Mesopore-Free Hollow Silica Particles with Controllable Diameter and

May 15, 2012 - Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima...
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Mesopore-Free Hollow Silica Particles with Controllable Diameter and Shell Thickness via Additive-Free Synthesis Asep Bayu Dani Nandiyanto, Yui Akane, Takashi Ogi, and Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: Mesopore-free hollow silica particles with a spherical shape, smooth surface, and controllable diameter (from 80 to 300 nm) and shell thickness (from 2 to 25 nm) were successfully prepared using an additive-free synthesis method. Different from other hollow particle developments, a mesopore-free shell was produced because of the absence of additive. Although common reports pointed out the importance of the additional additive in pasting and growing silica on the surface of a template, here we preferred to exploit the effect of the template charge in gaining the silica coating process. To form the silica, basic amino acid (i.e., lysine) was used as a catalyst to replace ammonia or hydrazine, which is harmless and able to control the silica growth and produce hollow particles with smooth surfaces. Control of the particle diameter was drastically achieved by altering the size of the template. The flexibility of the process in controlling the shell thickness was predominantly attained by varying the compositions of the reactants (i.e., silica source and catalyst). The present mesopore-free hollow particles could be efficiently used for various applications, especially for thermal insulator and optical devices because of their tendency not to adsorb large molecules, as confirmed by adsorption analysis.

1. INTRODUCTION The synthesis of hollow silica particles has attracted a tremendous amount of attention.1 Their excellent performance makes this type of particle useful for many applications, for example, thermal insulators and optical devices, chromatography-related components, shields for enzymes or proteins, delivery vehicles for drugs, dyes, or inks, photonic crystals, artificial cells, waste removal, and large biomolecular release system.2 Silica is chemically inert, thermally stable, harmless, and inexpensive.3 Many preparation methods for hollow silica particles have been reported1−15 in which the organic template-driven synthesis process is commonly used.2 Although the previously reported methods are feasible for industry, they still have several disadvantages: (i) their limitation with respect to the production of hollow particles with mesoporous structure in the shell;4,5,16 (ii) their dependence on the use of additives (e.g., polymers, surfactants, salts, etc.),13 in which this additive sometimes cannot be removed completely17 and leads to the formation of mesoporous structure in the shell;16 (iii) no © 2012 American Chemical Society

detailed information about controlling the shell thickness, shell structure, and agglomeration phenomenon; (iv) their limitation of the production of hollow particles with rough surfaces; (v) the difficult preparation of particles with controllable sizes of less than 200 nm in diameter; and (vi) the requirements of harmful and difficult-to-handle chemicals (e.g., ammonia and hydrazine) that create conflicts with respect to safety and environmental problems.16 The synthesis of mesopore-free hollow particles with controllable size and shell thickness remains to be an unresolved issue. There is no report on the synthesis of hollow particles with mesopore-free shells because, as mentioned before, the current hollow developments cannot be separated with the use of additives to paste and to grow silica on the template surface. In fact, the mesopore-free material is important because of its ability to avoid mass transfer, Received: April 10, 2012 Revised: May 15, 2012 Published: May 15, 2012 8616

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Table 1. Adsorption Analysis of the Particles with Various Morphologies C/Co (%)b after treatment sample no adsorbent nonporous particlesd HP without mesoporous structurese HP with mesoporous structuresf porous particlesg

mean diameter (nm) 85 270 270 80

specific surface area (SBET)a

before additional adsorbent

30.21 77.85 552.33 600.82

100 100 100 100 100

as additional adsorbentc

1h 100 100 95 94 87

100 99 92 85 75

1 day 100 99 90 52 47

5 days 100 98 90 52 45

a

Specific surface areas were measured by nitrogen adsorption analysis. bCo and C are the concentrations of RhB at the initial and the measured times, respectively. cThe sample analyzed in the first time (t < 2 min) range after the adsorbent was added. dCommercially nonporous silica nanoparticles (ZL, Nissan Chemical Industry Co. Ltd., Japan). ePrepared hollow particles. fPrepared hollow particles with an additional 0.29 mg/mL cetyltrimethylammonium bromide (CTAB, Merck, Germany). gMesoporous silica nanoparticles (Hiroshima Mesoporous Material) were prepared using our previous method.15

could be efficiently used for various applications, especially for thermal insulator and optical devices, because of their tendency not to adsorb large molecules, compared to porous-structured material.

