Article Cite This: Langmuir 2018, 34, 622−629
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Aqueous Superparamagnetic Magnetite Dispersions with Ultrahigh Initial Magnetic Susceptibilities Yunping Fei,†,⊥ Muhammad Iqbal,†,⊥,# Seong D. Kong,† Zheng Xue,† Charles P. McFadden,† Jesse L. Guillet,‡ Linda H. Doerrer,‡ Esen E. Alp,§ Wenli Bi,§ Yi Lu,† Chola B. Dandamudi,† Prashant J. Ranganath,† Kevin J. Javier,† Mohsen Ahmadian,∥ Christopher J. Ellison,† and Keith P. Johnston*,† †
McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States § Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States ∥ Advanced Energy Consortium & Bureau of Economic Geology, University of Texas at Austin, Austin, Texas 78758, United States ‡
S Supporting Information *
ABSTRACT: Superparamagnetic nanoparticles with a high initial magnetic susceptibility χo are of great interest in a wide variety of chemical, biomedical, electronic, and subsurface energy applications. In order to achieve the theoretically predicted increase in χo with the cube of the magnetic diameter, new synthetic techniques are needed to control the crystal structure, particularly for magnetite nanoparticles larger than 10 nm. Aqueous magnetite dispersions (Fe3O4) with a χo of 3.3 (dimensionless SI units) at 1.9 vol %, over 3to 5-fold greater than those reported previously, were produced in a one-pot synthesis at 210 °C and ambient pressure via thermal decomposition of Fe(II) acetate in triethylene glycol (TEG). The rapid nucleation and focused growth with an unusually high precursor-to-solvent molar ratio of 1:12 led to primary particles with a volume average diameter of 16 nm and low polydispersity according to TEM. The morphology was a mixture of stoichiometric and substoichiometric magnetite according to X-ray diffraction (XRD) and Mössbauer spectroscopy. The increase in χo with the cube of magnetic diameter as well as a saturation magnetization approaching the theoretical limit may be attributed to the highly crystalline structure and very small nonmagnetic layer (∼1 nm) with disordered spin orientation on the surface.
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θμ0 πMd2Dp3 i dM yz zz χo = jjj = 18kBT k dH { H → 0
INTRODUCTION
Magnetic nanoparticles (NPs) with high initial magnetic susceptibilities are of interest in diverse fields including magnetic separations for water purification,1,2 MRI contrast agents for biomedical imaging,3 targeted drug delivery,4 magnetic hyperthermia treatment, and data storage.5,6 Recently magnetic NPs have been proposed as contrast agents in electromagnetic imaging of subsurface oil and gas reservoirs.7−9 The spatial distribution of the nanoparticle dispersions flowing through a reservoir may be determined from the perturbations they produce in applied electromagnetic fields with low field strengths. Whereas numerous studies have investigated the magnetization of dry superparamagnetic nanoparticles, relatively few have reported initial magnetic susceptibilities χo at low fields (below 13 Oe or 1000 A/m), especially for aqueous dispersions.9−11 For noninteracting particles, the initial magnetic susceptibility (χo) is described by the Langevin equation in the limit where the applied field strength (H) goes to 0,12−14 © 2017 American Chemical Society
(1)
where μ0 is the magnetic permeability in vacuum, Md is the bulk (solid) saturation magnetization, Dp is the magnetic diameter of the particle, θ is the particle volume fraction, kB is the Boltzmann constant, and T is the absolute temperature. At low particle concentrations as reported in this study (80%. 623
DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
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Langmuir Table 1. Magnetic Properties of TEG-Functionalized IONPs for Different Precursor Amounts for a Molar Ratio of Fe(OAc)2]:TEG of 1:12a Sample
Fe(OAc)2 (g)
Concn of Fe3O4 (mg/mL)
Vol % of Fe3O4
Hydrodynamic Size (nm)
A B C D E F
5.4 5.2 3.5 9.0 6.5 3.8
24.4 97.4 21.1 25.1 35.3 47.4
0.465 1.86 0.363 0.478 0.672 0.904
33 26 53 34 54 53
Primary Particle Diameter from TEM (nm)
Calculated Magnetic Diameter (nm)
± ± ± ± ± ±
16.3 18.8 18.2 17.5 19.5 16.9
14.3 17.6 14.9 16.0 17.5 14.6
2.2 3.5 2.1 2.8 3.1 2.6
χo (SI)
χo 2 vol % basis
Sat. Magnetization (emu/g Fe3O4)
Coating on the NPs
0.766 3.30 0.680 0.813 1.341 1.68
3.29 3.56 3.36 3.41 3.99 3.71
76.3 93.8 85.1 92.0 82.8 84.2
TEG TEG TEG Silica Silica Silica
a
The Fe3O4 concentration is from FAAS.
