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Laser Ablation of Silver in Liquid Organic Monomer: Influence of Experimental Parameters on the Synthesized Silver Nanoparticles/Graphite Colloids Maxime Delmée, Gregory Mertz, Julien Bardon, Adeline Marguier, Lydie Ploux, Vincent Roucoules, and David Ruch J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05409 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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The Journal of Physical Chemistry

Laser Ablation of Silver in Liquid Organic Monomer: Influence of Experimental Parameters on the Synthesized Silver Nanoparticles/Graphite Colloids

Authors : Maxime Delméea,b, Grégory Mertza, Julien Bardona, Adeline Marguierb, Lydie Plouxb, Vincent Roucoules*b, David Rucha a. Luxembourg Institute of Science and Technology (LIST), Materials Research and Technology, 5 avenue des Hauts-Fourneaux, L-4362Esch/Alzette, Luxembourg b. Institut de Science des Materiaux de Mulhouse, IS2M-C.N.R.S.-UMR7361, University of Haute Alsace, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France * Corresponding author: Vincent Roucoules : E-mail : [email protected] ; Phone : +33 3 8960 8782

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Abstract During the last decade, synthesizing silver nanoparticles (Ag NPs) by Liquid Phase-Pulsed Laser Ablation (LP-PLA) has attracted a lot of attention. Basically, this technique allows producing various metallic nanoparticles with controlled size, shape, composition or surroundings in several liquids (i.e.: water, ethanol, acetone, toluene,…). Recently, such processes have been studied in liquid organic monomer such as methyl methacrylate (MMA). However, the influence of the laser parameters on the materials synthesised in such reactive liquid and their features were not fully investigated so far. Here we investigate the LP-PLA of silver in two different but rather similar acrylate monomers: dodecylacrylate (DOCA) and 1H,1H,2H,2H perfluorodecylacrylate (PFDA). The influence of the fluence and the number of pulses on the production, size and morphology of the materials has been examined. First, a factorial design experiments have been achieved in order to determine the weight of the laser parameters in each precursor. This study shows two highly different behaviours in function of the monomer where the process took place. This has been explained by the plasma plume confinement and/or the “inter-pulses” self-absorption of the particles by the laser beam. The formation of graphite around the synthesized AgNPs has been highlighted by Raman spectroscopy at low number of pulses. Nevertheless, increasing the number of pulses could lead to three phenomenon depending on the fluence and the used monomer: degradation of the matrix, conservation of the matrix with changes in AgNPs size and distribution or sustainment of the matrix with any changes in the particles properties. So the surrounding, the size and stability could be triggered by adjusting these parameters. This paper does highlight that LP-PLA is a powerful technique to provide AgNPs in acrylate monomer with a good control of their features.


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1.

Introduction

Metallic nanoparticles have attracted a lot of attention this last decade due to their excellent intrinsic properties. They have been proving their efficiency in various applications fields such as catalysis1, 2, optoelectronics3, 4, antimicrobial5, 6 or sensing7, 8. Nevertheless, new safe synthesis routes remain the main issue. A huge number of innovative strategies has been developed based on biological production by bacteria9, fungi10 or plants11, chemical generation by metallic salt reduction12 or electrochemical process13. However, most of these new routes are not only based on using hazardous chemicals but also usually generates byproducts. Besides, a promising way, the so-called liquid phase – pulsed laser ablation (LPPLA) has been developed by Henglein et al.14 . This method allows a safe, in-situ and highly controllable synthesis of metallic particles without any further purification step.15, 16, 17 LP-PLA consists in focusing a pulsed laser beam on a metallic target immersed in a liquid. The interaction between the laser beam and the metallic target induces the formation of a plasma plume (i.e. elevated temperature and high pressure medium) composed of radicals, ions and atoms coming from the target species and the media. The nanoparticles growth occurs in this environment and more specifically during the cooling down step of the plasma phase. The mechanisms involved are mainly based on nucleation and growth, ejection of liquid droplet and solid fragmentation or aggregation18. Various metallic target could be used with the possibility to get a high control of the particles surface chemistry19. More interestingly, it is possible to finely adjust the size distribution of the particles by a judicious choice of the experimental parameters (wavelength, fluence, spot size,…)20. For example, Tsuji et al. have demonstrated that, switching the wavelength of the laser from 1064nm to 355nm, decreased the mean diameter of silver nanoparticles from 29 nm to 12 nm21,

22

.

