Evolution of the Local Structure in the Sol-Gel Synthesis of Fe3C Nanostructures.

Matthew S Chambers,Dean S Keeble,Dean Fletcher,Joseph A Hriljac,Zoe Schnepp

doi:10.1021/acs.inorgchem.0c03692

Matthew S Chambers, Dean S Keeble + Show 3 more

Open Access

https://doi.org/10.1021/acs.inorgchem.0c03692

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Abstract

The sol–gel synthesis of iron carbide (Fe3C) nanoparticles proceeds through multiple intermediate crystalline phases, including iron oxide (FeOx) and iron nitride (Fe3N). The control of particle size is challenging, and most methods produce polydisperse Fe3C nanoparticles of 20–100 nm in diameter. Given the wide range of applications of Fe3C 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 Fe3C 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 Fe3N at 600 °C. Structural refinement of the high-temperature data revealed local distortion of the NFe6 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.

Highlights

Iron forms a range of interstitial compounds with carbon and nitrogen, including ε-Fe3N and θ-Fe3C (Figure 1)
Sol−gel synthesis of Fe3N or Fe3C nanoparticles is achieved by mixing aqueous iron salts with organic molecules such as urea[11] or gelatin[12] as well as with CTAB and melamine.[7]
We report an in situ synchrotron total scattering study of the sol−gel synthesis of Fe3C

Summary

Introduction

Iron forms a range of interstitial compounds with carbon and nitrogen, including ε-Fe3N and θ-Fe3C (Figure 1). These have been widely studied due to their importance in steel but are receiving renewed attention for their potential as catalysts. In order to fully exploit the potential of Fe3N and Fe3C, it is important to have controlled routes to nanoparticles of these materials.[7] Various routes have been proposed to achieve this goal, including laser ablation, ammonolysis of iron oxide nanoparticles, nanocasting,[9] solvothermal synthesis[8,10] and sol−gel chemistry. In order to maximize the beneficial catalytic properties of iron nitrides and carbides and fully explore their potential, it is essential to gain a better understanding of how they are formed

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