Pristine Phase Research Articles

Oxide‐derived Silver Nanowires for CO2 Electrocatalytic Reduction to CO

AbstractSilver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide‐derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide‐derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3‐fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.

Si-Based High-Entropy Anode for Lithium-Ion Batteries.

Up to now, only a small portion of Si has been utilized in the anode for commercial lithium-ion batteries (LIBs) despite its high energy density. The main challenge of using micron-sized Si anode is the particle crack and pulverization due to the volume expansion during cycling. This work proposes a type of Si-based high-entropy alloy (HEA) materials with high structural stability for the LIB anode. Micron-sized HEA-Si anode can deliver a capacity of 971 mAhg-1 and retains 93.5% of its capacity after 100 cycles. In contrast, the silicon-germanium anode only retains 15% of its capacity after 20 cycles. This study has discovered that including HEA elements in Si-based anode can decrease its anisotropic stress and consequently enhance ductility at discharged state. By utilizing in situ X-ray diffraction and transmission electron microscopy analyses, a high-entropy transition metal doped Lix (Si/Ge) phase is found at lithiated anode, which returns to the pristine HEA phase after delithiation. The reversible lithiation and delithiation process between the HEA phases leads to intrinsic stability during cycling. These findings suggest that incorporating high-entropy modification is a promising approach in designing anode materials toward high-energy density LIBs.

A homogeneous composite approach for improved thermoelectric performance: Repressing lattice thermal conductivity in substoichiometric WO2.90 and WO2.72

Nanostructured phonon-glass thermoelectric materials are highly sought after for heat waste recovery applications. This work introduces a novel phonon-glass multi-phase ceramic material synthesized from sintered mixtures of two sub-stoichiometric tungsten oxides, WO2.90 and WO2.72. Notable measurements were recorded for our sample, denoted as 0.1WO2.72/0.9WO2.90, at 800 °C, with S2σ = 0.51 ± 0.03 mWm−1K−2, representing a tenfold increase compared to the base materials. The lattice thermal conductivity, already displaying phonon-glass characteristics, is further suppressed by the addition of the secondary WO2.72 pentacolumn phase, reaching its lowest value at 0.54 ± 0.25 Wm−1K−1. As a result, the ZT value is enhanced to twice the intrinsic value of the pristine phase. This study demonstrates a novel strategy for improving the thermoelectric performance of oxide-based materials by leveraging a synergistic relationship among several key factors, including phonon-glass nanostructures, tunable electrical properties, and the reduction of lattice thermal conductivity within a homogeneous composite system.

1D/0D heterostructured ZnIn2S4@ZnO S-scheme photocatalysts for improved H2O2 preparation

Solar photocatalysis is a promising, green, and sustainable technique for the synthesis of H2O2. In this study, low-dimensional ZnO/ZnIn2S4 S-scheme heterojunction photocatalysts are fabricated using electrostatic spinning and chemical bath deposition methods for the efficient photocatalytic production of H2O2. ZnO nanofibers loaded with 20 wt% ZnIn2S4 exhibit a superior H2O2 production rate of 928 μmol g−1 h−1, which is more than four times higher than that seen in pristine hexagonal phase ZnO and ZnIn2S4. First-principles calculations and in-situ X-ray photoelectron spectroscopy reveal the charge separation and transfer mechanisms in the S-scheme heterojunction. The construction of the S-scheme heterojunction facilitates the spatial separation of charge carriers, and electrons and holes with higher redox abilities are retained. Photoelectrochemical and photoluminescence tests further show that the formation of an S-scheme heterojunction is beneficial for the separation of photoinduced charge carriers. Electrochemical tests and electron paramagnetic resonance measurements indicate that H2O2 production is primarily via a two-step single-electron O2 reduction path. This study provides a new approach for the construction of S-scheme heterojunction materials that can efficiently produce H2O2 under solar irradiation.

Na4-xSn2-xSbxGe5O16, an Air-Stable Solid-State Na-Ion Conductor.

