THE KINETICS OF SILVER IODIDE FILM FORMATION ON THE SILVER ANODE

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1 Electrchmca Acta, Vl. 27. N., pp , 982. Prnted n Great Brtan , $ Pergamn Press Ltd. THE KINETICS OF SILVER IODIDE FILM FORMATION ON THE SILVER ANODE VIOLA I. BIRSS Chemstry Department. Unversty f Ottawa, Ottawa, Ontar, Canada and GRAHAM A. WRIGHT Chemstry Department, Unversty f Auckland, Auckland, New Zealand (Receved 8 January 982) Abstract The ptentdynamc frmatn f Aglflmsby reactn () n an Ag rtatng dsc electrde was studed and the results were cmpared t prevus wrk regardng the frmatn f andc AgBr and Ag 2 S flms, Ag + r ^Agl + f- () In dlute dde slutns, a very prus Aglflms frmed at a rate cntrlled by the dffusn f I ~ t the electrde surface. At hgher dde cncentratns, and partcularly n hgh cnductvty supprtng electrlytes, a prus Agl flm frms at a rate ntally lmted by nc mgratn n slutn fllwed by dffusn f I" n the pres f the Agl flm. In M Nal slutns, when Agl becmes rather sluble, thck granular Aglflmsfrm at a rate lmted by the sld-state mgratn f ns n the bulk f theflm.on the bass f the lw-feld mdel fflmgrwth, the nc cnductvty f these Agl flms has been determned t be 8.7 x (T 5 ScrcT' INTRODUCTION A study f the ptentdynamc frmatn and reductn f an Agl flm n an Ag substrate n aqueus dde slutns [reactn ()] was undertaken n rder t cmpare these results wth thse btaned frm a study f the andc frmatn f AgBr flms[l], Ag + I,; ql^agi (flm) + e-. () In[l], t was fund that the reactn between the slver electrde and brmde ns was fast and that physcal prcesses such as nc dffusn and mgratn lmted the rate f AgBr flm grwth. It was als shwn n[l] by Scannng Electrn Mcrscpy (SEM) that ptentdynamcally frmed AgBr flms remaned prus thrughut ther grwth, even fr flm thcknesses up t abut 2 /jm. It s the purpse f ths paper t crrelate the bserved E/I curves and the Agl flm mrphlgy, as seen by SEM, wth a mdel f andc Agl flm frmatn and t cmpare ths mdel wth that btaned fr AgBr[l] and Ag 2 S[2, 3] flm frmatn under smlar expermental cndtns. EXPERIMENTAL The basc expermental detals have been descrbed prevusly[l]. As befre, an Ag rtatng dsc electrde (rde) was utlzed, wth a rtatn rate f c, and a rughness factr, y, btaned by duble layer capactance measurements[4]. Intally, y was fund t be between 2 and 4 but culd reach r mre wth 439 repeated cycles f Agl flm frmatn and reductn. Because f ths large range n y, current denstes are reprted wth respect t the apparent electrde surface area n ths paper. RESULTS AND DISCUSSION () Agl flm frmatn n dlute dde slutns A typcal E/I curve fr Agl flm frmatn and reductn n dlute dde slutns s shwn n Fg. fr a slutn f 2.5 mm Nal +. M NaC 4. As n dlute brmde slutns[l], the andc current ccurs as a current plateau wth a magntude whch s ndependent f the ptental sweep rate, s, but whch vares lnearly wth w ' 2 and wth the dde cncen- 5 < ' ) E -a' \f ' ' ' - - / -5 : E/Vnhe) Fg. I. E/I curve f Ag n 2.5 mm Nal +.M NaC 4 at s = 5 mv s"' and c = Hz. Andc current plateau reflects rate lmtatn by the dffusn f dde n t Ag electrde.

