[Image above] Schematic from the original Drexel University press release announcing discovery of MXenes in 2011. The schematic shows an exfoliated Ti3AlC2 forming two OH-terminated MXene layers. Credit: M. Kurtoglu, Drexel University
It’s been a few months since we last talked about MXenes.
MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides that are made by selectively etching MAX phases. Originally discovered in 2011 by two groups of researchers at Drexel University, it wasn’t until 2017 that the MXene “gold rush” began and researchers started identifying new MXenes at an unprecedented rate.
(There are currently more than 30 different experimentally made MXenes and more than 100 theoretically predicted MXenes, including those with in-plane and out-of-plane ordering of metal atoms.)
MXenes have a wide variety of applications, from use in water treatment and environmental remediation to bone regeneration and ultrahigh-temperature ceramics, among others. With such a broad playing field, it can be difficult to know what research on MXenes has taken place—which is why a new review article published in Science last week is such a valuable resource.
The senior author of the new paper is ACerS Fellow Yury Gogotsi, Distinguished University and Charles T. and Ruth M. Bach Professor in the Department of Materials Science and Engineering at Drexel University. He wrote the paper along with Drexel research assistant professor Armin VahidMohammadi and Linköping University professor Johanna Rosén.
The three researchers wrote the review article as a way to mark the 10th anniversary of the discovery of these materials. In addition to discussing the fundamentals of MXenes, they give the article a future focus by including ways to overcome the hype accompanying new discoveries, as well as ways to address challenges related to synthesis, scale-up, and commercialization of these materials.
Below are a few highlights from the 14-page review. You can learn more about MXenes by joining a free webinar tomorrow (June 16) at 10 a.m. Eastern, where Gogotsi will talk about advanced 2D materials.
Though the basic definition of MXenes as 2D transition metal carbides and nitrides is straightforward, there is enough variety in structures and compositions that defining terminology for MXenes is necessary when discussing them in detail.
The general formula of MXenes is Mn+1XnTx, where M represents the transition metal site, X represents carbon or nitrogen sites, n can vary from 1 to 4, and Tx (where x is variable) indicates surface terminations on the outer transition metal layers.
The formula is written as (M′,M′′)n+1XnTx if there are two randomly distributed transition metals occupying M sites in the MXene structure (M′ and M′′ represent the two different metals).
Two-metal MXenes are called i-MXenes if the two metals have in-plane ordering and form alternating chains of M′ and M′′ atoms within the same M layer. Two-metal MXenes are called o-MXenes if the two metals are located in separate atomic planes and have out-of-plane ordering, with M′′ atoms constituting the inner metal layers and M′ atoms placed in outer layers.
MXenes are mainly metallic conductors that, as stated above, are being explored for use in a wide variety of applications. The review article goes into detail on some of the main applications, including the following.
Optical and electronic applications
Low sheet resistance and good transparency in the visible light range make thin films of the MXene titanium carbide (Ti3C2Tx) promising for optoelectronic applications where flexible, transparent, conductive electrodes are required, such as solar cells, liquid crystal displays, and organic light-emitting diodes.
MXenes also serve well in gas sensors because of their metallic core channels and surface functional groups, which allow the material to strongly adsorb and thereby detect volatile organic compounds and nonpolar gases, such as ammonia, ethanol, and acetone, at room temperature.
MXenes can serve in electromagnetic interference shielding applications as well thanks to their high metallic conductivity and abundant free electrons. The high conductivity and solution processability of Ti3C2Tx has helped it attract attention for microwave absorbance and terahertz shielding, as well as wireless communication, antennas, and radio frequency identification tags.
Energy storage, harvesting, and electrocatalysis
Use of MXenes for electrochemical energy storage, such as lithium-ion batteries and supercapacitors, was an early area of interest. (See reviews on this specific application area here and here.)
Two reasons that MXenes have so much potential for this application area are: 1) the transition metal core layers in MXenes facilitate rapid electron transport through the electrode, enabling charge storage at ultrahigh rates; and 2) a transition metal oxide-like surface provides redox-active sites for pseudocapacitive charge storage.
Biomedical and environmental applications
Biocompatibility and low cytotoxicity of most MXene compositions, such as Ti3C2Tx, Nb2CTx, and Ta4C3Tx, as well as their plasmon resonance and high photothermal conversion efficiency in the near-infrared and infrared range make these materials promising for cancer theranostics, i.e., the combination of using one radioactive drug to identify (diagnose) and a second radioactive drug to deliver therapy to treat the main tumor and any metastatic tumors.
MXenes also have higher resistance to biofouling and accumulation of microorganisms such as bacteria compared with graphene oxide, so researchers have explored using MXenes as filtration and desalination membranes and as implantable devices.
One of the most exciting possible biomedical applications is to use MXenes to build wearable dialysis systems (“artificial kidneys”), which would free millions of people from having to use stationary dialysis machines and could save lives of people where there is no access to dialysis facilities.
The authors acknowledge there are still many challenges that must be addressed to unleash the full potential of MXenes.
“Efficient, scalable, and cost-effective etching techniques and delamination routes need to be developed for MXenes beyond Ti3C2Tx,” they write. In addition, “Nitride MXenes have been predicted to have a variety of attractive properties, from ferromagnetism to higher conductivity than carbides or semiconducting properties. However, only a few nitrides have been made.”
Considering how much MXenes have to offer, the researchers “expect to see the pinnacle of MXene research in the next few years with wide commercialization and use of these materials,” they write in an email.
They conclude the paper by identifying some specific areas ripe for exploration, for example:
- Considering how MXenes complement properties of other 2D materials and how they can be used as building blocks to create hybrid materials and structures.
- Establishing methods and protocols to improve the chemical stability of MXenes.
- Facilitating scale-up and large-volume manufacturing of MXenes by better understanding the role of precursor structure and stoichiometry, as well as the etchant composition and postprocessing treatments on synthesis and properties.
- Exploring alternative synthesis methods, such as by using chemical or physical vapor deposition methods or by starting from non-aluminum-based MAX phases.
The paper, published in Science, is “The world of two-dimensional carbides and nitrides (MXenes)” (DOI: 10.1126/science.abf1581).