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A General Approach to the Synthesis and Detailed Characterization of Magnetic Ferrite Nanocubes Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

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Yaolin Xu, Jennifer Sherwood, Ying Qin, Robert A. Holler and Yuping Bao

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A general approach to the synthesis and detailed characterization of magnetic ferrite nanocubes were reported, where the nanocubes were synthesized by the thermal decomposition of metal-oleate complexes following a step-heating method. The doping ions were introduced during the precursor preparation by forming M2+/Fe3+ oleate mixed complex (M2+ = Fe2+, Mn2+, Zn2+, Cu2+, Ca2+, and Mg2+). The mechanistic studies showed that the presence of sodium oleate in combination with step-heating was critical for the formation of the cubic shapes for the doped magnetic ferrites. The nanocubes were extensively characterized, including morphology and crytsal structure by advanced transmission electron microscopy, doping level and distribution by energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy, cation distribution within the spinel structures by Fourier transform infrared and Raman spectroscopy, and magnetic properties by alternating gradient magnetometer at room temperature.

Introduction Magnetic ferrite nanoparticles have been explored for 1-3 numerous applications, such as catalysts for water splitting, 4-6 nanomedicine, and as matrices for matrix-assisted laser 7 desorption/ionization (MALDI) analysis. The fundamental magnetic properties are critically important to define their potential applications, such as hard magnets for data storage and soft magnets for magnetic switches. The properties of the 8 magnetic ferrite nanoparticles can be tuned by their size, 9 10-13 14, 15 16 17, 18 surface, shape, assembly, coupling, and doping. Recently, it has been reported that non-spherical iron oxide nanoparticles (NPs) could greatly improve their usefulness for biomedical applications.6, 19-21 Of particular importance, iron oxide nanocubes demonstrate extremely high r2 relaxivity as 20 negative magnetic resonance imaging (MRI) contrast agents and a high value of the specific absorption rate necessary for 21 hyperthermia cancer treatment. Further, doping of other inorganic ions offers a great way to adjust the physical properties of magnetic ferrite nanoparticles and their usefulness in applications. For example, the doped ions 22 directly affected the relaxivities of the nanoparticles. The energy gaps are directly related to the doped ions of magnetic

ferrite nanoparticles, which subsequently affect their 1 effectiveness as photocatalysts. Therefore, a general approach to the synthesis of cubic shaped magnetic ferrite nanoparticles is of great technical importance for various applications. Several strategies have been applied to synthesize magnetic ferrite nanocubes with a primary focus on iron oxide nanoparticles. For example, about 80 nm iron oxide nanocubes were synthesized by the thermal decomposition of Fe(acac)3 in the presence of oleic acid at 290 °C as a result of fast growth along 〈111〉 direction. A mixture of nanocubes (84%) and nanospheres (16%) was produced by heating Fe(acac)3 in the presence of decanoic acid at 200 °C for 2.5 h followed by reflux (258 °C) for 1 h , where the shape control was achieved by the selective interaction of surfactant molecules (decanoic acid) 23 with preferential growth facet. Some iron oxide nanocubes were observed when decomposing iron–oleate over 300 °C, where either the heating rate or additional sodium oleate was believed to facilitate the cube formation. Manganese ferrite nanocubes were synthesized by the thermal decomposition of a mixture of Fe(acac)3 and Mn(acac)2 with oleic acid and oleylamine as capping ligands, where the shape control was achieved by a step-heating method (200 °C, 1 h and 300 °C, 1 24 h). Cubic-shaped Mn and Zn ferrite nanoparticles were reported through either a seed-mediated method by thermal decomposition of Fe(acac)3 and Mn(acac)3 or Zn(acac)3 in the 25, 26 presence of oleic acid and oleylamine, or a hydrothermal method by autoclaving metal salts and NaOH in Telfon 27, 28 reactors at 180 °C. One report of copper ferrite nanocubes was found by co-precipitating CuCl2 and FeCl3 in poly-ethylene glycol at 80 °C for 6 h (pH=12), yielding a mixture of spheres

