Small Formation Energy Research Articles

First-Principles Studies of the Electronic and Optical Properties of Zinc Titanium Nitride: The Role of Cation Disorder.

Cation disorder is an established feature of heterovalent ternary nitrides, a promising class of semiconductor materials. A recently synthesized wurtzite-family ternary nitride, ZnTiN2, shows potential for durable photoelectrochemical applications with a measured optical absorption onset of 2 eV, which is 1.4 eV lower than previously predicted, a large difference attributed to cation disorder. Here, we use first-principles calculations based on density functional theory to establish the role of cation disorder in the electronic and optical properties of ZnTiN2. We compute antisite defect arrangement formation energies for one hundred 128-atom supercells and analyze their trends and their effect on electronic structures, rationalizing experimental results. We demonstrate that charge imbalance created by antisite defects in Ti and N local environments, respectively, broadens the conduction and valence bands near the band edges, reducing the band gap relative to the cation-ordered limit, a general mechanism relevant to other multivalent ternary nitrides. Charge-imbalanced antisite defect arrangements that lead to N-centered tetrahedral motifs fully coordinated by Zn are the most energetically costly and introduce localized in-gap states; cation arrangements that better preserve local charge balance have smaller formation energies and have less impact on the electronic structure. Our work provides insights into the nature of cation disorder in the newly synthesized semiconductor ZnTiN2, with implications for its performance in energy applications, and provides a baseline for the future study of controlling cation order in ZnTiN2 and other ternary nitrides.

The optimized size distribution and NIR persistent luminescence of Zn2Ga2.98-4x/3HfxGe0.75O8:Cr0.02 nanoparticles via high-valence ion doping

In this study, spinel-phase Zn2Ga4-4y/3GeyO8 (ZGO:Ge4+y) (y = 0–0.90) and Zn2Ga2.98-4x/3HfxGe0.75O8:Cr0.02 (ZGGO:Cr3+,Hf4+x) (x = 0–0.075) nanoparticles were prepared by a hydrothermal method combined with a subsequent vacuum annealing. For Cr3+doped ZGO:Ge4+y nanoparticles, it was found that the replacement of Ga3+ with high-valence Ge4+ can lead to more uniform morphology and enhanced afterglow. Meanwhile, for ZGGO:Cr3+,Hf4+x nanoparticles, the non-equivalent replacement of Ga3+ with high-valence Hf4+ can give rise to the decrease in standard deviation gradually from 12.3 to 7.0 nm and the afterglow intensity is increased to 3.4 times of the undoped ZGGO:Cr3+ nanoparticles. Based on the first principle calculations using an ideal model of Zn8Ga16O32 (ZGO) unit, it is found that the formation energy (11.32 eV) using a doped model of Zn8Ga12Ge3O32 unit (i.e. three Ge ions substitute for three Ga ions and one Ga vacancy (VGa′′′) is created in a unit due to the charge compensation effect) is lower than that (13.35 eV) using a doped model of Zn11Ga10Ge3O32 unit (i.e. three Ge ions and three Zn ions substitute for six Ga ions without the charge compensation effect). Combined with the spectral data, we can confirm that, in the case of the same Ge doping concentration (y = 0.75), the formula of ZGGO host is Zn2Ga4-4y/3GeyO8 rather than Zn2+yGa4-2yGeyO8. Based on the above ZGGO (Zn8Ga12Ge3O32) model, it is found that the smaller formation energies (−1.49 to −3.34 eV) of the Hf4+ doped systems further prove that high-valence Hf4+ ions prefer to substitute for octahedral and tetrahedral Ga3+ and Ge4+ sites and promote the creation of Zn’Ga–Ga°Zn and Ga°Zn defects and it leads to the improved afterglow. Furthermore, contrary to other isovalent ions (such as Nd3+) doped ZGGO nanoparticles, ZGGO:Cr3+,Hf4+x nanoparticles exhibit excellent dispersion stability in an acidic fluid. These findings suggest ZGGO:Cr3+,Hf4+x nanoparticles might be potential for bioimaging in an acidic body fluid.

