Environmental Transmission Electron Microscopy for Catalysis Research: the Example of Carbon Nanotubes
Crucial to the application of nanostructured materials is control over their nucleation and growth, as these aspects determine their structure andthus properties. I will describe how we can exploit the unique capabilities of in-situ environmental cell transmission electron microscopy(ETEM) to observe multiple aspects of these processes. With this approach wecan directly visualize how the catalysts that mediate nanotube growth respond to various changes in the growth environment, and correlate these changes with the resulting nanotube structures.
In the first part of the presentation, we will investigate how dynamic changes in the catalyst morphology are correlated with the termination of growth in vertically aligned SWNT arrays. In particular, we haveinvestigate how the processes of catalyst coarsening, Ostwald ripening and diffusion into the catalyst support can lead to growth termination, and we will describe how changes in the growth feedstock – in particular the incorporation of controlled amounts of water vapor – can alter the catalyst evolution. In the second portion of the presentation, we will describe how altering other aspects of the growthfeedstock – in this case the carrier gas, in combination with the water vapor content – can not only affect catalyst morphological evolution, but can also significantly bias the chiral distribution of the resulting nanotubes.
We will correlate the changes in growth ambient with a faceting / defacting transition, as well as a resulting change in the rate of Ostwald ripening. Finally, ongoing developments of the ETEM technique will be presented, focusing on control of gas streams, improvements in dataacquisition and correlative studies with x-ray absorption spectroscopy. Extension of the observations of morphological changes in CNT growth to broader studies in catalysis will beoutlined.