Influence of Water Temperature on the Hydrodynamic Diameter of

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J. Phys. Chem. C 2010, 114, 2499–2504

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Influence of Water Temperature on the Hydrodynamic Diameter of Gold Nanoparticles from Laser Ablation Ana Mene´ndez-Manjo´n, Boris N. Chichkov, and Stephan Barcikowski* Laser Zentrum HannoVer, Hollerithallee 8, D-30419 HannoVer, Germany ReceiVed: October 15, 2009; ReVised Manuscript ReceiVed: December 22, 2009

Defined hydrodynamic properties of nanoparticle colloids are required for applications in dosimetry, rheology, or biosensing studies. During the generation of nanoparticles by laser ablation of a solid target in liquids, the temperature of the liquid increases, which may effect cavitation bubble and particle formation. We demonstrate that this temperature variation influences the hydrodynamic diameter of the resulting colloidal nanoparticles when a gold target is ablated by an IR femtosecond laser in water at different stabilized liquid temperatures in the range of 283-353 K. The maximum hydrodynamic diameter was observed at 330 K, the temperature at which the compressibility of water reaches its minimum. The formation of particles by condensation of ablated species in the liquid matrix or inside the confined cavitation bubble is discussed, as well as the influence of the physical properties of the liquid that vary with temperature, such as viscosity and compressibility. The reduction of the hydrodynamic particle diameter at the higher compressible state of water indicates that a lower number of agglomerates are dispersed in the liquid, reducing the polydispersity index of the gold colloid. Introduction Metal nanoparticles are nowadays applied in biological sensors,1 catalysis electrodes,2 or hydrogen fuel cells.3 Laser ablation enables the production of nanomaterials with an almost unlimited variety of matter (metals, semiconductors, ceramics, etc.) without any chemical precursors or additives.4-6 Its flexibility and productivity potential due to the evolution of new laser sources had pointed it to an alternative technique in the nanomaterial production.7 The generation of nanoparticles by laser ablation can be done in gaseous or liquid ambience. The first technique provides nanoparticles that are typically collected as powders or deposited on a substrate. Even though a control on nanoparticle size can be achieved by adjusting laser parameters and ambient atmosphere, agglomerates are usually formed after particle collection, and the redispersion of the particles is difficult.8 Many applications, such as drug delivery9 and toxicology,10 production of more efficient coolants based on enhanced heat-transfer nanofluids11,12 or electrophoretic deposition on neuronal electrodes,13 require nanoparticles dispersed in liquids. The hydrodynamic diameter of the primary particles or agglomerates in the liquid is of fundamental importance for the characterization of the behavior of nanoelements in a fluid. Laser ablation in liquids provides particles safely dispersed in a liquid matrix, but the polydispersity of the size distribution of these colloidal particles is usually higher as compared to other techniques, such as wet chemical precipitation, unless laser fragmentation14 or in situ quenching15 is involved. Many experimental efforts are performed to produce narrower size distributions of nanoparticles by laser ablation of solid targets in liquids. The intrinsic effects accompanying the ablation in liquids, such as plume confinement,16 cavitation,17 or laserinduced fragmentation,14 still need to be deeply examined to understand their influence on the formation of nanoparticles. Because of the high temperature, pressure, and density states * Corresponding author. Phone: +49 511 2788 377. Fax: +49 511 2788 100. E-mail: [email protected].

reached in the ablation zone, a detailed dynamic study of the ablation process in liquids is difficult. Whether the particles originate as expelled droplets from the melt pool18 or condensation19 of the ablated material is still under discussion. The ablation process is determined by the physical properties of both the solid and the liquid; however, the influences of physical properties of the liquid environment on the quality of the lasergenerated colloid has been scarcely investigated. When a laser beam is focused in a liquid, the high intensities reached in the focal volume may induce plasma formation, and the liquid absorbs part of the laser radiation20 so that part of the energy delivered to the liquid dissipates as heat. Focusing through the liquid on the surface of a solid target increases the liquids’ temperature. In addition, colloidal nanoparticles formed during the ablation absorb and scatter the incident radiation, causing heating of the liquid. The change in the solvent temperature produces a continuous variation of properties such as viscosity or compressibility (see Figure S1 in the Supporting Information), possibly affecting the ablation and subsequent particle generation. This continuous variation in the ambient conditions may result in nanoparticle colloids of high polydispersity. A better understanding of the ablation boundary conditions during the process of laser ablation in liquids may contribute to controlling the formation of particles in liquid confinement. We have analyzed the effect of temperature of the surrounding liquid to study the influence of the physical properties of the liquid on the resulting hydrodynamic diameter of the nanoparticles, without adding molecules or changing the solvents’ chemistry. In this way, chemical additives used for size quenching by ligands or surfactants,15 which may affect the purity of the colloid and could cause cross-effects on hydrodynamic size, are avoided. Experimental Setup Nanoparticles were prepared by femtosecond-pulsed laser ablation of a gold target (99.99%, Goodfellow) in deionized

