Mercury-Plati nu rn Optically Trra nsparent Electrode Willism R . Heineman and Theodore Kuwana D ~ p u r t m t n int C'he~i;i~tr.v, C m e Western Resrrce linivrrsitv, Cleueimd, Ohio 44106
A mercury-platinum optically transparent electrode (Mg-Pt QTE) was prepared by electrochemically reducin mercurous ions at a Pt OTE to form a thin coating o! mercury. Optical transparency could be retained by depositing less than 50 mC/cm2 of mercury. An increase in the usable cathodic potential range of up to 400 mV resulted from the greater hydrogen overvoltage OR mercury as compared to the platinum surface A typicirl Hg-PP QTE retained its overvoltage characteristics for two to three days. The utility ofthis electrode fot spectroelectrachemical applications was illustrated using cyclic voltammetry and chronoabsorptomatry far the reduction of methyl vi010 en dicatinn and cadmium ion. The optical response ?or the latter i s due to optical properties of the Hg--Pt OTE changing as metal i s deposited.
THERECENT D w F L o P M w r af optically transparent electrodes (OTE) has enabled spectra! monitoring of chemical reactions which immediate!y follow heterogeneous electron transfer at the electrode-solution interface (1-5). Optically transparent electrodrs corriposed of glass or quartz plates coated with ilc1pe3 tin oxide (6, 7) ~r thin films of platinum or gold (7-101, of gernianiurri plates; ( I ? : 12), and o f gold minigrids (13) have all been uwd. We wish now to report :he development of a mercuryplatinum OTE (Hg P t OTE). This clectrode makes possible an increaced cathodic potential range because of its greater hydrogen overvoltage, thereby enabling spectral observation of cathodic processes whicb arc norrnally obscured by hydrogen evolution a t the Pt OTI3. The Hg-Pt OTE was formed by electrochemical reduction of niercurous ions at a platinum OTE previously c!earred in a plasma discharge. By contiolling the amount of mercwy deposited, an electrode codd be prepared which retairied optical transparency yet (caused a 300-400 mV increase in hydrogen overvoltage. Deposition of z thin film of mercury on bulk phtinum elcctrodes h a s bee?! reported by investigators who used various approache? such as electrodepositing mercury by reduction of inercurous (14) or mercuric ion (14, /6), plunging .
.
-
~
-
(1) J. W. Strojek, T. Kuwana, and S. W. Feldberg, J . A v e r . Chem. SOC.,YO, 1353 (1968). (2) J. W. Strojek, f:j.A. Grurer, and T. Kuwana, ANAL, CHEM., 41, 481 (1969). (3) N. Winograd acd 1'. Kuwana, J . Amer. C h m . Soc., 92, 224 (1970). (4) W. Winograd, H. N. Blourit, and T. Kuwana, J . Phys. Chem., 73, 3456 (1969). ( 5 ) H. N. Blotrnt, N. Winograd, and T. Kuwana, ibid., 74,3231 (1970). ( 6 ) J. W. Strojek and T. Kuwans, J . Ekctroanal. Chem., 16, 471 (1968). (7) T. Os8 and T. Kuwana. ibid., 22, 389 (1969). (8) W. vnn Benkrn and T. Kuviana, ANAL. Grim., 42, 1114 (19'70). (9) A. Yildiz. P. T Kissinger, and C. N. Reilley, ibid., 40, I018 (19681. (10) 1-1. Prostak. H. B. Mark, Jr., and W'. N, Hansen, J. Phys. Chem., 72, 2576 (1968). (11) K. 5 hlork, Jr., and B. S. Pons, ANAL.CHEM., 38, 119 (1966). (12) D. R . Tallant and D. 13. Evans, ibid.,41, 835 (1969). (13) R . W. Murray, W. R. Heheinan, and G. W. O'Dom. ibid.. 39, 1666 (1967). (14) A. M. Hartley, A . G. Hiebert, and J. A. Cox, J . E/ec!roarid. Chex,, 17, 81 (1.968). (15) T. L. Marple and L. B. Rogers, ANAL.CHPM.. 25, 1351 (1953). (16) R. Neeh. Z. A d . CcIem.,180, 161 (1961).
