Cation-eutaxy-enabled III-V-derived van der Waals crystals as memristive semiconductors

Novel two-dimensional semiconductor crystals can exhibit diverse physical properties beyond their inherent semiconducting attributes, making their pursuit paramount. Memristive properties, as exemplars of these attributes, are predominantly manifested in wide-bandgap materials. However, simultaneously harnessing semiconductor properties alongside memristive characteristics to produce memtransistors is challenging. Herein we prepared a class of semiconducting III-V-derived van der Waals crystals, specifically the HxA1-xBX form, exhibiting memristive characteristics. To identify candidates for the material synthesis, we conducted a systematic high-throughput screening, leading us to 44 prospective III-V candidates; of these, we successfully synthesized ten, including nitrides, phosphides, arsenides and antimonides. These materials exhibited intriguing characteristics such as electrochemical polarization and memristive phenomena while retaining their semiconductive attributes. We demonstrated the gate-tunable synaptic and logic functions within single-gate memtransistors, capitalizing on the synergistic interplay between the semiconducting and memristive properties of our two-dimensional crystals. Our approach guides the discovery of van der Waals materials with unique properties from unconventional crystal symmetries.

Construction of Hierarchical Films via Layer-by-Layer Assembly of Exfoliated Unilamellar Zeolite Nanosheets

Zeolites have been widely applied as versatile catalysts, sorbents, and ion exchangers with unique porous structures showing molecular sieving capability. In these years, it is reported that some layered zeolites can be delaminated into molecularly thin 2-dimensional (2D) nanosheets characterized by inherent porous structures and highly exposed active sites. In the present study, two types of zeolite nanosheets with distinct porous structures with MWW topology (denoted mww) and ferrierite-related structure (denoted bifer) are deposited on a substrate through the solution process via electrostatic self-assembly. Alternate deposition of zeolite nanosheets with polycation under optimized conditions allows the layer-by-layer growth of their multilayer films with a stacking distance of 2-3 nm. Furthermore, various hierarchical structures defined at the unit-cell dimensions can be constructed simply by conducting the deposition of mww and bifer nanosheets in a designed sequence. Adsorption of a dye, Rhodamine B, in these films, is examined to show that adsorption is dependent on constituent zeolite nanosheets and their assembled nanostructures. This work has provided fundamental advancements in the fabrication of artificial zeolite-related hierarchical structures, which may be extended to other zeolite nanosheets, broadening their functionalities, applications, and benefits.

The Grafting of Hydroxyaromatic Organics within Layered Perovskites via a Microwave-Assisted Method

A new series of inorganic-organic hybrid perovskite materials were prepared by microwave-assisted grafting reactions. Simple carboxylic acids, acetic acid, and propionic acid, as well as hydroxyaromatic carboxylic acids, 3,5-dihydroxy benzoic acid (DBA), 5-hydroxyisophthalic acid (HPA), 4-hydroxybenzoic acid (HBA), and 4-hydroxy-4-biphenyl carboxylic acid (HBCA), were reacted with the Dion-Jacobson double-layered perovskite, HLaNb2O7, and its alcoxy derivatives. Grafting was found to not occur with simple carboxylic acids, while those molecules with hydroxyls were all attached to the perovskite interlayers. Reactivity of the hydroxyaromatic carboxylic acids varied with the different layered perovskite hosts where reactions with HLaNb2O7 did not occur, and those with n-propoxy-LaNb2O7 were limited; the greatest extent of reactivity was seen with n-decoxy-LaNb2O7. This is attributed to the larger interlayer spacing available for the insertion of the various hydroxyaromatic carboxylic acid compounds. The loading exhibited by the grafting species was less than that seen with well-known long-chain alkoxy grafting groups. It is expected that the width of the molecules contributes to this where, due to the benzyl groups, the interlayer volume of the grafted moieties occupies a larger horizontal fraction, therefore minimizing the loading to the below half. X-ray powder diffraction and transmission electron microscopy studies found that grafting of the n-decoxy-LaNb2O7 intermediates with the series of hydroxyaromatics resulted in a reduction in crystallinity along with a disruption of the layer structure. Raman data on the series show little variation in local structure except for HBCA, where there appears to be a lengthening of the Nb-O apical linkage and a possible reduction in the distortion of inner-layer NbO6 octahedra. The optical properties of the hydroxyaromatic carboxylic acid grafted perovskites were also investigated using diffuse-reflectance UV-Vis spectroscopy. The band gaps of DBA, HPA, and HBA were found to be similar to the parent (Eg ≈ 3.4 eV), while the HBCA was significantly less by ca. 0.6 eV. This difference is attributed to electron withdrawal from the perovskite block to the HBCA ligand, leading to a lower band gap for the HBCA compound. The methods described herein allow for the formation of a new series of inorganic-organic hybrid materials where the products are of interest as precursors to more complex architectures as well as models for band gap modification of metal oxide photocatalysts.

Hydrothermally Synthesized ZnCr- and NiCr-Layered Double Hydroxides as Hydrogen Evolution Photocatalysts

The layered double hydroxides (LDHs) of transition metals are of great interest as building blocks for the creation of composite photocatalytic materials for hydrogen production, environmental remediation and other applications. However, the synthesis of most LDHs is reported only by the conventional coprecipitation method, which makes it difficult to control the catalyst's crystallinity. In the present study, ZnCr- and NiCr-LDHs have been successfully prepared using a facile hydrothermal approach. Varying the hydrothermal synthesis conditions allowed us to obtain target products with a controllable crystallite size in the range of 2-26 nm and a specific surface area of 45-83 m2∙g-1. The LDHs synthesized were investigated as photocatalysts of hydrogen generation from aqueous methanol. It was revealed that the photocatalytic activity of ZnCr-LDH samples grows monotonically with the increase in their average crystallite size, while that of NiCr-LDH ones reaches a maximum with intermediate-sized crystallites and then decreases due to the specific surface area reduction. The concentration dependence of the hydrogen evolution activity is generally consistent with the standard Langmuir-Hinshelwood model for heterogeneous catalysis. At a methanol content of 50 mol. %, the rate of hydrogen generation over ZnCr- and NiCr-LDHs reaches 88 and 41 μmol∙h-1∙g-1, respectively. The hydrothermally synthesized LDHs with enhanced crystallinity may be of interest for further fabrication of their nanosheets being promising components of new composite photocatalysts.

