**1. Introduction**

Population growth and rapid urbanization/industrialization are among the greatest causes of environmental pollution and consumption of a large amount of energy [1]. Synthetic organic dyes that are used in various industries such as pharmaceuticals, textiles, cosmetics, paper, and plastic factories have led to severe environmental pollution as a result of their discharging contaminated and colored wastewater into the water stream [2]. They adversely affect the quality of water, prevent light penetration and diminish photosynthetic reactions. Moreover, some dyes are both poisonous and cancer-causing [3]. To address the above issues, several treatment methods have been commonly used. The treatment techniques including adsorption, chemical precipitation and coagulation show reduced efficiency and generate other pollutants like toxic gases and slurry that require additional purification [4,5]. Therefore, advanced technology-based treatments have been suggested for the removal of these pollutants.

Advanced oxidation processes (AOPs) have attracted much attention as a substitute for traditional treatment routes for the removal of toxic organic pollutants into harmless products. AOPs have benefits such as degradation of organic pollutants to green products and capability of operating at normal temperature and pressure [6]. Among AOPs, heterogeneous photocatalysis is an emerging technique, which is valuable for environmental and energy applications. It is a photochemical reaction which takes place on the surface of the solid catalyst and encompasses oxidation from photogenerated holes and reduction from photogenerated electrons at the same time [7]. TiO2, ZnO, CdS, ZnS and Fe2O3 are some of the outstanding semiconductors used as photocatalysts [8]. Among these, TiO2 and ZnO are the top applicable as photocatalysts [9,10]. The energy levels of these semiconductors are nearly comparable. However, ZnO is easily obtainable; it absorbs a large portion of solar light and has great photocatalytic performance than TiO2 [11,12].

Zinc oxide-based materials are used in the area of multifunctional electrode for both energy conversion and storage applications, like lithium-ion batteries and Dye-sensitized Solar Cells [13], gas sensors, monitoring air quality and optical devices due to its exceptional properties, for example, being inexpensive, photoconductive response, pyroelectricity and surface functionalization [14], high binding energy and electron mobility [15]. This metal oxide based semiconductor also has great application in the field of optoelectronic devices such as light-emitting diodes, flat panel displays, transparent semiconductors and conductive oxides, due to its good optical properties [16]. Different research investigated sensing and photovoltaic applications of ZnO-based materials such as ammonia gas sensing using Ag/ZnO flower and Cu-doped ZnO nanostructures [15,17], tin-doped ZnO thin films as a NO2 gas sensor [18], ZnO@In2O3 coreshell nanofibers for ethanol vapor sensing [19], ZnO-based quaternary transparent conductive oxide materials for solar cells [16], and CdO-ZnO nanocones for efficient electrode materials [18]. The application of lithium ion batteries has been a major success in small electronic devices. However, the shortage of lithium resources challenges its application in large-scale electrical energy storage systems [20]. The Layered Sodium Transition-Metal Oxides are promising materials that can minimize the challenges of lithium batteries due to excellent cyclic stability and rate performance which significantly contribute to the development of large-scale electrical energy storage systems [21].

However, ZnO absorbs only in the ultraviolet section of electromagnetic radiation for the reason that its bandgap energy is large. As a result, its photocatalytic activity is low under solar radiation as the ultraviolet (UV) constituent of solar energy that touches the Earth is only 3–5% [22,23]. Nowadays, the existing photocatalysts such as ZnO are modified by doping or co-doping with metals and non-metals to enrich their photocatalytic activity [24,25]. Moreover, compositing of dissimilar nanostructured semiconductors develops their photocatalytic performance by sharing of their charge carriers to each other [26].

Cuprous oxide (Cu2O) is a narrow bandgap semiconductor which has been thought of as a possible visible light photocatalyst. Electrons of Cu2O can undergo a transition from the valence band to the conduction band using visible light as a source of energy. However, the photo-induced electrons and holes recombine within microseconds after their generation, which can influence its photocatalytic action negatively. Up to the present time, graphene and selected metals were coupled with Cu2O to delay the recombination of the photoinduced electrons and holes. Cu2O joined with large bandgap metal oxides such as ZnO is expected as an operational means to control the problem of recombining the charge carriers [27]. In this work, we synthesized a N-doped Cu2O/ZnO nanocomposite via co-precipitation and thermal decomposition methods and tested its photocatalytic activity in the degradation of methyl red (Scheme 1), which is considered as model dye pollutant.

**Scheme 1.** Structure of methyl red.

#### **2. Experimental**
