Electrolysis of Water in CuO/ZnO Heterocontact Humidity Sensor

Abstract

The oxygen partial pressure dependence of current in CuOIZnO heterocontact has been investigated to confirm its humidity sensing mechanism. A CuO/ZnO heterocontact was fabricated by physically pressing the sintered disks of CuO and ZnO, and its current was measured at various oxygen partial pressures under wet and dry conditions at room temperature.The current of the heterocontact was strongly dependent on oxygen partial pressure only at forward bias and wet atmosphere. Such a phenomenon originates from the humidity sensing mechanism, i.e., electrolysis of water physisorbed at the vicinity of the heterocontact. The current measurement under air with ammonia water vapor also provided one more evidence for the electrolysis function of the heterocontact. gen partial pressure up to about 0.01 atm and then decreased smoothly. The maximum current change from the N2 to 02 atmosphere was large by about a factor of 100.

The Effect of Synthesis Conditions and Humidity on Current−Voltage Relations in Electrodeposited ZnO-Based Schottky Junctions

“…The third term counteracts the forward bias current and is due to the oxygen partial pressure of the system. 13 The rectifying behavior of the Schottky contact ensures that there is little reverse bias current, even in the presence of water electrolysis. We note that relative magnitudes of the protonic conduction, based on electrolysis and charge movement Figure 6: Schematic diagrams illustrate (a) the water electrolysis reaction that can occur between two metal electrodes, and (b) where the water electrolysis reaction occurs in the electrodeposited Schottky junctions (in the water adsorbed at the ZnO/air interface).…”

Focused-Ion-Beam-Assisted Magnet Fabrication and Manipulation for Magnetic Field Detection Applications

“…5,7,8 Different mechanisms to explain the origin of humidity effects for metal oxide materials have been proposed, including temperature-dependent or voltage-dependent chemisorption. 7,[9][10][11] Here we show that electrochemically prepared ZnO rectifying (Schottky) junctions have current-voltage relationships that are influenced by relative humidity changes at ambient temperatures due to a water electrolysis mechanism, 12,13 and that their responses can be qualitatively changed by varying electrolyte pH during electrodeposition or by adding post-deposition coatings. These findings highlight the effect that wet or humid environments can have on electron transport materials.…”

“…13 The humidity-dependent responses of this and similar systems 7,11 has been attributed to water electrolysis at the positive and negative sides of the junction. A schematic diagram of the water electrolysis reaction, given in Figure 6a, depicts the oxidation and reduction reactions that occur at the anode and cathode, respectively.…”

“…5,7,8,10 When used in rectifying pn heterojunction humidity sensors, it has been proposed that the electrolytic decomposition of water at the junction continually refreshes the surface to prolong device operation. 13 In contrast, other studies have shown that electrolysis of condensed water can contribute to undesirable degradation in ZnO-based varistors 14 that are exposed to water vapor during the course of their normal operation.…”

Inorganic structures as materials for gas sensors

“…Individual CuO and ZnO layers are insensitive to moisture content. A feature of these heterojunctions is a maximum in the dependence of sensitivity I H 2 O /I dry (I H 2 O is the current in moist air and I dry is the current in dry air) on the potential, which was associated 50 with electrolysis of water molecules. The mechanism of gas sensitivity is schematically illustrated by Fig.…”