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Preparation of Y2O3 Particles by Flame Spray Pyrolysis with Emulsion Shin Ae Song,† Kyeong Youl Jung,‡ and Seung Bin Park*,† Department of Chemical and Biomolecular Engineering, KAIST, 335, Gwahangno, Yuseong-gu, Daejeon, Republic of Korea, and Department of Chemical Engineering, Kongju National UniVersity, 275, Budae-dong, Cheonan, Chungnam, Republic of Korea ReceiVed October 29, 2008. ReVised Manuscript ReceiVed December 31, 2008 Y2O3 particles of various sizes have been prepared by flame spray pyrolysis using water-in-oil emulsion precursor solutions. We found that by varying the emulsion droplet size, the particle size of the prepared Y2O3 powder can easily be varied from 30 to 700 nm. In conventional spray pyrolysis, each droplet generated by the atomizer is converted to one particle. Thus, particle size can only be controlled by varying the concentration of the precursor, which results in a decrease in the generation rate. As in conventional spray pyrolysis, the flame spray pyrolysis of the emulsions was found to result in the conversion of one emulsion droplet to one particle. Control of particle size was achieved by varying the emulsion droplet size, which results in no decrease in the particle generation rate.
1. Introduction Particle size is one of the most important factors affecting the material properties of ceramic particles. Many researchers have investigated the development of preparation methods that enable control of particle size1-3 and the effects of particle size on adsorption/desorption, magnetic, photoluminescence, and mechanical properties.4-7 Of the many recognized particle preparation methods (the solid state method, sol-gel, coprecipitation, hydrothermal synthesis, etc.), spray pyrolysis is a convenient dispersion phase technique for producing fine powders and materials. In spray pyrolysis, particles with spherical and nonagglomerated morphologies are prepared through the atomization of a precursor solution into discrete atomized droplets; the mean particle size and size distribution of the resulting particles are determined by the size and size distribution of the atomized droplets and the initial concentration of the solution.8 The range of particle sizes that can be obtained with conventional spray pyrolysis is from approximately submicron to micron, given one-atomized-droplet-to-one-particle conversion and the droplet sizes in the range 2-20 µm that are obtained with general atomization equipment. To prepare particles with below submicron size by using spray pyrolysis, low precursor concentrations and small atomized droplets are required, but low precursor concentrations result in poor particle generation rates. Thus, the control of atomized droplet size in spray pyrolysis is crucial to obtaining particles with the desired size and distribution. An ultrasonic nebulizer with various frequencies is needed to * Corresponding author. Tel: +82-42-350-3928. Fax: +82-42-350-3910. E-mail:
[email protected]. † KAIST. ‡ Kongju National University.
(1) Mathieu, Y.; Lebeau, B.; Valtchev, V. Langmuir 2007, 23, 9435. (2) Kobayashi, M.; Juillerat, F.; Galletto, P.; Bowen, P.; Borkovec, M. Langmuir 2005, 21, 5761. (3) Barauskas, J.; Johnsson, M.; Joabsson, F.; Tiberg, F. Langmuir 2005, 21, 2569. (4) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Langmuir 2004, 20, 4696. (5) Panda, R. N.; Gajbhiye, N. S. IEEE Trans. Magn. 1998, 34, 542. (6) Wang, W.-N.; Widiyastuti, W.; Ogi, T.; Lenggoro, L. W.; Okuyama, K. Chem. Mater. 2007, 19, 1723. (7) Luo, J.; Dornfeld, D. A. IEEE Trans. Semiconduct. Manufact. 2003, 16, 469. (8) Lenggoro, I. W.; Itoh, Y.; Iida, N.; Okuyama, K. Mater. Res. Bull. 2003, 38, 1819.
