Synthesis of Gold Nanoparticles in Liquid Environment by Laser

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Synthesis of Gold Nanoparticles in Liquid Environment by Laser Ablation with Geometrically Confined Configurations: Insights to Improve Size Control and Productivity Stefano Scaramuzza, Mirco Zerbetto, and Vincenzo Amendola J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00161 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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The Journal of Physical Chemistry

Synthesis of Gold Nanoparticles in Liquid Environment by Laser Ablation with Geometrically Confined Configurations: Insights to Improve Size Control and Productivity Stefano Scaramuzza,+ Mirco Zerbetto,+ Vincenzo Amendola*

Department of Chemical Sciences, University of Padova, via Marzolo 1, I-35131 Padova, Italy +

S.S. and M.Z. contributed equally to this work.

*Correspondence: [email protected]; Tel +390498275673

Abstract. Laser ablation of solid targets in liquid environment allows the generation of nanoparticles (NPs) with several useful properties such as high purity, easily functionalizable surface, metastable composition or complex structure, including doped nanocrystals, core-shells, hollow microspheres, nanotruffles or nanocrescents. However, the mechanisms of NPs formation is still not well understood, and challenges remain in size control and productivity. Here, we investigate how the asymmetry intrinsic of laser-matter interaction can influence the structure and yield of gold NPs produced with nanosecond pulses. In particular, we confined the geometry of the laser ablation configuration in three ways: by reducing the thickness of the solid target from bulk size to few tens of nanometers, by reducing the size of the laser spot on the solid target and, finally, by reducing the lateral size of the bulk target. The interpretation of results was supported with numerical simulations of heat distribution inside the metal target in the three configurations. Surprisingly, we found that only the average size of NPs is affected by target thickness, whereas NPs polydispersity is reduced by confining the ablation geometry in transversal direction to the light propagation axis, i.e. by decreasing transversal target size or laser spot size. In addition, we observed a strong dependence of yield versus target thickness, suggesting that targets below ~0.1 mm should be avoided for optimal ablation rate. Taken together, these findings indicate that NPs formation mechanism changes with the depth of the ablated layer inside the bulk target and with the spatiotemporal temperature gradient in the material. By adding another piece to the puzzle of laser ablation synthesis in liquid solution, this study provides useful indications to improve the size distribution and productivity of laser-generated NPs.

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Introduction Large scale production of nanoparticles by “clean” and cheap technologies is highly desired for the rapid translation of nanostructures to real-life applications.1,2 This is possible especially when nanomaterials with innovative functions and structure are achieved with relatively simple synthetic approaches, such as doped crystals, core-shells structures or metastable phases.2-4 For these reasons, laser ablation synthesis in solution (LASiS) attracted a growing interest in the last decade. LASiS contemplates the production of NPs by focusing laser pulses at high repetition rate on a bulk target dipped in liquid environment.3,5 This approach is very versatile, because nearly all the materials forming a solid phase can be converted into a colloidal solution by the same methodology.3,6 Besides, LASiS does not require chemical precursors, with several advantages concerning the purity of products, the environmental impact of the procedure and the cost for unit mass of nanomaterials produced.3,5 Laser ablation in liquid environment is a multistage process with an intrinsically asymmetric geometry, consisting in the propagation of a single laser pulse toward a solid target dipped in a liquid solution.3,5,6 Therefore, it is possible to identify discontinuous synthetic conditions in longitudinal and transversal directions to the beam propagation axis.3 In particular, the energy of photons is transferred only to the portion of the target located at the solid/liquid interface.3 This inevitably rules to the formation of four relevant gradients during the ablation and subsequent formation of nanomaterials, which are interrelated and vary with space and time: temperature, pressure, concentration of material species and concentration of solution species.3 The fast kinetics of the process and the low amount of material generated per single pulse complicate the precise investigation of NPs formation mechanism in LASiS.3,7 In addition, the main synthesis stages are partially overalpping in time and space, with a concentric geometry where the inner hot core can be probed only passing through the colder external layers surrounding the ablation spot.3 Despite these difficulties, several brilliant attempts in recent times afforded the problem of LASiS mechanism, such as the observation of shockwave generation during pulse absorption,8,9 the rise, expansion and cooling of the plasma plume of ablated material,8,10 and the growth and collapse of the cavitation bubble produced by the release of absorbed energy from the hot target material to the surrounding liquid solution.11 To date, however, several important pieces are missing in the puzzle of LASiS. In particular, the role played by the asymmetry of the laser/matter interaction has been scarcely studied, also due to the difficulty of operating on the intrinsic geometry of this synthetic technique. Indeed, both laser ablation in liquid environment or in gas buffer are known to give polydisperse NPs, suggesting that the gradients created by the laser beam may play a relevant role in the control of NPs size.3


