Magnetic Fluids Based on γ-Fe2O3 and ... - ACS Publications

It is worth mentioning that Rubim et al. have recently observed that the formation of stable dispersions of Ag nanoparticles in BMIBF4 is only possibl...
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J. Phys. Chem. C 2009, 113, 8566–8572

Magnetic Fluids Based on γ-Fe2O3 and CoFe2O4 Nanoparticles Dispersed in Ionic Liquids Flavia C. C. Oliveira,† Liane M. Rossi,‡ Renato F. Jardim,§ and Joel C. Rubim*,† Laborato´rio de Materiais e Combustı´Veis (LMC-INCT-CMN), Instituto de Quı´mica da UniVersidade de Brası´lia, CP 04478, 70904-970, Brası´lia, DF, Brazil, Instituto de Quı´mica, UniVersidade de Sa˜o Paulo, CP 26077, 05513-970, Sa˜o Paulo, SP, Brazil, and Instituto de Fı´sica, UniVersidade de Sa˜o Paulo, CP 66318, 05315-970, Sa˜o Paulo, SP, Brazil ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: March 31, 2009

The disclosure of magnetic ionic liquids (MILs) as stable dispersions of surface modified γ-Fe2O3 or CoFe2O4 nanoparticles (NPs) in the 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) ionic liquid is reported. The magnetic NPs were characterized by X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopy. The surface modified NPs have proved to form stable dispersions in BMIBF4 in the absence of water and behave like a magnetic ionic liquid. The MILs have been characterized by Raman spectroscopy, magnetic measurements, and DSC. The stability of the magnetic NPs in BMIBF4 is consistently explained by assuming the formation of a semiorganized protective layer composed of supramolecular aggregates in the form of [(BMI)2(BF4)3]-. A superparamagnetic behavior and saturation magnetization of ca. 18 emu/g for a sample containing 30% w/w maghemite NPs/BMIBF4 have been inferred from static and dynamic magnetic measurements. DSC results have shown that the MIL composed of 30% w/w CoFe2O4 NPs/BMIBF4 remains a liquid phase down to -84 °C. Introduction Ionic liquids (ILs), mainly those derived from the 1,3dialkylimidazolium (DIM+) cation and anions such as tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), methanesulfonate (MeSO3-), triflate (CF3SO3- or Tf-), and bistrifluoromethanesulfonyl imide ([NTf]2-) (see Figure 1), display very interesting physical-chemical properties such as low melting temperatures, high thermal and chemical stabilities, very low vapor pressures, low viscosities, and low toxicity.1 These substances are excellent solvents for transition metal compounds, and they exhibit high electrical conductivities and large electrochemical windows (ca. 6 V).2 As a result of the interesting physical-chemical features, the ILs have been used as “green” solvents in biphasic catalysis,3,4 organic synthesis,5 extractions,6 electrochemical applications,7 biopolymers,8 and self-assembled molecular films9 as well as in the synthesis and stabilization of metal10-12 and metal oxide nanoparticles.13-15 Very recently, efforts have been done to obtain ILs with suitable magnetic properties.16-20 The pioneering study on ILs with magnetic properties has been done by Hayashi and Hamaguchi.16 The authors mixed FeCl3 and 1-n-butyl-3methylimidazolium chloride (BMICl) for obtaining the IL BMIFeCl4 with magnetic properties. Although this kind of IL has been previously described in the literature,21 its magnetic properties were unexplored. In addition to this, Clavel et al. obtained cyano-bridged molecular magnets in ILs with compositions M3[Fe(CN)6]2/[RMIM][BF4] (M2+ ) Ni, Cu, Co) and Fe4[Fe(CN)6]3/[RMIM][BF4].18 More recently, the synthesis and stabilization of magnetic Ni(0) †

Instituto de Quı´mica da Universidade de Brası´lia. Instituto de Quı´mica, Universidade de Sa˜o Paulo. § Instituto de Fı´sica, Universidade de Sa˜o Paulo. ‡

Figure 1. Structure of ionic liquids derived from the 1,3-dialkylimidazolium cation.