Effect of p-block metal doping on the optical properties of blue-phosphorene phase monolayer GeSe

Expanding the optical absorption of blue-phosphorene phase GeSe and designing it as infrared light driven photocatalysts and photodetectors is of particular significance. In this work, the mechanism of p-block metals (Al, Ga, In, An, Pb, Sb and Bi) doping enhancing the optical performance of monolayer blue-phosphorene phase GeSe was investigated. The effect of all p-block metal doping on the lattice constant of GeSe is very weak. Pb doped system has the smallest formation energy (−4.82 eV). Except for Sn and Pb, other p-block metal doping can introduce deep impurity levels into the band gap of monolayer GeSe, and they can significantly absorb infrared light, especially Ga doped systems. This is due to the emergence of impurity levels that assist the photon transition. The results of this work provide guidance for promoting the application of blue-phosphorene phase GeSe in the field of photocatalysts and photodetectors.

Transition metal doped and strain engineered VS−MoS2 as electrocatalysts for N2 electroreduction: A DFT study

S vacancy activated MoS2 (VS−MoS2) materials are greatly attractive as electrocatalysts in electrocatalytic N2 reduction reaction (NRR). Herein, we investigate the electrocatalytic NRR performance of transition metal (Cr, Mn, V, W) doped and strain engineered VS−MoS2 using first-principles calculations. For all the designed MoS2s, VS is easy to form due to the small defect formation energy (≤3.48 eV) and determined as active site for N2 adsorption and electrocatalytic NRR. Calculation results show that doping and strain regulate the electrocatalytic activity of VS−MoS2 by altering its electronic properties. When strain is absent, Cr-, Mn-, and V-VS−MoS2 exhibit better electrocatalytic NRR activity than undoped VS−MoS2, while W-VS−MoS2 shows slightly worse activity. However, the above situation changes with applying strain. Moreover, the catalytic activity of all the catalysts firstly decreases and then increases with increasing strain. Further comprehensive considering limiting potential (UL) and Faraday efficiency (FE), Cr-VS−MoS2 under 7 % strain is identified as the best NRR electrocatalyst with both excellent activity (UL = -0.18 V) and high selectivity (FE = 99.44 %). In addition, 5 % and 6 % strained Mn-VS−MoS2 are also good electrocatalysts for NRR with low UL and (-0.45 V and -0.23 V, respectively) and substantial FE (37.69 % and 19.29 %, respectively). Therefore, the electrocatalytic NRR performance of VS−MoS2 could be optimized through transition metal doping and strain engineering. Relevant experimental explores will be encouraged based on our theoretical findings.

Polarons: Energetics and their structural and electronic effects in ATiO3 perovskite systems

Understanding the formation of a polaron near vacancies in oxides is of vital importance for controlling defect properties and related functionalities. We examine the energetics and local structure of polarons in the vicinity of oxygen vacancies using density functional theory with the Hubbard on-site Coulombic correction. We systematically consider several $\mathrm{ATi}{\mathrm{O}}_{3}$ perovskite systems: cubic $\mathrm{SrTi}{\mathrm{O}}_{3}$ (c-STO), cubic $\mathrm{BaTi}{\mathrm{O}}_{3}$ (c-BTO), cubic $\mathrm{PbTi}{\mathrm{O}}_{3}$ (c-PTO), tetragonal $\mathrm{BaTi}{\mathrm{O}}_{3}$ (t-BTO), and tetragonal $\mathrm{PbTi}{\mathrm{O}}_{3}$ (t-PTO). The polaron formation energies vary by several orders of magnitude across the systems, where PTO systems have large polaron formation energies $(\ensuremath{\sim}\ensuremath{-}1.688\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$, c-BTO is intermediate (\ensuremath{-}0.234 eV), and c-STO and t-BTO have considerably smaller formation energies $(<\ensuremath{-}0.01\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$. The formation of a polaron is found to influence the charge density, atomic displacement, and electronic structure, with higher polaron energies corresponding to larger charge density changes and greater displacements of the ions surrounding the oxygen vacancy, especially the displacement of the first nearest neighbor A-site. Finally, a comparison of the projected band structures shows fictitious in-gap states present in the delocalized picture of the system moving to the conduction and valence bands in the polaron solution of the system, indicating an incompleteness of the description of a vacancy without considering charge localization as in the polaron picture. In this paper, we provide fundamental properties of polarons near vacancies, which can be used to control and tune defect, charge transport, magnetoelectric, and multiferroic properties of perovskite oxides.

