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Collinear double pulse laser ablation in water for the production of silver nanoparticles Marcella Dell’Aglio,a Rosalba Gaudiuso,ab Remah ElRashedy,b Olga De Pascale,a Gerardo Palazzob and Alessandro De Giacomo*ab Experiments of collinear Double Pulse Laser Ablation in Liquid (DP-LAL) were carried out for studying the production mechanisms of nanoparticles (NPs) in water, which revealed the fundamental role of the cavitation bubble dynamics in the formation of aqueous colloidal dispersions. In this work, DP-LAL was used to generate silver nanoparticles (AgNPs) from a silver target submerged in water at atmospheric pressure and room temperature, by using the second harmonic (532 nm) of two Nd:YAG lasers. The second laser pulse was shot at different delay times (i.e. interpulse delay) during the bubble temporal evolution of the first laser induced bubble. Optical Emission Spectroscopy, Shadowgraph Images,

Received 4th October 2013, Accepted 21st October 2013

Surface Plasmon Resonance absorption spectroscopy and Dynamic Light Scattering were carried out to

DOI: 10.1039/c3cp54194k

to monitor the generation of AgNPs under different conditions, and for characterization of NPs. The

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results of DP-LAL were always compared with the corresponding ones obtained with Single Pulse Laser Ablation in Liquid (SP-LAL), so as to highlight the peculiarities of the two different techniques.

study the behaviour of laser-induced plasma and cavitation bubbles during the laser ablation in liquid,

1. Introduction Laser Ablation in Liquids (LAL) for the production of colloidal solutions has been widely investigated in the last decade because of several promising advantages, which include: environmental sustainability, easy set-up, and long stability of the produced nanoparticles (NPs).1,2 Fundamental studies have been carried out to improve the LAL process for what concerns the yield of production and for reducing the size distribution of the colloidal solution.3,4 Recent studies demonstrate the importance of the cavitation bubble dynamics in the nucleation and aggregation processes leading to the formation of nanoparticles in the solution.5,6 Based on this knowledge, in this work Double Pulse Laser Ablation in Liquid (DP-LAL) has been applied in order to investigate the effect of the delay time between two consecutive pulses (i.e. interpulse delay) on the time scale of the oscillation period of the bubble produced by the first pulse (hundreds of microsecond depending on laser energy). DP-LAL has been investigated previously both for the production of nanostructures7 and for analytical chemistry applications, i.e. LIBS (Laser-Induced Breakdown Spectroscopy).8 Most double pulse experiments for

a

CNR-IMIP, Via Amendola 122/D, 70126, Bari, Italy. E-mail: [email protected]; Fax: +39 080 5929520; Tel: +39 080 5929506 b University of Bari, Dipartimento di Chimica, Via Orabona 4, 70125, Bari, Italy. Fax: +39 080 5442024; Tel: +39 080 5929511

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production of NPs reported in the literature have been carried out in orthogonal configuration, with the aim of inducing fragmentation and improving the NP size control.9 In orthogonal configuration DP-LAL, the two laser beams are sent with an angle of 901 between each other so that the first pulse ablates the target, while the second pulse, opportunely delayed, irradiates the ejected material. In spite of its promising preliminary results, especially for what concerns the production yield,10 DP-LAL in collinear configuration for production of NPs has been poorly investigated, and a deeper insight into the fundamental aspects of this technique is needed to improve the LAL process. In this frame, it can be beneficial to underline here what are the differences between collinear DP-LAL and high repetition rate ablation (in the kHz regime). These are mainly related to the time interval between subsequent pulses. In the first technique, the second pulse is shot on the time scale of the first oscillation of the cavitation bubble (about 0–200 ms, depending on the laser energy) without affecting the repetition rate of the ablation. On the other hand, in high repetition rate ablation, typical temporal separation between subsequent pulses is 0.1–1 ms and usually it is not finely optimized with respect to the phenomena induced by the previous laser pulse. As a consequence, the production yield may increase but on the other hand the control of the NP size is poor and heating of the target and of the solution is induced, which in turn can affect the distribution size of the colloidal solution. In some of our previous works the evolution of thermodynamic parameters such as pressure and temperature inside

