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Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 39–46

Porous nanoballs formed through an in situgenerated “framework” template

Fei Teng, Li Wang, Yongfa Zhu ∗Department of Chemistry, Tsinghua University, Beijing

100084, China

Received 23 February 2007; received in revised form 10 June 2007; accepted 14 June 2007Available online 19 June 2007

bstract

The unique hierarchic porous nanoballs were constructed through a controllable hydrolysis process in 40–60 vol.% ethanol aqueous solution.he effects of ethanol concentration, hydrolysis duration, solvent, heating way on the formation of the porous structures have been investigated.

he samples were characterized by TEM, EDX, SEM, XRD, TG, and pH apparatus, etc. An in situ generated “framework” template was proposed

o explain the formation of porous structure. The results showed that ethanol molecules enclosed in the “framework” played an important role inhe formation of the porous structures. Importantly, the formation process of such nanoballs is useful for the structure control of porous materials. 2007 Elsevier B.V. All rights reserved.

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eywords: Porous; Nanoballs; Framework template; In situ

. Introduction

Recently, great progress has been made in research on hol-ow nanostructures, which could be applied as biomaterials [1],ano-reactors [2], and lubricates [3]. Particularly, mesoporouslter and nano-reactor play important roles in biologic chip [4],hemical detector [5], chemical sensor [6], and high-efficiencyatalyst/support [7]. Consequently, nanoballs with mesoporesill be the good candidates for manufacturing a variety of nano-evices. In all of the methods reported, mesoporous structurend nanoballs cannot be obtained simultaneously. There is noood way to prepare porous nanoballs using general templates,ince the templates with secondary structure are hard to be foundo far as we know. Fortunately, the work performed by Mannnd co-workers [8–11] provides us with some ideas to design aew way to fabricate the mesoporous structure. In their works,he porous structure was obtained using foams as templates:he pore size of the obtained sample was larger than 50 nm [8].

evertheless, it is difficult to accurately control the pore size.s a result, understanding the formation process of the pores is

mportant to control the pore structure effectively.

∗ Corresponding author. Fax: +86 10 62787601.E-mail address: zhuyf@mail.tsinghua.edu.cn (Y. Zhu).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.06.021

Herein, we reported, for the first time, a facile synthesis ofAl4O7 (M = Sr, Ba) nanoballs with porous structures through

n in situ generated “framework” template. The nanoballs withnique hierarchic nanopores were constructed by the control-able hydrolysis of MAl2O4 (M = Sr, Ba) spinel powders in0–60 vol.% ethanol aqueous solution, in which the pore sizesere handily controlled in the range of 10–30 nm. The effects of

thanol concentration, hydrolysis duration, drying ways and thentensity of electron beam on the pore structures were investi-ated. Besides, the annular arrangement of pores with a diameterf 10 nm resulted in hierarchic porous nanoballs. Most impor-antly, a “framework” template generated in situ was proposedo explain the formation of the porous structures, which is sig-ificantly different from the typical template process. The ideaould provide an important contribution to the structure control

or the porous structures.

. Experimental section

.1. Preparation of spinel SrAl2O4 and BaAl2O4 powders

The SrAl2O4 and BaAl2O4 spinels were prepared by a non-rystalline route, as reported in our previous research [12,13].ypically, the fresh Al(OH)3 colloid was prepared by droppingH3·H2O into an Al(NO3)3 solution, followed by washing for

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everal times. Then H5DTPA solution was mixed with freshl(OH)3 and SrCO3 in a calculated molar ratio, heated and

tirred with a magnetic stirrer to promote the formation of hetero-ucleus complexes. After the solution became transparent, it wasried at 60–80 ◦C to yield the precursor. X-ray diffraction (XRD)atterns illustrated that the precursor was amorphous. Nano-ized spinel SrAl2O4 was synthesized by calcining the precursort 600 ◦C for 3 h in air. A slow heating rate (5 ◦C min−1) wassed to increase temperature to various pre-set temperatures andaintained for a defined period of time to promote the forma-

ion of SrAl2O4 with spinel structure. BaAl2O4 spinel was alsorepared by the similar method.

.2. Preparation of porous nanoballs

The spinel-structured SrAl2O4 powder was dispersed in0–60 vol.% ethanol aqueous solution, in which the molar ratiof SrAl2O4 to the mixture solvents was controlled above 1:40;

nd then the mixture system was sonicated for 10 min to pro-ote the hydrolysis of SrAl2O4. The solids in the suspensionere removed by centrifuging at 500 r min−1. After the recov-

red topper solution was exposed to air for 60 min, the solids was

top2

ig. 1. The hollow sample prepared by the hydrolysis of SrAl2O4 powders in 50 vol.op images of SEM; (d) side images of SEM.

ochem. Eng. Aspects 312 (2008) 39–46

ecovered by high-speeded centrifugation (5000 r min−1) andhen dried naturally in air. BaAl2O4 spinel was used to preparehe porous counterpart.

