Fast detection and discriminative analysis of volatile organic compounds using Al-doped ZnO thin films

Aluminum-doped zinc oxide (AZO) thin films with different doping concentrations have been synthesized by simple spray pyrolysis technique. Precursor solution concentration was maintained ~ 50 mM throughout the fabrication process and dopant concentration was varied from 0 to 5 at. %. Prepared solution was sprayed on top of pre-heated glass plate to get highly adhesive AZO thin films. Various characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Visible spectroscopy were adopted to get an insight into the material formation. Electrical and gas-sensing characteristics were also recorded in detail to evaluate its potential application as a transparent conductor and gas sensor. As determined from XRD analysis, continuous decrease in grain size was observed with increase in aluminum doping concentration. Further, extracting the interplanar distance and lattice parameters, it was noticed that there was a negligible random variation. Aluminum doping also plays a significant role in modifying the surface morphology of thin films. Randomly arranged plate-like structures in undoped ZnO thin films transform to granular morphology with aluminum doping. Minimum resistivity of 0.517Ωm with ~ 80% transmittance in visible region was achieved at an optimal aluminum doping concentration of 3 at.%. Aluminum doping helps in increasing the sensitivity of ZnO thin films toward various volatile organic compound vapors such as acetone and ethanol. 3 at.% Al-doped thin films were capable of detecting 100 ppm of ethanol and acetone with a highest sensitivity of ~ 60%. Al incorporation to ZnO lattice is also supportive in bringing down the response and recovery time of the sensing material. A very short response time of 3 s and recovery time of 28 s was achieved at 100 ppm of ethanol. Principal component analysis shows proper discrimination between acetone and ethanol. Graphic abstract