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the laser-induced cavitation bubble has been studied and described in detail.11 Based on these previous results, in this work we investigated DP-LAL in collinear configuration from the fundamental point of view, with the aim of clarifying the main characteristics of the process and its possible advantages. In this set of experiments, the first pulse was used for plasma induction and cavitation bubble generation, while the second pulse was focused on the target surface, through the cavitation bubble, at specific interpulse delays corresponding to different bubble conditions (i.e. vapour pressure and temperature, bubble volume). Different techniques were used in order to understand the different phenomena occurring in the DP-LAL process upon changing the interpulse delay time. These include: Optical Emission Spectroscopy (OES) for studying the plasma induced by the second laser pulse; fast camera shadowgraph for the cavitation dynamics during single and double pulse techniques; Surface Plasmon Resonance (SPR) absorption spectroscopy and Dynamic Light Scattering (DLS) for the produced NPs. All these results were linked to obtain a comprehensive description of the main features of the DP-LAL processes, both from the fundamental and application points of view.

2. Experimental set-up and procedures In this work one or two nanosecond laser pulses are focused on a silver target submerged in water to perform Single Pulse or Double Pulse Laser Ablation in Liquids (SP-LAL and DP-LAL, respectively). The experimental set-up used for SP- and DP-LAL is shown in Fig. 1. It consisted of two nanosecond lasers, a stainless steel chamber (or a cuvette), a spectroscopic system and a shadowgraph one. The two laser sources (Quanta System, PILS-GIANT) were Nd:YAG lasers operating at the second harmonic (532 nm), with a repetition rate of 10 Hz and a nominal pulse duration of 8 ns, and they were synchronized using a pulse

Fig. 1 Experimental set-up. Dashed boxes show the two different detection systems (for OES and shadowgraph measurements), and the cuvette for the production of Ag colloidal solutions at controlled concentration.

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generator (Stanford inc. DG 535). The two lasers had cross polarization, thus dichroic mirrors were placed to match spatially the two beams, so that one was transmitted and the other reflected. For the spectroscopic and shadowgraph experiments a stainless steel chamber was filled with 500 ml of de-ionized water that was changed for each measurement. The chamber was equipped with spacer tubes (at an angle of 901 with each other), which hosted a focusing lens and/or a sapphire optical window and allowed adjustment of the lens-to-sample distance. The sample is placed inside the chamber onto a rotating target holder with the possibility of 10 mm axial movement (back and forth). To focus the laser pulses on the sample, a 4.0 cm focusing lens in air is fitted in the spacer tube opposite the target. Ag colloidal solutions at controlled concentration have been produced by focusing the laser with a 4.0 cm focusing lens directly on the sample, placed in a cuvette filled with 3 ml of de-ionized water. A Good fellow Ag pellet (99.95% purity, 6 mm thickness) was used as a target. The laser ablation time was 30 seconds for all the DP experiments and 30 seconds or 3 minutes for the SP experiments. In SP-LAL, the laser energy was always set at 33 mJ with a fluence of 105 J cm 2. In DP-LAL each laser pulse energy was 33 mJ with a fluence of 105 J cm 2. The Optical Emission Spectroscopy (OES) experiments for the plasma investigation were performed with a spectroscopic system consisting of: a spectrograph Jobin Yvon TRIAX 550, an ICCD Jobin Yvon i3000 and a pulse generator Stanford inc. DG 535 for the synchronization of laser pulses and ICCD. To collect the plasma emission a mirror was placed on top of the chamber and, with a biconvex 7.5 cm focal quartz lens, the plasma emission was coupled with the monochromator through an optical fiber. For the optical emission spectroscopy the sample was rotated during the measurements to limit the surface digging effect. All the emission spectra were acquired with 10 accumulations and 5 averages to optimize the signal-to-noise ratio. The employed acquisition time parameters were: delay time of 250 ns and gate width of 5 ms (with respect to the second laser pulse in DP experiments). Intensity and Stark broadening of the Ag I emission line at 520.90 nm were measured during DP experiments. To calculate the electron number density from the Stark broadening the values in ref. 12 were used. The shadowgraph set-up for studying the cavitation bubble dynamics consisted of a continuous light source, a set of lenses to reduce light divergence, a fast Camera (CITIUS IMAGING High Speed Video Camera C100 Centurio) with a teleobjective to acquire the bubble profile and a pulse generator to synchronize laser pulses with the camera. The shadow of the bubble was collected through a dedicated sapphire window set in the chamber spacer tube at 901 with respect to the laser pulse direction. For the shadowgraph analyses, the sample was kept fixed to avoid formation of bubbles along the optical path, but its position was changed after each measurement. The used speed frame was 35 461 frames per s and the temporal resolution was 28 ms for each frame. The Surface Plasmon Resonance (SPR) absorption spectroscopy of the colloidal solutions was performed using an Ocean Optics (USB2000 + XR) spectrometer equipped with a continuous

