Controllable Synthesis of Nanoparticles in Continuous Flow Microreactors

The accurate control of nanoparticle (NP) morphology, particle size and particle size distribution (PSD) is essential to achieve their accurate size/morphology-dependent properties. Continuous microflow reactors offer potential advantages over traditional batch processing in producing micro- and nanoparticles with excellent controllability, reproducibility with merit of superior mixing and mass/heat transfer performance. This thesis focuses on the synthesis of silver NPs (Ag NPs), tin dioxide NPs (SnO2 NPs), gold@tin dioxide NPs (Au@SnO2 NPs), and cuprous oxide NPs (Cu2O NPs) within continuous microfluidic systems, aiming to achieve controllable particle size, PSD and morphology.
While successful cases of micro- and milli-reactors for nanomaterials production have been reported in a lab scale, scaling up these home-made micro-channel / micro-tubular reactors with non-standardized components such as tubing, connectors, adaptors, etc. poses challenges for the large- scale production. One of the commercial examples of continuous flow reactors, Corning Advanced-Flow Reactor (AFR) has demonstrated success in homogeneous and heterogenous organic synthesis process. For the large-scale requirement, the Corning AFR provide a wide range of throughput by increasing the channel size and/or number of the reactor units, while retaining mixing, mass/heat transfer performance, which could helpful to achieve the seamless transition from laboratory-optimized process to industrial- scale production. However, limited studies have been reported on the nanomaterials production using Corning AFR. To evaluate the performance of the Corning AFR (Lab Reactor Module) for the nanomaterials production, the mixing characteristic (on the micro-, meso- and macroscale) at different flow rate and flow rate ratio were evaluated. A case study of colloidal Ag NPs was introduced to examine how flow dynamics and multiscale mixing performance inside the Corning AFR affect average particle size and PSD. It will pave the way for large scale production of size-tunable colloidal nanoparticles in the future work.
Furthermore, plasmonic metal@semiconductor (Au@SnO2) binary nanomaterials were synthesized in a simple capillary milli/micro-tube reactor. SnO2, a n-type semiconductor with a direct wide band gap (3.64 eV at 300 K), mainly absorb the UV light (< 5% in solar light). To enhance visible-light absorption, noble metal NPs are commonly used to improve solar-energy-conversion efficiency in composite photocatalysts through the surface plasmon resonance (SPR) effect and also prevent the hole-electron recombination by charge transfer. The solar energy conversion efficiency of this binary composite photocatalysts is determined by the type, material features, geometric arrangement of the building blocks. In conventional synthetic strategies, semiconductor and metal NPs were synthesised separately, and following sol-immobilisation, impregnation and co-precipitation are commonly used to combine this binary composite. These semi-continuous synthesis methods are challenging to scale up with precise nanostructures control. To address this, the study explores the synthesis of SnO2 NPs at atmospheric pressure, room temperature to make high-volume production more feasible. Herein, the study introduces a microfluidic way for the continuous synthesis of Au@SnO2 in a micro-capillary reactor. This study provides a versatile strategy with an economic way to continuously synthesis plasmonic metal/semiconductor with narrow size-distribution on a large scale.
Moreover, as a visible light-driven p-type semiconductor with a narrow bandgap of 1.9-2.2 eV, cuprous oxide (Cu2O) shows promising potential in photocatalytic applications due to its suitable electronic band structure, earth abundance and nontoxicity. With a face-centered cubic crystal structure (Pn3 ̅m), Cu2O polyhedron exhibit low-index and/or high-index facets. Achieving controllable crystal facets as well as narrow particle size distribution is vital for Cu2O NPs synthesis. While wet chemical reduction approaches are the cheapest and simplest methods to manipulate the morphology of Cu2O NPs in the liquid phase, however, the reported methods are mainly based on the batch-reactor systems in the laboratory settings. To bridge the gap between the academic research and engineering production process, in this study, we present a facile and continuous synthesis method for Cu2O NPs using a lab-made capillary reactor. The morphological evolution of Cu2O NPs from cubic to rhombic dodecahedra and from cubic to octahedra was first reported in a single system.
Overall, the results show continuous microfluidic reactor provide a facile method for synthesising nanoparticles with precise control over particle size, PSD and morphology.