Materials Research Bulletin 70 (2015) 184–189

Synthesis and characterization of highly reflective hollow silicaparticles

Jiwoong Kim a,b,*, Chongmin Lee b, Yong Jae Suh a,b, Hankwon Chang a,b, Ki-Min Roh a,Hee Dong Jang a,b,**aRare Metals Research Center, Korea Institute of Geoscience and Mineral Resources, Gwahang-ro 92, Yuseong-gu, Daejeon 305-350, Republic of KoreabKorea University of Science and Technology, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea

A R T I C L E I N F O

Article history:Received 22 January 2015Received in revised form 3 April 2015Accepted 20 April 2015Available online 22 April 2015

Keywords:Microporous materialsSurfacesChemical synthesisSol–gel chemistryOptical properties

A B S T R A C T

Hollow reflective silica particles with various features were synthesized from sodium silicate. The hollowarchitecture was obtained using organic templates synthesized using various initiator concentrationsand polymerization media. The concentration of sodium silicate was shown to affect the pore structureand shell thickness of the silica layers subsequently grown on the templates. Surface morphology wasaffected by the synthesis pH owing to its effects on the silica formation mechanism. The reflectivity of thehollow silica particles was measured in the UV–vis range. They were much better reflectors than acommercial reflection material and were more reflective than silica particles previously synthesized fromtetraethyl-orthosilicate, suggesting that sodium silicate is a more ecologically compatible and less costlyalternative material for the synthesis of reflective hollow silica particles.

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1. Introduction

The hollow architecture of nanometer and micrometer sizedparticles has several advantages, such as a high surface area due tothe ordered internal pores and the highly porous shell [1–5]. It ispotentially very useful in some specific applications, and thesynthesis of hollow architectures for various purposes has beenwidely studied using diverse materials. For example, they havebeen employed in the development of heat insulators [6],semiconductors [7], catalysts [8], and nanobiotechnologies[9–13]. Various inorganic materials with hollow architectureshave been prepared using ZnO, CdS, and TiO2 [14–20], but silica isone of the most popular choices due to its useful properties andabundance [21].

There are two representative silica sources: tetraethyl-ortho-silicate (TEOS) and sodium silicate (Na2SiO3). TEOS is anestablished silica precursor and is widely used, particularly toproduce highly ordered materials. However, it is more expensivethan sodium silicate, and its use leaves organic waste afterprocessing. Sodium silicate is a cheaper and less polluting silicasource; however, it contains salt ions that interfere with the

* Corresponding author. Tel.: +82 42 868 3927; fax: +82 42 868 3415.** Corresponding author. Tel.: +82 42 868 3612; fax: +82 42 868 3415.

E-mail addresses: jwk@kigam.re.kr (J. Kim), hdjang@kigam.re.kr (H.D. Jang).

http://dx.doi.org/10.1016/j.materresbull.2015.04.0460025-5408/ã 2015 Elsevier Ltd. All rights reserved.

movements of other ions in the reaction medium. Furthermore, therapid formation of silica induced by acid hydrolysis is difficult tocontrol in a typical colloidal process. Accordingly, despite theseadvantages, sodium silicate is rarely used in applications orresearch. However, its cost and environmental advantages make itdifficult to discount entirely as a silica precursor. Therefore, weinvestigated its use as a silica source for the controlled synthesis ofhollow silica particles. Highly ordered hollow silica particles wereconsistently more difficult to prepare using sodium silicate thanusing TEOS. Therefore, we sought not only effective synthesismethods but also suitable uses for the resulting less well-definedhollow silica particles, such as light reflection. This work presentsfirst a fundamental assessment of the synthesis from sodiumsilicate of hollow silica particles with various sizes, shellthicknesses, and morphologies, and secondly an exploration ofthe possibility of replacing TEOS in the production of hollow silicaparticles for light reflection applications.

2. Experimental

Styrene (99.5%, Junsei Chemical) with aqueous 2-(methacry-loyloxy)ethyltrimethylammonium chloride (MTC; 72%, Alfa Aesar)was employed as a template. 2,20-Azobis(isobutyronitrile) (AIBN;98%, Junsei) was used as the initiator for polymerization.Polyvinylpyrrolidone (PVP; mw = 30,000, Cica reagent) was usedas a stabilizer. Mono-dispersed, positively charged polystyrene

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(PS) particles with various sizes were prepared by dispersionpolymerization [22]. To synthesize the PS organic templates, theproper amounts of PVP, AIBN, H2O, ethanol, MTC, and styrene(monomer) were charged into a four-neck flask in an oil bath. Thereaction solution was deoxygenated by bubbling argon gas at roomtemperature for 30 min. It was then heated at 70 �C with stirring at100 rpm for 20 h to yield the mono-dispersed PS organic templateparticles. The stirring rate greatly influenced the final size of thetemplate particles: stirring that was too slow or too fast resulted inpoly-dispersed PS organic templates particles with a broad sizedistribution. Therefore, once optimized, the effect of stirring speedwas not further considered.

