method can be readit? adapted to automated data acquisition svctem with a tnnsquent additional reductior! in data handh?.
J. H . Harding who was respnstble for the maintenance of t h instrument and D. E. Allan for assisting with the computer pr0gEiW.
ACDiOWL.EDGM33NT
The authors thank H. E. Lumpkin for many nelpfui s u w rions and G. F.. litylor and J. L. Tayior for arrying out most of the experimental work described in this paper; also
RECEIVED for review August 31, 1966. Acsepted February 24, 1%7. Presented at May 1966 Meeting of ASTM Committee E14 on Mass Spectrometry, Dallas, Texas.
Application of Deposited Thin Metal Films as Optically Transparent Electrodes lor internat Reflection Spectrometric Observation of
Electrode Sdution interfaces SIR: The feasibility of using in$ernal refkxtance specprometry @Its!as a nlethod for monitoring electrochemical reaci.ions spectrophotometrically at the electrode surface has k n demonstrated previously (I, 2). Since ihat time, it h x been found that it would be highly advantageous to produce IKS crystal electrodes which have surfaces with better eiectrochemic21 characteristics than those Llsrd previously-i.e., those which employed doped tin oxide in the visible range of‘ ihe electromagneric spectrum (I) and germanium in the infrared region (2). The semiconductor properties of tin oxide and
germanium make these materials quite unsuitable for many experiments as their electrode properties are quite complex ( 3 , 4 ) . This, “rerefore, makes the characterization of this combination eiectrolysiss~trophotonletri~technique exrremrlv dirliciilt. Experimentalli;, it has been observed t h ; ! ~ the ebsorption base line of tin oxide coated glass eiectrock daes noi i ~ m a i nconsttant from one electro1,ysisIZI t:ir next (2;4) Clther investigators have also cbserved the same changes in tl1.r: opiicai properties OR electroiysis (5> 6 ) shich in55ates that the snrra:.: i s cirangir.~ IP, sxme way. k-or t n t w :asxi:;, thc possi%lhtyof using thin pktirwni SIt? p;j:&ii:m meid :ii;ns OR opticaliy ti-ansparerrl substrate.> v.hi~:-! LXI riren 0 2 empioyed as IRS e i e c t m k ~has b e s
sufficiently high temperatures. It should be noted that this material also contains gold, as well as other metais, so tk electrode is not pure pl3:inurn To lessen the viscosity of the availabie iiquid plat-inum (which enables a thin platinam film IO be produced). a smali amount (0.1 to 0.2 mi, depending on the thickness of film desired) is dissolved in 3 ml of dichloromethane, and the resulting soiution is painted on to the IRS plate using brush strokes parallel to the light path. Several coatings may be applied, again depending o n the thickness of film desired. The plate is then allowed to cure in air for about 1 hour. a: room temperature. It is then fired in an open oven for a b u i 4 hours. The firing temperature for borosilicate giass plares is between 650”C and 680” C (approximately the fusion point of the glass). The cell was essentially the same as employed by hiansen et a!. (I, 6:. Constar?t current chronopotentiometry was carried out on the previously characterized (I) o-tolidine system. The a g paratus is similar to that described earlier (!, 7). A 55.V solution oi’ o-toiidine in a pM 2.00 KCL-HC! buHer was piare.; in the reacticja 41, and on eiccrroiysis the f d i o w i In i , ~7t‘aiC?!on
iTiV5tig6te.i
3 5 like I(> report a relativeiy ranid and inexper r n e ~ ~ of t dprrxloc;np buck f i l m s and present some preiimi eiei:trour optis;;i c.:sractenstics oi these elecirodcs EXPERUMkXT.41.
Platioum G b IRS E h o d e . The method o i producing the platinum giass 1R.F eiectrodtt employs the use or ii sohtiun of an organic ligand complex of platinum, (Liquid Platinum No. :Lp Engelhard Industries Inc.. Hanovia LiqEid Goid Divisioc; East Newark, N.J.) which is readily reduceci to platinum mtal or, an inert substrate, such as glass, at ___
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( I ) W. S . Hansen, K A. Osterywng, and T. Kuwac8.J. Alr. C/ieni. S O C . , 88, 1062 (I!&>>” 12) ;-1. 8. Mark, Jr., and 3.S . Fons, ANAL.CHM.,38,119 (1966:. i i) “The Elecfrochec&xry 0:‘ Semiconductors,” Y. 3. Ho1mL.s. -.1., Academic pres:;, N% York. 19t.z. :4t 3. 5. Roc.;, L.. 0. Winstron. J . Mattson. and H. ’d. Mark, :;. unpubiished data. 9M. :Si 7 . kuwane. Cqerniswy De& Case institute of Techoiup,
Spec19 I is wlo-less In the visible range, but II atnorb? < + The. Cary Model IJ was set at t h ~ bwave 4380 -4 (A&) length and srrnultaneuus absorbance-time and potenual-tme plots of tne oxiciatior reaction were carried mt Tne resuit, are sh;twr, :n f r p r e Thristie ( 8 ) and Hansw et ai. ( l j have shown fur iorwzr? .
