Fabrication of copper nanowires via electrodeposition in anodic aluminum oxide templates formed by combined hard anodizing and electrochemical barrier layer thinning

Anodic aluminum oxide was formed by employing mild and hard anodizing in sulfuric acid followed by mild anodizing in oxalic acid without oxide removal in-between at 40 and 45 V. Such multi-step anodizing, combining hard anodizing in sulfuric acid with mild anodizing in oxalic acid allowed to form a highly-ordered nanoporous template with a barrier layer at the pore bottoms thin enough for further processing. Four different conditions of electrochemical barrier layer thinning, with varied voltage steps and their time durations, were investigated. Optimized conditions allowed to provide conductivity at the pore bottoms and made the nanoporous oxide templates suitable for electrodeposition. It was found that the most effective barrier layer thinning approach employs voltage steps U n + 1 = 0.75·U n with each step (n) being 10 s long. To check applicability of the formed templates, copper electrodeposition from sulfate-borate bath was done. Copper nanowires with average length of about 14–16 μm and diameter of about 35–40 nm were obtained by using through-hole AAO templates.