In this study, we report a rapid sonochemical synthesis of monodisperse nonaggregated Fe(3)O(4)@SiO(2) magnetic nanoparticles (NPs). We found that coprecipitation of Fe(II) and Fe(III) in aqueous solutions under the effect of power ultrasound yields smaller Fe(3)O(4) NPs with a narrow size distribution (4-8 nm) compared to the silent reaction. Moreover, the coating of Fe(3)O(4) NPs with silica using an alkaline hydrolysis of tetraethyl orthosilicate in ethanol-water mixture is accelerated many-fold in the presence of a 20 kHz ultrasonic field. The thickness of the silica shell can be easily controlled in the range of several nanometers during sonication. Mossbauer spectra revealed that nonsuperparamagnetic behavior of obtained core-shell NPs is mostly related to the dipole-dipole interactions of magnetic cores and not to the particle size effect. Core-shell Fe(3)O(4)@SiO(2) NPs prepared with sonochemistry exhibit a higher magnetization value than that for NPs obtained under silent conditions owing to better control of the deposited silica quantities as well as to the high speed of sonochemical coating, which prevents the magnetite from oxidizing.

Systematic Mn 2p XPS and Mn K-edge XAS analyses together with the electrochemical measurement have been carried out for the spinel LiMn 2 O 4 prepared at various sintering temperatures in order to elucidate an origin of the dependence of electrochemical properties on synthetic conditions. From the comparative experiments, it becomes clear that a lowering of synthetic temperature gives rise to an increase of structural disorder and of the average oxidation state of manganese, which is more prominent on the surface than in the bulk. Such results suggest that the modification of surface property induced by a decrease of particle size is closely related to the electrochemical performance. The nanocrystalline LiMn 2 O 4 prepared at 250 °C shows excellent cyclability at the 3 V region compared to that of microcrystalline LiMn 2 O 4 prepared at 700 °C. For the purpose of examining the evolution of the chemical bonding nature of inserted lithium, 7 Li MAS NMR studies have been performed for both the spinel compounds before and after Li + intercalation. While the intercalation of 0.2 mol Li + does not induce any remarkable spectral change for the microcrystalline LiMn 2 O 4 , it leads to a dramatic suppression of the NMR signal for the nanocrystalline LiMn 2 O 4 , indicating that the process of grafting Li into the latter phase results in significant modifications of the chemical environment of lithium. On the basis of present experimental findings, it can be concluded that the lowering of synthetic temperature modifies the surface properties, which facilitates the grafting process of Li + ion and, thereby, enhances the electrochemical properties for the 3 V region corresponding to the Li insertion.