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wSpcctmclcctrochcmicai studies of poly-c^-phcnylcnediamincPari 2. In situ UV-vis subtraclive rcUcctance spectroscopy
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ivvfiilcd miiiiily hy iisiiiy in silii ivsoiiiiMLV kiiniiin s|iL'Lliiiscopy. Ii h : . \ \ ;iK(i hi-cii shown Ihiii ilic SL'nii-o\icli/LHl sliilu of I'r^I'D is ilie nioslsl;ililc Miilc of l'r
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('it:. I . The possih lc react ion scheme o l ' l l ie redox process of VoVD n i tn in s t ron j ; uc i i l so lmion . (A l l o f l l ie i ion -ho i id in f ; e lec t ron p i i i rs o f n i t rogen a toms inthe I '^PD molecule are pro lonale i l bccii i ise the experiments were carr iet l out in sirony acid solut ion. Hut they are omitted in the react ion equations fors im p l i c i t y . )
Pr^PD to explain its electrical conductivity changes in theredox process. Martinusz ct al. [9] used the electrochemicalquartz crystal microbalancc technique to study the rorina-tion and redox behavior of Pr;PD film in acid media. Theyrecognized that the redox transformation of Pr;PD is rathercomplex and assumed three main reaction routes at different pH conditions . How ever, it is difficult for them to takethis study further because the quartz crystal microbalancccan give information on mass change only. Both Yano ctal and Martinusz et al have supposed the existence of freeradicals in the redox process of Pc^PD, but obviously, whatthey used to study in the redox processes of Pr^PD areindirect techniques. These are not sufficient to study thecomplex reaction process of Pr;PD and its ability to provide molecular level information is limited.
In the present paper, in situ UV-vis subtractive reflectance spectroscopy has been applied to study the moredetailed reaction mechanism of Pr;PD in strong acid solution. The use of multi-channel detection spectrophotometers allows absorption spectra to be obtained in real tim:^,with sampling limes of the order of tnicroseconds (seebelow). Multi-channel spectro.scopy allows the measurement time to be decreased and thus, allows the observationof intermediate states involved in electrochemical pro-
2. Experimental2.1. Materials
r^-phcnylenediaminc was of reagent grade and was purified by recrystallization from water. H-,S04 was an analargtade reagent. The preparation and the after-treatment ofthe Pr;PD film-coatcd electrode was exactly the same asdescribed previously [10]. The counter electrode was a
platinum wire arranged to give uniform current distributionwithout obstructing the light path, A saturated calomelelectrode (SCE) wa.' u.scd as the reference electrode. Allsolutions were prepared with doubly distilled water. Allpotentials were measured and reported with respect to theSCE.2.2. Inslnimenlation
The in situ UV-vis spectroscopic experiments werecarried out with the apparatus shown in Fig. 2 [12]. Thespectra are detected by an optical multichannel analyzer(OMA) EG&G PARC MI460 with a detector controllerEG&G PARC MI463, an intensified silicon photodiodearray detector EG&G PARC Ml421 (with 1024 elements)and a polychromator EG&G PARC Ml228. The visiblelight source was a 36 W tungsten-halogen lamp and theultraviolet light source was a 10 W deuterium dischargelamp. The electrochemical control unit includes a potentio-stat with a high slew rate of 10 V//zs (EG&G PARCM273) and a function generator, universal programmerEG&G PARC MI75, which can afford two kinds oftrigger pulses: frame sync and cycle .sync. The detector canwork in a continuously working version (CW ) in which thedetector is continuously light-sensitive, or a gating version(Gate) in which it is only light-sensitive for the duration of- 2 0 0 V pulse applied to the photocathode in the detector.The pulse train sync controls and timing unit contain theframe sync pulse, the cycle .sync pulse, a gate pulser withpulse delay setting function (EG&G PARC Ml303) and apul.se amplirier(EG&G PARC M1304).2.3. Procedures
The measurement system allows the acquisition of insitu time-resolved UV-vis spectra by either a continuously
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Fig. 2. Ulock diagram of the clcctrochcmically modulated lime-rcsolvcd UV -vis .spectroscopic m easurement .sy.stem.
