Positive impedance humidity sensors via single-component materials

Resistivity-type humidity sensors have been investigated with great interest due to the increasing demands in industry, agriculture and daily life. To date, most of the available humidity sensors have been fabricated based on negative humidity impedance, in which the electrical resistance decreases as the humidity increases, and only several carbon composites have been reported to present positive humidity impedance. However, here we fabricate positive impedance humidity sensors only via single-component WO3−x crystals. The resistance of WO3−x crystal sensors in response to relative humidity could be tuned from a negative to positive one by increasing the compositional x. And it was revealed that the positive humidity impedance was driven by the defects of oxygen vacancy. This result will extend the application field of humidity sensors, because the positive humidity impedance sensors would be more energy-efficient, easier to be miniaturized and electrically safer than their negative counterparts for their lower operation voltages. And we believe that constructing vacancies in semiconducting materials is a universal way to fabricate positive impedance humidity sensors.
Introduction
Resistivity-type humidity sensors, which can perceive and record the change in electrical resistance in response to that in environmental humidity, have been investigated with great interest due to the increasing demands in industry, agriculture and daily life. To date, most of the available humidity sensors have been fabricated based on negative humidity impedance, in which the electrical resistance decreases as the humidity increases. However, due to their lower operation voltages, positive humidity impedance sensors would be more energy-efficient, easier to be miniaturized and electrically safer than their negative counterparts. Thus they would have wider applications in protectors for integrated circuits from humidity, energy-efficient automatic air humidifiers, and so on. But so far only several carbon composites have been reported to present positive humidity impedance.
In sensing materials, semiconductor metal oxides are one of the most promising candidates for solid-state chemical sensors due to their high sensitivity, and quick response and recovery. Among them, tungsten oxides are very important semiconducting materials, finding applications in gas sensing together with photocatalysis and electrochromism. Focusing on gas sensors, tungsten oxides can be applied for a variety of gases, such as H2S, O2, NOx, COx, NH3 and so on. Particularly, the sensors for H2O (humidity) based on WO3 (the only reported tungsten oxide based sensors in literature) are WO3 nanowire humidity sensor on chip manufactured using CMOS-MEMS technique and WO3 thin-film sensor fabricated using deposition technology. But in most cases, they also functionalize in a composite, just like poly-2,5-dimethoxyaniline/WO3 composites, the mixture of Cr2O3 and WO3, and polyaniline/WO3 composites. And none of them exhibits positive-sensitive property to himidity. As for the sensing mechanism, the response of WO3 to relative humidity (RH) is generally attributed to the water dissociative chemisorptions process that would result in the formation of hydroxyl groups on the surface of WO3 crystals; and then, electrons are accumulated on the WO3 surface. As a result, the resistance of WO3 crystals decreases with increasing RH. To the best of our knowledge, no study focuses on the influence of oxygen vacancies density of metal oxides on humidity sensing property.
Furthermore, unlike most of the oxygen-deficient metal oxides, which are not stable (especially in humid condition), WO3−x crystals with a variety of oxygen-deficient stoichiometries, such as WO2.72, WO2.8, WO2.83 and WO2.9, can be easily prepared, since they are stable, ordered phases with precise stoichiometries. And the early studies revealed that oxygen vacancy can consistently account for the defect level and trap assisted conduction in semiconducting oxides. Among them, Gillet and co-workers even indicated that the density of oxygen vacancy in WO3 would be affected by water vapor when the experiments were performed in air. These facts inspire us to design and fabricate various WO3−x humidity resistors in which the different densities of oxygen vacancy might induce and modulate the humidity sensitivity.
Therefore, here we developed an approach to prepare oxygen-deficient tungsten oxides (WO3−x) nano-/micron-structures (NMS) only by heating WO3 powder in S atomsphere in a vacuum tube furnace, and with the structured WO3−x crystals, humidity sensors were fabricated simply by screen-printing them onto ceramic substrates with Ag-Pd interdigital electrodes. Surprisingly, a positive humidity-sensitive property was found in the sensors prepared by single-component WO3−x crystals with high density of oxygen vacancies. And the resistance of WO3−x crystal sensors in response to relative humidity could be tuned from a negative to positive one by increasing the compositional x. We believe that our method not only provides a new avenue for fabricating highly effective positive humidity sensors by various metal oxides, but also creates a powerful platform to understand and design desirable semiconducting oxides humidity sensors. In addition, the findings on the positive resistance characteristics of single-component material humidity sensors can not only extend the application of humidity-sensitive resistor in different types of miniaturized devices, but also enrich and compensate for the humidity-sensing principles.

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