capillary condensation of the materials stabilized at 60 and80 �C occurs at a relative pressure 0.4 < P/P0 < 0.6, which corre-sponds with an adsorption pore size diameter of 4.3 nm (Fig. 2B).The total pore volume and the specific surface area increase pro-portionally from 60 to 80 �C (Table 1), while the pore size andthe shape of the adsorption–desorption isotherm, including hyster-esis, remain identical. That means, both samples stabilized at 60and 80 �C exhibit a very similar pore structure (pore size, shape& geometry), but the sample stabilized at 80 �C most likely consistsof a higher amount of pores per gram of TiO2, leading to the ob-served higher pore volume and thus higher specific surface area(Table 1). At higher stabilization temperature, the capillary con-densation shifts to a higher and broader P/P0 range with the tem-perature. In other words, the pore size increases (5.7 nm at100 �C and 6.8 nm at 120 �C), and the pores become less uniformwith the temperature. Raising the thermal stabilization tempera-ture from 80 to 100 �C and 120 �C leads to an increase in porediameter and specific surface area, whereas the total pore volumedoubles (Table 1).

Fig. 3A shows the small angle X-ray diffractograms of the non-calcined samples stabilized at different temperatures during thethermal stabilization (24 h). Some of the non-calcined samples ex-hibit a hump at 2–4� 2h (�20–40 Å). The hump is due to randomlyarranged spheroidal pure titania particles in an amorphous matrixof impure titania in combination with the organic template, whichleads to a texture on a length scale of 30 Å.[33–36] Nevertheless,

Fig. 2. (A) N2-sorption isotherms of the calcined samples thermally stabilized atdifferent temperature, and (B) adsorption PSD of the calcined samples thermallystabilized at different temperature.

Fig. 3. (A) Small angle XRD patterns of the non-calcined samples thermallystabilized at different temperature, (B) Wide angle XRD patterns of the anatasereference, Brij 58 and the non-calcined samples thermally stabilized at differenttemperature for 24 h (Inset: wide angle XRD patterns with adjusted intensity toreveal the peak broadening, 60 �C with factor 3.47, 80 �C with factor 1.60 and 100 �Cwith factor 1.13).

the assignment of the peak is not unambiguous since it could alsooriginate from ordered pores or the combination of both. The sam-ples stabilized at 60 and 80 �C feature a clear peak around 1.2� 2h(d-spacing = 77 Å) due to the organization of the porous structure.A shift to lower 2h can be observed after thermal treatment at100 �C. The shift indicates an increase in unit cell size of the porousnetwork, which can be correlated to the enlarged pore diameter at100 �C as determined by N2-sorption. Furthermore, at 120 �C noclear peak can be observed indicating that the organization disap-pears at 120 �C. To conclude, an increase in thermal stabilizationtemperature leads to a decrease in the organization of the porousstructure. Nevertheless, after calcination, no clear higher order sig-nals can be observed for any of the applied temperatures. Subse-quently, the porous structure loses most of its long rangeorganization after calcination at 360 �C (supporting information,Fig. S2).

Fig. 3B displays the wide angle X-ray diffractograms of the non-calcined samples stabilized at different temperatures during thethermal stabilization (24 h) and the wide angle X-ray diffracto-grams of anatase and the template Brij 58. Clearly, all the samplesexhibit a broad band (20–35� 2h) due to the presence of amorphoustitania. Nevertheless, the samples stabilized at 80, 100, and 120 �C

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feature the characteristic peaks of the anatase crystal phase aswell. Indeed, the samples consist mainly of amorphous titania witha limited amount of small anatase crystals. With the temperature,the characteristic anatase peak around 25.3� becomes more narrow(Fig. 3B inset). This means, that the anatase crystals grow withtemperature, leading to larger crystals at higher temperatures.Table 2 summarizes the calculated crystal size of the non-calcinedsamples stabilized at different temperatures during the thermalstabilization (24 h). Obviously, the obtained values are not exactand should be used as estimates only since some of the valuesare just outside the applicable range of the Sherrer equation (i.e.,3–100 nm [37]). Nevertheless, the trend of increasing crystal sizeof the non-calcined samples with increasing thermal stabilizationtemperatures remains clear. The samples thermally stabilized at100 and 120 �C show additional peaks due to the crystallizationof the template Brij 58. Similar micelle crystallization of the tem-plate F127 in titania mesoporous materials was previously re-ported by Innocenzi et al. using in situ FTIR [23,38].

