Mechanosynthesized BiFeO₃ Nanoparticles with Highly Reactive Surface and Enhanced Magnetization

Abstract

A one-step synthesis of nanostructured bismuth ferrite (BiFeO3) via mechanochemical processing of a α-Fe2O3/Bi2O3 mixture at room temperature is reported. The mechanically induced phase evolution of the mixture is followed by XRD and 57Fe Mössbauer spectroscopy. It is shown that the mechanosynthesis of the rhombohedrally distorted perovskite BiFeO3 phase is completed after 12 h. Compared to the traditional synthesis route, the mechanochemical process used here represents a one-step, high-yield, low-temperature, and low-cost procedure for the synthesis of BiFeO3. High-resolution TEM and XRD studies reveal a nonuniform structure of mechanosynthesized BiFeO3 nanoparticles consisting of a crystalline core surrounded by an amorphous surface shell. The latter is found to exhibit an extraordinarily high metastability causing a rapid crystallization of nanoparticles under irradiation with electrons. In situ high-resolution TEM observations of the crystallization clearly show that the heterogeneous processes of nucleation and growth of bismuth iron oxide crystallites are spatially confined to the amorphous surface regions. This fact provides access to the elucidation of the mechanism of mechanosynthesis. It is demonstrated that the mechanosynthesized ferrite nanoparticles exhibit a partial superparamagnetism at room temperature. Quantitative information on the short-range structure and hyperfine interactions, provided by the nuclear spectroscopic technique, is complemented by an investigation of the magnetic behavior of nanostructured BiFeO3 on a macroscopic scale by means of SQUID technique. As a consequence of canted spins in the surface shell of nanoparticles, the mechanosynthesized BiFeO3 exhibits an enhanced magnetization, an enhanced coercivity, and a shifted hysteresis loop.

Hallmarks of mechanochemistry: from nanoparticles to technology

“…The amorphous phase is mostly located on the particle surface (core/shell structure). 177 The presence of a destruction limit in brittle materials is confirmed by Fig. 12.…”

Synthesis of ternary transition metal fluorides Li3MF6via a sol–gel route as candidates for cathode materials in lithium-ion batteries

“…[13][14][15] Besides activation, mechanochemistry has also successfully been used for the synthesis of a variety of materials from precursors at room temperature including oxides and fluorides. [16][17][18] However, in contrast to such a top-down approach, the preparation of nanostructured particles by precipitation from aqueous solution is much more beneficial when the shape and surface morphology of the crystallites have also to be controlled. By following such a route, the corresponding salts, for example nitrates or oxides, are reacted with hydrofluoric acid and subsequent dehydration is carried out by annealing at elevated temperatures.…”

Metastable nature of donor-doped BiFeO₃ obtained by mechanochemical synthesis

“…On the other hand, the lattice fringes observed on the crystalline particles showed an interplanar distance of ³4.0 ¡, corresponding with the (012) planes of the perovskite structure and in good agreement with the values reported in literature. 29) The atomic composition of the particles, as estimated from EDS analyses, evidenced a Bi/Fe ratio µ 1. It is worth to mention at this point that these EDS analyses were carried out with relatively short acquisition times, namely around 30 s; this is because, as a consequence of the far-from-equilibrium state of the surface shell in the mechanosynthesized nanoparticles, a prolonged exposure time (above 60 s) unavoidably promotes the crystallization of the particles under the electron beam.…”

“…But moreover, all particles seem to be surrounded by an amorphous layer with a width of just a few nanometers; as described in the literature, such morphology is characteristic of mechanosynthesized BiFeO 3 powders. 27), 29) Da Silva et al 29) suggested that the observed amorphous layer is associated with the area where the heterogeneous processes of nucleation and growth of BiFeO 3 crystallites are spatially confined: during high-energy milling the reaction precursors are mixed at the molecular level and the new mechanosynthesized phase nucleates at the interfaces between the solid reactants. On the other hand, the lattice fringes observed on the crystalline particles showed an interplanar distance of ³4.0 ¡, corresponding with the (012) planes of the perovskite structure and in good agreement with the values reported in literature.…”

“…It is worth to mention at this point that these EDS analyses were carried out with relatively short acquisition times, namely around 30 s; this is because, as a consequence of the far-from-equilibrium state of the surface shell in the mechanosynthesized nanoparticles, a prolonged exposure time (above 60 s) unavoidably promotes the crystallization of the particles under the electron beam. 29) Regarding the particles of the BiFe 0.95 Nb 0.05 O 3.05 nominal composition, they also show a nanometric size, evidencing as well a mixture of nanocrystalline areas and amorphous regions [ Figs. 3(c) and 3(d)].…”