1570
Anal. Chem. 1991, 63, 1570-1573
(91 Lindner. J. K. N.: te Kaat. E. H. Met. Res. Soc. SvmD. ~. R o c . 1988. 707, 275-280. (10) Schonborn, A.; Llndner, J. K. N.; te Kaat, E. H.; Bubert. H.; Grasserbauer, M.;Frledbacher, G. Fresenius 2. Anal. Chem. 1989, 333, 51 1-5 15. (11) Dearnaley, G.; Freeman, J. H.; Nelson, R. S.; Stephen, J. Ion Implantatlon; North Holland: Amsterdam, 1973; pp 4161. (12) McKenna, C. M. I n Ion Implantation Techniques; Ryssei, H.. GlawIschnlg, H., Eds; Springer Series In Electrophyslcs 10; Springer: Berlin, I
,
'
----.
1982.
DO 731 rr .
(13) Klockenkamper, R.; Becker, M.;Bubert, H.; Burba, P.: Palmetshofer, L. Anal. Chem. 1990. 6 2 . 1674-1676. (14) Seah, M. P.; Dench, W.' A. Surf. Interface Anal. 1979, 1, 2-11. (15) Bubert, H. Mlkrochim. Acta [Wlen] 1987, 7986 I l l , 367-406. (16) Behrlsch, R.; Sputtering by Particle Bomberdmnt I ; Springer-Verkg: Berlin, 1981. (17) Seah, M. P. I n Ractical Surface Analysis; Brlggs, D., Seah. M. P., Eds; John Wiley & Sons: Chichester, 1983; pp 1811. (18) Davis, L. E.; McDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger flectron Spectroscopy; PerkinIlmer Corp.: Eden Prairie, MN, 1976. (19) Ichlmura, S.; Shimlzu, R. Surf. Sci. 1981, 112. 386-408. (20) Shlmlzu, R. Jpn. J . Appl. Phys. 1983, 22, 1631-1642. (21) Werner, H. W. Acta Electron. 1976, 79, 53-66. (22) Morrlson. G. H. I n SIMS I I I ; Bennlnghoven, A,, et ai., Eds; SprlngerVerlag: Berlin, 1982; pp 244-250. (23) Scherzer, B. M. U.; Bay, H. L.; Behrlsch, R.; Borgesen, P.; Roth, J. Nucl. Instrum. Methods 1978, 157, 75.
(24) Robinson, M. T.; Oen, 0. S. Appl. phvs. Lett. 1963, 2 . 30-32. (25) Robinson, M. T. User's Gum to MARLOWE; Radiation Shielding Information Center (RSIC), Oak RMge National Laboratory: Oak RMge, TN, 1986; Version 12. (26) Ziegler, J. F.; Biersack. J. P.; Littmark, V. The Stopping and Range of Ions in Solids; Pergamon Press: Oxford, New York, 1985; Vol. 1. (27) Llndhard, J.; Scharff, M.; Schiott, H. E. Kgl. Dan. Vid. Selsk. Met. Fys. 1983, 33 (No. 14) 1-42. (28) Dekempeneer, E. H. A.; Zalm, P. C.; Vriezema, C. J.; Polltlek, J.; Llgthart, H. J. "9.Instrum. Methods 1989, 842, 155-161. (29) Hall, P. M.; Morablto, J. M.; Conley, D. K. Swf. Sci. 1977, 62, 1-20. (30) Schonborn, A.; Bubert, H.; te Kaat, E. H. Fresenlus J . Anal. Chem., in press. (31) Bubert, H.;Burba. P.; Klockenkamper, R.: Schbnborn. A,; Wielunski, M. Fresenius J . Anal. Chem., In press. (32) Schonborn, A.; Bubert, H.; te Kaat, E. H. Verhandlungen DPG ( V I ) 1990, 2 4 , HL 3.9.
RECEIVED for review December 26,1990. Accepted April 30, 1991. This work has partly been supported financially by the Austrian National Bank and by the "Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen" and the "Bundesministerium fiir Forschung und Technologie der Bundesrepublik Deutschland".
