Titania Hollow Nanoparticles and

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Synthesis of Hierarchical Silica/Titania Hollow Nanoparticle and Its Enhanced Electro-Responsive Activity Chang-Min Yoon, Jaehoon Ryu, Juyoung Yun, Yun Ki Kim, and Jyongsik Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18895 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

Synthesis of Hierarchical Silica/Titania Hollow Nanoparticle and Its Enhanced Electro-Responsive 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, Gwanakgu, Seoul 151-742, Korea

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) *

‡

Tel.: 82-2-880-7069; Fax: 82-2-888-7295; e-mail: [email protected] These authors equally contributed to this work.

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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 the W-HNPs was attained by using WSNs as a core template. Compared with SiO2 sphere-templated hollow SiO2/TiO2 nanoparticles (S-HNPs) with flat inner surfaces, the W-HNPs displayed distinctive surface areas, TiO2 loading amounts, and dielectric properties arising from the hierarchical internal surface. The unique properties of the W-HNPs were further investigated as an electrorheological (ER) material. The W-HNP-based ER fluids exhibited a ca. 1.9-fold enhancement in ER efficiency compared with S-HNP-based ER fluids. Such enhancement was attributed to the unique inner surface of the W-HNPs, which effectively enhanced the polarizability by increasing the number of charge accumulation sites, and according to the presence of the high-dielectric TiO2. This study demonstrated the advantages of hollow nanomaterials having uniquely arrayed internal spaces, including in terms of practical ER applications.

KEYWORDS Hierarchical, Wrinkled, Silica, Titania, Hollow, Electro-response, 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 One 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 solid-like states in the presence of an E field.5 Notably, ER materials can form fibril-like structures perpendicular to electrodes, and the resulting changes in the rheological properties are usually characterized by 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, dye-sensitized 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 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 the SiO2 particles.17,18 The various SiO2 materials reported to date include mesoporous, one-dimensional, core-shell, multi-shelled, 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 non-porous 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 uniform size because of nucleation and growth mechanisms that are more ACS Paragon Plus Environment

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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 uniform particle 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 large-scale production, controllability of size and structure, and facile modification of the material properties.23–25 These positive 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 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 improves the dielectric properties and dispersion stability of the final products, which consequently have better ER efficiency than 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 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 ACS Paragon Plus Environment

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etching and re-deposition (SMER) using ammonium hydroxide as the etchant solution. The resulting WHNPs were compared with SiO2 sphere-templated hollow SiO2/TiO2 nanoparticles (S-HNPs), synthesized by the same experimental method except using a flat-surfaced spherical type of SiO2 as the core template material. Notably, the W-HNPs exhibited increased porosity, surface area, TiO2 loading, and dielectric behavior compared with S-HNPs because of the unique internal surface of the W-HNPs, which resulted from the packing of TiO2 on the hierarchically-arrayed surface of the WSN core template. Furthermore, we investigated the electro-response of W-HNPs in a practical ER fluid. The W-HNPbased ER fluid had remarkable 5.1- and 1.9-fold enhanced ER activities compared with those of nonTiO2-coated 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%),

iso-propanol

(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 process. Fabrication of hierarchical wrinkled silica nanoparticles (WSN). 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) was dissolved into DI water (60 mL) by stirring. And then, a mixture of cyclohexane (30 mL) and iso-propanol (1.84 mL, 24 mmol) were added to the above 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 temperuture was raised to 70 °C and reaction was continuously proceeded for 1h. Thereafter, the reactor was suddenly transferred to the water bath for cooling process (10 °C) for 2 h. Subsequent to cooling process, the reaction was resumed at 70 °C with vigorous stirring, and this state was maintained for 12 h. Finally, WSNn were collected by centrifugation and washed several times with ethanol to remove the residues. The isolated WSNs were dried in the 90 °C condition for overnight. Finally, the dried WSNs were calcined at 550 °C for six hours in air. In addition, the scale of WSN synthesis process could be controlled by scaling down or scaling up the amounts of all reagents in ranges of 0.5 to 2.0 times. The relative yield percentage of WSN to TEOS precursor in mass ratio was about ca. 28.0 %. On the other hand, SSN with 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) was mixed by magnetic stirring for 10 min. Subsequently, TEOS (5.8 mL) was injected into the mixture, and reaction was processed for 12 h at room temperature. The resulting SSNs were collected by centrifugation and washed with ethanol for three times, and dried in oven (90 °C) for overnight. ACS Paragon Plus Environment

