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Photocatalytic Surface-Initiated Polymerization on TiO2 toward WellDefined Composite Nanostructures Xin Wang,†,‡ Qipeng Lu,‡ Xuefei Wang,‡ Jibong Joo,‡ Michael Dahl,‡ Bo Liu,*,† Chuanbo Gao,*,§ and Yadong Yin*,‡ †

Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, People’s Republic of China Department of Chemistry, University of California, Riverside, California 92521, United States § Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: We demonstrate the use of TiO2 nanospheres as the photoinitiator for photocatalytic surface-initiated polymerization for the synthesis of various inorganic/polymer nanocomposites with welldefined structures. The excitation of TiO2 by UV-light irradiation produces electrons and holes which drive the free radical polymerization near its surface, producing core/shell composite nanospheres with eccentric or concentric structures that can be tuned by controlling the surface compatibility between the polymer and the TiO2. When highly porous TiO2 nanospheres were employed as the photoinitiator, polymerization could disintegrate the mesoporous framework and give rise to nanocomposites with multiple TiO2 nanoparticles evenly distributed in the polymer spheres. Thanks to the well-developed sol−gel chemistry of titania, this synthesis is well-extendable to the coating of the polymers on many other substrates of interest such as silica and ZnS by simply premodifying their surface with a thin layer of titania. In addition, this strategy could be easily applied to coating of different types of polymers such as polystyrene, poly(methyl methacrylate), and poly(N-isopropylacrylamide). We expect this photocatalytic surface-initiated polymerization process could provide a platform for the synthesis of various inorganic/ polymer hybrid nanocomposites for many interesting applications. KEYWORDS: photocatalytic polymerization, titania nanospheres, core/shell nanostructure, inorganic/polymer nanocomposites, polymer coating

1. INTRODUCTION Inorganic/polymer nanocomposites have received extensive interest in recent years and are important materials for photonic crystals, coatings, pharmaceutical, biomedical, and cosmetic formulations.1−4 For example, polymer-coated magnetic nanoparticles can be used for magnetic recording, magnetic sealing, electromagnetic shielding, magnetic resonance imaging (MRI), drug targeting, and magnetic cell separation.5−10 Polymercoated Au/Ag nanoparticles are useful in selective catalysis,11 highly stable organic light-emitting diode (OLED) and organic photovoltaic (OPV) devices,12 and high dielectric constant (k) composites.13 Polymer-coated silica nanoparticles showed high colloidal stability, which facilitates their applications in optical/ electrical devices, sensors, catalysis, and controlled drug release.14−18 Various polymerization approaches have been developed to synthesize inorganic/polymer hybrid nanocomposites,19−26 among which photocatalytic surface-initiated polymerization represents an ideal one. This approach utilizes semiconductor nanoparticles as initiators, which are a family of materials © 2015 American Chemical Society

extensively studied in photocatalytic hydrogen production and environmental remediation. When a semiconductor is excited by a photon with high energy, an electron is promoted from the valence band into the conduction band of the semiconductor, leaving a photogenerated hole in the valence band. This process generates oxidative holes and free radicals which can initiate polymerization of monomers in a way similar to conventional free radical polymerization. Although great efforts have been made in earlier reports, photocatalytic surface-initiated polymerization was mainly employed as a methodology to afford polymer materials embedding or decorating with randomly distributed inorganic nanoparticles.27−34 Much less attention has been paid to well-defined inorganic/polymer nanostructures such as the core/shell ones,35,36 while these nanostructures may lead to peculiar properties of the nanocomposites and convenient applicability in many applicaReceived: October 8, 2015 Accepted: December 15, 2015 Published: December 15, 2015 538