adsorption, diffusivity, and the penetration of larger molecules into or out of the particles that can deter the material properties (i.e., thermal conductivity, refractive index, and density). Furthermore, in controlling the shell thickness, although some reports have proposed their strategies,5,8 their process is relatively difficult, and the increase in shell thickness is typically associated with the increase in roughness, the porosity, the agglomeration, and the irregular shape of the particle. In addition, the roughness of the shell was also disregarded in the current reports. Shells with smooth surfaces are important for their positive impact on the particle dispersity, the adsorption ability, the material strength, and other material performance.18−20 On the basis of our experience in the synthesis of particles with controllable size and morphology using a spray17,21−25 and a liquid-phase synthesis method,12,16,26,27 the purpose of this study was to describe the details of the synthesis of mesoporefree hollow silica nanoparticles with a spherical shape, smooth surface, and controllable diameter (from 80 to 300 nm) and shell thickness (from 2 to 25 nm) using an additive-free liquidphase synthesis with an organic colloidal templating method. A mesopore-free shell could be produced by the use of an additive-free process in which, to the best of our knowledge, this type of shell was the first in the current hollow particle developments. Although common reports noted the importance of additional additives to the pasting and growth of silica on the surface of the template,13 here the exploitation of the template charge to gain the silica coating process was explored. A basic amino acid (i.e., lysine) was used as a catalyst for forming silica, replacing ammonia or hydrazine. L-Lysine is harmless and able to control the silica formation rate,16 leading to the formation of a shell with a smooth surface. The control of particle diameter and shell thickness is also contained in this report. The control of particle diameter was obtained by changing the template size. The flexibility of the process in controlling the shell thickness was attained by varying the compositions of reactants (i.e., silica source and catalyst). Furthermore, the detailed investigation of hollow silica particle synthesis under successful and unsuccessful conditions (incomplete hollow, agglomeration, irregular shape, and the appearance of silica nanoparticles) was described, and this information is typically not considered in the current hollow-particle synthesis reports. Finally, the adsorption ability of the prepared hollow particles by large organic molecules (i.e., rhodamine B) was investigated. The result showed that the present mesopore-free hollow particles

2. EXPERIMENTAL SECTION The synthesis of hollow silica particles involved three major steps: (i) the preparation of surfactant-free PS particles as the template; (ii) the preparation of silica-coated PS particles; and (iii) a template-removal process. In the first step, the PS particles were synthesized using a surfactantfree liquid-phase synthesis method involving a simple polymerization of the styrene monomer (styrene, Aldrich, US) in the aqueous solution under 2,2′-azobis-(2-methylpropionamide) dihydrochloride (AIBA, Sigma-Aldrich, US) as the initiator, which was described in detail in our previous study.12 In addition, to investigate the effect of the charge of the PS particles, the AIBA concentration was varied from 0.008 to 0.42 mg/mL. In the second step, the synthesized PS particles were put into the reactor system. The reactor system itself consisted of a batch-glass reactor (a 100 mL two-necked reactor), a magnetic stirrer, a mantle heater, and a reactant inlet. To begin the silica coating process, the PS solution was diluted with an aqueous solution, vigorously stirred (500 rpm), and heated to 60 °C. The heating process was then maintained for 30 min to ensure that the solution was heated well and dispersed homogenously. After 30 min of stirring, L-lysine (lysine, Aldrich, US, as the silica catalyst) and tetraethyl orthosilicate (TEOS, 98%, Aldrich, US, as the silica source) were subsequently added to the heated PS solution. The system was then maintained for 6 h. To ensure that the process was reproducible, the total volume of solution was fixed at 70 mL. The mass ratio of TEOS/PS was varied in the range of 0.32 to 3.35. The lysine concentration was varied from 4.80 to 14.80 mmol/L. At the end of the hollow particle synthesis procedure, the reacted solution was purified using a centrifugation process (15 000 rpm, 30 min). The sedimented solution containing composite PS/silica particles was dried and then used in the template-removal process (i.e., heat treatment (500 °C)) to form hollow silica particles. The prepared particles were characterized with a scanning electron microscope (SEM, Hitachi S-5000 operated at 20 kV, Hitachi, Japan) and transmission electron microscopes (TEM, JEM-3000F, operated at 300 kV) and JEM-2010 (operated at 200 kV, JEOL, Japan) to examine the size, the morphology, and the structure of the particles. The elemental and chemical composition of the prepared particles were evaluated using energy-dispersive X-ray spectroscopy (EDS, attached to the TEM apparatus) and Fourier transform infrared spectroscopy analysis (FTIR, PerkinElmer, Spectrum One System, in the range of 600−4000 cm−1). To analyze the effectiveness of the template-removal process, the sample was also characterized using a thermogravimetric and differential thermal analyzer (TG-DTA, Exstar6000, Seiko Instruments Inc., Japan). In addition, the charge 8617