.3. Characterization

The morphology of the sample was characterized with aEM (Japan Hitachi H-800, accelerating voltage of 200 kV)quipped with a Link ISIS EDX analyzer. The same instrumentas involved in the recording of X-ray maps. The accelerationoltage in all cases was 15 keV and the current was 1.2 nA. Inhe recording of X-ray maps the current of 6.7 nA was used.he powders were dispersed in ethanol ultrasonically, and then

he samples were deposited on a thin amorphous carbon filmupported by copper grids. The morphology and compositionalistribution of the sample in the samples was determined byse of a SEM (Hitachi S-450). TG curve was performed onyris 1 TGA thermogravimeter (U.S. Perkin-Elmer Co.), and

he temperature rises from room temperature to 600 ◦C at a ratef 10 ◦C min−1. pH values of the solution were determined withH apparatus (PHS-25CW, Shanghai LIDA Instrument Co.) at5 ◦C.

% ethanol aqueous solution for 60 min: (a) TEM images; (b) EDX spectra; (c)

Physicochem. Eng. Aspects 312 (2008) 39–46 41

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. Results and discussion

.1. Effect of ethanol concentration on the porousanoballs

The hydrolysis of SrAl2O4 powders was carried out in0–60 vol.% ethanol aqueous solution at room temperatures toynthesize the hierarchic porous nanostructures (See supportingnformation of experimental section). Fig. 1a gives the typicalEM images of the obtained nanoballs in 50 vol.% ethanol aque-us solution. It is clear that the nanoballs of 80 nm in diameterre well uniform. A clair-obscure contrast between the centrend outer regions of the nanoballs can be observed clearly, con-rming that the interesting nanostructures with hollow interiorsf about 10 nm in size. The sample compositions were furtherharacterized by EDX analysis, as shown in Fig. 1b. The analysisesults show that the sample is comprised of Al and Sr, and thetomic ratio of Sr to Al is 1:4, since Cu signals resulted from cop-er grid. The sample was further characterized by SEM (Fig. 1cnd d). The top and side SEM images of the sample show theormation of three-dimensional ball-like structures with smoothurface. Since the C, O and H elements cannot be detected byDX spectra, TG analysis was performed to determine the sam-le compositions, as shown in Fig. 2. In the range of 30–600 ◦C,wo peaks of weight loss in TG curve can be observed. In theange of 30–250 ◦C, the weight loss (14 wt.%) of the samplean be ascribed as to the removal of water, ethanol, and part ofydroxyls; in the range of 250–600 ◦C, the weight loss (7 wt.%)f the sample can be caused by the removal of hydroxyls. Byalculation, the sample is comprised of SrAl4O5.5(OH)3 andontains 14 wt.% ethanol. The molar ratio (1:4) of Sr to Al isonsisted with the analysis result of EDX above. Combing with

he preparation and the used chemicals, we could deduce that theample may have the structure very similar to SrAl4O7, whichs a long afterglow phosphor [14]. This could be also confirmedy XRD patterns (See supporting information of Fig. S1).

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ig. 3. TEM micrographs of the porous structures prepared by the hydrolysis of SrAlolutions: (a) 40 vol.%, exposed to electron beam for 5 min; (b) 50 vol.%, exposed to

ig. 2. TG curves of the sample prepared in 50 vol.% ethanol aqueous solutionor 60 min.

The porous structures of the sample can be handily adjustedy controlling ethanol concentration, as typically shown inig. 3. The smooth and uniform mesoporous nanoballs (100 nm

n diameter) were obtained in 40 vol.% ethanol aqueous solu-ion (Fig. 3a). After the sample was exposed to electron beamor about 8 min, a large number of tiny pores less than 5 nmn size were formed. It is interestingly that the pores in theringe parts are larger than those in the centre region (the insertf Fig. 3a). These porous nanostructures show higher stabil-ty under exposed electron beam than those formed in 50 vol.%thanol aqueous solution. This probably means that there wereess solvents enclosed within the nanostructures. In 50 vol.%

thanol aqueous solution, the sizes of the formed nanoballs are0 nm; after being exposed to electron beam for 2 min, the largeores have the diameters of about 15 nm and the shell thickness isbout 5 nm (Fig. 3b); it is interesting that the multi-shelled struc-

2O4 powders for 60 min at different ethanol concentrations in ethanol aqueouselectron beam for 2 min; (c) 60 vol.%, exposed to electron beam for 2 min.

