An Unexpected Decrease in Vibrational Entropy of Multicomponent Rutile Oxides.

High-entropy oxides (HEOs), featuring infinite chemical composition and exceptional physicochemical properties, are attracting much attention. The configurational entropy caused by a component disorder of HEOs is popularly believed to be the main driving force for thermal stability, while the role of vibrational entropy in the thermodynamic landscape has been neglected. In this study, we systematically investigated the vibrational entropy of multicomponent rutile oxides (including Fe0.5Ta0.5O2, Fe0.333Ti0.333Ta0.333O2, Fe0.25Ti0.25Ta0.25Sn0.25O2, and Fe0.21Ti0.21Ta0.21Sn0.21Ge0.16O2) by precise heat capacity measurements. It is found that vibrational entropy gradually decreases with increasing component disorder, beyond what one could expect from an equilibrium thermodynamics perspective. Moreover, all multicomponent rutile oxides exhibit a positive excess vibrational entropy at 298.15 K. Upon examinations of configuration disorder, size mismatch, phase transition, and polyhedral distortions, we demonstrate that the excess vibrational entropy plays a pivotal role in lowering the crystallization temperature of multicomponent rutile oxides. These findings represent the first experimental confirmation of the role of lattice vibrations in the thermodynamic landscape of rutile HEOs. In particular, vibrational entropy could serve as a novel descriptor to guide the predictive design of multicomponent oxide materials.

Luminescence-Monitored Progressive Chemical Pressure Implementation Realized through Successive Y3+ and Mg2+ Doping into Ca10.5(PO4)7:Eu2.

Chemical pressure generated through ion doping into crystal lattices has been proven to be conducive to exploration of new matter, development of novel functionalities, and realization of unprecedented performances. However, studies are focusing on one-time doping, and there is a lack of both advanced investigations for multiple doping and sophisticated strategies to precisely and quantitatively track the gradual functionality evolution along with progressive chemical pressure implementation. Herein, high-valent Y3+ and equal-valent Mg2+ is successively doped to replace multiple Ca sites in Ca10.5(PO4)7:Eu2+. The luminescence evolution of Eu2+ serves as an optical probe, allowing step-by-step and atomic-level tracking of the site occupation of Y3+ and Mg2+, interassociation of Ca sites, and ultimately functionality improvement. The resulting Ca8MgY(PO4)7:Eu2+ displays a record-high relative sensitivity for optical thermometry. Utilization of the environment-sensitive emission of Eu2+ as a luminescent probe has offered a unique approach to monitoring structure-functionality evolution in vivo with atomic precision, which shall also be extended to optimization of other functionalities such as ferroelectricity, conductivity, thermoelectricity, and catalytic activity through precise control over atomic diffusion in other types of substances.

Ultrasonic reduction: an unconventional route to exsolute Ag from perovskite La(Ag)FeO3-δ for enhanced catalytic oxidation activity.

We explore an uncommon ultrasonic reduction method to exsolute Ag from perovskite La0.87Ag0.03FeO3-δ, forming a composite with enhanced catalytic oxidation activity. Such a mild exsolution is based on the coupling effect of ultrasonic cavitation and reducible BH4-, and holds great potential in the fields of energy and environment catalysis.

Multi-interface Combination of Bimetallic Selenide and V4C3Tx MXene for High-Rate and Ultrastable Sodium Storage Devices.

