Micron-Thick Graphene Oxide Films for the Selective Permeation of CO₂

In this study, we characterize the gas transport properties of micrometer-thick graphene oxide (GO) films to elucidate how to control the barrier and membrane properties using GO sheets as a model 2D material. GO films were prepared by the vacuum filtration method using different flake sizes ranging from 1 μm to 300 nm. The prepared GO films show distinct film properties in terms of d-spacing and crystallite size which affect the tortuosity and channel width. Although there were only slight differences in the aforementioned features, the gas transport characteristics dramatically changed, particularly the enhanced CO2 permeability of approximately 60 times. Furthermore, we found critical channel width changes in d-spacing in the range of 0.91–0.85 nm and crystallite size in the range of 23.5–20.5 nm. Consequently, this resulted in a shorter diffusion pathway ratio between vertical and horizontal transportation of gas molecules which is a transition region for switching from a barrier character without notable gas selectivity to a permeation property with reasonable gas selectivity

Preparation of organic-inorganic hybrid adhesion promoters for polyurethane-metal adhesion and their properties

This study reports the synthesis of organic-inorganic (O-I) hybrid adhesion promoters, which were synthesized by an in situ sol-gel process, using alkoxysilane functionalized polymer precursor (AFAP) and other alkoxysilanes such as (3-aminopropyl)triethoxysilane (APTES) and (3-glycidyloxypropyl)trimethoxysilane (GPTMS). These adhesion promoters were then applied between polyurethane (PU) and metal substrate for improving adhesive strength between them. In particular, by modifying the input parameters including molecular structure of AFAP, weight ratios between AFAP and alkoxysilane, and r value (molar ratio of H2O/Si), the adhesive strength can be significantly improved by more than twice as high as compared to that of pristine samples without application of O-I hybrid adhesion promoters. It was reported that the combination of (3-aminopropyl)triethoxysilane (APTES) and AFAP synthesized from glycerol propoxylate (Mw = 266 g/mol) and polyethylene glycol (Mw = 300 g/mol) with high r value showed the highest adhesive strength compared to other O-I hybrid adhesion promoters. Additionally, the properties of various hybrid materials such as particle size distribution, touch drying speed, degree of condensation, molecular weight, and adhesive strength were characterized by dynamic light scattering test, 29Si-NMR, MALDI-TOF, and shear strength test.</p

Dimensionality Control of Self-Assembled Azobenzene Derivatives on a Gold Surface

Well-defined nanostructures constructed with functional molecules provide a feasible route to realize molecular nanotechnology. Synthesis of selectively interacting molecules is essential to develop nanostructures with desired functionalities and dimensions. Substantial efforts have been devoted to achieve finely controlled supramolecular structures on surfaces using various interactions such as van der Waals (vdW), dipolar, hydrogen boning, and metal–ligand interactions. Yet, controlling the dimensions of a supramolecular assembly by changing the strength of the intermolecular vdW interactions, in particular through attaching alkyl chains of different lengths, has not been reported so far. Here, we present the dimensionality control of self-assembled azobenzene derivatives, from one-dimensional chain to two-dimensional island, on an Au(111) surface by exploiting vdW interactions assisted by hydrogen bonding. The designed azobenzene derivatives have alkoxy groups with different chain lengths (6, 8, and 10 carbons). Depending on the alkyl chain length, the molecules self-assemble into two different stacking structures, which determine the dimensionality of the superstructures. Furthermore, we demonstrate that the reconstructed herringbone structures of the substrate determine the stacking structure and growth direction at the elbow of the Au(111) surface. Our results provide a new perspective for engineering well-defined nanostructures with functional molecules as well as deeper insights into the mechanism of molecular self-assembly on surfaces

<i>Operando</i> Spectroscopic Observation of CO Oxidation on a Pt/TiO₂ Catalyst in NO–CO Mixtures under Dark and UV Illumination

The importance of catalytic CO oxidation for exhaust gas processing has inspired the development of efficient and durable catalysts that can be operated at low temperatures, as exemplified by supported noble metals. However, most of the related studies have neglected the potential effects of co-present exhaust gas components, such as NO. Herein, we compared the performances of platinized and bare TiO2 for the ambient-temperature (photo)­catalytic oxidation of CO by O2 in the co-presence of NO and used in situ Fourier transform infrared spectroscopy to elucidate the effects of platinization on the efficiency and mechanism of this oxidation. Catalytic CO oxidation was accompanied by competitive NO oxidation both in the dark and under ultraviolet (UV) illumination. In the dark, Pt/TiO2 achieved a higher CO removal efficiency than bare TiO2, which was ascribed to the enhanced adsorption of O2 and CO and the inhibition of NO overoxidation (mainly the oxidation of the monodentate nitrito form to the bidentate nitrate form). Under UV illumination, Pt/TiO2 efficiently promoted CO oxidation at photocatalytically active sites (Pt0–CO) by suppressing NO overoxidation and increasing the reduction of NO to N2O. Hence, the enhanced CO oxidation performance of Pt/TiO2 was due to its ability to control NO oxidation and reduction at ambient temperature, which presents a new strategy for the design of low-temperature CO oxidation catalysts

