Hybrid Density Functional Calculations Research Articles

O2 Activation over Ag-Decorated CeO2(111) and TiO2(110) Surfaces: A Theoretical Comparative Investigation

Periodic spin-polarized hybrid density functional theory calculations have been performed to investigate the reactivity of pristine, O-defective, and Ag-decorated CeO2(111) and TiO2(110) surfaces w...

Configurations of V4+ centers in the MoVO catalyst material. A systematic stability analysis of DFT results

The reactivity of a catalyst is in part determined by its geometric and electronic structure. Here we present a model that is able to describe the energy trend of the important oxidation catalyst material MoVO, as obtained from hybrid density functional calculations for various V4+/V5+ configurations. For an exemplary V/Mo occupancy, we systematically examined the universe of all V4+ distributions. The distribution of these V4+ centers, in combination with the induced lattice distortions, plays a key role in determining the stability of the material, entailing energy variations of up to ~140 kJ mol−1 per unit cell. Hence, for this kind of catalyst, it is crucial to account for the V4+ distributions. To this end, we are proposing novel predictive models based on features like the number of Mo centers with two reduced neighbors V4+ and the locations of potentially reducible centers V5+. For the V/Mo occupancy chosen, these models are able to describe the energy variation due to the V4+ distribution with root mean square errors as low as 6 kJ mol−1. Accordingly, catalytically selective sites featuring pentameric units with a single polaron center are among the most of stable configurations. Another aspect of this work is to understand energy contributions of polaron arrangements bracketing Mo centers.

(INVITED) First-principles calculations of quantum transitions at local centers

Fundamental quantum transitions at local centers are the basis for numerous technologies, including LED, laser, photodetection, and quantum computing. However, accurate theoretical calculations of basic parameters related to these processes remain challenging. In this paper, we review recent developments in first-principles calculations of electronic transitions at local centers in solid-state materials. Specifically, we discuss (1) promising results on hybrid density functional calculations of self-trapped excitons in low-dimensional metal halides and 5d-4f transitions at rare-earth dopants in solids as well as (2) the more challenging case of intra-d-shell transitions at transition-metal ions, which require a higher-level many-body theory to better account for the electron correlation effect. Diffusion quantum Monte Carlo calculations of Mn4+ emissions in oxides and halides show excellent agreement with experimental results, demonstrating the potentially predictive power in calculating d-d transitions at transition-metal ions and other optical transitions at local centers.

Cause of Extremely Long-Lasting Room-Temperature Persistent Photoconductivity in SrTiO_{3} and Related Materials.

It has been recently revealed that strontium titanate (SrTiO_{3}) displays persistent photoconductivity with unique characteristics: it occurs at room temperature and lasts over a very long period of time. Illumination of SrTiO_{3} crystals at room temperature with sub-band-gap light reduces the electrical resistance by three orders of magnitude and persists for weeks or longer [Tarun etal., Phys. Rev. Lett. 111, 187403 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.187403]. Experiments indicate that oxygen vacancy and hydrogen play important roles, yet the microscopic mechanism responsible for this remarkable effect has remained unidentified. Using hybrid density functional theory calculations we show that an instability associated with substitutional hydrogen H_{O}^{+} under illumination, which becomes doubly ionized and leaves the oxygen site, can explain the experimental observations. H_{O} then turns into an interstitial hydrogen and an oxygen vacancy, leading to excess carriers in the conduction band. This phenomenon is not exclusive to SrTiO_{3}, but it is also predicted to occur in other oxides. Interestingly, this phenomenon represents an elegant way of proving the existence of hydrogen substituting on an oxygen site (H_{O}), forming an interesting, and rarely observed, type of three-center, two-electron bond.

