Understanding the Chemical Bond in Semiconductor/MXene Composites: TiO2 Clusters Anchored on the Ti2C MXene Surface.

First-principles calculations on titania clusters (TiO2)n (n=5 and 10) supported on the pristine Ti2C (0001) surface were carried out to understand the properties of semiconductor/MXene composites with implications in (photo)-catalysis. The reported results reveal a high exothermic interaction accompanied by a substantial charge transfer with a concomitant, notorious, deformation of the titania nanoclusters. The analysis of the density of states analysis of the composite systems evidences a metallic character with titania related states crossing the Fermi level. The picture of the chemical bonds is completed by the analysis of X-Ray Photoelectron Spectra (XPS) features, evidencing clear shifts of the C(1s) and O(1s) related peaks relative to the isolated systems that have a quite complex origin. This detailed analysis provides insights to experimentalists interested in the design and synthesis of these systems with possible applications in catalysis.

How does thickness affect magnetic coupling in Ti-based MXenes.

The magnetic nature of Ti2C, Ti3C2, and Ti4C3 MXenes is determined from periodic calculations within density functional theory and using the generalized gradient approximation based PBE functional, the PBE0 and HSE06 hybrids, and the on-site Hubbard corrected PBE+U one, in all cases using a very tight numerical setup. The results show that all functionals consistently predict a magnetic ground state for all MXenes, with spin densities mainly located at the Ti surface atoms. The analysis of solutions corresponding to different spin orderings consistently show that all functionals predict an antiferromagnetic conducting ground state with the two ferromagnetic outer (surface) Ti layers being antiferromagnetically coupled. A physically meaningful spin model is proposed, consistent with the analysis of the chemical bond, with closed shell, diamagnetic, Ti2+ like ions in inner layers and surface paramagnetic Ti+ like centers with one unpaired electron per magnetic center. From a Heisenberg spin model, the relevant isotropic magnetic coupling constants are extracted from an appropriate mapping of total energy differences per formula unit to the expected energy values of the spin Hamiltonian. While the numerical values of the magnetic coupling constants largely depend on the used functional, the nearest neighbor intralayer coupling is found to be always ferromagnetic, and constitutes the dominant interaction, although two other non-negligible interlayer antiferromagnetic terms are involved, implying that the spin description cannot be reduced to NN interaction only. The influence of the MXene thickness is noticeable for the dominant ferromagnetic interaction, increasing its value with the MXene width. However, the interlayer interactions are essentially due to the covalency effects observed in all metallic solutions which, as expected, decay with distance. Within the PBE+U approach, a U value of 5 eV is found to closely simulate the results from hybrid functionals for Ti2C and less accurately for Ti3C2 and Ti4C3.

The nature of the electronic ground state of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes.

A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes. Electronic band structure calculations in the framework of density functional theory (DFT), carried out with different types of basis sets and employing the generalized gradient approach (GGA) and hybrid functionals, provide strong evidence that Ti2C, Zr2C, Hf2C, and Cr2C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V2C, Nb2C, Mo2C, Ta2C, and W2C MXenes exhibit a diamagnetic conducting ground state. For Ti2C, Zr2C, Hf2C, and Cr2C, the analysis of the low-lying spin polarized solutions with different spin orderings indicates that their ground states are antiferromagnetic (AFM), consisting of two ferromagnetic (FM) metal layers coupled antiferromagnetically. For the diamagnetic MXenes, the converged spin polarized solutions are significantly less stable than the closed shell solution except for the case of V2C and Mo2C where those excited open shell solutions can be thermally accessible (less than 300 meV per formula unit). The analysis of charge and spin density distributions of the ground state of the MXenes reveals that, in all cases, the metal atoms have a net charge close to +1 e and C atoms close to -2 e. In the case of diamagnetic MXenes, the electronic structure of V2C, Nb2C, and Ta2C is consistent with metal atoms exhibiting a closed-shell s2d2 configuration whereas for Mo2C, and W2C is consistent with a low-spin s1d4 configuration although the FM solution is close in energy for V2C and Mo2C suggesting that they may play a role in their chemistry at high temperature. For the open shell MXenes, the spin density primarily located at the metal atoms showing one unpaired electron per Ti+, Zr+, and Hf+ magnetic center, consistent with s2d1 configuration of the metal atom, and of ∼3.5 unpaired electrons per Cr+ magnetic center interpreted as a mixture of s2d3 and high-spin s1d4 configuration. Finally, the analysis of the density of states reveals the metallic character of all these bare MXenes, irrespective of the nature of the ground state, with significant covalent contributions for Mo2C and W2C.

