Fabrication of Hollow Silica Microspheres with Orderly Hemispherical

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Fabrication of Hollow Silica Microspheres with Orderly Hemispherical Protrusions and Capability for Heat-Induced Controlled Cracking Makoto Takafuji, Nanami Hano, Md. Ashraful Alam, and Hirotaka Ihara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02223 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Fabrication of Hollow Silica Microspheres with Orderly Hemispherical Protrusions and Capability for Heat-Induced Controlled Cracking

Makoto Takafuji,†, ‡ *, Nanami Hano, † Md. A. Alam, †, § Hirotaka Ihara‡,⊥ *

†

Department of Applied Chemistry and Biochemistry, Kumamoto University,

2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 Japan ‡

Kumamoto Institute for Photo-electro Organics (PHOENICS),

3-11-38 Higashimachi, Higashi-ku Kumamoto 862-0901, Japan §

Department of Applied Chemistry and Chemical Engineering, Noakhali Science and

Technology University, Sonapur, Noakhali-3814 Bangladesh ⊥

Department of New Frontier Science, Kumamoto University, 2-39-1 Kurokami, Chuo-ku,

Kumamoto 860-8555 Japan.

ACS Paragon Plus Environment

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ABSTRACT

Hollow silica microspheres with orderly protrusions on their outer and inner surfaces were fabricated in three simple steps: (1) suspension polymerization of a polymerizable monomer containing silica nanoparticles to obtain polymeric microspheres with a layered shell of silica particles; (2) sol–gel reaction of tetraethoxysilane (TEOS) on the surface of the microspheres to connect the silica nanoparticles; (3) removal of polymer core by calcination. The shell composed of silica-connected silica nanoparticles remained spherical even after calcination, and the characteristic surface morphology with protrusions were obtained on both inner and outer surfaces. Measurements of the mechanical strength revealed that the compression modulus of the hollow microspheres increased with increasing thickness of the silica layer, which could be controlled by changing the concentration of TEOS in the sol–gel reaction. Rapid heating of the hollow silica microspheres with the thin silica-connected layer led to silica shell cracking, and the cracks were mostly observed in the connecting layer between the silica nanoparticles. The stress was probably concentrated in the connecting layer because of its lower thickness than the nanoparticles. Such characteristic of the hollow microspheres is useful for a capsule with capability for heat-induced controlled cracking caused by internal pressure changes.


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1. INTRODUCTION Hollow particles have been extensively studied because of their unique properties and high potential for application in various fields. Since the size, shape, and shell thickness of hollow spheres are important parameters in all applications, many studies have focused on the control of these parameters. For instance, nanometer-sized hollow particles have been applied as drug carriers,1 gas absorbents, catalysts.2 photonic crystals,3 encapsulating ingredients,4 photocatalysts,5 and gas sensors.6 The inner space of a hollow microsphere is useful for preservation or storage of substances in the solid, liquid, or gaseous states, and the release of such substances can be controlled by adjusting the thickness and microscopic structures of the shell. There are two approaches to the synthesis of hollow microspheres: the template-based approach and the template-free approach. The template-based approach can be categorized by the type of template: hard templates7–26 (e.g., inorganic, metal, and polymer particles) and soft templates27–34 (e.g., supramolecular assemblies of surfactants and polymers). As an alternative, template-free approaches35–41 based on different mechanisms were also developed to synthesize hollow spheres with more complicated structures. Template-based approaches have been commonly used to create hollow structures because of their simplicity and easy controllability; however, the ability to construct complicated structures is limited by the availability of templates.42–52 The sol–gel process has been widely used to fabricate layered shells comprising inorganic materials like silica and titania on the templateformed particles.20,53 Hollow particles can be obtained by removal of the template-formed core using methods such as calcination.13,54–56 As described above, the morphological and structural features of the shells are important contributors to the functions of the particles, including those of hollow particles. The surface morphology is directly related to surface properties like optical


