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J. Phys. Chem. C 2008, 112, 6220-6227
Synthesis of Core-Shell Ferrite Nanoparticles for Ferrofluids: Chemical and Magnetic Analysis Juliano de A. Gomes,†,‡ Marcelo H. Sousa,‡ Francisco A. Tourinho,‡ Renata Aquino,† Geraldo J. da Silva,† Je´ roˆ me Depeyrot,† Emmanuelle Dubois,*,§ and Re´ gine Perzynski§ Complex Fluids Group, Instituto de Fı´sica, UniVersidade de Brası´lia, CP 04455, CEP 70919-970 Brası´lia-DF, Brazil; Complex Fluids Group, Instituto de Quı´mica, UniVersidade de Brası´lia, CP 04478, CEP 70904-970 Brası´lia-DF, Brazil; and Laboratoire des Liquides Ioniques et Interfaces Charge´ es, UniVersite´ Pierre et Marie Curie, CNRS, ESPCI, UMR 7612, Baˆ timent F, Case 63, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed: October 5, 2007; In Final Form: February 14, 2008
A chemical core-shell strategy is developed here for the synthesis of ferrofluids based on nanoparticles of different ferrites with different mean sizes. A heterogeneity of chemical composition, associated with a superficial enrichment of iron, allows to obtain chemically stable ionic colloids. We propose here a coreshell model to describe the synthesized nanoparticles, which is tested by chemical and magnetic measurements performed at the various steps of the synthesis. The thickness of the superficial layer, rich in iron, is ranging between 0.4 and 1.3 nm, depending on the nanoparticle size and on the underlying ferrite. Its density is found close to that of maghemite, and its magnetization depends on the core ferrite. It is low with a cobalt ferrite core and larger for the three other ferrites investigated here (NiFe2O4, CuFe2O4, and ZnFe2O4). Magnetic measurements prove that there is a strong redistribution of Zn2+ ions inside the core of the synthesized nanoparticles based on ZnFe2O4.
Introduction Ferrite particles with sizes of the order of few nanometers are emerging as reliable materials able to solve complex engineering problems1 and, very recently, as promising materials for biomedical applications.2 At the nanometric scale, the size reduction leads to interesting magnetic properties, such as superparamagnetism, enhanced anisotropy, and surface effects,3 which are of great interest in biomedicine for magnetic resonance imaging or thermotherapy.4 Indeed, such nanoparticles, which have dimensions smaller than or comparable to biological entities, can be coated with biological molecules and dispersed in a liquid medium, leading to colloidal solutions called magnetic fluids or ferrofluids. Because of their liquid properties and sensitivity to an applied magnetic field, these materials can be made to deliver packages such as anticancer drugs or radionuclide atoms to a targeted region of the body, such as a tumor. The elaboration of a conventional ferrofluid involves at least two fundamental steps:5 first, the particle preparation, in which the method of synthesis determines the crystallographic structure of the grains, their shape, their mean size and their size distribution, their surface chemical state, and consequently their magnetic properties; second, the peptization of the particles into a carrier liquid, in order to obtain a magnetic sol. In this case, the colloidal stability depends first on the dimension of the particles, which should be sufficiently small to avoid precipitation due to gravity and, second, on the repulsion between * To whom correspondence should be addressed: Fax (+33) 1 44 27 32 28; e-mail
[email protected]. † Complex Fluids Group, Instituto de Fı´sica, Universidade de Brası´lia. ‡ Instituto de Quı´mica, Universidade de Brası´lia. § Universite ´ Pierre et Marie Curie.
particles, provided by steric and/or Coulombic interactions, to prevent agglomeration due to van der Waals and magnetic dipole forces. Nanoferrites are good candidates for biomedical purposes since they present a high magnetic moment (of the order of 104 Bohr magnetons, µB), they are chemically stable (there is no oxidation nor particle dissolution in the physiological conditions), and their surfaces are very reactive to attach biological molecules. Moreover, the presence of the divalent metallic ion (usually a d-block metal) can improve the determination of the in-vivo biodistribution of such nanoparticles in blood probes. Before binding, in the future, biomolecules on the particle surface and proceeding to medical applications,6 we investigate here some important characteristics such as the crystallographic structure, the chemical composition, and the magnetization of nanoparticles. In this work, we study ferrofluids based on ferrite nanoparticles MFe2O4 (M ) Co, Cu, Ni, Zn) that are chemically synthesized in a bottom-up process and then peptized using an appropriate surface treatment of the particles which, in aqueous media, leads to electrostatic repulsion between them, yielding the so-called electric double-layered magnetic fluids (EDL-MF). Indeed, the charges of the grains in an EDL-MF originate from aquation reactions of the metal ions on the particle surface. The superficial sites, which behave as a diprotic Bro¨nsted acid, are positively charged in strong acidic medium, negatively charged in strong basic medium, and neutral for a pH close to 7. However, in acidic medium, dispersions that are chemically stable with time cannot be obtained with the pure ferrite MFe2O4 because they tend to dissolve in acid.7 This point was solved using a hydrothermal treatment with boiling ferric nitrate, initially proposed to promote the oxidation of the poorly stable magnetite Fe3O4 nanoparticles (which dissolve in acidic medium and which are easily oxidized) into more stable maghemite
10.1021/jp7097608 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008
Synthesis of Core-Shell Ferrite Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6221 TABLE 1: Synthesis of Cobalt Ferrite Nanoparticles: Influence of the Experimental Conditions Used in Step 2b of the Synthesisa synthesis medium H2O [Co(NO3)2] ) 1 mol/L [Fe(NO3)3] ) 1 mol/L
T (°C)
XM
25 100 25 100 25 100
0.32 0.31 0.32 0.31 0.31 0.27
a First column: nature of the solution used during step 2b; second column: temperature during step 2b, which lasts here 60 min; third column: molar fraction XM of cobalt in the nanoparticles obtained after step 2b.
