Ningdong Huang

Molybdenum sulfide/graphene-carbon nanotube nanocomposite material for electrocatalytic applications in hydrogen evolution reactions

We report a three-dimensional hierarchical ternary hybrid composite of molybdenum disulfide (MoS2), reduced graphene oxide (GO), and carbon nanotubes (CNTs) prepared by a two-step process. Firstly, reduced GO–CNT composites with three-dimensional microstructuresare synthesized by hydrothermal treatment of an aqueous dispersion of GO and CNTs to form a composite structure via π–π interactions. Then, MoS2 nanoparticles are hydrothermally grown on the surfaces of the GO–CNT composite. This ternary composite shows superior electrocatalytic activity and stability in the hydrogen evolution reaction, with a low onset potential of only 35 mV, a Tafel slope of ~38 mV·decade−1, and an apparent exchange current density of 74.25 mA·cm−2. The superior hydrogen evolution activity stemmed from the synergistic effect of MoS2 with its electrocatalytically active edge-sites and excellent electrical coupling to the underlying graphene and CNT network.

The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene

Combining extensional rheology with in-situ synchrotron ultrafast x-ray scattering, we studied flow-induced phase behaviors of polyethylene (PE) in a wide temperature range up to 240 °C. Non-equilibrium phase diagrams of crystallization and melting under flow conditions are constructed in stress-temperature space, composing of melt, non-crystalline δ, hexagonal and orthorhombic phases. The non-crystalline δ phase is demonstrated to be either a metastable transient pre-order for crystallization or a thermodynamically stable phase. Based on the non-equilibrium phase diagrams, nearly all observations in flow-induced crystallization (FIC) of PE can be well understood. The interplay of thermodynamic stabilities and kinetic competitions of the four phases creates rich kinetic pathways for FIC and diverse final structures. The non-equilibrium flow phase diagrams provide a detailed roadmap for precisely processing of PE with designed structures and properties.

Toughening mystery of natural rubber deciphered by double network incorporating hierarchical structures

As an indispensible material for modern society, natural rubber possesses peerless mechanical properties such as strength and toughness over its artificial analogues, which remains a mystery. Intensive experimental and theoretical investigations have revealed the self-enhancement of natural rubber due to strain-induced crystallization. However a rigorous model on the self-enhancement, elucidating natural rubbers extraordinary mechanical properties, is obscured by deficient understanding of the local hierarchical structure under strain. With spatially resolved synchrotron radiation micro-beam scanning X-ray diffraction we discover weak oscillation in distributions of strain-induced crystallinity around crack tip for stretched natural rubber film, demonstrating a soft-hard double network structure. The fracture energy enhancement factor obtained by utilizing the double network model indicates an enhancement of toughness by 3 orders. Its proposed that upon stretching spontaneously developed double network structures integrating hierarchy at multi length-scale in natural rubber play an essential role in its remarkable mechanical performance.

Supramolecular Polymers Self‐Assembled from trans‐Bis(pyridine) Dichloropalladium(II) and Platinum(II) Complexes

Two structurally similar trans-bis(pyridine) dichloropalladium(II)- and platinum(II)-type complexes were synthesized and characterized. They both self-assemble in n-hexane to form viscous fluids at lower concentrations, but form metallogels at sufficient concentrations. The viscous solutions were studied by capillary viscosity measurements and UV/Vis absorption spectra monitored during the disassembly process indicated that a metallophilic interaction was involved in the supramolecular polymerization process. For the two supramolecular assemblies, uncommon continuous porous networks were observed by using SEM and TEM revealed that they were built from nanofibers that fused and crosslinked with the increase of concentration. The xerogels of the palladium and platinum complexes were carefully studied by using synchrotron radiation WAXD and EXAFS. The WAXD data show close stacking distances driven by π-π and metal-metal interactions and an evident dimer structure for the platinum complex was found. The coordination bond lengths were extracted from fitting of the EXAFS data. Moreover, close Pt(II) -Pt(II) (Pd(II) -Pd(II) ) and PtCl (PdCl) interactions proposed from DFT calculations in the reported oligo(phenylene ethynylene) (OPE)-based palladium(II) pyridyl supramolecular polymers were also confirmed by using EXAFS. The Pt(II) -Pt(II) interaction is more feasible for supramolecular interaction than the Pd(II) -Pd(II) interaction in our simple case.

The thermodynamic properties of flow-induced precursor of polyethylene

Flow-induced preordering or precursor (FIP) has been studied in a series of lightly cross-linked high-density polyethylene with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements. Based on the incipient strains of SAXS and WAXD signals during extension in a large temperature range, strain-temperature diagrams for flow-induced preordering and nucleation were constructed and revealed that flow-induced crystallization (FIC) undergoes two stages: melt-precursor transition (MPT) and precursor-nuclei transition (PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain; these embryos have shape and structure that are related to those of the corresponding critical nuclei. With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk free energy (ΔH) and a much lower surface free energy (σe). In extreme cases (high temperature), the σe of FIP can be negligible. The quantitative estimation of the thermodynamic parameters suggests the existence of variant FIPs, which plays a vital role for the subsequent progress of PNT and the whole process of FIC.

