Strong optical limiting of silver-containing nanocrystalline particles in stable suspensions

Article abstract of DOI:10.1021/jp9835014

Metal and metal sulfide nanoparticles are prepared using a method that is based on the rapid expansion of supercritical fluid solution (RESS) into a liquid solution and characterized using transmission electron microscopy and X-ray diffraction methods. The nanoparticles form solution-like stable suspensions in the presence of a stabilization agent such as poly(N-vinyl-2-pyrrolidone) (PVP) polymer. The stable suspensions allow systematic nonlinear optical measurements. The nanocrystalline silver metal and silver sulfide particles in PVP polymer-stabilized ethanol suspensions of high linear transmittance exhibit excellent optical limiting properties, with the optical limiting responses toward nanosecond laser pulses at 532 nm being much stronger than those of benchmark materials [60]fullerene and chloroaluminum phthalocyanine in solution. A comparison of the results with those of stable suspensions of other nanoparticles including cadmium sulfide, lead sulfide, and nickel suggests that the optical limiting properties are unique to the nanoscopic silver-containing materials. Mechanistic issues concerning the optical limiting performance of the silver-containing nanocrystalline particles are discussed, and a nonlinear absorption mechanism is proposed.

Full text of DOI:10.1021/jp9835014

J. Phys. Chem. B 1999, 103, 77-82
77
Strong Optical Limiting of Silver-Containing Nanocrystalline Particles in Stable
Suspensions
Ya-Ping Sun,* Jason E. Riggs, Harry W. Rollins, and Radhakishan Guduru
Department of Chemistry and Center for AdVanced Engineering Fibers & Films, Howard L. Hunter Chemistry
Laboratory, Clemson UniVersity, Clemson, South Carolina 29634-1905
ReceiVed: August 25, 1998; In Final Form: NoVember 2, 1998
Metal and metal sulfide nanoparticles are prepared using a method that is based on the rapid expansion of
supercritical fluid solution (RESS) into a liquid solution and characterized using transmission electron
microscopy and X-ray diffraction methods. The nanoparticles form solution-like stable suspensions in the
presence of a stabilization agent such as poly(N-vinyl-2-pyrrolidone) (PVP) polymer. The stable suspensions
allow systematic nonlinear optical measurements. The nanocrystalline silver metal and silver sulfide particles
in PVP polymer-stabilized ethanol suspensions of high linear transmittance exhibit excellent optical limiting
properties, with the optical limiting responses toward nanosecond laser pulses at 532 nm being much stronger
than those of benchmark materials [60]fullerene and chloroaluminum phthalocyanine in solution. A comparison
of the results with those of stable suspensions of other nanoparticles including cadmium sulfide, lead sulfide,
and nickel suggests that the optical limiting properties are unique to the nanoscopic silver-containing materials.
Mechanistic issues concerning the optical limiting performance of the silver-containing nanocrystalline particles
are discussed, and a nonlinear absorption mechanism is proposed.
Introduction
issues on the optical limiting behavior of the nanoparticles are
discussed.
Nonlinear optical materials for the manipulation of optical
beams in the passive method have received much recent
Experimental Section
1
attention. There is great current interest in the development of
organic and inorganic optical limiters for eye protection and
optical switching applications.1 Among potent optical limiters
are materials that show strong nonlinear absorptions (or reverse
Materials. Silver nitrate (AgNO3), cadmium nitrate tetra-
hydrate (Cd(NO3)2‚4H2O), and sodium sulfide (Na2S) were
purchased from Aldrich. Lead nitrate (Pb(NO3)2) and nickel
-4
3
5,6
chloride hexahydrate (NiCl ‚6H O) were purchased from Fisher
saturable absorbers), such as metallophthalocyanines,
2
2
fullerenes,7-14
15-17
Scientific. Nickel chloride hexahydrate (NiCl ‚6H O) was dried
and mixed metal complexes and clusters.
2
2
under vacuum at 100 °C for 16 h before use. [60]Fullerene (C60,
purity >99.5%) was obtained from BuckyUSA, and chloroalu-
minum phthalocyanine was purchased from Exciton. Both were
used without further purification. Hydrazine and sodium
borohydride (NaBH4) were obtained from Aldrich and used as
received. Anhydrous ammonia (purity >99.9999%) was
purchased from Air Products and filtered before use. Dimeth-
ylformamide (DMF) and methanol were obtained from Mallinck-
rodt and used as received. Absolute ethanol was purchased from
Fisher Scientific. It was distilled over molecular sieves and
filtered before use. Water was deionized and purified by being
passed through a Labconco WaterPros water purification system.
