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Flame synthesis of silica nanoparticles by adopting two-fluid nozzle spray
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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
Flame synthesis of silica nanoparticles by adopting two-fluid nozzle spray Hankwon Chang a , Jin-Ho Park a,b , Hee Dong Jang a,∗ a
Nano-Materials Group, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b Department of Chemical and Biomolecular Engineering, Sogan University, Seoul 121-742, South Korea Received 30 October 2006; accepted 25 April 2007 Available online 31 May 2007
Abstract Aggregation-free amorphous silica nanoparticles were synthesized from TEOS by flame pyrolysis with liquid-phase precursors fed by a two-fluid nozzle. The effects of key variables such as pressure of dispersion air, precursor feed rate, hydrogen flow rate, and precursor concentrations on the particle morphology and size were investigated by TEM and BET. The morphology of as-prepared silica nanoparticles was spherical, non-hollow and non-aggregated. The average particle diameters determined by BET analysis ranged from 9 to 68 nm and were proportional to the precursor feed rate, hydrogen flow rate, and precursor concentration. On the contrary, the average particle size was reversely proportional to the pressure of dispersion air. © 2007 Elsevier B.V. All rights reserved. Keywords: Silica nanoparticles; Flame spray pyrolysis; Two-fluid nozzle
1. Introduction Silica (SiO2 ) nanoparticles have been widely used in various industrial applications, such as catalysis, pigments, pharmacy, electronic and thin film substrates, electronic and thermal insulators, and humidity sensors [1]. SiO2 nanoparticles were also considered as nanometric fillers with potentially interesting reinforcing capabilities [2]. In these applications, the particle morphology, average particle size or specific surface area, and surface characteristics are considered as the key characteristics of powders that must be controlled [3]. Flame synthesis is commercially important because of its high production rates and relative low cost and it can be operated as continuous processes. It would be a novel technique of producing fine, pure and single-phase particles in the as-prepared state [4,5]. However, nanoparticles produced by flame methods with vapor-phase precursors are generally hardly agglomerated, which are undesirable in a lot of industrial fields. In addition, it is often difficult to synthesize multicomponent materials with homogeneous chemical composition because of differences in
∗
Corresponding author. Tel.: +82 42 868 3612; fax: +82 42 868 3418. E-mail address: [email protected] (H.D. Jang).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.083
the chemical reaction rate and the vapor pressure of reactants [6]. A flame assisted liquid droplet-to-particle conversion process called as flame spray pyrolysis (FSP) is an excellent method to prepare multicomponent nanoparticles directly from precursor solution composed of a lot of elements with controlled stoichiometry. An ultrasonic atomizer has been usually taken in spray pyrolysis for the generation of liquid droplets because the liquid droplet size is fine and uniform [7]. Unfortunately, when the concentration of raw materials is high, the ultrasonic atomizer cannot produce liquid droplets because of the high viscosity. Also, it is difficult to generate the liquid droplets with high production rates. Therefore, it is required to use a bundle of oscillators to apply the process to industrial field. Flame pyrolysis with liquid-phase precursors supplied by a two-fluid nozzle was employed in this study to synthesize aggregation-free silica nanoparticles. We chose an externalmixing nozzle because it is easy to control the pressure of dispersion air and precursor feed rate independently. The effects of key variables, such as the pressure of dispersion air, the feed rate of precursor solution, the flow rate of hydrogen, and precursor concentration on the particle morphology and size were investigated.
