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HYDROGEN ECONOMY
Gas analytical systems and hydrogen sensors
SEMICONDUCTOR NOX SENSORS
V. M. Aroutiounian
Member of International Editorial Board
Department of Physics of Semiconductors and Microelectronics at Yerevan State University 0025, 1, A. Manoukyan, Yerevan, Armenia, e-mail: kisahar@ysu.am
Vladimir M. Aroutiounian received his physics-engineer MS degree from Kiev Polytechnic Institute in 1964, the doctoral degree (Ph. D.) from Yerevan State University in 1970 and Dr. Sc. degree from Vilnius State University, Lithuania, in Physics of Semiconductors and Dielectrics in 1977. Academician of National Academy of Sciences and Engineering Academy of Armenia.
He is currently the full professor, head of Department of Physics of Semiconductors and Microelectronics and the research supervisor of Research Laboratory of Physics of Semiconductor Materials and Devices at Yerevan State University.
His current research interests include chemical sensors, physical properties of porous and quantum-well semiconductor structures, photoelectrochemical and photoelectrical conversion of solar energy.
Aroutiounian Vladimir M.
Necessity in nitrogen semiconductor oxide and dioxide sensors with numerical output is very important for science and technical systems. Present review paper gives information about sensitive semiconductor materials, technology of their manufacturing and main parameters of sensors.
Introduction
Last 30 years, research and development of gas-sensing devices is in the focus of activity of scientists and engineers in many countries. Such 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 precious technological processes in industry. Gas sensors can be manufactured using different materials, technologies and phenomena [1-9]. Sensing devices should be smaller and cheaper than the analytical devices currently used and have high sensitivities. We considered earlier [4, 5, 7] and below semiconductor sensors, which found soon remarkable positions in science and technology as 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 due to high consumed electric power and working temperatures, inaccuracies and inherent characteristics of the sensors themselves.
So, suitable semiconductor materials with the required surface and bulk having higher sensitivity, stability and selectivity are demanding today.
Hydrogen (H2), oxygen (O2), carbon monoxide (CO), hydrocarbon (including methane CH4) sensors were reviewed by author in [5, 7]. Nitrous oxide (NO) and nitrogen dioxide (NO2) gases are also among the important gaseous species. Solid state sensors (mainly metal oxide nitric devices) are considered below.
Most of combustion processes result in the formation of nitric oxides, commonly considered inside the global label NO^. NO2 is the main gas to be detected because NO is easily oxidized to NO2 in atmosphere. There is a complex relationship between emissions for NO^ and the resulting concentrations of NO2. It transforms in the air to form gaseous nitric acid and toxic organic nitrates. NO2 even at low concentration is a highly toxic gas. Nitrogen oxide NO^ (NO or NO2) can cause diseases of respiratory system and leads to pulmonary edema and death. NO2 catalyze the formation of toxic ozone in the troposphere, being one of the causes of urban photochemical smog. Cumulation of NO^ is the main reason of acid rains.
Лекция профессора Ереванского государственного университета, академика НАН Армении В. М. Арутюняна будет представлена во время торжественной церемонии награждения в Государственной Думе РФ 29 ноября 2008 г. в 1500. Lecture of professor of the Yerevan State University, academician of the NAS of Armenia V. M. Aroutiounian will be presented during rewarding ceremony in the RF State Duma November 29, 2008 at 1500.
This gas also participates in a chain reaction that destroys the protective stratospheric ozone layer, resulting in an increase of ultraviolet radiation reaching the surface of the earth. NO2 is often found at higher levels indoors compared with outdoors. Mainly this occurs in settings where gas stoves and kerosene heaters are being used. In this sense, the emission control and effective methods to detect nitrogen oxides are highly demanded today to prevent environmental and health problems [10-12]. Therefore, to optimize combustion and reduce emissions, it is necessary the precise monitoring of exhaust gasses in boilers, heaters, gas stoves, combustion furnaces, vehicles or automobile engines and their direction by controlling initial reactant mixtures.
Therefore the development of portable fast-response sensors that are robust, small sized, long lifetime, quick in response and with sufficient sensitivity for the detection of nitrogen dioxide in low concentrations, such as few ppm, in the ambient is necessary and demanded also in order to prevent irreversible changes in the global atmosphere.
For this reason efforts made nowadays by scientific research community in leading laboratories all over the world have been focused on the development and investigation of novel NO2 sensitive materials suited for solid-state gas sensors, consequently, their performances have to be improved dramatically by adopting preparation conditions and by controlling post-deposition processing.
Hence, there is a great interest in developing chemoresistive gas sensor based on metal oxides for the detection of low concentration of NO2 and NO^ for air-quality monitoring or exhaust-gas control. For the automotive exhaust monitoring, NO^ sensors should be able to detect NO^ concentration from 10 up to 2000 ppm in very harsh environment where the temperature fluctuates from about 500 °C up to 900 °C. The engine temperature may occasionally reach up to 900 °C during vehicle acceleration and may keep it for a while thereafter. It is therefore inevitable that for a practical implementation of the reliable NO^ sensor, high sensitivity in the range of 100-1000 ppm as well as high selectivity to NO^, long-term stability at high temperatures, fast response and recovery rates should be realized for demanded sensors. An ideal on-board NO^ sensor should attain stability over a long period of operating time (more than 10 years). Such compact and accurate sensors should measure NO levels in oxygen-rich exhausts to aid in the precise dispensation of reducing agents.
