MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti₃C₂T_x—the most important and widely used MXene, from a Ti₃AlC₂ MAX phase precursor. The procedure contains three main sections: synthesis of Ti₃AlC₂ MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti₃C₂T_x and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti₃C₂T_x. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti₂CT_x) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti₃C₂T_x with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Link: https://www.nature.com/articles/s41596-024-00969-1

Efficient catalysts with minimal content of catalytically active noble metals are essential for the transition to the clean hydrogen economy. Catalyst supports that can immobilize and stabilize catalytic nanoparticles and facilitate the supply of electrons and reactants to the catalysts are needed. Being hydrophilic and more conductive compared with carbons, MXenes have shown promise as catalyst supports. However, the controlled assembly of their 2D sheets creates a challenge. This study established a lattice engineering approach to regulate the assembly of exfoliated Ti₃C₂T_x MXene nanosheets with guest cations of various sizes. The enlargement of guest cations led to a decreased interlayer interaction of MXene lamellae and increased surface accessibility, allowing intercalation of Pd nanoparticles. Stabilization of Pd nanoparticles between interlayer-expanded MXene nanosheets improved their electrocatalytic activity. The Pd-immobilized K⁺-intercalated MXene nanosheets (PdKMX) demonstrated exceptional electrocatalytic performance for the hydrogen evolution reaction with the lowest overpotential of 72 mV (@10 mA cm^–2) and the highest turnover frequency of 1.122 s^–1 (@ an overpotential of 100 mV), which were superior to those of the state-of-the-art Pd nanoparticle-based electrocatalysts. Weakening of the interlayer interaction during self-assembly with K⁺ ions led to fewer layers in lamellae and expansion of the MXene in the c direction during Pd anchoring, providing numerous surface-active sites and promoting mass transport. In situ spectroscopic analysis suggests that the effective interfacial electron injection from the Pd nanoparticles strongly immobilized on interlayer-expanded PdKMX may be responsible for the improved electrocatalytic performance.

Link: https://pubs.acs.org/doi/full/10.1021/acsnano.3c09640

MXenes produced by Lewis acid molten salt (LAMS) etching of MAX phases have attracted the community’s attention due to their controllable surface chemistry. However, their delamination is challenging due to the hydrophobicity of the produced multilayer MXene and strong interactions between the halogen-terminated MXene sheets. The current delamination method involves dangerous chemicals such as n-butyllithium or sodium hydride, making scale-up difficult and limiting the practical application of this class of MXenes. In this work, we present a simple and efficient method for the delamination of MXenes from the LAMS synthesis while maintaining their surface chemistry. LiCl salt and anhydrous polar organic solvents are used for delamination. Films produced from the delaminated MXene are flexible and have an electrical conductivity of 8000 S/cm, which is maintained after a week of exposure to 95% humidity. This successful delamination, preservation of inherent surface properties, and stability under high-humidity conditions dramatically expand the range of MXene chemistries available for research and potential applications.

Link: https://pubs.acs.org/doi/full/10.1021/acs.chemmater.3c02872

We are entering the era of new materials: materials that can be assembled from nanoscale building blocks, unlike all previous material generations from the Stone Age to the Silicon Age. Two-dimensional (2D) materials provide nanometer and subnanometer-thin “bricks” for such assembly. If needed, organic molecules and polymers can serve as mortar, but van der Waals (vdW) or electrostatic forces can also provide a strong bonding between the 2D layers. To make this vision real and start assembling materials, structures, and devices from nanoparticles, we need many building blocks with a large variety of physical and chemical properties. … The advancement of MXenes toward industrial use would require the development of scalable, low-cost, safe, and environmentally friendly synthesis processes, as well as extensive and thorough studies of their toxicity and fate in the environment. All new materials follow this path. Based on the unique and tunable properties of MXenes as well as their enormous compositional diversity and tunability of properties, we are confident that the field of MXenes has a bright and exciting future.

Link: https://pubs.acs.org/doi/full/10.1021/acs.chemmater.3c02491

Since their discovery in 2011, the family of 2D transition metal carbides and nitrides, MXenes, has made remarkable progress. While their initial development was relatively slow compared to their current growth and some other 2D materials, MXenes have gained momentum over the past seven years. We can expect accelerating progress as the critical mass of scientists and engineers focusing on MXenes is being accumulated. Still, since MXenes are developing into the largest known family of inorganic low-dimensional materials with an enormously broad range of applications in almost every engineering field, as well as healthcare and analytical chemistry, there is plenty of room for discovery for any number of researchers entering the field of these fascinating materials. Moreover, considering the transformational role that MXenes are expected to play in the fields from communication to optoelectronics, healthcare, energy and environment, countries that seize the opportunity earlier may gain a major technological advantage.

Link: https://link.springer.com/article/10.1007/s41127-023-00067-1