Communications - Polymer

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Communications

Facile Synthesis of Hollow Anatase Titania Prepared by Charged Polymeric Nanosphere Template

Ju Hyun Oh, Kyu Bo Kim, Yoon Soo Ko, Han Sun Park, Min Sung Kim and Yong Ku Kwon*

Department of Polymer Science and Engineering, Inha University, Incheon, 402-751, Korea

Received

Revised

Introduction

Two main synthetic routes have been employed to produce inorganic-coated polymer nanoparticles.1-11 The first approach is to precipitate to form a thin inorganic shell of hydrolyzed metal oxide precursors onto the polymeric template.1-3 The second approach is a layer-by-layer(LBL) deposition technique to form alternate layers of oppositely charged inorganic and organic species on the core materials.2,12-13 Various inorganic materials, including silica, titania, zirconia, clay and iron oxide, are used as a coating material.14-17 Among them, titania has attracted a great deal of recent attention, due to their application in catalysis, photovoltaics and photoelectronics.18-21 Titania-coated particles are particularly useful as catalysts,22-23 white pigments24 and electrophoretic particles.25-27 These inorganic-coated polymer nanoparticles have been also used to prepare inorganic hollow nanoparticles, which are prepared by removal of the polymer core either by etching in solution or by calcination at high temperature.28-29

Hollow titania spheres have been prepared by the LBL manipulation of preformed inorganic nanoparticles onto polymeric colloidal.1,30 Recently, templated syntheses of hollow titania spheres was reported based on sulfonated polystyrene particles, which were prepared by seed emulsion polymerization, followed by treatment in concentrated sulfuric acid.31 We also reported the preparation of hollow titania nanospheres, based on the cationically-charged copolymer core, comprised of styrene, butyl acrylate and cationic [2-(methacryloxy) ethyl]trimethyl ammonium chloride (MOTAC), was prepared by soap-free emulsion polymerization. Cationically-charged polystyrene nanospheres were prepared by using an ionogenic initiator of 2,2’-azo bis(2-methylpropionamidine)dihydrochloride (AIBA). The formation of the organic-inorganic hybrids was achieved by adsorption of titania through the hydrolysis of titania precursor.

In the present study, to achieve the rapid and sufficient adsorption of inorganic precursor species onto the surfaces of polymer nanospheres, we synthesize positively-charged, monodisperse polymeric cores which are easily associated with negatively-charged inorganic titania precursors by charge density matching. To enhance the charge density of the polymer nanospheres, the polymer cores were prepared by surfactant-free emulsion copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDMA) and methacryloxyethyltrimethyl ammonium chloride (MOTAC) in the presence of azo bis(isobutylamidine) hydrochloride (AIBA) as an initiator. Unlike our previous study, the component monomers used in the present work are all acrylic, relatively-large amount of the cationic MOTAC monomer can be incorporated within the polymer backbone, which results in the increase in the charge density onto the surface of the polymer core in an attempt to achieve the sufficient adsorption of negatively-charged titania and obtain hollow titania nanospheres with adequate shell thickness.

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|* Corresponding Author. E-mail: ykkwon@inha.ac.kr |

Results and Discussion

Monodisperse polymer nanospheres, composed of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDMA) and methacryloxyethyltri- methyl ammonium chloride (MOTAC) [poly(MMA-co-EGDMA-co-MOTAC)], were successfully prepared by soap-free emulsion polymerization. The electrophoretic mobility measurement of the poly(MMA-co-EGDMA-co-MOTAC) nanospheres gave a (-potential of +80 mV and electrophoretic mobility of 9.2 mV and 7.69 ( 10-5 cm2/Vs at a neutral pH and below. To enhance the cationic charge density of the surfaces of the nanospheres, the aminolysis reaction was conducted via nucleophilic attack on the carbonyl carbon of poly(MMA-co-EGDMA-co-MOTAC) to form a positively charged tetrahedral intermediate. The nanospheres after the aminolysis reaction were denoted as NH2-poly(MMA-co-EGDMA-co-MOTAC) nanospheres. The (-potential and electrophoretic mobility of the NH2-poly(MMA-co-EGDMA-co-MOTAC) was 28.7 mV and 2.37( 10-4 cm2/Vs. In Figs. 1 (a) and (b), we observed the mono-disperse polymeric nanospheres with a diameter in the range of 170-220 nm. Dynamic light scattering (DLS) was also used to measure the size and distribution of the polymerized particles. The DLS data showed that the breadth of the distribution of the particles, obtained by the polydispersity value of the particles was relatively narrow, indicating that the particles were almost monodisperse.

Titania shell was successively deposited onto the positively-charged surfaces of the poly(MMA-co-EGDMA-co-MOTAC) nanospheres by the consecutive precipitation of the negatively-charged, hydrolyzed titania precursors. Poly(MMA-co-EGDMA-co-MOTAC) nanospheres with an average diameter of approximately 190 nm was used as a template.

