Probing UPD-Induced Surface Atomic Rearrangement of Polycrystalline Gold Nanofilms with Surface Plasmon Resonance Spectroscopy and Cyclic Voltammetry

Article abstract of DOI:10.1021/jp035851i

Silver underpotential deposition (UPD)-induced surface atomic rearrangement of polycrystalline gold nanofilms was probed with use of surface plasmon resonance spectroscopy (SPRs) as a novel probe tool in combination with cyclic voltammetry. Interestingly, upon repetitive electrochemical UPD and stripping of Ag, the surface structure of the resulting bare Au film is rearranged due to strong adatom-substrate interactions, which causes a large angle shift of SPR R-θ curves, in a good linear relationship with the number of UPDs, to a lower SPR angle. The n, K values of the surfacial Au monolayers before and after the repetitive Ag UPD and stripping for 27 times are found to be 0.133, 3.60 and 0.565, 9.39, respectively, corresponding to the huge shift of 1.61° to the left of the SPR minima. Cyclic voltammetry experiments in 0.10 M H₂SO₄ are carried out before and after the UPD treatment to examine the quality of the whole electrode surface and confirmed this change. To correlate the angle change in SPRs with the profile change in the cyclic voltammogram, the UPD treatment was also performed on a Au(111) textured thin film. It was therefore confirmed that the resonance position of the SPR spectrum is very sensitive to the surface crystallographic orientation of the bare Au substrates. Some surface atomic rearrangement can cause a pronounced SPR angle shift.

Full text of DOI:10.1021/jp035851i

J. Phys. Chem. B 2003, 107, 13969-13975
13969
Probing UPD-Induced Surface Atomic Rearrangement of Polycrystalline Gold Nanofilms
with Surface Plasmon Resonance Spectroscopy and Cyclic Voltammetry
Yongdong Jin and Shaojun Dong*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun, Jilin 130022, China
ReceiVed: June 28, 2003
Silver underpotential deposition (UPD)-induced surface atomic rearrangement of polycrystalline gold nanofilms
was probed with use of surface plasmon resonance spectroscopy (SPRs) as a novel probe tool in combination
with cyclic voltammetry. Interestingly, upon repetitive electrochemical UPD and stripping of Ag, the surface
structure of the resulting bare Au film is rearranged due to strong adatom-substrate interactions, which causes
a large angle shift of SPR R-θ curves, in a good linear relationship with the number of UPDs, to a lower
SPR angle. The n, K values of the surfacial Au monolayers before and after the repetitive Ag UPD and
stripping for 27 times are found to be 0.133, 3.60 and 0.565, 9.39, respectively, corresponding to the huge
shift of 1.61° to the left of the SPR minima. Cyclic voltammetry experiments in 0.10 M H2SO4 are carried
out before and after the UPD treatment to examine the quality of the whole electrode surface and confirmed
this change. To correlate the angle change in SPRs with the profile change in the cyclic voltammogram, the
UPD treatment was also performed on a Au(111) textured thin film. It was therefore confirmed that the
resonance position of the SPR spectrum is very sensitive to the surface crystallographic orientation of the
bare Au substrates. Some surface atomic rearrangement can cause a pronounced SPR angle shift.
Introduction
chemically evidenced by cyclic voltammetry. The n, K values
of the surfacial Au monolayers before and after the repetitive
Ag UPD and stripping for 27 times are found to be 0.133, 3.60
and 0.565, 9.39, respectively, corresponding to the huge shift
of 1.61° to the left of the SPR minima. It was therefore found
that the resonance position of the SPR spectrum is more sensitive
than cyclic voltammograms (CVs) of Ag UPD to the surface
crystallographic orientation of the bare Au substrates. Some
surface atomic rearrangement can cause a pronounced SPR angle
shift.
An understanding of structural transitions in nanocrystal
systems has sparked technological and fundamental interest
recently due to the ability to interrogate the structure-property
correlation and that the study of phase transitions can be greatly
simplified in these systems.1-3 For example, Hess et al.4 revealed
that apparently small changes in surface structure of CdSe
nanocrystals can have a dramatic effect on the coupling of
photoexcitations to surface states. Although surface effects are
prominent in nanosystems, few details are known about the
surface itself, to a great extent because of the lack of a suitable
model system and addressable in situ surface analysis tool,
which allows for a direct correlation of the structural charac-
teristics of the system with property measurements.
