Transformation of Mesostructured Silica Nanoparticles into Colloidal

‡Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Shin- juku-ku, Tokyo, 169-0051, Jap...
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Article Cite This: Chem. Mater. 2018, 30, 540−548

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Transformation of Mesostructured Silica Nanoparticles into Colloidal Hollow Nanoparticles in the Presence of a Bridged-Organosiloxane Shell Eisuke Yamamoto,† Saki Uchida,† Atsushi Shimojima,† Hiroaki Wada,† and Kazuyuki Kuroda*,†,‡ †

Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Tokyo, Shinjuku-ku 169-0051, Japan S Supporting Information *

ABSTRACT: Hollow siloxane-based nanoparticles (HSNs) have attracted significant attention because of their unique properties and applications. Recently, it was discovered that the simple covering of silica nanoparticles with an organosiloxane shell leads to the spontaneous formation of HSNs; however, the detailed mechanism of their formation has not yet been established. In this study, colloidal 30 nm HSNs were prepared by adding organically bridged alkoxysilane to an aqueous dispersion of mesostructured silica−surfactant composite nanoparticles, and the temporal changes of the morphology and chemical state of the nanoparticles were monitored to elucidate the formation mechanism. Core silica was dissolved after the formation of the core−shell structured nanoparticles, and almost all the dissolved silicate species were incorporated in the organosiloxane shell, changing the shell thickness. Two conditions were essential for silica dissolution induced by covering with organosiloxane: (i) presence of a sufficient amount of uncondensed Si−OH groups in the organosiloxane shell, and (ii) elevated temperature and pH for the promotion of the hydrolysis of silica. These findings will enable the fabrication of various HSNs through organosiloxane-induced silica dissolution and redeposition.

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INTRODUCTION Hollow nanoparticles have attracted significant attention because of their unique properties such as the ability to encapsulate functional materials in their void space.1−3 Preparation of the hollow nanoparticles through easy and green processes is a great issue for their practical applications. Transformation of dense nanoparticles into their hollow counterparts is a potential solution. Various hollow nanoparticles have been prepared through this transformation based on Ostwald ripening,4,5 galvanic replacement,6,7 and the Kirkendal effect.6,8−10 Although these phenomena proceed via complex processes, elucidation of a detailed mechanism would assist in the production of hollow nanoparticles with various shell components including metals and semiconductors.11−14 Hollow siloxane-based nanoparticles (HSNs) have great potential for applications such as in drug carriers15,16 and antireflective coatings.17−19 HSNs are typically prepared by using sacrificial templates,20,21 though this method requires a time-consuming core removal process. Transformation of silica nanoparticles into hollow structures has attracted significant attention as a facile alternative to the conventional templating method.22−24 It has been reported that nanoparticles containing © 2017 American Chemical Society

both silica and organosiloxane moieties could be transformed into HSNs.25−30 For example, the simple addition of organically bridged alkoxysilane to silica nanoparticles leads to the dissolution of the silica and the subsequent formation of HSNs.27−30 It was demonstrated that hydrolysis of the Si−O− Si bonds in the silica nanoparticles was promoted when covered with organosiloxanes.27 However, this hypothesis cannot fully explain the overall mechanism because this phenomenon does not always occur when silica nanoparticles are covered with organosiloxane. There have been many reports on the preparation of silica particles covered/functionalized with organosiloxane, and the silica was retained within their structures in almost all reports.31−39 Only a few reports claim to have succeeded in the dissolution of the core silica upon coverage with organosiloxane.27−30 Understanding the detailed mechanism and factors influencing this transformation is important for the generalization of this process and to control the structure and morphology of the resulting HSNs. Received: November 20, 2017 Revised: December 19, 2017 Published: December 24, 2017 540