10.1021/jp909897v  2010 American Chemical Society Published on Web 01/22/2010

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Figure 1. Experimental setup for the laser ablation in water at variable liquid temperatures.

water (0.05 µS/cm). The gold foil was placed vertically at a side of a quartz cuvette filled with 1 mL water. The water layer over the target was 10 mm. At the back, a Peltier element connected to a PID controller maintained the temperature of the sample constant with an accuracy of 0.1 K. The irradiation time was set to 1 min for each sample for a better stabilization of the temperature during the nanoparticle generation process. A digital thermometer, with its thermoelement immersed in the liquid, was used to determine the water temperature during the process (Figure 1). Laser ablation was performed with a Ti: sapphire laser source (Spitfire Pro, Spectra-Physics) delivering 120 fs pulses at 5 kHz and a wavelength of 800 nm. The laser beam was focused on the target surface with a 40 mm focal distance, achromatic lens. The relative position between lens and target was adjusted to obtain a laser fluence on the target of 0.17 J/cm2 (200 µJ pulse energy and 545 µm spot diameter). The ablation crater diameter was measured after the ablation with 5000 pulses on a silicon wafer. The scanning velocity was 76 mm/min, resulting in a pulse overlap of 99.91%. The spot diameter was measured at different water temperatures, and no significant variation was observed, so changes in propagation of the laser beam caused by a variation of the refractive index of water can be neglected (see Figure S2 in the Supporting Information). The energy density delivered to the target was constant in each experiment. Ablation was carried out for 1 min at different water temperatures from 283 to 353 K. During each nanoparticle generation experiment, a typical oscillation of 1.5 K around the set temperature was observed due to the heating effect of the laser beam. Particle size distributions prepared by this method were analyzed by dynamic light scattering (ZetaSizer ZS, Malvern Instruments) UV-vis spectroscopy (UV-1650PC, Shimadzu) and transmission electron microscopy (EM 10 C, Zeiss). The particle size is presented by the z-average of the colloid, the intensity-weighted mean hydrodynamic size of the ensemble collection of particles. Trend measurements in the particle size could be achieved with the same method. For this, a sample was prepared at room temperature, and the z-average was determined during the heating and cooling of the sample inside the measuring device. Results The high laser intensities reached in the focal volume initiate plasmas on the target and in the liquid.21 Moreover, the ablation

Mene´ndez-Manjo´n et al.

Figure 2. Increment of liquid temperature during irradiation with femtosecond laser beam focused inside a quarz cuvette filled with water at two different fluences (0.04 J/cm2, squares; and 0.13 J/cm2, circles and triangles) and with (triangles) or without (squares and circles) a gold target foil in the cuvette. The dashed lines represent the time at which water temperature stabilizes with the presence of a gold target (blue dashed line) and without target (red dotted line).

produces particles that stay dispersed in the liquid and along the beam path. For this reason, and even though ultrashort laser pulses are used, the liquid temperature increases considerably during the process. Of course, the thermalized energy is proportional to the input fluence, and the temperature increment is higher at bigger fluences (see Figure S3 in the Supporting Information). This effect can be observed in Figure 2, where the temperature increment of water at two different fluences, obtained by focusing inside the liquid (0.13 J/cm2) and behind the cuvette (0.04 J/cm2), is shown. At the beginning of the irradiation process, the solvent temperature rapidly increases and stabilizes after 15 min (red dotted line in Figure 2). At tight focusing conditions, a 2 K higher temperature is reached, as compared to defocused irradiation. When a target is placed inside the liquid and is ablated (0.13 J/cm2), the generated nanoparticles disperse completely in the liquid volume within a few seconds. The optical density of the solvent increases continuously and almost linearly with time until the concentration is high enough to reduce the energy reaching the target. At this moment, the ablation rate decreases, and fewer particles are produced. Under our experimental conditions, the ablation rate is reduced by a factor 360 after 160 min of ablation in relation to starting conditions (0.09 µg/min in the first minute). A direct proportional relation was found between the increment of temperature of the colloid and the gold concentration, indicating that, at first approximation, linear absorption of the laser radiation by the particles takes place (see the Supporting Information). The energy delivered by the laser source is absorbed and scattered by the nanoparticles so that the temperatures reached in the colloid are considerably higher than in neat water. In this case, at the same laser fluence, temperature increases 7 K more while ablating a gold target than while irradiating neat water. Because the optical density of the sample increases with irradiation time, the initial heating rate is higher, and the moment at which temperature stabilizes is delayed as compared to neat water, occurring under our experimental conditions 35 min after the start of the process (blue dashed line in Figure 2). This continuous and rapid change in the thermal conditions of the liquid during the laser irradiation process could affect the ablation and particle formation mechanism. To determine the possible effect of variations of the temperature of water during the laser ablation process on the nano-

