Anodic iridium oxide films: An UPS study of emersed electrodes

5urface 5c1ence 160 (1985) 517-530 N0rth-H011and, Am5terdam

1R1D1UM 0 X 1 D E ELEC7R0DE5

AN0D1C

E.R. K 0 7 2

517

F1LM5: AN UP5

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Rece1ved 25 January 1985; accepted f0r pu611cat10n 3 May 1985

F0rmat10n 0f an0d1c 1r1d1um0x1de f11m5ha5 6een m0n1t0red u51n9 U1trav101et Ph0t0em15510n 5pectr05c0py (UP5) 0f the emer5ed e1ectr0de5. 7he p0tent1a1 dependent va1ence 6and 5pectra c1ear1y 5h0w the 0n5et 0f 0x1de f0rmat10n at a60ut 0.6 V ver5u5 5CE. 7he den51ty 0f 5tate5 at the Ferm1 1eve1and the p051t10n 0f the Ferm1 1eve1w1th re5pect t0 the max1mum 0f the t 29 6and 0f the 0x1de 1nd1cate5 a tran51t10n fr0m meta1f1ct0 5em1c0nduct1n9 6ehav10ur 0f the 0x1de. Pr0t0nat10n 0f the 0x1de 15 a550c1ated w1th 1ncrea5ed em15510n fr0m 0 H 5pec1e5. A 11near c0rre1at10n 6etween e1ectr0de p0tent1a1 and w0rkfunct10n chan9e 15 065erved f0r the meta1 a5 we11a5 f0r the 0x1de. 0ur re5u1t5 c0nf1rm kn0wn 6and the0ry m0de15 and pr0v1de a fundamenta1 under5tand1n9 0f the e1ectr0chr0m15m 0f an0d1c 1r1d1um0x1de f11m5.

1.1ntr0duct10n

51nce the d15c0very 0f the e1ectr0chr0m1c effect 1n an0d1c 1r1d1um 0x1de f11m5 1n 1975 6y 8uck1ey a n d 8 u r k e [1] 5evera1 1nve5t19at10n5 were d e v 0 t e d t0 th15 effect wh1ch m19ht 6e u5efu1 f0r d15p1ay app11cat10n5 [2-9]. 1t 15 9enera11y a c c e p t e d t h a t dur1n9 the tran51t10n f r 0 m the 61eached t0 the c010red 5ta9e at a p0tent1a1 0f - + 0.7 V ver5u5 5 C E 1n ac1d1c med1a the 1r1d1um a t 0 m under90e5 a n 0x1dat10n 5tate chan9e f r 0 m 111 t0 1V. 51mu1tane0u51y the re515tance 0f the an0d1c 1r1d1um 0x1de f11m ( A 1 R 0 F ) decrea5e5 6 y 0rder5 0f ma9n1tude [10]. 7 h e char9e 5t0ra9e mechan15m 15 a5cr16ed t0 p r 0 t 0 n m19rat10n w1th1n the f11m. 7 h e m0rph0109y 0f the 0x1de f11m 15 u5ua11y de5cr16ed a5 p0r0u5 a n d a m 0 r p h 0 u 5 [111. De5p1te the a 9 r e e m e n t 6 e t w e e n 5evera1 auth0r5 0n the5e macr05c0p1c p r 0 p ert1e5 0f the A 1 R 0 F there 15 50me c0ntr0ver5y w1th re5pect t0 the under1y1n9 pr0ce55e5 0n a m1cr05c0p1c 5ca1e. 8 a n d t h e 0 r y m0de15 were 5u99e5ted 6 y 60tte5fe1d [3] a n d 6 y M 0 2 0 t a a n d C 0 n w a y [4] 1n 0 r d e r t0 exp1a1n the tran51t10n f r 0 m the 1n5u1at1n9 61eached 5ta9e (1r(111)) t0 the c0nduct1n9 c010red 5ta9e * Pre5ent addre55: Department 0f Chem15try, N0rth Car011na 5tate Un1ver51ty, Ra1e19h, N0rth

Car011na 27695-8204, u5A. 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 • E15ev1er 5c1ence Pu6115her5 8.V. (N0rth-H011and Phy51c5 Pu6115h1n9 D1v1510n)

518

E.R. K0t2, H. Neff / An0d1c 1r1d1um 0x1de f11m5

(1r(1V)). 7he5e m0de15 a55ume a 5h1ft 0f the Ferm1 1eve1 fr0m a p051t10n w1th1n a 6and t0 a 6and 9ap. 7he ar9ument5 were pu5hed even further 6y Yuen et a1. [6] wh0 tr1ed t0 exp1a1n the 5tructure 0f the v01tamm09ram 0f A1R0F5 0n the 6a515 0f e1ectr0n d15tr16ut10n curve5 (EDC). 7he va11d1ty 0f 6and 5tructure ar9ument5 wa5 4ue5t10ned 6y 8urke and Whe1an [9]. 1n the5e auth0r5• v1ew the maj0r d1fference 6etween the tw0 5ta9e5 0f the A 1 R 0 F 0ccur5 w1th re5pect t0 5t01ch10metry. 1n the c010red 5ta9e the f11m 15 a55umed t0 6e h19h1y n0n-5t01ch10metr1c, wh1ch make5 h0pp1n9 0f e1ectr0n5 fr0m reduced t0 0x1d12ed 51te5 p055161e. 7he 61eached f11m 5h0u1d 6e 5t01ch10metr1c and c0n5e4uent1y h0pp1n9 0f e1ectr0n5 15 n0 10n9er p055161e [9]. 1n 0rder t0 exam1ne the va11d1ty 0f 6and the0ry ar9ument5 we have app11ed UP5 t0 the emer5ed 1r1d1um e1ectr0de. Wh11e 6e1n9 the m05t 5ucce55fu1 techn14ue f0r 6and 5tructure 1nve5t19at10n5 at the 5011d-vacuum 1nterface, UP5 6ear5 the d15advanta9e 0f 6e1n9 an ex-51tu techn14ue 1f the 5011d/e1ectr01yte 1nterface ha5 t0 6e 1nve5t19ated. 0w1n9 t0 the extreme 5urface 5en51t1v1ty 0f UP5, c0ntam1nat10n5 accumu1at1n9 0n the 5urface dur1n9 the e1ectr0chem1ca1 exper1ment5 at atm05pher1c pre55ure 0r dur1n9 the tran5fer fr0m the e1ectr0chem1ca1 preparat10n cham6er 5h0u1d p1ay a cruc1a1 r01e. 1n a recent 1nve5t19at10n the app11ca6111ty 0f UP5 t0 emer5ed 901d e1ectr0de5 ha5 6een dem0n5trated [12]. A5 a c0n5e4uence 0f the h19h react1v1ty 0f the p1at1num meta15, h0wever, the 51tuat10n 15 much m0re cr1t1ca1 1n the ca5e 0f an 1r1d1um e1ectr0de. 7he purp05e 0f th15 c0mmun1cat10n 15 tw0f01d. F1r5t we want t0 dem0n5trate the p0wer 0f U1trav101et Ph0t0em15510n 5pectr05c0py (UP5) app11ed t0 emer5ed e1ectr0de5 1n 9enera1. 5ec0nd, the 1nf0rmat10n ava11a61e w1th UP5 1n the 5pec1a1 ca5e 0f 1r1d1um e1ectr0de5 pr0v1de5 new 1n519ht 1nt0 the mechan15m5 90vern1n9 9r0wth and e1ectr0chr0m15m 0f an0d1c 1r1d1um 0x1de f11m5 (A1R0F).

