Chemical bonding of termination species in 2D carbides investigated through valence band UPS/XPS of Ti 3 C 2 T x MXene (original) (raw)
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Chemical bonding in carbide MXene nanosheets
The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Ti n+1 C n-T x (n = 1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO 2 that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s – Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar + sputtering on the electronic structure is also discussed.
A comparative study of electronic structure and bonding in transition metal monocarbides
Journal of Physics and Chemistry of Solids, 2012
The structural, electronic, elastic and bonding properties of four transition metal carbides, ScC, YC (group III), VC and NbC (group V), have been investigated systematically using the first principles density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange correlation has been used for the calculation of the total energy. The ground state properties, such as equilibrium lattice constant, bulk modulus, are computed and compared with theoretical and experimental data. The electronic and bonding patterns of the two groups of compounds have been analyzed quantitatively and compared with the available data. It is clear from band structures that all the four transition metal monocarbides are metallic in nature. Analysis of elastic constants reveals that the carbides of group III are ductile in nature while those of group V are brittle.
Two-Dimensional Transition Metal Carbides
ACS Nano, 2012
Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti 2 AlC, Ta 4 AlC 3 , (Ti 0.5 ,Nb 0.5 ) 2 AlC, (V 0.5 ,Cr 0.5 ) 3 AlC 2 , and Ti 3 AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.
Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)
ACS Nano, 2015
The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. Herein, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M 0 2 M 00 C 2 and M 0 2 M 00 2 C 3 , where M 0 and M 00 are two different early transition metals. In these solids, M 0 layers sandwich M 00 carbide layers. By synthesizing Mo 2 TiC 2 T x , Mo 2 Ti 2 C 3 T x , and Cr 2 TiC 2 T x (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo 2 TiC 2 T x and Ti 3 C 2 T x. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.
Bonding and classification of nanolayered ternary carbides
Physical Review B, 2004
We have investigated the elastic properties of nanolayered M 2 AC, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and A = Al, Ga, Ge, Sn, by ab initio calculations. We suggest that M 2 AC can be classified into two groups: One where the bulk modulus of the binary MC is conserved and another group where the bulk modulus is decreased. This classification can be understood in terms of coupling between MC and A layers, which is defined by the valence electron population. These results may have implications for the understanding of properties and the performance of this class of solids.
Journal of Applied Physics, 2006
Metal carbides are good candidates to contact carbon-based semiconductors ͑SiC, diamond, and carbon nanotubes͒. Here, we report on an in situ study of carbide formation during the solid-state reaction between thin Ti or Mo films and C substrates. Titanium carbide ͑TiC͒ was previously reported as a contact material to diamond and carbon nanotubes. However, the present study shows two disadvantages for the solid-state reaction of Ti and C. First, because Ti reacts readily with oxygen, a capping layer should be included to enable carbide formation. Second, the TiC phase can exist over a wide range of composition ͑about 10%, i.e., from Ti 0.5 C 0.5 to Ti 0.6 C 0.4 ͒, leading to significant variations in the properties of the material formed. The study of the Mo-C system suggests that molybdenum carbide ͑Mo 2 C͒ is a promising alternative, since the phase shows a lower resistivity ͑about 45% lower than for TiC͒, the carbide forms below 900°C, and its formation is less sensitive to oxidation as compared with the Ti-C system. The measured resistivity for Mo 2 C is =59 ⍀ cm, and from kinetic studies an activation energy for Mo 2 C formation of E a = 3.15± 0.15 eV was obtained.
PHYSICAL REVIEW RESEARCH 2, 033516 (2020), 2020
The chemical bonding within the transition-metal carbide materials MAX phase Ti 3 AlC 2 and MXene Ti 3 C 2 T x is investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of M n+1 X n and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species T x will separate the M n+1 X n-layers forming two-dimensional (2D) flakes of M n+1 X n T x. For Ti 3 C 2 T x the T x corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D M n+1 X n-flake. The Ti K-edge (1s) XANES of both Ti 3 AlC 2 and Ti 3 C 2 T x exhibit characteristic preedge absorption regions of C 2p-Ti 3d hybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti 1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti 3 AlC 2 and Ti 3 C 2 T x are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750°C.
Journal of Physics and Chemistry of Solids, 1988
for x = 1 .O and 0.75, have been calculated self-consistently by the LMTGASA method. The equilibrium lattice constants, the bulk moduli, the cohesive energies, and the energies of vacancy formation have been determined. The available experimental data for the bulk moduli, cohesive energies and some electromagnetic data (Hall effect, magnetic susceptibility, thermopower) have been analyzed on the basis of the calculated results. Keywon& Band structure calculations, LMTO method, refractory carbides and nitrides.