The presence of the anatase crystals in the non-calcinedsamples thermally stabilized at 80, 100, and 120 �C is confirmedby Raman spectroscopy (supporting information, Fig. S3). Withincreasing thermal stabilization temperature, a decrease in blueshift of the peak around 144 cm�1 due to phonon confinement[39,40] is observed, confirming the trend of increasing nanocrystalsize of the non-calcined samples with increasing thermal stabiliza-tion temperatures. Although the stabilization temperature is farbelow the bulk crystallization temperature of anatase (Tc = 350–400 �C), anatase nanocrystals have been previously observed be-low Tc, even at room temperature [41].

After calcination at 360 �C, the broad band due to amorphoustitania disappears, and an anatase structure with average crystalsizes around 7.5 nm appears (Table 2 and supporting information,Fig. S4, traces of rutile could be present). In contrast to the non-cal-cined samples, the crystal size of the calcined samples shows norelation with the thermal stabilization temperature. In otherwords, although the crystal size of the non-calcined samplesstrongly depends on the thermal stabilization temperature, the fi-nal crystal size of the calcined samples is independent of the pre-ceding thermal stabilization and thus approximately identical forall calcined samples.

Fig. 4A and B displays TEM images of non-calcined samplesthermally stabilized at 80 �C or 120 �C (24 h). Both samples containanatase crystals although the main part consists of amorphous tita-nia. The sample stabilized at 80 �C has an average anatase crystalsize of 4.0 ± 0.4 nm, and the sample stabilized at 120 �C has anaverage anatase crystal size of 6.0 ± 0.8 nm. The crystal sizes ob-tained from XRD are smaller probably due to the limitation ofthe Scherrer equation, but the increasing crystal size with the ther-mal stabilization remains clear. Fig. 4C and D shows the TEMimages of a calcined sample (360 �C, 2 h) thermally stabilized at80 �C (24 h) showing mesopores of 4–5 nm, which is in good agree-ment with the N2-sorption measurements. The sample consists ofanatase crystals with an average diameter of 9.7 ± 1.0 nm, again abit larger than the values obtained with XRD.

Table 2Crystal size of the non-calcined and calcined (360 �C for 2 h) samples thermally stabilized

Non-calcined

dcrystal (24 h) (nm) dcrystal (4-6 h) (nm

60 �C <1.5–2a �1.5–2a

80 �C �1.5–2a �1.5–2a

100 �C �2.7 �2.2120 �C 3.3 3.3

a The values are only estimates due to the limitations of the Scherrer equation.

Fig. 5 shows the dependence of the pore diameters (calcinedsamples) and the crystal sizes (non-calcined samples, based onXRD) on the thermal stabilization temperature (24 h). During thethermal stabilization of the inorganic structure, a partial crystalgrowth occurs as a function of the temperature although themajority of the structure consists of amorphous titania. The partialcrystal growth during the thermal stabilization (before calcination)has practically no effect on the final crystal size after calcination(�7.6 nm for all samples), but it has a pronounced effect on the fi-nal pore structure after calcination (Fig. 5). The samples stabilizedat 60 and 80 �C have a small pore diameter, which is in agreementwith the expected pore size/micelle size of a good self-assembledBrij 58 structure [11]. Moreover, the formation of small crystalsis observed. Therefore, in this case, it seems that the self-assemblyof the Brij 58 template is not influenced by the presence of thesmall crystals during the thermal stabilization. Thus, a pure self-assembly process is observed meaning that the surfactant mole-cules organize themselves into micelles which are used as a tem-plate for the pores and the titania condenses around thesemicelles, resulting in a pore size with a similar diameter as the sizeof the template upon calcination. The small angle XRD data con-firm this statement, since an organized structure occurs after thethermal stabilization due to the good self-assembly of the tem-plate. The sample stabilized at 60 �C exhibits a lower porosity thanthe sample stabilized at 80 �C most likely due to the lower degreeof condensation of the inorganic source around the template priorto calcination.

