Oleate-based hydrothermal preparation of CoFe2O4 nanoparticles, and their magnetic properties with respect to particle size and surface coating

Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

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Oleate-based hydrothermal preparation of CoFe2O4 nanoparticles, and their magnetic properties with respect to particle size and surface coating Anton Repko a,n, Jana Vejpravová b, Taťana Vacková c, Dominika Zákutná a, Daniel Nižňanský a a b c

Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 128 43 Prague 2, Czech Republic Department of Magnetic Nanosystems, Institute of Physics AS CR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

art ic l e i nf o

a b s t r a c t

Article history: Received 8 July 2014 Received in revised form 13 April 2015 Accepted 22 April 2015 Available online 23 April 2015

We present a facile and high-yield synthesis of cobalt ferrite nanoparticles by hydrothermal hydrolysis of Co–Fe oleate in the presence of pentanol/octanol/toluene and water at 180 or 220 °C. The particle size (6– 10 nm) was controlled by the composition of the organic solvent and temperature. Magnetic properties were then investigated with respect to the particle size and surface modification with citric acid or titanium dioxide (leading to hydrophilic particles). The as-prepared hydrophobic nanoparticles (coated by oleic acid) had a minimum inter-particle distance of 2.5 nm. Their apparent blocking temperature (estimated as a maximum of the zero-field-cooled magnetization) was 180 K, 280 K and 330 K for the particles with size of 6, 9 and 10.5 nm, respectively. Replacement of oleic acid on the surface by citric acid decreased inter-particle distance to less than 1 nm, and increased blocking temperature by ca. 10 K. On the other hand, coating with titanium dioxide, supported by nitrilotri(methylphosphonic acid), caused increase of the particle spacing, and lowering of the blocking temperature by ca. 20 K. The CoFe2O4@TiO2 nanoparticles were sufficiently stable in water, methanol and ethanol. The particles were also investigated by Mössbauer spectroscopy and alternating-current (AC) susceptibility measurements, and their analysis with Vögel–Fulcher and power law. Effect of different particle coating and dipolar interactions on the magnetic properties is discussed. & 2015 Elsevier B.V. All rights reserved.

Keywords: Superparamagnetism Size effect Monodisperse nanocrystals Hydrothermal synthesis Cobalt iron oxide Titania

1. Introduction Cobalt ferrite and magnetite particles attracted considerable attention for their high application potential, e.g. in biomedicine [1], catalysis [2], microwave absorbers [3], magnetic inks and high-density magnetic memories [4]. Cobalt ferrite, as a material with high magnetocrystalline anisotropy, is well suited for the study of superparamagnetism and inter-particle interactions, since its blocking temperature varies in a wide interval up to room temperature for sub-10 nm nanocrystals [5]. However, the present synthesis methods have various drawbacks, such as low crystallinity and broad size distribution (coprecipitation [6]), low batch yield (microemulsion techniques [5]), aggregated particles (co-precipitation with subsequent hydrothermal treatment [7,8], and polyol method [9]), or n

Corresponding author. E-mail addresses: [email protected] (A. Repko), [email protected] (J. Vejpravová), [email protected] (D. Nižňanský). http://dx.doi.org/10.1016/j.jmmm.2015.04.090 0304-8853/& 2015 Elsevier B.V. All rights reserved.

involve complicated procedure with high-boiling-point solvents (thermal decomposition [10]). Therefore, it is desirable to develop a facile and green method with high yield and no toxic side products. The promising method in this direction is the hydrothermal synthesis utilizing fatty acids [11–14]. The main objective of the present work is to further improve the oleate-based hydrothermal method by separating the step of preparation of cobalt–iron oleate solution, leading to narrower size distribution of the prepared nanocrystals, better phase purity, high batch yield and good size control. As the magnetic properties of cobalt ferrite nanoparticles are strongly dependent on its degree of inversion, inter-particle distance and structural and spin disorder [15,16,8], we also addressed the influence of surface modification on their magnetic response. Magnetic properties of the monodisperse particles with different surface modification were further investigated. Since silica coating was already a subject of numerous previous investigations [16,17], we chose titania coating and compared the results to the as-prepared particles with oleic acid coating and those modified by the citric acid.

A. Repko et al. / Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

The use of titanium dioxide is also motivated by its photocatalytic and phosphate-adsorbing properties, and its coupling with iron oxide to achieve re-usability of the catalyst through magnetic separation [18–21]. Yet, it has proven difficulties in obtaining sub-100 nm core–shell particles with TiO2 as a shell [22,23]. Although the larger particle dimension may not be a problem in the field of catalysis, where sufficient response to magnetic separation is a crucial factor, we focused here on the preparation of particles with good dispersibility in water, which may be desirable in other applications where the sub-100 nm size is limiting parameter.