nanoparticles (NPs) with a mean diameter of ca. 5 nm stabilized in BMIBF4 and BMI(NTf)2 ILs have been reported.20 The preparation of magnetic rheological fluid (MRF) based on the dispersion of magnetite particles in eight different ILs, including BMIBF4, has been also accomplished.22 The authors used commercial magnetite powders containing particles with mean diameter 98%) of the ionic liquid was determined by 1H NMR using the intensity of the 13C satellites of the imidazolium N-methyl group as internal standard.33 The 1 H NMR spectrum was recorded on a Varian Mercury Plus spectrometer (300 MHz) at room temperature. Synthesis of γ-Fe2O3/BMIBF4 and CoFe2O4/BMIBF4 MILs. The γ-Fe2O3/BMIBF4 MIL was prepared by mixing an aqueous maghemite MF prepared as described above with BMIBF4. This mixture was kept under ultrasonic conditions for 2 min. The water was removed using a high vacuum pump, and stable MILs were obtained. A similar procedure was adopted to obtain the CoFe2O4/BMIBF4 MIL. MILs from 0.1 to 30% w/w (NPs/IL) concentration range were obtained without the observation of aggregation and precipitation. Aggregation was only observed for concentrations higher than 30% w/w. The concentration of magnetic nanoparticles in the MILs investigated in this work was 30% w/w. Instrumentation. Transmission electron microscopy (TEM) micrographs were taken on a Philips CM 200 microscope operating at an accelerating voltage of 200 kV. Samples for TEM observations were prepared by placing a drop of an aqueous solution containing the nanoparticles in a carbon coated copper grid. The metal particle size distribution was estimated from the measurement of about 300 particles, assuming spherical shape, found in an arbitrary chosen area in enlarged micrographs. The X-ray diffraction (XRD) patterns were obtained on a Rigaku Geigerflex Model D/MAX-2AC XRD instrument by using Cu KR radiation (λ ) 1.5406 Å). The Raman spectra were acquired on a Raman System 3000 from Renishaw at 632.8 nm excitation provided by a HeNe laser from SpectraPhysics. The spectra were collected in the backscattering configuration using a 50× objective from an Olympus microscope. The laser power on the sample was adjusted in order to avoid sample decomposition. Both static and dynamic magnetic measurements were carried out by using a SQUID (superconducting quantum interference device) magnetometer from Quantum Design. The temperature dependence of the dc magnetization M(T) under low applied magnetic fields (H < 1 kG) was obtained in both zero-fieldcooled (ZFC) and field-cooled (FC) modes and at temperatures ranging from 5 to 300 K. Hysteresis loops M(H) were also taken under applied magnetic fields varying from -70 to 70 kOe at selected temperatures. The measurements of the linear ac magnetic susceptibility were also performed in the SQUID

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Figure 2. XRD patterns: (a) γ-Fe2O3 and (b) CoFe2O4.

magnetometer in the temperature range 5 < T < 350 K under zero applied magnetic field. Both the in-phase (χ′(T)) and outof-phase (χ′′(T)) components of the linear ac magnetic susceptibility, χ(T) ) χ′(T) + iχ′′(T), were measured under an excitation ac magnetic field Hac ) 1 Oe and in a wide range of frequencies f (10-2 e f < 103 Hz). The phase transitions of BMIBF4 and magnetic ionic liquids were determined using a Shimadzu DSC-60 differential scanning calorimeter, equipped with a manual cooling unit. The DSC instrument was calibrated using zinc. An average sample weight of 10-20 mg was sealed in an aluminum pan. The samples were cooled to -120 °C and heated to 80 °C at a rate of 10 °C · min-1 under a N2 flow. The kinematic viscosity measurements of BMIBF4 and magnetic ionic liquids were performed at 30.0 ( 0.5 °C on an Ubbelohde viscosimeter using a Herzog-HVB 438 viscosimetric bath. The densities of these samples were obtained on an Anton Paar DMA35N digital densimeter calibrated with water at the temperature 30.0 ( 0.5 °C.

