Resistive Ion Sensors Based on Metal–Oxide–Semiconductor Nanostructures Formed by Heating Metal Foil

Resistive ion sensors based on CuO and ZnO nanostructures were demonstrated at room temperature. These sensors that exhibit good sensing performance with respect to chlorine, sodium, and calcium ions were fabricated by heating pure metal foils. Resistance of CuO- and ZnO-based sensors changed when exposed to ionic solutions and recovered to their initial values in air after their removal from ionic solutions. The sensitivity of the two sensors increased nearly proportionally to ion concentration. In particular, the ZnO-based sensor showed sensitivities of 1051, 5.8, and 10 to chlorine, sodium, and calcium ions at concentrations of 1 ppm, respectively. In addition, the two sensors exhibited excellent repeatability for ionic solutions with concentrations of 5 ppm. Furthermore, the sensing performance of the ZnO-based sensor was highly stable in the bent state (in convex and concave directions) regardless of ion species.