working version (CW) or a gating version (Gale). In theCW version as shown in Fig. 3a. a step .signal from thefunction generator iVI175 is sent to thic potentiostat thatcontrols the electrode potential (Fig. 3a(n)). At the sametime of the potential stepping, a frame sync pulse from thefunction generator is fed into the detector controller M1463to start data acquisition (Fig. 3a(I)). Because the detector isnow continuously light-sensitive, the time resolution of thespectra is limited by the shortest time for recording aspectrum for an entire spectral range. For a normal S-an,the detector controller M1463 needs at least 16.634 ms toread and reset ail of the 1024 elements, but if scans arefast-accessed or pixel grouped (gained at the expense of acorresponding decrease in detector resolution), the shortesttime for acquiring an entire spectrum will be shorter [13].Therefore, the CW version gives the limit of time resolution of the order of ms. Up to now, most in situ UV-visspectroelectrochemical experiments used CW or CW-likeversions to record the absorption spectra [14-19], and thereported shortest time resolution is 5 ms [16]. This is thelimitation of the CW version [17]. This time range wouldlimit the observation of intermediate electroactive species.
In the Gate version as shown in Fig. 3b, a series ofsuccessive double step waves from the function generatorMl75 is sent to the potentiostat M273 to control theelectrode potential (Fig. 3b(II)). Meanwhile, a cycle .syncpulse train from the function generator Ml75 (Fig. 3b(I)),which provides a start of each double step wave andtiming pul.se, is fed into the gate pulser Ml303 to triggerthe,timing puKses (Fig. 3b(ni)) which have fixed timeintei^vals and settled progressive delay. In the meantime,the timing pulses from the gate pulser MI303 go into thedetector controller MI463 to trigger the data acquisition(Fig. 3b(IV)), while the a.mplified timing pulses from thepulse amplifier M l30 4 control the exposure of the detector(Fig. 3b(III)). Thus, the data acquisition is separated fromthe exposure of the detector and the time resolution, whichis just equal to the exposure time of the detector, isindependent of the minimum time for data acquisition (seeCW version). The pulse amplifier M1304 monitors thegating of the microchannel plate intensifier and limits theilluminadon time of the photodiodes from 100 ns to 10 ms.However, the minimum time of the response of potentiostat M273 of the measurement system is 3 /xs. Therefore,
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ilck-cHM- or O M A ui lh It inu case i lo lay ; (ad lD ) ami (h( lV )): Da la acc| u i si i ii in l iming o l OM A.
ihc shoitcsi time ivsolulion of 3 ix
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/. .- /. . \Vtifi,il./.l H i l o J I'M I - I I ) ( I > J < I 7 I 17.1 hS 2LMisiiiv tluil llij clixliocliciiiiuiil syslL'iii IKICI iciichal iissleiidy siiiic. unci ihc Art/A' -- A was sicatly slalu- UV-vissiihliaclivc icrit'cUiMce spcctiuin.(c) A scries of W /;',) WLMC obtainucl when the uluulioclcpolL'iilial was siL'ppL'cl I'loin 1:^ lo li,, ilio lime inlcivallioiwecii ivvo successive spedra was ihe lime lesdlulion.and the A A '/A '-A were lime-resolved U V- vis suhtrac-live relleclance siiectra.
