PNL-SA-26095 CDDF-qSc/w5- -2

EVALUATION OF HYDROGEN AND AMMONIA GAS DEC 2 Cj 9995 EFFECT TRANSISTOR SENSOR ARRAY MIXTURES WITH THE SUSPENDED-GATE FIELD-

O S T I

K. Domansky H. Li M. Josowicza J. Janata

December 19 9 5

Presented at the 1995 International Chemi cal Congress o f

December 17-22 , 1995 Honol u l u , Hawai i

Pacific Basin Societies

Work supported by t h e U.S. Department of Energy under Contract DE-AC06-76RLO 1830

Paci f i c Northwest Laboratory Richland, WA 99352

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EVALUATION OF HYDROGEN AND AMMONIA GAS MIXTURES WITH THE

K. Domansky, HongShi Li, M. Josowicz and J. Janata, Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory*, Richland, WA 99352

SUSPENDED-GATE FIELD-EFFECT TRANSISTOR SENSOR ARRAY

Generation of hydrogen represents a severe industrial hazard primarily because the mixture of hydrogen with air in the ratio 4.0 - 74.2 vol.% is explosive. In some industrial applications, such as waste remediation, hydrogen, as a product of radiolysis and corrosion, occurs in the presence of ammonia, nitrous oxide, water vapor and other molecules. A low cost, reliable method for monitoring these gaseous mixtures is essential.

Palladium-based layers have been used successfully as hydrogen sensitive layers in several potentiometric sensors for many years [ 1-31. Since the sensing mechanism is based on the catalytic decomposition of hydrogen molecules, other hydrogen-bearing gases can also produce a response. From this viewpoint, using an array of sensing elements with catalytic and noncatalytic chemically selective layers in these applications can be highly effective. Moreover, integration of this array on a single chip can be routinely achieved.

microfabricated in silicon - see Fig. 1. The metal gate of the transistor is separated from the substrate by an air gap. The chemically sensitive layer is electrodeposited on the bottom of the suspended gate. Chemical species can penetrate into the gate area and interact with the sensing layer. This interaction modulates the work function of the layer. The change in the work function results in the shift of the transistor threshold voltage. The measured threshold voltage shift is a function of the gas concentration in the sensor vicinity. By passing a small current through the suspended gate,,it is possible to control the operating temperature of the sensing layer (up to 200 "C) and, therefore, to modulate the sensor sensitivity, selectivity, response and recovery times. Due to the very low thermal mass, the heat is localized on the gate so that many devices can be operated on a single chip, each with the gate at different temperature [.SI.

Catalytic and noncatalytic films (palladium and polyaniline) were used as chemically selective layers - Fig. 2a and Fig. 2b. First, polyaniline (PANI) film was electrodeposited on one suspended gate. This step was followed by the electrodeposition of palladium on top of the PANI film. Then, PANI film was electrodeposited on the second gate. PANI films were prepared from 0.1 M aniline in 1.0 M sulfuric acid by cycling the potential on the Pt gate between -0.1 V and +0.75 V vs. SCE. The waiting period at

The Suspended Gate Field-Effect Transistor (SGFET) [4] is

both potential limits was 10 seconds, the scan rate was 20 mV/s. Palladium film was prepared from the solution of 0.03M PdC12 in 0.05M NH4Cl acidified to pH 1 with HC1. A series of current pulses several seconds long were applied in order to deposit the Pd layer. The current density was 2 mA/cm2, the total deposition time was 5 minutes.

Fig. 3a and 3b illustrate the wide dynamic range of the SGFET with PANI/Pd sensing layer. In this case, the minimum detection limit at the signal-to-noise ratio of 10 was lower than 1 ppm of hydrogen in air. The responses of the SGFET with the same sensing layer to ammonia are illustrated in Fig. 4. The calibration curves for both gases are shown in Fig. 5 . The sensitivities to hydrogen and ammonia are -58 mV/decade and - 24 mV/decade, respectively.

