A semiconducting metal oxide sensor array for the detection of NOx and NH3

Sensors and Actuators B 77 (2001) 100±110

A semiconducting metal oxide sensor array for the detection of NOx and NH3 Brent T. Marquisa,*, John F. Vetelinob a

b

Sensor Research and Development Corporation, 5 Godfrey Drive, Orono, ME 04473, USA Laboratory for Surface Science and Technology, University of Maine, Orono, ME 04469, USA

Abstract In many fossil fuel burning systems, NOx emissions are minimized by a selective catalytic reduction (SCR) technique where NH3 is injected into the ¯ue gas stream to react with NOx to form environmentally safe gases, such as nitrogen and water vapor. Unfortunately, this process is usually incomplete, resulting in either NOx emissions or excess NH3 (NH3 slip). Therefore, a critical need exits for an in situ sensor array near the stack to provide real-time control of the NH3 injection, and hence, minimize the NOx emissions released into the environment. In the present work, semiconducting metal oxide (SMO) ®lm technology is used to engineer a small, robust, sensitive, and selective sensor array to detect NOx and NH3 emissions. Many thin ®lm tungsten trioxide (WO3) based sensing elements were tested in order to identify two ®lm recipes capable of sensitively and selectively detecting NOx and NH3. The critical parameters inherent in each ®lm recipe are type of substrate material, ®lm thickness, doping, deposition temperature, and operating temperature. The two element sensor array's response characteristics analyzed include the response and recovery times, rates of reaction, dynamic range, sensitivity, repeatability and selectivity. # 2001 Published by Elsevier Science B.V. Keywords: Chemiresistive sensor; WO3; NH3; Nox; Sensor array

1. Introduction The detection and measurement of ¯ue gases are critical not only for achieving real time process control of new clean combustion systems, but also to minimize their emissions of dangerous air pollutants. Among the most dangerous of these air pollutants are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx. Currently about one-half of all NOx emissions into the environment are due to power plants and industrial boilers. NOx gas, which is the precursor to nitric and nitrous acid, causes acid rain and photochemical smog and is a critical factor in the formation of ozone in the troposphere. Ground level ozone is a severe irritant, responsible for the choking, coughing and burning eyes associated with smog. Ozone often damages lungs, aggravates respiratory disorders, increases susceptibility to respiratory infections and is particularly harmful to children. Elevated ozone levels can also inhibit plant growth and cause widespread damage to trees and crops. Therefore, exceeding critical NOx levels poses immediate health and environmental problems. In fossil fuel combustion NOx is formed by high temperature chemical processes from both nitrogen present in *

Corresponding author.

0925-4005/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 6 8 0 - 3

the fuel and oxidation of nitrogen in air. Typically, the NOx emissions consist of 90±95% NO with the remainder being N2O and NO2 [1]. Several methods have been examined as potential control systems for the reduction of NOx emissions in combustion processes including combustion control and ¯ue gas treatment techniques, such as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR) [2,3]. SCR technology achieves the highest overall control ef®ciency of 60±80% NOx reduction. In this process, NH3 is uniformly injected into the ¯ue gas stream that passes through a catalyst bed to enhance the kinetics of the NOx/NH3 reaction. This can be further improved with air staging to provide a longer residence time allowing more NOx to react with the NH3, thus increasing NOx reduction and decreasing NH3 emissions (commonly called NH3 slip). Although there are several chemical reactions, that may take place in the catalyst bed, the most common reaction reduces NOx into harmless nitrogen and water vapor. If a judicious choice of NH3 injection levels is made, NOx emissions in SCR systems, as well as NH3 slip into the atmosphere can be reduced signi®cantly. In order to control the precise levels of NH3 injection, a critical need exists for an NOx/NH3 sensor system in the ¯ue gas exhaust prior to the stack emission. The output from the

