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pubs.acs.org/JPCC
Generation of AgCl Cubes by Excimer Laser Ablation of Bulk Ag in
Aqueous NaCl Solutions
Zijie Yan,† Giuseppe Compagnini,‡ and Douglas B. Chrisey*,†
†
‡
Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States
Dipartimento di Scienze Chimiche, Universita di Catania, Viale A.Doria 6, Catania 95125, Italy
ABSTRACT: AgCl cubes have been fabricated by excimer laser ablation
of a silver target in aqueous NaCl solutions with concentration CNaCl
e 0.01 M. Absorption spectra of the laser ablated solutions indicated
that Ag nanoclusters also formed during the ablation. The formation
of Ag and AgCl are dependent on CNaCl, laser fluence, and length of
ablation time (laser pulses). We consider that the laser ablation of Ag
in NaCl solution will produce Agþ ions that combine with Cl- and
form insoluble AgCl precipitates. The AgCl could catalyze the
reduction of excess Agþ ions resulting in Ag nanoclusters. When
CNaCl g 0.05 M, soluble complex anions of the type AgClmþ1m- will
form that reduce the formation of AgCl and Ag. Our research reveals the underlying mechanism of laser ablation of Ag in aqueous
NaCl solution and provides a facile route to fabricate cuboidal AgCl particles.
1. INTRODUCTION
The development of nanoparticle generation by pulsed laser
ablation in liquid (PLAL) is closely related to the preparation of
Ag colloids and nanoparticles by PLAL. Previous work produced
Ag colloids using a Q-switched Nd:YAG laser (1064 nm) to
ablate bulk Ag in water or aqueous solutions of NaCl and applied
the colloids in surface-enhanced Raman scattering spectroscopy.1-3 Later sodium dodecyl sulfate (SDS), an ionic surfactant
that can be used to control the size distribution of various
nanoparticles fabricated by PLAL, was first introduced to this
area by laser ablation of Ag in aqueous solutions of SDS.4,5 Both
NaCl and SDS provide anions (Cl- or DS-) that were influential
in the particle formation,1,5-7 but they are different in several
ways. The SDS could increase the stability of Ag colloids, but the
NaCl did not.1,4,6 Increasing the concentration of SDS is usually
preferred to decrease the average particle size of Ag,5 but one has
to select a proper concentration of NaCl (CNaCl). Past work has
revealed that although NaCl could increase the formation
efficiency of Ag nanoparticles, the NaCl would also inhibit or
even prevent the formation of Ag nanoparticles (as indicated by
the absorption spectra of the colloids) if CNaCl increased to a
certain value,3,6 for example, 0.07 M,3 while the reason for this
was unknown.
We have systematically studied the pulsed laser ablation of
bulk Ag in NaCl solutions using an excimer laser and proposed a
mechanism to explain the particle growth. Previous reports of
laser ablation of bulk Ag in NaCl solutions did not include
structural analyses of the products and omitted the possible
reaction of laser-generated Ag species with the electrolyte,1-3,6,7
while in our research, X-ray diffraction (XRD) analyses showed
the laser-fabricated products in NaCl solutions with CNaCl e 0.01
M were predominately AgCl instead of Ag. The AgCl particles
have cubic morphologies. The presence of the surface plasmon
r 2010 American Chemical Society
bands (SPBs) close to 400 nm in the absorption spectra of laserablated solutions indicate Ag nanoclusters also exist in the
solutions. We consider that the photochemical properties of
AgCl in an aqueous solution could benefit the formation of Ag
nanoclusters from Agþ ions under laser irradiation, which may be
the reason that NaCl could promote the formation efficiency of
Ag nanoparticles as previously speculated.6 Very recently, studies
have revealed that AgCl/Ag nanocomposites are plasmonic
photocatalysts.8-10 In particular, Sun et al. fabricated AgCl/Ag
hybrid nanoparticles that showed sunlight-driven photocatalytic
activity.8 The hybrid nanoparticles were produced by partial
reduction of AgCl nanocubes at an elevated temperature.8 The
research results herein reveal the particle formation mechanism
by laser ablation of Ag in NaCl solution and that the products
have potential applications for water treatment and solar energy
conversion.