Surface plasmon resonance (SPR) spectroscopy is a powerful
surface-sensitive technique. SPR, as a probe of film dielectric
properties, film thickness, and morphology, has been used to
probe the film nanostructure. On the other hand, the Ag
underpotential deposition (UPD) system to date has been probed
with use of a wide variety of analytical techniques including
low-energy electron diffraction (LEED),5 atomic force micros-
copy (AFM),6-8 scanning tunneling microscopy (STM),7,9-11
and X-ray photoelectron spectroscopy (XPS).12 However, to the
best of our knowledge, no study by SPR spectroscopy has been
used for probing the UPD process to date.
Experimental Section
Chemicals. All aqueous solutions were made with deionized
water, which was further purified with a Milli-Q system
(Millipore). The following materials were obtained from Ald-
rich: H2O2, HAuCl4‚3H2O, trisodium citrate dihydrate, (3-
aminopropyl)trimethoxysilane (APTMS), hydroxylamine hy-
drochloride, and NaBH4. CH3OH (spectrophotometric grade)
was obtained from EM Sciences. K2Cr2O7, H2SO4 (double
distilled, 98%), Ag2SO4 (c(Cl-) < 0.02%), and glass microscope
slides (∼1.8 × 1.8 cm2) were obtained from China. All of the
chemicals, unless mentioned otherwise, were of analytical grade
and were used as received.
Preparation of SPR Substrates. The SPR-active substrates
were prepared according to our newly developed method,13
which is completely solution-based and vacuum-free. In brief,
carefully cleaned glass microscope slides were placed in a dilute
solution of (3-aminopropyl)trimethoxysilane (APTMS) (0.3 mL
of APTMS in 3 mL of CH3OH) for 12 h and rinsed with copious
amounts of CH3OH upon removal. The APTMS-modified glass
slides were subsequently immersed in colloidal Au solution (2.5
nm in diameter) for ca. 12 h for Au nanoparticle assembling.
Then the colloidal Au monolayers were rinsed with water and
In the present work, we use SPR as a novel probe tool in
combination with electrochemistry to study in situ the effect of
Ag UPD on the SPR response of the electroless plated gold
thin-film electrode. The transition of electronic information on
the surface is photonically transduced by using in situ SPR
spectroscopy and the surface atomic rearrangement is electro-
* Author to whom correspondence should be addressed. E-mail: dongsj@
ns.ciac.jl.cn.
10.1021/jp035851i CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/20/2003

13970 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Jin and Dong
immediately immersed in 6 mL of aqueous 0.4 mM hydroxy-
lamine hydrochloride and 0.1% HAuCl4‚3H2O. The solution was
agitated for 10 ( 0.5 min. After plating, the substrates were
rinsed thoroughly with copious amounts of water to completely
remove weakly adsorbed Chloride. After being immersed in
freshly prepared piranha solution (4:1 H2SO4:30% H2O2) for
ca. 2 min to remove any organic impurities, the as-prepared
gold films were rinsed again thoroughly with copious amounts
of water and dried under a nitrogen stream, then they were ready
for Ag UPD and SPR experiments. All the experiments were
performed at room temperature.
In Situ SPR Measurements. In situ scanning SPR experi-
ments were performed to assess the effect of the repetitive UPD
and stripping of Ag on gold substrates. For these experiments,
the piranha solution cleaned SPR-active substrate was pressed
onto the base of a half-cylindrical lens (BaK4, n ) 1.566) via
an index-matching oil. Linearly p-polarized light having a
wavelength of 650 nm from a diode laser was directed through
the prism onto the gold film in the Kretschmann configuration.
The intensity of the reflected light was measured as a function
of the angle of incidence, θ, using a photodiode with a chopper/
lock-in amplifier technique. The small, single-compartment,
three-electrode Teflon cell was mounted against the gold surface
of the substrate with use of a Kalrez O-ring, which provided a
liquid-tight seal and an electrolyte contact. The Teflon cell
allowed for the simultaneous recording of SPR and electro-
chemical data and the application of a voltage to the sample.
