Ki Tae Nam

Free-floating ultrathin two-dimensional crystals from sequence-specific peptoid polymers

The design and synthesis of protein-like polymers is a fundamental challenge in materials science. A biomimetic approach is to explore the impact of monomer sequence on non-natural polymer structure and function. We present the aqueous self-assembly of two peptoid polymers into extremely thin two-dimensional (2D) crystalline sheets directed by periodic amphiphilicity, electrostatic recognition and aromatic interactions. Peptoids are sequence-specific, oligo-N-substituted glycine polymers designed to mimic the structure and functionality of proteins. Mixing a 1:1 ratio of two oppositely charged peptoid 36mers of a specific sequence in aqueous solution results in the formation of giant, free-floating sheets with only 2.7 nm thickness. Direct visualization of aligned individual peptoid chains in the sheet structure was achieved using aberration-corrected transmission electron microscopy. Specific binding of a protein to ligand-functionalized sheets was also demonstrated. The synthetic flexibility and biocompatibility of peptoids provide a flexible and robust platform for integrating functionality into defined 2D nanostructures.

Hydrated Manganese(II) Phosphate (Mn3(PO4)2·3H2O) as a Water Oxidation Catalyst

The development of a water oxidation catalyst has been a demanding challenge in realizing water splitting systems. The asymmetric geometry and flexible ligation of the biological Mn4CaO5 cluster are important properties for the function of photosystem II, and these properties can be applied to the design of new inorganic water oxidation catalysts. We identified a new crystal structure, Mn3(PO4)2·3H2O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its high catalytic performance under neutral conditions. The bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn3(PO4)2·3H2O. Computational analysis indicated that the structural flexibility in Mn3(PO4)2·3H2O could stabilize the Jahn-Teller-distorted Mn(III) and thus facilitate Mn(II) oxidation. This study provides valuable insights into the interplay between atomic structure and catalytic activity.

Peptide-mediated reduction of silver ions on engineered biological scaffolds.

Herein we report the spontaneous reduction of silver ions into nanostructures by yeast surface-displayed glutamic acid (E(6)) and aspartic acid (D(6)) peptides. Light spectroscopy and electron microscopy reveal that silver ions are photoreduced in the presence of the polycarboxylic acid-containing peptides and ambient light, with an increase in reduction capability of E(6) expressing yeast over D(6) yeast. The importance of tethering peptides to a biological scaffold was inferred by observing the reduced particle forming capacity of soluble peptides with respect to corresponding yeast-displayed peptides. This principle was further extended to the M13 virus for fabrication of crystalline silver nanowires. These insights into the spontaneous reduction of metal ions on biological scaffolds should help further the formation of novel nanomaterials in biological systems.

Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Stamped microbattery electrodes based on self-assembled M13 viruses.

The fabrication and spatial positioning of electrodes are becoming central issues in battery technology because of emerging needs for small scale power sources, including those embedded in flexible substrates and textiles. More generally, novel electrode positioning methods could enable the use of nanostructured electrodes and multidimensional architectures in new battery designs having improved electrochemical performance. Here, we demonstrate the synergistic use of biological and nonbiological assembly methods for fabricating and positioning small battery components that may enable high performance microbatteries with complex architectures. A self-assembled layer of virus-templated cobalt oxide nanowires serving as the active anode material in the battery anode was formed on top of microscale islands of polyelectrolyte multilayers serving as the battery electrolyte, and this assembly was stamped onto platinum microband current collectors. The resulting electrode arrays exhibit full electrochemical functionality. This versatile approach for fabricating and positioning electrodes may provide greater flexibility for implementing advanced battery designs such as those with interdigitated microelectrodes or 3D architectures.

Synthesis and Microcontact Printing of Dual End-Functionalized Mucin-like Glycopolymers for Microarray Applications†

Click to view: Glycopolymers can be used to display glycans on microarrays in native-like architectures. The structurally uniform alkyne-terminated mucin mimetic glycopolymers (see picture; TR = fluorophore) were printed on azide-functionalized chips by microcontact printing in the presence of a copper catalyst. The surface-bound glycopolymers bind lectins in a ligand-specific manner.

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we suggest that monolayer graphene can be used as a catalyst for solar-driven hydrogen evolution reaction on Si-photocathodes, and its catalytic activity is boosted by plasma treatment in N2-ambient. The plasma treatment induces abundant defects and the incorporation of nitrogen atoms in the graphene structure, which can act as catalytic sites on graphene. The monolayer graphene containing nitrogen impurities exhibits a remarkable increase in the exchange current density and leads to a significant anodic shift of the onset of photocurrent from the Si-photocathode. Additionally, monolayer graphene shows the passivation effect that suppresses the surface oxidation of Si, thus enabling the operation of the Si-photocathode in neutral water. This study shows that graphene itself can be applied to a photoelectrochemical system as a catalyst with high activity and chemical stability.

A new water oxidation catalyst: lithium manganese pyrophosphate with tunable Mn valency.

