Kyung Min Choi

Supercapacitors of Nanocrystalline Metal–Organic Frameworks

The high porosity of metal-organic frameworks (MOFs) has been used to achieve exceptional gas adsorptive properties but as yet remains largely unexplored for electrochemical energy storage devices. This study shows that MOFs made as nanocrystals (nMOFs) can be doped with graphene and successfully incorporated into devices to function as supercapacitors. A series of 23 different nMOFs with multiple organic functionalities and metal ions, differing pore sizes and shapes, discrete and infinite metal oxide backbones, large and small nanocrystals, and a variety of structure types have been prepared and examined. Several members of this series give high capacitance; in particular, a zirconium MOF exhibits exceptionally high capacitance. It has the stack and areal capacitance of 0.64 and 5.09 mF cm(-2), about 6 times that of the supercapacitors made from the benchmark commercial activated carbon materials and a performance that is preserved over at least 10000 charge/discharge cycles.

Heterogeneity within Order in Crystals of a Porous Metal–Organic Framework

Generally, crystals of synthetic porous materials such as metal-organic frameworks (MOFs) are commonly made up from one kind of repeating pore structure which predominates the whole material. Surprisingly, little is known about how to introduce heterogeneously arranged pores within a crystal of homogeneous pores without losing the crystalline nature of the material. Here, we outline a strategy for producing crystals of MOF-5 in which a system of meso- and macropores either permeates the whole crystal to make sponge-like crystals or is entirely enclosed by a thick crystalline microporous MOF-5 sheath to make pomegranate-like crystals. These new forms of crystals represent a new class of materials in which micro-, meso-, and macroporosity are juxtaposed and are directly linked unique arrangements known to be useful in natural systems but heretofore unknown in synthetic crystals.

Chemical Environment Control and Enhanced Catalytic Performance of Platinum Nanoparticles Embedded in Nanocrystalline Metal–Organic Frameworks

Chemical environment control of the metal nanoparticles (NPs) embedded in nanocrystalline metal-organic frameworks (nMOFs) is useful in controlling the activity and selectivity of catalytic reactions. In this report, organic linkers with two functional groups, sulfonic acid (-SO3H, S) and ammonium (-NH3(+), N), are chosen as strong and weak acidic functionalities, respectively, and then incorporated into a MOF [Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), termed UiO-66] separately or together in the presence of 2.5 nm Pt NPs to build a series of Pt NPs-embedded in UiO-66 (Pt⊂nUiO-66). We find that these chemical functionalities play a critical role in product selectivity and activity in the gas-phase conversion of methylcyclopentane (MCP) to acyclic isomer, olefins, cyclohexane, and benzene. Pt⊂nUiO-66-S gives the highest selectivity to C6-cyclic products (62.4% and 28.6% for cyclohexane and benzene, respectively) without acyclic isomers products. Moreover, its catalytic activity was doubled relative to the nonfunctionalized Pt⊂nUiO-66. In contrast, Pt⊂nUiO-66-N decreases selectivity for C6-cyclic products to <50% while increases the acyclic isomer selectivity to 38.6%. Interestingly, the Pt⊂nUiO-66-SN containing both functional groups gave different product selectivity than their constituents; no cyclohexane was produced, while benzene was the dominant product with olefins and acyclic isomers as minor products. All Pt⊂nUiO-66 catalysts with different functionalities remain intact and maintain their crystal structure, morphology, and chemical functionalities without catalytic deactivation after reactions over 8 h.

Titanium-embedded layered double hydroxides as highly efficient water oxidation photocatalysts under visible light

Here, we have synthesized the new titanium-embedded layered double hydroxides (LDHs), such as (Ni/Ti)LDH and (Cu/Ti)LDH. First of all, the formation of LDH structures and the bonding nature for a mixed oxide structure of LDHs are explored in this work. Also, it is determined that our LDHs show two absorption bands in the red and blue regions under visible light, thus different from those of a pure titanium oxide with absorption bands in only the UV region. We find that the (Ni/Ti)LDH with the high surface area showed a higher reaction rate, producing 49 μmol O2 in water oxidation by using 200 mg of the photocatalyst and 1 mmol of AgNO3 as a sacrificial agent. Also, the (Cu/Ti)LDH showed a good reaction rate and produced 31 μmol of O2 under the same condition. On the other hand, conventional TiO2 nanoparticles generated a very small amount of oxygen within the error range under this visible light irradiation. Consequently, these results imply that absorption bands in the visible range and the large surface area of an LDH could result in the high water oxidation photocatalytic activity under visible light.