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properties and wettability.57–59 There are only a number of reports in the literature on hollow microspheres with peculiar surface morphology.60–76 It was reported that hollow microspheres with shells consisting of a monolayer or bilayer of silica nanoparticles can be prepared using a water–toluene emulsion stabilized by hydrophobic silica nanoparticles as a template in combination with a liquid hyper-branched polyethoxysiloxane precursor as a binder.63 The preparation of hollow titania spheres with urchin-like morphology by a template-free approach was reported, in which a titania precursor, TiOSO4, was solvothermally reacted in glycerol, alcohol, and ethyl ether. The surface morphology, size, and interior structure were tuned by selecting the appropriate solvent and temperature for the reaction mixture.64 Finally, hollow hydroxyapatite microspheres were selectively synthesized by assembling one-dimensional hydroxyapatite nanorods through a water-soluble polyaspartic-acid-assisted hydrothermal route.65 We have previously reported core–shell microspheres with a layered shell of inorganic nanoparticles prepared by a simple process involving modified suspension polymerization of monomer droplets containing inorganic particles,77-79 microparticulation from phase-separated viscose containing inorganic particles,80 and hybridization of polymer microspheres and inorganic particles using supercritical carbon dioxide.81,82 Various inorganic particles composed of silicon dioxide (silica), iron oxide, titanium oxide (titania), boron nitride, diamond, and carbon materials can be immobilized on the surface of polymer microspheres as the shell component. These core–shell microspheres with a layered shell of inorganic nanoparticles can be used as a precursor of hollow microspheres. In this study, we created hollow silica microspheres with hemispherical protrusions on the outer and inner surfaces. This process was based on a previously reported method we used to


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prepare core–shell microspheres with a layered shell of inorganic nanoparticles.77-79 Polymer microspheres consisting of a layered shell of silica particles with dimensions ranging from nanometers to sub-micrometers were prepared by modified suspension polymerization of polymerizable monomer droplets containing silica particles. The silica nanoparticles on the surface were chemically cross-linked with the organic polymer. Therefore, hollow microspheres could not be obtained because of aggregation of the nanoparticles concomitant with shrinking of the core polymer by calcination. To avoid the layered shell of particles collapsing due to shrinking of the core, we connected the layers of inorganic nanoparticles with silica through the sol–gel reaction before calcination. The strategy for the fabrication of hollow silica microspheres with orderly protrusions is illustrated in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Materials. The silica nanoparticles (SiPs, SNOWTEX MP-4540M) were purchased from Nissan Chemical Industries, Ltd., Japan. SEM observation and particle-size measurement showed that the average diameter of the SiPs was 433 nm, with a coefficient of variation (CV) of 3.70%. Styrene and ethyleneglycol dimethacrylate (EGDMA) were purchased from Wako Pure Chemical Industries Ltd., Japan, and were used for copolymerization after adsorbent-based removal of their respective inhibitors, p-tert-butylcatechol and hydroquinone. α,α’Azobisisobutylonitrile (AIBN) was purchased from Nacalai Tesque Inc., Japan; and was used as a thermal radical initiator after recrystallization from methanol. Poly(vinyl alcohol) (PVA; MW = 66–79 kDa, degree of polymerization: 1,500–1,800, degree of deacetylation: 78–82%) was purchased from Wako Pure Chemical Industries Ltd., Japan, and was used as an aqueous


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suspension medium. 3-Methacryloxypropyl trimethoxisilane (M) was kindly provided by JNC Co., Japan. Tetraethoxysilane (TEOS) was purchased from Tokyo Chemical Industry Co., Ltd., Japan.

2.2. Preparation of Polymer Microspheres with Layered Shell of Silica Nanoparticles. Polymer microspheres with a layered shell of silica nanoparticles were prepared using the procedure described in our previous report.77-79 The preparation process included two steps: (1) surface modification of Si with M and (2) modified suspension polymerization of polymerizable monomers containing M-modified SiP (MSiPs). In brief, Si was suspended in 1-butanol and M was added to the suspension. The mixture was gently stirred at 90 °C for 18 h. The obtained particles were thoroughly washed with 1-butanol and methanol, after which they were vacuumdried. The grafting of M molecules onto the silica surface was evaluated by diffuse-reflectance Fourier-transform infrared (FTIR) spectroscopy and elemental analysis (EA). Next, 7.5 g of MSiPs was dispersed in 100 g of a monomer mixture of styrene and EGDMA (1:1, wt/wt) containing 1.0 wt% of AIBN. The dispersion was poured into 500 mL of an aqueous PVA (5 wt%) solution in a 1 L separable reaction vessel (round bottom). The mixture was suspended by stirring with a 4-blade propeller at 550 rpm. After stirring at 40 °C for 15 min, the suspension was heated to 60 °C to initiate radical polymerization and stirred for 6 h. The experimental set up for the suspension polymerization is displayed in Figure S1. The obtained particles were collected by filtration and washed with hot water and methanol, and then vacuum-dried.

