Generation of AgCl Cubes by Excimer Laser … of AgCl Cubes by Excimer Laser Ablation of Bulk Ag in...

Published: November 18, 2010

r 2010 American Chemical Society 5058 dx.doi.org/10.1021/jp109240s | J. Phys. Chem. C 2011, 115, 5058–5062

ARTICLE

pubs.acs.org/JPCC

Generation of AgCl Cubes by Excimer Laser Ablation of Bulk Ag inAqueous NaCl SolutionsZijie 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, Universit�a di Catania, Viale A.Doria 6, Catania 95125, Italy

ABSTRACT: AgCl cubes have been fabricated by excimer laser ablationof a silver target in aqueous NaCl solutions with concentration CNaCl

e 0.01 M. Absorption spectra of the laser ablated solutions indicatedthat Ag nanoclusters also formed during the ablation. The formationof Ag and AgCl are dependent on CNaCl, laser fluence, and length ofablation time (laser pulses). We consider that the laser ablation of Agin NaCl solution will produce Agþ ions that combine with Cl- andform insoluble AgCl precipitates. The AgCl could catalyze thereduction of excess Agþ ions resulting in Ag nanoclusters. WhenCNaCl g 0.05 M, soluble complex anions of the type AgClmþ1

m- willform that reduce the formation of AgCl and Ag. Our research reveals the underlying mechanism of laser ablation of Ag in aqueousNaCl solution and provides a facile route to fabricate cuboidal AgCl particles.

1. INTRODUCTION

The development of nanoparticle generation by pulsed laserablation in liquid (PLAL) is closely related to the preparation ofAg colloids and nanoparticles by PLAL. Previous work producedAg colloids using a Q-switched Nd:YAG laser (1064 nm) toablate bulk Ag in water or aqueous solutions of NaCl and appliedthe colloids in surface-enhanced Raman scattering spectros-copy.1-3 Later sodium dodecyl sulfate (SDS), an ionic surfactantthat can be used to control the size distribution of variousnanoparticles fabricated by PLAL, was first introduced to thisarea by laser ablation of Ag in aqueous solutions of SDS.4,5 BothNaCl and SDS provide anions (Cl- or DS-) that were influentialin the particle formation,1,5-7 but they are different in severalways. The SDS could increase the stability of Ag colloids, but theNaCl did not.1,4,6 Increasing the concentration of SDS is usuallypreferred to decrease the average particle size of Ag,5 but one hasto select a proper concentration of NaCl (CNaCl). Past work hasrevealed that although NaCl could increase the formationefficiency of Ag nanoparticles, the NaCl would also inhibit oreven prevent the formation of Ag nanoparticles (as indicated bythe absorption spectra of the colloids) if CNaCl increased to acertain value,3,6 for example, 0.07 M,3 while the reason for thiswas unknown.

We have systematically studied the pulsed laser ablation ofbulk Ag in NaCl solutions using an excimer laser and proposed amechanism to explain the particle growth. Previous reports oflaser ablation of bulk Ag in NaCl solutions did not includestructural analyses of the products and omitted the possiblereaction of laser-generated Ag species with the electrolyte,1-3,6,7

while in our research, X-ray diffraction (XRD) analyses showedthe laser-fabricated products in NaCl solutions withCNaCle 0.01M were predominately AgCl instead of Ag. The AgCl particleshave cubic morphologies. The presence of the surface plasmon

bands (SPBs) close to 400 nm in the absorption spectra of laser-ablated solutions indicate Ag nanoclusters also exist in thesolutions. We consider that the photochemical properties ofAgCl in an aqueous solution could benefit the formation of Agnanoclusters fromAgþ ions under laser irradiation, whichmay bethe reason that NaCl could promote the formation efficiency ofAg nanoparticles as previously speculated.6 Very recently, studieshave revealed that AgCl/Ag nanocomposites are plasmonicphotocatalysts.8-10 In particular, Sun et al. fabricated AgCl/Aghybrid nanoparticles that showed sunlight-driven photocatalyticactivity.8 The hybrid nanoparticles were produced by partialreduction of AgCl nanocubes at an elevated temperature.8 Theresearch results herein reveal the particle formation mechanismby laser ablation of Ag in NaCl solution and that the productshave potential applications for water treatment and solar energyconversion.

