Hydrogen Vacancies Research Articles

Preparation and luminescence of a new violet blue phosphor derived from proton-type zirconium phosphate

A violet blue phosphor was prepared by thermal treatment of HZr2(PO4)3 at 700°C in a reducing atmosphere and its PL was investigated. The phosphor shows violet blue emissions at around 392nm when excited by 254nm UV light. Its absorptivity, internal and external quantum efficiency are found to be 43, 41, and 18%, respectively, at room temperature when excited at 254nm. Analyses using non-dispersive infrared absorption, X-ray fluorescence, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction suggest that such an emission originates from the formation of hydrogen and oxygen vacancies in the phosphor during the thermal treatment of HZr2(PO4)3.

Sequestration of carbon dioxide in coal: Energetics and bonding from first-principles calculations

Sequestration of CO2 in unmineable coalbeds is one of the attractive ways being investigated to reduce the CO2 concentration in the atmosphere. Extensive experimental studies have demonstrated that storage of CO2 molecules in coal seam involves both chemisorption and physisorption, but the underlying mechanism at the atomic scale has remained elusive. We report results of first-principles density-functional-theory calculations for the interaction between molecular CO2 and coal. The calculations show that chemisorption of CO2 in defective coal is possible, and CO2 can form chemical bonding with defective coal, particularly at the defects involving carbon and hydrogen vacancies and epoxy functional groups. On the other hand, formation of chemical bonding of CO2 with defect-free coal network is unlikely.

Water Oxidation Catalysis with Fe2O3 Constrained at the Nanoscale

Photoelectrochemical cells containing iron(III) oxide (Fe2O3) have attracted extensive investigations due to their ability to convert solar energy into chemical energy by water splitting. Recently, fabrication of nanoscaled Fe2O3 has been adopted for photoelectrochemical cells to increase solar energy absorption and reduce slow diffusion length of charge carriers. To understand how nanoscaled confinement influences catalytic efficiency, we perform density functional theory + U calculations of water oxidation on a thin slab of Fe2O3(0001). We consider possible hydrogen vacancies that may appear at high pH and voltage and find that promoting hydrogen vacancy formation improves catalytic efficiency. We also analyze the effect of geometrical strain on the slab that may result from deposition on a substrate. We conclude that nano-Fe2O3 should be grown on a substrate with a similar lattice constant to reduce strain and improve catalytic efficiency.

Oxygen Vacancy Formation, Mobility, and Hydrogen Pick-up during Oxidation of Zirconium by Water

A comprehensive first principles understanding of the oxidation of zirconium alloys by water was reiterated. Two channels were taken to jointly constitute to the oxidation process: one according to classical oxidation theory involving hydrogen evolution and the second reflected by inwards transport of protons causing hydrogen pick-up. The two were associated with charged and uncharged oxygen vacancies, respectively. The purpose of the present study was to clarify the nature of the effective anode during oxidation of zirconium as to the detailed role of the metal. Oxygen dissolution in the alloy resulted in a “pre-anodic” property associated with the formation of oxygen vacancy VO in the oxide, i.e., preceding VO2+/2e− separation. Atomistic perspective on the metal/oxide interface before nucleation of VO was provided. The rapid convergence of the model interface to bulk properties in spite of the local structural variability provided new insight as to the nature of an amorphous metal/oxide interface.

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2.

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2 (low α: converted metal fraction and dα/dt) and the reasons for the occurrence of the incubation period at 320, 350, and 400 °C were investigated. Pre-existing Mg crystallites can enhance Mg nucleation during the incubation period, as suggested by the estimated activation energy for nucleation (12 ± 2 kJ per mol H). The released H-atoms enter MgH2 as interstitials, as indicated by the MgH2 unit-cell contraction, resulting in increased equatorial Mg-H bond length, decreased charge-density distribution in the interstitial region, as observed from the charge-density maps, and decreased H-H distance in the {001} plane up to the midway of the incubation period. Eventually, hydrogen vacancies are created, as indicated by the red shift in the Eg and A1g peaks of Raman spectra. The high estimated activation energy for the growth of Mg (209 ± 8 kJ per mol H) renders it difficult and explains the reason for the presence of an incubation period.