Designed two dimensional transition metal borides (TM2B12): Robust ferromagnetic half metal and antiferromagnetic semiconductor

Two dimensional transition metal borides have been attracting broad interest due to its rich electronic and magnetic properties. Here, using first-principles calculations, we predict two transition metal boride monolayers, Cr2B12 and Mn2B12, composed of B12 icosahedra and transition metal atoms. It is found that both structures are thermodynamically stable with large cohesive energies and small formation energies. The Cr2B12 monolayer is a ferromagnetic (FM) quasi-half metal, and the Mn2B12 monolayer is an antiferromagnetic (AFM) semiconductor with a bandgap of 0.41 eV. The critical temperature is found to be 145 and 135 K for the Cr2B12 monolayer and the Mn2B12 monolayer, respectively. Moreover, the electronic and magnetic properties of both systems can be tuned by applying external strains. Upon applying biaxial tensile/compressive strain, the (half metallic) bandgap of both systems increases/decreases, and a quasi-half metal to half metal transition is found for the Cr2B12 monolayer under 5% tensile and 4% compressive strain. Furthermore, the critical temperatures of both systems are found to increase with compressive strain and decrease with tensile strain, which reaches 165 and 510 K for the Cr2B12 monolayer and the Mn2B12 monolayer under 5% compressive strain, respectively. The results provide a strategy for designing 2D transition metal borides with potential applications in electronic and spintronic devices.

Preparation and thermoelectric properties of Sc-doped Ti<sub>1–<i>x</i></sub>NiSb half-Heusler alloys

The nominal composition TiNiSb with 19 valence electrons is demonstrated to be composed of off-stoichiometric half-Heusler phase and impurities. In this work, the Ti<sub>1–<i>x</i></sub>NiSb (<i>x</i> = 0, 0.10, 0.15, 0.20, 0.25) samples are prepared by ball milling and spark plasma sintering. The single-phase Ti<sub>0.9</sub>NiSb sample, deviating from the theoretical composition Ti<sub>0.75</sub>NiSb base on 18-electron rule, is obtained, which might be ascribed to the small defect formation energy of Ti filling the vacancy as well as our ball-milling preparation method. With the single-phase Ti<sub>0.9</sub>NiSb sample used as the base material, a small amount of Sc is used to partially replace Ti in order to further reduce the carrier concentration. Thus, the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb (<i>x</i> = 0.10, 0.15; <i>y</i> = 0.03, 0.05) samples are designed to investigate the effect of Sc doping on the thermoelectric properties. The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analysis confirm the single-phase nature of the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb samples. Energy-dispersive X-ray spectroscopy (EDS) results indicate that the actual compositions of the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb samples are consistent well with their nominal compositions, and all elements are distributed uniformly in the sample. Moreover, the doping of Sc can increase the content of Ti vacancy while maintaining the single-phase structure, which could be attributed to the higher binding energy between Sc and Sb because the electronegativity of Sc is less than that of Ti. Both the substitution of Sc for Ti and the increase of the Ti vacancies significantly reduce the carrier concentration, which decreases from ~13.6 × 10<sup>21</sup> cm<sup>–3</sup> for Ti<sub>0.9</sub>NiSb to ~3.4 × 10<sup>21</sup> cm<sup>–3</sup> for Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb. The reduced carrier concentration results in greatly increased Seebeck coefficient, therefore the Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb sample achieves a power factor as high as 17.7 μW·cm<sup>-1</sup>·K<sup>-2</sup> at 973 K. Although the lattice thermal conductivity of Sc-doped sample increases slightly due to the reduction of electron–phonon scattering and the enhancement of chemical bonds, the total thermal conductivity decreases dramatically due to the electronic thermal conductivity decreasing greatly. Finally, the Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb sample reaches a <i>ZT</i> value of ~0.42 at 973 K, which is 180% higher than that of Ti<sub>0.9</sub>NiSb sample. Despite the fact that the thermoelectric performance of our sample is still inferior to those of the state-of-the-art off-stoichiometric 19-electron half-Heusler alloys, this work demonstrates that the thermoelectric performance of Ti<sub>1–<i>x</i></sub>NiSb can be further improved by non-isoelectronic doping.

