Designed synthesis of Au/Ag/Pd trimetallic nanoparticle-based catalysts for sonogashira coupling reactions

Article abstract of DOI:10.1021/la101088d

Pdnp and Pd containing trimetallic nanoparticles (tnp) are synthesized by chemical methodwith cetyltrimethylammonium bromide as the capping agent. Compositionally, four different tnp are prepared and the particle sizes are characterized by UV-vis spectra,

Full text of DOI:10.1021/la101088d

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2010 American Chemical Society
Designed Synthesis of Au/Ag/Pd Trimetallic Nanoparticle-Based Catalysts
for Sonogashira Coupling Reactions
P. Venkatesan and J. Santhanalakshmi*
Department of Physical Chemistry, University of Madras, Guindy Campus, Chennai-600 025, Tamilnadu, India
Received March 22, 2010. Revised Manuscript Received April 16, 2010
Pdnp and Pd containing trimetallic nanoparticles (tnp) are synthesized by chemical method with cetyltrimethylam-
monium bromide as the capping agent. Compositionally, four different tnp are prepared and the particle sizes are
characterized by UV-vis spectra, HR-TEM, and XRD measurements. The catalytic activities of Pdnp and tnp
are tested using the Sonogashira C-C coupling reaction. The product yield and recyclability of the recovered catalysts
are studied. tnp (1:1:1) exhibited better catalysis than Pdnp, which may be due to the concerted electronic effects of the
Au-Ag core onto the Pd shell atoms.
The use of transition metal nanoparticles of Pt, Pd, Ru, Au, Cu,
Ag, and so forth as catalysts in organic C-C coupling reactions
has placed nanometal catalysis, an important frontier of research
Scheme 1. Sonogashitra C-C Coupling Reaction
1
in recent years. For this purpose, mono, bi, tri, and multimetallic
nanoparticles in solutions are prepared with appreciable stability
using a variety of capping agents such as surfactant micelles,
functional polymers, dendrimers, and so forth. The use of
2
for the nanoparticles of expensive noble metals such as palladium
and platinum. In view of “atom economy”, the synthesis of core/
shell nanoparticles having a cheap metal core and a noble metal
shell is desirable. Several palladium-containing bi- and trimetallic
nanoparticles including core/shell structures have been syn-
nanometal catalysts in many cases has simplified the reaction
procedures and has shown versatilities like product specificity,
3
recyclability of catalysts, high turnover, and so forth. Of late,
there have been many reports on the use of phosphine-free Pd
nanoparticles (Pdnp) as successful replacements of air and moist-
ure sensitive and inert atmosphere requiring conditions of Sono-
7
thesized. In most of these core/shell nanoparticles, however,
palladium is in the core and the other metal such as gold and or
silver covers the surface of palladium, which is not useful for the
catalytic reactions involving palladium as an active catalytic
4
gashira C-C coupling reactions (Scheme 1).
Palladium nanoparticles have been tested as catalysts for
8
5
,6
species. Luis M. Liz Marzan et al. synthesized core/shell Au-Ag
various reactions, because catalytic reactions occur on the
surface of the nanoparticles. This issue becomes very important
nanoparticles from the consecutive reduction of HAuCl and
4
AgNO ; Murali Sastry et al. synthesized core/shell Ag-Pd nano-
3
particles from the consecutive reduction of AgNO and H PdCl
*
jslakshmi@yahoo.co.in (J. Santhanalakshmi) and venkatesanorg@
gmail.com (P. Venkatesan).
1) (a) Narayanan, R.; E1-Sayed, M. A. Langmuir 2005, 21, 2027. (b) Li, Y.;
3
2
4
through the chemical method. In this study, the authors claimed
that palladium nanoparticles catalyzed the reduction of gold and
silver precursors and induced the deposition of Au-Ag atoms on
the surface of Pd nanoparticles. Herein, we report on the synthesis
of Au-Ag-Pd trimetallic nanoparticles (tnp) with Au-Ag rich
core/Pd-rich shell structure from the chemical method of gold,
silver, and palladium precursors.