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and cubes.29 Cobalt ferrite nanocubes were reported by thermal decomposition of iron oleate, or Fe(acac)3 and Co(acac)3 with oleic acid following a step-heating method (200 °C, 1 h and 320 °C, 40 min), where heating rate and growth 30 temperature were responsible for shape control. To the best of our knowledge, calcium and magnesium ferrite nanocubes have not been reported. In addition, no general approach is available for the synthesis of various ferrite nanocubes. All of these previous reports suggested the importance of reaction temperature and capping ligands to the nanocube formation. In this paper, we report a facile approach for the synthesis of magnetic ferrite nanoparticles (M2+ = Fe2+, Mn2+, Zn2+, Cu2+, Ca2+, and Mg2+) by the thermal decomposition of a mixture of metal-oleate complexes. The doping ions were introduced during the precursor preparation by forming M2+/Fe3+-oleate mixed complex (M2+ = Fe2+, Mn2+, Zn2+, Cu2+, Ca2+, and Mg2+). The combination of step-heating and sodium oleate is responsible for the formation of cubic shapes. The thermogravimetric analysis (TGA) of the metal-complexes provided important information about the temperature selections of step-heating, one of the key parameters for the nanocube formation. The magnetic ferrites were extensively characterized, including morphology and crystal structures by advanced transmission electron microscopy (TEM), doping level and cation distribution by energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS), cation distribution within the spinel structures by Fourier transform infrared (FTIR) and Raman spectroscopy, and magnetic properties by alternating gradient magnetometer (AGM) at room temperature.

Materials and methods Materials. All the chemical reagents were commercially purchased and used without further purification. These reagents included ferric chloride (FeCl3, ACROS, 98%), ferrous chloride tetrahydrate (FeCl2·4H2O, J. T. Baker, 99%), manganese chloride tetrahydrate (MnCl2·4H2O, Fisher, 99 %), zinc chloride (ZnCl2, Fisher, 99%), calcium chloride dehydrate (CaCl2·2H2O, Fisher, 99%), copper chloride (CuCl2, Sigma-Aldrich, 99%), magnesium chloride hexahydrate (MgCl2·6H2O, BDH, 99%), sodium oleate (TCI, 95%), oleic acid (OA, Fisher, 95%), trioctylphosphine oxide (TOPO, SigmaAldrich, 90%), and 1-octadecene (Sigma-Aldrich, 90%). Preparation of the mixed M2+/Fe3+ oleate precursor. Six types of 2+ 3+ 2+ M /Fe oleate precursor were prepared with a molar ratio of M 3+ to Fe ions at 1:2. In brief, MCl2 (13.4 mmol) and FeCl3 (4.35 g, 26.8 mmol) were mixed with sodium oleate (36.5 g) in a solvent mixture (hexane-140 mL, ethanol-80 mL, and de-ionized water-60 mL) at 65 °C for four hours. The entire process was conducted under inert gas protection, and each reaction solvent was degassed for 20 min before usage. After phase separation, the organic phase containing 2+ 3+ M –Fe oleate precursor was washed with degassed, de-ionized water, and subsequently sealed in degassed containers. The obtained pastes were used as precursors for the synthesis of various ferrite nanocubes.