2. EXPERIMENTAL SECTION

The experiments were conducted by pulsed laser ablation of asilver metal target (99.99%) in water or aqueous solutions ofNaCl (99.0%). The CNaCl ranged from 10-5 to 0.1 M and thevolume of the NaCl solution was 12 mL for each experiment. AKrF excimer laser with wavelength of 248 nm, pulse width of 30ns, and repetition frequency of 10 Hz was used for the ablationexperiments. The detailed experimental procedure has been pub-lished 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, 2010Revised: October 30, 2010

5059 dx.doi.org/10.1021/jp109240s |J. Phys. Chem. C 2011, 115, 5058–5062

The Journal of Physical Chemistry C ARTICLE

products were collected from the colloids by centrifugation. Theresulting deposits, with a small amount of residual solution, werecast onto glass or silicon substrates and dried at room tempera-ture for XRD and scanning electron microscopy (SEM) char-acterization, respectively. Some deposits were redispersed inwater and dropped on copper grids for transmission electronmicroscopy (TEM) analyses.

To study the influence of the UV laser irradiation on Agnanoparticles, a glass dish containing colloidal silver was irra-diated by the excimer laser with 12 000 pluses. The colloidalsilver, purchased from Purest Colloids, Inc., contains ∼75% Agnanoclusters (∼0.65 nm) and∼25% Agþ ions that are dispersedin water. The laser beam was unfocused with much lower fluenceof 8 � 10-3 J/cm2 to avoid the formation of plasma in thecolloid. For comparison, another Ag colloid with 0.001 M NaClwas irradiated by the excimer laser with the same laser para-meters.

SEM images were taken on JEOL JSM-6330F field emissionSEM equipped with energy-dispersive X-ray spectroscopy (EDS).TEM images and selected area electron diffraction (SAED) weretaken on Philip CM12 TEM at 120 kV. XRD patterns weremeasured on a X-ray diffractometer (Bruker D8) with Cu KRradiation. Absorption spectra were collected by PerkinElmerLAMBDA 950 Spectrophotometer.

3. RESULTS AND DISCUSSION

3.1. Formation Efficiency of Ag Nanoparticles. The for-mation efficiency of Ag nanoparticles prepared by laser ablationin a NaCl solution could be characterized by the maximumabsorbance of the typical SPB in the absorption spectrum.3,6

Figure 1a shows the absorption spectra of the colloids preparedin water and NaCl solutions with different concentrations. Thelaser fluence was 7.5 J/cm2 and the ablation time was 5 min. Anabsorption band at∼398 nm could be observed in each spectrumwhen the NaCl concentration is lower than 5 � 10-2 M. Theabsorption band corresponds to the surface plasmon resonanceof Ag nanospheres,12 indicating that Ag nanoclusters have formedin the solution. Another absorption band at ∼275 nm alsobecomes visible when CNaCl is 0.001 to 0.01 M, probably dueto the formation of AgCl in the solution. Similar absorptionbands around 250 nm has been observed in absorption spectra ofAgCl nanoparticles.10,13 The presence of NaCl, even at thelowest concentration of 10-5 M, could increase the formationefficiency of Ag nanoparticles compared with pure water. Themaximum absorbance of the SPB is dependent on the NaClconcentration and has a maximumwhen the NaCl concentrationis 0.001M as depicted in Figure 1b. This optimum concentrationdepends on the laser fluence and the length of ablation time.Figure 1b also depicts the results of two series of colloidsproduced with different ablation parameters. The optimumconcentration shifts to 0.005 M when the laser fluence isincreased to 15.0 J/cm2 with the same ablation time of 5 min,and further shifts to higher concentration when the ablation timeis increased to 20 min with the laser fluence of 15.0 J/cm2. In allthe three experimental series, the laser ablated solutions withCNaCl g 0.05 M do not show SPBs and particles could not becollected by centrifugation of these solutions.3.2. Structure of the Products. The centrifuged deposits

with ∼0.02 mL residual solution from samples prepared withlaser fluence of 15.0 J/cm2 were dropped onto glass substrates forXRD analyses. Figure 2 shows the XRD patterns of the samples.

