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Preparation of nanoparticles via spray route
Chemical Engineering Science 58 (2003) 537 – 547
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Preparation of nanoparticles via spray route Kikuo Okuyama∗ , I. Wuled Lenggoro Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan
Abstract Nanometer-sized particles (1–100 nm) are of considerable interest for a wide variety of applications, ranging from electronics via ceramics to catalysts, due to their unique or improved properties that are primarily determined by size, composition and structure. In this study, we report a simple, rapid and generalizable aerosol decomposition (spray pyrolysis) process for the continuous synthesis of nanoparticles with adjustable sizes, narrow size distribution, high crystallinity and good stoichiometry. The production of spherical-shaped porous particles with nanoscale ordering porosity and the zinc oxide quantum dots in silica nanoparticles matrix by means of a spray drying method using a colloidal mixture as the precursor and by the combined sol–gel and spray drying method were also reported. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Nanoparticle; Spray pyrolysis; Aerosol; Droplet; Spray drying; Colloids
1. Introduction Ultra9ne particles or nanoparticles (size between a few nanometer and 100 nm) are of interest, because the chemical and physical behavior of the particles is unprecedented and remarkably di:erent from those in bulk form. They have great potential for use in applications in the electronic, chemical or mechanical industries, as well as technologies with them, including superconductors, catalyst, drug carriers, sensors, magnetic materials, pigment, and in structural and electronic materials. It is well known that an unagglomerated spherical particle with a narrow size distribution (monodispersed) is the preferred state for applications and technologies, especially for the compacting and sintering of particles. Large number techniques for the preparations of ultra9ne particles that satisfy this requirement have been developed. Among these, a sol–gel process, a “build-up” type of liquid-phase method, has been most widely studied. Some of the other liquid phase chemical techniques using metal salts as a starting precursor, such as hydrothermal processes, homogeneous precipitation methods or reversed micelles, however, have several problems: the concentration of reacting species must be low and a long reaction time is required. ∗ Corresponding author. Tel.: +81-824-24-7716; fax: +81-824-24-7850. E-mail address: [email protected] (K. Okuyama).
It is important to develop a process in which particles having controlled characteristics including size, morphology, and composition can be produced. To be industrially relevant, the process needs to be low cost and involve both continuous operation and a high production rate. Functional materials with speci9c physical and chemical properties begin with molecular precursors that must be transformed into the product. The general method used to ful9ll this transformation is the aerosol process (gas-phase method). An aerosol is de9ned as a suspension of solid or liquid particles in a gas. The term “aerosol” is used in the 9elds of atmospheric, health and air pollutant, as well as in material processing. There are at least two routes for the preparation of ultra9ne particles by aerosol processes. The 9rst involves gas-to-particle conversion (a build-up method) and the second liquid-to-solid particle conversion (a break-down method). In gas-to-particle conversion, particles are generated by cooling a supersaturated vapor involving the use of methods called physical vapor deposition (PVD) or “evaporation–condensation method” and chemical vapor deposition (CVD) (Okuyama, Huang, Seinfeld, Tani, & Kousaka, 1991). In PVD, the evaporation of a solid or liquid is the source of the vapor. In the cooling stage, nucleation and condensation of the saturated vapor take place and solid particles are formed. In CVD, the vapor evaporated from the solution precursors is thermally decomposed or is reacted with another precursor vapor or a surrounding
0009-2509/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(02)00578-X
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gas. Finally, the solid particles are formed by nucleation, condensation and coagulation. The primary advantages of the gas-to-particle conversion method are the small particle size (a few nanometers to micrometer), narrow size distribution and high purity of the particles produced. However, as disadvantages, the formation of hard agglomerates in the gas phase leads to diHculties in preparing high-quality bulk materials. It is also diHcult to synthesize multicomponent materials (e.g., phosphors and superconductors), because of the di:erences in chemical reaction rate, vapor pressure, nucleation and growth rate which occur during the gas-to-particle conversion may lead to non-uniform composition. Many types of reactors, such as Jame, furnace, plasma, or laser, have been used to facilitate the gas-to-particle conversion route. Liquid-to-particle conversion (i.e., spray pyrolysis) is a representative “break-down” method for aerosol processing. Spray method is often classi9ed as a liquid-phase method because solutions or sols are used. This method has been used to prepare numerous types of functional particles. Multicomponent materials are also easily prepared by this route. Relative to the gas-to-particle conversion route, spray method is a simple and low-cost process, but our understanding of the process itself is not well understood (Lenggoro, Hata, Iskandar, Lunden, & Okuyama, 2000a). In this paper, a novel aerosol decomposition (spray pyrolysis) process for the continuous synthesis of nanoparticles as well as the electrospray pyrolysis was reviewed. The production of spherical-shaped porous silica particles with nanoscale ordering porosity by means of a spray drying method using a colloidal mixture as the precursor and zinc oxide nanoparticles in silica matrix prepared by the combined sol–gel and spray drying method were also reported. 2. Particle preparation by spray method 2.1. Spray pyrolysis and spray drying process To prepare 9ne particles by spray pyrolysis, a starting solution is prepared by dissolving, usually, the metal salt of the product in the solvent. The droplets, which are atomized from a starting solution, are introduced to the furnace. Evaporation of the solvent, di:usion of solute, drying, precipitation, reaction between precursor and surrounding gas, pyrolysis, or sintering may occur inside the furnace to form the 9nal product. Spray drying is similar to spray pyrolysis except the type of precursor. For spray drying method, the colloidal particles or sols are typically used as precursors. This method has also the capability of producing uniformly spherical particles from submicron to micron sizes. If the suspension consists of colloidal nanoparticles (primary particles), the resulting particles also comprise nanoparticles to form a nanostructured powder. Therefore, spray drying may be a suitable process for consolidating nanoparticles into
macroscopic compacts, and submicron spherical powders that have nanometer-scaled properties can be obtained. 2.2. Starting solutions The composition of the 9nal particle is determined by the solutes or reactants dissolved in the starting solution in predetermined stoichiometric ratio. As precursor solutions, usually inexpensive materials such as nitrate, chloride and acetates are typically used. It is necessary to eliminate solubility problems and phase segregation, where the di:erent components precipitate at di:erent times. Water or alcohol is usually used as a solvent. To prepare 9ne particles by spray drying method, colloidal suspension/sols are used as precursors (Iskandar, Lenggoro, Xia, & Okuyama, 2001a). The suspension, which contains liquid and solid particles, is sprayed and the liquid phase (the solvent) evaporates from the droplets. 2.3. Droplet generation The average size and size distribution of the 9nal particles can be roughly determined from the size of the atomized droplet and the initial concentration of the starting solution. Atomization is the production of droplets and their dispersion into the gas. The size or morphology of the 9nal particles produced can also be determined by the concentration and velocity of the droplet generated by the atomizers. A variety of atomization methods (Bayvel & Orzechowski, 1993) has been used in spray pyrolysis studies, such as air-assist (pneumatic) or a two-Juids nozzle, ultrasonic, vibrating ori9ce and spinning disk. 2.4. Particle morphology Properties of precursor, carrier gas Jow rate (i.e., residence time of heating or the solvent evaporation) and the temperature are the main parameters, which a:ect the morphology of particles generated by spray pyrolysis. Since this is a somewhat empirical process, morphology control seems to be the goal of most studies in the area of spray pyrolysis. Concerning the evolution of particle morphology during spray pyrolysis, in addition to solid (dense) and spherical particles, hollow or fragmented particles are often formed (Fig. 1). These morphological conditions are undesirable for most applications. Single-crystal or polycrystal particles are also formed, depending on the operation conditions. 2.5. Apparatus and procedure for the preparation of particles by spray method A representative system used for the preparation of 9ne particles by spray method is schematically shown in Fig. 2. The main equipment consists of: (i) an atomizer or nebulizer that converts the starting solution to droplets, (ii) carrier gas(es), (iii) tubular furnace/reactor, (iv) sampler or
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Fig. 1. Morphology of particle preparing by spray method.
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precipitator (e.g., 9lter, electrostatic precipitator, thermophoretic sampler). The furnace length is selected so as to satisfy the residence time of the droplets/particles during the heating. The furnace can include several independently controlled heating zones which enables the desired temperature distribution to be achieved. Instruments for measuring particle size such as a di:erential mobility analyzer (DMA) and a condensation nucleus/particle counter (CNC/CPC) can be placed at the inlet or the outlet of the furnace to measure the size distribution of the aerosols in real time/online. The size of the particles also can be analyzed o:-line by electron microscopy, after the particles are sampled by the precipitator. Other analysis such as for crystallinity (powder X-ray di:raction (XRD)), luminescent or electrical properties can be performed. In spray pyrolysis, a change in solution concentrations can be used to easily control the mean size of the 9nal particles at a constant size of the sprayed droplet. The overall solution concentration can typically be varied from 0.01 to above a few mols/litre for changing the mean size of particles. The precursors (e.g., metal nitrates, acetates or chlorides) are dissolved in distilled water to give the desired molar ratio of the 9nal product. All precursors were reagent grade and are usually used without further puri9cation. A small amount of acid (e.g., nitric) may be added during the preparation of the homogeneous solutions. The temperature of the hot-wall aerosol reactor was changed from 500◦ C to 1700◦ C. In some cases, in order to prevent oxidation of the materials, a mixture of N2 and H2 were used as carrier gas. The overall gas Jow rate was adjusted to several litres/minutes and the corresponding residence time of the particles inside the reactor was around several seconds at atmospheric pressure.
