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An “electrocapillary” study of the gold-perchloric acid solution interface
SURFACE
SCIENCE
46 (1974) 641-652 o North-Holland
AN “ELECTROCAPILLARY” PERCHLORIC
STUDY
ACID SOLUTION
Publishing Co.
OF THE GOLDINTERFACE
R. A. FREDLEIN Chemistry
Department,
University
of Newcastle,
New South
Wales, 2308, Australia
and J. O’M. BOCKRIS School of Physical Sciences, Flinders University, Bedford Park, South Australia, 5042, Australia
Received 5 March 1974; revised manuscript
received 27 August 1974
The bending experienced by a thin elastic electrode insulated on one side is used to obtain “electrocapillary” data on the gold-perchloric acid solution interface. The technique yields self-consistent and numerically reasonable values of the charge on the electrode and the cationic and anionic Gibbs’ surfaces excesses. The results are consistent with a specific adsorption of perchlorate ions which is represented by an isotherm of Temkin type. Hydroxonium ions are not specifically adsorbed. The data are limited in number, the numerical value of Young’s modulus for the system lacks accuracy: hence the uncertainty
of the data is high. 1. Introduction Of several methods used to investigate the solid metal electrode-solution interfacel-‘j), one of the more promising makes use of the bending experienced by a thin, one-sided, elastic electrode when its potential and the concentration of the solution are variede). A one-sided electrode is formed when metal is deposited on an insulating elastic substrate; glass being used as the substrate in the present work. The angular deflection, 8, measured at the end of the electrode of length I and thickness S is given by * : 8 = Aa 61/ES2 (1 - v), where Aa is the change in surface stress and E and v are Young’s
(1) modulus
* In eq. (l), B is not the angle, but the angular deflection or change of angle at the tip of the electrode. Consequently, we measure 19and obtain Au by substitution in eq. (1). For this reason, if there is an error in E, it will appear in do. Correspondingly, as E is a multiplier in (l), errors in E appear also in derivations of da and hence in the quantities r and qrn. 641
642
R. A. FREDLEIN
AND
J. O’M. BOCKRIS
and Poisson’s ratio of the electrode respectively*. The surface stress related to the interfacial tension, y, by the Shuttleworth equation 7):
is
o=y+dy/de,
(2)
where E is the surface strain. the surface strain,
Writing
the surface stress as a power series in
G=U+bE+CE’+...,
(3)
we have for small strains: ci=bE,
(4)
since 0 = 0 when E= 0. Using (4), eq. (2) may be written :
and integration
o=bdy/da+y,
(5)
y=a-b+Ce-“I”,
(6)
yields :
where the constant
of integration,
C, is given by: C=b+
where y0 is the interfacial
tension
yo,
(7)
at c =O. From
(6) we have that:
Ay = Ao + C e-uib (e-d0’b - 1). Differentiating
(4) and (6) with respect to E and making dylde - b YO
+
(8) use of (7) yields:
-a/b
b
For evaporated gold particles at 50 “C the values : IT= 1145 dyne cm-’ ; have been obtainedlo). dylde = - 706 dyne cm-’ y= 1851 dyne cm-‘; Since y. must be of the order of 1000 dyne cm-l, these values satisfy (9) only if b is sufficiently large for both sides of (9) to be - 1. Under this condition e-Aalb is unity and (8) becomes: Ay=Ao,
(10)
and (1) may be written: 0 = Ay 61/E6’ (1 - v), yielding
the differential
interfacial
tension.
AY=Y-Y’,
(11)
Now (12)
* This equation differs from that previously derived6) in that Aa was previously assumed to equal the differential interfacial tension and by the introduction of the plate bending correction term, (1 -v) *, 9.
ELECTROCAPILLARY
STUDY
OF GULRPERCHLORIC
ACID
SOLUTION
INTERFACE
643
where y” is the unknown interfacial tension of the reference state, i.e. the state, at some particular solution composition and electrode potential, from which all deflection measurements are made. Since y” is a constant, the usual electrocapillary equations can be applied to calculate the Gibbs’ surface excesses, F+, r- of the cations and anions and the electrode charge,
and (14) a, is the mean ion activity of the electrolyte, V+, V- the potential of the electrode on the scale of a reference electrode reversibie to the cation and anion respectively and p refers to chemical potential of all the species composing the solution. This paper reports the first attempt to study the doubielayer by making “electrocapillary” measurements at a solid metal-solution interface. 2. Experimental Electrodes: Gold was cathodically sputtered onto glass strips (Corning 0211) 120x 15 x 0.085 mm to a thickness of ca. 1000 A. To obtain good adhesion of the gold, transparently thin layers of tantalum and then platinum were first depositedll). Slow relaxation of the intrinsic stress in the deposited film caused significant deformation of the electrode during the course of experiments. Attempts to anneal the electrodes at 200°C increased the intrinsic stress, in accord with the other observations12)~ and the rate of deformation but stable electrodes were obtained by exposing them to lop3 M KC1 solution at 90°C for four weeks. Cell: The electrode was pressed against a flat surface formed by cutting a 20 mm long semi-cylindrical section from a S/S inch dia. quartz rod; the section cut from the rod being used as the pressure plate. Electrical contact to the electrode was made by goid deposited on the rod. The PFE cell head contained four ports (of which two are shown, fig. 1) used to introduce the bright platinum foil counter electrode (20 cm’), solution inlet and outlet tubes and a saturated calomel reference electrode. Two medium porosity frits separated the reference electrode from the main compartment. Water from a thermostat was circulated through the cell jacket. Solutions were prepared in a closed, external, thermostatted mixing chamber and circulated through the cell using all Teflon pump (Fluorocarbon Saturn 2000) and flexible Teflon tubes. Provision existed for passing the
644
R. A. FREDLEIN
AND
1. O’M. BOCKRIS
PC
PC
Fig. 1. Cell: (a) gold electrode, (b) optical window, (c) counter electrode, (d) gold coated quartz contact rod, (e) pressure plate, (f) solution inlet tube, (g) PFE cell head, (h) Teflon connectors (Fluorocarbon), (i) water jacket connected to circulating thermostat.
