Chem. Mater. 2002, 14, 2623-2627
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Nanoparticle Separation in Salted Droplet Microreactors Bin Xia, I. Wuled Lenggoro, and Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Received December 11, 2001. Revised Manuscript Received March 25, 2002
Nanoparticles were synthesized using a modified aerosol decomposition process. The goal of this work was to investigate the breakup of micrometer/submicrometer particles into nanosized particles in salt microreactors using NiO synthesis as a model. The influence of various operating conditions such as temperature, type of salt, salt composition, and concentration on the separation process were investigated. The results show that addition of inorganic salts leads to substantial changes in particles which are produced and that nanoparticles can be obtained under a variety of conditions. An optimum temperature range exists for nanoparticle separation. Different types of salts and precursors have great influences on the separation process as well as the characteristics of the products, which, along with synthesis temperature, can be used to control the properties of the final product.
Introduction Nanoparticles are of great interest due to their improved or unique properties and many techniques have been reported for their synthesis. Aerosol decomposition, also known as spray pyrolysis, which is a wellknown procedure for the rapid production of multicomponent or single-component materials in a continuous process, is typically used only for micrometer/submicrometer materials due to the relatively large droplet sizes produced.1,2 A precursor solution is atomized or misted into aerosol droplets that are transferred to a hot reactor where they undergo droplet drying, precursor decomposition, solid-state reactions, and sintering to form the product particles.3,4 Generally speaking, each micrometer/submicrometer particle consists of a number of crystallites which are nanometer in size, or nanocrystallites, but they are agglomerated/sintered together and virtually inseparable (as an example see Figure 1a).1,2 Recently we found that aerosol decomposition can also be used to synthesize nanosized particles by introducing salts into the process, and that many types of nanoparticles can be prepared.5 In our method, salts such as potassium and sodium nitrates were added to a precursor solution followed by decomposition in an aerosol reactor. Nanoparticles are obtained after washing the product to remove the salts or other soluble derivatives. Our method is denoted as salt-assisted aerosol decomposition (SAD), as opposed to conventional aerosol decomposition (CAD). In this work, we describe * Corresponding author. E-mail:
[email protected]. Telephone: 81-824-24-7716. Fax: 81-824-24-7850. (1) Messing, G. L.; Zhang, S. C.; Jayanthi, G. V. J. Am. Ceram. Soc. 1993, 76, 2707. (2) Kodas, T. T.; Hampden-Smith, M. J. Aerosol processing of materials; Wiley-VCH: New York, 1999. (3) Majumdar, D.; Kodas, T. T.; Glicksman, H. D. Adv. Mater. 1996, 8, 1020. (4) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (5) Xia, B.; Lenggoro, I. W.; Okuyama, K. Adv. Mater. 2001, 13, 1579.
experiments which were designed to better understand how nanoparticles are formed and separated during aerosol decomposition and how synthesis conditions affect the products. NiO, an important material used in solid oxide fuel cells, diodes, gas sensors, lithiumion batteries, etc.,6,7 was used as a typical example to demonstrate how nanoparticles in a large (micrometer/ submicrometer) particle are separated into suitable nanoparticles. Experimental Section The nickel precursor, Ni(NO3)2, NiCl2, or Ni(CH3COO)2, was dissolved in water. A salt mixture, KNO3-NaNO3, KCl-LiCl, or CH3COOK-CH3COONa, with a eutectic composition unless otherwise stated was then added to the respective precursor solution and mixed to homogeneity. The concentrations of nickel and the total salts were kept at 0.5 and 2.0 mol/L, respectively, unless otherwise stated. The solution was then atomized by means of an ultrasonic transducer (1.7 MHz, NEU11B, Omron, Kyoto, Japan) and the mist was carrier by a stream of clean air (1 L/min) into a tubular reactor (1 m in length and 13 mm inner diameter) maintained at predetermined temperatures, followed by heating for several seconds. The generated mist/droplets have average diameters of around 5 µm measured by a laser diffraction technique (Spraytec, Malvern Instruments Ltd., Malvern, Worcs., U.K.). Aerosol products were collected in an electrostatic precipitator kept at around 150 °C followed by washing and drying. The production rate of the process was on the order of a few grams per hour. The experimental apparatus and some details can be found in a previous report describing the conventional process.8 When comparing our method (SAD) with the conventional one (CAD), all operating parameters were kept the same except for the salts. All SAD data shown in this paper were collected from the washed product unless otherwise stated. Products were characterized by means of a field-emission scanning electron micrograph (FE-SEM, S-5000, Hitachi, (6) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.M. Nature (London) 2000, 407, 496. (7) Tietz, F.; Dias, F. J.; Simwonis, D.; Stover, D. J. Eur. Ceram. Soc. 2000, 20, 1023. (8) Xia, B.; Lenggoro, I. W.; Okuyama, K. J. Mater. Res. 2000, 15, 2157.