Two dimensional CdS/ZnO type-II heterostructure used for photocatalytic water-splitting

The electronic structures of two dimensional (2D) CdS/ZnO heterostructure (CdZnHT) consisting of CdS singlelayer (SL) and ZnO SL are explored based on hybrid density functional calculation. The negative interface formation energies suggest the formation of CdZnHT is exothermic. The bandgap of CdZnHT is favorable for absorbing visible light, and the decent band edge position makes it thermodynamically feasible for spontaneous generation of oxygen and hydrogen. The formed electric field across the interface induced by charge transfer will reduce photogenerated carrier recombination and promote carrier migration. Particularly, CdZnHT is a type-II heterostructure. Oxygen generation takes place at ZnO layer and hydrogen production occurs at CdS layer, which will also promote the effective separation and migration of phogogenerated carriers and enhance photocatalytic performance. These findings suggest that 2D CdZnHTs are possible candidates as water-splitting photocatalysts.

Two-Dimensional CdX/C2N (X = S, Se) Heterostructures as Potential Photocatalysts for Water Splitting: A DFT Study.

Global environmental issues, in addition to limited fossil fuel resources, are being addressed by quests in search of efficient visible-light-driven water splitting catalysts for hydrogen production. The photocatalytic water splitting activities of CdX/C2N (X = S, Se) heterostructures have been investigated here using hybrid density functional theory calculations. The calculated band gaps of CdS/C2N and CdSe/C2N heterostructures are 1.48 and 2.12 eV, respectively. These are ideal band gap values that make possible harvesting of more visible light from the solar spectrum, which will result in high solar to energy conversion efficiencies. Charge density difference analysis shows that the charge redistributions mainly occur in the interface regions and that the charges transfer from the C2N to CdX layers. It is interesting to note that the CdX/C2N heterostructures possess a type-II band alignment, where the relative band alignment of the C2N and CdX monolayers promotes a spatial separation of the electrons (that resides in C2N) and holes (that resides in CdX). Importantly, the band edges of the heterostructures straddle the water redox potential under different pH conditions. This study demonstrates that the CdS/C2N and CdSe/C2N heterostructures are suitable materials to split water (from various sources) in different ranges of pH values.

Supramolecular Energetic and Topological Study of Halogenated Aryl Carboxylic Acids

A supramolecular energetic and topological study of 12 monohalogenated aryl carboxylic acids (F, Cl, Br, I) in the ortho, meta, and para positions was performed. The energy of intermolecular interactions was determined from long-range corrected hybrid density functional theoretical computational calculations, using the ωB97X-D3 method. The surface of contact between the molecules and the molecular coordination numbers were determined. From these data, all intermolecular interactions were classified, and the robustness of the interactions was evaluated. Thus, the molecule (Mn) allocations around a central molecule (M1) are proposed. In addition, the energy from the intermolecular interactions present in the first coordination sphere, which involves the central molecule, the energy resulting from the interactions between the peripheral molecules and the total energy of the cluster provided by all interactions, was determined. The role of hydrogen bonding (O–H···O, C–H···O, and C–H···X), π···π interactions, and halogen bonding (X···X) were evaluated, in the formation of the first coordination sphere and in the nucleation process.

Towards intermediate-band photovoltaic absorbers: theoretical insights on the incorporation of Ti and Nb in In2S3

Creation of a partially filled intermediate band in a photovoltaic absorber material is an appealing concept for increasing the quantum efficiency of solar cells. Recently, we showed that formation of a partially filled intermediate band through doping a host semiconductor with a transition metal dopant is hindered by the strongly correlated nature of d-electrons and the antecedent Jahn–Teller distortion, as we have previously reported. In present work, we take a step forward and study the delocalization of a filled (valence-like) intermediate band throughout the lattice: a case study of Ti- and Nb-doped In2S3. By means of hybrid density functional calculations, we present extensive analysis on structural properties and interactions leading to electronic characteristics of Ti- and Nb-doped In2S3. We find that Nb creates an occupied doublet, which can become delocalized onto the crystal at high but feasible concentrations (around 2.5 at% and above). As a consequence, doping In2S3 with adequately high concentrations of Nb allows the subgap intermediate band to conduction band absorption, which leads to higher photocurrent densities compared to pure In2S3. Ti on the other hand forms an occupied singlet intermediate band, which remains strongly localized even at high concentration of 5 at%.