Can calculated harmonic vibrational spectra rationalize the structure of TiC-based nanoparticles?

Nanoscale titanium carbide (TiC) is widely used in composites and energy applications. In order to design and optimize these systems and to gain a fundamental understanding of these nanomaterials, it is important to understand the atomistic structure of nano-TiC. Cluster beam experiments have provided detailed infrared vibrational spectra of numerous TixCy nanoparticles with well defined masses. However, these spectra have yet to be convincingly assigned to TixCy nanoparticle structures. Herein, using accurate density functional theory based calculations, we perform a systematic survey of likely candidate nanoparticle structures with masses corresponding to those in experiment. We calculate harmonic infrared vibrational spectra for a range of nanoparticles up to 100 atoms in size, with a focus on systems based on removing either four carbon atoms or a single titanium atom from rocksalt-structured stoichiometric TiC nanoparticles. Our calculations clearly show that Ti-deficient nanoparticles are unlikely candidates to explain the experimental spectra as such structures are highly susceptible to C-C bonding, whose characteristic frequencies are not observed in experiment. However, our calculated infrared spectra for C-deficient nanoparticles have some matching features with the experimental spectra but tend to have more complex infrared spectra with more peaks than those obtained from experiment. We suggest that the discrepancy between experiment and theory may be largely due to thermally induced anharmonicities and broadening in the latter nanoparticles, which are not be accounted for in harmonic vibrational calculations.

Effect of nanostructuring on the interaction of CO2 with molybdenum carbide nanoparticles.

Transition metal carbides are increasingly used as catalysts for the transformation of CO2 into useful chemicals. Recently, the effect of nanostructuring of such carbides has started to gain relevance in tailoring their catalytic capabilities. Catalytic materials based on molybdenum carbide nanoparticles (MoCy) have shown a remarkable ability to bind CO2 at room temperature and to hydrogenate it into oxygenates or light alkanes. However, the involved chemistry is largely unknown. In the present work, a systematic computational study is presented aiming to elucidate the chemistry behind the bonding of CO2 with a representative set of MoCy nanoparticles of increasing size, including stoichiometric and non-stoichiometric cases. The obtained results provide clear trends to tune the catalytic activity of these systems and to move towards more efficient CO2 transformation processes.

Adsorption and Activation of CO2 on Nitride MXenes: Composition, Temperature, and Pressure effects.

The interaction of CO2 with nitride MXenes of different thickness is investigated using periodic density functional theory-based calculations and kinetic simulations carried out in the framework of transition state theory, the ultimate goal being predicting their possible use in Carbon Capture and Storage (CCS). We consider the basal (0001) surface plane of nitride MXenes with Mn+1 Nn (n=1-3; M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometry and also compare to equivalent results for extended (001) and (111) surfaces of the bulk rock-salt transition metal nitride compounds. The present results show that the composition of MXenes has a marked influence on the CO2 -philicity of these substrates, whereas the thickness effect is, in general, small, but not negligible. The largest exothermic activation is predicted for Ti-, Hf-, and Zr-derived MXenes, making them feasible substrates for CO2 trapping. From an applied point of view, Cr-, Mo-, and W-derived MXenes are especially well suited for CCS as the interaction with CO2 is strong enough but molecular dissociation is not favored. Newly developed kinetic phase diagrams are introduced supporting that Cr-, Mo-, and W-derived MXenes are appropriate CCS substrates as they are predicted to exhibit easy capture at mild conditions and easy release by heating below 500 K.

Generalized gradient approximation adjusted to transition metals properties: Key roles of exchange and local spin density.

