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We report for the first time on the synthesis of NiO/ZnO one-dimensional (1D) nanowire (NW) based heterostructures by applying a suitable methodology of transport and condensation. The synthesis involves, firstly the growth of NiO NWs on gold (Au) catalyzed alumina substrates using the vapor-liquid-solid (VLS) mechanism and then the formation of ZnO NWs directly on the NiO NWs using the vapor-solid (VS) mechanism. Sequential evaporation–condensation over Au-seeded alumina promotes the formation of NiO NWs, driven by the VLS growth mechanism. These NiO NWs act as backbones for the condensation of epitaxial ZnO nanostructures. The detailed morphological study of these heterostructures reveals that ZnO nanowires completely cover the whole NiO nanowires completely and growing out in the form of flat leaves from the NiO nanowire branches. The diameters of the NiO NWs have been found to vary from 15 nm to 60 nm. Selected area electron diffraction data (SAED) indicate an epitaxial growth of ZnO nanowires along (101)-planes on the strongly oriented NiO nanowires along (200) crystallographic planes. Finally, NiO NW and NiO/ZnO heterostructure based conductometric gas sensing devices have been fabricated and the comparison between their sensing performances have been compared. Interestingly, NiO/ZnO NWs heterostructure based sensing devices shows superior performance compared to NiO sensors toward volatile organic compounds (VOC).

a b s t r a c t This paper focuses on the capabilities and development prospects of nanostructured metal oxides (MOX) representing the most versatile and richest class of materials in terms of electronic structure and structural, chemical, and physical properties. MOX nano-materials with controlled composition, surface terminations, and crystalline structures to be used as chemical sensors as a new area in analytical chemistry and instrument engineering are discussed. Their principles of operation, and basic characteristics are outlined and main applications of MOX sensor technology are presented.

Focusing here on the effects of zinc doping in a nanocrystalline matrix of tin dioxide, inverse opal prototype sensors are presented and extensively studied as superior candidates for gas sensing applications. Courtesy of factors including controlled porosity, enhanced surface to volume ratio and homogeneous dispersion of species in the crystalline lattice assured by the sol-gel technique, prototype sensors were prepared with high dopant ratios in a range of new compositions. Exploiting their high sensitivities to low-gas concentrations at low working temperatures, and thanks to the presented templated sol-gel approach, the prepared sensors open up new frontiers in compositional control over the sensing oxide materials, consequently widening the possibilities available in on-demand gas sensor synthesis.

We present some recent trends about research on gas sensors based on semiconducting thin films based on Ti, W, and Mo mixed oxides. The structural, electrical and gas-sensing properties of these films, deposited by rf reactive sputtering from composite targets of W–Mo, Ti–W and Ti–Mo at different abundance, were investigated through electron microscopy and volt-amperometric techniques. All of these layers showed sensing capability toward CO, NO2 or ethanol. The thin films obtained were able to detect CO, NO2, and ethanol below the concentration limit for environmental-monitoring and breath-analysis application.

By means of the pulverisation of a solution of indium oxide nanoparticles, sensitive layers for gas detection were deposited in silicon micromachined sensors in order to define layers with a thickness range covering thin and thick technologies. Influence of electrodes geometry, interdigited and parallel, was studied and better sensor response was found in interdigited sensors. Continuously heated and temperature-pulsed working modes were also compared, showing that pulsed mode improve the sensor response. As an example, sensor response to low concentrations of NO2 in humid air has been studied and results confirmed the viability of this technique to obtain micromachined gas sensors.

Development of MoO 3 as a novel material for gas sensing was addressed. Thin films were produced by reactive rf sputtering assisted by annealing. Doping with Ti was performed to enhance the conductance of the MoO 3 film. It came out that the layers were two orders of magnitude more conductive than undoped material. Good and reversible response to CO was achieved at 300°C, which fell off at higher temperatures. An interesting feature of the films was a considerably fast response for both CO and NO 2 despite low operating temperature. Doped films were found to operate at best about 100°C below than for pure MoO 3 layers.

We report the characterization of thin films of MoO 3 and their implementation on a micromachined silicon-based structure to achieve considerably low power consumption. The sensing layer is capable of detecting NO 2 up to a few ppm with considerably short response and recovery times. Investigation of structural features of the films is carried out by X-ray diffraction and electron microscopy.

The authors report about preparation and characterization of thin films of MoO3 as a material for gas-sensing applications. Structural investigation of the films is carried out by electron microscopy and X-ray diffraction techniques. The sensing behavior of the MoO3 films was tested to NO2, showing capability to detect a few ppm of NO2 with considerably short response time.

Thin films were obtained by r.f. reactive sputtering from a Ti0.1–W0.9 target followed by heating in dry air. Their morphological and structural characteristics were studied by means of electron-microscopy techniques. Annealing at 600°C of a sample leads to a WO3 polycrystalline thin-film. A solution of Ti ions in the WO3 lattice causes the film to have higher surface-to-volume ratio and thereby higher gas-sensitivity with respect to pure WO3 sputtered thin films. This layer is highly sensitive to NO2 as concentrations lower than 1 ppm of NO2 are detected. Annealing the film at 800°C results in a TiO2-anatase nanostructured layer which is capable to detect few ppm of NO2 in dry air within the temperature range of 350–800°C.

WO 3 -TlO 2 thin films were prepared by r.f. reactive sputtering from a Ti 0.1 -W 0.9 target. Annealing in synthetic air-flow of the initial W-Ti-O deposited compound results in layers with different features, depending on the temperature at which the annealing is being carried out. Morphological and structural characteristics of the thin films have been studied through X-ray diffraction, electron microscopy and Raman techniques. After annealing at 600°C, a polycrystalline thin film isostructural to WO 3 is achieved. This layer is capable of detecting concentrations of NO 2 lower than 1 ppm. Annealing of the film at 800°C results in a nanosized structurally stable TiO 2 -anatase layer. This layer can detect a few ppm of NO 2 in dry air even at as high a temperature as 800°C.