On February 14, 2023, Beijing time, Professor Zeng Zhiyuan of City University of Hong Kong, Professor Ju Li of the Massachusetts Institute of Technology and others co-published a review article entitled “Synthesis of atomically thin sheets by the intercalation-based exfoliation of layered materials” in the journal Nature Synthesis.
In this review, the authors review the progress of intercalation stripping technology in the preparation of atomic thin-layer materials in the past ten years, introduce a variety of mature intercalation peeling strategies, analyze the factors affecting the peeling effect, summarize the application potential of stripped atomic thin-layer materials, and look forward to the future opportunities and challenges of this technology.
The corresponding authors of the paper are Zeng Zhiyuan and Li Ju; The first author is Yang Ruijie.
After the discovery of graphene in 2004, the reliable production of atomic thin-layer materials has become a common pursuit in academia and industry. These thin-layer atomic materials, especially monolayered materials, have attracted interest in many fields, including photonics, electronics, optoelectronics, energy storage, catalysis, environmental remediation, and bioengineering. Peeling is a “top-down” strategy for preparing atomic thin-layer materials, including micromechanical peeling, direct liquid phase peeling, and intercalation peeling. Micromechanical peeling technology using transparent tape to peel nanosheets from bulk crystals has the disadvantages of low yield and poor control of the thickness, size and shape of the target nanosheet. Direct liquid phase peeling in solvents is also hampered by low monolayer yield, small transverse dimensions of peeled flakes, and toxicity of the organic solvents used. Intercalation stripping is one of the most promising strategies for the large-scale production of atomic flakes, and is popular for its solution processability, scalability, and large transverse size and high single-layer yield. The last decade has seen the rapid development of intercalation stripping technology to produce thin-layer atomic materials.
Figure 1: Timeline of key advances in the preparation of atomic thin-layer materials by intercalation stripping technology.
▌The basic process of intercalation peeling to prepare atomic thin-layer materials
Typical procedures for the intercalation stripping process include object (alien species) intercalation and subsequent subject (lamellar material) stripping. Guest intercalation can be achieved by chemical or electrochemical approaches. Body peeling refers to the separation of atomic layers from their embedded compounds (subject + guest) spontaneously or with the help of sonication, stirring, or manual shaking, after which an opaque suspension of peeling nanosheets is formed. In most cases, this process is accompanied by the formation of bubbles. The stripped suspension usually contains some lamellar material that has not been completely peeled off as well as an intercalating agent. Therefore, to obtain a clean product, a purification process is required, which consists of a low-speed centrifugation process to remove larger particles (precipitated at the bottom of the vial), followed by multiple cycles of high-speed centrifugation to remove residual intercalation ions on the surface of the nanosheet. Subsequently, the collected 2D nanosheet-rich pellet (purified nanosheet) is typically re-dispersed in water or solvent (e.g., isopropanol, N,N-dimethylformamide) by sonication to form a printable ink for storage and subsequent use.
Figure 2: Schematic diagram of the process of intercalation peeling to prepare atomic thin-layer materials.
▌Mechanism of intercalation peeling to prepare atomic thin-layer materials
The interlayer force of layered materials is overcome by intercalation and post-intercalation effects, and then facilitate their subsequent peeling, which is the intrinsic mechanism of intercalation peeling to prepare atomic thin-layer materials. This post-intercalation effect may be an increase in the distance between layers, the release of bubbles, or an energy-favorable solvation process. This mainly depends on the type of intercalator and solvent used. The insertion of molecules, such as alkyl amines, is a process without charge transfer and often results in a significant increase in layer spacing. This post-intercalation effect weakens the van der Waals force of adhesion between layers, which in turn promotes the peeling off of atomic layers. The intercalation of ions, such as alkali metal ions, is always accompanied by a charge transfer between the intercalated ions and the layered crystal, resulting in the formation of a charged layer. This process reduces the van der Waals force between layers, but creates an additional electrostatic attraction (stronger than the van der Waals force) between the ions and layers of opposite charges. As a result, the overall attraction between the layers increases, the overcoming of which is often associated with the solvent used in the peeling process. Proton solvents, such as water, often result in the release of gases (such as hydrogen, sulfur dioxide, and oxygen), which generate a large force that pushes the individual layers apart and plays an important role in the stripping mechanism. Aprotic solvents can coordinate charged layers and ions, thus promoting an energy-advantageous solvation process, which also facilitates the dispersion of atomic layers.