2. Experimental 2.1. Preparation of Co–Fe oleate in pentanol All chemicals were of analytical grade and were used without further purification. Ethanol which we used was the absolute one (99%). First, 66 mmol (2.64 g) of sodium hydroxide was dissolved in 10 ml of water in a 250 ml round-bottom flask. Then, ethanol (20 ml) and oleic acid (68 mmol, 19.21 g) were added to create a transparent sodium oleate solution. A solution of iron(III) nitrate (16 mmol, 6.464 g) and cobalt(II) nitrate (8 mmol, 2.328 g) in water (10 ml) was added to the flask and stirred, while black droplets of metal oleates were formed. Addition of hexane (20 ml) then led to formation of two well-separated phases: upper black organic phase and lower water phase. This mixture was then boiled under reflux condenser for 60 min (with occasional manual stirring) to complete the reaction, i.e.

3Na (oleate) + Fe (NO3 )3 → Fe (oleate)3 + 3NaNO3 2Na (oleate) + Co (NO3 )2 → Co (oleate)2 + 2NaNO3 After cooling, the colorless water phase was removed by Pasteur pipette. The organic phase was washed by adding 20 ml of water, 5 ml of ethanol and 5 ml of hexane, refluxing for 30 min, and removing the water phase. The washing step was done twice. Then, 15 ml of 1-pentanol was added, and the flask was heated open for 30 min to evaporate hexane. The obtained product as a slightly viscous black liquid (Co–Fe oleate in pentanol) was moved to a 40 ml glass vial with teflon cap, with help of another 5 ml of pentanol. The composition of the product (concentration of Co–Fe oleate and the content of pentanol) was estimated from its weight, assuming quantitative yield from Co and Fe salts. The product sufficed for four hydrothermal preparations. 2.2. Hydrothermal preparation of nanoparticles The amount of Co–Fe oleate in pentanol, containing 2 mmol of Co and 4 mmol of Fe (e.g. 8.54 g; contains also 4.3 ml of pentanol), was poured into a 45 ml teflon liner. More pentanol was added to get a total of 20 ml of pentanol. After addition of water (10 ml), the free space was flushed with nitrogen, the liner was enclosed in a stainless steel autoclave (Berghof DAB-2), briefly shaken, and put vertically into oven, pre-heated to 180 °C, for 10 h (sample A). Other samples (B, C) were prepared using 10 ml of pentanol plus 10 ml of 1-octanol (or toluene, respectively), and 5 ml of water, and were treated at 220 °C for 10 h. After heat treatment, the autoclave was left to cool down to room temperature, and the liquid phases were discarded, while the sedimented product (CoFe2O4) was held by magnet. The particles were dispersed in hexane (10 ml), precipitated by ethanol (10 ml), separated by magnet, again dispersed in hexane (10 ml) and precipitated by ethanol (8, 7 and 6 ml for samples A, B and C, respectively). The precipitate isolated by magnet was dispersed in

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hexane (4 ml) and centrifuged at 3000 rpm for 5 min to remove any possible impurities, and the final dispersion of hydrophobic CoFe2O4 nanoparticles (denoted as A-oleic, B-oleic, and C-oleic, respectively) was stored in a 4 ml glass vial with teflon seal for further experiments. Part of the dispersion was dried at room temperature in a flow of air, to estimate the concentration and yield (ca. 550 mg for A and B, and 300 mg for C), and to get solid samples for magnetic measurements. 2.3. Surface modification with citric acid The amount of hexane dispersion containing 100 mg of particles was added to toluene (20 ml) in a 25 ml glass beaker. Then, citric acid (1 mmol, 192 mg) was dissolved in dimethylsulfoxide (5 ml) and added to the toluene dispersion. The mixture was sonicated 5 min and then mechanically stirred for 26 h. After that, the colorless supernatant was discarded, and the sedimented particles were washed twice with ethanol (10 ml) and separated by centrifugation (3000 rpm for 5 and 10 min). The particles were dispersed in water (8 ml), and 0.1 mmol (10 mg) of Na2CO3 in water (2 ml) was added. The water dispersion was sonicated for 5 min and dialyzed in water (1 l) for 48 h. The water was changed after 24 h. The particle dispersion was then centrifuged at 4500 rpm for 5 min to remove larger aggregates (the sediment, making up to 80% of the product, was discarded; the large losses are a trade-off for getting well-dispersed particles with minimal content of organic material). The prepared water-dispersed hydrophilic particles are denoted as A-citric, B-citric, and C-citric. 2.4. Surface modification with TiO2 The amount of hexane dispersion containing 50 mg of particles was added to toluene (20 ml) in a 25 ml glass beaker and the dispersion was mechanically stirred. Titanium isopropoxide (0.5 ml, ca. 1.7 mmol) was added drop-wise by a syringe with needle during 30 s. The dispersion was then sonicated for 5 min and mechanically stirred for 30 min. We assume partial hydrolysis of Ti isopropoxide by air humidity, which was 60% at 20 °C (when the time was prolonged to 1 h, we observed precipitation, therefore the shorter time was chosen). Then we added drop-wise a solution of nitrilotri (methylphosphonic acid) (1 mmol, 0.299 g) in 5 ml of dimethylsulfoxide (the solution was prepared in advance by heating to 50 °C for 2 h). The prepared mixture was sonicated for 1 min and then mechanically stirred for 24 h. The particles were then isolated by centrifugation (3000 rpm, 5 min) and washed twice by acetone (8 and 10 ml) and once by ethanol (10 ml). The sediment was redispersed in water (8 ml) and 80 mg of sodium hydroxide in water (2 ml) was added. The water dispersion was sonicated for 5 min and dialyzed in water (1 l) for 48 h. The water was changed after 24 h. The particle dispersion was then centrifuged at 4500 rpm for 5 min to remove larger aggregates (around 60% of the material, which was discarded). The prepared water dispersion (denoted as A-TiO2, B-TiO2, and C-TiO2, respectively) was stable in water, methanol and ethanol. The solid samples were prepared by drying the water dispersion in the flow of air at room temperature; they had a homogeneous black appearance. 2.5. Characterization Transmission electron microscopy (TEM): A drop of ca. 0.05% dispersion of the particles in cyclohexane (samples A/B/C-oleic) or methanol (A/B/C-citric), or ethanol (A/B/C-TiO2) was dried on carbon-coated copper grid. The particles were studied by Tecnai G2 Spirit (120 kV) in the bright field mode, and the composition was studied by EDX at 120 kV with SUTW-sapphire detector. The volume-weighted average particle size (diameter) was obtained by