Poly(N-vinyl-2-pyrrolidone) (PVP) of average molecular weight
Mw ≈ 360 000 was obtained from Sigma and used without
further purification.
Measurements. The apparatus for the preparation of nano-
particles through the rapid expansion of supercritical fluid
solution (RESS) into a liquid solution process is illustrated in
Figure 1. It consists of a syringe pump for pressure generation
and pressure maintenance during RESS process and a gauge
for monitoring the system pressure. The heating unit consists
of a cylindrical solid copper block of high heat capacity in a
tube furnace. The copper block is wrapped with stainless steel
tubing coil and inserted tightly into a stainless steel tube to
ensure close contacts between the tubing coil and the copper
block for efficient heat transfer. The copper block/tubing coil
Systems exhibiting strong transient light scattering due to
photoinduced thermal processes, such as carbon black suspen-
sion, have also been investigated extensively for their strong
1
8-20
optical limiting responses over a broad wavelength range.
Recently, there have been several studies of the optical limiting
and nonlinear absorption properties of semiconductor nano-
particles.21-24
21
For example, it was reported that silver bromide
particles trapped in nanosols exhibit interesting optical limiting
characteristics.
We have recently developed a versatile method for the
preparation of a series of polymer-protected metal and metal
2
5,26
sulfide nanoparticles.
The method is based on the rapid
27,28
expansion of supercritical fluid solution (RESS) into a liquid
The nanoparticles thus produced are relatively
solution.2
5,26
narrowly distributed in size and form highly stable suspensions
in the presence of polymer as a stabilization agent. The stable
suspensions of different metal and metal sulfide nanoparticles
have allowed a systematic investigation of optical limiting
properties of the nanoscopic materials under solution-like
conditions. The results show that silver-containing nanocrys-
talline particles in polymer-stabilized suspensions of high linear
transmittance strongly limit nanosecond laser pulses at 532 nm.
The optical limiting responses of the suspensions are in fact
significantly better than those of the benchmark materials
fullerene and metallophthalocyanine in solution. Mechanistic
1
0.1021/jp9835014 CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/11/1998

78
J. Phys. Chem. B, Vol. 103, No. 1, 1999
Sun et al.
Figure 1. Experimental setup for the preparation of nanoparticles based
on the rapid expansion of supercritical fluid solution (RESS) into a
liquid solution.
assembly is preheated to a set temperature before each RESS
experiment. For solutions of high critical temperature fluids,
the syringe pump can be preheated to ensure that the solution
reaches the designated temperature at the end of the tubing coil
and becomes thermally equilibrated before rapid expansion. The
expansion nozzle is a fused silica capillary hosted in a stainless
steel tubing, which is inserted into the RESS chamber containing
a room-temperature solution.
Figure 2. Absorption spectra of the nanocrystalline metal and metal
sulfide particles in stable suspensions.
into a room-temperature ethanol solution of Na2S (25 mL, 0.7
mg/mL concentration). A silica capillary of 77 µm inner
diameter hosted in a stainless steel tubing by applying epoxy
resin was used as an expansion nozzle. The system pressure
was maintained at 3000-3500 psia during the RESS process.
The room-temperature solution in the RESS chamber (Figure
UV/vis absorption spectra were measured on a computer-
controlled Shimadzu UV-2101PC spectrophotometer. Diffuse
reflectance spectra were obtained using a Shimadzu UV-3100
spectrophotometer with ISR-3100 diffuse reflectance attachment.
Powder X-ray diffraction measurements were carried out on a
Scintag XDS-2000 powder diffraction system. Transmission
electron microscopy (TEM) images were obtained using a
Hitachi 600AB 100 kV transmission electron microscope.
The experimental setup for optical limiting measurements
consists of a Continuum Surelite-I Q-switched Nd:YAG laser
operated in the single-shot mode. The second harmonic from
frequency doubling the infrared fundamental is isolated by use
of the Surelite harmonic separation package. The laser beam is
collimated, with the maximum energy at 532 nm of 160 mJ/
pulse and a 5 ns pulse width (fwhm). The laser pulse energy is
varied in the range of 10-160 mJ/pulse using a waveplate-
polarizer combination. With the laser beam diameter of 6 mm,
the corresponding input energy densities for optical limiting
1) also contains 5 mg/mL of poly(N-vinyl-2-pyrrolidone) (PVP)
polymer (Mw ≈ 360 000) as a stabilization agent. With the PVP
polymer protection, the nanocrystalline Ag2S particles form a
very stable suspension in ethanol, which is in fact indistinguish-
able from a typical homogeneous solution. The absorption
spectrum of the yellowish suspension is shown in Figure 2.
