The term “materials cycle” is generally used to describe the synthesis of substances from raw materials (sometimes including the synthesis of manufactured raw materials), the fabrication of shaped materials, their use, and their eventual disposal. It is well known that all materials are extracted from the earth, then are converted to functional products through various fabrication processes, usually involving a high expenditure of energy, which in turn contributes to environmental problems such as global warming.

Fine powders of semiconducting oxides loaded with deposited metal and/or metaloxide particles have been widely used as heterogeneous photocatalysts for innumerable chemical reactions. Among the many photocatalytic reactions, the splitting of water assisted by light has become one of the most active areas in heterogeneous photocatalysis, since it can be a promising chemical route for energy renewal and energy storage. The photocatalytic splitting of water on TiO2 electrodes, discovered by Fujishima and Honda in 1972, is a prototypic example of this technique, and there is a vast body of literature describing the potential application of TiO2-based photocatalysts for water decomposition. This has brought about a burst of research related to the development of many other photocatalytic systems.

Recently, inorganic/inorganic and organic/inorganic heterostructured materials have attracted considerable research interest, due to their unusual physicochemical properties, which cannot be achieved by conventional solid-state reactions. In order to develop new hybrid materials, various synthetic approaches, such as vacuum deposition, Langmuir–Blodgett films, selfassembly, and intercalation techniques, have been explored. Among them, the intercalation reaction technique—that is, the reversible insertion of guest species into the two-dimensional host lattice—is expected to be one of the most effective tools for preparing new layered heterostructures because this process can provide a soft chemical way of hybridizing inorganic/inorganic, organic/inorganic, or biological/inorganic compounds. In fact, the intercalation/deintercalation process allows us to design high-performance materials in a solution at ambient temperature and pressure, just as “soft solution processing” provides a simple and economical route for advanced inorganic materials by means of an environmentally benign, lowenergy method. These unique advantages of the intercalation technique have led to its wide application to diverse fields of the solid-state sciences, namely, secondary (rechargeable) batteries, electrochromic systems, oxidation–reduction catalysts, separating agents, sorbents, and so on. Through these extensive studies, many kinds of low-dimensional compounds have been developed as host materials for the intercalation reaction, including graphite, transition-metal chalcogenides, transitionmetal oxides, aluminosilicates, metal phosphates, metal chalcogenohalides, and so on. Recently, the area of intercalation chemistry has been extended to high-Tc superconducting copper oxides, resulting in remarkable structural anisotropy.

“Soft solution processing” is rapidly becoming a viable approach for the fabrication of advanced nanostructured materials. It involves the use of environmentally friendly chemicals and preparation methods with minimum energy input. Construction of ultrathin films by the room-temperature, layer-by-layer self-assembly of dilute aqueous solutions (or dispersions) of polyelectrolytes (or polymers), nanoparticles, and nanoplatelets is clearly soft solution processing (see the articles by Yoshimura et al. in this issue).

Metal oxides are important materials in industry. For example, there are many kinds of functional oxides: dielectric oxides such as BaTiO3 (perovskite), semiconductor oxides such as ZnO, ionic conductors such as Bi2O3, magnetic oxides such as Fe3−xMxO4 (spinel), and so on. These oxide materials must be prepared as films, nanodots, and layered films to increase their functionality for a wide variety of applications.

“Ferrite plating” is a typical “soft solution processing” (SSP) application; it enables the formation of oxide ferromagnetic films from an aqueous solution at T 24−100°C under atmospheric pressure. Using ferrite plating, we can grow crystallized ferrite films of spinel-type (MFe)3O4 (where M = Fe, Co, Ni, Zn, Al, Cr, etc.) in one step, requiring no heat treatment. This opens the door to fabricating novel ferritefilm devices using substrates of such nonheat-resistant materials as plastics and GaAs integrated circuits; conventional ferrite-film preparation techniques, such as sputtering, vacuum evaporation, molecularbeam epitaxy, liquid-phase epitaxy, and so on, require high temperatures (>∼600°C) for the crystallization of ferrites, which deteriorates the non-heat-resistant substrates. Ferrite plating is a unique technique that allows us to synthesize ferrite “films” by means of a wet chemical process. There are many techniques, for synthesizing ferrite “particles” from aqueous solutions, but no technique, to our knowledge, enables ferrite-film synthesis by a wet chemical process.