Electrochemical Study of Oxidation of Fractional Monolayer Gold Films on Platinum S . H. Cadlel and Stanley Bruckenstein* Chemistry Department, State University of New York at Buffalo, Buffalo, N . Y . 14214
IN THIS PAPER, we report the results obtained in studying the average oxidation state of submonolayer amounts of gold on a platinum electrode. Our initial interest in this problem arose in another context ( l ) , but seemed to be of general interest when we discovered that the electrochemical behavior of such films was remarkably similar to that of bulk gold. A number of metals form stable, thin oxide films on electrooxidation. Determining the average oxidation state of the metal in such films is not always simple since the electrode surface roughness and film thickness must be known. This problem becomes more complex when several possible oxidation states exist and film thicknesses are on the order of one to several atom layers. In such cases, we propose that the oxidation of known submonolayer and monolayer thick deposits of the metal on another metal may yield valuable information. Since it will frequently be possible to electrodeposit the metal quantitatively, and to determine quantitatively the amount of the metal oxidized from current-potential, current-time (constant potential), or potential-time (constant current) curves, the oxidation state as a function of potential and of atom layer thickness can be determined. The reliability of carrying such conclusions over to bulk metal studies can readily be judged by comparing the currentpotential, or other convenient electrochemical behavior of the thin film to that of the bulk metal. It has not been possible to determine the average oxidation state of a gold electrode directly because the roughness factor and depth of the oxidized layer have not been well characterized. One thorough study of the gold electrode oxidation state as a function of potential was made by S. B. Brummer and A. C. Makrides ( 2 ) . Two linear portions were found in their plot of the quantity of gold oxide formed (anodic charge) G S . the electrode potential. The change in slope occurred at -+1.45 V U S . RHE, and the anodic charge consumed corresponded to the equivalent of an adsorbed oxygen monolayer, i.e., one oxygen atom per one gold atom or an average gold oxidation state of Au(I1). Breiter (3) and Woods ( 4 ) studied the electrochemical properties of solid solutions of gold and platinum and reported that the current-potential curve for such alloys was the weighted sum of the current-potential curves of the pure materials. Electrochemically produced codeposits of gold and platinum were studied by Woods ( 5 ) . Untereker and Bruckenstein (6) have produced isopotential points at a platinum electrode with different initial submono-
* Present address, Chemistry Department, Vassar College, Poughkeepsie, N.Y. 12601. * To whom requests for reprints should be addressed. (1) S. Cadle and S. Bruckenstein, State University of New York at Buffalo, unpublished work, 1972.
(2) S. B. Brummer and A. C. Makrides, J . Electroc/zem. Soc., 111, 1122 (1964). (3) M. W. Breiter, J . Pl7ys. C/zem., 69, 901 (1965). (4) R. Woods, Elrctrochim. Acta, 16, 655 (1971). (5) Ibid.,p 533.
(6) D. Untereker and S. Bruckenstein, ANAL. CHEM.,44, 1009 (1972).
layer gold coverages in acid solution. The existence of these isopotential points established the electrochemical independence of the gold and platinum regions on the electrode surface. These and Breiter’s (3) and Woods’ ( 4 ) results suggested that there should be a close correlation between the potential dependence for the oxidation of fractional monolayer gold films on platinum with that of bulk gold. EXPERIMENTAL
Electrochemical Equipment. The circuit for the ring-disk potentiostat, the cell and the rotator have been previously described (7). An EA1 X-Y-Y’variplotter was used to record all data. The rotating platinum disk electrode and gold disk electrode had an area of 0.459 and 0.457 cm2, respectively. All potentials are reported cs. the SCE. Chemicals and Solutions. All solutions were prepared using triply distilled water. The 0.2M sulfuric acid supporting electrolyte was prepared from Baker reagent grade sulfuric acid which was fumed for 24 hours. A stock solution of millimolar gold sulfate was prepared by anodizing a gold electrode in concentrated sulfuric acid. All solutions were deoxygenated by passing nitrogen through and over the solution. The nitrogen was passed through a hot column of finely divided copper dispersed on diatomaceous earth to remove any traces of oxygen from the gas. Electrode Pretreatment. In order to obtain reproducible current-potential curves in the supporting electrolyte, an electrode pretreatment involving polishing followed by oxidation and reduction was necessary. First, the electrodes were polished with Buehler 0.05-p gamma micropolish before each experiment. The platinum electrode was introduced into solution, oxidized at f 1 . 3 V for 5 minutes, and then reduced at -0.3 V for an equal time. The electrode was then cycled repeatedly between +1.3 V and -0.3 V until a reproducible current-potential curve was obtained. A similar electrode pretreatment was used for the gold disk electrode. Gold Plating Procedure. At 0.0 V a plot of current cs. w1I2, the square root of the rotation speed, is linear in a 1.0 x 1 0 P M Au(III), 0.2M H2S04 solution. The convectivediffusion controlled limiting current in excess of the residual current at 2500 rpm was 1.44 FA for our electrode. No oxygen reduction current was detected in the supporting electrolyte under our experimental conditions. In the experiments described below, gold was deposited from this solution in successive intervals of either 30 or 60 seconds at 0.0 V and w = 2500 rpm. At the end of each interval, the electrode potential was stepped to +1.0 V, well off the limiting current for Au(II1) reduction, and electrode rotation was stopped. Oxidation and Reduction of Fractional Gold Monolayers. After stopping the electrode rotation at 1.O V, the potential was scanned between the limits +1.3 V 2 ED 2 -0.26 V at 100 mV/sec. We have shown ( 1 ) that some soluble gold is produced during the oxidation and reduction of gold electrodes. Thus, our scanning routine was chosen to minimize the production of any soluble gold, and the consequent rearrangement of the gold on the platinum surface that might occur via a solution/redeposition mechanism. The latter phenomena could produce gold regions more than one mono-
+
(7) D. T. Napp, D. C. Johnson, andS. Bruckenstein. ANAL.CHEM., 39, 481 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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Figure 1. Current-potential curves at a platinum electrode in 0.2MHzS04,l X 10-'MAu (111)
Figure 2. Relationship between number of pC of gold reduced, Q A ~ o , , at a gold film platinum disk (eAu< 0.5) and the number of p C of gold deposited, QAu
layer deep. The extent of oxidation of the gold surface was determined by integration of the oxidized gold reduction peak.
RESULTS AND DISCUSSION Average Oxidation State of Gold Film. Various amounts of gold were plated on a platinum disk electrode, and the current-potential curves were obtained by cycling the electrode from +1.30 V to -0.26 V and back. Typical currentpotential curves are shown in Figure 1, in which the maximum amount of gold plated was always less than 0.2 monolayers. The reduction peak associated with oxidized gold is well separated froin related processes of platinum. The charge required to reduce the gold oxidized at +1.30 V, Q A ~ o , is plotted us. the charge consumed in deposition of the gold by reduction of Au(III), Q A ~in, Figure 2. The data in Figure 2 are represented by Q A ~ o=~ 0.55Qau
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