Titania Hollow Nanoparticles and

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6570−6579

Synthesis of Hierarchical Silica/Titania Hollow Nanoparticles and Their Enhanced Electroresponsive Activity Chang-Min Yoon,† Jaehoon Ryu,† Juyoung Yun, Yun Ki Kim, and Jyongsik Jang* School of Chemical and Biological Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea S Supporting Information *

ABSTRACT: Wrinkled silica nanoparticle (WSN)-based hollow SiO2/TiO2 nanoparticles (W-HNPs) with hierarchically arrayed internal surfaces were prepared via the combination of sol−gel, TiO2 coating, and etching of core template techniques. The hierarchical internal surface of W-HNPs was attained using WSNs as a core template. Compared with SiO2 sphere-templated hollow SiO2/TiO2 nanoparticles (S-HNPs) with flat inner surfaces, W-HNPs displayed distinctive surface areas, TiO2 loading amounts, and dielectric properties arising from the hierarchical internal surface. The unique properties of W-HNPs were further investigated as an electrorheological (ER) material. W-HNP-based ER fluids exhibited ca. 1.9-fold enhancement in the ER efficiency compared to that of S-HNP-based ER fluids. Such enhancement was attributed to the unique inner surface of W-HNPs, which effectively enhanced the polarizability by increasing the number of charge accumulation sites, and to the presence of the high-dielectric TiO2. This study demonstrated the advantages, in terms of practical ER applications, of hollow nanomaterials having uniquely arrayed internal spaces. KEYWORDS: hierarchical, wrinkled, silica, titania, hollow, electroresponse, electrorheology

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INTRODUCTION Smart fluids, whose rheological properties are controllable by the application of external stimuli such as an electric field (E field), magnetic field (H field), and light (UV−visible), are of considerable interest.1−4 A type of smart fluid, an electrorheological (ER) fluid composed of polarizable materials in an insulating medium such as mineral or silicone oils, exhibits reversible transitions from liquid- to solidlike states in the presence of an E field.5 Notably, ER materials can form fibrillike structures perpendicular to electrodes, and the resulting changes in the rheological properties are usually characterized in reference to the shear stress and viscosity. The ER performance is greatly affected by the dielectric properties and dispersion stability of the materials.6 The former relates to the polarization ability of the material and the latter to the stability of the materials in the dispersing medium. Numerous polarizable materials, including organic, inorganic, metal, carbonaceous, and polymeric ones (and their composites), have been evaluated as ER materials.7−9 Silica, composed of a network of silicon and oxygen atoms, is widely used in drug delivery, adsorption, catalysis, dyesensitized solar cells, and ER applications.10−14 Following Stöber’s invention, uniformly sized SiO2 particles have been synthesized in various sizes, ranging from a few nanometers to hundreds of micrometers, by simply controlling the added amounts (i.e., molar ratio) of the reagents.15,16 Numerous modifications, such as the addition of surfactants (or swelling © 2018 American Chemical Society

agents), controlling the emulsion phases, and changing the reaction temperature, have been made to the typical Stöber method to modify the morphology, dimensions, and porosity of SiO2 particles.17,18 The various SiO2 materials reported to date include mesoporous, one-dimensional, core−shell, multishelled, and wrinkled (or hierarchical) particles.19,20 Among these, wrinkled SiO2 particles are currently of great interest because of their unique properties, originating from their hierarchical morphology. Compared with nonporous or other mesoporous SiO2 particles, wrinkled SiO2 particles have remarkably high surface areas, porosities, and accessibility to their inner surfaces.21,22 However, these wrinkled SiO2 particles are difficult to prepare with a uniform size because of nucleation and growth mechanisms that are more complex than those for Stöber-type particles or their modifications. Therefore, there remains a need to establish methods that can provide wrinkled SiO2 particles of a uniform size. Silica is not the optimal material for ER applications because it has low dielectric properties, which results in decreased polarizability. Nevertheless, many ER studies have used SiO2 because of its other advantages, which include ease of largescale production, controllability of size and structure, and facile modification of the material properties.23−25 These positive Received: December 12, 2017 Accepted: February 1, 2018 Published: February 1, 2018 6570

DOI: 10.1021/acsami.7b18895 ACS Appl. Mater. Interfaces 2018, 10, 6570−6579

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ACS Applied Materials & Interfaces

