2023, Ni, Bing, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

Conventional nanocrystal (NC) growth mechanisms have overwhelmingly focused on the final exposed facets to explain shape evolution. However, how the final facets are formed from the initial nuclei or seeds, has not been specifically interrogated. In this concept paper, we would like to concentrate on this specific topic, and introduce the symmetry based kinematic theory (SBKT) to explain the formation and evolution of crystal facets. It is a crystallographic theory based on the classical crystal growth concepts developed to illustrate the shape evolution during the NC growth. The most important principles connecting the basic NC growth processes and morphology evolution are the preferential growth directions and the properties of kinematic waves. On the contrary, the final facets are just indications of how the crystal growth terminates, and their formation and evolution rely on the NC growth processes: surface nucleation and layer advancement. Accordingly, the SBKT could even be applied to situations where non-faceted NCs such as spheres are formed.

2022-07-14, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

In this study, we analysed for the first time heterogeneous nucleation with anisotropic nanoparticles as a model system for non-spherical building units on the nanoscale. Gold nanorods were synthesised and assembled to investigate the phenomenon of heterogeneous nucleation. To determine the influence of the particle shape on heterogeneous nucleation, we utilised gold nanorods with varying aspect ratios, ranging from 3.00 and 2.25 to 1.75, while keeping the surface chemistry constant. First, the nucleation of the gold nanorod assemblies in solution and the process kinetics were analyzed with UV-vis-NIR spectroscopy followed by a microscopic examination of the gold nanorod-based superstructures formed heterogeneously on substrates. Here, positively charged cetyltrimethylammonium bromide (CTAB)-functionalized gold nanorods and negatively charged polystyrene sulfonate (PSS) functionalized substrates ensured the directed heterogeneous nucleation on the substrates. A combination of light microscopy with simultaneous UV-vis-NIR spectroscopy allowed us to observe the gold nanorod-based superstructure formation on the substrates in situ and to determine the nucleation rates of the process. We analysed the resulting data with the classical nucleation theory, which revealed a dominating kinetic term and a negligible thermodynamic term in contrast to ionic systems like calcium carbonate. Our studies consistently exhibit an influence of the aspect ratio on the nucleation behaviour resulting in faster nucleation of superstructures as the aspect ratio decreases. Hence our studies show unprecedented insight into the influence of particle anisotropy on the nucleation and growth of nanorod-based superstructures and reveal significant differences in the nucleation of nanoparticle building units compared to the nucleation of atoms or molecules as building units.

2022-05-09, Ni, Bing, Gonzalez-Rubio, Guillermo, Kirner, Felizitas, Zhang, Siyuan, Cölfen, Helmut

Das Wachstum von kristallinen Nanopartikeln (NP) umfasst im Allgemeinen drei Prozesse: Keimbildung, Wachstum und die Entwicklung der Form. Von diesen Prozessen ist die Formentwicklung trotz der Bedeutung der Morphologie für die Eigenschaften der NP am wenigsten verstanden. Hier beschreiben wir eine symmetriebasierte kinematische Theorie (SBKT) die auf klassischen Wachstumstheorien basiert, um den Prozess zu veranschaulichen. Auf Grundlage des Kristallgitters, der Keim- (oder Kern-) Symmetrie und der bevorzugten Wachstumsrichtungen unter den experimentellen Bedingungen kann die SBKT die Wachstumstrajektorien veranschaulichen. Die Theorie erfüllt die konventionellen Kriterien der wichtigsten bestehenden Theorien für das Kristallwachstum und bietet Werkzeuge zum besseren Verständnis des Symmetriebrechens während des Wachstums anisotroper Strukturen. Darüber hinaus wird das komplexe dendritische Wachstum sowohl theoretisch als auch experimentell nachgewiesen. Damit bietet sie einen Ansatz zur Erklärung der Formentwicklung und erweitert die Vorhersage der Morphogenese auf Fälle, die von anderen Theorien nicht erfasst werden können.

