Catalysis Sans Catalyst Loss: The Origins of Prolonged Stability of Graphene-Metal-Graphene Sandwich Architecture for Oxygen Reduction Reactions.

Over the past decades, the design of active catalysts has been the subject of intense research efforts. However, there has been significantly less deliberate emphasis on rationally designing a catalyst system with a prolonged stability. A major obstacle comes from the ambiguity behind how catalyst degrades. Several degradation mechanisms are proposed in literature,   but with a lack of systematic studies, the causal relations between degradation and those proposed mechanisms remain ambiguous. Here, a systematic study of a catalyst system comprising of small particles and single atoms of Pt sandwiched between graphene layers, GR/Pt/GR, is studied to  unravel the degradation mechanism of the studied electrocatalyst for oxygen reduction reaction(ORR). Catalyst suffers from atomic dissolution under ORR harsh acidic and oxidizing operation voltages. Single atoms trapped in point defects within the top graphene layer on their way hopping through toward the surface of GR/Pt/GR architecture. Trapping mechanism renders individual Pt atoms as single atom catalyst sites catalyzing ORR for thousands of cycles before washed away in the electrolyte. The GR/Pt/GR catalysts also compare favorably to state-of-the-art commercial Pt/C catalysts and demonstrates a rational design of a hybrid nanoarchitecture with a prolonged stability for thousands of operation cycles.

Dual-light emitting 3D encryption with printable fluorescent-phosphorescent metal-organic frameworks.

Optical encryption technologies based on room-temperature light-emitting materials are of considerable interest. Herein, we present three-dimensional (3D) printable dual-light-emitting materials for high-performance optical pattern encryption. These are based on fluorescent perovskite nanocrystals (NCs) embedded in metal-organic frameworks (MOFs) designed for phosphorescent host-guest interactions. Notably, perovskite-containing MOFs emit a highly efficient blue phosphorescence, and perovskite NCs embedded in the MOFs emit characteristic green or red fluorescence under ultraviolet (UV) irradiation. Such dual-light-emitting MOFs with independent fluorescence and phosphorescence emissions are employed in pochoir pattern encryption, wherein actual information with transient phosphorescence is efficiently concealed behind fake information with fluorescence under UV exposure. Moreover, a 3D cubic skeleton is developed with the dual-light-emitting MOF powder dispersed in 3D-printable polymer filaments for 3D dual-pattern encryption. This article outlines a universal principle for developing MOF-based room-temperature multi-light-emitting materials and a strategy for multidimensional information encryption with enhanced capacity and security.

Distribution and Transport of CO2 in Hyperbranched Poly(ethylenimine)-Loaded MCM-41: A Molecular Dynamics Simulation Approach.

Fossil fuel use is accelerating climate change, driving the need for efficient CO2 capture technologies. Solid adsorption-based direct air capture (DAC) of CO2 has emerged as a promising mode for CO2 removal from the atmosphere due to its potential for scalability. Sorbents based on porous supports incorporating oligomeric amines in their pore spaces are widely studied. In this study, we investigate the intermolecular interactions and adsorption of CO2 and H2O molecules in hyperbranched poly(ethylenimine) (HB-PEI) functionalized MCM-41 systems to understand the distribution and transport of CO2 and H2O molecules. Density Functional Theory (DFT) is employed to compute the binding energies of CO2 and H2O molecules with HB-PEI and MCM-41 and to develop force field parameters for molecular dynamics (MD) simulations. The MD simulations are performed to examine the distribution and transport of CO2 and H2O molecules as a function of the HB-PEI content. The study finds that an HB-PEI content of approximately 34 wt % is thermodynamically favorable, with an upper limit of HB-PEI loading between 45 and 50 wt %. The distribution of CO2 and H2O molecules is primarily determined by their adsorptive binding energies, for which H2O molecules dominate the occupation of binding sites due to their strong affinity with silanol groups on MCM-41 and amine groups of HB-PEI. The HB-PEI content has a considerable impact on the diffusion of CO2 and H2O molecules. Furthermore, a larger number of water molecules (higher relative humidity) reduces the correlation of CO2 with the MCM-41 pore surface while enhancing the correlation of CO2 with the amine groups of the HB-PEI. Overall, the presence of H2O molecules increases the CO2 correlation with the amine groups and also the CO2 transport within HB-PEI-loaded MCM-41, meaning that the presence of H2O enhances the CO2 capture in the HB-PEI-loaded MCM-41. These findings are consistent with experimental observations of the impact of increasing humidity on CO2 capture while providing new, molecular-level explanations for the macroscopic experimental findings.