Fabrication of Hierarchical Wrinkled Silica Nanoparticles (WSNs). Hierarchical WSNs were prepared according to the following process. In a typical synthesis process of WSN, CPB (2 g, 5.2 mmol) and urea (1.2 g, 20.0 mmol) were dissolved in deionized (DI) water (60 mL) by stirring. Then, a mixture of cyclohexane (30 mL) and isopropyl alcohol (1.84 mL, 24 mmol) was added to the abovementioned solution. Subsequently, TEOS (5 g, 24 mmol) was added to the mixture in a dropwise manner with vigorous stirring for 30 min at room temperature. After 30 min of reaction, the temperature was raised to 70 °C and the reaction was continuously proceeded for 1 h. Thereafter, the reactor was suddenly transferred to the water bath for the cooling process (10 °C) for 2 h. After the cooling process, the reaction was resumed at 70 °C with vigorous stirring, and this state was maintained for 12 h. Finally, WSNs were collected by centrifugation and washed several times with ethanol to remove the residues. The isolated WSNs were dried under 90 °C conditions overnight. Finally, the dried WSNs were calcined at 550 °C for 6 h in air. In addition, the scale of the WSN synthesis process could be controlled by scaling down or scaling up the amounts of all reagents in ranges of 0.5−2.0 times. The relative yield percentage of WSN to TEOS precursor in mass ratio was about 28.0%. On the other hand, SSN with a flat surface was prepared according to the typical Stöber method.15 In a typical synthesis of SSNs, ethanol (158 mL), DI water (2.8 mL), and NH4OH (7.8 mL) were mixed by magnetic stirring for 10 min. Subsequently, TEOS (5.8 mL) was injected into the mixture and the reaction was proceeded for 12 h at room temperature. The resulting SSNs were collected by centrifugation, washed with ethanol three times, and dried in an oven (90 °C) overnight. Synthesis of WSN-SiO2/TiO2 Hollow Nanoparticles (WHNPs). As-synthesized WSNs (1.0 g) were dispersed in the mixed solution of ethanol (79 mL), ammonia (3.9 mL), and water (1.4 mL). Then, acetonitrile (28 mL) was added to the colloidal WSN solution with vigorous stirring at 4 °C. The other solution containing ethanol (36 mL), acetonitrile (12 mL), and TTIP (4.6 mL) was prepared and added dropwise to the WSN solution. The resulting mixture was vigorously stirred for 12 h to result in a white cloudy solution containing TiO2 shell-coated W-STCSs. The fabricated W-STCSs were collected by centrifugation at 12 000 rpm for 20 min. The hollow-structured W-HNP was obtained by following the sonicationmediated etching and re-deposition (SMER) method. Dried W-STCSs (0.5 g) were well dispersed in DI water (15 mL) by stirring and sonication. The etchant solution of ammonium hydroxide (0.1 M, 5.0 mL) was added to the W-STCS solution, and the resulting mixture was placed under sonication for 12 h. The final W-HNP product was obtained by centrifugation at 12 000 rpm for 10 min and washed five times with DI water and ethanol to remove the organic residues and etchant solution. W-HNPs were collected by centrifugation and dried in an oven (90 °C) overnight. S-STCSs and S-HNPs were prepared by the same experimental procedure as that used for the preparation of W-STCS and W-HNP, except that the added amount of TTIP was lessened to 3.5 mL. Characterization. The morphological characteristics of SSN, SSTCS, S-HNP, WSN, W-STCS, and W-HNP were examined by transmission electron microscope (LIBRA 120, Carl Zeiss) and fieldemission scanning electron microscope (JSM-7800F Prime, JEOL) analyses. The Brunauer−Emmett−Teller (BET) surface area and pore volume of materials were attained by N2-sorption analysis (ASAP 2000, Micromeritics Co.) and the related calculation. Elemental compositions of materials were analyzed by a scanning transmission electron microscope (JEM-2100F, JEOL). The dispersion stability of materials dispersed in silicone oil was determined by calculating the sedimentation ratio (R). Dielectric characteristics of materials were determined by an impedance spectroscope (Solatron 1260) installed with an interface analyzer (Solatron 1296). Investigation of Electrorheological (ER) Properties. All ER fluids were formulated according to the following procedures. First, particles were dried in an oven (90 °C) overnight to completely remove the moisture. Dried particles (0.3 g) were ground by mortar and pestle and dispersed into silicone oil (11.0 mL, viscosity = 100 cSt). For complete dispersion, as-prepared ER fluids were vigorously