2. Exper1menta1 7he 1r1d1um w0rk1n9 e1ectr0de5 were prepared 6y RF 5putter1n9 0nt0 91a55 5u65trate5 w1th a f11m th1ckne55 0f 2000 A. 7he 5putter1n9 tar9et wa5 prepared f0rm 1r p0wder (J0hn50n-Matthey) 0f 4N pur1ty 6y 51nter1n9. A11 e1ectr0chem1ca1 exper1ment5 were perf0rmed 1n 0.1N HC104 made fr0m 5uprapure rea9ent5 (Merck) and h19h1y pur1f1ed 0r9an1c free water (8aker). 7he e1ectr0chem1ca1 preparat10n cham6er d1rect1y attached t0 the U H V 5y5tem ha5 6een de5cr16ed e15ewhere [13]. 7he 5tandard three e1ectr0de p0tent105tat1c arran9ement wa5 u5ed w1th a 5aturated ca10me1 e1ectr0de (5CE) a5 a reference and a Pt w1re a5 a c0unter e1ectr0de. 7he e1ectr0chem1ca1 ce11 wa5 c0n5tant1y pur9ed w1th pur1f1ed Ar. 7he 1r1d1um w0rk1n9 e1ectr0de wa5 c1eaned 6y Ar 10n 60m6ardment 6ef0re each e1ectr0chem1ca1 treatment. UP5 mea5urement5 were perf0rmed 1n Krat05 E5 300 e1ectr0n 5pectr0meter e4u1pped w1th a d1fferent1a11y pumped he11um d15char9e 1amp. 7he UP5 data

E.R. K~t2, H. Neff / An0d1c 1r1d1um 0x1de f11m5

519

were rec0rded at n0rma1 em15510n u51n9 unp01ar12ed He 11 (40.8 eV) rad1at10n. 7he em15510n 0n5et 0f the He 1 5pectra determ1ned the re1at1ve ph0t0e1ectr1c w0rkfunct10n. 7he p051t10n 0f the Ferm1 1eve1 wa5 determ1ned f0r the c1ean 1r 5amp1e and a55umed t0 6e c0n5tant. Char91n9 0f the 0x1d12ed 5amp1e5 wa5 exc1uded 0n the 6a515 0f XP5 mea5urement5. 7he 6a5e pre55ure 0f the 5y5tem wa5 1n the 10-10 70rr ran9e,

3. Re5u1t5

7he u1trav101et ph0t0em15510n 5pectrum 0f a c1ean p01ycry5ta111ne 1r1d1um e1ectr0de after Ar 5putter1n9 15 5h0wn 1n f19. 1. 7he 5pectrum 15 1n 900d a9reement w1th pu6115hed data [14]. After exp05ure t0 the e1ectr01yte at 0.0 and

1

1

1

He 11(40.8 eV) [ r (p0ty)

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c1.e0n A 1.U

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1

2 1--

~=.~.5V~

2 U.1 1•1

0.75v

Ct 04--

1

t

15

10

81ND1N6

t29

5 ENER6Y

EF=0 / ev

F19. 1. Ph0t0em15510n 5pectra 0f a c1ean p01ycry5ta111ne 1r e1ectr0de and after emer510n at d1fferent p0tent1a15 fr0m 0.1M HC10 4. 7 h e t29 6and 0f the 0x1de and m01ecu1ar 0r61ta15 0f C10 4 are 1nd1cated. 7 h e d1fferent p0tent1a15 c0rre5p0nd t0 the f1r5t an0d1c 5can up t0 1.25 V and 6ack t0 0.0 V.

