ZnO Gas Sensor Research Articles

Characteristics and Preparation of Gas Sensor Using ZnO Nanorods Grown by Hydrothermal Process

Abstract ZnO nanorods for gas sensors were prepared by a hydrothermal method. The ZnO gas sensors were fabricated onalumina substrates by a screen printing method. The gas-sensing properties of the ZnO nanorods were investigated for CH 4 gas. The effects of growth time on the structural and morphological properties of the ZnO nanorods were investigated by X-ray diffraction and scanning electron microscope. The XRD patterns of the nanocrystallized ZnO nanorods showed a wurtzitestructure with the (002) predominant orientation. The diameter and length of the ZnO nanorods increased in proportion to thegrowth time. The sensitivity of the ZnO sensors to 5 ppm CH 4 gas was investigated for various growth times. The ZnO sensorsexhibited good sensitivity and rapid response-recovery characteristics to CH 4 gas, and both traits were dependent on the growthtime. The highest sensitivity of the ZnO sensors to CH 4 gas was observed with the growth time of 7 h. The response andrecovery times were 13 s and 6 s, respectively.Key wordshydrothermal process, Zno nanorod, growth time, gas sensor.

Gas Sensitivity of MnO<sub>2</sub> Doped Nano ZnO Coplanar Plate-Type Gas Sensor Arrays

Based on the integrated technologies of screen printing and hydrothermal method, this paper introduced a new integrative method with materials microstructure controlled growth and gas sensor fabrication. For example,based on space positioning using screen printing and ZnO nanwall with large specific surface area induced by hydrothermal method on the surface of ZnO film, the MnO2 doped nano ZnO coplanar plate-type gas sensor arrays with a good gas sensitivity to formaldehyde were fabricated successfully. The results show that the sensitivity to formaldehyde of MnO2 doped nano ZnO gas sensor arrays fabricated by hydrothermal method in 0.01mol/L Zn(NO3)2 solution, 95°C for 5h on the surface of MnO2 doped nano ZnO film and sintering at 500°C for 2h is higher than that of in distilled water. The existence of ZnO nanwall with large specific surface area induced by hydrothermal method on the surface of MnO2 doped nano ZnO film is the important reason of improvement for the sensitivity to formaldehyde.

Preparation of ZnO nanorod arrays with tailored defect-related characterisitcs and their effect on the ethanol gas sensing performance

In this study gas sensors using ZnO nanorod arrays (NRs) with different surface-to-volume ratios and tailored defect-related properties are prepared and used to fabricate gas sensors. There are three kinds of ZnO gas sensors (ZnO seed layer and different ZnO NRs) grown by sol–gel route and subsequent aqueous chemical growth (ACG). The variation in sensing performance to ethanol gas of those gas sensors will be discussed in terms of morphology and the defect-related carrier behavior of ZnO NRs. Furthermore, the I– V characteristics (metal–semiconductor contact) of ZnO NRs are used to elucidate the defect-related properties affecting the electrical properties of ZnO as well as the sensitivities of those ZnO NRs. The ZnO NRs grown with Zn salt/KOH solution have a larger intensity of deep level emission and a higher Fermi level than that grown with Zn salt/HMTA solution, and sensitivity to ethanol gas of the former is superior. Therefore, it is proposed that more oxygen vacancy inducing more free carriers for gas sensing will result in a higher sensitivity to ethanol gas for sensors based on ZnO nanorod arrays.

Effect of low H2 concentrations on the current-voltage characteristic of ZnO gas sensor

A self-heated ZnO gas sensor was prepared through thermal oxidation of Zn metal for 30 and 60 min in oxygen atmosphere. The XRD results confirmed the conversion of Zn metal to ZnO. The current-voltage measurements showed changes in forward current in the presence of hydrogen gas, with the concentration ranging from 40 to 160 ppm. A better resolution of current response due to H2 concentrations was obtained for the samples with 30 min oxidation, although both samples showed enhanced response with the applied voltage. The barrier height of both samples was found to decrease with H2 concentrations, thus aiding in the increase in current response.

Electron transport mechanism of thermally oxidized ZnO gas sensors

ZnO gas sensor was fabricated by thermal oxidation of metallic Zn at different time periods. The sensors were characterized by I– V measurement with DC voltage, ranging from −2 to 2 volts, in both normal air and H 2 gas with concentration from 40 to 160 ppm. The transport mechanism of the carriers was found to be due to thermionic process through both the grain boundaries and the metal–semiconductor junctions. Resistance of the ZnO sensing film is independent of applied voltage in the range 0.5 V< V a<2 V; however, it is dependent on gas concentration, which makes it useful for gas sensing application.

