Sun-Woo Choi

Synthesis of SnO2–ZnO core–shell nanofibers via a novel two-step process and their gas sensing properties

SnO2-ZnO core-shell nanofibers were synthesized via a novel two-step process. First, SnO2 nanofibers were synthesized by electrospinning. In sequence, ZnO shell layers were deposited using atomic layer deposition on the electrospinning synthesized SnO2 nanofibers. To demonstrate the practical applications of the synthesized core-shell nanofibers, we investigated their sensing properties to O2 and NO2. The high sensitivity and dynamic repeatability observed in these sensors reveal that the core-shell nanofibers are promising as sensitive and reliable chemical sensors.

Fabrication of a Highly Sensitive Chemical Sensor Based on ZnO Nanorod Arrays.

We report a novel method for fabricating a highly sensitive chemical sensor based on a ZnO nanorod array that is epitaxially grown on a Pt-coated Si substrate, with a top–top electrode configuration. To practically test the device, its O2 and NO2 sensing properties were investigated. The gas sensing properties of this type of device suggest that the approach is promising for the fabrication of sensitive and reliable nanorod chemical sensors.

Dual Functional Sensing Mechanism in SnO2–ZnO Core–Shell Nanowires

We report a dual functional sensing mechanism for ultrasensitive chemoresistive sensors based on SnO2-ZnO core-shell nanowires (C-S NWs) for detection of trace amounts of reducing gases. C-S NWs were synthesized by a two-step process, in which core SnO2 nanowires were first prepared by vapor-liquid-solid growth and ZnO shell layers were subsequently deposited by atomic layer deposition. The radial modulation of the electron-depleted shell layer was accomplished by controlling its thickness. The sensing capabilities of C-S NWs were investigated in terms of CO, which is a typical reducing gas. At an optimized shell thickness, C-S NWs showed the best CO sensing ability, which was quite superior to that of pure SnO2 nanowires without a shell. The dual functional sensing mechanism is proposed as the sensing mechanism in these nanowires and is based on the combination of the radial modulation effect of the electron-depleted shell and the electric field smearing effect.

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.

Significant enhancement of the sensing characteristics of In2O3 nanowires by functionalization with Pt nanoparticles.

We report a significant enhancement in the gas sensing properties of In(2)O(3) nanowires by functionalizing their surfaces with Pt nanoparticles. For Pt-functionalization, In(2)O(3)-Pt core-shell nanowires are synthesized by the sputtering deposition of Pt layers on bare In(2)O(3) nanowires. Next, continuous Pt shell layers are transformed into Pt nanoparticles of cubic phase by heat treatment. In an O(2) gas sensing test, the Pt-functionalized In(2)O(3) nanowires reveal exceptionally higher sensitivity and faster response than bare In(2)O(3) nanowires.

A synthesis and sensing application of hollow ZnO nanofibers with uniform wall thicknesses grown using polymer templates

A novel approach is applied to fabricating hollow ZnO nanofibers (ZNFs) with uniform wall thicknesses. In this approach, polymers synthesized by electrospinning are used as sacrificial templates and ZnO is subsequently deposited on these templates using atomic layer deposition, which makes ZnO uniformly cover the round surface of the polymer nanofibers. Heat treatments result in the selective removal of the polymer templates and the formation of hollow ZNFs with very uniform wall thicknesses. To test a potential use of hollow ZNFs in chemical gas sensors, their sensing properties with regard to O(2), NO(2), and CO are investigated in a comparative manner with those of normal ZnO nanofibers. The excellent sensing properties observed in the hollow ZNF sensor are ascribed to (1) the more pronounced change in resistance due to the presence of nanograins and (2) the doubling of the surface-to-volume ratio due to the generation of inner surfaces.

