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.

First principles modeling of composites involving TiO2 clusters supported on M2C MXenes.

First-principles calculations based on density functional theory are performed to investigate the formation of titania/MXene composites taking (TiO2)5/M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) as cases of study. The present systematic analysis confirms a favorable, high exothermic interaction, which promotes important structural reconstructions of the (TiO2)5 cluster along with charge transfer from the MXene to titania. MXenes composed of d3 transition metals promote the strongest interaction, deformation energy, and charge transfer, followed by d4 and d5 M2C MXenes. We provide evidence that the formation of these (TiO2)5/M2C composites is governed by charge transfer, leading to scaling relationships. By using the electronegativity of the metal composing MXene and the MXene d-band center, we also establish linear correlations that can be used to predict the interaction strength of (TiO2)5/M2C composites just from the knowledge of the MXene composition. It is likely that the present trends hold for other TiO2/MXene composites.

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

Correction for 'The nature of the electronic ground state of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes' by Néstor García-Romeral et al., Phys. Chem. Chem. Phys., 2023, 25, 31153-31164, https://doi.org/10.1039/D3CP04402E.

Theoretical Prediction of Core-Level Binding Energies: Analysis of Unexpected Errors.

The analysis of the C(1s) and O(1s) core-level binding energies (CLBEs) of selected molecules computed by means of total energy Hartree-Fock (ΔSCF-HF) differences shows that in some cases, the calculated values for the C(1s) are larger than the experiment, which is unexpected. The origin of these unexpected errors of the Hartree-Fock ΔSCF BEs is shown to arise from static, nondynamical, electron correlation effects which are larger for the ion than for the neutral system. Once these static correlation effects are included by using complete active space self-consistent field (CASSCF) wave functions that include internal correlation terms, the resulting ΔSCF BEs are, as expected, smaller than measured values.

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.

Challenges of modeling nanostructured materials for photocatalytic water splitting.

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

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.

Assigning XPS features in B,N-doped graphene: input from ab initio quantum chemical calculations.

Ab initio quantum chemical calculations using large enough cluster models have been used to predict the core level binding energies of B(1s) and N(1s), including initial and final state effects, in several possible atomic arrangements in B,N-codoped graphene, such as isolated atoms, different types of B,N pairs and BN domains. To a large extent, the observed trends are dominated by initial state effects that support assigning the experimental features to the neutral samples. For the BN domains the present theoretical results are in full agreement with the experimental assignment thus providing support to the rest of the assignments. In particular, the present results strongly suggest that some of the features observed in the experiments are likely to correspond to isolated B or N atoms in graphene and, others fit well to the prediction corresponding to different types of B,N pairs. The importance of having an unambiguous, rigorous way to assign experimental features is emphasized.

Limitations of the equivalent core model for understanding core-level spectroscopies.

The equivalent core model, or the Z + 1 approximation, has been used to interpret the binding energy, BE, shifts observed in X-ray photoelectron spectroscopy, XPS; in particular to relate these shifts to their origin in the electronic structure of the system. Indeed, a recent paper has claimed that the equivalent core model provides an intuitive chemical view of XPS BE shifts. In the present paper, we present a detailed comparison of the electronic structure provided from rigorous core-hole theory and from the equivalent core model to assess the validity and the utility of the use of the equivalent core model. This comparison shows that the equivalent core model provides a qualitative view of the different properties of initial and core-hole electronic structure. It is also shown that a very serious limitation of the equivalent core model is that it fails to distinguish between initial and final state contributions to the shifts of BEs which seriously reduces the utility of the information obtained with the equivalent core model. Indeed, there is a danger of making an incorrect assignment of the importance of relaxation because the equivalent core model appears to stress the role of final state effects. Given the importance of the distinction of initial and final state effects, we provide rigorous definitions of these two effects and we discuss an example where an incorrect interpretation was made based on the use of the equivalent core model.

Investigating the character of excited states in TiO2 nanoparticles from topological descriptors: implications for photocatalysis.

Titanium dioxide (TiO2) nanoclusters (NCs) and nanoparticles (NPs) have been the focus of intense research in recent years since they play a prominent role in photocatalysis. In particular, the properties of their excited states determine the photocatalytic activity. Among the requirements for photocatalytic activity, low excitation energy and large separation of the charge carriers are crucial. While information regarding the first is straightforward from either experiment or theory, the information regarding the second is scarce or missing. In the present work we fill this gap through a topological analysis of the first singlet excited state of a series of TiO2 NCs, and anatase and rutile derived NPs containing up to 495 atoms. The excited states of all these systems in vacuo have been obtained from time-dependent density functional theory (TDDFT) calculations using hybrid functionals and the influence of water was taken into account through a continuum model. Three different topological descriptors based on the attachment/detachment one-electron charge density, are scrutinized: (i) charge transfer degree, (ii) charge density overlap, and (iii) distance between centroids of charge. The present analysis shows that the charge separation in the excited state strongly depends on the NP size and shape. The character of the electronic excitations, as arising from the analysis of the canonical Kohn-Sham molecular orbitals (MOs) or from natural transition orbitals (NTOs), is also investigated. The understanding and prediction of charge transfer and recombination in TiO2 nanostructures may have implications in the rational design of these systems to boost their photocatalytic potential.