Transmission electron microscopy (TEM) and powder X-ray
diffraction methods were used in the nanoparticle characteriza-
tion. The as prepared nanocrystalline Ag2S particles with the
protective PVP polymer were deposited on a collodion film on
a copper grid support for TEM measurements. A TEM image
of the nanocrystalline Ag2S particles is shown in Figure 3. A
statistical analysis of the TEM image consisting of 200 particles
yields an average Ag2S particle size of 7.3 nm, with a size
distribution standard deviation of 1.7 nm (Table 1). The powder
X-ray diffraction pattern of the solid sample of PVP polymer-
protected nanocrystalline Ag2S particles agrees well with that
of bulk Ag2S (monoclinic) documented in the X-ray diffraction
reference library. On the basis of the band broadening in the
2
measurements are in the range of 0.035-0.57 J/cm . For higher
2
energy densities of up to 2.2 J/cm , the laser beam diameter is
reduced to 3 mm using a galilean style telescope, which consists
of a plano-concave lens and a plano-convex lens. The detector
is a Scientech Mentor MC2501 calorimeter controlled by a
Scientech MD10 meter. A quartz cuvette of 2 mm optical path
length was used in the optical limiting measurements.
29
X-ray diffraction pattern, the average size of the Ag2S particles
in the solid sample is ∼20 nm, significantly larger than that for
the as prepared Ag2S particles.
A stable suspension of the PVP polymer-protected nano-
crystalline Ag2S particles in ethanol was measured for optical
limiting responses toward the second harmonic of a Q-switched
Nd:YAG laser at 532 nm. Dissolved oxygen and ammonia (from
the nanoparticle preparation process) in the suspension were
removed before optical limiting measurements by purging with
argon gas. The suspension was very dilute and indistinguishable
from a typical homogeneous solution, with a linear transmittance
at 532 nm of 90%. Optical limiting measurements of the
Results and Discussion
Nanocrystalline silver sulfide (Ag2S) particles were prepared
in the rapid expansion of supercritical fluid solution (RESS)
into liquid solution process using the setup shown in Figure 1.
In the preparation, a solution of AgNO3 in supercritical ammonia
(0.5 mg/mL) at 200 °C was rapidly expanded through a nozzle

Silver-Containing Nanocrystalline Particles
J. Phys. Chem. B, Vol. 103, No. 1, 1999 79
2
Figure 3. A TEM image of the nanocrystalline Ag S particles (1 mm
)
5 nm).
TABLE 1: Physical and Structural Parameters of the Metal
and Metal Sulfide Nanoparticles
TEM
RESS
RT
stabilization
agent
X-ray
size
σ
particle solution solution
diffraction (nm) (nm)a
2
Figure 4. Optical limiting responses of the nanocrystalline Ag S
Ag2S
CdS
PbS
ammonia ethanol
ammonia water
methanol methanol
PVP
gelatin
PVP
PVP
PVP
monoclinic
cubic
cubic
cubic
cubic
7.3 1.7
∼5
particles in a PVP polymer-stabilized ethanol suspension (o) of 90%
linear transmittance at 532 are compared with those of C60 in toluene
(0) and chloroaluminum phthalocyanine in DMF (3) of the same linear
transmittance and those of the CdS nanoparticle suspension (]) of 81%
linear transmittance and the PbS nanoparticle suspension (4) of 90%
linear transmittance.
6.6 1.0
5.6 0.78
5.8 0.54
Ag metal ammonia ethanol
Ni metal ethanol
DMF
a
Size distribution standard deviation.
suspension were performed as a function of input light fluence
IIN. Fresh sample was used for each IIN value to avoid possible
effects of laser radiation on the nanoparticle suspension. As
shown in Figure 4, the nanocrystalline Ag2S particles in the
PVP polymer-stabilized ethanol suspension exhibit strong optical
limiting responses toward 5 ns (fwhm) laser pulses at 532 nm.
The output light fluence IOUT reaches a plateau at IIN of only
Ag2S nanoparticles, though the difference is somewhat smaller
for the PbS nanoparticles. The comparison of optical limiting
results shown in Figure 4 seems to suggest that the excellent
optical limiting properties of the nanocrystalline Ag2S particles
are specific with respect to the silver (Ag) element. Similarly,
strong optical limiting responses were observed in a stable
suspension of Ag metal nanoparticles.
