Surface-enhanced Raman scattering from silver nanostructures with different morphologies

Abstract

Scanning electron microscopy and X-ray diffraction reveal that four different types of crystalline silver nanostructures including
nanoparticles, nanowires, nanocubes, and bipyramids are synthesized by a solvothermal method by reducing silver nitrate with
ethylene glycol using poly(vinylpyrrolidone) as an adsorption agent and adding different quantities of sodium chloride to
the solution. These nanostructures which exhibit different surface plasma resonance properties in the ultraviolet–visible
region are shown to be good surface-enhanced Raman scattering (SERS) substrates using rhodamine 6G molecules. Our results
demonstrate that the silver nanocubes, bipyramids with sharp corners and edges, and aggregated silver nanoparticles possess
better SERS properties than the silver nanowires, indicating that they can serve as high-sensitivity substrates in SERS-based
measurements.

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ISSN 0947-8396, Volume 100, Number 1

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Appl Phys A (2010) 100: 83–88
DOI 10.1007/s00339-010-5583-6
Surface-enhanced Raman scattering from silver nanostructures
with different morphologies
W.C. Zhang ·X.L. Wu ·C.X. Kan ·F.M. Pan ·
H.T. Chen ·J. Zhu ·Paul K. Chu
Received: 15 October 2009 / Accepted: 7 January 2010 / Published online: 24 February 2010
© Springer-Verlag 2010
Abstract Scanning electron microscopy and X-ray diffrac-
tion reveal that four different types of crystalline silver
nanostructures including nanoparticles, nanowires, nano-
cubes, and bipyramids are synthesized by a solvothermal
method by reducing silver nitrate with ethylene glycol using
poly(vinylpyrrolidone) as an adsorption agent and adding
different quantities of sodium chloride to the solution. These
nanostructures which exhibit different surface plasma reso-
nance properties in the ultraviolet–visible region are shown
to be good surface-enhanced Raman scattering (SERS) sub-
strates using rhodamine 6G molecules. Our results demon-
strate that the silver nanocubes, bipyramids with sharp cor-
ners and edges, and aggregated silver nanoparticles possess
better SERS properties than the silver nanowires, indicating
that they can serve as high-sensitivity substrates in SERS-
based measurements.
W.C. Zhang (?) · C.X. Kan · F.M. Pan
College of Science, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, People’s Republic of China
e-mail: weichun@nuaa.edu.cn
W.C. Zhang · X.L. Wu (?) · H.T. Chen · J. Zhu
National Laboratory of Solid State Microstructures and
Department of Physics, Nanjing University, Nanjing 210093,
People’s Republic of China
e-mail: hkxlwu@nju.edu.cn
H.T. Chen · J. Zhu
College of Physics Science and Technology, Yangzhou
University, Yangzhou 225002, People’s Republic of China
P.K. Chu
Department of Physics & Materials Science, City University
of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
1 Introduction
Since the advent of surface-enhanced Raman scattering
(SERS) in 1974 [1], the technique has become very impor-
tant in surface science and spectroscopy because of its high
surface sensitivity and extensive applications to structural
analysis, nanotechnology, and biological sensing [2–6]. Two
enhancement mechanisms, the primary one being the elec-
tromagnetic (EM) effect and the other being the chemical
(CHEM) effect, have been proposed to explain SERS. The
largest EM enhancement is achieved when the irradiation
wavelength matches the surface plasma resonance (SPR)
maximum. In general, in order to exploit the enhancement
phenomena, researchers have developed many substrate ma-
terials with different structures and shapes [7–10].
Recently, silver nanostructures have attracted much at-
tention due to their unique surface plasma features as well
as potential applications as active substrates for SERS. The-
oretical studies and experimental data have suggested that
the SPR of silver nanostructures can be tuned by control-
ling their morphology. Silver nanostructures with various
shapes have been produced by various methods such as a
modified polyol process [11, 12], microwave rapid heat-
ing [13], seed-mediated growth method [14], and so on.
