Controlling Oriented Attachment of Gold Nanoparticles by Size and Shape

Abstract

The concept of nanoparticles (NPs) as building blocks offers new possibilities to produce complex and tailored structures from the nano- to the mesoscale. In order to control a “polymerization” of particles, knowledge of the mechanism and kinetics of the reaction are necessary. We show that controlled assembly of cetylpyridinium chloride-stabilized gold NPs utilizing induced dipole–dipole interactions can lead to the formation of defined one-dimensional structures in solution. Three different shaped NPs (cubes, octahedra, and truncated cuboctahedra) were investigated individually. The assembly process is analogous to a step growth polymerization and is quantitatively describable with kinetics of a polyesterification. In situ kinetic studies reveal that there is an ideal particle size and shape for the induced dipole-driven assembly. Even small changes in size have remarkable effects on the assembly behavior. We further demonstrate that the transition from oriented assembly to oriented attachment requires a critical particle size (critical interface area) resulting from a size-dependent energy barrier for the crystallographic fusion. A combination of ideal size, shape, and degree of destabilization enables controlled oriented attachment of gold NPs in solution to chainlike structures under ambient conditions.

Au–Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications

“…According to this set of experimental evidence, the anisotropic growth of NCs may be divided in two stages (Figure G), with the first one occurring just after photofragmentation generating the initial NCs, followed by a second slower one taking place over several days in which the NCs coalesce and increase their aspect ratio. This is in agreement with previous observations of unidirectional self-assembly of metallic nanoparticles, which occurs with the fast formation of oligomers followed by their slower assembly into larger structures. ,,,, …”

“…Besides, in the NCs with appreciable broadband absorption, the longitudinal size extends well beyond the transversal size and, in part, also beyond the diameter of the initial NPs. This is a clear indication that, after photofragmentation, the small metal particles preferentially regrew in a unidirectional way, as a consequence of the balance between the attractive dipolar interactions in the colloidal system and the electrostatic repulsion forces, which are weaker along the axis of an elongated particle (e.g., after sticking of two nanospheres). ,,, …”

“…Unfortunately, the plasmon resonances of spherical or rod NPs are narrow and centered at specific wavelengths, which is not optimal for harvesting of solar energy, ,− even for the most effective plasmon heaters such as nanodoughnut . Symmetry reduction allows for tuning the number, position, and intensity of plasmons, which become broader and cover a wide spectral range when also the size of the nanostructure is increased over tens of nanometers. − ,,, Alternatively, new broad resonances from the visible to the NIR arise in large aggregates of NPs due to the mutual coupling of plasmon modes of the neighboring particles. ,,, In particular, several elongated or asymmetric networks of noble metal nanoparticles have been described for their multimodal plasmonic responses extending in the red and NIR. ,− However, the light-scattering component scales with the sixth power of object size and rapidly equals or overwhelms the absorption component in large objects, with a consequent loss of photothermal efficiency in big NPs or their aggregates. ,,, …”

“…51 Driven by the above considerations, here, we operated to achieve Au−Ag alloy NCs with the plasmonic quality factor and cost-affordability of Ag as well as the compatibility with the selfstanding, green and scalable laser-assisted synthetic procedure previously established with Au NCs. Laser irradiation lets metal particles spontaneously undergo a preferential unidirectional growth in solution, without external chemical agents or capping molecules, as a consequence of the balance between the electrostatic repulsion force and the attractive dipolar interactions in the colloidal system, 33,36,52,53 and the resulting Au−Ag alloy NCs have optimized broadband absorption for sunlight-driven thermoplasmonic applications. Numerical calculations elucidated the correlation of NCs morphology with the observed panchromism, also quantifying the predominance of the absorption contribution over scattering.…”

Influence of anisotropy on heterogeneous nucleation of gold nanorod assemblies

“…53 During the nucleation and self-assembly process, a shift and broadening of the longitudinal plasmon bands for all investigated nanorod colloids were observed, meaning new bands corresponding to the formation of aggregates emerged at longer wavelengths. 50,54,55 Overall, these results indicate the suitability of using ethanol to trigger the aggregation of CTAB-stabilised gold nanorods. Moreover, analysis of the destabilization kinetics showed that the reactions proceeded as a second-order reaction (ESI Table S1†).…”