Publications by authors named "Tae Joo Shin"

89 Publications

Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of the Oxygen Reduction Reaction.

Angew Chem Int Ed Engl 2021 Sep 6;60(37):20528-20534. Epub 2021 Aug 6.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Rational control of the coordination environment of atomically dispersed catalysts is pivotal to achieve desirable catalytic reactivity. We report the reversible control of coordination structure in atomically dispersed electrocatalysts via ligand exchange reactions to reversibly modulate their reactivity for oxygen reduction reaction (ORR). The CO-ligated atomically dispersed Rh catalyst exhibited ca. 30-fold higher ORR activity than the NH -ligated catalyst, whereas the latter showed three times higher H O selectivity than the former. Post-treatments of the catalysts with CO or NH allowed the reversible exchange of CO and NH ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. DFT calculations revealed that more reduced oxidation state of CO-ligated Rh site could further stabilize the *OOH intermediate, facilitating the two- and four-electron pathway ORR. The reversible ligand exchange reactions were generalized to Ir- and Pt-based catalysts.
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http://dx.doi.org/10.1002/anie.202108439DOI Listing
September 2021

Surface Electronic Modulation with Hetero-Single Atoms to Enhance Oxygen Evolution Catalysis.

ACS Nano 2021 Jun 29. Epub 2021 Jun 29.

School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST, Ulsan 44919, South Korea.

Oxygen evolution catalysis plays a crucial role in the solar-to-fuel conversion for green energy applications. However, developing efficient and stable catalysts for the oxygen evolution catalysis remains a great challenge. Here, we successfully activate an inefficient oxygen evolution catalyst using a simple single atom tailoring strategy. The Rh element with its unfilled 4d electron configuration was selected to atomically implant into a Cu oxide matrix, which has a filled 3d electron configuration. The hetero-Rh single atom (SA) migration was achieved by dispersing CuO nanocubes in a RhCl aqueous solution, enabling an ion exchange process. The activated catalyst (CuO-RhSA) exhibited significantly enhanced catalytic activity toward oxygen evolution in alkaline media, surpassing the performance of pristine CuO nanocubes and commercial IrO. A theoretical study further confirmed that the migrated hetero-Rh SAs can tailor the interaction between the Cu and O sites, modulating the electron density distribution on the surface of CuO, leading to a more favorable oxygen evolution process.
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http://dx.doi.org/10.1021/acsnano.1c02989DOI Listing
June 2021

Tailored Polymer Gate Dielectric Engineering to Optimize Flexible Organic Field-Effect Transistors and Complementary Integrated Circuits.

ACS Appl Mater Interfaces 2021 Jul 13;13(26):30921-30929. Epub 2021 Jun 13.

Advanced Functional Polymers Center, KRICT, Daejeon 34114, Republic of Korea.

The increasing demand for solution-processed and flexible organic electronics has promoted the fabrication of integrated logic circuits using organic field-effect transistors (OFETs) instead of fundamental unit devices. This has been made possible through the rapid development of materials and processes in the past few decades. It is important for the p- and n-type OFETs using different organic semiconductors (OSCs) to have complementarily matched electrical characteristics, which significantly improve the performance of organic logic circuits. In this study, an efficient strategy to optimize the performance of flexible organic electronics, such as OFETs and complementary inverters, is proposed using a combination of polymer insulators tailored to each OSC type. Photopatternable soluble copolyimides (SPIs), which exhibit excellent insulating properties and chemical resistance, are synthesized and applied as gate dielectric layers in the OFETs. The material and electrical properties are systematically investigated by varying the molecular ratio of SPIs to determine the optimal conditions for each OFET type. As a result, complementary inverters report 1.67 times higher integration density compared to the conventional ones while maintaining gain, switching threshold, and static noise margin of 23.7 V/V, 22.1 V, and 12.1 V, respectively, at a supply voltage of 40 V. The flexible complementary inverters are successfully demonstrated by fully exploiting the advantages of SPIs.
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http://dx.doi.org/10.1021/acsami.1c06293DOI Listing
July 2021

Solution-processed near-infrared Cu(In,Ga)(S,Se) photodetectors with enhanced chalcopyrite crystallization and bandgap grading structure via potassium incorporation.

Sci Rep 2021 Apr 9;11(1):7820. Epub 2021 Apr 9.

Center for Opto-Electronic Materials and Devices, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea.