2. EXPERIMENTAL SECTION
The experiments were conducted by pulsed laser ablation of a
silver metal target (99.99%) in water or aqueous solutions of
NaCl (99.0%). The CNaCl ranged from 10-5 to 0.1 M and the
volume of the NaCl solution was 12 mL for each experiment. A
KrF excimer laser with wavelength of 248 nm, pulse width of 30
ns, and repetition frequency of 10 Hz was used for the ablation
experiments. The detailed experimental procedure has been published elsewhere.10 The laser fluence was set to 7.5 or 15.0 J/cm2
and the ablation time was either 5 or 20 min. After ablation, the
Special Issue: Laser Ablation and Nanoparticle Generation in Liquids
Received: September 27, 2010
Revised:
October 30, 2010
Published: November 18, 2010
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products were collected from the colloids by centrifugation. The
resulting deposits, with a small amount of residual solution, were
cast onto glass or silicon substrates and dried at room temperature for XRD and scanning electron microscopy (SEM) characterization, respectively. Some deposits were redispersed in
water and dropped on copper grids for transmission electron
microscopy (TEM) analyses.
To study the influence of the UV laser irradiation on Ag
nanoparticles, a glass dish containing colloidal silver was irradiated by the excimer laser with 12 000 pluses. The colloidal
silver, purchased from Purest Colloids, Inc., contains ∼75% Ag
nanoclusters (∼0.65 nm) and ∼25% Agþ ions that are dispersed
in water. The laser beam was unfocused with much lower fluence
of 8 10-3 J/cm2 to avoid the formation of plasma in the
colloid. For comparison, another Ag colloid with 0.001 M NaCl
was irradiated by the excimer laser with the same laser parameters.
SEM images were taken on JEOL JSM-6330F field emission
SEM equipped with energy-dispersive X-ray spectroscopy (EDS).
TEM images and selected area electron diffraction (SAED) were
taken on Philip CM12 TEM at 120 kV. XRD patterns were
measured on a X-ray diffractometer (Bruker D8) with Cu KR
radiation. Absorption spectra were collected by PerkinElmer
LAMBDA 950 Spectrophotometer.
3. RESULTS AND DISCUSSION
3.1. Formation Efficiency of Ag Nanoparticles. The formation efficiency of Ag nanoparticles prepared by laser ablation
in a NaCl solution could be characterized by the maximum
absorbance of the typical SPB in the absorption spectrum.3,6
Figure 1a shows the absorption spectra of the colloids prepared
in water and NaCl solutions with different concentrations. The
laser fluence was 7.5 J/cm2 and the ablation time was 5 min. An
absorption band at ∼398 nm could be observed in each spectrum
when the NaCl concentration is lower than 5 10-2 M. The
absorption band corresponds to the surface plasmon resonance
of Ag nanospheres,12 indicating that Ag nanoclusters have formed
in the solution. Another absorption band at ∼275 nm also
becomes visible when CNaCl is 0.001 to 0.01 M, probably due
to the formation of AgCl in the solution. Similar absorption
bands around 250 nm has been observed in absorption spectra of
AgCl nanoparticles.10,13 The presence of NaCl, even at the
lowest concentration of 10-5 M, could increase the formation
efficiency of Ag nanoparticles compared with pure water. The
maximum absorbance of the SPB is dependent on the NaCl
concentration and has a maximum when the NaCl concentration
is 0.001 M as depicted in Figure 1b. This optimum concentration
depends on the laser fluence and the length of ablation time.
Figure 1b also depicts the results of two series of colloids
produced with different ablation parameters. The optimum
concentration shifts to 0.005 M when the laser fluence is
increased to 15.0 J/cm2 with the same ablation time of 5 min,
and further shifts to higher concentration when the ablation time
is increased to 20 min with the laser fluence of 15.0 J/cm2. In all
the three experimental series, the laser ablated solutions with
CNaCl g 0.05 M do not show SPBs and particles could not be
collected by centrifugation of these solutions.
3.2. Structure of the Products. The centrifuged deposits
with ∼0.02 mL residual solution from samples prepared with
laser fluence of 15.0 J/cm2 were dropped onto glass substrates for
XRD analyses. Figure 2 shows the XRD patterns of the samples.
