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Partial charge transfer and adsorption at metal electrodes. The underpotential deposition of Hg(I), Tl(I), Bi(III) and Cu(II) on polycrystalline gold electrodes
E&c-a
) Pergamon
Acta, Vol 39.No S/9.pi 1037-10611994 cqyn@ttQ 1994ebmasaallxLtd Pmted m Great Bntun. All n&s mmmd Wl3-4696/949700+0.09
PARTIAL CHARGE TRANSFER AND ADSORPTION AT METAL ELECTRODES. THE UNDERPOTENTIAL DEPOSITION OF Hg(I), Tl(I), Bi(II1) AND Cu(I1) ON POLYCRYSTALLINE GOLD ELECTRODES G SAL&* and K BARTELS~ + Fachhochschule Hamburg, Fb BPV. Labor fir Phyakahsche Chenue, D 21033 Hamburg, Germany t BIOTRONIK GmbH & Co, Ingemeurburo, Woermannkehre 1, D 12359 Berhn. Germany (Recnued 8 September 1993, m
remsed
fbrm
10 December
1993)
Abet-The absorption of H&, Tl+, B13+ and Cuz+ at polycrystalbne gold electrodes from aqueous solutions IS mvestigated by potentrostatic step expenme-nts at rotatmg rmgdBk electrodes The macroscoptc partml charge transfer coefiiuent 1u determmed from the uutud part of the current transients at rmg and tik which 1s diffuszon controlled for low reactant concentrations The measured I are 047 (Hg$+), 061 (Tl’), 073 (B?) and 092 (Cuz+) f 003 The potential dependence. of the measured I IS small and could m&a& that the mam part of 1IS causedby microscopic charge transfer A DdTusron controlled formation of the mam part of the partmlly charged adsorbate pomts to strong repulsive forces between the adsorbed particles With all of the mvesfigated systems, a second type of deposit has been detected which conasts of completely &charged metal
Key words partial charge transfer, underpotential deposttton, adsorption, rrde, gold
1. INTRODUCl’ION Underpotential deposition of metals contmues to be studted mtensrvely (for revrews see [l-4]) Especially, an abundant literature exists about the mvesttgatton of the structure and structural transformattons of the deposits wtth pure electrochemtcal[5, 61 and wtth in situ surface. techmques exC7, 83 and (elhpsometry[9, lo], X-ray spectroscoprc[l l-181 and drffractton techmques[19-211, LEED/RHEED and AES[22, 231, STM and atonuc force[24-281, see also the hterature cued there) Another interesting question m underpotentud deposmon is the charge of the deposns In general, the charge of electrochemtcal spectes chemtsorbed at an interface IS charactermed by the coefScrent of partml charge transfer (for revtews see[29, 303, collection of data also m[31-331). Recently, it has been shown that adsorptron of Hg(I) tons m the underpotenttal range is connected wtth non-integer pa&al charge transfer[34-37l Two dtfferent processes have been detected durmg the depositton of HgO Ions at polycrystalhne Au electrodes m the underpotentml range by rmg-disk measurements[34, 361 and impedance spectrometry[37j. The first process is the adsorptton of metal ions onto the substrate wrth part& charge transfer The substantmlly slower second process IS the formatton of completely discharged metal spectes. In the former process, single ions are adsorbed and chemical bonds are formed between the ton, the surroundmg solution components and the metal lattrce of the substrate The charge distrrbutron of thts chermcal bond is responsible for the
macroscopic pa&al charge on the ion The second process may be the begmmng or a prehmmary stage of the formatton of an alloy[35, 361 whose rate IS perhaps governed by the generally slow sohd state reacttons Thts part of the deposited metal spectes ts therefore termed alloy depostt[35) The whole underpotentud deposmon IS modelled by a multistep mechanism wtth chemlsorbed m&mediates at which m the u last Wstep completely dtscharged metal 1s formed At a gtven underpotenttal m equihbnum, both forms (parttally charged specres and alloy deposit) are present simultaneously These findmgs (re the exrstence of two forms of the deposited metal) are to be seen m connectron with the noneqmhbnum structural transformatrons investigated m[38, 393, where also adsorbed and stronger Incorporated specres are detected at the Ag(l1 l)/Pb*+ underpotenttal deposmon Formatton of alloy was also descrrbed at Pd(lOO)/Cu*+ (LEED and XPS)[40], at Au(poly)/Pb’+ (roughemng of the substrate after strrpping of the deposit has been observed wtth STM[41]), at Pt(poly)/Hg[42], at Ru(poly)/Ag+[43] and at Au(poly)/Pb*+[44] (see also Refs [40,42,72,95] m Ref [35]) Two &fferent deposrt layers have been found at Pt(lOO)/Cu*+ (4th EXAFS)[l6], at Pt(lll)/Ag+[23], &d at Ag(poly)/T1+ CW. Although there were some hmts on macroscopic partial charges of adsorbates m the past, like for Pt/ Cu* + [45], Au/Pb* + [46,47] and AufAg + [48], rt was wrdely accepted to assume the metal tons are completely discharged m the underpotentml depostts (see eg [l-3, 26, 28, 493) Recently, tt has been shown wtth XANESIEXAFS, that the Cu underpotentud
1057
1058
G
and K BARTELS
SAL&
deposits possess a charge smaller or near by + 1 at Au(100)/Cu2+[14], at Pt/W+[lS], at Pt[lll]/ Cuz+[13] and at Pt(lOO)/Cu’+[16] Ring-disk measurements at the system Pt(poly)/Cd2+ have yielded a partial charge transfer of I= 0 77[50] The quartz crystal microbalance (for reviews see [St, 521) has been used to determine the mass and the partial charge of underpotential depomts[51-561 However, an exact mass determination depends on roughness[57] and some of the results are hard to explam[56] The aim of this paper 1s to give further expenmental examples for partially charged species 1n the underpotential deposits and to strengthen the ev1dence for the above mentioned mechanism Therefore, 1n the followmg, attention should be focussed on the adsorption of ions and the determination of their partial charge transfer described by the macroscop1c Lcoeff1aent The Lcoeff1iaents and surface densities of deposited species have been determrned by ring-disk measurements as 1n [34, 36, 581 These measurements allow a direct and accurate deterrmnation of I-coeffiiaents 2. EXPERIMENTAL
SECTION
The ring-disk equipment has been descIlbed [36, 581 In general, the rotatron speed of the electrode has been varied between 100 and 25OOrpm (max1mal deviation 0 15%) The radii (m cm) of the electrode are 0 250 (disk), 0 275 (ring mt ), and 0 350 (ring ext ) The polycrystalllne gold electrodes have been prepared 1n the same way as m[36] Before measurements, the Au electrodes were polished with Al,O, , rmsed with d1st1lled water and cycled 1n the corresponding supporting electrolyte (sweep rate 1 V m1n - ‘) until reproducible current-potential curves were obtained The following systems have been investigated Au/Hgz+ 0 02-94 mM, NaClO, 0 8 M, HClO, 0 2 M Au/Hg’+ 0 02-80mM,
Au/Y+ 0 004-10 mM, NaClO,
“L -A1-Az-
substrate met., S
“I
phase boundary
as
“3 phase boundary
0
soIYllon
(1)
The adsorbed intermediates A, may be thought as changmg from adsorbed single species with relatively high partial charge at the O-side (eg here A2) to more or less structurrzed adsorbates at the R-side (eg here A,) The u, values are the corresponding react1on rates The transfer of the charge z e- 1n the overall reaction RcrO+z
e-
(2)
