Oxygen evolution reaction (OER) is reconciled with the bottleneck in hydrogen production via photo- or electrocatalytic water splitting. One of the remedies to overcome this shortcoming is to develop catalytically efficient anode materials. Strontium titanate (STO) is a potential candidate for the OER by referring to recent experimental reports on enhanced photocatalytic activity of faceted nanoparticles. In this paper, we perform electronic structure calculations in the density functional theory approximation to study the OER on flat and stepped surfaces, such as encountered in nano-sized STO. We demonstrate that stepped surfaces reveal higher OER activity than flat surfaces, which is consistent with experimental data. We also observe partial breaking of OER scaling relation on stepped surfaces, albeit this does not necessarily cause higher catalytic activity. Finally, we propose recommendations for thorough implementation of zero-point energy calculations as the calculation method may impact the free-energy landscape of the OER.

While the bulk strontium titanate (STO) crystal characteristics are relatively well known, ultrathin perovskites’ nanostructure, chemical composition, and crystallinity are quite complex and challenging to understand in detail. In our study, the DFT methods were used for modelling the Raman spectra of the STO bulk (space group I4/mcm) and 5–21-layer thin films (layer group p4/mbm) in tetragonal phase with different thicknesses ranging from ~0.8 to 3.9 nm. Our calculations revealed features in the Raman spectra of the films that were absent in the bulk spectra. Out of the seven Raman-active modes associated with bulk STO, the frequencies of five modes (2Eg, A1g, B2g, and B1g) decreased as the film thickness increased, while the low-frequency B2g and higher-frequency Eg modes frequencies increased. The modes in the films exhibited vibrations with different amplitudes in the central or surface parts of the films compared to the bulk, resulting in frequency shifts. Some peaks related to bulk vibrations were too weak (compared to the new modes related to films) to distinguish in the Raman spectra. However, as the film thickness increased, the Raman modes approached the frequencies of the bulk, and their intensities became higher, making them more noticeable in the Raman spectrum. Our results could help to explain inconsistencies in the experimental data for thin STO films, providing insights into the behavior of Raman modes and their relationship with film thickness.

Recent research suggests that photocatalytic activity toward water splitting of strontium titanate SrTiO 3 (STO) is enhanced by creating multifaceted nanoparticles. To better understand the source of this activity, a previously designed model is used for two types of surfaces of this nanoparticle, flat and double‐stepped. Density functional theory calculations of water adsorption on these surfaces are performed to gain insight into water adsorption and proton migration processes, as well as thermodynamics of hydrogen evolution reaction within the framework of computational hydrogen electrode. It is concluded that ridges of single‐ and double‐stepped surfaces are nearly identical in terms of adsorption configurations and energetics. Also, it is demonstrated that protons have migration barriers lower than 0.7 eV and that surface morphology impacts catalytic activity toward hydrogen evolution reaction, with flat surface demonstrating higher catalytic activity.

Various photocatalysts are being currently studied with the aim of increasing the photocatalytic efficiency of water splitting for production of hydrogen as a fuel and oxygen as a medical gas. A noticeable increase of hydrogen production was found recently experimentally on the anisotropic faces (facets) of strontium titanate (SrTiO3, STO) nanoparticles. In order to identify optimal sites for water splitting, the first principles calculations of the Raman vibrational spectrum of the bulk and stepped (facet) surface of a thin STO film with adsorbed water derivatives were performed. According to our calculations, the Raman spectrum of a stepped STO surface differs from the bulk spectrum, which agrees with the experimental data. The characteristic vibrational frequencies for the chemisorption of water derivatives on the surface were identified. Moreover, it is also possible to distinguish between differently adsorbed hydrogen atoms of a split water molecule. Our approach helps to select the most efficient (size and shape) perovskite nanoparticles for efficient hydrogen/oxygen photocatalytic production.

Recent experimental findings suggest that strontium titanate SrTiO3 (STO) photocatalytic activity for water splitting could be improved by creating multifaceted nanoparticles. To understand the underlying mechanisms and energetics, the model for faceted nanoparticles was created. The multifaceted nanoparticles’ surface is considered by us as a combination of flat and “stepped” facets. Ab initio calculations of the adsorption of water and oxygen evolution reaction (OER) intermediates were performed. Our findings suggest that the “slope” part of the step showed a natural similarity to the flat surface, whereas the “ridge” part exhibited significantly different adsorption configurations. On the “slope” region, both molecular and dissociative adsorption modes were possible, whereas on the “ridge”, only dissociative adsorption was observed. Water adsorption energies on the “ridge” ( −1.50 eV) were significantly higher than on the “slope” ( −0.76 eV molecular; −0.83 eV dissociative) or flat surface ( −0.79 eV molecular; −1.09 eV dissociative).

