Hollow Particles Formed on Laser-Induced Bubbles by Excimer Laser Ablation of Al inLiquid

Zijie Yan,† Ruqiang Bao,† Yong Huang,‡ and Douglas B. Chrisey*,†

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180,and Department of Mechanical Engineering, Clemson UniVersity, Clemson, South Carolina 29634

ReceiVed: February 16, 2010

Herein we show how the unique temporal and thermal events occurring during the pulsed excimer laserablation of an Al target in water result in hollow Al2O3 micro/nanoparticles with smooth surfaces and anamorphous structure. We demonstrate that the hollow particles are formed on laser-induced bubbles fromlaser ionized/evaporated liquid during the ablation. The fabrication of hollow particles can be improved bythe addition of ethanol to the water, and the particles contain crystalline Al. Our work and the associatedmechanism represent a new paradigm to fabricate hollow particles directly from bulk material; that is, theexcimer laser ablation produces nanoclusters from the target and bubbles from the liquid, and the bubbleinterfaces trap the nanoclusters, resulting in the formation of hollow particles.

1. Introduction

Laser ablation is a powerful technique to explore or evendiscover the growth of new materials and structures. A landmarkdemonstration of the power of laser ablation was the fabricationof C60 by laser vaporization of graphite into a helium flow,1

which has become well known partially by virtue of the massproduction of C60 using arcing graphite electrodes.2 Probablythe most extensive application of laser ablation is the pulsedlaser deposition of thin films in a controlled atmosphere.3 Withthe growth of nanotechnology, pulsed laser ablation of solidtargets immersed in liquid has been developed as a facileapproach to fabricate nanoparticles, albeit not especially smallor monodispersed.4-7 When pure liquid is used, this methodcould produce various nanoparticles at room temperature thatare free of surface agents and counterions,4 which are typicalissues for nanoparticle generation by other methods, and thereinlies the power of laser ablating targets in liquids; that is, wecan explore disparate regions of parameter space with outcomesthat are impossible to envision a priori.

Nanoparticles owe their novel properties largely to their highspecific surface areas. Hollow particles could also provide highspecific surface areas even at relatively large particle sizes andthus have attracted considerable interest.8 Although laser ablationin liquid has shown the ability to fabricate a large variety ofnanoparticles, they were limited in scope to solid ones.4,5 Untilrecently, some Al nanoparticles generated by femtosecond orpicosecond laser ablation of bulk Al in ethanol showed irregularpores, which were considered to be due to oversaturation ofdissolved gas or reaction products of molten Al with traces ofwater in ethanol.9 Quite recently, we found that unique hollowmicro/nanoparticles could be generated by excimer laser ablationof a permalloy target in sodium dodecyl sulfate aqueoussolution.10 We proposed that the formation of these hollowstructures was due to laser-induced bubbles, but the mechanismneeded further investigation.10 In this article, we report on theformation of hollow Al2O3 particles, mostly spherical in shape,

but also with nonspherical particles, by excimer laser (248 nm)ablation of bulk Al in water. Very recently, Liu et al. reportedthe Nd/YAG laser (1064 nm) ablation of Al in water; theproducts were Al2O3 nanoparticles, but they were solid.11 Wedemonstrate that the hollow particles were formed on laser-induced bubbles from excimer laser ionized/evaporated waterat the liquid-solid interface. The laser-induced bubbles trappedthe laser-produced nanoclusters, resulting in the formation ofhollow particles. A similar phenomenon has been recognizedin sonochemistry, that the acoustic cavitation bubbles inducedby ultrasonic irradiation can absorb nanoparticles and generatehollow particles.12,13 We further found that adding ethanol towater could improve the fabrication of hollow particles by laserablation. Our work and the associated mechanism show thatexcimer laser ablation in liquid provides the possibility tofabricate hollow micro/nanoparticles directly from bulk materials.

