Shohreh Hemmati

Silver nanowires (AgNWs) have garnered significant attention in nanotechnology due to their unique mechanical and electrical properties and versatile applications. This review explores the synthesis of AgNWs, with a specific focus on the utilization of millifluidic flow reactors (MFRs) as a promising platform for controlled and efficient production. It begins by elucidating the exceptional characteristics and relevance of AgNWs in various technological domains and then delves into the principles and advantages of MFRs by showcasing their pivotal role in enhancing the precision and scalability of nanowire synthesis. Within this review, an overview of the diverse synthetic methods employed for AgNW production using MFRs is provided. Special attention is given to the intricate parameters and factors influencing synthesis and how MFRs offer superior control over these critical variables. Recent advances in this field are highlighted, revealing innovative strategies and promising developments that have emerged. As with any burgeoning field, challenges are expected, so future directions are explored, offering insights into the current limitations and opportunities for further exploration. In conclusion, this review consolidates the state-of-the-art knowledge in AgNW synthesis and emphasizes the critical role of MFRs in shaping the future of nanomaterial production and nanomanufacturing.

Article Sina Kaabipour ¹, Finley Neal ², and Shohreh Hemmati ^2,* ¹ School of Chemical Engineering, Oklahoma State University, Stillwater, OK 74078, USA ² School of Mathematics and Natural Sciences, The University of Southern Mississippi, Hattiesburg, MS 39406, USA * Correspondence: shohreh.hemmati@usm.edu Received: 25 November 2024; Revised: 15 January 2025; Accepted: 11 February 2025; Published: 20 February 2025 Abstract: Silver nanowires (AgNWs) have garnered significant attention during the past decade thanks to their applications in conductive inks used for electronic applications. The polyol process, widely used for AgNW synthesis, is known for its effectiveness in producing high aspect ratio and high yield nanowires. However, this process suffers from drawbacks such as high energy consumption and use of unsustainable reagents derived from nonrenewable resources, which makes its large-scale utilization and economic feasibility challenging. In contrast, green synthesis methods offer potential solutions by employing environmentally friendly and cost-effective approaches. In this study, we offer a high-yield (90%) approach for the inexpensive, environmentally friendly, and sustainable synthesis of AgNWs, and show that the production cost per gram of AgNWs can be reduced by 31.72% compared to the polyol process. In addition, we investigate the rheological behavior of the synthesized AgNW-based conductive ink under screen printing and direct writing conditions using flow sweep, peak hold, and frequency sweep tests. The rheological behavior of the AgNWbased conductive ink provides valuable information regarding its use for various printing applications. The conductive ink demonstrated a shear-thinning thixotropic behavior for all silver nanostructure contents (2, 5, 10, and 20 wt.%), and all temperatures (25, 30, and 40 °C). It was observed that direct writing is better suited for printing inks with low colloidal content due to its lower shear rate, whereas screen printing is more effective for high-content, high-viscosity inks because it utilizes higher shear rates. The proposed cheaper and more sustainable method can serve as a promising alternative for industrial conductive ink manufacturing for printed electronic appliances such as printed circuit boards (PCBs) and flexible transparent conductive films (TCFs).

Nucleation (k₁) and growth (k₂) rate constants for the polyol silver nanowire synthesis were determined for batch and millifluidic flow reactors (MFRs). The MFR rate constants are larger in magnitude and synthesize AgNWs faster than batch reactors.

The challenge of synthesizing noble metal nanostructures sustainably has encouraged researchers to explore biological routes for nanostructure production, such as biotemplating. Plant viruses with rod-shape morphology, such as tobacco mosaic virus (TMV) and barley stripe mosaic virus (BSMV), offer promising biotemplates to produce metal nanorods. TMV and BSMV can be incubated in aqueous metal precursor solutions to mineralize metals on the coat proteins (CPs) of the viruses. Previous studies have primarily examined palladium (Pd) mineralization on TMV and BSMV using Na₂PdCl₄ as the Pd precursor. There is limited scientific literature on the effect of using alternative Pd precursor solutions besides Na₂PdCl₄ such as K₂PdCl₄ and PdCl₂ to mineralize Pd on TMV and BSMV. Past attempts at mineralizing other noble metals such as platinum (Pt) and gold (Au) required an initial layer of Pd to be deposited on the TMV and BSMV biotemplates. In this study, we aimed to expand the understanding of using alternative Pd precursor solutions to mineralize Pd on TMV and BSMV. Additionally, the deposition of Pt and Au onto TMV and BSMV without the need for an initial Pd mineralization layer was achieved using alternative Pt and Au precursors, including K₂PtCl₄ and AuCl₃, respectively. Pd, Pt, and Au were successfully deposited on TMV and BSMV by incubation in aqueous solutions of Na₂PdCl₄, K₂PdCl₄, PdCl₂, K₂PtCl₄, and AuCl₃. Kinetic studies were also conducted using ultraviolet-visible (UV-vis) spectroscopy to examine the rates at which Pd, Pt, and Au precursor ions were reduced during the mineralization process, mimicking their adsorption onto TMV and BSMV CPs. BSMV adsorbed noble metal precursor ions faster than TMV as determined by UV-vis spectroscopy. While palladium nanorods (PdNRs) offer high electrical conductivity desirable for electronic applications, Pd-coated TMV and BSMV may face limitations due to their organic cores, potentially compromising conductivity. To address this, one approach is to convert the organic core into conductive amorphous carbon through thermal annealing. In this study, in situ transmission electron microscopy was utilized to thermally anneal Pd-TMV2Cys, thereby transforming them into PdNRs with amorphous carbon cores.