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

sensor would be fed back to the SCR system via a real time control system to adjust NH3 injection levels thereby maximizing NOx reduction with minimal NH3 slip into the atmosphere. Current NOx and NH3 measurement techniques, such as chromatography, chemiluminescence and infrared adsorption are very expensive and too bulky for in situ operation. Chemiresistive semiconducting metal oxide (SMO) ®lms offer the technology to develop a small, inexpensive and reliable in situ sensor array for the simultaneous identi®cation of NOx and NH3 present in a ¯ue gas exhaust. The ®rst report on chemiresistive SMO ®lms for gas sensing appeared in 1962 [4]. Since that time, considerable efforts have been made [5] to study SMO ®lms for the detection of a variety of gases. Investigators have examined SMO ®lms, such as SnO2 [6±15], TiO2 [15,16], indium tin oxide (ITO) [17], ZnO [15,17,18] and WO3 [19±22] for detecting NO, as well as ZnO [23±28], MoO3/TiO2 [29] and WO3 [30±34] for detecting NH3. WO3 ®lms can operate at elevated temperatures for long periods of time and selectively detect NO and NH3 in the presence of interferent gases, such as H2, CO, CO2, CH4 and various other hydrocarbons. These ®lms are also electrically and structurally stable at elevated temperatures. The electrical conductivity of thin WO3 ®lms doped with metals, such as gold (Au) and ruthenium (Ru) change upon exposure to NO and NH3. The sensitivity of WO3 to NO and NH3 depends signi®cantly upon ®lm parameters, such as thickness, dopant type, doping method, deposition temperature, annealing procedure and operating temperature. Furthermore, the functional relationship between sensitivity and each of these parameters is different for both gases. 2. Theory The WO3 ®lm conductivity changes as a function of NO and NH3 gas concentrations. These ®lms exhibit very fast response and recovery times and, after initial ®lm conditioning, show no appreciable aging effects after repeated gas exposures. The basic chemical sensing mechanism involves the dissociative chemisorption of the target molecules and the formation of transitory concentrations of chemisorbed atoms on the WO3 ®lm surface [35]. The rate of dissociation of the target molecules on the surface can be greatly enhanced by the addition of catalytic metals, such as gold or ruthenium. Although the overall chemistry of the possible interactions of NO and NH3 with WO3 is complex and not well de®ned, certain primary interactions dominate. In the case of NH3, which acts as a reducing agent (an oxygen scavenger), the carrier concentration in the ®lm rises as a result of a decrease in adsorbed surface oxygen, as follows: 2NH3 ‡ 3O …ad† ! N2 ‡ 3H2 O ‡ 3e

(1)

This rise in carrier concentration within the film is then manifested as a decrease in resistivity.

101

Unlike NH3, NO acts as an oxidizing agent (oxygen donor) at the temperatures of interest. Thus, reactions with NO result in an increase in chemisorbed oxygen in the ®lm, decreasing the free carrier concentration, as follows: 2NO ‡ 2e ! N2 ‡ 2O …ad†

(2)

This decrease in carrier concentration causes the film resistivity to rise. This behavior is completely opposite to that observed with NH3. 3. Experimental setup The WO3 sensing ®lms were deposited on alumina and sapphire substrates by reactively sputtering a pure tungsten target in an 80/20 argon±oxygen atmosphere using an RF magnetron sputtering system. Alumina and sapphire substrates were used because they are good electrical insulators and thermodynamically stable. A dc magnetron gun was used to precisely dope the WO3 ®lm with gold or ruthenium to selectively catalyze the reaction with NO or NH3. After the ®lms were sputtered, they were subjected to an annealing process, which transformed the as-deposited amorphous ®lm into a polycrystalline ®lm. This results in a more stable ®lm that is chemically and conductiometrically inert to moisture and many potential interferent gases. Microheaters, which controlled the temperature of the sensing ®lm, were fabricated by depositing a thin chrome serpentine structure on alumina or sapphire substrate. Electrical connections to both the sensing ®lm and microheater are made with 100 mm aluminum bond wire to the sensor package. In order to determine the electrical conductivity of a large number of WO3 ®lms exposed to a wide range of gas concentrations, a system capable of simultaneously testing and controlling up to eight sensors was designed and built. This system improved testing ef®ciency and insured that all ®lms were subjected to the same gas environment during each test. A block diagram of this system is shown in Fig. 1. The sensing elements shown in Fig. 1, reside inside the sealed Te¯on gas chamber. Two-point conductivity measurements of each sensing ®lm are performed with an electrometer and read into the computer via a HPIB interface. The computer outputs an analog voltage to the microheater that is determined by feeding the measured ®lm temperature through a proportional integral differential (PID) temperature control algorithm. The support electronics include power stages to supply the necessary current to all the microheaters and ampli®ers that linearize the thermocouples' temperature measurements into a 10 mV/8C analog signal for the computer's temperature control algorithm. The delivery of the NO and NH3 gases to the sensor is achieved with a fully programmable gas delivery system which is capable of running multiple tests and collecting data without user input or intervention.