The development of a water oxidation catalyst has been a demanding challenge for the realization of overall water-splitting systems. Although intensive studies have explored the role of Mn element in water oxidation catalysis, it has been difficult to understand whether the catalytic capability originates mainly from either the Mn arrangement or the Mn valency. In this study, to decouple these two factors and to investigate the role of Mn valency on catalysis, we selected a new pyrophosphate-based Mn compound (Li2MnP2O7), which has not been utilized for water oxidation catalysis to date, as a model system. Due to the monophasic behavior of Li2MnP2O7 with delithiation, the Mn valency of Li(2-x)MnP2O7 (x = 0.3, 0.5, 1) can be controlled with negligible change in the crystal framework (e.g., volume change ~1%). Moreover, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, ex-situ X-ray absorption near-edge structure, galvanostatic charging-discharging, and cyclic voltammetry analysis indicate that Li(2-x)MnP2O7 (x = 0.3, 0.5, 1) exhibits high catalytic stability without additional delithiation or phase transformation. Notably, we observed that, as the averaged oxidation state of Mn in Li(2-x)MnP2O7 increases from 2 to 3, the catalytic performance is enhanced in the series Li2MnP2O7 < Li(1.7)MnP2O7 < Li(1.5)MnP2O7 < LiMnP2O7. Moreover, Li2MnP2O7 itself exhibits superior catalytic performance compared with MnO or MnO2. Our study provides valuable guidelines for developing an efficient Mn-based catalyst under neutral conditions with controlled Mn valency and atomic arrangement.

Redox Cofactor from Biological Energy Transduction as Molecularly Tunable Energy-Storage Compound†

Energy transduction and storage in biological systems involve multiply coupled, stepwise reduction/oxidation of energycarrying molecules such as adenosine triphosphate (ATP), nicotinamide, and flavin cofactors. These are synthesized as a result of oxidation during citric acid cycles in mitochondria or during photosynthesis in chloroplasts, and high energies stored in their chemical bonds are consequently harnessed for many biological reactions. Phosphorylation and protonation are key underlying mechanisms that allow for reversible cycling and regulate the molecule-specific redox potential. A sequential progression of electron transfer through the redox cascades as well as continuous recycling of the redox centers enables efficient energy use in biological systems. The biological energy transductionmechanism hints at the construction of a man-made energy storage system. Since the pioneering work by Tarascon and co-workers towards a sustainable lithium rechargeable battery received significant resonance, organic materials such as carbonyl, carboxy, or quinone-based compounds have been demonstrated to be bio-inspired organic electrodes. The imitation of redoxactive plastoquinone and ubiquinone cofactors through the use of redox-active C=O functionalities in organic electrodes is a significant step forward to biomimetic energy storage. However, the biological energy transduction is based on numerous redox centers of versatile functionalities available in nature, not limited to the simple redox active C=O functionalities. Consideration of how natural energy transduction systems function at organelle or cellular levels by elucidating the basic components and their operating principles selected by evolution will enrich the biomimetic strategy for efficient and green energy storage. Flavins are one of most structurally and functionally versatile redox centers in nature, catalyzing an enormous range of biotransformations and electron-transfer reactions, which occur over a wide potential range (> 500 mV). The extraordinary versatility of flavins stems from their ability to engage in either oneor twoelectron-transfer redox processes, accompanying proton transfer at the nitrogen atoms of diazabutadiene motif. In the respiratory electron transport chain, for example, electrons from reduced flavin adenine dinucleotide (FADH2) are transported along a group of proteins located in the inner membrane of mitochondria to induce proton pumping across the membrane, as illustrated in Figure 1a (left). This process generates an electrochemical proton gradient, which results in the formation of high-energy ATP. FAD is reduced again in the citric acid cycle of mitochondria, which enables continuous recycling of flavin redox centers. A close analogy exists between the key components, facilitating respiration and battery operation (Figure 1a); charged ions (H or Li) and electrons, which are derived from flavin redox centers, are unidirectionally transported in a stoichiometric manner using separated paths. This creates chemical gradients across membranes, and finally results in the formation of highenergy species such as ATP and metallic lithium. Herein, we report on the possibility of using the energystorage mechanism of flavin redox cycling in mitochondria to lithium rechargeable batteries. According to our results, flavin electrodes were capable of reversibly storing and releasing two lithium ions and two electrons per formula unit. Redox reactions in flavin electrodes were thoroughly investigated using the combined analyses of ex situ characterizations and density functional theory (DFT)-based calculations. We found that the flavin redox reaction occurs during battery operation at the nitrogen atoms of the diazabutadiene motif in flavin molecules using two successive single-electron transfer steps, in a similar way to the proton-coupled electron transfer in flavoenzymes. Molecular tuning by chemical substitution on the isoalloxazine ring significantly improved electrochemical performances in terms of an average redox potential, a gravimetric capacity, and stability, resulting in a high-energy density comparable to that of LiFePO4, the [*] M. Lee, D. H. Nam, Prof. C. B. Park Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology Daejeon 305-701 (Korea) E-mail: parkcb@kaist.ac.kr J. Hong, Dr. D.-H. Seo, Prof. K. T. Nam, Prof. K. Kang Center for Nanoparticle Research Institute for Basic Science (IBS) Department of Materials Science and Engineering Research Institute of Advanced Materials Seoul National University, Seoul 151-742 (Korea) E-mail: matlgen1@snu.ac.kr [] These authors contributed equally to this work.

Concave Rhombic Dodecahedral Au Nanocatalyst with Multiple High-Index Facets for CO2 Reduction

A concave rhombic dodecahedron (RD) gold nanoparticle was synthesized by adding 4-aminothiophenol (4-ATP) during growth from seeds. This shape is enclosed by stabilized facets of various high-indexes, such as (331), (221), and (553). Because it is driven thermodynamically and stabilized by 4-ATP ligands, the concave RD maintains its structure over a few months, even after rigorous electrochemical reactions. We discussed the mechanism of the shape evolution controlled by 4-ATP and found that both the binding energy of Au-S and the aromatic geometry of 4-ATP are major determinants of Au atom deposition during growth. As a possible application, we demonstrated that the concave RD exhibits superior electrocatalytic performance for the selective conversion of CO2 to CO in aqueous solution.