Figure 1. Standard diagram of the synthesis of an EDL-MF. Step 1 is the coprecipitation of the metals in alkaline medium. Here M2+ refers to Co2+, Ni2+, Cu2+, and Zn2+. Steps 2a and 2b correspond to the surface treatment of the nanoparticles before their peptization in acidic medium in step 3.
nanoparticles (γ-Fe2O3).8,9 However, this hydrothermal treatment gives rise to changes in the chemical composition of the particles, already reported for several kinds of grains such as cobalt and manganese,7 nickel, copper, and zinc,10 manganesezinc,11 and nickel-zinc ferrites.12 There is a gain in iron leading to a decrease of the molar ratio XM ) [M2+]/([Fe3+] + [M2+]) from its stoichiometric (MFe2O4) value of 0.33, which is assigned to the formation of a layer rich in iron that protects the particles from acid dissolution. Although this hydrothermal treatment with Fe(NO3)3 is an essential step to elaborate acidic sols based on ferrite nanoparticles, there are only few works investigating this step in the EDL-MF elaboration. Here, our main goal is to experimentally investigate the formation of this iron-rich shell, its composition, its thickness, and the changes induced in the global magnetic properties of the particles, using systems based on CoFe2O4, CuFe2O4, NiFe2O4, and ZnFe2O4 ferrites. Elaboration of the Samples and Experimental Methods A. General Procedure of Elaboration of EDL-MF. The acidic EDL-MF samples are prepared using the elaboration procedure schematized in Figure 1 which is detailed elsewhere.7,10 All the reagents used in this work are products of analytical purity. Nitrogen is passed through both prepared solutions and distilled water before their use in order to avoid air contamination. Step 1 summarizes the synthesis of the ferrite nanoparticles, which are obtained by alkalinizing 1:2 mixtures of M2+ and Fe3+ with NaOH under vigorous stirring (M ) Cu, Ni, Zn, and Co). Moreover, the synthesis must be done at 100 °C in order to yield particles of good crystallinity. Note that if the particles are dispersed in acid after step 1, they dissolve with time. Step 2 is subdivided in acid cleaning (2a) and surface treatment (2b). Step 2a consists in washing the precipitate twice with water and once in a HNO3 solution (2 mol/L) in order to reverse the charge of the nanoparticles and eliminate any undesirable less soluble byproduct formed during step 1. Then, the pH of the slurry becomes very low, and the dissolution of the precipitated nanoparticles starts. Thus, to ensure the thermodynamical stability of the grains and avoid particle degradation in acidic medium, the precipitate is hydrothermally treated with a 1 mol/L Fe(NO3)3 solution at 100 °C, a step indexed 2b. The excess of ferric nitrate is then removed by decantation, and the nanoparticles are twice washed with acetone. In step 3, after evaporating the acetone, the nanograins are redispersed in water at pH around 2, thus around an ionic strength of 10-2 mol/L.
B. Characterization. Chemical Analysis. During all the experiments, the pH of the solution is measured after each step by using a Metrohm 713 pH meter. The concentrations of Fe3+ and M2+ are determined by atomic spectroscopy absorption and/ or by inductively coupled plasma atomic emission. The molar fraction XM of divalent metal can thus be determined for the nanoparticles at the different steps of the synthesis. Morphological Characterization. Micrographs patterns are obtained by transmission electron microscopy (TEM) using a JEOL 100 CX2 apparatus. The samples are prepared by evaporating very dilute ferrofluid solutions onto carbon-coated grids. The particle size distribution is estimated by measuring the size of about 500 particles, using a log-normal law for the size distribution. Structural Characterization. The room temperature X-ray powder diffraction (XRD) patterns are measured at the Brazilian Synchrotron Light Laboratory (LNLS) using the D12A-XRD1 beamline. The sample holder is rotated to improve the randomization of the crystallites. Monochromatized 6.01 keV (λ ) 2.0633 Å) X-ray beam of ∼4 × 1.5 mm2 area is used. Diffraction patterns are obtained typically within 20° e 2θ e 130° interval, with 0.04° step and 10 s counting time. The average lattice parameters are calculated from the five most intense diffraction lines, while the mean crystal sizes (dXR) are deduced by means of the Scherrer formula from the width at half-maximum of the diffraction line (311). C. Study of Step 2b of the Synthesis. To better understand the chemical, structural, and magnetic modifications that occur during steps 2a and 2b of the synthesis procedure, a system based on cobalt ferrite nanoparticles is first investigated because this ferrite is easier to synthesize and was already studied.7 After step 1, the precipitate is fractionated in seven parts, which are then treated with different conditions. In order to keep unmodified nanoparticles with XM ) 0.33, one part is directly dispersed in basic medium. To obtain a stable colloidal dispersion, the precipitate is first washed with water to eliminate sodium ions. Then TMAOH (tetramethylammonium hydroxide) is added in order to charge the surface negatively, and the big counterion TMA+ allows stabilizing the nanoparticles. Note that these dispersions in alkaline medium easily carbonate with time and are complicated to manipulate, leading to badly defined dispersions. Therefore, we do not use this process further on. The other six parts are first washed with a 2 mol/L HNO3 solution for 15 min (step 2a), and the resulting nanoparticles are treated during step 2b using different compositions of the medium and different temperatures, summarized in Table 1. The precipitate is then washed with acetone and peptized at pH around 2 (corresponding to an ionic strength about 10-2 mol/ L). The pH of the solutions is followed for several weeks to investigate the resistance of the grains against dissolution in acidic medium.
