Adv. Sci.｜Efficient Exfoliation Strategies for Large Size/High Yield MXene Materials--Efficient Utilization of Vortex Kinetic Energy

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North Konami can provide MXene (can be customized)

Research abstract

As an emerging two-dimensional material, MXene has good application prospects in many fields such as energy storage, catalysis, superconductivity, and electromagnetic shielding. Ti3C2Tx is a new type of layered two-dimensional nanosheets, which exist in the form of multiple layers after the Al atomic layer is removed by hydrofluoric acid etching. At present, the synthesis scheme of single-layer MXene materials mostly relies on two steps of etching and stripping. The stripping mainly relies on the intercalation of organic bases or lithium ions into the multi-layer, and then assisted by ultrasound to delaminate the material in large quantities. Then, the use of ultrasound inevitably leads to the fragmentation of the single-layer MXene material, resulting in a large number of defects on the surface, which reduces the electrical conductivity and stability of the material. How to avoid the use of ultrasound has important research significance for improving the performance of MXene materials.

In order to solve the above problems, a common measure is to use a scheme such as hand shaking to exfoliate the monolayer MXene material from the bulk phase. However, there is still a lack of understanding of the mechanics involved in this process, and the hand-cranking or oscillating peeling strategies are less efficient. Therefore, it is of great significance to find a preparation scheme that can obtain clean, large-scale, low-defect, and high-yield 2D MXenes.

Introduction

Recently, the team of Professor Li Hui and Associate Professor Zhang Chenhao of Shanghai Normal University designed a power-focused delamination (PFD) by analyzing the resistance and dynamics of delamination during the delamination process of MXene, combined with the unique organ-like structure of MXene. ) program. In short, the MXene-etched multilayer structure is stacked layer by layer by centrifugal precipitation, which becomes easier to peel off. At the same time, the impact force generated by the vortex motion of the water flow is concentrated on a single surface of the MXene sheet, which can effectively overcome the interlayer force of the MXene sheet and do useful work. While in the MILD process, the energy generated by hand shaking is mainly transformed into the kinetic energy of the multilayer MXene bulk, which is rarely provided to the flake layers to overcome the interlayer force. During the PFD process, the impact force generated by the eddy currents is concentrated on the surface of the MXene deposits. Therefore, using this method, a focused shear force can be applied to exfoliate a single layer of MXene from the surface of the multilayer MXene. Yields obtained using this method are significantly higher than those obtained using the MILD method. After 5 PFD cycles, the yield of large-sheet defect-free Ti3C2Tx MXene nanosheets reached 61.2% with a colloidal concentration of 20.4 mg mL-1 without any sonication. The relatively large lateral dimensions of MXene sheets allow the facile fabrication of nanometer-thin MXene field-effect transistor (FET) devices with electrical conductivities up to 25,000 S cm-1. The as-prepared free-standing MXene film exhibits excellent absolute electromagnetic shielding performance of 35419 dB cm2 g-1, surpassing that of metals, graphene/carbon nanotubes, and MXene materials prepared by traditional methods. Such high-quality MXene nanosheets prepared using the PFD protocol are expected to facilitate the research on large-scale and size-dependent MXenes and expand the application scope of MXenes.

The result was published online in the top international journal Advanced Science (impact factor 17.521) with the title: Synthesis of Large-Area MXenes with High Yields through Power-Focused Delamination Utilizing Vortex Kinetic Energy.

Zhang Qingxiao and Fan Runze are the co-first authors of this article.

Graphical guide

Figure 1. Ti3C2Tx exfoliation: a) Schematic of conventional sonication, MILD and PFD methods for Ti3C2Tx exfoliation; scale bar: 2 μm. b) Yields of dispersed Ti3C2Tx MXenes after different treatments and c) Yields of dispersed Ti3C2Tx MXenes obtained after different durations of PFD. (d) Size and conductivity of MXenes prepared by different sonication times.

Figure 2. TEM image of the as-prepared LPFD-Ti3C2Tx

Figure 3. Conductivity measurements and performance comparison of MXenes. a) Schematic diagram of the conductivity test scheme of LPFD-Ti3C2Tx MXene nanosheets. b) Optical microscope image of the FET device with LPFD-Ti3C2Tx MXene nanosheets. c) IDS-VDS curve of the device with VG = 0. The inset is a schematic diagram of the error obtained in two tests at maximum voltage; the difference is almost negligible and can be considered a test error. d) Comparison of reported conductivity, size, and material yield of Ti3C2Tx MXene materials prepared by different processes.