▌Lithium-ion-based intercalation stripping strategy
Since the publication of groundbreaking work in 1986, lithium-ion (Li+) intercalators have been widely used in the stripping of atomic thin-layer materials. The article describes several of the most commonly used lithium-ion intercalators, including n-butyllithium (n-BuLi), lithium borohydride (LiBH4), lithium naphthalene (Nap-Li), pyrene lithium (Py-Li), lithium metal, and lithium-ion batteries. The most popular strategy in recent years has been the electrochemical intercalation stripping strategy based on lithium-ion batteries (Nat. Protoc. 2022, 17, 358-377；Angew. Chem. Int. Edit. 2021, 50, 11093-11097；Angew. Chem. Int. Edit. 2012, 51, 9052-9056)。 Through this strategy, a library of atomic thin-layer inorganic nanosheets has been established, including graphene, MoS2, WS2, TiS2, TaS2, ZrS2, h-BN, NbSe2, WSe2, Sb2Se3, Bi2Te3, etc. The yield of MoS2 and TaS2 monolayers can reach 92% and 93%, respectively. Another advantage of this method is the ability to monitor and finely control the degree of Li+ insertion by adjusting the cutoff voltage, which avoids inadequate or excessive lithium-ion insertion. The strategy is also extensible. The output of a single electrochemical cell has evolved from the initial milligram scale (using columnar or button cells) to the gram level (using flexible packaging batteries). If the technology allows, it is expected to achieve tonnage production by expanding the scale of electrochemical cells in the future.
Figure 3: Intercalation stripping strategy based on lithium-ion batteries.
▌Intercalation peeling strategy based on tetraalkyl ammonium cations
Tetraalkylammonium cation (R4N+; For example, TMA+, TEA+, TPA+, TBA+, THA+ and TOA+) are commonly used intercalators for electrochemical peeling of layered crystals. The advantage of this strategy lies in its universality. Through this strategy, a variety of atomic thin-layer materials can be produced, including graphene, TMD, BP, A2B3 (A: group III elements; B: Group VI elements; For example, In2Se3), AMX2 (A: unit valence metal; M: trivalent metals; X: Chalcogeniphagenic elements; such as AgCrS2) and so on. Electrochemical R4N+ intercalation peeling tends to produce graphene with fewer defects than intercalation peeling of anions (e.g., sulfate) due to its non-oxidative production pathway. However, the flaking graphene is unstable and easily re-stacks into graphite. Therefore, covalent functionalization of graphene is often required to improve its stability. Compared with TMD monolayers stripped based on Li+ intercalation technology (with defects, small size and phase transition), TMD monolayers produced based on R4N+ intercalation peeling are usually defect-free, phase change-free, large in size and environmentally stable.
Figure 4: Intercalation peeling strategy based on tetraalkyl ammonium cations, anions, and small molecules.
▌Anion-based intercalation peeling strategy
Various polyanions have also proven to be effective intercalation ions for the synthesis of atomic sheets, including SO42-, BF4-, ClO4-, PO43-, C2O42-, OH- and R-SO3-. They are commonly used in the production of graphene. The most widely used of these are SO42- and BF4-. Electrochemical peeling strategies based on sulfate ion (SO42-) intercalation are effective and versatile methods for graphene peel synthesis, but usually introduce substrate defects caused by surface oxidation. In addition to graphene, the strategy can also produce other inorganic nanosheets such as MoS2, BP and topological insulators (Bi2Se3 and Bi2Te3). However, the monolayer yield of the final product is relatively low; For example, the yield of MoS2 monolayer is less than 7%. Boron tetrafluoride (BF4-) is another commonly used anion for electrochemical stripping. This strategy is also mainly applied to the production of graphene. Impressively, the stripped graphene can functionalize fluorine, improving the thermal stability and transparency of graphene. In addition to graphene, BP monolayers can also be produced by an oxygen-driven peeling mechanism by this method. However, the final BP monolayer contains a large number of neutral defects and dangling oxygen and hydrogen bonds at the edges.
▌Small molecule-based intercalation stripping strategy
Atomic thin-layer materials can also be stripped by embedding small molecules such as 4,4′-dipyridyl disulfide and alkyl amines into the bulk material to expand their interlayer distance. 4,4′-Dipyridyl disulfide (DPDS) is a chemically unstable dipyridyl ligand that has proven to be an effective intercalator for stripping metal-organic frameworks (MOFs). The yield of 2D MOF flakes (thickness less than 1.2 nm) produced by this method is as high as 90%. Alkyl amines (e.g., alamines[C3H9N], butylamine[C4H11N]and hexylamine[C6H15N]It is also used as a molecular intercalator in the production of TiS2, ZrS2, NbS2 and MoS2 single-atom-layer nanosheets. The advantages of this strategy are also room temperature, safe, and gentle operating conditions, and no harmful gases (e.g. H2, SO2).
▌Factors that affect the peeling effect
Factors affecting the peeling effect include: characteristics of the initial bulk material (e.g., crystal phase, elemental composition), intercalator (type, concentration), solvent (polarity, surface tension, proton), applied voltage (size, time), peeling strength, and centrifugation speed. By adjusting these parameters, the thickness, transverse size, crystal phase, defect concentration, etc. of the final stripped atomic thin layer material can be adjusted on demand.
