Layered double hydroxides (LDHs) are a group of synthetic anion clays, characterized by the formula [MII1-xMIIIx(OH)2]x+(An-)x/n.yH2O (where M=metal and A=anion). The identities of the di- and trivalent cations (MII and MIII respectively) and the interlayer anion (An-) together with the value of the stoichiometric coefficient (x) may be varied widely, giving rise to a large class of isostructural materials. LDHs have a layered structure, with positively charged brucite-like sheets and anions, CO32-, SO42-, NO3-, in the interlayer. LDHs have attracted a great interest as anion exchangers, additives for various polymers and other functional materials. The mixed oxides obtained from the LDHs precursors can be used as adsorbents, catalysts and pigments and in sensors and magnetic technologies. Most of these advanced functions depend strongly on the composition, size and morphology of LDHs, therefore nanostructures of Ni/Al LDHs will be of particular interest for the applications.
A quaternary microemulsion, cetyltrimethylammonium bromide (CTAB)/water/n-hexane/n-hexanol, was selected for the synthesis of Ni/Al-LDHs. The structure, composition and morphology of the obtained Ni/Al-LDHs were investigated by X-ray diffraction (XRD), inductively coupled plasma emission spectroscopy (ICP), infrared spectrometer (IR) and transmission electron microscopy (TEM).
Ni/Al-LDHs with nanowire-, spherical-, rod- and tube-like morphologies were prepared via the microemulsion-mediated hydrothermal synthesis. The CTAB concentration played an important role in determining the morphology of Ni/Al-LDHs, Ni/Al LDHs with uniform nanowires were formed at the CTAB concentration of 3.5wt%. As increasing the CTAB concentration to 3.8wt%, spherical Ni/Al-LDHs were obtained. With the further increase of CTAB content to 4.2wt%, a clear rod-like structure was observed, and many Ni/Al-LDH nanorods self-assembled into well-ordered bundles of a few to tens of aligned rods. The tube structure was observed in the sample obtained at 4.4wt% of CTAB concentration. This leads to the conclusion that the morphology of Ni/Al-LDHs could be controlled by changing the CTAB concentration.
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