Although solution-processed Cu(In,Ga)(S,Se) (CIGS) absorber layers can potentially enable the low-cost and large-area production of highly stable electronic devices, they have rarely been applied in photodetector applications. In this work, we present a near-infrared photodetector functioning at 980 nm based on solution-processed CIGS with a potassium-induced bandgap grading structure and chalcopyrite grain growth. The incorporation of potassium in the CIGS film promotes Se uptake in the bulk of the film during the chalcogenization process, resulting in a bandgap grading structure with a wide space charge region that allows improved light absorption in the near-infrared region and charge carrier separation. Also, increasing the Se penetration in the potassium-incorporated CIGS film leads to the enhancement of chalcopyrite crystalline grain growth, increasing charge carrier mobility. Under the reverse bias condition, associated with hole tunneling from the ZnO interlayer, the increasing carrier mobility of potassium-incorporated CIGS photodetector improved photosensitivity and particularly external quantum efficiency more than 100% at low light intensity. The responsivity and detectivity of the potassium-incorporated CIGS photodetector reach 1.87 A W and 6.45 [Formula: see text] 10 Jones, respectively, and the - 3 dB bandwidth of the device extends to 10.5 kHz under 980 nm near-infrared light.
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http://dx.doi.org/10.1038/s41598-021-87359-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8035197PMC
April 2021

Molecular Orientation Control of Liquid Crystal Organic Semiconductor for High-Performance Organic Field-Effect Transistors.

ACS Appl Mater Interfaces 2021 Mar 25;13(9):11125-11133. Epub 2021 Feb 25.

Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics Co., Ltd., Samsung-ro 130, Yongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea.

The control of molecular orientation and ordering of liquid crystal (LC) organic semiconductor (OSC) for high-performance and thermally stable organic thin-film transistors is investigated. A liquid crystalline molecule, 2-(4-dodecyl thiophenyl)[1]dibenzothiopheno[6,5-:6',5'-]-thieno[3,2-]thiophene (C12-Th-DBTTT) is synthesized, showing the highly ordered smectic X (SmX) phase, demonstrating molecular reorganization via thermal annealing. The resulting thermally evaporated polycrystalline film and solution-sheared thin film show high charge carrier mobilities of 9.08 and 27.34 cm V s, respectively. Atomic force microscopy and grazing-incidence X-ray diffraction analyses prove that the random SmA-like structure (smectic monolayer) is reorganized to the highly ordered SmA-like structure (smectic bilayer) of C12-Ph-DBTTT at the crystal-SmX transition temperature region. Because of the strong intermolecular interactions between rigid DBTTT cores, the thin film devices of C12-Th-DBTTT show excellent thermal stability up to 300 °C, indicating that LC characterization of conventional OSC materials can obtain high electrical performance as well as superior thermal durability.
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http://dx.doi.org/10.1021/acsami.0c22393DOI Listing
March 2021

Tailored growth of graphene oxide liquid crystals with controlled polymer crystallization in GO-polymer composites.

Nanoscale 2021 Feb;13(4):2720-2727

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Republic of Korea. and School of Chemical and Biological Engineering, Seoul National University, Seoul, 08826, Republic of Korea.

Graphene Oxides (GOs) have been frequently employed as fillers in polymer-based applications. While GO is known to nucleate polymer crystallization in GO-polymer composites reinforcing the mechanical properties of semicrystalline polymers, its counter effect on how polymer crystallization can alter the microstructure of GO has rarely been systematically studied yet. In this work, we study the GO nematic liquid crystal (LC) phase during polymer crystallization focusing on their hierarchical structures by employing in situ small/wide-angle X-ray scattering/diffraction (SAXS/WAXD) techniques. We found that GO LC and polymer crystals co-exist in the GO/polymer complex, where the overall liquid crystallinity is influenced by polymer crystallization. While polymer crystallizes in bulk or at the interface depending on the cooling rate, the interfacial crystallization of poly(ethylene glycol) (PEG) on GO improves both GO alignment and orientation of PEG crystal. This work provides an opportunity to develop a hierarchical structure of GO-based crystalline polymer nanocomposites, whose directionality can be controlled by polymer crystallization under proper cooling rates.
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http://dx.doi.org/10.1039/d0nr07858aDOI Listing
February 2021

Directing Polymorphism in the Helical Nanofilament Phase.

Chemistry 2021 Apr 10;27(24):7108-7113. Epub 2021 Mar 10.

Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea.

