Radially Aligned Hierarchical Nickel/Nickel–Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting

Designing well-controlled hierarchical structures on micrometer and nanometer scales represents one of the most important approaches for upgrading the catalytic abilities of electrocatalysts. Although NiFe (oxy)­hydroxide has been widely studied as a water oxidation catalyst due to its high catalytic capability and abundance, its structural manipulation has been greatly restricted due to its inherent crystallographic stacking feature. In this work, we report for the first time the construction of a nanotube structure of NiFe (oxy)­hydroxide with an inner Ni-rich layer, which was radially aligned on a macroporous nickel foam. Such a hierarchically structured material realized several crucial factors that are essential for excellent catalytic behaviors, including abundant catalytic sites, a high surface area, efficient ionic and electronic transport, etc., and the designed catalyst exhibited competitive electrocatalytic activity for reaction of not only oxygen evolution but also hydrogen evolution, which is very rare. As a result, this novel material was well-suited for the use as a bifunctional catalyst in an integrated water-splitting electrolyzer, which could be even driven by a single AA battery or a 1.5 V solar cell, outperforming a benchmark catalyst of noble-metal ruthenium–platinum combinations and most state-of-the-art electrocatalysts. The work provided important suggestions for the rational modulation of catalysts with new structures targeted for high-performance electrodes used in electrochemical applications

Radially Aligned Hierarchical Nickel/Nickel–Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting

Designing well-controlled hierarchical structures on micrometer and nanometer scales represents one of the most important approaches for upgrading the catalytic abilities of electrocatalysts. Although NiFe (oxy)­hydroxide has been widely studied as a water oxidation catalyst due to its high catalytic capability and abundance, its structural manipulation has been greatly restricted due to its inherent crystallographic stacking feature. In this work, we report for the first time the construction of a nanotube structure of NiFe (oxy)­hydroxide with an inner Ni-rich layer, which was radially aligned on a macroporous nickel foam. Such a hierarchically structured material realized several crucial factors that are essential for excellent catalytic behaviors, including abundant catalytic sites, a high surface area, efficient ionic and electronic transport, etc., and the designed catalyst exhibited competitive electrocatalytic activity for reaction of not only oxygen evolution but also hydrogen evolution, which is very rare. As a result, this novel material was well-suited for the use as a bifunctional catalyst in an integrated water-splitting electrolyzer, which could be even driven by a single AA battery or a 1.5 V solar cell, outperforming a benchmark catalyst of noble-metal ruthenium–platinum combinations and most state-of-the-art electrocatalysts. The work provided important suggestions for the rational modulation of catalysts with new structures targeted for high-performance electrodes used in electrochemical applications

Versatile Cutting Method for Producing Fluorescent Ultrasmall MXene Sheets

As a recently created inorganic nanosheet material, MXene has received growing attention and has become a hotspot of intensive research. The efficient morphology control of this class of material could bring enormous possibilities for creating marvelous properties and functions; however, this type of research is very scarce. In this work, we demonstrate a general and mild approach for creating ultrasmall MXenes by simultaneous intralayer cutting and interlayer delamination. Taking the most commonly studied Ti₃C₂ as an illustrative example, the resulting product possessed monolayer thickness with a lateral dimension of 2–8 nm and exhibited bright and tunable fluorescence. Further, the method could also be employed to synthesize ultrasmall sheets of other MXene phases, for example, Nb₂C or Ti₂C. Importantly, although the strong covalent M–C bond was to some extent broken, all of the characterizations suggested that the chemical structure was composed of well-maintained host layers without observation of any serious damages, demonstrating the superior reaction efficiencies and safeties of our methods. This work may provide a facile and general approach to modulate various nanoscale materials and could further stimulate the vast applications of MXene materials in many optical-related fields

New Family of Lanthanide-Based Inorganic–Organic Hybrid Frameworks: Ln₂(OH)₄[O₃S(CH₂)_<i>n</i>SO₃]·2H₂O (Ln = La, Ce, Pr, Nd, Sm; <i>n</i> = 3, 4) and Their Derivatives

