One-Pot Synthesis of Highly Monodispersed Ferrite Nanocrystals

Sep 6, 2011 - Alvise Parma , Isidora Freris , Pietro Riello , Davide Cristofori , César de Julián ... Vincenzo Amendola , Moreno Meneghetti , Alvise...
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One-Pot Synthesis of Highly Monodispersed Ferrite Nanocrystals: Surface Characterization and Magnetic Properties Seema Verma* and D. Pravarthana Department of Chemistry, Indian Institute of Science Education and Research (IISER), 900, NCL Innovation Park, Dr Homi Bhabha Road, Pune 411 008, India

bS Supporting Information ABSTRACT: In the present study, a facile one-pot synthetic route, utilizing a strong polar organic solvent, N-methyl 2-pyrrolidone (NMP), is demonstrated to obtain highly monodispersed ferrite nanocrystals. The equimolar mixture of oleic acid, C17H33COOH (R-COOH), and oleylamine, C18H35NH2 (R0 -NH2), was used to coat the magnetic nanocrystals. Structural and magnetic properties of the ferrite nanocrystals were studied by a multitechnique approach including X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), and M€ossbauer spectroscopy. FTIR spectral analysis indicates oleylamine helps in deprotonation of oleic acid, resulting in the formation of an acidbase complex, R-COOh:NH3+-R0 , which acts as binary capping agent. Structural and coordination differences of iron were studied by XPS and M€ossbauer spectral analysis. XPS analysis was carried out to examine the oxidation state of iron ions in iron oxide nanocrystals. The presence of a magnetically dead layer (∼0.38 and ∼0.67 nm) and a nonmagnetic organic coating (∼2.3 and ∼1.7 nm) may substantially reduce the saturation magnetization values for CoFe2O4 and Fe3O4 nanocrystals, respectively. The energy barrier distribution function of magnetic anisotropy was derived from the temperature dependent decay of magnetization. A very narrow energy barrier distribution elucidates that the ferrite nanocrystals obtained in this study are highly monodispersed.

1. INTRODUCTION Over the past few years, there has been an increased emphasis on creating a wide range of nanostructures with potential bottom-up approaches to achieve advanced materials of high performance.1 Preparation of high quality nanocrystals of controlled size and monodispersity is a key in the formation of 2D and 3D self-assembled structures. Therefore, a search for a relatively simple and reproducible approach to synthesize these nanocrystals is of great fundamental and technological interest. During the past decade, synthesis of monodisperse magnetic nanocrystals has attracted special attention for its potential biomedical applications2 and its utilization in the emerging spintronics devices,3 magnetoresistive devices,4 spin-torque nano-oscillators,5 and high density data storage media.6 Most commonly used successful synthetic routes to develop good quality monodispersed magnetic particles involve thermal decomposition of organometallic compounds such as metal carbonyls, metal acetyl acetonates, and FeCup3.7 These procedures have been quite successful in obtaining good quality nanocrystals in organic solvents; however, the method involves the use of toxic chemicals, such as metal carbonyls, and reactions take place at very high temperature (up to 305 °C). Recently, Li et al. have demonstrated the possibility of producing magnetite nanocrystals by the thermal decomposition method using polar 2-pyrrolidone after refluxing at ∼250 °C.8 Though the synthetic strategy reported here is interesting to obtain water-soluble r 2011 American Chemical Society

magnetite nanocrystals, this method has not been extended to obtain other oxides. Importantly, the nanocrystals obtained are not monodispersed and hence are not suitable for assembly. Therefore, there is a need to look for a new simple strategy to obtain highly monodispersed good quality magnetic nanocrystals with high yield. In search of the new simple strategy to obtain highly monodispersed magnetic nanocrystals, commonly used nonpolar ether solvents of high boiling points can be readily replaced with other strong polar organic solvents. This can demonstrate a generic approach to systematically form stable dispersions of magnetic nanocrystals in both aqueous and nonaqueous media by appropriate surface functionalities. In the present investigation, a much convenient polar organic solvent, N-methyl 2-pyrrolidone (NMP), has been introduced for the first time to synthesize highly monodispersed magnetic nanocrystals. This is particularly due to its stability at ambient temperature, low flammability, low volatility, and its ready availability with no clear toxicity profile. Importantly, the reaction has been carried out at a comparatively much lower boiling temperature of ∼200 °C in ordinary laboratory equipment. Although, this thermal decomposition method has become the main approach to achieve good quality highly monodispersed magnetic nanocrystals, the choice of the surfactants plays an Received: September 24, 2010 Published: September 06, 2011 13189