(
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(
The current synthetic procedure is a modified version of the
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complexes. The colloidal dispersions of CTAB protected
(
Au-Ag bimetallic nanoparticles were prepared by refluxing of
4
1
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Langmuir 2010, 26(14), 12225–12229
Published on Web 05/13/2010
DOI: 10.1021/la101088d 12225

Article
Venkatesan and Santhanalakshmi
Figure 1. UV-vis absorption spectra of CTAB stabilized mono-
metallic Pdnp and trimetallic Au-Ag-Pd colloidal nanoparticles
at different time intervals in aqueous medium at 25 °C.
Figure 2. X-ray diffraction (XRD) spectra of the CTAB stabilized
trimetallic Au-Ag-Pd colloidal nanoparticles at different mole
ratios of a = 0.5:1:1, b = 1:0.5:1, c = 1:1:1, and d = 1:1:0.5,
respectively at 25 °C.
of monomer unit of CTAB against total metal (R) was kept 40 in
the present experiments. The colloidal dispersions of Pd nano-
particles are separately prepared by refluxing a H O/EtOH (1/1,
2
1
1
v/v) solution of PdCl in the presence of CTAB. The colloidal
2
dispersion of CTAB-protected Au-Ag (1/1) nanoparticle and
that of CTAB-protected Pd nanoparticles are mixed at room
temperature in the designated ratio. The mixed dispersions are
kept stirring at least for a 24 h at room temperature to complete
the self-organization reaction. The tnp with different molar ratios
of the precursor metal salts of gold, silver, and palladium could be
prepared without any precipitation. The solutions of all the
compositions are very stable over extended periods of time.
The completion of formation reactions of monometallic Pd,
bimetallic Au-Ag, and tnp was followed by UV-vis spectroscopy
as a function of time (Figure 1). The absorption spectra of the
mixtures changed as the duration time was increased, the surface
plasmon resonance (spr) bands moved closer to one another and
merged into a single peak over the course of the time intervals. The
bimetallic Au-Ag nanoparticles as conformed after 45 min, further
increased the time duration resulted in a continuous decrease in the
absorption band until, eventually, no absorption peak was apparent
after 60 min exposure. As the duration of the time was increased up
to 24 h (overnight), there is no distinct change in the shape or
magnitude of the absorbance. Finally, the color of the colloidal
nanoparticles is dark brown, confirming formation of palladium-
coated trimetallic nanoparticles. The Pd nanoparticle shows that a
sharp absorbance peak cannot be observed over the entire range
due to the brown color of the solution.
Figure 3. Transmission electron microphotographs (HRTEM) of
the CTAB-stabilized trimetallic Au-Ag-Pd colloidal nanoparti-
cles solution. (a) HRTEM (bar indicates 20 nm) and (b) electron
diffraction pattern spectra in aqueous medium at 25 °C.
Successive reduction strategies are an effective method to
prepare core-shell structure bi- and multimetallic nanoparticles.
One of the metal salts is reduced first to form the core, and then a
second metal is deposited on the surface of preformed mono-
metallic nanoparticles to form the shell. The XRD patterns of the
Figure 4. Scanning electron microscopy EDX spectra of CTAB
stabilized trimetallic Au-Ag-Pd nanoparticles at 25 °C.
Table 1. EDX Spectral Data of Trimetallic Nanoparticles in 1:1:0.5,
1:0.5:1, 0.5:1:1, and 1:1:1 Catalysis, Respectively
1
:1:1, 0.5:1:1, 1:0.5:1, and 1:1:0.5 stoichometric tnp fcc structures
are shown in Figure 2. This is in agreement with the electron
diffraction results shown, the presence of well-mixed Au-Ag-Pd
trimetallic nanoparticles. Also, the Palladium is present in the
shell region of tnp.
