Metallic nanoparticles from spontaneous reduction of silver(I) in DMSO. Interaction between nitric oxide and silver nanoparticles

Article abstract of DOI:10.1021/jp012670c

Here we report a novel pathway for the synthesis of silver nanoparticles. Spontaneous reduction of silver 2-ethylhexanoate [Ag(ethex)] takes place in dimethyl sulfoxide (DMSO) at room temperature. The reaction is slow and markedly depends on temperature l

Full text of DOI:10.1021/jp012670c

2482
J. Phys. Chem. B 2002, 106, 2482-2487
Metallic Nanoparticles from Spontaneous Reduction of Silver(I) in DMSO. Interaction
between Nitric Oxide and Silver Nanoparticles
G. Rodr´ıguez-Gattorno,† D. D´ıaz,*,† L. Rendo´n,‡ and G. O. Herna´ndez-Segura†
Facultad de Qu´ımica, UniVersidad Nacional Auto´noma de Me´xico, Coyoaca´n, Me´xico DF, 04510 Mexico, and
Instituto de F´ısica, Dpto. de Materia Condensada, UniVersidad Nacional Auto´noma de Me´xico,
Coyoaca´n, Me´xico DF, 04510 Mexico
ReceiVed: July 12, 2001; In Final Form: NoVember 29, 2001
Here we report a novel pathway for the synthesis of silver nanoparticles. Spontaneous reduction of silver
2-ethylhexanoate [Ag(ethex)] takes place in dimethyl sulfoxide (DMSO) at room temperature. The reaction
is slow and markedly depends on temperature leading to the formation of silver nanoparticles (NPs) with a
surface plasmon resonant band maximum centered at 424 nm. Colloidal silver is not stable in DMSO without
stabilizing agents. When sodium citrate (1 × 10-4 M) is utilized as a capping agent, the surface plasmon
shifts to 414 nm and the surface-modified silver nanoparticles are stable for more than 6 months. The resulting
nanoparticles are quite stable but at the same time reactive enough for catalytic purposes. An HR-TEM study
shows a nanoparticles size distribution centered in 4.4 nm of diameter (SD ) 1.2) and a considerable number
of defects such as stacking faults and twined particles. From ab initio quantum mechanical calculations, we
propose a possible precursor for the spontaneous reduction of [Ag(ethex)] in DMSO. In addition, the interaction
between NO and silver nanoparticles was tested. UV-visible spectra show the oxidation of silver and the
reduction of NO at room temperature. The most probable products of this reaction are N2O, molecular nitrogen,
and oxygen. Therefore, we have a simple catalytic colloidal system for NO.
Introduction
which many applications have been found (e.g. physical chem-
istry, medicine, and biology), there is little information about
the synthesis of metal nanoparticles in this medium. A previous
paper14 demonstrated the feasibility of using the 2-ethylhex-
anoate anion as a capping agent of CdS nanoparticles in DMSO;
this synthesis pathway is used in the present paper.
Different chemical methods have been used for the synthesis
of metallic nanoparticle (NP) dispersions, the most common
involving the use of an excess of reducing agents such as sodium
citrate1 or sodium borohydride.2,3 Other simple methods involve
the reduction of metallic salts by the solvent used to prepare
colloidal dispersions. For example, Toshima et al. have reported
a well-established method to obtain metallic and bimetallic
nanoclusters using alcohols in the presence of poly(N-vinyl-2-
pyrrolidone) (PVP).4-7 Vasan and Rao prepared Ag-Pd and
Cu-Pd nanoparticle alloys by using methanol or ethanol,8 and
Ayyappan et al.9 obtained Ag, Au, Pd, and Cu nanoparticles
by the reduction of metallic salts in dry ethanol. More recently,
Pastoriza and Liz-Marzan10 reported the preparation of silver
particles by the spontaneous reduction of AgNO3 in N,N′-
dimethylformamide. These contributions represent some of the
simplest ways to obtain colloidal dispersions because they
require a minimum number of components, thus avoiding un-
desirable excess of reducing agents and reaction byproducts.
