# Formation of ZnO/CuO Heterostructures Based on Quasi-One-Dimensional Nanomaterials
## 1 Introduction  
With the development of micro- and optoelectronics, sensor and microsystems technology, requirements for the functionality and energy reduction of designed devices are increasing while reducing the weight and size of components. Therefore, research on nanomaterials that contribute to the increasing demands of electronics, photonics, catalysis, sensorics, and other high-tech industries is becoming topical [1; 2; 3]. Nanostructured metal oxides (e.g., in the form of nanowires, nanorods, nanotubes), which are being actively researched and developed, are of great interest both for promising research and for a wide range of applications. Such oxide nanomaterials demonstrate unique properties, which may be significantly superior to those of their macroscale analogs [4; 5]. One of the fields for the application of metal oxides is heterogeneous catalysis, in which they are used as carriers as well as active phases [6]. Currently, in the chemical industry, there is an evident tendency for a transition from homogeneous to heterogeneous catalytic processes, so the proportion of homogeneous catalytic processes is decreasing and constitutes only about 10% [7]. In heterogeneous catalysis, unlike homogeneous catalysis, the active centres are not each molecule of the catalyst, but only the available atoms onthe surface. The requirement for a high concentration of catalytic centres in the adsorption process [8] motivates the use of nanostructured materials. The use of nanostructured metal oxides provides an opportunity to develop effective catalysts with high selectivity, stability and responsiveness [9]. Much attention has been paid to the application of synthesized one-dimensional (1D) metal oxide nanostructures. This is also facilitated by the ability to adapt the properties of metal oxides by controlling their shape, size and composition [10]. The use of metal oxide nanomaterials can also increase the selectivity of heterogeneous catalytic processes, i.e., the ability to selectively accelerate the target reaction without increasing the temperature and pressure. Due to their high surface area-to-volume ratio and inherent surface defects, these materials show the highest catalytic efficiency [11]. An example is the nanowire complex metal/metal oxide catalysts described in [12], which showed high catalytic activity in the CO oxidation reaction. Metal oxide-based semiconductor nanowires with unique electrical, mechanical and optical properties and a high surface-to-volume ratio have become a promising and attractive base for sensor devices [13; 14; 15]. This is facilitated by the fact that in addition to good thermal and chemical stability, such materials have excellent sensitivity to changes in environmental conditions. Their gas-sensitive properties have been widely studied to detect and measure the concentration of various substances [16; 17]. A smaller sensor size results in lower power consumption due to reduced thermal mass, faster heating and easier integration with small microchips. The larger effective area, which enhances gas adsorption phenomena, is a major advantage of nanostructured materials [18]. Resistive-type gas sensors are widely used to detect various toxic gases and volatile organic compounds [19; 20; 21]. Nanowires and metal oxide heterostructures are key components of future sensor devices due to their special properties [22; 23; 24]. New applications of sensors based on metal oxide semiconductor nanowires in disease diagnosis, environmental engineering, safety and security are being explored [25]. There are two main approaches for obtaining arrays of nanostructures on metal substrates. The first one involves the application and improvement of technologies developed in the semiconductor industry. Deposition and local removal of material on planar substrates is performed to reduce the size of structures to the nanometer scale. Such methods as nanoimprint [26; 27], electron beam [28; 29], ion beam [29; 30], and scanning probe lithography [31] are utilized for the selective removal of materials.