Faceted Controlled Synthesis of 2D Rare Earth Oxides

Recently, two-dimensional rare earth oxides (REO) have emerged as a unique and promising family of non-layered materials. Since graphene won the Nobel Prize in 2010, two-dimensional (2D) materials have continued to attract the attention of researchers in the fields of logic, storage, optoelectronics, and photonic 2D device fabrication due to their atomic thickness and excellent properties.

Building on graphene research, scientists have discovered several other 2D materials such as layered transition metal dichloride (TMDs), hexagonal boron nitride (h-BN), and non-layered group III-V semiconductors.

The unfilled 4f orbitals of rare earth elements are shielded by filled shells, so that the unpaired 4f electrons of rare earth ions are usually not involved in chemical reactions, resulting in excellent properties in terms of luminescence, magnetic, electronic, and catalytic activities. Combining the unique properties of rare earth elements, 2D REO has been widely used in optics, magnetism, efficient catalysts, transistors, biomedicine and other fields. In addition, crystal faces have been reported to influence the properties of two-dimensional materials.

Therefore, it is important to synthesize 2D materials with specific facets in a controlled manner. However, controlling the two-dimensional anisotropic growth of materials is challenging due to their non-laminar structure. Furthermore, since 2D materials expose the most stable facets with the lowest energy, controlling the thermodynamics of the material becomes particularly important.

To address these challenges, in a recent research article published in National Science Review, scientists at Wuhan University, China, have proposed a new paradigm and achieved a series of faceted controlled syntheses of non-layered 2D rare earth oxides. The introduction of a faceted control aid (FCA) allows for controlled 2D nucleation of predetermined facets, adjusting the growth pattern and orientation of the crystals.

The authors stated: "According to the hard-soft-acid-base (HSAB) theory, RE ions are hard acid and prefer to have affinities toward the base. We employed NH4X as the FCA and halide ions belonging to the base act as the active assistor. The introduction of FCA not only controls the 2D nucleation of the predetermined facets and promotes the 2D anisotropic growth of REOs, but also leads to the change in the relative surface energy of each facet with the increasing concentration of FCA and eventually determines the final exposing facet. The strategy can be extended to the facet controllable synthesis of a series of 2D REOs single crystals, including light REOs (CeO₂, Nd₂O₃), middle REOs (Sm₂O₃, Eu₂O₃), and heavy REOs (Dy₂O₃, Ho₂O₃, Y₂O₃), respectively."

Taking CeO2 as an example, they systematically investigated the atomic structure differences between the grown 2D CeO₂ (111) and CeO₂ (100) single crystals. At the same time, they performed experiments and DFT calculations to further confirm the mechanism. It was demonstrated that at low concentrations of FCA, the calculated surface energy of CeO₂ (111) was lower, preferentially obtaining 2D CeO₂(111). As the concentration of FCA increases, the calculated surface energy of CeO₂(100) is lower and the corresponding crystal morphology becomes square. They also explored the facet-dependent paramagnetic properties of 2D REO single crystals.

"Our versatile work brings forth new insights for realizing the anisotropic growth of non-layered 2D REOs materials and enriches the 2D material family," says Prof. Lei Fu. "Notably, the high maneuverability of this strategy opens up opportunities for designing new materials, studying their properties and potential applications in a wide range."

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