DOI: 10.1021/acsami.5b09551 ACS Appl. Mater. Interfaces 2016, 8, 538−546

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dispersed in 10 mL of water for subsequent photocatalytic surfaceinitiated polymerizations. 2.4. Synthesis of Eccentric and Concentric TiO2@PS Core/ Shell Nanospheres. In a typical process, MPS-modified TiO2 nanospheres (∼20 mg), 50 mL of water, 56 mg of SDS, and 0.5 mL of styrene were mixed and sonicated for 30 min, followed by degassing with nitrogen for another 30 min. The temperature of the solution was then raised to 70 °C, exposed to UV light (wavelength, 365 nm; 10 cm away from the 15 W light source, the same hereinafter) to initiate polymerization. Eccentric TiO2@PS core/shell nanospheres were thus obtained by centrifugation and washing with water for 3 times. The synthesis of concentric TiO2@PS core/shell nanospheres followed a similar recipe as that of eccentric TiO2@PS except that an additional cross-linker DVB (10−50 μL) was added before photocatalytic polymerization. 2.5. Synthesis of TiO2@PMMA Core/Shell Nanospheres. For PMMA coating, 20 mg of MPS-modified titania nanospheres was dispersed in 50 mL of water, followed by the addition of 56 mg of SDS and 0.5 mL of MMA. The solution was sonicated for 30 min and then degassed with nitrogen for another 30 min. The photocatalytic polymerization was performed at 70 °C under UV-light irradiation for 2 h. The TiO2@PMMA composites were obtained by centrifugation and washed with water. 2.6. Synthesis of TiO2@PNIPAM Core/Shell Nanospheres. A stock solution containing 0.17 g of NIPAM, 0.023 g of MBA, and 10 mL of water was degassed by bubbling nitrogen for 15 min. Then, 20 mg of the MPS-modified titania nanospheres was mixed with 3 mL of the stock solution in 50 mL of water, and the mixture was purged with nitrogen for 30 min. The temperature was raised to 70 °C, and UVlight irradiation was applied to the mixture to initiate the photocatalytic polymerization. After 2 h, the TiO2@PNIPAM nanospheres were collected and washed with water. 2.7. Synthesis of Eccentric and Concentric SiO2@TiO2@PS Nanospheres. SiO2@TiO2 nanoparticles were prepared based on our previous report.42,45 SiO2 nanospheres with diameters of ∼360 nm were prepared using a modified Stöber method, by rapidly adding TEOS (0.86 mL) into a mixture of ethanol (23 mL), deionized water (4.3 mL), and ammonia (0.62 mL, 28%). After stirring for 2 h at room temperature, the precipitated silica nanoparticles were collected by centrifugation, washed with ethanol, and re-dispersed in 5 mL of ethanol. The SiO2 nanospheres were then dispersed in a mixture of HPC (100 mg), ethanol (20 mL), and water (0.1 mL). After stirring for 30 min, an ethanolic TBOT solution (1 mL of TBOT in 5 mL of ethanol) was slowly added using a syringe pump at a rate of 0.5 mL/ min. After injection, the reaction system was heated to 85 °C and refluxed for 100 min. The precipitate was collected by centrifugation, washed with ethanol, and modified with MPS. After that, 3 mL of the modified SiO2@TiO2 nanospheres was mixed with water (50 mL), SDS (56 mg), and styrene (0.5 mL) and exposed to UV light at 70 °C for 3 h. The final eccentric product was obtained by centrifugation and washed with water 3 times. The synthesis of concentric TiO2@PS core/shell nanospheres followed a recipe similar to that of eccentric SiO2@TiO2@PS except that an additional cross-linker DVB (10 μL) was added before photocatalytic polymerization. 2.8. Synthesis of Eccentric ZnS@TiO2@PS Nanospheres. ZnS nanospheres were prepared by following a previously reported method.46,47 Typically, 0.8 mmol of Zn(Ac)2·2H2O and 20 mmol of thiourea were dissolved in 20 mL of water to form a clear solution. The solution was then transferred into an autoclave and maintained at 140 °C for 3 h. After the autoclave was cooled to room temperature, a white product was obtained, which was then washed with water and redispersed in 20 mL of ethanol. The subsequent coating of TiO2 and PS on the ZnS nanospheres was conducted following the same process as that used in the synthesis of SiO2@TiO2@PS nanospheres. 2.9. Characterizations. Transmission electron microscopy (TEM) images were taken on a Philips Tecnai 12 transmission electron microscope operating at 120 kV. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 diffractometer (Karlsruhe, Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). The nitrogen adsorption isotherm was obtained at 77 K using a