Understanding effect of distortions and vacancies in wurtzite AlScN ferroelectric memory materials: Vacancy-induced multiple defect state types and relaxation dependence in transition energy levels

Energy-efficient compact alternatives to fully digital computing strategies could be achieved by implementations of artificial neural networks (ANNs) that borrow analog techniques. In-memory computing based on crossbar device architectures with memristive materials systems that execute, in an analog way, multiply-and-accumulate operations prevalent in ANN is a notable example. Ferroelectric (FE) materials are promising candidates for achieving ANN thanks to their excellent down-scalability, improved electrical control, and high energy efficiency. However, it remains challenging to develop a crossbar device architecture using FE materials. The difficulty stems from decreasing the leakage current of FE hardware and, simultaneously, reducing the film thickness for achieving compact systems. Here, we have performed density-functional-theory calculations to investigate the electronic, energy-based, and structural signatures of wurtzite FE material Al0.75Sc0.25N with a nitrogen vacancy (VN) in different charge states. We find that VN can introduce two defect states, viz., the singlet state above the valence band maximum (VBM) and a triplet state below the conduction band minimum in wurtzite AlScN models. The calculations reveal that the group of transition levels E3+/2+/E2+/1+ with small formation energies occur at ∼0.78/1.03 eV above the VBM in the wurtzite AlScN with a relaxed configuration, which may shift by a large degree to lower energy levels if atoms surrounding the defect are not fully relaxed. Theoretical studies elucidate the vacancy-enhanced increase in the leakage current utilizing large AlScN supercells. These findings render atomistic insights that can provide a path forward for the design of next-generation portable low-power electronic systems.

Janus Ga2SSe nanotubes as efficient photocatalyst for overall water splitting

Using sunlight to decompose water into hydrogen and oxygen is one of the most important ways to solve the current global environmental issues and energy problems. In this paper, we use density functional theory to predict the photocatalytic performance of Janus Ga2SSe nanotubes (JGSSe NTs) for the first time. The result shows that the small formation energy and strain energy ensure the stability of the nanotubes. Compared with monolayers, the visible light absorption range of JGSSe NTs is wider, and the large radius (>26.60 A) nanotubes all meet the hydrolysis potential. Surprisingly, the hole mobility of JGSSe NT was estimated to be as high as 2.89 × 104 cm2 V−1 S−1. In conclusion, JGSSe nanotubes are expected to be an excellent photocatalyst due to their low electron–hole recombination rate, high hole mobility, solar absorption in the visible light range, and good oxidation capacity. In addition, the nanotube band gap can be effectively regulated by applying strain. It is hoped that our research will provide meaningful progress in the development of novel and efficient photocatalysts. We hope that our research will provide a possible way to develop novel and efficient photocatalysts.

