Single Nanoparticle Spectroscopy
Nanocatalysis: Well-defined nanofabricated model systems and taming catalytic nanoparticles at single particle level
Understanding the size effects in nanoparticles is of great interest and many studies have indicated that shape and size considerably affect selectivity and activity of reactions involving metal catalysts but the impact of confinement of nanoparticles on the active sites is ambiguous. 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 with X-ray absorption spectroscopy (XAS), under same conditions, during in-situ redox reaction using X-ray photoemission electron microscopy (PEEM) 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 implying that every size-effect seen in rate of reactions is not necessarily associated with change of intrinsic properties or increasing reactivity of catalytic metal nanoparticles. This advance of nanopatterning together with spatial-resolved single nanoparticle 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, and nanocatalysis.
- W. Karim, A. Kleibert, U. Hartfelder, A. Balan, J. Gobrecht, J. A. van Bokhoven, Y. Ekinci, Size-dependent redox behavior of iron observed by in-situ single nanoparticle spectro-microscopy on well-defined model systems, Nature Scientific Reports, 2015, accepted, in press.