2.3. Preparation of Hollow Silica Microspheres with Orderly Protrusions. Polymer microspheres with a layered shell of MSiPs (MSiP@pS) were then coated with silica by the


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surface sol–gel reaction of TEOS.83 In brief, 2 g of MSiP@pS was suspended in 50 mL of an ethanol solution containing 2 mL of 25% ammonia solution and 4 mL of water. A given amount of TEOS (0.5 mL, concentration: 0.04 mol/L; 1.0 mL, concentration: 0.08 mol/L; 2.0 mL, concentration: 0.16 mol/L; 4.0 mL, concentration: 0.32 mol/L; 6.0 mL, concentration: 0.48 mol/L) was added to the suspension, after which the suspension was stirred at 30 °C for 6 h. The obtained microspheres (hereafter abbreviated as Tx–MSiP@pS) (x = 0.04, 0.08, 0.16, 0.32, and 0.48) were collected by filtration and washed with methanol to remove the excess TEOS. After drying, the microspheres were calcinated to burn off the organic component (which served as the template for the empty core) by heating to 650 °C (heating rate: 5 °C/min) in air and then holding at that temperature for 30 min. After calcination, the resulting hollow microspheres (TxMSiP@void) were stored in a desiccator until they were used for the measurements.

2.4. Measurements. The functional groups and chemical composition of the particles were identified using diffuse-reflectance infrared Fourier-transform (DRIFT) spectroscopy (FT/IR4100, JASCO, Japan). The particles were also subjected to thermogravimetric analysis (TGA; TG/DTA6200, Seiko, Japan) and EA (Micro Corder JM10, J Science Co., Japan). The surface charge of the particles was measured using a zeta potential analyzer (Zetasizer Nano ZS, Malvern Instruments, UK). The size and surface morphology of the microspheres were characterized using optical microscopy (OM; CX31-P, Olympus, Japan) and scanning electron microscopy (SEM; JCM-5700, JEOL, Japan). The average size and size distribution were statistically analyzed from the OM and SEM images using A-Zoukun software (Asahi Kasei Engineering Corporation, Japan). Energy-dispersive X-ray spectroscopy (EDX) analysis was performed with a detector (Genesis APEX2, Ametek Co., Ltd., USA) installed on the field-


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emission scanning electron microscope (FE-SEM; SU-8000, Hitachi, Japan). Cracking of the particles during the heating process was observed with a digital microscope (KH-8700, Hirox Co., Ltd., Japan) equipped with a heating stage (HCS412W, Instec, Inc., USA)

3. RESULTS AND DISCUSSION 3.1. Preparation of Core–Shell Microspheres with Layered Shell of Silica Particles. The surface of the SiPs was chemically modified with a silane-coupling reagent consisting of the polymerizable methacryloyl group (M) through silane-coupling reactions. As described in our previous papers,78,79 the production of M-modified SiPs (MSiPs) was easily confirmed by observation of their extremely low dispersibility in an aqueous dispersion. The MSiPs were obviously repelled by water, indicating that the hydrophobic M was successfully grafted onto the silica particles. The contact angles of a water droplet on silica particles spread flat on a piece of glue tape before (using SiPs) and after (using MSiPs) surface modification with M were compared. Figure S2 shows that the water contact angle on a SiP was less than 5°, indicating that the SiP surface was extremely hydrophilic because of the surface silanol groups, which is a common property of silica nanoparticles. After surface modification with M, the water contact angle on a MSiP sharply increased to above 125°. According to elemental analysis, the amount of M molecules on the MSiPs was calculated from the weight percentage of carbon (0.19 wt%) to be 0.38 wt%. The MSiPs were then dispersed in an organic medium such as methanol, ethanol, or toluene, as well as a polymerizable monomer such as styrene or EGDMA. The zeta potential measurements in various organic solvents, including polymerizable monomers, indicated that the surface of the