Controlling the size of the organic templates is crucial to theproduction of suitable hollow inorganic particles, because it is thedominant determinant of their final size [23]. The initiatorconcentration affected the size of the mono-dispersed styreneorganic template particles. A pure distilled water reaction medium(120 ml) was compared against one comprising a water(5 ml)/ethanol (115 ml) mixture. An unvarying amount of styrenemonomer (11 ml, �~10 g) was used in the tests to maintainconsistency.

The prepared PS particles were washed twice in distilled waterby centrifugation at 8000 rpm for 15 min. A 60 ml aliquot of themono-dispersed PS organic template particles was injected into athree-neck flask at 50 �C. Aqueous Na2SiO3 solutions (60 ml, 0.08,0.05, and 0.02 M) were blended with the PS organic templateparticles and then stirred at 100 rpm for 5 min. HCl and NH3 werethen added to the mixture to adjust it to the desired pH (2, 5, and9); it was then stirred at 80 �C. The resultant was centrifuged andwashed two or three times with distilled water to yield the silica-coated PS core–shell particles. The core–shell particles were

Fig. 1. Size of polystyrene organic template particles with respect to content of initiamicrographs are SEM images of polystyrene organic template produced in the mixture

dispersed in tetrahydrofuran (THF, Samchun Chem., 99.5%) for 12 hand then washed.

The surface morphology and shell thickness of the preparedhollow silica particles and the shape of the organic templateparticles were observed by scanning electron microscopy (FE-SEM,S-4800, Hitachi, Japan) and transmission electron microscopy(TEM, JEM-3011, JEOL, Japan). The mean particle size wascalculated by measuring the diameters of over the three hundredparticles on the SEM micrographs. Fourier transform infraredspectrometry (FT-IR, Nicolet-380, Thermoelectron, Germany)revealed the structures of the core–shell and hollow silicaparticles. Surface and pore analyses were conducted by Brunauer-–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) meas-urements (Tristar 3000, Micromeritics, USA). Reflection by thehollow silica particles was measured by diffuse reflection UV–visspectrometry (S-4100, SCINCO). The relative reflectivity of the as-prepared particles was assessed using Spectralon (Labsphere,Model No. SRS-99) as a reflectivity standard [24]. The results werecompared against those taken using a commercial reflectivematerial Insuladd (�~100 mm, Insuladd Asia) and hollow silicaparticles synthesized previously using TEOS as a silica source [23].

3. Results & discussion

3.1. Size control of organic template particles

The size of the organic template particles initially increased asthe amount of initiator increased up to 0.1 g in pure water and up to0.5 g in the water/ethanol mixture. Further addition of initiator ledto smaller particles until eventually no organic particles formed ineither reaction medium (Fig. 1). The thermal decomposition of the

tor in (a) water/ethanol mixed medium and (b) aqueous medium. (The included (upper) and the aqueous (lower) medium, respectively.)

Fig. 2. Size of polystyrene organic template particles with respect to the water content of the water/ethanol reaction medium.

186 J. Kim et al. / Materials Research Bulletin 70 (2015) 184–189

initiator to generate active radicals is important in radicalpolymerization [25]. In this case, the subsequent condensationreaction of each formed radical with a styrene monomer generateda new radical on another site of the styrene. This process of formingradicals through repeated condensation reactions continued untilthe whole monomer was consumed, until radicals combined witheach other, or until disproportionation of the living styrene.Therefore, the concentrations of initiator and polymerizationsource (i.e., the amount of monomer in the medium) greatlyinfluenced the size of the resulting organic template particles.More initiator supplied more initiating radicals, which led to moreactive reactions using the monomer and thus to more activepolymerization. However, once a critical content of initiator wasreached, increasing the number of the radicals produced too manyactivated monomers, dimers, and polymeric chains, whicheventually consumed the monomer without being able to growsignificantly. With enough initiator, the particles were not

Fig. 3. FT-IR spectra of (a) hollow silica particles and (b)

sufficiently large to resist re-dissolution into the medium. Overall,the size of the resulting organic particles increased with increasinginitiator content until a critical level, after which ever smallerparticles were produced until they finally vanished.

