The rapid detection for methane of ZnO porous nanoflakes with the decoration of Ag nanoparticles

Realizing the real-time detection of CH₄ is important for the safety of human life. A facile hydrothermal method was used to synthesize Ag nanoparticles-decorated ZnO porous nanoflakes (PNFs) in this study. The characterization results confirmed that Ag nanoparticles had been decorated in ZnO nanoflakes with the thickness of ~10 nm. The gas-sensing properties of Ag-decorated ZnO nanoflakes were also investigated. While the gas-sensing performances of ZnO were remarkably improved by decorating Ag nanoparticles on the surface of ZnO nanoflakes, the response of the Ag-decorated ZnO sensor to 3000 ppm CH₄ is almost 1.3 times as high as that of pristine ZnO sensor. The obtained Ag/ZnO sensor exhibits better long-term stability and shorter response recovery time (5/38 s) in the comparison with pristine ZnO, demonstrating the possibility for the actual detection of CH₄. The enhanced CH₄ sensing performance can be attributed to the synergism between the unique hierarchical porous structure and the sensitizing actions utilized by the Ag nanoparticles.     

Room temperature solid-state synthesis and ethanol sensing properties of sea-urchin-like ZnO nanostructures

ZnO gas-sensing materials were prepared by one-step solid-state reaction at room temperature. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) were used to characterize the hierarchal ZnO nanocrystals. The results indicate that ZnO clusters are assembled with nanosheets when no surface-active agent is present. When surface-active agent PEG-10000 was added, sea-urchin-like ZnO are assembled from ZnO nodules. The rods have an average diameter of 33 nm and length of about 100 nm. The primary gas-sensing results show that sea-urchin-like ZnO have good gas sensitivity to ethanol vapor.     

CuO nanoparticle decorated ZnO nanorod sensor for low-temperature H2S detection

CuO nanoparticle decorated porous ZnO nanorods were synthesized via a two-stage solution process. First, porous ZnO nanorods were fabricated by a low-temperature hydrothermal method. Afterward, the porous ZnO nanorods were used as supports to load CuO nanoparticles by a non-aqueous solution method. The morphology and structure of the prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). To demonstrate the practical application of the synthesized heterostructured porous CuO/ZnO nanorod hybrid, the sensing properties for H₂S at low operating temperatures were investigated. The high sensitivity, reversible response and good selectivity indicated its potential application as a chemical sensor.     

Solvent thermal synthesis and gas-sensing properties of Fe-doped ZnO

In this study, pure ZnO microbullets, ZnO–ZnFe₂O₄ composite, and ZnO–Fe₂O₃–ZnFe₂O₄ composite with micron structured balloons, rods, and particles were prepared by a simple solvent thermal process using methanol or ethanol as solvents. The influence of solvents on the composition and morphology of the products was studied, and their gas-sensing properties were also investigated. The morphology of ZnO microbullets synthesized in ethanol is similar to but more uniform than that of ZnO microbullets synthesized in methanol. The Fe-doped ZnO synthesized in ethanol contains many micron particles homogeneously dispersing on the surface of the microbullets, which is composed of hexagonal wurtzite ZnO and franklinite ZnFe₂O₄, while Fe-doped ZnO prepared in methanol consists of micron structured balloons, rods, and particles, which is composed of hexagonal wurtzite ZnO, hematite Fe₂O₃, and franklinite ZnFe₂O₄. Compared with pure ZnO and ZnO–ZnFe₂O₄ composite, the ZnO–Fe₂O₃–ZnFe₂O₄ composite presented high response, rapid response/recovery characteristics, good selectivity, and excellent stability to acetone at relatively low operating temperature of 190 °C. This sensor could detect acetone in wide range of 1–1000 ppm, which was expected to be a promising gas sensor for detecting acetone.     

Acetone Sensors Based on La3+ Doped ZnO Nano-rods Prepared by Solvothermal Method

La3+ doped ZnO nano-rods with different doping concentration were prepared via solvothermal method.The doped ZnO nano-rods were characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM),respectively.The effect of La3+ doping on the gas-sensing properties was investigated.The results revealed that the sensor based on 6 mol% La3+ doped ZnO nano-rods exhibited high response to dilute acetone,and the responses to 0.01×10-6 acetone reached 2.4 when operating at 425 ℃.The response time and the recovery time for 0.01×10-6 acetone were only 16 and 3 s,respectively.     

ZnO thin films for VOC sensing applications

ZnO thin films were prepared by reactive RF sputtering on thermally oxidized Si for gas sensing applications. Three VOC vapors were chosen to investigate the response behavior of the prepared ZnO. Acetone, isopropanol and ethanol were tested, and the sensitivity of the sensor toward acetone was the highest (S ∼ 100) for 500 ppm acetone at 400 °C. The largest sensitivity was achieved at 400 °C for all the above vapors. The sensor shows a stable, reversible and repeatable behavior in the acetone concentration ranging from 15 up to 1000 ppm. The mechanism of the sensing was explained according to the ionosorption model.     