We report the synthesis and structure characterization of a new family of lanthanide-based inorganic–organic hybrid frameworks, Ln₂(OH)₄[O₃S­(CH₂)_<i>n</i>SO₃]·2H₂O (Ln = La, Ce, Pr, Nd, Sm; <i>n</i> = 3, 4), and their oxide derivatives. Highly crystallized samples were synthesized by homogeneous precipitation of Ln³⁺ ions from a solution containing α,ω-organodisulfonate salts promoted by slow hydrolysis of hexamethylenetetramine. The crystal structure solved from powder X-ray diffraction data revealed that this material comprises two-dimensional cationic lanthanide hydroxide {[Ln­(OH)₂(H₂O)]⁺}_∞ layers, which are cross-linked by α,ω-organodisulfonate ligands into a three-dimensional pillared framework. This hybrid framework can be regarded as a derivative of UCl₃-type Ln­(OH)₃ involving penetration of organic chains into two {LnO₉} polyhedra. Substitutional modification of the lanthanide coordination promotes a 2D arrangement of the {LnO₉} polyhedra. A new hybrid oxide, Ln₂O₂[O₃S­(CH₂)_<i>n</i>SO₃], which is supposed to consist of alternating {[Ln₂O₂]²⁺}_∞ layers and α,ω-organodisulfonate ligands, can be derived from the hydroxide form upon dehydration/dehydroxylation. These hybrid frameworks provide new opportunities to engineer the interlayer chemistry of layered structures and achieve advanced functionalities coupled with the advantages of lanthanide elements

Osmotic Swelling of Layered Compounds as a Route to Producing High-Quality Two-Dimensional Materials. A Comparative Study of Tetramethylammonium versus Tetrabutylammonium Cation in a Lepidocrocite-type Titanate

Osmotic swelling and exfoliation behaviors in a lepidocrocite-type titanate H_1.07Ti_1.73O₄·H₂O were investigated upon reactions with tetramethylammonium (TMA⁺) and tetrabutylammonium (TBA⁺) cations. The reaction products in various physical states (suspension, wet aggregate, and deposited nanosheets) were characterized by several techniques, including X-ray diffraction under controlled humidity, small-angle X-ray scattering, particle size analysis, and atomic force microscopy. As the ratio of tetraalkylammonium ion in a solution to exchangeable proton in a solid decreased, the predominant product changed from the osmotically swollen phase, having an interlayer spacing <i>d</i> of several tens of nanometers, to the exfoliated nanosheets. The different behaviors of two cations in the osmotic swelling were evident from the slope and the transition point in the <i>d</i> versus <i>C</i>^–1/2 plot, where <i>C</i> is the concentration of the cations. At a short reaction time, crystallites of a few stacks were obtained as a major product in the reaction with TMA⁺. On the other hand, a mixture of those crystallites and a significant portion of unilamellar nanosheets were obtained in the reaction with TBA⁺. In both cases, those stacks were ultimately thinned down at long reaction time to unilamellar nanosheets. The lateral size of the nanosheets could be controlled, depending on the type of the cations, the tetraalkylammonium-to-proton ratios, and the mode of the reaction (manual versus mechanical shaking). The nanosheets produced by TMA⁺ had large lateral sizes up to tens of micrometers, and the suspension showed a distinctive silky appearance based on liquid crystallinity. Our work provides insights into the fundamentals of osmotic swelling and exfoliation, allowing a better understanding of the preparation of nanosheets, which are one of the most important building blocks in nanoarchitectonics

Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double Hydroxide on NiCo₂O₄ Nanowire Arrays for Highly Efficient Overall Water-Splitting Activity

