# Formation of ZnO/CuO Heterostructures Based on Quasi-One-Dimensional Nanomaterials
## 2 CuO Nanowires: Fabrication Methods, Areas of Application  
Copper oxide nanowires have become important in optoelectronic devices and systems for energy conversion. The methods of CuO nanowires growing include wet-chemical [40; 41], solution-based, electrochemical [42] and hydrothermal methods. The methods of thermal and plasma oxidation are also used, which are considered to be the most prospective ones from the point of view of advanced synthesis of nanowires. A combined hybrid wet chemical method in which nanowires of copper hydroxide are transformed into copper oxide by plasma oxidation is promising. According to [43] it was shown that although the desired improved properties have been demonstrated, materials containing copper oxide nanowires cannot yet be produced in large quantities, for wider use. Therefore, the search must continue for the most efficient synthesis processes that provide not only high quality and improved properties of such materials but also higher productivity. One promising approach for the growth of nanomaterials is thermal oxidation. The creation of copper oxide II (CuO) nanowires by thermal oxidation of copper in the air appears to be a simple and relatively cost-effective strategy [44; 45], including the use of additional electrical or magnetic actions [46; 47]. Possibilities for the practical application of copper (II) oxide nanowires continue to extend actively. For example, ref. [48] reports that a highly sensitive electrochemical sensor with CuO nanowire electrodes was fabricated on the copper substrate and used for the non-enzymatic determination of glucose. The development of high-performance gas sensors based on CuO nanowire networks demonstrating fast response/recovery times with ultra-high response is of great importance for gas sensing applications. The sensors fabricated in [49] demonstrated improved properties in the detection of ethanol vapour. Sensors based on CuO nanowires with different layer morphologies have been used for hydrogen and ammonia detection. Changing the surface morphology of the nanowires has been shown to influence the magnitude of the electrical response [50]. In [51] copper oxide II (CuO) nanowires were synthesized by thermal oxidation of copper foil (Figure 1) and deposited on fluorine-doped tin oxide-coated glass substrates. The thermal oxidation method has been shown to improve surface contact, which is promising for solar cell and catalytic applications. The use of CuO nanowires as basic components of nanoelectromechanical switches was presented in [52]. In [53], CuO nanowires demonstrated unique characteristics with respect to the electrocatalytic oxygen extraction reaction. Arrays of CuO nanowires have been synthesized for use as the anode material in Li/Na-ion batteries [54]. Copper oxide (CuO) nanowires are an effective material for catalysis [55], including photocatalysts [56; 57; 58]. In most cases, nanowires can be synthesized either in the supercritical liquid phase, in the vapour phase, or by thermal oxidation. The morphology of CuO nanowires, for example, can be controlled by changing the annealing temperature during thermal oxidation [59]. Nevertheless, ref. [60] shows that the growth mechanism of CuO nanowires, like other metal oxides, has yet to be fully understood. In the fabrication of CuO nanowires by thermal oxidation, even the initial thickness and configuration of the copper substrate have an influence on the morphology of the resulting CuO nanowires and electrical contact properties [61]. Geometric optimization of nanowire arrays, which is considered to be a simple and important way to improve the characteristics of photoelectrocatalytic systems based on copper oxide nanowires, has also not yet been implemented [62]. Different variants to solve this problem have been proposed. For example, in [63] laser texturing was proposed for nanowires synthesis to increase adhesion to the copper substrate before thermal oxidation.