aspects offset the low dielectric properties. Moreover, various methods to improve the dielectric properties of SiO2 particles have been reported. These methods include using SiO2 as a hard template for metal doping, plasma treatments, coating with conductive polymers, and incorporation of metal oxides such as TiO2.26−31 Our group previously reported the improvement of ER-related and other characteristics of SiO2 particles by incorporating TiO2 layers to form SiO2/TiO2 core/ shell particles.32 Additionally, acid and basic etchants have been used to entirely or partially etch out the core SiO2 portion, resulting in hollow particles having different compositions of SiO2, TiO2, and additional materials depending on the type and amount of etchants used. Adding TiO2 layers on SiO2 particles and manipulating them to produce a hollow morphology greatly improve the dielectric properties and dispersion stability of the final products, which consequently have a better ER efficiency than that of those formed from pristine SiO2 templates. However, most of these SiO2 hard template methods have been carried out using the Stöber approach or a derivative thereof and relatively little attention has been paid to adapting them to more complex materials like wrinkled or hierarchical SiO2 particles. In this study, we report the first synthesis of wrinkled silica nanoparticle (WSN)-based hollow SiO2/TiO2 nanoparticles (W-HNPs) with hierarchically arrayed inner surfaces using WSNs as hard templates. Notably, uniform core WSN nanoparticles were prepared by a simple one-pot method featuring a bicontinuous microemulsion phase and a cooling step, which enabled the separation of nucleation and growth steps. A TiO2 shell was introduced onto prepared WSNs via a sol−gel method using titanium tetraisopropoxide (TTIP) as a precursor to obtain WSN-based SiO2/TiO2 core/shell nanoparticles (W-STCSs). Subsequently, the core SiO2 part was easily etched by sonication-mediated etching and re-deposition (SMER) using ammonium hydroxide as the etchant solution. The resulting W-HNPs were compared with SiO2 spheretemplated hollow SiO2/TiO2 nanoparticles (S-HNPs), synthesized by the same experimental method, except using a flatsurfaced spherical type of SiO2 as the core template material. Notably, W-HNPs exhibited increased porosity, surface area, TiO2 loading, and dielectric behavior compared to those of SHNPs because of the unique internal surface of W-HNPs, which resulted from the packing of TiO2 on the hierarchically arrayed surface of the WSN core template. Furthermore, we investigated the electroresponse of W-HNPs in a practical ER fluid. The W-HNP-based ER fluid had remarkable 5.1- and 1.9fold enhanced ER activities compared to those of non-TiO2coated WSN- and S-HNP-based ER fluids, respectively. Accordingly, this study successfully leveraged the unique characteristics of hollow nanoparticles with hierarchically arrayed internal spaces and demonstrated their potential in ER fluids.

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MATERIALS AND METHODS

Materials. Cetylpyridinium bromide (CPB, 98.0%), tetraethyl orthosilicate (TEOS, 98.0%), titanium(IV) isopropoxide (TTIP, 97.0%), cyclohexane (99.7%), isopropyl alcohol (99.5%), and poly(methylphenylsiloxane) (silicone oil, viscosity = 100 cSt) were purchased from Aldrich Chemical Co. Absolute ethanol (99.9%) was obtained from Fisher chemical Co. Urea (99.0%) and ammonium hydroxide solution (NH4OH, 28.0−30.0%) was purchased from Samchun Chemical Co. (Korea). All chemicals were used as received without further purification. 6571


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ACS Applied Materials & Interfaces stirred using a magnetic stirrer for 24 h. Also, no additives other than particles and silicone oil were added to the ER fluids. The ER activities of ER fluids were examined by a rheometer (AR2000, TA instruments) with accessories of a container cup (d: 30.0 mm and h: 30.0 mm), concentric cylinder conical geometry (d: 28.0 mm and h: 30.0 mm), and voltage generator (Trek 677B). The gap distance of geometry and cup was set to 1.0 mm on each side (total: 2.0 mm) without contacting and friction between them. For practical ER measurements, welldispersed ER fluids were added into the cup and geometry inserted. Before applying the E field, preshear of 10.0 s−1 was applied for 10 min to attain an equilibrium state of the fluid. Finally, the intended strength of E field and an appropriate testing mode were selected to investigate ER properties of ER fluids.

