Gram Scale Synthesis of Pure Ceramic Nanoparticles by Laser

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J. Phys. Chem. C 2010, 114, 2421–2427

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Gram Scale Synthesis of Pure Ceramic Nanoparticles by Laser Ablation in Liquid Csaba La´szlo´ Sajti,† Ramin Sattari, Boris N. Chichkov, and Stephan Barcikowski* Laser Zentrum HannoVer e.V., Hollerithallee 8, 30419 HannoVer, Germany ReceiVed: July 22, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

Scale-up criteria of laser ablation in the liquid phase with nanosecond pulses is studied for efficient generation of pure ceramic nanoparticles in an aqueous environment. Besides high laser fluence and low height of the applied liquid layer, specific pulse overlap and defined laser repetition rate are required for significant enhancement in nanoparticle productivity. The ablation rate increases by 350% by reducing the liquid film from 8 mm to 2.5 mm owing to reduced absorption and scattering of the incident laser beam by previously ablated nanoparticles. The controlled interpulse distance yields a further increase in material removal rate by another 300% compared to machining in the pulse overlap mode. The residual cavitation bubble from the previous laser pulse and the dispersed nanoparticle interaction with the following laser pulse and optimized temperature gradient in the lattice of the target are assumed to alter productivity. This hypothesis is confirmed by varying the repetition rate with equal laser fluence and pulse overlap, which causes a drastic rise in nanoparticle productivity by a factor of 65. A maximum corundum nanoparticle productivity of 1.3 g/h with Feret particle size of 30 nm is gained by 18.5 W of focused laser power at 4 kHz of repetition rate, providing 125 µm interpulse distance and liquid flow. Introduction Generation of a high amount of colloidal nanoparticles (metals, ceramics, semiconductors, etc.) is well established by conventional methods such as wet chemical synthesis or mechanical milling and grinding.1-3 In terms of purity, these techniques often represent severe limitations. Sol-gel or precipitation procedures require chemical precursors, surfactants or reducing agents,4 while during milling and grinding contaminations might occur by the employed abrasive tools, hence application prospects can be restricted. In the particular case of hard ceramics like Al2O3, aqueous chemical synthesis of nanostructures may trigger lower hardness of particles due to the high number of hydroxy groups introduced into the lattice leading to the significantly less hard AlO(OH) structure. Recently, laser ablation in liquid (LAL) environments has attracted more and more attention, enabling the generation of nanoparticle colloids of a great variety of materials with outstanding purity.5 This versatile physical preparation method provides size and stability control of nanoparticles under rigorous control of laser and process parameters, surfactants, and nanoparticle postprocessing.6 Taking advantage of simplicity and key advantages of the technique, a vast diversity of functional materials has been prepared by LAL including metal nanoparticles,6-9 semiconductors,10,11 ceramics and even alloys,12 or bioconjugates.13 But usually properties and functions of lasergenerated nanoparticles are in scope and not productivity of LAL, even if this factor has a major aspect on implementation of a method to industrial scale manufacturing. Hence only a few publications mention production related data. Ablation by femtosecond laser pulses is accompanied by significantly reduced thermal effects though at the expense of productivity. Maximum material removal rates of only 3.2 and 4.1 µg/s (11.5 and 14.7 mg/h) are achieved by femtosecond laser machining