The structure of a Na4Sn2Ge5O16 phase was established via single-crystal X-ray diffraction. Unusually large displacement parameters of Na atoms suggested the possibility of Na+ ionic conductivity. To create Na deficiencies and thus increase the Na+ mobility in Na4Sn2Ge5O16, Sn4+ cations were partially substituted with Sb5+. A series of Na4-xSn2-xSbxGe5O16 samples (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or 0.35) were prepared by solid-state reactions and characterized with electrical impedance spectroscopy in the range of 25-200 °C. The highest ionic conductivity value was achieved in the Na3.8Sn1.8Sb0.2Ge5O16 sample (1.6 mS cm-1 at 200 °C). Na+ migration pathways were calculated using the bond-valence energy landscape approach, and two-dimensional conductivity channels with low energy barriers (≈0.4 eV) were found in the structure. Three-dimensional conductivity can also be achieved in the structure; however, it has a much higher energy barrier. The pristine phase and Na3.8Sn1.8Sb0.2Ge5O16 sample were studied via 23Na and 119Sn solid-state nuclear magnetic resonance. A faster exchange between the Na sites was observed in the doped sample.

First-Principles Studies on the Structural and Electronic Properties of α-Na2FePO4F with Strong Antisite Disorder.

Polyanionic Na2FePO4F is one of the most important cathode materials for sodium-ion batteries. The orthorhombic β-Na2FePO4F material has been studied extensively and intensively since it was proposed. In this article, a novel monoclinic sodium phosphate fluoride α-Na2FePO4F is concerned. Kirsanova's experiment showed that Na and Fe ions in α-Na2FePO4F are prone to antisite, leading to strong antisite disorder. Through first-principle calculations, we show that the steric effect, the magnetic exchange and superexchange interactions between transition-metal cations are shown to be the main driving forces for Na+/Fe2+ antisite disorder. We first calculated the crystal structures, electronic properties, and cohesive energies of all the 10 antisite phases of α-Na2FePO4F and β-Na2FePO4F. Then, we compared the difference charge densities, magnetism, binding energies, and electrostatic potentials of α-Na2FePO4F and β-Na2FePO4F materials in the antisite and pristine phases. In α-Na2FePO4F, the binding energy of the antisite phase with the lowest binding energy is almost degenerate with that of the pristine phase. Moreover, only small differences of the electrostatic potential and the charge density distribution are found between the antisite (with lowest energy) and the pristine phases of α-Na2FePO4F, which also helped elaborate the facile formation of Na+/Fe2+ antisite in the α-Na2FePO4F material. Our research contributes to the understanding of the mechanism of Na+/Fe2+ antisite and the development of high-performance polyanionic cathode materials.

Coupled magnetic and structural transition in topological antiferromagnet EuAgAs under high pressure

Recently, EuAgAs and its derivatives have emerged as a promising platform for studying the interplay of transports, magnetism and topology. Herein, we report construction of a temperature-pressure phase diagram for antiferromagnetic Dirac semimetal EuAgAs, through electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements in diamond anvil cells at high pressures up to 48.6 GPa. At Pc1∼3–5 GPa, we found a pressure-induced magnetic phase transition with accompanying a structural P63/mmc→Pnma transition. In the pristine antiferromagnetic phase, negative colossal magnetoresistance is observed mainly below the Néel temperature TN along with a field-induced metamagnetic transition; in the emergent magnetic phase, however, it is evident both above and below the magnetic critical temperature TC yet with no field-induced metamagnetic transition. These distinct features lead us to argue a ferromagnetic-like state for EuAgAs above Pc1, which is further supported by observation of a T5/3 temperature dependence of low-temperature resistance, a characteristic of marginal Fermi-liquid state expected to exist close to a ferromagnetic instability by spin fluctuations in metallic ferromagnets. At Pc2∼20 GPa, a possible phase transition of electronic origin may take place, which is revealed jointly by change of a T5/3→T5/2 temperature dependence of low-temperature resistance and change in slope of linear-in-T resistance at high temperatures.

