Lutao Weng

Phase Cooperation and Remote-control Effects in Selective Oxidation Catalysts

This review paper concerns allylic oxidations and oxidative dehydrogenations. The corresponding catalysts often contain two or several oxide phases. The objective is to discuss the possible reasons why these phases cooperate (act synergetically). Different interpretations are reviewed. The authors discuss in detail the evidence showing that the synergy is often due to remote control. In such a mechanism, one phase, the donor, dissociates oxygen to form a surface mobile species which spills over to the other phase, or acceptor. The acceptor is the potentially active phase. This phase needs to be irrigated by spill-over oxygen to exhibit maximum activity and selectivity. The various modifications of the acceptor brought about by spill-over oxygen are discussed: maintaining the acceptor at a high oxidation state, preventing the destruction of the structure of the acceptor, and inhibiting the formation of carbonaceous deposits or coke precursors. Parallel experiments with the same two-phase catalysts catalysing an oxygen aided dehydration suggest that the role of spill-over oxygen is to protect some Bronsted acidity of the acceptor. This interpretation of the cooperation between phases permits definite roles to be attributed to the oxide phases present in multicomponent catalysts and to measure approximately their ability to act as donors or acceptors.

Characterization of chemical inhomogeneity in plasma-sprayed hydroxyapatite coatings.

Successful applications of plasma-sprayed hydroxyapatite (HA) coating for implants rely on understanding characteristics of the coatings microstructure, particularly its inhomogeneity. We explored three new techniques for characterizing the chemical inhomogeneity of sprayed HA coatings on titanium substrate: micro-Raman spectroscopy (MRS), positive and negative ion ratios of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the energy loss peaks of X-ray photoelectron spectroscopy (XPS). The results showed that MRS effectively revealed a chemical gradient in the direction of the coating thickness and a decrease in crystallinity from the surface to interface within the as-sprayed coatings. The post-spray treatment effectively promoted homogeneity between surface and the coating/Ti interfaces. Elucidating the chemistry of the sprayed HA coatings using the ion ratios of ToF-SIMS and the energy loss peaks of XPS remains a challenge, even though such techniques can be used to identify certain calcium phosphate phases in pure powder form.

Insights into the Aggregation/Deposition and Structure of a Polydopamine Film

Surface-adherent polydopamine (PDA) films as multifunctional coatings can be easily deposited onto a wide range of materials through dopamine self-polymerization. However, a lack of in-depth understanding of PDA aggregation and deposition processes and definite structure elucidation of PDA make it challenging to tailor the surface characteristic and functionality of the PDA films. Herein, we demonstrate that the surface characteristics of the PDA films can be readily tuned by controlling the competitive interplay between PDA aggregation in solution and deposition on the substrate. Moreover, a structural investigation of the PDA films using analytical tools such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) allows us to propose a new structure model for the PDA building block. The (DHI)2/PCA trimer complex, which consists of two 5,6-dihydroxyindole (DHI) units and one pyrrolecarboxylic acid (PCA) moiety, is definitely identified as a primary building block of PDA, and its formation is steered by covalent interactions in the initial stages of polymerization. In latter stages, the (DHI)2/PCA trimer complexes are further linked primarily through noncovalent interactions to build up the supramolecular structure of PDA. This study provides new insights into the mechanisms of PDA buildup.

A simple and scalable method for preparing low-defect ZIF-8 tubular membranes

Substrate modification by an ultrathin ZnO layer followed by surface activation promotes homogeneous surface nucleation and the growth of a low-defect ZIF-8 tubular membrane that exhibits superb gas permeation and permselectivity.

Mussel-Inspired Adhesive and Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization

Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents.

A Mussel-Inspired Conductive, Self-Adhesive, and Self-Healable Tough Hydrogel as Cell Stimulators and Implantable Bioelectronics

A graphene oxide conductive hydrogel is reported that simultaneously possesses high toughness, self-healability, and self-adhesiveness. Inspired by the adhesion behaviors of mussels, our conductive hydrogel shows self-adhesiveness on various surfaces and soft tissues. The hydrogel can be used as self-adhesive bioelectronics, such as electrical stimulators to regulate cell activity and implantable electrodes for recording in vivo signals.

Time of flight secondary ion mass spectrometry (ToF-SIMS)

Secondary ion mass spectrometry in the “static mode” is becoming a key technique for the surface characterization of organic materials. This is due to the very specific chemical information derived from characteristic molecular secondary ions. The present expansion of this technique is related to the development of high performance time-of-flight mass spectrometers. Indeed they combine high mass resolution allowing to resolve mass interferences between isobaric molecular secondary ions, unlimited mass range, high transmission allowing to reduce the total ion fluence per spectrum (< 1012 ions/cm2) and the molecular imaging capabilities in microscope and/or microprobe modes.

Super stretchable hydrogel achieved by non-aggregated spherulites with diameters <5 nm.

The scope of hydrogel applications can be greatly expanded by the improvement of mechanical properties. However, enhancement of nanocomposite hydrogels (NC gels) has been severely limited because the size of crosslinking nanoparticles is too large, at least in one dimension. Here we report a new strategy to synthesize non-aggregated spherulite nanoparticles, with diameters <5 nm, in aqueous solution, and their enhancement to hydrogel. The stress and stretch ratio at rupture of our NC gel are 430 and 121 KPa with only 40-p.p.m. nanoparticle content. The NC gel containing 200-p.p.m. nanoparticles can revert to 90% of its original size after enduring 100-MPa compressive stress. Our results demonstrate that the suppression of nanoparticle size without aggregation helps to establish a super stretchable and high-toughness hydrogel network at very low inorganic content.

Imaging of sub-surface nano particles by tapping-mode atomic force microscopy

Time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and tapping mode atomic force microscopy (TM-AFM) were used to study the surface of a poly(N-vinyl-2-pyrrolidone) thin film containing nano silica particles. ToF-SIMS results illustrate that the topmost layer of the thin film consists of PVP and a small amount of poly(dimethyl siloxane) (PDMS). Nano silica particles are localized underneath this layer. XPS results suggest that the concentration of the silica particles increases as the sampling depth increases from 5.3 to 7.2 nm. TM-AFM phase imaging is shown to be capable of detecting the presence of these sub-surface nano silica particles.

Surface characterization and quantitative study of poly(4-vinyl phenol) and poly(4-vinyl pyridine) blends by XPS and ToF-SIMS

Abstract Hydrogen bonding between poly(vinyl phenol) (PVPh) and poly(4-vinyl pyridine) (PVPy) was studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Systematic studies were performed on various blends of PVPh and PVPy in different solvents, including ethanol and N,N-dimethylformamide (DMF). Both X-ray photoelectron spectroscopy and contact angle results showed no surface segregation of any component for the blends and complexes of PVPy and a low molecular weight PVPh. Excess of PVPh was found at the surface of the blends when a high molecular weight PVPh was used. However, after annealing at 90°C in a vacuum oven for five days, the surface and bulk compositions are the same. These findings reveal that the surface of blends of high molecular weight polymers may not be in the thermodynamic equilibrium state. The peak intensity of the characteristic pyridyl ions of the blends, especially the PVPh/PVPy complexes, such as the peak at m/z=106, was greatly enhanced by the hydrogen bonding. The quantitative relationship between the ion intensity and the surface composition was studied. A linear relationship was established between the intensity ratio of some of the characteristic ions and the surface concentration determined by XPS.