solution through an anodic pre-electrolysis cell. The solutions were maintained under an atmosphere of purified argon and contacted only glass and Teflon surfaces. Optical lever: The deflection at the tip of the electrode was measured with a laser optical lever. The beam from a 1 mV He/Ne laser (Spectra-Physics, 132) was collumated by passage through a 2 mm hole positioned 1200 mm in front of the laser. The beam passed through a meniscus lens (f = + 500 mm) and was reflected from the electrode positioned close to the focal point. The reflected beam traversed a path of 2300 mm before entering the detector (fig. 2); long path lengths being obtained by reflection from
Fig. 2. Detector: (a) support bracket, (b) translation stage, (c) micrometer, (d) right prism, bright platinized front faces, (e) photocells, (f) interference filter, 6278-6378 A, (g) tube, (h) diffusion discs.
ELECTROCAPILLARY
surfaced optical
mirrors. components
STUDY
OF GOLD--PERCHLORlC
The cell (in a horizontal were mounted
ACID SOLUTION
position),
on two parallel
645
INTERFACE
laser, detector 1.3 m optical
and
benches
securely fastened together. In the detector (fig. 2), the reflected beam was split on the bright platinized front faces of a right prism and deflected through diffusion discs to photoconducting CdS cells (International Rectifier CS 120-C). Displacement of the beam due to bending of the electrode causes one photocell to receive more light and the other less. The output voltage was measured with a recorder (Esterline Angus). This detector design minimized the effect of slow variation in laser intensity (ca. 3%) and the time constant of the detector circuit (fig. 3) was made ca. 6 set to minimize the effects of building vibrations.
Fig. 3.
Detector circuit.
The interference filter minimized the stray light entering the detector. The detector was mounted on a translation stage (Lansing) to permit calibration and tests showed that the output was linear with beam displacement over the range encountered in this work. Procedure: All the components of the cell, pre-electrolysis cell, pump, mixing chamber and the electrode assembly were cleaned in nitric-sulfuric acid cleaning mixture and thoroughly rinsed in conductivity water. A dilute HClO, solution (ca. 3 x 10v3 M) was prepared in the mixing chamber with conductivity water and 5N8, 70% HClO, (Apache Chemicals). The solution of volume 1.5 1 was anodically pre-electrolysed on bright platinum gauze electrodes and flushed with purified argon for 24 hours while circulated through the cell and mixing chamber. Electrode deflections with change of potential and electrolyte concentration were measured relative to the electrode position at 540 mV (SHE) in the most dilute solution. At potentials higher than 540 mV the possibility of electrode oxidation exists; sputtered metal electrodes seem to be more
646
R. A.
FREDLEIN
AND
J. O’M. BOCKRIS
reactive than foils. Held at 540 mV (Tacussel PRT 20-2A potentiostat) the electrode underwent no deflection with time. When the potential was suddenly decreased the deflection obtained (fig. 4) decreased slowly and almost linearly with time. This decrease in the deflection with time was thought to be due to adsorption of organic impurities (vide infra) and the deflection was obtained by extrapolating the deflection against time curve back to zero time. The electrode was then brought back to 540 mV until the impurities had desorbed, as indicated by an unchanging deflection with time, before the deflection at a new potential was measured. In this way, a curve of deflection against potential was constructed. After desorption of impurities was complete, variations in the electrode position at 540 mV were small.
4
Fig. 4.
Typical deflection
against
Time
time curves showing
extrapolations
to zero time.
With the electrode held at 540 mV the concentration of the electrolyte was increased by adding HClO, to the mixing chamber and circulating the solution. The deflection caused by increasing the electrolyte concentration at this potential was measured and a curve of deflection against potential for this new concentration determined as previously described. The potentia1 of the electrode was never permitted to go below ca. 0 V SHE to prevent hydrogen deposition on the electrode. Over the potential range O-0.540 V only steady-state currents (< 10 -6 A cm-“) were observed. 3. Results Values of Ay were calculated from the observed deflections using (11) and the values: E=7.45 x 1O’l dyne cm-’ and v =0.22 for Corning 0211 glass and plotted in fig. 5. Each curve was drawn from three sets of measurements which had average data spread of ca. 5 dyne cm-‘. Only one uninterrupted series of measurements was completed. The charge on the electrode (table 1) was found by graphical differentiation of these curves [see eq. (14)].
ELECTROCAPILLARY
STUDY
OF GOLD-PERCHLORIC
Plots of differential
I
I
0.30
O-61
2.99
8.92
26.3
79.9
(V, SHE)
interfacial tension against potential acid concentrations at 30°C.
TABLE
647
INTERFACE
I
Double layer data for the gold-perchloric lo3 C HC104 (M)
SOLUTION
0 Potential
Fig. 5.