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Langmuir important role in enhancing the thermal and chemical stability of the nanocrystals. Moreover, the self-assembly of these nanocrystals is greatly influenced by the properties of their surfaces. Therefore, understanding the surface chemistry is of great importance for the synthesis and applications of the nanocrystals. Fatty acid, especially oleic acid, C17H33COOH (R-COOH), is a commonly used surfactant to stabilize the magnetic nanocrystals in the organic phase.9 Recently, synthesis of magnetite nanocrystals using oleylamine, C18H35NH2 (R0 NH2), as a key to provide both the reductive environment and a stabilizer has also been reported.10 However, a much used and well reported surfactant to synthesize chemically stable magnetic nanocrystals is the equimolar mixture of both oleylamine and oleic acid.11 Though few studies have been carried out on elucidating the chemical structure of the mixed surfactant adsorbed on the surface of magnetic particles,12 the present investigation gives more insight into the chemical coordination of the mixed surfactants on the surface of the magnetic nanocrystals. In particular, the spinel ferrites of composition MFe2O4 (M = Fe, Co, Ni, Mn, etc.) provide great opportunities for studying the mechanism of superparamagnetic properties. Superparamagnetism is a fundamental issue of magnetism which can be controlled by varying the atomic level magnetic couplings between the electron spin and its orbital angular momentum at a crystal lattice. Such couplings at an atomic level generate the anisotropy energy known as magnetocrystalline anisotropy, EA. Utilizing the StonerWohlfarth theory, the magnetocrystalline anisotropy EA of a single domain particle can be expressed as EA = KV sin2 θ, where K is the magnetocrystalline anisotropy constant, V is the volume of the nanocrystals, and θ is the angle between the magnetization direction and the easy axis of the nanocrystals.13 The superparamagnetic properties of the nanocrystals can be controlled by adjusting EA.14 Through a well controlled EA, one can design and control the superparamagnetic properties of nanocrystals for the specific applications. EA will change when the size of the nanocrystals V and/or the anisotropy constant K varies. Since K is determined by the strength of the LS couplings, it directly reflects the correlation between the LS coupling and the superparamagnetic properties of the nanocrystals. Certainly, understanding and controlling the mechanism of superparamagnetic behavior and nanoassemblies of magnetic nanocrystals is important to many technological and biological applications. Therefore, in the present investigation, we have demonstrated a facile one-pot synthetic route to obtain highly monodispersed superparamagnetic ferrite nanocrystals. The identification and chemical coordination of the surfactant to the surface of the nanocrystals have shed more insights into the function of oleic acid and oleylamine toward the formation of nanocrystals of enhanced stability. The energy barrier distribution of the magnetic anisotropy is deduced from the temperature decay of magnetization, confirming the formation of highly monodispersed magnetic nanocrystals. It is important to note that, though the present study provides a simple strategy to achieve highly monodispersed ferrite nanocrystals dispersible in nonaqueous solvent, introduction of NMP as a solvent facilitates further work on appropriate surface functionalities to obtain water dispersible nanocrystals, suitable for biomedical applications.

2. EXPERIMENTAL SECTION The chemicals iron acetylacetonate, cobalt acetylacetonate, oleic acid, oleylamine, and 1,2-hexadecanediol were purchased from Aldrich

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Chemicals. N-methyl 2-pyrrolidone (NMP) was purchased from Merck Chemicals. All the chemicals were of analytical grade or better. Iron acetylacetonate (0.15 M) dissolved in 10 mL of NMP was taken in a pressure equalizer attached to a three-neck round-bottom (RB) flask. In a RB flask, 1,2-hexadecanediol (0.3 M), oleic acid (0.3 M), and oleylamine (0.3 M) were mixed with 35 mL of NMP, and then the mixture was magnetically stirred under a flow of argon. The mixture was heated to reflux at 200 °C. To this solution, iron acetylacetonate solution was injected instantaneously and was further refluxed for 1 h. As illustrated in scheme in the Supporting Information, the process involves rapid injection of metal acetylacetonates to 1,2-hexadecanediol, oleic acid, and oleylamine dissolved in NMP. After 1 h, the heat source was removed, and the black brown mixture was further stirred for ∼12 h. The black material was precipitated after adding ethanol (50 mL) and stuck to the surface of the magnetic stirring bar. The black product was centrifuged for 10 min with the speed 10 000 rpm. The product obtained was dispersed in hexane in the presence of 100 μL of oleic acid and oleylamine. This procedure of precipitation and centrifugation was repeated two to three times to remove the solvent and excess surfactants. The precipitate was dried at 50 °C in a vacuum oven for further characterization. In order to obtain CoFe2O4 nanocrystals, similar workup procedures were followed starting with cobalt acetylacetonate (0.05 M) and iron acetylacetonate (0.10 M). The samples were characterized for their phase purity and crystallinity by powder X-ray diffraction (XRD) measurements (Panalytical Xpert Pro) with Cu Kα radiation using a Ni filter. Particle sizes were investigated by high resolution transmission electron microscopy (HRTEM; FEI Technai 30 system) operated at 300 kV, on a carbon coated copper grid after dispersing the powder in toluene. The hydrodynamic size of the nanocrystals was measured by dynamic light scattering (DLS) using a Nano ZS Malvern instrument employing a 4 mW He Ne laser (λ = 632.8 nm) and equipped with a thermostatted sample chamber. Thermogravimetric analysis (TGA) was done using a Perkin-Elmer STA 6000 simultaneous thermal analyzer. Fourier transform infrared (FTIR) spectra of the pure liquid of oleic acid, oleylamine, and the equimolar mixture of oleic acid and oleylamine were obtained by drop casting the liquids dissolved in chloroform onto the NaCl window (Thermo: Nicolet 6700 FTIR) and allowing the solvent to evaporate. The transmission FTIR spectra of surfactant coated magnetic nanocrystals were recorded in the 4004000 cm1 range (Perkin-Elmer system, spectrum One B) by preparing KBr (Merck, spectroscopy grade) pellets (0.1 wt % sample). X-ray photoelectron spectroscopy (XPS) studies were carried out using a VG Scientific ESCA-3 MK spectrometer at a base pressure of better than 1  109 Torr. The exciting radiation was Mg Kα X-rays (1253.6 eV), and the spectrometer was operated in the constant analyzer energy mode (CAE) at a pass energy of 50 eV, yielding an overall resolution of ∼1.1 eV. Low temperature M€ossbauer measurements were performed in a He bath cryostat using a 57Co source incorporated in a Rh matrix with a commercial spectrometer. The spectra were analyzed by the Win-normos software. Magnetic measurements were carried out using a Quantum Design PPMS system. The measurements were made between 5 and 300 K using zero-field-cooling (ZFC) and field-cooling (FC) protocols at 50 Oe, and the hysteresis loops were obtained in a magnetic field varied from +3 to 3 T.

