Formation of colloidal CuO nanocrystallites and their spherical aggregation and reductive transformation to hollow Cu2O nanospheres

In this work, we demonstrate that cuprous oxide Cu(2)O 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 Cu(2)O; and (iv) crystal aging and hollowing of Cu(2)O 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 Cu(2)O 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 Cu(2)O hollow spheres can be potentially useful for harvesting solar energy in the visible range. Possibility of fabrication of Cu-Cu(2)O nanocomposites has also been discussed.