Room-Temperature O3 Detection: Zero-Bias Sensors Based on ZnO Thin Films

ZnO thin films with a thickness of 300 nm were deposited on Si and Al2O3 substrates using an electron beam evaporation technique with the aim of testing them as low cost and low power consumption gas sensors for ozone (O3). Scanning electron microscopy and atomic force microscopy were used to characterize the film surface morphology and quantify the roughness and grain size, recognized as the primary parameters influencing the gas sensitivity due to their direct impact on the effective sensing area. The crystalline structure and elemental composition were studied through Raman spectroscopy and X-ray photoelectron spectroscopy. Gas tests were conducted at room temperature and zero-bias voltage to assess the sensitivity and response as a function of time of the films to O3 pollutant. The results indicate that the films deposited on Al2O3 exhibit promising characteristics, such as high sensitivity and a very short response time (<2 s) to the gas concentration. Additionally, it was observed that the films display pronounced degradation effects after a significant exposure to O3

Synthesis Characterization of Nanostructured ZnCo2O4 with High Sensitivity to CO Gas

In this work, nanostructured ZnCo2O4 was synthesized via a microwave-assisted colloidal method, and its application as gas sensor for the detection of CO was studied. Typical diffraction peaks corresponding to the cubic ZnCo2O4 spinel structure were identified at calcination temperature of 500°C by X-ray powder diffraction. A high degree of porosity in the surface of the nanostructured powder of ZnCo2O4 was observed by scanning electron microscopy and transmission electron microscopy, faceted nanoparticles with a pockmarked structure were clearly identified. The estimated average particle size was approximately 75 nm. The formation of ZnCo2O4 material was also confirmed by Raman characterization. Pellets fabricated with nanostructured powder of ZnCo2O4 were tested as sensors using CO gas at different concentrations and temperatures. A high sensitivity value of 305–300 ppm of CO was measured at 300°C, indicating that nanostructured ZnCo2O4 had a high performance in the detection of CO

Fast response and low temperature sensing of acetone and ethanol using Al-doped ZnO microrods

We report low temperature acetone and ethanol sensing properties of Al-doped ZnO microrods synthesized using hydrothermal technique. We observe the acetone detection at room temperature as well as ethanol and acetone detection at low temperature of 150 °C using Al-doped ZnO microrods. 3 wt% Al-doped ZnO microrods sensor exhibits the highest response of 231 toward 8100 parts per million (ppm) of ethanol at 150 °C. The response & recovery time are found to be ultrafast of 60 ms & 870 ms for ethanol and 110 ms & 330 ms for acetone of the Al-doped ZnO microrods at an operating temperature of 150 °C, respectively. In addition, sensing mechanism has explained to illuminate the improved sensing performances of Al-doped ZnO microrods. Thus it is revealed that Al-doped ZnO microrods are promising as an ultrafast gas sensor

Study of a QCM Dimethyl Methylphosphonate Sensor Based on a ZnO-Modified Nanowire-Structured Manganese Dioxide Film

Sensitive, selective and fast detection of chemical warfare agents is necessary for anti-terrorism purposes. In our search for functional materials sensitive to dimethyl methylphosphonate (DMMP), a simulant of sarin and other toxic organophosphorus compounds, we found that zinc oxide (ZnO) modification potentially enhances the absorption of DMMP on a manganese dioxide (MnO2) surface. The adsorption behavior of DMMP was evaluated through the detection of tiny organophosphonate compounds with quartz crystal microbalance (QCM) sensors coated with ZnO-modified MnO2 nanofibers and pure MnO2 nanofibers. Experimental results indicated that the QCM sensor coated with ZnO-modified nanostructured MnO2 film exhibited much higher sensitivity and better selectivity in comparison with the one coated with pure MnO2 nanofiber film. Therefore, the DMMP sensor developed with this composite nanostructured material should possess excellent selectivity and reasonable sensitivity towards the tiny gaseous DMMP species

High Energy Heavy Ion-Induced Structural Modifications in Binary Oxides.