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MSiPs was negatively charged (Figure S3). This was probably due to the silanol groups that remained on the MSiP surface after surface modification with M. Next, the MSiPs were dispersed in a mixture of styrene and EGDMA containing 1 wt% of AIBN, and then the dispersion was poured into an aqueous solution containing 5 wt% PVA. Suspension polymerization of the MSiP-containing monomer mixture (oil phase) was carried out by pre-stirring at 40 °C for 15 min, followed by stirring at 60 °C for 6 h. The pre-stirring was essential for the formation of the layered shell of silica particles on the polymer microspheres. During pre-stirring, MSiPs migrated from the oil phase to the interface of the suspension droplet. While the MSiPs formed the shell, the monomers inside were polymerized to form the hybrid core–shell microspheres (MSiP@pS). Image analysis software (A-Zoukun, Asahi Kasei Engineering Co.) was used to analyze the diameter and size distribution of MSiP@pS on the SEM images. The average diameter and the coefficient of variation (CV) were 22 µm and 33.4% respectively. The CV value was within the usual range for suspension polymerization84. The particle size distribution is depicted in Figure 2b. TG analysis indicated that the amount of silica nanoparticles loaded in the MSiP@pS particles was 6.9 wt%, slightly less than the amount of MSiPs added to the monomer mixture. Since the diameter of the protrusions was approximately 400 nm and almost no MSiPs were found inside the MSiP@pS particles, it was certain that the MSiPs were stabilized on the surface of MSiP@pS particles. We have not yet clarified the effects of the size of the microspheres on the packing density of MSiPs. However we found that the surface of microspheres was waved, when excess amount of silica nanoparticles were used.79 In our previous study, we concluded that MSiPs migrated from inside a monomer droplet and collected at the oil–in–water (O/W) interface because of the optimal hydrophobic-hydrophilic balance of the MSiP surface.77-79 Zeta potential measurements confirmed that the MSiP surface


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carried a negative charge in organic solvents (Fig. S3). These results indicated that the silanol groups remained on the MSiP surface even after grafting of the M molecules. Consequently, MSiP preferred to accumulate at the O/W interface and was stabilized by the crosslinking with the monomers.

3.2. Sol–Gel Coating and Calcination of Fabricated Hollow Silica Microspheres. The silica-coated MSiP@pS particles were calcinated at 650 °C in a furnace to burn off the organic component, which originated mostly from the microspheres. When the MSiP@pS particles (without silica coating) were calcinated, the microspheres shrank and the obtained microspheres were composed of agglomerated silica nanoparticles without any hollow structure inside, as shown by the SEM image in Figure 1. Therefore, further silica coating was carried out on MSiP@pS before calcination to connect the silica particles through the sol–gel reaction in 56 mL of an aqueous suspension containing 2.0 g of MSiP@pS with different concentrations of TEOS—0.04, 0.08, 0.16, 0.32 and 0.48 mol/L—to produce Tx–MSiP@pS (x = 0.04, 0.08, 0.16, 0.32, and 0.48). SEM observations revealed that the surface of the MSiP@pS particles was covered after the sol–gel reaction. A silica layer was gradually deposited on the surface of each microsphere with increasing TEOS concentration in the suspension, and finally, the MSiPs were completely covered by a silica layer. Figure 3 shows that the original surface composed of orderly silica particles remained even after the sol–gel reaction with a small amount of TEOS (0.04, 0.08, and 0.16 mol/L, shown in Figures 3b, 3c, and 3d, respectively), but the surface became rough and irregular because of the uneven growth of the silica layer after the sol–gel reaction with a large amount of TEOS (0.32 and 0.48 mol/L, shown in Figures 3e and 3f, respectively). Typical EDX results with SEM images of Tx–MSiP@pS prepared with different