Fig. 2. Representative spray pyrolysis system used for particle preparation
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3. Preparation of nanoparticles via spray route The concept or the basis of the spray pyrolysis process assumes that one droplet forms one product particle. To date, submicrometer-to-micrometer-sized particles are typically formed in an spray pyrolysis process due to the ineffective generation of very 9ne droplets, although the process has been widely used to synthesize multicomponent materials in a simple process. A few attempts have been to synthesize nanoparticles using the spray pyrolysis process. Typically, fuels (e.g. alcohols, ureas and sucroses), dissolved in precursor solutions, were then misted and decomposed in a high-temperature Jame or combustion processes to form ceramic oxide powders. In these processes, the abrupt evolution of considerable heat and gas aids in breaking or fragmenting the large particles into smaller pieces, coupled with evaporation-derived particle formation process. Particles are formed from concurrent gas-to-particle and liquid-to-particle conversions. Electrospray (Section 3.1) is capable of generating 9ne droplets as well as nanoparticles, and low-pressure spray pyrolysis (Section 3.2) has also been used to fragment particles to give nanoparticles. Recently, we reported a new synthesis technique via spray pyrolysis process (salt-assisted aerosol decomposition (SAD); Section 3.3) for the continuous synthesis of nanoparticles with adjustable sizes, a narrow size distribution, high crystallinity, and good stoichiometry (Xia, Lenggoro, & Okuyama, 2002). The desired nanoparticles are formed and separated in salt microreactors (droplets) suspended in a gas. 3.1. Electrospray pyrolysis In spray pyrolysis, the average size of the 9nal solid particle can be determined from the droplet size of the solution sprayed. Most studies have used a variety of typical atomizers such as twin Juid or an ultrasonic nebulizer to generate the droplets. These atomizers are capable of producing droplets with an average size in the range of several microns. For a typical initial droplet with a diameter of 5 m to dry into a particle with a diameter of 100 nm, the initial volume fraction of dissolved involatile solute must be less than 0.0008%. In practice, these low solution concentrations may lead to a low rate of particle generation and may a:ect the purity of the particles ultimately. In the other words, the preparation of ultra9ne material particles with diameters of less than 100 nm via a conventional spray pyrolysis method remains a problem. The electrospray technique has been examined as a method for producing ultra9ne droplets. In this method, a meniscus of a spray solution at the end of the capillary tube becomes conical when charged to a high voltage (several kilovolt) with respect to a counter electrode. The droplets are stably formed by the continuous breakup of a jet extending from this liquid cone (Fig. 3), generally referred to as a “Taylor cone”. A variety of experimental studies have shown that the diameter of such jets and droplets may
Fig. 3. (Upper) Schematic of the formation of liquid meniscus (cone) and the exerted jet and droplets in electrospray. (Lower) Liquid meniscus at the capillary tip in a stable cone-jet mode with a 9ne jet (approximately a few micrometers in diameter). Methanol liquid, Jow rate 0:20 ml=h, applied voltage 3 kV.
be controlled by means of a nanometer up to hundreds of micrometers. The preparation of zinc sul9de (ZnS) particles below 50 nm in diameter by an electrospray pyrolysis method has been reported (Lenggoro, Okuyama, Fernandez de la Mora, & Tohge, 2000b). For a preparation of ZnS nanoparticles, it is necessary to control the spray liquid Jow rate, the applied voltage and the solute concentration, and to ensure that stable spray conditions in the so-called cone-jet mode are operational. The particles generated can be measured by a DMA and a CNC system, and their sizes are comparable with those expected after drying from available scaling laws. In addition, by sampling the particles, the suitability of electrospray pyrolysis for generating nanometer-sized spherical ZnS 9ne particle was investigated. Using the DMA–CNC system, the results showed that ZnS particles were generated in a size ranging from 20 to 30 nm. Fig. 4 shows TEM photographs of ZnS particles prepared from a solution concentration of 0:01 mol=l. The shapes of the particles are spherical, and their size distributions were similar to those obtained by DMA–CNC. Recently, we developed a simple technique for sizing colloidal particles by means of electrospray and aerosol techniques (Lenggoro, Xia, Okuyama, & Fernandez de la Mora, 2002). Size distribution of di:erent types of colloids (oxides, metals and polymers) with di:erent nominal sizes below 100 nm was determined online. Nanometer-sized particles were dispersed into the gas phase as an aerosol via electrosprays operating in the cone-jet mode of a colloidal solution followed by a charge reduction of the sprayed
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For multicomponent particles, such as rare-earth-doped phosphor materials, the fragmentation of particles into nanometer particles in the FEAG process did not easily occur. In addition, for preparing phosphor particles with an activator, the FEAG route requires post-annealing after spray pyrolysis at low pressure. This is due to the short residence time in the aerosol reactor of below 0:1 s (estimated). For example, in the preparation of Zn2 SiO4 : Mn phosphor particles by spray pyrolysis using the FEAG technique, a pure phase willemite (Zn2 SiO4 ) structure was obtained after post- annealing at 1200◦ C for 5 h. Fig. 4. TEM photographs of ZnS particles prepared by an electrospray pyrolysis at a temperature of 600◦ C. Magni9cation of (a) 50; 000× and (b) 250; 000×. Particle size was coincided well with those measured online by aerosol sizing technique (DMA-CNC system). (Reprinted with permission from I. W. Lenggoro et al. (2000). J. Aerosol Sci., 31, 121. c 2000 Elsevier Science)
droplets to unity and subsequent evaporation of the solvent. The size distribution of the generated aerosol particles was then determined by a DMA combined with a CNC. The proposed technique is capable of detecting the degree of dispersity of colloid samples and the measured values were comparable to results obtained by electron microscopy and the dynamic light scattering. 3.2. Low-pressure spray pyrolysis using 7lter expansion aerosol generator (FEAG) In low-pressure conditions, a two-step spray aerosol generator, referred to as FEAG, was developed by Kang and Park (1995). The FEAG is composed of two-Juid nozzle for dispersing a liquid, a porous glass 9lter, and a vacuum pump. Liquid is sprayed through a pneumatic nozzle by a carrier gas on to a glass 9lter surface where it forms a thin liquid 9lm. This liquid 9lm and carrier gas pass through the 9lter pores, aided by the carrier gas and are expanded into a low-pressure chamber. The mean droplet size was estimated to be around 2 m and the standard deviation of the droplet size distribution was 1.76. The FEAG process (low-pressure spray pyrolysis) was applied to the production of nanometer order particles (ZnO) and submicron phosphor particles such as Zn2 SiO4 : Mn. The morphology of particles prepared by the FEAG process was di:erent from those produced by ultrasonic spray pyrolysis. ZnO nanometer particles were formed via the FEAG route whereas submicron size ZnO particles were produced in the case of ultrasonic spray pyrolysis. This di:erence in size and morphology can be attributed to the di:erence in operating pressures and aerosol formation mechanisms between the two types of aerosol generators. However, no systematic investigation on the e:ect of reactor pressure on the morphology and size in spray pyrolysis has been reported. In the FEAG process, the generation rate of a droplet may be increased with increasing surface area of the glass 9lter and by scaling-up, and it is expectable that this process will be used industrially in the near future.
