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Electrochemical seed-mediated growth of SERS-active Au (111)-like nanoparticles on indium tin oxide electrodes Jun-Gang Wang, Xiao-Wei Cao, Ling Li, Tian Li, and Rong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400633k • Publication Date (Web): 25 Jun 2013 Downloaded from http://pubs.acs.org on June 27, 2013
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Electrochemical Seed-Mediated Growth of SERS-Active Au
2
(111)-Like Nanoparticles on Indium Tin Oxide Electrodes
3
Jungang Wang, Xiaowei Cao*, Ling Li, Tian Li, Rong Wang
4
Department of Chemistry, Shanghai Normal University, Shanghai, 200234, P. R. China
5
Abstract
6
In this article, a novel electrochemical seed-mediated method has been proposed for the
7
fabrication of surface enhanced Raman scattering (SERS) active gold nanoparticle films
8
deposited on indium tin oxide electrodes. Firstly, due to the fact that a high overpotential and a
9
short time of potentiostatic transient are applied in the nucleation stage, the high controllability
10
with the density and the size of gold nanoparticles on an indium tin oxide (ITO) electrode are
11
realized. Secondly, due to the fact that AuCl4‒ ions will be preferentially reduced and deposited
12
on the previously formed Au seeds in a subsequent cyclic voltammetrical process, we achieve
13
the size-controlled gold nanoparticles which are enriched in Au (111) facet orientation (ca.
14
80.7%). With the growth of the nanopaticles, the gap between the adjacent nanoparticles
15
gradually decreases and thus plenty of ‘hot spots’ are formed on the substrate for SERS
16
applications. Thirdly, by using 4-Mercaptobenzoic acid as a probe molecule, the enhancement
17
factor of the as-prepared substrate is estimated as ~1.3×106, and its reproducibility and stability
18
for SERS measurements are also evaluated. These works reveal its great potential in routine
19
SERS applications for microanalysis. The protocol for the fabrication of such a SERS substrate
20
is facile and cost-effective, without introducing any template or surfactant.
21
Keywords: electrodeposition, Au (111)-like nanoparticles, O2 reduction reaction, surface
22
enhanced Raman scattering 1
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1. Introduction
2
Surface-enhanced Raman scattering (SERS) technique has been proven to be a powerful tool
3
in obtaining vibrational information about molecules on or near a nanoscale metallic surface and
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it has been a potential technique for non-destructive, sensitive detection of chemical and
5
biological molecules.1-5 For the super sensitivity of the method, it has the capacity to investigate
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samples in a single molecular level.6 Having been served as a molecular spectroscopic technique,
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SERS possesses many excellent merits including high sensitivity and negligible interference
8
from water. In addition, compared to infrared absorption spectroscopy, it does not require any
9
special sample preparation procedure, and this facilitates its effective applications in biomedical
10
sensing, environmental monitoring and analytical sciences.7-9 At present, the widely accepted
11
theory about the SERS mechanism involves electromagnetic (EM) enhancement and chemical
12
(charge transfer or CT) enhancement.10-15
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There are numerous studies on the fabrication of noble metal nanostructures as SERS active
14
substrates.16-19 The quantitative applications of SERS are still confined by the fabrication of
15
sensitive, reproducible, and stable SERS active substrates. It is critical and difficult to control the
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parameters for the fabrication of SERS active substrate that influencing the size, shape,
17
morphology of nanoparticles and the gap between them. How to optimize these parameters to 2
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produce high uniformity and reproducibility SERS active substrates with controllable metallic
2
nanostructures will be a main topic as for the improvement of SERS performance. The common
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methods include wet-chemical reduction to produce gold and silver colloidal suspensions,20
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electrochemical roughing,21 and physical vapor deposition.22 In recent years, the nanosphere
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lithography coupled to film over nanospheres (NSL-FON)4,5 and the electron-beam lithography
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(EBL)23,24 have been also adopted to fabricate high SERS active substrate with high
7
reproducibility. However, the barriers such as high cost, necessity for advanced equipments, and
8
time-consuming procedures will restrict the popularisation and application of these methods in
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the future. Nevertheless, there are a lot of other interesting methods such as layer-by-layer (LBL)
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assembly, nanoparticle self-assembly, and electrochemical deposition that have been investigated
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to fabricate high stable and sensitive SERS substrates.7,25-27 Utilizing electrochemical deposition
12
to fabricate metallic nanostructures is attractive for its versatility, facility, low cost, facile and
13
simple preparation, and easy accessibility. Several strategies have been proposed to fabricate
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metallic nanoparticles on conductive substrates, such as utilization of a template, or employing
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double potential pulses, or employing a cyclic voltammetry scan.28,29,30 However, no paper about
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electrochemical deposition of gold nanoparticles by potential step followed by cyclic
17
voltammetry has been found available on known publications.
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Herein, we demonstrate a novel electrochemical seed-mediated method for the fabrication of
2
SERS active Au (111)-like nanoparticle array on an ITO electrode. This method improves
3
effectively the monodispersion of AuNPs in comparison with the Frens’ method, and it is a
4
valuable tool to control conveniently the growth of surface-confined AuNPs.31 By using the
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potentiostatic transient technique in the nucleation stage, i.e. a high overpotential is applied to a
6
bare ITO electrode for a short time; a great deal of gold seeds will be formed on the ITO
7
electrode with high monodispersity and homogeneous morphology. Then, due to the property of
8
preferential deposition and reduction of AuCl4− ions on the existing Au seeds, the size of the
9
AuNPs will consequently become large and the desired gaps between adjacent nanoparticles will
10
be obtained by utilizing a subsequent cyclic voltammetry with an adequate deposition time. It is
11
convenient for us to control reasonably the AuNPs gaps and size (expressed as a statistically
12
averaged value), which is essential for the emergence of ‘hot spots’ and is also critical for a high
13
SERS activity. In our conditions, we fabricate the gold nanoparticles array with controlled
14
particle size and density, tunable LSPR property, and enrichment in Au (111) facet orientation
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(ca. 80.7%). The as-prepared substrate shows relatively high sensitivity and stability in SERS
16
measurements. Moreover, the procedure is simple and low cost without using any linker
4
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molecules, or templates, or surfactants. These characteristics endow it with the tremendous
2
potential for routine SERS applications and LSPR sensing.
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2. Experimental Section
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2.1.