~
unpublished data, 15C.f.. ( 5 ) i? A . Osreryoung. 24onIi American Avia-;ialionCo.. Ine.. Ttou-
VOL 39, NO. 6 , MAY 1967
a
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- I
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20
40
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100
120
140
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pigure 1. Absorbance-time and p0tentiaHime curves for the ~ p o t e a t i o m e t r i c electrolysis of 5mM 0-tolidhe in a pH 2.00 HCI-KCI solution The current was 1.00 ma. Tbe electrode area is approximately 6 an* served which has a maximum at 4375 y. This corresponds @nost exactly to the normal transmission peak (A, = 4380 A) observed for the oxidation product (species I1 of Equation 1) of o--tolidine (Curve 1 of Figure 3 actually shows the transmission spectra of this compound prepared during electrolysis). It should be noted at this point that the IRS spectra obtained during electrolysis at platinum film electrodes are at least one order of magnitude better, with respect to both AA and resolution, than any obtained with tin oxide conducting glass electrodes (4-6,8). A replicate IRS cell containing supporting electrolyte was placed in the reference beam of the Cary 14 during all runs. RESULTS AND DISCUSSION
6
,seconds '/2
Figure 2. Absorbance -t''* plot of a typical cbronopotentiometric electrolysis of the dolidhe system a t pH 2.00
current chronopotentiometry that the absorbance observed is proportional to the fiprovided that the penetration depth of the electric field vector is small compared to the diffusion layer. Figure 2 shows a typical absorbance - t l l * plot. These plots are linear over a wide range of current densities and reactant concentrations. They have also been found to be reproducible for successive runs with no shift appearing in the base line which is what was originally desired. This is an absolutely necessary condition for scanning wavelength during an electrolysis to obtain a spectrum of the electrolysis product or intermediate. Typical IRS spectrum scan results during the electrooxidation of o-tolidine are shown in Figure 3. A 5.0 X 10-3M o-tolidine and 0.22M chloride (pH = 2.0) solution was potentiostated at +0.60 volt os. SCE. Curve 3 represents the spectrum (base line) of the solution at zero applied voltage and Curve 2 represents the spectrum obtained during actual electrolysis. A very pronounced absorbance peak is ob-
686 *
ANALYTlCALCHEMlSTRY
Experiments were also carried out varying the applied potential with cells containing only supporting electrolyte solution to see if the IRS absorption base line was affected or varied in any way (a possible change in refractive index in the solution at the electrode sl;-face might occur as the concentrations of the anions and cations of the supporting electrolyte in the compact and diffuse double layer vary with potential). As expected, however, no significant change in the base line with potentid was observed under the above conditions. The penetration or effectl-,e path length into aosolution of the electric field vector is approximately lo00 A in the visible range and the slight change in concentration of the various ions in the double layer (approx. 100 A) predicted by GuoyChapman theory does not appreciably alter the integrated or average environment seen by the electric vector in the lO00-A thick solution volume. However, whenever the potentials were sufficient to reach solvent or electrolyte breakdown large shifts in absorption are observed, especially with the ilm was found at halide solutions where corrosion of the f anodic breakdown. In designing an IRS electrode, such as the metal film type described above, it is desired to have the film as thin as possible yet thick enough to have good electrical conductivity. Transmission measurements on the above IRS cells have shown that the film is not completely Uniform in thickness in some small areas but appeared to be covered to the eye. The average optical absorbance of the films was about 0.40 ab-
O8t
4370 1
i
WAVFLENGM
Figure 3. Viable
i
IRS spectra of 5mM dolidhn 00 a pH 200 HCI-KQ solotioa
Chrr 1. Noapplied pdentkl Cmre 2. Appliea potentiale q d s +Om rOn os. SCE C-3. Normal. .on spednm of elertmlysis pmdrrt
