Charge-Transfer Reaction Inverse Photoemission at Gold(111) and

Chapter 16

Charge-Transfer Reaction Inverse Photoemission at Gold(111) and Polycrystalline Silver Electrodes R. McIntyre, D. K. Roe, J. K. Sass, and H. Gerischer

Downloaded by CORNELL UNIV on July 28, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch016

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Federal Republic of Germany Emission spectra have been recorded f o r electron i n j e c t i o n i n t o Au and Ag s p h e r i c a l e l e c t r o d e s and hole injection i n t o Au(111) planar electrodes. These processes were brought about in solutions of a c e t o n i t r i l e c o n t a i n i n g tetrabutylammonium hexafluorophosphate (TBAHP), u s i n g t h e t r a n s - s t i l b e n e r a d i c a l anion as the e l e c t r o n i n j e c t o r and t h e thianthrene r a d i c a l c a t i o n as hole i n j e c t o r . The spectrum f o r the hole i n j e c t i o n process i n t o p l a n a r Au(111) e l e c t r o d e s has been r e s o l v e d i n t o the P & S - p o l a r i s e d components o f the emitted light. A comparison o f t h e s p e c t r a l d i s t r i b u t i o n o f emitted l i g h t f o r t h e above e l e c t r o n injection process, occurring a t both Au and Ag e l e c t r o d e s , with t h a t obtained f o r t h e emission process occurring a t the e q u i v a l e n t A1/A1 O /metal tunnel j u n c t i o n is reported. 2

3

The emission o f l i g h t from metal and semi-metal s u r f a c e s i n contact with a condensed phase c o n t a i n i n g charge acceptors o r donors has r e c e n t l y been t h e s u b j e c t o f s e v e r a l papers /1-5/. In t h i s work, we a r e going t o d e a l with three new aspects which have r e c e n t l y emerged. F i r s t , an experimental m o d i f i c a t i o n t o the o r i g i n a l c e l l geometry has been made, t o avoid an unusual edge e f f e c t caused by a lower c e l l r e s i s t a n c e a t t h e edge o f t h e e l e c t r o d e than a t the centre. The consequence i s a nonuniformity o f p o t e n t i a l which i s enhanced during transient changes of potential with electronic compensation f o r the IR drop i n s o l u t i o n . Second, with t h i s new modified c e l l geometry and hence b e t t e r d e f i n e d c o n d i t i o n s , we have r e s o l v e d the emitted l i g h t from t h e Au(lll) planar surface into P and S-polarised components. The enhanced emission o f P - p o l a r i s e d l i g h t has been a t t r i b u t e d t o d i r e c t sp t o d-band t r a n s i t i o n s o c c u r r i n g i n the 1-L d i r e c t i o n f o r p l a n a r A u ( l l l ) electrodes. Third, by comparing the s p e c t r a l d i s t r i b u t i o n of the emitted l i g h t f o r the electron injection process from e l e c t r o n i c donor states i n s o l u t i o n i n t o p o l y c r y s t a l l i n e g o l d and s i l v e r e l e c t r o d e s with the spectrum f o r the analogous process o c c u r r i n g a t 0097-6156/88/0378-0233$06.00/0 * 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

234

ELECTROCHEMICAL SURFACE SCIENCE

the A l / A l 0 3 / m e t a l tunnel j u n c t i o n /6/, we propose t h a t information concerning the shape of the d i s t r i b u t i o n curve which d e s c r i b e s the e l e c t r o n i c s t a t e s i n s o l u t i o n can be obtained. 2