Sodium-ion batteries (SIBs) have great potential as electrochemical energy storage systems; however, their commercial viability is limited by the lack of anode materials with fast charge/discharge rates and long lifetimes. These challenges were addressed by developing a multi-interface design strategy using FCSe (FeSe2/CoSe2) nanoparticles on V4C3Tx MXene nanosheets as conductive substrates. The heterogeneous interface created between the two materials provided high-speed transport of sodium ions, suppressed the chalking-off of nanoparticles, and improved the cycling stability. Additionally, the Fe-Co bonds generated at the interface effectively relieved mechanical stress, further enhancing the electrode durability. The C@FCSe@V4C3 electrode exhibited high-speed charging and discharging characteristics, and maintained a high specific capacity of 260.5 mAh g-1 even after 15,000 cycles at 10 A g-1, with a capacity retention rate of 50.2% at an ultrahigh current density of 20 A g-1. Furthermore, the composite displayed a good cycling capability in the fast discharge and slow charge mode. This demonstrates its promising commercial potential. This multi-interface design strategy provides insights and guidance for solving the reversibility and cycling problems of transformed selenide anode materials.

Novel NASICON-Type Na-V-Mn-Ni-Containing Cathodes for High-Rate and Long-Life SIBs.

Partial substitution of V by other transition metals in Na3 V2 (PO4 )3 (NVP) can improve the electrochemical performance of NVP as a cathode for sodium-ion batteries (SIBs). Herein, phosphate Na-V-Mn-Ni-containing composites based on NASICON (Natrium Super Ionic Conductor)-type structure have been fabricated by sol-gel method. The synchrotron-based X-ray study, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) studies show that manganese/nickel combinations successfully substitute the vanadium in its site within certain limits. Among the received samples, composite based on Na3.83 V1.17 Mn0.58 Ni0.25 (PO4 )3 (VMN-0.5, 108.1 mAh g-1 at 0.2 C) shows the highest electrochemical ability. The cyclic voltammetry, galvanostatic intermittent titration technique, in situ XRD, ex situ XPS, and bond valence site energy calculations exhibit the kinetic properties and the sodium storage mechanism of VMN-0.5. Moreover, VMN-0.5 electrode also exhibits excellent electrochemical performance in quasi-solid-state sodium metal batteries with PVDF-HFP quasi-solid electrolyte membranes. The presented work analyzes the advantages of VMN-0.5 and the nature of the substituted metal in relation to the electrochemical properties of the NASICON-type structure, which will facilitate further commercialization of SIBs.

Yolk-Shell Gradient-Structured SiOx Anodes Derived from Periodic Mesoporous Organosilicas Enable High-Performance Lithium-Ion Batteries.

Gradient-structured materials hold great promise in the areas of batteries and electrocatalysis. Here, yolk-shell gradient-structured SiOx -based anode (YSG-SiOx /C@C) derived from periodic mesoporous organosilica spheres (PMOs) through a selective etching method is reported. Capitalizing on the poor hydrothermal stability of inorganic silica in organic-inorganic hybrid silica spheres, the inorganic silica component in the hybrid spheres is selectively etched to obtain yolk-shell-structured PMOs. Subsequently, the yolk-shell PMOs are coated with carbon to fabricate YSG-SiOx /C@C. YSG-SiOx /C@C is comprised of a core with uniform distribution of SiOx and carbon at the atomic scale, a middle void layer, and outer layers of SiOx and amorphous carbon. This unique gradient structure and composition from inside to outside not only enhances the electrical conductivity of the SiOx anode and reduces the side reactions, but also reserves void space for the expansion of SiOx , thereby effectively mitigating the stress caused by volumetric effect. As a result, YSG-SiOx /C@C exhibits exceptional cycling stability and rate capability. Specifically, YSG-SiOx /C@C maintains a specific capacity of 627 mAh g-1 after 400 cycles at 0.5 A g-1 , and remains stable even after 550 cycles at current density of 2 A g-1 , achieving a specific capacity of 519 mAh g-1 .

Flexible Ti3 C2 Tx MXene Bonded Bio-Derived Carbon Fibers Support Tin Disulfide for Fast and Stable Sodium Storage.

