Fabrication of ZnO/SiO2 Composite Nanospheres with High Core-Loading Levels

Jinfeng Wang, Takuya Tsuzuki, Lu Sun and Xungai Wang Centre for Material and Fibre Innovation, Institute for Technology Research and Innovation,

Deakin University, Geelong, Vic. 3217, Australia

Abstract—Monodispersed silica shell / zinc oxide core composite nanospheres were prepared in an oil-in-water microemulsion system. By using cyclohexane as the oil phase and Triton X-100 as the surfactant, nanospheres with a high core loading level and high monodispersity were obtained. The silica coating greatly reduced the photoactivity of ZnO nanoparticles, offering safe and durable applications of ZnO as UV screening agents.

Keywords - reverse microemulsion; core/shell nanoparticles; photocatalytic activity

I. INTRODUCTION ZnO has been widely used as an excellent UV absorber in

outdoor textile and cosmetics products [1]. Compared with organic UV absorbers, ZnO possesses the advantages of inorganic materials such as physical and chemical stability under both high temperature and UV irradiation, as well as low toxicity. However, the application of ZnO as a UV absorber is limited in many practical areas because of the inherent photocatalytic activity of ZnO. For example, the photocatalysis results in color fading of fabrics [2] and potential damage of the skin cells [3]. Hence, in order to utilize ZnO nanoparticles as UV absorbers in a safe and effective manner, it is of particular importance to develop the methods to reduce the photocatalytic activity of ZnO. One of the approaches to the reduction of the photocatalytic activity of ZnO is to build a non-conducting barrier, such as silica, between ZnO and the surrounding materials to prevent direct contact between them.

Silica-based composite nanospheres are of particular interest to many applications because of their biocompatibility, chemical stability and ease of surface modification with a wide range of functional groups [4]. By incorporating other types of materials, such as quantum dots or magnetic nanoparticles, the composite nanospheres can become multi-functionalized. In order to increase the functionality arising from the incorporated nanoparticles in the silica spheres, it is of interest to achieve high loading levels of nanoparticles in each silica sphere. At present, there are only a limited number of studies reported where particles have been formed with both a high loading level of particles and a relatively small particle size, i.e., ~ 50 nm. In this work, we investigated the preparation of monodispersed silica-coated ZnO composite nanoparticles having high loading levels of ZnO crystallites.

II. EXPERIMENTAL DETAILS

A. Synthesis of poly-vinyl pyrrolidone (PVP)-capped ZnO nanoparticles

Poly-vinyl pyrrolidone (PVP) are used as a size-limiting agent and a dispersant for the formation of ZnO nanoparticles. First, 0.27g of Zn(Ac)2•2H2O was dissolved into ethanol and heated to 60℃ under constant stirring for 2 hours. Then, 0.5 g of PVP was added to the solution. After the PVP was dissolved in ethanol, the solution was cooled down to room temperature. A NaOH solution was prepared separately by dissolving 0.27 g of NaOH·H2O in 50 ml of ethanol at room temperature in an ultrasonic bath. The NaOH solution was added dropwise into the Zn(Ac)2 solution under constant stirring to form PVP-capped ZnO nanoparticles. The solution mixture was then stirred continuously at room temperature for up to 2 hours. The PVP-capped ZnO particles were flocculated from ethanol by the addition of hexane (30 mL of hexane/10 mL of reaction mixture) and subsequently separated by centrifugation at 6000 rpm for 10 min. Then the nanoparticles were washed with ethanol 3 times and redispersed in water.

B. Synthesis of silica-coated ZnO nanoparticles (SCZNs) The PVP-capped ZnO nanoparticles were introduced to the

reverse microemulsion medium for silica coating. In the water-in-oil microemulsion system, cyclohexane was used as the oil phase with polyethylene glycol octylphenyl ether (Triton X-100) and n-hexanol as the surfactants. Tetraethyl orthosilicate (TEOS) was used to form silica shells and NH4OH was used as a polycondensation initiator. The effects of the amounts of surfactants and NH4OH on the morphology of particles were studied using different synthesis conditions, as shown in Table 1. First, Triton X-100 and n-hexanol were dispersed in cyclohexane (7.5 ml) by sonication. Then PVP-capped ZnO dispersion (2 mM in water) was added to the solution. The resulting mixture was vortexed, and NH4OH was added to form a transparent solution of reverse microemulsion. Finally, 100 l of TEOS was added and the reaction was continued for 24 hours. The resulting silica-coated ZnO composite nanoparticles were collected by centrifuging, followed by washing and redispersion in ethanol or deionized water. The whole process was carried out at room temperature.