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visual measurement of 300 particles for each sample, assuming log-normal distribution. Powder X-ray diffraction (XRD) was carried out on PANalytical X'Pert PRO using Cu Kα radiation, secondary monochromator and PIXcel position-sensitive detector. Calibration of peak position and instrumental width was done using powder LaB6 from NIST. The hexane dispersions A/B/C-oleic were dried on a glass plate and measured in the interval 10–80° with step 0.039° and integration time of 5000 s. The background was subtracted in PANalytical X'Pert HighScore. The peaks in the interval 28–66° were fitted in Gnuplot by Voigt function (convolution of Gaussian and Lorentzian) in reciprocal space:

q=

2 sin θ = λ

h2 + k 2 + l2 , a

1.10 . Γq

Sample Preparation of oleatecoated particles

A B

C

20 ml pentanol, 10 ml H2O, 180 °C 10 ml pentanol, 10 ml octanol, 5 ml H2O, 220 °C 10 ml pentanol, 10 ml toluene, 5 ml H2O, 220 °C

Size (nm) – oleic

TB (K), 7 3 K

TEM

sTEM

XRD Oleic Citric TiO2

6.2 7 1.2

0.20

5.7

179

194

155

9.1 72.1

0.23

8.6

283

295

263

10.4

331

338

309

10.5 7 1.6 0.15

(1)

and the crystallite size (diameter) was obtained from full width at half maximum of q (denoted as Γq) by

d=

Table 1 Summary of preparation and properties of the CoFe2O4 nanoparticles (size and blocking temperatures).

(2)

Peak widths (Γq) were constrained in groups 311-511-422-222 and 440-220-531, 400. The constant in the numerator of (2) was obtained by fitting the peak of Fourier-transformed solid spheres with volume-weighted log-normal size distribution with s¼ 0.18 – in this case, the peak shape almost coincides with Gauss function. Thermogravimetry was performed on SETARAM SETSYS evolution 1750 in the oxygen atmosphere (40 ml/min). Samples (3– 10 mg) were placed into 100 μl alumina crucible and mounted on a Pt/Rh DSC rod. Heating rate was 10 K/min. Magnetic measurements: Temperature dependences of zerofield-cooled (ZFC) and field-cooled (FC) magnetizations (at the field of 10 mT), hysteresis loops (at 10 and 300 K, maximum field of 7 T), and alternating-current (AC) susceptibilities (with amplitude of 0.3 mT and 5 K temperature step) were measured on Quantum Design MPMS7XL (SQUID magnetometer). Solid samples, prepared by drying the corresponding dispersions at room temperature, were put into a gelatine capsule and fastened by a polystyrene bead to avoid rotation of particles when being magnetized. Blocking temperature TB was obtained as the maximum of ZFC curve; more precisely, by fitting a cubic polynomial to the data points in the interval of 740 K around the maximum. A similar analysis (with an interval of 720 K) was applied to the AC susceptibility data and either the maximum of χ′ or inflection point of the χ″ was used. Mössbauer spectroscopy of 57Fe was done on Wissel spectrometer using transmission arrangement and scintillating detector ND-220-M (NaI:Tl þ ). An α-Fe foil was used as a standard, and fitting procedure was done using NORMOS program. Measurements at low temperature (4 K) under magnetic field of 6 T were done in perpendicular arrangement. Around 50 mg of dried sample was used each time. Transmittance data were corrected for saturation by taking a logarithm and shifting to original baseline before fitting.