2. Experimental Section

In a typical experiment, a solid Al target (99.999% pure) wasattached to the bottom of a rotating glass Petri dish. The dishwas then filled with distilled water; the distance from the targetto the water meniscus was 4 mm. A pulsed KrF excimer laser(248 nm, 10 Hz, 30 ns) was spatially filtered and focused ontothe surface of the target. The laser fluence was 2.3 J/cm2, andthe focal spot was ∼1.2 mm2. The ablation lasted for 5 min;the resulting particles were collected by centrifugation and dried.We also performed the experiments using water-ethanolmixture with various laser fluences and frequencies, which willbe specified in the text.

The morphology and structure of the products were character-ized with a field emission scanning electron microscope (FE-SEM, JEOL JSM-6330F) equipped with energy-dispersive X-rayspectroscopy (EDS) and a transmission electron microscope(TEM, Philip CM12) equipped with selected area electrondiffraction (SAED). Structures of the products were also studiedby X-ray diffraction (XRD) using a X-ray diffractometer (BrukerD8) with Cu KR radiation (λ ) 1.5406 Å). The Fouriertransform infrared (FT-IR) spectrum was measured with aPerkin-Elmer Spectrum One FTIR spectrometer using the KBrpellet technique.

* To whom correspondence should be addressed. E-mail: chrisd@rpi.edu.† Rensselaer Polytechnic Institute.‡ Clemson University.

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10.1021/jp104884x 2010 American Chemical SocietyPublished on Web 06/14/2010

3. Results and Discussion

Figure 1a shows the SEM image of the as-prepared particlesobtained by pulsed excimer laser ablation of the Al target inwater. Hollow spheres can be observed, as indicated by thebroken shells shown in the inset. EDS analysis of the particlesreveals both aluminum and oxygen, in a ratio indicating theformation of Al2O3 (Figure 1b). EDS pattern of the target isalso shown in Figure 1b, which only shows the peak fromaluminum. The structure of the products was studied by XRD.As shown in Figure 1c, no observable peak from the particlescan be identified in the XRD pattern, indicating that the particlesare amorphous. The broad peak around 24° was from the glasssubstrate. We also measured the FT-IR spectrum of the particles.As shown in Figure 1d, the spectrum has a broad band at 804cm-1, which can be assigned to the Al-O vibration of (AlO4)and features amorphous Al2O3.14 The band at 3434 cm-1 comesfrom the stretching mode of hydroxyl groups, whereas the 1624cm-1 band is due to the bending mode of water molecules.14

Figure 2a shows the TEM image of the particles. A mixtureof hollow spheres and solid spheres can be observed. The shellof a typical hollow sphere is shown in Figure 2b. It is smoothand continuous with thickness of ∼60 nm. This is much differentfrom the chemically synthesized hollow spheres, which aregenerally porous and formed from aggregated smaller nano-particles.8 The inset of Figure 2b shows the SAED pattern ofthe shell, with the halo rings indicating an amorphous structure.However, weak diffraction spots can be observed in the SAEDpatterns of some hollow particles possessing weak crystalliza-tion. Figure 2c shows such a SAED pattern with the weakdiffraction spots pointed out by the arrows. The spots can beindexed to the (400) and (440) planes of a γ-Al2O3 structurewith Fd3jm space group. The amorphous structure of the Al2O3

is due to the rapid quenching in water. Many oxide compoundsform amorphous structures under cooling rates as fast as 10K/s in air,15 and laser ablation in liquid provides an even greaterheat exchange environment to preserve such metastable phases.Amorphous Al2O3 has four-oxygen coordinated Al ion similarto that of γ-Al2O3 and can be transformed into the γ phase bythermal annealing. Figure 2d shows the size distributions of allparticles (solid and hollow ones) and the hollow particles inthem. The square symbols show the proportion of the hollowparticles in all particles in each size range. The proportion by

number increases with the increasing of particle size, revealingthat larger particles are more likely to be hollow.

