ISSN 19950780, Nanotechnologies in Russia, 2010, Vol. 5, Nos. 7–8, pp. 493–497. © Pleiades Publishing, Ltd., 2010. Original Russian Text © D.V. Lysov, D.V. Kuznetsov, A.G. Yudin, D.S. Muratov, V.V. Levina, D.I. Ryzhonkov, 2010, published in Rossiiskie nanotekhnologii, 2010, Vol. 5, Nos. 7–8.
ARTICLES
Preparation of Nickel Oxide Nanostructured Powders under the Action of Ultrasound D. V. Lysov, D. V. Kuznetsov, A. G. Yudin, D. S. Muratov, V. V. Levina, and D. I. Ryzhonkov Moscow Institute of Steel and Alloys (State Technical University), Leninskii pr. 4, Moscow, 119049 Russia email:
[email protected] Received January 12, 2010
Abstract—Nickel oxide nanopowders were obtained using a method that combines the pyrolysis of aerosols of a nickel acetate solution in an oxidizing atmosphere with their subsequent ultrasonic treatment. The inter mediate products, which result from the drying and pyrolysis of aerosol droplets, are hollow nanostructured NiO microspheres with a size of 0.5–5 μm whose walls are formed by nanoparticles with a size of 10–20 nm. It is shown that, by varying the reactionzone temperature and the initial saltsolution concentration, it is possible to control the size of nanoparticles and the wall thickness of microspheres. The destruction of micro spheres in an ultrasonic cavitation field results in the formation of nickel oxide nanoparticles with a size rang ing from 5 to 14 nm. DOI: 10.1134/S1995078010070098
INTRODUCTION The method for preparing nanodispersed materials based on the pyrolysis of organic and inorganic salts of metals is one of the most efficient ways of obtaining nanopowders in regards to their industrial applica tions. The main advantages of this method are the sim plicity of the equipment, the possibility of organizing a simultaneous sequence production, and the low energy consumption. These factors make it possible to significantly reduce costs for the nanomaterials and, accordingly, to expand the scope of their possible practical applications [1]. In this work, the nickel oxide nanopowders were prepared by a method that combines the pyrolysis of aerosols of a salt solution and their subsequent ultra sonic treatment. The intermediate products, which result from the drying and pyrolysis of aerosol droplets, are hollow nanostructured microspheres 0.5–5 μm in size whose walls are formed by nanoparticles with a size of 10–20 nm. It is shown that, by varying the reac tionzone temperature and the initial saltsolution concentration, it is possible to control the size of the nanoparticles and the wall thickness of the micro spheres. The destruction of microspheres in an ultra sonic cavitation field results in the formation of nickel oxide nanoparticles with a size ranging from 5 to 15 nm [2, 3].
solution of nickel acetate (AR). To obtain aerosols of the solution, we employed an ultrasonic piezoceramic transducer with an operating frequency of about 1.2 MHz; while it is in service, aerosol droplets are formed due to the annihilation of the cavitation bub bles that appear under the action of ultrasound near the surface of the liquid, i.e., mechanically. A resultant cavitation bubble floats to the surface of the solution, where the surface film breaks and, due to a shock wave, small film droplets (aerosol) and large jet drops are formed (Fig. 1). This method makes it possible to obtain both solvents and solutions of various salts in the form of aerosols. Another important reason for using ultrasonic cav itation spraying in this work is the possibility of obtain
EXPERIMENTAL In the capacity of the initial material for preparing the nickel nanopowder, we used a 10 wt % aqueous 493
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Fig. 1. Diagram illustrating the formation process of ultra sonic aerosol droplets: (1) floating gas bubble, (2) bubble near the surface of water, (3) breaking of the surface film with the formation of small film droplets, and (4) the for mation of large jet drops due to a shock wave [6].