Figure 3. TEM images of γ-Fe2O3 (a) and CoFe2O4 (c) prepared by the coprecipitation method. (b) and (d) are the corresponding particle size histograms.

Results and Discussion XRD and TEM. The powder XRD patterns of γ-Fe2O3 and CoFe2O4 samples are displayed in Figure 2. They exhibit the typical pattern of ferrites with six well-defined peaks occurring at 2θ ∼ 18.2°, 30.3°, 35.6°, 43.3°, 53.6°, 57.1°, and 62.6°. These peaks correspond to the (111), (220), (311), (400), (422), (511), and (440) Bragg planes, respectively, of the polycrystalline CoFe2O4 spinel structure.34,35 The average size of the γ-Fe2O3 and CoFe2O4 crystallite particles were determined from the full width at half-maximum of the (311) peak by using the Scherrer equation:36

d ) 0.89λ ⁄ (B cos θ)

(I)

where λ is the X-ray wavelength, θ is the angle of the Bragg diffraction, and B is the difference between the full width at half-maximum and the instrumental broadening. The average particle diameters for the γ-Fe2O3 and CoFe2O4 samples, as determined by eq I, were found to be 8.2 and 11.3 nm, respectively. Transmission electron microscopy (TEM) micrographs of the colloidal γ-Fe2OX3 and CoFe2O4 revealed the presence of nearly spherical nanoparticles (see Figure 3a,c). Analysis of the TEM micrographs, by measuring the diameter of ∼300 randomly selected particles in enlarged TEM images, resulted in the particle size distribution histograms shown in Figure 3b,d. The size distribution of both samples was found to be well described by a log-normal distribution function from which we obtained the median particle diameter of 10.6 nm and the distribution

Figure 4. Raman spectra excited at 632.8 nm of (a) BMIBF4; (b) γ-Fe2O3/BMIBF4 MIL; (c) CoFe2O4/BMIBF4 MIL; (d) difference spectrum (b) - (a); (e) γ-Fe2O3 (powder); (f) difference spectrum (c) - (a); (g) CoFe2O4 (powder).

width of 0.27 for γ-Fe2O3 and the median particle diameter of 15.3 nm and the distribution width of 0.37 for CoFe2O4. The log-normal distribution has been widely used for describing the polydispersity of small particles, mainly in samples where the lower particle sizes are difficult to measure and some aggregation occurs.37,38 The difference between these values and those obtained by XRD may be related to the presence of extra phases, as discussed below. Raman Measurements. As can be inferred from the X-ray diagrams displayed in Figure 2, the X-ray data from the investigated ferrites are insufficient to distinguish these ferrites from each other. In order to better characterize the ferrites as well as their dispersion in BMIBF4, i.e., the MIL, we have recorded their Raman spectra. The Raman spectrum in the 100-1000 cm-1 spectral range of BMIBF4 is displayed in Figure 4a. The peak at 764 cm-1 is the ν(BF4-) symmetric stretching.39 The other features are characteristic of the BMI+ cation.39 The Raman spectrum of the γ-Fe2O3/BMIBF4 MIL is shown in Figure 4b. In order to recover the Raman spectrum of the γ-Fe2O3 NPs dispersed in