ACID
for various perchloric
1 acid solution interface at 30°C
Electrode potential (V SHE)
105qhl (C cm-z)
105 Electrode capacitance (F cm-“)
105 z+FT-+ (C cm-2)
0.440 0.340 0.240 0.140 0.440 0.340 0.240 0.140 0.440 0.340 0.240 0.140 0.040 0.340 0.240 0.140 0.040
1.8 1.7 1.6 1.3 1.9 1.8 1.5 1.2 2.1 1.7 1.3 1.0 0.4 2.1 1.7 1.0 0.5
0.2 1 2 3 0.4 2 3 4 3 4 4 5 6 4 5 6 7
5.4 5.6 5.8 6.2 11.8 11.8 11.9 12.2 17.7 17.8 18.3 19.0 19.7 25.1 24.4 24.1 23.8
IO5 z_Ff_
(C cm-3 -
5.9 5.9 6.2 6.4 13.6 13.6 13.6 13.8 21.0 20.5 20.3 20.0 19.9 26.4 26.1 25.5 24.2
105 qs (C cm-2)
106 qli(C cm-2)
-1
0.3 0.3 0.3 0.3 0.6 0.6 0.6 0.6 1 .o 1.0 1.0 1.0 1.0 1.6 1.6 1.6 1.6
0 0 0 -2 -2 -2 -1 -3 -3 -2 -1 0 -1 -2 -1 0
648
R. A. FREDLELN
AND
J. O’M. BOCKRIS
-------H
7L.9 x 10-k
200
. . .
’ 26.3 x Wl+M
.
. 6’92 x lO-jM
100 c-
. 2.99
F ;
x
IO-‘M
0
%
~
_
_
_
.2.99x
10-3M
3 L -100 k c
-200
6.92 x lo_3M .
-
*
-
I
0
.
26.3x&M *
I
I
I
0.20
040
oal
Potential (V, SHE 1
Fig. 6. Surface excesses of hydroxonium ions (positive values) and perchlorate ions (negative values) at various potentials and concentrations of perchloric acid at 30°C.
Values of A? interpolated from the curves of fig. 5 were plotted against lna, at constant potential on the scale of a reversible hydrogen electrode in the solutions and on the scale of a reversible perchlorate electrode in the solutions. The potentials on these scales were calculated from the potentials measured against the saturated calomel reference electrode using the Nernst equation and neglecting liquid-junction potentials. Activity coefficients used in this work were calculated from the Debye-Htickel relation taking the ion size parameter as 5.1 AIs). These curves were graphically differentiated and the Gibbs’ surface excesses calculated [eq. (13)] and the results plotted in fig. 6 on the standard hydrogen electrode scale. 4. Discussions The decrease in the deflection with time observed when the potential of the electrode was suddenly reduced from + 540 mV (SHE) (fig. 4) is due to adsorption of organic impurities on the electrode since this effect was more
ELECTROCAPILLARY
pronounced
STUDY
for solutions
OF GOLD-PERCHLORIC
ACID
SOLUTION
that had not been subjected
INTERFACE
to long periods
649
of
anodic pre-electrolysis. Further, the effect was more pronounced as the potential was lowered towards the potential of zero charge (P.z.c.) (0 to - 100 mV, SHE; fig. 5). Organic impurities are most strongly adsorbed at the p.z.c. where the competition from adsorbed water dipoles is at a minimuml4). At + 540 mV adsorption of organic impurities would be minimal. The data reported here were all taken from the same series of experiments, each reading being repeated three times and the possibility of undetected errors of calibration exists in the Ay values *. Further, the possibility that the vacuum deposition of metal on the glass substrate may alter its Young’s modulus must not be overlooked. [For accurate results, Young’s modulus of the electrode should be directly determined; the technique making use of the natural frequency of flexural vibration being appropriates).] In addition, the reported values do not take into consideration the roughness of the electrode. Values referred to the true surfaces area are the reported values diminished by the unknown roughness factor. For these reasons, it would be wise to regard the absolute values of Ay, r+ , K, qM and the electrode capacitance as having a low (e.g., +50%) order of accuracy. However, since all these values would be in error by the same factor, the selfconsistency of the results and trends shown by the results are significant. There is also the possibility that the surface may be trace contaminated by platinum due to the pre-electrolysis or the method of preparation of the electrodesls) and the results may not be entirely applicable to gold foil electrodes. Estimates of the double-layer capacitances (table 1) were made by double graphical differentiation of Ay with respect to potential. Capacitances from the present work are in accord with galvanostatic pulse values obtained on gold single crystals [IO uF at 0.01 M HClO, and 15 uF at 0.01 M HClO,, 0 to 0.8 V16)] and on gold foil [60 uF at 1 M HCIO,, 0.45 V17)]. The Ay against potential curve (fig. 5) exhibits a p.z.c. in 74.9 x 10m3 M HClO, at ca. 0 V (SHE) which does not agree (200 to 240 mV less positive) with the values obtained from capacitance measurementsls). From table 1 and fig. 6, r+ increases with decreasing potential for the three more dilute solutions and decreases with decreasing potential only for the most concentrated solution. This type of behaviour is also found at mercury electrodes for some electrolytes at potentials more positive than P.Z.C. Thus, Wroblowa, Kovac and Bockrisrs) found that for mercury/ * The voltage output from the detector unit was calibrated for various deflections by moving the detector unit on its precision micrometer translation stage. A mistake in reading the micrometer would lead to a calibration error. Since a series of readings are taken, the probability of this type of error is small.