The formation of hollow spheres via laser ablation in waterwithout any additives is quite unusual. To reveal the formationmechanism, we examined the surface morphology of the targetafter laser ablation. Figure 3a shows the SEM image of the targetsurface before the ablation; it is smooth. Then, we shot a singlelaser pulse on the target in water and found that the surfacebecame porous and contained hollow structures, as shown inFigure 3b. For comparison, we also shot a single laser pulse onthe target in air. The SEM image of the surface after shootingis shown in Figure 3c; no pores can be observed. Therefore,we infer that the water on the target surface played an importantrole for the formation of the porous structures during the laserablation. We consider that the formation mechanism of theporous structures is related to the laser-induced bubbles in water,and the bubbles also induce the formation of hollow spheres.Research has shown that pulsed laser could produce bubbles inwater at the focal spot.16,17 In our experiments, the laser focalspot was adjusted to the target surface, and thus bubbles shouldbe produced at the solid-liquid interface. Because our KrFexcimer laser is not equipped with a CCD camera, we performedthe ablation experiment using an ArF excimer laser system andrecorded the ablation process. (See the video in the SupportingInformation.) The video confirms that bubbles could be createdon the target surface near the focal spot. A bubble originatesfrom laser evaporated/ionized water at the laser focus, rapidly

Figure 1. Characterization of the laser ablated products from the Altarget: (a) SEM image of the particles, (b) EDS patterns, (c) XRDpatterns of the target and particles, and (d) FT-IR spectrum of theparticles.

Figure 2. (a) TEM image of the as-prepared particles, (b) TEM imageof the shell of a hollow sphere and the inset shows the correspondingSAED pattern, (c) typical SAED pattern of a hollow particle with weakcrystallization, (d) size distributions of the Al2O3 particles and thehollow ones in them (left vertical axis), and the proportion of hollowparticles within each size range (right vertical axis).

Figure 3. SEM images of the Al target surface (a) before laser ablation,(b) after a single laser pulse ablation on the target in water, and (c)after a single laser pulse ablation on the target in air.

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expands up to a maximum radius, and then shrinks to aminimum size that may undergo a rebound.16-18 The bubblemay oscillate several times, meanwhile loses energy due todamping, such as sound radiation or shock wave emission, andfinally collapses.18 The bubble dynamics can be described byRayleigh-Plesset equation,18,19 and Tc, the time from the bubblecreation to the first collapse point, is determined by the Rayleighformula19

where F is the density of the liquid, P is the ambient pressure,Pv is the vapor pressure, and Rmax is the maximum radius ofthe bubble. In our experiment, F ) 998.2 kg/m3, P ) 100 kPa,and Pv ) 2.33 kPa at 20 °C. For a bubble with Rmax ) 1 µm,calculation shows that Tc ) 185 ns. One may wonder thatwhether such unstable bubbles could promote the formation ofhollow particles. However, the Rayleigh’s formula assumes thatthe bubble does not contain gas content,19 whereas in ourexperiment, the possible byproduct of H2 gas generated fromthe following reaction may be contained in the bubble

The excimer laser-induced optical breakdown of water mayalso generate O2 and H2 gases that could be involved in thebubble. The gas content will largely increase the stability ofthe bubble18,19 and thus provide a larger chance to form a hollowparticle.

The stability of the bubble can be also increased byattachment of impurities on the bubble interface;19 in the currentexperiment, this is mainly due to the trapping of Al or Al2O3

nanoclusters. Similarly, research has shown that silica nano-particles could be used to stabilize gas bubbles.20 Al speciesare generated by laser ablation and then are oxidized into Al2O3

nanoclusters because of the reaction of Al species with wateror soluble oxygen. The nanoclusters, including those producedby the former laser pulses, are dispersed near the solid-liquidinterface before they diffuse away via Brownian motion andform a local colloidal environment around the bubble. Duringthe expansion of the bubble, the nanoclusters in the volumethat the bubble has swept are absorbed by the bubble interface.The energy needed for the detachment of a nanocluster fromthe bubble interface is given by21

