Effects of laser fluence and liquid media on preparation of small Ag nanoparticles by laser ablation in liquid

Optics and Laser Technology 97 (2017) 20–28

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Effects of laser fluence and liquid media on preparation of small Ag nanoparticles by laser ablation in liquid Caroline Gomes Moura a,⇑, Rafael Santiago Floriani Pereira b,a, Martin Andritschky c, Augusto Luís Barros Lopes d, João Paulo de Freitas Grilo d, Rubens Maribondo do Nascimento e, Filipe Samuel Silva a a

CMEMS-UMinho, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal Ceramics and Composites Materials Research Center (CERMAT)-UFSC, Campus de Florianópolis, 88040-900 Florianópolis-SC, Brazil c Department of Physics, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal d Department of Materials and Ceramics Engineering, Universidade de Aveiro, Aveiro, Portugal e Materials Science and Engineering Post-Graduate Program, UFRN, 59078-970 Natal, Brazil b

a r t i c l e

i n f o

Article history: Received 20 January 2017 Received in revised form 24 April 2017 Accepted 9 June 2017

a b s t r a c t This study aims to assess a method for preparation of small and highly stable Ag nanoparticles by nanosecond laser ablation in liquid. Effect of liquid medium and laser fluence on the size, morphology and structure of produced nanoparticles has been studied experimentally. Pulses of a Nd:YAG laser of 1064 nm wavelength at 35 ns pulse width at different fluences were employed to irradiate the silver target in different environments (water, ethanol and acetone). The UV-Visible absorption spectra of nanoparticles exhibit surface plasmon resonance absorption peak in the UV region. STEM and TEM micrographs were used to evaluate the size and shape of nanoparticles. The stability of silver colloids in terms of oxidation at different liquid media was analyzed by SAED patterns. The results showed that characteristics of Ag nanoparticles and their production rate were strongly influenced by varying laser fluence and liquid medium. Particles from 2 to 80 nm of diameter were produced using different conditions and no oxidation was found in ethanol and acetone media. This work puts in evidence a promising approach to produce small nanoparticles by using high laser fluence energy. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Noble metal nanoparticles have been widely investigated due to their unique properties attributed to their small size [1–6]. Among them, silver nanoparticles (Ag NPs) have received much attention because of their high electrical and thermal conductivity, when compared to others NPs materials [7], besides Ag NPs present bactericidal effects [8], high catalytic activity [9] and important optical properties [7,10–14]. For instance, antibacterial activity of silver colloids is strictly related to their size - the smaller the silver nuclei, the higher the antibacterial activity [15,16]. Several techniques have been used to produce nanoparticles, including chemical and physical routes. Each method has disadvantages and restrictions, being the chemical route the most used. Pulsed laser ablation in liquid (PLAL) becomes quite suitable as an alternative to chemical route, since it avoids contamination and presence of impurities in the obtained products [1,17]. This ⇑ Corresponding author at: Center for MicroElectroMechanical Systems (CMEMS), University of Minho, Azurém, 4800-058 Guimarães, Portugal. E-mail address: [email protected] (C.G. Moura). http://dx.doi.org/10.1016/j.optlastec.2017.06.007 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

method allows to produce stable colloids and nanoparticles with size less than 5 nm, which can be interesting to reduce dramatically melting point of material [18,19]. Briefly, in this method, a laser beam is focused on a bulk target, ablating the material surface. Ablated mass (plume) expands under the liquid and releases many species, including nanoparticles. The liquid environment surrounds the ablated plume and nanoparticles are formed through the fast condensation of molten bubble, collisions between the plume species or nucleation of the clusters from free atoms. This latter mechanism is dominant and has been already discussed by others researchers [20]. More details about the chemical and physical mechanisms involved in this method are better explained in related literatures [16,21,22]. Silver nanoparticles suspensions exhibit distinctive color depending on their characteristics. At nanometer scale, electron cloud can oscillate on the particle surface and absorbs electromagnetic radiation at a particular energy. This resonance is known as surface plasmon resonance (SPR) of nanoparticles. The wavelength and width of SPR band depend on the size and morphology of particle as well as the liquid medium. SPR also has been used to monitor particle size, since allows determining the spectral position of