3. RESULTS AND DISCUSSION Figure 1 shows the powder X-ray diffraction patterns of the ferrite formed. The XRD peak positions and the relative intensity of all diffraction peaks match well with the simulated pattern of bulk CoFe2O4 (a = 8.3919 Å, JCPDS no. 22-1086) and Fe3O4 (a = 8.3967 Å, JCPDS no. 19-629). The Bragg reflection peaks are all relatively broad because of the extremely small size of the ferrite formed. 13190

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The average crystallite sizes were calculated using the Scherrer equation, d = (0.9λ/β cos θ), where d is the diameter in Ångstroms, β is the half-maximum line width, and λ is the wavelength of X-rays. The average crystallite sizes are estimated to be about 4.0 ( 1 and 4.6 ( 1 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals. The calculated average crystallite sizes for the samples are compared with particle sizes obtained from other measurements as listed in Table 1. The calculated cubic lattice parameters a are 8.390 ( 0.004 Å and 8.388 ( 0.005 Å, respectively, for the samples CoFe2O4 and Fe3O4 and are comparable to that obtained for the bulk samples. Figure 2 shows the representative TEM images of the CoFe2O4 and Fe3O4 nanocrystals. Analysis of the TEM micrographs clearly indicates formation of highly monodispersed spherical nanocrystals. Deposition of the toluene dispersion of the nanocrystals on a carbon-coated copper grid and the slow evaporation of the solvent in air atmosphere (relative humidity of ∼80%) favor the formation of interesting self-assembled superstructures with “breath figure voids”.15 During the drying process, with the evaporation of the solvent toluene, the fluid surface cools below the ambient temperature, leading to the condensation of water droplets on the evaporating surface. However, being on the surface of the organic solvent, water droplets do not coalesce immediately. With the slow evaporation of toluene, the magnetic nanocrystals assemble spontaneously due to the interplay of the intermolecular interactions resulting from the cooperation of electrostatic, dipoledipole, and van der Waals forces. Over time with the removal of the solvent and simultaneous assembly of magnetic nanocrystals, the water droplets grow and coalesce and leave an imprint of water droplets as hollow air filled semiordered hexagonal arrays. However, the formation of these voids can be avoided by depositing the nanocrystals under dry atmospheric conditions.15b The mechanism for the formation of highly monodispersed nanocrystals is a kinetically driven process, where the ratio between the rates of nucleation and growth is responsible for tuning the final nanocrystal sizes. Normally, hot rapid injection technique of the metal

organometallics to the reaction mixture demonstrates that the nucleation rate rises much faster than the growth rate. The crystal nucleation rate per unit volume (JN) is expressed by the classical nucleation theory as16 JN = BN exp(ΔGN/RT), where ΔGN is the activation energy for the homogeneous nucleation and BN is the pre-exponential factor. Usually, at high temperature, the activation energy ΔGN for the nucleation process to occur is much higher than that of the growth of the particles; therefore, at higher temperature, rapid addition of organometallics follows fast nucleation, slow growth, and higher particle concentration, favoring the formation of tiny monodisperse nanocrystals.16,17 The particle size distributions, obtained from the TEM micrographs, are shown in the insets of Figure 2A and B. The histograms show narrow particle size distribution. The mean particle sizes obtained from the Gaussian fit of the histograms are 4.4 ( 1.0 and 6.4 ( 1.0 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals, which is somewhat larger than that of the crystallite sizes, as estimated from the XRD line width analysis (see Table 1). Figure 2C and D shows the HRTEM micrographs with the inset showing the lattice fringes of the selected area of the nanocrystals marked in the same figure. The insets of the figures clearly shows nanocrystals with lattice fringes with interfringe distance measured to be around 0.255 and 0.258 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals, which matches with the {311} d-spacing of the spinel phase.

Figure 2. (A,B) TEM images of CoFe2O4 and Fe3O4 nanocrystals, respectively. Insets: histograms from TEM analysis. (C,D) HRTEM images of CoFe2O4 and Fe3O4 nanocrystals, respectively. Insets: lattice fringes of the square-marked nanocrystals.

Figure 1. XRD patterns for (a) CoFe2O4 and (b) Fe3O4 nanocrystals. S1 and S2 are the simulated patterns with a = 8.3919 Å and a = 8.3967 Å, respectively.

