Microemulsion mediated sol-gel synthesis of nano-scaled MAl2O4 (M=Co, Ni, Cu) spinels from single-source heterobimetallic alkoxide precursors

Nanocrystalline aluminate spinels with cobalt, nickel or copper as bivalent cations have been prepared in different particle sizes using single-source heterometal alkoxides of the type [M{Al(OR)(4)}(2)] (M-II=Co, Ni, Cu; R=Pr-i, Bu-t) in a microemulsion assisted sol-gel process. The compatibility of the metal stoichiometry in the precursor molecules with respect to the spinel requirement was established by elemental, spectroscopic and single crystal X-ray diffraction analyses. Synthesis of CoAl2O4 using both single- and multi-component routes gave different results: whereas the single-source precursor approach yielded monophasic high purity spinels, phase contamination was evident in the diffractograms of ceramics obtained using a mixture of individual components. The alkoxide precursors were hydrolysed in a homogeneous dispersion of colloidal water droplets in a non-aqueous phase. The diameter of water droplets in the microemulsion can be tuned, inter alia, by varying the hydrophilic chain length of the surfactant molecules which influences the size distribution of nanoparticles in the MAl2O4 ceramics. This parameter is used to obtain nano-spinels with particle sizes in the range 5-45 nm and reveals a qualitative correlation between the initial droplet size and the crystallite size of the resulting spinels. The TG-DTA measurements showed the thermal decomposition to be a three step process with crystallisation of the spinel phase occurring around 900 degrees C. Al-27 MAS NMR spectra reveal the inverse nature of the obtained spinels. XRD, TEM, SEM, EDX, FTIR and UV-VIS spectral studies were used for characterisation of the powders.