Two-dimensional GaTe/Bi2Se3 heterostructure: A promising direct Z-scheme water splitting photocatalyst

Among various photocatalytic materials, Z-scheme photocatalysts have drawn tremendous research interest due to high photocatalytic performance in solar water splitting. Here, we perform extensive hybrid density functional theory calculations to explore electronic structures, interfacial charge transfer, electrostatic potential profile, optical absorption properties, and photocatalytic properties of a proposed two-dimensional (2D) small-lattice-mismatched GaTe/Bi2Se3 heterostructure. Theoretical results clearly reveal that the examined het-erostructure with a small direct band gap can effectively harvest the broad spectrum of the incoming sunlight. Due to the relative strong interfacial built-in electric field in the heterostructure and the small band gap between the valence band maximum of GaTe monolayer and the conduction band minimum of Bi2Se3 nanosheet with slight band edge bending, these photogenerated carriers transfer via Z-scheme pathway, which results in the photogenerated electrons and holes effectively separating into the GaTe monolayer and the Bi2Se3 nanosheet for the hydrogen and oxygen evolution reactions, respectively. Our results imply that the artificial 2D GaTe/Bi2Se3 is a promising Z-scheme photocatalyst for overall solar water splitting.

Insight into enhanced visible-light photocatalytic activity of SWCNTs/g-C3N4 nanocomposites from first principles

Single-walled carbon nanotubes (SWCNTs) modified g-C3N4nanocompositeshave attracted extensive attentiondue to their good photocatalytic activity for water splitting in the visible region, but the theoretical research on the enhancement mechanism is still lacking. Using first-principles hybrid density functional calculations, the potential photocatalytic enhancement mechanism of SWCNTs-modified g-C3N4 has been studied in detail. For different metal (9,0) and semiconductor (10,0) SWCNTs modified patterns, the two components are always linked by van der Waals forces without exception. Obvious structural foldsin g-C3N4, large interlayer charge transfer, significantly enhanced visible light absorption, and smaller |ΔGH*|, these factors enhance the photocatalytic activity of SWCNTs/g-C3N4.Interestingly, (10,0) SWCNT have better coordination advantage with g-C3N4 than (9,0) SWCNT, resulting in (10,0)SWCNT/g-C3N4nanocomposite with more visible structural folding for more stable structures, greater charge transfer, and more efficient visible light absorption spectra. Moreover, we also studied the photocatalytic performance of g-C3N4 modified by metal (6,0) and semiconductor (7,0) SWCNTs. As a result, the photocatalytic performance of the latter is significantly better than the former, which further shows that the semiconductor SWCNTs-modified g-C3N4 maybe more superior. The findings pave the way for the development of highly active g-C3N4-based photocatalysts.

Insight into the Na adsorption on WSe2xS2(1−x) monolayers: a hybrid functional investigation

Monolayer WSe2 has aroused strong interest as a semiconducting 2D material. The tuning of its electronic properties is important for expanding the range of applications. In this work, by hybrid density functional theory calculations, we investigate the effect of Na adsorption on the electronic and magnetic properties of stoichiometric and defective WSe2 monolayers and their WSe2xS2(1−x) alloys. Our results suggest that Na adsorption on defective WSe2xS2(1−x) monolayers would split and partially fill the empty defect states in the gap region inducing a local magnetic moment of 1 μB per alkaline atom. The efficient tuning of the electronic and magnetic properties of WSe2 monolayer by Na adsorption combined with the Se vacancy and S alloying would benefit its wide applications.