2 44 VIOLA I. BIRSS AND GRAHAM A. WRIGHT tratn, C, as s expressed by the Levch equatn (2) fr a dffusn lmted reactn[5]. / =.62zFAD 2 3 Cv (2) where z s the number f electrns nvlved n reactn () ( = ), F s the Faraday cnstant, D s the dffusn ceffcent f I" (2. x ~ 5 cm 2 s" '[6]) and v s the knematc vscsty f the slutn (8.9 x " '[7]) The theretcal //c 2 relatnshp f (2) and the expermentally btaned pnts n several dlute dde slutns are shwn n Fg. 2. The prxmty f these tw plts ndcates that the reactn between Ag and ls dffusn cntrlled under these cndtns. -.4 /V (nhe) Fg. 3. E/I curves f Ag n 92 mm Nal + 83 mm NaC 4 c = Hz and s = (a), (b) 3 and (c) mv s" '. /(Hz) 2 Fg. 2. Expermental (e e e-) and theretcal ( ) (2) relatnshp between plateau current densty and c' 2 fr Agl frmatn n (a) 2.5 mm I" and (b mm I " slutns at s = SOmVs". At a ptental f abut.7 V/nhe n Fg., the andc current can be seen t ncrease but wthut a cathdc cunterpart t ths current. As n dlute brmde slutns[l], ths current has been attrbuted t the dsslutn f Ag by reactn (3), whch mples that the Ag surface s nt fully cvered by Agl flm under these cndtns. Ag ^ Ag + + e' (3) The reductn f the Agl flm s seen t ccur n a sngle sharp peak (Fg. ). In these dlute dde slutns, the charge cnsumed n the andc and n the cathdc sweeps are apprxmately equal, cnsstent wth the lw slublty f Agl at these dde cncentratns. () Agl flm frmatn n slutns f hgher dde cncentratn Wth an ncrease n the dde cncentratn, E/I curves such as n Fg. 3 are btaned. Fr a 92 mm Nal + 83 mm NaC 4 slutn, an andc current peak whch n lnger depends n w but rather depends lnearly n s" 2 s nw bserved. Therefre, the dffusn f dde ns t the electrde surface s n lnger lmtng the rate f Agl flm frmatn. At these hgher dde cncentratns, cmparable t the case f AgBr flm frmatn[l], the E/I curves f Fg. 3 nw dsplay a lnear leadng edge t the andc current peak, wth an nverse slpe ( ~ 3 Q) whch s apprxmately equal t the calculated hmc resstance f the slutn, R s (4), between the Ag electrde and the tp f the Luggn capllary leadng t the reference electrde[8]. R = tan (4) 2nha \a where z s the perpendcular dstance between the Luggn capllary and the Ag electrde surface, a s the electrde radus and K s the nc cnductvty f the slutn. Ths ndcates that n the regn f the E/I curves leadng t the andc peak, the mgratn f ns n slutn s nw lmtng the rate f Agl flm grwth. Ths nce agan mples that reactn () s very fast and that the prduct, the Agl flm, des nt behave as an bstructng layer. () Agl flm frmatn n hgh cnductvty. M dde slutns In rder t vercme the nc mgratn rate lmtatn, the grwth f Agl flms was then studed n the hgh cnductvty slutn f. M Nal -f5mnac 4. The dde cncentratn was nt ncreased t M because Agl becmes t sluble at hgher dde cncentratns. Fgure 4 shws sme typcal E/I curves at varus s fr Ag n ths hgh cnductvty slutn. It can be seen 2 / / b \ \ Ha/ / r*-* h r ^> ryf l c / / - Wd I l E/M{nhe) Fg. 4. E/I curve f Ag n hgh cnductvty dde slutn (. M Nal + 5 M NaC 4 ) at c = 8 Hz and s = (a) 3, (b) 5, (c) 5 and (d) 3mVs"'. -