C. Characterization The morphologies and structures of the samples were

investigated by transmission electron microscopy (TEM) on a JEM-2100 with an acceleration voltage of 200 kV. The optical absorption spectra of the samples were obtained using a Varian Cary 3E UV/Vis spectrophotometer. Fourier transform infrared (FT-IR) spectra were obtained with a Bruker Vertex 70 FT-IR Spectrophotometer, using the KBr method. Thermogravimetric analysis (TGA) was carried out using a Netzsch 409PC TG-DSC instrument under a constant air flow in the temperature

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range from room temperature to 530℃ at a heating rate of 10 ℃•min-1. D. Photocatalytic activity test

Rhodamine B (RhB) was used as a probe molecule to evaluate the photocatalytic activity of uncoated ZnO (PVP-capped ZnO) and silica-coated PVP-capped ZnO nanocomposites (SCZNs) in response to UV and visible light irradiation. The characteristic optical absorption peak of RhB at 554 nm was chosen to monitor the photocatalytic degradation process. The experiment was carried out according to the following procedure: First, 8 mg of dried uncoated ZnO powder was dispersed in 50 ml of RhB aqueous solution in a 100 ml beaker. The suspension was stirred in the dark for 1 hour to ensure the establishment of the adsorption and desorption equilibrium of RhB on particle surfaces. Subsequently the suspension was irradiated with simulated sunlight using a Suntest instrument (CPS+ ATLAS) equipped with a 150 W xenon lamp and a quartz filter. At given intervals, 3 ml of the suspension was extracted and then centrifuged at 6000 rpm for 10 min to separate nanoparticles from the supernatant. The UV/Vis absorbance spectra of the supernatant were measured using a Varian Cary 3E spectrophotometer.

III. RESULTS AND DISCUSSION

A. PVP-capped ZnO(uncoated ZnO) nanoparticles before silica coating Fig. 1 shows a TEM micrograph of the uncoated PVP-

capped ZnO after aging for 24 hours. It is evident that the uncoated ZnO particles had nearly spherical morphologies. The average diameter, that was estimated from the image analysis of TEM micrographs, was approximately 6.3 nm with a narrow size distribution. The corresponding high-magnification TEM image (inset of Fig. 1) revealed that lattice fringes run in the same direction across the whole particle, indicating that the particles were single crystals. B. Silica-coated PVP-capped ZnO nanoparticles (SCZNs)

Only 4 reaction conditions out of 9 in Table 1 resulted in discrete nanoparticles. The rest of the reaction conditions led to the formation of large porous structures. The TEM images of the particulate samples prepared under the reaction conditions 4, 5, 8 and 9, are shown in Fig. 2. As can be seen in Fig. 2A, the sample prepared under the condition 4 consisted of near-spherical particles of 50 – 100 nm in diameter. However, the particles did not appear to have a core/shell structure and the particle surface was not smooth.

When the concentration of NH4OH increased from 0.25 wt% (condition 4) to 0.5 wt% (condition 5), highly monodispersed spherical particles that had a core/shell

Figure 1. Transmission electron micrograph of PVP-capped ZnO

nanoparticles. A high resolution image of a particle is also shown in the inset.

structure were observed (Fig. 2B). All the ZnO nanoparticles appeared to be individually dispersed in the silica matrix and the nanospheres had shells consisting only of silica. TEM image analysis revealed that the nanospheres were quite uniform in size with an average diameter of 48.4 ± 2.1 nm and a size standard deviation of ~ 4.1%. The surface of the nanospheres appeared quite smooth.