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Synthesis of WSN-SiO2/TiO2 hollow nanoparticles (W-HNP). 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). And then, acetonitrile (28 mL) was added to the colloidal WSN solution with vigorous stirring at 4 °C. Other solution containing ethanol (36 mL), acetonitrile (12 mL), and TTIP (4.6 mL) was prepared and added to the WSN solution by dropwise addition. The resulting mixture was vigorously stirred for 12 h to result in white cloudy solution containing TiO2 shell-coated W-STCS. The fabricated W-STCSs were collected by centrifugation at 12,000 rpm for 20 min. The hollow structured W-HNP was obtained by following sonication-mediated etching and re-deposition (SMER) method. Dried W-STCS (0.5 g) was well-dispersed in DI water (15 mL) by stirring and sonication. Etchant solution of ammonium hydroxide (0.1 M, 5.0 mL) was added to the W-STCS solution and 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. The W-HNPs was collected by centrifugation and dried in oven (90 °C) for overnight. The S-STCS and SHNPs were prepared by same experimental procedure as preparation of W-STCS and W-HNP, but except added amount of TTIP was lessened to 3.5 mL. Characterization. The morphological characteristics of SSN, S-STCS, 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) analysis. Brunauer−Emmett−Teller (BET) surface area and pore volume of materials were attained by N2-sorption analysis (ASAP 2000, Micromeritics Co.) and related calculation. Elemental compositions of materials were analyzed by scanning transmission electron microscope (JEM-2100F, JEOL). Dispersion stability of materials dispersed in silicone oil was determined by calculating the sedimentation ratio (R). Dielectric characteristics of materials were determined by impedance spectroscope (Solatron-1260) installed with interface analyzer (Solatron-1296). Investigation of electrorheological (ER) properties. All ER fluids were formulated according to the following procedures. Firstly, particles were dried in oven (90 °C) for overnight to completely remove ACS Paragon Plus Environment

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the moistures. 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 stirred using a magnetic stirrer for 24 h. Also, no additive other than particles and silicone oil were added to the ER fluids. The ER activities of ER fluids were examined by 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 height: 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, well-dispersed ER fluids were added into the cup and geometry inserted. Prior to applying the E field, pre-shear of 10.0 s–1 was applied for 10 min to attain an equilibrium state of fluid. Finally, intended strength of E field and appropriate testing mode was selected to investigate ER properties of ER fluids.


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RESULTS AND DISCUSSION Preparation of WSN-SiO2/TiO2 hollow nanoparticles (W-HNP). The preparation process of the W-HNPs is schematically illustrated in Figure 1. First, uniform core WSNs were prepared based on the Winsor III system microemulsion method. In the preparation of WSN, the bicontinuous phase played a key role for formation of hierarchical and wrinkle structure. The bicontinuous phase is consisted of ternary systems of surfactant and equivalent amounts of water and oil.20 With the vigorous stirring on Winsor III systems, oil-in-water (o/w) types of macroemulsions systems were formed. And then, TEOS was injected into this emulsion system. At the beginning of the reaction, TEOS was dissolved in oillayer of a bicontinuous phase. As soon as TEOS contacts the water layer of bicontinuous phase, a continuous condensation and hydrolysis of TEOS were occurred to result in the formation of closed and spherical structured surfactant-silicate system.22 These surfactant-silicate systems were able to demulsify and gathered into repetitive mesophases, which serve as the seeds of WSN.33 Continuously, newly formed seeds of WSN grew with the chain reaction of hydrolysis and condensation of TEOS through the water layer. Eventually, wrinkle and hierarchical shape of WSN was able to form during the particle growth owing to the mixed distribution shape of water and oil within bicontinous phase. In the next place, a porous TiO2 shell was then coated onto the surface of the WSNs by a sol-gel method, using TTIP as a precursor to obtain the W-STCSs. Finally, the core WSNs were etched by sonication using NH4OH solution as the etchant to obtain W-HNPs. During the etching process, the basicity of the NH4OH and 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]6–2). And then, these re-coordinated silicic ions were diffuse into the TiO2 shell layer as well as the hollow cavity. Sequentially, small silicic fragments experience the condensation and resulting SiO2 materials were re-deposited within TiO2 materials to yield the 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 due to the chemical inertness of TiO2 compared with SiO2.35 For comparison, S-HNPs were synthesized by a similar experimental method, ACS Paragon Plus Environment

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except that the core SSNs were fabricated by the Stöber method using flat-surfaced SiO2 as the core template.

