Alternate layered nanostructures of metal oxides by a click reaction.

The nanoconjugation of two metal oxides is expected to enhance and innovate their functionalities and properties. One of the best candidates for nanoconjugation is a photocatalyst consisting of titanium and tungsten oxides. Conjugation between titanium oxide and tungsten oxide facilitates a charge separation, because excited electrons of titanium oxide will move to the conduction band of tungsten oxide. Nanoconjugations precisely arranged on a nanometer scale are crucial for improved function and the development of new properties. Nanostructural control using layered metal oxide is one of the most useful methods for arranging metal-oxide species on a nanometer scale. Layered metal oxide composed of various elemental species is expected to develop unique properties and applications. The interlayers of a layered metal oxide possess the cations for insertion of a bulky quaternary ammonium ion, thereby resulting in the formation of nanosheets by exfoliation of the layers. Nanosheets are utilized in the formation of alternate-layer structures using selforganization and the Langmuir–Blodgett method. However, these methods require step-by-step operations, which takes a lot of time and labor for multilayer structures. On the other hand, a multilayer structure can also be created in one step. Sasaki et al. reported that an alternating layered structure can be synthesized by simply mixing a liquid dispersion of a double-hydroxide nanosheet with a positively charged surface and that of a metal-oxide nanosheet with a negatively charged surface. However, in this case, cationic nanosheets must be combined, and there is no other choice than to choose layered double hydroxides as a cationic nanosheet. Therefore, the method of creating an alternating laminate structure using various metal-oxide nanosheets through a one-step process creates possibilities for varied chemical compositions. To realize this new methodology of alternating nanoconjugation, we focused on click chemistry. Click chemistry is a reaction used to easily form stable connections between specific functional groups. Sharpless suggested that the Huisgen reaction, which is a cycloaddition reaction of alkyne and azide groups, is the main reaction of click chemistry. Dinolfo et al. reported porphyrin multilayer films on inorganic substrates by the reaction of alkyne and azide groups, which indicated that click chemistry is suitable for the synthesis of multilayer structures. However, explosive sodium azide is commonly used for introducing an azido group, and the experimental operation is not easy. Therefore, the thiol–ene reaction of click chemistry was used. A thiol–ene reaction starts when a radical initiator reacts with a thiol group, and alkene and thiol groups react in good yields. The thiol–ene reaction was also applied to ferrocene multilayer films. Therefore, heterogeneous materials can be selectively connected. Herein, we propose a new methodology to realize alternating multilayer powders by using a thiol–ene reaction (Figure 1). Two kinds of metal-oxide species were each modified with alkene and alkyl thiol groups, and dispersed into organic solvent to exfoliate the layers. A radical initiator was added to the dispersion to induce a thiol–ene reaction, then the powder of hundreds of layers of alternate stacks was synthesized in one step. Titanium oxide–tungsten oxide alternating laminate was used as a photocatalyst. Recently, heterojunctions between titanium oxide and tungsten oxide nanoparticles obtained by click chemistry showed enhancement of the photocatalytic reaction. Alternate layers of metal oxides are a promising structure because of the large area of nanoconjugations.