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Figure 1. Electron microscope images and elemental analysis results of the prepared particles. (a−c) SEM images of PS, PS/silica, and hollow silica particles, respectively. (d, e) TEM images of PS/silica and hollow silica particles, respectively. (f) Low-magnification TEM image of hollow silica particles. (g, h) EDS analysis results of PS/silica particles before and after the template-removal process, respectively. Samples were prepared using a TEOS/PS mass ratio of 1.00 and a lysine concentration of 9.60 mmol/L. on the prepared PS was analyzed using a zeta potential measurement (Malvern Zetasizer, Nano ZS, U.K.). To determine the shell structure in the hollow silica particles, a nitrogen adsorption isotherm using a conventional volumetric apparatus (BET, BELSORP-Max, Bel Japan Inc., Japan; operated at 77 K) and a large-molecule adsorption analysis of the prepared particles were also measured. For the large-molecule adsorption analysis, the following experimental procedures were conducted. First, the calcined sample was softly ground to break the soft agglomeration after the template-removal process. Second, the sample was redispersed into the aqueous solution at a concentration of 30 ppm. Meanwhile, rhodamine B (RhB, Wako Pure Chemicals Industries Ltd., Osaka, Japan) was dissolved in the aqueous solution to a concentration of 60 ppm. After both suspensions were dispersed well, they were mixed in a volume ratio of 1:1 using vigorous stirring for 1 min. Then, the sample was immediately measured via spectroscopy (Shimadzu UV−vis spectrophotometer, UV-2450). The measurements were continuously repeated for 100 min and then for several hours and days. To determine the concentration of RhB in the solution, Beer’s law was adopted and the data were normalized to a red wavelength (555 nm).16 To investigate the effect of the shell structure on the ability of materials to adsorbing RhB, different types of particles (i.e., our prepared mesopore-free hollow particles, dense silica particles, mesoporous silica particles, and hollow silica particles with mesoporous structures) were used and compared, as described in Table 1.

monodisperse PS particles with a mean size of 89 nm could be prepared. By placing the PS into the silica coating process, composite PS/silica particles were produced (Figure 1b). When the prepared composite particles were put in the templateremoval process, hollow silica particles could be obtained (Figure 1c). Monodisperse and spherical particles were observed in all cases. The size and the shape of the silicacoated PS and the hollow silica particles were identical to those of the initial PS particles. In spite of analysis via SEM, the hollow structure was clearly detected in Figure 1c. The hollowstructured particles were transparent, which was not identified in either PS or PS/silica SEM images. The use of a high-voltage SEM apparatus (20 kV) was believed to be the main reason for this unique observation result. To verify the hollow structure inside the prepared particles, the samples were characterized using TEM analysis (Figure 1d,e). The TEM image in Figure 1d indicated that PS was coated completely by the silica, whereas the TEM image in Figure 1e confirmed that the addition of the template-removal process led to the production of hollow particles. The high resolution of TEM images for both samples showed no mesoporous structure in the shell, which was different from the current hollow particle developments.4,5,14 Ferret analysis revealed that the mean diameter and the shell thickness of the prepared particles were 86 and 3 nm, respectively. The lowmagnification TEM image in Figure 1f showed a group of collected hollow particles. The morphology and the dimensions of all particles were identical, verifying that the process was

3. RESULTS AND DISCUSSION The morphologies of the prepared particles characterized using electron microscopy and an EDS analysis are shown in Figure 1. The SEM image in Figure 1a showed that spherically shaped, 8618