The 0.005 M NaCl solution, after 5 min laser ablation, stillcontains NaCl as characterized by the diffraction peak located at2θ value of 31.7� in Figure 2a (JCPDS card no. 77-2064). A weakpeak at 32.2� belonging to the cubic phase of AgCl (JCPDS cardno. 31-1238) also appears. When the ablation time is increased to20 min, the amount of residual NaCl decreases and diffractionpeaks from AgCl in Figure 2b becomes more visible than that inFigure 2a. With 20 min ablation time, NaCl is nearly all con-sumed in the 0.001 MNaCl solution as the diffraction peak fromNaCl disappears in Figure 2c, but it is still abundant in the 0.01MNaCl solution as indicated by Figure 2d. It should be noted thatalthough Ag nanoclusters exist in the solution as indicated by theabsorption spectra, diffraction peaks from Ag could not beidentified in the XRD patterns, probably because there was muchless Ag than the amount of AgCl particles, or they were too smallto be centrifuged from the solutions. To reveal this, we centri-fuged half of the 0.005MNaCl solution after laser ablation for 20min with fluence of 15.0 J/cm2, and measured the absorptionspectra of the solution before and after centrifugation and thespectrum of the deposits redispersed in the same amount ofwater. Before centrifugation, the solution shows two absorptionbands at ∼280 and ∼398 nm as shown in Figure 3a. After

Figure 1. (a) Absorption spectra of colloids formed by excimer laserablation of Ag in water andNaCl solutions with different concentrations.The laser fluence was 7.5 J/cm2 and ablation time was 5 min. (b) Themaximum absorbance of the SPBs in the absorption spectra from threeseries of samples as a function of NaCl concentration with error bars.



centrifugation, the SPB band from Ag in the solution is strong asshown in Figure 3b, and the deposits mainly show an absorptionband at∼280 nm as depicted in Figure 3c, indicating that most ofthe Ag nanoclusters are still in the solution. Moreover, anadditional absorption band at ∼240 nm can be identified inFigure 3c, which corresponds to the direct bandgap of AgCl.8

The formation of AgCl and the consumption of Cl- anionsindicate that the laser ablation produced Agþ ions that combinedwith 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 from0.005 M NaCl solution after laser ablation for 5 and 20 min,respectively. Microcubes could be observed in Figure 4a, whichare NaCl crystals as indicated by Figure 2a and are furtherconfirmed by the EDS pattern shown in the inset. The peak of Sicame from the substrate used to hold the sample. In Figure 4b,

NaCl crystals cannot be observed. Instead, AgCl particles showup and the EDS pattern in the inset reveals the existence of Agand Cl elements. A magnified SEM image of the AgCl particles isshown in Figure 4c. The particles have cubic morphologiesalthough some of them lack well-defined facets. The cubes arepolydispersed with the sizes on the order of several hundrednanometers. Figure 4d shows the products from 0.001 M NaClsolution, similar cubes can be observed. Recently, AgCl nano-cubes with average edge length of 130 nm have been synthesizedby a precipitation reaction between Agþ and Cl- ions in polyolsynthesis.8 The AgCl in the laser-ablated solution should alsoform by the precipitation. It is worth noting that the generation ofAgCl is limited by the amount of Cl- ions, which in turn,depends on the amount of NaCl and the ablation time. Ourexperiments showed that increasing the ablation time from 20 to30 min in 0.001 M NaCl solution would start to produce Agspheres (with diameters of hundreds of nanometers) besidesAgCl cubes, indicating that Cl- ions could also prevent aggrega-tion and/or further growth of Ag nanoclusters. This effect of Cl-

