Reversible Structural Transformation Research Articles

Crystalline structures and optoelectronic properties of orthorhombic CsPbBr3 polycrystalline films grown by the Co-evaporation method

In order to fabricate pure-phase CsPbBr3 films with high crystalline quality, the stoichiometry between precursors is of great importance. However, the precise control of the stoichiometry is difficult for solution processed methods, which is hindered by the poor solubility of CsBr in commonly used solvents. On the other hand, in terms of co-evaporation method, the stoichiometry of CsPbBr3 films can be easily modulated by controlling the deposition rates of precursors. Herein, the impact of deposition rate ratio between CsBr and PbBr2 on the crystalline quality and optoelectronic properties of orthorhombic CsPbBr3 polycrystalline films has been investigated. CsPbBr3 polycrystalline films with high crystalline quality and preferential crystalline orientation have been fabricated using co-evaporation method at the optimized deposition rate ratio. The reversible structural transformation from orthorhombic to cubic phase CsPbBr3 films is confirmed using temperature-dependent XRD characterizations. At the deposition ratio of 0.5:0.7, largest grain size of 7.82 × 104 nm2 along with a high uniformity is observed for CsPbBr3 films. Moreover, the longest lifetime of 24.0 ns for photo-generated exciton within CsPbBr3 films has also been revealed at the optimized deposition rate ratio. Because of the high crystalline quality and prolonged lifetime for photo-generated exciton, a photoresponsivity of 20.2 A/W and external quantum efficiency of 48.7% are achieved for CsPbBr3 based photodetector. Our findings demonstrate a facile approach for the growth of CsPbBr3 polycrystalline films with high crystalline quality and promote their applications in optoelectronic devices.

High-Performance Selective CO2 Capture on a Stable and Flexible Metal-Organic Framework via Discriminatory Gate-Opening Effect.

Selective CO2 capture is of great significance for environmental protection and industrial demand. Here, we report a stable and flexible metal-organic framework (MOF) with excellent water/moisture stability, namely, ZnDatzBdc, that enables high-performance selective CO2 capture from N2 and CH4 via a discriminatory gate-opening effect. ZnDatzBdc shows reversible structural transformation between the open-phase (OP) state and the close-phase (CP) state, owing to the synergistic effect of breakage/re-formation of intraframework hydrogen bonds and the rotation of the phenyl rings. Significantly, ZnDatzBdc exhibits S-shaped isotherms toward CO2, resulting in a large CO2 theoretical working capacity of 94.9 cm3/cm3 under typical pressure vacuum swing adsorption (PVSA) operations, which outperforms other flexible MOFs showing the CO2 selective gate-opening effect except for the miosture-sensitive ELM-11. In addition, CO2 uptake of ZnDatzBdc is well maintained upon multiple water/moisture exposure, indicating its excellent stability. Moreover, ZnDatzBdc establishes remarkable CO2 selectivity with ultrahigh uptake ratios of CO2/N2 (107 at 273 K and 129 at 298 K) and CO2/CH4 (35 at 273 K and 44 at 298 K) at 100 kPa. The in situ gas sorption PXRD experiment verifies that the gate-opening effect takes place in the atmospheric environment of CO2 but not for N2 or CH4. Molecular simulation suggests the selective gate-opening of CO2 comes from its strong electrostatic interactions with the amino groups. Furthermore, effective breakthrough performance and easy regeneration are further confirmed. Hence, combined with excellent separation performance and remarkable stability, ZnDatzBdc can serve as a potential industrial adsorbent for selective CO2 capture.

Strain Glass State, Strain Glass Transition, and Controlled Strain Release

Strain glass is a new strain state discovered recently in ferroelastic systems that is characterized by nanoscale martensitic domains formed through a freezing transition. These nanodomains typically have mottled or tweed morphology depending on the elastic anisotropy of the system. Strain glass transition is a broadly smeared and high order–like transition, taking place within a wide temperature or stress range. It is accompanied by many unique properties, including linear superelasticity with high strength, low modulus, Invar and Elinvar anomalies, and large magnetostriction. In this review, we first discuss experimental characterization and testing that have led to the discovery of the strain glass transition and its unique properties. We then introduce theoretical models and computer simulations that have shed light on the origin and mechanisms underlying the unique characteristics and properties of strain glass transitions. Unresolved issues and challenges in strain glass study are also addressed. Strain glass transition can offer giant elastic strain and ultralow elastic modulus by well-controlled reversible structural phase transformations through defect engineering.

