Native defects in monolayer GaS and GaSe: Electrical properties and thermodynamic stability

Structural, electronic, and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab initio calculations. Based on their charge transition levels we assess the influence of the studied defects on the electrical properties of the monolayers. Specifically, we show that native defects do not behave as shallow dopants and their presence cannot account for the experimentally observed intrinsic doping. In addition, we predict that native defects are efficient compensation and recombination centers. Besides pointing out their detrimental nature, we also calculate the corresponding finite-temperature formation energies and provide a window of growth conditions able to reduce the concentration of all relevant native defects

Surface reactivity and cation non-stoichiometry in BaZr_1−xY_xO_3−δ (x = 0–0.2) exposed to CO₂ at elevated temperature

The reactivity of BaZr1−xYxO3−δ (x = 0–0.2) ceramics under 1 atm CO2 at 650 °C for up to 1000 h was investigated in order to elucidate possible degradation processes occurring when the material is applied as a proton-conducting electrolyte in electrochemical devices. The annealed ceramics were characterized by a range of techniques (SEM, TEM, GIXRD, XPS and SIMS) with respect to changes in the phase composition and microstructure. Formation of BaCO3 was observed on the surfaces of the annealed samples and the amount increased with time and was higher for the Y-doped compositions. The subsurface regions were found to be deficient in Ba and, in the case of the Y-doped compositions, enriched in Y in two distinct chemical states as identified by XPS. First-principles calculations showed that they were Y residing on the Zr and Ba-sites, respectively, and that local enrichment of Y both in bulk and on the surface attained a structure similar to Y2O3. Overall, it was substantiated that the reaction with CO2 mainly proceeded according to a defect chemical reaction involving transfer of Y to the Ba-site and consumption of BaZrO3 formula units. It was suggested that a similar degradation mechanism may occur in the case of Ba(OH)2 formation under high steam pressure conditions

Robust nanocomposites of a-Fe2O3 and N-doped graphene oxide: Interfacial bonding and chemisorption of H2O

Nanocomposites of α-Fe2O3 (hematite) and (N-doped) graphene oxide (GO) were investigated using first-principles calculations with focus on structure, chemical bonding, electronic structure and H2O adsorption. The nanocomposites were modeled as the interface between the α-Fe2O3 (0 0 0 1) surface and the basal plane of reduced graphene oxide, comprising epoxy groups (C:O ratio of 8) as well as graphitic and pyridinic nitrogen doping. The composite structures exhibited strong chemical bonding by the formation of a bridging Fe–O–C bond. The calculated binding energy between the materials was −0.56 eV per Fe–O–C bond for GO and up to −1.14 eV for N-doped GO, and the binding energies were found to correlate with the charge of the bridging oxide ion. The composites exhibited partly occupied carbon states close to or above the α-Fe2O3 valence band maximum. Dissociative adsorption of H2O was found to be more exothermic for the composites compared to the individual materials, ranging from about −0.9 to −1.7 eV for the most stable configurations with hydroxide species adsorbed to GO and protons forming NH groups or adsorbed to the α-Fe2O3 surface.publishedVersio

Influence of Ce3+ polarons on grain boundary space-charge in proton conducting Y-doped BaCeO3

Defect segregation and space-charge formation were investigated for a (0 2 1)[1 0 0] symmetric tilt grain boundary in Y-doped BaCeO3. Density functional theory calculations according to the PBE+U formalism were used to calculate segregation energies for protons, oxygen vacancies and Y-acceptor dopants from the bulk to the grain boundary core. Defect concentration and potential profiles across the grain boundary were obtained from thermodynamic space-charge models. Oxygen vacancies were found to exhibit a particularly exothermic segregation energy of up to −1.66 eV while protons exhibited segregation energies in the range of −0.47 eV to −0.93 eV. The grain boundary was determined to be predominated by protons below 800 K in 3% H2O and the corresponding space-charge potential was 0.4–0.7 V under the Mott–Schottky approximation. The role of electronic defects in the space-charge properties was evaluated, and it was substantiated that electron conduction along the grain boundary could become evident under reducing conditions.publishedVersio

Proton segregation and space-charge at the BaZrO3 (0 0 1)/MgO (0 0 1) heterointerface

Y-doped BaZrO3 (BZY) can be deposited epitaxially on MgO (0 0 1) and the considered interface serves as a model system for studying heterointerface properties of protonic conductors. In this study, the defect chemistry of the interface between ZrO2-terminated BaZrO3 (0 0 1) and MgO (0 0 1) was investigated by first-principles calculations and space-charge theory. Segregation energies from the BZY bulk to the interface ZrO2 and MgO layers were calculated for effectively charged protons, oxygen vacancies, Y-acceptors as well as cation vacancies. Protons were found to exhibit a strong tendency for segregating to the interface, particularly to an oxide ion in the MgO layer, rendering a net positive charge of the interface. According to the applied thermodynamic space-charge models, the interface potential could reach more than 1 V under the Mott-Schottky approximation, with depletion regions extending up to 2 nm into BZY. With fully equilibrated Y-segregation profiles, the interface potential was significantly diminished to about 0.2 V at 573 K and 0.025 bar H2O. While the interface was found to be close to saturated by protons under most condition, it was concluded that proton conduction along the interface could not contribute significantly to the in-plane conductivity of BZY films deposited on MgO substrate.acceptedVersio