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plasmon band absorption [23,24]. Concerning these aspects, several works have reported the influence of laser wavelength on production of silver nanoparticles [25,26]. Furthermore, the role of laser parameters and liquid medium on final characteristics of nanoparticles, as also the efficiency of PLAL method, have been widely investigated [26–30]. In Boutinguiza and co-authors [1] a laser fluence of 100 J/cm2 was used to produce Ag NP’s and they obtained particles with a mean size of 15 nm. However, in Vinod [31] and Al-Azawi [27], the authors have used lower laser fluence values and achieved average particles size around 10 – 15 nm, similar to Boutinguiza and co-authors results [1]. Recently, Solati and co-authors investigated the effects of laser laser wavelength and fluence on characteristics by laser ablation in acetone [25]. Effect of laser fluence and different liquid media (distilled water, acetone and ethanol) on the properties of nanoparticles by PLAL was also studied by Mahdieh and co-authors, however, the authors used different materials and did not focus on Ag NPs production [32]. Thus, from previous works, we conclude that there is still no linear behavior regarding the characteristics of nanoparticles, since several factors influence the ablation method. Despite the great amount of published studies on production of metal nanoparticles by laser ablation in liquid, there still remain difficulties to be overcome in order to improve this method. The purpose of this work is to investigate the effect of laser fluence and different liquid media on the characteristics of Ag nanoparticles and to correlate these parameters with the production rate of ablation process, since it has not been reported extensively.

peak on UV–vis was used to calculate nanoparticle concentration via Beer-Lambert law [47] (Eq. (1)):

A ¼ ebc

ð1Þ

where A is the absorbance, e is the molar extinction coefficient with the unit of M1 cm1, b is the path length of the sample (cm), and c is the concentration of nanoparticles in solution (M). To obtain the concentration of nanoparticle through the Eq. (1) it is necessary to know the molar extinction coefficient of silver nanoparticles. The relationship of their extinction coefficient and diameters has been reported in [48] by Eq. (2):

e ¼ Adc

ð2Þ

where A = 2.3  10 M cm and c = 3.48, when d  38 nm. Silver colloidal suspensions were characterized by optical absorbance spectroscopy, scanning electron microscope with a transmission electron detector (STEM) and a transmission electron microscope (TEM). Optical absorption spectra of the colloidal suspensions were recorded in the range from 300 to 900 nm of wavelength by using an UV–vis absorption spectrophotometer Model 2501 PC, Shimadzu. Observation of colloidal particles was performed by STEM (Nova 200 Nano SEM) operated at 20 kV and the electron diffraction patterns (SAED) were achieved by using a TEM (Hitachi 9000) at accelerating voltage of 300 kV. For both STEM and TEM characterization, samples were prepared by adding droplets of the colloidal solutions on carbon-coated copper grids. Two droplets of solution were deposited and the grids dried in air. 5