a cathdized pletinurr, electrode which is evolving hydrogen into a mercury pool (17), and abrading a platir:uni surface under mercury (18). 44erc!!iy.coatetl platir'um filrns have been prepared for use in 1hin.layer cells and for semi-infinite specular reflectiov experirnents (19,. No attempt to retain optical transpnrency has bepn repoi ted
F: XPERIME?Tl'AL Electrochemical and optical instrumentation far potential step with concurrent spectral monitoring through am OTE has been reported ( 4 ) . Spectral scaris were made on a Cary Model 15 spectrometer. Pt OTF siibstrate electrodes were prcpared by vapor deposition (8). Those which w v c iised in these experiments exhibited a resistant; of nbout 10 ohnxisquare. The spectroelectrochemical :ell W H S of the sandwich type (6) with an electrode area of 0.5 cm2. Mercurous solutions for the preparation of the Hg-Pt OTE weie made by the dissolutim of reagent Hg,(NO&. 2H20 (J. T. Rakcr Cheriiical Ccmparp) in 0 5M acetic: acid-sodium acetate buffer at pH 4,O. Cadmium solutions were prepared by disso!ving reqgent Cd(NO& 4H20 (Mallinckrodt Chemical Works) in 0.5Macetic acid -sodium acetate Suffer at pI.1 5.2. Reagent grade methyl viologen (K & K. Laboratories) was twice recrystallized as the chloride salt from cold methanol (20). The resulting MYCh wzs dissolved in 0.3M phosphate buffer at pH 7.05. Doubly distillcd w t e r was used for all solutions. Preparation of a Hg-Pt OTE invoked the following sequence of steps. The Pt OP'E was washed wi.!h Isopropanol and then distilled water, hlotted dry with a Kimwipe, placed in the discharge of a Harrick Plasnia Cleaner (Harrick Scienfor five n?i;7iites, and then tific Corporation, Ossining, N , Y.) incorporated into the speclroelectrocheniica) cell. A solution of ca. 0.5mM Hg2(N03):,in UH 4.0 acetic acid-sodium acetate buffer was immediately added t a the cell and deoxygenated by bubbling argon through the sdution for thirty minutes. The argon was pretreated by passing it over hot copper turnings and through a gas dispersion bottle containing distilled water. The: potential of the Pt OTE was then stepped from open circuit to 0.00 V c C'E, the optical response was moni., tored at 609 nrn! and the c u r r a t WBF integrated to determine the charge of mercury which was deposired. This total. charge was corrected for backyround charge in order to extract the charge contributing vrily tn formalion of the mercury film. The background charge was determined in separate potential step experiments o n the buffer only. The residual current which contributed to the backgrcund charge was found t o decrea.se as ihe mercury was deposited. The resulting change in the rate o f s~cumula!ion of background charge during the clepositiori of mercury syas taken into account, lmrnediately following the forrnatian of a Hg-Pt OTE, the P out of the cell wit,h dilute mercurous solution W ~ rinsed acetic acid and then distilled water. 'The d u t i o n to be studied was then introduced into the cell. -
-
(17) L. Ramalcy, 35, 1088 (1963)
.__.
.~ .-. . .
R.L. Sruhakw, snd C . G. Enke. A N ~ LCHEM., .
(18) S.A. Moros, i b X , 34, 1585 (1952). (13) P.T.Kissinget, Ph. I>. Thesis, LJnivrrsity of North Carolina, Chapel Hill, N C., 1970. (20) E. M. Kosower and J. L. Cotter, J. AmPr. Chem. Soc., 86, 5524 (1964). ANALYTIC4L CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1075
$ o’6
0.4
t
400
w’ 0
I-4
-I 0
i:
8 2
200
W z
0 W
a
> r a 20
0
5
10
I5
20
25
30 35 40
4 5 50
C H A R G E , mC/cmz
Figure 1. Change in absorbance with charge of mercury deposited on Pt OTE
40
60
C H A R G E , mC/cmt
Figure 2. Change in hydrogen overvoltage with charge of mercury deposited on P t OTE 0.5M acetate buffer pH 4.0
X = 609 nm. 0.45 mM Hg2(N03)2-0.5Macetate buffer pH 4.0. Potential step, open circuit to 0.00 V us. SCE
A wavelength of 609 nm was used for spectral observation of MVt and during other reduction processes. The wavelength could have been different for the monitoring of Cd and Hg formatioc since both behave similarly to a neutral density filter.