Synthesis of MFe2O4 ferrite nanocubes. MFe2O4 ferrite nanocubes were synthesized by the thermal decomposition of the mixed 2+ 3+ M /Fe -oleate precursors via a step-heating process (250 °C, 20 2+ 3+ min and 320 °C, varies). Specifically, M /Fe -oleate precursor (6.2 mmol) was heated to 250 °C in 10 mL of 1-octadecene in the presence of capping molecules (TOPO, 0.38 mmol and OA, 0.54 mmol). After 20 min, the reaction temperature was raised to 320 °C. Depending on the doping ions, the reaction time at 320 °C varied in order to achieve uniform MFe2O4 nanocubes, such as Fe3O4-30 min; MnFe2O4–30 min; ZnFe2O4–60 min; CuFe2O4–90 min; CaFe2O4–120 min; and MgFe2O4–120 min. The reaction was then cooled down to room temperature and the nanocubes were collected by centrifugation for analysis. Characterization. The morphology, structure, and chemical distribution of these MFe2O4 ferrite nanocubes were studied using a FEI Tecnai F-20 TEM, including bright field TEM, high resolution TEM, selected area electron diffraction (SAED), energy dispersive xray spectroscopy (EDX) line-scan, and high angle annular dark-field 2+ 3+ (HAADF) imaging. The metal (M , Fe ) and oxygen valance states of MFe2O4 nanoparticles were studied using x-ray photoelectron spectroscopy (XPS) on a Kratos AXIS 165 Multitechnique Electron Spectrometer, equipped with a monochromatic x-ray source (Al, hv = 1486.6 eV). The spinel structures and doping location of MFe2O4 nanocubes were studied by FTIR and Raman spectroscopy. The FTIR spectra were collected on a PerkinElmer Spectrum 100 spectrometer (Bucks, UK), equipped with an attenuated total reflectance (ATR) cell by accumulation of 4 scans, with a resolution -1 of 2 cm . The Raman spectra of MFe2O4 nanocubes were collected via a HORIBA Jobin Yvon system, equipped with 563 nm laser source at 10 mW laser power and 20X objective. The magnetic moment versus applied magnetic field (M-H) curves of MFe2O4 nanocubes were recorded by a Princeton alternating 65 gradient field magnetometer (AGM) at room temperature.

Results and discussion The preparation and characterization of MFe2O4 nanocubes. The (M2+ = Fe2+, Mn2+, Zn2+, Cu2+, Ca2+, and Mg2+) nanocubes were synthesized by the thermal decomposition of metal-oleate complexes via a step-heating process. The doping ions were introduced during precursor preparation with a 1:2 molar ratio of 2+ 3+ M and Fe ions in order to achieve the chemical composition of MFe2O4 nanocubes. Compared to the synthesis of our other shaped iron oxide nanoparticles,11, 31, 32 the key design for the synthesis of cubic nanoparticles is the step-heating (250 °C – 20 min, 320 °C – 30 min). Our thermogravimetric analysis (TGA, Fig. S1†) of metaloleate complexes suggested that all of ligands started decomposing around 250 °C, where the iron oxide nucleation initiated, similar to the thermal behaviors of pure iron-oleate suggested by Hyeon.33 The design of slow decomposition at 250 °C allowed for the formation of cubic ferrite structures, and then the heating at 320 °C led to the nanocube growth on the pre-formed seeds. Using magnetite as a model system, the reaction conditions were varied

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Fig. 1 TEM and HRTEM images (inset), electron diffraction pattern, nanoparticle compositional profiles, and HAADF images of Fe3O4 (a, b, none, and none), MnFe2O4 (c, f, e, and d), ZnFe2O4 (g, j, i, and h), CuFe2O4 (k, n, m, and l), CaFe2O4 (o, r, q, and q), and MgFe2O4 (s, v, u, and t). Fig. 1a, c, g, k, o, and s showed the TEM images of as-prepared MFe2O4 ferrite nanocubes (M2+ = Fe2+, Mn2+, Zn2+, Cu2+, Ca2+, and 2+ Mg ) from typical reactions. Despite a similar step-heating process, the sizes of the nanocubes varied depending on the divalent ions, such as 12 nm for Fe3O4 and MnFe2O4, 17 nm for ZnFe2O4, 16 nm for CuFe2O4, 13 nm for CaFe2O4, and 17 nm for MgFe2O4. We believe that the size variation resulted from the size and thermal behavior differences in doping ions. For example, the 2+ 3+ 2+ 3+ nucleation window for Zn -Fe oleate and Mg -Fe oleate were from around 250 °C to about 300 °C, which were longer than others (from around 250 °C close to 270 °C) (Fig. S1†). The longer nucleation window suggested a slower decomposition rate and subsequently lower nucleus concentration, leading to larger nanocubes. The clear lattice fringes of the HRTEM images suggest

single crystallinity of all the MFe2O4 ferrite nanocubes. The lattice spacing between lattice fringes (labelled on HRTEM images) of the analyzed nanocubes matched well with their corresponding planes of MFe2O4 crystal structures, including, 2.97 Å for Fe3O4 (220) plane, 2.13 Å for MnFe2O4 (400) plane, 2.97 Å for ZnFe2O4, (220) plane, 2.10 Å for CuFe2O4 (400) plane, 2.12 Å for CaFe2O4 (420) plane, and 34 2.09 Å for MgFe2O4 (400) plane. The select area electron diffraction (SAED) patterns of all the samples showed typical diffraction rings (220), (311), (400), (511), and (440) of spinel structures, as shown in Fig. 1b, f, j, n, r, and v. The ring patterns of SAED, instead of individual dots, were resulted from the random orientation of the individual crystalline nanocubes within the selected areas.