Thermoresponsive Conductivity of Graphene-Based Fibers.

Smart materials are versatile material systems which exhibit a measurable response to external stimuli. Recently, smart material systems have been developed which incorporate graphene in order to share on its various advantageous properties, such as mechanical strength, electrical conductivity, and thermal conductivity as well as to achieve unique stimuli-dependent responses. Here, a graphene fiber-based smart material that exhibits reversible electrical conductivity switching at a relatively low temperature (60 °C), is reported. Using molecular dynamics (MD) simulation and density functional theory-based non-equilibrium Green's function (DFT-NEGF) approach, it is revealed that this thermo-response behavior is due to the change in configuration of amphiphilic triblock dispersant molecules occurring in the graphene fiber during heating or cooling. These conformational changes alter the total number of graphene-graphene contacts within the composite material system, and thus the electrical conductivity as well. Additionally, this graphene fiber fabrication approach uses a scalable, facile, water-based method, that makes it easy to modify material composition ratios. In all, this work represents an important step forward to enable complete functional tuning of graphene-based smart materials at the nanoscale while increasing commercialization viability.

Graphene Nanoribbon Hybridization of Zeolitic Imidazolate Framework Membranes for Intrinsic Molecular Separation.

Zeolitic imidazolate frameworks (ZIFs) are promising for gas separation membrane, but their molecular cut-off differs from that expected from its intrinsic aperture structure because of their flexibility. Herein, we introduced graphene nanoribbons (GNRs) to rigidify the ZIF framework. Because the sp2 edge of the GNRs induces strong anchoring effects, the modified layer can be rigidified. Particularly, when the GNRs were embedded and distributed in the ZIF-8 layer, an intrinsic aperture size of 3.4 Šwas observed, resulting in high H2 /CO2 separation (H2 permeance: 5.2×10-6  mol/m2 Pa s, ideal selectivity: 142). The performance surpasses the upper bound of polycrystalline MOF membrane performance. In addition, the membrane can be applied to blue H2 production, as demonstrated with a simulated steam reformed gas containing H2 /CO2 /CH4 . The separation performance was retained in the presence of water. The fundamentals of the molecular transport through the rigid ZIF-8 framework were revealed using molecular dynamics simulations.

Cooperativity in the Aldol Condensation Using Bifunctional Mesoporous Silica-Poly(styrene) MCM-41 Organic/Inorganic Hybrid Catalysts.

This work explores the efficacy of silica/organic hybrid catalysts, where the organic component is built from linear aminopolymers appended to the silica support within the support mesopores. Specifically, the role of molecular weight and polymer chain composition in amine-bearing atom transfer radical polymerization-synthesized poly(styrene-co-2-(4-vinylbenzyl)isoindoline-1,3-dione) copolymers is probed in the aldol condensation of 4-nitrobenzaldehyde and acetone. Controlled polymerization produces protected amine-containing poly(styrene) chains of controlled molecular weight and dispersity, and a grafting-to thiol-ene coupling approach followed by a phthalimide deprotection step are used to covalently tether and activate the polymer hybrid catalysts prior to the catalytic reactions. Site-normalized batch kinetics are used to assess the role of polymer molecular weight and chain composition in the cooperative catalysis. Lower-molecular-weight copolymers are demonstrated to be more active than catalysts built from only molecular organic components or from higher-molecular-weight chains. Molecular dynamics simulations are used to probe the role of polymer flexibility and morphology, whereby it is determined that higher-molecular-weight hybrid structures result in congested pores that inhibit active site cooperativity and the diffusivity of reagents, thus resulting in lower rates during the reaction.

