Enhanced ethanol vapor sensing by 2D CuO-ZnO composite at ambient conditions

Abstract

Gas sensors find application in the fields of food packaging, medical, agriculture, industries, transport, storage, and warehousing, etc. An efficient gas sensor should be sensitive and selective towards analyte, have fast response and recovery time and operate under moderate conditions. The commercial sensors have an in-built heating element. Thus, there is scope for finding materials that can function as room temperature ethanol sensors. Here, a composite of two-dimensional copper oxide (CuO) with zinc oxide (ZnO) nanoparticles is reported as a potential ethanol (EtOH) vapor sensor under ambient conditions. CuO is synthesized with the help of liquid–liquid phase separation technique yielding architecturally aligned 2D structures. ZnO nanoparticles are synthesized via microwave assisted wet chemical route separately. X-ray diffraction studies show the phase formation of pure CuO and ZnO individually. The CuO and ZnO samples are ground mechanically in 1:1 wt ratio to form a composite referred to as CZC. The optical absorption studies show bandgap of CuO structures as 1.41 eV, ZnO as 3.13 eV, and of CZC as 1.24 eV. A slurry of the composite is then coated on a glass substrate to study the resistance changes after ethanol vapor exposure. It is observed that while pure CuO is almost unresponsive to EtOH vapors, ZnO has sluggish response, the CZC samples responded fast. The best average response time is observed to be ∼ 4 sec while recovery time is ∼ 12 sec. The sensors show good reversibility as they return to their initial state without applying any heat treatment. The change in EtOH sensing in composites is attributed to the formation of heterojunction between CuO-ZnO which facilitates faster electron transfer across the heterojunction and the presence of large surface area renders an increased number of adsorption sites.