Synthesis and assembly of zinc hydroxide sulfate large flakes: Application in gas sensor based on a novel surface mount technology

Assembly of 2D materials into hierarchical architectures can produce films with high orientation and special properties. Most research focus on the nano-sized 2D flakes. In this report, zinc hydroxide sulfate (ZHS) large flakes with lateral size up to 410 μm were synthesized by hydrothermal treatment of ZnSO 4·7H 2O solution at 150 °C. The large ZHS flakes were then used as building blocks to assemble the oriented thick films with 63 μm thickness and tightly overlapped structure. Herein, we present one of its potential applications in the gas sensor. According to the TG-DTA analysis, the ZHS films were converted into ZnO thick films with 3D inter-linked particles and pores. Based on the obtained ZnO thick films, a novel surface mount technology was developed to fabricate ZnO gas sensor with low resistance, high sensitivity and fast response.

High-performance NO2 gas sensor based on ZnO nanorod grown by ultrasonic irradiation

A facile and fast sonochemical route for the fabrication of a resistive-type ZnO gas sensor has been demonstrated. Vertically aligned ZnO nanorod arrays were grown on a Pt-electrode patterned alumina substrate under ambient conditions. The average diameter and length of the ZnO nanorods were 50 and 500nm, respectively. Sonochemically grown ZnO nanorod gas sensor was highly sensitive to NO2 gas with a very low detection limit of 10ppb at 250°C; further, its response and recovery time were short. Considering the advantageous properties of this sonochemical technique, we believe that it can be used to fabricate high-performance gas sensors.

NO Gas Sensing Properties of ZnO-Carbon Nanotube Composites

Abstract The NO gas sensing properties of ZnO-carbon nanotube (ZnO-CNT) composites fabricated by thecoaxial coating of single-walled CNTs with ZnO were investigated using pulsed laser deposition. Uponexamination, the morphology and crystallinity of the ZnO-CNT composites showed that CNTs were uniformlycoated with polycrystalline ZnO with a grain size as small as 5-10 nm. Gas sensing measurements clearlyindicated a remarkable enhancement of the sensitivity of ZnO-CNT composites for NO gas compared to thatof ZnO films while maintaining the strong sensing stability of the composites, properties that CNT-basedsensing materials do not have. The enhanced gas sensing properties of the ZnO-CNT composites are attributedto 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 novelsolid-state semiconducting gas sensors.Key words ZnO gas sensor, carbon nanotube, ZnO-CNT composite

ZnO gas sensors: A comparison between nanoparticles and nanotetrapods-based thick films

Nanostructured ZnO was obtained through two different synthetic processes: a traditional sol–gel (SG) route and a combination of vapor transport process and controlled oxidation methods. In SG synthesis, thermal decomposition of the precursor at 450 °C for 2 h in an oven led to pure zincite nanoparticles. Through the second method, instead, nanocrystalline structures in form of tetrapods (TP) were directly obtained during the synthesis process. Powder and film characterizations have been carried out by means of TG–DTA, SEM, TEM and XRD. Finally, electric measurements have been performed with the aim to compare conductive properties, surface barrier heights and gas sensing features, versus O 3, NO 2, CO and H 2S. Despite the significant difference in morphology, it turned out that both types of sensors offered large responses to oxidizing gases at concentration suitable for environmental monitoring.

Fabrication and CO Sensing Properties of Mesostructured ZnO Gas Sensors

This study presents the synthesis of mesostructured ZnO using a template replication method and the fabrication of a carbon monoxide sensor using the synthesized ZnO using the dielectrophoresis process. The mesoporous carbon, CMK-3, was employed for the template and zinc nitrite was used as the precursor for synthesizing the ordered porous ZnO. The mesostructured ZnO was analyzed using X-ray diffraction (XRD) patterns, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and adsorption–desorption isotherms. The XRD patterns indicated that the ZnO exhibited a highly ordered structure after the template was removed. The morphology of the ZnO was randomly oriented and the structure was polycrystalline. The surface area, pore volume, and pore size of the porous ZnO were , , and , respectively. When CO gas was injected, the optimum sensitivities at were 12.1, 14.6, 18.4, 26.0, and 60% when the CO concentrations were 10, 20, 30, 50, and , respectively.

