Controlled Synthesis of Layered Double Hydroxide Nanoplates Driven by Screw Dislocations

Layered double hydroxides (LDHs) are a family of two-dimensional (2D) materials with layered crystal structures that have found many applications. Common strategies to synthesize LDHs lead to a wide variety of morphologies, from discrete 2D nanosheets to nanoflowers. Here, we report a study of carefully controlled LDH nanoplate syntheses using zinc aluminum (ZnAl) and cobalt aluminum (CoAl) LDHs as examples and reveal their crystal growth to be driven by screw dislocations. By controlling and maintaining a low precursor supersaturation using a continuous flow reactor, individual LDH nanoplates with well-defined morphologies were synthesized on alumina-coated substrates, instead of the nanoflowers that result from uncontrolled overgrowth. The dislocation-driven growth was further established for LDH nanoplates directly synthesized using the respective metal salt precursors. Atomic force microscopy revealed screw dislocation growth spirals, and under transmission electron microscopy, thin CoAl LDH nanoplates displayed complex contrast contours indicative of strong lattice strain caused by dislocations. These results suggest the dislocation-driven mechanism is generally responsible for the growth of 2D LDH nanostructures, and likely other materials with layered crystal structures, which could help the rational synthesis of well-defined 2D nanomaterials with improved properties

Synthetic Collagen Hydrogels through Symmetric Self‐Assembly of Small Peptides

Abstract Animal‐sourced hydrogels, such as collagen, are widely used as extracellular‐matrix (ECM) mimics in tissue engineering but are plagued with problems of reproducibility, immunogenicity, and contamination. Synthetic, chemically defined hydrogels can avoid such issues. Despite the abundance of collagen in the ECM, synthetic collagen hydrogels are extremely rare due to design challenges brought on by the triple‐helical structure of collagen. Sticky‐ended symmetric self‐assembly (SESSA) overcomes these challenges by maximizing interactions between the strands of the triple helix, allowing the assembly of collagen‐mimetic peptides (CMPs) into robust synthetic collagen nanofibers. This optimization, however, also minimizes interfiber contacts. In this work, symmetric association states for the SESSA of short CMPs to probe their increased propensity for interfiber association are modelled. It is found that 33‐residue CMPs not only self‐assemble through sticky ends, but also form hydrogels. These self‐assemblies behave with remarkable consistency across multiple scales and present a clear link between their triple‐helical architecture and the properties of their hydrogels. The results show that SESSA is an effective and robust design methodology that enables the rational design of synthetic collagen hydrogels

Lattice-Matched Bimetallic CuPd-Graphene Nanocatalysts for Facile Conversion of Biomass-Derived Polyols to Chemicals

A bimetallic nanocatalyst with unique surface configuration displays extraordinary performance for converting biomass-derived polyols to chemicals, with potentially much broader applications in the design of novel catalysts for several reactions of industrial relevance. The synthesis of nanostructured metal catalysts containing a large population of active surface facets is critical to achieve high activity and selectivity in catalytic reactions. Here, we describe a new strategy for synthesizing copper-based nanocatalysts on reduced graphene oxide support in which the catalytically active {111} facet is achieved as the dominant surface by lattice-match engineering. This method yields highly active Cu-graphene catalysts (turnover frequency = 33–114 mol/g atom Cu/h) for converting biopolyols (glycerol, xylitol, and sorbitol) to value-added chemicals, such as lactic acid and other useful co-products consisting of diols and linear alcohols. Palladium incorporation in the Cu-graphene system in trace amounts results in a tandem synergistic system in which the hydrogen generated <i>in situ</i> from polyols is used for sequential hydrogenolysis of the feedstock itself. Furthermore, the Pd addition remarkably enhances the overall stability of the nanocatalysts. The insights gained from this synthetic methodology open new vistas for exploiting graphene-based supports to develop novel and improved metal-based catalysts for a variety of heterogeneous catalytic reactions

Porous Two-Dimensional Nanosheets Converted from Layered Double Hydroxides and Their Applications in Electrocatalytic Water Splitting

Porous materials are of particular interest due to their high surface area and rich edge sites, which are favorable for applications such as catalysis. Although there are well-established strategies for synthesizing porous metal oxides (e.g., by annealing the corresponding metal hydroxides), facile and scalable routes to porous metal hydroxides and metal chalcogenides are lacking. Here, we report a simple and general strategy to synthesize porous nanosheets of metal hydroxides by selectively etching layered double hydroxide (LDH) nanoplate precursors that contain amphoteric metal and to further convert them into porous metal chalcogenides by a solution method. Using NiGa LDH as an example, we show that the thin nanoplates with high surface accessibility facilitate the topotactic conversion of NiGa LDH to β-Ni­(OH)₂ and further to NiSe₂ with porous texture while preserving the sheet-like morphology. The converted β-Ni­(OH)₂ and NiSe₂ are highly active for electrocatalytic oxygen evolution reaction and hydrogen evolution reaction (HER), respectively, which demonstrates the applications of such high surface area porous nanostructures with rich edge sites. Particularly, the porous NiSe₂ nanosheets exhibited excellent catalytic activity toward HER with low onset overpotential, small Tafel slope, and good stability under both acidic and alkaline conditions. Overall electrochemical water splitting experiments using these porous β-Ni­(OH)₂ and NiSe₂ nanosheets were further demonstrated. Our work presents a new strategy to prepare porous nanomaterials and to further enhance their catalytic and other applications

