Gas sensors based on the oxide skin of liquid indium.

Various non-stratified two-dimensional (2D) materials can be obtained from liquid metal surfaces that are not naturally accessible. Homogenous nucleation on atomically flat interfaces of liquid metals with air produces unprecedented high-quality oxide layers that can be transferred onto desired substrates. The atomically flat and large areas provide large surface-to-volume ratios ideal for sensing applications. Versatile crucial applications of the liquid metal-derived 2D oxides have been realized; however, their gas-sensing properties remain largely underexplored. The cubic In2O3 structure, which is nonlayered, can be formed as an ultrathin layer on the surface of liquid indium during the self-limiting Cabrera-Mott oxidation process in the air. The morphology, crystal structure, and band structure of the harvested 2D In2O3 nanosheets from liquid indium are characterized. Sensing capability toward several gases, both inorganic and organic, entailing NO2, O2, NH3, H2, H2S, CO, and Methyl ethyl ketone (MEK) are explored. A high ohmic resistance change of 1974% at 10 ppm, fast response, and recovery times are observed for NO2 at an optimum temperature of 200 °C. The sensing fundamentals are investigated for NO2, and its performances and cross-selectivity to different gases are analyzed. The NO2 sensing response from room temperature to 300 °C has been measured and discussed, and stability after 24 hours of continuous operation is presented. The results demonstrate liquid metal-derived 2D oxides as promising materials for gas sensing applications.

Facile conversion of zinc hydroxide carbonate to CaO-ZnO for selective CO2 gas detection.

Tailored synthesis of heterostructures for low temperature (sub 200 °C) CO2 sensing continues to be a challenging task. The present study demonstrates CO2 sensing characteristics of CaO-ZnO heterostructures achieved by zinc hydroxide carbonate (Zn5(CO3)2(OH)6) conversion to ZnO using Ca(OH)2 at 50 °C. Control samples namely, Zn5(CO3)2(OH)6, Ca(OH)2, ZnO, and CaO integrated microsensors exhibited low sensitivity towards CO2 gas. However, CaO-ZnO heterostructures demonstrated significant sensitivity (26 to 91%) at 150 °C for gas concentration ranging from 100 to 10000 ppm, respectively. In this study, zinc hydroxide carbonate sensitized with 25 wt% Ca(OH)2 to form CaO-ZnO heterostructures (25CaZMS) displayed a promising sensitivity (77%) and selectivity (98%) towards 500 ppm CO2 gas. Moreover, the selectivity studies were conducted in the presence of 10 commonly found gases and their sensing performance was compared against CO2 gas in dry and humid conditions. The developed CaO-ZnO sensor exhibited faster kinetics in comparison to the control samples. Improved sensing performance observed here is attributed to the low-temperature synthesis route which resulted in a large number of active pores and high surface area morphology. Additionally, the high CO2 adsorption capacity of CaO combined with compatible n-type semiconductors in forming highly dynamic nano-interfaced heterostructure is a promising step towards developing a precise CO2 gas microsensor.

Efficient Heterostructures of Ag@CuO/BaTiO3 for Low-Temperature CO2 Gas Detection: Assessing the Role of Nanointerfaces during Sensing by Operando DRIFTS Technique.

Tetragonal BaTiO3 spheroids synthesized by a facile hydrothermal route using Tween 80 were observed to be polydispersed with a diameter in the range of ∼15-75 nm. Thereon, BaTiO3 spheroids were decorated with different percentages of Ag@CuO by wet impregnation, and their affinity toward carbon dioxide (CO2) gas when employed as sensitive layers in a microsensor was investigated. The results revealed that the metal nanocomposite-based sensor had an exceptional stability and sensitivity toward CO2 gas (6-fold higher response), with appreciable response and recovery times (<10 s) and higher repeatability (98%) and accuracy (96%) at a low operating temperature of 120 °C, compared to those of pure BaTiO3 and CuO. Such improved gas-sensing performances even at a very low concentration (∼700 ppm) is attributable to both the chemical and electrical contributions of Ag@CuO forming intermittent nanointerfaces with BaTiO3 spheroids, exhibiting unique structural stability. The CO2-sensing mechanism of CuO/BaTiO3 nanocomposite was studied by the diffuse reflectance infrared Fourier transform spectroscopy technique that established the reaction of CO2 with BaO and CuO to form the respective carbonate species that is correlated with the change in material resistance consequently monitored as sensor response.

Surface Water Dependent Properties of Sulfur-Rich Molybdenum Sulfides: Electrolyteless Gas Phase Water Splitting.

Sulfur-rich molybdenum sulfides are an emerging class of inorganic coordination polymers that are predominantly utilized for their superior catalytic properties. Here we investigate surface water dependent properties of sulfur-rich MoSx (x = 32/3) and its interaction with water vapor. We report that MoSx is a highly hygroscopic semiconductor, which can reversibly bind up to 0.9 H2O molecule per Mo. The presence of surface water is found to have a profound influence on the semiconductor's properties, modulating the material's photoluminescence by over 1 order of magnitude, in transition from dry to moist ambient. Furthermore, the conductivity of a MoSx-based moisture sensor is modulated in excess of 2 orders of magnitude for 30% increase in humidity. As the core application, we utilize the discovered properties of MoSx to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoSx as the water source. The catalyst is formulated as an ink that can be coated onto insulating substrates, such as glass, leading to efficient hydrogen and oxygen evolution from water vapor. The concept has the potential to be widely adopted for future solar fuel production.

Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance.

The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 µg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.