Figure 5: Factors affecting the quality of stripped atomic thin layer material.
▌Crystal phase transfer
During intercalation stripping (especially when lithium ions are used as intercalation agents), the insertion of cations usually involves simultaneous implantation of electrons.When electron injection exceeds a certain threshold, the stability of the 2H phase of TMDs is lower than that of 1T or 1T′ phase, resulting in a corresponding phase transition. For a theoretical explanation of this phenomenon, readers can refer to the following article on crystal field theory (Nat. Rev. Mater. 2021, 6, 829-846；Nat. Phys. 2017, 13, 931-937）。 Experimental verification of phase transitions can be achieved by scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and photoluminescence spectroscopy. Among them, STEM provides an opportunity to distinguish the fine atomic structure of the 2H, 1T, and 1T′ phases. The deconvolution of high-resolution XPS peaks provides feasibility for quantitative analysis of mixed phases.
The phase transition of 2H-to-1T due to electron injection can be restored by some means. These include: gentle annealing (above 300°C, 1 h), hydrothermal treatment (200°C), infrared (IR) laser-induced treatment.
Previous electrostatic gating studies have shown that the phase transition of 2H-to-1T can be avoided by reducing electron injection. In 2018, Duan’s team at UCLA avoided phase transitions by using larger cations (e.g., tetraalkyl ammonium cations) as intercalators to prepare high-purity 2H-phase MoS2 (Nature 2018, 562, 254-258).
Figure 6: Phase transfer during intercalation stripping.
▌Applications for stripping nanosheets
When the layered material peels off into thin layers of atomic nanosheets, its extraordinary and unique properties are unlocked and activated. Crucially, the atomically thin layer nanosheets prepared by intercalation peeling are compatible with solution-based deposition techniques (e.g., drop casting, spin coating, spraying, filtration) and printing techniques. This allows stripped nanosheets to be easily and scalable into a variety of customizable devices. Thus allowing flakling atomic flakes (e.g., graphene, TMDs, BP, h-BN, MOF, Sb) to be used as core building blocks of modern devices, which can be used as core building blocks in electrons (e.g., field-effect transistors, thin-film transistors), photons (e.g., optical switchers, Kerr shutters, beam shapers), photoelectrons (e.g., photodetectors), energy storage (e.g., supercapacitors, sodium-ion batteries, lithium-ion batteries), environmental remediation (e.g., nanofiltration and desalination devices), Areas such as bioengineering such as biosensors have yielded an impressive array of applications.
The atomically thin nature of the stripped nanosheets also gives them advantages over bulk crystals in a variety of catalytic applications. These advantages include quantum confinement, short carrier transport distances, large surface area-to-volume ratios, and abundant low-coordination surface atoms. For photocatalytic applications, the quantum confinement effect imparts a tunable band structure to atomically thin-layer semiconductor nanosheets. The short transport distance of the carriers from the inside to the surface of the material inhibits bulk recombination of charge carriers. The large surface area-to-volume ratio enables multi-point contact between the reactant and the catalytic site. Abundant low-coordination surface atoms provide abundant adsorption and activation sites for typical reactant molecules. All these features contribute to the excellent photocatalytic performance of the stripped atomic, thin-layer semiconductors. In addition to photocatalysis, stripped nanosheets are also superior to their bulk materials in terms of electrocatalysis (such as electrocatalytic hydrogen production and carbon dioxide reduction).
Although intercalation peeling technology has successfully prepared a series of atomic thin layer materials, from graphene, h-BN, BP, TMDs, A2B3 (such as In2Se3), to MOFs, Sb, ZnIn2S4, BiOCl, etc. However, there are more than 5,600 experimentally known lamellar materials, and the current stripped material is only the tip of the iceberg of the vast library of lamellar materials. Therefore, it is urgent to extend and promote intercalation peeling technology.
It is important and challenging to understand the deep mechanisms involved in the intercalation stripping process, such as defect formation mechanism, electron transfer mechanism, performance evolution mechanism, etc. The proliferation of modern in situ characterization toolkits, such as in situ liquid phase TEM, in situ X-ray absorption spectroscopy, in situ scanning transmission X-ray microscopy, etc., provides sophisticated techniques to elucidate these mechanisms. However, the most challenging direction is how to build a suitable measurement microplatform to meet both the operational requirements of characterization (e.g., vacuum environment for TEM imaging) and the liquid microenvironment of intercalation stripping.
Although intercalation stripping technology has made great progress in recent years, there is still a long way to go to achieve the lab-to-factory transition. The first and most common obstacle is scaling up production. This requires technical and cost efforts from researchers and engineers.
In this promising field, opportunities and challenges coexist. The authors anticipate that the intercalation stripping strategy will become one of the most effective methods for producing atomic thin-layer two-dimensional materials in the coming decades. (Source: Science Network)
Related paper information:https://doi.org/10.1038/s44160-022-00232-z