Monodispersed Co, Ni-Ferrite Nanoparticles with Tunable Sizes

Sanjeev Kumar , Vaishali Singh , Saroj Aggarwal , Uttam Kumar Mandal and Ravinder Kumar Kotnala. The Journal of Physical Chemistry C 2010 114 (14), ...
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J. Phys. Chem. C 2008, 112, 911-917

911

Monodispersed Co, Ni-Ferrite Nanoparticles with Tunable Sizes: Controlled Synthesis, Magnetic Properties, and Surface Modification Xiao Jia, Dairong Chen,* Xiuling Jiao,* Tao He, Hanyu Wang, and Wei Jiang School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, P. R. China ReceiVed: August 31, 2007; In Final Form: NoVember 1, 2007

Ferrite (CoxNiyFe3-x-yO4, x ) 0.6, 0.3, 0.2, 0, y ) 0, 0.6, 0.7, 0.8) nanoparticles monodispersed in nonpolar solvents have been prepared with different Co, Ni, and Fe compositions by heating mixtures of inorganic salts and sodium dodecylbenzenesulfonate (SDBS) in n-octanol. The as-prepared nanoparticles were characterized in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), selective area electron diffraction (SAED), infrared (IR), thermal-gravimetric (TG) analysis, and other techniques. The nanoparticle sizes could be tuned by adjusting the reaction time. The formation mechanism of the nanoparticles during the solvothermal process is proposed based on the experimental results. The effects of size and composition of the nanoparticles on magnetic properties were investigated, and the magnetic properties of nanoparticles with the same size but different compositions were compared. Furthermore, water-dispersible nanoparticles were obtained by simply washing the as-prepared nanoparticles with acetic acid.

1. Introduction Magnetic nanoparticles in the single-domain size range, often showing superparamagnetic character and high field saturation, have attracted much interest not only in answering basic research questions but also in technological applications in many fields such as data storage, color imaging, ferrofluids, biomedicine, and so forth.1 Many groups have developed fabrication methods for magnetically stable ferrite nanoparticles, including sonochemical reactions,2 sol-gel techniques,3 reverse micelles,4 host templates,5 coprecipitation,6 microemulsion procedures,7 thermal decomposition of organic complexes (precursor techniques),8 microwave,9 mechanochemical alloying,10 and hydrothermal or solvothermal routes.11 The magnetic properties of nanoparticles are sensitive to synthesis conditions, particle size, shape, and composition, so a large amount of experimental data has been collected with a large variance in the observed magnetic behavior.12 However, the unusual physiochemical properties of magnetic nanoparticles continue to evoke interest, and new experimental recipes for the controlled synthesis of nanomaterials continually come forth. Of all of the methods to date, the precursor technique and solvothermal route exhibit advantages in preparing high-quality monodispersed nanoparticles. In addition, the use of suitable reagents and solvents in the solvothermal method can make synthesis relatively safe and economical. Most reports of monodisperse magnetic nanoparticles have focused on monodisperse metals, metal alloys, and binary and ternary metal oxides, but there are few investigations on monodisperse compounds with more than three components.13 It would be meaningful to carry out a study of compositional variation using a single synthesis procedure to enrich and better understand the magnetism of the nanoparticles. In previous research, we reported a liquid-phase redox reaction that generates high-quality Co3O4 nanocrystals in which cobalt nitrate was used as a reagent, with n-octanol as the solvent, * Corresponding author. Tel: +86-0531-88364280. Fax: +86-053188364281. E-mail: [email protected].