Related also to environmental concern, a necessity in NO^ and ammonia sensors is demanded for combustion exhaust control in power plants, where NO^ is removed by a chemical treatment with NH3 over catalysts. Also the need of gas sensors suitable for safety and industrial process control applications has been raised.
In actual case, the optimum sensor performance is not achieved well due to insufficient understandings of the SE-gas interactions as well as the sensing mechanism. For instance, deposi-
tion techniques and processing parameters can affect the chemical composition, the microstructure and the morphology of metal oxide films, consequently influencing the gas sensing properties. Thus, it is important to study the microstructure of the SE film for fabrication of practical NO^ sensors as well as for understanding of the sensing mechanism.
Materials and Technologies
Promising materials for manufacture of sensors and their form, parameters (operating temperature range in °C), range of detection limit, response time) as well as measured physical parameter are listed in Tables 1-3.
SnO2, In2O3 and In2O3—SnO2 sensors
SnO2 thin films for high-sensitivity gas sensor structures were prepared in [36] using the rheotaxial growth and thermal oxidation (RGTO) method. Connections between agglomerates, so-called necks created during the final oxidation. A significant increase in size of the previously formed particles was observed what results in almost 30 % increase in the film thickness. That constituted the paths for the electric current. The sensing mechanism of such SnO2 thin films manufactured by RGTO method proposed in [36]. The sensor response of SnO2 thin film to NO2 is maximal at 200 °C working temperature for the Sn deposition on Al2O3 substrate at between 265 and 275 °C, what was in a good correlation with the value of surface coverage by Sn droplets obtained in the first step of RGTO process.
In2O3 and SnO2 have good stability and sensing properties to NO2 and NO gases and satisfactory selectivity suitable modifiable by noble metal/ catalyst or temperature modulated operation (see, [37-43] in the Table 1 and references herein). Both SnO2 and In2O3 oxide sensors are mainly based on electrical resistive changes. Let us discuss results obtained in last years.
In recent years, a great attention has been paid to the development of a new concept design for gas sensors based on the use of two different combined oxides instead of a single oxide as active material in order to exploit the synergic effect of the two different phases in the gas detection mechanism. The advantages that can derive from these mixed systems consist in the possibility of increasing the sensitivity and the selectivity towards specific gases, the availability of a double receptor-transducer function in the same materials system and the stabilization of the sensing material, ensuring, for example, the maintenance of a nanocrystalline structure.
Although both SnO2 and In2O3 thin films have already been studied as gas sensors, their sensing properties combined in a binary oxide are not completely known and, in particular, the potentials are not fully exploited for NO2 gas detection. In2O3 shows electrophysical and chemical properties essentially different compared to SnO2 and this different behavior seems to considerably modify the
Table 1
NO, NO2, NOx sensors
Operating Range of detection limit Sensing element form Sensor Response time
Material of work body temperature physical Reference
range (°C) parameter
1 2 3 4 5 6 7
131-313 0.01-0.25 T-ck F R ~1 h 13, 14
300-450 Up to 9 ppm TF R 15
300 100 ppb TF R 16
150-300 TF R 3 s 17
SnO2 200 25-100 ppb TF R ~2 min 18
130 up to 100 ppm nXl TF R ~10 min 19
150-200 up to 800 ppb TF WF 20
200-700 100 ppm Disks R ~30 s 21
100-350 5-800 ppb TF CVC ~30 min 22
350-550 50-3000 ppm ceramic R 20-60 s 23
SnOi.85 525 T-ck F WF&R 24
SnO2 <Pt, Au, Al> 250-400 1-10 ppm ThF, FET R <10 s 10, 25
SnO2 -Pt 35-160 FET R 26
150-300 TF R 27, 28
SnO2- Pt 300 10-1000 ppm JFET CVC <3 min 29
SnO2 -Pd 225-500 SchB & R CVC 30, 31
SnO2 -Rh/Pd 450-500 30-50 ppm TF R ~5 min 32
SnO2 (Au) 550 up to 45 ppm TF R <20 s 33
SnO2 (Pt, In, Au) 200-400 2-70 ppm TF R 1-2 min 34
Pt, Ni, Au, Mo coated with SnO2 20-280 up to 1000 ppm TF R 35
SnO2 200 RGTO F 36
0.5 wt. % SrO- SnO2 500 10-300 ppm ceramic R 20-60 s 23
SnO2 - CdO 250-300 18-100ppm TF CVC 1 37
Sn-Zn-Sb-O 400&600 up to 5 ppm TF R ~1 min 38
SnO2 - Al2O3 300 18-199 ppm TF CVC ~1 min 37
In2O3 250 0.7-7 ppm nXl TF R ~5 min 39
In2O3-SnO2 (NO2) 100-350 2-20 TF R 40
In2O3-SnO2 (NO2) RT-400 1-3 ppm TF R 10 min 41
In2O3 -SnO2 (NO) RT 60-2000 ppm TF FreqShif < 20 s 42
In2O3, In2O3-MoO3 150-350 400-800 ppb TF R 1-4 min 43
SnO2<Ru> RT nanowire R 46
In2O3 nanowire R 47
CoO- In2O3 129 100 ppm Disks C ~ 4 min 49
SnO2-Fe2O3 150-450 50 ppb-10 ppm nanoXl R 50
Fe2O3 (Au, Zn) 150-400 2-20 ppm TF t 2-5 min 51
ZnO 775 52
200-400 100 ppm Disk CVC <0.5 min 21
ZnO(Er) 150-400 5 ppm TF R 53
Zn-Sn-Sb-O 400&600 300-700 up to 5 ppm up to 3000 ppm TF ElChCell R Current 38 54
ZnO<Ir> 200-375 NO nano R 55
The following notations are made force, FET — field effect transistor, TF
in Tables: R — resistance, WF — — thin film, T — ck F — thick film,
work function, EMF — electromotive CVC — current-voltage characteristics.