Another way to adjust the distribution size of the particles consists in using the “secondary laser irradiation” process (SLI) which is based on irradiating the particles already available in



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solution. The particle size could be reduced by fragmentation23, 24 or increased by melting and coalescence25, 26. These processes have been entirely described by Zhang et al27 in a very interesting review where they pointed out the different mechanisms, influence of the synthesis parameters and the size control obtained. The broad range of liquids where the particles could be synthesized is an enormous advantage of this production way. The ablation process in aqueous solutions and organic solvents like ethanol28, acetone29 or toluene30, has been intensively studied. Liquid media have shown a strong influence on the generated nanoparticles (size, concentration, surface chemistry,…). For example, Amendola et al. have shown that silver ablation in dimethylformamide (DMF) gave rise to free AgNPs while the same condition of ablation lead to AgNPs embedded in a graphitic matrix when the experiments have been performed in dimethylsulfoxide (DMSO)31. The same observations have been done for gold nanoparticles in toluene and benzonitirile32. So, this parameter is of prime importance in LP-PLA since it could have a dramatic impact on the final properties such as optics33. More recently, studies have been done on the synthesis of nanoparticles in monomer and/or polymer solutions in order to provide new precursors route for nanoparticles/polymer composites. Two recent reviews highlight the state of the art and address the promising perspectives of this research area34, 35. It worth noting that few authors have shown the opportunity to synthesize nanoparticles directly in pure liquid monomer (without solvent) like methyl methacrylate (MMA)36, 37, 38. These works definitively open doors for developing new strategies for elaborating hybrid coatings, as it should be possible to polymerize directly in one step the nanoparticle-containing precursors, by plasma polymerisation for example39. This study focuses on nanosecond laser ablation of silver in two different acrylate precursors: dodecylacrylate

(DOCA)

and

1H,1H,2H,2H-perfluorodecylacrylate

(PFDA).

These

precursors have been chosen for their ability to well polymerize in atmospheric pressure 4 ACS Paragon Plus Environment

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plasma discharge40. We report the effect of LP-PLA parameters, namely the fluence of the laser, the number of pulses and the nature of the precursors on the particle size and concentration. Further, we provide evidence for the formation of graphitic matrix under specific conditions. Finally, we propose a mechanism for the formation of the nanoparticles during LP-PLA.

2. Experimental section

a. Chemicals Dodecyl acrylate (DOCA) (98 %) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97 %), were respectively purchased from Tokyo Chemical Industry and from Sigma Aldrich Company. The silver target (99.99 %) was provided by Kurt J. Lesker Company and was used as received.

b. Experimental apparatus The LP-PLA process was carried out in a home-made design (see Scheme 1). The laser is a Continuum Surelite I 10 pulsed Nd:YAG laser with a pulse width comprised between 5ns to 7ns working at a pulse frequency of 10 Hz. The spot size was not modified during the experiments and was given by the supplier to 6mm. The wavelength was chosen at 1064 nm to be sure that the precursors did not absorb at this wavelength. The LP-PLA process was designed in order to keep the physical dimensions of the plasma plume almost constant so that the influence of the hydrostatic pressure can be considered as negligible. To do that, the silver target was immersed in the liquid precursor (constant volume of 4 mL) and maintained at 1 cm from the bottom of the glass cell by using a rotative holder. The rotative rate was set



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to 350 rpm. The silver target was irradiating on the bottom surface. This allowed to obtain a very good reproducibility for a given precursor.

