Tracking the soft dynamic nature of halide perovskites is challenging. Theoretical calculations and experiment comparisons are complex. We discuss how different strategies for calculating averages in AIMD can be tuned to specific measurements.

Yttria-stabilized tetragonal zirconia (Y-TZP) ceramics have excellent mechanical properties. However, such materials cannot undergo plastic deformation at room temperature due to their high hardness and brittleness values, hindering machinability. To overcome these limitations, we propose a zirconia-yttria-titania ceramics, based on zirconia containing 3mol% yttria and up to 15mol% titania. The zirconia-yttria-titania powders were synthesized by co-precipitation method, uniaxially pressed and sintered at 1400?C/5 h. Sample characterizations were carried out by X-ray diffraction, scanning electron microscopy and mechanical properties through Vickers hardness and toughness measurements. Compared to the Y-TZP ceramics, the yttria stabilised tetragonal zirconia ceramics co-doped with 10mol%Ti showed noticeable increase of tetragonality parameter, higher toughness and lower hardness values, indicating plasticity at room temperature. Furthermore, the atomistic simulation by Density Functional Theory methodology suggests the occurrence of spatial arrangement of the atoms, explaining the proposed plasticity.

Doping of tungsten trioxide (WO3) and molybdenum trioxide (MoO3) materials with alkali atoms, leading to the formation of the so-called sodium bronzes, is a viable approach to achieve a precise control of their electronic, optical, and magnetic properties via electron band structure engineering. Driven by the ongoing trend for thickness reduction and the resulting new functionalities at the nanoscale, using a combination of state-of-the-art experimental and computational techniques, we investigate here the interaction of two isostructural two-dimensional (2D) WO3 and MoO3 layers, grown epitaxially onto a Pd(100) surface, with Na dopants. We identify two interaction regimes as a function of the Na coverage: a low-coverage regime up to 0.3 ML, which we describe in terms of doping interactions, and a reaction regime, where at higher Na coverages, the 2D WO3/MoO3 lattices become destroyed and several ordered 2D bronze-type phases form upon thermal activation. In the doping regime, Na initially decorates the oxide domain boundaries and later adsorbs in a (2 × 2) superstructure, filling the regular adsorption sites within the oxide domains. Further Na accommodation in the 2D oxide lattice is unfavorable due to the poor lateral electrostatic screening and elastic strain increase. In the reaction regime, the most prominent and energetically stable phase is the hexagonal 2D bronze-like layer, whose atomic details are resolved in a density functional theory (DFT) analysis and compared with the structure of the bulk counterpart.

The electroreduction of CO2 into value-added products is a significant step toward closing the global carbon loop, but its performance remains far from meeting the requirement of any practical application. The insufficient understanding of the reaction mechanism is one of the major causes that impede future development. Although several possible reaction pathways have been proposed, significant debates exist due to the lack of experimental support. In this work, we provide opportunities for experiments to validate the reaction mechanism by providing predictions of the core-level shifts (CLS) of reactive intermediates, which can be verified by the X-ray photoelectron spectroscopy (XPS) data in the experiment. We first validated our methods from benchmark calculations of cases with reliable experiments, from which we reach consistent predictions with experimental results. Then, we conduct theoretical calculations under conditions close to the operando experimental ones and predict the C 1s CLS of 20 reactive intermediates in the CO2 reduction reaction (CO2RR) to CH4 and C2H4 on a Cu(100) catalyst by carefully including solvation effects and applied voltage (U). The results presented in this work should be guidelines for future experiments to verify and interpret the reaction mechanism of CO2RR.

2D systems are an emerging class of material that was discovered only a decade ago. They consist of infinite slabs only a few-atoms wide, and as it occurs for other low-dimensional systems, such as nanoparticles and nanowires, they present unique and interesting properties. Being ultra-thin, they can also be used as building blocks to generate nanosheets or heterostructures, that have the potential of combining and even enhancing the different properties of its constituent 2D layers. In this chapter, two types of material will be discussed. The Van der Waals heterostructures, that present strong in-plane bonds of ionic or covalent character, and weak inter-plane interaction of Van der Waals character. More general heterostructures, where a 2D layer interacts more strongly with support or even with another 2D material. A few recent studies will be presented and the difficulties one usually finds from a computer simulation perspective will be highlighted. The current existing databases of theoretically predicted stable 2D materials and the properties they contain will also be mentioned.

