Facile Fabrication of Raspberry-like Composite Nanoparticles and

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J. Phys. Chem. C 2009, 113, 9063–9070

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Facile Fabrication of Raspberry-like Composite Nanoparticles and Their Application as Building Blocks for Constructing Superhydrophilic Coatings Xin Du,†,‡ Xiangmei Liu,†,‡ Hongmin Chen,†,‡ and Junhui He*,† Functional Nanomaterials Laboratory and Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: February 23, 2009; ReVised Manuscript ReceiVed: April 7, 2009

Monodisperse raspberry-like polystyrene@silica composite nanoparticles (RCNs) were readily prepared by a single-step sol-gel method, using oxygen plasma-treated monodisperse polystyrene (PS) spheres as cores. The fabricated composite particles have a well-defined structure and a surface morphology of dual-size roughness. The diameter of silica nanoparticles and their coverage degree on the surface of PS spheres could be easily tailored by adjusting the tetraethoxysilane concentration. Subsequently, porous silica hollow sphere coatings were fabricated by depositing the RCNs on polyelectrolyte-modified glass substrates via layer-bylayer assembly followed by calcination. Such porous silica hollow sphere coatings present unique superhydrophilic and antifogging properties even though the coverage degree of porous silica hollow spheres on substrates is only ca. 50%. Therefore, the obtained raspberry-like composite spheres are ideal building blocks for constructing superhydrophilic and superhydrophobic coatings. Introduction Raspberry-like organic@inorganic composite particles with hierarchical surface morphologies and well-defined structures have received increasing attention over past few years due to their high surface roughness and potential applications. For example, well-defined composite particles are promising as building blocks in constructing superhydrophobic films that mimic the surface topology of self-cleaning lotus leaves,1–3 and in fabricating superhydrophilic coatings with hierarchically porous structure and/or rough surface.4–7 They are also very important for fundamental research in colloid and interface science because these composite particles are often used as model particles to study phase behavior, rheology, and diffusion. Methods previously used for fabricating raspberry-like composite particles include the assembly of small particles on large particles by chemical bonding8 or via layer-by-layer assembly (LbL),6,7,9,10 pickering emulsion polymerization using Janus solid particles to stabilize the emulsion,11–14 and the sol-gel method on the surface of large spheres.15–17 Among them, the assembly method generally needs surface modification and multistep assembly by using polyelectrolytes as binders. The pickering emulsion method needs special amphiphilic Janus particles, and the fabricated nanometer-scale raspberry-like composite spheres usually have strong adhesion. The sol-gel method cannot fabricate well-defined monodisperse structures. Therefore, it is still a challenge to fabricate both readily and effectively raspberry-like composite particles of well-defined sizes and morphologies. Among organic@inorganic composite particles, polymer@silica composite spheres are especially interesting, from which silica hollow spheres can be subsequently fabricated. Recently, Yang and co-workers, by smart design, successfully fabricated mono* To whom correspondence should be addressed. Phone/fax: +86 10 82543535. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.

disperse conductive polyaniline@silica composite capsules and silica hollow spheres using sulfonated PS spheres.18 Very recently, Kim and co-workers reported on the preparation of hollow silica and titania microspheres using plasma-treated PS spheres as sacrificial templates.19 While the resulting silica shell of PS@SiO2 composite particles consisted of a large number of small silica nanoparticles of a broad size distribution, the titania shell of PS@TiO2 composite spheres was rather smooth. In the present work, we have succeeded in ready fabrication of monodisperse raspberry-like PS@SiO2 composite nanoparticles (RCNs) by a single-step sol-gel process using oxygen plasma-treated PS spheres as cores. The diameter of silica nanoparticles and their coverage degree on the surface of PS spheres could be easily tailored by adjusting the tetraethoxysilane (TEOS) concentration. To the best of our knowledge, there have been few reports on the fabrication of RCNs of welldefined structures by a single-step method. Subsequently, porous silica hollow sphere coatings were fabricated by depositing the RCNs on polyelectrolyte-modified glass substrates via LbL assembly followed by calcination.6,7,20–24 Surface properties of the coatings were investigated by measuring their water contact angles (WCAs) and antifogging properties. The results showed that such porous silica hollow sphere coatings have unique superhydrophilic and antifogging properties. Experimental Section Materials. Aqueous ammonia (25-28%), concentrated sulfuric acid (98%), hydrogen peroxide (30%), and absolute ethanol were purchased from Beijing Fine Chemicals. TEOS (99+%) and sodium poly(4-styrenesulfonate) (PSS, Mw ) 70 000) were obtained from Alfa Aesar. Poly(diallyldimethylammonium chloride) (PDDA, Mw ) 200 000-350 000, 20 wt %) were purchased from Aldrich. All chemicals were analytic grade, and were used without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ · cm was used in all experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic).