Scheme 1 : Experimental setup of the laser ablation production of silver nanoparticles colloidal solution

The experiments have been done in two different precursors (PFDA and DOCA) by varying the fluence (1.6 J/cm² and 0.8 J/cm²) and the number of pulses (10 and 600 pulses). The measurements of the energy have been done after the laser beam cross the liquids to cancel the effect of the change in refractive index between the two media.

c. Factorial analysis Laser ablation have a large number of input variables which have their own influence on the output characteristics. Changing the value of one factor at a time and noting its influence on a given characteristic of the final product is commonly done and could bring good results. However, this method has some disadvantages: it requires a large number of trials, and it does not reveal the possible interactions between factors. In contrast experimental designs can be efficient to obtain solutions41, 42, 43. To carry out a design of experiments, a representative variable (response) that will be evaluated must be 6 ACS Paragon Plus Environment

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identified in a first step. Then variables of the process (factors) and the values of these variables (level) have to be selected. It worth noting that the use of only, the highest and the lowest value of the level in the experimental range, is widespread. Finally, experiments must be carried out using all the possible levels and combinations of factors in order to determine the effect of the interactions among any number of factors. In the present case, the role of synthesis parameters, i.e media, fluence and pulses number on the concentration, mean diameter and size dispersion (see section 2.d.) were investigated by a 2-level full factorial design44,

45

. In a factorial design approach, it is postulated that the

measured properties of the considered system can be expressed as a linear function of experimental parameters. The experimental task will consist in obtaining the value of the coefficients associated with the considered variables in the expression of the measured response of the system. It is assumed that the measured property y (the response of the system) can be related with the experimental variables through the polynomial expression (1):

y=

b0 + b1 x Fluence + b2 x Nbrpulses + b12 x Fluence x Nbrpulses + ei

(1)

It would give access to all the values of the bi coefficients and to the interactive terms. y denotes the experimental data, b0 the mean value of y, bi and bij represent the respective coefficients of the experimental parameters (bi represents the first-order and bij represents interaction between two input variables) and ei the error term. The next step consists in determining the parameters bi and bij using the least squares method46. Results will be presented in graphs to highlight the part of each parameters and their interactions on the system response.



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d. Colloidal solutions characterization Atomic Absorption Spectroscopy measurements. The nanoparticles suspensions have been characterized by Atomic Absorption Spectroscopy (AAS) using a Perkin Elmer AA 600 spectrometer. 3 mL of the solutions has been introduced in an oven at 550°C for 30 minutes and at 1000°C for an hour. Then, the residues have been dissolved in HNO3 1 M and the total concentration of silver has been determined for each solutions. Raman spectroscopy measurements. The suspensions of the nanoparticles have been also characterized by Raman spectroscopy using a Renishaw inVia Raman microscope proceeding with Ar laser working at 442 nm. Drops of the solutions were deposited on silicon wafers and dried at 80°C under vacuum. Measurements have been performed on aggregates of nanoparticles observed by optical microscope. UV-Visible measurement. UV-Visible measurement have been carried out with a Perkin Elmer lambda 35 UV-Visible spectrophotometer. Each UV spectra have been recorded in a range of 350-800 nm. All the data have been normalized by the silver concentration determined by AAS. Transmission Electronic Microscopy measurements. Drops of nanoparticles suspensions have been deposited on TEM grids and analysed by a Jeol ARM 200 High Resolution Transmission Electronic Microscope (HRTEM). The size distribution of the nanoparticles have been determined using Image J software on 300 nanoparticles from these images. LogNormal fit has been used to determine the mean diameter. (All the histograms are resumed in the supporting information)


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3. Results and discussions

a. A chemometric investigation of the process parameters during LP-PLA nanoparticle synthesis: a 22 combinatorial approach LP-PLA has been proven to be a powerful technique for nanoparticle synthesis. This process involves a large amount of tunable parameters, which should be optimized. Beyond the influence of laser parameters, the nature of the liquid is of a major importance. Here, nanosecond laser ablation of silver have been performed in two different acrylate monomer: dodecylacrylate (DOCA) and 1H,1H,2H,2H-perfluorodecylacrylate (PFDA). The fluences were selected to 0.8 J/cm², just above the ablation threshold and 1.6 J/cm². The number of pulses were chosen to 10 and 600. With 10 pulses, the generated nanoparticles are supposed to did not cross the beam during further pulses while they do with 600 pulses. A 22 factorial design has been used to identify the effect of the laser parameters, namely the fluence and the number of pulses, on the concentration, the size and distribution size of the silver particles. This approach consisted in obtaining the values of coefficients bi, bij which were the measure of the respective weight of the experimental variables xi (here, fluence and number of pulses). The level of each parameter and the results of 2 x 4 experiments have been summarized in Table 1.

