A simple and facile template-free method has been developed for the fabrication of CdS hollow nanospheres via hydrothermal treatment of aqueous solutions of cadmium acetate and thiourea. Based on the detailed investigation on the influences of experimental parameters including the precursor chemical concentrations, reaction time, and reaction temperature, the formation mechanism of the hollow interiors by an Ostwald ripening process was the first time proposed for CdS hollow nanospheres. In particular, it was found that molar ratio of thiourea:Cd2+ in the starting solutions affected the sizes of the nanospheres and the hollowing process. At a high ratio of 25:1, hollow nanospheres with an average diameter of ca. 100 nm were obtained. In the presence of a large excess of thiourea, isotropic growth of CdS nanocrystallites occurred with their domain sizes in [100] and [001] directions similar at ca. 20 nm. At the same time, the hollowing process was promoted because of a fast mass transport. At lower molar ratios at 10:1 and 5:1, hollow interiors were not observed and the diameters of the nanospheres increased to 200 and 280 nm. The nanocrystallites in these larger nanospheres grew preferentially in [001] direction to ca. 40 nm. All the nanospheres formed have a hexagonal wurtzite structure and exhibit good size uniformity and regularity. Furthermore, the as-prepared hollow nanospheres demonstrated a good photocatalytic activity for methylene blue degradation by complete oxidation under visible light.

Uniform NiO, α-Fe2O3, ZnO, CuO and Ga2O3 hollow nanospheres consisting of a shell of closely packed nanoparticles with a shell wall thickness of a few tens of nanometers have been successfully synthesized on a large scale by controlled precipitation of metal cations with urea in the presence of carbonaceous saccharide nanospheres as hard templates. In addition, it is interesting to find that the single-phase transition metal oxides of NiO and α-Fe2O3 can form ball-in-ball hollow nanoarchitectures by this simple approach. Furthermore, it is demonstrated that direct size control of the oxide hollow nanospheres can be achieved by using carbon templates of different diameters. This method could be extended to the synthesis of many other metal oxide hollow nanospheres. The hollow nanostructured metal oxides might be found to have potential applications in many fields such as catalyst supports, catalysis, drug delivery, chemical/biological separation, sensing, etc.