Interlayer Engineering of Layered Materials for Efficient Ion Separation and Storage

Layered materials are characterized by strong in-plane covalent chemical bonds within each atomic layer and weak out-of-plane van der Waals (vdW) interactions between adjacent layers. The non-bonding nature between neighboring layers naturally results in a vdW gap, which enables the insertion of guest species into the interlayer gap. Rational design and regulation of interlayer nanochannels are crucial for converting these layered materials and their 2D derivatives into ion separation membranes or battery electrodes. Herein, based on the latest progress in layered materials and their derivative nanosheets, various interlayer engineering methods are briefly introduced, along with the effects of intercalated species on the crystal structure and interlayer coupling of the host layered materials. Their applications in the ion separation and energy storage fields are then summarized, with a focus on interlayer engineering to improve selective ion transport and ion storage performance. Finally, future research opportunities and challenges in this emerging field are comprehensively discussed.

Simple to Quadruple Perovskite Transformation by Coordination Switching upon Solid-State Ion Exchange of NaNbO3

Topotactic ion exchange is ubiquitous in the preparation of many metastable solids with layered structures. In recent times, the scope of chimie-douce ion exchange has been extended to quasi-2D and -3D structures including nanocrystals. The low-temperature solid-state exchange is yet another unique synthetic tool to access preconceived structures for the rational design of solids. Although rational synthesis using inorganic synthons is rare, few examples exist among inorganic solids with layered structures. Herein, we extend the scope further by transforming a simple perovskite (ABO3) into a high-pressure quadruple (AA'3B4O12) perovskite. The transformation is achieved at moderate temperatures and ambient pressure via a solid-state metathesis reaction, wherein the transition metal adopts a new A-cation coordination upon exchange. Such coordination switching upon ion exchange will open up possibilities for functionality-driven structural transformations and the rational design of new solids.

Quasi-2D Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, Eu): reversible iodine intercalation and their evaluation as the anode in the lithium-ion battery system

Layered materials with a robust structure and reversible intercalation behavior are highly sought-after in applications involving energy conversion and storage systems, energy converting devices, supercapacitors, batteries, superconductors, photonic materials, and catalysis involving hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), solar cells and sensors. In the current study, quasi-2D rhombohedral Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, and Eu) samples, synthesized by a solution combustion route, have been demonstrated to intercalate iodine reversibly. A solid-vapor reaction was employed to intercalate iodine at moderate temperatures, and deintercalation occurred on heating at higher temperatures. Expansion of the rhombohedral c-axis by ∼10 Å occurred, and the iodine between the interlayers existed as triiodide ions (I3-) in an unsymmetrical fashion. The amount of intercalated iodide has been determined from thermogravimetric analysis. Electron microscopic analysis confirmed these systems' intercalation and subsequent lattice expansion. In the diffuse reflectance spectra, charge transfer from the triiodide ions to the host oxide was noticed, and it caused the absorption edge to fall beyond the visible region for the intercalated samples. XPS analysis of iodine intercalated Bi0.775Pr0.225O1.5 has shown the mixed valence states for Pr and the existence of I3- along with some IO3- species. The quasi-2D structure was stable during the thermal deintercalation process. The evaluation of iodine intercalated Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, and Eu) samples as anode material in the lithium-ion battery system has given quite promising results, exhibiting fast Li+-ion diffusion, low charge transfer resistance, good reversible capacity, capacity retention (after cycling back to 10 mA g-1), and structural stability (after long cycles).

Incorporation of Photo- and Thermoresponsive N-Salicylidene Aniline Derivatives into Cobalt and Zinc Layered Hydroxides

In search of new multifunctional hybrid materials and in order to investigate the influence of chemical modification on the possible synergy between properties, the carboxylate and sulfonate derivatives of photo- and thermochromic N-salicylidene aniline were successfully inserted into Co(II)- and Zn(II)-based layered simple hydroxides, resulting in four novel hybrids: Co-N-Sali-COO, Co-N-Sali-SO3, Zn-N-Sali-COO, and Zn-N-Sali-SO3. All synthesized hybrids adopt a double organic layered configuration, which prevents the cis-trans photoisomerization ability of N-Sali-R molecules in the hybrids. However, the Zn hybrids exhibit fluorescence upon exposure to UV light due to the excited-state intramolecular proton transfer (ESIPT) mechanism. The thermally stimulated keto-enol tautomerization of N-salicylidene aniline in the hybrids was related with the changes in interlamellar spacings observed by temperature-dependent PXRD. This tautomerization process was prominently evident in the Co-N-Sali-SO3 hybrid (about 11% increase in d-spacing upon decreasing the temperature to -180 °C). Finally, the Co-N-Sali-R hybrids exhibit the typical magnetic behavior associated with Co(II)-based LSHs (ferrimagnetic ordering at TN = 6.8 and 7.7 K for Co-N-Sali-COO and Co-N-Sali-SO3, respectively). This work offers insights into isomerization in LSHs and the ESIPT mechanism's potential in new luminescent materials and prospects for designing new multifunctional materials.

Deposition of silver nanoparticles on nanoscroll-supported inorganic solid using incompletely rolled-up kaolinite

Nanoscroll-supported platy particles were prepared by incomplete rolling-up of kaolinite layers; when the rolling-up of the kaolinite layer followed by its exfoliation incompletely proceeds, kaolinite nanoscrolls were found at the edge of kaolinite platy particles. To assess the support property of these nanoscroll-supported platy particles, when the deposition of Ag nanoparticles was conducted, these nanoparticles were present on the surface of platy particles and in the tubular interior of nanoscrolls at the edge of platy particles but absent on the surface of ordinal kaolinites, as revealed by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. These results indicated the successful formation and support property of nanoscroll-supported platy particles.

Topochemical Anionic Subunit Insertion Reaction for Constructing Nanoparticles of Layered Oxychalcogenide Intergrowth Structures

Intergrowth compounds contain alternating layers of chemically distinct subunits that yield composition-tunable synergistic properties. Synthesizing nanoparticles of intergrowth structures requires atomic-level intermixing of the subunits rather than segregation into stable constituent phases. Here we introduce an anionic subunit insertion reaction for nanoparticles that installs metal chalcogenide layers between metal oxide sheets. Anionic [CuS]- subunits from solution replace interlayer chloride anions from LaOCl to form LaOCuS topochemically with retention of crystal structure and morphology. Sodium acetylacetonate helps extract Cl- concomitant with the insertion of S2- and Cu+ and is generalized to other oxychalcogenides. This topochemical reaction produces nanoparticles of ordered mixed-anion intergrowth compounds and expands nanoparticle ion exchange chemistry to anionic subunits.