H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
141
2. Experiment
3. Results and discussion
The precursor solution for the synthesis of SiO2 nanoparticles was prepared by dissolving 30 vol.% of tetraethyl orthosilicate (TEOS, Acros Organics, 98%) into ethanol (EtOH, >97%). A schematic diagram of the experimental setup for the synthesis of SiO2 nanoparticles was shown in Fig. 1. It consisted of a twofluid nozzle for the supply of liquid precursor, a burner composed of four-concentric stainless tube for diffusion flame, and a thermophoretic particle sampler. The diffusion flame burner was designed so that flame could be formed from the combustion of precursor + air + Ar, H2 , O2 , and air, separately introduced through four concentric stainless tubes. The liquid-phase precursor solution was sprayed by using the two-fluid nozzle with dispersion air and then the generated precursor droplets were introduced into the central area in the diffusion flame, in which the evaporation of solvent, the precipitation of solute, decomposition and oxidation occurred. Hydrogen (H2 ) is used as a fuel while oxygen (O2 ) and air are used as oxidants. As-prepared particles in the high temperature flame zone were thermophoretically collected on the surface of a cold glass tube (100 mm in diameter and 300 mm in length) which was maintained at 12 ◦ C by flowing cooling water through the inside of the glass tube. The particle morphology and crystalline phase were characterized by a transmission electron microscope (TEM, Philips Model CM12) operating at 120 kV and X-ray diffractometry (XRD, RTP 300 RC, Rigaku Co.) with CuK␣ target operated at 30 kV and 40 mA, respectively. The specific surface area of the particles was measured by using a nitrogen adsorption analyzer (ASAP 2400, Micrometrics) employing the BET equation. Assuming spherical particles, the average particle diameter, dp , was calculated from the specific surface area, A, and the density of silica (ρp = 2.2 g/cm3 ) by dp =6/(ρp A).
The crystalline phase of as-prepared silica nanoparticles generated by the flame spray pyrolysis was found to be amorphous from the analysis of X-ray diffraction pattern which was not shown here. Fig. 2(a–c) shows TEM micrographs of the asprepared silica nanoparticles by flame spray pyrolysis at the various pressures of the dispersion air, Pd , while keeping the feed rate of precursor, Qp , the flow rates of Ar, H2 , O2 , and air at 17.3 ml/min, 5 l/min, 10 l/min, 15 l/min and 40 l/min, respectively. The morphology of as-prepared silica nanoparticles was spherical, non-hollow and there was no neck formation when the pressure of dispersion air was less than 5.0 kgf /cm2 . However, when the pressure of dispersion air was as high as 5.0 kgf /cm2 , chain-like aggregates were generated. When the liquid droplets size is small enough, the droplets are evaporated instantly due to the Kelvin effect [8]. The evaporated vapors undergo typical vapor-phase reaction described by oxidation, nucleation, condensation, collision and coalescence, finally resulting in chain-like aggregates. Besides, the effect of precursor feed rate on the particle morphology and size was investigated by increasing the precursor feed rate from 11.0 to 24.6 ml/min while keeping the pressure of dispersion air and the other gas flow rates at constant (Fig. 2(b, d and e)). Even though precursor feed rate increased, the morphology of as-prepared powder kept spherical, non-hollow and non-aggregated, but particle size increased. The average particle diameters were calculated from the specific surface areas measured by BET analysis and are shown in Fig. 3. As the pressure of dispersion air increased from 1.0 to 5.0 kgf /cm2 at the fixed experimental conditions (Fig. 2(a–c)), the specific surface areas of the silica nanoparticles were increased from 45.0 to 250.4 m2 /g and the corresponding average particle diameters were decreased from 60.7 to 10.9 nm. In addition, with an increment in the feed rate of precursor, the average particle size increased from 24.1 to 50.7 nm while keeping the other experimental variables at constant. The mean droplet diameter (dd ) for liquid atomization using external-mixing nozzles is given by [9,10] 0.29 −0.18 ml dd = Cdo Re − 0.39We (1) mg where C is a scaling constant, do is the orifice diameter, and ml /mg is the liquid to gas mass flow rate ratio. Reynolds and Weber numbers are defined as Re = ρl do (ug − ul )/μl and We = ρl do (ug − ul )2 /σ l , where μl and σ l are liquid viscosity and surface tension, while ug and ul are the gas and liquid velocity at the nozzle exit. By reasonably assuming that ul can be ignored because it is usually much less than ug , the ratio of droplet diameters can be expressed as Eq. (2), while a same external-mixing nozzle is used for the generation of liquid droplets at different gas velocities, ug,0 , ug,2 , and liquid to gas mass flow rate ratios, ml,0 /mg,0 , ml,2 /mg,2 .