of Al2O3 and yttria-stabilized ZrO2 ceramic plates in air, respectively.14,15 Working in dense environments such as aqueous media results in a less efficient ablation process than the same ablation in air atmosphere or under vacuum conditions.16 Ablating gold and silver in water by ultrashort laser pulses gains nanoparticle productivity of 8.3 and 7.9 mg/h, respectively.17,18 In the picosecond time domain, maximum productivity increases because of higher available laser power and reaches 8.6 µg/s (31 mg/h) for silver in water surrounding18 despite that ablation efficiency is higher for femtosecond pulses (2 µg/J for femtosecond irradiation compared to 1.5 µg/J for picosecond ablation). Employing nanosecond lasers reveals that the ablation rate further increases under rigorous control of scan speed (pulse overlap) leading to a zirconia mass ablation rate of 195 µg/s (700 mg/h) in air, reported by Wang et al.19 Abdolvand et al. reported on continuous wave LAL of titanium in water resulting in 1000-1500 mg/h of nanoparticle productivity applying 250 W laser power.20 Unfortunately, this value was derived from the target mass loss after an extremely short irradiation time interval of a few seconds followed by linear extrapolation to 1 h (multiplying the measured ablated mass of 400 µg/s by 3600) not considering any laser-nanoparticle interactions or concentration effects. In this work we report on laser ablation in a liquid of one of the hardest ceramics to evaluate significant influences on nanoparticle productivity by laser and process parameters, whereas control of particle size, size distribution, morphology, and crystal structure are also in focus. In particular, we demonstrate the importance of cost-effective (laser fluence) and non-cost-effective process parameters such as material adapted laser frequency, controlled pulse overlap, and the role of liquid layer and fluid dynamics on nanoparticle productivity and size distribution of primary nanoparticulate ceramics. Experimental Section

* Corresponding author. E-mail: [email protected]. Phone: +49 511 2788 377. Fax: +49 511 2788 100. † E-mail: [email protected].

Laser ablation was carried out with a commercial Q-switched Nd:YLF laser (Trumpf 20-1 FQ) that provided 20-60 ns full

10.1021/jp906960g  2010 American Chemical Society Published on Web 01/25/2010

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Figure 1. (a) Al2O3 nanoparticle productivity as a function of the applied laser pulse energy. A fixed 6 mm liquid layer and 5 kHz laser repetition rate were utilized. (b) Calculated ablation efficiencies as a function of laser pulse energy. Extrapolated ablation threshold fluence (Fth) and critical fluence (Fcrit) values are also mentioned.

width at half-maximum (fwhm) pulses at a central wavelength of 1047 nm (maximum pulse energy 4.6 mJ, repetition rate 4-15 kHz). The irradiated corundum target (Deranox 975, Morgan Technical Ceramics) was prepared by hot isostatic pressing of R-Al2O3 powder with a purity of 97.5% and grain size of 5 µm. The beam was focused by a laser scanner (Scanlab, HurryScan II-14) through a telecentric 58 mm focal length F-θ lens (Sill Optics, S4LFT0058/126). All ablation experiments were performed at room temperature at atmospheric pressure in a distilled water environment; LAL experiments were similarly performed with the only modification being the laser beam was coupled horizontally into the process chamber (filled with 65 mL distilled water) through an antireflex coated sapphire window. The applied scan speed varied from 50 to 1400 mm/s over a fixed 8 mm × 8 mm multiline ablation pattern, providing 50 µm of interline distance. The position of the ceramic sample and the process chamber were separately controlled with two microcontrollers. This way allowed precise control of the liquid layer between the window and target, and control of laser fluence in parallel. Controlled liquid flow on the target surface was generated by a flexible tube pump (1 mm tube diameter) through tube connections at the side walls of the chamber. The utilized flow chamber yielded a stable and highly reproducible LAL process with variation in absolute material mass removal values of less than 3%. Accordingly, in previous studies it was shown that specific liquid flow allows elevated ablation process stability.18 Absolute values of nanoparticle productivity were measured by differential weighting of the target by a precision microbalance (Sartorius M3P) via the estimation that no material was attached on the process chamber wall or on the inner side of the tubes. Throughout the present study, 5 min of irradiation time was chosen after ensuring that no significant discrepancy from linearity is involved, comparing the amount of ablated material after 5, 10, 15, and 30 min of ablation time. Size measurements were performed by dynamic light scattering (DLS, Zetasizer ZS Malvern Instruments) and by transmission electron microscopy (TEM, EM 10C Zeiss), while the crystal structure of nanoceramics was investigated by X-ray diffractometry (XRD, Stadi P Stoe with Cu KR1 rays). Results and Discussion We assume that high reproducibility of the ablation rate (about 98%) was achieved due to axial separation of the laser beam and generated air bubbles, enabling higher ablation process

