Responsivity optimization of methane gas sensor through the modification of hexagonal nanorod and reduction of defect states (original) (raw)
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Chemical synthesis of zinc oxide nanorods for enhanced hydrogen gas sensing
Chinese Physics B, 2014
Zinc oxide (ZnO) nanorods are prepared using equimolar solution of zinc nitrate ((Zn(NO 3) 2) and hexamethylenetetramine (C 6 H 12 N 4) by the hydrothermal technique at 80 • C for 12 h. Epitaxial growth is explored by X-ray diffraction (XRD) patterns, revealing that the ZnO nanorods have a hexagonal (wurtzite) structure. Absorption spectra of ZnO are measured by UV-visible spectrometer. The surface morphology is investigated by field emission scanning electron microscopy (FESEM). The synthesized ZnO nanorods are used for detecting the 150 • C hydrogen gas with a concentration over 1000 ppm. The obtained results show a reversible response. The influence of operating temperature on hydrogen gas detecting characteristic of ZnO nanorods is also investigated.
High Sensitivity Methane Sensor by Chemically Deposited Nanocrystalline Zno Thin Film
International Journal on Smart Sensing and Intelligent Systems
Nanocrystalline n-ZnO thin films were deposited on SiO 2-coated (0.45 µm) p type Si substrates (10-20Ω-cm) by a low cost chemical deposition technique to fabricate ZnO-based resistive sensors for methane detection. The nanocrystalline ZnO needle like structures were grown on RCA cleaned p-Si<100> substrates by successive immersion (100-200 times) into a Sodium Zincate bath (0.125M) kept at room temperature and DI water maintained at 90 o C. The Sodium Zincate was prepared by reacting Zinc Sulphate and excess Sodium Hydroxide in aqueous solution. The film thickness of 1.5 µm (approx.) for 75 dippings was obtained. The dipping time is 1 second. The annealing was done at a lower temperature (150 o C) for 30 minutes in air. Structural characteristics were studied by FESEM and EDAX to indicate the formation of ZnO with vertical orientation. The hexagonal needle like structures of 0.3-0.5µm diameter and 1-1.5µm length were formed. The resistance of the ZnO films in ambient air (zero level for gas sensing) was found to be stable and reproducible after several thermal cyclings. Surface modification with palladium (0.01% PdCl 2 for 5 seconds) was done to enhance sensitivity; so that the ZnO thin films can give significant response to target gases at the operating temperature of as low as 130 o C, compared to the normal operating temperature range of 200-400 o C for zinc oxide resistive gas sensors. The planar gold contacts were deposited by vacuum evaporation technique. The device was then tested for its methane sensing property at different operating temperatures (150,175,200,250,300,350 o C) and at 5 different methane concentrations (0.01,0.05,0.1,0.5,1%) taking N 2 as a carrier gas. The response magnitude, response time and recovery time were studied in detail for both Pd modified and unmodified ZnO film. The maximum response of 99.76% and the lowest response time of 39 seconds were obtained at 200 o C for Pd modified sensor. A high sensitivity to methane even at low temperature (130 o C) was observed comparable to those obtained by more sophisticated and expensive deposition process e.g. MOCVD.
Hierarchically designed ZnO nanostructure based high performance gas sensors
RSC Adv., 2014
Rationally controlled multistage hydrothermal methods have been developed to prepare different types of hierarchical zinc oxide (ZnO) nanostructures with high surface-to-volume ratios and more exposed polar facets. Four types of hierarchical ZnO nanostructures, i.e. nanobrushes (ZNBs), nanoleaves (ZNLs), hierarchical nanodisks (HNDs) and nanoflakes (ZNFs), assembled from initial mono-morphological nanostructures, i.e. nanowires (ZNWs) and nanodisks (ZNDs), were produced from sequential nucleation and growth after a hydrothermal process. Hierarchical nanostructures with 1D nanowire and 2D nanodisk building blocks were realized using zinc nitrate and zinc sulphate as the source of zinc ions, respectively. Compared to their initial mono-morphological counterparts, the grown hierarchical nanostructures demonstrated superior gas sensing properties. ZNLs and ZNFs showed a significant improvement in the sensitivity and fast response to acetone. In addition to the high surface-to-volume ratio, due to the ultrathin sheet building blocks, the enhanced gas sensing properties of the ZNLs and ZNFs are chiefly ascribed to the increased proportion of exposed (0001) polar facets. The current study offers a path for the structure induced development of gas sensing properties by designing a necessary nanostructure, which could be used to fabricate high performance nanostructured gas sensors based on other metal oxides.
Fabrication of ZnO nanorod-based hydrogen gas nanosensor
Microelectronics Journal, 2007
We report a first work on nanofabrication of hydrogen nanosensor from single ZnO branched nanorods (tripod) using in-situ lift-out technique and performed in the chamber of focused ion beam (FIB) system. Self-assembled ZnO branched nanorod has been grown by a cost-effective and fast synthesis route using an aqueous solution method and rapid thermal processing. Their properties were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersion X-ray spectroscopy, transmission electron microscopy, and micro-Raman spectroscopy. These analyses indicate high quality ZnO nanorods. Furthermore, our synthesis technique permits branched nanorods to be easily transferred to other substrates. This flexibility of substrate choice opens the possibility of using FIB system for handling.
ZnO thin film as methane sensor
Methane (CH 4) sensitivity of zinc oxide (ZnO) thin film has been studied in the present work. The sensor element comprises of a chemically fabricated ZnO semiconducting layer and a layer of palladium (Pd) as catalyst. The catalyst layer was formed on the surface of semiconducting ZnO following a wet chemical process from palladium chloride (PdCl 2) solution. Fundamental features of a sensor element e.g. sensitivity, response time and recovery process has been studied. The effect of operating temperature on performance of the sensor material has been investigated and a choice of optimum temperature was made at around 200 o C. The sensor element exhibited reasonable sensitivity of about 86% at this temperature in presence of 1 vol% methane (CH 4) in air.