EXPERIMENTAL Experiments were performed with a conventional three e l e c t r o d e e l e c t r o c h e m i c a l system c o n t r o l l e d by a PGp o t e n t i o s t a t (HEKA E l e c t r o n i k ) modified t o g i v e +25V output with a maximum c u r r e n t of 2A. The s i n g l e c r y s t a l A u ( l l l ) e l e c t r o d e was mechanically p o l i s h e d , o r i e n t a t e d and f i n a l l y e l e c t r o c h e m i c a l l y p o l i s h e d u s i n g a procedure d e s c r i b e d i n d e t a i l elsewhere /7/. The g o l d and s i l v e r spherical electrodes were produced by heating the r e s p e c t i v e wires i n a reducing flame. To a v o i d exposing the wire t o the charge i n j e c t i o n process, i t was s e a l e d i n t o g l a s s a t the p o i n t of contact with the sphere. The A u ( l l l ) c r y s t a l was s e a l e d i n t o a Kel-F h o l d e r u s i n g a s i l i c o n rubber gasket m a t e r i a l . The p l a t i n i s e d Pt mesh counter e l e c t r o d e was then h e l d f i r m l y i n p l a c e , a g a i n s t the Kel-F holder, d i r e c t l y opposite the A u ( l l l ) working e l e c t r o d e such t h a t the volume of e l e c t r o l y t e between the working and counter e l e c t r o d e s could be d e s c r i b e d as a cone with the counter e l e c t r o d e as base (A = 0.64cm ) and the working e l e c t r o d e as top (A = 0.125cm ). The s e p a r a t i o n between the two e l e c t r o d e s was ^3mm. This h o l d e r s u b s t a n t i a l l y decreases the edge e f f e c t . The Kel-F holder was l o c a t e d i n the g l a s s c e l l such t h a t the A u ( l l l ) e l e c t r o d e was i n l i n e with the r o t a t i o n a x i s . The Luggin c a p i l l a r y was p o s i t i o n e d between the working and counter e l e c t r o d e s by means of a 0.5mm diameter hole i n the Kel-F holder. 2

2

The s o l v e n t a c e t o n i t r i l e , the supporting electrolyte, TBAHP, and the r e a c t a n t thianthrene were p u r i f i e d by well-known procedures d e s c r i b e d i n d e t a i l elsewhere /8/. The r e a c t a n t t - s t i l b e n e (Fluka Gmbh) was r e c r y s t a l l i s e d twice from a methanol water mixture. The optical arrangement consisted of focusing lenses, a high efficiency Bausch and Lomb monochromator and a polarising f i l t e r . The e l e c t r o c h e m i c a l c e l l was mounted on an X, Y, Z manipulator with c a l i b r a t e d r o t a t i o n facilities (Fritz-Haber-Institut). The detection equipment has been d e s c r i b e d i n d e t a i l elsewhere /1,2/. RESULTS It

has

previously

been

shown

that

the

trans-stilbene


16.

McINTYRE ET AL.

Charge- Transfer Reaction Inverse Photoemission

235

r a d i c a l anion i s a very s u i t a b l e s p e c i e s f o r i n j e c t i n g e l e c t r o n s i n t o Pt e l e c t r o d e s , i n a c e t o n i t r i l e s o l u t i o n s , with excess energies of up t o 2.2eV /5/. F i g 1 shows the voltaitimogram f o r the r e v e r s i b l e r e d u c t i o n of t stilbene (10 M) a t a Au surface (the reversible potential U = -2.5V vs Ag/Ag ). The p o t e n t i a l range p o s i t i v e of the wave i s shown t o be almost p e r f e c t l y p o l a r i s a b l e t o +1.0V. 3