102

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

Fig. 1. Block diagram of the experimental testing system.

4. Film engineering process The substrate material, ®lm thickness, ®lm doping, deposition temperature, and operating temperature are the critical ®lm parameters that have been chosen for examination in a ®lm engineering process to identify two ®lm recipes capable of sensitively and selectively detecting NH3 and NOx. The determination of unique ®lm parameters, which result in an optimum sensor, is a 5D optimization problem, and therefore, it is doubtful that a unique combination of the parameters can be determined. However, a judicious choice of the parameters may result in an acceptable NH3/NOx sensor albeit sub-optimum. The ®lm engineering process consists of two phases. In Phase I, recipes using three values of ®lm thickness (500, Ê ), two substrate materials (sapphire and 1000, and 5000 A alumina), four doping options, and one deposition temperature (2008C) were explored. The four doping options included: 1. undoped (film 1±6); Ê Au (film 2. surface doped, post-sputter (PS), with 16 A 7±12); Ê Au (film 13±18); 3. bulk doped, co-sputter (CS), with 16 A Ê Ru (only 4. surface doped, post-sputter (PS), with 16 A Ê 500 A WO3) (film 19±20). Table 1 summarizes the 20 different ®lm recipes prepared and tested in Phase I. Once these ®lms were sputtered, they were annealed at 4008 in air for 5 h. This annealing process transformed the as deposited amorphous ®lm into more stable polycrystalline ®lms. The ®lms were then exposed to the NH3 and NOx gas sequence shown in Fig. 2, at operating temperature of 200,

250, 300, 350 and 4008C. The NH3 and NOx concentrations are typical levels found in coal-®red power plants. Seven ®lms, namely 1, 3, 7, 8, 13, 17 and 18, that demonstrated fast, sensitive, and reversible electrical responses to NH3 and/or NOx were advanced to Phase II, which involved depositing the ®lm at 4008C rather than 2008C. The higher temperature deposition was performed to determine whether the ®lm's baseline stability and overall sensitivity to NH3 and/or NOx increases. All of these new ®lms were annealed and exposed again to the gas sequence shown in Fig. 2 at the aforementioned operating temperatures. Factoring in the ®ve operating temperatures, the number of possible ®lm recipes were 70. In order to determine the effect, the critical ®ve ®lm parameters had on the response to the gas sequence, several selected ®lm recipes were examined. For example, in the case of the operating temperature, the ®ve ®lm types examined are summarized in Table 2. The effect of different Ê WO3 operating temperatures on the sensitivity of a 1000 A Ê at 2008 on alumina to 10 ppm ®lm co-sputtered with 10 A NH3 and 75 ppm NOx is given in Fig. 3. It can be seen that this particular ®lm responded only to NOx and not NH3. Similar experiments were performed for the other four ®lm types. Examination of the data revealed that the optimum operating temperature is 3508C for the ®lms most sensitive to NH3 and 3008C for the ®lms most sensitive to NOx. Similar sequences of experiments were performed on selected ®lms to determine the effect of substrate, ®lm thickness, doping and deposition temperature. As a result of the experiments, several observations were made. In the case of the substrate, ®lms grown on alumina tended to be more sensitive to NH3 whereas ®lms grown on sapphire were more sensitive to NOx. For ®lm thickness, thinner ®lms were

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

103

Table 1 Twenty different film recipes tested in Phase I Film

Substrate material

Ê) Film thickness (A

Ê) Dopant (A

Doping method

Deposition temperature (8C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina Sapphire Alumina

500 500 1000 1000 5000 5000 500 500 1000 1000 5000 5000 500 500 1000 1000 5000 5000 500 500