Thin film chemical sensors based on p-CuO/n-ZnO heterocontacts

Previous work on bulk ceramic heterocontacts (n-ZnO/p-CuO) has indicated significant sensitivity to the presence of specific adsorbed chemical species. Here, these results are extended to thin film heterostructures fabricated via chemical solution methods. It is expected that thin film sensor architectures will possess significant advantages over their bulk counterparts. In this study, the desired properties of porosity and crystallinity have been optimized with respect to pyrolysis temperature for each ZnO and CuO sol-gel process. The results of microscopy and X-ray diffraction (XRD) indicated that an optimal balance of these two properties is achieved at a pyrolysis temperature of 250 °C. The CuO films were seen to possess a level of porosity significantly higher than that seen in the ZnO films, making them an ideal candidate for the top layer in a planar thin film heterostructure. Results of current-voltage measurements conducted in 4000 ppm hydrogen have confirmed that the inherent porosity of the CuO films led to an enhanced sensor response in CuO on ZnO heterostructures. Lastly, the fabrication and structural characterization of a mixed solution type heterostructure has been detailed. Atomic force microscopy and XRD data indicated the presence of ZnO pillars dispersed among a matrix of CuO.     

ZnFe2O4纳米球的制备及其对丙酮的气敏性能研究

丙酮被广泛应用于工业和实验室,对丙酮浓度的检测十分重要。ZnFe₂O₄是一种尖晶石型三元金属氧化物,气敏性能优良,可广泛应用于气体传感器。本文采用简单的一步水热法制备了球状的ZnFe₂O₄气敏材料。通过XRD、XPS、SEM、TEM、N2吸附-解析仪对材料的形貌结构、化学组成、比表面积等进行分析,并以丙酮为目标气体对其气敏性能进行了综合研究。结果表明,ZnFe₂O₄纳米球是由纳米粒子自组装而成,有较大的比表面积;该ZnFe₂O₄基气体传感器在最佳工作温度150℃下对丙酮的灵敏度为65.74,并具有出色的选择性、稳定性、重复性,但随着湿度的增加其气敏性能逐渐降低。     

Facile synthesis of Ag/ZnO nanorods using Ag/C cables as templates and their gas-sensing properties

A solid-state chemical reaction with the assistance of Ag/C nanocables was implemented for the preparation of Ag/ZnO nanorods. This is the first time Ag/ZnO nanorods are fabricated by using Ag/C cables as template. Compared with the traditional organic surfactant, Ag/C cable is a new and effective template to control the shape of precursors in the solid-state reaction under ambient conditions. The results of systematical gas-sensing studies demonstrate that the sensor based on Ag/ZnO nanorod materials has high sensitivity, good selectivity and short response and reversion time to ethanol. It demonstrates that Ag/ZnO nanorods can be used as gas-sensing material.     

The gas-sensing properties of thick film based on tetrapod-shaped ZnO nanopowders

Tetrapod-shaped ZnO nanopowders were prepared by the method of vapor-phase oxidation from metallic zinc as raw materials. The gas-sensing properties of thick film based on tetrapod-shaped ZnO nanopowders to volatile organic compounds (VOCs), benzene, toluene, xylene, alcohol and acetone were measured, and compared with that of commercial ZnO powders with granular shape. The results showed that tetrapod-shaped ZnO had the better gas-sensing properties: the maximum sensitivity temperature was reduced, the gas sensitivity was improved and the time of response–recovery was shortened. The differences in gas-sensing properties between the thick films were discussed in according to the morphological characteristics, size and agglomeration of raw powder as well as microstructure of sintered thick films.     

Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor

This paper reports the preparation and gas-sensing characteristic of ZnO:TiO₂-based hydrogen sulfide (H₂S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO₂ materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO₂ shows excellent electrical resistance response toward H₂S gas. The cross sensitivity was also checked for reducing gases like CH₄, CO and H₂ gas. The selectivity and sensitivity of ZnO:TiO₂-based H₂S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 °C.     

Fabrication of a CuO-infiltrated ZnO composite and its gas sensing properties

CuO–ZnO composites were fabricated by heating with infiltrating a cupric solution into a porous ZnO matrix. The composites possess a nonlinear, rectifying current–voltage character due to the presence of a p–n junction produced by the CuO and ZnO semiconductors. This junction is essential for the creation of voltage-dependent sensing properties of humidity and flammable gases. The forward current (CuO: positive bias) greatly increased with increasing the relative humidity, while the reverse current only slightly increased with an equivalent increase in the relative humidity. This asymmetric current change with the humidity is similar to that observed for conventional CuO and ZnO sintered specimens heterocontact produced by mechanically pressing the specimens together. The current was increased by the introduction of CO and H2 (4000 p.p.m.) at 250°C, with the current increase due to CO exceeding that of the H2 in the measured bias region within ±6 V. The utility of the new processing method for forming p–n semiconductor junctions open to the atmosphere has been shown. This revised version was published online in November 2006 with corrections to the Cover Date.     

