Ji Wook Yoon

Design of highly sensitive and selective Au@NiO yolk–shell nanoreactors for gas sensor applications

Au@NiO yolk-shell nanoparticles (NPs) were synthesized by simple solution route and applied for efficient gas sensor towards H₂S gas. Carbon encapsulated Au (Au@C core-shell) NPs were synthesized by glucose-assisted hydrothermal method, whereas Au@NiO yolk-shell NPs were synthesized by precipitation method using Au@C core-shell NPs as a template. Sub-micrometer Au@NiO yolk-shell NPs were formed having 50-70 nm Au NPs at the periphery of NiO shell (10-20 nm), which was composed of 6-12 nm primary NiO particles. Au@NiO yolk-shell NPs showed higher response for H2S compared to other interfering gases (ethanol, p-xylene, NH₃, CO and H₂). The maximum response was 108.92 for 5 ppm of H₂S gas at 300 °C, which was approximately 19 times higher than that for the interfering gases. The response of Au@NiO yolk-shell NPs to H₂S was approximately 4 times higher than that of bare NiO hollow nanospheres. Improved performance of Au@NiO yolk-shell NPs was attributed to hollow spaces that allowed the accessibility of Au NPs to gas molecules. It was suggested that adsorption of H₂S on Au NPs resulted in the formation of sulfide layer, which possibly lowered its work function, and therefore tuned the electron transfer from Au to NiO rather NiO to Au, which leaded to increase in resistance and therefore response.

One-pot synthesis of Pd-loaded SnO(2) yolk-shell nanostructures for ultraselective methyl benzene sensors

A continuous, single-step, and large-scale preparation of Pd-catalyst-loaded SnO2 yolk-shell spheres is demonstrated. These nanostructures show an unusually high response and selectivity to methyl benzenes, such as xylene and toluene, with very low cross-responses to various interfering gases, making them suitable for precise monitoring of indoor air quality.

Electronic sensitization of the response to C2H5OH of p-type NiO nanofibers by Fe doping

Pure and 0.18-13.2 at.% Fe-doped NiO nanofibers were prepared by electrospinning and their gas sensing characteristics and microstructural evolution were investigated. The responses ((Rg - Ra)/Ra, where Rg is the resistance in gas and Ra is the resistance in air) to 5 ppm C2H5OH, toluene, benzene, p-xylene, HCHO, CO, H2, and NH3 at 350-500 ° C were significantly enhanced by Fe doping of the NiO nanofibers, while the responses of pure NiO nanofibers to all the analyte gases were very low ((Rg - Ra)/Ra = 0.07-0.78). In particular, the response to 100 ppm C2H5OH was enhanced up to 217.86 times by doping of NiO nanofibers with 3.04 at.% Fe. The variation in the gas response was closely dependent upon changes in the base resistance of the sensors in air. The enhanced gas response of Fe-doped NiO nanofibers was explained in relation to electronic sensitization, that is, the increase in the chemoresistive variation due to the decrease in the hole concentration induced by Fe doping.

Role of Pd nanoparticles in gas sensing behaviour of Pd@In2O3 yolk–shell nanoreactors

Pd@In2O3 yolk–shell nanoparticles (NPs) were synthesized by a simple solution route using Pd@C core–shell NPs as template and applied for gas sensing. A glucose-assisted hydrothermal method was used for the synthesis of Pd@C core–shell NPs. Pd@In2O3 yolk–shell NPs were formed after calcination (450 °C for 3h) of Pd@C core–shell NPs containing indium precursor. In the Pd@In2O3 yolk–shell geometry, about 50–70 nm Pd NPs were present at the periphery of an In2O3 shell (10–20 nm thickness). The In2O3 shell was composed of ∼10 nm primary particles. The role of Pd NPs in gas sensing behavior of In2O3 has been investigated. The loading of In2O3 with Pd NPs improved the response for reducing gases, but reduced the response for oxidizing gases. The response of Pd@In2O3 yolk–shell NPs to ethanol was approximately 14 times higher than that of pure In2O3 hollow nanospheres at 350 °C. However, no response was recorded for NO2 for Pd@In2O3 as compared to In2O3 (resistance ratio Rs = 2.50) at 350 °C. The maximum response of Pd@In2O3 yolk–shell NPs to 5 ppm ethanol was 159.02 at 350 °C, which was approximately 2.5 times higher than those for other interfering gases (NO2, p-xylene, trimethylamine, HCHO, CO and H2). The effect of humidity on the gas sensing characteristics of Pd@In2O3 yolk–shell NPs suggested that the present sensor can be used to detect ppm-level ethanol even in highly humid atmosphere (80% RH). The improved gas sensing performance of Pd@In2O3 yolk–shell NPs was attributed to catalytic activity of Pd NPs as well as hollow spaces that allowed the accessibility of Pd NPs to gas molecules.