Multiple yet switchable hydrogen-bonded organic frameworks with white-light emission

The development of new strategies to construct on-demand porous lattice frameworks from simple motifs is desirable. However, mitigating complexity while combing multiplicity and reversibility in the porous architectures is a challenging task. Herein, based on the synergy of dynamic intermolecular interactions and flexible molecular conformation of a simple cyano-modified tetraphenylethylene tecton, eleven kinetic-stable hydrogen-bonded organic frameworks (HOFs) with various shapes and two thermo-stable non-porous structures with rare perpendicular conformation are obtained. Multimode reversible structural transformations along with visible fluorescence output between porous and non-porous or between different porous forms is realized under different external stimuli. Furthermore, the collaborative of flexible framework and soft long-chain guests facilitate the relaxation from intrinsic blue emission to yellow emission in the excited state, which represents a strategy for generating white-light emission. The dynamic intermolecular interactions, facilitated by flexible molecular conformation and soft guests, diversifies the strategies of construction of versatile smart molecular frameworks.

Enhancement of Output Performance of Triboelectric Nanogenerator by Switchable Stimuli in Metal-Organic Frameworks for Photocatalysis.

Precise control of the structure of crystalline materials is an efficient strategy to manipulate the fundamental performance of solids. In metal-organic framework (MOF) materials, this control can be realized by reversible cation-exchange through chemically driven changes in the crystalline state. Herein, we reported that the reversible structural transformations between an anionic Zn-MOF (1) and a topologically equivalent bimetallic Zn/Co-MOF (2) were accomplished. Both MOFs powders and their hybrid composites were used as positive electrode materials to assemble triboelectric nanogenerators (TENGs). The results demonstrated that the output performance of the Zn/Co-MOF-TENG was effectively improved because the introduction of Co ions makes electron transfer easier. Moreover, the output performance of the TENGs based on MOF@PVDF (PVDF = polyvinylidene fluoride) composite films showed that the Zn/Co-MOF@PVDF-TENG possessed much higher output than these corresponding film-based and MOF-based TENGs. As a practical application, the superior output of Zn/Co-MOF@PVDF-TENG was used to light an ultraviolet lamp plate for the [2 + 2] photochemical cycloaddition of organometallic macrocycles.

Programming polymorphable yet stiff truss metamaterials in response to temperature

Shape morphing in response to an external stimulus can create geometric transformations from initially flat materials. Current strategies rely on thin layers made of soft materials that are inherently floppy to provide morphability, but cannot support their own weight and must forgo structural resistance, hence limiting their use as structural materials. Here, we demonstrate thermally actuated shape morphing of a three-dimensional framework that can evolve into complex, lightweight shapes that are both stiff and morphable, hence resolving the conflict between rigidity and morphability. Our building block is a truss unit cell that leverages the mismatched thermal expansions of its rigid constituents to generate programmable spatial deformation. By encoding specific geometric and multimaterial attributes into the truss system, we realize shape morphing on target and generate reversible structural transformations with complex curvature combinations. We also explore cooling and localized heating, unveiling temperature-dependent morphological evolutions and heat-source tracking. The morphable, yet stiff, truss metamaterial here introduced has potential applications in thermally responsive smart buildings, thermal management adaptive devices, deployable shelters in extreme environments, and mirror optical systems in space.

Mitigating the voltage fading and air sensitivity of O3-type NaNi0.4Mn0.4Cu0.1Ti0.1O2 cathode material via La doping

The practical application of O3-type layered oxides cathode materials for sodium ion batteries (SIBs) is hindered by two major difficulties, including capacity decay rooted in multiphase transitions and storage issue originated from sensitivity to moisture. Herein, we report a series of O3-type layered oxides Na(Ni0.4Cu0.1Mn0.4Ti0.1)1-xLaxO2 (x = 0, 0.001, 0.003, 0.005, termed as NMCT, NMCT-Lax, respectively) cathode materials synthesized via spray pyrolysis followed by high-temperature sintering method, which manifest enhanced electrochemical performance and air stability as compared to the NaNi0.5Mn0.5O2 (termed as NM) material. Optimum performance is achieved in the NMCT-La0.001 material that delivers a high initial discharge capacity of 170.1 mAh g−1 in the voltage range of 2.0–4.4 V at 0.1C (1C = 150 mA g−1). NMCT-La0.001 exhibits a highly reversible structural transformation during charging/discharging as demonstrated by in-situ XRD, and it keeps the original crystal structure after being exposed to the air for 30 days. The capacity decay of NMCT is effectively decelerated at high voltage (>4.2 V) after La doping, as which can stabilize the structure of the material by reducing the Mn3+ content through suppressing the Jahn-Teller effect. The research with respect to La doping provides a facile and instructive way for the design of high-performance cathode materials for SIBs.

Component Engineering to Tailor the Structure and Optical Properties of Sb-Doped Indium-Based Halides.