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level, cation distribution, and effects on the chemical and physical properties of the original structures. Consequently, magnetic ferrite nanocubes were extensively characterized using a series of complementary characterization techniques.

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to investigate the roles of step-heating and NaOA (Fig. S2-S5†). The mechanistic studies suggested that the presence of sodium oleate in combination with step-heating was critical for the formation of nanocubes. Several critical questions need to be answered for a doping system, such as conformation of effective doping, doping

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Fe 2p 724 eV

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Cu to Fe, 1: 2.12 for Ca to Fe, and 1: 2.89 for Mg to Fe. The doping level for all types of doping were lower than the anticipated level of 1 to 2, in particular for the Mg doping. The lower doping level likely resulted from the ionic radius variation of the doped ions. For 2+ example, the ionic radius of Mg octahedral coordination is about 2+ 86 pm, which is much larger than that the ionic radius of Fe 2+ octahedral coordination of 75 pm. The co-existence of the M and 3+ Fe ions in the ferrite nanocubes were further evaluated with XPS analysis, which confirmed the valence states of the metal ions in the nanocubes.

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Fig. 2 XPS spectra of MFe2O4 nanoparticles: Fe3O4 (a-Fe2p, b-O1s), MnFe2O4 (c-Mn2p, d- f-Fe2p), ZnFe2O4 (e-Zn2p, f-Fe2p), CuFe2O4 (g-Ca2p, hFe2p), CaFe2O4 (i-Ca2p, j-Fe2p), MgFe2O4 (k-Mg1s, l-Fe2p). The core-level XPS spectra of M 2p3/2 and M 2p1/2 were shown in Fig. 2. The O 1s core level XPS spectra of the ferrite nanocubes were all similar with a single peak at 530 eV; therefore, only a representative spectrum of Fe3O4 was shown (Fig. 2b). The Fe 2p3/2 and Fe 2p1/2 core level peaks of Fe3O4 nanocubes were clearly observed at 711 and 724 eV (Fig. 2a). The absence of Fe 2p3/2 satellite peak at 718 eV was an indicator of magnetite (Fe3O4) formation, rather than maghemite (γ-Fe2O3).35, 36 The shoulder peak 2+ 37 at 709 eV also suggested the presence of Fe ion. In contrast, a satellite peak at 718 eV in Fe 2p regions were evident for the XPS spectra of all other MFe2O4 nanocubes (Fig. 2d, f, h, j and l),

2+

suggesting that the Fe was below the detection limit. The successful doping of MFe2O4 nanocubes was confirmed by the characteristic XPS spectra of the corresponding doped ions, including: Mn 2p3/2 and Mn 2p1/2 core level peaks at 641.2 eV and 653.1 eV (Fig. 2c),38 Zn 2p3/2 and Zn 2p1/2 core level peaks at 1022.2 39 eV and 1044.2 eV (Fig. 2e), Cu 2p3/2 and Cu 2p1/2 core level peaks 40 at 931.8 and 955.5 eV (Fig. 2g), Ca 2p3/2 and Ca 2p1/2 core level peaks at 345.9 eV and 349.7 eV (Fig. 2i),41 and 1303.7 eV for Mg 1s 42 (Fig. 2k). Vibrational spectrum of a spinel structure provides useful information about crystal structure, composition, and internal

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The doping effectiveness and cation distribution within the asprepared MFe2O4 nanocubes were studied by high angle annular dark-field (HAADF) imaging and corresponding energy dispersive xray spectroscopy (EDX) line-scan of that nanocube (Fig. 1 d and e, h and i, l and m, p and q, and t and u). The doped ions and Fe elements were homogeneously distributed throughout individual MFe2O4 nanocubes with a decreasing trend from nanocube center to edges. The intensity ratio between M and Fe elements from the EDX line-scan profiles revealed that the compositional ratios of M to Fe were roughly 1: 2.26 for Mn to Fe, 1: 2.17 for Zn to Fe, 1: 2.1 for

Intensity (a.u.)