2
2
0.23 J/cm . The saturated IOUT value at the plateau is 0.11 J/cm .
Nanocrystalline Ag particles were prepared in the RESS into
liquid solution process, coupled with chemical reduction. In the
preparation, a solution of AgNO3 in supercritical ammonia (0.5
mg/mL) at 160 °C and 4000 psia was rapidly expanded into a
room-temperature hydrazine solution in ethanol (5 µL/mL) to
form nanocrystalline Ag metal particles. Under the PVP polymer
protection, the Ag nanoparticles form a very stable suspension
in ethanol, which appears indistinguishable from a typical
homogeneous solution. The absorption spectrum of the yellow-
colored suspension is also shown in Figure 2. The spectrum
consists of an intense absorption band peaking at ∼410 nm,
The strong optical limiting responses are not associated with
the PVP polymer because no optical limiting was observed in
the solution of PVP in ethanol. For comparison, strong optical
limiters C60 and chloroaluminum phthalocyanine as benchmark
materials were investigated under the same experimental condi-
tions. As shown in Figure 4, the optical limiting responses of
the nanocrystalline Ag2S particles are considerably stronger than
those of C60 in toluene solution and chloroaluminum phthalo-
cyanine in DMF solution of the same linear transmittance of
9
0%.
Also, for comparison, other metal sulfide nanocrystalline
3
0,31
particles including cadmium sulfide (CdS) and lead sulfide (PbS)
characteristic of plasmon absorption.
The as prepared
were prepared using the same RESS into liquid solution
nanocrystalline Ag metal particles with the protective PVP
polymer were deposited on a collodion film on a copper grid
support for TEM measurements. A TEM image of the nano-
crystalline Ag metal particles is shown in Figure 5. A statistical
analysis of the TEM image consisting of 180 particles yields
an average Ag particle size of 5.6 nm, with a size distribution
standard deviation of 0.78 nm (Table 1). The PVP polymer-
protected nanocrystalline Ag metal particles in the solid state
were also characterized by powder X-ray diffraction method.
The diffraction pattern matches well with that of the bulk Ag
metal (face centered cubic) documented in the X-ray diffraction
reference library. In the solid sample, the average size of the
Ag particles was estimated to be ∼14 nm on the basis of the
method.2
5,26
The nanoparticles also form stable suspensions
under the protection of a stabilization agent. The physical and
structural parameters of the CdS and PbS nanoparticles from
TEM and powder X-ray diffraction characterizations are sum-
marized in Table 1. The absorption spectra of the metal sulfide
nanoparticles are shown in Figure 2. The nanocrystalline CdS
and PbS particles in stabilized methanol and water suspensions,
respectively, were measured for optical limiting responses
toward 5 ns laser pulses at 532 nm. The results are clearly
different from that of the nanocrystalline Ag2S particles, as
compared in Figure 4. The optical limiting responses of both
CdS and PbS nanoparticles are much weaker than those of the

80
J. Phys. Chem. B, Vol. 103, No. 1, 1999
Sun et al.
aluminum phthalocyanine in DMF solution of the same linear
transmittance (Figure 6).
For comparison, nickel (Ni) metal nanoparticles were pre-
pared using the RESS into liquid solution/chemical reduction
26
method for optical limiting measurements. A solution of NiCl2
in supercritical ethanol was used in the rapid expansion, and
sodium borohydride (NaBH4) was used as a chemical reducing
agent. The Ni metal nanoparticles thus prepared form a stable
suspension in DMF under the PVP polymer protection. The
physical and structural parameters of the Ni nanoparticles from
TEM and powder X-ray diffraction characterizations are also
summarized in Table 1. Optical limiting properties of the Ni
metal nanoparticles in PVP polymer-stabilized DMF suspension
were studied. As shown in Figure 6, the Ni metal nanoparticles
exhibit only marginal optical limiting responses toward 5 ns
laser pulses at 532 nm, significantly different from the nano-
crystalline Ag metal particles.
Figure 5. TEM image of the nanocrystalline Ag metal particles (1
mm ) 4 nm).
The optical limiting properties of the nanocrystalline Ag metal
particles and Ag2S particles are unique to these silver-containing
nanoscopic materials. However, no meaningful optical limiting
responses were observed in a homogeneous solution of silver
ions. The aqueous solution of AgNO3 hardly absorbs at 532
nm even at a high solution concentration, exhibiting no nonlinear
optical behavior.