In contrast to the samples stabilized at 60 and 80 �C, thermalstabilization at higher temperatures results in larger crystals,which seems to influence the template self-assembly causing thepore size to exceed the expected pore size for a Brij 58 self-assem-bled structure [11]. The self-assembly of Brij 58 appears to be dis-rupted by the presence of larger crystals in the inorganic networkformed during the thermal stabilization. This means, that duringthe thermal stabilization the inorganic network is still flexibleand contributes to the self-assembly of the template. In otherwords, a disrupted self-assembly occurs due to the formation ofthe anatase crystals in the modulable steady state during thermalstabilization. This leads to less uniform pores (broader PSD) and anincrease in pore size with the crystal size (and thus the tempera-ture). The disruption of the self-assembly of Brij 58 at too hightemperatures is confirmed by the small angle XRD data of thenon-calcined samples, indicating the decreased organization ofthe self-assembled structure and an increase unit cell size withthe temperature (and the crystal size). From the correlation be-tween both graphs in Fig. 5 (£ and d), it is clear that the growthof the crystal size above a value of �2 nm is the underlying reasonfor the existence of enlarged pores, larger than the surfactant/tem-plate size (�4 nm). Indeed, above crystal sizes of �2 nm (�half ofthe micelle size), the self-assembly process seems to be disruptedmeaning that surfactant molecules organize themselves into mi-celles, but the micelles are altered/enlarged due to the presenceof large anatase crystals with which they interact. This occurs dur-ing the modulable steady state (the inorganic network is still flex-

at different temperature and for different duration obtained from wide angle XRD.

Calcined

) dcrystal (24 h) (nm) dcrystal (4-6 h) (nm)

7.4 8.27.6 7.87.4 7.37.6 7.6

Fig. 4. TEM images of non-calcined and calcined samples thermally stabilized at different temperatures. (A) Non-calcined sample thermally stabilized at 80 �C (24 h) showinganatase crystals with an average diameter of 4.0 nm. (B) Non-calcined sample thermally stabilized at 120 �C (24 h) showing anatase crystals with an average diameter of6.0 nm. (C) Overview image of a calcined sample (360 �C, 2 h) thermally stabilized at 80 �C (24 h). (D) Calcined sample (360 �C, 2 h) thermally stabilized at 80 �C (24 h)showing the mesoporosity of the sample.

Fig. 5. The pore diameter (calcined samples) and the crystal size (non-calcinedsamples, based on XRD) correlated to the thermal stabilization temperature (24 h)(the dashed line and the hollow triangles represent the uncertainty on the crystalsize of the non-calcined samples due to the limitations of the Scherrer equation).

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ible, but the chemical composition is constant). These altered mi-celles are used as a template for the pores upon calcination, leadingto larger a less uniform pores in the final structure.

Although the crystal size has an influence on the final porediameter other underlying mechanisms, and further experimentalconditions such as the calcination temperature can also play animportant role (see further, Fig. 8 and others).

To sum up, the presence of crystals prior to calcination has apronounced effect on the template self-assembly and could shift

the formation mechanism from self-assembly to disrupted self-assembly. Consequently, the crystal growth should already be con-trolled during the thermal stabilization of the structure prior tocalcination. Therefore, a controlled thermal stabilization is essen-tial to fine tune the pore structure of the material to uniform poresor to a specific pore diameter.

3.2. Influence of the duration of the thermal stabilization

Fig. 6 shows again the N2-sorption isotherms of the calcinedsamples stabilized at different temperatures and a shorter thermalstabilization (4–6 h). In contrast to the previous samples, a shorterthermal stabilization was carried out to unravel the time influenceof the thermal stabilization. The calcination was done at 360 �C for2 h in all cases, as before.

Similarly, to the samples stabilized for 24 h, all the samples sta-bilized for 4–6 h are obviously mesoporous structures featuringtype IV isotherms (IUPAC classification [32]). The samples stabi-lized at 60 and 80 �C (4–6 h) have a similar isotherm obtained aftercalcination of the samples at 360 �C. The capillary condensation oc-curs in a broad relative pressure range (0.4 < P/P0 < 0.8), leading toa broad pore size distribution (Fig. 7). The pore size decreases from5.7 to 5.2 nm with the temperature, while the specific surface areaand total pore volume increase (Table 1). This indicates that the80 �C (4–6 h) sample consists of more but smaller pores. The cap-illary condensation of the 100 �C (4–6 h) sample occurs in a morenarrow relative pressure range (0.4 < P/P0 < 0.65), leading to a nar-

Fig. 7. (A) and (B) adsorption PSD of the calcined samples thermally stabilized atdifferent temperature and for different duration.

