SERS at structured palladium and platinum surfaces

Article abstract of DOI:10.1021/ja071269m

Palladium and platinum are important catalytic metals, and it would be highly advantageous to be able to use surface enhanced Raman spectroscopy (SERS) to study reactive species and intermediates on their surfaces. In this paper we describe the use of tem

Full text of DOI:10.1021/ja071269m

Published on Web 05/17/2007
SERS at Structured Palladium and Platinum Surfaces
Mamdouh E. Abdelsalam,† Sumeet Mahajan,† Philip N. Bartlett,*,†
Jeremy J. Baumberg,‡ and Andrea E. Russell†
Contribution from the School of Chemistry, UniVersity of Southampton, Southampton, SO17 1BJ,
U.K., and School of Physics and Astronomy, UniVersity of Southampton, Southampton,
SO17 1BJ, U.K.
Received February 22, 2007; E-mail: pnb@soton.ac.uk
Abstract: Palladium and platinum are important catalytic metals, and it would be highly advantageous to
be able to use surface enhanced Raman spectroscopy (SERS) to study reactive species and intermediates
on their surfaces. In this paper we describe the use of templated electrodeposition through colloidal templates
to produce thin (<1 µm) films of palladium and platinum containing close packed hexagonal arrays of
uniform sphere segment voids. We show that, even though these films are not rough, when the appropriate
film thickness and sphere diameter are employed these surfaces give stable, reproducible surface
enhancements for Raman scattering from molecules adsorbed at the metal surface. We report SERS spectra
for benzenethiol adsorbed on the structured palladium and platinum surfaces of different thicknesses and
void diameters and show that, for 633 nm radiation, enhancements of 1800 and 550 can be obtained for
palladium and platinum, respectively.
Introduction
Tian’s group have made extensive studies of surface roughen-
ing and surface structuring to create SERS active palladium and
Surface enhanced Raman spectroscopy (SERS) is a very
sensitive technique. It has been reported that molecules adsorbed
on an electrochemically roughened silver substrate produce a
Raman spectrum that is, in some cases, a million fold more
intense than expected.1-3 Therefore SERS has been widely used
to identify molecules adsorbed at metal surfaces and, for a range
of analytical applications,4,5 to study intermediates in electro-
chemical reactions6,7 and to study the structure of the electrode/
electrolyte interface.8,9 Unfortunately, surface enhancement at
roughened metal surfaces is only strong on the coinage metals,
Ag, Au, and Cu, and this limitation severely reduces the range
of applications of SERS. Although the Pt-group metals, e.g.,
palladium and platinum, have better surface stability and find
much wider application as electrodes and catalysts in electro-
chemistry and surface science, they have been commonly
considered as non-SERS-active substrates. There have been
considerable efforts over the past 20 years to expand the use of
SERS to the Pt-group metals, particularly by the groups of
Tian,10-12 Weaver,13-16 and Pe´rez.17,18
platinum surfaces using surface roughening by repetitive
potential cycling10,11,19 and by chemical etching.20 Low quality
SERS and resonance Raman spectra have been reported for
adsorbates with a large Raman cross section on roughened
platinum surfaces21 and mechanically polished Pt electrodes.22
The disadvantage of these roughened surfaces is that they have
a wide distribution of surface features that vary in shape and
size ranging from nanometers to microns. Thus, it is very hard
to judge which feature or size of surface geometry causes the
major part of the SERS activity. The observed enhancement is
thought to be due to nanoscale “hot spots” of tightly localized
plasmons which produce 1000-fold enhancements of the surface
electromagnetic field.23 The presence of this random distribution
of hot spots also accounts for the extreme variability of the
enhancement from place to place on the surface. Degradation
(11) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463-9483.
(12) Hu, J. W.; Zhang, Y.; Li, J. F.; Liu, Z.; Ren, B.; Sun, S. G.; Tian, Z. Q.;
Lian, T. Chem. Phys. Lett. 2005, 408, 354-359.
