Experimental and Theoretical Investigations on the Magnetic‐Field‐Induced Variation of Surface Energy of Co₃O₄ Crystal Faces

Abstract

Magnetic fields have long been reported to influence the structures and physical properties of materials, such as crystal structures, [1] electronic properties, [2] magnetic and mechanical properties.[3] Based on the interactions between materials and the magnetic fields themselves, there is potential for the design of many devices, including magnetic actuators, and in many systems for the investigation of electron-transport properties.[4] Although magnetic fields were, for a long time, considered to only have weak influence on the synthesis of materials, recent advances indicate that the real situation is not as previously assumed. It has been found that magnetic fields can induce the assembly of ferromagnetic (FM) nanocrystallites along magnetic lines of force, and also the anisotropic growths of single-crystal materials.[5] Moreover, materials synthesized under magnetic fields usually exhibit different properties, such as enhanced saturated magnetization and different ions arrangement in the lattice. [5] Although it is generally realized that magnetic fields can influence the materials growth, hitherto the pathways by which they work remain unknown. Theoretical work on the influence of magnetic fields on materials synthesis is rare. In this paper, Co 3 O 4 nanoparticles are selected as objects for investigating the effects of external magnetic fields on nanomaterial growth. Experimental results show that external magnetic fields can result in the morphology of Co 3 O 4 nanoparticles changing from irregular spheres to nanocubes. Then in the theoretical investigations, the effects of external magnetic fields on the competitive growth between {111} and {100} faces and on the morphology of Co 3 O 4 crystals are simulated. It is shown that the energy of both faces decrease under external magnetic fields; however, the energy of {100} faces decrease more rapidly, so that the energy of the {100} faces is lower than that of {111} faces and the growth rates along {100} and {111} faces are also different. As a result the {100} faces are the growth limiting form.Co 3 O 4 particles are chosen for straightforwardness in discussing the effect of magnetic fields. Since they are not ferromagnetic, interaction between their intrinsic magnetic fields and external magnetic fields can be eliminated. The possibility that the as-formed nuclei assemble by magnetic dipole-dipole interactions and form one-dimensional structure can also be ruled out. The synthesis is based on a recent publication and is further modified.[6] We denote the samples synthesized in the absence and presence of magnetic fields as S1 and S2, respectively. X-ray diffraction (XRD) patterns of S1 and S2 show that they are pure Co 3 O 4 (shown in Figure 1 a). The average diameter of particles calculated by Scherrers equation for both samples are about 15 nm, in accordance with transmission electron microscopy (TEM) observations, indicating these grains are of high-quality single-crystalline particles. TEM images of S1 and S2 (Figure 1 b and 1c) show that S2 prod...

Co3O4 Hexagonal Platelets with Controllable Facets Enabling Highly Efficient Visible‐Light Photocatalytic Reduction of CO2

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Synthesis of monocarboxylic acid-modified CeO2 nanoparticles using supercritical water

“…This journal is © The Royal Society of Chemistry 2014 The stability of ionic crystals has been theoretically and experimentally investigated. 53,54 In MO 2 -type (metal oxide) ionic crystals with uorite structure (cubic system), the surface energy of the {100} plane is larger than that of the {111} plane. A dipole moment arises perpendicular to the {100} plane.…”

Effects of high magnetic field annealing on FePt nanoparticles with shape-anisotropy and element-distribution-anisotropy

“…Recently, high-magnetic-eld-assisted (HMF-assisted) heat treatment method attracts more and more attentions with the developing of superconducting technology. The morphology of lms, 14,15 growth rates of crystals, [16][17][18][19] and solid-state phase transformation [20][21][22] are reported to be regulated by the HMF. For the FePt alloys, the HMF annealing has been reported to induce strain in Fe 3 Pt alloy, 23 promote disorder-order transformation of FePt nanomaterials, [24][25][26] and alignment of FePt spherical NPs.…”