520

E.R. K0t2, H. Neff/An0d1c 1r1d1um 0x1de f11m5

0.5 V ver5u5 5CE the 0vera11 em15510n 15 reduced and a 6r0ad feature deve10p5 6etween 5 and 12 eV 6e10w the Ferm1 1evel 7he p0tent1a15 0f 0.0 and 0.5 V fa11 w1th1n the 50-ca11ed d0u61e 1ayer re910n where n0 5urface react10n5 5h0u1d 0ccur (5ee f19. 1 1n ref. [7]). 1rre5pect1ve 0f whether the 5pectrum 0f the c1ean 5urface 0r that 0f the e1ectr0de at 0.0 V ver5u5 5CE 15 taken a5 a reference, 519n1f1cant chan9e5 0ccur when the e1ectr0de p0tent1a1 6ec0me5 m0re an0d1c. F0ur c1ear1y d15t1n9u15hed em15510n max1ma deve10p at 61nd1n9 ener91e5 0f 1.5, 3.8, 6.5 and 11.1 eV f0r e1ectr0de p0tent1a15 0f +0.75 and 1.25 V ver5u5 5CE. When the e1ectr0de p0tent1a1 15 5canned 6ack t0 0.0 V, 5tart1n9 at 1.25 V, the feature5 deve10ped dur1n9 the an0d1c 5can are 5t111ev1dent, 6ut 5h1fted t0 h19her 61nd1n9 ener91e5 6y 0.7 eV (f19. 1, 60tt0m 5pectrum). 1n add1t10n a new max1mum 15 065erved at 10.0 eV. 7he 5harpne55 and he19ht 0f the Ferm1 ed9e 0f the under1y1n9 meta1 1nd1cate5 that the 5urface 1ayer 91v1n9 r15e t0 the add1t10na1 em15510n feature5 15 5t111 rather th1n. C0u10metr1c mea5urement5 [15] and XP5 re5u1t5 [16,17] 5h0w that the 0x1de 1ayer f0rmed 0n c1ean 1r at a p0tent1a1 0f 1.25 V r0u9h1y c0rre5p0nd5 t0 a m0n01ayer w1th a th1ckne55 0f 4-6 A 0n1y. 7he UP5 data 1n f19. 1 c1ear1y 1nd1cate a p0tent1a1 dependent c0mp051t10n 0f the e1ectr0de 5urface and the 1rrever5161e f0rmat10n 0f a 5urface 1ayer at an0d1c p0tent1a15. 7h1ck an0d1c A1R0F5 can 6e 9r0wn 6y p0tent1a1 cyc11n9 0f an 1r1d1um e1ectr0de 6etween - 0 . 2 5 and + 1.3 V ver5u5 5CE w1th a cyc1e fre4uency 0f 1 H2 [18]. 7h15 5tandard pr0cedure wa5 app11ed t0 the e1ectr0de5 1n the U H V c0mpat161e e1ectr0chem1ca1 ce11. 7he re5u1t1n9 f11m5 5h0wed the kn0wn e1ectr0chr0m1c effect and the c0rre5p0nd1n9 v01tamm09ram. UP5 re5u1t5 0f the e1ectr0de 1n the c010red (1.25 V) and 1n the 61eached (0.0 V) 5ta9e are 91ven 1n f19. 2 . 7 h e 5pectra c105e1y re5em61e th05e 06ta1ned f0r the very f1r5t cyc1e (f19. 1) 6ut the em15510n feature5, 0ccurr1n9 at 1dent1ca1 ener91e5 f0r a p0tent1a1 0f 1.25 V, are m0re pr0n0unced. A 5h1ft 0f the max1ma t0 h19her 61nd1n9 ener91e5 6y 1.0 •+ 0.1 eV 15 065erved f0r an e1ectr0de p0tent1a1 0f 0.0 V ver5u5 5CE. 7he den51ty 0f e1ectr0n1c 5tate5 at the Ferm1 1eve1 15 5tr0n91y reduced f0r the e1ectr0de at 1.25 V ver5u5 5CE and v1rtua11y 2er0 at 0.0 V. N0 c0ntr16ut10n fr0m the meta111c 5u65trate 15 065erved and c0n5e4uent1y the 0x1de th1ckne55 15 a60ve the e1ectr0n e5cape depth. 1n 0rder t0 rem0ve p055161e 5urface c0ntam1nat10n5 0r re51dua1 e1ectr01yte fr0m the emer5ed e1ectr0de, we have exp05ed the 5urface t0 m11d 5putter1n9 ( - 2 5). 1n a 5eparate exper1ment the e1ectr0de wa5 r1n5ed w1th pure H 2 0 after emer510n w1th1n the e1ectr0chem1ca1 preparat10n cham6er. 7he re5u1t1n9 5pectra, c0rre5p0nd1n9 t0 th05e 0f f19. 2, are p10tted 1n f19. 3 f0r the r1n5ed e1ectr0de5. 5h0rt 5putter1n9 (tw0 5ec0nd5, 4 keV, 2.5 mA) re5u1ted 1n 51m11ar 5pectra, 6ut tend5 a150 t0 reduce the 0x1de. 06v10u51y, r1n51n9 the emer5ed e1ectr0de w1th pure H 2 0 rem0Ve5 three 0f the f0ur em15510n max1ma f0und f0r the A 1 R 0 F 1n f19. 2. 7he max1mum at 1.6 eV 6e10w E v, h0wever, 6ec0me5 m0re pr0n0unced. A5 w111 6e d15cu55ed 1n

E.R. K0t2, H. Neff / An0d1c 1r1d1um 0x1def11m5

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F19. 2. Ph0t0em15510n 5pectra 0f a th1ck an0d1c 1r1d1um 0x1de f11m emer5ed at 1.25 and 0.0 V. 7he f11m wa5 9r0wn 1n the e1ectr0chem1ca1 preparat10n cham6er 6y p0tent1a1 cyc11n9 6etween - 0 . 2 5 and + 1.3 V.

7

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F19. 3. 5ame a5 f19. 2 6ut the e1ectr0de5 were r1n5ed w1th tr1p1y d15t111ed water after emer510n.

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t29

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EF=0 ( eV )

F19. 4. D1fference 5pectra f0r a th1ck an0d1c 1r1d1um 0x1de f11m emer5ed at 1.25 and 0.0 V: (a) d1fference 6etween 5pectra 0f f19. 2; (6) d1fference 6etween 5pectra 0f f19. 3. 7he p051t10n 0f the t29 6and, 0 H 0r61ta15 and C104 0r61ta15 are 1nd1cated.