Parameters Sample

Laser Liquid

1 2 3

Responses

DOCA

Fluence (J/cm²) 0.8 1.6

Pulses number

Silver concentration (ppm)

10 600 10

0.12 1.06 1.91 9

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Silver nanoparticles Mean diameter (nm) 3.6 19.3 /

Size distribution (nm) 2.4 15.5 /


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4 5 6 7 8

0.8 PFDA 1.6

600 10 600 10 600

18.50 0.21 6.00 1.76 13.6

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3.7 4.5 5.1 4.6 4.1

1.0 1.8 2.7 3.5 3.2

Table 1 : The chemometric approach combining two 22 factorial designs.

Figure 1 shows the values of the coefficients for the concentration, the mean diameter and the size distribution of the silver particles. The higher the value is, the more important is the factor affecting the response. The ‘‘plus’’ or ‘‘minus’’ sign indicates an increase or a decrease of the response when the considered factor is changed.

Figure 1 : Coefficient values after the complete factorial analysis for the concentration of silver, the mean diameter and the size distribution of Ag NPs.


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The silver concentration is positively correlated with the fluence and the number of pulse. The first-order interactive term (i.e. bij) has also a significant effect. As it was expected, an increase in both fluence and number of pulse let to high values of concentration17. When the experiments were conducted in DOCA, the weight of the fluence and the number of pulses were measured at the same level (bi and bij ~ 4- 5). Changing the precursor from DOCA to PFDA ran to a decrease in the weight of the fluence whilst the effect of the number of pulse remained constant. The weight of the first-order interactive term also decreased. When the DOCA was replaced by the PFDA, the physicochemical properties of the media in which the silver target ablation occurred changed. It is well known that changing the liquid media properties impact strongly the ablation process, especially polarity, refractive index, viscosity and density27, 47, 48. However, DOCA and PFDA are nonpolar, so the impact of polarity could be consider as negligible in our experiments. Additionally, the specific configuration of the experimental setup used allowed to drastically decrease the influence of the refractive index. Then, viscosity and density were considered as the more influent physicochemical parameters on the AgNPs synthesis. The density and viscosity are respectively ρ=0.873 g/mL and υ=5 mPa.s in DOCA whilst they are equal to ρ=1.637 g/mL and υ=12 mPa.s in PFDA. During laser ablation of the silver target, the differences in density and viscosity affect hardly the confinement and the concentration of silver species in the plasma plume49. An increase in density and in viscosity (here by replacing DOCA by PFDA) enhanced the confinement of the plasma plume (which was directly correlated with the fluence), which provoked an increase in the concentration of ejected material inside the plasma phase. Another phenomenon that has to be taken into account is the so-called “intra-pulse” self-absorption, which occurs in the plasma phase (i.e. plasma plume)22. During our experimental conditions, the ejection of silver material occurred after few picoseconds50. Depending on their concentration, the new synthesized silver particles could act as a shield by absorbing a part of



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the laser beam (which runs in nanosecond regime). It has been demonstrated elsewhere that a high concentration of silver species confined in the plasma plume could avoid the latter part of the laser beam to reach the target and ablate it49. This was clearly demonstrated in our result where the effect of the fluence was lower in PFDA, in which the confinement of the plasma plume was amplified and the concentration of the ejected material inside the plasma plume was elevated. Besides, the diameter of the particles was directly correlated to the number of pulses but inversely correlated to the fluence in DOCA. This result was explained by considering the socalled “inter-pulse” self-absorption, or in other words, the secondary laser irradiation. This phenomenon concerns, here again, the absorption of a part of the laser beam by the silver particles, but this time existing in the liquid media (and no more in the plasma phase as mentioned during the “intra-pulse” self-absorption)22. At 1064 nm (which corresponds to our experimental conditions), this absorption could rise to the fusion and the coalescence of the silver particles in solution and an increase in their average diameter. Other studies have demonstrated also that the fusion of the particles (which means an increase in the average diameter) occur mainly at low fluence whilst the fragmentation of the particles (which means a decrease in the average diameter) happens mainly at elevated fluence51. The results presented here confirm these statements: the higher values of diameters were found in the case of low fluence levels and high number of pulses. But, very interestingly, the weight of both the fluence and the number of pulses dropped in intensity and became negligible in PFDA. So, all the aforementioned phenomena do not affect the particles size when the synthesis were performed in such liquid. This point will be clarified further in the paper. Finally, the fluence and the number of pulses had similar effect on the size distribution compare to what it has been observed with the average diameter. Once again, the coefficients were lower in PFDA. However, the weight of the fluence remained significant in PFDA and 12 ACS Paragon Plus Environment

