Goethite Nanorods: Synthesis and Investigation of the Size Effect on Their Orientation within a Magnetic Field by SAXS.

Stephan Hinrichs,Larissa Grossmann,Dieter Skroblin,Birgit Hankiewicz,Hannah Grotian Genannt Klages,Andreas Meyer,Eike Clasen,Christian Gollwitzer

doi:10.3390/nano10122526

Stephan Hinrichs, Larissa Grossmann + Show 6 more

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https://doi.org/10.3390/nano10122526

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Abstract

Goethite is a naturally anisotropic, antiferromagnetic iron oxide. Following its atomic structure, crystals grow into a fine needle shape that has interesting properties in a magnetic field. The needles align parallel to weak magnetic fields and perpendicular when subjected to high fields. We synthesized goethite nanorods with lengths between 200 nm and 650 nm in a two-step process. In a first step we synthesized precursor particles made of akaganeite (β-FeOOH) rods from iron(III)chloride. The precursors were then treated in a hydrothermal reactor under alkaline conditions with NaOH and polyvinylpyrrolidone (PVP) to form goethite needles. The aspect ratio was tunable between 8 and 15, based on the conditions during hydrothermal treatment. The orientation of these particles in a magnetic field was investigated by small angle X-ray scattering (SAXS). We observed that the field strength required to trigger a reorientation is dependent on the length and aspect ratio of the particles and could be shifted from 85 mT for the small particles to about 147 mT for the large particles. These particles could provide highly interesting magnetic properties to nanocomposites, that could then be used for sensing applications or membranes.

Highlights

Iron oxides offer a lot of different phases with widely varying shapes and magnetic properties
The needles align parallel to weak magnetic fields and perpendicular when subjected to high fields
small angle X-ray scattering (SAXS) was used to investigate the changes in orientation when the magnetic field strength is varied

Summary

Introduction

Iron oxides offer a lot of different phases with widely varying shapes and magnetic properties. While the bulk of the research focuses on the ferro- and ferrimagnetic properties of pure iron, magnetite or maghemite, there are the antiferromagnetic species that show interesting magnetic properties. Its extraordinary properties were first observed by Lemaire and coworkers [5,6,7,8] They showed that goethite forms a liquid crystalline phase above 8.5 wt.%, due to its elongated particle shape in the nanometer regime. This behavior is unique to nanoparticles, as the effect is attributed to the surface moments of the particles that are free to orient themselves in response to a magnetic field

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