achieve a wide range of atomized droplet sizes. However, in general, ultrasonic nebulizers have only one frequency, so control of atomized droplet size is difficult.9 Electrospray10,11 has also been used as an atomization method. But it has limited industrial applications because of its poor particle yield. In addition, there have been many attempts to prepare nanoparticles with improved spray pyrolysis techniques, such as salt-assisted spray pyrolysis,12 spray pyrolysis using a polymeric precursor solution,13 and low pressure spray pyrolysis.14 In these methods, nanoparticles are prepared by ways to break particles such as inserting an obstacle into the particle matrix or by producing many particles from atomized droplets when they disintegrate. However, the techniques used for the disintegration of particles or atomized droplets mean that control of a wide range of particle sizes is very difficult. Tani et al. have prepared metal oxide particles (e.g., Al2O3, TiO2, ZrO2, Y2O3) with hollow morphologies in flame using emulsion precursor solution.15,16 They showed that one particle was independently made from one emulsion droplet in a nozzle droplet. Especially, Y2O3 was prepared in flame using kerosene and hexa(2-hydroxy-1,3-propylene glycol) as oil phase and surfactant, respectively.16 We propose the use of flame spray pyrolysis of emulsion with the aim of achieving control of particle size via control of emulsion droplet size. Flame spray pyrolysis of emulsion is a good method for the preparation of particles, because it overcomes the limitations of spray pyrolysis in the control of droplet size and production rate. Figure 1 shows the preparation of particles via the flame spray pyrolysis of an emulsion. In flame spray pyrolysis of emulsion, an emulsion is used as the precursor solution. The precursor for the particles is dissolved in the water phase, and (9) Tohge, N.; Tatsumisago, M.; Minami, T.; Okuyama, K.; Adachi, M.; Kousaka, Y. Jpn. J. Appl. Phys. 1988, 27, L1086. (10) Lenggoro, I. W.; Okuyama, K.; Mora, J. F.; Tohge, N. J. Aerosol Sci. 2000, 31, 121. (11) Hong, S.-H.; Moon, J. H.; Lim, J.-M.; Kim, S.-H.; Yang, S.-M. Langmuir 2005, 21, 10416. (12) Panatarani, C.; Lenggoro, I. W.; Okuyama, K. J. Nanoparticle Res. 2003, 5, 47. (13) Kang, H. S.; Kang, Y. C.; Koo, H. Y.; Ju, S. H.; Kim, D. Y.; Hong, S. K.; Sohn, J. R.; Jung, K. Y.; Park, S. B. Mater. Sci. Eng., B 2006, 127, 99. (14) Kang, Y. C.; Park, S. B. J. Aerosol Sci. 1995, 26, 1131. (15) Tani, T.; Takatori, K.; Watanabe, N.; Kamiya, N. J. Mater. Res. 1998, 13, 1099. (16) Tani, T; Watanabe, N.; Takatori, K.; Pratsinis, S. E. J. Am. Ceram. Soc. 2003, 86, 898.
10.1021/la8035924 CCC: $40.75 2009 American Chemical Society Published on Web 02/13/2009
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Figure 1. A schematic diagram of the flame spray pyrolysis of emulsion.
the oil phase has the role of separating the water emulsion droplets containing the precursor. This water-in-oil (w/o) emulsion precursor solution is then atomized via a two-phase nozzle and flows into the flame. It should be emphasized that the droplets flowing out the nozzle are composed of many emulsion drops. In the flame, each emulsion droplet in a nozzle droplet is converted into a particle, and the surfactant and oil are decomposed into gas. This mechanism for conversion of a emulsion droplet to a particle is similar to that of the conversion of an atomized droplet to a particle in conventional spray pyrolysis; emulsion droplets also undergo evaporation, precipitation, drying, decomposition, and sintering to form particles in the flame like atomized droplets in spray pyrolysis. It is possible in flame spray pyrolysis of emulsion to control the particle size over a wide range, from nano- to submicron, by varying the emulsion droplet size due to one-emulsion-droplet-to-one-particle conversion. Emulsion droplet size is varied by varying the amount of water, oil, and surfactant. Further merits of this approach are that a modifying apparatus and lower precursor concentrations are not necessary for the control of particle size. Emulsions consist of a dispersed phase in a continuous phase and require a surfactant and two liquids that are not miscible. In order to create a stable emulsion, the selection of the type and amount of the surfactant and the emulsification method are very important.17-19 There are two main emulsification methods: highenergy emulsification and low-energy emulsification.18 The former method requires a high level of mechanical energy, such as generated by high-pressure homogenizers or ultrasound generators, to produce fine emulsion droplets. In contrast, the low-energy method uses the chemical energy stored in the ingredients by changing the temperature [the phase inversion temperature (PIT) method] or by changing the volume fraction of water or oil [the emulsion inversion point (EIP) method]. In this study, most emulsion precursor solutions were created using an ultrasonic processor, i.e., with high-energy emulsification. In this study, we investigated whether the size of Y2O3 particles synthesized via the flame spray pyrolysis of emulsion could be controlled by varying the emulsion droplet size. To vary the emulsion droplet size, several emulsion samples were prepared by varying the type of surfactant, the amount of surfactant, the ratio of water to oil, and the emulsion preparation method.
