Anal. Chem. 1991, 63, 2984-2985
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knowledge of the analyst. Currently, the sensitivity of the glow discharge system on which the DPD is mounted is limited to the low ppm range. It should also be noted that the location of the two electrodes somewhat off the central ion beam axis, along with the presence of the shield, reduces the observed ion signal intensity. We plan to use the DPD to study diffusion of sputtered species into the effective ionization volume of the ion source. Studies are currently underway to improve the sensitivity of the technique and to expand its applicability to samples with other matrices. Registry No. Co, 7440-48-4;Cu, 7440-50-8; Fe, 7439-89-6;Mo, 7439-98-7; Ni, 7440-02-0; Nb, 7440-03-1; Zr, 7440-67-7; steel, 12597-69-2.
LITERATURE CITED (1) King, F. L.; Harrison, W. W. Mass Spectrom. Rev. 1990, 9 , 285. (2) Kllngler, J. A.; Savickas, P. J.; Harrison, W. W. J. Am. SOC. Mass Spectrom. 1900, 7 , 138. (3) Klingler, J. A.; Barshick, C. M.; Hanlson, W. W. Anal. Chem., in press. (4) Sullhran, J. V.; Walsh, A. Spectrochim. Acta 1085, 27, 721. (5) Lowe, R. M. Spectrochim. Acta 6 1971, 26, 201. (6) Lowe, R. M. Spectrochim. Acta 6 1978, 37,257.
(7) (8) (9) (10)
Sullhran, J. V.; Gough, D. S. Analyst 1078, 703, 887. Harrison, W. W.; Bentz, B. C. Anal. Chem. 1079, 57, 1853. Loving, T. J.; Harrlson, W. W. Anal. Chem. 1089, 55, 1526. Bruhn, C. G.; Bentz, B. C.; Harrlson, W. W. Anal. Chem. 1978, 5 0 , 373. (11) Jakubowski, N.; Stuewer, D.; Vleth. W. Anal. Chem. 1987, 59, 1825. (12) Sanderson, N. E.; Hall, E.; Clark, J.; Charalambous, P.; Hall, D. Mikrochim. Acta 1987, 7 , 275. Corresponding author. Shell Development Co., Westhollow Research Center, Houston, TX.
' Current address:
J. A. Klingler' W. W. Harrison* University of Florida Department of Chemistry Gainesville. Florida 32611-2046
RECETVED for review June 10,1991. Accepted October 4,1991. This work was made possible by support from the Department of Energy, Basic Energy Sciences, for which we are most grateful.
Anomalous Surface Area Change at an Ultramicroelectrode during the Reduction of Molybdenum Oxide Powder in the Absence of a Solution Phase Sir: We previously reported the reduction of Moo3 to H,Mo03, with x = 0.4, in a biamperometry cell (1). In that study, powdered Moos was simply pressed between two glassy-carbon electrodes of conventional size (5-mm diameter). Along with other recent reports (2-5), that study demonstrated the feasibility of using common electroanalytical methods such as cyclic voltammetry and chronoamperometry on the study of solids in the absence of a solution phase or of an analyte that originates in a gas phase. The work was limited by three aspects of the cell design. The counter process a t the glassy-carbon electrode required small amounts of water as an anodic depolarizer to initiate the reduction of MOO> The current passed led to a large iR distortion of voltammograms. Perhaps the most severe problem was that with a biamperometry cell cyclic voltammetric peaks symmetrical about 0 V are inherent since a reduction at one electrode is indistinguishable from an oxidation a t the other. The present study employs a cell which consists of a modified glass-carbon electrode as the quasireference/counter and an ultramicroelectrode as the indicator. The sample powder is pressed between them. The above shortcomings of a biamperometric cell with electrodes of conventional size are alleviated; however, with a chemical system where the electrolysis product is much more conductive than the sample matrix and where mass transport is slow, the apparent area of the indicator electrode can greatly exceed that of the ultramicroelectrode surface after generation of some product. EXPERIMENTAL SECTION AU chemicals were reagent grade and were used without further purification. Moo3, PdC12,and NazIrC&were purchased from Aldrich Chemical Co., Morton Thiokol, and Strem Chemicals, respectively. Electrochemical measurements on the solid-state samples were performed with a Bioanalytical Systems, Inc. (BAS) CV 37 voltammograph. Measurements were carried out in the two0003-2700/91/0363-2984$02.50/0
electrode mode. Samples of fiiely-ground Mo03were sandwiched between the working electrode (BAS 10-pm-diameter glassycarbon ultramicroelectrode) and the auxiliary/reference electrode (BAS 3 mm diameter ghay carbon) which were vertically aligned in a glass tube. Both electrodes were sealed in Teflon rings to form pistons which fit the inside diameter of the tube. A mass of 100 g of mercury was used to force the two electrodes together, thereby providing reproducibility in the sample size and in the contact between the electrodes and samples. The reaction chamber was purged with an argon stream that was saturated with water. This procedure also removed oxygen from the sample. Unless stated otherwise, the electrode used as the auxiliary (GC-Pd/Ir) was modified with an iridium oxide film (0.75-pm thickness) containing palladium (6) by cycling it 50 times between +1.2 and -0.3 V vs SCE at 0.01 V/s in a solution containing 1.0 mM PdC12, 2.0 mM NazIrC&, 0.2 M KzS04,and 0.1 M HC1. Electron spectroscopy experiments showed that the palladium does not occur in the outer surface of the film, so the formation of super stoichiometricmolybdenum bronzes containing palladium is not likely. The ratio of Ir(II1) to Ir(1V) in the oxide film was established by applying 0.6 V vs SCE in 0.1 M HCl, which is about the formal potential for the Ir(II1,IV)couple. The electrode was removed from the solution and equilibrated in air for 24 h in a controlled humidity chamber (relative humidity of 64 k 3% and temperature of 20 k 0.5 "C) prior to use in the solid-state cell. Air oxidation of the Ir(II1)was negligible under these conditions.