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ures were formed (the insert of Fig. 3b). When the volume ratiof ethanol to water was increased to 60 vol.%, the formed hol-ow nanostructures immediately deformed to some extent evennder exposed to electron beam for 2 min. Differing from thoseormed in 40 vol.% ethanol aqueous solution, the small hollowalls connected with one another, whose average diameters andhickness of shells are about 50 and 5 nm, respectively. This maye ascribed to the high concentration of ethanol in the mixture.owever, only very fewer of hollow nanostructures contained

he tiny pores (Fig. 3c).

.2. Effect of hydrolysis duration on the porous nanoballs

The effect of hydrolysis duration on the prepared samplen 50 vol.% ethanol aqueous solution was further investigatedFig. 4). After hydrolysis for 10 min, the morphology of thebtained sample is similar to that of SrAl2O4 powders, prob-

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ig. 4. TEM images of the samples prepared by hydrolysis of SrAl2O4 powders in 520 min; (d) 300 min.

ochem. Eng. Aspects 312 (2008) 39–46

bly because little hydrolysis has taken placed (Fig. 4a); afterydrolysis for 60 min, the obtained sample was composed ofhe regular nanoballs, which were much different from the orig-nal SrAl2O4 sample (Fig. 4b); in the range of 80–120 min, itas found that the number of the regular nanoballs decreased

ignificantly and some nanowires were formed; after 120 min,rCO3 nanowires with the diameters of about 20 nm and the

engths of about 3–5 �m were obtained (Fig. 4c); after 300 min,he orthogonal SrCO3 nanorods were formed (Fig. 4d). Onase of our previous study [15], SrAl2O4 hydrolyzed in pureater to form the mixture of Al(OH)3 and SrCO3, where SrCO3

esulted from the reaction between Sr(OH)2 and CO2 in air. Theolloidal Al(OH)3 agglomerated severely to precipitate out of

he solution; therefore, it could be inferred that the hydroly-is of SrAl2O4 powders have completed after 120 min. It couldlso be concluded that the hydrolysis of SrAl2O4 in 50 vol.%thanol aqueous solution was much slower than that in pure

0 vol.% ethanol aqueous solution for different time: (a) 10 min; (b) 60 min; (c)

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ater, and the formed nanoballs may be an intermediate in therocess.

.3. Effect of solvent on the formation of the porousanoballs

The above results showed that ethanol played an importantole in the formation of the nanoballs. To research the sol-ent effect, the solvent was varied from ethanol to acetone andlycol. However, in acetone–water or glycol–water, we haveot obtained the uniform nanoballs with hollow interiors. (Seeupporting information of Figs. S2 and S3), suggesting that theormation of the hollow nanostructures may be correlated to theroperties of the solvents. Considering their similar polarities,e suggested that ethanol molecules stabilized the intermedi-

te structure. While Sr dissolved and entered the solution, someholes” or vacant sites” were formed. Ethanol molecules maydsorb on and/or fill into these “holes or vacant sites”, and thentermediate could exist stably for enough time. As a result,thanol could effectively stabilize the Al-enriched framework.or glycol or acetone molecule, it was probable that they can-ot adsorb on or fill into the “holes or vacant sites” due to theirtereo effects (their large molecule sizes). As a result, the inter-ediate was unstable and the Al-enriched framework collapsed.he proof is needed to research further. The above results havehown that the mixture compositions and the hydrolysis dura-ion have significant influences on the formation of the porousanostructures.

.4. Effect of heating way on the formation of the porousanoballs

In this work, an interesting phenomenon was also observedhile the hollow nanostructures were exposed to electron beamsith TEM. Fig. 5 shows the TEM images of the mesoporousanoballs, which were prepared by the hydrolysis of spinel

