Topotactic Research Articles

Pressure‐Enhanced Superconductivity and Structural Phase Transition in Layered Sn4P3

High pressure provides a unique tuning method depending on structure modulation to explore the structure–property relationship. Herein, the pressure‐induced structural phase transformation and enhanced superconductivity in a layered binary phosphide Sn4P3 are reported. Comprehensive measurements using in situ synchrotron X‐Ray diffraction and Raman spectroscopy reveal a structural phase transition with mild distortion of SnP3 building blocks and interlayer shrinkage under high pressure. This differs from a conventional trigonal SnAs(P)3 to square SnAs(P)4 topotactic transition in SnAs(P)‐based compound. Through this structure reconstruction under high pressure, electron distribution has been reorganized and phonons have softened, facilitating a high superconducting temperature (Tc) value of 7.8 K at 34.9 GPa, which is almost six times higher than its ambient value. The study introduces a new transition route in layered SnAs/SnP‐based intermetallic materials and provides insight into the structural and electronic changes under high pressure for Sn4P3.

A New Semicrystalline Polymer from Renewable Source: Poly-1,4-(3-methylene-cyclopentene) as an Example of Three-Dimensional Topotactic Transition Triggered by Temperature

A New Semicrystalline Polymer from Renewable Source: Poly-1,4-(3-methylene-cyclopentene) as an Example of Three-Dimensional Topotactic Transition Triggered by Temperature

Third harmonic generation in monoclinic 2D KNbO3 with high power endurance

Two-dimensional (2D) KNbO3 is prepared by topotactic transition from 2D KNbO2 at 530 °C in air, which has a single spontaneous polarization (Ps) direction along the [101¯]pc zone axis. The strong Ps anisotropy leads to a negligible second harmonic generation of 2D KNbO3 with laser vertically incident on the sample. However, a strong third harmonic generation is observed. The effective third-order susceptibility of 2D KNbO3 with the order of ∼10−19 m2 V−2 is estimated by comparing it with the χeff(3) of graphene. The robust power tolerance of 2D KNbO3 endows it great potential in the application of high-power third harmonic generation 2D devices.

Hydrogen-Driven Low-Temperature Topotactic Transition in Nanocomb Cobaltite for Ultralow Power Ionic-Magnetic Coupled Applications.

We reversibly control ferromagnetic-antiferromagnetic ordering in an insulating ground state by annealing tensile-strained LaCoO3 films in hydrogen. This ionic-magnetic coupling occurs due to the hydrogen-driven topotactic transition between perovskite LaCoO3 and brownmillerite La2Co2O5 at a lower temperature (125-200 °C) and within a shorter time (3-10 min) than the oxygen-driven effect (500 °C, tens of hours). The X-ray and optical spectroscopic analyses reveal that the transition results from hydrogen-driven filling of correlated electrons in the Co 3d-orbitals, which successively releases oxygen by destabilizing the CoO6 octahedra into CoO4 tetrahedra. The transition is accelerated by surface exchange, diffusion of hydrogen in and oxygen out through atomically ordered oxygen vacancy "nanocomb" stripes in the tensile-strained LaCoO3 films. Our ionic-magnetic coupling with fast operation, good reproducibility, and long-term stability is a proof-of-principle demonstration of high-performance ultralow power magnetic switching devices for sensors, energy, and artificial intelligence applications, which are keys for attaining carbon neutrality.

Construction of coordination bond electron bridge boosting interfacial electron transfer to enhance bifunctional photocatalytic activity of TiO2/g-C3N4 heterojunction

