General Trends in Core-Shell Preferences for Bimetallic Nanoparticles.

Surface segregation phenomena dictate core-shell preference of bimetallic nanoparticles and thus play a crucial role in the nanoparticle synthesis and applications. Although it is generally agreed that surface segregation depends on the constituent materials' physical properties, a comprehensive picture of the phenomena on the nanoscale is not yet complete. Here we use a combination of molecular dynamics (MD) and Monte Carlo (MC) simulations on 45 bimetallic combinations to determine the general trend on the core-shell preference and the effects of size and composition. From the extensive studies over sizes and compositions, we find that the surface segregation and degree of the core-shell tendency of the bimetallic combinations depend on the sufficiency or scarcity of the surface-preferring material. Principal component analysis (PCA) and linear discriminant analysis (LDA) on the molecular dynamics simulations results reveal that cohesive energy and Wigner-Seitz radius are the two primary factors that have an "additive" effect on the segregation level and core-shell preference in the bimetallic nanoparticles studied. When the element with the higher cohesive energy also has the larger Wigner-Seitz radius, its core preference decreases, and thus this combination forms less segregated structures than what one would expect from the cohesive energy difference alone. Highly segregated structures (highly segregated core-shell or Janus-like) are expected to form when both the relative cohesive energy difference is greater than ∼20%, and the relative Wigner-Seitz radius difference is greater than ∼4%. Practical guides for predicting core-shell preference and degree of segregation level are presented.

Continuous gas-phase synthesis of core-shell nanoparticles via surface segregation.

Synthesis methods of highly functional core@shell nanoparticles with high throughput and high purity are in great demand for applications, including catalysis and optoelectronics. Traditionally chemical synthesis has been widely explored, but recently, gas-phase methods have attracted attention since such methods can provide a more flexible choice of materials and altogether avoid solvents. Here, we demonstrate that Cu@Ag core-shell nanoparticles with well-controlled size and compositional variance can be generated via surface segregation using spark ablation with an additional heating step, which is a continuous gas-phase process. The characterization of the nanoparticles reveals that the Cu-Ag agglomerates generated by spark ablation adopt core-shell or quasi-Janus structures depending on the compaction temperature used to transform the agglomerates into spherical particles. Molecular dynamics (MD) simulations verify that the structural evolution is caused by heat-induced surface segregation. With the incorporated heat treatment that acts as an annealing and equilibrium cooling step after the initial nucleation and growth processes in the spark ablation, the presented method is suitable for creating nanoparticles with both uniform size and composition and uniform bimetallic configuration. We confirm the compositional uniformity between particles by analyzing compositional variance of individual particles rather than presenting an ensemble-average of many particles. This gas-phase synthesis method can be employed for generating other bi- or multi-metallic nanoparticles with the predicted configuration of the structure from the surface energy and atomic size of the elements.

The Role of Citric Acid in the Stabilization of Nanoparticles and Colloidal Particles in the Environment: Measurement of Surface Forces between Hafnium Oxide Surfaces in the Presence of Citric Acid.

The interactions between colloidal particles and nanoparticles determine solution stability and the structures formed when the particles are unstable to flocculation. Therefore, knowledge of the interparticle interactions is important for understanding the transport, dissolution, and fate of particles in the environment. The interactions between particles are governed by the surface properties of the particles, which are altered when species adsorb to the surface. The important interactions in the environment are almost never those between the bare particles but rather those between particles that have been modified by the adsorption of natural organic materials. Citric acid is important in this regard not only because it is present in soil but also as a model of humic and fulvic acids. Here we have studied the surface forces between the model metal oxide surface hafnia in the presence of citric acid in order to understand the stability of colloidal particles and nanoparticles. We find that citric acid stabilizes the particles over a wide range of pH at low to moderate ionic strength. At high ionic strength, colloidal particles will flocculate due to a secondary minimum, resulting in aggregates that are dense and easily redispersed. In contrast, nanoparticles stabilized by citric acid remain stable at high ionic strengths and therefore exist in solution as individual particles; this will contribute to their dispersion in the environment and the uptake of nanoparticles by mammalian cells.

Measurement of long range attractive forces between hydrophobic surfaces produced by vapor phase adsorption of palmitic acid.

Extensive research into the surface forces between hydrophobic surfaces has produced experimentally measured interaction forces that vary widely in range and in magnitude. This variability is attributed to interference from surface nanobubbles and the nature of the hydrophobic surface. Whilst the effects of nanobubbles are now recognised and can be addressed, the precise nature of the surface remains a confounding factor in measurements between hydrophobic surfaces. Here we show that a monolayer coating with hydrophobic properties is formed by exposing metal oxide surfaces to palmitic acid vapour. Surface forces measured between these smooth hydrophobic surfaces exhibited an exponential attraction. Neither patchy surface charges, nor surface nanobubbles could explain the measured forces. However, the observed interaction may be explained by the interaction of a single patch of bilayered palmitic acid molecules interacting with an exposed patch of the hafnia surface. Such an interaction is consistent with the observed exponential nature of the attraction and the agreement between the measured decay of the exponential attraction with the Debye length of the solution.

Roughness in Surface Force Measurements: Extension of DLVO Theory To Describe the Forces between Hafnia Surfaces.

The interaction between colloidal particles is commonly viewed through the lens of DLVO theory, whereby the interaction is described as the sum of the electrostatic and dispersion forces. For similar materials acting across a medium at pH values remote from the isoelectric point the theory typically involves an electrostatic repulsion that is overcome by dispersion forces at very small separations. However, the dominance of the dispersion forces at short separations is generally not seen in force measurements, with the exception of the interaction between mica surfaces. The discrepancy for silica surfaces has been attributed to hydration forces, but this does not explain the situation for titania surfaces where the dispersion forces are very much larger. Here, the interaction forces between very smooth hafnia surfaces have been measured using the colloid probe technique and the forces evaluated within the DLVO framework, including both hydration forces and the influence of roughness. The measured forces across a wide range of pH at different salt concentrations are well described with a single parameter for the surface roughness. These findings show that even small degrees of surface roughness significantly alter the form of the interaction force and therefore indicate that surface roughness needs to be included in the evaluation of surface forces between all surfaces that are not ideally smooth.