Controlling the structure of halide perovskites through component engineering, and thus revealing the changes in luminescence properties caused by the conversion of crystal structure, is of great significance. Herein, we report a controllable synthetic strategy of three-dimensional (3D) Cs2KInCl6 and zero-dimensional (0D) (Cs/K)2InCl5(H2O) halide perovskites by changing the Cs/K feed ratio. 3D Cs2KInCl6 double perovskites are obtained at the Cs/K feed ratio of 1:1, while 0D (Cs/K)2InCl5(H2O) perovskites are formed at the Cs/K feed ratio of 2:1. Further, a reversible crystal structure transformation between 3D Cs2KInCl6 double perovskites and 0D (Cs/K)2InCl5(H2O) perovskites can be achieved by subsequent addition of metal-salt precursors. In addition, the emission efficiency of two perovskite structures can be greatly boosted by breaking the forbidden transition through Sb doping, and as a result, a novel green/yellow reversible emission switch is generated. Meanwhile, the relationship between perovskite structure and luminescence mechanism has been systematically revealed. These environmentally stable halide perovskites have great potential to be applied in optoelectronic devices.

Flexible Vertex Engineers the Controlled Assembly of Distorted Supramolecular Tetrahedral and Octahedral Cages.

Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.

Stepwise assembly and reversible structural transformation of ligated titanium coated bismuth-oxo cores: shell morphology engineering for enhanced chemical fixation of CO2.

Herein, we report the stepwise assembly and reversible transformation of atomically precise ligated titanium coated bismuth-oxide core nanostructures. The soluble and stable Bi38O45@Ti6-oxo clusters with weakly coordinated surface salicylate ligands were first prepared as precursors. Owing to the high surface reactivity of the Bi38O45 inner core, its shell composition and morphology could be systemically modified by assembly with various Ti ions and auxiliary ligands (L), especially those with different flexibility, bridging ability and steric hindrance. As a result, a series of new core–shell Bi38O44/45@TixL-oxo (x = 14, 16, 18 or 20) clusters containing gradually increasing shell Ti atoms were successfully synthesized. Among them, the Bi38Ti20-oxo cluster is the largest one in the family of heterometallic Bi/Ti-oxo clusters to date. In addition, the sensitized titanium outer shell can effectively improve the photocurrent response under visible light irradiation. More remarkably, the obtained core–shell Bi38O44/45@TixL-oxo clusters can serve as stable and efficient catalysts for CO2 cycloaddition with epoxides under ambient conditions, whose activity was significantly influenced by the outer ligated titanium shell structure. This work provides a new insight into the construction of atomically precise heterometallic core–shell nanostructures and also an interesting shell engineering strategy for tuning their physicochemical properties.

Structural specificity of groove binding mechanism between imidazolium-based ionic liquids and DNA revealed by synchrotron-UV Resonance Raman spectroscopy and molecular dynamics simulations

The predicted capability of Ionic Liquids (ILs) in stabilizing the native structure of nucleic acids is relevant in biotechnology, especially for DNA storage and handling. In the present work, we implement a joint combination of advanced spectroscopic techniques such as synchrotron radiation-UV Resonance Raman spectroscopy (SR-UVRR) and molecular dynamics (MD) simulations for deepening insight into the sequence and structural specificity of the binding interactions between imidazolium-based ILs and both the phosphate groups and nucleobases in the minor and major grooves of double-stranded DNA. A 30-base pair double-stranded DNA structure has been chosen as a model of natural DNA. The experimental and simulation results give evidence of the predominance of a groove binding mechanism between ILs cations and DNA, with preferential interactions among guanine residues and the shorter alkyl-chain length on imidazolium cations. Raman experiments allowed us to detect both cooperative transition and reversible pre-melting structural transformations that involve specific tracts in the structure of DNA and are turned on at lower temperatures for guanine residues than for adenine ones. The more marked effect on the pre-melting states of adenine operated by the imidazolium-based ILs with chloride as anion suggests a selective strong interaction of this anion with the DNA’s adenine-rich tracts. MD simulation results reveal the influence of ILs on the structural properties of DNA and provide more details about the solvation, interaction, stability and flexibility of DNA in the hydrated ILs. According to MD analyses, simultaneous electrostatic and hydrophobic interactions drive the shorter alkyl-chain length of imidazolium cations to have greater interplays with the DNA major groove.

Flexibility of a Metal–Organic Framework Enhances Gas Separation and Enables Quantum Sieving

Flexible metal–organic frameworks (MOFs) undergo reversible structural transformations triggered by external stimuli. An interesting feature of some MOFs is their ability to flex in response to spe...

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

Pseudoelasticity of SrNi2P2 Micropillar via Double Lattice Collapse and Expansion.

