metal oxide semiconductor sensors for determination of reactive gas impurities in air

1、ISSN 1070-3632. Russian Journal of General Chemistry, 2008, Vol. 78, No. 12, pp. 2545–2555. © Pleiades Publishing. Ltd., 2008. Original Russian Text © L.A. Obvintseva, 2008, published in Rossiiskii Khimicheskii

2、 Zhurnal, 2008, Vol. 52, No. 2, pp. 113–121. 2545 Metal Oxide Semiconductor Sensors for Determination of Reactive Gas Impurities in Air L. A. Obvintseva Karpov Institute of Physical Chemistry, Vorontsovo pole 10, Mosco

3、w, 105064 Russia phone: (495)7356557, (495)9161719 fax: (495)9166025 e-mail: obvint@yandex.ru Received June 5, 2007 Abstract―Characteristics of metal oxide semiconductor sensors intended for measuring O3, NOx, Cl2, C1O2,

4、 and HCl microconcentrations were discussed. Specific features of detection of these microimpurities with semiconductor sensors were determined. The size of signal generated by sensors with WO3-, ZnO-, and In2O3- based

5、 sensing layers was examined in relation to the O3, NOx, Cl2, C1O2, and HCl concentration. The sensitivities exhibited by the semiconductor sensors with respect to target impurities make them suitable for measuring the

6、ir maximum permissible concentrations in sanitary zones and for monitoring background ozone level in atmosphere. Examples of application of gas analyzers based on semiconductor sensors in determination of gas impuritie

7、s in the open atmosphere were given. INTRODUCTION Research efforts aimed to elucidate how the electrophysical characteristics of semiconductors are affected by gas adsorption were initiated in the 1940s. Significant a

8、dvances achieved since then in semi- conductor engineering also posed an inverse problem, namely, that of detecting gas impurities from the change of the electrophysical characteristics of the semiconductor. However,

9、by contrast to semiconductor instruments that were promptly integrated into all spheres of science and technology, for semiconductor sensors it took an incomparably long time to cover the distance from laboratory pro

10、totypes to mass-produced gas analyzers. The progress in this sphere owes much to research activities supervised by I.A. Myasnikov from Karpov Institute of Physical Chemistry (NIFKHI). Those studies were focused o

11、n the elementary physicochemical processes occurring on the semiconductor metal oxide surface and on physico- chemical applications of semiconductor sensors as high-sensitivity gauges (for summary of findings for mos

12、t of those studies, see [1]). To this end, sensors with unique designs, manufactured in laboratories as one-of-a-kind instruments, were employed. This was paralleled by development (for the most part, abroad) of appli

13、ed research activities on designing semicon- ductor sensors for determination of gas impurities in air and development of an appropriate mass production commercial technology [2–7]. Those efforts culminated in manu

14、facture of some types of sensors on the commercial scale. The principal manufacturers of metal oxide semiconductor sensors are City Technology (UK) and Figaro Inc. (Japan). The major drawback suffered by semicond

15、uctor sensors is poor selectivity, but the advantages they offer, namely, high sensitivity, promptness, small size, and low cost in mass production, still make them extremely attractive for application in gas analysis

16、. As to selectivity of semiconductor sensors, much efforts is being devoted to its enhancement, and this problem is already solved for many of these applications [4, 8]. Semiconductor sensors offer much promise for de

17、termination, in particular via long-term monitoring, of reactive gas microimpurities in the atmosphere at background pollution monitoring stations (in the absence of anthropogenic emissions), as well as for air quali

18、ty control in industrial zones and residential areas. Since recently, ever increasing application has been DOI: 10.1134/S1070363208120347 METAL OXIDE SEMICONDUCTOR SENSORS FOR DETERMINATION RUSSIAN JOURNAL OF GENERAL

19、 CHEMISTRY Vol. 78 No. 12 2008 2547 Fig. 2. Signal generated by semiconductor sensors at variable ozone concentration in air. Sensor: (a) WO3, 250°C (working temperature) [15]; (b) WO3, 530°C [9], a

20、nd (c) In2O3:Fe2O3 (3%), 240°C [18]. Resistance, ? Analog signal Resistance, ? simulation of the sensor signal depending on the sensing layer parameters and preparation, under controlled conditions, of sensin

21、g layers with desired parameters. Specific Features of Detection of О3, NOx, Cl2, ClO2, and НСl Microconcentrations in Air with Metal Oxide Semiconductor Sensors Among the publications (especially the most recent ones)

22、dedicated to semiconductor sensors, the majority is concerned with ozone sensors (see, e.g., [2–4, 12, 15–20]); large number, with nitrogen dioxide [20–22]; much smaller number, with nitrogen monoxide [21– 22] and chl

23、orine [8, 23–26]; and only scarce publications, with chlorine dioxide [8, 24, 25, 27] and hydrogen chloride [28, 29]. It should be noted that research activities devoted to nitrogen oxide sensors were carried out mai

24、nly at fairly high (ca. 1 ppm) NOx concentrations with the aim to assess the suitability of these sensors for analysis of motor transport exhaust gases. Naturally, the interest in ozone sensors stems from their being

25、in high demand. At the same time, it is essential that, upon ozone exposure, semiconductor sensors generate large and completely reversible signals which can be reliably measured with high accuracy. Also, streamlined

26、 commercial production of good-quality ozone generators allows the necessary experiments to be fairly promptly arranged. The materials for sensing layers intended for semiconductor sensor detection of О3, NOx, Cl2, ClO

27、2, and HCl microconcentrations are represented primarily by the following oxides: In2O3 [4–6, 8, 18–20, 24–29], WO3 [2, 3, 9, 12, 15–17, 21, 23], ZnO [18, 19, 24, 28, 29], SnO2 [4, 22], both doped and undoped. Metal

28、oxide sensors with n-type conductivity typically generate an acceptor signal under exposure to O3, Сl2, СlО2, NO2, and NO microconcentrations; the appearance of these impurities in the gas phase causes the resistance

29、of the sensor to increase. This is ex- emplified by the kinetic curves for signals generated by WO3- and In2O3-based sensors at different ozone concentrations, presented in Fig. 2 [9, 15, 18]. All the sensors of inter

30、est generate completely reversible signals; the signals in the case of NO2, NO, СlО2, and Сl2 are qualitatively similar to, though weaker than, those in the case of О3 [20–27]. The signal from the sensor (In2O3- or Zn

31、O-based) under HСl exposure has specific features: Obvintseva et al. [28, 29] showed that, depending on air humidity and working temperature, the sensor can generate either acceptor or donor signal; specifically, the

32、resistance of the sensing layer can either decrease or increase. This phenomenon received an explanation under presumption that the donor signal is associated with adsorption of hydrogen chloride, and the acceptor si