3. Kesiilts and discussidnJ./. /// .v//(( sWaily stall' UV -ris siihiniclivc rcflccUiiifi'spectra
In redox processes of Tr-syslcms changes in ihe ahsorp-lion speclrum often occur. Fig. 4 siiows the potenlial-re-.solved UV-vis subtraclive reriectance spectra of VoVDlihii on a uold electrode in a 1.0 M 11,SO, sohuion. 'Ihe
inset sliows liie overlaid mode ol' thesi- sprclia. !i isoiivious that the spectra change with the cli:clrode poter-tial. One broad ab.sorption band emerges aroiiiid .iOO nni atthe potential ol' about - 1 2 0 mV, and becomes progressively stronger vviien the electrode potential changes I'roni- i2() inV to I.'IO mV. '['hen aliiiosl no change in Ihospectra can be observed from potential 130 mV to 200mV. However, when the electrode potential changes continuously to more positive values, two shoulder peaksappear on each side of the broad band. The spectra tend toremain unchangeable beyond a potential of 3K() mV.
The |)otenlial-res()lved UV-vis absorption spectra ofVdVD film indicate that the three states ol' PoI'D displaydilTerenl electronic absorption chaiacters in the visiblespectral region. It is universally accepted that the reducedstate ol' VoVQ is colorless and has no absorption band inthe visible range [1]. When the potential of the ViA^D filmelec trod e ch ang es to - 120 mV , the .semi-o.Kidized state ofPrW'D is produced inunediately and exhibits an absorption
800 700 600 500 400A,/nm
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l - ig . 5. The in .si lu .sleatly slate UV -v is sub ln ic l ivc rc l lccian cc spectra ofIhe Po PD nim elec trode in a 1.0 M I I , .SO ., solu l io n. (a) At potent ia l of0 .05 V ( / ; ,) w i th re le rence to 0 .50 V ( / ; , ) ; (b ) A t po ten t ia l o f -0 ,2 0 V( i : , ) wil l) reference lo 0.05 V (/; ) .
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Plots of the peak ci irre i i ls / a iwii ist potent ia l scan rate c. Theses were ealculate i l t ' roni the C'Vs o l ' ihe PoP D l l l n i e lec lnule in a11,,SO,, soln l io n (see Kef. [10]), (A) anod ic peak 1: ( l i) c athodic(C) cathodic peak .V
simi lar lo Ihe si ruclure uni t of PoPD e x h i b it s i m i la r U V - v i sabsorpt ion bands in ac id ic solut ion [21] , which ver i f ies th isproposal for the structure of PoPD to a cer lain degree.Ob viou sly , the resul ts f rom both the in s itu U V -v is sub-tract ive ref lectance spectra and the in si tu resonance Raman spect ra are consis lenl w i th each other .
On the other hand, an interest ing phenomenon can beobserved in a l l o f the cycl i c v ol l iunm ogram s (CV s) in f ig .2 of Re l ' . [10 ], in si tu resonance Ram an spectra in f ig. 1 ofPici'. [10] and in s i tu steady sta le UV -v is subt racl i veleneclance spectra Fig. 5; t l ie "si j inals' attr ibuted to thetoUi l l y -ox id ized sta le of PoPD are obviously .smal ler t i ianthose attr ibuted to the .semi-oxidized stale of P^;PD. I t maybe that only a par t of the semi -oxid ized state of PoPD canbe oxid ized to the tota l ly -ox id ized sta le of PoPD for somereason which is at present unknown. The quant i tat i vere lat ionship between them wi l l become c lear af ter thet realmenls of F igs. 2, 1 and 5 of Ref . [10] as shown below .
