Chemical bonding and electronic properties along Group 13 metal oxides.

CONTEXT: The present work provides a systematic theoretical analysis of the nature of the chemical bond in Al2O3, Ga2O3, and In2O3 group 13 cubic crystal structure metal oxides. The influence of the functional in the resulting band gap is assessed. The topological analysis of the electron density provides unambiguous information about the degree of ionicity along the group which is linearly correlated with the band gap values and with the cost of forming a single oxygen vacancy. Overall, this study offers a comprehensive insight into the electronic structure of metal oxides and their interrelations. This will help researchers to harness information effectively, boosting the development of novel metal oxide catalysts or innovative methodologies for their preparation. METHODS: Periodic density functional theory was used to predict the atomic structure of the materials of interest. Structure optimization was carried out using the PBE functional, using a plane wave basis set and the PAW representation of the atomic cores, using the VASP code. Next, the electronic properties were computed by carrying out single point calculations employing PBE, PBE + U functionals using VASP and also with PBE and the hybrid HSE06 functionals using the FHI-AIMS software. For the hybrid HSE06, the impact of the screening parameter, ω, and mixing parameter, α, on the calculated band gap has also been assessed.

Tuning electronic levels in photoactive hydroxylated titania nanosystems: combining the ligand dipole effect and quantum confinement.

Reducing the size of titania (TiO2) to the nanoscale promotes the photoactive anatase phase for use in a range of applications from industrial catalysis to environment remediation. The nanoscale dimensions of these systems affect the magnitude of the electronic energy gap by quantum confinement. Upon interaction with aqueous environments or water vapour, the surfaces of these systems will also be hydroxylated to some degree. In turn, this affects the electronic energy levels due to the cumulative electrostatic effect of the dipolar hydroxyl (-OH) ligands (i.e. the ligand dipole effect). Using accurate density functional calculations, we investigate the combined effects of quantum confinement and the hydration-induced ligand dipole effect on a set of realistic titania nanosystems over a wide range of hydroxylation. Our detailed investigation reveals that, contrary to previous models, the ligand dipole effect does not-linearly depend on the ligand coverage due to the formation of inter-ligand OH⋯OH hydrogen bonds. To account for the resulting effects, we propose a refined model, which describes the ligand dipole effect more accurately in our systems. We show that both hydroxylation (by the ligand dipole effect) and size (by quantum confinement) have significant but distinct impacts on the electronic energy levels in nanotitania. As an example, we discuss how variations in these effects can be used to tune the highest unoccupied energy level in nanotitania for enhancing the efficiency of the hydrogen evolution reaction. Overall, we show that any specific energy shift can be achieved by a range of different combinations of nanosystem size and degree of hydroxylation, thus providing options for energy-level tuning while also allowing consideration of practical constraints (e.g. synthetic limitations, operating conditions) for photochemical applications.

Comprehensive Density Functional and Kinetic Monte Carlo Study of CO2 Hydrogenation on a Well-Defined Ni/CeO2 Model Catalyst: Role of Eley-Rideal Reactions.

A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO2 → CO → HCO → CH → CH2 → CH3 → CH4 mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO2 (111) surface as potential good catalysts for high selective CO2 methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.

Estimating Nonradiative Excited-State Lifetimes in Photoactive Semiconducting Nanostructures.

The time evolution of the exciton generated by light adsorption in a photocatalyst is an important feature that can be approached from full nonadiabatic molecular dynamics simulations. Here, a crucial parameter is the nonradiative recombination rate between the hole and the electron that form the exciton. In the present work, we explore the performance of a Fermi's golden rule-based approach on predicting the recombination rate in a set of photoactive titania nanostructures, relying solely on the coupling of the ground and first excited state. In this scheme the analysis of the first excited state is carried out by invoking Kasha's rule thus avoiding computationally expensive nonadiabatic molecular dynamics simulations and resulting in an affordable estimate of the recombination rate. Our results show that, compared to previous ones from nonadiabatic molecular dynamics simulations, semiquantitative recombination rates can be predicted for the smaller titania nanostructures, and qualitative values are obtained from the larger ones. The present scheme is expected to be useful in the field of computational heterogeneous photocatalysis whenever a complex and computationally expensive full nonadiabatic molecular dynamics cannot be carried out.

Electrostatic Gating of Phosphorene Polymorphs.

The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.

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.

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.

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.

Role of N Doping in the Reduction of Titania Nanostructures.

