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Low-temperature activation of oxide semiconductor materials such as In-Ga-Zn-O (IGZO) is a key approach for their utilization in flexible devices. We previously reported that the activation temperature can be reduced to 150 °C by hydrogen-doped IGZO (IGZO:H), demonstrating a strong potential of this approach. In this paper, we investigated the mechanism for reducing the activation temperature of the IGZO:H films. In situ Hall measurements revealed that oxygen diffusion from annealing ambient into the conventional Ar/O2-sputtered IGZO film was observed at >240 °C. Moreover, the temperature at which the oxygen diffusion starts into the film significantly decreased to 100 °C for the IGZO:H film deposited at hydrogen gas flow ratio (R[H2]) of 8%. Hard X-ray photoelectron spectroscopy indicated that the near Fermi level (EF) defects in the IGZO:H film after the 150 °C annealing decreased in comparison to that in the conventional IGZO film after 300 °C annealing. The oxygen diffusion into the film during annealing plays an important role for reducing oxygen vacancies and subgap states especially for near EF. X-ray reflectometry analysis revealed that the film density of the IGZO:H decreased with an increase in R[H2] which would be the possible cause for facilitating the O diffusion at low temperature.

The scale-up deployment of ruthenium (Ru)-based oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane water electrolysis (PEMWE) is greatly restricted by the poor stability. As the lattice-oxygen-mediated mechanism (LOM) has been identified as the major contributor to the fast performance degradation, impeding lattice oxygen diffusion to inhibit lattice oxygen participation is imperative, yet remains challenging due to the lack of efficient approaches. Herein, we strategically regulate the bonding nature of Ru-O towards suppressed LOM via Ru-based high-entropy oxide (HEO) construction. The lattice disorder in HEOs is believed to increase migration energy barrier of lattice oxygen. As a result, the screened Ti 23 Nb 9 Hf 13 W 12 Ru 43 O x exhibits 11.7 times slower lattice oxygen diffusion rate, 84% reduction in LOM ratio, and 29 times lifespan extension compared with the state-of-the-art RuO 2 catalyst. Our work opens up a feasible avenue to constructing stabilized Ru-based OER catalysts towards scalable application.

Legumes establish endosymbiotic associations with nitrogen-fixing rhizobia, which they host inside root nodules. Here, specific physiological and morphological adaptations, such as the production of oxygen-binding leghemoglobin proteins and the formation of an oxygen diffusion barrier in the nodule periphery, are essential to protect the oxygenlabile bacterial nitrogenase enzyme. The molecular basis of the latter process remains elusive as the identification of required genes is limited by the epistatic effect of nodule organogenesis over nodule infection and rhizobia accommodation. We overcame this by exploring the phenotypic diversity of Lotus japonicus accessions that uncouple nodule organogenesis from nodule infection when inoculated with a subcompatible Rhizobium strain. Using comparative transcriptomics, we identified genes with functions associated with oxygen homeostasis and deposition of lipid polyesters on cell walls to be specifically up-regulated in infected compared to noninfected nodules. As hydrophobic modification of cell walls is pivotal for creating diffusion barriers like the root endodermis, we focused on two Fatty acyl-CoA Reductase genes that were specifically activated in the root and/or in the nodule endodermis. Mutant lines in a Fatty acyl-CoA Reductase gene expressed exclusively in the nodule endodermis had decreased deposition of polyesters on this cell layer and increased nodule permeability compared to wild-type plants. Oxygen concentrations were significantly increased in the inner cortex of mutant nodules, which correlated with reduced nitrogenase activity, and impaired shoot growth. These results provide the first genetic evidence for the formation of the nodule oxygen diffusion barrier, a key adaptation enabling nitrogen fixation in legume nodules.

Discovering new energy materials is a key step toward satisfying the needs for next-generation energy conversion and storage devices. Among the various types of oxides, Ruddlesden–Popper (RP) oxides (A2BO4) are promising candidates for electrochemical energy devices, such as solid oxide fuel cells, owing to their attractive physicochemical properties, including the anisotropic nature of oxygen migration and controllable stoichiometry from oxygen excess to oxygen deficiency. Thus, understanding and controlling the kinetics of oxygen transport are essential for designing optimized materials to use in electrochemical energy devices. In this review, we first discuss the basic mechanisms of oxygen migration in RP oxides depending on oxygen nonstoichiometry. We then focus on the effect of changes in the defect concentration, crystallographic orientation, and strain on the oxygen migration in RP oxides. We also briefly review their thermal and chemical stability. Finally, we conclude with a perspective on potential research directions for future investigation to facilitate controlling oxygen ion migration in RP oxides.

