Highly selective and sensitive xylene sensors using Ni-doped branched ZnO nanowire networks

Branched ZnO nanowires (NWs) doped with Ni were grown by a three-step vapor phase method for the sensitive and selective detection of p-xylene. ZnO NWs were directly grown on sensor substrates with Au electrodes, which were transformed into NiO NWs by the thermal evaporation of NiCl₂ powder at 700 °C. ZnO branches doped with Ni were grown from NiO NWs by the thermal evaporation of Zn metal powder at 500°C. The stem NiO NWs played the role of catalyst for the growth of ZnO branches through vapor-liquid-solid mechanism. The Ni-doped branched ZnO NWs showed enhanced gas response (S = resistance ratio) to methyl benzenes, especially to 5 ppm p-xylene (S = 42.44) at 400°C. This value is 1.7 and 2.5 times higher than the responses to 5 ppm toluene (S = 25.73) and C₂H₅OH (S = 16.72), respectively, and significantly higher than the cross-responses to other interfering gases such as benzene, HCHO, trimethylamine, H₂, and CO. The selective detection of xylene was attributed to the catalytic role of the Ni component. This novel method to form catalyst-doped hierarchical ZnO NWs provides a promising approach to accomplish superior gas sensing characteristics by the synergetic combination of enhanced chemiresistive variation due to the increased number of branch-to-branch Schottky barrier contacts and the catalytic function of the Ni dopant.