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Generally, the IR spectra of all spinel ferrites exhibit two major broad metal-oxygen bands. In contrast to the octahedral sites, the metal-oxygen (M-O) bonds in the tetrahedral sites have a lower bond length with expected higher stretching frequency. The intrinsic stretching vibrations of the metal ions at the tetrahedral -1 sites are normally observed around 600 cm , which correspond to the vibration of the entire tetrahedral unit, while the octahedral-1 metal stretching is normally around 400 cm with a much weaker 46 intensity. The octahedral units are linked together, which makes the entire unit vibration limited. Instead, asymmetric and symmetric stretching of M-O bonds were used to explain the -1 absorption around 400 cm . Because of broken symmetry near nanoparticle surfaces, surface defects and oxidation, sometimes, lead to additional M-O vibration around 700 cm -1.47 Fig. 3 shows the FTIR spectra of MFe2O4 nanocubes in the range of 400 - 800 cm 1 . All of the magnetic ferrite nanocubes exhibited the two characteristic bands of the spinel structure with slight variation in the high frequency band position and connecting shoulder peak. The difference in the low frequency peaks was difficult to differentiate because of the low intensities. The high frequency -1 peak of the Fe3O4 absorption around 574 cm was the tetrahedral -1 unit vibration, while the shoulder peaks around 650 and 720 cm 2+ were assigned to the partial oxidization of Fe in the octahedral 3+ 47 sites to Fe , as observed in previous report. The low frequency -1 peak showed several features at 470, 440, and 410 cm , where the former two were related to the two types of Fe-O bonds (Fe2+-O 3+ -1 and Fe -O) in the octahedral site and the 410 cm is likely due to the surface-oxidized ions. The lower frequency peak splitting was 12 also attributed to the Jahn-Teller distortion on octahedral sites, 2+ 3+ which was caused by the co-presence of M and Fe elements 48, 49 2+ with different vibration frequencies. The Mn doping did not cause much variation in the IR spectrum in terms of both peak positions and shapes, indicating that Mn2+ ions likely replaced Fe2+ in the octahedral sites, forming inverse phase spinel structures. In contrast, for the ZnFe2O4 nanocubes, the vibrational stretching bonds of tetrahedral sites around 570 cm-1 are much broader and the surface shoulder peaks became less evident. In addition, the vibrational bonds at the octahedral sites became one broader peak, suggesting that the Zn2+ ions primarily doped in tetrahedral sites but with some octahedral site occupation. Mixed ferrite structures 28, 50 of ZnFe2O4 were commonly observed in nanostructured phases. 2+ For Cu doped ferrite nanocubes, significant peak broadening of -1 tetrahedral site vibrational band was observed around 577 cm . -1 Along with the strong surface peaks around 660 and 730 cm , the IR spectrum suggested that the doped Cu2+ ions were likely located at the octahedral sites and were close to the surface, because it has been shown that the doped ions were located at the defect sites on the surface, causing peak broadening. The FTIR spectra of CaFe2O4 and MgFe2O4 were very similar with a broadened peak around 580 -1 cm , which suggested the Ca and Mg were likely doped at both tetrahedral and octahedral sites. The frequency shifts in the peak 3+ positions were likely due to the ionic radius difference between Fe 2+ 2+ and Ca /Mg , which caused the vibrational shifts of the tetrahedral units. Normally, the displacement with smaller ions will

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bonds. The cubic crystal symmetry of a spinel structure is expected to have defined infrared (IR) and Raman absorption bands. In addition, the vibrational bands are very sensitive to their coordination environments, cation distributions, and metal oxidation states. FTIR offers a very effective tool to study the cubic 2+ spinel structure and the effect of the doped M ions on the 43 structures. In a spinel structure, 32 oxygen ions (O2-) form a cubic close packed sublattice with two different crystallographic sites: 8 44, 45 tetrahedral (A) sites and 16 octahedral (B) sites. In a normal 3+ spinel structure, trivalent cations (e.g., Fe ) are in the B sites and divalent cations (e.g., Fe2+) are located in A sites. In contrast, the inverse phase spinel structure has divalent cations in the B sites and trivalent ions in the A and B sites. Depending on the doping ions, a mixed ferrite structure can be formed, in particular, for ferrites. Therefore, the vibrational modes of the ferrites are sample specific, strongly depending on the cation distributions, defects, and nonstoichiometry.

