Electronic to protonic conduction switching in Cu2O nanostructured porous films: the effect of humidity exposure

In this paper, we present the first experimental evidence for electronic to protonic conduction switching in p-type semiconducting nanostructured cuprous oxide (Cu2O) porous films when exposed to humidity. We also present a linear response at low relative humidity (RH < 48%). The Cu2O nanostructured porous films were synthesized by spray pyrolysis of a Cu/Cu2O colloidal solution obtained by laser liquid medium ablation of a Cu target in water. The as-prepared and annealed Cu2O films were extensively characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction analysis, and Raman spectroscopy. The chemiresistor response to RH values from 7.5 to 84% was examined at a temperature of 22 °C and a pressure of 760 mmHg. At RH values below 48%, recombination of the majority charge carrier holes and electrons occurred owing to the dissociation of water molecules near the surface; the RH level was used to quantify the increase in resistance response (RRes). Both devices revealed linear responses to RH (7.5–48%), with a maximum rate of 4.38 ± 0.16%/RH. As the RH increased beyond 48%, proton hopping between the physisorbed water molecules had a larger effect than the electronic conduction, and the response showed the opposite effect. The response exhibited a linear log-normal relationship with higher RH values (56–84%), with a maximum rate of −0.0694 ± 0.002 log(%)/RH. The mechanism for switching the resistive response trend of the Cu2O films is discussed.