# Formation of ZnO/CuO Heterostructures Based on Quasi-One-Dimensional Nanomaterials
## 4 Synthesis of ZnO/CuO Heterostructures Based on Quasi-Dimensional Nanomaterials  
Unlike metals, the appearance of charge carriers in semiconductors is defined by many factors, among which the most important are the purity of the semiconductor and its  
Figure 5: Field-emission scanning electron microscopy (FE-SEM) images of ZnO nanowires grown on ITO-coated glass substrates: (**a,b**) non annealed, (**c,d**) annealed. The inset images (**a,c**) show the side views of the ZnO nanowires [112]. temperature [114]. Controlling the conductivity in ZnO remains an important issue. Even relatively small concentrations of intrinsic point defects and impurities can significantly affect the electrical and optical properties. As shown in [115], understanding the role of intrinsic point defects and unintentionally included shallow-donor impurities is the key to controlling the conductivity in ZnO. In optoelectronics, some progress has been made towards creating emitting p-n junctions based on ZnO nanowires by doping them. There are reports of p-type doping in ZnO [116; 117]. The properties of zinc oxide-based materials can be improved by creating heterojunctions with other metal oxides. ZnO n-type nanowires combined with crystalline or polymeric p-type semiconductors are used. For instance, photocatalytic activity may be enhanced due to an increase in light absorption in the active material, a shift in absorption into the visible region of the spectrum and suppression of recombination of photoinduced charge carriers [118]. The synthesis of materials with such improved characteristics is important for the development of solar photovoltaics [119; 120]. Copper oxide II (CuO), which is an indirect-gap semiconductor with p-type conductivity, can be used as a second material for heterojunction formation. As shown in [121], the formation of the ZnO/CuO p-n junction leads to an increase in photocatalytic activity due to changes in the spectral characteristics of the formed materials and more effective separation of the generated charge carriers. The self-cleaning activity of the CuO/ZnO heterostructure during photocatalysis has been demonstrated in [122]. Increased photocatalytic activity is associated with efficient separation of charge carriers due to the exploitation of the heterostructure [123]. Examples of the use of ZnO/CuO heterostructures and prospects for solar energy are presented in [124; 125; 126]. Figure 6 shows an X-ray diffraction pattern of samples from a CuO, ZnO and CuO/ZnO heterostructure synthesized by the microwave-assisted hydrothermal method [125]. In order to improve the efficiency of ZnO/CuO heterojunction photocells instead of a thin layer in [127] the use of one-dimensional ZnO structures was proposed, which provides an opportunity to increase the specific contact area of the heterojunction. In [128], a photodetector based on an array of axial n-ZnO/p-CuO heterostructure nanowires was fabricated on a Si silicon substrate by sliding angle deposition. A UV-A photodetector operating in the long-wave ultraviolet region from 320 to 400 nm showed a high rectification ratio, low dark current and fast photoresponse [129]. In [130], high-performance formaldehyde gas sensors based on ZnO/CuO heterostructure have been developed and the detection mechanism has been investigated. It has been shown that the performance of the ZnO sensor can be effectively improved by performing a heterojunction. Figure 7 shows high-resolution transmission electron microscopy images of the ZnO/CuO heterostructure [130]. ZnO/CuO heterojunction has also shown good selectivity for ethanol [131]. There is a probability that the ZnO/CuO interface is not chemically sharp. In such cases, some of the observed properties can be attributed to unintentional doping rather than heterojunction. It is assumed that CuO doping introduces an additional impurity level into the bandgap zone of ZnO.