Chemical Imaging of MXenes with X-ray Microscopy

Chemical imaging combines microscopy with spectroscopy to achieve a chemical contrast, enabling the identification of different chemical species with high spatial resolution. This capability is particularly valuable for the characterization of composite and heterogeneous materials, as well as for probing chemical reactions that are spatially segregated at the micro- or nanoscale. Representative examples include catalytic processes that locally modify the state of a catalyst, or battery particles that charge nonuniformly during cycling. Chemical imaging can be implemented using scanning probe, optical, electron, or X-ray spectromicroscopy techniques, each providing distinct forms of chemical contrast. Scanning probe and optical methods primarily probe valence electrons or vibrational modes, offering high chemical sensitivity, whereas electron and X-ray techniques typically probe core-level excitations, providing element-specific information. The latter also enable higher spatial resolution, as electrons and X-rays have much shorter wavelengths than visible light (typically <10 nm versus ∼400 nm). Among these approaches, X-ray-based methods are particularly powerful for direct, label-free chemical imaging due to their high elemental and site specificity. They are also significantly less invasive than electron-beam techniques. The soft X-ray range (ca. 100-2000 eV) is especially relevant, as it enables the selective excitation of the K-edge of light elements such as carbon, oxygen, and nitrogen, as well as the L- or M-edge of transition metals. Despite these relatively low photon energies, spatial resolutions down to ∼10 nm can be routinely achieved using standard X-ray optics.Moreover, the information depth can be tuned through the detection mode. For instance, photoemission-based techniques such as X-ray photoemission electron microscopy (X-PEEM) provide high surface sensitivity, whereas transmission-based methods such as scanning transmission X-ray microscopy (STXM) probe the bulk of the material that is though limited to thin samples. Finally, the comparatively large penetration depth of X-rays, relative to electrons, enables the use of in situ cells to investigate chemical processes under realistic conditions. In this Account, we present recent advances in X-ray chemical imaging, highlighting our work on two-dimensional transition-metal carbides known as MXenes. These materials are particularly well-suited for such approaches due to their rich surface chemistry, flat geometry, and micrometer-sized flakes. MXenes have attracted growing interest across a wide range of applications, including electromagnetic interference shielding, sensing, and energy storage. They adopt a Mn+1XnTx layered structure, where M is a transition metal, X is carbon and/or nitrogen, and Tx denotes mixed surface terminations, which can be selectively imaged by (soft) X-ray techniques. This account focuses mainly on Ti3C2Tx characterization because it is the most studied MXene to date. Leveraging the elemental sensitivity of soft X-rays, we imaged both confined water and the MXene framework itself, in vacuum and in various aqueous environments. We further present recent results on the spontaneous and electrochemically driven intercalation of protons and alkali cations in Ti3C2Tx, investigated by in situ STXM. These studies reveal that changes in the metal oxidation state strongly depend on the nature of the intercalant and highlight pronounced subflake variations in surface chemistry. This new chemical picture completely upended the conventional understanding of MXene chemistry, offering new keys to address challenges in their implementation for energy devices. Finally, we discuss recent developments in the field and outline promising directions for future research.