Elettra | ESCA Microscopy An innovative platform for NO2 detection for cleaner air and safer cities The detection of nitrogen dioxide (NO2), a highly reactive and toxic pollutant, presents a substantial challenge in environmental monitoring. This is due to the necessity of sensors that can achieve high sensitivity and selectivity in volatile conditions. In industrial and urban environments, it is essential to have reliable and stable sensors that can detect low concentrations and prevent interference from other gases. This is especially true for effective NO2 sensing. Among emerging materials, indium sulfide (InS) offers a promising solution. As a layered van der Waals (VdW) semiconductor, InS combines several advantageous characteristics for gas sensing. Its structure, characterized by strong in-plane bonds and weak interlayer interactions, facilitates exfoliation into nanosheets. This process significantly increases the InS surface area and enhances interactions with target gases. Taking advantage of its surface chemistry, high carrier mobility, and large surface area, InS represents an ideal candidate for NO2 sensing applications that require a consistent and selective response. A key discovery in this research is that exposure to an oxidative environment induces the formation of an oxygendeficient In2O3-x layer on the InS surface through sulfur abstraction. Initially considered a limitation, this transformation instead creates active sites that significantly enhance NO2 adsorption. The use of advanced synchrotron techniques at Elettra, specifically Scanning PhotoEmission Microscopy (SPEM) at the ESCA Microscopy beamline under operando conditions, was essential in capturing these nanoscale surface transformations in real time. SPEM enabled high spatial resolution imaging and provided precise photoemission data on specific InS nanosheets, allowing the team to observe the sulfur abstraction process directly. This localized removal of sulfur leads to oxygen-deficient regions crucial for NO2 sensitivity. Through the use of SPEM in operando conditions, it was possible to measure the chemical state shifts occurring on the InS surface as it actively interacted with NO2. The measurements established a direct connection between the formation of the oxygen-deficient In2O3-x layer through sulfur abstraction and the enhancement of the sensor performance. This indium oxide layer modifies the electronic environment near the Fermi level, increasing selectivity for NO2 while effectively reducing cross-sensitivity to other gases. The combination of high spatial resolution and operando measurement in SPEM was essential to obtain these transformations in real time, enabling detailed observation of localized electronic and chemical changes at the nanoscale. This approach provided comprehensive insight into the role of oxygen-deficient regions in optimizing the response of InS, demonstrating the effectiveness of the material for selective NO2 detection. HR-TEM confirmed the structural characteristics of the In2O3-x layer, showing a 3.5 nm amorphous oxide layer on the InS nanosheet. The formed oxide layer induced by the ambient atmosphere not only stabilize the sensor but also provides an active site for NO2 interaction, enhancing the charge transfer and optimizing the sensor response. The sensor made based on InS demonstrated a detection threshold as low as 180 ppb for NO₂ at 350°C, with a high resistance to water adsorption that ensures stable operation under varying humidity conditions, a common limitation in conventional sensors. To evaluate the selectivity of the InSbased sensor, we exposed the device to several common dangerous gases, including H2, CO, CO2, CH2, ethanol, and acetone, under standard operational conditions. The response of the sensor to these substances was minimal or substantially lower to that of NO2, therefore validating its exceptional selectivity. For example, at 5 ppm NO2, the reaction surpassed a factor of 30 compared to hydrogen and exceeded six times relative to ethanol, even in the presence of far larger concentrations of the interfering gases. The dynamic response curves showed rapid, reversible signal changes when exposed to varying NO2 concentrations, demonstrating high sensitivity. The linear association between sensor response and NO2 concentration enables accurate quantification across environmental exposure levels. The InS sensor maintained performance in high-humidity settings (up to 90% RH), with slight response decrease. These results highlight a significant advantage of InS over conventional oxide sensors, which generally experience considerable humidity interference. The integration of high selectivity, rapid response, minimal detection limit, and humidity resilience makes InS-based sensors appropriate for practical application in urban air quality monitoring systems and safety-sensitive industrial settings. Finally, using Density Functional Theory (DFT) calculations, we elucidated the mechanism behind the enhanced performance of the sensor. Our results show that oxygen-deficient sites lower the energy barrier for NO2 adsorption. This finding supports the experimental observations and highlights the crucial role of sulfur abstraction and oxygen vacancies in improving NO2 sensitivity. In conclusion, This study underscores the significant potential of InS nanosheets as an economical and highperformance material for NO2 detection. By exploiting the intrinsic oxidation characteristics of InS, it contributes in the construction of advanced sensors essential for industrial safety and environmental monitoring. Furthermore, the research illustrates the role of advanced synchrotron techniques at Elettra, particularly SPEM and operando XPS, in advancing the development of next-generation gas sensors through novel insights into surface mechanisms. Elettra Highlights 2024–2025 112
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