From scratch to scale-up: high-temperature syntheses of inorganic nanoparticles at different progress stages

The potential application and implementation of nanomaterials represent the central goal of nanotechnology. However, strategies for the production of nanomaterials with precisely tunable properties must first be developed. Subsequently, both the materials themselves and their synthesis processes need to be optimized. Finally, these processes must be scaled up to provide the required material quantities. These three aspects can be summarized under the term “nanoparticle synthesis engineering”. This dissertation addresses three inorganic nanoparticle systems, each representing one of these phases, with a focus on high-temperature synthesis strategies. Studies in the development phase were conducted on the semiconductor Sb2S3. Existing synthesis strategies produce either amorphous, spherical nanoparticles or crystalline, elongated structures in the micrometer range. In this work, a synthesis was developed that yields spherical, crystalline sub-microparticles. Investigations of the formation mechanism using the hot-injection method showed that the crystalline Sb2S3 structures arise through hierarchical growth of several amorphous intermediate stages. By employing L-cysteine and SbCl3 as dispersed but undissolved precursors in the heat-up method, one of these intermediate stages could be stabilized until crystallization occurred. The cysteine forms a waxy matrix that prevents the growth of larger structures, resulting in spherical, crystalline particles with diameters of 300-500 nm In the optimization phase, a hot-injection synthesis of silver nanocubes via the formation of an AgCl intermediate in non-polar solvents was refined. The original synthesis procedure is based on the injection of a AgNO3 precursor solution into a hot solvent mixture containing a chloride source. The present study demonstrates that the choice of chloride source has a decisive impact on the control and reproducibility of the synthesis. Moreover, by varying the chloride concentration, reaction temperature, and nitrate content, the edge length, yield, and edge sharpness of the Ag nanocubes can be precisely tuned. In addition, iron ions were identified as catalytically active key components. Based on the results of this work, an optimized reaction mechanism was proposed in which the iron ions are involved in two distinct reaction steps. As a complementary side project, the third part of this dissertation contributes to completing the concept of nanoparticle synthesis engineering. It focuses on the scale-up of a heat-up synthesis of superparamagnetic iron oxide nanoparticles. Using an oleate-based batch protocol, a millireactor for its continuous synthesis was developed, and its fundamental feasibility was demonstrated. The reactor, featuring two temperature zones corresponding to the pre-nucleation as well as the nucleation and growth phases of the reaction, was characterized with respect to flow behavior and mixing. Initial continuous experiments yielded iron oxide nanoparticles similar to those from the batch synthesis, though not yet fully equivalent. Despite fouling that led to reactor blockage, a functional experimental millireactor was realized, and the fundamental feasibility of the continuous synthesis approach demonstrated. Overall, this dissertation highlights the complexity and demands of nanoparticle synthesis engineering. The results contribute to a fundamental understanding of the synthesis processes of inorganic nanoparticles and provide strategies for their targeted development, optimization, and scale-up.