ReviewMetal oxide hydrogen, oxygen, and carbon monoxide sensors for hydrogen setups and cells
Introduction
The very exciting concept of hydrogen economy is developing more than 30 years. A. Fujishima, J. Bockris, T. Nejat Veziroglu, and many other researchers were at the beginning of solar–hydrogen economy. They and other scientists are working also on other methods of hydrogen production. Some researchers try to solve the problems of storage of hydrogen energy and production of electric energy using fuel cells, etc. [1], [2], [3]. Many achievements in all these directions were adequately presented in the Journal of Hydrogen Energy. Several National Programs on hydrogen are acting currently; Hydrogen Energy Economics is in the focus of many states [4], [5]. I think that it is high time to draw attention of the JHE to also hydrogen, oxygen, and carbon oxide semiconductor sensors, which allow controlling electronically the content of these gases in operation of many hydrogen setups, cells and devices. The present review-paper summarizes the achievements in this field.
Research and development of gas-sensing devices is in the focus of activity of scientists and engineers in many countries in the last 20–30 years. Such detectors (including smoke-detectors) can be used for different applications—continuous monitoring of the concentration of gases in the environment and premises, detection of toxic dangerous gases, drugs, smoke and fire, energy saving, anti-terrorist defense, health, amenity, control of automotive and industrial emissions, as well as various technological processes in industry. Gas sensors can be manufactured using different materials, technologies and phenomena [6], [7], [8], [9], [10], [11], [12], [13]. Sensing devices should be smaller and cheaper in comparison with analytical devices currently used which have sensitivities in the range from tens to several hundreds of ppm. I consider below only metal oxide semiconductor sensors, which got remarkable positions in science and technology, since they allow producing fast, reliable, non-expensive, low-maintenance devices using modern electron technologies. However, many gas-sensing micro-systems have not yet reached commercial viability because of high consumed electric power and working temperatures, inaccuracies, and the inherent characteristics of the sensors. So, suitable semiconductor materials are currently needed, which have the required surface and bulk properties and high sensitivity, stability, and selectivity.
Oxygen (O2), hydrogen (H2), carbon monoxide (CO), nitrous oxide (NO), nitrogen dioxide (NO2), carbon dioxide (CO2), methane (CH4), ammonia (NH3), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), smoke and many others are among the important gaseous species.
Concentration levels of typical gas components are shown in Fig. 1 in Ref. [10]. They are limited in leading countries by corresponding standards(Environmental Standard, Ordinance on Health Standards in the Office, Offensive Odor Control Law, Working Environment Measurement Law, and Ordinance by Ministry of Health, Labour and Welfare, etc.) Gases like O2 and N2, and the humidity should be kept at adequate levels in living atmospheres, while hazardous gases should be controlled to be under the designated levels. One-tenth of lower explosion limit for lower hydrocarbons and H2 gases is usually taken as an alarming level. Standards for toxic gases, volatile organic compounds (VOCs), odors, and other air pollutants are different. Some standards of VOCs such as benzene are seen to be less than 0.1 ppm, far out of the level reached by the present gas sensors.
Current trends in and promising materials for the manufacture of metal oxide hydrogen, oxygen, and carbon monoxide sensors, and different technologies are described in more details below after some general comments.
Section snippets
Semiconductor materials for gas sensors
First semiconductor gas sensors were made of tin oxide (SnO2) and demonstrated in early sixties. Since that time, SnO2 is the main material in sensorics. Sensors made of tin oxide became commercially available for detecting fuel gas, carbon monoxide, general purpose combustible gases, ammonia, water vapour, etc.
The demand for better environmental control and safety intensified the research into solid-state sensors of gases such as hydrogen and toxic gases, which have caused major accidents of
Mechanisms of sensitivity of semiconductor sensors to different gases. Measurement of sensitivity
Physics, chemistry, and technology of semiconductor sensors require a better understanding of both the bulk and surface properties of the sensing materials. Although many different sensor arrays have been fabricated, the sensing mechanism is similar for many elements.
Adsorption/desorption processes determining gas sensitivity have activation nature. The general mechanism for vapour sensing metal oxide is a change in the resistance (or conductance) of the sensor when it is exposed to a gas in
Integrated gas sensors
It is evident that single crystal silicon (Si) wafers, pre-oxidized on both sides for electrical isolation, can serve as a better substrate for the gas sensor micro-systems, whereas silicon can be used as a construction material for transducer integrated circuits (IC). Integration minimizes mismatch among sensors, decreases the power consumed by sensor and increase the transducer signal-to-noise ratio. As the signal processing is carried out in sensors themselves, less noise is generated. Note
Hydrogen sensors
Hydrogen is the most attractive and ultimate candidate for future fuel and energy carrier. Its production may be realized by a variety of methods, including reforming of natural gas and alcohols (methanol, etc.), electrolysis and photoelectrolysis of water and biomass, as well as chemical decomposition of hydrogen containing compounds. Result of H2 burning is water, which is decomposed again into hydrogen and oxygen. The electric energy is obtained in hydrogen-based fuel cells from the
Oxygen sensors
Oxygen sensors are produced in a large scale exceeding several tens of million sets yearly. Oxygen sensors are widely used as sensors for automotive applications. To reduce the exhaust emissions from gasoline internal combustion engine automobiles, the air/fuel ratio is monitored with lambda oxygen sensors. Each car powered by a gasoline engine should be equipped with at least one lambda sensor measuring the air-to-fuel ratio to control engine operation. The second lambda sensor is used for
Carbon monoxide sensors
The CO detection is one of the most important. It is well known that CO is produced due to the incomplete combustion of fuels, it is commonly found in the emission of automobile exhaust. Such a toxic and dangerous gas is colorless and odorless. For CO detection, the German legislation fixes the maximum tolerance concentration in working place at 30 ppm and alarm level at 60 ppm, while in Italy an attention level is 12.9 ppm and alarm level is 25.8 ppm [183].
Promising materials for manufacture of CO
New results on hydrogen and carbon monoxide sensors obtained in Yerevan State University
Investigations of tin oxide thin-film hydrogen sensors made using sol–gel technique were carried out [185]. A solution of Na2SnO3 was used as a precursor. The analysis of the surface morphology of specimens obtained by TEM images have shown that the dimensions of SnO2 particles were about 4–6 nm and practically do not increase even after calcinations at .
The measurements of the gas sensors performances were carried out by means of the automated DAQ system developed by us, which allows
Conclusion
- 1.
Basic semiconductor materials for the manufacturing of hydrogen and carbon monoxide sensors are tin dioxide and indium oxide. Zirconia is main material for oxygen sensors. TiO2, ZnO, Fe2O3, NiO are promising oxides for sensors considered in the review.
- 2.
Many of metal oxide sensors need pre-heating and high operating temperatures, which lead to high consumed power.
- 3.
It is very difficult to envisage high selectivity of metal oxide sensors, although some success is achieved.
- 4.
New powerful method of
Acknowledgements
This work is supported by ISTC A-1232 and CRDF-IPP ARE-10838-YE-05 Projects as well as in the framework of Armenian National Program in semiconductor nanoelectronics. The author is thankful to R. Abelyan, H. Hovhannissyan and A. Poghosyan for their assistance.
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