Anion-exchange Mediated Synthesis of Hollow 2D Layered Materials and Heterostructures: Mechanism and Room-Temperature Gas Sensing Properties

The designing of nanostructures with unique morphologies and enhanced functionalities is a cornerstone of modern materials science. Ion exchange reactions in inorganic crystals offer a versatile approach for precisely controlling the composition, morphology and properties of the materials through stepwise transformations. In this study, we report the anion exchange-mediated conversion of 2D layered material SnS2 into SnSe2, with SnS2-SnSe2 lateral heterostructures as intermediates. This transformation, driven by the disparate diffusion rates of S²⁻ and Se²⁻ ions, leads to the generation of hexagonal nanorings of SnSe2 (inaccessible by direct synthetic routes) via the Kirkendall effect. By carefully balancing the diffusion kinetics through concentration control, we also successfully synthesized continuous SnSe2 nanosheets. To elucidate the anion exchange mechanism, we conducted a comprehensive investigation using electron microscopy techniques, varying parameters such as time, precursor concentration, and reagents. Our findings revealed that the exchange process initiates at the edges of the template SnS2 nanosheets and progresses inward. Cross-sectional atomic-resolution electron microscopy of the interfaces and layer stacking in the SnS2-SnSe2 heterostructure uncovered numerous defects, attributed to ion migration and lattice mismatch, which were not detectable in planar views. Furthermore, as-synthesized materials are explored for gas-sensing applications. Our anion-exchange-derived SnS2-SnSe2 heterostructure and SnSe2 exhibited exceptional selectivity and sensitivity towards NO2 gas (response > 700%) at room temperature comparable to state-of-art sensors, significantly outperforming the pristine SnS2 material, which required elevated temperatures (150°C) for optimal response. This study underscores the potential of anion exchange as a powerful tool for designing novel nanomaterials with tailored properties and applications, particularly in the realm of gas sensing.