2020-07-09, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

Nucleation phenomena play an important role in our world, and understanding them is of major interest. However, we lack analytical methods with the sufficient temporal and spatial resolution to analyze nucleation processes. In this work we used CTAB-stabilized gold nanocubes as a model system for nucleation, meaning the nanoparticles act like ions or atoms and built up larger superstructures comparable to normal nucleation phenomena. Thereby we analyzed the heterogeneous nucleation of the gold nanocubes on hydrophobized and negatively charged mica surfaces with a combination of UV–vis–NIR spectroscopy and light microscopy. With the plasmon resonance of the gold nanocubes we gained valuable information about the early nucleation of the particles and their concentration in solution via UV–vis–NIR spectroscopy. The combination with a light microscope enabled the simultaneous detection of nucleated species on the surfaces and opened the possibility to analyze the kinetics of the heterogeneous nucleation process. With this, we were able to determine the nucleation rates. While the hydrophobized surfaces did not influence the nucleation of the gold nanocubes, the negatively charged surfaces greatly promoted the nucleation. Thereby, we could demonstrate that the combination of simple and commonly available light microscopy and optical spectroscopies in general is a suitable and easy strategy to analyze heterogeneous nucleation processes directly in solution on a relevant statistical basis.

2023, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Schnitzlein, Simon, Cölfen, Helmut

Heterogeneous nucleation processes are involved in many important phenomena in nature, including devastating human diseases caused by amyloid structures or the harmful frost formed on fruits. However, understanding them is challenging due to the difficulties of characterizing the initial stages of the process occurring at the interface between the nucleation medium and the substrate surfaces. This work implements a model system based on gold nanoparticles to investigate the effect of particle surface chemistry and substrate properties on heterogeneous nucleation processes. Using widely available techniques such as UV–vis–NIR spectroscopy and light microscopy, gold nanoparticle-based superstructure formation was studied in the presence of substrates with different hydrophilicity and electrostatic charges. The results were evaluated on grounds of classical nucleation theory (CNT) to reveal kinetic and thermodynamic contributions of the heterogeneous nucleation process. In contrast to nucleation from ions, the kinetic contributions toward nucleation turned out to be larger than the thermodynamic contributions for the nanoparticle building blocks. Electrostatic interactions between substrates and nanoparticles with opposite charges were crucial to enhancing the nucleation rates and decreasing the nucleation barrier of superstructure formation. Thereby, the described strategy is demonstrated advantageous for characterizing physicochemical aspects of heterogeneous nucleation processes in a simple and accessible manner, which could be potentially explored to study more complex nucleation phenomena.

2022-06-21, Ni, Bing, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

Conspectus: Biominerals are unique materials found in many living organisms that often display outstanding functionalities attributed to their mesocrystalline structure. Mesocrystals are nanocrystal superstructures with mutual crystallographic alignment of the building units. One could thus imagine these optimized evolutionary systems as archetypes to fabricate advanced materials. The main advantage of such systems relies on their ability to combine the features of the nanocrystals with those of single crystalline microscopic structures, yielding assemblies with directional, enhanced, and potentially emergent properties. Moreover, fueled by the promises of multifunctional materials with unprecedented and tunable properties, the rational design of mesocrystals assembled from two distinct colloidal nanocrystal ensembles has become a recent focus of research. However, the combination of dissimilar nanocrystals into ordered binary superstructures is still a major scientific challenge due to the nature of the coassembly process.

We focus this Account on the growth of tridimensional (3D) binary mesocrystals and the understanding of the self-assembly of two colloidal nanocrystal ensembles with the ultimate goal to serve as a basis for more rational mesocrystal syntheses in the future. The formation of mesocrystals demands nanocrystals with defined surface faceting, the primary factor influencing their oriented self-assembly. Notably, such a process cannot be successfully afforded without functionalized nanocrystals with high and, in many cases, tunable colloidal stability. Besides, the nature and solvation degree of the surface ligand shell influences the effective shape of the nanocrystals and the kinetics of self-assembly. If the assembly is triggered by reducing the colloidal stability with nonsolvents, 3D single-component mesocrystals are often grown. Here, the different magnitude of the van der Waals attraction forces between nanocrystals with differing compositions, dimensions, and morphologies generally favors the segregation and growth of single component mesocrystals. This phenomenon was illustrated during the successful preparation of 3D binary mesocrystals composed of iron oxide and platinum nanocubes. Although the building blocks possessed comparable sizes and were stabilized by similar ligands, the amount of the second component could only be arbitrarily tuned up to some extent, even when the assembly conditions were rationally optimized to achieve 3D binary mesocrystals. Only a small amount of it was effectively incorporated into the matrix of the initial mesocrystal. The 3D binary mesocrystal growth process demands a delicate control over the size, shape, and surface chemistry of the nanocrystals, the solvent nature, and the self-assembly process. Hence, the improvement of our ability to control the synthesis of 3D binary mesocrystalline materials is critical to exploit their potential toward technological applications in catalysis, energy storage, or structural materials.

