Heterogeneous Nucleation of Anisotropic Particles

Abstract

Nucleation processes play a significant role throughout nature, science, and industry, as they constitute the first step in the spontaneous formation of distinct thermodynamic phases and structures. They often occur around us, with the formation of clouds through the condensation of water vapor into liquid droplets being among the most elegant examples. In industry, the quality and properties of pharmaceutical drugs can depend on the nucleation conditions used during their production. Further, an unwanted aspect of nucleation constitutes the precipitation of calcium carbonate minerals in pipes for water conduction. These examples show how gaining insight into nucleation can help to understand countless processes occurring in nature and industry. Furthermore, this understanding should help to manipulate systems in a targeted manner to eventually deliver optimized methods for desired applications or functional materials with tailor-made properties. Despite the importance of nucleation processes in nature and technology, the complex system still poses many unanswered questions. This fact can be partially explained by a lack of analytical methods with sufficient spatial and temporal resolution to characterize the nucleation of critical germs, which possess dimensions in the nanometer range. This thesis used anisotropic nanoparticles as a model system to study heterogeneous nucleation. Nanoparticles as model system means that instead of atoms or molecules nucleating into larger structures, nanoparticles were used as building blocks and assembled into larger superstructures in a process comparable to the nucleation of atomic, ionic or molecular systems. This approach brought the system to a larger scale and thus simplified the analysis of nucleation processes. Herein, the thesis dealt explicitly with heterogeneous nucleation as this is the most common form compared to homogeneous nucleation. Moreover, the thesis worked with anisotropic particles, since anisotropic building blocks are omnipresent in nature and technology, unlike isotropic particles commonly utilized for nucleation experiments. With the model system of anisotropic particles, the thesis analyzed the impact of surface chemistry and particle shape on the nucleation of the nanoparticles into superstructures to gain insight into the complex phenomenon of heterogeneous nucleation. To study the effects of these parameters, the thesis first focused on substrates where the heterogeneous nucleation of the nanoparticles took place (Chapter 3), then on the nanoparticles used as building blocks for the nucleation (Chapter 4), and finally, on analytical methods, which allowed observing the nucleation process and measuring its kinetic behavior (Chapter 5). Accordingly, Chapter 3 was dedicated to investigate the substrates used for heterogeneous nucleation experiments. Here, mica was identified as a promising substrate for the analysis of heterogeneous nucleation due to its smooth atomically surface. A variety of surface chemistry properties were introduced, by varying their hydrophilicity and surface charge, while preserving the smooth substrate’s surface, the prerequisite for the investigation of heterogeneous nucleation events. The applied functionalization reactions using silanes only depended on free hydroxyl groups on the substrate’s surface. Thus, not only mica but additionally silicon and quartz could be functionalized in the same way. Having three different substrates was advantageous to investigate and characterize nucleation of nanoparticle superstructures. For instance, mica substrates were employed for optical microscope analysis and SEM experiments and the surface of quartz cuvettes to analyze kinetics in situ via UV-vis-NIR spectroscopy. Chapter 4 focused on the nature of the colloidal nanoparticle systems used as building blocks for heterogeneous nucleation experiments. Here, three different anisotropic nanoparticle systems were successfully prepared and their nucleation and assembly into larger superstructures was investigated: zinc oxide nanorods, gold nanocubes, and gold nanorods. Zinc oxide nanorods were functionalized via ligand exchange with siloxanes carrying three different polar groups (-NH2, -OH, -COOH). Furthermore, gold nanocubes were functionalized with four different polar groups (-NH2, -OH, -SO3H, -COOH). With these two nanoparticle systems, it was possible to analyze the influence of the surface chemistry on heterogeneous nucleation by using the same nanoparticle, which only differs in its surface chemistry, as a building block for heterogeneous nucleation (Chapter 6). To further study the influence of the nanoparticle shape (Chapter 7), three sets of gold nanorods with distinct aspect ratios (3.00, 2.25, and 1.75) were investigated. However, to properly examine the nucleation behavior of nanoparticle-based superstructures, it also was necessary to design an appropriate experimental setup that allowed to observe and quantify the heterogeneous nucleation in situ (Chapter 5). For this reason, a suitable and easily accessible system was designed and