RESULTS AND DISCUSSION Characteristics of the Hg-Pt OTE. Since the layer of vapor-deposited platinum on the Pt OTE which were used in this investigation was very thin (cu. 400 A), abrasion or evolution of hydrogen damaged the Pt substrate sufficiently to render fabrication of a Hg-Pt OTE difficult by techniques which involved these procedures. Consequently, a method of electrochemical reduction of mercurous ion without concurrent hydrogen evolution was pursued. The Hg-Pt OTE was prepared by the electrochemical reduction of mercurous ions in pH 4.0 acetate buffer at an applied potential of 0.00 V vs. SCE. Figure 1 shows the change in absorbance a t 609 nm as a function of the electrochemical charge for the deposition of mercury on the Pt OTE. The AA-Q curve was found to be linear for charge below about 7-8 mC, indicating adherence to Beer’s law for the deposition of very thin films. Additional mercury deposition was characterized by deviation from linearity of the AA-Q curve (Figure 1). This behavior is believed to be a result of droplet formation as opposed to the deposition of a uniform film of mercury. The formation of tiny islands of mercury which increase in depth as well as in radius with increased deposition would be expected to give negative departure from Beer’s law, since absorbance changes would not be homogeneous over the electrode surface. This is true even if a homogeneous sample of the deposited material does obey Beer’s law. The formation of droplets was also observed by microscopic examination of the Hg-Pt OTE surface immediately after its formation. Reproducible Hg-Pt OTE could be prepared by depositing a given amount of mercury as determined by either direct measurement of the mC used for the mercury deposited or measurement of the change in absorbance, using the curve in Figure 1 to correlate this to M C of Hg. The depth of the mercury film which was formed could be estimated from the mC/cm2 of mercury which was electrochemically deposited, assuming the density of mercury to be 13.5 g/cma. Such a calculation showed that a 10.0 mq/cm2 deposition of mercury gave a film of thickness of 154 A. Thus, it can be 1076
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
seen that the thickness of the mercury films used varied from about 50 to 500 A. These thicknesses are only approximate, since the calculation is for the formation of a uniformly thick film. Figure 2 shows the increase in hydrogen overvoltage (compared to that for a Pt OTE) as a function of the number of mC of mercury deposited. It is apparent that a greater deposition of mercury gave a greater hydrogen overvoltage. However, the change in hydrogen overvoltage per mC of mercury deposited decreased as the total amount of mercury deposited increased. Deposition of mercury in excess of about 30 mC/cm2, a n estimated thickness of 450 A, gave little additional improvement in hydrogen overvoltage, but further attenuated the transmittance of the OTE. This hydrogen overvoltage on the Hg-Pt OTE is greater than that on pure platinum by 300-400 mV, but is less than the hydrogen overvoltage on pure mercury. This is illustrated in Figure 4A which shows the current from hydrogen evolution a t pH 4.0 for the Pt OTE, the Hg-Pt OTE, and mercury. The cause of this intermediate overvoltage is the accumulation of mercury on the surface as droplets rather than as a continuous film. Formation of an “interaction compound” at the Pt/Hg interface has been observed for the electrodeposition of mercury on bulk platinum (14) and identified as PtHg4 (21). The hydrogen overvoltage of this compound compared t o bright platinum is ca. 100-150 mV (14). In the thin films of mercury which constitute the Hg-Pt OTE, it is quite likely that some Pt and PtHg4 remain exposed to solution. Also, since droplets tend to form, further reduction of mercurous ions merely accumulates more Hg on the droplets. Thus, the actual background current on a Hg-Pt OTE is probably a composite of that o n Pt and PtHg, and the relatively pure Hg on the surface of the droplets, the total current being determined by the relative exposed areas of these constituents and their characteristic hydrogen overvoltages. The technique of vigorous evolution of hydrogen to coalesce the droplets of mercury on bulk platinum (14) when applied to the Hg-Pt OTE resulted in a rapid increase in absorbance of the OTE followed by an abrupt decrease corresponding to the sudden disintegration of the Hg-Pt film. Once prepared, a typical electrode retains its overvoltage characterisics for two to three days. This lifetime of effec(21) G. D. Robbins and C . G. Enke, J . Elecrroanal. Chem., 23, 343 (1969).
