Size-dependent redox behavior of iron observed by in-situ single nanoparticle spectro-microscopy on well-defined model systems

Understanding the chemistry of nanoparticles is crucial in many applications. Their synthesis in a controlled manner and their characterization at the single particle level is essential to gain deeper insight into chemical mechanisms. In this work, single nanoparticle spectro-microscopy with top-down nanofabrication is demonstrated to study individual iron nanoparticles of nine different lateral dimensions from 80 nm down to 6 nm. The particles are probed simultaneously, under same conditions, during in-situ redox reaction using X-ray photoemission electron microscopy elucidating the size effect during the early stage of oxidation, yielding time-dependent evolution of iron oxides and the mechanism for the inter-conversion of oxides in nanoparticles. Fabrication of well-defined system followed by visualization and investigation of singled-out particles eliminates the ambiguities emerging from dispersed nanoparticles and reveals a significant increase in the initial rate of oxidation with decreasing size, but the reactivity per active site basis and the intrinsic chemical properties in the particles remain the same in the scale of interest. This advance of nanopatterning together with spatially-resolved single nanoparticle X-ray absorption spectroscopy will guide future discourse in understanding the impact of confinement of metal nanoparticles and pave way to solve fundamental questions in material science, chemical physics, magnetism, nanomedicine and nanocatalysis.

Real Time Determination of the Electronic Structure of Unstable Reaction Intermediates during Au2O3 Reduction

Chemical reactions are always associated with electronic structure changes of the involved chemical species. Determining the electronic configuration of an atom allows probing its chemical state and gives understanding of the reaction pathways. However, often the reactions are too complex and too fast to be measured at in situ conditions due to slow and/or insensitive experimental techniques. A short-lived Au2O compound has been detected for the first time under in situ conditions during the temperature-programmed reduction of Au2O3. A time-resolved resonant inelastic X-ray scattering experiment (RIXS) allowed the determination of changes in the Au electronic structure, enabling a better understanding of the reaction mechanism of Au(III) reduction. On the basis of time-resolved RIXS data analysis combined with genetic algorithm methodology, we determined the electronic structure of the metastable Au2O intermediate species. The data analysis showed a notably larger value for the lattice constant of the intermediate Au as compared to the theoretical predictions. With support of DFT calculations, we found that such a structure may indeed be formed and that the expanded lattice constant is due to the termination of Au2O on the Au2O3 structure.