Materials
5
ITO-coated glass (sheet resistance 20-30 Ω sp-1.; 1 mm thickness; size, 7 mm × 4 mm) was
6
purchased from Shenzhen Laibao Technology Co. Ltd (Shenzhen China). Hydrogen
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tetrachloroaurate (HAuCl4) and sodium perchlorate (NaClO4) are purchased from Sinopharm
8
Chemical Reagent Co., Ltd (Shanghai China). 4-Mercaptobenzoic acid (p-MBA) was purchased
9
from Sigma-Aldrich Co. All reagents were analytical grade and used as received without any
10
further purification. Ultrapure water, filtered by a Milli-Q reagent water system at a resistivity
11
of >18 MΩ·cm, was used throughout the experiments. The ITO-coated glass was cleansed by
12
ethanol first, and then it was washed successively in acetone, isopropanol, pure water for at least
13
20 min with sonication. Last, it was dipped in a mixture of pure water, hydrogen peroxide and
14
ammonium hydroxide (volume ratio, 5:1:1) and boiling it for at least 30 minutes.22 It can be used
15
in the following procedure after drying with nitrogen gas.
16
2.2.
17
2.2.1. Electrochemistry
Instrumentation
18
All electrochemical experiments were performed with a CHI750B electrochemical
19
workstation (CH Instruments Inc.) in a conventional one-compartment cell in conjunction with a
20
standard three-electrode system. Experiments were carried out with NaClO4 as the supporting 5
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electrolyte, and if necessary the solutions were purged with nitrogen 20 min before
2
electrochemical experiments. Moreover, a nitrogen atmosphere was kept over the solutions.
3
2.2.2. UV-vis Spectrophotometer
4
UV-vis absorption spectra were recorded by using a 760-CRT double beam UV-vis
5
spectrophotometer (Shanghai Precision and Scientific Instrument Co. Ltd., China) and using a
6
quartz cuvette of 1cm width. The recorded wavelength range was from 350 to 800 nm against
7
bare ITO-coated glass as the reference.
8
2.2.3. Field Emission Scanning Electron Microscope
9
Field emission scanning electron microscopy (FESEM) images were obtained by using
10
Hitachi S-4800 Field emission scanning electron microscopy (Hitachi, Ltd., Japan) at
11
accelerating voltage 15.00 KV and using an in-lens ion annular electron detector.
12
2.2.4. X-Ray Diffraction
13
X-ray diffraction (XRD) analysis was carried out by D-MAX2000 X-ray powder
14
diffractometer (Rigaku Co., Japan) with a Cu anticathode (40 kV, 30 mA). The range of the
15
data are between 10° and 90°, with a step angel of 0.01°
16
2.2.5. Raman Spectroscopic Apparatus
and a scan rate of 0.15° min-1.
17
Raman spectroscopic measurements were performed by using a confocal microscopic Raman
18
system (SuperLabRam II, Dilor, France). A liquid nitrogen-cooled 1024×800 pixels
19
charge-coupled device was used as a detector and an exciting line of 632.8 nm was supplied by a
20
He-Ne laser with power ca. 5 mW. A 50× long-working-length objective was used for focusing 6
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the laser spot onto the substrates surface. The slit and pinhole are set at 100 and 1000 µm,
2
respectively. Each spectrum was measured three times and the acquisition time was 15 s. The
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Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer.
4
2.3.
5
2.3.1.
Electrode Modification Au Nucleation
6
A saturated calomel reference electrode (SCE) was used as the reference electrode and a Pt
7
foil is used as the counter electrode. The ITO slides (geometry area ca. 0.28 cm-2, the skimmed
8
tape was wrapped around the slides leaving a certain area), which were obtained according to the
9
above method, serve as the working electrode for all electrochemical experiments. All the
10
potentials were reported in this article with respect to the SCE. The deposition solution contains
11
0.1 mM HAuCl4 and 0.1 M NaClO4. After being applied a potential step from +0.89 V to –0.8 V
12
(vs. SCE), the ITO electrode showed a great change as to the open circuit potential which varies
13
from 0.74 V to 0.89 V. It was demonstrated that the Au seeds had been formed on the surface of
14
the electrode. In fact, its subsequent cyclic voltammogram and the FE-SEM images could
15
provide strong evidence for the nucleation of AuNPs on the bare ITO electrode under such a
16
circumstance.
17
2.3.2. Au NPs Growth
18
The as-prepared AuNPs array deposited on an ITO electrode (signified as AuNPs/ITO
19
electrode) was used as the working electrode for the growth of AuNPs in the same
20
electrochemical cell. The growth of AuNPs was performed by cyclic voltammetric mode at room
21
temperature 25 ± 2 ℃. The potential range from +0.3 to ‒0.04 V was applied for different 7
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cycles at 50 mV/s. Afterward, the working electrode was removed from the deposition solution
2
and rinsed copiously with ultrapure water. Then the as-prepared AuNPs/ITO electrode was dried
3
under a gentle stream of nitrogen before its utilization. Otherwise, it will be stored in H2O at 4 ℃
4
when it is not being immediately used.
5
2.3.3. Preparation of SERS-Active Substrates
6
The AuNPs/ITO electrodes with different deposition cycles n (n=0, 50, 150, 300, 500) and a
7
bare ITO electrode were immersed into 1 mM 4-mercaptobenzoic acid (p-MBA) ethanol solution
8
for 24 hr. Residual or excess of p-MBA was removed by rinsing the electrodes with ethanol for
9
three times, and then with copious amounts of ultrapure water. Before SERS measurement, the
10
electrodes were dried under a gentle stream of nitrogen.
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3.
12
Results and Dicussion 3.1.
Electrochemical Deposition of AuNPs on ITO
13
Our aims here were focused on controlling the size, density, and crystal orientation of the
14
AuNPs electrodeposited on a bare ITO electrode to fabricate high quality SERS-active substrate.
15
The factors like the overpotential, the concentration of electroactive species and the
16
electrodeposition time (nucleation time and growth time) had great influence on the
17
characteristics of the electrodeposit such as granularity, thickness, preferential crystallographic
18
orientation.32 For the sake of simplicity, our experiment was carried out in 0.1 M NaClO4 + 0.1
19
mM HAuCl4 which was significantly lower than those used in previous studies, and the seeding
20
overpotential only used one value (the potential is stepped from OCP 740 mV to 890 mV vs.
21
SCE).33 The effect of seeding time and growing time on the geometry and crystallography of 8
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resultant AuNPs would be investigated in this article. The electrochemical deposition method
2
used here included two steps, as schematically shown in schematic S1 (Shown in Supporting
3
Information). First, a short nucleation pulse of high cathodic polarization (En) was applied on a
4
bare ITO electrode. Then, the resultant AuNPs/ITO electrode, in which the AuNPs formed in
5
first step were regarded as gold seeds for further growth in the following step, was carried out in
6
a specific potential range (Eg1~Eg2) with relatively low overpotential and different voltammetric
7
cycles. This procedure was essentially different from the traditional double pulse method in
8
which a much longer growth pulse at low overvoltage followed after a nucleation pulse.34,35 It
9
was obvious that as for the growth of AuNPs, a variable driving force was used here instead of
10
one constant driving force used in the traditional double pulse procedure. This change would be
11
verified to produce great influence on the structure of AuNPs deposited on ITO electrodes.