+


r

To c r e a t e the e x c i t e d s t a t e s we employed square-wave p o t e n t i a l modulation from -2.6V where the r a d i c a l anion i s produced t o v a r i o u s p o s i t i v e p o t e n t i a l s w i t h i n the p o l a r i s a b l e region. The o s c i l l o s c o p e t r a c e of the current-time and photon-time data are shown i n F i g 2, u s i n g e l e c t r o n i c compensation f o r about 90% of the c e l l r e s i s t a n c e and a modulation amplitude of 3.1V ( i e . -2.6V to 0.5V). We see c l e a r l y t h a t l i g h t i s produced only during the half-cycle when the radical anion is oxidised. During the negative h a l f - c y c l e p r o d u c t i o n of the t-stilbene radical anion occurs, on stepping p o s i t i v e t h e r e i s i n i t i a l l y no l i g h t s i n c e the time constant of the c e l l i s about 20/AS due t o the r e s i d u a l uncompensated r e s i s t a n c e . A f t e r 30-50 s, the d e s i r e d p o t e n t i a l change i s s u f f i c i e n t and we observe emission throughout the remaining p e r i o d of the p o s i t i v e c y c l e . On r e t u r n i n g t o the negative h a l f - c y c l e , the e x c i t a t i o n energy i s reduced and the photon p u l s e s disappear. The s p i k e s observed on the current-time curves are an unfortunate a b e r r a t i o n due t o the high impedance of the junction between reference and working electrode compartments. The curvature of the photon s p i k e s i s simply due t o the f i n i t e time constant a s s o c i a t e d with the inherent capacitance of the coax c a b l e and the l a r g e load resistor (10k Q ) necessary to register the p h o t o m u l t i p l i e r c u r r e n t on the o s c i l l o s c o p e . The s p e c t r a recorded f o r the e l e c t r o n i n j e c t i o n process by the t - s t i l b e n e r a d i c a l anion i n t o both Au and Ag s p h e r i c a l e l e c t r o d e s are shown i n F i g 3. The e x c i t a t i o n energy E i s d e f i n e d as ( U - U ) / e where Up i s the positive potential limit. For the s i l v e r e l e c t r o d e s U was l i m i t e d t o -0.1V t o avoid a p p r e c i a b l e d i s s o l u t i o n ox the e l e c t r o d e . Two observations can be made, both of which have p r e v i o u s l y been i d e n t i f i e d with the CTRIPS process /1-4/. F i r s t , s p e c t r a l peak v a l u e s ( E ) and the high energy t h r e s h o l d values ( E ) are observed t o shift t o lower energies with decreasing excitation energy. Second, the emission i n t e n s i t y i s observed t o f a l l with decreasing values of In t h i s case, however, we no longer observe the high energy l i g h t ( i e . photons with energies>0.5eV higher than expected from e x

p

r

R

s p

t h


236



Current/pA

\

\

Ja.opA

1

1 1

1

1**——

1

/z.s /

'

r

•

—

-1.0 P o t e n t i a l / V v s Ag/Ag+

Figure 1. The c u r r e n t - v o l t a g e r e v e r s i b l e reduction of t - s t i l b e n e sphere e l e c t r o d e (area = 0.125cm ) c o n t a i n i n g TBAHP (0.2M) f o r a sweep 2

curve f o r t h e (10~ M) a t a Au i n acetonitrile r a t e = O.IVs"" . 3


1

16. McINTYRE ET AL.

Charge-Transfer Reaction Inverse Photoemission


(e-injectlon) Positive half cycle

— | 20|is

— I

20«A

Negative half-cycle (anion production)

F i g u r e 2. Current-time and photon-time data f o r the e l e c t r o n i n j e c t i o n process by t - s t i l b e n e i n t o a Au sphere e l e c t r o d e . These data were obtained u s i n g e l e c t r o n i c compensation f o r the r e s i d u a l IR drop i n s o l u t i o n , with a modulation amplitude o f 3.1V ( i e . -2.6V t o 0.5V).


237

238


100


Inoise level

a 1x0.125)

1.85

2.05 2.25 2.45 2.65 2.85

3.05

Photon Energy lev) F i g u r e 3. CTRIPS s p e c t r a recorded f o r t h e system, metal/MeCN, TBAHP (0.2M) t - s t i l b e n e (0.07M) f o r t h e e l e c t r o n i n j e c t i o n process by the r a d i c a l anion i n t o a s p h e r i c a l Au e l e c t r o d e (dashed l i n e ) , area = 0.22cm , a) E = 2.6eV; b) E =2.6eV; c) E = 2.4eV and a s p h e r i c a l Ag e l e c t r o d e area = 0.125cm ; E = 2.0eV. d) e x * ) 2