Figure 3a shows a typical TEM image of tnp (1:1:1) nanopar-
ticles, and the particles are monodispersed with an average
diameter of 4.2 ( 0.2 nm and a narrow size distribution.
Figure 3b shows the electron diffraction pattern obtained from
a selected area of Figure 3a. The bright rings with occasional
bright spots are due to the presence of polycrystals. Regarding the
variation of particle sizes of tnp with different compositions, there
is no appreciable change in the size and the size growth is only
within 10% of the 1:1:1 tnp.
initial atomic
composition
Au:Ag:Pd
atomic composition
in nanoparticles
Au:Ag:Pd
structure of
nanoparticle
0
1
1
1
.5:1:1
:0.5:1
:1:1
fcc
fcc
fcc
fcc
0.42:0.90:0.94
0.98:0.48:0.96
0.94:0.96:0.98
0.96:0.98:0.48
:1:0.5
Energy-dispersive X-ray analyses are also conducted by focus-
ing the electron beam on several different selected regions on each
sample (each region area 1 μm  1 μm) of the unsupported
Au-Ag-Pd nanoparticles. An EDX spectrum of Au-Ag-Pd
nanoparticles is shown in Figure 4. The compositions of
Au-Ag-Pd trimetallic nanoparticles in different regions are in
close agreement and no significant deviations (Table 1).
(
11) Tan, H.; Zhan, T.; Fan, W. Y. Chem. Phys. Lett. 2006, 428, 352.
1
2226 DOI: 10.1021/la101088d
Langmuir 2010, 26(14), 12225–12229

Venkatesan and Santhanalakshmi
Article
that the bases used have dramatic effects on the yields of cross-
coupling product in the Sonogashira reaction. Among the bases
screened, K CO and Cs CO gave the best results (Table 2, entry
2
3
2
3
3
, 6). In addition, a profound solvent effect on the reaction was
observed. DMF-water (3:1) was found to be the best solvents
Table 2, entry 12). Variation in mole percentage of the catalysts
(
for trimetallic Au-Ag-Pd indicates 0.5 mol % catalyst loading
produced maximum percentage yield, compared to 0.6 mol % of
the catalysts (Table 2, entry 3, 15 to 17). The temperature effects
studies show that the optimum reaction temperature is found at
1
20 °C, since 140 °C reaction temperature also produced the same
percentage yield (Table 2, entries 3, 12 to 14).
Under optimized reaction conditions (trimetallic Au-Ag-Pd
as catalyst, K CO as base and DMF-water), a series of aryl
2
3
bromides or iodides and phenyl acetylene were tested in the
reaction conditions, and results were summarized in Table 3.
The catalytic activity of trimetallic Au-Ag-Pd nanoparticle
toward the various aryl substituents reacting with phenyl acet-
ylene inthe Sonogashira reaction keeping the reactions conditions
of entry 3 in Table 2 constant has been performed separately. The
results are given in Table 3. It is found that reaction time varied
slightly for different substituents.
Figure 5. FTIR spectra of CO adsorbed on the CTAB-stabilized
monometallic palladium (a = Pdnp) and trimetallic (b = 1:1:0.5,
c = 1:1:1) nanoparticles, respectively, at 25 °C.
Infrared spectroscopy has been widely used to study the surface
chemistry of small, adsorbed molecules. The vibrational fre-
quency of adsorbed CO changes with the metal substrate and
binding structure. Hence, IRspectroscopy of CO on the surface of
trimetallic nanoparticles is expected to give information about the
surface of the nanoparticle. On gold nanoparticles, however, CO
The reaction occurred exclusively with aryl iodides and bro-
mides but not with aryl chlorides, and produced the correspond-
ing products of biphenyl acetylene with excellent yield (entries 1 to
1
2,13
3, Table 3). Various aryl halides and phenyl acetylene are
is known to be very weakly and reversibly adsorbed.
adsorbed CO is generally observed only at low temperatures.