Formation and evolution of defect levels in the electronic structure of silicon nitride with cubic spinel structure, gamma-Si3N4, after the irradiation with He+ ions was investigated using spectroscopic techniques. Strong changes of cathodoluminescence (CL), photoluminescence (PL), photoluminescence excitation (PLE) and Raman spectra were detected. In particular, excitonic PL was significantly inhibited and a new near-IR band appeared with the band gap excitation h nu >= E-g = 5.05 eV. This was explained by an effective trapping of photoinduced electrons and holes by charged defects. The spectral shift of PL with the excitation photon energy indicated heterogeneous nature of the defect sites. The energetic position of near-IR and visible PL bands correlate, suggesting an interaction with the common cation defect to be an origin. The visible PL of exciton bound to a neutral defect Si-x was red shifted, which was attributed to the permutations between empty and occupied octahedral and tetrahedral sites, inherent to the spinel structure, after collisions with He+ ions. The positively charged cation sites in the spinel structure are compensated by V-N'''nion vacancies. The local deformation of the spinel lattice affects PL intensity of the self-trapped exciton at 4.35 eV.

Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters.

Three cubic bromide perovskites CsMBr3 (M = Ca, Ge, Sn) with two different surface terminations (CsBr and MBr2) were studied in this work using the first principles method. A wide range of physical properties, including electronic band structures, atom-projected density of states for each layer, surface relaxation effects, and surface energy, were evaluated for each considered surface termination. Differences between the properties of the bulk and slab models were highlighted. It was shown that surfaces with the CsBr termination have a lower energy and a more pronounced surface rumpling than those with the MBr2 termination. As a main result of this study, it was demonstrated that the CsBr-terminated surfaces appear to be energetically more stable in each of these three considered perovskites.

The structural, electronic, optical and elastic properties of the CdGeAs2 and CdSiAs2 chalcopyrites and their CdSi1-xGexAs2 mixtures were calculated using the first-principles methods. Good agreement with the available experimental data was achieved for both neat compounds. Based on this, the mixed CdSi1-xGexAs2 compounds were explored in details. Two different models -the virtual crystal approximation (VCA) and the supercell (SC) model -were used to analyze the impact of the second cation substitution on the above-mentioned properties of the mixed materials in the whole range of the Ge/ Si concentration from x = 0 to x= 1. It has been shown that the lattice constants, band gaps, refractive indexes, elastic constants and Debye temperature all exhibit linear variation with the second cation composition. The linear dependences of all these parameters allow for their numerical estimation for any composition in the studied range.

Bi-benzene - chemically bound two benzene molecules in stuck position is studied both analytically and numerically. There are several allotropes of bi-benzene having different geometry. The reason of the existence of sundry distorted structures is the pseudo-Jahn-Teller effect. The parameters of vibronic couplings causing distortions are found. For the calculation of these parameters both, the vibronic coupling of carbon atoms in different C-6 rings and the vibronic coupling in the rings are considered. The contribution of the distortion of C-6-planes to the latter coupling is also found. The energies of all the electronic states of pi-electrons in all bi-benzene allotropes are determined by using the calculated vibronic interaction parameters.

Chemically bound states of benzene molecules with graphene are studied both analytically and numerically. The states are formed by switching off intrabonds of π-electrons in C6 rings to interbonds. A number of different undistorted and distorted structures are established both with aligned and with transversal mutual orientation of benzene and graphene. The vibronic interactions causing distortions of bound states are found, by using a combination of analytical and numerical considerations. This allows one to determine all electronic transitions of π-electrons without explicit numerical calculations of excited states, to find the conical intersections of potentials, and to show that the mechanism of distortions is the pseudo-Jahn-Teller effect. It is found that the aligned distorted benzene molecule placed between two graphene sheets makes a chemical bond with both of them, which may be used for fastening of graphene sheets together.

The composition-induced “direct–indirect” band gap transition in the AlxIn1−xP alloys was studied theoretically. Two different approaches – virtual crystal approximation and supercell-based calculations, both in the general gradient and local density approximations – were used to model the influence of the chemical composition on the structural and electronic properties of the AlxIn1−xP system in the whole range of Al concentration x from 0 to 1. The band gap crossover from the direct to indirect band gap was shown to take place at x=0.406, in excellent agreement with the experimental result x=0.408 of Beaton et al. It was also demonstrated that the supercell-based calculations are better suited to describe variation of the composition-driven structural properties, whereas the virtual crystal approximation is better to be employed for the analysis of the electronic properties of the title system.

Chemically bound structures of several benzene molecules are studied both analytically and numerically. The bonds in these structures are established by replacing the intra pi-bonds of C-6 rings by the inter sigma-bonds. The rearrangements of different pi-bonds may take place differently which causes distortions of C-6 rings. The reason of the distortions is the pseudo-Jahn-Teller effect (PJTE). In the case of two chemically bound benzene molecules in a hollow position (with direct on-top stacking), called as bi-benzene, three different distorted states in addition to the undistorted one are found to exist. PJTE leads also to a chemical bonding of three and more benzene molecules stacked directly on-top of each other. These bound structures do not exist for undistorted molecules. All bound states, although being metastable are separated from the unbound ones by energy barriers greater than or similar to 2 eV. It is found that the height of the barrier for symmetric bi-benzene is essentially affected by the Jahn-Teller effect. (C) 2015 Elsevier B.V. All rights reserved.