3.3. SAD method In the conventional route, each spray pyrolyzed particle has multiple nanosized crystallites under typical process conditions, but they are virtually inseparable due to the formation of a three-dimensional network. The salt-assisted spray pyrolysis (i.e., SAD) method proposed by Xia, Lenggoro, and Okuyama (2001a) focused on a strategy for separating these nanocrystallites by introducing some compounds that can distribute on the nanocrystallite surfaces to prevent them from agglomerating and are then easy to remove. During the SAD process, when the particle temperature exceeds the melting point of the salts, the salts melt and act as high-temperature solvents. The material or its components can then dissolve, undergo reactions, and, upon exceeding the solubility limit, precipitate in the solvent. These processes, absent in the conventional spray pyrolysis (conventional aerosol decomposition (CAD)), can remarkably enhance mass transfer due to the liquid-state solvent, in contrast to the very small solid-state di:usion coeHcients in the case of the CAD processes. Within an aerosol particle, the dissolution/precipitation cycle can lead to the dissolution of some nanocrystallites and the growth of other crystallites by precipitation. This may break up the three-dimensional network and disintegrate the nanocrystallites, which has been observed in the experiments. Fig. 5 illustrates the experimental setup and the particle formation processes. To demonstrate the feasibility of this method, eutectic mixtures or single salts, e.g., chlorides or nitrates of Li, Na, K, were dissolved in an aqueous precursor solution. The aerosol particles passed through the reactor in less than 5 s, then they were cooled and particles, which contained the 9nal nanoparticles within a salt matrix, were collected. The nanoparticles were obtained after washing the product with water to remove the salts. The spherical and dense submicrometer to micrometer particles, typical particle morphologies formed in the conventional spray pyrolysis processes, remain unchanged after washing. In contrast, the SAD sample shows a substantial change after washing and nanoparticles are obtained. Fig. 6 shows scanning and transmission electron microscopy (SEM, TEM) images of two Y2 O3 –ZrO2 samples, one synthesized using the CAD and the other using the SAD
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process. The spherical and dense submicrometer to micrometer CAD particles, typical particle morphologies formed in AD processes, remain unchanged after washing. In contrast, the SAD sample shows a substantial change after washing and nanoparticles are obtained. The CAD particle sizes are broadly distributed over a range of 0:11 ± 2:12 m with a mean size dp of 0:66 m and a geometric standard deviation g of 1.76, showing a typical polydispersity (1:3 ± 2:0) of a material formed in aerosol processes. Interestingly, the SAD sample (Fig. 6d), formed in the same aerosol process, has a remarkably sharpened size distribution (close to the normal distribution) in the range of 6:3 ± 18:7 nm with a
Fig. 5. Schematic illustration of the experimental apparatus and an illustration of particle formation both for the CAD and the SAD processes. Solution droplets were generated by an atomizer and were carried by a gas into a tube reactor typically operated between 300◦ C and 1600◦ C. The resulting particles were collected in an precipitator. (Reprinted with c 2001 permission from B. Xia et al. (2001). Adv. Mater., 13, 1579. Wiley-VCH)
dp of 12:8 nm and a g of 1.19. In other words, the size distribution of the SAD particles appears to be independent of that of their parent solution droplets. Addition of the salts causes particle size reduction by a factor (dCAD =dSAD ) of over 50. A variety of other materials such as NiO, Ag–Pd, ZnS and CeO2 with nanometer sizes have been synthesized using the SAD method (Xia, Lenggoro, & Okuyama, 2001b). Figs. 7a and b show CeO2 particles synthesized by SAD technique (temperature of 800◦ C) with the addition of the eutectic salts to the precursor solution prior to the aerosol decomposition. The SAD product (Fig. 7a) is composed of isolated nanoparticles (mean size 51 nm) while the CAD product consists of submicron-to-micron-sized particles (mean size 0:74 m) containing sintered nanocrystallites. Powder XRD patterns of the CAD and the SAD products, shown in Figs. 8a and b, illustrate the well-de9ned CeO2 cubic phase. The SAD product has a much higher crystallinity than the CAD product, as shown from the sharp peaks in Fig. 8b. The crystallite size of the SAD 800◦ C sample is 54:4 nm. This is much larger than the corresponding CAD sample (13:8 nm) and even than the CAD 1000◦ C sample (29:9 nm); it is also bigger than the CeO2 crystallites (about 40 nm) that were heated at 800◦ C for 2 h, while the SAD product in this work was heated for only a few seconds. The SAD CeO2 particles are single crystals while the CAD CeO2 particles are polycrystalline. The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones at all synthesis temperatures. The typical crystal lattice image shown in Fig. 7b con9rms the presence of single-crystalline particles. In addition to the above
Fig. 6. Y2 O3 –ZrO2 samples synthesized by the CAD process (a,b) and the SAD process (c,d). (a,c) SEM images of the samples before washing and (b,d) TEM images after washing. Note that the unwashed SAD particles (c) are larger than the unwashed CAD particles (a) due to the presence of salts. c 2001 Wiley-VCH) (Reprinted with permission from B. Xia et al. (2001). Adv. Mater., 13, 1579.
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Fig. 7. CeO2 nanoparticles synthesized by the SAD method at (a) 800◦ C, and (b) a typical high-resolution TEM image of sample (a), showing the crystal lattice of a particle; (c) 900◦ C, and (d) 800◦ C with addition of acetic acid to the solution prior to aerosol decomposition. (Reprinted with c 2001 The Royal Society of Chemistry) permission from B. Xia et al. (2001). J. Mater. Chem., 13, 2925.
Fig. 8. Powder XRD patterns of products synthesized at (a) CAD, 800◦ C (CeO2 ); (b) SAD, 800◦ C (CeO2 ). The inset shows that the addition of HAc to the precursor solution causes a decrease in crystallite size. The SAD pattern (b) has much sharper peaks than the corresponding CAD pattern (a). The XRD patterns were recorded at room temperature using Cu Ka radiation operated at 40 kV and 20 mA. (Reprinted with permission from B. Xia et c 2001 The Royal Society of Chemistry) al. (2001). J. Mater. Chem., 13, 2925.
mentioneddi:erences,itisalsoworthnotingthatthegeometric standard deviation (g ) of the CAD particles is determined as 1.63, a typical polydispersity of aerosol particles produced by ultrasonic aerosol decomposition, while it is only 1.25 for the corresponding SAD particles. Clearly, the particle size distribution of the SAD product has been remarkably narrowed in comparison to the CAD product (Fig. 6).
The CAD CeO2 particle sizes change slightly with synthesis temperature, because it is well understood that the CAD particle size is primarily determined by the droplet size (mainly depending on the transducer vibration frequency) and concentration of the precursor solution. In the SAD process, however, we found that many factors such as precursor(s), inert salts, additives and process parameters can be
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used to control particle size and morphology. As shown in Fig. 7c, for example, the mean particle size of the SAD product increases from 51 nm (Fig. 7a) to 119 nm when the synthesis temperature is increased from 800◦ C to 900◦ C. Fig. 7d shows that the addition of acetic acid as an additive to the precursor solution resulted in a reduction in the 9nal particle size from 51 to 21 nm. The broadened XRD peak is clearly seen in the inset of Fig. 8. The SAD process expands the concept of the spray pyrolysis as a one-step process and, thus, its applications to materials synthesis, namely that one droplet can produce multiple 9ner particles. This route can o:er good controllability of particle size, chemical composition, and material crystallinity,allofwhichareimportantforadvancedmaterials. 4. In situ production of spherical silica particles containing self-organized mesopores by spray drying The synthesis of mesostructures porous silica represents a fascinating and intellectually challenging problem due to its potential for applications in catalysts, chromatography, the controlled release of drugs, low dielectric constant 9llers, pigments, microelectronics, and electro-optics. When supramolecular surfactant micelles are used in the synthesis, the pore sizes are in the range of 10 nm or less. Synthesis via colloid crystallization allows the pore sizes to be controlled in the range of nanometers to micrometers. Iskandar, Mikrajuddin, and Okuyama (2001b, 2002) reported the production of spherical-shaped porous silica particles with nanoscale ordering porosity by means of a spray drying method using a colloidal mixture of silica and polystyrene (PS) latex as the precursor. The setup used in the work consists of a nebulizer, a vertical tube reactor and a 9lter for sampling. Colloidal silica particles (particle size around 5 nm) were used as a precursor, which was mixed with PS latex (various particle sizes), in a certain fraction in water to form a dilute solution, which was then atomized to generate droplets. The aerosol reactor (an inner diameter of 13 mm and a length of 1000 mm) consisted of two heating zones, the 9rst having a 9xed temperature of 200◦ C and the second, a 9xed temperature of 450◦ C. The 9rst zone is used to evaporate the solvent in the droplet, resulting in large particles of composite consisting of primary silica particles and PS latex particles. The second zone was used to evaporate the PS latex particles, which resulted in the formation of porous silica particles. The Jow rate of the nitrogen carrier gas was maintained at 1 l=min. Scanning electron micrographs in Fig. 9 reveals the ordered arrangement of pores on the surface of spherical particles. The pore sizes are similar to the PS latex particle sizes. This result is the 9rst observation of such a self-organization process of particles in the environment formed by other colloidal particles. The spacing between the pores is very homogeneous, approximately two primary silica particles in thickness. This result indicates that the spacing between the
Fig. 9. SEM images of the surface morphology of silica powders using 79 nm PS latex particles. (Reprinted with permission from F. Iskandar c 2001 American Chemical Society) et al. (2001). Nano. Lett., 1, 231.
