Jae-Hyoung Lee

Extraordinary Improvement of Gas-Sensing Performances in SnO2 Nanofibers Due to Creation of Local p–n Heterojunctions by Loading Reduced Graphene Oxide Nanosheets

We propose a novel approach to improve the gas-sensing properties of n-type nanofibers (NFs) that involves creation of local p-n heterojunctions with p-type reduced graphene oxide (RGO) nanosheets (NSs). This work investigates the sensing behaviors of n-SnO2 NFs loaded with p-RGO NSs as a model system. n-SnO2 NFs demonstrated greatly improved gas-sensing performances when loaded with an optimized amount of p-RGO NSs. Loading an optimized amount of RGOs resulted in a 20-fold higher sensor response than that of pristine SnO2 NFs. The sensing mechanism of monolithic SnO2 NFs is based on the joint effects of modulation of the potential barrier at nanograin boundaries and radial modulation of the electron-depletion layer. In addition to the sensing mechanisms described above, enhanced sensing was obtained for p-RGO NS-loaded SnO2 NFs due to creation of local p-n heterojunctions, which not only provided a potential barrier, but also functioned as a local electron absorption reservoir. These mechanisms markedly increased the resistance of SnO2 NFs, and were the origin of intensified resistance modulation during interaction of analyte gases with preadsorbed oxygen species or with the surfaces and grain boundaries of NFs. The approach used in this work can be used to fabricate sensitive gas sensors based on n-type NFs.

Nanostructural and photoluminescence features of nanoporous silicon prepared by anodic etching

The nanostructural and photoluminescence (PL) features of nanoporous Si (NPS) were investigated in terms of various process parameters such as current density, etching time and oxidation conditions. The NPS was prepared by electrochemical anodic etching of p-type (0 0 1) Si wafers of 4 Ω cm resistivity in HF solution. The pores are of polygon-type columns with 5, 6 and 7 side walls. The average diameter of the column-shaped pores is critically determined by the current density, while the etching time plays an important role on the pore depth; in particular, when the current densities of 30 and 100 mA/cm 2 were applied, the pore diameters were ∼9 nm and 3.3 μm, respectively. The variation in the PL characteristics of the NPS with oxidation condition and etching current density was measured and then related with their structural changes. The aging and thermal treatments produce oxidation and lattice distortion in the NPS. The degree of deviation from the as-prepared NPS during aging or thermal treatment seems to depend on the nanostructure as well as morphology of the NPS. It is found in this study that etching current density plays an important role on such structural features of the NPS.

Synthesis and gas sensing properties of membrane template-grown hollow ZnO nanowires

One-dimensional, hollow nanostructured materials are among the most promising materials for sensing applications owing to their high surface area that facilitates the adsorption of target gases. Accordingly, for gas sensing studies, hollow ZnO nanowires (NWs) with different surface areas were successfully synthesized herein by using polycarbonate membranes with different pore sizes as templates, and deposition of ZnO via the atomic layer deposition technique. The sensing properties of the synthesized hollow ZnO NWs were examined for CO and NO2, revealing their comparative sensing performances with ZnO nanomaterials-based sensors reported in literature. This study highlights a novel way of synthesizing hollow ZnO NWs by using membrane template and their promising sensing properties as well.

Electrowetting on dielectric (EWOD) properties of Teflon-coated electrosprayed silica layers in air and oil media and the influence of electric leakage

This paper reports the electrowetting (EW) properties of silica layers. Rough silica layers were deposited on a Si substrate by the electrospraying of SiO2 for different times (10 s–10 min) and subsequently Teflon coated with different thicknesses (35 and 625 nm). The surface roughness measurements revealed an increase in roughness with increasing electrospraying time. A thinner (35 nm) Teflon-coated layer showed better EW properties and was selected for subsequent studies. By applying different voltages (0–200 V) to the thinner Teflon-coated rough SiO2 layers, the water contact angle (WCA) decreased continuously and the SiO2 layer that had been electrosprayed for 20 s showed the maximum decrease in WCA in air, whereas the wetting behavior changed from hydrophobic to hydrophilic under a voltage of 200 V. In addition, the EW behavior of the SiO2 layers in oil ambient was studied and it was found that under the maximum applied voltage (150 V), both 20 and 30 s electrosprayed layers exhibited hydrophilic behavior. These results showed that electric leaking currents, which are largely ignored, can be used to tailor the relationship between the WCA and applied voltage: from a non-linear (V2) to a linear (V) function. In particular, a decrease in the WCA equals the applied voltage normalized to the maximum (or breakdown) external voltage. This study opens a new research field in materials chemistry with regards to the coupling of electrochemical, thermodynamic and mechanical interactions.

High-Performance Nanowire Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and Metal–Organic Framework Membranes

Herein, we report the fabrication of hydrogen gas sensors with enhanced sensitivity and excellent selectivity. The sensor device is based on the strategic combination of ZnO nanowires (NWs) decorated with palladium nanoparticles (Pd NPs) and a molecular sieve metal-organic framework (MOF) nanomembrane (ZIF-8). The Pd NPs permit the sensors to reach maximal signal responses, whereas the ZIF-8 overcoat enables for an excellent selectivity. Three steps were employed for the fabrication: (i) coating of a miniaturized sensor with vapor-grown ZnO NWs, (ii) decoration of these NWs with Pd NPs by atomic layer deposition, and (iii) partial solvothermal conversion of the tuned NWs surface to ZIF-8 nanomembrane. The microstructure and composition investigations of the ZIF-8/Pd/ZnO nanostructured materials confirmed the presence of both metallic Pd NPs and uniform ZIF-8 thin membrane layer. The integration of these nanomaterials within a miniaturized sensor device enabled the assessment of their performance for H2 detection at concentrations as low as 10 ppm in the presence of various gases such as C6H6, C7H8, C2H5OH, and CH3COCH3. Remarkably high-response signals of 3.2, 4.7, and 6.7 ( Ra/ Rg) have been measured for H2 detection at only 10, 30, and 50 ppm, whereas no noticeable response toward other tested gases was detected, thus confirming the excellent H2 selectivity obtained with such a sensor design. The results obtained showed that the performance of gas sensors toward H2 gas can be greatly increased by both the addition of Pd NPs and the use of ZIF-8 coating, acting as a molecular sieve membrane. Furthermore, the presented strategy could be extended toward the sensing of other species by a judicious choice of both the metallic NPs and MOF materials with tuned properties for specific molecule detection, thus opening a new avenue for the preparation of highly selective sensing devices.