Dimethyl sulfoxide (DMSO) is a relatively inexpensive, stable,
environmental compatible, and very useful solvent in organic
and inorganic synthesis due to its specific chemical and physical
properties. The dielectric constant of DMSO (46.7)11 is high
enough to allow charge separation; consequently, it becomes a
good solvent for ionic solids, polar and polarizable mole-
cules.12 DMSO is miscible with water in all proportions. In
general, sulfoxides are potential reductors by oxidizing itself
to sulfones.13 Despite DMSO being one of the few solvents for
Here we investigated the preparation of silver nanoparticle
dispersions by the simple dissolution of silver 2-ethylhexanoate
in DMSO. To ensure stability of the silver dispersions, sodium
citrate was used as an additional stabilizing agent. The ad-
vantages of such a nanoparticle synthesis method can be found
in the applications of silver nanoclusters as catalysts (most
organic reactions take place in nonaqueous solvents), and in
the study of the changes in surface chemistry the adsorption of
different moieties depends on the surrounding medium. Even
more, it has been known since ancient times that metallic silver
colloids act as an efficient bactericide, while DMSO has the
ability to easily penetrate the biological membranes; both com-
bined properties (i.e. silver nanoparticles suspended in DMSO)
could find application in the preparation of very effective
disease-fighting drugs.15-18
In addition, the novel reaction presented here opens new
possibilities in the field of solvent exchange, which represents
one of the main challenges in metal colloids with nanometer
size range. This should allow for dispersions of the particles in
both polar and nonpolar solvents thus avoiding particle ag-
gregation during the process.
Our results suggest the formation of a silver precursor species
with a convenient LUMO energy, which favors the silver
reduction process. To support the proposed mechanism for silver
reduction in DMSO, we performed ab initio quantum mechan-
ical computations arising to a simple silver complex structure
as the most probable precursor species. We also explored the
* To whom correspondence should be addressed. E-mail: david@
servidor.unam.mx.
† Facultad de Qu´ımica.
‡ Instituto de F´ısica, Dpto. de Materia Condensada.
10.1021/jp012670c CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/19/2002

Metallic Nanoparticles from Reduction of Ag(I)
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2483
interaction between silver dispersions and nitric oxide (NO)
under anhydrous, anaerobic conditions and at room temperature,
by means of UV-visible spectroscopy.
Experimental Section
Materials. Silver 2-ethylhexanoate {[Ag(ethex)] (Strem
Chemical, 99%)}, trisodium citrate dihydrated [Na3Cit‚2H2O
(Aldrich, 99%)], sodium 2-ethylhexanoate {Na[ethex] (Aldrich,
97%)}, sodium borohydride [NaBH4 (Aldrich, 98%)], silver
nitrate [AgNO3 (Aldrich, 99%)], silver citrate hydrate [AgO2-
CCH2C(OH)(CO2Ag)CH2CO2Ag‚xH2O (Aldrich, 99%)], silver
perchlorate monohydrate [AgClO4‚H2O (Strem Chemical, 99%)],
silver metavanadate [AgVO3 (Aldrich, 98%)], FeSO4‚7H2O
(Baker, 99.6%), NaNO2 (Baker, 99.99%), and argon (Praxair
Inc., 99.998%) were used as starting materials. These chemical
reagents were used without further purification. DMSO used
was Sigma (ACS reagent). It is strongly recommended to check
the purity of DMSO, especially to avoid the presence of volatile
contaminants. A procedure to eliminate the majority of the
volatile contaminates from DMSO includes the use of a rotary
evaporator at 50 °C during 2-3 h, under vacuum (10 mmHg).
The electronic absorption spectra were collected in a Hewlett-
Packard 8452A UV-visible spectrophotometer. An HR-TEM
study was carried out in a JEOL 4000EX at 400 KV, depositing
a drop of the sample over a carbon/collodion coated copper grid,
no special care being taken to avoid aggregation.
Figure 1. Evolution of UV-visible spectrum during the formation of
silver NPs in DMSO, from the spontaneous reduction of [Ag(ethex)]
(1 × 10-4 M) at room temperature. Numbers indicate the spectra for
different times: (1) recently dissolved; (2) 10 min; (3) 27 min; (4) 80
min; (5) 120 min; (6) 240 min; (7) 420 min; (8) 24 h.
Here R ) -(C2H5)CH(CH2)3CH3.