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Given the increasing demand for LASiS of NPs with plasmonic,12-16 magnetic,17-21 catalytic22-26 and photonic27-30 properties, further efforts of research and optimization are required in particular to improve the control of product structure while maintaining acceptably high yield,3,5,31,32 which is not possible without an exhaustive comprehension of LASiS mechanism.3,33 Here, we studied how structure and yield of gold NPs is influenced by the asymmetry intrinsically present in laser ablation synthesis, by acting on the transversal and longitudinal confinement of LASiS configuration. We selected gold NPs as case study because they are very important tools in nanotechnology, in particular due to their optical and catalytic properties, the multiple possibilities of surface (bio)conjugation, the chemical stability and the biocompatibility.5,34 Besides, laser assisted synthesis of gold NPs has been frequently investigated in recent years, and this type of NPs can be considered as a reference nanomaterial for LASiS.5,35-41 We monitored LASiS of gold NPs in three different configurations: (I) by reducing the thickness of the solid target from bulk size to few tens of nanometers, (II) by reducing the size of the laser spot on the solid target, and (III) by reducing the lateral size of the bulk target. Experimental data were supported with numerical simulations of the heat distribution inside the metal target in the three configurations. Interestingly, results provide useful insights for the control of NPs size and yield, because we found that longitudinal confinement of LASiS geometry mostly affected the average NPs size, whereas transversal confinement has a remarkable effect on NPs polydispersity. Besides, the efficiency of the ablation process depends on both types of confinement, with the strongest effect observed when target size falls below a critical threshold of ~0.1 mm. Overall, this study expands the knowledge about the scarcely investigated aspect of LASiS intrinsic asymmetry, thus contributing to clarify the puzzle of NPs formation during laser ablation in liquid environment.

Materials and Methods Materials. Au films, each with a different thickness varying in the 50 nm – 1 mm range, were used for the experiment. Films in the 50 – 250 nm range were obtained by sputtering in Ar atmosphere on a quartz substrate. No intermetallic adhesion layers were used. Films in the 800 nm – 1 mm range, obtained by milling, were purchased from “Anna Pavan Pure Gold and Silver Foils Inc.”. Films thickness was evaluated with a profilometer model Tencor P10 (below 1 µm) or with a digital spessimeter model LTF Italmachines (above 1 µm). LASiS. LASiS was carried out by placing the metal target in a glass cell filled with 8.0 mL of an aqueous solution of NaCl (10-4 M). In the first experimental configuration (configuration I), pulses with a fluence of 4.3 J/cm2 from a Quantel Brilliant 50 Q-switched Nd-YAG laser (1064 nm, 6 ns of FWHM, 5 Hz) were focused on the target with an f 150 mm lens. The metal target was at a distance


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of 3.7 cm from the focal point of the f 150 mm lens (1.6 mm of laser spot size on the target). Ablation was performed on an area of 1.10 ± 0.05 cm x 1.10 ± 0.05 cm by placing the glass cell on an automatic translating stage connected to a personal computer and controlled via a Labview software. Each synthesis was repeated at least 3 times. In the second experimental configuration (configuration II), LASiS was performed on a 1 mm thick Au foil by placing a diaphragm after the focusing lens, while maintaining unaltered all the other parameters. By acting on the diaphragm, the beam area was changed from the 100% of its value at the output from the laser (6.1 mm) to the 20% of the output value, corresponding respectively to a spot diameter on the Au target varying from 1.6 mm to 0.7 mm. In this experiment the pulse energy at laser output (before the diaphragm) was maintained at 61 mJ (corresponding to a fluence of 3.0 J/cm2). All the other experimental parameters were kept unchanged with respect to configuration I. In the third experimental configuration (configuration III), LASiS was performed by focusing laser pulses on a gold cylindrical target with diameter 0.7 mm and thickness of 10 mm, by maintaining laser spot size at 1.6 mm and pulse energy at 61 mJ (corresponding to a fluence of 3.0 J/cm2, i.e. same laser parameters as in the configuration II). In this experiment, the top face of the gold cylinder, where focusing conditions are set equal to configuration II, has a smaller diameter than laser spot size. The gold cylinder is located at the center of the laser spot, i.e. aligned with the beam propagation direction. All other parameters are kept unchanged with respect to configurations I and II. Characterization. Optical absorption spectra (OAS) of Au NPs solutions were collected with a Varian Cary 5 UV-visible spectrometer in 2 mm optical path quartz cells. The concentration of Au NPs was evaluated by the analysis of OAS according to previously published protocols.15,42 Transmission electron microscopy (TEM) analysis was performed with a FEI Tecnai G2 12 operating at 100 kV and equipped with a TVIPS CCD camera. The samples for TEM analysis were prepared by evaporating NPs suspensions on a copper grid coated with an amorphous carbon holey film. Size histograms are obtained by counting more than 400 NPs for each sample. Numerical calculations. Calculations of heating profiles in Au films were performed with the COMSOL 4.2 multiphysics package.43 A cylindrical geometry has been adopted for the gold plate, so that axial symmetry allowed to perform calculations in only two dimensions (see Figure S1 in S.I.). In this way, only two variables are required in our setup, namely z ∈ [0, t ] , where t is the target thickness, and x ∈ [0, x max ] , where x max is 1.2 mm for configurations I and II (dimension that ensures that the lateral surface of the cylinder is at room temperature for any z), while it is fixed at 0.35 mm in configuration III. The heat equation to be solved is ACS Paragon Plus Environment

