An innovative platform for NO₂ detection for cleaner air and safer cities

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 (Figure 1).

Figure 1: operando µ-XPS measurements of In-3d and S-2p core levels: (black) after air annealing at 350°C, (red) during exposure to NO₂ gas (1 kL dose, 1L = 10^-6 Torr·s^-1) at 350°C, and (yellow) after exposure to NO₂. (c) In-3d image of the InS nanosheets.

Operando SPEM experiments carried out at the operation temperature of gas sensors (350°C) further revealed that sulfur abstraction induced by NO₂ plays a fundamental role in the sensing mechanism. The altered electronic charge density at the indium-oxide/indium-sulfide heterostructures overall enhances the interaction with NO₂, while simultaneously reducing cross-sensitivity, a limitation that is frequently encountered in conventional sensors. By promoting selective interactions with NO₂, this oxide skin enhances the reliability of InS-based sensors in practical, real-world applications.

Theoretical calculations supported these experimental observations, providing insight into the electronic modifications induced by NO₂ adsorption on oxygen-deficient InS surfaces. The synergy between experimental findings and theoretical insights confirmed the critical role of the InO_3-x layer in enhancing sensing efficiency. Together, these results suggest that InS holds significant potential as a high-performance gas sensor, offering a promising alternative to traditional materials in environmental safety applications.

This research underscores InS as an ideal candidate for selective NO₂ sensing, with important implications for air quality monitoring and industrial safety systems. Definitely, InS-based sensors display high potential for real-world implementation to satisfy critical needs in urban air quality management and workplace safety. As air pollution remains a major global issue, innovative materials enabling ultrasensitive and selective detection of noxious gases pave the way for safer, more sustainable environments.

This research was conducted by the following research team:

G. D’Olimpio¹, D. W. Boukhvalov², V. Galstyan³, Jessica Occhiuzzi¹, Michael Vorochta⁴, M. Amati⁵, Z. Milosz⁵, L. Gregoratti⁵, M. C. Istrate⁶, C-N. Kuo⁷, C. S. Lue⁷, C. Ghica⁶, E. Comini³, A. Politano^1
1 Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila, Italy.
² College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, P. R. China.
³ Sensor Lab, Department of Information Engineering, University of Brescia, Brescia, Italy.
⁴ Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
⁵ Elettra - Sincrotrone Trieste SCpA, Trieste, Italy.
⁶ National Institute of Materials Physics, Magurele, Romania.
⁷ Department of Physics, National Cheng Kung University, Tainan, Taiwan.

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