Development of Two-Dimensional Functional Nanomaterials for Biosensor Applications: Opportunities, Challenges, and Future Prospects

New possibilities for the development of biosensors that are ready to be implemented in the field have emerged thanks to the recent progress of functional nanomaterials and the careful engineering of nanostructures. Two-dimensional (2D) nanomaterials have exceptional physical, chemical, highly anisotropic, chemically active, and mechanical capabilities due to their ultra-thin structures. The diversity of the high surface area, layered topologies, and porosity found in 2D nanomaterials makes them amenable to being engineered with surface characteristics that make it possible for targeted identification. By integrating the distinctive features of several varieties of nanostructures and employing them as scaffolds for bimolecular assemblies, biosensing platforms with improved reliability, selectivity, and sensitivity for the identification of a plethora of analytes can be developed. In this review, we compile a number of approaches to using 2D nanomaterials for biomolecule detection. Subsequently, we summarize the advantages and disadvantages of using 2D nanomaterials in biosensing. Finally, both the opportunities and the challenges that exist within this potentially fruitful subject are discussed. This review will assist readers in understanding the synthesis of 2D nanomaterials, their alteration by enzymes and composite materials, and the implementation of 2D material-based biosensors for efficient bioanalysis and disease diagnosis.

Advanced growth of 2D MXene for electrochemical sensors

Over the last few years, electroanalysis has made significant advancements, particularly in developing electrochemical sensors. Electrochemical sensors generally include emerging Photoelectrochemical and Electrochemiluminescence sensors, which combine optical techniques and traditional electrochemical bio/non-biosensors. Numerous EC-detecting methods have also been designed for commercial applications to detect biological and non-biological markers for various diseases. Analytical applications have recently focused significantly on one of the novel nanomaterials, the MXene. This material is being extensively investigated for applications in electrochemical sensors due to its unique mechanical, electronic, optical, active functional groups and thermal characteristics. This study extensively discusses the salient features of MXene-based electrochemical sensors, photoelectrochemical sensors, enzyme-based biosensors, immunosensors, aptasensors, electrochemiluminescence sensors, and electrochemical non-biosensors. In addition, their performance in detecting various substances and contaminants is thoroughly discussed. Furthermore, the challenges and prospects the MXene-based electrochemical sensors are elaborated.

Multiscale structural control of thiostannate chalcogels with two-dimensional crystalline constituents

Chalcogenide aerogels (chalcogels) are amorphous structures widely known for their lack of localized structural control. This study, however, demonstrates a precise multiscale structural control through a thiostannate motif ([Sn₂S₆]^4-)-transformation-induced self-assembly, yielding Na-Mn-Sn-S, Na-Mg-Sn-S, and Na-Sn(II)-Sn(IV)-S aerogels. The aerogels exhibited [Sn₂S₆]^4-:Mn²⁺ stoichiometric-variation-induced-control of average specific surface areas (95-226 m² g^-1), thiostannate coordination networks (octahedral to tetrahedral), phase crystallinity (crystalline to amorphous), and hierarchical porous structures (micropore-intensive to mixed-pore state). In addition, these chalcogels successfully adopted the structural motifs and ion-exchange principles of two-dimensional layered metal sulfides (K_2xMn_xSn_3-xS₆, KMS-1), featuring a layer-by-layer stacking structure and effective radionuclide (Cs⁺, Sr²⁺)-control functionality. The thiostannate cluster-based gelation principle can be extended to afford Na-Mg-Sn-S and Na-Sn(II)-Sn(IV)-S chalcogels with the same structural features as the Na-Mn-Sn-S chalcogels (NMSCs). The study of NMSCs and their chalcogel family proves that the self-assembly principle of two-dimensional chalcogenide clusters can be used to design unique chalcogels with unprecedented structural hierarchy.

Photocatalytic Hydrogen Generation from Aqueous Methanol Solution over n-Butylamine-Intercalated Layered Titanate H2La2Ti3O10: Activity and Stability of the Hybrid Photocatalyst

The stability of platinized n-butylamine-intercalated layered titanate H2La2Ti3O10 during the process of photocatalytic hydrogen production from aqueous methanol under UV irradiation has been thoroughly investigated by means of XRD, CHN, TG, 13C NMR, BET, SEM and GC-MS analysis. It was revealed that n-butylamine completely abandons the interlayer space and transforms into n-butyraldehyde within 3 h of the reaction, while the particle morphology and specific surface area of the photocatalyst are preserved. The resulting solid phase contains carbon in at least two different oxidation states, which are attributed to the intermediate products of methanol oxidation bound to the perovskite matrix. The activity of the photocatalyst formed in this way is stable in time and strongly depends on the medium pH, which is not typical of either the parent H2La2Ti3O10 or TiO2. An approximate linear equation φ ≈ 29−2∙pH holds for the apparent quantum efficiency of hydrogen production in the 220–340 nm range at 1 mol. % methanol concentration. In the acidic medium, the photocatalyst under study outperforms the platinized H2La2Ti3O10 by more than one order of magnitude. The variation in methanol concentration allowed a maximum quantum efficiency of hydrogen production of 44% at 10 mol. % to be reached.

Band-Gap Engineering of Layered Perovskites by Cu Spacer Insertion as Photocatalysts for Depollution Reaction

A multi-step ion-exchange methodology was developed for the fabrication of Cu(LaTa2O7)2 lamellar architectures capable of wastewater depollution. The (001) diffraction line of RbLaTa2O7 depended on the guest species hosted by the starting material. SEM and TEM images confirmed the well-preserved lamellar structure for all intercalated layered perovskites. The UV–Vis, XPS, and photocurrent spectroscopies proved that Cu intercalation induces a red-shift band gap compared to the perovskite host. Moreover, the UV–Vis spectroscopy elucidated the copper ions environment in the Cu-modified layered perovskites. H2-TPR results confirmed that Cu species located on the surface are reduced at a lower temperature while those from the interlayer occur at higher temperature ranges. The photocatalytic degradation of phenol under simulated solar irradiation was used as a model reaction to assess the performances of the studied catalysts. Increased photocatalytic activity was observed for Cu-modified layered perovskites compared to RbLaTa2O7 pristine. This behavior resulted from the efficient separation of photogenerated charge carriers and light absorption induced by copper spacer insertion.

