Panda Sri Aurobindo, Choudhary Sumita, Barala Sushil, Hazra Arnab, Jena Suchit Kumar, Gangopadhyay Subhashis
Department of Physics, Birla Institute of Technology & Science Pilani Rajasthan India.
Department of Electrical and Electronics Engineering, Birla Institute of Technology & Science Pilani Rajasthan India
RSC Adv. 2024 Sep 3;14(38):28086-28097. doi: 10.1039/d4ra03128h. eCollection 2024 Aug 29.
Formation of highly crystalline zinc oxide (ZnO) nanowires with an extremely high aspect ratio (length = 60 μm, diameter = 50 nm) is routinely achieved by introducing an intermediate step-oxidation method during the thermal oxidation process of thin zinc (Zn) films. High-purity Zn was deposited onto clean glass substrates at room temperature using a vacuum-assisted thermal evaporation technique. Afterwards, the as-deposited Zn layers were thermally oxidized under a closed air ambient condition at different temperatures and durations. Structural, morphological, chemical, optical and electrical properties of these oxide layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoemission spectroscopy (XPS). It was noticed that the initial thermal oxidation of Zn films usually starts above 400 °C. Homogeneous and lateral growth of the ZnO layer is usually preferred for oxidation at a lower temperature below 500 °C. One-dimensional (1D) asymmetric growth of ZnO started to dominate thermal oxidation above 600 °C. Highly dense 1D ZnO nanowires were specifically observed after prolonged oxidation at 600 °C for 5 hours, followed by short-step oxidation at 700 °C for 30 minutes. However, direct oxidation of Zn films at 700 °C resulted in ZnO nanorod formation. The formation of ZnO nanowires using step-oxidation is explained in terms of surface free energy and compressive stress-driven Zn adatom kinetics through the grain boundaries of laterally grown ZnO seed layers. This simple thermal oxidation process using intermittent step-oxidation was found to be quite unique and very much useful to routinely grow an array of high-density ZnO nanowires. Moreover, these ZnO nanowires showed very high sensitivity and selectivity towards formaldehyde vapour sensing against few other VOCs.
通过在薄锌（Zn）膜的热氧化过程中引入中间步骤氧化法，通常可以制备出具有极高纵横比（长度 = 60 μm，直径 = 50 nm）的高度结晶氧化锌（ZnO）纳米线。使用真空辅助热蒸发技术在室温下将高纯度锌沉积到干净的玻璃基板上。之后，在封闭的空气环境条件下，于不同温度和持续时间对沉积后的锌层进行热氧化。使用各种表面表征技术，如X射线衍射（XRD）、扫描电子显微镜（SEM）、拉曼光谱和X射线光电子能谱（XPS），对这些氧化层的结构、形态、化学、光学和电学性质进行了研究。值得注意的是，锌膜的初始热氧化通常在400℃以上开始。在低于500℃的较低温度下氧化时，通常更倾向于氧化锌层的均匀横向生长。在600℃以上，氧化锌的一维（1D）不对称生长开始主导热氧化过程。在600℃下长时间氧化5小时，随后在700℃下短时间氧化30分钟后，特别观察到了高度致密的1D氧化锌纳米线。然而，在700℃下直接氧化锌膜会导致氧化锌纳米棒的形成。使用分步氧化法形成氧化锌纳米线是根据表面自由能和压缩应力驱动的锌吸附原子通过横向生长的氧化锌籽晶层的晶界的动力学来解释的。发现这种使用间歇分步氧化的简单热氧化过程非常独特，并且对于常规生长高密度氧化锌纳米线阵列非常有用。此外，这些氧化锌纳米线对甲醛蒸汽传感表现出非常高的灵敏度和选择性，相对于其他几种挥发性有机化合物（VOC）而言。
