Defect-driven selective metal oxidation at atomic scale

Defects endow materials with fascinating chemical reactivities while the underlying dynamics remain elusive. Here we provide an atomistic visualization of oxidation dynamics driven by planar defects in metals.

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Background in structure-property relation in metals and alloys, we are more than familiar with the significant role of defect, especially coherent boundaries, in tuning the mechanical properties of metallic materials. Meanwhile, we are also fascinated by the superior physiochemical properties driven by these typical lattice defects (such as chemical reactivities and catalytic performances) . However, compared with well-established understanding of the strengthening and softening mechanisms, the underlying origin of defect-assisted chemical reactivity in nanomaterials have been largely elusive. We are, therefore, inspired to bridge the gap between the structure and property of materials by rationalizing the real-time atomistic behaviours associated with lattice defects during chemical reactions.

Equipped with Cs-corrected transmission electron microscope (TEM), we are able to directly visualize the complex chemical reactions at the atomic scale. Here, we delve into oxidation process and unveil the critical influences of planar defects, which may hold implications in a range of chemical reactions. This seemingly complex task is in fact realized via an easy and accessible experimental methodology, in which fractured Ag and Pd rods with high-density defects at the fracture surface were oxidized in situ inside the TEM. It is interesting to find that site-selective oxidation behaviours occurred frequently at the junctions between the surface and the coherent twin boundary (TB) or stacking fault (SF).

Both in situ experimental observation and density function theory based calculations demonstrate a higher oxygen binding energy Eb at the junctions between TB/SF and the surface, which is closely related with a lower coordination number (CN). This means that the free oxygen atoms are prone to be absorbed at these sites on the surface, resulting in preferential oxide nucleation over neighboring surface steps or kinks. In general, SF is more favourable than TB for oxide nucleation, and Eb is dependent on the spacing between planar defects.

Figure 1. Site-selective oxide nucleation at the surface-planar defect junctions.

Beyond site-selective nucleation, subsequent oxide growth is assisted by the planar defects as well. In specific, the oxygen ions absorbed at the junction site are provided with an accessible TB or SF freeway to travel into the metal matrix, generating a new step dipole beneath the oxide/matrix interface; this pair of steps move laterally along the interface, resulting in a continuous layer-by-layer inward grow of the oxide. As the oxide thickness accumulates, few O ions are pumped towards the oxidation front (i.e., short of oxygen supply), leading to a gradually decreased oxide growth rate.

Figure 2. Schematic showing the layer-by-layer oxide growth assisted by planar defect.

This selective oxide nucleation with synergistic diffusive oxide growth should essentially accelerate the oxidation process at the TB/SF. In contrast, homogeneous oxidation normally initiates from multiple sites simultaneously in defect-free nanocrystals, and the migration of the oxide/metal interface is mainly dominated by conventional bulk diffusion, leading to significantly slower inward growth.

Figure 3. Homogeneous and site-selective oxidation processes.

Given that nanoscale twins and SFs are common in metallic nanocrystals with low stacking fault energies, this site-selective mechanism should play important roles during the catalytic/chemical/electrochemical reactions in a wide range of materials. Therefore, our mechanistic understanding of defect-driven reaction dynamics holds both fundamental and technological implications for the development of advanced nanomaterials through defect engineering.

More details of this work can be found at:

Qi Zhu

PhD Candidate, Zhejiang University