12
Figure 1a illustrated a cyclic voltammogram (CV) of a bare ITO electrode measured from 1.4
13
to -0.2 V at 50 mV/s in a solution of 0.1 mM HAuCl4 in 0.1 M NaClO4. We observed a reduction
14
peak of AuCl4- at around +0.35 V on the first cyclovoltammetric scan. The corresponding
15
reaction occurring on the surface of a bare ITO electrode was shown here36:
16
AuCl4- + 3e-
Au0 + 4Cl-
17
Due to H2 evolution by reduction of water, there was a second cathodic peak at around ca.
18
-0.2 V. In the reverse scan, there was a cathodic current flowed until ca. 830 mV which
19
represented the reduction of AuCl4- to Au0 and the reductive current was lower than the current
20
in the first scan. Due to the nucleation and growth of the nuclei, it was clear that the reduction of
21
AuCl4- occurred at about 780 mV on the forward scan and remained up until 830 mV in the
22
subsequent scan. Beyond that, in the subsequent second scan the reduction peak potential was
E0= 0.994 V vs. NHE
(1)
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approximately 670 mV and it would bring great effect on the further reduction of AuCl4- on the
2
Au seeds formed on the bare ITO electrode. As for a bare ITO electrode, it was necessary for the
3
formation of gold nuclei on it to overcome an energy barrier by applying a relatively high
4
overpotential. Once the inoculating crystal was formed, the formation of Au seeds would
5
catalyze the reduction of AuCl4- and make it thermodynamically more facile, thus the reduction
6
would occur at a more positive potential (830 mV). Therefore, a characteristic ‘nucleation loop’
7
which signified metal nucleation and growth could be observed in the first scan.
8
Figure 1b represented the cyclic voltammograms for the ITO electrode modified with Au
9
seeds (signified as Au-seeds/ITO electrode) in a solution of 0.1 mM HAuCl4 + 0.1 M NaClO4.
10
There was no an anodic current ‘crossover’ in the cyclic voltammogram due to the presence of
11
Au seeds. Moreover, the reduction peak potential (730 mV) for it shifted along the positive
12
direction by ca. 380 mV in comparison with that for a bare ITO electrode. This behavior was
13
very similar to that for a fluorine-doped tin oxide (FDTO) electrode modified with an
14
3-aminopropyldimethoxysilane (ADMMS) monolayer.37 The remarkable change of the reduction
15
peak potential could be ascribed to the existence of small Au nuclei. The presence of Au seeds
16
reduced the activation energy of the reduction of AuCl4- due to the fact that the Au seeds possess
17
prodigious surface-to-volume ratio and relatively high surface energy. Once the Au nuclei were
18
formed on a bare ITO electrode, the growth of AuNPs immediately proceeded on the surface of
19
these Au nuclei. We could further take full advantage of this effect to fabricate the AuNPs on an
20
Au seeds/ITO electrode with controlled size and distribution in a low overpotential range (-0.04
21
~ +0.3 V) since the depletion layer around Au seeds could be reduced by lowering the applied
22
potential and the growth rate of AuNPs, which would eliminate diffusion zone coupling
23
effectively and produce higher size homogeneity and better controllability.38 10
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a
15
Start point Loop
0 -15 -30
Scan 2 -45
Scan 1 ∆E
-60 -0.4
1
Current Density / (µ A cm-2)
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Current Density / (µ µA cm-2)
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0.0
0.4
50
b Start point
25 0 -25
Scan 1 -50
Scan 2 -75
-100
0.8
Potential vs. SCE / V
1.2
1.6
-0.4
0.0
0.4
0.8
1.2
1.6
Potential vs. SCE / V
2 3 4
Figure 1. Cylic voltammograms for gold electrodeposition on a bare ITO electrode (a) and Au seeds/ITO electrode (b). Potential was scanned with the first cyclic (solid line, black) and the second cyclic (dash line, red) at a scan rate of 50 mV/s in 0.1 mM HAuCl4 in 0.1 M NaClO4.
5
A single nucleation pulse was applied to form Au seeds on a bare ITO electrode. Figure S1
6
(Supporting Information) represented a current transient recorded for electrodeposition of AuNPs
7
by applying a potential step from 0.89 to -0.8 V vs. SCE. There was no initial sharp high current
8
appearing that was caused by the charging of double electric layer. A current decay was observed
9
which was linear with t−1/2, and this indicated that a planar diffusion regime arising due to the
10
overlapping of growing hemispherical diffusion layers which provided mass transport for
11
nanocrystal growth in the process of gold nucleation.32 The diffusion coefficient could be
12
obtained from the slop of Q versus t1/2 plot according to Cottrell equation:39
13
Q=2nFACoD1/2π-1/2t1/2
14
Where Co=0.1 mM, A=0.28 cm2 and n=3.
(2)
15
In our experimental conditions, the calculated diffusion coefficient for Au electrodeposition
16
on a bare ITO electrode under -0.8 V was 2.59×10-4 cm2/s and it was 0.67 times as high as that
17
for a polydopamine (Pdop)/ITO electrode (1.65×10-4 cm2/s).33 The results demonstrated that the
11
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electrodeposition under the potential of -0.8 V was more facile than on a biopolymer coated ITO
2
electrode. No apparent humps could be observed in Figure S1, and it could be concluded that the
3
growth of Au nuclei was relatively rapid. Because the rising portion of the peak was not
4
experimentally accessible under our conditions, estimation of the nucleation mechanism (i.e.
5
instantaneous or progressive) was not possible from current transient.37,40 It was reasonable to
6
infer that the electrochemical reduction process of AuCl4- described here undergoes an
7
instantaneous nucleation process on bare ITO electrodes since there was no noteworthy charging
8
phase.37,41,42 The so formed Au seeds would achieve higher size monodispersity than progressive
9
nucleation which would lead to the coupling of the diffusion layer of the adjacent AuNPs and the
10
inhomogeneity of the particle size. Figure S2a illustrated the influence that different scan rates
11
had on the reduction peak of AuCl4- in 0.1 M NaClO4 +0.1 mM HAuCl4. It could be observed
12
the reduction peak of AuCl4- shifted along the negative direction with increasing of the reductive
13
current intensity. A linear relationship was shown when plotting the anodic peak currents as a
14
function of the square roots of scan rates (Figure S2b), which indicated diffusion-controlled mass
15
transport in all cases. It was more interesting that as for the AuNPs growth, the cyclic
16
voltammetric process resulted in the predomination of the Au (111) facet (Figure 5) which was
12
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different from the previously reported electrochemical preparation methods but was similar to
2
the hydroxylamine seeding method.43,44
3
3.2.
