Exploring Anomalous Polarization Dynamics in Organometallic Halide Perovskites.

Organometallic halide perovskites (OMHPs) have attracted broad attention as prospective materials for optoelectronic applications. Among the many anomalous properties of these materials, of special interest are the ferroelectric properties including both classical and relaxor-like components, as a potential origin of slow dynamics, field enhancement, and anomalous mobilities. Here, ferroelectric properties of the three representative OMHPs are explored, including FAPbx Sn1-x I3 (x = 0, x = 0.85) and FA0.85 MA0.15 PbI3 using band excitation piezoresponse force microscopy and contact mode Kelvin probe force microscopy, providing insight into long- and short-range dipole and charge dynamics in these materials and probing ferroelectric density of states. Furthermore, second-harmonic generation in thin films of OMHPs is observed, providing a direct information on the noncentrosymmetric polarization in such materials. Overall, the data provide strong evidence for the presence of ferroelectric domains in these systems; however, the domain dynamics is suppressed by fast ion dynamics. These materials hence present the limit of ferroelectric materials with spontaneous polarization dynamically screened by ionic and electronic carriers.

One-Step Synthesis of Nb2 O5 /C/Nb2 C (MXene) Composites and Their Use as Photocatalysts for Hydrogen Evolution.

Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition-metal carbides (MXenes) have recently drawn attention as potential co-catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb2 O5 /C/Nb2 C) as photocatalysts for hydrogen evolution from water splitting. The Nb2 O5 /C/Nb2 C composites were synthesized by one-step CO2 oxidation of Nb2 CTx . Nb2 O5 grew homogeneously on Nb2 C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb2 O5 /C/Nb2 C showed the highest hydrogen generation rate (7.81 µmol h-1 gcat-1 ), a value that was four times higher than that of pure Nb2 O5 . The enhanced performance of Nb2 O5 /C/Nb2 C was attributed to intimate contact between Nb2 O5 and conductive Nb2 C and the separation of photogenerated charge carriers at the Nb2 O5 /Nb2 C interface; the results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.

Reduction-Triggered Self-Assembly of Nanoscale Molybdenum Oxide Molecular Clusters.

Understanding the formation mechanism of giant molecular clusters is essential for rational design and synthesis of cluster-based nanomaterials with required morphologies and functionalities. Here, typical synthetic reactions of a 2.9 nm spherical molybdenum oxide cluster, {Mo132} (formula: [Mo(VI)72Mo(V)60O372(CH3COO)30(H2O)72](42-)), with systematically varied reaction parameters have been fully explored to determine the morphologies and concentration of products, reduction of metal centers, and chemical environments of the organic ligands. The growth of these clusters shows a typical sigmoid curve, suggesting a general multistep self-assembly mechanism for the formation of giant molecular clusters. The reaction starts with a lag phase period when partial Mo(VI) centers of molybdate precursors are reduced to form {Mo(V)2(acetate)} structures under the coordination effect of the acetate groups. Once the concentration of {Mo(V)2(acetate)} reaches a critical value, it triggers the co-assembly of Mo(V) and Mo(VI) species into the giant clusters.

An Air-Stable Na3 SbS4 Superionic Conductor Prepared by a Rapid and Economic Synthetic Procedure.

All-solid-state sodium batteries, using solid electrolyte and abundant sodium resources, show great promise for safe, low-cost, and large-scale energy storage applications. The exploration of novel solid electrolytes is critical for the room temperature operation of all-solid-state Na batteries. An ideal solid electrolyte must have high ionic conductivity, hold outstanding chemical and electrochemical stability, and employ low-cost synthetic methods. Achieving the combination of these properties is a grand challenge for the synthesis of sulfide-based solid electrolytes. Design of the solid electrolyte Na3 SbS4 is described, realizing excellent air stability and an economic synthesis based on hard and soft acid and base (HSAB) theory. This new solid electrolyte also exhibits a remarkably high ionic conductivity of 1 mS cm(-1) at 25 °C and ideal compatibility with a metallic sodium anode.

X-ray and Neutron Scattering Study of the Formation of Core-Shell-Type Polyoxometalates.

A typical type of core-shell polyoxometalates can be obtained through the Keggin-type polyoxometalate-templated growth of a layer of spherical shell structure of {Mo72Fe30}. Small-angle X-ray scattering is used to study the structural features and stability of the core-shell structures in aqueous solutions. Time-resolved small-angle X-ray scattering is applied to monitor the synthetic reactions, and a three-stage formation mechanism is proposed to describe the synthesis of the core-shell polyoxometalates based on the monitoring results. New protocols have been developed by fitting the X-ray data with custom physical models, which provide more convincing, objective, and completed data interpretation. Quasi-elastic and inelastic neutron scattering are used to probe the dynamics of water molecules in the core-shell structures, and two different types of water molecules, the confined and structured water, are observed. These water molecules play an important role in bridging core and shell structures and stabilizing the cluster structures.

