11370
J. Phys. Chem. C 2010, 114, 11370–11374
Hollow Particles Formed on Laser-Induced Bubbles by Excimer Laser Ablation of Al in Liquid 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 laser ablation of an Al target in water result in hollow Al2O3 micro/nanoparticles with smooth surfaces and an amorphous structure. We demonstrate that the hollow particles are formed on laser-induced bubbles from laser ionized/evaporated liquid during the ablation. The fabrication of hollow particles can be improved by the addition of ethanol to the water, and the particles contain crystalline Al. Our work and the associated mechanism represent a new paradigm to fabricate hollow particles directly from bulk material; that is, the excimer laser ablation produces nanoclusters from the target and bubbles from the liquid, and the bubble interfaces trap the nanoclusters, resulting in the formation of hollow particles. 1. Introduction Laser ablation is a powerful technique to explore or even discover the growth of new materials and structures. A landmark demonstration of the power of laser ablation was the fabrication of C60 by laser vaporization of graphite into a helium flow,1 which has become well known partially by virtue of the mass production of C60 using arcing graphite electrodes.2 Probably the most extensive application of laser ablation is the pulsed laser deposition of thin films in a controlled atmosphere.3 With the growth of nanotechnology, pulsed laser ablation of solid targets immersed in liquid has been developed as a facile approach to fabricate nanoparticles, albeit not especially small or monodispersed.4-7 When pure liquid is used, this method could produce various nanoparticles at room temperature that are free of surface agents and counterions,4 which are typical issues for nanoparticle generation by other methods, and therein lies the power of laser ablating targets in liquids; that is, we can explore disparate regions of parameter space with outcomes that are impossible to envision a priori. Nanoparticles owe their novel properties largely to their high specific surface areas. Hollow particles could also provide high specific surface areas even at relatively large particle sizes and thus have attracted considerable interest.8 Although laser ablation in liquid has shown the ability to fabricate a large variety of nanoparticles, they were limited in scope to solid ones.4,5 Until recently, some Al nanoparticles generated by femtosecond or picosecond laser ablation of bulk Al in ethanol showed irregular pores, which were considered to be due to oversaturation of dissolved gas or reaction products of molten Al with traces of water in ethanol.9 Quite recently, we found that unique hollow micro/nanoparticles could be generated by excimer laser ablation of a permalloy target in sodium dodecyl sulfate aqueous solution.10 We proposed that the formation of these hollow structures was due to laser-induced bubbles, but the mechanism needed further investigation.10 In this article, we report on the formation of hollow Al2O3 particles, mostly spherical in shape, * To whom correspondence should be addressed. E-mail:
[email protected]. † Rensselaer Polytechnic Institute. ‡ Clemson University.
but also with nonspherical particles, by excimer laser (248 nm) ablation of bulk Al in water. Very recently, Liu et al. reported the Nd/YAG laser (1064 nm) ablation of Al in water; the products were Al2O3 nanoparticles, but they were solid.11 We demonstrate that the hollow particles were formed on laserinduced bubbles from excimer laser ionized/evaporated water at the liquid-solid interface. The laser-induced bubbles trapped the laser-produced nanoclusters, resulting in the formation of hollow particles. A similar phenomenon has been recognized in sonochemistry, that the acoustic cavitation bubbles induced by ultrasonic irradiation can absorb nanoparticles and generate hollow particles.12,13 We further found that adding ethanol to water could improve the fabrication of hollow particles by laser ablation. Our work and the associated mechanism show that excimer laser ablation in liquid provides the possibility to fabricate hollow micro/nanoparticles directly from bulk materials. 2. Experimental Section In a typical experiment, a solid Al target (99.999% pure) was attached to the bottom of a rotating glass Petri dish. The dish was then filled with distilled water; the distance from the target to the water meniscus was 4 mm. A pulsed KrF excimer laser (248 nm, 10 Hz, 30 ns) was spatially filtered and focused onto the surface of the target. The laser fluence was 2.3 J/cm2, and the 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-ethanol mixture with various laser fluences and frequencies, which will be specified in the text. The morphology and structure of the products were characterized with a field emission scanning electron microscope (FESEM, JEOL JSM-6330F) equipped with energy-dispersive X-ray spectroscopy (EDS) and a transmission electron microscope (TEM, Philip CM12) equipped with selected area electron diffraction (SAED). Structures of the products were also studied by X-ray diffraction (XRD) using a X-ray diffractometer (Bruker D8) with Cu KR radiation (λ ) 1.5406 Å). The Fourier transform infrared (FT-IR) spectrum was measured with a Perkin-Elmer Spectrum One FTIR spectrometer using the KBr pellet technique.