Co/N co‐doped flower‐like carbon‐based phase change materials toward solar energy harvesting

AbstractThe photothermal conversion capacity of pristine organic phase change materials (PCMs) is inherently insufficient in solar energy utilization. To upgrade their photothermal conversion capacity, we developed bimetallic zeolitic imidazolate framework (ZIF) derived Co/N co‐doped flower‐like carbon (Co/N‐FLC)‐based composite PCMs toward solar energy harvesting. 3D interconnected carbon framework with low interfacial thermal resistance, abundant carbon defects and high content of nitrogen doping, excellent localized surface plasmon resonance (LSPR) effect of Co nanoparticles, and light absorber Co3ZnC in Co/N‐FLC synergistically upgrade the photothermal capacity of (polyethylene glycol) PEG@Co/N‐FLC composite PCMs with an ultrahigh photothermal conversion efficiency of 94.8% under 0.16 W/cm2. Uniformly anchored Co and Co3ZnC nanoparticles in carbon framework guarantee excellent photon capture ability. Bridging carbon nanotubes (CNTs) in 2D carbon nanosheets further accelerate the rapid transport of phonons by constructing cross‐connected heat transfer paths. Additionally, PEG@Co/N‐FLC exhibits a thermal energy storage density of 100.69 J/g and excellent thermal stability and durable reliability. Therefore, PEG@Co/N‐FLC composite PCMs are promising candidates to accelerate the efficient utilization of solar energy.

Multi-state structural modulation of hydrogenated VO2 revealed by in situ x-ray diffraction

The generation and control of multiple phases via hydrogen insertion open up avenues for tuning the properties of transition metal oxides. Here, by employing both in situ x-ray diffractions and in situ electrical measurements, we accurately probed the full structural phase transitions during the reversible process of hydrogen insertion into and extraction from the vanadium dioxide lattice. Repeatable switches between the hydrogenated VO2 phases and the pristine VO2 phase were demonstrated, implying potential applications for hydrogen detection/storage and multi-state information memorizers. Moreover, different phases were further discussed by synchrotron x-ray absorption spectroscopy and theoretical first-principles calculations, which reveal that hydrogen insertion greatly affects the filling of the d-band as well as the electrical properties. This work will provide fundamental insight into the comprehensive understanding of hydrogen-induced phase transition in metal oxides and may guide the development of proton-based sensors and devices.

Phase transformation of cold-sintered doped barium titanate ceramics during the post-annealing process

The cold sintering technique has been developed to densify electroceramics at significantly low temperatures to tackle high energy consumption as well as compatibility of electroceramic components in modern electronics. However, obstacles still exist since, to obtain desired electrical properties, a post-annealing procedure at elevated temperatures must follow to trigger compulsory phase formation and/or grain growth. This work investigates the BaTiO3 and (Ba,Sr)TiO3 ceramics via both the cold and conventional sintering routes, and then compares their phase transformation behaviors during the post-annealing process. Results suggest that, given similar particle and grain sizes, the cold-sintered ceramics tend to retain a substantial part of the pristine tetragonal phase at annealing temperatures of <800 °C, while the cubic phase tends to dominate when annealed at >800 °C. This indicates a process-stimulated phase transformation that may guide or limit the selection of annealing temperature in practice.

Rhombohedral distortion induced structural, magnetic, optical phase transitions and photocatalytic activity in Sm and Sm-Cr co-substituted bismuth ferrite nanoparticles

