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Control of size and morphology in NiO particles prepared by a low-pressure spray pyrolysis
Materials Research Bulletin 38 (2003) 1819–1827
Control of size and morphology in NiO particles prepared by a low-pressure spray pyrolysis I. Wuled Lenggoroa, Yoshifumi Itoha, Noritaka Iidab, Kikuo Okuyamaa,* a
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan b Technical Division, Noritake Kizai Co., Ltd., Higashiyama, Miyoshi-cho, Aichi 470-0293, Japan Received 5 June 2002; received in revised form 23 March 2003; accepted 10 August 2003
Abstract Nickel oxides (NiO) nano- and submicron particles were synthesized by a low-pressure spray pyrolysis system. Conversion of micrometer droplets into particles in the gas phase using NiO synthesis as a model was studied. Different types of precursors (aqueous Ni(NO3)26H2O, NiCl2, and Ni(HCO2)22H2O) and temperatures from 400 to 900 8C have great influences on controlling the size and morphology of final particles. The results show that operation of a low-pressure system leads to substantial changes in size and morphology of particles, and that the NiO particles having size around 20 nm can be generated under a limited of conditions. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; A. Oxides; B. Chemical synthesis; C. Electron microscopy; D. Microstructure
1. Introduction It is important to develop a process, in which particles having controlled characteristics including size, morphology, and composition can be produced. One of the promising methods used to fulfill these conditions is the spray pyrolysis [1–3]. Particle synthesis by spray pyrolysis involves the atomization of a precursor solution into discrete droplets. These droplets are subsequently transported through a furnace where the solvent is evaporated from the droplets and the dissolved species react to form the particulate product. Each droplet has the same composition, thus the desired materials particles can be easily synthesized by controlling the chemistry of the precursor solutions. In spray pyrolysis, the average size and size distribution of the final particles can be roughly determined from the size of the atomized droplet and the initial concentration of the starting solution. A variety of atomization methods *
Corresponding author. Tel.: þ81-824-24-7716; fax: þ81-824-24-7850. E-mail address: [email protected] (K. Okuyama). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.08.005
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has been used in spray pyrolysis studies [1,3] and generally, those atomizers are able to produce droplets in sizes between 1 and 100 mm. For instance, using the ultrasonic spray pyrolysis method, we have prepared various functional particles such as metal oxides [4], metals [5], and metal sulfides [6] in submicrometer order. The concept or the basis of the spray pyrolysis process assumes that one droplet forms one product particle. To date, submicrometer sized particles are typically formed in a spray pyrolysis process due to the difficulty on generation of fine droplets (below 1 mm). On the other hand, the preparation of nanoparticles (size below 100 nm) are of interest because the chemical and physical behavior of the particles are unprecedented and remarkably different from those in bulk form [7,8]. A few attempts have been reported to synthesize nanoparticles via spray pyrolysis route, such as electrospray pyrolysis [9] and salt-assisted aerosol decomposition [10,11]. They are capable to generate the nanoparticles but at a low efficiency. Kang and Park developed a low-pressure spray pyrolysis system using a filter expansion aerosol generator (FEAG) [12] and applied this process for the production of submicron sized oxide particles [12] as well as nanoparticles [13]. ZnO nanoparticles were formed via the low-pressure spray pyrolysis route [14], whereas submicron size ZnO particles were produced in the case of atmospheric pressure (ultrasonic) spray pyrolysis [13]. This difference in size may be attributed to the difference in operating pressures and aerosol formation mechanisms between the two routes, however, no systematic investigation on the effect of operation parameters on the particle size in the low-pressure spray pyrolysis has been reported. In this study, we describe systematic experiments that were designed to better understand how nanoparticles are generated during a low-pressure spray pyrolysis, and how experimental conditions affect the final products. NiO, an important material used in gas sensors [15], or battery materials [16], was used as a typical example to demonstrate how nanoparticles are converted from the solution droplets atomized by using the FEAG.
2. Experimental A schematic of the experimental apparatus of a low-pressure spray pyrolysis used to produce and collect the particles is shown in Fig. 1. The main equipment consists of (i) an atomizer that converts the starting solution into droplets, (ii) the carrier gas, (iii) a tubular electric furnace, (iv) a sampler or precipitator, and (v) vacuum controller. The atomizer used consists of a two-fluids nozzle (Ohkawara Kakohki, Co. Ltd., Yokohama) and a modified glass filter (Shibata Scientific Technology Ltd., Tokyo) having the diameter of 150 mm, the thickness 10 mm, and the pore size 5.5 and 16 mm. Liquid is sprayed through the two-fluids nozzle by a carrier gas on to a glass filter surface where it forms a thin liquid film. Water was used as the solvent. This liquid film and carrier gas passes through the filter pores, aided by the carrier gas and are expanded to the liquid jet, and then converted to the droplets into a low-pressure chamber. Kang and Park estimated that the mean size of solution droplets generated by FEAG was around 2 mm by measuring the size distributions of final particles [12]. The tubular furnace was a quartz tube of 38 mm inner diameter and about 800 mm-long. The furnace consists of four independently-controlled heating zones, each 200 mm in length, enabling flexibility in the production of the experimental temperature distributions. The carrier gas used was nitrogen. The pressure of the system was maintained at 40 or 10 Torr using a vacuum pump having a capacity of 15 m3/h. Residence time of the droplets or particles in the gas phase along the aerosol reactor could be controlled by
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Fig. 1. Experimental setup for preparing NiO particles by low-pressure spray pyrolysis using the filter expansion aerosol generator.