1s divided into the portion 1, for the corresponding steps 1 However, the measurable quantities are the macroscopic partial charge transfer coelliclents I,[29], Including the double layer terms @q/X,), The macroscopic partial charge 1 of the adsorbed metal species 1s determined from potentlostatlc relaxation expenments at ring-disk electrodes The procedure (for details see [35, 361) 1s described shortly 1n the following Imtmlly, the disk potentral E, 1s held 1n the far positive region, where adsorption (1e underpotential deposltlon) 1s practically absent At the time t = 0, E, 1s stepped to the desired value 1n the underpotential regon and the disk current I~ 1s reglstrated as a time function During the whole time, the ring potential E, IS held at a potential 1n the region of the cathodic 11m1t1ngcurrent of reaction (2) Thus, the charge of the deposited 0 1s measured at the disk, whereas the mass of the deposited 0 1s measured at the ring as difference 1Rbetween the momentary ring current I,,“* and the 11m1t1ngring current I,,.~ ,,,,, flowing at the end of (and before) the experiment IR = G,“g- Lp, Irm In the initial part of the current transients, only step 3 (the step with which the reduction of 0 starts) of reaction (1) proceeds and 1t 1s[35] I&) = -zF
&,I
dr,/dt
(3)
and (II)
1 M, HClO, 1 mM (III)
Au/B13+ 0 001-l mM, HClO, 1 M
(IV)
Au/Cu2+ 1O-7-1O-4 M, H2S04 1 M
(V)
The solutions were prepared from p a chemicals and b1dnt1lled water (once over KMnO,) The measurements have been performed 1n deaerated solutions (argon) at 25 0 f 0 1°C All potentials are referred to the ssce (NaCl saturated calomel electrode) The concentrations of the Hg(1) solution are expressed 1n mol 1-l referred to the entity Hg:+ 3. THEORETICAL
Rt
(I)
NaClO, 0 8 M, HClO, 0 2 M
the general theory of chemlsorptlon reactlons[29] a multi-step process (here three steps)[35]
SECl-ION
The react1on of the metal 0 at the substrate metal S 1n the underpotential range 1s wrttten according to
1~0) = l,inp - l,inp urn = N
zF
A,
dr,/dt
(4)
with the area A, of the disk electrode and the collection elliclency N Here, the rndex “3” of the coelliclent I 1s omitted for convenience, because only the first chemlsorbed state accessible from the O-side 1s consIdered explicitly The terms containing v1 (reaction of A, to R), dr,/dt (change of the amount of A,) and (8q/8)cR(x = O)/dt (change of double layer charge q by appearance of R with concentration cR(x = 0) at the surface) are neglected 1n equations (3) and (4), because the adsorption density of A, and the mass of completely dscharged metal R IS zero or small at the begmnmg (see Ref [35] Chaps IV and V) Thus, the quotient -to(t)
N/I&) = I
for t-*0
(5)
1s constant, as long as no remarkable amounts of other chemlsorptlon states (A,) or completely dls-
Partml charge transfer and adsorption
charged metal R are formed Because the potential IS constant durmg the adsorption process, 1 IS obtamed at a defined potential, and it IS possible to measure the potential dependence of I If one of the steps of reaction (1) IS rate determmmg m the course of the time, the quotient -lo N/i, IS constant and equal to the corresponding 1, durmg that tlmq35] The determination of 1 wth equation (5) does not depend on the true area of the disk electrode For sufficient large times, the adsorption densities of the mtermedlates successively attam eqmhbrmm and the whole process consists m the formation of R according to equation (2) Thus it IS -I&)
N/z&) = 1
for t + 00
1059
Hgp9.4x
W5M
1 ~=+O.lm%v 2.~=+02731
4
V
3 q)=+O473lV 4 en=+0673l V
:3
2
1
(6)
0
if the exchange rates tl,,, of the steps of reaction (1) are nearly equal or increase m the order ulA < vl,, < uJA what seems plausible because stronger incorporation into the ordered solid phase leads to lower reaction rates as uncorrelated single particle chenusorptlon has The expenments confirm the slower formation of R, see below (-on 5 2)
3 ? 0
K I*
5
0
A RESULTS 4 1 Disk and ring current transients The Qsk and rmg current transients of potential step expetrments at polycrystallme Au electrodes m Hgz+, Hg2+, Tl+, B13+ and Cu2+ containing solutions are exhlblted m Figs l-5 The shown transients are charactenstrc for the range of low reactant concentrations, they are dlffuslon controlled at the begmmng The diffusion control 1s venfied by the dependence of the current upon the rotation rate, see [34,36] for the Hg(I) and Hg(Ik) system and [58] for Tl(I), Bi(III), and Cu(I1) The MWon plateau IS well marked m the Hg(I) and B@II) systems, whereas m the case of very short [Fig 3, Tl(I)] and very long [Fig 5, Cu(II)] measurmg times the plateau IS a little dlstorted In some cases [Hg systems, Bi(III)], the most part of the adsorbate IS obviously formed under dlffuslon control It has been shown that this behavlour reqwres high rates of the reamon steps and adsorption isotherms wth strong repulsive forces between the particles[35] Model calculations for the Hg(I) system wth Frumkm type isotherms lead to very high repulsion constants (>20)[35, 361 This 1s m agreement with adsorbed species bemg not completely discharged but carrymg a partml charge For higher reactant concentrations, the current transients exhibit a maximum and after that they decrease monotonously, Hrlthout reachmg dlffuslon control (curve 3 m Fig 3, curve 4 m Fig 4 and Fig 6) 4 2 Determmatzon of the 1 coejicrents The quotient - I~ N/I, IS plotted as tune function for the five systems (I)-(V) Typical curves are pra sented m Figs 7-9 In all cases, the quotient M constant at the begmnmg. Hence, according to equation
I
‘:
15
t/s Fig 1 Jhk (IJ
and nng current (ra) transients of potential steu exuerunents for Au/HRU) 94 x 10-‘M. NaCIO. 08-M, HClO, 0 2M Im&’ potential 07hOV (no’ adsorption), step to E, = 0 1806V (l), 02731V (2),
04731 V (3), 06731 V (4) at t = 0 Rmg potenbal E, = 0 OOOV Electrode III, revolution rate 1230 rpm
(5), m this regon the values of the coeficlents 1 are obtamed All systems exhibit 1 more or less dlffermg from 1 In solutions contammg higher reactant concentrations where the current transients do not offer &&ion controlled behavlour, -tu N/I,, IS constant at the begmmng, too, and hence 1 IS measurable In this manner, 1 IS determmed for the systems (I)-(V) at vmous potentials and concentrations Figure 10 shows the patial charge coefficent 1 of all mvestlgated systems as function of the potential The potential dependence of 1 IS small, It IS < 0 01 V- ’ for Hg(I), +OOl V-’ for Cu(II), +0 06 V-’ for Tl(I), and -0 07V-’ for Bi(II1) The data selected 111 Fig 10 belong to a single reactant concentration for each system, 1 does not depend upon concentration m the error hmlts (about +O 02 0 04) The Hg(II) ion is reduced to Hg(1) at the mterestmg potentials m the underpotential range Thus the interpretation of the expenments anth HgO IS not as strrughtforward as m the case of Hg(I), for &tads see [36] There are two possible won paths for underpotentml deposition from Hg(I1) solutions. Hg(II) reacts unmdately to an adsorbed parWle or it IS reduced first to the Hg(IO_state and then adsorbed The evaluation of the l-type charge of the adsorbed species from the i& data wth the ad of equations (23) and (24) in Ref [35] does not depend on the rea&on mechamsm d the rea&ons are fast as it 1s the case for the dfluslon controlled Hg(I) and Hg(I1) underpotential deposition In [36] it was shown that the l-type charge of adsorbed
G SAL&and K BARTELS
1060
1-
.