bicontinuous phase. In the next step, a porous TiO2 shell was then coated onto the surface of WSNs by a sol−gel method, using TTIP as a precursor to obtain W-STCSs. Finally, core WSNs were etched by sonication using NH4OH solution as the etchant to obtain W-HNPs. During the etching process, the basicity of NH4OH and the intense energy derived from the sonication broke the siloxane bond of the SiO2 core into small Si fragments. These Si fragments were continuously dissolved and coordinated with hydroxide ions (OH−) to form silicic ions (Si[OH]62−). Then, these re-coordinated silicic ions were diffused into the TiO2 shell layer as well as in the hollow cavity. Sequentially, small silicic fragments experienced condensation and the resulting SiO2 materials were re-deposited within TiO2 materials to yield SiO2/TiO2 composite hollow materials according to the process of Ostwald ripening.34 Unlike the core SiO2, the TiO2 shell did not dissolve during the SMER process because of the chemical inertness of TiO2 compared to that of SiO2.35 For comparison, S-HNPs were synthesized by a similar experimental method, except that the core SSNs were fabricated by the Stöber method using a flat-surfaced SiO2 as the core template. The morphologies of WSNs, W-STCSs, and W-HNPs were examined by transmission electron microscopy (TEM) to confirm their successful preparation (Figure 2). The core WSNs were synthesized with a hierarchical structure and had a uniform diameter of ca. 100 nm. By adding the cooling step, WSNs were uniformly synthesized. In the case of W-STCSs, the hierarchical surface of a WSN was completely covered by a TiO2 shell to provide ca. 120 nm sized particles, which indicated that the TiO2 shell thickness was ca. 10 nm on each end. The final W-HNPs were monodispersed with a size of ca. 140 nm and increased shell thickness of ca. 20 nm, which confirmed the successful re-deposition of SiO2 by the SMER process. Figure 2d shows that W-HNPs were hollow, with hierarchically arrayed inner surfaces. Also, SSNs, S-STCSs, and S-HNPs (hereafter referred to as the series of SSN materials) were successfully fabricated with particle sizes of ca. 100, 120, and 140 nm, respectively (Figure S1). In contrast, S-HNPs did not have a hierarchical surface but rather a smooth internal surface (Figure S1d). This difference between the inner surfaces of W-HNPs and S-HNPs was attributed to the dissimilarity between the core WSN and SSN templates. For W-HNPs, the TTIP precursor could permeate into the hierarchical pores of WSNs and thereby provide the uniquely packed internal surface, but the smooth inner surface of S-HNPs developed from the nonporous surface of SSNs. Focused ion beam (FIB) milling was used to prepare samples of W-HNPs for a closer examination of the internal surface by scanning electron microscopy (SEM) (Figure S2). It was clearly observed that W-HNPs showed a bumpy, uneven, and spindly internal surface because of the formation of the hierarchical array. Furthermore, SEM observations confirmed uniform particle sizes of WSNs, W-STCSs, W-HNPs, and the series of SSN materials (Figures S3 and S4). Under our experimental conditions, the particle size of the core WSNs and the amount of TTIP added were the key to attaining the hierarchical internal hollow surface of W-HNPs. The WSN core size must be at least ca. 100 nm to obtain WHNPs with clear internal surfaces. We have synthesized and examined W-HNPs made from 80 nm sized core WSNs, but the hierarchical structure was not observed within W-HNPs because of the lower porosity of the core WSNs, which hindered the formation of the hierarchical array of TiO2 shells.

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RESULTS AND DISCUSSION Preparation of WSN-SiO2/TiO2 Hollow Nanoparticles (W-HNPs). The preparation process of W-HNPs is schematically illustrated in Figure 1. First, uniform core WSNs were

Figure 1. Schematic illustration of the fabrication of wrinkled silica nanoparticle (WSN)-based hollow SiO2/TiO2 nanoparticles (WHNPs) via sol−gel and sonication-mediated etching and re-deposition (SMER) methods.

prepared on the basis of the Winsor III system microemulsion method. In the preparation of WSN, the bicontinuous phase played a key role in the formation of a hierarchical and wrinkled structure. The bicontinuous phase consists of ternary systems of surfactant and equivalent amounts of water and oil.20 With the vigorous stirring in Winsor III systems, oil-in-water (o/w) types of macroemulsion systems were formed. Then, TEOS was injected into this emulsion system. At the beginning of the reaction, TEOS was dissolved in the oil layer of a bicontinuous phase. As soon as TEOS contacted the water layer of the bicontinuous phase, a continuous condensation and hydrolysis of TEOS occurred to result in the formation of a closed and spherical structured surfactant-silicate system.22 These surfactant-silicate systems were able to be demulsified and gathered into repetitive mesophases, which serve as the seeds of WSNs.33 Continuously, newly formed seeds of WSN grew with the chain reaction of hydrolysis and condensation of TEOS through the water layer. Eventually, the wrinkled and hierarchical shape of WSNs was formed during the particle growth owing to the mixed distribution shapes of water and oil within the 6572


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Figure 2. TEM micrographs of (a) WSNs, (b) W-STCSs, (c) W-HNPs, and (d) a W-HNP at high magnification.