DOI: 10.1021/acs.chemmater.7b04860 Chem. Mater. 2018, 30, 540−548

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Chemistry of Materials Scheme 1. Illustration of Preparation Method of Colloidal HSNs

process: TEA (0.11 g) and C16TMABr (0.5 g) were dissolved in deionized water (60 mL) in a 200 mL flask, followed by the addition of 60 mL of the MSNs-as to this solution. Next, BTEE (1.4 mmol) was added, and this mixture was stirred at 60 °C for several days to form a hollow structure. The molar composition of the final solution was 0.50 TEOS:0.25 BTEE:1200 H2O:0.25 TEA:0.50 C16TMABr. The resulting sample was labeled “MSNs-BTEE-as”. Removal of Surfactants from MSNs-BTEE-as. The MSNsBTEE-as sample was transferred to a cellulose dialysis membrane tube with a molecular weight cutoff in the range 12 000−14 000 Da and was dialyzed for 24 h with a mixture of 2 M aqueous acetic acid and ethanol (1:1 v/v, 250 mL) to remove residual TEA and surfactant. This process was repeated five times. Next, the sample was dialyzed with deionized water (250 mL) to remove the acetic acid and ethanol, and this process was repeated four times. The resulting sample was denoted as “MSNs-BTEE-dia”. Characterization. Dynamic light scattering (DLS) measurements were conducted on a Horiba nanopartica SZ-100-S instrument at 25 °C. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 microscope operating at 200 kV. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were obtained on a JEOL JEM-2100F microscope operating at 200 kV. Scanning electron microscopy (SEM) images were obtained on a Hitachi S5500 electron microscope operating at 3.0 kV. The samples used for the TEM and SEM measurements were prepared by dropping colloidal solutions on a carbon-coated microgrid (Okenshoji Co.) and drying the grids under vacuum. The mean particle (void) sizes were obtained by measuring the long axes of 150 nanoparticles within the TEM images because the shapes of particles and voids were somewhat distorted. Nitrogen gas adsorption−desorption measurements were performed with an Autosorb-iQ instrument (Quantachrome Instruments) at −196 °C. Samples were preheated to 120 °C for 4.5 h under vacuum. Brunauer− Emmett−Teller (BET) relative surface areas were calculated using Rouquerol method.41 The pore-size distributions were evaluated using the nonlocal density functional theory (NLDFT) method assuming nitrogen adsorption in cylindrical silica pores. For the hysteresis scanning, four segments of nitrogen adsorption−desorption isotherms were measured under different ranges of relative pressures. In segment 1, the gas adsorption−desorption measurement was performed in the relative pressure range of 0.40−0.97. In segments 2, 3, and 4, the minimum relative pressures were 0.40, while the maximum relative pressures were 0.87, 0.84, and 0.80, respectively. FT-IR spectra were obtained using Jasco FT/IR-6100. CHN analysis data were obtained using a PerkinElmer 2400 Series II instrument. Thermogravimetric (TG) curves were obtained on a Rigaku Thermo Plus 2 instrument under a dry air flow at a heating rate of 10 °C min−1 up to 900 °C. The 29 Si magic-angle spinning (MAS) NMR spectra were recorded on a JEOL JNM-CMX-400 spectrometer at a resonance frequency of 79.42 MHz with a 90° pulse and a recycle delay of 430 s. Chemical shifts in the 29Si NMR spectra were referenced to polydimethylsilane at −33.8 ppm. Solid-state two-dimensional 1H−29Si heteronuclear correlation solid state (HETCOR) NMR spectra were recorded on the same spectrometer at resonance frequencies of 400.0 MHz for 1H and 79.4 MHz for 29Si with a contact time of 5 ms. The samples for NMR measurement were prepared by drying the colloidal solutions after removal of the surfactants. The concentration of Si species dissolved in aqueous solutions, separated from colloidal solutions by centrifugation using a centrifuge tube normally used for colloidal solutions (Amicon

