LAUSR

Cu nanoparticles have attracted much attention due to their optical, catalytic, and electrical properties. Their syntheses, however, have required hazardous reducing agents. In addition, they are easily oxidized, which destroys their properties. It has also not been possible to control the diameter and the dispersibility for further applications in functional materials. To solve these problems, we have recently synthesized oxide-free Cu nanoparticles adsorbed on kaolinite, a non-dispersive layered clay, by environmentally friendly photoreduction. Focusing on the layer charge of the clay, we report diameter- and dispersibility-controlled Cu nanoparticles on saponite, a dispersive clay, in the present study. The obtained oxide-free Cu nanoparticles were stable under N2 atmosphere, while they exhibited a fast back reaction to Cu2+ upon exposure to fresh air. The diameter of the Cu nanoparticles could be controlled simply with [Cu2+], because Cu2+ was adsorbed on saponite before the photoreduction. The dispersibility of the Cu nanoparticles was controlled without changing the diameter using the size of saponite aggregates: these were stably dispersed at low [Cu2+], and flocculated at high [Cu2+]. The flocculates could be collected by filtration and the obtained viscous paste was useful for preparing a film with the optical properties of the Cu nanoparticles without aggregation. Therefore, the present study revealed that combining the adsorption of Cu2+ on saponite with photoreduction is a novel and environmentally friendly synthetic method for controlling the diameter and the dispersibility of Cu nanoparticles, and it has the potential to further their applications in functional materials.