Yang et al. [15] synthesized silver nanoparticles with dif-
ferent structural architectures and found that triangular sil-
ver plates with sharp corners and edges exhibit enhanced
SERS properties. Zhang et al. [16] also compared the SERS
spectra of rhodamine B on three kinds of silver colloids
and concluded that the surface morphology and crystal
planes of silver affect the Raman enhancement. In the work
reported in this paper, different silver nanostructures, in-
cluding nanoparticles, nanowires, nanocubes, and bipyra-
mids are produced using a solvothermal method by reduc-
ing silver nitrate (AgNO3) with ethylene glycol (EG) us-
ingpoly(vinylpyrrolidone)(PVP)asanadsorptionagentand
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84 W.C. Zhang et al.
adding sodium chloride (NaCl) of different concentrations
into the solution. The silver nanostructures which possess
different SPR properties exhibit outstanding SERS charac-
teristics using rhodamine 6G molecules as the probe. The
Raman spectra show that the field enhancement is notice-
able in the vicinity of the sharp point or between aggregated
silver nanoparticles. This work increases our understanding
of the SERS mechanism.
2 Experimental procedure
All of the chemical reagents used in our experiments were
of the analytical grade. The synthetic details of silver nanos-
tructures with different morphologies have been reported
previously [17]. In our experiments, a 20 ml EG solution of
NaCl with different concentrations (0, 0.2, 2, and 20 mM)
was vigorously stirred after addition of 0.888 g of PVP.
The mixed solution was introduced dropwise using a sy-
ringe into 20 ml of a magnetically stirred EG solution of
AgNO3(0.1 M) for 5 min. Afterwards, the solution was put
into two separate 25 ml Teflon-lined autoclave tubes. The
tubes were sealed and maintained at 160oC for 2 h (or 1 h),
followed by natural cooling to room temperature. Finally,
the products were washed with acetone and deionized water
and centrifuged at 8000 rpm for 20 min. The supernatants
containing redundant EG and PVP were removed by a sy-
ringe. This process was repeated three times in order to im-
prove the purity of the products. The final samples were pre-
served in deionized water prior to microstructural and spec-
troscopic characterization.
The morphologies and chemical compositions of the
samples were determined using a FEG JSM 6335 field-
emission scanning electron microscope (SEM, JEOL Com-
pany, Tokyo, Japan) equipped with an EDAX PV7715/
89ME energy dispersive X-ray (EDS) spectrometer. Pow-
der X-ray diffraction (XRD) measurements were carried out
on a Rigaku 3015 type diffractometer using Cu Kα radi-
ation to determine the crystal structures. The samples for
XRD and SEM measurements were prepared by dropping
the solutions onto polished silicon substrates and dried at
60oC. The ultraviolet–visible (UV–Vis) absorption spectra
were acquired on a Lambda35 apparatus directly from the
solutions with silver nanostructures or using the same sub-
strates with silver coatings. We found that the two kinds of
absorption spectra have no obvious difference in peak po-
sition but intensity. Prior to SERS measurements, the sub-
strates with the silver nanostructures were dipped in a 4 mL
aqueous solution of rhodamine 6G (10−6M) for 30 min and
then dried at room temperature. Raman measurements were
performed on a Jobin-Yvon T64000 triple Raman system
with the 514.5 nm laser excitation line. The probed area was
1–2 µm in diameter and the incident power was 5 mW. The
spectrawereobtainedin10s,andallthemeasurementswere
performed at room temperature.
3 Experimental results and analyses
Figure 1 displays the XRD patterns of the samples synthe-
sized with different NaCl concentrations. When the NaCl
concentration is 0 or 0.2 mM, five diffraction peaks can
be seen and indexed to the (111), (200), (220), (311), and
(222) planes of the face-centered-cubic silver crystals. If the
NaCl concentration is higher than 0.2 mM, besides the silver
diffraction peaks, new diffraction peaks marked with aster-
isks appear. They are associated with silver chloride (AgCl).
