The hydrogen evolution reaction (HER) is an important energy conversion process that underpins many clean energy technologies including water splitting. Herein, we report for the first time the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Our computational screening study of 2D layered M 2 XT x (M = metal; X = (C, N); and T x = surface functional groups) predicts Mo 2 CT x to be an active catalyst candidate for the HER. We synthesized both Mo 2 CT x and Ti 2 CT x MXenes, and in agreement with our theoretical predictions, Mo 2 CT x was found to exhibit far higher HER activity than Ti 2 CT x . Theory suggests that the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied MoS 2 , in which only the edge sites of the 2H phase are active. This work paves the way for the development of novel 2D layered materials that can be applied in a multitude of other clean energy reactions for a sustainable energy future.

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Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T x ) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti 3 C 2 T x produced by different fluorinecontaining etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo 2 CT x with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm −2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS 2 , in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed.

We use density functional theory calculations to study a group of 2D materials known as MXenes toward the electrochemical nitrogen reduction reaction (NRR) to ammonia. So far, all computational studies have only considered the NRR chemistry on unfunctionalized (bare) MXenes. In this study, we investigate a total of 65 bare and functionalized MXenes. We establish free energy diagrams for the NRR on the basal planes of 55 different M 2 XT x MXenes (M = Ti, V, Zr, Nb, Mo, Ta, W; X = C, N) to span a large variety of possible chemistries. Energy trends with respect to the metal as well as nonmetal constituent of the MXenes are established for both bare and functionalized MXenes. We determine the limiting potentials and find that either the formation of NH 3 from *NH 2 or the formation of *N 2 H is the potential limiting reaction step for bare and functionalized MXenes, respectively. We find several Mo-, W-, and V-based MXenes (Mo 2 C, Mo 2 N, W 2 N, W 2 NH 2 , and V 2 N) to have suitable theoretical overpotentials for the NRR. Importantly, calculated Pourbaix stability diagrams combined with selectivity analysis, however, reveal that all bare MXenes are not stable under relevant NRR operating conditions. The only functionalized MXene with the three minimum required properties (i) having a low theoretical overpotential, (ii) being stable under NRR conditions, and (iii) having selectivity toward NRR rather than the parasitic HER is W 2 CH 2 , which is a H-terminated MXene. Finally, on the basis of our findings, we explore other routes for improving the NRR chemistry by studying 10 additional MXenes with the chemical formula M 3 X 2 T x and MXenes with other functional groups (T x = S, F, Cl). This opens up a larger variety and tunability of MXenes to be considered for the NRR.

Two-dimensional transition metal carbides and nitrides, also known as MXenes, represent an attractive class of materials for a multitude of electrochemical and other applications. While single sheets of MXenes have been widely studied theoretically, there have been much fewer studies on layered bulk MXenes, which are more representative of multi- or few-layer MXenes used in actual applications. Herein, we investigate the structural and electronic effects of water intercalation, multiple functional groups and applied potential on layered bulk Ti2C and Mo2C MXenes using density functional theory. The out-of plane lattice parameter, c, was found to vary significantly with the functional group, and is greatly increased upon intercalation of water. Experimental results confirm the change in lattice constant due to addition or removal of intercalated water. Under zero applied potential, both Ti2C and Mo2C were found to be functionalized by one monolayer of O; bare MXenes were never found to be stable, regardless of the applied potential. Applying a potential changed the adsorbate coverage, changing the systems from O covered to H covered at negative potentials and, in some cases, giving rise to a metal–insulator transition. Understanding of the effects of surface functionalization and water intercalation of MXenes provides a better insight of their use for catalytic and electronic applications.

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect-carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO 3 grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO 3 polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO 3 tetragonality and the absence of any low-permittivity interlayer at the interface with Ge. The non-volatile, switchable nature of the single-domain out-of-plane ferroelectric polarization of BaTiO 3 is confirmed using piezoelectric force microscopy. The effect of the polarization switching on the conductivity of the underlying Ge is measured using microwave impedance microscopy, clearly demonstrating a ferroelectric field effect.

We investigate theoretically the interface between a ferroelectric BaTiO 3 film and a non-polar insulating SrTiO 3 substrate. We find that thin BaTiO 3 (under 5 nm) can stabilize a non-polarized state, and an additional metastable polarized state. While the non-polarized state is insulating, for the polarized heterostructure, we discover the existence of two-dimensional charge carrier gases. In this case, the heterostructure undergoes an electronic reconstruction in order to prevent the polar catastrophe. The two-dimensional gases, formed as a result, screen the polarization, leading to a substantially reduced potential drop across the ferroelectric film. We emphasize that the twodimensional electron and hole gases are created by the polarization of the sample, and are not due to polar nature of the material or to doping.

Difference in electronic structure between tetragonal and cubic Sr Nb O 2 N J. Appl. Phys. 98, 043706 (2005); 10.1063/1.2032612High-resolution synchrotron-radiation photoemission characterization for atomically-controlled SrTiO 3 ( 001 ) substrate surfaces subjected to various surface treatments High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to study the surface electronic structure of Nb-doped SrTiO 3 (STO) single crystals prepared using a variety of surface preparations. ARPES measurements show that simple degreasing with subsequent anneal in vacuum is not an adequate surface preparation of STO, rather, preparations consisting of etching with buffered HF or HCl, and to a lesser extent, simple water leaching resulted in surfaces with much less disorder. A non-dispersing, mid-gap state was found $800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present for vacuum-annealed and water-leached samples, as well as for STO thin films grown using molecular beam epitaxy. Theoretical modeling using density functional theory suggests that this mid-gap state is not related to the SrO-and TiO 2 -terminated surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching. V C 2013 AIP Publishing LLC.