Materials Letters 109 (2013) 257–260

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Materials Letters

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Synthesis of three-dimensionally ordered macroporous silica spheresby evaporation-induced assembling template process

Chong Cheng, Tong-Zai Yang n, Yu-Hui He, Liang Yang, Li-Xiong WangInstitute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China

a r t i c l e i n f o

Article history:Received 31 March 2013Accepted 18 May 2013Available online 23 July 2013

Keywords:Colloidal crystalEvaporation-induced self-assemblySiO2 spheresThree-dimensionally ordered macropores

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esponding author. Tel.: +86 816 2494384; faxail address: chengchong2010@sina.com (T.-Z.

a b s t r a c t

Three-dimensionally ordered macroporous silica microspheres were fabricated by colloidal crystaltemplating method. The colloidal crystals were assembled through the crystallization of monodispersecharged polystyrene (PS) particles during the evaporation of suspension droplets in the self-assemblyprocess. Viscous silicone oil was used as dispersion medium to confine specific geometry. Followed byinfusing silica precursor into colloidal crystals and a final calcination for PS removal, silica skeleton wasformed in the interstices and macropores were left at PS′s sites. The macroporous silica made by thismethod was macroscopically spherical with a diameter of few tens of micrometers and the pores wereclose-packed and well-ordered in three-dimensions. This paper provides a facile technique to fabricateordered colloidal crystal microballs and macroporous materials.

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1. Introduction

Three-dimensionally ordered macroporous (3DOM) materialsexhibit significant properties such as well-defined porosity, uniquesurface structure and high surface area. These advantages provide3DOM materials with great potential applications, such as inphotonic band-gap materials, chemical and biochemical sensors,catalysts, and chromatographic separation [1–6]. Especially,ordered macroporous silica spheres with a pore size of a fewhundred nanometers, displaying excellent temperature and sol-vent resistance, high mechanical strength, easy pore functionali-zation, and prompt mass transfer ability, are the most appreciatedmaterials for the chromatographic packing [7,8]. Regularly shapedmaterials with multiple macro-/mesoporous structure have theadvantages of exhibiting shorter diffusion path length, higherfilling density, and lower back pressure during usage. Thusmacroporous supports can be designed to maintain high chroma-tography efficiency and excellent trapping ability of large guestmolecules [9].

Recently, the techniques of highly ordered colloidal crystaltemplating for macroporous materials have been studied by anumber of researchers [10,11]. However, the macroporous materi-als are generally in the form of macroscopically irregular shapessuch as films or blocks [12,13]. Considering their practical applica-tions and unique properties, it is desirable to synthesize them aswell-shaped objects with good uniformity in size and morphology,

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such as photonic balls. Herein, we report on the fabrication ofthree-dimensionally ordered macroporous silica microspheresfrom colloidal crystals, which are in good sphericity and possessnarrow pore size distribution with highly ordered packing. Thestructures of as-obtained colloidal crystals and macroporous silicaspheres have been characterized in detail.

2. Experimental

Fig. 1 is the schematic of the three-step preparation method for3DOM SiO2 spheres. First, non-cross-linked monodisperse PSparticles synthesized by emulsifier-free emulsion polymerization[14] were assembled into colloidal crystal templates. A suspensionsystem was prepared by suspending PS latex droplets in theviscous silicone oil to confine geometries [15]. The concentrationof PS particles was adjusted to 3–5 wt% and the volume ratio of PSlatex to oil phase was 1:20. The suspension system was vigorouslystirred, and then colloidal crystallization of PS particles graduallyoccurred in droplets when the water was evaporated at 80 1C. Theoil soaked into the structure was washed with petroleum etherand dried in the air. Second, the colloidal crystals were immersedin silica precursor solution for one week. The precursor solutionwas composed of tetraethylorthosilicate (TEOS) and ethanol(volume ratio was 1:1). The infiltrated spheres were washed withethanol and then hydrolyzed with moisture from the atmosphere.Finally, the PS particles were removed by calcination at 600 1C for5 h. The heating rate was fixed at 2 1C/min.

The structure of colloidal crystal templates and macroporoussilica spheres was characterized by a scanning electron microscope

Fig. 1. Schematic of the fabrication method for 3DOM silica spheres.

C. Cheng et al. / Materials Letters 109 (2013) 257–260258

(SEM: Carl Zeiss, Ultra 55). The Brunauer–Emmett–Teller (BET)specific surface area and pore volume by N2 adsorption/desorptionat 77 K were performed by an automated surface area and poresize analyzer (Quantachrome instruments, Quadrasorb SI).

3. Results and discussions

The first step of the process to obtain 3DOM silica balls is theself-assembly of PS particles into a well-ordered colloidal crystalthat will be used as template. The experiments were conductedwith the charged monodisperse PS latexes in the size range of400–500 nm as shown in Fig. 2a. A good uniformity of PS micro-spheres can be found. The inset of size distribution indicates thatPS latex spheres have an average diameter of 423 nm and themonodispersity is about 2.5%, which is crucial for the self-arrangement into ordered colloidal crystals.

