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Gas sensing properties of devices fabricated using ZnO nanowires/nanobelts (NS) aligned between two electrodes using dielectrophoresis technique were investigated. ZnO nanostructures were synthesized by carbothermal method. The devices were characterized as gas sensors and showed high sensitivity for detection of NO gas at room temperature. Typical sensor response, defined as the relative change in resistance, due to the introduction of the gas was found to be approximately 500% for 40 ppm of NO gas. The devices showed average response and recovery times of about 30 s and 1 min respectively. The results demonstrate the potential of fabricating nanosized sensors using single nanowires/nanobelts.

Abstract: Hydrothermally grown zinc oxide (ZnO) nanowires network has been modified with thin Au layer (∼10nm) to realize an ethanol sensor. The sensor films detected ethanol with maximum response at an operating temperature of 325°C. Incorporation of Au not only enhances the sensor response but also improves the reaction kinetics towards ethanol. The response and recovery times for Au modified sensor films towards 50ppm of ethanol at 325°C were 5 and 20s, respectively. This faster reaction kinetics is attributed to the role of Au in improving the sensing properties as per the electronic sensitization mechanism and its nano-Schottky barriers type junction with ZnO. Nano-Schottky barrier type junction has been attributed further to an increase in resistance for Au modified ZnO. Work function measurements performed using Kelvin probe technique further corroborates the findings. [Copyright &y& Elsevier]

Room temperature gas sensing properties of CuO nanowires synthesized by thermal oxidation of copper foils was studied in different configurations: (i) isolated nanowires aligned between two electrodes, (ii) as grown CuO foil consisting of nanowires and (iii) CuO nanowire films. Sensors were studied for response to different gases. Different sensors showed qualitatively different response on exposure to H2S. Isolated nanowires showed high sensitivity, (~200% for 10 ppm of gas) and fast response (30 s) and recovery times (60 s). In these samples, the resistance mainly decreased on exposure to H2S (though a small initial increase was observed). In CuO foils, resistance increased for low concentrations (5-10 ppm) but decreased at high concentrations. In the case of CuO nanowire films, resistance only increased on exposure of H2S (upto 400 ppm). Since CuO is a p-type semiconductor, on exposure to H2S an increase in resistance is expected due to oxygen adsorption related process. Decrease in resistance in some of the sensors was understood in terms of reaction of CuO with H2S resulting in the formation of CuS. [ABSTRACT FROM AUTHOR]

Sensing characteristics of the drop-cast thin films of poly(3-hexylthiophene) (P3HT), ZnO nanowires, and P3HT:ZnO-nanowire hybrids with different compositions have been studied for various gases, namely, NO2, H2S, NH3, CH4, and CO. At room temperature, pure P3HT and ZnO-nanowire films were highly sensitive to NO2 and H2S, moderately sensitive to CO, and nearly insensitive to NH3 and CH4. P3HT:ZnO-nanowire hybrid films, with increasing ZnO-nanowire content, exhibited gradual enhancement in NO2 sensitivity, while reducing sensitivity for H2S. P3HT:ZnO-nanowire (1:1 ratio by weight) films were found to be highly selective for NO2. Plausible mechanisms explaining the enhanced NO2 selectivity by hybrid films are discussed. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, significant interest has emerged in the synthesis of nanoscale materials owing to their superior and enhanced functional properties. And metal oxides based nanostructures are considered to be the most attractive class of materials for realizing functional nanodevices. Of the various nanostructures, nanowires (NWs) are particularly useful for gas sensing application as they offer various advantages. These includes high surface area-to-volume ratio, pathway for electron transfer (length of NWs), enhanced and tunable surface reactivity implying possible room temperature operation, faster response and recovery time, dimensions comparable to the extension of the surface charge region, relatively simple preparation methods allowing large-scale production, convenient to use, ease of fabrication and manipulation, high integration density, smaller size and low power consumption. All the above mentioned features definitely make NWs a promising candidate for nanosensors. The present review deals with the progress made toward the effective use of NWs in particular metal oxide based for achieving superior sensing performance. The promising approaches contributing toward the further development of this field have been discussed and elaborated. Various issues related to successful utilization of NWs for commercial sensor application is discussed citing examples from the literature. In particular, different sensor configurations like single-NW based, multiple-NW based, NW films/as-grown NWs films have been critically addressed. The important steps taken toward commercialization of ultimate sensor device and the major obstacles involved are also discussed. [Copyright &y& Elsevier]

Abstract: Random networks of ZnO nanowires (ZnO-NWs) have been fabricated on the Si/SiO2 substrates by hydrothermal process using ZnO-nanoparticles as seed. The ZnO-NWs networks have been modified by depositing ultrathin CuO layers (nominal thickness: 10nm) using thermal evaporation of Cu followed by oxygen annealing at 600°C. The results of photoluminescence measurements on ZnO-NWs and CuO:ZnO-NWs networks suggest formation of a p–n junction between p-CuO and n-type ZnO. The gas sensing characteristics of both ZnO-NWs and CuO:ZnO-NWs networks have been investigated. It has been demonstrated that CuO:ZnO-NWs network has selective response toward H2S with a sensitivity of ∼40 (for 10ppm) at an operating temperature of 200°C. A sensing mechanism based on the destruction of p–n junctions due to the formation of metallic CuS (product of chemical reaction between CuO and H2S) has been proposed. [Copyright &y& Elsevier]

Abstract: Hierarchical heterostructures consisting of W18O49 nanowires grown on SnO2 nanowires have been prepared by sequential thermal evaporation of tin and tungsten under partial oxygen atmosphere. Sensors made using isolated heterostructure nanowires showed superior selectivity for the detection of chlorine in comparison to isolated pure SnO2 nanowires as well as mat type films prepared using heterostructure nanowires. Improved response compared to mat type films arises due to different sensing mechanisms of chlorine arising from its adsorption or replacement of oxygen. On the other hand better selectivity in comparison to isolated SnO2 nanowires has been attributed to transfer of electrons across hetero-junction from W18O49 to SnO2 and lower sensitivity of W18O49 to H2S. The results show potential of tailored hierarchical nanoheterostructures for the fabrication of gas sensors. [Copyright &y& Elsevier]