3. Results and discussion 3.1. Synthesis and characterization of oleic-acid coated CoFe2O4 nanoparticles In this work, nanoparticles of cobalt ferrite were prepared by hydrothermal hydrolysis of mixed cobalt–iron oleate in a mixture of pentanol/octanol/toluene þwater. The details of the preparation and basic parameters of the obtained products are summarized in Table 1. Typical TEM micrographs for the bare particles and those

modified by citric acid and titania are shown in Fig. 1. Compared to our previous work [14,24] and similar work of other authors [11,2], the present procedure does not utilize in situ generated oleates; they were prepared separately and subsequently introduced into the reaction mixture. We found that such approach leads to significantly improved purity of the product, large batch yield, with no other impurity phases as revealed by X-ray diffraction analysis (see Fig. 2). The particles also exhibit better size distribution in comparison to those obtained by previously reported routes based on hydrothermal technique. Also the hydrothermal step is greatly simplified, involving only three components (metal oleate, organic solvent and water), and thus suitable for large-scale industrial application. Preparation of larger particles (up to 10 nm) was facilitated by not only the less-polar organic phase [14], but also higher temperature, which supposedly enhanced growth rate over nucleation, so the seed-mediated growth [24] was not necessary. It should be noted that in both steps (Co–Fe oleate synthesis, and nanoparticle preparation), no unreacted precursors were observed at the end (neither water coloration due to Co/Fe salts nor unreacted oleate), so the Co/Fe ratio is fixed to 0.5 by the composition of the precursor. This assumption was also supported by EDX measurement (Table 2), which agrees with the expectations (considering the measurement uncertainties). The only side-products were the particles of smaller size present in organic phase after the preparation of the sample C-oleic (and to a smaller extent for Boleic). These particles were isolated to prove the absence of unreacted metal oleates; nevertheless, they were then discarded due to their inferior size distribution – they are an inevitable by-product in this type of synthesis, which proceeds by polarity-driven precipitation of the particles during the hydrothermal treatment [14]. It should be emphasized that, compared to other authors, our approach does not use any base to increase the pH (e.g. amine [13,2]); hydrolysis of oleates occurs spontaneously at hydrothermal conditions, avoiding the creation of intermediate hydroxides due to high pH. Somewhat similar high-temperature hydrolysis of metal acetates occurs also in polyol method [9]. Regarding the reproducibility, some researchers have claimed that aging of cobalt–iron oleate influences the outcome of synthesis [25]. Although the cited work makes use of the thermal decomposition approach and solid oleates, we took care to process the prepared oleates (as a pentanol solution) within one week. The oleic acid was also fresh, since the older oleic acid (exhibiting also transient orange coloration during the neutralization by NaOH) led to particles larger by 1–2 nm (see Fig. 3a). 3.2. Surface modification of the CoFe2O4 nanoparticles by citric acid and titania The surface modification of the CoFe2O4 nanoparticles was carried out with the aim of keeping sufficient dispersibility of the

A. Repko et al. / Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

nanoparticles in polar solvents, which extends the range of their potential application. As a consequence of varying coatings, both different inter-particle separation and possibility of chemical modification were investigated. Oleic acid capping of the as-prepared particles provides uniform 2.5 nm separation of the particles, as observed by the TEM. Its amount in the product was estimated by thermogravimetry in oxygen atmosphere (see Fig. 4), and it decreases with increasing particle size. Replacement with smaller carboxylic acid decreases the inter-particle distance; and to get stable water dispersion, it is desirable to have more carboxylic groups, which increase the surface charge. However, small dicarboxylic acids (such as succinic and tartaric) did not yield

145

stable water dispersion, with the exception of dimercaptosuccinic acid [26,27,14], which probably creates S–S links, stabilizing the surface layer. Dimercaptosuccinic acid, which otherwise leads to excellent dispersions with nearly quantitative yield, was not used in the present study, to avoid the influence of sulfur atoms, and the higher content of organic material in the product. The 3-phosphopropionic acid (containing one carboxylic and one phosphonic group) also appeared promising, however, the modified particles in water flocculated during the dialysis. Therefore, we finally chose citric acid (tricarboxylic acid), which provided water dispersion stable for one week. The stability of the dispersion was improved to over a month by dilution with ≥70%

Fig. 1. TEM micrographs of 6, 9 and 10.5 nm CoFe2O4 nanoparticles coated with either oleic acid (up), citric acid (middle) or titanium dioxide (bottom).

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Fig. 4. Thermogravimetry of oleate-coated CoFe2O4 particles in the oxygen atmosphere.

Fig. 2. Powder X-ray diffraction patterns of as-prepared samples (coated with oleic acid) confirm pure CoFe2O4 phase. Crystallite size was obtained by profile fitting in the range 28–66°. Table 2 Co/Fe ratio of CoFe2O4 particles coated by oleic acid and citric acid as determined by EDX. The uncertainty is up to 0.04 (with statistical variation of 0.01). Sample