High energy density and flexible electrodes, which have high mechanical properties and electrochemical stability, are critical to the development of wearable electronics. In this work, a free-standing MXene bonded SnS2 composited nitrogen-doped carbon fibers (MXene/SnS2 @NCFs) film is reported as a flexible anode for sodium-ion batteries. SnS2 nanoparticles with high-capacity properties are covalently decorated in bio-derived nitrogen-doped 1D carbon fibers (SnS2 @NCFs) and further assembled with highly conductive MXene sheets. The addition of bacterial cellulose (BC) can further improve the flexibility of the film. The unique 3D structure of points, lines, and planes can not only offset the disadvantage of low conductivity of SnS2 nanoparticles but also expand the distance between MXene sheets, which is conducive to the penetration of electrolytes. More importantly, the MXene sheets and N-doped 1D carbon fibers (NCFs) can accommodate the large volume expansion of SnS2 nanoparticles and trap polysulfide during the cycle. The MXene/SnS2 @NCFs film exhibits better sodium storage and excellent rate performance compared to the SnS2 @NCFs. The in situ XRD and ex situ (XRD, XPS, and HRTEM) techniques are used to analyze the sodiation process and to deeply study the reaction mechanism of the films. Finally, the quasi-solid-state full cells with MXene/SnS2 @NCFs and Na3 V2 (PO4 )3 @carbon cloth (NVP@CC) fully demonstrate the application potential of the flexible electrodes.

The Organic Ligand Etching Method for Constructing In Situ Terraced Protective Layer Toward Stable Aqueous Zn Anode.

The stability of aqueous Zn-ion batteries (AZIBs) is highly dependent on the reversibility of stripping/plating Zn anode. In this work, an organic ligand etching method is proposed to develop a series of in situ multifunctional protective layers on Zn anode. Particularly, the 0.02 m [Fe(CN) 6]3- etching solutions can spontaneously etch the Zn anode, creating an in situ protective layer with unique terraced structure, which blocks the direct contact between the electrode and electrolyte and increases the area for Zn2+ ions deposition. Interestingly, all elements in the organic ligands (i.e., C, N, Zn, and Fe) exhibit strong zincophilic, significantly promoting zinc deposition kinetics and enhancing 3D nucleation behavior to inhibit zinc dendrite growth. As a result, the etched Zn anode can provide as high a Coulombic efficiency of 99.6% over 1000 cycles and sustain over 400 h long-term stability at a high current density of 10 mA cm-2 . As general validation, the small amount of metal cations additives (e.g., Ni2+ , Mn2+ , and Cu2+ ) can accelerate the synthesis of artificial interface layers with 3D structures and also regulate zinc deposition behavior. This work provides a new idea from the perspective of etching solution selection for surface modification of Zn metal anode.

Protocol in Evaluating Capacity of Zn-Mn Aqueous Batteries: A Clue of pH.

In the literature, Zn-Mn aqueous batteries (ZMABs) confront abnormal capacity behavior, such as capacity fluctuation and diverse "unprecedented performances." Because of the electrolyte additive-induced complexes, various charge/discharge behaviors associated with different mechanisms are being reported. However, the current performance assessment remains unregulated, and only the electrode or the electrolyte is considered. The lack of a comprehensive and impartial performance evaluation protocol for ZMABs hinders forward research and commercialization. Here, a pH clue (proton-coupled reaction) to understand different mechanisms is proposed and the capacity contribution is normalized. Then, a series of performance metrics, including rated capacity (Cr ) and electrolyte contribution ratio from Mn2+ (CfM), are systematically discussed based on diverse energy storage mechanisms. The relationship between Mn (II) ↔ Mn (III) ↔ Mn (IV) conversion chemistry and protons consumption/production is well-established. Finally, the concrete design concepts of a tunable H+ /Zn2+ /Mn2+ storage system for customized application scenarios, opening the door for the next-generation high-safety and reliable energy storage system, are proposed.

Confinement chemistry of FeOx centers for activating molecular oxygen under ambient conditions.

