Surface X-Ray Studies of Rare Earth Group Ion Transport Pathway on Functionalized Bilayer-Single-Channel MoS₂

Rare earth elements (REEs) have been identified as critical elements with short-term supply risks. They are essential in modern technologies and devices such as permanent magnets, optical fiber, and medical imaging agents. REEs exhibit very similar chemical properties but varied electronic properties, and their application requires high purity of every single element. Therefore, achieving effective and efficient separation among REEs from one another has been the challenge since their discovery and this task is especially critical now to enable recycling to secure the REs supply. Currently, the solvent extraction used in industrial productions induces negative impact on the environment due to drawback of energy and chemical intensiveness.<br/>Based on the two important properties of REs: ionic radius (decreasing with atomic number) and Lewis acidity (increasing with atomic number), we use 2D materials (e.g., cysteine molecule functionalized MoS2 membrane) that can modulate the dehydration, transport, and hydration of REEs. Concentration gradient is the only driving force here that does not need extra energy or hazardous chemicals. To achieve selective transport of REs by rational design, a better understanding of the binding and conduction of REs ions through the 2D channels is required. Hence, we used both single layer and bilayer-single-channel MoS2 with functionalized surface as model systems. By combining surface X-ray diffraction (crystal truncation rod) and gracing incidence X-ray absorption spectra, we could provide a precise local coordination configuration. This allows us to create an accurate molecular-level structural model for the electronic structure computation and modeling, which would be better to reconstruct the ion transport pathway and realize the selectivity control among REEs. Our results indicate that functionalized groups not only increase the interlayer space of MoS2 but also interact with rare earth elements (REEs), altering the dehydration process. After functionalization, different REEs insertion further expand the single-channel interlayer space to varying degrees to achieve dehydration. Additionally, bilayer single-channel MoS2 exerts a constraining force similar to that of bulk membrane systems, unlike single-layer MoS2. However, a single layer provides a simpler model system for understanding fundamental molecular interactions. The outcomes will have immediate impact to enabling new energy efficient separation methods, especially transport-based separation technologies (membrane separation), to be applied to REs extraction, separation, and recycling.