Spectral Instability of Layered Mixed Halide Perovskites Results from Anion Phase Redistribution and Selective Hole Injection.

Despite the ability to precisely tune their bandgap energies, mixed halide perovskites (MHPs) suffer from significant spectral instability, which obstructs their utilization for the rational design of light-emitting diodes. Here, we investigate the origin of the electroluminescence peak shifts in layered MHPs containing bromide and iodide. X-ray diffraction and steady-state absorption measurements prove effective integration of iodide into the cubic lattice and the spatially uniform distribution of halides in the ambient environment. However, the applied electric field during the device operation is found to drive the systematic halide migration. Quantum mechanical density functional theory calculations reveal that the different activation energies required for directional ion hopping lead to the redistribution of anions. In-depth analyses of the electroluminescence spectra indicate that the spectral shifting rate is dependent on the drift velocity of halides. Finally, it is suggested from our study that the dominant red emission is ascribed to the thermodynamically favorable selective hole injection. Our mechanistic study provides insights into the fundamental reason for the spectral instability of devices based on MHPs.

Electron-Transport Characteristics through Aluminum Oxide (100) and (012) in a Metal-Insulator-Metal Junction System: Density Functional Theory-Nonequilibrium Green Function Approach.

Al2O3 is commonly used in modern electronic devices because of its good mechanical properties and excellent electrical insulating property. Although fundamental understanding of the electron transport in Al2O3 is essential for its use in electronic device applications, a thorough investigation for the electron-transport mechanism has not been conducted on the structures of Al2O3, especially in nanometer-scale electronic device settings. In this work, electron transport via Al2O3 for two crystallographic facets, (100) and (012), in a metal-insulator-metal junction configuration is investigated using a density functional theory-based nonequilibrium Green function method. First, it is confirmed that the transmission function, T(E), decreases as a function of energy in (E - E F) < 0 regime, which is an intuitively expected trend. On the other hand, in the (E - E F) > 0 regime, Al2O3(100) and Al2O3(012) show their own characteristic behaviors of T(E), presenting that major peaks are shifted toward lower energy levels under a finite bias voltage. Second, the overall conductance decay rates under zero bias are similar regardless of the crystallographic orientation, so that the contact interface seemingly has only a minor contribution to the overall conductance. A noteworthy feature at the finite bias condition is that the electrical current drastically increases as a function of bias potential (>0.7 V) in Al2O3(012)-based junction compared with the Al2O3(100) counterpart. It is elucidated that such a difference is due to the well-developed eigenchannels for electron transport in the Al2O3(012)-based junction. Therefore, it is evidently demonstrated that at finite bias condition, the contact interface plays a key role in determining insulating properties of Al2O3-Pt junctions.

Enhanced Selectivity for CO2 Adsorption on Mesoporous Silica with Alkali Metal Halide Due to Electrostatic Field: A Molecular Simulation Approach.

Since adsorption performances are dominantly determined by adsorbate-adsorbent interactions, accurate theoretical prediction of the thermodynamic characteristics of gas adsorption is critical for designing new sorbent materials as well as understanding the adsorption mechanisms. Here, through our molecular modeling approach using a newly developed quantum-mechanics-based force field, it is demonstrated that the CO2 adsorption selectivity of SBA-15 can be enhanced by incorporating crystalline potassium chloride particles. It is noted that the induced intensive electrostatic fields around potassium chloride clusters create gas-trapping sites with high selectivity for CO2 adsorption. The newly developed force field can provide a reliable theoretical tool for accurately evaluating the gas adsorption on given adsorbents, which can be utilized to identify good gas adsorbents.