and sodium dodecylbenzenesulfonate (SDBS) as an additive.14 Our further experiments indicated that this method could be readily extended to the synthesis of ferrite (MFe2O4) nanoparticles by simply adding a different metal inorganic salt precursor. Herein, we present detailed syntheses and characterization of high-quality monodisperse Co, Ni-ferrite (CoxNiyFe3-x-yO4, x ) 0.6, 0.3, 0.2, 0, y ) 0, 0.6, 0.7, 0.8) nanoparticles with multiple components without further size-selection. The sizes and compositions can be controlled by adjusting the reaction time and the molar ratio of inorganic salts in the initial mixture, respectively. Many investigations have focused on not only the controlled synthesis of monodisperse ferrite nanoparticles but also the correlations between material properties and either particulate properties or synthetic conditions.15 For example, Zhang’s group synthesized single-phase CoCrxFe2-xO4 (0 < x < 1 or x ) 1) nanoparticles with a controlled size by use of normal and reverse micelle microemulsion methods and characterized their sizedependent magnetic properties;13 furthermore, they prepared monodisperse CoFe2O4 nanocrystals with highly controllable, nearly spherical or almost perfectly cubic shapes by combining a nonhydrolytic process and seed-mediated growth and compared the magnetic properties with respect to the structure’s shape and size.8g Sun et al. prepared monodisperse MFe2O4 (M ) Fe, Co, Mn) nanoparticles with tunable sizes ranging from 3 to 20 nm through the reaction of metal acetylacetonate and 1,2hexadecanediol and investigated the effects of composition on magnetic properties.8f Hyeon and co-workers prepared highly crystalline and monodisperse MFe2O4 (M ) Mn, Co) nanocrystals with different sizes via the thermal decomposition of metal-surfactant complexes followed by mild chemical oxidation and investigated their magnetic properties.8c, 8d Although significant progress has been made in studies on the correlations between magnetic properties and composition, size, shape, or synthesis parameters of the nanoparticles, systemic and profound understanding, especially of the magnetic properties of nanoparticles with the same size and morphology but different

10.1021/jp077019+ CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008

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Figure 1. TEM images and the corresponding XRD patterns of Co0.6Fe2.4O4 nanoparticles with sizes of (a) 9.5 nm, (b) 11.5 nm, and (c)13.5 nm. TEM samples were deposited from their cyclohexane dispersion on a copper grid and dried at room temperature. All of the scale bars in the TEM images are 100 nm.

components, remains challenging. In this study, the blocking temperature, saturation magnetization, and coercivity of magnetic nanoparticles with nearly the same size and morphology but different Co, Ni, and Fe compositions were compared. Monodispered nanoparticles prepared in nonaqueous solvents are usually well-dispersed in nonpolar solvents but aggregate in polar solvents such as water. For magnetic nanoparticles, a hydrophobic surface will limit their applications in many fields. For example, water-dispersible magnetic nanoparticles can be applied in cancer treatment by attaching cytotoxic drugs to the particles and then guiding and concentrating the drugs at tumor locations.16 Nanoparticles with hydrophobic surfaces cannot be used in such biological applications. Thus, surface modification is important because of the direct effect of the surface species on the dispersion in different media. A series of surface modification experiments has been carried out to adjust the dispersion of nanoparticles in polar solvents;17 however, it is desirable to develop strategies for the fabrication of waterdispersible magnetic nanoparticles with nontoxic surfaces. In the present work, water-dispersible Co-, Ni-, and Fe-ferrite nanoparticles are obtained by washing the as-prepared hydrophobic nanoparticles described above with acetic acid. The acetate-modified nanoparticles might have broader and more significant applications. The objective of this work is to develop a route to synthesize monodispersed ferrite nanoparticles with multiple compositions and precisely controlled sizes in both nonpolar and polar solvents using low-cost inorganic salts as precursors. We also investigated the effects of nanoparticle size and composition on magnetic properties, especially the magnetic properties of nanoparticles with the same size but different compositions, which might be different from the corresponding bulk materials.

