Do Hong Kim

Self-activated ultrahigh chemosensitivity of oxide thin film nanostructures for transparent sensors

One of the top design priorities for semiconductor chemical sensors is developing simple, low-cost, sensitive and reliable sensors to be built in handheld devices. However, the need to implement heating elements in sensor devices, and the resulting high power consumption, remains a major obstacle for the realization of miniaturized and integrated chemoresistive thin film sensors based on metal oxides. Here we demonstrate structurally simple but extremely efficient all oxide chemoresistive sensors with ~90% transmittance at visible wavelengths. Highly effective self-activation in anisotropically self-assembled nanocolumnar tungsten oxide thin films on glass substrate with indium-tin oxide electrodes enables ultrahigh response to nitrogen dioxide and volatile organic compounds with detection limits down to parts per trillion levels and power consumption less than 0.2 microwatts. Beyond the sensing performance, high transparency at visible wavelengths creates opportunities for their use in transparent electronic circuitry and optoelectronic devices with avenues for further functional convergence.

Wafer-scale transferable molybdenum disulfide thin-film catalysts for photoelectrochemical hydrogen production

We demonstrate that wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide a low onset potential of 1 mA cm−2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA cm−2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown via a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction.

Vertically ordered hematite nanotube array as an ultrasensitive and rapid response acetone sensor

Vertically ordered nanotube array is a desirable configuration to improve gas sensing properties of the hematite which is the most abundant and cheapest metal oxide semiconductor on earth but has low and sluggish chemiresistive responses. We have synthesized a vertically aligned, highly ordered hematite nanotube array directly on a patterned SiO2/Si substrate and then it used as a gas sensor without additional processing. The nanotube array sensor shows unprecedentedly ultrahigh and selective responses to acetone with detection limits down to a few parts per billion and response time shorter than 3 s.

Au-decorated WO3 cross-linked nanodomes for ultrahigh sensitive and selective sensing of NO2 and C2H5OH

Au-decorated WO3 cross-linked nanodomes are fabricated using soft templates composed of highly ordered polystyrene beads and self-agglomeration of Au. The distribution and size of Au nanoparticles on the surface of WO3 cross-linked nanodomes are controlled by varying the thickness of the initial Au film. The responses of Au-decorated WO3 cross-linked nanodomes to various gases such as NO2, CH3COCH3, C2H5OH, NH3, CO, H2, and C6H6 are at least 5 times higher than those of bare WO3 cross-linked nanodomes. The response enhancement by Au decoration is dependent on the target gas, which is attributed to an interplay between electronic and chemical sensitizations. In particular, the Au-decorated WO3 cross-linked nanodomes exhibit extremely high sensitivities and selectivities, and ppt-level detection limits to NO2 and C2H5OH at 250 °C and 450 °C, respectively. These results suggest that Au-decorated WO3 cross-linked nanodomes are very promising for use in breath analysers to diagnose both asthma and lung cancer from exhaled human breath.

Vertically ordered SnO2 nanobamboos for substantially improved detection of volatile reducing gases

Vertically ordered SnO2 nanorods with Au nanoparticles deposited in multiple steps, namely, SnO2 nanobamboos are synthesized by a glancing-angle deposition technique. The highly ordered porous structures enable us to detect sub-ppb levels of volatile reducing gases with a fast response speed by maximizing the sensitization effects of the Au nanoparticles.

