We present the preparation and characterization of a novel copper(II)oxide (CuO) nanofiber based sensor with very high sensitivity and selectivity to hydrogen sulfide (H 2 S). The working principle is based on the phase transition of semiconducting p-type CuO to strongly degenerated p-type copper sulfide (CuS) with metallic conductivity. Electrospun polymer fiber networks of polyvinyl butyrate (PVB) and Cu(NO 3 ) 2 were attached on standard gas sensing substrates and calcined to CuO at 600 8C in ambient air for 24 h. Continuous exposure to H 2 S (0.1-5 ppm) as well as a sequence of 1 ppm H 2 S pulses result in a dosimeter type behavior of the nanofiber sensors. Triggered by a certain dose (gas concentration multiplied by time) a steep conductance increase of the sensitive layer over several orders of magnitude is observed. After reaching this percolation threshold only small conductance changes were observed. These fiber based sensors show a remarkably high specificity, there is no response to carbon monoxide, hydrogen, and methane at 160 8C. The fiber network can be regenerated by raising the operating temperature to 350 8C for 30 min in absence of H 2 S.Conductance change under exposure to various gases. Continuous recording with one measurement per second.

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We present a new concept for the detection of hydrogen sulfide (H2S) doses based on percolation effects in semiconducting (p‐type) copper (II) oxide (CuO) thin films. Under H2S exposure at 180 °C CuO undergoes a chemical reaction to metallic conducting copper (II) sulfide (CuS). Reaching a certain dose of H2S (concentration × exposure time) the conductance increases rapidly by two orders of magnitude which is attributed to the formation of CuS percolation paths. This study focuses on the reproducibility of this effect as well as on theoretical modeling of the assumed underlying percolation mechanism. Analysis of conductance data reveals a behavior that is qualitatively very similar to standard scaling theory, but with a lower conductance exponent of µ ≈ 0.85 (instead of 1.3 for 2D systems). The deviation can be explained by a superimposing diffusion process and by deviations of the experimental systems from standard percolation systems. Nevertheless, the CuO thin films exhibit intrinsic structure controlled thresholds for H2S doses, which allows the utilization as H2S dosimeter. Conductance behavior of a CuO thin film exposed to 20 ppm H2S at 180 °C. The percolation threshold pc is reached 1054 s after start of the measurement.

Mesoporous SiO2 nanofibers were prepared by electrospinning dispersions of preformed SiO2 nanoparticles and infiltrated with soluble Cu compounds, forming CuO/SiO2 nanocomposites after heat treatment in ambient air. These composites were used as sensor material in H2S dosimeters, which is based on the formation of conductive CuS, allowing for the dosimetric detection of H2S via the significant increase in conductance. The use of CuO/SiO2 nanocomposites targeted the improvement of the morphological stability of CuO nanostructures during the H2S detection, being confined in a rigid nanoscopic scaffold. Also, we aimed at understanding the parameters determining the decrease in sensing performance commonly observed for CuO-based H2S sensors. As a main result, these CuO/SiO2 nanofibers can be used for gas sensing of H2S in a quasi-continuous and dosimetric detection mode for up to 130 cycles for a concentration of 5 ppm, which exceeds the performance of pure CuO materials. As a concentration of 5 ppm is regarded as a level above which H2S is considered to be harmful, the materials show potential for H2S sensors with long-term stability. The deterioration in the sensing properties is attributed to the irreversible formation of CuSO4. Building on these insights, our study indicates strategies to further improve CuO-based sensors for H2S.

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Abstract.Copper oxides, such as CuO and Cu 2 O, are promising materials for H 2 S detection because of the reversible reaction with H 2 S to copper sulfides (CuS, Cu 2 S). Along with the phase change, the electrical conductance increases by several orders of magnitude. On CuO x films the H 2 S reaction causes the formation of statistically distributed Cu x S islands. Continuous exposition to H 2 S leads to island growth and eventually to the formation of an electrical highly conductive path traversing the entire system: the so-called percolation path. The associated CuO x / Cu x S conversion ratio is referred to as the percolation threshold. This pronounced threshold causes a gas concentration dependent switch-like behaviour of the film conductance. However, to utilize this effect for the preparation of CuO-based H 2 S sensors, a profound understanding of the operational and morphological parameters influencing the CuS path evolution is needed.Thus, this article is focused on basic features of H 2 S detection by copper oxide films and the influence of structural parameters on the percolation threshold and switching behaviour. In particular, two important factors, namely the stoichiometry of copper oxides (CuO, Cu 2 O and Cu 4 O 3 ) and surface morphology, are investigated in detail. CuO x thin films were synthesized by a radio frequency magnetron sputtering process which allows modification of these parameters. It could be shown that, for instance, the impact on the switching behaviour is dominated by morphology rather than stoichiometry of copper oxide.

We present a sensor concept based on copper(II)oxide (CuO) nanofibres for the detection of hydrogen peroxide (H 2 O 2 ) vapour in the percent per volume (% v/v) range. The fibres were produced by using the electrospinning technique. To avoid water condensation in the pores, the fibres were initially modified by an exposure to H 2 S to get an enclosed surface. By a thermal treatment at 350 8C the fibres were oxidised back to CuO. Thereby, the visible pores disappear which was verified by SEM analysis. The fibres show a decrease of resistance with increasing H 2 O 2 concentration which is due to the fact that hydrogen peroxide is an oxidising gas and CuO a p-type semiconductor. The sensor shows a change of resistance within the minute range to the exposure until the maximum concentration of 6.9% v/v H 2 O 2 . At operating temperatures below 450 8C the corresponding sensor response to a concentration of 4.1% v/v increases. The sensor shows a good reproducibility of the signal at different measurements. CuO seems to be a suitable candidate for the detection of H 2 O 2 vapour at high concentrations.Resistance behaviour of the sensor under exposure to H 2 O 2 vapours between 2.3 and 6.9% v/v at an operating temperature of 450 8C.