Rhombohedral distortion induced structural, magnetic, optical phase transitions and photocatalytic activity of BiFeO3 (BFO), samarium (Sm)-substituted and Sm-Cr co-substituted bismuth iron oxide nanoparticles were prepared using the tartaric acid modified wet chemical sol–gel route. XRD and FESEM techniques confirmed the pristine phase and nanocrystalline property of BFO, Bi0.85Sm0.15FeO3 and BSFCO5 samples. XRD as well as Raman spectra confirmed structural phase transformations from the rhombohedral (pristine BFO) to rhombohedral (Bi0.85Sm0.15FeO3 and Bi0.85Sm0.15Fe0.95Cr0.05O3), in case of Sm-Cr co-doping there is indication of more change of crystal structure than single Sm-doped. Sm-Cr co-doped into BFO could enhance magnetism, more interestingly; M−H loops reveal the highest saturation magnetization value compared to Sm-doped and BFO. An obvious blue shift is noted in the substituted samples as well as the reduction of bandgap compared to pristine BFO. The co-substitution enhanced the photocatalytic degradation activity as compared to Sm-doped and pure BiFeO3.

Investigating the Electromechanical Behavior of Unconventionally Ferroelectric Hf0.5Zr0.5O2‐Based Capacitors Through Operando Nanobeam X‐Ray Diffraction

AbstractUnderstanding various aspects of ferroelectricity in hafnia‐based nanomaterials is of vital importance for the development of future nonvolatile memory and logic devices. Here, the unconventional and weak electromechanical response of epitaxial La0.67Sr0.33MnO3/Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 ferroelectric capacitors is investigated, via the sensitivity offered by nanobeam X‐ray diffraction experiments during application of electrical bias. It is shown that the pristine rhombohedral phase exhibits a linear piezoelectric effect with piezoelectric coefficient (|d33|) ≈ 0.5–0.8 pmV−1. It is found that the piezoelectric response is suppressed above the coercive voltage. For higher voltages, and with the onset of DC conductivity throughout the capacitor, a second‐order effect is observed. The work sheds light into the electromechanical response of rhombohedral Hf0.5Zr0.5O2 and suggests its (un)correlation with ferroelectric switching.

Effects of niobium doping on the charge density wave and electronic correlations in the kagome metal Cs(V1−xNbx)3Sb5

The transport and optical properties of the Nb-doped Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$ with x = 0.03 and 0.07 have been investigated and compared with those of the undoped CsV$_{3}$Sb$_{5}$. Upon Nb doping, the charge-density wave (CDW) transition temperature $T_{\text{CDW}}$ is suppressed, and the superconducting temperature $T_{c}$ rises. The residual resistivity ratio decreases with Nb doping, suggesting an increase of disorder. For all compounds, the optical conductivity in the pristine phase reveals two Drude components (D1 and D2). The substitution of Nb causes an increase of D1 alongside a reduction of D2 in weight, which implies a change of the Fermi surface. The total Drude weight is reduced with increasing Nb content, signifying an enhancement of electronic correlations. Below $T_{\text{CDW}}$, while the optical conductivity clearly manifests the CDW gap in all materials, the gapped portion of the Fermi surface shrinks as the Nb content grows. A comprehensive analysis indicates that the change of the Fermi surface, the enhancement of electronic correlations, the shrinkage of the removed Fermi surface by the CDW gap, and the increase of disorder may all have a considerable impact on the interplay between the CDW and superconductivity in Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$.

Shear-induced phase transition in the aqueous solution of an imidazolium-based ionic liquid.

An ionic liquid (IL) is a salt in the liquid state that consists of a cation and an anion, one of which possesses an organic component. Because of their non-volatile property, these solvents have a high recovery rate, and, hence, they are considered as environment-friendly green solvents. It is necessary to study the detailed physicochemical properties of these liquids for designing and processing techniques and find suitable operating conditions for IL-based systems. In the present work, the flow behavior of aqueous solutions of an imidazolium-based IL, 1-methyl-3-octylimidazolium chloride, is investigated, where the dynamic viscosity measurements indicate non-Newtonian shear thickening behavior in the solutions. Polarizing optical microscopy shows that the pristine samples are isotropic and transform into anisotropic after shear. These shear thickened liquid crystalline samples change into an isotropic phase upon heating, which is quantified by the differential scanning calorimetry. The small angle x-ray scattering study revealed that the pristine isotropic cubic phase of spherical micelles distort into non-spherical micelles. This has provided the detailed structural evolution of mesoscopic aggregates of the IL in an aqueous solution and the corresponding viscoelastic property of the solution.