That is to say, the sample is a mixture solution contain-
ing AgCl and silver nanostructures. As the concentration of
NaCl is further increased to 20 mM, all the diffraction peaks
can be attributed to AgCl crystal when the reaction time is
1 h. After 2 h, some diffraction peaks of AgCl disappear,
and diffraction peaks corresponding to the silver crystal can
be observed. Furthermore, the intensity of the (200) peak of
silver is significantly higher than that of the (111) peak. This
implies that more silver nanostructures grow along the ?100?
direction parallel to the supporting substrate [17–19]. Based
on the XRD results, it can be inferred that the NaCl concen-
tration in the reaction process plays an important role in the
formation of the final products.
Figure 2 depicts the SEM images of the silver nanos-
tructures fabricated with different NaCl concentrations. If
there is no NaCl in the solution, irregular nanoparticles with
sizes ranging from 40 to 150 nm are obtained, as shown in
Fig. 2(a). These particles display a polyhedral structure. If
the NaCl concentration is 0.2 mM, large quantities of silver
nanowires are produced, as shown in Fig. 2(b). The longest
silver nanowire can reach about 20 µm, and its diameter is
around 150 nm. The SEM image of a single nanowire dis-
closes a pentagonal crystalline structure (not shown here)
[17] indicative of seeds derived from twinned bicrystalline
Fig. 1 XRD pattern obtained from the silver nanostructures synthe-
sized using different NaCl concentrations and reaction time
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Surface-enhanced Raman scattering from silver nanostructures with different morphologies 85
Fig. 2 SEM images of the
silver nanostructures
synthesized using different NaCl
concentrations: (a) 0 mM, 2 h,
(b) 0.2 mM, 2 h, (c) 2 mM, 2 h,
(d) 20 mM, 2 h (the insert is the
sample with 1 h reaction time)
particles with a decahedral shape [17, 20, 21]. If the NaCl
concentration is increased to 2 mM, large quantities of sil-
ver nanowires can still be seen. Besides the nanowires,
some grains comprising small nanoparticles are formed (see
Fig.2(c)).ThecorrespondingEDSspectrumshows thepres-
ence of chlorine and silver components indicating that bulk
quantities of AgCl colloids are formed and the data are con-
sistent with the aforementioned XRD results. Figure 2(d)
exhibits the SEM image of the sample after reaction for
2 h when the concentration of NaCl is 20 mM. Many sil-
ver nanocubes and bipyramids can be observed and some
nanowires with small diameters are also found. The insert in
Fig. 2(d) shows the sample undergoing the reaction for 1 h.
The sample resembles the grains in Fig. 2(c). According to
the XRD spectrum, it is believed that the sample contains
bulkquantitiesofAgClcolloids,asreportedpreviously[17].
When the EG solution with the PVP and AgNO3 is
heated to 160oC in the autoclave tubes, the silver ions are
reduced to atoms by the EG solution and then the atoms ac-
cumulate forming particles when its concentration reaches a
critical value [22]. Due to the relatively high reaction tem-
perature, Brownian motion and mobility of surface atoms
increase. This enhances the probability of particle collision,
adhesion, and coalescence [23]. However, PVP is added to
protect the particles from agglomeration and selectively ad-
sorb on the crystal plane of the silver particles via nitrogen
and oxygen atoms of the pyrrolidone unit, thereby mainly
playing the role of a capping agent [12, 22]. Because the
temperature of the reactants increases gradually and reduc-
tion of silver ions proceeds slowly, the long nucleation step
yields nanoparticles that are larger and polydispersed [23].
When a small amount of NaCl is added to the AgNO3so-
lution, small amounts of AgCl colloids are formed in the
initial stage, acting as the initial nuclei for further growth
of the silver nanowires [11, 17]. The role of these colloids
resembles that of gold catalysts in the Vapor–Liquid–Solid
process in which the diameter of the nanowires depends on
the sizes of the AgCl colloids [24]. As the NaCl concentra-
tion is increased to 2 mM, some small AgCl colloids act as
the nuclei, leading to the growth of nanowires and other un-
reacted, and larger colloids are left and mixed with the final
sample. When the NaCl concentration is 20 mM, large AgCl
colloids in large quantities are produced. If the solution is
kept for a long time under 160oC, large amounts of AgCl
colloids decompose into silver and chloride ions, yielding a
high local concentration of free silver ions. Under this con-
dition, the single-crystal and single-twinned silver seeds nu-
cleate rapidly and grow by reducing silver ions with EG, and
consequently, large amounts of silver nanocubes and bipyra-
mids are produced [11, 17, 22]. When the reaction time is
1 h, AgCl colloids decompose insufficiently and few sam-
ples are formed. After purify of the products, only AgCl
colloids are retained. In comparison, if the reaction time is
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86W.C. Zhang et al.
