Al-doped zinc oxide thin films for liquid petroleum gas (LPG) sensors

Undoped and Al-doped zinc oxide thin films were prepared by chemical spray pyrolysis technique using Zn(CH3COO)2 as a precursor solution without or with AlCl3 as a doping solution, respectively. The dopant concentration (at% Al to Zn) was varied from 0 to 1.5 at%. Grazing incidence X-ray diffraction (GIXRD) analyses of the films confirm that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. The films show the change in preferential orientation depending on the doping concentration of Al. The crystallite size in the 0.5 at% Al-doped film is found to be minimum (∼103 nm) among all the films. It is observed that the resistivity of the Al-doped films decreases with the Al dopant concentration up to 1 at%. At a higher doping concentration of 1.5 at%, the disorder produced in the lattice causes an increase in the resistivity of the film. It is found that compared to the undoped ZnO film, Al-doped films show high response to LPG. Among all the doped films studied, the 0.5 at% Al-doped ZnO film shows the maximum response (∼89%) to 1 vol% of LPG in air at 325 °C. Further, the response and recovery times of the films to LPG become shorter at higher operating temperatures. A possible mechanism of LPG sensing has been explained.