In Situ Encapsulation of Ultrasmall CuO Quantum Dots with Controlled Band-Gap and Reversible Thermochromism

Silica encapsulated ultrasmall CuO quantum dots (QDs; CuO@SiO2) were synthesized by reverse microemulsion. The CuO QDs with sizes ranging from 2.0 to 1.0 nm with corresponding band gaps of 1.4 to 2.6 eV were prepared simply by varying the concentration of the Cu(2+) precursor. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV-vis spectroscopy. The CuO@SiO2 composite displayed reversible thermochromism which resulted from the strong electron-phonon coupling of ultrasmall CuO in the confined space of SiO2 and the enhanced band-gap shift in the visible light region depending on temperature. Besides, the as synthesized CuO@SiO2 was found to be highly stable for reversible thermochromism due to the micropore structure of silica matrix and local confinement of the QDs.