Figure 2. A schematic diagram of the flame spray pyrolysis apparatus.
2. Experimental Section Yttrium nitrate hexahydrate [Y(NO3)3 · 6H2O] was used as the Y2O3 precursor, and sorbitan monooleate (Span 80, HLB 4.3) and dioctyl sulfosuccinate sodium salt (AOT) were used as stabilizers of the w/o emulsions, and both were purchased from Aldrich Chemical Co. Heptane (C7H16, 99%) was used as the medium phase of the w/o emulsion and was obtained from Junsei Chemical Co. (17) Sajjadi, S. Langmuir 2006, 22, 5597. (18) Liu, W.; Sun, D.; Li, C.; Liu, Q.; Xu, J. Colloid Interface Sci. 2006, 303, 557. (19) Abismail, B.; Canselier, J. P.; Wilhelm, A. M.; Delams, H.; Gourdon, C. Ultrason. Sonochem. 1999, 6, 75.
Figure 3. The XRD pattern of Y2O3 particles synthesized with flame spray pyrolysis of emulsion.
The w/o emulsion precursor solution was prepared as follows. Yttrium nitrate hexahydrate (1 mol/L) was dissolved in 20 g of deionized water and 6 wt % of Span 80 was dissolved in 136.8 g of heptane. After complete dissolution, the w/o emulsion was prepared by the addition of the as-prepared aqueous yttrium nitrate solution to the as-prepared Span 80 heptane solution with a high-power ultrasonic processor (Vibra-Cell VCX750, 20 kHz, Sonics & Materials Inc.) for 5 min.
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Figure 4. The relationship between the mean emulsion droplet diameter and the mean diameter of the Y2O3 particles prepared with emulsion flame spray pyrolysis.
To control the emulsion droplet size, we used two types of surfactant. First, we used Span 80, which is a nonionic surfactant, and varied its amounts at 0.8, 1.6, 3.1, and 6.0 wt %. Second, we used AOT, which is an anionic surfactant, and varied its concentration at 3.1 and 6 wt %. We also used a homogenizer (Heidolph DIAX900) to produce larger w/o emulsion droplets. The ratio of water to heptane was also varied at 3.5, 13, and 42 wt %, and the concentration of the precursor was varied from 0.5 to 3 M. The flame spray pyrolysis apparatus consists of a precursor injection nozzle, a flame generation nozzle, and a particle collector.
Song et al. Figure 2 shows a schematic diagram of the apparatus. A two-phase nozzle with the center capillary diameter of 0.6 mm and the outer gap of 0.14 mm was used in this study. The two-phase nozzle was placed inside the center tube of the flame nozzle. The flame nozzle consists of four concentric tubes with inner diameters of 19, 27, 35, and 43 mm and a wall thickness of 2 mm. The emulsion precursor solution was atomized with the two-phase nozzle at a flow rate of 1 mL/min with 5 L/min of carrier gas (O2). Fifteen liters/minute of O2 was introduced into the second tube of the flame nozzle to ensure the complete combustion of heptane in the emulsion precursor solution. One liter/minute of propane was injected into the third tube of the flame nozzle as a fuel gas. Twenty liter/minute of O2 flowed into the fourth tube of the flame nozzle for combustion of the fuel gas. The emulsion droplets were converted into particles in the flame, and the as-prepared particles were collected in a bag filter. It should be noted that sampling point is fixed at the end of the reactor, because the morphology would be different if the sampling point is in the middle of the flame or on the wall.20 The production rate of the current experimental reactor system is around 12 g/h. The adiabatic flame temperature was approximately 2234 K. The morphology and size of the as-prepared particles were investigated with transmission electron microscopy (TEM, Philips F20). The crystal structures of the as-prepared particles were investigated with X-ray diffraction (XRD, Rigaku D/Max-IIIC) at 2θ (Cu Ka) ) 20°-60°. The size distributions of the water droplets in the w/o emulsion solutions were determined with a laser light scattering analyzer (Microtrac NanoTrac 150).
3. Results and Discussion Figure 3 shows the XRD pattern of the Y2O3 particles synthesized by the flame spray pyrolysis of emulsion. The prepared particles consist of monoclinic phase with small cubic
Figure 5. The TEM images of Y2O3 prepared with emulsion flame spray pyrolysis for various concentrations of Span 80 (wt %): (a) 0.8, (b) 1.6, (c) 3.1, and (d) 6.0.
