Highly stable aluminum nanoparticles (NPs) are generated via ablation of bulk Al in ethanol using either femtosecond (fs) or picosecond (ps) laser sources. The colloidal NPs solutions obtained with fs pulses exhibit a yellow coloration and show an increased optical absorption between 300 and 400 nm, tentatively assigned to the plasmon resonance of nanosized Al. The corresponding solutions after ps ablation are gray colored and opalescent. The average size of the NPs formed ranges from 20 nm for the fs case to 60 nm for the ps case, while a narrower distribution is obtained using the shorter pulses. High Resolution Transmission Electron Microscopy (HRTEM) studies indicate that the NPs are mostly amorphous with single crystalline inclusions. Al NPs generated with short laser pulses slowly react with air oxygen due to the presence of a native oxide cladding, which efficiently passivates their surface against further oxidation.
©2009 Optical Society of America
Aluminum NPs are considered as a possible fuel in advanced energetic materials applications such as propellants and pyrotechnics, as they feature high surface area which provides enhanced heat release during their exothermal oxidation [1–3]. Furthermore, aluminum nanostructures have recently attracted interest as building blocks for high-capacity hydrogen storage materials [4–6]; this is due to the ability of metallic aluminum to form hydrogen upon reaction with water. Finally, Al NPs are found to exhibit distinctive optical characteristics for optoelectronic applications, such as surface-enhanced Raman scattering (SERS) , metal-enhanced fluorescence for the label-free detection of biomolecules  and broad-band wire-grid polarizers . Several methods for the generation of Al NPs have been tested so far, among them is wet-chemical synthesis , arc discharge, laser ablation in vacuum and calcinations of Al hydride dispersed on fibrous substrates at elevated temperature . One of the major impediments, especially for energetic applications, is that bare Al is highly reactive while oxide coated Al significantly decreases overall performance. This effect becomes more pronounced as particle size decreases, since the oxide layer represents a significant fraction of its mass. Much effort has been devoted to the synthesis of small aluminum nanoparticles that are stabilized by coatings or organic surface passivation agents for protection from rapid oxidation to aluminum oxide under ambient conditions [12–15]. Nevertheless, synthesis of stable Al NPs exhibiting minute oxide thickness has remained a challenge.
Our approach to synthesis of metallic Al NPs is the use of laser ablation of bulk Al immersed into liquid. This technique provides the possibility of generating a large variety of NPs that are free of both surface-active substances and counter-ions [16,17]. During laser ablation in liquid a quenching of the ejected hot plasma from the target to the liquid occurs. As a result the plasma formed reacts with both the liquid and air oxygen dissolved in it. However, the high reactivity of Al with ambient oxygen can partially be compensated by ablating using short laser pulses due to faster quenching which could minimize the formation of either oxide or hydroxide on the particle surface. Furthermore, oxidation of NPs generated by this technique can be additionally suppressed by carefully outgassing the air trapped in liquid and/or replacement of it by a neutral gas.
Formation of nanostructures on a bulk Al target under its ablation in liquids with short laser pulses has been recently reported, which leads to visible yellow coloration of the exposed area . The coloration was assigned to the plasmon oscillation of electrons in the Al nanostructures formed on the target. At the same time it is observed that the liquid in which the ablation was carried out also takes on a yellow colour due to the ejection of Al NPs into the liquid. The present paper presents a systematic study of the generation of Al NPs colloids in ethanol using fs and ps laser pulses. It is shown that this technique may be an efficient way towards producing highly stable colloidal solutions of nanosized aluminum.
The generation of Al NPs was performed by laser ablation of an Al target supplied by Fluka with 99.9% purity. The surface of Al pellet was mechanically flattened and grinded with sandpaper to a μm roughness. The target was placed into a Pyrex cell and covered by a layer of absolute ethanol. The typical thickness of the liquid above the target was 1-2 mm. The cell was mounted on a computer-driven X-Y stage and translated during laser exposure. The ablation was performed using the beam of a fs Ti:sapphire laser emitting at 800nm, focused onto the target through the ethanol layer. The free surface of ethanol was exposed to air. Details of the experimental setup are given elsewhere [16,17]. It was found that the rate of NPs generation with 40 fs laser pulses at 800 nm is extremely low. This is mostly due to the fact that the peak power is so high that laser radiation is strongly absorbed by the liquid itself through non-linear processes. As a result, the fraction of laser energy that finally reaches the surface of the target is very small. Therefore, most of the experiments described below have been performed using 200 fs pulses. The typical spot size on the target was approximately 500 μm, corresponding to laser fluence of 0.2 J/cm2 at the target surface. Finally the exposure time was about 10 minutes at the laser repetition rate of 1 kHz.
