**3. Laser ablation mechanism and metal nanoparticles formation**

The mechanism of laser ablation depends on physical properties of metals and environment medium. Therefore, the ablation of metals is an intricate subject [42–44]. Ablation of metal target commences with the sorption of laser beam energy. When the laser beam interacts with the metal target, the heat can generate and the photoionization of the metal target can occur. After that, metal nanoparticles will be released from the metal plate as the different phase that depends on the absorbed energy E [42], and plasma plume expands [43–45]. Hence, if the duration of laser ablation is much higher than the laser pulse duration, the ablation depth (*La* ) could be obtained as follows:

$$L\_a \approx E^{\frac{2}{3}}, \quad t\_a \approx E^{\frac{1}{2}},\tag{1}$$

Where *ta* , *Te* , and *t*<sup>1</sup> are the time of the ablation process, the electronic temperature during the ablation process, and the laser pulse duration, respectively [6, 42].

In accordance prior report, when the energy of laser beam is high enough to generate a plasma plume, an acceptable ablation rate can be obtained. During laser ablation of metal plates, the plasma plume can be formed with the generation of photon and sound [46, 47]. This phenomenon is confined near the metal plate [44, 46, 47]. The face of the metal plate face in the plasma plume remains at high temperature and high pressure during the ablation of the metal plate [44, 46–48]. During the laser ablation of the metal plate, some thermodynamic phenomena occur near the metal target. Therefore, a face of the target in the plasma plume observes energy and the physical parameters are not constant on the whole target area [44, 49, 50]. Moreover, sometimes the deportation of particles from the metal targets due to photoionization [51]. So, the concentration and distribution of metal nanoparticles become large in the liquids by using the laser ablation of the metal plate [50]. The formation of metal nanoparticles based on laser ablation of the metal plate can be explained as bubbling of metal molecules [43, 44, 49]. Some physical properties including of size and concentration of nanoparticles in the liquid are a function of the phase homogeneity of the material released into the liquid during the irradiation of metal target. In accordance with the literature, the changes of morphology were observed around the ablated area on the surface of Au or Ag plates, when the high-power laser (110 fs to 800 nm) was used to ablate the gold and silver plate at different fluences [52]. The high-power laser with the fluence of 60 and 1000 J cm−<sup>2</sup> can generate sharp and irregular craters [52]. Therefore, the variety of particle size and concentration of nanoparticles were achieved with different fluences [6, 50, 52, 53]. For example, the metal nanoparticles with small size have regular craters and low fluences [6, 50, 52]. The nanoparticles with the average size larger than 10 nm can generate the irregular craters and high fluences that related to boiling of metal plates [6].

plasmon resonance or inter band transitions in the UV-visible spectrum; hence, the laser beam can be interacted and absorbed by metal nanoparticles which were generated in liquid. This phenomenon occurred with two negative effects including the damping of ablation rate and altering the concentration and distribution of particle size [54]. Plasma plume has the tendency to absorb the laser beam at short wavelengths during the ablation process [44]. Therefore, the relation between the absorption coefficient and ablation efficiency appear in nonlinear, and these phenomena are minimum when the laser beam wavelength in the infrared range [54]. The nucleation of nanoparticles is formed during the plasma plume cooling. The nuclei growth and coalescence are the predominant mechanisms in the formation of metal nanoparticles using laser ablation technique [44, 55]. This procedure was confirmed with microscopy image of the polycrystalline structure of metal nanoparticles [56–59]. Consequently, during the formation of particles, ion metal and nanoparticles that formed in the liquid have interaction [44, 54, 55]. The nanoparticles can grow without any agent and ligand for some day after preparation in liquid because metal ions remain in an aqueous solution few days, and the formation of metal nanoparticles such as Pt and Ag continue with high affinity with the liquid such as water [55, 60]. As mention above, the size of metal nanoparticles depends on the density of metal atoms and temperature [44, 55], and sometimes, the atomic density and the temperature are not homogeneous in the plasma plume since two boundary regions exist with the surrounding liquid and the metal target [44, 46]. Many researchers reported that an energy threshold exists for the formation of nanoparticles using laser ablation technique [48, 50, 52, 54, 55]. The preparation of metal nanoparticle using laser ablation of metal target requires the presence of a plasma plume, and the minimum metal atom density should be provided to form the metal nanoparticles in the solvent [49, 50, 54, 55, 61]. The evidence of strong reactivity between metal and solvent in the plasma plume are consequences of the extreme pressure and temperature conditions [44]. Consequently, the plasma plume is quenched one order of magnitude faster in liquids than in gas or vacuum and the cooling process may not be considered an adiabatic process [44, 62]. This is useful for out-of-equilibrium reactions and

Laser Ablation Technique for Synthesis of Metal Nanoparticle in Liquid

http://dx.doi.org/10.5772/intechopen.80374

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the synthesis of metastable phases formed at high pressures and temperatures [44].