Enhanced hydrogen evolution reaction performance of MoS2 by dual metal atoms doping

Molybdenum disulfide (MoS2) has been considered a promising high-efficiency, low-cost hydrogen evolution reaction (HER) catalyst in acidic and alkaline media. However, the lack of active sites in the basal plane become the most significant obstacle hindering the widespread application of MoS2. Here, we systematically studied the HER performance of MoS2 plane or edge by co-doping Co atom and other 3d transition metals (TM = Ti–Fe, Ni) by density functional theory calculation methods. Interestingly, the dual atoms doping in both the basal plane and edges of MoS2 is a feasible fabrication with small or negative formation energies. Compared with the pristine MoS2 electrocatalyst, the HER performance in these doped systems is largely enhanced in both basal plane and edges due to the effective charge regulation on the S site by dual atom doping. Remarkably, close to zero H adsorption free energy (ΔGH = −0.161–0.119 eV) is identified for the TM-Co co-doped MoS2 basal, indicating that they are potential alternate HER electrocatalysts of Pt. Our study provides a new strategy to design highly efficient non-noble metal electrocatalysts accessibility for energy-related applications.

First-principles study on the effect of biaxial strain on the carrier lifetime and absorption spectrum redshift of (S, Se, Te) double-doped ZnO

Considering the mismatch of lattice constants between the substrate and the (S, Se, Te) double-doped ZnO system and of the thermal expansion coefficients between the substrate and the doped system, the doped system was subject to external strain. Previous studies have neglected this issue. In the current research, we first applied biaxial strain on the model and then performed density functional theory calculation. The effects of applying different strains on the stability of the doped system, the red shift of the absorption spectrum, the trap effect, and the carrier lifetime were determined. The research results showed that Zn36SSeO34, Zn36STeO34, and Zn36SeTeO34 systems had smaller formation energies and better stability under the Zn-rich condition than under the O-rich condition. All doped systems had negative cohesive energies and good stability. With the synergistic effect of strain and doping, when the biaxial compressive strain of the Zn36STeO34 system was − 5%, the hole–electron separation was good, the trap effect was substantial, the hole life increased, the absorption spectrum had a better red shift, and the oxidation reaction improved. These findings had a certain theoretical guiding effect on photocatalysts for the decomposition of water for oxygen production.

Computational investigation of structural, magnetic, electronic and optical properties of the cluster Mn-X6 (X=P, S, Cl, Br or Te) doped monolayer WSe2

• The Mn-S 6 doped monolayer WSe 2 under the W-rich condition is the most stable. • The Mn-S 6 , Mn-Cl 6 and Mn-Br 6 doped monolayer WSe 2 systems become magnetic half-metal. • The Mn-P 6 and Mn-Te 6 doped monolayer WSe 2 systems turn into magnetic semiconductor. • The Mn-X 6 doped monolayer WSe 2 systems exhibit red shift and enhanced absorption. We perform systematic first-principles calculations to clarify the effects of the cluster Mn-X 6 (X=P, S, Cl, Br or Te) doping on the structural, electronic, magnetic and optical properties of monolayer WSe 2 . The smaller formation energy shows that all doping systems are more stable under W-rich condition than Se-rich condition. The cluster Mn-S 6 doped monolayer WSe 2 has the smallest formation energy may be due to S and Se elements having similar electronegativity, while the maximum formation energy of the cluster Mn-Te 6 doping may be due to the lowest electronegativity of the Te element. Although pure monolayer WSe 2 is a non-magnetic semiconductor, the Mn-P 6 and Mn-Te 6 doped monolayer WSe 2 become magnetic semiconductor, while the Mn-S 6 , Mn-Cl 6 and Mn-Br 6 doped monolayer WSe 2 turn into magnetic half-metal. Compared with the pure monolayer WSe 2 , the cluster Mn-X 6 doped monolayer WSe 2 have not only a red-shift phenomenon but also an enhanced absorption in infrared and far-ultraviolet region and thus a potential application in infrared and far-ultraviolet light detectors.