± ± ± ± ± ± 16 16 16 16 16 16 16 16 16 16 16 16 16 16

± ± ± ± ± ± Surface Surface Surface Surface Surface Surface Bulk Bulk Bulk Bulk Bulk Bulk Surface Surface

200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200

Au Au Au Au Au Au Au Au Au Au Au Au Ru Ru

more sensitive to NOx whereas thicker ®lms were more sensitive to NH3. Relative to doping, Ru doped ®lms shown no sensitivity to either NH3 or NOx. Undoped ®lms were less sensitive than Au doped ®lms and ®lms doped throughout the bulk (co-sputtered) were more sensitive to NH3. In the case of ®lm deposition temperature, the ®lms grown at 2008C were more sensitive to NH3, whereas the ®lms grown at 4008C were more sensitive to NOx. Obviously, it is dif®cult to make a unique choice of the ®ve ®lm parameters. However, by performing a large

number of tests, some general trends can be identi®ed with each ®lm parameter. The two ®lm recipes chosen are, one to detect NH3 and the other to detect NOx, are given in Table 3. 5. Sensor array characterization The two ®lms selected as a result of the ®lm engineering process were carefully examined relative to critical sensing

Fig. 2. NH3 and NOx gas exposure sequence used throughout the film engineering process for every operating temperature.

104

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

Table 2 Summary of the five different film recipes (each operated at 200, 250, 300, 350 and 4008C) to study the effect that operating temperature has on each film's electrical response to NH3 or NOx Film type

Operating temperature (8C)

Ê Ê WO3 film CS with 16 A 5000 A Au on alumina at 2008C

200 250 300 350 400

Ê Ê WO3 film CS with 16 A 1000 A Au on alumina at 2008C

200 250 300 350 400

Ê Ê WO3 film PS with 16 A 1000 A Au on alumina at 2008C

200 250 300 350 400

Ê WO3 undoped film 1000 A sputtered on sapphire at 4008C

200 250 300 350 400

Ê WO3 film PS with 16 A Ê 500 A Au on sapphire at 2008C

200 250 300 350 400

parameters. The parameters of interest included response and recovery time, response rate, response magnitude, dynamic range, response repeatability, stability, selectivity and ®lm growth reproducibility.

Table 3 Summary of the two film recipes chosen from the film engineering process to selectively detect NH3 and NOx Film

Film type

NOx Film

Ê WO3 undoped film sputtered at 1000 A 4008C on sapphire operating at 3008C Ê WO3 film CS with 16 A Ê Au at 5000 A 2008C on alumina operating at 3508C

NH3 Film

The sensor response time is de®ned as the time it took the sensor to reach 90% of its steady value after exposure to a target gas while the recovery time is de®ned as the time it took the sensor to be within 10% of its baseline value. The response times of the NH3 ®lm ranged for about 7 min for a 1 ppm NH3 concentration to about 4 min for a 50 ppm NH3 concentration. The NOx ®lm's response times ranged from about 6 min for a 1 ppm NOx concentration to <2 min for a 75 ppm concentration. The larger response times for the NH3 ®lm may be attributable to the fact that the NH3 molecule is smaller than the NOx molecule, and therefore, diffuses deeper into the ®lm. The recovery times were more than twice the response times for each ®lm. The response of the NH3 and NOx ®lms to a wide range of gas concentrations was determined. In Fig. 4, the NH3 sensor response to a wide range of NH3 gas concentrations is presented. The response to each NH3 concentrations was reversible and repeatable. The response magnitude of the NH3 sensor from 1 to 50 ppm displayed a linear variation. The NOx ®lm's response to gas concentrations varying from 1 to 75 ppm was also reversible and repeatable. However, the response magnitude was not linear over the entire range of gas concentrations. In particular, the slope was constant up

Ê WO3 film co-sputtered with 16 A Ê Au at 2008C on alumina to 10 ppm NH3 and 75 ppm Fig. 3. Sensitivity as a function of operating temperature for a 1000 A NOx.

