Aerosol and Air Quality Research, 13: 415–420, 2013 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2012.08.0230

Hollow SiO2 Nanospheres: One-Step Synthesis by Introducing Guest Ag Nanoparticles and an Irradiating Electron Beam under Ambient Condition Jin Hyoung Kim1, Minsoo Son1, Youngku Sohn2, Weon Gyu Shin1* 1 Department of Mechanical Engineering, Chungnam National University, Daejeon 305-764, Korea 2 Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Korea ABSTRACT

A novel one step gas phase synthesis method for hollow SiO2 nanospheres under ambient condition was developed using an electron beam irradiation system. Ag nanoparticles were used as a guest-template material. The inner structure of the hollow SiO2 nanospheres can be controlled by adjusting the electron beam intensity and precursor concentration. The average diameter and shell thickness of the produced particles were 56 nm and 10 nm, respectively. Keywords: Hollow SiO2 nanosphere; Electron beam irradiation; One-step gas phase synthesis. INTRODUCTION

The fabrication of hollow nanoparticles has recently attracted great attentions in that hollow nanosystems have very large specific surface area and low density (Gao et al., 2007; Du et al., 2011). Thus, hollow nanoparticles have been widely used with applications such as drug delivery (Yang et al., 2008), photocatalysis (Chen et al., 2008; Lin et al., 2011), gas sensors (Sun et al., 2012), lithium batteries (Wang et al., 2012), and solar cells (Koo et al., 2008). Especially, hollow SiO2 nanoparticles show good thermal stability and biocompatibility as contrasted with other materials. Such properties are very useful for various applications in antireflection coatings (Du et al., 2010), catalysis (Chai et al., 2004) and microvessels for drug delivery (Li et al., 2010).

In general, hollow nanoparticles have been synthesized in four different ways (Lou et al., 2008): (1) hard templating synthesis methods including layer-by-layer (LBL) method (Wang et al., 2004; Martinez et al., 2005; Yuan et al., 2011) and sol-gel process (Guo et al., 2009) (2) sacrificial templating synthesis methods using a kirkendall effect (Nakamura et al., 2007) and a galvanic replacement (Au et al., 2008) (3) soft templating synthesis methods (Zoldesi et al., 2006; Wang et al., 2007) and (4) template-free synthesis methods (Lou et al., 2006; Lam et al., 2008).

In the abovementioned methods hollow structures were formed in the liquid phase except for hard templating * Corresponding author. Tel.: 82-42-821-5647; Fax: 82-42-822-5642 E-mail address: wgshin@cnu.ac.kr

synthesis methods using layer-by-layer (LBL) method (Martinez et al., 2005), a sacrificial templating method using a kirkendall effect (Nakamura et al., 2007) and a template-free synthesis method using Metal organic chemical vapor deposition (MOCVD) (Lam et al., 2008). However, those three methods also require multiple steps for the preparation of precursor solutions.

The templating synthesis methods involve the use of various removable or sacrificial templates, including hard ones such as CaCO3 microparticles (Yuan et al., 2011) or Polymethyl methacrylate (PMMA) beads (Wang et al., 2004) and latex microspheres (Martinez et al., 2005) as well as soft ones, for example, silicone oil-in-water emulsion droplets (Zoldesi et al., 2006) and phenyltrimethoxysilane (PTMS) droplets (Wang et al., 2007). In order to remove template materials, various methods such as using EDTA solution (Yuan et al., 2011), heating or UV irradiation (Wang et al., 2004; Martinez et al., 2005), calcination of the precursors at the elevated temperature (Guo et al., 2009) were used. The template synthesis methods have a drawback that the size of the hollow shell is greatly dependent on the dimensions of template materials. On the other hand, template free synthesis method can be used to make the size of the hollow metal shell smaller (Lam et al., 2008).