10.1021/jp9016344 CCC: $40.75  2009 American Chemical Society Published on Web 04/24/2009

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SCHEME 1: Schematic Illustration of the Fabrication of Monodisperse Raspberry-like PS@SiO2 Composite Particles with Oxygen Plasma Treated PS Spheres As Core and of Hierarchically Structured Porous Coating of Silica Hollow Spheres

Fabrication of RCNs by a Single-Step Sol-Gel Process. Polyvinylpyrrolidone (PVP)-functionalized monodisperse PS spheres of varied diameters were synthesized by PVP-mediated emulsifier-free emulsion polymerization (for synthesis details, see the Supporting Information).25,32 We chose those 270 and 615 nm in diameter as cores to fabricate RCNs. The PS spheres were first treated by oxygen plasma, giving hydroxyl groups on their surface. In a typical process,19,26,27 dry PS samples were subjected to grinding, and then transferred into the chamber of a plasma cleaner (PDC-M, Chengdu Mingheng Science and Technology Co., China). Oxygen was admitted into the chamber via a flow controller, and the flow rate was maintained at 800 mL/min. After the chamber was evacuated to low-pressure residual air (∆P ) -0.1 MPa), the PS samples were subject to oxygen plasma treatment at 60 W (600 V × 0.1 A) for 5 min. The treatment was repeated three times to introduce hydroxyl groups homogeneously on the surface of PS spheres. Subsequently, the oxygen plasma-treated PS spheres (0.1 g) were transferred to a 150 mL conical flask containing 100 mL of absolute ethanol. After sonication (120 W) for 30 min to fully separate and disperse the oxygen plasma-treated PS spheres in ethanol, 5 mL of aqueous ammonia was added to the suspension under gentle stirring. After 30 min of stirring, an appropriate volume of TEOS was added to the mixture under vigorous stirring. After the mixture was stirred at room temperature for 3 h, a white precipitation was obtained and purified twice by centrifugation, ethanol washing, and redispersion. Finally, it was dried at 60 °C overnight. Fabrication of Porous Silica Hollow Sphere Coatings on Slide Glasses by Using RCNs as Building Blocks via LbL Assembly. The fabrication procedure of porous silica hollow sphere coatings on slide glasses is illustrated in Scheme 1. First, the as-prepared undried RCNs were redispersed in ultrapure water by sonication (100 W) for 30 min. Second, the RCNs were assembled onto glass substrates. Briefly, glass or silicon substrates were cleaned with Piranha solution (98 wt % H2SO4/ 30 wt % H2O2, 7/3, v/v) and then washed with pure water. Sequential adsorption of polyelectrolyte mutlilayers and RCNs was performed on either slide glasses or silicon wafers by dip coating followed by rinsing with water. The dipping time for both polyelectolytes and RCNs was 5 min. The concentrations of PDDA and PSS aqueous solutions were 2 mg · mL-1. After a substrate was primed with (PDDA/PSS)5/PDDA multilayers, it was dipped into the RCNs dispersion, and then rinsed with pure water. The as-prepared coating was blown dry with air and calcined (heating rate: 1 deg · min-1) at 550 °C for 3 h to remove both the PS core and polyelectrolytes. Characterization. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-4300 field emission