Summary 17 I. Introduction 17 II. Oxygen diffusion and the barriers to gas exchange 18 III. Metabolic indicators of seed hypoxia 19 IV. High-resolution mapping of oxygen distribution in the developing seed 20 V. The effect of environmental factors on steady-state oxygen concentrations 23 VI. Gene expression and metabolism under low oxygen 24 VII. Mechanisms of oxygen sensing and balancing in seeds 25 VIII. Might low internal oxygen levels be advantageous for seed development? 27 Acknowledgements 27 References 28 Recent applications of oxygen-sensitive microsensors have demonstrated steep oxygen gradients in developing seeds of various crops. Here, we present an overview on oxygen distribution, major determinants of the oxygen status in the developing seed and implications for seed physiology. The steady-state oxygen concentration in different seed tissues depends on developmental parameters, and is determined to a large extent by environmental factors. Photosynthetic activity of the seed significantly diminishes hypoxic constraints, and can even cause transient, local hyperoxia. Changes in oxygen availability cause rapid adjustments in mitochondrial respiration and global metabolism. We argue that nitric oxide (NO) is a key player in the oxygen balancing process in seeds, avoiding fermentation and anoxia in vivo. Molecular approaches aiming to increase oxygen availability within the seed are discussed.

Cyanobacterial diazotrophs are considered to be the most important source of fixed N₂ in the open ocean. Biological N₂ fixation is catalyzed by the extremely O₂-sensitive nitrogenase enzyme. In cyanobacteria without specialized N₂-fixing cells (heterocysts), mechanisms such as decoupling photosynthesis from N₂ fixation in space or time are involved in protecting nitrogenase from the intracellular O₂ evolved by photosynthesis. However, it is not known how cyanobacterial cells limit O₂ diffusion across their membranes to protect nitrogenase in ambient O₂-saturated surface ocean waters. Here, we explored all known genomes of the majormarine cyanobacterial lineages for the presence of hopanoid synthesis genes, since hopanoids are a class of lipids that might act as an O₂ diffusion barrier. We found that, whereas all non−heterocyst-forming cyanobacterial diazotrophs had hopanoid synthesis genes, none of the marine Synechococcus, Prochlorococcus (non−N₂-fixing), and marine heterocyst-forming (N₂-fixing) cyanobacteria did. Finally, we conclude that hopanoid-enriched membranes are a conserved trait in non−heterocyst-forming cyanobacterial diazotrophs that might lower the permeability to extracellular O₂. This membrane property coupled with high respiration rates to decrease intracellular O₂ concentration may therefore explain how non−heterocyst-forming cyanobacterial diazotrophs can fix N₂ in the fully oxic surface ocean.

The mixed ionic and electronic oxide LaNi0.6Fe0.4O3−δ (LNF) is a promising ceramic cathode material for solid oxide fuel cells. Since the reaction rate of oxygen interaction with the cathode material is extremely important, the present work considers the oxygen exchange mechanism between O2 and LNF oxide. The kinetic dependence of the oxygen/oxide interaction has been determined by two isotopic methods using 18O-labelled oxygen. The application of the isotope exchange with the gas phase equilibrium (IE-GPE) and the pulsed isotope exchange (PIE) has provided information over a wide range of temperatures (350–800 °C) and oxygen pressures (10–200 mbar), as each method has different applicability limits. Applying mathematical models to treat the kinetic relationships, the oxygen exchange rate (rH, atom × cm−2 × s−1) and the diffusion coefficient (D, cm2/s) were calculated. The values of rH and D depend on both temperature and oxygen pressure. The activation energy of the surface exchange rate is 0.73 ± 0.05 eV for the PIE method at 200 mbar, and 0.48 ± 0.02 eV for the IE-GPE method at 10–20 mbar; for the diffusion coefficient, the activation energy equals 0.62 ± 0.01 eV at 10–20 mbar for the IE-GPE method. Differences in the mechanism of oxygen exchange and diffusion on dense and powder samples are observed due to the different microstructure and surface morphology of the samples. The influence of oxygen pressure on the ratio of contributions of different exchange types to the total oxygen exchange rate is demonstrated. For the first time, the rate-determining step in the oxygen exchange process for LNF material has been identified. This paper discusses the reasons for the difference in the mechanisms of oxygen exchange and diffusion.