Highly porous gallium oxide with a high CO2 affinity for the photocatalytic conversion of carbon dioxide into methane

Highly porous gallium oxide was synthesized by reconstructing its surface and body with mesopores and macropores. For the first time, the efficient photocatalytic conversion of CO2 into a high energy carrier, CH4, using the porous gallium oxide was realized without any co-particle or sacrificial reagent. The enhanced photocatalytic activity is mainly attributed to the 300% higher CO2 adsorption capacity, as well as the 200% increased surface area, compared to the bulk nanoparticles. Furthermore, we propose the new reaction pathway based on the result that the carbon dioxide was converted directly into methane without going through carbon monoxide intermediates.

Covalent organic frameworks for extremely high reversible CO2 uptake capacity: a theoretical approach

We report that the novel covalent organic frameworks (COFs) are capable of reversibly providing an extremely high uptake capacity of carbon dioxide at room temperature. These COFs are designed via the combination of ab initio calculations and force-field calculations. For this goal, we explore the adsorption sites of carbon dioxide on COFs, their porosity, as well as carbon dioxide adsorption isotherms. We identify the binding sites and energies of CO2 on COFs using ab initio calculations and obtain the carbon dioxide adsorption isotherms using grand canonical ensemble Monte Carlo calculations. Moreover, the calculated adsorption isotherms are compared with the experimental values in order to build the reference model in describing the interactions between the CO2 and the COFs and in predicting the CO2 adsorption isotherms of COFs. Finally, we design three new COFs, 2D COF-05, 3D COF-05 (ctn), and 3D COF-05 (bor), for the high capacity CO2 storage. The carbon dioxide adsorption values of the new 3D COFs are about six times larger than that of MOF-177. This suggests that 3D COFs are very promising candidates for high capacity CO2 storage.

Cooperative effects at the interface of nanocrystalline metal–organic frameworks

Controlling the chemistry at the interface of nanocrystalline solids has been a challenge and an important goal to realize desired properties. Integrating two different types of materials has the potential to yield new functions resulting from cooperative effects between the two constituents. Metal–organic frameworks (MOFs) are unique in that they are constructed by linking inorganic units with organic linkers where the building units can be varied nearly at will. This flexibility has made MOFs ideal materials for the design of functional entities at interfaces and hence allowing control of properties. This review highlights the strategies employed to access synergistic functionality at the interface of nanocrystalline MOFs (nMOFs) and inorganic nanocrystals (NCs).

Metal–organic frameworks for visible light absorption via anion substitution

We report that the band gap of metal–organic frameworks (MOFs) could be tuned to absorb visible light via anion substitution. First of all, when an oxygen anion in the metal oxide core of IRMOF-1 is substituted with sulphur anions as Zn4S or selenium anions as Zn4Se to form IRMOF-1-S and IRMOF-1-Se, respectively, they result in density-of-states and molecular orbitals for the band gap that can utilize visible light through shifting of the Fermi level due to their electron-rich properties. This implies that the tailored band gap could provide a new route to allow the series of IRMOFs and other MOFs to be used for visible light absorption.

