Metal-oxide nanostructures for low-power gas sensors

Doctoral thesis (2022)

For gas sensing applications, metal oxide (MOx) nanostructures have demonstrated attractive properties due their large surface-over-volume ratio, combined with the possibility to use multiple materials ... [more ▼]

For gas sensing applications, metal oxide (MOx) nanostructures have demonstrated attractive properties due their large surface-over-volume ratio, combined with the possibility to use multiple materials and multi-functional properties. For MOx chemiresistive gas sensors, the temperature activated interaction of atmospheric oxygen with MOx surface plays a major role in the sensor kinetics as it leads to oxygen adsorption-desorption reactions, that eventually affects the gas sensing performance. Thus, MOx sensors are operated at high temperatures to achieve the desired sensitivity. This high temperature operation of MOx sensors limits their application in explosive gas detection, reduces the sensor lifetime and causes power consumption. To overcome these drawbacks of MOx sensors, researchers have proposed the use of heterostructures and light activation as alternatives. In this thesis, we aim to develop low power consuming MOx sensors using these solutions. We show the template-free bottom-up synthesis and shape control of copper hydroxide-based nanostructures grown in liquid phase which act as templates for formation of CuO nanostructures. Precise control over the pH of the solution and the reaction temperature led to intended tuning of the morphology and chemical composition of the nanostructures. We contemplate upon the rationale behind this change in shape and material as CuO nanostructures are further used in a heterostructure. We discuss synthesis and characterisation of CuO bundles and Cu2O truncated cubes, former of which lead to very interesting gas sensing properties and application. Devices made from CuO bundles network are investigated for their electrical and oxygen adsorption- desorption properties as a gas sensor. It was observed that the sensor has faster response and recovery in as deposited condition in comparison to annealed sensor. A detailed inspection of response and recovery curves enabled us to derive parameters like time constants, reaction constants and diffusion coefficients for CuO bundles, an analysis that is scarcely performed on p-type materials. Investigation of the derived parameters, role of network junctions and a hydroxylated CuO surface leads us to discuss the hypotheses for the contributing processes. CuO bundles show conduction transients upon exposure to reducing gas H2 and temperature-based inversion of response upon exposure to reducing gas CO. This has not been reported in literature for CuO exposed to H2 and/or CO. Armed with this fundamental knowledge of gas sensing, we choose ZnO, n type transducer material, and CuO, p type materials with lower band gap and higher absorption in the visible range to synthesise a heterostructure. However, sol-gel synthesis of ZnO and CuO nanostructures have different reactions parameters, like temperature, pH, etc., and do not show natural affinity to grow on the other material. These challenges are overcome by implementing a stepped synthesis procedure to fabricate a heterostructure with Cu-based nanoplatelets on ZnO Nanorods, also represented as CuO@ZnO heterostructure in this thesis. We finally demonstrate electrical and functional characterisation of CuO@ZnO heterostructure. The heterostructure responds differently to tested gasses as compared to its constituent nanostructure ZnO nanorods and a reference CuO nanostructure, CuO bundles. This is an unexpected result as heterostructures usually show response type similar to their base material but with an enhanced sensor response. We present a possible application of e-nose that can differentiate qualitatively between CO, NO2 and ethanol, using the heterostructure, ZnO nanorods and CuO bundles together. [less ▲]