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concentrations of TEOS are shown in Figure 4. The spaces between the silica particles were gradually filled with increasing TEOS concentration, and the silica particles were finally connected to each other. Detailed observation of the surface revealed that the diameter of the silica particles gradually increased, indicating that the sol–gel reaction occurred on the surface of silica particles as well as in the space between the silica particles. The EDX spectra show that the C signal decreased, while the O and Si signals increased on the surface of hybrid microspheres with increasing amount of TEOS. These results confirm that the sol–gel reaction with TEOS occurred on the surface of the microspheres to form the second silica layer. In the initial stage of the sol–gel reaction, smaller silica particles were observed in the polymer region of the surface (Figure S4). The hydrophobic TEOS was expected to be adsorbed on the polymer region, and the formation of the silica layer was probably initiated by TEOS adsorbed on the surface. Our results of TG analysis indicate that the amounts of residue after calcination of Tx–MSiP@pS (x = 0.04, 0.08, 0.16, 0.32 and 0.48 mol/L) with different shell thicknesses were 0.10, 0.18, 0.46, 0.90, and 1.35 µm, respectively. Since the amount of residual silica before silica coating was 6.9 wt%, the increases in concentration of incombustible residues were 4.8, 9.8, 18.2, 28.9, and 38.4 wt% for T0.04–MSiP@pS, T0.08–MSiP@pS, T0.16–MSiP@pS, T0.32–MSiP@pS, and T0.48– MSiP@pS, respectively. Figure 5 shows that the amount of thermal residue had a linear relationship with the concentration of TEOS in the reaction mixture. After TG analysis, the color of the residual particles changed from white to gray with increasing TEOS concentration. Slight carbonization of the polymer core probably occurred inside the core–shell microspheres because of oxygen deficiency. The DRIFT spectra (Figure S5) proved that the thermal residue consisted mostly of silica (most of the organic chemical bonds in the polymer core, such as C=O bonds from EGDMA and C=C bonds from styrene, disappeared) and the carbonized component was


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extremely small. Based on the TG analysis results, the calcination of Tx–MSiP@pS was performed at 650 °C for 2 h. Almost no organic component was found, indicating that the polymer was burned off from the inside of the core–shell microspheres. SEM observations demonstrated that the spherical shape of T0.08–MSiP@pS remained without shrinking, even after calcination at 650 °C; however, some of microspheres were broken during the calcination process (Figure 6a). Similar results were obtained for the other Tx–MSiP@pS samples, implying that the silica shell contained pathways for small gas molecules. The number of broken particles were higher in Tx–MSiP@void prepared with lower TEOS concentrations, which was expected because the fragile thin shell could not withstand the shrinking of the polymer core during the calcination process. The broken particles confirmed the hollow structure inside Tx–MSiP@void (Figure 6b). As described above, MSiP@pS was shrunken by burning at 650 °C and hollow structures could not be formed. Therefore, it can be emphasized that the second coating layer, formed by the sol–gel reaction of TEOS, connected the silica particles on MSiP@pS. As shown in Figure 2a–d, the size of the microspheres was mostly constant in the preparation processes of pre-stirring (21.6 µm (CV = 32.3%)), suspension polymerization (22.0 µm (CV = 33.4%), MSiP@pS) and sol-gel reaction (22.0 µm (CV = 29.4%), T0.16-MSiP@pS), but slightly decreased in the calcination process (20.0 µm (CV = 29.1%), T0.16-MSiP@void). These results indicate that the microspheres (T0.16-MSiP@pS) shrink slightly by heating even after coating with second silica layer. 3.3. Morphologies of Inner and Outer Surfaces of the Hollow Silica Microspheres. Most of the MSiPs remained and the protrusion structures were found on the inner surface of the hollow microspheres (Figure 6c), confirming that the MSiPs were chemically connected by the second silica coating. As described above, the silanol groups on the SiPs remained on its surface even