where r is the radius of the nanocluster, γ is the surface tensionof the bubble interface, and θ is the contact angle of thenanoparticle with the liquid. For an amorphous Al2O3 nano-cluster with r ) 1 nm in water at 20 °C, γ ) 72.9 × 10-3 N/m,and θ ≈ 38°,22 calculation shows E ) 12 kBT (kB is theBoltzmann constant and T is temperature), which is larger thanthe thermal energy of the nanocluster, and thus it has littlechance to escape from the bubble interface. Then, during itsshrinking, the areal density of nanoclusters on the interfaceincreases because the interface area is decreasing, and when itshrinks to a certain size, the nanoclusters encounter each otherand a network or a rigid layer may form because of the bondingof the nanoclusters and restricts the interface motion, whichcould provide a template for further nucleation and growth of

a hollow particle because a large number of Al2O3 nanoclusterswas formed during the ablation process. It should be noted thatin this scenario, not every bubble could induce a hollow particle,and most bubbles may collapse finally. The possibility to trapenough nanoclusters and form hollow particles strongly dependson the concentration and oscillation time. Another possibilityexists for the formation of hollow particles, that is, thedetachment of hollow particles from the target surface due tothe interaction of water vapors with the Al melt, because thesizes of the cavities shown in Figure 3b are similar to those ofthe hollow particles. However, it is hard for most of the hollowparticles to maintain spherical shapes following this mechanism,and nanobumps will remain on the surface.4,9 In the recentreport,9 the hollow Al nanoparticles and their inside cavitiespossibly formed via this route were quite irregular, and the targetsurface showed mushroom-like nanostructures. We consider thatthe cavities shown in Figure 3b are more likely due to cavitationdamage from the interaction of bubbles with the target surface.Research has shown that cavitation bubbles could pierce similarholes into the surface of bulk Al.23

Some phenomena could be used to support the formation ofhollow particles on laser-produced bubbles. First, the possibilityto obtain a hollow particle increases with its size, as indicatedby Figure 2d. This may relate to the lifetime of a bubble, whichis directly proportional to its maximum radius, as shown by eq1. Second, research has shown that relatively large laser spotsize would produce nonspherical bubbles;16 similar nonsphericalhollow microparticles can be seen in Figure 4a. Specifically,for a laser-induced bubble near a rigid boundary, a protrusionmay form during its oscillation.16 Figure 4b exhibits an Al2O3

hollow particle just with such protrusion. Third, hollow sphereswith double cavities could be also observed, such as the oneshown in Figure 4c, which may be formed on two neighboringbubbles. It is similar to the hollow permalloy nanospheres withmulticavities in our previous report.10 Finally, a shell formedon the bubble interface will restrict the interface motion andstabilize the bubble. Figure 4d shows the SEM image of a

Tc ) 1.83� FP - Pv

Rmax (1)

2Al + 3H2O f Al2O3+3H2 (2)

E ) πr2γ(1 - cos θ)2 (3)

Figure 4. (a) TEM image of an oval-shaped hollow particle, (b) TEMimage of an aspherical particle with a protrusion, (c) TEM image of ahollow particle with double cavities, and (d) SEM image of a brokenhollow sphere with a new thin layer grown on the hole.

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broken hollow sphere. A layer thinner than the shell formed onthe broken hole, indicating that even after the broken of theshell, a bubble interface still existed on the hole and promotedthe formation of a new shell.

