Investigation of nanostructured semiconducting metal oxide and conducting polymer thin films for gas sensing applications

In this thesis, the author developed and investigated nanostructured materials based Surface Acoustic Wave (SAW) and conductometric devices for gas sensing applications. The nanostructured material synthesis, device fabrication and their gas sensing performance have been undertaken. The investigated SAW structures are based on 64° YX LiNbO3 and 36° YX LiTaO3 substrates, with a piezoelectric zinc oxide (ZnO) intermediate layer. The conductometric structures are based on sapphire substrates. A variety of nanostructured materials were investigated as gas sensing elements, these are: ZnO nanorods and nanobelts, polyaniline nanofibers and polyaniline/metal oxide nanocomposites fibers. The developed nanostructured materials based sensors have high surface to volume ratio and achieved high sensitivity towards different gas species. Thermal evaporation and radio frequency (RF) sputtering methods were used to synthesize and deposit ZnO nanobelts whereas ZnO nanorods were fabricated using a hydrothermal method. Polyaniline nanofibers and its nanocomposites with metal oxides were synthesized with rapidly mixed reaction method. All of the nanostructured materials synthesized by the author have at list one lateral dimension less than 100 nm. Nanostructural characterization techniques were employed to extract important information of gas sensitive films such as their structural morphology, surface topography, and material orientation. Structural characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction spectroscopy (XRD) revealed that the developed ZnO nanobelts and nanorods thin films have single crystal, one-dimensional nanostructures. The gas sensing results reveal that ZnO nanorod based sensors are more sensitive towards H2, NO2 and CO gas species than ZnO nanobelt based sensors due to nanorods well-aligned orientation to the substrate and nanorod based films high porosity. Hydrochloric acid (HCl) and camphor sulfonic acid (CSA) were used in the synthesis to obtain 30 and 50 nm average diameter polyaniline nanofibers. These polyaniline nanofibers were employed to developed polyaniline nanofibers/ZnO/SAW and conductometric sensors to operate at room temperature. It is the author’s best knowledge that he was the first to investigate and link polyaniline nanofibers diameters towards hydrogen gas sensing responses and found that camphor sulphonic acid (CSA) doped polyaniline nanofiber (average 50 nm diameter) based sensors have higher sensitivity towards H2 than HCl doped polyaniline nanofiber (average 30 nm diameter) based sensors. However, the response and recovery were faster for the 30 nm diameter HCl doped nanofibers than that of the 50 nm diameter nanofibers. Additionally, to the author’s best knowledge, for the first time comparison between doped and dedoped nanofiber sensors responses have been investigated during this PhD work. Doped polyaniline nanofibers based sensor has higher sensitivity than dedoped. The author also extended his research to successfully develop polyaniline/metal oxide nanocomposites/ZnO/SAW structures for room temperature gas sensing applications. The author’s investigations revealed that polyaniline/In2O3 nanocomposite based sensors produced stable and stronger response towards gas species than other polyaniline/metal oxide nanocomposite based sensors. The gas sensing performance of the investigated nanostructured materials/SAW and conductometric structures provide a way for further investigation to future commercialization of these types of sensors.