Water is required for mass balance reasons; enough of it
comes from the hydrated solvent. This reaction seems to take
place conforming to a reported mechanism by initial complex-
ation of the (CH3)2SO: to the metal ion species,
[AgOOCR] + (CH3)2SO: h [(CH3)2SOAgOOCR]
The size distribution was obtained from a digitalized amplified
micrograph by averaging the larger and smaller axis diameters
measured for each particle. Ab initio quantum mechanical
calculations of the type restricted closed-shell, Hartree-Fock
with a 3-21G(*) split valence basis set were carried out; all the
structures were previously optimized at the same level of theory,
HF/STO3-21G(*)//HF/STO3-21G(*). These computations were
performed using the version 1.5.2 of the PC-Spartan Plus
program.19
Colloid Preparation. The required quantity of [Ag(ethex)]
(0.001 25 g) was mixed with 50 mL of DMSO (previously
saturated with argon) in an Erlenmeyer flask to a final
concentration of 1 × 10-4 M. Stirring ensured the rapid
solubilization of the silver salt. The colloidal dispersions were
protected from light. Preparations at 60 °C were performed in
a hot plate thermostat.
followed by electron transfer to form a sulfoxide radical cation
([(CH3)2SO‚]+),
[(CH3)2SOAgOOCR] f
Ag0 + [(CH3)2SO•]+1 + [:OOCR]-
which then transfers a second electron,
[(CH3)2SO•]+ + [AgOOCR] f
[(CH3)2SO]2+ + Ag0 + [:OOCR]-
Therefore, the cationic species ([(CH3)2SO]+2) is attacked by
water forming the sulfone:
[(CH3)2SO]2+ + H2O f
[(CH3)2SOOH2]2+ f (CH3)2SO2 + 2H+
Glassware was cleaned with moderately concentrated nitric
acid and then washed with Alconox detergent and wiped away
with plenty of ultrapure water (18 MΩ, obtained from a Barnsted
E-pure deionization system). Experiments of silver colloid
dispersion interactions with NO were carried out in a sealed
UV-visible cell filled with 3 mL of 2.5 × 10-4 M silver
dispersion. NO was generated from the reaction between
FeSO4‚7H2O and NaNO2.20 The gas and silver dispersions were
dried with previously activated Linde 4 Å sieves at 500 °C for
16 h.21 NO was dragged with an argon gas flow of 0.6 L/min.
The quantity of NO was approximately controlled by careful
additions of known quantities of the ferrous salt over the nitrite
salt, yielding NO in an amount lower than 1 × 10-7 mol.
This happens in the same way with other transition metals (for
example, Ce4+) used to oxidize sulfoxides to sulfones.13
In addition, under darkness and in anaerobic conditions, the
silver reduction reaction in DMSO takes place.
The silver reduction reaction becomes apparent after 10-15
min at room temperature (approximately 23 °C); the colorless
and transparent DMSO solution turns from light yellow to
yellowish-orange. Figure 1 shows the time evolution spectra of
the typical surface plasmon resonant band for silver NPs
resulting from the reduction of 1 × 10-4 M [Ag(ethex)] in
DMSO. As can be seen, the process is slow, and after 24 h the
relative increase of the half-width at the maximum of the peak
becomes more significant than its increase in intensity. This
implies an increment in particle size, easily verified by the
gradual appearance of a silver precipitate. The reaction, however,
cannot be observed at room temperature, after 1 week, when
the silver salt concentration is as low as 0.5 × 10-4 M, probably
due to the slowness of the reaction. Though raising the
temperature can accelerate the progress of the reaction, the
agglomeration of nanoparticles is also favored.
Results and Discussion
Since the oxidation of DMSO implies the formation of
dimethyl sulfone,22 the following total reaction might take place
during the reduction of [Ag(ethex)]:
2[AgOOCR] + (CH3)2SO: + H2O f
2 Ag0 + (CH3)2SO2 + 2H+ + 2[:OOCR]-

2484 J. Phys. Chem. B, Vol. 106, No. 10, 2002
Rodr´ıguez-Gattorno et al.
Figure 2. (a) Diagram of calculated HOMO and LUMO energies resulting from ab initio quantum mechanics computations HF/STO3-21G(*)//
HF/STO3-21G(*) for several possible precursor species in the reduction of silver 2-ethylhexanoate in DMSO. In parentheses are the resulting point
groups for each molecule. (b) Image shows a projection of the calculated LUMO surface for the [Ag(ethex)(DMSO)] species.