The weak interfacial coupling of heterojunction catalysts leads to a large barrier, which inhibits interfacial electron transfer and limits the catalytic efficiency. Herein, a unique Ti-N coordination electron bridge has been designed and constructed in TiO2/g-C3N4 heterojunction with a tightly-coupled interface by using a newly designed topotactic transition and in-situ pyrolysis method. Experimental data and characterization analyses indicate that abundant stacking faults and surface oxygen vacancies change the surface atomic coordination environment and induce the formation of Ti-N coordination bond electron bridges. XPS, TA-PL probing experiment, and EPR analysis indicate a S-scheme electron transport mechanism. The synergistic effect of internal electric field and Ti-N electron bridge promote the efficient S-scheme electron transport, which facilitates the spatial separation of strong oxidizing holes and strong reducing electrons. The hierarchical nanoflower structure benefits electrolyte diffusion and reaction species adsorption. The above virtues endowed the catalyst with enhanced bifunctional photocatalytic activity for hydrogen evolution (1.89 mmol·g−1·h−1) and methyl blue degradation (0.114 min−1), which are 3.2 and 1.9 times that of the bare TiO2. This work opens a new avenue for the construction of heterojunctions and the optimization of interface carrier transfer, providing important insights for synthesizing photocatalysts with excellent activity and stability.

Topotactic Transition: A Promising Opportunity for Creating New Oxides

AbstractTopotactic transition is a structural phase change in a matrix crystal lattice mediated by the ordered loss/gain and rearrangement of atoms, leading to unusual coordination environments and metal atoms with rare valent states. As early as in 1990s, low temperature hydride reduction is utilized to realize the topotactic transition. Since then, topological transformations are developed via multiple approaches. Especially, the recent discovery of the Ni‐based superconductivity in infinite‐layer nickelates has greatly boosted the topotactic transition mean to synthesizing new oxides for exploring exotic functional properties. In this review, a detailed and generalized introduction is provided to oxygen‐related topotactic transition. The main body of the review includes four parts: the structure‐facilitated effects, the mechanism of the topotactic transition, some examples of topotactic transition methods adopted in different metal oxides (V, Mn, Fe, Co, Ni) and the related applications. This study is to provide timely and thorough strategies to successfully realize topotactic transitions for researchers who are eager to create new oxide phases or new oxide materials with desired functions.

Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

AbstractFine, pure Ti3AlC2 powder is prepared in a very mild condition via Ti3Al alloy and carbon black with the assistance of molten salts. X‐ray diffraction, scanning electron microscopy, TG‐DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti3AlC2 can be obtained at 900°C with the 1:1 salt‐to‐material ratio. The formation mechanism of Ti3AlC2 through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti3Al and C → TiAl and TiC → Ti2AlC and TiC → ψ and TiC → Ti5Al2C3 and TiC → Ti3AlC2, where the key ψ, a modulated Ti2AlC structure, is determined for the first time containing alternate‐displacement Al layers along (0 0 0 2) of Ti2AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti5Al2C3 through topotactic transition, followed by Ti5Al2C3 converting into Ti3AlC2 by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti2AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti5Al2C3 (0 0 0 3), Ti5Al2C3 (0 0 0 3)//Ti3AlC2 (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.

Topotactic transition of Ti4AlN3 MAX phase in Lewis acid molten salt

MAX phases and its derived two-dimensional MXenes have attracted considerable interest because of their rich structural chemistry and multifunctional applications. Lewis acid molten salt route provides an opportunity for structure design and performance manipulation of new MAX phases and MXenes, Although a series of new MAX phases and MXenes were successfully prepared via Lewis acid melt route in recent years, few work is explored on nitride MAX phases and MXenes. Herein, a new copper-based 413-type Ti4CuN3 MAX phase was synthesized through isomorphous replacement reaction using Ti4AlN3 MAX phase precursor in molten CuCl2. In addition, it was found that at high temperature Ti4N3Clx MXene will transform into two-dimensional cubic TiNα nanosheets with improved structural stability.

Methods and strategies for producing porous photocatalysts: Review

The desire to improve photocatalytic activity is keep growing. The Porous photocatalyst been synthesized with significant improvement in its surface area and reduction in the electron-hole pair recombination especially in semiconductors. This paper reviewed recent works on porous photocatalyst with a focus on its synthesis and fabrication methods. According to this study, the topotactic transition technique is the best method to fabricate metal oxide porous photocatalyst, while self-organizing blocks are the best method to fabricate polymeric porous photocatalyst. Particularly in growing and fixing of 1D semiconductor nanomaterials on 3D, and 2D semiconductors on 2D. The hard template method is the one who provides more capability for controlling photocatalyst particle’s shape and size, but its template removal is non-ecofriendly, due to that; a soft template is considered more favorable. On the other hand, the etching method is appropriate for fabricating porous photocatalyst through a membrane, using the electrical charge created by moving electrons in the electrolyte as a driving force reaction.