determining the pressure difference between the upper and lower surface or space of the glass filter as well as the carrier gas flow rate. For calculating more precise residence time, we also consider the thermal expansion of gas by measuring temperature profile along the reactor and monitoring pressure at the end of the reactor. The gas flow rate was kept at 2.0 l/min, unless otherwise stated. The nickel precursors, i.e. nitrates Ni(NO3)26H2O, chlorides NiCl2, and formate Ni(HCO2)22H2O dissolved in water were used as the precursors for producing NiO particles. The concentrations of nickel salts were kept at 0.15 mol/l, unless otherwise stated. The physical properties, i.e. solubility in water, viscosity and density, of these solutions are shown in Table 1. The solution was then atomized by means of an FEAG system and the droplet was carried by a stream of gas into a tubular reactor maintained at predetermined temperatures, followed by heating for several milliseconds up to 0.2 s. Aerosol products were collected in an electrostatic precipitator kept at around 150 8C. Products were characterized by means of a field-emission scanning electron micrograph (FE-SEM, S-5000, Hitachi, Tokyo) operated
Table 1 Physical properties of precursor solutions used
Ni(NO3)26H2O NiCl2 Ni(HCO2)22H2O a b
Compounds solubilitya Solutions (measured at 20 8C) (g/100 g H2O) Concentration (mol/l) Densityb (g/cm3)
Viscosityb (103 Pa s)
243.0 (0 8C) 1 (56.7 8C)
0.15 0.75
1.023 1.112
1.004 1.223
53.8 (0 8C) 87.6 (100 8C)
0.15
1.017
1.016
0.15
1.019
1.046
Slightly
Cited from ‘‘Rikagaku Jiten, Physical and Chemical Dictionary’’, Iwanami-Shoten, Tokyo, 1998. Measured in laboratory.
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at 20 kV and X-ray diffraction (XRD, Rint 2200 V, Rigaku, Tokyo) with Cu Ka radiation operated at 40 kV and 20 mA.
3. Results and discussion First, the effect of the type of precursor on particle size and morphology were examined. XRD analysis showed that the particles prepared from chloride (NiCl26H2O), nitrate (Ni(NO3)2), and formate (Ni(HCO2)22H2O) precursors by a low-pressure spray pyrolysis in the nitrogen atmosphere and below the pyrolysis temperature of 1000 8C were composed of NiO. Che et al. reported preparation of NiO submicron particles from nitrate solution by an ultrasonic spray pyrolysis in a N2 atmosphere and temperature between 400 and 1200 8C with residence time 30–150 s in the reactor [17]. Recently, we have discussed the solution chemistry and the reaction pathway of preparations of metallic nickel and NiO submicron particles from chloride, nitrate or formate precursor solutions via ultrasonic spray pyrolysis route in the absence and presence of H2 gas in the reduction atmosphere [5,18]. As shown in Fig. 2, nanoparticles were obtained only for the case of precursor of nickel nitrate. Most particles synthesized from nickel formate (Fig. 2a) were in the submicron order. The particles obtained from nickel chloride (Fig. 2b) were a mixture of submicron particles and nanoparticles of several 10 nm in size. At 700 8C (not shown), the chloride precursors produced only submicron NiO particles. Hence, size and morphology of the particles are greatly dependent on the starting metal solution. To more clearly elucidate the effect of experimental conditions on size and morphology of NiO nanoparticles, the nitrate precursor was used as the NiO source in all subsequent experiments. Fig. 3 shows FE-SEM images of the particles derived from the nitrate precursor (0.15 mol/l) at temperatures of 400, 700, and 900 8C, respectively, at an operating pressure of 40 Torr using a filter with the pore size of 5.5 mm. Residence time of the droplet/particle processing time was settled to be about 0.08 s. The submicron particles were observed at a reaction temperature of 400 8C (Fig. 3a). A large number of nanometer sized particles were agglomerated on the surface of submicron particles prepared at 700 8C (Fig. 3b). The number of submicron order particles greatly decrease, if the temperature is increased to 900 8C and the aggregate nanoparticles of about 20 nm were mainly observed, as can be seen from Fig. 3c. Again, this is obviously different from the particles obtained by
Fig. 2. NiO particles prepared from different precursors: formate (a), chloride (b), and nitrate (c) at temperature of 1000 8C. Other conditions: operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l; carrier gas flow rate, 2 l/min; glass filter size, 5.5 mm.
