Facile Tailoring of Surface Terminations of MXenes by Doping Nb Element: Toward Extraordinary Pseudocapacitance Performance

MXenes show promising potential in supercapacitors due to their unique two-dimensional (2D) structure and abundant surface functional groups. However, most studies about MXenes have focused on tailoring surface structures by alternating synthesis methods or post-etch treatments, and little is known about the inherent relationship between surface groups and M elements. Herein, we propose a simple and novel strategy to adjust the surface structure of few-layered MXene flakes by adding a small amount of Nb element. Because of the strong affinity between Nb and O elements, the as-received V_1.8Nb_0.2CT_x and Ti_2.7Nb_0.3C₂T_x MXenes have much fewer -F functional groups and a higher O content than V₂CT_x and Ti₃C₂T_x MXenes, respectively. Thus, both V_1.8Nb_0.2CT_x and Ti_2.7Nb_0.3C₂T_x MXenes show enhanced pseudocapacitance performance. Especially, V_1.8Nb_0.2CT_x delivers an ultrahigh volumetric capacitance of 1698 F/cm³ at a scan rate of 2 mV/s. Moreover, benefiting from the high activity of MAX precursors obtained through a fast self-propagating high-temperature synthesis, the etching time to produce V-based MXenes is much shorter than that in previous reports. Therefore, the results presented here are applicable to the surface engineering and rational design of 2D MXene materials and develop them into promising, cost-effective electrode materials for supercapacitors or other energy-storage equipment.

Freeing Fluoride Termination of Ti3C2Tx via Electrochemical Etching for High-Performance Capacitive Deionization

Ti3C2Tx MXene is a promising Faradic capacitive deionization (CDI) electrode for high salt removal in future desalination, whereas the surface termination group of fluoride (-F) significantly impedes ion access to Ti3C2 and charge-transfer efficiency. Herein, we propose an electrochemically etched strategy to synthesize -F-free Ti3C2Tx through three-electrode cyclic voltammetry scanning within a narrowed potential window in an alkaline electrolyte. The resulting assembly of an asymmetric electrochemical-etched Ti3C2Tx//activated carbon CDI device can deliver an excellent salt removal capacity of 20.27 mg·g-1 with an adsorption rate of 1.01 mg g-1 min-1 owing to the enhanced hydrophilicity and ion transport. The tiny CDI device is demonstrated, which can generate an electric current during the electrosorption of salt ions, thus facilitating the powering of a red light-emitting diode. This study opens a new avenue for the surface chemistry of Ti3C2Tx and is expected to achieve future applications in desalination and renewable energy.

Switching the Nonlinear Optical Absorption of Titanium Carbide MXene by Modulation of the Surface Terminations

Surface terminations of two-dimensional materials should have a strong influence on the nonlinear optical (NLO) properties, but the relationship between surface terminations and NLO properties has not yet been reported. In this work, switching the NLO properties of MXenes (Ti₃C₂T_x) via "surface terminations modulation" is explored. The surface terminations of Ti₃C₂T_x are modulated by electrochemical treatment, resulting in different states (viz., Ti₃C₂T_x(pristine), Ti₃C₂T_x(═O rich), and Ti₃C₂T_x(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β_eff) change with the surface terminations. Ti₃C₂T_x(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (β_eff = -1020 ± 136.2 cm GW^-1), while reverse saturable absorption (RSA) is found in Ti₃C₂T_x(pristine) and Ti₃C₂T_x(-OH rich). The RSA of Ti₃C₂T_x(pristine) and Ti₃C₂T_x(-OH rich) is attributed to excited-state absorption, while the SA of Ti₃C₂T_x(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β_eff of Ti₃C₂T_x(-OH rich) is 113 ± 3.2 cm GW^-1, 1.68 times that of Ti₃C₂T_x(pristine); the RSA is caused by photon-induced absorption. Our results reveal a correlation between surface terminations and NLO properties, highlighting the potential of MXenes in photoelectronics.

