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.

This study on future ultrahigh density magnetic recording devices aims to develop a technique for arranging FePt nanocubes on a substrate by using organosilane as an interlayer. An array of FePt nanocubes was formed by chemical synthesis, utilizing the Pt-S binding between the SH functional group in ͑3-mercaptopropyl͒trimethoxysilane and Pt in the FePt nanocubes. The morphology of the FePt nanocube array on the substrate was characterized using atomic force microscopy and plan-view scanning electron microscopy, which revealed that the array was a monolayer. After SiO 2 was coated by chemical vapor deposition as a protective layer for sintering prevention, annealing was carried out at 900°C for 3 h in a reducing atmosphere to transform the crystal structure of the FePt nanocubes to the L1 0 phase. X-ray diffraction results revealed that the annealed FePt nanocube array had a FePt͑110͒ orientation and L1 0 phases such as ͑001͒ and ͑110͒. The Pt-S binding did not obstruct the L1 0 ordering of FePt. Consequently, a method for arranging nanoparticles by using the organic molecules with functional groups that can interact with nanoparticle selectivity as an interlayer can be applied as a formation technique for arranging a wide range of nanoparticles. L1 0 -FePt alloys have a very high uniaxial magnetic crystalline anisotropy energy ͑K u = 7 ϫ 10 7 erg/cm 3 ͒. Because the superparamagnetic fluctuation of room-temperature magnetization can be suppressed even for grains with a diameter of 3.0 nm, the appropriate arrangement of FePt nanoparticles composed of one recording bit ϫ one particle is a promising candidate for next-generation-type ultrahigh density magnetic recording media with capacities of Ͼ10 Tbit in. −2 . Since the preparation of monodisperse FePt nanoparticles by chemical synthesis was reported by Sun et al., 1-4 many researchers have paid attention to such nanoparticles for a wide range of applications including magnetic recording media, 5-8 ferrofluids, 9-11 magnetic hyperthermia, 12 magnetic resonance imaging contrast agents, 13 magnetic cell separation, 14 and environmental remediation. 15 Techniques for arranging nanoparticles and controlling the magnetic easy axis in two dimensions ͑2D͒ are very important for the development of ultrahigh density magnetic recording media. Some methods of 2D arrangement such as self-organization and Langmuir-Blodgett ͑LB͒ 16-19 have been reported. It is difficult to use the self-assembly and LB techniques for controlling better organized monolayers on a substrate. This study aims to develop a technique for arranging FePt nanoparticles on a substrate by using organosilane as an interlayer. Previously, we succeeded in fabricating cubic FePt nanoparticles ͑FePt nanocubes͒ with partial L1 0 phases by controlling the growth temperature during chemical synthesis. 20 Self-assembled FePt nanocubes have a ͑100͒ texture on a Cu grid with a carbon-supporting film. This shape-controlled synthesis and self-assembly offers a promising approach to fabricate magnetically aligned FePt na...

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