The electroless plated gold film on the glass slide was used for
the excitation of surface plasmon modes and also served as the
working electrode. A series of SPR R-θ curves were obtained
in situ before, during, and after the repetitive UPD and stripping
of Ag on the gold film substrate.
Figure 1. XRD pattern of (a) a typical electroless plated SPR-active
Au nanofilm (ca. 50 nm) and (b) a Au(111) textured Au thin film
prepared by thermal vaccum evaporation on a glass slide.
slightly below its resonance frequency (typically, 200-300 kHz)
and raster scanning across the surface.
Results and Discussion
A gold film of about 50-nm thickness prepared on a glass
slide by the wet-chemical method13a is used both as the working
electrode and for the excitation of surface plasmon modes in
the common Kretschman configuration. The as-prepared gold
surfaces are polycrystalline in nature13b and reproducible, which
was confirmed by X-ray diffraction characterization (Figure 1a).
In all experiments, solutions were purged and blanketed with
pure N2. A solution of 0.6 mM Ag2SO4 and 0.1 M H2SO4 in
deionized water (Millipore, 18.2 MΩ) was used to deposit the
Ag adlayer on gold by the UPD method. The repetitive Ag UPD
and stripping on a polycrystalline gold substrate was performed
at ambient temperature by cycling the potential between 0 and
+0.70 V (vs the KCl-saturated Ag/AgCl reference electrode)
at a scan rate of 20 mV/s for the desired time. The Ag adlayers
were removed before each SPR characterization by scanning
unilaterally from 0 to +0.70 V, and held at +0.70 V for ca. 5
s. All SPR experiments were performed at open circuit.
Upon UPD of Ag monolayers on the substrate, a decrease
(ca. 6.4%) in percent reflectivity, a 0.65° shift (to a lower angle)
in plasmon minimum, and an overall narrowness of the curve
are observed. Upon stripping of Ag adlayers from the Au
surface, the shape of the SPR curve is almost recovered.
However, the dip of the SPR curve still left-shifts pronouncedly
from 66.05° to a lower angle, 65.77°, after the UPD treatment,
as shown in Figure 2. Although more information, such as
reflectance oscillations during potential cycling, would be
recorded in the SPR R-t form when performed at a fixed angle,
our main concern herein is the influence of Ag UPD treatment
on the SPR response of the bare Au substrates, and thereby all
SPR signals were collected at open circuit potential to avoid
electrical interference and recorded in the R-θ form to simplify
the SPR data analysis. Interestingly, the dip left-shifts continu-
ously to a lower angle with the increasment of the Ag UPD
cycles, as shown in Figure 3A. The dip of the SPR curves left-
shifts as much as 1.61° after the repetitive Ag UPD and stripping
for 27 times. The inset in Figure 3A shows the effect of
repetitive Ag UPD and stripping on the SPR responses of the
resulting bare Au films. In this case, the angle shift of the SPR
curves shows a good linear relationship (R ) 0.993, N ) 7)
with the number of UPD treatments. The results were reproduc-
ible. Importantly, because the Au film electrodes were cleaned
by piranha solution before SPR measurements and all aqueous
solutions were prepared with deionized water, and no detectable
Electrochemistry Experiments. Electrochemistry experi-
ments were carried out with a PAR 370 electrochemical system
(EG&G, PAR; USA) and an X-Y recorder in a small, single-
compartment, three-electrode Teflon cell. The as-prepared SPR-
active gold thin-film substrate acted as a working electrode and
was pressed at the opening of the carefully cleaned Teflon cell
with a Kalrez O-ring (the apparent electrode area was 0.38 cm2).
A Pt wire counter electrode and a KCl-saturated Ag/AgCl
reference electrode were used. Electrochemical measurements
were all recorded and reported vs the KCl-saturated Ag/AgCl
reference electrode. In all experiments, solutions were purged
and blanketed with pure N2. A solution of 0.6 mM Ag2SO4 and
0.1 M H2SO4 in deionized water (Millipore, 18.2 MΩ) was used
to deposit the Ag adlayer on gold by the UPD method. The
repetitive electrochemical UPD and stripping of Ag on elec-
troless plated polycrystalline gold was performed by cycling
the potential between 0 and +0.70 V at a scan rate of 20 mV/s
for ca. 30 min. The Ag adlayer was then removed by scanning
unilaterally from 0 to +0.70 V, and holding at +0.70 V for ca.