650
R. A. FREDLEIN AND J. O’M. BOCKRIS
NaClO, solutions, r+ increases with potential in the dilute solutions, but for solutions of 0.3 M or greater r+ first decreases with decreasing potential before passing through a minimum and then increasing. At high concentrations, it is possible that the data would also pass through a minimum were it available over a wider concentration range. The surface excesses are an order of magnitude higher than those found for the mercury-perchlorate solution interface, although the electrode charge is of the same orderI*). Some evidence exists in favour of high adsorptions at solid metal electrodes. It has been concluded from ellipsometric studies on gold electrodes that anionic surface excesses are large (e.g., - 140 uC cm-2)1a). Further, large adsorptions have been found for a variety of large ions on other solid metal electrodes using radio-tracer techniques 2).
250 -
"
200-
6 ii _o 150? a 2
100 -
L: N
-5.0
-3.0
I -1.0
Fig. 7. Typical plot of total surface excess of per-chlorate ions plotted against the logarithm of concentration of perchlorate ions at 30°C. Potential is + 0.240 V on SHE.
These adsorptions follow the Temkin isotherm giving linear r against log C plots at constant potential. This relation is obeyed by the present data at all potentials studied and only the slope would be affected by errors in calibration and Young’s modulus. A typical plot is shown in fig. 7. A large body of evidence exists to show that at the mercury-solution interface small cations are not specifically adsorbed 2ol 2i). Assuming this to be the case at the gold-perchloric acid solution interface, the anionic diffuse layer charge, qd_, can be calculated from diffuse layer theory using the relations : z+FT+ = (snRT/27t)* [exp ( - z+F$,/2RT)
- 11,
(1%
ELECTROCAPILLARY
STUDY
OF GOLD-PERCHLORIC
qd- = - (s&T/2x)+
ACID
SOLUTION
[exp ( - z_P$,/2RT)
INTERFACE
- 11,
651
(16)
where E is the dielectric constant of the medium and IZ the concentration of the ions (mole cm-j). The potential at the outer Helmholtz plane, tjO, is calculated from (15) and substituted in (16). Values of qd-, which are sensitive only to the order of z+FT+, are listed in table 1 and seen to be negligibly small compared to the total anionic charge. Thus, the adsorbed perchlorate ions are all contact adsorbed on the electrode; a result in keeping with the observation that the total adsorption of perchlorate follows the Temkin isotherm and suggests that hydroxonium ions are not specifically adsorbed at this interface. The most stringent test of the technique comes from the condition of electrical neutrality of the interface which demands that the net charge in the solution, qs=zFT+ +z_Fr_, be equal and opposite to that on the electrode, qM. Errors in qs, being the small difference between relatively large quantities, are greater than in qw, but the agreement (table 1) is nevertheless satisfactory and testifies to the worth of the technique. A possible source of error in this work is the neglect of the effect of the pH on the glass reverse surface of the electrode. A decrease in the pH of the solution would suppress the dissociation of surface hydroxyl groups on the glass surface. Si-OH P SiO- + Hf , leading to a decrease in the charge and a change in the surface stress of the reverse surface. However, this effect is possibly small over the narrow range of pH studied (1.4 pH units). The observed agreement between q, and -qM would not result if the pH change produced significant changes in the surface stress of the reverse surface. It would, however, be advisable to use constant pH when studying an interface over a wide range of electrolyte concentration. Acknowledgement This work was performed at the University of Pennsylvania, Electrochemistry Laboratory. The work was supported by the Office of Saline Water, Department of the Interior, Washington, D.C. (Grant No. 14-30-2771).
1) M. Bonnemay, G. Bronoel, P. J. Jonville and E. Levant, Compt. Rend. (Paris) 260 (1965) 5262. 2) N. A. Balashova and V. E. Kazarinov, in: Electroanalytical Chemistry, Vol. 3, Ed. A. J. Bard (Dekker, New York, 1969). 3) T. R. Beck, J. Phys. Chem. 73 (1969) 446.
R. A. FREDLEIN
652 4) A. Y. Gokhstein.
5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
AND J. O’M. BOCKRIS
Electrochim. Acta 15 (1970) 219. I. Morcos, J. Colloid Interface Sci. 37 (1971) 410. R. A. Fredlein, A. Damjanovic and J. O’M. Bockris, Surface Sci. 25 (1971) 261. R. Suttleworth, Proc. Phys. Sot. (London) A63 (1950) 444. R. W. Hoffman, in: Physics of Thin Films, Vol. 3, Eds. G. Hass and R. E. Thun (Academic Press, New York, 1966). J. W. Wilcock and D. S. Campbell, Thin Solid Films 3 (1969) 3. J. S. Vermaak and D. Kuhlmann-Wilsdorf, J. Phys. Chem. 72 (1968) 4150. B. D. Cahan, Thesis, University of Pennsylvania (1968). A. Kinbara and H. Haraki, J. Appl. Phys. Japan 4 (1965) 243. R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 1955). J. O’M. Bockris, E. Gileadi and K. Muller, Electrochim. Acta 12 1301 (1967). R. Adzic, J. Horkans, B. D. Cahan and E. Yeager, J. Electrochem. Sot. 120 1216 (1973). G. M. Schmid and N. Hackerman, J. Electrochem. Sot. 109 (1962) 243. H. A. Laitinen and M. S. Chao, J. Electrochem. Sot. 108 (1961) 726. H. Wroblowa, Z. Kovac and J. O’M. Brockris, Trans. Faraday Sot. 61 (1965) 1523. W. K. Paik and J. O’M. Brockris, Surface Sci. 27 (1971) 191. D. C. Grahame, Chem. Rev. 41 (1947) 441. P. Delahay, Double-Layer and Electrode Kinetics (Interscience, New York, 1965).