Guidelines for Arranging 2D Nanosheets into Neatly Tiled Monolayer Films by a Spin-Coating Process

Neat (dense and nonoverlapped) monolayer tiling of 2D nanosheets on a substrate surface is very important because we can conduct artificial lattice-engineering by repeating the tiling process in a designed sequence to tailor various hierarchical nanostructures, leading to a range of sophisticated functions. It is recently reported that a facile spin-coating technique realizes the neat monolayer tiling of various 2D nanosheets. Establishing universal guidelines to neatly tile 2D nanosheets on substrates of various materials and size/shape is of essential importance to fully apply this technique, but the mechanism of how the nanosheets are tiled has not been clarified yet. In the present study, we have systematically examined the nanosheet deposition process at various rotation speeds by microscopic observations and found that the neat monolayer tiling of nanosheets is attained on the solvent surface during the spin-coating, and then the monolayer film is deposited onto the substrate surface from its center toward the edges upon evaporation of the solvent. Furthermore, we have clarified how the rotation speed and the nanosheet concentration govern the deposition behaviors in terms of neat tiling, overlap, or noncoverage in a such process. On the basis of the guidelines, we can predict the optimum spin-coating conditions for attaining the neat monolayer tiling of various nanosheets over an entire surface of the substrate.

Photocatalytic Hydrogen Production from Aqueous Solutions of Glucose and Xylose over Layered Perovskite-like Oxides HCa2Nb3O10, H2La2Ti3O10 and Their Inorganic-Organic Derivatives

Nowadays, the efficient conversion of plant biomass components (alcohols, carbohydrates, etc.) into more energy-intensive fuels, such as hydrogen, is one of the urgent scientific and technological problems. The present study is the first one focused on the photoinduced hydrogen evolution from aqueous D-glucose and D-xylose using layered perovskite-like oxides HCa2Nb3O10, H2La2Ti3O10, and their organically modified derivatives that have previously proven themselves as highly active photocatalysts. The photocatalytic performance was investigated for the bare compounds and products of their surface modification with a 1 mass. % Pt cocatalyst. The photocatalytic experiments followed an innovative scheme including dark stages as well as the control of the reaction suspension's pH and composition. The study has revealed that the inorganic-organic derivatives of the layered perovskite-like oxides can provide efficient conversion of carbohydrates into hydrogen fuel, being up to 8.3 times more active than the unmodified materials and reaching apparent quantum efficiency of 8.8%. Based on new and previously obtained data, it was shown that the oxides' interlayer space functions as an additional reaction zone in the photocatalytic hydrogen production and the contribution of this zone to the overall activity is dependent on the steric characteristics of the sacrificial agent used.

Renaissance of Topotactic Ion-Exchange for Functional Solids with Close Packed Structures

Recently, many new, complex, functional oxides have been discovered with the surprising use of topotactic ion-exchange reactions on close-packed structures, such as found for wurtzite, rutile, perovskite, and other structure types. Despite a lack of apparent cation-diffusion pathways in these structure types, synthetic low-temperature transformations are possible with the interdiffusion and exchange of functional cations possessing ns2 stereoactive lone pairs (e. g., Sn(II)) or unpaired ndx electrons (e. g., Co(II)), targeting new and favorable modulations of their electronic, magnetic, or catalytic properties. This enables a synergistic blending of new functionality to an underlying three-dimensional connectivity, i. e., [-M-O-M-O-]n , that is maintained during the transformation. In many cases, this tactic represents the only known pathway to prepare thermodynamically unstable solids that otherwise would commonly decompose by phase segregation, such as that recently applied to the discovery of many new small bandgap semiconductors.

Recent Advances in Layered Metal-Oxide Cathodes for Application in Potassium-Ion Batteries

To meet future energy demands, currently, dominant lithium-ion batteries (LIBs) must be supported by abundant and cost-effective alternative battery materials. Potassium-ion batteries (KIBs) are promising alternatives to LIBs because KIB materials are abundant and because KIBs exhibit intercalation chemistry like LIBs and comparable energy densities. In pursuit of superior batteries, designing and developing highly efficient electrode materials are indispensable for meeting the requirements of large-scale energy storage applications. Despite using graphite anodes in KIBs instead of in sodium-ion batteries (NIBs), developing suitable KIB cathodes is extremely challenging and has attracted considerable research attention. Among the various cathode materials, layered metal oxides have attracted considerable interest owing to their tunable stoichiometry, high specific capacity, and structural stability. Therefore, the recent progress in layered metal-oxide cathodes is comprehensively reviewed for application to KIBs and the fundamental material design, classification, phase transitions, preparation techniques, and corresponding electrochemical performance of KIBs are presented. Furthermore, the challenges and opportunities associated with developing layered oxide cathode materials are presented for practical application to KIBs.

Fluorine-Free Fabrication of MXene via Photo-Fenton Approach for Advanced Lithium-Sulfur Batteries

The mainstream synthesis method for MXene is using aqueous fluorine-containing acidic solutions to eliminate the A-element layers from their MAX phases. However, this strategy is environmentally hazardous and impairs the material performance (e.g., supercapacitor and Li-S batteries) owing to the presence of -F terminations. Herein, we exploit a low-temperature "soft chemistry" approach based on photo-Fenton (P.F.) reaction for the fabrication of F-free Ti₃C₂ (Ff-Ti₃C₂) with high purity of 95%. It is confirmed that the continuous generation of highly reactive oxygen species (HO^• and O₂^•- radicals) during the P.F. reaction weakens the metallic Ti-Al bonds in the MAX phase and promotes the formation of high concentration OH^- anions, which are conducive to the sequential topochemical deintercalation of Al layers. Moreover, the strengthened charge accumulation on the Ff-Ti₃C₂ surface creates rich electron "reservoirs" for actuating the Li-S chemistry, which not only strengthens the host-guest interactions but also propels the kinetics of the polysulfide conversion. Taking advantage of the superior mechanical robustness, better electrolyte wettability, and improved electrocatalytic activity, the resultant Ff-Ti₃C₂ can be used as an ideal sulfur host and Li-S chemistry mediator for advanced flexible Li-S batteries.