sorbance unit which would indicate an average thickness of about 75 & . (9). The electrical resistance of such a thin film would be exceedingly high, however. Actual measurements of the contact resistance of the actual films was about 25 Q-cm. To resolve this disxrepancy, electron photomicrw graphs were made of the surface of the films. Two distinct characteristics were ohserved that olTer a possible explanation as to the operation of these plates. Figure 44 is a portion of the enlarged (45,000 diameters) surface which shows that there are a large number of small holes (estimated to be about 1.5 x 10-5 cm in diameter on the average). These holes apbear to penetrate to the glass surface which explains why the film has such good optical transparency. They are made u p essentially of bare areas and thick opaque interconnected areas of metal. The size of the holes is small compared to the diffusion layer thickness, of course. so all IRS measurements appear diffusioncontrolled to a planeelectrode. The second anomaly in the optical behavior of these films is the experimental fact that absorbances obtained using these electrodes are a n order of magnitude larger than those o b served at conducting glass IRS electrodes [compare Figure 3 to Figure 16 of Reference (711 even though the cell geometries were essentially the same which would lead one to expect the same number of internal reflections. Figure 46 shows a second type of struciure of the films. These are long parallel tunnels (rather than grooves as can be distinguished by the shadowing and other features of the piciure) (10) in the filmwhich are a-b parallel t o the brush strokes. I t is postulated that
(9) 0. S. Heaven%,'Wptical Propdes of Thin Solid Film," p. 162. Academic Fxs. New York, 1955. (IO) 3. Rosen, Dept. of Metallurgical Engineering, University of Michigan, private communication, 1966.
Figure 4. Eleetroo pbotoi platinum coated IRS electrr a. 4SspoOdinmetw,sImwiq dace
b. 7spoO dinmetw, sbmhq cham& VOL 39,
these tunnels, perhaps f o d in some w a y by the initiai mtrapment of the liquid phtinun? soivent, a d as tiny “‘waw channels” 01). The Lght beam entering one of these wa?e ctaanaeh will undergo an extremeiy high number of reflections because of tire small dimension in traveling down the tunnel. Thus, the overall effective number of reflections is very large c o r n p a d to those calculated from the gross geonet~ of the cr)stal. The average width of these tunnels was a b u t 1 x 10-5 cm. It should be noted ai this pomt that t k c m s pcobabiy do not contain any of the eiectmlysis solution This concept is further substanriatea cv the fact tha: IKS ekctrodes made in the exact same nadnn: exceot that the fiim was applied w t h ~mi! strc.h l~ve’: dimfarto the light path were essentially opaque wth respect tu the solution ohase. At present this theor?. is onlv speculaxion. however, detailed investigations of tiie film construction and optical theory of ti= wave channels is being mvestigaated and w3l be reported in the near future.
difhsion theory well. Further investigauon of other metal surfacese methods of film deposition, characteristic: of ’.:le deposited layer, and optical characteristics of the film aie fi, progress and will be reported at an early date. Also. mtd film deposits on quartz plates are being studied in the ultmviolet region and those on AgCi and KRS-5are being studied in the IR region. Preliminary experiments have shown tbci palladnun films on KRS-5 crystals have a much broader IR wndow (5 to I4 microns) than conducting germanium c r y ta!s (2 to 10 microns) used in previous in situ IRS electlolvslstudies ACKNOWLEDUMKW
The authors thank D. I. Meyer of the Physics Department of The University of Michigan for hs helpfu! suggctions, and John Rosen for taking the electron photoilli’crographs B. STANLCY pO?uyS JAhm s. M n m h LEohi 0. wM!XROhi H A R R Y 13. MARK. jk?.
coNcLLIsToh.‘s
It is felt that t k e results show that it is possible to p r d t i c e suitab!e thin metal surfaces on glass that are optically traxparent as far as 1RS is concerned and that enable onz to study the IRS ektrochemical techniq=e and eiectrode reaction mechanisms without serious ctmnges of the eieztrotk itself CP the absorbance background during electrolysis. This conciusion k supported by the tact that the data in the vsibie region diffusion of the spectrum were reproducible and fiiied f) D. 4. Meyer, Physics lkpt. University of Michigan. i>F:vate communhon, 196E
Bpartment of Chemistry The University of Michigan Ann Arbor, hilkh
RECEIVED for review J d y 27, 196s. Accepted December ‘2, 1566. Research supported by a grant from The U . S. Anrig Research Oftice, Durham, Contract No. DA-31-124-ARO D-284 Divisior of Fuel Ghemistrj, 153d Natiocal MeelI%, ACS, Aprii 1%7, Miami Beach, Fia
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Modification Po increase Sensitivity of Barium Chforaniialte Method for Sulfate,
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