traditional gas detection mechanism based on the mediation of surface chemisorbed oxygen species. An interaction mechanism manly based on redox reactions reversibly changing the surface composition of the metal oxide film seems to be a more realistic situation.
Systematic gas-sensing tests towards NO2 in dry air have been carried out for In2O3-SnO2 oxide based sensors. The gas-sensing properties of these sensors to different NO2 concentrations at different working temperatures have been analyzed and compared with the corresponding properties
of the single oxide SnO2 and In2O3 based sensor. All the sensors showed high responses to NO2. In general, the best performances in terms of response, sensitivity and low detection limit were found in In2O3-SnO2 based sensor.
In paper [40] nanocrystalline SnO2, In2O3 and In2O3-SnO2 (molar ratio 1:1) thin films have been prepared by modified sol-gel methods. The structural and morphological properties of SnO2, In2O3 and In2O3-SnO2 oxide have been investigated in order to characterize their sensing properties as NO2 gas sensor based on electrical resistive changes. Their performances in the detection of nitrogen dioxide (2-20 ppm in dry air) have been analyzed by electrical characterization in controlled atmosphere.
The mean crystallite size in the heat-treated powders was found equal to 3.5 nm for In2O3. Drop-coating of the powders onto alumina substrates allowed the fabrication of gas-sensing devices.
The SnO2 and In2O3 based devices were tested to NO2, displaying outstanding responses even at temperatures as low as 100 °C, and fast responses and recovery times. Results of measurements are shown in Figs. 1 and 2.
Gas-sensing nanocomposites of SnO2-In2O3 were prepared in [44] using a chemically controlled co-precipitation method. Through manipulating the Sn/In cation ratio, metal salt total concentration, precipitation pH value and aging conditions, the nanocrystalline composite powder was successfully derived with chemical homogeneity and superior thermal stability compared to the single-component oxides. The experimental results showed that these nanocomposites exhibited high sensitivity and selectivity for the detection of CO and NO^, and the sensitivity depended on the composition of the composites, calcination temperature and operating temperature. The 40 % InO1.5 content, calcination at 600 °C and operation at 250 °C and 200 °C were most optimal for CO and NO2 gas-sensing, respectively. The incorporation of In2O3 as a secondary component suppressed the grain growth of SnO2, thus resulting in the increased sensitivity. Particularly, additives of Pd and Al2O3 as a dopants and a surface coating, respectively, greatly improved the sensitivity and selectivity of the optimized nanocomposite.
Mixed oxide (Sn-In)O + Pt with various composition Sn-In = 1:1, 2:1, 5:1 and 10:1 for sensing materials were prepared in [45]. Fabricated micro-gas sensor shows low resistance; gas sensing characteristics have linearity for wide range (0.5-20 ppm) of gas concentration.
Tin oxide has been synthesized in various forms like whiskers, nanorods, nanotubes, nano-diskettes, nanobelts and nanowires [46]. Such a new generation of metal oxide semiconducting na-nostructures has attracted the interest due to their potential applications in sensor field. Because of their peculiar structural characteristics, the effects arising from size reduction result in novel physical properties for these materials. Newly developed metal oxide nano-belts and nanorings are potential candidates for fabrication of nanoscale
100 t
10-5 10-6 10-7 10 10-9
450 400350 -300250200150 100 50 0
3
Time, h a
10
NO.