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Gomes et al.
TABLE 2: Influence of the Surface Treatment (Steps 2a and 2b of the Synthesis) on the Characteristics of the Nanoparticles Based on the Different Ferritesa bulk sample NiFe2O4
CuFe2O4
ZnFe2O4
CoFe2O4
step
mps
mcs
mss
dXR (nm)
a (Å)
XM
φs/φp
e (nm)
1 2a 2b
4.8 5 5.2
8.32 8.32 8.31
0.33 0.2 0.13
0 0.42 0.63
0 0.42 0.74
96 153 216
96 274 587
241 285
1 2a 2b
7.7 8.2 8.0
8.37 8.34 8.34
0.33 0.19 0.15
0 0.45 0.57
0 0.74 0.98
132 220 213
132 398 495
327 274
1 2a 2b
10.6 11.7 11.3
8.42 8.41 8.40
0.33 0.33 0.24
0 0 0.29
0 0 0.62
208 210 291
208 210 413
1 2a 2b
12.9 13.6 13.5
8.33 8.35 8.36
0.33 0.33 0.27
0 0 0.22
0 0 0.54
257
257
214
275
(kA/m)
(kA/m)
(kA/m)
aASTM (Å)
ms 293K (kA/m)
8.34
270
8.35
135
8.44
∼0
8.33
425
490
64
a
dXR is the crystalline size of the grain obtained from XRD experiments (see text for details); a is the lattice parameter obtained from XRD experiments; XM is the molar fraction of the divalent metal in the particle; φs/φp is the ratio of the volume of the shell to the volume of the whole particle after step 2b; e is the thickness of the shell; mps , mcs , and mss are obtained from magnetization measurements at saturation (see text): mps is the effective magnetization considering “homogeneous” nanoparticles; mcs is the magnetization of the core with the hypothesis of a nonmagnetic shell; mss is the magnetization of the shell with the hypothesis that the magnetization of the core equals the value determined after step 1. The last two columns give the characteristics of the bulk material: lattice parameter aASTM and magnetization ms of the material at 293 K.
TABLE 3: Influence of the Surface Treatment on Different Ferrites for Size-Tailored Synthesisa NiFe2O4
supernatant
precipitate
CuFe2O4
ZnFe2O4
dV/dt [mL/s] dXR [nm]b
125 4.2
10 7.1
3 8.2
125 7.5
3 9.5
0.5 11.6
125 7.4
7 8.0
5 10.0
XM dXR [nm] d0 [nm] sd φs/φp e [nm]
0.15 3.8
0.18 5.5 6.7 0.23 0.48 0.54
0.12 5.8 5.3 0.25 0.66 0.87
0.15 8.3 7.3 0.22 0.57 1.02
0.19 9.7
0.20 6.5
0.21 7.1
0.26 9.0
0.57 0.47
0.17 5.3 5.9 0.18 0.51 0.57
0.45 0.88
0.42 0.53
0.39 0.53
0.23 0.38
0.17 4.5
0.17 7.3
0.18 8.1
0.12 7.9
0.20 8.3
0.26 9.8
0.51 0.78
0.48 0.8
0.66 1.19
0.19 11.4 10.2 0.25 0.45 1.03
0.19 6.7
0.51 0.48
0.15 8.7 8.3 0.22 0.57 1.07
0.45 0.6
0.42 0.68
0.23 0.41
XM dXR [nm] d0 [nm] sd φs/φp e [nm]
a dV/dt is the addition rate of the reagents during the step 1 of the particle synthesis. dXR is the crystalline size of the grain obtained from XRD experiments. XM is the molar fraction of the divalent metal. d0 and sd are the mean diameter and polydispersity obtained from TEM analysis. φs/φp is the ratio of the volume of the shell to the volume of the whole particle after step 2b; e is the thickness of the shell. b These X-ray determinations are obtained with a conventional diffractometer using the Cu KR line at λ ) 0.154 nm on the samples after step 2b.
