Jae Hwa 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.

Luminescent Li-Based Metal–Organic Framework Tailored for the Selective Detection of Explosive Nitroaromatic Compounds: Direct Observation of Interaction Sites

A luminescent lithium metal-organic framework (MOF) is constructed from the solvothermal reaction of Li(+) and a well-designed organic ligand, bis(4-carboxyphenyl)-N-methylamine (H(2)CPMA). A Li-based MOF can detect an explosive aromatic compound containing nitro groups as an explosophore, by showing a dramatic color change with concurrent luminescence quenching in the solid state. The detection sites are proven directly through single-crystal-to-single-crystal transformations, which show strong interactions between the aromatic rings of the electron-rich CPMA(2-) molecules and the electron-deficient nitrobenzene.

A transformative route to nanoporous manganese oxides of controlled oxidation states with identical textural properties

Nanoporous nanocrystalline metal oxides with tunable oxidation states are crucial for controlling their catalytic, electronic, and optical properties. However, previous approaches to modulate oxidation states in nanoporous metal oxides commonly lead to the breakdown of the nanoporous structure as well as involve concomitant changes in their morphology, pore size, surface area, and nanocrystalline size. Herein, we present a transformative route to nanoporous metal oxides with various oxidation states using manganese oxides as model systems. Thermal conversion of Mn-based metal–organic frameworks (Mn-MOFs) at controlled temperature and atmosphere yielded a series of nanoporous manganese oxides with continuously tuned oxidation states: MnO, Mn3O4, Mn5O8, and Mn2O3. This transformation enabled the preparation of low-oxidation phase MnO and metastable intermediate phase Mn5O8 with nanoporous architectures, which were previously rarely accessible. Significantly, nanoporous MnO, Mn3O4, and Mn5O8 had a very similar morphology, surface area, and crystalline size. We investigated the electrocatalytic activity of nanoporous manganese oxides for oxygen reduction reaction (ORR) to identify the role of oxidation states, and observed oxidation state-dependent activity and kinetics for the ORR.

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.

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.

Solvent-induced single-crystal to single-crystal transformation of a Zn4O-containing doubly interpenetrated metal-organic framework with a pcu net

Guest molecules in a metal–organic framework (MOF), [Zn4O(CPMA)3]·6DMF (DMF = N,N-dimethylformamide), with an interpenetrating pcu net were exchanged with benzene, n-hexane, and methanol in a single-crystal to single-crystal (SC–SC) manner, which involved the sliding motion of the interpenetrating network, as well as the dynamic movements of the molecular components, and was triggered by host–guest interactions.

Crystal-Size Effects on Carbon Dioxide Capture of a Covalently Alkylamine-Tethered Metal-Organic Framework Constructed by a One-Step Self-Assembly

To enhance the carbon dioxide (CO2) uptake of metal-organic frameworks (MOFs), amine functionalization of their pore surfaces has been studied extensively. In general, amine-functionalized MOFs have been synthesized via post-synthetic modifications. Herein, we introduce a one-step construction of a MOF ([(NiLethylamine)(BPDC)] = MOFNH2; [NiLethylamine]2+ = [Ni(C12H32N8)]2+; BPDC2− = 4,4‘-biphenyldicarboxylate) possessing covalently tethered alkylamine groups without post-synthetic modification. Two-amine groups per metal centre were introduced by this method. MOFNH2 showed enhanced CO2 uptake at elevated temperatures, attributed to active chemical interactions between the amine groups and the CO2 molecules. Due to the narrow channels of MOFNH2, the accessibility to the channel of CO2 is the limiting factor in its sorption behaviour. In this context, only crystal size reduction of MOFNH2 led to much faster and greater CO2 uptake at low pressures.

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.