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row pore size distribution with an average pore size of 4.2 nm.Only the 100 �C (4–6 h) sample exhibits uniform and small pores,whereas the samples stabilized at other temperatures (4–6 h) exhi-bit less uniform and larger pores. In brief, with increasing temper-ature, first of all (60 �C–80 �C), the pore diameter and PSD decrease,while the surface area and total pore volume increase, indicatingthat the structure stabilized at 80 �C consists of more but smallerand more uniform pores. Then, the pore diameter and PSD of thesample at 100 �C are the lowest, and the specific surface area andtotal pore volume of this sample are lower as well. Finally, the porediameter and PSD of the material stabilized at 120 �C have increasedrastically as compared to the other materials, leading to an in-crease in specific surface area and total pore volume.

When comparing the samples with the same thermal stabiliza-tion temperature but variable duration (4–6 h vs. 24 h), a fewtrends become clear (Fig. 7). At the lower temperatures of 60 and80 �C, the pore size decreases with time (Fig. 7A). After 24 h ofthermal stabilization, a mesoporous structure with more uniformpores of 4.3 nm appears. At 100 �C, very similar uniform 4.2 nmpores already appear after 4 h thermal stabilization. However,the pore size increases again to 5.7 nm, and the pores become lessuniform after 24 h (Fig. 7B). Finally, at 120 �C, less uniform and lar-ger pores (6.8 nm) emerge, irrespective of the duration of the ther-mal stabilization.

The small angle diffraction patterns (supporting information,Fig. S5) of the samples thermally stabilized for 4–6 h resemblethose of the samples thermally stabilized for 24 h at the same tem-perature. A shift to higher 2h values during the short thermal sta-bilization at 100 �C compared 24 h at 100 �C is observed. The shiftto higher 2h values indicates a decrease in unit cell during the shortthermal stabilization, which can be correlated to the decreasedpore diameter during the short thermal stabilization. The organiza-tion of the structure decreases with the temperature. In conclusion,the organization of the mesoporous structure is independent of theduration of the thermal stabilization in the investigated range.

Fig. 8. The pore diameter (calcined samples) and the crystal size (non-calcinedsamples, based on XRD) correlated to the thermal stabilization temperature (4–6 h)and each other.

Fig. 6. (A) N2-sorption isotherms of the calcined samples thermally stabilized at60 �C or 80 �C for different duration, and (B) N2-sorption isotherms of the calcinedsamples thermally stabilized at 100 �C or 120 �C for different duration.

The wide angle X-ray diffraction data of the samples thermallystabilized at different temperatures for 4–6 h show the same corre-lation to those stabilized for 24 h (supporting information, Fig. S6).All the samples thermally stabilized for 4–6 h mainly consist ofamorphous titania with a limited amount of small anatase crystals.Indeed, the X-ray diffraction patterns exhibit a broad band due toamorphous titania on the one hand and the characteristic peaksof the anatase crystal phase on the other hand. In the same wayas the samples stabilized for 24 h, the samples stabilized for 4–6 h show an increasing crystal size with the temperature. In otherwords, at higher temperature, the anatase phase grows moreextensively, resulting in larger crystals. Comparing the samplesstabilized at the same temperature but with different duration (Ta-ble 2), as expected an increasing crystal size with the duration ofthe thermal stabilization is observed. Only the sample thermallystabilized at 120 �C shows the same crystal size independent ofthe duration of the thermal stabilization. Moreover, it is clear thatat shorter duration (4–6 h) the crystal size at 100 �C is smaller thanat 24 h. As before, the crystal sizes below 3 nm should be used as

Fig. 9. TGA and DTA of a non-calcined sample.

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guidelines and not as exact values due to the limits of the Scherrerequation. Despite the fact that the thermal stabilization tempera-ture and duration exhibit a pronounced effect on the crystal sizeof the non-calcined samples, the crystal sizes of the calcined sam-ples (at 360 �C) show no correlation with the thermal stabilization(Table 2). Even, after calcination (at 360 �C), very similar crystalsizes (�7.6 nm) are obtained independent of the duration of thethermal stabilization (4–6 h vs. 24 h). Thus, the calcination (at360 �C) determines the final crystal size of the samples indepen-dent of the nature of the crystals present before calcination.