resulted in the decreased surface area and porosity of the WSTCS material. Additionally, the TiO2 shell formed as a mesoporous material with a uniform pore size of ca. 2.5 nm, which permitted effective transport of the NH4OH etchant and silicic ions during the etching and re-deposition processes. The final W-HNPs displayed a surface area and pore volume of 501.3 m2 g−1 and 0.84 cm3 g−1, respectively. Importantly, the surface area and porosity of W-HNPs, which were higher than those of previous W-STCS materials, were attributed to etching of the core WSN and the formation of the hollow structure. Moreover, pore sizes of ca. 2, 5, 7.5, 10, and 80 nm were observed in the BJH plot of W-HNPs. The different sizes of the pores corresponded to the morphological characteristics of WHNPs as follows: 2 nm diameter pores, porosity of the TiO2 shell; 5−10 nm diameter pores, porosity created by the hierarchically arrayed inner surface of W-HNPs; and 80 nm diameter pores, porosity arising from the internal hollow space. In the case of the series of SSN materials, the surface area and porosity increased with the sequential experimental steps. Detailed physical characteristics of WSNs, W-STCSs, W-HNPs, and the series of SSN materials are listed in Table S1. The elemental compositions of WSNs, W-STCSs, W-HNPs, and the series of SSN materials were determined by energydispersive X-ray spectroscopy (EDS) (Table S2). WSNs and SSNs were mainly composed of Si and O, and a slight amount of C was detected because of organic residues. W-STCSs and SSTCSs showed additional Ti that originated from the outer TiO2 shell. In the case of W-HNPs and S-HNPs, the lower amount of the Si feature compared with that of precursor materials was attributed to the etching of the core SiO2. Notably, W-STCSs and W-HNPs contained more Ti than SSTCSs and S-HNPs. These phenomena were attributed to the high surface area of the WSN core template compared with that

Moreover, the amount of TTIP added was controlled under *0.5, *0.75, *1, and *2 conditions (where *1 denotes the standard TTIP amount corresponding to 4.32 times the WSN mass), as shown in Figure 3. For the *0.5 TTIP condition, complete TiO2 shells were unable to form on the WSN cores because of insufficient TTIP. In the *0.75 TTIP case, TiO2 shells covered the surface of WSNs, but the resulting W-HNPs displayed a deformed structure and ruptured shells because of the formation of relatively thin TiO2 shells; these fragile shells could not withstand the sonication treatment. With the excess *2.0 TTIP case, a hierarchical and hollow structure was not obtained because of the thick coating of the TiO2 shell, which blocked the pathway of the etchant solution to the inner space; only partial etching of the core SiO2 occurred. Consequently, it was essential to select the appropriate WSN core size (>100 nm) and the amount of TTIP precursor (4.32 times the WSN mass) to successfully synthesize W-HNPs. To gain further insight into the structure of the materials, N2sorption curves of WSNs, W-STCSs, and W-HNPs were measured and the related Brunauer−Emmett−Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) pore distributions were calculated. All of the materials exhibited typical type IV hysteresis curves, indicating their porous nature (Figure 4a−c). The surface area and porosity of WSNs and derived materials changed with the sequential experimental steps of TiO2 coating and SMER processing. WSNs had a surface area, pore volume, and major pore size of 533.4 m2 g−1, 1.77 cm3 g−1, and ca. 12.5 nm, respectively. The high surface area and porosity of WSNs originated from the hierarchically arrayed outer surface. In the case of W-STCSs, the surface area, pore volume, and major pore size were 463.4 m2 g−1, 0.45 cm3 g−1, and ca. 2.5 nm, respectively. Notably, the TiO2 shell covered the high surface area of the hierarchical WSNs, which 6573


