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ZnO nanotetrapods have been synthesized by carbothermal method. The structure, phase, morphology of the synthesized sample were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, Scanning electron microscopy, Transmission electron microscopy and Selected area electron diffraction. The gas-sensing characteristics of thick films of pure and CuO-functionalized ZnO Nanotetrapods have been compared. Pure ZnO nanotetrapod films were found to be sensitive to both H2S and NO with similar sensitivities, at a temperature of 250–300 °C. It is demonstrated that functionalization of ZnO nanotetrapods with CuO, results in selectivity towards H2S at a lower temperature of 50 °C. [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]

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]

In the present work, gas sensing properties of Calcium-doped Bismuth Ferrite (BFO) nano-particulate films have been investigated as a function of doping concentration, operating temperature and nature of the exposure gases. Pure Bismuth ferrite films were found to be sensitive towards H 2 S gas at 350 °C. With increase in calcium concentration, besides an increase in sensor response, from ∼4.5 in BFO to ∼226 in Bi 0·7 Ca 0·3 FeO 3 on exposure to 50 ppm of H 2 S, a non-ideal p-type behavior is observed in response curves. Such a large enhancement has not been observed earlier in a p-type semiconductor. In BFO and Bi 0·9 Ca 0·1 FeO 3 , resistance of films is observed to increase in accordance to p-type behavior, followed by a small decrease before saturation in gas ambience. In samples with higher calcium content, on exposure to gas, resistance of the sample is observed to increase initially followed by a decrease in resistance while still in gas ambience (saturation in gas ambience is not at all observed). In order to investigate the reason for (i) the large enhancement in sensor response usually not observed in a p-type sensor, and (ii) origin of non-ideal p-type behavior, X-ray photoelectron spectroscopy analysis of Bi 0·7 Ca 0·3 FeO 3 films was carried out, after gas exposure. Based on analysis of XPS results, an additional sensing mechanism involving chemical reaction between the cations of sensing material and H 2 S, was found; which could satisfactorily explain the non-ideal behavior as well as enhancement in sensor reponse. • H 2 S gas sensing properties of calcium-doped Bismuth Ferrite films have been investigated. • With increase in calcium concentration, sensor response is observed to increase from ∼4.5 in BFO to ∼226 in Bi 0·7 Ca 0·3 FeO 3. • Response curves observed, exhibit non-ideal p-type behavior. • Due to samples inherent p-type nature, enhanced sensor response cannot be attributed solely to decrease in particle size or to aliovalent doping or due to induced strain in lattice. The high sensor response is attributed to an additional sensing mechanism due to formation sulfides and sulfates on exposure to gas. [ABSTRACT FROM AUTHOR]

Hydrogen sulphide (H2S) sensors, exhibiting high sensitivity (>104 in 50 ppm of H2S) and short response time (80 s), have been developed employing tin oxide:copper oxide (SnO2:CuO) composite thin films. The films were prepared by thermal evaporation of Sn and Cu metals on recrystallized alumina substrates, followed by oxidation at high temperatures. Results of various studies, such as, scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) reveal that SnO2 and CuO are mutually non-reactive. The CuO grains, which in turn reside in the inter-granular regions of SnO2, inhibit grain growth of SnO2 as well as form a network of p–n junctions. Results of XPS measurements revealed that the formation of Cu(OH)2 due interaction of sensor film with ambient moisture causes drifts in sensor characteristics (low sensitivity and slow response) on prolonged operation in the industrial environment. The sensitivity and response time of degraded sensors could be restored either by exposing the sensor to a high H2S dose (>100 ppm) or by heating it to an elevated temperature (>400 °C). [Copyright &y& Elsevier]