5 s.
Before and after the repetitively electrochemical UPD and
stripping of Ag, electrochemical measurements at various basal
Au film electrodes in aqueous 0.10 M H2SO4 were carried out
to examine the quality of the whole surface. The potential was
cycled at the gold electrode between -0.2 and +1.6 V at 100
mV/s.
AFM Characterization. Surface images of gold film sub-
strates were acquired in the tapping mode under ambient
conditions (Nanoscope a; Digital Instruments, Inc.) Si3N4
cantilevers having integral tips (spring contant, 20-100 N/m)
were used. Images were obtained by oscillating the cantilever

Atomic Rearrangement of Polycrystalline Gold Nanofilms
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13971
Figure 2. In situ SPR spectra of the polycrystalline Au film recorded
in 0.6 mM Ag2SO4/0.1 M H2SO4 (aq) before (1) and after the UPD (2)
and stripping (3) of Ag one time.
Figure 4. CVs for the Ag UPD and stripping on polycrystalline Au
over a period of five successive scans. The scan rate is 20 mV/s. After
removing the Ag adlayers, CV running in Ag-free solutions exhibited
only a double-layer charging current (bottom).
gradually back to the right due to readsorption of organic
impurities and Cl-.
Cyclic voltammograms for Ag UPD on polycrystalline Au
film in 0.10 M H2SO4 over a period of 5 successive scans are
shown in Figure 4. It shows one set of adsorption (labeled A1)
and desorption (labeled A1′) peaks. The cyclic voltammogram
(CV) curve differs significantly from that of UPD on the Au-
(111) electrode and is somewhat similar to that of UPD on
polycrystalline Au both reported by Laibins.14 The difference
in the number and sharpness of CV peaks may be due to the
use of polycrystalline electroless Au film here and the purity
of chemicals. On the other hand, Laibins et al. found that trace
amounts of chloride in the electrolyte have a dramatic effect
on the repetitive UPD of Ag onto Au substrates. The increase
of Cl- ions due to accumulate on the electrode surface induces
a quantitative and reversible shift of the initial Ag UPD layer
to a more noble state by roughly 80 mV. In our case the
deposition peak (A1) at 0.395 V and the stripping peak (A1′) at
0.475 V almost persisted during the continuous cycling, except
for a fractional decrease in peak current due to some surface
smoothing caused by electrochemical annealing. This might
suggest that the cleanliness of the Au surface and the cell used
here and the influence of chloride is negligible.14 Although silver
UPDs on Au(111) terraces of a well-ordered electrode show
almost no defects nor surface alloy, as characterized by the
presence of a sharp intense UPD peak, Buess-Herman et al.15
recently reported that defects such as steps, kinks, and grain
boundaries can play a critical role in the initial stages of metal
deposition and tend to form a Ag-Au surface alloy via place
exchange processes between silver adatoms and gold atoms at
defect sites. The some broadness and asymmetry of peak may
reflect this process, and stem from the influence of the grain
feature (such as size distribution and edges) of the nanofilms
on the electrochemical behaviors and be due to overlapping of
another couple of peaks (labeled A2 and A2′), similar to the
case of Gewirth.16 As a control, CVs running over the entire
potential range in Ag-free electrolytes exhibit only double-layer
charging currents and no similar SPR angle shift was observed
after successively cycling for 27 times. The modification in
shape of SPR curves, as is representatively shown in Figure 5,
Figure 3. (A) In situ SPR spectra recorded in 0.6 mM Ag2SO4/0.1 M
H2SO4 (aq) after stripping of Ag adlayers: (1) 0, (2) 1, (3) 2, (4) 7, (5)
12, (6) 17, (7) 22, (8) 27 times. The inset shows the linear relationship
between the cycling number of UPD and the resulting SPR angle shift.
(B) Experimental angular scan curves (dotted lines) and corresponding
Frensel fitting results (solid lines) of the polycrystalline Au film
recorded in 0.6 mM Ag2SO4/0.1 M H2SO4 aqueous solution before
(curve 1) and after the UPD and stripping of Ag 27 times (curve 8).
The parameters used for Frensel fitting are listed in Table 1.
