Synthesis of Zinc Ferrite Nanoparticles with Tunable Size, Composition, and Magnetic Properties

Corey Kubat, Z. Yan, O.T. Mefford
Departmental of Materials Science and Engineering, Clemson University

Abstract

The mechanisms responsible for the decomposition of metal-containing precursors for the synthesis and formation of metal iron oxide nanoparticles are not well understood; therefore, in this investigation, series of reaction parameters including reagents, their concentrations, reaction temperature, and reaction stages were tested in order to continue research developing a synthetic route to produce zinc ferrite nanoparticles with tunable size, composition, and magnetic properties, with focus toward heightened monodispersity of these properties. Non-stoichiometric spherical zinc ferrite nanoparticles with <10% size dispersity have been synthesized employing a one-pot thermal decomposition of iron and zinc acetylacetonates in the presence of the non-binding solvent , octadecene, and the surfactant, oleic acid. Using these reaction parameters, a zinc ferrite series ranging from magnetite to stoichiometric zinc ferrite, with targeted composition ZnxFe3−xO4 (x=0, 0.25, 0.5, 0.75,1), was synthesized and is currently awaiting magnetic characterization via vibrating sample magnetometry and elemental analysis via inductively coupled plasma-optical emission spectroscopy and energy-dispersive x-ray spectroscopy.

Introduction

Iron oxide nanoparticles for applications in various fields including energy and electronics, the environment, and medicine, have long been a subject of intense research. Specific biomedical applications of these materials can be seen in Figure 1. below. The magnetic properties of these iron oxide nanoparticles pertinent to these fields are highly dependent upon their size, shape, and chemical composition. Because there is an inherent lack of variability in the composition of these simple iron oxides, which may be required or prove more advantageous for different applications, successful efforts have been made to substitute the divalent iron atoms in these materials with other various transition metals including manganese, nickel, cobalt, zinc, etc., to yield metal iron oxide, or metal ferrite, nanoparticles with enhanced magnetic, electrical, thermal, and other properties.1−6 Zinc-substituted ferrite nanoparticles in particular have attracted interest due to zinc’s low toxicity relative to other potential substituent metals4 and its potential as a theranostic agent, namely for acting as a T1 non-lanthanide MRI contrast agent and a magnetophotothermal agent.1,3−5

Figure 1. Biomedical applications of iron oxide nanoparticles. Kwon, H. J., Shin, K., Soh, M., Chang, H., Kim, J., Lee, J., et al. (2018). Advanced Materials, 115, 1704290–24.

Materials and Methods

The control reaction employed for comparison purposes was a one-pot thermal decomposition of 0.84 mmol zinc acetylacetonate (acac) hydrate and 4.2 mmol iron (III) acetylacetonate metal precursors in the presence of 5 mL oleic acid (OAC) and 5 mL octadecene, under nitrogen atmosphere at 350 °C in metal bath for 1 h; average particle size (13.9 nm) and particle size dispersity (5.55%) were determined from transmission electron microscopy (TEM) image analysis. Related spectra and micrographs can be seen in Figure 2. below. Zn0.5Fe2.5O4 was the composition targeted and Zn0.33Fe2.67O4 was the composition observed via energy-dispersive x-ray spectroscopy (EDX) spectra analysis. Series of reaction parameters were tested individually and some were tested together to determine their combined effects on the size and composition of particles. All else other than the described change remained the same as what was performed in the control reaction. The changes made to reaction parameters were as follows:

The following parameter changes were only tested individually-

Figure 2. TEM micrograph, particle size histogram, XRD diffraction pattern compared to the spinel zinc ferrite diffraction pattern calculated by the Materials Project,8 and EDX spectra for control sample

The following parameter changes were tested individually and together in deliberate combinations-

The replacement of Zn(acac)2 · H2O with ZnCl2 to determine the effects of incorporating metal chlorides as metal precursors in the presence of a non-binding solvent (most substituted ferrite synthesis methods utilize binding solvents such as octyl ether or benzyl ether)

The replacement of a portion of Fe(acac)3 with FeCl2 to avoid relying on some Fe3+ to reduce to Fe2+ during the reaction, as well as the reasoning stated in “ZnCl2” above

The addition of another surfactant, oleylamine (OAM), to reach a 3:1 OAC/OAM molar ratio9 to determine effects in these particular reaction parameters and sets of reagents

The addition of a small amount (1 mmol) of 1,2-hexadecanediol (HDD), an accelerating agent,6 in OAC/OAM = 3 reactions to validate effects on particles of smaller size while using this particular reaction parameters and sets of reagents

Results

All TEM micrographs and XRD diffraction patterns obtained, as well as EDX spectra on select samples, for the samples other than those of the ZnxFe3−xO4 series can be observed in Figures 4. and 5. The effects of each parameter change on the synthesis of zinc ferrite nanoparticles are as follows-