Optimization of Doping CdTe with Group-V Elements: A First-Principles Study

Group-V substitution at the $\mathrm{Te}$ site, ${X}_{\mathrm{Te}}$ (X = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$), under $\mathrm{Cd}$-rich conditions is an effective way to enhance the hole density and, in the meantime, suppresses the dominant nonradiative carrier recombination center in $\mathrm{Cd}\mathrm{Te}$, thus, improving the performance of $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells. However, it is not clear which group-V dopant, X, is the most effective dopant, because it is expected that ${\mathrm{P}}_{\mathrm{Te}}$ will have the shallowest acceptor level due to its high electronegativity, whereas ${\mathrm{Sb}}_{\mathrm{Te}}$ will have the smallest formation energy due to its small size mismatch with $\mathrm{Te}$. Our systematic first-principles study shows that the hole concentration contributed by the acceptor ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ is limited by the related compensating $A{X}^{+}$ center that increases simultaneously with ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ as the chemical potential of dopant X increases. However, the ratio of ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ acceptors to the $A{X}^{+}$ donors can be significantly increased if the sample is grown at high temperature and then annealed to room temperature, achieving a high hole density and low Fermi level $({E}_{F})$. We find that all group-V ($\mathrm{P}$, $\mathrm{As}$, and $\mathrm{Sb}$) dopings can achieve maximum hole densities of about ${10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which are consistent with previous experimental results. Despite the relatively deep acceptor level of 150 meV, $\mathrm{Sb}$ doping can achieve a considerable hole density due to the low formation energy of substituting $\mathrm{Sb}$ for $\mathrm{Te}$ with similar atomic radii. $\mathrm{P}$ doping can achieve a higher hole density than that of $\mathrm{Sb}$ due to its shallow transition energy at \ensuremath{\epsilon}(0/\ensuremath{-}) = 70 meV. However, the highest hole density is achieved through $\mathrm{As}$ doping, which is attributed to its balanced defect level at \ensuremath{\epsilon}(0/\ensuremath{-}) = 80 meV and relatively small formation energy.

Influence of (Li/Na/K) doping and point defect (VAl, Hi) on the magnetic and photocatalytic performance of AlN: A first-principles study

Although extensive reports on the influence of Li/Na/K doping on the physical properties of AlN systems exist, the effects of Li/Na/K doping and point defects (VAl, Hi) coexisting in AlN systems on magneto-optical properties are often neglected. Experimentally, Li/Na/K doping and point defects (VAl, Hi) cannot be precisely controlled. In this study, the generalized gradient approximation (GGA) plane wave ultrasoft pseudopotential + U method based on spin density functional theory is adopted to solve the abovementioned problem. The first-principles method is utilized to study the effects of point defects (VAl, Hi) and Li/Na/K doping on the photocatalytic performance of AlN systems. Compared with the Al34MN36 (M = Li/Na/K) system, the Al34MHiN36 (M = Li/Na/K) system presents reduced formation energy and improved stability. The Al34NaHiN36 system has the smallest formation energy and the highest stability. In particular, compared with all doped systems, the Al34NaHiN36 system has the best absorption spectrum redshift, the best carrier activity, the longest carrier lifetime, and the strongest oxidation capacity, that is, the best photocatalytic performance. The first-principles study indicates that N atoms in all Al34MN36 (M = Li/Na/K) and Al34MHiN36 (M = Li/Na/K) systems have acceptor and donor characteristics. The magnetic source of all doped systems is related to the double exchange of carriers as mediators, the itinerant electron polarization of unpaired N2− atoms and electric polarization. This study has certain theoretical guidance for the experimental design and preparation of new AlN magneto-optical functional materials.

