N. Bârsan

Micromachined metal oxide gas sensors: opportunities to improve sensor performance

Abstract This review deals with gas sensors combining a metal oxide based sensing layer and a substrate realized by using micromachining. It starts by giving an overview of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects. Both kinds of micromachined substrates, closed-membrane-type and the suspended-membrane-type, are discussed. The deposition of the sensing layer is complicated by the mechanical fragility of the micromachined substrates. Different approaches used for the formation of the sensing layer such as thin film and thick film deposition techniques are reviewed. Finally, the gas sensing function of the sensitive layer is analyzed and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.

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.

Influence of the catalytic introduction procedure on the nano-SnO2 gas sensor performances: Where and how stay the catalytic atoms?☆

Abstract The role and activity of catalytic additives on solid-state gas sensors are determined by the additive chemical state, aggregation form and interaction with the semiconductor oxide. All these parameters depend on the technological steps involved in the element introduction and the treatments applied to the sensor material. The aim of this work is to analyse the influence of the additive introduction procedure on the gas sensor performance. In order to achieve this objective, two sets of different palladium, platinum or gold modified tin oxide materials have been prepared. In a first set of samples, additives were introduced by impregnation of the previously thermally stabilised oxide. In the second set, catalyst addition was carried out before any thermal treatment was applied. The study of the catalytically modified materials, calcined at different treatment temperatures between 250 and 1000°C, has been performed by means of HRTEM, XRD, XPS, and Raman spectroscopy. The influence of both processes on additive surface concentration, chemical state, nanoparticle growth and resistivity values are presented and discussed. Moreover, electrical characterisation of the sensors prepared from these materials has been carried out.

CO sensing with SnO2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO2 based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H2O) and oxygen (O2) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO2. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R0 and the resistance in the presence of the target gases, R, namely R0/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O2 and H2O related surface species and, as a result, different sensing mechanisms can be observed for CO.

An n- to p-type conductivity transition induced by oxygen adsorption on α-Fe2O3

The simultaneous measurements of conductance and work function changes induced by gaseous ambient have been performed on α-Fe2O3 thick film polycrystalline samples kept at 280 °C and exposed to different gaseous atmospheres. The switching from n- to p-type conductivity on α-Fe2O3 is shown to have an electronic origin, which is due to the oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type. The modeling of the n–p switching is described in terms of conductance dependence on the band bending induced by gaseous ambient.The simultaneous measurements of conductance and work function changes induced by gaseous ambient have been performed on α-Fe2O3 thick film polycrystalline samples kept at 280 °C and exposed to different gaseous atmospheres. The switching from n- to p-type conductivity on α-Fe2O3 is shown to have an electronic origin, which is due to the oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type. The modeling of the n–p switching is described in terms of conductance dependence on the band bending induced by gaseous ambient.

Making environmental sensors on plastic foil

With the emergence of the printed electronics industry, the development of sensing technologies on non conventional substrates such as plastic foils is on-going. In this article, we review the work performed and the trends in the development of environmental sensors on plastic and flexible foils. Our main focus is on the integration of temperature, humidity, and gas sensors on plastic substrates targeting low-power operation for wireless applications. Some perspectives in this dynamic field are also provided showing the potential for the realization of several types of transducers on substrates of different natures and their combination with other components to realize smart systems.

Influence on the gas sensor performances of the metal chemical states introduced by impregnation of calcinated SnO2 sol–gel nanocrystals

Abstract The effects of the introduction of Pt and Pd by impregnation in sol–gel fabricated SnO 2 nanoparticles after calcination are reported in this paper. The differences in base resistance and sensitivity of sensors prepared using these powders are presented and explained — taking into account the chemical states of the metal additives and the generated surface states in the band gap of the SnO 2 .