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ρ ( T ) Cp ( T )

∂T ( x,z,s ) ∂s

tr

 ∂ / ∂x  ∂ / ∂x  = K (T)    T ( x,z,s ) + Q ( x,z,s )  ∂ / ∂z   ∂ / ∂z 

(1)

where T ( x, z,s ) is temperature field in space and time ( s ) for which equation 1 is solved, ρ ( T ) is the density of gold, Cp ( T ) its specific heat capacity, K ( T ) is Au thermal conductivity, and Q ( x,z,s ) is an inward heat flux describing heat transfer from the laser source to the gold plate.

Finally, in equation 1, “tr” stands for matrix transposition. Thermal properties and density of gold are taken as functions of the temperature field (specific equations and parameterizations for gold are provided in S.I.). To solve the heat diffusion equation, the homogeneous field T ( x, z,0 ) = T0 = 293.15 K has been chosen as initial condition. For what concerns boundary conditions (BC), with reference to Figure S1: i) Neumann (insulating) BC is applied to surfaces 1, 2, and 4; ii) Robins BC is applied to surface 3, to simulate heat exchange with the glass support; iii) surface – ambient radiation is applied to surfaces 1, and 3. Summarizing, at the boundaries:  i = 2, 4 0   ∂ / ∂x  4 4 −ni ⋅ K ( T )  i =1  T = εσSB T0 − T ∂ ∂ / z    εσ T 4 − T 4 + h ( T0 − T ) i = 3  SB 0

( (

) )

(2)

where ni is the normal to the i-th surface, ε is the emissivity of gold, σSB the Stefan-Boltzmann constant and h the convective heat transfer coefficient between Au and SiO2. In all calculations, the gold plate surface in contact with water rapidly overcomes the water vaporization temperature. At that point, we checked that a temperature dependent exchange with the h constant falling to 0 as the gold surface reaches water vaporization temperature in a first numerical test, or 1000 K in a second numerical test, did not give different results with respect to the case of completely excluding this effect on the surface labelled 1 in Figure S1. Hence, we neglected the Au-H2O heat transfer above water vaporization temperature. The laser source term in equation 1 was modelled as Q ( x,z,s ) = (1 − R )

E 0α

(

Sspot 1 − e−αt

)

e−αz H ( x )

2 1 − s −s /2σ2 e ( 0) 2πσ

(3)

where R is the reflectivity of gold, E0 is the total energy per laser pulse, Sspot the surface of the gold plate affected by the pulse, α the skin depth, σ = FWHM / 2 2ln ( 2 ) , s 0 = 4σ , and H ( x ) a smoothed hat function that reproduces the radial profile of the laser pulse. In particular, it is a step function centred at x = 0, ranging from –Rspot to +Rspot (with Rspot being 0.8 mm or 0.35 mm based on the


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experimental configuration), and a smoothing factor of 4.5.10-4 for the spot of 0.8 mm radius, and 9.0.10-5 for the smaller spot. All the functions and their parameterization are given in S.I.. Equation 1 has been solved numerically by applying a regular mesh, dividing both the x and z dimensions in 30 equally spaced intervals. Time integration has been carried out in a range of 20 ns, dumping results every 100 ps. We set the time τ = 0 ns (where τ = s − s 0 ) when the center of the laser pulse with Gaussian temporal profile and FWHM of 6 ns is at the liquid/metal interface (as described in Figure S2 in S.I.). Simulations have been performed on a desktop workstation equipped with a Xeon QuadCore E5606, 2.13 GHz.