Jia et al. grade. Anhydrous FeCl3, SDBS, and n-octanol were used without further purification, but Ni(Ac)2‚4H2O and Co(NO3)2‚ 6H2O were dehydrated before use. The anhydrous Co(NO3)2 was prepared using the method reported previously by our group.18 Co(NO3)2‚6H2O was dissolved in n-octanol, forming a red solution (ca. 0.1 g‚mL-1), and then the solution was refluxed for 10 h under reduced pressure to give Co(NO3)2‚ 7C8H15OH. Ni(Ac)2‚4H2O was dehydrated as follows: Ni(Ac)2‚ 4H2O was dissolved in acetic anhydride, at a concentration of 0.5 g‚mL-1. The solution was refluxed for 5 h, and the superfluous acetic anhydride was distilled, leaving a green powder. The powder was dried under vacuum at 80 °C for 3 h. The chemical formula of the dried salt should be Ni(Ac)2‚ 1.1H2O based on thermal-gravimetric (TG) and elemental analyses. 2.2. Synthesis. In a typical synthesis, a set quantity of Co(NO3)2‚7C8H15OH and/or Ni(Ac)2‚1.1H2O (total 1.147 mmol), 0.372 g (2.294 mmol) of FeCl3 and 3.30 g (8.58 mmol) of SDBS were added into 20.0 mL of n-octanol under magnetic stirring. When the precursors were completely dissolved, the solution was poured into a 25 mL autoclave with a Teflon liner. The autoclave was first heated at 90 °C for 6 h and then reacted at 240 °C for different lengths of time. Then the autoclave was cooled to room temperature quickly by dipping it into water. After cooling, a black-brown solution was obtained. Anhydrous ethanol was then added to the solution to precipitate a black powder, which was separated by centrifugation (8500 rpm, 10 min). The product was composed of nanoparticles that can be monodispersed in nonpolar and weakly polar solvents. To produce water-dispersible nanoparticles, the product was first washed with ethanol twice, then with water 5 times, and finally with acetic acid (10 wt %). After each washing, the particles were recovered by centrifugation at 8500 rpm for 10 min. The molar compositions of the products were determined by energy dispersive spectroscope (EDS) analysis. 2.3. Characterization. The crystal structure of the product was determined from X-ray diffraction (XRD) patterns collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu KR radiation (λ ) 0.15418 nm). The morphology and microstructure were characterized using a transmission electron microscope (TEM, JEM-100CXII), and a high-resolution TEM (HR-TEM, GEOL JEM-2100). Thermalgravimetric (TG) analysis was carried out on a Mettler Toledo SDTA851e thermal gravimetric analyzer at a heating rate of 10 °C‚min-1 under an air atmosphere. The infrared (IR) spectra were measured on a Nicolet 5DX Fourier transform infrared (FT-IR) spectrometer using the KBr pellet technique. Elemental analysis was determined by the EDS on a scanning electron microscope (SEM). The magnetic properties of the samples were determined on a SQUID (Quantum Design, MPMSXL-7) magnetometer. For XRD, TG, IR, and the characterization of magnetic properties, the precipitate was washed with water after washing with ethanol and then dried at 90 °C for 30 min. Gas chromatography-mass spectrometry (GC/MSD, Agilent 6890N5973N) was used to detect the species on the particle surfaces. Before GC/MS analysis, the product was separated and washed with ethanol twice and then redispersed in cyclohexane. UVvis absorption spectra (Lambda-35, Perkin-Elmer) were applied to track the progress of the reaction.

2. Experimental Section 2.1. Chemicals. Anhydrous FeCl3 (Alfa Aesar Chemical Co.), Ni(CH3COO)2‚4H2O (Shanghai Chemical Co.), SDBS (>90 wt %, Shanghai Chemical Co), Co(NO3)2‚6H2O (Shanghai Chemical Co.), and n-octanol (Tianjin Reagent Co.) were all analytical

3. Results and Discussion 3.1. CoxNiyFe3-x-yO4 Nanoparticles. The size of the CoxNiyFe3-x-yO4 nanoparticles increased with reaction time and could be controlled by changing the reaction time, but no

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Figure 2. TEM (a-d) and HR-TEM (e-h) images of 11.5 nm ferrite nanoparticles. (a and e) Co0.6Fe2.4O4, (b and f) Co0.3Ni0.6Fe2.1O4, (c and g) Co0.2Ni0.7Fe2.1O4, and (d and h) Ni0.8Fe2.2O4. The insets are the corresponding SAED patterns, and the scale bars in the TEM images are 100 nm.