\
en=-04893V
I
s
c
:
;
4
2
3
0
I
*,O P
c( CI ‘:
Tl+4xlo~w
r-z I
2
3
1 tD=-04883V 2 cD=-O4OOV 3 rD=+OlOOV
I
\
\
c
&*+2x
10”M
q=+01591v r
f
21 cD=+t,b%,t v cD=+02026V 3 b=+O4526V 4q,=+O6526V f
f
3 ?0 * LL
4
2
3
1
-
0
16
7
”
4
tls
Fig 2 Disk (tu) and rmg current (1s) transients of potential step expenments for Au/Hg(II) 200 x lOA M, NaCIO, OSM, HCIO, 02M Imtial potential 1 OOOV (no adsorption), step to E, = 0 1606V (l), 0 2026 V (2), 04526V (3), 0 6526V (4) at t = 0 Rmg potential E, = -0 1OOV Electrode III, revolution rate 1230rpm The current of the discharge Hg(I1) + Hg(1) has been subtracted from to, 1s IS corrected correspondmgly for t < 0
mercury species IS 1 - I = 0 53 f 0 02 for the Hg(1) system and 1 - I= 0 61 + 0 05 for the Hg(I1) system The correspondmg I-coefficients are 047 and 0 39, respectively, rf one uses the reaction scheme I l-1 Hg(1) + eHg Hg,,, + (1 - W- 0 Hg(I1) + 2 em equation (3) of Ref [36] I, must be replaced by 1 - A) The mterpretatlon of the results of the Hg(I1) system has to be reserved for further mvestlgatlons 4 3 Adsorption densrtles ofthe mtermedlate r and of the completely discharged metal ions m, Figures 7-9 show the the quotient -zD N/z, 1s uutlally constant and rases near or some time after the end of the dlffuslon hmlted part of the current trannents In this time regon, step 3 of reaction (1) 1s no longer the prevadrng reaction m the overall process For all mvestlgated systems, -lD N/I~ reaches a further plateau m the final range of the experiment at longer times at 100 f 004 Thus, completely dacharged material 1s formed here m accordance with equation (6) Because there 1s no mdlcatlon of further plateaux or other stnkmg lrregulmtles m the time behavior of
4
IL
”
tlms Rg 3 Disk (I~) and rmg current (l,J transients of potential step expenments for Au/Tl(I) 4 00 x 10e6 M, NaCIO, 1 M, HCIO, 1 mM Imtlal potential 0 3000 V (no adsorptlon), step to E, = -04883V (l), -04OOOV (2), -0lOOOV (3), at t = 0 Rmg potential E, = -0 55OV Electrode IV, revoiutlon rate 1250rpm
313+1 X 10dM :u=00671V
lD=omV
1
2 rD=0135V 3eD=O250V 4cD=O400V
l&a 4
3
4
3
2
I
100
2
3 t0 *
lzU
I 0
1
15
$
I
50
1
tls
Ftg 4 Disk (I~) and rmg current (1s) transients of potential step experiments for Au/Bl(III) 100 x 10e6 M, HClO, 1 M Imtlal potential 07OOV (no adsorptlon), step to E, = 007OOV (I), 0 135OV (2), 025OOV (3), 04OOOV(4) at t = 0 Rmg potential Es = -025OV Electrode IV, revolution rate 1230rpm
Partud charge transferand adsorptlon
r
40 c
1061
1
1 2
20
\
-
1
~=0205Mv
50-
9=0.2104v I 0
2
I 05
0
I 10
I 15
t ‘n/P
300
3000 t/s
Fig 5 D&c (z,,) and nng current (tR)transients of potential step expenments at two revolution rates for Au/Cu(II) 2 0 x lo-’ M, H,SO, 1 M Imtial potential 0 600V (no adsorptlon), step to E, = -0 1OOOVat t = 0 Rmg potential & = -0 200 V, revolution rate 2500 (1), 1250 rpm (2)
the quotient -lo Nfix , It seems improbable that mtermedlates other than A, exist TOWmeans that ma&on (1) comusts, for the mvestigated systems, of a two-step mechamsm with one adsorbed mtermedmte and the completely dmcharged metal species R if the
Rg 7 I)lsk current I~, rmg current Qfference tx, and quotient I~ N/I, for potenti step expenments Imbally drffunon controlled case Au/Hg(I) 9 4 x 10m4M, NaClO, 08M,HCl0,02M SteptoE,=02104V Rmgpotentlal E,, = 0 000 V Electrode III, revolution rate 1230 rpm
amount of completely discharged matenal 1s small m compatrson to the partially charged intermediate, mtegratlon of the rmg current difference I~ (m the time domam where the quotient -lo N/t, IS constant and equal to I) yields the adsorption density of the mterme&ate r In the other case, assummg that only one partmlly charged m&me&ate exists, the adsorption density r and the amount of fully kharged matenal m, can be computed mth the ad of the equations (7)-(11) and the computed 1(for detads see Ref [36])
0
-16
-16
”
Rg. 6 Jhk (ID)and rmg currant (f,J tranments of potenttal step experunents at two revoltion rates for Au/II(I) 400 x lo-“M, NaClO,l M, IWO, 1mM huhal potentml &WV (no adsorptmn), step to E, = -04292V at t-o RmgpotentlalEa= -0 55OV, revolution rate 2500 (----hOrpn(--).
I t
4
lnls’n
F& 8 Dtsk current I~, rmg current ddference C, and quobent aN/i, of a potenti step expenment for Au/II(I) 40 x 10e6 M, NaClO, 1 M, HClO, 1 mM 1n1tu1Ipotenti O3ooV Step to E, = -04883V at t = 0 Rmg potenhal En = -0 55OV Revolution rate 1250rpm
G SALI~and K BARTEXS
1062
s -05
P 9
0
50
100
t ‘~I,‘~ Fig 9 Disk current I~, nng current difference I~, and quotient lDN/I, of a potential step experunent for Au/Cu(II) 2 0 x lo-’ M, H,SO, 1 M Conditions as m Fig 5, revolution rate 1250rpm
l- = Al- -
‘[~&4,IF)]
dt
s AI- =
CQ.c4-"Qd411CW - 01 mn=CQR(=JW-KI Qdm)= j?tdAddt 1
Qdm)=
5z
mbd~ADN)~
dt,
-E/v
va ssce
Fig 11 Surface densities of adsorbed species A [ = r( +)I and of completely discharged metal species R [ = m,(m)] as potentml functions for Au/n(I), concentration of Tl(1) 40 x 10-6M
(7)
(8) adsorbed species 1s m the same order of magnitude as that of completely discharged species, for Tl(1) the amount of partially charged particles IS small m (10)comparison to the amount of completely discharged ones (9)
(11)
5. DISCUSSION
where 7 IS the time where the rmg and disk currents begm to deviate from the constant mltml value, A, 1s the area of the disk electrode Figures 11 and 12 show the Isotherms for the surface density of the adsorbed intermediate I’ and of the fully discharged matenal m, (as mol/area) for the Au/Y(I) and Au/Bl(III) system Whereas for Hg(I) (see [36]), Bl(III), and Cu(I1) the amount of
1 q
5 1 Parka1 charge transfer
The measured I values are nearly potential mdependent for all Investigated systems (Fig 10) Two conclusions can be drawn from this fact First, the detected intermediate 1s umform, le it 1s a single one and not a mixture of two states with different 1, because a mixture of two different states leads to 1 being strongly dependent on the potentlal[59] Second, It seems probable that the @q/i?& part of
00
-0
1
c
I
all/v
02 -E/v
VI ssce
i
Fig 10 Partial charge transfer coetliclents I for the systems Au/Hg(I) (A), Au/n(I) ( x ), Au/Bi(III) (O), and Au/Cu(II) Fig 12 Surface dcnsitlcs of adsorbed species A [ =lJ+)] (0) as function of potentml (disk potential En) Concentra- and of completely dwharged metal spews R [ = ms(m)] as tlons Hg(I) 94 x lo-‘M, Tl(1) 8 0 x 10s6 M, B1(111) potential funtions for Au/Bi(III), concentration of Bl(II1) 100 x 10e6 M, and Cu(I1)40 x lo-’ M 20 x lO_‘M
Partial charge transfer and adsorptton
1063
crystal faces (Au(ll1) HClO, 1 M: I= 0 8 m [S]) In XAS investigations of Au(lOO)/Cu(II)[14] copper i = A- (aq/ar)e (12)spectes with an oxidation state near to + 1 have been observed All these findings support our results because it is surely potential dependent Therefore, about the existence of partrally charged Cu spectes m the main part of I seems to be 1 k 1 for the mvesugated systems. Thrs statement is equivalent to the underpotenttal deposits In contrast to the cued mvesttgauons, m the present work the amounts of C,(r) = C,(r = 0x59], where C, = (aq/a& IS the the different spectes have been estimated quantttatdouble layer capauty in the presence of the reaction ively system R, A,, 0 Thts is supported by unpedance measurements[37J (c) Au/TV) The perchlorate tons do not form strong complexes with the metal tons mvesugated here. The adsorpUp to date no hmts of parttal charge transfer in tton of ClO; tons (used m the Hg(I), Hg(II), Tl(1) the underpotenttal deposthon of Tl+ onto Au (as and Bi(II) system) and SO:- ions (used m the Cu(I1) well as Ag and Pt) electrodes have been reported m system) at Au is neghgible or low Thus it 1s very the literature. The surface density of partially improbable that a kmd of mduced adsorption causes charged Tl found m the present work IS relahvely the chennsorptron of these metal ions The reason for small in compartson to the completely discharged the chermsorpuon of Hg, Tl, Bt and Cu IS the formaone, see Fig 11 uon of a bond between the substrate Au and the correspondmg metal ton 5 2 Reaction rates Whereas only a few metal cations (Tl+, Cs+, Rb+, The breakdown of the mittally diffusion controlled K+) are known to be adsorbed at the mercury elec- current gives hmts on the rate of the correspondmg trode wtth a &rect bond between Hg and the correstep l’here step 3 at the