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had an opposite effect. An increase in the fluence induced a widening of the size distribution. As it has been underlined before, “intra-pulse” self-absorption was amplified in PFDA and also at high fluence leading to an increase in the size distribution of the particles due to the absorption by the ejected materials of the latter part of the laser beam.

b. Characterization of the silver nanoparticles solutions Raman spectroscopy measurements have been performed in order to determine the chemical fingerprint of the Ag NPs synthesized by LP-PLA. Figure 2 shows the spectra of silver particles generated in DOCA (Spectrum A) and in PFDA (spectrum B). The conditions of LP-PLA experiments were the same than the ones selected during the factorial analysis. The spectra are zoomed between 1100 cm-1 and 1750 cm-1. It is worth noting here that no Raman signal was detected for nanoparticles aggregates synthesized in the condition of experiment 4 (i.e. in DOCA with 1.6 J/cm² and 600 pulses, spectrum not shown). However, for all the other conditions, two characteristic peaks can be clearly identified and belong to carbon species: the first one centred at 1365 cm-1 corresponds to D band resulting from disorder sp2 hybridized carbon and the second one, centred at 1590 cm-1, corresponds to G band and is due to the in plane vibrational mode E2g of sp2 carbon cycle52.



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Figure 2 : Raman spectra of colloidal solutions of Ag NPs/graphite in A) DOCA and B) PFDA

Very interestingly, according to A. C. Ferrari, these bands correspond to the fingerprint of graphite and confirm also the observations made by other authors which have shown the possible formation graphitic matrix around the nanoparticles during LP-PLA process. Furthermore, the respective position of these two bands and their relative intensity provide information on the size of the graphite crystallites. By applying the relation of Tuinstra and Koening53 (see equation 2) the graphitic crystallite length (Lc) can be estimated :

=

(2)

where C(λ) is the coupling Raman constant, which was estimated to 2.0 nm in our work (λ = 442 nm)54. The results are shown in Table 2. The lengths of crystallites were estimated from 1.5 nm to 5 nm according to the experimental conditions used. These values were in good agreement with other results found in the literature for similar synthesis30. Our results also showed a direct correlation between the number of pulses and the graphitic crystallite length Lc. An increase in the number of pulses readily induced an increase in Lc. This trend could be due to “inter-pulse” self-absorption that could induce a laser thermal annealing as explained in previous studies55,

56

. However, the absence of graphite for experiment 4 remained

questionable at this stage of the study.

Sample 1 2 3 4 5 6 7

Media

Fluence (J/cm²) 0.8

DOCA 1.6 PFDA

0.8 1.6

Pulses number

I(D)/I(G)

Lc (nm)

10 600 10 600 10 600 10

0.53 ± 0.03 0.36 ± 0.03 0.37 ± 0.03 / 1.32 ± 0.03 0.37 ± 0.03 0.69 ± 0.03

3.6 – 4.0 5.1 - 6.1 5.0 – 5.9 / 1.6 – 1.5 5.0 - 5.9 2.8 – 3.0


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8

600

0.41 ± 0.03

4.5 - 5.3

Further characterization of the AgNPs solutions have been done by UV-Visible spectroscopy. Spectra in Figure 3 A show typical surface plasmon resonance (SPR) centred at 400 nm and characteristic of the AgNPs (here, the AgNPs have been synthesized in DOCA). The intensity of such UV bands is proportional to the concentration of AgNPs and its shape and position is Table 2 : Summary of the I(D)/I(G) ratio and crystallite length (Lc) obtained respectively from Raman spectra and Tuinstra and Koening relation

strongly correlated with their size, shape and surrounding. In our case, not only the size and shape of the AgNPs could impact the UV spectra but also the graphitic species around AgNPs or freely dispersed in the solution. This has been already demonstrated by few authors. For example, V. Amendola et al. demonstrated that the graphitic matrix surroundings particles could lower, shift or totally extinct the SPR signal in function of the size of the graphitic layer33, whilst graphite freely dispersed in solution with AgNPs could increase the intensity without affecting the shape of the SPR. In order to evaluate the dispersion state of the graphite and the AgNPs/graphite ratio, all the UV-Visible spectra reported in Figure 3 have been normalized by the corresponding silver concentration (which was determined by AAS). This normalization allowed to analyse the SPR intensities or the intensities at 400 nm in case of lack of SPR.