The
1
2,13
employed in the Sonogashira reaction under the same catalysts
conditions, as shown in Table 3 (entries 4 to 8). The iodobenzene
with electron-donating ring substituents proceed with good yield
1
0
1
10
1
Gold and silver have fully occupied d-orbitals (5d 6s , 4d 5s ,
respectively) and exhibit weak coordination ability toward CO. It
is reported that only weak Raman and IR bands are observed for
(
entry 5, Table 3) unlike the reported yield values. When trime-
1
2,13
tallic Au-Ag-Pd nanoparticles serve as catalyst in the Sonoga-
shira reaction, better yields crop up.
The turnover frequency of the catalysts in the Sonogashira
reaction has been measured by recovering the mono-, bi-, and
trimetallic nanoparticle precipitates upon addition of dilute HCl
CO on gold and silver surfaces.
The monometallic Pd particle
1
4
IR spectra are reported by Toshima and co-workers. The IR
spectra of CO adsorbed on mono- and trimetallic nanoparticles
stabilized in CTAB is shown in Figure 5. A broad peak is seen at
-
1
2
1
030 cm and another strong absorption band is observed at
910 cm . The former band can be assigned to the CO adsorbed
-1
(pH < 3.0) to the reaction mixture remaining after the removal of
the product into the CH Cl phase. The precipitates are subjected
on the Pd surface at the terminal site (linear adsorption site) and
the latter to CO adsorbed at the bridging site (bridging adsorption
2
2
to Sonogashira reaction catalysis. It is found that 99.0%, 92%,
8%, and 46% product yields existed corresponding to fresh and
1
4
6
site). The relative intensities of these two peaks have been
1
5
first, second, and third reuses, respectively. Catalyst reuse the
fourth time showed that the product yield is low, but in the case
of monometallic Pdnp precipitated during the reaction. The cata-
lytic activity of the Pd colloidal solution decreased during the
reaction as the metal was precipitated. Similar observations were
made by Herrmann and co-workers for the Heck reaction between
reported to vary depending on the particle size constituted of
three distinct metals. Such compounds often showed better
activity and selectivity compared to mono- and bimetallic
1
6
colloids. In the present work, the catalytic activity of the Au/
Ag/Pd trimetallic nanoparticles and similarly sized monometallic
palladium nanoparticle for Sonogashira coupling reactions using
1
8
1
7
aryl halides and styrene using palladium colloid as catalyst. This
instability of the colloid might be due to the use of high tempera-
tures. We report that the tnp was no formation of any precipitate at
the end of the reaction. Therefore, further improvements on the
catalyst stability are necessary for practical applications.
an equal amount of palladium in the reaction mixtures.
We initially studied the reaction of iodobenzene with phenyl
acetylene in DMF-water (3:1) as a model reaction under gradual
heating from 20 to 120 °C in the presence of 0.5 mol % of
trimetallic Au-Ag-Pd nanoparticle and 3 equiv of various bases.
The results are summarized in Table 2. These results suggested
The metallic nanoparticles are definitely much less catalytically
activeduringthe secondcycle of thecouplingreaction. The reason
is a lower amount of nanoparticles present in the solution if
caused by precipitation of larger nanoparticles. Another possibi-
lity is that, if the number density has not changed, but only the size
is getting smaller, the smaller particles might not be as catalyti-
cally active as the larger particles. As a result, the lower catalytic
activity observed during the second cycle is due to a lower amount
of nanoparticles present in the solution because of larger nano-
particles aggregating and precipitating out of solution. In addi-
tion, surface poisoning by the products could be another reason.
(
12) Sun, S.-G.; Cai, W.-B.; Wan, L.-J.; Osawa, M. J. Phys. Chem. B 1999, 103,
2
460.
(
(
(
13) Bradshaw, A. M.; Pritchard, J. Proc. R. Soc. London, Ser. A 1970, 316, 169.
14) Wang, Y.; Toshima, N. J. Phys. Chem. B 1997, 101, 5301.
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Chem. Mater. 1992, 4, 1234.