stability. Moreover, transmitting the laser beam through an entrance media allows accurate control of the liquid layer, likewise allowing elimination of the liquid meniscus, which considerably reduces inaccuracy in focalization. Laser Fluence. In good accordance with the literature, the mechanism of ablation, hence the productivity, strongly depends on the laser energy deposited into the lattice of the target material. Figure 1a demonstrates the influence of the laser pulse energy on nanoparticle productivity using a 6 mm liquid layer and a laser focal spot 80 µm in diameter. In the range 0.7-3 mJ (14-50 J/cm2 of laser fluence) nanoparticle productivity increased almost linearly with pulse energy. Exciting nonlinear phenomena seem to appear when pulse energies higher than 3.55 mJ (laser fluence of 71 J/cm2) are employed. The productivity increases no more linearly with pulse energy, showing rising quadratic dependence. Using a 3.9 mJ pulse energy at a 5 kHz laser repetition rate, 445 mg/h nanoparticle productivity was reached, providing a 6 mm liquid layer and a 120 mm/s scan speed. Calculating ablation efficiencies from Figure 1a clearly reveals several ablation regimes, presented in Figure 1b. According to the literature, increasing the laser fluence above the threshold fluence leads to first the linear ablation domain followed by the saturation domain appearing in the low-middle fluence regime.21-23 We presume that normal vaporization is the major material removal process in this regime. Extrapolation of the linear part of the curve denotes the threshold fluence of Al2O3 of 14.5 J/cm2. Exceeding 2 mJ focused pulse energy, the ablation efficiency reaches the saturation plateau, clearly indicating that in this regime the same ablation mechanism takes place. Regarding the saturation plateau in more detail revealed a slight decrease in ablation efficiency with increasing laser fluence, indicating that absorption and scattering of the laser beam on previously ablated nanoparticles becomes more and more significant in the high fluence regime. Considering 20 ns of laser pulse duration and the pulse energy of 3.9 mJ results in a laser intensity of 3.9 × 109 W cm-2. Referring to the literature, laserinduced breakdown by focused nanosecond laser irradiation in water-confined media occurs when the laser intensity exceeds 4 × 1011 W cm-2, using 1064 nm laser irradiation and 6 ns laser pulse duration.24 We thus assume that in the pulse energy regime investigated, laser-induced breakdown cannot play an important role or limit the actual material removal and nanoparticle generation processes.

Gram Scale Synthesis of Pure Ceramic Nanoparticles

Figure 2. Al2O3 nanoparticle production rate in distilled water carried out in various thicknesses of the water layer. A fixed laser pulse energy of 3.8 mJ at 5 kHz repetition rate and 120 mm/s scan speed were employed.