SCIENCE
46 (1974) 641-652 o North-Holland
AN “ELECTROCAPILLARY” PERCHLORIC
STUDY
ACID SOLUTION
Publishing Co.
OF THE GOLDINTERFACE
R. A. FREDLEIN Chemistry
Department,
University
of Newcastle,
New South
Wales, 2308, Australia
and J. O’M. BOCKRIS School of Physical Sciences, Flinders University, Bedford Park, South Australia, 5042, Australia
Received 5 March 1974; revised manuscript
received 27 August 1974
The bending experienced by a thin elastic electrode insulated on one side is used to obtain “electrocapillary” data on the gold-perchloric acid solution interface. The technique yields self-consistent and numerically reasonable values of the charge on the electrode and the cationic and anionic Gibbs’ surfaces excesses. The results are consistent with a specific adsorption of perchlorate ions which is represented by an isotherm of Temkin type. Hydroxonium ions are not specifically adsorbed. The data are limited in number, the numerical value of Young’s modulus for the system lacks accuracy: hence the uncertainty
of the data is high. 1. Introduction Of several methods used to investigate the solid metal electrode-solution interfacel-‘j), one of the more promising makes use of the bending experienced by a thin, one-sided, elastic electrode when its potential and the concentration of the solution are variede). A one-sided electrode is formed when metal is deposited on an insulating elastic substrate; glass being used as the substrate in the present work. The angular deflection, 8, measured at the end of the electrode of length I and thickness S is given by * : 8 = Aa 61/ES2 (1 - v), where Aa is the change in surface stress and E and v are Young’s
(1) modulus
* In eq. (l), B is not the angle, but the angular deflection or change of angle at the tip of the electrode. Consequently, we measure 19and obtain Au by substitution in eq. (1). For this reason, if there is an error in E, it will appear in do. Correspondingly, as E is a multiplier in (l), errors in E appear also in derivations of da and hence in the quantities r and qrn. 641
642
R. A. FREDLEIN
AND
J. O’M. BOCKRIS
and Poisson’s ratio of the electrode respectively*. The surface stress related to the interfacial tension, y, by the Shuttleworth equation 7):
is
o=y+dy/de,
(2)
where E is the surface strain. the surface strain,
Writing
the surface stress as a power series in
G=U+bE+CE’+...,
(3)
we have for small strains: ci=bE,
(4)
since 0 = 0 when E= 0. Using (4), eq. (2) may be written :
and integration
o=bdy/da+y,
(5)
y=a-b+Ce-“I”,
(6)
yields :
where the constant
of integration,
C, is given by: C=b+
where y0 is the interfacial
tension
yo,
(7)
at c =O. From
(6) we have that:
Ay = Ao + C e-uib (e-d0’b - 1). Differentiating
(4) and (6) with respect to E and making dylde - b YO
+
(8) use of (7) yields:
-a/b
b
For evaporated gold particles at 50 “C the values : IT= 1145 dyne cm-’ ; have been obtainedlo). dylde = - 706 dyne cm-’ y= 1851 dyne cm-‘; Since y. must be of the order of 1000 dyne cm-l, these values satisfy (9) only if b is sufficiently large for both sides of (9) to be - 1. Under this condition e-Aalb is unity and (8) becomes: Ay=Ao,
(10)
and (1) may be written: 0 = Ay 61/E6’ (1 - v), yielding
the differential
interfacial
tension.
AY=Y-Y’,
(11)
Now (12)
* This equation differs from that previously derived6) in that Aa was previously assumed to equal the differential interfacial tension and by the introduction of the plate bending correction term, (1 -v) *, 9.
ELECTROCAPILLARY
STUDY
OF GULRPERCHLORIC
ACID
SOLUTION
INTERFACE
643
where y” is the unknown interfacial tension of the reference state, i.e. the state, at some particular solution composition and electrode potential, from which all deflection measurements are made. Since y” is a constant, the usual electrocapillary equations can be applied to calculate the Gibbs’ surface excesses, F+, r- of the cations and anions and the electrode charge,
and (14) a, is the mean ion activity of the electrolyte, V+, V- the potential of the electrode on the scale of a reference electrode reversibie to the cation and anion respectively and p refers to chemical potential of all the species composing the solution. This paper reports the first attempt to study the doubielayer by making “electrocapillary” measurements at a solid metal-solution interface. 2. Experimental Electrodes: Gold was cathodically sputtered onto glass strips (Corning 0211) 120x 15 x 0.085 mm to a thickness of ca. 1000 A. To obtain good adhesion of the gold, transparently thin layers of tantalum and then platinum were first depositedll). Slow relaxation of the intrinsic stress in the deposited film caused significant deformation of the electrode during the course of experiments. Attempts to anneal the electrodes at 200°C increased the intrinsic stress, in accord with the other observations12)~ and the rate of deformation but stable electrodes were obtained by exposing them to lop3 M KC1 solution at 90°C for four weeks. Cell: The electrode was pressed against a flat surface formed by cutting a 20 mm long semi-cylindrical section from a S/S inch dia. quartz rod; the section cut from the rod being used as the pressure plate. Electrical contact to the electrode was made by goid deposited on the rod. The PFE cell head contained four ports (of which two are shown, fig. 1) used to introduce the bright platinum foil counter electrode (20 cm’), solution inlet and outlet tubes and a saturated calomel reference electrode. Two medium porosity frits separated the reference electrode from the main compartment. Water from a thermostat was circulated through the cell jacket. Solutions were prepared in a closed, external, thermostatted mixing chamber and circulated through the cell using all Teflon pump (Fluorocarbon Saturn 2000) and flexible Teflon tubes. Provision existed for passing the
644
R. A. FREDLEIN
AND
1. O’M. BOCKRIS
PC
PC
Fig. 1. Cell: (a) gold electrode, (b) optical window, (c) counter electrode, (d) gold coated quartz contact rod, (e) pressure plate, (f) solution inlet tube, (g) PFE cell head, (h) Teflon connectors (Fluorocarbon), (i) water jacket connected to circulating thermostat.