mixture versus time and analyzed using TEM and DLS as shown in Figure S2 and Table S2, respectively. Before the color change at 150 °C, NPs were not yet present. When the reaction reached 210 °C, the small primary particle size of 6.5−8.5 nm and low polydispersity indicated a high nucleation rate and limited growth. An hour later it reached 10−12 nm and then 15−16 nm after 2 h. The rapid nucleation was followed by growth and size focusing during the annealing process at 210 °C. The relatively low polydispersity of the unusually large 15.8 nm primary nanoparticles was favored by the short nucleation time window, during which the color changed. In addition, at high supersaturation, the higher growth rates for the smaller versus larger NPs leads to focusing of the size distribution to a moderate polydispersity.30 Furthermore, the rapid growth rate of primary nuclei suppresses formation of secondary nucleation.29 The larger size may also be attributed to a lower ratio of capping agent TEG-to-Fe(OAc)2 as the capping agent arrests growth in addition to providing steric stabilization against aggregation. In contrast, the lower supersaturation for lower precursor-to-TEG ratios as presented in Figure 1a−c and Table S1 would lead to
As shown by TEM (Figures 1a−c), the diameter of the primary IONPs increased with increasing the precursor-tosolvent ratio. For the two lower molar ratios of Fe(OAc)2-toTEG, the primary particle size was only ∼10.8 nm, comparable to those reported previously at similar precursor:TEG ratios.25,26 For the highest molar ratio of 1:12 the volume average diameter reached 15.8 ± 1.5 nm (Figure 1c), still small enough to be in the superparamagnetic regime. For a range of Fe(OAc)2 initial masses, the primary particle diameter ranged from 14.3 to 17.6 for samples A−C in Table 1 and Figure S4. Furthermore, the hydrodynamic diameters obtained from DLS varied from 26 to 53 nm indicating the formation of small aggregates. The decomposition of mechanism of Fe(OAc)2 in hydrophobic organic solvents goes through black wüstite (FeO) nanoparticles which are oxidized to form magnetite (Fe3O4).21,32,33 In the case of TEG, the mechanism is more complex, as TEG displaces acetate ions from the surface and adds hydroxyl groups that subsequently undergo condensation.25,29 In order to investigate the nucleation and growth stages, sample aliquots were withdrawn from the reaction
Figure 2. Characterization of IONPs coated with silica. (a) Zeta potential of IONPs@SiO2 at different pH’s. Electron microscopy imaging and elemental analyses of IONPs. (b) Bright-field HR-TEM image of IONPs@SiO2 (c) HAADF-STEM image of silica coated IONPs, (d) STEM-EDS mapping of silica coated IONPs and (e−g) elemental mapping of the Fe, O, and Si. 624
DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
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Langmuir
magnetic sextets (Figure 3) to yield hyperfine parameters listed in Table 2 and Table S4. The reproducibility was very good as
generation of a large number of nuclei and greater quenching of growth due to capping with TEG, thus resulting in the observed primary particles no larger than 11 nm.25,26,29 An increase in particle size was also observed with an increase in precursor-to-stabilizer ratio in the synthesis of iron oxide in benzyl ether, whereby the aliphatic amine stabilizing ligand was a mild reducing agent that increased the nucleation rate.31 Additional examples of this precursor-to-stabilizer ratio effect include citrate stabilized precipitation of FeSO432 and coprecipitation of iron chlorides in aqueous media.9 Powder XRD and Mössbauer spectroscopy measurements were performed to elucidate the crystal structure of the IONPs. The powder XRD revealed (Figure 1f) an inverse spinel magnetite crystalline phase as identified with the International Center for Diffraction Data (ICDD) card No. 19-0629. The diffraction pattern was indexed to a cubic spinel Fe3O4 structure, showing (220), (311), (400), (422), (511), and (440) reflections. The relatively sharp scattering peaks given the small particle size indicate a highly crystalline structure. According to the Scherrer equation for the (311) peak, τ=
Figure 3. Mössbauer spectroscopy of silica coated IONPs (Sample D in Table 1). Additional spectra are given in the Supporting Information.
