Sang Sub Kim

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.

MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity

Gas sensors are of a great interest for applications including toxic or explosive gases detection in both in-house and industrial environments, air quality monitoring, medical diagnostics, or control of food/cosmetic properties. In the area of semiconductor metal oxides (SMOs)-based sensors, a lot of effort has been devoted to improve the sensing characteristics. In this work, we report on a general methodology for improving the selectivity of SMOx nanowires sensors, based on the coverage of ZnO nanowires with a thin ZIF-8 molecular sieve membrane. The optimized ZnO@ZIF-8-based nanocomposite sensor shows markedly selective response to H2 in comparison with the pristine ZnO nanowires sensor, while showing the negligible sensing response to C7H8 and C6H6. This original MOF-membrane encapsulation strategy applied to nanowires sensor architecture pave the way for other complex 3D architectures and various types of applications requiring either gas or ion selectivity, such as biosensors, photo(catalysts), and electrodes.

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.

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.

Investigations on the structural, optical and electronic properties of Nd doped ZnO thin films

We report the synthesis and characterization of Nd doped ZnO thin films grown on Si (1 0 0) substrates by the spray pyrolysis method. The surface morphology of these thin films was investigated by scanning electron microscopy and shows the presence of randomly distributed structures of nanorods. Grazing angle x-ray diffraction studies confirm that the doped Nd ions occupied Zn sites and these samples exhibited a wurtzite hexagonal-like crystal structure similar to that of the parent compound, ZnO. The micro-photoluminescence measurement shows a decrease in the near band edge position with Nd doping in the ZnO matrix due to the impurity levels. The near-edge x-ray absorption fine structure (NEXAFS) measurements at the O K edge clearly exhibit a pre-edge spectral feature which evolves with Nd doping, suggesting incorporation of more charge carriers in the ZnO system and the presence of strong hybridization between O 2p‐Nd 5d orbitals. The Nd M5 edge NEXAFS spectra reveal that the Nd ions are in the trivalent state. (Some figures in this article are in colour only in the electronic version)

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.

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.

A model for the enhancement of gas sensing properties in SnO2–ZnO core–shell nanofibres

Sensing properties of the sensors fabricated with SnO2–ZnO core–shell nanofibres were investigated in terms of CO in a comparative manner with the sensors with normal ZnO and SnO2 nanofibres. The results clearly demonstrated that the sensing properties were sharply enhanced by forming a ZnO shell layer on the SnO2 fibres. The mechanism for the enhancement seems to be related to the formation of a fully depleted shell layer due to the presence of heterojunction between the ZnO shell and the SnO2 core fibre. This fully depleted shell layer is likely to pronounce the change in resistance during adsorption and desorption of gaseous species.