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Liquid methanol has the potential to be the hydrogen energy carrier and storage medium for the future green economy. However, there are still many challenges before zero-emission, affordable molecular H2 can be extracted from methanol with high performance. Here, we present noble-metal-free Cu-WC/W plasmonic nanohybrids which exhibit unsurpassed solar H2 extraction efficiency from pure methanol of 2,176.7 µmol g-1 h-1 at room temperature and normal pressure. Macro-to-micro experiments and simulations unveil that local reaction microenvironments are generated by the coperturbation of WC/W's lattice strain and infrared-plasmonic electric field. It enables spontaneous but selective zero-emission reaction pathways. Such microenvironments are found to be highly cooperative with solar-broadband-plasmon-excited charge carriers flowing from Cu to WC surfaces for efficient stable CH3OH plasmonic reforming with C3-dominated liquid products and 100% selective gaseous H2. Such high efficiency, without any COx emission, can be sustained for over a thousand-hour operation without obvious degradation.
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Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. However, interfacial mixing can readily occur and significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here, we show a two-step sequence involving the growth of an epitaxial LiCoO2 cathode layer followed by the deposition of a binary transition metal oxide. Orientation-controlled epitaxial synthesis of the model solid-state-electrolyte Li2WO4 and anode material Li4Ti5O12 occurs as WO3 and TiO2 nucleate and react with Li ions from the underlying cathode. We demonstrate that this lithiation-assisted epitaxy approach can be used for energy materials discovery and exploring different combinations of epitaxial interfaces that can serve as well-defined model systems for mechanistic studies of energy storage and conversion processes.
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Heterointerfaces have led to the discovery of novel electronic and magnetic states because of their strongly entangled electronic degrees of freedom. Single-phase chromium compounds always exhibit antiferromagnetism following the prediction of the Goodenough-Kanamori rules. So far, exchange coupling between chromium ions via heteroanions has not been explored and the associated quantum states are unknown. Here, we report the successful epitaxial synthesis and characterization of chromium oxide (Cr_{2}O_{3})-chromium nitride (CrN) superlattices. Room-temperature ferromagnetic spin ordering is achieved at the interfaces between these two antiferromagnets, and the magnitude of the effect decays with increasing layer thickness. First-principles calculations indicate that robust ferromagnetic spin interaction between Cr^{3+} ions via anion-hybridization across the interface yields the lowest total energy. This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.
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The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.
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Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.
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Infinite-layer Nd_{1-x}Sr_{x}NiO_{2} thin films with Sr doping level x from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome x between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x≥0.18, the normal state Hall coefficient (R_{H}) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing x from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.
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Using wide spectral range in situ spectroscopic ellipsometry with systematic ultrahigh vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS_{2} isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the band structure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This Letter unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS_{2}.
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As an iron oxyhydroxide, nanosized ferrihydrite (Fh) is important in Earth science, biology, and industrial applications. However, its basic structure and origin of its magnetism have long been debated. We integrate synchrotron-based techniques to explore the chemical structures of 2-line ferrihydrite and to determine the origin of its magnetism during hydrothermal aging in air. Our results demonstrate that both the magnetism and X-ray magnetic circular dichroism (XMCD) signal of 2-line ferrihydrite are enhanced with aging time, and that XMCD spectral patterns resemble that of maghemite (γ-Fe2O3) rather than magnetite (Fe3O4). Fe L-edge and K-edge X-ray absorption spectroscopy (XAS) further indicate formation of both maghemite and hematite (α-Fe2O3) with increasing concentrations with longer hydrothermal aging time. Thus, magnetic enhancement with longer hydrothermal aging time is attributed to increasing maghemite concentration instead of a magnetically ordered ferrihydrite as previously reported. Moreover, L-edge and K-edge XAS spectra with different probing depths yield different ratios of these Fe oxides, which suggest the formation of a core (ferrihydrite-rich)-shell (with a mixture of both allotropes; α-Fe2O3 and γ-Fe2O3) structure during hydrothermal aging. Our results provide insights into the chemical evolution of 2-line ferrihydrite that reveal unambiguously the origin of its magnetism.
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Compuestos Férricos/química , Suspensiones , Magnetismo , Espectroscopía de Absorción de Rayos X , Rayos XRESUMEN
We report the first observation of the coexistence of a distinct midgap state and a Mott state in undoped and their evolution in electron and hole-doped ambipolar Y_{0.38}La_{0.62}(Ba_{0.82}La_{0.18})_{2}Cu_{3}O_{y} films using spectroscopic ellipsometry and x-ray absorption spectroscopies at the O K and Cu L_{3,2} edges. Supported by theoretical calculations, the midgap state is shown to originate from antiferromagnetic correlation. Surprisingly, while the magnetic state collapses and its correlation strength weakens with dopings, the Mott state in contrast moves toward a higher energy and its correlation strength increases. Our result provides important clues to the mechanism of electronic correlation strengths and superconductivity in cuprates.