Hydrogen divacancy diffusion: a new perspective on H migration in MgH2 materials for energy storage.

The formation and diffusion of pairs of hydrogen vacancies (divacancies) in magnesium hydride is modeled using density functional theory. Compared to the commonly studied case of single hydrogen vacancies, it is found that divacancies are energetically favored over two isolated vacancies. Also, as a function of the diffusion axis considered, the calculated diffusion barriers of divacancies are either smaller or of comparable magnitude with respect to the diffusion barriers of a single vacancy. These findings shed new light on hydrogen transport in MgH2, which is of crucial importance to understand the kinetics of hydrogen take-up and release in this storage material.

Enhanced Water Oxidation Catalysis of Nickel Oxyhydroxide through the Addition of Vacancies

Enhancing catalytic efficiency for the oxygen evolution reaction (OER) is of extreme importance for the future of sustainable energy. One of the best-performing catalysts reported to date is nickel oxyhydroxide (NiOOH). However, during operating conditions, NiOOH has varying hydrogen content. To understand how hydrogen vacancies affect catalytic efficiency, we use density functional theory + U calculations that model oxygen evolution reaction catalysis for NiOOH with hydrogen vacancies. Our calculations reveal that these defects destabilize the surface by altering local oxidation states and therefore reduce the overpotential. Our results agree with a very recent experiment that shows enhanced OER activity of NiOOH upon deprotonation.

Bubble growth from clustered hydrogen and helium atoms in tungsten under a fusion environment

Bubbles seriously degrade the mechanical properties of tungsten and thus threaten the safety of nuclear fusion devices, however, the underlying atomic mechanism of bubble growth from clustered hydrogen and helium atoms is still mysterious. In this work, first-principles calculations are therefore carried out to assess the stability of tungsten atoms around both hydrogen and helium clusters. We find that the closest vacancy-formation energies of interstitial hydrogen and helium clusters are substantially decreased. The first-nearest and second-nearest vacancy-formation energies close to vacancy–hydrogen clusters decrease in a step-like way to ∼0, while those close to vacancy–helium clusters are reduced almost linearly to ∼−5.46 eV when atom number reaches 10. The vacancy-formation energies closest to helium clusters are more significantly reduced than those nearest to hydrogen clusters, whatever the clusters are embedded at interstitial sites or vacancies. The reduction of vacancy-formation energies results in instability and thus emission of tungsten atoms close to interstitial helium and vacancy–helium clusters, which illustrates the experimental results, that the tungsten atoms can be emitted from the vicinity of vacancy–helium clusters. In addition, the emission of unstable tungsten atoms close to hydrogen clusters may become possible once they are disturbed by the environment. The emission of tungsten atoms facilitates the growth and evolution of hydrogen and helium clusters and ultimately the bubble formation. The results also explain the bubble formation even if no displacement damage is produced in tungsten exposed to low-energy hydrogen and helium plasma.

Refinement of the structure of hydrogen–vacancy complexes in titanium by the Rietveld method

The VT1-0 titanium alloy (phase α-Ti) with various hydrogen and hydrogen-vacancy concentrations has been studied. The stability of the 32-atom Ti–nV–mH supercell (n is the number of the V vacancies, and m is the number of hydrogen atoms H) with varying numbers of vacancies and hydrogen atoms has been calculated from the first principles. The structural state of the α-Ti phase has been identified by the Rietveld method based on the calculations of the supercell stability and the data on the defect concentration obtained using positron spectroscopy. The complete structural information on the considered states of the α-Ti phase (the lattice parameters, spatial distribution of titanium and hydrogen atoms and vacancies) has been obtained.

