Abstract
We show how the kinetics of a fast and irreversible chemical reaction in a nanocrystalline material at high temperature can be studied using nanosecond electron pulses in an electron microscope. Infrared laser pulses first heat a nanocrystalline oxide layer on a carbon film, then single nanosecond electron pulses allow imaging, electron diffraction and electron energy-loss spectroscopy. This enables us to study the evolution of the morphology, crystallography, and elemental composition of the system with nanosecond resolution. Here, NiO nanocrystals are reduced to elemental nickel within 5 µs after the laser pulse. At high temperatures induced by laser heating, reduction results first in a liquid nickel phase that crystallizes on microsecond timescales. We show that the reaction kinetics in the reduction of nanocrystalline NiO differ from those in bulk materials. The observation of liquid nickel as a transition phase explains why the reaction is first order and occurs at high rates.
Highlights
We show how the kinetics of a fast and irreversible chemical reaction in a nanocrystalline material at high temperature can be studied using nanosecond electron pulses in an electron microscope
We show the presence of a short-lived liquid transition phase that decisively determines key pathways of the reaction kinetics
Time-resolved TEM experiments are carried out in an ultrafast TEM12,22 where nanosecond electron pulses are used for imaging, electron diffraction, and energy-loss spectroscopy (EELS)
Summary
We show how the kinetics of a fast and irreversible chemical reaction in a nanocrystalline material at high temperature can be studied using nanosecond electron pulses in an electron microscope. Infrared laser pulses first heat a nanocrystalline oxide layer on a carbon film, single nanosecond electron pulses allow imaging, electron diffraction and electron energy-loss spectroscopy This enables us to study the evolution of the morphology, crystallography, and elemental composition of the system with nanosecond resolution. The large magnetization of some transition metal nanoparticles is lost immediately upon oxidation, and the rapid reduction can be an indispensable reconditioning process for such applications Understanding these transformations needs detailed knowledge about the reaction kinetics[4,5,6] and the appearance of transition states during the reaction. This problem has recently been solved by using a modified photo-gun and TEM electron optical configuration to filter the electron pulses and improve their energy resolution while providing for highcollection efficiency in the spectrometer[12]
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