solution through an anodic pre-electrolysis cell. The solutions were maintained under an atmosphere of purified argon and contacted only glass and Teflon surfaces. Optical lever: The deflection at the tip of the electrode was measured with a laser optical lever. The beam from a 1 mV He/Ne laser (Spectra-Physics, 132) was collumated by passage through a 2 mm hole positioned 1200 mm in front of the laser. The beam passed through a meniscus lens (f = + 500 mm) and was reflected from the electrode positioned close to the focal point. The reflected beam traversed a path of 2300 mm before entering the detector (fig. 2); long path lengths being obtained by reflection from
Fig. 2. Detector: (a) support bracket, (b) translation stage, (c) micrometer, (d) right prism, bright platinized front faces, (e) photocells, (f) interference filter, 6278-6378 A, (g) tube, (h) diffusion discs.
ELECTROCAPILLARY
surfaced optical
mirrors. components
STUDY
OF GOLD--PERCHLORlC
The cell (in a horizontal were mounted
ACID SOLUTION
position),
on two parallel
645
INTERFACE
laser, detector 1.3 m optical
and
benches
securely fastened together. In the detector (fig. 2), the reflected beam was split on the bright platinized front faces of a right prism and deflected through diffusion discs to photoconducting CdS cells (International Rectifier CS 120-C). Displacement of the beam due to bending of the electrode causes one photocell to receive more light and the other less. The output voltage was measured with a recorder (Esterline Angus). This detector design minimized the effect of slow variation in laser intensity (ca. 3%) and the time constant of the detector circuit (fig. 3) was made ca. 6 set to minimize the effects of building vibrations.
Fig. 3.
Detector circuit.
The interference filter minimized the stray light entering the detector. The detector was mounted on a translation stage (Lansing) to permit calibration and tests showed that the output was linear with beam displacement over the range encountered in this work. Procedure: All the components of the cell, pre-electrolysis cell, pump, mixing chamber and the electrode assembly were cleaned in nitric-sulfuric acid cleaning mixture and thoroughly rinsed in conductivity water. A dilute HClO, solution (ca. 3 x 10v3 M) was prepared in the mixing chamber with conductivity water and 5N8, 70% HClO, (Apache Chemicals). The solution of volume 1.5 1 was anodically pre-electrolysed on bright platinum gauze electrodes and flushed with purified argon for 24 hours while circulated through the cell and mixing chamber. Electrode deflections with change of potential and electrolyte concentration were measured relative to the electrode position at 540 mV (SHE) in the most dilute solution. At potentials higher than 540 mV the possibility of electrode oxidation exists; sputtered metal electrodes seem to be more
646
R. A.
FREDLEIN
AND
J. O’M. BOCKRIS
reactive than foils. Held at 540 mV (Tacussel PRT 20-2A potentiostat) the electrode underwent no deflection with time. When the potential was suddenly decreased the deflection obtained (fig. 4) decreased slowly and almost linearly with time. This decrease in the deflection with time was thought to be due to adsorption of organic impurities (vide infra) and the deflection was obtained by extrapolating the deflection against time curve back to zero time. The electrode was then brought back to 540 mV until the impurities had desorbed, as indicated by an unchanging deflection with time, before the deflection at a new potential was measured. In this way, a curve of deflection against potential was constructed. After desorption of impurities was complete, variations in the electrode position at 540 mV were small.
4
Fig. 4.
Typical deflection
against
Time
time curves showing
extrapolations
to zero time.
With the electrode held at 540 mV the concentration of the electrolyte was increased by adding HClO, to the mixing chamber and circulating the solution. The deflection caused by increasing the electrolyte concentration at this potential was measured and a curve of deflection against potential for this new concentration determined as previously described. The potentia1 of the electrode was never permitted to go below ca. 0 V SHE to prevent hydrogen deposition on the electrode. Over the potential range O-0.540 V only steady-state currents (< 10 -6 A cm-“) were observed. 3. Results Values of Ay were calculated from the observed deflections using (11) and the values: E=7.45 x 1O’l dyne cm-’ and v =0.22 for Corning 0211 glass and plotted in fig. 5. Each curve was drawn from three sets of measurements which had average data spread of ca. 5 dyne cm-‘. Only one uninterrupted series of measurements was completed. The charge on the electrode (table 1) was found by graphical differentiation of these curves [see eq. (14)].
ELECTROCAPILLARY
STUDY
OF GOLD-PERCHLORIC
Plots of differential
I
I
0.30
O-61
2.99
8.92
26.3
79.9
(V, SHE)
interfacial tension against potential acid concentrations at 30°C.
TABLE
647
INTERFACE
I
Double layer data for the gold-perchloric lo3 C HC104 (M)
SOLUTION
0 Potential
Fig. 5.