Kλ , K = 0.9, λ = 0.15418 nm, β = 0.0108, θ β cos θ
Table 2. Mössbauer Parameters Determined from the Room Temperature Spectra of Sample D in Figure 4
= 17.7°
the calculated crystalline size was 13.4 nm, which is in good agreement with the primary particle size observed by TEM. To provide electrostatic stabilization, the IONPs were coated with a thin shell of silica via sol−gel processing with TEOS. The silica shell helps prevent oxidation and provides a versatile platform for further functionalization.33 The increase in the magnitude of the zeta potential of silica coated IONPs with pH in Figure 2a followed the expected behavior as silanol groups were deprotonated.34 A pH of 7 was chosen for studies of the silica-coated magnetite, such that the highly negative zeta potential provided strong electrostatic repulsion. The hydrodynamic diameter of the silica-coated particles typically changed less than 10% after storage for more than one month in deionized water. The approximate thickness of the silica layer determined by TEM was only ∼1−2 nm (Figure 2b, c) such that the hydrodynamic diameter did not change significantly (Table 1 and Figure S4). The complementary suite of electron microscopy techniques and associated spectroscopic methods provide valuable insight into the core−shell structure of the silica coated IONPs. STEM-EDX mapping revealed the Fe-rich core with a Si-rich shell while O was present in both the core and shell with greater prevalence in the core (Figure 2b−f). Crystalline Structure Characterization. It is known that the bulk magnetite (Fe3O4) and maghemite (γ-Fe2O3) have nearly identical spinel structures with only about 1% difference in the cubic lattice constant, which is not discernible from the XRD patterns.35 Therefore, room temperature Mössbauer spectroscopy measurements were performed to further distinguish between magnetite (Fe3O4), substoichiometric magnetite, and maghemite (γ-Fe2O3). The narrower sextet with the smaller isomer shift of 0.34 mm/s and the larger hyperfine field of 49.4 T was associated with the high spin Fe3+ ions occupying the tetrahedral A sites in the inverse spinel structure of magnetite. The broader sextet with the higher isomer shift of 0.55 mm/s and lower hyperfine filed of 45.3 T corresponded to octahedral B sites occupied by Fe3+ and Fe2+ ions.36 The spectra reported here were fit to two discrete
Site A (tetrahedral Fe3+) B (octahedral Fe3+, Fe2+) a
Isomer shift (mm/s)a
Hyperfine field (T)
Area (%)
0.34 0.55
49.4 45.3
56 44
The isomer shift was calibrated to α-Fe foil.
