Ultra-sensitive H₂S sensing at ppb concentrations by barely crystalline WOx/W thin film

This work describes the preparation of thin films of tungsten oxide (WO₃) and a semiconducting composite of tungsten-containing tungsten oxide (WO_x/W), for sensing hydrogen sulfide. These thin films were prepared on oxidized Si substrates, respectively by RF magnetron sputtering and DC reactive sputtering. DC reactive sputtering was used to optimize the stoichiometry of thin films to achieve tungsten-containing tungsten oxide films for sensing hydrogen sulfide. Scanning electron microscopy (SEM), x-ray diffractometry (XRD), and x-ray photoelectron microscopy (XPS) were employed to analyze the structure and the morphology of the thin films. An inter-digitated platinum electrode structure was patterned using optical lithography to define the sensor device. In sensing hydrogen sulfide, the reactive DC-sputtered WO_x/W film sensors exhibit superior response (conductometric response of ∼3100% to 10 ppb concentration, at operating temperature of 250 °C) and a high degree of selectivity. On the other hand, RF-sputtered tungsten oxide thin films show no response when H₂S concentration is lower than 500 ppb. (paper)