Table 1. Particle Sizes Determined from Different Methods for Oleic AcidOleylamine Stabilized Ferrite Nanocrystals magnetic particle size (nm) from magnetization data sample

XRD ((1.0 nm)

TEM ((1.0 nm)

DLS ((1.0 nm)

low field

high field

CoFe2O4

4.0

4.4

6.6

3.9 ((0.2)

3.6 ((0.2)

Fe3O4

4.6

6.4

8.2

3.9 ((0.4)

3.7 ((0.3)

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Figure 4. Thermogravimetric curves of the oleic acidoleylamine coated samples (a) CoFe2O4 and (b) Fe3O4 nanocrystals. Inset: Derivative of the weight loss as a function of temperature. Figure 3. FTIR spectra of (a) CoFe2O4 and (b) Fe3O4 nanocrystals coated with 1:1 mixture of oleic acid and oleylamine.

To further investigate the influence of the oleic acidoleylamine coating on overall size of the nanocrystals (ferrite core + organic coating), we performed DLS measurements. DLS cannot discriminate between the ferrite core and organic shell and therefore measures the hydrodynamic diameter of the nanocrystals in their dispersion state. The mean hydrodynamic diameter as obtained from the Gaussian fit of the histograms (see Supporting Information Figure S1) are 6.6 ( 1.0 and 8.2 ( 1.0 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals. It is evident that the hydrodynamic particle sizes are larger than the TEM particle sizes (Table 1). The increased values compared to that of the TEM analysis can be explained by oleic acidoleylamine layer present around each nanocrystal. In order to demonstrate the coating and understand the adsorption mechanism of the surfactant (equimolar mixture of oleic acid and oleylamine) on the surface of ferrite nanocrystals, FTIR spectroscopic measurements were first performed on the pure oleic acid (R-COOH), oleylamine (R0 -NH2), and the equimolar mixture of oleic acid and oleylamine (see Supporting Information Figure S2). Our FTIR analysis matches well with the earlier reports,9a,1821 and the details are given in the Supporting Information. Based on our FTIR spectral analysis, we conclude that oleylamine helps in enhancing the deprotonation of oleic acid, resulting in the formation of acidbase complex, which can act as binary capping agent, and the free surfactants are washed away during the washing steps.12b Enhanced deprotonation of the oleic acid and its strong electron donating ability favors the binding of surfactant on the nanoparticle surface, thereby enhancing their stability. The capping of ferrite nanocrystals by the surfactant has been further confirmed from FTIR spectroscopy. Figure 3 illustrates the FTIR spectra of the surfactant coated ferrite nanocrystals taken in 0.1 wt % of the sample in KBr. No spectral features corresponding to free oleic acid (RCOOH) and free oleylamine (R0 NH2) are found in the FTIR spectra of surfactant coated magnetic nanocrystals. There is no shift in methylene asymmetric (νas (CH2)) and symmetric (νs (CH2)) (2924, 2853 cm1) stretching bands. Interestingly, there is a weak broad absorption peak corresponding to the antisymmetric deformation of the NH3+ group at around 1625 cm1 which is superimposed with that of CdC stretching mode at 1648 cm1. This clearly reveals the presence of the protonated amines.12a Also, a peak at 3005 cm1 indicates the presence of CH bonds adjacent to the CdC bond. Unlike the

earlier report on Fe rich FePt nanoparticles12c and CoFe2O4SiO2 coreshell nanoparticles,12a where the dehydrogenation and isomerization of the surfactant from the cis to the trans isomer of oleic acid called elaidic acid were noticed, in the present investigation, no isomerization of the oleic acid is observed. Importantly, the absence of the 1710 cm1 band corresponding to CdO stretch bond of the carboxyl group (of pure oleic acid) and the appearance of two new bands characteristics of asymmetric (νas (COO)) and symmetric (νs (COO)) stretch at 1562 and 1428 cm1 (curve a) for CoFe2O4 nanocrystals and at 1554 and 1426 cm1 (curve b) for Fe3O4 nanocrystals confirm the formation of deprotonated carboxylate. Moreover, strong bands at 1094, 1023, and 1260 cm1 arising from the CO stretching reveal that two oxygen atoms in the carboxylate are coordinated onto the surface of the nanocrystal. A wavenumber difference (Δ) between the νas (COO) and νs (COO) of value less than 145 cm1 9a and weak νas(COO) bands suggests the formation of a bidentate (COOM) mode where interactions between the COO group and the surface atoms of metals are ionic.22 Interestingly, there is no characteristic peak for oleylamine indicating the absence of NH2 bonds on the surface of the magnetic nanocrystals ,and our result matches with the result obtained by Bu et al. with NaLa(MoO4)2 nanocrystals coated with mixed oleic acid and oleylamine surfactants.20 The characteristic spinel absorption bands at 605 and 613 cm1 confirm the formation of CoFe2O4 and Fe3O4 ferrite phases.23 On the basis of IR analysis, the qualitative pictorial representation of the coating of the magnetic nanocrystals by the equimolar mixture of oleic acid and oleylamine is represented in the scheme in the Supporting Information. In order to quantify the coverage of surfactant molecules surrounding each nanocrystal, TGA analysis was performed. Figure 4 shows the TGA curves of the as prepared ferrite powders. In this experiment, the ferrite powders were heated (10°/min) between room temperature and 600 °C under flowing N2 and the change in the weight of the samples due to loss of surfactants was recorded. The desorption steps are comparable to that reported earlier in the literature.2426 Derivatives of weight loss as a function of temperature (see inset Figure 4) show two distinct transitions for both the samples. The first derivative peak with small weight loss may be due to the dynamics of chain reorganization, and as the heating progresses the second derivative peak with large weight loss may be attributed to the decomposition of the binary surfactants.25,26The derivative peaks, corresponding to percentage weight loss, surfactant coverage, and thickness of the 13192