First-principles XANES simulation for oxygen-related defects in Si-O amorphous materials

To understand the chemical reaction during the initial charging in an amorphous negative SiO (a-SiO) electrode for Li-ion batteries, we simulated the O K-edge X-ray absorption near-edge structure (XANES) spectra for oxygen-excess defects in silicon oxide crystals and glasses using the first-principles projector augmented-wave (PAW) method. Oxygen-excess defects, including the oxygen bridge-bonded (OBB) and peroxy linkage (POL) configurations, reproduce the pre-edge peak observed in the experimental spectrum of the as-prepared a-SiO material. It was found that the electronic states contributing to the pre-edge peaks are the σ∗ state of the O-O bond for the OBB and POL configurations, and the π∗ state of the O-O bond for an O2 molecule. The formation energy of these defects is distributed within the amorphous structure, and POL configurations with small formation energies were found in a-SiO. In addition, O2 molecules can exist in a-SiO2, whereas those in a-SiO are incorporated into the bond network. Through the insertion of Li+ ions, the pre-edge peak disappears, and weak peaks appear in the higher-energy region. These correspond to the experimental results in which the pre-edge peak in the O K-edge XANES of the as-prepared a-SiO disappeared during the initial charging. The chemical reaction between POL configurations in a-SiO and Li+ ions during the initial charging is thought to play an important role in improving cycle characteristics.

First-principles of Be/Mg/Ca doping and point defects of VZn and Hi in the magnetic and optical properties of ZnO

The effects of Be/Mg/Ca doping and the coexistence of Zn vacancies in the magnetic and optical properties of ZnO have been widely reported, but the previous studies have neglected the influence of the H interstitial on the doping system. The source and mechanism of magnetism caused by O ions in the doping system remain unclear, and no reasonable theoretical explanation has been proposed thus far. As a point defect, Zn vacancies are hard to precisely control in experiments. In view of solving this problem, this study uses the first principles under the framework of density functional theory to investigate the effects of the single doping of Be, Mg, and Ca and the coexistence of Zn vacancies and H interstitial in the magnetic and optical properties of ZnO. Findings indicate that the bulk moduli of all doped systems are reduced with respect to that of the undoped Zn36O36. Apart from the O2− ions, some O1− ions exist in all doping systems. The O1− ions containing itinerant electrons are the main source of O ion magnetism in the doping system. The formation energy of the Zn34MHiO36 (M = Be/Mg/Ca) system is lower than that of the Zn34MO36 (M = Be/Mg/Ca) system. Findings indicate that the doping of the H interstitial can reduce the formation energy of the Zn34MO36 (M = Be/Mg/Ca) system, allowing the system to become even more stable. The Zn34BeHiO36 system has the smallest formation energy. The Zn34BeHiO36, Zn34MgHiO36, and Zn34CaHiO36 systems all exhibit magnetism. The carriers’ activity in the photocatalyst, the visible light effect, the separation and lifetime of the carriers, and the photocatalytic oxidation ability are also considered in this research. Compared with the other configurations, the Zn34MgHiO36 system is more stable than the other systems, and it has a longer electron lifetime, stronger electric dipole moment, more obvious absorption spectral red shift, and stronger reduction ability. Therefore, the Zn34MgHiO36 system is the best choice for photocatalysis in H production. This research offers certain theoretical reference value for the design and preparation of new magneto-optical functional materials.