High performance chemiresistive H2S sensors using Ag-loaded SnO2 yolk–shell nanostructures

SnO2 yolk–shell spheres uniformly loaded with Ag nanoparticles were prepared by a facile one-pot ultrasonic spray pyrolysis of the source solution and the H2S sensing characteristics were investigated. The Ag-loaded SnO2 yolk–shell spheres showed ultrahigh and reversible response (Ra/Rg − 1 = 613.9, where Ra is the resistance in air and Rg is the resistance in gas) to 5 ppm H2S with negligible cross-responses (0.6–17.3) to eight other interference gases at 350 °C. In contrast, pure SnO2 spheres with dense inner structures and yolk–shell morphologies did not exhibit a high response/selectivity to H2S nor reversible H2S sensing. The highly sensitive, selective, and reversible H2S sensing characteristics were explained in terms of the gas-accessible yolk–shell morphology and uniform loading of catalytic Ag nanoparticles. Namely, the gas-accessible yolk–shell morphology facilitated the rapid and effective diffusion of the analyte/oxygen gases and the uniform loading of Ag nanoparticles promoted the H2S sensing reaction.

A New Concept for Obtaining SnO2 Fiber‐in‐Tube Nanostructures with Superior Electrochemical Properties

Tin oxide (SnO2 ) nanotubes with a fiber-in-tube structure have been prepared by electrospinning and the mechanism of their formation has been investigated. Tin oxide-carbon composite nanofibers with a filled structure were formed as an intermediate product, which were then transformed into SnO2 nanotubes with a fiber-in-tube structure during heat treatment at 500 °C. Nanofibers with a diameter of 85 nm were found to be located inside hollow nanotubes with an outer diameter of 260 nm. The prepared SnO2 nanotubes had well-developed mesopores. The discharge capacities of the SnO2 nanotubes at the 2nd and 300th cycles at a current density of 1 A g(-1) were measured as 720 and 640 mA h g(-1), respectively, and the corresponding capacity retention measured from the 2nd cycle was 88 %. The discharge capacities of the SnO2 nanotubes at incrementally increased current densities of 0.5, 1.5, 3, and 5 A g(-1) were 774, 711, 652, and 591 mA h g(-1), respectively. The SnO2 nanotubes with a fiber-in-tube structure showed superior cycling and rate performances compared to those of SnO2 nanopowder. The unique structure of the SnO2 nanotubes with a fiber@void@tube configuration improves their electrochemical properties by reducing the diffusion length of the lithium ions, and also imparts greater stability during electrochemical cycling.

Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures with giant and ultraselective chemiresistivity to xylene and toluene.

Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures consisting of nanosheets have been prepared by solvothermal self-assembly. The nanostructures exhibited an ultrahigh response and selectivity towards p-xylene and toluene. The responses (resistance ratio) of the palladium-loaded Co3O4 hollow hierarchical nanostructures to 5 ppm of p-xylene and toluene were as high as 361 and 305, respectively, whereas the selectivity values (response ratios) towards p-xylene and toluene over interference from ethanol were 18.1 and 16.1, respectively. We attributed the giant response and unprecedented high selectivity towards methylbenzenes to the abundant adsorption of oxygen by Co3O4, the high chemiresistive variation in the Co3O4 nanosheets (thickness≈11 nm), and the catalytic promotion of the specific gas-sensing reaction. The morphological design of the p-type Co3O4 nanostructures and loading of the palladium catalyst have paved a new way to monitoring the most representative indoor air pollutants in a highly selective, sensitive, and reliable manner.