Activating molecular oxygen under mild conditions is highly important for developing advanced green technologies and for understanding the origin and running of life as well, which still remains a challenge. In this work, we report on the confinement chemistry for activating molecular oxygen over oxides under mild conditions by presenting the synthesis and characterization of FeOx species confined to the pores of support CeO2 nanospheres. Active catalytic materials are obtained by a controllable three-step method via the formation of porous CeO2 nanospheres that have an average diameter of 120 nm and exhibit a large surface area of 168 m2 g-1 and a pore size of 18.7 nm, confining FeOx in intimate contact with ultra-small Pt particles in pores. The optimized PtOy-FeOx/CeO2-H catalyst showed an excellent performance in the preferential oxidation of CO reactions, as featured by 100% CO conversion at room temperature with almost no attenuation in a prolonged operation, which could not be accessible without pore-confined FeOx centers. Mechanical studies prove that the reaction progresses via abnormal non-competitive adsorption associated with synergistic roles from uniform loading, stabilization of divalent Fe species, surface oxygen activation on CeO2 supports, and the reduced H2 spillover effect on Pt0, making the CO species adsorbed on Ptδ+ easier to be desorbed. The methodology demonstrated here may inspire one to explore more advanced catalysts with high activity at room temperature essential for a wide range of applications.

Fabrication of VO Nanorings on a Porous Carbon Architecture for High-Performance Li-Ion Batteries.

Vanadium monoxide (VO) is a promising candidate as an anode for lithium-ion batteries due to its high theoretical capacity, low cost, and considerable electronic conductivity. Unfortunately, a large volume change during electrochemical processes obstructs its practical application. In this work, a composite of VO nanorings grown on a porous carbon architecture is prepared via a topochemical self-reduction approach. When used as an anode for lithium-ion batteries, improved redox kinetics from enhanced electronic conduction and the corresponding fast lithium-ion diffusion is observed to greatly promote the electrochemical performance of lithium-ion batteries. The resulting composite delivered a reversible capacity of 336 mA h g-1 after 400 cycles at 10 A g-1 with a capacity retention of 85%, owing to the synergistic effect of VO nanorings and porous carbon in alleviating volume changes that result in a long-term cycling ability at a high current density. At 20 A g-1, the composite anode exhibited a rate capability of 235 mA h g-1, superior to all VO-based electrodes reported in the literature. Furthermore, a full cell was first fabricated by employing VO@C-2 as the anode and LiFePO4 as the cathode, which exhibited a capacity of 213 mA h g-1 after 100 cycles at 0.1 A g-1, indicating the potential of VO as an anode for practical application.

Layered Cu1-zMn1+zO2 Crednerite: Mapping the Phase Stabilization Region via Precise Compositional Control for Optimum Supercapacitor Performance.

CuMnO2 is a prototype ABO2-type crednerite compound featured by transition metal ions of variable valence states essential for creating novel properties and optimum performance. However, the phase stabilization region of CuMnO2 has not yet been well established, restricting one's ability in comprehending this unique structure for functional applications. Here, layered Cu1-zMn1+zO2 crednerite was systematically synthesized and characterized by accurately regulating the reaction parameters of hydrothermal conditions, which led to a first demonstration of the phase diagram for CuMnO2 crednerite. The pure phase layered structure was uncovered to be stabilized under hydrothermal conditions as the temperature varies between 85 and 175 °C and the molar ratio of Cu to (Cu + Mn) varies between 0.45 and 0.55. For Cu1-zMn1+zO2, there appeared non-stoichiometric occupation of transition metal ions. Strikingly, different from many other layered oxides, the samples at a molar ratio of Cu:(Cu + Mn) = 0.55 showed a special structure, in which excess Cu2+ occupied the position of the Mn3+ site to form a Cu2+ (3d9)/Mn4+ (3d3) ionic pair and traces of corresponding cationic ordered phases. Such a configuration (3d9/3d3 ionic pair) gives rise to an optimum super-capacitor performance, as represented by a highest mass specific capacitance of 428.4 F/g at a current density of 1 A/g. The strategy reported in this work for mapping the phase diagram of layered CuMnO2 crednerite is fundamentally important, which may offer guidance to explore the potentials of other ABO2-type compounds for functional applications.

Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO4 by Hydrothermal Conditions.

Doping chemistry has become one of the most effective means of tuning materials' properties for diverse applications. In particular for scheelite-type CaWO4, high-oxidation-state doping is extremely important, since one may expand the scheelite family and further create prospective candidates for novel applications and/or useful spectral signatures for nuclear forensics. However, the chemistry associated with high-valence doping in scheelite-type CaWO4 is far from understanding. In this work, a series of scheelite-based materials (Ca1-x-y-zEuxKy□z)WO4 (□ represents the cation vacancy of the Ca2+ site) were synthesized by hydrothermal conditions and solid-state methods and comparatively studied. For the bulk prepared by the solid-state method, occupation of high-oxidation-state Eu3+ at the Ca2+ sites of CaWO4 is followed by doping of the low-oxidation-state K+ at a nearly equivalent molar amount. The Eu3+ local symmetry is thus varied from the original S4 point group symmetry to C2v point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu3+ was incorporated in CaWO4 at two distinct sites, and its amount is higher than that of the low-oxidation-state K+ even though KOH was used as a mineralizer, creating a certain amount of cation vacancies. Consequently, an apparent split emission of 5D0 → 7F0 was first demonstrated for (Ca1-x-y-zEuxKy□z)WO4. The doping chemistry of high oxidation states uncovered in this work not only provides an explanation for the commonly observed spectral changes in rare-earth-ion-modified scheelite structures, but also points out an advanced direction that can guide the design and synthesis of novel functional oxides by solution chemistry routes.

Size-encoded hierarchical self-assembly of nanoparticles into chains and tubules.

Hierarchical structures with sophisticated patterns allow the emergence of challenging properties. However, the highly cooperative and specific interactions needed for assembly spanning different length scales are typically lacking in inorganic nanoparticles (NPs). Here we show that size can be a common structural driving force for controlling hierarchical assembly of inorganic NPs into anisotropic superstructures. It involves first the self-limiting assembly of small CdS NPs into large supraparticles and their subsequent spontaneous organization into chains and tubules hundreds of nanometers long. Our quantitative calculations based on DLVO theory reveals an intrinsic size effect relating to the dimension change of assembly units in accordance with a negative cooperativity. It is shown that the size increase in building blocks creates an effective kinetic barrier contrast at different attachment sites due to the increase of interparticle electrostatic repulsion, switching the assembly from thermodynamically preferred 3D to kinetically favored 1D pathway. The size-encoded hierarchical assembly is accompanied by the ligand-controlled Oswald ripening process, which is responsible for the variation of hierarchical patterns from chains to tubules. The general principle in governing multistage inorganic NP ordering represents an important guideline toward the complex mesoscale structures that may surpass traditional materials in both design and functionality.

CsCu2I3 Nanocrystals: Growth and Structural Evolution for Tunable Light Emission.

CsCu2I3 mixed with Cs3Cu2I5 has shown potential applications as white-light-emitting materials, while their growth, structural evolution behaviors, and their impact on photoluminescence of CsCu2I3 nanocrystals (NCs) are still not known. In this work, we investigated the growth and structural evolution of CsCu2I3 nanocrystals with increasing reaction temperature. At low temperature and in the presence of a high dosage of oleic acid and oleylamine, Cs3Cu2I5 nanoparticles, rather than CsCu2I3 NCs, preferred to form in the hot-injection reaction system. Increasing the reaction temperature promoted the formation of CsCu2I3 nanorods. Phase-pure CsCu2I3 nanorods were steadily obtained at 180 °C. Structural evolution from less copper-containing NCs to copper-rich ones in the low-temperature reaction condition is highly related to the coordination of copper ions with OAm. More importantly, accompanying the growth of nanorods and structural evolution from Cu3Cs2I5 to CsCu2I3, the color of photoluminescence emission of NCs changed from blue to nearly white and to yellow, but their photoluminescence quantum yield decreased from 36.00 to 9.86%. The finding in this work would give a view to the structural evolution of copper-containing perovskite-like halides, being helpful for adjusting their photoluminescence in white LEDs.

