Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy

Despite much technical progress achieved so far, theexact surfaceand shape evolution during wet chemical etching is less unraveled,especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, arepeated two-stage anisotropic and pulsating periodic etching dynamicis discovered during the pencil shape evolution of a single crystalZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopenciltip shrinks at a slower rate along [0001 ] than that along the & LeftAngleBracket;101 0 & RightAngleBracket; directions, resulting in a sharper ZnOpencil tip. Afterward, rapid tip dissolution happens due to acceleratedetching rates along various crystal directions. Concurrently, thevicinal base region of the original nanopencil tip emerges as a newtip followed by the repeated sequence of tip shrinking and removal.The high-index surfaces, such as {101 m} (m = 0, 1, 2, or 3) and {21 1 n} (n = 0, 1, 2, or 3), are found to preferentiallyexpose in different ratios. Our 3D electron tomography, convergentbeam electron diffraction, middle-angle bright-field STEM, and XPSresults indicate the dissociative Cl- species werebound to the Zn-terminated tip surfaces. Furthermore, DFT calculationsuggests the preferential Cl- passivation over the{101 1} and (0001) surfaces of lower energy than others, leadingto preferential surface exposures and the oscillatory variation ofdifferent facet etching rates. The boosted reactivity due to high-indexnanoscale surface exposures is confirmed by comparatively enhancedchemical sensing and CO2 hydrogenation activity. Thesefindings provide an in-depth understanding of anisotropic wet chemicaletching of ionic nanocrystals and offer a design strategy for advancedfunctional materials.