10.1021/cm011684s CCC: $22.00 © 2002 American Chemical Society Published on Web 05/01/2002
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Figure 1. Scanning (a-e) and transmission (f) electron micrographs of particles synthesized at various temperatures using the CAD and SAD methods: (a) CAD, 600 °C; (b-e) SAD, 400, 500, 550, and 600 °C, respectively; (f) TEM image of sample d. Tokyo) operated at 20 kV, field-emission transmission electron micrograph (FE-TEM, HF-2000, Hitachi, Tokyo) operated at 200 kV and X-ray diffraction (XRD, Rint 2200V, Rigaku, Tokyo) with Cu KR radiation operated at 40 kV and 20 mA.
Results Figure 1 shows the products synthesized from Ni(NO3)2 without (CAD) and with the (SAD) K-Na-NO3 mixture in the solution at various temperatures. The particles produced by the CAD method at 600 °C are micrometer/submicrometer sized hollow particles around 1.0 µm in size with crinkled shells. From the inset, it is clear that the micrometer/submicrometer particles are composed of 5-10 nm nanocrystallites which are bound tightly together. The particles shown here are typical of CAD products.1,2 After addtion of the nitrate salts to the precursor solution, however, the SAD products give rise to particles that are broken into smaller ones (Figure 1b-e). At 400 °C, nanocrystallites are still strongly agglomerated as observed by TEM, and at 500 °C, agglomeration becomes weaker. At 550 °C and above, small and homogeneous agglomerates are obtained. What is more important is that particle agglomeration becomes much weaker. From the TEM image (Figure 1f), it is clear that nanoparticles, about 11 nm in size, were obtained at 550 °C. This is obviously different from the CAD particles (Figure 1a), in which a number of the nanocrystallites strongly agglomerate into larger particles. Figure 2 shows XRD patterns of products synthesized at 400 and 700 °C, showing the NiO phase. Before the product was washed, the phases of salts or derivatives were found to coexist with the desired NiO phase, and were removed by washing. From TEM observation and powder X-ray difrraction spectra, it is clear that the crystallite/particle size increases with
Figure 2. XRD patterns of the SAD products synthesized at (a) 400 and (b) 700 °C. The inset shows product a before washing, showing that the NiO phase coexists with (K,Na)NO3 or derivatives. The impurity phases are soluble and can be removed by washing. The reference is a cubic NiO phase.
the synthesis temperature, i.e., 24 nm for 700 °C and 10 nm for 400 °C, respectively. It is also noteworthy that the crystallite sizes of the SAD particles are larger than those of the CAD products synthesized under the same operating conditions, which is similar to that used in our previous report.5 For example, the crystallite sizes (obtained by powder X-ray diffraction spectra) are 12 and 19 nm for the CAD and the SAD products respectively, synthesized at 600 °C, clearly demonstrating the acceleration of crystal growth in the salt melt. Therefore, products with high crystal-
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Figure 3. (a) SEM image and (b) XRD pattern of NiO particles synthesized at 650 °C from the chloride system. The inset is a TEM image.