Once the most efficient surface treatment is determined, powders of zinc, copper, and nickel ferrites nanoparticles are studied at each step of the synthesis (results given in Table 2). D. Variation of the Nanoparticles Size. The nanoparticle size can be varied either during the step of nanograins coprecipitation and/or after the step of peptization. During the step of coprecipitation, the velocity of mixing of the reagents dV/dt is varied from a quick to a slow dropping procedure while the temperature, reagent concentrations, and stirring rate are maintained constant. Once the particles are peptized, the synthesized sols are centrifuged at 4000 rpm for 15 min, and both the resulting precipitate and supernatant are redispersed, forming two EDL-MF samples from the same synthesis. Table 3 lists the velocity of mixing of the reagents as well as the characteristics of the nanoferrite samples then obtained. All the crystalline mean sizes in Table 3 correspond to particle diameters measured after step 2b of the synthesis. Another way of refining sizes and polydispersities is to use the phase transitions which occur when ionic strength is increased in the solutions.13 The dispersions are separated in two phases, the upper phase containing smaller particles than
the bottom phase. This method is used here on one dispersion based on copper ferrite nanoparticles and another based on nickel ferrite nanoparticles to study more finely the influence of the nanoparticle size on the nanoparticle inhomogeneities after step 2b. E. Magnetization. The magnetization MS at high magnetic field is measured for the samples of Table 2 after the different steps of the synthesis. For cobalt ferrite nanoparticles, the measurements are performed on stable colloidal dispersions with a vibrating magnetometer at H ) 7.6 × 102 kA/m. In this case, MS is normalized using the volume fraction of the particles. For the other ferrites (MFe2O4, with M ) Ni, Cu, Zn), the measurements are performed on powders using a SQUID (superconducting quantum interference device) at H ) 4 × 103 kA/m. Here, MS is normalized by the mass of the ferrite which is the mass-weighted corrected from the mass of water. Results and Discussion A. Influence of the Surface Treatment in the Elaboration of Ferrofluids Based on Cobalt Nanoferrites. The influence of steps 2a and 2b on the chemical composition of ferrite
Synthesis of Core-Shell Ferrite Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6223
Figure 2. Evolution of the pH as a function of the time for solutions of nanoparticles of cobalt ferrite treated using different conditions in step 2b: (b) with H2O at 25 °C, (O) with H2O at 100 °C, (0) with Co(NO3)2 at 100 °C, and (4) with Fe(NO3)3 at 100 °C (same samples as in Table 1).
Figure 3. Evolution of the M2+ molar fraction, XM, as a function of the heating time for different Fe(NO3)3 concentrations during the hydrothermal treatment of the nanograins of cobalt ferrite (step 2b). The synthesis conditions are pH ) 2, temperature ) 100 °C, and ferric nitrate concentration ) (O) 0.5 mol/L and (0) 1 mol/L.
nanoparticles and on their dispersion in aqueous medium is studied with cobalt ferrite nanoparticles of diameter 12.9 nm after step 1. The different conditions for the realization of step 2b are given in Table 1 as well as the composition of the nanoparticles after step 2b. After step 1 the precipitate presents a M2+ molar fraction (XM ) [M2+]/[M2+] + [Fe3+]) equal to 0.33 as expected from the ideal ferrite stoichiometry. After step 2a, a chemical analysis on precipitate and supernatant shows that, due to the acidic condition, partial nanograins dissolution starts during step 2a. However, the release of Fe3+ and Co2+ does not happen exactly at the stoichiometric ratio of 2:1 as expected from the ferrite formula. As the release of metals is very low, it does not affect the value of XM, which still equals 0.33. After step 2b, the nanoparticles which are treated at 25 °C have slightly changed their composition, XM, as shown in Table 1. On the other hand, in samples hydrothermally treated at 100 °C, the value of XM after step 2b is lower than after a treatment at 25 °C. The lowest value of XM is obtained in the precipitate treated with Fe(NO3)3 at 100 °C. In order to evaluate the efficiency of the treatment employed on the solubility characteristics of the nanoparticles prepared using the different treatments, each sample described in Table 1 is peptized in water at pH around 2. The resistance of the grains against dissolution in acidic medium is checked by following the variations of the pH (Figure 2) of the slurry as a function of time over 18 days. As the dissolution of these oxides consumes H3O+ ions, the pH increases while the metal concentration in the solution increases. For the sample treated with H2O at 25 °C, the whole amount of available H3O+ ions reacts with the particles which are partially dissolved, resulting in an increase of the pH up to 4.5 shown in Figure 2. The charge of the remaining particles is thus low, and the colloidal stabilization is no longer ensured. In the supernatant (free of particles), cobalt and iron ions are not in stoichiometric proportions: the concentration of cobalt ions is higher than what is expected, meaning that cobalt ions are preferentially released and that CoFe2O4 is not simply dissolved in ions. For the samples treated at 25 °C with Co(NO3)2 or with Fe(NO3)3 (data not shown), the same evolution of the pH is observed as with the one treated at room temperature with water. On the contrary, hydrothermal treatments at 100 °C using water or Co(NO3)2 bring a certain degree of stability, but not sufficient to prevent particle dissolution and colloidal destabilization on long periods.