Oleic

Citric

A B C

0.47 0.47 0.45

0.41 0.44 0.42

methanol, whereas ethanol caused precipitation. Moreover, the citric acid provided similar chemical coupling to the surface as the oleic acid (i.e. carboxylic groups), while decreasing the inter-particle distance to less than 1 nm. All steps were done at room temperature, so any dramatic change of intrinsic crystallo-chemical properties of the CoFe2O4 is not expected. To further check this assumption, we measured EDX (Table 2), which suggests that

the partial leaching of Co2 þ ions from the surface layer occurs during the surface modification. Visual evaluation of TEM micrographs indicates also the size reduction of about 0.3 nm, although the precise number cannot be assessed due to mutual overlaps of the particles. Modification with titanium dioxide was supplemented by the addition of tri-phosphonic acid to obtain water dispersible particles (otherwise only non-dispersible composite precipitate is obtained). Titanium dioxide was present in an amorphous form (as confirmed by the XRD study), and the coating of the ferrite particles was not completely regular (as demonstrated in Fig. 1). To make the titania-coated particles suitable for further measurements (and also as a potential candidate for magnetically separable photocatalyst), the titania should be tightly attached to the nanoparticles. This fact was supported by homogeneous color of the solid material, obtained by drying or by prolonged centrifugation, and also by TEM pictures and EDX analysis (see Table 3). However, the ethanol dispersion exhibited a slow (in the order of weeks) recrystallization and sedimentation of a white powder (see Fig. 3b), which apparently contained Na2Ti4O9 (by XRD: JCPDS 33-1294), and perhaps also crystalline anatase (tetragonal TiO2). The recrystallization was not observed in water dispersions neither after one-month aging at room temperature, nor under hydrothermal conditions (180 °C for 10 h), as was evidenced by powder X-ray analysis which revealed only the familiar Bragg reflections of cobalt ferrite over amorphous background (not shown here). 3.3. Mössbauer spectroscopy Mössbauer spectroscopy at room temperature was used to demonstrate magnetic structure of the samples A-oleic and C-oleic

Fig. 3. TEM micrographs of some irregular products: (a) particles similar to C-oleic, but prepared using a three-year-old oleic acid, with the size of 12.5 nm; (b) sample C-TiO2 aged in ethanol, showing crystallization of large thin plates (apparently Na2Ti4O9).

A. Repko et al. / Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

Table 3 EDX analysis of the CoFe2O4@TiO2 samples (carbon and oxygen were excluded from the sum). Element

Na P Ti Fe Co

C-TiO2

A-TiO2 Atomic (%)

Weight (%)

Atomic (%)

Weight (%)

16 36 30 13 5

9 28 36 19 7

7 41 32 13 6

4 31 37 18 9

Fig. 5. Room temperature Mössbauer spectroscopy of (a) 6 nm and (b) 10.5 nm CoFe2O4 nanoparticles with oleate coating.

in a time window of 10  7 s (excited level of 57Fe at 14.413 keV has a half-life of 98 ns [28]). Fig. 5 shows measurements at room temperature and zero external magnetic field, indicating that sample A-oleic is almost superparamagnetic, just near the blocking temperature, while sample C-oleic is still in the blocked state, although partial relaxation of magnetic moments prevented us from getting reliable information about site occupancy and hyperfine field. Moreover, the signal intensity was quite small, most probably due to inability of recoil-less γ absorption by loosely fixed nanoparticles [14]. Therefore, additional measurements were done at 4 K and in external magnetic field of 0 T and 6 T (see Fig. 6). Fits to the experimental data are summarized in Table 4. Site occupancy, isomer shift (δ), and quadrupole splitting (ΔEQ ) of the measurement at 0 T were set equal to the data at 6 T. Besides one tetrahedral position (denoted as A), it is necessary to assume several octahedral positions with various hyperfine fields, due to statistical (binomial) distribution of Co2 þ and Fe3 þ in neighboring tetrahedral sites, giving rise to asymmetric peak shape [29]. We mimicked this effect by assuming two effective octahedral sites: B1, fitted with ordinary Lorentzian sextet; and B2, fitted with a broad sextet, which takes into account Gaussian distribution of hyperfine fields. Canting angles obtained from hyperfine fields are small, and can be explained by incomplete alignment of magnetic moments due to deviation of the easy axis from the direction of external field [30,31]. However, increased canting angles for the smaller particles (6 nm) suggest additional contribution of spin disorder in that case. This effect can also be related to almost statistical distribution of iron and cobalt in tetrahedral and octahedral sites (i.e. nearly 33% Fe3 þ in A site), whereas sample C shows some tendency towards inverted spinel (theor. 50% Fe3 þ in A site). Deviation from inverted-spinel occupancy also explains larger lattice parameter, as compared to the bulk value (see Fig. 2) [15].