(13) Zou, S.; Weaver, M. J.; Ren, B.; Tian, Z. Q. J. Phys. Chem. B 1999, 103,
4218-4222.
† School of Chemistry.
(14) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953-5960.
(15) Leung, L. W. H.; Weaver, M. J. Langmuir 1988, 4, 1076-1083.
(16) Leung, L. W. H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113-
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‡ School of Physics and Astronomy.
(1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974,
26, 163-166.
(2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1-20.
(3) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215-
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(17) Go´mez, R.; Solla-Gullo´n, J.; Pe´rez, J. M.; Aldaz, A. J. Raman Spectrosc.
2005, 36, 613-622.
(18) Go´mez, R.; Pe´rez, J. M.; Solla-Gullo´n, J.; Montiel, V.; Aldaz, A. J. Phys.
Chem. B 2004, 108, 9943-9949.
(4) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Fel, M. S. Chem. ReV.
1999, 99, 2957-2975.
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F. M.; Mao, B. W. J. Raman Spectrosc. 1998, 29, 703-711.
(20) Huang, Q. J.; Yao, J. L.; Mao, B. W.; Gu, R. A.; Tian, Z. Q. Chem. Phys.
Lett. 1997, 271, 101-106.
(5) Laserna, J. J. Anal. Chim. Acta 1993, 283, 607-622.
(6) Shi, C.; Zhang, W.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990, 94,
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(8) Zhang, Z.; Imae, T. Langmuir 2001, 17, 4564-4568.
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(10) Tian, Z. Q.; Yang, Z. L.; Ren, B.; Li, J. F.; Zhang, Y.; Lin, X. F.; Hu, J.
W.; Wu, D. Y. Faraday Discuss. 2006, 132, 159-170.
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1990, 46, 285-294.
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J. AM. CHEM. SOC. 2007, 129, 7399-7406
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A R T I C L E S
Abdelsalam et al.
Figure 1. Scanning electron micrographs of structured palladium films produced using 600 nm diameter template spheres with thicknesses of (A) 160, (B)
250, and (C) 500 nm and of platinum films produced using 700 nm diameter template spheres with thicknesses of (D) 80, (E) 300, and (F) 520 nm. The scale
bar is 2 µm in each case.
made by manipulating the reduction kinetics of a polyol process.
The SPR properties of these triangular and hexagonal nanoplates
have been investigated, and the viability of using them as SERS
substrates has been demonstrated.26 The control parameters of
all these syntheses have been largely limited to temperature,
the concentration of precursor, and the choice of surfactant or
polymer. The ability to control and fine-tune the shape and size
has been modestly successful.
Nanoparticles are usually synthesized in solution, and it is
very hard to assemble them in a well ordered and controllable
way to produce the well-defined electrodes required for in situ
electrochemical SERS experiments. Nevertheless Tian’s group
has recently reported the preparation and assembly of platinum
nanothorn structures with sharp edges and used these to
demonstrate good SERS enhancements.27
A further concern when using roughened surfaces or surfaces
Figure 2. Spectrum of benzenethiol adsorbed on a structured palladium
made from arrays of nanoparticles, where the SERS originates
film produced using template spheres of 600 nm diameter and thickness
from molecules located at hot spots, is that the environment of
the molecules which produce spectra is very different from the
environment of the majority of the molecules on the surface
and therefore is unrepresentative of the species which are
important in electrode or catalytic processes.
Palladium and platinum SERS active surfaces have also been
produced using the technique of borrowed SERS. In this case
ultrathin films of palladium or platinum are electrodeposited
on SERS-active Ag or Au surfaces such as monodisperse Au
nanoparticles or roughened surfaces.10,15,16,28 Tian has recently
reported the utilization of the borrowed SERS technique to make
SERS substrates using gold core, platinum shell nanoparticles.