Figure 3. Particle diameter histogram of 11.5 nm ferrite nanoparticles. (a) Co0.6Fe2.4O4, (b) Co0.3Ni0.6Fe2.1O4, (c) Co0.2Ni0.7 Fe2.1O4, and (d) Ni0.8Fe2.2O4.

Figure 4. XRD patterns of 11.5 nm ferrite nanoparticles. (a) Co0.6Fe2.4O4, (b) Co0.3Ni0.6Fe2.1O4, (c) Co0.2Ni0.7 Fe2.1O4, and (d) Ni0.8Fe2.2O4.

obvious linear correlation was observed. For example, Co0.6Fe2.4O4 nanoparticles with uniform sizes of 9.5 ( 0.2, 10.5 ( 0.3, and 13.5 ( 0.3 nm (Figure 1a-c) could be obtained with reaction times of 6, 8, and 12 h, respectively. However, when the reaction time was longer than 24 h, the nanoparticle size distribution broadened noticeably. The corresponding XRD patterns show that the peaks became narrower with longer

reaction times, indicating that the particle size increased. The calculated lattice constants from the (311) reflection are nearly the same for the different-sized nanoparticles, averaging a ) 0.8392 nm, a little smaller than that of the pure Fe3O4 phase (a ) 0.8396 nm, JCPDS No.19-0629), a result of the smaller radius of Co2+ cations compared to Fe2+. Further experiments showed that other ferrite nanoparticles with different sizes could also be obtained. Under the same conditions, uniform Co0.3Ni0.6Fe2.1O4, Co0.2Ni0.7Fe2.1O4, and Ni0.8Fe2.2O4 nanoparticles with sizes ranging from 7.0 to 11.5 nm (Supporting information, Figure S1-3) could be prepared. The results indicate that the size of the product decreased with Ni increasing content under the same reaction conditions. The Ni might affect the relative rate of nucleation and crystal growth by accelerating the nucleation rate and/or decelerating the crystal growth.19 After a reaction time of more than 24 h, the size distribution of CoxNiyFe3-x-yO4 nanoparticles broadened due to Ostwald ripening that starts after the NO3- and Ac- are consumed,20 which was confirmed by UV-vis and chemical precipitation. EDX analysis of the CoxNiyFe3-x-yO4 nanoparticles shows that Co/ Ni, Fe, and O were the main elemental components and about 25% of the metal ions still existed in the filtrate. To evaluate the composition uniformity of the nanoparticles, we carried out further EDX analysis in different regions of the product. EDX results from different regions are nearly consistent within error, which confirms the composition uniformity of the as-obtained samples. All of the above samples were monodispersed in nonpolar or weakly polar solvents. A series of uniform and monodispersed nanoparticles with different elemental compositions and the same size (CoxNiy Fe3-x-yO4) could be obtained. For example, as shown in Figure 2, 11.5 nm ferrite nanoparticles were prepared as described above. Their corresponding selected area electron diffraction (SAED) patterns (insets of Figure 2) indicate their high crystallinity, and the HR-TEM images illustrate their singlecrystal structure. The lattice space of 0.295 nm corresponds to the {220} planes of spinel Co0.6Fe2.4O4, the spaces of 0.293 nm for Co0.3Ni0.6Fe2.1O4 and 0.294 nm for Co0.2Ni0.7Fe2.1O4 are also attributed to the {220} planes, and the spaces of 0.486 and 0.301 nm for Ni0.8Fe2.2O4 correspond to the {111} and {220} planes. All of the exhibited facets of the nanoparticles with

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Figure 5. IR spectra (A) and TG curves (B) of ferrite nanoparticles. (a) Co0.6Fe2.4O4, (b) Co0.3Ni0.6Fe2.1O4, (c) Co0.2Ni0.7Fe2.1O4, and (d) Ni0.8Fe2.2O4.