When the water-to-surfactants ratio was increased from 10.8 (condition 5) to 11.2 (condition 8) while the amount of ammonia concentration was fixed, hollow structures appeared inside of the particles (Fig. 2C). The shape of the particles became less spherical but the smoothness of the particle surface appeared unchanged. When the amount of ammonia concentration was further increased from 0.5 wt% (condition 8) to 0.75 wt% (condition 9) while the water-to-surfactants ratio was fixed, the particle shape became more spherical but hollow structures remained inside of the particles (Fig. 2D).

TABLE 1. COMPOSITION OF MICROEMULSIONS AND RESULTING

MORPHOLOGY.

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Figure 2. Transmission electron microscopy images of silica-coated ZnO nanoparticles prepared using different concentrition of surfactants and

NH4OH; (A) condition 4, (B) condition 5, (C) condition 8 and (D) condition 9.

The reaction condition 5 gave the best ZnO-core /silica-shell structure. Hence further investigation was conducted using the samples prepared under the reaction condition 5 only.

To prove that the dark spots inside the particles in Fig. 2B are in fact ZnO crystallites, TEM dark field imaging and X-ray powder diffraction (XRD) analyses were performed. Fig. 3A shows the dark field version of the bright field micrograph in Fig. 3B. The dark filed micrograph showed the crystalline ZnO nanoparticles as bright spots due to the diffraction of electrons by the crystallites. The XRD spectra of both uncoated and silica-coated ZnO showed the diffraction peaks that correspond to the Wurtzite ZnO (JCPDS Card No. 89-1397). These results indicated that the coating procedure did not affect the crystal structure of ZnO.

Fig. 4 shows FT-IR spectra of uncoated ZnO, pure SiO2 and SCZNs nanoparticles. In the spectrum of uncoated ZnO nanoparticles (curve a), the peaks labeled with 1-4 that are located at 2930, 1675, 1420, and 1290 cm-1, are assigned to CH2 asymmetrical stretching vibration, C=O stretching vibration, CH2 bending vibration and C-N stretching vibration band of PVP, respectively [5, 6]. In the spectra of SCZNs (curve c), the peak position of C=O stretching band in PVP shifted from 1675 cm-1to 1653 cm-1, indicative of the formation of intermolecular hydrogen bonds between silica and PVP.

Figure 3. Transmission electron microscopy bright field (A) and dark field (B) images of silica-coated ZnO composite nanoparticles that were prepared under

the condition 5.

Figure 4. Infrared spectra of uncoated ZnO nanoparticles (a), silica

nanoparticles (b) and silica-coated ZnO composite nanoparticles (c).

The intensity of the CH2 symmetrical and unsymmetrical stretching vibration of the SCZNs became weaker compared to that of uncoated ZnO, which may be caused by the low content of PVP in SCZNs. The FT-IR results indicated that TEOS was hydrolyzed on PVP molecule. This result is consistent with early findings that the amphiphilic character of PVP enables the coupling (and thus deposition) of silica monomers or oligomers to ZnO surfaces [8].

In order to estimate the amount of ZnO in SCZNs composite particles, thermogravimetric analysis (TGA) was carried out. Fig. 5 shows the TGA curves of uncoated ZnO and SCZNs. Both coated and uncoated samples showed a slight weight loss at ~ 120 ℃ due to the removal of adsorbed water.

Figure 5. TGA curves of uncoated ZnO nanoparticles (a) and silica-coated

ZnO composite nanoparticles (b).

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Figure 6. Absorption spectra of Rhodamine B solution as a function of ultraviolet light irradiation time in the presence of uncoated (A) and silica-coated (B) ZnO nanoparticle dispersion.

The significant weight loss in both samples in the temperature range between 300 and 450 ℃ was attributed to the decomposition of PVP [7]. The weight loss of uncoated ZnO particles was 9% at ~120 ℃ and 48% between 300 and 450 ℃ and the remaining weight is assigned to ZnO. Hence the weight ratio between PVP and ZnO in the uncoated sample was 48% : 43%. SCZNs showed a weight loss of 9% between 300 and 450 ℃, which corresponds to the amount of PVP in SCZNs. Assuming that the weight ratio between PVP and ZnO remained the same before and after silica coating, the amount of ZnO in SCZNs is calculated to be 8 wt%.