e

2

E

=

2

2 e V

a

n

d

e

e x


Charge-Transfer Reaction Inverse Photoemission

16. McINTYRE ET AL.

239

the maximum excitation energy) when compensating electronically for the I R drop in solution, as p r e v i o u s l y reported /3,4/. Furthermore, with this p a r t i c u l a r geometry, the p r e c i s i o n with which E and E move as a f u n c t i o n of E i s much c l o s e r t o t h a t expected from simple energetic considerations, than p r e v i o u s l y observed /1-4/. t n


s p

e x

As a consequence of having performed the above experiments with s p h e r i c a l e l e c t r o d e s we suspected t h a t the o r i g i n of the high energy l i g h t , p r e v i o u s l y observed with p l a n a r e l e c t r o d e s , was a r e s u l t of a non-uniform p o t e n t i a l d i s t r i b u t i o n at the p l a n a r e l e c t r o d e due t o the lower s o l u t i o n r e s i s t a n c e at the unshielded edge compared t o the c e n t r a l area of the e l e c t r o d e . We t e s t e d t h i s hypothesis with a p l a n a r g o l d d i s c e l e c t r o d e evaporated onto a mica s u b s t r a t e , s i m i l a r t o t h a t used previously /2-4/. By having two redox couples benzophenone ( U = -2.0V vs Ag/Ag ) and t h i a n t h r e n e ( U = +0.95V vs Ag~/Ag ) present i n s o l u t i o n and then s e t t i n g the modulation l i m i t s t o -1.5V and +0.4V ( i e . 500mV away from the r e v e r s i b l e p o t e n t i a l of e i t h e r couple) we c o u l d not expect t o observe l i g h t from e i t h e r the electrogenerated chemiluminescence or inverse photoemission processes. However, by applying e l e c t r o n i c compensation we were able t o observe orange l i g h t o r i g i n a t i n g from the edges of the g o l d e l e c t r o d e . T h i s observation can be explained i f we c o n s i d e r t h a t f o r a given volume element at the edge of the e l e c t r o d e , the r e s i s t a n c e between working and counter e l e c t r o d e s i s s l i g h t l y l e s s than f o r the same volume element a t the centre, simply because more c u r r e n t paths e x i s t a t the edge than i n the c e n t r e . Therefore, d u r i n g the p o s i t i v e feedback process, used t o compensate f o r the I R drop, i t i s p o s s i b l e t o s u s t a i n higher p o t e n t i a l s a t the edge than i n the centre, thus exceeding the s e t p o t e n t i a l limits at the edge and hence giving rise to e l e c t r o g e n e r a t e d chemiluminescence a t the edges only. A q u a n t i t a t i v e a n a l y s i s which attempts t o e x p l a i n the high energy l i g h t reported e a r l i e r /3,4/ will be given elsewhere /9/. To avoid t h i s s i t u a t i o n with p l a n a r e l e c t r o d e s , i t i s necessary t o e l i m i n a t e the e x t r a c u r r e n t paths a v a i l a b l e at the edges of the e l e c t r o d e s . T h i s was achieved f o r the A u ( l l l ) e l e c t r o d e u s i n g the Kel-F holder d e s c r i b e d i n S e c t i o n 2. +

r

r

+

The s p e c t r a recorded f o r the hole i n j e c t i o n process by the thianthrene radical cation into the Au(lll) e l e c t r o d e are shown i n F i g 4, f o r three different e x c i t a t i o n energies. Once again, square wave modulation was employed with a f i x e d p o s i t i v e value of +1.0V where the c a t i o n r a d i c a l i s produced /4/ t o negative values



240


.23 5

5

Photon Energy lev) F i g u r e 4. CTRIPS s p e c t r a recorded f o r the system A u ( l l l ) / MeCN, TBAHP (0.2M), t h i a n t h r e n e (10" M) f o r the h o l e i n j e c t i o n process with excitation = 2.6eV; b) = energies, a) E = 2; Y ex = 3.0eV f o r an e l e c t r o d e area ex = 0.125cm' 2

e x

8e

a

n

d

C

)

E


16.

