1996 J Chem Soc Ag Roughening

8/10/2019 1996 J Chem Soc Ag Roughening

1/7

Roughening

of

thin silver

films

in aqueous electrolytes

Sara A. Bilmes

I N Q U I M A E , Facultad de C iencias Exactas, Universidad de Buenos Aires. Ciudad Universitaria

Pab I I . 1428)Buenos Aires, Argentina

The reactivity of silver surfaces in aqueous electrolytes, i.e. corrosion and roughening, is studied by monitoring simultaneously

changes in resistivity and transmittance of thin silver films. Atomic-scale and large-scale roughening are discriminated by the

different sensitivity of the tran sm ittan ce and resistance responses. Results for different surface conditions produced on a thin silver

film by adsorption of water, pyridine and Cl- ions, as well as the potential dependence, are interpreted by considering that

adsorb ates induce roughening

of

the surface.

The morphology and texture of silver surfaces has attracted

much attention in connection with the use of surface-

enhanced spectroscopy for the resolution of adsorbate-

sub strate structures. Surface-enhanced Ram an scattering

(SERS), second h armo nic generation and fluorescence have

allowed one to probe the electronic and geometric configu-

ration of the surface complexes formed upon chemisorp-

tion.' ** These surface-enhanced phenomena require a metallic

substrate with some degree of roughness to support optical

resonances. Owing to their unique optical and electronic

properties, thin silver films are suitable substrates for surface-

enhanced processes either in ultra-high vacuum (UHV) or in

electrolytes, and for a variety of additional applications such

as solar

cell^ ^ ^

In aqueous electrolytes, the morphology and texture of

silver surfaces can be modified by changing the potential of

the silver/electrolyte interface by electrochemical method^.

The modification of the surface occurs as a consequence of a

variety of potential-dependent processes such as adsorption-

desorption of anions, oxidation-reduction of silver. In this

way, an initially smooth silver surface can be roughened by

dissolution and redeposition of metallic ions at different

surface site^.^-''

The surface of silver electrodes which generate SERS has

been extensively studied by scanning electron microscopy

(SEM)6,7and by scanning tunneling microscopy (STM).7-'o

Recent STM work reveals that electrochemically rough silver

electrodes exhibit a nodular structure after the oxidation-

reduc t ion of the ~urface ,~nd the neighbouring nodules col-

lapse in the further relaxation of the surface. In

situ

S TM

observations of silver electrodes also indicate that C1- adso rp-

tion strongly influences surface morphology. l o ,

Sm ooth an d rough surfaces in vacuum are characterized by

their different optical properties and different specific

resistivities. Bumps on rough surfaces generate changes in the

scattering and absorption cross-sections.' 2 , 1 3 Experiments

with Ag overlayers on sm ooth silver films demonstrate large-

scale modifications of surface plasmon polariton (SPP)

reso-

nances which are partially restored to the original shape by

warming to room temperature.14-16 Bumps on the surface

also produce an increase in the electron surface scattering,

increasing the resistivity of the sample.' 7- Fo r thin Ag over-

layers on smooth silver films the change in a simple Fuchs-

type specularity parameter indicates an almost perfectly

diffuse scattering of electrons at the surface. l Adsorbates

produce changes both in the UV-VIS spectrum and in the

resistivity of thin silver Th e latter has been

recently related to th e electron-hole pair da mp ing of the frus-

trated tra nslation of adsorb ates.22

Thin silver film electrodes exhibit different behaviour than

massive silver electrodes. As soo n as the m ean free pat h of the

conduction electrons is comparable to the film thickness, elec-

tron scattering at th e surface is the dom inan t effect in thin film

electrodes, the chemical reactivity of thin films is enhanced

when compared to that of the massive metal. One of the

major problems for the use of thin silver films in aqueous

systems is their chemical instability, through dissolution and

changes in the surface roughness. These corrosion effects may

be enhanced by anions or attenuated by organic molecules.

