In Situ Phase-Transformation Forming a Bicontinuous (Ni, Co, Ti, and Al)-Oxide/Li2CO3 Interpenetrating Network Electrolyte for Solid Oxide Fuel Cells.

Designing heterogeneous electrolytes with superior interface charge transfer is promising for low-temperature solid oxide fuel cells (LT-SOFCs). However, a rational construction with optimal interfaces to maximize ionic conduction remains a challenge. Here an in situ phase-transformation strategy is demonstrated to prepare a highly conductive heterogeneous electrolyte. A pristine LiNiO2-TiO2 nanocomposite precursor undergoes chemical reactions and phase-transformation upon heating and feeding H2, destroying the original phases, and forming new species, including an amorphous Li2CO3 scaffold within a (Ni, Co, Al, and Ti)-oxide (NCAT) matrix. It creates an intertwining and continuous network inside the electrolyte with plentiful interfaces. The in situ formed NCAT/Li2CO3 heterogeneous electrolyte displays superior ionic conductivity and impressive fuel cell performance. This work emphasizes the potential of rational heterogeneous structure design and interface engineering for LT-SOFC electrolyte through an in situ phase-transform approach. The generated interfaces enhance ion transport, presenting an opportunity for further optimizing electrolyte candidates, and lowering the operating temperatures of SOFCs.

Nanoscale-Femtosecond Imaging of Evanescent Surface Plasmons on Silver Film by Photon-Induced Near-Field Electron Microscopy.

Surface plasmons on silver nanostructures have a broad range of tunable resonance properties in visible and near-infrared regimes, which possess wide applications in nanophotonics and optoelectronics. Here we use a femtosecond laser to excite surface plasmons on a silver film and trace the subsequent transient dynamics via photon-induced near-field electron microscopy (PINEM). A polarization experiment of PINEM demonstrates a conspicuous polarization dependence of the transient surface plasmon field on the silver film; however, unlike silver nanowires and nanorods, there is no polarization dependence for the PINEM intensity. This compelling finding suggests a thin film platform can be more easily used to identify the temporal and spatial overlaps between the pump laser and probe electron pulses in 4D ultrafast electron microscopy (UEM). Our work illustrates the femtosecond excitation and transient behavior of the surface plasmons on silver film and paves a universal, simple way for identifying the time zero in 4D UEM.

Enriching Nano-Heterointerfaces in Proton Conducting TiO2-SrTiO3@TiO2 Yolk-Shell Electrolyte for Low-Temperature Solid Oxide Fuel Cells.

A challenging task in solid oxide fuel cells (SOFCs) is seeking for an alternative electrolyte, enabling high ionic conduction at relatively low operating temperatures, i.e., 300-600 °C. Proton-conducting candidates, in particular, hold a significant promise due to their low transport activation energy to deliver protons. Here, a unique hierarchical TiO2-SrTiO3@TiO2 structure is developed inside an intercalated TiO2-SrTiO3 core as "yolk" decorating densely packed flake TiO2 as shell, creating plentiful nano-heterointerfaces with a continuous TiO2 and SrTiO3 "in-house" interfaces, as well the interfaces between TiO2-SrTiO3 yolk and TiO2 shell. It exhibits a reduced activation energy, down to 0.225 eV, and an unexpectedly high proton conductivity at low temperature, e.g., 0.084 S cm-1 at 550 °C, confirmed by experimentally H/D isotope method and proton-filtrating membrane measurement. Raman mapping technique identifies the presence of hydrogenated HO─Sr bonds, providing further evidence for proton conduction. And its interfacial conduction is comparatively analyzed with a directly-mixing TiO2-SrTiO3 composite electrolyte. Consequently, a single fuel cell based on the TiO2-SrTiO3@TiO2 heterogeneous electrolyte delivers a good peak power density of 799.7 mW cm-2 at 550 °C. These findings highlight a dexterous nano-heterointerface design strategy of highly proton-conductive electrolytes at reduced operating temperatures for SOFC technology.