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tion ratio (R) for 2.7 wt % dispersions in silicone oil (Figure 6). The dispersion stability of the series of SSN materials was also assessed for comparison. Notably, all materials of the WSN and SSN series displayed abrupt sedimentation behavior for the first 20 h; after that, sedimentation gradually subsided and equilibrium was reached by 90 h. Each material sedimented slightly differently because of their different characteristics, such as surface area, pore volume, composition, and morphology. WSNs had the highest dispersion stability of 0.89 (i.e., ca. 85% of the particles were stably dispersed in the final state); they also had the highest surface area and pore volume, as determined from BET and BJH analyses. After the TiO2 coating process, the dispersion stability of W-STCS decreased slightly to 0.77 because of the addition of the relatively heavy TiO2 shell onto the hierarchical WSN morphology, which resulted in the decreased surface area and pore volume. After the etching process, the dispersion stability of the final WHNPs increased to 0.83, which showed the successful formation of the internal hollow space and the porous structure of the material. Accordingly, it was concluded that the final WHNPs were highly dispersive because of the creation of the inner hollow space, despite the coating of the relatively heavy TiO2. The series of SSN materials displayed a similar, but slightly different, dispersion stability trend to that of the WSN series. Specifically, SSNs showed a dispersion stability of 0.79, which was lower than that of WSN dispersions. The higher surface area and pore volume of WSNs resulted in a better dispersion stability than that of the nonporous SSNs. The dispersion stability of S-STCSs was lower at 0.74, which was the same trend as that observed for W-STCSs. However, the SHNP dispersion stability increased to 0.85, which was slightly higher than that of W-HNPs. The differences between dispersion stability trends of the final hollow W-HNPs and SHNPs stemmed from the different internal structures of the materials. As discussed previously, the WSN surface had a hierarchical structure with a high surface area, enabling more TiO2 to be loaded onto the surface compared with that onto a flat-surfaced SSN. Furthermore, S-HNPs lost a large amount of their core SiO2, leaving only TiO2 shells behind after etching, whereas W-HNPs remained with their TiO2 shells and a hierarchically arrayed TiO2 inner surface. Therefore, the dispersion stability of S-HNPs was higher than that of WHNPs because of the relatively low TiO2 loading. The dispersion stabilities of both W-HNPs and S-HNPs were thus effectively improved by the removal of the core SiO2, but different amounts of the leftover TiO2 shell caused slight variations in the dispersion stabilities of the final hollow materials. As discussed earlier, TiO2 is a high-dielectric material; thus, the loading amount of TiO2 could be expected to affect the dielectric properties of materials. To gain insight into the dielectric properties, the dielectric constant (ε′) and loss factor (ε″) of WSNs, SSNs, and their derived materials were examined as a function of the E field frequency (Figure 7). Generally, the dielectric properties of materials can be enhanced by manipulations such as increasing the surface area for charge accumulation, introducing a highdielectric material, or decreasing the low-dielectric portion.32,39,40 Dielectric characteristics are closely related to the polarization abilities of ER materials under an applied E field.41,42 Specifically, the achievable polarizability (Δε, polarization tendency) and relaxation time (λ, polarization rate) can be interpreted from the ε′ and ε″ curves.43 Previous studies have reported that materials with large Δε and short λ exhibit

Figure 3. TEM micrographs of various W-HNPs prepared by adding TTIP amounts of (a) *0.5, (b) *0.75, and (c) *2.0.

of the flat-surfaced SSN. Because of the high surface area, the loadings of TiO2 were higher for the series of WSN materials than for the SSN-derived materials. Scanning transmission electron microscopy (STEM) was used to clearly visualize the elemental composition of W-HNPs (Figure 5). In the final WHNPs, Ti, Si, and O were well distributed over the entire region, which indicated the successful formation of the SiO2 and TiO2 composite materials via the SMER process. These experimental results confirmed that W-HNPs having a hierarchically arrayed internal surface were successfully synthesized. Two Key Parameters Affecting the ER Activities. Two key intrinsic characteristics of materials that affect their ER performance are their dispersion stability and dielectric properties.36,37 Generally, materials with high dispersion stability exhibit an enhanced ER performance because of the improved particle mobility in the dispersing medium.38 In the present study, the dispersion stabilities of WSNs, W-STCSs, and W-HNPs were examined by determining their sedimenta6574


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Figure 4. N2-sorption isotherms of (a) WSNs, (b) W-STCSs, and (c) W-HNPs. BJH pore distribution plots of (d) WSNs, (e) W-STCSs, and (f) WHNPs.

better ER performances under an applied E field.6,9 Specifically, Δε can be determined by referring to the differences between fictitious (ε0, f → 0) and static (ε∞, f → ∞) dielectric constants, thus providing information on the polarizability of materials. The determined Δε values for WSNs, W-STCSs, and W-HNPs were 1.12, 2.11, and 2.42, respectively. By introducing the high-dielectric TiO2 shell, W-STCSs manifested enhanced polarizability compared with that of WSNs constituted only of SiO2. W-HNPs had higher Δε because of the combined effects of increased surface area for charge accumulation and the decreased amount of low-dielectric core SiO2 remaining after the etching process. On the other hand, the Δε values for SSNs, S-STCSs, and S-HNPs were 0.70, 1.35, and 1.81, respectively. Noticeably, W-HNPs displayed a higher polarization tendency than S-HNPs. This suggested that the hierarchically arrayed internal structure and increased loading of TiO2 positively affected the dielectric properties of W-HNPs. The λ of the materials was also estimated using the following equation44

Figure 5. STEM elemental-mapping analysis of W-HNPs (detected elements: Ti, Si, and O).