The shell thickness was one to five monolayers of platinum
320 nm. The spectrum was taken using a 633 nm HeNe laser, 3 mW power,
single 60 s accumulation.
of the surface enhancement is observed when cleaning of the
surfaces is attempted: the sharp nanoscale features that localize
the plasmons are easily etched or deformed, and these changes
in the surface lead to changes in the surface enhancement.
Exploiting the dependence of the surface plasmon resonance
(SPR) of nanoparticles on their size and shape, SERS active
surfaces have been made out of nanoparticles of various shapes
for a number of metals (Pt, Pd, Ru, and Co) using synthesis
procedures mainly based on the reduction of the metal salt or
thermal decomposition of organometallic precursors in the
presence of surfactants, polymers, and coordinating ligands.24,25
Triangular and hexagonal nanoplates of palladium have been
(26) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z. Y.; Xia, Y. J. Am.
Chem. Soc. 2005, 127, 17118-17127.
(24) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000,
287, 1989-1992.
(25) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am.
Chem. Soc. 2003, 125, 13914-13915.
(27) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Cui, L.; Ren, B.; Tian, Z. Q. Chem.
Commun. 2006, 4090-4092.
(28) Park, S.; Yang, P.; Corredor, P.; Weaver, M. J. J. Am. Chem. Soc. 2002,
124, 2428-2429.
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SERS at Structured Palladium and Platinum Surfaces
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Figure 3. SER spectra of benzenethiol recorded at different film thicknesses for sphere templated Pd films (A) 400, (B) 500, (C) 600, and (D) 700 nm. The
spectra were taken with a 633 nm HeNe laser, 3 mW power, single 60 s accumulation.
atoms.29 In borrowed SERS activity arises from the long-range
effect of the electromagnetic enhancement created by the SERS-
active substrate underneath. The borrowed SERS activity
decreases rapidly with increasing thickness of the overlayer.
Consequently the coated film has to be deposited with a
thickness of only a few atomic monolayers, and this leads to
the potential problem of effects from pinholes where the
underlying SERS active substrate is exposed. The experimental
difficulty of preparing pinhole free ultrathin films and the
possible instability of the film, especially when using them for
the in situ electrochemical SERS experiments or during
prolonged measurements, means that these ultrathin film
substrates are not ideal. In addition there are potential problems
arising from electronic interactions between the substrate metal
and the thin overlayer which could lead to differences between
the catalytic behavior of these thin overlayers and the bulk metal.
Thus, despite the success of using roughened surfaces,
nanoparticulate arrays, or borrowed SERS approaches, it is still
desirable to find new ways to obtain SERS directly from
palladium and platinum surfaces. We recently described a new
form of nanostructured metal surface in which the plasmon
modes can be engineered with precision30-33 through control
of the film thickness and void diameter, and we have shown
that for Au and Ag these surfaces show very significant SERS
enhancements.34-37 The plasmons observed on these nanostruc-
tured surfaces correspond to localized electromagnetic fields
that can be excited by incident light and can be confined strongly
in the region of the metal surface. Such surfaces are distin-
guished from other SERS plasmon substrates by their low
surface roughness, by their regularity, and by their sphere
(30) Kelf, T. A.; Sugawara, Y.; Baumberg, J. J.; Abdelsalam, M. E.; Bartlett,
P. N. Phys. ReV. Lett. 2005, 95, 116802.
(31) Kelf, T. A.; Sugawara, Y.; Cole, R. M.; Baumberg, J. J.; Abdelsalam, M.
E.; Cintra, S.; Mahajan, S.; Russell, A. E.; Bartlett, P. N. Phys. ReV. B
2006, 74, 245415.
(32) Cole, R. M.; Sugawara, Y.; Baumberg, J. J.; Mahajan, S.; Abdelsalam, M.
E.; Bartlett, P. N. Phys. ReV. Lett. 2006, 97, 137401.
