Atomic‐Scale Mechanism of Unidirectional Oxide Growth

Abstract

A fundamental knowledge of the unidirectional growth mechanisms is required for precise control on size, shape, and thereby functionalities of nanostructures. The oxidation of many metals results in oxide nanowire growth with a bicrystal grain boundary along the axial direction. Using transmission electron microscopy that spatially and temporally resolves CuO nanowire growth during the oxidation of copper, herein, direct evidence of the correlation between unidirectional crystal growth and bicrystal grain boundary diffusion is provided. Based on atomic scale observations of the upward growth at the nanowire tip, oscillatory downward growth of atomic layers on the nanowire sidewall and the parabolic kinetics of lengthening, it is shown that bicrystal grain boundary diffusion is the mechanism by which Cu ions are delivered from the nanowire root to the tip. Together with density‐functional theory calculations, it is further shown that the asymmetry in the corner‐crossing barriers promotes the unidirectional oxide growth by hindering the transport of Cu ions from the nanowire tip to the sidewall facets. The broader applicability of these results in manipulating the growth of nanostructured oxides by controlling the bicrystal grain boundary structure that favors anisotropic diffusion for unidirectional, 1D crystal growth for nanowires or isotropic diffusion for 2D platelet growth is expected.

Defect-driven selective metal oxidation at atomic scale

“…4b ). This specific event suggests that interstitial O atoms are more rapidly diffused along the TB to bind with freshly exposed Ag atom columns, during which short-cut diffusions along the TB plane is likely to occur 29 . In the following seconds, each newly formed step moves away on the Ag/Ag 2 O interface (Fig.…”

Surface-reaction induced structural oscillations in the subsurface

“…In-situ TEM imaging of structural oscillations in CuO. Our insitu TEM experiments include the in-place CuO preparation via the oxidation of metallic Cu into CuO by exposing clean Cu to O 2 flow 13,14,[24][25][26][27] , followed by subsequently switching to H 2 flow. Figure 1 shows a time sequence of in-situ high-resolution TEM (HRTEM) images extracted from the Supplementary TEM Movie 1 revealing the reaction dynamics spanning from the outermost surface to the subsurface and then to the bulk during the exposure to H 2 flow at pH 2 ≈ 0.5 Pa and T ≈ 300°C.…”

In-situ Atomic-scale Visualization of Autocatalytic Reduction of CuO with H2

“…Using environmental transmission electron microscopy (E-TEM) to visualize hydrogen-induced oxide reduction, here we find that the reaction follows an auto-catalytic process, which differs fundamentally from the two aforementioned kinetic models. E-TEM allows to observe oxide reduction dynamics by flowing a reducing gas in the sample area while simultaneously probing atomic structure evolution from the outermost surface layer to deeper atomic layers [4,5]. In this work, we employ E-TEM to dynamically resolve atomic structure changes in both the surface and deeper regions of the CuO lattice in response to H2 induced oxide reduction.…”

“…E-TEM allows to observe oxide reduction dynamics by flowing a reducing gas in the sample area while simultaneously probing atomic structure evolution from the outermost surface layer to deeper atomic layers [4,5]. In this work, we employ E-TEM to dynamically resolve atomic structure changes in both the surface and deeper regions of the CuO lattice in response to H2 induced oxide reduction.…”