It is critical to determine the detailed structure and formation processes of HSNs as well as the factors essential for the induction of silica dissolution to elucidate the transformation mechanism. Previous studies used dense silica particles (230 nm in size) or mesostructured silica−surfactant composite nanoparticles (MSNs, 100 nm) as the core silica particles and successfully demonstrated the formation of hollow structures by adding organoalkoxysilanes under basic conditions. However, the compositions and structures of the shell were not thoroughly characterized. For example, the location of the silica, whether they remain in solution or are incorporated into the shell, was not fully clarified due to lack of quantitative data. In addition, the essential factors that govern the dissolution of silica remain unclear. The following transformation processes were proposed: (i) formation of the organosiloxane shell as a protective layer and subsequent etching and replacement of the core silica by organosiloxane27 and (ii) formation of the organosiloxane shell co-condensed with silica and the subsequent organosiloxane growth-induced etching of the remaining silica.29 It should be noted that these proposed mechanisms are only speculative. In this study, colloidal HSNs 40 nm in size were prepared by adding organically bridged alkoxysilane to an aqueous dispersion of MSNs. The overall preparation method is shown in Scheme 1. Temporal changes in the structure and composition of the particles were investigated to clarify the process of organosiloxane-induced silica dissolution. We found that the HSNs are formed not simply by the dissolution of silica by etching, but by the reconstruction of the organosiloxane shell accompanied by redeposition of the silica. In addition, the essential factors for the formation of the HSNs were revealed by varying the reaction conditions. These findings will guide new approaches for the construction of organosiloxane-based nanoparticles with various compositions.

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EXPERIMENTAL SECTION

Materials. Hexadecyltrimethylammonium bromide (C16TMABr), triethanolamine (TEA) as a base catalyst, and acetic acid were purchased from Wako Pure Chem. Ind., Ltd. Tetraethoxysilane (TEOS: Si(OC2H5)4) was purchased from Kishida Chemical Co., Ltd. 1,2-bis(triethoxysilyl)ethylene (BTEE) was purchased from Gelest, Inc. All compounds were used as-received without further purification. Preparation of HSNs. MSNs were prepared according to our previously developed method.40 In brief, TEA (0.42 g) and C16TMABr (2.0 g) were dissolved in deionized water (240 mL) in a 300 mL flask. The mixture was stirred for 30 min at 80 °C using a magnetic stirrer. Then TEOS (2.46 mL, 11 mmol) was added to the solution, and the mixture was allowed to react while stirring (1300 rpm) at 80 °C for several days. The obtained colloidal solution was slowly cooled to room temperature with continued stirring and filtered to remove the very small number of formed flocs. The obtained sample was denoted as “MSNs-as.” The molar ratio of the precursor solution was 1 TEOS:1200 H2O:0.25 TEA:0.50 C16TMABr. Further, to use the prepared MSNs-as as the core nanoparticles, they were covered with an organosiloxane shell by the following 541

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Chemistry of Materials Ultra purchased from Merck), was determined using an inductively coupled plasma (ICP) optical emission spectrometer with a 5100SVDV ICP-OES (Agilent Technologies).

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RESULTS AND DISCUSSION Characterization of HSNs. The TEM image of the MSNsas (Figure 1a) shows nanoparticles 30 nm in size (hydro-

Figure 2. (a) Nitrogen adsorption and desorption isotherms of MSNsBTEE-dia, (b) pore size distribution calculated using the NLDFT method. (c) Isotherms obtained by hysteresis scans, and (d) pore size distributions obtained by segments.

mesopores arising from the core nanoparticles (3 nm-sized pore40) almost completely disappeared and 10−15 nm-sized mesopores were formed. To further investigate the pore structure, a hysteresis scan was conducted. The scan indicated that the nanoparticles had about 13 nm-sized voids and broad sized interparticle pores (Figure 2c,d). The cavity size calculated from the nitrogen adsorption isotherms should be smaller than that obtained from the TEM images because of their surface roughness. Although the obtained HSNs had almost no mesopores in the shell part, they can be formed by increasing the pH of the core colloidal solution from 8.5 to 10.5 before adding the organosiloxane source (BTEE). The TEM image and nitrogen adsorption and desorption isotherms show that the HSNs have clear 4 nm-sized pores upon increase in pH to 10.5 (Figures S4 and S5). The increase in pH leads to the generation of silanolate groups in the silicate and organosilicate species, which enhances their electrostatic interaction with the cationic surfactant. Therefore, mesopores were formed by templating with surfactant micelles at higher pH. The CHN analysis of the MSNs-BTEE-dia shows that the sample contains 0.1 wt % nitrogen, which indicates that almost all surfactants were removed. The FT-IR spectrum (Figure 3a) shows three major bands at 1190, 1580, and 2960 cm−1 that can be attributed to Si−C, C=C, and C−H, respectively. In addition, the 13C CP/MAS NMR spectrum (Figure S6) shows signals at 147 and 139 ppm arising from trans- and cisethenylene groups, respectively. The TG-DTA measurements (Figure 3b) shows a 4.0% weight loss at 400−600 °C due to the degradation of the ethenylene group and dehydration condensation of the silanol groups. After heating to 900 °C, 87% of the dry weight remained as white powder (SiO2). In combination with the carbon content (8.2 wt %) measured by CHN analysis, the ethenylene/Si ratio of the sample was calculated to be 0.24. The 29Si MAS NMR spectrum (Figure 3d) shows signals at −110 and −100 ppm corresponding to the