Fi g . 6 shows the plots o f the ano dic peak cu rren t, / | , , ( 1) ,and the cathod ic peak curren ts, / | , ,. (2) and /p,.(3) , ca lculated from f ig . 2 o f Ref. [10] against electrode pote nt ialscan rate r. The |) lols for al l of three cuiTcnts fal l on astraight l ine that passes near ly through the or igin, indicating that three currents are control led by surface processsimi lar to the redox processes of adsorbales. Under idealcondi t ions, the re lat ionship between 1 , and r is give n byl^, = irl-''AI'ji/4lir for non- interact ing s i tes. As can beseen in Fig. 6. the values of y 1) and / , (3 ) , al thoug hl inear wi th r. are not equal as predicted. This fad suggestsnon- ideal behavior due to the d i f ferent in teract ion betweenactive sites in the P^^PD f i l m . Thus. / , , = irr-Aryr/RT[4- ( / + / ) / ' . | . ] , where /,, and /-^ are the interact ion parameters [22] . W i th the data of the s lopes (calculated f romthe plot / | , against r ) and the lota! surface concentrat ionsof the electroact ive substance l \ (see next page), thevalue of the interac t ion param eters ( ' ' + '"/ (^ ^' ' ' " ^ ' "^ "l a i ncd . They are ( ; ; , + / ) , = - 1.06 X 10' ' c i i r mo l ' forthe reduct ion process (1) (cor responding to peak 2) , and
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^/"''ini,ii,.v/^'^'' 'H'iiii-..v' ""- ''"'" i-'iilL'uhiifd I'lom I'iL!. 5 is;ib()iM. 1/ 3 . w hic h la ilius qtiiti; well w ill) ihu ic su iis IVoinilic (.'Vs and ll)o in siiii IVSOIUUIUL' Kaii ia i i spcci ia . a i i t lpro ve s oiiCL- ayai i i l l ia l only a bo ul o ne t l i i rd of Ihc s eni i-o .\ i i l i / jc l s la te of \'ii\'D i s ox id i / ed lo l li e lo ia i iy -o x i i i i / edslate ol ' I'ol'D. Ti i i s may be heca i i se tha i t i i e s lu ie lu ie o l 'Hi.- tu ta l ly ' .n id ixe d s lute ol ' I ' r .P D is less s tabl e ( lOl , l luis ,the r eac t ion be iween the sen i i -o \ id i / ed s t a l e o f P rW'D andthe to ia l ly -ox id i / .ed s t a t e o f l^;!-"!) i s l e s s r eve r s ib le . ' Ih i smay be a l so the r et i son tha i a lmo s t a l l o l' t he inves t iga to r son VdVD e,\cep l for a few [9j lend lo co ns ide r that onlytwo redox s t a le s o f \'/iVD ex i s t in i t s r edox p rocess .
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piocliicud in tliu liiiiu SIIOIILT IIKIII 0.2 ms. When llie limeivsoiuiion (.'hiMigcs lo 5 ins, tlii; iihos.' 4(i5 nni iiuil 330 iiniahsorption liiinds clisiippuiir unit only ihc 625 nni handL',\isls in die t'ii'si six sjiccira siiown in l-'is.!, S. whicli showsdial anolhcr inicinicdiaic siiuciurc of \'(i\'D is pioduced.ConsaiiiLMilly. die in siiu linic-rcsolved U V -v is siihiraclivclericclaiice spectra r.\' die ivduciion pincess (1) show ihalal Icasl iwi) sUiL'es of struelure ciiange exisl during diercduclion process of die lolaliy-oxidi/.cd slale ol' I'oPD ti)die scnii-oxidi/.ed slale of WA'D.
When ii conies lo the reduction process (2). Ihe spectradilTered rroiii the steady state spectrum, '-'ig. 3h can beohscrved clearly w itii a time resohilion ol' 0.2 m s. Threenew absorption hands located near 445 nni. 520 nni and(i85 mil appear in the spectra as shown in Fig. 9. The resultalso indicates thai one interiiiediale structure change existsin Ihe reduction of the .semi-oxidi/ed stale of \'(>l'D to thereduced slate of Pom.
The in situ lime-resolved UV -vis sub; ve re-riectaiice spectra ol' the reduction process of Pi-. ) showthat al least three inlermediale siruciures of I'oVD areproduced in tile ledox |irocess of Pr^FD. They can beobserved only under raliier iiigh lime resolution and thenare m ore unstable than the three steady slates ol' Fr^PD.The exact structures remain unknown and need lo bestudied lurdier.
