Introduction

In recent years, the synthesis of hollow microspheres has attracted a great deal of attention [1–5]. The special structures of hollow microspheres made them ideal candidates for such applications as catalyst, controlled drug delivery, paints, and electronics materials [6–9]. A general approach to prepare the hollow structure is based on the layer-by-layer deposition that allows template particles to be coated by various polymeric, inorganic, and metallic materials. Other physical and chemical methods, such as colloidal crystal templating [10] and surface-initiated atom transfer radical polymerization [11], have also been developed for preparing functional hollow particles with various different materials. Based on these technologies, the design and fabrication of inorganic/polymer hybrid microspheres have attracted considerable attention recently. These hybrid particles not only exhibit special properties of inorganic components but also lead to materials with improved comprehensive properties. Strohm et al. [12] prepared polyamide hollow spheres coated with TiO2 by liquid-phase deposition. Caruso et al. [13] had obtained inorganic hybrid hollow spheres through the colloid templated electrostatic layer-by-layer self-assembly followed by removal of the templated core. Similarly, Zhang et al. [14] synthesized SiO2/polymer hybrid hollow microspheres via double in situ miniemulsion polymerization. Armes et al. [15] prepared the silica/polymer microspheres with a well-defined core–shell morphology by using the commercially available glycerol-functionalized ultrafine silica sol reported that using commercially available glycerol-functionalized ultrafine silica sol without surfactant or nonaqueous cosolvents.

Among all the methods mentioned, Pickering emulsions [16] were of great practical interest due to their widespread use in many areas including cosmetics, food, pharmaceuticals, oil recovery, and wastewater treatment. Pickering emulsion usually made robust emulsions, and it was generally difficult to break Pickering emulsions by changing the chemical or physical parameters, such as the pH, salt concentration, temperature, and composition of the oil phase. Hasell et al. [17] prepared the magnetic microparticles with poly(methyl methacrylate) core and Fe3O4 nanoparticle shell by Pickering suspension polymerization. He et al. [18] fabricated the novel Janus Cu2(OH)2CO3/CuS microspheres via a Pickering emulsion route. Zhang et al. [19] made the magnetic polymer enhanced hybrid capsules from a novel Pickering emulsion polymerization. Bon et al. [20, 21] investigated solid-stabilized, or Pickering, miniemulsion polymerization using Laponite clay disks as the stabilizer for a variety of hydrophobic monomers such as styrene, lauryl methacrylate, butyl methacrylate, octyl acrylate, and 2-ethyl hexyl acrylate.

Semiconductor nanocrystals have been investigated during the past decades owing to their novel optical, electrical, and catalytic properties. The quantum confinement effect on the nanometer scale [22, 23] allowed the semiconductor nanocrystals applied in such various different fields as optoelectronics, photocatalysts, and photonic crystals [24–26]. In recent years, much attention has been paid to the zinc oxide (ZnO) nanoparticles because of its outstanding characteristics such as wide band gap of 3.37 eV at room temperature [27], photoluminescent properties [28], and fairly good conductivity [29]. Because of its unique chemical and physical properties, the ZnO nanostructure has drawn much attention in many applications, such as chemical sensors, biosensors, solar cells, photocatalysts, and optoelectronics. In this study, we prepared the polystyrene (PS)/ZnO hybrid microcapsules by a facile approach using ZnO nanoparticles as stabilizer in o/w Pickering emulsion. The unique multifarious hollow structures could be obtained by choosing the monomer and solvent as the oil phase and controlling the solubility of PS in the oil phase. Variations in morphology and property of PS/ZnO hybrid microcapsules were investigated in terms of the polymerization conditions.

Experimental

Materials

ZnO nanoparticles with mean diameter of 50 nm were obtained from Aldrich and used as stabilizer. Styrene monomer was purified by vacuum distillation. 1, 2-Divinylbenzene (DVB) as cross-linking agent, 2, 2′-azobisisbutyronitrile (AIBN) as initiator, and n-octadecane as solvent were purchased from Aldrich. These were all of analytical grade and used without any further purification.

Synthesis of PS/ZnO hybrid microcapsules

The synthetic procedures of PS/ZnO hybrid microcapsules are presented in Scheme 1. ZnO nanoparticles (0.5 g) were dispersed in 50 ml deionized water and ultrasonicated with Branson ultrasonic cleaner (Model 2210R-DTH) at a frequency of 40 kHz for 20 min. The oil phase solutions consisting of AIBN, styrene, DVB, and n-octadecane, at the compositions shown in Table 1, were mixed with the ZnO dispersion. Pickering emulsion was prepared by vigorous stirring for 5 min at 20,000 rpm with Ika Werka Ultra Turrax T25 Basic agitator. The resultant emulsion was transferred into 100 ml round-bottomed flask. Nitrogen gas was bubbled through the emulsion for 30 min followed by heating to 55°C for the polymerization. PS/ZnO hybrid microcapsules were dried by either conventional drying at ambient temperature or freeze-drying.

