This paper reviews important research on chemical and electrochemical synthesis and application of nanoparticles, especially our recent results in this field: (i) catalytic metal nanoparticles for micro‐fuel cells, (ii) magnetic oxide nanoparticles for drug delivery systems, and (iii) magnetic metal nanoparticles for magnetic recording media. To fulfill the requirements of each application, we chose and modified those synthetic methods for obtaining suitable properties, e.g., morphology, catalytic activity, and magnetic properties. (i) For micro‐fuel cells, electrodeposition is attractive because of its selective deposition onto current collectors and possible elimination of an annealing process. As a result, we have successfully synthesized Pt, PtRu alloy, and PdCo alloy, which consisted of dendritic structures macroscopically and of interconnected nanoparticles microscopically. (ii) For drug delivery systems, since magnetic nanoparticles should possess ferromagnetism, be dispersible in water, and be nontoxic, Fe3O4 nanoparticles synthesized by hydrolysis in aqueous media are suitable. As a result, we have successfully controlled the size (10–40 nm in diameter) and the magnetic properties of Fe3O4 nanoparticles by means of adjusting the molar ratio of ferrous to ferric ions in the precursor solution. (iii) For magnetic recording materials, since magnetic nanoparticles should possess high coercivity, a controlled shape, and a uniform small size, we have modified a chemical method for synthesizing FePt by adjusting the growth temperature. As a result, we have succeeded in synthesizing FePt nanoparticles with a controlled shape (cubic) and a uniform size (ca. 5.6 nm).

The process of preparation of FePt nanoparticles was investigated with emphasis on the effect of “growth temperature,” at which the atomic diffusion between Pt-rich and Fe-rich phases leads to the formation of uniform nanoparticles. Consequently, it was demonstrated that by controlling the growth temperature, the shape and crystallinity of FePt nanoparticles can be controlled. On the other hand, the size and composition were almost invariable at 5.6±0.5 nm and Fe53Pt47, respectively.

Au-Ni-C alloys films electrodeposited by a pulsed current method were investigated to assess the crystalline structure, sheet resistance, and wear resistance, for use as an electronic contact material. In particular, we have analyzed the effects of citric acid concentration and pulse off-time on the mechanical and electrical properties of the electrodeposited Au-Ni-C alloy films. The electrodeposition bath used in this study was composed of K[Au(CN) 2 ] and NiSO 4 · 6H 2 O as precursors, and citric acid as the complexing agent. The film microstructure and composition were controlled by adjusting the interval of pulse off-time and the concentration of citric acid. With the prolongation of the pulse off-time interval, XRD results indicated that the amorphous structure with high Ni and C contents was transformed into a nanocrystalline structure, followed by the formation of crystals with small Ni and C contents. The amorphous and nanocrystalline films showed a high Knoop hardness of ca. 500 kg mm −2 , while that of the crystalline films was found to be 300 kg mm −2 . The wear resistance of the film electrodeposited by the pulsed current method was remarkably good compared to that of the direct-current electrodeposition films, even though both films exhibited essentially identical microstructure and composition. The wear property was considered to be relevant to the restoration of the surface flatness of the film by the galvanic displacement deposition of Au during the pulse off time.

Nanoparticles of FePt were prepared in two different ionic liquids (ILs). Because IL serves not only as a solvent but also as a surfactant, the particles prepared could be dispersed stably in hexane without adding other surfactants. The size distribution of the particles was found to be narrow without depending on a size-selection process such as centrifugation. In addition, the particle sizes and composition were found to depend on the kind of IL employed.

L1 0 -FePt nanoparticle having high magnetic anisotropy is a promising candidate for the future ultra high magnetic recording material. We have succeeded in the fabrication of cubical FePt nanoparticles. It is necessary for high areal recording density to develop a homogeneous assembled technique of the cubical nanoparticle with its magnetic easy axis in the vertical direction on a substrate. It has been known the selective chemical reaction between metal species and functional group of organic molecules. We focused on the selective chemical reaction to control the direction of the magnetic easy axis in the cubical nanoparticle. In this study, the modification of the organic molecule with various functional groups to the surface of the cubical nanoparticle was examined. Consequently, the amino groups were replaced by thiol groups and carboxyl groups on the surface ligands of the nanoparticle were residual, and then, we confirmed bonding thiol group end to Pt atom selectively.