Optical identification of point defects in monolayer beryllium oxide by ab initio calculations

Triggered by recent experimental observation of monolayer (1L) beryllium oxide (BeO), we systematically investigate the formation energies, thermodynamic stabilities and optoelectronic properties of point defects in 1L BeO by the hybrid density functional calculations and ab initio molecular dynamics simulations. Under Be rich(poor) condition, we find that the most favorable vacancy and antisite defects are respectively the VO(VBe) and the OBe(BeO). Most of these defects are found to be thermodynamically stable at room temperature. The VBe and VO show strong attractive interaction, which tends to result in a cooperative effect. Moreover, we reveal the magnetism and the tunable optoelectronic properties induced by these defects. Specifically, due to the defect states involved interband transitions, the new characteristic peaks and the improved absorptions with large absorption coefficients (105 cm−1) are demonstrated in the visible and near-UV region, which further help identify the defects. The microscopic understanding about the point defects would provide guidance for experimental design and synthesis of 1L BeO based optoelectronic devices via defect engineering.

Electronic Properties and Carrier Trapping in Bi and Mn Co-doped CsPbCl3 Perovskite.

Metal halide perovskites exhibit impressive optoelectronic properties with applications in solar cells and light-emitting diodes. Co-doping the high-band gap CsPbCl3 perovskite with Bi and Mn enhances both material stability and luminescence, providing emission on a wide spectral range. To discuss the role of Bi3+ and Mn2+ dopants in tuning the CsPbCl3 perovskite energy levels and their involvement in carrier trapping, we report state-of-the-art hybrid density functional theory calculations, including spin–orbit coupling. We show that co-doping the perovskite with Bi and Mn delivers essentially the sum of the electronic properties of the single dopants, with no significant interaction or the preferential mutual location of them. Furthermore, we identify the structural features and energetics of transitions of electrons trapped at Bi and holes trapped at Mn dopant ions, respectively, and discuss their possible role in determining the optical properties of the co-doped perovskite.

Hybrid density functional studies for the effect of oxygen vacancy on the visible light photocatalytic activity in M (M=Li, Na, K, Rb, Cs)-doped [formula omitted]-Bi2O3

In the work, the relative stability of the single substitutional defect MBi, single oxygen vacancy defect VO and the complex defect MBi-VO (M = Li, Na, K, Rb, Cs) in α-Bi2O3 as well as their influences on the electronic properties and optical absorption spectrum of α-Bi2O3 are investigated using hybrid density functional calculations. It is shown that the complex defect MBi-VO is easier to form than the single defect (MBi or VO). The calculated electronic structures and optical absorption spectrum for the supercell with complex defect are also closer to the available experimental data than these with the single defect. Further, the charge compensation effect in the M monodoped α-Bi2O3 case is supported. Particularly, the induced impurity level in the band gap by the single defect (MBi or VO) disappears when the complex defect MBi-VO is presented. This makes the possible recombination center of the photogenerated electrons and holes disappear. At the same time, the theoretical band gap in the complex defect case decreases, and the absorption capacity of the visible light enhances with increasing doping concentration. These calculations well understand the significant improvement of the photocatalytic activity in the visible light range for Na and Cs-dopedα-Bi2O3 in very recent experiments. Besides, it could be a good strategy to effectively improve the visible light photocatalytic activity of α-Bi2O3, by preparing α-Bi2O3 with oxygen vacancies.

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.

Energy transfer, structural and luminescent properties of the color tunable phosphor Y2WO6:Sm3+

Abstract Inorganic phosphors based on monoclinic Y2WO6 doped with Sm3+ ions were prepared via conventional solid-state reactions at high temperature. A total of six samples were obtained with different Sm3+ concentrations (0–9%). The purity of the as-prepared phases was checked by powder X-ray diffraction (PXRD). The excitation, emission, and time-resolved emission spectra were examined in detail. The experimental decay curves were fitted to the Inokuti–Hirayama model, showing that the Sm3+ ions form clusters at all compositions. Periodic hybrid density functional theory calculations were also carried out on the undoped material and on 144-atom supercells of stoichiometry Y1.9375Sm0.0625WO6 and Y1.875Sm0.125WO6. The different coordination environments at the Y1, Y2 and Y3 sites are analysed in detail. The calculated structure and band gap of Y2WO6 are in good agreement with experiment with one potentially important discrepancy in a Y3–O bond length. The thermodynamically favoured substitution sites for Sm, Y2 and Y3, in the supercell are not those observed under the preparation conditions used here, since the experimental Rietveld analysis suggests occupation of Y1 at all concentrations. Analogous calculations for the Eu-doped system highlight marked differences between Eu and Sm despite their similar ionic sizes. The calculated densities of states show the position of the 4f levels in the band gap depend on the sites occupied by the dopants and thus expect marked differences in the luminescence spectra, opening up possibilities for tuning device performance.

