Tailoring Surface Morphology of ZnO Nanowires by Confinement Effects: Implications for Gas Sensing

Abstract In this work, we report an anodic aluminum oxide (AAO) template-assisted vacuum die-casting approach for fabricating Zn nanowires, followed by thermal oxidation to obtain ZnO nanowires. Oxalic acid and phosphoric acid-derived AAO templates were employed to produce Zn nanowires with distinct diameters, enabling systematic investigation of diameter-dependent stress evolution under confinement imposed by the native ZnO shell during oxidation. The as-released Zn nanowires exhibited average diameters of approximately 97.1 and 252.7 nm for the O-AAO and P-AAO templates, respectively. During oxidation at 450 °C, the nanowires derived from O-AAO retained smooth, cylindrical morphologies with a gradual diameter increase, whereas those derived from P-AAO underwent pronounced radial expansion accompanied by surface nanoflake formation. These contrasting behaviors are attributed to confinement-dependent diffusion kinetics, volumetric expansion, and stress accumulation within the oxide shell during oxidation. In larger diameter nanowires, the observed morphology is consistent with stress-assisted shell rupture and possible extrusion of softened Zn, leading to localized oxide burst-like nanoflake formation. The resulting nanoflake decorated ZnO nanowires possess surface-enriched architectures that are promising for surface-dominated applications such as chemiresistive gas sensing.