Hollow structures prepared from templating routes usually suffer from disadvantages related to high cost and tedious synthetic procedures, which may prevent them from being used in large-scale applications. Ideally, one would prefer a one step template-free synthesis method for controlled preparation of hollow structures in a wide range of sizes. But, to our knowledge there are almost no previous studies on the one step template free synthesis method. Lou et al. (2006) reported a simple one-pot template-free synthesis of SnO2 hollow nanostructures, based on an unusual insideout

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Ostwald ripening mechanism. Lam et al.(2008) developed a template free nano-wrapping with a four-step mechanism for the synthesis of copper hollow nanoparticles with the smallest size of 15 nm using MOCVD. Overall, template free synthesis methods reported in the literature also require multiple steps for the formation of hollow structure, which is not preferred for the mass production of nanomaterials.

In this study, a new one-step gas phase synthesis process of hollow SiO2 nanoparticles using an electron beam irradiation system is presented. In order to synthesize hollow SiO2 nanoparticles, tetraethylorthosilicate (TEOS) and Ag nanoparticles were chosen as a shell and a guest template material, respectively. In our method, silver nanoparticles are used as template materials and continuously irradiated by uniform electron beam until they exit a reaction chamber so that the template materials can be evaporated resulting in the hollow silica nanoparticles. It is thought that other kind of metal nanoparticles can be used as a guest template material as long as the metal nanoparticles can evaporate under continuous electron beam irradiation. Using an electron beam irradiation system for the gas phase synthesis of hollow nanoparticles has several advantages over liquid phase synthesis processes: (1) Hollow nanoparticles can be continuously produced under an ambient condition through one-step process (2) A variety of precursors can be chosen as materials of a shell structure using high electron beam energy (3) Since the process is carried out in gas phase, it is not necessary to manage environmentally hazardous solvents (Girshick, 2008).

EXPERIMENTAL

Fig. 1 shows a schematic diagram of the gas phase synthesis system of hollow SiO2 nanospheres using an electron accelerator. This system consists of an electron accelerator (100–200 keV, EB-TECH, Model: LEB-2), an electron beam reaction chamber, and a bubbler to introduce reactant materials into the electron beam reaction chamber.

The transmittance property of electron beam is adjustable with the physical properties of a chamber window. In this study, Kapton foil (thickness: 7.5 μm) was used as a chamber window. In order to synthesize hollow SiO2 nanospheres, a mixture of TEOS liquid and Ag nanoparticles was prepared. Ag nanoparticles (Sigma Aldrich, < 100 nm) were well dispersed in the 40 mL TEOS liquid under 20 minutes ultrasonication. Using the bubbler both TEOS vapor and Ag nanoparticles were introduced into an electron beam reaction chamber with nitrogen gas (N2, bubbler) as a carrier. A heating tape (DAIHAN Scientific Co., Ltd, Model: WHM12314) was installed to prevent the condensation of the TEOS vapor on the transport line located between the electron beam reaction chamber and the bubbler. The temperature of the vapor transport line was maintained at 25°C higher than that of the bubbler during the experiments. As TEOS vapor and Ag nanoparticles enter the electron beam reaction chamber, they are exposed to the electron beam. In our experiments, electron beam was irradiated to the electron beam reaction chamber and the electron beam was generated by the electron accelerator operated at the electron beam voltage of 150 kV and the electron beam current of 6 to 15 mA.

The mixture of the TEOS vapor and Ag nanoparticles introduced to the electron reaction chamber was affected by electron beam energy for 12–24 seconds depending on the experimental conditions. The electron beam energy (150 keV) is much higher than the binding energies of C-O (3.8 eV) and C-H (4.2 eV). As soon as exposed to the electron beam, the TEOS vapor dissociates and then reacts on the surface of Ag nanoparticles creating a SiO2 surface layer onto Ag nanoparticles. Then, silver nanoparticles are removed from Ag-SiO2 core-shell nanoparticles by electron beam energy as particles flow down the chamber. During the process, silver nanoparticles play a role as a guest-template. In addition, the extent of dissociation of TEOS is also affected by the residence time of TEOS in the electron beam reaction chamber and the operating conditions of electron accelerator. The residence time and concentration of the

Fig. 1. Schematic diagram of the gas phase synthesis system of hollow SiO2 nanospheres using electron beam.

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TEOS vapor and Ag nanoparticles in the electron beam reaction chamber can be controlled by adjusting the flow rate of the bubbler gas and the dilution gas (N2, dilution).

