Enhanced sensitive and selective sensors made with p-NiO nanowire/n-ZnO Nanowire heterostructures

Abstract

A great deal of research has been focused on the synthesis of metal oxide-based nanomaterials because of their superior and enhanced functional properties for realizing functional nanodevices. Among different nanostructures, nanowires (NWs) are considered as next-generation gas sensors. They offer various advantages, including high surface area-to-volume ratio, effective pathway for electron transfer (length of NWs), dimensions comparable to the extension of the surface charge region, enhanced and tunable surface reactivity implying possible room-temperature operation, faster response and recovery time. Further advantages include relatively simple preparation methods allowing large-scale production, convenient to use, ease of fabrication and manipulation, and low power consumption. ZnO NWs, in particular, are easily synthesized using physical/chemical processes, and display wide band gap, high thermal stability, and easy control over morphology. They are often modified with sensitizers and have been used in sensors with improved response kinetics towards gases like CO, C2H5OH, and H2S. Metal oxide based sensors have low cost, low power consumption, and high compatibility with microelectronic processing. However, they suffer from the drawback of poor selectivity, long response and recovery times, less stability and less sensor response values. To overcome these drawbacks the host matrix is often modified with metal sensitizers such as Pd, Pt, Ag, Au, Fe and Cu, or with other metal oxides, viz. NiO, CuO, SnO2, to achieve selective sensors towards desired gases with enhanced response characteristics. This project will focus on establishing nano p-n junctions between p-type NiO and n-type ZnO to reach enhanced, selective response towards target gases such as H2S, NH3, C2H5OH and CO. To accomplish this main objective, we shall: i) Develop low cost, highly selective and room temperature operable and portable nanostructured metal oxide gas sensors. ii) Prepare metal oxide nanowires such as ZnO, NiO, In2O3 and WO3 by different wet chemical techniques, including sol-gel, hydrothermal and Pechini routes and thin film deposition by thermal evaporation, sputtering, electro spinning, chemical vapor deposition and spray pyrolysis. iii) study interfaces of NiO/ZnO random nanowires; iv) study sensor design and fabrication for commercialization. The gas sensors will be made with metal oxides and conducting polymers. Different characterization techniques will be used to investigate materials properties to optimize the sensors, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. p-n junctions with metal oxides will be designed by modifying the sensor surface with a thin layer of NiO. The formation of p-n junctions causes depletion of NWs wherein modification results in the formation of p-n junctions/potential barriers distributed over the surface of the sensor films. The unique interaction leads to the collapse of the potential barrier between the p-type NiO and n-type ZnO thereby causing a drastic change in the sensor resistance. We may also investigate YMn2O5 oxide as a sensing material.