(33) Bartlett, P. N.; Baumberg, J. J.; Coyle, S.; Abdelsalam, M. E. Faraday
Discuss. 2004, 125, 117-132.
(34) Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Cintra, S.; Abdelsalam, M. E.;
Bartlett, P. N.; Russell, A. E. Nano Lett. 2005, 5, 2262-2267.
(35) Mahajan, S.; Abdelsalam, M. E.; Sugawara, Y.; Cintra, S.; Russell, A. E.;
Baumberg, J. J.; Bartlett, P. N. Phys. Chem. Chem. Phys. 2007, 9, 104-
109.
(36) Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Kelf, T. A.;
Sugawara, Y.; Russell, A. E. Faraday Discuss. 2006, 132, 191-199.
(37) Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Cintra, S.; Kelf, T. A.;
Russell, A. E. Electrochem. Commun. 2005, 7, 740-744.
(29) Li, J. F.; Yang, Z. L.; Ren, B.; Liu, G. K.; Fang, P. P.; Jiang, Y. X.; Wu,
D. Y.; Tian, Z. Q. Langmuir 2006, 22, 10372-10379.
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Abdelsalam et al.
Figure 4. SER spectra of benzenethiol recorded at different film thicknesses for sphere templated Pt films (A) 400, (B) 500, (C) 600, and (D) 700 nm. The
spectra were taken with a 633 nm HeNe laser, 3 mW power, single 60 s accumulation.
as described previously.33,38 In order to avoid any possible borrowed
segment void geometry which allows confinement of localized
SERS effects all of the structured palladium and platinum films
plasmons within the cavities. Here we extend our work to show
presented here were prepared by electrodeposition onto evaporated
that electrodeposition of palladium and platinum through
palladium substrates. Other experiments using evaporated gold sub-
colloidal templates can be used to produce thin (<1 µm) films
strates rather than palladium gave identical results.
containing close packed hexagonal arrays of uniform sphere
The evaporated palladium substrates were prepared by evaporating
segment voids and that these surfaces show surface enhancement
30 nm of palladium onto 1 mm thick glass microscope slides. The
for Raman scattering from molecules adsorbed on them which
are comparable to or larger than those found for roughened
surfaces or borrowed SERS on these metals. The reproducibility
of the SERS enhancement across the sample is reported. We
also show that this enhancement depends upon engineering the
structure of the surface through the choice of template sphere
diameter and the thickness of the film for the particular exciting
laser wavelength used to record the Raman spectra. Because
the films we produce do not have high surface roughness and
are stable and reproducible, we believe that they will be useful
in studies of electrode processes and catalysis on representative
palladium and platinum surfaces.
palladium coated microscope slides were then cleaned in isopropanol
(analytical grade, Fisher Scientific) in an ultrasonic bath (CE, Ultrawave
Limited, Cardiff, UK) for 90 min and then chemically modified with
cysteamine (Aldrich) by soaking for 3 days in a 10 mM solution in
ethanol (HPLC grade, Rathburn) before assembling the templates (more
details can be found in our earlier publication33).
Templates were assembled from monodisperse polystyrene latex
spheres (diameters 400, 500, 600, and 700 nm, Duke Scientific, 1 wt
% aqueous suspension). The monolayer template was assembled by
the capillary evaporation method to produce a well-ordered hexagonal
monolayer of spheres on the palladium coated glass slide.
Palladium or platinum were deposited from 50 mM solutions of
ammonium tetrachloropalladate (purity 99.99%, Aldrich) or hexachlo-
roplatinic acid (purity 99.9%, Aldrich). Electrochemical deposition was
performed in a thermostated cell at 25 °C using a conventional three-
electrode system controlled by an Autolab PGSTAT30. The template-
coated palladium substrate was the working electrode with a large area
Experimental Section
The structured palladium and platinum substrates were prepared by
electrodeposition of palladium or platinum through a template of self-
assembled polystyrene latex spheres onto a suitable conducting surface
(38) Bartlett, P. N.; Baumberg, J. J.; Birkin, P. R.; Ghanem, M. A.; Netti, M.