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Table 2. Percentage of Surfactants and Thickness of the Coating Measured via TGA for Oleic AcidOleylamine Stabilized Ferrite Nanocrystals first weight

second weight

loss (%)

loss (%)

derivative weight derivative weight surfactant/cm2 thickness (1014)

(nm)

27.8

4.4

∼2.3

15.71

1.2

∼1.7

sample

peak

loss

peak

loss

CoFe2O4

259

7.84

348

Fe3O4

257

5.53

385

film formed are summarized in Table 2. For the equimolar mixture of oleic acid and oleylamine forming the binary capping agent, the number of surfactant molecules per particle (N) can be calculated using the following formula:26 4 wNA F πR 3  1023 3 N ¼ MM where R is the radius of the ferrite nanoparticles, as obtained from TEM analysis, F is the density of the nanoparticles, NA is Avogadro’s number, MM is the molar mass of the binary surfactant molecules, and w is the weight loss in percentage as obtained from TGA analysis. The number of binary surfactant molecules per particle is around 269 and 165, respectively, for CoFe2O4 and Fe3O4 ferrites. Using the above equation, we can also calculate the typical surfactant coverage of around 4.4  1014 and 1.2  1014 molecules/cm2 with thickness of around 2.3 and 1.7 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals (Table 2).The surface coverages and the thickness of the surfactants as calculated from TGA are in well agreement with the results reported by earlier groups.26 From TGA, DLS, and TEM analysis, we conclude that the oleic acidoleylamine binary surfactant forms the continuous monolayer or quasidouble layer around each nanocrystals. Figure 5 illustrates the magnetic properties of the nanocrystals, investigated with a quantum design physical property measurement system (PPMS). For the Fe3O4 and CoFe2O4 nanocrystals, the magnetization does not saturate, even for the applied field of 30 kOe, and no hysteresis is observed. The MH characteristics are typical of superparamagnetic behavior for the case of both CoFe2O4 and Fe3O4 nanocrystals. In the superparamagnetic region, the magnetization of the magnetic nanocrystals is represented by the Langevin function,13 M = Ms[coth(mH/kT)  kT/mH], where M is the specific magnetization at field H, Ms is the saturation magnetization, m is the magnetic moment per particle at temperature T, H is the field applied, and k is the Boltzmann constant. For H f 0, the initial magnetization is mainly determined by the larger particles, whereas for H f ∞, the magnetization mainly corresponds to the smaller particles. Therefore, the magnetic particle sizes can be calculated from the initial slope and the slope at high magnetic fields. The detailed discussion on the calculation of magnetic particle size distribution on other magnetic oxides is reported elsewhere.27 From the fit of the Langevin function for an assembly of superparamagnetic particles, the magnetic particle size distributions of 3.6 ((0.2) to 3.9 ((0.2) nm and 3.7 ((0.3) to 3.9 ((0.4) nm are obtained from high-field and low-field limits, respectively, for CoFe2O4 and Fe3O4 nanocrystals. The

Figure 5. Field-dependent magnetization behavior of (a) CoFe2O4 and (b) Fe3O4 nanocrystals at 300 K. Insets A and B are the corresponding M versus 1/H magnetization.

identical values of magnetic particle sizes obtained for CoFe2O4 and Fe3O4 nanocrystals in the low-field and high-field limits indicate narrow particle size distribution. Also the average magnetic particle sizes obtained from magnetic data is comparable to that of average XRD crystallite sizes of CoFe2O4 and Fe3O4 nanocrystals (see Table 1). TEM measurements showed slightly larger particle sizes of 4.4 ( 1.0 and 6.4 ( 1.0 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals when compared to both magnetic particle sizes and XRD crystallite sizes. This indicates the presence of magnetically dead layers on the surface of each particle. The presence of magnetically dead layers and the nonmagnetic organic coating as seen from TGA analysis may substantially reduce the saturation magnetization values.27,28 The estimates of the room-temperature saturation magnetization (Ms) value for these samples are obtained by the extrapolation of M versus 1/H curves to the limit 1/H f 0 (see insets A and B of Figure 5). The room temperature saturation magnetization values obtained are 38.0 and 34.0 emu/g which are less than the bulk values of 80 and 92 emu/g for CoFe2O4 and Fe3O4 ferrites, respectively. Reduced magnetization of the nanocrystals can be well explained by the coreshell morphology of the nanocrystals where the magnetic order parameter (MOP) is originating only from the core of particles and the noncollinear magnetically dead layer is present predominantly on the surface of each particles.29,30 Knowing the particle size d and the saturation magnetization values Ms(d), the thickness t of the magnetically dead layer can be calculated from the equation Ms(d) = Ms(bulk)[1  (6t/d)]. By substituting the magnetization data and the particle sizes thickness t for CoFe2O4 and Fe3O4, nanocrystals are ∼0.38 and ∼0.67 nm, respectively, which implies the magnetic nature of first crystalline layer is destroyed due to canted surface spins. It is however important to note that each particle is further coated with a nonmagnetic organic layer of thickness ∼2.3 and ∼1.7 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals. The results are supported from the hydrodynamic particle sizes of nanocrystals which are larger than that of the TEM particle sizes (see Table 1). Usually, superparamagnetic particles are characterized by a maximum in the temperature variation of zero-field-cooled (ZFC) magnetization measured in a small dc magnetic field. For the ZFC measurements, the sample was cooled from room temperature to 5 K, without any external magnetic field, and the magnetization was recorded while warming the sample in the applied field of 50 Oe. For field-cooled (FC) measurements, the 13193

