In this study, laser-induced graphene oxide (LIGO) was synthesized through a facile liquid-based process involving the introduction of deionized (DI) water onto polyimide (PI) film and subsequent direct laser irradiation using a CO2 laser (λ = 10.6 μm). The synthesized LIGO was then evaluated as a sensing material for monitoring changes in humidity levels. The synthesis conditions were optimized by precisely controlling the laser scribing speed, leading to the synthesis of LIGO with different structural characteristics and varying oxygen contents. The increased number of oxygen-containing functional groups contributed to the hydrophilic properties of LIGO, resulting in a superior humidity sensing capabilities compared with laser-induced graphene (LIG). The LIGO-based sensors outperformed LIG-based sensors, demonstrating approximately tenfold higher sensing responsivity when detecting changes at each humidity level, along with 1.25 to 1.75 times faster response/recovery times, making LIGO-based sensors more promising for humidity-monitoring applications. This study demonstrated laser ablation in a renewable and natural precursor as an eco-friendly and energy-efficient approach to directly synthesize LIGO with controllable oxidation levels.

We report the synthesis and gas sensing properties of bare and ZnO decorated TeO2 nanowires (NWs). A catalyst assisted-vapor-liquid-solid (VLS) growth method was used to synthesize TeO2 NWs and ZnO decoration was performed using an Au-catalyst assisted-VLS growth method followed by a subsequent heat treatment. Structural and morphological analyses using X-ray diffraction (XRD) and scanning/transmission electron microscopies, respectively, demonstrated the formation of bare and ZnO decorated TeO2 NWs with desired phase and morphology. NO2 gas sensing studies were performed at different temperatures ranging from 50 to 400 oC towards 50 ppm NO2 gas. The results obtained showed that both sensors had their best optimal sensing temperature at 350 oC, while ZnO decorated TeO2 NWs sensor showed much better sensitivity towards NO2 relative to a bare TeO2 NWs gas sensor. The reason for the enhanced sensing performance of the ZnO decorated TeO2 NWs sensor was attributed to the formation of ZnO (n)/ TeO2 (p) heterojunctions and the high intrinsic gas sensing properties of ZnO.

The gas response characteristic toward C2H5OH has been demonstrated in terms of copper-vacancy concentration, hole density, and microstructural factors for undoped/Li(I)-doped CuO thin films prepared by sol-gel method. For the films, both concentrations of intrinsic copper vacancies and electronic holes decrease with increasing calcination temperature from 400 to 500 to 600 oC. Li(I) doping into CuO leads to the reduction of copper-vacancy concentration and the enhancement of hole density. The increase of calcination temperature or Li(I) doping concentration in the film increases both optical band gap energy and Cu2p binding energy, which are characterized by UV-vis-NIR and X-ray photoelectron spectroscopy, respectively. The overall hole density of the film is determined by the offset effect of intrinsic and extrinsic hole densities, which depend on the calcination temperature and the Li(I) doping amount, respectively. The apparent resistance of the film is determined by the concentration of the structural defects such as copper vacancies, Li(I) dopants, and grain boundaries, as well as by the hole density. As a result, it is found that the gas response value of the film sensor is directly proportional to the apparent sensor resistance.

In2O3 doped WO3 powders were prepared by a polymer solution route and their NO2 gas sensing properties were analyzed. The synthesized powders showed nano-sized particles with specific surface areas of 6.01~21.5 m2/g and the particle size and shape changed according to the content of In2O3. The gas sensors fabricated with the synthesized powders were tested at operating temperatures of 400~500 oC and 100~500 ppm concentrations of NO2 atmosphere. The particle size and In2O3 content affected on the initial sensor resistance in an air atmosphere. The highest sensitivity (8.57 at 500 oC), which was 1.77 higher than the sensor consisting of the pure WO3 sample, was measured in the 0.5 mol% In2O3 doping sample. In addition, the response time and recovery time were improved by the addition of In2O3.

Polyacrylonitrile/pitch nanofibers were prepared by electrospinning as a precursor for a gas sensor material. Pitch nanofibers were properly fabricated by incorporating polyacrylonitrile as an electrospinning supplement component. Polyacrylonitrile/pitch nanofibers were activated with steam at various temperatures followed by subsequent carbonization to make carbon nanofibers with a highly conductive graphitic structure. Steam activation was effective in facilitating gas adsorption onto the carbon nanofibers due to the increased surface area. The carbon nanofibers activated at 800°C had a larger surface area and a lower micro pore fraction resulting in a higher variation in electrical resistance for improved CO gas sensing properties.