Performing LAL with a pulse energy higher than 3.5 mJ points out the early stage of enhanced ablation efficiency, indicating the change in ablation mechanism. Phase explosion might be the responsible mechanism for such a violent increase in ablation efficiency.23 However, influence of the liquid surrounding should not be excluded. Collapse of the laserinduced cavitated vapor bubble might also generate a powerful shockwave, provoking alterations in production efficiencies. Further investigations are necessary for better understanding such an increase in nanoparticle production efficiency in the high fluence regime during LAL. Liquid Layer. The thickness of the applied liquid layer is the second major parameter determining nanoparticle productivity. Figure 2 shows the influence of the water layer on nanoparticle productivity using a fixed laser pulse energy of 3.8 mJ with a 5 kHz laser repetition rate, 120 mm/s scan speed, and 80 µm focal diameter. A 116 mL/min liquid flow was utilized throughout these experiments. Using 8 and 2.5 mm liquid layers above the target surface resulted in nanoparticle production rates of 172 and 592 mg/h, respectively. It is clearly observed that adequate selection of the liquid layer can lead to an enhancement in productivity by 350% using equal laser parameters. Increased absorption and scattering of laser irradiation on previously ablated nanoparticles is assumed to occur with such a diminished ablation rate in the large liquid layer regime. We presume further reduction in the liquid layer might enable additional enhancement in the ablation rate; nevertheless a liquid layer smaller than 2.5 mm in our experiments overcame the intensity threshold of the entrance window and damaged it. Besides absorption/scattering effects, Zhu et al. reported that ablation in a water-confined environment is also affected by the generated high-temperature/pressure plasma expanding adiabatically at supersonic velocity, triggering shockwave formation in the surroundings.25 They assumed that the material removal rate is highly enhanced when a specific water layer (1.1 mm using Si target and pulsed KrF laser) is adjusted above the target surface due to the formation of optimal high-pressure and high temperature plasma strongly depending on laser parameters and process conditions. Liquid Flow. According to our study, the flow rate of the applied liquid media has an impact on productivity due to elimination of ablated particles and generated air bubbles from the ablation zone. The increase in flow rate from the stationary

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2423 liquid to 190 mL/min using equal laser parameters enhances the nanoparticle productivity by 20%. A further rise in liquid flow rate did not lead to a higher ablation rate. Without liquid circulation, the ablated nanomaterials disperse into the entire liquid volume only with slow diffusion and Brownian motion. Thus after each ablation sequence a dense particle cloud is ejected, usually characterized by a relatively long residual period in front of the target, leading to significant absorption of the subsequent laser beam. We assume that the use of liquid flow permits the removal of processed gas bubbles from the ablation zone as well as leads to rapid dispersion of nanoparticles into the whole liquid volume. A liquid flow of 190 mL/min permits the effective dispersion of ablated species and removes gas bubbles at the fluence regime studied. Every further measurement was carried out with this specific liquid flow rate. However, we have to note that the optimal flow rate may alter with the chamber form and dimensions as well as laser fluence and repetition rate. Pulse Overlap. Inspired from the work of Wang et al.19 and Barcikowski et al.16 influence of pulse overlap on ceramic nanoparticle productivity was deeply investigated. Both reference studies concluded that the interpulse distance has a crucial effect on the material removal rate. Amplified productivity in the case of laser ablation in a liquid is related to increased radiation absorbed by the material due to increased beam absorption in the premachined spot area, whereas in nanosecond ablation thermal effects play an important role. Utilization of the laser scanner allows precise variation and monitoring of pulse overlap controlled over the range 50-1400 mm/s in this experiment. Figure 3 depicts Al2O3 nanoparticle productivity as a function of scan speed and interpulse distance using a 4 mm liquid layer and 4.6 mJ pulse energy at 4 kHz repetition rate using a laser focal spot of 50 µm. Calculated pulse overlaps in percentage are also mentioned in the figure. The term interpulse distance defines the distance between two pulses, from one pulse center to the center of the neighboring pulse. As expected, the material removal rate, thus nanoparticle productivity, strongly depends on scan speed owing to different pulse overlaps involved. A lower scan speed results in a higher overlap and therefore results in a higher local temperature. It was clearly observed that the interpulse distance between 75 and 250 µm allows higher particle production rates than machining in the pulse overlap mode. Adjusting the scan speed to 500 mm/s provides an optimal interpulse distance of 125 µm for the utilized repetition rate of 4 kHz. In this case a maximum nanoparticle productivity of 1265 mg/h [efficiency ) 0.08 g/(J · s)] was determined, which is about 300% higher than an nonoptimized scan speed of, e.g., 100 mm/s. In the first range of the curve in Figure 3 the pulse overlap changes from 68.5% to 0% (scan speed ) 50 to 200 mm/s). Using a scan speed higher than 200 mm/s involves no pulse overlap. Nanoparticle productivity increases constantly in the scan speed range 50-500 mm/s from 402 to 1265 mg/h. However, when the 500 mm/s scan speed is exceeded, this tendency reverses and the ablation rate continuously decreases. We assume that two competitive effects tend to affect nanoparticulate production efficiency in the case of ceramics. First, in the high overlap region, interaction of the laser beam with the previously ablated nanoparticles and the previously generated cavitation bubble could be the principal mechanism preventing the material from higher ablation. Referring to the literature, laser ablation in liquids generates a cavitation gas bubble26-28 that lasts around 300 µs using Nd:YAG nanosecond laser irradiation of 36 J/cm2 laser fluence.27,28 This cavitation bubble

