Journal of Colloid and Interface Science 316 (2007) 211–215www.elsevier.com/locate/jcis

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Rapid evaporation-induced synthesis of monodisperse budded silica spheres

Hongmin Chen a,b, Junhui He a,∗

a Functional Nanomaterials Laboratory and Key Laboratory of Organic Optoelectronic Functional Materials, Technical Institute of Physics and Chemistry,Chinese Academy of Sciences (CAS), Beijing 100080, PR China

b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China

Received 10 April 2007; accepted 23 August 2007

Available online 28 August 2007

Abstract

Budded silica spheres have been synthesized by a novel rapid evaporation-induced self-assembly combined with the well-known Stöber method.The morphology of budded silica spheres were examined by transmission electron microscopy, and their mean size and size distribution were alsoestimated. Both the temperature of the sol–gel reaction and following post-treatment were found to play crucial roles in determining the surfacemorphology of obtained silica spheres and the yield of budded silica spheres. The possible formation mechanism was also proposed on the basisof experimental observations. The budded silica spheres would have higher surface areas than smooth silica spheres, and significant potentials forcatalyst supports, building blocks of photonic crystals, and for constructing superhydrophobic and superhydrophilic surfaces.© 2007 Elsevier Inc. All rights reserved.

Keywords: Self-assembly; Budded silica spheres; Evaporation-induced synthesis; Hierarchical structures; Stöber method

1. Introduction

Colloidal silica spheres have been attractive for their uniqueoptical, biological and other technological significances, as wellas their important industrial applications [1–4]. Synthetic para-meters, such as pH value, temperature and amounts of reactants,and mechanisms behind the formation and growth of silica par-ticles have been investigated and discussed [5–8]. For exam-ple, Stöber et al. prepared size-controlled monodisperse silicaspheres of smooth surfaces by controlling basic conditions inreaction mixtures [5]. Recently, various silica particles of hier-archical structures have been reported. For example, Xia et al.introduced fresh tetraethylorthosilicate (TEOS) into a colloidalsuspension of silica spheres and prepared dimers of colloidalsilica spheres in higher yields [9]. Chen et al. prepared buddedsilica hollow spheres by an emulsion-templating approach inwhich the sodium salt of N -lauroylsarcosine was used as botha surfactant and an oil phase after acidification [10]. Sun et al.deposited silica nanoparticles on silica spheres by alternate ad-sorption of sodium silicate and poly(diallyldimethylammonium

* Corresponding author. Fax: +86 10 8254 3535.E-mail address: jhhe@mail.ipc.ac.cn (J. He).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.08.046

chloride) [11], and the coatings of the obtained silica spheresshowed superhydrophobic property after modification with flu-oroalkylsilane [12]. On the other hand, we fabricated raspberry-like nanostructures (RNs) by assembly of small silica nanopar-ticles on large silica spheres. Coatings of such RNs exhibitedsuperhydrophilic and antifogging properties [13]. Meanwhile,drying-mediated self-assembly of nanoparticles has recentlybecome an attractive approach for preparing nanomaterials withunique hierarchical structures and desired properties [14,15].

Previous methods of preparing budded silica spheres dependon complicated, time-consuming processes, and much simplerapproaches are desirable. In this work, we reported a facile ap-proach to preparation of hierarchically structured monodispersebudded silica spheres, which has advantages of both the static-solution process and the drying process [16,17]. Mechanismsthat had not been uncovered previously in the Stöber methodwere also discussed.

2. Experimental

2.1. Materials

Ammonia solution (25%, A.R.) and ethanol (�99.7%, A.R.)were purchased from Beijing Chemical Reagent Company.

212 H. Chen, J. He / Journal of Colloid and Interface Science 316 (2007) 211–215

Tetraethoxysilane (TEOS, 99.9%) was purchased from AlfaAesar. All chemicals were used without further purification.Pure water used in all preparations had a resistivity of 18.2M� cm, and was obtained from a Milli-Q system (Millipore).

2.2. Preparation of colloidal silica spheres

Syntheses of colloidal silica spheres generally followedthe Stöber method. However, different conditions and post-treatments were used, resulting in different morphologies.

In a typical procedure, 6.36 mL of water, 0.9 mL of ammo-nia solution and 14.76 mL of ethanol were mixed, and stirredfor 30 min to form a homogeneous solution. Then, 0.98 mLof TEOS was dripped into the solution. To control the hydrol-ysis process of TEOS, the temperature was kept below 18 ◦C.The resulting solution was aged for a certain period of time atthe same temperature. It was divided into four parts, each ofwhich was post-treated in a different way. The first part wascentrifuged (4000 rpm, 10 min) and washed three times withethanol. The obtained silica spheres were dispersed again in10 mL of ethanol by sonication for 20 min. A droplet of the sus-pension was transferred onto a carbon-coated copper grid, andwas dried at room temperature for 24 h (Product A). The sec-ond part was evaporated at 60 ◦C in air, and the obtained solidproduct was again dispersed in 10 mL of ethanol by sonicationfor 20 min. A droplet of the suspension was transferred onto acarbon-coated copper grid, and was dried at room temperaturefor 24 h (Product B). The third part was directly transferredonto a carbon-coated copper grid, and was dried in air at roomtemperature for 24 h (Product C). The forth part was also di-rectly transferred onto a carbon-coated copper grid, and dried,however, in vacuum at room temperature for 24 h (Product D).