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Figure 6. ZFC and FC magnetization measured at 50 Oe and energy barrier distribution of the magnetic anisotropy for 6.4 ( 1.0 nm Fe3O4 nanocrystals. Inset: corresponding field-dependent magnetization behavior measured at 5 K.

samples were cooled from room temperature to 5 K under a field of 50 Oe and magnetization was measured while warming the sample in the presence of a field. For an assembly of noninteracting particles with random distribution of anisotropy axis, ZFC and FC measurements show distinct features. A maximum is observed in the ZFC magnetization curve at a temperature, Tmax.31 When the sample is cooled under a magnetic field (FC), the magnetization shows its maximum value at 5 K and then steadily decreases while increasing the temperature. The bifurcation between ZFC and FC magnetization starts at the irreversibility temperature (Tirr), above which the two curves coincide. Tirr is related to the blocking of the largest particles, and it is defined as the temperature at which the difference between MFC and MZFC, normalized to its maximum value at 5 K becomes smaller than 3%.31b Tmax obtained from the ZFC maximum is directly proportional to the average blocking temperature (TB) with a proportionality constant (β = 12) that depends upon the type of size distribution. Therefore, Tmax is related to the blocking of the particles with mean particle size.31c The difference between Tmax and Tirr provides a qualitative measure of the width of blocking temperature distribution.31 Below Tmax, the magnetic anisotropy energy is larger than the thermal energy and therefore blocks the magnetic moments from orienting in the direction of the small magnetic field.The bifurcation between ZFC and FC magnetization curves is due to the existence and the distribution of energy barriers of the magnetic anisotropy and the slow relaxation of the magnetic nanocrystals below Tmax. Such temperature dependence of ZFC and FC magnetization and the divergence of ZFC and FC magnetizations below Tmax are characteristic features of superparamagnetism.32 The ZFC and FC magnetization, measured at 50 Oe, and the energy barrier distribution of the magnetic anisotropy are illustrated in Figures 6 and 7 for Fe3O4 and CoFe2O4 nanocrystals, respectively. The maximum in magnetization for the Fe3O4 nanocrystals measured at 50 Oe is at 8 K (see Figure 6). It is usually expected that Tirr and Tmax increases with increasing size. However, Tmax of CoFe2O4 nanocrystals of size 4.4 ( 1.0 nm is at 70 K (see Figure 7) which is about 62 K higher to 6.4 ( 1.0 nm sized Fe3O4 nanocrystals. Higher Tmax indicates that EA is much higher in CoFe2O4 nanocrystals compared to Fe3O4 nanocrystals. For randomly oriented assemblies of CoFe2O4 and Fe3O4 nanocrystals and similar arrangement of the easy axes of magnetization relative to the field direction, the higher EA value suggests that the anisotropy constant K is much higher in CoFe2O4 nanocrystals.

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Figure 7. ZFC and FC magnetization measured at 50 Oe and energy barrier distribution of the magnetic anisotropy for 4.4 ( 1.0 nm CoFe2O4 nanocrystals. Inset: corresponding field-dependent magnetization behavior measured at 5 K.