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

105

Ê WO3 film co-sputtered with 16 A Ê Au at 2008C on alumina operating at 3508C) during exposure to Fig. 4. Electrical conductivity of the NH3 sensor (5000 A 10, 1, 50, 5, 25, 10, 1, 50, 5, and 25 ppm NH3.

to 10 ppm and then decreased and remained constant from 10 to 75 ppm. The rate of response, de®ned as the derivative of the response, was determined for both the NH3 and NOx ®lms. In Figs. 5 and 6, the rate of response of each ®lm as a function of time for a wide range of gas exposures is presented. The response and recovery times associated with the rate of

response decreased. In particular, for a 10 ppm exposure to NH3, the NH3 ®lm reaches steady-state in 2 min compared to a 3 min response time. Once the NH3 is removed, it took only 1 min and 45 s to reach the steady value while the recovery time was about 7 min. These results clearly point out that a senor which operates on the rate of response will exhibit faster response and recovery times.

Ê WO3 film co-sputtered with 16 A Ê Au at 2008C on alumina operating at 3508C) during exposure to 10, 1, Fig. 5. Rate of response of the NH3 sensor (5000 A 50, 5, 25, 10, 1, 50, 5, and 25 ppm NH3.

106

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

Ê WO3 undoped film sputtered at 4008C on sapphire operating at 3008C) during exposure to 1, 5, 10, 25, 50, Fig. 6. Rate of response of the NOx sensor (1000 A and 75 ppm NOx.

The NH3 ®lm exhibited excellent repeatability for both the sensor response and the rate of response as shown in Figs. 7 and 8. Except for the ®rst exposure, the ®lm responded to the other nine 10 ppm exposures with a

response magnitude within 3%. The ®lm's rate of response magnitude was also repeatable and within 6%. In the case of the NOx ®lm, tests were performed for a 75 ppm exposure. After ®ve exposures the response

Ê WO3 film co-sputtered with 16 A Ê Au at 2008C on alumina operating at 3508C) during exposure to Fig. 7. Electrical conductivity of the NH3 sensor (5000 A 10 pulses of 10 ppm NH3.

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

107

Ê WO3 film co-sputtered with 16 A Ê Au at 2008C on alumina operating at 3508C) during exposure to 10 Fig. 8. Rate of response of the NH3 sensor (5000 A pulses of 10 ppm NH3 (same film previously shown in Fig. 7).

and rate of response magnitude were with 3 and 4%, respectively. Both the short and long-term stability of each ®lm was tested during exposure to air in 15% relative humidity. The NH3 ®lm was found to be more stable exhibiting baseline variations of <3% over a 30 min period and <8% variation over a 10 h period. The NOx ®lm exhibited an excellent

short-term stability of <0.6%. However, the long term stability was about 20%. This indicated a gradual drift of baseline which eventually stabilized after several days. Selectivity is perhaps one of the most critical sensing parameters. Ideally each sensor in the two-sensor array should be selective to either NH3 or NOx and exhibit no cross selectivity to the other gas. Since 1±10 ppm NH3 and

Fig. 9. Electrical conductivity of both the NH3 and NOx sensors during exposure to five pulses of 10 ppm NH3 followed by five pulses of 75 ppm NOx.

108

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

5±75 ppm NOx are the levels emitted from coal-®red power plants, the two sensor array's selectivity was evaluated to 10 ppm NH3 and then 75 ppm NOx. The results are summarized in Fig. 9. It can be seen that the NH3 sensor's conductivity increases upon exposure to each 10 ppm NH3 pulse while responding very little if any to the 75 ppm NOx exposures. The NOx sensor on the other hand exhibits a decrease in conductivity during each NOx exposure while responding very little to the 10 ppm NH3 pulses. Also, both NH3 and NOx sensors display stable baseline conductivities as well as reversible and repeatable responses to 10 ppm NH3 and 75 ppm NOx, respectively. Since H2S often occurs as a gaseous emission in fossil fuel burning systems, it was examined as an interferent. The NH3 sensor displayed no noticeable response to H2S. However, the NOx sensor responded to H2S. The response of the NOx sensor to H2S was opposite to the corresponding response to NOx which may be useful in processing the sensor array data. A large number of ®lms were grown and tested to evaluate the ®lm growth reproducibility within a batch and batch-tobatch. It was found that in order to produce results, which are reproducible, very precise control must be maintained in ®lm sputtering, electrode deposition and ®lm packaging. 6. Principal component analysis Once a sensor array is exposed to a particular gas environment, the array's output must be correlated to the gas types and concentrations present. The sensor array has characteristics that change depending on gas type, concentration, ambient conditions (i.e. humidity, temperature, and pressure), and sensor aging. Among these characteristics are response and recovery times, rate of reaction, sensitivity,

