Solid-state Gas Research Articles

Ordered ZnO Nanorods Synthesized by Combustion Chemical Vapor Deposition

Aligned single-crystal ZnO rod arrays with diameters ranging from 50 nm to 300 nm and length up to 10 microm have been synthesized on silicon substrate under open-air conditions using a combustion chemical vapor deposition (CVD) method. The ZnO rods grew in the direction of (0001) with six [1120] peripheral surface planes. A single ZnO rod solid-state gas sensor fabricated on an interdigitated electrode pattern demonstrated prompt response to ethanol vapor at 400 degrees C.

Titanium oxide thin film gas sensors

Solid-state gas sensors based on metal-oxide thin films offer unique advantages such as compatibility with microfabrication processes and stability at high temperatures and under harsh conditions. At the same time, the sensitivity and selectivity of metal oxide sensors should be improved to compete with alternative technologies. This paper describes titanium oxide thin films for gas sensors. Titanium oxide is particularly attractive due to its high dielectric constant, which leads to extended gas-modulated space charge effects well into the film—resulting in high sensitivity, as well as for its compatibility with Si-based microprocessing technologies. The sensing properties of polycrystalline titanium oxide films (∼70 nm thick) deposited having small grain size (tens of nanometers), are described. A unique technique, using laser-induced melting and solidification of the films, that enables controlling the grain size in thin films will be discussed as a method to study grain size effects on sensing properties of titanium oxide films.

Solid state gas sensor for fast carbon dioxide detection

Promising results on the behaviour of BaTiO3–CuO sputtered thin films as a sensing material under carbon dioxide atmospheres are presented. The response to carbon dioxide of BaTiO3–CuO is due to a reversible reaction based on the presence of BaCO3 in the material. The measurement of impedance changes allows to monitor the variation in CO2 concentration. Resistance and capacitance changes are closely related to work function changes in the p–n heterojunction between BaTiO3 and CuO. The sensing layer thickness influence on the response and the influence of Ag as an additive in the film are also reported.

Electrochemical solid state gas sensors: An overview

Gas sensors has been currently in great demand because of serious concern over environmental pollution and public health considerations resulting from tremendous growth of industrialisation. Concurrently, there have been continuous efforts to obtain sensors with improved performance. The performance of any solid-state electrochemical gas sensor has been always rated on its response time, thermodynamic stability, operating temperature, gas sensing ability, sensitivity and gas concentration range, which is to be sensed. This article reviews the factors contributing towards a gradual development of electrochemical solid-state gas sensors in terms of a continuous tailoring of its two basic components, i.e. solid electrolyte and reference electrode.

High-resolution photoemission study of organic systems at the CAMD 3 m NIM beamline

The 3 m normal incidence monochromator (NIM) VUV beamline at CAMD was designed for novel high-resolution photoemission experiments. Both solid-state samples and gas phase resolution tests demonstrate that high-resolution photoemission is possible below 10 meV, and vibronic fine structure in the photoemission final state from the crystalline copolymer poly-vinylidenefluoride with trifluoroethylene can be observed: the first direct experimental identification of symmetry dependence in electron–phonon coupling in photoemission from a solid-state system.

Detection of individual gas molecules adsorbed on graphene

The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

Metal Oxide “Nanosponges” as Chemical Sensors: Highly Sensitive Detection of Hydrogen with Nanosponge Titania

Semiconducting metal oxides are widely considered as the most promising platform for solid-state gas sensors. Due to enhanced responsiveness of conductance to surface effects in low dimensional nanostructures, various forms of nanostructured metal oxides have been synthesized and their sensing properties studied.

Acetone and ethanol solid-state gas sensors based on TiO2 nanoparticles thin film deposited by matrix assisted pulsed laser evaporation

Matrix assisted pulsed laser evaporation (MAPLE) is a new promising laser-based technique thought for polymer or biomaterial thin films deposition. In this work, the MAPLE technique has been used for the deposition of titania (TiO 2 ) nanoparticle thin films to be used for gas sensor applications. For this purpose, an aqueous solution of TiO 2 nanoparticles, synthesized by a novel chemical route, was frozen at the liquid nitrogen temperature and introduced in a vacuum chamber to be irradiated with a pulsed ArF excimer laser. The volatile solvent was pumped away while the TiO 2 nanoparticles were deposited on Si and Al 2 O 3 substrates. A uniform distribution of TiO 2 nanoparticles with an average size of about 10 nm was obtained, as demonstrated by high resolution scanning electron microscopy (SEM-FEG) inspection. We realized gas-sensing devices based on resistive transduction mechanism by using the TiO 2 nanoparticles thin films deposited by the MAPLE technique onto suitable rough alumina substrates equipped with interdigitated electrical contacts (IDC) and heating elements. Electrical characterization measurements were carried out in controlled environment. The results of the gas-sensing tests towards low concentrations of ethanol and acetone vapors are reported. Typical gas sensor parameters (gas responses, response/recovery time, sensitivity, and low detection limit) towards ethanol were extracted and compared with to the corresponding parameters towards acetone.