A density functional theory and neutron diffraction study of the ambient condition properties of sub-stoichiometric yttrium hydride

Several mechanical and thermophysical properties are required as a function of non-stoichiometry for the successful implementation of YH2-x for nuclear reactor moderator applications. Density functional theory calculations, in combination with neutron diffraction experiments, were used to study the structural and mechanical properties of YH2-x. Point defect analysis indicated H occupation primarily at the tetrahedral site within an fcc Y sub-lattice, confirming a fluorite YH2 structure. The small positive formation energy for H vacancies under Y-rich conditions predicted that hypo-stoichiometry is accommodated by Y+YH2 at ambient conditions and by H vacancies in the YH2-x single phase that is relevant to high temperatures. Neutron diffraction studies were used to confirm both the occupation of H on tetrahedral sites and the near-stoichiometric composition of the hydride phase in the two-phase Y+YH2 region of the phase diagram that dominates at room temperature. Energy minimized special-quasirandom-structures of H vacancies were used to calculate lattice parameters, elastic constants, and several other properties as a function of composition for the single phase YH2-x. The lattice parameter of YH2-x decreased by only 0.004 Å with increasing H/Y for the range 1.31≤H/Y≤2.0 indicating a negligible effect on lattice parameters due to vacancy formation. In the two-phase region, however, calculations predicted the density of zY+(1−z)YH2−x to increase with decreasing H/Y at lower temperatures due to the increased fraction of high-density Y metal. For the high temperature single phase, decreasing H/Y reduced the density as a consequence of the lattice expansion associated with vacancy formation. All elastic constants and moduli increased with increasing hydrogen content in single-phase YH2-x.

First-principles study on the dissolution and diffusion properties of hydrogen in α-Al2O3

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. α-Al2O3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk α-Al2O3. The calculation results show that the most energetically stable form of hydrogen in a perfect α-Al2O3 crystal under H2 gas annealing treatment is the H2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in α-Al2O3, these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in α-Al2O3 is 2Hi, with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H2 molecule. We have derived the temperature-dependent diffusivity of the H2 molecule in α-Al2O3 as D(T)=(3.65×10−7m2/s)exp(−2.27eV/KT), which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H2 molecule are so high that dissolution and diffusion of the H2 molecule in α-Al2O3 are very difficult, resulting in low hydrogen permeability.

Titanium-doped MoS2 monolayer as highly efficient catalyst for hydrogen evolution reaction

The effects of Ti dopants on the catalytic activities of the hydrogen evolution reaction (HER) in monolayer MoS2 basal plane were investigated using density functional theory. Our study shows that Ti dopants that substituted Mo atoms in MoS2 have small formation energy, and the complex dopant structures involving multiple Ti sites are energetically more stable than the isolated ones. Doping Ti atoms greatly improves the catalytic performances of MoS2 with a near-ideal hydrogen-adsorption Gibbs free energy. Projected density of states analysis revealed that the mechanism for the improvement of catalytic activity is due to formation of the density of states near the Fermi energy level upon increasing dopant concentrations. It can be expected from the current results that excellent catalytic performance can be obtained by atomic composition modifications, such as Ti doping in the HER reaction.

Modulating geometric, electronic, gas sensing and catalytic properties of single-atom Pd supported on divacancy and N-doped graphene sheets

The formation geometries, gas sensing and catalytic properties of single Pd atom supported on divacancy graphene (555-777-graphene-Pd) and Nx-doped graphene (Nx-graphene-Pd, x = 1, 2, 3) are systemically investigated using the first-principle calculations. Compared with 555-777-graphene-Pd, the formation of Nx-graphene-Pd configurations have the smaller formation energies and the N3-graphene-Pd is more easily formed than other ones. The N3-graphene-Pd substrate exhibits the higher adsorption sensitivity for detecting gas reactants than that on 555-777-graphene-Pd. Besides, the single gas molecule and two coadsorbed gas molecules can control the electronic structures and magnetic properties of single-atom Pd anchored graphene systems. In the sequential oxidation reactions for NO and CO, the termolecular Eley–Rideal (TER) reaction as initial state has the relatively lower energy barriers (<0.2 eV) on N3-graphene-Pd than that of Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms. Moreover, these oxidation reactions (TER, ER and LH) through the rate-limiting steps have the small energy barriers (<0.7 eV), which demonstrates that the doped Pd and N atoms within graphene may be an efficient catalyst for toxic gases removal.