oxtdahon srde of the reaction sponding ton, the present mvesttgauon suggests that (l)] One obtams by inspection of Figs l-5 under at gold electrodes, direct chemrsorpuon of cations 1s considerahon of the dtfferent reaction concentration more frequent that the chemtsorphon rates mcrease m the order The detectton of two different types of spectes in Cu(I1) c Bi(III) < HgO < Tl(I) Obvrously, the rates the underpotentml deposits throws new hght on the of the followmg processes formmg the completely statements of the charge of the underpotentml deposQscharged metals are lower but have a similar graits If the time resoluuon of a measurmg method is dation not high or the tmtially flowmg part of the charge is The overall reactron rates of the correspondmg not detected, one obtams an average of the charge or tons at Hg electrodes increase m the same order In even the charge of the completely discharged spectes perchlonc solutions, Bt(II1) and Hg(1) are discharged only Perhaps this explams why the partrally charged m 1 (unresolved) step, whereas the Tl(I) reacttons part of the adsorbates 1s dlffcult to detect and the m 2 steps wtth an adsorbed proceeds (mhomogeneous) deposits are constdered as being mtermedlate[29] The rate constants are completely discharged In this respect, a weak Bt(Hg)/Bt(III) HCIO, 2 M method 1s cychc voltammetry and other linear sweep methods The time behavtour is nearly inseparably k = 6 x 10-5cms-1 [62,63] mixed Hnth the effects induced by potential changes (mcludmg isotherm form), rf the sweep rate 1s not Hg/Hg(I) HClO, 1 M: k = OMcms-’ [64] low enough Tl(Hg)/Tl(I) NaClO, 1 M (a) Au/Bt(III) k,, k, > 1 ems-’ [29,65] Indications to non-integer partial charge transfer Thrs confirms that the behavtour of Bt(II1) at the has been gamed from specular reflectance spectromsoluhon side of the phase boundary causes the slow etry combined with linear sweep voltammetry[60] reactron rate, obvtously the separation of the hydraand electrochermcal quartz crystal rmcrobalance tion sphere measurements with cyclovoltammetry[61] Concernmg the potential sweep the above menuoned 5 3 Completelydrscharged specres restnctrons are valid (besides the above menuoned For all inveshgated systems it has been checked d dtfficultms of mass determmauon wtth the EQCM), however The ume and potenual dependence IS the completely dtscharged metal Qffuses mto the substrate Correspondmg current plots agamst t-‘12 nnxed leading to average values of 1 Tlus could yield no mdicahon for detectable diffusion The drfexplam why the I estimated m [60] for the system fusion coetlictent for Hg into Au has been estimated Au/Bt(III) HClO,, 1 M (I = 087) and m [61] for the from this plot to be smaller than 10-20cm2 s-‘[36]. system Au/Bt(III) HClO, 0 1 M (1= 0 9) are nearer Thts constderatton assumes that the Qffuston coeffito 1 as the value measured in the present work ctent is independent of concentratton, an assumphon (HClO, 1 M I = 0 73 f 0.03) which IS to some degree questtonable in solid metals, But without doubt, the completely discharged (b) Au/Cu(W spectes are located m the lint layers of the substrate. The Au/Cu(II) system (0 1 M HClO, I- 0.7 in Thus tt 1s possible to formally descrtbe these spectes [61]) IS not tmmedrately comparable, because m the either as R, as is done here, or as an mtermedtate [eg present work HsSO, solutions have been used A, m reaction scheme (l)] In the first case, the Further hints on I # 0 or 1 are found at smgle charge balance contams the term (aq/dc,&,[35] In the macroscoptc partial charge coetlicrent I IS small
1064
G
SAL&
and K BARTELS
the second case, the correspondmg charge appears as (%Prl)E. r, m the coetliclent I, The (8q/dc,),, ,-I term has been omitted m the evaluation of the quotient -I~N/I~ For Au/Hg it was estimated that this term IS neghgble For the other systems the same holds true A devlatlon of c I, from 1 would indicate a detectable contrlbutlon of this term In all investigated cases, completely discharged metal species have been detected This 1s m agreement with thermodynamic conslderatlons which reqmre the formatlon of an alloy with the substrate, under conslderatlon of the rmsclblhty and the mlsc1blhty gaps[35] In the time scale of experiments, this begmmng of alloy formatlon ~111only concern the top layer(s) of the substrate lattice m general The main difference to isolated adsorbed metal ions 1s the partial charge of the latter which exerts strong repulsive forces and prevents co-operative phenomena The more the ions are discharged the stronger their mcorporatlon mto the substrate ~11 be Our expenmental findings are m general accordance or at least not in conflict with the known facts on the structures of the underpotential deposits, m particular with stronger mcorporatlon mto the substrateC38, 391 also indicated by roughemng of the surface after strlppmg[41], with alloy formatlon[40, 42, 43, 441, and with formatlon of different deposit structures or layers[ 16,23,20] The measurements show that the adsorbed species coexist with the completely discharged ones, because there 1s no charge l-low compensatmg the partial charges z(l - r) It 1s assumed that the formation of the completely discharged metal occurs via the intermediate(s) A, of mechamsm (1) 6. CONCLUSIONS The analysis of the current transients at rotating rmg-disk electrodes for the mvestlgated systems Au(poly)/Hg(I), Tl(I), Bl(III), Cu(I1) has shown that m equlhbrmm two different forms of metal species m the underpotenttal deposits exist partially charged metal ions (adsorbed species) and completely dlscharged metal species (alloy deposit) The partial charge transfer (f) and the adsorption isotherm for the two species as functions of potential and concentration of bulk metal ions have been determmed The partial charge transfer 1s nearly potential mdependent The uncharged alloy deposit IS located in the top layer(s) of the substrate lattice, because substantial diffusion mto the lattice could not be detected The form of the transients mdlcates large expulslve forces between the partially charged species The reaction rates for the steps of the electrochemical reaction m the underpotentlal deposltlon increases m the order Cu(I1) c Bl(III) < Hg(I) < Tl(1) It seems that the adsorption of partially charged metal ions m the underpotentlal range 1s a rather genera1 phenomenon REFERENCES 1 D M Kolb, m Aduances m Electrochemrstry and Electrochem Engmeermg (Edited by H Genscher and C W Tobias), Vol 11, p 125 Wdey, New York (1978)
2 R Ad&, m Advances m Electrochenustry and Electrothem Engmeermg (Edited by H Genscher and C W Toblas), Vol 13, p 159 Wiley, New York (1984) 3 K Juttner and W J Lorenz, Z physlk Chem N F 122, 163 (1980) 4 D M Kolb, K Al Jaaf-Golze and M S Zel, Dechema Monoar. Vol 102. p 53 VCH, Wanhelm (1986) 5 J W” Schultze anh D Dlckertmann, Faraday Symp Chem Sot 12.36 (1977). Surf SCI 54.489 (1976) 6 A BewIck and B yho&s, j electroonal bhem’ 65,911 (1975), 70,239 (1976) 7 H 0 Beckmann, H Genscher, D M Kolb and G Lehmpfuhl, Faraday Symp Chem Sot 1552 (1977) 8 A T Hubbard, Chem Reo 88,633 (1988) 9 R H Muller and J C Farmer, Surf Set 135, 521 (1983) 10 W Visscher and A P Cox, Electrochlm Acta 37, 2245 (1992) 11 L Blum, H D Abruiia, J White, J G Gordon, G L Borges, M G Samant and 0 R Melroy, J them Phys 85,6732 (1986) 12 J H White, M J Albarelh, H D Abruiia, L Blum, 0 R Melroy, M G Samant, G L Borges and J G Gordon II, J phys Chem 92.4432 (1988) H S Yee and H D Abruiia, J phys Chem 97, 6278 (1993) G Tounllon, D Guay and A TadJeddme, J electroanal Chem 289,263 (1990) J McBreen, W E O’Grady, G Tounllon, E Dartyge and A Fontaine, J electroanal Chem 307,29 (1991) R Durand, R Faure, D Aberdam, C Salem, G Tourdlon, D Guay and M Ladouceur, Electrochlm Acta 37, 1977 (1992) 17 M G Samant, G L Borges, J G Gordon II, 0 R Melroy and L Blum, .! Am them Sot 109, 5970 (1987) 18 R Michaelis, M S Zq R S Zhai and D M Kolb, J electroanal Chem 339,299 (1992) 19 R G Barradas, I G Hettgen and M Gqps, J electroanal Chem 243,435 (1988) 20 M Flaschmann and B W Mao, J electroanal Chem 247,297 (1988) 21 G Materhk, M Schmah, J Zegenhagen and W Uelhoff, Ber Bunsenges 91, 292 (1987) 22 M S Zq G Qao, G Lehmpfuhl and D M Kolb, Ber Bunsenges 91,349 (1987) 23 J F Rodnguez, D L Taylor and H D Abruiia, Electrochlm Acta 38,235 (199j) 24 R J Nichols. D M Kolb and R J Behm. J electroanal Chem 3i, 109 (1991) 25 T Hacluya, H Honbo and K Itaya, J electroanal Chem 315,275 (1991) 26 W J Lorenz, L M Gassa, U Schmidt, W Obretenov, G Stiukov, V Bostanov and E Budevski, Electrochmm Acta 37,2173 (1992) 27 H J Pauhng and K Juttner, Electrochlm Acta 37, 2237 (1992) 28 W Obretenov, U Schmidt, W J Lorenz, G Staikov, E Budevski, D Carnol, U Muller, H Slegenthaler and E Schmidt, J electrochem Sot 140,692 (1993) 29 W Lorenz and G Saht, J electroanal Chem 80, 1 (1977) 30 W Lorenz, J phys Chem 95, 10566(1991) 31 J W Schultze and F Koppltz, Electrochlm Acta 21, 327 (1976) 32 J W Schultze and F Kopltz, Electrochlm Acta 21, 337 (1976) 33 F Koppitz, J W Schultze and E Rolle, J electroanal Chem 170,5 (1984) 34 G Sahe and K Bartels, Z Chem 27,451 (1987) 35 G Sahe, J electroanal Chem 245, 1 (1988) 36 G Saht and K Bartels, J electroanal Chem 245, 21 (1988)
