Ki-Young Kwon

Systematic analysis of palladium-graphene nanocomposites and their catalytic applications in Sonogashira reaction

Graphene has been modified with palladium nanoparticles (Pd NPs) to develop high performance catalysts for the Sonogashira cross coupling reaction. In this research, graphite oxide (GO) sheets exfoliated from graphite were impregnated with Pd(OAc)2 to prepare Pd(2+)/GO. Thermal treatments of the Pd(2+)/GO in H2 flow at 100°C produced Pd/graphene (Pd/G) nanocomposites. TEM images show that Pd NPs were distributed quite uniformly on the graphene sheet without obvious aggregation, and the mean size of Pd NPs was determined to be less than 2 nm in diameter. Morphological and chemical structures of the GO, Pd(2+)/GO, and Pd/G were investigated using FT-IR, XRD, XPS, and XAFS. The resulting Pd/G showed excellent catalytic efficiency in the Sonogashira reaction and offers significant advantages over inorganic supported catalysts such as simple recovery and recycling. Finally, deactivation process of the Pd/G in recycling was investigated. We believe that the remarkable reactivity of the Pd/G catalyst toward the Sonogashira reaction is attributed to the high degree of the Pd NP dispersion and thus the increased low coordination numbers of smaller Pd NPs.

Fluorescent Composite Hydrogels of Metal–Organic Frameworks and Functionalized Graphene Oxide

GO MOFs! Azobenzoic acid functionalized graphene (A-GO) can act as a structure-directing template that influences hydrogel formation together with metal-organic frameworks (MOFs). Zn(2+) MOFs of pyridine derivatives work as framework linkers between the A-GO sheets (MOF-A-GO, see figure). MOF-A-GO exhibits a strong fluorescence enhancement upon gel formation. In addition, MOF-A-GO selectively recognizes trinitrotoluene.

Hydroxyapatite supported cobalt catalysts for hydrogen generation

The controlled generation of H(2) from storage materials by using an efficient catalytic support is a highly sought after technology; however, the majority of successes utilize expensive materials considered unfeasible. In our report on the creation of a novel, durable, and inexpensive catalytic support material for hydrogen generation, we examine a critical surface modification of hydroxyapatite (HAP) with cobalt ions to provide the necessary catalytic transition metal for the fast hydrolysis of the hydrogen storage material, sodium borohydride (NaBH(4)). By altering the morphology and composition of the HAP crystal supports, we revealed novel methods for enhancing the hydrogen generation rates. Particularly, lowering the Ca composition during synthesis of the HAP crystals afforded a Ca deficient HAP capable of exhibiting a higher surface coverage of cobalt, thereby eliciting faster hydrolysis reaction rates in comparison with the amorphous HAP control having the characteristic Ca content for HAP. A more significant increase in hydrogen generation was observed when using single crystal HAP in comparison with amorphous and calcium deficient HAP supports. Despite the smaller surface area of the hydrothermally prepared single crystal HAP, it provided significantly faster hydrogen generation. Each of the HAP supports exhibit repeatability with catalytic efficiency decreasing by approximately 25% over 3 weeks upon repeated daily exposure to solutions of the hydrogen storage material NaBH(4). Through these experiments, we proved that altering the composition and morphology of cobalt ion exchanged HAP supports can offers a useful means for increasing the rate of controlled hydrogen generation.

Ultraviolet Patterned Calixarene-Derived Supramolecular Gels and Films with Spatially Resolved Mechanical and Fluorescent Properties

Supramolecular assemblies have in the past been considered mechanically weak and in most cases unable to withstand their own weight. Calixarene-derived networks can, however, provide robust supramolecular gels. Incorporating a photoreactive stilbene moiety, we show that the aggregation state of the material can be tuned by heating and UV exposure in order to control the mechanical as well as the fluorescent properties. Regulating the extent of heating to control the proportion of H-aggregates and J-aggregates and further cross-linking of H-aggregates by control over UV exposure allows for adjustable photopatterning of the fluorescence as well as the material stiffness in the range from ∼100 to 450 kPa. We expect this straightforward supramolecular system will be suitable for advanced prototyping in applications where modulus and shape are important design criteria.

A facile method to fabricate hydrogels from DMSO polymer gels via solvent exchange

A mixture of the bipyridine, phenyl and/or cyclohexanediamine-based building blocks 1, 2, and/or 3, having hydrazide, aldehyde or amine moieties, respectively, formed DMSO polymer gels by the hydrazone reaction under acidic conditions. These DMSO gels were transformed into hydrogels without a change in shape by solvent exchange. Interestingly, G′ and G′′ values of the hydrogels increased 9–12-fold due to the formation of a three-dimensional network structure by intermolecular hydrogen bonding interactions. In sweep measurements, the γ values of the hydrogels prepared without NaCl were enhanced from 18.5% to 46–55%, which was attributed to the three-dimensional network with additional intermolecular hydrogen-bonding interactions. Furthermore, a gel-to-quasi-liquid transition was completely reversible, where the G′ and G′′ values were recovered rapidly within 60 s. This reversibility was a thixotropic response.