Enhanced acetone detection using Au doped ZnO thin film sensor

Zinc oxide (ZnO) thin films are prepared using sol–gel method for acetone vapor sensing. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was taken as starting material and a stable and homogeneous solution was prepared in ethanol by deliquescing the zinc acetate and distinct amount of monoethanolamine as a stabilizing agent. The prepared solution was then coated on silicon substrates by spin coating method and then annealed at 650 °C for preparing ZnO thin films. The thickness of the film was maintained at 410 nm. The structural, morphological and optical studies were done for the synthesized ZnO thin films. The operating temperature and sensor response is considered to be an important parameter for the gas sensing behavior of any material. Therefore, the present study examined the effect of sensing behavior of 3% v/v gold (Au) doped ZnO thin films as a sensor. The response characteristics of 410 nm ZnO thin film for temperature ranging from 180 to 360 °C were determined for the acetone vapors. The reported study provides a significant development towards acetone sensors, where a very high sensitivity with rapid response and recovery times are reported with lowered optimal operating temperature as compared to bare ZnO nano-chains like structured thin films. In comparison to the bare ZnO thin films giving a response of 63 at an operating temperature of 320 °C, a much better response of 132.3 was observed for the Au doped ZnO thin films at an optimised operating temperature of 280 °C for a concentration of 500 ppm of acetone vapors.     

MWCNTs/天然纤维素壳聚糖导电复合材料的制备及其气敏性能

为了对空气中含毒性的有机气体进行检测,以1-丁基-3-甲基咪唑氯盐离子（[BMIm]Cl）液体溶解棉纤维素和壳聚糖得到均相混合溶液,以十二烷基磺酸钠（SDS）改性的多壁碳纳米管（MWCNTs）为导电填料,通过溶液涂覆方法,制备了MWCNTs/天然纤维素-壳聚糖气敏导电复合材料。结果表明：当壳聚糖与棉纤维素的质量比为1：7,MWCNTs含量在逾渗值（2.8wt%）附近时,该复合材料对甲醇、乙醇、氯仿和丙酮等极性有机溶剂蒸气显示出较好的气敏性和重复使用稳定性,其气敏响应行为表现为典型的负蒸气系数（NVC）效应。     

Performance evaluation of ZnO–CuO hetero junction solid state room temperature ethanol sensor

A semiconductor ethanol sensor was developed using ZnO–CuO and its performance was evaluated at room temperature. Hetero-junction sensor was made of ZnO–CuO nanoparticles for sensing alcohol at room temperature. Nanoparticles were prepared by hydrothermal method and optimized with different weight ratios. Sensor characteristics were linear for the concentration range of 150–250 ppm. Composite materials of ZnO–CuO were characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR) and high-resolution transmission electron microscopy (HR-TEM). ZnO–CuO (1:1) material showed maximum sensor response (S = R_air/R_alcohol) of 3.32 ± 0.1 toward 200 ppm of alcohol vapor at room temperature. The response and recovery times were measured to be 62 and 83 s, respectively. The linearity R² of the sensor response was 0.9026. The sensing materials ZnO–CuO (1:1) provide a simple, rapid and highly sensitive alcohol gas sensor operating at room temperature.     

Detection of Nitric Oxide from Living Cells Using Polymeric Zinc Organic Framework‐Derived Zinc Oxide Composite with Conducting Polymer

Sensitive and selective detection of nitric oxide (NO) in the human body is crucial since it has the vital roles in the physiological and pathological processes. This study reports a new type of electrochemical NO biosensor based on zinc‐dithiooxamide framework derived porous ZnO nanoparticles and polyterthiophene‐rGO composite. By taking advantage of the synergetic effect between ZnO and poly(TTBA‐rGO) (TTBA = 3′‐(p‐benzoic acid)‐2,2′:5′,2″‐terthiophene, rGO = reduced graphene oxide) nanocomposite layer, the poly(TTBA‐rGO)/ZnO sensor probe displays excellent electrocatalytic activity and explores to detect NO released from normal and cancer cell lines. The ZnO is immobilized on a composite layer of poly(TTBA‐rGO). The highly porous ZnO offers a high electrolyte accessible surface area and high ion–electron transport rates that efficiently catalyze the NO reduction reaction. Amperometry with the modified electrode displays highly sensitive response and wide dynamic range of 0.019–76 × 10^?6m with the detection limit of 7.7 ± 0.43 × 10^?9m . The sensor probe is demonstrated to detect NO released from living cells by drug stimulation. The proposed sensor provides a powerful platform for the low detection limit that is feasible for real‐time analysis of NO in a biological system.     