Atomic-scale observation of strain-dependent reversible topotactic transition in La0.7Sr0.3MnOx films under an ultra-high vacuum environment

Reversible topotactic phase transition between perovskite (PV) and oxygen-vacancy-ordered brownmillerite (BM) structures provides an effective platform for realizing the control of physical properties in complex transition metal oxides. However, such reversibility always requires extreme external conditions, that is, a high temperature and vacuum environment during the PV-BM transition, while an oxidizing atmosphere and relatively low temperature vice versa. Here, we experimentally observe the reversible process in strained La0.7Sr0.3MnOx films at atomic scale by using in-situ heating aberration-corrected scanning transmission electron microscopy (STEM). Apart from the conventional reduction reaction of creating oxygen-vacancy-ordered frameworks after heating in the TEM, the inverse process of BM-to-PV transition is unexpectedly discovered under such an ultra-high vacuum atmosphere (∼10−9 Torr) at room temperature. Moreover, this abnormal behavior is strain-dependent. The large compressive strain is found to be detrimental to the inverse phase transition. The density functional theory (DFT) calculations show that the high oxygen affinity of La0.7Sr0.3MnO2.5 is responsible for the reversible transitions. Our findings provide a new insight into the redox reactions of manganite and might be further utilized for potential applications in solid fuel cells, oxygen sensors or resistive switching memories.

High-surface-area corundum nanoparticles by resistive hotspot-induced phase transformation

High-surface-area α-Al2O3 nanoparticles are used in high-strength ceramics and stable catalyst supports. The production of α-Al2O3 by phase transformation from γ-Al2O3 is hampered by a high activation energy barrier, which usually requires extended high-temperature annealing (~1500 K, > 10 h) and suffers from aggregation. Here, we report the synthesis of dehydrated α-Al2O3 nanoparticles (phase purity ~100%, particle size ~23 nm, surface area ~65 m2 g−1) by a pulsed direct current Joule heating of γ-Al2O3. The phase transformation is completed at a reduced bulk temperature and duration (~573 K, < 1 s) via an intermediate δʹ-Al2O3 phase. Numerical simulations reveal the resistive hotspot-induced local heating in the pulsed current process enables the rapid transformation. Theoretical calculations show the topotactic transition (from γ- to δʹ- to α-Al2O3) is driven by their surface energy differences. The α-Al2O3 nanoparticles are sintered to nanograined ceramics with hardness superior to commercial alumina and approaching that of sapphire.

Atomic-scale operando observation of oxygen diffusion during topotactic phase transition of a perovskite oxide

<h2>Summary</h2> Topotactic phase transition of perovskite oxides enables fast, reversible oxygen transport with minimal volume change, which is advantageous for applications in solid oxide fuel cells. However, the oxygen-diffusion mechanism remains elusive due to the lack of direct atomic-scale observations. Here, we report operando atomic-scale observation and simulation revealing the diffusion mechanism during the topotactic transition of perovskite SrFeO₃ to brownmillerite SrFeO_2.5. Hyper-stoichiometric brownmillerite phase containing excess oxygen emerges at the phase boundary facilitates oxygen diffusion; oxygen diffuses predominantly along the FeO₄ tetrahedral chains via sequential modification of oxygen coordination between FeO₄ and FeO₅. A steady-state oxygen diffusion is attained through interstitialcy diffusion across the fast-diffusion channels, which accommodates excess oxygen at the interstitial sites between SrO columns. The flexibility of multivalent Fe ions in accommodating various oxygen coordination and the rigidity of Sr lattice framework embracing excess oxygen are key to the fast, anisotropic oxygen diffusion.