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after grafting of the M molecules. The remaining silanol groups probably acted as reactive sites for the sol–gel reaction with TEOS, and the MSiPs were connected with silica as a second layer. Similar protrusions were observed on the inner surface in all Tx–MSiP@pS samples regardless of the shell thickness. The morphology of the outer surface was changed by varying the amount of TEOS used for the second silica coating. The protrusions sturdily expanded with growth of the silica layer, and the original arrangement of silica particles on the surface finally disappeared due to the randomly occurring sol–gel reactions. Figure 6d shows that the outer surface of T0.08– MSiP@void had dense protrusion structures with larger diameters when compared with the MSiPs. These results indicate that the sol–gel reaction of TEOS was dominant on the surface of MSiP@pS. The size distribution of Tx-MSiP@void (x = 0.16, 0.32 and 0.48) and SEM images were shown in Figure 2d–f. The average diameters (CV values) of the hollow microspheres were 20.0 µm (CV = 29.1%), 23.8 µm (30.8%) and 25.4 µm (20.8%) respectively. It is understandable that the size of hollow microspheres increases with increasing the thickness of shell layer (Figure 2). Figures 7a–e show that the thickness of the silica layer increased linearly with increasing amount of TEOS in the reaction mixture for the silica-coating process. The silica shell fabricated in this study reached a maximum of 1.2–1.3 µm, which could not be obtained in MSiP@pS (without the second silica coating) after calcination (Figure 7f). Figure 8 shows TEM images of the cross-section of the microspheres before (T0.08–MSiP@pS) and after (T0.08–MSiP@void) calcination. The results indicated that the second silica layer was coated on the surface of MSiP@pS without any space. Furthermore, the image of the silica layer was brighter than that of the silica nanoparticles after calcination, probably due to the formation of a porous structure upon the removal of organic components from TEOS.


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3.4. Mechanical Properties of Hollow Silica Particles. The compressive mechanical strength was tested using a micro-compression testing machine (MCT-W, Shimadzu Co. Ltd., Japan) The diameter of the hollow silica microspheres used for the compression test was approximately 25 µm. As shown in Figure 9a, the compressive stress increased linearly and all Tx-MSiP@void samples except T0.04-MSiP@void were broken at the point that was compressed to nearly 1 µm (the compression ratio was about 4–5 %). SEM images of the cross-section of the broken hollow silica microspheres indicated that the second silica layer between the MSiPs, which was the thinner part of the shell, was predominantly broken (Figure 9b). The compressive strength at the breaking point increased with increasing thickness of the silica shell. For T0.48-MSiP@void, the strength of the breaking point was more than 14 MPa, which is extremely large compared with reported results for hollow silica microparticles.85 This was probably due to the growth of the silica layer from the regularly spaced silica particles, which served as a scaffold.

3.5. Heat-Induced Cracking Property. As described above, the silica shells of core–shell microspheres cracked during calcination. In particular, the core–shell microspheres coated with smaller amounts of TEOS were mostly crushed. Figure 10a shows the TG curve of T0.08– MSiP@pS; weight loss occurred between 280 and 430 °C because the organic components inside the core–shell microspheres were burned off. Direct observations of core–shell microspheres were carried out on the heating stage at temperatures from room temperature to 400 °C. Cracks were generated when the temperature was over 300 °C, and the microspheres were crushed at 350–400 °C (OM images in Figure 10a and SEM images in Figure 10b), which corresponds to the temperature range for the polymer core to burn off. These results indicate that the silica shell was broken by the pressure change associated with expansion, with gasification of the interior


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polymer core. Such heat-induced crushing was not observed in core–shell microspheres with a thick shell layer, such as T0.48–MSiP@pS. Figure 9b shows that cracks were observed in the thinner part of the silica layer between the silica particles. It is believed that the non-uniform thickness of the silica shell may be exploited for controlled cracking. Therefore, the hollow silica microspheres with orderly protrusions may potentially be applied to heat-sensitive capsules that are crushed by internal pressure changes.

4. CONCLUSIONS Hollow silica microspheres with orderly hemispherical protrusions on the outer and inner surfaces were fabricated by calcination of polymer microspheres consisting of a silica-connected layered shell of silica particles. Similar protrusion structures are commonly found in nature, and they often perform definite functions. For instance, a lotus leaf with papillae 10–20 µm in height and 10–15 µm in width shows super-hydrophobic properties with self-cleaning functions. Protuberance arrays on the eyes of some species of moths, butterflies, and flies exhibit antireflection properties. Such specific functions can be realized by mimicking the morphologies of natural creatures on the nanoscale or microscale. Under specially arranged conditions, unique functions like anti-reflection, sound absorption, and superhydrophobicity functions should be obtained using the ordered hemispherical protrusions on hollow microspheres fabricated in this study. Internal-pressure-induced cracking of hollow microspheres can be realized by stress concentration to the connecting silica layer between silica nanoparticles in the shell. Such heatinduced explosion of the hollow particles can be useful for triggering the beginning and end of chemical reactions. Very recently, core–shell microfibers with thermally triggered flameretardant properties were reported for lithium-ion batteries; the microfibers encapsulated a flame


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retardant inside a protective polymer shell designed to melt at higher temperatures to release the organophosphorus-based flame retardant in the core.86 Further studies are being conducted on the practical use of core–shell silica microspheres with well-controlled surfaces and orderly hemispherical protrusions.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]

ACKNOWLEDGMENT This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO), Japan and Grants-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS).