tions. In this regard, albeit challenging, it becomes highly desirable to gain precise control of the polymer coating on inorganic cores, yielding many well-defined and interesting nanostructures. In this work, we take titania nanospheres as an example and demonstrate the versatility of the photocatalytic surfaceinitiated polymerization in producing inorganic/polymer nanocomposites with controlled structures and morphologies. We chose titania as the core material because it has been widely studied as a photocatalyst with relatively high activity and stability and low toxicity, and is widely used in the polymer industry as an inorganic additive.37−40 More importantly, sol− gel chemistry of TiO2 has been well-developed in recent years,41,42 which enables quick expansion of the technique to the polymer coating of many other substrates, such as SiO2, Fe3O4, and ZnS, by simply precoating the substrates with a thin layer of TiO2. Through the photocatalytic surface-initiated polymerization, various polymers such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(N-isopropylacrylamide) (PNIPAM) have been successfully coated on TiO2 nanospheres, forming inorganic/polymer nanocomposites with well-controlled eccentric and concentric core/shell nanostructures. We also show that by weakening the connection between the crystal grains within the TiO2 nanospheres, the photocatalytic polymerization process could drive the disintegration of the nanospheres and move them along with the growth of the polymer, producing unique composite spheres with TiO2 nanocrystals evenly distributed within. We believe these novel composites with various structures may find interesting applications in many fields.

2. EXPREIMENTAL SECTION 2.1. Chemicals. Sodium fluoride, silver nitrate, sodium hydroxide, sodium dodecyl sulfate (SDS), ammonia (28%), and ethanol (denatured) were purchased from Fisher Scientific. Aeroxide P25, thiourea, zinc acetate dihydrate, 3-(trimethoxysilyl)propyl methacrylate (MPS), titanium butoxide (TBOT), and tetraethyl orthosilicate (TEOS) were purchased from Acros. Divinylbenzene (DVB, 80%), polyvinylpyrrolidone K30 (PVP, MW 40,000), hydroxypropyl cellulose (HPC), styrene, N-isopropylacrylamide (NIPAM, 97%), N,N-methylenebis(acrylamide) (MBA), and methyl methacrylate (MMA) were purchased from Sigma-Aldrich. All chemicals were used as received. 2.2. Synthesis of TiO2 Nanospheres. Colloidal titania nanospheres were prepared by a previously reported method.43,44 Typically, 0.15 g of HPC was dissolved in 50 mL of ethanol and 0.3 mL of deionized water. After stirring for 30 min, 0.85 mL of TBOT was added to the system, which was mixed vigorously for 15 min and then aged for 3 h. The product was collected by centrifugation, washed with ethanol and water, and re-dispersed in water. The as-prepared titania nanospheres were then crystallized by silica-protected calcination that was previously developed in our group.43,44 In a typical process, 0.2 g of PVP was added to the dispersion of titania nanospheres and the system was allowed to stay static overnight. The nanospheres were collected by centrifugation, re-dispersed in 10 mL of ethanol, and then mixed with 4.3 mL of water, 13 mL of ethanol, 0.62 mL of ammonia (28%), and 0.86 mL of TEOS. After the solution was stirred for 3 h, the TiO2@SiO2 core/shell nanospheres were collected by centrifugation, washed with ethanol, calcined at desired temperatures for 2 h to crystallize the amorphous TiO2 core, and finally etched in a NaOH solution to remove the silica layer. 2.3. Surface Modification of the TiO2 Nanospheres with MPS. The crystalline TiO2 nanospheres were dispersed in 20 mL of ethanol, mixed with 0.2 mL of MPS for 48 h, collected by centrifugation, washed with ethanol and water twice, and then re539


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ACS Applied Materials & Interfaces Quantachrome NOVA 4200e surface area and pore size analyzer. Particle size was measured by dynamic light scattering (DLS) using a Delsa Nano C particle analyzer (Beckman Coulter, Brea, CA, USA). Xray photoelectron spectroscopy (XPS) characterization was carried out using a Kratos AXIS ULTRADLD XPS system equipped with an Al Kα monochromated X-ray source and a 165 mm electron energy hemispherical analyzer. A probe-type Ocean Optics HR2000CG-UVNIR spectrometer was used to measure the UV−vis spectra.

which are highly active oxygenation agents and could assist in the photodegradation of organic pollutants. It is therefore expected that photocatalytic surface-initiated polymerization may also proceed in the presence of oxygen as long as it can be rapidly depleted by the photogenerated electrons. To confirm this assumption, we first investigated the photocatalytic polymerization of PS on the commercial photocatalysts of Degussa P25 TiO2 nanocrystals under various conditions. The size of nanocomposites was measured by DLS. As shown in Figure 2, polymerization of PS occurs in both ambient and