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light source (Mini Deuterium Halogen Light Source DT-Mini2-GS). For the calculation of the extinction coefficient of AgNPs in water solution, that was used to calculate the concentration, a calibration curve was drawn by using Ag colloidal standard solutions with known concentration and particle size distribution (10 nm Citrate NanoXactt Silver, 0.022 g l 1). The last was chosen because of its similarity to the particle size distribution obtained in this work with SP and DP-LAL and determined with DLS measurements. Dynamic Light Scattering (DLS) measurements were performed using a Zetasizer-Nano S from Malvern operating with a 4 mW He–Ne laser (633 nm wavelength) and a fixed detector angle of 1731 (non invasive backscattering geometry NIBSt) and with the cell holder maintained at 25 1C by means of a Peltier element. Measurements were performed directly in the same cuvettes in which nanoparticles were prepared by LAL. Data were collected leaving the instrument free to optimize the instrumental parameters (attenuator, optics position and number of runs13,14). Usually the time autocorrelation function (ACF) of scattered light intensity was the average of 10–12 consecutive runs of 10 s each. Three intensity ACFs collected every 5 min were subsequently examined by the operator and averaged. The size distribution by number was recovered by taking the inverse Laplace transform of the ACF and subsequent application of Stokes–Einstein equation assuming the viscosity of the water solution at 25 1C using the software implemented by the manufacturer.

3. Results and discussion During SP-LAL, a laser-induced plasma is generated by the interaction of a laser pulse with the target and initially shows high temperature (6000–10 000 K) and ionization degree close to unity.15 While expanding at supersonic speed, the plasma releases its energy to the surrounding liquid and generates a thin layer of vapor at the plasma border, which evolves into a cavitation bubble.5 In a previous work of ours,5 laser-induced plasmas of different metallic targets in liquid, and the consequent cavitation bubble generation, have been described in detail. We report in Fig. 2 the typical evolution of the radius of a cavitation bubble induced on a solid surface, as determined in this work by fast shadowgraph. This trend is characterized by an expansion stage, a maximum and a compression stage. Fig. 2 shows that under the employed experimental conditions the bubble radius significantly increases in the first 40 ms and then, once reached the maximum of expansion, it holds approximately similar values up to 80 ms, because the vapor in the bubble is close to equilibrium with the surrounding liquid.16 After this stage, since at the maximum expansion pressure inside the bubble is lower than the external one, the bubble starts to shrink and decreases its size again, at high speed, until an inversion point occurs at 170 ms and the bubble rebounds. Based on the temporal evolution of the radius it is possible to estimate pressure and temperature inside the bubble, as suggested in ref. 5. This analysis shows that initially

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Fig. 2 Temporal evolution of the radius of a laser-induced bubble (produced from a silver target in water) as measured from shadowgraph images during a SP experiment.

the pressure is in the order of GPa, reaches a minimum, equal to the saturation pressure at the maximum expansion, when the bubble is in equilibrium with the surrounding liquid, and finally increases again during the collapse phase. Immediately after the plasma quenching, ablated silver species grow in spherical nanoparticles of various sizes that diffuse in the bubble interior. The dynamics of NP formation was studied in ref. 17 by X-ray mapping of the particulate matter during cavitation and in ref. 5 by shadowgraph and laser scattering of the cavitation bubble. In the early stage of the bubble expansion small size NPs are formed and diluted during bubble expansion, while larger agglomerates are formed in the late stage of the bubble compression. Moreover as the bubble volume changes in time, the NP concentration is modified accordingly. Initially, the bubble volume is small, so the NP concentration is high and decreases in time as the bubble volume increases during the expansion, in agreement with the discussion about Fig. 2. When the collapse stage begins, and since the shrinking dynamics is very fast, most AgNPs accumulate behind the bubble border and the local concentration increases again. In this scenario, when the subsequent pulse is shot, very different phenomena can be expected to occur according to the chosen interpulse delay, i.e. to the moment of the bubble evolution.