SPR angle shift occurred after incubation of the UPD-treated
Au electrode in the reaction solution for ∼30 min, the huge
shift of 1.61° to the left of the SPR minima cannot be the result
of the washing away of organic material and chlorine from the
Au film during UPD. Otherwise the SPR angle would shift

13972 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Jin and Dong
Figure 5. Control SPR spectra recorded before (1) and after (2)
successive cycling in 0.1 M H2SO4 blank solution (Ag-free) for 27
times.
is ascribed to some morphological improvement of the gold film
resulting from an electrochemical annealing during the repetitive
cycling process.14 We found that the electrochemical annealing-
induced modification of SPR curves occurred mostly within the
first several potential cyclings and no further pronounced
variation of the SPR response was observed. Such a change
would not result from the cleaning of the Au surface because
both the Au surface and cell were carefully cleaned just before
use and no impurity was introduced during the process. Indeed,
similar chemical annealing-induced variation of SPR responses
has been reported previously.17
Figure 6. CVs of polycrystalline Au film in 0.1 M H2SO4: (a) before
and (b) after the repetitive Ag UPD and stripping for 27 times. The
scan rate is 100 mV/s.
TABLE 1: The Parameters Used for Frensel Fitting of SPR
Angular Scan Curves
before UPD
treatment
after UPD treatment
for 27 times
Note that UPD is a surface-limited process and the Ag UPD
adlayer does not alloy with the underlying bulk gold. In addition,
as all SPR spectra were recorded under open circuit, the potential
dependence of the optical properties for bare gold SPR substrates
is negligible. No pronounced change in width and depth of the
spectra (Figure 3A), which are mostly a function of the refractive
index, n, and the thickness of the metal film, t, respectively,18
indicates that both the value of the refraction index and the
effective thickness of the gold film were hardly affected by the
repetitive Ag UPD treatment. It is noteworthy to mention that
Ag-Au surface alloying takes place, although mainly at
defective sites (such as grain edges)15,19 due to the polycrys-
talline and granular surface nature of the Au films. Indeed, some
broadness of the UPD peaks in Figure 4 may be indicative of
Au-Ag surface alloying at defective sites.19 However, all SPR
curves were recorded after complete removal of Ag adlayers
(confirmed by XPS characterization). Thus the Ag contamination
is negligible. There is therefore no doubt that, after the repetitive
UPD treatment, the physical properties, especially the extinction
coefficient, K, of the resulting bare Au film are changed, thus
causing the distinct position shift of the SPR angle.18 This in
turn implies that the extinction coefficient, K, is largely
dominated by the surface state of the substrate, since there is
no alternation in bulk properties during the UPD treatment.20
We also give a comparison of SPR experimental data calculated
with use of Fresnel equations. The fitted optical parameters n
and K of the metal films will be the best indicators of the metal
layer quality in terms of homogeneity and packing density. The
optical constants can be extracted by nonlinear least-squares
fitting to a four-layer (prism/bulk Au film/surface Au monolayer/
water) Frensel optical mode. It is reasonable to fit and set the
refractive index, n, and the extinction coefficient, K, of the bulk
Au films to be -0.133 and 3.60, respectively, and as constants
during further fitting because UPD is a surface-limited process
and the changes in the n-value not only cause a shift of the
layer
(λ ) 650 nm)
thickness/
thickness/
nm
nm
n
K
n
K
BaK4 prism
bulk Au film
surface Au
monolayer
water
1.566
0.133 3.60
0.133 3.60
0
1.566
0.133 3.60
0.565 9.39
0
41.5
0.4
39.0
0.4
1.33
0
1.33 0
minimum position of the SPR resonance, but also produce an
alteration in the shape of the spectrum.18 The n, K values of
the surfacial Au monolayers before and after the repetitive Ag
UPD and stripping for 27 times are found to be 0.133, 3.60
and 0.565, 9.39, respectively, corresponding to the huge shift
of 1.61° to the left of the SPR minima. The other parameters
used for Frensel fitting are listed in Table 1. Figure 3B shows
experimental angular scan curves and corresponding Frensel
fitting results of the polycrystalline Au film recorded in 0.6 mM
Ag2SO4/0.1 M H2SO4 aqueous solution before (curve 1) and
after the UPD and stripping of Ag for 27 times (curve 8).
