Morphology Controlled 1-D Iron Oxide Nanochains with Tunable Magnetic Properties

Magnetic 1-dimension (1-D) ordered nanostructures has stimulated great interests attributed to their potential applications including magnetic resonance imaging, data storage, magnetic sensor, and drug delivery. Self-assembly of 1-D hierarchical nanostructures with aids of surfactant using bottom-up wet chemistry method can provide controllable magnetic properties. Herein, an unusual polymeric additive- polypropylene grafted maleic anhydride (PP-g-MA, 2500 g/mole) has demonstrated its unique capability, i.e., to synthesize 1-D ferromagnetic hard (292.7 Oe) γ-Fe2O3 nanochains made of 24 nm nanoparticles vs. PP-g-MA with 8000 g/mole for the synthesis of 1-D ferromagnetic soft (70.5 Oe) γ-Fe2O3 nanochains (30 nm) made of flowerlike nanoparticles (NPs).

Experimental

The synthesis procedures of γ-Fe2O3 nanochains were demonstrated as below. First, solid PP-g-MA (0.25 g, Molecular weight, Mn≈8000 or ≈2500) was added into a three-neck flask with 100 mL xylene and heated to reflux (140 °C) for 30 min. Second, brown Fe(CO)5 (3.50 g) was injected into the hot mixed solution (solution turned from transparent to yellow). Then, the solution was further refluxed for 3 hours at 140 °C constantly to form a black colloid. After cooling, the final colloidal solution was used for morphology characterization, while the powder product from drying the colloidal was used for magnetic property measurement.

 

Results and Discussion

By using PP-g-MA (Mn≈ 8000), the formation of 1-D nanochains (diameter: ~30.0 nm) was observed in the TEM image, Fig. 1a. The enlarged TEM image in Fig. 1b demonstrates that the building blocks of these nanochains are consisted of unique flower-shape NPs, Selected area electron diffraction (SAED) ring pattern confirm the sole

existence of γ-Fe2O3 in the nano-chains.17 The self-assembled 1-D nanochains can be realized via replacing PP-g-MA (Mn≈ 8000) with PP-g-MA (Mn≈ 2500). When PP-g-MA (Mn ≈ 2500) was used, the formed colloidal NPs were observed to assemble into 1-D nanochains consisting of quasi-spherical NPs instead of the flower shape NP building blocks, when drop-cast on the carbon coated copper TEM grid, the average diameter was ~24.0 nm, Fig. 1c and d. SAED patterns also indicate the sole existence of γ-Fe2O3.¹

Figure 1 EM images of γ-Fe₂O₃ nanochains with a, b) PP-g-MA (M_n≈8000); and c, d) PP-MA (M_n≈2500). Average chain diameter is: a, b) ~30.0 nm, and c, d) ~ 24.0 nm, respectively. 

The difference in nanochain morphologies are caused by the different capping strength from varying the Mn of PP-g-MA on the magnetic NPs. The MA group in PP-MA can be tightly chemisorbed onto the magnetic NPs² similar to the carboxylic acid surfactant. ³Larger Mn PP-MA resulted in low MA density in the reaction system, and a lower capping strength on the surface of magnetic NPs was thus observed. At low concentrations, for example, 0.25 g PP-MA (Mn≈ 8000), the aggregation of small magnetic clusters led to the formation of flower-shape NPs grown from the highly concentrated nuclei upon thermal-decomposition of Fe(CO)5. The further assembly of these flower ‘‘aggregates’’ is driven by the balance between attractive forces (magnetic dipolar attractions) and the steric hindrance from the coordinating PP-MA backbones.

Beside the morphological difference, different magnetic properties were also observed. Materials with coercivity (H_c) greater than 200 Oe is defined as ferromagnetic hard; while H_c smaller than 200 Oe is defined as ferromagnetic soft. Room temperature magnetic property reveals that the γ-Fe₂O₃ nanochains (30 nm diameter) have higher saturation magnetization (M_s) than that of γ-Fe₂O₃ nanochains (24 nm diameter), More importantly, the H_c of 30 nm γ-Fe₂O₃ nanochains is about 70.5 Oe, reflecting a ferromagnetic soft material; while the H_c of 24 nm γ-Fe₂O₃ nanochains is 292.7 Oe, corresponding to a ferromagnetic hard material. It can be concluded that the magnetic property such as coercivity (ferromagnetic soft vs. hard) of thus synthesized 1-D nanochains can be easily controlled by only changing the molecular weight of PP-MA. It is believed that the size and aspect ratio, shape anisotropy and magnetization reversal mechanism are responsible for the observed different magnetic property.  

Conclusion

A facile one-pot bottom up approach has been demonstrated to synthesize well defined 1-D γ-Fe₂O₃ nanochains with easily controlled building block configurations, self-assembly morphologies towards the further manipulation of magnetic property. PP- MA with proper MA grafting density and reaction concentration is of key importance to achieve these evolved different 1-D γ-Fe₂O₃nanochains. These 1-D nanochains are promising for a variety of applications such as high-density magnetic storage and sensors.

References

1. Q. He, T. Yuan, Z. Luo, N. Haldolaarachchige, D. P. Young, S. Wei and Z. Guo, Chem. Commun., 2014, 50, 201-203.

2.  Q. He, T. Yuan, S. Wei, N. Haldolaarachchige, Z. Luo, D. P. Young, A. Khasanov, Z. Guo, Angew. Chem. Int. Ed., 2012, 51, 8842-8845.

3. S. Sun and C. Murray, J. Appl. Phys., 1999, 85, 4325-4330.