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Figure 3. (Left) productivity of Al2O3 nanoparticle as a function of interpulse distance and scan speed using a fixed laser pulse energy of 4.6 mJ at 4 kHz repetition rate and a 4 mm liquid layer. Calculated laser pulse overlaps (PO) are also shown in percentage. (Right) simplified scheme of the processes taking place under laser ablation of a target material in a liquid, demonstrating the impact of the cavitation bubble and heat affected zone on subsequent laser irradiation.

contains primary nanoparticles of extremely high local concentration that can scatter, reflect, or absorb the subsequent laser pulse. This absorption and scattering effect decreases with increasing interpulse distance due to the limited size of the cavitation bubble and leads theoretically to a final saturation plateau in the ablation rate. Accordingly, we observed a rapid enhancement in nanoparticle productivity for the scan speed increasing to 500 mm/s. On the other hand, an increasing interpulse distance with equal laser parameters results in a significant variation in temperature gradient deposited into the lattice, as investigations of laser-induced stress and amorphization of irradiated silicon wafers concluded.29 The heat of evaporation decreases with increasing lattice temperature due to energy input supplied by the incident laser irradiation, whereas increasing temperature (heat accumulation) in the target lattice at constant density supports phase explosion.21,22 We assume that the temperature gradient in the ceramic lattice may also have an influence on absorptivity; hence when the favorable scan speed is exceeded, insufficient overlap leads to thermally isolated ablation areas, resulting in less effective material removal, and thus nanoparticle productivity. Laser Repetition Rate. This hypothesis of cavitation bubble and heat accumulation affected laser ablation was confirmed by changing the laser repetition rate possessing equal interpulse distances and laser fluence, plotted in Figure 4 for a constant pulse energy of 2.4 mJ (laser fluence of 123 J/cm2 and pulse duration of 20 to 35 ns fwhm). Investigations revealed an extensive decrease in nanoparticle production rate when the pulse frequency was increased from 4 to 9 kHz. Using 4 and 9 kHz laser frequency, 7.6 and 1 mg/min ceramic ablation rates were reached, respectively. Investigations were confirmed with another nanosecond laser system (Rofin Sinar, RS-Marker 100D, 1064 nm, 40-55 ns fwhm) where a similar exponential drop in productivity was observed for the frequency range 0.5-20 kHz using 3.3 mJ pulse energy, providing a 6 mm liquid layer and 125 µm interpulse distance (see Figure 4 inset). Investigations revealed an enhancement factor of 65 in material removal rate when the laser frequency was decreased from 20 to 0.5 kHz, resulting in 0.26 and 17 ng/pulse,

Figure 4. Influence of the laser repetition rate on nanoparticle productivity providing same pulse overlap of about 40%. A pulse energy of 2.4 mJ and liquid layer of 4 mm were used. Presented fit represents a monoexponential function. The inset image shows a similar measurement obtained with a 1064 nm Nd:YAG laser with 3.3 mJ pulse energy and 40-55 ns fwhm pulse duration using a fixed interpulse distance of 125 µm.