On the other hand, the sample hydrothermally treated at 100 °C with Fe(NO3)3, the surface of which is the richest in Fe3+ with respect to those above-mentioned, is very stable against dissolution, as shown in Figure 2. Note that the colloidal stability in the long term (which means months or years) is achieved only in this latter sample. This comparative study shows that the hydrothermal treatment with Fe(NO3)3 is the most efficient in order to avoid dissolution of the ferrites and to obtain colloidal stability in the long term. In order to better understand what happens during this treatment, additional experiments varying the concentration of ferric nitrate and the heating time during step 2b are performed. The results plotted in Figure 3 show that the quantity of Fe3+ incorporated in the nanoparticles increases as the Fe(NO3)3 concentration and heating time increase during the hydrothermal treatment (XM decreases); however, the decrease of XM seems to have a tendency to saturation. As there is no metal oxidation during step 2b in this kind of ferrite, this extra resistance to dissolution can be associated with the presence of an iron oxide-hydroxide superficial layer with high chemical stability and low solubility, achieved after the hydrothermal treatment. In the experimental conditions used here, several kinds of iron oxides could be formed. Comparative studies14,15 have shown that, in acid medium at 25 °C, the rate of dissolution of more common oxides follows the order goethite < hematite < maghemite , akaganeite < magnetite < lepidocrocite , ferrihydrite. As an attempt to precise the nature of the shell material, X-ray diffraction measurements are performed using a synchrotron source because of its high resolution and intensity. The diffractograms of the cobalt ferrite samples that correspond to the precipitates obtained after each step of the synthesis are presented in Figure 4. For all the spectra, the analysis is made from the indexation of the diffracted lines and by comparing with bulk standard data. After step 1, the obtained pattern leads us to identify only one system of lines, characteristic of the spinel structure. The XRD results also show that there is no significant difference between patterns of nanoparticles obtained after passing through the steps 2a and 2b (see Figure 4). Besides, as listed in Table 2, the lattice parameter determined after the different synthesis steps are identical within the error bars. Thus, from the XRD patterns analysis it is not possible to detect the presence of a new phase different from the underlying spinel one and which could originate from this surface treatment. On the other hand, one observes that the crystalline mean diameter
6224 J. Phys. Chem. C, Vol. 112, No. 16, 2008
Figure 4. XRD patterns (measurements at LNLS, Campinas, Brazil) of cobalt ferrite nanoparticles after each step of the synthesis: from top to bottom, steps 2b, 2a, and 1. The position of the peaks for the bulk material is indicated at the bottom of the graph.
Figure 5. XRD patterns (measurements at LNLS, Campinas, Brazil) of ferrite nanoparticles after step 2b of the synthesis.
dXR, determined from Scherrer analysis, slightly increases after surface treatment as listed in Table 2. However, these variations are close to the error bar, and the mean diameter should anyway slightly increase due to the dissolution of the smaller particles of the distribution during step 2. B. Influence of the Surface Treatment in the Composition of the Nanoferrites of Different Mixed Oxides. After this detailed study on cobalt ferrite, we check the influence of the most efficient hydrothermal treatment ([Fe(NO3)3] ) 1 mol/L, 100 °C, and 60 min) on the composition of the other types of nanoferrites. In this way, nanoparticles of copper, nickel, and zinc ferrite are collected and analyzed at the different steps of the synthesis described in Figure 1. The results are given in Table 2. As observed with the cobalt ferrite, after step 1, the precipitates present XM equal to 0.33 as expected from the ideal ferrite stoichiometry, whatever the divalent metal used. However, after step 2a, XM ) 0.33 for ZnFe2O4, as for CoFe2O4, but XM < 0.33 for NiFe2O4 and for CuFe2O4, which means that the release of the divalent metal in acidic medium depends on the nature of the ferrite. After step 2b, XM is lower than after step 2a for all the ferrites. The XRD patterns for samples of copper, nickel, and zinc ferrites (Figure 5) present the same features as the one of cobalt ferrite: there is no new phase, no
Gomes et al. change of the lattice parameter, and the size of the particles at the different steps does not evolve significantly. The hydrothermal treatment at 100 °C with ferric nitrate is efficient for all kinds of ferrites studied here: the particles do not dissolve in acidic medium, and the colloidal stability of the dispersions obtained is good for long periods. Differences on the evolution of XM are observed in Table 2 between the four ferrites; however, the mean size of the particles is different, and it may have an influence. Therefore, for each ferrite, particles of various sizes are now studied. C. Influence of the Surface Treatment in Sols with SizeTailored Nanoferrites. In this section we study the influence of the hydrothermal treatment by ferric nitrate on the nanoparticles characteristics, for nanoparticles of different mean sizes and for different ferrites. Table 3 shows that the size can be controlled by varying the rate of mixing dV/dt of the reagents during the coprecipitation step: slow rates of addition yield larger particles while increasing dV/dt generates smaller particles. Indeed, the formation of these nanoparticles in aqueous medium involves nucleation and crystal growth, the nucleation occurring spontaneously above a critical concentration of precursor. Modifying dV/dt, one controls the concentration of precursors: by increasing dV/dt, the nucleation process is favored, and therefore nanoparticles of smaller sizes are formed.16 The following step of crystal growth is a slower process, which is controlled by the concentration of the precursor, but also by the transport processes and