Freezing-tolerant and conductive hydrogels have attracted tremendous interest as promising materials for flexible sensors under the gelid condition. In this work, a freezing-tolerant, dual-responsive hydrogel sensor was developed by using an ionic–glycerol hybrid hydrogel. The fabricated hydrogel sensor was demonstrated to detect efficiently different temperatures ranging from −30 to 100 °C or to detect both small strains (e.g., 1%, 2%, 4%) and large strains (e.g., 10%, 20% and 30%) as a freezing-tolerant strain sensor. A systematic investigation was conducted to explore the thermal- and strain-sensitive mechanisms of the obtained hydrogel. It was found that the resistance of the hydrogel rapidly increased as the temperature (T) decreased. In addition, the strain sensitivity increased as T decreased from 0 °C to −30 °C. Surprisingly, an abrupt resistance increase was observed when the tensile strain of the gel reached a critical value (e.g., 2620% at 0 °C) or upon moving the hydrogel from 25 to −10 to −30 °C. Such a sharp increase in resistance was found to be mainly caused by the abrupt fracture of a vast amount of the inner hydrogel network at large strains or the appearance of microcrystals in the gel network at low temperatures. This work provides fundamental and practical insights into fabricating functional freezing-tolerant hydrogel sensors for engineering applications.
In this paper, WO 3 nanosheets were prepared by an acidification method and Ru catalyst was loaded on the surface of nanosheets through a conventional impregnation process. NH 3 gas sensors based on the neat and Ru-loaded WO 3 nanosheets were fabricated by the scree-printing technique and their sensing properties to NH 3 were investigated. The morphologies and surface structures of neat and Ru-loaded WO 3 nanosheets were analyzed. It is revealed that WO 3 nanosheets have an irregular flake shape and Ru catalyst is found on the surface of nanosheets with an oxided state. It is also found that the sensor response of WO 3 is greatly improved by the loading of Ru. Based on the power-law response of oxygen and TPR results, the sensitization mechanism of Ru is explained. It is proposed that the introduction of Ru promotes the catalytic activity of WO 3 and in turn modulates the reaction route of NH 3 with oxygen adsorbates on the surface, which inhibit the generation of nitrogen oxides and consequently reduces the competitive adsorption with oxygen during the sensor responses.
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