A recent article in Advanced Materials Interfaces presented a method to more accurately determine the surface chemistry of Ti3C2Tx MXenes. The authors used energy-dependent X-ray photoelectron spectroscopy (XPS) combined with depth profile modeling.
This approach allowed them to separate and quantify signals from the MXene itself and from adsorbed species. In contrast, uncorrected lab-based XPS was found to systematically overestimate titanium vacancies and underestimate surface terminal groups.
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Background
MXenes are a family of two-dimensional transition metal carbides and nitrides known for their distinctive chemical and physical properties.
These characteristics make them valuable in fields such as energy storage, water purification, catalysis, and gas separation. In such applications, understanding the surface chemistry and internal composition of MXenes is crucial, yet still difficult to achieve with high precision.
A range of techniques—including infrared and Raman spectroscopy, nuclear magnetic resonance (NMR), and secondary ion mass spectrometry—are typically used to study MXene chemistry. XPS is especially common for identifying surface composition and bonding. However, it often produces biased results due to the presence of surface adsorbates like etching residues or adventitious carbon.
To address this issue, the authors used synchrotron-based XPS, which allows the probing energy to be tuned. This made it possible to distinguish between signals from true MXene surfaces and external contaminants.
Methods
Ti3C2Tx MXenes were synthesized from Ti3AlC2 MAX phases using a mixture of hydrofluoric acid (HF) and hydrochloric acid (HCl), with HF concentrations set at either 5 % or 30 % by weight. These samples were referred to as 5HF- and 30HF-Ti3C2Tx, respectively.
Additional batches were made using commercially sourced Ti3AlC2 under the same etching conditions to investigate how synthesis variables affect surface chemistry.
For structural and surface analysis, Ti3C2Tx MXene and Ti3AlC2 MAX phase films were fabricated using vacuum-assisted filtration. Freestanding MXene films were created from colloidal suspensions containing single to few-layer delaminated flakes. These suspensions were filtered through a porous membrane to form the films.
For X-ray absorption spectroscopy (XAS) and synchrotron-based XPS, films were instead prepared by spray-coating onto gold-coated p-type silicon substrates.
XPS measurements were conducted at an experimental station with an electron energy analyzer. The energy of the incident photons was adjusted using a monochromator. XAS was carried out at the same facility by sweeping the X-ray wavelength through the monochromator while recording absorption data simultaneously.
The optical properties of the MXenes were characterized using ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy. To support the experimental observations, density functional theory (DFT) simulations were also performed.
Results and Discussion
XPS analysis enabled the quantification of atomic ratios by measuring the intensity of photoelectrons emitted from core electron levels. The study compared results from synchrotron XPS at 750 eV and traditional Al Kα XPS at 1486 eV. Higher photon energies were found necessary for the reliable detection of fluorine.
Signals from oxygen, carbon, and titanium were affected by overlapping contributions from surface adsorbates, complicating the identification of true surface terminations and the core stoichiometry. These surface contaminants likely came from synthesis or storage, altering measured elemental ratios and absorbing photoelectrons to varying degrees. This was particularly evident in the fluorine 1s region.
To correct for these effects, the researchers accounted for electron inelastic mean free paths and absorption by surface contamination. This allowed them to isolate the signal from MXene components and derive corrected elemental concentrations.
The experimental XAS data aligned well with DFT simulations. The L2,3-edge spectra for both 5HF- and 30HF-Ti3C2Tx MXenes samples resembled known signatures of pristine Ti3C2Tx. The F and O K-edge spectra were consistent with XPS results and appeared less sensitive to surface contamination.
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Conclusion
This study demonstrates a refined approach to quantifying the surface composition and stoichiometry of Ti3C2Tx MXenes.
By isolating signals from MXene materials and correcting for surface artifacts, the authors achieved more accurate XPS-based analysis. The comparison with conventional lab-based XPS underscored the importance of excitation energy in obtaining reliable measurements.
XAS was also shown to be a powerful technique for studying surface chemistry in these materials. While the study focused on MXenes synthesized using HF/HCl etching, the same methodology could be applied to samples prepared via molten salt etching, LiF/HCl methods, or electrochemical techniques.
Journal Reference
Dessoliers, Z., et al. (2025). Combining X‐Ray Photoelectron and Absorption Spectroscopies for Determining Surface Chemistry and Composition of Ti3C2Tx MXene. Advanced Materials Interfaces. DOI: 10.1002/admi.202500391, https://advanced.onlinelibrary.wiley.com/doi/10.1002/admi.202500391
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