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Fig. 3. NiO particles prepared from nitrate precursor at different temperatures: 400 8C (a), 700 8C (b), and 900 8C (c). Other conditions: operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l; carrier gas flow rate, 2 l/min; glass filter size, 5.5 mm.
conventional spray pyrolysis, in which a number of the nanocrystallites virtually agglomerate into larger (submicron) particles. Also, what is more important is that a few nanoparticles appeared together with submicrometer particles in Fig. 2b (chloride sample) and Fig. 3b (nitrate sample, 700 8C) and these are clearly demonstrating that in the low-pressure spray pyrolysis, the abrupt evolution of considerable heat and gas aids in fragmenting larger particles into smaller pieces, coupled with evaporation-derived particle formation process. It is also noteworthy that the nanoparticles can be obtained in a single step process. Fig. 4 shows XRD spectra of the particle synthesized from nitrate precursor at different pyrolysis temperatures. Below 900 8C, the crystallinity of NiO submicron particles was increased with increasing
Fig. 4. Powder XRD of particles prepared from nitrate precursor at different temperatures: 900 8C (a), 700 8C (b), and 400 8C (c).
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reactor temperatures (from 400 to 700 8C). The particles prepared at 900 8C showed broad peaks of NiO structures comparing with that of 700 8C. In general the sharpness of the XRD peak (i.e. high crystallinity) is increased as the pyrolysis temperature increases. However, it should be noted that the 900 8C sample is nanoparticles in which the comparison between the crystallinity of the nano- and submicron particles cannot be distinguished easily from XRD powder patterns, even no structural change, beside the decrease of the NiO crystallite size. In our low-pressure system, NiO nano- and submicron particles were well-crystallized at a very short residence time (<0.1 s). This operation time is remarkably shorter than that of conventional spray pyrolysis [17] or the other chemical synthesis techniques [19] reflecting the advantage of low-pressure spray pyrolysis. Using Scherrer’s equation, the crystallite size of the sample was estimated from the XRD spectra. The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones in case of nanoparticles formation. The crystallite size is around 20 nm for the nitrate products synthesized at 900 8C, clearly demonstrating the acceleration of crystal growth in the very short heating time. Fig. 5a and b shows the products synthesized from 0.15 mol/l nitrate precursor at various residence time of the droplet/particles along the aerosol reactor using a filter with the pore size of 5.5 mm. The product prepared at a residence time of 0.02 (Fig. 5a) and 0.08 s (Fig. 3c) are nanometer sized particles around 20–30 nm in size with a spherical shape. From Fig. 5b, it is clear that the particles produced at a longer residence time are submicron sized spherical ones containing a few nanoparticles. Submicron particles shown in Fig. 2a (formate sample) or in Fig. 5b are typical of conventional spray pyrolysis products, based on the concept of the spray pyrolysis in which assumes that one droplet forms one product particle and also by assuming the size of droplet generated by a glass filter in the FEAG was around 2 mm [12]. With decreasing the residence time, i.e. accelerating the heating rate to the solution droplets, however, the low-pressure system products give rise to particles that are broken into smaller ones. At 0.2 s, nanocrystallites are still strongly agglomerated as observed by FE-SEM images and below 0.1 s the agglomeration becomes weaker. In order to determine the effect of initial stage (before the drying stage) as well as the droplet generation, two glass filters with different pore sizes (5.5 and 16 mm) were used in this study. The pressure loss measured between the upper and lower space of the glass filter of 5.5 and 16 mm were
Fig. 5. NiO particles prepared at different residence time: 0.02 s (a) and 0.2 s (b) using 5.5 mm glass filter. (c) Particles prepared using 16 mm glass filter. Other conditions: nitrate precursor operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l.
