[Image above] Graphene and graphene oxide are not the only 2D materials that can effectively remove pollutants. Credit: Pixabay
It has been just over one year since I wrote my first article on MXenes last December. Since that time, I have come to know a lot more about these 2D transition metal carbides, nitrides, and carbonitrides, such as how to protect them from oxidation.
One thing I still do not know a lot about, though, is MXene applications. A lot of research focuses on understanding MXene structure and properties, but how MXenes work in application is something I’m always looking to learn more about.
Lucky for me, applications is exactly the topic of a new review paper published last month in Materials Today.
In the 23-page review, researchers from Hamad Bin Khalifa University in Qatar and Drexel University in the U.S. discuss a very timely and relevant application of MXenes: water treatment and environmental remediation.
“Water and environment are among the most important subjects to pursue for any material scientist, as those are super important problems for humanity,” Yury Gogotsi, article coauthor and professor and director of the A.J. Drexel Nanomaterials Institute at Drexel University, says in an email. “My student and myself are experts in MXenes, while our QEERI [Qatar Environment and Energy Research Institute] co-authors are experts in water and environment … So, this was a natural choice for us to write jointly a review.”
Main points of the review are highlighted in the sections below.
Pollutant removal: Why MXenes?
Historically, graphene and graphene oxide derivatives are the most extensively studied class of 2D materials for removal of different pollutants due to their ultrahigh water flux, selective molecular and ion sieving, and strong resistance to biofouling. Since the discovery of MXenes in 2011, though, interest in using MXenes for pollutant removal has grown.
MXenes hold promise for environmental remediation applications because of several desirable properties, including hydrophilicity (readily absorbed/dissolved in water), high surface area, activated metallic hydroxide sites, and nontoxicity. Additionally, MXenes’ characteristic “conductive clay-like” properties allow them to be easily processed by various methods.
To date, close to 30 MXenes have been successfully synthesized. Titanium-based MXenes are most promising for environmental applications due to element abundance and nontoxic decomposition products. In particular, titanium carbide (Ti3C2Tx) is the most widely studied MXene.
At this point, it is difficult to produce nitride MXenes by selective acid etching methods, which limits wide exploration of nitride and carbonitride MXenes in environmental remediation applications. As such, the review focuses on carbide MXenes.
MXenes in water treatment applications
MXene applications in water and wastewater treatment fall into three main categories: adsorbents, membranes, and electrode materials for electrochemical separation and deionization.
Adsorbents
Adsorption, or adhesion of atoms to a surface, is a widely used water purification technique because of its ease of operation, comparatively low cost, and high removal efficiency.
MXenes work well as adsorbents for several reasons, including high specific surface area and presence of functional groups on the MXene surface, which not only provide sites for direct ion exchange but also reduce some organic molecules and cations. This in situ reduction ability coupled with adsorption is considered advantageous over many other nanomaterials.
MXenes have demonstrated the ability to adsorb several heavy metals, such as lead, copper, chromium, and mercury. Additionally, MXenes have shown significant resistance to radiation-induced damage and good thermal stability, which means they may work well as adsorbents for radionuclide removal from nuclear waste.
Membranes
MXenes’ high surface area, flexibility, hydrophilicity, and high strength of 2D layers make MXenes useful as water purification membranes.
The first MXene water purification membrane was prepared by a vacuum-assisted filtration in 2015. By this method, MXene sheets restack themselves to form a membrane perfect for ion-sieving due to a well-defined interlayer distance and slit-like channels.
Currently, the majority of research focuses on Ti3C2Tx nanosheets. However, it would be beneficial to explore ion-sieving performances of other MXenes.
Electrochemical separation
Capacitive deionization (CDI) is an energy-efficient method that uses an electrical potential difference over two electrodes to selectively remove low concentrations of multivalent ions from aqueous or organic media.
In contrast to conventional CDI and electrical double-layer capacitors, MXenes can capitalize on their impressive pseudocapacitive properties and Faradaic ion intercalation in their layered structure to perform energy-efficient desalination of seawater or industrial wastewater.
The first MXene-based CDI electrode was fabricated in 2016 by drop-casting ML Ti3C2Tx onto both sides of a commercial porous separator. That electrode and following studies on other MXene-based CDI electrodes show promising capacity for salt removal.
MXenes in other environmental applications
In addition to reviewing MXene potential in water treatment applications, the researchers also discuss use of MXenes in various other environmental applications, such as photocatalysts, sensors, electromagnetic interference (EMI) shielding, and antibacterial and antibiofouling materials. Some highlights from this section include
- Efficient light-to-heat conversion—MXenes, including Ti3C2Tx, absorb light in both UV and visible ranges and show plasmon resonance peaks in visible or infrared range, which makes them efficient in light-to-heat conversion.
- Superior chemiresistive gas sensors— The signal-to-noise ratio of a MXene-based chemiresistive gas sensor developed in 2018 was two orders of magnitude higher than that of other 2D materials, superior to the best sensors known so far.
- Effective EMI shields—Although MXenes have high hydrophilicity, MXene foams and composites can exhibit hydrophobic properties, which make them effective EMI shields.
- Increased antibiofouling properties—In a 2016 study on MXenes for antimicrobial coatings, it was discovered that partial oxidation of Ti3C2Tx membrane surface to TiO2 increases bactericidal activity, likely due to synergy between the two materials.
The researchers also briefly discuss toxicity and environmental impact of MXenes, noting that although several studies demonstrate biocompatibility or nonacute toxicity of different MXenes, a 2016 study showed Ti3C2Tx has the ability to pass through bacterial membranes, meaning it may potentially distress physiology of cells in the body.
“The known hazards of other nanomaterials on living organisms suggests that more research is needed to safely design, use and dispose of MXene-containing products,” they write in the conclusion. “Understanding toxicity and environmental impact of MXenes would considerably expand the range of their applications in environmental remediation and pave a way for the construction of novel MXenes composites satisfying environmental safety requirements.”
The paper, published in Materials Today, is “Water treatment and environmental remediation applications of two-dimensional metal carbides (MXenes)” (DOI: 10.1016/j.mattod.2019.05.017).
More reviews, continuing research
Gogotsi sees reviews as a good way to show how “[n]ew materials can help in solving important problems that materials evaluated in the past could not solve.”
In addition to the water treatment and environmental remediation review article, Gogotsi has been involved with reviews on MXene applications in energy storage, MXene electronic and optical properties, and MXene synthesis methods.
On a larger scale, Gogotsi and Indiana University-Purdue University Indianapolis assistant professor Babak Anasori recently published a book on the structure, properties, and applications of 2D metal carbides and nitrides. “We explore synthesis of new MXenes (beyond 30 published already) and particularly focus on solid solution MXenes,” Gogotsi adds.
In terms of research, Gogotsi and his research group are actively investigating both MXenes and other nanomaterials in a multitude of areas, including synthesis of nanomaterials, electrochemical energy storage, water desalination, carbon nanopipettes for single-cell studies, and nanodiamonds for drug delivery applications.