Dongkyu Cha

Substrate Dependent Self-Organization of Mesoporous Cobalt Oxide Nanowires with Remarkable Pseudocapacitance

A scheme of current collector dependent self-organization of mesoporous cobalt oxide nanowires has been used to create unique supercapacitor electrodes, with each nanowire making direct contact with the current collector. The fabricated electrodes offer the desired properties of macroporosity to allow facile electrolyte flow, thereby reducing device resistance and nanoporosity with large surface area to allow faster reaction kinetics. Co(3)O(4) nanowires grown on carbon fiber paper collectors self-organize into a brush-like morphology with the nanowires completely surrounding the carbon microfiber cores. In comparison, Co(3)O(4) nanowires grown on planar graphitized carbon paper collectors self-organize into a flower-like morphology. In three electrode configuration, brush-like and flower-like morphologies exhibited specific capacitance values of 1525 and 1199 F/g, respectively, at a constant current density of 1 A/g. In two electrode configuration, the brush-like nanowire morphology resulted in a superior supercapacitor performance with high specific capacitances of 911 F/g at 0.25 A/g and 784 F/g at 40 A/g. In comparison, the flower-like morphology exhibited lower specific capacitance values of 620 F/g at 0.25 A/g and 423 F/g at 40 A/g. The Co(3)O(4) nanowires with brush-like morphology exhibited high values of specific power (71 kW/kg) and specific energy (81 Wh/kg). Maximum energy and power densities calculated for Co(3)O(4) nanowires with flower-like morphology were 55 Wh/kg and 37 kW/kg respectively. Both electrode designs exhibited excellent cycling stability by retaining ∼91-94% of their maximum capacitance after 5000 cycles of continuous charge-discharge.

High‐Surface‐Area Silica Nanospheres (KCC‐1) with a Fibrous Morphology

The past decade has seen significant advances in the ability to fabricate new porous solids with ordered structures from a wide range of different materials, with silica being the most common. Porous materials and their nanoscopic version now seem set to contribute to the developments in areas ranging from microelectronics to medical diagnosis or targeting of drugs. The realm of mesoporous materials was extended after the emergence of Kresge s innovative method for the preparation of mesoporous silica materials (MCM-41) through the use of surfactants as organizing agents. After the inception of template-directed synthesis of silica, extensive research was conducted to control their morphologies, pore sizes, and structures. These templated techniques led to the synthesis of a variety of mesoporous and nanoscale silica materials with a wide range of morphologies that have been successfully used as supports in heterogeneous catalysis. The effectiveness of these materials as catalyst supports is mainly due to their microstructures, which allow active catalytic sites to disperse on the large internal surfaces and pores, which in turn improve the activity of the catalyst system. However, poor accessibility to these active sites inside the pores sometimes limits their applications for which significant mass transport is essential. Silica supports with easily accessible high surface areas (that is, not in the pores) are therefore needed. In quest of nanocatalysis by surface organometallic chemistry (SOMC), herein we present the synthesis of fibrous silica nanospheres (KCC-1) with high surface areas. To the best of our knowledge, silica nanospheres with this type of fibrous morphology (Figures 1 and 2) is unprecedented. The high surface area is due to the presence

Vertically Aligned Ta3N5 Nanorod Arrays for Solar‐Driven Photoelectrochemical Water Splitting

A vertically aligned Ta(3)N(5) nanorod photoelectrode is fabricated by through-mask anodization and nitridation for water splitting. The Ta(3)N(5) nanorods, working as photoanodes of a photoelectrochemical cell, yield a high photocurrent density of 3.8 mA cm(-2) at 1.23 V versus a reversible hydrogen electrode under AM 1.5G simulated sunlight and an incident photon-to-current conversion efficiency of 41.3% at 440 nm, one of the highest activities reported for photoanodes so far.

Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency

Spurred by the decreased availability of fossil fuels and global warming, the idea of converting solar energy into clean fuels has been widely recognized. Hydrogen produced by photoelectrochemical water splitting using sunlight could provide a carbon dioxide lean fuel as an alternative to fossil fuels. A major challenge in photoelectrochemical water splitting is to develop an efficient photoanode that can stably oxidize water into oxygen. Here we report an efficient and stable photoanode that couples an active barium-doped tantalum nitride nanostructure with a stable cobalt phosphate co-catalyst. The effect of barium doping on the photoelectrochemical activity of the photoanode is investigated. The photoanode yields a maximum solar energy conversion efficiency of 1.5%, which is more than three times higher than that of state-of-the-art single-photon photoanodes. Further, stoichiometric oxygen and hydrogen are stably produced on the photoanode and the counter electrode with Faraday efficiency of almost unity for 100 min.