Developing efficient but nonprecious bifunctional electrocatalysts for overall water splitting in basic media has been the subject of intensive research focus with the increasing demand for clean and regenerated energy. Herein, we report on the synthesis of a novel hierarchical hybrid electrode, NiFe-layered double hydroxide molecularly ultrathin sheets grown on NiCo₂O₄ nanowire arrays assembled from thin platelets with nickel foam as the scaffold support, in which the catalytic metal sites are more accessible and active and most importantly strong chemical coupling exists at the interface, enabling superior catalytic power toward both oxygen evolution reaction (OER) and additionally hydrogen evolution reaction (HER) in the same alkaline KOH electrolyte. The behavior ranks top-class compared with documented non-noble HER and OER electrocatalysts and even comparable to state-of-the-art noble-metal electrocatalysts, Pt and RuO₂. When fabricated as an integrated alkaline water electrolyzer, the designed electrode can deliver a current density of 10 mA cm^–2 at a fairly low cell voltage of 1.60 V, promising the material as efficient bifunctional catalysts toward whole cell water splitting

Synergy of W₁₈O₄₉ and Polyaniline for Smart Supercapacitor Electrode Integrated with Energy Level Indicating Functionality

Supercapacitors are important energy storage technologies in fields such as fuel-efficient transport and renewable energy. State-of-the-art supercapacitors are capable of supplanting conventional batteries in real applications, and supercapacitors with novel features and functionalities have been sought for years. Herein, we report the realization of a new concept, a smart supercapacitor, which functions as a normal supercapacitor in energy storage and also communicates the level of stored energy through multiple-stage pattern indications integrated into the device. The metal-oxide W₁₈O₄₉ and polyaniline constitute the pattern and background, respectively. Both materials possess excellent electrochemical and electrochromic behaviors and operate in different potential windows, −0.5–0 V (W₁₈O₄₉) and 0–0.8 V (polyaniline). The intricate cooperation of the two materials enables the supercapacitor to work in a widened, 1.3 V window while displaying variations in color schemes depending on the level of energy storage. We believe that our success in integrating this new functionality into a supercapacitor may open the door to significant opportunities in the development of future supercapacitors with imaginative and humanization features

Semiconductor SERS enhancement enabled by oxygen incorporation

<p>Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates represent a new frontier in the field of SERS. However, the application of semiconductor materials as SERS substrates is still seriously impeded by their low SERS enhancement and inferior detection sensitivity, especially for non-metal-oxide semiconductor materials. Herein, we demonstrate a general oxygen-incorporation-assisted strategy to magnify the semiconductor substrate–analyte molecule interaction, leading to significant increase in SERS enhancement for non-metal-oxide semiconductor materials. Oxygen incorporation in MoS₂ even with trace concentrations can not only increase enhancement factors by up to 100,000 folds compared with oxygen-unincorporated samples, but also endow MoS₂ with low limit of detection below 10^-7 M. Intriguingly, combined with the findings in previous studies, our present results indicate that both oxygen incorporation and extraction processes can result in SERS enhancement, probably due to the enhanced charge-transfer resonance as well as exciton resonance arising from the judicious control of oxygen admission in semiconductor substrate.</p

Flexible Lithium-Ion Fiber Battery by the Regular Stacking of Two-Dimensional Titanium Oxide Nanosheets Hybridized with Reduced Graphene Oxide

Increasing interest has recently been devoted to developing small, rapid, and portable electronic devices; thus, it is becoming critically important to provide matching light and flexible energy-storage systems to power them. To this end, compared with the inevitable drawbacks of being bulky, heavy, and rigid for traditional planar sandwiched structures, linear fiber-shaped lithium-ion batteries (LIB) have become increasingly important owing to their combined superiorities of miniaturization, adaptability, and weavability, the progress of which being heavily dependent on the development of new fiber-shaped electrodes. Here, we report a novel fiber battery electrode based on the most widely used LIB material, titanium oxide, which is processed into two-dimensional nanosheets and assembled into a macroscopic fiber by a scalable wet-spinning process. The titania sheets are regularly stacked and conformally hybridized in situ with reduced graphene oxide (rGO), thereby serving as efficient current collectors, which endows the novel fiber electrode with excellent integrated mechanical properties combined with superior battery performances in terms of linear densities, rate capabilities, and cyclic behaviors. The present study clearly demonstrates a new material-design paradigm toward novel fiber electrodes by assembling metal oxide nanosheets into an ordered macroscopic structure, which would represent the most-promising solution to advanced flexible energy-storage systems