Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles. Katalin Sinkó1â, Géza Szabó1, and Miklós ZrÃnyi2. 1Institute of Chemistry, L. Eötvös University, H-1117 ...
Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 11, 1–9, 2011
Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles Katalin Sinkó1 ∗ , Géza Szabó1 , and Miklós Zrínyi2 2
1 Institute of Chemistry, L. Eötvös University, H-1117 Budapest, Hungary Department of Biophysics, Semmelweis University, H-1085 Budapest, Hungary
Various liquid-phase syntheses of CoO and Co3 O4 nanoparticles have been studied. The experiments focus on two synthesis routes: the coprecipitation and the sol–gel methods combined with thermal decomposition. The effect of synthesis route, the type of precursors (cobalt nitrate/chloride) and precipitation agent (carbonate, hydroxide, oxalic acid, and ammonia), the chemical compositions, pH, application of surfactants (PDMS, Triton X-100, NaDS, NaDBS, TTAB, ethyl acetate, citric acid), and the heat treatments on the properties of particles were investigated. The particle size and distribution have been determined by dynamic light scattering (DLS). The phases and the morphology of products have been analysed by XRD and SEM. The coprecipitation technique is less able to shape the particles than sol–gel technique. PDMS can be applied efficiently as surfactant in preparation methods. The finest particles (around 85 nm) with narrow polydispersity (70–100 nm) and spherical shape could be achieved by using sol–gel technique in medium of 1-propanol and ethyl acetate.
Keywords: Cobalt Oxide, Nanoparticles, Coprecipitation, Sol–Gel Method.
Recently, nanostructured transition metal oxides have attracted a lot of attention due to their technological applications and outstanding properties. The properties (such as magnetic, optic, catalytic, and electronic) of nanomaterials depend strongly on their size, structure, and shape. Co3 O4 nanoparticles exhibit weak ferromagnetic behavior.1 CoO nanocrystals display superparamagnetism or weak ferromagnetism, whereas bulk CoO is antiferromagnetic.2 3 Co3 O4 is a magnetic p-type semiconductor. Co3 O4 has a cubic spinel crystal structure in which the Co2+ ions occupy the tetrahedral sites and the Co3+ ions the octahedral sites.4 The Co3+ ions at the octahedral sites are diamagnetic in the octahedral crystal field. The Co2+ ions at the tetrahedral sites form an antiferromagnetic sublattice with a diamond structure. The cobalt spinel compounds can act as efficient catalysts in a lot of heterogeneous chemical processes.5–8 Nanoparticles of Co3 O4 are promising materials for electronic devices,9 gas sensors,10 magnetic materials,11 electrochromic devices,12 electrochemical systems,13 and high-temperature solar selective absorbers.14 CoO also shows interesting properties and has
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applications as gas sensors15 and as anodes of lithium-ion batteries.16 17 Many different synthesis techniques give access to nanomaterials with a wide range of compositions, well-defined and uniform crystallite sizes. The liquid-phase syntheses offer a good technique and control for tailoring the structures, the compositions, and the morphological features of nanomaterials. The liquid-phase routes include the coprecipitation, the hydrolytic as well as the nonhydrolytic sol–gel processes, the hydrothermal or solvothermal methods, the template synthesis and microemulsion-based processes. The coprecipitation method offers some advantages. Applying this simple and rapid technique, the control of particle size and composition is easy. The precipitation method provides several possibilities to modify the particle surface and shape. Precipitation of various cobalt salts (nitrate,18–20 chloride,21 22 acetate20 23 ) from aqueous or alcoholic-aqueous solutions yields cobalt oxide nanoparticles. The particle size of the coprecipitated materials is strongly dependent on the pH of medium and the concentration of the initial materials. The precipitation agent may be oxalic acid or oxalate,19 20 24 carbonate,19 25 ammonia,26 and sodium hydroxide.25 Organic surfactants (e.g., sodium dodecyl sulphate19 ) can be used during the precipitation in
1533-4880/2011/11/001/009
doi:10.1166/jnn.2011.3875
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RESEARCH ARTICLE
1. INTRODUCTION