High performance supercapacitors using metal oxide anchored graphene nanosheet electrodes

Metal oxide nanoparticles were chemically anchored onto graphene nanosheets (GNs) and the resultant composites—SnO2/GNs, MnO2/GNs and RuO2/GNs (58% of GNs loading)—coated over conductive carbon fabric substrates were successfully used as supercapacitor electrodes. The results showed that the incorporation of metal oxide nanoparticles improved the capacitive performance of GNs due to a combination of the effect of spacers and redox reactions. The specific capacitance values (with respect to the composite mass) obtained for SnO2/GNs (195 F g−1) and RuO2/GNs (365 F g−1) composites at a scan rate of 20 mV s−1 in the present study are the best ones reported to date for a two electrode configuration. The resultant supercapacitors also exhibited high values for maximum energy (27.6, 33.1 and 50.6 W h kg−1) and power densities (15.9, 20.4 and 31.2 kW kg−1) for SnO2/GNs, MnO2/GNs and RuO2/GNs respectively. These findings demonstrate the importance and great potential of metal oxide/GNs based composite coated carbon fabric in the development of high-performance energy-storage systems.

Solution‐Processed Small Molecule‐Polymer Blend Organic Thin‐Film Transistors with Hole Mobility Greater than 5 cm2/Vs

Using phase-separated organic semiconducting blends containing a small molecule, as the hole transporting material, and a conjugated amorphous polymer, as the binder material, we demonstrate solution-processed organic thin-film transistors with superior performance characteristics that include; hole mobility >5 cm(2) /Vs, current on/off ratio ≥10(6) and narrow transistor parameter spread. These exceptional characteristics are attributed to the electronic properties of the binder polymer and the advantageous nanomorphology of the blend film.

Enhanced Rate Performance of Mesoporous Co3O4 Nanosheet Supercapacitor Electrodes by Hydrous RuO2 Nanoparticle Decoration

Mesoporous cobalt oxide (Co3O4) nanosheet electrode arrays are directly grown over flexible carbon paper substrates using an economical and scalable two-step process for supercapacitor applications. The interconnected nanosheet arrays form a three-dimensional network with exceptional supercapacitor performance in standard two electrode configuration. Dramatic improvement in the rate capacity of the Co3O4 nanosheets is achieved by electrodeposition of nanocrystalline, hydrous RuO2 nanoparticles dispersed on the Co3O4 nanosheets. An optimum RuO2 electrodeposition time is found to result in the best supercapacitor performance, where the controlled morphology of the electrode provides a balance between good conductivity and efficient electrolyte access to the RuO2 nanoparticles. An excellent specific capacitance of 905 F/g at 1 A/g is obtained, and a nearly constant rate performance of 78% is achieved at current density ranging from 1 to 40 A/g. The sample could retain more than 96% of its maximum capacitance even after 5000 continuous charge-discharge cycles at a constant high current density of 10 A/g. Thicker RuO2 coating, while maintaining good conductivity, results in agglomeration, decreasing electrolyte access to active material and hence the capacitive performance.

Fibrous Nano‐Silica (KCC‐1)‐Supported Palladium Catalyst: Suzuki Coupling Reactions Under Sustainable Conditions

We thank the King Abdullah University of Science and Technology (KAUST) for financial and logistic support. Thanks are also due to Prof. Jean-Marie Basset, Director of the KAUST Catalysis Center (KCC), for his encouragement and support.

High performance In2O3 thin film transistors using chemically derived aluminum oxide dielectric

We report high performance solution-deposited indium oxide thin film transistors with field-effect mobility of 127 cm2/Vs and an Ion/Ioff ratio of 106. This excellent performance is achieved by controlling the hydroxyl group content in chemically derived aluminum oxide (AlOx) thin-film dielectrics. The AlOx films annealed in the temperature range of 250–350 °C showed higher amount of Al-OH groups compared to the films annealed at 500 °C, and correspondingly higher mobility. It is proposed that the presence of Al-OH groups at the AlOx surface facilitates unintentional Al-doping and efficient oxidation of the indium oxide channel layer, leading to improved device performance.

High performance solution-deposited amorphous indium gallium zinc oxide thin film transistors by oxygen plasma treatment

Solution-deposited amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) with high performance were fabricated using O2-plasma treatment of the films prior to high temperature annealing. The O2-plasma treatment resulted in a decrease in oxygen vacancy and residual hydrocarbon concentration in the a-IGZO films, as well as an improvement in the dielectric/channel interfacial roughness. As a result, the TFTs with O2-plasma treated a-IGZO channel layers showed three times higher linear field-effect mobility compared to the untreated a-IGZO over a range of processing temperatures. The O2-plasma treatment effectively reduces the required processing temperature of solution-deposited a-IGZO films to achieve the required performance.