FE-SEM Characterization of Au NPs-Modified ITO Electrodes
4
To get more information about morphology of AuNPs on ITO electrodes, their visual
5
representation were obtained with FE-SEM. Figure S3 (Supporting Information) and Figure 2
6
contained representative images depicting respectively the effect of seeding time (Figure S3) or
7
growing time (Figure 2) on the electrodeposition morphology of AuNPs. Gold nanoparticles had
8
been deposited on the surface of ITO electrodes using electrochemical seed-mediated growth
9
method. This approach involved the electrochemical deposition of nanoscale Au seeds on a bare
10
ITO electrode and further growth of the Au seeds by electrochemical deposition rather than by
11
hydroxylamine, which was different from the method adopted by Raj.45 Figure S2a showed
12
typical FE-SEM images of Au seeds formed by electrodeposition from 0.1 mM HAuCl4 in 0.1 M
13
NaClO4 at the potential of -800 mV over a period of 10 s. The dark, grainy background
14
corresponded to the bare ITO glass and the bright, somewhat rough and spherical features
15
corresponded to the AuNPs deposited on the ITO electrode. As could be seen, the Au
16
nanoparticles with a quite symmetric distribution were appeared on the ITO electrode and the Au
17
seeds with hemispheric morphology had the size distribution between 3 and 19 nm with a
18
standard deviation 2 nm. When the seeding time reached to 80 s, there were large particles
19
formed on the ITO electrode and they were randomly distributed on the ITO surface. The size
20
distribution of the AuNPs (Relative Standard Deviation, RSD 31 %) was larger than that in the
21
10 s (RSD 24 %). We considered that this might be related to the coupling of the diffusion zones 13
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of the adjacent particles and the formation of new nucleation sites throughout the deposition
2
process, which lead to appearance of some new seeds and their consequent growth. Forster and
3
Zamborini et al. had obtained same conclusion from their works.36,41 Due to the high density of
4
the Au seeds (Figure S2), the diffusion shielding effect of the nanoelectrode ensembles probably
5
occurred which would result in an inhomogeneous distribution of AuCl4- around the small
6
AuNPs surfaces in subsequent electrochemical reactions.46 Though adjusting the duration of the
7
electrodeposition could control the size of the gold nanoparticles, the long seeding time would
8
lead to the heterogeneity of particle size. Compton’ group came to the conclusion that an
9
increase in the deposition time would result in the enhancement of the average particle size of Au
10
nanoparticles, but their corresponding RSD would also increase with prolonging of the
11
deposition time.28 Meanwhile, it was very important to slack the effect of non-uniform
12
distribution of AuCl4- around AuNPs in the diffusion process. Therefore, we selected a relatively
13
short duration (10 s) as the seeding time in this work for the purpose of obtaining high density
14
and size monodispersity.
15
Zamborini and co-workers had discussed the different deposition potentials of AuCl4- on an
16
ITO electrode.36 With increasing electrode potential from -0.2 to 0.8 V, the size of the AuNPs
17
increased and the density of the AuNPs decreased. The electrochemical deposition of metal went
18
through the nucleation of metal clusters and subsequent growth.36,38 High overpotential could
19
activate a bare ITO electrode and alleviate its significant surface roughness to achieve narrow
20
particles distributions.47 At a low overpotential relative to the thermodynamically favored
21
potential for AuNPs deposition, the energy was not sufficient for the rapid AuNPs nucleation all
22
over the surface and the energy was significant relative to the electron energy (governed by
23
potential).36 In our condition, as expected, the AuNPs with high density (1800 particles/µm2) and 14
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1
low particle size distribution were obtained at high overpotential (-0.8 V) (Figure S2a). The
2
particle size distribution was not especially large (RSD 24 %) and the mean particle size was ca.
3
9 nm. It was significant that the Au seeds with acceptably narrow size distribution had great
4
influence on the growth process.41 When the nucleation event occurred, the nucleation sites
5
catalyzed further AuCl4‒ ions reduction, leading to their preferential growth over the formation
6
of new nucleation events on the bare ITO electrode by using the slow growth approach.34,36,41 It
7
had same effect on the growth of Au nanocrystals relative to the chemical seeding method using
8
hydroxylamine.42-44,48 For the preferential growth of the Au seeds, we could control the size and
9
density of the Au seeds in the seeding process by using different overpotentials and the changes
10
of the conditions of the seeding process would have great influence on the electrochemical
11
growth process. In order to fabricate SERS-active substrate, a relatively high overpotential (-0.8
12
V) were chosen to produce high particle density and size monodispersity for the high
13
overpotential nucleation pulse was favorable to instantaneous nucleation. As shown in Figure 2,
14
after the seeding process, the size of the Au seeds would become large and the distance between
15
the adjacent AuNPs would decrease which could produce tremendous local electromagnetic field
16
enhancement when the gap was smaller than 10 nm.49,50 They would contribute to the excellent
17
SERS-active performance of the substrates.
