Synthesis of copper based nanowires for optoelectronics and electrochemical applications

This thesis introduces new methodologies for the synthesis of copper Nanowires (CuNW) with high aspect ratio as well as copper nickel (CuNi) alloyed and core-shell NW with enhanced resistance to oxidation. The potential applications of Cu-based NW as Transparent Conductors (TC) and as electrocatalysts for the Hydrogen Evolution Reaction (HER) are also investigated.
The effect of the addition of alkane and alkene non-coordinating solvents with a range of carbon chains- from C8 to C18- on the NW aspect ratio was investigated, using Scanning Electron Microscopy (SEM) and Fourier Transform Infra-Red (FTIR) spectroscopy. It was also established that non-coordinating solvents such as alkanes and alkenes tune the reactivity between the copper precursor and the capping ligand oleylamine (OLA). It was found that the chain length, the bond saturation as well as the concentration of the solvent have an effect on the NW aspect ratio. The addition of middle chain C12 alkane, dodecane, and long chain C18 alkene 1-octadecene (ODE) produced NW with the best aspect ratios reaching 800 and 1100 respectively. An optimal aspect ratio was attained for an ODE:OLA ratio of 1:4.
CuNW transparent conductors (TC) films were fabricated. Treatments including lactic acid ligand exchange on NW, plasma and anneal pre- and post-treatments on thin films were carried out. It was found that a combination of a lactic acid ligand exchange followed by an H2/N2 plasma treatment gave the best optoelectronic performances with films presenting a sheet resistance Rs of 43 Ω/□ for a transmittance of 93%. The addition of a top layer was investigated to improve the resistance to oxidation of CuNW films. Different layers of coatings were compared, and it was found that 2 layers of ZnO stabilise the NW electrical properties for 50 days with a Rs of 27.5 Ω/□ for 76% transmittance.
CuNW were turned into bimetallic CuNi NW by transforming the original seed-mediated growth synthesis into a co-reduction method, using Zn2+ salts as catalyst for the reduction of both Cu and Ni precursors. It was found that the concentration of Zn precursor was critical to reduce Ni precursor into metallic Ni effectively. The counterion effect was also investigated: it was found that the presence of acetylacetonate ions resulted in the formation of core-shell NW preferentially while chloride ions favoured the formation of alloys. A mechanism for both CuNi core-shell and alloy formation was proposed. Structural differences between core-shell and alloys were studied using Scanning Transmission Electron Microscopy (STEM), TEM and X-Ray Powder Diffraction (XRD) analyses. The STEM study also showed that both bimetallic NW possess an oxide that reveals to be of a diffusive nature for core-shell NW and a protective nature for alloyed NW.
NW were investigated as potential electrocatalysts for HER. It was shown that core-shell NW have activities similar to NiNW ones. 1:1 Cu:Ni NW activity surpassed both CuNW and NiNW activities with a Tafel slope as low as 21 mV/decade and a current density of 0.31 mA/cm2, values similar to the ones exhibited by Platinum (Pt), therefore hailing a milestone in low-cost electrocatalyst for HER applications.

Keywords: Copper
Nanoalloys
Nanowires
Advisor: Watt, A