Changes in the resistivity of thin metal films in electrochemi-

cal interface^^^ ^^ reveal that adsorp tion of anions leads to the

reconstruction of the surface with increasing roughness,

accompanied by an increase in the surface re~ is tan ce .'~ his

effect can be avoided

if

an organic molecule such as pyridine

(Py) is first adsorbed on the surface to act as a corrosion

inhibitor.23 Th e study of th e massive silver electrode/

electrolyte interface by differential reflectance, electro-

reflectance, and attenuated total reflectance (ATR),25-28

reveals that the optical properties are dependent on th e poten-

tial, the electrolyte composition and the surface roughn ess.

In this work, the reactivity of silver surfaces in aqueous

electrolytes, i.e. corrosion and roughening, is studied. By mon-

itoring simultaneously the changes in resistivity and transmit-

tance for different surface conditions produced on a thin silver

film by ad sor ptio n of water, pyridine and C1- ions, as well as

the potential dependence of these parameters, it is possible to

describe the surface chemical processes taking place at the

interface. The sensitivity of transmittance and resistivity

towards surface roughness is discussed; this allows us to

separate atomic-scale and large-scale roughness co ntribution s

which, in tur n, are related to SE RS activity.

Experimental

Apparatus

The transmittance apparatus consisted of a deuterium-

halogen duplex lam p focused on the electrode with a concave

mirror in order to avoid chromatic aberrat ion. Transmitted

light was focused on a slit (also by concave mirrors) prior to

discrimination by a grating (250 lines mm -'), an d detected

with a d iode arra y (E G& G M 1412). All experiments w ere per-

formed with normal incidence. Simultaneous resistance mea-

surem ents w ere m ad e using the three-contact m e t h ~ d ~ ~ , ~

with a Wheatsto ne bridge and dc current.

The electrochemical apparatus consisted of a potentiostat

(PA R 173) and a scan g enerator (PAR 175). Th e electrochemi-

cal cell was designed with both adjustable counter electrode

J .

Chem.

SOC.,

Faraday Trans . , 1996,92(13), 2381-2387

2381

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2/7

(Pt) and reference electrode (saturated calomel electrode, SCE )

in order to optimize current-potential distribution dur ing

oxidation-reduction cycles. Fo r transmission experiments

these electrodes were kept away from the optical path. All

chemicals employed w ere p.a. or better and water w as bidistil-

led on q uar tz. Unless otherwise specified, the electrolyte was

pre-saturated with N, before contacting the silver film.

Preparation

of

thin silver film electr odes

Silver was evaporated at room temperature onto a circular

Suprasil optical substrate of 50 mm diameter at P

< 5

x

Pa. The substrate was cleaned by sonication in acetone,

immersion in hot alkaline KM n0,-satura ted solution, rinsed

in water and sonicated in acid H,O, solution. The substrate

was finally rinsed with water and exposed to water vapour.

This cleaning process ensures the total removal of organic

matter. The adhesion

of

silver onto this optical substrate is

good enough to make it unnecessary to improve adherence

with previous deposition of Cr . Before the condensation of the

working electrode film, 100 nm silver contacts were evapo-

rated. The probe was grown at 0.2 8, s - l for the first

10

nm

thickness and at 1 8, s - l up to the specified thickness, which

was typically 30-35 nm. Exp erimental results have a repro -

ducibility better than

lo ,

independent of the thickness of the

film over the above-mentioned range. Contact wires were con-

nected with silver-loaded paint and masked with Epoxigelb@.

The substrate-film assembly was then annealed at

60C

for

2

h to ensure the smoothness (on a macroscopic scale)

of

the

sample.

Results

The transmittance spectrum of a thin silver film in N, P 1

atm) exhibits an asymmetric peak a t 320 nm due t o the high

transmittance of the metal at the bulk plasma frequency,

up

as

shown in Fig.

l(a).

