Selective room-temperature sensing of NO2 by WO3 film/graphene layers

This is one of the first attempts of making WO₃ thick film selective at low operating temperature by placing graphene sheets directly on the film surface. The WO₃ film was prepared by DC magnetron reactive sputtering of a pure tungsten target and annealing in air. The thickness of the film averaged 507 nm from FIB cross-sectioning and STEM measurement. The film was characterized with XRD, XPS and Raman spectroscopy. Graphene was synthesized on copper foil using plasma enhanced chemical vapor deposition of methane gas, which was found to produce three monolayers of graphene sheets according to Raman spectroscopic analysis. The graphene was transferred onto the WO₃ film and annealed. It was found that the pure WO₃ sensed low concentrations of nitrogen dioxide at 30°C and 100°C operating temperature. The selectivity towards nitrogen dioxide over ammonia and carbon monoxide was achieved with graphene on the tungsten trioxide film at the compromise of sensitivity at both operating temperatures. The analysis of the graphene/WO₃ composite orientation may lead to improved sensor capabilities as well as better understanding of the sensing mechanism.