m0re deta11 1ater, we a5519n the max1ma at 3.8, 6.5 and 11.1 eV and the hump at 14.6 eV t0 ad50r6ed C104 [22,23] wh11e the feature at 1.6 eV 15 a5519ned t0 the t29 6and 0f the 0x1de [25-27,30]. 7he d1fference5 6etween the 5pectra rec0rded f0r 1.25 and 0.0 V ver5u5 5CE w1th re5pect t0 em15510n 1nten51ty at the Ferm1 1eve1 and t0 the 5h1ft 0f the t29 6and are 4ua11tat1ve1y unaffected after r1n51n9 the e1ectr0de. 7he rema1n1n9 6r0ad feature5 1n the ener9y ran9e fr0m 3 t0 12 eV m05t pr06a61y 0r191nate fr0m 0 2p em15510n 0f the hydr0u5 0x1de. 7he 4ue5t10n ar15e5, whether the 1nf0rmat10n c0ncern1n9 the p0tent1a1 dependent chan9e5 0f the va1ence 6and 5tructure 15 5t111 ava11a61e after the re1at1ve1y 111 def1ned pr0cedure 0f r1n51n9. 7he an5wer 15 91ven 1n f19. 4 where the d1fference 5pectra c0rre5p0nd1n9 t0 f195. 2a, 26 and 3a, 36 are 91ven. 7he 4ua11tat1ve 1nf0rmat10n 0f 5pectrum 4a and 46 15 1dent1ca1 f0r 61nd1n9 ener91e5 c105e t0 E F. Chan91n9 the e1ectr0de p0tent1a1 fr0m 1.25 t0 0.0 V 91ve5 r15e t0 1ncrea5ed em15510n 0f the t29 6and at 2.5 eV and t0 a c0n51dera61e decrea5e 0f the den51ty 0f e1ectr0n1c 5tate5 at the Ferm1 1eve1. F0r h19her 61nd1n9 ener91e5 5pectrum 4a 15 d0m1nated 6y a der1vat1ve 11ke 5tructure due t0 the 5h1ft 0f the C104 em15510n feature5 w1th p0tent1a1, wh11e 5pectrum 46 5h0w5 tw0 add1t10na1 max1ma at 6.4 and 10.3 eV. 7he chan9e 1n w0rkfunct10n 0f the emer5ed e1ectr0de 15 p10tted 1n f19. 5 a5 a funct10n 0f the p0tent1a1 app11ed dur1n9 emer510n.

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ELEC7R0DE

F19. 5. w0rkfunct10n chan9e a5 a funct10n 0f e1ectr0de p0tent1a1. F111edc1rc1e5 refer t0 the f1r5t an0d1c 5can. A5ter15k den0te5 f1r5t cath0d1c 5can fr0m 1.25 t0 0.0 v. 0pen c1rc1e5refer t0 a th1ck an0d1c 0x1de f11m f0rmed 6y p0tent1a1 cyc11n9. 7he w0rkfunct10n chan9e5 are referred t0 the w0rkfunct10n 0f the c1ean 1r 5amp1e. W h e n a c1ean, 0x1de free 1r e1ectr0de 15 exp05ed t0 the e1ectr01yte at e1ectr0de p0tent1a15 1ncrea51n9 f r 0 m - 0 . 2 t0 + 1.5 V ver5u5 5 C E a 11near 1ncrea5e 0f the w0rkfunct10n 15 065erved after emer510n 6 e t w e e n + 0.25 a n d 1.5 V ver5u5 5 C E w1th a 510pe 0f 1.0 + 0.1. A t the cath0d1c e n d 0f the p0tent1a1 r a n 9 e the 5catter 0f the d a t a p01nt5 15 t00 1ar9e f0r re11a61e c0nc1u510n5. A f t e r exp05ure 0f the e1ectr0de t0 an an0d1c p0tent1a1 0f 1.3 V ver5u5 5 C E the 5u65e4uent cath0d1c 6ack5can t0 0.0 V d0e5 n0t rec0ver the 1n1t1a1 w 0 r k f u n c t10n a5 1nd1cated 6 y the tw0 a5ter15k5 1n f19. 5. F 0 r a th1ck 0x1de ( A 1 R 0 F ) , 9 e n e r a t e d 6 y the 5 t a n d a r d cyc11n9 p r 0 c e d u r e , a 11near re1at10n 6 e t w e e n e1ect r 0 d e p0tent1a1 a n d w 0 r k funct10n chan9e 15 06ta1ned a9a1n, 6 u t 5h1fted t0 h19her w0rkfunct10n5 6 y - 1 eV. 1n 0 r d e r t0 re501ve 5tep5 0r m1n0r dev1at10n5 f r 0 m 510pe 1 1n f19. 5, a 519n1f1cant reduct10n 0f d a t a 5catter1n9 w0u1d 6e nece55ary.

4. D15cu5510n 4.1. 6enera1 a5pect5 0 f emer510n 7 h e U P 5 5pectra 0f p01ycry5ta111ne A u 0r Pt e1ectr0de5 emer5ed at a p0tent1a1 w1th1n the d0u61e 1ayer re91me 5h0w n0 519n1f1cant d1fference5 c 0 m p a r e d t0 the 5pectra 0f the re5pect1ve c1ean meta1 [19]. 1n part1cu1ar, n0 em15510n feature5 0f H 2 0 c0u1d 6e d e t e c t e d [12]. 1n c0m61nat10n w1th the

