Journal article
Layer-by-layer mechanism of the MAX phase to MXene transformation
Matter, v 8(6), 102092
04 Jun 2025
Abstract
MXenes are the fastest growing family of two-dimensional (2D) materials with potential for applications from energy storage to biomedicine, sensing, and electromagnetic shielding. Despite significant progress in MXene synthesis through selective etching of layered MAX phase precursors, limited understanding of the fundamental etching mechanism and kinetics hinders rational optimization of the process. Here, we monitored the etching process using in situ and ex situ techniques at the single-particle and ensemble levels. Our work shows that etching nucleation is instantaneous and etching occurs layer by layer. Through analytical modeling, we found that etching of V2AlC is diffusion-limited. In contrast, etching of Ti2AlC and Ti3AlC2 is reaction-interface-limited, with an additional surface reaction limitation for Ti3AlC2 accounting for more than one-quarter of the total etching time. Overall, our work provides significant insights into the MAX phase etching mechanism and kinetics and an overview of the tools and techniques available to investigate etchable layered materials.
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•Etching of MAX phases in aqueous fluoride solutions initiates on particle surfaces•TiCx, TiOx, and AlOx adlayers on particle surfaces delay the MAX phase etching•Etching occurs layer-by-layer•V2AlC etching is diffusion-limited, while Ti2AlC and Ti3AlC2 are interface-limited
Layered materials are plentiful, from silicate clays to metal double hydroxides, transition metal oxy-halides, electrides, and MAX phases. These materials can serve as precursors to two-dimensional (2D) materials. Selective etching is a promising approach to remove specific atomic layers, thereby decoupling the remaining layers and allowing them to be exfoliated into freestanding 2D sheets. MXenes are the fastest growing family of 2D materials, comprising transition metal carbides and nitrides, formed by selective etching from precursor MAX phases. They are promising candidates for applications ranging from energy storage to biomedicine, molecular sensing, electromagnetic shielding, and others. However, they are prone to oxidation and hydrolysis because of atomic defects created from inadvertent non-selective etching. Here, we studied how the etching process of MAX phases happens so that in the short term we can improve MXene synthesis and in the long term gain some insight that would be useful to etch other layered materials. We discovered that although MAX phase particles are submerged in a harsh etchant, hydrofluoric acid, etching starts from the two basal surfaces of the particles and proceeds layer by layer, akin to being forced to read a book in order from the beginning (or end!) rather than at some other random point. The longer that already-formed MXene sheets remain in the etchant, the more likely it is for non-selective etching to happen and defects to form. Therefore, our discovery prompts the research community to figure out how to remove MXene sheets from the etchant as soon as they are created. Our experiments also showcased how in situ monitoring of etching can be done effectively using differential contrast with optical microscopy and collection of by-product evolving gases, which will make it easier to probe etching of other layered materials.
MXenes are two-dimensional materials created by the selective etching of atomic layers from precursor-layered MAX phases. We discovered that etching starts from the basal planes of particle surfaces and proceeds layer by layer, prompting further research to remove MXene sheets from the etchant as soon as they are produced, to minimize atomic defects from overetching and improve their stability.
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Details
- Title
- Layer-by-layer mechanism of the MAX phase to MXene transformation
- Creators
- Mark Anayee - A.J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USARuocun (John) Wang - Drexel University, Materials Science and EngineeringMarley Downes - A.J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USAStefano Ippolito - Drexel University, A.J. Drexel Nanomaterials InstituteYury Gogotsi - Drexel University, Materials Science and Engineering
- Publication Details
- Matter, v 8(6), 102092
- Publisher
- Elsevier
- Resource Type
- Journal article
- Language
- English
- Academic Unit
- Materials Science and Engineering; A.J. Drexel Nanomaterials Institute
- Web of Science ID
- WOS:001566292500002
- Scopus ID
- 2-s2.0-105002229585
- Other Identifier
- 991022048283604721
InCites Highlights
Data related to this publication, from InCites Benchmarking & Analytics tool:
- Web of Science research areas
- Materials Science, Multidisciplinary