Although the origins of changes in the extinction coefficient
and then the SPR angle shift during the rearrangement are not
yet clear, qualitatively we confirmed that some surface atom
rearrangement alters the surface physical properties (electron
distribution, orientation, packing density), and therefore alters
the optical properties of the metal nanofilms as revealed by the
optically sensitive SPR technique.
Direct evidence of some surface atomic rearrangement comes
from electrochemical characterizations. Electrochemical studies
of gold single crystals of various surface orientation have shown
that the shape of CVs is very sensitive to the surface structure
of the electrode,21,22 and proved to be a good method to examine
the quality of the whole gold film surface. Figure 6 shows CVs
of the polycrystalline bare Au film before and after 27 times of
successive Ag UPDs and stripping measured in 0.10 M H2SO4.
In all cases, the reduction wave at ca. +0.89 V is observed.

Atomic Rearrangement of Polycrystalline Gold Nanofilms
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13973
Figure 7. AFM phase images of the same polycrystalline Au film before (A) and after (B) the repetitive UPD and stripping of Ag 27 times.
The freshly prepared Au film exhibits only one broad oxidation
wave centered at ca. +1.36 V. In the control experiment, there
is no observable change in the shape after continuous cycling
in blank solution (Ag-free) for 0.5 h. Thus, it is reasonable to
assume that the influence of anions on the voltammetric change
is negligible. But, after the repetitive Ag UPD and stripping
for 27 times (ca. 0.5 h), the resulting basal Au film shows a
pronounced change in the anodic branch of the shape. The main
oxidation peak appears at +1.22 V, accompanied by a small
prewave as a shoulder at +0.99 V and a postwave centered at
ca. +1.35 V, which are quite similar to the CVs of the Au-
(111) single-crystal electrode. The first oxidation peak may be
a measure of the defect density of the Au(111) surface,22 and
the peaks at 0.99 and 1.22 V are completely consistent with
the formation of straight (100)×(111) edges. Although atomic
force microscopy (AFM) images with atomic resolution (such
as Au(111) terraces) were difficult to obtain due to the granular
morphology features of the film, the AFM phase images (Figure
7) show evidence of some surface smoothing and well-arranged
mound arrangement after the treatment. The rms roughness of
the same Au film before and after the UPD treatments is 4.3
and 4.0 nm, respectively. It was proposed previously15 that two
competitive processes are involved during the repetitive UPD
treatment: the Ag-Au surface alloying (a consequence of the
place exchange mechanism) and the self-annealing of the gold
surface (surface smoothing). From the electrochemistry point
of view, the voltammetric change provides evidence that the
repetitive UPD treatment does induce the surface atom crystal-
line rearrangement and thus results in changing SPR angles.
To correlate the angle change in SPRs with the profile change
in CV, we also performed the experiments using an evaporation-
prepared and anneal-treated Au(111) textured thin film instead
of electroless plated polycrystalline Au thin films. The nearly
Au(111) single-crystal nature of the textured thin film was
confirmed by X-ray diffraction characterization (Figure 1b). The
CV of the Ag UPD on the textured Au film shows a similar

13974 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Jin and Dong
on Au,15,23,24 surface atomic rearragement induced by UPD
treatment has not been probed by the electrochemical SPR
(ESPR) technique to date. In this study, we found that the
resonance position is very sensitive to the surface crystal-
lographic orientation of the bare Au substrates. Some surface
atomic rearrangement can cause a pronounced signal change.
At the same time, there are no significant changes in CV curve
recorded in the UPD system while cycling, although it is well-
known that CV of UPD Ag on Au single crystal terraces is
extremely sensitive to the surface structure of the metal. We
repeated the experiments and the results were similar. The CVs
of UPD Ag on electroless plated polycrystalline Au films show
no pronounced change whereas the SPRs do show an effect,
indicating that the surface atomic rearrangement herein is not
sensitive to discrimination by the CV curve of UPD and that
SPR is a more sensitive tool for probing surface atomic
rearrangement. Although SPR probes both bulk and surface
layers of gold nanofilms, the changes of SPRs herein mainly
reflect the variation of the superficial state of the substrate due
to the fact that UPD is a surface-limited process. In this case,
the transition of electronic information of the UPD-treated gold
surface is photonically transduced by using more sensitive SPRs.