C. Chem. Mater. 2002, 14, 2199-2208.
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SERS at Structured Palladium and Platinum Surfaces
A R T I C L E S
A Philips XL30 Environmental Scanning Electron Microscope
(ESEM) was used to image the structured metal films. All Raman
spectra were recorded on a Renishaw Raman 2000 system using a 633
nm HeNe laser with 5 µm diameter spot size and 3 mW power using
a single 60 s accumulation unless otherwise stated. Benzenethiol (purity
99.99%, Aldrich) was adsorbed onto the palladium and platinum
surfaces by soaking in a 10 mM solution in ethanol for 1 h. The samples
were then rinsed with ethanol and left to dry in air for 15 min before
measurement. The absence of unbound benzene thiol on the surface
was confirmed by the absence in the SER spectra of the -SH band at
2600 cm-1
.
The reflectivity of the nanostructured films was measured using an
optical microscopy arrangement (BX51TRF Olympus) using an inco-
herent white light source to illuminate the samples.33 Images were
recorded using a CCD camera (DP2 Olympus). Spectra from selected
areas of the sample (50 µm) were recorded at normal incidence using
a 20× IR objective with a numerical aperture of 0.40 and a fiber-
coupled spectrometer (Ocean Optics, spectral range 300-1000 nm,
resolution l nm) placed in the focal plane of the image. All spectra are
normalized with respect to that recorded for a flat vapor deposited
palladium film or a flat electrodeposited platinum film as appropriate.
Results and Discussion
Structured palladium and platinum surfaces were prepared
by electrodeposition through close packed monolayers of
polystyrene spheres with diameters between 400 and 700 nm
assembled onto evaporated palladium substrates. The thickness
of the electrodeposited film was controlled by varying the charge
passed during electrodeposition. After deposition the polystyrene
template was removed to leave the thin structured films
containing a regular hexagonal array of uniform segment sphere
voids. These surfaces are strongly colored, the precise color
depending on the viewing angle, void diameter, and film
thickness.33 Figure 1 shows typical SEM images of templated
palladium and platinum films of various heights grown through
templates made from monolayers of 600 nm or 700 nm diameter
polystyrene spheres, respectively. The micrographs show that
the spherical segment voids left after the removal of the
polystyrene spheres are smooth and uniform. The pore mouth
diameter and the topography of the spherical cavities change
with the film thickness.
Figure 5. SERS of benzenethiol adsorbed on structured (A) palladium and
(B) platinum substrates as a function of sphere diameter. The spectra shown
represent the maximum enhancements for each given sphere size; the film
thicknesses and template sphere diameters are indicated next to each
spectrum.
Electrochemical measurements of the surface area of these
structured surfaces were achieved by measuring the oxide
stripping and proton adsorption/desorption charge in acidic
solution for palladium and platinum respectively.39,40 A rough-
ness factor, that is the ratio of the electrochemically determined
surface area to the projected area, of 3.65 was obtained for a
platinum film made using 500 nm template spheres and grown
to 0.5D (where D is the diameter of the sphere), while a
roughness factor of 3.85 was obtained for a palladium film made
using 600 nm template spheres and grown to 0.6D. The
predicted increases in surface area due to the presence of the
sphere segment voids within the films, calculated on the basis
of the geometry and assuming that the insides of the sphere
segment cavities are perfectly smooth, are 2.3 for the palladium
film and 1.9 for the platinum film. Thus the modest increases
in the experimentally determined surface area are consistent with
a smooth, yet sculpted, surface. These results confirm that the
templated surfaces do not have high surface roughness compared
to the typical values of up to 500 found for the electrochemically
roughened surfaces conventionally used for SERS.11
platinum gauze counter electrode and a homemade saturated calomel
(SCE) reference electrode. Palladium and platinum films were deposited
under potentiostatic conditions at 0.25 V or 0.05 V vs SCE, respectively.