by the rate of surface chemical reaction. Nevertheless, although this route of size tailoring is effective to yield nanoparticles in a wide range of sizes, it does not permit to finely vary the grain size and its associated polydispersity (around 0.4 after the synthesis).17 Thus, in order to refine the size-tailoring, the sols are centrifuged at 4000 rpm for 15 min. After centrifugation, the nanograins in precipitate and supernatant present distinct crystalline sizes as listed in Table 3. Furthermore, the analysis of TEM pictures shows that the polydispersity is reduced to values under 0.25 after centrifugation as listed for some of the samples in Table 3. Note that the values of the size distributions obtained from TEM analysis compare well with the crystalline mean sizes determined from XRD measurements using the relation dXR ) d0 exp(2.5sd2).18 After the whole synthesis, the XM values experimentally determined from chemical analysis are inferior to 0.33 for all the synthesized samples, whatever the size of the nanoparticles, as shown in Table 3. For a given ferrite, XM decreases with the size of the particles since the ratio of the surface of the nanoparticles, which is in contact with the Fe(NO3)3 solution during step 2b, to their volume increases as the size of the particle decreases. Even if the structural XRD characterization shows no extra contribution which could be due to this protective iron-rich shell, the chemical analysis shows that some iron is incorporated in the nanoparticles during the hydrothermal treatment, whatever the nature of the mixed ferrite and the size of the nanoparticles. The particles obtained at the end of the whole process are thus inhomogeneous from the point of view of their internal composition. In the next section we thus elaborate a chemical core-shell nanoparticle model. D. Chemical Core-Shell Model of Nanoparticle. We propose to take into account the modifications of the chemical composition of our nanoparticles within the chemical coreshell model detailed in the following. Indeed, the hydrothermal treatment (step 2b of Figure 1), performed to protect the particles from acidic decomposition, creates a superficial layer of iron
Synthesis of Core-Shell Ferrite Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6225
oxide onto the stoichiometric ferrite core. Then, the volume fraction φc of the core is proportional to the content of divalent metal while the volume fraction φs of the iron oxide layer is proportional to the iron content of the shell. In this context, the volume fraction of the whole particles φp is given by φp ) φc + φs, where
φc ) [M2+]V cM
(1a)
φs ) 0.5([Fe3+] - 2[M2+])V sM
(1b)
V cM and V sM being respectively the molar volumes of the core material and of the shell one. The value of this molar volume is well-known for the core, made of a stoichiometric ferrite. For the surface layer, the value that we use is deduced from a mean density equal to 5 g/cm3 (equivalent molar mass of the shell M sM ∼ 160 g/mol and V sM ∼ 32 cm3/mol which corresponds to maghemite, the only iron oxide with a spinel structure compatible with XRD patterns of section A and a density compatible with measurements of section E). Thus, using the content of metallic cations measured by chemical titrations, we obtain the relative proportions of core and shell volumes and the thickness e of the iron oxide surface layer, deduced by using the crystalline size dXR. The calculated values of φs/φp and e are listed in Tables 2 and 3 for nanoparticles of different ferrites and various mean sizes. One can note that the shell, the thickness of which is ranging between 0.4 and 1.2 nm in these tables, represents a significant proportion of the total volume of the particle, between 22% and 66%. The relative volume fraction φs/φp of the iron oxide surface layer, which only depends on the molar fraction of divalent metal XM, increases as the nanoparticle size decreases. The latter result can be seen as a mark of nanoscale behavior since the surface to volume ratio becomes larger for smaller particles. Moreover, one can therefore wonder whether these large values of the shell proportion would induce some changes in the magnetization value; the next section will try to enlighten this issue. Table 2 also shows that, in the case of nickel and copper ferrite nanocrystals, the proportion of the iron oxide surface layer becomes large already at step 2a of the synthesis. It means, at least for these samples, that the main part of this layer is not coming from a deposited layer during step 2b onto the particle surface. The values reported for the thickness of the iron oxide shell are always of the order of the spinel unit cell. No relevant variation of this thickness e can be safely identified in Table 3 because e depends on several parameters, which are here modified in parallel in the different syntheses of the table (underlying ferrite, nanoparticle size, dV/dt, the quantity of ferrite dissolved in step 2a, conditions during step 2b). However, for the supernatant and for the precipitate obtained from the same synthesis, e is always larger in the precipitate, i.e., for the larger particles. In order to investigate this point in more details, one sample based on copper ferrite and one based on nickel ferrite are size-sorted by a colloidal phase separation (section D), leading to samples of increasing mean size. Figure 6 plots the dXR dependence of the thickness e obtained for these samples issued from the same initial synthesis. e is here clearly increasing with dXR from 0.6 to 1.3 nm for the CuFe2O4 sample and from 0.4 to 0.7 nm for NiFe2O4 sample. A larger thickness for nickel than for copper ferrites is also observed on average in Table 3. Indeed, for each of these two ferrites, the averaged thickness e and the variations of e in Table 3 are close to the averaged thickness and the variations of e seen in Figure 6.
Figure 6. Thickness e of the external shell of the nanoparticles of copper ferrite (O) and of nickel ferrite (0) as a function of their diameter for particles issued from the same synthesis after fractionation of the size distribution. The full lines are the guides for the eye.