Improving the heterointerface in hybrid organic-inorganic perovskite solar cells by surface engineering: Insights from periodic hybrid density functional theory calculations.

A periodic hybrid density functional theory computational strategy is presented to model the heterointerface between the methylammonium lead iodide (MAPI) perovskite and titanium dioxide (TiO2 ), as found in perovskite solar cells (PSC), where the 4-chlorobenzoic acid (CBA) ligand is used to improve the stability and the band alignment at the interface. The CBA ligand acts as a bifunctional linker to efficiently connect the perovskite and the oxide moieties, ensuring the stability of the interface through Ti-O and Pb-Cl interactions. The computed density of states reveals that the perovskite contributes to the top of the valence band while the oxide contributes to the bottom of the conduction band with a direct bandgap of 2.16 eV, indicating a possible electron transfer from MAPI to TiO2 . Dipole moment analysis additionally reveals that the CBA ligand can induce a favorable effect to improve band alignment and thus electron transfer from MAPI to TiO2 . This latter has been quantified by calculation of the spin density of the reduced MAPI/CBA/TiO2 system and indicates an almost quantitative (99.94%) electron transfer from MAPI to TiO2 for the surface engineered system, together with an ultrafast electron injection time in the femtosecond timescale. Overall, the proposed DFT-based computational protocol therefore indicates that surface engineering and the use of a bifunctional linker can lead to a better stability, together with improved band alignment and electron injection in PSC systems.

Enhanced photocatalytic efficiency of layered CdS/CdSe heterostructures: Insights from first principles electronic structure calculations

Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. In this class of materials, CdS with a direct band gap of 2.4 eV, has gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strategy for improving the photocatalytic performance. In this work we have done a detail theoretical investigation based on hybrid density functional theory calculation to get insight into the energy band structure, mobility and charge transfer across the CdS/CdSe heterojunction. The results indicate that CdS/CdSe forms type-II heterostructure that has several advantages in improving the photocatalytic efficiency under visible light irradiation.

Achieving indirect‐to‐direct band gap transition and enhanced photocatalytic performance in blue phosphorene through doping and strain

AbstractRecently, blue phosphorene (BP) has demonstrated great potential in the field of photocatalytic water splitting due to the ultrahigh carrier mobility. However, the practical application of BP as an efficient photocatalyst is greatly limited by its indirect band gap. In this work, we investigate the synergistic effect of substitutional doping and biaxial strain on the electronic and photocatalytic properties of BP using hybrid density functional calculations. The results show that As/Sb doping not only reduces the band gap of BP without introducing any midgap states but also turns it into direct band gap semiconductor, which can be ascribed to the p states of the dopants appearing around the band edges. For these As/Sb‐doped BP systems, the band gaps, band edge positions, and optical absorption abilities can be further tuned by applying a biaxial strain. In particular, we predict that compressive strains are more propitious for the doped systems than the tensile strains since the requirements for water splitting are satisfied, meanwhile preserving the direct band gap characteristics. Besides, our calculations also show that the band gap and the reducing and oxidizing power of multilayer BP are highly dependent on the layer thickness. These results suggest feasible modulation strategies for enabling BP to be a visible‐light‐driven photocatalyst for water splitting.

Double-centers of V,Ce–Codoped LiNbO3 from Hybrid Density Functional Theory Calculations: Electron Trapping and Excitation between the Defect Levels

The double-center holographic recording method appears as one of the most promising methods for the storage of persistent holograms in LiNbO3. In the study, we theoretically proposed that V and Ce ...