Herein, it is reported that the polymorphism in the helical nanofilament (HNF, B ) liquid-crystalline phase depends on the fabrication methods, that is, UV-driven formation and template-assisted self-assembly in the nanoconfined geometry. As a result, uniaxially oriented HNFs with different helical structures were obtained, in which generation of the twisted-ribbon and cylindrical-ribbon polymorphs showed that even the molecular lattice has a different orientation. The detailed structures were directly observed by SEM and grazing-incidence X-ray diffraction with synchrotron radiation. The resultant polymorphs could be used in chiro-optical applications due to the capability for fine control of the helical structures.
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http://dx.doi.org/10.1002/chem.202005221DOI Listing
April 2021

Defect-Induced Atomic Doping in Transition Metal Dichalcogenides via Liquid-Phase Synthesis toward Efficient Electrochemical Activity.

ACS Nano 2020 Dec 7. Epub 2020 Dec 7.

Department of Materials Science and Engineering, Perovtronics Research Center, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea.

Transition metal dichalcogenides (TMDs), due to their fascinating properties, have emerged as potential next-generation semiconducting nanomaterials across diverse fields of applications. When combined with other material systems, precise control of the intrinsic properties of the TMDs plays a vital role in maximizing their performance. Defect-induced atomic doping through introduction of a chalcogen vacancy into the TMDs lattices is known to be a promising strategy for modulating their characteristic properties. As a result, there is a need to develop tunable and scalable synthesis routes to achieve vacancy-modulated TMDs. Herein, we propose a facile liquid-phase ligand exchange approach for scalable, uniform, and vacancy-tunable synthesis of TMDs films. Varying the relative molar ratio of the chalcogen to transition metal precursors enabled the modulation of the chalcogen vacancy concentrations without necessitating additional post-treatments. When employed as the electrocatalyst in the hydrogen evolution reaction (HER), the vacancy-modulated TMDs, exhibiting a synergetic effect on the energy level matching to the reduction potential of water and optimized free energy differences in the HER pathways, showed a significant enhancement in the hydrogen production the improved charge transfer kinetics and increased active sites. The proposed approach for synthesizing tunable vacancy-modulated TMDs with wafer-scale synthesis capability is, therefore, promising for better practical applications of TMDs.
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http://dx.doi.org/10.1021/acsnano.0c06783DOI Listing
December 2020

Morphological-Electrical Property Relation in Cu(In,Ga)(S,Se) Solar Cells: Significance of Crystal Grain Growth and Band Grading by Potassium Treatment.

Small 2020 Dec 4;16(48):e2003865. Epub 2020 Nov 4.

Clean Energy Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea.

Solution-processed Cu(In,Ga)(S,Se)  (CIGS) has a great potential for the production of large-area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum-processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron-hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution-processed CIGS solar cells.
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http://dx.doi.org/10.1002/smll.202003865DOI Listing
December 2020

Orientation Control of Semiconducting Polymers Using Microchannel Molds.

ACS Nano 2020 Oct 16;14(10):12951-12961. Epub 2020 Oct 16.

Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea.

The molecular orientation of organic semiconductors (OSCs) is of fundamental importance to anisotropic electrical behavior as well as superior properties in practical applications. Here, a simple and effective method is demonstrated to fabricate highly oriented semiconducting polymers, poly(3-hexylthiophene) (P3HT) and poly{[,'-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]--5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)), by mass transfer effect under microchannel molds by diffusion and convection. Furthermore, parallel or perpendicular molecular arrangements relative to the channel direction were achieved by varying the widths of the microchannels, which are directly observed using polarized optical microscopy and two-dimensional grazing-incidence X-ray diffraction experiments. The method could enable the fabrication of organic field-effect transistors that exhibit anisotropic electrical properties indicating inter- or intrachain charge transport. The resulting platform will provide a simple approach for multidirectional orientations of anisotropic OSCs.
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http://dx.doi.org/10.1021/acsnano.0c04138DOI Listing
October 2020

Ordered Mesoporous Carbons with Graphitic Tubular Frameworks by Dual Templating for Efficient Electrocatalysis and Energy Storage.

Angew Chem Int Ed Engl 2021 Jan 12;60(3):1441-1449. Epub 2020 Nov 12.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Ordered mesoporous carbons (OMCs) have attracted considerable interest owing to their broad utility. OMCs reported to date comprise amorphous rod-like or tubular or graphitic rod-like frameworks, which exhibit tradeoffs between conductivity and surface area. Here we report ordered mesoporous carbons constructed with graphitic tubular frameworks (OMGCs) with tunable pore sizes and mesostructures via dual templating, using mesoporous silica and molybdenum carbide as exo- and endo-templates, respectively. OMGCs simultaneously realize high electrical conductivity and large surface area and pore volume. Benefitting from these features, Ru nanoparticles (NPs) supported on OMGC exhibit superior catalytic activity for alkaline hydrogen evolution reaction and single-cell performance for anion exchange membrane water electrolysis compared to Ru NPs on other OMCs and commercial catalysts. Further, the OMGC-based full-carbon symmetric cell demonstrates excellent performances for Li-ion capacitors.
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http://dx.doi.org/10.1002/anie.202012936DOI Listing
January 2021

Lyotropic Chromonic Liquid Crystals and Their Impurities Reveal the Importance of the Position of Functional Groups in Self-Assembly.