From these results, we can conclude that there is an influence of the ferrite on the thickness e of the iron-rich shell. Although the same conditions are used in steps 2a and 2b, the reaction rates can differ with the nature of the ferrite (in particular, the rate of dissolution in step 2a and of deposition in step 2b). This thickness e can thus be modified by changing the conditions of step 2b (concentration of ferric nitrate and/or duration of the heating). The thickness of the shell thus increases with the particle size and depends on the underlying ferrite in equivalent conditions of synthesis. E. Density of the Nanoparticles. The density of the nanoparticles can be extracted from measurements of the density of diluted magnetic fluid solutions as a function of the nanoparticle volume fraction. The density of the ferrofluid solution therefore is written as
FFF ) Fwater + φp(Fp - Fwater)
(2)
where Fwater is the density of the liquid carrier, taken here equal to 0.998. Thus, the slope of a linear representation FFF vs φp allows determining the nanoparticle density. From another side, the density of the particle Fp can be calculated in the framework of the chemical core-shell model using the general expression of the density F of a heterogeneous material made of n phases: n
F)
χ iMM iM ∑ i)1 n
(3)
χ iMV iM ∑ i)1 where χiM, M iM, and V iM are respectively the molar fraction n χiM ) 1), the molar mass and the molar volume of each (∑l)1 phase i (i ) c: core; i ) s: shell). In our case, the molar fraction of the core and shell phases can be expressed as a function of the molar fraction of divalent metal XM of the particle, and Fp is written as
Fp )
(1 - 3XM)M SM + 2XMM CM (1 - 3XM)V SM + 2XMV CM
(4)
6226 J. Phys. Chem. C, Vol. 112, No. 16, 2008
Figure 7. Variation of the density of the nanoparticles as a function of XM for MFe2O4 with M ) Ni, Cu, and Zn. Open symbols: deduced from measurements of the ferrofluid density and eq 2. Full, dashed, and dotted lines are best fits to the measurements using eq 4 (see text).
The content of metallic cations measured by chemical titrations is thus sufficient to determine the nanoparticle density, M sM and V SM being adjustable parameters. Figure 7 presents the variations of the nanoparticle density Fp as a function of the molar fraction of divalent metal XM: symbols are associated with the values deduced from measurements of the fluid density, and the full, dashed, and dotted lines correspond to values obtained using eq 4. The best agreement between both determinations is obtained with a molar mass of the shell M sM ) 162 g/mol and V SM ) 32.4 cm3/mol, values close to what is expected for maghemite. This result indicates that our chemical core-shell model of nanoparticle, composed of a stoichiometric ferrite core and an iron oxide surface layer works well to describe the chemical composition heterogeneities induced by the surface hydrothermal treatment. F. Magnetization Measurements. The magnetization in high field at 300 K is measured on the powders at the different steps of the synthesis in order to quantify the influence of the chemical treatments on the magnetic properties, these treatments being necessary to obtain stable colloidal dispersions in the long term. Table 2 presents the results after the coprecipitation (step 1), after the treatment with nitric acid (step 2a), and after the hydrothermal treatment with ferric nitrate (step 2b). After step 1, the oxide is stoichiometric, and we consider the material as homogeneous. The values of the nanoparticle magnetization mps determined at 300 K can be compared to the values for bulk materials. Very different results are obtained for the different ferrites. In the case of zinc ferrite, mps is rather large and very different from the zero value expected for the bulk material. For copper ferrite mps is close to the bulk value, and for cobalt and nickel ferrite, it is smaller than the bulk values.19 In such nanoparticles, several phenomena can occur: first, because of finite size effects,20 the magnetization of the nanograins, which are here all smaller than a Bloch wall, can be substantially increased by eventual cation redistributions;21,22 second, finite-size and surface effects23 (such as the formation of a magnetically dead layer on the surface, the existence of random canting of surface spins, and nonsaturation effects due to imperfect orientation of the particle magnetic moment under the applied field) usually induce a reduction of the nanograin magnetization even observed at room temperature.24,25 Here a redistribution of the divalent ions Zn2+ is obvious in zinc ferrite nanoparticles26 (mps (nanoparticles) > ms (bulk)). It leads to a ferrimagnetic behavior which is consistent with EXAFS measurements and Rietveld refinement of XRD patterns performed with similar ZnFe2O4 nanoparticles.27,28 On the contrary, for
Gomes et al. NiFe2O4 and CoFe2O4, we observe mps (nanoparticles) < ms (bulk): finite size effects are thus obvious in these two materials and cannot be excluded with the two other materials. For CoFe2O4 particles a strong reduction of the magnetization saturation has been previously observed and attributed to finite size effects.29 Moreover, redistributions have been evidenced in NiFe2O4 and CuFe2O4 by in-field Mo¨ssbauer spectroscopy at low temperature.30,31 It means that if the mps value obtained for CuFe2O4 after step 1 is close to the bulk value, it is very likely thanks to a compensation effect between cationic redistribution, surface, and finite size effects. Concerning the next steps of the synthesis, it is shown above that a shell rich in iron is formed. Therefore, the particle is not chemically homogeneous. An effective magnetization mps can however be determined from an average on the whole particle. Moreover, a magnetization of the core and of the shell can be also determined