is reason, the surface of the p-GaN was treated in hydrofluoric acid followed by the deposition of Au catalytic nanoparticles on the p-GaN. The ZnO nanowires with high density were prepared on the treated p-GaN layer by the VLS synthesis method \[[@B42-sensors-20-00579]\]. The NiO/ZnO nanowire-based heterostructures were obtained by VLS and vs. procedures separately. At first, NiO nanowires were prepared via the VLS mechanism using Au as a catalyst. The ZnO nanowires were directly synthesized on NiO based on vs. mechanism \[[@B43-sensors-20-00579]\].

Thermal oxidation of metallic thin films is another approach that may be used to grow metal oxide nanowires in a furnace at elevated temperatures. In this case, the metallic films may be deposited on the substrates by radio frequency (RF) magnetron sputtering \[[@B44-sensors-20-00579]\]. Then, the samples are placed in the furnace and the synthesis procedure of nanowires is carried out under the oxygen flow. The WO~3~ nanowires were obtained by thermal oxidation of tungsten metallic films at 600 °C for 1 h \[[@B22-sensors-20-00579]\].

A few methods, such as the electrochemical anodization \[[@B36-sensors-20-00579],[@B37-sensors-20-00579],[@B38-sensors-20-00579],[@B39-sensors-20-00579]\], atomic layer deposition (ALD) \[[@B40-sensors-20-00579],[@B41-sensors-20-00579],[@B42-sensors-20-00579]\] and hydrothermal synthesis \[[@B43-sensors-20-00579],[@B44-sensors-20-00579],[@B45-sensors-20-00579]\] have been developed for the preparation of metal oxide nanotubes \[[@B45-sensors-20-00579],[@B46-sensors-20-00579],[@B47-sensors-20-00579],[@B48-sensors-20-00579],[@B49-sensors-20-00579],[@B50-sensors-20-00579],[@B51-sensors-20-00579]\]. ALD is a template-assisted technique for the synthesis of nanotubular arrays and requires post-processing separation of the obtained tubes from the template \[[@B52-sensors-20-00579]\]. The fabrication of well-ordered metal oxide nanotubes with the homogeneous tube-size distribution over the substrate by the hydrothermal growth method is diff