# Formation of ZnO/CuO Heterostructures Based on Quasi-One-Dimensional Nanomaterials
## 3 Applications and Peculiarities of Synthesis of ZnO-Based Quasi-One-Dimensional Nanostructures  
Zinc oxide is a semiconductor compound, which, due to its piezo- and segnetoelectric properties, has many practical and very promising applications [65, 66, 67]. Of particular importance is the fabrication of structures based on nanoelements such as nanowires, nanorods, nanofibres and nanofilms [68, 69, 70, 71, 72]. The Atomic force microscopy (AFM) images of ZnO nanofilms are shown in Figure 3[72]. Nanostructures based on zinc oxide are used in opto- and microelectronics, as well as in microsystems technology. ZnO has potential applications in the production of light-emitting diodes and lasers in the ultraviolet spectrum, as well as in the manufacturing of solar cells, scintillators, piezoelectric devices and others [73, 74, 75, 76, 77]. Potential applications of ZnO nanowires in various types of solar cells have been intensively studied [78, 79, 80]. Figure 4 shows the morphological characteristics and dimensions of an ideal network of ZnO nanowires for optimal solar cell assembly [79]. Nanostructured zinc oxide has been used as a wide-band semiconductor in photoelectrochemical cells [81, 82]. One-dimensional nanocrystal arrays of ZnO are a promising material for creating piezoelectric nanogenators [83, 84, 85]. ZnO has photocatalytic efficiency and allows the degradation of contaminants in both alkaline and acidic environments [86, 87]. Specific conductivity is an important factor in the photocatalysis process. Surface defects, especially oxygen defects, directly affect the photocatalytic activity of metal oxide semiconductors. Arrays of ZnO nanorods with high surface-to-volume ratios were shown in [88, 89, 90] to be  
Figure 2: SEM images after heat treatment of three surfaces: copper foil (**a**), vapour deposited copper (**b**), and sputtered copper (**c**) [64]. promising photocatalysts for water purification both obtained by chemical bath deposition and under hydrothermal synthesis conditions. A miniature fibre thermometer based on ZnO nanorods embedded into the tip of a ZnO nanowire has been experimentally demonstrated in [91]. Such a temperature sensor offers the advantages of compactness, stability and ease of fabrication. As a quasi-one-dimensional oxide wide-band semiconductor nanomaterial, zinc oxide nanowires, due to their high specific surface area and sufficient electrical conductivity, are used in sensor devices for the detection of hydrogen sulphide [92], hydrogen [93, 94], ethanol [95, 96, 97], benzene and toluene [98]. Such devices gain distinct advantages over commercially available sensors [99, 100, 101, 102] and provide increased selectivity and reduced energy consumption. Possible implementations of these structures include metal-oxide layered materials that can be used as materials for electrical contacts. A sensitive characteristic of ZnO nanowires in relation to the composition of the surrounding atmosphere  
Figure 4: Morphological characteristics and dimensions of an ideal network of ZnO nanowires (ZnONW) for optimal solar cell assembly: ZnONW length \(\sim\)200 nm, ZnONW diameter \(\sim\)30 nm, ZnONW spacing \(\sim\)100 nm (**left**) and cross-section, XY plan (**right**): HTL—hole transport layer; ITO—oxide layer consisting of a mixture of 90 wt% indium oxide In\({}_{2}\)O\({}_{3}\) and 10 wt% tin oxide SnO\({}_{2}\)[79]. Figure 3: Atomic force microscopy (AFM) images of ZnO nanofilms (**a**) image 3 \(\times\) 3 \(\upmu\)m\({}^{2}\), (**b**) rough profile, (**c**) 3D image, (**d**) Zoomed image 1 \(\times\) 1 \(\upmu\)m\({}^{2}\), (**e**) rough profile, (**f**) 3D image [72]. is electrical conductivity. The most important properties of sensors such as sensitivity, response time, and operating temperature are determined by both surface quality and structure of ZnO nanowires [103; 104; 105].