18
Seed-mediated growth approach using electrochemical method was applied for the growth of
19
monodisperse Au nanoparticles. Figure 2 showed a series of FE-SEM images to estimate the
20
effect of the growing time on the AuNPs. It was clearly seen that particle density decreased;
21
meanwhile, the average size of the AuNPs was reached to 50 nm after 500 cycles. We noted that
22
the size of the AuNPs depended on the electrodeposition cycles. The average size of the AuNPs
23
increased from about 9 ± 2 nm to 50 ± 8 nm. We could adjust the number of electrodeposition 15
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1
cycles to control the size of the AuNPs on the ITO electrode. It was worth while mentioning that
2
the distance between the adjacent AuNPs became shorter after different deposition cycles and it
3
was less than 10nm after 500 cycles (in Figure 2a). For the short distance between AuNPs, it was
4
expected that under such a circumstance abundance of ‘hot spots’ would be provided which
5
could produce high performance of SERS effect. When the gap between adjacent Au
6
nanoparticles reached to sub-10 nm, which had been demonstrated that in this range, the
7
substrate modified with noble metal nanoparticles would produce a lot of “hot spots” which
8
would result in interacting among the surface plasmon resonances though dipole-dipole EM
9
interactions.72 Though the distance between AuNPs after 50 cycles approached to 10 nm, the size
10
of the nanoparticles only reached to 16 nm which could not contribute to the high performance of
11
SERS. On the basis of FE-SEM analysis, the average diameters of the AuNPs on the ITO
12
electrode were 16 ± 3, 18 ± 4, 32 ± 6, 50 ± 9 nm, respectively, for electrodeposition of 50, 150,
13
300, 500 cycles. The relative standard deviations of the average AuNPs diameter did not show
14
statistically significant differences (17, 19, 18, 17 % for deposition cycles of 50, 150, 300, 500
15
cycles, respectively). However, there was significant improvement of standard deviations of the
16
average AuNPs diameter compared to that for the Au seeds (RSD 24 %). The particle density
17
decreased with increasing of the deposition cycles (Figure 3a). It could be due to (1) interparticle
18
diffusion coupling that occurred with higher density of AuNPs34,36,38,46 and (2) the Ostwald
19
electrochemical mechanism that might be involved in our experiment to explain the change of
20
the particle density.51 There was a concentration gradient due to the size disparities and it
21
allowed the diffusion of AuCl4- toward large AuNPs. Larger Au particle accepted three electrons,
22
for the reduction of AuCl4- ions onto its surface. It would accept three electrons from a
23
neighboring smaller AuNPs through the conducting substrate to reestablish electrical equilibrium. 16
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1
The Ostwald electrochemical coarsening leaded to the particle density decrease due to the large
2
particles scavenged smaller ones. However, we did not observe the broadening of AuNPs size
3
distribution with increasing of deposition cycles. In our conditions, when the cycles of
4
electrodeposition reached ca. 200, there was no significant variation of the density of the AuNPs
5
on the ITO substrate (Figure 3a). Furthermore, we inferred that the system of the deposition was
6
kinetically and thermodynamically biased toward the formation of large single crystals, for the
7
different deposition rate of AuCl4- at polycrystalline and single crystal surface. Brus and Ohsaka
8
et al. got the same conclusion in their previous works.43,51 The characterization of the lattice
9
plane of the AuNPs/ITO electrode obtained here was shown in Figure S4 (Supporting
10
Information). The results from FE-SEM image analysis for electrochemically deposited gold
11
nanocrystals on ITO electrode were shown in Table 1. 140 mean=50nm σdia.=8.5nm
Count
120 RSD dia.=17% 100 Density=225 particles µm-2 80 60 40 20 0
0
10
20
30
40
50
60
70
80
Particle Diameter / nm
12
mean=32nm σdia.=5.9nm
140 120
RSDdia.=18% Density=230 particles µm-2
100
Count
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80 60 40 20 0
13
0
10
20
30
40
50
60
70
80
Particle Diameter / nm
17
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mean=18nm σdia.=3.5nm
140 120
RSDdia.=19% Density=750 particles µm-2
Count
100 80 60 40 20 0
0
10
20
30
40
50
60
70
80
Particle Diameter / nm
1 160
mean=16nm σdia.=2.9nm
140
RSDdia.=18%
120
Densitry=1250 particles µm-2
Count
100 80 60 40 20 0
0
10
2
2000
9 10
30
(a)
1600 1200 800 400 0
0
150
300
Cycles
6 7 8
30
40
50
60
70
80
Figure 2. Scanning electron micrographs and particles size histograms for Au NPs on ITO glass at deposition potential of -800 mV and a deposition duration of 10s, followed by cyclic voltammetry, a: 500 cycles b: 300 cycles; c: 150 cycles; d: 50 cycles; at potential range of +0.3 to -0.04 V.
Nanogap distance / nm
3 4 5
20
Particle Diameter / nm
Density / (particles / µ m2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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450
(b)
25 20 15 10 5 0 0
100
200
300
Cycles
400
500
Figure 3. (a) The AuNPs density dependence of the cycles of electrodeposition; (b) the nanogap distance between AuNPs with the different deposited cycles. (300 nanogaps are analyzed).
3.3.
XRD Characterization and Electrochemical Characterization of AuNPs-Modified ITO Electrodes
18
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Au(111)
200
Intensity (a.u)
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150 ∗ ∗
100
c
50
b a
0 0
20
40
60
80
100
2−Theta (degrees)
1 2 3 4 5
Figure 4. XRD patterns of bare ITO (a), AuNPs seed/ITO (b) prepared from a solution of 0.1 M NaClO4 +0.1 mM HAuCl4 via applying a potential step from +0.89 V to –0.8 V (vs. SCE) , and AuNPs/ITO (c) electrodeposited by cyclic voltammmetry from +0.3 to -0.04 V(vs. SCE) for 500 cycles at scan rate of 0.05 V s-1 based on the AuNPs seed/ITO in 0.1 M NaClO4 +0.1 mM HAuCl4 (peaks of ITO are marked with asterisks).
6
The crystalline structures of the AuNPs on ITO electrode were further investigated by XRD.
7
XRD patterns of the AuNPs/ITO electrodes described above were illustrated in Figure 4. The
8
peaks around 30.2º and 35.2º were assigned to ITO (222) and ITO (400) facet (reference to the
9
JCPDS file 06-0416), respectively.
42,52
The peak at 38.2º was ascribed to the Au (111) facet
10
(reference to the JCPDS file 04-0416).44,53 It was interesting that there was only Au (111) crystal
11
facet in the AuNPs/ITO, and the pattern of Au (200), Au (220), and Au (311) were so weak that
12
we could not recognize them in the Figure 4. The higher intensity of the peak at 2θ =38.2º
13
indicated the higher population of Au (111) facet in the AuNPs/ITO electrode. In all the cases,
14
the ITO electrode was used as the substrate for the electrochemical deposition of AuNPs and it
15
should be noted that since the path length of the X-ray photons is of the order of micrometer, the 19
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1
X-ray beam might penetrate the substrate in the case.53 There were clear patterns of ITO.
2
Furthermore, it could be observed in the FE-SEM that nanoarray of AuNPs were formed on the
3
ITO substrate instead of a compact Au film. The AuNPs deposited on the ITO electrode showed
4
an Au (111) facet preferential orientation that could be related to the electrochemical deposition
5
potential range used in the fabrication of the nanoarray. In fact, Ohsaka’ group had ever utilized
6
a chemical seeding approach followed by an electrochemical means to fabricate Au electrode
7
with a high surface area (ca. 7.5×10-3 cm2) and enriched with Au (111) facet (ca. 60%).53 Dong’s
8
group had introduced a simple approach for preparation of Au (111) single-crystal
9
nanoisland-arrayed ensembles which was based on fine colloidal Au monolayer-directed seeding
10
growth.44 The common characteristic of these methods lied in the employment of colloidal gold
11
surface-catalyzed reduction of Au3+ by hydroxylamine. In our condition, we used an
12
electrochemical seeding approach rather than hydroxylamine seeding followed by an
13
electrochemical mean. Nevertheless, it seemed that they had the same effect on the growth of the
14
Au nanoparticles. In the potential range of deposition from +0.3 to -0.04 V after 500 cycles, we
15
found the Au nanoparticales predominantly oriented in the Au (111) crystal facet (Figure 4). The
16
XRD patterns of Au-seed/ITO were very weak and almost absent. It might be that for applying
17
high overpotential in deposition, though there were numerous Au nuclei formed on the ITO
20
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1
substrate, the size of Au seeds only reaches ca. 9 nm, resulting in the absence of the information
2
of the XRD patterns of such a Au-seed/ITO.