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phase impurities, as previously reported by Kang et al.21 and Camenzind et al.22 Kang et al. prepared Y2O3:Eu3+ phosphors with flame spray pyrolysis. Although the as-prepared particles consist of monoclinic phase with small cubic phase impurities, the post-treated particles consist of purely cubic phase after heat treatment at 1200 °C. Camenzind et al. also prepared Y2O3:Eu3+ phosphors with flame spray pyrolysis. The as-prepared Y2O3: Eu3+ powders were found to only contain the monoclinic phase or a mixture of monoclinic and cubic phases for various precursor solution feed rates and oxygen dispersion gas flows. The mean diameters of the emulsion droplets in emulsion precursor solution prepared under various conditions were measured with a laser light scattering analyzer (NanoTrac 150, Microtrac). The mean particle diameters of the Y2O3 particles prepared with flame spray pyrolysis of emulsion were also determined from TEM images. The relationship between the measured mean emulsion droplet diameter and the measured mean particle diameter is shown in Figure 4. The mean diameters of the Y2O3 particles synthesized with flame spray pyrolysis of emulsion range from 30 to 700 nm for the various emulsion droplet diameters. In general spray pyrolysis, by assuming that one dense particle is formed from one atomized droplet, the mean particle diameter is calculated with the following equation
Dparticle ) Ddroplet
(CM)1⁄3 F1⁄3
(1)
where Dparticle, Ddroplet, C, M, and F are the mean diameter of the final particles (µm), the mean diameter of the droplets (µm), the concentration of the precursor solution (mol/m3), the molecular mass of final particles (kg/mol), and the density of final particles (kg/m3), respectively. The mean Y2O3 particle diameters were calculated with this equation for various w/o emulsion droplet diameters and are plotted in Figure 4. The mean particle diameter increases linearly with the mean emulsion droplet diameter. As shown in Figure 4, the experimental results are in good agreement with the calculated results. Thus, the particle preparation mechanism in flame spray pyrolysis of emulsion involves oneemulsion-droplet-to-one-particle conversion. Further, the preparation of particles with a particular submicron size is possible without modifying the apparatus but instead by changing the preparation conditions of the emulsion precursor solution. In a general spray pyrolysis, it is possible to prepare particles with a particular diameter of submicron size by lowering the precursor concentration. Suppose that Y2O3 particles are to be prepared from droplets with a mean diameter of 5 µm generated with an ultrasonic nebulizer. According to eq 1, when the precursor concentration is 0.2 M, the mean particle diameter is 0.75 µm. Further, when the precursor concentration is 0.0005 M, the mean particle diameter is 0.10 µm. Thus, if we want to lower the mean particle diameter from 0.75 to 0.10 µm, the processing time is increased by a factor of 400 times in order to obtain the same amount of Y2O3 particles. Thus, although the control of particle size with general spray pyrolysis is theoretically possible, the control of particle size by lowering the concentration of the precursor is not practical in industry. In contrast, the control of particle size in flame spray pyrolysis of emulsion is possible without varying the particle generation rate. (20) Tani, T.; Takatori, K.; Prastsinis, S. E. J. Am. Soc. 2004, 87, 523. (21) Kang, Y. C.; Seo, D. C.; Park, S. B.; Park, H. D. Jpn. J. Appl. Phys. 2001, 40, 4083. (22) Camenzind, A.; Strobel, R.; Pratsinis, S. E. Chem. Phys. Lett. 2005, 415, 193.
Figure 6. The relationship between the mean particle diameter and (a) the surfactant type, surfactant amount, emulsion preparation method, (b) water ratio, and (c) precursor concentration.
Figure 5 shows TEM images of Y2O3 particles prepared with emulsion flame spray pyrolysis for various amounts of the surfactant Span 80. All the samples obtained with flame spray pyrolysis of emulsion are spherical with nonagglomerated morphology. These particles are also relatively solid Y2O3, which is consistent with Tani et al.’s results in which solid alumina particles are produced when pure oxygen is used as dispersion/ oxidant gas.20 A few nanoparticles were observed at TEM images.