3 Knetcs f Agl flm frmatn n Ag ande that the current ncreases lnearly frm the frmatn ptental t the peak wth an nverse slpe f Q. Ths s nce agan very smlar t the calculated slutn resstance, Rs, [ = 2 f by equatn (4)] whch ndcates that nc mgratn n slutn s stll rate lmtng. At the andc peak, the peak current densty s ndependent f t but vares lnearly wth s 2 (Fg. 5). After the andc peak n Fg. 4, the current decreases gradually wth ncreasng ptental, althugh at a ptental f abut.75 V, evdence f reactn (3) can stll be bserved. Ths supprts the cntentn that the Ag surface s stll nt ttally cvered wth Agl flm and that the Agl flm s prbably prus n nature. Ths has been substantated by an SEM examnatn f such a flm surface (Fg. 6) whch clearly shws a very prus and granular type f flm. ]! JOO E u <E \ 2 ^S*.^a S^ 4 8 I 2 svlmvs")^ /s-/2' Fg. 5. Expermental p relatnshp fr Agl flm frmatn under cndtns f Fg. 4 at m = 8 Hz. \ tztw^mn^mmfe * JL# *... w -w w, &&&L.-M 44 current n the andc peak regn f the curves (Fg. 4) by the slutn resstance, by the bserved lnear p / s " 2 relatnshp, by the gradual decrease f the current after the andc peak ndependent f the magntude f the ptental and by the bserved flm prsty. In the case f AgBr flm frmatn[l], all f these bservatns were lnked and the expermentally bserved andc E/I curves were quanttatvely smulated by cmputer n the bass f the fllwng mechansm[9]. AgBr flm ntatn was cnsdered t ccur by the nucleatn f small slands f flm t a crtcal nucleus heght, whch then spread and thcken untl nly small spaces, r pres, reman between them. Because the electrn transfer reactn between Ag and brmde n was bserved t be fast, ths grwth regme n hgh cnductvty brmde slutns was cnsdered t be cntrlled by the resstance f the bulk f the slutn and by the resstance f the electrlyte n the pres f the flm[]. When the resstance f the pres becmes large enugh, the current peak develps, and a lnear p/s' '2 relatnshp s then theretcally antcpated. After the current peak, the cncentratn f the brmde n was fund t becme equal t zer at the pre base and then further flm thckenng was fund t be cntrlled by the dffusn f brmde ns wthn the pres f the flm. In accrdance wth ths verall mdel f flm grwth, Agl flm frmatn n hgh cnductvty. M dde slutns s als characterzed by a slutn resstance cntrlled current n the regn f the andc peak f the E/I curves (Fg. 4), a lnear p/sl:'2 relatnshp (Fg. 5) and a prus Agl flm mrphlgy (Fg. 6). Hwever, the bserved E/I curves fr Agl flm grwth have nt been smulated t date fr a fnal verfcatn f ths mdel f flm grwth. Nevertheless, t appears that enugh smlartes exst between the bserved electrchemcal relatnshps, the flm mrphlges and the basc characterstcs f AgBr and Agl that the same pre resstance and pre dffusn mdels apply t bth Agl and AgBr flm frmatn under ptentdynamc cndtns n hgh cnductvty slutns. (v) Agl flm frmatn n. M dde slutns I 'f 3 ftm Fg. 6. SEM vew f Agl flm frmed n. M Nal + 5MNaC 4 at s = mvs -, w = 8 Hz, where the sweep was termnated at abut.4 V/nhe (see Fg. 4). These results n hgh cnductvty dde slutns bear a strng resemblance t thse fr the andc frmatn f AgBr flms n hgh cnductvty M brmde slutns[l]. Ths s seen by the cntrl f the In. M dde slutns, the slublty f Agl rses sharply t abut 2 M[] frm a slublty f abut " 5 M n. M dde slutns. Therefre, the dsslutn f Agl as sluble Agl _< " _ " speces wuld be an antcpated feature f the mechansm f Agl flm frmatn n. M dde slutns. Fgure 7 shws a typcal E/I curve fr Ag n M Nal slutn at s = 5 m V s _. As n. M dde slutns, the andc current denstes are all ndependent f w but vary lnearly wth s / 2 (Fg. 8). The nverse f the leadng slpe t the andc peak nce agan depends n the slutn cnductvty, whch ndcates that Agl flm grwth s lmted by the hmc resstance f the slutn n ths regn f the curve (4). Hwever, the E/I curve f Fg. 7 s ntceably dfferent frm all f the prevus Agl and AgBr[l] E/I curves at hgh halde cncentratns, and s nstead smewhat remnscent f the E/I curves fr Ag2S flm frmatn]^]. Cntrary t the curves f Fgs, 3 and 4 n