2021-05-28, Gonzalez-Rubio, Guillermo, Hilbert, Holger, Rosenberg, Rose, Ni, Bing, Fuhrer, Lisa, Cölfen, Helmut

Analytical ultracentrifugation (AUC) is a powerful technique to observe colloidal nanocrystals (NCs) directly in solution and obtain critical information about their physical-chemical properties. Nevertheless, a more comprehensive implementation of AUC for the characterisation of such a class of crystalline colloids has been traditionally impaired by the requirement of having a priori knowledge of the complex, multilayered structure formed by NC in solution. This includes the nature (density and mass) of the surface ligands (SLs) that provide NC colloidal stability and the shell of solvent molecules formed on it. Herein, we propose a methodology to determine the NCs size by using SLs with a density equal to that of the solvent. Thereby, the buoyancy force of the SL shell is neutral, and the density of the NCs is sufficient a priori knowledge to calculate their related mass and size distributions. The simplicity and reliability of the method are evaluated with cetyltrimethylammonium bromide (CTAB) stabilized spherical gold NCs (AuNCs) of dimensions ranging from 1 to 17 nm. The proposed method has great potential to be transferred to any non-crystalline and crystalline colloids of different nature and composition, which have a density that is equal to the bulk and can be stabilized by SLs having a density that matches that of the solvent.

2023, Ni, Bing, Mychinko, Mikhail, Gómez-Graña, Sergio, Morales-Vidal, Jordi, Obelleiro-Liz, Manuel, Heyvaert, Wouter, Gonzalez-Rubio, Guillermo, Cölfen, Helmut, Bals, Sara, Liz-Marzán, Luis M.

A robust and reproducible methodology to prepare stable inorganic nanoparticles with chiral morphology might hold the key to the practical utilization of these materials. We describe herein an optimized chiral growth method to prepare 4-fold twisted gold nanorods, where the amino acid cysteine is used as a dissymmetry inducer. Four tilted ridges were found to develop on the surface of single-crystal nanorods upon repeated reduction of HAuCl4 , in the presence of cysteine as the chiral inducer and ascorbic acid as a reducing agent. From detailed electron microscopy analysis of the crystallographic structures, we propose that dissymmetry results from the development of chiral facets in the form of protrusions (tilted ridges) on the initial nanorods, eventually leading to a twisted shape. The role of cysteine is attributed to assisting enantioselective facet evolution, which is supported by density functional theory simulations of the surface energies, modified upon adsorption of the chiral molecule. The development of R-type and S-type chiral structures (small facets, terraces, or kinks) would thus be non-equal, removing the mirror symmetry of the Au NR and in turn resulting in a markedly chiral morphology with high plasmonic optical activity. This article is protected by copyright. All rights reserved.

2022-05-09, Ni, Bing, Gonzalez-Rubio, Guillermo, Kirner, Felizitas, Zhang, Siyuan, Cölfen, Helmut

The growth of crystalline nanoparticles (NPs) generally involves three processes: nucleation, growth, and shape evolution. Among them, the shape evolution is less understood, despite the importance of morphology for NP properties. Here, we propose a symmetry-based kinematic theory (SBKT) based on classical growth theories to illustrate the process. Based on the crystal lattice, nucleus (or seed) symmetry, and the preferential growth directions under the experimental conditions, the SBKT can illustrate the growth trajectories. The theory accommodates the conventional criteria of the major existing theories for crystal growth and provides tools to better understand the symmetry-breaking process during the growth of anisotropic structures. Furthermore, complex dendritic growth is theoretically and experimentally demonstrated. Thus, it provides a framework to explain the shape evolution, and extends the morphogenesis prediction to cases, which cannot be treated by other theories.

2020-11-19, Keßler, Sascha, Gonzalez-Rubio, Guillermo, Reinalter, Elrike R., Kovermann, Michael, Cölfen, Helmut

The citrate-assisted growth of nickel hexacyanoferrate (NiHCF) nanocubes was investigated. Control over the complexation of Ni²⁺ ions with citrate at different temperatures enabled fine tuning of the nanocrystal (NC) dimensions and their self-assembly into mesocrystals. Our results introduce new concepts towards the synthesis of NiHCF NCs, potentially applicable to other members of the Prussian blue analogues family.