Femtosecond-resolved imaging of a single-particle phase transition in energy-filtered ultrafast electron microscopy.

Using an energy filter in transmission electron microscopy has enabled elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respectively. Here, we use an energy filter in ultrafast electron microscopy to enhance the temporal resolution toward the domain of atomic motion. Visualizing transient structures with femtosecond temporal precision was achieved by selecting imaging electrons in a narrow energy distribution from dense chirped photoelectron packets with broad longitudinal momentum distributions and thus typically exhibiting picosecond durations. In this study, the heterogeneous ultrafast phase transitions of vanadium dioxide (VO2) nanoparticles, a representative strongly correlated system, were filmed and attributed to the emergence of a transient, low-symmetry metallic phase caused by different local strains. Our approach enables electron microscopy to access the time scale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale.

Nonequilibrium Phonon Dynamics and Its Impact on the Thermal Conductivity of the Benchmark Thermoelectric Material SnSe.

Thermoelectric materials play a vital role in the pursuit of a sustainable energy system by allowing the conversion of waste heat to electric energy. Low thermal conductivity is essential to achieving high-efficiency conversion. The conductivity depends on an interplay between the phononic and electronic properties of the nonequilibrium state. Therefore, obtaining a comprehensive understanding of nonequilibrium dynamics of the electronic and phononic subsystems as well as their interactions is key for unlocking the microscopic mechanisms that ultimately govern thermal conductivity. A benchmark material that exhibits ultralow thermal conductivity is SnSe. We study the nonequilibrium phonon dynamics induced by an excited electron population using a framework combining ultrafast electron diffuse scattering and nonequilibrium kinetic theory. This in-depth approach provides a fundamental understanding of energy transfer in the spatiotemporal domain. Our analysis explains the dynamics leading to the observed low thermal conductivity, which we attribute to a mode-dependent tendency to nonconservative phonon scattering. The results offer a penetrating perspective on energy transport in condensed matter with far-reaching implications for rational design of advanced materials with tailored thermal properties.

Influence of strain on an ultrafast phase transition.

The flexibility of 2D materials combined with properties highly sensitive to strain makes strain engineering a promising avenue for manipulation of both structure and function. Here we investigate the influence of strain, associated with microstructural defects, on a photo-induced structural phase transition in Td-WTe2. Above threshold photoexcitation of uniform, non-strained, samples result in an orthorhombic Td to a metastable orthorhombic 1T* phase transition facilitated by shear displacements of the WTe2 layers along the b axis of the material. In samples prepared with wrinkle defects WTe2 continue its trajectory through a secondary transition that shears the unit cell along the c axis towards a metastable monoclinic 1T' phase. The time scales and microstructural evolution associated with the transition and its subsequent recovery to the 1T* phase is followed in detail by a combination of ultrafast electron diffraction and microscopy. Our findings show how local strain fields can be employed for tailoring phase change dynamics in ultrafast optically driven processes with potential applications in phase change devices.

Manipulation of Stacking Order in Td-WTe2 by Ultrafast Optical Excitation.

Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td-WTe2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td-WTe2 following ultrafast photoexcitation by time-resolved electron diffraction. A 0.23 THz shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ∼1 mJ/cm2 result in formation, with an ∼5 ps time constant, of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm. We demonstrate that the excitation of the shear phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by observation of a transition also in samples with a pinned shear phonon. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where transient local disorder is prominent before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of how optical fields can be employed to manipulate the stacking order in 2-dimensional transition metal dichalcogenides.

Novel K2Ti8O17 Anode via Na+/Al3+ Co-Intercalation Mechanism for Rechargeable Aqueous Al-Ion Battery with Superior Rate Capability.