12
2 PPm b
1 SO 2<y 150
T, °C
Fig. 1. NO2-response curve to 5 ppm NO2 for the In2O—SnO2 thin film based sensors as function of operating temperature
10 10-4 "I
Fig. 2. (a) Transient response of SnO2 and In2O—SnO2 thin film based sensors to different concentration of NO2 in dry air at T = 250 °C; (b) corresponding calibration curve
devices. Their extraordinary sensing properties have been recently shown for ultra-sensitive gas and DNA detection. Yang et al. have reported photochemical gas sensing properties of single crystalline SnO2 nanowires towards NO2, where high sensitivity and reversibility was achieved only at elevated temperature or by exposure to the UV light corresponding to energy near the SnO2 band-gap. The synthesis of Ru-doped SnO2 nanowires and their unique response towards NO2 and LPG was reported in [46]. The incorporation of Ru in SnO2 thin film results in the low temperature crystallization implying the role of Ru as a promoter/ nucleating aid besides sensitizer. The gas sensing properties of such wires were demonstrated by studying their response towards NO2 in air at room
1
2
4
5
6
temperature. Furthermore, Ru being one of the excellent sensitizers towards LPG, these wires was studied for their gas sensing behavior towards LPG, which interestingly exhibited maximum sensitivity towards LPG at 250 °C in agreement to the results obtained for Ru-doped SnO2 thin film.
The SnO2 nanobelt sensor response was 900% for 200 ppb 2NO2 at 300 °C [47]. It has been studied the variation of the response as a function of the density of the nanobelts. The results demonstrate the potential of fabricating nanosize sensors using the integrity of a single nanobelt with sensitivity at the level of a few ppb and the necessity to control nanobelts density to optimize the sensing performances.
Nano-wires of indium oxide for gas sensing were proposed in [48]. Main attention has focused on the influence of the deposition conditions (pressure, evaporation temperature, gas flux and substrates temperature) of these structures on the morphological and electrical properties of the nanos-tructures, highlighting that the response is strictly dependent on the lateral dimensions of the wire.
The CoO-In2O3 system was also used for the manufacture of NOx sensors [49].
Nanocomposites SnO2-Fe2O3 have been obtained in [50] in whole concentration range (0100 mol. % Fe2O3) using wet chemical synthesis. The gas sensor properties of nanocomposites towards CO (40-150 ppm), ethanol (10-200 ppm), H2S (2-10 ppm) and NO2 (50 ppb-10 ppm) have been studied by conductance measurements in tem-
perature range 150-450 °C. Nanocomposites of SnO2-Fe2O3 have been synthesized by wet chemical method and tested as gas sensors. Results highlight that balancing the SnO2-Fe2O3 molar ratio sensors performances can be tailored to obtain materials suitable for different applications. At x = 0.015 sample revealed the most sensitive towards NO2. Fe2O3 <Au, Zn> sensing materials were investigated in [51].
ZnO and tungsten trioxide sensors
ZnO has rather good stability and sensing properties to NO2 and NO gases and satisfactory selectivity (see, [21, 38, 52-54] in the Table and References herein). The Ir-doped ZnO sample shows no reaction towards NO2 over the whole temperature range [55]. A significant signal upon exposure of NO was observed from 200 to 325 °C. This behavior makes Ir-doped ZnO a promising NO2-tolerant NO sensing material in the temperature range from 200 to 375 °C.
NO2 can be detected with a high sensitivity on samples Al-doped ZnO [56]. [57] is a first report describing the formation of conductive tin-doped zinc oxide thin films by SILAR technique and rapid photothermal processing (RPP) and their NO2 gas sensing applications. Gas sensor elements based on the Sn-doped zinc oxide films were sensitive to NO2 at room temperature. It was experimentally demonstrated that tin impurities in ZnO films improved sensors gas-sensing properties to NO2 and produce a shorter response time. It is possible
NO, NO2, NOx sensors
Table 2
Material of work body
Operating temperature range ( °C)
Range of detection limit
Sensing element form
Sensor physical parameter
Response time
Reference
1
2
4
5
WO3-TiO2 W-Ti-Mo-O
350-800 l75-380 350-800
l-20 ppm up to 45 ppm l-l0 ppm
nXl TF T-ck F TF
R R R
1-2 min ~2 min
59
60 6l
WO3 <Au, Pd, Pt>
25o-35o
l-l0 ppm
Thick F
R
<l0 s
lo
WO3
l00-250
NO and NO2 Up to l00 ppm
SAW delay lines
Output phase
-1-2 min
62, 63
WO3
2oo-45o 25-35o l00-300
0.7-5.4 ppm 2-300 ppm up to 5 ppm
TF T-ck F TF
R R R
-5 min 3-5 min -lo s
64 65, 66 67, 68
WO3 (Ag) WO3 (Al, Ti)
25o-3oo 2oo-3oo
40 ppm 4 ppm
Pellet
TF
R R
20-30 m -l min
69
70
WO3 (with 1 % Metal Oxide)
350
40-80 ppm
Powder onto Al Tu
R
~2 min
7l
WO3 -Bi2O3 WO3 -SiO2 WO3 -TiO2
25-350 25G-450 350-800 l75-380
2-300 ppm 0.l-2 ppm l-20 ppm up to 45 ppm
T-ck F TF n TF T-ck F
R R R R
3-5 min 5-7 min
1-2 min
65 72
59
60
WO3 -ZrO2
500-700
5-l000 ppm
ElChCell
El-de
1 min
73
WO3 < In
l00-200
n-part TF
R
75
WO3-TiO2
30G-500
0.07-6 ppm NO2 0.07-5 ppm NO
TF
R
79
3
6
7
to tune these parameters by varying the film porosity and Sn concentrations in the solution: highly porous film sensors have a high sensitivity, but a longer response time. In situ patterned zinc oxide thin films were prepared in [58] by precipitation of Zn(NO3)2/urea aqueous solution and by microcontact printing of self-assembled monolayers on Al/SiO2/Si substrates. The ZnO gas sensor was exposed to the concentrations of NO (1000 ppm). The optimum operating temperature of the NO sensor was 200 °C.
