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1980 (13) (14) (15) (16) (17)
Pallister. D. C u r . S e p . 1987, 8 , 53-57. Oates, M. D.; Jorgenson, J. W. Anal. Chem. 1989, 61,432-435. Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1988, 6 0 , 1521-1524. Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1989, 61,436-441. Knecht, L. A.; Guthrie, E. J.; Jorgenson. J. W. Anal. Chem. 1984, 56,
479-482. (18) St. Claire, R. L., 111 Ph.D. Thesis, University of North Carolina, Chapel Hill, North Carolina, 1986. (19) White, J. G.: St. Claire. R. L.. 111: Jorwnson. J. W. Anal. Chem. 1986, 58, 293-298. (20) Whle, J. G.; Jorgenson, J. W. Anal. Chem. 1986, 58,2992-2995. (21) Roach, M. C.; Harmony, M. D. Anal. Chem. 1887, 59,411-415. (22) de Montigny, F.; Stobugh, J. F.; Givens, R. S.; Carlson, R. G.; Srinivasachar, K.; Sternson, L. A.; Hlguchi, T. Anal. Chem. 1987, 59, 1096-1101. (23) Matuszewski, B. K.: Givens, R. S.;Srinivasachar, K ; Carison, R. G :
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1980-1983 Higuchi, T. Anal. Chem. 1987, 59, 1102-1105.
Mary D. Oates J a m e s W. Jorgenson* Department of Chemistry University of North Carolina Chape1 North 27599-3290 RECEIVED for review December 12, 1988. Accepted May 19, 1989. Support for this work was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
Crystal-Face-Specific Response of a Single-Crystal Cadmium Sulfide Based Ion-Selective Electrode Sir: The approach to characterization of ion-selective electrodes (ISEs) has been to pull together thermodynamic and kinetic principles and apply them to response interpretation. The extensive studies by Buck, Eisenman, Simon, Morf, Pungor, Cammann and others have solved major thermodynamic and kinetic problems in the theory and mechanism of ISE responses (1-6). However, not much has been achieved in the way of microscopic characterization of ISEs, particularly of solid membrane ISEs. Although ionexchange reaction is considered to be one of the processes responsible for permselectivity and charge separation for solid ISE/electrolyte interfaces, conventional solid membrane ISEs such as pressed-pellet types do not necessarily satisfy rigorous analysis of experimental results in terms of the relation between surface structure on the atomic scale and potential response. It appeared to us that the study of the ion-selectiveresponse of a well-defined surface of a single-crystal ISE would provide direct atomic/molecular information for understanding the basic principle of potential generation. Since the introduction of LaF3 single-crystal ISEs (9, several attempts have been made to use single crystals for ISEs. However, the purpose for utilizing a single-crystal membrane has been limited to improving bulk electrical conductance and reducing the dissolution of solid matrix. Thus, no control of the crystallographic polarity has been attempted aiming a t the atomic/ molecular basis for the potential response (8-12). Here we report for the first time the crystal-face-specific response of single-crystal ISEs. As a representative example of the system, we have chosen CdS, which has been known for a long time as an active component of Cd(I1) ISEs (see, for example, ref 13). The lack of inversion symmetry in the [OOOl] direction of wurtzite CdS single crystals gives rise to a crystallographic polarity of this compound (14). One face (0001) terminates with Cd and the other (0001) with S. Fortunately, crystal-face controlled single crystals of CdS are easily available because of its use as a photocell component. Also, the historical background that CdS has been well studied in terms of solid-state physics, semiconductor electrochemistry, and photoelectrochemistry further justifies the choice of CdS for the above objective. Striking differences between these two faces have been observed in some of the physical, electrical, and chemical properties (15-20). To correlate the surface compositions with the response characteristics, the surface of ISE membranes should be characterized. X-ray photoelectron spectroscopy (XPS),
Auger, and Fourier transform infrared (FT-IR) techniques are often employed (21, 22). Another potential technique for characterization of ISEs seems to be ion-scattering spectroscopy (ISS) (23-25), particularly impact collision ionscattering spectroscopy (ICISS) (26,27). ISS gives stoichiometric information about an exclusively top layer (monolayer) of solid surfaces and is most suitable for this study. Here we demonstrate for the first time the use of ICISS for the surface characterization of a CdS single-crystal ISE. Theoretical analysis of the potential response of the (0001)Cd and (0oOi)S surfaces of a single-crystal CdS ISE suggests the imperfection of surface composition, which is confirmed by ICISS measurements. The results and approach of this study will be of value for the atomic/molecular level characterization and design of solid membrane ISEs.