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Figure 3 : UV-Visible spectra of Ag NPs colloidal solutions in A) DOCA and B) PFDA

These intensities varied according to the experimental conditions used. First, it was shown that the SPR intensities were inversely correlated with the fluence during LP-PLA. This trend was observed in both precursors (DOCA and PFDA) and was attributed to the competition between the ejection rate of silver species in the solution and the pyrolysis rate of the precursor. At low fluence, the ejection rate was lower than the pyrolysis rate, which could be considered in a first approximation as independent of the experimental condition used. As a result, the concentration of AgNPs in solution was low, the graphite species was high and finally, the intensity of the detected UV-Visible band was strong. In a second time, it could be also noticed that, except for the experiments in DOCA at 1.6 J/cm², the intensity was inversely correlated to the number of pulses. This statement was attributed to the growth rate of the graphite layer around the AgNPs. The decreasing of the SPR intensity came from a thicker graphitic crystalline matrix. Indeed, as demonstrated by the Raman measurements, when the number of pulses was high, “inter-pulse” self-absorption should promote the graphitic crystallites growth around the AgNPs. At the opposite, when the experiments were performed in DOCA at 1.6 J/cm², the results showed an increase in the intensity with an increase of the number of pulses. A reasonable explanation was the destruction of the graphitic matrix by the “inter-pulses” self-absorption itself when the numbers of pulses were elevated. This hypothesis was confirmed by the disappearance of graphite signal in Raman spectra for such experimental conditions and the enhancement of the SPR intensity at higher number of pulses. Besides the influence of the laser irradiation conditions, the general shape of the UV-Visible spectra of AgNPs was impacted from DOCA to PFDA. In PFDA, the SPR was not observable in Figure 3 B (only a weak bump for experiment 5). This was attributed to a thick 16 ACS Paragon Plus Environment

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graphitic matrix on the surface of the particles. Indeed, as previously mentioned, the presence of graphite species around AgNPs could hindered the plasmonic resonance. In DOCA monomer, the SPR is clearly visible despite the presence of graphite around the particles highlighted by the Raman signal around particles aggregates (except for experiment 4). This difference between DOCA and PFDA is assumed to be due to a thinner graphitic matrix around AgNPs in the case of DOCA. At low fluence, it is worth noting that the maximum intensity of the SPR shifted from 388 nm to 423 nm at higher number of pulses. This could be attributed to the generation of bigger particles by “inter-pulses” self-absorption. In the other hand, at high fluence, the increase of the number of pulses did not change dramatically the SPR but remove the graphite of the vicinity of the particles so the graphitic matrix seems to have a limited effect on the SPR. In order to confirm the aforementioned results, TEM analyses have been performed. Figure 4 shows TEM images of AgNPs synthesized under high number of pulses. All the other images with the corresponding size distribution are reported in the supporting information.

A



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B

C


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D

Figure 4 : TEM images of Ag NPs in DOCA at A) 0.8 and B) 1.6 J/cm² and in PFDA at C) 0.8 and D) 1.6 J/cm²

The results show dramatic changes in size, distribution size and surrounding of the AgNPs when the synthesis occurred in DOCA. First, experiments using high fluence led to huge differences between low and high number of pulses. A low number of pulses induced large AgNPs aggregates covered with a thin graphitic layer (Figure S 3). Very interestingly, accumulating pulses (which provoked “inter-pulse” self-absorption) led to a total removal of the graphitic layer and a release of AgNPs (~ 3.7 nm in diameter) from the graphitic matrix (Figure 4 B ; S 4). However, bigger particles were observable in the samples (which could be clearly distinguished in TEM images) but it composed less than 5 % of the materials. It worth



