Synthesis of anisotropic gold nano-micro crystals using by ionic liquid and water interface

A novel synthetic technique for gold nanosheets and hollow gold nanocapsules is developed. When aqueous HAuCl₄ (pH 10, 1 mL) layered on C₄mimPF₆ (1-butyl-3-methylimidazolium hexafluorophosphate, 1 mL) was illuminated vertically with a super high pressure mercury lamp, gold nanosheets with thickness of ca. 30 nm are formed. The gold nanosheets are face-centered cubic (fee) single crystals with an atomically flat (111) surface as determined by electron diffraction analysis. Photoreduction of Au(III) ions did not proceed when aqueous HAuCl₄ solution (pH 10) was photoirradiated in the absence of ionic liquid. Therefore, photoreduction occurs at the ionic liquid/water interface, and photo-excited C ₄mimPF₆ molecules may act as sacrificial electron donors to reduce Au(OH)₄^- ions. We also conducted the photoreduction by using microdroplets of C₄mimPF₆ in water. Ultrathin gold nanosheets with thickness of ca. 10 nm are obtained by photoilluminating mixture of C₄mimPF₆ (1 mL) and aqueous HAuCl₄ (pH 10, 1 mL) under vigorous stirring. To our surprise, in the TEM observation of ultrathin gold nanosheets, moiré fringes are observed for overlapped portions. The observation confirms that the ultrathin gold nanosheets are single crystalline with homogeneous thickness. On the other hand, when a mixture of C₄mimPF₆ (1.8 mL) and aqueous HAuCl₄ (pH 10, 0.2 mL) was illuminated under vigorously stirring, hollow gold microspheres with diameter less than 3 μm are obtained. In this case, water microdroplets in C₄mimPF₆ seem to act as template for the formation of hollow gold spheres.