pores can be easily controlled by changing the size of the primary silica particles. From experiments using di:erent sizes of PS latex particles, the hexagonal packing of the pores inside the produced particles is still observed. In addition, the ordering of the pores increases with the increase in the PS latex particles size. Indeed, this result is in agreement with the commonly observed ones that ordering of larger particles in general is easier than ordering of the smaller particles. Fig. 10 shows data on the variation of pore spacing for distinct fractions of PS latex and primary silica particles. As expected, the spacing between the pores increases when the fraction of PS latex is decreased. However, the long-range ordering of pores on the particle surface is reduced. A further increase in the PS latex fraction results in the formation of brittle particles, many of which are broken. Fig. 11 shows combinations which produced successful (open circle) and unsuccessful (solid three angle) organizations. Consider three contacting PSL particles developing hexagonal structure. The maximum diameter, d, of a silica particle 9lling the√space between the three contacting PSL particles is d = (2 3=3 − 1)D with D denoting the diameter of a PSL particle. Therefore, the maximum ratio of silica nanoparticle and PSL sizes to produce organization arrangement is d=D = 0:155. This value is represented by a straight line in Fig. 11. Most of our experiment results are consistent with this calculation. The successful combinations lay below the line and the unsuccessful combinations lay above the line. The mesopores was observed to be arranged into a hexagonal packing, indicating the self-organization process occurred spontaneously during the solvent evaporation. The
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pore size was controlled by changing the size of the PSL particles. By adding an additional zone (third zone) maintained at high temperatures, the produced powders could be in situ annealed. A comparison of the average volume of the powder before and after annealing (at 1500◦ C) indicated that the porosity of the powder was about 70%, indicating that pores were present both on surface of the powders and inside the powder particles. By exploiting colloidal PSL nanoparticles templating and “over-sputtering”, we reported a novel method for simultaneously producing three types of structural organized layers on a mesoscopic scale: domes, dots, and pores (Mikrajuddin, Iskandar, & Okuyama, 2002). 5. Zinc oxide nanoparticles in silica particles matrix prepared by the combined sol--gel and spray drying
Fig. 10. SEM images of a spherical silica powder produced by varying the content ratio of PS latex and silica: 10 ml of silica colloids (0.05%) mixed with (a) 0:5 ml, (b) 1 ml and (c) 2 ml of PS latex colloid (3:6 × 1013 particles=ml). The size of PS latex particles were maintained at 79 nm. (Reprinted with permission from F. Iskandar et al. (2001). c 2001 American Chemical Society) Nano. Lett., 1, 231.
Fig. 11. Combination of PS latex sizes, and silica nanoparticles sizes that exhibited successful arrangement (open circle) and unsuccessful arrangement (solid three angle). (Reprinted with permission from F. Iskandar c 2002 American Chemical Society) et al. (2002). Nano. Lett., 2, 389.
It is well known that the sol–gel method allows the production of colloids (ZnO) with particle sizes between 3 and 6 nm within a few minutes. However, a fresh colloid that showed a prominent green luminescent at ∼ 500 nm suffered a signi9cant red-shift caused by the increase in cluster size after aging for several days. A shift from 500 to 560 nm was observed when the colloid was aged for over 5 days. This transformation must be avoided in applications. Mikrajuddin, Iskandar, and Okuyama (2001) reported an approach to avoid the growth of ZnO nanoparticles produced by a sol–gel process to maintain the photoluminescence (PL) spectra by trapping these particles at localized positions in a solid matrix host. They used silica (SiO2 ) particles, produced by sol-spraying as a host matrix. The SiO2 matrix plays the role of a separator for the nanoparticles which result in nanometer separated ZnO nanoparticles occupying 9xed positions. This separation can be controlled by simply changing the sizes of the starting SiO2 particles. SiO2 is transparent in the visible region and has almost no e:ect on the green luminescence intensity of ZnO. In the fabrication process, two types of nanoparticles in the spraying solution (SiO2 and ZnO) remain homogeneously mixed due to the presence of an ultrasonic wave so that they are also homogeneously distributed in the droplets. The surfaces of particles derived from sols-spraying are not perfectly smooth, but contain pores with a size and separation of ∼ 5 nm. These sizes are very similar to those found in ZnO nanoparticles. ZnO nanoparticles occupy these pores so that their distances are separated only by primary particles, thus developing larger SiO2 nanoparticles. When the concentration of the ZnO nanoparticles is relatively high, all pores of the SiO2 are occupied and the remaining ZnO nanoparticles cover the surface of SiO2 as well as the surface of the powder to form a “continuous 9lm”. Fig. 12 displays PL spectra using an excitation wavelength of 250 nm as well as the excitation spectra detected at 500 nm for the produced particles measured at room temperature. The PL spectra of a powder consisting only of SiO2 nanoparticles is also displayed, exhibiting a very weak
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Fig. 12. PL spectra of samples with di:erent SiO2 concentration; (a) 4, (b) 0.4, (c) 0.2, and (d) 0:1 mol=l, using a 250 nm Xe laser source and excitation spectra detected at 500 nm taken just after the preparation of the particles. ZnO concentration was 9xed at 0:1 mol=l. The PL spectra of particles consisting of only SiO2 is also displayed. Temperature of a aerosol reactor was increased gradually from 250◦ C to 450◦ C. (Reprinted with permission from Mikrajuddin et al. (2001). J. Appl. Phys. 89, 6431. c 2001 American Institute of Physics)
intensity so that its contribution to the PL spectra of the powder can be ignored. Except at the region between 300 and 400 nm, all samples exhibited similar PL pro9les with prominent peaks at 500 nm, indicating that the size distribution of ZnO nanoparticles in powders is similar, independent of the concentration ratio of components in the precursor. ZnO nanoparticles do not have suHcient time to collide with each other to form larger clusters (the resident time of droplets/particles in a reactor is around 2 s). In addition, the presence of SiO2 nanoparticles instead of merely a continuum liquid around the ZnO nanoparticles reduces the chance of ZnO nanoparticles meeting each other. No change in the PL spectra, either the shape or position, was observed for the four samples, even after being aged for over 30 days, indicating that the PL spectra of the powders are time independent. This result strongly suggests that the production of ZnO nanoparticles in SiO2 matrix by sol-spraying is a simple way to maintain the luminescence spectra of the ZnO nanoparticles. 6. Conclusion Clearly, the novel spray route, i.e. salt-assisted aerosol decomposition (SAD) process, expands the concept of aerosol decomposition process and, thus, its applications to materials synthesis, namely that one droplet can produce multiple 9ner particles. This route can o:er good controllability of particle size, chemical composition and material crystallinity, all of which are important to advanced materials. On the other hand, the electrospray pyrolysis and a low-pressure spray pyrolysis using the 9lter expansion aerosol generator are capable of generating nanoparticles and fragmenting particles to give nanoparticles.
A spray drying method was used to produce a silica powder which contained ordered mesopores. The mesopores was observed to be arranged into a hexagonal packing, indicating the self-organization process occurred spontaneously during the solvent evaporation. The entire process was completed in only several seconds, which is the contrary to currently available methods that require several hours or up to several days to complete this self-organization process. The combined sol–gel and spray drying method have developed for obtaining stability for luminescence of ZnO nanoparticles by forming ZnO=SiO2 nanocomposite. The new technique employs SiO2 particles as the matrix for the ZnO nanoparticles. SiO2 nanoparticles can also reduce the tendency for the ZnO nanoparticles to agglomerate during spray drying in the reactor.
Acknowledgements The authors wish thank to Dr. F. Iskandar for his assistance in preparing this manuscript. This work was funded by Grant-in-Aid sponsored by the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan and also supported by the New Energy and Industrial Technology Development Organization (NEDO)’s “Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade and Industry (METI), Japan.
References Bayvel, L., & Orzechowski, Z. (1993). Liquid atomization. Washington, DC: Taylor & Francis. Iskandar, F., Lenggoro, I. W., Xia, B., & Okuyama, K. (2001a). Functional nanostructured silica powders derived from colloidal suspensions by sol spraying. Journal of Nanoparticle Research, 3, 263–270. Iskandar, F., Mikrajuddin, & Okuyama, K. (2001b). In situ production of spherical silica particles containing self-organized mesopores. Nano Letters, 1, 231–234. Iskandar, F., Mikrajuddin, & Okuyama, K. (2002). Controllability of pore size and porosity on self-organized porous silica particles. Nano Letters, 2, 389–392. Kang, Y. C., & Park, S. B. (1995). A high-volume spray aerosol generator producing small droplets for low pressure applications. Journal of Aerosol Science, 26, 1131–1138. Lenggoro, I. W., Hata, T., Iskandar, F., Lunden, M. M., & Okuyama, K. (2000a). An experimental and modeling investigation of particle production by spray pyrolysis using a laminar Jow aerosol reactor. Journal of Material Research, 15, 733–743. Lenggoro, I. W., Okuyama, K., Fernandez de la Mora, J., & Tohge, N. (2000b). Preparation of ZnS nanoparticles by electrospray pyrolysis. Journal of Aerosol Science, 31, 121–136. Lenggoro, I. W., Xia, B., Okuyama, K., & Fernandez de la Mora, J. (2002). Sizing of colloidal nanoparticles by electrospray and di:erential mobility analyzer methods. Langmuir, 18, 4584–4591. Mikrajuddin, Iskandar, F., & Okuyama, K. (2002). Single route for producing organized metallic domes, dots, and pores by colloidal templating and over sputtering. Advanced Materials, 14, 930–933.