Perfluorosulfonate Ionomer-Phosphorus Pentoxide Composite Thin Films as Amperometric Sensors for Water Huiliang Huang and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Stanley Ronchinsky EG&G Inc., Environmental Equipment Division, Burlington, Massachusetts 01803
Incorporatlng HsPO4 In solutlons of perfluorosulfonate lonom81s (PFSI) slgnlfkantly Improves the Sensnlvlty of electrdytk thln-fllm PFSI molsture sensors. Stable fllms are formed wlth PFSI:H,P04 ratlos %:1, and sensors based on such fllms have been successfully used to measure moisture levels as low as 2 ppm (dew polnt -70 "C) wlth an operatlng voltage of 15 V. The sensors exhlblt fast response tlmes to moisture surge, have no slgnlflcant hysteresis or fbw rate dependence, and are Immune to prolonged exposure to very hlgh humldtty levels. At hlgh humldltles, the sensors also exhlblt unusual current-voltage characterlstlcs. I n terms of overall cost and performance, such sensors are not only cmpetltlve wlth exlstlng alternatives for bw-level mdsture measurement but are also the first such devices capable of measuring molsture from low-ppm to saturatlon levels.
INTRODUCTION In a previous paper (I), a pertluorosulfonate ionomer (PFSI) thin-film amperometric sensor for water was described. A survey of the techniques presently used to measure water also appeared therein and will not be repeated here. Briefly, PFSI films have a high affinity for water and, if such a film is in direct contact with two electrodes and sufficiently high voltage is applied across them, the water that partitions into the film from the surrounding environment is electrolytically decomposed. The magnitude of the current is used as a measure of the water content of the environment surrounding the 0003-2700/9 1/0363-1570$02.50/0
sensor. This principle of operation is quite similar to amperometric sensors coated with a P205film described by Keidel in 1959 ( 2 ) ;the Pz05film absorbs water (Pz05!@+ 2HP03 2H3P04),and the process is reversed electrolytically. Such P205-based moisture sensors are presently in wide commercial use and available from a number of manufacturers in different physical configurations for moisture measurement in gases. There are a number of disadvantages to the Pz05 sensor. The sensor is fabricated by initially coating the electrodes with syrupy H3P04and electrolytically removing the associated water. This "drying" process requires several days and sometimes weeks. Further it is uncontrollable and results in low and unpredictable sensor yield. With continuous use, the pastelike P206film tends to become nonuniform over the electrodes, causing the current output to decrease with time (3). Even more importantly, P205sensors are intolerant of high moisture surges that can occur in real measurement situations: liquid H,PO, is formed and runs off the electrodes. Obviously, it is not possible to expose such a sensor to ambient air without damage and this complicates installation procedures. Repeated exposure to relatively high moisture levels that are not high enough to cause catastrophic failure nevertheless dramatically reduce the lifetime of microfabricated sensors due to high current densities. Despite these drawbacks, the P,05film sensing technology is in wide use because of its ability to measure very low levels of moisture and relatively low cost. Sensors based on PFSI films (I)can also be inexpensive and free of the drawbacks exhibited by a P 0 sensor. However, they are significantly less sensitive; in5 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, A W S T 1, 1991
identical physical configurations (total active area, total electrode area, interelectrode separation, etc.), the observed current levels are typically 2 orders of magnitude lower, with a proportionate effect on the attainable limit of detection. In the present communication, we deacribe an electrolytic sensor bearing a hybrid PFSI-P206film that effectively combines the advantages of each type. EXPERIMENTAL SECTION Reagents and Supplies. Nafion was obtained as a 5 wt % solution (equivalent weight (EW) 1100,diswlved in lower alcohols, Aldrich). The Dow PFSI (4,5), 1,1,2,2-tetrafluoro-2-((trifluoroetheny1)oxy)ethanesulfonic acid (EW 850),was obtained as a 2.5 wt % solution in alcohol as a gift from the Dow Chemical Co., Freeport, TX. Phosphoric acid (85%)was diluted with water to produce a 10 wt % solution. Thii acid solution was added to the alcoholic PFSI solutions in different proportions to provide composition ratios