Review of Fabrication Methods, Physical Properties, and Applications of Nanostructured Copper Oxides Formed via Electrochemical Oxidation

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.

Robust Inorganic Hole Transport Materials for Organic and Perovskite Solar Cells: Insights into Materials Electronic Properties and Device Performance

“…It shows a strong quantum confinement effect below that thickness range. [87,88] Its electron configuration has an open 3d cell (3d 9 ), which is considered to significantly affect the ionization states and its band structure. While cuprous oxide (Cu 2 O) has a cubic structure, CuO is monoclinic with lattice a, b, and c parameters approximately equal to 4.68, 3.42, and 5.13 Å, respectively, with each atom neighboring four atoms of the other kind ( Figure 5d).…”

The Use of Anodic Oxides in Practical and Sustainable Devices for Energy Conversion and Storage

“…The former case refers to barrier-type oxide films [ 2 ]. The latter case involves the formation of nanostructured oxide films with different morphologies, such as nanopores, nanotubes, nanowires, nanorods, nanopetals, and other nanogeometries [ 1 , 2 , 3 , 41 , 42 ]. The type of oxide formed depends on the experimental conditions.…”

“…The composition of the oxide surface can be also be tuned in this step. The anodization of Cu in alkaline solutions, for instance, leads to the formation of copper hydroxides that can be converted into CuO or Cu 2 O species after an annealing step [ 42 ]. Kang et al [ 51 ] utilized a CO atmosphere to convert anodized WO 3 into WC for a cathode in PEC applications.…”

Developments of the Electroactive Materials for Non-Enzymatic Glucose Sensing and Their Mechanisms

“…Most of the commercialized glucose sensors are enzymatic ones [8]. However, nonenzymatic glucose sensors are expected to have high potential because of their relatively high environmental stability [53][54][55], good sensitivity [56,57], high long-term stability [58], and time-efficient manufacture [42,59]. Literature reporting the non-enzymatic glucose sensor published between 2000 and 2020 is shown in Figure 1.…”