The addition of a drip stage after allowing particle nucleation in a seed-mediated stage increased particle size dispersity without increasing particle size, contrary to expectations (see Figure 4.); this unexpected finding still requires a more in-depth understanding of the mechanisms pertaining to the synthesis of these materials

Increased input amount of Fe(acac)3 and oleic acid times 1.5 increased average particle size to 16.8 nm and with increased particle size dispersity. Increased input times 2.5 demonstrated a bimodal distribution of particle sizes where one mode showed a larger particle diameter (~20-25 nm) than the control and the other displayed particle diameters lower than the control (~7-10 nm), with noticeably high dispersity. This could indicate that a critical size (based on amount of precursor) of present nuclei was reached and thus decomposition of still-present precursor formed new nuclei, but not enough precursor remained to fuel the continued growth of these new nuclei (see Figure 5.)

Increased input amount of Zn(acac)2 times 4 displayed an increase in average particle size to 24.6 nm with a very small increase in dispersity to 6.19%. Increased input times 8 increased particle size and dispersity, but in addition to zinc ferrite nanoparticles, hexagonal particles confirmed to be zinc oxide were observed and there is potential that core-shell structures with zinc ferrite core and zinc oxide shell were formed (see Figure 5.)

Using ZnCl2 and FeCl2 as metal precursors in the presence of octadecene was observed to yield zinc ferrite nanoparticles that were extremely large relative to the previously observed particles, as well as nanoparticles of previously observed sizes (see Figure 5.), which is most likely due to the differing decomposition temperatures between these metal chlorides and Fe(acac)3 and the interactions between these reagents and octadecene as a non-binding solvent

Contradictory to findings of another previous publication on iron oxide nanoparticles,altering the used surfactant ratio to OAC/OAM = 3 was observed to greatly increase particle size and slightly increase dispersity (25.4 nm and 7.39%) in the presence of octadecene as a solvent (see Figure 5.)

Perhaps due to the larger-than-expected particle sizes observed in the surfactant ratio syntheses, the addition of HDD was observed to vastly increase particle size dispersity, and was also observed to potentially promote the formation of some triangular (or tetrahedral) particle structures (see Figure 5.)

The reaction parameters that were observed to produce zinc ferrite nanoparticles with the lowest size dispersity that demonstrate spinel crystal structure are as follows:

  • Fe(acac)3 and Zn(acac)2 ⋅ H2O were used as metal precursors
  • 5 mL octadecene, 5 mL oleic acid
  • one-pot synthesis at 350 °C for 1 h

This synthesis was repeated using varying amounts of Fe(acac)3 and Zn(acac)2 ⋅ H2O to synthesize a zinc ferrite series of targeted composition ZnxFe3−xO4 where x = 0, 0.25, 0.5, 0.75, and 1, to be magnetically characterized via vibrating sample magnetometry (VSM); TEM micrographs, particle size histograms, XRD diffraction patterns, and other relevant information pertaining to this zinc ferrite series can be seen in Figure 3. and Table 1. below. Performing VSM, as well as elemental analysis, on these zinc ferrite nanoparticles will allow better understanding of the role of zinc substitution during nanoparticle synthesis in affecting the magnetic properties that determine the efficacy of these materials in various applications.

Figure 3. TEM micrographs, particle size histograms, and the XRD diffraction patterns of the ZnxFe3−xO4 series compared to that of zinc ferrite calculated by the Materials Project,8 in order of increasing Zn concentration

Table 1. Information pertinent to VSM measurements and magnetic properties
Targeted Zinc Content (x)Particle Diameter (nm)Standard DeviationParticle Size Dispersity (%)
12.31.613.18
0.2512.91.813.81
0.512.90.86.53
0.7514.51.28.44
114.90.96.03

Conclusions

Reaction parameters for the synthesis of zinc ferrite nanoparticles were optimized to minimize particle size dispersity while incorporating significant concentrations of zinc into these particles’ crystal lattices. Monodisperse ferrite nanoparticles with significant incorporation of zinc and that demonstrate spinel crystal structure have been synthesized. Insights into the mechanisms of how reagents decompose and interact with solvents and how iron oxide nanoparticles are formed and interact with surfactants as compared to findings from other researchers in this field were gained; the groundwork for incorporating different substituent transition metals and combinations of their substitutions into these materials has been laid.

Figure 4. Drip stage temperature series TEM micrographs and particle size histograms

Figure 5. TEM micrographs, particle size histograms, XRD diffraction patterns compared to the diffraction pattern for zinc ferrite calculated by the Materials Project,8  and relevant EDX spectra and SEM micrographs of select samples outside of the drip stage temperature series and the zinc ferrite series

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