Exploring Local Structure in Iron Oxide Nanoparticles During Chemical Vapor Synthesis by in Situ and Operando X-Ray Absorption Spectroscopy.

Abstract

Early stages of nanoparticle (NP) formation in gas-phase synthesis processes, such as chemical vapor synthesis (CVS), involve (trans-)formations of molecular species (precursors), small clusters (nuclei), or amorphous intermediates before forming the final NP product. These transformations may be probed using in situ and operando X-ray absorption spectroscopy (XAS), enabling the observation of local structure evolution under CVS conditions. However, this is challenging due to the very dilute sample system, that is, NPs in the gas phase, coupled with high temperatures required for particle formation, deteriorating the signal-to-noise ratio. Here, we present an experimental setup specially tailored to overcome these challenges, enabling in situ and operando XAS measurements of sufficient quality to perform quantitative data analysis. The major focus is a comprehensive understanding of the formation of iron oxide NPs in CVS. Quantitative XANES analysis reveals controlled changes of oxidation states of iron under varying oxygen partial pressures, while EXAFS analysis using reverse Monte Carlo (RMC) simulations provides detailed insight into local structure around the iron atom probed as a function of process temperatures. The mean square displacements available from RMC analysis may be used to estimate the temperature of NPs in the gas phase or to determine their Debye temperatures.