’
’
400 h
5 :‘
to 500 A
Figure 3. Schematic representation of the mercury-platinum optically transparent electrode
tiveness is apparently limited by the slow amalgamation of the thin film of substrate platinum. A schematic representation of the structure of the Hg-Pt OTE is shown in Figure 3. A layer of PtHg, interfaces the electrodeposited coating of mercury with the underlying film of platinum which was previously vapor deposited on the glass substrate. The thickness of this layer is expected to vary during the lifetime of the electrode as slow amalgamation proceeds. Since the mercury deposits in the form of droplets, the mercury-solution boundary is represented by a dashed line which shows the average thickness of the mercury layer. The visible spectrum of the Hg-Pt OTE was essentially featureless. The mercury behaves as a neutral density filter which is superimposed on the spectrum of the Pt OTE. Demonstration of the Hg-Pt OTE. A significant feature of the Hg-Pt OTE is its hydrogen overvoltage which enables the spectroelectrochemical observation of redox processes which are normally obscured by hydrogen evolution on the Pt OTE. This capability is illustrated for the reduction of methyl viologen (MV2+)and Cd2+. The well-defined currentvoltage curve for the reduction of MVZ+in pH 7.05 phosphate buffer on a Hg-Pt OTE is shown by the solid-lined curve in Figure 4B. The first reduction wave corresponds to conversion of MV2+to the radical cation MVf ; the second wave is for the reduction of the radical to the neutral species MV”. These reduction waves are similar to those obtained on a pure mercury surface. The distorted wave for the reoxidation of MV” is caused by adsorption a t the electrode surface, since the neutral species has limited solubility in aqueous solution. The dashed line in Figure 4 8 represents current from hydrogen evolution at pH 7.05 on a Pt OTE. It is evident that the two reduction waves for methyl viologen would be obscured by the wave for hydrogen evolution on a Pt OTE. Reduction of Cd2+ in p H 5.2 buffer is also clearly observable on a Hg-Pt OTE as illustrated by the cyclic voltammogram in Figure 4C. This reduction would also be obscured at this pH on a Pt OTE by current resulting from hydrogen evolution as indicated by the dashed line. Thus, the Hg-Pt OTE widens the spectroelectrochemical “window” to include cathodic processes in aqueous solution which were previously screened out by the low hydrogen overvoltage on platinum. The small anodic wave at -0.4 V in Figure 4C is characteristic of C d ” being oxidized from a surface other than the thin mercury coating of the Hg-Pt OTE. The prominence of this wave was found to depend upon the extent of coverage of the substrate Pt layer by mercury. Electrodes with incomplete coverage exhibited a larger peak at this potential than electrodes with better coverage of mercury for which
-0.8
-0.4
E, V
VI.
-1.2
-1.6
SCE
Figure 4. Current-potential curves. Sweep rate 0.01 V/sec A . Residual current in 0.5M acetate buffer pH
4.0 a. Pt
om
b. Hg-ROTE c. Hg
B. -1.43mM MV2+-Q.3M phosphate buffer pH 7.05. Hg-Pt OTE with IS mC/cm2 of Hg deposited -0.3M phosphate buffer pH 7.05. Pt OTE -1.44mM Cd2+-O.SM acetate buffer pH 5.2. Hg-Pt OTE with 15 mC/cm*of Hg deposited 4 . S M acetate buffer pH 5.2. Pt OTJ3
e.
---
---
this peak was absent. Frequently, an Hg-Pt OTE which exhibited such a peak immediately after its preparation would not have this peak after the electrode had aged for 24 hours in a pH 4.0 acetate buffer. Possibly the slow amalgamation process leads to better coverage of the Pt substrate. In a chronoamperometric investigation of an electroactive species which exists as the oxidized form in bulk solution, the potential of the electrode is stepped to a value a t which the species is reduced at a diffusion-controlled rate at the electrode surface.
O f e e R
(1)
If the product R of the electron transfer absorbs electromagnetic radiation, its formation can be spectrally monitored by passing light of the appropriate wavelength through the OTE and the bulk solution. The absorbance-time relationship for such a diffusion-controlled reaction with no kinetic complications is given by Equation 2 AR =
2
ERC~O(D~~)’/’ =1/2
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
(2) 1077
1
0.030,
c
0.020
U
a
0.010
02
04
06
0 8
IO
t"'.
