Layered Precursors Research Articles

Phase diagram exploration of Tc-Al-B: from bulk Tc2AlB2 to two-dimensional Tc2B2.

In this study, the ternary phase diagram of the Tc-Al-B system is constructed by a combination of an evolutionary algorithm and density functional theory calculations. Four novel phases are predicted, including three binary compounds (P1̄ Al7B15, Cmcm TcAl2, and C2 TcAl3) and one ternary compound (Cmmm Tc2AlB2). All predicted structures are mechanically, dynamically, and thermodynamically stable. Among the predicted phases, P1̄ Al7B15 resembles the experimental structure of Al0.93B2 and Cmmm Tc2AlB2 corresponds to the 212-type MAB phase. Due to the in- and out-of-plane anisotropic chemical bonding in Cmmm Tc2AlB2, a tetragonal two-dimensional (2D) Tc2B2 structure could be possibly exfoliated by chemical removal of Al atoms. The electronic structure calculations indicate that the 2D Tc2B2 structure and its potential layered precursors are all metallic. Furthermore, the chemical reactivity of H, F, O and, OH ligands with the 2D Tc2B2 surface is studied and the associated 2D surface-functionalized Tc2B2 derivatives are found to be metallic. It is revealed that the F and O functional groups strengthen the surface atomic layer of 2D Tc2B2 and enhance the Young's moduli.

Transformation Control of a Layered Zeolite Precursor by Simple Cation Exchange

The topotactic transformation of layered zeolite precursors to three-dimensional zeolite frameworks is of major importance because it is a viable option for finding new zeolite structures. Here, we...

Controllable fabrication of two-dimensional layered transition metal oxides through electrochemical exfoliation of non-van der Waals metals for rechargeable zinc-ion batteries

Electrochemical exfoliation of layered van der Waals solids is becoming a promising method for the fabrication of two-dimensional (2D) materials beyond graphene. Nevertheless, most of bulk layered starting solids are semiconductive or even nonconductive, which will reduce the exfoliation efficiency. In addition, the strong bonding interaction between the layers of van der Waals crystal will lead to the incomplete exfoliation. These deficiencies will hinder the application of this method in the synthesis of two-dimensional semiconductive and insulating materials. Herein, one-pot electrochemical potential-cycles exfoliation has been introduced to fabricate nonconductive 2D layered birnessite-type manganese oxide utilizing non-layered metal as starting precursor, demonstrating that monolayer and few layer birnessite sheets can be obtained through tuning the potential sweep rate. More importantly, this strategy promotes a high exfoliation yield up to 96%, which is much higher than that of electrochemical exfoliation of layered precursors. Additionally, this method can overwhelm the drawbacks such as hindrance of intrinsic/contact high resistance and incomplete exfoliation of the layered starting material presented by electrochemical exfoliation of bulk layered van der Waals solids. When utilized as cathode material for Zn-ion batteries, the exfoliated birnessite can deliver a capacity of 325 mAh g−1 at a current density of 200 mA g−1 and achieved capacity retention of 84% over 1000 cycles at 1000 mA g−1. As exfoliation parameters can be manipulated, this effective method is applicable to producing various 2D layered transition metal oxides and layered double hydroxides regardless of electric conductivity, which will contribute to plentiful applications.

Structural characterization and photoluminescence of (Gd1-xErx)2O3 nanophosphors synthesized by co-precipitation of layered precursors

In this paper, the layered (Gd1-xErx)2(OH)5NO3·nH2O (LGEH) with a variable content of erbium were synthesized by controlled double-jet precipitation (CDJP) at a constant pH value. The effect of pH co-precipitation on the morphology, crystallinity, porosity of the LGEH, as well as on the spectral-luminescent properties of oxide nanophosphors obtained by their heat treatment is significant. We have found that the samples with the lowest disordering and the highest of photoluminescence (PL) intensity of Er3+ ions are obtained from amorphous sponge-like precursor co-precipitated at pH 9.5. The PL of nanostructural (Gd1-xErx)2O3 depends on both structural-morphology properties and samples composition. It was also found that efficiency of radiative and non-radiative relaxation channels for excited Er3+ ions mainly depends on three factors: nanocrystals size, specific surface area and erbium concentration. For explanation of the PL behavior in the green and red spectral ranges, the energy diagram showing the intracenter energy transitions and intercenter energy migration in synthesized nanophosphors was proposed.

