An international collaboration involving scientists from Italy, China, Czech Republic, Romania, Taiwan has highlighted how indium sulfide (InS), with its moderate band gap and layered structure, holds great promise for NO2 gas sensing.
Nitrogen dioxide, a harmful gas linked to respiratory and cardiovascular issues, is particularly challenging to detect due to the need for sensors that combine high sensitivity, precise selectivity, and stability under diverse conditions. Traditional materials, such as metal-oxide semiconductors, are widely used but often lack the required sensitivity and selectivity, especially for NO2 detection. In contrast, InS meets these requirements, also showing an evolution of its surface under oxidative conditions (e.g., in the air), implying chemical transformations that improve sensing performance. The sub-stoichiometric metal oxide formed upon oxidation results to be ideal for gas adsorption, with the ultimate obtainment of an ultrasensitive NO2 detection. Moreover, when exfoliated into nanosheets, 2D InS gets an intrinsically higher amount of active sites that enhances interaction with gases, making it particularly suitable for selective detection of NO2 in real-time air quality monitoring applications, as demonstrated by gas-sensing tests carried out with an operation temperature of 350°C.
To investigate chemical transformations in InS under oxidative conditions, Scanning Photoemission Microscopy (SPEM) at the ESCA Microscopy beamline of Elettra allowed real-time observation of the material as it actively interacted with NO2. Under these operando conditions, the surface of InS develops an oxygen-deficient In2O3-x layer, with nanometric thickness detected by transmission electron microscopy, through a sulfur abstraction process. This reaction, which removes sulfur atoms from the structure, creates highly active sites on the InS surface. The high spatial resolution of SPEM enabled direct observation of these nanoscale chemical changes on the surface of InS nanosheets, providing real-time visualizations of active sites as they formed.
Read more on Elettra website