Disentangling Second Harmonic Generation from Multiphoton Photoluminescence in Halide Perovskites using Multidimensional Harmonic Generation

Metal halide perovskites are an intriguing class of semiconductor materials being explored for their linear and non-linear optical, and potentially ferroelectric properties. In particular, layered two-dimensional Ruddlesden-Popper (RP) halide perovskites have shown non-linear optoelectronic properties. Optical second harmonic generation (SHG) is commonly used to screen for non-centrosymmetric and ferroelectric materials, however, SHG measurements of perovskites are complicated by their intense multiphoton photoluminescence (mPL) which can be mistaken for SHG signal. In this work, we introduce multidimensional harmonic generation as a method to eliminate the complications caused by mPL. By scanning and correlating both excitation and emission frequencies, we un-ambiguously assess whether a material supports SHG by examining if an emission feature scales as twice the excitation frequency. Careful multidimensional harmonic generation measurements of a series of n=2 and n=3 RP perovskites reveal that, contrary to previous belief, n-butylammonium (BA) RP perovskites display no SHG, thus they have inversion symmetry; but RP perovskites with phenylethylammonium (PEA) and 2-thiophenemethylammonium (TPMA) spacer cations display SHG. Multidimensional harmonic generation is also able to confirm the SHG and thus non-centrosymmetry of a recently reported ferroelectric RP perovskite even in the presence of an obscuring mPL background. This work establishes multidimensional harmonic generation as a definitive method to measure the SHG properties of materials and demonstrates that tuning organic cations can allow the design of new non-centrosymmetric or even ferroelectric RP perovskites.<br /

Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe⁴⁺ by Mössbauer Spectroscopy

Nickel–iron oxides/hydroxides are among the most active electrocatalysts for the oxygen evolution reaction. In an effort to gain insight into the role of Fe in these catalysts, we have performed operando Mössbauer spectroscopic studies of a 3:1 Ni:Fe layered hydroxide and a hydrous Fe oxide electrocatalyst. The catalysts were prepared by a hydrothermal precipitation method that enabled catalyst growth directly on carbon paper electrodes. Fe⁴⁺ species were detected in the NiFe hydroxide catalyst during steady-state water oxidation, accounting for up to 21% of the total Fe. In contrast, no Fe⁴⁺ was detected in the Fe oxide catalyst. The observed Fe⁴⁺ species are not kinetically competent to serve as the active site in water oxidation; however, their presence has important implications for the role of Fe in NiFe oxide electrocatalysts

Tuning Mixed Nickel Iron Phosphosulfide Nanosheet Electrocatalysts for Enhanced Hydrogen and Oxygen Evolution

Highly efficient earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of great importance for renewable energy conversion systems. Herein, guided by theoretical calculations, we demonstrate highly efficient water splitting in alkaline solution using quarternary mixed nickel iron phosphosulfide (Ni_1–<i>x</i>Fe_<i>x</i>PS₃) nanosheets (NSs), even though neither NiPS₃ nor FePS₃ is a good HER (or OER) electrocatalyst. With tuned electronic structure and improved electrical conductivity induced by mixing appropriate amount of Fe into NiPS₃, Ni_0.9Fe_0.1PS₃ NSs display excellent HER activity (an overpotential of 72 mV vs reversible hydrogen electrode (RHE) at a geometric catalytic current density of −10 mA cm^–2 and a Tafel slope of 73 mV dec^–1), which is among the best HER catalysts under alkaline conditions. Ni_0.9Fe_0.1PS₃ NSs also show a good apparent OER activity (an overpotential of 329 mV vs RHE at a catalytic current density of 20 mA cm^–2 and a Tafel slope of 69 mV dec^–1), although structural investigation indicates the formation of Ni­(Fe)­OOH and Ni­(Fe)­(OH)₂ layers on the catalyst surface after OER reactions as likely the real active species. These mixed nickel iron phosphosulfide non-precious-metal electrocatalysts with enhanced intrinsic activity and long-term stability and durability should have great potential in overall water-splitting applications

Single-Crystal Thin Films of Cesium Lead Bromide Perovskite Epitaxially Grown on Metal Oxide Perovskite (SrTiO₃)

High-quality metal halide perovskite single crystals have low defect densities and excellent photophysical properties, yet thin films are the most sought after material geometry for optoelectronic devices. Perovskite single-crystal thin films (SCTFs) would be highly desirable for high-performance devices, but their growth remains challenging, particularly for inorganic metal halide perovskites. Herein, we report the facile vapor-phase epitaxial growth of cesium lead bromide perovskite (CsPbBr₃) continuous SCTFs with controllable micrometer thickness, as well as nanoplate arrays, on traditional oxide perovskite SrTiO₃(100) substrates. Heteroepitaxial single-crystal growth is enabled by the serendipitous incommensurate lattice match between these two perovskites, and overcoming the limitation of island-forming Volmer–Weber crystal growth is critical for growing large-area continuous thin films. Time-resolved photoluminescence, transient reflection spectroscopy, and electrical transport measurements show that the CsPbBr₃ epitaxial thin film has a slow charge carrier recombination rate, low surface recombination velocity (10⁴ cm s^–1), and low defect density of 10¹² cm^–3, which are comparable to those of CsPbBr₃ single crystals. This work suggests a general approach using oxide perovskites as substrates for heteroepitaxial growth of halide perovskites. The high-quality halide perovskite SCTFs epitaxially integrated with multifunctional oxide perovskites could open up opportunities for a variety of high-performance optoelectronics devices