Other silver salts such as the nitrate, perchlorate, and
metavanadate were also tested as silver sources, but DMSO does
not reduce them even upon heating to 80 °C. However, silver
citrate starts to reduce at 70 °C in DMSO. In this case, silver
citrate yields silver nanoparticles that are stable for less than
12 h. To stabilize them it is necessary to add at least a 4-fold
excess of citrate anions to the total silver concentration. An
explanation can be that citrate acts both as reducing and
stabilizing agent. During these preparations, it was found that
silver nitrate was slowly reduced to silver nanoparticles in
DMSO at room temperature but only in the presence of
trisodium citrate, which suggests that it proceeds by the direct
reaction between Ag+ and the citrate anion. The optical response
in this case is similar to that reported by Rivas et al.23 for
particles ranging between 30 and 60 nm in diameter, where the
absorption electronic spectrum displays an asymmetric plasmon
resonant band centered in 434 nm. In water this reduction
reaction is not observed at room temperature.
For the purpose of finding out which silver species acts as a
precursor in the reduction process of this metal, we carried out
the following experiment: A AgNO3 solution (with final
concentration 1 × 10-3 M) was mixed with equimolar Na[ethex]
solution, in DMSO, at room temperature. After 5 min, the
starting colorless solutions turned into a brownish-orange
transparent dispersion. A wide plasmon band centered at 418
nm was developed in the electronic absorption spectra. These
results suggest the formation of a silver-ethex coordination
compound as the precursor species for the silver reduction
reaction.
To support the silver-ethex precursor formation we carried
out several ab initio quantum mechanic geometry optimizations
at the restricted Hartree-Fock level and with the 3-21G (*)
basis set, i.e., at the HF/3-21G(*)//HF/3-21G(*) level of theory,
for the following molecules: [Ag(DMSO)4]+; [Ag(DMSO)2]+;
[Ag(H2O)(DMSO)]+; [Ag(ethex)2]-; [Ag(ethex)(H2O)]; [Ag-
(ethex)(DMSO)]. All the calculated structures were previously
optimized using the same calculation level. Some of the more
relevant results are shown in Figure 2. We took in account the
following experimental facts. It is well-known that the Ag+ ion
shows a pronounced tendency to exhibit linear, 2-fold coordina-
tion.24 However, the silver cation in DMSO can form di- and
tetrasolvated species [Ag(DMSO)2]+ and [Ag(DMSO)4]+, as
reported by Ahrland.25 Since other silver salts soluble in DMSO
(e.g. AgNO3) are not reduced spontaneously in this solvent, it
can be deduced that the anion 2-ethylhexanoate should be part
of the most probable precursor species such as [Ag(ethex)-
(DMSO)]. It is well-known that DMSO interacts with many
different transition cations generating stable complex species,
as [Cu(DMSO)2]2+, [Hg(DMSO)6]2+, [Cd(DMSO)6]2+, [Mn-
(DMSO)6]2+, and many others.26
In aqueous solutions, silver salts may generate [Ag(H2O)2]+;
however, the addition of DMSO to water may cause dehydration
of the Ag+ ion to leave free coordination sites for DMSO
molecules.27 On the other hand, the interaction DMSO-H2O
is stronger than Ag+-H2O; therefore, we reject the idea of
possible formation of a complex containing coordinated water.
On the basis of Koopmans’ theorem, the negative values of
the HOMO and LUMO energies can be related to the ionization
potentials and the electron affinity, respectively.28,29 From the
results of our calculations, it is possible to propose the species
that would act as precursor for silver reduction, on the basis of
the fact that this precursor should have the highest LUMO
energy and the lowest difference between HOMO-LUMO gap
(hardness). In Figure 2, we show an energy scheme in which
the calculated HOMO and LUMO energies were plotted for
each possible precursor.
As can be seen, the highest LUMO energy corresponds to
the [Ag(ethex)2]- molecule but its formation is very unlikely
because there are not sufficient ethex anions to complete this
composition. The molecule [Ag(ethex)(DMSO)] is the next with
an adequate value for the LUMO energy and is the only
molecule for which we do not have arguments to discard as the
most possible precursor for silver reduction in DMSO from
silver 2-ethylhexanoate. At the right side of the energy diagram
it is shown the LUMO surface of this molecule where one can
observe that the charge acceptor site is centered over the silver
atom, as can be expected.