Investigating the Formation of MoSe2 and TiSe2 Films from Artificially Layered Precursors.

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe2 crystallites that nucleated during deposition and a sharper intensity maximum resulting from the composition modulation created from the alternating deposition of Mo and Se layers. In contrast, as-deposited Ti|Se films around a 1:2 ratio of Ti to Se contain narrow and intense 00l reflections from TiSe2 crystallites and do not contain a Bragg reflection from the sequence of deposited Ti|Se layers. The as-deposited TiSe2 crystallites have a larger c-axis lattice parameter than was previously reported for TiSe2, however, which suggests a poor vertical interlayer registry and/or high defect densities including interstitial atoms. In-plane XRD patterns show the nucleation of both TiSe2 and Ti2Se during deposition, with the Ti2Se at the substrate. For both systems, annealing the precursors decreases the peak width and increases the intensity of reflections from crystalline TiSe2 and MoSe2. Optimized films consist of a single phase after the annealing and show clear Laue oscillations in the specular XRD patterns, which can only occur if a majority of the diffracting crystallites in the film consist of the same number of unit cells. The highest quality films was obtained when an excess of ∼10% Se was deposited in the precursor, which presumably acts as a flux to facilitate diffusion of metal atoms to crystallite growth fronts and compensates for Se loss to the open system during annealing.

Combining Soft- and Hard-Templating Approaches in MWW-Type Zeolites.

A combination of hard-templating (HT) and soft-templating (ST) approaches was studied to obtain MWW-type materials with intermediate physicochemical properties. The HT methodology involved the introduction of carbon particles as hard templates during gel synthesis to obtain a layered zeolitic precursor (LZP) with particles possessing a microspherical morphology. The LZP obtained was treated with surfactants as soft templates to expand the layers of the LZP, followed by a pillaring procedure. The materials were characterized by X-ray diffraction, transmission and scanning electron microscopy, elemental analysis and N2 adsorption. The results demonstrate that the obtained material possesses intermediate properties from both approaches, with interparticle mesopores/macropores and pore sizes between 18 and 46 Å. However, the ST procedure causes a partial disruption of some microspheres, forming small crystallite aggregates, and results in a decrease in the number of interparticle mesopores/macropores previously formed by the HT method.

Conversion of Intercalated MoO3 to Multi-Heteroatoms-Doped MoS2 with High Hydrogen Evolution Activity.

Lack of effective strategies to regulate the internal activity of MoS2 limits its practical application for hydrogen evolution reactions (HERs). Doping of heteroatoms without forming aggregation or an edge enrichment is still challenging, and its effect on the HER needs to be further explored. Herein, a two-step method is developed to obtain multi-metal-doped H-MoS2 , which includes intercalation of the layered MoO3 precursor with a following sulfurization. Benefiting from the capability of the intercalation method to uniformly and simultaneously introduce different elements into the van der Waals gap, this method is universal to obtain multi-heteroatoms co-doped MoS2 without forming clusters, phase separation, and an edge enrichment. It is demonstrated that the doping of adjacent cobalt and palladium monomers on MoS2 greatly enhances the HER catalytic activity. The overpotential at 10 mA cm-2 and Tafel slope of Co and Pd co-doped MoS2 is found to be 49.3 mV and 43.2 mV dec-1 , respectively, representing a superior acidic HER catalytic activity. This intercalation-assisted method also provides a new and general strategy to synthesize uniformly doped transition metal dichalcogenides for various applications.