J Phys Chem B 2020 10 5;124(41):9246-9254. Epub 2020 Oct 5.

Department of Physics, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea.

We study the effect of purification and impurities on the self-assembly and phase behavior of lyotropic chromonic liquid crystals (LCLCs). LCLC molecules in water stack to form aggregates; then, the elongated nanoaggregates align to make liquid crystalline phases. Utilizing multiple experimental techniques, we unveil impurities in commercial Sunset Yellow FCF (SSY), a representative LCLC, and how the precipitation-based purification promotes the formation of the aggregates and mesophase. We further explore the roles of intrinsic impurities, i.e., byproducts of the SSY synthesis, whose molecular structures are almost identical to that of SSY but differ only in the number and position of sulfonate groups. Combining quantum chemical calculations of molecular structures and experimental investigation of aggregate structures and phase behavior, we propose that the impurities of the planar shapes behave as planar SSY, i.e., participating in aggregate formation, whereas the nonplanar one disrupts the nematic phase. These results highlight the critical roles of the impurities and deepen our understanding of self-assembled aggregates and their aligned mesophases.
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http://dx.doi.org/10.1021/acs.jpcb.0c07163DOI Listing
October 2020

Revealing Isolated M-N C Active Sites for Efficient Collaborative Oxygen Reduction Catalysis.

Angew Chem Int Ed Engl 2020 Dec 23;59(52):23678-23683. Epub 2020 Oct 23.

School of Energy and Chemical Engineering/Center for Dimension Controllable Organic Frameworks, Ulsan National Institute of Science and Technology, South Korea.

Single atom catalysts (SACs) are of great importance for oxygen reduction, a critical process in renewable energy technologies. The catalytic performance of SACs largely depends on the structure of their active sites, but explorations of highly active structures for SAC active sites are still limited. Herein, we demonstrate a combined experimental and theoretical study of oxygen reduction catalysis on SACs, which incorporate M-N C site structure, composed of atomically dispersed transition metals (e.g., Fe, Co, and Cu) in nitrogenated carbon nanosheets. The resulting SACs with M-N C sites exhibited prominent oxygen reduction catalytic activities in both acidic and alkaline media, following the trend Fe-N C > Co-N C > Cu-N C . Theoretical calculations suggest the C atoms in these structures behave as collaborative adsorption sites to M atoms, thanks to interactions between the d/p orbitals of the M/C atoms in the M-N C sites, enabling dual site oxygen reduction.
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http://dx.doi.org/10.1002/anie.202008325DOI Listing
December 2020

Gradient tantalum-doped hematite homojunction photoanode improves both photocurrents and turn-on voltage for solar water splitting.

Nat Commun 2020 Sep 15;11(1):4622. Epub 2020 Sep 15.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

Hematite has a great potential as a photoanode for photoelectrochemical (PEC) water splitting by converting solar energy into hydrogen fuels, but the solar-to-hydrogen conversion efficiency of state-of-the-art hematite photoelectrodes are still far below the values required for practical hydrogen production. Here, we report a core-shell formation of gradient tantalum-doped hematite homojunction nanorods by combination of hydrothermal regrowth strategy and hybrid microwave annealing, which enhances the photocurrent density and reduces the turn-on voltage simultaneously. The unusual bi-functional effects originate from the passivation of the surface states and intrinsic built-in electric field by the homojunction formation. The additional driving force provided by the field can effectively suppress charge-carrier recombination both in the bulk and on the surface of hematite, especially at lower potentials. Moreover, the synthesized homojunction shows a remarkable synergy with NiFe(OH) cocatalyst with significant additional improvements of photocurrent density and cathodic shift of turn-on voltage. The work has nicely demonstrated multiple collaborative strategies of gradient doping, homojunction formation, and cocatalyst modification, and the concept could shed light on designing and constructing the efficient nanostructures of semiconductor photoelectrodes in the field of solar energy conversion.
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http://dx.doi.org/10.1038/s41467-020-18484-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7493915PMC
September 2020

Ultralow-dielectric-constant amorphous boron nitride.

Nature 2020 06 24;582(7813):511-514. Epub 2020 Jun 24.

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, South Korea.

Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics.
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http://dx.doi.org/10.1038/s41586-020-2375-9DOI Listing
June 2020

Unveiling the Relationship between the Perovskite Precursor Solution and the Resulting Device Performance.

J Am Chem Soc 2020 Apr 18;142(13):6251-6260. Epub 2020 Mar 18.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea.

For the fabrication of perovskite solar cells (PSCs) using a solution process, it is essential to understand the characteristics of the perovskite precursor solution to achieve high performance and reproducibility. The colloids (iodoplumbates) in the perovskite precursors under various conditions were investigated by UV-visible absorption, dynamic light scattering, photoluminescence, and total internal reflection fluorescence microscopy techniques. Their local structure was examined by in situ X-ray absorption fine structure studies. Perovskite thin films on a substrate with precursor solutions were characterized by transmission electron microscopy, X-ray diffraction analysis, space-charge-limited current, and Kelvin probe force microscopy. The colloidal properties of the perovskite precursor solutions were found to be directly correlated with the defect concentration and crystallinity of the perovskite film. This work provides guidelines for controlling perovskite films by varying the precursor solution, making it possible to use colloid-engineered lead halide perovskite layers to fabricate efficient PSCs.
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http://dx.doi.org/10.1021/jacs.0c00411DOI Listing
April 2020

A General Strategy to Atomically Dispersed Precious Metal Catalysts for Unravelling Their Catalytic Trends for Oxygen Reduction Reaction.

ACS Nano 2020 Feb 6;14(2):1990-2001. Epub 2020 Feb 6.

Department of Chemistry , Korea Advanced Institute of Science and Technology (KAIST) , 291 Daehak-Ro , Daejeon 34141 , Republic of Korea.

Atomically dispersed precious metal catalysts have emerged as a frontier in catalysis. However, a robust, generic synthetic strategy toward atomically dispersed catalysts is still lacking, which has limited systematic studies revealing their general catalytic trends distinct from those of conventional nanoparticle (NP)-based catalysts. Herein, we report a general synthetic strategy toward atomically dispersed precious metal catalysts, which consists of "trapping" precious metal precursors on a heteroatom-doped carbonaceous layer coated on a carbon support and "immobilizing" them with a SiO layer during thermal activation. Through the "trapping-and-immobilizing" method, five atomically dispersed precious metal catalysts (Os, Ru, Rh, Ir, and Pt) could be obtained and served as model catalysts for unravelling catalytic trends for the oxygen reduction reaction (ORR). Owing to their isolated geometry, the atomically dispersed precious metal catalysts generally showed higher selectivity for HO production than their NP counterparts for the ORR. Among the atomically dispersed catalysts, the HO selectivity was changed by the types of metals, with atomically dispersed Pt catalyst showing the highest selectivity. A combination of experimental results and density functional theory calculations revealed that the selectivity trend of atomically dispersed catalysts could be correlated to the binding energy difference between *OOH and *O species. In terms of 2 e ORR activity, the atomically dispersed Rh catalyst showed the best activity. Our general approach to atomically dispersed precious metal catalysts may help in understanding their unique catalytic behaviors for the ORR.
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http://dx.doi.org/10.1021/acsnano.9b08494DOI Listing
February 2020

Synthesis, Molecular Packing, and Electrical Properties of New Regioisomeric n-type Semiconducting Molecules with Modification of Alkyl Substituents Position.

ACS Appl Mater Interfaces 2019 Dec 3;11(50):47170-47181. Epub 2019 Dec 3.

Department of Chemistry , Korea University , Seoul 136-713 , Republic of Korea.

We design and synthesize a series of regioisomeric n-type small molecules, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by two-dimensional NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed organic field-effect transistors (OFETs). Interestingly, a minor change in the substitution position of the octyl side chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 °C, the kinked DPP(EH)-INCNO1 (EH = 2-ethylhexyl) structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ∼0.6 cmV s compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 (HD = 2-hexyldecyl) molecules become more crystalline with thermal treatments, but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.
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http://dx.doi.org/10.1021/acsami.9b17664DOI Listing
December 2019

Superb water splitting activity of the electrocatalyst FeCo(PO) designed with computation aid.

Nat Commun 2019 11 15;10(1):5195. Epub 2019 Nov 15.

Center for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 689-798, Korea.

