Ethanol Gas Sensors Using Semi-Hedgehog-like CuO Nanostructures: Studying the Role of Oxygen Vacancies, Unsaturated Cu-Sites, and Hole Accumulation Layer

Shape engineering, such as designing unique hierarchical morphologies with special geometric attributes, has been considered to be one of the effective ways to tailor the gas sensing abilities on transition-metal oxide nanostructured surfaces. Herein, we report a significant enhancement in the ethanol sensing performance of p-type CuO by tailoring their microstructure to a unique semihedgehog-like nanostructure (SHN, named sample A) having stiff spiky nanowires. SHNs were prepared by calcinating precipitates (at 400 °C) obtained from refluxing aqueous solutions of copper acetate, l-tartaric acid, SDS, and NaOH. The ethanol sensing performance of SHNs was compared to marigold-like nanoflowers and the existing literature. SHNs showed higher selectivity toward ethanol vapors against other VOCs and a sensitivity of 75% with response percentages of 3342.2 and 23.77 for 500 and 1 ppm, respectively, at 260 °C and a response time as low as ∼4-10 s. The study also investigated the structure-activity relationship between hole accumulation layer (HAL), doubly positive oxygen vacancy (OV) defects, exposed facets, coordinatively unsaturated Cu-3-fold sites, and grain size of SHNs (Debye length) with ethanol gas sensing performances. The study also quantified the mechanistic role of the local suppression of HAL and positive surface oxygen defects with the ethanol sensing reaction.