Figure 5. AA tometry
L.S.
t"2 plots for chronoabsorp-
A . 1.07mM MV2+--0.3Mphosphate buffer pII 7.05. Potential step -0.55 to -0.85 V us. SCE. Hg-Pt OTE with 15 mC/cm* of IIg deposited. X = 609 nm B. 1.30mM Cd(N0,). -0.5M acetate buffer pH 5.0. Potential step -0.50 to -0.95 V I S SCE. Hg-Pt OTE with 40 mC/cm* of Hg deposited. X = 609 nm
where A X is the change in absorbance caused by the formation of R, t~ is the molar absorptivity of R, Do is the difusion coefficient of 0 and Co" is the bulk concentration of 0 in ~ moles/l. It is apparent from this equa tion that a plot of ' 4 us. t l i z is a straight line. The quantity t~.\/%, can be extracted from the slope. Since the diffusion coefficient Do can be determined electrochemically, the molar absorptivity of the electrogenerated species R can be calculated. Such a "chronoabsorptometric" experiment was used to deriionstrate the utility of the Hg-Pt OTE for spectrally monitoring products of heterogeneous electron transfer reactions. Formation of the methyl viologen cation radical MVf was observed at a wavelength of 609 nm during a potential step to -0.85 V us. SCE in aqbeous pH 7.05 buffer. As shown in Figure 5A a plot of A A us. t l ! ? was linear as expected for diffusion-controlled generation of M V t . The quantity E%'% was found to be 24.5. For a diffusion coefficient of 0.4 X 10-j cmlisec for MV*+ in this aqueous solution (22), the molar absorptivity of MV' was calculated to be 12.3 X loa l./mole/cm. Equation 2 predicts an intercept of zero for a plot of A A us. for the diffusion-controlled generation of (22) M. Ito and T.Kuwana. J E'lectroanul. Chem., in press.
1078
ANALYTICAL CHEMISTRY. VOt.. 43, NO. 8,JULY 1971
MV:. '41though the reason. for t t r tiegatirc intercept in Figure 5A has not been codusively determined, it is prob... ably caused by a change in surface tmsion or a perturbation of the absorptivity of the Hy film resulting from the hetero.. geneaus electron transfer process. Diuing the reduction of Cd2+ at the Hg--Pr Oi'E, thr product Cd" dissolves in the thin film of mercwy to form Cd-Hg amalgam. Since Cd" attenuates visible radiation, its formation during the reduction process could be observed spectrally by chronoabsorptometry. Spectral monitoring at 609 nm through the Hg-Pt OTE during a potential step tu -0.95 V us. SCE in 1.30mMCd2+in pH 5.0 buffer sliowed an increase in absorbance as non-absorbing Cd*+-was reduced to "absorbing" Cd". As illustrated 5y curve B in Figure 5 , a plot of A.4 us. t i l 2 rvas linear, which indicates adherence to Beer's law by dilute solutions of Cd" In mercury. Since the parameters CW+ and &dz+ were known, the rnolar absorptivity of Cd" dissolved in mercury could be calculated from the slope of curve B, according to Equation 2. (Equation 2 is valid regardless of whether the spectrally monitored product remains in solution as in the case of MVT forination or is retained in the thin film of mercury as in the case of Cd" formation.) For a diffusion coefficient of 8 >: 10-6 cma/sec the molar absorptivity of Cd" in Hg was calculated to be 7 X l o a I./mole/cm at 609 nm. Preliminary experiments have also been performed with other metal ions (e.g., Ph2+, Zn2f) for which linear A -- t 1 f 2 behavior has also been obtained. It is apparent that the Hg-Pt OTE provides a unique means of observing electrode processes which involve reduction to M '. Strong surfa.ce adsorption of the oxalate anion on n mercury electrode at anodic potentials immediately prior to mercury dissolution has been reported (23). If one anodically scans the potential of a Hg-Pt OTE in 0.SM oxalic acid, a sharp spike of non-Faradaic current is observed at +0.275 V cs. SCE which denotes the rapid adsorption of C?O,*-.. Several nlillivolts anodic of this spike, a larger Faradaic current arises from the oxidation of Hg to form mercurous oxalate. By repeatedly stepping the potential just slightly anodic of the non-Faradaic spike and signal averaging the absorbance response, a small difference in the amount of light transmitted by the Hg-Pt OTE with oxalate adsorbed as compared to no adsorbed oxalate has been detected. Thus, it appears that adsorption can be monitored indirectly by its effect on the optical constants of the thin mercury film. The technique of signal averaging (3,5)which can detect absorbance changes of less than 10-5 unit enables observation of the small changes in absorption involved here RECEIVED for review January 18, 1971. Accepted April 9, 1971. The authors gratefully acknowledge the financial support provided by the National Science Foundation Grant Number GP9306 and the IJ. S. Army Electronics Command, Contract DA ARO7-68-C-0278 (Fort Monmouth, N. J.). (23) W. P. Race, J . Elecfroarral.Chcm., 24, 315 (1970).