These conditions lead to the deposition of smooth metal films on the
template coated substrates. Following electrodeposition, the latex
spheres were removed by dissolving in dimethylformamide (DMF,
analytical reagent grade, Fisher Scientific) to leave an ordered array of
interconnected sphere segment voids.
Electrochemical measurements of the active surface area of the films
was carried out by recording cyclic voltammograms in 1 M H2SO4.
Before each experiment the solution was purged for 15 min with a
stream of argon gas to displace dissolved oxygen. The electrochemically
active surface areas of the nanostructured palladium films were
estimated by integrating the charge passed in the surface oxide stripping
peak and using the conversion factor of 424 µC cm-2 39
. For platinum
the active surface areas were estimated by integrating the charge
associated with the proton adsorption/desorption peaks and using the
conversion factor of 210 µC cm-2 40
. All solutions were freshly prepared
using reagent-grade water (18 MΩ cm) from a Whatman RO80 system
coupled to a Whatman “Still Plus” system.
The SER spectrum of benzenethiol adsorbed on a structured
palladium film (thickness 320 nm, template sphere diameter 600
(39) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 31, 29-38.
(40) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 353-376.
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Abdelsalam et al.
Figure 6. Normal incidence reflectance spectra for a stepped, structured palladium film grown through a 600 nm sphere diameter template together with
the corresponding optical and SEM images. The spectra are offset for clarity.
nm) is shown in Figure 2. Similar spectra were obtained using
templated platinum films. The spectra agree well with those in
the literature for benzene thiol on silver and gold substrates in
terms of both band positions and relative intensities.41,42 The
absence of the ν(S-H) stretching vibration peak confirms that
the benzenethiol is chemisorbed on the palladium substrate
through the Pd-S bond. In control experiments, no SERS
spectra are observed for benzenethiol adsorbed on the flat
evaporated palladium and platinum substrates or for nonstruc-
tured palladium and platinum films electrodeposited on the
evaporated palladium substrates under identical conditions. The
surface enhancement on the structured palladium and platinum
surfaces is reproducible from place to place across the surface.
Standard deviations of less than 12.5% were found for spectra
(n ) 10) recorded at different places on the samples. In addition,
the structured palladium and platinum surfaces were found to
be very stable: the palladium and platinum substrates can be
cleaned and reused following the removal of the benzenethiol
by electrochemical reduction in 0.5 M KOH solution, and
reproducible results were obtained using substrates up to 6
months after cleaning followed by the assembly of a fresh
monolayer of the thiol.
The effect of film thickness on the SERS intensity for 400,
500, 600, and 700 nm sphere templated substrates is shown in
Figures 3 and 4. These spectra were obtained using films that
were grown with a series of steps in thickness by withdrawing
the substrate in a series of steps (each ∼500 µm) from the
solution during electrodeposition. In this way the film height
was systematically increased in steps from 0 to 1D. It is clearly
evident that the SERS intensity varies with the film thickness
and the template sphere size indicating that the precise geometry
of the structured film is an important factor in determining the
enhancement. These results agree well with results previously
obtained for templated gold substrates.34-36 For palladium and
platinum surfaces made using various sphere sizes and using
633 nm excitation, the maximum enhancement always occurs
between 0.7 and 0.9 D, the precise value depending on the
template sphere diameter. For all the palladium and platinum
samples investigated, the most intense SERS spectra for
adsorbed benzenethiol obtained using 633 nm excitation are
shown in Figure 5 (the normalized film thicknesses are quoted
next to each spectrum). For both palladium and platinum the
largest SERS enhancements were obtained with 600 nm sphere-
templated nanovoid film at around 0.8 D. Enhancement factors
were calculated for both palladium and platinum films using
the method described in the literature assuming a surface
coverage of 0.45 nmol cm-2 for benzenethiol on platinum and
the surface roughness of 3.0.11,43 The calculated values for
palladium and platinum are 1800 and 550, respectively. The
enhancement factors reported here on the structured palladium
and platinum surfaces are reproducible from place to place
across the surface and from sample to sample when prepared
under identical conditions. Xia et al. have reported an enhance-
ment factor of 1.3 × 104 for 4-mercaptopryidine adsorbed on
Pd nanoboxes. The enhancement critically depends on the way
in which the nanoboxes aggregate upon drying, and therefore
the enhancement changes significantly from sample to sample
and from place to place within the same sample.44 For pyridine
adsorbed on the electrochemically roughened Pd Tian et al. have
reported an enhancement factor of 1800.45 For platinum Tian
has reported an enhancement factor as high as 2000 for adsorbed
pyridine on a platinum nanothorn substrate.27 Among all the
samples prepared in our study the smallest enhancements were
(43) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A.