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approximately 330 and 140 Torr, respectively. Fig. 5c shows FE-SEM image of the NiO particles prepared using 16 mm glass filter. Nanometer order particles can be obtained from the two types of glass filters used. In this study, the initial droplet size as well as the size and morphology of final product had expected to be changed by changing the filter pore size. However, from 5.5 to 16 mm pore diameter, the nanoparticles did not show notable changes in size or morphologies. This result was confirmed by Kang and Park [14] by changing the solvent from water to methyl alcohol. They found that there was no significant change in the size of ZnO nanoparticles and claimed that the generation of nanoparticles in the FEAG process, was not influenced by the evaporation rate of the solvent. As shown in Table 1, there are no great differences on the density and viscosity of the solution precursors, however the solubility of nickel nitrate in water was the highest among the precursors used. One can suggest that the solubility plays an important role in the low-pressure spray pyrolysis and greatly influences the final product particles. The particle formation is regarded to be affected not only by the solubility of starting precursor, but also by the solvent evaporation rate, pyrolysis rate of droplet/precipitate, diffusion rate of precipitate, as well as the phase transition. Until now, however, it is clear that the formation of particles by spray pyrolysis is a complex process and is still difficult to accurately model [2,3]. The mechanism of agglomerated particle formation in a low-pressure spray pyrolysis will be described in the following. One can assume that in a conventional (atmospheric pressure) spray pyrolysis, homogeneous nucleation would occur when the solute concentration at the surface of the droplet reached the critical supersaturation. After nucleation of the solid, precipitation occurs only in the part of the droplet where the solute concentration is higher than the equilibrium saturation. From micron droplets, hollow (submicron) particles result if the solute concentration at the center of the droplet is less than the equilibrium saturation of the solute. Our numerical simulation model of conventional spray pyrolysis considered the effect of parameters such as process temperature and the initial solute concentration on the morphology of particles [2]. Lower process temperatures and higher initial solute concentrations favored the formation of dense (i.e. aggregate of nanocrystallites) particles. When the solvent evaporation rate is too high relative to the diffusion rate of the precipitation, hollow particles were obtained as in the case of conventional spray pyrolysis. In low-pressure spray pyrolysis as well as in its condition of predetermined high-temperature, it is predicted that the solvent evaporation rate is extremely higher than the precipitation diffusion rate. In this case, then, the nanoparticles or weak agglomerated particles are formed prior to the solid-state reaction without the formation of hollow particles. Therefore, in this study, it is also predicted that the operating pressure (i.e. low pressure) highly influences the solvent evaporation rate of the atomized droplet. The mechanisms of nanoparticle formation is showed schematically in Fig. 6. In case of a solute with a low water solubility (e.g. formate precursor), precipitates are abundantly generated that promote the agglomeration by solvent evaporation and thermal decomposition. It is because the supersaturation state is easily formed in the droplet by solvent evaporation, even if the solvent sufficiently exist in the droplet. Then, hollow or spherical particles were obtained. In case of a solution with a high solubility (e.g. nitrate precursor), the agglomeration is suppressed and the crystallite grow at a low number concentration. When the solvent evaporation rate is higher (i.e. short residence time) than precipitate diffusion rate, nanoparticles are formed. On the other hand, aggregated particles (in submicron) are obtained, when solvent evaporation rate is low, i.e. long residence time. Accordingly, the described particle formation mechanism in low-pressure environment that we suggested can demonstrate well our experimental result.
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Fig. 6. A schematic diagram of the droplet-to-particles conversion process in low-pressure spray pyrolysis.
4. Conclusions Conversion of droplet-to-nanoparticle in the gas phase using NiO synthesis as a model was investigated. It is clear that various operating conditions such as temperature, residence time, and type of salt influenced the size and morphology of the final particle. Controlling the solubility of the salt, the rate of solvent evaporation and precipitate diffusion are of importance to obtain nanoparticles and avoid the agglomeration of final particles in the low-pressure spray pyrolysis. It seems that the technique is one of the promising methods to synthesis nanoparticles with controlled morphology and size in large scale.
Acknowledgements The authors thank to Yutaka Fujita and Katsuaki Kurata for their assistance in the experiments and Dr. Yun Chan Kang for his valuable advice on building the apparatus. Grant-in-Aid sponsored by the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Society for the Promotion of Science are gratefully acknowledged. This work was also supported by the NEDO’s ‘‘Nanotechnology Particle Project’’ based on funds provided by the Ministry of Economy, Trade, and Industry, Japan.
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G.L. Messing, S.C. Zhang, G.V. Jayanthi, J. Am. Ceram. Soc. 76 (1993) 2707. I.W. Lenggoro, T. Hata, F. Iskandar, M.M. Lunden, K. Okuyama, J. Mater. Res. 15 (2000) 743. T.T. Kodas, M.J. Hampden-Smith, Aerosol Processing of Materials, Wiley, New York, 1999. Y.C. Kang, I.W. Lenggoro, K. Okuyama, S.B. Park, Mater. Res. Bull. 35 (2000) 789. B. Xia, I.W. Lenggoro, K. Okuyama, J. Am. Ceram. Soc. 84 (2001) 1425. I.W. Lenggoro, Y.C. Kang, T. Komiya, K. Okuyama, N. Tohge, Jpn. J. Appl. Phys. Part 2: Lett. 37 (1998) L288.