λ= 6575

1 2πfmax DOI: 10.1021/acsami.7b18895 ACS Appl. Mater. Interfaces 2018, 10, 6570−6579

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Table 1. Dielectric Properties of the Series of WSN- and SSN-Based ER Fluidsa sample

ε0

ε∞

Δε = (ε0 − ε∞)

f maxb (Hz)

λc (s)

SSN S-STCS S-HNP WSN W-STCS W-HNP

3.55 4.36 4.89 3.95 5.23 5.75

2.85 3.01 3.08 2.83 3.12 3.40

0.70 1.35 1.81 1.12 2.11 2.35

48.4 98.0 238.9 88.3 249.1 998.2

0.0033 0.0016 0.00066 0.0018 0.00063 0.00015

a Dielectric properties of ER fluids were determined by an impedance analyzer (Solatron 1260) and a dielectric interface analyzer (Solatron 1296). bThe local frequency of the peak from dielectric loss factor ε″ and the f max values were determined by the nonlinear regression plot using OriginPro. cThe relaxation time was calculated by λ = 1/(2πf max) relation (f max was the maximum frequency of the ε″ graph).

confirm the unique hierarchical morphology of WSNs, as well as the importance of each synthetic step, from TiO2 coating to SiO2 etching, to enhance the dispersion stability and dielectric properties of the final W-HNPs and render them suitable for ER applications. Investigation of Electrorheological (ER) Activities of a Series of WSN Materials. The ER activities of sequentially prepared WSNs, W-STCSs, W-HNPs, and a series of SSNbased ER fluids were investigated by controlling various parameters including the shear rate, E field strength, and E field on−off condition. Shear stress curves (i.e., flow curves) of ER fluids were examined as a function of the shear rate under an applied E field of strength 3.0 kV mm−1 (Figure 8). With the applied E field strength, all ER fluids exhibited shear stresses because of the formation of fibril-like structures induced by the electrostatic forces between particles. In the low-shear-rate region (before the critical shear rate, τc), the shear stresses of all of the ER fluids displayed Bingham plasticlike behavior marked by plateau curves.45 Such behavior was attributed to balancing between the formation and deformation of fibril-like structures because of the competing electrostatic and hydrodynamic forces. After passing the τc, the shear stresses showed Newtonian fluidlike behavior, that is, proportionally increasing with the increasing shear rate, indicating the domination of hydrodynamic forces over electrostatic forces.46 The ER activities increased in the order of WSNs < W-STCSs < WHNPs, which was in accordance with the sequential preparation steps. The highest ER performances of WSN-, W-STCS-, and W-HNP-based ER fluids were determined as ca. 8.2, 27.3, and 40.9 Pa, respectively. Notably, W-STCSs had a better ER performance than that of WSNs because of the improved dielectric properties stemming from the incorporation of a high-dielectric TiO2 layer as the outermost shell. Furthermore, the hollow W-HNPs exhibited the highest shear stress; the etching of core SiO2 simultaneously improved the dispersion stability and dielectric properties. The highest ER performances of SSN-, S-STCS-, and S-HNP-based ER fluids were measured as ca. 3.2, 13.3, and 25.8 Pa, respectively. A similar trend in ER activity was observed for SSN-, S-STCS-, and S-HNP-based ER fluids. The improved dielectric properties and dispersion stability of S-HNPs resulted in better ER performance than that of the precursor SSN and S-STCS materials. Notably, WSN-derived materials exhibited a greater improvement in ER activity than that of SSN-derived materials. These different ER activities of two sequential sets of WSN and SSN materials were attributed to differences in their morphologies and other

Figure 6. (a) Sedimentation ratio (R) of the series of WSN- and SSNbased electrorheological (ER) fluids in silicone oil (2.7 wt %) and (b) graphical illustration and definition of the sedimentation ratio.

Figure 7. Dielectric constant (ε′) and loss factor (ε″) of the series of WSN- and SSN-based ER fluids as a function of the electric field frequency ( f).

where f max is obtained from the maximum E field frequency of the ε″ curve. The calculated λ values of WSNs, W-STCSs, and W-HNPs were 2.0 × 10−3, 6.0 × 10−4, and 1.0 × 10−4 s, respectively. These results also suggest that W-HNPs had faster polarization rates than those of the precursor materials of WSNs and W-STCSs. This was attributed to the increased amount of high-dielectric TiO2 and the decreased proportion of the SiO2 material. Also, the λ values of SSNs, S-STCSs, and SHNPs were 3.0 × 10−3, 1.7 × 10−3, and 7.0 × 10−4, respectively. Therefore, it was confirmed that the polarizability and polarization rate of W-HNPs were higher than those of SHNPs. The detailed dielectric characteristics of the WSN series and the SSN series are summarized in Table 1. These results 6576