10.1021/jp104884x 2010 American Chemical Society Published on Web 06/14/2010
Hollow Particles Formed on Laser-Induced Bubbles
Figure 1. Characterization of the laser ablated products from the Al target: (a) SEM image of the particles, (b) EDS patterns, (c) XRD patterns of the target and particles, and (d) FT-IR spectrum of the particles.
3. Results and Discussion Figure 1a shows the SEM image of the as-prepared particles obtained by pulsed excimer laser ablation of the Al target in water. Hollow spheres can be observed, as indicated by the broken shells shown in the inset. EDS analysis of the particles reveals both aluminum and oxygen, in a ratio indicating the formation of Al2O3 (Figure 1b). EDS pattern of the target is also shown in Figure 1b, which only shows the peak from aluminum. The structure of the products was studied by XRD. As shown in Figure 1c, no observable peak from the particles can be identified in the XRD pattern, indicating that the particles are amorphous. The broad peak around 24° was from the glass substrate. We also measured the FT-IR spectrum of the particles. As shown in Figure 1d, the spectrum has a broad band at 804 cm-1, which can be assigned to the Al-O vibration of (AlO4) and features amorphous Al2O3.14 The band at 3434 cm-1 comes from the stretching mode of hydroxyl groups, whereas the 1624 cm-1 band is due to the bending mode of water molecules.14 Figure 2a shows the TEM image of the particles. A mixture of hollow spheres and solid spheres can be observed. The shell of a typical hollow sphere is shown in Figure 2b. It is smooth and continuous with thickness of ∼60 nm. This is much different from the chemically synthesized hollow spheres, which are generally porous and formed from aggregated smaller nanoparticles.8 The inset of Figure 2b shows the SAED pattern of the shell, with the halo rings indicating an amorphous structure. However, weak diffraction spots can be observed in the SAED patterns of some hollow particles possessing weak crystallization. Figure 2c shows such a SAED pattern with the weak diffraction spots pointed out by the arrows. The spots can be indexed to the (400) and (440) planes of a γ-Al2O3 structure with Fd3jm space group. The amorphous structure of the Al2O3 is due to the rapid quenching in water. Many oxide compounds form amorphous structures under cooling rates as fast as 10 K/s in air,15 and laser ablation in liquid provides an even greater heat exchange environment to preserve such metastable phases. Amorphous Al2O3 has four-oxygen coordinated Al ion similar to that of γ-Al2O3 and can be transformed into the γ phase by thermal annealing. Figure 2d shows the size distributions of all particles (solid and hollow ones) and the hollow particles in them. The square symbols show the proportion of the hollow particles in all particles in each size range. The proportion by
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11371
Figure 2. (a) TEM image of the as-prepared particles, (b) TEM image of the shell of a hollow sphere and the inset shows the corresponding SAED pattern, (c) typical SAED pattern of a hollow particle with weak crystallization, (d) size distributions of the Al2O3 particles and the hollow ones in them (left vertical axis), and the proportion of hollow particles 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.