Sensing properties of WO3 to NO2 and NO gases has investigated very intensive (see, [10, 59-73] in the Table 2 and References herein). Consider results obtained in last years.
Pure and activated (doped) nanocrystalline WO3 films, produced by advanced reactive gas deposition, were investigated for gas sensing applications [74]. Activation took place by co-evaporation of Al or Au with tungsten oxide as the particles were produced. Test gases of H2S, NO2, and CO were used at different concentrations on the ppm level. Any overlap in its maximum gas-specific sensitivities, which implies that chemical selectivity, can be obtained when the sensor is operated at 400, 525, and 700 K (see Fig. 3).
140 120 100
£ 80
5000
4000
- 3000 n
2000
1000
0
300
400
700
800
500 600 Temperature, K
Fig. 3. Dependencies of sensitivity on operating temperature [74]
The gas sensing properties of pure and indium-doped nanoparticle WO3 thick films were studied in [75]. As usual, the reduction of grain size in metal oxide films was one of the key factors to enhance the gas sensing properties of semiconductor layers. Thin metal oxide films with small grain size by using a special regime of rf sputtering from either metallic or metal oxide targets were manufactured. Sensors were prepared using commercial WO3 nanopowders and powder mixtures with different concentrations of In (1.5, 3.0 and 5.0 wt. %). It was found that the pure WO3 sensors responded to nitrogen dioxide even when operated at room temperature. These sensors showed a maximum sensitivity to NO2 when working at 100m%C. The fact that pure WO3 sensing layers show response to NO2 at low temperatures is due to the small grain size and high surface area of the WO3 nanopowders. Indium-doped WO3 sensors
were selective to NO2 and CO when operated at 200 and 300 °C, respectively. In this case, the selectivity of the sensors is determined by the defects generated by indium impurities.
Other methods have been also used to prepare nanometric WO3 thin films, including sol-gel, dc and rf magnetron sputtering and thermal evaporation. It has been reported that the sensitivity of the WO3 thin film to NO2 was enhanced by the addition of metals, such as Au, Al or Pt. It was established that the WO3 thin film was monoclinic and showed high sensitivity to low concentration of NO2 [76, 77].
WO3 sensors can detect 0.5 ppm dilute NO2 with high sensitivity at 57-200 °C using a micro-electrode [78]. Micro-gap electrode with various gap sizes was fabricated on Si substrate by using MEMS techniques (photolithography and FIB) and deposited WO3 thick film on it to be micro-gas sensor.
In the last years the interest in micromachined gas sensors has become more and more important.
It is already known that the sensitivity to NO2 is many times higher than the one to NO and most of the reported promoters are also known to be good oxidation catalysts. As a result, one of the roles of the noble metal additives may be to provide a surface for the catalytic conversion of NO to NO2, which is responsible for the high sensitivity associated with the promotion.
Micromachined four-element integrated thick-film gas sensors based on pure and modified with noble metal additives WO3 and SnO2 were fabricated on double-side polished p-type <100> Si substrates [10]. The structure of the device basically consists of the gas sensing layer, the electrodes, insulating layers and a polysilicon heater. Different noble metals (Pd, Pt and Au) and quantity levels (1, 2 and 4 wt. %) were tested in order to fabricate a reliable sensor with high sensitivity and selectivity to NO2. The WO3 + 2 % Au sensors showed very promising results for NO2 gas detection. Their responsiveness was high, while their selectivity was as high as 0.97. Therefore, resistive gas sensors based on WO3 + 2 % Au screen-printed on micro-hotplates have a very interesting potential for inexpensive hand-held gas monitors.
A screen printed n-type WO3/TiO2 mixed oxide layer shown high sensitivity to NOx [79].