K. Okuyama, I. Wuled Lenggoro / Chemical Engineering Science 58 (2003) 537 – 547 Mikrajuddin, Iskandar, F., Okuyama, K., & Shi, F. G. (2001). Stable photoluminescence of zinc oxide quantum dots in silica nanoparticles matrix prepared by the combined sol–gel and spray drying method. Journal of Applied Physics, 89, 6431– 6434. Okuyama, K., Huang, D. D., Seinfeld, J. H., Tani, N., & Kousaka, Y. (1991). Aerosol formation by rapid nucleation in the preparation of SiO2 thin 9lm from SiCl2 and O2 gases using CVD process. Chemical Engineering Science, 46, 1545–1560.
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Xia, B., Lenggoro, I. W., & Okuyama, K. (2001a). Novel route to nanoparticle synthesis by salt-assisted aerosol decomposition. Advanced Materials, 13, 1579–1582. Xia, B., Lenggoro, I. W., & Okuyama, K. (2001b). Synthesis of CeO2 nanoparticles by salt-assisted ultrasonic aerosol decomposition. Journal of Materials Chemistry, 13, 2925–2927. Xia, B., Lenggoro, I. W., & Okuyama, K. (2002). Nanoparticle separation in salted droplet micro-reactors. Chemistry of Materials, 14, 2623–2627.
www.elsevier.com/locate/ces
Preparation of nanoparticles via spray route Kikuo Okuyama∗ , I. Wuled Lenggoro Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan
Abstract Nanometer-sized particles (1–100 nm) are of considerable interest for a wide variety of applications, ranging from electronics via ceramics to catalysts, due to their unique or improved properties that are primarily determined by size, composition and structure. In this study, we report a simple, rapid and generalizable aerosol decomposition (spray pyrolysis) process for the continuous synthesis of nanoparticles with adjustable sizes, narrow size distribution, high crystallinity and good stoichiometry. The production of spherical-shaped porous particles with nanoscale ordering porosity and the zinc oxide quantum dots in silica nanoparticles matrix by means of a spray drying method using a colloidal mixture as the precursor and by the combined sol–gel and spray drying method were also reported. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Nanoparticle; Spray pyrolysis; Aerosol; Droplet; Spray drying; Colloids
1. Introduction Ultra9ne particles or nanoparticles (size between a few nanometer and 100 nm) are of interest, because the chemical and physical behavior of the particles is unprecedented and remarkably di:erent from those in bulk form. They have great potential for use in applications in the electronic, chemical or mechanical industries, as well as technologies with them, including superconductors, catalyst, drug carriers, sensors, magnetic materials, pigment, and in structural and electronic materials. It is well known that an unagglomerated spherical particle with a narrow size distribution (monodispersed) is the preferred state for applications and technologies, especially for the compacting and sintering of particles. Large number techniques for the preparations of ultra9ne particles that satisfy this requirement have been developed. Among these, a sol–gel process, a “build-up” type of liquid-phase method, has been most widely studied. Some of the other liquid phase chemical techniques using metal salts as a starting precursor, such as hydrothermal processes, homogeneous precipitation methods or reversed micelles, however, have several problems: the concentration of reacting species must be low and a long reaction time is required. ∗ Corresponding author. Tel.: +81-824-24-7716; fax: +81-824-24-7850. E-mail address: [email protected] (K. Okuyama).
It is important to develop a process in which particles having controlled characteristics including size, morphology, and composition can be produced. To be industrially relevant, the process needs to be low cost and involve both continuous operation and a high production rate. Functional materials with speci9c physical and chemical properties begin with molecular precursors that must be transformed into the product. The general method used to ful9ll this transformation is the aerosol process (gas-phase method). An aerosol is de9ned as a suspension of solid or liquid particles in a gas. The term “aerosol” is used in the 9elds of atmospheric, health and air pollutant, as well as in material processing. There are at least two routes for the preparation of ultra9ne particles by aerosol processes. The 9rst involves gas-to-particle conversion (a build-up method) and the second liquid-to-solid particle conversion (a break-down method). In gas-to-particle conversion, particles are generated by cooling a supersaturated vapor involving the use of methods called physical vapor deposition (PVD) or “evaporation–condensation method” and chemical vapor deposition (CVD) (Okuyama, Huang, Seinfeld, Tani, & Kousaka, 1991). In PVD, the evaporation of a solid or liquid is the source of the vapor. In the cooling stage, nucleation and condensation of the saturated vapor take place and solid particles are formed. In CVD, the vapor evaporated from the solution precursors is thermally decomposed or is reacted with another precursor vapor or a surrounding
0009-2509/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(02)00578-X
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gas. Finally, the solid particles are formed by nucleation, condensation and coagulation. The primary advantages of the gas-to-particle conversion method are the small particle size (a few nanometers to micrometer), narrow size distribution and high purity of the particles produced. However, as disadvantages, the formation of hard agglomerates in the gas phase leads to diHculties in preparing high-quality bulk materials. It is also diHcult to synthesize multicomponent materials (e.g., phosphors and superconductors), because of the di:erences in chemical reaction rate, vapor pressure, nucleation and growth rate which occur during the gas-to-particle conversion may lead to non-uniform composition. Many types of reactors, such as Jame, furnace, plasma, or laser, have been used to facilitate the gas-to-particle conversion route. Liquid-to-particle conversion (i.e., spray pyrolysis) is a representative “break-down” method for aerosol processing. Spray method is often classi9ed as a liquid-phase method because solutions or sols are used. This method has been used to prepare numerous types of functional particles. Multicomponent materials are also easily prepared by this route. Relative to the gas-to-particle conversion route, spray method is a simple and low-cost process, but our understanding of the process itself is not well understood (Lenggoro, Hata, Iskandar, Lunden, & Okuyama, 2000a). In this paper, a novel aerosol decomposition (spray pyrolysis) process for the continuous synthesis of nanoparticles as well as the electrospray pyrolysis was reviewed. The production of spherical-shaped porous silica particles with nanoscale ordering porosity by means of a spray drying method using a colloidal mixture as the precursor and zinc oxide nanoparticles in silica matrix prepared by the combined sol–gel and spray drying method were also reported. 2. Particle preparation by spray method 2.1. Spray pyrolysis and spray drying process To prepare 9ne particles by spray pyrolysis, a starting solution is prepared by dissolving, usually, the metal salt of the product in the solvent. The droplets, which are atomized from a starting solution, are introduced to the furnace. Evaporation of the solvent, di:usion of solute, drying, precipitation, reaction between precursor and surrounding gas, pyrolysis, or sintering may occur inside the furnace to form the 9nal product. Spray drying is similar to spray pyrolysis except the type of precursor. For spray drying method, the colloidal particles or sols are typically used as precursors. This method has also the capability of producing uniformly spherical particles from submicron to micron sizes. If the suspension consists of colloidal nanoparticles (primary particles), the resulting particles also comprise nanoparticles to form a nanostructured powder. Therefore, spray drying may be a suitable process for consolidating nanoparticles into
macroscopic compacts, and submicron spherical powders that have nanometer-scaled properties can be obtained. 2.2. Starting solutions The composition of the 9nal particle is determined by the solutes or reactants dissolved in the starting solution in predetermined stoichiometric ratio. As precursor solutions, usually inexpensive materials such as nitrate, chloride and acetates are typically used. It is necessary to eliminate solubility problems and phase segregation, where the di:erent components precipitate at di:erent times. Water or alcohol is usually used as a solvent. To prepare 9ne particles by spray drying method, colloidal suspension/sols are used as precursors (Iskandar, Lenggoro, Xia, & Okuyama, 2001a). The suspension, which contains liquid and solid particles, is sprayed and the liquid phase (the solvent) evaporates from the droplets. 2.3. Droplet generation The average size and size distribution of the 9nal particles can be roughly determined from the size of the atomized droplet and the initial concentration of the starting solution. Atomization is the production of droplets and their dispersion into the gas. The size or morphology of the 9nal particles produced can also be determined by the concentration and velocity of the droplet generated by the atomizers. A variety of atomization methods (Bayvel & Orzechowski, 1993) has been used in spray pyrolysis studies, such as air-assist (pneumatic) or a two-Juids nozzle, ultrasonic, vibrating ori9ce and spinning disk. 2.4. Particle morphology Properties of precursor, carrier gas Jow rate (i.e., residence time of heating or the solvent evaporation) and the temperature are the main parameters, which a:ect the morphology of particles generated by spray pyrolysis. Since this is a somewhat empirical process, morphology control seems to be the goal of most studies in the area of spray pyrolysis. Concerning the evolution of particle morphology during spray pyrolysis, in addition to solid (dense) and spherical particles, hollow or fragmented particles are often formed (Fig. 1). These morphological conditions are undesirable for most applications. Single-crystal or polycrystal particles are also formed, depending on the operation conditions. 2.5. Apparatus and procedure for the preparation of particles by spray method A representative system used for the preparation of 9ne particles by spray method is schematically shown in Fig. 2. The main equipment consists of: (i) an atomizer or nebulizer that converts the starting solution to droplets, (ii) carrier gas(es), (iii) tubular furnace/reactor, (iv) sampler or
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Fig. 1. Morphology of particle preparing by spray method.