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icochem. Eng. Aspects 312 (2008) 39–46 43

rAl2O4 powders in 50 vol.% ethanol aqueous solution. TheEM sample was then exposed to electron beam for differenturations. After exposed for 2 min, the nanostructures are highlyispersed, while the bare surfaces kept smooth (Fig. 5a). TheEM micrograph with a high magnification of single nanoball

s shown in the insert of Fig. 5a, indicating that the nanoball hashe multi-shelled structure. There are many tiny pores with theiameters of less than 10 nm near the wall of the nanoball, as wells the connected shells with the thickness less than 5 nm. Whileeing exposed for 5 and 10 min (Fig. 5b and c), the sizes of theores within the nanoballs increased to about 40 nm; the neigh-ouring nanoballs merged into one another, probably indicatinghat the more ethanol molecules remained in nanoballs wereeleased. As mentioned above, the release of volatile moleculesould be attributed to thermal effect by electron beams. Whilelectron beams irradiated the sample, the tiny pores immedi-tely merged together and the shapes of nanoballs deformedignificantly. At a low electron beam intensity (i.e. ×50,000agnification), the surfaces of nanostructures are smooth and

lean-cut; at a high electron beam intensity (i.e. ×100,000 mag-ification), the nanostructures deformed obviously. After beingxposed to electron beams, the tiny pores within the ball innerrstly emerged, then merged and grew bigger with the dura-

ion gradually. As a result, the mesoporous nanostructures werenally formed. It was found that while the intensity of electroneam was lowed, the deformation process would become slownd then stopped; but the process took place again while thentensity of electron beam increased again. This can be ascribedo the heating effect caused by electron beam. The deforma-ion process of nanostructures under electron beam led to aew concept for controlling porous structures. Summarily, theore structures in nanoballs could be controlled by the exposure

uration of electron beam.

To investigate the heating effect, the prepared samples wereried in oven at 60 ◦C for 10 h and in microwave oven, respec-ively. The microwave drying was used to simulate the case of

tion for 60 min but exposed to electron beam for different time: (a) 2 min; (b)

44 F. Teng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 39–46

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ast heating by electron beam. As shown in Fig. 6a, the porousanostructures were well dispersed and slightly deformed undericrowave drying. Under fast heating, the volatile solventsould expand and release quickly, leading to the nanoballs.hown in Fig. 6b, upon drying for 10 h, the nanoballs with largeollow interiors were obtained, which resulted from the merg-ng of the tiny pores. It could also be found that the thick shells loose and the outer surface is not smooth. It is clear that therying way also has a significant effect on the pore structure ofhe sample.

.5. The formation of porous BaAl4O7 nanoballs

Similarly, the hollow BaAl4O7 nanostructures were also pre-ared by the hydrolysis of BaAl2O4 in ethanol aqueous solution,s shown in Fig. 7. In 40 vol.% ethanol aqueous solution, the hol-ow nanostructures have the diameter of 100 nm and the hollownterior of 30 nm in size; in 50 vol.% ethanol aqueous solution,he majority of nanoballs with hierarchic nanopores have theiameter of 50 nm; and a few large balls with 100 nm in size cane also observed, which may result from the connection of theanoballs to one another. EDX spectra also show the molar ratiof Ba to Al is 1:4. The result is similar to that of the preparedrAl4O7.

.6. The proposed formation mechanism of the porousanoballs

To explore the formation mechanism of the nanoballs withierarchic nanopores, the hydrolysis process of SrAl2O4 pow-

ers was followed basing on the variations of pH values of theolution with hydrolysis time, as shown in Fig. 8. In pure water,H values of the system rapidly reached 12 at t = 20 min and keptonstant at t ≥ 50 min, indicating that the hydrolysis of SrAl2O4

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anol aqueous solution by different drying: (a) microwave drying for 3 min; (b)

n water was very fast and completed at 50 min. In 25 vol.%thanol aqueous solution, pH values of the system increasedontinuously and reached 12 at t = 70 min and the hydrolysisf SrAl2O4 was completed at t = 100 min, meaning that theydrolysis rate of SrAl2O4 decreased upon adding of ethanol. In0 vol.% ethanol aqueous solution, it is interesting that two plat-orms of pH values appeared. At t = 10 min, pH value reached 11;n the range of 10–80 min, pH values nearly kept a constant of 11;ith elongating the time from 80 to 110 min, pH value reached2. The results probably indicate that a metastable intermediateay be formed in the process. After 24 h, the obtained sample

s composed colloidal Al(OH)3 and wire-like SrCO3, signinghat the hydrolysis reaction stopped. XRD patterns showed thathe product was composed of crystalline SrCO3, Al(OH)3 andrAl4O7, in which SrCO3 crystals resulted from the reaction ofr(OH)2 with CO2 from air. (See supporting information of Fig.4). It is important to note that SrAl4O7 was the main phase,ut the peaks of SrCO3 and Al(OH)3 were weak. In 75 vol.%thanol aqueous solution, pH values of the system varied very lit-le from 8.2 to 8.5 within 180 min, indicating that the hydrolysisf SrAl2O4 have not almost taken place in the high-concentrationthanol aqueous solution. The effect of ethanol concentrationn the pore structure of the product has been introduced above.he results showed that after hydrolysis for 60 min, the nanoballsith hierarchic nanopores could be prepared only in the range of0–60 vol.% ethanol. At x < 40 vol.% ethanol, SrCO3 nanowiresere formed, just as in pure water; at x > 60 vol.%, the samplesere same as the original SrAl2O4, due to the very slow hydrol-sis rate in the presence of the high-concentration ethanol. In therocess, it may be that the hydrolysis of SrAl2O4 was fast in pure