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Figure 8. (a) Electrorheological activities of WSN- and SSN-based ER fluids as a function of the shear rate (2.7 wt %, 3.0 kV mm−1). (b) Proposed mechanisms for the polarization of W-HNPs and S-HNPs under an applied E field. (c) Yield stresses of various ER fluids measured as a function of the E field strength (2.7 wt %, 0.1 s−1). (d) Real-time on−off test results for various ER fluids.

state when the E field was turned off. A repeated cycle of the on−off test also displayed similar increasing and decreasing yield stresses, which demonstrated the reversibility and reproducibility of the ER fluids. The fibril-like formations in W-HNP- and S-HNP-based ER fluids were analyzed by optical microscopy (OM) at an applied E field of strength 1.0 kV mm−1. Representative OM images of two materials are shown in Figure 9. In the absence of an E

characteristics. In particular, WSNs were synthesized as unique hierarchical structures with highly polarizable surface areas and pore volumes; these characteristics resulted in ER activity that was better than that of the flat-surfaced SSNs. Furthermore, WHNPs manifested a higher ER performance than that of SHNPs because of their hierarchically structured internal surface. The hierarchically arrayed inner surfaces and increased loading amount of TiO2 provided more polarizable sites on W-HNPs than on S-HNPs, which led to stronger interactions between particles and an outstanding ER performance. Figure 8b provides a tentative mechanism for the polarization and particle interactions of W-HNPs and S-HNPs. The ER activities were further investigated by measuring the yield stresses of ER fluids as a function of the E field strength at the fixed shear rate of 0.1 s−1 (Figure 8c). All of the ER fluids manifested increased yield stresses with the increasing E field strength. Yield stresses increased by approximately the square of the E field (τy ∝ E2) strength up to 1.0 kV mm−1 of the E field strength. After passing the 1.0 kV mm−1 region, yield stresses increased by approximately 1.5-power of the E field strength (τy ∝ E1.5). These strongly increasing trends of yield stresses at intervals on either side of 1.0 kV mm−1 were in good agreement with the typical behavior of ER fluids under various E field strengths.9 Stable yield stresses measured up to 4.0 kV mm−1 verified the stability of all of the ER fluids. The reversibility and reproducibility of the ER fluids were assessed by a real-time on−off test (Figure 8d). All of the ER fluids showed zero ER activity (less than 1.0 Pa) in the absence of an applied E field (off-state). With an applied E field (on-state), the ER fluids manifested a sudden increase in the yield stress. All yield stresses instantly returned to the initial zero ER activity

Figure 9. Optical microscopy (OM) images of practical fibril-like structure formation of (a) W-HNP- and (b) S-HNP-based ER fluids under an applied E field of strength 1.0 kV mm−1. 6577



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field, W-HNPs and S-HNPs were well distributed in the silicone oil medium. When an E field was applied, both materials displayed a sudden formation of fibril-like structures, which were responsible for the ER activity. Noticeably, the fibril-like structures of W-HNPs were stronger and more rigid than those of S-HNPs; this was in good agreement with ER behavior. These experimental results clearly illustrate the potential of W-HNPs as practical ER materials, given their unique morphology.

CONCLUSIONS W-HNPs were fabricated by the sequential experimental steps of WSN synthesis, TiO2 coating, and SMER. The resulting WHNPs were prepared as hollow SiO2/TiO2 composite materials with a unique hierarchically arrayed inner surface. Compared to similarly synthesized S-HNPs without such hierarchical internal surfaces, W-HNPs manifested various distinguishing characteristics that included increased surface area, high TiO2 loading, and improved dielectric properties. In a practical ER application, W-HNP-based ER fluids showed ca. 5.1-fold improved ER activity compared with that of starting WSN materials. Furthermore, compared with S-HNP-based ER fluids, W-HNP-based ones had ca. 1.9-fold better ER performance. These practical ER results revealed that the hierarchically arrayed internal structure of W-HNPs positively affected the creation of fibril-like structures and ER activity by providing more polarizable sites and increased loading of a high-dielectric TiO2 material. This study clearly demonstrated the advantages of manipulating the inner surface of nanomaterials and the potential advantage of using uniquely structured nanomaterials as ER fluids. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18895. TEM micrographs of the series of SSN materials, SEM micrograph of the highly magnified internal surface of WHNPs, SEM micrographs of the series of WSN materials, SEM micrographs of the series of SSN materials, various physical parameters of the series of WSN and SSN materials, and elemental compositions of the series of WSN and SSN materials (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 82-2-880-7069. Fax: 82-2888-7295. ORCID

Jyongsik Jang: 0000-0002-0415-802X Author Contributions †

C.-M.Y. and J.R. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Development Fund of Seoul National University funded by Dongjin Semichem Co., South Korea (0668-20150016). 6578


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