number increases with the increasing of particle size, revealing that larger particles are more likely to be hollow. The formation of hollow spheres via laser ablation in water without any additives is quite unusual. To reveal the formation mechanism, we examined the surface morphology of the target after laser ablation. Figure 3a shows the SEM image of the target surface before the ablation; it is smooth. Then, we shot a single laser pulse on the target in water and found that the surface became porous and contained hollow structures, as shown in Figure 3b. For comparison, we also shot a single laser pulse on the target in air. The SEM image of the surface after shooting is shown in Figure 3c; no pores can be observed. Therefore, we infer that the water on the target surface played an important role for the formation of the porous structures during the laser ablation. We consider that the formation mechanism of the porous 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 in water at the focal spot.16,17 In our experiments, the laser focal spot was adjusted to the target surface, and thus bubbles should be produced at the solid-liquid interface. Because our KrF excimer laser is not equipped with a CCD camera, we performed the ablation experiment using an ArF excimer laser system and recorded the ablation process. (See the video in the Supporting Information.) The video confirms that bubbles could be created on the target surface near the focal spot. A bubble originates from laser evaporated/ionized water at the laser focus, rapidly
11372
J. Phys. Chem. C, Vol. 114, No. 26, 2010
Yan et al.
expands up to a maximum radius, and then shrinks to a minimum size that may undergo a rebound.16-18 The bubble may oscillate several times, meanwhile loses energy due to damping, such as sound radiation or shock wave emission, and finally collapses.18 The bubble dynamics can be described by Rayleigh-Plesset equation,18,19 and Tc, the time from the bubble creation to the first collapse point, is determined by the Rayleigh formula19
Tc ) 1.83
F R P - Pv max
(1)
where F is the density of the liquid, P is the ambient pressure, Pv is the vapor pressure, and Rmax is the maximum radius of the 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 that whether such unstable bubbles could promote the formation of hollow particles. However, the Rayleigh’s formula assumes that the bubble does not contain gas content,19 whereas in our experiment, the possible byproduct of H2 gas generated from the following reaction may be contained in the bubble
2Al + 3H2O f Al2O3+3H2
(2)
The excimer laser-induced optical breakdown of water may also generate O2 and H2 gases that could be involved in the bubble. The gas content will largely increase the stability of the bubble18,19 and thus provide a larger chance to form a hollow particle. The stability of the bubble can be also increased by attachment of impurities on the bubble interface;19 in the current experiment, this is mainly due to the trapping of Al or Al2O3 nanoclusters. Similarly, research has shown that silica nanoparticles could be used to stabilize gas bubbles.20 Al species are generated by laser ablation and then are oxidized into Al2O3 nanoclusters because of the reaction of Al species with water or soluble oxygen. The nanoclusters, including those produced by the former laser pulses, are dispersed near the solid-liquid interface before they diffuse away via Brownian motion and form a local colloidal environment around the bubble. During the expansion of the bubble, the nanoclusters in the volume that the bubble has swept are absorbed by the bubble interface. The energy needed for the detachment of a nanocluster from the bubble interface is given by21
E ) πr2γ(1 - cos θ)2
(3)
where r is the radius of the nanocluster, γ is the surface tension of the bubble interface, and θ is the contact angle of the nanoparticle with the liquid. For an amorphous Al2O3 nanocluster 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 the Boltzmann constant and T is temperature), which is larger than the thermal energy of the nanocluster, and thus it has little chance to escape from the bubble interface. Then, during its shrinking, the areal density of nanoclusters on the interface increases because the interface area is decreasing, and when it shrinks to a certain size, the nanoclusters encounter each other and a network or a rigid layer may form because of the bonding of the nanoclusters and restricts the interface motion, which could provide a template for further nucleation and growth of
Figure 4. (a) TEM image of an oval-shaped hollow particle, (b) TEM image of an aspherical particle with a protrusion, (c) TEM image of a hollow particle with double cavities, and (d) SEM image of a broken hollow sphere with a new thin layer grown on the hole.