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

The morphologies of the 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 the 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 ends. The final W-HNPs were monodispersed with a size of ca. ACS Paragon Plus Environment

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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 the W-HNPs were hollow, with hierarchicallyarrayed inner surfaces. Also, the 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 in the inner surface between W-HNPs and S-HNPs was attributed to the dissimilarity between the core WSN and SSN templates. For the W-HNPs, the TTIP precursor could permeate into the hierarchical pores of the WSNs and thereby provide the uniquely-packed internal surface, but the smooth inner surface of the S-HNPs developed from the non-porous surface of the SSNs. Focused ion beam (FIB) milling was used to prepare samples of the W-HNPs for examination by scanning electron microscopy (SEM) for closer examination of internal surface (Figure S2). It was clearly observed that W-HNP showed a bumpy, uneven, and spindly internal surface due to the formation of the hierarchical array. Furthermore, SEM observations confirmed uniform particle sizes of the WSNs, W-STCSs, W-HNPs, and series of SSN materials (Figures S3 and S4).


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

Under our experimental conditions, the particle size of the core WSNs and amount of added TTIP were key to attaining the hierarchical internal hollow surface of the W-HNPs. The WSN core size must be at least ca. 100 nm to obtain W-HNPs 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 the W-HNPs due to the lower porosity of the core WSNs, which hindered formation of the hierarchical array of TiO2 shells. Moreover, the amount of added TTIP was controlled under *0.5, *0.75, *1, and *2 conditions (where *1 denotes the standard TTIP amount corresponding to 4.32-times the ACS Paragon Plus Environment

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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 the WSNs, but the resulting W-HNPs displayed a deformed structure and ruptured shells due to 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 due to 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 amount of TTIP precursor (4.32-times the WSN mass) to successfully synthesize the W-HNPs.


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

To gain further insight into the structure of the materials, the N2-sorption 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 the ACS Paragon Plus Environment

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WSNs and derived materials changed with the sequential experimental steps of TiO2 coating and SMER processing. The WSNs had a surface area, pore volume, and major pore size of 533.4 m 2 g–1, 1.77 cm3 g–1, and ca. 12.5 nm, respectively. The high surface area and porosity of the 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 resulted in the decreased surface area and porosity of the W-STCS 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 the W-HNPs, which was higher than those of previous W-STCS materials, was attributed to etching of the core WSN and 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 the W-HNPs. The different sizes of the pores corresponded to the morphological characteristics of the W-HNPs 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 the WHNPs; and 80-nm-diameter pores: porosity arising from the internal hollow space. In case of the series of SSN materials, surface area and porosity increased with the sequential experimental steps. Detailed physical characteristics of the WSNs, W-STCSs, W-HNPs, and the series of SSN materials are listed in Table S1.


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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) W-HNPs.

The elemental compositions of the WSNs, W-STCSs, W-HNPs, and series of SSN materials were determined by energy-dispersive X-ray spectroscopy (EDS) (Table S2). The WSNs and SSNs were mainly composed of Si and O, and a slight amount of C was detected due to organic residues. The WSTCSs and S-STCSs showed additional Ti that originated from the outer TiO2 shell. In the case of the ACS Paragon Plus Environment

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W-HNPs and S-HNPs, the lower amount of Si feature compared with the precursor materials was attributed to the etching of the core SiO2. Notably, the W-STCSs and W-HNPs contained more Ti than the S-STCSs and S-HNPs. These phenomena were attributed to the high surface area of the WSN core template compared with 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 the W-HNPs (Figure 5). In the final W-HNPs, Ti, Si, and O were well-distributed over the entire region, which indicated successful formation of the SiO2 and TiO2 composite materials via the SMER process. These experimental results confirmed that W-HNPs were successfully synthesized with the following features: hollow SiO2/TiO2 composite nanoparticles having a hierarchically arrayed internal surface.

