Surface X-Ray Studies of Rare Earth Group Ion Transport Pathway on Functionalized Monolayer MoS₂

Rare earth elements (REs) 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. [1,2] REs 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 REs 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 a negative impact on the environment due to the drawbacks of energy and chemical intensiveness. [3-5]<br/>Based on the two important properties of REs: ionic radius (decreasing with an atomic number) and Lewis acidity (increasing with atomic number), [2] we proposed to create two-dimensional (2D) solid ionic channels with stacked 2D materials (such as MoS2) that are able to modulate the dehydration, transport, and hydration of REs. 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 established an understanding of binding thermodynamics on the single monolayer MoS2 with the functionalized surface as a model system to determine how lanthanide RE ions contact and interact with pristine and surface functionalized monolayer 2D materials. We combined surface X-ray diffraction (crystal truncation rod) and gracing incidence X-ray absorption spectra to 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 REs. 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.