Unraveling Acid Etching-Induced Anisotropic Morphology Evolution of ZnO Nanorods by In Situ Liquid Cell TEM

Wet chemical etching of metals, semiconductors, and insulators is a significant technique that enables surface cleaning, micromachining, shaping and engineering of various structural and functional surface and bulk features [1-3]. However, the microscale shape and size evolution of ionic semiconductors such as zinc oxide during the anisotropic wet etching process is less understood owing to the challenge of complex etching mechanism and limited resolution for visualization of the etching process in acidic liquid medium [4]. In this work, via <i>in situ </i>liquid cell transmission electron microscopy (LCTEM), we directly observe the shape transformation of ZnO nanorod to pencil-shaped nanostructure during the hydrochloride (HCl) acid etching process. Time-lapse LCTEM images revealed a dynamic etching process along the nanopencil tip front. Specifically, the nano-pencil tip shrinks with a slower rate along [000-1] than that along &lt;10-10&gt; directions, with a rapid cyclic tip dissolution and removal in the acid due to accelerating etching rates along various crystal directions. Thus, the base region of the original nano-pencil tip is revealed as a new tip followed by the repeating of the nano-pencil tip angle shrinking step. The high index surfaces, such as {10-1<i>m</i>} (<i>m</i>=0, 1, 2, 3) and {2-1-1<i>n</i>} (<i>n</i>=0, 1, 2, 3), are found to preferentially expose in different ratios, as attributed to selective Cl^- passivation on Zn-terminated surfaces. The surface energetics calculation suggests the preferential Cl^- passivation over the {10-11} and (0001) surfaces of lower energy than others, leading to preferential surface exposures. These investigations shed light on the understanding of anisotropic wet chemical etching of wurtzite-type semiconductor crystals and the design of new functional materials.<br/><br/><b>References:</b><br/>1. Cai ZX, et al. Tailored Catalytic Nanoframes from Metal-Organic Frameworks by Anisotropic Surface Modification and Etching for the Hydrogen Evolution Reaction. <i>Angew. Chem. Int. Ed. </i><b>60</b>, 4747-4755 (2021).<br/>2. Kyeremateng NA, Brousse T, Pech D. Microsupercapacitors as miniaturized energy-storage components for on-chip electronics. <i>Nat. Nanotechnol.</i> <b>12</b>, 7-15 (2017).<br/>3. Ye X, et al. Single-particle mapping of nonequilibrium nanocrystal transformations. <i>Science</i> <b>354</b>, 874-877 (2016).<br/>4. Sun M, Tian J, Chen Q. The studies on wet chemical etching via in situ liquid cell TEM. <i>Ultramicroscopy</i> <b>231</b>, 113271 (2021).