Scientists have long been dedicated to applying two-dimensional ultra-thin materials to renewable energy technologies to enhance efficiency. Recently, a breakthrough in the research of low-dimensional compound MXene was achieved by Texas A&M University in the United States, revealing its ability to convert air into ammonia in a more sustainable way, paving the way for future fertilizer production and transportation fuel.
MXene is a two-dimensional material composed of transition metals and carbides, nitrides, or carbonitrides, possessing outstanding characteristics such as high conductivity, high specific surface area, and controllable surface groups. Particularly, the electrocatalytic performance of “nitrogenated MXene” is significantly superior to commonly seen carbide MXenes. These features allow for optimization in various renewable energy applications and have the potential to replace traditional expensive metal catalysts.
Despite the excellent performance of nitrogenated MXene, the scientific community still lacks fundamental understanding of how its structure evolves during the electrocatalytic process. Specifically, the reactivity of “lattice nitrogen,” how it affects the vibration properties of the material in solvents, and its specific impact on electrocatalysis remain mysteries to be solved.
To elucidate the mechanism of MXene, a team from the Department of Chemical Engineering at Texas A&M University utilized confocal Raman spectroscopy technology for in-depth research, ultimately unveiling the reaction mechanism of titanium-based nitrogenated MXenes at the atomic level. This research outcome was published in the “Journal of the American Chemical Society.”
The research team discovered that when titanium-based nitrogenated MXenes were immersed in “polar solvents” such as water, acetone, and ethanol, their Raman vibration modes experienced significant decay, even disappearing completely, leaving only the background signals of the solvent on the spectrum. However, once the solvent evaporated and the material dried, the Raman characteristic peaks would reappear.
Furthermore, if the material was immersed in “non-polar” hydrocarbon solvents like hexane and octane, the Raman characteristic peak signals remained clearly visible. These two scenarios confirmed that the disappearance of the Raman signals was not simply due to “liquid coverage” or optical refraction, but closely related to the chemical nature (polarity) of the solvent.
To further verify that “lattice nitrogen” is the primary reason for this phenomenon, the research team synthesized carbonitride MXene and nitrogen-doped carbide MXene for comparative testing. The results showed that once nitrogen atoms were introduced into the carbide structure, the material exhibited the same signal disappearance in polar solvents as pure nitrides. This demonstrates that “lattice nitrogen” is the key switch controlling the interaction of MXene with light.
The team speculated that this is because lattice nitrogen atoms form a strong hydrogen bond network with polar solvent molecules (oxygen and hydrogen in water), altering the material’s surface electronic structure and polarizability, thus leading to changes in the Raman signals.
This characteristic is crucial for ammonia synthesis. During the research, they used nuclear magnetic resonance (NMR) to discover that when nitrogenated MXenes undergo nitrogen reduction reactions (NRR), it is not just surface-adsorbed nitrogen gas participating but rather lattice nitrogen directly involved. This is because after lattice nitrogen is protonated to form released ammonia gas, it leaves nitrogen vacancies on the surface, which are subsequently filled by nitrogen gas from the air to complete the catalytic cycle.
The researchers stated that this is the first systematic revelation of the vibration properties of titanium-based nitrogenated MXenes, which can be controlled through solvent polarity. Future studies will continue in hopes of finding more relevant applications, as understanding these mechanisms contributes to enhancing catalytic efficiency and potentially replacing expensive metal catalysts, achieving a low-cost and low-carbon emission “ammonia economy.”
Abdoulaye Djire, a professor of chemical engineering at Texas A&M University, expressed to the university newsroom, “This research deepens our understanding of the material’s operational mechanism. We have demonstrated that ammonia electrochemical synthesis can be achieved through protonation and supplemental lattice nitrogen, and we plan to apply this knowledge to chemical and fuel production.”
Ray Yoo, a doctoral candidate in the Department of Chemical Engineering at Texas A&M University, stated, “MXene is an ideal transition metal-based alternative material, especially the performance of nitrogenated MXene in the field of electrocatalysis, has significantly improved compared to the widely researched carbide MXene.”
This research was funded by the U.S. Army Research Office’s Energy Sciences Strength Building Program (Project Number: W911NF-24-1-208).