Perdew-Burke-Ernzerhof (PBE) and PBE adapted for solids (PBEsol) are exchange-correlation (xc) functionals widely used in density functional theory simulations. Their differences are the exchange, µ, and correlation, ß, coefficients, causing PBEsol to lose the Local Spin Density (LSD) response. Here, the µ/ß two-dimensional (2D) accuracy landscape is analyzed between PBE and PBEsol xc functional limits for 27 transition metal (TM) bulks, as well as for 81 TM surfaces. Several properties are analyzed, including the shortest interatomic distances, cohesive energies, and bulk moduli for TM bulks, and surface relaxation degree, surface energies, and work functions for TM surfaces. The exploration, comparing the accuracy degree with respect experimental values, reveals that the found xc minimum, called VV, being a PBE variant, represents an improvement of 5% in mean absolute percentage error terms, whereas this improvement reaches ~11% for VVsol, a xc resulting from the restoration of LSD response in PBEsol, and so regarded as its variant.

Explaining Cu@Pt Bimetallic Nanoparticles Activity Based on NO Adsorption.

Cu@Pt nanoparticles (NPs) are experimentally regarded as improved catalysts for NOx storage/reduction, with higher activities and selectivities compared with pure Pt or Cu NPs, and with inverse Pt@Cu NPs. Here, a density functional theory-based study on such NP models with different sizes and shapes reveals that the observed enhanced stability of Cu@Pt compared with Pt@Cu NPs is due to energetic reasons. On both types of core@shell NPs, charge is transferred from Cu to Pt, strengthening the NP cohesion energy in Pt@Cu NPs, and spreading charge along the surface in Cu@Pt NPs. The negative surface Pt atoms in the latter diminish the NO bonding owing to an energetic rise of the Pt bands, as detected by the appliance of the d-band model, although other factors, such as atomic low coordination or the presence of an immediate subsurface Pt atom do as well. A charge density difference analysis discloses a donation/back-donation mechanism in the NO adsorption.

Bulk (in)stability as a possible source of surface reconstruction.

A density functional theory based study is presented with the aim of addressing the surface energy stabilization mechanisms of transition metal carbide and nitride surfaces from a crystal structure different from that of the most stable polymorph. To this end, we consider the MoC(001), MoN(001), WC(001), and WN(001) surface of rocksalt structures, which, for these compounds, is not the most stable one. The geometry optimization of suitable slab models shows that all these surfaces undergo a sensible reconstruction. The energy difference per formula unit between the rock salt and the most stable polymorph seems to be the driving force behind the observed reconstruction. A note of caution is given in that certain small periodic boundary conditions can artificially restrain such reconstructions, for which at least (2×2) supercells are needed. Also, it is shown that neglecting such a surface reconstruction can lead to artifacts in the prediction of the chemical activity and/or reactivity of these surfaces.

Boosting the activity of transition metal carbides towards methane activation by nanostructuring.

The interaction of methane with pristine surfaces of bulk MoC and Mo2C is known to be weak. In contrast, a series of X-ray photoelectron spectroscopy (XPS) experiments, combined with thermal desorption mass spectroscopy (TDS), for MoCy (y = 0.5-1.3) nanoparticles supported on Au(111)-which is completely inert towards CH4-show that these systems adsorb and dissociate CH4 at room temperature and low CH4 partial pressure. This industrially-relevant finding has been further investigated with accurate density functional theory (DFT) based calculations on a variety of MoCy supported model systems. The DFT calculations reveal that the MoCy/Au(111) systems can feature low C-H bond scission energy barriers, smaller than the CH4 adsorption energy. Our theoretical results for bulk surfaces of Mo2C and MoC show that a simple Brønsted-Evans-Polanyi (BEP) relationship holds for C-H bond scission on these systems. However, this is not the case for methane activation on the MoCy nanoparticles as a consequence of their unique electronic and chemical properties. The discovery that supported molybdenum carbide nanoparticles are able to activate methane at room temperature paves the road towards the design of a new family of active carbide catalysts for methane activation and valorisation, with important implications in climate change mitigation and carbon cycle closure.

Room Temperature Methane Capture and Activation by Ni Clusters Supported on TiC(001): Effects of Metal-Carbide Interactions on the Cleavage of the C-H Bond.