Gas Diffusion through Nanoporous Channels of Graphene Oxide and Reduced Graphene Oxide Membranes

Recently, graphene oxide (GO) has been investigated as a class of molecular filters for selective gas and ion transport. However, detailed transport mechanisms have been poorly understood thus far. Here, we report the gas transport behavior of noninterlocked GO and reduced GO (rGO) membranes, which contain nanoporous gas diffusion channels generated by the adjacent edges of GO and rGO sheets. Both membranes exhibited Knudsen gas diffusion behavior; however, the separation factors of these membranes exceeded the theoretical Knudsen separation factors for gas/CO2 selectivities of various gas mixtures owing to extremely low CO2 permeance. The unique transport features of the low CO2 permeance were explained by the blocking effect of CO2 adsorbed in the nanoporous diffusion channels because of the high CO2 affinity of the edges of GO and rGO sheets. Furthermore, the rGO lamellar structure generally shows impermeable interlayer spacing, indicating that the only gas diffusion channel is the nanopores created by neighboring the edges of the rGO sheets. Notably, both membranes maintained a higher H2/CO2 separation factor than the theoretical Knudsen selectivity, including the measurements of mixed-gas permeation experiments. This study provides insight that further GO modification may improve the gas separation performance suitable for specific separation processes

One-Dimensional Molecular Zippers

We synthesized an azobenzene derivative to demonstrate a one-dimensional molecular zipper. The formation and underlying mechanism of the molecular zipper formed by combined hydrogen-bonding and van der Waals interactions between adjacent molecules were investigated on a Au(111) surface using scanning tunneling microscopy and density functional theory calculations

Oxygen Concentration Control of Dopamine-Induced High Uniformity Surface Coating Chemistry

Material surface engineering has attracted great interest in important applications, including electronics, biomedicine, and membranes. More recently, dopamine has been widely exploited in solution-based chemistry to direct facile surface modification. However, unsolved questions remain about the chemical identity of the final products, their deposition kinetics and their binding mechanism. In particular, the dopamine oxidation reaction kinetics is a key to improving surface modification efficiency. Here, we demonstrate that high O₂ concentrations in the dopamine solution lead to highly homogeneous, thin layer deposition on any material surfaces via accelerated reaction kinetics, elucidated by Le Chatelier’s principle toward dopamine oxidation steps in a Michael-addition reaction. As a result, highly uniform, ultra-smooth modified surfaces are achieved in much shorter deposition times. This finding provides new insights into the effect of reaction kinetics and molecular geometry on the uniformity of modifications for surface engineering techniques

Combining Experiment and Theory To Unravel the Mechanism of Two-Electron Oxygen Reduction at a Selective and Active Co-catalyst

We present a combination of comprehensive experimental and theoretical evidence to unravel the mechanism of two-electron oxygen reduction reaction (ORR) on a catalyst composed of mildly reduced graphene oxide supported on P50 carbon paper (mrGO/P50). This catalyst is unique in that it shows >99% selectivity toward H2O2, the highest mass activity to date, and essentially zero overpotential in base. Furthermore, the mrGO catalytically active site is unambiguously identified and presents a unique opportunity to investigate mechanisms of carbon-based catalysis in atomistic detail. A wide range of experiments at varying pH are reported: ORR onset potential, Tafel slopes, H/D kinetic isotope effects, and O2 reaction order. With DFT reaction energies and known thermodynamic parameters, we calculate the potential and pH-dependent free energies of all possible intermediates in this ORR and propose simple kinetic models that give semiquantitative agreement with all experiments. Our results show that mrGO is semiconducting and cannot support the conventional mechanism of coherently coupled proton–electron transfers. The conducting P50 provides electrons for initiating the ORR via outer sphere electron transfer to O2(aq), while the semiconducting mrGO provides the active catalytic sites for adsorption of O2–(aq) or HO2(aq), depending upon electrolyte pH. Due to this unique synergistic effect, we describe the mrGO/P50 as a co-catalyst. This concept implies departure from the traditional picture of predicting catalytic activity trends based on a single descriptor, and the co-catalyst design strategy may generally enable other semiconductors to function as electrocatalysts as well