Researchers Reveal Atomic Positions and Local Electronic Properties of 2D MXene

Xiahan Sang (left) and Raymond Unocic of Oak Ridge National Laboratory used scanning transmission electron microscopy and electron energy loss spectroscopy to reveal atomic positions and local electronic properties of 2D MXene that had been etched and exfoliated from a 3D crystal. Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; photographer Carlos Jones

Scientists have long been searching for electrically conductive materials for cost-effective energy-storage devices. Two-dimensional (2D) ceramics known as MXenes are candidates. MXenes, unlike most 2D ceramics, have inherently excellent conductivity, because they are molecular sheets fabricated from the nitrides and carbides of transition metals like titanium.

Michael Naguib, who is now a Wigner Fellow at the Department of Energy’s Oak Ridge National Laboratory, co-discovered MXenes while pursuing his PhD degree at Drexel University in 2011. MXene layers can be easily combined to develop batteries, electronics, supercapacitors, catalysts, and sensors. Since then, about 20 MXenes have been reported.

ORNL researchers recently used state-of-the-art scanning transmission electron microscopy (STEM) to provide the first direct proof of the atomic-defect configurations in a titanium-carbide MXene created at Drexel University. The study, published in ACS Nano, a journal of the American Chemical Society, combined electrical property measurements and atomic-scale characterization with theory-based simulation.

Using atomic-resolution scanning transmission electron microscopy imaging, we visualized defects and defects clusters in MXene that are very important for future nano electronic devices and catalytic applications.

Xiahan Sang, Center for Nanophase Materials Sciences (CNMS)

“Atomic-level defects can be engineered into materials to enable new functionalities,” said senior author Raymond Unocic of CNMS. “Understanding these defects is critical for advancing materials.”

Atomic imaging from multiple perspectives was the key to revealing the structure of MXene. When the sample is arranged in line with the electron beam inside a STEM instrument, the viewer will not be able to tell how many sheets lie under the top layer.

However, when the sample is tilted, differences can be seen. For instance, a multi-sheet layer is composed of stacked atoms, a structure that creates a blurred image when the layer is tilted. The appearance of clear atomic images under different tilting conditions explicitly confirmed the single-layer structure of the MXene.

Easy Mass-Production of a Good 2D Conductor

MXenes are fabricated from a three-dimensional (3D) bulk crystal known as MAX (the “M” indicates a transition metal; “A,” an element, such as silicon or aluminum, from a particular chemical group; and “X,” either nitrogen or carbon). In the MAX lattice, from which the MXene examined in this study emerged, three titanium carbide layers are inserted between aluminum layers.

The researchers from Drexel University enhanced a method, which was developed and altered in 2011 and 2014, respectively, to create MXene from the bulk MAX phase by using acids. The enhanced method is known as minimally intensive layer delamination (MILD).

By going with MILD, we ended up with large flakes of high-quality MXene.

Mohamed Alhabeb, PhD Student, Drexel University

Alhabeb achieved this feat with Katherine Van Aken, another PhD student, under the leadership of one of the co-discoverers of MXenes, Yury Gogotsi, Distinguished University Professor and Director of the A.J. Drexel Nanomaterials Institute.

In order to create free-standing MXene flakes, the Drexel researchers first treated bulk MAX with a hydrochloric acid and fluoride salt to remove unwanted layers of aluminum present between titanium carbide layers. After that, they manually shook the etched material in order to separate and gather the titanium carbide layers.

Each layer has a thickness of five atoms and is made up of carbon atoms combining three titanium sheets. Etching and exfoliating MAX generates lots of free-standing MXene layers. This simple method may allow manufacturing-scale production.

Etching can create defects - vacant spaces that appear as titanium atoms are removed from surfaces. Actually, “defects” are good in many applications of materials. They can be easily introduced into a material and manipulated to improve the optical, catalytic, or electronic properties.

The study found that when the concentration of etchant is greater, the number of defects created will be larger.

We have the capability to tune the defect concentration, which could be used to tailor physicochemical properties for energy storage and conversion devices.

Xiahan Sang, Center for Nanophase Materials Sciences (CNMS)

Additionally, many defects did not strongly affect the electrical conductivity of MXene. Kai Xiao and Ming-Wei measured physical properties, including electrical conductivity, of different promising 2D materials at CNMS. They found that MXene was a single order of magnitude less conductive than an ideal graphene sheet, but two orders of magnitude are more conductive than metallic molybdenum disulfide.

ORNL’s Paul Kent and Yu Xie used modeling and simulation to calculate the energy required to produce atomic configurations of defects that Sang’s STEM confirmed were prevalent.

As a next step, the researchers plan to alter defects to the atomic level to modify specific behaviors. The title of the paper is “Atomic Defects in Monolayer Titanium Carbide (Ti3C2Tx) MXene.”

The Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center led by ORNL and funded by DOE’s Office of Science, supported the study. FIRST plans to develop scientific understanding and authenticated predictive models of the nanoscale environment at fluid–solid interfaces significant in electrocatalysis and electrical energy storage.

Aberration-corrected STEM imaging as well as device fabrication and measurements were performed at CNMS, a DOE Office of Science User Facility at ORNL. The study also used resources from the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory.

UT-Battelle manages ORNL for DOE’s Office of Science. As the single largest supporter of fundamental research in the physical sciences in the United States, the Office of Science is working to tackle some of the most vital challenges of our time.

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