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Article

Photocatalytic Surface-Initiated Polymerization onTiO2 toward Well-Defined Composite Nanostructures

Xin Wang, Qipeng Lu, Xuefei Wang, Ji Bong Joo, Michael Dahl, Bo Liu, Chuanbo Gao, and Yadong YinACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09551 • Publication Date (Web): 15 Dec 2015

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Photocatalytic Surface-Initiated Polymerization on TiO2 toward

Well-Defined Composite Nanostructures

Xin Wang,a,b Qipeng Lu,

b Xuefei Wang,

b Jibong Joo,

b Michael Dahl,

b Bo Liu,*

a Chuanbo Gao,*

c

Yadong Yin*b

a Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044 (P.

R. China)

b Department of Chemistry, University of California, Riverside, CA 92521 (USA)

c Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong

University, Xi’an, Shaanxi 710054 (P. R. China)

KEYWORDS: photocatalytic polymerization, titania nanospheres, core/shell nanostructure,

inorganic/polymer nanocomposites, polymer coating

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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

well-defined 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

eventually 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 pre-modifying 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 (PS), poly(methyl methacrylate) (PMMA) and poly(N-isopropylacrylamide)

(PNIPAM). 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.

1. INTRODUCTION

Inorganic/polymer nanocomposites have received extensive interest in recent years, which 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

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recording, magnetic sealing, electromagnetic shielding, magnetic resonance imaging (MRI), drug

targeting, and magnetic cell separation.5-10

Polymer-coated 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 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 applications. 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.

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In this paper, we take titania nanospheres as an example, and demonstrate the versatility of the

photocatalytic surface-initiated 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, low toxicity,

and is widely used in polymer industry as an inorganic additive.37-40

More importantly, sol-gel

chemistry of TiO2 has been well developed in the 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 pre-coating 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

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.

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Divinylbenzene (DVB, 80%), polyvinylpyrrolidone K30 (PVP, Mw 40,000), hydroxypropyl

cellulose (HPC), styrene, N-isopropylacrylamide (NIPAM, 97%), N,N-methylenebisacrylamide

(MBA) and methyl methacrylate (MMA) were purchased from Sigma-Aldrich. All chemicals

were used as received.

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.

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 redispersed in 10 mL of water

for subsequent photocatalytic surface-initiated polymerizations.

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

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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 eccentric TiO2@PS except that an additional

crosslinker DVB (10−50 µL) was added before photocatalytic polymerization.

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.

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 were 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 UV light irradiation was applied to the mixture to initiate

the photocatalytic polymerization. After 2 h, the TiO2@PNIPAM nanospheres were collected

and washed with water.

Synthesis of eccentric and concentric SiO2@TiO2@PS nanospheres. SiO2@TiO2

nanoparticles were prepared based on our previous report42, 45

. SiO2 nanospheres with diameters

of ~360 nm were prepared using a modified Stöber method, by rapidly adding TEOS (0.86 mL)

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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 redispersed 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 3h. The final eccentric product was obtained by centrifugation and washed

with water three times. The synthesis of concentric TiO2@PS core/shell nanospheres followed a

similar recipe as eccentric SiO2@TiO2@PS except that an additional crosslinker DVB (10 µL)

was added before photocatalytic polymerization.

Synthesis of eccentric ZnS@TiO2@PS nanospheres. ZnS nanospheres were prepared by

following a previously reported method.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

down to room temperature, a white product was obtained, which was then washed with water

and redispersed in 20 mL 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.

Characterizations. Transmission electron microscopy (TEM) images were taken on a Philips

Tecnai 12 transmission electron microscope operating at 120 kV. X-ray diffraction (XRD)

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patterns were recorded on a Bruker D8 diffractometer (Germany) with Ni-filtered Cu Kα

radiation (40 kV, 40 mA). The nitrogen adsorption isotherm was obtained at 77 K using a

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,

USA). X-ray 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-UV-

NIR spectrometer was used to measure the UV-vis spectra.

3. RESULTS AND DISCUSSION

3.1. Principles of the photocatalytic surface-initiated polymerization

The general principle of our photo-initiated 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

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

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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

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 (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.

Figure 1. A schematic illustration showing the photocatalytic surface-initiated polymerization

with TiO2 nanospheres as the photoinitiator.

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

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because oxygen may serve as an efficient photogenerated electron scavenger to form superoxide

O2– species, 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 firstly 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 oxygen-free conditions, albeit the 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 synthesis.

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Figure 2. Plots of the size 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.

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 firstly 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 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 PS layer homogeneously coated on the surface of the titania

nanospheres. The TEM image in Figure 3d clearly reveals the formation of 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 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 copolymerization with the

surface double bonds. After absorbing hydrophobic monomers, the swollen polystyrene shell

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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.

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 high-magnification images of the

nanospheres. The scale bars in all insets are 100 nm.

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

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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 crosslinker, DVB, to control the

eccentric degree of the TiO2 cores during the polymerization, because it can significantly

increase the viscosity of 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.

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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.

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 °C and 800 °C). The TiO2 nanospheres before

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calcination were amorphous, and their crystallinity readily increased with the calcination

temperature. Although both calcined samples showed anatase phase of titania, the grain size was

increased after raising the calcination temperature from 500 °C 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 5 c−e) further confirmed the different polymerization

rates of the PS as a function of the crystallinity of the titania nanospheres. The higher

crystallinity of the titania, 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.

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Figure 5. (a) XRD patterns of the titania nanospheres without calcination (Amorphous TiO2)

and calcined at different temperatures: 800 °C (TiO2-800) 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.

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 of the neighboring grains and thus robust

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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 amorphous phase transformed into 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 multi-core/shell

nanostructures, as clearly shown in Figure 6d.

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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 multi-core/shell nanocomposites versus UV

light irradiation time. Insets: TEM images of the TiO2/PS multi-core/shell nanocomposites

prepared after 0.5, 2, 6 h of polymerization. All scale bars are 100 nm. (c−d) TEM images of

the mesoporous titania nanospheres and the TiO2/PS multi-core/shell nanocomposites,

respectively.

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On the basis of the above observation, a plausible mechanism is proposed for the formation of

the TiO2/PS multi-core/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 multi-core/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 multi-core/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 (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.

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Figure 7. TEM images of the TiO2/PS multi-core/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 substrate 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).

Figure 8a−b shows the TEM images of the PS-coated SiO2 nanospheres with a TiO2

overlayer as the initiator. The image contrast indicates a 3-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 crosslinker

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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 these results confirmed that the photocatalytic surface-

initiated polymerization method is general and well extendable to many other substrates.

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.

Coating different polymer on 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.

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

PMMA.

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 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

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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.

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 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.

ASSOCIATED CONTENT

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Supporting Information

Additional TEM images, nitrogen adsorption isotherms, UV-vis spectra and XPS data. This

material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

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

[email protected] (C.G.).

Author Contributions

The manuscript was written through contributions of all authors. All authors have given

approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

We are grateful for the financial support from the U.S. Department of Energy (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

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Scholarship Council. C.G. acknowledges support by the National Natural Science Foundation of

China (Grant No. 21301138).

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