147

3.4. Magnetization measurements The apparent blocking temperatures, TB of the particles, obtained as maxima of the ZFC curves (see Fig. 7), are summarized in the Table 1. As can be seen, modification by citric acid raised the TB by ca. 10 K, whereas covering by TiO2 lowered it by ca. 20 K. The effect of replacement the oleic acid by the citric acid appears to be stronger for smaller particles (15 K vs. 12 K vs. 7 K), probably due to the higher distance-to-size ratio. The slight decrease of Co/Fe ratio during the surface modification (Table 2) can have only a minor influence, as it would lead to decreased blocking temperature, considering the magnetic softness of magnetite/maghemite [24]. Decreased TB would be expected also in the case of size reduction by partial etching. Since the chemical environment is changed by TiO2 coating, and the inter-particle spacing is not completely uniform, the changes of magnetic properties cannot be unambiguously attributed to smaller dipolar interactions, nevertheless, the observed trend (including the citrate-coated particles) strongly supports the prevalent importance of dipolar interactions for the change of magnetic properties. The parameters of magnetization isotherms at 10 K and 300 K are summarized in Table 5. Saturation magnetizations of bare nanoparticles can be estimated from oleate-coated particles by subtraction of organic content as found by thermogravimetry (Fig. 4), which was however not performed on citrate- and TiO2-coated particles due to low amount of the former, and corrosive properties of the phosphorus in the latter. Nevertheless, the value of the magnetization at 7 T, Mmax , for CoFe2O4@TiO2 can be used to estimate the content of magnetic phase in the prepared composite, and amounts to 16% for all samples, which further supports our assumption of homogeneous composition. The decreased coercitive field of citrate-coated particles (mainly for Acitric, which exhibits a constricted hysteresis loop) may arise from several effects. It can be explained by the surface depletion in Co2 þ content (Table 2), since magnetite and maghemite have much lower coercivity; or as a consequence of certain amount of directly touching particles, facilitating their spin–flip. The dominant contribution of the magnetic anisotropy can be estimated inspecting the Mr/Ms ratio. For the larger particles (B and C series), the value is very close to the theoretical value expected for cubic anisotropy (0.8). For the small particles (A series), it decreases down to 0.4–0.6 suggesting that uniaxial anisotropy dominates as expected for CoFe2O4 nanoparticles with size below 5 nm [32]. On the other hand, cubic term is preferred by the crystallite shape (which appears to be an intermediate between octahedral, cubic, and spherical), and also the intrinsic structure could not develop uniaxial anisotropy, which is achieved only by a long-term annealing of CoFe2O4 in external magnetic field at increased temperature [33]. The available data (including AC measurements) do not allow us clearly distinguishing the magnetocrystalline, surface and shape contribution. 3.5. AC susceptibility The relation of magnetic properties with respect to different coating and strength of inter-particle dipolar interactions was finally investigated by AC-susceptibility measurements. The effective blocking temperature related to a frequency-dependent anomaly is the key parameter in determination of the superparamagnetic and related collective properties from the AC-susceptibility data. A typical temperature dependence of the real and imaginary part of the AC-susceptibility is shown in Fig. 8. Therefore, those temperatures (determined as maxima on the real part of the AC susceptibility, χ′, see Fig. 8) were used in the analysis. It would be more appropriate to use inflection point of imaginary

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Fig. 6. Mössbauer spectroscopy of 10.5 nm CoFe2O4 nanoparticles (C-oleic) at 4 K in external field of 0 T and 6 T. Fitted components, from top to bottom, are B2, B1 (octahedral Fe3 þ ), and A (tetrahedral Fe3 þ ). Table 4 Mössbauer parameters of the samples A-oleic (6 nm) and C-oleic (10.5 nm) at 4 K: isomer shift (δ), quadrupole splitting (ΔEQ ), hyperfine field (Bhf), and canting angles (θ) calculated from cosine rule. Fitting assumed one tetrahedral (A) and two octahedral (B1, B2) positions of Fe3 þ , where B2 was fitted with a broad sextet. Sample

A-oleic

C-oleic

Site

A

B1

B2

A

B1

B2

Occupancy (%) δ (mm/s) ΔEQ (mm/s)

35 0.37  0.01

36 0.49 0.03

29 0.49  0.02

40 0.37 0.00

35 0.50 0.01

25 0.50  0.02

Bhf@0T (T) Bhf@6T (T) θ (deg)

50.95 56.44 25

53.97 48.85 30

51.58 46.58 32

51.02 56.68 20

54.79 49.24 21

52.51 47.14 25

susceptibility ( χ″) [34] rather than the maximum of χ′. However, the low amplitude of χ″ and somewhat noisy data hindered us to perform an accurate analysis of χ″. It was done only for A-oleic, whose data had the best quality (see Fig. 8, and also the first line in Table 6). In the first step, we inspected the spin dynamics by applying a simple formula, which relates a shift of the AC-susceptibility maxima Tf per a frequency decade:

δTf =

ΔTf Tf Δ log10f

.