MILs Based on γ-Fe2O3 and CoFe2O4 NPs the IL, the spectrum of Figure 4a was subtracted from the one of Figure 4b. This difference spectrum is displayed in Figure 4d. For comparison purposes, Figure 4e shows the Raman spectrum of the γ-Fe2O3 NPs used to prepare the γ-Fe2O3/ BMIBF4 MIL. The broad features observed at 362, 493, and 712 cm-1, including the shoulder near 680 cm-1 and their relative intensities, are those expected for γ-Fe2O3.40,41 A similar procedure was used to recover the Raman spectrum of CoFe2O4 NPs dispersed in BMIBF4. The Raman spectrum of the CoFe2O4/BMIBF4 MIL is shown in Figure 4c. After the subtraction of the BMIBF4 spectrum, the recovered Raman spectrum of the CoFe2O4 NPs dispersed in the IL is presented in Figure 4f and agrees very well with the Raman spectrum of the CoFe2O4 NPs shown in Figure 4g. The main features observed at 183, 322 (broad), 475, 632, and 681 cm-1 and their relative intensities are quite similar to those reported for tetragonal CoFe2O4 crystals.42 The normal Raman spectra of the γ-Fe2O3 and CoFe2O4 NPs provide little or no information on the interface structure formed by the NPs and the IL. Most of the Raman signal comes from the bulk of the particle. In a recent study, the spectroelectrochemical features of γ-Fe2O3 NPs, synthesized as described in the present work, were investigated.43 The combination of surface-enhanced Raman scattering (SERS) and cyclic voltammetry data were used to gain information about the composition of the species present on the γ-Fe2O3 surface. The results indicated that the actual morphology of the maghemite is unusual. It is comprised of a γ-Fe2O3 core and an external layer of a protonated nonstoichiometric oxyhydroxide, [FeOx(OH)3-2x]H+ (x < 1).43 In addition to these two components, another layer of δ-FeOOH was found to be localized between the core and the external layer. Thus, the occurrence of these two extra phases is sufficient to account for the differences in the NP mean diameters as estimated by TEM and XRD analyses. It is worth mentioning that we were not able to obtain a stable dispersion of magnetite (Fe3O4) in BMIBF4. It seems that the Fe(NO3)/HNO3 treatment of magnetite for producing a positively charged maghemite NP is an important step to generating a stable MIL. However, we also argue that the dispersion of maghemite in BMIBF4 showed to be stable only after the removal of the excess of water by vacuum pumping. There is considerable evidence in the literature for the occurrence of such a protective layer in NPs dispersed in IL.44 For example, it has been proposed that the positively charged Pt(0) NPs, dispersed in an IL such as BMIBF4 and BMIPF6, are surrounded by a protective layer.44 The authors argued that such a layer was comprised of semiorganized anionic species in the form of supramolecular aggregates of the type [(BMI)x(X)x+1]- with x ) 2 for X ) BF4- and x ) 3 for X ) PF6-. In the materials discussed here, we consider that γ-Fe2O3 NPs may also have a positively charged external layer. Therefore, it is reasonable to assume that similar semiorganized anionic supramolecular aggregates, as proposed in ref 43, would be responsible for the formation of a surrounding protective layer for stabilizing the γ-Fe2O3 NPs in BMIBF4. If this were the case, a schematic representation for the γ-Fe2O3 NP and the anionic supramolecular aggregates would have the morphology of the drawing (sketch) displayed in Figure 5. Note that we have used the mean diameter values of the NPs, as estimated from both XRD and TEM measurements. The presence of such a protective layer is then responsible for the stability of the maghemite NPs in BMIBF4 in the absence of water. In the

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Figure 5. Schematic representation of the maghemite nanoparticle and the protective layer formed by [(BMI)2(BF4)3]- supramolecular aggregates.

Figure 6. Stability of dry (left) and wet (right) CoFe2O4/BMIBF4 MIL in the presence of an applied magnetic field. The permanent magnet is placed bellow the flasks containing the MIL. (a), (b), and (c) refer to pictures taken at t ) 0, 5, and 10 min, respectively.

presence of water, the positive charge of the surface layer is screened by the water molecules and, mainly due to the BF4water affinity, the semiorganized layer is destroyed. This results in aggregation of the NPs and the consequent instability of the colloidal solution. A similar nanostructure can be envisioned for the CoFe2O4 since the same approach was used for the stabilization of the MIL. It is worth mentioning that Rubim et al. have recently observed that the formation of stable dispersions of Ag nanoparticles in BMIBF4 is only possible if water is completely removed from the colloidal solution.45 Therefore, we believe that water plays an important role in the stabilization of NPs in BMIBF4. We have observed that dry MILs remain stable (against aggregation) in the presence of a magnetic field. In a sealed tube, the MIL remains stable for at least 60 days. However, in the presence of water, probably due to aggregation, the magnetic field induces the sedimentation of the particles (see Figure 6). We have also tried to promote the dispersion of the positively charged ferrites on BMIPF6 and BMI(NTf)2 ILs. However, these dispersions were unstable. Contrary to BMIBF4, those ILs are hydrophobic ILs and the anions PF6- and (NTf)2- are larger than BF4-. Therefore, we believe that the affinity of the IL for water and the size of the anions are important parameters affecting the stabilization of the nanoparticles in the IL.