Versatile Coating Platform for Metal Oxide Nanoparticles: Applications to Materials and Biological Science

In this feature article, we provide an overview of our research on statistical copolymers as a coating material for metal oxide nanoparticles and surfaces. These copolymers contain functional groups enabling noncovalent binding to oxide surfaces and poly(ethylene glycol) (PEG) polymers for colloidal stability and stealthiness. The functional groups are organic derivatives of phosphorous acid compounds R-H₂PO₃, also known as phosphonic acids that have been screened for their strong affinity to metals and for their multidentate binding ability. Herein we develop a polymer-based coating platform that shares features with the self-assembled monolayer (SAM) and layer-by-layer (L-b-L) deposition techniques. The milestones of this endeavor are the synthesis of PEG-based copolymers containing multiple phosphonic acid groups, the implementation of simple protocols combining versatility with high particle production yields, and the experimental evidence of the colloidal stability of the coated particles. As a demonstration, coating studies are conducted on cerium (CeO₂), iron (γ-Fe₂O₃), aluminum (Al₂O₃), and titanium (TiO₂) oxides of different sizes and morphologies. We finally discuss applications in the domain of nanomaterials and nanomedicine. We evaluate the beneficial effects of coatings on redispersible nanopowders, contrast agents for in vitro/vivo assays, and stimuli-responsive particles.

Vanadium oxides obtained by chimie douce reactions: The influences of transition metal species on crystal structures and electrochemical behaviors in zinc-ion batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) have received increasing attention due to cost-effectiveness and inherent safety. A wide variety of advanced cathode materials have been revealed with promising performance in RAZIBs. However, these materials usually require sophisticated procedures at high temperatures, which greatly limit further practical application. Herein, a chimie douce approach is adopted to prepare vanadium oxides from V₂O₅ suspension with the addition of various transition metal cations (Mn²⁺, Zn²⁺, Ag⁺, and Fe³⁺) by simple liquid-solid mixing under ambient conditions. For the cases of Mn²⁺ and Zn²⁺, the dissolution-recrystallization process takes place leading to layered Mn_0.31V₃O₇·1.40H₂O (MnVO) and Zn_0.32V₃O₇·1.52H₂O (ZnVO). The use of Ag⁺ forms tunneled Ag_0.33V₂O₅ (AgVO), and the present of Fe³⁺ stays mainly unreacted V₂O₅. The underlying reaction chemistries are proposed, for which the pH values of precursor solutions are found to be a key factor. Among the prepared materials, layered vanadium oxides exhibit promising battery performance. Particularly, MnVO delivers 340 and 217 mAh g^-1 at 1 and 8 A g^-1, respectively. A specific capacity of 164 mAh g^-1 can be retained after 500 cycles at 1 A g^-1. By contrast, AgVO and FeVO demonstrate inferior performance with retaining only 89 and 20 mAh g^-1 after 500 cycles.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]- layers

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite "charge-neutral perovskite layers" by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative reaction based on anionic [CeIIITa2O7]- layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [CeIIITa2O7]- layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7]- layers. Furthermore, the anionic [CeIIITa2O7]- layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the "charge-neutral" perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

Combined Role of Biaxial Strain and Nonstoichiometry for the Electronic, Magnetic, and Redox Properties of Lithiated Metal-Oxide Films: The LiMn2O4 Case

Understanding the interplay between strain and nonstoichiometry for the electronic, magnetic, and redox properties of LiMn₂O₄ films is essential for their development as Li-ion battery (LIB) cathodes, photoelectrodes, and systems for sustainable spintronics applications as well as for emerging applications that combine these technologies. Here, density functional theory (DFT) simulations suggest that compressive strain increases the reduction drive of (111) LiMn₂O₄ films by inducing >1 eV upshift of the valence band edge. The DFT results indicate that, regardless of the crystallographic orientation for the LiMn₂O₄ film, biaxial expansion increases the magnetic moments of the Mn atoms. Conversely, biaxial compression reduces them. For ferromagnetic films, these changes can be substantial and as large as over 4 Bohr magnetons per unit cell over the simulated range of strain (from -6 to +3%). The DFT simulations also uncover a compensation mechanism whereby strain induces opposite changes in the magnetic moment of the Mn and O atoms, leading to an overall constant magnetic moment for the ferromagnetic films. The calculated strain-induced changes in atomic magnetic moments reflect modifications in the local electronic hybridization of both the Mn and O atoms, which in turn suggests strain-tunable, local chemical, and electrochemical reactivity. Several energy-favored (110) and (111) ferromagnetic surfaces turn out to be half-metallic with minority-spin band gaps as large as 3.2 eV and compatible with spin-dependent electron-transport and possible spin-dependent electrochemical and electrocatalytic properties. The resilience of the ferromagnetic, half-metallic states to surface nonstoichiometry and compositional changes invites exploration of the potential of LiMn₂O₄ thin films for sustainable spintronic applications beyond state-of-the-art, rare-earth metal-based, ferromagnetic half-metallic oxides.

Kinetic 2D Crystals via Topochemical Approach

The designing of novel materials is a fascinating and innovative pathway in materials science. Particularly, novel layered compounds have tremendous influence in various research fields. Advanced fundamental studies covering various aspects, including reactants and synthetic methods, are required to obtain novel layered materials with unique physical and chemical properties. Among the promising synthetic techniques, topochemical approaches have afforded the platform for widening the extent of novel 2D materials. Notably, the synthesis of binary layered materials is considered as a major scientific breakthrough after the synthesis of graphene as they exhibit a wide spectrum of material properties with varied potential applicability. In this review, a comprehensive overview of the progress in the development of metastable layered compounds is presented. The various metastable layered compounds synthesized from layered ternary bulk materials through topochemical approaches are listed, followed by the descriptions of their mechanisms, structural analyses, characterizations, and potential applications. Finally, an essential research direction concerning the synthesis of new materials is indicated, wherein the possible application of topochemical approaches in unprecedented areas is explored.

Structural and Compositional Characteristics of Ball-Milled Lepidocrocite Alkali Titanate and the Correlation to Its Surface Acidic-Basic Properties

The studies on mechanical treatments of layered alkali metal oxides are limited despite their diverse compositions/structures and potential for property tuning. In this work, we vibratory mill Cs_0.7Zn_0.35Ti_1.65O₄, K_0.8Zn_0.4Ti_1.6O₄, and Cs₂Ti₆O₁₃ for up to 4 h, during which the lepidocrocite-type structure and the plate-like morphology are well preserved. X-ray diffraction (XRD) indicates a tiny (≤0.6 Å) interlayer expansion accompanied by the enhancement of the preferred orientation along the stacking direction. Chemical analyses across multiple length scales suggest Cs deintercalation, elemental redistributions, and bulk-to-surface (or crystal edge) Cs migration. This ball-milling-induced Cs-rich moiety partially blocks the surface acid sites, although the solids still show a dominating acidic character. The ball-milled samples Cs_0.7-pZn_0.35-qTi_1.65O_4-δ contain vacancies between the sheets (p) and at the sheets (q and δ). It is deduced from Sanderson's electronegativity equalization principle and experimentally verified by X-ray photoelectron spectroscopy (XPS) that ball milling increases (decreases) the partial charge at the surface acidic Ti⁴⁺/Zn²⁺ (basic O^2-) sites. These nonporous solids (≤20 m²·g^-1) contain water sorbed on the external surface as high as 1.1 mol·mol^-1, which is comparable to that in a water-intercalated sample. Our work expands the current understanding of the reactivity vs robustness in layered alkali titanates under physically demanding conditions, complementing knowledge gathered via the soft chemistry approach.