Due to silver nanoparticles dispersions being unstable, it is
necessary to use capping agents to stabilize them. Among the
known stabilizers, trisodium citrate is a simple molecule with
a well-defined composition, which is recognized as a good silver
stabilizer.30
Other stabilizers such as sodium dodecyl sulfate (SDS) and
ammonium tetrathiomolybdate were tested, but the former does
not work and, especially, the latter blocks the surface plasmon
resonant band. Also was tested poly(N-vinyl-2-pirrolidone)
(PVP) as capping agent, which shows a plasmon resonant band
centered in 401 nm that shifts to higher wavelength in few days;

Metallic Nanoparticles from Reduction of Ag(I)
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2485
Figure 3. Electronic absorption spectra of silver nanoparticles in
DMSO obtained from AgNO3 (1 × 10-4 M) using a stoichiometric
quantity of sodium borohydride and sodium citrate as a stabilizing agent
(1 × 10-4 M): (1) after mixing; (2) 12 min; (3) 43 min; (4) 120 h.
Top insert shows the spectral changes at different times of naked silver
nanoparticles in DMSO obtained from AgNO3 (1 × 10-4 M) by
reduction with a stoichiometric quantity of sodium borohydride.
Figure 4. Spectral variations with time during the silver NPs synthesis
in DMSO from Ag(ethex) (1 × 10-4 M) in the presence of sodium
citrate (1 × 10-4 M), heated at 60° C for 20 min: (1) 7 s after mixing;
(2) 18 s; (3) 34 s; (4) 2 h; (5) 44 h. Top plot shows the variation of the
intensity at the maximum of the plasmon resonance band versus time.
Sodium citrate acts as a stabilizer, but when combined with
the starting material, 2-ethylhexanoate, the silver nanoparticles
are stable for longer periods (compare Figures 3 and 4).
stability appears to be low because after 1 week the plasmon
becomes broad and asymmetric. Finally, after 8 days the PVP-
capped silver nanoparticles start to aggregate, which could be
a result of the stronger interaction of PVP-DMSO in compari-
son with PVP-silver nanoparticles interactions. PVP is a good
stabilizing agent for metallic nanoparticles; consequently, we
do not discard the possibility that the stability, in the case of
PVP-silver nanoparticles, could dependent on PVP concentra-
tion.
To test the stabilizing effect of DMSO on the particles we
decided to carry out the reduction of silver nitrate (1 × 10-4
M) using a conventional reductor, sodium borohydride;31 in 1:1
stoichiometric ratio, it produces NPs that are stable for less than
40 min (see insert in Figure 3). In addition, any attempt to
stabilize the nanoparticles thus obtained using sodium citrate
as a stabilizer resulted in dispersions containing larger particles,
which precipitated after 5 days.
Otherwise, the absorbance at the maximum of the plasmon
resonance band was plotted against time for further clarity; see
the upper insert in Figure 4. The absorbance increased up to a
maximum value of 1.5 after 40 h. After this time the absorbance
decreased by 17% of the maximum value, becoming constant
after 11 days of preparation. No substantial changes in width
at half-height are noticed in the UV-visible spectra (88 ( 2
nm). This means that no important variation occurs in size or
size distribution of the silver NPs. These variations could be
related with two possible events: small displacement in size
distribution or slow equilibrium of absorption over the surface
of the nanoparticles. To validate this interpretation concerning
with such spectral variations it is necessary to carry out more
experiments supported by HR-TEM.
To rationalize our spectroscopic results with the nanoparticle
size distribution we performed Mie’s standard calculations to
simulate the electronic absorption spectrum by using the Drude
correction of the dielectric constants32 for the damping of
electrons at particle size near or under 5 nm. The calculations
were performed with the Mietab v.6.38 software.33 Routines of
this calculations have been previously described in detail.34
Contributions from particle size distribution were taken into
account, by assuming that HR-TEM micrographs approximately
describe our dispersion in this sense.35 The calculated absorption
maximum appears at 411 nm, differing by only 5 nm from the
experimental one (416 ( 2 nm), while the width at half-height
is 57 nm compared to 88 nm for the experimental value. These
differences have been attributed to the nature of the molecular
interactions of the surrounding medium with the surface of the
particle.36,37 The red shift of the peak position in our dispersions
with respect to classical aqueous silver colloids is predictable
since the higher refractive index of DMSO (1.4170) can be
compared to 1.333 for water at 20 °C.38 Additionally, Wang et
al. reported the same optical behavior (plasmon centered at 413
nm and a full width at half-maximum of 90 nm) for nanopar-
ticles of similar sizes capped with unsaturated carboxylates;39
then, we cannot discard the possibility that the position of the
peak is significantly influenced by the interaction of metallic
silver with the carboxylic groups of citrate and 2-ethylhexanoate
anions. Therefore, the silver molecular surroundings, solvent
and capping agents, determinate the plasmon resonance band
position.