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precipitator (e.g., 9lter, electrostatic precipitator, thermophoretic sampler). The furnace length is selected so as to satisfy the residence time of the droplets/particles during the heating. The furnace can include several independently controlled heating zones which enables the desired temperature distribution to be achieved. Instruments for measuring particle size such as a di:erential mobility analyzer (DMA) and a condensation nucleus/particle counter (CNC/CPC) can be placed at the inlet or the outlet of the furnace to measure the size distribution of the aerosols in real time/online. The size of the particles also can be analyzed o:-line by electron microscopy, after the particles are sampled by the precipitator. Other analysis such as for crystallinity (powder X-ray di:raction (XRD)), luminescent or electrical properties can be performed. In spray pyrolysis, a change in solution concentrations can be used to easily control the mean size of the 9nal particles at a constant size of the sprayed droplet. The overall solution concentration can typically be varied from 0.01 to above a few mols/litre for changing the mean size of particles. The precursors (e.g., metal nitrates, acetates or chlorides) are dissolved in distilled water to give the desired molar ratio of the 9nal product. All precursors were reagent grade and are usually used without further puri9cation. A small amount of acid (e.g., nitric) may be added during the preparation of the homogeneous solutions. The temperature of the hot-wall aerosol reactor was changed from 500◦ C to 1700◦ C. In some cases, in order to prevent oxidation of the materials, a mixture of N2 and H2 were used as carrier gas. The overall gas Jow rate was adjusted to several litres/minutes and the corresponding residence time of the particles inside the reactor was around several seconds at atmospheric pressure.
Fig. 2. Representative spray pyrolysis system used for particle preparation
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3. Preparation of nanoparticles via spray route The concept or the basis of the spray pyrolysis process assumes that one droplet forms one product particle. To date, submicrometer-to-micrometer-sized particles are typically formed in an spray pyrolysis process due to the ineffective generation of very 9ne droplets, although the process has been widely used to synthesize multicomponent materials in a simple process. A few attempts have been to synthesize nanoparticles using the spray pyrolysis process. Typically, fuels (e.g. alcohols, ureas and sucroses), dissolved in precursor solutions, were then misted and decomposed in a high-temperature Jame or combustion processes to form ceramic oxide powders. In these processes, the abrupt evolution of considerable heat and gas aids in breaking or fragmenting the large particles into smaller pieces, coupled with evaporation-derived particle formation process. Particles are formed from concurrent gas-to-particle and liquid-to-particle conversions. Electrospray (Section 3.1) is capable of generating 9ne droplets as well as nanoparticles, and low-pressure spray pyrolysis (Section 3.2) has also been used to fragment particles to give nanoparticles. Recently, we reported a new synthesis technique via spray pyrolysis process (salt-assisted aerosol decomposition (SAD); Section 3.3) for the continuous synthesis of nanoparticles with adjustable sizes, a narrow size distribution, high crystallinity, and good stoichiometry (Xia, Lenggoro, & Okuyama, 2002). The desired nanoparticles are formed and separated in salt microreactors (droplets) suspended in a gas. 3.1. Electrospray pyrolysis In spray pyrolysis, the average size of the 9nal solid particle can be determined from the droplet size of the solution sprayed. Most studies have used a variety of typical atomizers such as twin Juid or an ultrasonic nebulizer to generate the droplets. These atomizers are capable of producing droplets with an average size in the range of several microns. For a typical initial droplet with a diameter of 5 m to dry into a particle with a diameter of 100 nm, the initial volume fraction of dissolved involatile solute must be less than 0.0008%. In practice, these low solution concentrations may lead to a low rate of particle generation and may a:ect the purity of the particles ultimately. In the other words, the preparation of ultra9ne material particles with diameters of less than 100 nm via a conventional spray pyrolysis method remains a problem. The electrospray technique has been examined as a method for producing ultra9ne droplets. In this method, a meniscus of a spray solution at the end of the capillary tube becomes conical when charged to a high voltage (several kilovolt) with respect to a counter electrode. The droplets are stably formed by the continuous breakup of a jet extending from this liquid cone (Fig. 3), generally referred to as a “Taylor cone”. A variety of experimental studies have shown that the diameter of such jets and droplets may
Fig. 3. (Upper) Schematic of the formation of liquid meniscus (cone) and the exerted jet and droplets in electrospray. (Lower) Liquid meniscus at the capillary tip in a stable cone-jet mode with a 9ne jet (approximately a few micrometers in diameter). Methanol liquid, Jow rate 0:20 ml=h, applied voltage 3 kV.
be controlled by means of a nanometer up to hundreds of micrometers. The preparation of zinc sul9de (ZnS) particles below 50 nm in diameter by an electrospray pyrolysis method has been reported (Lenggoro, Okuyama, Fernandez de la Mora, & Tohge, 2000b). For a preparation of ZnS nanoparticles, it is necessary to control the spray liquid Jow rate, the applied voltage and the solute concentration, and to ensure that stable spray conditions in the so-called cone-jet mode are operational. The particles generated can be measured by a DMA and a CNC system, and their sizes are comparable with those expected after drying from available scaling laws. In addition, by sampling the particles, the suitability of electrospray pyrolysis for generating nanometer-sized spherical ZnS 9ne particle was investigated. Using the DMA–CNC system, the results showed that ZnS particles were generated in a size ranging from 20 to 30 nm. Fig. 4 shows TEM photographs of ZnS particles prepared from a solution concentration of 0:01 mol=l. The shapes of the particles are spherical, and their size distributions were similar to those obtained by DMA–CNC. Recently, we developed a simple technique for sizing colloidal particles by means of electrospray and aerosol techniques (Lenggoro, Xia, Okuyama, & Fernandez de la Mora, 2002). Size distribution of di:erent types of colloids (oxides, metals and polymers) with di:erent nominal sizes below 100 nm was determined online. Nanometer-sized particles were dispersed into the gas phase as an aerosol via electrosprays operating in the cone-jet mode of a colloidal solution followed by a charge reduction of the sprayed
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For multicomponent particles, such as rare-earth-doped phosphor materials, the fragmentation of particles into nanometer particles in the FEAG process did not easily occur. In addition, for preparing phosphor particles with an activator, the FEAG route requires post-annealing after spray pyrolysis at low pressure. This is due to the short residence time in the aerosol reactor of below 0:1 s (estimated). For example, in the preparation of Zn2 SiO4 : Mn phosphor particles by spray pyrolysis using the FEAG technique, a pure phase willemite (Zn2 SiO4 ) structure was obtained after post- annealing at 1200◦ C for 5 h. Fig. 4. TEM photographs of ZnS particles prepared by an electrospray pyrolysis at a temperature of 600◦ C. Magni9cation of (a) 50; 000× and (b) 250; 000×. Particle size was coincided well with those measured online by aerosol sizing technique (DMA-CNC system). (Reprinted with permission from I. W. Lenggoro et al. (2000). J. Aerosol Sci., 31, 121. c 2000 Elsevier Science)
droplets to unity and subsequent evaporation of the solvent. The size distribution of the generated aerosol particles was then determined by a DMA combined with a CNC. The proposed technique is capable of detecting the degree of dispersity of colloid samples and the measured values were comparable to results obtained by electron microscopy and the dynamic light scattering. 3.2. Low-pressure spray pyrolysis using 7lter expansion aerosol generator (FEAG) In low-pressure conditions, a two-step spray aerosol generator, referred to as FEAG, was developed by Kang and Park (1995). The FEAG is composed of two-Juid nozzle for dispersing a liquid, a porous glass 9lter, and a vacuum pump. Liquid is sprayed through a pneumatic nozzle by a carrier gas on to a glass 9lter surface where it forms a thin liquid 9lm. This liquid 9lm and carrier gas pass through the 9lter pores, aided by the carrier gas and are expanded into a low-pressure chamber. The mean droplet size was estimated to be around 2 m and the standard deviation of the droplet size distribution was 1.76. The FEAG process (low-pressure spray pyrolysis) was applied to the production of nanometer order particles (ZnO) and submicron phosphor particles such as Zn2 SiO4 : Mn. The morphology of particles prepared by the FEAG process was di:erent from those produced by ultrasonic spray pyrolysis. ZnO nanometer particles were formed via the FEAG route whereas submicron size ZnO particles were produced in the case of ultrasonic spray pyrolysis. This di:erence in size and morphology can be attributed to the di:erence in operating pressures and aerosol formation mechanisms between the two types of aerosol generators. However, no systematic investigation on the e:ect of reactor pressure on the morphology and size in spray pyrolysis has been reported. In the FEAG process, the generation rate of a droplet may be increased with increasing surface area of the glass 9lter and by scaling-up, and it is expectable that this process will be used industrially in the near future.