Alternatively, two ps Nd:YAG lasers at a wavelength of 1.06 μm, 10 Hz repetition rate were used for generation of Al NPs in either identical to the fs or anaerobic conditions. Their pulse widths were 30 and 150 ps, the former is used for ablation in anaerobic conditions and the latter for ablation in air. For the ablation in anaerobic conditions the target was placed into a sealed-off Pyrex cell, filled with ethanol and outgassed with a vacuum pump following several cycles of cell freezing with liquid nitrogen. The cell was filled with Ar prior to laser exposure minimizing thus the air content both in the cell and liquid. Laser radiation was focused through the Pyrex cap and liquid layer on the target and the typical exposure time was approximately 1 hour.
The morphology of NPs was characterized by Transmission Electron Microscopy (TEM) with a Philips CM20 equipment. High Resolution Field Emission Transmission Electron Microscopy (HRTEM) was performed on a Jeol JEM 2100F UHR at 200 kV equipped with a field emission electron source. The optical absorption spectra of the different colloidal solutions were recorded in the range of 200-800 nm with the help of a Perkin-Elmer spectrophotometer.
3. Results and discussion
The colloidal solutions obtained by ablation of an Al target with two types of lasers are different in appearance; the colloids produced with fs laser ablation look yellow in transmission, while those obtained with ps radiation look gray. In both cases the solutions are slightly opalescent and their opalescence increases upon increasing the laser fluence. All the colloids prepared were stable against sedimentation for at least several months with no addition of any surface-active substances. Ablation of the same Al target in purified water, at fluencies of 0.2 – 0.5 J/cm2, leads to opalescent colorless solutions. This should be attributed to chemical interaction of molten Al with water.
The optical density of the solutions is presented in Fig. 1(a) , showing that NPs obtained with fs pulses exhibit a wide absorption “shoulder” between 300 and 400 nm. Additionally, the absorption edge protrudes to the visible region resulting in the yellowish coloration of the corresponding solution. On the contrary, no shoulder is observed in case of ps laser ablation; the peaks in the UV region are probably due to products of ethanol decomposition (Fig. 1(a),curve 3). NPs produced with fs pulses are hardly oxidized by air oxygen as indicated by the evolution of the corresponding absorption spectrum over several months (Fig. 1(b)). The same is true for those produced with ps radiation, however only if the solution is saved without contact with air in a sealed-off container.
A TEM image of the Al NPs produced by fs laser ablation is presented in Fig. 2 indicating that a significant number of nanoparticles feature a tail. Although this tail has lower contrast in TEM compared to the high-contrast core, a high number of such tails are shown on the image background. The NPs show no distinct diffraction pattern in TEM, so that they are mostly amorphous. This is confirmed by HRTEM characterization presented below. On the other hand, NPs generated using ps laser pulses are more round in shape. The colloidal solution in this case contains a significant amount of nanometer-sized debris that envelope NPs. Such debris have low contrast upon imaging and are probably non-metallic.
Size distribution of NPs obtained with different laser sources is presented in Fig. 3 . One can see that the size distribution is rather wide with the majority of NPs having size between 10 and 60 nm. It should be mentioned that the size of NPs determined using the dynamic light scattering (DLS) technique is always higher than that visible in Fig. 3 and for the same colloids is around 100-120 nm. This is attributed to the complex shape of NPs, shown in Fig. 2. Therefore, the presence of low-contrast tails of NPs contributes to DLS measurements, while distributions in Fig. 3 correspond to high-contrast metallic NPs. A higher magnification TEM image, shown in Fig. 4 , reveals that a distinct feature of Al NPs is the presence of a well-defined roundish area into their body. In most cases this area is situated close to the center of the particle and can be either brighter or darker than the rest of it. In some cases the same NP contains both dark and bright areas.