stability, response repeatability, and ®lm growth reproducibility. In sensor array evaluation, it is important to simplify the problem by reducing the dimension of representation while preserving as much of the original information as possible. Principle component analysis (PCA) offers a convenient way to control the trade-off between losing information and simplifying the problem. PCA can be used to eliminate redundant information (i.e. similar sensors) and reduce the multidimensional data set (i.e. n sensor responses) into two or three dimensions. PCA projects the sensor array's responses in feature space for easy visualization. The spatial separation of the sensor array's responses to different gases is a measure of its selectivity or ability to discriminate. PCA can also be used to evaluate a sensor array's response repeatability and variations from one array to another (®lm growth reproduction). The NH3 and NOx sensor array's electrical response, selected in the ®lm-engineering process previously discussed, was characterized with graphical analysis techniques. PCA techniques were employed to evaluate this twosensor array's response repeatability, selectivity, and ®lm growth reproducibility. The array's sensor responses are preprocessed into response sensitivities and fed into the PCA algorithm written in MATLAB. Fig. 10 shows the PCA plot of sensitivities of three ``identical'' sensor arrays (1st and 2nd sensor arrays grown in the same batch and the 3rd senor array grown in a separate batch) to 10 ppm NH3 (1s, 2s, and 3s) followed by 75 ppm NOx (4s, 5s, and 6s). The sensitivities of the 1st and 2nd sensor arrays to 10 ppm NH3 and 75 ppm NOx fall into two tight clusters with signi®cant spatial separation for discrimination. The tight clusters imply relatively small variations between each sensor array's response to 10 ppm NH3 or 75 ppm NOx. The 3rd sensor array's responses slightly

Fig. 10. PCA plot of three ``identical'' two-sensor arrays to two pulses of 10 ppm NH3 and two pulses of 75 ppm NOx demonstrating response repeatability, selectivity, and film growth reproducibility using the sensor array's response sensitivity to each gas exposure.

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

broaden the 10 ppm NH3 and 75 ppm NOx clusters, but the clusters are still relatively tight with adequate spatial separation for selectivity. These results suggest that the sensors can be reproduced within a batch, as well as from batch to batch. 7. Conclusions A tungsten trioxide based sensor array for the sensitive and selective detection of NOx and NH3 has been engineered as a result of an exhaustive examination of critical sensing ®lm parameters, such as a substrate material, ®lm thickness, ®lm doping, deposition temperature and operating temperature. Sensing parameters, such as response and recovery time, response magnitude and rate, response repeatability and stability, selectivity and ®lm growth reproducibility were carefully examined. An evaluation of the array response using PCA analysis clearly demonstrated that the array can selectively and repeatedly detect NH3 and NOx. Acknowledgements The authors wish to thank the Environmental Protection Agency for their ®nancial support (Grant No. GAD#R826164). Also, thanks goes out to Ron Schmitt for writing the PCA algorithm in MATLAB. References [1] J.H.A. Kiel, A. Edelaar, W. Prims, W. Van Swaajj, Selective Catalytic Reduction of Nitric Oxide by Ammonia, Appl. Catal. B: Environ. 1 (1992) 41±60. [2] J. A. Eddinger, Status of EPA regulatory development program for revised NOx new source performance standards for utility and nonutility units Ð performance and costs of control options, in: Environmental Protection Agency, Office of Air Quality Planning & Standards, Research Triangle, North Carolina. No. 328 pp. 1±13, Dec 5 1996. [3] K. J. Fewel, J. H. Conroy, Design Guidelines for NH3 Injection Grids Optimize SCR NOx Removal, Oil & Gas Journal Nov. 29 (1993) 56±64. [4] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseous components using semiconductive thin films, Anal. Chem. 34 (1962) 1502. [5] See, for example, in: Proceedings of the First, Second, Third, Fourth and Fifth International Meetings on Chemical Sensors, 1983, 1986, 1990, 1992 and 1994 and references contained therein, Elsevier, Amsterdam (1983, 1992), Unpublished (1986, 1990, 1994). [6] K. Tanaka, S. Morimoto, S. Sonoda, S. Matsuura, K. Moriya, M. Egashira, Combustion monitoring sensor using tin dioxide semiconductor, Sens. Actuators B 3 (1991) 247. [7] S. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli, Methods for the preparation of NO, NO2 and H2 sensors based on tine oxide thin films grown by means of RF magnetron sputtering techniques, Sens. Actuators B 8 (1992) 79. [8] G. Williams, G.S.V. Coles, NOx response of tin dioxide based gas sensors, Sens. Actuators B 15/16 (1993) 349. [9] F.J. Gutierrez, L. AreÂs, J.I. Robla, M.C. Horillo, I. Sayago, J.M. Getino, J.A. de Agapito, NOx tin dioxide sensor activities as a function of doped materials and temperature, Sens. Actuators B 15/16 (1993) 354.