McINTYRE E T AL.


chosen t o g i v e the r e q u i r e d e x c i t a t i o n energy. The c h a r a c t e r i s t i c s h i f t s of E ^ and E t o lower energies with d e c r e a s i n g values of E are observed. Also, i n c o n t r a s t t o the e l e c t r o n i n j e c t i o n process, where l i g h t p u l s e s are observed i n the p o s i t i v e h a l f - c y c l e (see F i g 2) , we observed emission only d u r i n g the negative h a l f cycle i n accordance with expectations f o r a hole i n j e c t i o n process. F i g 5 shows an emission peak f o r Pp o l a r i s e d l i g h t from the A u ( l l l ) s u r f a c e . T h i s spectrum was obtained by r e s o l v i n g the emitted l i g h t i n t o P and S - p o l a r i s e d components f o r an emission angle = 30°. The P and S s p e c t r a were then normalised t o account f o r the wavelength dependent t r a n s m i s s i o n of the monochromator and the photo-response of the p h o t o m u l t i p l i e r tube, then s u b t r a c t e d t o g i v e the P - p o l a r i s e d emission spectrum. The emission peak i s centred a t about 2.1eV f o r an e x c i t a t i o n energy E = 3.0eV. t


e

e

l

x

x

x

DISCUSSION With t h i s new c e l l geometry f o r p l a n a r e l e c t r o d e s the t h r e s h o l d energy i s no longer dependent on the degree of e l e c t r o n i c compensation f o r the IR drop and always coincides closely with the excitation energy. Therefore, these s p e c t r a are more l i k e l y t o represent the t r u e j o i n t o p t i c a l d e n s i t y of s t a t e s f o r the system than those r e p o r t e d p r e v i o u s l y /1-4/. Consequently, t h i s data does merit more r i g o r o u s i n t e r p r e t a t i o n with r e s p e c t t o the s p e c t r a l d i s t r i b u t i o n of the emitted l i g h t and the p o l a r i s a t i o n dependence of the emission. The emission of P - p o l a r i s e d l i g h t , f o r h o l e i n j e c t i o n i n t o A u ( l l l ) e l e c t r o d e s , occurs predominantly i n the energy range 1.7 t o 2.5eV. I f we c o n s i d e r t h a t f o r o r i e n t a t i o n P the t r a n s m i t t i n g a x i s of the f i l t e r was p a r a l l e l t o the plane of the e l e c t r o d e but r o t a t e d 90° about the l i n e of o b s e r v a t i o n , then by r o t a t i n g the e l e c t r o d e t o make a f i n i t e angle ( i n our case 30°) with the normal, the emission of P - p o l a r i s e d l i g h t would be enhanced i f the source were a dipole radiating p e r p e n d i c u l a r t o the s u r f a c e of the g o l d e l e c t r o d e . I n s p e c t i o n of the bulk band s t r u c t u r e f o r Au /10/ shows t h a t d i r e c t sp t o d-band t r a n s i t i o n s , f u l f i l l i n g the above requirements, are p o s s i b l e i n the *1-L d i r e c t i o n with energies s i m i l a r t o those observed above. The p r i n c i p l e of the l i g h t - e m i t t i n g t u n n e l j u n c t i o n warrants p a r t i c u l a r mention because of the s i m i l a r i t i e s which e x i s t between the CTRIPS process f o r e l e c t r o n i n j e c t i o n and the emission process which occurs i n tunnel j u n c t i o n s / l l , 1 2 / . In the f i r s t case e l e c t r o n s


241

242


Au(lll)


eiission angle -30°

.7

P - polarised .5 .4 .3

1.55

_L _L _L 1.75 1.95 2.15 2.35 2.55 2.75 2.95

3.15

Photon Energy (ev) Figure 5. The P - p o l a r i s e d emission spectrum obtained f o r hole i n j e c t i o n by the t h i a n t h r e n e r a d i c a l c a t i o n i n t o A u ( l l l ) f o r emission angle = (NB. 30° and e x c i t a t i o n energy = 3.0eV. electrochemical c o n d i t i o n s as f o r spectrum C, F i g 4).


