Zinc Nanoparticles Electrodeposited on TiO2 Nanotube Arrays Using Deep Eutectic Solvents for Implantable Electrochemical Sensors

This work reports on the electrodeposition of zinc on nanoporous metal oxide titania nanotube arrays (TiO2 NTAs) using a zinc-containing deep eutectic solvent (Zn2+–DES). The effects of substrate morphology and crystallinity, deposition temperature, zinc concentration, and deposition time on the morphology and electrochemical properties of the Zn/TiO2 NTAs were investigated. Two-dimensional zinc nanohexagons (NHexs) with a mean diameter of ∼300 nm and a thickness of 10–20 nm were decorated onto the TiO2 NTAs (with a pore diameter of ∼80 nm and a tube length of ∼5 μm) via electrodeposition at −1.6 V using Zn2+–DES. Cyclic voltammetry tests on the Zn2+–DES electrolyte revealed an electrochemical window of ∼3.5 V, and the diffusion coefficient of Zn2+ was found to be 4.29 × 10–10 cm2 s–1 at room temperature and 7.10 × 10–10 cm2 s–1 at 40 °C. Zinc nucleation on TiO2 NTA substrates followed an instantaneous model. Using a higher electrodeposition temperature increased the nucleation and growth rate of zinc NHexs, while annealing TiO2 NTAs was found to improve their uniformity and morphology. During the initial stage of deposition, hexagonal close-packed zinc NHexs were found to preferentially grow on tetragonal anatase TiO2 NTAs. The electrodeposition of zinc resulted in lowering the impedance and improving the overall electrochemical properties of TiO2 NTAs. The Zn/TiO2 NTAs developed in this study offer a promising electrocatalyst material system for implantable electrochemical sensors.