Results In LASiS configuration I (Figure 1a), we explored the effect of longitudinal confinement in the laser ablation set up, by reducing target thickness (t) along the direction of incidence of the laser spot on the bulk metal target. For all thicknesses from 1 mm to 50 nm, the aqueous solution after laser ablation assumed a purple colour, clearly indicating that Au NPs were successfully generated. OAS spectra showed that Au NPs absorption is correlated to the thickness of the metal target (Figure 1b), i.e. that the mass of ablated material scales with Au film thickness. In case of the target with millimetric depth, we calculated from OAS that an effective layer of 490 ± 80 nm was converted into NPs. This represents the maximum amount of material achievable in our experimental conditions and is very close to the thermal diffusion length of gold (550-600 nm), which is given by LTh=(2τLδ)1/2,44,45 where δ is the film thermal diffusivity, usually of the order of 0.3-0.25 cm2 s-1 for Au films, and τL is pulse duration (6 ns). In fact, for ns laser pulses the heat transport is governed by heat diffusion into the bulk target, and the characteristic penetration depth is of the order of LTh.44-46 In Figure 1c we plotted the mass of Au NPs versus the logarithm of film thickness, and compared the experimental results with the amount of material theoretically achievable by ablation of gold films. Interestingly, the linear increase of Au mass with film thickness is not observed for all samples, while yield remains below the theoretical threshold for t between ~200 nm and ~0.1 mm. Below and above this range, yield is in agreement with the theoretical prediction.


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(a)

(b)

(c)

Configuration I 0.4

Liquid solution

1.3 0.9

Au film thickness 0.2

Au mass (mg)

Laser pulse

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.3

t Au film

0.0 400 600 800 Wavelength (nm)

0.0 1 10

2

3

4

5

6

10 10 10 10 10 Film thickness (t/nm)

Figure 1. a) Sketch of LASiS configuration I, concerning the laser ablation of Au films with variable thickness. b) OAS of Au NPs colloid obtained by laser ablation of Au films with different thickness. c) Au mass versus film thickness (red dots) and theoretical yield (black line). The dashed area in the theoretical trend is the interval of confidence in our experimental conditions.

We performed TEM analysis in order to investigate the possible effects on the structure of Au NPs obtained from different targets (Figure 2). The average size, the relative standard deviation and the size histograms are similar for the 1 mm and 226 nm thick films (Figure 2). Instead, the size distribution shifts at larger sizes for film thickness below 226 nm, with the consequent increase of average Au NPs diameter (Figure 2). On the other hand, the relative standard deviation (sd) is only slightly affected by the film thickness, since it goes from 54% in the 1 mm target to 49% in the 50 nm film (Figure 2c). This indicates that film thickness has a scarce influence on NPs polydispersity.


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Figure 2. Dimensional analysis of Au NPs obtained by laser ablation (with configuration I) of the 1 mm (black), 226 nm (red), 154 nm (green) and 50 nm (blue) films. (a) TEM images. (b) Size histograms. Histograms area is proportional to the total mass of Au NPs produced in each case. (c) Average size (d, black squares) and relative standard deviation (sd, red circles) of the four samples.

In LASiS configuration II (Figure 3a), we explored the effect of transversal confinement in the laser ablation set up, by maintaining unaltered the pulse energy at the laser output, while reducing the portion of beam reaching the target with a diaphragm. In our case we have laser pulses with top-hat spatial profile, then diaphragm area is proportional to the total energy delivered to the target. Moreover, beam divergence occurs at the pulse borders,47 therefore diaphragm also helps in selecting the inner part of the laser beam, where energy spatial distribution is more homogeneous. This instrument parameter is especially relevant with millimeter laser spot size, i.e. when laser ablation is performed by placing the target out of lens focus to avoid solvent breakdown, which is ACS Paragon Plus Environment

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the common operating condition in LASiS.3 As shown in Figure 3b, we observed a correlation between the Au NPs concentration and the area of the laser spot in configuration II. Interestingly, the yield expressed as Au mass after each synthetic run, grows with the square of the laser spot area (Figure 3c), i.e. with the fourth power of spot diameter. Besides, the dimensional analysis shows that the average size of Au NPs is largest (15.8 nm) when the spot size is the smallest (Figure 3d-e). On the other hand, Au NPs standard deviations decreased from 59% to 47% reducing the spot diameter from 1.6 mm to 0.7 mm (Figure 3e).

(a) Configuration II

(b)

Au target t = 1 mm

Spot size 1.60 mm 1.25 mm 1.10 mm 0.70 mm Au wire

15 10 5

10

20

30 d (nm)

40

50

16 15 14 13 12 11

Au wire

0

400 600 800 Wavelength (nm)

(e)

20

0

Au mass (mg)

0.0

(d)

0

0.2

60 55 50 45 40

0

(f)

1 2 Spot area (mm )

2

Configuration III

sd (%)

spot size

%

Parabolic Fit 2 y=A+Bx+Cx A=0.005+/-0.009 B=-0.006+/-0.021 C=0.27+/-0.01 2 R =0.9995

1

Absorbance

Liquid solution

(c)

0.4

Laser pulse

d (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Laser pulse

1.6 mm Liquid solution

0.7 mm Au wire

1.6

1.25 1.1 0.7 Au wire Spot size (mm)

Figure 3. (a) Sketch of LASiS configuration II, concerning laser ablation with different laser spot sizes on the target (1.60 mm: black, 1.25 mm: red, 1.10 mm: green, 0.70 mm: blue). (b) OAS of Au NPs colloids. (c) Au mass versus spot area. (d) Size histograms. (e) Average size (d, black squares) and relative standard deviation (sd, red circles) of the four samples. (f) Sketch of LASiS configuration III, concerning Au target with lateral size smaller than laser spot. OAS spectrum, Au mass, size histogram, average size and relative standard deviation are reported in magenta in figures (b), (c), (d) and (e) respectively.