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C.N.R. Rao, J. Mater. Chem. 9 (1999) 1. C.M. Niemeyer, Angew. Chem. Int. Ed. 40 (2001) 4128. I.W. Lenggoro, K. Okuyama, J. Fernandez de la Mora, N. Tohge, J. Aerosol Sci. 31 (2000) 121. B. Xia, I.W. Lenggoro, K. Okuyama, Adv. Mater. 13 (2001) 1579. B. Xia, I.W. Lenggoro, K. Okuyama, Chem. Mater. 14 (2002) 2623. Y.C. Kang, S.B. Park, J. Aerosol Sci. 26 (1995) 1131. Y.C. Kang, S.B. Park, J. Mater. Sci. 16 (1996) 2409. Y.C. Kang, S.B. Park, J. Mater. Sci. Lett. 16 (1997) 131. A. Neubecker, T. Pompl, T. Doll, W. Hansch, I. Eisele, Thin Solid Films 310 (1997) 19. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. S.L. Che, K. Takada, K. Takashima, O. Sakurai, K. Shinozaki, N. Mizutani, J. Mater. Sci. 34 (1999) 1313. B. Xia, I.W. Lenggoro, K. Okuyama, J. Mater. Res. 15 (2000) 2157. C.L. Carnes, J. Stipp, K.J. Klabunde, Langmuir 18 (2002) 1352.
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Control of size and morphology in NiO particles prepared by a low-pressure spray pyrolysis I. Wuled Lenggoroa, Yoshifumi Itoha, Noritaka Iidab, Kikuo Okuyamaa,* a
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan b Technical Division, Noritake Kizai Co., Ltd., Higashiyama, Miyoshi-cho, Aichi 470-0293, Japan Received 5 June 2002; received in revised form 23 March 2003; accepted 10 August 2003
Abstract Nickel oxides (NiO) nano- and submicron particles were synthesized by a low-pressure spray pyrolysis system. Conversion of micrometer droplets into particles in the gas phase using NiO synthesis as a model was studied. Different types of precursors (aqueous Ni(NO3)26H2O, NiCl2, and Ni(HCO2)22H2O) and temperatures from 400 to 900 8C have great influences on controlling the size and morphology of final particles. The results show that operation of a low-pressure system leads to substantial changes in size and morphology of particles, and that the NiO particles having size around 20 nm can be generated under a limited of conditions. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; A. Oxides; B. Chemical synthesis; C. Electron microscopy; D. Microstructure
1. Introduction It is important to develop a process, in which particles having controlled characteristics including size, morphology, and composition can be produced. One of the promising methods used to fulfill these conditions is the spray pyrolysis [1–3]. Particle synthesis by spray pyrolysis involves the atomization of a precursor solution into discrete droplets. These droplets are subsequently transported through a furnace where the solvent is evaporated from the droplets and the dissolved species react to form the particulate product. Each droplet has the same composition, thus the desired materials particles can be easily synthesized by controlling the chemistry of the precursor solutions. In spray pyrolysis, the average size and size distribution of the final particles can be roughly determined from the size of the atomized droplet and the initial concentration of the starting solution. A variety of atomization methods *
Corresponding author. Tel.: þ81-824-24-7716; fax: þ81-824-24-7850. E-mail address: [email protected] (K. Okuyama). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.08.005
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has been used in spray pyrolysis studies [1,3] and generally, those atomizers are able to produce droplets in sizes between 1 and 100 mm. For instance, using the ultrasonic spray pyrolysis method, we have prepared various functional particles such as metal oxides [4], metals [5], and metal sulfides [6] in submicrometer order. The concept or the basis of the spray pyrolysis process assumes that one droplet forms one product particle. To date, submicrometer sized particles are typically formed in a spray pyrolysis process due to the difficulty on generation of fine droplets (below 1 mm). On the other hand, the preparation of nanoparticles (size below 100 nm) are of interest because the chemical and physical behavior of the particles are unprecedented and remarkably different from those in bulk form [7,8]. A few attempts have been reported to synthesize nanoparticles via spray pyrolysis route, such as electrospray pyrolysis [9] and salt-assisted aerosol decomposition [10,11]. They are capable to generate the nanoparticles but at a low efficiency. Kang and Park developed a low-pressure spray pyrolysis system using a filter expansion aerosol generator (FEAG) [12] and applied this process for the production of submicron sized oxide particles [12] as well as nanoparticles [13]. ZnO nanoparticles were formed via the low-pressure spray pyrolysis route [14], whereas submicron size ZnO particles were produced in the case of atmospheric pressure (ultrasonic) spray pyrolysis [13]. This difference in size may be attributed to the difference in operating pressures and aerosol formation mechanisms between the two routes, however, no systematic investigation on the effect of operation parameters on the particle size in the low-pressure spray pyrolysis has been reported. In this study, we describe systematic experiments that were designed to better understand how nanoparticles are generated during a low-pressure spray pyrolysis, and how experimental conditions affect the final products. NiO, an important material used in gas sensors [15], or battery materials [16], was used as a typical example to demonstrate how nanoparticles are converted from the solution droplets atomized by using the FEAG.
