Surface energy and stress driven growth of extremely long and high-density ZnO nanowires using a thermal step-oxidation process

• Published in: RSC advances 2024-08, Vol.14 (38), p.2886-2897
• Main Authors: Panda, Sri Aurobindo, Choudhary, Sumita, Barala, Sushil, Hazra, Arnab, Jena, Suchit Kumar, Gangopadhyay, Subhashis
• Format: Article
• Language: English
• Subjects: Chemistry; Compressive properties; Electrical properties; Free energy; Glass substrates; Grain boundaries; High aspect ratio; High density; Nanorods; Nanowires; Optical properties; Oxidation; Photoelectric emission; Raman spectroscopy; Reaction kinetics; Room temperature; Spectrum analysis; Structural analysis; Surface energy; Surface properties; Thin films; Vacuum thermal evaporation; X ray photoelectron spectroscopy; Zinc oxide; Zinc oxides
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/d4ra03128h

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. Formation of ZnO nanowires during thermal oxidation of thin Zn films.