Y-doped ZnO films for acetic acid sensing down to ppb at high humidity

Abstract

Acetic acid is a potential breath marker for cystic fibrosis and gastroesophageal reflux. Also, it is a key tracer for aroma development in the food industry for chocolate and coffee processing. However, its selective detection down to relevant parts per billion (ppb) concentrations under realistic relative humidity (RH) is most challenging, especially with low-cost sensors. Here, highly porous Y-doped ZnO films for sensing acetic acid down to 10 ppb within 2 min at 90% RH are prepared by single-step flame-aerosol deposition with close control over their composition. X-ray diffraction and energy-dispersive X-ray spectroscopy reveal Y traces inside the ZnO wurtzite nanoparticles assuring small (below 25 nm) and thermally stable crystal sizes even upon annealing at 500 °C for 5 h in air. Separate Y2O3 nanoparticles are formed at elevated Y-contents. At an optimal Y-content of 2.5 mol% and sensing at 350 °C, remarkable selectivities of acetic acid over H2 (200), acetone (15), ethanol (5) and isoprene (3) are obtained. This is attributed to the high surface basicity of Y-doped ZnO featuring up to three orders of magnitude higher acetic acid selectivity than less basic SnO2 and WO3 sensors. Additionally, the influence of RH (10 - 90%) on acetic acid sensing is examined. This sensor is compact and inexpensive, thus promising for integration into hand-held and low-cost food processing or breath monitors.