Fabrication of Nanostructured Functional Materials Using Exfoliated Nanosheets as a Building Block

Abstract

A variety of layered transition metal oxides and layered double hydroxides have been exfoliated into unilamellar nanosheets such as Ti_0.91O₂, Ca₂Nb₃O₁₀, MnO₂, and M_1-x^IIAl_x(OH)₂ (M^II=Mg, Co, Zn, Ni). The exfoliation was promoted via the high degree of swelling driven by intercalation of quaternary ammonium ions for the layered oxides and solvation with formamide for the hydroxides. Various characterizations revealed very high two-dimensional anisotropy for the nanosheets thus obtained; a thickness of around 1 nm vs. a lateral size of submicrometers or larger. These nanosheets can be assembled or organized through various solution-phase processings to produce a range of nanostructured materials. Addition of electrolytes into a colloidal suspension of nanosheets promotes their restacking, yielding a flocculate. The combination of nanosheets and some counterions through this route led to a range of nanocomposites which exhibited fluorescent, photocatalytic, and electrochemical properties. On the other hand, the nanosheets can be deposited layer-by-layer with suitable polyelectrolytes to produce multilayer ultrathin films, in which the nanoarchitecture can be precisely controlled. The multilayer films of Ti_0.91O₂ and MnO₂ nanosheets showed photoinduced superhydrophilicity and electrochromic properties, respectively. Furthermore, the films of ferromagnetic titania nanosheets such as Ti_0.8Co_0.2O₂ and Ti_0.6Fe_0.4O₂ were found to show very high magneto-optical properties at room temperature.