Fig. 6 displays the absorption spectra of RhB aqueous solutions during the irradiation of simulated sunlight in the presence of uncoated ZnO and SCZNs. For uncoated ZnO, the characteristic absorption peaks of RhB became weaker as the irradiation time was prolonged, and disappeared almost completely after irradiation for 120 min (Fig. 6A). Fig. 6B shows that SCZNs had much lower efficiency to photodegrade RhB than uncoated ZnO nanoparticles. In the presence of uncoated ZnO, 96.7% of the dye was degraded at an irradiation time of 120 min, while there was only 22.9% degradation for SCZNs. Thus the coating of silica on the surface of ZnO nanoparticles could markedly suppress the photodegradation of the RhB solution under the irradiation of simulated sunlight.

IV. CONCLUSION In this study, monodispersed silica-coated ZnO

composite nanoparticles with high loading levels of ZnO nanoparticles were successfully prepared using a reverse microemulsion method. Photodegradation experiments demonstrated that the silica coating can greatly decrease the photocatalytic activity of ZnO nanoparticles. Consequently, reduced cytotoxicity could also be expected from the silica-

coated ZnO nanoparticles, which is of particular importance for the use of ZnO in personal-care products.

REFERENCES [1] L. Sun, J. A. Rippon, P. G. Cookson, O. Koulaeva, and X. G. Wang,

‘‘Effects of undoped and manganese-doped zinc oxide nanoparticles on the colour fading of dyed polyester fabrics’’ , Chem. Eng. J. Vol 147, pp. 391-398, 2009.

[2] (a)A. Becheri, M. Dürr, P. L. Nostr, and P. Baglioni, “Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers” , J. Nanopart. Res., Vol. 10, pp 679–689, 2008. (b)R. H. Wang, J. H. Xin, and X. M. Tao, “UV-blocking property of dumbbell-vshaped ZnO crystallites on cotton fabrics” , Inorg. Chem. Vol 44, pp 3926-3930, 2005.

[3] R. Dunford, A. Salinaro, L. Cai, N. Serpone, S. Horikoshi, H. Hidaka, and J. Knowland, “Chemical oxidation and DNA damage catalysed by inorganic sunscreen ingredients” , FEBS Lett. Vol 418, pp 87–90, 1997.

[4] (a)Y. Lu, Y. D. Ying, B. T. Mayer, and Y. N. Xia, “Modifying the surface properties of superparamagnetic iron oxide nanoparticles through a sol-gel approach” , Nano Lett. Vol 12, pp.183-186, 2002. (b) T. Selvan, S. T. Tan, and J. Y. Ying, “Robust, non-cytotoxic, silica-coated CdSe quantum dots with efficiency photoluminacence”, Adv. Mater. Vol 17, pp. 1620-1625, 2005. (c) D. K. Yi, T. Selvan, S. S. Lee, G. C. Papaefthymiou, D. Kundaliya, and J. Y. Ying. “Silica-coated nanocomposites of magnetic nanoparticles and quantum dots ” , J. Am. Chem. Soc. Vol. 127, pp. 4990-4491, 2005.

[5] X. M. Sui, C. L. Shao, Y. Liu, and C. S. Xu, “Structural and photoluminescent properties of ZnO hexagonal nanoprisms synthesized by microemulsion with polyvinyl pyrrolidone served as surfactant and passivant”, Chem. Phys. Lett., Vol. 424, pp 340-344, 2006.

[6] S. Maensiri, and V. Promarak, “Synthesis and optical properties of nanocrystalline ZnO powders by a simple method using zinc acetate dihydrate and poly(vinyl pyrrolidone)”, J. Cryst. Growth, Vol. 289, pp 102-106, 2006.

[7] Q. Zhang, J. Ge, and Y. D. Yin, “Permeable Silica Shell through Surface-Protected Etching”, Nano Lett., Vol. 8, pp 2867-2871, 2008.

[8] H. Zou, S. Wu, Q. P. Ran, and J. Shen “A Simple and Low-Cost Method for the Preparation of Monodisperse Hollow Silica Spheres”, J. Phys. Chem. C, Vol. 112, pp 11623–11629, 2008.

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