2. Experimental A schematic of the experimental apparatus of a low-pressure spray pyrolysis used to produce and collect the particles is shown in Fig. 1. The main equipment consists of (i) an atomizer that converts the starting solution into droplets, (ii) the carrier gas, (iii) a tubular electric furnace, (iv) a sampler or precipitator, and (v) vacuum controller. The atomizer used consists of a two-fluids nozzle (Ohkawara Kakohki, Co. Ltd., Yokohama) and a modified glass filter (Shibata Scientific Technology Ltd., Tokyo) having the diameter of 150 mm, the thickness 10 mm, and the pore size 5.5 and 16 mm. Liquid is sprayed through the two-fluids nozzle by a carrier gas on to a glass filter surface where it forms a thin liquid film. Water was used as the solvent. This liquid film and carrier gas passes through the filter pores, aided by the carrier gas and are expanded to the liquid jet, and then converted to the droplets into a low-pressure chamber. Kang and Park estimated that the mean size of solution droplets generated by FEAG was around 2 mm by measuring the size distributions of final particles [12]. The tubular furnace was a quartz tube of 38 mm inner diameter and about 800 mm-long. The furnace consists of four independently-controlled heating zones, each 200 mm in length, enabling flexibility in the production of the experimental temperature distributions. The carrier gas used was nitrogen. The pressure of the system was maintained at 40 or 10 Torr using a vacuum pump having a capacity of 15 m3/h. Residence time of the droplets or particles in the gas phase along the aerosol reactor could be controlled by
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Fig. 1. Experimental setup for preparing NiO particles by low-pressure spray pyrolysis using the filter expansion aerosol generator.
determining the pressure difference between the upper and lower surface or space of the glass filter as well as the carrier gas flow rate. For calculating more precise residence time, we also consider the thermal expansion of gas by measuring temperature profile along the reactor and monitoring pressure at the end of the reactor. The gas flow rate was kept at 2.0 l/min, unless otherwise stated. The nickel precursors, i.e. nitrates Ni(NO3)26H2O, chlorides NiCl2, and formate Ni(HCO2)22H2O dissolved in water were used as the precursors for producing NiO particles. The concentrations of nickel salts were kept at 0.15 mol/l, unless otherwise stated. The physical properties, i.e. solubility in water, viscosity and density, of these solutions are shown in Table 1. The solution was then atomized by means of an FEAG system and the droplet was carried by a stream of gas into a tubular reactor maintained at predetermined temperatures, followed by heating for several milliseconds up to 0.2 s. Aerosol products were collected in an electrostatic precipitator kept at around 150 8C. Products were characterized by means of a field-emission scanning electron micrograph (FE-SEM, S-5000, Hitachi, Tokyo) operated
Table 1 Physical properties of precursor solutions used
Ni(NO3)26H2O NiCl2 Ni(HCO2)22H2O a b
Compounds solubilitya Solutions (measured at 20 8C) (g/100 g H2O) Concentration (mol/l) Densityb (g/cm3)
Viscosityb (103 Pa s)
243.0 (0 8C) 1 (56.7 8C)
0.15 0.75
1.023 1.112
1.004 1.223
53.8 (0 8C) 87.6 (100 8C)
0.15
1.017
1.016
0.15
1.019
1.046
Slightly
Cited from ‘‘Rikagaku Jiten, Physical and Chemical Dictionary’’, Iwanami-Shoten, Tokyo, 1998. Measured in laboratory.
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at 20 kV and X-ray diffraction (XRD, Rint 2200 V, Rigaku, Tokyo) with Cu Ka radiation operated at 40 kV and 20 mA.
