Oxidation Conversion of Carbon-Encapsulated Metal Nanoparticles to Hollow Nanoparticles

We synthesized α-Fe 2 O 3 hollow nanoparticles by directly oxidizing the carbon-encapsulated iron carbide (Fe 3 C@C) nanoparticles in air. In this paper, the conversion mechanism of Fe 3 C@C to hollow nanoparticles was deduced in detail by comparatively investigating the morphologies and compositions of the oxidized products at different oxidation stages using transmission electron microscope (TEM), high resolution TEM (HRTEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It was found that both oxygen and carbon play important roles in the formation of hollow nanostructures, wherein oxygen is the driving force for the outward diffusion of core species and the carbon shell not only provides the diffusion vacancies but also effectively moderates the interdiffusion rates of metal core materials and oxygen. A growth model was proposed: during the oxidation process, three diffusion processes occur including the inward diffusion of oxygen along the carbon shell, outward diffusion of core materials, and inward diffusion of vacancies from carbon shell to core. The outward diffusion of core species involves two steps: the first step is the diffusion of Fe 3 C from core to carbon shells, which is only a physical change (single-crystal Fe 3 C was changed to multicrystal Fe 3 C); and the second one is the diffusion and chemical reactions of Fe 3 C in carbon shells with oxygen (the multicrystal Fe 3 C was oxidized to Fe 3 O 4 and then to α-Fe 2 O 3 ). The two-step diffusion is a theoretical extension to the nanoscale Kirkendall effect, which is expected to be valid in other diffusion couples and theoretical simulation.