Nanoparticle samples were collected on a piece of stainless steel mesh (200 mesh size) with 6 mm diameter placed downstream of the electron beam reaction chamber. For analysis by Field Emission Transmission Electron microscope (FE-TEM) and Energy Dispersive Spectrometer (EDS), the collection stainless mesh was immersed in ethanol and the hollow nanoparticles were dispersed under 20 minutes ultrasonication. A sample for imaging was prepared by dropping a few drops of the dispersed solution on a lacey carbon-coated Cu grid. A Tecnai G2 F30 FE-TEM operating at 300 kV was used for FE-TEM/EDX. RESULTS AND DISCUSSION

During the synthesis process of hollow SiO2 nanospheres Ag nanoparticles are coated with SiO2 and then Ag core is evaporated creating a hollow structure. Thus, according to the degree of SiO2 coating on Ag core two different mechanisms for the formation of hollow spheres are suggested: (1) Ag nanoparticles are not fully coated with SiO2 and then Ag nanospheres are completely evaporated as they are exposed to electron beam. Downstream the reaction chamber, the surface of hollow SiO2 nanospheres becomes non-porous by the additional reaction of the decomposed TEOS vapor on the particle surface. (2) Ag nanoparticles are fully coated with SiO2, and then Ag nanoparticles are evaporated by electrons which penetrate into the SiO2 layer, and the Ag vapor finally diffuses outside the SiO2 layer creating a hollow structure. Also, downstream the reaction chamber the decomposed TEOS vapor can diffuse into the hollow spheres depending on experimental conditions.

The temperature of the core-shell nanoparticles can be increased by increasing electron beam energy. Since the temperature rise depends on the type of materials, the core and shell materials will show different amounts of temperature rise. The temperature rise by electron beam is related to the absorbed dose of the electron beam and the specific heat capacity of the material. It can be expressed as ΔT = D/c (°C), where ΔT is the temperature rise (°C), D is the average dose (kGy), and c is the specific heat capacity of materials (J/g-°C) (Cleland et al., 2003). Accordingly, the temperature rise depends on the specific heat capacity of a material for a given absorbed dose.

The specific heat capacities of silver and silica are 0.235 J/g-°C and 0.741 J/g-°C, respectively. The temperature rises are 4.26°C for the silver and 1.41°C for silica per 1 kGy for the case of bulk materials. In our experiments, the absorbed doses of the electron beam were 57.4 kGy/s for the electron beam current of 15 mA and 23.0 kGy/s for that of 6 mA as the electron beam accelerator was operated at 150 kV. Silver nanoparticles used as a template experienced temperature rises of 224 °C/s for 15 mA and 97.6 °C/s for 6 mA, and SiO2 experienced temperature rises of 77.3 °C/s for 15 mA and 30 °C/s for 6 mA. Melting temperatures of Ag and SiO2 are 961.7°C and 1725°C for bulk materials, respectively. When the mixture of the TEOS vapor and Ag

nanoparticles is affected by electron beam for 12 seconds, the temperature of Ag nanoparticles can be increased up to 2930°C for 15 mA and up to 1172°C for 6 mA exceeding the melting temperature. The temperatures of SiO2 are also increased until 928°C for 15 mA and 371°C for 6 mA, but do not reach the melting temperature. For this reason, Ag nanoparticles are evaporated fast within a very short time, but SiO2 shell still remains. Although it is difficult to know the exact evaporation temperature of silver nanoparticles, the evaporation temperature of nanoparticles is much lower than bulk material owing to large surface-to-volume ratio of nanoparticles (Nanda et al., 2002).

Fig. 2 shows the TEM images of the hollow nanoparticles synthesized with electron beam according to the experimental conditions. As shown in Fig. 2, the hollow particles have spherical morphology, and the Ag nanoparticles used as a guest-template material are not seen. In Fig. 2(a), a complete cavity is seen inside the hollow nanoparticles. In Fig. 2(b) and 2(c), the cavity is partially filled with SiO2. In Fig. 2(d), the cavity of the particles disappeared and the compact spherical SiO2 particles were formed. It indicates that the hollow structure can be controlled by varying experimental conditions. It was found that the hollow structure can be affected by electron beam intensity (compare a and b, c and d) and precursor concentration (compare a and c, b and d). It was found that (1) for a given TEOS vapor concentration inside the electron beam reaction chamber, the cavity volume inside the produced particles decreases as the electron beam intensity decreases (2) for a given electron beam intensity inside the electron beam reaction chamber, the cavity volume of the produced particles decreases as the TEOS vapor concentration increases. Even after hollow nanoparticles are generated, the dissociated TEOS vapor can still react on the surface of hollow nanoparticles depending on experimental conditions. Thus, the cavity volume of the produced particles can decrease as SiO2 material diffuses into hollow SiO2 nanospheres.

