ZnO-based Gas Sensors Research Articles

Two-dimensional ZnO nanowalls for gas sensor and photoelectrochemical applications

Zinc oxide (ZnO) nanowalls were vertically grown on a glass substrate through a fast, low-temperature, catalyst-free process in a tube furnace. The morphology and microstructure of ZnO nanowalls were characterized by field-emission scanning electron microscopy, x-ray diffraction analysis, transmission electron microscopy, and photoluminescence measurements. The ZnO nanowall films showed strong UV emission and preferential c-axis orientation, with a hexagonal structure. Potential applications of the ZnO nanowalls were further investigated through experiments. The sensitivity of ZnO-based carbon monoxide gas sensors increased with increasing temperature and reached a maximum value at 300°C. In photoelectrochemical experiments, direct photoelectrolysis of water to generate hydrogen was performed using the ZnO nanowalls as the working photoelectrode. For an external bias of 1.2 V, the photocurrent densities reached 0.037 A/cm2 under Xe lamp illumination.

Synthesis and Gas-Sensing Properties of ZnO Porous Microflowers

Three-dimensional (3D) porous nanomaterials with large surface area and abundant surface defects have promis- ing potential in gas sensing due to the following merits: (i) large contact area of gaseous species with the materials, and more adsorption sites on the highly defective surface, (ii) good accessibility to gaseous species of the 3D open structure, (iii) fast electron transport among the 3D network. Rational design and controlled synthesis of the 3D porous nanomaterials with spe- cific morphology and microstructure are essential for improving the performance of gas sensing. In this work, 3D ZnO po- rous microflowers were prepared by directly calcining the zinc-based flowerlike precursor at 400 ℃ in air. The precursor was preformed through a simple coprecipitation method, i.e., by refluxing the aqueous solution of zinc nitrate and co-precipitators of hexamethylenetetramine and oxalic acid at 90 ℃ for 4 h. The unique ZnO porous microflowers were composed of porous nanosheets of 10~50 nanometers in thickness and 1~2 micrometers in width, which inherited from the zinc-based precursor except for the randomly distributed pores on the nanosheet "petals". The 3D porous microstructures endowed ZnO with large specific surface area of 31.3 m 2 •g -1 and abundant surface defects. ZnO porous microflowers were used as active materials to fabricate gas sensors, which exhibited low working temperature, high sensitivity and fast response (recovery) characteristic against ethanol vapor, at the advanced level in comparison with the reported ZnO-based gas sensors for ethanol. The superior performance of the gas sensors could be attributed to the unique microstructures of the ZnO porous nanomaterials. In addition, the sensitivity of the gas sensors showed an exponential relationship with the concentration of ethanol vapor, indicating their capability of quantitative detection within the ethanol volume ratio of 1×10 -6 ~500×10 -6 .

Effect of Embedded Pd Microstructures on the Flat-Band-Voltage Operation of Room Temperature ZnO-Based Liquid Petroleum Gas Sensors

Three methods were used to fabricate ZnO-based room temperature liquid petroleum gas (LPG) sensors having interdigitated metal-semiconductor-metal (MSM) structures. Specifically, devices with Pd Schottky contacts were fabricated with: (1) un-doped ZnO active layers; (2) Pd-doped ZnO active layers; and (3) un-doped ZnO layers on top of Pd microstructure arrays. All ZnO films were grown on p-type Si(111) substrates by the sol-gel method. For devices incorporating a microstructure array, Pd islands were first grown on the substrate by thermal evaporation using a 100 μm mesh shadow mask. We have estimated the sensitivity of the sensors for applied voltage from –5 to 5 V in air ambient, as well as with exposure to LPG in concentrations from 500 to 3,500 ppm at room temperature (300 K). The current-voltage characteristics were studied and parameters such as leakage current, barrier height, reach-through voltage, and flat-band voltage were extracted. We include contributions due to the barrier height dependence on the electric field and tunneling through the barrier for the studied MSM devices. The Pd-enhanced devices demonstrated a maximum gas response at flat-band voltages. The study also revealed that active layers consisting of Pd microstructure embedded ZnO films resulted in devices exhibiting greater gas-response as compared to those using Pd-doped ZnO thin films or un-doped active layers.

ZnO를 기반으로한 에탄올 가스 센서

ZnO-based ethanol gas sensors were fabricated by the painting method and their electrical and ethanol gas sensing characteristics were investigated. The ZnO-Pt (1 wt.%) film heat treated at <TEX>$400^{\circ}C$</TEX>, for 2 hrs. in air showed the highest sensitivity to ethanol gas in air at an operating temperature of <TEX>$250^{\circ}C$</TEX> The sensitivity of the gas sensors to 1000 ppm ethanol in air at <TEX>$250^{\circ}C$</TEX> was 8.7 and rising time and falling time of the gas sensors were 3.12 minutes and 25 minutes, respectively.