Interaction of Hydrogen Atoms with Vacancies and Divacancies in bcc Iron

The paper presents the results of both ab initio and thermodynamic analysis of vacancy and divacancy formation and hydrogen interaction with them in alpha (bcc) iron. Ab initio calculations were performed by DFT method using LAPW in WIEN2k package. Monovacancy formation energy was found to be 2.15 eV and divacancy binding energy 0.22 ± 0.01 eV. Equlibrium fraction of vacancies bound into divacancies is of the order of 10–5 even at the highest temperatures close to bcc → fcc transformation point. Hydrogen has a strong interaction with monovacancies (vacancy-hydrogen binding energy decreasing from 0.60 to 0.31 eV for the first–fifth H atom inside a single vacancy) but has only a small effect on divacancy formation energy that is equal to 0.28, 0.19 and 0.17 for the case of joining of VH + V, VH + VH and VH2 + VH2, respectively. This means that the presence of hydrogen cannot significantly increase the equilibrium concentration of divacancies.

Surface Chemistry of tert-Butylphosphine (TBP) on Si(001) in the Nucleation Phase of Thin-Film Growth.

We combine density functional theory calculations and scanning tunneling microscopy investigations to identify the relevant chemical species and reactions in the nucleation phase of chemical vapor deposition. tert-Butylphosphine (TBP) was deposited on a silicon substrate under conditions typical for surface functionalization and growth of semiconductor materials. On the activated hydrogen-covered surface H/Si(001) it forms a strong covalent P-Si bond without loss of the tert-butyl group. Calculations show that site preference for multiple adsorption of TBP is influenced by steric repulsion of the adsorbate's bulky substituent. STM imaging furthermore revealed an anisotropic distribution of TBP with a preference for adsorption perpendicular to the surface dimer rows. The adsorption patterns found can be understood by a mechanism invoking stabilization of surface hydrogen vacancies through electron donation by an adsorbate. The now improved understanding of nucleation in thin-film growth may help to optimize molecular precursors and experimental conditions and will ultimately lead to higher quality materials.

High-temperature superconductivity at high pressures for H3SixP1−x, H3PxS1−x, and H3ClxS1−x

Recent experimental and computational works have established the occurrence of superconducting temperatures, Tc, near 200K when the pressure is close to 200GPa in hydrogen-based sulfur compounds. In this work we investigate the effects of phosphorus and chlorine substitutions of sulfur on Tc, as well as the effect of hydrogen vacancies. In addition, we explore the superconductivity-relevant parameters in the H3SixP1−x system.In executing this work we used the virtual crystal approximation and performed a systematic set of linearized augmented plane wave calculations (LAPW) for many different concentrations of the sulfur component. From the densities of states and the scattering phase shifts at the Fermi level, we calculated electron-ion matrix elements and estimated the electron-phonon coupling constants for different concentrations, as well as Tc. We find that the highest value of Tc=197K corresponds to a phosphorus concentration of x=0.15, or 8.85 valence electrons in a H3P0.15S0.85 alloy. From a detailed analysis of the results given by a Gaspari–Gyorffy (GG) determination of the Hopfield parameter, we identify the role of each term in the GG equation that produce the maximum Tc. In addition, we present a non-orthogonal tight-binding parameterization of the band structure of H3S which fits very well with the LAPW results.

Hydrogen-related complexes in Li-diffused ZnO single crystals

Zinc oxide (ZnO) is a wide band gap semiconductor and a potential candidate for next generation white solid state lighting applications. In this work, hydrogen-related complexes in lithium diffused ZnO single crystals were studied. In addition to the well-known Li-OH complex, several other hydrogen defects were observed. When a mixture of Li2O and ZnO is used as the dopant source, zinc vacancies are suppressed and the bulk Li concentration is very high (>1019 cm−3). In that case, the predominant hydrogen complex has a vibrational frequency of 3677 cm−1, attributed to surface O-H species. When Li2CO3 is used, a structured blue luminescence band and O-H mode at 3327 cm−1 are observed at 10 K. These observations, along with positron annihilation measurements, suggest a zinc vacancy–hydrogen complex, with an acceptor level ∼0.3 eV above the valence-band maximum. This relatively shallow acceptor could be beneficial for p-type ZnO.

