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Size-Controlled Synthesis of Quasi-Monodisperse Transition-Metal Ferrite Nanocrystals in Fatty Alcohol Solutions Shiva Adireddy,†,‡ Cuikun Lin,†,‡ Vadim Palshin,§ Yuming Dong,‡ Richard Cole,‡ and Gabriel Caruntu*,†,‡ AdVanced Materials Research Institute, UniVersity of New Orleans, 2000 Lakeshore DriVe, New Orleans, Louisiana, Department of Chemistry, UniVersity of New Orleans, 2000 Lakeshore DriVe, New Orleans, Louisiana 70148, and J. Benett Johnston Sr. Center of AdVanced Microstructures and DeVices, 6980 Jefferson Hwy., Baton Rouge, Louisiana 70806 ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: October 11, 2009
Quasi-monodisperse hydrophobic transition-metal ferrite MFe2O4 (M ) Mn, Fe, Co, Ni, Zn) nanocrystals were synthesized by the thermolysis of transition-metal acetates in oleyl alcohol solutions under aerobic conditions. The proposed reaction protocol is simple, rapid, and highly versatile because it takes advantage of the multiple roles of the oleyl alcohol molecules, namely, solvent for the precursors, reaction medium, and capping ligand for the metal oxide nanoparticles. A systematic FT-IR spectroscopy study has indicated that the transition-metal ferrite nanoparticles are precipitated in oleyl alcohol solutions via a thermal decomposition process with no evidence about a potential esterification reaction, involving the long-chain alcohol and the metal acetate salt. A detailed characterization of the oleyl alcohol capped ferrite nanoparticles was performed by XRD, TEM, SAED, EXAFS, FT-IR, and SQUID measurements. The as-prepared transition-metal ferrite nanocrystals are spherically shaped, and their average diameter can be conveniently tuned between 4 and 15 nm by increasing the heating rate of the solution. The surface composition of the nanoparticles can be modified via ligand-exchange reactions through which the nanocrystals can be rendered soluble in polar solvents without altering their morphology. The oleyl alcohol capped ferrite nanocrystals typically exhibit a superparamagnetic behavior with blocking temperatures depending on both the nature of the transition metal and the size of the nanocrystals. 1. Introduction Spinel-type transition-metal ferrites have attracted an increasing interest because of their wide area of technological applications in energy conversion and storage,1 magnetic recording,2 gas sensing,3 catalysis,4 and biotechnology.5-7 In particular, dispersions of surface-stabilized iron oxide (γ-Fe2O3 and Fe3O4) nanoparticles in a carrier fluid are very attractive for biomedical applications in drug delivery, blood regeneration, cell labeling, tracking and separation of cells, and magneto-hyperthermia.8-11 For biomedical applications, magnetic nanoparticles are required to have a well-defined composition, controlled size/shape, density of reactive surface groups, and dispersibility in desired solvents because these features control effectively their intrinsic chemical and physical properties. Among the most successful chemical approaches for the controlled synthesis of magnetic nanoparticles is the thermolysis of molecular precursors in high-boiling-point organic solvents. In a typical reaction, a metalorganic precursor, such as a cupferronate,12,13 carbonyl,14,15 acetylacetonate,16 or fatty metal salt,17 is dissolved in a long-chain ether,18 amine,19 or alkene20 and thermally decomposed in the presence of a surface-capping agent, such as an amine or a carboxylic acid. The thermolysis reaction is performed between 200 and 300 °C under a protective atmosphere that prevents the oxidation of the transition-metal * To whom correspondence should be addressed. Tel: +1-(504)-2803221. Fax: +1-(504)-280-3185. E-mail:
[email protected]. † Advanced Materials Research Institute, University of New Orleans. ‡ Chemistry Department, University of New Orleans. § J. Benett Johnston Sr. Center of Advanced Microstructures and Devices.