Since lateral confinement of spot size induced a remarkable variation of Au NPs size distribution and yield, we considered an additional LASiS configuration (configuration III, Figure 3f) where target lateral size is reduced below the spot size, which is maintained at the maximum value of configuration II, while keeping unaltered all the other ablation parameters. Interestingly, Au NPs yield is ~12 times larger than in configuration II, at parity of the overlapped area between target and laser spot (magenta dot in Figure 3c). Au NPs size distribution is centered at larger dimensions (magenta line in Figure 3d), although the average size is comparable to that obtained in configuration II (magenta dot in the bottom graph of Figure 3e). Remarkably, the relative standard ACS Paragon Plus Environment

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deviation of 40% (magenta dot in the top graph of Figure 3e) is lower than the best value of 47% obtained with configuration II (spot size of 0.70 mm). Overall, results obtained with configurations II and III suggest that lateral confinement of LASiS geometry has a direct influence on NPs polydispersity.

Discussion The experimental investigation of 3 different LASiS configurations, corresponding to 3 different types of geometric confinement of the laser ablation process, evidenced several differences in NPs formation mechanism. Modelling the whole process of nanoparticles formation by laser ablation in liquid environment would require a multiscale approach which goes from the molecular dynamics of ejected material to the thermodynamics of the cavitation bubble, and must consider also photoionization phenomena connected to the high density of photons in the ablation area. The latter point is especially important when nanosecond pulses are used, because the plasma plume forms 0.1 ns after the beginning of laser matter interaction48-52 and, consequently, there is spatial and temporal overlap with the incident photon flux and direct absorption of laser light from the plasma.3,53-58 Besides, steady state models such as continuum hydrodynamics and classical nucleation theory are inapplicable in LASiS conditions.33 A multiscale approach with the required level of complexity is currently beyond state of the art computational ability,3,49,52 and modelling usually has been limited to the molecular dynamics (MD) of the hot matter in the ablation crater as a function of material properties and density of absorbed energy.48,49,59 This type of MD calculations, however, implies that the energy density throughout the metal target is well known, which is not the case of our experiments. Therefore, to obtain more insights about our experimental results, we considered a simplified model of metal target for which we calculated the spatio-temporal temperature gradients as a function of the different geometries studied in our experiments. Although this is an approximate calculation which does not provide direct data on nanoparticles formation, its results are very useful to obtain semi-quantitative and comparative information on the temperature gradient in the various experimental conditions.48,60,61 In fact, temperature gradients are crucial in LASiS, because they determine the ablation mechanism and, in cascade, the concentration and state of ejected material in the plasma plume, the thermodynamics for NPs formation, the cavitation bubble dynamics and, ultimately, the structure of NPs.33,51,59,62

Au NPs formation: configuration I. In configuration I we found that Au NPs average size increased by reducing target thickness (see Figure 2), without appreciable changes in NPs polydispersity. This suggests that average NPs size depends on the depth of the ablated layer in the metal target, with smaller particles coming from the deepest layers.