This is likely because the Co2+ cations have high spin ligand fields and out of seven d electrons, three are unpaired. The quantum origin of the magnetocrystalline anisotropy is the LS coupling at the Co2+ crystal lattices and the value of anisotropy constant K indicates the strength of such couplings.33 The total effective magnetic anisotropy constant (Keff) value can be deduced from the StonerWohlfarth expression:13 KeffV = 25kBTB, where kB is the Boltzmann constant and V is the volume of the particles. The calculated Keff values are 6.0  106 and 0.5  106 erg/cm3 for the CoFe2O4 and Fe3O4 nanocrystals, respectively. These values are larger than the values reported for the bulk powders (1.8  106 erg/cm3 for CoFe2O4 and 0.12  106 erg/cm3 for Fe3O4).13 Similar large differences in the value of magnetic anisotropy for the superparamagnetic particles of other spinels have been observed.27 Though the presence of dipolar interaction has been earlier invoked as a source of additional anisotropy,27a,34 the nature of FC magnetization curves, which gradually increases below TB, rules out the presence of strong dipolar interactions in the case of Fe3O4 nanocrystals. However, in the case of CoFe2O4 nanocrystals, almost constant values of FC magnetization below ∼25 K is an indication for the presence of dipolar interactions.31b The energy barrier distribution of the magnetic anisotropy is calculated with the method adopted by Zhang et al.32a The distribution function of the magnetic anisotropy energy barriers, f(T) is represented by the derivative of the magnetic decay plot, f(T) = (dMTD/dT), where MTD is the temperature dependent decay of the FC magnetization representing the number of the nanocrystals, whose energy barriers are overcome at a given temperature by kBT and whose magnetization starts to flip randomly. In the FC magnetization process, MFC consists of the contribution from the total nanocrystals at each temperature whereas in the zero-field-magnetization process MZFC reflects the contribution from the nanocrystals of which the energy barriers are overcome by the thermal energy at the measuring temperature.33 The calculated magnetic anisotropy energy distributions for the Fe3O4 and CoFe2O4 nanocrystals are represented in Figures 6 and 7. It is clear that most of the nanocrystals have overcome their energy barriers at their corresponding Tmax values. Normally, the anisotropy energy distribution depends upon the shape and size of the nanocrystals. Very narrow anisotropy energy distributions clearly indicate formation of highly monodispersed magnetic nanocrystals in the present study. However, due to stronger LS coupling at the Co2+ 13194

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Figure 8. M€ossbauer spectra of (a) CoFe2O4 and (b) Fe3O4 nanocrystals measured at 4.2 K.

crystal lattices, a higher anisotropy energy value with higher TB for CoFe2O4 nanocrystals is expected.32,33 The behavior of the opening up of the magnetic hysteresis below the blocking temperature is also expected. This is clearly seen from the field-dependent magnetization behavior shown in the insets of Figures 6 and 7. The Fe3O4 nanocrystals (6.4 ( 1.0 nm) display a thin hysteresis loop with a small coercivity of 70 Oe (see inset of Figure 6). However, CoFe2O4 nanocrystals of 4.4 ( 1.0 nm size display a significantly high coercivity of 9150 Oe at 5 K (see inset of Figure 7). The value corroborates with the results observed by others for CoFe2O4 particles of similar size synthesized by other routes.35 It is important to note that, in the XRD spectrum (as shown in Figure1), we cannot exclude the possibility of the formation of maghemite (γ-Fe2O3 phase, a = 8.346 Å, JCPDS file 39-1346), especially in the nanocrystalline phase where the characteristic reflections are broad. Another more specialized technique such as M€ossbauer spectrometry is useful for elucidating this aspect. In order to distinguish two iron oxide phases and study the possibility of structural and coordination differences of iron in the superparamagnetic CoFe2O4, 57Fe M€ossbauer spectroscopic measurements were performed at 4.2 K. Figure 8 represents the M€ossbauer spectra of nanocrystals in the absence of magnetic field. Below TB, the samples exhibit sextets demonstrating ferrimagnetic behavior. It is known that, for Fe3O4 particles, there is a dynamic disorder between the ferric and ferrous ions in the octahedral B sites associated with the fast electron exchange between ferric and ferrous states above the Verwey temperature (∼120 °C), whereas below the Verwey temperature the ferrous and ferric ions in the octahedral sites are separately ordered.36 Therefore, below the Verwey temperature, the ferric and ferrous ions in the octahedral sites of magnetite nanoparticles will produce distinct and uniquely defined effective magnetic fields at the iron nuclei.36 M€ossbauer spectrum of Fe3O4 is shown in Figure 8b. It is interesting to note that the acceptable fit of the spectra was obtained only when the data were fitted with three sextets. Sextet with smallest isomer shift value of δtet = 0.30 (0.03) mm/s is assumed to arise from Fe3+ ions located in tetrahedral A sites. Sextets with isomer shifts values of