ions and the formation of unwanted secondary phases.21,22 Aromatic and long-chain alcohols are attractive as reaction media and/or capping ligands in the organic phase synthesis of metal oxide nanoparticles. This is because they have relatively high boiling points, are inert toward the formation of metal oxides, and can effectively dissolve various metal salt precursors. Also, they can be easily chemisorbed onto the surface of metal oxides, thereby eliminating the need of capping ligands. Last, but not least, the reducing power of the alcohol group prevents the oxidation of the transition-metal ions, which allows for the stabilization of the spinel structure under aerobic conditions and simplifies considerably the reaction protocol. Despite these advantages, alcohols have been rarely used in the hightemperature solution phase synthesis of metal oxide nanoparticles. Sun and co-workers tested the ability of long-chain alcohols to replace 1,2-hydrocarbon diols for the colloidal synthesis of monodisperse Fe3O4 nanocrystals and found that the resulted nanocrystals possess a lower quality than those obtained in the presence of diols.23 Niederberger et al. used benzyl alcohol in the synthesis of Fe3O4 nanoparticles by a nonaqueous sol-gel from Fe(acac)3 under solvothermal conditions.24,25 In this case, the mechanistic pathway involves an esterification reaction between the metal salt and the alcohol. The size of the nanoparticles was varied from 12 to 25 nm upon increasing the reaction temperature from 175 to 200 °C. Recently, benzyl alcohol was used for the preparation of a barium alkoxide, which serves as a precursor in the synthesis of BaTiO3 nanocrystals at 320 °C under an inert gas.26 We herein report on the preparation of monodisperse transition-metal ferrite MFe2O4 nanoparticles from transition-metal
10.1021/jp905955k CCC: $40.75 2009 American Chemical Society Published on Web 11/11/2009
Synthesis of Transition-Metal Ferrite Nanocrystals
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acetate salts in oleyl alcohol solutions. This approach is highly versatile, easily scalable, and yields high-quality hydrophobic magnetic nanocrystals. The size of the nanoparticles can be mainly controlled by varying the heating rate of the reaction solution. The oleyl alcohol capped ferrite nanoparticles are spherically shaped and form highly stable colloidal solutions in nonpolar solvents. By performing ligand-exchange reactions, the surface composition of the nanocrystals can be altered in such a way that they can be rendered dispersible in polar solvents. 2. Experimental Details 2.1. Chemicals. Reagent grade divalent transition-metal acetates M(ac)2 and technical grade oleyl alcohol C18H35OH (85%) were purchased from Alfa Aesar and used as received without further purification. To minimize the exposure to the air and/or the atmospheric moisture, the metal salt precursors were stored and handled in a glovebox under a protective atmosphere of argon. During the synthesis of the metal oxide nanoparticles, each heating/cooling cycle was performed with a Keithley temperature controller connected to a computer system. Because of the reducing properties of the alcohol molecules under an inert atmosphere, the divalent acetates lead to the formation of mixtures of FeO and metals (Cu, Ni) and binary oxides (MnO, CoO, and ZnO); to ensure the stabilization of the spinel structure, the subsequent syntheses were carried out in a flask initially exposed to the air and then closed without circulating an inert gas. Unlike otherwise specified, the experimental results presented in this paper correspond to spinel-type transition-metal ferrite nanopowders obtained under static aerobic conditions. 