Adsorption of CO2 and Facile Carbonate Formation on BaZrO3 Surfaces

The adsorption of CO2 and CO on BaZrO3 (0 0 1) was investigated by first-principles calculations with a focus on the BaO termination. CO2 was found to strongly chemisorb on the surface by formation of carbonate species with an adsorption enthalpy of up to −2.25 eV at low coverage and −1.05 eV for a full monolayer. An adsorption entropy of −8.8 × 10–4 eV K–1 was obtained from the vibrational properties of the adsorbates. Surface coverages were evaluated as a function of temperature and CO2 partial pressure, and the obtained coverage under 1 bar CO2 was more than 0.8 at 1000 K (conditions relevant for steam methane reforming). The fully saturated surface was stable up to about 400 K under ambient atmosphere, i.e., 400 ppm of CO2. The initial stage of BaCO3 formation was evaluated according to migration of barium to the carbonate overlayer, which was found to result in a significant stabilization of the system. The barium migration was found to be essentially unobstructed with a barrier of only ∼5 meV. In light of the stability of carbonate adsorbates at the surface, the prospect of bulk dissolution of carbonate species was evaluated but ultimately found to be negligible in acceptor-doped BaZrO3.acceptedVersio

Robust nanocomposites of a-Fe2O3 and N-doped graphene oxide: Interfacial bonding and chemisorption of H2O

Nanocomposites of α-Fe2O3 (hematite) and (N-doped) graphene oxide (GO) were investigated using first-principles calculations with focus on structure, chemical bonding, electronic structure and H2O adsorption. The nanocomposites were modeled as the interface between the α-Fe2O3 (0 0 0 1) surface and the basal plane of reduced graphene oxide, comprising epoxy groups (C:O ratio of 8) as well as graphitic and pyridinic nitrogen doping. The composite structures exhibited strong chemical bonding by the formation of a bridging Fe–O–C bond. The calculated binding energy between the materials was −0.56 eV per Fe–O–C bond for GO and up to −1.14 eV for N-doped GO, and the binding energies were found to correlate with the charge of the bridging oxide ion. The composites exhibited partly occupied carbon states close to or above the α-Fe2O3 valence band maximum. Dissociative adsorption of H2O was found to be more exothermic for the composites compared to the individual materials, ranging from about −0.9 to −1.7 eV for the most stable configurations with hydroxide species adsorbed to GO and protons forming NH groups or adsorbed to the α-Fe2O3 surface

Thermochemically stable ceramic compositemembranes based on Bi2O3 for oxygen separationwith high permeability

Ceramic oxygen separation membranes can be utilized to reduce CO2 emissions in fossil fuel power generation cycles based on oxy-fuel combustion. State-of-the-art oxygen permeable membranes based on Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) offer high oxygen permeability but suffer from long-term instability, especially in the presence of CO2. In this work, we present a novel ceramic composite membrane consisting of 60 vol% (Bi0.8Tm0.2)2O3−δ (BTM) and 40 vol% (La0.8Sr0.2)0.99MnO3−δ (LSM), which shows not only comparable oxygen permeability to that of BSCF but also outstanding long-term stability. At 900 °C, oxygen fluxes of 1.01 mL min−1 cm−2 and 1.33 mL min−1 cm−2 were obtained for membranes with thicknesses of 1.35 mm and 0.75 mm, respectively. Moreover, significant oxygen fluxes were obtained at temperatures down to 600 °C. A stable operation of the membrane was demonstrated with insignificant changes in the oxygen flux at 750 °C for approx. one month and at 700 °C with 50% CO2 as the sweep gas for more than two weeks.publishedVersio

Nanocomposites of few-layer graphene oxide and alumina by density functional theory calculations

The atomistic and electronic structure and oxygen stoichiometry of nanocomposites between alumina and graphene oxide were investigated by density functional theory calculations. The nanocomposite was described as interfaces between α-Al2O3 (0001) surfaces and graphene oxide; the latter was defined with oxygen bound as epoxy groups and a C:O atomic ratio of 4:1. The optimized composite structure with 1–3 layers of graphene oxide in between Al2O3 contains bridging Alsingle bondOsingle bondC bonds at the interface. Reduction of the composite was investigated by removal of oxygen from the interface Alsingle bondOsingle bondC bonds, within the graphene oxide layers and in Al2O3. It was found that removal of oxygen within the graphene oxide layers is essentially independent of the Al2O3 interface, i.e., the same as in pure graphene oxide. Oxygen was, however, more strongly bound in the interface Alsingle bondOsingle bondC bonds by 0.80 eV, and reduction of graphene oxide to graphene is accordingly preferred within the graphene oxide layers rather than at the oxide interface.acceptedVersio