The colloidal balls of latex particles were obtained by theevaporation-induced assembling process in a suspension systemin which silicone oil was used as continuous phase and PS latexdroplets suspending in the oil as disperse phase. A sufficientlyhigh oil/water volume ratio and constant stirring rate wereessential for the stable suspension system, as the droplet aggrega-tion or destruction caused by low phase ratio and stirringdisturbance should be omitted. With the combined effect ofshearing force, surface tension and electrostatic repulsion force,the suspension droplets were in dynamic equilibrium and shapedinto microballs. The ball size decreased monotonically with timeduring the evaporation of water. As the PS concentration exceededa certain transition value, the colloidal crystallization occurred andPS latex spheres were automatically arranged into a closelypacking structure. Fig. 2 shows the SEM images of crystal ballsobtained under these conditions. Colloidal crystal balls with goodsphericity can be observed. The surface of the crystal ball is in ahexagonally close-packed arrangement, even though some lineardefects or vacancies exist throughout the crystal ball. These defectscan be attributed to several reasons, such as size deviation of PSspheres, or plane dislocations. There are three possible stackingstructures of closely packed plane: hexagonal close packing (hcp),face-centered cubic (fcc) and a mixture of hcp and fcc. The internalstacking of PS particles (Fig. 2d) displays that each PS particle is

surrounded by six neighbors in one plane and by three otherneighbors from upper and lower planes, forming an ABC type asfcc structure. The experimental results are well agreeable withtheoretical calculations that fcc structure is more stable than hcpstructure [16].

It is concluded that the droplet-assisted method can provide aconvenient and controllable way for well-shaped colloidal crystalswith high ordering, which is also regarded as the most productivemethod to obtain colloidal crystals [1]. The size of the crystal ballscan be controlled by changing the experimental conditions, suchas the solid content of suspension droplet, stirring speed and theratio of oil phase to aqueous latex. The percentage of crystal ballswith diameter of 100–150 μm accounted for over 60% undercertain conditions.

The as-prepared PS latex crystal balls were immersed in silicaprecursor solution, which was diffused into the interspacesbetween PS latex particles by capillary force. After fully infiltrationfor several days, the crystal balls were filtrated out and exposed tothe air. With moisture from the air, the silica precursor washydrolyzed, and the organic–inorganic composites were obtained.It deserves to pay special attention that the excess silica precursorremaining on the surface of crystal balls should be washed.Otherwise, the residual precursor solution will gel into a thickfilm on the macroporous balls. In this study, we treated the latex/silica composite balls with isopropanol. Finally, the PS latextemplates inside the silica were removed by calcination at600 1C for 5 h, leaving ordered void space at their sites as shownin Fig. 3. From the low-magnification SEM image of macroporoussilica (Fig. 3a), the macroporous silica maintains well sphericalshapes. Fig. 3b shows that the voids’ size is in the range from 320to 360 nm in diameter, which is 15–24% smaller than the PS latexparticles. Samples undertake shrinkage during calcinations. Thewall thickness of the voids is about 48 nm. As noted from SEMimages, the macroporous silica spheres are the conversely dupli-cated production of crystal balls, which consist of uniform close-packed spherical pores and hexagonal silica ring skeleton, andchannels connecting pores of different planes indicate that thesample is an open-pore structure. There is a clear evidence thatthe pores are interconnected in three dimensions and the orderingextends to the whole volume of the silica balls (Fig. 3c). Themacropores traverse the whole balls, which may realize rapid

Fig. 2. SEM images of (a) the monodisperse PS particles (the inset shows the size distribution of PS), (b) colloidal crystal ball, (c) top view of the PS colloidal crystal ball and(d) the fractured section of a broken colloidal crystal ball.

Fig. 3. SEM images of (a) 3DOM silica spheres, (b) top view of the silica surface and (c) the cross-section of broken 3DOM SiO2 sphere.

C. Cheng et al. / Materials Letters 109 (2013) 257–260 259

C. Cheng et al. / Materials Letters 109 (2013) 257–260260

mass transfer ability and unique optical properties. Meanwhile,plenty of mesopores spread over the skeleton, which provide highsurface area. The BET surface area of silica balls was calculated tobe 466.6 m2/g and the total mesopore volume was 0.75 cm3/g.

4. Conclusion

In this study, we synthesized ordered macroporous silicaspheres by utilizing colloidal crystals as templates. The templateswere assembled by colloidal crystallization process of negativelycharged monodisperse PS latex particles. The shape of the colloidaltemplates was turned into sphere using evaporation-inducedsuspension system. The colloidal balls displayed a face-centeredcubic structure extending to the whole material with a diameter ofa few hundred nanometers. The prepared macroporous sphereswere composed of silica frameworks and uniform close-packedspherical pores interconnected with each other. The pore size andwall thickness of macroporous spheres could be tuned by usingdifferent templates. The ordering of macropores endows thematerials with outstanding properties such as large internal sur-face areas and high porosity, which can find potential applicationsin optical materials, catalysis, and chromatographic separation.

Acknowledgment

This work was financially supported by the National NaturalScience Foundation of China (Grant no. 91026022).

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