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The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.
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Owing to its inherent non-trivial geometry, the unique structural motif of the recently discovered kagome topological superconductor AV3 Sb5 (A = K, Rb, Cs) is an ideal host of diverse topologically non-trivial phenomena, including giant anomalous Hall conductivity, topological charge order, charge density wave (CDW), and unconventional superconductivity. Despite possessing a normal-state CDW order in the form of topological chiral charge order and diverse superconducting gaps structures, it remains unclear how fundamental atomic-level properties and many-body effects including Fermi surface nesting, electron-phonon coupling, and orbital hybridization contribute to these symmetry-breaking phenomena. Here, the direct participation of the V3d-Sb5p orbital hybridization in mediating the CDW phase transition in CsV3 Sb5 is reported. The combination of temperature-dependent X-ray absorption and first-principles studies clearly indicates the inverse Star-of-David structure as the preferred reconstruction in the low-temperature CDW phase. The results highlight the critical role that Sb orbitals play and establish orbital hybridization as the direct mediator of the CDW states and structural transition dynamics in kagome unconventional superconductors. This is a significant step toward the fundamental understanding and control of the emerging correlated phases from the kagome lattice through the orbital interactions and provides promising approaches to novel regimes in unconventional orders and topology.
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Well-ordered spin arrays are desirable for next-generation molecule-based magnetic devices, yet their synthetic method remains a challenging task. Herein, we demonstrate the realization of two-dimensional supramolecular spin arrays on surfaces via halogen-bonding molecular self-assembly. A bromine-terminated perchlorotriphenylmethyl radical with net carbon spin was synthesized and deposited on Au(111) to achieve two-dimensional supramolecular spin arrays. By taking advantage of the diversity of halogen bonds, five supramolecular spin arrays form and are probed by low-temperature scanning tunneling microscopy at the single-molecule level. First-principles calculations verify that the formation of three distinct types of halogen bonds can be used to tailor supramolecular spin arrays via molecular coverage and annealing temperature. Our work suggests that supramolecular self-assembly can be a promising method to engineer two-dimensional molecular spin arrays.
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We report the observation of superconductivity in infinite-layer Ca-doped LaNiO2 (La1-xCaxNiO2) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1-xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient RH changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the RH-sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.
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The demand for biodegradable and renewable UV-shielding materials is ever increasing due to the rising concern for the environment. In this paper, biobased lignin was functionalized by polyhedral oligomeric silsesquioxane (POSS) with an epoxy substituent. Then the POSS decorated lignin (lignin-POSS) was mixed with polylactide (PLA) to act as UV-shielding filler by melt compounding. The SEM observation revealed that the presence of POSS contributed to improving the homogeneous dispersion of lignin-POSS in the PLA matrix with good compatibility when the content of lignin-POSS was lower than 5 wt%. The synergistic effects of lignin and POSS endowed PLA composite films with a good balance of UV-shielding ability and transparency in the visible light region. With the addition of 5 wt% lignin-POSS, the PLA composite film absorbed almost all UV irradiation across the entire UV spectrum. In addition, the presence of lignin-POSS could serve as a nucleating agent to increase the degree of crystallinity of PLA. The dynamical rheological tests revealed that the lignin-POSSS reduced the complex viscosity and storage modulus of PLA composites, improving the flowability of PLA composites. This work presents a viable pathway to prepare biodegradable and renewable UV-shielding materials for potential packaging applications.
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Plásticos Biodegradables/química , Lignina/química , Poliésteres/química , Lignina/ultraestructura , Compuestos de Organosilicio/química , Rayos UltravioletaRESUMEN
The titanomagnetites (Fe2-xTixO4,x⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.
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An atomically dispersed structure is attractive for electrochemically converting carbon dioxide (CO2) to fuels and feedstock due to its unique properties and activity. Most single-atom electrocatalysts are reported to reduce CO2 to carbon monoxide (CO). Herein, we develop atomically dispersed indium (In) on a nitrogen-doped carbon skeleton (In-N-C) as an efficient catalyst to produce formic acid/formate in aqueous media, reaching a turnover frequency as high as 26771 h-1 at -0.99 V relative to a reversible hydrogen electrode (RHE). Electrochemical measurements show that trace amounts of In loaded on the carbon matrix significantly improve the electrocatalytic behavior for the CO2 reduction reaction, outperforming conventional metallic In catalysts. Further experiments and density functional theory (DFT) calculations reveal that the formation of intermediate *OCHO on isolated In sites plays a pivotal role in the efficiency of the CO2-to-formate process, which has a lower energy barrier than that on metallic In.
