The outstanding structural and physical properties of carbon nanostructures can be extended by decoration with metallic nanoparticles (NPs) to offer a wide range of potential applications. However, such absorbed metal nanoparticles are generally metastable, and the functional properties of such NPs may be strongly affected by their size and shape. It is therefore necessary to obtain a full understanding of the thermal stability of supported nanoparticles, and how it is affected by the structure of the nanocarbon support.

In-situ TEM observations of thermally and electron beam activated transformations of preformed Au nanoparticles absorbed on a range of selected nanocarbon support structures can provide valuable insight into the NP growth mechanisms. Conventional TEM imaging with a high intensity beam can be used to induce ripening of NPs via beam energy only. Conversely, by using an in-situ heating holder and scanning TEM imaging, with the dwell time per pixel set to be less than the thermal equilibrium time, the effect of thermal heating can be studied. Subsequent to heating, HAADF-STEM combined with tilt-series acquisition enables the 3-dimensional distribution of the AuNPs to be determined in relation to the nanocarbon support without inducing further ripening.

A range of nanocarbon supports, providing differing surfaces environments for Au NPs, have been thus studied. Graphitised carbon nanofibres (GNF) offer two differing surfaces for NP growth. Internally, the surface is corrugated, with typical step-edge heights of ~3nm. Externally, the surface is a smooth graphitic layer. Secondly, multi-walled carbon nanotubes (MWNT) on few-layer graphene (FLG) or amorphous carbon support films provides model systems to appraise the relative importance of the effects of electrostatic interaction and structural factors on the nanoscale organisation of metallic NPs. In particular, the intersection between the convex surface of the nanocarbon exterior and the flat surface of the support film delineates a 1D channel along the nanotube growth axis, with the potential to stabilise NP growth as the proportion of the total surface area of the NP in contact with the carbon support is directly related to the nanoparticle size.

The growth mechanism of Au NPs is observed to be unaffected by the nature of the energy source, with beam induced and thermally induced ripening forming similar size distributions. However, different size distributions internally and externally for AuNPs on GNFs show that the structural influences of the carbon support control the growth mechanism, with growth of AuNPs limited on the corrugated internal surface. In addition, differing size distributions are observed between AuNPs on MWNTs on FLG, and AuNPs on MWNTs on amorphous carbon, with the FLG sample showing a highly ordered structure. Hence the growth mechanism is controlled by the structural and electrostatic influences of the carbon support.

Figures:

HAADF STEM image of Au nanoparticles on a graphitic nanofibre after in-situ thermal annealing. Tilt series analysis confirms that NPs larger than 6nm are on the exterior, smooth graphitic surface. NPs on the interior surface are limited to ~6nm

HAADF STEM image of Au nanoparticles on a multi-walled carbon nanotube on few-layer graphene, after in-situ thermal annealing. Tilt series analysis reveals that exterior NPs present after heat-induced growth and ripening are predominantly located at the 1D channel delineated by the intersection between the nanotube exterior and few layer graphene support.

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-tem-analysis-of-heating-of-gold-nanoparticles-on-nanocarbon-supports-and-implications-for-control-of-ripening/