We present the rectifying and nitrogen monoxide (NO) gas sensing properties of an oxide semiconductor heterostructure composed of n-type zinc oxide (ZnO) and p-type copper oxide thin layers. A CuO thin layer was first formed on an indium-tin-oxide-coated glass substrate by sol-gel spin coating method using copper acetate monohydrate and diethanolamine as precursors; then, to form a p-n oxide heterostructure, a ZnO thin layer was spin-coated on the CuO layer using copper zinc dihydrate and diethanolamine. The crystalline structures and microstructures of the heterojunction materials were examined using X-ray diffraction and scanning electron microscopy. The observed current-voltage characteristics of the p-n oxide heterostructure showed a non-linear diode-like rectifying behavior at various temperatures ranging from room temperature to 200 oC. When the spin-coated ZnO/CuO heterojunction was exposed to the acceptor gas NO in dry air, a significant increase in the forward diode current of the p-n junction was observed. It was found that the NO gas response of the ZnO/CuO heterostructure exhibited a maximum value at an operating temperature as low as 100 oC and increased gradually with increasing of the NO gas concentration up to 30 ppm. The experimental results indicate that the spin-coated ZnO/CuO heterojunction structure has significant potential applications for gas sensors and other oxide electronics.

The development of glucose biosensors has been attracting much attention because of their importance in monitoring glucose in the human body; such sensors are used to diagnose diabetes and related human diseases. Thanks to the high selectivity, sensitivity to glucose detection, and relatively low-cost fabrication of enzyme-immobilized electrochemical glucose sensors, these devices are recognized as one of the most intensively investigated glucose sensor types. In this work, ZnO nanofibers were synthesized using an electrospinning method with polyvinyl alcohol zinc acetate as precursor material. Using the synthesized ZnO nanofibers, we fabricated glucose biosensors in which glucose oxidase was immobilized on the ZnO nanofibers. The sensors were used to detect a wide range of glucose from 10 to 700 M with a sensitivity of 10.01 nA/cm2- μM, indicating that the ZnO nanofiber-based glucose sensor can be used for the detection of glucose in the human body. The control of nanograins in terms of the size and crystalline quality of the individual nanofibers is required for improving the glucose-sensing abilities of the nanofibers.

Impedancemetric NOx (NO and NO2) gas sensors were designed with a stacked-layer structure and fabricated using LaCrxCo1-xO3 (x = 0, 0.2, 0.5, 0.8 and 1) as the receptor material and Li1.3Al0.3Ti1.7(PO4)3 plates as the solid-electrolyte transducer material. The LaCrxCo1-xO3 layers were prepared with a polymeric precursor method that used ethylene glycol as the solvent, acetyl acetone as the chelating agent, and polyvinylpyrrolidone as the polymer additive. The effects of the Co concentration on the structural, morphological, and NOx sensing properties of the LaCrxCo1-xO3 powders were investigated with powder Xray diffraction, field emission scanning electron microscopy, and its response to 20~250 ppm of NOx at 400 oC (for 1 kHz and 0.5 V), respectively. When the as-prepared precursors were calcined at 700 oC, only a single phase was detected, which corresponded to a perovskite-type structure. The XRD results showed that as the Co concentration of the LaCrxCo1-xO3 powders increased, the crystal structure was transformed from an orthorhombic phase to a rhombohedral phase. Moreover, the LaCrxCo1-xO3 powders with 0 ≤ x < 0.8 had a rhombohedral symmetry. The size of the particles in the LaCrxCo1-xO3 powders increased from 0.1 to 0.5 μm as the Co concentration increased. The sensing performance of the stack-structured LaCrxCo1-xO3/Li1.3Al0.3Ti1.7(PO4)3 sensors was found to divide the impedance component between the resistance and capacitance. The response of these sensors to NO gas was more sensitive than that to NO2 gas. Compared to other impedancemetric sensors, the LaCr0.8Co0.2O3/Li1.3Al0.3Ti1.7(PO4)3 sensor exhibited good reversibility and reliable sensingresponse properties for NOx gases.

We present the detection characteristics of nitrogen monoxide(NO) gas using p-type copper oxide(CuO) thin film gas sensors. The CuO thin films were fabricated on glass substrates by a sol-gel spin coating method using copper acetate hydrate and diethanolamine as precursors. Structural characterizations revealed that we prepared the pure CuO thin films having a monoclinic crystalline structure without any obvious formation of secondary phase. It was found from the NO gas sensin measurements that the p-type CuO thin film gas sensors exhibited a maximum sensitivity to NO gas in dry air at an operating temperature as low as 100 oC. Additionally, these CuO thin film gas sensors were found to show reversible and reliable electrical response to NO gas in a range of operating temperatures from 60 oC to 200 oC. It is supposed from these results that the ptype oxide semiconductor CuO thin film could have significant potential for use in future gas sensors and other oxide electronics applications using oxide p-n heterojunction structures.

We report the effect of the fabric of the surface microstructure on the CO gas sensing properties of SnO2 thin films deposited on self-assembled Au nanodots (SnO2/Au) that were formed on SiO2/Si substrates. We characterized structural and morphological properties, comparing them to those of SnO2 thin films deposited directly onto SiO2/Si substrates. We observed a significant enhancement of CO gas sensing properties in the SnO2/Au gas sensors, specifically exhibiting a high maximum response at 200˚C and quite a low detection limit of 1 ppm level in dry air. In particular, the response of the SnO2/Au gas sensor was found to reach the maximum value of 32.5 at 200˚C, which is roughly 27 times higher than the response (~1.2) of the SnO2 gas sensor obtained at the same operating temperature of 200˚C. Furthermore, the SnO2/Au gas sensors displayed very fast response and recovery behaviors. The observed enhancement in the CO gas sensing properties of the SnO2/Au sensors is mainly ascribed to the formation of a nanostructured morphology in the active SnO2 layer having a high specific surface-reaction area by the insertion of a nanodot form of Au nucleation layer.