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Figure 7. Temperature dependence of magnetization for the maghemite/ BMIBF4 MIL. The inset shows hysteresis loops at 300 K.

Magnetization Measurements. Most of the magnetic measurements were performed in the γ-Fe2O3/BMIBF4 MIL. The temperature dependence of the magnetization measured under an applied magnetic field of 50 Oe from 5 to 300 K, displayed in Figure 7, exhibits a maximum at ca. 136 K in the ZFC branch of the curves. Such a maximum is identified as the blocking temperature, TB, in a system comprised of superparamagnetic (SPM) particles. In fact, other features of SPM particles have been also observed in the magnetic field dependence of the magnetization (M × H) curve (see the inset of Figure 7). Note that both the coercivity and remanent magnetization are essentially absent in the M × H at 300 K, a temperature much higher than TB. The inset of Figure 7 indicates that the saturation magnetization MS at 300 K and H ) 2 kOe is found to be ∼18 emu/g. This corresponds to ca. 30% of the MS measured in the magnetic precursor γ-Fe2O3 NPs (65 emu/g at T ) 300 K and H ) 2 kOe). Considering that the MIL contains ca. 30 wt % maghemite, the magnetization (18 emu/g of MIL) corresponds to ca. 60 emu/g of maghemite. Therefore, the magnetization of the NPs dispersed in the IL was preserved since it is very close to the value measured for the maghemite NPs before the addition of the IL (65 emu/g). A similar discussion may be applied to CoFe2O4 nanoparticles, since a dispersion of ca. 30 wt % CoFe2O4 NPs in BMIBF4 also presents a superparamagnetic behavior; that is, in the presence of a magnetic field the fluid is attracted to the walls of the glass tube while in its absence the colloidal solution behaves like a normal fluid (see Supporting Information). To gain further insight on the magnetic nature of our samples, we have performed measurements of temperature dependence of the magnetic susceptibility χ(T) (χ′ and χ′′) under different frequencies f. The data for both components of χ(T), shown in Figure 8 for the γ-Fe2O3 nanoparticles, display two expected

Figure 8. Temperature dependence of the components of magnetic susceptibility χ′(T) and χ′′(T) (inset) at indicated frequencies.

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Figure 9. Plot of ln τ vs T-1. Open circles represent experimental data as obtained from Figure 8, and the dotted line is the fitting of data to eq 1.

features of SPM systems: (i) the occurrence of a maximum in temperature, Tmax, for both components; (ii) that such a maximum is shifted toward higher temperatures with increasing f. The freezing of the magnetic moments from the superparamagnetic to a blocked state occurs at Tmax ≈ TB. At this temperature, χ′′ peaks and the relaxation time τ of the γ-Fe2O3 nanoparticles is believed to be equal to the measuring time window τexp ) 1/f of the ac measurements. Thus, the dynamic response of a system comprised of very small particles is determined by the measuring time τexp of a particular technique. Since the reversion of the magnetic moment in isolated single-domain particles over the anisotropy energy barrier Ea is assisted by phonons, the relaxation time τ is activated by temperature and given by the Ne´el-Arrhenius law:

τ ) τ0 exp(Ea⁄kBT)