The effect of N-doping of titania (TiO2) nanoparticles (NPs) on their reduction through neutral O vacancy (Ovac) formation is investigated using all electron density functional theory-based calculations, including hybrid density functionals, and taking the bipyramidal anatase (TiO2)84 NP as a realistic model. The location of the N dopant is systematically analyzed, including O substitution in the (TiO2)84 structure and N occupying interstitial regions. Our computational study concludes that interstitial N doping is more favorable than N substituting O atoms and confirms that the presence of N reduces the energy gap. In the N-doped NP, Ovac formation is more favored than in undoped NP but less than in the N-doped bulk, which has important consequences.

Ceftiofur treatment of sows results in long-term alterations in the nasal microbiota of the offspring that can be ameliorated by inoculation of nasal colonizers.

BACKGROUND: The nasal microbiota of the piglet is a reservoir for opportunistic pathogens that can cause polyserositis, such as Glaesserella parasuis, Mycoplasma hyorhinis or Streptococcus suis. Antibiotic treatment is a strategy to control these diseases, but it has a detrimental effect on the microbiota. We followed the piglets of 60 sows from birth to 8 weeks of age, to study the effect of ceftiofur on the nasal microbiota and the colonization by pathogens when the treatment was administered to sows or their litters. We also aimed to revert the effect of the antibiotic on the nasal microbiota by the inoculation at birth of nasal colonizers selected from healthy piglets. Nasal swabs were collected at birth, and at 7, 15, 21 and 49 days of age, and were used for pathogen detection by PCR and bacterial culture, 16S rRNA amplicon sequencing and whole shotgun metagenomics. Weights, clinical signs and production parameters were also recorded during the study. RESULTS: The composition of the nasal microbiota of piglets changed over time, with a clear increment of Clostridiales at the end of nursery. The administration of ceftiofur induced an unexpected temporary increase in alpha diversity at day 7 mainly due to colonization by environmental taxa. Ceftiofur had a longer impact on the nasal microbiota of piglets when administered to their sows before farrowing than directly to them. This effect was partially reverted by the inoculation of nasal colonizers to newborn piglets and was accompanied by a reduction in the number of animals showing clinical signs (mainly lameness). Both interventions altered the colonization pattern of different strains of the above pathogens. In addition, the prevalence of resistance genes increased over time in all the groups but was significantly higher at weaning when the antibiotic was administered to the sows. Also, ceftiofur treatment induced the selection of more beta-lactams resistance genes when it was administered directly to the piglets. CONCLUSIONS: This study shed light on the effect of the ceftiofur treatment on the piglet nasal microbiota over time and demonstrated for the first time the possibility of modifying the piglets' nasal microbiota by inoculating natural colonizers of the upper respiratory tract.

Minimum conditions for accurate modeling of urea production via co-electrolysis.

Co-electrolysis of carbon oxides and nitrogen oxides promise to simultaneously help restore the balance of the C and N cycles while producing valuable chemicals such as urea. However, co-electrolysis processes are still largely inefficient and numerous knowledge voids persist. Here, we provide a solid thermodynamic basis for modelling urea production via co-electrolysis. First, we determine the energetics of aqueous urea produced under electrochemical conditions based on experimental data, which enables an accurate assessment of equilibrium potentials and overpotentials. Next, we use density functional theory (DFT) calculations to model various co-electrolysis reactions producing urea. The calculated reaction free energies deviate significantly from experimental values for well-known GGA, meta-GGA and hybrid functionals. These deviations stem from errors in the DFT-calculated energies of molecular reactants and products. In particular, the error for urea is approximately -0.25 ± 0.10 eV. Finally, we show that all these errors introduce large inconsistencies in the calculated free-energy diagrams of urea production via co-electrolysis, such that gas-phase corrections are strongly advised.

Evaluating Adsorbate-Solvent Interactions: Are Dispersion Corrections Necessary?

Incorporating solvent-adsorbate interactions is paramount in models of aqueous (electro)catalytic reactions. Although a number of techniques exist, they are either highly demanding in computational terms or inaccurate. Microsolvation offers a trade-off between accuracy and computational expenses. Here, we dissect a method to swiftly outline the first solvation shell of species adsorbed on transition-metal surfaces and assess their corresponding solvation energy. Interestingly, dispersion corrections are generally not needed in the model, but caution is to be exercised when water-water and water-adsorbate interactions are of similar magnitude.

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis.

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated PtII species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous PtII catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of PtII moieties on the single-atom catalysts beyond ideally four-coordinated PtII-N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtII species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d8 metal ions.