ACID
for various perchloric
1 acid solution interface at 30°C
Electrode potential (V SHE)
105qhl (C cm-z)
105 Electrode capacitance (F cm-“)
105 z+FT-+ (C cm-2)
0.440 0.340 0.240 0.140 0.440 0.340 0.240 0.140 0.440 0.340 0.240 0.140 0.040 0.340 0.240 0.140 0.040
1.8 1.7 1.6 1.3 1.9 1.8 1.5 1.2 2.1 1.7 1.3 1.0 0.4 2.1 1.7 1.0 0.5
0.2 1 2 3 0.4 2 3 4 3 4 4 5 6 4 5 6 7
5.4 5.6 5.8 6.2 11.8 11.8 11.9 12.2 17.7 17.8 18.3 19.0 19.7 25.1 24.4 24.1 23.8
IO5 z_Ff_
(C cm-3 -
5.9 5.9 6.2 6.4 13.6 13.6 13.6 13.8 21.0 20.5 20.3 20.0 19.9 26.4 26.1 25.5 24.2
105 qs (C cm-2)
106 qli(C cm-2)
-1
0.3 0.3 0.3 0.3 0.6 0.6 0.6 0.6 1 .o 1.0 1.0 1.0 1.0 1.6 1.6 1.6 1.6
0 0 0 -2 -2 -2 -1 -3 -3 -2 -1 0 -1 -2 -1 0
648
R. A. FREDLELN
AND
J. O’M. BOCKRIS
-------H
7L.9 x 10-k
200
. . .
’ 26.3 x Wl+M
.
. 6’92 x lO-jM
100 c-
. 2.99
F ;
x
IO-‘M
0
%
~
_
_
_
.2.99x
10-3M
3 L -100 k c
-200
6.92 x lo_3M .
-
*
-
I
0
.
26.3x&M *
I
I
I
0.20
040
oal
Potential (V, SHE 1
Fig. 6. Surface excesses of hydroxonium ions (positive values) and perchlorate ions (negative values) at various potentials and concentrations of perchloric acid at 30°C.
Values of A? interpolated from the curves of fig. 5 were plotted against lna, at constant potential on the scale of a reversible hydrogen electrode in the solutions and on the scale of a reversible perchlorate electrode in the solutions. The potentials on these scales were calculated from the potentials measured against the saturated calomel reference electrode using the Nernst equation and neglecting liquid-junction potentials. Activity coefficients used in this work were calculated from the Debye-Htickel relation taking the ion size parameter as 5.1 AIs). These curves were graphically differentiated and the Gibbs’ surface excesses calculated [eq. (13)] and the results plotted in fig. 6 on the standard hydrogen electrode scale. 4. Discussions The decrease in the deflection with time observed when the potential of the electrode was suddenly reduced from + 540 mV (SHE) (fig. 4) is due to adsorption of organic impurities on the electrode since this effect was more
ELECTROCAPILLARY
pronounced
STUDY
for solutions
OF GOLD-PERCHLORIC
ACID
SOLUTION
that had not been subjected
INTERFACE
to long periods
649
of
anodic pre-electrolysis. Further, the effect was more pronounced as the potential was lowered towards the potential of zero charge (P.z.c.) (0 to - 100 mV, SHE; fig. 5). Organic impurities are most strongly adsorbed at the p.z.c. where the competition from adsorbed water dipoles is at a minimuml4). At + 540 mV adsorption of organic impurities would be minimal. The data reported here were all taken from the same series of experiments, each reading being repeated three times and the possibility of undetected errors of calibration exists in the Ay values *. Further, the possibility that the vacuum deposition of metal on the glass substrate may alter its Young’s modulus must not be overlooked. [For accurate results, Young’s modulus of the electrode should be directly determined; the technique making use of the natural frequency of flexural vibration being appropriates).] In addition, the reported values do not take into consideration the roughness of the electrode. Values referred to the true surfaces area are the reported values diminished by the unknown roughness factor. For these reasons, it would be wise to regard the absolute values of Ay, r+ , K, qM and the electrode capacitance as having a low (e.g., +50%) order of accuracy. However, since all these values would be in error by the same factor, the selfconsistency of the results and trends shown by the results are significant. There is also the possibility that the surface may be trace contaminated by platinum due to the pre-electrolysis or the method of preparation of the electrodesls) and the results may not be entirely applicable to gold foil electrodes. Estimates of the double-layer capacitances (table 1) were made by double graphical differentiation of Ay with respect to potential. Capacitances from the present work are in accord with galvanostatic pulse values obtained on gold single crystals [IO uF at 0.01 M HClO, and 15 uF at 0.01 M HClO,, 0 to 0.8 V16)] and on gold foil [60 uF at 1 M HCIO,, 0.45 V17)]. The Ay against potential curve (fig. 5) exhibits a p.z.c. in 74.9 x 10m3 M HClO, at ca. 0 V (SHE) which does not agree (200 to 240 mV less positive) with the values obtained from capacitance measurementsls). From table 1 and fig. 6, r+ increases with decreasing potential for the three more dilute solutions and decreases with decreasing potential only for the most concentrated solution. This type of behaviour is also found at mercury electrodes for some electrolytes at potentials more positive than P.Z.C. Thus, Wroblowa, Kovac and Bockrisrs) found that for mercury/ * The voltage output from the detector unit was calibrated for various deflections by moving the detector unit on its precision micrometer translation stage. A mistake in reading the micrometer would lead to a calibration error. Since a series of readings are taken, the probability of this type of error is small.