Epitaxially Self-Assembled Alkane Layers for Graphene Electronics.

The epitaxially grown alkane layers on graphene are prepared by a simple drop-casting method and greatly reduce the environmentally driven doping and charge impurities in graphene. Multiscale simulation studies show that this enhancement of charge homogeneity in graphene originates from the lifting of graphene from the SiO2 surface toward the well-ordered and rigid alkane self-assembled layers.

Edge-selenated graphene nanoplatelets as durable metal-free catalysts for iodine reduction reaction in dye-sensitized solar cells.

Metal-free carbon-based electrocatalysts for dye-sensitized solar cells (DSSCs) are sufficiently active in Co(II)/Co(III) electrolytes but are not satisfactory in the most commonly used iodide/triiodide (I(-)/I3 (-)) electrolytes. Thus, developing active and stable metal-free electrocatalysts in both electrolytes is one of the most important issues in DSSC research. We report the synthesis of edge-selenated graphene nanoplatelets (SeGnPs) prepared by a simple mechanochemical reaction between graphite and selenium (Se) powders, and their application to the counter electrode (CE) for DSSCs in both I(-)/I3 (-) and Co(II)/Co(III) electrolytes. The edge-selective doping and the preservation of the pristine graphene basal plane in the SeGnPs were confirmed by various analytical techniques, including atomic-resolution transmission electron microscopy. Tested as the DSSC CE in both Co(bpy)3 (2+/3+) (bpy = 2,2'-bipyridine) and I(-)/I3 (-) electrolytes, the SeGnP-CEs exhibited outstanding electrocatalytic performance with ultimately high stability. The SeGnP-CE-based DSSCs displayed a higher photovoltaic performance than did the Pt-CE-based DSSCs in both SM315 sensitizer with Co(bpy)3 (2+/3+) and N719 sensitizer with I(-)/I3 (-) electrolytes. Furthermore, the I3 (-) reduction mechanism, which has not been fully understood in carbon-based CE materials to date, was clarified by an electrochemical kinetics study combined with density functional theory and nonequilibrium Green's function calculations.

Carbon nanobuds based on carbon nanotube caps: a first-principles study.

Based on density functional theory calculations, we here show that the formation of a fullerene C60 carbon "nanobud" (CNB) on carbon nanotube (CNT) caps is energetically more favorable than that on CNT sidewalls. The dominant CNB formation mode for CNT caps is found to be the [2 + 2] cycloaddition reaction as in the conventional CNT sidewall case. However, it is identified to be exothermic in contrast to the endothermic reaction on CNT sidewalls. Computed reaction pathways further demonstrate that the formation (dissociation) barrier for the CNT cap-based CNB is slightly lower (significantly higher) than that of the CNT sidewall-based CNB, indicating an easier CNB formation as well as a higher structural stability. Additionally, performing matrix Green's function calculations, we study the charge transport properties of the CNB/metal electrode interfaces, and show that the C60 bonding to the CNT cap or open end induces resonant transmissions near the Fermi level. It is also found that the good electronic linkage in the CNT cap-C60 cycloaddition bonds results in the absence of quantum interference patterns, which contrasts with the case of the CNB formed on an open-ended CNT that shows a Fano resonance profile.

Interaction of C60 with water: first-principles modeling and environmental implications.

The nature of fullerene-water interactions has been the subject of much research and debate. Specifically, the presence of a stabilizing, negative surface potential on colloidal aggregates of C60 in water is unexpected, given the neutral nature of pure carbon, and is not well understood. Previous simulation efforts have focused on the C60-water interaction using molecular dynamics simulations that lacked the ability to account for charge transfer and distribution interactions. In this study, first-principles density functional theory was used to analyze the fundamental electronic interactions to elucidate the polarization and charge transfer between water and C60. Simulations show that charge is inductively transferred to the C60 from water molecules, with subsequent polarization of the C60 molecule. In a case with two neighboring C60 molecules, the charge polarization induces a charge onto the second C60. Simulation suggests that this charge transfer and polarization may contribute at least partly to the observed negative surface potential of fullerene aggregates and, combined with hydrogen bonding network formation around C60, provides a fundamental driving force for aggregate formation in water.

