Two-dimensional (2D) rare earth nanomaterials

(Nanowerk Spotlight) Despite their name, rare earth elements, which are all metals, actually are not that rare. But while they aren't necessarily tricky to find, the elements often occur together and are extremely difficult to separate and extract.
Abundant in mines around the world, rare earth elements are a group of chemical elements that occur together in the periodic table. The group consists of yttrium and the 15 lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element, bringing the total to 17.
Rare earth metals and alloys that contain them are used in many devices that people use in their everyday lives such as computer memory, DVDs, rechargeable batteries, supercapacitors, cell phones, catalytic converters, magnets, fluorescent lighting and much more.
Whereas the research on two-dimensional (2D) nanomaterials such as graphene, hexagonal boron nitride, MXenes, transition metal dichalcogenides, or metal-organic frameworks moves full-steam ahead, overview reports on 2D rare earth materials are, well, rare.
A report in Advanced Materials ("Ultrathin 2D Rare-Earth Nanomaterials: Compositions, Syntheses, and Applications") is the first comprehensive review of the compositions, syntheses, and applications of all families of ultrathin 2D rare-earth nanomaterials.
The review does not include numerous nanomaterials in which rare-earth elements are only performed as dopants or charge-balancing interlayer counter ions, although the rare-earth ions often play key roles in these materials.
In their first section – composition and syntheses – the authors summarize the compositions of known ultrathin 2D rare-earth nanomaterials. To date, only a small fraction of rare-earth materials is obtained as ultrathin 2D nanomaterials, the majority of which are oxides and layered rare- earth hydroxides (LREHs).
In general, the rare-earth materials suitable for ultrathin 2D nanostructures consist of atomic layers held via either strong forces (e.g., ionic bonds) or weak forces (e.g., van der Waals forces).
In the first case, a bottom-up synthetic strategy is usually employed for synthesis. In contrast, when weak forces are present, a top-down synthetic strategy can be effective. Here, swelling layered bulk rare-earth materials in a solvent and then delaminating the layers by mechanic forces such as sonification (liquid exfoliation approach), provides an alternative route to ultrathin 2D rare-earth nanomaterials.
Schematic illustration of two common approaches for the synthesis of ultrathin 2D rare-earth nanomaterials
Schematic illustration of two common approaches for the synthesis of ultrathin 2D rare-earth nanomaterials: a) thermal decomposition and b) liquid exfoliation approaches. The blue and red spheres in (b) denote two different intercalated ions. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
With the electron confinement within mono- or multi-layers, the optical, magnetic, electric, catalytic, and adsorptive properties of ultrathin 2D rare-earth nanomaterials may vary remarkably from those of the bulk phase, opening up great opportunities for applications in numerous areas.

Optical applications

Ultrathin rare-earth nanomaterials might be utilized to tune the performances of current luminescent materials and design new luminescent materials Novel display devices have been reported on the basis of layered rare-earth hydroxides. Layer-by-layer assembly of nanosheets on quartz glass yielded an antireflection and antifogging film.
UV absorbents: Cerium dioxide is an important UV absorbent as a result of its bandgap energy. When obtained as nanomaterials, the bandgap of CeO2 increases due to a quantum size effect.

Magnetic applications

Research reports have unambiguously demonstrated the surface and quantum size effects on the magnetic properties of ultrathin 2D rare-earth nanomaterials.
Layered gadolinium hydroxides have been demonstrated as excellent candidates for cryogenic magnetic refrigeration, as they exhibit an enhanced magnetocaloric effect.
Monodispersed 2D Gd2O3 nanomaterials and exfoliated layered gadolinium hydroxides are promising candidates as Magnetic Resonance Imaging (MRI) agents.

Electric applications

Consisting of Se or Te sheets, many rare-earth selenide and telluride compounds are 2D charge density wave conductor that may possess unique electronic transport properties and could be great candidates for light filters and solar cells.

Catalytic applications

The easy conversion between CeO2 and CeO2-x makes CeO2 a promising CO oxidation catalyst. Understanding the detailed structure and role of catalytic active sites is still challenging, though.

Adsorptive applications

Due to the low dimensions and high percentage of coordinatively unsaturated surface atoms, ultrathin 2D rare-earth nanomaterials are excellent adsorbents for various applications such as drug-delivery and waste water treatment.
In their review, the authors point out that the mechanism of crystal nucleation and anisotropic growth processes during the bottom-up synthesis is still not well studied, with the optimization of reaction conditions relying on the cost-ineffective and time-consuming trial-and-error procedure.
Furthermore, many works were published after the synthesis of ultrathin 2D rare-earth nanomaterials without the exploration of the applications.
"Extensive investigations of ultrathin 2D nanostructures of rare-earth nanomaterials may result in the discovery of novel physical phenomenon and chemical properties, shedding lights on the applications that have never been described," they conclude.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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