Yang Yang Li

An XPS investigation of the oxidation/corrosion of melt-spun Mg

Abstract The oxide films formed on the surfaces of melt-spun Mg exposed to air, immersed in distilled water or 3% NaCl solution saturated with Mg(OH)2 have been investigated by X-ray photoelectron spectroscopy (XPS). High resolution XPS spectra revealed two distinct oxygen species on the surface films: one assigned to O2− in MgO, the other to OH− in Mg(OH)2. Depth profiling revealed that the two species had different depth distributions in the films. The oxide film formed in air comprised a contamination outer layer and a relatively thick (5–6 nm) predominantly MgO inner layer. The film formed in distilled water or 3% NaCl solution saturated with Mg(OH)2 was mainly a mixture of Mg(OH)2 and MgO. Mg(OH)2 was predominant at the top layer and decreased gradually with depth while MgO exhibited the opposite behavior. The corrosion product formed in 3% NaCl solution was more hydrated and much thicker that the films formed in the other two conditions. Cl− ion was incorporated in the oxide film formed in 3% NaCl solution. There exists both partial and complete dissociation of adsorbed water when melt-spun pure Mg ribbons are immersed in distilled water or 3% NaCl solution saturated with Mg(OH)2.

Electrochemical doping of anatase TiO2 in organic electrolytes for high-performance supercapacitors and photocatalysts

In this article, we report a facile electrochemical method to modify anatase TiO2 by cathodically biasing TiO2 in an ethylene glycol electrolyte. The resulting black TiO2 is highly stable with a significantly narrower bandgap and higher electrical conductivity. Furthermore, largely improved photoconversion efficiency (increased from 48% to 72% in the visible region, and from nearly 0% to 7% in the UV region), photocatalytic efficiency, and charge-storage capability (∼42 fold increase) are achieved for the treated TiO2.

Growth of TiO2 nanorod arrays on reduced graphene oxide with enhanced lithium-ion storage

We demonstrate the synthesis of a sandwich-like nanocomposite by planting rutile TiO2 nanorods onto reduced graphene oxide (RGO) via a modified seed-assisted hydrothermal growth method. The synthetic process consists of functionalization of graphene oxide (GO), followed by hydrolytic deposition of TiO2 nanoparticles on GO and reduction, and finally hydrothermal growth of rutile TiO2 nanorods on RGO. The resultant nanocomposite, i.e. rutile TiO2 nanorod arrays on RGO (TONRAs–RGO), exhibits largely enhanced reversible charge–discharge capacity and rate capability compared to bare TiO2 nanorods (TONRs) due to its unique structure and superior conductivity. The rate performance of the nanocomposite is also better than that of anatase TiO2 nanoparticles. This study will inspire better design of RGO-based nanocomposites for high energy density lithium-ion battery applications.

pH-triggered release of vancomycin from protein-capped porous silicon films

OBJECTIVE An in vitro model system for pH-triggered release of the antibiotic vancomycin from porous Si films is studied. METHOD Vancomycin is infused into a mesoporous Si film from a mixed aqueous/acetonitrile solution and trapped by a capping layer containing the protein bovine serum albumin (BSA). The protein effectively traps vancomycin in the porous nanostructure at pH 4.0; the protein dissolves and vancomycin is released into solution when the pH increases to 7.4. The surface chemistry of porous Si exerts a substantial effect on the efficacy of drug loading. The amount of drug loading is larger in freshly-etched (hydrophobic, hydrogen-terminated) porous Si and smaller in methyl-modified, undecylenic acid-modified and thermally oxidized samples. The quantity of drug loaded in a freshly etched porous Si chip is proportional to the thickness of the porous layer, which exhibits a constant volume loading efficiency of 31% (v/v). Flow-cell experiments designed to mimic the transition from pH 4 to 7 that occurs when material moves from the stomach to the upper intestinal tract were performed on the freshly etched films and vancomycin- and BSA-release rates were quantified from the effluent of the flow cell by high-pressure liquid chromatography analysis. RESULTS & CONCLUSION There is a small, constant rate of vancomycin release at pH 4 that is independent of the amount of drug loaded in the pores. This is attributed to diffusion of vancomycin from the BSA-capping layer. The release rate increases five- to tenfold when the pH of the solution in the flow cell increases to 7.4; 100% of the drug is released within 3 h of this increase.

Highly Stable Porous Silicon–Carbon Composites as Label-Free Optical Biosensors

A stable, label-free optical biosensor based on a porous silicon-carbon (pSi-C) composite is demonstrated. The material is prepared by electrochemical anodization of crystalline Si in an HF-containing electrolyte to generate a porous Si template, followed by infiltration of poly(furfuryl) alcohol (PFA) and subsequent carbonization to generate the pSi-C composite as an optically smooth thin film. The pSi-C sensor is significantly more stable toward aqueous buffer solutions (pH 7.4 or 12) compared to thermally oxidized (in air, 800 °C), hydrosilylated (with undecylenic acid), or hydrocarbonized (with acetylene, 700 °C) porous Si samples prepared and tested under similar conditions. Aqueous stability of the pSi-C sensor is comparable to related optical biosensors based on porous TiO(2) or porous Al(2)O(3). Label-free optical interferometric biosensing with the pSi-C composite is demonstrated by detection of rabbit IgG on a protein-A-modified chip and confirmed with control experiments using chicken IgG (which shows no affinity for protein A). The pSi-C sensor binds significantly more of the protein A capture probe than porous TiO(2) or porous Al(2)O(3), and the sensitivity of the protein-A-modified pSi-C sensor to rabbit IgG is found to be ~2× greater than label-free optical biosensors constructed from these other two materials.