Hydrogen Gas Sensor Based on ZnO Nanoroads Grown on Si by Thermal Evaporation
Journal of Materials Science and Engineering A, 2016
High-quality ZnO (Zinc oxide) nanorods grown on Si substrate have been synthesized for hydrogen gas sensor application through a low-cost catalyst-free process by thermal evaporation at 800 º C. The morphological, structural and optical properties of the ZnO nanorods have been examined. In this study, Pd/ZnO/Pd MSM (Metal-semiconductor-metal) gas sensor has been fabricated based on the ZnO nanorods. The absence of a seed layer and the coalescence of ZnO nanorods were the key factors responsible for the high sensitivity of the gas sensor at room temperature. The sensitivity of ZnO nanorods is measured at different concentrations from 25 ppm to 150 ppm of H 2 gas at room temperature. The highest response of the ZnO/Si sensor was 110% in the presence of 500 ppm of H 2. This high sensitivity can be attributed to the high surface-area-to-volume ratio of the nanorods between the Pd contacts of the MSM configuration.
Highly efficient Zinc Oxide nanostructure based gas sensor for domestic application
Journal of the Pakistan Institute of Chemical Engineers
In this work, Zinc Oxide (ZnO) nanorods with high surface to volume ratio were fabricated through the hydrothermal synthesis process on a glass slide and highly conductive alumina ceramic based gold interdigitated electrode (IDE). The ZnO nanorods structure on substrates were characterized through X-ray diffraction (XRD) and UV absorption spectroscopy followed by growth verification by Scherrer’s equation. The sensitivity characterization of fabricated sensor was determined for 2000 ppm and 4000 ppm natural gas in the air through high resistance electrometer at room temperature. The 2000 ppm concentration of gas shows 11.3% sensitivity, response time of 66 seconds and recovery time of 92 seconds to the sensor. The 4000 ppm concentration of gas shows 64% sensitivity, the response time of 106 seconds and a recovery time of 174 seconds to the sensor. The higher sensitivities with slow response and recovery times exhibit the behavior of redox reactions of sensor surface to the higher ...
2013
Novel cacti-like structure and nanoneedles of zinc oxide (ZnO) were grown onto glass substrate using chemical route at comparatively low temperature (90 • C), and employed for the application of gas sensor. The grown nanostructures were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM) and photoluminescence (PL) spectroscopy. FE-SEM and TEM images showed that vertically aligned ZnO nanoneedles were formed on substrate and secondary branches emanated from primary aligned nanoneedles. PL spectra showed distinctively different peaks for nanoneedles and cacti-like structure where the peak intensities for cacti structures are as high as the one compared for aligned nanoneedles. Also, the intense visible peak detected for cacti structure confirmed the presence of defects due to oxygen vacancy in the grown nanostructures. Further, gas sensing behaviors were studied for these two different nanostructures against nitrogen dioxide (NO 2 ) gas, and their selectivities toward reducing gases such as hydrogen disulfide, ethanol and liquefied petroleum gas were compared. It was found that cacti-like structure exhibited high gas response at 200 • C and is selective for NO 2 gas as compared with that for nanoneedles. The improved gas response is due to high surface area of cacti structure and presence of oxygen vacancies. Moreover, the contact between two adjacent cacti branches creates barrier potential at junction, which controls its resistance of sensors resulting in high gas sensing. Therefore, novel cacti-like nanostructure demonstrated to be the best candidate as NO 2 gas sensor at low cost and temperature. (C.S. Lee).
The superior performance of the electrochemically grown ZnO thin films as methane sensor
Sensors and Actuators B: Chemical, 2008
Pd-Ag/ZnO/Zn and Rh/ZnO/Zn MIM (metal-insulator-metal) gas sensors were fabricated using nanoporous ZnO thin films, obtained by an electrochemical deposition method in the absence and presence of UV light. A high-purity Zn anode, a Pt cathode, a calomel reference electrode and a 0.3-M oxalic acid electrolyte were used for deposition. Pd-Ag (26%) and Rh were used separately as the catalytic metal electrodes to fabricate the two different types of MIM configurations. A gas response of the order of 3.85 ± 2, a response time of 5 ± 0.5 s and a recovery time of 16 ± 0.5 s were obtained with the Pd-Ag contact, while the Rh contact showed a response of the order of 4.82 ± 2, a response time of 24 ± 0.5 s and a recovery time of 72 ± 0.5 s, at the optimum temperature of 220 • C, which is the lowest temperature so far reported for metal oxide sensors to sense 1% methane in a N 2 carrier gas. The undoped zinc oxide thin films grown by UV-assisted electrochemical anodization of high-purity Zn demonstrated a better performance for methane sensing. The experiments were repeated in synthetic air and a somewhat reduced performance was observed. The selectivity in the presence of hydrogen and the stability of the sensors were studied.
Development of gas sensors using ZnO nanostructures
Journal of Chemical Sciences, 2010
Different ZnO nanostructures such as nanowires, nanobelts and tetrapods have been grown and used for preparation of thick film (with random grain boundaries) as well as isolated nanowire/nanobelt gas sensors. Sensitivity of different type of sensors has been studied to H2S and NO gases. The results show that the response of ZnO sensors to H2S arises from grain boundary only whereas both grain boundaries and intragrain resistances contribute towards response to NO. In addition, oxygen vacancies in the lattice were also seen to help in improvement of sensor response. Room temperature operating H2S and NO sensors based on ZnO nanowires have been demonstrated. Further, sensors based on isolated nanobelts were found to be highly selective in their response to NO.