3.3. SAD method In the conventional route, each spray pyrolyzed particle has multiple nanosized crystallites under typical process conditions, but they are virtually inseparable due to the formation of a three-dimensional network. The salt-assisted spray pyrolysis (i.e., SAD) method proposed by Xia, Lenggoro, and Okuyama (2001a) focused on a strategy for separating these nanocrystallites by introducing some compounds that can distribute on the nanocrystallite surfaces to prevent them from agglomerating and are then easy to remove. During the SAD process, when the particle temperature exceeds the melting point of the salts, the salts melt and act as high-temperature solvents. The material or its components can then dissolve, undergo reactions, and, upon exceeding the solubility limit, precipitate in the solvent. These processes, absent in the conventional spray pyrolysis (conventional aerosol decomposition (CAD)), can remarkably enhance mass transfer due to the liquid-state solvent, in contrast to the very small solid-state di:usion coeHcients in the case of the CAD processes. Within an aerosol particle, the dissolution/precipitation cycle can lead to the dissolution of some nanocrystallites and the growth of other crystallites by precipitation. This may break up the three-dimensional network and disintegrate the nanocrystallites, which has been observed in the experiments. Fig. 5 illustrates the experimental setup and the particle formation processes. To demonstrate the feasibility of this method, eutectic mixtures or single salts, e.g., chlorides or nitrates of Li, Na, K, were dissolved in an aqueous precursor solution. The aerosol particles passed through the reactor in less than 5 s, then they were cooled and particles, which contained the 9nal nanoparticles within a salt matrix, were collected. The nanoparticles were obtained after washing the product with water to remove the salts. The spherical and dense submicrometer to micrometer particles, typical particle morphologies formed in the conventional spray pyrolysis processes, remain unchanged after washing. In contrast, the SAD sample shows a substantial change after washing and nanoparticles are obtained. Fig. 6 shows scanning and transmission electron microscopy (SEM, TEM) images of two Y2 O3 –ZrO2 samples, one synthesized using the CAD and the other using the SAD
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process. The spherical and dense submicrometer to micrometer CAD particles, typical particle morphologies formed in AD processes, remain unchanged after washing. In contrast, the SAD sample shows a substantial change after washing and nanoparticles are obtained. The CAD particle sizes are broadly distributed over a range of 0:11 ± 2:12 m with a mean size dp of 0:66 m and a geometric standard deviation g of 1.76, showing a typical polydispersity (1:3 ± 2:0) of a material formed in aerosol processes. Interestingly, the SAD sample (Fig. 6d), formed in the same aerosol process, has a remarkably sharpened size distribution (close to the normal distribution) in the range of 6:3 ± 18:7 nm with a
Fig. 5. Schematic illustration of the experimental apparatus and an illustration of particle formation both for the CAD and the SAD processes. Solution droplets were generated by an atomizer and were carried by a gas into a tube reactor typically operated between 300◦ C and 1600◦ C. The resulting particles were collected in an precipitator. (Reprinted with c 2001 permission from B. Xia et al. (2001). Adv. Mater., 13, 1579. Wiley-VCH)
dp of 12:8 nm and a g of 1.19. In other words, the size distribution of the SAD particles appears to be independent of that of their parent solution droplets. Addition of the salts causes particle size reduction by a factor (dCAD =dSAD ) of over 50. A variety of other materials such as NiO, Ag–Pd, ZnS and CeO2 with nanometer sizes have been synthesized using the SAD method (Xia, Lenggoro, & Okuyama, 2001b). Figs. 7a and b show CeO2 particles synthesized by SAD technique (temperature of 800◦ C) with the addition of the eutectic salts to the precursor solution prior to the aerosol decomposition. The SAD product (Fig. 7a) is composed of isolated nanoparticles (mean size 51 nm) while the CAD product consists of submicron-to-micron-sized particles (mean size 0:74 m) containing sintered nanocrystallites. Powder XRD patterns of the CAD and the SAD products, shown in Figs. 8a and b, illustrate the well-de9ned CeO2 cubic phase. The SAD product has a much higher crystallinity than the CAD product, as shown from the sharp peaks in Fig. 8b. The crystallite size of the SAD 800◦ C sample is 54:4 nm. This is much larger than the corresponding CAD sample (13:8 nm) and even than the CAD 1000◦ C sample (29:9 nm); it is also bigger than the CeO2 crystallites (about 40 nm) that were heated at 800◦ C for 2 h, while the SAD product in this work was heated for only a few seconds. The SAD CeO2 particles are single crystals while the CAD CeO2 particles are polycrystalline. The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones at all synthesis temperatures. The typical crystal lattice image shown in Fig. 7b con9rms the presence of single-crystalline particles. In addition to the above
Fig. 6. Y2 O3 –ZrO2 samples synthesized by the CAD process (a,b) and the SAD process (c,d). (a,c) SEM images of the samples before washing and (b,d) TEM images after washing. Note that the unwashed SAD particles (c) are larger than the unwashed CAD particles (a) due to the presence of salts. c 2001 Wiley-VCH) (Reprinted with permission from B. Xia et al. (2001). Adv. Mater., 13, 1579.
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Fig. 7. CeO2 nanoparticles synthesized by the SAD method at (a) 800◦ C, and (b) a typical high-resolution TEM image of sample (a), showing the crystal lattice of a particle; (c) 900◦ C, and (d) 800◦ C with addition of acetic acid to the solution prior to aerosol decomposition. (Reprinted with c 2001 The Royal Society of Chemistry) permission from B. Xia et al. (2001). J. Mater. Chem., 13, 2925.
Fig. 8. Powder XRD patterns of products synthesized at (a) CAD, 800◦ C (CeO2 ); (b) SAD, 800◦ C (CeO2 ). The inset shows that the addition of HAc to the precursor solution causes a decrease in crystallite size. The SAD pattern (b) has much sharper peaks than the corresponding CAD pattern (a). The XRD patterns were recorded at room temperature using Cu Ka radiation operated at 40 kV and 20 mA. (Reprinted with permission from B. Xia et c 2001 The Royal Society of Chemistry) al. (2001). J. Mater. Chem., 13, 2925.
mentioneddi:erences,itisalsoworthnotingthatthegeometric standard deviation (g ) of the CAD particles is determined as 1.63, a typical polydispersity of aerosol particles produced by ultrasonic aerosol decomposition, while it is only 1.25 for the corresponding SAD particles. Clearly, the particle size distribution of the SAD product has been remarkably narrowed in comparison to the CAD product (Fig. 6).
The CAD CeO2 particle sizes change slightly with synthesis temperature, because it is well understood that the CAD particle size is primarily determined by the droplet size (mainly depending on the transducer vibration frequency) and concentration of the precursor solution. In the SAD process, however, we found that many factors such as precursor(s), inert salts, additives and process parameters can be
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used to control particle size and morphology. As shown in Fig. 7c, for example, the mean particle size of the SAD product increases from 51 nm (Fig. 7a) to 119 nm when the synthesis temperature is increased from 800◦ C to 900◦ C. Fig. 7d shows that the addition of acetic acid as an additive to the precursor solution resulted in a reduction in the 9nal particle size from 51 to 21 nm. The broadened XRD peak is clearly seen in the inset of Fig. 8. The SAD process expands the concept of the spray pyrolysis as a one-step process and, thus, its applications to materials synthesis, namely that one droplet can produce multiple 9ner particles. This route can o:er good controllability of particle size, chemical composition, and material crystallinity,allofwhichareimportantforadvancedmaterials. 4. In situ production of spherical silica particles containing self-organized mesopores by spray drying The synthesis of mesostructures porous silica represents a fascinating and intellectually challenging problem due to its potential for applications in catalysts, chromatography, the controlled release of drugs, low dielectric constant 9llers, pigments, microelectronics, and electro-optics. When supramolecular surfactant micelles are used in the synthesis, the pore sizes are in the range of 10 nm or less. Synthesis via colloid crystallization allows the pore sizes to be controlled in the range of nanometers to micrometers. Iskandar, Mikrajuddin, and Okuyama (2001b, 2002) reported the production of spherical-shaped porous silica particles with nanoscale ordering porosity by means of a spray drying method using a colloidal mixture of silica and polystyrene (PS) latex as the precursor. The setup used in the work consists of a nebulizer, a vertical tube reactor and a 9lter for sampling. Colloidal silica particles (particle size around 5 nm) were used as a precursor, which was mixed with PS latex (various particle sizes), in a certain fraction in water to form a dilute solution, which was then atomized to generate droplets. The aerosol reactor (an inner diameter of 13 mm and a length of 1000 mm) consisted of two heating zones, the 9rst having a 9xed temperature of 200◦ C and the second, a 9xed temperature of 450◦ C. The 9rst zone is used to evaporate the solvent in the droplet, resulting in large particles of composite consisting of primary silica particles and PS latex particles. The second zone was used to evaporate the PS latex particles, which resulted in the formation of porous silica particles. The Jow rate of the nitrogen carrier gas was maintained at 1 l=min. Scanning electron micrographs in Fig. 9 reveals the ordered arrangement of pores on the surface of spherical particles. The pore sizes are similar to the PS latex particle sizes. This result is the 9rst observation of such a self-organization process of particles in the environment formed by other colloidal particles. The spacing between the pores is very homogeneous, approximately two primary silica particles in thickness. This result indicates that the spacing between the
Fig. 9. SEM images of the surface morphology of silica powders using 79 nm PS latex particles. (Reprinted with permission from F. Iskandar c 2001 American Chemical Society) et al. (2001). Nano. Lett., 1, 231.