Centimeter-scale perovskite SrTaO2N single crystals with enhanced photoelectrochemical performance

Large-scale single crystals have potential applications in many fields, such as in ferroelectric and photoelectric energy conversion devices. Perovskite oxynitrides have also attracted attention in photoelectrochemical water splitting systems because of their high theoretical solar-to-hydrogen efficiencies. Nevertheless, the synthesis of perovskite oxynitride single crystals requires the coupling of cation exchange and ammonization processes, which is exceptionally challenging. The present study demonstrates an inorganic vapor method that provides, for the first time ever, high-quality epitaxial perovskite SrTaO2N single crystals on the centimeter scale. Assessments using Raman spectroscopy, crystal structure analysis and density functional theory determined that the conversion mechanism followed a topotactic transition mode. Compared with conventional SrTaO2N particle-assembled films, the SrTaO2N single crystals made in this work were free of interparticle interfaces and grain boundaries, which exhibited extremely high performance during photoelectrochemical water oxidation. In particular, these SrTaO2N single crystals showed the highest photocurrent density at 0.6 V vs. RHE (1.20 mA cm−2) and the highest photocurrent filling factor (47.6%) reported to date, together with a low onset potential (0.35 V vs. RHE). This onset potential was 200 mV less than that of the reported in situ SrTaO2N film, and the photocurrent fill factor was improved by 2 to 3 times.

Ultralow contents of AgNbO3 fibers induced high energy storage density in ferroelectric polymer nanocomposites

Polymer dielectric films have been widely used in electronic and power systems due to their unique dielectric properties, processing properties, and excellent cost performance. Although the dielectric constant of dielectric polymers can be improved by adding high contents of ceramic fillers, this approach comes at the expense of the breakdown strength (Eb). This work is inspired by the idea that high-aspect-ratio fibers can induce a larger electric dipole moment without sacrificing too much Eb compared to zero-dimensional nanoparticles, thereby effectively improving the energy storage performance of the composites. We synthesized antiferroelectric AgNbO3 (ANO) fibers by using an in situ topotactic transition reaction which were then introduced into the polyvinylidene fluoride (PVDF) matrix for energy storage applications. The results show that the nanocomposite film with an ultralow loading of 0.4 wt. % ANO fibers achieves a high discharge energy density of 12.97 J cm−3 at 490 MV m−1. Nanocomposites based on ultralow content ANO fibers show great promise as one-dimensional antiferroelectric fillers for high energy density capacitor applications.

Constructing Anatase-Brookite TiO2 Phase Junction by Thermal Topotactic Transition to Promote Charge Separation for Superior Photocatalytic H2 Generation.

Phase junctions of photocatalysts can promote the separation of photogenerated charge carriers for efficient utilization of the carriers. Construction of phase junctions and establishing their structure-performance relationship are still required. Herein, polycrystalline TiO2 decahedral plates with different phases were synthesized by thermal treatment-induced topotactic transition of titanium oxalate crystals. The phase of TiO2 evolved from pure anatase to anatase-brookite, anatase-brookite-rutile, and then to anatase-rutile, while the morphology of the decahedral plates was well maintained. The biphase anatase-brookite was found to be most efficient in photocatalytic hydrogen generation. Specifically, the hydrogen generation rate of the biphase anatase-brookite TiO2 was nearly 2.4 times greater than that of the biphase anatase-rutile TiO2. The spatially resolved surface photovoltage measurements indicate the more efficient separation of photogenerated charge carriers and thus greater photocatalytic activity of the former. This work provides a strategy for developing efficient phase-junction photocatalysts.

Selenous Acid in an Aromatic Framework: Insights Into a Temperature-Sensitive Internal Redox System from the Solid State.

We report on a new compound composed of a phenanthroline network in which emerging channels are alternately occupied by selenous acid (H2SeO3) and dioxane molecules. The material undergoes a variety of structural changes due to both its redox activity as well as its thermal decomposition. We investigate an internal redox system of the incorporated selenous acid and the aldehyde groups of the phenanthroline framework. The reduction process of the selenium species was further elucidated by cyclic voltammetry, while the oxidation process was also monitored by 1H NMR spectra. The thermal behavior reveals that the material can undergo a reversible, topotactic transition due to dioxane and water (de)intercalation.

