One-step flame method for the synthesis of coated composite nanoparticles
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- Sheen, S., Yang, S., Jun, K. et al. J Nanopart Res (2009) 11: 1767. doi:10.1007/s11051-009-9596-z
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A simple in situ flame coating method has been developed by designing a new type of coflow diffusion flame burner having a sliding unit. The sliding unit was shown to be very effective in finding a right position where the precursor for coating layer should meet with core particles. SiO2-coated TiO2 nanoparticles were first prepared and whether most surfaces of particles were coated was examined by both direct observation of particles through a transmission electron microscope and Zeta potential measurements. Mean core sizes varied from 28 to 109 nm and mean coating thickness was about 2.4 nm for silica-coated titania particles. By simply changing chemical precursors, we demonstrated that SiO2-coated SnO2, SnO2-coated TiO2, SiO2–SnO2-coated TiO2 nanoparticles could be also synthesized.
KeywordsFlame methodAerosol coatingCoated particlesComposite particles
Synthesis of composite nanoparticles attracts much attention since composite nanoparticles may not only exhibit unique properties that could not be expected from each constituent of the composites, but may also be necessitated for ensuring the stability of particles. In particular, coated composite nanoparticles have shown enhanced properties compared to non-coated particles. For example, more efficient photoluminescence and stability were reported from silica-coated CdSe quantum dots (Selvan et al. 2005) and silica-coated magnetic particles were shown to be more easily dispersed in organic and aqueous solutions (Santra et al. 2001). Layer by layer assembly of silica-coated Fe3O4 nanoparticle was proposed and coated magnetic particles showed reduction of cooperative magnetization switching between particles due to the barrier effect of silica layer (Aliev et al. 1999).
Liquid phase methods were mostly used to produce oxide-coated oxide nanoparticles. Water-in-oil microemulsion method was applied to produce silica-coated ceria nanoparticles for cosmetic applications (Tago et al. 2003) and silica-coated iron oxide nanoparticles for magnetic application (Santra et al. 2001). Precipitation was also used to produce silica-coated titania nanoparticles (Lin et al. 2002). Gas phase methods were also developed. Microwave plasma process was reported to prepare various coated composite oxide nanoparticles (Szabó and Vollath 1999). A continuous flow hot wall reactor was utilized to produce silica or alumina-coated titania particles (Fotou and Kodas 1997; Powell et al. 1997). Lee et al. (2002) also used a tubular furnace for the synthesis of silica-coated titania nanoparticles. Counterflow diffusion flame was used to form TiO2/SiO2 composite nanoparticles and depending on the flame condition, SiO2-coated TiO2 particles were obtained (Hung and Katz 1992). A premixed flat flame burner was also utilized to generate SiO2/TiO2 particles including coated particles (Ehrman et al. 1998). Recently, a coflow diffusion flame reactor was employed to prepare silica-coated titania nanoparticles (Teleki et al. 2005). Flame aerosol method has the capability to continuously produce high purity nanoparticles at mass quantity scale and its target is now moving toward the large scale production of size and crystalline phase controlled nanoparticles (Jiang et al. 2007), as well as more sophisticated composite nanoparticles (Stark and Pratsinis 2002; Akurati et al. 2006). Previous attempts for the synthesis of coated oxides in flames used the difference of reaction rates between mixed precursors. Therefore, this kind of approach has limitation for choosing precursors.
Here, we report a robust one-step flame method for the synthesis of versatile kinds of coated oxide nano-composite particles. We designed a new type of coflow diffusion flame burner having a sliding unit and demonstrated that this design could produce different kinds of oxide-coated oxide nanoparticles by simply changing chemical precursors. The sliding unit was shown to be very effective in finding a right position where the precursors for coating layer should meet with the core particles growing from the lower position near the burner surface. SiO2-coated TiO2 nanoparticles were first prepared and the particle sizes and the coating layer thickness were examined. The direct observation of a few hundred particles via a transmission electron microscope together with the comparison of Zeta potential measurements confirmed that most surfaces of particles were coated. Fourier transform infrared spectroscopy and X-ray Photoelectron Spectroscopy were also done to study the bonding between the coating layer and core particle. We also demonstrated that SiO2-coated SnO2, SnO2-coated TiO2, and SiO2–SnO2-coated TiO2 nanoparticles could be synthesized by simply changing chemical precursors.