(1)

where τ0 is the attempt time and assumes values in the 10-9-10-12 s range for SPM systems. Under zero external applied magnetic field, Ea ) KeffV sin2 θ, where V is the volume of the particle, Keff is an effective anisotropy constant, and θ is the angle between the magnetic moment of the small particle and its easy magnetization axis. The linear behavior displayed in Figure 9, where ln τ is plotted against 1/TB, clear indicates that the Ne´el-Arrhenius model correctly describes the behavior of our γ-Fe2O3 nanoparticles. From the fitting of the data using eq 1, we have obtained τ0 ) 2.3 × 10-18 s and Ea ) 7.65 × 10-13 erg. Assuming that Ea ) KeffV and the average radius of our nanoparticles is 4.1 nm (from XRD data), one obtains Keff ) 2.6 × 104 erg/cm3. The resulting effective anisotropy is in excellent agreement with the magnetocrystalline anisotropy constant of bulk maghemite Kbulk ) 4.7 × 104 erg/cm3.46 On the other hand, our very small value of τ0 is out of the range expected in SPM systems (10-9-10-12 s) and resembles the ones commonly observed in those with magnetic interaction between nanoparticles. Such a kind of collective phenomenon deserves to be better discussed.47 We first mention that the relaxation time τ is affected by the magnetic interaction between SPM nanoparticles and the system may display features of spin glasses. In classical spin glasses instead the system is believed to undergo a thermodynamic phase transition to an ordered phase at low temperature TC. Within this context, much of the experimental data have been analyzed within the framework of critical phenomena and scaling laws were suggested for the relaxation time in the vicinity of a phase transition. For finite temperatures TC * 0, a power law has been proposed:

MILs Based on γ-Fe2O3 and CoFe2O4 NPs

τ ) τ0[Tf ⁄ (Tmax - Tf)]β -8

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(2)

-13

where τ0 is in the range 10 -10 s, Tf is the freezing (glass) temperature, and β, varying from 4 to 8, is the critical exponent.48 Another approach for a transition at TC ) 0, known as the generalized Arrhenius law, predicts that τ varies with Tf as49

τ ) τ0 exp(Ea ⁄ kBTfδ)

(3)

where the theoretical values (Ising model) of δ for spin glasses are 2 and 4 for two and three dimensions, respectively.49 In order to check if our samples display spin-glass-like behavior, we have carefully analyzed the frequency dependence of Tmax following the predictions given by the two models, or more appropriately, given by eqs 2 and 3. By fitting the data to eq 2, we have found τ0 ) 1.7 × 10-5 s, Tf ) 96 ( 7 K, and β ) 12 ( 2 (see Supporting Information). On the other hand, fitting the experimental data with eq 3 yielded τ0 ) 2.5 × 10-9 ( 9.6 × 10-9 s, Ea ) 1.2 × 10-10 ( 3.2 × 10-11 erg, and δ ≡ 2.2 ( 0.5 (see Supporting Information). The values obtained from the fitting procedure to the power law (eq 2) are out of the range expected for spin glasses at the very high values of both τ0 and β. A similar conclusion can be drawn for the values obtained for the generalized Arrhenius model, where the errors were very high and Ea ) KeffV will result in unphysical values of Keff ∼ 106-107 erg/cm3. On the other hand, it is tempting to classify our γ-Fe2O3 nanoparticles by using an empirical and model-independent criterion for sorting a transition to a frozen state. Such a criterion is the relative shift of the maximum of the temperature in χ′′(T) data, Tmax, with the measuring frequency f:

Θ ) (∆Tmax) ⁄ [Tmax(∆ log(f))]

(4)

where ∆Tmax is the difference between Tmax measured in the ∆ log(f) interval of frequency. Θ values in the range ∼0.10-0.13 are frequently attributed to diluted superparamagnetic systems,50,51 while a much smaller dependence of Tmax with frequency f is usually observed in spin glasses (Θ ∼ 5 × 10-3-5 × 10-2).52 We also mention that intermediate values of Θ (0.05-0.13) may correspond to nondiluted particulate systems where dipolar interaction between granules may not be neglected. Our value of Θ ∼ 0.07, obtained from the fitting procedure of the γ-Fe2O3 nanoparticles, indicated (i) that the progressive increasing of Tmax with increasing f may correspond to a thermally activated Ne´el-Arrhenius model for superparamagnetic nanoparticles and (ii) that dipolar interactions may be relevant for our analysis. A way to quantify the effect of dipolar interactions in SPM systems is by using the so-called Vogel-Fulcher relationship given by52

τ ) τ0 exp(Ea ⁄ kB(T - T0))