ABBREVIATIONS AIBN, α,α’-Azobisisobutylonitrile; CV, coefficient of variation; DRIFT, diffuse-reflectance infrared Fourier-transform; EA elemental analysis; EDX, energy-dispersive X-ray; EGDMA, ethyleneglycol dimethacrylate; FE-SEM, field-emission scanning electron microscopy; FTIR, Fourier-transform infrared; M, 3-methacryloxypropyl trimethoxisilane; MSiP, 3methacryloxypropyl-trimethoxisilane-modified silica nanoparticle; MSiP@pS, polymer


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microspheres with layered shell of silica nanoparticles; OM, optical microscopy; PVA, poly(vinyl alcohol); SEM, scanning electron microscopy; Tx–MSiP@pS, MSiP@pS coated with silica through reaction with x mol/L of TEOS; Tx-MSiP@void, hollow Tx–MSiP@pS after calcination; SiP, silica nanoparticle; TEOS, tetraethoxysilane; TGA, thermogravimetric analysis.

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Table 1 Silica coating conditions of MSiP@pS through sol–gel reactions with TEOS Reaction mixturea Yield (g)

Residue (wt%)b

MSiP@pS (g)

TEOS (mL (mol/L))

–

–

–

6.9

T0.04–MSiP@pS

0.5 (0.04)

2.08

11.7

T0.08–MSiP@pS

1.0 (0.08)

2.20

16.7

2.0 (0.16)

2.53

25.1

T0.32–MSiP@pS

4.0 (0.32)

3.11

35.8

T0.48–MSiP@pS

6.0 (0.48)

3.53

45.3

MSiP@pS

T0.16–MSiP@pS

2.0

a

Reaction conditions: Temperature = 25–30 ˚C, Time = 6h, Solvent = water / ethanol / 25% NH3 aqueous solution (4 mL / 50 mL / 2 mL). bThe residues were determined by TG analysis.


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Figure 1 Strategy for the fabrication of hollow microspheres with orderly protrusions on the outer and inner surfaces.

Figure 2 OM and SEM images and particle size distributions in the preparation processes of prestirring (a), suspension polymerization (b), sol-gel reaction (c) and calcination (d, e, f).

Figure 3 SEM images of silica-coated MSiP@pS. Silica coatings were carried out on MSiP@pS by the sol–gel reaction with TEOS at various concentrations: (a) no coating (MSiP@pS); (b) 0.04 mol/L (T0.04–MSiP@pS); (c) 0.08 mol/L (T0.08–MSiP@pS); (d) 0.16 mol/L (T0.16– MSiP@pS); (e) 0.32 mol/L (T0.32–MSiP@pS); (f) 0.48 mol/L (T0.48–MSiP@pS).

Figure 4 SEM images and EDX analysis results of the outer surface of hollow microspheres (Tx–MSiP@pS). Scale bars: 5 µm.

Figure 5 Effects of TEOS concentration in the reaction mixture for sol–gel coating of MSiP@pS on the total amount of silica. Open circles: MSiP@pS; closed circles: Tx–MSiP@pS.

Figure 6 SEM images of T0.08–MSiP@void: (a) over-view image; (b) broken microsphere; (c) inner surface; (d) outer surface.


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Figure 7 SEM images of the cross-sections of the shells of (a-e) Tx–MSiP@void and (f) aggregated silica nanoparticles obtained by the calcination of MSiP@pS: (a) T0.04–MSiP@void; (b) T0.08–MSiP@void; (c) T0.16–MSiP@void; (d) T0.32–MSiP@void; (e) T0.48–MSiP@void.

Figure 8 TEM images of the cross-sections of T0.08–MSiP@pS (a) and T0.08–MSiP@void (b).