3.1

Optical investigation of the DP-LAL process

OES of the plasma produced by the second pulse allows estimating excitation temperature (Tex) and electron number density (Ne) as a function of the interpulse delay, respectively, with the techniques of the Boltzmann plot and Stark broadening. Silver emission lines are not appropriate to draw a Boltzmann plot18 for some useful criteria, thus the excitation temperature was only estimated as a comparison with the one measured by OES of a titanium plasma produced in water under the same experimental conditions used for silver. The estimated Tex of the second laser-induced plasma varies in the range 5500–7500 K, according to the interpulse delay. Instead, plasma emission intensity and electron number density are

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Fig. 3 OES measurement of plasma emission produced by the second laser emission intensity of pulse during a DP experiment on an Ag target in water: electron number density as a function of the Ag I spectral line (520.9 nm) and the delay time between the two laser pulses. The temporal evolution of the ’ bubble radius (Fig. 2) is reported for a comparison.

directly measured during the DP-LAL of an Ag target by using the Ag I emission line at 520.90 nm. The results are shown in Fig. 3, where the radius of the bubble induced by the first pulse is reported for facilitating the discussion. Firstly, it can be observed that the emission intensity trend follows the bubble dynamics. Considering that the excitation temperature during DP-LAL is just slightly different at different interpulse delays,8 this result can be interpreted in terms of different ablation yield at different interpulse delay, so that around the maximum of bubble expansion the number density of Ag emitters is highest. This suggests that when the second pulse is shot during the early expansion and late collapse phases, a considerable portion of the laser energy cannot reach the target surface, mainly because of its interaction with the NPs produced by the first pulse. It is anyway worth underlining that the bubble surface itself can deflect and scatter part of the second laser beam. In the collinear DP case this effect is minimized, since the second laser beam is always perpendicular to the bubble surface. It is interesting to note that electron number density (Fig. 3) shows an inverse trend with respect to that of intensity. This is due to the pressure in the bubble being higher in the early expansion stages and in the late collapse ones, which causes strong confinement of the second pulse plasma and an increase of the electron density due to an increased collision frequency, as discussed in ref. 5 and 15. To elucidate the effect of the second pulse as a function of the interpulse delay in DP-LAL technique on the time scale of cavitation bubble evolution (i.e. after the plasma extinction), time-resolved shadowgraph was employed. Selected shadowgraph imaging frames are shown in Fig. 4 at representative interpulse delays, corresponding to different phases of the bubble induced by the first pulse, namely, the early expansion stage, maximum of expansion and the collapse phase. By the analysis of the images it is clear that for small bubble volumes, i.e. at the early expansion and late collapse stages, the

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second laser pulse is strongly absorbed by the NPs behind the front head of the cavitation bubble, thus inducing their optical breakdown. It is interesting to note that in the case of the early expansion stage (i.e. interpulse delay = 40 ms), due to this breakdown and to the induction of a plasma of the NPs, a secondary cavitation bubble is generated at the main bubble front. In the case of the collapse stage the secondary cavitation bubble can be observed only after that the first laser-induced bubble actually disappears. This can be rationalized considering that the point where the secondary breakdown takes place is moving towards the target, due to the drag effect of the collapsing bubble. Thus, even if the secondary bubble is formed, it cannot be imaged by shadowgraph, because it is hidden by the shadow of the residual first bubble. This also confirms that, during the early expansion and later collapse stages, most of the produced NPs are close to the bubble boundary. When the second breakdown occurs on NPs produced by the first pulse, most laser energy does not reach the target surface and, in agreement with the OES observation, its ablation is strongly reduced and so is the production of Ag emitters. In contrast, when the bubble is at its maximum expansion, NPs have the time to distribute evenly in the whole volume, thus decreasing their concentration. Therefore, at interpulse delay corresponding to this situation, the energy of the second laser pulse is maintained, with the exception of the scattering losses, and efficient ablation of the bulk target can occur. As reported in Fig. 3, the highest optical emission intensity is observed at this interpulse delay.

