Tailoring Copper Nanostructures on TiO ₂ Nanotube Arrays via Controlled Electrochemical Deposition and Diazonium Surface Chemistry

The advancement of nanostructured TiO 2 catalytic systems requires the tailoring of self-organized TiO 2 nanotube arrays decorated with morphologically controlled copper materials. In this work, a comprehensive study of the electrolytic deposition of copper onto anodized TiO 2 electrodes─consisting of vertically oriented, individual nanotubes─in H 2 SO 4 /CuSO 4 media is presented. The results shed light on the complexity of the two-step cathodic processes, involving an initial formation of mono- and/or adlayers of copper on the surface, followed by a second bulk reduction and deposition step. The kinetics of these two distinct reduction processes are governed by not only diffusion but also mass-transfer phenomena. The fundamental insights obtained provide practical strategies for size- and shape-controlled deposition of metallic copper onto TiO 2 nanotube electrodes by tuning either the electrochemical parameters of cyclic voltammetry (working potential range or scan rate) or the concentration of copper cations in the electrolyte solution. In addition to direct electrodeposition, a one-step spontaneous approach based on diazonium chemistry is reported for the covalent grafting of 4-sulfophenyl groups onto the surface of TiO 2 nanotubes. Owing to the strong electrostatic interaction between sulfonate functionalities and copper ions, the modified surface is readily functionalized with copper cations, which are subsequently converted into metallic copper via a simple chemical reduction using a reducing agent. Stripping voltammetry performed on pristine TiO 2 nanotube arrays and copper-loaded samples, together with XPS and SEM analyses, confirms the presence of metallic copper on the modified surfaces and contributes to a better understanding of the mechanisms governing copper deposition.