Synthesis of Core@Shell Nanostructures in a Continuous Flow Droplet Reactor: Controlling Structure through Relative Flow Rates

Mixing Characteristic and High-Throughput Synthesis of Cadmium Sulfide Nanoparticles with Cubic Hexagonal Phase Junctions in a Chaotic Millireactor

A four-stage oscillating feedback millireactor with splitters (S-OFM) was designed to improve the mixing performance based on chaotic advection. Three-dimensional CFD simulations were used to investigate its flow characteristics and mixing performance, and the generation mechanisms of secondary flows were examined. The results show that the mixing index (MI_cup) increased with the increase in the Reynolds number (Re), and MI_cup could reach 99.8% at Re = 663. Poincaré mapping and Kolmogorov entropy were adopted to characterize the chaotic advection intensity, which indicates that there is a intensity increase with the increase in Re. In addition, the results of Villermaux-Dushman experiments demonstrate that S-OFM performs excellently, and the mixing time could reach 1.04 ms at Re = 2764. Finally, S-OFM was successfully used to synthesize CdS nanoparticles with cubic hexagonal phase junctions. At a flow rate of 180 mL/min, the average particle size was 10.5 nm and the particle size distribution was narrow (with a coefficient of variation of 0.14).

Nanocatalysts in electrosynthesis

Electrosynthesis is to use electricity to drive chemical reactions for chemical synthesis and is potentially a green approach to fuel and energy sustainability. Nanostructured catalysts play an important role in promoting electrochemical reactions under green chemistry conditions. This perspective first provides a brief tutorial on electrosynthesis and the roles the nanocatalysts play in the synthesis. It then outlines the common strategies used to develop nanocatalysts for hydrogen evolution reaction, CO2 reduction reaction, and biomass upgrading. The perspective further summarizes the current methodologies that have been developed for scaling-up synthesis of nanocatalysts, which will be essential for the electrosynthesis to become a viable industry approach.

Centrifugal microfluidic device for the high-throughput synthesis of Pd@AuPt core-shell nanoparticles to evaluate the performance of hydrogen peroxide generation

We propose a novel high-throughput screening platform using a centrifugal microfluidic device for producing combinatorial tri-metallic catalysts. The centrifugal device was designed to perform 60 reactions under different conditions on a single device. As a model to search for an optimal tri-metallic catalyst, we synthesized a variety of Pd@AuPt nanoparticles (NPs), in which Pd nanocubes served as a core, and Au and Pt atoms formed a shell. The centrifugal microfluidic device was etched on the top and bottom sides, in which two zigzag-shaped microchannels were patterned on the top side, and 60 reaction chambers were fabricated on the bottom side. Through the sophisticated zigzag-shaped microchannels, Pt²⁺ ion and Pd nanocube solutions were injected into the channel in one shot, and the centrifugal force equally and automatically divided the injected solutions into 60 aliquots during the rotation. By controlling the sophisticated channel dimensions and designing the passive valve structure, the Pt²⁺ ion, Pd nanocube, and Au³⁺ solutions were loaded into the reaction chamber in sequential order depending on the programmed rotational direction and speed. Therefore, the ratio of Au to Pt to synthesize Pd@AuPt core-shell NPs was changed from 0.028 : 1 to 12 : 1, and accordingly, the resultant 60 types of Pd@AuPt catalysts presented with different ratios of metal atom compositions. Then, we screened the catalytic activity of the Pd@AuPt NPs for generating H₂O₂ according to the degree of coating of Au and Pt, and the Pd@AuPt catalyst with the Au/Pt ratio at 0.5 turned out to be the most effective.