1

1

3. Results and discussion 2. Experimental The laser ablation of the Ag target (polished rectangular plate, 1.27 mm of thickness and purity 99.99%) was carried out using a high power Nd:YAG laser (OEM Plus, Italy) with an output power of 6 W, a spot size of 3 mm, a pulse width
35 ns, operated at the repetition rate of 20 kHz. The laser power was 0.3 mJ/pulse and the fundamental wavelength of 1064 nm was utilized as the laser beam was focused on the Ag surface using a fused quartz lens (f = 160 mm). Silver target was placed inside a glass vessel filled with a liquid level of 7 mm above the target in a volume of 10 mL of liquid. During the ablation, the liquid was stirred to keep the ablated particles out of the beam path. The duration of ablation process was 11 min in all experiments and the scanning of the target surface is carried out by means of a XY translation stage. Three different liquids (double distilled water - DDW, acetone and ethanol) were used in these experiments. In the case of the ablation process in double distilled water medium, sodium dodecyl sulfate - SDS (C12H25SO4Na, Alfa-Aesar) was used as a surfactant. The aqueous suspension was prepared by adding pure SDS powder to DDW and shaked carefully to obtain a molar concentration of 0.025 mol/L. Two separated experiments were performed to investigate the effect of different laser fluences and ambient liquids on characteristics of produced nanoparticles, regarding to size distribution and shape. In the first experiment, silver target in liquid medium composed by DDW and surfactant SDS was irradiated by laser with fluences of 4244.0, 3183.3 and 2122.2 J/cm2, which correspond to 6.0, 4.5 and 3.0 W of power, respectively. In the second experiment, the laser fluence was kept at 4244.0 J/cm2, corresponding to 6 W of power and silver target was irradiated by laser beam in three different liquids media (double distilled water, acetone and ethanol). The ablated mass was obtained through the difference in target weight measured before and after the ablation process, using a high precision weighing scales. In order to evaluate the effect of laser fluence on the production of nanoparticles, the absorption

As previously stated, the characteristics of produced nanoparticles by laser ablation method are related to many factors. It has been shown that in PLAL, both laser fluence and liquid medium have a significant role in mean size and size distribution of nanoparticles, as also the production rate of the process. Experimental results showed that laser fluence also influence on characteristics of final generated nanoparticles. The next sections present details about the experimental results. 3.1. Effect of laser fluence The ablation process of silver target was performed at different laser fluences: 4244.0, 3183.3 and 2122.2 J/cm2, in a double distilled water environment with a wavelength of 1064 nm laser light. The absorption spectra of suspended Ag nanoparticles were measured in a 300–850 nm wavelength range, with respect to double distilled water absorption as the base line. The spectra demonstrate visible absorption peaks from the surface plasmon resonance (SPR) absorption of silver nanoparticles at 398–401 nm. As a result of the ablation process, the solutions acquired a yellow color, which is characteristic of the presence of Ag nanoparticles. The yellow color intensity depends on the laser energy, energy density and ablation time. Displacements in absorption peak position may represent changes in particle size and its intensity depends on the amount of produced nanoparticles [33]. However, according the absorption spectra in Fig. 1, there was no significant peak displacement. It is also observed that the absorption peaks intensity tends to be higher when the laser energy is increased. A higher peak absorbance suggests a higher concentration of silver nanoparticles in suspension. In Fig. 2, STEM images and the diameter distribution of various suspensions of nanoparticles obtained by ablation at different laser fluences are showed. Regardless the variation of laser fluence all generated particles are approximately spherical. Considerable aggregation is not seen, what is expressed by the narrow particle

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Fig. 1. UV–vis absorption spectra of silver nanoparticles prepared by laser ablation at different laser fluence in DDW + SDS liquid medium.

size distribution. According STEM images, smaller nanoparticles (in average size) are obtained by using higher laser fluence values (3183.3 and 4244.0 J/cm2) (Table 1). According to obtained results (Table 1), although it has not been verified a linear behavior, in general, the increase of laser fluence promoted a particle size reduction. This behavior has already been mentioned by others authors in literature [34,35]. In the ablation mechanism, the laser beam is able to produce both melted and evaporated mass. This last one, in turn, allows producing soft nanoparticles by aggregation of evaporated atoms. In case of melting mass, small metal droplets are ejected and nanoparticles formed as fragment of these droplets. At high energy, the ablation process produced melting mass of the target surface with less evaporation and also the inter-absorption of laser light is taken place. This absorption leads to formation of smaller particles due to the fragmentation mechanism of larger particles [36–40]. The strong effect of the laser intensity on the particle size and size distribution observed by our present study suggests that these parameters depend strongly on the plasma conditions, mainly temperature, pressure and species density [41]. However, is relevant to mention that some authors also reported that using a fundamental wavelength the average particle size remained practically independent of the laser fluence, as can be seen in Nikolov and co-authors [42]. Notwithstanding, in this work the laser fluence shown to have major influence on average size of NPs and the interaction between laser light and colloidal particles in suspension may have influenced the size distribution of nanoparticles. This aspect has been shown in other works [26,43]. Previous works have reported that laser wavelength can influence the particle size in terms of ablation efficiency, selfabsorption and penetration depth of laser beam [26,28]. These works showed that using longer wavelength (1064 nm) promotes a high ablation efficiency when compared to those shorter. This fact can be explained because self-absorption can reduce the intensity of laser light which reach onto the surface of target [26]. However, if particles have a low extinction coefficient at wavelength of laser light the self-absorption can be negligible. In the case of 1064 nm the extinction coefficient is <0.1. Tsuji and co-authors 2002 compared silver nanoparticles produced by laser ablation with different wavelength (355, 532 and of 1064 nm) and verified that the use of 1064 nm wavelength allowed the production of larger nanoparticles with a broader size distribution [26]. In our work, we confirmed this finding, which is possible to see in STEM analysis.