The objective of this work was to determine the relation between materials properties and their effect on the structural response of binary oxides to high energy heavy ion irradiation. The Group 14 oxides offered an ideal system of study due to the gradual change in materials properties from SiO2 to PbO2, while their electronic configurations remain consistent; this series facilitated the association of specific materials properties with their effect on radiation response. SnO2 and PbO2 were investigated experimentally in order to complete the body of data for this system. For comparative purposes, Ta2O5 was investigated under the same conditions due to the contrast in physical and chemical characteristics it offers, as well as its unusually large and complicated unit cell. SnO2, PbO2, and Ta2O5 were irradiated by 2.2 GeV 197Au ions (11.1 MeV/u) at room temperature. Samples were analyzed with synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy, small-angle X-ray scattering, and X-ray photoelectron spectroscopy. Irradiation of SnO2 led to the formation of a crystalline SnO phase with trace quantities of metallic Sn, indicating the loss of oxygen and cation reduction during irradiation. Irradiation of PbO2 resulted in the formation of seven distinct structures with compositions of Pb2O3, Pb3O4, PbO, and Pb. Gradual cation reduction was measured. Irradiation of Ta2O5 induced amorphous ion tracks with core-shell morphologies. Oxygen loss was evidenced, increasing with fluence to an estimated final stoichiometry of Ta2O4.2. Using the Group 14 oxide system, the following relations were made: (i) increased susceptibility to amorphization has been attributed to high enthalpy of formation, bandgap, electrical resistivity, and cation electronegativity (relative to those resistant to amorphization), as well as relatively low bond ionicity and bond lengths; (ii) increased susceptibility to oxygen loss during irradiation has been attributed to relatively low bond dissociation energy, bandgap, and electrical resistivity, as well as relatively large bond lengths; (iii) increased susceptibility to cation reduction has been attributed to relatively high bond ionicity as well as low enthalpy of formation, melting temperature, resistivity, and cation electronegativity. Materials property value thresholds are presented for all properties that show correlations to each radiation effect.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113452/1/acusick_1.pd

Synthesis, Characterization and Sensing Properties of AZO and IZO Nanomaterials

Al-doped ZnO (AZO) and In-doped ZnO (IZO) nanopowders were prepared by a sol-gel route and subsequent drying in ethanol under supercritical conditions. The morphological and microstructural properties were investigated by transmission electron microscopy (TEM) analysis and X-ray powder diffraction (XRD). The characterization study showed that the AZO and IZO nanoparticles were crystalline and exhibited the hexagonal wurtzite structure. Chemoresistive devices consisting of a thick layer of synthesized nanoparticles on interdigitated alumina substrates have been fabricated and their electrical and sensing characteristics were investigated. The sensor performances of the AZO and IZO nanoparticles for carbon monoxide (CO) were reported. The results indicated that both doped-sensors exhibited higher response and quick response/recovery dynamics compared to a ZnO-based sensor. These interesting sensing properties were discussed on the basis of the characterization data reported

A Review on Preparation of ZnO Nanorods and Their Use in Ethanol Vapors Sensing

The devices of polycrystalline film have small sensitivity that can be overthrown by using high aspect ratio of 1D nanostructures, such as ZnO nanostructures. Sensors based on 1D nanostructures show very quick response time and high sensitivity for their high impact factor. The purpose of this article is to provide a comparison of different methods and the quality of the sensors thus produced. Currently, metal oxide 1D nanoarchitectures like ZnO have great attraction due to their applications in sensors. Metal oxide nanostructures have high aspect ratio, with small consumption of power and low weight, however, keeping excellent chemical and thermal dependability. Different techniques have been adopted to fabricate metal oxide one-dimensional nanostructures like hydrothermal, electro-spinning, sol-gel, ultrasonic irradiation, anodization, solid state chemical reaction, molten-salt, thermal evaporation, carbothermal reduction, aerosol, vapor-phase transport, chemical vapor deposition, RF sputtering, gas-phase-assisted nanocarving, molecular beam epitaxy, dry plasma etching, and UV lithography. The sensitivity depends upon the materials; synthesis technique and morphology of the sensor performance toward a particular gas have different range of success. This article estimates the efficiency of ZnO 1D nanoarchitectures, gas sensors. Finally, in this review, we had mentioned the future directions of investigations in this field

Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

Climate change and global warming are two huge current threats due to continuous anthropogenic emissions of greenhouse gases (GHGs) in the Earth’s atmosphere. Accurate measurements and reliable quantifications of GHG emissions in air are thus of primary importance to the study of climate change and for taking mitigation actions. Therefore, the detection of GHGs should be the first step when trying to reduce their concentration in the environment. Throughout recent decades, nanostructured metal oxide semiconductors have been found to be reliable and accurate for the detection of many different toxic gases in air. Thus, the aim of this article is to present a comprehensive review of the development of various metal oxide semiconductors, as well as to discuss their strong and weak points for GHG detection

Scalable production and applications of metal oxide nanowires.