Thermal evaporation-induced anhydrous synthesis of Fe3O4-graphene composite with enhanced rate performance and cyclic stability for lithium ion batteries.

We present a high-yield and low cost thermal evaporation-induced anhydrous strategy to prepare hybrid materials of Fe3O4 nanoparticles and graphene as an advanced anode for high-performance lithium ion batteries. The ~10-20 nm Fe3O4 nanoparticles are densely anchored on conducting graphene sheets and act as spacers to keep the adjacent sheets separated. The Fe3O4-graphene composite displays a superior battery performance with high retained capacity of 868 mA h g(-1) up to 100 cycles at a current density of 200 mA g(-1), and 539 mA h g(-1) up to 200 cycles when cycling at 1000 mA g(-1), high Coulombic efficiency (above 99% after 200 cycles), good rate capability, and excellent cyclic stability. The simple approach offers a promising route to prepare anode materials for practical fabrication of lithium ion batteries.

Nitrogen-Doped Nanoporous Carbon Membranes with Co/CoP Janus-Type Nanocrystals as Hydrogen Evolution Electrode in Both Acidic and Alkaline Environments

Self-supported electrocatalysts being generated and employed directly as electrodes for energy conversion has been intensively pursued in the fields of materials chemistry and energy. Herein, we report a synthetic strategy to prepare freestanding hierarchically structured, nitrogen-doped nanoporous graphitic carbon membranes functionalized with Janus-type Co/CoP nanocrystals (termed as HNDCM-Co/CoP), which were successfully applied as a highly efficient, binder-free electrode in the hydrogen evolution reaction (HER). Benefited from multiple structural merits, such as a high degree of graphitization, three-dimensionally interconnected micro/meso/macropores, uniform nitrogen doping, well-dispersed Co/CoP nanocrystals, as well as the confinement effect of the thin carbon layer on the nanocrystals, HNDCM-Co/CoP exhibited superior electrocatalytic activity and long-term operation stability for HER under both acidic and alkaline conditions. As a proof-of-concept of practical usage, a 5.6 cm × 4 cm × 60 μm macroscopic piece of HNDCM-Co/CoP was prepared in our laboratory. Driven by a solar cell, electroreduction of water in alkaline conditions (pH 14) was performed, and H2 was produced at a rate of 16 mL/min, demonstrating its potential as real-life energy conversion systems.

Triple-layer Fabry-Perot absorber with near-perfect absorption in visible and near-infrared regime

A simple absorber design which enables near-perfect absorption in the visible and near-infrared regions is presented. The absorber is an unpatterned metal/dielectric/metal triple-layer, e.g., a 20 nm-thick metal film as the top layer, a 250 nm-thick dielectric film as the middle layer, and a 200 nm-thick metal film as the bottom layer. It was found that the high-efficiency absorption at specific wavelengths is mainly due to the Fabry-Perot (FP) resonances in the dielectric middle layer which result in trapping of the resonant light in the middle layer and thus enhanced absorption efficiency.

Perovskite Photovoltachromic Supercapacitor with All-Transparent Electrodes

Photovoltachromic cells (PVCCs) are of great interest for the self-powered smart windows of architectures and vehicles, which require widely tunable transmittance and automatic color change under photostimuli. Organolead halide perovskite possesses high light absorption coefficient and enables thin and semitransparent photovoltaic device. In this work, we demonstrate co-anode and co-cathode photovoltachromic supercapacitors (PVCSs) by vertically integrating a perovskite solar cell (PSC) with MoO3/Au/MoO3 transparent electrode and electrochromic supercapacitor. The PVCSs provide a seamless integration of energy harvesting/storage device, automatic and wide color tunability, and enhanced photostability of PSCs. Compared with conventional PVCC, the counter electrodes of our PVCSs provide sufficient balancing charge, eliminate the necessity of reverse bias voltage for bleaching the device, and realize reasonable in situ energy storage. The color states of PVCSs not only indicate the amount of energy stored and energy consumed in real time, but also enhance the photostability of photovoltaic component by preventing its long-time photoexposure under fully charged state of PVCSs. This work designs PVCS devices for multifunctional smart window applications commonly made of glass.

Synthesis of single-crystal-like nanoporous carbon membranes and their application in overall water splitting

Nanoporous graphitic carbon membranes with defined chemical composition and pore architecture are novel nanomaterials that are actively pursued. Compared with easy-to-make porous carbon powders that dominate the porous carbon research and applications in energy generation/conversion and environmental remediation, porous carbon membranes are synthetically more challenging though rather appealing from an application perspective due to their structural integrity, interconnectivity and purity. Here we report a simple bottom–up approach to fabricate large-size, freestanding and porous carbon membranes that feature an unusual single-crystal-like graphitic order and hierarchical pore architecture plus favourable nitrogen doping. When loaded with cobalt nanoparticles, such carbon membranes serve as high-performance carbon-based non-noble metal electrocatalyst for overall water splitting.