650
R. A. FREDLEIN AND J. O’M. BOCKRIS
NaClO, solutions, r+ increases with potential in the dilute solutions, but for solutions of 0.3 M or greater r+ first decreases with decreasing potential before passing through a minimum and then increasing. At high concentrations, it is possible that the data would also pass through a minimum were it available over a wider concentration range. The surface excesses are an order of magnitude higher than those found for the mercury-perchlorate solution interface, although the electrode charge is of the same orderI*). Some evidence exists in favour of high adsorptions at solid metal electrodes. It has been concluded from ellipsometric studies on gold electrodes that anionic surface excesses are large (e.g., - 140 uC cm-2)1a). Further, large adsorptions have been found for a variety of large ions on other solid metal electrodes using radio-tracer techniques 2).
250 -
"
200-
6 ii _o 150? a 2
100 -
L: N
-5.0
-3.0
I -1.0
Fig. 7. Typical plot of total surface excess of per-chlorate ions plotted against the logarithm of concentration of perchlorate ions at 30°C. Potential is + 0.240 V on SHE.
These adsorptions follow the Temkin isotherm giving linear r against log C plots at constant potential. This relation is obeyed by the present data at all potentials studied and only the slope would be affected by errors in calibration and Young’s modulus. A typical plot is shown in fig. 7. A large body of evidence exists to show that at the mercury-solution interface small cations are not specifically adsorbed 2ol 2i). Assuming this to be the case at the gold-perchloric acid solution interface, the anionic diffuse layer charge, qd_, can be calculated from diffuse layer theory using the relations : z+FT+ = (snRT/27t)* [exp ( - z+F$,/2RT)
- 11,
(1%
ELECTROCAPILLARY
STUDY
OF GOLD-PERCHLORIC
qd- = - (s&T/2x)+
ACID
SOLUTION
[exp ( - z_P$,/2RT)
INTERFACE
- 11,
651
(16)
where E is the dielectric constant of the medium and IZ the concentration of the ions (mole cm-j). The potential at the outer Helmholtz plane, tjO, is calculated from (15) and substituted in (16). Values of qd-, which are sensitive only to the order of z+FT+, are listed in table 1 and seen to be negligibly small compared to the total anionic charge. Thus, the adsorbed perchlorate ions are all contact adsorbed on the electrode; a result in keeping with the observation that the total adsorption of perchlorate follows the Temkin isotherm and suggests that hydroxonium ions are not specifically adsorbed at this interface. The most stringent test of the technique comes from the condition of electrical neutrality of the interface which demands that the net charge in the solution, qs=zFT+ +z_Fr_, be equal and opposite to that on the electrode, qM. Errors in qs, being the small difference between relatively large quantities, are greater than in qw, but the agreement (table 1) is nevertheless satisfactory and testifies to the worth of the technique. A possible source of error in this work is the neglect of the effect of the pH on the glass reverse surface of the electrode. A decrease in the pH of the solution would suppress the dissociation of surface hydroxyl groups on the glass surface. Si-OH P SiO- + Hf , leading to a decrease in the charge and a change in the surface stress of the reverse surface. However, this effect is possibly small over the narrow range of pH studied (1.4 pH units). The observed agreement between q, and -qM would not result if the pH change produced significant changes in the surface stress of the reverse surface. It would, however, be advisable to use constant pH when studying an interface over a wide range of electrolyte concentration. Acknowledgement This work was performed at the University of Pennsylvania, Electrochemistry Laboratory. The work was supported by the Office of Saline Water, Department of the Interior, Washington, D.C. (Grant No. 14-30-2771).
1) M. Bonnemay, G. Bronoel, P. J. Jonville and E. Levant, Compt. Rend. (Paris) 260 (1965) 5262. 2) N. A. Balashova and V. E. Kazarinov, in: Electroanalytical Chemistry, Vol. 3, Ed. A. J. Bard (Dekker, New York, 1969). 3) T. R. Beck, J. Phys. Chem. 73 (1969) 446.
R. A. FREDLEIN
652 4) A. Y. Gokhstein.
5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
AND J. O’M. BOCKRIS
Electrochim. Acta 15 (1970) 219. I. Morcos, J. Colloid Interface Sci. 37 (1971) 410. R. A. Fredlein, A. Damjanovic and J. O’M. Bockris, Surface Sci. 25 (1971) 261. R. Suttleworth, Proc. Phys. Sot. (London) A63 (1950) 444. R. W. Hoffman, in: Physics of Thin Films, Vol. 3, Eds. G. Hass and R. E. Thun (Academic Press, New York, 1966). J. W. Wilcock and D. S. Campbell, Thin Solid Films 3 (1969) 3. J. S. Vermaak and D. Kuhlmann-Wilsdorf, J. Phys. Chem. 72 (1968) 4150. B. D. Cahan, Thesis, University of Pennsylvania (1968). A. Kinbara and H. Haraki, J. Appl. Phys. Japan 4 (1965) 243. R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 1955). J. O’M. Bockris, E. Gileadi and K. Muller, Electrochim. Acta 12 1301 (1967). R. Adzic, J. Horkans, B. D. Cahan and E. Yeager, J. Electrochem. Sot. 120 1216 (1973). G. M. Schmid and N. Hackerman, J. Electrochem. Sot. 109 (1962) 243. H. A. Laitinen and M. S. Chao, J. Electrochem. Sot. 108 (1961) 726. H. Wroblowa, Z. Kovac and J. O’M. Brockris, Trans. Faraday Sot. 61 (1965) 1523. W. K. Paik and J. O’M. Brockris, Surface Sci. 27 (1971) 191. D. C. Grahame, Chem. Rev. 41 (1947) 441. P. Delahay, Double-Layer and Electrode Kinetics (Interscience, New York, 1965).