MoO3 and mixed oxide sensors
MoO3 films demonstrated to have good NO2 sensing capability [80, 81]. MoO3 exhibits a low evaporating temperature, permitting only low operating temperatures and it has a very high resistivity. A sintered sensor element based on molybdenum oxide (MoO3) has been fabricated. This sensor showed a good sensitivity towards NO and NO2 gases in the temperature range of 200-500 °C. The resistance of the sensor element increased upon the introduction of NO and NO2 gases. The sensor was also found to be suitable for methane gas; however, it showed a decrease in the resistance upon introduction of the gas. Satisfactory per-
NO, NO2, NO sensors
Table 3
Material of work body Operating temperature range ( °C) Range of detection limit Sensing element form Sensor physical parameter Respons e time Reference
1 2 3 4 5 6 7
MoO3 200-400 5-10 ppm TF CVC ~2 min 80
MoO3 -TiO2 150-370 1 ppm TF R 1 min 83
MoO,-SnO2 150-400 20 mbar Pellet R 2-10 min 85
Cr2O3-TiO2 350-400 100-10000ppm TF R 1-3 min 85
TiO2- Nb 600 100-300 ppm TF R ~1 min 86, 87
Ga2O3 -Ta2O5 600-1000 300 ppm TF R <1 min 88
CdO-ZnO 100-330 3-300 ppm TF R 89
NiO, 30 100 ppm MBE R 90
ZrO2(Au, Pt, Cr) 300-700 up to 3000 ppm ElChCell Current 54
NaSiCON 400 10-1000 ppm ElChCell EMF 30 s 98
Ba2WO5 300-600 80&200 ppm Chips R 3-5 min 109
BScCO 20-50 up to 10000ppm TF, FET CVC 24 s 110
Pt-NaNO3-Ag 250 0.1-1000 ppm ElChCell EMF 2-10 s 111
YCuO 550 up to 500 ppm Powder R 10 min 112
YBCO 20-50 up to 1000 ppm TF FET 24 s 113
formance of the present sensor to NO2, NO and CH4 at concentrations of 100, 250 and 500 ppm, respectively, was obtained [81].
The addition of a second element may improve the gas sensor performances, varying the morphology of the film, the grain size and the electrical properties. MoO3 and WO3 are transition metal oxides with similar physical and chemical properties; in particular, they show n-type semiconducting properties related to the presence of lattice defects, mainly oxygen defects. They are extensively studied for their potential applicability in chemical gas sensor devices. Recent studies focused on MoO3-WO3 mixed oxides report on their promising gas sensing potential [82]. Parameters of sensors made of MoO3-TiO2, V2O5-MoO3 and MoOx-SnO2 mixed oxides are reported in [83-85]. Other mixed oxide sensors have parameters shown in Table 3 [86-90]. The VO2 samples with MoO3 were found to be sensitive to NOx [91]. In paper [76], micro NO2 gas sensors were fabricated with WO3-based thin films on silicon-based substrates. Vanadium pentoxide (V2O5) was sputtered on the WO3 thin film as an additive. The effect of the thickness of V2O5 layer on the NO2 response has been investigated in order to develop a thin film sensor that is more sensitive to NO2. The grain size of WO3 thin film annealed at 600 °C is smaller than that of WO3 thin film annealed at 400 °C. Coating with the V2O5 layer changed the surface structure of WO3 thin film. The WO3 thin film with the 20 nm V2O5 coating layer annealed in air at 600 °C for 4 h was porous than that of the pureWO3 thin films. Coating with the 20 nm V2O5 layer improved the response of WO3 thin film to NO2.
Cuprous oxide layers are used in [192] as NO2 gas sensitive material in a novel gas sensor element. They were fabricated by chemical deposition
and rapid photothermal processing (RPP) method. Such technology not only allows green materials preparation but also improves the performance and reliability over conventional methods of the production of sensors for continuous environmental monitoring. A good sensitivity and response time was found towards NO2 gas in the concentration range of 1-2 ppm at relative low temperatures 150 ± 20 °C for the samples treated by RPP.
By introduction of various dopants onto the CoTiO3<La> surface changes in sensitivity were observed in [93] with regard to the working temperature of the sensor. Especially with Au-doping a NO tolerant NO2-sensor can be achieved. Thin films of barium strontium titanate deposited by sol-gel spin coating technique have been found sensitive to NO2 and ammonia [94].
Solid electrolyte based NOx sensors
ZrO2, YSZ (formulae: (ZrO2)0 92(Y2O3)0.08), other solid electrolyte materials and high temperature NOx sensors made of them are produced on large-scale level many years. Usually Pt catalyst dispersed onto their surface. Usually the traces of NO in air oxidated on the sensing layer of the sensor by reaching the thermodynamic equilibrium over the whole working temperature range (optimum temperature is equal to 600 °C) [54, 95-98].