4 442 VIOLA I. BIRSS AND GRAHAM A. WRIGHT EIM(nhe) Fg. 7. Ell curve fr Ag n M Nal slutn at s = 5mVs ',tu = 7 Hz. [ S 5 'pa 'a OyS, 5 j r O Fg. 8. Expermental relatnshp fr Agl flm grwth between the andc peak current (^) and the pst-peak current (' ) vs s 2 n M Nal at 7 Hz. Fg. 9. SEM vew f Agl flm frmed n M Nal at s = 5 mv s" ', t = 3 Hz. when ptental sweep termnated at.67 V/nhe (see Fg. 7). bserved value f the nverse f the leadng slpe f the andc current peak t the calculated value f the slutn resstance (4). Als, the bserved lnear relatnshp between / p and s 2 (Fg. 8) s predcted fr the frmatn f a prus flm at a rate lmted by the resstance f the bulk electrlyte and the electrlyte n the pres f the flm[l]. Hwever, t s mprtant t nte that at hgh s. n andc peak s bserved n M Nal slutns, and then L = g, whch ndcates that the peak seen at lw s may be due prmarly t the dsslutn f Agl. After the current peak n Fg. 7, the current (g) remans cnstant wth ncreasng ptental but vares lnearly wth s 2. Wth sweep reversal, g drps rapdly wth the decreasng ptental, whch s cnsstent wth the lw-feld apprxmatn fr flm grwth lmted by sld-state nc transprt acrss the flm[2]. In the case f the lw-feld apprxmatn, (5) (Ohm's law) apples[2] mre dlute dde cncentratns, the andc current after the andc peak regn,., nw remans large and relatvely cnstant wth ncreasng ptental rather than decreasng cntnuusly even wth ncreasng ptental. Upn sweep reversal t the cathdc drectn, the current can be seen n Fg. 7 t drp apprxmately lnearly t zer as wuld be characterstc f flm grwth cntrlled by a feld-dependent prcess[2], rather than exhbtng the gradual declne f current f Fg. 4.», = Krf (5) When the ptental s extended t abut.75 V n V M Nal slutns, n andc current attrbutable t where = (6) the ccurrence f reactn (3) s bserved (Fg. 7). Instead, the current remans cnstant and ndependent f the ptental untl the drectn f ptental sweep s and where K, s the nc cnductvty f the Agl flm, I reversed. Ths may ndcate that n free Ag s avalable s the electrc feld acrss the flm, V s the vltage fr reactn (3) t ccur and that the pres n the flm dstrbuted acrss the flm and / s the flm thckness. If t s assumed that nly flm thckenng s ccurrng are n lnger carryng the current. An SEM vew f an Agl flm frmed n M Nal after the andc peak, then Faraday's law can be slutns s gven n Fg. 9 and a very granular type f expressed as n (7) flm can be seen. The gaps r the pres whch are seen _ zfp d/ n Fg. 9 are prbably sealed further n twards the Ag (7) g ' "^Td7' base. The surface appears qute dfferent frm that n Fg. 6 fr an Agl flm frmed n. M Nal slutns where p s the flm densty (5.7 gem 3) and M s the and may ndcate that a dfferent mechansm f flm flm mlecular weght (235 g mle" '). frmatn nw exsts, perhaps nvlvng Agl dsslu(8) dv = sdt tn and precptatn. Ths effect f dsslutn can be As seen partcularly at lw s where the rat f the andc. (KjZFp) '* (9) then t the cathdc charge denstes can be up t M A mdel f Agl flm grwth whch wuld best ft the expermental bservatns n M dde slutns s as Frm (9), K, fr andcally frmed Agl flms can be fllws. Intally, the rate f Agl flm grwth s btaned frm the expermentally btaned js' 2 plt cntrlled by the rate f nc mgratn n the bulk f (Fg. 8). Frm the slpe, K, s fund t be equal t 8.7 the slutn. Ths s seen by the prxmty f the x l O ~ 5 S c m ~ '. Ths s a reasnable value when