109

[10] G.B. Barbi, J.S. Blanco, Structure of tin oxide layers and operating temperature as factors determining the sensitivity to NOx, Sens. Actuators B 15/16 (1993) 372. [11] C. DiNatale, A. D'Amico, F.A.M. Davide, G. Faglia, P. Nelli, G. Sberveglieri, Performance evalation of an SnO2-based sensor array for the quantitative measurement of mixtures of H2S and NO2, Sens. Actuators B 20 (1994) 217. [12] G. Wiegleb, J. Heitbaum, Semiconductor gas sensor for detecting NO and CO traces in ambient air of road traffic, Sens. Actuators B 17 (1994) 93. [13] I. Sayago, J. GutieÂrrez, L. AreÂs, J.I. Robla, M.C. Horillo, J. Getino, J. Rino, J.A. Agapito, The effect of additives in tin oxide on the sensitivity and selectivity to NOx and CO, Sens. Actuators B 26/27 (1995) 19. [14] I. Sayago, J. GutieÂrrez, L. AreÂs, J.I. Robla, M.C. Horillo, J. Getino, J. Rino, J.A. Agapito, Long-term reliability of sensors for detection of nitrogen dioxides, Sens. Actuators B 26/27 (1995) 56. [15] K. Satake, A. Kobayashi, T. Inoue, T. Nakahara, T. Takeuchi, NOx sensors for exhaust monitoring, in: Proceedings of the Third International Meeting on Chem. Sensors, 24±26 September 1990, Cleveland, OH, 1990, pp. 334±337. [16] J. Huusko, V. Lantto, H. Torvela, TiO2 thick film gas sensors and their suitability for NOx monitoring, Sens. Actuators B 15/16 (1993) 245. [17] G. Sberveglieri, P. Benussi, G. Coccoli, S. Groppelli, P. Nelli, Reactively sputtered indium tin oxide polycrystalline thin films as NO and NO2 gas sensors, Thin Solid Films 186 (1990) 349. [18] S. Matsushima, D. Ikeda, K. Kobayashi, G. Okada NO2 gas sensing properties of Ga-Doped ZnO thin films, in: Proceedings of the Fourth International Meeting on Chem. Sensors, 13±17 September 1992, Tokoyo, Japan, 1992, pp. 704±705. [19] M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxidebased semiconductor sensor highly sensitive to NO and NO2, Chem. Lett. (1991) 1611. [20] M. Akiyama, Z. Zhang, J. Tamaki, T. Harada, N. Miura, N. Yamazoe, Tungsten oxide-based sensor for detection of nitrogen oxides in combustion exhaust, Sens. Actuators B 13/14 (1993) 619. [21] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yamazoe, Grain-size effects in tungsten oxide-based sensor for nitrogen oxides, J. Electrochem. Soc. 141 (1994) 2207. [22] G. Sberveglieri, L. Depero, S. Gropelli, P. Nelli, WO3 sputtered tin films for NOx monitoring, Sens. Actuators B 26/27 (1995) 89. [23] H. Nanto, H. Sokooshi, T. Usuda, Smell sensor using aluminumdoped zinc oxide thin film prepared by sputtering technique, Sens. Actuators B 10 (1993) 79±83. [24] H. Nanto, T. Minami, S. Takata, Ammonia gas sensor using sputtered zinc oxide thin film, in: Proceedings of the 5th Sensor Symposium, 1985, pp. 191±194. [25] Hidehito Nanto, Tadatugu Minami, Shinso Takata, Zinc-oxide thinfilm ammonia gas sensors with high sensititivity and excellent selectivity, J. Appl. Phys. 60 (2) (1986) 482±484. [26] Arya, D'Amico, Verona, Study of sputtered ZnO±Pd thin films as solid state H2 and NH3 gas sensors, Thin Solid Films 157 (1988) 169±174. [27] H. Nanto, H. Sokooshi, T. Kawai, T. Usuda, Zinc-oxide thin-film trimethylamine sensor with high sensitivity and excellent selectivity, J. Mater. Sci. Lett. 11 (1992) 235±237. [28] G. Sberveglieri, S. Groppelli, P. Nelli, A novel method for the preparation of ZnO±In thin films for selective NH3 detection, in: Proceedings of the 5th International Meeting on Chemical Sensors, Rome, Italy 1994, pp. 748±751. [29] A.R. Raju, C.N.R., MoO3/TiO2 and Bi2MoO6 as ammonia sensors, Sens. Actuators B 21(1994) 23±26. [30] A. Bryant, M. Poirer, D. Lee, J.F. Vetelino, Gas detection using surface acoustic wave delay lines, Sens. Actuators B 4 (1983) 105±111.