The mechanism of antibacterial activity of phlorofucofuroeckol-A against methicillin-resistant Staphylococcus aureus.

To find more effective ways of overcoming methicillin-resistant Staphylococcus aureus (MRSA), there has been considerable interest in the use of marine-derived constituents as alternatives to control pathogenic microorganisms. In this study, we investigated whether phlorofucofuroeckol-A (PFF) isolated from the edible brown alga Eisenia bicyclis suppressed production or function of penicillin-binding protein 2a (PBP2a). The antimicrobial mode of action of PFF in MRSA was identified by measuring cell membrane integrity and using the time-kill curve method. We attempted to determine the antimicrobial effects of PFF on the expression level of the resistance determinants mecA and its regulatory genes mecI and mecR1 in MRSA by reverse transcriptase polymerase chain reaction. PFF suppressed mecI, mecR1, and mecA gene expression in a dose-dependent manner. In addition, we revealed PFF mediates the suppressive effect of PBP2a expression in MRSA by Western blot analysis. PFF suppressed production of the PBP2a protein, suggesting that PFF probably acts by controlling the methicillin resistance-associated genes involved in the cell wall and production of PBP2a. These results demonstrate that PFF isolated from E. bicyclis significantly suppressed the expression of the methicillin resistance-associated genes and production of PBP2a, which is considered the primary cause of methicillin resistance.

A facile synthesis of multi metal-doped rectangular ZnO nanocrystals using a nanocrystalline metal-organic framework template.

A multi metal (M: Fe, Co, and Ni)-doped rectangular ZnO nanocrystal (r-ZnO:M) was synthesised using nanocrystalline metal-organic framework-5 (n-MOF-5). After calcination in air, M-inserted n-MOF-5 led to r-ZnO:M of the wurtzite crystal structure with a small amount (<1%) of spinel ZnM2O4 phase. The inserted metal atoms of r-ZnO:M, replacing the Zn atoms of the wurtzite ZnO structure, were well-dispersed throughout the nanocrystal. Density functional theory calculations not only confirm the structural stability of wurtzite r-ZnO:M and negligible contribution of spinel ZnM2O4 but also elucidate the experimentally observed increase of visible light absorbance and appearance of ferromagnetism upon metal atom doping.

Mechanism of Li adsorption on carbon nanotube-fullerene hybrid system: a first-principles study.

The lithium (Li) adsorption mechanism on the metallic (5,5) single wall carbon nanotube (SWCNT)-fullerene (C(60)) hybrid material system is investigated using first-principles method. It is found that the Li adsorption energy (-2.649 eV) on the CNT-C(60) hybrid system is lower than that on the peapod system (-1.837 eV) and the bare CNT (-1.720 eV), indicating that the Li adsorption on the CNT-C(60) hybrid system is more stable than on the peapod or bare CNT system. This is due to the C(60) of high electron affinity and the charge redistribution after mixing CNT with C(60). In order to estimate how efficiently Li can utilize the vast surface area of the hybrid system for increasing energy density, the Li adsorption energy is calculated as a function of the adsorption positions around the CNT-C(60) hybrid system. It turns out that Li preferably occupies the mid-space between C(60) and CNT and then wraps up the C(60) side and subsequently the CNT side. It is also found that the electronic properties of the CNT-C(60) system, such as band structure, molecular orbital, and charge distribution, are influenced by the Li adsorption as a function of the number of Li atoms. From the results, it is expected that the CNT-C(60) hybrid system has enhanced the charge transport properties in addition to the Li adsorption, compared to both CNT and C(60).