Surfactant Assisted Solid-State Synthesis and Gas Sensor Application of a SWCNT/SnO2 Nanocomposite Material

Although tin oxide has been the most widely investigated metal oxide material for gas detection, it suffers from the large resistance and high operating temperature. This could be overcome by hybridization with nanostructured carbon. In this work, tin oxide nanoparticles with ultrasmall sizes of 1-3 nm have been uniformly coated onto bundles of single-walled carbon nanotubes by a surfactant assisted solid state synthesis approach for the first time. Gas sensor properties of the as-synthesized nanocomposite material toward NO2 (from 5 to 60 ppm) are measured at 150 degrees C. Compared to the pure carbon tubes gas sensors, the nanocomposite gas sensor responds to NO2 in low concentrations with good linearity, high sensitivity, and fast recovery, while working at a relatively low temperature.

Room-Temperature Hydrogen Sensors Based on an In3 + -Doped SnP2O7 Proton Conductor

A potentiometric solid-state gas sensor was fabricated using a proton-conducting electrolyte with an active working electrode in order to study its sensing properties for small quantities ( ) of in air at room temperature. The sensor showed electromotive force (emf) response in the negative direction to changes in the concentration. Furthermore, the emf value varied linearly with the logarithm of the concentration, while it was minimally affected by the water-vapor concentration. The sensing mechanism was shown to be based on the mixed potential at the working electrode through measurements of the polarization curves of and air. The electrolyte was also applied in two single-chamber sensors, wherein and carbon were used as active and inactive electrodes, respectively; these electrodes were attached on the opposite surfaces or on the same surface of the electrolyte. Both single-chamber sensors could exhibit comparable sensitivities, compared to the dual-chamber sensor.

Metal oxides for solid-state gas sensors: What determines our choice?

The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.

A Simple and Effective Route to Annihilate Defects in Nanocrystalline SnO2 Thin Films Prepared by Pulsed Laser Deposition

AbstractThe microstructural defects of nanocrystalline SnO2 thin films prepared by pulsed laser deposition have been investigated using transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Defects inside nanocrystalline SnO2 thin films could be significantly reduced by annealing the SnO2 thin films at 300 °C for 2 h. High-resolution transmission electron microscopy showed that stacking faults and twins were annihilated upon annealing. In particular, the edges of the SnO2 nanoparticles demonstrated perfect lattices free of defects after annealing. Raman spectra also confirmed that annealing the specimen was almost defect-free. By using thermal annealing, defect-free nanocrystalline SnO2 thin films can be prepared in a simple and practical way, which holds promise for applications as transparent electrodes and solid-state gas sensors.

Nanomaterials hold promise in natural gas industry

The gas industry represents a major prospect for substantial, near-term adoption of nanoscale technologies with sustained benefits. Importantly, the gas industry employs materials and processes particularly suited to improvement through application of nanoscale technologies. Designing and creating materials at the nanoscale often leads to products that achieve exceptional performance ? possibly enabling more efficient and less expensive manufacturing processes. The commercialisation of new nanomaterials promises to have a profound effect on multiple industries in the coming decades. Depending on the application, nanomaterials are incorporated into a wide variety of hydrocarbon extraction, gas separations, solid-state gas sensors for air pollution monitoring, nanoadsorbent materials for environmental separations, and corrosion inhibitors that are used in a broad range of gas industry markets. In this paper, recent advances in nanomaterial-derived technologies are presented as examples of how nanomaterials might be used to create new developments in the natural gas industry.

Transparent and flexible carbon nanotube/polypyrrole and carbon nanotube/polyaniline pH sensors

AbstractThin film Carbon Nanotube (CNT) networks are used as a conductive, transparent and flexible electrode for electrochemically depositing a conducting polymer on it, polypyrrole or polyaniline in the present work. We analyse the properties of the device as an electrochemical sensor, measuring his pH dependence by recording the open circuit potential in various buffer solutions, ranging from pH 1 to 13. The results show a good sensitivity, linearity and stability in both cases. In the case of CNT/polyaniline, it can be used simply as an optical sensor, as the colour of polyaniline changes with pH. The CNT/polypyrrole and CNT/polyaniline devices could have applications as solid state gas sensor or biosensor deposited on any shape of surface that can be transparent and flexible. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solid‐State Electrochemical Gas Sensors

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Synthesis of nanostructured SnO 2 materials by reactive ball-milling

Tin dioxide (SnO 2) is an important material in the field of solid-state gas sensors and the performances are strongly linked to the microstructure. Particularly, a decrease in the crystallite grain size leads to an increase in the sensing properties. Many chemical and physical techniques have been used to prepare nanosized SnO 2 particles, but few papers report the use of a ball-milling process that promote nanostructured grains. In this work, tetragonal phase SnO 2 powders were produced at room temperature (RT) by reactive milling of metallic tin (Sn) powder under pure oxygen atmosphere in a vertical planetary ball-mill. Powder samples were characterized by using X-ray diffraction (XRD), secondary electron microscopy (SEM), electron microprobe. Experimental results indicate that the formation of SnO 2 is a mechanically induced self-sustained reaction. Very short milling time is needed to complete the reaction and the obtained powder is made of aggregates of SnO 2 crystallites, the mean size of which is inferior to 10 nm according to XRD analyses. However, the strong agglomeration led to a decrease of the specific surface area. The reasons and the possible ways to overcome these phenomena are discussed.

