• Korean Journal of Materials Research
• /
• v.19 no.11
• /
• pp.607-612
• /
• 2009

We investigated the effects of Co doping on the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with ZnO using pulsed laser deposition. Structural examinations clearly confirmed a distinct nanostructure of the CNTs coated with ZnO nanoparticles of an average diameter as small as 10 nm and showed little influence of doping 1 at.% Co into ZnO on the morphology of the ZnO-CNT composites. It was found from the gas sensing measurements that 1 at.% Co doping into ZnO gave rise to a significant improvement in the response of the ZnO-CNT composite sensor to NO gas exposure. In particular, the Co-doped ZnO-CNT composite sensor shows a highly sensitive and fast response to NO gas at relatively low temperatures and even at low NO concentrations. The observed significant improvement of the NO gas sensing properties is attributed to an increase in the specific surface area and the role as a catalyst of the doped Co elements. These results suggest that Co-doped ZnOCNT composites are suitable for use as practical high-performance NO gas sensors.

• Korean Journal of Materials Research
• /
• v.18 no.12
• /
• pp.655-659
• /
• 2008

The NO gas sensing properties of ZnO-carbon nanotube (ZnO-CNT) composites fabricated by the coaxial coating of single-walled CNTs with ZnO were investigated using pulsed laser deposition. Upon examination, the morphology and crystallinity of the ZnO-CNT composites showed that CNTs were uniformly coated with polycrystalline ZnO with a grain size as small as 5-10 nm. Gas sensing measurements clearly indicated a remarkable enhancement of the sensitivity of ZnO-CNT composites for NO gas compared to that of ZnO films while maintaining the strong sensing stability of the composites, properties that CNT-based sensing materials do not have. The enhanced gas sensing properties of the ZnO-CNT composites are attributed to an increase in the surface adsorption area of the ZnO layer via the coating by CNTs of a high surface-to-volume ratio structure. These results suggest that the ZnO-CNT composite is a promising template for novel solid-state semiconducting gas sensors.

• Bulletin of the Korean Chemical Society
• /
• v.30 no.10
• /
• pp.2387-2392
• /
• 2009

A composite film of multi-walled carbon nanotube (MWCNT)/sol-gel-derived zinc oxide(ZnO)/Nafion has been utilized as an efficient immobilization matrix for the construction of a highly sensitive and stable tris(2,2'-bipyridyl) ruthenium(II) (Ru(${bpy)_3}^{2+})$ electrogenerated chemiluminescence (ECL) sensor. The electrochemical and ECL behaviors of Ru(${bpy)_3}^{2+})$ ion-exchanged into the composite film were strongly dependent upon the sol-gel preparation condition, the amount of MWCNT incorporated into the ZnO/Nafion composite film, and the buffer solution pH. The synergistic effect of MWCNTs and ZnO in the composite films increased not only the sensitivity but also the long-term stability of the ECL sensor. The present ECL sensor based on the MWCNT/ZnO/Nafion gave a linear response ($R^2$ = 0.999) for tripropylamine concentration from 500 nM to 1.0 mM with a remarkable detection limit (S/N = 3) of 15 nM. The present ECL sensor showed outstanding long-term stability (94% initial signal retained for 5 weeks). Since the present ECL sensor exhibits large response towards NADH, it could be applied as a transduction platform for the ECL biosensor in which the NADH is produced from the dehydrogenase-based enzymatic reaction in the presence of NA$D^+$ cofactor.

• Korean Journal of Materials Research
• /
• v.22 no.5
• /
• pp.237-242
• /
• 2012

We report on the NO gas sensing properties of Al-doped zinc oxide-carbon nanotube (ZnO-CNT) wire-like layered composites fabricated by coaxially coating Al-doped ZnO thin films on randomly oriented single-walled carbon nanotubes. We were able to wrap thin ZnO layers around the CNTs using the pulsed laser deposition method, forming wire-like nanostructures of ZnO-CNT. Microstructural observations revealed an ultrathin wire-like structure with a diameter of several tens of nm. Gas sensors based on ZnO-CNT wire-like layered composites were found to exhibit a novel sensing capability that originated from the genuine characteristics of the composites. Specifically, it was observed by measured gas sensing characteristics that the gas sensors based on ZnO-CNT layered composites showed a very high sensitivity of above 1,500% for NO gas in dry air at an optimal operating temperature of $200^{\circ}C$; the sensors also showed a low NO gas detection limit at a sub-ppm level in dry air. The enhanced gas sensing properties of the ZnO-CNT wire-like layered composites are ascribed to a catalytic effect of Al elements on the surface reaction and an increase in the effective surface reaction area of the active ZnO layer due to the coating of CNT templates with a higher surface-to-volume ratio structure. These results suggest that ZnO-CNT composites made of ultrathin Al-doped ZnO layers uniformly coated around carbon nanotubes can be promising materials for use in practical high-performance NO gas sensors.

• Korean Journal of Materials Research
• /
• v.19 no.11
• /
• pp.631-636
• /
• 2009

We investigated the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with a thin layer of 1 wt% Al-doped ZnO using rf magnetron sputtering deposition. Morphological studies clearly revealed that the ZnO appeared to form beadshaped crystalline nanoparticles with an average diameter as small as 30 nm, attaching to the surface of the nanotubes. It was found that the NO gas sensing properties of the ZnO-CNT layered composites were dramatically improved over Al-doped ZnO thin films. It is reasoned from these observations that an increase in the surface-to-volume ratio associated with the numerous ZnO “nanobeads” on the surface of the CNTs results in the enhancement of the NO gas sensing properties. The ZnO-CNT layered composite sensors exhibited a maximum sensitivity of 13.7 to 2 ppm NO gas at a temperature of 200${^{\circ}C}$ and a low NO gas detection limit of 0.2 ppm in dry air.