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Figure 3. (a) Primary particle size distribution and maximal Feret diameter distribution of agglomerates observed in the TEM micrograph of the gold colloid generated at 313.5 K. (b) The cumulative frequency of hydrodynamic diameter measured for gold colloids generated in water at 282.4 (squares), 313.5 (circles), 337.2 (triangles), and 354.1 K (stars).

Figure 4. (a) Hydrodynamic diameter of gold nanoparticles generated by laser ablation in water at different temperatures and (b) the plasmon resonant wavelength of the gold colloid obtained at different liquid temperatures. (c) Absorption spectra of two gold colloids generated in water at 282.5 and 354.1 K.

particle size, a gold target was ablated at different stabilized solvent temperatures. The hydrodynamic diameter and plasmon absorption band of the colloid were measured at room temperature (293 K). The Feret diameter of primary particles presented in each case a monomodal log-normal distribution (Figure 3a). However, the big difference between the two measurement techniques (about 10 nm Feret diameter and 90 nm hydrodynamic diameter) indicates that agglomerates were preferably detected by dynamic light scattering. This technique is based on Mie-scattering of a laser beam by nanoparticles, and the scattered intensity scales with d6, where d is the particle diameter. The signal produced by 105 primary nanoparticles of diameter 10 nm would be just 10% of a scattering signal produced by only one particle of 100 nm diameter. For this reason, big primary nanoparticles or agglomerates of particles mask the weak signal coming from small primary particles. Some agglomerates can be seen in the transmission electron micrographs, even though it is not possible to determine if they are formed during the drying process for sample preparation or were already contained in the colloids. Correspondingly, Figure 3a displays gray bars showing the size distribution of the agglomerates observed in the micrographs and that reach the hydrodynamic size range analyzed by dynamic light scattering. Figure 3b shows the cumulative frequency of the intensityweighted hydrodynamic diameter at four different temperatures. In the insert of Figure 3b, it can be clearly seen that liquid temperature has affected the cumulative hydrodynamic size. In the following, the trends of the hydrodynamic particle diameter for variable temperature of water and its calculated viscosity and compressibility are discussed in detail.

The mean hydrodynamic diameter of the colloids increases linearly with temperature until 330 K (Figure 4a), where a maximum is reached. At higher temperatures, it decreases again. Similar dependency is found for the plasmon resonant wavelength of the colloid (Figure 4b). The plasmon shift to longer wavelengths and a higher absorption of the red tail of the spectrum indicate the presence of bigger secondary particles in the solution (Figure 4c). Variations of temperature in the liquid may cause changes in the colloidal state of gold nanoparticles in water. This might be the origin of the different hydrodynamic particle size distributions of the generated colloids. To analyze whether the effect on the particle size is produced after or during nanoparticle generation, the hydrodynamic diameter and optical properties of a laser-generated aqueous gold colloid was measured while heating and cooling the prepared colloid. It can be seen in Figure 5 that a reversible change is produced in the hydrodynamic diameter of the laser-generated nanoparticles when the colloid is heated to 353 K and cooled to 283 K. No irreversible change in the hydrodynamic diameter, as strong aggregation, agglomeration, or particle growth, is induced by modifications in the temperature of the liquid containing laser-generated gold nanoparticles. The plasmon absorption band also remains constant during the heating and cooling of the sample (see Figure S4 in the Supporting Information). This confirms that the initial colloid is hydrodynamically stable against temperature changes in the range of 283-353 K and that the phenomenon observed is a result of temperature impact on the formation process of the nanoparticles.

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Mene´ndez-Manjo´n et al. exponential decay trend is observed (Figure 7). Low compressible states of water (occurring at high and low temperatures) produce smaller volumes into which the matter within the cavitation bubble expands, resulting in a higher collision probability and bigger nanoparticles than when the ambient medium is more compressible. The decrease in the plasmon band absorption wavelength (inset in Figure 7) confirms the variation of hydrodynamic particle size observed by dynamic light scattering. Compressibility of water reaches a minimum at 330 K, and at this temperature, a maximum in the hydrodynamic diameter of the particles is found. Discussion

Figure 5. Variation of the hydrodynamic diameter of gold nanoparticles in water while heating and cooling the sample after laser generation of the colloid.