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Figure 1. SEM images of (a) poly(MMA-co-EGDMA-co-MOTAC) nanoparticles, (b) NH2-poly(MMA-co-EGDMA-co-MOTAC) particles, (c) titania-coated polymer nanospheres (d) hollow titania nanospheres. The insets represent the TEM images of each sample.

During the adsorption of titania precursor on the surfaces of the polymer cores, we added PVP to prevent the particles from coagulating each other. By exploring the positively-charged polymer nanospheres, the negatively-charged titania precursors can be rapidly adsorbed to form titania shells with a uniform thickness. The deposition and condensation reaction of the titania precursor were carried out for 24 hrs to ensure their uniform coating onto the polymer surfaces. During the reaction, the negatively-charged titania precursors were deposited onto the positively-charged surfaces of polymer nanospheres by electrostatic interactions and the hydrolyzed titania precursors were then thermally condensed to form a three-dimensional infinite network-like gel of -Ti-O- bonds through a solid rearrangement reaction. Because a low concentration of titanium butoxide was used to make the sol, its gelation proceeded slowly in a bulk solution. However, the adsorbed sol particles located on the surfaces of the polymer nanospheres increased their local concentration, which led to an increase of the gelation speed of the titania sols to form the titania shells on the polymer nanospheres.32 In Figs. 1 (c), the average diameter of the titania-coated polymer nanospheres was approximately 230 nm, which estimated the average thickness of the titania shell as approximately 25 nm. It was also found that the thin layer of the titania shells was composed of fine titania particles.

These polymer core/titania shell nanospheres were transferred to a furnace and heat-treated in air at 500 oC to remove the polymeric core by calcination. The average diameter of the hollow titania nanospheres was approximately 180 nm which was reduced by 24%, as compared with one measured from the titania-coated polymer core/shell nanospheres. The main reason of this shrinkage may be due to the crystallization of the adsorbed titania precursors during the heat-treatment, together with the removal of the polymer cores.

Figure 2 showed the wide angle X-ray diffraction (WAXD) patterns of the (a) titania-coated polymer nanospheres and (b) hollow titania, calcined at 500oC. In Fig. 2(a), no peak was observed in the range of 20 o ( 2((degree) ( 80 o. Since the polymer core was purely amorphous, the XRD data indicated that the titania shells coated onto polymer cores were also amorphous. In Fig. 2(b), we found a series of sharp and strong Bragg peaks which appeared due to the anatase phase of the titania crystals in the shells.

Figure 2. Wide angle X-ray diffraction (WAXD) patterns of the (a) titania-coated polymer nanospheres and (b) hollow anatase titania, calcined at 500oC.

The data indicated that the amorphous titania gels, adsorbed on the polymer core surfaces was crystallized during the calcination at a high temperature. We also found the trace of Bragg’s peaks due to the rutile crystalline phase of titania crystal, but its amount was almost negligible. Based on the peak width of the most intense peak at 2( ~ 25o, which was assigned as (101) peaks of anatase type titania crystal, we estimated the apparent size of the titania crystal using Scherrer equation, which was about 13.8 nm. Upon annealing at 800oC, rutile phase became predominant. However, most of the hollow spheres were ruptured because the thin titania shells were not robust at extremely high temperatures.

The polymer cores could be also removed by immersing in a selective solvent, e.g. tetrahydrofuran, but the morphology of the solvent-etched hollow titania was slightly distorted during solvent etching because the titania shells was too thin to be maintained without any deformation. The shell of the calcined hollow titania was composed of small titania crystallites, as clearly demonstrated in Fig. 1(d).

In summary, the nanoparticles with the organic core and the inorganic shells have been synthesized by using stepwise reactions such as the soap-free emulsion copolymerization of the cationic colloidal core and the thermal condensation of the titania shell. Monodisperse and stable cationic nanoparticles were prepared by the introduction of cationic comonomer as a template. The positive charge on the spheres thus ensures not only the rapid capture of the negatively charged titania precursor to form titania shell, but also the thin-layer deposition of the titania precursors onto the polymer surface with uniform thickness.

Acknowledgments. This research was supported by Inha University Research Grant.

Supporting Information: Experimental procedure for the syntheses of poly(MMA-co-EGDMA-co-MOTAC), NH2-poly(MMA-co-EGDMA-co-MOTAC), polymer core/titania shell, hollow titania, their FTIR and TGA, DLS data. The materials are available via the Internet at .