Methane is an extremely stable molecule, a major component of natural gas, and also one of the most potent greenhouse gases contributing to global warming. Consequently, the capture and activation of methane is a challenging and intensively studied topic. A major research goal is to find systems that can activate methane, even at low temperatures. Here, combining ultrahigh vacuum catalytic experiments, X-ray photoemission spectra, and accurate density functional theory (DFT) based calculations, we show that small Ni clusters dispersed on the (001) surface of TiC are able to capture and dissociate methane at room temperature. Our DFT calculations reveal that two-dimensional Ni clusters are responsible for this chemical transformation, confirming that the lability of the supported clusters appears to be a critical aspect in the strong adsorption of methane. A small energy barrier of 0.18 eV is predicted for CH4 dissociation into adsorbed methyl and atomic hydrogen species. In addition, the calculated reaction free energy profile at 300 K and 1 atm of CH4 shows no effective energy barriers in the system. Comparison with other reported systems which activate methane at room temperature, including oxide and zeolite-based materials, indicates that a different chemistry takes place on our metal/carbide system. The discovery of a carbide-based surface able to activate methane at low temperatures paves the road for the design of new types of catalysts which can efficiently convert this hydrocarbon into other added-value chemicals, with implications in climate change mitigation.

Thickness biased capture of CO2 on carbide MXenes.

The synthesis of two-dimensional transition metal carbides (MXenes) with a predefined number of atomic layers offers a possible way to tune these nanomaterials chemical activity. MXenes have been theoretically predicted to be able to store CO2 even at high temperatures and low CO2 partial pressures, a prediction which has been experimentally confirmed afterwards. In the present work, the influence of the number of atomic layers on CO2 adsorption is systematically investigated by means of density functional theory based calculations, using suitable periodic models representing the (0001) surface of a series of these materials with formula Mn+1Cn (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W) and n = 1-3. The interaction of CO2 with the MXene surfaces is always favorable with the adsorption energy decreasing as the transition metal electronic configuration goes from d2 through d3 to d4, in agreement with previous work for n = 1. The influence of the thickness is found to be rather small, yet noticeable, although somewhat erratic. Nevertheless, the adsorption energy seems to converge to a defined clear limit for sufficiently thick MXenes. Interestingly, this value is close to that corresponding to the (111) surface of bulk Transition Metal Carbides (TMCs). The close structural similarity between the MXene (0001) and TMC (111) surfaces strongly suggests that the former provide a practical way to approach this otherwise unstable surface. The possibility to tune the CO2 interaction based on the MXene thickness is further investigated by means of kinetic phase diagrams. These provide additional evidence that carbide MXene surfaces are promising materials for CO2 capture even at low CO2 partial pressures, and that the MXene thickness can be used to fine tune this appealing behavior.

Double-well potential energy surface in the interaction between h-BN and Ni(111).

Density functional theory calculations with non-local correlation functionals, properly accounting for dispersion forces, predict the presence of two minima in the interaction energy between h-BN and Ni(111). These can be described as a physisorbed state with no corrugation of the h-BN structure, and a chemisorbed state exhibiting noticeable corrugation and a shorter distance of h-BN to the metallic support. The latter corresponds indeed to the one reported in most experiments. The relative stability of the two minima depends on the specific density functional employed: of those investigated here only optB86b-vdW yields the correct order of stability. We also demonstrate that the effect of the metal support on the Raman frequency of the chemisorbed boron nitride monolayer cannot be reduced to the associated strain. This is important because the Raman frequency has been proposed as a signature to identify h-BN monolayers from multilayered samples. Our analysis shows that such signatures would be strongly dependent on the nature of the interaction between the support and h-BN.

Implicit solvent effects in the determination of Brønsted-Evans-Polanyi relationships for heterogeneously catalyzed reactions.

Heterogeneously catalyzed reactions take place at the catalyst surface where, depending on the conditions and process, the reacting molecules are either in the gas or liquid phase. In the latter case, computational heterogeneous catalysis studies usually neglect solvent effects. In this work, we systematically analyze how the electrostatic contribution to solvent effects influences the atomic structure of the reactants and products as well as the adsorption, activation, and reaction energy for the dissociation of water on several planar and stepped transition metal surfaces. The solvent effects were accounted for through an implicit model that describes the effect of electrostatics, cavitation, and dispersion on the interaction between the solute and solvent. The present study shows that the activation energy barriers are only slightly influenced by the inclusion of the electrostatic solvent effects accounted for in a continuum solvent approach, whereas the adsorption energies of the reactants or products are significantly affected. Encouragingly, the linear equations corresponding to the Brønsted-Evans-Polanyi relationships (BEPs), relating the activation energies for the dissociation reaction with a suitable descriptor, e.g. the adsorption energies of the products of the reaction on the difference surfaces, are similar in the presence or in the absence of the solvent. Despite the associated uncertainties, this suggests that BEP relationships derived without the implicit consideration of the solvent are still valid for predicting the activation energy barriers of catalytic reactions from a reaction descriptor.