(3)

The corresponding values are 0.05, 0.03 and 0.05 for the A-oleic, Acitric and A-TiO2, respectively, which are about half order lower than the value expected for an ideal superparamagnet with no inter-particle interaction (0.1). However, smaller values of the parameter δTf suggest significance of inter-particle interactions or collective spin-glass-like behavior in the nanoparticle system [35– 37]. In order to investigate more deeply the nature of the collective state, the phenomenological Vögel–Fulcher law [38,39] is usually applied as follows:

−Ea f = f0 exp , k B (Tf − TVF )

(4)

where f is the applied frequency, f0 ¼ 1/τ0 f0 = 1/τ0 is the characteristic attempt frequency, Ea is the activation energy, and TVF is the Vögel–Fulcher temperature. The typical values of the τ0 range from 10  9 to 10  10 s [40], or even down to 10  12 s [41]. In our

Fig. 7. Zero-field-cooled and field-cooled magnetizations for 6 nm (A), 9 nm (B), and 10.5 nm (C) CoFe2O4 nanoparticles with various coatings. Small vertical lines show positions of apparent blocking temperatures, obtained as maxima of the ZFC curves. The y-scale of TiO2-coated samples was magnified three times for clarity.

case, the magnetic moment of the particle has an activation energy Ea = Keff V , taken as a product of effective anisotropy (Keff ) and particle volume (calculated from the XRD size, assuming spherical shape). The XRD size was used as it reflects better the magnetic response compared to the TEM size, as it was evaluated from representative volume of the sample and represents better the magnetic size (magnetically aligned volume of the individual nanoparticle). The temperature TVF is often related to the strength of interparticle interactions. However, there remains a problem of underestimation of parameters in (4) due to narrow experimental window of AC measurements and distribution of relaxation times. Therefore, we decided to choose τ0 ¼ 10  11 s τ0 = 10−11 s, and demonstrate the variation of fitted parameters with τ0 ranging from 10  10 to 10  12 s (Fig. 9, Table 6), since the value of 10  12 s, although somewhat unphysically small, is preferred by the Mössbauer measurement of the sample A-oleic, as illustrated in Fig. 8c. Chosen scale can be justified also by the available thermal energy: all the measured blocking temperatures are above 100 K, so the safe interval for the attempt frequency lies below:

f0 < k B × 100 K/= = 1.3 × 1013 Hz.

(5)

When strong interactions are present in a superparamagnetic system (as is typically the case for dried powders), the dynamics may be better accounted for by critical slowing down as the system approaches a phase transition. One possibility to analyze the critical behavior in this process is to measure the divergence of the relaxation times τ as the temperature is close to the critical temperature Tg , where the phase transition takes place. The conventional result of dynamical scaling relates the relaxation time for

A. Repko et al. / Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

149

Table 5 Parameters of magnetization isotherms of the CoFe2O4 nanoparticles. Instead of true saturation magnetization, we give Mmax at 7 T. Bare magnetizations can be calculated from oleate-coated particles, subtracting the organic layer weight (see Fig. 4). All samples show zero coercivity at 300 K. Sample

A B C

Mr (Am2/kg) @10 K

μ0 Hc (T) @ 10 K

Mmax (Am2/kg)@10 K

Citric

TiO2

Oleic

Citric

TiO2

Oleic

Citric

TiO2

Oleic

Citric

TiO2

Oleic

Citric

TiO2

1.3 1.8 2.0

0.3 1.6 1.9

1.2 1.9 2.1

40 61 65

35 62 66

6.8 10.5 11.2

68 79 80

79 86 82

13.9 14.9 15.0

54 67 69

62 72 70

9.2 11.3 11.6

0.59 0.78 0.81

0.44 0.72 0.81

0.49 0.70 0.74

Table 6 Vögel–Fulcher and power-law parameters obtained from the AC susceptibility measurements, with τ0 ¼ 10  11 s, and with τ0 ¼ 10  10 s (lower index) or τ0 ¼ 10  12 s (upper index). The maxima of χ ′ were used as Tf, except the first line, where inflection points of χ ″ were taken.

A-oleic ( χ ″) A-oleic

Mr/Mmax @ 10 K

Oleic

Fig. 8. Real ( χ ′) and imaginary ( χ ″) parts of the AC susceptibility of the 6 nm CoFe2O4 particles evaluated in the alternating field of 0.3 mT. Temperatures Tf were obtained as maxima of χ ′ and then fitted to Vögel–Fulcher law with two different τ0 (10  9 s and 10  12 s), see inset (c). A point at 300 K (not included in the fit) corresponds to Mössbauer measurement, shown in Fig. 5(a). Marks on χ ″ show the inflection points.

Sample

Mmax (Am2/kg)@300 K

Vögel–Fulcher

Power law

Keff (J/m3)

TVF (K)

zν

Tg (K)

+0.8 5 3.6− 0.7 × 10

13 84− +12

+7 22− 5

14 126− +12

+0.6 3.1− 0.6

×

105

9 108− +9

+4 17− 3

8 150− +7

×

105

7 154− +7

+2 12− 2

5 194− +4

×

105

10 93− +9

+5 19− 4

9 133− +8

A-citric

+0.5 2.4− 0.5

A-TiO2

+0.7 3.1− 0.6

B-oleic

+0.18 5 0.89− 0.17 × 10

9 223− +9

+2.0 11. 2− 1.8

6 275− +5

B-citric

+0.17 0.81− 0.15

8 238− +8

10.

+1.7 4− 1.6

4 287− +4

B-TiO2

+0.20 5 0.00− 0.19 × 10

10 189− +10

+2.5 13. 1− 2.2

7 243− +6

C-oleic

+0.09 0.43− 0.08

105

8 271− +8

9.

