On-line monitoring of CO2 quality using doped WO3 thin film sensors

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.