Environmental transmission electron microscopy (ETEM) enables dynamic investigation of chemical reactions with atomic resolution in the gaseous environment. Therefore, it provides a powerful tool to study fundamental chemical reactions in-situ [1]. In this regard, understanding the oxidation mechanism of catalytically active metals is important to increase their efficiency [2]. In this work, we have investigated the effects of high-energy electron irradiation on the oxidation reaction of Cu nanoparticles (NPs). The hemispherical-shaped NPs were oxidized in 3 mbar of O2 and at temperatures 100, 125, 150, 175, and 200 °C. The formation and evolution of the copper oxide layer were recorded with sub-nanometer spatial resolution in the environmental scanning transmission electron microscope (ESTEM). The oxidation process encompasses the outward diffusion of Cu ions, which causes the outer oxide shell growth and the inner oxide shell growth arising from the inward diffusion of the O ions. The results reveal that the electron beam actively influences the reaction, and overall, accelerates the oxidation of the NPs when compared to particles oxidized without exposure to the electron beam. However, we observed that while the beam exposure accelerates oxidation due to the outward diffusion of Cu ions, it inhibits oxidation due to the inward diffusion of O ions. Moreover, the extent of the beam influence on the reaction is not constant but decreases with increasing temperature. In this regard, we quantified this electron beam induced increase in reaction rate in terms of an apparent rise in temperature; and obtained values ranging from 30 – 6 °C for the nominal temperatures 100 – 200 °C. In order to check the charging and ionization effects in the ESTEM environment, we performed the experiment at half of the previously applied electron dose and observe a considerably lower effect of the beam on the reaction rate. In summary, we can conclude that the role of the electron beam in ESTEM oxidation reactions is mostly related to charging and ionization effects. In general, the beam can both enhance and suppress the oxidation reaction depending on the direction of the diffusing ions and the induced electrostatic field.

References

[1] T. W. Hansen, J. B. Wagner and R. E. Dunin-Borkowski, Mater. Sci. Technol., 2010, 26, 1338–1344.

[2] S. Nilsson, M. R. Nielsen, J. Fritzsche, C. Langhammer and S. Kadkhodazadeh, Nanoscale, 2022, 8332–8341.