Partud charge transfer and adsorption 37 G Salle, J electroanal Chem 259, 315 (1989) 38 A Popov, N Dmutrov, 0 Velev, T Vttanov, E Budevskl, E Schmidt and H Slegenthaler, Electrochlm Acto 34,265 (1989) 39 A Popov, N Dtmltrov, D Kashchev, T Vltanov and E Budevskl, EIectrochtm Acta 34,265 (1989) 40 T Solomun, J electroanal Chem Ml, 229 (1989) 41 M P Green and K J Hanson, Su$ SCI 259, 1743 (1991) 42 R C Salvarezza and A J Arvia, Electroclum Acta 33, 1031(1988) 43 M A Qmroz, L Salgado, I Gonzalez, Y Meas, A Raps-Hemindez and A Vela, J electroanal Chem 261,409 (1989) 44 H-J Schmidt, U Plttermann, H Schnader and K G Wed, Anal Chum Acta 273,561 (1993) 45 J W Schultze, Bm Bunsenges Phys Chem 74, 705 (1970) 46 R AdZIC, E Yeager and B D Cahan, J electrochem sot 121,474 (1974) 47 J Harkans, B D Cahan and E Yeager, J electrochem Sot 122,1585 (1975) 48 T M bedhammer, L S Melmckl and S Bruckenstem, Z phys~k Chem NF 111,177(1978) 49 B E Conway and S Marshall, Electrochtm Acta 28, 1003 (1983)
1065
50 S A S Machado, A A Tanaka and E R Gotiez, Electrochtm Acta 37,2559 (1992) 51 M R Deakm and D A Buttry, Anal Chem 61, 1147A
(1989) 52 R Schumacher, Angew Chem 102,347 (1990) 53 S Bruckenstem and S SwathlraJan, Electrochun Ada 30,851(1985)
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62 G Salrb, Z physzk Chem (Lelpzlg) 256, 545 (1975) 63 G Salk, J electroad Chem 273, 1,31 (1989) 64 H Gerlscher and M Krause, Z physdc Gem. N F 14, 184 (1958) 65 G Said, J electroad Chem 262, 1 (1989)
) Pergamon
Acta, Vol 39.No S/9.pi 1037-10611994 cqyn@ttQ 1994ebmasaallxLtd Pmted m Great Bntun. All n&s mmmd Wl3-4696/949700+0.09
PARTIAL CHARGE TRANSFER AND ADSORPTION AT METAL ELECTRODES. THE UNDERPOTENTIAL DEPOSITION OF Hg(I), Tl(I), Bi(II1) AND Cu(I1) ON POLYCRYSTALLINE GOLD ELECTRODES G SAL&* and K BARTELS~ + Fachhochschule Hamburg, Fb BPV. Labor fir Phyakahsche Chenue, D 21033 Hamburg, Germany t BIOTRONIK GmbH & Co, Ingemeurburo, Woermannkehre 1, D 12359 Berhn. Germany (Recnued 8 September 1993, m
remsed
fbrm
10 December
1993)
Abet-The absorption of H&, Tl+, B13+ and Cuz+ at polycrystalbne gold electrodes from aqueous solutions IS mvestigated by potentrostatic step expenme-nts at rotatmg rmgdBk electrodes The macroscoptc partml charge transfer coefiiuent 1u determmed from the uutud part of the current transients at rmg and tik which 1s diffuszon controlled for low reactant concentrations The measured I are 047 (Hg$+), 061 (Tl’), 073 (B?) and 092 (Cuz+) f 003 The potential dependence. of the measured I IS small and could m&a& that the mam part of 1IS causedby microscopic charge transfer A DdTusron controlled formation of the mam part of the partmlly charged adsorbate pomts to strong repulsive forces between the adsorbed particles With all of the mvesfigated systems, a second type of deposit has been detected which conasts of completely &charged metal
Key words partial charge transfer, underpotential deposttton, adsorption, rrde, gold
1. INTRODUCl’ION Underpotential deposition of metals contmues to be studted mtensrvely (for revrews see [l-4]) Especially, an abundant literature exists about the mvesttgatton of the structure and structural transformattons of the deposits wtth pure electrochemtcal[5, 61 and wtth in situ surface. techmques exC7, 83 and (elhpsometry[9, lo], X-ray spectroscoprc[l l-181 and drffractton techmques[19-211, LEED/RHEED and AES[22, 231, STM and atonuc force[24-281, see also the hterature cued there) Another interesting question m underpotentud deposmon is the charge of the deposns In general, the charge of electrochemtcal spectes chemtsorbed at an interface IS charactermed by the coefScrent of partml charge transfer (for revtews see[29, 303, collection of data also m[31-331). Recently, it has been shown that adsorptron of Hg(I) tons m the underpotenttal range is connected wtth non-integer pa&al charge transfer[34-37l Two dtfferent processes have been detected durmg the depositton of HgO Ions at polycrystalhne Au electrodes m the underpotentml range by rmg-disk measurements[34, 361 and impedance spectrometry[37j. The first process is the adsorptton of metal ions onto the substrate wrth part& charge transfer The substantmlly slower second process IS the formatton of completely discharged metal spectes. In the former process, single ions are adsorbed and chemical bonds are formed between the ton, the surroundmg solution components and the metal lattrce of the substrate The charge distrrbutron of thts chermcal bond is responsible for the
macroscopic pa&al charge on the ion The second process may be the begmmng or a prehmmary stage of the formatton of an alloy[35, 361 whose rate IS perhaps governed by the generally slow sohd state reacttons Thts part of the deposited metal spectes ts therefore termed alloy depostt[35) The whole underpotentud deposmon IS modelled by a multistep mechanism wtth chemlsorbed m&mediates at which m the u last Wstep completely dtscharged metal 1s formed At a gtven underpotenttal m equihbnum, both forms (parttally charged specres and alloy deposit) are present simultaneously These findmgs (re the exrstence of two forms of the deposited metal) are to be seen m connectron with the noneqmhbnum structural transformatrons investigated m[38, 393, where also adsorbed and stronger Incorporated specres are detected at the Ag(l1 l)/Pb*+ underpotenttal deposmon Formatton of alloy was also descrrbed at Pd(lOO)/Cu*+ (LEED and XPS)[40], at Au(poly)/Pb’+ (roughemng of the substrate after strrpping of the deposit has been observed wtth STM[41]), at Pt(poly)/Hg[42], at Ru(poly)/Ag+[43] and at Au(poly)/Pb*+[44] (see also Refs [40,42,72,95] m Ref [35]) Two &fferent deposrt layers have been found at Pt(lOO)/Cu*+ (4th EXAFS)[l6], at Pt(lll)/Ag+[23], &d at Ag(poly)/T1+ CW. Although there were some hmts on macroscopic partial charges of adsorbates m the past, like for Pt/ Cu* + [45], Au/Pb* + [46,47] and AufAg + [48], rt was wrdely accepted to assume the metal tons are completely discharged m the underpotentml depostts (see eg [l-3, 26, 28, 493) Recently, tt has been shown wtth XANESIEXAFS, that the Cu underpotentud
1057
1058
G
and K BARTELS
SAL&
deposits possess a charge smaller or near by + 1 at Au(100)/Cu2+[14], at Pt/W+[lS], at Pt[lll]/ Cuz+[13] and at Pt(lOO)/Cu’+[16] Ring-disk measurements at the system Pt(poly)/Cd2+ have yielded a partial charge transfer of I= 0 77[50] The quartz crystal microbalance (for reviews see [St, 521) has been used to determine the mass and the partial charge of underpotential depomts[51-561 However, an exact mass determination depends on roughness[57] and some of the results are hard to explam[56] The aim of this paper 1s to give further expenmental examples for partially charged species 1n the underpotential deposits and to strengthen the ev1dence for the above mentioned mechanism Therefore, 1n the followmg, attention should be focussed on the adsorption of ions and the determination of their partial charge transfer described by the macroscop1c Lcoeff1aent The Lcoeff1iaents and surface densities of deposited species have been determrned by ring-disk measurements as 1n [34, 36, 581 These measurements allow a direct and accurate deterrmnation of I-coeffiiaents 2. EXPERIMENTAL
SECTION
