Two-dimensional (2D) nanomaterials like graphene have received great research attention for several years, but a new generation of atom-thick materials – MXenes – has recently captured the interest of materials science researchers and nanotechnology developers alike.
Synthesizing MXene Nanomaterials
MXenes are 2D nanomaterials that exhibit high conductivity, high surface area, high functionalization, a site of hydroxide, and many other interesting properties. They were only discovered in 2011, but have been the subject of many studies ever since.
MXenes are inorganic compounds measuring only a few atoms in thickness and are made up of metal carbides, nitrides, or carbonitrides. These materials combine the metallic conductivity of the transition metal carbides with hydrophilic features imparted by their hydroxyl- or oxygen-terminated surfaces.
The first MXene nanomaterial to be reported in 2011 was Ti4N3. Though, this compound is synthesized in a slightly different manner to most of the other MXenes developed since.
Ti4N3 is synthesized by mixing TI4AlN3 with a molten eutectic fluoride salt made up of lithium fluoride, sodium fluoride, and potassium fluoride. This mixture is treated at a high temperature, causing the Al to etch out and yield multilayered Ti4N3.
This material is then delaminated into single layers by immersion in tetrabutylammonium hydroxide and a subsequent sonication step.
Most MXene nanomaterials, however, are synthesized in a top-down (subtractive) selective etching process. This process is scalable without exhibiting any loss or change in the nanomaterials’ properties when batch sizes increase.
The top-down etching process uses strong etching solutions with a fluoride ion such as hydrofluoric acid (HF), ammonium bifluoride (NH4HF2), or a mixture of lithium fluoride (LiF) and hydrochloric acid (HCI).
For example, aqueous HF at room temperature is used to etch the MXene material Ti3AlC2. The HF selectively removes the Al atoms so that the surface of the carbide layers becomes terminated by O or F atoms.
MXene is also sometimes obtained from Lewis acid molten salts like ZnCl2. This method synthesizes a Cl-terminated MXene structurally stable up to 750 °C.
Rapid Progress in MXene Research
So far, 27 MXene materials have been synthesized. These are formed from elements like titanium, zirconium, molybdenum, niobium, tungsten, and hydrogen, along with carbon.
There have been numerous applications for the materials already. Mxenes are employed as conductive layered materials with tunable surface terminations in energy storage applications, both in Li-ion batteries and supercapacitors.
Applications as photocatalytic cells, transparent conducting electrodes, and neural electrodes also capitalize on Mxenes’ unique electrochemical properties.
The nanomaterials can be used in water purification, gas sensing, triboelectric nanogenerators (TENGs), and electrochromic devices.
Key Applications for the Future of Mxene Nanomaterials
The remarkable and unique properties of Mxene nanomaterials have led researchers tackling science’s major challenges to consider how the materials could be useful in their work. As such, Mxene’s have been investigated for use in environmental projects, cleaner energy production, and lifesaving medical technologies.
Environmental Clean Up and Water Sustainability
Mxene’s high surface area, biological compatibility (non-toxicity), robust electrochemistry, and high hydrophilicity make them ideal candidates for advanced environmental clean technologies.
Nanoarchitectures based on Mxene materials can mitigate the role of inorganic pollutants in interfacial chemical transformation and sorption in ecosystems.
They achieve this through three main mechanisms: surface complexation and sorption, catalytic activation and removal, and radical generation-based photocatalytic degradation.
When applied to drinking water sources and water waste, MXene-based filtration systems can have significant impacts on both local communities’ health and wellbeing, as well as the overall health of their surrounding environments.
Electrocatalysts for Hydrogen Energy
According to many forecasters, hydrogen will play a crucial role in transitioning from a fossil-fuel based economy to clean renewable energy. Generating hydrogen with water electrolysis is the preferred solution at present, being the cleanest and most sustainable method.
The development of non-noble metal electrocatalysts is a hot topic of research in hydrogen power. MXene nanomaterials have recently been explored as potential electrocatalysts for hydrogen evolution reaction (HER) electrocatalysis.
The materials display good HER activity and remarkable stability and may help us to break free from fossil fuels.
MXenes may even save lives, as they are currently being explored for numerous biomedical applications. Their non-toxicity and unique properties make them interesting to researchers working on antibacterial safety, advanced medical imaging technologies, biosensing devices, tissue regeneration, and even cancer therapies.
Still, MXenes’ appropriateness and potential for biomedical applications are still not fully understood. As such, many are invested in characterizing the unique nanomaterials. Doing so would aid in understanding their interactions with other materials, especially organic matter, and fully estimating their potential for use in lifesaving medical technology.
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