Compagnini et al.22 showed that viscosity affects the aspect ratio of elongated gold nanoparticles generated in liquid alkanes at high laser fluences, arguing that the laser-induced plume expands in the liquid environment. Further influence of physical properties, such as density or compressibility, has not been considered because of the lower variation with the carbon atoms in the chain of the alkanes considered.23 Hence, if the particle formation process would take place mainly in the surrounding cold liquid matrix, the density or viscosity of the water in which the particles propagate and agglomerate should affect the final hydrodynamic size distribution. The formation and growth of particles in the liquid phase is a diffusion-limited reaction,24 and therefore, the reaction rate constant is proportional to the temperature and inversely proportional to the viscosity. To analyze such a possible correlation, the hydrodynamic diameter is represented as function of the diffusion coefficient and viscosity of the water in Figure 6a and b. The Lifshitz-SlyozovWagner (LSW) theory25,26 implies that fast reactions will lead to bigger particles and the final particle size is related to the viscosity trough the reaction constant k ) AT/η for diffusionlimited reactions, as follows:

T R3 ) R03 + A t η where T is the Temperature, η is the dynamic viscosity, A is a constant, t is the growth time, and R0 is the initial radius. Such a relation can be fitted to the experimental results in Figure 6; however, the correlation is quite inaccurate, and the maxima reached at 330 K (corresponding viscosity 0.48 mPa s) cannot be explained by this viscosity-correlated approximation. This indicates that different liquid properties and formation mechanisms may be more accurate to describe the effect of temperature on the hydrodynamic particle size. Several authors have pointed out that particles are already formed in a vaporized volume close to the ablated spot33,32 and not in the surrounding liquid. The ejected matter is trapped within the first 300 µs in the laser-induced cavitation bubble and cools as the bubble expands.17 After its collapse, the matter may be already condensed in clusters and is released into the surrounding liquid. In this case, the effect of the viscosity or diffusion into the cold liquid matrix should not be significant for the particle formation, but should be for parameters of nucleation inside the cavitation bubble, such as temperature, density, or pressure. When the hydrodynamic diameter of the nanoparticle population obtained at different temperatures is plotted versus the corresponding compressibility of water, an

Liquids are almost incompressible. In the case of water, compressibility ranges over 4 × 10-10 Pa-1, requiring high pressures to observe an effect of compressibility variations. High energetic states are present inside the cavitation bubble formed above the ablation pool in the target. The thermodynamic conditions reached during ablation of solid targets in liquids are much more critical than ablation in gases or vacuum due to the strong confinement of the plasma plume by the liquid.27 Material is ejected from the target at about 50 ns after the laser pulse in the form of atoms and ions, forming a plasma that lasts around 10 µs. Fast pressure gradients induce the formation of a cavitation bubble 1 µs after the pulse.17,33 This bubble expands, shrinks, and collapses in a time scale of typically 300 µs,17,33 releasing the ablated matter inside and eventually supporting a secondary mechanical ablation. It has been observed via emission spectroscopy that material ablated from the target is trapped inside the cavitation bubble.28 Double pulse experiments showed that a second delayed pulse ablates more matter, which is trapped inside the cavitation bubble induced by the first pulse.29 The density inside the bubble increases considerably at high repetition rates and high pulse overlap (that we have used in our experiments). The formation of primary particles and agglomerates should be initiated inside this compressed volume if the characteristic nucleation time is shorter than the bubble lifetime. Wang et al.30 proposed a theoretical kinetic approach to nucleation and growth of nanocrystals formed by laser ablation in liquids. In the particular case of ablation of a graphite target in water, they found nucleation times of 10-10-10-9 s. This time range is 3 orders of magnitude shorter than the cavitation bubble lifetime (µs), giving enough time for nucleation, growth, and aggregation to bigger hydrodynamic sizes. The expansion of the bubble in the liquid leads to fast cooling of the material inside. Cavitation dynamics will determine the volume and time for nucleation inside the vaporized medium. Collision theory31 gives an exponential relation for the collision probability, p, and element density (atoms, ions, particles, clusters, etc.). Considering that the ablated mass per pulse is confined inside the bubble and that, because the same fluence was applied, the same mass is ablated per pulse, the density inside the cavitation bubble thus depends on only its total volume. At the same fluence, the final volume of the bubble is determined by the compressibility, β, of the liquid. Thus, a relation can be described between the particle collision probability inside the cavitation bubble and compressibility of the water,

p)A

where A is a constant.

e-A/1-β 1-β

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Figure 6. Hydrodynamic diameter of gold nanoparticles generated by laser ablation in water at different (a) diffusion coefficients and (b) viscosities of water. In the insets, the corresponding plasmon resonance peak wavelength is shown.