Figure 1. (a) Absorption spectra of colloids formed by excimer laser
ablation of Ag in water and NaCl solutions with different concentrations.
The laser fluence was 7.5 J/cm2 and ablation time was 5 min. (b) The
maximum absorbance of the SPBs in the absorption spectra from three
series of samples as a function of NaCl concentration with error bars.
The 0.005 M NaCl solution, after 5 min laser ablation, still
contains NaCl as characterized by the diffraction peak located at
2θ value of 31.7° in Figure 2a (JCPDS card no. 77-2064). A weak
peak at 32.2° belonging to the cubic phase of AgCl (JCPDS card
no. 31-1238) also appears. When the ablation time is increased to
20 min, the amount of residual NaCl decreases and diffraction
peaks from AgCl in Figure 2b becomes more visible than that in
Figure 2a. With 20 min ablation time, NaCl is nearly all consumed in the 0.001 M NaCl solution as the diffraction peak from
NaCl disappears in Figure 2c, but it is still abundant in the 0.01 M
NaCl solution as indicated by Figure 2d. It should be noted that
although Ag nanoclusters exist in the solution as indicated by the
absorption spectra, diffraction peaks from Ag could not be
identified in the XRD patterns, probably because there was much
less Ag than the amount of AgCl particles, or they were too small
to be centrifuged from the solutions. To reveal this, we centrifuged half of the 0.005 M NaCl solution after laser ablation for 20
min with fluence of 15.0 J/cm2, and measured the absorption
spectra of the solution before and after centrifugation and the
spectrum of the deposits redispersed in the same amount of
water. Before centrifugation, the solution shows two absorption
bands at ∼280 and ∼398 nm as shown in Figure 3a. After
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Figure 2. XRD patterns of the laser fabricated products and residues
from NaCl solutions.
Figure 3. Absorption spectra of 0.005 M NaCl solution after laser
ablation for 20 min with fluence of 15.0 J/cm2 (a) before and (b) after
centrifugation, and (c) the spectrum of the deposits redispersed in the
same amount of water.
centrifugation, the SPB band from Ag in the solution is strong as
shown in Figure 3b, and the deposits mainly show an absorption
band at ∼280 nm as depicted in Figure 3c, indicating that most of
the Ag nanoclusters are still in the solution. Moreover, an
additional absorption band at ∼240 nm can be identified in
Figure 3c, which corresponds to the direct bandgap of AgCl.8
The formation of AgCl and the consumption of Cl- anions
indicate that the laser ablation produced Agþ ions that combined
with the Cl- and formed AgCl precipitates.
3.3. Morphology and Stability of AgCl Particles. Figure 4a,
b shows the SEM image of products and residues obtained from
0.005 M NaCl solution after laser ablation for 5 and 20 min,
respectively. Microcubes could be observed in Figure 4a, which
are NaCl crystals as indicated by Figure 2a and are further
confirmed by the EDS pattern shown in the inset. The peak of Si
came from the substrate used to hold the sample. In Figure 4b,
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Figure 4. SEM images of products and residues obtained by laser
ablation of Ag in 0.005 M NaCl solutions for (a) 5 min and (b) 20 min,
and magnified images of AgCl cubes obtained in (c) 0.005 M and (d)
0.001 M NaCl solution with ablation time of 20 min. The insets in (a)
and (b) are the corresponding EDS patterns. The laser fluence was set to
15.0 J/cm2.
NaCl crystals cannot be observed. Instead, AgCl particles show
up and the EDS pattern in the inset reveals the existence of Ag
and Cl elements. A magnified SEM image of the AgCl particles is
shown in Figure 4c. The particles have cubic morphologies
although some of them lack well-defined facets. The cubes are
polydispersed with the sizes on the order of several hundred
nanometers. Figure 4d shows the products from 0.001 M NaCl
solution, similar cubes can be observed. Recently, AgCl nanocubes with average edge length of 130 nm have been synthesized
by a precipitation reaction between Agþ and Cl- ions in polyol
synthesis.8 The AgCl in the laser-ablated solution should also
form by the precipitation. It is worth noting that the generation of
AgCl is limited by the amount of Cl- ions, which in turn,
depends on the amount of NaCl and the ablation time. Our
experiments showed that increasing the ablation time from 20 to
30 min in 0.001 M NaCl solution would start to produce Ag
spheres (with diameters of hundreds of nanometers) besides
AgCl cubes, indicating that Cl- ions could also prevent aggregation and/or further growth of Ag nanoclusters. This effect of Clions has been reported elsewhere.7 The cubic morphology may
be partly caused by the Cl- ions. Several studies have shown that
Cl- ions have the ability to promote the formation of cubic
morphology, including during the PLAL,14,15 probably due to the
preferred absorption of Cl- ions to {100} facets of a cubic
structure as it lowers the surface energy of these facets and
stabilizes a cubic morphology. We also laser ablated Ag in aqueous
solutions of some surface capping agents, which are effective for
shape control of silver nanocrystals in polyol synthesis, such as
poly(vinyl pyrrolidone) and sodium citrate,16 but could only
obtain Ag spheres. These results will be published elsewhere.
