Kyung Joo Lee

Nanoporous Metal Oxides with Tunable and Nanocrystalline Frameworks via Conversion of Metal–Organic Frameworks

Nanoporous metal oxide materials are ubiquitous in the material sciences because of their numerous potential applications in various areas, including adsorption, catalysis, energy conversion and storage, optoelectronics, and drug delivery. While synthetic strategies for the preparation of siliceous nanoporous materials are well-established, nonsiliceous metal oxide-based nanoporous materials still present challenges. Herein, we report a novel synthetic strategy that exploits a metal-organic framework (MOF)-driven, self-templated route toward nanoporous metal oxides via thermolysis under inert atmosphere. In this approach, an aliphatic ligand-based MOF is thermally converted to nanoporous metal oxides with highly nanocrystalline frameworks, in which aliphatic ligands act as the self-templates that are afterward evaporated to generate nanopores. We demonstrate this concept with hierarchically nanoporous magnesia (MgO) and ceria (CeO2), which have potential applicability for adsorption, catalysis, and energy storage. The pore size of these nanoporous metal oxides can be readily tuned by simple control of experimental parameters. Significantly, nanoporous MgO exhibits exceptional CO2 adsorption capacity (9.2 wt %) under conditions mimicking flue gas. This MOF-driven strategy can be expanded to other nanoporous monometallic and multimetallic oxides with a multitude of potential applications.

Preparation of Co3O4 electrode materials with different microstructures via pseudomorphic conversion of Co-based metal–organic frameworks

To develop high-performance nanostructured metal oxide electrodes, it is important to understand their structural effects on electrochemical performances. Thus, the preparation of metal oxide materials that have well-tailored nanostructures is crucial for studies. However, while synthetic strategies to control the size of metal oxide nanoparticles are well-developed, the control of the higher level structures, namely microstructure, is not very well established. Herein, we present the synthesis of the two kinds of Co3O4 nanomaterials through pseudomorphic conversion so that the macroscopic morphologies of parent MOFs, such as plate-like and rod-like shape, are well-maintained. Both Co3O4 nanomaterials are composed of almost identical 10 nm-sized primary nanocrystals but with different nanoporous secondary structures and macroscopic morphologies such as plate and rod shapes. These Co3O4 nanomaterials were utilized as an electrode in lithium ion batteries (LIBs), and their electrochemical properties were comparatively investigated. It was revealed that the different cyclability and rate capability are attributed to their different microstructures. The pseudo-monolithic integration of primary and secondary structures at higher level was the governing factor, which determined the electrochemical performances of the Co3O4 electrode.

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.

In situ-generated metal oxide catalyst during CO oxidation reaction transformed from redox-active metal-organic framework-supported palladium nanoparticles

The preparation of redox-active metal-organic framework (ra-MOF)-supported Pd nanoparticles (NPs) via the redox couple-driven method is reported, which can yield unprotected metallic NPs at room temperature within 10 min without the use of reducing agents. The Pd@ra-MOF has been exploited as a precursor of an active catalyst for CO oxidation. Under the CO oxidation reaction condition, Pd@ra-MOF is transformed into a PdOx-NiOy/C nanocomposite to generate catalytically active species in situ, and the resultant nanocatalyst shows sustainable activity through synergistic stabilization.

Carboxylated Pillar[5]arene-Coated Gold Nanoparticles with Chemical Stability and Enzyme-like Activity

A facile synthesis of gold nanoparticles (AuNPs) covered with a multidentate macrocycle, carboxylated pillar[5]arene (CP), via a one-pot hydrothermal process is reported. The resulting AuNPs are highly stable against salts and pH variations, while their traditional counterparts are not stable at the same conditions. For the stabilization, multiple carboxylate groups of CP might contribute to electrostatic or steric stabilization. In addition, we found that CP-coated AuNPs exhibit greater peroxidase-like activity than citrate-stabilized AuNPs in the presence of silver cations. The system presented herein would provide a new scheme to fabricate unique sensory systems in combination with enzymes, which can bind to the pocket of CP.

Metal–organic frameworks constructed from flexible ditopic ligands: conformational diversity of an aliphatic ligand

The solvothermal reaction of adipic acid as a flexible ditopic ligand and the metal ions MnII, CoII, and TbIII afforded three novel metal–organic frameworks (MOFs), {[Mn2(adipate)2(DMA)]} (1), {[Co2(adipate)2(DMF)]·1DMF·1.5H2O} (2), and {[Tb3(adipate)4.5(DMF)2]} (3) (DMA = N,N-dimethylacetamide; DMF = N,N-dimethylformamide), respectively, which were structurally characterized by single-crystal X-ray diffraction. Depending on the kind of metal ion and solvent system, the conformations and coordination modes of the adipate ligands were diverse and governed the entire MOF structure. Compound 1 consists of the secondary building units (SBUs) of Mn–O chains that were linked by adipate ligands extending in two-dimensional sheets, which were infinitely stacked in a layer-by-layer manner. Compound 2 presented a three-dimensional MOF constructed from Co–O chains and bridging adipate ligands extending in four different directions. Compound 3 also had a three-dimensional structure which was formed by Tb–O chains connected with adipate ligands in six directions. From these structures, ten different adipate ligands with diverse conformations were found, and the potential energy of each conformation was calculated by the first-principles density function. In addition, the luminescence properties of the Tb-based MOF 3 were investigated in the solid state at room temperature.

Multi-core MgO NPs@C core–shell nanospheres for selective CO2 capture under mild conditions

The core–shell structures have attracted attention in catalysis, because the outer shells isolate the catalytically active NP cores and prevent the possibility of sintering of core particles during catalytic reaction under physically and chemically harsh conditions. We aimed to adopt this core–shell system for CO2 sorption materials. In this study, a composite material of multi-core 3 nm-sized magnesium oxide nanoparticles embedded in porous carbon nanospheres (MgO NPs@C) was synthesized by a gas phase reaction via a solvent-free process. It showed selective CO2 adsorption capacity over N2 under mild regeneration conditions.