W. Göpel

SnO2 sensors: current status and future prospects☆

Abstract A survey is given on the current status and future prospects in research and development of SnO2-based sensors. Atomistic models of molecular recognition are discussed first. They include physisorption, chemisorption, surface reaction, catalytic reaction, grain boundary reaction, bulk reaction and three-phase boundary reaction steps. The influence of contact geometry and crystallinity on the sensor response signal is outlined. A brief summary is given of the current status of sensor research and development with emphasis on ceramic, thick-film and thin-film sensors based on crystalline, polycrystalline and nanocrystalline SnO2. Three different aspects are mentioned in the outline which are expected to lead to significantly improved performances of future sensors: the improved selectivity through the modulation frequency in a.c. measurements, the improved stability through the better control of structures, and the improved selectivity and drift compensation through pattern recognition.

Oxide ion conducting solid electrolytes based on Bi2O3

The high oxide ion conductivity of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. The bismuth oxide based compounds are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

Surface defects of TiO2(110): a combined XPS, XAES and ELS study

Abstract Intrinsic defects were produced at TiO 2 (110) by thermal treatment and ion bombardment. High-temperature low-oxygen-pressure treatment leads to the formation of point defects with characteristic shifts in the cation (Ti 2p and Ti 3p) core levels of 1.7 eV towards lower binding energies and in the anion (O 1s) core level towards higher binding energy. Simultaneously, attenuation of nonbonding O 2p valence band states, an additional auger peak at 5.3 eV higher energy than the L 3 M 23 V transition of ideal surfaces without point defects, and a pronounced electronic loss feature at 0.8 eV are observed. From these results, a defect-related gap state with mainly Ti3d contribution 0.3 eV below the conduction band edge can be determined.

IN2O3 AND MOO3-IN2O3 THIN FILM SEMICONDUCTOR SENSORS : INTERACTION WITH NO2 AND O3

Abstract Semiconductor sensors based on nanocrystalline In2O3 and MoO3–In2O3 thin films are found to be very sensitive to detecting low concentrations (100–200 ppb) of ozone and nitrogen dioxide. In this work, the sensitive layers were prepared by a sol–gel method. Mo-loading (MoO3–In2O3 samples) was performed by coprecipitation of In–Mo mixed hydroxides and subsequent drying and annealing (700°C, air). A simple adsorption model for target gases (NO2, O3) is proposed. According to this model O2− and O− are the predominant species at the In2O3 surface during the ozone interaction. NO2 interaction with In2O3 is dissociative and leads to the formation of atomic oxygen species at the surface.

Gas identification by modulating temperatures of SnO2-based thick film sensors

Abstract A new method is presented to identify the presence of two gases in the ambient atmosphere. The method employs only one SnO2-based gas sensor in a sinusoidal temperature mode to perform the quantitative analysis of a binary gas mixture (CO/NO2) in air.

Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO2 sol–gel nanocrystals for gas sensors

In order to clarify the role of the noble metal additives in the gas sensing mechanisms, three of the most common catalytic additives, such as Pd, Pt and Au, have been introduced in a sol–gel obtained tin oxide base material. The additives nominal weight concentrations used were 0.2% and 2%, and they were introduced in the precipitated tin oxide. A posterior calcination treatment was carried out, during 8 h, at the temperatures of 250°C, 400°C, 450°C, 600°C, 800°C and 1000°C. Structural and surface analysis of these nanopowders have been performed. Identification and localisation of metallic, 2+ and 4+ oxidised states of the used noble metals are discussed, and experimental evidences about their effects on the sensor performance are presented. Likewise, effects of their presence on the nanoparticle characteristics, and also on the material sensitivity to CO and CH4, are analysed and discussed.

Grain size control in nanocrystalline In2O3 semiconductor gas sensors

Abstract In2O3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG). SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher sensor resistance in air and higher response to NO2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C. The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In2O3 layer prepared by using low agglomerated IHS is smooth and porous, In2O3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO2 sensitivity. Sensor resistance and NO2 sensitivity increase with the decrease of the grain sizes in In2O3.

Conductance, work function and catalytic activity of SnO2-based gas sensors

Abstract We have studied the geometric microstructure as well as the elemental composition and distribution of SnO2-based polycrystalline sensors with X-ray diffraction and surface spectroscopic techniques. Electrical properties and responses against reducing gases are characterized by a.c. and d.c. measurements of resistances and/or conductances. The models explaining the high sensitivity of polycrystalline SnO2 against reducing gases in terms of changes of the intergrain conductivity have been completed. The Schottky-barrier mechanism of the electron transport across the grain boundaries is valid only for SnO2 grains larger than the Debye length of electrons. For grains smaller than the Debye length, the band bending at the surface can be neglected compared with the overall shift of the Fermi level in the grains during gas exposure. From combined measurements of conductances, work function changes and catalytic activities as functions of temperature, CO and H2O partial pressures, we deduce that OH dipoles, which do not influence the oxidation kinetics of carbon monoxide, are formed during interaction with water at 673 K.

Odor, drug and toxin analysis with neuronal networks in vitro: extracellular array recording of network responses.

Neurons, by virtue of intrinsic electrophysiological mechanisms, represent transducers that report the dynamics of cell death, receptor-ligand interactions, alterations in metabolism, and generic membrane perforation processes. In cell culture, mammalian neurons form fault-tolerant, spontaneously active systems with great sensitivity to their chemical environment and generate response profiles that are often concentration- and substance-specific. Changes in action potential patterns are usually detected before morphological changes and cell damage occur, which provides sensitivity and reversibility. Such biological systems can be used to screen rapidly for novel pharmacological substances, toxic agents, and for the detection of certain odorants. Existing simple culture preparations can already be employed effectively for the detection of chemical compounds. So far, three strategies have been investigated in pilot experiments: (1) Substance-dependent major changes in spontaneous native activity patterns. All synaptically active agents (e.g. glutamate, strychnine, N-methyl D-aspartic acid) as well as metabolic poisons generate such changes. (2) Substance-dependent changes in network oscillations via disinhibition. The regularized, oscillatory activity is altered by synaptically and metabolically active substances, ion channel blockers, and toxins. (3) Detection of paroxysmal responses indicating major, pathological membrane currents in large subpopulation of cells. We have explored these three strategies via 64 channel array recordings using spontaneously active murine spinal cord cultures. The glycine receptor blocker strychnine reliably generated increased multichannel bursting at 5-20 nM and regular, coordinated bursting above 5 microM. During biculline-induced network oscillations many compounds alter oscillation frequencies or terminate activity in a substance-specific manner. Finally, the gp120 protein of the AIDS virus (at 1 microgram/ml) produces massive, unique paroxysmal discharges that may last as long as 2 min. These results indicate that cultured neuronal networks are practical systems that can be used for the detection and identification of a great variety of chemical substances. The concept of dynamic fingerprinting to identify specific compounds is discussed.

Phthalocyanines as Sensitive Materials for Chemical Sensors

This paper provides a review of phthalocyan- ines suitable for the development of chemical sensors. Phthalocyanines may be utilized for different types of chemical sensors, including in particular electronic conductance sensors [such as semiconductive, field-effect transistor (FET), solid-state ionic and capacitance sensors], mass-sensitive sensors utilizing a quartz crystal microbalance (QCM) and surface acoustic-wave (SAW) sensors, and optical sensors. The phthalocyanines used are discussed in terms of their physical and chemical properties, as well as their sensitivity, selectivity and reversibility towards the detection of NO2 and organic solvent vapours. The interaction mechanism between phthalocyanine films and analyte molecules is also discussed.