Fig. 3 UV–Vis absorption spectra acquired from the silver nanos-
tructures synthesized using different NaCl concentrations and reaction
times
2 h, many silver nanocubes and bipyramids remain in the
sample.
Silver nanostructures exhibit interesting optical proper-
ties directly related to SPR, which is highly dependent on
the morphology of the samples [25, 26]. UV–Vis absorp-
tion spectrophotometry is commonly used to investigate the
phenomenon. Figure 3 shows the UV–Vis absorption spectra
acquired from the silver nanostructures produced using dif-
ferent NaCl concentrations. It is clear that the morphologies
of the particles have a great impact on the absorption peak.
When no NaCl is introduced, the SPR peak at 432 nm origi-
nates from silver nanoparticles [22, 27]. Herein, the unsym-
metrical SPR peak is due to the different size and shape of
the particles. The nanowires show two main peaks attribut-
able to the quadrupole resonance excitation mode (350 nm)
and transverse plasma resonance mode (390 nm) [17, 25].
There is a broad absorption peak at around 650 nm related to
the large and asymmetric AgCl grains [17]. The absorption
spectrum of the as-prepared silver nanocubes and bipyra-
mids shows three obvious resonance peaks, two at 347 and
390 nm and another wide band at 460–530 nm [28, 29].
Broadening of the band is believed to be due to the asym-
metrical size of the nanocubes and bipyramids.
According to the above analysis of the morphologies and
optical properties, it is reasonable that these nanostructure
substrates show large differences in the SERS activities of
organic molecules. In order to eliminate errors arising from
the asymmetrical substrates, multiple sites on the substrate
surface are randomly selected to acquire the SERS spec-
tra and the spectra are also averaged. Figure 4 shows the
SERS spectra of R6G absorbed on the different nanostruc-
ture substrates. Because the R6G fluorescence and Raman
spectra often overlap in the range of 540–580 nm (Stokes
shift, 920–2200 cm−1) when excited with the 514 nm line
of an argon laser, the Raman spectra have a fluorescence
background [2, 10]. On the fluorescence background, many
distinctive peaks can be clearly observed. The most pro-
nounced ones are at 1364, 1508, 1575, and 1651 cm−1,
Fig. 4 SERS spectra of R6G molecule adsorbed on different nanos-
tructures coated on the silicon substrate
which can be assigned to the aromatic stretching vibrations,
whereas the peak at 613 cm−1is due to the in-plane de-
formation vibration of the ring and the peak at 774 and
1185cm−1correspondtoaromaticC–Hbending[7, 30, 31].
Comparison among the spectra reveals different SERS en-
hancements (please refer to the inset showing a quantita-
tiveintensitycomparisonof theselectedband(1651cm−1)).
It can be observed that the Raman enhancement achieved
on the silver nanoparticles is superior to that on the silver
nanowires. Moreover, the SERS enhancement from the sil-
ver nanocubes and bipyramids mixed AgCl colloids is more
obvious than that from the silver nanowires mixed AgCl,
and that from the AgCl colloids is the worst.
It is widely accepted that SERS enhancement is the re-
sult of a combination of intense localized fields arising from
SPR in metallic nanostructures and CHEM effects [16]. In
our samples, the SERS enhancements also arise from both
the EM and CHEM effects. When these irregular nanoparti-
clesaccumulateonthesiliconsubstrate,theiruniquegeome-
tries provide numerous contacts and large interstitial sites.
Theoretical prediction suggests that field enhancement can
become extremely large in the gap region between aggre-
gated metal nanoparticles which are often referred to as “hot
spots” in the film [32, 33]. Experiments have also demon-
strated that these hot spots can lead to high Raman sensitiv-
ity and even single molecule detection limit [2, 10]. Fur-
thermore, the nanoparticle substrates expose more crystal
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