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LYSOV et al. Filter Quartz reactor
Al2O3 Threezone furnace T = 850°C Solution feeding Compressor
Ultrasonic generator Fig. 2. Diagram of the experimental installation for obtaining nanostructured ceramic microspheres.
ing liquid droplets reaching 10 μm in size with a nar row size distribution, which is impossible using other methods [4, 5]. The diagram of the installation for the preparation of nanostructured microspheres by pyrolysis is shown in Fig. 2. A solution is continuously fed via a peristaltic pump into a glass vessel, at the bottom of which an ultrasonic transducer is placed. The distance from the surface of the solution to the ultrasonic transducer was 20 mm and was maintained by feeding the working solution via the peristaltic pump. An air current induced by an air compressor with a capacity of 2 l/min involves the “fog” that is formed under the action of ultrasonic vibrations into a quartz tube with an inner diameter of 40 mm, which is placed in a threezone tube furnace. The length of the constant temperature heated zone of the furnace was 400 mm; the temperature in the reaction zone was set to between 650 and 1150°C. A filter element for trapping particles was mounted at the other end of the quartz tube. The resultant powders were washed with distilled water until residual salt was completely washed off. To prepare the nickel oxide nanopowder of microspheres, we subjected the resultant suspension to treatment with an ultrasonic homogenizer for 5 min, centrifuga tion, separation of sediment, and drying at 50°C. The specific surface area (Ssp) of the samples was measured using the BET method according to low temperature nitrogen adsorption using a Nova 1200e analyzer produced by the Quantachrome Instruments Co. (United States). The average effective particle size (Dav) was calculated by the formula Dav = 6/ρSsp.
In this calculation we assumed that the shape of all particles is close to spherical. The morphology of the materials was studied using a Hitachi TM 1000 scanning electron microscope. The Xray phase and Xray diffraction analyses of the resultant materials were carried out using a Rigaku dif fractometer (αFe radiation). The particle size distributions were measured using a FRITSCH analysette 22 NanoTec instrument. RESULTS AND DISCUSSION The time of flight of a droplet of the solution through the reaction zone, which was calculated with regard for the operating parameters of the experimen tal installation, was 0.5 s. The studies of the reaction intermediates revealed that the process was accompa nied by the moisture removal from the droplets and the decomposition of acetate in the air atmosphere by the reaction Ni(CH3COO)2 + 4O2 = NiO + 4CO2 + 3H2O. The electronmicroscopic studies of the samples showed that the powder particles are hollow micro spheres ranging in size from 0.1 to 10 μm. The wall thickness of the microspheres is 20–100 nm. The wall consists of nanoparticles with a size of 10–20 nm, which can be clearly seen on both the inner and outer walls of the microspheres (Figs. 3a, 3b). This structure is formed because, in the process of drying of a drop let, the nucleation of nickel acetate crystals begins in its surface layer. In addition, as was noted by the authors of [5], the basic condition for the formation of a cavity inside a microsphere is the fairly low concen tration of the salt in the solution used for preparing the powder.
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PREPARATION OF NICKEL OXIDE NANOSTRUCTURED POWDERS
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Fig. 3. Micrographs of NiO obtained by the pyrolysis of ultrasonic aerosols at a reactor temperature of 850°C at various magnifi cations.