Scheme 1
scheme 1

Schematic illustration of the preparation of PS/ZnO hybrid microcapsules by Pickering emulsion polymerization

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Table 1 Processing parameters of Pickering emulsions for polymerization
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Characterization

Fourier transform infrared (FTIR) spectra were recorded on a Bio-Rad FTS 175C spectrometer using KBr pellets. Thermogravimetric analysis (TGA) thermograms were obtained with a TA Instrument 2050 thermogravimetric analyzer at a heating rate of 10°C/min from 25°C to 600°C under a nitrogen atmosphere. Field emission scanning electron microscopy was performed using a JEOL JSM-7000F instrument. Transmission electron microscopy (TEM) was performed using a JEM 2100F instrument. The TEM samples were prepared by dropping the dilute suspension of PS/ZnO colloid on a copper grid covered with a perforated carbon film. The room-temperature photoluminescence (PL) measurement was carried out on a Cary Eclipse fluorescence spectrophotometer and a xenon flash lamp with an excitation wavelength of 340 nm at room temperature. Energy-dispersive X-ray (EDX) microanalysis of the samples was performed during SEM measurements.

Results and discussion

Stabilization of o/w emulsions prepared with ZnO as surfactant

In order to investigate the stability of the o/w emulsions prepared with ZnO as surfactant over an extended period of time, an emulsion sample (run 1 in Table 1) was kept on the laboratory bench at room temperature for 200 h after preparation. The size and size distribution of droplets were analyzed at several different time intervals by dynamic light scattering. As shown in Fig. 1, relatively constant number averaged droplet diameter was maintained at around 12 μm during the time period of 200 h, indicating there were no severe coalescence and destabilization processes. In addition, the insets in Fig. 1 also showed that the size distribution of o/w emulsion remained relatively constant after 200 h. Hence, ZnO worked as the useful emulsifier to prepare a stable o/w emulsion, which was beneficial to the following microcapsule fabrication.

Fig. 1
figure 1

Variations in the size of PS/ZnO hybrid microcapsules (run 1) depending on the time period (insets: size distribution of run 1 as-prepared and kept for 200 h, respectively)

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Morphology of PS/ZnO hybrid microcapsules

PS/ZnO hybrid microcapsules were successfully synthesized via o/w Pickering emulsion polymerization. ZnO nanoparticles as stabilizer were dispersed initially in the water phase. The o/w Pickering emulsion was formed by adding the oil phase consisting of styrene, DVB, AIBN, and n-octadecane in the aqueous ZnO dispersion. The polymerization was carried out inside the oil phase droplets. Because n-octadecane was the nonsolvent for PS, the styrene–DVB copolymer phase was separated from the reaction medium resulting in the deposition on the interface of ZnO colloidosomes to form the multifarious hollow structure. After the evaporation of solvent inside, PS/ZnO hybrid microcapsules were obtained. PS/ZnO hybrid microcapsules were shown in Fig. 2, although some of them were shrunken owing to the exhalation of solvent during the drying process. The diameters of microcapsule were in the range from 5 to 30 μm. The shell of microcapsules with ZnO nanoparticles ranging from 40 to 60 nm on the surface was clearly observed in Fig. 2b. The successful formation of PS/ZnO hybrid microcapsules was very dependent on the composition of reaction ingredients. With all the same conditions but the solvent, the solvent content was varied from 1.5 to 2.5 g. The various different morphologies of PS/ZnO hybrid microcapsules are shown in Fig. 2. As the solvent content increased in the oil phase, most of the PS/ZnO hybrid microcapsules were broken because the thinner PS walls were formed inside the ZnO colloidosomes from the diluted styrene concentration. EDX analysis was used to determine the local chemical composition of our samples. EDX analysis was also carried out to determine the chemical species on the shell of PS/ZnO hybrid microcapsules. Fig. 2e shows the EDX spectrum, which exhibits the presence of Zn, O, and C elements on PS/ZnO hybrid microcapsules.

Fig. 2
figure 2

SEM images of PS/ZnO hybrid microcapsules prepared from various different compositions: a run 1, b magnification of run 1, c run 2, d run 3, and e EDX spectrum of PS/ZnO hybrid microcapsules

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The drying method also played an important role in maintaining the microcapsule structure as shown in Fig. 3. PS/ZnO hybrid microcapsules were damaged severely by conventional drying at ambient temperature due to the pressure difference across the shell. On the other hand, the freeze-drying was effective to alleviate the stress on the shell of microcapsules during the drying process and to give most of microcapsules intact.