Machine Learning-Driven Discovery of Key Descriptors for CO2 Activation over Two-Dimensional Transition Metal Carbides and Nitrides.

Fusing high-throughput quantum mechanical screening techniques with modern artificial intelligence strategies is among the most fundamental ─yet revolutionary─ science activities, capable of opening new horizons in catalyst discovery. Here, we apply this strategy to the process of finding appropriate key descriptors for CO2 activation over two-dimensional transition metal (TM) carbides/nitrides (MXenes). Various machine learning (ML) models are developed to screen over 114 pure and defective MXenes, where the random forest regressor (RFR) ML scheme exhibits the best predictive performance for the CO2 adsorption energy, with a mean absolute error ± standard deviation of 0.16 ± 0.01 and 0.42 ± 0.06 eV for training and test data sets, respectively. Feature importance analysis revealed d-band center (εd), surface metal electronegativity (χM), and valence electron number of metal atoms (MV) as key descriptors for CO2 activation. These findings furnish a fundamental basis for designing novel MXene-based catalysts through the prediction of potential indicators for CO2 activation and their posterior usage.

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.

Toward a Rigorous Theoretical Description of Photocatalysis Using Realistic Models.

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

A general but still unknown characteristic of active oxygen evolution electrocatalysts.

The unsatisfactory electrocatalysis of the oxygen evolution reaction (OER) is a major hurdle for the sustainable production of hydrogen using water electrolyzers. Besides, most state-of-the-art catalysts are based on expensive and scant elements such as Ru and Ir. Hence, it is paramount to establish the features of active OER catalysts to make well-informed searches. Here, an affordable statistical analysis exposes a general yet unnoticed characteristic of active materials for the OER: they frequently have three out of four electrochemical steps with free energies above 1.23 eV. For such catalysts, the first three steps (abbreviated as: H2O → *OH, *OH → *O, *O → *OOH) are statistically prone to be over 1.23 eV, and the second step is often potential limiting. Finally, "electrochemical symmetry", a recently introduced concept, is shown to be a simple and convenient criterion for the in silico design of enhanced OER catalysts, as materials with three steps over 1.23 eV tend to be highly symmetric.

Extracting Features of Active Transition Metal Electrodes for NO Electroreduction with Catalytic Matrices.

Electrocatalytic reduction of oxidized nitrogen compounds (NOx) promises to help rebalance the nitrogen cycle. It is widely accepted that nitrate reduction to NH4+/NH3 involves NO as an intermediate, and NO hydrogenation is the potential-limiting step of NO reduction. Whether *NO hydrogenates to *NHO or *NOH is still a matter of debate, which makes it difficult to optimize catalysts for NOx electroreduction. Here, "catalytic matrices" are used to swiftly extract features of active transition metal catalysts for NO electroreduction. The matrices show that active catalysts statistically stabilize *NHO over *NOH and have undercoordinated sites. Besides, square-symmetry active sites with Cu and other elements may prove active for NO electroreduction. Finally, multivariate regressions are able to reproduce the main features found by the matrices, which opens the door for more sophisticated machine-learning studies. In sum, catalytic matrices may ease the analysis of complex electrocatalytic reactions on multifaceted materials.

Theoretical Analysis of Magnetic Coupling in the Ti2C Bare MXene.

The nature of the electronic ground state of the Ti2C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti2C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

Crystal properties without crystallinity? Influence of surface hydroxylation on the structure and properties of small TiO2 nanoparticles.

Titania (TiO2) nanoparticles (NPs) are widely employed in applications that take advantage of their photochemical properties (e.g. pollutant degradation, photocatalysis). Here, we study the interrelation between crystallinity, surface hydroxylation and electronic structure in titania NPs with 1.4-2.3 nm diameters using all electron density functional theory-based calculations. We show how the distribution of local coordination environments of the atoms in thermally annealed quasi-spherical non-crystalline NPs converge to those in correspondingly sized faceted crystalline anatase NPs upon increasing hydroxylation. When highly hydroxylated, annealed NPs also possess electronic energy gaps with very similar energies and band edge orbital characters to those of the crystalline anatase NPs. We refer to the crystallite-mimicking non-crystalline annealed NPs as "crystalikes". Small stable crystalike NPs could allow for photochemical applications of titania in the size range where crystalline anatase NPs tend to become thermodynamically unfavoured (<3-5 nm). Our work implies the anatase crystal structure may not be as essential as previously assumed for TiO2 NP applications and generally suggests that crystalikes could be possible in other nanomaterials.