Colloidal Stability of Metal Nanoparticles in Engine Oil under Thermal and Mechanical Load

Stabilization of Apolar Nanoparticle Dispersions by Molecular Additives

We study the effect of additives on the colloidal stability of alkanethiol-coated gold nanoparticles. Cyclic amines and sulfides of different sizes were added to dispersions in decane at additive concentrations below 128 mM. Small-angle X-ray scattering (SAXS) indicated that tetrahydrothiophene reduced the agglomeration temperature, Tagglo, by up to 29 °C, a considerable increase in colloidal stability. Amines had a much weaker stabilizing effect of up to 2.5 °C. We found an unexpected maximum of stabilization for low additive concentrations, where Tagglo increased at concentrations above 64 mM. Molecular dynamics simulations were used to correlate these observations with the ligand shell structure. They excluded the physisorption of additives as a stabilization mechanism and suggested that sulfides replace hexadecanethiol on the AuNP surfaces by chemisorption. This hinders ligand ordering, thereby reducing Tagglo, which explains the stabilizing effect. Clustering of chemisorbed additive molecules at high concentration restabilized the ligand ordered state, explaining the detrimental effect of higher additive concentrations. The predictions of the simulations were confirmed by using thermogravimetric analyses and SAXS measurements of washed samples that indicated that the structure of the ligand shell itself, not the presence of physisorbed additives, changes Tagglo. Finally, we calculated potentials of mean force, which show that larger sulfide-based additives have a weaker affinity for the gold surface than smaller ones due to stronger steric hindrance. This explains why smaller cyclic sulfides were the most efficient stabilizers.

How the Physicochemical Properties of the Bulk Material Affect the Ablation Crater Profile, Mass Balance, and Bubble Dynamics During Single-Pulse, Nanosecond Laser Ablation in Water

Understanding the key steps that drive the laser-based synthesis of colloids is a prerequisite for learning how to optimize the ablation process in terms of nanoparticle output and functional design of the nanomaterials. Even though many studies focus on cavitation bubble formation using single-pulse ablation conditions, the ablation efficiency and nanoparticle properties are typically investigated under prolonged ablation conditions with repetition rate lasers. Linking single-pulse and multiple-pulse ablation is difficult due to limitations induced by gas formation cross-effects, which occur on longer timescales and depend on the target materials' oxidation-sensitivity. Therefore, this study investigates the ablation and cavitation bubble dynamics under nanosecond, single laser pulse conditions for six different bulk materials (Au, Ag, Cu, Fe, Ti, and Al). Also, the effective threshold fluences, ablation volumes, and penetration depths are quantified for these materials. The thermal and chemical properties of the corresponding bulk materials not only favor the formation of larger spot sizes but also lead to the highest molar ablation efficiencies for low melting materials such as aluminum. Furthermore, the concept of the cavitation bubble growth linked with the oxidation sensitivity of the ablated material is discussed. With this, evidence is provided that intensive chemical reactions occurring during the very early timescale of ablation are significantly enhanced by the bubble collapse.

Discrimination of ablation, shielding, and interface layer effects on the steady-state formation of persistent bubbles under liquid flow conditions during laser synthesis of colloids

Over the past decade, laser ablation in liquids (LAL) was established as an innovative nanoparticle synthesis method obeying the principles of green chemistry. While one of the main advantages of this method is the absence of stabilizers leading to nanoparticles with “clean” ligand-free surfaces, its main disadvantage is the comparably low nanoparticle production efficiency dampening the sustainability of the method and preventing the use of laser-synthesized nanoparticles in applications that require high amounts of material. In this study, the effects of productivity-dampening entities that become particularly relevant for LAL with high repetition rate lasers, i.e., persistent bubbles or colloidal nanoparticles (NPs), on the synthesis of colloidal gold nanoparticles in different solvents are studied. Especially under batch ablation conditions in highly viscous liquids with prolonged ablation times both shielding entities are closely interconnected and need to be disentangled. By performing liquid flow-assisted nanosecond laser ablation of gold in liquids with different viscosity and nanoparticle or bubble diffusivity, it is shown that a steady-state is reached after a few seconds with fixed individual contributions of bubble- and colloid-induced shielding effects. By analyzing dimensionless numbers (i.e., Axial Peclet, Reynolds, and Schmidt) it is demonstrated how these shielding effects strongly depend on the liquid’s transport properties and the flow-induced formation of an interface layer along the target surface. In highly viscous liquids, the transport of NPs and persistent bubbles within this interface layer is strongly diffusion-controlled. This diffusion-limitation not only affects the agglomeration of the NPs but also leads to high local densities of NPs and bubbles near the target surface, shielding up to 80% of the laser power. Hence, the ablation rate does not only depend on the total amount of shielding matter in the flow channel, but also on the location of the persistent bubbles and NPs. By comparing LAL in different liquids, it is demonstrated that 30 times more gas is produced per ablated amount of substance in acetone and ethylene glycol compared to ablation in water. This finding confirms that chemical effects contribute to the liquid’s decomposition and the ablation yield as well. Furthermore, it is shown that the highest ablation efficiencies and monodisperse qualities are achieved in liquids with the lowest viscosities and gas formation rates at the highest volumetric flow rates.

Manipulation of the Size and Phase Composition of Yttrium Iron Garnet Nanoparticles by Pulsed Laser Post-Processing in Liquid

Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties.

The Effect of Addition of Nanoparticles, Especially ZrO2-Based, on Tribological Behavior of Lubricants

The aim of the paper was to discuss different effects, such as, among others, agglomeration of selected nanoparticles, particularly those from zirconia, on the tribological behavior of lubricants. The explanation of the difference between the concepts of ‘aggregation’ and ‘agglomeration’ for ZrO2 nanoparticles is included. The factors that influence such an agglomeration are considered. Classification and thickeners of grease, the role of additives therein, and characteristics of the lithium grease with and without ZrO2 additive are discussed in the paper. The role of nanoparticles, including those from ZrO2 utilized as additives to lubricants, particularly to the lithium grease, is also discussed. The methods of preparation of ZrO2 nanoparticles are described in the paper. The agglomeration of ZrO2 nanoparticles and methods to prevent it and the lubrication mechanism of the lithium nanogrease and its tribological evaluation are also discussed. Sample preparation and a ball-on disc tester for investigating of spinning friction are described. The effect of ZrO2 nanoparticles agglomeration on the frictional properties of the lithium grease is shown. The addition of 1 wt.% ZrO2 nanoparticles to pure lithium grease can decrease the friction coefficient to 50%. On the other hand, the agglomeration of ZrO2 nanoparticles in the lithium grease can increase twice the friction coefficient relative to that for the pure grease.

Determining the role of redox-active materials during laser-induced water decomposition

The decomposition of water and the formation of molecular hydrogen, oxygen, and hydrogen peroxide during laser ablation of redox-active materials is systematically studied and related to the ablation rate and oxidation degree of the nanoparticles.