The AgCl cubes are unstable under electron beam irradiation
especially under the TEM.8 Figure 5a shows the SEM image of
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Figure 6. Absorption spectra of commercial Ag colloid (a) before and
(b) after excimer laser irradiation, (c) spectrum of Ag colloid with 0.001
M NaCl after excimer laser irradiation.
Figure 5. (a) SEM and (b) TEM images of the AgCl cubes obtained in
0.002 M NaCl solution with laser fluence of 7.5 J/cm2 and ablation time
of 20 min. (c) The corresponding SAED pattern. (d) TEM image of a
decomposed AgCl cube.
AgCl particles prepared in 0.002 M NaCl solution with laser
fluence of 7.5 J/cm2 and ablation time of 20 min, and Figure 5b
shows the TEM image of the same sample. It can be observed
that the AgCl particles have decomposed into smaller nanoparticles in the TEM image, and the corresponding SAED pattern
reveals that the decomposed nanoparticles contain crystalline Ag.
The decomposition occurred within several seconds under the
TEM and showed hollow interior during the process. Figure 5d
shows a sequential image of the decomposed AgCl nanocube
marked by an arrow in Figure 5b. The hollow interior became
larger and more Ag nanoparticles grew out of the cubes.
3.4. Formation Mechanism of Ag and AgCl Particles in
NaCl Solution. Although the previous reports did not consider
the formation of AgCl in NaCl solution during the laser ablation,1-3,6,7
our experimental results indicate that AgCl should form in a
NaCl solution with proper concentration if Agþ ions exist. AgCl
is almost insoluble in water (the solubility product [Agþ][Cl-] =
2 10-10).17 A previous study on laser ablation of Ag in aqueous
electrolyte solutions, including NaCl solutions, proposed that
electric bilayers, such as Agþ/Cl-, may build up on Ag nanoclusters that prevent aggregation and/or further growth.7 While the
build-up of a Agþ/Cl- electric bilayer is less likely considering
the low solubility of AgCl, the formation of a AgCl thin layer on a
Ag nanocluster may play similar role, that is, preventing further
growth. This may be the reason why Ag nanoclusters were
difficult to be centrifuged from the solutions in our experiments.
Two routes could generate Agþ ions during the PLAL. First, the
Agþ ions can form in the laser-induced plasma and react with the
Cl- at the boundary regions of the plasma; and second, the
photoejection of electrons from Ag nanoclusters by the laser
irradiation.18 The first process relies on the laser fluence and
could occur even using a laser with long excitation wavelength,
for example, the 1064 nm laser used in the previous studies.1-3,6
The second process depends on the photon energy and will be
more significant when a UV laser is used. Photoejection of an
electron from a (Ag)n nanocluster is a monophotonic process
under 248 nm (5.0 eV) laser irradiation since the work function
of Ag is ca. 4.3 eV, and then an Agþ ion could be released from
the (Ag)nþ nanocluster.18 To confirm this, we irradiated a dish of
commercial Ag colloid using unfocused excimer laser with
fluence of 8 10-3 J/cm2. Figure 6a,b shows the absorption
spectrum of the colloid before and after irradiation of 12000 laser
pulses. It can be observed that the absorbance of SPB at 398 nm
from Ag nanoclusters has decreased after laser irradiation,
indicating that some Ag has changed into Agþ ions due to
photoionization. Therefore, during the laser ablation of Ag in
liquid, a certain amount of Agþ ions should be produced, that in
turn, results in some AgCl nanoclusters with the existence of Clanions. The AgCl in the solution with excess Agþ will promote
the formation of Ag under light irradiation by the following
reaction19
hv;AgCl
4Agþ þ 2H2 O f 4Ag þ 4Hþ þ O2
ð1Þ
AgCl is photoactive such that it absorbs photons and creates
electrons and holes which in turn induce the formation of Ag
from Agþ and O2 from H2O. This process was confirmed by the
study of photochemical O2 evolution from water with thin AgCl
layers.19 A feature of PLAL is that the laser-generated particles
are dispersed in the solution and will be randomly reirradiated by
the subsequent laser pulses. When a certain amount of Agþ exists
in solution, the formation of AgCl will cause reduction of Agþ
ions under laser irradiation that finally results in Ag nanoclusters.