3
To evaluate the quality of the AuNPs on the ITO electrode, the electrochemical
4
characterization of the as-prepared substrates that were measured in a deoxygenated (i.e.,
5
N2-saturated) 0.5 M H2SO4 solution at a potential scan rate of 0.1 V s-1 was shown in Figure S4
6
(Supporting Information). The CV of the AuNPs/ITO showed the peaks were similar to the Au
7
(111) single-crystal in the 0.5 M H2SO4 and were different from the peaks that observed at the
8
polycrystalline Au electrode in the positive region (1.1∼1.5 V, Figure S4), indicating that the
9
fabrication protocol herein could populate the Au surface to grow with Au (111) facet.54
10
In order to get more information of the crystallographic orientation of the Au nanoparticles on
11
the ITO glass for deposition 500 cycles under potential 0.3 to -0.04 V, the reductive thiol
12
desorption experiments were undertaken (Figure S5). Different single-crystalline domain
13
exhibits different binding strengths toward an attached self-assembled thiol. For instance, for a
14
short-chain thiol species like cysteine, a multiple reductive desorption pattern was recorded for
15
the reductive desorption from a polycrystalline Au electrode.43, 55 The cathodic peak observed at
16
ca. -0.75 V corresponding to desorption of L-cysteine from Au (111) facet and a broad cathodic
17
peak at ca. -0.95 V∼-1.0 V for desorption of L-cysteine from the Au (100) facet and Au (110)
21
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1
facet were also recognized. In the AuNPs/ITO electrode, Au nanoparticles enriched with ca.
2
80.7 % Au (111) facet were founded.
A
a B
d
b
e f g
c
h
50µ µA cm-2
-0.8 -0.6 -0.4 -0.2 0.0
0.2
Potential vs. SCE / V
-2 250µ µA cm -0.8 -0.6 -0.4 -0.2 0.0
0.2
Potential vs. SCE / V
3 4 5 6 7
Figure 5. CVs obtained at: (A) :(a) AuNPs/ITO (500 cycles); (b) polycrystalline Au electrodes; (c) bare ITO electrode; (B) (d) Au seed/ITO electrode; (e) AuNPs/ITO (50 cycles); (f) AuNPs/ITO (150 cycles); (g) AuNPs/ITO (300 cycles); (h) AuNPs/ITO electrode (500 cycles) in (a) N2- and (b, c, d, e, f, g and h ) O2-saturated 0.5 M KOH solution at a scan rate of 0.1 V s-1. (Note: spectra are stacked for clarity)
8
Moreover, the O2 reduction reaction (ORR) could reflect the surface state of the Au
9
electrode and this reaction exhibited a strong relationship with the crystallographic orientation of
10
the Au electrode.56-58 It had been shown that ORR proceeds via a quasi-reversible pathway at the
11
Au (111) electrodes in alkaline media. Hence, the ORR would furnish further evidence for the
12
Au (111) orientation-enriched AuNPs/ITO electrode without any additive during the
13
electrodeposition process.59 Figure 5 showed a typical set of CVs for the ORR at a bare ITO
14
(curve a), polycrystalline Au (curve b), Au-seed/ITO (curve d), AuNPs/ITO (50, 150, 300, 500 22
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1
cycles; curve d, e, f, g, h) electrodes in O2-saturated 0.5 M KOH solution. The curve-a showed
2
the corresponding response of the AuNPs/ITO electrode (500 cycles) in N2-saturated 0.5 M KOH
3
solution and there was no any peak appearing in the cycles and there was a cathodic peak at -0.8
4
V for the reduction of the O2 at the bare ITO electrode in curve-c. It was clear from curve-b that
5
the ORR proceeded irreversibly at the polycrystalline Au electrode where occurred four-electron
6
reduction of O2 for the ratio of the Au(100) was relatively high in the poly-Au electrode, while a
7
quasi-reversible two-elecron behavior was observed for the ORR at the Au-seed/ITO or the
8
AuNPs/ITO electrodes (50, 150, 300, 500 cycles). Ohsaka and co-workers had demonstrated that
9
(1) the Au (111) was the least active basal plane, whereas Au (100) was shown to be the most
10
active and the catalysis performance to the reduction of O2 of these crystallographic planes was
11
in the order Au (111) < Au (110) < Au (100); (2) the ORR was completely irreversible at Au
12
(100) electrode and a quasi-reversible behavior was undergone at Au (111) and Au (110)
13
electrodes.53,56,60-62 The cathodic peak at the Au (111) was related to the catalysis of the O2 to
14
hydrogen peroxide. Nevertheless, the anodic peak was correspond to the reoxidation of hydrogen
15
peroxide (HO2- in 0.5 M KOH) (which was confirmed by the fact that the quasi-reversible
16
reduction pathway of O2 is not affected by the presence of the SOD, while in the case of the
17
presence of catalase, a significant increase of the cathodic peak current was observed and the
23
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1
anodic peak current was severely suppressed)56. Interestingly, there was the anodic peak at 0 V
2
and that was to say that the ratio of the Au (111) in the Au-seeds/ITO electrode was higher than
3
that in the natural polycrystalline Au electrode. With the increasing of the electrodeposition
4
cycles, it was seen clear from the curve-B that there was a gradual positive shift of the cathodic
5
peak potential and small increase in the peak current. The ratio of the cathodic to anodic peak
6
was higher at AuNPs (50, 150, 300, 500 cycles) compared to that at single Au (111) electrode,
7
which could be considered as the consequence of the partial involvement of the other facets of
8
Au in ORR.53 In Figure S5, there was 19.3% Au (100) and Au (110) facet in the AuNPs/ITO
9
electrode (500 cycles). The peak potential for the ORR at AuNPs/ITO (500 cycles) appeared at
10
ca.-0.45 V different with that at the bare ITO electrode in the same medium (ca. -0.8 V). Thus, it
11
could be consider that the bare ITO substrate did not participate in reducing the O2 to HO2-. The
12
cathodic peak shifted from ca.-0.7 to ca.-0.45 V (Au seeds/ITO -0.7 V, 50 -0.65 V, 150 -0.50 V,
13
300 -0.48 V and 500 cycles -0.45 V) in curve-B. It could be concluded that this might be
14
attributed to the enrichment of the Au (100) facets in the AuNPs, which leaded to the positive
15
shift of the cathodic peak.60
16
gradually increasing of the real active surface of the electrode with increasing in
17
electrodeposition cycles (Table 1).