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Figure 2. Effect of AIBA concentration on the formation of hollow silica particles. (a−c) SEM analysis images of hollow particles prepared using AIBA with concentrations of 0.42, 0.17, and 0.008 mg/mL, respectively. (d, e) TEM images of hollow particles prepared using AIBA with concentrations of 0.42 and 0.17 mg/mL, respectively. (f) Zeta measurement analysis of PS particles prepared using various AIBA concentrations. Hollow particles were prepared using a TEOS/PS mass ratio of 1.00 and a lysine concentration of 9.60 mmol/L.

reliable with respect to the quality of the product. In addition, a small difference between the hole and the PS sizes existed, which was probably caused by two factors: (i) the small dissolution of PS when in contact with the organic solution (i.e., TEOS) during the silica coating process and (ii) the thermal treatment effect during the PS removal process. To confirm the elemental composition of the prepared particles, EDS analysis was conducted, as shown in Figure 1g,h. Figure 1g shows the EDS analysis result of as-prepared PS/ silica particles (before the template-removal process), and Figure 1h reveals the result of particles after the templateremoval process. The ratio intensity of Si and O was the same for both samples, confirming no change in the silica composition during the template-removal process. The decrease in C intensity was found, verifying the impact of the template-removal process in the elemental composition of the particles. The loss of some carbon components was caused by the removal process of the PS. The compositions of PS/silica and PS particles during the template-removal process were investigated by means of TGDTA analysis, as shown in Figure S1 in the Supporting Information. The result showed that a loss of mass was detected for both samples. The weight loss below 270 °C was due to the evaporation of physically adsorbed water and residual solvent in the samples. The major weight loss of the samples was identified from 270 °C and completed at around 330 °C. After reaching more than 330 °C, the mass was relatively stable. The final masses of the PS/silica and PS samples were close to 30 and 0 wt %, respectively, confirming that all of the PS particles were completely removed in the template-removal process. FTIR patterns of the prepared particles are presented in Figure S2 in the Supporting Information. Different peaks were observed among the PS, the composite PS/silica, and the hollow silica samples. The intensities of alkyl (CnH2n+1) groups (in the range of 500−1000, 1400−1700, and 2800−3200 cm)12

were identified for the PS and composite PS/silica samples. However, these peaks completely disappeared after the template-removal process, with a Si−O−Si peak (in the range of 1000−1200 cm) remaining.16 These FTIR results indicated that the PS particles and some organic components were selfassembled during the formation of composite PS/silica particles and completely removed after the template-removal process, with silica remaining as the final product. To confirm the fundamental reason for the successful silica coating process, the effect of the PS charge on the formation of hollow particles was investigated (Figure 2). To control the charge of PS, the concentrations of AIBA during the PS synthesis were varied. Spherical hollow particles were produced using an AIBA concentration of 0.42 mg/mL (Figure 2a). Decreases in the AIBA concentration allowed the production of hollow particles with an aspherical shape (Figure 2b). Some particles with broken and irregular shapes were found. Further decreases in the AIBA concentration resulted in unsuccessful hollow particle formation (Figure 2c). A porous filmlike structure was observed. The sizes of the larger pores were identical to those of the initial PS. TEM images of hollow particles prepared using different AIBA concentrations are shown in Figure 2d,e. Hollow particles with good structure were obtained when using PS with 0.42 mg/mL AIBA (Figure 2d). A decrease in the AIBA concentration led to the formation of aspherically shaped hollow particles with a thin shell (Figure 2e). To confirm the effect of AIBA on the charge of PS, zeta potential analysis of the prepared PS was conducted (Figure 2f). Different concentrations of AIBA impacted the charge of PS. When 0.42 mg/mL AIBA was used, a positive zeta value (of about +45 mV) was obtained. A decrease in the AIBA concentration down to 0.17 mg/mL reduced the zeta value (about +35 mV). Further decreases in the amount of AIBA with a concentration of down to 0.008 mg/mL allowed the 8619

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Figure 3. TEM images with the size distribution of hollow silica particles prepared from different PS sizes: (a) 262, (b) 157, and (c) 86 nm. Samples were prepared using a TEOS/PS mass ratio of 1.00 and a lysine concentration of 9.60 mmol/L.