different compositions were {110} planes, indicating that the slowest growth direction was [110]. This result is different from the thermally controlled crystal growth of cubic structures, in which the exhibited facet is usually {100}, implying that the crystal growth in the present experiment might be a kinetically controlled process.21 The particle size-distribution histograms (Figure 3) demonstrate that the particle sizes of these four samples with different components were uniform. The reflections in the XRD patterns of these 11.5 nm ferrite nanoparticles can be indexed to the (220), (311), (400), (422), (511), and (440) crystal planes of the spinel structure (Figure 4), exhibiting no impurity. The calculated lattice constant of a ) 0.8392 nm for Co0.6Fe2.4O4 is similar to that of CoFe2O4 (a ) 0.8391 nm, JCPDS No.22-1086), and the broad shape of the XRD reflections results from the small size of the nanoparticles. The reflections show high intensity for the Co0.6Fe2.4O4 and Ni0.8Fe2.2O4 nanoparticles, while the reflection intensities of Co0.3Ni0.6Fe2.1O4 and Co0.2Ni0.7Fe2.1O4 samples are comparatively low, indicating that crystallization was more difficult in the presence of both Co and Ni because of their different atomic radii and electronegativities. As the Ni content in the products increased, the XRD peaks shifted slightly to high 2θ angles and the lattice constant decreased to a ) 0.8368 nm for Co0.3Ni0.6Fe2.1O4, a ) 0.8356 nm for Co0.2Ni0.7Fe2.1O4, and a ) 0.8348 nm for Ni0.8Fe2.2O4, a little larger than that of NiFe2O4 (a ) 0.8339 nm, JCPDS No.10-0325) because of the smaller atomic radius of Ni(II) compared to Co(II). The XRD results are not so consistent with the lattice spacings measured from the corresponding HR-TEM images, which might be caused by individual differences among the nanoparticles during HR-TEM observation. All of the average particle sizes for these samples are estimated from Scherrer’s formula to be 11.5 nm, which is in agreement with the TEM observations. Figure 5 shows the corresponding IR (A) and TG (B) spectra of 11.5 nm ferrite nanoparticles with different compositions. The absorptions at 3370 and 1630 cm-1 are ascribed to the -OH vibrations of the adsorbed water or the surface hydroxyls. The peaks at 2923 and 2850 cm-1 are attributed to υ(C-H), and those from 970 to 1200 cm-1 are ascribed to the vibrations of C-O, or SO42- anions resulting from the decomposition of SDBS during the solvothermal process.22 The absorption at ca. 590 cm-1 ascribed to υ(Fe-O) shifted gradually from 581 to 592, 596, and 601 cm-1 as the Co content decreased in curves a-d.5 The shift is caused by the difference in the electronegativity of Co, Ni, and Fe atoms. To further detect the species on the particle’s surface, we conducted GCMS measurements, which revealed many caprylic ether groups on the surface of the nanoparticles. Further experiments indicated that the IR absorptions at 3300-340 and 2850-3000 cm-1 disappeared

Figure 6. UV-vis spectra of the reagent solutions after heating at 90 °C without further reaction at high temperature. (a) SDBS, (b) FeCl3/ Co(NO3)2/SDBS, (c) FeCl3/Ni(Ac)2/SDBS, (d) Co(NO3)2/SDBS, (e) Ni(Ac)2/ SDBS, and (f) FeCl3/SDBS.