Subsurface Carbon: A General Feature of Noble Metals.

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble-metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low-coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low-coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity.

On the prediction of core level binding energies in molecules, surfaces and solids.

Core level binding energies, directly measured by X-ray photoelectron spectroscopy (XPS), provide unique information regarding the chemical environment of atoms in a given system. However, interpretation of XPS in extended systems may not be straightforward and requires assistance from theory. The different state-of-the-art theoretical methods commonly used to approach core level binding energies and their shifts with respect to a given reference are reviewed and critically assessed with special emphasis on recently developed theoretical methods and with a focus on future applications in materials and surface sciences.

Correction: Two-dimensional nitrides as highly efficient potential candidates for CO2 capture and activation.

Correction for 'Two-dimensional nitrides as highly efficient potential candidates for CO2 capture and activation' by Raul Morales-Salvador et al., Phys. Chem. Chem. Phys., 2018, 20, 17117-17124.

Robustness of surface activity electronic structure-based descriptors of transition metals.

Efficient yet simple electronic structure-based descriptors of transition metal surfaces are key in material design for many scientific fields in research and technology. Density functional theory-based methods provide the framework to systematically explore the performance and transferability of such descriptors. Using appropriate surface models and the Vosko-Wilk-Nussair (VWN), Perdew-Burke-Ernzerhof (PBE), PBE adapted for solids (PBEsol), revised PBE (RPBE), and Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation functionals, we study the transferability of three descriptors: the d-band centre, the width-corrected d-band centre, and the Hilbert transform highest peak, among the low-index Miller surfaces for the metals of transition elements. We show that the d-band centre and the width-corrected d-band centre descriptors are almost independent of the functional used whereas a dependency is seen in the Hilbert transform highest peak. Moreover, it is seen that the differences between the surface descriptor values and predictions from the bulk ones are affected by the presence of surface states. Interestingly, a direct relation between the surface coordination number and the d-band centre electronic descriptor is found when surface states are absent.

On the H2 interactions with transition metal adatoms supported on graphene: a systematic density functional study.

The attachment of H2 to the full set of transition metal (TM) adatoms supported on graphene is studied by using density functional theory. Methodology validation calculations on the interactions of H2 with benzene and graphene show that any of the vdW corrections under study, the Grimme D2, D3, D3 with Becke-Jonson damping (D3BJ), and Tkatchenko-Scheffler methods, applied on the PBE functional, are similarly accurate in describing such subtle interactions, with an accuracy of almost 2 kJ mol-1 compared to experiments. The PBE-D3 results show that H2 physisorbs on especially stable d5 or d10 TMs. In other 5d metals, and the rightmost 3d and 4d ones, H2 dissociates, and only for Y, Mn, Fe, and Zr the H2 binds strongly enough for its storage in the so-called Kubas mode, where the H2 bond is sensibly elongated. Other metals (Co, Ni, Ru, Rh and Pd) feature also an elongated Kubas mode, interesting as well for H2 storage. Sc and Ti display a Kubas modes especially suited, given their lightness, for meeting the gravimetric requirements. The H2 interactions with TM adatoms imply a TM → H2 charge transfer, although the magnetic moment of the system tends to remain intact, except for the early 5d TMs, where the unpaired electron transfer seems to be associated with the H2 bond breakage.

Two-dimensional nitrides as highly efficient potential candidates for CO2 capture and activation.

The performance of novel two-dimensional nitrides in carbon capture and storage (CCS) is analyzed for a broad range of pressures and temperatures. Employing an integrated theoretical framework where CO2 adsorption/desorption rates on the M2N (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) surfaces are derived from transition state theory and density functional theory based calculations, the present theoretical simulations consistently predict that, depending on the particular composition, CO2 can be strongly adsorbed and even activated at temperatures above 1000 K. For practical purposes, Ti2N, Zr2N, Hf2N, V2N, Nb2N, and Ta2N are predicted as the best suited materials for CO2 activation. Moreover, the estimated CO2 uptake of 2.32-7.96 mol CO2 kg-1 reinforces the potential of these materials for CO2 abatement.