CFD-PBM Simulation for Continuous Hydrothermal Flow Synthesis of Zirconia Nanoparticles in a Confined Impinging Jet Reactor

Computational fluid dynamics (CFD) and population balance models (PBM) were coupled together for the first time to simulate the synthesis of zirconia nanoparticles in a continuous hydrothermal flow synthesis (CHFS) system with a self-designed confined impinging jet mixing (CJM) reactor. The hydrodynamic and thermodynamic behaviors within the CJM reactor strongly influenced the formation of the ZrO₂ nanoparticles. Crucial parameters, such as velocities, temperatures, mixing conditions, and reaction rates, were analyzed under various supercritical conditions. Temperature and velocity measurements as functions of distance were also investigated. Normal particle size distribution (PSD) patterns were observed in all cases. The mean particle sizes in this study were calculated and compared using PBM aggregation analysis.

Conducting ITO Nanoparticle-Based Aerogels—Nonaqueous One-Pot Synthesis vs. Particle Assembly Routes

Indium tin oxide (ITO) aerogels offer a combination of high surface area, porosity and conductive properties and could therefore be a promising material for electrodes in the fields of batteries, solar cells and fuel cells, as well as for optoelectronic applications. In this study, ITO aerogels were synthesized via two different approaches, followed by critical point drying (CPD) with liquid CO2. During the nonaqueous one-pot sol–gel synthesis in benzylamine (BnNH2), the ITO nanoparticles arranged to form a gel, which could be directly processed into an aerogel via solvent exchange, followed by CPD. Alternatively, for the analogous nonaqueous sol–gel synthesis in benzyl alcohol (BnOH), ITO nanoparticles were obtained and assembled into macroscopic aerogels with centimeter dimensions by controlled destabilization of a concentrated dispersion and CPD. As-synthesized ITO aerogels showed low electrical conductivities, but an improvement of two to three orders of magnitude was achieved by annealing, resulting in an electrical resistivity of 64.5–1.6 kΩ·cm. Annealing in a N2 atmosphere led to an even lower resistivity of 0.2–0.6 kΩ·cm. Concurrently, the BET surface area decreased from 106.2 to 55.6 m2/g with increasing annealing temperature. In essence, both synthesis strategies resulted in aerogels with attractive properties, showing great potential for many applications in energy storage and for optoelectronic devices.

Crystal engineering of nanomaterials: current insights and prospects

Nanocrystal engineering has evolved into a dynamic research area over the past few decades but is not properly defined. Here, we present select examples to highlight the diverse aspects of crystal engineering applied on inorganic nanomaterials.

Crystal growth of nanomaterials

Georg Garnweitner, Kwangyeol Lee and Dongfeng Xue introduce the CrystEngComm themed issue on the crystal growth of nanomaterials.

Secondary Particle Formation during the Nonaqueous Synthesis of Metal Oxide Nanocrystals

This study aims to elucidate the aggregation and agglomeration behavior of TiO₂ and ZrO₂ nanoparticles during the nonaqueous synthesis. We found that zirconia nanoparticles immediately form spherical-like aggregates after nucleation with a homogeneous size of 200 nm, which can be related to the metastable state of the nuclei and the reduction of surface free energy. These aggregates further agglomerate, following a diffusion-limited colloid agglomeration mechanism that is additionally supported by the high fractal dimension of the resulting agglomerates. In contrast, TiO₂ nanoparticles randomly orient and follow a reaction-limited colloid agglomeration mechanism that leads to a dense network of particles throughout the entire reaction volume. We performed in situ laser light transmission measurements and showed that particle formation starts earlier than previously reported. A complex population balance equation model was developed that is able to simulate particle aggregation as well as agglomeration, which eventually allowed us to distinguish between both phenomena. Hence, we were able to investigate the respective agglomeration kinetics with great agreement to our experimental data.

Continuous synthesis of gold nanoparticles in micro- and millifluidic systems

Gold nanomaterials have diverse applications ranging from healthcare and nanomedicine to analytical sciences and catalysis. Microfluidic and millifluidic reactors offer multiple advantages for their synthesis and manufacturing, including controlled or fast mixing, accurate reaction time control and excellent heat transfer. These advantages are demonstrated by reviewing gold nanoparticle synthesis strategies in flow devices. However, there are still challenges to be resolved, such as reactor fouling, particularly if robust manufacturing processes are to be developed to achieve the desired targets in terms of nanoparticle size, size distribution, surface properties, process throughput and robustness. Solutions to these challenges are more effective through a coordinated approach from chemists, engineers and physicists, which has at its core a qualitative and quantitative understanding of the synthesis processes and reactor operation. This is important as nanoparticle synthesis is complex, encompassing multiple phenomena interacting with each other, often taking place at short timescales. The proposed methodology for the development of reactors and processes is generic and contains various interconnected considerations. It aims to be a starting point towards rigorous design procedures for the robust and reproducible continuous flow synthesis of gold nanoparticles.

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Impact of nanoparticle surface modification on the mechanical properties of polystyrene-based nanocomposites

Nanocomposites consisting of metal oxide nanoparticles in a polymeric matrix enable the improvement of material properties and have become highly relevant for numerous applications, such as in lightweight structures with an enhanced Young's modulus for automotive and aircraft applications. The mechanical properties can be adjusted by controlling the amount of particles, their degree of agglomeration and their direct interaction with the matrix. Whilst the latter aspect is particularly promising to achieve high reinforcement at low filler contents, the mechanisms behind this effect are still not fully understood, preventing the rational design of a particle–polymer system with customized properties. In this work, a two-step modification strategy is used to tailor the particle–matrix interface via chemical groups bound to the surface of zirconia nanoparticles. Two modifications featuring terminal vinyl functions as potentially polymerizable groups are compared. Moreover, an inert reference modification is used to determine the influence of the terminal vinylic groups. In contrast to previous studies, all groups are covalently linked to the particle surface, thereby excluding effects such as detachment or weak coordination and ensuring that changes in the mechanical properties can be correlated to chemical groups on the particle surface. After embedding modified particles in polystyrene, the mechanical properties as well as the cross-linkage between the particles and the matrix are characterized, clearly showing the significant impact of a covalent particle–matrix linkage, with an increase of the Young's modulus by up to 28% with only 3 wt% filler content.

A two-step modification strategy is applied to tailor the particle–matrix interface in zirconia nanoparticle–polystyrene composites, achieving strongly enhanced mechanical properties.