Evolution of the Local Structure in the Sol-Gel Synthesis of Fe 3 C Nanostructures.

The sol-gel synthesis of iron carbide (Fe ₃ C) nanoparticles proceeds through multiple intermediate crystalline phases, including iron oxide (FeO _x ) and iron nitride (Fe ₃ N). The control of particle size is challenging, and most methods produce polydisperse Fe ₃ C nanoparticles of 20-100 nm in diameter. Given the wide range of applications of Fe ₃ C nanoparticles, it is essential that we understand the evolution of the system during the synthesis. Here, we report an in situ synchrotron total scattering study of the formation of Fe ₃ C from gelatin and iron nitrate sol-gel precursors. A pair distribution function analysis reveals a dramatic increase in local ordering between 300 and 350 °C, indicating rapid nucleation and growth of iron oxide nanoparticles. The oxide intermediate remains stable until the emergence of Fe ₃ N at 600 °C. Structural refinement of the high-temperature data revealed local distortion of the NFe ₆ octahedra, resulting in a change in the twist angle suggestive of a carbonitride intermediate. This work demonstrates the importance of intermediate phases in controlling the particle size of a sol-gel product. It is also, to the best of our knowledge, the first example of in situ total scattering analysis of a sol-gel system.