Fig. 3
figure 3

SEM images of PS/ZnO hybrid microcapsules (run 5) prepared by a conventional drying at ambient temperature and b freeze-drying

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Effect of monomer concentration on the shell thickness of PS/ZnO hybrid microcapsules

Several different styrene monomer contents were used for the preparation of PS/ZnO hybrid microcapsules in order to study the effect of monomer concentration on the shell thickness. The monomer concentration played an important role in determining the shell thickness as shown in Fig. 4. As the monomer content decreased from 1.0 to 0.5 g in the oil phase, the shell thickness decreased from 500 to 200 nm with keeping the shape of hybrid microcapsules unaltered. Therefore, the shell thickness could be controlled easily by changing the monomer concentration in the oil phase.

Fig. 4
figure 4

SEM images of the shell of PS/ZnO hybrid microcapsules prepared with different monomer concentration: a run 5, b run 4, and c run 1

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Characterization of PS/ZnO hybrid microcapsules

FTIR spectra of ZnO nanoparticles and PS/ZnO hybrid microcapsules were shown in Fig. 5. For the pristine ZnO nanoparticles, the peak at 432 cm−1 was the characteristic absorption of Zn–O bond. The broad absorption peak at 3,437 cm−1 was attributed to the characteristic absorption of hydroxyl groups, and the absorptions at 1,585 and 1,423 cm−1 were attributed to the vibration peaks of carboxyl groups adsorbed on the surface. For the PS/ZnO hybrid microcapsules, some new vibration absorption peaks were observed at 698, 758, 1,453, 1,497, 1,601, 2,854, 2,930, and 3,016 cm−1. These peaks were the typical PS absorption bands.

Fig. 5
figure 5

FTIR spectra of a pristine ZnO nanoparticles and b PS/ZnO hybrid microcapsules (run 5)

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The thermal properties of PS/ZnO hybrid microcapsules were characterized by the TGA as shown in Fig. 6. The pristine ZnO nanoparticles showed a weight loss of about 5.8% between 25°C and 200°C due to the removal of physically adsorbed water and the decomposition of hydroxide groups. On the other hand, PS/ZnO hybrid microcapsules showed the subsequent loss of 83.7% above 200°C due to the decomposition of PS component. Furthermore, compared with the TGA plot of pure PS in Fig. 6c, the degradation temperature of PS in PS/ZnO hybrid hollow microspheres was a little higher which was resulting from the chemical or physical interaction between ZnO nanoparticles and styrene during the polymerization process.

Fig. 6
figure 6

TGA thermograms of a pristine ZnO nanoparticles, b ZnO/PS hybrid microcapsules (run 5), and c PS

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Photoluminescence behavior of PS/ZnO hybrid microcapsules

The PL spectra of pristine ZnO nanoparticles and PS/ZnO hybrid microcapsules with both higher and lower ZnO contents were measured with an excitation wavelength of 340 nm at room temperature as shown in Fig. 7. PL spectrum of pristine ZnO showed four emission bands. A UV strong emission peak at 387 nm, corresponding to the near band edge emission, was attributed to the recombination of free excitons [30]. Peaks centering at 431, 443, and 461 nm were caused by the transition between the vacancy of oxygen and the interstitial oxygen [31]. In addition, some very weak peaks between 480 and 550 nm were attributed to the recombination of photoexcited electrons with deeply trapped holes in the singly ionized oxygen vacancy [32–34], which were responsible for the surface defects at the interface of ZnO. The PL properties of PS/ZnO hybrid microcapsules could be maintained without any noticeable variation because ZnO properties did not vary with the formation of microcapsules. ZnO worked successfully as surfactant in the preparation of PS/ZnO microcapsules without structural variation. Compared with the PL spectrum of pristine ZnO, PS/ZnO hybrid microcapsules showed nearly no change except a red shift at UV strong emission peak, which might be due to the defects and the probable shallow energy level near valence band by the introduced PS. TEM microphotographs of PS/ZnO hybrid microcapsules with different ZnO content were shown together in Fig. 7. The more ZnO nanoparticles located on the surface of microcapsules resulted in the higher intensity of PL spectra.

Fig. 7
figure 7

PL spectra of pristine ZnO nanoparticles and PS/ZnO hybrid microcapsules. (higher ZnO content: run 6, lower ZnO content: run 1)

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Conclusions

The hybrid hollow microcapsules with PS inner shell and ZnO nanoparticles outer shell were easily prepared via a Pickering emulsion reaction. The hybrid microcapsules consisting of PS and ZnO were characterized by FTIR and TGA. The various different morphologies of PS/ZnO hybrid microcapsules were observed by controlling the oil phase composition. The shell thickness of microcapsules increased as the relative monomer concentration in the oil phase increased. Compared with the pristine ZnO nanoparticles, PS/ZnO hybrid microcapsules did not show any noticeable difference in the PL spectra and maintained the good photoluminescent property. Hence, PS/ZnO hybrid microcapsules could be applied as not only chemical sensors and optoelectronics but also drug delivery systems due to the functional inorganic nanoparticles on the surface and the multifarious hollow structures.