The structural, electronic, and elastic properties of three mixed transition metal carbonitrides TiNxC1-x, ZrNxC1-x, and HfNxC1-x (0 <= x <= 1) with the rock-salt structure were calculated at ambient and elevated up to 50 GPa hydrostatic pressures in the framework of the density functional theory methods. The lattice constants, densities, and bulk moduli of the considered compounds were shown to behave as linear functions of the nitrogen concentration x. The obtained linear dependencies of all these parameters allow for getting their estimates at any value of x in the range from 0 to 1. Gradual enhancement of the ionicity of the chemical bonds with gradual replacement of carbon by nitrogen was demonstrated by calculating the bond orders and electron density difference distributions. (C) 2013 Elsevier Masson SAS. All rights reserved.

CdIn2S4 thiospinel was studied by means of first-principles calculations in both generalized gradient and local density approximations (GGA and LDA). One of the main results of this paper is that the controversy regarding the character of the CdIn2S4 band gap previously encountered in the literature was clearly resolved in favor of the indirect gap. The calculated density of states was compared with the experimental XPS spectrum; very good agreement was demonstrated. The structural, electronic, optical and elastic properties were calculated in the pressure range of 0 to 10 GPa, below the pressure of phase transition for this compound. The estimated pressure coefficient of the band gap of 0.071/0.063 eV GPa(-1) (GGA/LDA) is in excellent agreement with the experimental data of 0.076 or 0.069 eV GPa(-1) found in the literature. The calculated pressure dependence of the unit cell volume follows the experimental results very closely. The dependence of the interionic distances, lattice parameter and all elastic constants on pressure was calculated. Refined estimations of the Debye temperature for CdIn2S4 are given as 280 K (LDA) and 252 K (GGA). The elastic anisotropy of CdIn2S4 was visualized by plotting the three-dimensional dependence of the Young's modulus on a direction in the crystal lattice; it was established that the lowest Young's moduli are realized if the external stress is applied along the crystallographic axes.

Density functional calculations are presented for various properties of the elpasolite crystals Cs2NaYX6 (X=F, Cl, Br) using the CASTEP module, either in the generalized gradient approximation (GGA) or in the local density approximation (LDA). Specifically, the calculated properties are lattice parameter, density, band gap, elastic constants, bulk modulus, sound velocity, Debye temperature, Grüneisen constant, phonon frequencies and phonon dispersion. The variations of some of these properties with applied pressure have also been calculated. Comparison with experimental data is made where available.

Hybrid materials built from conjoined structures of graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) have important properties for novel applications. In this communication we have performed a numerical study of these structures and have found two types: (i) CNT and GNR structures formed by van der Waals forces with a distance close to 0.35 nm and (ii) CNT and GNR structures interconnected by short (0.17 nm) and strong chemical bonds. It appears that the latter bonds essentially perturb conjoined carbon C6 rings. The reason for the perturbation is the pseudo-Jahn–Teller effect.

A survey of the literature data on the adsorption of benzene on graphene or carbon nanotubes indicates that the distance between the graphene sheet and benzene molecule is determined from weak van der Waals forces (similar to 3.40 angstrom). In our theoretical study, it was found that the benzene/graphene structure (in a specific configuration with carbon atoms located at the atop positions, stacked directly on the top of each other) forms strong covalent bonds, if the distance between the graphene and benzene is about 1.60 angstrom. Such a short distance corresponds to about a half of the usual separation between the graphite layers. It was also shown that at such a short distance the carbon atoms of the benzene molecule move towards the graphene sheet, whereas the hydrogen atoms move in a different direction, thus breaking the benzene planar structure.In addition to the structural optimization, the calculated electronic and optical properties (significantly modified by the adsorbed benzene molecule) are presented as well. (C) 2012 Elsevier B.V. All rights reserved.

Six transition metal monocarbides (TiC, VC, CrC, NbC, MoC, HfC) with the rock-salt structure were chosen for a detailed comparative ab initio study of their structural, electronic, elastic, and thermodynamic properties at ambient and elevated up to 50 GPa hydrostatic pressures. Special attention was paid to the relation between the elastic and bonding properties and the number of valence electrons in each compound. Elastic anisotropy of the considered carbides was analyzed; the directions in the crystal lattice corresponding to the greatest and smallest Young's moduli values were identified. The calculated values of the elastic constants were used for further estimations of the Debye temperatures, Grüneisen parameters, specific heat capacities and linear coefficients of thermal expansion. Comparison of the calculated results with available experimental and theoretical data for TiC, VC, NbC, HfC yielded good agreement. The specific heat capacities and thermal expansion coefficients for CrC and MoC were calculated for the first time, to the best of the authors' knowledge.