The higher of the cathodic peak current might be produced by the
24
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1
We could estimate the real surface area of the electrodeposited Au particles on ITO glass by
2
calculating the amount of charge consumed during the reduction of the gold oxide monolayer
3
(Figure S6). A reported value of 400 µC cm-2 was adopted in these calculations.28,63 The results
4
were presented in the Table 1. The real surface area (500 cycles) was higher than those of the
5
AuNPs/ITO electrodes fabricated by Compton’s group and Ohsaka’s group.28,53
6 7
Table 1. Effect of Nucleating and Growing Time on the Mean Diameter and Density of Gold Nanoparticles Electrodeposited on ITO Glass En a /mV -800 -800 -800 -800 -800 -800 a
tnb /s 10 80 10 10 10 10
Range of Eg/mV ---40~300 -40~300 -40~300 -40~300
Cycles of Total surface depositio area of gold
Mean Density diameter/ (particles
n
naoparticle/cm2
nm
/µm2)
--50 150 300 500
0.26 -0.87 1.3 2.9 3.1
9±2 13±4 16±3 18±4 32±6 50±9
1800±200 1700±150 1250±100 750±60 230±50 225±30
RSD EF (%) (105) 24 31 17 18 19 18
0.31 0.54 1.8 5.3 9.5 13
The potential for the formation of Au seeds on the indium tin oxide electrode. bThe time for the formation
of Au seeds on the indium tin oxide electrode.
8 9
3.5.
Optical Properties of the AuNPs-Modified ITO Electrodes
10
Figure 6A showed the UV-visible absorption spectroscopy of the Au nanoparticles deposited
11
on the ITO substrate. The AuNPs on the ITO substrate with nanometer-scale dimensions
12
absorbed light in the visible and/or near-infrared region due to localized plasmon resonance,
13
which was light-induced collective oscillation of conduction electrons at the metal surface, and
14
that the resonance wavelength depended on particle size, shape, and local dielectric 25
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1
environment.64,65 The absorbance peaks of the UV-vis spectra corresponding to the localized
2
surface plasmon resonance band were observed to increase linearly with the number of cycles
3
(Figure 6B). Figure 6C showed the corresponding photographs of the AuNPs/ITO with different
4
deposition cycles. The electrochemically formed AuNPs on the ITO substrate with spherical
5
shape and high monodispersity possessed especially interesting optical properties.42 Depending
6
on the growing time, the interfaces of the ITO electrode exhibited different colors ranging from
7
colorless (Au seed) to red (50 cycles) and blue-purple (500 cycles) (Figure 6C). After
8
electrochemical deposition of 500 cycles, one broad absorption peak was observed at about 625
9
nm and the LSPR signals were not very intense. With increasing the deposition cycles, we could
10
observe the obvious red shift of the peak of the absorbance. It should be related to that with
11
increasing of the growing time, the size of the AuNPs became larger and the distance between
12
the adjacent Au nanoparticles diminished which would produce the enormous local
13
electromagnetic enhancement. It demonstrated that our method had the capacity to adjust the
14
location of the LSPR peaks of the SERS-active substrates by changing the number of the
15
electrodeposition cycles. The substrate’s performance for Raman scattering enhancement should
16
be related to the excitation of surface plasmon within the nanostructure.66 We used 632 nm laser
17
as excitation line to evaluate the SERS performance of the AuNPs/ITO substrates and found that
18
the substrate with electrodeposition 500 cycles showed the best SERS performance (Figure 7).
19
The wavelength of the absorption maximum for the AuNPs/ITO after 500 cycles was ca. 625 nm.
20
The λmax of the absorption spectrum was close to the excitation line (632 nm) of SERS
21
spectroscopic measurement, this would further facilitate EM enhancement. It had been shown
22
that when the λmax corresponded to the excitation wavelength, it would produce the largest SERS
23
enhancement.66 26
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A
B
640
1.2
a
λmax=503.6+0.227n R=0.964
λmax / nm
Absorption
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The Journal of Physical Chemistry
600
0.9
b c d e f g
0.6
0.3
1
400
600
Wavelength / nm
800
560
520
0
100
200
300
400
500
Number of cycles / n
2 3 4 5 6 7 8 9 10
Figure 7. (A) UV-vis spectra of the AuNPs/ITO, a: 500 cycles; b: 300 cycles; c: 200 cycles; d: 150 cycles; e: 70 cycles; f: 50 cycles; g: seed/ITO (0 cycle); at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry. (Note: spectra are stacked for clarity) (B) The absorbance peak of the UV-vis spectra versus the number of cycles of deposition (statistics based on one measurement for three samples). (C) Photographs taken on a white paper sheet of the samples prepared by electrochemical seed-mediated growth. a: bare ITO; b: seeds; c: 50 cycles d: 70 cycles; e: 100 cycles; f:150 cycles; g: 200 cycles; h: 300 cycles; i: 500 cycles; at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry.
3.6.
SERS Application of the AuNPs-Modified ITO Electrodes
11
The AuNPs/ITO with controlled nanostructure made them attractive for use as SERS-active
12
substrates. 4-Mercaptobenzoic acid was chosen as the SERS probing molecule because it was
13
well studied and its thiol group was easily cleaved to form a metal-S bond when adsorbed on a
14
metal surface.1,67 SERS spectra of p-MBA adsorbed on the AuNPs/ITO electrodes with different
15
electrodeposition cycles were shown in Figure 8. We could observe the SERS signal became
16
larger with increasing in the electrodeposition cycles. It was noted that AuNPs/ITO with 500
17
cycles (Figure 8f) gave the strongest Raman signals compared with the others in Figure 8. There
18
were no characteristic p-MBA vibrational signals for the bare ITO substrate. However, as the 27
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1
growing time of AuNPs increased, the characteristic vibrational modes of p-MBA began to
2
appear and increase in their intensities. We could observe the ring breathing and axial
3
deformation modes at 1075 and 1585 cm-1, the bending of the CH groups on the ring at 1178
4
cm-1, and the stretching associated with the carboxylate group at 1283 cm-1.1,68,69 We chose the
5
intensity of the CC ring-breathing mode (1074 cm-1) to calculate the enhancement factor (EF) of
6
the AuNPs/ITO substrates. The EF value for AuNPs/ITO (500 cycles) was calculated to be about
7
1.26×106 (The calculation method had been shown in Supporting Information, Figure S7). It was
8
lower than the p-MBA SAMs which served as the linker molecules in the sandwich structure
9
between gold substrates and immobilized gold nanocubes.1 The calculated enhancement factors
10
apparently increased with increasing the growing time (Table 1). The difference of the metal
11
nanostructures contributed to the variation of the Raman spectra. For the increase of the growing
12
time, the size of the Au nanoparticles became larger and began to form the junctions between the
13
adjacent Au nanoparticles to increase the SERS signals and enhancement factor, which could
14
attribute to electromagnetic (EM) enhancement mechanism. For the frequency of excitation light
15
source corresponding to that of the localized surface plasmon resonance in the AuNPs, it would
16
produce extremely intense local electromagnetic fields in the gaps between adjacent Au NPs (hot
17
spots). When the interparticle spacing was in the sub-10 nm range, it could provide large SERS
18
enhancement in the junction geometry.70 We concluded that the strong SERS-active effect for the
19
AuNPs/ITO (500 cycles) could attribute to their particular geometry (the size of Au NPs and the
20
spacing between the adjacent Au NPs).