Fig. 1. A schematic diagram of experimental apparatus for the synthesis of silica nanoparticles by flame spray pyrolysis using two-fluid nozzle spray.
dd,0 = dd,2
ug,2 ug,0
0.75
(ml,0 × mg,2 ) (ml,2 × mg,0 )
0.29 (2)
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H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
Fig. 2. TEM micrographs of SiO2 nanoparticles prepared at different pressures of the dispersion air, Pd , and feed rates of precursor, Qp , while keeping the flow rates of Ar, H2 , O2 , and air at 5, 10, 15 and 40 l/min, respectively. (a) Pd = 1.0 kgf /cm2 , Qp = 17.3 ml/min; (b) Pd = 3.0, Qp = 17.3; (c) Pd = 5.0, Qp = 17.3; (d) Pd = 3.0, Qp = 11.0; (e) Pd = 3.0, Qp = 24.6.
It can be known from above equation that the droplet diameter decreases with the increment in dispersion air supply while keeping the precursor feed rate constant. In reverse, higher feed rate of precursor leads to increase in the droplet diameter at a constant pressure of dispersion air. These are also consistent with TEM micrographs shown in Fig. 2. In order to investigate the effect of hydrogen flow rate on the particle size, hydrogen flow rate into the flame was reduced from 10 to 2 l/min while keeping the pressure of dispersion air and precursor feed rate at 3.0 kgf /cm2 and 11 ml/min, respectively (Fig. 4). As the hydrogen flow rate decreased from 10 to 2 l/min, the average particle diameter decreased from 45 to 18 nm. More hydrogen flow rate causes higher flame temperature where the collision and coalescence of particles are enhanced, resulting in relatively larger particles.
The effect of TEOS concentration in precursor solution on the particle size was also investigated while keeping the pressure of dispersion air, the feed rate of precursor solution, and hydrogen flow rate at 3.0 kgf /cm2 , 11 ml/min, and 2.0 l/min, respectively (Fig. 5). When the TEOS concentration was 30, 50, and 70 vol.% in EtOH solution, the average particle diameters determined by BET analysis were 18, 26, and 32 nm, respectively. By assuming that one precursor droplet generates one particle, the particle diameter can be calculated from the droplet diameter, dd , as following [11,12]: cpr Mp 1/3 dp = dd (3) ρp Mpr where cpr is the precursor concentration, Mp is the particle molecular mass, ρp is the particle density, and Mpr is the molec-
H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
143
ular mass of the precursor. From Eq. (3), the ratio of particle diameters prepared at different precursor concentrations, cpr,0 , cpr,2 , while keeping the droplet diameter at constant, can be expressed as following: cpr,0 1/3 dp,0 = (4) dp,2 cpr,2
Fig. 3. The average particle diameter of silica nanoparticles prepared by FSP at various pressures of dispersion air and feed rates of precursor.
Relatively higher concentration of precursor will lead to larger particle size, which is well consistent with our results. While the precursor concentrations were increased from 30 to 50 and from 50 to 70 vol.%, the ratios of particle diameters were 0.69 and 0.81. According to Eq. (4), the ratios of particle diameters were expected to be 0.84 and 0.89, respectively. The differences between the measured and calculated ratios increased when the precursor concentration became relatively low. These discrepancies describe that more particles were generated from one precursor droplet when the precursor concentration was relatively low. 4. Conclusion
Fig. 4. Effect of the hydrogen flow rate on the specific surface area and the average particle diameter.