ranging from 501 to 2:l pure PFSI:H3P04 solution (by weight). Enough ethanol or water was then added to the mixture to provide a solvent medium of 50% alcohol These solutions were used for impregnating the sensors as described below. Sensor Fabrication. Two basic physical designs were investigated with the new sensor-impregnatingsolution. Both the 'needle" and the 'wafer" designs have been previously described in detail (I). Unless otherwise stated, data are reported for the needle sensor. This sensor comprises a 18-gauge blunt-end stainless steel hypodermic needle serving as the cathode. The needle encloses a concentric 100 pm diameter Pt wire, insulated by an appropriately sized Teflon sleeve. The film is formed by applying a 2-pL aliquot of the PFSI-H3P04 solution at the needle tip, drying for 2 h at -70 "C, and repeating the coating process twice more. The importance of heat treatment in casting such films has been pointed out by Moore and Martin (6,7). It was reported previously ( 1 ) that with the pure PFSI polymer films, the Dow PFSI yields sensors that are significantly more sensitive than the corresponding devices fabricated from Nafiion. In contrast, although the data reported here represent sensors based on the Dow PFSI only, no significant or consistent differences were found between the performance of sensors fabricated from Ndion or Dow PFSI solution containing identical amounts of &Pod. The wafer device (I) was a 1 cm square alumina wafer with a vapor-deposited interdigitated grid of rhodium film electrodes with an interelectrode separation of 150pm. The sensor was dip-coated with a 101 PFSIH3P04solution and thermally cured; this cycle was repeated a total of five times. Test Arrangements. Experimental arrangement and instrumentation were similar to those previously used (I). A Harrison 6106A dc power supply (Hewlett-Packard)was used in conjunction with a Model 427 current amplifier (Keithley Instruments) for the majority of the measurements. The response data for the needle sensors were obtained by recording the sensor output as the device was exposed alternately between Mg(C104)2-driedN2and a test relative humidity stream (generated with saturated LiCl solutions or H 8 0 4solutions of known composition; see ref B), utilizing switching valves controlled by a programmable process controller (Micromaster LS, Minarik Electric, Los Angeles, CA). For the low-humidity measurements with the wafer sensor, the sensor was put in series with a 10-or 100-kQprecision resistor and 15-V dc applied across the assemblage. The voltage across the resistor was measured with a Fluke Model 8060A digital multimeter and recorded with 10-pV resolution. Under all test conditions, the value of the measuring series resistor was negligible compared to the resistance of the device. Thus, the measured voltage was directly related to the sensor current at constant applied voltage. The low-humiditystreams were generated by blending ultradry air (compressed air put through a heatless air dryer) with a controlled small flow of completely moist air (bubbled through water thermcatated at 15 "C)by a dynamic mixing instrument based on high-precision masa flow controllers (EnvironicaInc., West Willington, CT). The entire mixing and test manifold was of stainless steel, and the actual moisture content of the stream was measured with a chilled mirror hygrometer (Model 300,EG&G, Inc., Burlington, MA) as the reference instrument.
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RESULTS AND DISCUSSION Stability of Sensor Output a n d Film Formation. Macroscopically continuous films are formed down to a PFSIH3P04ratio of 2:l. These films exhibit relatively poor integrity, however, and useful films are formed with PFSIH3P04ratios 1 5 1 . Microscopic investigations of such films .at ambient humidities indicate that microdroplets of HSP04are randomly distributed, encapsulated within the PFSI film. The current output of a sensor containing a hybrid PFSI-H$04 film exposed to a constant humidity atmosphere stops decreasing within 2-3 h of applying the voltage; presumably the H3P04 P205electrolytic dehydration reaches its steady-state condition within this period. Thereafter, the sensor output is stable and reproducible in regard to ita response to variations in relative humidity (RH). As an example, the output of a 5:l f i (hereinafter, film composition ratios quoted in this manner indicates the ratio of PFSI to HPOJ needle sensor showed a relative standard deviation of