In this figure, the transmittance of the

cell without film T,) is taken as a reference. These kind of

films h ave a resistivity of ca. 2.5 n il m a t room temperature, in

good agreement wi th da ta in the l i t e ra t~re . ~ .~oth resist-

ance and transmittance are time-independent in N, atmo-

sphere.

Ag/water and AglO.1 mol dm-

KCI

at open circuit

The transmittance of a silver film exhibits a time-dependent

transmission spectrum when the cell is filled with

N,-

saturated water, as shown in Fig.

l(b).

The normalized trans-

mittance spectra, ATJT,

,

epresents the difference between the

transmittance at time

t

after addition of water

17;)

and that

corresponding to

1

min after the cell is filled with water

T ) .

This reference was chosen in order to monitor the changes in

the film, independent of the refractive index of the surround-

ing medium. The spectra dep icted in Fig.

l(b)

exhibit two defi-

nite regions: at il< 600 nm there is a decrease in

transmittance whereas transmitted intensity increases for

1 > 600

nm. The limit between these two regions,

ATJT

= 0,

is red-shifted with increasing time. In the low-wavelength

region a broad peak centred at

ca.

540 nm is noticed. A sta-

tionary situation is achieved after 60-90 min. Addition of 2

mol dm- KCl up to a

0.1

mol dm- final concentration pro-

duces further changes, and the system continues to evolve

with the above-described tren d [Fig. l(c)].

Dc resistance changes, AR/R, recorded simultaneously with

transmittance changes, indicate that, after contact with water,

there is an increase in the film resistivity with time (Fig.

2).

However, at longer times the resistivity is continuously

increasing, even though no significant changes in transmit-

0.6

kv 0 7

I=

0.2

I

800

700

600

500

LOO 300

r I

A g ( 3 S n m

H z o

* * l o t

0.1

0

I I

800

600

4

k

I

d

c) -

-

155 m i n ( 1 m i n )

O e 2 k

\8 9

m i n ( 3 5 m i n l

0.2

800 600 4

00

A h m

Fig. 1 a)Transmittance of a

30

nm silver film in

N,

,measured rela-

tive to the transmittance of the cell. (b) Time evolution

of

the relative

transmittance of the same film in contact with water. (c) Same

as 6) after the addition of KCl final concentration

0.1

mol dm-3).

Time in parenthesis are referred to the

KCI

addition.

A T / T =

W

- ~(O)I/T(O).

tance are noticed. The same results are obtained, with respect

to both transmittance and resistance, when the same experi-

ment is carried out in air-saturated water instead of N,-

saturated w ater.

2382

J .

Chem.

SOC.,

Faraday Trans., 1996, Vol . 92

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3/7

2.5

I

0.4

2.0

-

1.5

-

U

50 1 0 0 1 5 0 200 250

timelmin

Fig.

2 Relative resistance ARIR)

(-)

and relative transmittance

AT/T)

---), at

L

= 800 A) nd 400 nm

a),

f a 35 nm silver film

in

water. KCl is added at t

= 155

min.

a1

, , , I ,

Ag

30

myo.1mol dm-3 KCI

-2

-

0

- 1

2

- 3

- c

am 600 LOO

800

600 LOO

2nrn

Aglpyridine

A

thin silver film exposed t o the vapour of 0.1 cm3 of pure

liquid pyridine add ed to the N,-saturated cell exhibits a

change both in transmittance and resistance with a time

0.6

I

I

- 1 . 3 -1 .1 0.9 -0.7 -0.5 0.3 -0.1

EN

i=

a

800 600

L O O

AJnm

3.5 I I I 3.0

k u

- - - ,

1 6

32 48 64 80

tirne/min

Fig. 3 (a) Time evolution of the relative transmittance of a

35

nm

silver film exposed to pyridine vapour. (b) Relative resistance

ARIR)

(-)

and relative transmittance AT/T) ---), at L = 800 A) nd

400 nm

a),

f a

35

nm silver film exposed to pyridine vapour.