Flame synthesis of silica nanoparticles by a two-fluid nozzle was successfully conducted from TEOS solution. By controlling the experimental variables such as the pressure of dispersion air, the feed rate of precursor solution, the hydrogen flow rate, and the precursor concentration, the average particle size ranged from 9 to 68 nm and the morphology of as-prepared silica nanoparticles was spherical, non-aggregated, and non-hollow. The average particle size was proportional to the precursor feed rate, hydrogen flow rate, and precursor concentration. On the contrary, the average particle size was reversely proportional to the pressure of dispersion air. The chain-like aggregates originated from vapor-phase reaction where the vapor was generated by the evaporation of relatively small droplets due to the Kelvin effect. The enhanced collision and coalescence by high flame temperature led to larger particles. In addition, it was known that more particles were generated from one precursor droplet. Acknowledgments This research was supported by the Resource Recycling R&D Center of Korea and the Ministry of Science and Technology, Korea. References [1] [2] [3] [4] [5]
Fig. 5. Effect of the precursor concentration on the specific surface area and the average particle diameter prepared by the flame spray pyrolysis.
G. Herbert, J. Eur. Ceram. Soc. 14 (1994) 205. M. Preghenella, A. Pegoretti, C. Migliaresi, Polymer 46 (2005) 12065. H.D. Jang, Powder Technol. 119 (2001) 102. H.D. Jang, Aerosol Sci. Technol. 30 (1999) 477. L. M¨adler, H.K. Kammler, R. Mueller, S.E. Pratsinis, J. Aerosol Sci. 33 (2002) 369. [6] R. Mueller, L. M¨adler, S.E. Pratsinis, Chem. Eng. Sci. 58 (2003) 1969. [7] B. Xia, I.W. Lenggoro, K. Okuyama, Adv. Mater 13 (2001) 1579. [8] S.K. Friedlander, Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics, second ed., Oxford, New York, 2000.
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H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
[9] A.H. Lefebvre, Atomization and Sprays, Taylor & Francis, London, 1989. [10] M.C. Heine, L. M¨adler, R. Jossen, S.E. Pratsinis, Combust. Flame 144 (2006) 809.
[11] G.L. Messing, S.C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. [12] V. Jokanovi´c, A.M. Spasi´c, D. Uskokovi´c, J. Colloid Interface Sci. 278 (2004) 342.
Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
Flame synthesis of silica nanoparticles by adopting two-fluid nozzle spray Hankwon Chang a , Jin-Ho Park a,b , Hee Dong Jang a,∗ a
Nano-Materials Group, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b Department of Chemical and Biomolecular Engineering, Sogan University, Seoul 121-742, South Korea Received 30 October 2006; accepted 25 April 2007 Available online 31 May 2007
Abstract Aggregation-free amorphous silica nanoparticles were synthesized from TEOS by flame pyrolysis with liquid-phase precursors fed by a two-fluid nozzle. The effects of key variables such as pressure of dispersion air, precursor feed rate, hydrogen flow rate, and precursor concentrations on the particle morphology and size were investigated by TEM and BET. The morphology of as-prepared silica nanoparticles was spherical, non-hollow and non-aggregated. The average particle diameters determined by BET analysis ranged from 9 to 68 nm and were proportional to the precursor feed rate, hydrogen flow rate, and precursor concentration. On the contrary, the average particle size was reversely proportional to the pressure of dispersion air. © 2007 Elsevier B.V. All rights reserved. Keywords: Silica nanoparticles; Flame spray pyrolysis; Two-fluid nozzle