RESULTS AND DISCUSSION Figure 1illustrates a cyclic voltammogram of Moos at an ultramicroelectrode. The reduction of Moo3 to hydrogen molybdenum bronze (HMB) is initiated a t -0.5 V vs the quasireference. The anodic process at 0.2 V is the reoxidation of HMB. This is substantiated by the fact that the peak current increases if the negative-going scan is delayed for various times at -1.0 V before reversing the direction. This mechanism is consistent with that in our report on potential scan biamperometry of MooBpowders (1). A cathodic peak is not developed because on the time scale of the experiment Moo3 is not depleted in the vicinity of the working electrode. 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991 30 r
I
2 ...............
.-
........,..*....' -30'
1.o
.
'
0.5
.
'
0.0
I
-0.5
I
-1.0
E / V
Figure 1. Cyclic voltammetry of MOO, at an ultramicroelectrode with potential scan stopped at -1 .O V for (-) 0 s, (- - -) 20 s, and (.) 40 s. Conditions: auxiliaryheference, GC-Pd/Ir scan rate, 0.1 V s-'.
i-i
Flgure 2. Double-step chrmmperometry of Moo,. C o n d i i s : initial potential, 1.0 V; first step potential, -1.0 V; final mtential. 1.0 V. Three consecutive experiments are shown. Between experiments the electrode was polarized at 1.0 V until the current dropped to 0.05 nA.
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behavior. First, the outer portion of the HMB produced during the cathodic half-cycle can act as the apparent electrode surface because the HMB is much more conductive than Moo3 (7). As a result, the electrode grows into the matrix during the reduction. Second, because the growth is irregular, leading to the formation of fiigers of HMB extending into the Moo3 during the cathodic half-cycle (I),electrical contact to these outer domains can be broken when HMB is oxidized, thereby accounting for the low charge ratio during a given cycle. Subsequently, contact to these isolated domains is reestablished during reduction of Moo3, so the apparent electrode surface area increases with cycle number because the quantity of HMB present is the sum of the newly formed bronze and the amount remaining from the previous cycle. That the cathodic current increases with time during any cycle supports this model (seeFigure 2) in that a growth in apparent electrode area with time is strongly suggested. Direct evidence for this model is shown in Figure 3. After 30 min of continuous cycling, the region of HMB around the 10-pm-diameter electrode after an anodic half-cycle is 180 pm in diameter. Irregularity in the growth pattern is also shown. These irregularities will be relatively more important after shorter experiments as they will then comprise a greater fraction of the HMB produced. Ultramicroelectrodes will certainly be important to the voltammetric study of solid phases in the absence of a solution phase. The present results illustrate a potential limitation for cases where the matrix is electrolyzed to a product with greater conductivity.
LITERATURE CITED Jaworski, R. K.; Cox, J. A. Electrochim. Acta, in press. Feldman, B. J.; Murray, R. W. Znorg. Chem. 1987, 26, 1702-1708. Kulesza, P. J. J . Electrmnal. Chem. Znterfackl Electrochem. 1990, 289, 103-116. Kuiesza, P. J. Inorg. Chem. 1990, 2 9 , 2395-2397. Kuiesza, P. J.; Gaius, 2. J . Electrmnal. Chem. Znterfaclal Ektrochem . 1989, 269, 455-460. Cox, J. A.; Gadd. S. E.; Das, B. K. J . Electrmnal. Chem. Interfacial Electrochem. 1988, 256, 199-205. Barbara, T. M.; Gammie, G.; Lyding, J. W.; Jonas, J. J . SolM State Chem. 1988, 75, 183-187. Corresponding author. 'On leave from the University of Warsaw, Biatystok Branch, InstRute of Chemistry, Pilsudskiego 1114, 15-443 Bialystok, Poland.
Robert K. Jaworski' James A. Cox*
Figure 3. Photograph of a molybdenum oxide sample in contact with the working electrode after a 30-min cyclic voitammetry experiment at 0.1 V s-'. The dark area, corresponding to the bronze, has a diameter of 180 pm. Potential limits are +1.0 and -1.0 V.
Department of Chemistry Miami University Oxford, Ohio 45056
Continuous double potential step chronoamperometry (Figure 2) yields two interesting results. In any given cycle, the ratio of cathodic-banodic charge passed is always much more than 1. Second, the currents become greater with cycle number. We hypothesized that two factors account for this
RECEIVEDfor review August 5, 1991. Accepted October 9, 1991. This work was supported in part by the Ohio Board of Regents through a Center-of-Excellence grant to the Analytical Chemistry Program.