The ring-disk equipment has been descIlbed [36, 581 In general, the rotatron speed of the electrode has been varied between 100 and 25OOrpm (max1mal deviation 0 15%) The radii (m cm) of the electrode are 0 250 (disk), 0 275 (ring mt ), and 0 350 (ring ext ) The polycrystalllne gold electrodes have been prepared 1n the same way as m[36] Before measurements, the Au electrodes were polished with Al,O, , rmsed with d1st1lled water and cycled 1n the corresponding supporting electrolyte (sweep rate 1 V m1n - ‘) until reproducible current-potential curves were obtained The following systems have been investigated Au/Hgz+ 0 02-94 mM, NaClO, 0 8 M, HClO, 0 2 M Au/Hg’+ 0 02-80mM,
Au/Y+ 0 004-10 mM, NaClO,
“L -A1-Az-
substrate met., S
“I
phase boundary
as
“3 phase boundary
0
soIYllon
(1)
The adsorbed intermediates A, may be thought as changmg from adsorbed single species with relatively high partial charge at the O-side (eg here A2) to more or less structurrzed adsorbates at the R-side (eg here A,) The u, values are the corresponding react1on rates The transfer of the charge z e- 1n the overall reaction RcrO+z
e-
(2)
1s divided into the portion 1, for the corresponding steps 1 However, the measurable quantities are the macroscopic partial charge transfer coelliclents I,[29], Including the double layer terms @q/X,), The macroscopic partial charge 1 of the adsorbed metal species 1s determined from potentlostatlc relaxation expenments at ring-disk electrodes The procedure (for details see [35, 361) 1s described shortly 1n the following Imtmlly, the disk potentral E, 1s held 1n the far positive region, where adsorption (1e underpotential deposltlon) 1s practically absent At the time t = 0, E, 1s stepped to the desired value 1n the underpotential regon and the disk current I~ 1s reglstrated as a time function During the whole time, the ring potential E, IS held at a potential 1n the region of the cathodic 11m1t1ngcurrent of reaction (2) Thus, the charge of the deposited 0 1s measured at the disk, whereas the mass of the deposited 0 1s measured at the ring as difference 1Rbetween the momentary ring current I,,“* and the 11m1t1ngring current I,,.~ ,,,,, flowing at the end of (and before) the experiment IR = G,“g- Lp, Irm In the initial part of the current transients, only step 3 (the step with which the reduction of 0 starts) of reaction (1) proceeds and 1t 1s[35] I&) = -zF
&,I
dr,/dt
(3)
and (II)
1 M, HClO, 1 mM (III)
Au/B13+ 0 001-l mM, HClO, 1 M
(IV)
Au/Cu2+ 1O-7-1O-4 M, H2S04 1 M
(V)
The solutions were prepared from p a chemicals and b1dnt1lled water (once over KMnO,) The measurements have been performed 1n deaerated solutions (argon) at 25 0 f 0 1°C All potentials are referred to the ssce (NaCl saturated calomel electrode) The concentrations of the Hg(1) solution are expressed 1n mol 1-l referred to the entity Hg:+ 3. THEORETICAL
Rt
(I)
NaClO, 0 8 M, HClO, 0 2 M
the general theory of chemlsorptlon reactlons[29] a multi-step process (here three steps)[35]
SECl-ION
The react1on of the metal 0 at the substrate metal S 1n the underpotential range 1s wrttten according to
1~0) = l,inp - l,inp urn = N
zF
A,
dr,/dt
(4)
with the area A, of the disk electrode and the collection elliclency N Here, the rndex “3” of the coelliclent I 1s omitted for convenience, because only the first chemlsorbed state accessible from the O-side 1s consIdered explicitly The terms containing v1 (reaction of A, to R), dr,/dt (change of the amount of A,) and (8q/8)cR(x = O)/dt (change of double layer charge q by appearance of R with concentration cR(x = 0) at the surface) are neglected 1n equations (3) and (4), because the adsorption density of A, and the mass of completely dscharged metal R IS zero or small at the begmnmg (see Ref [35] Chaps IV and V) Thus, the quotient -to(t)
N/I&) = I
for t-*0
(5)
1s constant, as long as no remarkable amounts of other chemlsorptlon states (A,) or completely dls-
Partml charge transfer and adsorption
charged metal R are formed Because the potential IS constant durmg the adsorption process, 1 IS obtamed at a defined potential, and it IS possible to measure the potential dependence of I If one of the steps of reaction (1) IS rate determmmg m the course of the time, the quotient -lo N/i, IS constant and equal to the corresponding 1, durmg that tlmq35] The determination of 1 wth equation (5) does not depend on the true area of the disk electrode For sufficient large times, the adsorption densities of the mtermedlates successively attam eqmhbrmm and the whole process consists m the formation of R according to equation (2) Thus it IS -I&)
N/z&) = 1
for t + 00
1059
Hgp9.4x
W5M
1 ~=+O.lm%v 2.~=+02731
4
V
3 q)=+O473lV 4 en=+0673l V
:3
2
1
(6)
0
if the exchange rates tl,,, of the steps of reaction (1) are nearly equal or increase m the order ulA < vl,, < uJA what seems plausible because stronger incorporation into the ordered solid phase leads to lower reaction rates as uncorrelated single particle chenusorptlon has The expenments confirm the slower formation of R, see below (-on 5 2)
3 ? 0
K I*
5
0
A RESULTS 4 1 Disk and ring current transients The Qsk and rmg current transients of potential step expetrments at polycrystallme Au electrodes m Hgz+, Hg2+, Tl+, B13+ and Cu2+ containing solutions are exhlblted m Figs l-5 The shown transients are charactenstrc for the range of low reactant concentrations, they are dlffuslon controlled at the begmmng The diffusion control 1s venfied by the dependence of the current upon the rotation rate, see [34,36] for the Hg(I) and Hg(Ik) system and [58] for Tl(I), Bi(III), and Cu(I1) The MWon plateau IS well marked m the Hg(I) and B@II) systems, whereas m the case of very short [Fig 3, Tl(I)] and very long [Fig 5, Cu(II)] measurmg times the plateau IS a little dlstorted In some cases [Hg systems, Bi(III)], the most part of the adsorbate IS obviously formed under dlffuslon control It has been shown that this behavlour reqwres high rates of the reamon steps and adsorption isotherms wth strong repulsive forces between the particles[35] Model calculations for the Hg(I) system wth Frumkm type isotherms lead to very high repulsion constants (>20)[35, 361 This 1s m agreement with adsorbed species bemg not completely discharged but carrymg a partml charge For higher reactant concentrations, the current transients exhibit a maximum and after that they decrease monotonously, Hrlthout reachmg dlffuslon control (curve 3 m Fig 3, curve 4 m Fig 4 and Fig 6) 4 2 Determmatzon of the 1 coejicrents The quotient - I~ N/I, IS plotted as tune function for the five systems (I)-(V) Typical curves are pra sented m Figs 7-9 In all cases, the quotient M constant at the begmnmg. Hence, according to equation
I
‘:
15
t/s Fig 1 Jhk (IJ
and nng current (ra) transients of potential steu exuerunents for Au/HRU) 94 x 10-‘M. NaCIO. 08-M, HClO, 0 2M Im&’ potential 07hOV (no’ adsorption), step to E, = 0 1806V (l), 02731V (2),
04731 V (3), 06731 V (4) at t = 0 Rmg potenbal E, = 0 OOOV Electrode III, revolution rate 1230 rpm
(5), m this regon the values of the coeficlents 1 are obtamed All systems exhibit 1 more or less dlffermg from 1 In solutions contammg higher reactant concentrations where the current transients do not offer &&ion controlled behavlour, -tu N/I,, IS constant at the begmmng, too, and hence 1 IS measurable In this manner, 1 IS determmed for the systems (I)-(V) at vmous potentials and concentrations Figure 10 shows the patial charge coefficent 1 of all mvestlgated systems as function of the potential The potential dependence of 1 IS small, It IS < 0 01 V- ’ for Hg(I), +OOl V-’ for Cu(II), +0 06 V-’ for Tl(I), and -0 07V-’ for Bi(II1) The data selected 111 Fig 10 belong to a single reactant concentration for each system, 1 does not depend upon concentration m the error hmlts (about +O 02 0 04) The Hg(II) ion is reduced to Hg(1) at the mterestmg potentials m the underpotential range Thus the interpretation of the expenments anth HgO IS not as strrughtforward as m the case of Hg(I), for &tads see [36] There are two possible won paths for underpotentml deposition from Hg(I1) solutions. Hg(II) reacts unmdately to an adsorbed parWle or it IS reduced first to the Hg(IO_state and then adsorbed The evaluation of the l-type charge of the adsorbed species from the i& data wth the ad of equations (23) and (24) in Ref [35] does not depend on the rea&on mechamsm d the rea&ons are fast as it 1s the case for the dfluslon controlled Hg(I) and Hg(I1) underpotential deposition In [36] it was shown that the l-type charge of adsorbed
G SAL&and K BARTELS
1060
1-
.