Low-temperature topotactic oxidation using the solid-state oxidant Zr-doped CeO2

A new topotactic oxidation was developed using the solid-state oxidant Zr-doped CeO₂. For the anti-ThCr₂Si₂ type Y₂O₂Bi, which became superconducting via oxygen intercalation during solid-state oxidation at 1000 °C, the topotactic oxidation enabled not only the oxygen intercalation at a much lower temperature of 200 °C, hampering the segregation of impurity phases, but also the highest superconducting transition temperature for Y₂O₂Bi.

Atomically Thin Materials for Next-Generation Rechargeable Batteries

Atomically thin materials (ATMs) with thicknesses in the atomic scale (typically <5 nm) offer inherent advantages of large specific surface areas, proper crystal lattice distortion, abundant surface dangling bonds, and strong in-plane chemical bonds, making them ideal 2D platforms to construct high-performance electrode materials for rechargeable metal-ion batteries, metal-sulfur batteries, and metal-air batteries. This work reviews the synthesis and electronic property tuning of state-of-the-art ATMs, including graphene and graphene derivatives (GE/GO/rGO), graphitic carbon nitride (g-C₃N₄), phosphorene, covalent organic frameworks (COFs), layered transition metal dichalcogenides (TMDs), transition metal carbides, carbonitrides, and nitrides (MXenes), transition metal oxides (TMOs), and metal-organic frameworks (MOFs) for constructing next-generation high-energy-density and high-power-density rechargeable batteries to meet the needs of the rapid developments in portable electronics, electric vehicles, and smart electricity grids. We also present our viewpoints on future challenges and opportunities of constructing efficient ATMs for next-generation rechargeable batteries.

Photocatalytic Activity of n-Alkylamine and n-Alkoxy Derivatives of Layered Perovskite-like Titanates H2Ln2Ti3O10 (Ln = La, Nd) in the Reaction of Hydrogen Production from an Aqueous Solution of Methanol

Two series of hybrid inorganic-organic derivatives, obtained via the modification of protonated Ruddlesden–Popper phases H2Ln2Ti3O10 (Ln = La, Nd) with intercalated n-alkylamines and grafted n-alkoxy groups, have been systematically investigated in relation to photocatalytic hydrogen production from a model of 1 mol % aqueous solution of methanol for the first time. Photocatalytic measurements were performed both for bare samples and for their composites with Pt nanoparticles as a cocatalyst using an advanced scheme, including dark stages, monitoring of the volume concentration of the sample in the reaction suspension during the experiment, shifts of its pH and possible exfoliation of layered compounds into nanolayers. It was found that the incorporation of organic components into the interlayer space of the titanates increases their photocatalytic activity up to 117 times compared with that of the initial compounds. Additional platinization of the hybrid samples’ surface allowed for achieving apparent quantum efficiency of hydrogen evolution of more than 40%. It was established that the photocatalytic activity of the hybrid samples correlates with the hydration degree of their interlayer space, which is considered a separate reaction zone in photocatalysis, and that hydrogen indeed generates from the aqueous methanol solution rather than from organic components of the derivatives.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Fast and efficient shear-force assisted production of covalently functionalized oxide nanosheets

HYPOTHESIS

While controlled and efficient exfoliation of layered oxides often remains a time consuming challenge, the surface modification of inorganic nanosheets is of outmost importance for future applications. The functionalization of the bulk material prior to exfoliation should allow the application of tools developped for Van der Waals materials to directly produce functionalized oxide nanosheets.

EXPERIMENTS

The Aurivillius phase Bi₂SrTa₂O₉ is functionalized by a linear aliphatic phosphonic acid via microwave-assisted reactions. The structure of the hybrid material and the coordination of the phosphonate group is scrutinized, notably by Pair Distribution Function. This functionalized layered oxide is then exfoliated in one hour in organic solvent, using high shear force dispersion. The obtained nanosheets are characterized in suspension and as deposits to check their chemical integrity.

FINDINGS

The covalent functionalization decreases the electrostatic cohesion between the inorganic layers leading to an efficient exfoliation in short time under shearing. The functionalization of the bulk material is preserved on the nanosheets upon exfoliation and plays a major role to enable liquid-phase exfoliation and in the stability of the resulting suspensions. This strategy is very promising for the straighforward preparation of functionalized nanosheets, paving the way for versatile design of new (multi)functional hybrid nanosheets for various potential applications.

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

ConspectusMany small bandgap semiconductors have been discovered or predicted to exist beyond the edges of stability, that is, accessible only as metastable solids that are thermodynamically unstable. In many cases, these metastable semiconductors have been revealed to have technologically promising properties for solar energy conversion, such as in photocatalysis or in photovoltaics. This Account presents a review of research results selected from my group and others in recent years on these semiconductors. Notably, these include the chemical systems of mixed-metal oxides (i.e., M'MOx; M = Ti(IV), Nb(V), or Ta(V) cation; M' = Ag(I), Cu(I), Sn(II), Pb(II), or Bi(III) cation), which have diverse structure types and compositions. High photocatalytic activities have been found for the light-driven reduction or oxidation of water as p- or n-type photoelectrodes, respectively, or as suspended powders in aqueous solutions. These have exhibited new combinations of favorable semiconductor properties, such as deep visible-light absorption, near-optimal band edge energies, defect tolerance, and functional carrier mobilities and charge separation. As described herein, this set of properties is inextricably linked to their metastable nature, that is, the crystalline structures and compositions needed for these characteristics lead naturally to thermodynamic instabilities.This Account focuses on current research efforts that have begun unlocking the potential of these semiconductors via new recent advances in (1) synthetic approaches that enable their preparation and (2) the understanding of structure-property relationships discovered at the precipices of stability that lead to the improved semiconductor properties. For example, low-temperature reactions have been developed to facilitate greater kinetic control, such as with the use of molten salts, and have been a key factor in preparing many of these semiconductors. As a result, a plethora of promising new mixed-metal oxide systems have been uncovered that exhibit band gaps spanning the range of photon energies from ∼1.3 to >3.0 eV. Especially relevant for visible-light applications are the Cu(I)- and Sn(II)-containing semiconductors. For example, n-type Sn(II)-titanates and p-type Cu(I)-niobates can be synthesized by flux methods and exhibit some of the smallest known visible-light band gaps that also maintain suitable conduction and valence band edges for driving the water-splitting half reactions. Kinetic stabilization of these metastable semiconductors against thermally driven phase segregation is increased with the formation of solid solutions for both the M and M' cation sites, leading to effective strategies to more finely tune their band gaps, band edge energies, and photoelectrochemical properties. Many unique and useful relationships are emerging between the synthesis and structures of metastable semiconductors and their physical properties, leading to more efficient solar energy conversion.