scanning electron microscope operated at 10 kV. All the samples were dispersed in ethanol by sonication for 10 min. They were then dropped onto the surface of a silicon wafer and dried at 60 °C overnight. The specimens were coated with a layer of gold by ion sputtering before SEM observations. The size distributions of partial products were measured by Malvern Zetasizer 3000HS. Surface hydroxyl groups of PS spheres before and after oxygen plasma treatment were examined by Fourier transform infrared (FTIR) spectrometry on an Excalibur 3100 FTIR spectrometer. WCAs of surfaces were measured at ambient temperature on a JC2000C contact angle/interface system (Shanghai Zhongchen Digital Technique Apparatus Co., China). Water droplets of ca. 3.0 µL in volume were dropped carefully onto the sample surfaces. Once a water droplet contacted the film surface, the machine began to take photographs at a speed of 20 frames · .s-1, and the interval between the contact moment and the first image was 50 ms. Measurements were carried out at three different locations on the sample surface. For the examination of the antifogging property, a control slide glass and a slide glass coated with porous silica hollow spheres were cooled at ca. -18 °C in a refrigerator overnight and exposed to hot water vapor above a beaker containing boiling water. Photos were taken immediately after the exposure. Results and Discussion Fabrication of RCNs by a Single-Step Sol-Gel Process. Figure 1 shows SEM images of PS spheres with mean diameters of ca. 615 nm (Figure 1a) and ca. 270 nm (Figure 1b), respectively. They are both spherical, uniform in size, and have smooth surfaces. After treatment by oxygen plasma (60 W) for 15 min, neither the size nor the morphology of PS spheres had significant changes (Figure S2a, Supporting Information). When the time of oxygen plasma treatment was extended to 50 min, however, PS spheres became less spherical (Figure S2b, Supporting Information). Clearly, extended oxygen plasma treatment caused the inhomogeneous etching of PS spheres. It is well-known that oxygen plasma can react with polymer surfaces, producing a variety of surface oxygen-contained functional groups, such as CsO, CdO, OsCdO, and CO3 at the polymer surface.26,27 In the oxygen plasma treatment, two processes occur simultaneously. One is the etching of polymer surface through the reaction of atomic oxygen with surface carbon atom, giving volatile reaction products (e.g., CO2). The other is the formation of oxygen-contained functional groups at polymer surfaces through the interaction between active species in oxygen plasma and surface atoms.26 FTIR analyses of PS spheres before and after oxygen plasma treatment (Figure

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Figure 1. SEM images of monodisperse PS spheres of ca. 615 nm (a) and ca. 270 nm (b) in diameter, respectively.

S3, Supporting Information) showed a large increase in the intensity of broadband at ca. 3442 cm-1 (O-H stretching vibration), indicating that hydroxyl groups had been successfully generated on the surface of PS spheres by oxygen plasma treatment.19,26,27 The sharp peak at ca. 1680 cm-1 has a minor decrease after oxygen plasma treatment. Before oxygen plasma treatment, this peak is attributed to the CdO stretching vibration of the PVP macromolecules located at the surface of PS spheres. The peak decrease indicates the decrease of CdO content at the surface of PS spheres. Thus, the active atomic oxygen in oxygen plasma might paritally destroy the CdO group of the pyrrolidone ring. Clearly, this destructing reaction was dominant, although some CdO contained functional groups might also be produced by oxygen plasma treatment. PS spheres ca. 615 nm in size were first used as core to fabricate RCNs in the single-step sol-gel process. Figure 2 shows SEM images of RCNs obtained at varied TEOS concentrations. When the TEOS concentration is 0.35 vol %, SiO2 nanoparticles of ca. 50 nm in size appear on the surface of PS spheres. The distribution and coverage degree of SiO2 nanoparticles is inhomogeneous and low on the surface of PS spheres (Figure 2a,b). When the TEOS concentration increases to 0.5 vol %, the size of RCNs becomes ca. 650 nm (twice the distance from the center of the sphere to the outermost edge) and the diameter of SiO2 nanoparticles on the surface of PS spheres (SNs@PS) remains ca. 50 nm. Although the diameter increase from PS sphere to RCN is only 35 nm, the coverage degree and distribution homogeneity increase significantly (Figure 2c,d). The particle sizes of PS spheres and RCNs were also measured by dynamic light scattering (DLS), and the results are shown in Figure S3 in the Supporting Information. They all have a narrow size distribution with a Z average mean of ca. 596.7 nm and ca. 618.1 nm, respectively. The diameter increase from PS sphere to RCN is 21.4 nm. It is noted that the average hydrodynamic sizes of PS spheres and RCNs measured by DLS are smaller than those estimated by SEM. The even smaller hydrodynamic diameter of RCNs may be attributed to their protruding surface morphology.28,29 When the TEOS concentration further increases to 0.8 vol %, the diameter of SNs@PS increases to ca. 100 nm, and their surface distribution remains homogeneous (Figure 2e,f). We also observed the morphologies of products obtained using untreated PS spheres as core under otherwise identical conditions. When the TEOS concentration is 0.35 vol %, the diameter of SNs@PS is ca. 200 nm in size, and their surface distribution is inhomogeneous (Figure 3a). When the TEOS concentration increases to 0.7 vol %, the diameter of SNs@PS only increases slightly, and their surface distribution remains

