Thermal Decomposition Synthesis of Iron Oxide Nanoparticles with Diminished Magnetic Dead Layer by Controlled Addition of Oxygen

Abstract

Decades of research focused on size and shape control of iron oxide nanoparticles have led to methods of synthesis that afford excellent control over physical size and shape but comparatively poor control over magnetic properties. Popular synthesis methods based on thermal decomposition of organometallic precursors in the absence of oxygen have yielded particles with mixed iron oxide phases, crystal defects, and poorer than expected magnetic properties, including the existence of a thick "magnetically dead layer" experimentally evidenced by a magnetic diameter significantly smaller than the physical diameter. Here, we show how single-crystalline iron oxide nanoparticles with few defects and similar physical and magetic diameter distributions can be obtained by introducing molecular oxygen as one of the reactive species in the thermal decomposition synthesis. This is achieved without the need for any postsynthesis oxidation or thermal annealing. These results address a significant challenge in the synthesis of nanoparticles with predictable magnetic properties and could lead to advances in applications of magnetic nanoparticles.

Keywords: iron oxide; magnetic dead layer; magnetic diameter; magnetic nanoparticles; oxygen; thermal decomposition.

Figure 1

Physical (red solid line) and magnetic (green dashed line) diameter distributions, representative transmission electron micrographs, and equilibrium magnetization curves for iron oxide nanoparticles obtained from two different thermal decomposition synthesis routes and commercially available nanoparticles from Ocean Nanotech, the corresponding TEM images and the Langevin fit to the equilibrium magnetization curves. a) Nanoparticles obtained from the heating-up thermal decomposition synthesis of an iron oleate precursor in trioctylamine at 350°C. b) Nanoparticles obtained by the Extended LaMer synthesis mechanism by dripping an iron oleate/1-octadecene precursor into a solvent system of oleic acid/docosane at 350°C. c) Commercial nanoparticles from Ocean Nanotech with a nominal diameter of 20 nm according to the manufacturer. Note how in all cases the magnetic diameter distributions are much smaller and much broader than the physical diameter distributions.

Figure 2

Effect of post-synthesis thermal treatment on the magnetic diameter distribution for iron oxide nanoparticles obtained by the thermal decomposition synthesis under anaerobic conditions, for particles of two physical diameters. a) 14.77 nm b) 26.34 nm. The red solid line indicates the physical diameter distribution, which was relatively narrow for both nanoparticle samples. Green dashed lines indicate the magnetic diameter distributions before (BO, long dash) and after (AO, short dash) thermal treatment in an oxygen atmosphere for both samples. The corresponding TEM images and the Langevin fit to the equilibrium magnetization curves are laid out.

Figure 3

Physical (red solid line) and magnetic diameter (green dashed line) distributions for iron oxide nanoparticles obtained by the Extended LaMer mechanism based synthesis in the absence and presence of molecular oxygen and corresponding TEM images and the Langevin fit to the equilibrium magnetization curves for nanoparticles synthesized under a) Argon and b) Nitrogen atmosphere.

Figure 4

Effect of adding molecular oxygen on physical and magnetic diameters for iron oxide nanoparticles obtained by the heating up thermal decomposition synthesis. a) For particles synthesized in the absence of oxygen the magnetic diameter distribution (Green dashed line) is smaller and broader than the physical diameter distribution (Red solid line), b) For nanoparticles synthesized under identical conditions but with molecular oxygen added to the reactor the magnetic and physical diameters have similar distributions.

Figure 5

Controlled growth of physical but not magnetic diameter for iron oxide nanoparticles obtained using the Extended Lamer thermal decomposition synthesis in the absence of molecular oxygen for different time points of the reaction a) 2hr b) 3hr c) 4hr d) 5hr.

Figure 6

Controlled growth of both physical and magnetic diameter for iron oxide nanoparticles obtained using the Extended LaMer mechanism based synthesis thermal decomposition synthesis in the presence of molecular oxygen for different time points of the reaction a) 2hr b) 3hr c) 4hr d) 5hr.

Figure 7

XRD powder diffractograms of iron oxide nanoparticles synthesized in the absence (w/o) and in the presence (with) of molecular oxygen (O₂) for both the Extended LaMer (EL) and heating up (Hp) method help understand the iron oxide phases present in the particle.

Figure 8

HAADF-STEM images of iron oxide nanoparticles synthesized (a) in the absence of oxygenshow polycrystalline structure comprised of many smaller crystallites, highlighted in the high magnification insets. In contrast, nanoparticles synthesized (b) in the presence of oxygen show single crystal structure throughout the particles.

Figure 9

Saturation magnetization of iron oxide nanoparticles synthesized in the absence (red, open circles) and in the presence (green, closed circles) of molecular oxygen through the Extended LaMer thermal decomposition method.

Figure 10

ZFC (solid line)-FC (dashed line) curves of the particles synthesized in the absence (red, open circles) and presence of oxygen (green, closed circles).

Figure 11

In-phase susceptibility vs temperature at different frequencies for particles synthesized a) without oxygen, b) with oxygen.

Figure 12

Evidence of exchange bias in particles synthesized in the absence of oxygen (a) but not in particles synthesized in the presence of oxygen (b). FC – magnetization curves were obtained after cooling the sample from 298K to 5K in the presence of a 70 kOe magnetic field. ZFC – magnetization curves were obtained after cooling the sample from 298K to 5K in the absence of a magnetic field. The panels on the left show show comparisons between magnetization curves obtained under FC and ZFC conditions after a single magnetization cycle. The panels on the right show comparisons between magnetization curves obtained under FC and ZFC conditions after a second magnetization cycle.

Figure 13

Comparison of thermal energy dissipation by magnetic nanoparticles in alternating magnetic fields for nanoparticles synthesized in the absence (red, dashed line) and presence of oxygen (green, solid line)

Figure 14

Comparison of a) FFT amplitude as a function of Harmonic number, b) point spread function of the particles synthesized in the presence and absence of oxygen.