Figure 1. (a) TEM image of the core nanoparticles (MSN-as), (b) TEM image, (c) HAADF-STEM image, and (d) SEM image of the MSNs-BTEE-dia.

dynamic diameter was 36 nm, shown in Figure S1 in the Supporting Information), which indicates that the MSNs-as were dispersed as primary nanoparticles. In addition, the colloidal solution had high transparency. The TEM image (Figure 1b) and HAADF-STEM image (Figure 1c) of the MSNs-BTEE-dia show a clear contrast between the central and the shell regions of the nanoparticles. In addition, intact spheres without open pores on the surfaces were observed in the SEM image (Figure 1d). These results suggest that the hollow structure was successfully formed. The mean and hydrodynamic diameters were both about 40 nm, which indicated that the nanoparticles retained their dispersibility as primary nanoparticles even after the addition of BTEE (Figure S2). The mean thickness of the shells was 8 nm, and the nanoparticles had voids 22 nm in size on average (Figure 1b). The size of the voids is generally smaller than the diameter of the original core nanoparticles (30 nm). It should be noted that the nanoparticles before being subjected to dialysis (MSNs-BTEE-as) already exhibited a hollow structure (Figure S3). In addition, the ICP measurements showed that the silicate species were barely dissolved in the dialysate solutions. This indicated that the dialysis process did not affect the formation of the hollow structure. Nitrogen adsorption and desorption isotherms of the MSNsBTEE-dia (Figure 2a) show a type IV isotherm with an H5 type hysteresis, which indicates the nanoparticles have both open and partially blocked pores. The BET area of the HSNs was 820 m2g−1, with a total pore volume of 1.2 cm3g−1. The pore size distribution (Figure 2b) strongly suggests that the 542

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Figure 3. (a) IR spectrum of the MSNs-BTEE-dia, (b) TG-DTA curve of the MSNs-BTEE-dia, and nanoparticles, and (d) MSNs-BTEE-dia.

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Si MAS NMR spectra of (c) core

Figure 4. HAADF-STEM images of (a−e) samples after 2, 4, 8, 12, and 24 h after the addition of BTEE, respectively, (f) the concentration of dissolved silicate species at each time point, and (g) 29Si MAS NMR spectra of the sample after 2 h (above) and 24 h (below).

MSNs.44 The core MSNs-as contained a sufficient amount of the cationic surfactant to disperse the nanoparticles, which contributed to the formation of the colloidal HSNs. In addition, the use of organically bridged alkoxysilane with an ethenylene group as the organic substituent is effective for obtaining colloidal nanoparticles. Formation of HSNs was observed when organosiloxanes with other organic substituents, such as ethylene and phenylene, were used (Figure S7). Unfortunately, using organosiloxanes with these organic substituents leads to the formation of HSN aggregates, which is most likely due to their hydrophobicity. Formation Processes of HSNs. The formation process of HSNs must be fully characterized for the complete understanding of the mechanism. Although the temporal changes of the morphology and composition of the HSNs were investigated in previous studies,27−30 the detailed structure of the HSNs, such as the chemical states of the silicates in the shell, was not reported. In addition, the precise location of the Si derived from the core silica nanoparticles is not clear. In this section, (a) the temporal change of the morphology and the location of Si derived from the core, (b) the temporal change of