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δoct1 = 0.50 (0.02) mm/s are assigned to Fe3+ ions in the octahedral B sites. The third sextet with the highest value of isomer shift δoct2 = 0.70 (0.04) mm/s arises due to the presence of Fe2+ cations confirming the formation of Fe3O4 phase of magnetic oxide.37 The magnetic hyperfine field values for tetrahedral A sites (ΔHhf)tet = 48.2 (0.2) T and for octahedral B sites (ΔHhf)oct1 = 42.9 (0.2) T, (ΔHhf)oct2 = 48.2 (0.2) T are smaller than that of bulk values but comparable to the values reported earlier for magnetite nanoparticles.37d Reduction in the magnetic hyperfine field for the nanoparticles was earlier explained by Morup38 in terms of collective magnetic excitation. The ratio of the relative intensities due to the iron ions at the tetrahedral A site to octahedral B site is 0.67, which is higher than that of the theoretical value of 0.50. It is important to note that the intensity ratio of Fe3+/Fe2+, Fe3+ is very sensitive to the stoichiometry of magnetite.39 The higher value may be attributed to the slight surface oxidation of Fe2+ in the octahedral sites to Fe3+ accompanied by vacancy formation in B sites, giving the general formula, Fe3+A [Fe2.5+26δFe3+5δeδ]BO42 .39 In order to examine the oxidation state of iron ions in these nanocrystals, XPS analysis was carried out which is very sensitive to core electron lines of Fe2+ and Fe3+ cations. XPS spectra (see Supporting Information Figure S3) display characteristic bands assigned to Fe3O4. The peaks at 711.6 and 724.8 eV can be attributed to the levels of Fe 2p3/2 and Fe 2p1/2, respectively. The broadening of the peaks due to the presence of Fe2+(2p3/2) and Fe2+(2p1/2) and the respective peak positions are in agreement with the literature.40 The shoulder peak at 710.2 eV provides further evidence for the formation of Fe3O4 nanoparticles.40b It is however interesting to note a very weak satellite of the 2p3/2 peak at around 719 eV (arrow in Figure S3, Supporting Information). This peak is characteristics of Fe3+ in γ-Fe2O3. Therefore, some surface oxidation of Fe3O4 nanocrystals to the γ-Fe2O3 phase cannot be ignored. The M€ossbauer spectrum of CoFe2O4 nanocrystals is shown in Figure 8a. The zero field spectrum at 4.2 K exhibits a magnetically ordered state, similar to those of bulk. The acceptable fit of the spectrum was obtained only when the data were fitted with three sextets corresponding to one tetrahedral and two octahedral sites. The sextet with the smallest isomer shift of 0.39 (0.05) mm/s and magnetic hyperfine field of 40.4 (0.4) T is assumed to arise from the Fe3+ ions occupying A sites. The sextet with the isomer shift value of 0.47 (0.01) mm/s and magnetic hyperfine field value of 50.8 (0.2) T is assigned to Fe3+ in the octahedral sites. The third sextet with the isomer shift value of 0.42 (0.02) mm/s and magnetic hyperfine field value of 46.7 (0.3) T indicates the presence of second octahedrally coordinated Fe3+ in the octahedral sites.The presence of an additional octahedral site apart from the tetrahedral and octahedral sites has been reported earlier.31b,41,42 This contribution from an additional site has been assigned to the frustrated surface contribution,4143 resulting in different environments. This hypothesis of randomly canted spins on the surface of CoFe2O4 nanoparticles is supported by in-field M€ossbauer results as reported earlier.31b,4143 Direct evidence on the change in coordination environments when the particle size of the spinel-type ferrite nanoparticles is reduced to the nanometer range has been reported earlier by utilizing27Al magic-angle spinning (MAS) NMR spectroscopy.44 Therefore, third sextet in the B site of CoFe2O4 nanocrystals is introduced to represent the ions with canted spins. As a result, a different cation distribution in the A and B sites is expected. Generally, in the nanocrystalline particles, 13195

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Langmuir the ratio between Fe3+ in A and B sites is found to be in the range of 0.670.34 for different synthetic routes.31b,41 In the present investigation, this ratio is around 0.35 (3) and the obtained cation distribution is (Co0.48Fe0.52) [Co0.52Fe1.48]O4, which is comparable to the earlier report on CoFe2O4 nanoparticles of similar size.31b,41 It is therefore important to note that presence of noncollinear spin structures predominantly on the surface of the particles drastically reduces the saturation magnetization values.

4. CONCLUSIONS The present study demonstrates a simple strategy to achieve highly monodispersed good quality CoFe2O4 and Fe3O4 nanocrystals by a thermal decomposition method utilizing a strong polar organic solvent, N-methyl 2-pyrrolidone (NMP), at a much lower temperature of 200 °C. Hot rapid injection of the metal organometallics to the reaction mixture favors faster nucleation rate than that of the growth, resulting in the formation of highly monodispersed size-controlled tiny nanocrystals. FTIR analysis confirms that oleylamine promotes deprotonation of the oleic acid, favoring the formation of an acidbase complex which can be considered as new binary capping agent. Substantial loss of the saturation magnetization value is attributed to the magnetically dead layer of thickness ∼ 0.38 and ∼ 0.67 nm and nonmagnetic coating of thickness ∼2.3 and ∼1.7 nm, respectively, for CoFe2O4 and Fe3O4 nanocrystals. The effective magnetic anisotropy energy value for CoFe2O4 nanocrystals is about 10-fold higher and the Tmax value is at least 62 K higher than that of the similar sized Fe3O4 nanocrystals. This is due to the stronger magnetic couplings originating from Co2+ lattice sites. Narrow anisotropy energy distribution of the magnetic nanocrystals implies that the ferrite nanocrystals obtained in this study is highly monodispersed. M€ossbauer spectroscopic analysis allows gaining a better insight into the structural and coordination differences of iron in the ferrite nanocrystals. The present investigation on the facile one-pot synthesis and 2D self-assemblies of magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) nanocrystals of similar sizes but having varied magnetic properties illustrates that controlling superparamagnetism by adjusting the total magnetic anisotropy energy will certainly find potential applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: 91-20-2590 8072. Fax: 91-20-25899790.

’ ACKNOWLEDGMENT S.V. gratefully acknowledges Department of Science and Technology (DST) India, Sanction No. SR/FTP/CS-09/2007 for research grant. Research support from DST nanoscience unit of IISER, SRNM/NS-42/2009 is also acknowledged. S.V. is thankful to National Chemical Laboratory (NCL), Pune for HRTEM images and VSM measurements. UGC-DAE Consortium for Scientific Research, Indore is also acknowledged for M€ossbauer spectra.

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