after the nanoparticles were washed thoroughly with ethanol and water and dried at 160 °C for several hours, while the absorptions between 970 and 1200 cm-1 still remained, indicating the absence of water, hydroxyls, n-octanol, and caprylic ether but the presence of SO42- anions. Thus, it is concluded that the species directly attached to the metal atoms is SO42-, with caprylic ether, n-octanol, or water adsorbed to the sulfates. The presence of caprylic ether and n-octanol molecules on the surfaces of the as-prepared nanoparticles led to their excellent dispersibility in nonpolar or weakly polar solvents such as cyclohexane and n-octanol. There are three weight loss steps in the TG curves of the ferrite nanoparticles. The first mass loss of ca. 7-8% below 280 °C is attributed to the removal of adsorbed water and n-octanol on the nanoparticle surfaces. The boiling point of n-octanol is 195 °C, but the elimination of the species may be delayed by the rapid heating rate during TG analysis. The second mass loss, ranging from 280 to 410 °C, which is the most precipitous among the three, is a result of the elimination of caprylic ether, indicating that a large amount of caprylic ether is adsorbed on the particle surface. The weight loss at temperatures higher than 410 °C may be ascribed to the removal of the surface SO42- anions and oxidation of carbon from the incomplete decomposition of caprylic ether. As a result, the total oxide content in the as-prepared nanoparticles is in the range of 80-90%. The TG curves also demonstrate that the caprylic ether content on the Co0.6Fe2.4O4 and Ni0.8Fe2.2O4 nanoparticles’ surface was much smaller than that on the surface of the Co0.3Ni0.6Fe2.1O4 and Co0.2Ni0.7Fe2.1O4 nanoparticles and that of the four types of nanoparticles the least caprylic ether was coated on Co0.6Fe2.4O4. Combining the TG and XRD analysis results, it is proposed that the high crystallinity of Co0.6Fe2.4O4 and Ni0.8-

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Figure 7. Zero-field-cooling (ZFC, solid line) and field-cooling (FC, open line) magnetization curves of the 11.5 nm nanoparticles with variable compositions under 100 Oe applied magnetic field (A) and their corresponding field-dependent magnetization at 300 K (B). (a) Co0.6Fe2.4O4, (b) Co0.3Ni0.6Fe2.1O4, (c) Co0.2Ni0.7Fe2.1O4, and (d) Ni0.8Fe2.2O4.

Fe2.2O4 compared to Co0.3Ni0.6Fe2.1O4 and Co0.2Ni0.7Fe2.1O4 might result in smaller amounts of caprylic ether molecules on the particle surfaces. In the present experiment, the synthesis of the complex oxides CoxNiyFe3-x-yO4 can be divided into two steps: the mixture of inorganic salts, SDBS, n-octanol heated at 90 °C for 6 h; and the solvothermal treatment of the mixture at 240 °C. XRD analysis indicates that crystalline NaNO3 and NaCl, which are nearly insoluble in n-octanol, were formed after the heattreatment at 90 °C (data not shown). Therefore, it is proposed that the DBS anions entered the solution and coordinated to the transition metal cations. NaAc was not detected in the precipitates, so the acetate species must still remain in the solution as a result of their relatively high solubility. After solvothermal treatment at 240 °C for several hours, ferrite was formed as shown by the XRD analysis. At the same time, the NaNO3 disappeared, but NaCl was still detected in the precipitates. If the sodium salt deposited at the bottom of the autoclave after the first step at 90 °C was removed, and only the reaction liquid was heated at 240 °C for several hours, hardly any ferrite nanoparticles were obtained. UV-vis spectra show that SDBS only absorbs around 255 nm (Figure 6a) and FeCl3 has an absorbance maximum at 340 nm. Figure 6f is a superposition of the SDBS spectrum with FeCl3, showing that no complexation occurs between Fe3+ and SDBS. The UV-vis spectra of the reaction solutions after heating at 90 °C show that the absorption from the NO3- anions at 300 nm disappeared (Ac- has no absorption from 250 to 500 nm), suggesting that Co(NO3)2 and/ or Ni(Ac)2 was coordinated with SDBS molecules. The GCMS peak is consistent with the presence of caprylic ether, which could be formed during the dehydration reaction of n-octanol at 240 °C, along with a small amount of water. The formation of ferrites was accompanied by the decomposition of the anions in the presence of the small amount of water. Although little water existed in the system, it was an important factor that led to the formation of the monodispersed nanoparticles, which differed from our previous synthesis of Co3O4 nanoparticles where different-sized particles were obtained by adding a small quantity of water or other polar solvents.18 In this system, when a small amount of extra water was added, the course of the reaction was directly changed, and the byproduct R-Fe2O3 was formed by the characteristic hydrolysis and decomposition of FeCl3 and ferrite nanoparticles were not formed. Hence, the limited water in our reaction system mainly increased the solubility of the salts and further promoted the decomposition of nitrates and acetate instead of the hydrolysis and decomposition of FeCl3. So in order to prepare magnetic ferrite nanoparticles, the water content of all the reactants needed