524

E.R. K0t2, H. Neff / An0d1c 1r1d1um 0x1de f11m5

pre5ervat10n 0f the p0tent1a1 1nduced w0rkfunct10n chan9e [12,19,20] th15 re5u1t wa5 taken a5 ev1dence f0r the pre5ervat10n 0f the effect1ve e1ectr1ca1 d0u61e 1ayer up0n emer510n and tran5fer t0 the U H V 5y5tem at r00m temperature. 7 h e 5ma11 am0unt 0f c0unter 10n5 nece55ary t0 pre5erve the w0rkfunct10n chan9e 5tay 0n the 5urface, wh1ch can 6e 5h0wn 6y XP5 [19]. 7he water pr06a61y 105t dur1n9 emer510n and tran5fer 06v10u51y c0ntr16ute5 0n1y 11tt1e t0 the p0tent1a1 1nduced w0rkfunct10n chan9e. 7he 51tuat10n appear5 t0 6e d1fferent f0r 1r where the UP-5pectra 0f the e1ectr0de emer5ed at 0.0 and 0.5 V 5h0w d15t1nct em15510n feature5 at 61nd1n9 ener91e5 6etween 3 and 10 eV. 1n c0ntra5t t0 Au and Pt, the d0u61e 1ayer re91me, where n0 farada1c react10n5 0ccur, 1f at a11 ex15t1n9, 15 extreme1y narr0w 0n 1r [4]. 7hu5 the 065erved feature5 f0r emer510n p0tent1a15 0f 0.0 and 0.5 V m a y 6e c0rre1ated t0 the ear1y 5ta9e5 0f 0x1de f0rmat10n (M ---, M x ( 0 H ) 1n ref. [4]) 6ef0re p1ace exchan9e react10n5 0ccur. 7he except10na1 5ta6111ty 0f 0 H 9r0up5 0n an 1r(110) 5urface up t0 r00m temperature ha5 6een dem0n5trated u51n9 therma1 de50rpt10n after exp05ure 0f the 5amp1e t0 H 2 0 at 10w temperature5 [21]. 7he ad50rpt10n 0f C10 4 10n5 pr06a61y due t0 re51dua1 e1ectr01yte may 6e an a1ternat1ve exp1anat10n 0f the 065erved 5tructure5. A c0mp1ete under5tand1n9 0f the 0r191n 0f the5e em15510n freature5 de5erve5 further re5earch, prefera01y 0n 51n91e cry5ta1 5urface5. Wh11e the c0ntr16ut10n 0f c0unter 10n5 (C104) t0 the UP 5pectra 0f the 1r e1ectr0de emer5ed at 0.0 and 0.5 V 15 de6ata61e, the f0urf01d em15510n feature5 at 3.8 6.5, 11.1 and 14.2 eV can 6e c1ear1y a5519ned t0 perch10rate f0r emer510n p0tent1a15 0f 0.75 and 1.25 V. P051t10n a5 we11 a5 re1at1ve 1nten51t1e5 0f the f0urf01d 5tructure 15 1n 900d a9reement w1th pu6115hed data f0r L1C104 [22] 0r 1nterca1ated C104 [23]. A1th0u9h an 1ncrea5e 1n 5urface c0vera9e 0f ne9at1ve1y char9ed 10n5 15 expected up0n an0d1c p01ar12at10n 0f the e1ctr0de, the t0ta1 am0unt 5h0u1d 6e a fract10n 0f m0n01ayer 0n1y [12] and hard1y detecta61e 1n UP5. U p 0 n 1n5pect10n 0f f19. 1 1t 6ec0me5 ev1dent that an10n ad50rpt10n 15 d1rect1y c0rre1ated w1th the f0rmat10n 0f the an0d1c 0x1de. 7he em15510n max1mum 1.5 eV 6e10w E F, wh1ch 15 a5519ned t0 the t29 6and 0f the 0x1de [25-27,30], and th05e feature5 a5519ned t0 C10 4 appear and 9r0w 51mu1tane0u51y. 1n fact, p10tt1n9 the 1nten51ty 0f th05e feature5 due t0 C10 4 a5 a funct10n 0f the e1ectr0de p0tent1a1 revea15 a c0mm0n 0n5et p0tent1a1 f0r 60th, 0x1de f0rmat10n and an10n ad50rpt10n. 7heref0re, 1t can 6e 5tated 5afe1y that 0x1de f0rmat10n 0n 1r1d1um 15 d1rect1y a550c1ated w1th 1ncrea51n9 ad50rpt10n 0f C10 4 .

7he 0r191n 0f th15 effect, h0wever, 15 n0t c1ear. An 1ncrea5e 1n hydr0ph111c1ty 0f the e1ectr0de 5urface dur1n9 0x1de f0rmat10n may 91ve r15e t0 exce55 e1ectr01yte 0n the e1ectr0de 5urface after emer510n. 0 n the 0ther hand, an10n ad50rpt10n 0n 0x1de5 15 we11 kn0wn 1n ac1d1c e1ectr01yte5 [24]. 7hu5 the f0rmat10n 0f an 0x1de-an10n 5urface c0mp1ex may exp1a1n the 1ncrea5e 1n C10 4

E.R. K0t2, 11. Neff / An0d1c 1r1d1um0x1def11m5

525

ad50rpt10n. 7he 1atter a55umpt10n 15 5upp0rted 6y the fact that the 11near c0rre1at10n 6etween the e1ectr0de p0tent1a1 and the chan9e 1n w0rkfunct10n 15 pre5erved up t0 1.5 V where 519n1f1cant am0unt5 0f C104 are ad50r6ed 0n the 0x1de. 7he5e ne9at1ve1y char9ed 10n5 are pr06a61y needed 1n 0rder t0 c0mpen5ate f0r a reduced w0rkfunct10n 0f the e1ectr0de 5urface due t0 0x1de f0rmat10n. 7he 11near1ty 0f the w0rkfunct10n-e1ectr0de p0tent1a1 re1at10n even after 0x1de f0rmat10n and an10n ad50rpt10n - 5h0w5 the de11cate 6a1ance 0f d1fferent d1p01e c0ntr16ut10n5 at the e1ectr0de/e1ectr01yte 1nterface. Chan9e5 1n 5urface 1ayer c0mp05t10n 11ke 0x1de f0rmat10n, wh1ch w0u1d 91ve r15e t0 519n1f1cant chan9e5 0f the w0rkfunct10n at the meta1/vacuum 1nterface, are c0mpen5ated at the meta1/e1ectr01yte 1nterface 6y a rearran9ement 0f the d0u61e 1ayer5, the 0vera11 w0rkfunct10n 6e1n9 0n1y determ1ned 6y the app11ed p0tent1a1. 7he 5catter 0f data p01nt5 1n 0ur mea5urement5, h0wever, 15 t00 1ar9e 1n 0rder t0 re501ve p055161e f1ne 5tructure 11ke 5tep5 0r m1n0r chan9e5 1n 510pe. 7he kn0wn 1rrever516111ty 0f an0d1c 0x1de f0rmat10n 0n 1r1d1um 15 n1ce1y ref1ected 1n the UP5 data 0f f19. 1. A11 0f the em15510n feature5 065erved after the f1r5t an0d1c 5can up t0 + 1.25 V and a550c1ated w1th 0x1de f0rmat10n and an10n ad50rpt10n are 5t111pre5ent when the p0tent1a1 15 5canned 6ack t0 0.0 V. 7he 5ame 1rrever516111ty15 f0und f0r the w0rkfunct10n chan9e a5 1nd1cated 1n f19. 5 6y the tw0 a5ter15k5, when 5tepp1n9 the e1ectr0de p0tent1a1 6ack t0 0.0 V after the f1r5t an0d1c 5can. 0 n c e a th1ck 0x1de 15 f0rmed a 11near w0rkfunct10n-p0tent1a1 re1at10n 15 re5t0red, the mea5ured w0rkfunct10n 6e1n9 n0w 1 eV h19her than 0n the meta1 5urface. 7he 0r191n f0r the dev1at10n 1n mea5ured w0rkfunct10n 6etween 1n1t1a1 an0d1c 5can and the f0110w1n9 p0tent1a1 cyc1e5 15 n0t under5t00d. 1t m19ht 6e due t0 a 1055 0f a we11 def1ned reference a5 a c0n5e4uence 0f the f0rmat10n 0f a th1ck 0x1de 1ayer where the e1ectr0de/e1ectr01yte 1nterface 15 0n1y 6ad1y def1ned. A1ternat1ve1y, the p0r0u5 0x1de 1ayer may c0nta1n 50me re51dua1 e1ectr01yte after emer510n wh1ch c0ntr16ute5 t0 the 0vera11 w0rkfunct10n 1ndependent 0f the app11ed e1ectr0de p0tent1a1. 7he 1atter ar9umentat10n 15 5upp0rted 6y the fact that r1n51n9 0f the e1ectr0de5 w1th pure H 2 0 (5ee f195. 2 and 3) rem0ve5 c0unter-10n5 fr0m the e1ectr0de 5urface and 51mu1tane0u51y reduce5 the w0rkfunct10n 0f the A 1 R 0 F emer5ed at 0.0 and 1.25 V 6y 0.6 and 1.0 eV re5pect1ve1y. 1t 5h0u1d 6e kept 1n m1nd, h0wever, that the r1n51n9 pr0ce55 15 rather 111def1ned and may 1ntr0duce add1t10na1 c0mp11cat10n5. 4.2. Va1ence 5tate chan9e5 and e1ectr0chr0m15m

