Sungeun Jeoung

Exploration of Gate-Opening and Breathing Phenomena in a Tailored Flexible Metal–Organic Framework

Flexible metal-organic frameworks (MOFs) show the structural transition phenomena, gate opening and breathing, upon the input of external stimuli. These phenomena have significant implications in their adsorptive applications. In this work, we demonstrate the direct capture of these gate-opening and breathing phenomena, triggered by CO2 molecules, in a well-designed flexible MOF composed of rotational sites and molecular gates. Combining X-ray single crystallographic data of a flexible MOF during gate opening/closing and breathing with in situ X-ray powder diffraction results uncovered the origin of this flexibility. Furthermore, computational studies revealed the specific sites required to open these gates by interaction with CO2 molecules.

Transformation of Metal–Organic Frameworks/Coordination Polymers into Functional Nanostructured Materials: Experimental Approaches Based on Mechanistic Insights

Nanostructured materials such as porous metal oxides, metal nanoparticles, porous carbons, and their composites have been intensively studied due to their applications, including energy conversion and storage devices, catalysis, and gas storage. Appropriate precursors and synthetic methods are chosen for synthesizing the target materials. About a decade ago, metal-organic frameworks (MOFs) and coordination polymers (CPs) emerged as new precursors for these nanomaterials because they contain both organic and inorganic species that can play parallel roles as both a template and a precursor under given circumstances. Thermal conversions of MOFs offer a promising toolbox for synthesizing functional nanomaterials that are difficult to obtain using conventional methods. Although understanding the conversion mechanism is important for designing MOF precursors for the synthesis of nanomaterials with desired physicochemical properties, comprehensive discussions revealing the transformation mechanism remain insufficient. This Account reviews the utilization of MOFs/CPs as precursors and their transformation into functional nanomaterials with a special emphasis on understanding the relationship between the intrinsic nature of the parent MOFs and the daughter nanomaterials while discussing various experimental approaches based on mechanistic insights. We discuss nanomaterials categorized by materials such as metal-based nanomaterials and porous carbons. For metal-based nanomaterials transformed from MOFs, the nature of metal ions in the MOF scaffolds affects the physicochemical properties of the resultant materials including the phase, composite, and morphology of nanomaterials. Organic ligands are also involved in the in situ chemical reactions with metal species during thermal conversion. We describe these conversion mechanisms by classifying the phase of metal components in the resultant materials. Along with the metal species, carbon is a major element in MOFs, and thus, the appropriate choice of precursor MOFs and heat treatment can be expected to yield carbon-based nanomaterials. We address the relationship between the nature of the parent MOF and the porosity of the daughter carbon material-a controversial issue in the synthesis of porous carbons. Based on an understanding of the mechanism of MOF conversion, morphologically or compositionally advanced materials are synthesized by adopting appropriate MOF precursors and thermolysis conditions. Despite the progressive understanding of conversion phenomena of MOFs/CPs, this research field still has rooms to be explored and developed, ultimately in order to precisely control the properties of resultant nanomaterials. In this sense, we should pay more attention to the mechanism investigations of MOF conversion. We believe this Account will facilitate a deeper understanding of MOF/CP conversion routes and will accelerate further development in this field.

Direct conversion of coordination compounds into Ni2P nanoparticles entrapped in 3D mesoporous graphene for an efficient hydrogen evolution reaction