Figure 8. Field-dependent magnetization of different sized Co0.6Fe2.4O4 nanoparticles at 300 K. (a) 9.0 nm, (b) 11.0 nm, (c) 13.6 nm, and (d) 16.0 nm.

to be restricted to ensure that the Co2+ or Ni2+ cations coordinated with SDBS initially, thus avoiding hydrolysis and decomposition of FeCl3 at high temperature. 3.2.MagneticProperties.Asdiscussedabove,theCoxNiyFe3-x-yO4 nanoparticles with the same size but different compositions could be obtained, and the study of their magnetic properties is significant for fundamental research. Figure 7A shows the temperature-dependent zero-field-cooling (ZFC) and fieldcooling (FC) curves for 11.5 nm nanoparticles with different Co, Ni, and Fe contents. Generally, the ZFC magnetization increased slowly as the temperature increased up to a maximum (Tmax) and then decreased slowly. The Tmax temperatures of Co0.6Fe2.4O4, Co0.3Ni0.6Fe2.1O4, Co0.2Ni0.7Fe2.1O4, and Ni0.8Fe2.2O4 were 372, 251, 207, and 119 K, respectively, which decreased with increasing Ni content. By adjusting the Ni content, Tmax would change from above room temperature to a low temperature. Meanwhile, the FC magnetization increased steadily from high temperature, 390 to 10 K, and deviated from the ZFC magnetization near Tmax. Field-dependent magnetization measurements (Figure 7B) show the superparamagnetic characteristics of the as-prepared nanoparticles, except for the Co0.6Fe2.4O4 sample. The magnetization rose rapidly as the applied field increased and reached a saturation point at ca. 2.0 T. There was no appreciable distinction between the saturation magnetizations of the three superparamagnetic nanoparticles, but as the Ni content decreased the superparamagnetic behavior showed a slowly increasing slope as the field increased up to 3000 Oe. This may be caused by different thermal agitations and surface spin canting of small particles with different surface structures;23 however, it is currently difficult to fully understand the complicated quantum

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Figure 9. (a) TEM image of 7.0 nm Co0.3Ni0.6Fe2.1O4 nanoparticles dispersed in water (the scale bar is 100 nm) and IR spectra of (b) as-synthesized nanoparticles and (c) that after surface modification.

