Tuning the shape, size, phase composition and stoichiometry of iron oxide nanoparticles: The role of phosphate anions
Graphical abstract
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.
Section snippets
Synthesis of α-Fe2O3 and Fe3O4
The α-Fe2O3 nanotubes (NTs), nanorods (NRs) and (NDs) were prepared by a microwave assisted hydrothermal reaction of iron chloride (FeCl3) with addition of sodium phosphate (NaH2PO4) and sodium sulfate (Na2SO4) as additives to control shape and morphology. Briefly, 0.06 mol L−1 (∼370 mg) of FeCl3 with 35 mL of distilled water were stirred for 15–20 min. The additives NaH2PO4, and Na2SO4 mixed with 3 mL of distilled water separately and finally mixed with FeCl3 solution to make mixture of final
Formation mechanism of α-Fe2O3 nanoparticles
Several studies have shown that concentration of phosphate ion has a significant impact to control the NPs shape, from NTs, NRs to NDs, whereas the concentration of iron cations in solution is responsible for the final nanoparticle size. Higher the ferric cation concentration, larger becomes the size of the synthesized nanoparticles [3,14]. In addition, sulfate anions allows to control the growth of α-Fe2O3 nanocrystals [14]. Hydrolysis of the metal cation precursor (FeCl3), which involves
Conclusion
In this work we have studied the impact of phosphate anions in impacting size, shape, phase composition and stoichiometry of Fe3O4 nanoparticles. Analysis of the structural evolution and formation mechanism of the nanoparticle systems revealed that by using a controlled ratio of and microwave irradiation we can effectively produce in water hollow nanotubes, nanorods and nanodisks, with tunable phase composition and stoichiometry. The magnetic properties of these morphologically
CRediT authorship contribution statement
Gopal Niraula: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Jose A.H. Coaquira: Conceptualization, Validation. Fermin H. Aragon: Writing - review & editing. Bianca M. Galeano Villar: Investigation. Alexandre Mello: Formal analysis. Flavio Garcia: Formal analysis, Validation. Diego Muraca: Writing - review & editing. Giorgio Zoppellaro: Writing - review & editing. Jose M. Vargas: Writing - review & editing. Surender K. Sharma:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
GN is thankful to Brazilian funding agency CAPES and PPGF-UFMA for providing doctorate fellowship and other financial support to visit University of Brasilia, CBPF, Rio de Janerio to perform the experiment, respectively. GZ thanks the support of the ERDF project “Development of pre-applied research in nanotechnology and biotechnology” (No. CZ.02.1.01/0.0/0.0/17_048/0007323).
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