A sensing element consisting of Nd2CuO4 and Pt electrodes on opposite sides of an yttria-stabi-lized zirconia (YSZ) disk. The presence of NOx produced an increase in DC current at 400 °C. As electrode is usually used LaFeO3 or CdCr2O4-coat-ed Pt electrode. In [99] La0 85Sr0 15CrO3 is used in high-temperature NOx sensing elements. A solidstate planar sensor consisting of scandia-stabilised zirconia (8 mol. % Sc2O3-ZrO2) solid electrolyte sandwiched between CuO + CuCr2O4 mixed-oxide
and Au foil electrodes has been tested in NO2 containing gaseous atmosphere in the range of 100500 ppm NO2 at 611 and 658 °C [100]. The sensor has been found to respond consistently to change in concentration of NO2 in the oxygen rich gas mixture within 8 s. The sensor showed negligible cross-sensitivity to O2, CO and CH4 in the gas stream. Mixed potential phenomena occurring at the oxide/YSZ electrode in the presence of NOx -containing air is explained in [101] in terms of the absolute potential model, which demonstrates how maintaining a non-equilibrium state with gas phase, through catalytic inactiveness of the electrode, is important to obtain a noticeable EMF response to NO2 and NO. The YSZ-based NOx sensor attached with NiO-SE was fabricated and tested in [102] for detection of NO2 in different environments. The sensing characteristics were examined in the temperature range of 800-900 °C under dry condition as well as in the presence of 5 vol. % water vapor. It was found that NiO-SE provided the highest sensitivity to NO2 at such high temperatures among known single-oxides SEs. Proposed sensor showed quicker recovery and higher sensitivity to NO2 at 850 °C, if water vapor was present in the sample gas. The sensor capable to detect NO2 on-board at high temperature in car exhausts.
A new solid electrolyte type nitrogen monoxide gas sensor which can operate in the intermediate temperature region was fabricated in [103] by the combination of trivalent aluminum cation conducting (Al0.2Zr0.8)20/19Nb(PO4)3 and divalent oxide anion conducting yttria stabilized zirconia (YSZ) with LiNO3-doped (Gd0 9La0 1)2O3 as the sensing auxiliary electrode. The present sensor shows such a practical performance of a rapid, stable, continuous, and reproducible response as low as at 523 K, and the linear relationship, which obeys the Nernst theoretical relationship, was clearly observed between the sensor EMF output and the logarithm of the NO concentration. Since the present sensor using the LiNO3-doped (Gd0 9La0 1)2O3 auxiliary electrode shows such a high sensing performance, it is expected to be a new type of the NO gas sensing device applicable in the intermediate temperature range of around 523 K.
Mixed potential sensors using dense, thin film metal oxide working electrodes, Pt counter electrodes, and thin film YSZ electrolytes on Al2O3 polycrystalline and sapphire substrates were prepared and studied between 450 and 650 °C [104]. Their response to NO, NO2, CO, and C1 and C3 hydrocarbons in 10.4 % O2/N2 balance and in air atmospheres was characterized. The lanthanum chr-omite-based sensors showed preferential sensitivity to NO2 with cross sensitivity to CO and non-methane hydrocarbons such as C3H6 and C3H8 with the highest NOx sensitivity and minimal CO/HC cross sensitivity exhibited by a sensor prepared with a La0.8Sr0.2CrO3 working electrode. Studies of the Mg-doped LaCrO3 devices conducted for up to 800 h at 600 °C showed minimal aging in these devices.
A new capacitive-type gas sensor for NO2 detection was manufactured in [105, 106] by depos-
iting NaNO2-based solid electrolyte layer together with an Au electrode over insulator-semiconductor structure. The resulting device exhibited expected metal-insulator-semiconductor (MIS) characteristics at 130-160 °C in air. In NO2 containing air, the voltage at constant capacitance was found to shift in negative direction in a well-controlled manner as the NO2 concentration increased. The voltage necessary to keep the capacitance was found to decrease linearly with an increase in the logarithm of NO2 concentration.
The activities of Pt/YSZ, PtAu/YSZ and RhPtAu/YSZ thick film electrodes in O2, N2 and NO, N2 gas mixtures at high temperatures were investigated in [107]. A new amperometric NO sensor with only one working electrode made of RhPtAu mixture was tested in simulated gas mixtures containing NO, O2, N2, resulting in a linear response to the NO concentration, which is nearly independent of the O2 concentration.
YSZ-based electrochemical sensors with two parallel Pt finger electrodes, one coated with WO3 thick film as sensing electrode, were fabricated [108]. Measurements at an engine bench test at air/fuel stoichiometric value and under lean and rich conditions at different operating temperatures, up to 700 °C, showed good performance in terms of sensitivity, stability, reproducibility and response time.
Other materials for NOx sensors
Many studies to detect low NO and NO2 concentration have been carried out on other oxides, materials containing phthalocyanine coupled mainly with SAW devices, high temperature superconducting oxides and nanotubes. Some references are given in Table. Sensors based on semiconductor oxides are generally low in cost and show high stability, even in corrosive environments. The physical property to be transduced into the electronic signal is the resistance change caused by gas adsorption over the sensor surface. The disadvantages of these sensors are: 1) a working temperature of hundreds of Celsius degrees and 2) the interference with other gases. A stable, sensitive sensor working at room temperature has not been developed yet.
Low temperature annealing was performed in
[114] on CuPc films to evaluate the possible structure transformation of the sensing films during the gas sensing period. The effects of heat annealing, as well as the doping time of NO2, on the sensing characteristics of copper phthalocyanine films were investigated at a temperature as low as 100 °C. The structure transformation causes a decrease in film resistance and sensitivity to NO2, but an increase in response rate. After the NO2 doping period, the CuPc films cannot recover completely to the original resistance.