2.2. Synthesis of the MFe2O4 Nanoparticles. In a typical synthesis, 1 mmol of Fe(ac)2 and 0.5 mmol of transition-metal acetate M(ac)2 (M ) Mn, Fe, Co, Zn) were added to 20 mL of oleyl alcohol in a three-neck round flask and kept under vigorous stirring. To promote the dissolution of the acetate salts and eliminate the hydrating water molecules, the initial solution was heated rapidly to 150 °C and maintained at this temperature for 15 min. The temperature was subsequently increased to 300 °C. After 30 min, the reaction was stopped by removing the heating source and allowing the solution to cool naturally to room temperature. The resulting powders were recovered from the solution by magnetic separation and then washed several times with ethanol and acetone, separated again, and extracted in toluene. For the complete removal of the excess oleyl-alcohol, this purification protocol was repeated three times. Oleyl alcohol capped magnetic nanocrystals were isolated and then dispersed in various nonpolar solvents (chloroform, toluene, hexane, etc.), yielding brown clear colloids that are stable against aggregation for months at room temperature. 2.3. Characterization of the MFe2O4 Nanoparticles. The thermal stability of the carboxylate salts was studied by recording their TGA profiles with an SDT Q600 thermal analysis system. Samples were heated to 600 °C at the constant rate of 5 °C/min under ultrahigh purity argon, followed by their natural cooling to room temperature. The residuals obtained were analyzed by powder X-ray diffraction. The stability of oleyl alcohol, the composition of the postsynthesis solutions, and the surface composition of the nanoparticles were studied by FTIR spectroscopy with a Nexus 870 spectrometer in the 400-4000 cm-1 spectral range and a resolution of 2 cm-1. The FT-IR spectra of the oleyl alcohol were recorded before and after a heat treatment at 300 °C for 30 min under a protective atmosphere of argon. Prior to analysis, the oleyl alcohol and the MFe2O4 samples were mixed with KBr powder, then ground
Figure 1. TGA profiles of bulk transition-metal acetates M(ac)2 under flowing N2.
and compacted into thin disk-shaped pellets. Microstructural characterization was performed by transmission electron microscopy (TEM) with a JEOL 3010 microscope with an accelerating voltage of 300 kV. For the TEM experiments, clear toluene solutions of the as-prepared MFe2O4 nanoparticles were cast onto carbon-coated Cu TEM grids and evaporated naturally. No selective precipitation of the nanoparticles was used to narrow their size distribution. The phase purity and crystal structure of the nanopowders were performed by X-ray diffraction with a Panalytical X’Pert diffraction system using the Cu KR radiation. The data were collected in step scanning mode from 15° to 75° in 2θ with a 0.02° step size and 5 s/point. Measurements were carried out with a 0.4° divergence slit and a 0.5 mm programmable receiving slit. Commercial Si powder was used to determine the instrumental resolution of the X-ray diffractometer. The structure refinement was performed in the Fd3jm space group by using the Fullprof suite of programs.27The background was refined by using a 6-parameter polynomial function, whereas the peak profiles were modeled by using a pseudo-Voigt function (convolution of a Gaussian and Lorentzian function). In the final stage of the refinement, 24 parameters, including the zero shift; scale factor; background polynomial parameters; peak profile parameters U, V, W, and h (Lorentzian/ Gaussian distribution); unit cell parameters; atomic positions; site occupancies; and the isotropic displacement factors were refined simultaneously. The chemical composition of the ferrite samples dissolved in a 1 M HCl solution was determined by using inductive coupled plasma (ICP) spectroscopy with a Varian FT220s flame absorption spectrometer. Magnetic measurements of the nanopowders were carried out with a MPMS5S SQUID susceptometer from Quantum Design working in the temperature range from 5 to 300 K. Iron K-edge XANES spectra were collected at the Double-Crystal Monochromator (DCM) beamline at the 1.3 GeV electron energy storage ring synchrotron radiation facility of the Center for Advanced Microstructures & Devices (CAMD) at Louisiana State University. 3. Results and Discussion 3.1. Thermal Behavior of the Metal Acetate Precursors. The TGA profiles of the metal acetate salts are broadly similar and indicate that each acetate decomposes in a specific temperature range, depending on the nature of the transition metal (Figure 1) Two distinct trends are observed in the weight loss
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of the samples, depending on the water content of the acetate salt precursors. Specifically, the anhydrous acetates M(ac)2 (M ) Mn, Fe, Co, Zn) present a slight mass loss (
