Browsing by Author "Choudhary, Sumita"

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Growth and Characterization of ZnO Nanostructures: Materials for CO and Ethanol Sensing
(Springer, 2021) Hazra, Arnab; Choudhary, Sumita; Gangopadhyay, Subhashis
Controlled growth of ZnO-based nanostructures, starting from a vertical nanowall surface morphology to laterally grown highly anisotropic nanorods/wires formation has successfully been achieved by controlled thermal oxidation of thin Zn films for a temperature range of 100–700 °C. The as-grown ZnO nanorods were further used for carbon monoxide gas sensing at low temperatures (down to 150 °C) as well as ethanol vapour sensing at room temperatures. Thin films of Zn were deposited on glass and silicon substrate at room temperature, using a vacuum-assisted thermal evaporation technique. Structure, morphology and chemical property of ZnO layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. The XRD and SEM results are in very good correlation and showed vertical growth morphology of ZnO nanowall/sheet structures at a relatively lower oxidation temperature up to 400 °C. However, at higher oxidation temperature, lateral growths started to dominate over the vertical growth. Oxidation at 700 °C appeared with laterally grown one-dimensional (1D) ZnO nanowires/rods of high density. Raman spectroscopy and XPS results suggested that the vertical growth is mainly initiated by the metallic Zn film morphology, whereas the lateral growth is strongly dominated by the oxide (ZnO) formation. Finally, laterally grown ZnO nanorods could successfully sense CO gas and ethanol vapour. A drastic enhancement in CO gas sensitivity for a concentration of 230 ppm was clearly observed in dynamic gas flow mode even for a wide range of operating temperature.
Highly selective formaldehyde sensing using ZnO nano-rods
(AIP, 2023-03) Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, Subhashis
Early detection of formaldehyde emission from any household materials is technologically very demanding as it can be a serious human health hazard. Even indirect inhaling of formaldehyde may cause significant harm to our eyes, skin, mouth or any other organs. Hence, fabrication of a simple and sensitive formaldehyde sensor would be of high practical importance. Within this work, formation of ZnO nano-rods by controlled thermal oxidation of vacuum deposited thin Zn films in air ambient, followed by fabrication of formaldehyde sensor operating at relatively lower operating temperature are reported. The crystal structure, surface morphology and optical properties of the as grown ZnO nano-rods have been investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectroscopy, respectively. The XRD patterns of ZnO suggested the formation of highly crystalline oxide films whereas FESEM images have revealed its nano-rods surface morphology with significantly high (length to diameter) aspect ratio. Raman spectroscopy confirms the thermal oxidation of the Zn thin films. As-grown ZnO nano-rods were then subsequently used to fabricate the chemi-resistive formaldehyde sensors. These sensors showed an extremely high formaldehyde sensing performance at a relatively lower operating temperature of 200°C. In a static measurement mode, the sensor exhibited a gas response of about 53% toward 100 ppm of formaldehyde, with a reasonable fast response and recovery time. Moreover, these ZnO nano-rod based sensors have also been tested with similar type of VOCs such as benzene, xylene, alcohols and acetone and appeared with an excellent selectivity towards formaldehyde over the other VOCs.
Semiconducting Metal (Cu, Zn) Oxide Nano-structures: Growth Mechanism, Properties and Gas Sensing Application
(BITS Pilani, 2020) Choudhary, Sumita
Surface energy and stress driven growth of extremely long and high-density ZnO nanowires using a thermal step-oxidation process
(RSC, 2024-09) Panda, Sri Aurobindo; Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, Subhashis
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.