Hierarchical Sphere-Like ZnO–CuO Grown in a Controlled Boundary Layer for High-Performance H2S Sensing

A highly sensitive sensor for hydrogen sulfide based on a p–n junction between metal-oxide semiconductors is reported herein. Uniform ZnO–CuO hollow spheres were synthesized by a combination of chemical and physical methods. To deposit a uniform ZnO layer, the silicon substrate was tilted from 0° to 40° relative to the gas flow direction during the growth process. Next, a low concentration of CuO nanoparticles was decorated onto the ZnO nano/microstructure using physical vapor deposition (PVD). The ZnO–CuO heterojunction sensor showed a remarkable response of 112 at 100 ppm H2S\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{H}}_{2} {\text{S}}$$\end{document} and 158°C. The response time and recovery time were calculated to be 8 s and 35 s, respectively. The response to H2S concentration increases of 100 ppm was as high as 3.5 times compared with ZnO alone. Finally, a sensing mechanism is proposed and discussed.