To further test the validity of the current approach for thesynthesis of budded silica spheres, the amounts of the precur-sors were all increased tenfold. In a typical procedure, 63.6 mLof water, 9.0 mL of ammonia solution and 147.6 mL of ethanolwere mixed, and stirred for 30 min to form a homogeneous so-lution. Then, 9.8 mL of TEOS was dripped into the solution.Other conditions were identical to those used in the preparationof Product D.

2.3. Characterization

Transmission electron microscopy (TEM) measurementswere carried out on a JEOL JEM-200CX transmission electronmicroscope at an acceleration voltage of 150 kV. Samples weretransferred onto carbon-coated copper grids and dried in vac-uum or in air at room temperature for 24 h before observation.Histograms of silica nanoparticles were obtained by samplingmore than 100 particles. From these histograms, their mean sizeand size distribution were estimated.

3. Results and discussion

Several parameters in the sol–gel process, such as co-solvents and water, pH value and temperature, are already

Fig. 1. Effects of post-treatments of reaction mixture on the surface mor-phologies of as-synthesized colloidal silica spheres: centrifugation and washing(A and B), evaporation of reaction mixture at 60 ◦C in air (C and D), below18 ◦C in air (E and F), and below 18 ◦C in vacuum (G and H), respectively.

known to influence the size and morphology of colloidal sil-ica spheres [5–8]. However, the effects of post-treatments havenot been studied yet. On the basis of these considerations, wetried to explore the effects of post-treatments on the size andmorphology of colloidal silica spheres. By centrifugation ofthe reaction mixture and following washing the product withethanol, only silica spheres of smooth surfaces were obtained(Product A, Figs. 1A and 1B). By evaporation at 60 ◦C in air,also only silica spheres of smooth surfaces were obtained (Prod-

H. Chen, J. He / Journal of Colloid and Interface Science 316 (2007) 211–215 213

Fig. 2. TEM images of budded silica spheres obtained by evaporation below18 ◦C in vacuum, and histograms of the buds on their surfaces.

uct B, Figs. 1C and 1D). By drying in air at room temperature,however, silica spheres of rough surfaces were mostly obtained(Product C, Figs. 1E and 1F). By drying in vacuum at roomtemperature, surprisingly, budded silica spheres (BSS) of silicananoparticles on silica spheres were mostly obtained (Prod-uct D, Figs. 1G and 1H).

Figs. 2C and 2D show the histograms of the silica nanopar-ticles (pointed by arrows) on the surfaces of the silica spheresin Figs. 2A and 2B, respectively. The mean diameters of silicananoparticles were estimated to be ca. 11.2 and 18.9 nm, re-spectively. Fig. 3 shows a plot of the BSS yield versus the tem-perature of the sol–gel process (see the curve of filled cubes).The yield of BSS decreased abruptly from ca. 90% at below18 ◦C to ca. 20% at 22.3 ◦C. Thus, the temperature of the sol–gel reaction must be kept below 18 ◦C to control the hydrolysisprocess of TEOS. Fig. 3 also shows the effect of post-treatment.Even if the sol–gel reaction was carried out at below 18 ◦C,no BSSs were obtained after post-treatment at 60 ◦C (see thepoint of filled triangle). Clearly, both the temperature of thesol–gel process and the post-treatment play crucial roles in de-termining the surface morphology of silica spheres. To furthertest the validity of the current approach for the synthesis ofbudded silica spheres, the amounts of the precursors were allincreased tenfold. As shown in Fig. 4, budded silica spheres ofidentical morphology were also obtained. Therefore, the cur-rent approach is applicable for enlarged production of buddedsilica spheres, and would provide sufficient building blocks forfuture studies.

The above experimental results suggest that silica spheresof varied morphologies could be prepared by applying variedpost-treatments, as depicted in Scheme 1. Each process shouldinvolve, first, the preparation of a solution containing ethanol,water, ammonia and TEOS. The alcohol acts as a co-solvent,

Fig. 3. Dependence of the yield of budded silica spheres on the temperatureof the sol–gel process (2). The triangle (Q) indicates the BSS yield afterpost-treatment of identical reaction mixture at 60 ◦C in air.

