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Abstract: Thin films of either pure or doped tungsten oxide were grown by radiofrequency (rf) sputtering onto silicon micromachined substrates. Up to 7 different dopant materials (noble metals or metal oxides) were deposited by rf sputtering or by evaporation onto the tungsten oxide films. The responsiveness of the resulting micromachined sensors towards sulfur dioxide and hydrogen sulfide was studied. Other pollutants in CO2 such as ethylene and methane were also tested. It was found that Au-doped tungsten oxide sensors were highly sensitive to H2S, poorly sensitive to SO2 and almost insensitive to hydrocarbons. On the other hand, Pt-doped tungsten oxide was highly sensitive to SO2, poorly responsive to H2S and nearly insensitive to hydrocarbons. By applying a principal component analysis (PCA), we show that it would be possible to selectively detect traces of H2S and SO2 in a CO2 stream using doped WO3 microsensors. These sensors could be used in a low-cost analyzer of beverage-grade CO2. [Copyright &y& Elsevier]

Abstract: Active layers consisting of rf sputtered WO3 were deposited on microhotplate substrates. The films were doped with seven different materials (Pt, Au, Ag, Ti, SnO2, ZnO and ITO (indium tin oxide)). The eight types of sensors (including pure tungsten oxide ones) were tested in the presence of ammonia, hydrogen sulphide, nitrogen dioxide, carbon monoxide and methane. It was found that gold improved the sensitivity to H2S. On the other hand, doping with Ag and Pt led to higher responses to NO2 and NH3, respectively. No response to CH4 was observed. The sensitivity to CO was very low. The influence of the working temperature on the sensor response was also studied. Our study proves that selective gas detection is possible combining a few tungsten oxide sensors with different dopants. [Copyright &y& Elsevier]

The sensing properties of WO3 thin films deposited by rf sputtering onto silicon substrates have been investigated. After deposition, the films underwent an annealing process in dry air that lasted 24 h. Three different annealing temperatures (350, 400 and 450 °C) were used to analyze the influence of this parameter on the sensor response. The morphology and composition of the films were studied by AFM and XPS. The films were found to be essentially inhomogeneous. The oxygen-to-tungsten atomic ratio of the active layers reveals that, irrespective to the annealing temperature used, the WO3 active layers were close to their stoichiometric formulation, although a tendency to loose oxygen atoms at the higher annealing temperatures tested was inferred. The sensitivity of the films to ammonia, nitrogen dioxide, ethanol, benzene, methane and carbon monoxide was studied at three different operating temperatures (300, 350 and 370 °C). By selecting the appropriate annealing and operating temperatures, it was found possible to selectively detect nitrogen dioxide in the presence of ethanol and ammonia, or to detect ammonia in the presence of ethanol. [Copyright &y& Elsevier]

Active layers of pure and Pt doped WO3 were deposited by rf magnetron sputtering on micro-hotplate substrates. SEM was used for active layer morphology characterization. The obtained devices can be used to detect traces of pollutant gases in a CO2 stream, showing reversible responses. The pure WO3-based sensors were sensitive to low concentrations of SO2 (1 ppm) in carbon dioxide while the Pt doped detected very low concentrations of H2S (100 ppb). The influences of the operating temperature and the electrode geometry have been analyzed. The results are promising for using these sensors in a low cost beverage grade carbon dioxide monitoring system. [Copyright &y& Elsevier]