We report the nitrogen monoxide (NO) gas sensing properties of p-type CuO-nanorod-based gas sensors. We synthesized the p-type CuO nanorods with breadth of about 30 nm and length of about 330 nm by a hydrothermal method using an as-deposited CuO seed layer prepared on a Si/SiO2 substrate by the sputtering method. We fabricated polycrystalline CuO nanorod arrays at 80˚C under the hydrothermal condition of 1:1 morality ratio between copper nitrate trihydrate [Cu(NO2)2·3H2O] and hexamethylenetetramine (C6H12N4). Structural characterizations revealed that we prepared the pure CuO nanorod array of a monoclinic crystalline structure without any obvious formation of secondary phase. It was found from the gas sensing measurements that the p-type CuO nanorod gas sensors exhibited a maximum sensitivity to NO gas in dry air at an operating temperature as low as 200˚C. We also found that these CuO nanorod gas sensors showed reversible and reliable electrical response to NO gas at a range of operating temperatures. These results would indicate some potential applications of the p-type semiconductor CuO nanorods as promising sensing materials for gas sensors, including various types of p-n junction gas sensors.

[ LaMeO3 ](Me = Cr, Co) powders were prepared using the polymeric precursor method. The effects of the chelating agent and the polymeric additive on the synthesis of the LaMeO3 perovskite were studied. The samples were synthesized using ethylene glycol (EG) as the solvent, acetyl acetone (AcAc) as the chelating agent, and polyvinylpyrrolidone (PVP) as the polymer additive. The thermal decomposition behavior of the precursor powder was characterized using a thermal analysis (TG-DTA). The crystallization and particle sizes of the LaMeO3 powders were investigated via powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and particle size analyzer, respectively. The as-prepared precursor primarily has LaMeO3 at the optimum condition, i.e. for a molar ratio of both metal-source (a : a) : EG (80a : 80a) : AcAc (8a) inclusive of 1 wt% PVP. When the as-prepared precursor was calcined at 700˚C, only a single phase was observed to correspond with the orthorhombic structure of LaCrO3 and the rhombohedral structure of LaCoO3. A solid-electrolyte impedance-metric sensor device composed of Li1.5Al0.5Ti1.5(PO4)3 as a transducer and LaMeO3 as a receptor has been systematically investigated for the detection of NOx in the range of 20 to 250 ppm at 400˚C. The sensor responses were able to divide the component between resistance and capacitance. The impedance-metric sensor for the NO showed higher sensitivity compared with NO2. The responses of the impedance-metric sensor device showed dependence on each value of the NOx concentration.

Ni 8 wt.%-doped tin oxide (SnO2) thick films were fabricated into gas sensors by the method of screen printing onto alumina substrates. The particle size of SnO2 was controlled by changing the ball-mill time between 0~120 h. The structural and morphological properties of these thick films were investigated using X-ray diffraction and scanning electron microscopy. The structural properties of SnO2 powders showed a tetragonal phase with (110) dominant orientation. The particle size of the SnO2:Ni powders after ball-mill of 120 h was about 0.05 μm. The gas sensitivity (S = Rg/Ra) to 5 ppm CH4 gas and CH3CH2CH3 gas was measured at room temperature by comparing the resistance in air (Ra) with that of the target gases (Rg). The sensitivity of the SnO2 gas sensors was enhanced by increasing the ball-mill time. There was an association between the sensitivity of both the CH4 gas and the CH3CH2CH3 gas and the particle size of the SnO2. SnO2 gas sensors prepared by 72 h ball-mill showed a sensitivity of about 13 to 5 ppm CH4 gas and CH3CH2CH3 gas. The response time of the SnO2:Ni gas sensors to the CH4 gas was about 20 seconds.

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 SiO2 substrates followed by sputter deposition of Zn and thermal oxidation at 400˚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˚C. The highest sensor responses were observed at 300˚C in ZnO film and 250˚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˚C.

A cobalt oxide - tin oxide nanocomposite based gas sensor on an SiO2 substrate was fabricated. Granular thin film of tin oxide was formed by a rheotaxial growth and thermal oxidation method using dc magnetron sputtering of Sn. Nano particles of cobalt oxide were spin-coated on the tin oxide. The cobalt oxide nanoparticles were synthesized by polymer-assisted deposition method, which is a simple cost-effective versatile synthesis method for various metal oxides. The thickness of the film can be controlled over a wide range of thicknesses. The composite structures thus formed were characterized in terms of morphology and gas sensing properties for reduction gas of H2. The composites showed a highest response of 240% at 250˚C upon exposure to 4% H2. This response is higher than those observed in pure SnO2 (90%) and Co3O4 (70%) thin films. The improved response with the composite structure may be related to the additional formation of electrically active defects at the interfaces. The composite sensor shows a very fast response and good reproducibility.

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.

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.