HRTEM images show crystalline inclusions within the Al nanoparticles (Fig. 5 ). Qualitatively, the fraction of crystallized areas of Al NPs is higher in case of ablation with ps laser (Fig. 6 ) under anaerobic conditions. The lattice plane distances measured on the HRTEM images are 0.233 and 0.202 nm which correspond to the (111) and (200) planes of metallic aluminum. It suggests that the nanoparticles are mostly amorphous but contain metal crystalline inclusions the proportion of which depending on the quenching rate of the nanoparticles. Longer pulses interact with NPs and therefore alter their quenching time.
The darker areas inside the NPs on the HRTEM images showed in Fig. 4 can correspond to a diffraction contrast of the crystallized inclusions or to the absorption contrast of thicker zones. On the other hand the bright areas visible inside the NPs in Fig. 4 can only correspond to a thinner zone and suggest a porosity of the Al NPs. A possible reason for appearance of these pores inside the Al NPs is the dissolution of the surrounding gas in the molten NP during laser ablation.The solubility of gases in metals increases by a factor of ten upon their melting . Fast solidification of NP ejected from the target quenches the dissolved gas, so that it becomes over-saturated. This over-saturation results in the formation of a gas-filled cavity inside the NP. Alternatively, these cavities can also be due to the chemical interaction of molten Al with traces of water in ethanol. In this case the cavities are filled with hydrogen, though this hypothesis requires further studies. In any case, the observation of this cavity is unique for laser-assisted generation of Al NPs and has never been reported so far for NP of other metals .
Electron Energy Loss Spectroscopy (EELS, not presented here) data confirm the metallic Al nature of the NPs obtained. The Al K-edge located at 1560 eV presents a near edge structure consistent with aluminium metal. In case of alumina particles the Al K-edge would be very different with a sharp line that characterizes aluminium oxide . A broad peak located at 531 eV corresponds to the O K-edge. The weak intensity of this peak confirms that the particles are mainly metallic certainly with a thin oxide layer at the surface, also indicated by the slow evolution of the absorption spectrum with time. This layer is not clearly visible in TEM and HRTEM images of NPs produced using fs pulses, indicating that its thickness does not exceed the one of natural Al oxide and passivates Al NPs against further oxidation. On the other hand presence of an oxide layer is well seen on TEM images of NPs produced by ablation with ps pulses without using anaerobic conditions. This is illustrated in Fig. 7 where the NPs generated using 150 ps pulses have a core-shell structure with metallic core. The low-contrast shell is better visible on large NPs though is apparently presented on smaller ones.
One may deduce that laser ablation of a bulk Al target in ethanol results in the formation of metallic Al NPs. These NP are characterized by a plasmon resonance in near UV range. Absence of resonance peak in case of NP generated with ps laser radiation may be assigned to their larger size. In general, the average size of NPs is determined by the laser fluence on the target and not by the duration of the laser pulse . The oxidation of NPs generated by laser ablation is closely related to the laser pulse width. Indeed, sufficiently short laser pulses produce mostly metallic NPs without the oxide shell observable with TEM. The NPs are oxidized slowly due to diffusion of air oxygen into the colloidal solution. On the contrary, the longer pulse duration of 150 ps leads to visible oxidation of as-generated NPs. However, in the latter case this oxide shell may provide better stability of NPs against further oxidation by air oxygen serving as a protective layer.
The elongated shape of Al NPs (tails) observed in case of ablation with a fs laser is closely related to the formation of nanostructures (NSs) on bulk Al targets under their ablation in liquids with short laser pulses [18,21]. NSs are formed under the recoil pressure of the vapours of the liquid that surrounds the target. At the same time, a fraction of the molten layer of the target is dispersed into the liquid as NPs. Accordingly the tails of NPs observed in the present work are due to detachment of liquid nano-drops from the molten layer. Finally, the nano-drops that fail to detach from the melt remain on the solidified surface as mushroom-like structures . Such structures are apparent in FESEM images of nanostructured Al surfaces, an example of which is shown in Fig. 8 . The close relationship between Al NSs and Al NPs can also be deduced from the proximity of the plasmon resonance of Al NPs reported in this work and the maximum of absorption of nanostructured Al surface .