1n the f0110w1n9 we want t0 d15cu55 the 1nf0rmat10n 06ta1na61e w1th UP5 and w0rkfunct10n mea5urement5 a60ut va1ence 5tate chan9e5 and the e1ectr0chr0m1c effect 1n an0d1c 1r1d1um 0x1de f11m5 (A1R0F). 1n 0rder t0 pr0ve 0r d15pr0ve 6and the0ry m0de15 wh1ch have 6een 5u99e5ted 5h0rt1y after d15c0very 0f the e1ectr0chr0m1c effect 6y 60tte5fe1d [3] and 6y M020ta and C0nway [4]

526

E.R. K0t2, H. Neff/An0d1c 1r1d1um0x1def11m5

and wh1ch have 6een pu5hed recent1y 50 far a5 t0 exp1a1n the 5tructure 0f the v01tamm09ram [6], we have mea5ured the ener9y d15tr16ut10n curve5 after emer510n. 7 h e 5pectrum 065erved after emer510n 0f the A 1 R 0 F at 1.25 V and r1n51n9 (f19. 3) 15 1n 900d a9reement w1th kn0wn 5pectra fr0m 1r0 2 [25-27]. At 1.25 V the A 1 R 0 F 15 1n 1t5 c010red 5tate, exh161t5 a 900d e1ectr0n1c c0nduct1v1ty [10] and the meta1 at0m 15 6e11eved t0 6e ma1n1y 1n the tetrava1ent 0x1dat10n 5tate. 7 h e 10cat10n 0f the t29 6and 0f the A 1 R 0 F 1n the c010red 5tate 15 1.5 eV 6e10w E v and c0mpare5 we11 w1th data 0f R19a et a1. [26] and Dan1e15 et a1. [27] wh0 f0und 1.7 and 1.5 eV re5pect1ve1y. F0r the 0 2p em15510n 0f 1r0 2 max1ma have 6een determ1ned at 5.6, 7.8 and 10.6 eV [26] wh11e the 5pectrum 0f the A 1 R 0 F exh161t5 a max1mum at 4.6 eV and a 6r0ad 5h0u1der ar0und 9 eV. 7h15 d15crepancy, h0wever, 15 n0t t00 5urpr151n9 1n v1ew 0f the am0rph0u5 5tructure 0f the A 1 R 0 F and the fact that the f11m 15 6etter de5cr16ed a5 an 0xy hydr0x1de. 5w1tch1n9 the A 1 R 0 F fr0m the c010red (1.25 V) t0 the 61eached (0.0 V) 5tate 91ve5 r15e t0 519n1f1cant chan9e5 1n the UP 5pectrum 0f f195. 2 and 3 . 7 h e p051t10n 0f the t29 6and 15 5h1fted 6y 1.0 t0 2.5 eV 6e10w the Ferm1 1eve1. 51mu1tane0u51y, the den51ty 0f 5tate5 at the Ferm1 1eve1 15 reduced t0 2er0 f0r the A 1 R 0 F emer5ed at 0.0 V. 7he 10wer den51ty 0f e1ectr0n1c 5tate5 at the Ferm1 1eve1 f0r the f11m 1n the 61eached 5tate (0.0 V) a1ready pr0v1de5 a rea50na61e exp1anat10n f0r the 10wer e1ectr0n1c c0nduct1v1ty 0f the 61eached f11m and c0nf1rm5 the a55umpt10n5 u5ed f0r the 6and the0ry m0de15 [3,4]. Wh11e n0n-5t01ch10metry cann0t 6e ru1ed 0ut 0n the 6a515 0f XP5 [7] and UP5 re5u1t5, 1t 15 n0t a nece55ary a55umpt10n 1n 0rder t0 exp1a1n the d1fference5 1n f11m c0nduct1v1ty [10]. Further chan9e5 1n the va1ence 6and re910n 6ec0me 06v10u5 up0n 1n5pect10n 0f the d1fference 5pectra 1n f19. 4. F0r the f11m 1n the 61eached 5tate an 1ncrea5e 1n em15510n at 6.8 and 10.8 eV 6e10w E F 15 065erved. 7he5e feature5 are attr16uted t0 the M - 0 ~r and 0 H 0 0r61ta15 0f 0 H 5pec1e5 [28,29]. 51mu1tane0u51y, an em15510n 1ncrea5e 0f the t29 6and 15 065erved. 7 h e 1nf0rmat10n 06ta1ned 50 far 15 c0nden5ed 1n f19. 6 where the ca1cu1ated den51ty curve N ( E ) 0f 1r0 2 [30] 15 c0mpared t0 the mea5ured curve f0r an A 1 R 0 F . 7 h e p051t10n 0f the Ferm1 1eve1, the t29 6and and the h19he5t 1eve1 0f the 0 2p re910n 15 a150 p10tted a5 a funct10n 0f the e1ectr0de p0tent1a1. 7 h e a9reement 6etween the ca1cu1ated and mea5ured N ( E ) curve5 15 4ua11tat1ve1y 900d. 7he f1ne 5tructure 0f the ca1cu1ated curve, h0wever, 15 n0t re501ved. 1n part1cu1ar, the 5tructure w1th1n the t29 6and, wh1ch wa5 re501ved 1n the XP5 mea5urement5 6y R19a et a1. [26], 15 5meared 0ut f0r the m1cr0cry5ta111ne 0r even am0rph0u5 A1R0F. Chan91n9 the p0tent1a1 0f the A 1 R 0 F e1ectr0de re5u1t5 1n a 5h1ft 0f the Ferm1 1eve1 w1th re5pect t0 the max1mum 0f the t29 6and. At the cath0d1c end 1.e. f0r the 61eached f11m - the Ferm1 1eve1 fa115 w1th1n the 9ap 6etween the t29 and the % 6and, 91v1n9 r15e t0 the 5em1c0nduct1n9 pr0pert1e5 0f the f11m. -