Fig. 4 (a)

Differential transmittance spectra

of

a 30 nm Ag thin film

in 0.1 mol dm-3 KCl, and in 0.1 mol dm- 3 KC1-0.04 mol dm-3

Pyridine. Spectra are recorded at the indicated potentials during a

potential sweep at

u

= 0.01 V s - l from - .3 to -0.2 V.

(b)

Relative

resistance change during a potential sweep,

u

=

0.01

V

s - ,

from

-

.3

to

-0.2

V and back to 1.3 V.

ATJT

= [ T E )- T -0.2

V)]/

T -

.2

V).

dependence similar to that described for liquid water. This

situation is shown in Fig. 3. The same trend has been found

for the resistance variation of annealed silver films during

exposure to Py a t 40 K in UHV.30

Although adsorption of pyridine on the bare surface modi-

fies both the resistivity and the transmittance of the sample, as

depicted in Fig. 3, the addition of pyridine to the Ag/H,O

stabilized interface did not produce significant changes. On

the other hand, the addition of pyridine inhibits the effect of

C l- ions show n in Fig. l(c). This effect has already been dis-

cussed by Korwer

et

aLZ3

AglO.1 mol dm-3 KCl under potential control

Tra nsm ittanc e spectra are influenced by the potential applied

to the interface, as can be seen in Fig. qa) . The difference

between the transmittance at potential E and that a t

E

=

-0.2

V,

ATE,

normalized to that at E

=

-0.2 V is

recorded during a potential sweep at 10mV s-' , starting from

-0.2

V. An absorption band at

380

nm is the main feature of

the relative transmission spectra with a small structure at 340

nm. As the interface is polarized to more negative potentials

the intensity of the main feature increases and the structure at

higher energy is more clearly defined. There are no significant

differences between the spectra taken with and without dis-

solved p yridine.

The relative resistance change of the same film is shown in

Fig. 4 b) in the potential range where the silver surface is not

oxidized ( - 0.2 =- E/V > - .3). For potentials more positive

J . Chem. SOC.,

Faraday Trans.,

1996, V o l .

92 2383

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4/7

0

- 2

4

- 6

=

- a

= o

e :

G?

? - 2

N

4

- 6

- 8

0.02

1

I

I

I

(4

Ag

35 nm)/O.l mot dmJ

KCI

+

0 02

mot dm Py

- -

Ag

35

m)/O.l mol

dm4

KCI

00

700 600

500

400 300

&nrn

Fig.

5 Transmittance spectra of a 35 nm silver film in

0.1

mol dm-3

KCI at - .2 V after ORC between - .2 and +0.2 V (TAR) relat ive to

that of the same film at the same potential before ORC (T R).

Q A

z

m C cm -2 .

than the potential of zero charge E p z cx -0.9 V) the adsorp-

tion of C1- induces an increase in t he resistivity of the film of

ca.

10

as the surface is positively charged, whereas for

potentials more negative than the pzc no resistivity changes

are observed. This result is coincident with that of Korwer et

01.23

0.01

I

1

0.011

0

-

0.0

0.02

800 700

600 500 400 300

Unm

r

1

I I

P

Ag 35 nm)/O.l mot

d m 3 KCI

+

0.02 mot dm3

Py

E= -O .8V

OL

I

I

0 10 20

3 0

40 5 0 60

Urnin

Fig. 6

(a)

Relative transmittance spectra of a 35 nm silver film in 0.1

rnol dm-3 KCI-0.04 mol dm-3

Py,

at -0 .8 V after a cathodic pulse

of 2 s to E

=

- .5 V TAg ) .

TBQ

s the transmittance at the same

potential before the cathodic pulse. b ) Relative resistance change in

the same conditions as a).

SERS active silver surfaces

Electrochemical roughening of the silver surface was per-

formed by oxidation-reduction cycles (OR C), sweeping the

potential between -0.2 V and +0.2

V

at 10 mV s-'. The

anodic charge involved in this process

(ca.

