Continuous flow synthesis of metal nanowires: protocols, engineering aspects of scale-up and applications

This review comprehensively covers the translation from batch to continuous flow synthesis of metal nanowires (i.e., silver, copper, gold, and platinum nanowires) and their diverse applications across various sectors. Metal nanowires have attracted significant attention owing to their versatility and feasibility for large-scale synthesis. The efficacy of flow chemistry in nanomaterial synthesis has been extensively demonstrated over the past few decades. Continuous flow synthesis offers scalability, high throughput screening, and robust and reproducible synthesis procedures, making it a promising technology. Silver nanowires, widely used in flexible electronics, transparent conductive films, and sensors, have benefited from advancements in continuous flow synthesis aimed at achieving high aspect ratios and uniform diameters, though challenges in preventing agglomeration during large-scale production remain. Copper nanowires, considered as a cost-effective alternative to silver nanowires for conductive materials, have benefited from continuous flow synthesis methods that minimize oxidation and enhance stability, yet scaling up these processes requires precise control of reducing environments and copper ion concentration. A critical evaluation of various metal nanowire ink formulations is conducted, aiming to identify formulations that exhibit superior properties with lower metal solid content. This study delves into the intricacies of continuous flow synthesis methods for metal nanowires, emphasizing the exploration of engineering considerations essential for the design of continuous flow reactors. Furthermore, challenges associated with large-scale synthesis are addressed, highlighting the process-related issues.

Direct coating of gold nanolayers to enhance the oxidation resistance of copper nanowire flexible transparent conductive films

Copper nanowire-based transparent conductive films have garnered extensive attention owing to their cost-effectiveness and comparable electrical properties. However, the inherent instability of copper nanowires (Cu NWs) has curtailed their extensive utility and applicability. Herein, we present durable Cu@Au NW/PET films exhibiting elevated photoelectric attributes and remarkable flexibility. After preparing Cu NWs, the purification operation allows the purity of the Cu NWs to reach about 98%. Subsequently, Cu@Au NWs/PET flexible transparent conductive films (FTCFs) were prepared through vacuum filtration of Cu NWs and direct treatment with chloroauric acid. The resulting Cu@Au NW-based FTCFs exhibit impressive attributes including a low sheet resistance of 30 ohms per square and a high optical transmittance of 90%, resulting in an exceptional figure of merit (FOM) of 99. Remarkably, the Cu@Au NWs/PET film showed remarkable flexibility, retaining its properties after 10 000 cycles of continuous bending. Stability assessments further affirm the sheet resistance of the Cu@Au NW FTCFs remains nearly unchanged over 75 days at ambient temperature. The strategic integration of a gold nanolayer, serving as a protective coating on the Cu NWs, yields substantial enhancements in both electrical conductivity and overall stability within the Cu NW FTCF architecture. Furthermore, the obtained Cu@Au NW films exhibit rapid heating capabilities, reaching a temperature of 67 °C within 30 seconds at 3.5 V and subsequently returning to room temperature at the same rate. In summary, the introduction of a Au protective layer can effectively enhance the oxidation resistance of Cu NWs, which has great application potential in FTCFs in the field of film heaters.

Photoluminescence Investigations and Band Gap Engineering in Environment Friendly ZnO Nanorods: Enhanced Water Treatment Application and Defect Model

The inadvertent discharge of industrial effluents, mainly textile, contributes to the complex contamination load in water bodies. Textile dyes are the critical effluents and recalcitrant to traditional remediation procedures. Therefore, energy viable and environment friendly solutions are needed. In this study, we have synthesized zinc oxide nanorods (NRs) at various temperatures using modified thermal decomposition and evaluated its photocatalytic activities. Field effect scanning electron microscopy has confirmed rod-like morphology till TS = 500 °C and spherical morphology from TS = 600 °C onward. Photoluminescence spectra have shown a prominent defect peak in the synthesized ZnO, except for the NRs synthesized at 300 °C. Synthesized ZnO NRs and NPs have been employed to degrade crystal violet (CV) and congo red (CR) dyes. ZnO NRs have shown impressive photocatalytic performance with faster treatment time as compared to the earlier reports. Synthesis parameters are well correlated with the observed high efficiency and the band gap tailoring. Based on our findings, for the first time, we have proposed (i) defect model correlating synthesis parameters with defect states, (ii) systematic correlation of defect states with photocatalytic efficiency, and (iii) ZnO nanorods synthesized at 300 °C via an improved synthesis method as a promising photocatalytic solution to degrade the CV and CR dyes in contaminated water.