providing some assumptions. Two extreme cases can be considered: (i) the shell is not magnetic, i.e., only the core is magnetic with a magnetization mcs ; (ii) the magnetization of the core being equal to the magnetization of the particle after step 1 (the stoichiometric oxide formed), the shell is magnetic with a magnetization mss. These three values mps , mcs , and mss are reported in Table 2. In order to check the influence of the thickness of the shell on the magnetic properties, measurements are done on the cobalt ferrite particles after step 2b with conditions that lead to a thicker shell compared to the data reported in Table 2 (last line of the table). Increasing the duration of step 2b (using the same batch of particles, from step 1), the thickness increases from 0.54 to 0.88 nm and while mss varies from 64 to 82 kA/m, thus keeping the same order of magnitude. Concerning the influence of the size of the particles, we already know that the effective magnetization (mps ) after step 2b usually increases with particle size.32 Thus, these values for the different ferrites in Table 2 should not be directly compared. However, the variation of mps from step 1 to step 2b can be analyzed. Between step 1 and step 2b, the magnetization mps increases in Table 2 for all the ferrites, except for the cobalt ferrite. Even if, from the chemical point of view, the cobalt ferrite nanoparticles well compare to the three other ones, from the magnetic point of view, they present here a singular behavior. The effective magnetization is much lower than that of the bulk, in agreement with what was found in ref 29, meaning that the shell of CoFe2O4 particles is very weakly magnetic. This could be associated for CoFe2O4 to a weak coupling of the spins of the shell with those of the core, leading to a weak contribution to the magnetization. Note that the main magnetic difference between CoFe2O4 and the three other ferrites is that this material is magnetically hard while the three other materials are magnetically soft. For the NiFe2O4, CuFe2O4, and ZnFe2O4 nanoparticles, the hypothesis of a nonmagnetic shell leads to an unlikely huge magnetization of the core after step 2b. It would mean a strong redistribution of the metallic ions inside the core structure induced by the thermal treatment. However, EXAFS measurements on ZnFe2O4 nanoparticles show that the redistribution of Zn2+ ions present at step 1 remains unmodified after step 2b.28 At step 2a, the molar fraction XM being still equal to 0.33 for ZnFe2O4, the hypothesis (i) is adapted for a description of ZnFe2O4 nanoparticles and the value of mcs ) 210 kA/m found is in perfect agreement with unmodified redistribution with respect to the step 1. At step 2b, XM being smaller, a shell of finite thickness is present, and hypothesis (i) is no longer valid.
Synthesis of Core-Shell Ferrite Nanoparticles Hypothesis (ii) (with a core magnetization at step 2b equal to the core magnetization at step 1) is the only one compatible with EXAFS results. It leads to a very magnetic shell of magnetization 490 kA/m. Extending this hypothesis to NiFe2O4 and CuFe2O4 would also lead to a magnetic shell, of magnetization of the order of 280 kA/m. The observation of a strong magnetic contribution of the shell in the NiFe2O4, CuFe2O4, and ZnFe2O4 nanoparticles implies a better coupling between the spins of the shell with the magnetic core than in the CoFe2O4 nanoparticles. This coupling itself, anyway, demonstrates that hypothesis (ii) is probably an oversimplification. Conclusion Thanks to a hydrothermal surface treatment during the synthesis process, nanoparticles based on MFe2O4 (M ) Ni, Cu, Zn, and Co) are rendered chemically stable and dispersible in acidic media. It is then possible to obtain aqueous magnetic dispersions, with a long-term colloidal stability, but the counterpart of this treatment is that the chemical composition of the nanoparticles becomes heterogeneous. The core-shell model (core of stoichiometric ferrite and shell of iron oxide) proposed here well describes the evolution of this chemical composition at each step of the synthesis. This model is moreover tested by density measurements. Magnetization measurements show that the shell, the thickness of which depends both on the nanoparticle size and on the core ferrite, has different magnetic properties depending on this underlying ferrite. In the future, in order to better understand the role of the shell in the magnetic properties of the particles, it would be interesting to control the shell thickness, at constant nanoparticle size. This necessitates to explore in more details the step of the surface treatment in the synthesis (step 2b). However, magnetization measurements alone are insufficient to understand the redistribution of metallic ions inside the nanoparticles. It is necessary to bring into play several techniques in parallel. In particular, we plan to perform several kinds of measurements: EXAFS measurements at the various steps of the synthesis on NiFe2O4, CuFe2O4, and CoFe2O4; Mo¨ssbauer spectroscopy on copper and zinc ferrites; and Rietveld refinement on diffractograms of neutron with cobalt and nickel ferrites, the X-rays contrast in these ferrites being low. Acknowledgment. We greatly acknowledge Delphine Talbot from LI2C-Paris 6 for help in the chemical titrations, the L.N.L.S. for the experimental beamtime obtained on D12AXRD1 beamline (proposals D12A-XRD2 # 2860/04 and D12AXRD1 # 3563/04), and the CAPES-COFECUB cooperation program nο. 496/05. J. de Andrade Gomes thanks the same program for his personal grant. J. Depeyrot and F. A. Tourinho thank the program MCT/CNPq 31/2005 no. 490171/2005-2 for financial support. Sousa thanks the program CNPq 019/2004473224/2004-6 and MCT/CNPq 28/2005-555161/2005-6 for financial support.
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