1000 ppm of ammonia in air at several operating temperatures. This result suggests that the temperature has a significant effect on the recovery time but only a negligible effect on the response time.

After the initial exposure to individual gases, the SGFETs with PANI/Pd and PANI chemically selective layers were exposed to the mixtures of hydrogen and ammonia. Fig. 7 and 8 show the responses of the SGFETs with PANI/Pd and PANI sensing layers to the mixtures of hydrogen and ammonia. The PANI/Pd sensing layer produces large negative responses to both hydrogen and ammonia, as illustrated in Fig. 7. The sensitivity to ammonia reaches about 60% of the sensitivity to hydrogen. It can be seen from Fig. 8 that the selectivity of the PANI film is much higher. PANI alone gives a relatively large negative response to ammonia and a small positive response to hydrogen.

Fig. 6 shows the response of the SGFET with PANI sensing layer to

* Pacific Northwest Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.

References: 1) I. Lundstrom, in: Chemical Sensor Technology, (T. Seiyama, ed), v01.2, Kodansha: Tokyo, p.1 (1989). 2) I. Lundstrom, A. Spetz, F. Winquist, U Ackelid and H. Sundgren, Sensors and Actuators, B 1 (1990) p.15. 3) J. Cassidy, S. Pons and J. Janata, Anal. Chem., Vol. 58 (1986) p. 1757. 4) T. Zhang, D. Petelenz and J. Janata, Sensors and Actuators, B, 12 (1993) p. 175. 5 ) K. Domansky and J. Janata, in Proceedings of Symposium on Microstructures and Microfabricated Systems, (P. J. Hesketh, H. G. Hughes and J. N. Zemel, eds), Vol. 94-14, The Electrochemical Society, San Francisco (1994) p. 243.

Figure captions:

-. ___^ -4 .., Neither the United States Government norany agency thereof, nor any of their

akes any warranty, express or implied, or assumes my legal liability or responsi- accuracy, completeness, or usefulness of any information, apparatus, product, or ,sed, or represents that its use would not infringe privately owned rights. Refer- D any specific commercial product. prooess, or service by trade name, trademark, recorn- ., or otherwise does not necessarily constitute or imply its endorsement, lr favoring by the United States Government or any agency thereof. The views

I,, VY,"aV.Y of authors expressed herein do not necessarily state or reflect those of the united States Government or any agency thereof. /' I -

Fig. 1

Fig. 2a

Fig. 2b

Fig. 3a

Fig. 3b

Fig. 4

Fig. 5

Fig. 6

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Fig. 7

Fig. 8

Size comparison of a SGFET sensor to a Lincoln Penny.

Cross-section of the SGFET with PANI/Pd sensing layer.

Cross-section of the SGFET with PANI sensing layer.

Response of the SGFET with PANI/Pd sensing layer to 0.2,0.5, 1, 5 and 9.17 ppm of hydrogen in air at 110 "C.

Response of the SGFET with PANI/Pd sensing layer to 100, 1000 and 10000 ppm of hydrogen in air at 110 "C.

Response of the SGEFT with PANI/Pd sensing layer to 100, 500, 1000,5000 and 10 000 ppm of ammonia in air at 110 "C.

Calibration curves of the SGFET with PANI/Pd sensing layer for hydrogen and ammonia in air at 110 "C.

Response of the SGFET with PANI sensing layer to 1000 pprn of ammonia in air at several operating temperatures. Flowrate was 80 sccm, vds = 3 V and Id = 350 pA.

Response of the SGFET with PANVPd sensing layer to the mixture of hydrogen and ammonia. The operating temperature of the layer was 120"C, flowrate was 80 sccm, Vds = 3 V and Id = 350 PA.

Response of the SGFET with PANI sensing layer to the mixture of hydrogen .and ammonia. The operating temperature of the layer was12O0C, flowrate was 80 sccm, Vds = 3 V and Id = 350 PA.

I DISCLAIMER

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Government. employees, m b i t y for the process discla ence herein tc manufacturer mendation, o

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