(5)

where T0 is introduced as an additional parameter with respect to the Arrhenius-Ne´el expression (see eq 1) and represents the temperature for which the relaxation time diverges. Such an approach adds an extra contribution to the energy barrier and is valid when Tmax . T0. By using eq 5 we have successfully fitted our ln τ vs Tmax data, as displayed in Figure 10, and have found τ0 ) 3.1 × 10-12 s, Ea ) 3.1 × 10-13 erg, and T0 ∼ 52 K. The value of τ0 is within the range expected for SPM systems, and T0 ∼ 52 K is much smaller than Tmax ∼ 150 K. In addition to this, it is appealing to estimate the effective anisotropy constant Ea ≈ KeffV for this weak interacting system. By using the volume ∼ 28.8 × 10-18 cm3 of our γ-Fe2O3, we estimate Keff ∼ 1.1 × 104 erg/cm3, a value very close to that expected

Figure 10. Plot of ln τ vs T. The circles are experimental data as obtained from Figure 8, and the dotted line is the fitting to eq 5.

TABLE 1: Glass Transition Temperatures (Tg), Kinematic (ν) and Dynamic (η) Viscosities, and Densities (G) for BMIBF4 and BMIBF4/CoFe2O4 Measured at 30 °C sample

Tg (°C)

ν30 (cSt)

η30 (cP)

F30 (g/cm3)

BMIBF4 -86 (-86)59 187.8 (199.1)57 219.7 (233)57 1.17 (1.17)57 BMIBF4/ -84 207.1 296.2 1.43 CoFe2O4a a

30% (w/w) in CoFe2O4.

for the bulk γ-Fe2O3 of 4.7 × 104 erg/cm3. The combined results discussed above strong indicate that our γ-Fe2O3 nanoparticles are weak coupled due to dipolar interaction and display features of superparamagnetism. Based on the magnetic properties discussed here, we argue that these materials are suitable for practical applications as magnetic seals for ultrahigh vacuum and/or magnetic supports for metal (core-shell) NPs in catalysis.53-56 As suggested by reviewers differential scanning calorimetric (DSC) and viscosity measurements of BMIBF4 and of a magnetic ionic liquid were performed. The results are compiled in Table 1. The values of viscosity and density obtained for BMIBF4 are in good agreement with those reported in the literature.57 However, when the CoFe2O4 NPs are dispersed in the IL, the density and viscosity values increase. It has already been reported that ILs of the type [1-n-alkyl-3-methylimidazolium][BF4] (n ) 2-9) usually show glass transitions and no crystallization.58 The Tg value of -86 °C obtained for BMIBF4 is in good agreement with the one found in the literature.59 The DSC curve for the BMIBF4/CoFe2O4 sample shows a Tg at -84 °C, indicating that the presence of the CoFe2O4 NPs in the IL does not affect the glass transition temperature despite the observed increase in the viscosity. Therefore, this kind of material can be used as a magnet seal even at very low temperatures. Conclusion Positively charged γ-Fe2O3 and CoFe2O4 nanoparticles as obtained by an acidic treatment form stable dispersions in dry BMIBF4. These colloidal solutions behave like magnetic fluids, i.e., magnetic ionic liquids (MILs) with superparamagnetic features. The stability of the NPs in BMIBF4 in the absence of water is fully understood by considering the formation of a semiorganized protective layer surrounding the NPs. The supramolecular structure is proposed to be similar to the one suggested for the stabilization of Pt(0) NPs in ionic liquids consisting of anionic species of the type [(BMI)2(BF4)3]-. The presence of water causes the screening of the surface positive charges and the instability of the protective layer resulting in