a hollow particle because a large number of Al2O3 nanoclusters was formed during the ablation process. It should be noted that in this scenario, not every bubble could induce a hollow particle, and most bubbles may collapse finally. The possibility to trap enough nanoclusters and form hollow particles strongly depends on the concentration and oscillation time. Another possibility exists for the formation of hollow particles, that is, the detachment of hollow particles from the target surface due to the interaction of water vapors with the Al melt, because the sizes of the cavities shown in Figure 3b are similar to those of the hollow particles. However, it is hard for most of the hollow particles to maintain spherical shapes following this mechanism, and nanobumps will remain on the surface.4,9 In the recent report,9 the hollow Al nanoparticles and their inside cavities possibly formed via this route were quite irregular, and the target surface showed mushroom-like nanostructures. We consider that the cavities shown in Figure 3b are more likely due to cavitation damage from the interaction of bubbles with the target surface. Research has shown that cavitation bubbles could pierce similar holes into the surface of bulk Al.23 Some phenomena could be used to support the formation of hollow particles on laser-produced bubbles. First, the possibility to obtain a hollow particle increases with its size, as indicated by Figure 2d. This may relate to the lifetime of a bubble, which is directly proportional to its maximum radius, as shown by eq 1. Second, research has shown that relatively large laser spot size would produce nonspherical bubbles;16 similar nonspherical hollow microparticles can be seen in Figure 4a. Specifically, for a laser-induced bubble near a rigid boundary, a protrusion may form during its oscillation.16 Figure 4b exhibits an Al2O3 hollow particle just with such protrusion. Third, hollow spheres with double cavities could be also observed, such as the one shown in Figure 4c, which may be formed on two neighboring bubbles. It is similar to the hollow permalloy nanospheres with multicavities in our previous report.10 Finally, a shell formed on the bubble interface will restrict the interface motion and stabilize the bubble. Figure 4d shows the SEM image of a
Hollow Particles Formed on Laser-Induced Bubbles
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11373
Figure 5. (a) SEM image and (b) TEM image of the particles produced in water-ethanol mixture (Vwater/Vethanol 3:1). The inset of part a is the EDS pattern. (c) TEM image of a hollow sphere and (d) the corresponding SAED pattern.
broken hollow sphere. A layer thinner than the shell formed on the broken hole, indicating that even after the broken of the shell, a bubble interface still existed on the hole and promoted the formation of a new shell. Although hollow Al2O3 can be fabricated by laser ablation in water, only a small proportion of the products are hollow. Therefore, increasing the proportion of hollow spheres appears to be a challenge. One idea is to increase the generation of bubbles during the laser ablation. We found that laser could produce a large number of bubbles in ethanol. However, no particles could be obtained, maybe because that the laser energy was almost consumed by the production of bubbles or too many bubbles screened the laser beam. Then, we used water-ethanol mixture and found that the proportion of hollow spheres could be largely increased. Figure 5a,b shows the SEM and TEM images of particles fabricated by the KrF excimer laser ablation of 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 are hollow spheres. EDS analysis revealed that the particles were still oxidized. However, SAED analysis indicated that Al nanocrystals or nanoclusters existed in the particles. Figure 5c shows the TEM image of a hollow sphere; small nanoparticles can be observed in the shell, indicating the trapping ability of the bubble interface. Figure 5d is the corresponding SAED pattern. The rings are from cubic Al with Fm3jm space group, indicating that the ethanol, even in a small proportion, could prevent the oxidation of laser-produced Al nanoclusters. Figure 6a shows the size distributions of the hollow particles produced in water-ethanol mixtures under different conditions; data of hollow particles produced in water are also shown here for comparison: (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. The proportion is based on all particles including solid ones. Figure 6b shows the proportion of hollow particles in each size range. The Figure
Figure 6. (a) Size distributions of the hollow particles produced in liquid 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 hollow particles within each size range.
shows that for particles with sizes
