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

• Ni-doped branched ZnO nanowires were grown using three-step vapor phase reaction.
• Gas response was significantly enhanced by the formation of Ni-doped ZnO branches.
• Doped Ni acted as catalyst for the selective detection of p-xylene.
• A novel method to grow catalyst-doped hierarchical ZnO nanowires is suggested.

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.

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