Chemical Substitution—An Alternative Strategy for Controlling the

Communication pubs.acs.org/crystal

Chemical SubstitutionAn Alternative Strategy for Controlling the Particle Size of Barium Ferrite Darja Lisjak*,† and Miha Drofenik†,‡ †

Jožef Stefan Institute, Department for Materials Synthesis, Ljubljana, Slovenia University of Maribor, Faculty for Chemistry and Chemical Technology, Maribor, Slovenia

‡

S Supporting Information *

ABSTRACT: Barium ferrite nanoplates with the nominal composition BaFe12−xMxO19, where M = Sc, In, or Cr and x = 0−0.5, were hydrothermally synthesized at 160 and 240 °C. The incorporation of the substituent ions into the barium ferrite particles was confirmed with energy-dispersive X-ray spectroscopy. The effects of the temperature, the substituent ion, and the degree of the substitution on the final particle size and its distribution were studied with transmission electron microscopy. The magnetic properties of the synthesized particles were affected much more significantly by the particle size than by their chemical composition.

B

Therefore, the synthesis of BaFe nanoparticles with diameters in the range of several tens of nanometers remains a challenge. We selected a hydrothermal synthesis for this study, since it enables us to directly disperse as-synthesized particles in a solvent, while other methods result in dry powders and so their dispersion is problematic. During the hydrothermal synthesis, the diffusion during the dissolution and deposition of the precursor on the crystal surface is one of the main processes responsible for the grain growth. The basic idea is that, by substituting the Fe3+ with more voluminous ions, the overall mass transport during the particle growth and/or the deposition rate will decrease. Consequently, the particles will grow homogeneously, exhibiting a monomodal particle size distribution at high enough temperatures to ensure suitable crystallinity and magnetic properties. At the same time, the nucleation rate can also be significantly affected by the addition of more voluminous ions. Namely, in solution, the nucleation rate is supposed to determine the crystallization rate of the nanoparticles.12 Since the BaFe crystals and particles typically grow in a thin, platelike shape with a large hexagonal face (the ab crystal plane), it is crucial to be able to control the growth of this specific crystallographic face. To achieve this, we explored more voluminous ions, Sc3+ and In3+, which could substitute for the smaller Fe3+. Their ionic radii are 80, 75, and 64 pm, respectively.13 For a comparison, Cr3+ was selected as a substituent with a similar ionic radius to that of Fe3+. Powders with the chemical composition BaMxFe12−xO19 (M = Cr, In, or Sc and x = 0.0, 0.35, 0.5, or 1.0) were synthesized

arium ferrite (subsequently referred to as BaFe) with the chemical formula BaFe12O19 is a widely used permanentmagnet material that is suitable for high-density magnetic recording, as an absorber of microwave radiation and for the next generation of magnetic microwave devices that will be planar and self-biased, while operating at a frequency of 40 GHz and more.1−3 In addition, it can be used for advancing cancer therapies, as nanodiscs, which, when an alternating magnetic field is applied, will create an oscillation transmitting a mechanical force to the cell and so destroying it in vivo.4 Here, the required magnetic field is much lower than in the case of magnetic hyperthermia. The synthesis of BaFe nanoparticles with technically applicable properties is a demanding task. For most of the stated applications, the BaFe nanoparticles must be prepared, at least in the first step, in the form of a stable dispersion. Wet chemical methods demand, in most cases, an additional thermal treatment yielding strong agglomeration,2,5,6 while an in situ synthesis under hydrothermal conditions results in a broad or bimodal particle size distribution.7 The growth of larger particles can be suppressed by a drastic decrease of the synthesis temperature and/or using a proper surfactant.8,9 Both modified procedures lead to a monomodal size distribution of superparamagnetic particles with poor magnetic properties. The latter are a consequence of the very small particle size (