A simple and easily scalable process for the formation of metal oxide hollow nanospheres using nanoscale Kirkendall diffusion called the "all-in-one beaker method" is introduced. The Fe2O3, SnO2, NiO, and Co3O4 hollow nanospheres are successfully prepared by the all-in-one beaker method. The detailed formation mechanism of aggregate-free hematite hollow nanospheres is studied. Dimethylformamide solution containing Fe acetate, polyacrylonitrile (PAN), and polystyrene (PS) transforms into aggregate-free Fe2O3 hollow nanospheres. The porous structure formed by the combustion of PS provides a good pathway for the reducing gas. The carbon matrix formed from PAN acts as a barrier, which can prevent the aggregation of metallic Fe nanopowders by surrounding each particle. The Fe-C bulk material formed as an intermediate product transforms into aggregate-free Fe2O3 hollow nanospheres by the nanoscale Kirkendall diffusion process. The mean size and shell thickness of the hollow Fe2O3 nanospheres measured from the TEM images are 52 and 9 nm, respectively. The discharge capacities of the Fe2O3 nanopowders with hollow and dense structures and the bulk material for the 200th cycle at a current density of 0.5 A g(-1) are 1012, 498, and 637 mA h g(-1), respectively, and their capacity retentions calculated compared to those in the second cycles are 92, 45, and 59%, respectively. Additionally, Fe2O3 hollow nanospheres cycled at 1 A g(-1) after 1000 cycles showed a high discharge capacity of 871 mA h g(-1) (capacity retention was 80% from the second cycle). The Fe2O3, SnO2, NiO, and Co3O4 hollow nanospheres show excellent cycling performances for lithium-ion storage because they have a high contact area with the liquid electrolyte and space for accommodating a huge volume change during cycling.
Keywords: Kirkendall diffusion; anode material; hollow nanosphere; iron oxide; lithium-ion battery.