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to notice that the aggregation of the AgNPs cannot be avoided because of the loss of the graphite matrix which acted as a stabilizing agent. Then, at low number of pulses, a decrease of the fluence led to AgNPs of ~3.6 nm in diameter and well dispersed in a large graphitic matrix. This result confirmed the UV-Visible results described before (high amount of graphite versus AgNPs) (Figure S 1). At higher number of pulses (i.e. 600), the graphitic matrix remained around the Ag NPs and the Ag NPs diameter increased to ~20 nm due to “inter-pulses” self-absorption (Figure 4 A ; S 2). At low fluence, the laser beam was not energetic enough to remove the graphitic matrix. However, the graphitic layer was thin enough to allow the melting and the coalescence of the AgNPs by thermal absorption from laser irradiation. The graphitic matrix remained visible for all the experimental conditions performed in PFDA, and the AgNPs did not show significant variation in their average size (Figure 4 C ; S 6 and D ; S 8). The graphitic matrix surrounding the AgNPs was able to absorb a sufficient quantity of energy coming from the laser beam to avoid changes in the AgNPs morphology. However, it could be clearly pointed out that, at high fluence, the size distribution was affected. An increase in the size distribution was observed for small values of number of pulses. As explained before, the “intra-pulse” self-absorption was at the origin of coalescence of AgNPs in the plasma plume and led to broadening of size distribution.

4. Conclusion The LP-PLA synthesis of AgNPs in two acrylate precursors, DOCA and PFDA, have been investigated. The influence of the fluence and the number of pulses have been examined. It has been demonstrated that during the ablation of the target at low number of pulses, the precursor and the fluence strongly influenced the concentration and the size of the AgNPs synthesized. The different behaviour between DOCA and PFDA at low number of pulses has 20 ACS Paragon Plus Environment

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been explained in term of 1) variation in confinement of the plasma plume (by increasing the viscosity and density of the media) and 2) “intra-pulse” self-absorption which limited the nanoparticles production. The formation of a graphitic matrix has been highlighted for each experimental condition. Increasing the number of pulses led to “inter-pulses” self-absorption phenomenon. To sum-up, three different scenario have been observed for high number of pulses and explained by “inter-pulse” self-absorption (Scheme 2) : i)

in PFDA, the graphitic matrix was thick. This layer acted like a protective barrier during laser ablation. No significant change has been observed according to variations in experimental parameters during LP-PLA.

ii)

in DOCA at low fluence, the graphitic layer was relatively thin and allowed thermal absorption. This absorption led to an increase in the AgNPs diameter without removing the graphitic matrix.

iii)

in DOCA at high fluence, the graphitic species were diluted in the liquid precursor. Removing the graphitic layer from the surrounding of the AgNPs induced their release in the solution but affected dramatically the stability of the suspension.

Scheme 2 : Summary of the mechanism and obtained materials of LP-PLA of silver in liquids organic monomer



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Then, by a judicious choice of the LP-PLA parameters, it was possible to drive the final characteristics of the AgNPs such as their size, surrounding and concentration. Indeed, particles size varied from 4 to 20 nm by using “inter-pulses” self-absorption with appropriate fluence and pulse number. The surrounding could also be triggered from native to embedded in graphitic matrix of different thickness and crystallinity. This had a direct impact on the stability of particles: particles in graphitic matrix were stable for months while the native were quickly aggregating. For these last the use of capping agent could be consider to increase the stability. So, this paper highlights the successful synthesis of Ag NPs and the powerful control on the generated material by laser ablation in organic liquids such as acrylate precursors.

Supporting information description HRTEM images and size distribution determined by Image J of all the samples

Acknoweledgment The authors would like to thank the Luxembourg funding organization ‘‘Fonds National de la Recherche’’ (FNR) for financial support through the FNR CORE HABaC project. The authors would also thanks Marie-France Canisius and Nicolas Humblet from ISSEP (Belgium) for AAS meausrement, Brahime El Adib from LIST for Raman analysis and Loïc Vidal from IS2M for TEM pictures. Finally, we kindly thank Eric Millon from the University of Orléans for his positive remarks and advices.

References 22 ACS Paragon Plus Environment

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