Fig. 3 shows an overview of the mechanism suggested in this study. Ag nanoparticles and TEOS vapor generated from the bubbler are introduced into electron beam reaction chamber. TEOS vapor dissociated by the electron beam reacts on the surface of Ag nanoparticles creating a SiO2 surface layer onto Ag nanoparticles. Then, Ag nanoparticles are removed from Ag-SiO2 core-shell structures by electron beam as particles flow down the chamber. In Figs. 3(a)–3(d) show the structures of the hollow SiO2 nanoparticles can be varied depending on an experimental condition. The volume and structure of cavity inside the particles can be controlled by adjusting electron beam intensity and precursor concentration, as seen in Fig. 3.

Image analysis was conducted to measure the shell thickness and particle size range of the hollow SiO2 nanospheres synthesized under the experimental condition (a) in Fig. 2 using the Image J software. Fig. 4 shows the size distributions of outer diameter and inner diameter of hollow SiO2 nanoparticles. The shell thickness was measured to be in the range of 6–19 nm with an average thickness of 10 nm, and the particle sizes in the range of 35–138 nm with an average diameter of 56 nm.

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(a) (b)

(c) (d)

Fig. 2. TEM images of the hollow SiO2 nanoparticles generated in each experimental condition; (a) voltage: 150 kV, current: 15 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0.5 L/min. (b) voltage: 150 kV, current: 6 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0.5 L/min. (c) voltage: 150 kV, current: 15 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0 L/min. (d) voltage: 150 kV, current: 6 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0 L/min.

Fig. 3. Mechanism of one step gas phase synthesis system of SiO2 hollow nanoparticles under ambient condition using electron beam and possible structures of the hollow SiO2 nanoparticles depending on an experimental condition.

An EDX line scan image of a hollow nanoparticle

synthesized under the experimental condition (a) and (b) in Fig. 2 is shown in Fig. 5. In Fig. 5, both silicon and oxygen are seen over the particle. But, the central area of the hollow nanoparticle shows lower counts in Fig. 5 because it is empty,

as shown in the EDX spectra of typical hollow nanoparticles. In addition, Ag materials used as the guest-template material were not detected by EDX. It confirms that all the evaporated Ag nanoparticles passed through the reaction chamber as a gas vapor.

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(a) (b)

Fig. 4. Size distributions of (a) outer diameter and (b) inner diameter (voltage: 150 kV, current: 15 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0.5 L/min).

Fig. 5. EDX line scan result of a hollow SiO2 nanoparticle; voltage: 150 kV, current: 15 mA, N2, bubbler: 0.5 L/min, N2, dilution: 0.5 L/min.

CONCLUSIONS

In summary, a new one-step gas phase synthesis method of achieving hollow SiO2 nanospheres in an ambient condition was developed using an electron beam irradiation system. The Ag nanoparticles used as a guest-template material were selectively removed by electron beam induced heating of the material and the elimination of the Ag nanoparticles was confirmed through the EDX line scans and TEM images. By changing electron beam intensity and precursor concentration with other conditions fixed, the size and the cavity of the nanospheres can be controlled. The average diameter and shell thickness of the produced particles were 56 nm and 10 nm, respectively. This method can be further extended to other hollow nanosystems, and be an extremely useful technique for ultrapure large-scale synthesis of hollow nano-oxides. The mass production rate of our method can be reached up to several grams per hour similar to that of flame synthesis method. In addition, we show a new mechanism of the formation of hollow nano-structures. ACKNOWLEDGMENTS

This study was financially supported by research fund of Chungnam National University in 2011. The authors

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Received for review, August 30, 2012 Accepted, November 9, 2012