Ultrafast Interlayer Electron Transfer in Incommensurate Transition Metal Dichalcogenide Homobilayers

Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe₂ occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS₂ and WSe₂. The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures

Mixing Assisted Direct Formation of Isotactic Poly(1-butene) Form I′ Crystals from Blend Melt of Isotactic Poly(1-butene)/Polypropylene

The influence of mixing of iPB-1/iPP blend on the polymorphism of iPB-1 under processing-relevant conditions is studied with emphasis on the competition between the thermodynamically stable form I′ crystal and the kinetically favored form II. <i>In situ</i> optical microscopy measurements reveal that the upper critical solution temperature (UCST) of iPB-1/iPP blend locates in the range of 180–200 °C. Unexpectedly, by quenching mixed iPB-1/iPP melt down to temperatures below UCST and melting point, form I/I′ can be produced directly which is further identified as form I′ by temperature-dependent WAXS and DSC. The formation of form I′ is promoted by increasing the annealing time above UCST, while is suppresses by raising the quenching temperature. In addition, the crystallization of iPP also displays a similar trend as iPB-1 does. The correlated crystallization of each constituent with dependence on the initial mixing degree suggests that the crystallization behavior of the binary blends is determined by the interplay between simultaneous processes concomitant with the liquid–solid transition. The experimental results reveal the possibility to modify the crystallization pathway of iPB-1 in iPB-1/iPP blend through the mixing degree which is initially controlled by annealing but is subject to evolve during the subsequent thermal treatment. Possible mechanisms are discussed including the roles of phase separation and concentration fluctuation in crystallization

Coupling of Multiscale Orderings during Flow-Induced Crystallization of Isotactic Polypropylene

The sequence and coupling of intra- and interchain orderings in flow-induced crystallization (FIC) of partially cross-linked isotactic polypropylene (iPP) is studied with <i>in situ</i> Fourier transform infrared spectroscopy (FTIR) and synchrotron radiation X-ray scattering techniques, which reveal that multiscale structural intermediates emerge prior to the onset of crystallization. Upon imposing flow, intrachain conformational ordering or coil–helix transition (CHT) occurs first, which is directly correlated with external stress. As helical content is built up at large strain, density fluctuation happens, and sufficient long helices may result in orientation ordering before FIC. The results demonstrate that stress induced intrachain CHT is the essential structural intermediate in FIC, which can be further coupled with interchain orientation and density providing either helical content or length meets the criterions for the phase transitions. We propose that coupling among external stress, intrachain conformational, and interchain orientation and density orderings to be the molecular mechanism for FIC of polymer forming helical structures

Insight into the Structure of Single Antheraea pernyi Silkworm Fibers Using Synchrotron FTIR Microspectroscopy

Synchrotron FTIR (S-FTIR) microspectroscopy was used to monitor both protein secondary structures (conformations) and their orientations in single cocoon silk fibers of the Chinese Tussah silk moth (Antheraea pernyi). In addition, to understand further the relationship between structure and properties of single silk fibers, we studied the changes of orientation and content of different secondary structures in single A. pernyi silk fibers when subjected to different strains. The results showed that the content and orientation of β-sheet was almost unchanged for strains from 0 to 0.3. However, the orientation of α-helix and random coil improved progressively with increasing strain, with a parallel decrease in α-helix content and an increase in random coil. This clearly indicates that most of the deformation upon stretching of the single fiber is due to the change of orientation in the amorphous regions coupled with a conversion of some of the α-helix to random coil. These observations provide an explanation for the supercontraction behavior of certain animal silks and are likely to facilitate understanding and optimization of postdrawing used in the conjunction with the wet-spinning of silk fibers from regenerated silk solutions. Thus, our work demonstrates the power of S-FTIR microspectroscopy for studying biopolymers

Highly Active Cathode Achieved by Constructing Surface Proton Acid Sites through Electronic Regulation of Heteroatoms

For proton-conducting solid oxide fuel cells (PCFCs), accelerating the kinetics of the proton involved oxygen reduction reaction (P-ORR) occurring primarily on the surface of cathodes is of key importance. To this end, developing simple, efficient, and economical strategies to optimize the gas–solid interface is crucial but full of challenges. Herein, the heteroatom boron (B) is first introduced to modify the PCFC cathode surface (Pr4Ni3O10+δ, PN) by mechanical mixing method (0.5B-PN). Combined with in situ/ex situ characterizations and DFT calculation, it is found that the CO2 resistance, surface hydration ability, and surface electrocatalytic activity toward P-ORR are significantly improved by B decoration. Importantly, the B element is found to raise the surface Brønsted acid (−OH) concentration yet depress the surface Lewis acidity, both of which are conducive to P-ORR reaction. At 600 °C, the maximum power density of the cell using 0.5B-PN as the cathode improved by 149.5% compared with that using the PN cathode. This work opens up a new avenue for developing novel PCFC cathodes via nonmetallic regulation of surface

DRIFTS Evidence for Facet-Dependent Adsorption of Gaseous Toluene on TiO₂ with Relative Photocatalytic Properties