Through STEM images, it is also possible to observe that there was no aggregation, although metal nanoparticles tend to agglomerate when dispersed in solution. This behavior can be attributed to the presence of SDS as surfactant, which plays an important role in stability of nanoparticles. This process occurs due to electrostatic repulsion and hydrophobic interaction among the stabilizers chains. Furthermore, nanoparticles stability through ionic stabilizers is generally explained in terms of surfactant bilayer formation on the nanoparticles, which allows that the alkyl groups be held together by a hydrophobic bond [44,45]. These processes inhibit clusters aggregation, promoting nanoparticles stabilization [46]. In order to evaluate the effect of laser fluence on the production of nanoparticles, the absorption peak on UV–vis was used to calculate nanoparticle concentration, using Eqs. (1) and (2), and considering their absorption peaks and average diameters (Table 2): Fig. 3 shows the correlation between the ablated mass and the concentration of nanoparticles. As can be seen, the laser fluence that allowed a higher efficiency in the production of nanoparticles is 3183.4 J/cm2, as a result of smaller nanoparticles formation. Then, when the laser fluence increased the average particles number decreased due to the formation of large nanoparticles, although the ablated mass in this fluence has been superior. 3.2. Effect of liquid environment As already mentioned, composition of nanoparticles prepared by PLAL strongly depends on the liquid medium. The ablation process of silver target was performed at different liquid environments, double distilled water (DDW), acetone and ethanol, with 4244.0 J/cm2 of laser fluence. Absorption spectra of suspended Ag nanoparticles was measured in the 300–850 nm wavelength range, using absorption peaks of liquid medium as the base line. The spectra demonstrate visible absorption peaks from the SPR absorption of silver nanoparticles at 396–405 nm (Fig. 4). Peaks position was kept around 400 nm, but it is found that for both acetone and ethanol there was a slight peak deviation to larger wavelengths, as a result of an increase in average particles size, comparing to DDW peak. Ablation process performed in DDW resulted in a higher peak intensity in comparison to acetone and ethanol environments. The ablation in ethanol resulted in the lowest peak, which means a low ablation efficiency in this medium. This can be attributed to the ethanol decomposition during ablation process, which promotes the formation of permanent gases. The bubbles gas in solution of ethanol, in combination with ablated plasma and formed nanoparticles, may act as an obstacle to the laser path, reducing the energy that reaches the target [49]. Due to the fast growth and aggregation of nanoparticles, all measurements were performed immediately after ablation process. Both acetone and ethanol media silver colloidal suspensions presented gray or light gray color and precipitated after few days, although acetone took a longer to precipitate, as a result of its greater stabilization, when compared to ethanol. Acetone is preserving good dispersity of the nanoparticles, which comes from the interaction between the acetone carbonyl group and the metal nanoparticle surface. When the acetone molecules are adsorbed around the metal nanoparticle, is developed a protective surface dipole layer in the more external plane, becoming the interaction between nanoparticles repulsive in acetone medium [5,50]. According the STEM and histograms of size distribution (Fig. 5), the acetone and ethanol environments resulted in a production of larger nanoparticles, when compared to those produced in DDW medium. Despite the similarity among histograms, the high frequency of small particles in aqueous medium contributed to the achievement of the lower average particle size in this environment. The larger nanoparticles in some conditions may not necessarily be formed

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Fig. 2. STEM images and corresponding histograms of silver colloidal nanoparticles prepared by laser ablation at different laser fluences (a) 2122.2 J/cm2, (b) 3183.3 J/cm2 and (c) 4244.0 J/cm2.