16. McINTYRE ET AL.

243

are i n j e c t e d from a d i s t r i b u t i o n o f e l e c t r o n i c s t a t e s i n s o l u t i o n i n t o a metal e l e c t r o d e with t y p i c a l e x c i t a t i o n energies o f 2.0-3.0eV and c u r r e n t s o f t h e order o f 100mA. In the second case e l e c t r o n s t u n n e l from one metal t o another with e x c i t a t i o n energies o f 2.0-3.7eV and t y p i c a l c u r r e n t values between 8-100mA. Data e x i s t s f o r A1-A1 0 tunnel j u n c t i o n s with top Au and Ag f i l m s /6/. The s p e c t r a obtained from these j u n c t i o n s , f o r t h e top e l e c t r o d e s b i a s e d 3.5V p o s i t i v e with r e s p e c t t o t h e aluminium, show high energy t h r e s h o l d v a l u e s a t about 3.5eV, which i s c o n s i s t e n t with t h e maximum e x c i t a t i o n energy. The spectrum f o r the Ag top e l e c t r o d e shows almost constant emission intensity down t o 1.5eV. S i m i l a r behaviour i s observed f o r the Au t o p e l e c t r o d e with t h e exception o f a s l i g h t d i p a t 2.5eV which i s r e p o r t e d l y due t o a Au interband t r a n s i t i o n . The CTRIPS s p e c t r a f o r e l e c t r o n i n j e c t i o n , shown i n F i g 3 f o r both Au and Ag, have high energy t h r e s h o l d v a l u e s which compare favourably with the maximum e x c i t a t i o n energy, however, the emission i n t e n s i t y i n both cases peaks and f a l l s o f f quite rapidly. T h i s i s expected s i n c e the e l e c t r o n i c s t a t e s i n s o l u t i o n a r e not continuous as they are f o r t h e metal injector. However, t h e exact d i s t r i b u t i o n f u n c t i o n which d e s c r i b e s t h e s t a t e s i n s o l u t i o n has been t h e s u b j e c t o f controversy f o r a number o f years. U n f o r t u n a t e l y , the e x i s t i n g t u n n e l j u n c t i o n data i s not d i r e c t l y comparable w i t h e x i s t i n g CTRIPS data because o f a miss-match o f t h e e x c i t a t i o n energies. However, by c a r r y i n g out CTRIPS and t u n n e l j u n c t i o n experiments, experimental i n f o r m a t i o n c o u l d be obtained t o compare with t h e o r e t i c a l d e s c r i p t i o n s o f t h e distribution function f o r the e l e c t r o n i c states i n solution. 2



3

REFERENCES 1.

R McIntyre and J K Sass, J E l e c t r o a n a l . Chem., 196,199 (1985)

2.

R McIntyre and J K Sass, Phys. Rev. L e t t . 56,651 (1986)

3.

R McIntyre, D K Roe, J K Sass and H G e r i s c h e r , Ber.Bunsenges. Phys. Chem. 91,488 (1987)

4.

R McIntyre, D K Roe, J K Sass and W Storck, J E l e c t r o a n a l Chem. 228,293 (1987)

5.

J Ouyang and A J Bard, J Phys. Chem. 91,4058 (1987)

6.

A Adams and P K Hansma, Phys. Rev. B, 23,3597 (1981)




244

7.

H Laucht, J K Sass, E 80,141 (1979)

Piltz

and H Neff Surface Sci.

8.

R McIntyre, B Smandek and H G e r i s c h e r , Ber. Bunsenges. Phys. Chem. 89, 78 (1985)

9.

D K Roe, R McIntyre, J K Sass and H G e r i s c h e r , t o be p u b l i s h e d

10.

N E Christensen and B O Seraphin, Phys. Rev. B 4,3321 (1971)

11.

J Lambe and S J McCarthy, Phys. Rev. L e t t . 37,923 (1976)

12.

T i e n - L a i Hwany, S E Schwarz and R K J a i n , Phys. Rev. L e t t . 36,379 (1976)

R E C E I V E D May 17, 1988


Charge-Transfer Reaction Inverse Photoemission at Gold(111) and

Recommend Documents