3. RESULTS AND DISCUSSION 3.1. Principles of the Photocatalytic Surface-Initiated Polymerization. The general principle of our photoinitiated polymerization involves the photocatalytic excitation of TiO2 particles under UV light. It is well-known that when TiO2 absorbs photons with energy greater than its band gap, some electrons are excited to the conduction band and holes are left in the valence band, which may separate and then migrate to the surface if they do not recombine on their way out. Once reaching the surface, the electrons and holes may directly or indirectly initiate redox reactions, for example, in this case the radical polymerization. While hydrated electrons might involve in the radical polymerization directly, holes usually transform into hydroxide or other radicals and then participate in the initiation of polymerization. As schematically shown in Figure 1, to ensure binding of the resulting polymer chains to the TiO2

Figure 2. Plots of the sizes of the P25@PS nanocomposites as a function of irradiation time under different temperature and oxygen conditions. Here “oxygen free” refers to the condition where oxygen is minimized by bubbling N2 through the system.

oxygen-free conditions, albeit with different shell growth rates. This highlights that photocatalytic surface-initiated polymerization could be performed under more flexible reaction conditions than conventional chemically induced radical polymerization. However, the results in Figure 2 also clearly indicate that the polymerization is much favored in the absence of oxygen and at a relatively high temperature (70 °C), and thus such a condition is employed as the standard process in all our syntheses. 3.2. Eccentric and Concentric TiO2@PS Core/Shell Nanospheres. To demonstrate the synthesis of eccentric and concentric TiO2@PS core/shell nanospheres, titania nanospheres with good crystallinity and uniform size were first synthesized by the “silica-protected calcination” method, which typically includes the preparation of amorphous TiO2, coating of the silica shell, calcination, and silica etching.43,44 As shown in Figure 3, the as-synthesized amorphous TiO2 nanospheres are dense and nearly monodisperse with an average size of ∼190 nm. After silica coating, mesopores emerged due to the leaching of titania oligomers. The subsequent calcination at 800 °C enabled crystallization of titania into the anatase phase. The morphology of the titania nanospheres has been preserved thanks to the effective protection by the silica shell. When these titania nanospheres were used as initiators under UV-light irradiation at 70 °C, the polymerization of styrene monomers was initiated and continued, which eventually gave rise to a PS layer homogeneously coated on the surface of the titania nanospheres. The TEM image in Figure 3d clearly reveals the formation of the TiO2@PS core/shell structure, and the average size of composite nanospheres increased to ∼500 nm after 2 h of polymerization. It is observed that the TiO2@PS nanospheres are in eccentric configuration, which can be

Figure 1. Schematic illustration showing the photocatalytic surfaceinitiated polymerization with TiO2 nanospheres as the photoinitiator.

surface and the formation of a uniform coating, we first modified the particle surface with MPS through the Si−O−Ti linkages. The MPS moieties on the titania nanocrystals provides CC double bonds for capturing photogenerated radicals and supporting continuous polymerization initiated from them. Furthermore, the presence of the MPS improves titania/ polymer affinity and thus limits their phase separation, again benefiting the uniform coating of the polymer on the TiO2 surface.9,19 A control experiment of the synthesis without MPS modification confirmed that only rough coating of polymers can be achieved, mainly due to dewetting of the polymers on the titania surface (Supporting Information Figure S1). While the UV-light illumination can support continuous growth of the polymer shell on the titania particles, the shell thickness can be conveniently controlled by simply controlling the UV irradiation time. Free radical initiated polymerization or oxidation of monomers generally proceeds only in inert environments because oxygen can act as an inhibitor or retarder of polymerization to arrest the propagation of monomer radicals. It is therefore a common practice to minimize the oxygen content during normal radical polymerization. However, it is inspiring that photocatalytic decomposition of organic pollutants generally takes place in the presence of oxygen. This is because oxygen may serve as an efficient photogenerated electron scavenger to form superoxide O2− species, 540