On the fluoride-mediated synthesis and amine intercalation of large lamellar crystals of layered titanosilicate Na/K-JDF-L1

Fluoride-mediated synthesis in the inorganic hydrothermal system has been proposed for preparing large crystals of layered Na/K-JDF-L1 with lamellar morphology. The protonated JDF-L1 crystals derived from their layered precursors are structurally modified by 1,6-hexanediamine intercalation under mild hydrothermal conditions. All the materials mentioned are characterized by XRD, FESEM, EDX/ICP-AES, and TGA–DSC. It is concluded that the combination of fluorides with low alkalinity (pH ca.10.0) and crystallization temperature (200 °C) facilitates the oriented growth of metastable layered phase Na/K-JDF-L1, yielding regular lamellar crystals with square-sheet structure in two-dimensions of ca. 15 × 15 μm. Data on Na/K-JDF-L1 structure are presented for identification of the most probable tetragonal space group, P4212, corresponding to the highest crystallographic symmetry. It is also revealed that amine intercalation of relevant H-JDF-L1 by the hydrothermal process leads to the formation of layered titanosilicate materials with an ordered redistribution of the dominant interlayer space (d001) shifting from 10.674 to 15.620 A.

Effect of delamination on active base site formation over nitrided MWW-type zeolite for Knoevenagel condensation

Abstract High surface area zeolite is advantageous in base catalysis for large molecule synthesis via C–C bond formation. Delamination of layered zeolite precursors is one of the effective approaches to modify the microporous zeolite nature. In this study, a MWW-type zeolite precursor was delaminated under controlled treatment temperature and time and subsequently nitrided by ammonia. All nitrided products were employed to catalyze a model reaction, Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate. Among several catalytic properties, the initial reaction rate was proportional to the external surface area. This indicates that there is no relation between the number of active sites and the amount of nitrogen on the entire surface. Together with the experimental results and the structural aspect of the MWW framework, it is proposed that delamination increases the external surface and eventually increases the unique Si–OH groups, which might be a precursor to the active Si–NH2 sites for the base-catalytic reaction.

The next big thing for silicon nanostructures - CO2 photocatalysis.

Silicene is a relatively new member of the growing family of two-dimensional single-element materials. Both top-down and bottom-up approaches provide access to silicene, the former via vapor deposition on a substrate and the latter via exfoliation of the layered CaSi2 precursor. Most top-down research has been concerned with understanding the various electronic, optical, magnetic, mechanical, electrical, thermal transport and gas-adsorption properties of silicene. By contrast, the focus on bottom-up silicene has primarily been on its synthesis, structure and chemical properties as they relate to its function and utility. Herein, emphasis is placed on the bottom-up strategy because of its scalability and the ease of subsequent silicene modification, with both qualities being important prerequisites for heterogeneous catalysis applications. In this context, synthetic freestanding silicene exists as single sheets or multilayer assemblies, depending on the CaSi2 exfoliation synthesis conditions. The structure of a sheet comprises three connected chair-configuration silicon 6-rings. This connectivity creates buckled sheets in which the hybridization around the unsaturated silicon atoms is sp2-sp3. By adjusting the CaSi2 exfoliation synthesis conditions, either layered silane (Si6H6) or siloxene (Si6H3(OH)3) nanosheets can be obtained. In our studies, we have explored the nucleation and growth of different transition metal nanoparticles on and within the layer spaces of these nanosheets, and explored their thermochemical and photochemical reactivity in CO2 hydrogenation reactions. An overview of these findings, related works and a new-and-optimized catalyst are provided in this article.

Facile fabrication of titanosilicate zeolites with an unprecedented wide range of Si/Ti ratios by employing transition metal dichalcogenides as metal precursors

Herein JDF-L1 titanosilicate zeolites with ultra-low Si/Ti ratios in the framework were facilely prepared via hydrothermal treatment of layered TiS2 precursors along with effective suppression of an undesired TiO2 impurity phase.

Synthesis and characterization of a layered aluminosilicate NUD-11 and its transformation to a 3D stable zeolite.

Aluminosilicate zeolites are a well-known class of crystalline materials that have wide applications in various industrial fields due to their selective adsorption, acidic sites, and stable hydrothermal stability. Great efforts have been devoted to discovering new zeolite structures. As one of the effective methods, layered silicates have been used as precursors to produce stable zeolites through topotactic transformation. Herein, a new layered aluminosilicate, named NUD-11, was hydrothermally synthesized using N,N-dimethylbenzimidazolium as the structure directing agent (SDA). It was then converted into a stable crystalline zeolite by linking the interlayer Si-OH groups with a silylation agent, diethoxymethylsilane. Studies showed that the resulting NUD-11S consisted of alkylsilicate -O-Si(CH3)2-O- linkages between the adjacent layers to form interconnecting 10- and 12-membered ring channels. The calcined NUD-11S possessed micropores of 0.74 nm and 1.2 nm in diameter with a large specific surface area of 314 m2 g-1. The abundant microporosity would make NUD-11S useful as adsorbents or catalysts.