inhomogeneous (Figure 3b). Thus, oxygen plasma treatment results in better control over RCNs morphologies. Oxygen plasma treated PS spheres ca. 270 nm in size were also used as core to fabricate RCNs. When the TEOS concentration is 0.2 vol %, the diameter of SNs@PS is ca. 20 nm, and they do not fully cover the surface of PS spheres (Figure 4a,b). When the TEOS concentration increases to 0.35 vol %, the diameter of SNs@PS does not change significantly (Figure 4c,d). However, SiO2 nanoparticles become more homogenously distributed on the surface of PS spheres, and nearly reach the full surface coverage. When the TEOS concentration increases to 0.5 vol %, the diameter of SNs@PS increases to ca. 50 nm, keeping their homogeneous distribution on the surface of PS spheres. However, some SiO2 nanopartilces also appear on the substrate (Figure 4e,f). The above results show that the fabricated composite spheres are uniform in size and do not have strong adhesion among them, no matter if oxygen plasma treated PS spheres of either 615 or 270 nm are used as core. When the TEOS concentration increases between 0.35 and 0.5 vol % for 615 nm PS spheres and between 0.2 and 0.35 vol % for 270 nm PS spheres, the diameter of SNs@PS does not change significantly, but the coverage degree of SNs@PS increases and reaches saturation. When the TEOS concentration further increases, SNs@PS grow larger in size. The as-obtained composite particles present a novel raspberry-like protruding surface morphology when the TEOS concentration is between 0.5 and 0.8 vol % for 615 nm PS spheres and between 0.35 and 0.5 vol % for 270 nm PS spheres. It consists of a submicrometer-level structure of PS spheres, and a finer structure at the nanometer level on the submicrometer-level structure. The diameter and coverage degree of SNs@PS could be tailored simply by increasing the concentration of TEOS. Apparently, the current results provide a general approach to fabricate RCNs of varied sizes by using oxygen plasma treated PS spheres of corresponding sizes as cores. The morphology of the dual-size rough surface makes the fabricated RCNs ideal building blocks for the fabrication of superhydrophobic coatings.8 The morphology of RCNs after calcination at 550 °C was observed. When a PS spheres of ca. 615 nm were used as core, amorphous fragments and particles were produced (not shown). When PS spheres of ca. 270 nm were used as core and the TEOS concentration was 0.35 vol %, the calcination process ruptured the silica shell, and a macroporous material was obtained (Figure 5a). The pore diameters were ca. 200 nm, and apparent shrinkage occurred during the PS removal. When the TEOS concentration increased to 0.5 vol %, silica hollow spheres were obtained (Figure 5b). Surprisingly, little size shrinkage of RCNs occurred.

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Figure 2. SEM images of RCNs obtained with different volumes of TEOS: 0.35 (a, b), 0.5 (c, d), and 0.8 mL (e, f), respectively. Oxygen plasma treated PS spheres of ca. 615 nm were employed as core.

Figure 3. SEM images of RCNs obtained with different volumes of TEOS: 0.35 (a) and 0.7 mL (b), respectively. Untreated (i.e., as-prepared) PS spheres of ca. 615 nm were employed as core.