Li2OHCl Crystalline Electrolyte for Stable Metallic Lithium Anodes.

In a classic example of stability from instability, we show that Li2OHCl solid electrolyte forms a stable solid electrolyte interphase (SEI) layer with a metallic lithium anode. The Li2OHCl solid electrolyte can be readily achieved through simple mixing of LiOH and LiCl precursors at a mild processing temperature <400 °C. Additionally, we show that continuous, dense Li2OHCl membranes can be fabricated at temperatures <400 °C, standing in great contrast to current processing temperatures of >1600 °C for most oxide-based solid electrolytes. The ionic conductivity and Arrhenius activation energy were explored for the LiOH-LiCl system of crystalline solid electrolytes, where Li2OHCl with increased crystal defects was found to have the highest ionic conductivity and reasonable Arrhenius activation energy. The Li2OHCl solid electrolyte displays stability against metallic lithium, even in extreme conditions past the melting point of lithium metal. To understand this excellent stability, we show that SEI formation is critical in stabilizing the interface between metallic lithium and the Li2OHCl solid electrolyte.

Strong and electrically conductive graphene-based composite fibers and laminates.

Graphene is an ideal candidate for lightweight, high-strength composite materials given its superior mechanical properties (specific strength of 130 GPa and stiffness of 1 TPa). To date, easily scalable graphene-like materials in a form of separated flakes (exfoliated graphene, graphene oxide, and reduced graphene oxide) have been investigated as candidates for large-scale applications such as material reinforcement. These graphene-like materials do not fully exhibit all the capabilities of graphene in composite materials. In the current study, we show that macro (2 inch × 2 inch) graphene laminates and fibers can be produced using large continuous sheets of single-layer graphene grown by chemical vapor deposition. The resulting composite structures have potential to outperform the current state-of-the-art composite materials in both mechanical properties and electrical conductivities (>8 S/cm with only 0.13% volumetric graphene loading and 5 × 10(3) S/cm for pure graphene fibers) with estimated graphene contributions of >10 GPa in strength and 1 TPa in stiffness.

Nanostructure enhanced ionic transport in fullerene reinforced solid polymer electrolytes.

Solid polymer electrolytes, such as polyethylene oxide (PEO) based systems, have the potential to replace liquid electrolytes in secondary lithium batteries with flexible, safe, and mechanically robust designs. Previously reported PEO nanocomposite electrolytes routinely use metal oxide nanoparticles that are often 5-10 nm in diameter or larger. The mechanism of those oxide particle-based polymer nanocomposite electrolytes is under debate and the ion transport performance of these systems is still to be improved. Herein we report a 6-fold ion conductivity enhancement in PEO/lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-based solid electrolytes upon the addition of fullerene derivatives. The observed conductivity improvement correlates with nanometer-scale fullerene crystallite formation, reduced crystallinities of both the (PEO)6:LiTFSI phase and pure PEO, as well as a significantly larger PEO free volume. This improved performance is further interpreted by enhanced decoupling between ion transport and polymer segmental motion, as well as optimized permittivity and conductivity in bulk and grain boundaries. This study suggests that nanoparticle induced morphological changes, in a system with fullerene nanoparticles and no Lewis acidic sites, play critical roles in their ion conductivity enhancement. The marriage of fullerene derivatives and solid polymer electrolytes opens up significant opportunities in designing next-generation solid polymer electrolytes with improved performance.

PS-b-P3HT copolymers as P3HT/PCBM interfacial compatibilizers for high efficiency photovoltaics.

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

Water soluble complexes of chitosan-g-MPEG and hyaluronic acid.

Novel water soluble, biocompatible, and highly viscoelastic polyelectrolyte complexes were prepared by mixing of positively charged chitosan grafted with poly (ethylene glycol) monomethyl ether (CS-g-MPEG) and negatively charged hyaluronic acid (HA). CS-g-MPEGs having different degrees of substitution were synthesized by reacting chitosan with MPEG-aldehyde. The molecular structure, thermal and rheological properties, as well as biocompatibility of CS-g-MPEG/HA complexes were characterized. Rheological results showed that a small amount of HA could greatly enhance the viscosity of CS-g-MPEG solution. The highest viscosity was obtained when the charge ratio of CS-g-MPEG/HA was close to 1.0. Small-angle X-ray scattering measurements provided some insights into the lamellar structure of the CS-g-MPEG/HA complex. The CS-g-MPEG/HA complex system offers promising potentials in pharmaceutical, cosmetic, and biotechnology applications (e.g., cell scaffold, artificial synovial fluid, and drug/gene delivery medium).