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Journal Name tetrahedral units, bands in the range of the 450-620 cm-1 dominated by the octahedral sites, which include T2g(2) for 450-490 -1 -1 51 cm and T2g(3) for around 540 cm . The T2g(2) band is from the asymmetric stretch of Fe-O bonds and T2g(3) is asymmetric bending of oxygen with respect to Fe. Fig. 4 shows the Raman spectra of -1 various ferrite nanocubes in the range of 400-800 cm , which were expected to show the A1g, T2g(2) and T2g(3) modes.

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Complementarily, Raman spectra of various doped magnetic ferrite nanocubes were collected, which provided useful information about the doping sites and bonding. Typically, the ferrites exhibit several strong Raman features: A1g band in the range -1 of 670-710 cm corresponding to the vibrational stretching of the

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Fig. 4 Raman spectra of MFe2O4 nanoparticles: (a) Fe3O4, (b) MnFe2O4, (c) ZnFe2O4, (d) CuFe2O4, (e) CaFe2O4, (f) MgFe2O4. The Raman spectrum of Fe3O4 in Fig. 4a showed the as-1 -1 expected A1g band at 671 cm , T2g(2) band around 490 cm and -1 -1 T2g(3) band at 545 cm . The shoulder peak near 715 cm was assigned to the oxidation of Fe(II) ions at the octahedral sites. Defects near surface also lead to shoulder peaks in that range. In contrast, the Raman spectrum of the MnFe2O4 nanocubes showed significant peak broadening of the A1g band and a great shift to -1 lower frequency around 615 cm , which was assigned to the motion of octahedral units (Fig. 4b).51 The peak shift and -1 2+ broadening near 459 cm also suggested Mn doping into the octahedral sites, which is in agreement with the IR spectrum observation. For Zn2+ doped nanocubes, defined peaks were -1 observed at 670 and 495 cm (Fig. 4c). Compared to the spectrum -1 of Fe3O4 nanocubes, T2g(3) band at 545 cm almost disappeared and a small oxidation shoulder peak around 750 cm-1 were 2+ observed, suggesting that Zn ions were primarily incorporated into tetrahedral sites with some occupation at octahedral sites. For CuFe2O4 ferrite nanocubes, the T2g(2) band was split into two -1 defined peaks at 440 and 475 cm , and the T2g(3) band was shifted

2+

to higher frequency (Fig. 4d), suggesting that Cu ions were most likely located at the octahedral sites. This observation was also consistent with the IR spectrum. Interestingly, the Raman spectrum of the CaFe2O4 exhibited very different features with bands at 438, -1 525, 583, and 649 cm (Fig. 4e). Compared to the spectrum of Fe3O4, the vibrational stretching bands shifted to lower frequencies -1 for both tetrahedral (671 to 649 cm ) and octahedral (490 to 438 -1 cm ) units. The asymmetric bending band showed double features at 525 and 583 cm-1. All these observations suggested that the Ca2+ ions were likely located at both tetrahedral and octahedral sites. The Raman spectrum of MgFe2O4 (Fig. 4f) showed significant peak broadening of the vibrational stretching for both tetrahedral -1 -1 (around 657 cm ) and octahedral (around 485 cm ) units. In addition, the surface oxidation peak or doping (at the defect sites) peak was significant, suggesting the Mg2+ ions were doped at both sites and likely concentrated near the surface. In addition to the crystal structures, the magnetic properties of the ferrites are strongly dependent on their doped ions and cation 52 distributions. In a spinel structure, the cations at A and B sites

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result in decrease in metal-oxygen bond length and consequently an increase in the vibrational bond frequency.