3. Results and discussion First, the effect of the type of precursor on particle size and morphology were examined. XRD analysis showed that the particles prepared from chloride (NiCl26H2O), nitrate (Ni(NO3)2), and formate (Ni(HCO2)22H2O) precursors by a low-pressure spray pyrolysis in the nitrogen atmosphere and below the pyrolysis temperature of 1000 8C were composed of NiO. Che et al. reported preparation of NiO submicron particles from nitrate solution by an ultrasonic spray pyrolysis in a N2 atmosphere and temperature between 400 and 1200 8C with residence time 30–150 s in the reactor [17]. Recently, we have discussed the solution chemistry and the reaction pathway of preparations of metallic nickel and NiO submicron particles from chloride, nitrate or formate precursor solutions via ultrasonic spray pyrolysis route in the absence and presence of H2 gas in the reduction atmosphere [5,18]. As shown in Fig. 2, nanoparticles were obtained only for the case of precursor of nickel nitrate. Most particles synthesized from nickel formate (Fig. 2a) were in the submicron order. The particles obtained from nickel chloride (Fig. 2b) were a mixture of submicron particles and nanoparticles of several 10 nm in size. At 700 8C (not shown), the chloride precursors produced only submicron NiO particles. Hence, size and morphology of the particles are greatly dependent on the starting metal solution. To more clearly elucidate the effect of experimental conditions on size and morphology of NiO nanoparticles, the nitrate precursor was used as the NiO source in all subsequent experiments. Fig. 3 shows FE-SEM images of the particles derived from the nitrate precursor (0.15 mol/l) at temperatures of 400, 700, and 900 8C, respectively, at an operating pressure of 40 Torr using a filter with the pore size of 5.5 mm. Residence time of the droplet/particle processing time was settled to be about 0.08 s. The submicron particles were observed at a reaction temperature of 400 8C (Fig. 3a). A large number of nanometer sized particles were agglomerated on the surface of submicron particles prepared at 700 8C (Fig. 3b). The number of submicron order particles greatly decrease, if the temperature is increased to 900 8C and the aggregate nanoparticles of about 20 nm were mainly observed, as can be seen from Fig. 3c. Again, this is obviously different from the particles obtained by
Fig. 2. NiO particles prepared from different precursors: formate (a), chloride (b), and nitrate (c) at temperature of 1000 8C. Other conditions: operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l; carrier gas flow rate, 2 l/min; glass filter size, 5.5 mm.
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Fig. 3. NiO particles prepared from nitrate precursor at different temperatures: 400 8C (a), 700 8C (b), and 900 8C (c). Other conditions: operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l; carrier gas flow rate, 2 l/min; glass filter size, 5.5 mm.
conventional spray pyrolysis, in which a number of the nanocrystallites virtually agglomerate into larger (submicron) particles. Also, what is more important is that a few nanoparticles appeared together with submicrometer particles in Fig. 2b (chloride sample) and Fig. 3b (nitrate sample, 700 8C) and these are clearly demonstrating that in the low-pressure spray pyrolysis, the abrupt evolution of considerable heat and gas aids in fragmenting larger particles into smaller pieces, coupled with evaporation-derived particle formation process. It is also noteworthy that the nanoparticles can be obtained in a single step process. Fig. 4 shows XRD spectra of the particle synthesized from nitrate precursor at different pyrolysis temperatures. Below 900 8C, the crystallinity of NiO submicron particles was increased with increasing
Fig. 4. Powder XRD of particles prepared from nitrate precursor at different temperatures: 900 8C (a), 700 8C (b), and 400 8C (c).
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reactor temperatures (from 400 to 700 8C). The particles prepared at 900 8C showed broad peaks of NiO structures comparing with that of 700 8C. In general the sharpness of the XRD peak (i.e. high crystallinity) is increased as the pyrolysis temperature increases. However, it should be noted that the 900 8C sample is nanoparticles in which the comparison between the crystallinity of the nano- and submicron particles cannot be distinguished easily from XRD powder patterns, even no structural change, beside the decrease of the NiO crystallite size. In our low-pressure system, NiO nano- and submicron particles were well-crystallized at a very short residence time (<0.1 s). This operation time is remarkably shorter than that of conventional spray pyrolysis [17] or the other chemical synthesis techniques [19] reflecting the advantage of low-pressure spray pyrolysis. Using Scherrer’s equation, the crystallite size of the sample was estimated from the XRD spectra. The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones in case of nanoparticles formation. The crystallite size is around 20 nm for the nitrate products synthesized at 900 8C, clearly demonstrating the acceleration of crystal growth in the very short heating time. Fig. 5a and b shows the products synthesized from 0.15 mol/l nitrate precursor at various residence time of the droplet/particles along the aerosol reactor using a filter with the pore size of 5.5 mm. The product prepared at a residence time of 0.02 (Fig. 5a) and 0.08 s (Fig. 3c) are nanometer sized particles around 20–30 nm in size with a spherical shape. From Fig. 5b, it is clear that the particles produced at a longer residence time are submicron sized spherical ones containing a few nanoparticles. Submicron particles shown in Fig. 2a (formate sample) or in Fig. 5b are typical of conventional spray pyrolysis products, based on the concept of the spray pyrolysis in which assumes that one droplet forms one product particle and also by assuming the size of droplet generated by a glass filter in the FEAG was around 2 mm [12]. With decreasing the residence time, i.e. accelerating the heating rate to the solution droplets, however, the low-pressure system products give rise to particles that are broken into smaller ones. At 0.2 s, nanocrystallites are still strongly agglomerated as observed by FE-SEM images and below 0.1 s the agglomeration becomes weaker. In order to determine the effect of initial stage (before the drying stage) as well as the droplet generation, two glass filters with different pore sizes (5.5 and 16 mm) were used in this study. The pressure loss measured between the upper and lower space of the glass filter of 5.5 and 16 mm were
Fig. 5. NiO particles prepared at different residence time: 0.02 s (a) and 0.2 s (b) using 5.5 mm glass filter. (c) Particles prepared using 16 mm glass filter. Other conditions: nitrate precursor operating pressure, 40 Torr; starting solution concentration, 0.15 mol/l.