It is clear from Figure 3 (see insert) that citrate stabilizes the
nanoparticles (in comparison with the same system without
citrate) and also contributes considerably to the increase of the
plasmon resonance band not only by raising the intensity but
also by broadening it.
We prefer to introduce the capping agent at the very beginning
of the reaction, keeping in mind that it is important to disperse
the nanoparticles before they grow. A disadvantage of sodium
citrate is its low solubility in DMSO, a difficulty that can be
overcome by heating or dissolving it in a minimal amount of
water. We decided to raise the temperature to 60 °C for 20 min
to allow the rapid formation of silver nuclei. Under these con-
ditions, the reduction of silver ethex would proceed via a simul-
taneous and cooperative reaction with DMSO and citrate ions.
Figure 4 shows the UV-visible spectra at different times for
the formation of silver nanoparticles in DMSO using [Ag(ethex)]
(1 × 10-4 M) as a starting material and trisodium citrate(1 ×
10-4 M) as a capping agent. The stability of these NPs is
outstanding since they remain stable for over 6 months. By
examining the results already presented, one can summarize as
follows: 2-ethylhexanoate behaves as a stabilizer since the
resulting NPs are more stable than either of the naked ones
obtained via the stoichiometric borohydride reduction.
Silver nanoparticles from silver citrate, with an excess of
citrate, are as interesting as those obtained from 2-ethylhex-
anoate but with less stability, which leads us to the next
statement.

2486 J. Phys. Chem. B, Vol. 106, No. 10, 2002
Rodr´ıguez-Gattorno et al.
Figure 7. Electronic absorption spectra of silver NPs (2.5 × 10-4 M)
after additions of equal quantities of NO. The top insert shows the
spectrum of NO dissolved in anhydrous DMSO. Key: (1) dispersion
as prepared; spectra 2-7 correspond to equal successive additions of
NO.
Figure 5. HR-TEM micrographs of the citrate- and 2-ethylhexanoate-
stabilized silver nanoparticles in DMSO from Ag(ethex). The arrows
point out some defects present in nanoparticles. The sample stabilized
after 11 days. The insertions show the corresponding diffraction pattern
(dhkl calculated match with metallic silver) and a magnification of a
single silver nanoparticle oriented along the 110 direction.
silver concentration is 1 × 10-4 M (i.e. quantitative reaction),
and the cell parameters correspond to those known for bulk
silver (a ) 4.06 Å).
In accordance with the report of Henglein,30 we found in our
samples a considerable number of particles that present massive
defects, such as multiple twinning and stacking faults, which
gives strong support to Henglein’s proposition. In general, many
of the defects are destroyed during the sample observation as a
consequence of the unavoidable increase in temperature during
the observation process. Thus, it is not possible to get a
quantitative relationship between defects and optical response;
however, we do not discard the possibility that citrate species
allow the generation of such defects, this statement so far
remaining only a suggestion.
Interaction of Silver Nanoparticles with NO. The adsorp-
tion and reactions of NO on various macrocrystalline transition
metal surfaces has received considerable attention. The presence
of an unpaired electron in the 2π molecular orbital of NO makes
its behavior much more complicated and less understood than
that of CO. NO is also technologically important in air pollution
and as a strong oxidizing agent. Besides, the majority of the
chemical model systems to study the interaction between
metallic silver and NOx utilize silver macrocrystals under high
vacuum and low temperatures, or else the silver is deposited
over a solid support as alumina or zirconia. In the previous case,
theinteractionwithNOx takesplaceunderhighertemperatures.41-44
Reports related with the interaction of NOx and colloid disper-
sions are too scarce in the literature. In 1999, Henglein reported
the interaction of N2O with metallic lead nanoparticles at room
temperature.45
Figure 6. Size distribution in the sample illustrated in Figure 6. A
total of 250 particles were counted. Particles not well defined in the
largest lumps were not considered.