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Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles
order to tailor the size and shape of nanoparticles and to hinder the aggregation. In sol–gel synthesis, a soluble precursor molecule is hydrolyzed to form a dispersion of nano-sized particles. Inorganic cobalt salts are subjected to hydrolysis and condensation generally in ethanol at 50–80 C.20 27–29 Monodisperse tetrapod-shaped CoO nanocrystals were prepared via sol–gel alcoholysis of cobalt(III) oleate in dodecanol at high temperature. The dodecanol solvent acts as a reducing and a morphology controller reagent.30 The heat treatment of wet gels has a strong effect on the surface area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels.31 Nanoparticles can be also formed on liquid–liquid interfaces. Among the chemical methods the microemulsion process involving reverse micelles has been demonstrated as a versatile method for obtaining a wide variety of nanocrystalline oxides.32–35 Functionalized reverse micelles may be used to control the size and the polydispersity. The size of particles can be affected either by the volume of water, or by the solvent used to form reverse micelles, or by adding surfactants.32 In the microemulsion (reverse micellar) route, the first step is the synthesis of cobalt salt (e.g., oxalate23 ) nanoparticles from precursors in the presence of a surfactant (e.g., cetyltrimethylammonium bromide23 ) and a cosurfactant (e.g., n-butanol23 ). Among all the kinds of synthetic techniques, hydrothermal/solvothermal method has been achieved some success due to its simplicity, low-cost and different morphologies of products.35–40 The hydrothermal/solvothermal methods can be applied to form crystalline materials (films, nanoparticles, single-crystals etc.) from aqueous solutions, under high vapor pressure. The control of solutionrecrystallization and growth of particles yields all kinds of nanostructured materials. Co3 O4 crystals with various morphologies, such as nanospheres,41 nanorods,42 43 nanocrystalline,44 nanocubes,45 hollow spheres,46 47 nanorods bunches48 and urchin-like nanocrystals49 can be successfully prepared by a hydrothermal or solvothermal process. The aim of the present study was to synthesize CoO and Co3 O4 nanoparticles by various liquid-phase syntheses. The experiments were mostly carried out by two synthesis routes; by coprecipitation and sol–gel methods. These techniques were combined with thermal decomposition. We have monitored the effect of synthesis route, the type of precursors, the chemical compositions, the concentration of initial materials, the pH, the application of surfactants, and the heat treatments on the properties of particles. The particle size and distribution were determined by dynamic light scattering (DLS). The identification and characterization of the phases and the morphology of products were performed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The 2
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processes and the weight loss occurred by heat treatments were recorded by thermal analysis in a controlled atmosphere.
2. EXPERIMENTAL DETAILS 2.1. Preparation Methods In the experiments of the coprecipitation method, cobalt nitrate and chloride were provided as cobalt precursors. The aqueous solution of precipitation agents (sodium carbonate, oxalic acid, and ammonia) was dropped to the aqueous solution of cobalt precursors. The concentration of cobalt precursor varied from 0.01 to 5 mol dm−3 , the ratio of precipitator/Co(II) ions from 1.0 to 5.0 (Table I). The mixtures were stirred for 2 h at room temperature with or without a surfactant. Pink (Co oxalate), violet (Co carbonate), or green (Co hydroxide) precipitates were obtained. The dried precipitates were subjected to heat treatment at various temperatures resulting in CoO in reductive atmosphere and Co3 O4 in oxidative atmosphere. Polydimethylsiloxane (PDMS); polyethylene glycol tertoctylphenyl ether (Triton X-100); sodium dodecyl sulfate (NaDS); sodium dodecylbenzene sulfonate (NaDBS); and tetradodecylammonium bromide (TTAB) were used as surfactants in the concentration of 0, 1, 5, and 10 w/w% (Table I). In the experiments of the sol–gel method, only Co(NO3 2 could be employed as initial material, because CoCl2 has poor solubility in alcohol solvents. The Co2+ ions were allowed to hydrolyze for 2 h at 50 C in first Table I.
Summary of preparation experiments.