Rescaling of metal oxide nanocrystals for energy storage having high capacitance and energy density with robust cycle life

Significance The combined study of experiments and molecular dynamics simulations demonstrates that metal oxide nanocrystals on graphene can be rescaled into atomic clusters. It is notable that the capacitance of 3,023 F per the mass of NiO, matching the measured capacitance of 2,231 per the total electrode mass, exceeds the theoretical gravimetric capacitance of 2,618 F available via ion-to-atom redox reactions. This approach thus provides a new pathway to realize full capacitance via ion-to-atom Faradaic redox reactions. Furthermore, assembly with a rescaled metal oxide positive electrode shows that further development of high-capacity negative counter electrode materials can pave a new route to address challenging energy storage issues. Nanocrystals are promising structures, but they are too large for achieving maximum energy storage performance. We show that rescaling 3-nm particles through lithiation followed by delithiation leads to high-performance energy storage by realizing high capacitance close to the theoretical capacitance available via ion-to-atom redox reactions. Reactive force-field (ReaxFF) molecular dynamics simulations support the conclusion that Li atoms react with nickel oxide nanocrystals (NiO-n) to form lithiated core–shell structures (Ni:Li2O), whereas subsequent delithiation causes Ni:Li2O to form atomic clusters of NiO-a. This is consistent with in situ X-ray photoelectron and optical spectroscopy results showing that Ni2+ of the nanocrystal changes during lithiation–delithiation through Ni0 and back to Ni2+. These processes are also demonstrated to provide a generic route to rescale another metal oxide. Furthermore, assembling NiO-a into the positive electrode of an asymmetric device enables extraction of full capacitance for a counter negative electrode, giving high energy density in addition to robust capacitance retention over 100,000 cycles.

A Facile Way to Control the Number of Walls in Carbon Nanotubes through the Synthesis of Exposed‐Core/Shell Catalyst Nanoparticles

There is currently great interest in the controlled synthesis of carbon nanotubes (CNTs) with unique structures. Much of this attraction lies in the fact that the functionality of CNTs can be significantly tailored by control of their composition, diameter, and number of walls. CNTs are known to act as either metals or semiconductors, depending on their diameters and chiralities, for example; therefore, precise control of their nanostructures is essential for various applications, such as field-emitter tips in displays, transistors, interconnection and memory elements in integrated circuits, scan tips for atomic force microscopy, and energy-storage media. One conventional method for growing vertically aligned CNTs involves the use of chemical vapor deposition (CVD). In this case, CNTs can be grown selectively on catalytic sites and their properties depend to a large extent on the nanostructure of the catalyst, particularly the particle size, interparticle distance, and composition. 12] It is therefore necessary to develop improved catalysts using highly innovative methods in order to obtain desired CNT functionalities. Traditional methods for designing and preparing catalyst particles to control the properties of CNTs include a metal film sputtering method, 14] an organic silica mesoporous template method, a nanoparticle method, and a selfassembled block copolymer template method. These methods, however, only focus on controlling the diameter or interparticle distance of CNTs by controlling the size or interparticle distance of the catalyst particles. While it has been reported that controlling the number of walls in CNTs is possible by adjusting the catalyst size, some limitations remain with respect to controlling the diameter and number of walls simultaneously. This suggests that CNT nanostructures with required diameters and interlayer walls cannot be adequately controlled with current methods, and that a new innovative technique for controlling the diameter and the number of CNT walls simultaneously is required. Herein we report a facile way of controlling the number of CNT walls and their diameters simultaneously by using exposed-core/shell (ECS) catalysts composed of catalytically active iron in the shell layer and inactive iron nitride in the exposed core area. As these ECS catalysts were prepared using an Fe-loaded diblock copolymer micelle to pattern the nanoparticles in a controllable manner, this approach provides a total solution for controlling the CNT alignment and pattern as well as its nanostructure simultaneously. The synthetic process is illustrated in Scheme 1. Thus, after synthesizing the diblock copolymer micelle solution, Fe precursors were loaded into the micelle core. This Fe-loaded micelle solution was then coated onto a Si/SiO2 substrate in a spin-coater to give a hexagonally arrayed pattern of micelles. Subsequent low-temperature plasma treatment resulted in the formation of metal particles upon reduction of the metal precursors in the micelle core and removal of the micelle polymers. The resulting Fe nanoparticles patterned on a substrate were placed in a nitrogen plasma to precipitate the iron nitride inside each particle at high temperature. After this precipitation step, a chemical etching process was followed to remove the outer part of the iron shell and expose the iron nitride core. The resulting ECS catalysts, which are composed of an exposed core area (catalytically inactive) and a shell layer (catalytically active), were found to be arranged on the substrate with a constant size and interparticle distance.