Investigation Of Cobalt Ferrite Nanosheets Derived From Layered Double Hydroxides Precursor

Layered double hydroxides (LDHs), also known as anionic or hydrotalcite-like clays, are one of the most important types of layered compounds. Because of their lamellar features and high versatility of composition, LDHs are interesting in various fields of application, such as anion exchangers, catalysts, drug and drug delivery. Due to uniform distribution of metal ions in the layers at atomic level, the compositions of their calcined products are also uniform. Recently, spinel oxides fabricated by calcining LDHs have attracted considerable attention. Cobalt ferrite (CoFe2O4) is one of the most important spinel ferrite, which is considered to be a good candidate for magnetic recording materials, catalysts, magnetic fluids and electronic devices because of its large magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, large magnetostrictive coefficient, chemical stability, and mechanical hardness. However, the preparation of two-dimensional (2-D) nanosheets of ferrites remains a challenge. Because the high symmetric spinel structure and inherent magnetism of spinel ferrites, which do not favor the 2-D growth without extra restriction.In this paper, FeCl2, CoCl2 and ammonia were used as a reaction solution. Co-Fe-Al-LDHs nanosheets with homogeneous morphology and high crystallinity have been synthesized by a facile chemical process route. The LDHs precursors were calcined at different temperatures. And the structure and magnetic properties of the products were discussed.The as-obtained LDHs were characterized by means of XRD, SEM, TEM, FT-IR, XPS and VSM. The SEM image clearly indicates that the LDHs films consist of a large quantity of hexagonal nanosheets, most of which approximately perpendicular to the substrates. All the nanosheets form hierarchically network-like film. The nanosheets have a thickness of about 50 nm and width about 1 ?2 ?m.The effects of synthetic conditions such as the type of metal ions, the source of aluminum ions, the concentration of alkali, and the type of alkaline on the morphology of the LDHs were investigated. The results indicate that the Co-Fe-Al-LDHs nanosheets could be synthesized in a wide range of the metal ions concentration and ammonia concentration. The aluminum foil and porous anodic alumina template (AAO) have been used as the substrates in Co-Fe-Al-LDHs synthesis process. The results indicate that the aluminum and AAO were not only substrates but also sources of Al3+ in the LDHs synthesis process. The aluminum substrate and AAO should be gradually dissolved in alkaline solution, which provide a stable source of Al3+ for the solution. The effect of the type of alkaline was also investigated. The metal ions tended to directly generate CoFe2O4 particles when using NaOH as the alkali. Urea and ammonia can provide a steady OH– ion supply through their hydrolysis, which is favorable for the growth of high crystallinity LDHs.The growth mechanism of the LDHs was investigated based on the study of the evolution of the structure and the morphology of LDHs with increasing reaction time. The results indicate that the formation process of the LDH films on Al substrate includes a first nucleation of primary particles, followed by a fast grow and crystallization of the primary particles into nanosheets. From a thermodynamics perspective, the surface energy of an individual nanosheet is quite high with two main exposed planes, thus the neighboring nanosheets tend to interconnect to decrease the surface energy by reducing exposed areas.The Co-Fe-Al-LDHs precursors were calcined at different temperatures, and the structure and magnetic properties of the products were investigated. The results indicated that the LDHs precursors transformed into amorphous oxides and the layered structures were destroyed when the precursors were calcined at 400 oC. When the calcination temperature rose to 500 oC, the spinel phase appeared. when the calcination temperature was 600 oC, the cobalt ferrite nanosheets with spinel phase were obtained. After the calcinations temperature was raised to 800 oC, the results cobalt ferrite. There were no diffraction peaks of aluminum-containing material in the XRD and TEM patterns of calcined LDHs. So the Al3+ should tranform amorphous Al2O3. The products show a small coercivity and squareness when the LDHs precursors were calcined at 400 oC. After calcinations at 500 oC, the products show a coercivity of 1240 Oe and squarenessof about 0.4. When the calcination temperature rose to 600 oC, the coercivity increased to about 1490 Oe, and the largest squareness of 0.45 was achieved. However at higher calcinations temperature, both the squareness and the coercivity decreased. So, the best calcination temperature for the formation of cobalt ferrite nanosheets from LDHs precursors was about 600 oC.