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experience A-A ferromagnetic and A-B antiferromagnetic coupling, resulting in a net magnetic moment aligned in the same direction at A sites and opposite direction between A and B sites. According to the Neel’s two sublattice model of ferrimagnetism, magnetic moments per subunit in Bohr magneton (µB), can be expressed as where the MB and MA are the B and A site

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Fig. 5 shows the magnetization versus applied magnetic field (M-H) curves of MFe2O4 nanocubes recorded with AGM at room temperature. As expected, Fe3O4 shows typical superparamagnetic behaviors with a saturation magnetization about 40 emu/g. The relatively low magnetization, in comparison to bulk, was mainly due to the presence of the surface ligands. In agreement with the IR and

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nB = M B − M A ,

magnetic moments (µB). The cation distribution between A and B sites, the spin magnetic moments of the cations, and the doping level offer excellent opportunities to tune the properties of magnetic ferrites. Each metal cation has specific spin magnetic 3+ moments depending on their electronic structures, such as Fe (5 2+ 2+ 2+ 2+ 2+ µB), Fe (4 µB), Mn (5 µB), Zn (0 µB), Cu (1 µB), Ca (0 µB), and Mg2+ (0 µB). When the doped cations carried no magnetic 2+ 2+ 2+ moments, such as Zn , Ca , and Mg , the magnetic coupling purely originated from the Fe ions, and is relatively weak.

Raman structural analysis, Mn2+, mainly replaced the Fe2+ at octahedral sites, which resulted in only a slight magnetic moment decrease, but did not significantly affect the magnetic properties (Fig. 5b). A significant decrease in magnetic moment (26 emu/g) was observed for ZnFe2O4 nanocubes (Fig. 5c), likely due to the 2+ majority doping of Zn ions at the tetrahedral sites, which greatly affected the ferromagnetic coupling. Based on the IR and Raman 2+ analysis, Cu was mainly doped at the octahedral site, which led to little effect on the magnetic properties. A typical superparamagnetic behavior was observed with saturation magnetization of 39 emu/g (Fig. 5d). Very interestingly, compared to Fe3O4, the saturation magnetization of CaFe2O4 nanocubes (Fig. 5e) showed a marginal increase (47 emu/g), similar to the unexpected Raman spectrum observation. The exact reason was not completely understood at this point. The increase of saturation magnetization with calcium-doping was possibly due to uneven distribution of calcium ions at A and B sites of the spinel structures, which may lead to an unexpected enhancement on ferromagnetic coupling and decrease in anti-ferromagnetic coupling. The mixed doping position of Mg2+ led to a decrease in magnetic moment to 33 emu/g for MgFe2O4 (Fig. 5f).

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Field (kOe) Field (kOe) Field (kOe) Fig. 5 Room-temperature M-H curves with close views (-0.5 to 0.5 kOe, inset) of MFe2O4 nanoparticles: (a) Fe3O4, (b) MnFe2O4, (c) ZnFe2O4, (d) CuFe2O4, (e) CaFe2O4, (f) MgFe2O4.

Conclusions In summary, we reported a general approach to the synthesis and detailed characterization of magnetic ferrite nanocubes. The mechanism studies indicated that step-heating and sodium oleate were both important to the formation of magnetic ferrite nanocubes. Our detailed characterization

suggested that the doping ions greatly affect the crystal structure and magnetic properties. For example, FTIR and Raman spectroscopy analysis suggested that Zn2+ ions primarily occupied the tetrahedral sites, leading to significant decrease in magnetization. In contrast, Mn2+ and Cu2+ were mainly incorporated into octahedral sites, causing minimal effects on the magnetic properties. Ca2+ and Mg2+ were likely doped into both tetrahedral and octahedral sites, forming

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mixed ferrites. Our studies will greatly benefit various application fields that requires magnetic ferrite nanoparticles with great physical and chemical property control.

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Acknowledgements This work was supported in part by DMR1149931. We acknowledge the UA Central Analytical Facility (CAF) and the Biological Science Department for the use of TEM.

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Table of contents entry

Doped magnetic ferrite nanocubes were synthesized in the presence of sodium oleate in combination with a step-thermal decomposition of metal-oleate complexes.

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