indicated in the additional data for the other five samples provided in Table S4 and Figure S3. The sample is, therefore, identified as a mixture of Fe3O4 and substoichiometric magnetite. The lower isomer shift of sextet for site B (0.55 mm/s) of Sample D in the present study of magnetite as compared to the previously reported bulk value for stoichiometric Fe3O4 (0.66 mm/s)37 could imply that the samples are nonstoichiometric with a lower content of Fe2+ ions than stoichiometric Fe3O4, or be attributed to the small size of synthesized nanoparticles.38,39 The line broadening for room temperature spectra can be attributed to the size distribution of the nanoparticles. Magnetic Properties of Silica Coated IONPs. The magnetic properties of the silica coated IONPs were measured by vibrating sample magnetometry (VSM) (Figure 4) where the sample was vibrated in a direction perpendicular to the magnetic field. The aqueous IONP dispersions were examined under both high (−10 kOe to +10 kOe) and low magnetic field to investigate the saturation magnetization and initial susceptibility, respectively. For the lowest precursor:TEG ratio of 1:33 and a diameter of 10.8 nm, the saturation magnetization was only 45 emu/g Fe3O4 and the normalized χo was 0.23 at 2 vol % (Table S1). At an unusually high molar ratio of Fe(OAc)2:TEG of 1:12 the average saturation magnetization was 85.8 ± 6.5 emu/g Fe3O4, approaching the theoretical value of bulk magnetite (92.7 emu/g Fe3O4) in contrast to the much smaller value for maghemite (γ-Fe2O3) of 70 emu/g.40 Thus, the crystal structure indicated by the saturation magnetization was consistent with the Mössbauer results. A relatively uniform crystallinity over the entire cross section of the nanoparticles was further confirmed by HR-TEM with a measured d-spacing 625
DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
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The magnetization for an extremely low magnetic field sweep from −3 to +3 Oe on sample D at a concentration of 0.478 vol % (Figure 4b) reached 8.0 emu/g Fe3O4 for the high 1:12 Fe(OAc)2:TEG ratio. It is remarkable that the magnetization was about one-tenth of the saturation value under such a weak external field. From the slope of the magnetization curve and the correction for the volume fraction of 0.00478, χo was 0.813 (dimensionless SI units). In Table 1, a plot of χo versus particle volume fraction exhibited linear behavior, consistent with a previous study in this moderate concentration range,15 indicating the absence of dipolar interactions, which appear at much larger concentrations. The linear correlation shown in Figure 4c validates our extrapolation of susceptibility data measured at different concentrations to the 2 vol % basis for comparison with previous studies. For all of the samples in Table 1, the average χo, normalized to 2 vol %, was 3.55 ± 0.26 (dimensionless). As the Fe(OAc)2:TEG ratio increased from 1:22 to 1:11 (Table S1), the increase in χo from ∼1 to ∼3.5 is in semiquantitative agreement with the predicted value of (15.8 nm/10.8 nm)3 = 3.1 according to the Langevin equation (eq 1). The ratio of 1:12 was near the optimum for χo as for a ratio of 1:6 the particle size increased above 25 nm leading to ferromagnetic behavior above the size limit for superparamagnetism.14 The saturation magnetization and χo did not change significantly with the cluster hydrodynamic diameter (Table 1) indicating a limited influence of dipolar interactions, perhaps due to the electrostatic stabilization by silica coating as well as the low concentration. As shown in Table 3, χo is >6-fold higher than values obtained at similar concentrations for two of the best commercial ferrofluids, and over 3−5-fold higher than for previously reported magnetite dispersions at 2 vol %.11,15 Given the various characterization techniques for the IONPs, we attribute the high χo to (i) the large diameters (15.8 ± 1.5 nm by TEM) of primary particles within the superparamagnetic regime, (ii) highly crystalline substoichiometric and magnetite phase as evidenced by XRD, Mössbauer spectroscopy, and HR-TEM, and (iii) the small nonmagnetic layer given the similarities in Dp versus the diameters from TEM (Table 1), within experimental uncertainty. The silica coating provides high colloidal stability, yet the magnetic susceptibility per g of Fe3O4 does not decrease upon adding the silica layer (Table 1), as would often be the case with organic surfactants.44 To explore application of these core−shell IONPs for subsurface electromagnetic imaging of oil reservoirs, the silica coated IONPs were functionalized with amine using 3triethoxysilylpropylamine (APTES) and then grafted with a random copolymer of poly(AMPS-co-AA) (poly(2-acrylamido3-methylpropanesulfonate-co-acrylic acid)) by amide bond formation with the carboxylate AA groups using an approach with characterization reported previously.45,46 The polymer grafted IONPs exhibited unprecedented stability in high
Figure 4. Magnetization curve of IONPs dispersion at (a) 10 kOe and (b) 3 Oe by VSM (Sample D in Table 1) for Fe(OAc)2:TEG of 1:12 at a concentration of 0.478 vol % (c) linear correlation of initial susceptibility versus IONPs concentration.
