title: Altering nanoparticle dispersion parameters for structural control of spray-dried supraparticles
title: Änderung der Dispersionsparameter von Nanopartikeln zur Strukturkontrolle von sprühgetrockneten Suprapartikeln
Creators: name: Zhou, Huanhuan
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025

description: Supraparticles, which are assemblies of nano/microparticles (building blocks), offer the possibility of incorporating a diverse range of building blocks to form versatile structures. Supraparticles often reveal collective or synergistic properties, such as enhanced optical, magnetic, or catalytic behaviors. Crucially, their functionalities and potential applications are notably dependent on their structures, making structural control of high interest for accessing desired properties. Spray-drying is a popular forced-assembly technique to synthesize supraparticles, offering numerous advantages, including chemical versatility, simplicity, rapid operation, continuous processing, scalability, and highly reproducible results. In this thesis, it is demonstrated that supraparticles can be designed via spray-drying to achieve versatile structures by tuning the nanoparticle parameters of the feeding dispersions, including size, weight concentration, size ratio, weight ratio, and colloidal stability. To systematically explore these parameters, silica (SiO2) nanoparticles are used as model materials in the first three sections of this thesis, due to their easy synthesis of monodisperse dispersions with varied sizes, and the simplicity of its surface (only hydroxyl groups). Section 3.1 demonstrates the simultaneous influence of the nanoparticle size and weight concentration on the final supraparticle structure. Monomodal dispersions were spray-dried to mushroom-cup-like, spherical, and colloidal cluster-like supraparticles and non-connected nanoparticles. Depending on the nanoparticle size, the buckling of supraparticles is increased with decreasing nanoparticle diameter. The supraparticle sizes are increased with an increased particle weight concentration. The bimodal and trimodal dispersions are found to form core-shell, colloidal cluster-like, and well-mixed structures with either spherical or non-spherical morphology, depending on the size combination and concentration. Fragmented particles, resulting from mechanical breakage during the spray-drying process, were characterized to reveal their internal structures. The mechanism behind the formation of distinct structures is proposed: The buckling of supraparticles is a result of the collapse of the flexible shell of small nanoparticles and the size difference of nanoparticles leads to their size segregation within supraparticles because of thermophoretic effects influencing the mobility of nanoparticles. Based on the findings about the size segregation and mechanical breakage from Section 3.1, Section 3.2 presents a one-step method to achieve platelet-shaped Janus supraparticles. These Janus supraparticles feature two distinct regions with different chemical or physical properties, similar to the concept of Janus particles. To successfully form a Janus structure, the size ratio and weight ratio of the two types of building blocks were optimized, and the device parameters such as cyclone types were adjusted. Crucially, the stability of nanoparticles is found to be essential for forming a core-shell structure, a prerequisite to producing Janus supraparticles. In contrast, the introduction of salt is suboptimal in the formation of Janus supraparticles, as it causes pre-agglomeration of nanoparticles and hinders segregation. Furthermore, this section shows the transferability of this Janus supraparticle fabrication method to a wide range of other metal oxide materials. In Section 3.3, the influence of the salt addition on the colloidal stability of nanoparticles in bimodal dispersions and the resulting supraparticulate structures is shown. While the use of salts is a well-studied approach to manipulating the colloidal stability of nanoparticles, studies about the influence of salt on the structure formation of supraparticles, especially those formed from bimodal dispersions, remain rare. Section 3.3 demonstrates that the addition of salts can induce nanoparticle instability, leading to the formation of pre-agglomeration, which significantly influences the resulting supraparticulate structures. Different salts such as NaCl, CaCl2, and AlCl3, were introduced to the feeding dispersions before spray-drying. Depending on the cation valency and concentration, the degree of pre-agglomeration can be varied, producing distinct supraparticle structures, including core-shell, Janus-like, and well-mixed types. These variations also lead to corresponding changes in the textural properties, such as surface roughness and pore systems. Building on the method of adding salt to control the structure of supraparticles, Section 3.4 focuses on the adjustability of a functional property in a structure-controlled supraparticle. Instead of using bare SiO2 nanoparticles as building blocks, superparamagnetic iron oxide nanoparticles (SPIONs) were added to provide magnetic functionality. A structural variety of supraparticles was produced, and the relationship between the SPION cluster distance and magnetic interaction was explored. It is shown that the method of adding salt to alter the supraparticulate structure can be applied not only to tune the structure but also to the magnetic properties of supraparticles. The magnetic interaction is shown to depend on the distance between SPION clusters, as detected by magnetic particle spectroscopy. This thesis demonstrates that the structure of spray-dried supraparticles can be systematically designed by altering the nanoparticle parameters of the feeding dispersions. These adjustments enable control over the morphology and textural properties of the supraparticles to achieve specific structures. Notably, the one-step method to fabricate platelet-shaped Janus supraparticles and the approach of adding salt to tune the supraparticulate structures are shown to be transferable to other materials beyond SiO2.