Figure 9 Compression strength of the hollow silica microspheres (a): (i) T0.04–MSiP@void; (ii) T0.08–MSiP@void; (iii) T0.16–MSiP@void; (iv) T0.32–MSiP@void; (v) T0.48–MSiP@void. The inset is the enlarged view of the broken-lined square. SEM images of the cracked shell of T0.08–MSiP@void after breaking (b).

Figure 10 (a) Thermogravimetrical analysis and optical microscopic images of T0.08–MSiP@pS at varying temperature; heating rate: 10 °C/min. (b) SEM images of T0.08–MSiP@pS before and after thermogravimetrical analysis.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1

Sol-gel reaction of TEOS at surface Polymer microsphere with layered-shell of nanosilica

(MSiP@pS)

Calcination at 650 ˚C

Silica-coated polymer microsphere with layered shell of nanosilica

(Tx–MSiP@pS) Calcination at 650 ˚C

Versatile shell thickness controlled by sol-gel reaction of TEOS

Nanosilica-coaggulated microsphere Hollow silica microsphere with orderly hemispherical projections

(Tx–MSiP@void)


33

Langmuir

Figure 2 (b) MSiP@pS

(a) Suspension droplets

Proportion (%)

50 µm

(c) T0.16-MSiP@pS

50 µm

50 µm

60

60

50

d: 21.6 µm CV: 32.3% 50

40

40

40

30

30

30

20

20

20

10

10

10

0

60

d: 22.0 µm CV: 33.4%

50

d: 22.0 µm CV: 29.4%

0

0 0 0 10102020303040405050606070

0 0 10102020303040405050606070

0 0 10102020303040405050606070

Diameter (µm)

Diameter (µm)

Diameter (µm)

(d) T0.16-MSiP@void

(f) T0.48-MSiP@void

(e) T0.32-MSiP@void

50 µm

Proportion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 µm

50 µm

60

60

50

d: 20.0 µm CV: 29.1% 50

40

40

40

30

30

30

20

20

20

10

10

10

0

60

d: 23.8 µm CV: 30.8%

50

0

d: 25.4 µm CV: 20.8%

0

0 0 10102020303040405050606070

0 0 10102020303040405050606070

0 0 10102020303040405050606070

Diameter (µm)

Diameter (µm)

Diameter (µm)


34

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Langmuir

Figure 3

a

b

10 µm

d

c

10 µm

e

10 µm

10 µm

f

10 µm

10 µm


35

Langmuir

Figure 4

C

T0.16-MSiP@pS

O

O 1000

Si 2000

C 3000

T0.04-MSiP@pS

1000

T0.32-MSiP@pS

O C

2000

3000

Si

Si Counts

Counts

Si

Counts

Counts

MSiP@pS

O

C 1000

2000

3000

Si

T0.08-MSiP@pS

1000

2000

3000

Si

T0.48-MSiP@pS

O

O Counts

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

C

C 2000 3000 1000 Energy (eV)

2000 3000 1000 Energy (eV)


36

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20

T0.48-MSiP@pS

30

T0.32-MSiP@pS

T0.04-MSiP@pS

40

T0.08-MSiP@pS

50

T0.16-MSiP@pS

Figure 5

Residual silica (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

10 MSiP@pS 0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

TEOS (mL)


37

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6

b

a

50 µm

10 µm

d

c

1 µm

1 µm


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Langmuir

Figure 7

a

b

d e

c

f


39

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 8

a

b

200 nm

200 nm


40

Page 41 of 43

Figure 9

a

20

v

0.6

18 16

iv iii ii

0.4

14 Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.2

12

v

i

0.0

10

0

0.1

0.2

iv

0.3

8 6 4

iii

2

ii

0 0.0

0.2

0.4

0.6 0.8 Displacement (µm)

1.0

1.2

b

1 µm

1 µm


41

Langmuir

Figure 10

a

100 ˚C

300 ˚C

330 ˚C

100

380 ˚C

90 Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

80 70 60 50 40 400 ˚C

30 20 10 0 100

200

300

400

500

600

Temperature (˚C)

b

20 µm

20 µm


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Langmuir

119x70mm (300 x 300 DPI)


Fabrication of Hollow Silica Microspheres with Orderly Hemispherical

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