Synthesis and Applications of Copper Nanowires and Nanoplates

Abstract

<p>Research on the unique properties of metal nanocrystals sparked the advent of powerful nanomaterial-based technologies that improve the performance of catalysts, electronics, and cancer therapies, to name a few. Due to their promise, extensive progress has been made in tailoring the properties of metal nanocrystals by controlling their size, shape, composition, and structure. However, these nano-scaled materials remain largely within the confines of research laboratories. One of the biggest hindrances to the widespread use of metal nanocrystals is cost. Precious metals such as Ag and Au exhibit compelling properties in the nano scale, yet the astronomical cost of bulk materials and low throughput of synthetic methods make them impractical for applications that require more than a few milligrams of nanocrystals. This work addresses this issue by furthering our knowledge of Cu nanocrystal synthesis and presenting synthetic methods that lower the cost of its production.</p><p>Cu is significantly cheaper and more abundant compared to other metals, and much like Au and Ag nanocrystals, Cu nanocrystals exhibit conductive and catalytic properties. In this work, we present efforts to increase the impact of Cu nanowires by first developing a multigram-scale synthesis of stable Ag-coated Cu nanowires (Cu-Ag nanowires). The two-step process starts with the production of 4.4 g of Cu nanowires in 1 h, followed by a Ag-coating procedure that yields 22 g of Ag-coated Cu nanowires (Cu-Ag nanowires) in 1 h. Due to the large diameters of Cu nanowires (≈240 nm) produced by this synthesis, a Ag:Cu mol ratio of 0.04 is sufficient to coat the nanowires with a protective, oxidation-resistant shell. This multigram synthesis of Cu and Cu-Ag nanowire production process enabled the development of the first nanowire-based conductive polymer composite for 3D printing with a resistivity of 0.002 Ω cm, which is >100 times more conductive than commercially available graphene-based 3D printing filaments. Furthermore, a felt-like material consisting of Cu-Ag nanowires was also used to create a stretchable conductor that has a conductivity of 1000 S/cm and is stretchable up to 300%. Finally, Cu nanowires were annealed to create a porous flow-through electrode (FTE). Compared to other commercially available FTEs, Cu nanowires are significantly thinner, resulting in a 15x increase in surface area compared to carbon paper. This translates to a 4.2x increase in the productivity of an electroorganic intramolecular cyclization reaction. </p><p>We also developed a self-heating synthesis of Cu nanowires, which produced shorter, thinner nanowires than was previously possible. Self-heating is accomplished through NaOH dilution and glucose degradation, which allows us to tune the length of the Cu nanowires in the range of 1.0 to 5.5 µm by simply changing the concentration of glucose, and thereby the temperature of the reaction. The self-heating aspect of this synthesis decreases the energy cost associated with producing Cu nanowires, simplifies the reaction procedure, as well as increases its scalability. </p><p>Finally, we employed single-crystal electrochemistry to elucidate the growth mechanism of Cu nanoplates. Cu nanoplates have also been used in electronic and catalytic applications, yet their growth is not as well-understood as Cu nanowires. Thus, before they can be produced at quantities similar to that of Cu nanowires, we need to understand how they grow. Single-crystal electrochemistry allows us to replicate the crystal structure on the surfaces of Cu nanoplates using single crystal electrodes. The current density (jmp) of Cu(111) and Cu(100) single crystal electrodes were measured in the nanoplate growth solution, which showed that Cu reduction is 2.6x higher on Cu(100) compared to Cu(111) in the presence of iodide ions (I-) in a solution containing a CuCl2 precursor and hexadecylamine (HDA). Increasing the concentration of I- results in a higher ratio jmp(100)/jmp(111), which alludes to the passivation of the {111} basal surfaces of Cu nanoplates, resulting in lateral growth. This work clarifies the role of halide ions on the growth of Cu nanoplates, giving us insight into how size-control can be achieved. </p><p> As a whole, this work aims to increase the impact of Cu nanocrystal-based innovations by increasing throughput, simplifying reaction procedures, and achieving control over the size and dimensions of the synthesized product.</p>