1. Introduction Silica (SiO2 ) nanoparticles have been widely used in various industrial applications, such as catalysis, pigments, pharmacy, electronic and thin film substrates, electronic and thermal insulators, and humidity sensors [1]. SiO2 nanoparticles were also considered as nanometric fillers with potentially interesting reinforcing capabilities [2]. In these applications, the particle morphology, average particle size or specific surface area, and surface characteristics are considered as the key characteristics of powders that must be controlled [3]. Flame synthesis is commercially important because of its high production rates and relative low cost and it can be operated as continuous processes. It would be a novel technique of producing fine, pure and single-phase particles in the as-prepared state [4,5]. However, nanoparticles produced by flame methods with vapor-phase precursors are generally hardly agglomerated, which are undesirable in a lot of industrial fields. In addition, it is often difficult to synthesize multicomponent materials with homogeneous chemical composition because of differences in
∗
Corresponding author. Tel.: +82 42 868 3612; fax: +82 42 868 3418. E-mail address: [email protected] (H.D. Jang).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.083
the chemical reaction rate and the vapor pressure of reactants [6]. A flame assisted liquid droplet-to-particle conversion process called as flame spray pyrolysis (FSP) is an excellent method to prepare multicomponent nanoparticles directly from precursor solution composed of a lot of elements with controlled stoichiometry. An ultrasonic atomizer has been usually taken in spray pyrolysis for the generation of liquid droplets because the liquid droplet size is fine and uniform [7]. Unfortunately, when the concentration of raw materials is high, the ultrasonic atomizer cannot produce liquid droplets because of the high viscosity. Also, it is difficult to generate the liquid droplets with high production rates. Therefore, it is required to use a bundle of oscillators to apply the process to industrial field. Flame pyrolysis with liquid-phase precursors supplied by a two-fluid nozzle was employed in this study to synthesize aggregation-free silica nanoparticles. We chose an externalmixing nozzle because it is easy to control the pressure of dispersion air and precursor feed rate independently. The effects of key variables, such as the pressure of dispersion air, the feed rate of precursor solution, the flow rate of hydrogen, and precursor concentration on the particle morphology and size were investigated.
H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
141
2. Experiment
3. Results and discussion
The precursor solution for the synthesis of SiO2 nanoparticles was prepared by dissolving 30 vol.% of tetraethyl orthosilicate (TEOS, Acros Organics, 98%) into ethanol (EtOH, >97%). A schematic diagram of the experimental setup for the synthesis of SiO2 nanoparticles was shown in Fig. 1. It consisted of a twofluid nozzle for the supply of liquid precursor, a burner composed of four-concentric stainless tube for diffusion flame, and a thermophoretic particle sampler. The diffusion flame burner was designed so that flame could be formed from the combustion of precursor + air + Ar, H2 , O2 , and air, separately introduced through four concentric stainless tubes. The liquid-phase precursor solution was sprayed by using the two-fluid nozzle with dispersion air and then the generated precursor droplets were introduced into the central area in the diffusion flame, in which the evaporation of solvent, the precipitation of solute, decomposition and oxidation occurred. Hydrogen (H2 ) is used as a fuel while oxygen (O2 ) and air are used as oxidants. As-prepared particles in the high temperature flame zone were thermophoretically collected on the surface of a cold glass tube (100 mm in diameter and 300 mm in length) which was maintained at 12 ◦ C by flowing cooling water through the inside of the glass tube. The particle morphology and crystalline phase were characterized by a transmission electron microscope (TEM, Philips Model CM12) operating at 120 kV and X-ray diffractometry (XRD, RTP 300 RC, Rigaku Co.) with CuK␣ target operated at 30 kV and 40 mA, respectively. The specific surface area of the particles was measured by using a nitrogen adsorption analyzer (ASAP 2400, Micrometrics) employing the BET equation. Assuming spherical particles, the average particle diameter, dp , was calculated from the specific surface area, A, and the density of silica (ρp = 2.2 g/cm3 ) by dp =6/(ρp A).
