Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating

CeO2-doped ZnO thin-film gas sensors with different Ce/Zn ratios have been fabricated by dip-coating method, starting from zinc acetate dihydrate, cerium nitrate hexahydrate (Ce(NO3)3·6H2O) and anhydrous ethanol. Each layer was fired at 180 °C in a conventional oven for 30 min and the final coatings were sintered at 500 °C in a muffle furnace for 60 min. The microstructure and morphology of the films were characterized by XRD and FESEM, respectively. The resistance and sensitivities to volatile organic compounds were investigated on the static testing chamber. The X-ray diffraction (XRD) analysis of the films reveals the appearance of CeO2, tetravalent cerium dioxide whose valency is different from cerium nitrate hexahydrate. The results also show that as-prepared thin films with thickness of about 5 μm are polycrystalline with the structure of hexagonal wurtzite type. They consist of almost spherical particles with size ranging from 40 to 65 nm. Pure ZnO and Ce-doped ZnO thin-film sensors were prepared and tested for specific sensitivity to alcohol, acetone and benzene. It is observed that 1 at.% Ce–ZnO and 5 at.% Ce–ZnO are more sensitive to volatile organic compounds (VOCs), compared with other films with the different dopant concentration. The sensitivity of 5 at.% Ce–ZnO thin-film sensors to 100 ppm alcohol reaches 80 or so at 320 °C. 5 at.% Ce–ZnO thin-film sensors show good selectivity to alcohol, and thus can serve as alcohol-sensing sensors. A new physical model of the CeO2 dopant influence on the gas-sensing properties of ZnO thin films is proposed. The addition of Ce to ZnO modified the particles size distribution, electrical conductivity, the catalytic activity and thus affected gas-sensing property to some extent.