NH3 Sensor Based on 3D Hierarchical Flower-Shaped n-ZnO/p-NiO Heterostructures Yields Outstanding Sensing Capabilities at ppb Level

Abstract

Hierarchical three-dimensional (3D) flower-like <i>n</i>-ZnO/<i>p</i>-NiO heterostructures with various Zn_xNi_y molar ratios (Zn<b>₅</b>Ni<b>₁</b>, Zn<b>₂</b>Ni<b>₁</b>, Zn<b>₁</b>Ni<b>₁</b>, Zn₁Ni<b>₂</b> and Zn<b>₁</b>Ni<b>₅</b>) were synthesized by a facile hydrothermal method. Their crystal phase, surface morphology, elemental composition and chemical state were comprehensively investigated by XRD, SEM, EDS, TEM and XPS techniques. Gas sensing measurements were conducted on all the as-developed Zn_xNi_y-based sensors toward ammonia (NH<b>₃</b>) detection under various working temperatures from 160 to 340 °C. In particular, the as-prepared Zn<b>₁</b>Ni<b>₂</b> sensor exhibited superior NH<b>₃</b> sensing performance under optimum working temperature (280 °C) including high response (25 toward 100 ppm), fast response/recovery time (16 s/7 s), low detection limit (50 ppb), good selectivity and long-term stability. The enhanced NH<b>₃</b> sensing capabilities of Zn<b>₁</b>Ni<b>₂</b> sensor could be attributed to both the specific hierarchical structure which facilitates the adsorption of NH<b>₃</b> molecules and produces much more contact sites, and the improved gas response characteristics of <i>p-n</i> heterojunctions. The obtained results clear demonstrated that the optimum <i>n</i>-ZnO/<i>p</i>-NiO heterostructure is indeed very promising sensing material toward NH<b>₃</b> detection for different applications.