Uniform ZnO nanorods can be used to improve the response of ZnO gas sensor

Uniform ZnO nanorods were synthesized in high-yield by using metal zinc powder as zinc source via a one-step facile hydrothermal process under mild conditions, in which cetyltrimethylammonium bromide (CTAB) with ordered chain structures acted as the conversion of Zn powder into ZnO nanorods. The characterization results show that the as-synthesized products were structurally uniform and have diameters of 40–80 nm. Gas sensing properties studies show that ZnO nanorods exhibit more excellent response and stability to ethanol than that of ZnO nanoparticles. After working continuously for 50 days, the sensitivity of ZnO nanorods still retained 7.3, whereas, the ZnO nanoparticles showed only 1.0. The facile preparation method and the improved properties derived from typical rods-like nanostructure are significant for the future applications of gas sensing material.

Effects of Palladium Loading on the Response of Thick Film Flame-made ZnO Gas Sensor for Detection of Ethanol Vapor

ZnO nanoparticles doped with 0-5 mol% Pd were successfully produced in asingle step by flame spray pyrolysis (FSP) using zinc naphthenate and palladium (II)acetylacetonate dissolved in toluene-acetonitrile (80:20 vol%) as precursors. The effect ofPd loading on the ethanol gas sensing performance of the ZnO nanoparticles and thecrystalline sizes were investigated. The particle properties were analyzed by XRD, BET,AFM, SEM (EDS line scan mode), TEM, STEM, EDS, and CO-pulse chemisorptionmeasurements. A trend of an increase in specific surface area of samples and a decrease inthe dBET with increasing Pd concentrations was noted. ZnO nanoparticles were observed asparticles presenting clear spheroidal, hexagonal and rod-like morphologies. The sizes ofZnO spheroidal and hexagonal particle crystallites were in the 10-20 nm range. ZnOnanorods were in the range of 10-20 nm in width and 20-50 nm in length. The size of Pdnanoparticles increased and Pd-dispersion% decreased with increasing Pd concentrations.The sensing films were produced by mixing the particles into an organic paste composedof terpineol and ethyl cellulose as a vehicle binder. The paste was doctor-bladed ontoAl2O3 substrates interdigitated with Au electrodes. The film morphology was analyzed bySEM and EDS analyses. The gas sensing of ethanol (25-250 ppm) was studied in dry air at400°C. The oxidation of ethanol on the sensing surface of the semiconductor wasconfirmed by MS. A well-dispersed of 1 mol%Pd/ZnO films showed the highest sensitivityand the fastest response time (within seconds).

Nanopillar ZnO gas sensor for hydrogen and ethanol

Aligned zinc oxide nanorods were synthesized directly via a two-step solution approach on an Al 2O 3 tube, and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The zinc oxide nanorods prepared were uniform with diameters of 10–30 nm and lengths about 1.4 μm. The response S r (= R a/ R g) of the aligned zinc oxide nanorod sensor reached 18.29 and 10.41 to 100 ppm ethanol and hydrogen, respectively, which was a two-fold increase compared with that reported in literature, demonstrating the potential for developing stable and sensitive gas sensors.

An ethanol sensor based on cataluminescence on ZnO nanoparticles

A novel gas sensor for the determination of ethanol was proposed in the present work, which was based on the generated cataluminescence emission from catalytic oxidation of ethanol on the surface of ZnO nanoparticles. The cataluminescence characteristics and the effect of different parameters on the signal intensity, such as morphology of synthesized ZnO, temperature and flow rate, were discussed in detail. Under the optimized experimental conditions, the calibration curve of cataluminescence intensity versus ethanol vapor concentration was linear in the range 1.0–100 ppm, and with a detection limit of 0.7 ppm (S/N = 3). Compared with the traditional electrical conductivity-based ZnO gas sensor for the determination of ethanol, the proposed ethanol sensor showed the advantages of high sensitivity, high selectivity and low working temperature.

Gas-sensing properties of well-crystalline ZnO nanorods grown by a simple route

Well-crystalline ZnO nanorods were synthesized by a simple solution route employing dodecyl benzene sulfonic acid sodium salt (DBS) as a modifying agent. ZnO gas sensors were fabricated from ZnO nanorods with an average diameter of around 95nm and their gas-sensing properties were investigated. It was found that the sensors based on ZnO nanorods exhibit high responses and good selectivities to benzene and ethanol gas. Our results indicate that ZnO sensors will be promising candidates for practical detectors for dilute benzene and ethanol, respectively.

Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications

C-axis vertically aligned ZnO nanorod arrays were synthesized on a ZnO thinfilm through a simple hydrothermal route. The nanorods have a diameterof 30–100 nm and a length of about several hundred nanometres. The gassensor fabricated from ZnO nanorod arrays showed a high sensitivity toH2 from room temperature to a maximum sensitivity at250 °C and a detection limit of 20 ppm. In addition, the ZnO gas sensor also exhibited excellent responsesto NH3 and CO exposure. Our results demonstrate that the hydrothermally grown verticallyaligned ZnO nanorod arrays are very promising for the fabrication of cost effective and highperformance gas sensors.

LPG sensing properties of ZnO films prepared by spray pyrolysis method: Effect of molarity of precursor solution

Nanocrystalline ZnO films were deposited onto glass substrates by spray pyrolysis of zinc nitrate solutions and used as a liquid petroleum gas (LPG) sensor. The dependence of the LPG sensing properties on the molar concentration of zinc nitrate solutions was investigated. The ZnO films were oriented along (0 0 2) with the hexagonal crystal structure. The grain size and grain density increased with an increase in molar concentration of zinc nitrate solutions. The gas sensing properties for LPG of the ZnO films for LPG with different grain sizes were measured at different temperatures. The maximum sensitivity of 43% at the operation temperature of 673 K was found for the ZnO film prepared by spraying a 0.1 M solution. The ZnO thin films exhibited good sensitivity and rapid response–recovery characteristics to LPG. Further, it has been shown the gas sensitivity of the ZnO gas sensor depends upon its grain size.

Surface photocurrent gas sensor with properties dependent on Ru(dcbpy) 2(NCS) 2-sensitized ZnO nanoparticles

An original photoelectric gas sensor, fabricated by Ru(dcbpy) 2(NCS) 2 (dcbpy = 2,2′-bipyridyl-4,4′-dicarboxylate)(RuN3)-sensitized ZnO on a comb-like electrode by surface photocurrent technique with excitation of visible light (545 nm), is presented for CO and O 2 detection at room temperature. The surface photocurrent ( I SPC) of a sample can be dramatically enhanced and diminished depending on the properties of the gases. As a reductive gas, CO can make I SPC increase and the sensor response is linear to the change in CO concentration with a detection limit of 0.62 Torr. However, I SPC decreases upon O 2 adsorption, since it can capture electrons from the surface. There is no linear dependency between I SPC and concentration of O 2. In this article a microscopic mechanism of the photoelectric gas sensor has also been discussed in detail. This research should be valuable for the practical application of the ZnO gas sensor.

A novel method for improving the performance of ZnO gas sensors

A novel ZnO porous nanosolid has been prepared by hydrothermal hot-press method, and the measurements indicated that it had very uniform pore size. Compared with the sensor fabricated from conventional ZnO nanoparticles (denoted as S-1), the sensor from ZnO porous nanosolid (denoted as S-2) showed much better sensing performance towards several organic vapors; the intrinsic resistance of S-2 in air dramatically decreased and its selectivity was improved. The optimum working temperature of S-2 was 370 °C, whereas that of S-1 was as high as 420 °C. The differences in gas sensing performance between the two kinds of sensors were analyzed based on the scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) measurements.

Identification and pattern recognition analysis of Chinese liquors by doped nano ZnO gas sensor array

In this paper, ZnO nanoparticles were prepared by the renovated hybrid induction and laser heating. The sensitivities of gas sensors can be greatly improved by doping MnO 2, TiO 2 and Co 2O 3. Five different Chinese liquors, namely, Baiyunbian, Beijing Erguotou, Red Star Erguotou, Zhijiangdaqu and Jianliliangjiu, alcohol and diluted alcohol (forged liquor) were measured. Though the main ingredient is alcohol in liquors, Chinese liquors mainly differ from their flavour types, which rely on their trace components. Flavour type of liquors is a very important factor in Chinese liquor identification. Principal component analysis incorporating with discriminant analysis (PCA-DA), back-propagation artificial neural network (BP-ANN) and learning vector quantization (LVQ) were compared for their classification ability. The accuracy of PCA-DA, BP-ANN and LVQ in terms of predicting tested samples was 76.8, 71.4 and 89.3%, respectively. The LVQ is the most suitable pattern recognition algorithm in present experiment. This work shows the potential application of the gas sensor arrays for monitoring the quality of Chinese liquors.