Methods Coprecipitationa Coprecipitation Coprecipitation Coprecipitation Coprecipitation Coprecipitationa Coprecipitationa Coprecipitation Coprecipitation Coprecipitation Coprecipitationa Coprecipitation Coprecipitation Coprecipitation Coprecipitation Coprecipitationa Sol–gelc Sol–gelc Sol–gelc Sol–gelc
Solvent
Precipitator
Surfactants
Surfactants (w/w%)
Water Water Water Water Water Water Water Water Water Water
Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3 Na2 CO3
— Triton X-100 NaDS NaDBS TTAB PDMS (550)b PDMS (5600)b PDMS (5180)b PDMS (42500)b PDMS (83000)b
— 1, 5, 10 1, 5, 10 1, 5, 10 1, 5 1, 5, 10 1, 5, 10 5 5 5
Oxalic Oxalic Oxalic Oxalic Oxalic Oxalic
acid — acid Triton X-100 acid NaDS acid NaDBS acid TTAB acid PDMS (5600)2
— 1 1 1 1 1, 5, 10
— — — —
— Citric acid PDMS (550)2 Ethyl acetate
Water Water Water Water Water Water Ethanol Ethanol Ethanol 1-propanol
— 0.1–1.0 0.1–2.5 10, 20, 40
Concentration of Co nitrate varied from 0.01 to 2 mol dm−3 . b Molecular weight, g mol−1 . c Concentration of Co nitrate was 0.4 mol dm−3 .
a
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step and for 2 h at 85 C in the second step in ethanol or 1-propanol solution with (citric acid, PDMS, or ethyl acetate) or without a surfactant (Table I). The sol–gel technique produced different basic Co nitrate precipitates depending on the used solvent and surfactant. The precipitates were dried at 80 C and heated at different temperatures under oxidative atmosphere. In order to obtain also CoO, the dried samples were heat treated at 1000 C under reductive (N2 atmosphere. 2.2. Characterisation Methods
3. RESULTS AND DISCUSSION 3.1. Surfactant-Assisted Precipitation Method The cobalt nitrate is able to provide the Co precursor rather than cobalt chloride due to its much better solubility in aqueous or alcoholic solutions. The solubility of Co nitrate is ≈7 mol in 1 l water and that of Co chloride is ≈3 mol/l. J. Nanosci. Nanotechnol. 11, 1–9, 2011
Regarding the results of investigations with initial solutions of different concentrations, the precipitation from the concentrated solutions leads to the finest particles. The concentrated solutions promote the nucleation. The favorable concentration for Co nitrate solution is 5 mol dm−3 (close to the saturated value) and 2 mol dm−3 for Co chloride solution, respectively. The use of oxidative (air for Co3 O4 or inert (N2 for CoO) atmospheres does not result in a remarkable difference in the size and distribution, rather the effect of heat treatment at 1000 C is dominant. In the study on the surfactant-assisted precipitation techniques, the precipitation agents, the surfactants, their concentration, and the temperature of heat treatment were varied. The precipitations with sodium hydroxide or ammonia yield coarse and large particles (>1 in aqueous solutions. Thus the experiments concentrated on the application of carbonate and oxalate precipitators. The sizes of particles obtained by oxalate precipitators are larger and more polydisperse (100–2000 nm) than that of carbonate precipitates (60–1 in aqueous solutions. The oxalate precipitates are polydisperse (100–2000 nm). The carbonate agent yields fine and less polydisperse particles (60–700 C increases again the size. In the sol–gel method, the cobalt precursor (Co nitrate) hydrolysis in alcoholic (1-propanol or ethanol) solution and produces OH groups required for condensation. These processes provide the nucleation and growth of the crystalline particles. In the sol–gel procedures, different surfactants (citric acid, PDMS of various molecular weights, and ethyl acetate) have been also applied in order to improve the dispersion of powders. Application of ethyl acetate in concentration of 40 w/w% yielded the finest particles with mean diameter of 85 nm and narrow polydispersity (70–100 nm). The lower polarity of propanol and ethyl acetate supports the decomposition of nitrate ions. The escape of nitrous gases increases the pH, which promotes the hydrolysis and condensation reactions of cobalt salt. The sol–gel derived precipitate synthesized by ethyl acetate contains less nitrate ions than that obtained by other surfactants in ethanol. The structural transformations (basic Co salt to Co3 O4 and Co3 O4 to CoO) induce an increase of particle sizes, hence, the temperature of the J. Nanosci. Nanotechnol. 11, 1–9, 2011
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heat treatment must be set between or above the temperatures of the transformations. The sol–gel precipitates consist of more spherical shaped and uniform particles, than those produced by coprecipitation. Acknowledgments: This study has been supported by OTKA NK 68750 funds and I-04-009 EU in HASYLAB, DESY. The European Union and the European Social Fund have provided financial support to the project under the grant agreement no. TÁMOP 4.2.1./B-09/KMR-20100003.
References and Notes
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Received: 24 July 2010. Accepted: 5 October 2010.
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Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles
