Browsing by Author "Gatsi, Nyepudzai Charsline"

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    Optimization of gallium oxide (ga2o3) nanomaterials for gas sensing applications
    (University of the Witwatersrand, Johannesburg, 2024) Gatsi, Nyepudzai Charsline
    Gas sensors are needed for monitoring different gases in indoor and outdoor environments, food quality assessment, and health diagnostics. Among materials studied for these applications, semiconducting metal oxides (SMOs) have generated a lot of interest due to their excellent sensitivity, simple circuit, and low cost. One-dimensional (1𝐷) 𝐺𝑎2𝑂3 nanomaterials are part of the promising candidates explored for the sensing of different gases due to their excellent electrical conductivity, high catalytic behavior, and chemical and thermal stability. This study reports the optimization of crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for use in the detection of various gases. A set of unmodified and noble metal modified 1𝐷 𝐺𝑎2𝑂3 nanomaterials were synthesized by microwave-assisted hydrothermal method followed by heat-treatment at different temperatures and their gas sensing performances were systematically studied. The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman analysis, scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), photoluminescence (PL), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) methods. The effects of heat-treatment temperatures on phase transformations and gas sensing performances of various 𝐺𝑎2𝑂3 polymorphs were investigated. The 𝛼 − 𝐺𝑎2𝑂3, 𝛽 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures were synthesized and evaluated for gas sensing. The 𝛽 − 𝐺𝑎2𝑂3 sensing layers presented selective response coupled with fast response/recovery times towards carbon monoxide (𝐶𝑂) compared to the 𝛼 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures. The observed variations in the gas sensing performances of these three crystal structures were attributed to controlled properties of different 𝐺𝑎2𝑂3 polymorphs. Furthermore, the 𝛽 − 𝐺𝑎2𝑂3 polymorph was prepared in the form of regular and hierarchical nanorod-based morphological features which demonstrated different gas sensing behaviors. The 𝛽 − 𝐺𝑎2𝑂3 regular nanorods showed better capabilities of detecting isopropanol than the nanobundle-like and nanodandelion-like features, and these differences were attributed to changes in textural, porosity, and compositional properties related to different morphologies. The effects of incorporating 𝐴𝑔 and 𝐴𝑢 noble metal nanocrystals on regular 𝛽 − 𝐺𝑎2𝑂3 nanorods surfaces on their gas sensing behaviour were also investigated. The results revealed that surface modification of 𝛽 − 𝐺𝑎2𝑂3 nanorods with 0.5 and 1.0 𝑚𝑜𝑙% 𝐴𝑔 and 𝐴𝑢 noble metals significantly lowered the sensor operating temperature compared to that of unmodified 𝛽 − 𝐺𝑎2𝑂3 nanorods towards the detection of ethylene. In addition, surface incorporation of 1.0 𝑚𝑜𝑙% 𝐴𝑔 dramatically increased the sensor sensitivity and selectivity and reduced the response/recovery times towards ethylene gas, and these positive changes were attributed to the electronic and chemical sensitization effects stimulated by the catalytic activity of 𝐴𝑔 nanocrystals incorporated on the surface of 𝛽 − 𝐺𝑎2𝑂3 nanorods. This study unambiguously optimized the crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for the detection of carbon monoxide, ethylene and isopropanol gases. These sensors may potentially be used in real-time detection of carbon monoxide and isopropanol for indoor air quality monitoring to improve human health. In additional they have also demonstrated capabilities for the precise and economical detection of ethylene around plants and fruits, which could be beneficial to the horticultural and agricultural industries
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    Spectroscopy of trivalent neodymium ions (Nd3+) in zinc oxide (ZnO) thin films and powders
    (2019) Gatsi, Nyepudzai Charsline
    Intrinsic defects with energy levels which lie within the wide band gap of ZnO play an important role in the excitation of incorporated trivalent rare-earth (RE3+) ions, in ZnO:RE3+. In the present study, low-temperature (10 K – 75 K) photoluminescence studies of trivalent neodymium (Nd3+) ions doped into ZnO were conducted using argon-ion laser lines (457.9 nm, 476.5 nm, 488.0 nm and 514.5 nm) and a dye-laser tunable in the 570 nm – 640 nm region, as excitation sources. ZnO:Nd3+ samples in the form of films and compressed powders were used, with Nd3+ concentration of 1.5mol% and 1mol%, respectively. Nd3+-doped ZnO films suitable for photoluminescence studies were deposited on quartz substrates by the radio-frequency magnetron sputtering method using 100 W power and a 30%O2:70%Ar chamber-gas ratio. Deposition times of 3 hours were used in order to achieve acceptable photoluminescence signal to noise levels and some of the films were annealed at 600 ℃. For powder samples, the ZnO–Nd2O3 mixture was ground and compacted into pellets which were then sintered at 600 ℃, 750 ℃ and 950 ℃, in air. Some of the powder samples were co-doped with Nd3+ and lithium (Li+) in the ratios 1:1, 1:5 and 1:10. X-ray diffraction (XRD) peak positions confirmed the hexagonal wurtzite structure for both the films and the powder samples. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) results revealed an even distribution of Nd3+ ions in the films. In the powder samples, the Nd3+ distribution was even only in samples sintered at 600 ℃ and 750 ℃ whilst in samples sintered at 950 ℃, the Nd3+ ions aremainly situated at grain boundaries. The higher sintering temperature also encouraged grain growth. Photoluminescence spectra of both films and powder samples showed a broad emission band spanning the 500 nm – 875 nm range. This emission arises from transitions between energy levels of the ZnO intrinsic defects, which include vacancies, interstitials and antisites. In ZnO:Nd3+ powders, Nd3+ ions give rise to narrow and discrete absorption bands superposed on the broad emission band. The absorption bands are a result of the Nd3+ ions absorbing specific wavelength portions of the broadband radiation emitted by intrinsic defects. Coincidence of the defect transition energies and Nd3+ multiplet energy positions is testament to this. In addition, sharp Nd3+ emission transitions were observed in the 880 nm to 920 nm (11 364 cm-1 – 10 670 cm-1) range for the films as well as for the Nd3+:Li+ co-doped powders sintered at 950 ℃. This emission is attributed to the 4F3/2 → 4I9/2 transitions of Nd3+ ions. Two different Nd3+ centres were identified in both films and powder samples. The Nd3+ centre present in annealed films is different from that in the unannealed films, showing a changed crystal-field environment for the Nd3+ ion. However, two different Nd3+ centres co-exist in ZnO:Nd3+:Li+ powder samples, with distinctly different fluorescence life-times of 202 μs and 380 μs. Crystal-field energy levels of the D (2G7/2;4G5/2), R (4F3/2) and Z (4I9/2) Nd3+ multiplets have been deduced for each of the centres and comparisons were made. The site-selective technique was key in the separation of the two centres. The near-infrared Nd3+ emission studied in this work has potential applications in bio-imaging because biological tissues have maximum transmittance in the near-infrared region while the re-absorption characteristic can be applied on welding goggles to block the yellow-flame glow, for example. Additionally, doped ZnO pellets such as prepared and used for this study have potential use as cost-effective targets for thin-film deposition by the pulsed laser technique.