Green synthesis of zinc oxide nanoparticles using Plectranthus cyaneus leaf extract for ethanol sensing

Abstract

In this work, zinc oxide (ZnO) nanoparticles (NPs) were green-synthesized using Plectranthus cyaneus leaf plant extract as a reducing agent. The synthesized ZnO NPs were characterized using X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM), and Ultraviolet-Visible spectrophotometer. The XRD confirmed the crystal as a hexagonal wurtzite crystal structure with space group P63mc. The crystallite size was determined and found to increase with calcination temperature from 37.80 nm at 300 ◦C to 49.5 nm at 700 ◦C. The crystalline strain and dislocation density decreased from 1.538 × 10− 4 to 1.144 × 10− 4 and from 3.386 × 10− 2 nm− 2 to 2.919 × 10− 2 nm-− 2 respectively. The SEM images of the ZnO NPs reveal fine spherical particles with less agglomeration at higher temperature, thus enhancing surface area. The optical band gap decreased with an increase in temperature from 3.26 eV at 500 ◦C to 3.22 eV at 700 ℃. The synthesized ZnO NPs were used to fabricate an ethanol gas sensor on an interdigitated Indium doped tin oxide (ITO) coated glass substrate, which was exposed to various concentrations of ethanol, isopropanol, acetone, and methanol. The fabricated sensor exhibited high sensitivities of 27.6%, 10.7%, 9.9%, and 8.2% for ethanol, isopropanol, acetone, and methanol, respectively, at a working temperature of 200 ℃. The sensor exhibited fast response and recovery times of 17.90 s and 21.12 s for ethanol, respectively. The sensors’ selectivity of ethanol in the presence of other analyte were 36.16%, 38.77%, and 29.57% for isopropanol, methanol, and acetone, respectively. Therefore, our study provides a green route for synthesizing ZnO nanostructures as potential candidate for ethanol gas sensing.