The interaction between light impurities and vacancies in titanium and aluminum metals: A DFT study

In this paper, we present binding energies between hydrogen (H), carbon (C), nitrogen (N) and oxygen (O) atoms and a vacancy in the hexagonal closed-packed (HCP) lattice of titanium (Ti) and the face centered cubic (FCC) lattice of aluminum (Al), calculated using the density functional theory (DFT). We have also investigated the trapping of up to five hydrogen atoms by a vacancy and the reduction of the vacancy formation energy, due to the formation of a hydrogen–vacancy complex. We used the molecular-dynamics modeling with consecutive relaxation at 0K to obtain an atomic configuration of the vacancy–impurity complex, corresponding to the global energy minimum. According to our calculations, C–V, H–V, C– (H–V), N–(H–V) complexes are stable in the Al lattice with only H–V complex being stable in Ti. The formation of C–(H–V) and N–(H–V) complexes in the Al lattice results in the negative vacancy formation energy. The formation of H–V complex decreases the vacancy formation energy by 0.26eV in the Ti lattice. A vacancy in the Ti lattice can trap up to four hydrogen atoms.

High hydrogen retention in the sub-surfaces of tungsten plasma facing materials: A theoretical insight

A high hydrogen concentration at the sub-surface layers of tungsten plasma facing materials has been previously observed by experiments. We have performed the first-principles calculations to understand this experimental phenomenon. The layer-dependent hydrogen dissolution energies, vacancy formation energies and hydrogen trapping energies are calculated for the W(110) surface. Our results show that hydrogen dissolving into the vacancy-free sub-surfaces is energetically unfavorable. A surface tungsten vacancy is easier to form than in the bulk and has a large trapping effect on hydrogen dissolving. The present work demonstrates the important role of surface vacancies on the hydrogen retention in the tungsten sub-surfaces.

Surface reactivity of Ge[111] for organic functionalization by means of a radical-initiated reaction: A DFT study

The study of interfacial chemistry at semiconductor surfaces has become an important area of research. Functionalities such as molecular recognition, biocompatibility of surfaces, and molecular computing, could be achieved by the combinations of organic chemistry with the semiconductor technology. One way to accomplish this goal is by means of organic functionalization of semiconductor surfaces such as the bulk-terminated germanium surfaces, more specifically the Ge[111]. In this work, we theoretically study, by applying density functional theory, the surface reactivity of the bulk-terminated Ge[111] surface for organic functionalization by means of a radical-initiated reaction of unsaturated molecules such as acetylene, ethylene and styrene with a hydrogen vacancy on a previously hydrogen-terminated Ge[111] surface. Results derived from this work are compared with those obtained in our previous calculations on the germanene surface, following the same chemical route. Our calculations show an accumulation of electronic charge at the H-vacancy having as a result electron pairing due to strong lattice-electron coupling and therefore a diminished surface reactivity. Calculation of the transition states for acetylene and ethylene indicates that the surface reactivity of the hydrogen-terminated Ge[111] surface is less promising than its two-dimensional analogue, the hydrogen-terminated germanene.

Point-defect kinetics in α- and γ-MgH2

The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH2, desorption rates may be higher in the crystalline γ phase compared to the equilibrium bulk α phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856–3865] that this effect is responsible for enhanced desorption from ball-milled MgH2, since smaller particles contain a higher proportion of the metastable γ phase. We investigate hydrogen transport kinetics in these phases of MgH2 by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in γ-MgH2 is smaller by 0.032 eV compared to α-MgH2. Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in γ-MgH2 is 0.02 eV lower than in α-MgH2. We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the γ phase than in the α phase, thus explaining the faster dehydrogenation kinetics of γ-MgH2.