A novel ethanol gas sensor based on ZnO-microwire

One-dimensional (1D) ZnO microwires were successfully synthesized by chemical vapor deposition and their structural and morphological properties were analyzed by X-ray diffraction and scanning electron microscopy, demonstrating that the microwires were single crystalline with perfect hexagonal structure and smooth surface. Using these 1D microstructures, we fabricated a novel ZnO-based ethanol gas sensor. Operating at room temperature, the sensor was found to have good sensing characteristics. The reliability and stability of the sensor could be improved by connecting multiple 1-wire devices (1-WD) in parallel into a multi-wires device. In interior natural lighting environment and under 3 V bias, the response and recovery time of the 1-WD to 200 ppm ethanol gas were <10 s and about 300 s, respectively, and the minimum and maximum detection limit were about 2 and 200 ppm, respectively. A sensing model was proposed for discussing the performance of the sensor. The simplicity in fabrication, low power consumption and low cost make the sensor suitable for practical application in many fields, especially in identifying driving under the influence and chemical industry monitoring.

Synthesis of mesoporous Bi2WO6 architectures and their gas sensitivity to ethanol

Uniform hierarchical multilayered Bi2WO6 architectures were synthesized by a facile template-free hydrothermal process, and their synthesis conditions and formation mechanism were carefully investigated. XRD, XPS, FE-SEM, and HR-TEM techniques were employed to determine their phase composition, morphology and microstructure. Nitrogen adsorption and desorption isotherms were conducted to examine the specific surface area and the pore nature of the as-prepared sample. The results show that the as-prepared Bi2WO6 architectures consist of secondary nanoplates and have a mesoporous nest-like morphology with a diameter of 3–4 μm, and have very large specific surface areas. The largest surface area of 47.72 m2 g−1 is achieved when synthesized at 190 °C for 2.0 h. Furthermore, the Bi2WO6 samples were fabricated into a gas sensor, and the experimental results showed that the samples exhibited high sensitivity and a fast response–recovery to ethanol gas at lower temperatures (300 °C). For 100 ppm ethanol, the sensitivity of the best sensor (GS2) was 34.6, which is about 3-fold higher than the reported mesoporous ZnO based gas sensor because its mesoporous structure provided a high surface-to-volume ratio and surface accessibility for the ethanol. A plausible enhancement gas responding mechanism of the nest-like Bi2WO6 sensors was also proposed based on the structure and response–recovery properties.

Preparation and Performance of ZnO Based Gas Sensors Working at Room Temperature

In this presentation, we present the preparation and performance of ZnO gas sensors on glass substrates. The phase and microstructures of the prepared ZnO nanocrystals were characterized with the X-ray diffractometer and transmission electron microscope. The transient response characteristics of the ZnO based sensors were recorded at room temperatures in saturated ethanol gas. It has been observed that the ZnO sensors can be operated at room temperature.

Template-free hydrothermal synthesis of ZnO microrods for gas sensor application

Zinc oxide microrods with controlled diameter were prepared without the addition of template and additive by a simple hydrothermal route only using Zn(CH3COO)2·2H2O as a precursor. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron diffraction (ED). The crystal structure of prepared ZnO microrods is hexagonal phase polycrystalline with zincite structure. With the increase of the precursor concentration from 0.05 M to 0.6 M, the diameter of the ZnO microrods increased from 1 μm to 5 μm. A localized oriented attachment mechanism was prepared to account for the formation of ZnO microrods. The gas-sensing performance experiments indicated that the prepared ZnO microrods exhibited highly sensitive, selective gas-sensing properties, and good stability to acetone vapor. The response and recovery time of ZnO-based gas sensor to 100 ppm acetone vapor are 12 s and 18 s, respectively. The mechanism of the ZnO-based sensor was investigated.

Development of ZnO-based surface plasmon resonance gas sensor and analysis of UV irradiation effect on NO2 desorption from ZnO thin films

In order to perform a high throughput exploration of sensor materials using surface plasmon resonance (SPR), the gas sensing property of a ZnO/Au/SiO2 chip with SPR and the enhancing effect of UV irradiation on the desorption rate of NO2 from the ZnO surface were investigated. When the ZnO/Au/SiO2 chip was exposed to a high concentration of NO2 (1000 ppm), a large peak shift was observed in the SPR curve. However, this sensing signal for NO2 gas did not recover to the baseline. In the case of low-concentration NO2 (10 ppm), the peak shift of the SPR curve was lower than that in the case of the high-concentration gas, but recovery to the baseline was observed. From the X-ray photoelectron spectra for N 1s of the ZnO thin films exposed to 1000- and 10-ppm NO2, two chemisorption states—NO2- (403.7 eV) and NO3- (407 eV)—were confirmed. After the ZnO film was irradiated by UV rays, exposed to 10-ppm NO2, all peaks related to N 1s disappeared. However, in the case of the ZnO film exposed to 1000-ppm NO2, adsorbed NO3- remained on the surface of ZnO. From these results, it was found that UV irradiation effectively assisted NO2 desorption from the surface of the ZnO thin film exposed to 10-ppm NO2.