5 Knetcs f Agl flm frmatn n Ag ande 443 cmpared t thers n the lterature, such as a cnductvty f between ' 6 and " 4 Scm' ' fry-agi[3] and between " 5 and lo^scm" fr -AgI[4]. Therefre, t appears that n M dde slutns, the Agl flm s essentally a cmpact ne s that current mves va a sld-state feld dependent path and the lw-feld apprxmatn s vald. In mre dlute dde slutns, the pre mrphlgy seems t be mantaned and hence current fllws the pres and s then lmted by nc mgratn and dffusn n the electrlyte wthn the pres f the Agl flms. SUMMARY A ptentdynamc study f the andc frmatn f Agl flms n an Ag electrde n aqueus dde slutns was carred ut n cnjunctn wth SEM examnatns f the mrphlgy f the Agl flms. In general, t was fund that the electrn transfer reactn between Ag and I ~ s fast s that physcal prcesses lmt the rate f Agl flm frmatn. In dlute dde slutns, the rate lmtng step t the flm grwth prcess s the dffusn f I" n slutn t the Ag electrde surface. At hgher I" cncentratns up t abut. M, the rate f Agl flm grwth s ntally lmted by nc mgratn n slutn and a very prus Agl flm develps. As the pres f the flm lengthen, further flm grwth then becmes lmted by the dffusn f I" n the pres f the flm t the Ag substrate. SEM examnatns have supprted ths mechansm f grwth by revealng very prus and hgh area Agl flm surfaces. In IMF slutns, the Agl flm becmes qute sluble and t appears that a dfferent mechansm f Agl flm grwth prevals. Nw a granular but mre cmpact Agl flm s seen by SEM and the rate f flm grwth s nw lmted by the nc mgratn f Ag + under the nfluence f the feld n the Agl flm. On the bass f the lw feld mdel f flm grwth, the nc cnductvty f the Agl flms has been determned t be 8.7 x " 5 Scm" ', whch s n clse agreement wth prevus determnatns f the nc cnductvty f Agl. Acknwledgements V.I.B. gratefully acknwledges the recept f a Cmmnwealth Schlarshp frm Canada t New Zealand frm the New Zealand grants cmmttee. REFERENCES. V. I. Brss and G. A. Wrght. Electrchm. Acta 27, 429 (982). 2. V. I. Brss and G. A. Wrght, Electrchm. Acta 26, 89 (98). 3. V. I. Brss and G. A. Wrght, bd 27, (982). 4. M. Baba, T. Tshernkvsk and E. Glead, J. electrchem. Sc. 9, 8 (972). 5. L. Brcumshaw and A. Rddfrd, Q. Rev. chem. Sc. 6, 57 (952). 6. R. Parsns, Handbk f Electrchemcal Cnstants. Butterwrths, Lndn (959). 7. N.R.C. f Amerca, Internatnal Crtcal Tables, (Edted by E. W. Washburn) Vl. 5. McGraw-Hll, New Yrk (929). 8. J. Newman, J. electrchem. Sc. 3, 5 (966). 9. W. J. Mueller, Trans. Faraday Sc. 27, 737 (93).. A. J. Calandra, N. R. de Taccn, R. Perer and A. J. Arva, Electrchm. Acta 9, 9 (974).. J. Bjerrum, Stablty Cnstants f Metal-In Cmplexes, Chem. Sc. Lndn, Specal Publcatn N N. Cabrera and N. F. Mtt, Rep. Prg. Phys. 2, 63 (949). 3. T. Takahash, K. Kuwabara and O. Yamamt, J. electrchem. Sc. 6, 357 (969). 4. G. Cchrane and N. Fletcher, Physcs Chem. Slds 32, 2557 (97).