3.2

Optical characterization of AgNPs produced by DP-LAL

AgNPs produced by DP-LAL at different interpulse delays were investigated by Surface Plasmon Resonance (SPR) absorption and Dynamic Light Scattering (DLS). These techniques were preferred to microscopic ones because they are non-invasive and allow probing the particles in solution (in particular their possible aggregation), thus giving more useful information to be correlated with the processes described above. Fig. 5 shows typical SPR absorption spectra of AgNP aqueous dispersions obtained with SP-LAL and with DP-LAL at interpulse delays corresponding to late collapse. The SPR technique can be a fast technique for the characterization of particle shape and size in the case of silver, gold and copper nanoparticles.19 In the case of NPs with spherical shape, the absorption spectra show a sharp plasmon band (as in Fig. 5) with SPR peak wavelength directly proportional to the size of the NPs.21 Moreover it is well known that the NPs obtained by LAL, in particular the metallic ones (Ag, Au, Cu), have prevalently spherical shape, as demonstrated in several papers1,4,20 by TEM micrographs and SPR absorption spectroscopy. The absorption spectra of all the colloidal solutions obtained in this work at different interpulse delays show the same features of those in Fig. 5. Therefore, it was inferred that a reasonable fraction of AgNPs is spherical at all interpulse delays. The wavelength of the SPR peak and the absorbance of the solutions prepared by DP-LAL of silver immediately after 30 s of ablation

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Fig. 4 Time-resolved shadowgraph images of bubbles induced by double laser pulses on an Ag target in water. The image in a dashed frame represents the time of arrival of the second pulse (I.D. is the interpulse delay).

Fig. 5 SPR absorption spectra of the AgNPs in water obtained by SP-LAL and by DP-LAL at interpulse delays of 160 ms.

processes are shown in Fig. 6 and 7 as a function of interpulse delay. It should be noted that in Fig. 7 Ag mass concentration, Ag ablated mass per laser shot and the molar concentration of NPs were calculated from absorbance as explained below. In both

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Fig. 6 SPR peak wavelength of AgNPs in water as a function of interpulse delay prepared by DP-LAL. The first point corresponds to an AgNP solution prepared with SP-LAL under the same experimental conditions of DP-LAL. The temporal evolution of the bubble radius (Fig. 2) is reported for a comparison.

figures the first point, separated by others, corresponds to a solution prepared with SP-LAL of a silver target obtained under the same experimental conditions of DP-LAL, so as to provide a direct comparison between the two experiments. These conditions

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Fig. 7 Absorbance (a) of AgNPs in water, silver mass concentration (b), Ag ablated mass per laser shot (c) and molar concentration of nanoparticles (d) of solutions obtained by SP-LAL and DP-LAL as a function of interpulse delay.

were: ablation time of 30 s in both cases and laser energy of 33 mJ in SP-LAL and of 33 mJ for each laser pulse in DP-LAL. Moreover in Fig. 6 the bubble radius evolution is reported to facilitate the discussion. Two main observations can be done based on Fig. 6 and 7:  The wavelength of the SPR peak shows the same trend of the bubble radius, that is, it increases in the bubble expansion stage, reaches a maximum at the maximum of bubble expansion, and then decreases in the shrinking phase. Considering that wavelength and NP size are directly proportional, we can conclude that at interpulse delay corresponding to the maximum of bubble expansion, the obtained NPs are comparatively larger than at interpulse delay corresponding to expansion and collapse stages.  The absorbance of the SPR peak as a function of interpulse delay follows the trend of cavitation bubble dynamics too. As is well known, absorbance obeys the Lambert Beer law and is related to NP concentration. Considering that the scattering contribution can be neglected for NPs smaller than 20 nm22,23 such as those produced in these experiments, Fig. 7 suggests that higher concentration of NPs is obtained with DP-LAL, when the laser pulses are shot at interpulse delay corresponding to the maximum of bubble expansion. In Fig. 7 the Ag mass concentration, the molar concentration of NPs (calculated as in ref. 24) and the Ag ablated mass per laser shot (ng per laser shot) are reported for solutions obtained by SP and DP-LAL. The whole set of concentrations is calculated from the absorbance value, and keeps therefore the same trend as a function of the interpulse delay. The extinction coefficient, necessary to calculate the molar concentration, was obtained as described in Section 2 for 10 nm NPs, and considering that for a qualitative discussion its variation for NPs of similar size could be neglected. Connecting SPR, OES and shadowgraph results clearly shows that the larger the second laser pulse energy fraction arriving to the target, the higher the yield of the process. In contrast, the lower the pulse energy reaching the target, the higher the fraction spent in interacting with the first pulse-produced NPs,