For efficient water splitting, it is essential to develop inexpensive and super-efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, we report a phosphate-based electrocatalyst [FeCo(PO)@reduced-graphene-oxide(rGO)] showing outstanding OER performance (much higher than state-of-the-art Ir/C catalysts), the design of which was aided by first-principles calculations. This electrocatalyst displays low overpotential (237 mV at high current density 100 mA cm in 1 M KOH), high turnover frequency (TOF: 0.54 s), high Faradaic efficiency (98%), and long-term durability. Its remarkable performance is ascribed to the optimal free energy for OER at Fe sites and efficient mass/charge transfer. When a FeCo(PO)@rGO anodic electrode is integrated with a Pt/C cathodic electrode, the electrolyzer requires only 1.45 V to achieve 10 mA cm for whole water splitting in 1 M KOH (1.39 V in 6 M KOH), which is much smaller than commercial Ir-C//Pt-C electrocatalysts. This cost-effective powerful oxygen production material with carbon-supporting substrates offers great promise for water splitting.
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http://dx.doi.org/10.1038/s41467-019-13050-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6858335PMC
November 2019

Unassisted solar lignin valorisation using a compartmented photo-electro-biochemical cell.

Nat Commun 2019 11 12;10(1):5123. Epub 2019 Nov 12.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, Republic of Korea.

Lignin is a major component of lignocellulosic biomass. Although it is highly recalcitrant to break down, it is a very abundant natural source of valuable aromatic carbons. Thus, the effective valorisation of lignin is crucial for realising a sustainable biorefinery chain. Here, we report a compartmented photo-electro-biochemical system for unassisted, selective, and stable lignin valorisation, in which a TiO photocatalyst, an atomically dispersed Co-based electrocatalyst, and a biocatalyst (lignin peroxidase isozyme H8, horseradish peroxidase) are integrated, such that each system is separated using Nafion and cellulose membranes. This cell design enables lignin valorisation upon irradiation with sunlight without the need for any additional bias or sacrificial agent and allows the protection of the biocatalyst from enzyme-damaging elements, such as reactive radicals, gas bubbles, and light. The photo-electro-biochemical system is able to catalyse lignin depolymerisation with a 98.7% selectivity and polymerisation with a 73.3% yield using coniferyl alcohol, a lignin monomer.
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http://dx.doi.org/10.1038/s41467-019-13022-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6851146PMC
November 2019

Initial Solvent-Driven Nonequilibrium Effect on Structure, Properties, and Dynamics of Polymer Nanocomposites.

Phys Rev Lett 2019 Oct;123(16):167801

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil Ulsan 44919, Republic of Korea.

Unusual structures and dynamic properties found in polymer nanocomposites (PNCs) are often attributed to immobilized (adsorbed) polymers at nanoparticle-polymer interfaces, which are responsible for reducing the intrinsic incompatibility between nanoparticles and polymers in PNCs. Although tremendous effort has been made to characterize the presence of immobilized polymers, a systematic understanding of the structure and dynamics under different processing conditions is still lacking. Here, we report that the initial dispersing solvent, which is not present after producing PNCs, drives these nonequilibrium effects on polymer chain dynamics at interfaces. Employing extensive small-angle scattering, proton NMR spectroscopy, and rheometry experiments, we found that the thickness of the immobilized layer can be dependent on the initial solvent, changing the structure and the properties of the PNC significantly. In addition, we show that the outcome of the initial solvent effect becomes more effective at particle volume fractions where the immobile layers begin to interact.
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http://dx.doi.org/10.1103/PhysRevLett.123.167801DOI Listing
October 2019

Photothermal Polymer Nanocomposites of Tungsten Bronze Nanorods with Enhanced Tensile Elongation at Low Filler Contents.

Polymers (Basel) 2019 Oct 24;11(11). Epub 2019 Oct 24.

School of Chemical Engineering and Materials Science, Institute of Energy Converting Soft Materials, Chung-Ang University, Seoul 06974, Korea.

We present polymer nanocomposites of tungsten bronze nanorods (TBNRs) and ethylene propylene diene monomers (EPDM). The combination of these components allows the simultaneous enhancement in the mechanical and photothermal properties of the composites at low filler contents. The as-synthesized TBNRs had lengths and diameters of 14.0 ± 2.4 nm and 2.5 ± 0.5 nm, respectively, and were capped with oleylamine, which has a chemical structure similar to EPDM, making the TBNRs compatible with the bulk EPDM matrix. The TBNRs absorb a wide range of near-infrared light because of the sub-band transitions induced by alkali metal doping. Thus, the nanocomposites of TBNRs in EPDM showed enhanced photothermal properties owing to the light absorption and subsequent heat emission by the TBNRs. Noticeably, the nanocomposite with only 3 wt% TBNRs presented significantly enhanced tensile strain at break, in comparison with those of pristine EPDM, nanocomposites with 1 and 2 wt % TBNRs, and those with tungsten bronze nanoparticles, because of the alignment of the nanorods during tensile elongation. The photothermal and mechanical properties of these nanocomposites make them promising materials for various applications such as in fibers, foams, clothes with cold weather resistance, patches or mask-like films for efficient transdermal delivery upon heat generation, and photoresponsive surfaces for droplet transport by the thermocapillary effect in microfluidic devices and microengines.
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http://dx.doi.org/10.3390/polym11111740DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6918126PMC
October 2019

Stretchable batteries with gradient multilayer conductors.