The crystalline phase of as-prepared silica nanoparticles generated by the flame spray pyrolysis was found to be amorphous from the analysis of X-ray diffraction pattern which was not shown here. Fig. 2(a–c) shows TEM micrographs of the asprepared silica nanoparticles by flame spray pyrolysis at the various pressures of the dispersion air, Pd , while keeping the feed rate of precursor, Qp , the flow rates of Ar, H2 , O2 , and air at 17.3 ml/min, 5 l/min, 10 l/min, 15 l/min and 40 l/min, respectively. The morphology of as-prepared silica nanoparticles was spherical, non-hollow and there was no neck formation when the pressure of dispersion air was less than 5.0 kgf /cm2 . However, when the pressure of dispersion air was as high as 5.0 kgf /cm2 , chain-like aggregates were generated. When the liquid droplets size is small enough, the droplets are evaporated instantly due to the Kelvin effect [8]. The evaporated vapors undergo typical vapor-phase reaction described by oxidation, nucleation, condensation, collision and coalescence, finally resulting in chain-like aggregates. Besides, the effect of precursor feed rate on the particle morphology and size was investigated by increasing the precursor feed rate from 11.0 to 24.6 ml/min while keeping the pressure of dispersion air and the other gas flow rates at constant (Fig. 2(b, d and e)). Even though precursor feed rate increased, the morphology of as-prepared powder kept spherical, non-hollow and non-aggregated, but particle size increased. The average particle diameters were calculated from the specific surface areas measured by BET analysis and are shown in Fig. 3. As the pressure of dispersion air increased from 1.0 to 5.0 kgf /cm2 at the fixed experimental conditions (Fig. 2(a–c)), the specific surface areas of the silica nanoparticles were increased from 45.0 to 250.4 m2 /g and the corresponding average particle diameters were decreased from 60.7 to 10.9 nm. In addition, with an increment in the feed rate of precursor, the average particle size increased from 24.1 to 50.7 nm while keeping the other experimental variables at constant. The mean droplet diameter (dd ) for liquid atomization using external-mixing nozzles is given by [9,10] 0.29 −0.18 ml dd = Cdo Re − 0.39We (1) mg where C is a scaling constant, do is the orifice diameter, and ml /mg is the liquid to gas mass flow rate ratio. Reynolds and Weber numbers are defined as Re = ρl do (ug − ul )/μl and We = ρl do (ug − ul )2 /σ l , where μl and σ l are liquid viscosity and surface tension, while ug and ul are the gas and liquid velocity at the nozzle exit. By reasonably assuming that ul can be ignored because it is usually much less than ug , the ratio of droplet diameters can be expressed as Eq. (2), while a same external-mixing nozzle is used for the generation of liquid droplets at different gas velocities, ug,0 , ug,2 , and liquid to gas mass flow rate ratios, ml,0 /mg,0 , ml,2 /mg,2 .
Fig. 1. A schematic diagram of experimental apparatus for the synthesis of silica nanoparticles by flame spray pyrolysis using two-fluid nozzle spray.
dd,0 = dd,2
ug,2 ug,0
0.75
(ml,0 × mg,2 ) (ml,2 × mg,0 )
0.29 (2)
142
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Fig. 2. TEM micrographs of SiO2 nanoparticles prepared at different pressures of the dispersion air, Pd , and feed rates of precursor, Qp , while keeping the flow rates of Ar, H2 , O2 , and air at 5, 10, 15 and 40 l/min, respectively. (a) Pd = 1.0 kgf /cm2 , Qp = 17.3 ml/min; (b) Pd = 3.0, Qp = 17.3; (c) Pd = 5.0, Qp = 17.3; (d) Pd = 3.0, Qp = 11.0; (e) Pd = 3.0, Qp = 24.6.
It can be known from above equation that the droplet diameter decreases with the increment in dispersion air supply while keeping the precursor feed rate constant. In reverse, higher feed rate of precursor leads to increase in the droplet diameter at a constant pressure of dispersion air. These are also consistent with TEM micrographs shown in Fig. 2. In order to investigate the effect of hydrogen flow rate on the particle size, hydrogen flow rate into the flame was reduced from 10 to 2 l/min while keeping the pressure of dispersion air and precursor feed rate at 3.0 kgf /cm2 and 11 ml/min, respectively (Fig. 4). As the hydrogen flow rate decreased from 10 to 2 l/min, the average particle diameter decreased from 45 to 18 nm. More hydrogen flow rate causes higher flame temperature where the collision and coalescence of particles are enhanced, resulting in relatively larger particles.