Q4 and Q3 units (Qn: Si(OSi)n(OH)4‑n), respectively (Figure 3c). In addition, signals at −80 and −70 ppm originating from the T3 and T2 units (Tm: CSi(OSi)m(OH)3‑m), respectively, were also observed. The integral intensity ratio of the T/Q units was calculated to be approximately 1, which indicated that the SiO4 units derived from the core silica were incorporated into the shell part. The T/(T + Q) ratio of about 0.5 is in good agreement with the C/Si ratio (0.48) determined from the CHN and TG analysis. Such quantitative discussion has never been reported in previous literature. It should be noted that the preparation of HSNs with high colloidal stability has been challenging, as only a few groups have succeeded in preparing HSNs with high colloidal stability.36,42,43 HSNs prepared previously by covering silica nanoparticles with organosiloxane were either aggregates or their dispersibility was not reported.27−30 Our results demonstrated that organosiloxane-induced silica dissolution can be applied for the preparation of colloidal HSNs by covering the well dispersed MSNs with an organosiloxane shell in the presence of a cationic surfactant. It is well-known that the relatively large amount of cationic surfactant micelles can act as templates for the mesostructure and as the dispersant of 543

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when the nanoparticle morphology changed. The condensation degree of the T units changed from 0.88 (2 h) to 0.94 (24 h) when the nanoparticle structure changed from core−shell to hollow. This increase in the condensation degree through the transformation indicates that the presence of a sufficient amount of the T2 units in the shell is important for the formation of the hollow structure. In fact, the core−shell structured nanoparticles were formed when T3 rich bridgedorganosiloxane mesoporous shell covered the silica nanoparticles, as previously reported,36 which supports the present findings. In addition, the core−shell structured nanoparticles were not transformed into the hollow structure when the T3 rich organosiloxane shell derived from nonbridged organoalkoxysilane was formed (Figure S10). The importance of the T2 unit in dissolving of the core silica nanoparticles is a major finding of this study. The role of the T2 units in the transformation is discussed later. In addition, the distributions of silicate species in the nanoparticles obtained at 2 and 24 h after the addition of BTEE were investigated by 2D-NMR spectra (Figures S11). The 2D-NMR spectrum of the sample recovered at 2 h showed that the Q units (δSi = −100 and −110 ppm) did not interact with the protons of ethenylene group (δH = 6.5 ppm), which indicates that Q units were not present in the shell part (Figure S11a). In contrast, the spectrum of the sample recovered at 24 h showed that the Q units interacted with the protons of ethenylene group (Figure S11b), which indicates the dissolved silicate species were distributed in the shell part homogeneously. Effects of Various Parameters on the Formation of HSNs. It is important to understand the effects of other factors that could possibly influence the dissolution of the core silica. Thus, the effects of temperature, core silica nanoparticles, surfactants, and pH were investigated. Importantly, HSNs were not formed upon the addition of the organosiloxane source at room temperature, and core−shell structured nanoparticles, 36 nm in size were maintained even at 10 days after coverage with the organosiloxane shell (Figure S12a). HSNs were formed after the core−shell nanoparticles were left for 9 months at room temperature (Figure S12b), which indicates that the formation of the HSNs was accelerated by heating. It should also be noted that HSNs were not obtained when dense core silica nanoparticles were used (Figures S13a). The dense silica remained in the void of the HSNs after several days at 60 °C, and rattle- and core−shell structured nanoparticles were observed even when the composition of the solution and the T/Q ratio were the same as MSNs-BTEE-dia (Figures S13b). It is well-known that mesostructured silica can dissolve faster than its dense counterpart.40 Hydrothermal treatments have been used to form hollow particles when dense silica particles were used as the core,29 whereas relatively mild conditions were used when core MSNs were used. Therefore, using MSNs is not essential, but is effective for obtaining HSNs rapidly. In addition, it is known that surfactants can affect the hydrolysis rate of silica, and surfactant-induced etching has also been reported.46 To investigate the surfactant effect, the core− shell structured nanoparticles were prepared by adding BTEE at room temperature. After removal of the surfactant by dialysis, the colloidal solution was heated, which led to the formation of HSNs (Figure S14). This strongly indicates that the silica dissolution by organosiloxane is not affected by the surfactant.