Fig. 4. TEM images of budded silica spheres obtained by increasing the precur-sors tenfold.

as TEOS itself does not dissolve in water. Ammonia catalyzesboth hydrolysis and condensation of TEOS, and provides thesilica particles with a negative, stabilizing charge [8]. In thesecond step (Process A), hydrolysis and condensation of TEOStake place in the solution, and gradually formed a suspensionof colloidal silica spheres. In this step, it is very important tokeep the reaction temperature below 18 ◦C. Otherwise, the yieldof final BSSs will decrease. After a period of time, there ex-ist in the suspension silica spheres, oligomers and unreactedmonomers [1,4]. Thus, in the following step (Process B), variedmorphologies of silica spheres are obtained by selecting variedpost-treatments. Clearly, oligomers and unreacted monomerswere removed from the suspension by centrifugation and wash-ing with ethanol. Thus, only silica spheres of smooth surfaces(Product A) were obtained (Figs. 1A and 1B).The colloidalsilica spheres (Product B) that were obtained by evaporationat 60 ◦C in air also had a smooth surface (Figs. 1C and 1D).In fact, the evaporation of solvent at 60 ◦C in air was a so-called ripening process. In this process, oligomers and unre-acted monomers dissolved from the surface of silica spheres

214 H. Chen, J. He / Journal of Colloid and Interface Science 316 (2007) 211–215

Scheme 1. Schematic illustration of the formation mechanism of colloidal silica spheres: (A) hydrolysis and condensation of TEOS, (B) post-treatment. (Scale bar:200 nm.)

and deposited on this surface to create a new surface. Eventu-ally, spheres of rough surfaces changed to spheres of smoothsurfaces [4]. When the reaction mixture was directly droppedto a carbon-coated copper grid and allowed to evaporate below18 ◦C in air, silica spheres of rougher surfaces (Product C) weremostly obtained (Figs. 1E and 1F). Clearly, a rough layer ap-peared on the sphere surfaces. When the reaction mixture wasdirectly dripped onto a copper grid and dried in a vacuum, sil-ica spheres covered with many smaller spherical nanoparticles(Product D) were produced (Figs. 1G and 1H). The differencebetween Products C and D might result from different rates ofsolvent evaporation in air and in vacuum. Rapid evaporationof solvent by vacuum would slow down the sol–gel reactiondramatically, inducing the formation of monodisperse buddedsilica spheres [18,19]. In contrast, much slower solvent evapo-ration in air only resulted in rough surfaces.

It was surprising that budded silica spheres could be ob-tained directly by evaporating the reaction mixture in vacuum atroom temperature. Thus, it is important to understand the under-lying formation mechanism. When the suspension was droppedonto the substrate and evaporated in vacuum, both the static-solution process and the solvent evaporation process would oc-cur at the air–solution interface [16,17,20]. The colloidal silicaspheres came together due to the evaporation of solvent. As thesolvent layer thickness decreased, the attractions between silicaspheres became sufficient to induce condensation while smallersilica nanoparticles have weak enough attractions to remain

fluidized. When the last of the solvent evaporated, the inter-sphere van der Waals attractions further increased and the silicaspheres eventually became sticked to one another. The smallernanoparticles spontaneously adsorbed on the surface of silicaspheres, forming budded silica spheres. And van der Waalsforces subsequently cemented the silica spheres and smallernanoparticles together [21], as shown in Figs. 2A and 2B. Thedissociation of surface charges on these particles prevented theBSSs from aggregating with each other [22–24]. Close look re-veals that some buds are in fact hemispheroidal, as circled inFig. 1H. As pointed out above, the evaporation process in vac-uum was very fast. When the silica nanoparticles adsorbed tothe surface of the silica spheres, the initially narrow “necks”formed at the joint of particles [4]. The surface of an individ-ual particle has a positive radius of curvature, whereas that ofthe narrow neck between particles has a small negative radiusof curvature. The solubility (S) of a curved surface of radius ofcurvature (r) of a solid with interfacial tension (γSL) is relatedto the solubility of a flat surface (S0) by the following equation:

(1)S = S0 exp(2γSLVm/RT r),

where Vm is the molar volume of the solid and T is the temper-ature. Thus, smaller particles would have higher solubility, andwill generally dissolve and deposite on the narrow necks [4].Thus, the adsorbed smaller nanoparticles changed gradually tohemispheroidal particles. After 20 days, the BSSs on copper

H. Chen, J. He / Journal of Colloid and Interface Science 316 (2007) 211–215 215

grid were observed again, they still kept the hierarchical struc-ture.

4. Conclusion

In summary, we have developed a rapid evaporation-inducedsynthetic approach to fabrication of budded silica spheres. Thetemperature of the sol–gel reaction and the following post-treatment were found to play crucial roles in determining thesurface morphology of obtained silica spheres. The currentapproach combines the advantages of both the static-solutionprocess and the drying self-assembly process [16,17,25,26].The static-solution process provides appropriate conditions forformation of the budded morphology, and the evaporation ofsolvents drives both small and large spheres to form the denseparticle assemblies, i.e. budded colloidal silica spheres. Furtheroptimization of the approach is now being carried out to providefiner control over the size and morphology of the budded silicaspheres. The budded silica spheres would have higher surfaceareas than smooth silica spheres, and may be used as catalystsupports [27], building blocks of photonic crystals [28–30], andfor constructing superhydrophobic and superhydrophilic sur-faces [31,32].

Acknowledgments

We are grateful to the “Hundred Talents Program” ofCAS, the National Basic Research Program of China (GrantNo. 2006CB933000), and the National Natural Science Foun-dation of China (Grant No. 20471065) for financial supports.

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