Figure 4. Effect of TEOS concentration on the formation of hollow silica particles. (a) SEM image of a sample prepared using a TEOS/PS ratio of 0.325. (b−d) TEM images of particles prepared using TEOS/PS ratios of 1.00, 1.68, 2.00, and 3.35, respectively. (e) Average shell thickness as a function of the TEOS/PS mass ratio. All samples were prepared using a lysine concentration of 9.60 mmol/L.

investigation were prepared using an AIBA concentration of 0.42 mg/mL. (The zeta values of the PS particles with sizes of about 260, 150, and 80 nm were about +45, +45, and +48 mV, respectively.) Spherical, monodisperse, mesopore-free hollow particles with smooth surfaces were obtained for all cases. The use of PS spheres with different sizes allowed the change in the hollow particle diameter. Hollow particles with a mean size of 262 nm could be obtained when PS with a size of 263 nm was used (Figure 3a). Decreases in the PS size resulted in the production of smaller hollow silica particles (Figure 3b,c). Ferret diameters of the prepared hollow particles were 262, 157, and 86 nm, corresponding to the use of template sizes of 263, 156, and 89 nm, respectively. This result verified that the diameter of hollow particles could be effectively controlled by the change in the template size. Figure 4 shows the effect of TEOS concentration on the formation of hollow silica particles. Figure 4a−e corresponds to TEM micrographs of hollow silica particles prepared using different TEOS/PS mass ratios, and Figure 4f shows the relationship between the TEOS/PS ratio and the thickness of the shell. The use of a smaller ratio (TEOS/PS mass ratio = 0.325) resulted in the formation of incomplete hollow particles (Figure 4a). The addition of higher concentrations of TEOS

production of PS particles with a negative zeta value (about −23 mV). From the above results, the successful silica coating process was due to the attraction and interaction phenomena between the reaction components (i.e., silica and PS). Because of the tendency of silica charge in the range of negative zeta values,3 the successful coating process could be achieved only when PS particles with a positive zeta value were used. The use of PS with a negative zeta value led to the unsuccessful silica coating process. However, in addition to the necessity of the opposite charge between silica and PS, the value of the PS charge is also important. This value affects the attraction and the typical growth of silica on the PS surface, in which this growth influences the strength and the structure of the silica shell. In general, when using small charge value of PS, the adsorption and growth of silica on the surface of PS would not be optimal. As a consequence, the silica shell easily collapsed after the heat treatment, forming hollow particles with broken and irregular structures. Consistent with the above hypothesis that silica grew on the surface of PS, the effect of the initial size of PS on the diameter of hollow particles was investigated (Figure 3). To make a successful silica coating process, all PS particles used in this 8620

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allowed the production of hollow particles with good structure (Figure 4b,c). The shell thickness increased (from 5 to 10 nm) along with the increases in the amount of TEOS (TEOS/PS ratio from 1.00 to 1.70). Meanwhile, further increases in the TEOS composition resulted in the creation of silica nanoparticles in addition to the formation of hollow silica particles (Figure 4d). Moreover, if the concentration of TEOS was too high (TEOS/PS ratio of 3.25), then agglomerated hollow particles and free silica nanoparticles were produced (Figure 4e). The increase in the TEOS concentration provided an additional silica source that could be used for the further growth of silica on the PS. However, when using an excess concentration of TEOS, two phenomena existed: the uncontrolled growth of silica and the dissolution of the template. The growth of silica particles on the outside of the targeted area (surface of PS) was found because the silica monomers provided by the hydrolysis of TEOS would rather form new nuclei and increase their weight than combine with the silica on the surface of PS. As a consequence, in addition to the formation of hollow particles, free silica nanoparticles existed. Related to the second phenomena, when using too high an amount of TEOS, the PS was unstable and dissolved easily. Some of the dissolved PS was then easily contacting and coalescing with other PS, forming larger and irregular PS. When the coalesced PS was coated with silica, composite PS/silica with irregular forms were produced. Furthermore, using heat treatment to remove the PS brought about the formation of hollow silica particles with irregular structures. The relationship between the concentration of TEOS and the thickness of the shell in different hollow particle diameters is shown in Figure 4e. The thickness increased (from several to tens of nanometers) along with the increases in TEOS concentration. However, when maintaining the TEOS concentration but changing the size of the initial PS, different shell thicknesses were obtained. This was due to the available surface area on the PS to grow the silica. Figure 5 shows the effect of lysine concentration on the thickness of the shell. Figure 5a,b shows the TEM micrographs of hollow silica particles prepared using different lysine concentrations, and Figure 5c shows the relationship between the concentration of lysine and the thickness of the shell. The shell without mesoporous structures was obtained for all cases. Different from the TEOS effect on the control of the shell thickness in Figure 4, the lysine tuned the thickness on the nanometer scale. A lower concentration of lysine (4.80 mmol/ L) resulted in the formation of a thick shell (6.83 nm, Figure 5a), whereas a higher concentration (14.40 mmol/L) allowed the formation of a thin shell (5.84 nm, Figure 5b). With the increase in the amount of catalyst, the silica conversion rate should be increasing. This high rate resulted in good progress in the consumption of the silica source and the formation of the silica monomer but retarded the growth of silica. For this reason, a thin shell was obtained when employing a high concentration of lysine. In contrast, when using a small concentration of catalyst, the conversion rate should be lower, causing less progress in the formation of silica monomers and nuclei. The produced silica monomers were typically caught by the formed silica, which were attached to the surface of PS, forming a thick shell. Although the change in the amount of catalyst altered the conversion of silica, the control of shell thickness was only in the range of nanometers because the reaction and the formation of silica were limited to the