the chemical states of Si, and (c) the factors for inducing silica dissolution will be discussed. Temporal Change of Morphology and Dissolved Silicate Concentration. The formation process of the HSNs was investigated by HAADF-STEM (Figure 4) and through TEM observations (Figure S8). The TEM images show that the mesostructure of core became unclear at 2 h after the addition of BTEE, which indicates deposition of organosiloxane on/in the MSNs-as. The mean particle size of the sample recovered at 2 h after the addition of BTEE was 33nm, and was larger than that of the core MSNs-as (30 nm). Deposition of almost all organosiloxane at 2 h after the addition of BTEE was confirmed by the CHN analysis and TG curves (Figure S9). Although a clear difference in the image contrast was not observed at the interface of the core and shell parts because of their similar electron density, the increase of the diameter indicates that the core nanoparticles were covered with organosiloxane to form a core−shell structure (Figure 4a). Some voids were observed in the nanoparticles 4 h after the addition of BTEE (Figure 4b). Almost all nanoparticles were transformed into their hollow structure counterparts after 12 h (Figure 4d). Images a−e show that the particle sizes gradually became slightly larger with time after the formation of the hollow structure. The particle and void sizes of the sample after 24 h were 37 and 17 nm, respectively, and further increased to 38 and 22 nm, respectively, after several days (Figure 1b). This size change after the formation of the HSNs has not previously been reported. Furthermore, the TEM and HAADF-STEM images also show that the cores disappeared after covering the MSNs-as with organosiloxane, although the location of the silica was unclear. We proposed two possibilities regarding the location of the dissolved core silica: (i) the dissolved silicate species are located in the solution or (ii) almost all dissolved silicates are incorporated into the organosiloxane shell and a negligible amount of silica is dissolved in solution. To determine the location of the silica, the concentrations of the dissolved silicates were measured by ICP (Figure 4f). The initial concen tration of silicates (46 ppm corresponding to the 3.6% of total Si amount) was lower than the saturated concentration of silicate in a weakly basic/neutral solution (ca. 100 ppm),45 which is likely due to the presence of C16TMABr. The concentration of the soluble silicates decreased from 46 to 10 ppm (0.8% of total Si amount) 2 h after the addition of BTEE. Subsequently, the silicate concentration increased slightly to 34 ppm (2.7% of total Si amount) in the final solution. The final silicate concentration (34 ppm) was lower than the initial concentration (46 ppm). If the core silica was fully dissolved in solution, the concentration will be about 1300 ppm, which suggests that the core silica and organosiloxane were not dissolved. These findings indicate that core silica is not dissolved in the solution like in an etching process, which was suggested in previous reports.27−30 Temporal Changes in Local Structures. The shell part must play an important role in the process of the silica dissolution. To clarify the role of the organosiloxane shell in the dissolution of the core silica, the chemical states of Si in the samples recovered at 2 h and 24 h after the addition of BTEE were investigated by NMR spectroscopy (Figure 4g). The condensation degrees were calculated using the equation S1 in the Supporting Information. The integral ratios of T/Q were almost the same in the two samples (1.0), which indicates that the nanoparticles had the same elemental composition even 544

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Figure 5. (a) Proposed structural model of the prepared HSNs, (b) diameter of the obtained core−shell structured nanoparticles and HSNs. The diameter and volume of the core−shell structured nanoparticles were taken from the samples recovered 2 h after the addition of BTEE (Figure 3a). The diameter and volume of the HSNs were used from the HSNs-BTEE-as sample. (c) Illustration of the mesostructural change of the HSNs through the transformation process. (d) High magnification and (e) low magnification TEM images, and (f) SEM image of the MSNs prepared by transforming the core−shell structured nanoparticles at pH 10.5.

