Self-anchored catalysts for substrate-free synthesis of metal-encapsulated carbon nano-onions and study of their magnetic properties

We demonstrate the synthesis of a novel self-anchored catalyst structure containing a Fe-Ni alloy nanosheet generated by phase separation for the substrate-free synthesis of carbon nanostructures. Fast Fourier transform analysis was carried out in order to investigate both the phase and structural evolution of the alloy nanosheet during reduction and chemical vapor deposition (CVD) growth. γ-Fe-Ni (Fe_0.64Ni_0.36) and α-Fe-Ni (kamacite) phases were formed and separated on the NiFe₂O₄ nanosheet catalyst precursor during H₂ reduction, forming self-anchored mono-dispersed γ-Fe-Ni nanocrystals on a α-Fe-Ni matrix. The Fe-Ni alloy nanosheet serves both as a catalyst for growing metal-encapsulated carbon nano-onions (CNOs), and as a support for anchoring these preformed nanoparticles, yielding mono-dispersed catalyst nanoparticles with no requirement of additional substrates for the CVD growth. This synthesis is capable of mitigating the coalescence and Ostwald ripening without the assistance of an additional substrate. This structure allows for the growth of uniform-sized CNOs despite the aggregation, crumbling, and stacking of the alloy sheet. This study provides a promising design for novel catalyst structures by phase separation towards the substrate-free synthesis of carbon nanostructures in large scale. Finally, the ferromagnetic Fe_0.64Ni_0.36@CNOs particles demonstrate their application in both magnetic storage and water purification, as a non-toxic water treatment material.