CNRS scientists have studied the evolution of the shape of gold nano-tetrapods and have revealed a two-stage kinetics: the four arms – initially pointed – quickly become blunt, before beginning a slow retraction towards a final quasi-spherical shape.
Figure: Schematic of the temporal evolution (indicated by the arrow) of a nano-tetrapod, from its initial pointed state, to a truncated shape, with arms delimited by {110} facets and finally to a final rounded and quasi-spherical state. This evolution was followed in parallel in solution by X-ray scattering (SAXS) and optical absorption spectroscopy (UV-Vis). A complementary study by electron microscopy (TEM) was carried out in the dry state.
A considerable force is required to deform macroscopic metal objects. On the nanoscale, however, things are different: often, surface forces are sufficient to deform nanoparticles and make them adopt the most favorable shape. Understanding this “nano-metallurgy” is essential to advance our fundamental knowledge but also to guide the progress of nanotechnology. However, the processes involved are difficult to follow in real time.
A team bringing together researchers from several French and European laboratories [1] recently published in the journal Nano Letters a detailed study of the path followed by metastable gold particles, initially equipped with four pointed arms, towards a quasi-spherical particle. Thanks to their well-defined geometry, these “nano-tetrapods” are easier to study than the “nano-stars” that resemble them but whose morphology is more varied. Combining electron microscopy, X-ray scattering and optical spectroscopy techniques and advanced data processing, they were able to highlight a complex kinetics, which starts with a rapid flattening of the tip towards a {110} facet of the gold crystal lattice and continues with a slower step of remodeling towards an isometric shape.
Beyond the conceptual understanding of their evolution, this study will facilitate the use of these nano-objects in nonlinear optics applications [2], thanks to their particular shape which does not have an inversion center.
[1] Damien Alloyeau au Laboratoire Matériaux et Phénomènes Quantiques, (Université Paris Cité/CNRS), le Laboratoire de physique des solides (CNRS/Université de Paris-Saclay), Doru Constantin à l’Institut Charles Sadron (CNRS/Université de Strasbourg) et le synchrotron européen ESRF (Grenoble).
[2] Jieli Lyu et al., Shape-Controlled Second-Harmonic Scattering from Gold Nanotetrapods, J. Phys. Chem. C 126, 9831-9835 (2022).
Référence : Jieli Lyu et al., Two-step reshaping of acicular gold nanoparticles, Nano Letters (2025). DOI: 10.1021/acs.nanolett.4c05601
Contacts : Cyrille Hamon