The pattern of the size distribution of microspheres was determined by laser diffraction. The resultant dis tribution histograms are depicted in Fig. 4. Figures 4a and 4b show that, as the temperature in the reactor increases from 650 to 950°C, the range of size distributions of microspheres (0.5–12 μm) hardly changes at all, which indicates the similarity of the mechanism of a particle formation in this temperature range: the microsphere size is governed by the initial diameter of the droplet. At a higher temperature (Figs. 4c, 4d), larger and smaller particles appear in the composition of the material; this is probably due to the destruction of droplets in the process of formation, which is a result of the high rate of water removal from its volume. The photomicrographs of the samples of nickel oxide microspheres obtained at a temperature of 750°C before and after treatment with an ultrasonic homogenizer (Figs. 5a and 5b, respectively) show that, under the action of ultrasound, a deaggregation takes place and the material is transformed into a homoge neous mass consisting of nanoparticles (Fig. 5b). At higher temperatures, no destruction of microspheres occurred, which counts in favor of the formation of stable contacts between the particles. NANOTECHNOLOGIES IN RUSSIA
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The values of Ssp and Dav of the particles constitut ing the microsphere walls, which are calculated from the values of Ssp, are listed in the table. The average nanoparticle sizes vary from 5 to 14 nm and are appar ently governed by the reactor temperature. The results of the Xray phase analysis of the sam ples obtained at temperatures of 650, 750, 850, and 1150°C are shown in Fig. 6. The samples are a single phase system based on nickel oxide; the lines of other phases in the diffraction patterns are absent. As the operating temperature decreases, the Xray patterns of the samples exhibit a peak broadening due to a decrease in the crystallite size. The calculation showed no contribution to the broadening from microstresses. Values of Ssp of the NiO samples obtained at various tem peratures and Dav of particles Pyrolysis temperature, °C
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Fig. 4. Size dependence of the relative volume fraction of the NiO microspheres obtained at the different temperatures: (a) 650, (b) 950, (c) 1050, and (d) 1150°C.
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Fig. 5. Micrographs of the NiO samples (the reactor temperature of 750°C) obtained by the pyrolysis of nickel acetate solution (a) before and (b) after treatment in an ultrasonic cavitation field.
The calculated average sizes of the crystallites were 6, 8, and 11 nm for samples obtained at temperatures of 650, 850, and 1050°C, respectively. These values are in agreement with the data of electron microscopy
and the values of Dav calculated from the adsorption data, and they indicate that the particles composing the nanopowders of nickel oxide are individual crys tals.
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PREPARATION OF NICKEL OXIDE NANOSTRUCTURED POWDERS I, pulse 3000 2500
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Fig. 6. Xray patterns of the NiO nanopowders obtained by the pyrolysis of a nickel acetate solution.
CONCLUSIONS
REFERENCES
In this work we show the possibility of preparing nanostructured nickel oxide microspheres (0.5–5 μm) by the pyrolysis of nickel acetate solutions in an oxi dizing atmosphere with their subsequent destruction in an ultrasound field and the formation of nanoparti cles with 5–14 nm in size. The methods of lowtemperature nitrogen adsorp tion, Xray diffraction, laser diffraction, and electron microscopy are used to study the morphological char acteristics of the nickel oxide nanopowders at various temperatures in the reaction zone. By integrated research, we examine the character istics and dispersivity of nickel oxide nanoparticles and show ways to control them.
1. D. I. Ryzhonkov, V. V. Levina, and E. L. Dzidziguri, Nanomaterials: An Educational Manual (BINOM Lab oratoriya Znanii, Moscow, 2008) [in Russian]. 2. V. P. Rodionov, Hydrodynamics of Jet Efflux and the Phenomenon of Cavitation in Liquid (Krasnodar, 2000) [in Russian]. 3. R. G. Sarukhanov, Development of the Technology and Acoustic Installation for the Performance of the Process of Disaggregation of PowderLike Materials (Moscow Institute of Steel and Alloys, Moscow, 1981) [in Rus sian]. 4. A. D. Zimon, Colloid Chemistry (Agar, Moscow, 2001) [in Russian]. 5. N. A. Fuchs, The Mechanics of Aerosols (Pergamon, New York, 1964; Academy of Sciences of the Soviet Union, Moscow, 1985). 6. B. A. Agranat, M. N. Dubrovin, N. N. Khavskii, and G. I. Eskin, Fundamentals of the Physics and Technolo gies of Ultrasound: An Educational Manual for the Insti tute of Higher Education (Vysshaya Shkola, Moscow, 1987) [in Russian].
ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project no. 07080027a.
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