SiO2-coated TiO2 particles, SiO2-coated SnO2 particles, and SnO2-coated TiO2 particles were synthesized from SiCl4, TiCl4, and SnCl4 precursors. These three liquid precursors were contained in bubblers. A bubbler containing SiCl4 liquid was maintained at 20 °C, whereas bubblers for TiCl4 and SnCl4 were kept at 60 °C. Carrier gas (N2) flow rates for the three precursors were 200 cc/min each. Carrier gases were assumed to be saturated with the precursor for given temperature and carrier gas flow rates (Choi et al. 2002, Akhtar et al. 1991). In order to control the titania core particle size, the feeding rate of TiCl4 was changed from 6.2 × 10−4 to 4.7 × 10−5 mol/min and the feeding rate of SiCl4 was fixed as 2.7 × 10−3 mol/min. In the case of SnO2- and SiO2-coated TiO2, vaporized SiCl4 and SnCl4 gases carried by N2 (150 cc/min, each) were mixed before being injected through the coating unit. Synthesized nanoparticles were characterized by a transmission electron microscope (TEM), Philips CM-20 operated at 200 kV, high resolution transmission electron microscope (HR-TEM), JEOL JEM-3000F operated at 300 kV, selected area electron diffraction (SAD) attached to CM-20, energy dispersive X-ray analysis (EDS) attached to CM-20, and JEM-3000F. Particles in the flame were sampled onto formvar-coated nickel grid by a local thermophoretic sampling device. The local thermophoretic sampling device consists of a sampling probe holding a TEM grid, a shield covering the TEM grid, two air cylinders, and three timers to control the insertion times of the probe and the shield independently (Choi et al. 1999; Cho and Choi 2000). The shield was designed to retract itself to expose the TEM grid only for the duration of sampling at the local position within a flame. Particles were also thermophoretically sampled onto a water-cooled quartz tube (6 mm in diameter) for zetapotential analysis to confirm the uniformity of particle coating. TEM samples were also made from the collected powder and particle morphology was double checked. Zeta-potential of particles was measured using a ZetaPlus potential analyzer (Brookhaven Instruments). The collected powder was dispersed in aqueous 0.01 M NaCl solutions and the colloidal solutions were treated in an ultrasonic bath for more than 30 min to break up powders into non-agglomerated particles. The pH values of the solutions were adjusted by adding NaOH (1.0 N solution in water, Aldrich) or HCl (1.0 N solution in water, Aldrich). The crystalline phase of coated particles was determined by X-ray powder diffraction (MXP18XHF-22, MAC/Science) using CuKα radiation (λcu = 1.54056 Å). Voltage and current outputs for measurements are 40.0 kV and 200.0 mA, respectively. Samples were measured at a step width of 0.02° and scanning speed was 0.5° or 2.0°/min. The chemical bonding of coated nanoparticles was examined by FTIR spectroscopy using a spectrometer (DA 8, Bomen). The powders were diluted with KBr (99+%, Aldrich) and the spectra were collected with a spectral resolution of 2 cm−1. X-ray Photoelectron Spectroscopy (XPS) studies were performed using an Electron Spectroscopy for Chemical Analysis (ESCA) instrument (ARIESARSC 10MCD 150, Vacuum Science Workshop). MgKα (1253.6 eV) X-ray radiation was used. Powders with or without the coating layer were pressed into compacted pellets (10 mm in diameter) for measurements. Charging effects were corrected by adjusting the main C 1 s peak to 284.5 eV binding energy. The vacuum in the main analytical chamber during measurement was about 5 × 10−9 Torr. The spectra of O 1 s, Si 2p, Ti 2p3/2, and C 1 s were recorded. The obtained spectra were deconvoluted into Gaussian curves. The width, intensity, and position of peaks were determined by applying least mean square algorithm using PeakFitTM. (PeakFitTM 4.0, 1997, SPSS Inc., USA).
Results and discussion
Zeta-potential values of TiO2, SiO2, and SiO2-coated TiO2 nanoparticles collected at various heights from the burner surface
Zeta potential (mV)
−33.3 ± 0.5
−71.6 ± 2.5
−45.3 ± 1.2
−54.7 ± 0.8
−76.3 ± 1.3
−72.2 ± 1.2
−72.7 ± 1.3
In summary, we report a novel in situ flame coating method by modifying the widely used H2/O2 diffusion flame burner with the addition a sliding type coating unit that allows us to find a right position for producing coated composite nanoparticles. This coating method could produce various kinds of coated nanoparticles in one-step, particularly, oxide-coated oxide nanoparticles, at atmospheric condition. SiO2-coated TiO2 nanoparticles were first synthesized to show the validity of the present method. TEM examination and Zeta-potential measurements confirmed that most surfaces of the particles were well coated. Size distributions of particles and coating thicknesses were also measured. By simply changing the chemical precursors, we also demonstrated that other coated particles such as SiO2-coated SnO2, SnO2-coated TiO2, and SiO2- and SnO2-coated TiO2 could be synthesized.
This work was supported by the acceleration research program and BK21 program of the Ministry of Education, Science and Technology, Korea.