Layered terbium hydroxides for simultaneous drug delivery and imaging

Layered rare-earth hydroxides have begun to gather increasing attention as potential theranostic platforms owing to their extensive intercalation chemistry combined with magnetic and fluorescent properties. In this work, the potential of layered terbium hydroxide (LTbH) as a platform for simultaneous drug delivery and fluorescence imaging was evaluated. LTbH-Cl ([Tb2(OH)5]Cl·yH2O) was loaded with three nonsteroidal anti-inflammatory drugs (diclofenac, ibuprofen, and naproxen) via ion-exchange. Drug release studies in phosphate buffered saline (pH = 7.4) revealed all three formulations release their drug cargo rapidly over the course of approximately 5 hours. In addition, solid state fluorescence studies indicated that fluorescence intensity is strongly dependent on the identity of the guest anion. It was postulated that this feature may be used to track the extent of drug release from the formulation, which was subsequently successfully demonstrated for the ibuprofen loaded LTbH. Overall, LTbH exhibits good biocompatibility, high drug loading, and a strong, guest-dependent fluorescence signal, all of which are desirable qualities for theranostic applications.

Synthesis of n-Alkoxy Derivatives of Layered Perovskite-Like Niobate HCa2Nb3O10 and Study of Their Photocatalytic Activity for Hydrogen Production from an Aqueous Solution of Methanol

A series of hybrid inorganic–organic niobates HCa2Nb3O10×ROH, containing n-alkoxy groups of primary alcohols (R = Me, Et, Pr, Bu, Hx, and Dc) grafted in the interlayer space, has been studied for the first time in relation to photocatalytic hydrogen generation from a model 1 mol % aqueous solution of methanol under ultraviolet irradiation. Photocatalytic activity was measured both for bare samples and for their composites with Pt nanoparticles as a cocatalyst. The advanced measurement scheme allowed monitoring the volume concentration of a sample in a suspension during the experiment, its pH, and possible exfoliation of layered compounds into nanolayers. In the series of n-alkoxy derivatives, the maximum rate of hydrogen evolution was achieved over a Pt-loaded ethoxy derivative HCa2Nb3O10×EtOH/Pt. Its apparent quantum efficiency of 20.6% in the 220–350 nm range was found not to be caused by changes in the light absorption region or specific surface area upon ethanol grafting. Moreover, the amounts of hydrogen released during the measurements significantly exceeded those of interlayer organic components, indicating that hydrogen is generated from the reaction solution rather than from the hybrid material.

Soft chemistry of metastable metal chalcogenide nanomaterials

The metastable nature of metal chalcogenide nanomaterials (MCNs) provides us with fresh perspectives and plentiful grounds in the search of new strategies for physicochemical tuning. In the past decade, numerous efforts have been devoted to synthesizing and modifying diverse emerging MCNs based on their "soft chemistry", that is, gently regulating the composition, structure, phase, and interface while not entirely disrupting the original features. This tutorial review focuses on design principles based on the metastability of MCNs, such as ion mobility and vacancy, thermal and structural instability, chemical reactivity, and phase transition, together with corresponding soft chemical approaches, including ion-exchange, catalytic growth, segregation or coupling, template grafting or transformation, and crystal-phase engineering, and summarizes recent advances in their preparation and modification. Finally, prospects for the future development of soft chemistry-directed synthetic guidelines and metastable metal chalcogenide-derived nanomaterials are proposed and highlighted.

Topochemical Deintercalation of Li from Layered LiNiB: toward 2D MBene

The pursuit of two-dimensional (2D) borides, MBenes, has proven to be challenging, not the least because of the lack of a suitable precursor prone to the deintercalation. Here, we studied room-temperature topochemical deintercalation of lithium from the layered polymorphs of the LiNiB compound with a considerable amount of Li stored in between [NiB] layers (33 at. % Li). Deintercalation of Li leads to novel metastable borides (Li_∼0.5NiB) with unique crystal structures. Partial removal of Li is accomplished by exposing the parent phases to air, water, or dilute HCl under ambient conditions. Scanning transmission electron microscopy and solid-state ⁷Li and ¹¹B NMR spectroscopy, combined with X-ray pair distribution function (PDF) analysis and DFT calculations, were utilized to elucidate the novel structures of Li_∼0.5NiB and the mechanism of Li-deintercalation. We have shown that the deintercalation of Li proceeds via a "zip-lock" mechanism, leading to the condensation of single [NiB] layers into double or triple layers bound via covalent bonds, resulting in structural fragments with Li[NiB]₂ and Li[NiB]₃ compositions. The crystal structure of Li_∼0.5NiB is best described as an intergrowth of the ordered single [NiB], double [NiB]₂, or triple [NiB]₃ layers alternating with single Li layers; this explains its structural complexity. The formation of double or triple [NiB] layers induces a change in the magnetic behavior from temperature-independent paramagnets in the parent LiNiB compounds to the spin-glassiness in the deintercalated Li_∼0.5NiB counterparts. LiNiB compounds showcase the potential to access a plethora of unique materials, including 2D MBenes (NiB).