A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self-Refreshing of In2O3Sensing Surface Assisted by Layer-by-Layer Coated CeO2Nanoclusters

The humidity dependence of the gas sensing characteristics of metal oxide semiconductors has been one of the greatest obstacles for gas sensor applications during the last five decades because ambient humidity dynamically changes with the environmental conditions. Herein, a new and novel strategy is reported to eliminate the humidity dependence of the gas sensing characteristics of oxide chemiresistors via dynamic self-refreshing of the sensing surface affected by water vapor chemisorption. The sensor resistance and gas response of pure In2 O3 hollow spheres significantly change and deteriorate in humid atmospheres. In contrast, the humidity dependence becomes negligible when an optimal concentration of CeO2 nanoclusters is uniformly loaded onto In2 O3 hollow spheres via layer-by-layer (LBL) assembly. Moreover, In2 O3 sensors LBL-coated with CeO2 nanoclusters show fast response/recovery, low detection limit (500 ppb), and high selectivity to acetone even in highly humid conditions (relative humidity 80%). The mechanism underlying the dynamic refreshing of the In2 O3 sensing surfaces regardless of humidity variation is investigated in relation to the role of CeO2 and the chemical interaction among CeO2 , In2 O3 , and water vapor. This strategy can be widely used to design high performance gas sensors including disease diagnosis via breath analysis and pollutant monitoring.

Impedance spectroscopic analysis on effects of partial oxidation of TiN bottom electrode and microstructure of amorphous and crystalline HfO2 thin films on their bipolar resistive switching

The electrical resistance switching (RS) properties of amorphous HfO2 (a-HfO2) and crystalline HfO2 (c-HfO2) thin films grown on a TiN substrate via atomic layer deposition were examined using DC current-voltage (I-V) sweep and AC impedance spectroscopic (IS) analyses. The rapid thermal annealing of the a-HfO2 film at 500 °C under a N2 atmosphere for 5 min crystallized the HfO2 film and induced an interfacial TiON barrier layer. The a-HfO2 sample showed fluent bipolar RS performance with a high on/off ratio (∼ 500), whereas the c-HfO2 sample showed a much lower on/off ratio (<∼ 10), but its switching uniformity was better than that of a-HfO2. Such critical differences can be mainly attributed to the absence and presence of the TiON barrier layer in the a-HfO2 and c-HfO2 samples, respectively. The AC IS especially enabled the resistance states of the HfO2/Pt interface and the HfO2/TiN interface to be separately examined during one complete switching cycle of each sample. Although the Pt/c-HfO2 interface has a Schottky barrier in the pristine state, it disappeared once the c-HfO2 was electroformed and was not recovered even after the reset step. In contrast, the Pt/a-HfO2 interface partly recovered the Schottky barrier after the reset.

Ultra-selective detection of sub-ppm-level benzene using Pd–SnO2 yolk–shell micro-reactors with a catalytic Co3O4 overlayer for monitoring air quality

Ultra-selective and sensitive detection of carcinogenic benzene vapor with negligible interferences from other major indoor pollutants is not only critical but also challenging because the BTX gases (benzene, toluene, and xylene) are generally less reactive to a majority of n-type oxide semiconductor gas sensors and the similar chemical structures of BTX gases hamper their discrimination by chemiresistive variation. Through this paper, we propose a new strategy to detect sub-ppm-level benzene vapor in a highly selective manner using oxide semiconductor chemiresistors. A Pd-loaded SnO2 yolk–shell sensing film coated with a thin catalytic Co3O4 overlayer showed an ultrahigh response (resistance ratio = 88) to 5 ppm benzene with negligibly low cross-responses to the other representative and ubiquitous indoor pollutants such as toluene, xylene, HCHO, CO, and ethanol. The response to benzene vapor was significantly enhanced by reforming of highly stable benzene into more active and smaller species. The reforming was synergistically assisted by the Co3O4 catalytic overlayer and sensing layer consisting of Pd-loaded SnO2 micro-reactors, while the cross-responses to the other indoor pollutants became low because of the catalytic oxidation of the gases into less- or non-reactive species. This method will pave a new way to the precise monitoring of critically harmful benzene in both indoor and outdoor atmospheres.