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It is expected that these nanoparticles were made from vaporized precursor. The particle size decreases with the increase of the amounts of Span 80. The mean diameters of Y2O3 particles prepared with 0.8, 1.6, 3.1, and 6.0 wt % of span 80 are 0.396, 0.212, 0.145, and 0.094 µm. Polydispersity indexes are 1.36, 1.28, 1.13, and 1.20, respectively. These results show that particles of various diameters can be prepared by controlling the emulsion droplet size. Figure 6 shows the relationship between the mean particle diameter and the surfactant type, surfactant amount, emulsion preparation method, water ratio, and precursor concentration. Two types of surfactant were used as emulsifiers in the preparation of the emulsion precursor solutions: Span 80, a nonionic surfactant, and AOT, an anionic surfactant. For the same surfactant concentration, the particle size is much smaller when AOT is used as the emulsifier than when Span 80 is used. This difference in emulsion size is likely to be caused by the difference in the structure of surfactants. The bulky hydrophilic part of Span 80 is responsible for the formation of large droplets. AOT with the smaller hydrophilic part tends to form a smaller ball of water emulsion in oil. The ionic head groups of AOT have stronger interaction with water molecules than the nonionic head groups of Span 80, which results in the smaller emulsion droplets. As shown in Figure 6a, the particle diameter decreases with increasing surfactant concentration for both Span 80 and AOT; this trend arises because the emulsion droplet diameter decreases with increases in the surfactant concentration. When Span 80 is used as the emulsifier, the particle diameter varies from approximately 100 nm to approximately 400 nm; when AOT is used as the emulsifier, the particle diameter varies from approximately 30 nm to approximately 70 nm. Two different emulsion preparation devices were also used: a high-power ultrasonic processor, which delivers a high ultrasonic energy to the medium to overcome the interfacial energy between the oil and water, and a homogenizer, which delivers a high shear energy to the medium. For the same amount of surfactant, the particle diameter is much smaller when the high-power ultrasonic processor is used as emulsification equipment instead of the homogenizer. According to Abismail et al., ultrasonic processors generally produce smaller emulsion droplets than homogenizers, because the mechanical agitation method loses more energy to heat than the ultrasound method.19 The Y2O3 particle diameters are smallest when AOT is used as the surfactant and the ultrasonic processor is used in emulsion preparation; the Y2O3 particle diameters are largest when Span 80 is used as the surfactant and the homogenizer is used in emulsion preparation.
Song et al.
To control the emulsion droplet size, w/o emulsion precursor solutions were prepared with various water ratios of 3.5, 13, and 42 wt % when the water/Span 80 weight ratio was fixed at 8:1. Figure 6b shows the relationship between the Y2O3 particle diameter and the ratio of water to heptane. The Y2O3 particle diameters were found to be 0.28, 0.78, and 1.19 µm for water ratios of 3.5, 13, and 42 wt %. The particle diameter increases with increases in the water ratio when the ratio of surfactant to water is fixed, because the emulsion droplet diameter increases with increases in this ratio. To investigate the effects of varying the precursor concentration, emulsion precursor solution samples with various concentrations of yttrium nitrate in the water phase of the w/o emulsion (0.5, 1, 2, and 3 M) were prepared. Figure 6c shows the variations in the Y2O3 particle diameter and the w/o emulsion droplet diameter with the precursor concentration. As the yttrium nitrate concentration in the water phase of the emulsion increases, the emulsion droplet diameter also increases, because the emulsion stability decreases as the yttrium nitrate concentration increases. In emulsion flame spray pyrolysis, the precursor concentration has significant effects on the emulsion droplet diameter and particle diameter.
4. Conclusion In this study, Y2O3 particles with a particle size of approximately 30-700 nm were prepared with flame spray pyrolysis of emulsion. In conventional spray pyrolysis, it is difficult to prepare submicron-size particles without compromising the particle generation rate. In flame spray pyrolysis of emulsion, the control of particle size can be achieved over a wide range of sizes by simply varying the emulsion droplet size rather than the precursor concentration. The emulsion droplet size can be varied by changing the surfactant concentration, the type of surfactant, the emulsion generation method, the ratio of water to oil, and the concentration of the precursor. Increasing the surfactant concentration, decreasing the water to oil ratio, and decreasing the precursor concentration reduce the emulsion droplet size and the Y2O3 particle size. It is also experimentally confirmed that a collection of emulsion droplets of 100 nm in diameter keeps their identity as droplets at high flame temperature and transforms into individual particles. Acknowledgment. This project was supported by Center for Ultramicrochemical Process Systems sponsored by KOSEF (Korea Science and Engineering Foundation). LA8035924
