Kyoungsuk Jin

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.

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.

In vitro and in vivo evaluation of the bioactivity of hydroxyapatite-coated polyetheretherketone biocomposites created by cold spray technology.

Polyetheretherketone (PEEK) is a material that is widely used in medicine because its mechanical properties show excellent similarity to those of human bone. However, because it is bioinert, PEEK shows limited ability to bind to natural bone tissue. Here, we applied a cold spray method to make a hydroxyapatite (HA)-coated PEEK hybrid material and evaluated its osteointegration in vitro and in vivo. With the cold spray method, the HA coating formed a homogeneous layer and adhered strongly to the PEEK disk implant. When the material was tested in vitro, early cell adhesion and viability improved. Alkaline phosphatase (ALP) activity and calcium concentration were also higher in cells cultured on HA-coated PEEK disks. In addition, the expression of osteoblast differentiation markers, such as ALP, bone sialoprotein and runt-related transcription factor 2, increased in these cells. For the in vivo test, we designed and implanted HA-coated PEEK cylinders into a rabbit ilium model by the press-fit method. The bone-implant contact ratio, trabecular number and trabecular thickness were determined using either three-dimensional microcomputed tomography or general two-dimensional histomorphometric analysis. This report demonstrates that the HA coating on the PEEK implant added with the cold spray method increased biocompatibility in vitro and promoted osteointegration in vivo, which suggests that the HA coating may improve the biofunctionality of various medical devices used in clinical applications.

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.

Partially Oxidized Sub-10 nm MnO Nanocrystals with High Activity for Water Oxidation Catalysis

The oxygen evolution reaction (OER) is considered a major bottleneck in the overall water electrolysis process. In this work, highly active manganese oxide nano-catalysts were synthesized via hot injection. Facile surface treatment generated Mn(III) species on monodisperse 10 nm MnO nanocrystals (NCs). Size dependency of MnO NCs on OER activity was also investigated. Surprisingly, the partially oxidized MnO NCs only required 530 mV @ 5 mA cm−2 under near neutral conditions.

Heteroepitaxial growth of ZnO nanosheet bands on ZnCo2O4 submicron rods toward high-performance Li ion battery electrodes

AbstractWe report the direct synthesis of ZnCo2O4 and ZnO/ZnCo2O4 submicron rod arrays grown on Ni foil current collectors via an ammonia-evaporation-induced method by controlling the ratio of Zn to Co. These three-dimensional (3D) hierarchical self-supported nanostructures are composed of one-dimensional (1D) ZnCo2O4 rods and two-dimensional (2D) ZnO nanosheet bands perpendicular to the axis of the each ZnCo2O4 rod. We carefully deal with the heteroepitaxial growth mechanisms of hexagonal ZnO nanosheets from a crystallographic point of view. Furthermore, we demonstrate the ability of these high-surface-area ZnO/ZnCo2O4 heterostructured rods to enable improved electrolyte permeability and Li ion transfer, thereby enhancing their Li storage capability (∼900 mA·h·g−1 at a rate of 45 mA·h·g−1) for Li ion battery electrodes.

An iron oxide photoanode with hierarchical nanostructure for efficient water oxidation

Hematite (α-Fe2O3) has been attracting attention for photoelectrochemical water oxidation due to its visible light photon absorption capacity and high chemical stability, but the short-diffusion length of holes and the large overpotential are still challenging to overcome. Here, in an effort to address these challenges, we develop a hierarchically nanostructured photoanode composed of iron-oxides; Ti-doped hematite nanorods are decorated with an undoped hematite underlayer and β-FeOOH nano-branches. The Ti-doped hematite nanorod array is prepared by hydrothermal synthesis, and this nanostructure offers enhanced separation of photogenerated charges. The underlayer not only increases the photocurrent density but also improves the onset potential. The photocurrent further increases by the epitaxially grown β-FeOOH nano-branches on the hematite, but the onset potential is positively shifted by the β-FeOOH due to increasing flat-band potential. The analyses of the photocurrent transients and electrochemical impedance spectra reveal that β-FeOOH improves the photocurrent by decreasing the resistance to charge transfer through the anode/electrolyte. This study demonstrates a new possibility for improving the efficiency of a hematite photoanode with the interface of other iron-oxides.

Tyrosine-mediated two-dimensional peptide assembly and its role as a bio-inspired catalytic scaffold

In two-dimensional interfacial assemblies, there is an interplay between molecular ordering and interface geometry, which determines the final morphology and order of entire systems. Here we present the interfacial phenomenon of spontaneous facet formation in a water droplet driven by designed peptide assembly. The identified peptides can flatten the rounded top of a hemispherical droplet into a plane by forming a macroscopic two-dimensional crystal structure. Such ordering is driven by the folding geometry of the peptide, interactions of tyrosine and crosslinked stabilization by cysteine. We discover the key sequence motifs and folding structures and study their sequence-specific assembly. The well-ordered, densely packed, redox-active tyrosine units in the YYACAYY (H-Tyr-Tyr-Ala-Cys-Ala-Tyr-Tyr-OH) film can trigger or enhance chemical/electrochemical reactions, and can potentially serve as a platform to fabricate a molecularly tunable, self-repairable, flat peptide or hybrid film.

Revisiting whitlockite, the second most abundant biomineral in bone: nanocrystal synthesis in physiologically relevant conditions and biocompatibility evaluation.

The synthesis of pure whitlockite (WH: Ca18Mg2(HPO4)2(PO4)12) has remained a challenge even though it is the second most abundant inorganic in living bone. Although a few reports about the precipitation of WH in heterogeneous phases have been published, to date, synthesizing WH without utilizing any effects of a buffer or various other ions remains difficult. Thus, the related research fields have encountered difficulties and have not been fully developed. Here, we developed a large-scale synthesis method for pure WH nanoparticles in a ternary Ca(OH)2-Mg(OH)2-H3PO4 system based on a systematic approach. We used excess Mg(2+) to impede the growth of hydroxyapatite (HAP: Ca10(PO4)6(OH)2) and the formation of other kinetically favored calcium phosphate intermediate phases. In addition, we designed and investigated the synthesis conditions of WH under the acidic pH conditions required to dissolve HAP, which is the most thermodynamically stable phase above pH 4.2, and to incorporate the HPO4(2-) group into the chemical structure of WH. We demonstrated that pure WH nanoparticles can be precipitated under Mg(2+)-rich and acidic pH conditions without any intermediate phases. Interestingly, this synthesized nano-WH showed comparable biocompatibility with HAP. Our methodology for determining the synthesis conditions of WH could provide a new platform for investigating other important precipitants in aqueous systems.

Mechanistic Investigation of Water Oxidation Catalyzed by Uniform, Assembled MnO Nanoparticles

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.