Nanoscale silver exhibits intrinsic self-healing capabilities without external intervention

As an innovative concept in materials science and engineering, the inspiration for self-healing materials comes from living organisms that have the innate ability to self-repair. With this in mind, the search for self-healing materials has typically focused on “soft” materials such as polymers and hydrogels. For solid-state metals, one can intuitively imagine that any form of self-repair will be much more difficult to achieve.

Although some previous studies have highlighted the self-healing behavior of metals that requires more or less the help of external triggers (e.g., by heating, mechanical stimulus, or electron beam irradiation), the question of whether autonomous self-healing can occur in metallic solids without any external intervention remains a scientific curiosity.

Now, in a new study published in MatterResearchers from the Institute of Physics (IOP) of the Chinese Academy of Sciences have discovered that such an intrinsic and autonomous self-healing phenomenon can occur in silver (Ag) at the nanoscale.

This study, which combines advanced in situ transmission electron microscopy (TEM) with molecular dynamics (MD) simulations, reveals that nanoscale Ag can autonomously repair structural damage, such as nanocracks and nanopores, without external intervention.

This remarkable ability is observed not only at room temperature, but also at freezing temperatures as low as 173 K. Notably, on the same damaged area, repeated reversible self-healing cycles can also be achieved with the same level of efficiency.

The experiments were performed in an atomic-resolution TEM using single-crystal Ag nanosheets as test samples. The nanopores and nanocracks were deliberately fabricated by in situ TEM electron beam drilling. To avoid any possible intervention in the healing process, the Ag nanosheet sample was then kept in a “beam-off” state until each time point for time-lapse TEM imaging.

An interesting and perhaps surprising result is that both representative types of structural damage were observed to undergo rapid and autonomous self-healing within tens of minutes, with the healed regions perfectly restoring the Ag crystal lattice with atomically precise order.

Unlike Ag, gold (Au) did not show similar self-healing behavior at room temperature, despite the fact that Au is the most relevant element to Ag in the periodic table and they share many similarities in physical and chemical properties.

Further MD simulation results reproduced the experimental observations, particularly regarding the difference in healing behavior between Ag and Au. What distinguishes Ag from Au is its high surface diffusion mobility, a feature not generally found in other metallic solids.

Using transmission electron microscopy (TEM), the researchers were able to follow the trajectories of the Ag healing process at the atomic level in situ. Through a combination of atomistic imaging and theoretical simulation results, the research highlights that self-healing is enabled by surface-mediated self-diffusion of Ag atoms, driven by a chemical potential imbalance due to the Gibbs-Thomson effect.

When an incipient damaged structure (a nanopore or nanocrack) begins to exist in an Ag nanosheet, a concave site with negative local curvature is created. Due to the general dependence of the chemical potential on the curvature, the concave damage site will thus have a lower chemical potential compared to the undamaged areas of the nanosheet. This built-in chemical potential imbalance drives the Ag atoms to migrate and repair the damage autonomously, highlighting a sophisticated form of material self-maintenance.

The ability of Ag to autonomously self-repair nanoscale damage at room temperature and below shows a promising possibility for the development of submicron-scale damage-tolerant components and devices.

Perhaps more importantly, in a broader sense, this unusual discovery at the mechanistic level may provide a guiding framework for a deeper understanding of self-healing phenomena and concepts in metallic solids in general.