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Focus on MXenes, Materials, and Scanning Electron Microscopy

MXenes are a new family of 2D crystalline nanomaterials explored for energy applications using SEM microscopy

4 MIN READ
Flower power; Vanadium oxide nano structure synthesized from two-dimensional vanadium carbide MXenes; Yeonjin Baek, Auburn University; JEOL JSM-7000F SEM – November 2023 Contest Winner
MXene Golden Swan; In a moonlight canvas, presenting a multilayered titanium carbide Ti3C2Tx MXene visualized as a golden swan. This MXene is synthesized by the selective etching of the optimized Ti3AlC2 MAX phase. The moon is a digital photo of a MXene film. MXene Swan’s serenade, where beauty and moonlight dance in perfect harmony! Team: Nithin Chandran B S, Dr. Anupma Thakur and Prof. Babak Anasori ; CREDIT: Nithin Chandran B S, IUPUI and Purdue University; METHOD/INSTRUMENT: JEOL JSM-7800F
You might imagine a familiar shape when you look at any SEM image: Polymer “needles” become a field of Christmas trees; a tomato pollen tube resembles a lean Siamese cat. It’s even easier to visualize when the microscopist adds color to define shapes in the image. But one subject in particular seems to suggest likenesses when imaged in the SEM. For this reason MXene images are frequent contestants in the JEOL Image Contest. 
MXenes are a relatively new family of 2D crystalline nanomaterials that are just a few atoms thick and have jagged accordion shapes that when colored, can look like goldfish, bouquets of flowers, dinosaurs, and even sunsets. This November one MXene image “Flower Power” submitted by Yeonjin Baek, Graduate Research Assistant at Auburn University, won the JEOL contest.
Another sci-art “MXene Golden Swan,” submitted by Dr. Anupma Thakur and Nithin Chandran from Indiana University-Purdue University in Indianapolis (IUPUI), won the Science as Art contest during the Materials Research Society (MRS) Fall 2023 conference in Boston.

Synthesizing MXenes

AFM (Atomic Force Microscopy) image of Single layered Niobium carbide MXenes after delamination
We asked Yeonjin Baek about her work at Auburn University, where she focuses on synthesizing and developing different types of MXenes for energy storage applications.
She explained that more than 100 possible MXene compositions have been predicted and more than 30 have been experimentally obtained.
 “They are easy to synthesize in the lab,” says Yeonjin. Her work is helping to create a library of MXene materials to study for their applications with potential use in sensors, water purification membranes, and energy storage, including cathodes in lithium-ion batteries, and supercapacitors.
“The property of MXenes can be tuned by the choice of M,X element and surface terminal groups. Multilayer MXenes are typically imaged in the SEM, but “if you want to look at a single layer of MXene flakes you need to delaminate (or exfoliate) it and then look at it in the AFM. It looks like plate-like sheets.”
MXenes are indeed molecular sheets obtained from carbines and nitrides of transition metals, i.e. Titanium, Zirconium, and Molybdenum, that are etched with aqueous fluoride to selectively remove layers. (MXenes: trends, growth, and future directions)

The first 2D MXenes were discovered in 2011 at Drexel University, creating an entire family of new materials with a chemical formula of Mn+1XnTx (M covers groups 3 to 6 transition metals, X is carbon or nitrogen, and T represents the surface terminations). (https://link.springer.com/article/10.1007/s41127-022-00053-z)

Yeonjin shared a visual explanation of how MXenes are made:
What is MXenes?
Synthesis of MAX Phase and MXenes

Rise in Populartiy of MXenes Due to Energy Applications

There is no doubt a rise in popularity of MXenes in the world of material science, where all the MXene researchers seem to know one another.
Why are MXenes showing promise for future applications? Yeonjin Baek at Auburn University explains that they have a large surface area, excellent electrical conductivity, hydrophilicity, unique mechanical and thermal properties, an easily accessible structure, and versatile compositions.
Dr. Anupma Thakur, a Postdoctoral Research Associate at IUPU, notes that, “The focus of MXenes at universities is expanding due to its exciting material properties. The field of MXene synthesis is dynamic, with ongoing efforts to explore new precursors, improve scalability, and develop novel functionalization strategies to expand the range of applications for these 2D materials.”
MXene Chronicle; A multilayered titanium carbide. MXene is visualized as a chronicle.
MXene Chronicle; A multilayered titanium carbide. MXene is visualized as a chronicle. The two-dimensional (2D) Ti3C2Tx MXene layers were formed by selective etching of Al layers from the Ti3AlC2 MAX phase using hydrofluoric (HF) acid. Scale:10 micrometers; CREDIT: Nithin Chandran BS, Dr. Anupma Thakur, and Prof. Babak Anasori Indiana University-Purdue University Indianapolis, IUPUI, FE SEM – April 2023 Contest Winner
MXene Nanosaurus; A two-dimensional (2D) multilayered titanium carbide (Ti3C2Tx) MXene is visualized as a nanosaurus dinosaur.

MXene Nanosaurus; A two-dimensional (2D) multilayered titanium carbide (Ti3C2Tx) MXene is visualized as a nanosaurus dinosaur. The Ti3C2Tx MXene was synthesized by selective etching of Al layers from the Ti3C2Tx MAX phase using hydrofluoric acid. With the growing family of MXenes as engineered and tunable 2D ceramics, the future for the field of ceramic engineering is at the nanoscale. Our Team: Anupma Thakur, Nithin Chandran B S, and Prof. Babak Anasori; Anupma Thakur, Purdue University; JEOL FESEM

Dr. Thakur and Nithin Chandran B S sent us an explanation of how MXenes are made and also shared their recent research on the step-by-step guide for the optimized synthesis of Ti3C2Tx MXene:

MXenes are typically synthesized through a process involving the selective etching of MAX phases, which are layered ternary carbide precursors. The most common MAX phase used is titanium aluminum carbide (Ti3AlC2). The synthesis process involves several key steps:
  • MAX Phase Synthesis: The process begins with the synthesis of MAX phases, which are layered compounds with a composition of M(n+1)AXn, where M is a transition metal, A is an A-group element (usually aluminum), and X is either carbon or nitrogen. 
  • Selective Etching: The MAX phase is then subjected to a selective etching process, where the A element (typically aluminum) is removed by using strong acids or other etchants. This leaves behind a layered structure with the transition metal and carbon/nitrogen layers, forming the MXene. 
  • Delamination: After etching, the MXene is often delaminated to increase the interlayer spacing and enhance its properties. Delamination can be achieved through mechanical or chemical methods, such as ultrasonication, intercalation with specific molecules, or other exfoliation techniques. 
  • Surface Functionalization: To tailor the properties of MXenes for specific applications, researchers often perform surface functionalization by introducing different functional groups to the MXene surface. This can enhance stability, improve compatibility with other materials, or modify electronic properties. The choice of precursors, etching agents, and additional processing steps can influence the properties of the resulting MXene, allowing researchers to customize MXenes for various applications, from energy storage to sensing and catalysis. 

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