respectively. The maximum ablation rate achieved a value of 17 ng/pulse at 2 kHz repetition rate and did not further increase with reduced laser repetition rate. Referring to the literature, nanosecond laser ablation initiates a complex sequence of events occurring both during and after the ablating laser pulse. After the first laser photons absorbed on the target surface in the subpicosecond time domain, surface melting and vaporization of the upper layer starts to develop, generating the rapid temperature rise of the target lattice after a few hundred picoseconds.30 When the temperature of the melt reaches the critical temperature, an explosive phase change develops and mass expulsion in both liquid and vapor phase occur in the nanosecond time scale, generating a broad continuous emission spectrum attributed to bremsstrahlung, electron-ion recombination, and blackbody radiation with some discrete peaks related to transition of target atoms in the plume.31 This

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Figure 5. (a) Hydrodynamic size distribution of laser-generated Al2O3 nanoparticles using 202 J/cm2 laser fluence at 5 kHz laser repetition rate and a 6 mm liquid layer. (b) TEM micrograph image and corresponding Feret size distribution of laser-generated Al2O3 nanoparticles.

emission is combined with the generation of a first shockwave propagating with the estimated speed of 2.6 km/s and having an estimated pressure of 1200 MPa.27 Nanoparticle formation starts in the submicrosecond scale during plume expansion characterized by intensive heat transfer at the plume liquid boundary. Due to cooling that can occur more rapidly than condensation, exceptionally high saturation ratios are achieved, thus even at high laser fluences relatively small nanoparticles can be generated by laser ablation in liquids. For nanoparticle production efficiency, the subsequent events are even more critical, since in the time scale of a few microseconds a cavitation bubble is forming and growing on the target surface due the local heating of the solvent in the vicinity of the ablated spot. In the highly confined region of the cavitation bubble a large amount of primary nanoparticles is formed and then ejected when the bubble collapses in the time scale of 200-300 µs27,28 (or even later depending on utilized pulse energy), generating a secondary weaker shockwave. During the lifetime period of the cavitation bubble, the target cannot be ablated effectively, as such a bubble might act as a mirror and scatter, reflect or absorb the incident laser beam. As the inset of Figure 4 shows, a maximum nanoparticle productivity of 17 ng/pulse is gained at the specific laser repetition rate of 2 kHz, which is equal to 500 µs time delay between two laser pulses. This particular time delay is exactly in the order of the cavitation bubble lifetime. It is thus likely that, when the time delay between two laser pulses is high enough (2 kHz or lower), the cavitation bubble collapses before the subsequent laser pulse impacts; thus the ablation efficiency and nanoparticle production rate are not affected, resulting in a saturation plateau in absolute productivity. However, we note that in the case of short time interval between two pulses the ejected material could also interfere with the following laser pulse as described above. Particle Size. The laser-generated particle size, size distribution, and morphology are key parameters that determine the applicability of synthesized nanoparticles. DLS measurements, shown in Figure 5a, revealed that LAL of Al2O3 using optimal scan speed, repetition rate, liquid flow, and a high laser fluence of 202 J/cm2 give rise to medium hydrodynamic particles of size 77 nm (log normal fit) with a dispersion of 115 nm fwhm. Figure 5a represents also percentile 50 (d50) and percentile 90 (d90) likewise calculated particle mass concentrations. The TEM micrographs in Figure 5b confirmed the presence of a great majority of small spherical nanoparticles and only a

