Tuning the shape, size, phase composition and stoichiometry of iron oxide nanoparticles: The role of phosphate anions

Highlights

• Microwave synthesis of iron oxide hollow nanotubes, solid nanorods and nanodisks.

• Influence of phosphate anions in impacting size, shape, phase composition and stoichiometry.

• Shape-dependent magnetic properties and existence of magnetic vortex in nanodiscs.

• Mechanistic description of the chemical processes and core@shell models.

Abstract

This work describes a microwave synthetic approach for the controlled assembly of α-Fe₂O₃ nanosystems with defined morphologies, such as hollow nanotubes (NTs), solid nanorods (NRs) and nanodisks (NDs). The morphological control is aided during the crystallization processes by using phosphate anions as key surfactants in solution. Furthermore, the thermal reduction under H₂ atmosphere of these NTs, NRs and NDs α-Fe₂O₃ systems to the correspondent Fe₃O₄ nanomaterials preserved their initial morphologies. It was observed that the concentration of phosphate anions and volume of solvent had significant impact not only on controlling the shapes and sizes, but also phase composition and stoichiometry of the NTs, NRs and NDs nanoparticles. X-ray Rietveld refinement analysis of the NTs, NRs and NDs systems, after reduction in H₂, revealed the presence of zero-valent iron (Fe⁰) in the final materials, with Fe⁰ fractions that decreased gradually in % from NTs (∼16%), NRs (∼11%) to NDs (∼0%) upon increasing amount of phosphate anions. Bulk magnetic susceptibility measurements showed clear alterations of the Verwey transition temperatures (T_V) and the development of unusual magnetic phenomena, such as magnetic vortex states in NDs, which was subsequently verified by micro-magnetic simulations. From the combination of XRD analysis, bulk magnetic susceptibility and Mössbauer results, we provide herein a detailed mechanistic description of the chemical processes that gated the development of shape-controlled synthesis of NTs, NRs and NDs and give a detailed correlation between specific morphology and magneto-electronic behaviors.

Introduction

The synthesis and shape-controlled assembly of iron oxide nanoparticles (NPs), such as hematite, magnetite, and maghemite, that are arranged into 1-D, 2-D, and 3-D assemblies is an emerging field of research with potential applications that span from biomedicine, magnetic fluids, magnetic recording and spin-controlled electronics [[1], [2], [3], [4], [5]]. The nanoparticle’s morphology, e.g. hollow nanotubes (NTs), nanorods (NRs) and nanodiscs (NDs), impact severely the magnetic/electronic behavior of these systems; the NPs shape-effect provides in fact a viable route for the generation of systems with well-defined spin-polarized currents, an effect that can be used in nano-electronics [3,4,[6], [7], [8], [9]]. Spin polarized currents induce formation of magnetic vortexes, which are an in-plane circulation of magnetization around a nanometer-scale central core with out-of-plane magnetization pointing either up or down [10]. Magnetic vortex states have shown promising applications in spin based devices such as non-volatile memory [11], logic gates [12], vortex based transistor [13], in biomedicine and cancer treatments [6,9].

Several synthetic strategies have been tested in order to gain access to shape-controlled synthesis of iron oxide nanoparticles. For example, co-precipitation, sol-gel, and the hydrothermal solution-based methods use phosphate and/or sulfate ions as additives during the metal oxide nanoparticles assembly [3,7,[14], [15], [16], [17]], providing a route to the fair access to 2-D and 3-D organized nanostructures, but at the cost of long reaction times needed for their effective synthesis. In contrast, the microwave assisted hydrothermal (MAH) route offers great advantages compared to conventional hydrothermal synthesis, because it is faster, simpler, and energy efficient due to very high rates of microwave heating [18,19]. To the best our knowledge, there is a very limited number of reports describing the shape/size-controlled synthesis of iron oxide NPs, following the MAH route.

In this paper, we present the shape-controlled synthesis of α-Fe₂O₃ nanotubes (NTs), nanorods (NRs) and nanodisks (NDs) achieved by the MAH route, using cooperative action of sodium phosphate and sodium sulfate ions. Furthermore, we describe the conversion of these shape-controlled NPs into magnetite (Fe₃O₄) NPs by subsequent reduction processes that were capable to preserve the initial system’s morphologies. The FESEM, XRD, FTIR, Mössbauer spectroscopy analysis complemented by bulk magnetic measurements and micro-magnetic simulations are given in detail, validating the potential of the MAH approach towards assembly of carefully engineered magnetic nanostructures with tailored electronic properties.