+1.5 5− 1.4

4 319− +4

C-citric

+0.09 5 0.42− 0.08 × 10

7 293− +7

+1.4 9. 1− 1.3

4 341− +3

C-TiO2

+0.09 5 0.42− 0.08 × 10

8 252− +8

+1.6 9. 4− 1.4

4 300− +4

×

×

105

the decay of the fluctuations to the spin (or in the case of superparamagnet the superspin) correlation length, ξ, as τ ∝ ξ z with z being the dynamic critical exponent. As ξ diverges with temperature as [T /(T − Tg )]ν , the relaxation time τ can be expressed as [42]

Fig. 9. Frequency dependent blocking temperatures, obtained as maxima of χ ′ in AC measurements, were fitted with Vögel–Fulcher and power law, using τ0 ¼ 10  11 s τ0 = 10−11 s (the shaded area behind Vögel–Fulcher fit represents the range of τ0 from 10  10 to 10  12 s). The obtained parameters are summarized in Table 6.

⎛ Tf ⎞zν ⎟⎟ . 1/f = τ = τ0 ⎜⎜ ⎝ Tf − Tg ⎠

(6)

In our case, the characteristic relaxation time was fixed at 10  11 s, and the same arguments apply as in the VF approach. The result of fitting is presented in Fig. 9 and Table 6. The zν should fit into the fragile regime given by the interval: 5 < zν < 11 proposed in [42]. This is satisfied only for the larger particles, where we can expect stronger interactions. As can be seen, Tg is then close to the ZFC blocking temperature (Table 1). On the other hand, the Vögel–Fulcher law, besides giving useful measure of inter-particle interactions by means of TVF , also provides reasonable values of effective anisotropy, which agree well with the bulk values [43] (factor 1/4 comes from the direction cosines):

Keff =

1 4

× 1.96 × 106 exp ( − 1.90 × 10−5T 2) J/m3

≈ 4.9 × 105 J/m3 for 10 K

(7a)

≈2.3 × 105 J/m3 for 200 K

(7b)

≈0.48 × 105 J/m3 for 350 K

(7c)

as compared to high values, usually obtained from Néel–Arrhenius law [17,44]. However, somewhat lower Keff for sample C may point to inadequacy of VF model for larger particles, overestimating inter-particle interactions (i.e. too high TVF ) on account of magnetocrystalline anisotropy. The values of Keff also appear to be nearly independent of the surface coating, except A-citric (and partially

150

A. Repko et al. / Journal of Magnetism and Magnetic Materials 390 (2015) 142–151

B-citric), where the surface Co2 þ depletion can have similar effect as for Hc . Let us finally conclude that both spin-glass models should be taken with caution as the VF law is valid for weakly interacting systems, while the critical scaling fits to strongly interacting limit. Because our system lacks another sign of a collective superspin glass, e.g. a low-temperature shallow minimum on the FC magnetizations (Fig. 7) [40], its presence is disputable. The utilized models also did not take into account non-negligible temperature dependence of τ0 [41], and mainly of Keff , as indicated by bulk experimental data (7) [43]. Nevertheless, the obtained results confirm the overall trend in decreasing inter-particle interactions with increased spacing. Moreover, our parameters from the Vögel– Fulcher law can be directly compared to the results of Coskun [17], τ0 = 10−12 s): (and who found (using TVF = 89 K Keff = 3.5 × 105 J/m3) for oleate-coated 5.4 nm CoFe2O4, gradually changing to TVF = 63 K (and Keff = 3.8 × 105 J/m3) for CoFe2O4 uniformly coated by a layer of SiO2 up to 14 nm thick.

[7]

[8]

[9]

[10]

[11] [12]

[13]

[14]

4. Conclusions

[15]

Cobalt ferrite nanoparticles of various size and good monodispersity were prepared by a facile hydrothermal method from cobalt–iron oleate, using a simple solvent (pentanol/octanol/toluene and water) and no other auxiliary reagents. Besides original oleic-acid coating, which provided hydrophobicity and uniform 2.5 nm inter-particle spacing, they were surface-modified by citric acid and titanium dioxide, which led to decreased and increased inter-particle distance, respectively. Corresponding changes of the magnetic properties were investigated by magnetization and AC susceptibility measurements, which suggest the significant influence of dipolar interactions on the apparent blocking temperature, while the variation of other factors, such as surface chemistry, crystallinity, Co/Fe ratio, inversion degree, was found to have less significant impact. Interpretation of the magnetic measurements was supplemented by electron microscopy, powder X-ray diffraction analysis and Mössbauer spectroscopy.

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Acknowledgments

[23] [24]

This work was supported by the Grant Agency of the Czech Republic under Project no. P108/10/1250. Magnetization experiments were performed in MLTL (see http://mltl.eu), which is supported within the program of Czech Research Infrastructures (Project no. LM2011025). Electron microscopy at IMC was supported through Grant TACR (TE01020118).

[26]

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