implemented based on the combination of light microscopy with simultaneous UV-vis-NIR spectroscopy measurements. With this setup, it was possible to analyze heterogeneous nucleation processes in situ, directly in dispersion, and on a relevant statistical basis. Therefore, the controlled destabilization of the nanoparticle systems was established by altering the solubility of the nanoparticles in the solvent through the addition of ethanol. Thereby, it was possible to successfully analyze the homogeneous nucleation of CTAB stabilized gold nanocubes in dispersion, the acceleration of the process by the presence of a favorable surface (sulfonate-functionalized substrates), and the heterogeneous nucleation of the gold nanocubes on these surfaces resulting in nanoparticle-based superstructures. The plasmon resonance of the gold nanocubes gave 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 substrates and opened the possibility to analyze the kinetics of the heterogeneous nucleation process. This approach opened the possibility to calculate the effective interfacial energy and the nucleation barrier and to characterize the heterogeneous nucleation process more precisely. Thereby, it could be demonstrated that the combination of light microscopy and UV-vis-NIR spectroscopy is a suitable and easy to handle system to analyze heterogeneous nucleation processes in situ directly in solution on a relevant statistical basis with simple and commonly available equipment. Hence, this approach was used in the next steps to analyze the influence of surface chemistry and particle shape on heterogeneous nucleation by allowing a comparison of the variations in the nucleation systems. In Chapter 6, the impact of surface chemistry on heterogeneous nucleation could be successfully analyzed using the presented model system of nanoparticles as building blocks. The simple experiments with zinc oxide nanorods showed that depending on the interaction of the nanoparticle surface with the substrate surface, structures could be varied from layer-like to three-dimensional superstructures. In the following, the impact of surface chemistry was analyzed in more detail with the system of gold nanocubes, wherein 18 different surface chemistry combinations could be studied. First, the impact of the substrate’s surface chemistry on the destabilization kinetics of the gold nanocubes in solution was analyzed by UV-vis-NIR spectroscopy. Here, the destabilization of the gold nanocubes in solution did not seem to follow exclusively a homogeneous nucleation pathway. Electrostatic interactions between substrates and nanoparticles with opposite charges were crucial for enhancing the nucleation rates and decreasing the nucleation barrier of superstructure formation. Unfavorable interactions between nanoparticles and substrates did not influence the destabilization kinetics at all. Second, the superstructures formed on the substrates were analyzed. The results fit well the kinetic studies, and favorable interactions resulted not only in the enhancement of the destabilization kinetics but also in huge superstructures on the substrate’s surface. The favorable interaction of CTAB gold nanocubes with sulfonate- and carboxyl-functionalized substrates and of PAA gold nanocubes with amine-functionalized substrates allowed a quantitative analysis of the heterogeneous nucleation process using time-resolved light microscopy and simultaneous UV-vis-NIR spectroscopy. Thereby, the nucleation rates could be determined successfully and allowed further the calculation of the interfacial energies, the nucleation barriers, and the thermodynamic and kinetic terms. The results suggested that the interfacial energies and the nucleation barriers of the investigated nanoparticle systems are significantly lower than that of atoms and molecules. Moreover, the kinetic component plays a more significant role than the thermodynamic one, most probably due to the lower diffusion constants of nanoparticles in comparison to atoms or molecules. Next to the influence of surface chemistry on heterogeneous nucleation, Chapter 7 analyzed the influence of particle anisotropy in terms of the aspect ratio. It was possible to quantitatively investigate the heterogeneous nucleation phenomenon of superstructures based on anisotropic nanoparticles using a set of gold nanorods with different aspect ratios as a model system. Time-resolved light microscopy and UV-vis-NIR spectroscopy made it possible to determine the nucleation rates as well as the nanoparticle concentration in dispersion at each time. The studies consistently exhibited an influence of the aspect ratio on the nucleation behavior resulting in faster nucleation of superstructures as the aspect ratio decreased. In addition, evaluation of the nucleation rates in dependence of the supersaturation revealed a much larger kinetic nucleation barrier than the thermodynamic one. This dominating kinetic term is in agreement with the results obtained for the gold nanocubes (Chapter 6) and is in opposition to the situation reported for ionic or molecular systems. The cause might be the much faster diffusion of ions and