Layer-by-layer assembly into bulk-like g-C3N4via artificial manipulation of electrostatic forces.

For the first time, bulk-like g-C3N4 was achieved through initiating layer-by-layer assembly, which involves the important process of artificially manipulating protonated and oxygen doped g-C3N4 nanosheets. When acting as a photocatalyst under UV-visible light irradiation, bulk-like g-C3N4 exhibited an excellent photocatalytic H2 production rate of 1538 µmol h-1 g-1, about 8 times higher than that of the bulk g-C3N4 counterpart.

Topological transformation of LDH nanosheets to highly dispersed PtNiFe nanoalloys enhancing CO oxidation performance.

Highly dispersed nanoalloys with a tailored metal-oxide interface are pivotal in developing advanced catalysts with superior performance for applications. Herein, a series of highly dispersed Pt/NiFeAl nanoalloys on amorphous supports were successfully fabricated by a topological transformation of layered-double-hydroxide nanosheets. With increasing reduction temperature, samples Pt/NiFeAl-x (x = reduction temperature) showed a progressive transformation from Pt/NiFeAl-LDH to a mixture (Pt, NiFe alloys, FeOy, and NiOy) supported on amorphous Al2O3, which eventually transformed to atomically dispersed PtNiFe alloys supported on amorphous Al2O3. Systematic sample characterization demonstrates that amorphous alumina-supported PtNiFe nanoalloys are merited by excellent redox ability, outstanding O2 activation ability, and moderate CO adsorption strength. When tested as catalysts for CO oxidation, all samples have demonstrated an apparent interfacial effect on catalytic performance, among which Pt/NiFeAl-600 shows a strikingly high CO oxidation activity at low-temperatures coupled with a broader operation temperature window (i.e. CO conversion >99.0%, 100-400 °C). Such a topological transformation strategy has proven applicable for generating atomically dispersed nanoalloys on amorphous supports for catalytic applications.

New phenomena of photo-luminescence and persistent luminescence of a Eu2+,Tb3+ codoped Ca6BaP4O17 phosphor under high hydrostatic pressure.

In this study, we present a photo-luminescence (PL) and persistent luminescence (PersL) investigation of Ca6BaP4O17:Eu2+,Tb3+ (CBPO:Eu,Tb) at high hydrostatic pressure in the range of 0-11.04 GPa. More importantly, there is a significant increase of PL intensity and extension of PersL duration time at a pressure point of ∼0.15 GPa.

Evidence for the influence of polaron delocalization on the electrical transport in LiNi0.4+xMn0.4-xCo0.2O2.

Polaron delocalization in layered transition-metal oxides can considerably impact their physical properties and technological applications. Herein, we present the evidence for the influence of polaron delocalization on the electrical transport of layered oxides LiNi0.4+xMn0.4-xCo0.2O2, an active cathode material, by controlling the chemical compositions. We find that the chemical composition at x = 0.3 exhibits a sharp increment in electronic conductivity of four orders of magnitude at room temperature with respect to that at x = 0. We attribute the increased electronic conductivity to a low hopping energy in addition to a weak electron-phonon interaction. The weakened electron-phonon interaction is the source of polaron delocalization in LiNi0.4+xMn0.4-xCo0.2O2, which became improved with increasing x due to the increased polaron sizes. Moreover, it is also suggested that the polaron delocalization may have a relationship with the strong Jahn-Teller distortion induced by Ni3+. The analysis of temperature dependent electrical transport within the framework of the small polaron hopping conduction model enables us to comprehend the influence of polaron delocalization on the electrical transport pertinent to the applications of layered oxide materials.

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction.

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.