The maximum recoverable strain of most crystalline solids is less than 1% because plastic deformation or fracture usually occurs at a small strain. In this work, we show that a SrNi2P2 micropillar exhibits pseudoelasticity with a large maximum recoverable strain of ∼14% under uniaxial compression via unique reversible structural transformation, double lattice collapse-expansion that is repeatable under cyclic loading. Its high yield strength (∼3.8 ± 0.5 GPa) and large maximum recoverable strain bring out the ultrahigh modulus of resilience (∼146 ± 19 MJ/m3), a few orders of magnitude higher than that of most engineering materials. The double lattice collapse-expansion mechanism shows stress-strain behaviors similar to that of conventional shape-memory alloys, such as hysteresis and thermo-mechanical actuation, even though the structural changes involved are completely different. Our work suggests that the discovery of a new class of high-performance ThCr2Si2-structured materials will open new research opportunities in the field of pseudoelasticity.

Photomechanical polymer hydrogels based on molecular photoswitches

AbstractAs intelligent materials responsive to light, photomechanical hydrogels not only possess high‐water content, excellent softness and biocompatibility, but also can accomplish various mechanical motions upon spatiotemporal stimulation of external light, which exhibit great potential in biomedical and underwater bionic fields. Molecular photoswitches have been used broadly in preparation of photomechanical hydrogels owing to their high photosensitivity and reversible molecular structure transformations induced by light. Herein, the current progress of photomechanical hydrogels based on typical molecular photoswitches such as spiropyran, azobenzene, and hexaarylbiimidazole (HABI) are introduced. Especially, as a promising building unit for photomechanical hydrogels, HABI has been highlighted due to the unique molecular structures and reversible photoswitching capability. HABI‐derived polymer hydrogels demonstrate flexible mechanical behaviors upon localized light irradiation. The characteristics and challenges of photomechanical hydrogels based on molecular photoswitches are also prospected.

Stimuli-responsive switchable halide perovskites: Taking advantage of instability

<h2>Summary</h2> Halide perovskites have recently shown potential as stimuli-responsive materials (SRMs) for a range of important technologies such as smart windows, memory devices, data storage, and sensors. Here, we overview these emerging SRMs that are based on halide perovskite systems of switchable optical and electrical properties as well as discuss their switching characteristics and practical applications. We summarize the reversible chemical and structural transformations of these materials and categorize them by their switching mechanisms. Furthermore, to guide the community's search for new designs of stimuli-responsive halide perovskites, we outline several important criteria for effective switchable materials. Finally, we provide our perspective on the current challenges and future developments of these emerging materials.

Tunable phase transition, band gap and SHG properties by halogen replacement of hybrid perovskites [(thiomorpholinium)PbX3, X = Cl, Br, I

By the replacement of halogen anion, three new multifunctional organic-inorganic hybrid perovskites (thiomorpholinium)PbX3 (X = Cl, Br, I) were successfully synthesized and underwent reversible structural transformation above room temperature, accompanied by the anomalous change of dielectric constant. With the adjustment of the halogen anion from Cl to I in the inorganic skeleton, the space group is transformed from centrosymmetric space group P21/c ((thiomorpholinium)PbCl3) to chiral one P212121 ((thiomorpholinium)PbBr3, (thiomorpholinium)PbI3) at room temperature. The ordered-disordered transition of organic cations and the change of hydrogen bonds with the increase of temperature lead to above-room-temperature phase transitions. Ultraviolet absorption and second-harmonic generation (SHG) measurements confirmed that both the band gap and SHG activity of (thiomorpholinium)PbX3 (X = Cl, Br, I) crystals were tunable. The band gaps reveal a broadening trend with 3.532 eV, 3.410 eV and 3.175 eV along the Cl → Br → I series. This work provides an effective molecular design for multifunctional organic-inorganic perovskites.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces.

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10-7 mbar of O2, and a D2/O2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D2/O2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Light-Enabled Reversible Shape Transformation of Block Copolymer Particles

Confined self-assembly of block copolymers (BCPs) is effective to manipulate various shapes of particles. In emulsion confined self-assembly, reversibly light-trigged switchable BCP particles are extremely expected, yet rarely reported. Herein, a novel strategy is developed to realize reversibly light-responsive shape-transformation of BCP particles by constructing functional surfactants with light-active azobenzene (azo) groups in the confined self-assembly of BCPs within emulsion droplet. Ultraviolet and visible lights can reversibly modulate the amphiphilicity and interfacial affinity of the surfactants to different blocks, triggering the reversible microphase structure transformation of BCP particles with high temporal-spatial resolution. We can realize shape and morphological transitions of BCP particles from onion-shaped spherical particles to striped ellipsoids and, ultimately, to inverse onion-like particles by ultraviolet irradiation. More importantly, this shape transformation is reversible by the irradiation of visible light, attributed to the reversible trans-cis isomerization of azo groups. We also demonstrate that the light-triggered shape transformation of BCP particles can be employed in a controllable drug release through a noncontacted and programmed manner, showing promising potential in clinic and biomedicine.