Hybrid Ni-MoS2 electrocatalysts are one of the most promising materials for the generation of hydrogen in an alkaline medium. This paper presents a simple and economical method for the rational synthesis of Ni-MoS2 nanocomposites, maximizing the contact area and reducing the contact resistance between MoS2 and the nickel surface. In this way, it is possible to maximize the synergistic effect between both materials, obtaining a hybrid nanomaterial with high electroactivity toward the generation of hydrogen. A conventional nickel catalyst (NWts) was compared with the one obtained by dispersing a small amount of MoS2 (0.1425 mu g cm(-2)) over the surface denoted as NMS, and with the same type of catalyst after a 10 s electrodeposition of Ni (NMSN), to have a Ni-MoS2-Ni laminar structure. Thus, the NMSN catalyst shows a current density value of 59% higher than the observed value on the NMS catalyst and 113% higher than that found in the conventional NWts catalyst. Finally, these results were analyzed using DFT theoretical studies. DFT calculations predict a charge transfer between MoS2 and nearby Ni atoms, which becomes more important when a second Ni layer is placed on MoS2 explaining the increase in catalytic activity in the NMSN catalyst. Furthermore, the high hydrophobicity of the MoS2 plays an important role in the electrochemically active surface when comparing NMS and NMSN catalysts.

We report three paradigmatic examples of atomic-level computational modeling of catalytic processes of hydrogen use in the energy and environmental fields: ammonia synthesis (in particular the Haber-Bosch, HB, process), oxygen reduction reaction (ORR), and the carbon dioxide reduction reaction (CO2RR). Reaction mechanisms for these processes are illustrated first in a general overview and then in the specific cases of: a single-crystal Fe bcc(111) surface for HB, nanostructured systems (nanoporous particles, a small cluster, and nanowire) for ORR, and subnanometer (or ultranano) Ni3 and Ni2Cu clusters for CO2RR, respectively. Our goal is to show the potentialities of predictive computational modeling in this field as a basis for progress and possible breakthroughs in the rational design of catalysts satisfying the stringent societal requirements to H2 utilization for sustainable energy and matter cycles.

Zirconium dioxide, or zirconia, is a common and useful ceramic with a wide range of applications, from fuel cells to odontology. Its phase diagram is simple and well understood, having a structure which is monoclinic at temperatures up to 1500 K, tetragonal up to 2700 K and cubic up to 3000 K. Zirconia is rarely used in its pure form, being typically doped with ${\text{Y}}_{2}{\text{O}}_{3}$, MgO or ${\text{TiO}}_{2}$, and in this regime its phase diagram becomes much more complex. In this context, ab initio molecular dynamics (AIMD) can provide a detailed atomistic description of the phase diagram of this system, accurately describing its stable phases and transition regions. In this work, 3 mol-% ${\text{Y}}_{2}{\text{O}}_{3}$ (3YSZ) crystals doped with different Ti contents were studied at the density-functional level. For Ti contents varying from 0 to 30 at%, a global search algorithm was first used to explore the 0 K potential-energy surface and determine the most stable sites for the added Ti atoms. It was found that, at low Ti compositions ${X}_{\text{Ti}}$, small ${\text{TiO}}_{2}$ clusters form, followed by ${\text{TiO}}_{2}$ channels and infinite ${\text{TiO}}_{2}$ planes at larger ${X}_{\text{Ti}}$ values, and that the highest stability is achieved at 9% Ti. AIMD simulations within the isothermal-isobaric NPT ensemble were then performed to characterize the temperature-dependent phase changes as a function of the Ti content, where it was found that the Ti-doped structures presented considerably smaller volume changes near the phase-change critical temperatures. These findings suggest that YSZ materials doped with a small amount of Ti are both energetically and kinetically more stable than the undoped counterparts, in the ideal proportion of 3% ${\text{TiO}}_{2}$ for every 1% ${\text{Y}}_{2}{\text{O}}_{3}$ doping.

Defect complexes play critical roles in the dynamics of water molecules in photoelectrochemical cell devices. For the specific case of hematite (α-Fe2O3), iron and oxygen vacancies are said to mediate the water splitting process through the localization of optically-derived charges. Using first-principles methods based on density-functional theory we show that both iron and oxygen vacancies can be observed at the surface. For an oxygen-rich environment, usually under wet conditions, the charged iron vacancies should be more frequent. As sea water would be an ideal electrolyte for this kind of device, we have also analyzed the effect of additional chlorine adsorption on this surface. While the chlorine adatom kills the charged oxygen vacancies, entering the void sites, it will not react with the iron vacancies, keeping them active during water splitting processes.

We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS2 layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material. The MoS2 material, when used as a catalyst in operative conditions, was found to be mainly composed of thin and short 1-5 layer sheets instead of a poorly crystalline structure, as it was previously assumed. Notably wrinkled structures with S-vacancies were also found through transmission electron microscopy. Atomistic simulations revealed a natural connection between sulfur-vacancies, wrinkling and folding. Density functional calculations further revealed that such curved structures present a lower electronic band-gap and a higher reactivity towards thiophene compared to the planar MoS2 counterpart.