Figure 4. Hydrodynamic simulation of solution exfoliation. Changes in water flow and block motion as a function of water flow velocity and c) pressure when a) the multilayer MXene material is unanchored during conventional shaking and b) when anchored in the PFD method. d) The elastic potential energies obtained by the surface nanosheets during the delamination process under non-anchored and anchored conditions. e) Kinetic and elastic potential energies acquired by the surface layer during PFD.

Experimental part: raw material selection

For the preparation of MXene materials, the selection of raw materials is particularly important. The raw materials used in this paper are the commercial Ti3AlC2 raw materials purchased in the same batch, see the SI part of the original text. Note: For non-commercial promotion, raw materials from other companies may be better prepared.

The authors experience: Whether it is the scheme in this article or the very mature synthesis scheme in other articles, if it is repeated many times and still cannot be done, it is recommended to change the Ti3AlC2 raw material and try again. I and several friends who prepare MXene materials have suffered from this, and the experiments have been delayed for a long time, because I have been exposed to the rain, I hope I can help you hold an umbrella.

Here @SJTU Hu, DUT Tan, NUS Tian, SDUT Dong, come on!

Experimental part: LiF/HCl etching

Etching raw material: Ti3AlC2

Etching mass: 3 grams

Etching solvent: 27% HCl, 60 ml (*Note, the concentration can be slightly higher)

Etching temperature: 45 degrees (*Note, here is the actual temperature of the etching solvent, it is recommended to replace it with water before the experiment, and test the temperature)

Etching time: 24 hours

Stirring speed: about 450 rpm (general parameters, little effect on etching)

The authors experience: When etching, there must be a certain gap, and it should not be airtight. It can be a small hole with a diameter of about 2-3mm, or a small gap (personal experience). This detail is rarely mentioned in the article, but it should be noted that it is mentioned in the experimental part of this article, and citations are welcome.

Experimental part: cleaning after etching

Step 1: Pickling: Wash twice with dilute hydrochloric acid, which is beneficial to remove impurities. It is mentioned in the experimental part of this article, please refer to it.

Step 2: Wash with water until the pH of the upper liquid is neutral, or separate layers automatically. Spontaneous stratification tends to approach pH ≈ 4-6.

The authors experience: There are two possible misunderstandings about the raw materials after etching (in the exchange group, people often discuss these two issues, and I have participated a few times to talk about my own understanding).

1. It is possible to think that this material should have an accordion structure. Here I remind you that this is not a necessary shape. You can refer to the article Progress in Materials Science 120 (2021) 100757, page 10.

2. The pure MXene material obtained here is actually uncertain, especially the LiF/HCl etching scheme. After etching, washing and drying, it is actually (multi-layer MXene + incompletely etched MAX + impurities) The mixture, which we discussed in the article, can be used as an argument if it is necessary to write an article in the future.

The above description is only for the materials that have not been delaminated after etching. Once peeled off, the single layer of MXene and other impurities can be well separated by centrifugation, so students who want to prepare a single layer do not need to care about the morphology and purity of the multilayer. question.

Experimental part: peeling after washing

According to everyones needs, there are actually many options for peeling after washing. I venture to briefly talk about my understanding here:

Authors experience: In fact, peeling is very simple now, and even some articles can be carried out without any ultrasound and vibration. Students who have just studied MXene must first try ultrasonic peeling, and then choose other solutions according to their own needs, which will help to understand the properties of the material.

Summarize

This paper reports a fast and efficient PFD strategy to aid in the preparation of Ti3C2Tx MXene materials. Through controlled experiments, we demonstrate that the shear of the vortex fluid generated by oscillation is concentrated on the surface layer of the Ti3C2Tx MXene precipitate during PFD. This promotes the uncoordinated motion between the surface Ti3C2Tx MXene nanosheets and blocks. Compared with the ultrasonic stripping and MILD methods, this protocol greatly improves the stripping efficiency and increases the yield by 6.4 times. Furthermore, the MXene layers prepared by the PFD process have large lateral dimensions and can prevent the morphological damage and performance loss observed when using the ultrasonic lift-off method. Finally, the SSE/t value of the Ti3C2Tx MXene material prepared by the PFD method is much higher than that of the material prepared by ultrasonic treatment (35419 dB cm2 g-1). In conclusion, this new assisted exfoliation strategy significantly improves the exfoliation efficiency of large-area monolayer Ti3C2Tx MXene materials, which is expected to open new avenues for various applications of Ti3C2Tx MXene materials.

Literature link

https://onlinelibrary.wiley.com/doi/10.1002/advs.202202748

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