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the NP aggregation. The MILs at the concentration of 30% w/w (maghemite NPs/BMIBF4) have shown good stabilities in the presence of a magnetic field and were found to have a saturation magnetization of 18 emu/g. The dynamic measurements (dependence of the magnetic susceptibility on the temperature) have shown that the SPM behavior of the γ-Fe2O3 NPs in the BMIBF4 IL can be better described by the Vogel-Fulcher relationship strongly indicating a weak coupling between the γ-Fe2O3 NPs. This kind of material is suitable to be used both as a magnetic seal for ultrahigh vacuum, from room to lower temperatures (e.g., -80 °C), and as a magnetic support for metal (core-shell) NPs in catalysis, facilitating the catalyst recovery after using. Acknowledgment. The authors thank the Laborato´rio de Espectroscopia Molecular do Instituto de Quı´mica da USP, where the Raman measurements were performed. We also thank Prof. Pedro K. Kiyohara (IF-USP) for TEM images. F.C.C.O., L.M.R., R.F.J., and J.C.R. thank CNPq for research fellowships. This work was supported by the Brazilian agencies CAPES, CNPq, FAPESP, and FAPDF. The financial support of CNPq through the Instituto Nacional de Cieˆncia e Tecnologia de Cata´lise em Sistemas Moleculares e Nanoestruturados (INCTCMN) is also acknowledged. Supporting Information Available: Photographs of a glass tube containing the CoFe2O4/BMIBF4 MIL in the absence and presence of a magnetic field (Figure 1S). Fittings of experimental data to eq 2 (Figure 2S) and eq 3 (Figure 3S). DSC curves of BMIBF4 and BMIBF4/CoFe2O4 samples (Figure 4S). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667–3691. (2) Suarez, P. A. Z.; Selbach, V. M.; Dullius, J. E. L.; Einloft, S.; Piatnicki, C. M. S.; Azambuja, D. S.; deSouza, R. F.; Dupont, J. Electrochim. Acta 1997, 42, 2533–2535. (3) Dupont, J.; Consorti, C. S.; Spencer, J. J. Braz. Chem. Soc. 2000, 11, 337–344. (4) Welton, T. Chem. ReV. 1999, 99, 2071–2083. (5) Sheldon, R. Chem. Commun. 2001, 2399–2407. (6) Scovazzo, P.; Visser, A. E.; Davis, J. H.; Rogers, R. D.; Koval, C. A.; DuBois, D. L.; Noble, R. D. Ionic Liq. 2002, 818, 69–87. (7) Cheek, G. T.; O’Grady, W. E.; El Abedin, S. Z.; Moustafa, E. M.; Endres, F. J. Electrochem. Soc. 2008, 155, D91–D95. (8) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974–4975. (9) Kim, J. Y.; Kim, J. T.; Song, E. A.; Min, Y. K.; Hamaguchi, H. Macromolecules 2008, 41, 2886–2889. (10) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228–4229. (11) Schrekker, H. S.; Gelesky, M. A.; Stracke, M. P.; Schrekker, C. M. L.; Machado, G.; Teixeira, S. R.; Rubim, J. C.; Dupont, J. J. Colloid Interface Sci. 2007, 316, 189–195. (12) Ott, L. S.; Finke, R. G. Coord. Chem. ReV. 2007, 251, 1075–1100. (13) Jacob, D. S.; Bitton, L.; Grinblat, J.; Felner, I.; Koltypin, Y.; Gedanken, A. Chem. Mater. 2006, 18, 3162–3168. (14) Wang, Y.; Maksimuk, S.; Shen, R.; Yang, H. Green Chem. 2007, 9, 1051. (15) Bilecka, I.; Djerdj, I.; Niederberger, M. Chem. Commun. 2008, 886– 888. (16) Hayashi, S.; Hamaguchi, H. O. Chem. Lett. 2004, 33, 1590–1591. (17) Hayashi, S.; Hamaguchi, H. Chem. Lett. 2005, 34, 740–740. (18) Clavel, G.; Larionova, J.; Guari, Y.; Guerin, C. Chem.sEur. J. 2006, 12, 3798–3804. (19) Hayashi, S.; Saha, S.; Hamaguchi, H. O. IEE Trans. Magn. 2006, 42, 12–14. (20) Migowski, P.; Teixeira, S. R.; Machado, G.; Alves, M. C. M.; Geshev, J.; Dupont, J. J. Electron Spectrosc. Relat. Phenom. 2007, 156, 195–199.

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