E.R. K~t2, H. Neff / An0d1c 1r1d1um 0x1de f11m5 -4

>-21

[]

1

1

1

527

1

0--0--0~0~

t0 =. UJ

0

0-0--

0-0--0-0..0.0--

2

;[ ¢= 2

4 n0

6

1 N(E)

0--

1

0.0

,

1

1

1.0

P07EN71AL / V v5. 5 C E

F19. 6. C0mpar150n 6etween the ca1cu1ated EDC 0f 1r0 2 6y Matthe155 [30] and the mea5ured EDC f0r an an0d1c 1r1d1um 0x1de f11m at 0.0 V. A150 5h0wn 15 the p051t10n 0f the Ferm1 1eve1, the max1mum 0f the t29 6and and the h19he5t 1eve1 0f the 0 2p re910n a5 a funct10n 0f the e1ectr0de p0tent1a1. F0r the p10t 0n the r19ht the 61nd1n9 ener9y 15 re1ated t0 the vacuum 1eve1 6y tak1n9 1nt0 acc0unt the chan9e 1n w0rkfunct10n w1th e1ectr0de p0tent1a1.

Dur1n9 tran51t10n fr0m the 61eached t0 the c010red 5tate the Ferm1 1eve1 15 5h1fted 1nt0 the t29 6and thu5 ach1ev1n9 meta1 11ke c0nduct1v1ty. F1111n9 0r empty1n9 0f the upper part 0f the t29 6and 15 a c0n5e4uence 0f a chan9e 1n va1ence 5tate 0f the 1r at0m acc0rd1n9 t0 [7] 1r111(0H)3 ~ 1r1v0(0H)2 + e - + H + 7he add1t10na1 e1ectr0n f1115the t29 6and wh11e the add1t10na1 pr0t0n f0rm5 an0ther 0 H 60nd. Hence the va11d1ty 0f 51mp1e 6and the0ry m0de15 [31] 1n 0rder t0 exp1a1n the e1ectr0chem1ca1 perf0rmance 0f A1R0F5, 15 n1ce1y c0nf1rmed 6y the UP5 exper1ment5. C0rre1at10n 0f current max1ma 1n the v01tamm09ram 0f an A 1 R 0 F t0 5tructure5 1n the den51ty 0f e1ectr0n1c 5tate5 w1th1n the t25 6and [6], h0wever, 15 n0t rea115t1c. 7he5e 5tructure5 0ccur 0n1y f0r the ca1cu1ated N(E) curve5 and are 5meared 0ut f0r the an0d1c 1r1d1um 0x1de f11m5. 7he new 1n519ht 1nt0 the va1ence 6and 5tructure 0f an0d1ca11y 9enerated 1r1d1um 0x1de f11m5 pr0v1ded 6y UP5 mea5urement5 0n emer5ed e1ectr0de5 ena61e5 a 6etter under5tand1n9 0f the e1ectr0chr0m1c effect. Up t0 n0w the chan9e 1n c010r wa5 ma1n1y attr16uted t0 0pt1ca1 tran51t10n5 6ec0m1n9 p055161e 1n the c010red 5tate a5 a c0n5e4uence 0f n0n-5t01ch10metry. N0n-5t01ch10metry 15 h19h1y pr06a61e 0n the 6a515 0f kn0wn XP5 re5u1t5 [7,17]. 7 h e 4ue5t10n rema1n5, h0wever, what are the actua1 tran51t10n5 and 15 the a55umpt10n 0f n0n-5t01ch10metry nece55ary. 0 u r UP5 mea5urement5 5h0w that, de5p1te 0f 50me f1ne 5tructure, the