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Recent discovery of superconductivity in Nd0.8Sr0.2NiO2 motivates the synthesis of other nickelates for providing insights into the origin of high-temperature superconductivity. However, the synthesis of stoichiometric R 1-x Sr x NiO3 thin films over a range of x has proven challenging. Moreover, little is known about the structures and properties of the end member SrNiO3 Here, we show that spontaneous phase segregation occurs while depositing SrNiO3 thin films on perovskite oxide substrates by molecular beam epitaxy. Two coexisting oxygen-deficient Ruddlesden-Popper phases, Sr2NiO3 and SrNi2O3, are formed to balance the stoichiometry and stabilize the energetically preferred Ni2+ cation. Our study sheds light on an unusual oxide thin-film nucleation process driven by the instability in perovskite structured SrNiO3 and the tendency of transition metal cations to form their most stable valence (i.e., Ni2+ in this case). The resulting metastable reduced Ruddlesden-Popper structures offer a testbed for further studying emerging phenomena in nickel-based oxides.
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Epitaxial Fe3O4 thin films grown on single crystal MgO(001) present well-defined model systems to study fundamental multivalent ion diffusion and associated phase transition processes in transition-metal-oxide-based cathodes. In this work, we show at an atomic scale the Mg2+ diffusion pathways, kinetics, and reaction products at the Fe3O4/MgO heterostructures under different oxygen partial pressures but with the same thermal annealing conditions. Combining microscopic, optical, and spectroscopic techniques, we demonstrate that an oxygen-rich environment promotes facile Mg2+ incorporation into the Fe2+ sites, leading to the formation of Mg1-xFe2+xO4 spinel structures, where the corresponding portion of the Fe2+ ions are oxidized to Fe3+. Conversely, annealing in vacuum results in the formation of a thin interfacial rocksalt layer (Mg1-yFeyO), which serves as a blocking layer leading to significantly reduced Mg2+ diffusion to the bulk Fe3O4. The observed changes in transport and optical properties as a result of Mg diffusion are interpreted in light of the electronic structures determined by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Our results reveal the critical role of available anions in governing cation diffusion in the spinel structures and the need to prevent formation of unwanted reaction intermediates for the promotion of facile cation diffusion.
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Enhancing catalytic activity by decorating noble metals in catalysts provides an opportunity for promoting the electrocatalytic hydrogen evolution reaction (HER) application. However, there are few systematic studies on regulating the structures of noble metals in catalytic materials and investigating their influence on HER. Herein, Pt catalysts with different structures including single atoms (SAs), clusters, and nanoparticles well-controllably anchored on VS2 nanosheets through a cost-effective optothermal method are reported, and their HER performance is studied. The most efficient Pt-decorated VS2 catalyst (with both Pt SAs and clusters) delivers an overpotential of 77 mV at 10 mA cm-2, close to that of Pt/C (48 mV). However, the optimal mass activity of Pt (normalizing to Pt content) is obtained from only SA Pt-decorated VS2 (i.e., 22.88 A mgPt-1 at 200 mV) and is 12 times greater than that of the Pt/C (1.87 A mgPt-1), attributed to the greatly enhanced Pt utilization. Additionally, the theoretical simulations reveal that Pt SA decoration makes the adsorption free energy of H* closer to the thermoneutral value and improves the charge-transfer kinetics, significantly enhancing HER activity. This work offers a pathway to prepare the desired catalyst based on synergy of Pt structures and VS2 and reveals the intrinsic mechanism for enhancing catalytic activity, which is important for HER applications.
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A controllable electronic manipulation in a frustrated magnetic system such as solution-based two-dimensional (2D) all-inorganic perovskites offers a possible route for their integrations with electronic and magnetic devices for their advanced applications. Here, we perform element-specific investigations of an emergent class of quasi-2D all-inorganic perovskites Cs2CuCl4-xBrx with (0 ≤ x ≤ 4) using a combination of synchrotron-radiation photoelectron techniques. Surface- and element-sensitive X-ray absorption spectroscopy spectra of Cu L2,3 edges indicate strong electronic transition that is largely influenced by their halogen content at room temperature. This implies that site-selective occupation largely dominates the electronic transition across the unoccupied states of these series since chlorine atoms possess a stronger electronegative character than bromine atoms. Moreover, the implication of halogen site is reflected in the valence band of Cl-rich copper perovskite in which the valence band edge is closer to Fermi energy (EF) than that of the Br-rich compound. Furthermore, X-ray magnetic circular dichroism spectra of mixed ratio and Br-rich compounds exhibit antiferromagnetism at room temperature. These site-specific magnetic-spectroscopic results are corroborated by density functional theory calculations. The strong electronic modulation and the local magnetic spectroscopy results in these solution-based and low-temperature-growth materials will pave the way toward energy- and cost-efficient perovskite devices.