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

Abstract

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.

Revealing microscopic dynamics: in situ liquid-phase TEM for live observations of soft materials and quantitative analysis via deep learning

“…Taking advantage of its ability to achieve remarkable temporal and spatial precision while immersed in a liquid medium, liquid cell TEM technology offers a valuable means to explore diverse phenomena. These include the nucleation, growth, 7–9 and etching mechanisms of individual nanoparticles, 6,10–14 dynamic motion of nanoparticles in liquids, 15–18 electrochemical deposition and lithiation of electrode materials, 19–23 as well as imaging of biomaterials in liquid environments. 24–26 As in situ liquid-cell TEM techniques continue to mature and their application scope expands, there has also been a gradual emergence of studies focusing on soft matter.…”

All-Solution-Based, Low-to-Room-Temperature Fabrication of Position-Controlled Metal-Nanodot-Decorated Semiconductor Nanorods for Enhanced Optoelectronic Transducers

“…Metal–semiconductor hybrid nanostructures have emerged as promising candidates for a wide range of applications, including photodetectors, photocatalysts, plasmonic sensors, and environmental care. − In particular, Ag-nanodot-decorated semiconducting ZnO nanorod (ZNR) structures have shown great potential for high-performance UV sensing, among other applications, owing to the synergistic effect of localized surface plasmon resonance (LSPR) and enhanced charge carrier transport. − However, conventional means of fabricating ZnO and Ag nanostructures, such as chemical and physical vapor deposition or electron-beam lithography, often require high-temperature processing (especially for chemical vapor deposition) and/or complex vacuum equipment while also incurring high cost and slow serial processes, all of which impede their practical applications. Notably, the precise positioning of ZnO nanodots on specific regions of Ag nanostructures on diverse substrates is beneficial for application-specific uses, yet it is uneasy in conventional methods. − …”