Transformation of Ni-containing birnessite to tectomanganate: Influence and fate of weakly bound Ni(II) species

The geochemical behavior of nickel, an essential trace metal element, strongly depends on its interactions with Mn oxides. Interactions between the phyllomanganate birnessite and sorbed or structurally incorporated Ni have been extensively documented together with the fate of Ni along the transformation of these layered species to tunnel Mn oxides (tectomanganates). By contrast, interactions of phyllomanganates with weakly bound Ni species [hydrated Ni, Ni (hydr)oxides], that possibly prevail in natural Ni-rich (>10% NiO) manganates, have received little attention and the influence of these Ni species on the phyllomanganate-to-tectomanganate transformation remains essentially unknown. A set of phyllomanganate precursors with contrasting contents of Ni was thus prepared and subjected to a reflux treatment mimicking the natural phyllomanganate-to-tectomanganate conversion. Layered precursors and reflux products were characterized with a combination of diffractometric, spectroscopic, thermal, and chemical methods. Ni is essentially present as hydrated Ni(II) and Ni(II) (hydr)oxides in Ni-rich layered precursors whereas Ni(II) sorbed at particle edges prevail at low Ni content. No Ni sorbed at layer vacancy sites or structurally incorporated was detected in the initial vacancy-free layered precursors. Consistent with the high content (≈1/3) of Jahn-Teller distorted Mn(III) octahedra in layered precursors, which is favorable to their conversion to tectomanganates, Ni-free samples fully convert to an a-disordered todorokite, a common tectomanganate with a 3 × 3 tunnel structure. Contrastingly and despite similar high Mn(III) contents in Ni-rich precursors, hydrolysis of interlayer Ni2+ and polymerization of Ni(OH)2 in phyllomanganate interlayers is kinetically favored during reflux process. Asbolane, a phyllomanganate with an incomplete – island-like – octahedral layer of metal (hydr)oxides, is thus formed rather than todorokite. A nitric acid treatment, aiming at the dissolution of the island-like interlayer Ni(OH)2 layer, allows an easy and unambiguous differentiation between asbolane and todorokite, the latter being unaffected by the treatment. Both compounds exhibit indeed similar interplanar periodicities and can be confused when using X-ray diffraction, despite contrasting intensity ratios. Migration rate of Mn(III) out of the MnO2 layer relative to the metal hydrolysis and polymerization rate determines the formation of todorokite or asbolane. Here, Ni(OH)2 polymerization hampers the formation of tectomanganates and likely contributes to the prevalence of phyllomanganates over tectomanganates in natural Ni-rich environments. Most Ni is retained during the reflux process, part of Ni (≈20%) being likely structurally incorporated in the reaction products, thus enhancing the sequestration of Ni in Mn oxides.

Synthesis of unsupported two-dimensional molybdenum carbide nanosheets for hydrogen evolution

A method was developed for the synthesis of multi-layered and single-layered molybdenum carbide (β-Mo2C) nanosheets through exfoliation and centrifugation of a layered oxide precursor. The single-layer materials displayed higher intrinsic activity for the hydrogen evolution reaction (59.3 ± 6 mA/mg vs 5.3 ± 2 mA/mg) compared to the multi-layered carbide nanosheets. This effect was attributed both to high surface areas and to a more reactive surface where the binding energy of *H is optimized for the HER.