Anisotropic Growth of Copper Nanorods Mediated by Cl- Ions

Anisotropic growth to form Cu particles of rod and wire shapes has been obtained typically in a complex system that involves both organic capping agents and Cl- ions. However, the sole effect of Cl- ions on the formation of Cu wires has yet to be fully understood, especially in an organic system. This present work determines the effect of Cl- ions on the morphologies of Cu particles in an organic phase without any capping agents. The results revealed that anisotropic Cu rods could be grown with the sole presence of Cl- ions. The rods have the (011) facets as the long axis, the (111) facets as the tip, and the (100) facets as the side surface. By increasing the Cl- ion concentration, more Cu atoms contributed to the formation of Cu rods and the kinetic growth of the length and the diameter of the rods varied. This suggests that Cl- ions have preferential adsorption on the (100) Cu surfaces to promote the anisotropic growth of Cu. Meanwhile, the adsorption of Cl- to the (111) and (100) surfaces at high Cl- concentrations regulates the relative growth of the particle length and diameter.

All-Solution-Processed, Oxidation-Resistant Copper Nanowire Networks for Optoelectronic Applications with Year-Long Stability

Copper nanowires (Cu NWs) hold promise as they possess equivalent intrinsic electrical conductivity and optical transparency to silver nanowires (Ag NWs) and cost substantially less. However, poor resistance to oxidation is the historical challenge that has prevented the large-scale industrial utilization of Cu NWs. Here, we use benzotriazole (BTA), an organic corrosion inhibitor, to passivate Cu NW networks. The stability of BTA-passivated networks under various environmental conditions was monitored and compared to that of bare Cu NW control samples. BTA passivation greatly enhanced the stability of networks without deteriorating their optoelectronic performance. Moreover, to demonstrate their potential, BTA-passivated networks were successfully utilized in the fabrication of a flexible capacitive tactile sensor. This passivation strategy has a strong potential to pave the way for large-scale utilization of Cu NW networks in optoelectronic devices.

Optimized Microwave-Based Synthesis of Thermally Stable Inverse Catalytic Core-shell Motifs for CO2 Hydrogenation

The rational synthesis of Cu@TiO₂ core@shell nanowire (NW) structures was thoroughly explored using a microwave-assisted method through the tuning of experimental parameters such as but not limited to (i) controlled variation in molar ratios, (ii) the effect of discrete Ti precursors, (iii) the method of addition of the precursors themselves, and (iv) time of irradiation. Uniform coatings were obtained using Cu/Ti molar ratios of 1:2, 1:1, 2:1, and 4:1, respectively. It should be noted that although relative molar precursor concentrations primarily determined the magnitude of the resulting shell size, the dependence was nonlinear. Moreover, additionally important reaction parameters, such as precursor identity, the means of addition of precursors, and the reaction time, were individually explored with the objective of creating a series of optimized reaction conditions. As compared with Cu NWs alone, it is evident that both of the Cu@TiO₂ core-shell NW samples, regardless of pretreatment conditions, evinced much better catalytic performance, up to as much as 20 times greater activity as compared with standard Cu NWs. These results imply the significance of the Cu/TiO₂ interface in terms of promoting CO₂ hydrogenation, because TiO₂ alone is known to be inert for this reaction. Furthermore, it is additionally notable that the N₂ annealing pretreatment is crucial in terms of preserving the overall Cu@TiO₂ core@shell structure. We also systematically analyzed and tracked the structural and chemical evolution of our catalysts before and after the CO₂ reduction experiments. Indeed, we discovered that the core@shell wire motif was essentially maintained and conserved after this high-temperature reaction process, thereby accentuating the thermal stability and physical robustness of our as-prepared hierarchical motifs.

Core/shell copper nanowire networks for transparent thin film heaters

Copper nanowires (Cu NWs) appear as the strongest alternative to silver nanowires (Ag NWs) in transparent conductors. Cu NWs; however, are more prone to oxidation compared to Ag NWs even at room temperature. This problem becomes more severe when Cu NWs are used as transparent thin film heaters (TTFHs). In this work, we have utilized ALD deposited zinc oxide (ZnO) shell layers, and provide a comparison with typically used aluminum oxide (Al₂O₃) shell layers to improve the TTFH performance. While Cu NW network TTFHs barely withstood temperatures around 100 °C, critical thickness of ALD deposited Al₂O₃ and ZnO layers were determined to find out TTFH limits. Maximum stable and reproducible temperatures of 273 °C and 204 °C were obtained for Al₂O₃ and ZnO deposited Cu NW network TTFHs, respectively. An extensive parametric study on the NW density and oxide type in conjunction with the electrical conductivity and optical transmittance was conducted. A remarkably high heating rate of 14 °C s^-1 was obtained from the fabricated core/shell networks with improved oxidation stability under ambient and high humidity conditions. Finally, these high performance core/shell Cu NW network TTFHs were utilized as efficient defrosters.