Figure 5. The effect of lysine concentration on the shell thickness of hollow silica particles. (a, b) TEM images of particles prepared using lysine concentrations of 4.80 and 14.40 mmol/L, respectively. (c) Average shell thickness as a function of lysine concentration. All samples were prepared using a TEOS/PS (120 nm) mass ratio of 1.00.

amount and solubility of TEOS. In addition, different from other catalytic effects (e.g., ammonia),8 the change in the shell morphology was not identified for all cases. Lysine has the ability to cover the formed silica,16 resulting in a lowered silica formation rate, the controlling of silica growth, the prevention of agglomeration phenomena, and the creation of a shell with a smooth surface. The relationship between the concentration of lysine and the thickness of shell in different hollow particle diameters is shown in Figure 5c. The thickness decreased along with the increase in lysine concentration. Similar to the above TEOS effect on the control of shell thickness in Figure 4, the size of initial PS influenced the shell thickness (when using the same amount of lysine). To confirm the effect of shell structure on the adsorption ability of material, a nitrogen adsorption analysis was conducted (Figure S3 in the Supporting Information). The result showed that our hollow particles exhibited characteristics of a type II curve. The curve of our hollow particles was similar to the curve of commercially nonporous silica particles but different from the curves of hollow particles with mesoporous structure and porous particles. The specific surface area (SBET) of our hollow particles was 78 m2/g, which was double from the SBET of commercial nonporous particles (30 m2/g) but less than the SBET of both porous particles and hollow particles with mesoporous structure. This occurred because almost no pores were detected and the measured area was only the surface of the outer and inner diameters of the hollow particles. The large-molecule adsorption ability of particles with different structures (i.e., our mesopore-free hollow particles, commercially nonporous silica nanoparticles, hollow particles with mesoporous structure, and porous particles) is shown in Figure 6 and Table 1. The concentration of RhB decreased gradually over time because of the adsorption of RhB by the particles. The rate of RhB adsorption changed along with the use of different adsorbents. Porous silica particles exhibited the 8621

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Figure 6. Adsorption analysis of particles with different morphologies. Co and C are the concentrations of RhB at the initial and measured times, respectively.

Figure 7. Summary of the preparation of hollow silica particles with different morphologies.

fastest RhB adsorption rate, and dense particles exhibited the slowest. The hollow particle samples had an intermediate rate, which was in the range of between the porous and the nonporous particles. When comparing both hollow particles, the hollow particles without mesoporous structure had less

adsorption ability than those with mesoporous structure. In addition, when the adsorption time was extended to 5 days, it was found that the remaining RhB adsorbed by the mesoporefree hollow particles was about 90 wt % whereas by porous particles and hollow particles with mesoporous structure the 8622

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and shell thickness has succeeded, several disadvantages/ exceptions (e.g., the good hollow particles had a shell in the range of 4−12 nm) were found. For this reason, it underscores the need for further studies, which will be performed in our future work (i.e., control of shell thickness via multiple additions of reactant, evolution of the silica shell as a function of time, etc.). However, we believe that further insights gained from research such as the present study should make other fabrication innovations possible.