An approach to detecting a reducing gas by radial modulation of electron-depleted shells in core–shell nanofibers

Based on the radial modulation of electron-depleted shell layers in SnO2–ZnO core–shell nanofibers (CSNs), a novel approach is proposed for the detection of very low concentrations of reducing gases. In this work, SnO2–ZnO CSNs were synthesized by a two-step process: core SnO2 nanofibers were first prepared by electrospinning, followed by the preparation of ZnO shell layers by atomic layer deposition. The radial modulation of electron depletion in the CSN shells was accomplished by controlling the shell thickness. The sensing capabilities of CSNs were investigated with respect to CO and NO2 that represent typical reducing and oxidizing gases, respectively. In the case of CO at a critical shell thickness, the CSN-based sensors showed greatly improved sensing capabilities compared with those fabricated on the basis of either pure SnO2 or pure ZnO nanofibers. In sharp contrast, CSN sensors revealed inferior sensing capabilities for NO2. The results can be explained by a model based on the radial modulation of the electron-depleted CSN shells. The model suggests that CSNs comprising dissimilar materials having different energy-band structures represent an effective sensing platform for the detection of low concentrations of reducing gases when the shell thickness is equivalent to the Debye length.

Functionalization of selectively grown networked SnO2 nanowires with Pd nanodots by γ-ray radiolysis.

γ-ray radiolysis is applied to synthesizing Pd nanodots on networked SnO(2) nanowires. The growth behavior of Pd nanodots is systematically investigated as a function of the precursor concentration, illumination intensity, and exposure time of the γ-rays. These factors greatly influence the growth behavior of the Pd nanodots. Selectively grown networked SnO(2) nanowires are uniformly functionalized with Pd nanodots by the radiolysis process. The NO(2) sensing characteristics of the Pd-functionalized SnO(2) nanowires are compared with those of bare SnO(2) nanowires. The results indicate that γ-ray radiolysis is an attractive means of functionalizing the surface of oxide nanowires with catalytic Pd nanodots. Moreover, the Pd-functionalization greatly enhances the sensitivity and response time in SnO(2) nanowire-based gas sensors.

Prominent Reducing Gas-Sensing Performances of n-SnO2 Nanowires by Local Creation of p–n Heterojunctions by Functionalization with p-Cr2O3 Nanoparticles

A novel approach to improving the reducing gas-sensing properties of n-type nanowires (NWs), by locally creating p-n heterojunctions with p-type nanoparticles (NPs), is proposed. As a model system, this work investigates the sensing behaviors of n-SnO2 NWs functionalized with p-Cr2O3 NPs. Herein, n-SnO2 NWs demonstrate greatly improved reducing gas-sensing performance when functionalized with p-Cr2O3 NPs. Conversely, such functionalization deteriorates the oxidizing gas-sensing properties of n-SnO2 NWs. These phenomena are closely related to the local suppression of the conduction channel of n-type NWs, in the radial direction, beneath the p-n heterojunction by the flow of charge carriers. The approach used in this work can be used to fabricate sensitive reducing-gas sensors based on n-type NWs.

Bi-functional mechanism of H2S detection using CuO–SnO2 nanowires

In this study, a bi-functional mechanism is proposed and validated, which may be used to explain all of the reported experimental observations and to predict new sensing control parameters. Fast response and recovery in H2S sensing was then realized by using bi-functional SnO2 nanowires which have been radially modulated with CuO. Firstly, Cu metal nanoparticles were synthesized by applying γ-ray radiolysis. The Cu nanoparticles (attached to the surface of the SnO2 nanowires) were oxidized to the CuO phase by a thermal treatment at 500 °C in air. The H2S sensing characteristics of the CuO-functionalized SnO2 nanowires were compared with those of bare SnO2 nanowires. The results demonstrated that γ-ray radiolysis is an effective means of functionalizing the surface of oxide nanowires with CuO nanoparticles, and CuO functionalization greatly enhanced the ability of the SnO2 nanowires to detect H2S in terms of the response and recovery times. In addition, two control parameters, a 0.5 CuO to SnO2 surface ratio and a sensing temperature range of 80–220 °C, are predicted. The radially modulated nanostructures achieve two functions: (1) the formation and break-away of p–n (CuO–SnO2) junctions, and (2) the formation and dissolution of CuS using CuO–SnO2 solid solutions.