NaGaSe2: A Water-Loving Multifunctional Non-van der Waals Layered Selenogallate.

A missing member of well-known ternary chalcometallates, a sodium selenogallate, NaGaSe2, has been synthesized by employing a polyselenide flux and stoichiometric reaction. Crystal structure analysis using X-ray diffraction techniques reveals that it contains supertetrahedral adamantane-type Ga4Se10 secondary building units. These Ga4Se10 secondary building units are further connected via corners to form two-dimensional (2D) [GaSe2]∞- layers stacked along the c-axis of the unit cell, and the Na ions reside in the interlayer space. The compound has an unusual ability to absorb water molecules from the atmosphere or a nonanhydrous solvent to form distinct hydrated phases, NaGaSe2·xH2O (where x can be 1 and 2), with an expanded interlayer space, as verified by X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), desorption, and Fourier transform infrared spectroscopy (FT-IR) studies. The in situ thermodiffractogram indicates the emergence of an anhydrous phase before 300 °C with the decrease of interlayer spacings and reverting to the hydrated phase within a minute of re-exposure to the environment, supporting the reversibility of such a process. Structural transformation induced through water absorption results in an increase of Na ionic conductivity by 2 orders of magnitude compared to that of the pristine anhydrous phase, as verified by impedance spectroscopy. Na ions from NaGaSe2 can be exchanged in the solid-state route with other alkali and alkaline earth metals in a topotactic or nontopotactic way, leading to 2D isostructural and three-dimensional networks, respectively. Optical band gap measurements show a band gap of ∼3 eV for the hydrated phase, NaGaSe2·xH2O, which is in good agreement with the calculated band gap using a density functional theory (DFT)-based method. Sorption studies further confirm the selective absorption of water over MeOH, EtOH, and CH3CN with a maximum water uptake of 6 molecules/formula unit at a relative pressure, P/P0, of 0.9.

High-Performance Heat Storage Phase Change Composite by Highly Aligned N-Doped Mesoporous Carbon for Efficient Battery Thermal Management

Limited by leakage from inherent liquid to solid phase transition and inferior heat transfer, pristine organic phase change materials (PCMs) possess extremely low thermal storage performance. Thus, establishing shape-stable PCM composites with enhanced thermal conductivity and latent heat is a key task for practical application. Herein, we constructed N-doped mesoporous carbon monoliths with a highly aligned structure by directional freeze-drying to encapsulate polyethylene glycol. Benefiting from the mesoporous carbon matrix and pyridinic N from N-doping, the PCM composite possessed large latent heat (140 J·g–1) and leakage proof at 80 °C. Meanwhile, the highly aligned structure created a fast heat transfer pathway that improved thermal conductivity of the PCM composite by 1500%. Being different from most research studies, highly aligned carbon was utilized as the support directly without skeletons, which could increase PCM loading capacity and thermal conductivity as well. The PCM composite can be employed as promising candidates for thermal management working efficiently to decrease battery pack temperature by 13 °C and improve discharge capacity by 28%.

Compositional defects in a MoAlB MAB phase thin film grown by high-power pulsed magnetron sputtering.

Various compositional defects such as Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering. An overall film composition of Mo0.29Al0.33B0.38 is measured by time-of-flight elastic recoil detection analysis. The concurrent formation of these compositional defects in the MoAlB matrix occurs during the synthesis without using any chemical reagent, and their coexistence with the MAB phase is thermodynamically possible, as elucidated by density functional theory simulations. These defect phases are imaged at the atomic scale by aberration-corrected scanning transmission electron microscopy. A rough estimation of defect populations of 0.073, 0.037, 0.042 and 0.039 nm-1 for Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo compositional defects, respectively, is performed within the MoAlB matrix. The calculated energies of formation reveal that the Mo4Al3B4 and Mo6Al5B6 defect phases form spontaneously in the MoAlB host matrix, while the energy barrier towards the formation of the metastable Mo3Al2B4 phase is approx. 20 meV per atom. The small magnitude of this barrier is easily overcome during vapor phase condensation, and the surface diffusion of adatoms during deposition leads to local compositional variations and the coexistence of the defect phases in the host matrix. Additionally, at grain boundaries, the presence of MoB MBene is observed, with an interlayer spacing between two Mo2B2 units increasing up to ∼50% compared to the pristine MoAlB phase.