ions has been reported elsewhere.7 The cubic morphology maybe partly caused by the Cl- ions. Several studies have shown thatCl- ions have the ability to promote the formation of cubicmorphology, including during the PLAL,14,15 probably due to thepreferred absorption of Cl- ions to {100} facets of a cubicstructure as it lowers the surface energy of these facets andstabilizes a cubic morphology. We also laser ablated Ag in aqueoussolutions of some surface capping agents, which are effective forshape control of silver nanocrystals in polyol synthesis, such aspoly(vinyl pyrrolidone) and sodium citrate,16 but could onlyobtain 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

Figure 2. XRD patterns of the laser fabricated products and residuesfrom NaCl solutions.

Figure 3. Absorption spectra of 0.005 M NaCl solution after laserablation for 20 min with fluence of 15.0 J/cm2 (a) before and (b) aftercentrifugation, and (c) the spectrum of the deposits redispersed in thesame amount of water.

Figure 4. SEM images of products and residues obtained by laserablation 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 to15.0 J/cm2.



AgCl particles prepared in 0.002 M NaCl solution with laserfluence of 7.5 J/cm2 and ablation time of 20 min, and Figure 5bshows the TEM image of the same sample. It can be observedthat the AgCl particles have decomposed into smaller nanopar-ticles in the TEM image, and the corresponding SAED patternreveals that the decomposed nanoparticles contain crystalline Ag.The decomposition occurred within several seconds under theTEM and showed hollow interior during the process. Figure 5dshows a sequential image of the decomposed AgCl nanocubemarked by an arrow in Figure 5b. The hollow interior becamelarger 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 considerthe formationofAgCl inNaCl solutionduring the laser ablation,1-3,6,7

our experimental results indicate that AgCl should form in aNaCl solution with proper concentration if Agþ ions exist. AgClis almost insoluble in water (the solubility product [Agþ][Cl-] =2� 10-10).17 A previous study on laser ablation of Ag in aqueouselectrolyte solutions, including NaCl solutions, proposed thatelectric bilayers, such as Agþ/Cl-, may build up on Ag nanoclus-ters that prevent aggregation and/or further growth.7 While thebuild-up of a Agþ/Cl- electric bilayer is less likely consideringthe low solubility of AgCl, the formation of a AgCl thin layer on aAg nanocluster may play similar role, that is, preventing furthergrowth. This may be the reason why Ag nanoclusters weredifficult to be centrifuged from the solutions in our experiments.Two routes could generate Agþ ions during the PLAL. First, theAgþ ions can form in the laser-induced plasma and react with theCl- at the boundary regions of the plasma; and second, thephotoejection of electrons from Ag nanoclusters by the laserirradiation.18 The first process relies on the laser fluence andcould 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 anelectron from a (Ag)n nanocluster is a monophotonic processunder 248 nm (5.0 eV) laser irradiation since the work functionof Ag is ca. 4.3 eV, and then an Agþ ion could be released fromthe (Ag)n

þ nanocluster.18 To confirm this, we irradiated a dish ofcommercial Ag colloid using unfocused excimer laser withfluence of 8 � 10-3 J/cm2. Figure 6a,b shows the absorptionspectrum of the colloid before and after irradiation of 12000 laserpulses. It can be observed that the absorbance of SPB at 398 nmfrom Ag nanoclusters has decreased after laser irradiation,indicating that some Ag has changed into Agþ ions due tophotoionization. Therefore, during the laser ablation of Ag inliquid, a certain amount of Agþ ions should be produced, that inturn, results in some AgCl nanoclusters with the existence of Cl-

anions. The AgCl in the solution with excess Agþ will promotethe formation of Ag under light irradiation by the followingreaction19