linity can be easily produced in the SAD process, in comparison with the typical low crystallinities of the CAD products due to the short heating times used in the aerosol processes.5,6 At very high temperatures, nanoparticles cannot be produced. At 1000 °C, for example, separated nanoparticles were not obtained and well-faceted crystallites around 100 nm in size were formed and agglomerated into micrometer/submicrometer particles. To examine the issue of whether other salts can also be used, chlorides and acetates of K and Na were used with their corresponding nickel precursors. It was found that all of the three salts could be used to synthesize nanoparticles, although the acetates are less effective than the other two. In the case of the chloride system (Ni-K-Na-Cl), nanoparticles were also obtained at 550 °C while the separation temperature was around 700 °C for the acetates. Figure 3 shows SEM and TEM images and an XRD pattern. In addition, particle sizes and morphologies were also found to be different from the nitrate products. For example, from TEM observation, particles are nearly spherical or round-angled with an average size of 85 nm for the 550 °C product, which is much larger than the nitrate 550 °C product (11 nm) and even the 700 °C (24 nm) product. The use of chlorides tends to result in spherical particles while the nitrates tend to give faceted ones with a sufficient crytallinity. It is clear that the characteristics of a product are strongly dependent on the salts and precursors; therefore, they can be used to tailor the properties of the product. To understand the influence of salt composition, we synthesized NiO at 650 °C by changing the molar ratio of KCl/LiCl at 0.82/1.18 (eutectic), 1.08/0.92, 1.5/0.5, and 1.8/0.2 with a melting point of 355 (eutectic), 500, 650, and 730 °C, respectively while keeping the total amount of the salts unchanged. The results show that with an increase in KCl/LiCl ratio, the nanocrystallites become less separable. By using the same eutectic Ni-K-NaCl solution (K/Li ) 0.82/1.18), we added aqueous ammonia, which forms complex ions [Ni(NH3)x]2+ ions in the solution that changes the formation pathway of NiO.8 The results showed that nanoparticles (50-100 nm) could also be obtained. In the case of the (K,Li) chlorides, we replaced LiCl by NaCl and used a KCl-
NaCl eutectic mixture (50/50). In this case, the nanoparticles could be formed around 700 °C or higher temperatures. To determine whether the salt concentration is another parameter that affects the separation of nanocrystallites, we changed the total concentration of K-Na-NO3 salts to 0.25, 0.5, 1.0, 2.0, and 4.0 mol/L. Nanoparticles were obtained at concentrations of 2.0 and 4.0 mol/L, while at the other concentrations they were not effectively separated. We also found at lower nickel precursor concentrations, a lower salt concentration could be used to effectively separate NiO nanocrystallites. Discussion As shown in Figure 1, the separation of nanocrystallites is strongly dependent on the synthesis temperature. In our experiments since all synthesis temperatures were higher than the eutectic temperatures (e.g., the eutectic points of K-Na-NO3 and K-Li-Cl are 222 and 355 °C),9 the salt mixtures exist in the liquid state in the reactor. In addition, salt melts are able to dissolve many types of compounds such as oxides because they function as high-temperature solvents.10 For example, NiO has solubilities of 6.76 × 10-3 and 3.07 × 10-5 mol/ kg at 700 °C in KCl-LiCl and KCl-NaCl respectively, which increase with temperature.11,12 Therefore, all chemical and physical changes, such as precursor decomposition, chemical reactions, solid-state reactions, crystallization, sintering, etc., take place in the aerosol micrometer/submicrometer particles which are suspended in gas and, therefore, can be regarded as microreactors. Because of the accelerated mass transfer in the liquidstate solvent compared to dry particles used in the conventional process, crystallite growth is greatly enhanced in the SAD process. Some nanocrystallites grow (9) Weast, R. C.; Astle, M. J.; Beyer, W. H. CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press: Boca Raton, FL, 198889. (10) Ito, Y.; Nohira, T. Electrochim. Acta 2000, 45, 2611. (11) Cherginets, V. L.; Rebrova, T. P. Electrochim. Acta 1999, 45, 469. (12) Hayashi, Y.; Kimura, T.; Yamaguchi, T. J. Mater. Sci. 1986, 21, 2876.
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Figure 4. TEM images of NiO particles heated with K-Na-NO3 in a muffle furnace at (a) 400 and (b) 600 °C. The inset TEM image shows the unruptured particles coexisting with nanoparticles.
