The Growth Morphology of MnFe2S4 Single Crystals
MnFe2S4 is a compound that belongs to the spinel family and is commonly known as molybdenum ferrite. It has an orthorhombic crystal structure with eight atoms, containing two antiparallel spins and two tetrahedrally coordinated Mn and Fe ions. As a result, it has attracted a great deal of research attention as a promising anode material for Li-ion batteries due to its high Li-ion diffusivity and reversibility.
MnFe2S4 can be either synthesized via hydrothermal approaches, such as aqueous solution routes, solid state techniques, or wet chemical methods. Each approach has its advantages and disadvantages, yet hydrothermal synthesis is more frequently used due to its low costs, versatility, and simplicity. Of all the hydrothermal processes, the top-seeded solution growth (TSSG) method is one of the most widely used techniques for synthesizing MnFe2S4 single crystals.
TSSG is a complex, yet simple process that involves two main steps: 1) the formation of a seed crystal seed of proper morphology, size and crystallinity; and 2) the subsequent growth and deposition of crystals on the seed. As the seed crystal is usually epitaxial, the orientation and crystallinity of the single crystals that grow on it will be identical to those of the seed crystal, making reproducibility and uniformity possible. The use of single-crystal seeds also eliminates the need for pre-growth of large crystals prior to the growth process.
MnFe2S4 single crystals grown by TSSG tend to exhibit a range of morphologies. Generally, these single crystals manifest as three-dimensional plates which have either a sheet-like or an inverse pyramid-like form. The sheet-like forms typically have a wavy sheath structure, with the MnFe2S4 arrangement showing up on one side only. This occurs because the cations of the MnFe2S4 lattice prefer to be surrounded mainly by anions in the crystal structure. The cations are on the sides of the sheets, with the anions located in between them. In contrast, the inverse pyramid-like form consists of multiple thin layers of MnFe2S4, with the next layer perpendicular to the previous layer.
The growth morphologies of MnFe2S4 single crystals are largely attributed to the reaction between the anion and cation in the solution, as well as to the preferential adsorption of the cations onto the crystal surfaces. These processes determine the crystallization rate and orientation of the single crystals. The crystallization rate is in turn dependent on the composition of the solution, the pH value, the temperature, and other factors such as the penetration rate of oxygen, the type of anion, and the concentration of the reactants.
Moreover, nanosheaths of MnFe2S4 single crystals can also form due to the presence of organic molecules in the bulk solution of MnFe2S4 materials. These organic molecules have a greaterelectrostatic attraction to the MnFe2S4 crystals than the bulk solution components. This adsorption of molecules at the crystal surface increases the crystallization rate of the MnFe2S4 single crystals, resulting in a nanosheath morphology.
By understanding the growth mechanisms of MnFe2S4 single crystals, scientists can optimize the reaction conditions to obtain crystals of desired morphology and size. This is important for the development of efficient Li-ion batteries, in which MnFe2S4 is commonly used as an anode material.