Furthermore, the effect of pH was also investigated because it is well-known that pH strongly affects the rate of hydrolysis and condensation of silicate species. After the preparation of the core−shell nanoparticles, the pH of the solution was changed from 8.5 to 4 and 11. Although it took a week to transform the core−shell nanoparticles into the HSNs under acidic conditions (Figure S15), hollow structures were observed at all pH values tested. The difference in the time required for the transformation into HSNs is likely due to the difference in the hydrolysis rates of silica. It is known that silica dissolve slower in acidic solution than in the base solution,45 which results in the slow formation of HSNs. Figure S16 shows a summary of the present study. The following two factors are important for the dissolution of the core silica: (i) coverage of MSNs with the organosiloxane having a sufficient amount of uncondensed T2 units and (ii) promotion of the hydrolysis of silica by adjusting the solution conditions. Of course, there are still other possible factors involved in silica dissolution, such as porosity of the shell, type of organosiloxanes, and these are currently under investigation. Structural Model. It is important to establish an exact structural model of HSNs for proposing a mechanism. Figure 5a shows the proposed structural model; the HSNs have an organosiloxane shell incorporating almost all the dissolved silicates through a reconstruction process. The validity of the model was examined by considering the differences of geometrical volumes during structural change. The changes of

diameters and geometrical volumes of the nanoparticles are summarized in Figure 5b. Nevertheless, the total compositions of both nanoparticles were nearly same, and the diameter was increased from 33 to 38 nm in the structural transformation process. In addition, the geometrical volume of the shell was increased from 0.5 × 104 to 2.3 × 104 nm3, which is higher than the total geometrical volume of core−shell structured nanoparticles (1.9 × 104 nm3). This structural change cannot be explained by the previously proposed mechanisms.27−30 The increase of the particle diameter in the transformation process from the core−shell to the HSNs suggests that the HSNs were formed through reconstruction of the shell. To further examine the validity of the model, the hollow structure was investigated to determine whether it was formed through reconstruction of the siloxane networks. It is known that reconstruction of the silica sphere allows for the changing of mesostructure when additives are introduced before the transformation.47 It is expected that the HSNs develop clear mesopores by introducing additives before the process of transformation, as shown in Figure 5c, despite the dense structure of the core−shell nanoparticles. To test this hypothesis, dense core−shell structured nanoparticles were prepared by covering MSNs with an organosiloxane shell at room temperature (Figure S12a). After the pH of the colloidal solution was increased to 10.5, the mixture was heated, which resulted in the formation of HSNs. Although core−shell structured nanoparticles did not have clear mesopores, TEM 545

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Figure 6. Proposed formation mechanism of the HSNs.

S18a,b and Figure S19a,b). The molar ratio of Si/water in each colloidal solution was adjusted to 1:1200 by dilution. The mixing ratios of the solution volume were 1:3, 1:1, 3:1, and 9:1 (volume of colloidal solution containing organosiloxane nanoparticles/volume of colloidal solution containing MSNs). These samples are denoted as solution (X) (X means the mixing ratio of nanoparticles). When the solution contained a larger volume of MSNs than that of organosiloxane nanoparticles (in the case of mixing ratio = 1:3 or 1:1), MSNs remained with retaining their structure. Although some MSNs were partially dissolved, the TEM images of solution (1:3) and (1:1) show that the morphology and structure of both of the MSNs and organosiloxane nanoparticles were retained (Figure S20a,b). The DLS result of solution (1:3) shows only one peak at 92 nm because the intensity of light scattering derived from the organosiloxane nanoparticles should be smaller than that of the MSNs (Figure S21a). Although the hydrodynamic diameter of solution (1:1) was slightly smaller than that of the solution (3:1), this suggests that the MSNs were partially dissolved (Figure S21b). In contrast, when larger amounts of organosiloxane nanoparticles were mixed, almost all the MSNs were dissolved. The TEM images and DLS results of solutions (3:1) and (9:1) show that the MSNs disappeared, and only the organosiloxane nanoparticles remained (Figure S20c,d and Figure S21c,d). The concentrations of silica in all the samples were below the level of the solubility of silica, which was confirmed by ICP measurements (Table S1). These results strongly indicate that dissolved silicates derived from the MSNs were incorporated into the organosiloxane nanoparticles, which supports the proposed mechanism. It is expected that various unique nanoparticles can be obtained by applying this mechanism to other systems. For example, pot-like nanoparticles were formed through anisotropic covering of the MSNs with BTEE according to a previous report37 (Figure S22). These findings provide a new approach for constructing organosiloxane-based nanoparticles with various compositions.