Effective adsorption is of great importance to the photocatalytic degradation of volatile organic compounds. Herein, we succeeded in the preparation of anatase TiO₂ with clean dominant {001} and {101} facets. By using <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) equipped with a homemade reaction system and a coupling gas-dosing system, we found that TiO₂ with dominant {001} facets exhibits higher toluene adsorption capacity than TiO₂ with dominant {101} facets, which may be attributed to the different number of unsaturated 5c-Ti capable of forming the main active adsorption sites (terminal Ti–OH species). TiO₂ with dominant {001} facets shows a significantly high photocatalytic degradation performance, with its degradation rate being 6 times higher than that of dominant {101} facets. Combined with simulation results, it is suggested that the synergetic effects of the formation of specific active adsorption sites, the low adsorption energy for toluene, and preservation of the free molecularly adsorbed water on the surface promote the degradation of gaseous toluene on the dominant {001} facets. This study exemplifies that the facet-dependent adsorption of volatile organic compounds is one of the most important factors to effectively engineer photocatalysts for air purification

High Light Yield of Sr₈(Si₄O₁₂)Cl₈:Eu²⁺ under X‑ray Excitation and Its Temperature-Dependent Luminescence Characteristics

In this work, we first investigate the relationship between temperature and lattice parameters by means of Rietveld refinement and then demonstrate its impact on the luminescence peak position of Eu²⁺ in Sr₈(Si₄O₁₂)­Cl₈. It is found that with increases in temperature, lattice expansion takes place without significant distortion of the coordination around Eu²⁺. As a result, the crystal field splitting of the Eu²⁺ 5d state decreases. At the same time, with the experimental data of the full width at half-maximum of Eu²⁺ emission at different temperatures and the infrared spectrum, the effective phonon frequency is evaluated and the main vibration motions are determined using first-principles calculation. Due to the high light yield under X-ray excitation and the excellent thermal stability of luminescence intensity and decay, a further optimized sample Sr_7.7Eu_0.3(Si₄O₁₂)­Cl₈ could be a potential scintillation material

Quantum Control of Graphene Plasmon Excitation and Propagation at Heaviside Potential Steps

Quantum mechanical effects of single particles can affect the collective plasmon behaviors substantially. In this work, the quantum control of plasmon excitation and propagation in graphene is demonstrated by adopting the variable quantum transmission of carriers at Heaviside potential steps as a tuning knob. First, the plasmon reflection is revealed to be tunable within a broad range by varying the ratio γ between the carrier energy and potential height, which originates from the quantum mechanical effect of carrier propagation at potential steps. Moreover, the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ, which defines the degrees of the carrier confinement in the potential wells. These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating, which are indispensable processes in building plasmon circuitry

Isolation of Cu Atoms in Pd Lattice: Forming Highly Selective Sites for Photocatalytic Conversion of CO₂ to CH₄

Photocatalytic conversion of CO₂ to CH₄, a carbon-neutral fuel, represents an appealing approach to remedy the current energy and environmental crisis; however, it suffers from the large production of CO and H₂ by side reactions. The design of catalytic sites for CO₂ adsorption and activation holds the key to address this grand challenge. In this Article, we develop highly selective sites for photocatalytic conversion of CO₂ to CH₄ by isolating Cu atoms in Pd lattice. According to our synchrotron-radiation characterizations and theoretical simulations, the isolation of Cu atoms in Pd lattice can play dual roles in the enhancement of CO₂-to-CH₄ conversion: (1) providing the paired Cu–Pd sites for the enhanced CO₂ adsorption and the suppressed H₂ evolution; and (2) elevating the <i>d</i>-band center of Cu sites for the improved CO₂ activation. As a result, the Pd₇Cu₁–TiO₂ photocatalyst achieves the high selectivity of 96% for CH₄ production with a rate of 19.6 μmol g_cat^–1 h^–1. This work provides fresh insights into the catalytic site design for selective photocatalytic CO₂ conversion, and highlights the importance of catalyst lattice engineering at atomic precision to catalytic performance

Refining Defect States in W₁₈O₄₉ by Mo Doping: A Strategy for Tuning N₂ Activation towards Solar-Driven Nitrogen Fixation

Photocatalysis may provide an intriguing approach to nitrogen fixation, which relies on the transfer of photoexcited electrons to the ultrastable NN bond. Upon N₂ chemisorption at active sites (e.g., surface defects), the N₂ molecules have yet to receive energetic electrons toward efficient activation and dissociation, often forming a bottleneck. Herein, we report that the bottleneck can be well tackled by refining the defect states in photocatalysts via doping. As a proof of concept, W₁₈O₄₉ ultrathin nanowires are employed as a model material for subtle Mo doping, in which the coordinatively unsaturated (CUS) metal atoms with oxygen defects serve as the sites for N₂ chemisorption and electron transfer. The doped low-valence Mo species play multiple roles in facilitating N₂ activation and dissociation by refining the defect states of W₁₈O₄₉: (1) polarizing the chemisorbed N₂ molecules and facilitating the electron transfer from CUS sites to N₂ adsorbates, which enables the NN bond to be more feasible for dissociation through proton coupling; (2) elevating defect-band center toward the Fermi level, which preserves the energy of photoexcited electrons for N₂ reduction. As a result, the 1 mol % Mo-doped W₁₈O₄₉ sample achieves an ammonia production rate of 195.5 μmol g_cat^–1 h^–1, 7-fold higher than that of pristine W₁₈O₄₉. In pure water, the catalyst demonstrates an apparent quantum efficiency of 0.33% at 400 nm and a solar-to-ammonia efficiency of 0.028% under simulated AM 1.5 G light irradiation. This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity