Reaction of Pyrrole and Chlorauric Acid: A New Route to Composite Colloids

Article abstract of DOI:10.1149/1.1405802

Composite colloids of gold and polypyrrole were prepared using two different methods: 1, using pyrrole colloid, created by the oxidation of pyrrole by ferric chloride, to subsequently reduce chlorauric acid and, 2, oxidizing pyrrole monomer with chlorauric acid in a sodium dodecylbenzene sulfonate solution. In each case, the polypyrrole colloid consisted of irregularly shaped particles approximately 500 nm in diameter. The gold produced in each case was in the form of irregular spheres, approximately 407 nm in diameter in method 1 and 13 nm in method 2. X-ray photoelectron spectroscopy was used to determine the oxidation state of the species present. Transmission electron microscopy and light scattering data were used to determine the particle sizes of both gold and polypyrrole colloids. Energy dispersed spectrum X-ray analysis and electron diffraction were used to confirm the presence of metallic gold in the composite colloids. The second-order rate constant for the reaction of chlorauric acid with pyrrole in dilute solution was found to be 13 M ¹ s ¹. Aqueous solutions of palladium, platinum, rhodium, cobalt, tin, silver, zinc, nickel, titanium, cadmium, mercury, arsenic, and selenium were also examined for their potential to act as oxidants to produce composite polypyrrole colloids. Palladium, platinum, and rhodium salts were suitable oxidants, producing polypyrrole in less than 12 h.

Full text of DOI:10.1149/1.1405802

Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
D155
0
013-4651/2001/148͑11͒/D155/8/$7.00 © The Electrochemical Society, Inc.
Reaction of Pyrrole and Chlorauric Acid
A New Route to Composite Colloids
Matthew C. Henry, Chen-Chan Hsueh, Brian P. Timko, and Michael S. Freunda,z
Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18105, USA
Composite colloids of gold and polypyrrole were prepared using two different methods: 1, using pyrrole colloid, created by the
oxidation of pyrrole by ferric chloride, to subsequently reduce chlorauric acid and, 2, oxidizing pyrrole monomer with chlorauric
acid in a sodium dodecylbenzene sulfonate solution. In each case, the polypyrrole colloid consisted of irregularly shaped particles
approximately 500 nm in diameter. The gold produced in each case was in the form of irregular spheres, approximately 407 nm
in diameter in method 1 and 13 nm in method 2. X-ray photoelectron spectroscopy was used to determine the oxidation state of
the species present. Transmission electron microscopy and light scattering data were used to determine the particle sizes of both
gold and polypyrrole colloids. Energy dispersed spectrum X-ray analysis and electron diffraction were used to confirm the
presence of metallic gold in the composite colloids. The second-order rate constant for the reaction of chlorauric acid with pyrrole
Ϫ1 Ϫ1
in dilute solution was found to be 13 M
s . Aqueous solutions of palladium, platinum, rhodium, cobalt, tin, silver, zinc, nickel,
titanium, cadmium, mercury, arsenic, and selenium were also examined for their potential to act as oxidants to produce composite
polypyrrole colloids. Palladium, platinum, and rhodium salts were suitable oxidants, producing polypyrrole in less than 12 h.
©
2001 The Electrochemical Society. ͓DOI: 10.1149/1.1405802͔ All rights reserved.
Manuscript submitted April 13, 2001; revised manuscript received June 28, 2001. Available electronically September 21, 2001.
Methods for the preparation of colloidal gold have been known
pyrrole monomer. In addition, the nanoparticles thus produced are
surrounded by a shell of polypyrrole. In the aqueous system used in
our work, the colloidal gold is produced either as small ͑13 nm͒ or
large ͑407 nm͒ particles supported on a polypyrrole matrix. A mix-
ture of small and large particles is also theoretically possible if an
excess of chlorauric acid is used.
1
for hundreds of years. Over this time, colloidal metals have found
2
3
4
5
applications in biochemistry, cytology, microscopy, and catalysis
as described in recent reviews. Most of the procedures used to pre-
pare colloidal metals rely on micelles as a stabilizer containing ei-
ther metal salts or a reductant, thereby acting as microreactors to
control the size distribution of the metal particles produced. The
various morphologies that can be produced by this method have
been described as ‘‘cherries’’ ͑one central metal particle͒, ‘‘raspber-
ries’’ ͑multiple internal particles͒, ‘‘strawberries’’ ͑a layer of metal
particles outside the micelle boundary͒, and ‘‘red currant’’ or ‘‘gin-
ger root’’ ͑a dendritic type with strings of metal particles radiating
away from the central micelle͒.6
The literature gives several accounts of the ability of polymers to
act as reducing agents for metals in solution. For example it has
been reported that chlorauric acid (HAuCl ) can be reduced by
4
22,23
sheets of polypyrrole
and polyethylene oxide-polyethyleneimine
24
block copolymer. In the latter case, the oxidation of nitrogens in
the polyimine polymer accompanies the reduction of chlorauric acid
to metallic gold.
Under conditions in which the reductant is a monomer that un-
dergoes oxidative polymerization, a polymer/metal composite col-
loid can be formed. These are unique materials, with interesting
25
The production of polypyrrole colloids from aqueous and
26
nonaqueous media is also well known. For example, the oxidation
of the pyrrole monomer can be carried out rapidly using ferric chlo-
ride, which has been used for many years as an agent for preparing
7
8,9
10
11
electronic, magnetic, optical, and nonlinear optical properties,
depending on the metal and polymer employed. The metal thus sup-
ported can also be catalytically6,12,13 or electrochemically14-16 use-
ful, especially with a conducting polymer as part of the system.
Especially in terms of catalysis, polymer-supported precious metals
have many applications. For example, polymer-supported platinum
and rhodium catalysts have been used to carry out hydrogenation
‘
‘pyrrole blacks’’ ͑suspensions of pyrrole polymer particles͒ in
27
solution. The mechanism of this reaction is shown in Fig. 1 and
involves a one-electron oxidation of the pyrrole monomer to gener-
ate a radical cation. Two such radicals react to form a bond linking
the two pyrrole rings. Subsequent loss of two protons yields an
28
overall two-proton, two-electron oxidation. The formation of the
1
2
reactions. The polymer allows the control of the environment
around the metal center, thus influencing selectivity of the hydroge-
nation reaction.17,18 In terms of engineering applications, conducting
polymer-supported catalysts are attractive materials for fuel cell de-
polymer is favored by the fact that the oxidation potentials of the
n-mers decreases from ϩ1.30 V vs. SCE for the monomer to Ϫ0.10
29,30
V vs. SCE for the bulk polymer.
The polymer particles produced
31
32
19
in this fashion may be either spherical or irregular. The formal
potentials of the various oxidants used in this study are shown in
sign. For example, direct alcohol and proton exchange membrane
2
0
cell electrocatalysts based on conducting polymers have been stud-
3
3-35
Fig. 2.
The redox potentials of pyrrole and its oligomers are
ied.
shown for comparison.
Gold/polypyrrole composites have been prepared using both
Our goal is to investigate methods of producing polypyrrole/
noble metal colloids. Due to the previously cited reactivity of poly-
pyrrole with chlorauric acid, polypyrrole/gold composite colloids
were chosen as our model system. If these methods can be general-
ized to other polypyrrole/noble metal colloids, these materials may
find an application as catalysts.
solution-phase and vapor-phase polymerization in inverse micelles
in toluene.21 The stabilizer used in this case was a block copolymer
͓
polystyrene-block-poly͑2-vinyl
pyridine͔͒
and
produced
polypyrrole-coated gold particles in spherical, cubic, tetrahedral, and
octahedral shapes in solution phase and dendritic particles from the
gas-phase monomer. The micelle, in this case, acts as a microreactor
and the particle morphology is partially constrained by the phase
behavior of the surfactant system used. In the work cited, deviations
from well-controlled, spherical particle distributions reportedly oc-
curred around 13 nm, due to swelling of the reversed micelles by
In this study, we used preformed polypyrrole colloid, generated
by reacting the monomer with ferric chloride in a sodium dodecyl-
benzenesulfonate ͑SDBS͒ solution, to subsequently reduce chlorau-
ric acid. The particle size distribution and morphology of the result-
ing gold particles and polymer matrix has been characterized using
transmission electron microscopy ͑TEM͒. In order to verify that the
polymer was reducing the gold salt in solution, we used X-ray pho-
toelectron spectroscopy ͑XPS͒ to probe the oxidation states of
a
Present address: Molecular Materials Research Center, Beckman Institute, Califor-
nia Institute of Technology, Pasadena, CA 91125.
E-mail: msf@caltech.edu
z
36
polypyrrole. Energy dispersed X-ray analysis ͑EDS͒ detected the
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Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
for the reaction between pyrrole and chlorauric acid in dilute solu-
tions was determined by ultraviolet-visible spectroscopy and com-
pared to that reported for ferric chloride.
In an additional experiment, we conducted a screening test to
determine if other metals are capable of producing a polypyrrole
colloid in aqueous solution. Palladium, platinum, rhodium, cobalt,
tin, silver, zinc, nickel, titanium, cadmium, mercury, arsenic, and
selenium were examined as potential oxidants. Palladium, rhodium,
and platinum worked as oxidants, producing a polypyrrole black in
12 h or less.
Experimental
Reagents.—Pyrrole ͑Avocado Chemical͒ was distilled over zinc
dust prior to use. Solutions of pyrrole were either used immediately,
or stored under nitrogen and refrigerated in the dark. Chlorauric acid
(
HAuCl ) and palladium chloride were used as received from Ald-
4
rich. Potassium hexachloroplatinate was used as received from
Acros. Sodium dodecylbenzene sulfonate ͑SDBS͒ was used as ob-
tained from Aldrich. Ferric chloride and hydrochloric acid ͑ACS
grade͒ were used as received from Fischer scientific. Palladium͑II͒
chloride, platinum͑II͒ chloride, rhodium͑III͒ chloride, cobalt͑II͒ ni-
trate, tin͑II͒ chloride, silver͑I͒ nitrate, zinc͑II͒ chloride, nickel͑II͒
nitrate, titanium͑III͒ fluoride, cadmium͑II͒ nitrate, mercury͑II͒ ni-
trate, arsenic͑III͒ nitrate, and selenium͑IV͒ nitrate atomic absorption
͑AA͒ standard solutions ͑1000 mg/L͒ were used as obtained from
Fisher Scientific. The standards also contained ca. 5% HNO , HCl,
3
or HF stabilizer as appropriate.
Preparation
of
polypyrrole
colloids
with
ferric
Figure 1. Reaction mechanisms for the oxidation of pyrrole and polypyrrole
by chlorauric acid: ͑A͒ formation of radical cation, ͑B͒ coupling of radical
cations and elimination of two protons, ͑C͒ formation of dimer radical cat-
ion, ͑D͒ chain propagation, ͑E͒ polymer oxidation by HAuCl4 .
chloride.—Polypyrrole colloids were prepared as follows, based on
2
5
the method of Armes and co-workers, with concentrations
and surfactant modified as indicated. The polypyrrole colloid was
made by adding 1.6 mL of a 10% ͑w/w͒ solution of pyrrole in 0.5 M
HCl and 24.0 mL of a 5% ͑w/v͒ solution of SDBS in 0.5 M HCl, to
48.0 mL of 0.5 M HCl in a 250 mL Erlenmeyer flask. A total volume
presence of gold, but could not give speciation information. Electron
diffraction images provide additional evidence of the formation of
metallic gold.
of 1.6 mL of a 455 g/L ferric chloride solution was then added
dropwise using a syringe over a 5 min period, while stirring on a
magnetic stir plate. The mole ratio was 2:1, and there was 1 equiv
pyrrole for 1 equiv ferric chloride. The solution reached a uniform
black color within 1 h, however stirring was continued overnight to
ensure complete reaction. Occasionally, flocculation was observed
on the addition of the ferric chloride, but disappeared on stirring.
This may have been due to the formation of the less soluble ferric
dodecylbenzene sulfonate from the reaction of SDBS and some of
the ferric chloride. A gelatinous brown layer in the centrifuged prod-
Chlorauric acid was also reacted with pyrrole monomer under
conditions similar to those used to produce polypyrrole colloids with
ferric chloride, with the oxidant being present in slight excess in
both cases. The particles produced were characterized using the
same techniques as the colloids produced in the first part of the
study. We also examined the gold particles closely to determine if
the particles produced differ from those reported for the inverse
micelle system of Selvan and co-workers.21 Since chlorauric acid
uct was attributed to the formation of Fe͑DBS͒ , consistent with
3
37
39
has a chromophore that absorbs in the ultraviolet region, and poly-
previous reports.
pyrrole has an absorbance in the visible region,38 the rate constant
The colloidal polypyrrole produced by the above method was
subjected to multiple wash steps to remove any residual ferric and
ferrous chloride. The sample was placed in several 14 ϫ 89 mm
polyallomer ultracentrifuge tubes and spun at 70,000g in a Beckman
ultracentrifuge at 25°C for 30 min. The clear supernatant was de-
canted, and the pellet was resuspended in deionized ͑DI͒ water by
shaking and ultrasonication. The solution was washed twice in this
manner to remove FeCl , unreacted FeCl , and SDBS. At this point,
2
3
the density and viscosity of the solution were sufficiently low to
allow the samples to be spun in a bench-top centrifuge for 1 h at
5000g. The pellet was treated three more times by adding 1 mL of
a 10% ͑w/w͒ KCl solution to help remove excess surfactant. The
sample tube was then filled with DI water, and the sample was
resuspended and centrifuged. The theoretical yield is 12 mg, and a
single reaction typically gave 8-10 mg of product. The actual yield
is undoubtedly lower because this makes no allowance for doping of
the polymer by the HCl solution, counterions, or adsorbed surfac-
tant. The other colloid systems gave similar yields.
Preparation of polypyrrole/gold colloids by reacting polypyrrole
colloids with chlorauric acid.—The washed sample, prepared as de-
scribed above, was resuspended in 24 mL of 5.0% SDBS in 0.5 M
Figure 2. Formal potentials of pyrrole, pyrrole trimer, and the various oxi-
dants used to produce colloids in this study. All values are reported vs. SCE
in HCl ͑concentration as noted͒.
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Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
D157
0.1990 eV/step͒. All curve fitting was performed using a Voigt dis-
HCl, and 48 mL of 300 mg/L HAuCl solution in 0.5 M HCl was
4
tribution and a fixed linewidth. The data output of the instrument
was corrected for relative sensitivities of the elements studied. The
correction factors for the particular instrument were determined by
the electron spectroscopy for chemical analysis ͑ESCA͒ facility staff
and incorporated into the data analysis software.
added and stirred overnight using a magnetic stir plate. The sample
was again centrifuged and washed, using the protocol described
above.
Preparation of polypyrrole/gold colloids by reacting pyrrole with
chlorauric acid.—The samples prepared using chlorauric acid as an
oxidant were synthesized as follows. 1.6 mL of a 10% solution of
pyrrole in 0.5 M HCl and 24.0 mL 5% SDBS in 0.5 M HCl, were
added to a 250 mL Erlenmeyer flask. 48.0 mL of a 300 mg/L
UV-vis spectroscopy.—UV-vis spectroscopy was performed us-
ing a Shimadzu UV-2101 double beam spectrophotometer. The
chlorauric acid concentration was followed at its absorbance maxi-
mum at 3.2 eV ͑315 nm͒. The formation of the polypyrrole was
followed using its valence-to-conduction band transition absorbance
HAuCl solution in 0.5 M HCl was added dropwise over a 5 min
4
2
9,38
period with stirring. The mole ratio of pyrrole to HAuCl was 50:1,
4
at 1.5 eV ͑800 nm͒.
The reaction was carried out by placing 1.0
and there was a ratio of 33 equiv pyrrole to each equivalent of
mL of 0.05% ͑w/v͒ SDBS in 0.5 M HCl solution and 1.0 mL of 100
mg/L, chlorauric acid in 0.5 M HCl in 1 cm quartz cuvette. A vol-
ume of 36 ␮L of a 1% ͑w/v͒ pyrrole solution in water was added to
initiate the reaction. The sample was mixed by inversion, and placed
in the beam path in a thermostated cuvette holder set to 25°C. Read-
ings were taken four times per second for 5 min vs. a 0.025% SDBS
in 0.5 M HCl blank. The reaction was run twice to collect data at
both wavelengths.
This system was rerun using a 205.9 mg/L solution of ferric
chloride in place of the chlorauric acid solution. Since the molar
absorptivity of the ferric chloride solution was much lower than the
molar absorptivity of the chlorauric acid, it was not possible to
follow the change in ferric chloride concentration with time.
HAuCl . The solution was stirred overnight. The solution was cen-
4
trifuged using the protocol described above to produce the final
washed sample. No gelatinous precipitate was observed for these
samples, so sample clean-up was considerably easier.
Preparation of polypyrrole/metal colloids by reacting pyrrole
with Pd, Pt, and Rh metal salts.—Solutions were prepared as de-
scribed above, except that the oxidant was changed as follows. For
palladium, 48.0 mL of a 235 mg/L solution of PdCl in 0.5 M HCl
2
was used ͑mole ratio 35:1, equivalence ratio 35:1͒. The platinum
solution was 48.0 mL of 321.7 mg/L solution of potassium
hexachloroplatinate in 0.5 M HCl ͑mole ratio 71:1, equivalence ratio
3
6:1͒. The rhodium colloid was prepared from 4.36 mL of a com-
Rapid screen of potential oxidants.—1.0 mL of palladium͑II͒
chloride, platinum͑II͒ chloride, rhodium͑III͒ chloride, cobalt͑II͒ ni-
trate, tin͑II͒ chloride, silver͑I͒ nitrate, zinc͑II͒ chloride, nickel͑II͒
nitrate, titanium͑III͒ fluoride, cadmium͑II͒ nitrate, mercury͑II͒ ni-
trate, arsenic͑III͒ nitrate, and selenium͑IV͒ nitrate atomic absorption
standards were added to individual vials. 1.0 mL of a 1% solution of
pyrrole in water was added to each vial, and the mixture was al-
lowed to stand overnight. The samples were then visually examined
for the formation of polypyrrole black.
mercial 1000 ppm RhCl standard solution and 43.6 mL of 0.5 M
HCl ͑mole ratio 53:1, equivalence ratio 35:1͒.
3
Transmission electron microscopy (TEM).—Samples were ultra-
centrifuged ͑1/2 h at 70,000g͒, dried overnight, and then resus-
pended by ultrasonicating in chloroform, followed by dilution to a
ca. 5% solution. The samples were dispersed on carbon-coated cop-
per TEM grids ͑Ted Pella, Inc.͒. Grids can be damaged by the rapid
evaporation of a large amount of solvent. To avoid this, a single drop
was placed on the grid, and the excess solvent was removed by
capillary action by touching a corner of a laboratory wipe to the grid
edge. The TEM used was a Philips FEI 400T equipped with an
Oxford Link energy dispersed spectrum ͑EDS͒ X-ray analyzer.
Results and Discussion
TEM micrographs.—The oxidative polymerization of pyrrole
2
7,29
has been achieved by using a variety of chemical oxidants.
2
7,31,32
21,24
Through the addition of surfactants
it is possible to generate colloidal suspensions of polypyrrole. Figure
shows the TEM micrographs obtained for the oxidative
polymerization of pyrrole ͑30 mM͒ by chlorauric acid
n ϭ 3, 0.5 mM͒ and ferric chloride (n ϭ 1, 60 mM͒ under iden-
and block copolymers
Particle size analysis.—Particle size analysis was carried out on
a Coulter N4MD particle size analyzer. Sample colloids were diluted
with Nanopure deionized water until the instrument indicated that
the count rate was low enough to permit analysis. The samples were
measured in 1 cm disposable polystyrene cuvettes. The instrument
was operated in the simultaneous data processing mode to permit the
analysis of multimodal particle size distributions. The analysis was
performed at 25°C, the solution viscosity was set to 0.0089 P, and a
refractive index of 1.333 was assumed for the solution.
The particle size was also determined from the TEM. The NIH
Image V1.59 program ͑National Institutes of Health, Bethesda, MD,
USA͒ was used to determine the dimensions of the particles from
the TEM images. Since the particles of interest are nearly spherical,
the maximum particle dimension was taken as the diameter of the
particle. Particles touching the edge of the image were rejected from
the count.
3
(
tical solution conditions ͑0.5 M HCl and 0.16% SDBS͒. The TEM
results show that both reactions result in similar amorphous con-
ducting polymer structures, however in the case where chlorauric
acid is used, small clusters of metal particles, as evidenced by EDS
analysis and XPS ͑see below͒, are observed as shown in Fig. 3A.
Figure 4A shows some of the gold particles produced under the
conditions in Fig. 3A under higher magnification. Due to the small
size of these particles, it was difficult to obtain clear diffraction
patterns, however the spots observed in Fig. 4B suggest the presence
of crystalline metal, where the presence of gold was indicated by
EDS. EDS analysis did not indicate the presence of iron in any of
the samples. Figure 3A shows some circular, dense areas within the
polymer that are likely gold nanoparticles, or clusters of nanopar-
ticles. However the only clear images of these gold particles were
obtained from isolated clusters of metal particles located away from
the larger polypyrrole particles.
X-ray photoelectron spectroscopy (XPS).—XPS measurements
were obtained using a Scienta ESCA 300, with a monochromatic
source consisting of an Al K␣ line at 1486 eV from a rotating anode
operating at 1.0 kW. Analyses were performed at a photoelectron
takeoff angle of 90° relative to the sample surface with an analyzer
pass voltage of 150 eV. The centrifuged and dried pellet, prepared as
described in the TEM section above, was crushed and applied to a
carbon tape backing on a sample stud and placed in the instrument.
A survey scan was obtained from 5-1100 eV ͑one sweep at 0.991
eV/step͒. High-resolution measurements were also performed in the
gold 4f region from 80-92 eV ͑five sweeps at 0.0499 eV/step͒, the
nitrogen 1s region from 389-415 eV ͑four sweeps at 0.0498 eV/
step͒, and the iron 2p3/2 region from 700-740 eV ͑two sweeps at
In contrast, Fig. 5A shows the colloidal gold particles produced
upon exposing 2.1 g/L colloidal polypyrrole ͑produced under the
conditions described in Fig. 3B͒ to 1.5 mM HAuCl . These particles
4
represent either large gold particles or the clustering of a number of
smaller gold particles on top of the amorphous colloidal polypyrrole.
They were always found in intimate contact with polymer. The
nodular structure of these particles is similar to metals deposited on
carbon and conducting polymer electrodes. In particular, the resem-
blance to platinum particles electrochemically grown on polyaniline
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Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
Figure 3. TEM images of polypyrrole produced by the oxidation of 3.0 mM
pyrrole in 1.6% ͑w/v͒ SDBS/0.5 M HCl with ͑A͒ 0.5 mM chlorauric acid
Figure 4. Gold particles produced by the oxidation of pyrrole by chlorauric
acid under the conditions described in Fig. 3. ͑A͒ TEM image ͑410,000
times͒; ͑B͒ electron diffraction pattern obtained from the gold particle indi-
cated by arrow in Fig. 4A.
͑28,000 times͒ and ͑B͒ 7.7 mM ferric chloride ͑80,000 times͒. Arrows indi-
cate gold nanoparticles/clusters in the polymer matrix.
is striking.40,41 The large size of the particles made characterization
by electron diffraction difficult, however the diffraction patterns
could be observed from isolated bumps on the surface of the larger
particles as shown in Fig. 5B. This morphology is in contrast to the
previously reported rod-like structure at similar ratios of monomer
to oxidant. However, the gold/polymer colloids reported by Selvan
and co-workers21 were prepared in a system using a block co-
polymer in toluene in an inverse micelle system instead of SDBS in
an aqueous system. Since the previous work relies on the inverse
micelle to form a microreactor, the particle morphology is subject to
greater control, but it may also be altered by the phase behavior of
the micelle system. A given particle size/morphology combination
may be impossible to produce due to the formation of nonspherical
micelles at a given surfactant concentration. This effect can also be
exploited, and the rod-like micelle phase of a cationic micelle sys-
tem has been used to produce rod-like gold particles.4
system is also chemically simpler, using only a conventional surfac-
tant instead of the block copolymers.
Particle size.—Table I shows the particle-size data obtained from
light scattering experiments and from TEM micrographs. From the
table it is evident that the light scattering measurements overlook the
gold nanoparticles formed by the oxidation of pyrrole monomer by
HAuCl , reporting the larger, more numerous polypyrrole particles
4
instead. The TEM images therefore provide a more reliable measure
of the gold particle size. On the other hand, the polypyrrole particles
are highly irregular and tend to clump when deposited on TEM
grids. Since it was difficult to find an isolated particle for measure-
ment with the TEM, light scattering gave a better estimation of the
size of the polypyrrole colloid. Because of this difficulty, the TEM
value shown in Table I for the ferric chloride-only sample represents
a measurement performed on a single isolated particle ͑not shown͒.
2
The system described by Selvan and co-workers has the unique
property of producing gold particles surrounded by a shell of poly-
pyrrole. The system described in this paper produces gold particles
of various sizes exposed on the surface of a polypyrrole colloid. Our
UV spectroscopy.—Since the reaction of the chlorauric acid with
the pyrrole monomer was rapid, it was possible to observe the
change in concentration of the chlorauric acid as it was reduced by
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Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
D159
Figure 6. UV-vis spectra of pyrrole ͑1%͒, chlorauric acid ͑0.25 mM͒ ͑A͒,
polypyrrole as formed ͑B͒, and pyrrole monomer ͑150 mM͒͑C͒.
the pyrrole by monitoring the absorbance at 3.1 eV ͑315 nm͒ ͑see
Fig. 6͒. Under these experimental conditions, pyrrole, which had a
minimum absorbance throughout the spectral range of interest, was
present in excess. Therefore, the reaction was expected to be first
order with respect to chlorauric acid. The spectra in Fig. 6 shows the
absorbance of the reactants and the polypyrrole product following
the reaction in dilute solution. Note the valence-to-conduction band
transition at 1.5 eV ͑800 nm͒ and the ␲ → ␲* transition at 2.6 eV
3
8
͑477 nm͒, which is characteristic of polypyrrole.
Figure 7 shows the change in absorbance at 1.5 eV ͑800 nm͒ as
a function of time associated with the loss of chlorauric acid and the
generation of polypyrrole ͑due to the absorbance of the polymer and
scattering associated with the colloid formed͒. The inset in Fig. 7
shows the pseudo-first order plot for the reaction during the begin-
ning of the reaction where the spectra are dominated by chlorauric
Figure 5. Colloidal gold particles on polypyrrole produced from the reduc-
tion of 0.5 mM chlorauric acid by 2100 mg/L colloidal polypyrrole in a 0.5
M HCl solution containing 1.6% SDBS as a surfactant. ͑A͒ TEM image
͑13,000 times͒; ͑B͒ electron diffraction pattern obtained from the gold par-
ticle indicated in Fig. 5A.
Table I. Particle size data for polypyrroleÕgold colloids produced
by oxidation of 30 mM pyrrole in a 0.5 M HCl solution contain-
ing 1.6% SDBS as a surfactant, using the oxidant„s… indicated.
Oxidant concentrations were 0.5 mM for HAuCl and 60 mM for
4
FeCl3.
Oxidation method
FeCl3
Particle size ͑nm͒a
413 Ϯ 63
Particle size ͑nm͒b
500 Ϯ NA (n ϭ 1)
͑PPy only͒
͑PPy only͒
HAuCl4
368 Ϯ 63
12.8 Ϯ 9.4 (n ϭ 27)
͑Au only͒
͑Au&PPy͒
FeCl /HAuCl
422 Ϯ 35
407 Ϯ 212 (n ϭ 39)
͑Au only͒
3
4
͑Au&PPy͒
a
Determined with dynamic light scattering measurements, tolerance
Figure 7. Time course of the formation of polypyrrole from pyrrole and
HAuCl4 in dilute solution. Inset: first-order plot for the oxidation of pyrrole
Ϯ2 SD.
b
Determined from computer analysis of TEM micrographs, tolerance
Ϯ2 SD.
by HAuCl ͑0.13 mM͒.
4
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D160
Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
Figure 9. XPS spectra of the nitrogen 1s region of polypyrrole/gold colloids
produced by oxidation of 30 mM pyrrole in a 0.5 M HCl solution containing
Figure 8. XPS survey scan of polypyrrole/gold colloids produced by oxida-
tion of 30 mM pyrrole in a 0.5 M HCl solution containing 1.6% SDBS as a
surfactant, using FeCl ͑A͒, HAuCl ͑B͒, and FeCl followed by HAuCl ͑C͒.
0.5% SDBS as a surfactant, using FeCl3 ͑A͒, HAuCl4 ͑B͒, and FeCl3 fol-
3
4
3
4
lowed by HAuCl ͑C͒. Oxidant concentrations are 0.5 mM for HAuCl and
Oxidant concentrations are 0.5 mM for HAuCl and 60 mM for FeCl . Spec-
4
4
4
3
6
0 mM for FeCl . Spectra were taken from 398 to 415 eV at a 90° takeoff
tra were taken from 0 to 1200 eV at a 90° takeoff angle with a 150 eV pass
energy.
3
angle with a 120 mV pass energy.
acid. The corresponding pseudo-first-order rate constant was deter-
mined to be 0.034 sϪ1. Using the second-order rate equation
weight of the sample, it is surface-active and is over represented in
the XPS data. Figure 9 shows the high-resolution spectra of the
nitrogen 1s region. The broad peak is an indication that nitrogen
exists in several oxidation states. The observed spectrum could be fit
with three peaks matching assignments previously reported for
chemically oxidized pyrrole produced with ferric chloride.49 These
peaks include an amine nitrogen peak centered at 399.8 eV, a peak at
Rate ϭ k͓HAuCl ͔͓Py͔
͓1͔
4
The pseudo-first-order rate constant (kЈ) can be related to the
second-order-rate constant ͑k͒ ͑under conditions where ͓Py͔initial
Ϸ ͓Py͔final͒ by
400.4 eV due to protonated imine nitrogens, and a peak at 401.7 eV
k ϭ kЈ/͓Py͔
͓2͔
due to protonated amine nitrogens.
As shown in Fig. 9, changes in the nitrogen 1s region illustrate a
possible mechanism for the reduction of the gold from the oxidized
to the metallic state. The samples of pyrrole oxidized with chlorauric
acid and with ferric chloride show similar spectra, with a prominent
peak for amine nitrogen and smaller peaks for imine and amine.
However, when a chlorauric acid solution was applied to the pre-
formed polymer, there was an increase in the relative peak area
corresponding to the imine, indicating that it is the oxidation of
nitrogen from an amine to an imine that provides the source of
electrons for the reduction of the gold. A similar shift has been
reported for the XPS spectra of polypyrrole sheets used to reduce
For the 2.7 mM pyrrole solution used, a second-order rate constant
Ϫ1 Ϫ1
of 13 M
s
was determined. This rate is considerably faster than
Ϫ1 Ϫ1
43
that observed using ferric chloride ͑0.01 M
s
; in 0.3 M HCl͒
or potassium persulfate ͑no rate was reported by the author for per-
sulfate in aqueous solution, however the time-course data presented
44
was similar to that cited for FeCl3͒. This difference is likely due to
4
5
the higher standard potential of HAuCl ͑E° ϭ 0.754 V vs. SCE͒
4
35
relative to FeCl ͑E° ϭ 0.529 V vs. SCE͒. Persulfate has a higher
oxidation potential ͑E° ϭ 2.010 V vs. SCE͒, however, it acts as
3
46
both a free radical initiator and retarder thereby complicating the
22
47
chlorauric acid, however, the effect was less pronounced, probably
overall rate of polymerization. The reactive intermediate is thought
4
5,48
due to the smaller surface-to-volume ratio of the sheet form of poly-
pyrrole. Figure 10 shows the high resolution spectra obtained in the
gold 4f region. The 4f5/2 /4f7/2 doublet at 88 and 84 eV for the
pyrrole oxidized with chlorauric acid and for the chlorauric acid
reduced by preformed polypyrrole colloids is consistent with the
presence of metallic gold. The colloid sample formed by the oxida-
tion of pyrrole with ferric chloride show no peaks in the gold region,
as expected.
Table II shows the sensitivity-corrected absolute count rates ob-
tained for the gold/polypyrrole colloids. The N 1s count rates of the
three samples are of approximately equal magnitude, indicating that
the amount of sample exposed to the beam in each sample is roughly
equivalent. This data was used to calculate the relative count rates
shown in Table III.
to be a sulfate radical
rather than the persulfate ion itself, so the
higher oxidation potential of persulfate may not directly influence
the reaction rate.
X-ray photoelectron spectroscopy.—The XPS spectra support the
evidence from electron diffraction and EDS data that the features
observed in the TEM images are due to the presence of metallic
gold. Figure 8 shows survey scans for colloids formed by the meth-
ods described in Fig. 3. The XPS spectra show that there is gold
associated with the surface of the polypyrrole particles. Close ex-
amination of the iron region ͑not shown͒ indicates that there is no
iron present to participate in a redox cycle with the chlorauric acid.
It was therefore concluded that any oxidation of the preformed poly-
pyrrole colloid upon exposure to chlorauric acid was due to the
action of the chlorauric acid alone.
Examination of the relative count rates from the nitrogen 1s re-
gion and gold 4f region in Table III reveal several features. The
relative amounts of amine, protonated amine, and protonated imine
are similar for the samples oxidized with ferric chloride and chlo-
rauric acid only. However, the sample in which the preformed poly-
The survey scan also shows carbon and nitrogen from the poly-
mer, potassium, and chlorine associated with residual KCl from the
wash solution, and carbon, sulfur, and oxygen from the SDBS solu-
tion. Although residual SDBS is an insignificant part of the total
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Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
D161
Table III. Relative XPS intensities for various nitrogen oxidation
states, expressed as a percentage of the total nitrogen counts, for
polypyrroleÕgold colloids produced by oxidation of 30 mM pyr-
role in a 0.5 M HCl solution containing 1.6% SDBS as a surfac-
tant, using the oxidant„s… indicated. Oxidant concentrations are
0
.5 mM for HAuCl and 60 mM for FeCl .
4
3
Amine
N 1s
͑
Imine
Amine
Total gold
͑4f5/2 & 4f7/2
͑% total
ϩ
ϩ
N
1s
N
1s
͒
% total ͑% total ͑% total
Oxidation method
N cps͒
Ncps͒
Ncps͒
N cps͒
FeCl only
66.8
65.1
49.6
20.5
16.4
35.8
12.6
18.6
14.5
0.0
26.4
29.7
3
HAuCl only
4
FeCl , then HAuCl4
3
chloride, silver͑I͒ nitrate, zinc͑II͒ chloride, nickel͑II͒ nitrate, tita-
nium͑III͒ fluoride, cadmium͑II͒ nitrate, mercury͑II͒ nitrate, ar-
senic͑III͒ nitrate, and selenium͑IV͒ nitrate were examined as poten-
tial oxidants for pyrrole. Palladium͑II͒ chloride, platinum͑II͒
chloride, rhodium͑III͒ chloride were useful oxidants, producing a
polypyrrole black in 12 h or less. The reactivity of the metals was as
expected based on their oxidation potentials. The only unusual cases
are silver and mercury, which have redox potentials higher than
Figure 10. XPS spectra of the gold 4f region for polypyrrole/gold colloids
produced by oxidation of 30 mM pyrrole in a 0.5 M HCl solution containing
0
.5% SDBS as a surfactant, FeCl ͑a͒, HAuCl ͑b͒, and FeCl followed by
3 4 3
46
rhodium, yet failed to produce a pyrrole black within the 12 h
HAuCl ͑c͒. Oxidant concentrations are 0.5 mM for HAuCl and 60 mM for
4
4
period. This is likely due to the fact that the reaction, although
thermodynamically favorable, is kinetically very slow for these met-
als.
FeCl . Spectra were obtained at a 90° takeoff angle with a 120 mV pass
3
energy.
Detailed characterization of the colloids formed by platinum,
palladium, and rhodium represent avenues for further research.
Based on these results, platinum, palladium, and rhodium colloids
were produced as described in the Experimental section and tested
for catalytic activity.
pyrrole is oxidized by the chlorauric acid shows a decrease in the
amine nitrogen counts and a concomitant increase in the imine ion
peak. This change is expected since there is no more monomer
available, and any oxidation that takes place must involve the exist-
ing polymer.
Conclusions
Based on the stoichiometry of the pyrrole polymerization reac-
Ϫ
ϩ
ϩ
We have shown that gold colloids can be produced in an aqueous
solution via two different routes, using SDBS as a surfactant. These
methods yield two different particle size regimes. In either case, the
gold particles formed are accessible on the surface of the polymer
colloid, instead of trapped within a polymer coating as they are
when formed in an inverse micelle system.
tion ͑2e , 2H ͒ and the valence of gold in chlorauric acid (3 ), a
ratio of 3:2 for nitrogen to gold would be expected. In light of this,
the observed ratio of 2:1 for nitrogen to gold is low in the polymer
formed by reduction of HAuCl by the monomer. This probably
4
represents a surface effect, but it may also mean that some chlorau-
ϩ
ric acid is reduced only to the aurous (1 ) state and is removed in
Pyrrole can be oxidized by chlorauric acid, leading to the forma-
tion of polypyrrole particles with sizes on the order of 500 nm and
metallic gold nanoparticles with sizes on the order of 13 nm. The
gold particles have an irregular, amorphous structure associated with
a porous matrix of larger polypyrrole particles. The gold particles
produced are not coated with polypyrrole. If this technique is gen-
erally applicable to other noble metals these composite materials are
potentially useful as catalysts. XPS, EDS, and electron diffraction
confirm the presence of metallic gold, but we cannot rule out the
subsequent washing steps.
The ratio of gold to pyrrole nitrogens in the sample in which
chlorauric acid is reduced by the preformed polypyrrole colloid is
also consistent with the oxidation of a pyrrole nitrogen providing the
electrons for reduction of the metal. The ratio of nitrogen counts to
gold counts is 3.4:1. This ratio is low, but it is not less than the
lowest possible ratio of 2:1 with one positive charge carried by two
pyrrole rings in the polymer. Here again, the actual ratio is probably
much higher. The gold is formed only at the surface of the polypyr-
role, while the oxidized nitrogens may be either at the surface or
deep in the polymer particle. This leads to an overestimation of the
ratio of gold to nitrogen by XPS.
ϩ
possibility that some gold is only reduced to the aurous (1 ) state
and removed by subsequent washing. This reaction has been ob-
served in dilute solution and was shown to fit a second-order-rate
Ϫ1 Ϫ1
law. The reaction proceeded with a rate constant of 13 M
s .
Screening of other potential oxidants.—Palladium͑II͒ chloride,
platinum͑II͒ chloride, rhodium͑III͒ chloride, cobalt͑II͒ nitrate, tin͑II͒
Platinum, palladium, and rhodium salts were shown to be ca-
pable of producing pyrrole blacks in aqueous solution. Colloids pro-
Table II. XPS count rates for gold and various oxidation states of nitrogen, corrected for relative sensitivity, for polypyrroleÕgold colloids
produced by oxidation of 30 mM pyrrole in a 1.5 M HCl solution containing 1.6% SDBS as a surfactant, using the oxidant„s… indicated. Oxidant
concentrations are 1.5 mM for HAuCl and 60 mM for FeCl .
4
3
Total
N 1s
͑cps͒
Amine
N 1s
͑cps͒
Amine
Total gold
͑4f5/2 & 4f7/2
͑cps͒
ϩ
ϩ
Imine N 1s
N
1s
͒
Oxidation method
͑cps͒
͑cps͒
FeCl only
847
970
1128
566
631
560
174
159
404
107
180
164
0
256
335
3
HAuCl only
4
FeCl , then HAuCl
3
4
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D162
Journal of The Electrochemical Society, 148 ͑11͒ D155-D162 ͑2001͒
duced from these metals were screened for their ability to catalyze
the hydrogenation of nitrobenzene. The palladium colloid was found
to be catalytically active. The procedure described for gold may be
generally applicable to the production of a wide variety of
polypyrrole-supported precious metal catalysts. Preformed polypyr-
role colloids can also reduce chlorauric acid in aqueous solution.
Again, XPS, EDS, and electron diffraction confirm the presence of
metallic gold. The gold particles produced by this method are rela-
tively large ͑407 nm͒ and irregular, with a knobby, bumpy surface,
similar to metals grown electrochemically. Theoretically, this tech-
nique could be combined with the formation of polymer by polypyr-
role to give a mixture of large and small particle on the same poly-
pyrrole support.
XPS analysis reveals that the oxidation state of the pyrrole nitro-
gens in the polypyrrole produced by oxidation with chlorauric acid
is the same as in the pyrrole formed by oxidation with ferric chlo-
ride. When preformed polypyrrole colloids are used to reduce
chlorauric acid, a shift from amine to imine nitrogen is observed,
indicating that oxidation of the polypyrrole is responsible for the
reduction of the chlorauric acid. Gold-to-nitrogen count ratios are
consistent with the stoichiometry of the oxidative polymerization
and polymer oxidation reactions.
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The authors of this research paper acknowledge the allocation of
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California Institute of Technology assisted in meeting the publication
costs of this article.
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