Enhancing Dissociative Adsorption of Water on Cu(111) via Chemisorbed Oxygen

We have used X-ray photoelectron spectroscopy to study the dehydrogenation of H₂O molecules on the clean and oxygenated Cu(111) surfaces. The clean surface does not show reactivity toward H₂O dehydrogenation. By contrast, H₂O molecules on the oxygenated Cu(111) dissociate into OH species by reacting with chemisorbed oxygen until the complete consumption of the chemisorbed oxygen at which the surface loses its reactivity toward H₂O dehydrogenation. Increasing the temperature to 200 °C and above decreases molecularly adsorbed H₂O for dehydrogenation, thereby resulting in less loss of chemisorbed O. In conjunction with density-functional theory calculations, a three-step reaction pathway is proposed to account for the chemisorbed O assisted dehydrogenation of H₂O molecules and the net loss of surface oxygen. These results provide insight into understanding the elemental steps of the dehydrogenation of H₂O molecules and the controllable conditions for tuning H₂O dissociation on metal surfaces

Atomic-Step-Induced Local Nonequilibrium Effects on Surface Oxidation

By temperature-, time-, and pressure-resolved imaging of the dynamics of surface steps on NiAl(100) during its oxidation, we provide direct evidence of the significant effects of atomic steps in controlling the local thermodynamic driving force for oxidation. Our results show that the inherent barriers associated with step crossing by surface species of oxygen cause a heterogeneous oxygen concentration across the crystal surface, giving rise to <i>local</i> nonequilibrium effects governing oxidation even for surfaces that are <i>globally</i> in equilibrium. The asymmetry in the step-crossing barriers for oxygen atoms crossing up or down steps is such that descendant steps exert a local driving force that favors oxidation, whereas ascendant steps locally destabilize the surface oxide in their vicinity. The local differences in the thermodynamic driving force for oxidation due to atomic steps and step bunches give rise to novel phenomena, such as nonmonotonous oxide growth and the net translation motion of surface oxide stripes by growing on one end while receding on the other end

Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

Tuning the Deoxygenation of Bulk-Dissolved Oxygen in Copper

Using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy, we report the tuning of the deoxygenation process of bulk dissolved oxygen in copper via a combination of H₂ gas flow and elevated temperature. We show that a critical temperature of ∼580 °C exists for driving segregation of bulk dissolved oxygen to form chemisorbed oxygen on the Cu surface, which subsequently reacts with hydrogen to form OH species and then H₂O molecules that desorb from the surface. This deoxygenation process is tunable by a progressive stepwise increase of temperature that results in surface segregation of oxygen from deeper regions of bulk Cu. Using atomistic simulations, we show that the bulk-dissolved oxygen occupies octahedral sites of the Cu lattice and the deoxygenation process involves oxygen migration between octahedral and tetrahedral sites with a diffusion barrier of ∼0.5 eV

Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi_0.80Co_0.15Al_0.05O₂ Cathode Particles

Layered LiNi_0.80Co_0.15Al_0.05O₂ (NCA) holds great promise as a potential cathode material for high energy density lithium ion batteries. However, its high capacity is heavily dependent on the stability of its layered structure, which suffers from a severe structure degradation resulting from a not fully understood layered → spinel phase transformation. Using high-resolution transmission electron microscopy and electron diffraction, we probe the atomic structure evolution induced by the layered → spinel phase transformation in the NCA cathode. We show that the phase transformation results in the development of a particle structure with the formation of complete spinel, spinel domains, and intermediate spinel from the surface to the subsurface region. The lattice planes of the complete and intermediate spinel phases are highly interwoven in the subsurface region. The layered → spinel transformation occurs via the migration of transition metal (TM) atoms from the TM layer into the lithium layer. Incomplete migration leads to the formation of the intermediate spinel phase, which is featured by tetrahedral occupancy of TM cations in the lithium layer. The crystallographic structure of the intermediate spinel is discussed and verified by the simulation of electron diffraction patterns

Atomically Visualizing Elemental Segregation-Induced Surface Alloying and Restructuring

Using in situ transmission electron microscopy that spatially and temporally resolves the evolution of the atomic structure in the surface and subsurface regions, we find that the surface segregation of Au atoms in a Cu­(Au) solid solution results in the nucleation and growth of a (2 × 1) missing-row reconstructed, half-unit-cell thick L1₂ Cu₃Au­(110) surface alloy. Our in situ electron microscopy observations and atomistic simulations demonstrate that the (2 × 1) reconstruction of the Cu₃Au­(110) surface alloy remains as a stable surface structure as a result of the favored Cu–Au diatom configuration

Coincidence-Site-Lattice Twist Boundaries in Bicrystalline α‑Fe₂O₃ Nanoblades

Bicrystals are usually artificially designed to correlate the coincidence-site-lattice (CSL) configurations with the grain boundary property. Here, we report on CSL twist boundaries in bicrystalline α-Fe₂O₃ nanoblades, possessing only three distinct Σ values of 7, 13, and 19. The existence of CSL boundaries with various Σ values in the two-dimensional (2D) α-Fe₂O₃ nanoblades provides a good opportunity to investigate the effects of different grain boundaries on their physical properties. It is shown that the electrical resistivity of individual nanoblade decreases with increasing Σ values of the CSL boundaries. Such 2D nanoblades may have practical appeal because their 2D geometries facilitate integration into devices with realistic pathways to manufacturing

<i>In Situ</i> Atomic-Scale Probing of the Reduction Dynamics of Two-Dimensional Fe₂O₃ Nanostructures

Atomic-scale structural dynamics and phase transformation pathways were probed, <i>in situ,</i> during the hydrogen-induced reduction of Fe₂O₃ nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe₂O₃ → γ-Fe₂O₃ phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe₂O₃ and γ-Fe₂O₃. The progression of the phase transformation into the other half of the bicrystalline Fe₂O₃ across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe₂O₃ with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe₃O₄. Further reduction resulted in the coexistence of Fe₃O₄, FeO, and Fe <i>via</i> the transformation pathway Fe₃O₄ → FeO → Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides <i>via</i> tuning the phase transformation pathway

<i>In Situ</i> Atomic-Scale Probing of the Reduction Dynamics of Two-Dimensional Fe₂O₃ Nanostructures

Atomic-scale structural dynamics and phase transformation pathways were probed, <i>in situ,</i> during the hydrogen-induced reduction of Fe₂O₃ nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe₂O₃ → γ-Fe₂O₃ phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe₂O₃ and γ-Fe₂O₃. The progression of the phase transformation into the other half of the bicrystalline Fe₂O₃ across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe₂O₃ with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe₃O₄. Further reduction resulted in the coexistence of Fe₃O₄, FeO, and Fe <i>via</i> the transformation pathway Fe₃O₄ → FeO → Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides <i>via</i> tuning the phase transformation pathway