ater, the Al-enriched framework could not be remained in the

ystem, and but form Al(OH)3 precipitate before reconstruct-ng to form SrAl4O7. As a result, reaction (1) may take placeelow. It is reasonable to believe that SrAl4O7 may exist as the

F. Teng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 39–46 45

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ig. 7. TEM photographs and EDX spectrum of BaAl4O7 hollow structures prb) 50 vol.%; (c) EDX spectrum of sample b.

ntermediate. In the presence of ethanol, the Al-enriched spinelramework may generate after part of Sr dissolved into the solu-ion. The “holes” or “vacant sites” left by Sr would combine withthanol molecules, so the Al-enriched spinel framework wouldxist for enough time. As a result, the Al-enriched frameworknd the remained Sr ions in the framework would reconstruct toorm SrAl4O7. The ethanol molecules were enclosed in frame-ork during reconstruction process, as shown in reaction (2)elow:

rAl2O4 + H2O → Sr(OH)2 + Al(OH)3 (1)

2SrAl2O4 + H2O + x (ethanol) → Sr(OH)2

+ (ethanol)x-SrAl4O7 (2)

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d at different ethanol concentrations in ethanol aqueous solution: (a) 40 vol.%;

Basing on the results above, a self-generated in situ “frame-ork template” mechanism was proposed to explain the

ormation of porous hollow nanostructures. During the process,he presence of ethanol molecules reduced greatly the interac-ion between SrAl2O4 and water molecules. As a result, theomplete hydrolysis of SrAl2O4 was refrained to some extent.

hile SrO units gradually left off from SrAl2O4 frameworknd dissolved into the solution under the attack by water; a verymall amount of Al ions hydrolyzed to form Al(OH)3 whichould be precipitated out of the solution; but most of Al ions

till retained in the framework due to the restrained hydrol-sis by ethanol. As a result, some “holes” or “vacant sites”

ould generate in the framework because of the dissolution ofr. The present ethanol molecules may prefer to adsorb on theholes” and/or combine with the “vacant sites” in the frame-ork; the Al-enriched framework was stabilized by ethanol

46 F. Teng et al. / Colloids and Surfaces A: Physic

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ig. 8. Variations of pH values with hydrolysis time at different ethanol con-entration.

olecules. Then, the remained Sr and Al-enriched frameworkeconstructed to form the SrAl4O7 structure, in which theon-cracked framework acted as the template in situ. In the reor-anization process, a large number of ethanol molecules werenwrapped into the nanostructures. While irradiated by electroneam with a low intensity or dried at mild condition, ethanololecules were slowly released, and then a large number of

iny pores generated. Under bombed by high intensity of elec-ron beam, ethanol molecules evaporated quickly and the tinyores tended to expand and/or merged into the large pores. As aesult, the nanoballs with hierarchic nanopores were formed. Theormation process of such porous structure is useful for the struc-ure control of porous nanoball, although more proofs are stilleeded.

. Conclusions

The unique porous nanoballs were constructed by a facilend controllable hydrolysis route, which are mostly depen-ent on the intrinsic properties of the chemicals; the porous

tructures were formed through an in situ generated “frame-ork” template. Importantly, this formation process of theorous nanoballs is useful for the structure control of porousaterials.

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ochem. Eng. Aspects 312 (2008) 39–46

. Supporting information available

XRD patterns of the prepared sample by hydrolyzing ofrAl2O4 in 50 vol.% ethanol aqueous solution, TEM pho-

ographs of the prepared samples by the hydrolysis of SrAl2O4n acetone–water, TEM photographs of the prepared samplesy the hydrolysis of SrAl2O4 in glycerol–water (50:50 volumeatio) for different time and XRD patterns of the prepared sam-le by the hydrolysis of SrAl2O4 in 50 vol.% ethanol aqueousolution for 24 h. This material is available free of charge via thenternet at http://www.elsevier.com.

cknowledgment

This work is supported by Chinese National Science Foun-ation (Nos. 20433010, 20571047).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.colsurfa.2007.06.021.

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