Investigations of NOx sensors made of carbon nanotube (CNT) are at a beginning stage. In
[115] it is shown an efficient method for NOx detection in single wall carbon nanotubes ordered by mean of dielectrophoretical process after dis-
persion in clorophormium solution over gold inter-digitates. The NOx flux induces resistance change. It is well known that WO3 sensors are usually operated at temperatures well above 250 °C. In [116] it has shown that the addition of a suitable quantity of MWCNTs in a WO3 film can lower the sensor operating temperature. The response of the hybrid films to NO2 was found to increase dramatically when only a few MWCNTs were added into the WO3 films. For example, low concentrations of hazardous gases such as 500 ppb of NO2 or 10 ppm of CO could be detected even when the sensors were operated at ambient temperature.
The Al/CNT sensor response to NO2 gas was characterized by fast and large resistance increase at the moment of NO2 exposure, whereas the resistance of the other metal/CNT sensors monotonously decreased [117]. It was suggested that the adsorbed NO2 molecules might alter the Schottky barrier at the Al/CNT interface as well as the positive hole density in the p-type semiconducting CNT. The Al/CNT sensor response could be interpreted as a superposition of the Schottky contact resistance and the CNT resistance, which were differently influenced by the NO2 adsorption and contributed to the overall sensor response. The Schottky response of the Al/CNT sensor was approximately one order of magnitude faster than the CNT response obtained using the other metal electrodes. Sensitivity of MWNT films to NO2 were investigated in [118].
Porous silicon and silicon carbide sensors
Physical phenomena in porous silicon (PS) and its possible applications of it are in the focus of many investigators (see for example [119]). Optical properties of PS have been considered more extensively due to the intense visible photoluminescence. Recently, more interest has been attracted to the sensor properties of PS. The large surface to volume ratio of PS gives PS the ability to react with different gases and sense them (see our publications [120-128] and many others. The fabrication (utilizing CMOS fabrication methods) of porous silicon based sensors was discussed in literatures, which, while operating at room temperature, are selective to a wide variety of polluting gases. A highly sensitive, rapidly responding room temperature device based upon a porous silicon interface that is able to repeatedly detect different gases was manufactured. The advantages of PS gas sensors are the low cost, low power consumption compared with other type gas sensors due to working without pre-heating of its work body (at room temperature), and its compatibility with silicon device fabrication technology.
Several problems still exist for practical applications. The main problem is non-stability of sensors in time due to oxidation of PS in time relative humidity (RH) level interference significantly affects the sensor behavior. Response times are in the order of tens of minutes.
Nonetheless, the compatibility of the fabrication process with silicon standard technology has not been investigated in detail. Note that single crystal wafers of Si are very expensive materials
for gas sensing applications. So far, most studies have been performed on the porous Si formation on single crystal and poly-silicon substrate.
In particular, the electrical resistance of PS samples has been observed to change in the presence of NO and NO2 gases [129-132]. Quenching in photoluminescence of n-type PS in presence of very low concentration of NO and NO in inert gas (nitrogen) was also detected.
Recently, a new device — the adsorption porous silicon FET (APSFET) — where a PS layer was integrated in a FET structure by exploiting an industrial process has been reported [132]. Differently from other devices proposed in literature, in the APSFET the measured quantity is a current flowing not in the PS layer, but in the inversion channel of the FET. Surface and drain were fabricated before the PS formation, thus avoiding re-producibility problems. Moreover, integration technology allows the miniaturization of sensor dimensions and then the fabrication of a sensor array on the same chip as well as the integration, on the same chip, of conditioning/driving electronic circuits. In this paper, detection of NO with the APSFET is demonstrated for the first time. NO concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. No degradation on the sensitivity of oxidized samples, with respect to non-oxidized ones, has been observed. While non-oxidized sensors show a high sensitivity only for fresh devices, which degrades with the aging of the sample, oxidation of the PS film improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the RH level.
Conclusion
Intensive research and development of nitrogen oxide and dioxide sensitive materials were carried out. Tin dioxide, zinc oxide, In2O3, tungsten and molybdenum trioxide as well as mixed oxide solid solutions In2O3-SnO2, MoOx-SnO2, MoO3-
2 3 2 x 2 3
TiO2 and Cr2O3-TiO2 were recommended as promising materials for NOx sensors working in work body pre-heating temperatures range 200-400 °C.
Near-room temperature NOx sensors made of SnO2<Pt>, In2O3-SnO2, WO3-Bi2O3, NiOx and high temperature superconductors were realized.
Solid electrolyte based NOx sensors can successfully work at 500-850 °C. x
Porous silicon and silicon carbide sensors are very interesting for realization of gas sensitive integral circuits. It is necessary to improve the stability of porous silicon sensors in time.
Interesting possibilities open possible use different nanosized systems, including nanotubes, as NOx sensing materials.
Acknowledgements
This work was carried out in the framework of the ISTC A-1232 project and 041030 "Semiconductor Nanoelectronics" National Scientific Program.
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