Designed Synthesis and Structure–Property Relationships of Kinetically Stable [(PbSe)1+δ]m(VSe2)1 (m = 1, 2, 3, 4) Heterostructures

Understanding structure–function relationships is essential to guide the designed synthesis of novel materials with emergent properties. In this work, we targeted the metastable heterostructures [(PbSe)1+δ]m(VSe2)1, where m = 1–4, to test if the charge density wave (CDW) transition temperature increases as the layer thickness separating the VSe2 monolayers increases, as was observed when SnSe was the separating layer. The modulated elemental reactant approach was used to make the targeted products. This approach involves depositing elemental layers in which the number of atoms of each element per square angstrom in Pb|Se and V|Se bilayers equals the number calculated for a rock salt-structured PbSe bilayer and a CdI2-structured VSe2 slab, respectively. Layered elemental precursors with the correct composition and nanoarchitecture for each of the targeted compounds were prepared by repeatedly depositing a single V|Se bilayer followed by m Pb|Se bilayers. Precursors close to the targeted number of atoms per unit area were determined via X-ray fluorescence and the correct nanoarchitecture self-assembled to the targeted compounds during a low-temperature anneal. Resistivity measurements show that the number of PbSe layers per repeat unit (m) does not change the charge density transition onset temperature as previously reported for the analogous [(SnSe)1+δ]m(VSe2)1 compounds. The temperature dependence and absolute values of the resistivity of the m = 3 and 4 heterostructures scale as expected for composite behavior. The difference in the thickness dependence of the CDW transition between the PbSe- and SnSe-containing compounds highlights that the identity of the intervening rock salt layer plays a more important role in modifying the CDW onset temperature than the separation of the VSe2 layers.

Controlling dispersion and accessibility of Pd nanoparticles via 2D-to-3D zeolite transformation for shape-selective catalysis: Pd@MWW case

Metal nanoparticles (NPs) can be encapsulated into zeolites to prepare active catalysts. However, when using conventional encapsulation methods, the location of metal NPs cannot be controlled, which limits the dispersion of metal precursors. Here, we report a new strategy for the encapsulation of Pd NPs into an MWW zeolite during its 2D-to-3D transformation by introducing diethylenediamine palladium (II) acetate, Pd(en)2(ac)2, when swelling the layered zeolite precursor at room temperature. The resulting Pd@MCM-22 catalyst was investigated by X-ray diffraction, nitrogen sorption, ICP-OES, FTIR, and electron microscopy. The results showed a uniform distribution of NPs in the Pd-doped sample, which had the same nanoporous character as the ‘standard’ MWW zeolite. Moreover, the Pd@MCM-22 catalyst showed high shape selectivity and activity for the hydrogenation of 3-nitrotoluene and almost no activity for 1-nitronaphthalene. In total, 15 zeolites with different topologies have layered representatives that can be functionalized using this strategy, thereby enabling us to prepare numerous tunable microporous structures, with different zeolite-encapsulated metal NPs specifically designed for shape-selective hydrogenations.

Fabrication of (Y0.95Eu0.05)2O3 phosphors with enhanced properties by co-precipitation of layered rare-earth hydroxide

This work is devoted to a simple method for synthesizing phosphors based on layered rare earth hydroxonitrates (LRH). Layered precursors of (Y0.95Eu0.05)2(OH)5(NO3)·xH2O with different morphologies were obtained via co-precipitation at a constant pH and hydrothermal methods. It was established that LRHs co-precipitated at pH 7 are highly ordered close-packed micron-sized plates with low porosity. With an increase in the pH of co-precipitation to 9.5, the phase composition is retained, but the isotropy and porosity of the particles increases sharply: the morphology changes from micron flat plates to curved nanosheets and nanoflakes. Co-precipitated LRHs crystallize more intensively into cubic (Y0.95Eu0.05)2O3 during heat treatment compared to precursors obtained under mild hydrothermal conditions (120 °C, 24 h). The structural characteristics of the material laid at the stage of synthesis directly determine its optical properties and the fundamental parameters of photoluminescence. The phosphors that co-precipitate at pH 7 demonstrate the highest yield of luminescence due to a more ordered structure, larger crystallite size, lower asymmetry factor and surface to volume ratio. The proposed co-precipitation method with a constant pH value of 7 allows one to obtain (Y0.95Eu0.05)2O3 phosphor with enhanced structural and optical properties compared to materials synthesized in hydrothermal conditions. The LRHs co-precipitated in this work are a promising precursor for the preparation of crystal-oriented and highly-efficient luminescent coatings and nanofilms.

