High-productivity synthesis of ligand-free nanoparticles by laser ablation in liquids

In typical wet chemical synthesis methods for nanoparticles (such as precipitation and sol-gel techniques) organic adsorbates are used, or marks of the residual reagents are left. Unfortunately, both these factors cause the deactivation of nanoparticle surfaces. That is, the chemisorbed or physisorbed ligands weaken the catalytic activity, interfere with surface-sensitive spectroscopy, and hinder surface-assisted mass spectrometry. In contrast, the physico-chemical synthesis method of pulsed laser ablation in liquid (PLAL) produces ligand-free, highly pure nanoparticles. In recent years, this technique has thus emerged as a way to produce a variety of nanoparticles with unique properties. Research in this field has progressed rapidly with regard to the design of innovative composites^1–3 and their application in laser-assisted material processing techniques (e.g., rapid prototyping⁴ and laser lithography⁵). In comparison to ligand-coated nanoparticles, the laser-generated nanoparticles exhibit higher conjugation efficiency,⁶ higher grafting density,⁷ high electroaffinity toward charged biomolecules,⁸ and novel preparation routes for heterogeneous catalysts.⁹ With the current laser-based synthesis methods, however, it is difficult to achieve the large amounts of nanoparticles required for industrial applications.

In PLAL methods, a laser pulse interacts with a target material and ignites a plasma, which in turn leads to the formation of a cavitation bubble (of vapor gas). Nanoparticles form inside this bubble¹⁰ and then disperse into the liquid after its collapse. In addition, the cavitation bubble causes the scattering of consecutive laser pulses if they arrive before the bubble collapses (which usually takes about 200 microseconds). The lifetime, as well as the dimensions (diameter of up to 1mm), of the cavitation bubble therefore limit the energy input and the nanoparticle productivity. Accordingly, temporal bypassing of the cavitation bubble is only possible for low (i.e., kHz) repetition-rate laser systems.

In our work, we have therefore performed PLAL with a novel ablation setup (see Figure 1).¹¹ Indeed, we take advantage of recent advances in high-power pulsed-lased systems and laser scanning technologies in our unique upscaling concept for nanoparticle production.^{12, 13} In particular, our setup consists of a high-power ultrafast laser system (500W, 0.9–5ps, 1.2-40.5MHz, 1030nm), a polygon scanner, and a customized ablation chamber. With the polygon scanner, we scan in the ‘fast axis’—see Figure 1(b)—with an ultrasonic speed of 500m/s. The slow axis is controlled by a galvanometer scanner, which scans the target material at a rate of about 10m/s. We produce nanoparticles as the laser beam is directed onto the target material, which is placed in the water-filled ablation chamber. Furthermore, to enable continuous nanoparticle synthesis, we pump liquid through the chamber at a flow volume of up to 25 liters per hour. With our PLAL concept we can thus spatially bypass the cavitation bubbles with interpulse distances of up to 400μm (at a repetition rate of 1.2MHz), as shown in Figure 1(b).

Figure 1. (a) Schematic illustration of the novel pulsed laser ablation in liquid (PLAL) setup used to produce nanoparticles. The apparatus consists of a high-power laser system coupled with a polygon scanning system (which can reach ultrasonic speeds) in a specially designed liquid flow reactor. v_focus: Scanning speed of the focus. (b) Schematic diagram of the flow-through reactor, showing how the laser pulses spatially bypass the cavitation bubbles with interpulse distances of up to 400μm.

The productivities we achieve with our PLAL system—for several different nanomaterials—are as high as 4g/h (see Figure 2). In addition, we reach 265 times higher ablation rates (with an increase in average laser power of only a factor of 10) compared with previous results recorded in the literature.¹⁴ Although the ablation of noble metals (e.g., platinum, gold, and silver) leads to the formation of metal nanoparticles, other metals (such as aluminum, copper, and titanium) react more strongly with the solvent in our approach and form oxide and suboxide nanoparticles.

Figure 2. Productivity of the PLAL synthesis approach for a number of different metal (left) and metal oxide (right) nanoparticles, after one hour of laser ablation. Pt: Platinum. Au: Gold. Ag: Silver. Ni: Nickel. NiO: Nickel oxide. Al₂O₃: Aluminum oxide. CuO: Copper oxide.

We have also analyzed the nanoparticle quality that we produce during the one-hour ablation period. Our results show that the ablation process is stable, in terms of crystallite size (see Figure 3) and surface composition (see Figure 4).¹³ Interestingly, we also find that by increasing the repetition rate of the laser system (to more than 1MHz), we can increase the ablation efficiency and simultaneously decrease the ablation threshold by a significant level. Such observations can be explained by incubation effects.¹⁵

Figure 3. Crystallite size of (a) Ni and NiO, and (b) Pt nanoparticles (as determined from x-ray diffraction measurements) at different points during the one-hour ablation period.

Figure 4. Oxidation state of the (a) Ni and (b) Pt nanoparticles (determined from x-ray photoelectron spectroscopy measurements) at different points during the one hour of ablation. Ni(HO)₂: Nickel hydroxide.

Furthermore, our experimental results show that—at high ablation speeds—the scaling factors of ablation productivity in liquid start to converge with those of laser ablation in air. Indeed, we have used a model¹⁶—which is based on the ablation law, as developed for ablation in air—to show that the ablation rate can be optimized with a maximum specific ablation rate (defined as the ablate rate per average power) at a specific optimum fluence. As laser manufacturing—after automation and efficiency optimization—has led to industrial series production, we believe that our method for decoupling ablation from cavitation effects in the liquid may lead to the same maturity for PLAL as for laser ablation in air.

In summary, we have developed a novel PLAL setup for nanoparticle production in which we use a high-power pulsed-laser system, an ultra-high-speed laser scanner, and a customized ablation chamber. In this way, we can spatially bypass the cavitation bubbles (with interpulse distances of up to 400μm) and have thus addressed the current major drawback with PLAL methods, i.e., the productivity. With our work, we can therefore scale up the synthesis process to the multigram/hour regime. Our experimental results show that we can achieve productivities (for a variety of metal and metal oxide nanoparticles) of up to 4g/h. The difference in productivities for the different materials, however, cannot be explained by material or laser parameters (e.g., thermal conductivity, heat capacity, or optical penetration depth). The ablation phenomenon in liquid is therefore a combination of different—probably competing—parameters in the laser–target–liquid system. In our future studies we will thus focus on the identification of the relevant parameters and the theoretical prediction of productivities for different material systems.