A wafer-scale antireflective protection layer of solution-processed TiO2 nanorods for high performance silicon-based water splitting photocathodes

Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and reduces the consumption of fossil fuels, which are mainly responsible for the rise in global temperature. Solar water splitting using semiconductors, such as silicon, is promising to satisfy the global energy demand by producing hydrogen molecules. However, the solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and/or instability in liquid electrolytes. Herein, we report the synthesis of multifunctional solution-processed TiO2 nanorods on a 4-inch p-silicon wafer with controllable heights and diameters for highly efficient water splitting photocathodes. The solution-processed passivation layer of TiO2 nanorods reduces the overpotential of the silicon photocathode due to its catalytic properties. The TiO2 NRs also dramatically improves the light absorption of silicon due to the antireflective ability of the nanorods. The reflectance of silicon is decreased from 37.5% to 1.4% and enhances the saturated photocurrent density. The Pt-decorated (1–2.5 nm diameter) TiO2 nanorods/p-Si photocathodes show a short circuit current density of up to 40 mA cm−2, an open circuit voltage ∼440 mV and incident photon to current conversion efficiency of >90% using 0.5 M H2SO4 electrolyte with simulated 1 sun irradiation. The heterostructure photocathodes are stable for more than 52 h without noticeable degradation and an ideal regenerative cell efficiency of 2.5% is achieved.

Monolayer Co3O4 Inverse Opals as Multifunctional Sensors for Volatile Organic Compounds

Monolayers of periodic porous Co3 O4 inverse opal (IO) thin films for gas-sensor applications were prepared by transferring cobalt-solution-dipped polystyrene (PS) monolayers onto sensor substrates and subsequent removal of the PS template by heat treatment. Monolayer Co3 O4 IO thin films having periodic pores (d≈500 nm) showed a high response of 112.9 to 5 ppm C2 H5 OH at 200 °C with low cross-responses to other interfering gases. Moreover, the selective detection of xylene and methyl benzenes (xylene+toluene) could be achieved simply by tuning the sensor temperature to 250 and 275 °C, respectively, so that multiple gases can be detected with a single chemiresistor. Unprecedentedly high ethanol response and temperature-modulated control of selectivity with respect to ethanol, xylene, and methyl benzenes were attributed to the highly chemiresistive IO nanoarchitecture and to the tuned catalytic promotion of different gas-sensing reactions, respectively. These well-ordered porous nanostructures could have potential in the field of high-performance gas sensors based on p-type oxide semiconductors.

Toward High-Performance Hematite Nanotube Photoanodes: Charge-Transfer Engineering at Heterointerfaces

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm(2) at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm(2) at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts.

Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection

Ammonia (NH3) is an irritant gas with a unique pungent odor; sub-parts per million-level breath ammonia is a medical biomarker for kidney disorders and Helicobacter pylori bacteria-induced stomach infections. The humidity varies in both ambient environment and exhaled breath, and thus humidity dependence of gas-sensing characteristics is a great obstacle for real-time applications. Herein, flexible, humidity-independent, and room-temperature ammonia sensors are fabricated by the thermal evaporation of CuBr on a polyimide substrate and subsequent coating of a nanoscale moisture-blocking CeO2 overlayer by electron-beam evaporation. CuBr sensors coated with a 100 nm-thick CeO2 overlayer exhibits an ultrahigh response (resistance ratio) of 68 toward 5 ppm ammonia with excellent gas selectivity, rapid response, reversibility, and humidity-independent sensing characteristics at room temperature. In addition, the sensing performance remains stable after repetitive bending and long-term operation. Moreover, the sensors exhibit significant response to the simulated exhaled breath of patients with H. pylori infection; the simulated breath contains 50 ppb NH3. The sensors thus show promising potential in detecting sub-parts per million-level NH3, regardless of humidity fluctuations, which can open up new applications in wearable devices for in situ medical diagnosis and indoor/outdoor environment monitoring.

Temperature and Gas Sensing Multifunctional Ceramic Sensors

Multifunctional structures with two kinds of materials have been intensively investigated in order to improve their electrical characteristic with two functions simultaneously. However, the research regarding of multifunctional ceramic sensor is still in a preliminary stage and how to integrate them with low-cost and high-yield mass production process remains a challenge issue. In this study, we fabricated the multifunctional ceramic sensor composed of temperature and gas sensors. Moreover, we investigated the CO sensing properties of three dimensional nanostuctured thin film gas sensors fabricated with silica ( nanosphere (