One-Pot Green Process to Synthesize MXene with Controllable Surface Terminations using Molten Salts

Surface terminations of two-dimensional MXene (Ti₃ C₂ T_x ) considerably impact its physicochemical properties. Commonly used etching methods usually introduce -F surface terminations or metallic impurities in MXene. We present a new molten-salt-assisted electrochemical etching method to synthesize fluorine-free Ti₃ C₂ Cl₂ . Using electrons as reaction agents, cathode reduction and anode etching can be spatially isolated; thus, no metallics are present in the Ti₃ C₂ Cl₂ product. The surface terminations can be in situ modified from -Cl to -O and/or -S, which considerably shortens the modification steps and enriches the variety of surface terminations. The obtained -O-terminated Ti₃ C₂ T_x are excellent electrode materials for supercapacitors, exhibiting capacitances of 225 F g^-1 at 1.0 Ag^-1 , good rate performance (91.1 % at 10 Ag^-1 ), and excellent capacitance retention (100 % after 10000 charge/discharge cycles at 10 Ag^-1 ), which is superior to multi-layered Ti₃ C₂ T_x prepared by other etching methods.

Gold nanostructures on iron oxide surfaces and their interaction with CO

We review results of density functional theory calculations of the adsorption of single gold atoms and formation of sub-nanometer Aunstructures (n= 2 to 5) on most stable iron oxide surfaces: hematite (0001), and magnetite (111) and (001). Structural, energetic, and electronic properties of Aunstructures on both Fe- and O-rich oxide terminations are discussed. Different chemical character of the two oxide terminations is reflected in distinctly stronger binding of gold at the oxygen- than at the iron-terminated surface, and in different changes of the adsorption binding energy with the size of the Auncluster. On the iron-terminated oxide surface the binding energy increases whereas on the oxygen-rich termination it decreases with the number of Au atoms in the structure. Upon CO adsorption on magnetite surface all Aunstructures have a net positive charge and CO binds to the most cationic Au atom of a cluster. Interactions of Aunand CO with magnetite (111) show many similarities with those on hematite (0001) surface. The influence of the substrate relaxation effects on adsorption energy is also discussed.

Effects of Surface Terminations of 2D Bi2WO6 on Photocatalytic Hydrogen Evolution from Water Splitting

Two-dimensional (2D)-structured photocatalysts with atomically thin layers not only have the potential to enhance hydrogen generation efficiency but also allow more direct investigations of the effects of surface terminations on photocatalytic activity. Taking 2D Bi₂WO₆ as a model, we found that the configuration of bilayer Bi₂O₂ sandwiched by alternating WO₄ layers enabled the thermodynamic driving potential for photocatalytic hydrogen evolution. Without Pt deposition, the H₂ generation efficiency can reach to 56.9 μmol/g/h by 2D Bi₂WO₆ as compared with no activity of Bi₂WO₆ nanocrystals under simulated solar light. This configuration is easily functionalized by adsorption of Cl^-/Br^- to form Bi-Cl/Bi-Br bonds, which leads to the decrease of recombination in photogenerated charge carriers and narrower band gaps. This work highlights an effective way to design photocatalysts with efficient hydrogen evolution by tuning the surface terminations.

Enhancement of Dielectric Permittivity of Ti3C2Tx MXene/Polymer Composites by Controlling Flake Size and Surface Termination

We report a strong effect of the MXene flake size and surface termination on the dielectric permittivity of MXene polymer composites. Specifically, poly(vinylidene fluoride-trifluoro-ethylene-chlorofluoroehylene) or P(VDF-TrFE-CFE) polymer embedded with large (ca. 4.5 μm) Ti₃C₂T_x flakes achieves a dielectric permittivity as high as 10⁵ near the percolation limit of 15.3 wt % MXene loading. In comparison, the dielectric permittivity of MXene/P(VDF-TrFE-CFE) using small (ca. 1.5 μm) Ti₃C₂T_x flakes (S-MXene) achieves a dielectric permittivity of 10⁴ near the percolation limit of 16.8 wt %. Meanwhile, increasing the concentration of surface functional groups on the MXene surface (-O, -F, and -OH) by extending the etching time gives a dielectric constant of 2204 near the percolation limit of 15.7 wt %. The ratio of permittivity to the loss factor of our large flake composite is superior to that of the small flake composite, and to all previously reported carbon-based fillers in P(VDF-TrFE-CFE). We show that the dielectric permittivity enhancement is strongly related to the charge accumulation at the surfaces between the two dimensional (2D) MXene flakes and the polymer matrix under an external applied electric field.

2D Transition Metal Carbides (MXenes) for Carbon Capture

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever-increasing CO₂ emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO₂ . However, challenges remain because of limited CO₂ /N₂ selectivity and long-term stability. The effective adsorption of CO₂ gas (≈12 mol kg^-1 ) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N₂ gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO₂ /N₂ selectivity, together with a chemical and thermal stability, identifies the archetype Ti₃ C₂ MXene as a new material for carbon capture (CC) applications.