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Indeed, remarkable differences are observed in the snapshot of heating profiles, corresponding to the time when the highest temperature is reached in each target type (Figure 4). In particular, the temperature in the 50 nm film is almost uniformly above 104 K in the whole irradiated area, whereas in the thickest film we found a decay from the maximum value of ~6.103 K to room temperature moving away from the liquid/metal interface. The difference in peak temperature for different film thickness is clearly due to the efficient heat dissipation by the low lying metal layers in thick films. As a first approximation, we can assume that matter is ejected from a portion of target when it reaches the vaporization threshold of ~3150 K. In this case, according to Figure 4, matter ablated from the low lying target portion will expand toward an already formed hot plasma region at higher temperature, when target thickness exceeds ~100 nm. This is in agreement with the well known effect of plasma confinement on the ablation crater, due to the presence of the liquid buffer.3,10,59 This effect is responsible for the sensible increase of target erosion, and hence of the ablation yield, in liquid compared to gas environment.3,10,59 Besides, in case of nanosecond laser ablation, plasma heating is alimented also by the spatial and temporal overlap with the laser pulse.3,53,54 These effects lead to the ablation of inner layers of the target by the simultaneous action of laser pulse absorption (whose intensity exponentially decays with the distance travelled in the target, initially due to metal skin depth and, after ~0.1 ns, due to plasma shielding) and plasma erosion (which acts as the heat sink transferring the thermal energy to the inner part of the target).3,10,49,53,54,59 In this context, one can conclude that smaller Au NPs are obtained with thicker targets because of the combined operation of laser absorption and plasma confinement. For instance, plasma may induce the size reduction of liquid droplets emitted during the phase explosion process.63 In the 50 nm film there is no underlying material to be etched by the plasma and material ejection takes place only by laser-induced fragmentation. In fact, we calculated that the bottom of the 50 nm Au film reaches the vaporization threshold (T = 3150 K) already at time τ = –3.9 ns, whereas the bulk Au film reaches the same threshold in its deepest point at τ = +6.0 ns, when the plasma plume is well formed (see Figure S2 for designation of τ = 0 ns). According to what reported several times in literature, the size of NPs obtained by laser ablation in gas phase increases with film thickness,48,60,64-66 which is the opposite of what is observed in our study and in a recent report about laser ablation of wet gold films.61 In the gas phase, the trend in NPs size is explained with the higher density of ejected material when target thickness increases, because thicker films absorb a larger amount of laser energy in the whole process.33,51,59,62 Besides, MD calculations showed that slower and larger molecular fragments are obtained at lower temperature such as that in the low lying layers of the ablation crater,48,60 although this is not ACS Paragon Plus Environment

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necessarily an indication that larger NPs will be formed from these atomic clusters when the gas buffer is reached.33,49,62,67 Three main differences exist between laser ablation in gas and liquid phases: the strong confinement of the plasma plume on the target, the formation of a cavitation bubble and the interaction of target species with solution species.3 For what concerns the cavitation bubble, there are no specific studies of this phenomenon as a function of target thickness, although some recent investigations considered laser ablation of metal wires with different diameter in the 1.500 - 0.125 mm range.11,32 In these experiments, no dependence between NPs structure and the features of the cavitation bubble were observed.11,32 Besides, in our experimental conditions, bubble size depends on the amount of energy absorbed from the target, which increases with target thickness.6,68-70 Some recent reports evidenced that a smaller cavitation bubble is associated with smaller NPs with larger polydispersity, because there is a larger fraction of particles whose growth is rapidly quenched by the liquid phase, instead of prosecuting in the gas phase inside the bubble.71,72 Since we observed the opposite trend in NPs size, namely larger particles formed whit lower amount of energy absorbed from the target (smaller target thickness), and in addition polydispersity is almost constant in all cases, we conclude that cavitation bubble is not responsible for the differences in NPs size observed in our experiments. For what concerns the interaction of target species with solution species, this plays a relevant role in the structure of final products.3,6,73,74 For instance, it has been reported that the type of liquid influences the stoichiometry of laser-generated Au-Fe nanoalloys16,19 and the phase of Fe NPs.21 In case of Au NPs obtained in aqueous solutions of electrolytes, like in our case, the latter influence both average size and polydispersity of final NPs at the same time.3,5,35-37 However, in our study, we only observed a change of average size, without a variation in polydispersity. This, suggests that solution species are not involved in the observed trend. In summary, from the opposite trend in NPs size observed by laser ablation of thin films in gas and liquid phase, one can conclude that plasma plume confinement on the ablation crater is the main factor for the different average size of Au NPs obtained in configuration I. In addition, since the standard deviation of Au NPs size does not change with target thickness, we conclude that this aspect of particles formation is not affected by the longitudinal temperature gradient inside the metal target.


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Figure 4. Laser pulse (a) and temperature (b) spatial profiles for Au films with different thickness in configuration I. Heating profiles are referred to the time when the highest temperature is reached in each film.

Au NPs formation: configuration II and III. In configuration II and III, we observed a decrease of NPs polydispersity (i.e. of sd) by transversal confinement of LASiS geometry. The effect was observed when the laser spot size was reduced with a diaphragm (sd changed from 59% to 47%), ACS Paragon Plus Environment