TABLE 1: Approximate Dispersion Characteristics
Estimated from the Size Distribution in Figure 6, Assuming
Spherical Particles
concn
cluster concn
tot. surf
(m2)
surf/vol
% of atoms
at the surf
(atoms/mL) (particles/mL)
ratio (m-1
)
6.02 × 1016
8 × 1011
5 × 103
1 × 108
24
Another explanation for the overenlarged plasmon bands has
been proposed by Henglein, who argues that this is due to the
lattice imperfections that would act as potential barriers for
electrons in the conduction bands by shortening its pathway,
i.e., by increasing the damping of the electrons.40
Figure 5 shows HR-TEM micrographs of silver nanoparticles
capped with sodium citrate in DMSO, prepared from [Ag(ethex)]
and measured after 11 days. Roughly spherical nanoparticles
can be observed. By counting over 250 particles, we were able
to obtain the particle size distribution shown in Figure 6. Large
lumps, probably formed during sample preparation, were not
considered.
Very stable capped silver nanoparticles have been synthe-
sized; it was an important question to evaluate their catalytic
activity. Are they reactive enough for catalytic purposes? To
test silver NPs reactivity with NO it was necessary to remove
any remaining water in the reaction system. To keep NO gas
and silver colloids dry, pretreated 4 Å zeolites were used.
Nitric oxide in water generates an acidic medium, which leads
to spontaneous silver NP agglomeration. Figure 7 shows the
electronic absorption spectra of a silver colloid dispersion
reacting with NO. In this case, the silver concentration was 2.5
× 10-4 M, allowing the collection of several spectra on
consecutive additions of NO. When water is removed from
dispersions by using zeolites, the silver nanoparticles react with
the NO until the silver metal is completely dissolved. The
From the size distribution in Figure 6, it is possible to obtain
the features of the dispersions that are summarized in Table 1.
Parameters are approximate because they were calculated on
the assumption that the particles were spheres. The metallic

Metallic Nanoparticles from Reduction of Ag(I)
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2487
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plasmon resonance band decreases with each NO addition; the
area under the curve diminishes proportionally to NO additions.
After six additions of NO, the spectrum is similar to that of
NO in DMSO as a consequence of an excess of the dissolved
gas (for comparison purposes, the spectrum of NO in DMSO
was inserted in the upper part of Figure 7).
The colloid is not regenerated upon heating the dispersion
used in this experiment. Under our reaction conditions, it was
not possible to determine the NO and silver stoichiometric
coefficients exactly because the NO concentration was not
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silver.42
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Conclusions
A new and reproducible method for the synthesis of silver
nanoparticles is reported in this work. Very stable capped silver
nanoparticles with average diameter close to 4.4 nm and narrow
size distribution are prepared by addition of silver 2-ethylhex-
anoate to DMSO in the presence of trisodium citrate as a
stabilizer. Probably DMSO and citrate ions act as simultaneous
reducers of this silver salt. Our experimental and theoretical
results suggest that, in these colloidal DMSO dispersions, the
silver reduction takes place through a precursor species such
as [Ag(DMSO)ethex]. High concentration of defects, such as
stacking faults and twins, and citrate species interacting in
nanoparticles surface would be responsible for the modification
in the optical response of silver nanoparticles by enhancing
damping during plasmon resonance, thus overenlarging the
width of electronic absorption spectra.
UV-visible spectra show that silver nanoparticles in DMSO
react with NO, thus causing the dissolution of metal when the
dispersion is dry. Therefore, we have found a novel, simple,
and inexpensive chemical model system to study the interaction
of NO with metallic silver under anhydrous, anaerobic, and
room-temperature conditions. Ongoing research is concerned
with the reduction of different silver salts in DMSO, in the
presence of different solid supports.
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Acknowledgment. The authors thank the DGAPA-UNAM
for financial support through projects IN-100398 and IN-107700.
G.R.-G. thanks the DGEP-UNAM for a scholarship. The authors
express their gratefulness to Drs. A. R. Va´zquez-Olmos, E.
Zeller, J. Robles-Garc´ıa, and S. E. Castillo-Blum for their
revision and suggestions to this work. Also, we want to thank
to Mr. L.-A. Ortiz-Frade for his help in the experimental part.
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