A Core-Shell Strategy for Improving Alloy Catalyst Activity for Continual Growth of Hollow Carbon Onions

Herein, we report the controlled synthesis of hollow carbon nano-onions (HCNOs) through using core shell Fe-Ni alloy catalysts. The core shell structure is formed through the phase transition during the thermal treatment under hydrogen. The inner gamma-Fe-Ni core shows the catalytic activity in growing HCNOs, whereas the outer shell is catalytically inert and acts as the protective layer, which can also be converted to the inner core during the growth. This continuous supply of active metallic atoms from the outer shell to the inner catalyst can in situ maintain the catalytic activity, avoiding the use of additional oxygen-containing oxidative agents. In addition, the shell-to-core conversion can be well controlled by tuning hydrogen flow rates during the chemical vapor deposition process to enable a continual growth of HCNOs from one catalyst. Furthermore, the size-controlled growth can be realized by controlling the size of the catalyst nanoparticles. This controlled shell-to-core conversion allows for the improvement of catalytic activity for the continual growth of HCNOs before deactivation. To demonstrate the advantage of the hollow structure in energy storage, the performance of HCNOs as the supercapacitor electrode material has been tested. This study offers a possibility for catalyst engineering by a core shell strategy toward lifetime extending in the structure controlled and scalable growth of desirable carbon nanostructures.