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Figure 3. TEM images showing the preparation process of the eccentric TiO2@PS nanocomposites: (a) Colloidal amorphous TiO2 nanospheres; (b) TiO2@SiO2 nanospheres after silica coating; (c) crystallized TiO2 nanospheres after calcination and silica etching; (d) TiO2@PS nanocomposites after polymerization. The insets are highmagnification images of the nanospheres. The scale bars in all insets are 100 nm.

ascribed to the interfacial tension between the hydrophilic titania nanospheres and the hydrophobic styrene monomers.9 It is proposed that, at the initial stage of polymerization, a thin layer of polystyrene is deposited on the TiO2 surface through co-polymerization with the surface double bonds. After absorbing hydrophobic monomers, the swollen polystyrene shell becomes highly hydrophobic, leading to increased tension at the TiO2/PS interface. Thus, the polystyrene shell is prone to contraction so that the interface area and thus the free energy can be further reduced, giving rise to asymmetric distribution of the polymer around the TiO2 nanospheres and consequently the eccentric core/shell nanostructure. Besides the interfacial tension, the degree of contraction also depends on the viscosity of the monomer-swollen shell polymers. The viscosity of linear polymers is relatively low so that the contraction of the polymer shells leads to the eccentric location of core particles after completion of polymerization. Therefore, it is possible to tune the TiO2@PS core/shell nanostructures by tuning the viscosity of the polymers.9,47,48 We here introduced a cross-linker, DVB, to control the eccentric degree of the TiO2 cores during the polymerization, because it can significantly increase the viscosity of the monomer-swollen shell and limit the degree of contraction. As shown in Figure 4, with the increase of DVB contents from 2% to 15%, the titania cores in the TiO2@PS nanospheres gradually shift from the edge to the center of nanospheres, eventually leading to concentric nanostructures. It is also found that the formation rates of the PS shells can be controlled by the crystallinity of the titania nanospheres. Figure 5a shows the XRD patterns of TiO2 nanospheres obtained at different calcination temperatures (500 and 800 °C). The TiO2 nanospheres before calcination were amorphous, and their crystallinity readily increased with the

Figure 4. TEM images of TiO2@PS core/shell nanospheres synthesized with different contents of DVB: (a) TiO2@PS-2%DVB; (b) TiO2@PS-10%DVB; (c) TiO2@PS-15%DVB.

calcination temperature. Although both calcined samples showed the anatase phase of titania, the grain size was increased after raising the calcination temperature from 500 to 800 °C, judging from the decreasing full width at half-maximum (fwhm) of the XRD peaks. When these titania nanospheres were employed as the initiator for the polymerization, the size of the polymer shell increased at different rates (Figure 5b). It is clear that based on amorphous titania nanospheres no obvious polymerization of PS can be observed because no electron−hole pairs can be excited. In clear contrast, PS nanospheres formed on crystalline titania nanospheres, showing continuously increased particle size with the reaction time. The TEM images of the final TiO2@PS core/shell nanospheres (Figure 5c−e) further confirmed the different polymerization rates of the PS as a function of the crystallinity of the titania nanospheres. The higher the crystallinity of the titania is, the faster the polymerization. It is expected that the photogenerated electron−hole pairs are less recombined in highly crystalline TiO2 nanospheres, which improves the efficiency of photocatalytic initiated polymerization. 3.3. Disintegration of TiO2 Framework during Polymerization. In previous investigations, crystallization of the titania was achieved by silica-protected calcination. In this process, high temperature is usually employed for better crystallinity of the titania nanospheres, which leads to sintering 541


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Figure 5. (a) XRD patterns of the titania nanospheres without calcination (amorphous TiO2) and calcined at different temperatures: 800 °C (TiO2800) and 500 °C (TiO2-500). (b) Plots of the size of the TiO2@PS nanocomposites versus irradiation time for the three samples. (c−e) Growth of PS on the titania nanospheres under UV irradiation for 120 min. The titania nanospheres are amorphous TiO2 (c), TiO2-500 (d), and TiO2-800 (e), respectively.

Figure 6. (a) XRD patterns of the titania nanospheres before and after reflux at 75 °C in the presence of NaF. (b) Plot of the size of the TiO2/PS multicore/shell nanocomposites versus UV-light irradiation time. Insets: TEM images of the TiO2/PS multicore/shell nanocomposites prepared after 0.5, 2, and 6 h of polymerization. All scale bars are 100 nm. (c, d) TEM images of the mesoporous titania nanospheres and the TiO2/PS multicore/shell nanocomposites, respectively.

