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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.

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]

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]

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]