528

E.R. K0t2, 14. Neff / An0d1c 1r1d1um 0x1de f11m5

ener9y d15tr16ut10n curve5 are 1n 900d a9reement w1th th05e kn0wn f0r 1r0 2. 0pt1ca1 1nve5t19at10n5 0n 51n91e cry5ta111ne 1r0 2 have 5h0wn that a650rpt10n 1n an ener9y ran9e hu < 3.5 eV 15 e1ther due t0 Drude 11ke 6ehav10r, t0 1ntra6and tran515t10n5 0r t0 tran51t10n5 fr0m 10w f111ed 1eve15 t0 the Ferm1 1eve1, wh1ch fa115 w1th1n the t29 6and [32]. F1111n9 0f the t29 6and a5 a c0n5e4uence 0f a chan9e 1n va1ence 5tate fr0m 1r(1V) t0 1r(111) acc0rd1n9 t0 e4. (1) render5 a11 the5e tran51t10n5 1mp055161e. C0n5e4uent1y the f11m w111 appear tran5parent 1n the v15161e ran9e 0f the 5pectrum. W1th0ut u51n9 the ar9ument 0f n0n-5t01ch10metry the chan9e 1n c010rat10n 0f an A 1 R 0 F can 6e ea511y exp1a1ned 6y a 5h1ft 0f the Ferm1 1eve1 0f the 0x1de 91v1n9 r15e t0 an a1terat10n 0f the tran51t10n matr1x e1ement5 f0r ph0t0n ener91e5 hu < 3.5 eV. 4.3. 0x1de 9r0wth 7he UP5 re5u1t5 fu11y c0nf1rm th05e m0de15 f0r the an0d1c 0x1de 9r0wth 0n 1r dur1n9 cyc11n9 wh1ch were 5u99e5ted 6y 60tte5fe1d [3] and 6y M020ta and C0nway [4]. When 5tart1n9 fr0m a c1ean 1r e1ectr0de dur1n9 the f1r5t an0d1c 5can an 0x1de 1ayer 15 f0rmed f0r p0tent1a15 a60ve + 0.6 V. 7h15 15 ref1ected 1n the UP 5pectra 6y the appearance 0f the t29 6and (f19. 1). Dur1n9 the 5ucceed1n9 cath0d1c 5can the 1ayer 15 n0t reduced t0 the meta1 6ut 0n1y t0 50me hydr0x1de w1th a f111ed t29 6and (1a5t 5pectrum 1n f19. 1). 0w1n9 t0 the f111ed t29 6and the e1ectr0n1c c0nduct1v1ty 1n th15 1ayer 15 10w and 10n1c c0nduct1v1ty w111 d0m1nate. 7hu5 the app11ed p0tent1a1 dr0p w111 0ccur at the meta1/0x1de 1nterface and n0t at the 0x1de/e1ectr01yte 1nterface. Chan91n9 the e1ectr0de p0tent1a1 t0 an an0d1c va1ue a60ve + 1.0 V (5ec0nd cyc1e) w111 91ve r15e t0 tw0 pr0ce55e5. F1r5t, at the meta1/0x1de 1nterface meta111c 1r, 6e1n9 exp05ed t0 the e1ectr01yte v1a p0re5 w111 6e 0x1d12ed a5 10n9 a5 the f11m 15 1n the 10w c0nduct1v1ty 5tate. 5ec0nd, the reduced 0x1de w111 5w1tch t0 the c010red, h19h c0nduct1v1ty 5tate. Dur1n9 each 5ucceed1n9 an0d1c 5can there w1116e f0rmat10n 0f a new 0x1de 1ayer at the meta1/0x1de 1nterface f0110wed 6y an 0utward5 m0vement 0f the 60undary 6etween c010red and 61eached 0x1de. Pr0v1ded the e1ectr0chem1ca1 c0nd1t10n5 are appr0pr1ate dur1n9 p0tent1a1 cyc11n9 [18] each an0d1c 5can c0ntr16ute5 t0 the 1ayer 9r0wth 6y the f0rmat10n 0f a new 0x1de 1ayer at the meta1/0x1de 1nterface.

5. C0nc1u510n5

7he re5u1t5 0f 0ur 1nve5t19at10n 0f emer5ed 1r1d1um e1ectr0de5 u51n9 ex-51tu UP5 and w0rkfunct10n mea5urement5 have dem0n5trated the mer1t5 0f 5uch ex-51tu 5tud1e5 and the 5tr0n9 c0rre1at10n 6etween data 06ta1ned under U H V 20nd1t10n5 and th05e 06ta1ned w1th the e1ectr0de 1mmer5ed.

E.R. K~t2, H. Neff / An0d1c 1r1d1um 0x1de f11m5

529

0x1de f0rmat10n 0 n the meta1 e1ectr0de 0ccur5 at a p0tent1a1 0f 0.6 V ver5u5 5CE. 7h15 wa5 c0nc1uded fr0m the a p p e a r a n c e 0f the t29 6 a n d 1n the ener9y d15tr16ut10n curve5. 7 h e k n 0 w n 1rrever516111ty 0f 0x1de f0rmat10n 15 ref1ected 1n the U P 5pectra a5 weU a5 1n the w0rkfunct10n mea5urement5. A 11near re1at10n 6 e t w e e n the e1ectr0de p0tent1a1 app11ed 6ef0re emer510n a n d the w0rkfunct10n after emer510n 15 06ta1ned even at th05e p0tent1a15 where m0d1f1cat10n5 0f the e1ectr0de 5urface 0ccur. Chan9e5 0 n the 5011d 51de 0f the 1nterface, 11ke 0x1de f0rmat10n 0r va1ence 5tate chan9e5, are c0mpen5ated f0r 0 n the e1ectr01yte 51de thu5 pre5erv1n9 the effect1ve w0rkfunct10n. F0rmat10n 0f the an0d1c 0x1de a n d an10n ad50rpt10n are d1rect1y c0rre1ated. 7 h e c0nc1u510n wa5 6a5ed 0 n the 51mu1tane0u5 a p p e a r a n c e 0f the t 29 6 a n d a n d em15510n feature5 attr16uted t0 the C10 4 an10n. 1n the 61eached 5tate the t29 6 a n d 0f the 0x1de 15 f111ed wh1ch m0ve5 the Ferm1 1eve1 t0 the 6 a n d 9ap 6 e t w e e n t29 a n d e9 6and5. 7 h e 5h1ft 0f the Ferm1 1eve1 0f the 0x1de w1th e1ectr0de p0tent1a1 exp1a1n5 the e1ectr0chr0m1c effect a n d the chan9e 1n f11m c0nduct1v1ty. 7 h e U P 5 data, 1n c0m61nat10n w1th prev10u5 XP5 d a t a [1], 1nd1cate that the add1t10na1 p r 0 t 0 n enter1n9 the f11m 1n 1t5 61eached 5tate f0rm5 a n 0 H 6 0 n d .

Ackn0w1ed9ement5 7 h e expert techn1ca1 a5515tance 0f W. F0d1t5ch a n d P. U n t e r n ~ h r e r 15 9ratefu11y ackn0w1ed9ed. W e t h a n k Dr. 5. 5tuck1 f0r m a n y enc0ura91n9 d15cu5510n5.

Reference5 [1] [2] [3] [4] [5]

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