Yang Mulan, Schoop Leslie M
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
Acc Chem Res. 2024 Sep 3;57(17):2490-2499. doi: 10.1021/acs.accounts.4c00295. Epub 2024 Aug 16.
ConspectusTwo-dimensional materials have been a focus of study for decades, resulting in the development of a library of nanosheets made by a variety of methods. However, many of these atomically thin materials are exfoliated from van der Waals (vdW) compounds, which inherently have weaker bonding between layers in the bulk crystal. Even though there are diverse properties and structures within this class of compounds, it would behoove the community to look beyond these compounds toward the exfoliation of non-vdW compounds as well. A particular class of non-vdW compounds that may be amenable to exfoliation are the ionically bonded layered materials, which are structurally similar to vdW compounds but have alkali ions intercalated between the layers. Although initially they may have been more difficult to exfoliate due to a lack of methodology beyond mechanical exfoliation, many synthesis techniques have been developed that have been used successfully in exfoliating non-vdW materials. In fact, as we will show, in some cases it has even proven to be advantageous to start the exfoliation from a non-vdW compound.The method we will highlight here is chemical exfoliation, which has developed significantly and is better understood mechanistically compared to when it was first conceived. Encompassing many methods, such as acid/base reactions, solvent reactions, and oxidative extractions, chemical exfoliation can be tailored to the delamination of non-vdW materials, which opens up many more possibilities of compounds to study. In addition, beginning with intercalated analogues of vdW materials can even lead to more consistent and higher quality results, overcoming some challenges associated with chemical exfoliation in general. To exemplify this, we will discuss our group's work on the synthesis of a 1T'-WS monolayer ink. By starting with KWS, the exfoliated 1T'-WS nanosheets obtained were larger and more uniform in thickness than those from previous syntheses beginning with vdW materials. The crystallinity of the nanosheets was high enough that films made from this ink were superconducting. We will also show how soft chemical methods can be used to make new phases from existing compounds, such as HCrS from NaCrS. This material was found to have alternating amorphous and crystalline layers. Its biphasic structure improved the material's performance as a battery electrode, enabling reversible Cr redox and faster Na-ion diffusion. From these and other examples, we will see how chemical exfoliation of non-vdW materials compares to other methods, as well as how this technique can be further extended to known compounds that can be deintercalated electrochemically and to quasi-one-dimensional crystals.
概述
几十年来，二维材料一直是研究的重点，由此开发出了一系列通过各种方法制备的纳米片库。然而，这些原子级薄的材料大多是从范德华（vdW）化合物中剥离出来的，而块状晶体中层间键合本就较弱。尽管这类化合物具有多样的性质和结构，但该领域也应超越这些化合物，关注非vdW化合物的剥离。一类可能适合剥离的非vdW化合物是离子键合的层状材料，其结构与vdW化合物相似，但层间插层有碱金属离子。虽然最初由于缺乏除机械剥离之外的方法，它们可能更难剥离，但现已开发出许多成功用于剥离非vdW材料的合成技术。事实上，正如我们将展示的，在某些情况下，从非vdW化合物开始剥离甚至已证明具有优势。
我们在此重点介绍的方法是化学剥离，与最初构想时相比，它已取得显著发展，并且在机理上有了更深入的理解。化学剥离涵盖多种方法，如酸碱反应、溶剂反应和氧化萃取，可针对非vdW材料的分层进行定制，从而为研究开辟了更多化合物的可能性。此外，从vdW材料的插层类似物开始，甚至可以得到更一致、更高质量的结果，克服一般化学剥离所面临的一些挑战。为举例说明这一点，我们将讨论我们团队在合成1T'-WS单层油墨方面的工作。从KWS开始，得到的剥离1T'-WS纳米片比之前从vdW材料开始合成的更大且厚度更均匀。纳米片的结晶度足够高，以至于用这种油墨制成的薄膜具有超导性。我们还将展示如何使用软化学方法从现有化合物制备新相，例如从NaCrS制备HCrS。发现这种材料具有交替的非晶态和晶态层。其双相结构改善了该材料作为电池电极的性能，实现了可逆的Cr氧化还原和更快的Na离子扩散。从这些及其他例子中，我们将看到非vdW材料的化学剥离与其他方法相比如何，以及该技术如何进一步扩展到可通过电化学脱嵌的已知化合物和准一维晶体。