The effect of TEOS concentration in precursor solution on the particle size was also investigated while keeping the pressure of dispersion air, the feed rate of precursor solution, and hydrogen flow rate at 3.0 kgf /cm2 , 11 ml/min, and 2.0 l/min, respectively (Fig. 5). When the TEOS concentration was 30, 50, and 70 vol.% in EtOH solution, the average particle diameters determined by BET analysis were 18, 26, and 32 nm, respectively. By assuming that one precursor droplet generates one particle, the particle diameter can be calculated from the droplet diameter, dd , as following [11,12]: cpr Mp 1/3 dp = dd (3) ρp Mpr where cpr is the precursor concentration, Mp is the particle molecular mass, ρp is the particle density, and Mpr is the molec-
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143
ular mass of the precursor. From Eq. (3), the ratio of particle diameters prepared at different precursor concentrations, cpr,0 , cpr,2 , while keeping the droplet diameter at constant, can be expressed as following: cpr,0 1/3 dp,0 = (4) dp,2 cpr,2
Fig. 3. The average particle diameter of silica nanoparticles prepared by FSP at various pressures of dispersion air and feed rates of precursor.
Relatively higher concentration of precursor will lead to larger particle size, which is well consistent with our results. While the precursor concentrations were increased from 30 to 50 and from 50 to 70 vol.%, the ratios of particle diameters were 0.69 and 0.81. According to Eq. (4), the ratios of particle diameters were expected to be 0.84 and 0.89, respectively. The differences between the measured and calculated ratios increased when the precursor concentration became relatively low. These discrepancies describe that more particles were generated from one precursor droplet when the precursor concentration was relatively low. 4. Conclusion
Fig. 4. Effect of the hydrogen flow rate on the specific surface area and the average particle diameter.
Flame synthesis of silica nanoparticles by a two-fluid nozzle was successfully conducted from TEOS solution. By controlling the experimental variables such as the pressure of dispersion air, the feed rate of precursor solution, the hydrogen flow rate, and the precursor concentration, the average particle size ranged from 9 to 68 nm and the morphology of as-prepared silica nanoparticles was spherical, non-aggregated, and non-hollow. The average particle size was proportional to the precursor feed rate, hydrogen flow rate, and precursor concentration. On the contrary, the average particle size was reversely proportional to the pressure of dispersion air. The chain-like aggregates originated from vapor-phase reaction where the vapor was generated by the evaporation of relatively small droplets due to the Kelvin effect. The enhanced collision and coalescence by high flame temperature led to larger particles. In addition, it was known that more particles were generated from one precursor droplet. Acknowledgments This research was supported by the Resource Recycling R&D Center of Korea and the Ministry of Science and Technology, Korea. References [1] [2] [3] [4] [5]
Fig. 5. Effect of the precursor concentration on the specific surface area and the average particle diameter prepared by the flame spray pyrolysis.
G. Herbert, J. Eur. Ceram. Soc. 14 (1994) 205. M. Preghenella, A. Pegoretti, C. Migliaresi, Polymer 46 (2005) 12065. H.D. Jang, Powder Technol. 119 (2001) 102. H.D. Jang, Aerosol Sci. Technol. 30 (1999) 477. L. M¨adler, H.K. Kammler, R. Mueller, S.E. Pratsinis, J. Aerosol Sci. 33 (2002) 369. [6] R. Mueller, L. M¨adler, S.E. Pratsinis, Chem. Eng. Sci. 58 (2003) 1969. [7] B. Xia, I.W. Lenggoro, K. Okuyama, Adv. Mater 13 (2001) 1579. [8] S.K. Friedlander, Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics, second ed., Oxford, New York, 2000.
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H. Chang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 140–144
[9] A.H. Lefebvre, Atomization and Sprays, Taylor & Francis, London, 1989. [10] M.C. Heine, L. M¨adler, R. Jossen, S.E. Pratsinis, Combust. Flame 144 (2006) 809.
[11] G.L. Messing, S.C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. [12] V. Jokanovi´c, A.M. Spasi´c, D. Uskokovi´c, J. Colloid Interface Sci. 278 (2004) 342.