Researchers have shown the gas sensing properties of nanobelts, but there are other structures of TiO2 that have not been studied thoroughly which are in need of further investigations. Metal oxides nanoparticles and nanostructures have proven to be suitable in many electronics devices including gas sensors. TiO2 nanoparticles exhibit promising properties for gas sensing applications; although, there are not enough studies on the impacts of its various structures on the gas sensing behavior of TiO2. It is widely known that TiO2 nanoparticles can form nanostructures with high surface area and aspect ratio just by a simple hydrothermal treatment. Another aspect of this work is to examine the effects of carbon doping on TiO2 nanowires for hydrogen gas sensing applications at room temperature. In the past couple of decades, carbon has attracted a great deal of attention as a result of its amazing properties. Furthermore, it has been theoretically shown that a single layer of graphene has the ability to uptake hydrogen molecules. It is only logical to speculate that introducing carbon to TiO2 nanostructures would improve its functionality for gas sensing applications. In addition, this work will shed some light on why TiO2 nanotubes synthesized via hydrothermal treatment would not be a suitable candidate for gas sensing applications.
Various morphologies of TiO2 nanostructures are synthesized via alkali hydrothermal treatment of 10 M NaOH and titanium dioxide nanoparticles. The formation, effects of hydrothermal duration and temperature, annealing and annealing atmosphere, post-acid treatment and phase transition are studied. The samples are characterized using XRD, Raman spectroscopy, FESEM, EDS, TEM and UV /Vis spectrophotometer. The 1-V characteristics and gas sensing behavior of each structure is investigated. Furthermore; the synthesized nanowires are doped with RGO via azeotrope solution mixing method and then, I-V characteristics and gas sensing behavior of nanocomposite is examined.
The optical bandgap of the prepared samples is shown to be greatly manipulated as a result of alkali hydrothermal treatment, for at least 0.88 eV for nanotubes, 1.077 eV for nanowires and 0.073 eV for nanobelts. In addition samples with tubular morphology exhibit both direct and indirect bandgap. Although synthesized nanostructures exhibit superior electrical properties in compared with nanoparticles, we have come to realize that such improvement are mainly as a result of adsorbed water molecules on the surface and cavities of nanostructures. The nanostructures in compared with nanoparticles; as a result of their superior hydrophilicity exhibit higher conductance for at least two orders of magnitude. Nonlinear resistance variation at room temperature is observed in all samples, suggesting the presence of Schottky barrier between the metal and semiconductor contact. P-type behavior is observed in fabricated sensors due to the influence of water molecules. This is further confirmed by elevating the measurement temperature and observing n-type behavior. In addition the sensitivity of TiO2 (B) at high H2 concentration (1000 ppm to 500 ppm) is the highest, with no response in concentrations below 300 ppm.
The bandgap of final RGO/Nanowires nanocomposite is reduced for 0.34 eV and the redshift in optical absorbance suggest a bigger particle size in compared with nanowires. The addition of RGO to the nanowires proves to retard the hydrophobicity of nanostructure and eliminate the adsorption of water molecules on the surface of nanowires. This has allowed the nanostructure to behave as n-type semiconductor even at room temperature. The increase in RGO dopant from 2.5 wt% to 10 wt% increases the sensitivity from 0.75 ppm-1 to 2.5 ppm-1 although, unlike their nanowire counterpart the RGO doped TiO2 nanowires exhibit nonlinear sensitivity. Furthermore, the response time of 10 wt% RGO doped sample and a recovery time of 2.5 wt% RGO doped sample is the lowest. The 10 wt% RGO doped samples are able to detect H2 at a concentration of 100 ppm with a sensitivity of 0.09 ppm-1 which is a slight improvement compared with pure nanowire with a sensitivity of 0.08 ppm-1 at H2 concentration of 100 ppm although, the response and recovery time were increased.
FESEM images clearly display tubular morphologies (samples prepared at 130C)
*Abstract of the thesis (Doctor of Philosophy) by Saman Azhari
For further information please contact:
Professor Mohd Nizar Hamidon, PhD
Date of Input: 11/06/2020 | Updated: 18/09/2020 | nursyahirah
Institute of Advanced Technology,
Universiti Putra Malaysia,
Selangor Darul Ehsan Malaysia