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


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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 enhanced ER performance due to improved particle mobility in the dispersing medium.38 In this study, the dispersion stabilities of the WSNs, W-STCSs, and W-HNPs were examined by determining their sedimentation 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. The 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 TiO 2 coating process, the dispersion stability of the W-STCS decreased slightly to 0.77, due to the addition of the relatively heavy TiO2 shell onto the hierarchical WSN morphology, which resulted in decreased surface area and pore volume. After the etching process, the dispersion stability of the final W-HNPs 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 W-HNPs 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, the SSNs showed a dispersion stability of 0.79, which was lower than that of the WSN dispersions. The higher surface area and pore volume of the WSNs resulted in a better dispersion stability than that of the non-porous SSNs. The dispersion stability of S-STCSs was lower at 0.74, which was the same trend as observed for the W-STCSs. However, the S-HNP dispersion stability increased to 0.85, which was slightly higher than that of the W-HNPs. The differences in dispersion stability trends between the final hollow W-HNPs and S-HNPs stemmed from the different internal structures of the materials. As discussed previously, the WSN surface had a hierarchical structure with a ACS Paragon Plus Environment

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high surface area, enabling more TiO2 to be loaded onto the surface compared with a flat-surfaced SSN. Furthermore, the S-HNPs lost a large amount of their core SiO2, leaving only TiO2 shells behind after etching, while the W-HNPs remained with their TiO2 shells plus a hierarchically arrayed TiO2 inner surface. Therefore, the dispersion stability of the S-HNPs was higher than that of the W-HNPs because of the relatively low TiO2 loading. The dispersion stabilities of the W-HNPs and S-HNPs were thus both effectively improved by the removal of the core SiO2, but different amounts of 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.

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


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To gain insight into the dielectric properties, the dielectric constant (ε') and loss factor (ε'') of the WSNs, SSNs, and their derived materials were examined as a function of 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 high dielectric material, or decreasing the lowdielectric 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 (△ε, 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 △ε and short λ exhibit better ER performances under an applied E field.6,9 Specifically, △ε can be determined by reference to the differences between fictitious (ε0, f → 0) and static (ε∞, f → ∞) dielectric constants, thus providing information on the polarizability of materials. The determined △ε for the WSNs, W-STCSs, and W-HNPs was 1.12, 2.11, and 2.42, respectively. By introducing the high-dielectric TiO2 shell, the W-STCSs manifested enhanced polarizability compared with that of WSNs constituted only of SiO2. The W-HNPs had higher △ε due to 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 △ε for SSNs, SSTCSs, and S-HNPs was 0.70, 1.35, and 1.81, respectively. Noticeably, the 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 the W-HNPs. The λ of the materials were also estimated using the following equation:44 𝜆=

1 2𝜋𝑓max

where fmax is obtained from the maximum E field frequency of the ε'' curve. The calculated λ of the WSNs, W-STCSs, and W-HNPs was 2.0 × 10–3, 6.0 × 10–4, and 1.0 × 10–4 s, respectively. These results also suggest that the W-HNPs had faster polarization rates than the precursor materials of WSNs and WSTCSs. This was attributed to the increased amount of high-dielectric TiO2 and decreased proportion of SiO2 material. Also, the λ of the SSNs, S-STCSs, and S-HNPs was 3.0 × 10–3, 1.7 × 10–3, and 7.0 × 10–4, ACS Paragon Plus Environment

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respectively. Therefore, it was confirmed that the polarizability and polarization rate of the W-HNPs were higher than those of the S-HNPs. The detailed dielectric characteristics of the WSN series and SSN series are summarized in Table 1. These results confirm the unique hierarchical morphology of the 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.


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

Table 1. Dielectric properties of series of WSN- and SSN-based ER fluidsa Sample ε0 ε∞ Δε =(ε0–ε∞) fmaxb (Hz) λ c (s) SSN 3.55 2.85 0.70 48.4 0.0033 S-STCS 4.36 3.01 1.35 98.0 0.0016 S-HNP 4.89 3.08 1.81 238.9 0.00066 WSN 3.95 2.83 1.12 88.3 0.0018 W-STCS 5.23 3.12 2.11 249.1 0.00063 W-HNP 5.75 3.40 2.35 998.2 0.00015 a Dielectric properties of ER fluids were determined by impedance analyzer (Solatron 1260) and dielectric interface analyzer (Solatron 1296). b

The local frequency of the peak from the dielectric loss factor ε" and the fmax values were determined

by the non-linear regression plot using OriginPro. c

The relaxation time was calculated by λ=1/(2πfmax) relation (fmax was denoted by the maximum

frequency of the ε" graph).