Since the silver-containing nanocrystalline particles are in
suspensions, optical limiting contributions from transient scat-
terings that are associated with photothermal processes should
be considered. For a suspension of light-absorbing carbon black
particles, strong optical limiting responses have been
1
8-20
observed.
The optical limiting of carbon black suspension
has been explained in the literature in terms of the optical
breakdown of absorbing carbon black particles and the associ-
1
9,20
ated liquid solvent.
In such a mechanism, the attenuation
of nanosecond laser pulses is attributed to the formation of
scattering shock waves as a result of the optical breakdown.
However, for the optical limiting results of the nanoparticles in
stable suspensions reported here, it seems difficult to explain
the specificity in the strong optical limiting with silver-
containing nanocrystalline particles within the mechanistic
framework for carbon black suspensions. A more likely scenario
is that the observed optical limiting responses in suspensions
of the other nanoparticles, especially the Ni metal nanoparticles,
are contributed by similar mechanistic processes to those in
carbon black suspensions. The optical limiting properties of the
silver-containing nanocrystalline particles in stable suspensions
are probably dominated by a different mechanism.
Figure 6. Optical limiting responses of the nanocrystalline Ag metal
particles in PVP polymer-stabilized ethanol suspension (O) of 90%
linear transmittance at 532 are compared with those of C60 in toluene
It is well-documented that silver nanoparticles have interesting
(0), chloroaluminum phthalocyanine in DMF (3), and the Ni metal
3
1,32
nanoparticles in DMF suspension (4) of the same linear transmittance.
photoelectrochemical properties.
Recently, Kamat and co-
2
3
workers reported that silver colloids of particle diameter 40-
60 nm exhibit a strong and broad transient absorption in the
visible and near-infrared wavelength region when subjected to
laser pulse excitation. The absorption growth monitored at 600
nm follows a single-exponential kinetic equation, with a lifetime
of 1.5 ( 0.1 ns. The broad transient absorption has been
assigned to a transient state that is generated in a photoinduced
intraparticle charge separation process, namely (Ag e )x. Similar
photoinduced redox processes were proposed to be responsible
for the optical limiting characteristics of silver bromide nanosols
band broadening in the X-ray diffraction pattern,29 which is also
significantly larger than that for the as prepared Ag particles.
The nanocrystalline Ag metal particles in PVP polymer-
stabilized ethanol suspension were studied for optical limiting
properties. As shown in Figure 6, the suspension of 90% linear
transmittance exhibits strong optical limiting responses toward
2
3
+
-
5
ns laser pulses at 532 nm. Before optical limiting measure-
ments, the suspension was purged with argon gas to remove
dissolved oxygen and ammonia (from the nanoparticle prepara-
tion process). The output light fluence IOUT reaches a plateau
2
1
comprising ∼6 nm particles of silver bromide. In addition,
2
at input fluence IIN of 0.37 J/cm , and the saturated IOUT value
strong nonlinear absorption at 532 nm was observed in Ag2S/
2
22
at the plateau is 0.18 J/cm . Again, the optical limiting responses
CdS nanocomposites of ∼10 nm in diameter. The nonlinear
of the PVP polymer-protected nanocrystalline Ag metal particles
are stronger than those of C60 in toluene solution and chloro-
absorption was attributed to the free-carrier absorption that is
associated with the coating of CdS particles with Ag2S.22

Silver-Containing Nanocrystalline Particles
J. Phys. Chem. B, Vol. 103, No. 1, 1999 81
Similarly, the strong optical limiting responses of the nano-
crystalline Ag metal and Ag2S particles toward nanosecond laser
pulses (Figures 4 and 6) may be dominated by a nonlinear
absorption mechanism. The photoinduced electron ejection
produces electron holes in the nanoparticle structure, resulting
in strong free-carrier absorption on the nanosecond time scale.
That the optical limiting responses of the Ag metal nanoparticles
are somewhat weaker than those of the Ag2S nanoparticles
Supporting Information Available: Statistical analyses of
particle sizes and size distributions from TEM results and
powder X-ray diffraction patterns of the Ag2S and Ag metal
nanoparticles (5 pages). Ordering information is given on any
current masthead page.
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2
5
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Advanced Engineering Fibers & Films, a National Science
Foundation Engineering Research Center at Clemson University,
is gratefully acknowledged.
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