Eunyong Seo

Hybrid gold nanoparticle-reduced graphene oxide nanosheets as active catalysts for highly efficient reduction of nitroarenes

We demonstrate a simple, one-step synthesis of hybrid gold nanoparticle–graphene oxide nanosheets (Au–GO) through electrostatic self-assembly. This method affords a facile means of controlling the effective concentration of the active Au nanoparticles on the graphene sheets, but also offers the necessary stability of the resulting Au–GO nanostructure for catalytic transformation. Furthermore, this hybrid Au–GO is successfully employed in the catalytic reduction of a series of nitroarenes with high catalytic activity. Through careful investigation of the catalyst, we find the synergistic catalytic effect of Au nanoparticles and GO, further highlighting the significance of hybrid Au–GO nanostructure. Considering the wide potential applications of a two-dimensional graphene sheet as a host material for a variety of nanoparticles, the approach developed here may lead to new possibilities for the fabrication of hybrid nanoparticle–graphene nanosheet structures endowed with multiple functionalities.

Carbon-based layer-by-layer nanostructures: from films to hollow capsules

Over the past years, the layer-by-layer (LbL) assembly has been widely developed as one of the most powerful techniques to prepare multifunctional films with desired functions, structures and morphologies because of its versatility in the process steps in both material and substrate choices. Among various functional nanoscale objects, carbon-based nanomaterials, such as carbon nanotubes and graphene sheets, are promising candidates for emerging science and technology with their unique physical, chemical, and mechanical properties. In particular, carbon-based functional multilayer coatings based on the LbL assembly are currently being actively pursued as conducting electrodes, batteries, solar cells, supercapacitors, fuel cells and sensor applications. In this article, we give an overview on the use of carbon materials in nanostructured films and capsules prepared by the LbL assembly with the aim of unraveling the unique features and their applications of carbon multilayers prepared by the LbL assembly.

Interfacing Living Yeast Cells with Graphene Oxide Nanosheaths

The first example of the encapsulation of living yeast cells with multilayers of GO nanosheets via LbL self-assembly is reported. The GO nanosheets with opposite charges are alternatively coated onto the individual yeast cells while preserving the viability of the yeast cells, thus affording a means of interfacing graphene with living yeast cells. This approach is expanded by integrating other organic polymers or inorganic nanoparticles to the cells by hybridizing the entries with GO nanosheets through LbL self-assembly. It is demonstrated that incorporated iron oxide nanoparticles can deliver magnetic properties to the biological systems, allowing the integration of new physical and chemical functions for living cells with a combination of GO nanosheets.

Versatile double hydrophilic block copolymer: dual role as synthetic nanoreactor and ionic and electronic conduction layer for ruthenium oxide nanoparticle supercapacitors

The facile synthetic approach to ruthenium oxide nanoparticles using double hydrophilic block copolymers (DHBCs) and their application toward the supercapacitor are presented. Nanostructured hydrous ruthenium oxide (RuO2) nanoparticles are synthesized using a double hydrophilic block copolymer of poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA) as a template, forming a micelle upon addition of the ruthenium precursor, which then transformed into RuO2 nanoparticles of controlled dimension with reducing agents. The synthesized hydrous RuO2·xH2O nanoparticles are very stable for several months without any noticeable aggregates. Furthermore, we have demonstrated their utility in application as supercapacitors. Through annealing at 400 °C, we found that the crystallinity of RuO2 nanoparticles increases considerably with a simultaneous transformation of the surrounding double hydrophilic block copolymer into ionic and electronic conducting buffer layers atop RuO2 nanoparticles, which contribute to the significant enhancement of the overall specific capacitance from 106 to 962 F g−1 at 10 mV s−1. The RuO2 nanoparticles annealed at 400 °C also exhibit a superior retention of capacitance over 1000 cycles at very high charge–discharge rates at 20 A g−1. We envision that the double hydrophilic block copolymer will provide a facile and general tool in creating functional nanostructures with controlled dimensions that are useful for various applications.

Highly stable Au nanoparticles with double hydrophilic block copolymer templates: correlation between structure and stability

We herein report a facile synthetic method for preparing gold nanoparticles (Au NPs) with superior colloidal stability using a series of double hydrophilic block copolymers (DHBC), poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA), as a template (Au@DHBC NPs). Due to the presence of a well-defined polymeric shell around the Au NPs, this DHBC-based synthetic method provides superior stability when compared to conventional citrate-based synthesis. We have investigated NP performance by systematically varying the molecular weight of the interacting PAA block from 5000 g mol−1 to 27 000 g mol−1. Interestingly, the size of the Au NPs did not significantly depend on the molecular weight of the PAA block and the density of DHBC present around a single NP decreased upon an increase in the molecular weight of the PAA block. Cyanide etching of Au@DHBC NPs further confirmed the presence of DHBC with different densities around the NPs, resulting in tunable stability. Considering the structural variability of DHBCs, it is expected that the approach presented in this study will offer a new means for creating Au NPs with enhanced colloidal stability for potential biological and biomedical applications.

Bifunctional hydrous RuO 2 nanocluster electrocatalyst embedded in carbon matrix for efficient and durable operation of rechargeable zinc–air batteries

Ruthenium oxide (RuO2) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO2 can be also efficiently used as an oxygen reduction reaction (ORR) electrocatalyst, thereby serving as a bifunctional material for rechargeable Zn–air batteries. We found two forms of RuO2 (i.e. hydrous and anhydrous, respectively h-RuO2 and ah-RuO2) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO2 required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO2 triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO2 while improving its drawbacks, we designed a unique structure (RuO2@C) where h-RuO2 nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO2 nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO2 by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO2@C catalyst demonstrated a very low potential gap (ΔEOER-ORR = ca. 1.0 V) at 20 mA cm−2. The Zn||RuO2@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).