Fig. 8 summarizes the effect of the thermal stabilization on thenon-calcined crystal size and final pore structure. The observed re-sults could be explained based on two principles. (1) The crystalsize has to be small enough to prevent disruption of the self-assembly process on the one hand. From the results of 24 h and4–6 h, specifically 24 h at 100 �C (broad PSD and large pore diam-eter) compared to 4–6 h at 100 �C, it seems that the critical crystalsize for this is between �2.2 and �2.7 nm. A thermal stabilizationat 80 �C for 24 h seems to be optimal for template self-assemblywithout disruption. In that case, the crystal size is approximatelyhalf of the micelle size. (2) On the other hand, the thermal stabil-ization needs to be sufficient to prevent structural collapse orexcessive shrinking during calcination. A short thermal stabiliza-tion at 60 and 80 �C seems insufficient to form a stable inorganicnetwork which withstands the successive thermal template re-moval, resulting in a (partial) structural collapse during calcina-tion. From the results of 60 and 80 �C, it becomes clear that amore consolidated network is formed at 80 �C resulting in a moreuniformly structured material consisting of more and smallerpores that seems to be less sensitive to structural collapse uponcalcination. The structure can therefore be regarded as more stableat 80 �C. The two above principles, which control the final porestructure, can be directed by adjusting the temperature and dura-tion of the thermal stabilization which determine the crystal size/degree of condensation in the modulable steady state and there-fore the stability of the self-assembled structure during the subse-quent calcination. A 24 h thermal stabilization at 60 and 80 �Cleads to a controlled crystal growth (no disruption of the templateself-assembly) and sufficient condensation to form a stable inor-ganic network. In contrast to the short thermal stabilization at60 and 80 �C where a structural collapse occurs during calcinationdue to insufficient condensation of the inorganic network. At100 �C, a stable inorganic network is already formed after 4 hdue to more extensive condensation at higher temperature. Fur-thermore, the limited crystal growth does not disrupt the templateself-assembly. Therefore, the final pore size of the material is direc-ted by the template leading to an expected pore size around 4 nm.By elongating the thermal stabilization at 100 �C to 24 h, too exten-sive crystal growth occurs which disrupts the template self-assem-bly leading to an increased pore diameter and broader PSD. At120 �C, too large crystals are formed independent of the durationof the thermal stabilization in the investigated range. Thus, thetemplate self-assembly is disrupted by the presence of the crystalsleading to larger (�6.8 nm) and less uniform pores.

In conclusion, on the one hand, a thermal stabilization is ap-plied to consolidate the inorganic framework prior to template re-moval, so that the self-assembled mesoporous structure ismaintained upon calcination. But, on the other hand, a too exten-sive thermal stabilization (temperature and duration) could leadto too large anatase crystals, which disrupt template self-assembly.Furthermore, all the calcined samples consist of anatase crystalsaround 7.6 nm although the crystal size prior to calcination andthe final pore size greatly depend on the thermal stabilization.Therefore, the crystal size should be controlled during the thermalstabilization in order to direct the pore size of the final structuretoward the desired specifications. As an indication, the crystal size

should not exceed more than half the micelle size of the templatein order to direct the pore size solely by the template self-assem-bly. Of course, also, the disrupted self-assembly due to the pres-ence of larger crystals during the thermal stabilization could beused to direct the pore size to higher values in an easy way withoutchanging the template. However, this would also broaden the PSD.Obviously, also, all other parameters in the process (solvent, H2Ocontent, pH, type of acid, type of surfactant, etc.) that were keptconstant in this study have an influence on the pore size andself-assembly process and on the exact optimal thermal stabiliza-tion (temperature and duration).

3.3. Template removal by means of calcination

Thermogravimetric and differential thermal analysis was per-formed on a non-calcined sample in order to get a clear view onthe thermal behavior of the sample during calcination. The TGA-DTA curves (Fig. 9) show four distinct regions. First of all, thermaldesorption of physical adsorbed water and isopropanol occurs be-low 200 �C (11.5%). Then, the TGA-curve shows a steep decreasedue to the decomposition of the template Brij 58 between 200and 350 �C (28.2%). Next, between 350 and 490 �C, titanol conden-sation of the inorganic structure occurs possibly accompanied byresidual template (Brij 58) decomposition (10%). Finally, above490 �C, no further mass loss occurs indicating that a fully con-densed titania structure is formed and that all the template is re-moved from the structure. As the template is removed around350 �C, the samples were calcined at 360, 400, or 440 �C. Indeed,the calcined samples show minor mass loss above their calcinationtemperature indicating that the template is removed from thestructure (supporting information, Fig. S7).