A promising aqueous aluminum ion battery (AIB) was assembled using a novel layered K2Ti8O17 anode against an activated carbon coated on a Ti mesh cathode in an AlCl3-based aqueous electrolyte. The intercalation/deintercalation mechanism endowed the layered K2Ti8O17 as a promising anode for rechargeable aqueous AIBs. NaAc was introduced into the AlCl3 aqueous electrolyte to enhance the cycling stability of the assembled aqueous AIB. The as-designed AIB displayed a high discharge voltage near 1.6 V, and a discharge capacity of up to 189.6 mAh g-1. The assembled AIB lit up a commercial light-emitting diode (LED) lasting more than one hour. Inductively coupled plasma-optical emission spectroscopy (ICP-OES), high-resolution transmission electron microscopy (HRTEM), and X-ray absorption near-edge spectroscopy (XANES) were employed to investigate the intercalation/deintercalation mechanism of Na+/Al3+ ions in the aqueous AIB. The results indicated that the layered structure facilitated the intercalation/deintercalation of Na+/Al3+ ions, thus providing a high-rate performance of the K2Ti8O17 anode. The diffusion-controlled electrochemical characteristics and the reduction of Ti4+ species during the discharge process illustrated the intercalation/deintercalation mechanism of the K2Ti8O17 anode. This study provides not only insight into the charge-discharge mechanism of the K2Ti8O17 anode but also a novel strategy to design rechargeable aqueous AIBs.

Influence of cathode geometry on electron dynamics in an ultrafast electron microscope.

Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.

Localized committed differentiation of neural stem cells based on the topographical regulation effects of TiO2 nanostructured ceramics.

In this study, a porous-flat TiO2 micropattern was fabricated with flat and nanoporous TiO2 ceramics for investigating the effect of topography on neural stem cell (NSC) differentiation. This finding demonstrates that localized committed differentiation could be achieved in one system by integrating materials with different topographies.

High-Energy Faceted SnO2-Coated TiO2 Nanobelt Heterostructure for Near-Ambient Temperature-Responsive Ethanol Sensor.

A SnO2 gas sensor was prepared by a two-step oxidation process whereby a Sn(II) precursor was partially oxidized by a hydrothermal process and the resulting Sn3O4 nanoplates were thermally oxidized to yield SnO2 nanoplates. The SnO2 sensor was selective and responsive toward ethanol at a temperature as low as 43 °C. This low sensing temperature stems from the rapid charge transport within SnO2 and from the presence of high-energy (001) facets available for oxygen chemisorption. SnO2/TiO2 nanobelt heterostructures were fabricated by a similar two-step process in which TiO2 nanobelts acted as support for the epitaxial growth of intermediate Sn3O4. At temperatures ranging from 43 to 276 °C, the response of these branched nanobelts is more than double the response of SnO2 for ethanol detection. Our observations demonstrate the potential of low-cost SnO2-based sensors with controlled morphology and reactive facets for detecting gases around room temperature.

Synthesis of scaly Sn3O4/TiO2 nanobelt heterostructures for enhanced UV-visible light photocatalytic activity.

A novel scaly Sn3O4/TiO2 nanobelt heterostructured photocatalyst was fabricated via a facile hydrothermal route. The scaly Sn3O4 nanoflakes can be synthesized in situ and assembled on surface coarsened TiO2 nanobelts through a hydrothermal process. The morphology and distribution of Sn3O4 nanoflakes can be well-controlled by simply tuning the Sn/Ti molar ratio of the reactants. Compared with single phase nanostructures of Sn3O4 and TiO2, the scaly hybrid nanobelts exhibited markedly enhanced photoelectrochemical (PEC) response, which caused higher photocatalytic hydrogen evolution even without the assistance of Pt as a co-catalyst, and enhanced the degradation ability of organic pollutants under both UV and visible light irradiation. In addition to the increased exposure of active facets and broad light absorption, the outstanding performance is ascribed to the matching energy band structure between Sn3O4 and TiO2 at the two sides of the heterostructure, which efficiently reduces the recombination of photo-excited electron-hole pairs and prolongs the lifetime of charge carriers. Both photocatalytic assessment and PEC tests revealed that Sn3O4/TiO2 heterostructures with a molar ratio of Sn/Ti of 2/1 exhibited the highest photocatalytic activity. This study provides a facile and low-cost method for the large scale production of Sn3O4 based materials in various applications.