Rapid Catalytic Reduction of 4-Nitrophenol and Clock Reaction of Methylene Blue using Copper Nanowires

Copper nanowires (CuNWs) with a high aspect ratio of ~2600 have been successfully synthesized by using a facile hydrothermal method. The reductions of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and methylene blue (MB) to leucomethylene blue (LMB) by using sodium borohydride (NaBH4) were used as models to test the catalytic activity of CuNWs. We showed that by increasing the CuNWs content, the rate of reduction increased as well. The CuNWs showed an excellent catalytic performance where 99% reduction of 4-NP to 4-AP occurred in just 60 s by using only 0.1 pg of CuNWs after treatment with glacial acetic acid (GAA). The rate constant (kapp) and activity factor (K) of this study is 18 and ~1010 fold in comparison to previous study done with no GAA treatment applied, respectively. The CuNWs showed an outstanding catalytic activity for at least ten consecutive reusability tests with a consistent result in 4-NP reduction. In clock reaction of MB, approximately 99% of reduction of MB into LMB was achieved in ~5 s by using 2 μg CuNWs. Moreover, the addition of NaOH can improve the rate and degree of recolorization of LMB to MB.

Advancements in Copper Nanowires: Synthesis, Purification, Assemblies, Surface Modification, and Applications

Copper nanowires (CuNWs) are attracting a myriad of attention due to their preponderant electric conductivity, optoelectronic and mechanical properties, high electrocatalytic efficiency, and large abundance. Recently, great endeavors are undertaken to develop controllable and facile approaches to synthesize CuNWs with high dispersibility, oxidation resistance, and zero defects for future large-scale nano-enabled materials. Herein, this work provides a comprehensive review of current remarkable advancements in CuNWs. The Review starts with a thorough overview of recently developed synthetic strategies and growth mechanisms to achieve single-crystalline CuNWs and fivefold twinned CuNWs by the reduction of Cu(I) and Cu(II) ions, respectively. Following is a discussion of CuNW purification and multidimensional assemblies comprising films, aerogels, and arrays. Next, several effective approaches to protect CuNWs from oxidation are highlighted. The emerging applications of CuNWs in diverse fields are then focused on, with particular emphasis on optoelectronics, energy storage/conversion, catalysis, wearable electronics, and thermal management, followed by a brief comment on the current challenges and future research directions. The central theme of the Review is to provide an intimate correlation among the synthesis, structure, properties, and applications of CuNWs.

Synthesis of Ultralong Copper Nanowires for High-Performance Flexible Transparent Conductive Electrodes: The Effects of Polyhydric Alcohols

Copper nanowires (Cu NWs) have become a promising material for flexible transparent conductive electrodes (FTCEs) owing to their outstanding transparency and conductivity properties. In this work, ultralong Cu NWs with an average length over 250 μm and a diameter of around 50 nm (aspect ratio ∼5000) were synthesized in a water/polyhydric alcohol cosolvent. The effects of polyhydric alcohols (including ethanol, ethylene glycol, and glycerol) on the aspect ratio of Cu NWs were investigated. The diameter of Cu NWs decreased with the increased number of hydroxyl groups of polyhydric alcohols. In addition, the capping ligands (oleylamine and oleic acid) and glucose also exhibit important effects on the dispersity and morphology of Cu NWs. The ultralong Cu NW-based poly(dimethylsiloxane) (PDMS) FTCEs exhibit high performance with a low sheet resistance of 92.1 Ω sq^-1 at a transmittance of 91.524%. Inspired by the stretchable ability of PDMS, wearable sensors were fabricated to detect the movement of the finger joint through the chronoamperometry method. The prepared sensors exhibit high sensitivity and a fast response time. The excellent performance of FTCEs and wearable sensors suggests that the ultralong Cu NWs have a bright future in the application of the next generation of flexible optoelectronic devices.