• Korean Journal of Materials Research
• /
• v.20 no.11
• /
• pp.623-627
• /
• 2010

Semiconducting metal oxides have been frequently used as gas sensing materials. While zinc oxide is a popular material for such applications, structures such as nanowires, nanorods and nanotubes, due to their large surface area, are natural candidates for use as gas sensors of higher sensitivity. The compound ZnO has been studied, due to its chemical and thermal stability, for use as an n-type semiconducting gas sensor. ZnO has a large exciton binding energy and a large bandgap energy at room temperature. Also, ZnO is sensitive to toxic and combustible gases. The NO gas properties of zinc oxide-single wall carbon nanotube (ZnO-SWCNT) composites were investigated. Fabrication includes the deposition of porous SWCNTs on thermally oxidized $SiO_2$ substrates followed by sputter deposition of Zn and thermal oxidation at $400^{\circ}C$ in oxygen. The Zn films were controlled to 50 nm thicknesses. The effects of microstructure and gas sensing properties were studied for process optimization through comparison of ZnO-SWCNT composites with ZnO film. The basic sensor response behavior to 10 ppm NO gas were checked at different operation temperatures in the range of $150-300^{\circ}C$. The highest sensor responses were observed at $300^{\circ}C$ in ZnO film and $250^{\circ}C$ in ZnO-SWCNT composites. The ZnO-SWCNT composite sensor showed a sensor response (~1300%) five times higher than that of pure ZnO thin film sensors at an operation temperature of $250^{\circ}C$.

• Korean Journal of Materials Research
• /
• v.18 no.10
• /
• pp.529-534
• /
• 2008

The hydrogen gas sensing properties of a zinc oxide nanowire structure were studied. Porous zinc oxide nanowire structures were fabricated by oxidizing zinc deposited on a single-wall carbon nanotube (SWNT) template. This revealed a porous ZnO-SWNT composite due to the porosity in the SWNT film. The gas sensing properties were compared with those of zinc oxide thin films deposited on SiO2/Si substrates in sensitivity and operating temperature. The composite structure showed higher sensitivity and lower operating temperature than the zinc oxide film. It showed a response even at room temperature while the film structure did not.

• Journal of Sensor Science and Technology
• /
• v.30 no.6
• /
• pp.369-375
• /
• 2021

In this study, heterogeneous ZnO/CNTF composites were developed to improve the NO₂-sensing response, facilitated by the self-heating property. Highly conductive and mechanically stable CNTFs were prepared by a wet-spinning process assisted by the liquid crystal (LC) behavior of CNTs. Metal-organic frameworks (MOFs) of ZIF-8 were precipitated on the surface of the CNTF (ZIF-8/CNTF) via one-pot synthesis in solution. The subsequent calcination process resulted in the formation of the ZnO/CNTF composites. The calcination temperatures were controlled at 400, 500, and 600 ℃ in an N₂ atmosphere to confirm the evolution of the microstructures and NO₂-sensing properties. Gas sensor characterization was performed at 100 ℃ by applying a DC voltage to induce Joule heating through the CNTF. The results revealed that the ZnO/CNTF composite after calcination at 500 ℃ (ZnO/CNTF-500) exhibited an improved response (R_air/R_gas = 1.086) toward 20 ppm NO₂ as compared to the pristine CNTF (R_air/R_gas = 1.063). Selective NO₂-sensing properties were demonstrated with negligible responses toward interfering gas species such as H₂S, NH₃, CO, and toluene. Our approach for the synthesis of MOF-driven ZnO/CNTF composites can provide a new strategy for the fabrication of wearable gas sensors integrated with textile materials.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.29 no.5
• /
• pp.312-316
• /
• 2016

In this study, we fabricated $NO_x$ gas sensor by using multi-walled carbon nanotubes(MWCNT)/zinc oxide(ZnO) composite film. Carbon nanotubes (CNTs) have good electronic, chemical-stability, and sensitivity characteristics. And zinc oxide (ZnO) is a wide band gap and large exciton binding energy semiconductor. In particular, gas sensors require characteristics such as high speed, sensitivity, and selectivity. The fabricated gas sensor was used to detect $NO_x$ gas for different values of the $NO_x$ gas concentrations. The gas sensor that absorbed$NO_x$ gas molecules showed a increasing in resistance. The sensitivity of the gas sensor was increased by increasing the gas concentrations. Additionally, while changing the temperature inside the chamber for the MWCNT/ZnO composite film gas sensor, we obtained the sensitivity. And the comparison analysis to ZnO film gas sensor for detecting $NO_x$ gas. From the experiment result, we confirmed improvement of $NO_x$ gas detection characteristics using the MWCNT/ZnO composite film.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.31 no.3
• /
• pp.188-192
• /
• 2018

In this study, we fabricated an $NO_X$ gas sensor using a composite film of multi-walled carbon nanotubes (MWCNT)/zinc oxide (ZnO). Carbon nanotubes (CNTs) show good electronic conductivity and chemical-stability, and zinc oxide (ZnO) is a wide band gap semiconductor with a large exciton binding energy. Gas sensors require characteristics such as high speed, sensitivity, and selectivity. The fabricated gas sensor was used to detect $NO_X$ gas at different $NO_X$ concentrations. The sensitivity of the gas sensor increased with increasing gas concentrations. Additionally, while changing the temperature inside the chamber containing the MWCNT/ZnO gas sensor, we obtained the sensitivity and normalized responses for detecting $NO_X$ gas in comparison to ZnO and MWCNT film gas sensors. From the experimental results, we confirmed that the gas sensor sensing mechanism was enhanced in the composite-film gas-sensor and that the electronic interaction between MWCNT and ZnO contributed to the improved sensor performance.