Synthesis and hydrogen gas sensing properties of ZnO wirelike thin films

The authors investigated the hydrogen gas sensing properties of the ZnO wirelike films synthesized by two consecutive steps: thermal oxidation of sputtered Zn metallic films in dry air. Structural characterization revealed that the authors synthesized polycrystalline wurtzite ZnO films of a wirelike structure with a width of less than 100nm and a length of several micrometers, possibly consisting of a chain of ZnO nanocrystallites with a mean granular size of 15nm. It was found from the gas sensing measurements that the ZnO wirelike films exhibited a maximum sensitivity, defined as a ratio of the change in conductance upon exposure to H2 gas in dry air over the conductance in dry air, ΔG∕Ga, of 283% to 200ppm H2 gas at a temperature of 200°C, indicating quite a possibility of much reduction in the operating temperature of the ZnO-based H2 gas sensors. The ZnO wirelike film sensors showed a fast response to hydrogen gas in comparison with conventional ZnO films. These results suggest that the ZnO wirelike films can be used as the gas sensing materials for low-cost and high-performance gas sensors.

CBD grown ZnO-based gas sensors and dye-sensitized solar cells

Chemical bath deposition (CBD) is used for depositing different metal chalcogenide and oxide thin films through an ion-by-ion or by adsorption of colloidal particles from the solution on the substrate. In this review, we focused our intension on CBD zinc oxide (ZnO) films synthesis with different proposed deposition mechanisms and their integration into gas sensor and dye-sensitized solar cells. Due to the use of CBD ZnO films in gas sensors first time by our group, we presented film thickness, gas concentration and palladium sensitizer effect on the gas sensing properties in details. Review on CBD grown ZnO implication in dye-sensitized solar cells is also discussed in brief.

Studies on alcohol sensing mechanism of ZnO based gas sensors

Nanosized ZnO powder was synthesized by using a chemical precipitation method, and loaded with different dopants through impregnation. The as-prepared ZnO powder was characterized by XRD and TEM. The characterization results show that the as-prepared sample is wurtzite polycrystalline ZnO, the mean grain size is 30–40 nm, and there are three types of adsorbed oxygen (O2−, O22−, and O2−) on the surface of the sample. The as-prepared ZnO powder shows excellent gas responses to alcohol and acetaldehyde, but no response to ethene. The sensing mechanism of ZnO was further studied with the help of gas chromatography (GC) associated with a fixed-bed reactor. The studies show that acetaldehyde, carbon dioxide and water are the only oxidation products of C2H5OH over ZnO. The gas response to C2H5OH is strongly dependent on the conversion ratio of C2H5OH to acetaldehyde. In addition, among all the dopants tested, Ru is the optimal dopant which can increase the response to C2H5OH, but cannot increase the conversion ratio of C2H5OH to acetaldehyde. Thus we suggest that the gas sensing mechanism of ZnO to C2H5OH is the mode controlled by chemisorption of negatively charged oxygen, and the sensitizing role of Ru in the ZnO sensor belongs to the electronic sensitization mechanism.

Zinc oxide ceramic semi-conductor gas sensor for ethanol vapour

Sensitivity characteristics of ZnO-based semi-conductor gas sensors to ethanol vapours have been investigated with lanthanum oxide and palladium. The sensitivity of the sintered elements were promoted by the addition of La 2O 3 and Pd. Further, the sensor operating temperature was reduced with the noble metal. These elements have been tested for cross sensitivity for different gases and it has been confirmed that these are highly sensitive and selective for ethanol vapours at 175°C in air atmosphere. The promoting effect of the sensitivity of the sensor elements with lanthanum oxide seems to be related to the selectivity to oxidation of ethanol vapours.

Zinc-oxide-based semiconductor sensors for detecting acetone and capronaldehyde in the vapour of consomme´soup

ZnO-based gas sensors have been exploited for the selective detection of acetone and capronaldehyde in the vapour of consomme´soup. The sensitivity to acetone is much improved by the addition (5 wt.%) of two acidic oxides, MoO 3 and WO 3, the highest sensitivity being attained at 500 and 550 °C, respectively. These sensors show fairly good selectivity to acetone over other important flavour components, methylpyrazine, dimethyldisulfide and capronaldehyde. The promoting effects are suggested to result from changes in rate and path of the catalytic oxidation of acetone. For the detection of capronaldehyde, Gd 2O 3- and Er 2O 3-ZnO are found to show excellent sensitivities and selectivities at 300 °C.