This paper reports a simple preparation route to a composite of small Ni2P nanoparticles (NPs) entrapped in 3D mesoporous graphene by the thermal conversion of a coordination compound followed by phosphidation. Recently, transition metal phosphides (TMPs) have gained increasing attention owing to their promising potential as non-precious metal catalysts in the hydrogen evolution reaction (HER). In order to enhance the catalytic activity of TMPs, researchers have sought to synthesize small TMP NPs to increase the catalytically active surface area. Although surfactant-mediated syntheses can produce small TMP NPs, a cumbersome surfactant removal step is necessary to generate catalytically active clean surfaces. Interfacing TMP NPs with carbon nanomaterials is another promising approach to boost the catalytic performance by providing high electrical conductivity and durability. However, the synthesis of composites of TMP NPs and carbon demands multiple synthetic steps, including the preparation of TMP NPs, synthesis of carbon nanomaterials, and dispersion of TMP NPs onto the carbon support. The essence of our approach toward the 3D graphene encapsulating Ni2P NPs (Ni2P@mesoG) lies in the utilization of the conversion phenomenon of [Ni2(EDTA)] (EDTA = ethylenediaminetetraacetate). The thermolysis of [Ni2(EDTA)] at 600 °C produces a composite of single-crystalline 5 nm-sized Ni NPs individually entrapped in 3D mesoG (Ni@mesoG), and the following phosphidation completely converts the Ni NPs to single-crystalline Ni2P NPs in mesoG (Ni2P@mesoG) without agglomeration. This solvent-free thermal conversion route to the Ni2P@mesoG composite is simple and scalable. Notably, graphitic shell layers in Ni2P@mesoG stabilize small Ni2P NPs possessing a large active surface area, and facilitate the electron transfer due to the intimate contact between them. Consequently, the use of Ni2P@mosoG exhibits superior electrocatalytic HER activity and durability in both strong acidic and basic media.

Upcycling of nonporous coordination polymers: Controllable-conversion toward porosity-tuned N-doped carbons and their electrocatalytic activity in seawater batteries

Herein, we report the preparation of highly porous N-doped carbons (PNCs) via thermolysis of nonporous Zn-based coordination polymers (CPs) constructed with nitrogen-containing ligands. So far, the thermal conversion of Zn-CPs, including metal–organic frameworks (MOFs), has mainly yielded microporous carbon materials, and to change the textural properties of end carbons, new CPs/MOFs with different properties were introduced. However, present studies show that just varying the conversion conditions of a parent CP results in porosity-tuned PNCs, in which especially mesoporosity is developed, and this is applicable for even nonporous CPs. This is conducted based on the understanding of conversion phenomena which is that during thermal conversion of Zn-based CPs, the in situ generated Zn metal species act as porogens and their agglomeration can be controlled by the reaction conditions. Different reaction temperatures, ramping rates and retention times allow control over the ratio of micro- to meso-pore volume, while a slower ramping rate and longer retention time at lower heating temperature induced the agglomeration of the porogens, yielding greater mesoporosity, and holding the Zn-CPs at high temperature for a short period afforded the micropore-dominant PNCs due to rapid porogen elimination. The superiority of the mesopore-developed PNCs as electrocatalysts, attributed to greater mass-transport-accessible surfaces, was examined for the electrodes in a rechargeable seawater battery system as an example of a practical application. Therefore, our synthetic approach provides a facile method for the preparation of PNCs with suitable hierarchical pore distributions for use as energy-related materials without exerting significant effort in the design of coordination compounds.

Single-crystal-to-single-crystal transformation of a coordination polymer from 2D to 3D by [2 + 2] photodimerization assisted by a coexisting flexible ligand

A 2D interdigitated [Ni2(adipate)2(spy)4(H2O)2] (spy = 4-styrylpyridine) was transformed to a 3D coordination polymer through [2 + 2] photodimerization between the olefin bonds in spy upon UV-irradiation in a single-crystal-to-single-crystal (SC–SC) manner. During this transformation, a coexisting flexible adipate ligand changes its conformation, and helps in withstanding the conversion strain and retaining the single crystallinity.

Hierarchically porous adamantane-shaped carbon nanoframes

Hollow carbons have emerged as a new class of porous carbon materials, showing promise in a variety of areas. However, their morphology has been limited to spherical shapes, with the carbon shell possibly limiting access to their inner surfaces and utilization. Herein, we report a new type of hollow carbon material consisting of non-spherical, adamantane-shaped, hierarchically micro- and macro-porous, N-doped carbon nanoframes (mM-NCs) by exploiting selective etching and pseudomorphic thermal conversion of zeolitic imidazolate framework-8 (ZIF-8). The mM-NCs showed superior performance as adsorbents for large dye molecules and as catalysts for the oxygen reduction reaction relative to macropore-free N-doped carbons, which can be attributed to the presence of macropores, fully utilizable pore surfaces, and nitrogen species.