few irregular-shaped (probably) cracked particles (not presented in Figure 5b) independently from the laser fluence used. TEM micrographs reveal smaller nanoparticles (Feret diameter) than DLS measurements (hydrodynamic diameter) as the DLS technique is affected more by polydispersity and molecule solvatization around the nanoparticle. This tendency did not change even if the colloids were diluted, filtered with 0.45 µm pore size filter (Whatman, cellulose acetate), or centrifuged before measurements. From the DLS correlation function, investigations assumed no significant agglomeration, as it was also confirmed by TEM. We thus presume that no particle aggregates resulted in the slightly bigger size obtained by light scattering measurements and it is due to a solvent molecule present on the particle surface. The TEM size distribution revealed an average particle size of 30 nm with 29 nm fwhm. Figure 5a,b shows that nanoparticle productivity on the order of 1 g/h by LAL is associated with the generation of nearly monomodal nanoparticles. However, particle size and morphology will be further investigated in detail, since laser-induced fragmentation may take place, or can be introduced during nanoparticle synthesis, and yield even a smaller particle size with a narrower size distribution, reported for metal nanoparticles32 that have a much higher absorption cross-section than alumina. Besides particle size and morphology, the crystal structure of laser-ablated nanoceramics was investigated, as the stoichiometry might indicate ablation procedures, plasma temperatures, and cooling rates involved. Crystal Structure. The crystal structure of the laser-generated nanoparticles was investigated in a glass capillary. Diffraction patterns of the solid R-Al2O3 target and the synthesized Al2O3 nanoparticles at 2θ angle are shown in Figure 6. It was clearly proved that laser generated nanoparticles have identical trigonal Bravais lattice structure as the solid target, corresponding to corundum or alpha phase aluminum oxide with spacegroup R3cj. This is not surprising, since different metastable Al2O3 phases tend to undergo a phase transition around 1000 K33 and result in the thermodynamically most stable corundum, R-Al2O3, lattice structure. The maximum temperature in nanosecond laserinduced plasma is generally on the order of 10 000 K,34 hence hot enough to trigger such a phase transition. However, in the nanometer scale, conventional synthesis of nanocrystalline Al2O3 often results in γ-Al2O3. Referring to the literature based on molecular dynamics simulations and thermochemical data, McHale et al.36,37 predicted that γ-Al2O3 should become the energetically stable

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Sajti et al. the ζ-potential is directly related to the stability of particles in the suspension. Laser-fabricated ceramic-based nanoparticle colloids flocculated in the time regime of a few hours without adding any stabilizing agent (prior to laser ablation), whereas with electrostatic stabilization using nitric acid, the particle sedimentation time increased into the time range of about 10 days. Conclusion

Figure 6. Crystal structure: X-ray diffraction pattern of Al2O3 nanoparticles synthesized by laser ablation in a liquid and the reference Al2O3 spectrum35 indicating the R-Al2O3 phase of the nanoparticles with the corresponding R3cj space group.

crystal structure for a specific particle surface area exceeding about 125 m2 g-1 at room temperature. At 800 K (a temperature typical of oxyhydroxide decomposition) γ-Al2O3 might become thermodynamically stable when the generated particles have specific surface areas greater than 75 m2 g-1. We thus estimated the total surface area of laser generated Al2O3 nanoparticles, using the approximation that all nanoparticles have the 30 nm Feret diameter. A single spherical particle of 30 nm possesses an effective surface of Ap ) 2.8 × 10-15 m2, and single particle volume of Vp ) 1.4 × 10-17 cm3. From these values the total surface of ablated particles can be estimate using the following equation:

ATot )