110

B.T. Marquis, J.F. Vetelino / Sensors and Actuators B 77 (2001) 100±110

[31] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Gold-loaded tungsten oxide sensor for detection of ammonia in air, Chem. Lett. (1992) 639±642. [32] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Promoting effects of noble metals on the detection of ammonia by semiconducting gas sensor, New Aspects Spillover Effects Catal. (1993) 421±424. [33] G. Sberveglieri, L. Depero, S. Groppelli, P. Nelli, WO3 sputtered thin films for NOx monitoring, Sens. Actuators B 26/27 (1995) 89±92. [34] H. Meixner, J. Gerblinger, U. Lampe, M. Fleischer, Thin-film gas sensors based on semiconducting metal oxides, Sens. Actuators B 23 (1995) 119±125. [35] M.J. Madou, S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, Boston, 1989.

Biographies Brent T. Marquis was born in Fort Kent, Maine on 6 February 1974. He received BS and MS degrees in electrical engineering from the University of Maine in 1996 and 2000, respectively. He began working for Sensor Research and Development Corporation in 1995, where he is currently a senior engineer and program manager. John F. Vetelino (S'63-M'80SM'93) was born in Westerly, RI on 17 October 1942. He received the BS, MS, and PhD degrees in electrical

engineering from the University of Rhode Island in 1964, 1966, and 1969, respectively. He has been with the University of Maine since 1969 and is currently a professor of Electrical and Computer Engineering. He was one of the founding members of the Laboratory for Surface Science and Technology at the University of Maine and currently is the leader of the solid state sensor group. Dr. Vetelino's research group is working on chemical and biological sensors based on acoustic wave and chemiresistive technology. In 1976, he was at the Max Plank Institute Fuer Festkorperforschung in Stuttgart, west Germany, working on solid state properties of piezoelectric crystals. In 1980, he was awarded the Presidential Research Achievement Award, given annually to the outstanding researcher at the University of Maine. From 1983 to 1987, he was chairman of the Electrical Engineering Department at the University of Maine. He has given invited talks at many universities both in the US and abroad, at national and international conferences. Dr. Vetelino also has published invited papers in Sensors and Actuator and received outstanding paper awards at the International Chemical Sensors meeting and the society of Plastic Engineers Conference. He was a guest editor of the special issue of the IEEE UFFC Transactions on sensor and actuators in 1998. Three sensor companies Ð the BIODE Corporation in Westbrook, ME, Microsensor Conversion Technology in Brookings, SD, and Sensor Research and Development Corporation in Orono, ME Ð have been incubated from his research group. He is a member of Phi Kappa Phi, Sigma Xi, Tau Beta Pi, and Eta Kappa Nu. Dr. Vetelino has advised over 35 graduate students to MS and PhD degrees and has published over 150 scientific papers.