\
en=-04893V
I
s
c
:
;
4
2
3
0
I
*,O P
c( CI ‘:
Tl+4xlo~w
r-z I
2
3
1 tD=-04883V 2 cD=-O4OOV 3 rD=+OlOOV
I
\
\
c
&*+2x
10”M
q=+01591v r
f
21 cD=+t,b%,t v cD=+02026V 3 b=+O4526V 4q,=+O6526V f
f
3 ?0 * LL
4
2
3
1
-
0
16
7
”
4
tls
Fig 2 Disk (tu) and rmg current (1s) transients of potential step expenments for Au/Hg(II) 200 x lOA M, NaCIO, OSM, HCIO, 02M Imtial potential 1 OOOV (no adsorption), step to E, = 0 1606V (l), 0 2026 V (2), 04526V (3), 0 6526V (4) at t = 0 Rmg potential E, = -0 1OOV Electrode III, revolution rate 1230rpm The current of the discharge Hg(I1) + Hg(1) has been subtracted from to, 1s IS corrected correspondmgly for t < 0
mercury species IS 1 - I = 0 53 f 0 02 for the Hg(1) system and 1 - I= 0 61 + 0 05 for the Hg(I1) system The correspondmg I-coefficients are 047 and 0 39, respectively, rf one uses the reaction scheme I l-1 Hg(1) + eHg Hg,,, + (1 - W- 0 Hg(I1) + 2 em equation (3) of Ref [36] I, must be replaced by 1 - A) The mterpretatlon of the results of the Hg(I1) system has to be reserved for further mvestlgatlons 4 3 Adsorption densrtles ofthe mtermedlate r and of the completely discharged metal ions m, Figures 7-9 show the the quotient -zD N/z, 1s uutlally constant and rases near or some time after the end of the dlffuslon hmlted part of the current trannents In this time regon, step 3 of reaction (1) 1s no longer the prevadrng reaction m the overall process For all mvestlgated systems, -lD N/I~ reaches a further plateau m the final range of the experiment at longer times at 100 f 004 Thus, completely dacharged material 1s formed here m accordance with equation (6) Because there 1s no mdlcatlon of further plateaux or other stnkmg lrregulmtles m the time behavior of
4
IL
”
tlms Rg 3 Disk (I~) and rmg current (l,J transients of potential step expenments for Au/Tl(I) 4 00 x 10e6 M, NaCIO, 1 M, HCIO, 1 mM Imtlal potential 0 3000 V (no adsorptlon), step to E, = -04883V (l), -04OOOV (2), -0lOOOV (3), at t = 0 Rmg potential E, = -0 55OV Electrode IV, revoiutlon rate 1250rpm
313+1 X 10dM :u=00671V
lD=omV
1
2 rD=0135V 3eD=O250V 4cD=O400V
l&a 4
3
4
3
2
I
100
2
3 t0 *
lzU
I 0
1
15
$
I
50
1
tls
Ftg 4 Disk (I~) and rmg current (1s) transients of potential step experiments for Au/Bl(III) 100 x 10e6 M, HClO, 1 M Imtlal potential 07OOV (no adsorptlon), step to E, = 007OOV (I), 0 135OV (2), 025OOV (3), 04OOOV(4) at t = 0 Rmg potential Es = -025OV Electrode IV, revolution rate 1230rpm
Partud charge transferand adsorptlon
r
40 c
1061
1
1 2
20
\
-
1
~=0205Mv
50-
9=0.2104v I 0
2
I 05
0
I 10
I 15
t ‘n/P
300
3000 t/s
Fig 5 D&c (z,,) and nng current (tR)transients of potential step expenments at two revolution rates for Au/Cu(II) 2 0 x lo-’ M, H,SO, 1 M Imtial potential 0 600V (no adsorptlon), step to E, = -0 1OOOVat t = 0 Rmg potential & = -0 200 V, revolution rate 2500 (1), 1250 rpm (2)
the quotient -lo Nfix , It seems improbable that mtermedlates other than A, exist TOWmeans that ma&on (1) comusts, for the mvestigated systems, of a two-step mechamsm with one adsorbed mtermedmte and the completely dmcharged metal species R if the
Rg 7 I)lsk current I~, rmg current Qfference tx, and quotient I~ N/I, for potenti step expenments Imbally drffunon controlled case Au/Hg(I) 9 4 x 10m4M, NaClO, 08M,HCl0,02M SteptoE,=02104V Rmgpotentlal E,, = 0 000 V Electrode III, revolution rate 1230 rpm
amount of completely discharged matenal 1s small m compatrson to the partially charged intermediate, mtegratlon of the rmg current difference I~ (m the time domam where the quotient -lo N/t, IS constant and equal to I) yields the adsorption density of the mterme&ate r In the other case, assummg that only one partmlly charged m&me&ate exists, the adsorption density r and the amount of fully kharged matenal m, can be computed mth the ad of the equations (7)-(11) and the computed 1(for detads see Ref [36])
0
-16
-16
”
Rg. 6 Jhk (ID)and rmg currant (f,J tranments of potenttal step experunents at two revoltion rates for Au/II(I) 400 x lo-“M, NaClO,l M, IWO, 1mM huhal potentml &WV (no adsorptmn), step to E, = -04292V at t-o RmgpotentlalEa= -0 55OV, revolution rate 2500 (----hOrpn(--).