4Agþ þ 2H2Ofhv;AgCl

4Ag þ 4Hþ þ O2 ð1Þ

AgCl is photoactive such that it absorbs photons and createselectrons and holes which in turn induce the formation of Agfrom Agþ and O2 from H2O. This process was confirmed by thestudy of photochemical O2 evolution from water with thin AgCllayers.19 A feature of PLAL is that the laser-generated particlesare dispersed in the solution and will be randomly reirradiated bythe subsequent laser pulses. When a certain amount of Agþ existsin 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 behigher in NaCl solution with CNaCl e 0.01 M than that in purewater. The reduction of Agþ ions should occur at the surface ofAgCl particles and agglomeration of the Ag atoms results innanoclusters, which could then disperse into the solution bydiffusion. The AgCl-induced reduction of Agþ into Ag could bealso confirmed by excimer laser irradiation of the commercial Agcolloid with 0.001 M NaCl. Figure 6c depicts the absorptionspectrum of the colloid after laser irradiation. The absorbance ofSPB 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

Figure 5. (a) SEM and (b) TEM images of the AgCl cubes obtained in0.002 MNaCl solution with laser fluence of 7.5 J/cm2 and ablation timeof 20 min. (c) The corresponding SAED pattern. (d) TEM image of adecomposed AgCl cube.

Figure 6. Absorption spectra of commercial Ag colloid (a) before and(b) after excimer laser irradiation, (c) spectrum of Ag colloid with 0.001M NaCl after excimer laser irradiation.



some Agþ into Ag. Also, it could be further inferred that duringthe excimer laser ablation of Ag in NaCl solution, the photo-ionization of Ag into Agþ and the AgCl-induced reduction ofAgþ 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 explainedconsidering the extra Cl- anions. With the increasing of CNaCl,complex anions of the type AgClmþ1

m- (e.g., AgCl2-) will form

by the reaction17

AgCl þ mCl- a AgClm þ 1m- ð2Þ

The reaction will shift to the right with the increasing of Cl-

concentration. The complex anions are soluble in water,17 thusthe amount of AgCl will decrease and the reduction of Agþ to Agwill be limited. And further considering the photoionization ofAg by laser irradiation, any Ag nanoclusters in the solution willfinally dissolve and form complex anions, thus the SPB from Agnanoclusters will disappear. The probability for formation ofAgCl or AgClmþ1

m- depends on the ratio of [Agþ]/[Cl-].Increasing the laser fluence and ablation time will produce moreAgþ ions, thus the optimum NaCl concentration will shift tohigher values as depicted in Figure 1b. Recent work has shownthat AgCl particles could be fabricated into AgCl/Ag plasmonicphotocatalysts by heat treatment8 or light irritation of AgClparticles in a solution of methyl orange dye.10 While the laser-fabricated AgCl cubes could be also treated by these routes, it ispreferred to directly produce AgCl/Ag photocatalysts by amodified laser ablation procedure, and further investigation isunderway.

4. CONCLUSIONS

In summary, we have studied the UV laser ablation of bulk Agin NaCl solution with different NaCl concentrations, laserfluences, and ablation times. Absorption spectra of the laser-ablated solution CNaCl e 0.01 M indicate Ag nanoclusters couldform in the solutions, but XRD and SEM/TEM analyses revealedthat the products were mainly AgCl submicrometer cubes. Weconsider that the laser ablation of Ag in NaCl solution willproduce Agþ ions, which could combine with Cl- and forminsoluble AgCl nanoclusters that grow into submicrometercubes. The AgCl can improve the formation of Ag nanoclustersby photochemical reduction of Agþ ions. When CNaClg 0.05 M,soluble complex anions of the type AgClmþ1

m- will form thatreduce the formation of AgCl and Ag. The excimer laser ablationof Ag in NaCl solution provides a facile route to fabricate AgClcubes, and the colloids of AgCl with Ag/Agþ may find applica-tions in organic pollutants treatment and solar energy utilization.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

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Generation of AgCl Cubes by Excimer Laser … of AgCl Cubes by Excimer Laser Ablation of Bulk Ag in...

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