Comparison of topotactic and magnetic structures for manganite oxide films

Deliberate manipulation of topotactic transformation via oxygen insertion/extraction offers a feasible way to precisely tune oxygen concentration, B–O coordination occupation and resultant electronic functionalities of perovskite (ABO3)-based oxides. Herein, two methods, a post-annealing CaH2 treatment and an in-situ oxygen-vacancy diffusion using oxygen getter layer, were conducted to achieve topotactic transitions from perovskite to brownmillerite of La0·7Sr0·3MnO3-δ (LSMO, 0 ≤ δ ≤ 0.5) films. The results revealed that delicate control of oxygen composition is the crucial factor for evolutions of crystalline structure and subsequent magnetic properties of LSMO films. In particular, the brownmillerite LSMO film treated by the in-situ oxygen getter layer possesses desirable structural stability over time. In contrast, the topotactic LSMO films using reduction agents CaH2 exhibit compositional and structural inhomogeneities. These findings suggest that adjustable topotactic transformation through anionic oxygen provides a facile strategy for designing multifunctional perovskite-derivative oxide materials.

Phase controlled synthesis of transition metal carbide nanocrystals by ultrafast flash Joule heating

Nanoscale carbides enhance ultra-strong ceramics and show activity as high-performance catalysts. Traditional lengthy carburization methods for carbide syntheses usually result in coked surface, large particle size, and uncontrolled phase. Here, a flash Joule heating process is developed for ultrafast synthesis of carbide nanocrystals within 1 s. Various interstitial transition metal carbides (TiC, ZrC, HfC, VC, NbC, TaC, Cr2C3, MoC, and W2C) and covalent carbides (B4C and SiC) are produced using low-cost precursors. By controlling pulse voltages, phase-pure molybdenum carbides including β-Mo2C and metastable α-MoC1-x and η-MoC1-x are selectively synthesized, demonstrating the excellent phase engineering ability of the flash Joule heating by broadly tunable energy input that can exceed 3000 K coupled with kinetically controlled ultrafast cooling (>104 K s−1). Theoretical calculation reveals carbon vacancies as the driving factor for topotactic transition of carbide phases. The phase-dependent hydrogen evolution capability of molybdenum carbides is investigated with β-Mo2C showing the best performance.

Optimization of the multi-mem response of topotactic redox La1/2Sr1/2Mn1/2Co1/2O3−x

Memristive systems emerge as strong candidates for the implementation of resistive random access memories and neuromorphic computing devices, as they can mimic the electrical analog behavior or biological synapses. In addition, complementary functionalities, such as memcapacitance, could significantly improve the performance of bio-inspired devices in key issues, such as energy consumption. However, the physics of mem systems is not fully understood so far, hampering their large-scale implementation in devices. Perovskites that undergo topotactic transitions and redox reactions show improved performance as mem systems, compared to standard perovskites. In this paper, we analyze different strategies to optimize the multi-mem behavior (memristive and memcapacitive) of topotactic redox La1/2Sr1/2Mn1/2Co1/2O3−x (LSMCO) films grown on Nb:SrTiO3. We explored devices with different crystallinities (from amorphous to epitaxial LSMCO), out-of-plane orientation [(001) and (110)], and stimulated either with voltage or current pulses. We found that an optimum memory response is found for epitaxial (110) LSMCO stimulated with current pulses. Under these conditions, the system efficiently exchanges oxygen with the environment minimizing, at the same time, self-heating effects that trigger nanostructural and chemical changes that could affect the device integrity and performance. Our work contributes to pave the way for the integration of multi-mem topotactic redox oxide-based interfaces in multiple device architectures, in order to exploit their memristive and memcapacitive properties for data storage or neuromorphic computation.

Migration Kinetics of Surface Ions in Oxygen‐Deficient Perovskite During Topotactic Transitions (Small 51/2021)

Topotactic Transitions In article number 2104356, Lei Cao, Oleg Petracic, and co-workers report the first observation of a novel surface phase in La0.7Sr0.3MnO3-δ epitaxial thin films during the topotactic transition from perovskite to brownmillerite. Given the anisotropic vacancy channels found in the surface phase, the studies not only provide a deeper understanding of oxygen-transport behavior in oxides, but also offer optimal guidance to engineer coordination polyhedra towards desired properties.