Gas sensing properties of ZnO thin films prepared by microcontact printing

In situ patterned zinc oxide (ZnO) thin films were prepared by precipitation of Zn(NO3)2/urea aqueous solution and by microcontact printing of self-assembled monolayers (SAMs) on Al/SiO2/Si substrates. The visible precipitation of Zn(OH)2 from the urea containing Zn(NO3)2 solution was enhanced by increasing the reaction temperature and the amount of urea. The optimized condition for the ZnO thin films was found to be the Zn(NO3)2/urea ratio of 1/8, the precipitation temperature of 80 °C, the precipitation time of 1 h and the annealing temperature of 600 °C, respectively. SAMs are formed by exposing Al/SiO2/Si to solutions comprising of hydrophobic octadecylphosphonic acid (OPA) in tetrahydrofuran and hydrophilic 2-carboxylethylphosphonic acid (CPA) in ethanol. The ZnO thin film was then patterned with the heat treatment of Zn(OH)2 precipitated on the surface of hydrophilic CPA. The ZnO gas sensor was exposed to different concentrations of C3H8 (5000 ppm), CO (250 ppm) and NO (1000 ppm) at elevated temperatures to evaluate the gas sensitivity of ZnO sensors. The optimum operating temperatures of C3H8, CO and NO gases showing the highest gas sensitivity were determined to be 350, 400 and 200 °C, respectively.