Metal oxide nanowires are materials of interest in number of applications such as lithium ion batteries, solar cells, catalyst support, and gas sensing due to their unique charge transport properties and short diffusion length scales. To incorporate nanowires for any applications, one would need hundreds of grams to kilograms of these nanowires. However, state-of-the-art methods for producing metal oxide nanowires are limited to producing only milligrams to a gram in a batch. Hence, there is a need to develop scalable and cost effective processes and reactors to address this challenge. Direct gas phase oxidation of zinc metal powders using a downward atmospheric microwave plasma allowed producing 50-100 grams of zinc oxide nanowires per day. The downstream plasma reactor has certain limitations in terms of scalability: short residence time and plasma instability. Thus, a more improved reactor design is needed for continuous production of nanowire materials at commercially viable production rates. In this dissertation, a fluidized bed reactor is designed and studied for scalable production. The key feature of the reactor involves feed particles being fluidized in the flame to increase residence time. The reactor is equipped with cyclone and bag house filter housing enabling efficient powder collection and allows continuous production. Experiments using fluidized bed reactor produced single crystalline nanowires of about 30-200 nm in diameter and 0.5 – 2 mm in length. The nanowire morphology could be controlled by gas flow rate, powder feeding rate and flame type. A production rate of 1.2 kg of nanowires per hour and yield of about 90% has been demonstrated. The zinc oxide nanowires prepared in this work have been tested as catalyst support for hydro-desulfurization of diesel. The sulfur content was reduced from 200 ppm to less than 1 ppm and the catalyst was active for over 100 hours. Another concept termed as solvo-plasma oxidation has been demonstrated earlier with producing nanowires of titania and related transition metals. The technique involves the synthesis of nanowires using oxidation of metal containing precursors in the presence of alkali salts. The reaction time scales were on the order of few seconds to a minute which is about 3 to 4 orders of magnitude faster than that using a hydrothermal method. Growth rates higher than 1 mm/min were obtained. Here, the concept is studied with tin oxide first to see the ability to produce nanowires and then to understand the mechanism responsible for one-dimensional growth. Experiments reveal that the intermediate phase of potassium stannate could be held responsible for the 1D growth. In addition, experiments also confirm that the solvo-plasma technique is generic for synthesizing most of metal oxides nanowires including titania, cobalt oxide, manganese oxide, tungsten oxide, zinc oxide, and tin oxide nanowires. A simple lab-scale roll-to-roll setup can produce up to 300 grams per hour. Tin oxide nanowires find applications in lithium ion batteries. However, as-produced tin oxide nanowire powders are not chemically stable with respect to cycling. Previously, the nanowires were shown to be stable when they were reduced to produce decoration of tin clusters on their surfaces. Here, the tin oxide nanowires were treated with ultra-thin layers of titania or alumina coating as thin as 1 nm to understand the stability with respect to lithium ion cycling. No initial capacity loss due to SEI formation was found which increased the reversible capacity retention. Both titania- and alumina-coated tin oxide nanowires exhibited tin migration through the coatings to form tin nanoclusters. The compressive stress build-up during lithium intercalation and the enhanced diffusion of tin during lithium de-intercalation allowed for migration of tin to outside of coatings. The results obtained with tin should be applicable to other high capacity materials such as silicon. In summary, two types of scalable production for metal oxide nanowires were studied: A fluidized bed involving plasma and other types of flames for implementing direct oxidation of low-melting metals is studied with great success. In the second concept, the plasma oxidation of tin oxide materials dissolved in alkali salts is studied to understand the intermediate steps responsible for one-dimensional growth. Studies further showed that the second concept could also be implemented using fluidized bed reactor for scalable production. Finally, the bulk produced zinc oxide nanowires and tin oxide nanowires have been tested in hydro-desulfurization and lithium ion battery applications, respectively. In the case of lithium ion battery application, the bulk produced nanowires exhibited stability with cycling when coated with ultra-thin layers of tinania and alumina

Enhancing the sensitivity of nanoplasmonic thin films for ethanol vapor detection

Nanoplasmonic thin films, composed of noble metal nanoparticles (gold) embedded in an oxide matrix, have been a subject of considerable interest for Localized Surface Plasmon Resonance (LSPR) sensing. Ethanol is one of the promising materials for fuel cells, and there is an urgent need of a new generation of safe optical sensors for its detection. In this work, we propose the development of sensitive plasmonic platforms to detect molecular analytes (ethanol) through changes of the LSPR band. The thin films were deposited by sputtering followed by a heat treatment to promote the growth of the gold nanoparticles. To enhance the sensitivity of the thin films and the signal-to-noise ratio (SNR) of the transmittance–LSPR sensing system, physical plasma etching was used, resulting in a six-fold increase of the exposed gold nanoparticle area. The transmittance signal at the LSPR peak position increased nine-fold after plasma treatment, and the quality of the signal increased six times (SNR up to 16.5). The optimized thin films seem to be promising candidates to be used for ethanol vapor detection. This conclusion is based not only on the current sensitivity response but also on its enhancement resulting from the optimization routines of thin films’ architectures, which are still under investigation.This research was funded by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019 and by the projects NANOSENSING: POCI-01-0145-FEDER-016902, with FCT reference PTDC/FIS-NAN/1154/2014, and NANO4BIO: POCI-01-0145-FEDER-032299, with FCT reference PTDC/FIS-MAC/32299/201