Synthesis of recyclable, chemically cross-linked, high toughness, high conductivity ion gels by sequential triblock copolymer self-assembly and disulfide bond cross-linking

In this article, we report the synthesis of a disulfide bonded reversibly chemically cross-linked ion gel with high toughness and conductivity by sequential triblock copolymer self-assembly and the subsequent oxidation of thiol groups. Through reversible thiol-disulfide exchange, the ion gels had both high toughness of chemicals and recyclability of physical cross-linked ion gels. The triblock copolymer (SOS-SH) was prepared as follows: first, the RAFT copolymerization of styrene and 4-vinylbenzyl chloride (VBC) using CTA–PEO–CTA as a bi-functional macroRAFT agent was performed to obtain a triblock copolymer (SOS-Cl); then, the chloride group of SOS-Cl was replaced by an azido group to obtain SOS-N3; and finally, the click reaction of SOS-N3 with O-ethyl-S-prop-2-ynyl carbonodithioate and subsequent aminolysis were conducted to obtain SOS-SH. The disulfide bonded reversibly chemically cross-linked ion gel could be re-dissolved when mixed with a little amount of mild reducing agent (e.g., DTT) in CH2Cl2 with vigorous stirring, which reformed again after the removal of solvent and oxidation of thiol groups. The ion gels could undergo the reduction–oxidation cycle at least twice with a little loss of ionic conductivity and toughness (less than 25%), exhibiting good recyclability. Raman measurements were performed to confirm the existence and the key role of disulfide bond on the recyclability.

Specific ion effects induced by mono-valent salts in like charged aggregates in water.

While salt mediated association between similarly charged poly-electrolytes occurs in a broad range of biological and colloidal systems, the effects of mono-valent salts remains little known experimentally. In this communication we systematically study influences of assorted mono-valent salts on structures of and interactions in two dimensional ordered bundles of charged fibrils assembled in water using Small Angle X-ray Scattering (SAXS). By quantitatively analyzing the scattering peak features, we discern two competing effects with opposite influences due to partitioning of salts in the aqueous complex. While electrostatic effects from salts residing between the fibrils suppress attraction between fibrils and expand the bundles, it is compensated by external osmotic pressure from peripheral salts in the aqueous media. The balance between the two effects varies for different salts and gives rise to ion-specific equilibrium behavior as well as structure of ordered bundles in salty water. The specific ions effects in like charged aggregates can be attributed to preferential distribution of ions inside or outside the bundles, correlated to the ranking of ions in Hofmeister series for macromolecules. Unlike conventional studies on Hofmeister effects by thermodynamic measurements relying on modeling for data interpretation, our study is based directly on structural analysis and is model-insensitive.

Metallogels Self‐Assembled from Linear Rod‐Like Platinum Complexes: Influence of the Linkage

Two linear rod-like platinum complexes, which only differed in the linkage, were prepared. They both self-assemble into metallogels in nonpolar solvents; however, a very big contrast was observed. Unexpectedly, a much weaker gel was acquired upon replacing the ester linkage by an amide group. The intermolecular hydrogen bonding offered by the amide motif leads to a different stacking fashion and mechanism. The results demonstrated herein contribute to the rational design of metallogels as well as other functional supramolecular materials.

Highly ordered, ultra long nanofibrils via the hierarchical self-assembly of ionic aromatic oligoamides

Highly ordered templates are of great importance in fabricating well-arranged nanomaterials. Inspired by the hierarchical assembly of bio-macromolecules, we designed a water soluble three-arm ionic aromatic oligoamide which can spontaneously self-assemble into bundles of nanofibrils. Unlike conventional assemblies formed by synthetic oligoamides individually dispersed in solution, the nanofibrils in our present study tend to further arrange into orthorhombic arrays and form microfibers that can be up to millimeters in length. The surrounding ions play a key role in assembly behavior and possess a high metal-ion binding capacity, which provides a strategy for tuning ordered structures of functional materials and possibilities for understanding the assembly mechanism of biological counterparts.

Imaging the strain induced carbon black filler network structure breakage with nano X-ray tomography

Aiming to study the mechanical enhancement by the filler network in a rubber composite, three-dimensional images are acquired with in situ full field transmission X-ray microscopy (TXM), and the network structure of carbon black (CB) aggregates in a rubber matrix are studied with and without strain. Statistical analysis shows that the frequency of similar-sized aggregates decreases with the increase of aggregate size as well as the inter-aggregate distance monotonically without strain. An oscillation of the frequency-size plot is induced by strain on top of the damping trend, which is interpreted as stretch-induced breakage and re-aggregation of CB aggregates. Calculations adopting a soft-hard network model, predict a reduction of the contribution of the CB network to the mechanical property of the rubber composite by about 60%, caused by the breakage and re-aggregation of CB aggregates compared to those without strain. The experimental results directly prove the structural origin of the Payne effect and also show that TXM is a valuable tool to study the mechanical enhancement mechanism of filled rubber composites.