Table 1 Mean particle size of colloidal silver nanoparticles prepared by PLAL at different laser fluences.

Table 2 Data for the concentration calculation and the concentrations obtained according to laser fluence.

Laser fluence (J/cm2)

Mean particle size (nm)

Laser fluence (J/cm2)

d (nm)

Abs

concentration (M)

2122.2 3183.3 4244.0

18.9 ± 10 7.4 ± 5 11.8 ± 6

2122.2 3183.3 4244.0

18.9 7.4 11.8

0.338 1.961 3.369

0.53 80.51 27.27

during the interaction between laser light and target, but can be due to agglomeration effect. It is important to mention that in the case of ablation in ethanol environment, nanoparticles with

sizes larger than 100 nm were generated, although the average particle size is low. Nikov and co-authors 2017 reported silver ablation in ethanol medium and they obtained particles with a

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Fig. 3. Relation between the ablated mass and concentration of nanoparticles, according laser fluence.

Fig. 4. UV–vis absorption spectra of silver nanoparticles prepared by laser ablation in different liquid medium at 4244.0 J/cm2 of laser fluence and pure liquid media.

mean size around 20 nm. Moreover, their results showed that it is possible to obtain NPs up to 5 nm using toluene [51]. Nanoparticles aggregation in ethanol can be more intense, since ethanol is a low polar solvent. Their dipole-dipole interactions and the formation of weak electrical double layer have been already reported in the literature [52,53]. In acetone environment, size distribution of nanoparticles was narrower than ethanol medium and presented less average particle size (Table 3). To evaluate the effect of liquid medium on the production of nanoparticles, the concentration of nanoparticles was calculated based on Eqs. (1) and (2) (Table 4): The relation between ablated mass and concentration of nanoparticles are presented in Fig. 6. Acetone and ethanol environments resulted in a low production of particles and a higher mean

size of nanoparticles. Therefore, in PLAL process, not only the laser fluence is important, but also the liquid environment has a significant role in the size properties of produced nanoparticles. Additionally, because the different liquid species present dissimilar optical characteristics, the interaction among these parameters and laser beam in a target material has a strong influence on the ablation rate. Regarding the optical properties, for a given target materials and laser parameters, the production rate and nanoparticles formation depends on the transmission coefficient and refractive index of liquid medium at the laser wavelength. The refractive index and transmittance of three liquid media at applied 1064 nm of laser wavelength are shown in Table 3. Before reaching the target, the laser beam has two reflection processes: (i) reflection from the

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Fig. 5. STEM images and corresponding histograms of colloidal nanoparticles prepared by laser ablation at different liquid medium (a) DDW, (b) acetone and (c) ethanol.

Table 3 Mean particle size of colloidal silver nanoparticles prepared by PLAL at different liquid medium.

Table 4 Data for the concentration calculation and the concentrations obtained according to liquid medium.

Liquid medium

Mean particle size (nm)

Liquid medium

d (nm)

Abs

concentration (M)

DDW Acetone Ethanol

11.8 ± 6 13.9 ± 5 17.7 ± 10

DDW Acetone Ethanol

11.8 13.9 17.7

1.708 0.790 0.324

13.82 3.62 0.64

air–liquid interface and (ii) reflection from liquid-target surface interface [32]. As can be seen in Table 3, the refractive index of three liquids concerned is almost the same, thus, the energy por-

tion of the reflected laser beam from air–liquid interface in these three liquids would be similar. However, since these liquids have different transmission characteristics, the energy portion that is

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Fig. 6. Relation between the ablated mass and concentration of nanoparticles, according liquid medium.