T. Langmuir 1991, 7, 955-963.
(44) McLellan, J. M.; Xiong, Y. J.; Hu, M.; Xia, Y. N. Chem. Phys. Letters
2006, 417, 230-234.
(41) Han, S. W.; Lee, S. J.; Kim, K. Langmuir 2001, 17, 6981-6987.
(42) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir
1998, 14, 3570-3579.
(45) Liu, Z.; Yang, Z. L.; Cui, L.; Ren, B.; Tian, Z. Q. J. Phys. Chem. C 2007,
111, 1770-1775.
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as the film thickness increases, causing the dramatic changes
in the observed color of the sample.
The features observed in the reflectance are similar to those
previously reported for gold and silver.33 We have recently
shown that the absorbance features observed in the reflectance
spectra of template structured gold surfaces can be described
in terms of the relative contributions of the various plasmon
modes.34-37 When the film is very thin the surface takes the
form of an array of shallow dishes, ordered in a close packed
hexagonal lattice, with the top surface consisting of flat areas
of metal separating the dishes. Plasmons freely propagate on
the flat surface and multiply scatter off the rims of the dishes
resulting in plasmonic band gaps following a Bragg dispersion.46
As the film thickness increases the surface becomes strongly
corrugated, and in addition to the (Bragg) surface plasmons
travelling across the top surface, other plasmons are trapped
within the spherical cavities. For fully spherical voids, i.e., if
the top surface were to close over, the electromagnetic solutions
to an isolated spherical dielectric cavity within an infinite
expanse of metal correspond to Mie scattered modes from a
sphere.47 These are calculated from Maxwell’s equations
expanded in spherical coordinates, by matching boundary
conditions at spherical coordinates; more details can be found
in our recent paper.31 The Mie and Bragg modes interfere with
the incoming light, and this interference produces the series of
absorbance features observed in the reflectance spectra of the
thicker films. In recent studies we have used angle-resolved
reflectivity measurements of templated gold films to allow us
to fully map the spectral and angular dispersion of the different
types of plasmons and their interactions.31 Moreover we were
able to map the SERS signal intensity at different angles of
both the incident pump laser and the Stokes scattered photons
and thus clearly demonstrated the role of resonant plasmon
enhancement for both the incoming and outgoing light on the
SERS signal intensity.34 Sharp enhancements occur when the
laser is scanned through a plasmon resonance (ingoing) and also
when individual Raman scattered lines coincide with plasmon
resonances (outgoing); the Stokes scattered photons are not
emitted isotropically from the templated structure but come out
at particular angles. Consequently, to achieve maximal surface
enhancement templated substrates have to be carefully designed
bearing in mind not only the wavelength of the exciting laser
but also the geometry and numerical aperture of the spectrom-
eter.
Figure 7. Reflectance spectra of graded in thickness palladium substrates
made using (A) 600 and (B) 500 nm template spheres. The dashed red line
denotes the ingoing laser excitation at 633 nm and the solid blue line denotes
the out coming radiation at 703 nm corresponding to the 1570 cm-1 SERS
band of benzenethiol.
obtained with the 500 nm sphere templated structures, with
enhancements of 500 for palladium and 200 for platinum.