Two-dimensional (2D) WO3 nanosheets exhibit a range of novel properties and functionalities that render them attractive for advanced nanotechnologies. However, at the ultimate 2D limit of single-layer thickness, the structural properties of WO3 are unclear. Here, we fabricated, using molecular beam epitaxy techniques, a crystalline 2D WO3 overlayer on a Ag(100) surface and unveiled its geometric, electronic, and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO3 phase forms a bilayer with a staggered arrangement of WO6 octahedra, linked together by corner- and edge-sharing, which is significantly different from the cubic and monoclinic WO3 bulk structures, but resembles a bilayer of the alpha-MoO3 layered bulk lattice. Such a 2D WO3 bilayer on Ag(100) is a robust nonpolar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and, according to the density functional theory calculations, should survive as a stable freestanding layer, that is, as a nanosheet.

Supported Pt nanoparticles are key components in heterogeneous catalysis for energy and environment applications that involve vapor and wet conditions. In the latter case, the reaction proceeds at the catalyst-water interface where the solvent actively participates in the reaction mechanism. In this work, ab initio molecular dynamics simulations shed light on the effects of solvation on the reactivity and electronic properties of Pt-6 nanocatalysts supported by ceria (CeO2), a highly reducible oxide. The calculated trajectories show that H2O molecules spontaneously dissociate at both the supported Pt-6 cluster and at the ceria surface already at T = 350 K. Water dissociation leads to hydroxylation of the ceria surface and, most importantly, to the selective decoration of the metal-oxide periphery with hydroxide ions, which are stabilized by solvent induced electronic effects and which quickly diffuse to the interfacial Pt sites via Grotthus-like proton chains. The periphery of the metal-oxide interface is thus identified as the active region of ceria-supported Pt clusters in wet environments. Solvation is shown to drive dynamic charge transfers across the metal/oxide interface that modify the cluster charge, a key parameter of the catalyst reactivity.

The growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800-1100 degrees C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.

Water-mineral interfaces are important for several environmental, industrial, biological, and geological processes. Gypsum, CaSO4·2H2O, is a widespread mineral of high technological, medical, and environmental relevance, but little is known about its surface structure and its interaction with water. A molecular-level understanding of gypsum/water interface is given here by a combined experimental/theoretical study. We investigate the structure and dynamics of water adsorbed from vapor on the gypsum (010) single-crystal surface at room temperature, combining sum-frequency generation (SFG) vibrational spectroscopy experiments and ab initio molecular dynamics (AIMD) simulations. The SFG spectra of gypsum at low relative humidity (RH) show an anisotropic arrangement of structural water molecules and the presence of dangling OH groups. The AIMD simulations allow a detailed assignment of the SFG spectra and show that the cleaved (010) surface rearranges to have only 25% of the OH groups pointing away from the surface. At higher RHs, the first adsorbed water layer binds to these OH groups and forms an anisotropic arrangement, but with the amount of free OH groups significantly suppressed and without any significant diffusion. Upon adsorption of a second water layer, although the topmost layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath because it has a much reduced amount of free OH groups with respect to the free surface of water, and a slower surface diffusion with respect to bulk water. The theoretical results corroborate the experimental ones and provide an accurate atomic characterization of the surface structure.

Haematite (α-Fe2O3) is a potential candidate for photo-electrochemical water splitting. It is abundant and its electronic properties fit those needed for this kind of device. However, very little is known about the intermediate steps of this photon-induced water splitting process. We propose here that surface iron vacancies can be the main defects responsible for the activity of haematite in the photoelectrochemical reaction. We perform DFT+U calculations and explicitly add holes to show that these defects are common in iron-terminated (0001) surfaces. As holes tend to be localized at these centers, they should be available for the dissociation of water under sunlight. Our calculations also reveal that the water adsorption energy close to the vacancy is 1 eV stronger than far from it, and when the formation of multi-holes is considered, a thermodynamically stable water dissociation mechanism can be developed. We determined that both Fe[double bond, length as m-dash]O and Fe-OOH intermediate steps are stable, although Fe-OOH quickly leads to the formation of O2, having therefore a very short lifetime. Phonon calculations on these structures reveal the appearance of peaks in the 800-900 cm-1 frequency range only for the intermediate steps, connected to Fe[double bond, length as m-dash]O vibrations, in agreement with recent measurements.

Bridging the support gap in heterogeneous ultrananocatalysis.