MAbl ·A Vp · F p

where MAbl is the total ablated mass and F ) 3.97 g cm-3 is the density of corundum. The term Mabl/(VpF) is the estimation of the total number of particles in the solution. Calculating with 30 nm particle size and 1 g ablated target material provides a total particle surface of ATot ) 50 m2 g-1. This means that lasergenerated nanoparticles have the R-phase, corundum, lattice structure not just because they are generated from hot plasma but because the process also provides a specific particle surface area at which the corundum phase is thermodynamically the most favorable. The origin of the minor reflections in the XRD pattern is under investigation since it is likely that they are not corresponding to Al2O3 diffraction peaks. However, we estimated the nanoparticle size from the peak broadenings of the XRD spectra using the Scherrer equation. All major peaks (identified as the R corundum) are related to the particle size of about 53 nm with very small dispersion. The few nonidentified peaks are slightly broadened and give a particle size of 33 nm. Both sizes are in the size range identified by TEM and DLS. Stabilization. Nanoparticle stabilization was not the aim of this study; nevertheless, we should mention that Al2O3 nanoparticles in distilled water media are not stable: they flocculate and sedimentation occurs with time. Electrostatic stabilization of nanoparticles was easily achieved by adjusting (and buffering during ablation) the pH of the liquid by nitric acid prior to laser ablation. Using a pH value of 3 led to the maximum stability of nanoparticles gaining a ζ-potential of -20 to -40 mV, where

Laser ablation in aqueous media is represented as a powerful, full physical method to synthesize pure colloidal ceramic nanoparticles in a highly controlled manner. Deriving ideal ablation conditions, significant enhancement in material removal rate is achieved, leading to nanoparticle productivity of several factors higher than published before by laser ablation in liquids. Reducing the utilized liquid layer resulted in a 350% increase in productivity due to the reduced absorption and/or scattering of laser irradiation by previously ablated suspended nanoparticles. Combination of specific interpulse distance and laser repetition rate gave rise to a further increase in productivity of factors 3 and 65, respectively, whereas previously generated cavitation bubble and dispersed nanoparticle interaction with the following laser pulse and optimized temperature gradient in the lattice of the target are assumed to alter nanoparticle productivity. Under rigorous combination of favorable laser and process parameters, nanoparticle productivity of 1.3 g/h was achieved with an average Feret particle size of 30 nm and defined corundum crystal structure, using a 4.6 mJ pulse energy at a 4 kHz repetition rate, 4 mm liquid layer, 190 mL/min of liquid flow, and adjusting the 125 µm distance separation between neighboring laser pulse centers. Reaching a gram scale nanoparticle production rate with only 18.5 W laser power allows us to envisage potential mass production by laser ablation as a laser power of 720 W with 38 ns pulse duration is already commercially available, providing the same range of repetition rate as studied here. Acknowledgment. We gratefully acknowledge the help of Kerstin Rohn (Institut fu¨r Pathologie Tiera¨rztliche Hochschule Hannover) for the transmission electron microscopy measurements and Dr. Michael Wiebcke (Institut fu¨r Anorganische Chemie Leibniz Universita¨t Hannover) for the X-ray diffraction measurements. The work was funded by the German Federal Ministry of Education and Research (BMBF) and supported by Forschungszentrum Karlsruhe GmbH (PTKA-PFT) in the framework of the research project NANO-PART and by the Deutsche Forschungsgemeinschaft (CH 179/9-1). References and Notes (1) Vasylkiv, O.; Sakka, Y. J. Am. Ceram. Soc. 2001, 84, 2489. (2) Vasylkiv, O.; Sakka, Y.; Skorokhod, V. V. Powder Metall. Met. Ceram. 2005, 44, 228. (3) Mende, S.; Stenger, F.; Peukert, W.; Schwedes, J. J. Mater. Sci. 2004, 39, 5223. (4) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998, 160. (5) Dahl, J. A; Maddux, B. L. S.; Hutchison, J. E. Chem. ReV. 2007, 107, 2228. (6) Mafune´, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2001, 105, 5114. (7) Kabashin, A. V.; Meunier, M. J. Appl. Phys. 2003, 94, 7941. (8) Kabashin, A. V.; Meunier, M.; Kingston, C.; Luong, J. H. T. J. Phys. Chem. B 2003, 107, 4527. (9) Simakin, A. V.; Voronov, V. V.; Kirichenko, N. A.; Shafeev, G. A. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 1127. (10) Sajti, L.; Giorgio, S.; Khodorkovsky, V.; Marine, W. Appl. Surf. Sci. 2007, 253, 8111.

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