This might be attributed to the combination of high SNs@PS coverage degree with slow heating rate. The inset in Figure 5b shows the morphology of one silica hollow sphere. Clearly, the silica hollow spheres have rough surface and ca. 30 nm pores

in the shell. This special morphology resulted from the raspberry-like structure of RCNs. It would be very suitable for constructing superhydrophilic coatings,7 and was used below as building blocks for fabrication of superhydrophilic coatings.

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Figure 4. SEM images of RCNs obtained with different volumes of TEOS: 0.2 (a, b), 0.35 (c, d), and 0.5 mL (e, f), respectively. Oxygen plasma treated PS spheres of ca. 270 nm were employed as core.

Figure 5. SEM images of calcined RCNs obtained with varied volumes of TEOS: 0.35 (a) and 0.5 mL (b), respectively. Oxygen plasma treated PS spheres of ca. 270 nm were used as core. The inset in panel b is a magnified SEM image (scale bar: 100 nm).

On the basis of the above results, the formation of RCNs with protruding surface morphologies may result from combined effects of the nucleation and growth processes of [email protected]

Factors that affect these processes include the pH value of medium, the temperature of reaction, the concentration of precursor, the surface charge and surface functional groups on

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SCHEME 2: Schematic Illustration of the Fabrication of PS@SiO2 Composite Particles with Different Kinds of Morphology with Different PS Spheres As Core

polymer beads, the deposition time, and so on.16,17,30–36 Much work has been done recently to investigate these factors. Some related cases are summarized in Scheme 2 together with the current results. When nonmodified PS spheres with a negatively charged surface were used as core, small silica beads were deposited on the surface of PS spheres inhomogeneously by random collision, and a large number of free silica particles also appeared in the product.16,35 When the surface of PS spheres was positively charged or functionalized with silanol groups (Si-OH), silica sols could easily nucleate on the surface of PS spheres and eventually merge and grow into a thin shell of uniform thickness by attractive electrostatic interaction and cocondensation reaction, respectively.16,36 Our experimental results show that RCNs with a homogeneous coverage of SiO2 particles were produced by using oxygen plasma treated PS spheres as core, while RCNs with an inhomogeneous coverage of SiO2 particles were obtained by using PVP-modified PS spheres as core. In the former, hydroxyl groups had been successfully introduced onto the surface of PS spheres by means of oxygen plasma treatment. These hydroxyl groups could co-condense with TEOS via ammonia-catalyzed hydrolysis in the sol-gel process. However, thin silica shells of uniform thickness were not produced in the current reaction system. This may be because the hydroxyl groups (C-OH) attached to polymer chains are less reactive than silanol groups (Si-OH). Thus, the co-condensation reaction between C-OH groups attached to polymer chains and TEOS is slower than that between Si-OH groups and TEOS. In the sol-gel reaction, condensation between the hydroxyl groups on the surface of PS spheres and TEOS and among TEOS occurs simultaneously. Thus, we could conclude that when the condensation between hydroxy groups (e.g., Si-OH) on the surface of PS spheres and TEOS is fast, silica shells of uniform thickness would be produced. Conversely, RCNs of a protruding surface would be obtained (e.g., when the hydroxy groups are C-OH). Preparation of Porous Silica Hollow Sphere Coatings on Glass Substrates. As described in our previous work,6,7,20 polycation (PDDA) and polyanion (PSS) can be readily

