K.D. Schierbaum

Low and high temperature TiO2 oxygen sensors

We have studied two types of TiO2-based oxygen sensors operating at different temperatures with different detection principles. At high temperatures, TiO2 devices can be used as thermodynamically controlled bulk defect sensors to determine oxygen over a large range of partial pressures. Their intrinsic behaviour can be controlled by carefully directed doping with tri- or pentavalent cations. At low temperatures, we find that Pt/TiO2 Schottky diodes make extremely sensitive oxygen detection possible. The latter show reversible shifts of current-voltage curves, which are determined by interface states formed by chemisorbed oxygen.

Multicomponent gas analysis: An analytical chemistry approach applied to modified SnO2 sensors

Abstract Commonly used electronic conductivity sensors show significant cross-sensitivities for different gases and usually irreproducible results above or below certain temperatures and partial pressures of critical components in the gas phase. The present paper characterizes conditions for reversible sensor response with respect to partial pressures of CO, CH 4 and H 2 O and temperature. Sensitivities and selectivities of the SnO 2 -based sensors are modified chemically in a systematic way. The results are discussed by using analytical expressions describing conductivity changes upon partial pressure changes. After introducing briefly the general concept of multicomponent analysis to identify the concentration of n components with m sensors in analytical chemistry, we present a simple application to identify CO and CH 4 partial pressures in gas mixtures by using chemically modified sensors at different temperatures.

Pattern recognition methods for gas mixture analysis: Application to sensor arrays based upon SnO2

Abstract This paper demonstrates the possibility of determining single gas components, such as H 2 , CH 4 and CO, in air from specific patterns of chemically modified SnO 2 -based sensors. We also present results from two different multicomponent analysis approaches. In the first approach we measure conductivities of different chemically modified sensors and in the second one we measure different parameters of only one sensor.

Prototype Structure for Systematic Investigations of Thin-Film Gas Sensors

Note: 36 Reference SAMLAB-ARTICLE-1990-011 Record created on 2009-05-12, modified on 2016-08-08

Quartz microbalance sensors for gas detection

Abstract We have studied different chemically sensitive coatings such as amines, siloxanes and supramolecular compounds for quartz microbalance sensors to detect polar and apolar organic and inorganic compounds such as CO 2 , CO, NO 2 , Cl 2 C CHCl, Cl 2 C CCl 2 and C 8 H 18 in the gas phase.

Microstructured Solid-State Ion-Sensitive Membranes by Thermal Oxidation of Ta

Note: 39 Reference SAMLAB-ARTICLE-1990-005 Record created on 2009-05-12, modified on 2016-08-08

Visualization of precipitation induced crystallographic shear planes as one-dimensional structures on surfaces: an STM and RHEED study on TiO2(110)

Abstract Surface sensitive techniques [scanning tunneling microscopy (STM), reflection high-energy electron diffraction (RHEED), and Auger electron spectroscopy (AES) have been used to study the effects of changes in the bulk on the surface structure in TiO 2 . Annealing of TiO 2 (110) at elevated temperatures leads to straight steps on the surface with lengths in excess of 500 A. The structure of these steps has been investigated in detail by voltage dependent STM and electron diffraction. The steps are caused by crystallographic shear (CS) during annealing creating areas of different crystallographic structure. STM and RHEED studies showed that the dominant directions of the steps are along [110], [111] and [111]. From these experimental finding and the tetragonal symmetry of rutile it was concluded that the CS planes belong to the close packed family {112} of planes. CS is correlated to Ca precipitation. Caue5f8O diffusion to the surface may be an important process in the creation of oxygen vacancies. The calcium precipitates induce a (1 × 3) surface reconstruction which is visible both in STM images and electron diffraction patterns.

The electronic structure of CeO2 thin films: the influence of Rh surface dopants

Abstract The influence of Rh surface doping on the electronic structure of stoichiometric CeO 2 surfaces is studied using detection-angle dependent XPS, UPS and HREELS. Rh surface doping leads to an increase of the Ce 3+ concentration localized at the surface as shown by the Ce 3d core level emissions. Upon deposition of Rh in the submonolayer range UPS shows Rh 4d associated states to develop between the oxide O 2p band and the Fermi level E F . These states broaden with increasing coverage towards E F , and finally metallic Rh is formed. Besides the typical loss structure of undoped, stoichiometric CeO 2 an additional transition is observed in the HREELS spectra of Rh doped CeO 2 at 1.6 eV loss energy which is interpreted as a charge transfer transition from Rh 4d derived states to empty Ce 4f states. For coverages below 1.5 monolayers Rh is found to reduce the work function of clean, stoichiometric CeO 2 from 4.69 eV to a minimum value of 4.35 eV at a coverage of 0.5 monolayers.

Prototype chemical sensors for the selective detection of O2 and NO2 in gases

Thin-film structures of inorganic and organic electron-, hole- and ion-conducting materials were optimized for their use as chemical sensors to detect selectively O2 and NO2 in the gas phase. Comparative electrical and spectroscopic studies make possible an atomistic interpretation of the sensing mechanisms of our optimized sensor structures. Particular emphasis is given to the controlled mixed (i.e., electron as well as ion) conduction during the preparation and/or operation of long-term stable devices.

Defect chemistry of tin(iv)-oxide in bulk and boundary layers

Abstract We have investigated the defect chemistry of SnO 2 over a wide temperature range between 300 and 1100 K. In the high-temperature region fully ionized oxygen vacancies V .. O and conduction electrons e predominate in pure SnO 2 , whereby this region can be divided into two parts for acceptor (A) or donor (Ḋ) doped materials. At very high temperatures and/or low oxygen partial pressures P O2 the intrinsic case 2[V .. O =[e] is realized. At comparably lower temperatures and/or higher pressures P O2 the concentration of intrinsic defects is determined by doping and 2[V .. O ]=[A] or [e]=[D′] holds (“extrinsic case”). In all cases the concentration of the electronic charge carriers is controlled by the oxygen partial pressure. The conductivity behaviour of SnO 2 at low temperatures ( 2 with oxygen from the gas phase leads only to a chemisorption of O 2 with trapping of conduction band electrons in the chemisorption complexes (O - 2 ) ad and a charge compensating positive subsurface region in the semiconducting SnO 2 . At intermediate temperatures the reaction of oxygen can be understood in terms of an annealing and segregation of ionic subsurface defects. Diffusion processes can be enhanced by the electric field built up during chemisorption.