AbstractAn impedance model for a metal surface corroding under a thin electrolyte layer is presented. The model describes the oxygen diffusion in the electrolyte, the cathodic current via the oxygen reduction reaction (ORR) reaction and the anodic current via metal dissolution reaction (MDR) at the metal/electrolyte interface under the pseudo-steady approximation. The results for the impedance are obtained in terms of the thickness of electrolyte, the diffusion coefficient of oxygen, the concentration of dissolved oxygen and the anodic/cathodic reaction rates. The impedance characteristic are analysed through Bode and Nyquist plots which unveils six distinctive frequency regimes viz., (i) purely oxygen diffusion controlled regime, (ii) electrolyte film thickness controlled regime, (iii) activation controlled regime, (iv) mixed diffusion-kinetic controlled regime, (v) capacitive electric double layer controlled regime and (vi) solution Ohmic controlled regime. The impedance response shows two asymmetrical depressed arc on the Nyquist plots indicating the Faradaic charge transfer controlled regime and purely electrolyte thickness diffusion controlled regime with an intervening straight Warburg line. The arc at low frequencies is strongly dependent on the concentration and diffusion coefficient of dissolved oxygen indicative of pseudo-steady state behaviour of interface whereas the high frequency arc represents Faradaic regimes due to MDR and ORR which is indicative of the dynamic nature of corrosion reaction rates at the interface. At thick electrolyte layer, the interface shows a mass transport controlled kinetic regime with a finite length Warburg type impedance whereas at thin electrolyte layer the interface is activation controlled with a finite diffusion Randles type impedance response. A comparison of the model with the experimental data of corroding metal shows reasonable agreement.Graphical abstractAtmospheric corrosion of metal in contact with a thin electrolyte film via diffusion of dissolved oxygen coupled with oxygen reduction reaction and metal dissolution reaction in pseudosteady state.

This study focuses on the diffusion of oxygen in titanium alloys during high-temperature oxidation. In particular, the model used to obtain thermokinetic coefficients from microhardness profiles was investigated. A literature review shows that microhardness profiles are modeled by a simple error function in the same way as oxygen concentration profiles obtained by microprobe analysis (EPMA). The analysis of literature shows that the hypothesis of a linear relationship between microhardness and oxygen content is not true over the entire oxygen concentration range and that a parabolic relationship is empirically more accurate. A new modeling equation taking into account this parabolic law is proposed as well as a simplified and easier to use form. The relative error of the diffusion coefficients obtained using the simplified equation was then determined. This new model was applied to the experimental microhardness profile of a Ti-6242s sample oxidized at 625 °C. The resulting oxygen diffusion coefficient is in excellent agreement with the one determined from EPMA profile using the classic error function model. Finally, other data from literature were analyzed with the new model to obtain an Arrhenius diagram of oxygen diffusivity in Ti-64 alloy between 550 and 850 °C. This diagram gives thermokinetic coefficients D0=1.1×10-5exp-191kJ/molRT that are close to those reported for pure α-Ti in the temperature range 550–850 °C.

Thermal ageing of cross-linked polyethylene (XLPE) cable insulation is an important issue threatening the safe operation of power cables. In this paper, thermal ageing of XLPE was carried out at 160 °C in air for 240 h. The influence of oxygen diffusion on thermal ageing of XLPE was investigated by Ultraviolet–visible spectrophotometer (UV–Vis), tensile testing, and Fourier transformed infrared spectroscopy (FTIR). It was observed that the degradation degree not only depended on ageing time but also on sample positions. The thermally aged samples were more oxidized in the surface region, presented a darker color, more carbon atoms appeared in the conjugate cluster, had smaller elongation at break and tensile strength, and a larger carbonyl index. As ageing time increased, the non-uniform oxidation of the XLPE samples became more prominent. The degree of non-uniform oxidation caused by oxygen diffusion was quantitatively studied by first order oxidation kinetic. The calculated results demonstrated that carbonyl index measured by FTIR was more sensitive to non-uniform oxidation with a shape parameter in the range of 1–2. The result shown in this paper is helpful for interpreting and predicting the non-uniform ageing behavior of high voltage XLPE cables.