Influence of pH on the synthesis and characterization of CuO powder for thick film room-temperature NH3 gas sensor

CuO films of 51 μm thickness have been fabricated from nanocrystalline powder, which has been synthesized by a sol–gel auto-combustion method at different pH values of the precursor solution. Studies reveal that the pH value of the precursor solution strongly affects the decomposition rate of the metal–citrate complex formed by precursors (cupric nitrate and citric acid). Structural characterization of the powder samples shows a considerable change in agglomeration behavior, crystallite size and strain with variation in pH value of the precursor solution. Studies show that high pH reaction conditions results in the production of highly porous CuO nanoparticles with lowest crystallite size of 27 nm. Thick films of the synthesized material show an extremely high response of 0.941 to few parts per million level of ammonia at room temperature as well as possesses good stability for a long period of time. The adsorption of ammonia on the sensor surface obeys Elovich equation and the reaction kinetics followed is of first order. The lowest potential barrier of 0.50 MΩ and highest rate constant of 0.0136 s⁻¹ have been found for ammonia adsorption on the sensor surface in case of film fabricated from CuO powder synthesized at high pH value of precursor.     

Indium Tin Oxide Thin-Film Sensor for Detection of Volatile Organic Compounds (VOCs)

Indium tin oxide (ITO) (In₂O₃ + 17% SnO₂) thin films were grown on glass substrate by direct evaporation method. Two thick gold pads were deposited to take out contacts. The response of these films at different operating temperatures, when exposed to various volatile organic compounds (VOCs) such as methanol, ethanol, butanol, and acetone in the concentration range 200-2500 ppm was evaluated. Additionally, the effect of film thickness on the response charateristics of methanol and acetone was studied. The linearity and sensitivity of the sensors were measured. The ITO thin-film sensors showed a sensitivity of 0.256 ohms/ppm to acetone vapors, which was almost linear in the range 200-2500 ppm. In order to improve sensitivity and selectivity, a thin layer of various metal and metal oxides such as Cu and PbO was deposited on the sensor surface to work as catalytic layer and the effect on the performance of the sensor was studied. The response and recovery times of the sensor were determined for acetone vapors and were found to be 155 sec and 110 sec, respectively.     

Preparation and gas-sensing properties of SnO<Subscript>2</Subscript>/graphene quantum dots composites via solvothermal method

SnO₂/graphene quantum dots (GQDs) nano-composites were prepared via solvothermal method (160 °C, 10 h), in which graphene quantum dots were synthesized from graphene oxide by one-step solvothermal method. The nano-composites were characterized by means of HRTEM, XRD, SEM, FTIR, XPS and N₂ adsorption–desorption, respectively. The sensor devices were fabricated using SnO₂/GQDs nano-composites as sensing materials. The effect of the GQDs content on the gas-sensing responses and the gas-sensing selectivity was investigated. The experimental results showed that the sensor based on SnO₂/GQDs nano-composite (S-2) exhibited good response and good selectivity to acetone vapor. When operating at 275 °C, the responses of the sensor based on SnO₂/GQDs nano-composite (S-2) to 1000 and 0.1 ppm acetone reached 120.6 and 1.3, respectively; the response time and the recovery time for 1000 ppm acetone were 17 and 13 s, respectively.     

Pt-functionalized Amorphous RuOx as Excellent Stability and High-activity Catalysts for Low Temperature MEMS Sensors

The unsaturated coordination and abundant active sites endow amorphous metals with tremendous potential in improving metal oxide semiconductors’ gas-sensing properties. However, the amorphous materials maintain the metastable status and easily transfer into the lower-active crystals during the gas-sensing process at high working temperatures, significantly limiting their further applications. Here, a bimetal amorphous PtRu catalyst is developed by accurately regulating the introduction of Pt species into amorphous RuO_x supports to realize the highly active and stable H₂S gas-sensing detection. It is found that incorporation of low-concentration Pt species can effectively maintain the amorphous state of initial RuO_x and delay the crystallization temperature as high as 100 °C. Further, ex situ XPS and in situ Raman spectroscopy analysis confirm that active Pt species can facilitate H₂S adsorption by strong Pt-S coordination and dissociate the sulfur species to the surrounding support, which contribute to the chemisorption and sensitization of H₂S. Meanwhile, electron transport at the interface between Pt, RuO_x and ZnO further activates the reaction process at the surface of the gas-sensitive material. The final PtRu-modified ZnO (PtRu/ZnO) sensor enables the detection of H₂S in the ultra-low concentration range of 15–2000 ppb with remarkable stability.