concentration of remaining RhB was about 50 wt %. This result confirmed that the mesopore-free hollow particles had a lower tendency to adsorb large molecules than did the porous particles and the hollow particles with mesoporous structure. Because almost no adsorption was detected for the commercially nonporous silica particles, the differences in the load of organic compounds in the adsorbents were caused by the existence of porous structure. The size of the RhB was 1.44 nm × 1.09 nm × 0.64 nm.16 This size actually can penetrate the pore when the pore itself is larger than 1.50 nm. However, to increase the adsorption of RhB, the more spaces with appropriate pore sizes are required to put RhB inside the pore without blockage and hold against the desorption process.16 In general, when the pore size is too small, adsorption occurs only on the outer surface. For this reason, when employing our mesopore-free hollow particles as the adsorbent, a small adsorption of RhB was detected in the initial time and using more adsorption time was not useful for further adsorption because RhB was adsorbed only on the outer surface of the particle. A summary of the preparation of hollow silica particles is described in Figure 7. The preparation method explained in this paper involves the synthesis of surfactant-free PS particles, the silica coating process, and the template-removal process. The surfactant-free PS particles are synthesized from the polymerization of styrene under AIBA as the initiator. Because of the tendency of silica charge in the range of negative zeta charge,3 the charge on PS should be a positive zeta value. With this opposite charge condition, the silica can be adsorbed on the surface of the PS, growing and forming silica-coated PS particles. The composite PS/silica particles are then put into the template-removal process to produce hollow silica particles. On the basis of the above experimental results, five synthesis routes in the synthesis of hollow particles can be obtained, as described in the following: The first route shown as R1 in Figure 7 is the formation of an incomplete silica coating (broken hollow particle). This can be obtained when using a low mass ratio of TEOS/PS (ratio 3.25) permits the formation of irregular hollow particles, as illustrated by the fifth route (Figure 7, R5). A concentration of inorganic solution (i.e., TEOS) that is too high disturbs the stability of the PS. The PS is dissolved and some of them are coagulated, forming irregular polymers. When these irregular polymers are coated with silica, hollow particles with an irregular shape are produced. Although the preparation of mesopore-free hollow particles with a spherical shape, smooth surface, and controllable size

4. CONCLUSIONS Mesopore-free hollow silica particles with a spherical shape, smooth surface, and controllable particle diameter and shell thickness were effectively prepared via additive-free liquid-phase synthesis with organic colloidal templating. The present method involved the hydrolytic condensation of TEOS to form silica, in which this silica was grown on the surface of the PS spheres (as the organic-template particles). To hydrolyze TEOS, lysine was used as a catalyst that led to the formation of hollow silica particles with a smooth surface. Whereas in most reports an additive was utilized to bridge and paste the silica on the template, here the effect of charge in the template to make the mesopore-free silica shell was exploited. The ability to control the particle diameter was achieved by varying the size of the template. The shell thickness was controlled by the change in the reactant compositions (i.e., silica source and catalyst). Because of their excellent ability to function as less-adsorbing large molecules, the present hollow particles could be promising for various applications, especially for thermal insulator and optical devices.

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ASSOCIATED CONTENT

S Supporting Information *

Thermal analysis of PS and PS/silica particles. FTIR analysis of PS, PS/silica, and hollow silica particles. Nitrogen adsorption analysis of particles (i.e., mesopore-free hollow particles, commercially nonporous particles, hollow particles with mesoporous structure, and mesoporous particles). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-82-424-7850. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A fellowship provided for A.B.D.N. by the Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged. We acknowledge the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for supporting this research (Young researcher, no. 23-01070). The authors thank Dr. Eishi Tanabe of the Hiroshima Prefectural Institute of Industrial Science and Technology and Dr. Makoto Maeda of the Natural Science Center for Basic Research and Development (N-BARD) at Hiroshima University for his help with the TEM and for his consultation. We also thank Tuswadi from Hiroshima University for his assistance with preparing the manuscript and English consultation. 8623

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dx.doi.org/10.1021/la301457v | Langmuir 2012, 28, 8616−8624