Analysis of Solid-State Reaction Mechanisms with Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy

The utility of two-dimensional generalized correlation spectroscopy (2D-COS) for tracking complex solid-state reactions is demonstrated using infrared spectra acquired during a photochemically induced decomposition reaction. Eleven different thin films, consisting of six monometallic and five bimetallic 2-ethylhexanoate complexes, were tracked as a function of photolysis time. Overlapping peaks in the infrared fingerprint region are readily discriminated using 2D-COS, enabling individual vibrational components to be used to distinguish whether carboxylate ligands are free/ionic or bound in a chelating, bridging, or monodentate fashion. This classification enables the decomposition mechanism to be tracked for all 11 samples, revealing that ligands bound in monodentate and bridging fashions are first converted to chelates before being lost as volatile products for all samples. The magnitude of the measured first-order rate constants for loss of chelated ligands is found to correlate linearly to the asymmetric stretching frequency of monodentate ligands but exhibits a V shape when plotted against the electronegativity of the metal center. We propose that loss of chelated ligands proceeds via C-O scission for highly electronegative transition metals but M-O scission for transition metals with low electronegativity. These results establish 2D-COS as a powerful tool to deconvolute and correlate individual components, enabling mechanistic analysis of complex chemical reactions.

Honeycomb layered oxides: structure, energy storage, transport, topology and relevant insights

The advent of nanotechnology has hurtled the discovery and development of nanostructured materials with stellar chemical and physical functionalities in a bid to address issues in energy, environment, telecommunications and healthcare. In this quest, a class of two-dimensional layered materials consisting of alkali or coinage metal atoms sandwiched between slabs exclusively made of transition metal and chalcogen (or pnictogen) atoms arranged in a honeycomb fashion have emerged as materials exhibiting fascinatingly rich crystal chemistry, high-voltage electrochemistry, fast cation diffusion besides playing host to varied exotic electromagnetic and topological phenomena. Currently, with a niche application in energy storage as high-voltage materials, this class of honeycomb layered oxides serves as ideal pedagogical exemplars of the innumerable capabilities of nanomaterials drawing immense interest in multiple fields ranging from materials science, solid-state chemistry, electrochemistry and condensed matter physics. In this review, we delineate the relevant chemistry and physics of honeycomb layered oxides, and discuss their functionalities for tunable electrochemistry, superfast ionic conduction, electromagnetism and topology. Moreover, we elucidate the unexplored albeit vastly promising crystal chemistry space whilst outlining effective ways to identify regions within this compositional space, particularly where interesting electromagnetic and topological properties could be lurking within the aforementioned alkali and coinage-metal honeycomb layered oxide structures. We conclude by pointing towards possible future research directions, particularly the prospective realisation of Kitaev-Heisenberg-Dzyaloshinskii-Moriya interactions with single crystals and Floquet theory in closely-related honeycomb layered oxide materials.

Intercalation-exfoliation processes during ionic exchange reactions from sodium lepidocrocite-type titanate toward a proton-based trititanate structure

Topochemical reactions involving ionic exchange have been used to assess a large number of metastable compositions, particularly in layered metal oxides. This method encompasses complex reactions that are poorly explored, yet are of prime importance to understand and control the materials' properties. In this work, we embark on investigating the reactions involved during the ionic exchange between a layered Na-titanate (lepidocrocite-type structure) and an acidic solution (HCl), leading to a protonic (H3O+) titanate (trititanate structure). The reactions involve an ionic exchange provoking a structural change from the lepidocrocite-type to the trititanate structure as shown by real-space refinements of ex situ pair distribution function data. Mobile Na+ ions are exchanged by hydronium ions inducing high proton mobility in the final structure. Moreover, the reaction was followed by ex situ23Na and 1H solid-state MAS NMR which allowed, among other things, confirming that the Na+ ions are in the interlayer space and specifying their local environment. Strikingly, the ionic exchange reaction induces progressive exfoliation of the Na-titanate particles leading to 2-5 nm thin elongated crystallites. To further understand the different steps associated with the ionic exchange, the evolution of the electrolytic conductivity, using conductimetric titration, has been monitored upon HCl addition, enabling characterization of the intercalation(H+)/de-intercalation(Na+) reactions and assessing kinetic parameters. Accordingly, it is hypothesized that the exfoliation of the particles is due to the accumulation of charges at the particle level in relation to the rapid intercalation of protons. This work provides novel insights into ionic exchange reactions involved in layered oxide compounds.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.

Two-Dimensional Molecular Sheets of Transition Metal Oxides toward Wearable Energy Storage

Flexible and wearable electronics have recently sparked intense interest in both academia and industry because they can greatly revolutionize human lives by impacting every aspect of our daily routine. Therefore, developing compatible energy storage devices has become one of the most important research frontiers in this field. Particularly, the development of flexible electrodes is of great significance when considering their essential role in the performance of these devices. Although there is no doubt that transition metal oxide nanomaterials are suitable for providing electrochemical energy storage, individual oxides generally cannot be developed into freestanding electrodes because of their intrinsically low mechanical strength.Two-dimensional sheets with genuine unilamellar thickness are perfect units for the assembly of freestanding and mechanically flexible devices, as they have the advantages of low thickness and good flexibility. Therefore, the development of metal oxide materials into a two-dimensional sheet morphology analogous to graphene is expected to solve the above-mentioned problems. In this Account, we summarize the recent progress on two-dimensional molecular sheets of transition metal oxides for wearable energy storage applications. We start with our understanding of the principle of producing two-dimensional metal oxides from their bulk-layered counterparts. The unique layered structure of the precursors inspired the exploration of their interlayer chemistry, which helps us to understand the processes of swelling and delamination. Rational methods for tuning the chemical composition, size/thickness, and surface chemistry of the obtained nanosheets and how physicochemical properties of the nanosheets can be modulated are then briefly introduced. Subsequently, the orientational alignment of the anisotropic sheets and the origins of their liquid-crystalline characteristics are discussed, which are of vital importance for their subsequent macroscopic assembly. Finally, macroscopic electrodes with geometric diversity ranging from one-dimensional macroscopic fibers to two-dimensional films/papers and three-dimensional monolithic foams are summarized. The intrinsically low mechanical stiffness of metal oxide sheets can be effectively overcome by wisely designing the assembly mode and sheet interfaces to obtain decent mechanical properties integrated with superior electrochemical performance, thereby providing critical advantages for the fabrication of wearable energy storage devices.We expect that this Account will stimulate further efforts toward fundamental research on interface engineering in metal oxide sheet assembly and facilitate wide applications of their designed assemblies in future new-concept energy conversion devices and beyond. In the foreseeable future, we believe that there will be a big explosion in the application of transition metal oxide sheets in flexible electronics.