Sci Adv 2019 Jul 26;5(7):eaaw1879. Epub 2019 Jul 26.

Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea.

Stretchable conductors are essential components in next-generation deformable and wearable electronic devices. The ability of stretchable conductors to achieve sufficient electrical conductivity, however, remains limited under high strain, which is particularly detrimental for charge storage devices. In this study, we present stretchable conductors made from multiple layers of gradient assembled polyurethane (GAP) comprising gold nanoparticles capable of self-assembly under strain. Stratified layering affords control over the composite internal architecture at multiple scales, leading to metallic conductivity in both the lateral and transversal directions under strains of as high as 300%. The unique combination of the electrical and mechanical properties of GAP electrodes enables the development of a stretchable lithium-ion battery with a charge-discharge rate capability of 100 mAh g at a current density of 0.5 A g and remarkable cycle retention of 96% after 1000 cycles. The hierarchical GAP nanocomposites afford rapid fabrication of advanced charge storage devices.
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http://dx.doi.org/10.1126/sciadv.aaw1879DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6660205PMC
July 2019

Long-Term Chemical Aging of Hybrid Halide Perovskites.

Nano Lett 2019 Aug 22;19(8):5604-5611. Epub 2019 Jul 22.

Perovtronics Research Center, Department of Energy Engineering, School of Energy and Chemical Engineering , Ulsan National Institute of Science and Technology , 50 UNIST-gil, Eonyang-eup, Ulju-gun , Ulsan 44919 , Republic of Korea.

Because the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a number of ionized iodide (triiodide: I) ions during perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the long-term chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging of PSCs in the dark based on formamidinium lead triiodide by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in the PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference using grazing incidence wide-angle X-ray scattering, deep-level transient spectroscopy, scanning photoelectron microscopy, and high-resolution photoemission spectroscopy. The addition of I was found to reduce the amounts of hydroxide and O in the halide perovskites (HPs), affecting the migration of defects and the structural transformation of the HPs.
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http://dx.doi.org/10.1021/acs.nanolett.9b02142DOI Listing
August 2019

Shear-solvo defect annihilation of diblock copolymer thin films over a large area.

Sci Adv 2019 Jun 14;5(6):eaaw3974. Epub 2019 Jun 14.

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea.

Achieving defect-free block copolymer (BCP) nanopatterns with a long-ranged orientation over a large area remains a persistent challenge, impeding the successful and widespread application of BCP self-assembly. Here, we demonstrate a new experimental strategy for defect annihilation while conserving structural order and enhancing uniformity of nanopatterns. Sequential shear alignment and solvent vapor annealing generate perfectly aligned nanopatterns with a low defect density over centimeter-scale areas, outperforming previous single or sequential combinations of annealing. The enhanced order quality and pattern uniformity were characterized in unprecedented detail via scattering analysis and incorporating new mathematical indices using elaborate image processing algorithms. In addition, using an advanced sampling method combined with a coarse-grained molecular simulation, we found that domain swelling is the driving force for enhanced defect annihilation. The superior quality of large-scale nanopatterns was further confirmed with diffraction and optical properties after metallized patterns, suggesting strong potential for application in optoelectrical devices.
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http://dx.doi.org/10.1126/sciadv.aaw3974DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6570509PMC
June 2019

Ultrastiff, Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order.

Adv Mater 2019 Jul 3;31(29):e1903039. Epub 2019 Jun 3.

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan, 44919, Republic of Korea.

A macroscopic film (2.5 cm × 2.5 cm) made by layer-by-layer assembly of 100 single-layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat-treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near-perfect in-plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in-plane thermal conductivity is exceptionally high, 2292 ± 159 W m K while in-plane electrical conductivity is 2.2 × 10 S m . The high performance of these films is attributed to the combination of the high in-plane crystalline order and unique stacking configuration through the thickness.
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http://dx.doi.org/10.1002/adma.201903039DOI Listing
July 2019

Machine-Washable Smart Textiles with Photothermal and Antibacterial Activities from Nanocomposite Fibers of Conjugated Polymer Nanoparticles and Polyacrylonitrile.