Investigation of electrorheological (ER) activity of a series of WSN materials. The ER activities of sequentially prepared WSNs, W-STCSs, W-HNPs, and a series of SSN-based ER fluids were investigated by controlling various parameters including shear rate, E field strength, and E field on-off condition. Shear stress curves (i.e., flow curves) of the ER fluids were examined as a function of shear ACS Paragon Plus Environment

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rate under an applied E field strength of 3.0 kV mm–1 (Figure 8). With the applied E field strength, all ER fluids exhibited shear stresses due to the formation of fibril-like structures induced by the electrostatic forces between particles. In the low shear rate region (prior to the critical shear rate, τc), the shear stresses of all of the ER fluids displayed Bingham plastic-like behavior marked by plateau curves.45 Such behavior was attributed to balancing between the formation and deformation of fibril-like structures due to competing electrostatic and hydrodynamic forces. After passing the τc, the shear stresses showed Newtonian fluid-like behavior, i.e., proportionally increasing with increasing shear rate, indicating the domination of hydrodynamic forces over electrostatic forces. 46 The ER activities increased in the order of WSNs < W-STCSs < W-HNPs, which was in accordance with the sequential preparation steps. The highest ER performance of WSN-, W-STCS-, and W-HNP-based ER fluids were determined as ca. 8.2, 27.3, and 40.9 Pa, respectively. Notably, the W-STCSs had better ER performance than the WSNs due to 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 performance 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 the SSN-, S-STCS-, and S-HNP-based ER fluids. The improved dielectric properties and dispersion stability of the S-HNPs resulted in better ER performance than that of the precursor SSN and S-STCS materials. Notably, the WSN-derived materials exhibited a greater improvement in ER activity than the 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 characteristics. In particular, the 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, W-HNPs manifested higher ER performance than S-HNPs because of their hierarchically structured internal surface. The hierarchically arrayed inner surfaces and increased loading amount of TiO2 provided more polarizable sites on the W-HNPs than on the S-HNPs, which led to stronger ACS Paragon Plus Environment

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interactions between particles and 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 the ER fluids as a function of 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 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 E field strength. After passing the 1.0 KV mm–1 region, yield stresses increased at approximately the 1.5-power of 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 strength (off-state). With an applied E field (on-state), the ER fluids manifested a sudden increase in yield stress. All yield stresses instantly returned to the initial zero ER activity 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.


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Figure 8. a) Electrorheological activities of WSN- and SSN-based ER fluids as a function of 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 E field strength (2.7 wt%, 0.1 s–1). d) Real-time on-off test results for various 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 strength of 1.0 kV mm–1. Representative OM images of two materials are shown in Figure 9. In the absence of an E field, the 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 the W-HNPs were stronger and more rigid than those of the S-HNPs; this was in good


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agreement with the ER behaviors. These experimental results clearly illustrate the potential of W-HNPs as practical ER materials, given their unique morphology.

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


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CONCLUSION W-HNPs were fabricated by the sequential experimental steps of WSN synthesis, TiO2 coating, and SMER. The resulting W-HNPs 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 a ca. 5.1-fold improved ER activity compared with the starting WSN materials. Furthermore, compared with S-HNP-based ER fluids, the W-HNP-based ones had ca. 1.9-fold better ER performance. These practical ER results revealed that the hierarchically arrayed internal structure of the W-HNPs positively affected the creation of fibril-like structures and ER activity by providing more polarizable sites and increased loading of 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.


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ACKNOWLEDGEMENT This work was supported by the Development Fund of Seoul National University funded by Dongjin Semichem Co., South Korea (0668-20150016). Supporting Information Available: The contents of Supporting Information may include the following: (1) TEM micrographs of the series of SSN materials, (2) SEM micrograph of high-magnified internal surface of W-HNPs, (3) SEM micrographs of the series of WSN materials, (4) SEM micrographs of the series of SSN materials, (5) Various physical parameters of the series of WSN and SSN materials, (6) Elemental compositions of the series of WSN and SSN materials, This material is available free of charge via Internet at http://pubs.acs.org.


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SYNOPSIS TOC


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Titania Hollow Nanoparticles and

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