Investigation of nanostructured metal oxide based H2, NOx & C2H5OH conductometric and optical sensors

The author has developed and investigated nanostructured materials based conductometric and optical devices for gas sensing applications. The nanostructured material synthesis, device fabrication and their gas sensing performance have been undertaken. A variety of nanostructured materials were investigated as gas sensing elements, these are: ZnO nanowires, 1D ZnO nanorods, ZnO nanoflowers, nanosheets and nanopyramids, In₂O₃ nanoparticles, WO₃ nanorods, nanoplatelets, and nanoporous, gold (Au) activated WO₃, MoO₃ nanoplates, and Cu₂O and CuO nanograins. The developed nanostructured materials based sensors have high surface- to-volume ratio and achieved high sensitivity towards different gas species.

SILAR and FCVA deposited ZnO seed layers were subsequently followed by thermal decomposition technique to synthesize ZnO nanowires and 1D ZnO nanorods respectively. ZnO nanoflowers and nanosheets were synthesized using the hydrothermal method. Meanwhile, ZnO nanopyramids were synthesized employing the non-aqueous solvothermal method. In₂O₃ nanoparticles were synthesized using the same solvothermal method which employed the aminolysis reaction. WO₃ nanocrystallines and nanorods as well as Cu₂O and CuO nanograins were deposited via radio frequency (RF) sputtering and the WO₃ nanoplatelets and nanoporous were synthesized through the acid etching of RF sputtered W metal layers. MoO₃ nanoplates were deposited via the use of pulsed laser deposition (PLD) technique. All of the nanostructured materials synthesized by the author have at least one lateral dimension less than 100 nm. Micro- nanostructural characterization techniques were employed to extract important information of gas sensitive films such as their structural morphology, surface topography, and material orientation. This information was required to understand the properties of nanostructured materials and link its properties to their gas sensing characteristics.

Structural characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction spectroscopy (XRD) revealed that the developed ZnO nanowires and 1D ZnO nanorods thin films have 1 dimensional nanostructures. The gas sensing results revealed that the ZnO nanowires and 1D ZnO nanorods based sensors are sensitive towards C₂H₅OH, ZnO nanopyramids towards both C₂H₅OH and NO₂ at different optimized operating temperatures, ZnO nanoflowers and nanosheets towards NO₂. The developed sensors have lower optimum operating temperature and higher sensor response sensitivity than conventional polycrystalline ZnO based gas sensors. The particle or grain size of a conventional polycrystalline zinc oxide gas sensor is considerably greater than the depth of the surface space charge region, thus, electrical conduction is controlled by the grain boundaries. However, as synthesized 1D ZnO nanorods and nanowires have greatly reduced dimensions (ZnO nanorods: average diameters 40 nm, ZnO nanowires: average diameter 20 nm) which are comparable to the depletion layer depth. Thus, oxygen adsorption to the nanorods and nanowires has led to an almost complete depletion of conduction-band electrons with a large variation in resistance.

Micro-nano characterisation results revealed that acid etched prepared WO₃ film contains several morphological structures corresponding to different acid used. The vapour sensing results revealed that nanostructured WO₃ based thin films have increased the sensor response towards C₂H₅OH by 5 fold if compared to regular RF sputtered WO₃. The developed sensors showed good stability and repeatability towards C₂H₅OH at optimized operating temperature.