Clearly, after calcination at different temperatures (360, 400, or440 �C), the samples display the characteristic type IV isotherm(Fig. 10A) for all temperatures. Therefore, all the samples exhibita mesoporous structure. Furthermore, a similar total pore volumeis obtained, independent of the calcination temperature. The sam-ples calcined at 360 or 400 �C display an almost identical isothermand consequently an almost identical mesostructure with a highspecific surface area (Table 3). The capillary condensation occursbetween 0.4 < P/P0 < 0.6, which corresponds with an adsorptionpore diameter around 4 nm (Fig. 10B). Whereas the sample cal-cined at 440 �C displays capillary condensation in a higher andbroader range (0.45 < P/P0 < 0.75) indicating the formation of largerand less uniform pores with an adsorption pore diameter of6.2 nm. Moreover, the sample calcined at 440 �C exhibits a muchlower surface area although the total pore volume remainsidentical.

The small angle X-ray diffractograms (supporting information,Fig. S8) of all the calcined samples show no clear signals indicatingthat a large part of the organization is lost during calcination. The

Fig. 10. (A) N2-sorption isotherms of the calcined samples at different temperaturefor 2 h, and (B) adsorption PSD of the calcined samples at different temperature for2 h.

Table 3Adsorption pore size, apparent specific surface area and total pore volume (obtainedfrom N2-sorption) and the crystal size (obtained from wide angle XRD) of the calcinedsamples at different temperature and for 2 h. (The structure was thermally stabilizedat 80 �C for 24 h.).

360 �C 400 �C 440 �C

Øads (nm) 4.0 4.1 6.2mBET (m2/g) 130 137 92Vtot (cc/g) 0.13 0.13 0.14dcrystal,calcined (nm) 7.2 7.8 8.8

S.J.F. Herregods et al. / Journal of Colloid and Interface Science 391 (2013) 36–44 43

wide angle X-ray diffractograms (supporting information, Fig. S9)exhibit the characteristic peaks of the anatase crystal phase,although minor traces of other crystal phases could be present. Ta-ble 3 shows the crystal sizes of the samples calcined at differenttemperatures. Clearly, the crystal size increases with the calcina-tion temperature, especially between 400 and 440 �C, a significantincrease in crystal size occurs.

With the temperature, the crystal size of the calcined structureincreases as expected. The increase in crystal size during calcina-tion between 360 and 400 �C has no pronounced effect on the finalpore structure. However, from 400 to 440 �C, the crystal size aftercalcination increases more extensively. Furthermore, at 440 �C, asudden increase in pore size of the final structure is observedaccompanied by a broader pore size distribution (less uniformpores) and drastic decrease in specific surface area while the totalpore volume remains constant. As reported in literature, and anuncontrolled crystallization can result in a structural collapsewhich is harmful for the formed pore structure [5,17,26,27]. Inother words, uncontrolled crystallization must be avoided in orderto obtain a porous structure directed by the template. Therefore,the calcination temperature should be well below 440 �C unlessthe crystal size is controlled by other methods [5,42–44].

4. Conclusions

Our results show that the thermal stabilization is crucial for theformation and stabilization of small pore inorganic networks with

in situ nanoparticle formation. Moreover, a controlled thermal sta-bilization is also necessary to prevent disruption of the self-assem-bly process required to obtain ordered uniform small porestructures (dpore � 4.3 nm for Brij 58). The presence of a limitedamount of too large anatase crystals in the amorphous titaniaframework disrupts the template self-assembly. We showed thatby controlling the crystal size during the thermal stabilization(temperature and duration), one can direct the uniformity and sizeof the pores. In brief, a few principles should be kept in mind in or-der to maintain the formed mesostructure upon thermal stabiliza-tion and calcination: (1) the crystals size during thermalstabilization has to be small enough to prevent disruption of theself-assembly process, (2) the thermal stabilization needs to besufficient to prevent structural collapse or excessive shrinking dur-ing calcination, and (3) calcination under the here reported condi-tions and without extra stabilization methods should be done atmax 400 �C to keep a stable well-formed structure obtained duringthermal stabilization.

Acknowledgments

The authors thank Prof. Dr. A. Ayral and Prof. Dr. A. Julbe (IEMMontpellier) for the fruitful discussions. The authors also thankA. De Wilde and Dr. S. Mullens for the TGA measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.07.098.

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