Therefore, the formation efficiency of Ag nanoclusters will be
higher in NaCl solution with CNaCl e 0.01 M than that in pure
water. The reduction of Agþ ions should occur at the surface of
AgCl particles and agglomeration of the Ag atoms results in
nanoclusters, which could then disperse into the solution by
diffusion. The AgCl-induced reduction of Agþ into Ag could be
also confirmed by excimer laser irradiation of the commercial Ag
colloid with 0.001 M NaCl. Figure 6c depicts the absorption
spectrum of the colloid after laser irradiation. The absorbance of
SPB from Ag is higher than that of the colloid without NaCl
(Figure 6b), and an absorption band at ∼275 nm also appears,
indicating that AgCl clusters have formed and they have reduced
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some Agþ into Ag. Also, it could be further inferred that during
the excimer laser ablation of Ag in NaCl solution, the photoionization of Ag into Agþ and the AgCl-induced reduction of
Agþ to Ag will be two competitive processes.
The decrease and eventual disappearance of Ag nanoclusters
in a NaCl solution with CNaCl g 0.05 M could be explained
considering the extra Cl- anions. With the increasing of CNaCl,
complex anions of the type AgClmþ1m- (e.g., AgCl2-) will form
by the reaction17
AgCl þ mCl- a AgClm þ 1 mð2Þ
The reaction will shift to the right with the increasing of Clconcentration. The complex anions are soluble in water,17 thus
the amount of AgCl will decrease and the reduction of Agþ to Ag
will be limited. And further considering the photoionization of
Ag by laser irradiation, any Ag nanoclusters in the solution will
finally dissolve and form complex anions, thus the SPB from Ag
nanoclusters will disappear. The probability for formation of
AgCl or AgClmþ1m- depends on the ratio of [Agþ]/[Cl-].
Increasing the laser fluence and ablation time will produce more
Agþ ions, thus the optimum NaCl concentration will shift to
higher values as depicted in Figure 1b. Recent work has shown
that AgCl particles could be fabricated into AgCl/Ag plasmonic
photocatalysts by heat treatment8 or light irritation of AgCl
particles in a solution of methyl orange dye.10 While the laserfabricated AgCl cubes could be also treated by these routes, it is
preferred to directly produce AgCl/Ag photocatalysts by a
modified laser ablation procedure, and further investigation is
underway.
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4. CONCLUSIONS
In summary, we have studied the UV laser ablation of bulk Ag
in NaCl solution with different NaCl concentrations, laser
fluences, and ablation times. Absorption spectra of the laserablated solution CNaCl e 0.01 M indicate Ag nanoclusters could
form in the solutions, but XRD and SEM/TEM analyses revealed
that the products were mainly AgCl submicrometer cubes. We
consider that the laser ablation of Ag in NaCl solution will
produce Agþ ions, which could combine with Cl- and form
insoluble AgCl nanoclusters that grow into submicrometer
cubes. The AgCl can improve the formation of Ag nanoclusters
by photochemical reduction of Agþ ions. When CNaCl g 0.05 M,
soluble complex anions of the type AgClmþ1m- will form that
reduce the formation of AgCl and Ag. The excimer laser ablation
of Ag in NaCl solution provides a facile route to fabricate AgCl
cubes, and the colloids of AgCl with Ag/Agþ may find applications in organic pollutants treatment and solar energy utilization.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
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