Micro-scale organic template particles were easily synthesizedin the mixed medium, but were difficult to detect in the aqueousmedium despite the otherwise identical reaction conditions. Thisindicates that the reaction medium was another crucial parameterthat determined the size of the organic template particles. Fig. 2shows the sizes of particles synthesized in different ethanol/waterratios. Increasing the water content of the reaction medium led tosmaller particles.

The polarity of the reaction medium is important in polymeri-zation [22,25]—in this case because it determined the solubility ofthe styrene monomer. The increased polarity of the medium withits increasing water content decreased the solubility of themonomer, because the solubility of the styrene monomer is

silica-coated organic template (core–shell) particles.

Fig. 4. (a), (b) SEM and (c), (d) TEM images of the morphology of hollow silica particles synthesized at (a), (c) pH 5 and (b), (d) pH 9. (The concentration of Na2SiO3 was kept at0.08 M.)

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inversely proportional to the polarity of the medium [26].Accordingly, the amount of polymerization sources in the mediumincreased with decreasing water content, thus inducing theformation of organic particles with relatively high molecularweights. This interpretation directly accounts for the differentcritical initiator contents observed in the different media (Fig. 1).Despite identical total amounts of styrene in both cases, theamount available to participate in the polymerization was limitedin the highly polar aqueous medium. Therefore, the critical contentof initiator in water was lower than that in the mixed medium.

3.2. Synthesis of hollow silica particles for light reflection

The chemical structures of the core–shell particles of silica-coated organic templates and of the hollow silica particles werecharacterized by FT-IR (Fig. 3). The absorption peaks at 1492, 1451,754, and 695 cm�1 were attributed to the benzene rings of theorganic template, and the peaks at 2920 and 2848 cm�1 wereassigned to methylene of the template and PVP. The FT-IR spectraof the hollow silica particles show that these benzene andmethylene characteristic peaks were either substantially reducedor absent. The characteristic peaks of Si��O��Si (1063 cm�1) andSi��OH (957 cm�1) remained in both spectra, indicating that THFtreatment removed effectively the organic template.

Fig. 4 shows SEM and TEM images of template-containingcore–shell particles and hollow silica particles in which the silicaformed at pH 5 and at pH 9. The TEM micrographs show that ahollow architecture was successfully formed in both cases. Thesilica layers formed in the early stage of the shell formation by awater-releasing condensation reaction of RSi��OH�� monomerson the surface of the organic templates. This layer showed a well-defined structure and uniform thickness. The rough surface formedafter the formation of the uniform silica layers. It consisted of a fewto tens of nanometers of silica particles.

Different surface roughness of the hollow silica particles alsoemerged under the different pH conditions owing to the formationmechanism of the silica nanoparticles. The SEM micrographs inFig. 4 show that rougher surfaces formed at the higher pH than atthe lower. This was due to the different sizes of the silicananoparticles comprising the outermost layers. Given that theparticles at the outermost layers nucleated and grew separately,their growth mechanism was governed by Ostwald ripening [27].Because pH can affect the surface charge of particles, it would be an

important determinant of the final size and structure of silicaparticles electrostatically grown from sodium silicate precursor. Ahigher pH would give a more negative surface charge to the silicaparticles in the reaction medium. The resulting increased repulsionbetween particles would accelerate the Ostwald ripening, and theparticle size was increased [28].

The thickness of the silica shell was modified at constant pH byvarying the concentration of sodium silicate precursor, or moreprecisely the ratio of sodium silicate precursor to organic template(the S/O ratio). SEM images (Fig. 5) show no noticeable differencesin the surface morphology of the core–shell particles dependentupon the concentration of precursor; however, TEM revealed thatshell thickness remarkably increased with its increasing concen-tration. The thickening shell also changed from having a smooth toa rough surface, suggesting the presence of a separate growthmechanism of silica nanoparticles for the rough surface at a highconcentration of the silica precursor. This strongly supports a two-step formation mechanism of the rough surface, indicating thatsufficient silica source is essential to determine the surfaceroughness.

3.3. Light reflection

The reflectivity of particle depends on several features such assize, shape, shell thickness, pore structure, and surface morpholo-gy [29]. We varied the concentration of sodium silicate and the pHto achieve hollow silica particles with differing shell thickness andsurface morphology, and thus reflectivity. For simplicity, all theparticles were assumed to be of similar shape (i.e., spherical).

