Cu2O-CuO layers were prepared in situ on copper foam substrates by thermal oxidation at 400 °C in air using different pretreatments with acetone, HCl and NaOH. The effect of the pretreatment in the shape and physicochemical properties of the Cu2O-CuO layers, as well as in the growth or inhibition of the copper oxide nanostructures was studied, and a growth mechanism is proposed. It was found that the pretreatment modulates the nucleation and growth of the copper oxide nanostructures, being the process with NaOH the most suitable to promote the formation of well-defined nanoneedles, while in the case of the samples pretreated with acetone and HCl, copper oxide layers with irregular shape microstructures were obtained. The composition, structural, morphological and optical properties of the copper oxide structures were determined by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance and photoluminescence spectroscopy. The results showed that in all cases, the presence of both copper oxides, Cu2O and CuO was observed, with an optical band gap of 1.0 and 1.3 eV. The copper oxide structures exhibited photoluminescence emission centered at 551 nm, related to the recombination of the electron-hole pairs in the samples. The materials prepared with a NaOH pretreatment showed the lower emission and recombination rate.
Moreover, the 3D Cu-Cu2O-CuO based materials were evaluated as photocathodes in a 0.5 M Na2SO4 solution and under Xe lamp illumination. The photoelectrode where 1D nanostructures were grown, exhibited the lower resistance to the charge transference in the Nyquist plots, the highest current density in the linear voltammetry and the highest photoresponse in the on-off light experiments. The improved electrical and physicochemical properties of the samples pretreated with NaOH was related to the particular 1D nanoneedle morphology, which promoted higher conductivity and photoresponse, lower resistance to the charge transference and lower recombination of free charge carriers, demonstrating the potential use of these electrodes for photoelectrochemical applications. Finally, this work proved that it is possible to grow well-defined and highly crystalline CuO nanoneedles on copper foam porous substrates through a simple, fast and clean method.
1D nanostructured copper oxides have attracted attention recently due to their potential use in several applications such as cathodes for lithium-ion batteries [1,2], gas sensors  and biosensors [4,5], catalysts  and photocatalysts , as well as photocathodes for hydrogen generation from water [8,9], since cuprous and cupric oxides are usually p-type semiconductors, with a direct band gap of approximately 2.0–2.5 eV and 1.3–1.7 eV, respectively . Moreover, these nanostructures are obtained from copper, an earth-abundant metal, and can be synthesized through facile and scalable methods such as simple thermal oxidation of the copper substrate [3,7,11]. Typically, copper oxide structures in the form of nanowires or nanoneedles are grown on copper substrates by thermal annealing at temperatures between 400 °C and 600 °C in air or oxygen atmospheres . CuO wires, sheets and flower nanostructures have been grown on copper foam substrates and studied as supercapacitors , while in other work, highly porous CuO structures for batteries and fuel cells were sculptured tunning the pore sizes through the variation of the deposition conditions . The scalable and facile anodization-calcination process was used also for the preparation of copper oxide anodes for ion-lithium batteries . Moreover, the copper-based nanostructures prepared by this method have exhibited high selectivity, sensitivity and stability for gas sensing applications . The shape of the nanostructures has been also modified through the use of surfactants, such as Triton X-100, for obtaining cabbage rose CuO structures on copper foam substrates , and core-shell CuO-NiO hybrid structures with enhanced supercapacitance performance have been prepared .