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ACS Applied Materials & Interfaces of the neighboring grains and thus a robust framework of titania. As a result, eccentric or concentric TiO2@PS core/shell nanospheres have been obtained. We here found that the amorphous titania nanospheres can also be crystallized by simply stirring in water at 75 °C in the presence of NaF, resulting in more loosely connected anatase grains as no calcination process is involved, which may lead to different behavior in photocatalytic surface-initiated polymerization. In this method, water plays an important role in crystallizing amorphous TiO2 by dissolving and recrystallizing randomly distributed TiO62− octahedra via a dissolution− precipitation process, which leads to formation of mesoporous titania structures with loosely packed grains.49 The presence of F− not only suppresses the formation of brookite and rutile, leaving only anatase phase, but also promotes crystallite growth by adsorbing on the surface of TiO2 particles and enhances the crystallization degree of the anatase.50,51 The XRD patterns (Figure 6a) showed that the amorphous phase transformed into the anatase phase after 2 h of the reaction. The high porosity of the titania nanospheres can be further confirmed by TEM (Figure 6c) and nitrogen adsorption experiment (Figure S2), with the surface area measured to be ∼576 m2 g−1. When these mesoporous titania nanospheres were employed in the photocatalytic surface-initiated polymerization of polystyrene, continuous increase in the size of the titania/polymer composite nanospheres can be observed (Figure 6b). Different from the eccentric or concentric TiO2@PS nanospheres, it is surprising that the polymer disintegrates the titania nanospheres into individual grains, giving rise to the TiO2/PS multicore/shell nanostructures, as clearly shown in Figure 6d. On the basis of the preceding observation, a plausible mechanism is proposed for the formation of the TiO2/PS multicore/shell nanostructures. The mesoporous titania nanospheres obtained by refluxing were composed of loosely connected anatase grains. Under UV-light irradiation, photocatalytic polymerization occurred at individual anatase grains. Styrene monomers swelled through the PS shell onto the surface of the grains for polymerization, with the PS polymers diffusing out after the reaction. The continuous diffusion of PS polymers served as the driving force to break down the mesoporous titania framework and give rise to TiO2/PS multicore/shell nanostructures. The proposed mechanism involves continuous diffusion of the styrene onto the surface of individual anatase grains, the polymerization of the styrene at the surface, and the transfer of the polymer leaving the surface. To confirm the possibility of the mass transfer, a control experiment was designed to mix the TiO2/PS multicore/shell nanocomposites with a solution of AgNO3. Under UV-light irradiation, the white suspension turned brown due to the formation of Ag nanoparticles. The growth of Ag nanoparticles can be confirmed by UV−vis spectroscopy with light absorption at ∼400 nm, as well as XPS with typical core-level peaks from Ag (Supporting Information Figure S4). TEM imaging (Figure 7b) revealed that Ag nanoparticles are formed on the surface of the titania nanoparticles inside the PS shell, which indicates that Ag+ can diffuse through the PS shell and reach the surface of the titania nanoparticles where reduction occurred by photogenerated electrons. Under dark condition, no significant reduction of Ag can be observed (Figure 7a). These results are in good agreement with our proposed mechanism for the photocatalytic surface-initiated polymerization.

Figure 7. TEM images of the TiO2/PS multicore/shell composites nanospheres with addition of AgNO3 under dark condition (a) and under UV-light irradiation (b).