21
To obtain the information of the uniformity, we successively measured 30 spots randomly on
22
the AuNPs/ITO substrate (Figure 9a). Obviously, the Raman spectra of pMBA were enhanced
23
greatly at each point, indicating excellent reproducibility of the AuNPs/ITO substrate. 28
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The Journal of Physical Chemistry
1
Meanwhile, we still found that the fluctuation of the Raman signals could be attributed to the
2
variation of the nanostructures over the as-prepared substrate. It demonstrated that the substrate
3
was not complete uniform and existed spatial non-uniformity of the nanostructure, which was
4
regarded as the dominating factor affecting the degree of enhancement of Raman scattering. In
5
our condition, the ratio of the 1074 and 1588cm-1 band with different points did not have
6
significant variation and the standard deviation only reached to 0.047 indicated that the pMPA
7
had the same adsorption geometry over the substrate’s surface (Figure 9b) and the SERS
8
intensity deviation of the 1075cm-1 band of the 30 spectra was shown in Figure 9c and the
9
standard
deviation
only reached
to
0.092.
These
results
demonstrated
the
good
10
SERS-performance and relative uniformity of the substrate across the AuNPs/ITO substrate. It
11
was sufficient to provide reproducible SERS signals that made the as-prepared substrate possess
12
great potential in quantitative SERS analysis application. It should be mentioned that the stability
13
of the AuNPs/ITO substrate had been tested and the substrate stability could be retained for at
14
least 3 weeks without markedly attenuation of the SERS signals (Figure 9d). The statistical data
15
of the successively measured SERS spectra of 30 spots randomly of 1mM pMBA on the
16
AuNPs/ITO substrate (500 cycles) after three weeks had been shown in Figure S8.
29
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400cps
ν(C-C) ring ν(COO-) δ(CH)
500 cycles
Bare ITO f e d c b a
500
1 2 3
1000
1500
2000
Raman shift / cm-1
Figure 8. SERS spectra taken at 632.8 nm of the p-MBA on the AuNPs/ITO: a: bare ITO; b: Au seed/ITO; c: 50 cycles; d: 150 cycles; e: 300 cycles; f: 500 cycles; at potential range of +0.3 to -0.04 V utilizing cyclic voltammetry.
b
a
tn um be r
I1074 / I1588
0.8
30
25
20
800
1200
1600
2000
Raman shift / cm-1
Sc a
400
0.6 0.4 0.2
n
5
po in
15 10
0.0 0
5
35000
10
15
20
30
c
d
30000 25000 20000
20
10000
10
5
10
15
20
25
Scan point number
30
400
800
1200
1600
Raman shift / cm-1
2000
Sc an
5000
po in t
15
nu
15000
m be r
30
25
5
5 6 7 8 9 10 11 12
25
Scan point number
4 I1074 / Intensity(a.u)
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Page 30 of 38
Figure 9. The uniformity of the SERS signals on the AuNPs array-based substrate. (a) Successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles). (b) The SERS intensity distribution of the ratio of the 1074 and 1588 cm-1 band. The black line represents the average ratio of the 30 spectra and the standard deviation is 0.047. The blue lines show the standard deviation of the ratio. (c) The SERS intensity distribution of the 1074 cm-1 band. The red bands represent the deviation of the average intensity of the 30 spectra. (d) Successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles) after three weeks.
30
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1
The Journal of Physical Chemistry
4.
Conclusions
2
This work demonstrates an economical, feasible yet robust method for electrochemical
3
deposition gold nanoparticles on indium tin oxide (ITO) glass as SERS active structure. To
4
thoroughly consider the nucleation and the growth of the nanoparticles, the controlled particle
5
size, distribution, the distance between the adjacent gold nanoparticles which produce numerous
6
‘hot spots’ on the substrate. The nanoparticles are predominantly oriented in the Au (111) crystal
7
plane, showing a good face-selective adsorption of molecular with the thiol moiety. For the
8
as-prepared SERS active substrate with high reproducibility, stability, and cost advantage, this
9
electrochemical seed-mediated method has great potential to be commercialized and the
10
substrate could serve as the commercially portable SERS active substrate for on-site
11
environmental monitoring and other sensing applications in the near future. Through using the
12
different growth potential range to adjust the gold crystallographic orientation as SRES active
13
substrate is under study.
14
Associated Content
15
Supporting Information
16 17 18 19
Electrochemical characteristics, calculation of Enhancement Factor data and the statistical data of the successively measured SERS spectra of 30 spots randomly of 1 mM p-MBA on the AuNPs/ITO substrate (500 cycles) after three weeks. This material is available free of charge via the Internet at http://pubs.acs.org. 31
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Page 32 of 38
1
Author Information
2
Corresponding Author
3
*E-mail:
[email protected].
4
Notes
5
The authors declare no competing financial interest.
6
Acknowledgements
7
Financial support from Chinese National Foundation of Natural Science Research
8 9
(No.20775049, 20973114) and the Program of Shanghai Normal University (DXL122) is gratefully acknowledged. We also thank Mr. H.Jin for FE-SEM measurements.
10 11
References
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Text and graphics for Table of Contents:
3 4
Herein, a novel electrochemical seed-mediated method for the fabrication of Au (111) like
5
nanoparticles on indium tin oxide thin electrode was demonstrated. This method improves the
6
monodispersion of Au nanoparticles in comparison with the Frens’ method and it is a valuable
7
tool to control the growth of surface-confined Au nanoparticles. Utilizing the decrease of the gap
8
between the adjacent Au nanoparticles to produce numerous ‘hot spots’ in the process of
9
electrodeposition, the as-prepared substrate has the promising potential in route SERS
10
applications for the high uniformity and sensitivity of the SERS signal.
38
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