Formation of Colloidal CuO Nanocrystallites and Their Spherical Aggregation and Reductive Transformation to Hollow Cu2O Nanospheres

Abstract

In this work, we demonstrate that cuprous oxide Cu₂O nanospheres with hollow interiors can be fabricated from a reductive conversion of aggregated CuO nanocrystallites without using templates. A detailed process mechanism has been revealed: (i) formation of CuO nanocrystallites; (ii) spherical aggregation of primary CuO crystallites; (iii) reductive conversion of CuO to Cu₂O; and (iv) crystal aging and hollowing of Cu₂O nanospheres. In this template-free process, Ostwald ripening is operative in (iv) for controlling crystallite size in shell structures and thus for precisely tuning the optical band gap energy (E_g) of resultant semiconductor nanostructures. For the first time, a wealth of colorful Cu₂O hollow nanospheres (outer diameters in 100−200 nm), with variable E_g in the range of 2.405−2.170 eV, has been fabricated via this novel chemical route. Considering their unique hollow structure and facile tuning in band gap energy, the prepared Cu₂O hollow spheres can be potentially useful for harvesting solar energy in the visible range. Possibility of fabrication of Cu−Cu₂O nanocomposites has also been discussed.