The repetitive Ag UPD and stripping here act as “collective
atom tapping”, which can tap the surfaces of the polycrystalline
gold nanofilms gradually pulling out high-quality ones with
partially rearranged crystallographic surface and well-arranged
grain edges. This process is slow and varies from electrode to
electrode. After about 0.5-1 h of cycling, the SPR angle shift
terminated and no further rearrangements were observed by SPR
measurements, indicating the formation of a stable film in the
end. Although the detailed mechanism is not clear yet, from
the electrochemistry point of view, we assume that the strong
adatom-substrate (Ag-Au) interactions, such as defect-medi-
ated Ag-Au surface alloying and self-annealing of the Au
surface,15 during the repetitive Ag UPD treatment facilitate
mobility, and then surface atomic rearrangement of the super-
ficial Au atoms, and finally obtain a more energetic stable (111)
crystal orientation25 on the nanofilm surface. Detailed examina-
tions of the influence factors, such as scan rate and anions, on
the rearrangement behavior need further experiments.
Figure 8. CVs of evaporation-prepared Au(111) textured Au thin film
in 0.1 M H2SO4: (a) before and (b) after the repetitive Ag UPD and
stripping for ca. 0.5 h. The scan rate is 100 mV/s.
Figure 9. In situ SPR spectra of a evaporation-prepared Au(111)
textured Au thin film recorded in 0.6 mM Ag2SO4/0.1 M H2SO4 (aq)
(1) before and (2) after repetitive UPD and stripping of Ag for ca.
0.5 h.
Conclusion
We have investigated the effect of the repetitive UPD and
stripping of Ag on the electroless plated SPR-active polycrys-
talline Au thin-film electrode, using SPRs as a novel probe,
and found that the repetitively electrochemical UPD and
stripping of Ag have a dramatic effect on both the optical and
electrochemical properties of Au nanofilms, as evaluated by
using both in-situ SPR and electrochemical measurements. The
work reported herein is important for two reasons. First, the
Ag UPD-induced surface atomic rearrangement of polycrys-
talline Au nanofilms was reassessed in situ by using the novel
technique of SPR spectroscopy and cyclic voltammetry. Second,
we found for the first time that the resonance position of the
SPR spectroscopy is very sensitive to the surface crystal-
lographic orientation of the bare Au substrates. Some surface
atomic rearrangement can cause a pronounced signal change, a
phenomenon not observed before, to our knowledge. The n, K
values of the surfacial Au monolayers before and after the
repetitive Ag UPD and stripping for 27 times are found to be
0.133, 3.60 and 0.565, 9.39, respectively, corresponding to the
huge shift of 1.61° to the left of the SPR minima. The ability
of the method to process surface and to examine surface
reconstruction at unprecedented levels of detail is interesting
profile as Figure 4 (figure not shown). Figure 8 shows CVs of
the Au(111) textured Au film before and after successive Ag
UPD and stripping for ca. 0.5 h measured in 0.10 M H2SO4. In
this case, the freshly evaporation-prepared Au(111) textured Au
film exhibits three oxidation peaks centered at ca. 0.99, 1.22,
and 1.35 V, respectively, which is very similar to that of
electroless plated Au thin films after repetitive UPD treatment
for ca. 0.5 h. After the repetitive Ag UPD and stripping for ca.
0.5 h, the resulting basal Au film shows no pronounced change
in peak positions, except for some evolution in peak profile
caused mainly by morphology modification after the treatment.
At the same time, the dip of the SPR curves left-shifts only
∼0.30° after the repetitive Ag UPD and stripping for ca. 0.5 h,
as shown in Figure 9, which is much smaller than that of
electroless plated polycrystalline Au films. The results further
confirm that the repetitive UPD treatment does induce some
surface atom crystalline rearrangement and thus results in the
pronounced variation of SPR angles.
Although it has been known for many years that some surface
reconstruction and structural transformations occur as a conse-
quence of repetitive potential cycling in the UPD region of Ag

Atomic Rearrangement of Polycrystalline Gold Nanofilms
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13975
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from a fundamental point of view and can find applications in
electrochemistry and nanofilm community.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Nos. 20211130506 and
20075028) and we thank Mr. Y. Song for AFM measurements.
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