Although hollow Al2O3 can be fabricated by laser ablationin water, only a small proportion of the products are hollow.Therefore, increasing the proportion of hollow spheres appearsto be a challenge. One idea is to increase the generation ofbubbles during the laser ablation. We found that laser couldproduce a large number of bubbles in ethanol. However, noparticles could be obtained, maybe because that the laser energywas almost consumed by the production of bubbles or too manybubbles screened the laser beam. Then, we used water-ethanolmixture and found that the proportion of hollow spheres couldbe largely increased. Figure 5a,b shows the SEM and TEMimages of particles fabricated by the KrF excimer laser ablationof Al in water-ethanol mixture (Vwater/Vethanol 3:1), respectively.The laser fluence was still 2.3 J/cm2, and frequency was 10 Hz.The TEM image shows that the majority of the products arehollow spheres. EDS analysis revealed that the particles werestill oxidized. However, SAED analysis indicated that Alnanocrystals or nanoclusters existed in the particles. Figure 5cshows the TEM image of a hollow sphere; small nanoparticlescan be observed in the shell, indicating the trapping ability ofthe bubble interface. Figure 5d is the corresponding SAEDpattern. The rings are from cubic Al with Fm3jm space group,indicating that the ethanol, even in a small proportion, couldprevent the oxidation of laser-produced Al nanoclusters. Figure6a shows the size distributions of the hollow particles producedin water-ethanol mixtures under different conditions; data ofhollow particles produced in water are also shown here forcomparison: (1) water, laser fluence of 2.3 J/cm2, frequency of10 Hz; (2) Vwater/Vethanol 3:1, 2.3 J/cm2, 10 Hz; (3) Vwater/Vethanol

1:1, 2.3 J/cm2, 10 Hz; (4) Vwater/Vethanol 3:1, 2.3 J/cm2, 20 Hz;and (5) Vwater/Vethanol 3:1, 4.6 J/cm2, 10 Hz. The proportion isbased on all particles including solid ones. Figure 6b showsthe proportion of hollow particles in each size range. The Figure

shows that for particles with sizes <600 nm, in which mostparticles exist, water-ethanol mixture will largely increase theproportion of hollow particles. For all products, the proportiongenerally increases with the increasing of particle size, furtherdemonstrating the role of bubble lifetime. It has been knownthat the bubble oscillation undergoes less damping in liquidswith higher viscosity and thus has longer oscillation time.18,24

The ethanol-water mixture has higher viscosity than water.25

Therefore, the bubbles in the mixture have longer lifetimes andprovide a greater chance for the formation of hollow particles.

4. Conclusions

In summary, we have demonstrated that hollow particles canbe fabricated by excimer laser ablation of bulk Al in water.The addition of ethanol to the water can improve the fabricationof hollow particles. The particles obtained in water areamorphous Al2O3, which may have weak crystallization, andthe particles obtained in water-ethanol mixture contain crystal-line Al, although they are partially oxidized. The formation ofhollow particles is assisted by excimer laser-induced bubblesthat trap the laser-produced nanoclusters. Further work will beneeded to verify the versatility of the method to other materials.Once developed, the method will provide a unique approach tofabricate hollow structures.

Supporting Information Available: Video recording thegeneration of laser-induced bubbles on the Al target in waterusing an ArF excimer laser. The frame size is 474 µm × 352µm. The wavelength of the laser is 193 nm, frequency is also10 Hz, and fluence is about 5 J/cm2. This material is availablefree of charge via the Internet at http://pubs.acs.org.

Figure 5. (a) SEM image and (b) TEM image of the particles producedin water-ethanol mixture (Vwater/Vethanol 3:1). The inset of part a is theEDS pattern. (c) TEM image of a hollow sphere and (d) thecorresponding SAED pattern.

Figure 6. (a) Size distributions of the hollow particles produced inliquid with different conditions: (1) water, laser fluence of 2.3 J/cm2,frequency of 10 Hz; (2) Vwater/Vethanol 3:1, 2.3 J/cm2, 10 Hz; (3)Vwater/Vethanol 1:1, 2.3 J/cm2, 10 Hz; (4) Vwater/Vethanol 3:1, 2.3 J/cm2, 20 Hz;and (5) Vwater/Vethanol 3:1, 4.6 J/cm2, 10 Hz. (b) Proportion of hollowparticles within each size range.

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