Table 5 Refractive index of three different liquid media (at 1064 nm) and optical transmittance (through 1 cm of the medium). Liquid medium

Refractive index

Transmittance

Distilled water [54] Acetone [55] Ethanol [55]

1.326 1.361 1.364

0.546 1 1

absorbed at the target surface is different. According to detailed optical characteristics of these three liquids, acetone and ethanol have similar transmissions, which are almost perfect, (at 1064 nm) while distilled water absorbs the laser energy significantly at the same wavelength [22,32] (see Table 5). Stafe and co-authors reported that the colloidal nanoparticles can be formed in two stages. Firstly, the nuclei are formed and

Fig. 7. TEM images of the Ag NPs produced by laser ablation using a fluence of 4244.4 J/cm2 in three different liquid media (a) ethanol, (b) acetone and (c) DDW. (e), (f) and (g) are presented the electron diffraction patterns with the indexation of the main reflections of the samples a, b and c, respectively.

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then, the nanoparticles grow over the formed nuclei by a condensation mechanism. Thus, an important parameter that control the nanoparticle production process is the cooling speed of plasma, which influences the size distribution of nanoparticles. In a colloid (in PLAL), a faster cooling speed may result in a higher number of particles (resulted from condensation) and smaller mean size. Thereby, the thermodynamic and optical properties of liquid media strongly influence on size distribution and mean size of produced nanoparticles [56]. 3.3. SAED analysis To elucidate the crystalline phases of the obtained Ag nanoparticles and to verify the presence of silver oxide, a careful analysis of the SAED electron diffraction patterns was performed on various groups of particles. For this, three samples produced in different liquid media (DDW, acetone and ethanol) using a laser fluence of 4244.2 J/cm2 were selected. As shown in Fig. 7, the majority of interplanar distances from the main reflections can be assigned to family planes of metallic Ag. The results showed that the measured interplanar distances of 0.240 (in all the samples) and 0.147 nm (in sample 5) show good agreement with the (111) and (220) planes respectively of metallic Ag. However, in sample 3 the interplanar distance of 0.136 nm agrees with the plane (222) of Ag oxide (Ag2O). Although, the results suggest no oxide formation for samples produced in both ethanol and acetone media, the presence of silver oxide shouldn’t be ruled out, since the interplanar distances are very close. However, it could suggest that acetone and ethanol are good stabilizing power, serve as a superior liquid media that keep the metal nanoparticles free from precipitation and oxidation. 4. Conclusion Preparation of silver NPs by laser ablation method at different fluences and different liquid media is investigated. The generated NPs in this experimental conditions are almost spherical. The size of NPs is decreased when used higher laser fluence values due to the inter-absorption of laser light by ablated NPs. The concentration of NPs in solution was achieved by absorption peak intensity and the results showed the lowest concentration of NPs in higher laser fluence. When different liquid media were compared, ethanol and acetone showed be good stabilizers environments to keep nanoparticles free from precipitation and oxidation. From the presented results, it is clear that both laser fluence and the nature of the surrounding liquid environment, during the ablation process, affects directly the size distribution, stability of nanoparticles produced and the production rate. This work brings a novel approach in terms of laser and materials parameters that allowed obtaining nanoparticles with different sizes, since 2 to 120 nm of diameter. Acknowledgement This work was supported by the project NORTE 010145_FEDER-000018 and by project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941. Thank the Cnpq and CAPES for financial support. References [1] M. Boutinguiza, R. Comesaña, F. Lusquiños, A. Riveiro, J. Del Val, J. Pou, Production of silver nanoparticles by laser ablation in open air, Appl. Surf. Sci. 336 (2015) 108–111, http://dx.doi.org/10.1016/j.apsusc.2014.09.193. [2] P.V. Kamat, Photophysical, photochemical and photocatalytic aspects of metal nanoparticles, J. Phys. Chem. B. 106 (2002) 7729–7744.

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