Polymers (Basel) 2018 Dec 22;11(1). Epub 2018 Dec 22.

School of Chemical Engineering and Materials Science, Institute of Energy Converting Soft Materials, Chung-Ang University, Seoul 06974, Korea.

Smart textiles based on conjugated polymers have been highlighted as promising fabrics that can intelligently respond to environmental stimuli based on the electrical properties of polymer semiconductors. However, there has been limited interest in the photothermal properties of conjugated polymers that can be applied to smart textiles. We prepared nanoparticles by assembling a conjugated polymer with a fatty acid via an emulsion process and nanocomposite fibers by distributing the conjugated polymer nanoparticles in a polyacrylonitrile matrix. We then fabricated the textiles using the fibers. The resulting fabrics based on nanocomposite fibers show a temperature increase to 50 °C in 10 min under white light irradiation because of efficient photothermal conversion by the conjugated polymer light harvester, while the temperature of a pristine polyacrylonitrile fabric increases to only 35 °C. In addition, excellent antimicrobial activity was confirmed by a 99.9% decrease in the populations of and over 24 h because of the effect of the fatty acid in the nanocomposite films and fabrics. Furthermore, the fabric showed efficient durability after a laundry test, suggesting the usefulness of these smart textiles based on conjugated polymer nanoparticles for practical applications.
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http://dx.doi.org/10.3390/polym11010016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6402031PMC
December 2018

Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene).

Nature 2019 03 27;567(7749):511-515. Epub 2019 Mar 27.

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), Daejeon, South Korea.

Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells: poly(triarylamine) (PTAA) and 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). However, these materials have several drawbacks in terms of commercialization, including high cost, the need for hygroscopic dopants that trigger degradation of the perovskite layer and limitations in their deposition processes. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties, low cost and ease of fabrication, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)-which are fabricated using a scalable bar-coating method for the deposition of P3HT-and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research.
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http://dx.doi.org/10.1038/s41586-019-1036-3DOI Listing
March 2019

Manipulation of Supramolecular Columnar Structures of H-Bonded Donor-Acceptor Units through Geometrical Nanoconfinement.

Chemphyschem 2019 03 27;20(6):890-897. Epub 2019 Feb 27.

Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea.

Ambipolar organic semiconductors are considered promising for organic electronics because of their interesting electric properties. Many hurdles remain yet to be overcome before they can be used for practical applications, especially because their orientation is hard to control. We demonstrate a method to control the orientation of columnar structures based on a hydrogen (H)-bonded donor-acceptor complex between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acid, using physicochemical nanoconfinement. The molecular configuration and supramolecular columnar assemblies in a one-dimensional porous anodic aluminium oxide (AAO) film were dramatically modulated by controlling the pore-size and by chemical modification of the inner surface of the porous AAO film. In situ experiments using grazing-incidence X-ray diffraction (GIXRD) were carried out to investigate the structural evolution produced at the nanometer scale by varying physicochemical conditions. The resulting highly ordered nanostructures may open a new pathway to effectively control the alignment of liquid crystal ambipolar semiconductors.
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http://dx.doi.org/10.1002/cphc.201801042DOI Listing
March 2019

Ferroelectrically Switching Helical Columnar Assembly Comprising Cisoid Conformers of a 1,2,3-Triazole-based Liquid Crystal.

Angew Chem Int Ed Engl 2019 Feb 25;58(9):2749-2753. Epub 2019 Jan 25.

Department of Chemistry, Dankook University, 119, Dandae-ro, Chungnam, 448-701, Korea.

The 1,2,3-triazole molecule, which is a product of click chemistry, possesses a high dipole moment and can be a useful polar motif for ferroelectric columnar liquid crystal (LC) materials-though it has not been used to date. Herein, we report the helical assembly and ferroelectric switching properties of a columnar liquid crystal comprising a naphthalene core and 1,2,3-triazolyl linkages. The molecule assembles into a double-stranded helical columnar LC structure (Col ). The X-ray simulations of cisoid and transoid columnar models suggest that the helical assembly comprises cisoid conformers with a non-zero dipole moment. The helical columns in the Col phase are aligned homeotropically under an electric field. The ferroelectric switching of the axial polarization can be observed in the temperature range of 105-115 °C in the Col phase, wherein the triazolyl hydrogen bonding along the column axis is weakened. The ferroelectric switching event is attributed to the rotation of the polar triazolyl units in response to the electric field.
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http://dx.doi.org/10.1002/anie.201813195DOI Listing
February 2019
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