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approximately 330 and 140 Torr, respectively. Fig. 5c shows FE-SEM image of the NiO particles prepared using 16 mm glass filter. Nanometer order particles can be obtained from the two types of glass filters used. In this study, the initial droplet size as well as the size and morphology of final product had expected to be changed by changing the filter pore size. However, from 5.5 to 16 mm pore diameter, the nanoparticles did not show notable changes in size or morphologies. This result was confirmed by Kang and Park [14] by changing the solvent from water to methyl alcohol. They found that there was no significant change in the size of ZnO nanoparticles and claimed that the generation of nanoparticles in the FEAG process, was not influenced by the evaporation rate of the solvent. As shown in Table 1, there are no great differences on the density and viscosity of the solution precursors, however the solubility of nickel nitrate in water was the highest among the precursors used. One can suggest that the solubility plays an important role in the low-pressure spray pyrolysis and greatly influences the final product particles. The particle formation is regarded to be affected not only by the solubility of starting precursor, but also by the solvent evaporation rate, pyrolysis rate of droplet/precipitate, diffusion rate of precipitate, as well as the phase transition. Until now, however, it is clear that the formation of particles by spray pyrolysis is a complex process and is still difficult to accurately model [2,3]. The mechanism of agglomerated particle formation in a low-pressure spray pyrolysis will be described in the following. One can assume that in a conventional (atmospheric pressure) spray pyrolysis, homogeneous nucleation would occur when the solute concentration at the surface of the droplet reached the critical supersaturation. After nucleation of the solid, precipitation occurs only in the part of the droplet where the solute concentration is higher than the equilibrium saturation. From micron droplets, hollow (submicron) particles result if the solute concentration at the center of the droplet is less than the equilibrium saturation of the solute. Our numerical simulation model of conventional spray pyrolysis considered the effect of parameters such as process temperature and the initial solute concentration on the morphology of particles [2]. Lower process temperatures and higher initial solute concentrations favored the formation of dense (i.e. aggregate of nanocrystallites) particles. When the solvent evaporation rate is too high relative to the diffusion rate of the precipitation, hollow particles were obtained as in the case of conventional spray pyrolysis. In low-pressure spray pyrolysis as well as in its condition of predetermined high-temperature, it is predicted that the solvent evaporation rate is extremely higher than the precipitation diffusion rate. In this case, then, the nanoparticles or weak agglomerated particles are formed prior to the solid-state reaction without the formation of hollow particles. Therefore, in this study, it is also predicted that the operating pressure (i.e. low pressure) highly influences the solvent evaporation rate of the atomized droplet. The mechanisms of nanoparticle formation is showed schematically in Fig. 6. In case of a solute with a low water solubility (e.g. formate precursor), precipitates are abundantly generated that promote the agglomeration by solvent evaporation and thermal decomposition. It is because the supersaturation state is easily formed in the droplet by solvent evaporation, even if the solvent sufficiently exist in the droplet. Then, hollow or spherical particles were obtained. In case of a solution with a high solubility (e.g. nitrate precursor), the agglomeration is suppressed and the crystallite grow at a low number concentration. When the solvent evaporation rate is higher (i.e. short residence time) than precipitate diffusion rate, nanoparticles are formed. On the other hand, aggregated particles (in submicron) are obtained, when solvent evaporation rate is low, i.e. long residence time. Accordingly, the described particle formation mechanism in low-pressure environment that we suggested can demonstrate well our experimental result.
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Fig. 6. A schematic diagram of the droplet-to-particles conversion process in low-pressure spray pyrolysis.
4. Conclusions Conversion of droplet-to-nanoparticle in the gas phase using NiO synthesis as a model was investigated. It is clear that various operating conditions such as temperature, residence time, and type of salt influenced the size and morphology of the final particle. Controlling the solubility of the salt, the rate of solvent evaporation and precipitate diffusion are of importance to obtain nanoparticles and avoid the agglomeration of final particles in the low-pressure spray pyrolysis. It seems that the technique is one of the promising methods to synthesis nanoparticles with controlled morphology and size in large scale.
Acknowledgements The authors thank to Yutaka Fujita and Katsuaki Kurata for their assistance in the experiments and Dr. Yun Chan Kang for his valuable advice on building the apparatus. Grant-in-Aid sponsored by the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Society for the Promotion of Science are gratefully acknowledged. This work was also supported by the NEDO’s ‘‘Nanotechnology Particle Project’’ based on funds provided by the Ministry of Economy, Trade, and Industry, Japan.
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