EXPERIMENTAL SECTION CdS single-crystal slices (thickness 1 mm) cut perpendicularly to the c axis were purchased from Teikoku Tsushin Kogyo Co., Ltd. (Kanagawa, Japan). The crystals were not intentionally doped, and their carrier density was 1017~ m - Two ~ . types of electrodes were prepared. One has a (0001)Cd face and the other has a (000T)Sface as electrode surfaces, respectively. Two faces were distinguished by chemical etching in 6 M HC1 in this study. The difference in etching characteristics of these two faces has been noted many times (16-20). The (0001)Cd face is etched much faster than the (G00T)Sface and gives a brighter face by the etching (17). The faces thus determined were confiied by many methods including X-ray reflection (16), ISS (14), and piezoelectric (15) techniques. An ohmic contact was obtained by indium metal at the back face. Except for the front face, all other parts were covered with epoxy resin and shielded in a glass tubing. Sample solutions were prepared by using reagent grade chemicals (Wako Pure Chemicals Co., Ltd., Tokyo) and purified water (Milli-Qwater purification system, Millipore Corp.). Solutions used were to 10+ M CdS04 plus 0.1 M NaN03 and lo-* to lo4 M NazSplus 0.3 M CHBCOONHl (pH = 9.0). Since the pKa1 and pKd for sulfur species are 7.0 and 12.9, respectively, the solution species at pH = 9.0 is almost exclusively (>99%) SH-. Potentiometric measurements were carried out at room temperature in complete darkness by using a millivolt meter (Model COM-BOR, Denki Kagaku Keiki Co., Ltd., Tokyo) with Ag/AgCl as a reference electrode. Prior to each run, the electrode surface was etched in 6 M HCl followed by exhaustive rinsing. The measurements were carried out from dilute t o concentrated solutions unless otherwise stated. Flat band potentials were determined by analyzing measured impedance data (28). Impedance measurements were carried out
0003-2700/89/0361-1980$01.50/00 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
1901
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Flgure 2. Potential response of (0001)s face of a CdS single-crystal electrode toward Cd2+ ions. The potentiis were measured from dilute to concentrated (0)and then back from concentrated to dilute (A). The solution composition was CdS04 in 0.1 M NaNO,.
b
/-a
40
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-6
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Flgure 1. Crystal-face specific response of single-crystal CdS eiectrodes toward SH-and Cd2+ ions: 0, (0001)Cdface; 0 ,(0OOl)Sface. (a) Response to SH- ions. Solution composition was Na,S in 0.3 M CH,COONH, (pH = 9). (b) Response to Cd2+ ions. Solution composition was CdS0, in 0.1 M NaNO,.
by using a potentiostat (Model NPG-301S, Nikko Keisoku Co., Ltd., Tokyo) and a frequency response analyzer (Model S-5720B, NF Electronic Co., Ltd., Tokyo) which was controlled by a personal computer (Model PC-8801 mkII, NEC Corp., Tokyo) via a GP-IB interface. All data were transferred to and stored on a floppy disk. In these measurements, the three-electrode configuration with a large Pt auxiliary electrode and a Ag/AgCl reference electrode was employed. ISS and ICISS measurements were carried out by a Model ADES 400 spectrometer (V.G. Co., U.K.) operating at 5 X lo-" Torr vacuum. Attempts were made to remove surface impurities from the CdS single-crystalsamples, such as Zn, Na, and 0 atoms that still existed at negligible levels after the chemical etching in HC1, by flash-heating at 400-500 "C.
RESULTS AND DISCUSSION Figure l a shows a remarkable preferential response of SHions (pH = 9) to the (0001)Cd plane over the (000i)Splane of a CdS single-crystal ion-selective electrode. The slope of the log c vs E curve was found to be 53 mvfdecade at the (0001)Cd plane, which is close to the theoretical value (=59 mV) at 25 "C, whereas the (ooOT)Splane exhibits a very poor response to SH- ions (> f , eq 3 becomes dAVH/d In (a) = R T / Z F (5) which is the Nernst equation. Typical values of dAVH/d In (a),i.e., responses to concentration changes, are calculated by using values of 2 = 1, CH = 20 pF cm-2, 0 = 0.5, and various values of r' and f. The results are shown in Figure 3 as a function off. The larger the J?,the larger the response. When the number of adsorption sites is close to that of the exposed atom (- loi4 cm-2) at the solid surface, d VH/d In ( a ) is ca. 50 mV and is close to Nernstian slope. When r is 10l2, and 10" cm-2, the slope is ca. 20, a few, and almost 0 mV, respectively. Since values of CH at semiconductor electrodes are not known (29), a typical value at metal electrodes (CH = 20 p F cm-2) was used for the calculation shown in Figure 3. It is generally believed that values of CH at semiconductor electrodes are not much different from those at metal electrodes (29). The larger CH value gives the smaller slope, but the values shown above are not much affected as far as 10-30 pF cm-*, which are typical values of metal electrodes and are used as Cw The effect of 8 on the slope is quite small in the region of 0.2 < 0 < 0.8. When 0 approaches either 0 or 1,the slope approaches zero. This happens when the concentration
100
Figure 3. Calculated value of dA?lH/d In ( a )as a function of f based on eq 3. CH = 20 pF cm-', 0 = 0.5, and 2 = 1 were used for the calculation. Key: A, = 1014cm-'; B, = loi3cm-'; C, = 10'' cm-*; D, = 10" cm-'.
r
r
-
r
r
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is very high (fl 1) or very low (8 0). Based on this calculation, the experimental results are interpreted as follows. At the (0001)Cd face, exposed Cd atoms acted as adsorption sites for SH- and Nernstian behavior was observed. Due to surface imperfections, there were a small number of S atoms at the (0001)Cd face which acted as adsorption sites for Cd2+ and the (0001)Cd face responded to Cd2+although poorly. A similar explanation is valid for the (0001)sface. A comparison of results of Figure l a and Figure 3 (see curve B) suggests that about 10% of the (000i)S surface is actually Cd atoms. To confirm the above, surface atomic composition was analyzed by ISS. The best available means for determining surface composition has been known to be ISS (23-25). Here the term "surface" shall denote that region of a solid which is, at most, a few atomic dimensions in depth (38). ISS utilizes low-energy (