molecules than of nanorods and, therefore, a larger kinetic hindrance for the nucleation of the nanorod building units. On the other hand, the thermodynamic barrier did effectively not exist for the investigated nanorods as it was found to be smaller than 1 kBT already for supersaturations higher than 2 and only reached a few kBT at its’ maximum for supersaturations approaching 1. The reason could be the electrostatic attraction between the negatively charged mica substrates and the positively charged nanorods, which almost eliminated the thermodynamic barrier, while the kinetic one was still significant. Overall, the obtained data set on heterogeneous nucleation of monodisperse nanorods with different anisotropy degrees into superstructures provided important insights into the formation of superstructures from anisotropic nanoparticles. Moreover, the reported experiments can also serve as experimental data for theoretical descriptions of nucleation using non-spherical building units. That a theory describing nucleation with anisotropic building units is important can already be seen from the fact that molecules are rarely spherical and commonly exhibit anisotropy. Therefore, understanding their nucleation and crystallization better could be truly beneficial for several applications ranging from nanoparticle superstructures like mesocrystals via macromolecular crystals in protein crystallization to the crystallization of small molecules in pharmacy. In summary, with the combination of widely available techniques such as UV-vis-NIR spectroscopy and light microscopy, this thesis established a straightforward method to analyze heterogeneous nucleation processes on the model system of nanoparticles. With this approach, it was possible to qualitatively and quantitatively investigate the heterogeneous nucleation phenomenon of superstructures based on anisotropic gold nanoparticles. Thereby, the influence of the surface chemistry, as well as particle anisotropy, were studied successfully. This fact demonstrates the potential of using nanoparticle systems with different physicochemical features to investigate heterogeneous nucleation phenomena, a strategy that should eventually serve to study and unveil the mechanism behind complex nucleation processes observed in nature and industry. This potential opens the possibility to explore many more aspects of heterogeneous nucleation. Subsequent to this thesis, it would be interesting, for example, to extend the analytical setup. UV-vis-NIR spectroscopy is limited to the concentration measurement of light-absorbing samples and only in the concentration range where the Lambert-Beer law is valid. One can imagine the implementation of other optical techniques such as the very sensitive fluorescence spectroscopy after the introduction of fluorescence tags to the nanoparticles. A more universal approach would be a refractive-index measurement or turbidity detection. The latter would be possible already with the UV-vis-NIR spectroscopy setup and could take advantage of dynamic sensitivity in multiwavelength detection since light scattering intensity is proportional to the inverse light wavelength to the power of four.[248] The turbidity τ of the nanoparticle dispersion is related to the nanoparticle number concentration c (particles/ml) via equation 15.[249] τ=π/4 Kd^2 c (15) Here, d is the nanoparticle diameter and K the scattering coefficient being a function of nanoparticle size, light wavelength, and the refractive index difference of nanoparticles and medium. This dependence means that the nanoparticle size and scattering coefficient need to be determined in advance and that very small particles with negligible light scattering cannot be investigated anymore. But on the other hand, this concentration detection method is universal and opens the possibility to analyze various nanoparticle systems. Next to the extension of the analytical setup, further studies on the role of the substrates in heterogeneous nucleation would be of interest. Here, one could imagine functionalizing patterned substrates with different surface chemistries. Another possibility would be to use other nanoparticles or small aggregates instead of a macroscopic substrate and to analyze the heterogeneous nucleation on the surface of these particles. Moreover, the reported experiments can also serve as experimental data for theoretical descriptions of nucleation using non-spherical building units. A combination of experimental data and simulations would be promising. The first results for the particle anisotropy presented here are limited, but a more systematic future investigation using the established experimental methodology can potentially yield larger data sets to systematically reveal the influence of the building units’ shape on nucleation processes. There are still countless mysteries and fascinating aspects in the field of nucleation and probably every question that has been solved raises numerous new ones. However, every small finding allows more control over existing processes, the production of materials with improved properties and thereby takes us one step further.