deposited on Piranha solution treated glass substrates. As a result, a thin film of desired surface charges can be built up on the substrates. There are many silanol groups on the surface of RCNs, and the point of zero charge (PZC) is 2.1 for SiO2,37 which renders the surface of RCNs negatively charged in aqueous solution. Thus, RCNs can be deposited alternately with PDDA. The attractive long-range van der Waals forces, repulsive electrostatic interaction between the like-charge RCNs, and the PDDA layer are the critical terms governing the adsorption process. After calcination, a silica coating was obtained on the substrate. Figure 6a shows an SEM image of the obtained coating. Porous silica hollow spheres (their detailed morphology is shown in Figure 5b) are distributed on the substrate nearly in the form of a monolayer. However, they do not cover the substrate homogeneously, and the coverage degree was estimated to be ca. 50%. The obtained coating is a hierarchically structured porous coating with nanopores in the shell of silica hollow spheres, submicrometer-sized hollow spaces formed by removing PS spheres, and micrometer-sized voids enclosed by the porous silica hollow spheres. The functionalities of the coating were studies in terms of superhydrophilic and antifogging properties. The superhydrophilicity of the coating was characterized by WCAs and the speed of water spreading on its surface. As reported in our previous work,6 a glass substrate cleaned with Piranha solution had an immediate WCA of ca. 8°, which became less than 5° after a spreading time of 6 s. The WCA of (PDDA/PSS)5/PDDA multilayer primed substrate was measured to be ca. 40°, indicating that the polyelectrolyte multilayers were less hydrophilic than Piranha solution cleaned glass substrate. The RCNs coating on the substrate before calcination had a WCA of 22°. In contrast, the immediate WCA on the RCNs coating after calcination was ca. 6°, and the water droplet became a sheetlike water film with a WCA of ca. 0.6° (Figure 6b) and 0° (Figure S4, Supporting Information) after a spreading time of 0.5 and 1.8 s, respectively. Clearly, the polyelectrolyte multilayers lowered the hydrophilicity of the coating, and removal of them could largely enhance its hydrophilicity. It is very interesting and surprising that the

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Figure 6. (a) SEM image of as-prepared porous silica hollow sphere coating on slide glass and digital images of water contact angle (b) on the coating, and antifogging effects (c) of a slide glass coated with porous silica hollow spheres (top) and a control slide glass (bottom).

fabricated porous coatings present excellent superhydrophilic property even though the coverage degree of porous silica hollow spheres is only ca. 50% on slide glass. The excellent superhydrophilicity and fast water droplet spreading of the porous coating could be attributed to its characteristic nanostructure with both high surface roughness and high hierarchical surface porosity. The property-structure relationships have been discussed in detail in our previous work.7 It is believed that higher coverage degree would result in even better superhydrophilic property,7 and such improvement of coverage degree of silica hollow spheres is currently being carried out in our laboratory. The instantaneous spreading and sheet-like wetting by water would doubtlessly give self-cleaning and antifogging properties, which are extremely useful for various applications. To examine the antifogging property of the porous silica hollow sphere coating, a control slide glass and a slide glass with the superhydrophilic coating were cooled simultaneously at ca. -18 °C in a refrigerator overnight, and then exposed to hot water vapor above a beaker containing boiling water. Photos were taken immediately after the exposure. The results of the antifogging property are displayed in Figure 6c. The control slide glass fogged immediately (lower part of Figure 6c). In sharp contrast, the slide glass with the superhydrophilic coating remained clear (upper part of Figure 6c).

spheres could be readily tailored by adjusting the TEOS concentration. Subsequently, porous silica hollow sphere coatings were fabricated by depositing RCNs on polyelectrolytemodified glass substrates via electrostatic LbL assembly followed by calcination. The porous silica hollow sphere coatings present excellent superhydrophilic and antifogging properties even though the coverage degree of porous silica hollow spheres is only ca. 50% on the substrate. Clearly, the fabricated raspberry-like composite particles are ideal building blocks for constructing superhydrophilic and superhydrophobic coatings. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10776034 and 20871118), the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS) (Grant No. KGCX2-YW111-5), the “Hundred Talents Program” of CAS, and the “Graduate Science and Social Practice Special Funding Innovative Research Program” of CAS. Supporting Information Available: Details of synthesis of PS nanospheres, SEM images of PS spheres plasma-treated for different periods of time, FTIR spectra of PS spheres before and after plasma treatment, particle size distribution histograms of RCNs by DLS, and WCA image after a spreading time of 1.8 s. This material is available free of charge via the Internet at http://pubs.acs.org.

Conclusions Monodisperse RCNs have been facilely fabricated by using oxygen plasma-treated PS spheres of ca. 615 and 270 nm, respectively, as core in a single-step sol-gel process. The presence of surface hydroxyl groups as produced by oxygen plasma treatment mediated the formation of RCNs with a protruding surface morphology. The fabricated composite spheres have a well-defined structure and a novel surface morphology of dual-size roughness. The diameter of silica nanoparticles and their coverage degree on the surface of PS

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