Electron Beam Assisted Gas Phase Synthesis of SiO2 Nanoparticles in an Ambient Condition

This study presents a novel use of an electron beam irradiation system for the gas phase synthesis of nanoparticles, and demonstrates that SiO2 nanoparticles can be synthesized in ambient conditions. The formation of SiO2 nanoparticles is confirmed by the vibrational modes shown in FT-IR spectra, and the binding energy of Si 2p in the X-ray photoelectron spectrum. In this work, nanoparticles with average diameters of 210 nm and 73 nm were produced. The average particle size was controlled by adjusting the residence time of the precursor vapor.

SiO 2 nanoparticles have been applied to various fields such as chemical mechanical polishing and additives to drugs, cosmetics, printer toners, varnishes and food (Wang et al., 2009).In recent times, the use of SiO 2 nanoparticles has been extended to a host of biomedical and biotechnological applications including cancer cell imaging, ultrasensitive single bacterium detection, barcoding tags, gene/drug delivery, and DNA microarray detection (Wang et al., 2008).
In this study, a new gas phase synthesis route of SiO 2 nanoparticles is proposed.The new experimental system utilizes an electron beam source for the decomposition of the gaseous precursor material tetraethylorthosilicate (TEOS) for the formation of SiO 2 nanoparticles.Previously metal nanoparticles were produced by direct irradiation of electron beam onto ionic liquid precursors at room temperature (Tusda et al., 2012).Recently, electron beam accelerator has also been used for water treatment and air purification (Strokin, 2007;Won et al., 2002).
When an electron beam irradiation system is used for the gas phase synthesis of nanoparticles, it is expected to have the following advantages: (1) It is possible to manufacture nanoparticles in an ambient condition similar to the existing gas phase method using an electric furnace or a diffusion flame burner.(2) High electron beam energy enables us to choose a variety of precursors for the material synthesis.In addition, the gas phase synthesis method, in comparison with the liquid phase, has many advantages which include higher purity, greater throughputs, the ability to produce nanoparticles in continuous rather than batch mode, and avoiding the need to manage environmentally hazardous solvents (Girshick, 2008).

EXPERIMENTAL
A schematic of the gas phase synthesis system of SiO 2 nanoparticles using the electron beam, consisting of an electron beam reaction chamber for the dissociation of the precursor, a bubbler where the liquid TEOS is passed through by a nitrogen (99.9%) flow to create a TEOS enriched gas flow, and an electron accelerator (100-200 keV, EB-TECH, Model: LEB-2), is shown in Fig. 1.The transmittance property of an electron beam window is determined depending on the physical properties of a Fig. 1.Schematic of the gas phase synthesis system of SiO 2 nanoparticles using an electron beam.chamber window.In this study, Kapton foil (thickness: 7.5 μm) was used as a chamber window.The Kapton foil has high mechanical and thermal stability as well as its high transmittance to electron beam.TEOS in the liquid phase was bubbled by nitrogen (N 2 , 99.9%, 0.5 L/min).All experiments were performed in temperature-controlled room so that the TEOS delivery was stable, controllable, and predictable.At a constant bubbler temperature, the precursor feed rate was controlled by the flow rate of N 2 through the bubbler.In order to prevent the condensation of the precursor vapor, a heating tape (DAIHAN Scientific Co., Ltd, Model: WHM12314) was installed between the electron beam reaction chamber and the bubbler together with a temperature controller (Wise-Therm, Model: HM-C10D).The temperature of the vapor transport line was controlled to be at about 25°C higher than that of the bubbler at 20°C.During the experiments the precursor vapor with concentration of 8.54  10 −6 mol/L was introduced into the electron beam reaction chamber, and then exposed to the electron beam originated from an electron accelerator with operating conditions of 150 keV and 3 to 20 mA.Although the electron beam energy is much higher than the chemical binding energies of C-O and C-H bonds in TEOS (3.8 eV for C-O and 4.2 eV for C-H), the extent of dissociation of TEOS is also affected by the residence time of TEOS in the electron beam reaction chamber.The residence time of the TEOS in the electron beam reaction chamber can be controlled by adjusting the flow rate of the carrier gas as shown in Fig. 1.SiO 2 nanoparticles were collected on Si wafer substrates installed in the electron beam reaction chamber for 10 minutes using the combined effects of diffusion and thermophoresis.Particle density on the substrate was high and particle deposition was uniform.
To analyze the chemical composition of the nanoparticles, FT-IR spectra (Bruker Optic GmbH, Alpha-P) were taken for the particle samples collected on a Si substrate, and Xray photoelectron spectroscopy (XPS, Thermo-VG Scientific MultiLab 2000) was performed with a Al K X-ray source (1486.6 eV), a pass energy of 20.0 eV, and a hemispherical energy analyzer.Particles were imaged using a scanning electron microscope (SEM, Hitachi SE-4800).