of 4.8 Å (Figure 1d), corresponding to the (111) planes of the Fe3O4 single crystal with cubic inverse spinel structure.41−43 The magnetic diameters (Table 1), calculated using the method from Chantrell et al. by assuming a log-normal size distribution for the ferrofluid12 as summarized recently,9 are comparable to the sizes measured by TEM indicating a very small magnetic dead layer on the surface (70% of cluster volume. After silica coating the hydrodynamic diameter changed less than 10% in one month of storage at pH above 7.5 Zeta Potential Measurements. Electrophoretic mobility was measured with a Brookhaven Zeta PALS instrument at a 15° scattering angle at room temperature in 10 mM KCl solution (Debye length = 3 nm). Ten measurements with 30 electrode cycles for each run were performed and averaged. The zeta potential was obtained from the electrophoretic mobility using the Smoluchowski model. X-ray Diffraction (XRD). X-ray diffraction was obtained on a Rigaku R-Axis Spider diffractometer with an image plate detector using Cu Kα radiation (λ = 1.54 Å). Samples were prepared on nylon loops and scanned for 10 min rotated at a rate of 10°/min. Transmission Electron Microscopy (TEM). TEM images were acquired on an FEI Tecnai Spirit Bio Twin TEM operated at 80 kV. The samples were prepared by drop casting a dilute aqueous suspension of IONPs onto 400 mesh Formvar coated copper grids, blotting, and drying in air. Image analysis was done with ImageJ from 200 primary particles at various spots on the grid, being careful to avoid cases where primary particles on top of each other produced larger sizes that were not included. The diameter was averaged on a volume basis. High-resolution transmission microscopy (HR-TEM) and HAADF-STEM images were obtained on a field emission JEOL 2010F TEM operated at 200 kV. Scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDX) mapping was done on a JEOL 2010F equipped
CONCLUSIONS Aqueous magnetite dispersions (Fe3O4) with an average χo of 3.3 at 1.9 vol %, over 3- to 5-fold greater than those reported previously, were synthesized at 210 °C and ambient pressure via thermal decomposition of Fe(II) acetate in triethylene glycol (TEG). For the unusually high precursor-to-solvent ratio of 1:12, the high supersaturation produced rapid nucleation and focused growth leading to relatively uniform large primary particles with an average diameter of 15.8 nm and high crystallinity, which enabled the large χo. High crystallinity was present throughout the nanoparticle cross section with a very small nonmagnetic layer on the surface based on the following: (1) the similarity in the geometric (TEM) and magnetic diameters Dp, (2) direct observation by HR-TEM, (3) the saturation magnetization, which approached the theoretical value for bulk magnetite, and (4) the increase in χo with Dp3. From Mössbauer spectroscopy, a substoichiometric phase was identified along with magnetite. The silica shells on the nanoparticle surfaces provided colloidal stabilization and prevented loss in χo from spin disorder, as well as a versatile platform for further functionalization. Aqueous dispersions of superparamagnetic nanoparticles with extremely high values of χo and saturation magnetization are of interest in a wide variety of practical applications.