pores can be easily controlled by changing the size of the primary silica particles. From experiments using di:erent sizes of PS latex particles, the hexagonal packing of the pores inside the produced particles is still observed. In addition, the ordering of the pores increases with the increase in the PS latex particles size. Indeed, this result is in agreement with the commonly observed ones that ordering of larger particles in general is easier than ordering of the smaller particles. Fig. 10 shows data on the variation of pore spacing for distinct fractions of PS latex and primary silica particles. As expected, the spacing between the pores increases when the fraction of PS latex is decreased. However, the long-range ordering of pores on the particle surface is reduced. A further increase in the PS latex fraction results in the formation of brittle particles, many of which are broken. Fig. 11 shows combinations which produced successful (open circle) and unsuccessful (solid three angle) organizations. Consider three contacting PSL particles developing hexagonal structure. The maximum diameter, d, of a silica particle 9lling the√space between the three contacting PSL particles is d = (2 3=3 − 1)D with D denoting the diameter of a PSL particle. Therefore, the maximum ratio of silica nanoparticle and PSL sizes to produce organization arrangement is d=D = 0:155. This value is represented by a straight line in Fig. 11. Most of our experiment results are consistent with this calculation. The successful combinations lay below the line and the unsuccessful combinations lay above the line. The mesopores was observed to be arranged into a hexagonal packing, indicating the self-organization process occurred spontaneously during the solvent evaporation. The
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pore size was controlled by changing the size of the PSL particles. By adding an additional zone (third zone) maintained at high temperatures, the produced powders could be in situ annealed. A comparison of the average volume of the powder before and after annealing (at 1500◦ C) indicated that the porosity of the powder was about 70%, indicating that pores were present both on surface of the powders and inside the powder particles. By exploiting colloidal PSL nanoparticles templating and “over-sputtering”, we reported a novel method for simultaneously producing three types of structural organized layers on a mesoscopic scale: domes, dots, and pores (Mikrajuddin, Iskandar, & Okuyama, 2002). 5. Zinc oxide nanoparticles in silica particles matrix prepared by the combined sol--gel and spray drying
Fig. 10. SEM images of a spherical silica powder produced by varying the content ratio of PS latex and silica: 10 ml of silica colloids (0.05%) mixed with (a) 0:5 ml, (b) 1 ml and (c) 2 ml of PS latex colloid (3:6 × 1013 particles=ml). The size of PS latex particles were maintained at 79 nm. (Reprinted with permission from F. Iskandar et al. (2001). c 2001 American Chemical Society) Nano. Lett., 1, 231.
Fig. 11. Combination of PS latex sizes, and silica nanoparticles sizes that exhibited successful arrangement (open circle) and unsuccessful arrangement (solid three angle). (Reprinted with permission from F. Iskandar c 2002 American Chemical Society) et al. (2002). Nano. Lett., 2, 389.
It is well known that the sol–gel method allows the production of colloids (ZnO) with particle sizes between 3 and 6 nm within a few minutes. However, a fresh colloid that showed a prominent green luminescent at ∼ 500 nm suffered a signi9cant red-shift caused by the increase in cluster size after aging for several days. A shift from 500 to 560 nm was observed when the colloid was aged for over 5 days. This transformation must be avoided in applications. Mikrajuddin, Iskandar, and Okuyama (2001) reported an approach to avoid the growth of ZnO nanoparticles produced by a sol–gel process to maintain the photoluminescence (PL) spectra by trapping these particles at localized positions in a solid matrix host. They used silica (SiO2 ) particles, produced by sol-spraying as a host matrix. The SiO2 matrix plays the role of a separator for the nanoparticles which result in nanometer separated ZnO nanoparticles occupying 9xed positions. This separation can be controlled by simply changing the sizes of the starting SiO2 particles. SiO2 is transparent in the visible region and has almost no e:ect on the green luminescence intensity of ZnO. In the fabrication process, two types of nanoparticles in the spraying solution (SiO2 and ZnO) remain homogeneously mixed due to the presence of an ultrasonic wave so that they are also homogeneously distributed in the droplets. The surfaces of particles derived from sols-spraying are not perfectly smooth, but contain pores with a size and separation of ∼ 5 nm. These sizes are very similar to those found in ZnO nanoparticles. ZnO nanoparticles occupy these pores so that their distances are separated only by primary particles, thus developing larger SiO2 nanoparticles. When the concentration of the ZnO nanoparticles is relatively high, all pores of the SiO2 are occupied and the remaining ZnO nanoparticles cover the surface of SiO2 as well as the surface of the powder to form a “continuous 9lm”. Fig. 12 displays PL spectra using an excitation wavelength of 250 nm as well as the excitation spectra detected at 500 nm for the produced particles measured at room temperature. The PL spectra of a powder consisting only of SiO2 nanoparticles is also displayed, exhibiting a very weak
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Fig. 12. PL spectra of samples with di:erent SiO2 concentration; (a) 4, (b) 0.4, (c) 0.2, and (d) 0:1 mol=l, using a 250 nm Xe laser source and excitation spectra detected at 500 nm taken just after the preparation of the particles. ZnO concentration was 9xed at 0:1 mol=l. The PL spectra of particles consisting of only SiO2 is also displayed. Temperature of a aerosol reactor was increased gradually from 250◦ C to 450◦ C. (Reprinted with permission from Mikrajuddin et al. (2001). J. Appl. Phys. 89, 6431. c 2001 American Institute of Physics)
intensity so that its contribution to the PL spectra of the powder can be ignored. Except at the region between 300 and 400 nm, all samples exhibited similar PL pro9les with prominent peaks at 500 nm, indicating that the size distribution of ZnO nanoparticles in powders is similar, independent of the concentration ratio of components in the precursor. ZnO nanoparticles do not have suHcient time to collide with each other to form larger clusters (the resident time of droplets/particles in a reactor is around 2 s). In addition, the presence of SiO2 nanoparticles instead of merely a continuum liquid around the ZnO nanoparticles reduces the chance of ZnO nanoparticles meeting each other. No change in the PL spectra, either the shape or position, was observed for the four samples, even after being aged for over 30 days, indicating that the PL spectra of the powders are time independent. This result strongly suggests that the production of ZnO nanoparticles in SiO2 matrix by sol-spraying is a simple way to maintain the luminescence spectra of the ZnO nanoparticles. 6. Conclusion Clearly, the novel spray route, i.e. salt-assisted aerosol decomposition (SAD) process, expands the concept of aerosol decomposition process and, thus, its applications to materials synthesis, namely that one droplet can produce multiple 9ner particles. This route can o:er good controllability of particle size, chemical composition and material crystallinity, all of which are important to advanced materials. On the other hand, the electrospray pyrolysis and a low-pressure spray pyrolysis using the 9lter expansion aerosol generator are capable of generating nanoparticles and fragmenting particles to give nanoparticles.
A spray drying method was used to produce a silica powder which contained ordered mesopores. The mesopores was observed to be arranged into a hexagonal packing, indicating the self-organization process occurred spontaneously during the solvent evaporation. The entire process was completed in only several seconds, which is the contrary to currently available methods that require several hours or up to several days to complete this self-organization process. The combined sol–gel and spray drying method have developed for obtaining stability for luminescence of ZnO nanoparticles by forming ZnO=SiO2 nanocomposite. The new technique employs SiO2 particles as the matrix for the ZnO nanoparticles. SiO2 nanoparticles can also reduce the tendency for the ZnO nanoparticles to agglomerate during spray drying in the reactor.
Acknowledgements The authors wish thank to Dr. F. Iskandar for his assistance in preparing this manuscript. This work was funded by Grant-in-Aid sponsored by the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan and also supported by the New Energy and Industrial Technology Development Organization (NEDO)’s “Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade and Industry (METI), Japan.
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