Recent studies have demonstrated the effectiveness of Pd nanorods (PdNRs) in a wide variety of applications, which makes them highly desired nanocrystals. Green synthesis methods have resulted in very low PdNR yields (12% to 15%) in the past, therefore, making them less attractive for PdNR synthesis compared to conventional, less sustainable synthesis methods such as the polyol process. The lower yield of PdNRs is partly caused by oxidative etching that reduces the proportion of multiple-twinned-structures produced during nucleation, which act as the seeds for the growth of PdNRs. The rate of reduction also plays an essential role in determining the morphology of synthesized palladium (Pd) nanostructures. In this study, PdNRs were synthesized in segmented millifluidic flow reactors (SMFRs) using L-ascorbic acid as a green reducing agent. Application of SMFRs minimizes oxidative etching, while improves mass and heat transfer control to maximize the PdNR yield. Pd precursors containing different halides demonstrate varying levels of oxidative etching. Each precursor is also reduced at a different rate by L-ascorbic acid. Both oxidative etching and the rate of reduction can be adjusted by using different Pd precursors as a novel approach to further increase the yield of PdNRs in SMFRs. Reactions were conducted using Pd(NO₃)₂, PdCl₂, Na₂PdCl₄, and Na₂PdBr₄ and their combination as Pd precursors for the first time in SMFRs to the best of our knowledge. The synthesized Pd nanostructures were characterized using transmission electron microscopy (TEM) to compare the PdNR yields resulting from each reaction. The rate of reduction in each reaction was determined using kinetic studies by measuring the remaining palladium species concentration in the aqueous reaction solution using ultraviolet-visible (UV-vis) spectroscopy. It was found that the combination of two palladium precursors at an appropriate molar ratio would provide a better control of oxidative etching and the reduction rate, simultaneously. The synthesis process using Pd(NO₃)₂ and Na₂PdBr₄ in a 1 : 1 molar ratio as palladium precursors yielded the highest percentage of PdNRs over 90%.

Pd nanorods (PdNRs) have recently come to attention due to their wide array of applications. The green synthesis of PdNR with a relatively high yield and high aspect ratio is challenging. A continuous millifluidic flow reactor (CMFR) has been explored to precisely control mass and heat transfer as well as mixing in the PdNR synthesis processes. CMFRs demonstrate a few drawbacks, such as the presence of parabolic velocity profile in the laminar flow of the reaction solution, causing uneven axial residence time distribution. The CMFRs are likely to show irreversible fouling, which may cause the product quality to deteriorate or result in the channel being clogged. These shortcomings can be avoided or minimized using a segmented millifluidic flow reactor (SMFR) that consists of the solution forming a train of individual segments in another inert medium. This study explores the use of a sustainable reducing agent (l-ascorbic acid) in the presence of potassium bromide (KBr) as the capping agent and poly(vinyl pyrrolidone) (PVP) as the stabilizing agent for PdNR synthesis in an SMFR employing compartmentalized flow of a reaction solution, in which liquid segments consisting of a reaction solution will be immersed in the steam generated by boiling of the solvent water. The effect of reaction parameters such as reagent concentration has been studied on the size and morphology of synthesized Pd nanostructures. A kinetic study has been conducted to calculate the rate of reduction that can be used as a quantitative measure for manipulation of the type and relative concentration of initially formed seeds. It has been shown that the initial reduction rate during the first 45 min of residence time of the millifluidic reactor is about 66% faster compared to the rest of the reaction. A filtration procedure has been utilized to separate Pd nanostructures other than nanorods synthesized in the SMFR.

The significance of silver nanostructures has been growing considerably, thanks to their ubiquitous presence in numerous applications, including but not limited to renewable energy, electronics, biosensors, wastewater treatment, medicine, and clinical equipment. The properties of silver nanostructures, such as size, size distribution, and morphology, are strongly dependent on synthesis process conditions such as the process type, equipment type, reagent type, precursor concentration, temperature, process duration, and pH. Physical and chemical methods have been among the most common methods to synthesize silver nanostructures; however, they possess substantial disadvantages and short-comings, especially compared to green synthesis methods. On the contrary, the number of green synthesis techniques has been increasing during the last decade and they have emerged as alternative routes towards facile and effective synthesis of silver nanostructures with different morphologies. In this review, we have initially outlined the most common and popular chemical and physical methodologies and reviewed their advantages and disadvantages. Green synthesis methodologies are then discussed in detail and their advantages over chemical and physical methods have been noted. Recent studies are then reviewed in detail and the effects of essential reaction parameters, such as temperature, pH, precursor, and reagent concentration, on silver nanostructure size and morphology are discussed. Also, green synthesis techniques used for the synthesis of one-dimensional (1D) silver nanostructures have been reviewed, and the potential of alternative green reagents for their synthesis has been discussed. Furthermore, current challenges regarding the green synthesis of 1D silver nanostructures and future direction are outlined. To sum up, we aim to show the real potential of green nanotechnology towards the synthesis of silver nanostructures with various morphologies (especially 1D ones) and the possibility of altering current techniques towards more environmentally friendly, more energy-efficient, less hazardous, simpler, and cheaper procedures.