Surface Termination Dependent Work Function and Electronic Properties of Ti₃C₂T_x MXene

Abstract

MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown promise in various applications, such as energy storage, electromagnetic interference shielding, conductive thin films, photonics, and photothermal therapy. Their metallic nature, wide range of optical absorption, and tunable surface chemistry are the key to their success in those applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. In this study we investigated the electronic properties of Ti3C2Tx for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy, and density functional theory calculations. We find that fluorine occupies solely the face-centered cubic adsorption site, whereas oxygen initially occupies at least two different adsorption sites, followed by a rearrangement after fluorine desorption at high annealing temperatures. The measured electronic structure of Ti3C2Tx showed strong dispersion of more than 1 eV, which we conclude to stem from Ti-O bonds by comparing it to calculated band structures. We further measured the work function of Ti3C2Tx as a function of annealing temperature and found that is in the range of 3.9-4.8 eV, depending on the surface composition. Comparing the experimental work function to detailed density functional theory calculations shows that the measured value is not simply an average of the work function values of uniformly

Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells

“…We also observe that the WF reduction induced by the OH terminated surface is much more pronounced than the WF increase induced by O terminations. This is due to a rather strong non-linear behaviour already observed in literature 35 and also obtained in our calculations. Indeed, the perovskite WF using MXene with a OH:O termination ratio of 50:50 reduces to 3.1 eV, whereas for a ratio of 75:25 it is close to 1.9 eV, namely close to the value obtained for 100% OH.…”

“…32 As density functional theory (DFT) predicts, surface termination strongly influences the density of states 33 and the WF of MXenes 34 which can range from 1.6 eV (for OH-termination) to 6.25 eV (for Otermination). 34,35 This opens new opportunities for MXenes applications in optoelectronics and in particular in photovoltaics where already some initial studies have been presented for organic solar cells 36 , Si solar cells, 37 dye-synthesized solar cells 38 and PSCs. 39,40 In the PSC case, Ti3C2Tx MXenes have been incorporated into the perovskite absorber showing an improved morphology and an enhanced PCE (+12%) with respect to the reference cell without MXenes 39 or into the SnO2 electron transporting layer (ETL) 40 to provide superior charge transfer paths that permits to enhance the PCE (+6.5%) with respect to the reference cells.…”

Pristine Titanium Carbide MXene Films with Environmentally Stable Conductivity and Superior Mechanical Strength

“…Chemical composition and functional groups were also characterized through X‐ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) (Figures S7 and S8, Supporting Information). There is no prominent difference in XPS spectra after low concentration proton acid treatment, and the proportion of terminal groups such as O, OH, and F remained nearly identical in which the difference is within the uncertainty of XPS method . Slightly higher Ti/O ratio revealed by both XPS and EDS is probably due to the reduced amount of H 2 O in acid‐induced Ti 3 C 2 T x samples.…”

Beyond Gold: Spin‐Coated Ti3C2‐Based MXene Photodetectors

“…Finally, MXenes, including Ti 3 C 2 T z used here, have the unique attribute that their work function can be adjusted from 2.14 to 5.65 eV by different means, such as hole injection or surface termination using oxygen, fluorine, or chlorine . This provides a wider range compared to all metals used in MSM detectors .…”