Ppb-level NO2 sensing properties of Au-doped WO3 nanosheets synthesized from a low-grade scheelite concentrate

Abstract

Pure and Au-doped WO3 nanosheets were synthesized using a low-grade scheelite concentrate as the tungsten source by a combined process, namely NaOH leaching to decompose scheelite, chemical precipitation to form CaWO4 precursor, and sintering treatment of CaWO4 precursor under acid condition to obtain WO3 nanosheets. The crystal structure and morphology of the samples were examined by applying a series of characterization methods such as XRD, SEM, TEM, FTIR and XPS. Microstructure characterization indicated that the loose and irregular WO3 nanosheets with a single monoclinic structure were 10–30 nm in thicknesses and several hundreds of nanometers in lengths and widths. NO2 sensing measurements demonstrated that appropriate Au doping significantly improved the sensor response compared with pure WO3 nanosheets. The peak responses upon exposure to 5 ppm NO2 of 62.5 and 212.3 for pure and 0.7 mol% Au-doped WO3 nanosheets at 175 °C were achieved, respectively. Au-doped WO3 nanosheets also exhibited fast response/recovery speed, good repeatability, excellent long-term stability and high selectivity. Especially, the sensors could detect ppb-level NO2, which is beneficial to the practical application. The enhanced sensing mechanism of Au-doped WO3 nanosheets to NO2 could be ascribed to the electrical sensitization and chemical sensitization induced by noble metals.