Size and shape control of metal nanoparticles in millifluidic reactors

Engineered metal nanoparticles (metal NPs) possess unique size -dependent optical and electronic properties that could enable new applications in biomedicine, energy generation, microelectronics, micro-optics, and catalysis. For metal NPs to make a mark in these fields, however, new synthetic strategies must be developed that permit NP synthesis on the kilogram scale, while maintaining precise control over NP physiochemical properties (size, shape, composition, and surface chemistry). Currently, NP batch syntheses produce product on the milligram scale and rely on synthetic strategies that are not readily amenable to scale-up. Flow reactor systems (including lab-on-a-chip devices) provide a synthesis platform that can circumvent many of the traditional limitations of batch-scale NP syntheses. These reactors provide more uniform reagent mixing, more uniform heat transfer, opportunities to interface in situ monitoring technology, and allow product yield to be scaled up simply by running multiple reactors in parallel. While many NP syntheses have been successfully transferred to microfluidic reactor systems, microfluidic reactor fabrication is time intensive and typically requires sophisticated lithography facilities. Consequently, millifluidic flow reactors (reactors with channel dimensions of 0.5–10.0 mm) are gaining popularity in NP synthesis. These millifluidic reactors provide many of the same synthetic advantages as microfluidic devices, but are simpler to construct, easier to reconfigure, and more straightforward to interface with in situ monitoring techniques. In this chapter, we will discuss the progress that has been made in developing millifluidic reactors for functionalized metal NP synthesis. First, we will review the basic wet-chemical strategies used to control metal NP size and shape in batch reactors. We will then survey some of the basic principles of millifluidic device design, construction, and operation. We will also discuss the potential for incorporating in situ monitoring for quality control during synthesis. We will conclude by highlighting some particularly relevant examples of millifluidic metal NP synthesis that have set new standards for metal NP size, shape, and surface chemistry control.

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LSPR Tuning from 470 to 800 nm and Improved Stability of Au-Ag Nanoparticles Formed by Gold Deposition and Rebuilding in the Presence of Poly(styrenesulfonate)

Stability and precise control over functional properties of metal nanoparticles remain a challenge for the realization of prospective applications. Our described process of shell formation and rebuilding can address both these challenges. Template silver nanoparticles (AgNPs) stabilized by poly(styrenesulfonate) are first transformed with gold deposition, after which the resulting shell rebuilds with the replaced silver. The shell formation and rebuilding are accompanied by large shifts in localized surface plasmon resonance (LSPR) peak position, which enables LSPR tuning in a range from 470 to 800 nm. Furthermore, chemical stability of Au-AgNPs is significantly improved compared to AgNPs due to gold stability. Silver templates of different shapes and sizes were demonstrated to transform to AuAg composite NPs to further extend the accessible LSPR range tuning. Stabilization of template AgNPs with poly(styrenesulfonate), in contrast to commonly used poly(vinylpyrrolidone), was found to be a key factor for shell rebuilding. The developed Au-AgNPs were shown to be advantageous for surface plasmon resonance (SPR) detection and surface-enhanced Raman spectroscopy (SERS) owing to their tunable LSPR and enhanced stability.

Kinetically controlled synthesis of bimetallic nanostructures by flowrate manipulation in a continuous flow droplet reactor

We show that different Au–Pd nanoparticles, ranging from sharp-branched octopods to core@shell octahedra, can be achieved by inline manipulation of reagent flowrates in a microreactor for seeded growth.

Controllable synthesis of nanocrystals in droplet reactors

In recent years, a broad range of nanocrystals have been synthesized in droplet-based microfluidic reactors which provide obvious advantages, such as accurate manipulation, better reproducibility and reliable automation. In this review, we initially introduce general concepts of droplet reactors followed by discussions of their main functional regions including droplet generation, mixing of reactants, reaction controlling, in situ monitoring, and reaction quenching. Subsequently, the enhanced mass and heat transport properties are discussed. Next, we focus on research frontiers including sequential multistep synthesis, intelligent synthesis, reliable scale-up synthesis, and interfacial synthesis. Finally, we end with an outlook on droplet reactors, especially highlighting some aspects such as large-scale production, the integrated process of synthesis and post-synthetic treatments, automated droplet reactors with in situ monitoring and optimizing algorithms, and rapidly developing strategies for interfacial synthesis.