by depleting their adjacent nuclei by transferring the mass of the dissolved nuclei onto their surfaces, referred to as dissolution-precipitation processes. In the meantime, salt melts can spontaneously diffuse onto crystallite surfaces to become a barrier between particles due to the ionic nature of both the melt and the desired materials,13 which can prevent the re-agglomeration of the newly formed crystallites. Therefore, the crystallites are separated/isolated and trapped into the salt matrix, and nanoparticles are obtained after removing the soluble salts by washing. Because of the accelerated mass transfer in the SAD process, it would be expected that the crystallinity of SAD product would be higher than that of the corresponding CAD product under the same synthesis conditions; i.e., SAD crystals have larger sizes than CAD crystals, as seen in the above results. Since dissolving is largely dependent upon temperature, particle separation is thus related to synthesis temperature. In addition, different salts or solvents have different physical and chemical characteristics, e.g., dissolving ability, solvent viscosity, reactivity, etc. Therefore, crystallite separation and growth would be expected to be different in different salt mixtures, e.g., nitrate, chloride, and acetate; thus products with different characteristics such as crystallite size and morphology would also be expected. Even for the same salts, e.g., the K,Li chlorides, increasing K/Li molar ratio leads to an increased melting point (when K/Li > eutectic composition) and a decreased solubility,11,12 and thus results in less effective separation. Replacing Li with Na can also result in a decreased solubility and an increased melting point, thus leading to less effective separability and a lower crystallinity. All of these have been observed in our experimental results as shown above. The decomposition of the salt mixture at high synthesis temperature will add more complexity to particle separation and growth due to the altered environment for particle formation and growth. Therefore, in our method product characteristics and properties can be affected by many operating parameters and thus are adjustable. As shown above, the introduction of salts in the SAD process causes tremendous changes compared to the (13) Xie, Y. C.; Tang, Y. Q. Adv. Catal. 1990, 37, 1.
CAD process. In these two processes, aerosol particles are in different states (solid or liquid), and mass transfer and the chemical and physical environment are significantly different. It is clear that by using salts, nanocrystallites are separated in situ and formed in a rapid aerosol process similar to the conventional one. The issue of whether nanocrystallites that have been formed in the conventional process can also be separated if they are heated with salts is of interest. Therefore, two-step experiments were conducted. First, NiO particles were prepared by using the CAD method at 600 °C. Second, the NiO product was mixed with K-Na-NO3 salts and heated at 400 and 600 °C respectively in a muffle furnace for 2 h. In both cases, the NiO particle shells were ruptured and could not able to maintain their previous morphologies on heating with the salts, demonstrating that nanocrystallite agglomerates can also be broken down in the subsequent heating in the salt melt. As shown in Figure 4, at 400 °C some nanocrystallites were separated but some shells remained (inset). Longer heating plus stirring might aid in separating the crystallites. It was also observed that product crystallinities were higher than those of the products synthesized at the same temperatures (400 and 600 °C) in the SAD process. NiO particle separation in the two-step processes, however, is not as effective as that in the SAD process; i.e., agglomeration is stronger in the former case. Conclusions The introduction of salts to the conventional aerosol decomposition causes substantial changes to the product particles. This paper shows how nanoparticles are formed in the modified aerosol decomposition procedure and how synthesis conditions affect the products. Agglomerated nanocrystallites, which are formed in the aerosol decomposition process, can be in situ separated on heating with salts. This work shows that the crystallite separation process is dependent on a number of factors. Synthesis temperature and types of salts are important in the separation process. 1. NiO nanoparticles can be synthesized using various precursors and salts, e.g., nitrates, chlorides, and ac-
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etates. NiO nanoparticles around 10 nm in size were obtained at 550 °C from nitrates, while they were also obtained from chlorides at 650 °C in sizes around 100 nm. Acetates require higher temperatures. Therefore, different salts and precursors may form remarkably different products. There is a suitable temperature range exists for crystallite separation. 2. Particle growth is enhanced in the SAD process due to enhanced mass transfer, and the crystallinity of the SAD product is higher than that of the corresponding CAD product. 3. Changing the molar ratio of salts leads to changes in melting point and solubility, and thus results in different product characteristics. The use of a eutectic mixture is recommended. 4. Large (micrometer/submicrometer) particles that have been formed and which consist of nanocrystallites can also be ruptured to form nanoparticles when heated in a salt melt.
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Acknowledgment. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO)’s “Nanotechnology Materials Program - Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI). Grant-in-Aid for Exploratory Research (K.O. and I.W.L.), Grant-in-Aid for Encouragement of Young Scientists (I.W.L.) and a Postdoctoral Fellowship (B.X.) sponsored by the Japan Society for the Promotion of Science and the Ministry of Education, Culture, Sports, Science, and Technology of Japan are gratefully acknowledged. The authors wish thank to Hiroyuki Mori and Yutaka Fujita for their assistance in these experiments and the Research Center for Nanodevices and Systems for TEM instruments. CM011684S