Many researchers have used the laser ablation setup in different forms to syntheses the metal nanoparticles in a liquid. Basically, the laser ablation setup contains a lens, a high power pulsed laser, a liquid container, a stirrer, and a linear positioner. The metal targets such as gold, silver, or copper (99.99%) were submersed in liquid. A Nd:YAG pulsed laser beam of 532 nm or 1064 nm ablated the metal plate. **Figure 1** shows the laser ablation setup, which contains a Q-switched Nd:YAG laser, a solution container, a metal plate, a lens (f = 30 cm), a travel linear stage, and a stirrer. The duration of pulse and the laser ablation time can change from 10 to 60 Hz and 5–60 min, respectively. To prevent the absorption of energy of laser beam in the liquid solution, the path length which the laser beam passes through the liquid must be adjusted to shortest length. So, the distance between the target and entrance windows is a significant factor to achieve the best energy of laser beam on the surface of the target. In order to make sure the metal nanoparticles disperse evenly in the liquid solution, stirring of

**4. Laser ablation setups**

The energy transfer to the electron on the surface of metal plates depends on the time duration of high power lasers such as femtosecond, picosecond, and nanosecond. It is time duration of laser ablation, and it is a significant factor for synthesis of nanoparticles. Hence, femtosecond laser pulses can release the electron from the metal plates faster than the thermal action of electron-photon phenomena. But, picosecond and nanosecond laser pulses can transfer the energy on a time scale longer than femtosecond laser pulse, so the thermal relaxation processes of the target [42, 54] can appear, and photothermal or photomechanical phenomena can be observed during the laser ablation of the metal plate. Hence, the ablation of the metal target will commence between 10 and 80 ps and plasma plum will form after 10 ns [42, 44, 46, 52], and the plasma appears after twice the pulse duration time [44, 46]. Therefore, when the metal plates are ablated with an ultra-short pulse laser (10−15 and 10−<sup>12</sup> s), the delay between ejection of nanoparticles and interaction of laser beam with the metal target is not observable [42]. The concept of this delay is significant for the ablation mechanism [43, 46]. The plasma plume can absorb part of the incoming laser energy when the long laser pulse will be used for ablation of the metal target. The absorption of laser beam increases the temperature of the plasma and favors the atomization of the material contained in the plume [6, 44, 46]. Consequently, the phase materials released from the metal target are homogenized [6]. On the other hand, the laser energy absorbed by the target decreases due to the optical shielding of the plasma plume, while target ablation by interaction with the plasma plume is enhanced due to the increased plasma temperature [6, 44] and the absorption cross section of bulk metals is increased with a decreased laser wavelength [49]. Nevertheless, the ablation efficiency depends on absorption effects or the irradiation at short wavelengths. Metal nanoparticles usually have strong plasmon resonance or inter band transitions in the UV-visible spectrum; hence, the laser beam can be interacted and absorbed by metal nanoparticles which were generated in liquid. This phenomenon occurred with two negative effects including the damping of ablation rate and altering the concentration and distribution of particle size [54]. Plasma plume has the tendency to absorb the laser beam at short wavelengths during the ablation process [44]. Therefore, the relation between the absorption coefficient and ablation efficiency appear in nonlinear, and these phenomena are minimum when the laser beam wavelength in the infrared range [54].

The nucleation of nanoparticles is formed during the plasma plume cooling. The nuclei growth and coalescence are the predominant mechanisms in the formation of metal nanoparticles using laser ablation technique [44, 55]. This procedure was confirmed with microscopy image of the polycrystalline structure of metal nanoparticles [56–59]. Consequently, during the formation of particles, ion metal and nanoparticles that formed in the liquid have interaction [44, 54, 55]. The nanoparticles can grow without any agent and ligand for some day after preparation in liquid because metal ions remain in an aqueous solution few days, and the formation of metal nanoparticles such as Pt and Ag continue with high affinity with the liquid such as water [55, 60]. As mention above, the size of metal nanoparticles depends on the density of metal atoms and temperature [44, 55], and sometimes, the atomic density and the temperature are not homogeneous in the plasma plume since two boundary regions exist with the surrounding liquid and the metal target [44, 46]. Many researchers reported that an energy threshold exists for the formation of nanoparticles using laser ablation technique [48, 50, 52, 54, 55]. The preparation of metal nanoparticle using laser ablation of metal target requires the presence of a plasma plume, and the minimum metal atom density should be provided to form the metal nanoparticles in the solvent [49, 50, 54, 55, 61]. The evidence of strong reactivity between metal and solvent in the plasma plume are consequences of the extreme pressure and temperature conditions [44]. Consequently, the plasma plume is quenched one order of magnitude faster in liquids than in gas or vacuum and the cooling process may not be considered an adiabatic process [44, 62]. This is useful for out-of-equilibrium reactions and the synthesis of metastable phases formed at high pressures and temperatures [44].