The theoretical position of the plasmon resonance of Al NPs of 10 nm in diameter in water calculated in Ref , and the maximum of absorption lies around 200 nm. However, it is red shifted for NPs of higher diameters [22,23]. Oxidation of Al NPs would also cause a red shift of their plasmon resonance, since its oxide, Al2O3, has higher refractive index in the UV range than water. Indeed, theoretical calculations performed by Lukyanchuck et. al.  are in good agreement with our results, as they calculated a large absorption peak between 300 and 400 nm for Al NPs with size 30-60 nm. The same is shown by Hornyak et al , utilizing dynamical Maxwell-Garnett effective medium theory. Both laser wavelengths used in this study (800 and 1064 nm) are largely detuned from the plasmon resonance of Al NPs. This is an advantage for the generation of metallic Al NPs, since NPs formed can be weak absorbers at the laser wavelength. On the contrary, UV wavelengths are close to the plasmon resonance of generated Al NPs, which can thus be efficiently heated by the ablation beam. This leads to enhanced interaction with the environment and eventual oxidation. Therefore, UV laser sources are not appropriate for generation of Al NPs.
The data on the morphology of Al NPs generated by laser ablation of an Al target shed new light on the mechanism of their formation. A number of experiments on laser-assisted formation of NPs have been reported and the modeling of NPs growth is described in terms of nucleation of the target material in the expanding plasma plume . Such approach shows good agreement with experimental data [27,28], especially in case of carbon ablation in liquid environment with nanosecond laser pulses. Presence of a molten layer on the target made of carbon is still under question. Al is a metal with low melting point, and the presence of a melt layer during laser ablation affects the size distribution of ejected NPs. Indeed, the specific shape of Al NPs observed in the present work (see Fig. 4) and nano-protrusions left on the surface of the target (Fig. 8) resemble each other. At a given laser fluence some protrusions leave the molten surface and become nanoparticles. Therefore NPs leave the melt as nano-entities and are not condensed in the plasma plume from smaller clusters or atoms. The dispersion of the melt occurs due to hydrodynamic instability that develops at the melt- vapor interface of the liquid that surrounds the target . The maximal amount of the target material that can be dispersed is determined by the melt thickness and is of order of (atp)1/2, where a stands for the heat diffusion coefficient of the target material, tp is the laser pulse duration. Melt thickness also depends on the laser fluence in a linear way as soon as it exceeds the threshold fluence needed to melt the target material. In case of femtosecond laser the melting and dispersion of the melt starts when the laser pulse is over, after completion of the electron-phonon relaxation. The melt thickness in this case is determined by the absorption depth of laser radiation in the target material and the size of NPs is almost independent on the pulse duration, since the diffusion length of electrons excited in the material into the target is negligible.
In summary, laser ablation of a bulk Al targets in ethanol leads to the formation of metallic Al NPs. In case of fs laser ablation the process can be carried out in open air. Generated Al NPs exhibit minimal oxide cladding and are pretty stable as they become slowly oxidized by air oxygen. Both are highly desirable properties for advanced energetic applications. Laser ablation with longer pulses in anaerobic conditions also results in metallic NPs, while NPs generated in open air are of core-shell type due to partial oxidation. The average size of Al NPs formed lies between 10 and 60 nm, depending on the experimental conditions. In all cases, the NPs produced are mostly amorphous with few single crystalline inclusions per NP. Ultrafast laser structuring of bulk Al in liquids may be potentially a promising technique for efficient production of surface passivated Al NPs.
The work was partially supported by Russian Foundation for Basic Research, grants ## 07-02-00757, 07-02-12209, 08-07-91950 and by Scientific School 8108.2006.2. It was additionally supported, in part, by the European Commission through the Research Infrastructures activity of FP6 (‘Laserlab-Europe’ RII3-CT-2003-506350). M. Pugnet is acknowledged for his assistance in work with a 30 ps laser. E.V. Barmina is thanked for the help in TEM data processing. V. Collière is acknowledged for HRTEM experiments and B. Warrot-Fontrose for the EELS.
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