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which leads to increased fragmentation. In this frame, and in agreement with the trend of the SPR peak wavelength as a function of the interpulse delay, smaller NPs are obtained at interpulse delay times corresponding to the early expansion and late collapse of the cavitation bubble. Fig. 6 and 7 display a different behavior of SP-LAL results under the same laser experimental conditions as DP-LAL. The wavelength of the SPR peak of the solution prepared by SP-LAL is generally shorter than those corresponding to DP-LAL because of the lower concentration of the colloidal solution produced after 30 s of ablation (with the exception of those produced in the late collapsing phase, when the fragmentation efficiency is very high, as previously discussed). On the other hand the yield of DP-LAL, as shown in Fig. 7 in terms of molarity of AgNPs, is higher than SP-LAL at all the interpulse delays, reaching an improvement of about 3 times at the maximum of bubble expansion. The latter results can be better appreciated by determining the effective amount of matter removed from the target by each laser shot, in the case of SP and DP-LAL at different interpulse delay as reported in Fig. 7. Since the ablation time and the water volume are fixed and the laser repetition rate is known, the Ag ablated mass per laser shot (ng per laser shot) was directly calculated from the Ag mass concentration of the solution (g l 1). Finally to confirm the discussion above, dynamic light scattering was employed for analyzing the NP solution 48 hours after the ablation process. Representative size distribution functions are reported in Fig. 8. At any interpulse delay the average size is between 6 and 10 nm and the distribution is narrow enough to display a trend in the dependence of the particle size on the interpulse delay. The mean hydrodynamic diameter is plotted as a function of the interpulse delay in Fig. 9a. For comparison purpose on the same graph is also shown the dependence of the wavelength of the SPR already shown in Fig. 6. It is clear from Fig. 9a that the two quantities, which were measured with independent techniques for the same sample, follow the same trend. This confirms the interpretation of the wavelength shift corresponding to the maximum in the plasmon absorption in terms of larger size of the nanoparticles. An even more

Fig. 8 Number size distributions obtained by DLS experiments for AgNP solutions obtained by SP-LAL and by DP-LAL at two representative interpulse delays.

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Fig. 10 (a) SPR peak wavelength and (b) SPR peak width of AgNP solutions obtained by ’ SP and DP-LAL at three representative interpulse delays 10 ms, 80 ms and 160 ms, respectively, as a function of time after laser ablation.

Fig. 9 (a) Hydrodynamic diameter and (b) intensity of scattered light for AgNPs prepared by DP-LAL as a function of interpulse delay. The relevant solution prepared with SP-LAL under the same experimental conditions of DP-LAL is indicated by an arrow on the ordinate axis. SPR peak wavelength temporal evolution (Fig. 6) is reported for a comparison.

stringent agreement was found between the SPR maximum wavelength and the intensity of the scattered light. Within the Rayleigh regime, i.e. when the particle’s size is much smaller than the light wavelength, the intensity of light scattered by a solution of monodispersed particles is proportional to the square of the particle molecular weight (M1) times the number density of the particle (N1), i.e., IS p M12N1. Therefore, as long as the mass concentration of NPs (M1N1) does not change wildly among samples, the intensity of scattered light is a sensitive indicator of the particle mass and thus size. The dependence of scattered light intensity on interpulse delay (Fig. 9b) closely follows the trend discussed for the position of SPR maximum. It should be stressed that the light scattering intensity and the hydrodynamic particle size, although both are measured in a DLS experiment, are fully independent quantities. Together with the position of the SPR peak they form a group of three independent experimental proofs pointing towards a scenario in which smaller particles are produced in the early expansion and late collapse stages of the cavitation bubble. Finally, the SPR absorbance spectra of the AgNPs produced by SP- and DP-LAL were monitored throughout 3 weeks to check the stability of the solutions in time. In Fig. 10a and b the SPR peak wavelengths and peak width are reported as a