Designing water splitting catalysts using rules of thumb: advantages, dangers and alternatives.

Thermodynamic analysis of the oxygen evolution reaction (OER) hints toward an intrinsic overpotential caused by the nonoptimal adsorption-energy scaling relation between OH and OOH. Consequently, nowadays it is a widely accepted yet unverified rule of thumb that breaking such a scaling relation results in enhanced catalytic activity. In this perspective, we show that breaking the OH-OOH scaling relation does not per se lower the OER overpotential. Instead, electrocatalytic symmetry and ease of optimization are shown to be key factors when screening for enhanced OER catalysts. The essence of electrocatalytic symmetry is captured by a descriptor called the electrochemical-step symmetry index (ESSI). In turn, the ease of optimization and whether it should be scaling-based or scaling-free is provided by a procedure called δ-ε optimization. Finally, taking the search for bifunctional catalysts for oxygen electrocatalysis as an example, we show that the alternative analysis can be straightforwardly extended to other electrocatalytic reactions.

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.

Structural, electronic, and magnetic properties of Ni nanoparticles supported on the TiC(001) surface.

Metals supported on transition metal carbides are known to exhibit good catalytic activity and selectivity, which is interpreted in terms of electron polarization induced by the support. In the present work we go one step further and investigate the effect that a titanium carbide (TiC) support has on the structural, electronic, and magnetic properties of a series of Ni nanoparticles of increasing size exhibiting a two- or three-dimensional morphology. The obtained results show that three-dimensional nanoparticles are more stable and easier to form than their homologous two-dimensional counterparts. Also, comparison to previous results indicates that, when used as the support, transition metal carbides have a marked different chemical activity with respect to oxides. The analysis of the magnetic moments of the supported nanoparticles evidences a considerable quenching of the magnetic moment that affects mainly the Ni atoms in close contact with the TiC substrate indicating that these atoms are likely to be responsible for the catalytic activity reported for these systems. The analysis of the electronic structure reveals the existence of chemical interactions between the Ni nanoparticles and the TiC support, even if the net charge transfer between both systems is negligible.

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.

Nature of SrTiO3/TiO2 (anatase) heterostructure from hybrid density functional theory calculations.

In this work, we investigate the structural and electronic properties of the SrTiO3/TiO2 (anatase) heterostructure by means of hybrid density functional theory calculations. The work is motivated by several experiments that pointed to SrTiO3/TiO2 as a good system for photocatalytic applications, due to the small lattice mismatch between these two oxides and their favorable band alignment, leading to a type-II heterojunction, favoring the charge-carrier separation. The present results provide insights into the nature of the contact region and an estimation of the band offsets in the composite system. Our results are also compared with the available experimental values and with previous theoretical reports. The calculated offsets quantitatively agree with experimental measurements. In addition, we found significant interfacial effects that make the band offsets slightly increase with respect to those of the separated components. Last, we also discuss the role of point defects such as oxygen vacancies, finding that they do not remarkably affect the band alignment.

On the use of DFT+U to describe the electronic structure of TiO2 nanoparticles: (TiO2)35 as a case study.

One of the main drawbacks in the density functional theory (DFT) formalism is the underestimation of the energy gaps in semiconducting materials. The combination of DFT with an explicit treatment of the electronic correlation with a Hubbard-like model, known as the DFT+U method, has been extensively applied to open up the energy gap in materials. Here, we introduce a systematic study where the selection of the U parameter is analyzed considering two different basis sets: plane-waves and numerical atomic orbitals (NAOs), together with different implementations for including U, to investigate the structural and electronic properties of a well-defined bipyramidal (TiO2)35 nanoparticle. This study reveals, as expected, that a certain U value can reproduce the experimental value for the energy gap. However, there is a high dependence on the choice of basis set and on the U parameter employed. The present study shows that the linear combination of the NAO basis functions, as implemented in Fritz Haber Institute ab initio molecular simulation (FHI-aims), requires, requires a lower U value than the simplified rotationally invariant approach, as implemented in the Vienna ab initio simulation package (VASP). Therefore, the transfer of U values between codes is unfeasible and not recommended, demanding initial benchmark studies for the property of interest as a reference to determine the appropriate value of U.

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.

Effect of electron correlation in the decomposition of core level binding energy shifts into initial and final state contributions.

The influence of electron correlation into the decomposition of core level binding energy shifts, measured by X-ray photoelectron spectroscopy (XPS), into initial and final effects is analysed for a series of molecules where these effects are noticeable. Moreover, the series of molecules is chosen in such a way that electron delocalization and increasing number of electrons may provide a large screening of the core hole. A detailed analysis shows that the Hartree-Fock decomposition is biased whereas a physically meaningful decomposition is obtained when electron correlation effects are taken into account. The results show that in this case, trends in core level binding energy shifts are driven by initial state effects thus providing further support to the use of these observable quantities to interpret changes in the chemical bond in the neutral molecule rather than on the core ionized cation. Consequences for the theoretical interpretation of XPS data in materials and surface science are discussed.