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and it was even more evident when the target lateral size was reduced below the laser spot size (sd of 40%). In our experimental conditions, laser pulse is characterized by an intensity decay over a length which is ca. 10% of the beam diameter at the instrument output (Figure 5).47 In fact, temperature profiles calculated for configuration II without (Figure 5a) and with (Figure 5b) the diaphragm clearly indicate that the transversal heating profile is more homogeneous when laser beam passes through the diaphragm before reaching the target. Besides, calculations showed a very homogeneous temperature profile in transversal direction for configuration III, where laser spot size exceeds target size and there are no interfaces between irradiated and not irradiated portions of the target. When irradiation is more homogenous, also the transversal temperature profile is sharper, with a better homogeneity of the conditions for ablation and NPs formation. This is in agreement with results of MD simulations suggesting that the ablation mechanisms and the consequent NPs formation are strongly dependent on target temperature.3,49,75 In fact, target temperature influences the concentration and the thermodynamic phase of ejected material, as well as the thermodynamic parameters of the plasma plume formed at longer times.3,49,75 For instance, at the borders of laser spot, where temperature is lower, matter is ejected by spallation and vaporization-like processes, which are compatible with a nanoparticles formation by nucleation and growth.70 Conversely, in the inner part of the laser spot, explosive boiling and the ejection of melted target drops are more probable.49,51,75,76 The advantages of laser beams with homogeneous energy distribution to improve the control of NPs size distribution is in well agreement with previous reports concerning laser ablation in the gas phase.48 It is interesting to point out that transversal temperature inhomogeneity (configuration II and III) have a direct influence on nanoparticles standard deviation, but there are no clear evidences of a similar effect on nanoparticles average size, while the opposite is found for longitudinal temperature inhomogeneity (configuration I).


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Figure 5. Laser spot (top) and temperature (bottom) spatial profiles in configuration II for spot size of 1.60 mm, i.e. without diaphragm (a), and 0.70 mm, i.e. with diaphragm (b), and in configuration III (c). Heating profiles are referred to the time when the highest temperature is reached in each case.

Interestingly, according to our calculations, the inhomogeneity of temperature profile in the target is observed also in case of double pulse ablation (see Figure S3 in S.I.), although the maximum temperature in the target increases.


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In addition, we considered the temperature profile in case of surface inhomogeneity on the target, such as the presence of roughness or, more frequently, craters produced by repeated laser ablation in a fixed point of the target. Calculations performed on target with the shape described in Figure S4 in S.I. show that, in case of a smooth ablation crater, the temperature profile is basically the same as in case of a metal with flat surface (Figure 6b). Conversely, in the opposite case of a crater with sharp borders, a remarkable temperature difference is observed between the bottom and the walls of the crater, with consequent inhomogeneity of the laser ablation condition along the liquid/solid interface (Figure 6c). The same finding is observed in case of a crater with sharp borders and crests (Figure 6d).


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Figure 6. Laser pulse (a) and temperature spatial profiles for Au films with different surface structure: smooth crater (b), crater with sharp borders (c), crater with sharp borders and crests (d). Details of craters surface structure are in Figure S4 of S.I.. Heating profiles are referred to the time when the highest temperature is reached in the metal.

Au NPs yield. In configuration I, we found that film thickness has a strong influence on Au NPs yield, and three distinct regions can be identified in the plot of Au mass versus target height (Figure 1c). For target with millimetre and sub-millimetre thickness, we obtained the maximum yield in our experimental conditions. For thickness in the 0.1 mm – 200 nm range, yield is well below the theoretical maximum, which is reached only for films below 200 nm. Indeed, when yield falls below the theoretical limit, we also observed the formation of sub-millimetre Au debris. These fragments were not included in the calculation of total mass of laser-generated Au nanoparticles,


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which instead considered only the metal collected as a stable colloidal dispersion. The presence of Au debris clearly indicates that the lower yield is due to mechanical fragmentation of the metal films, with a consequent significant alteration also of the ablation crater. This can be the effect of the first shockwave generated by laser pulse absorption, as well as of the cavitation bubble expansion and collapse. It has been calculated that up to 10% - 50% of the absorbed laser energy is converted in the shockwave, which generates pressures as high as 108 Pa on a timescale of 102 ps after the beginning of laser beam absorption.50 On the other hand, the collapse of the cavitation bubble generates a pressure of the order of 1010 Pa and takes place at a time of ~102 µs after laser absorption.3 Since we observed the recovery of theoretical yield when film thickness was below 200 nm, in these cases the irradiated metal is already transformed in Au NPs when fragmentation occurs. Actually, this is in agreement with the typical timescale of shockwave or of cavitation bubble collapse, which exceeds in both cases the time for ejection of the first ~102 nm of material from the target.33,48,50,51,59,62 Our experiments confirmed that LASiS yield is maximum with macroscopic targets, suggesting that complete target consumption should be avoided by interrupting the process before reaching a thickness of ∼0.1 mm (Figure 7), according to our results. One can conclude that, in our experimental conditions, the forces responsible for target rupture exceed the mechanical resistance of the Au film only when its size is below the threshold of ∼105 nm. This can be of interest for the pioneering investigations of last years, showing the advantages of laser ablation with continuously fed metal wires of millimetre size, where a favourable cavitation bubble dynamics is achieved in comparison to flat targets.11,32 In configuration II, we found a parabolic dependence of Au NPs yield on the laser spot area, while maintaining constant fluence. Since incident laser energy scales linearly with spot area, but temperature profiles are comparable in the central part of the irradiated target (see Figures 5a-b), other effects must be involved in material ejection from the target. In particular, it is known from literature that laser spot size affects yield in liquid and gas phase, and the reason is usually ascribed to the increasing mass of hot material in the plasma plume when the irradiated area is increased at constant fluence.55-58,77 In fact, it is well known that plasma confinement increases the portion of target volume reaching the threshold for material detachment, compared to laser ablation in gas buffer.3,8-10 Although thorough quantitative description of this process has not been reported yet,5558,77