Size greatly determines the way in which particles scatter light.Generally, Rayleigh and Mie scattering respectively describe thescattering by nanometer and micrometer sized particles. Rayleighscattering theory states that volumetric scattering by particles isexpected to be proportional to the cube of their diameter (i.e., �d3).In the Mie scattering region, however, smaller particles scattermore greatly, with a relationship related to the inverse of thediameter (�d�1). Consequently, particles of a critical diameterbetween the two regimes will show the greatest scattering [30].Accordingly, only particles with diameters of 0.9–1.2 mm, situatedbetween the two scattering regions, were considered here.

The hollow silica particles reflect UV–vis light well. Thereflectivity of the prepared hollow silica particles was comparedagainst that of a TEOS [31,32] and a commercial reflector, Insuladd

Fig. 5. Morphology and shell thickness of hollow silica particles with respect to the concentration of Na2SiO3 used in synthesis: (a), (b) 0.08 M, (c), (d) 0.05 M, and (e), (f)0.02 M. (The pH of the reaction medium was kept at �~5.)

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(Fig. 6(a)) [33]. The Spectralon was used as a standard referencematerial [24,34,35]. Results from a commercial reflective materialInsuladd (�~100 mm, Insuladd Asia) are included for comparison.They clearly showed much better reflection of UV–vis light thanthe commercial material. Also, the hollow silica particles showedcomparable reflectivity to well-defined hollow silica particlespreviously synthesized using a TEOS source [23,33]. Theirreflectivity in the UV range was significantly greater than thatachieved in previous investigations [33,36].

To elucidate the variation of the reflectivity of the particles,their BET specific area and average BJH pore diameter weredetermined and listed in Table 1. The specific surface area and

Fig. 6. UV–vis spectra of hollow silica particles synthesized from sodium silicate and from(b) constant amounts of Na2SiO3 at different pH conditions. The Spectralon was used

reflective material Insuladd (�~100 mm, Insuladd Asia) are included for comparison.

average pore volume increased with the increasing concentrationof sodium silicate precursor at the constant pH 5. The scattering oflight by particles is greatly influenced by their surface area anddegree of porosity. A large surface area and high pore volumeprovide a number of scattering spots, which leads to increasedlight scattering [29]. Though the light scattering is directlyproportional to pore size [37] in the nanoscale, the reflectivityof sample b is slightly lower than that of sample c. It seems thatpore size has a lesser effect than other properties such as surfacearea. In addition, the effect of surface roughness on the reflectivityof the particles is examined in Fig. 6(b). While synthesis at differentpH caused different surface roughness (morphology) (Fig. 4), it did

TEOS: relative reflectivity of the particles (a) with different precursors at pH 5 andas a standard reference material (R = RHSPs/RSpectralon). Results from a commercial

Table 1Effects of Na2SiO3 concentration on specific surface area, pore diameter, and volume. (The pH of the medium was kept at �~5.)

Sample BET BJH

Specific surface area (m2/g) Pore diameter (nm) Pore volume (cm3/g)

a (0.02 M) 1.71 (�1.71) 8.52 (�0.42) �0.01b (0.05 M) 36.32 (�6.02) 14.59 (�0.36) 0.11 (�0.01)c (0.08 M) 232.71 (�17.10) 9.49 (�0.38) 0.54 (�0.12)d (TEOS) 216.22 (�8.05) 7.42 (�0.27) 0.41 (�0.15)

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not greatly affect the particles’ reflectivity of UV–vis lightcompared with the surface area and pore volume. The resultsshow that the key factors are the large surface area and the porevolume, which lead to the high reflectivity of hollow silicaparticles.

Overall, these results indicate that TEOS, an expensive silicasource, might be replaceable by less costly sodium silicate tosynthesize hollow silica particles, especially for reflection appli-cations.

4. Conclusions

Hollow silica particles for light reflection were synthesizedusing sodium silicate, a less costly silica source with a lowerenvironmental impact than those currently often used [38].Organic templates of various sizes were synthesized and used toform hollow silica particles of various sizes. The size of thetemplate depended significantly upon the concentration of theinitiator and the polarity of reaction medium. The shell thicknessand surface roughness (morphology) of the particles weremodified using various concentrations of sodium silicate andvarious pH conditions, respectively. As synthesized hollow silicaparticles represented the extraordinary reflectance not only invisible but also in UV light region. This indicates the hollow silicaparticles can be used as UV blocking applications, such ascosmetics and finishing materials for buildings. Also, we expectthis study to provide a basis for synthesis using organic templatesof other inorganic nanometer and micrometer sized particles withhollow architectures.

Acknowledgements

This work was supported by the Basic Research Project of theKorea Institute of Geoscience and Mineral Resources (KIGAM)funded by the Ministry of Knowledge Economy.

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