couplings between electron spin and orbital angular momentum,13b which may arise from interactions between Co, Ni, and Fe. Magnetic measurements (Figure 8) show that Co0.6Fe2.4O4 nanoparticles with different sizes exhibited ferromagnetic behavior. The saturation magnetizations (Ms) of 9.0, 11.0, 13.6, and 16.0 nm nanoparticles were 79.0, 74.0, 68.0, and 61.0 emu/ g, respectively, which decreased with increasing particle size. Our results are contrary to literature in which the saturation magnetization increased with increasing particle size.8,12,15 Nanoparticles prepared by various methods have different surface environments, and the surface coordination often has a great influence on the magnetic properties of the nanoparticles.15a The IR and TGA results discussed above confirmed that the SO42- anions resulting from the decomposition of SDBS were bonded to the metal atoms; the smaller the particle, the more SO42- anions coordinated with the metal atoms. As a result, the proportion of the surface disorder decreased and the saturation magnetization increased. The corresponding coercivities (Hc) of the above nanoparticles were 230, 435, 723, and 610 Oe, respectively, which first increased (a-c) as a result of the increasing magnetic anisotropy at smaller particle size, and then decreased (c-d) due to the lower surface anisotropy as the particle size increased past a certain value.8g 3.3. Surface Modification. All of the monodispersed nanoparticles described above exhibited a hydrophobic surface. To modify the surface into a hydrophilic surface, the ferrite nanoparticles were treated with 10% acetic acid. After modification with acetate, all of the ferrite nanoparticles described above could be dispersed in water easily and the suspensions were stable. Ferrite nanoparticles containing Ni could exist stably in water for several months, while Co0.6Fe2.4O4 nanoparticles were not so stable, aggregating after several days. The as-synthesized nanoparticles were well-dispersed in nonpolar cyclohexane and, after simple surface modification, also showed good dispersion in water. When 10% acetic acid was added directly without washing with water first, the nanoparticles were not so waterdispersible. The surface species of the nanoparticles affected their dispersive properties. To understand the relationship between dispersion and the surface species, the IR technique was applied to characterize the nanoparticles. The as-synthesized nanoparticles were washed thoroughly with ethanol and water, the precipitate was separated by centrifugation and dried at 160 °C for IR analysis (curve b in Figure 9), and then the powder was washed with acetic acid and dried at 160 °C to remove volatile components, producing surface-modified nanoparticles (curve c in Figure 9). The IR spectra of the cleaned and dried nanoparticles before surface modification showed no absorption

around 3300-3400 and 2850-3000 cm-1, indicating that there was no caprylic ether, n-octanol, water, or hydroxyls on the particles’ surfaces. The strong absorptions between 970 and 1200 cm-1 revealed that SO42- anions were bonded to metal atoms. After washing with acetic acid and drying, the intensity of the absorptions between 970 and 1200 cm-1 (curve c in Figure 9) decreased significantly and peaks appeared at 1558 and 1420 cm-1, which were attributed to the absorption characteristics of the asymmetric and symmetric vibrations of the carboxyl group.17c At the same time, a peak around 3400 cm-1 also appeared. Thus, most of the SO42- anions were substituted by acetate anions during the acid treatment due to the stronger coordination ability of acetate anions to metal atoms. Further experiments revealed that the SO42- anions could not be replaced by acetate anions without washing thoroughly with ethanol and water before surface modification. It can be concluded that on the surface of the prepared ferrite nanoparticles caprylic ether and n-octanol molecules produce a coating that leads to excellent dispersion in nonpolar or weakly polar solvents. After washing thoroughly with ethanol and water, the n-octanol and caprylic ether molecules are removed and only SO42- anions are left on the surface. The SO42- anions are then replaced by acetate anions during the acid treatment, and waterdispersible nanoparticles are formed. Such benefits of Acanions have also been found in the synthesis of water-dispersible Fe3O4, Pt particles, and so forth.24 4. Conclusions In summary, monodispersed CoxNiyFe3-x-yO4 nanoparticles have been solvothermally prepared and the sizes of the nanoparticles can be adjusted by changing the reaction time. Nanoparticles with different Co, Ni, and Fe compositions were obtained by changing the proportion of the reactants. The solvothermal process could be applied inexpensively for practical uses. During the formation of ferrite nanoparticles, thermal decomposition of nitrate and/or acetate occurs. Along with an increase in the Ni content, the temperature maximum observed in the ZFC magnetization curve (Tmax) decreased significantly for the CoxNiyFe3-x-yO4 ferrite nanoparticles, from 372 to 119 K, while the saturation magnetization was minimally affected. The monodispersed multicomponent ferrite nanoparticles could provide a novel perspective on the interparticle interactions between different cations (Co2+, Ni2+, or both), which play a major role in magnetic properties. Additionally, washing with acetic acid to prepare water-dispersible nanoparticles is a simple, cheap, and easy method that could broaden the applications of these nanoparticles greatly.

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