Conclusions

Figure 7. Hydrodynamic diameter of gold nanoparticles generated by laser ablation in water at different compressible states of water.

Assuming that high collision probability leads to bigger nanoparticles and a higher number of agglomerates, the exponential decay of the hydrodynamic diameter with the compressibility (Figure 7) can be justified. First implications of laser-induced cavitation dynamics on ablation efficiency and particle properties were investigated recently.32,33 Manipulation of the cavitation bubble dynamics was already induced by Sasaki et al.32 while ablating in pressurized water. They observed a bubble radius reduction of 75% for ablation in water at 106 Pa as compared to water at ambient atmosphere. It is clear that the physical conditions of the surrounding media affects the properties and behavior of the cavitation bubble and not only laser parameters such as pulse duration or pulse energy. Assuming that a high percentage of the ablated matter is confined in the cavitation bubble, it is plausible that variations in the cavitation dynamics and bubble size will affect the nanoparticle formation and aggregation process. If the ablated matter can expand in a bigger volume (high compressibility), the particle growth is depressed, and the final particle size is determined by the critical nucleation radius and growth process time. These factors are related to the material and its thermodynamic state and the lifetime of cavitation bubble. The pressure reached in the cavitation is on the order of 10 GPa.34 Accordingly, a change in the compressibility in the range of the experiments would theoretically produce at least a 16% change in the volume of the cavitation bubble and, therefore, the density of ablated material in it. Because compressibility of water varies very rapidly between 273 and 313 K, a small oscillation in the temperature may produce significant changes in the collision probability and the final hydrodynamic particle size.

Variation in water temperature during laser ablation affects the quality of the colloid in terms of resulting hydrodynamic nanoparticle size distribution. The hydrodynamic diameter is an important parameter of nanoparticle dispersions that are applied in liquid phase, such as colorimetric gold nanoparticle assays based on nanoparticle bioconjugates35,36 or unmodified gold nanoparticles.37 Laser-generated nanoparticles start to form by nucleation of the ablated matter inside the cavitation bubble, with collisions leading to the formation of agglomerates. Properties of the cavitation, such as volume, expansion velocity, and lifetime, may affect the nucleation and growth mechanism of the particles, determining final hydrodynamic particle size. The cavitation dynamics are influenced not only by laser parameters but also by physical properties of the fluid, such as surface tension, viscosity, or compressibility. We found the best correlation for an exponential decay of hydrodynamic nanoparticle diameter with the compressibility of water, which varies with the solvent temperature. Acknowledgment. We thank the German Research Foundation for financial support within the project DFG CH197-8 and the excellence cluster REBIRTH, as well as the ZFM - Center for Solid State Chemistry and New Materials, Leibniz University Hannover, Hannover, Germany. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442–453. (2) Nirmala Grace, A.; Pandian, K. Electrochem. Commun. 2006, 8, 1340–1348. (3) Bogdanovi, B.; Felderhoff, M.; Kaskel, S.; Pommerin, A.; Schlichte, K.; Schu¨th, F. AdV. Mater. 2003, 15, 1012–1015. (4) Barcikowski, S.; Hahn, A.; Kabashin, A. V.; Chichkov, B. N. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 47–55. (5) Barcikowski, S.; Mene´ndez-Manjo´n, A.; Chichkov, B. N.; Marijus Brikas, M.; Racıˇukaitis, G. Appl. Phys. Lett. 2007, 91 (8), 3–5. (6) Sylvestre, J.-P.; Kabashin, A. V.; Sacher, E.; Meunier, M. Appl. Phys. A, 2005, 80, 753–758. (7) Ba¨rsch, N.; Jakobi, J.; Weiler, S.; Barcikowski, S. Nanotechnology 2009, 20, 445603. (8) Ganeev, R. A.; Chakravarty, U.; Naik, P. A.; Srivastava, H.; Mukherjee, C.; Tiwari, M. K.; Nandedkar, R. V.; Gupta, P. D. Appl. Opt. 2007, 46, 1205–1210. (9) Mornet, S.; Vasseur, S.; Grasset, F.; Veverka, P.; Goglio, G.; Demourgues, A.; Portier, J.; Pollert, E.; Duguet, E. AdV. Funct. Mater. 2006, 34, 237–247.

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