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|Facile Synthesis of Hollow Anatase Titania |Monodisperse, nanometer-size hollow titania nanospheres were successfully prepared by using a |

|Prepared by Charged Polymeric Nanosphere |template of charged polymeric nanoparticle. The monodisperse, charged polymeric nanoparticles were |

|Template |prepared by emulsifier-free emulsion copolymerization of acrylic monomers in the presence of |

|Ju Hyun Oh, Kyu Bo Kim, Yoon Soo Ko, Han Sun |azobis(isobutylamidine)hydrochloride (AIBA) as an initiator. Negatively-charged titania precursors|

|Park, Min Sung Kim, and Yong Ku Kwon* |were adsorbed and rapidly hydrolyzed onto the cationic surfaces of colloidal particles. These |

|Macromol. Res., |polymer core/titania shell nanoparticles were then heated to 500◦C for calcination of the polymer |

| |cores and crystallization of the amorphous titania into anatase crystals. |

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| |[pic] |

Supplementary Information

Facile Synthesis of Hollow Anatase Titania Prepared by Using a Charged Polymeric Nanosphere Template

Ju Hyun Oh, Kyu Bo Kim, Yoon Soo Ko, Han Sun Park,

Min Sung Kim and Yong Ku Kwon,*

Department of Polymer Science and Engineering,

Inha University, Incheon 402-751, Korea

Corresponding author: Yong Ku Kwon

Department of Polymer Science and Engineering

School of Engineering, Inha University

253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Korea

Telephone: +82-32-860-7482

Fax: +82-32-865-5178

E-mail: ykkwon@inha.ac.kr.

1. Synthesis of Cationic Polymeric Nanospheres.

Polymeric nanospheres, poly(MMA-co-EGDMA-co-MOTAC)s, were prepared by emulsifier-free emulsion copolymerization using AIBA (0.08 g) and cationic comonomer MOTAC (0.06 g), together with EGDMA (0.8 g) and MMA (15 g). The polymerization reaction was carried out in nitrogen at 80oC for 12 hrs and the stirring speed was adjusted at 300 rpm. After the reaction, the nanospheres were purified by centrifugation/redispersion cycles in distilled water. The final spheres were obtained by freeze drying.

2. Synthesis of Cationic Polymeric Nanospheres

As-prepared PMMA nanospheres with a diameter in the range of 170-220 nm were dispersed in water. Hexamethylene diamine was added in a large molar excess to functional carbonyl group of the surfaces of the PMMA nanospheres. The functionalization reaction was performed at room temperatures and efficiently stirred with a magnetic stirrer for 12 hrs. The surface-modified particles were purified by several washing cycles.

3. Preparation of Hollow Titania Nanospheres

The polymer core/titania shell nanospheres were prepared by two successive steps such as the deposition of PVP on the cationic polymeric colloids and the adsorption of the hydrolyzed titania precursor on the PVP-adsorbed polymeric colloids. The cationic PVP was added to stabilize the polymer colloids and facilitate the rapid adsorption of the negatively-charged titania precursors in ethanol. The ethanolic solution of titanium(IV) butoxide was added in the solution of the PVP-adsorbed polymeric colloids under vigorous stirring. Titania precursors were rapidly adsorbed onto the positively-charged poly(MMA-co-EGDMA-co-MOTAC) colloids by charge density matching. The titania-coated polymer colloids were then washed with ethanol and dried in vacuum at 40 oC. To remove the polymer cores, these titania-coated polymer colloids were placed in a furnace, which was heated to 500 oC or 800 oC in air at a heating rate of 2 oC/min. These core-shell nanoparticles were calcined at 500 oC for 3 hrs and then cooled to room temperature at a rate of 10 oC/min.

4. Size distribution of poly(MMA-co-EGDMA-co-MOTAC) (a) and NH2-poly(MMA-co-EGDMA-co-MOTAC) (b) nanospheres, measured by DLS measurements

Dynamic light scattering (DLS) was also used to measure the size and distribution of the polymerized particles. The DLS data showed that the breadth of the distribution of the particles, obtained by the polydispersity value of the particles was relatively narrow, indicating that the particles were almost monodisperse.

a

b

Figure S1

5. FTIR data of (a) poly(MMA-co-EGDMA-co-MOTAC) and (b) NH2-poly(MMA-co-EGDMA-co-MOTAC) (c) hollow titania nanospheres.

The peaks at 1731 cm-1 and 1485 cm-1 arose from the carbonyl groups of the MMA and EGDMA segments and the CH2-N stretch of MOTAC, respectively. After aminolysis, the primary group (-NH2) and the peak at 1533cm-1 was from the amide group (-CONH-).

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Figure S2

6. TGA data of (a) polymer cores and (b) titania-coated polymer nanoparticles, measured during heating from room temperature to 500oC

In both figures, a large decrease of weight occurred in the range of 300 ( T (oC) ( 450 was caused by thermal decomposition of the polymer cores. In Fig. S3(b), obtained from the titania-coated polymer nanospheres, a gradual decrease in weight up to around 200 oC was due to the removal of oligomeric materials, as well as the desorption of water, mostly attached onto the titania shells. In Fig. S3(b), the final weight loss of the titania-coated nanoparticles was approximately 66 %. Based on these data, the weight ratio of TiO2 to polymer cores was 0.84.

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Figure S3

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