Enhanced piezo-humidity sensing of a Cd–ZnO nanowire nanogenerator as a self-powered/active gas sensor by coupling the piezoelectric screening effect and dopant displacement mechanism

Highly sensitive humidity sensing has been realized from a Cd-doped ZnO nanowire (NW) nanogenerator (NG) as a self-powered/active gas sensor. The piezoelectric output of the device acts not only as a power source, but also as a response signal to the relative humidity (RH) in the environment. The response of Cd-ZnO (1 : 10) NWs reached up to 85.7 upon exposure to 70% relative humidity, much higher than that of undoped ZnO NWs. Cd dopant can increase the number of oxygen vacancies in the NWs, resulting in more adsorption sites on the surface of the NWs. Upon exposure to a humid environment, a large amount of water molecules can displace the adsorbed oxygen ions on the surface of Cd-ZnO NWs. This procedure can influence the carrier density in Cd-ZnO NWs and vary the screening effect on the piezoelectric output. Our study can stimulate a research trend on exploring composite materials for piezo-gas sensing.