3.4. Extension to Other Substrates and Polymers. The photocatalytic surface-initiated polymerization method can be well-extendable to many other substrates besides titania, which can be achieved by simply modifying the substrates of interest with a thin layer of titania to serve as the initiator of the photocatalytic polymerization. It can be largely attributed to the well-established sol−gel chemistry of titania so that a thin titania layer can be readily deposited on many types of substrates, for example, Au, SiO2, Fe3O4, and ZnS.41,52,53 Here we demonstrate the synthesis of PS-coated SiO2 and ZnS nanospheres in eccentric and concentric configurations (Figure 8). Panels a and b of Figure 8 show the TEM images of the PScoated SiO2 nanospheres with a TiO2 overlayer as the initiator. The image contrast indicates a three-layer structure of the nanospheres, which are supposed to be SiO2, TiO2, and PS, respectively. Eccentric and concentric SiO2@TiO2@PS core/ shell nanospheres can be obtained by adjusting the amount of cross-linker DVB, consistent with our previous results. Figure 8c shows the TEM image of the PS-coated ZnS nanospheres synthesized by a similar method. The nanospheres showed eccentric nanostructure in the absence of DVB. All of these results confirmed that the photocatalytic surface-initiated polymerization method is general and well-extendable to many other substrates. Coating different polymers on the titania surface is an efficient strategy for increasing the structural complexity and functionality of colloidal particles. The process developed in this work is also applicable to the coating of many other polymers, for example, PNIPAM and PMMA, as long as these polymers can be obtained by free radical polymerization or oxidative polymerization. Figure 9 demonstrates the TiO2@ PNIPAM and TiO2@PMMA core/shell nanospheres with almost the same structure as that in the PS case, which shows that coating of PNIPAM and PMMA can be easily achieved on the TiO2 nanospheres. 3.5. Potential Applications of the TiO2/Polymer Nanocomposites with Well-Defined Structures. We believe these TiO2/polymer nanocomposites with well-defined structures may open up great opportunities in a diversity of applications, such as textile engineering, sensing, and analysis, to name a few. For example, by taking advantage of the hydrophobicity of the PS shell and the effective absorption of UV light by the TiO2 core, the TiO2@PS core/shell nanospheres can be easily incorporated into different fabrics to fabricate many UV-blocking products. In addition, monodisperse TiO2 nanospheres represent an ideal material 543


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Figure 8. TEM images of the PS-coated SiO2 and ZnS nanospheres by modifying their surface with a thin layer of titania: (a) eccentric SiO2@ TiO2@PS nanospheres; (b) concentric SiO2@TiO2@PS nanospheres; (c) eccentric ZnS@TiO2@PS nanospheres.

possibility to generalize this synthesis strategy to many other core materials such as SiO2 and ZnS and polymer coatings such as PMMA and PNIPAM. It is therefore expected to open up new opportunities in the design and synthesis of inorganic/ polymer composite nanomaterials for a variety of applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09551. Additional TEM images, nitrogen adsorption isotherms, UV−vis spectra, and XPS data (PDF)

Figure 9. TEM images of TiO2 nanospheres coated with different polymers: (a) PNIPAM; (b) PMMA.

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for photonic crystals due to their high refractive index and transparency in the visible range of the spectrum. By growing a polymer shell with controlled thickness, the color of the phonic crystals can be effectively tuned. Further with PNIPAM as the shell, which swells and shrinks with temperature, the resulting photonic crystal can serve as a colorimetric probe for the temperature. Moreover, many TiO2@polymer core/shell nanospheres, for example TiO2@PMMA, could be employed as an effective sorbent for solid-phase extraction and chromatography, thanks to the great rigidity of the TiO2 nanospheres and readily established interactions between the polymer shell and guest molecules. The TiO2@polymer core/shell nanospheres are thus versatile building blocks for a variety of interesting functions.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.). *E-mail: [email protected]. Tel.: +1-951-827-4965 (Y.Y.). *E-mail: [email protected] (C.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the U.S. Department of Energy (Grant DE-SC0002247). The support from the UCR Center for Catalysis and the UCR Office for Research and Economic Development is also acknowledged. X.W. acknowledges the fellowship support by the China Scholarship Council. C.G. acknowledges support by the National Natural Science Foundation of China (Grant No. 21301138).

4. CONCLUSIONS In summary, photocatalytic surface-initiated polymerization has been developed in this work to synthesize well-defined inorganic/polymer nanocomposites with various eccentric and concentric core/shell structures. TiO2 nanospheres were employed as typical photoinitiator, which was believed to be extendable to many other substrates. The excitation of TiO2 by UV-light irradiation produces electrons and holes which drive the free radical polymerization near the nanosphere surface, producing core/shell composite nanospheres with eccentric/ concentric structures that can be tuned by controlling the surface compatibility between the polymer and the TiO2. Further, it was discovered that when the TiO2 spheres of loosely packed anatase grains were used as the initiator, the polymerization process could disintegrate the TiO2 framework and move the photocatalyst grains along with the expansion of the polymer, producing unique composite spheres with even distribution of TiO2 nanocrystals. We also explored the

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