a more vigorous plasma plume, combined with the efficient confinement by the liquid buffer, it

is likely to contribute positively to the ablation rate. This hypothesis is further substantiated by the increase of yield in configuration III (see Figure 3c), in which laser spot size exceeds the heated target area. In fact, this is the ideal condition for alimenting plasma heating by direct absorption of ACS Paragon Plus Environment

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laser light even when plume diameter exceeds the heated target area. Indeed, only in configuration III the plasma is sustained by the absorption of the laser beam also when the plume expands beyond the heated target area, while in all the other configurations plasma expansion will just accelerate the heat release to the surrounding environment. It is worth to point out also that, in configuration II, the reduction of laser spot size also increases the rate of heat transfer from the plasma to the surrounding environment. In fact, if we assume as a first approximation that plasma plume has the shape of an oblate spheroid with transversal size proportional to the laser spot size, its surface-tovolume ratio increases of 50% when the laser spot diameter is reduced from 1.6 mm to 0.7 mm, with a consequent sensible increment in the heat transfer rate to the surrounding environment.49

Laser pulse

t

Laser pulse

Au film

Laser pulse

Au film

Au fragments

Au film t < 102nm

102nm < t < 0.1mm

t > 0.1mm

Target thickness (t)

Figure 7. Three different ablation regimes are observed by reducing target thickness (t): for t < 102 nm, all the irradiated area is converted in NPs; for 102 mm < t 0.1 mm, fragmentation is inhibited by the surrounding bulk metal, and conversion efficiency is maximum.


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Conclusions In summary, we have investigated the role of asymmetry intrinsic of laser-matter interaction on the structure and yield of gold NPs obtained by laser ablation in liquids. We confined the geometry of the laser ablation by reducing the thickness of the solid target from bulk size to few tens of nanometers (configuration I), by reducing the size of the laser spot on the target (configuration II) and, finally, by reducing the lateral size of the target (configuration III). The interpretation of results was supported with numerical simulations of heat distribution inside the metal target in the three cases. By confining LASiS geometry in longitudinal direction (configuration I), we found that Au NPs average size increases when target thickness is reduced below ~200 nm. Since the opposite trend has been reported for laser ablation of thin films in gas, and longitudinal temperature gradient is flat for thin films, we concluded that plasma plume confinement on the ablation crater is the main factor determining the decrease of average Au NPs size when target thickness is increased. In addition, since the standard deviation of Au NPs size does not change with target thickness, the kinetics of particles formation is not affected by the longitudinal temperature gradient inside the metal target. By confining LASiS geometry in transversal direction (configurations II and III), we found a reduction of Au NPs standard deviation from 59% to 40%. Instead, we have not found a clear trend on NPs size versus transversal confinement of LASiS geometry. The experimental findings and the calculated temperature profiles in the metal suggested that different ablation mechanisms take place as the effect of photon density decay at the border of laser pulse, and this is reflected in the polydispersity of laser generated NPs. However, this effect can be reduced by using a metal target smaller than the spot size. Besides, our experiments showed that yield is negatively influenced by the use of metal targets with sub-millimetre thickness, due to target fragmentation competing with nanoparticles formation. Conversely, we found the increase of NPs yield with the square of laser pulse diameter, likely due to the formation of a more vigorous plasma plume confined on the ablation crater. Overall, this study adds another piece to the wide puzzle of laser ablation synthesis in liquid solution, and provides useful indications to improve the control and productivity of laser-generated nanoparticles.

Supporting Information. Electronic supporting information available: Figure S1 about geometry setup in the COMSOL calculations, additional modelling and parametrization details, Figure S2 about calculation


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timescale, Figure S3 about temperature profiles with double pulse laser ablation, Figure S4 about surface morphology of targets with craters.

Acknowledgments. Authors would like to thank E. Napolitani for support with profilometer measurements and A. Pavan for support with gold films. Financial support from University of Padova (PRAT no. CPDA114097/11

and

Progetto

Strategico

STPD11RYPT_001)

and

MIUR

(PRIN

MULTINANOITA no. 2010JMAZML_001) is gratefully acknowledged.

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ToC graph:

Au NP

Au NPs size distribution

Au NPs size

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Longitudinal confinement Laser pulse

Transversal confinement

Laser pulse

Laser pulse

Laser pulse

Laser pulse

Au

Au

Au

Au

Au


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Synthesis of Gold Nanoparticles in Liquid Environment by Laser

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