Evolution of Thermoelectric and Oxidation Properties in Lu-Substituted Yb14MnSb11

Yb14MnSb11 is one of the state-of-the-art high-temperature p-type thermoelectric materials with reported zTs of 1.2–1.3 at 1273 K. Site substitution of Yb14MnSb11 provides a means to control carrier concentration and impacts the oxidation kinetics. Substitution of Lu3+ for Yb2+ in Yb14MnSb11 single crystals has been shown to provide faster oxidation kinetics compared with the pristine phase, and it has been speculated that this may lead to surface passivation. Polycrystalline samples of Yb14–xLuxMnSb11 were synthesized from the elements and through a route utilizing YbH2, MnSb, and Yb4Sb3 as reactive precursors. The solubility of Lu was found to be less than x = 0.5, and at that composition and above LuSb impurities are observed in the powder X-ray diffraction. Because Lu3+ is substituting for Yb2+ in a p-type system, there is an increase in both the electrical resistivity and Seebeck coefficient with Lu substitution. As Yb14MnSb11 has been predicted to be optimally doped ∼1.5 × 1021 h+ cm–3, this reduction in carrier concentration in the solid solution of Yb14–xLuxMnSb11 decreases the peak zT. Oxidation of Yb13.7Lu0.3MnSb11 as a function of temperature was studied. At low temperatures, the Lu-substituted sample may have improved oxidation resistance through forming a passivating oxide film on the surface.

Electrochemical characterizations of carbon decorated tin doped lithium titanate for lithium-ion battery anode applications

Lithium titanate (Li4Ti5O12) anode materials are widely popular because of their exceptional structural properties that could offer extremely long life and high current rate with safe operation. In-depth research works have been going on to mitigate the limiting factors like poor conduction and diffusion of lithium titanate for the electrode applications in the lithium-ion battery. Elemental doping and surface coating were found to be promising methods to overcome these challenges and enhance the capacity for widespread application. Herein, ex-situ carbon-coated tin (Sn4+) doped lithium titanate was synthesized via conventional solid-state synthesis. Its electrochemical performances were analyzed and compared with its pristine phase by making half cells with lithium. The results were highly promising as the Li4Sn0.05Ti4.95O12 with 5 wt% carbon coating exhibited significantly reduced cell impedance (Rct = 49.48 Ω), enhanced electrode lithium-ion diffusion coefficient (D = 2.40×10−11 cm2 s−1), and superior specific capacity (Q = 173.1 mAh/g at 0.1 C) over pristine lithium titanate (Rct = 274.2 Ω, D = 2.71×10−12 cm2 s−1, and Q = 145 mAh/g). The cell could also run for 1000 cycles with minimum capacity fade (82 % retention) at a 1 C rate.

Effective six-band model and unconventional spin-singlet pairing in Kagome superconductor CsV3Sb5

Recently synthesized Kagome compounds AV3Sb5 attract great attention due to the unusual coexistence of the topology, charge density wave, and superconductivity. In this work, based on the band structures for CsV3Sb5 in the pristine phase, we construct an effective six-band model for the low-energy processes; utilizing the random phase approximation on the effective six-band model, we show that the (p wave) pairing dominates in the region of eV, the (I wave) in the region of eV and the (d wave) in the region of eV. Considering that the correlation in AV3Sb5 is weak or intermediate, these results suggest that pairing symmetry is (I-wave) in the CsV3Sb5, which can explain some recent experiments about pairing symmetry.