(Invited) MXenes for High-Energy, High-Power Batteries and Electrochemical Capacitors

Two-dimensional (2D) nanomaterials have been attractive candidates for a plethora of applications since the successful isolation and demonstration of intriguing electronic properties of graphene in 2004 by the Manchester group. The ‘beyond graphene’ march led to the discovery of new types of conductive 2D materials, expanding the use of low-dimensional nanomaterials in electrochemical energy storage and conversion. MXenes are a relatively new and very large family of 2D transition metal carbides, nitrides, and carbonitrides, which were discovered at Drexel in 2011 and became popular due to their diverse chemistries and unique physicochemical properties. These materials are produced by wet chemical or electrochemical extraction of interleaved ‘A’ atomic layers from the layered ternary carbide or nitride precursors. The resulting MXenes with surface terminations (-O, -F, -OH, -Cl) impart negative surface charges, key for stable MXene dispersions in water and organic solvents, which enable solution processing. More than 30 different MXenes have been experimentally synthesized, with dozens of new MXenes and their properties predicted computationally. The compositional versatility, metallic conductivity, control of surface chemistry, ordered structures versus solid solutions, etc., make MXenes suitable for a wide spectrum of applications in energy storage, ranging from battery and supercapacitor electrodes to current collectors and conductive binders. The available for ion transport 2D gallery spaces, redox chemistry at the transition metal oxide-like surface and metallic conductivity provided by electrons of the transition metals, make MXenes promising as candidates for high-rate energy storage applications. The ability of MXenes to spontaneously intercalate a variety of cations, from Li+ to Al3+, while promoting fast charge transfer rates, endorses them for hybrid metal-ion capacitors and battery applications. Unlike most of the 2D materials produced in small quantities, pilot scale synthesis of MXenes is a promising aspect for future commercialization. This presentation will describe the potential use of MXenes in metal-ion capacitors, Li-ion and Li-S batteries as materials for anodes and cathodes. The use of MXenes to eliminate polymer binders and metal current collectors will also be addressed.

Monodispersed platinum nanoparticles embedded in Ni3S2-containing hollow carbon spheres with ultralow Pt loading and high alkaline hydrogen evolution activity

The design and fabrication of efficient Pt-based electrocatalysts with ultralow-loading but high-mass-activity for hydrogen evolution reaction (HER) in alkaline media is of significant importance. Herein, we report the monodispersed platinum nanoparticles (PtNPs) with an ultralow Pt loading (3.6 μg per electrode area (cm2)) embed in Ni3S2-containing hollow carbon spheres as HER catalysts. Notably, the as-prepared electrocatalyst exhibits a mass activity of 7.6 A gPt−1 at an overpotential of 70 mV, which is 8.5 times higher than that of commercial Pt/C and is among the highest levels of reported Pt-based HER electrocatalysts in alkaline conditions. This electrocatalyst also exhibits excellent HER performance with an overpotential of 42 mV reaching an area current of 10 mA cm−2 and significantly improved stability. The outstanding HER performance is attributed to the hollow and porous carbon spherical structure, as well as highly dispersed PtNPs that provide more active sites and facilitate proton/electron transport. Meanwhile, the nickel-containing layered precursors convert to some Ni3S2 NPs which guarantees the formation of Pt-Ni3S2 heterostructures on the carbon substrate and thus further enhance the activity of PtNPs. This work provides an effective strategy to simultaneously regulate the nanostructure and active site of Pt-containing carbon-based HER electrocatalysts.

Cryogenic Exfoliation of Non‐layered Magnesium into Two‐Dimensional Crystals

AbstractPhysical exfoliation of layered precursors is one of the most prevailing techniques to prepare two‐dimensional (2D) crystals, which, however, is considered to be intrinsically inapplicable to non‐layered bulks. Now, plane cleavage differentiation is identified in metallic magnesium at cryogenic temperature (CT), and a cryogenic exfoliation strategy of non‐layered magnesium into 2D crystals is developed. The cleavage anisotropy of the Mg lattice in response to the external mechanical stress originates from the CT‐induced specific inactivation of basal slip, which results in the basal cleavage perpendicular to c axis. The exfoliated novel 2D Mg crystals exhibit remarkable localized surface plasmon resonances, holding great promise for the applications in harvesting and converting solar energy. Beyond creating a new member for the burgeoning 2D family, this study may provide a useful tool for the physical exfoliations of various non‐layered materials.