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Different ZnO nanostructures such as nanowires, nanobelts and tetrapods have been grown and used for preparation of thick film (with random grain boundaries) as well as isolated nanowire/nanobelt gas sensors. Sensitivity of different type of sensors has been studied to H2S and NO gases. The results show that the response of ZnO sensors to H2S arises from grain boundary only whereas both grain boundaries and intragrain resistances contribute towards response to NO. In addition, oxygen vacancies in the lattice were also seen to help in improvement of sensor response. Room temperature operating H2S and NO sensors based on ZnO nanowires have been demonstrated. Further, sensors based on isolated nanobelts were found to be highly selective in their response to NO. [ABSTRACT FROM AUTHOR]

Room temperature sensing characteristics of the polypyrrole (PPy) films modified with different weight percentages of ZnO nanowires (ZnO-NWs) have been studied for various oxidizing (NO2 and Cl2) as well as reducing (H2S, NH3, CH4, and CO) gases. We demonstrate that ZnO-NW (50 wt %):PPy composite films are highly selective to Cl2 along with high-sensitivity (40 at 10 ppm), fast-response (55 s), and highly reproducible response curves. It has been shown that negatively charged O2- surfaces of ZnO-NW transfer electrons to PPy, making it in a highly reduced form. A strong localization of charge carriers in the reduced PPy makes composite film highly resistive (>1010 Ω cm) as well insensitive to interaction with most of the gases except Cl2. Cl2 being highly oxidizing gas interacts with composite films and causes a sharp reduction in its resistivity. [ABSTRACT FROM AUTHOR]

CuO-modified ZnO random nanowire networks have been demonstrated to enhance the sensitivity and selectivity towards H2S. CuO being p-type and ZnO being n-type semiconductors, modification with CuO results in the formation of random nano p-n junction distributed over the nanowire surface, thereby leading to depleted nanowires. The enhanced response has been attributed mainly to the interaction of CuO with H2S forming CuS, a degenerated semiconductor with a metallic conductance behaviour, causing a drastic change in the resistance. The governing sensing mechanism can be envisaged to have contributions from the different regions namely nanowires bulk (depleted), junctions among nanowires and the interface between sensor and Au contact electrode, respectively. To establish the governing sensing mechanism, it becomes critical to isolate the contribution arising from each of them. In the present work, we report the impedance and the Kelvin probe studies of CuO/ZnO random nanowire network sensor films. Impedance studies indicate that the contributions arising from the bulk and the nanowire-electrode contact is negligible. A drastic variation in the resistance of the sample arises mainly due to the band bending. The extent of band bending depends on the ambient oxygen and the interaction with the test gas. Temperature- and gas concentration-dependent studies clearly indicated that the CuS formation is the major cause for such bending. Work function measurements further corroborates the finding of impedance studies. [ABSTRACT FROM AUTHOR]

Co-doped SnO2 nanowires were grown by thermal evaporation of a mixture of Tin metal powder and CoCl2 · 6H2O in a tubular furnace. The growth occurs by vapor-solid growth mechanism. Nanowires grew along the sidewall of the alumina boat placed in the tubular furnace and the diameter of these nanowires (as was evident from scanning electron microscopy), was found to be in the range of 50 nm to 200 nm. Co was successfully doped in SnO2 lattice as revealed by X-ray diffraction data and was found to be in Co2+ state in the nanowires as detected by X-ray Photoelectron Spectroscopy measurements. Room temperature magnetic measurements, carried out using Vibrating Sample Magnetometer, indicated ferromagnetic behavior of the nanowires indicating their potential for spintronics applications. With increasing Co doping (upto 1%), the lattice parameters of SnO2 decreased and the saturation magnetization increased, suggesting a strong structure-magnetic property relationship. [ABSTRACT FROM AUTHOR]

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]

SnO2-WOX nanowire heterostructure were grown by physical deposition method. Characterization by XRD, UV-Vis and TEM shows that both the nanowire systems do not interact with each other, and retain their individual characteristic. Sensors fabricated from both, mat films and single isolated nanowire shows response to chlorine as well as H2S gas at room temperature. Single nanowire sensors are found to be better sensitive as well as selective Cl2 sensor. [ABSTRACT FROM AUTHOR]

Low temperature electrical transport properties of composite polypyrrole (PPy) films having ZnO-nanowires (ZnONWs) in the range of 0–50 wt. % have been investigated. It has been found that pure PPy film is in the critical regime of the metal-to-insulator transition, and adding ZnONWs drive composite films into the insulating regime. For PPy films, the transport is governed by Motts three-dimensional-variable range hopping (VRH) (ln ρ∼T-1/4) conduction and with increasing ZnONWs content above 10 wt. % results in a crossover to one-dimensional-VRH conduction (ln ρ∼T-1/2). It has been observed that films having ZnONWs content ≥10 wt. % exhibit reproducible hysteresis in the current-voltage characteristics. [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]