To relate the dependence of the SERS enhancement to the
geometry of the structured film, reflectance spectra were
recorded for the palladium and platinum surfaces as a function
of film thickness at the same positions on the graded substrates
at which the SERS were measured. Reflection spectra for a
templated palladium film are shown in Figure 6 together with
corresponding optical and SEM images (similar results were
obtained for platinum). The optical images show the striking
range of colors of the film when viewed at normal incidence,
changing from gray-green through red, orange, and blue, and
then back to gray-green as the film thickness increases from 0
to 1D. The corresponding SEM images show the associated
change in pore mouth diameter. Individual reflection spectra
are plotted on a log scale (off-set from one another for clarity)
with the sample thickness increasing from top to bottom. Dips
in reflectivity, corresponding to localized Plasmon absorption,
are observed which shift to progressively longer wavelengths
Comparing the SERS enhancement for the palladium and
platinum samples, the 500 nm sphere templated films gave the
smallest SERS enhancement while the 600 nm sphere templated
films gave the largest enhancement. This can be rationalized
by comparing their reflectance spectra, Figure 7. The SERS
enhancement depends on the matching of the incident and
exiting radiation with plasmons on the substrate. The wave-
lengths corresponding to the laser excitation, 633 nm (red line),
and Stokes scattered photons for the 1571 cm-1 SERS peak at
703 nm (blue line), are shown in Figure 6. As can be seen, for
the 600 nm sphere templated films both the wavelength of the
excitation laser and the Stokes scattered photons overlap with
a strong absorption feature in the reflectance spectra for the
thicker film. Therefore a large SERS signal is expected. For
(46) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824-830.
(47) Teperik, T. V.; Popov, V. V.; Garcia de Abajo, F. J. Phys. ReV. B 2005,
71, 085408.
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A R T I C L E S
Abdelsalam et al.
the 600 nm sphere templated film this overlap is maximized
for film heights of ∼0.8D. In contrast, in the case of the 500
nm sphere templated films the incident and Stokes scattered
radiation do not match well with the strong plasmon absorptions
in the spectra, and consequently these surfaces are expected to
produce much smaller surface enhancements. This analysis is
somewhat simplistic, since it does not take into account any
variation in the strength of the enhancement for the different
types of plasmon (Bragg or Mie) or the full angular dependence
of the reflectance spectra and the collection angle (numerical
aperture) of the spectrometer. A more detailed analysis is beyond
the scope of the present paper but will be the subject of a future
publication.
properties of the substrates. The surfaces can therefore be
precisely tailored to tune the surface plasmon modes to match
the requirements of the SERS experiment. The maximum
surface enhancement was obtained when the incident and Stokes
scattered radiation match plasmonic resonances of the surface.
The enhancements observed for the structured palladium and
platinum surfaces reported here are reproducible from place to
place on the sample (unlike SERS substrates produced by
electrochemical roughening or colloidal nanoparticles). The
structured surfaces are smooth, uniform, and very stable (unlike
the SERS substrates produced by ultrathin films), and excep-
tionally these surfaces can be cleaned and reused. Given the
relative ease of preparation and the robust and reproducible
nature of these surfaces, we believe that they have great promise
for applications in electrochemistry and analysis.
Conclusions
In this study we have shown that the templated electrodepo-
sition technique can be used to produce structured Pd and Pt
films that give SERS active substrates with significant signal
enhancements. An advantage of the structured surfaces reported
here is that both the template sphere diameter and the film
thickness can be varied to allow control over the optical
Acknowledgment. We thank Suzanne Cintra and Robin Cole
for assistance with the Raman and reflectance spectrometers
and the EPSRC (Grant number EP/C511786/1) for funding this
work.
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