Enhanced oxidation activity from modified ceria: MnOx–ceria, CrOx–ceria and Mg doped VOx–ceria

Ceria is an important component of catalysts for oxidation reactions that proceed through the Mars-van Krevelen mechanism, promoting activity. A paradigm example of this is the VOx–CeO2 system for oxidative dehydrogenation reactions, where vanadium oxide species are supported on ceria and a special synergy between them is behind the enhanced activity: reduction of the catalyst is promoted by ceria undergoing reduction. This leads to favourable oxygen vacancy formation and hydrogen adsorption energies—useful descriptors for the oxidation activity of VOx–CeO2 catalysts. In this paper, we examine if this promoting effect on ceria-based catalysts holds for other metal oxide modifiers and we investigate MnOn– and CrOn–CeO2(111) (n=0−4) as examples. We show, combining density functional theory calculations and statistical thermodynamics that similarly to the vanadia modifier, the stable species in each case is MnO2– and CrO2–CeO2. Both show favourable energetics for oxygen vacancy formation and hydrogen adsorption, indicating that VO2–CeO2 is not the only system of this type that can have an enhanced activity for oxidation reactions. However, the mechanism involved in each case is different: CrO2–CeO2 shows similar properties to VO2–CeO2 with ceria reduction upon oxygen removal stabilising the 5+ oxidation state of Cr. In contrast, with MnO2–CeO2, Mn is preferentially reduced. Finally, a model system of VO2–Mg:CeO2 is explored that shows a synergy between VO2 modification and Mg doping. These results shed light on the factors involved in active oxidation catalysts based on supported metal oxides on ceria that should be taken into consideration in a rational design of such catalysts.

Improvement of Hydrogen Vacancy Diffusion Kinetics in MgH2 by Niobium- and Zirconium-Doping for Hydrogen Storage Applications

Transition metal (TM) catalytic dopants are broadly used in hydrogen storage materials to increase H2 desorption and absorption kinetics. We have studied H vacancy formation energy in pure, Nb- or Zr- doped bulk magnesium hydride using density functional theory based calculations. The preferential dopant location was determined by means of occupation energy analysis. Both TM species prefer substitutional locations, Zr being more stable than Nb. Five different sites for H vacancy formation have been considered, all of them near the dopant. The vacancy formation energy decreases, especially in the interstitial Zr system in comparison with a pure one (from 1.35 to 0.51 eV). Concerning diffusion, we consider four paths in the pure and doped systems: the doping with TMs diminishes the activation energy barriers improving the diffusion kinetics, being more considerable for Zr. Effects of a possible spin polarization induced in the system by TM atoms and H vacancies generation have been considered as well.

Two-dimensional cyanates: stabilization through hydrogenation.

According to first-principles calculations, it should be possible to grow two-dimensional (2D) forms of copper thio-cyanate (CuSCN) and copper seleno-cyanate (CuSeCN) since their energies are only marginally higher than those of their most stable three-dimensional (3D) wurtzite structures. Here we show using the same theoretical approach that chemisorption reactions of hydrogen molecules with the above-mentioned 2D CuSCN and CuSeCN systems enhance their stability as they decrease the energy difference with respect to the corresponding hydrogenated forms of the wurtzite crystals. Hydrogenation causes a sizeable decrease in the energy band gap by 0.56 eV and 0.65 eV for hydrogenated 2D-CuSCN (CuSCNH2) and 2D-CuSeCN (CuSeCNH2), respectively. Finally, we describe the stability of hydrogen vacancies in CuSCNH2 and CuSeCNH2 and show that the presence of isolated single H vacancies or di-vacancies does not affect significantly the electronic properties of the host systems close to the valence and conduction band edges.