Abstract
Typically, anodic oxidation of metals results in the formation of hexagonally arranged nanoporous or nanotubular oxide, with a specific oxidation state of the transition metal. Recently, the majority of transition metals have been anodized; however, the formation of copper oxides by electrochemical oxidation is yet unexplored and offers numerous, unique properties and applications. Nanowires formed by copper electrochemical oxidation are crystalline and composed of cuprous (CuO) or cupric oxide (Cu2O), bringing varied physical and chemical properties to the nanostructured morphology and different band gaps: 1.44 and 2.22 eV, respectively. According to its Pourbaix (potential-pH) diagram, the passivity of copper occurs at ambient and alkaline pH. In order to grow oxide nanostructures on copper, alkaline electrolytes like NaOH and KOH are used. To date, no systemic study has yet been reported on the influence of the operating conditions, such as the type of electrolyte, its temperature, and applied potential, on the morphology of the grown nanostructures. However, the numerous reports gathered in this paper will provide a certain view on the matter. After passivation, the formed nanostructures can be also post-treated. Post-treatments employ calcinations or chemical reactions, including the chemical reduction of the grown oxides. Nanostructures made of CuO or Cu2O have a broad range of potential applications. On one hand, with the use of surface morphology, the wetting contact angle is tuned. On the other hand, the chemical composition (pure Cu2O) and high surface area make such materials attractive for renewable energy harvesting, including water splitting. While compared to other fabrication techniques, self-organized anodization is a facile, easy to scale-up, time-efficient approach, providing high-aspect ratio one-dimensional (1D) nanostructures. Despite these advantages, there are still numerous challenges that have to be faced, including the strict control of the chemical composition and morphology of the grown nanostructures, their uniformity, and understanding the mechanism of their growth.
Highlights
Nanostructured anodic oxides have attracted the attention of researchers due to their ease of fabrication and tailored ordered morphology on the nanometric scale
The majority of the nanostructures obtained by transition metals anodization are composed of oxide, where the metallic element is at one fixed oxidation state
Fluoroalkyl-silane (FAS-17) was chemically bonded to CuO nanoneedles, increasing the contact angle up to 169◦; the corrosion performance was significantly improved Nanowires‘ surface was modified by the chemical bonding of 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (FAS-17) in order to increase the wetting contact angle to 154◦ Nanostructures enhanced photocatalytic water splitting; the best results were achieved for nanowires made of both Cu2O and CuO
Summary
Nanostructured anodic oxides have attracted the attention of researchers due to their ease of fabrication and tailored ordered morphology on the nanometric scale. The only reported exception is anodically grown ZnO: in this case, the grown oxide is made of nanowires [26,27] Another promising metal for oxide nanostructures fabrication via self-organized anodization is copper. The anodization of copper, leading to the formation of nanostructures, has not been as intensively explored as other methods and was not even included in the numerous review reports focusing on the formation of cupric and cuprous oxide nanostructures. Self-organized anodization seems to be a promising method in copper oxides formation, providing high-surface area nanostructures with a band gap tunable by operating conditions (size of the nanostructures) and chemical composition (in situ doping). 3 of 19 3 of 19 ocopntidmitiizoantsio. nMoofrethoeveorp, erercaetinntgacdovnadnicteios nins.AMl aonredoTviearn, roedcieznattiaodnv, aans cweeslilnasArleacnendtThiiganho-tdecizhaatipopnl,icaastwioenlsl, amsaryecpernotvhidigehs-otemche ainpsppliircaattiioonnsf,omr ealyecptrroocvhideme siocaml ecoinpsppeirraotxioidnaftoiorne.lectrochemical copper oxidation
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