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function of time after laser ablation for SP-LAL and for DP-LAL at three characteristic interpulse delay times (10, 80 e 160 ms). To compare the stability of AgNP solutions, the laser ablation times for SP and DP-LAL experiments were set to 3 min and 30 seconds, respectively, in order to obtain similar concentration solutions. It is indeed well known that concentration plays a fundamental role in the aggregation of colloidal solutions without stabilizing agents. It is interesting to observe that in the case of DP-LAL the size of NPs (related to SPR peak wavelength) and their size distribution (related to the SPR peak width) rearrange in the first 500 minutes decreasing their values, then after 1 day they stabilize at a given value for a long time. A small increase of these values is observed after two and three weeks. In contrast, the NPs produced by SP-LAL after the first 100 minutes increase slowly their size as a consequence of the thermodynamic trend of the colloidal solution. In any case both colloidal solutions show an acceptable stability in the time interval considered in this observation. The better stability of DP-LAL colloidal solutions may be ascribed to the excess of free electrical charge generated by the laser-induced breakdown, which increases the charging effect of NPs, thus inhibiting the kinetics of growth of the colloidal solution and stabilizing their size distribution for over 3 weeks.

4. Conclusions Plasma emission spectroscopy, shadowgraph images, absorption spectroscopy and dynamic light scattering were used to study the effect of laser induced plasma and cavitation bubbles on the NP formation. The collinear double pulse technique was used to investigate how the different stages of the bubble evolution can influence the formation of NPs. In this work, a submerged silver target was employed to generate AgNP colloidal dispersions. The first laser pulse generates the first bubble and the NPs; the second laser pulse is shot with different interpulse delay, thus at different evolution times of the first bubble, and it can indirectly reveal where the NPs due to the

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first pulse were formed. On the other hand the second laser pulse can generate other NPs if it reaches the target surface. The resulting NP characteristics were found to be highly dependent on the delay between the two laser pulses. OES and shadowgraph clearly showed that the larger the energy fraction of the second laser pulse arriving to the target, the higher the yield of the process. In contrast, the lower the pulse energy reaching the target, the higher the fraction spent in interacting with the NPs produced by the first pulse, which led to increased fragmentation. Based on these observations, two main conclusions were drawn: (i) smaller NPs were produced at interpulse delay times corresponding to the early expansion and late collapse stages of the cavitation bubble; (ii) higher concentration of NPs was obtained with DP-LAL when the laser pulses were shot at interpulse delay corresponding to the maximum of bubble expansion. Moreover, following the surface plasmon resonance spectra of NP dispersion after the end of the laser ablation process, it appeared clear that DP-LAL enabled producing AgNPs with better stability. Moreover, although DLS showed that the size of NPs obtained with SP and DP-LAL was essentially comparable immediately after the end of the laser ablation, the aggregation process of NPs in time appeared to be favorably reduced for the NPs produced with DP-LAL.

Acknowledgements This research is partially supported by MIUR (PRIN Project 2010ERFKXL_007) and by CLaN (Combined Laser Nanotechnology) project co-financed by the Operational Programme ERDF Basilicata 2007–2013.

Notes and references 1 V. Amendola and M. Meneghetti, Phys. Chem. Chem. Phys., 2013, 15, 3027. 2 H. Zeng, X. Du, S. C. Singh, S. A. Kulinich, S. Yang, J. He and W. Cai, Adv. Funct. Mater., 2012, 22, 1333–1353. ´, J. Kohno, Y. Takeda, T. Kondow and H. Sawabe, 3 F. Mafune J. Phys. Chem. B, 2000, 104, 9111–9117. 4 V. Amendola and M. Meneghetti, Phys. Chem. Chem. Phys., 2009, 11, 3805–3821. 5 A. De Giacomo, M. Dell’Aglio, A. Santagata, R. Gaudiuso, O. De Pascale, P. Wagener, G. C. Messina, G. Compagnini and S. Barcikowski, Phys. Chem. Chem. Phys., 2013, 15, 3083.

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20875

Collinear double pulse laser ablation in water for the production of silver nanoparticles.

Experiments of collinear Double Pulse Laser Ablation in Liquid (DP-LAL) were carried out for studying the production mechanisms of nanoparticles (NPs)...
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