(Figure 5d,e) and SEM images (Figure 5f) of the samples after heating showed HSNs with clear mesopores as well as some aggregates. In contrast, the HSNs prepared by heating the core−shell structured nanoparticles without adjusting the pH did not have clear mesopores (Figure S17). These results suggest that the HSNs were prepared through reconstruction of the siloxane networks. The reconstruction of the shell might be ascribed to the flexibility of the shell when it has a sufficient amount of T2 units. Formation Mechanism of HSNs. The results described above regarding the formation of the hollow structure led us to propose the following mechanism. At the initial stage of the formation, the core−shell structured nanoparticles were formed by covering the MSNs with the organosiloxane shell (Figure 6i). The dissolved silicates were incorporated into the organosiloxane shell homogeneously (Figure 6ii), as confirmed by the ICP, NMR, and 2D-NMR analyses (Figure 4f,g and Figures S11). Although the driving force for incorporation of the silicate moieties is still unclear, organosiloxane leads to the incorporation of the dissolved silicate into the shell to stabilize the interface. We assume that the organosiloxane in the shell has a higher interfacial energy with water than with silica because of the hydrophobicity of the organic substituents including ethenylene, ethylene, and phenylene groups. This change of equilibrium through the incorporation of the dissolved silicates leads to further dissolution of the core silica nanoparticles (Figure 6iii) and the processes (ii) and (iii) simultaneously occur to form the stable organosiloxane-silica composite shell. The dissolution and deposition of silica continue when the amount of the T2 units is sufficient to allow the silica core to reconstruct the shell, and the HSNs can finally be obtained (Figure 6iv). To investigate the generality of the aforementioned mechanism, we applied these conditions to other systems. Discrete organosiloxane nanoparticles and MSNs were mixed and heated as a model system. A colloidal solution of organosiloxane nanoparticles with 20 nm in size48 and MSNs with 80 nm in size49 was prepared separately. TEM images and DLS results of the obtained nanoparticles show that MSNs and organosiloxane were dispersed as primary nanoparticles (Figure 546

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CONCLUSION HSNs with high colloidal stability were prepared by covering MSNs with ethenylene-bridged organosiloxane. The formation process of the HSNs was elucidated by investigating the temporal changes of the morphology and the chemical state of the silicates in the shell. Disappearance of the core nanoparticles proceeded not by an etching or a replacement process, as proposed in previous reports, but by the reconstruction of the shell through dissolution and redeposition of the silica. Almost all dissolved silica species were incorporated into the organosiloxane shell. By changing various factors such as the amount of BTEE added, temperature, pH, and the porosity of the core, it was found that the organosiloxane must have a sufficient amount of the T2 unit to dissolve the core silica and reconstruct the shell. This novel insight will be valuable for the design of siloxane-based materials through a strategy based on the dissolution and redeposition of silica induced by organosiloxane.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04860.

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Additional figures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atsushi Shimojima: 0000-0003-2863-1587 Kazuyuki Kuroda: 0000-0002-1602-0335 Author Contributions

The manuscript was written through contributions of all authors. E.Y. contributed to the design of concept and preparation of manuscript under the supervision of K.K. E.Y. and S.U. carried out the experiment. A.S., H.W., and K.K. discussed the data and revised important contents in the manuscript. Funding

This work was supported in part by two grants of JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research (No. 15K13809) and Grant-in-Aid for JSPS Fellows (No. 15J06919)). Grant-in-Aid for Strategic International Collaborative Research Program (SICORP) “France-Japan Joint Call on MOLECULAR TECHNOLOGY” from the Japan Science and Technology Agency is also acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. T. Shibue (Materials Characterization Central Lab., Waseda University), S. Enomoto, Y. Ishikawa, K. Nagata, and M. Yoshikawa (Waseda University) for their experimental assistance and fruitful discussion.

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