9 mC ern-,) allows

one to estimate that ca. 3 of the total Ag atom s are oxidized.

Com parison of anodic and cathodic charges during OR C

demo nstrates that Ag' ions are reduced back on to the film

during the cathodic sweep. This procedure is the usual one t o

obtain SERS-active silver surfaces in electrochem-

The relative change in the transmittance spectrum for this

process, TAR/ TBR is shown in Fig. 5. TAR is the transmittance of

the rougher surface generated by the ORC; T B R , the corre-

sponding value before the ORC. Immediately after the ORC

the ratio TAR/TBR exhibits a broad structure at

400

nm, and

2 decrease in transmittance for 2 > 600 nm. The band at

400 nm is still present even after 20-30 min O RC , whereas the

transmittance at lower energy is comparable to that of the

smooth surface throughout this time interval.

In SERS experiments a potential pulse at E

600

nm immediately after the ORC has also been found in

reflectance studies of massive silver electrode^,^^.^' and arises

from the change in optical constants of the surface by the for-

matio n of layers of [Ag(Py),]Cl. Th e brea thing m ode of Py in

this planar coo rdination com poun d is located at 1020 cm - ,

and is detected in SERS experiments at -0.2 V after the acti-

vation of the surface by ORC. Owing to the applied poten-

tial, a reduction process occurs:

[Ag(Py),]Cl

+

e

-+

Ago + C1-

+

2Py(ad)

(4)

increasing the transmitted intensity. In SERS experiments, the

1020

cm - band, characteristic of the [Ag(Py),]Cl complex

also decreases with time at -0.2

V . 3 5

The differences found for the transmission spectra between

adsorbate-induced and electrochemically roughened silver sur-

faces can be related to the m acroscopic roughn ess necessary

to p roduce SER S active surfaces. Roug h silver surfaces gener-

ated by a dsorb ed polar molecules can be thoug ht of as being

formed by randomly distributed hills and valleys with a broad

distribution of size and shape. These surfaces are not SERS

active. On the other hand, Ag films roughed by ORC are

SERS active, and exhibit nodular deposits of ca.

20-50

nm

diameter.6,7

SERS activity is mainly related to the existence of atomic-

scale roughness which is clearly monitored by resistivity

changes but not by optical methods. The different sensitivity

of resistance and transmittance is noticed in the relaxation of

the interface after the SE RS quenching pulse (Fig. 6). This per-

turbation of the surface produces a decrease in the resistance

due to digestion of small silver nuclei or clusters and/or

desorption of Py and C1-. This process involves short-scale

roughness chang es which are no t detected as differences in the

transmitted intensity. Moreover, the increase in resistance

after the quenching pulse is not accompanied by any change

in the transmittance spectrum, indicating that small clusters

are growing at the surface. These small nuclei of silver being

formed at sites onto a macroscopic rough surface are

undoubtedly associated with the recovering of SERS signal

after quenching.

Conclusion

Annealed thin silver films undergo roughening upon contact

with adsorbates. This effect is further enhanced by the pres-

ence of aggressive anions such as Cl-. This roughening is a

consequence of nucleation and growth of silver atoms at sites

on the surface with high reduction potential. Roughening of

the surface producing spheres with radius higher than some

critical value can be followed either by resistivity or transm it-

tance changes in the sample. Atomic-scale roughening can be

detected by the increase in the resistivity, the transmittance

being insensitive to these changes in the surface. The simulta-

neous monitoring of resistivity and transmittance allows one,

therefore, to demonstrate the operation of atomic-scale and

large-scale roughening as separate phenom ena.

This research project was financially supported by the Uni-

versity of Buenos Aires and the Fu nda cion An torchas.

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J . Chern.

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1996,

V o l . 92

2387

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Documents

1996 J Chem Soc Ag Roughening