MEMS-based hydrogen gas sensors

The widespread use of hydrogen as both an industrial process gas and an energy storage medium requires fast, selective detection of hydrogen gas. This paper reports the development of a new type of solid-state hydrogen gas sensor that couples novel metal hydride thin films with a micro-electro-mechanical system (MEMS) structure known as a micro-hotplate. Micro-hotplate structures are fabricated via surface silicon micro-machining, resulting in a suspended platform with an embedded heater and thermally isolated from the substrate. A rare earth metal thin film, overcoated with a palladium capping layer, is deposited on this structure and serves as the active hydrogen-sensing layer. The change in electrical resistance of this bilayer when exposed to hydrogen gas is the sensor output. Sub-second response times to 0.25% hydrogen in air and sensitivity to hydrogen concentrations below 200 ppm are measured. Rise and decay times of the response depend on the effective temperature of the micro-hotplate and decrease with increasing heater power. These results suggest that this device has the potential to meet the sensing requirements of advanced hydrogen applications.

Impedance spectroscopy of Na + conducting zeolite ZSM-5

The electrochemical impedance of Na + ion conducting zeolite Na-ZSM-5 contacted with gold electrodes is measured. Complementary impedance measurements of the same material contacted with Na + ion conducting Na 2CO 3 enabled the separation of volume and electrode parts of the impedance. Both volume and electrode impedances consisted of multiple components. Possible underlying mechanisms are discussed in view of the application of zeolites as sensitive materials in solid-state ionic gas sensors.

Characterization and quantification of biomarkers from biomass burning at a recent wildfire site in Northern Alberta, Canada

The composition of organic matter (OM) in pine vegetation and soil samples from a pine forest which was charred by a wildfire was analyzed using solid-state nuclear magnetic resonance ( 13C NMR) and gas chromatography–mass spectrometry (GC–MS) of solvent extracts to study the effects of thermal alteration on soil organic matter (SOM). The NMR data revealed the presence of unaltered biomolecules (cellulose, proteins) and low contents of aromatic C (15%) in the charred pine wood and cones while the charred soil samples exhibited higher contents of aromatic C (39–56%). The solvent extraction of charred and uncharred plant and soil samples yielded diterpenoids, triterpenoids, steroids, a series of aliphatic lipids, phenols and carbohydrates indicating the predominant input of higher plant OM and minor contributions from microorganisms and/or fauna. The lower yield of solvent extractable aliphatic lipids in the charred samples versus the uncharred samples suggests that these compounds are thermally degraded during a wildfire. Molecular markers for the burning of cellulose (levoglucosan, mannosan, galactosan) were detected in all charred samples. The comparison of charred and uncharred samples allowed the identification of unaltered pine derived biomolecules and their thermal alteration products in the charred samples. Terpenoid and steroid biomolecules were in part altered during incomplete combustion to aromatic, unsaturated and polar derivatives (“pyromolecules”) that still retained the characteristic skeleton of their precursors. Since some of the polar degradation products found in the charred soils can be generated either from thermal or microbial degradation, the aromatic and unsaturated hydrocarbon products are preferred as molecular markers for SOM burning. Ratios of biological precursors to aromatic (diterpenoids) or unsaturated products (steroids) indicate that the cyclic lipids in the pine wood and the soil surface horizon were highly altered. In conclusion, the solvent extractable lipids and carbohydrates in charred SOM are valuable, source-specific molecular markers for the burning of plant biomass and for tracing the biogeochemistry of charred residues in soils.

Multi-sensor technique and solid-state electrochemical sensor system for real-time and dynamic monitoring of multi-component gases

According to the kinetics of cross reactions for monitored multi-component gases and the transient electrochemical principles, we have proposed and established a novel multi-sensor technique for real-time and dynamic monitoring of multi-component gases, using a single microelectrode and by fusing a computer-controlled fast potential modulation method as well as functions of data acquisition, conversion and processing. Based on development of this multi-sensor technique and a new type solid polymer electrolyte, a kind of all-new solid-state electrochemical multi-component gas sensor system has been constructed, which means integrating more than two gas sensors on a single gas-sensing microelectrode. The mechanism for transient electrochemical response, detection accuracy, measuring rate and long-term stability of continuous monitoring are also discussed in this paper.