I t
4
lnls’n
F& 8 Dtsk current I~, rmg current ddference C, and quobent aN/i, of a potenti step expenment for Au/II(I) 40 x 10e6 M, NaClO, 1 M, HClO, 1 mM 1n1tu1Ipotenti O3ooV Step to E, = -04883V at t = 0 Rmg potenhal En = -0 55OV Revolution rate 1250rpm
G SALI~and K BARTEXS
1062
s -05
P 9
0
50
100
t ‘~I,‘~ Fig 9 Disk current I~, nng current difference I~, and quotient lDN/I, of a potential step experunent for Au/Cu(II) 2 0 x lo-’ M, H,SO, 1 M Conditions as m Fig 5, revolution rate 1250rpm
l- = Al- -
‘[~&4,IF)]
dt
s AI- =
CQ.c4-"Qd411CW - 01 mn=CQR(=JW-KI Qdm)= j?tdAddt 1
Qdm)=
5z
mbd~ADN)~
dt,
-E/v
va ssce
Fig 11 Surface densities of adsorbed species A [ = r( +)I and of completely discharged metal species R [ = m,(m)] as potentml functions for Au/n(I), concentration of Tl(1) 40 x 10-6M
(7)
(8) adsorbed species 1s m the same order of magnitude as that of completely discharged species, for Tl(1) the amount of partially charged particles IS small m (10)comparison to the amount of completely discharged ones (9)
(11)
5. DISCUSSION
where 7 IS the time where the rmg and disk currents begm to deviate from the constant mltml value, A, 1s the area of the disk electrode Figures 11 and 12 show the Isotherms for the surface density of the adsorbed intermediate I’ and of the fully discharged matenal m, (as mol/area) for the Au/Y(I) and Au/Bl(III) system Whereas for Hg(I) (see [36]), Bl(III), and Cu(I1) the amount of
1 q
5 1 Parka1 charge transfer
The measured I values are nearly potential mdependent for all Investigated systems (Fig 10) Two conclusions can be drawn from this fact First, the detected intermediate 1s umform, le it 1s a single one and not a mixture of two states with different 1, because a mixture of two different states leads to 1 being strongly dependent on the potentlal[59] Second, It seems probable that the @q/i?& part of
00
-0
1
c
I
all/v
02 -E/v
VI ssce
i
Fig 10 Partial charge transfer coetliclents I for the systems Au/Hg(I) (A), Au/n(I) ( x ), Au/Bi(III) (O), and Au/Cu(II) Fig 12 Surface dcnsitlcs of adsorbed species A [ =lJ+)] (0) as function of potentml (disk potential En) Concentra- and of completely dwharged metal spews R [ = ms(m)] as tlons Hg(I) 94 x lo-‘M, Tl(1) 8 0 x 10s6 M, B1(111) potential funtions for Au/Bi(III), concentration of Bl(II1) 100 x 10e6 M, and Cu(I1)40 x lo-’ M 20 x lO_‘M
Partial charge transfer and adsorptton
1063
crystal faces (Au(ll1) HClO, 1 M: I= 0 8 m [S]) In XAS investigations of Au(lOO)/Cu(II)[14] copper i = A- (aq/ar)e (12)spectes with an oxidation state near to + 1 have been observed All these findings support our results because it is surely potential dependent Therefore, about the existence of partrally charged Cu spectes m the main part of I seems to be 1 k 1 for the mvesugated systems. Thrs statement is equivalent to the underpotenttal deposits In contrast to the cued mvesttgauons, m the present work the amounts of C,(r) = C,(r = 0x59], where C, = (aq/a& IS the the different spectes have been estimated quantttatdouble layer capauty in the presence of the reaction ively system R, A,, 0 Thts is supported by unpedance measurements[37J (c) Au/TV) The perchlorate tons do not form strong complexes with the metal tons mvesugated here. The adsorpUp to date no hmts of parttal charge transfer in tton of ClO; tons (used m the Hg(I), Hg(II), Tl(1) the underpotenttal deposthon of Tl+ onto Au (as and Bi(II) system) and SO:- ions (used m the Cu(I1) well as Ag and Pt) electrodes have been reported m system) at Au is neghgible or low Thus it 1s very the literature. The surface density of partially improbable that a kmd of mduced adsorption causes charged Tl found m the present work IS relahvely the chennsorptron of these metal ions The reason for small in compartson to the completely discharged the chermsorpuon of Hg, Tl, Bt and Cu IS the formaone, see Fig 11 uon of a bond between the substrate Au and the correspondmg metal ton 5 2 Reaction rates Whereas only a few metal cations (Tl+, Cs+, Rb+, The breakdown of the mittally diffusion controlled K+) are known to be adsorbed at the mercury elec- current gives hmts on the rate of the correspondmg trode wtth a &rect bond between Hg and the correstep l’here step 3 at the oxtdahon srde of the reaction sponding ton, the present mvesttgauon suggests that (l)] One obtams by inspection of Figs l-5 under at gold electrodes, direct chemrsorpuon of cations 1s considerahon of the dtfferent reaction concentration more frequent that the chemtsorphon rates mcrease m the order The detectton of two different types of spectes in Cu(I1) c Bi(III) < HgO < Tl(I) Obvrously, the rates the underpotentml deposits throws new hght on the of the followmg processes formmg the completely statements of the charge of the underpotentml deposQscharged metals are lower but have a similar graits If the time resoluuon of a measurmg method is dation not high or the tmtially flowmg part of the charge is The overall reactron rates of the correspondmg not detected, one obtams an average of the charge or tons at Hg electrodes increase m the same order In even the charge of the completely discharged spectes perchlonc solutions, Bt(II1) and Hg(1) are discharged only Perhaps this explams why the partrally charged m 1 (unresolved) step, whereas the Tl(I) reacttons part of the adsorbates 1s dlffcult to detect and the m 2 steps wtth an adsorbed proceeds (mhomogeneous) deposits are constdered as being mtermedlate[29] The rate constants are completely discharged In this respect, a weak Bt(Hg)/Bt(III) HCIO, 2 M method 1s cychc voltammetry and other linear sweep methods The time behavtour is nearly inseparably k = 6 x 10-5cms-1 [62,63] mixed Hnth the effects induced by potential changes (mcludmg isotherm form), rf the sweep rate 1s not Hg/Hg(I) HClO, 1 M: k = OMcms-’ [64] low enough Tl(Hg)/Tl(I) NaClO, 1 M (a) Au/Bt(III) k,, k, > 1 ems-’ [29,65] Indications to non-integer partial charge transfer Thrs confirms that the behavtour of Bt(II1) at the has been gamed from specular reflectance spectromsoluhon side of the phase boundary causes the slow etry combined with linear sweep voltammetry[60] reactron rate, obvtously the separation of the hydraand electrochermcal quartz crystal rmcrobalance tion sphere measurements with cyclovoltammetry[61] Concernmg the potential sweep the above menuoned 5 3 Completelydrscharged specres restnctrons are valid (besides the above menuoned For all inveshgated systems it has been checked d dtfficultms of mass determmauon wtth the EQCM), however The ume and potenual dependence IS the completely dtscharged metal Qffuses mto the substrate Correspondmg current plots agamst t-‘12 nnxed leading to average values of 1 Tlus could yield no mdicahon for detectable diffusion The drfexplam why the I estimated m [60] for the system fusion coetlictent for Hg into Au has been estimated Au/Bt(III) HClO,, 1 M (I = 087) and m [61] for the from this plot to be smaller than 10-20cm2 s-‘[36]. system Au/Bt(III) HClO, 0 1 M (1= 0 9) are nearer Thts constderatton assumes that the Qffuston coeffito 1 as the value measured in the present work ctent is independent of concentratton, an assumphon (HClO, 1 M I = 0 73 f 0.03) which IS to some degree questtonable in solid metals, But without doubt, the completely discharged (b) Au/Cu(W spectes are located m the lint layers of the substrate. The Au/Cu(II) system (0 1 M HClO, I- 0.7 in Thus tt 1s possible to formally descrtbe these spectes [61]) IS not tmmedrately comparable, because m the either as R, as is done here, or as an mtermedtate [eg present work HsSO, solutions have been used A, m reaction scheme (l)] In the first case, the Further hints on I # 0 or 1 are found at smgle charge balance contams the term (aq/dc,&,[35] In the macroscoptc partial charge coetlicrent I IS small
1064
G
SAL&
and K BARTELS
the second case, the correspondmg charge appears as (%Prl)E. r, m the coetliclent I, The (8q/dc,),, ,-I term has been omitted m the evaluation of the quotient -I~N/I~ For Au/Hg it was estimated that this term IS neghgble For the other systems the same holds true A devlatlon of c I, from 1 would indicate a detectable contrlbutlon of this term In all investigated cases, completely discharged metal species have been detected This 1s m agreement with thermodynamic conslderatlons which reqmre the formatlon of an alloy with the substrate, under conslderatlon of the rmsclblhty and the mlsc1blhty gaps[35] In the time scale of experiments, this begmmng of alloy formatlon ~111only concern the top layer(s) of the substrate lattice m general The main difference to isolated adsorbed metal ions 1s the partial charge of the latter which exerts strong repulsive forces and prevents co-operative phenomena The more the ions are discharged the stronger their mcorporatlon mto the substrate ~11 be Our expenmental findings are m general accordance or at least not in conflict with the known facts on the structures of the underpotential deposits, m particular with stronger mcorporatlon mto the substrateC38, 391 also indicated by roughemng of the surface after strlppmg[41], with alloy formatlon[40, 42, 43, 441, and with formatlon of different deposit structures or layers[ 16,23,20] The measurements show that the adsorbed species coexist with the completely discharged ones, because there 1s no charge l-low compensatmg the partial charges z(l - r) It 1s assumed that the formation of the completely discharged metal occurs via the intermediate(s) A, of mechamsm (1) 6. CONCLUSIONS The analysis of the current transients at rotating rmg-disk electrodes for the mvestlgated systems Au(poly)/Hg(I), Tl(I), Bl(III), Cu(I1) has shown that m equlhbrmm two different forms of metal species m the underpotenttal deposits exist partially charged metal ions (adsorbed species) and completely dlscharged metal species (alloy deposit) The partial charge transfer (f) and the adsorption isotherm for the two species as functions of potential and concentration of bulk metal ions have been determmed The partial charge transfer 1s nearly potential mdependent The uncharged alloy deposit IS located in the top layer(s) of the substrate lattice, because substantial diffusion mto the lattice could not be detected The form of the transients mdlcates large expulslve forces between the partially charged species The reaction rates for the steps of the electrochemical reaction m the underpotentlal deposltlon increases m the order Cu(I1) c Bl(III) < Hg(I) < Tl(1) It seems that the adsorption of partially charged metal ions m the underpotentlal range 1s a rather genera1 phenomenon REFERENCES 1 D M Kolb, m Aduances m Electrochemrstry and Electrochem Engmeermg (Edited by H Genscher and C W Tobias), Vol 11, p 125 Wdey, New York (1978)
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