4. Cc,!. lusioiiThe stable state behavior ol' Fr'PD film in its redox
piDcess has been siuilied rurlher by in situ UV-vis subtrac-live relleciance spectroscopy. The results support the conclusions about the structure and the redox process of PnPDI'roni die in situ resonance Raman s|iectrosco|iy. 'Ihe irans-Tormaiion processes of three states of IV^FD were illus-tiated by in situ pi)teiitial-resolved UV-vis suhtractivereflectance spectra. All ol' the in situ steady stale UV-visspectra, the cyclic voltammograms and the in situ resonance Uaniaii spectra demonstraie that only about one thirdof Ihe .semi-oxidi/ed state of VoVD can be oxidi/.ed to thetotally-oxidi/.ed state of P(>FD.
The ill ^iln lime-re.solved UV-vis suhtractive le-riectaiK. , scopy with a rather high lime resohilionhas been u.se > .ludy the dynam ic process of Po FD I'ilmin strong acid sou.Jon. The results show thai at least threeintermediate structures of Fr^FD are produced in the dilTer-eiii stages ol' the redox process of PoVD.
Acknitwled^enieiit!)The authors are gratel'ul hn the financial support from
the National Natural Science Foundation of China.
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(II J. Ymio. K. Tciiiyi iiini, S. Yaiinisiiki, J, Minor. Sci. 31 (I' W. ) .I7S.S.[ 2 | T. Ohsiika. T. Walaiiahf, I'. Kilainiii a, N. Oyaina, K. Tokuda , .1.
Chciii. So c, Chum. Oimnnin. (I'WI ) p. 1072.[.1| K. Ogiira, II. Shiif. i, M. Nakayania. .1. Hla-lrdcJKMii. .Sue. 14.1 (I Wi )
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2Xf,.'i.{(}] I.. Xianyciiii, '/,. Il()ni!i|iang, I'lcclrochiin. Aula 41 (IWf)) 2()I'J.[71 K. Oyiira. M. Kokiua. J. Yano. II. .Sliiiyi. I'lccirothi m. A da 40
(l'W.'i)2707.[X) K. Marli mis/, (!. Iri/cll. (i. Iloraiiyi, J. l-lcLlioanal. CIIL-III. y)5
(\W5)2')X[y] K. MarilnusA li. C/.imk, G. Iii/cli. J. likci roanal. Chcm. .17y (IW 4)
4.17.[10) l,.L. Wii. J. Liio, /.. H. I.in. J. lilcclroanal. Chcm. 417 (l'J%) 5 3.[11] J. Yano. A. Shimoyania, T. Nagaoka , K. Ogura, Dcnki Kagakii W)
(I'W) IIOI.
[12] J. Lmi. /. II . I.in. I,.l.. Wii, Y. Huang, Z.W. Tian, Chcm. Kcs.Chinese Univ. 12 (I 'W. ) 270.
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[14] C.A. l.iMKlgien, U.W. Mnrray, liioig. Clieiii. 27 (I'JHK) 'J.ll.[!.'>] A.I). Monveriiay, I'.C. Laca/,e. A. Cherigiii, .1. lileclni anal. Chem.
260(I';S';)7.S.[If. ] T. Onikuho, K..I. I.in, M. Kaiieko, J. lilectruanal. Chem. .V.l ( l'W3)
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[21] M.A. Goyctlc, M. l.cclerc, J. lileelroanal. Chem. 382 (I'J'JS) 17.[22] U.W. Murray, in: A.J. Bard (I'd.), lilcclroanalylical Chemistry. Vol.
13 . Marcel Dekker, New York and Basel. New York (l')84) p. 200.[23 ] C. Barhero. J.J. Silber. 1,. .Screno, J. I'leclroanal . Chem. 2';i (I'J'JO)
XI .