RESULTS AND DISCUSSION
Using our experimental set-up we can supply sufficient electron beam energy to dissociate the TEOS gas into fragmentation species followed by the formation of SiO 2 nanoparticles.Homogeneous nucleation more likely occurs in the gas phase SiO 2 synthesis system using electron beam irradiation presented in this study.Gas-phase intermediate species with very low vapor pressures form liquid-like droplets by homogeneous nucleation that polymerize in the gas-phase to become silica-like particles (Okuyama et al., 1997).
Fig. 2(a) shows the FTIR spectra of the SiO 2 nanoparticles synthesized with 6 mA, 16 mA, and 20 mA electron beams, which were generated by an electron beam accelerator under operating conditions of 150 keV.The three spectra are very similar to each other.The major FTIR signal near 1085 cm −1 is related to Si-O-Si stretching mode (Hu et al., 2003).And the peaks at 960 cm −1 correspond to the Si-OH stretching mode (Buso et al., 2008).The peaks near 600 cm −1 are connected with the gas phase intermediates, although the identity of the peak is unknown (Adachi et al., 1999).As shown in Fig. 2 2) are very similar to those in Fig. 2(a).Comparison of the spectrum (1) with the spectrum (2) indicates that that there are no intermediates on the particle surface at the relatively high electron beam current of 16 mA despite that the residence time was decreased by a factor of two.Also, the comparison of the spectrum (3) with the spectrum (1) shows that when the carrier gas with 0.5 L/min was introduced the electron beam current of 3 mA is insufficient for SiO 2 nanoparticles to be formed, compared to 16 mA.
A high resolution XPS scan for the nanoparticle samples, generated by electron beam with a current of 16 mA and a voltage of 150 keV, is shown in Fig. 3.In Fig. 3 2)).chemical states on Si 2p are observed.The strongest counts peak, located at 103 eV, corresponds to Si(IV)O 2 .And the other peak on the right, located at 99 eV, corresponds to zerovalent Si.The result is in a good agreement with the literature (Mirji et al., 2006).For the XPS scan, the appearance of the peak related to Si is attributed to the Si substrate used for sampling nanoparticles.On the basis of the XPS and FTIR results, we come to a conclusion that TEOS vapor generated by the bubbler can be decomposed by the irradiated electron beam, and then SiO 2 nanoparticles were formed by nucleation and condensation from the decomposed TEOS vapor.
Fig. 4 shows SEM images of the SiO 2 nanoparticles synthesized with an electron accelerator under operating conditions of 6 mA and 16 mA.Image analysis was conducted to measure the size range of the synthesized particles using the ImageJ software.Single particles and primary particles in agglomerates were taken into account to obtain particle size distribution as shown in Fig. 4. For particles in agglomerates the particle size was measured by drawing a circle on each primary particle with a distinct boundary similar to Shin et al. (2009)'s study.As shown in Fig. 4, it was observed that the magnitude of electron beam current has no significant effect on the average diameter of SiO 2 spheres.For the same operating conditions as in Figs.4(c) and 4(d), the diameter of SiO 2 nanoparticles was measured to be in the range of 74-373 nm with an average diameter of 214 nm and a standard deviation of 64 nm.It is expected for the gas phase synthesis that decreasing residence time leads to a decrease of particles size.Thus, when the residence time of carrier gas in the reaction chamber was reduced by a factor of two, the size range was decreased to 27-212 nm.The average diameter of the SiO 2 nanoparticles was measured to be 74 nm with a standard deviation of 31 nm as shown in Fig. 5.

CONCLUSIONS
In summary, a new gas phase synthesis method of SiO 2 nanoparticles in an ambient condition was developed using the electron beam system.Electron beam irradiation was used to produce SiO 2 nanoparticles by decomposing TEOS vapor followed by nucleation and condensation of the decomposed TEOS vapor.The formation of SiO 2 nanoparticles was confirmed by the FTIR spectra, which show that Si-O-Si and Si-OH stretching mode exist.A peak corresponding to Si(IV)O 2 composition was also observed in the XPS scan results.And, when the residence time of the precursor vapor in the reaction chamber was decreased by a factor of two, TEOS vapor was not decomposed with a lower electron beam current of 3 mA while with the higher electron beam current of 16 mA, SiO 2 nanoparticles were formed.Furthermore, the average diameter of the produced particles was decreased from 210 nm to 73 nm as the residence time of the TEOS vapor was reduced by a factor of two indicating that particles size can be controlled by adjusting the residence time of the precursor vapor.
(a), C-O (near 1700 cm −1 ) and C-H (near 2900 cm −1 ) related peaks are weakly observed, indicating that both C-O and C-H bonds with the chemical binding energy of the 3.8 eV and 4.2 eV, respectively, were almost dissociated.Fig. 2(b) shows the FTIR spectra for varying carrier gas flow rates (0-0.5 L/min) and different electron beam currents (3-16 mA).The spectra (1) and (

Fig. 4 .
Fig. 4. SEM images of SiO 2 nanoparticles produced by electron beam with currents of 6 mA [a and b] and 16 mA [c and d] at an operating voltage of 150 keV, and bubbler gas with 0.5 L/min.

Fig. 5 .
Fig. 5. Size distribution of SiO 2 nanoparticles produced by electron beam with current of 16 mA at an operating voltage of 150 keV, bubbler gas with 0.5 L/min, and carrier gas with 0.5 L/min.