Investigation of metal oxide nanostructured thin films based optical hydrogen sensors
Creators: name: Yaacob, Mohd Hanif
Publisher: name: RMIT University
Year: 2024

description: In this PhD research, novel metal oxide nanostructured thin film based optical H<sub>2</sub> sensors were developed and investigated. The nanotechnology enabled gas sensors had been reported to improve substantially their performance compared to the conventional sensors towards target analytes. Nevertheless, the previous investigations were mostly concentrated on the electrical based sensors and less focused on the optical based sensors. Therefore, the author embarked into this project to explore the sensing potential of the optical devices and comprehensively study a various kinds of metal oxide nanostructured thin films as the gas sensing layers. This was undertaken with the aspirations of enhancing the performance of the nanostructured thin film based optical sensors as compared to the conventional based sensors. To the best of author’s knowledge, the author developed several novel metal-oxide nanostructured based optical sensors with morphologies that have not been reported previously. The nanostructured materials under investigation are tungsten trioxide (WO<sub>3</sub>), molybdenum trioxide (MoO<sub>3</sub>), titanium dioxide (TiO<sub>2</sub>) and nickel oxide (NiO). The developed optical gas sensors require a thin (≈25Å) catalytic metal layer such as palladium (Pd), platinum (Pt) or gold (Au) to dissociate the H<sub>2</sub> molecules into the nanostructured thin films. The gasochromic properties of these nanostructures were investigated towards H<sub>2</sub> with low concentration (&lt;1%). Micro-nanocharacterisation techniques such as SEM, TEM, XPS, XRD, EDX, Raman and UV-vis-NIR spectroscopies were employed to obtain complete structural properties of the nanostructured materials in order to fundamentally understand their functionalities with respect to the optical sensors’ performance. In the case of sputtered WO<sub>3</sub>, the 500 nm films were deposited onto different transparent substrates consisting of quartz, glass, ITO and FTO conductive glasses to study the effect of the different WO<sub>3</sub> nanograin sizes on the H<sub>2</sub> sensing performance. The absorbance response increase of 110% was recorded for the Pd/sputtered WO<sub>3</sub> films with similar grain sizes (30 - 60 nm) on quartz, glass and ITO as compared to the small response (5% increase) for the films on FTO glass with large grains (300 – 500 nm) upon exposure to H<sub>2</sub> of 1% concentration. The H<sub>2</sub> sensing performance was also compared for the WO<sub>3</sub> films with different catalyst. Pd was proven to be highly efficient in improving the optical response as compared to Pt and Au. For the first time, the author successfully developed optical sensors based on Pd/MoO<sub>3</sub> nanorod films which are sensitive towards H<sub>2</sub> with low concentrations &lt;1% at temperature below 120°C. The p-type metal oxide shows the opposite gasochromic properties than the n-type metal oxides which is proven by the Pd/400 nm p-type NiO nanostructured film reduced its absorbance upon H<sub>2</sub> exposure. It was found out that amongst the sensors investigated, the devices based on the Pd/WO<sub>3</sub> sputtered films and Pd/WO<sub>3</sub> nanoplatelet films produced the largest absorbance response compared to the other metal oxide nanostructured films by increasing the response to 110% and 99%, respectively, towards H<sub>2</sub> with 1% concentration. Finally, the author deposited the Pd/WO<sub>3</sub> sputtered films and Pd/WO<sub>3</sub> nanoplatelet films onto different optical transducing platforms such as optical fiber and channel waveguide.