Despite much technical progress achieved so far, the
exact surface
and shape evolution during wet chemical etching is less unraveled,
especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a
repeated two-stage anisotropic and pulsating periodic etching dynamic
is discovered during the pencil shape evolution of a single crystal
ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil
tip shrinks at a slower rate along [0001̅] than that along the
⟨101̅0⟩ directions, resulting in a sharper ZnO
pencil tip. Afterward, rapid tip dissolution happens due to accelerated
etching rates along various crystal directions. Concurrently, the
vicinal base region of the original nanopencil tip emerges as a new
tip followed by the repeated sequence of tip shrinking and removal.
The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅ 1̅n} (n = 0, 1, 2, or 3), are found to preferentially
expose in different ratios. Our 3D electron tomography, convergent
beam electron diffraction, middle-angle bright-field STEM, and XPS
results indicate the dissociative Cl– species were
bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation
suggests the preferential Cl– passivation over the
{101̅1} and (0001) surfaces of lower energy than others, leading
to preferential surface exposures and the oscillatory variation of
different facet etching rates. The boosted reactivity due to high-index
nanoscale surface exposures is confirmed by comparatively enhanced
chemical sensing and CO2 hydrogenation activity. These
findings provide an in-depth understanding of anisotropic wet chemical
etching of ionic nanocrystals and offer a design strategy for advanced
functional materials.
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