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MATERIALS AND METHODS
Materials. Fe(OAc)2 was purchased from Alpha Aesar and stored under argon. Triethylene glycol (C6H14O4) and tetraethyl orthosilicate (TEOS) [Si(OC2H6)4] were purchased from Sigma-Aldrich, USA, and used without further purification. Synthesis of TEG-Coated Iron Oxide Nanoclusters. IONPs were synthesized by modifying the procedure of Günay et al.26 IONPs were prepared by the thermal decomposition of Fe(OAc)2 (light brown powder) in the presence of TEG at 210 °C in an inert argon atmosphere with an overhead mechanical stirrer (15 mm × 46 mm PTFE blade, PTFE stirrer bearing). The mechanical stirrer was used to avoid aggregation that could take place in the case of a submerged magnetic stir bar. In a typical synthesis, 5 g (28.8 mM) of iron(II) acetate were mixed with 50 mL (366 mM) of TEG (molar ratio of 1:12) in a 250 mL three-neck round-bottom flask equipped with a condenser, mechanical stirrer, and heating mantle. The mixture was purged with argon gas for at least 20 min before ramping the temperature up to 210 °C in 20 min and then refluxing at 210 °C for 2 h. After cooling to room temperature, the IONPs were purified by copious washing with a mixture of DI water/ethanol/ethyl acetate followed by centrifugation to separate particles from solution. The procedure was repeated at least three times to remove excess TEG. The pellets were redispersed in 1 wt % nitric acid to aid dispersion well below the isoelectric point, immediately precipitated with centrifugation at 10 000 rpm, and purified as described immediately above. The loosely agglomerated pellets were redispersed with DI water to form a stable colloidal dispersion at pH 3.5 (from residual acid) at a final concentration of ∼10 mg/mL. Silica Coating of IONPs. The silica coating was performed by modifying the procedure of Kralj et al.33 TEG coated IONPs were dispersed in pH 12 alkaline media at a concentration of 0.1 wt %. TEOS was diluted with ethanol at a volumetric ratio of 1:9, and the mixture was added to the IONPs dispersion over 3.5 h by a syringe pump during vigorous stirring. The mass ratio of IONPs to TEOS was 0.05. The particles were collected with a strong magnet and washed 627
DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
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Energy. It was also supported by the Department of Energy (DOE) Center for Subsurface Energy Security and the Welch Foundation (F-1319). We thank Karalee Jarvis, at Texas Materials Institute (TMI), for assistance with HRTEM and STEM-EDX.
with an Oxford X-MaxN 80TLE solid state detector. The STEM probe size was ∼1 nm, and drift correction was performed during the mapping using AutoLock in Oxford’s Aztec software while the EDS maps were obtained with acquisition times of more than 1 min. The microscope was operated at 200 kV accelerating voltage. Vibrating Sample Magnetometer (VSM). The initial susceptibility of magnetic fluid samples was measured with a VSM (Microsense model EZ7) under a magnetic field with intensity swept between −3 to +3 Oe at a rate of 0.15 Oe/s at ambient temperature. Saturation magnetization was measured under a −10 kOe to +10 kOe range swept at a rate of 250 Oe/ s. For each measurement, 40 μL of magnetic particle solution were loaded into cylindrical crucibles and measured at 300 K at a DC field with a vibration frequency of 75 Hz. Values reported do not include normalization by the demagnetization factor due to the shape of the sample holder. Mössbauer Spectroscopy. Room temperature Mössbauer spectroscopy measurements were performed with the facilities at both Argonne National Lab and MIT. The Argonne National Lab facilities had a 57Co source in Rh matrix mounted on a constant acceleration drive. The spectrometer was calibrated using an iron foil. A VORTEX detector with 150 eV resolution was used to discriminate the 14.4 keV radiation. Data analysis was performed using in-house software. Facilities at MIT had a 57Co source in the Rh matrix mounted on a constant acceleration drive. 57Fe-enriched metallic iron foil was used for velocity calibration. Fits of the data were calculated by the WMOSS plot and fit program, version 2.5. All samples of IONPs were dried, and the sample powders were placed between two layers of Kapton tape. All isomer shifts are given with respect to metallic α-Fe at room temperature.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03702. Synthesis results for different precursor ratios, effect of time on particle size, Mossbauer parameters of various samples, and TEM of volume distributions (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Linda H. Doerrer: 0000-0002-2437-6374 Christopher J. Ellison: 0000-0002-0393-2941 Keith P. Johnston: 0000-0002-0915-1337 Present Address #
Michelman Inc., 9080 Shell Road, Cincinnati, OH 45040.
Author Contributions ⊥
Y.F. and M.I. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Advanced Energy Consortium: http://www.beg.utexas.edu/aec/. Member companies include BHP Billiton, ExxonMobil, Repsol, Sandia National Laboratories, Shell, Total, and US Department of 628
DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
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DOI: 10.1021/acs.langmuir.7b03702 Langmuir 2018, 34, 622−629