16) (a) Matsushita, T.; Shiraishi, Y.; Horiuchi, S.; Toshima, N. Bull. Chem. Soc.
(
Jpn. 2007, 80, 1217. (b) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.;
Lee, J.; Hyeon, T. J. Am. Chem. Soc. 2004, 126, 5026. (c) Gholap, A. R.; Venkatesan,
K.; Pasricha, R; Daniel, T; Lahoti, R. J.; Srinivasan, K. V. J. Org. Chem. 2005, 70, 4869.
(
d) Li, P.; Wang, L.; Li, H. Tetrahedron 2005, 61, 8633. (e) Thathagar, M. B; ten Elshof,
J. E; Rothenberg Angew. Chem., Int. Ed. 2006, 45, 2886. (f) Sawoo, S.; Srimani, D.;
Dutta, P.; Lahiri, R.; Sarkar, A. Tetrahedron 2009, 65, 4367.
(
17) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 467.
(
b) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46. (c) Tykwinski, R. R. Angew.
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J. Organomet. Chem. 1996, 520, 257.
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Langmuir 2010, 26(14), 12225–12229
DOI: 10.1021/la101088d 12227

Article
Venkatesan and Santhanalakshmi
a
Table 2. Sonogashira Coupling Reaction Using Mono-, Bi-, and Trimetallic Nanoparticles with Various Effects
entry
catalyst type
catalysts (mol %)
base
solvent
temp
product yield (%)
1
2
3
4
5
6
7
8
9
Pdnp
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.3
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF
2
O
O
O
O
O
O
O
O
O
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
140 °C
100 °C
80 °C
94.0
96.0
99.5
99.0
98.0
99.0
66.0
82.0
56.0
76.0
45.0
99.0
94.0
92.0
88.0
90.0
99.0
46.0
---
Au-Pdnp
tnp (1:1:1)
tnp (0.5:1;1)
tnp (1:1:0.5)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
tnp (1:1:1)
---
2
2
2
2
2
2
2
2
Cs
NaOH
Et
NaOAC
K
K
K
K
K
K
K
K
K
2
CO
3
3
N
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
3
3
CH CN
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
120 °C
120 °C
120 °C
120 °C
120 °C
0.6
without cat
without cat
3
9
---
Na
CO
2 3
a
[
Phenyl acetylene] = 1.05 mmol, [Iodobenzene] = 1.00 mmol, 3:1 molar ratio of DMF-water, and 0.5 mol % catalyst at 2 h period.
Table 3. Catalytic Activity of Mono- and Trimetallic Nanoparticles
for Sonogashira Reaction
a
b
entry
catalysts
R
X
yield (%)
1
2
3
4
5
6
7
tnp (1:1:1)
tnp (1:1:1)
Tnp (1:1:1)
Tnp (1:1:1)
Tnp (1:1:1)
Tnp (1:1:1)
Tnp (1:1:1)
Tnp (1:1:1)
H
H
H
I
99.0
95.0
30.0
92.0
96.0
70.0
86.0
92.0
Br
Cl
I
I
I
4 - NO
4 - OCH
4 - CH
2 - OCH
2
3
3
3
I
Br
8
a
4 - COCH
3
Figure 6. Comparison of the catalytic activity of 4.2 nm Au/Ag/
Pd trimetallic nanoparticles (a = first cycle, b = third cycle) and
Reaction conditions: Phenyl acetylene (1.05 mmol), iodobenzene
2
CO (3.00 mmol), and 0.5 mol % catalysis, DMF-H O
(
(
1.00 mmol), K
2
3
4
.0 nm monometallic Pd nanoparticles (c = first cycle). Reac-
tion conditions: Phenyl acetylene (1.05 mmol), iodobenzene
1.00 mmol), K CO (3.00 mmol), and 0.5 mol % catalysis,
DMF-H O (3:1 v/v) 120 °C, for 2 h.
b
3:1 v/v) 120 °C, for 2 h. Isolated yield. All compounds were fully
characterized (see Supporting Information).
(
2
3
2
Recovery and Recycling of Nanoparticle Catalysts
After the completion of the reaction, 0.1 M HCl solution and
-fold excess of dichloromethane (CH Cl ) are added so that the
2 2
nanoparticles derived from the core/shell structure (an equal
amount of nanoparticles was used). The nanoparticle catalyst
can be recycled and reused at least three times without losing the
catalytic activity (entry 3 in Table 2).
3
trimetallic nanoparticle (Au-Ag-Pd) catalyst and the products
are correspondingly separated into the pH-responsive aqueous
and organic phases, respectively. The precipitate of pH-respon-
sive metallic nanoparticle catalyst was collected by ultracentrifu-
gation. The collected catalyst is dried and reused for the next cycle
of reaction. When the recovered catalyst is UV-visible spectra
recoated, the spr peaks are not as prominent as in the first cycle.
The same reaction mixture solution was used for recycling after
the addition of fresh amounts of the reactants. For recycling,
iodobenzene (1 mmol), phenyl acetylene (1.05 mmol), and po-
tassium carbonate (3 mmol) were added without addition of any
nanoparticle. The reaction mixture was then refluxed for another
The product yields were determined by gas chromatography.
Each catalytic reaction was carried out in at least two runs. Both
plots reveal that using 0.5 mol % Au-Ag-Pd (1:1:1) was suffi-
cient to catalyze the coupling with conversion and product yield
of g95%. We have successfully fabricated the first example of a
Au-Ag-Pd trimetallic nanoparticle using wet chemical method
of mixtures consisting of Au, Ag, and Pd colloids. Figure 6 shows
that the present Au-Ag-Pd trimetallic nanoparticles demon-
strated good catalytic properties on the product formation test
platform, performing better than a traditional Pd containing
monometallic catalyst.
2
h to complete the second cycle.
As clearly shown in Table 2, the trimetallic nanoparticles (tnp)
showed much better catalytic activity than mono- and bime-
tallic nanoparticles, which resulted from the larger number of
In conclusion, we have synthesized a novel family of Au-
Ag-Pd core/shell trimetallic nanoparticles from the wet chemical
method of metal-surfactant complexes. Surface plasmon peaks
1
2228 DOI: 10.1021/la101088d
Langmuir 2010, 26(14), 12225–12229

Venkatesan and Santhanalakshmi
Article
in UV-vis spectroscopy have confirmed the formation of mono-,
bi-, and trimetallic nanoparticles. The HRTEM photographs of
trimetallic nanoparticles showed the sizes as 4.2 ( 0.5 nm. The
particle size results from HRTEM and XRD agree well with each
other. SEM-EDX spectra confirmed the presence of the three
metals. Their catalytic activities were tested in the Sonogashira
coupling reactions. These novel palladium-containing trimetallic
nanoparticles are air- and moisture-stable, and the reaction can
be readily conducted under aerobic conditions. Trimetallic
Au-Ag-Pd nanoparticle catalysis of Sonogashira reaction pro-
duced better results than mono- and bimetallic catalysts. In fact,
the presence of metal nanoparticles as catalysts replaces environ-
mentally detrimental catalytic strong bases being used in
Sonogashira reaction. Metal nanoparticles as catalysts also gave
appreciably high yield of the products. The high catalytic activity
of trimetallic nanoparticles is probably due to the sequential
electronic effect between elements in a particle. More detailed
investigations of nanoparticle structure effects on the catalytic
activity and their applicability in other synthetic transformations
are currently underway.
Acknowledgment. The author thank to IIT Madras, India, for
recording TEM of samples recovered from different stage of
preparations. Financial support to the Department from DST-
FIST is also acknowledged.
Supporting Information Available: Experimental proce-
1
13
dures of Sonogashira reaction and H and C NMR spectra
of all compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
Langmuir 2010, 26(14), 12225–12229
DOI: 10.1021/la101088d 12229