## 1 Introduction  
Currently, the pollution crisis is one of the major global problems. In order to overcome this problem, several research groups have turned their attention towards various kinds of nanomaterials, because such materials exhibit outstanding performance in gas sensor applications for controlling the emission of polluting gases. A ternary metal oxide and composites such as NiMn\({}_{2}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\)/ZnO and Ag-SnO\({}_{2}\)/\(\delta\)C\({}_{3}\)N\({}_{4}\)[1; 2; 3] have been developed as promising materials for use in gas sensing applications. Other materials, such as metal stannates including Ba\({}_{2}\)SnO\({}_{4}\), Ca\({}_{2}\)SnO\({}_{4}\), Zn\({}_{2}\)SnO\({}_{4}\), Cd\({}_{2}\)SnO\({}_{4}\), Mg\({}_{2}\)SnO\({}_{4}\), Mn\({}_{2}\)SnO\({}_{4}\) and Co\({}_{2}\)SnO\({}_{4}\), have also been developed by research groups [4; 5; 6; 7; 8; 9; 10]. These materials have been found to have interesting properties and demonstrated good optical and electrical performance, and have been widely used in the study of solar cells, super capacitors, etc. [11; 12; 13; 14; 15; 16; 17; 18]. So far, there have been very few studies on their gas sensing performance.  
In this context, special attention has been paid to the cobalt stannate Co\({}_{2}\)SnO\({}_{4}\) (CTO), which has the potential for many applications in various fields, including thermally stable capacitors [19]. Nevertheless, little study has been undertaken in the field of Co\({}_{2}\)SnO\({}_{4}\) synthesis, or on its gas detection properties. Thus, to the best of our knowledge, synthesis of Co\({}_{2}\)SnO\({}_{4}\) by the technique of spray pyrolysis and the study of its sensing properties under ethanol vapor (EtOH) has not been yet reported. In addition, there has been no study of the sensing properties of Co\({}_{2}\)SnO\({}_{4}\)/Co\({}_{3}\)O\({}_{4}\) composite under ethanol vapor. Among the most commonly used materials for ethanol detection are Co\({}_{3}\)O\({}_{4}\)[20; 21; 22] and its composites, such as ZnO/Co\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\)@Co\({}_{3}\)O\({}_{4}\), Co\({}_{3}\)O\({}_{4}\)-TiO\({}_{2}\) and Co\({}_{3}\)O\({}_{4}\)/Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\)[23; 24; 25; 26]. Most sensing devices for various kinds of gases are based on metallic oxide layers [27]. In the present work, the low-cost technique of spray pyrolysis followed by an annealing process was successfully used to synthetize ternary Co\({}_{2}\)SnO\({}_{4}\) and also Co\({}_{2}\)SnO\({}_{4}\)/Co\({}_{3}\)O\({}_{4}\) sensing layers for ethanol (EtOH) vapor detection. The Co\({}_{2}\)SnO\({}_{4}\)/Co\({}_{3}\)O\({}_{4}\) composite was the resultant product from an annealing treatment, as we will show later in this work.  
Ethanol vapor was chosen as the target gas in this study, due to its toxicity [28] and its application in other safety sectors. Thus, there is a necessity to develop more effective sensors for ethanol detection [29]. Therefore, many ethanol sensing methods based on physical and chemical processes have been developed [30]. In all these methods, different materials were tested in order to find an appropriate material with the best sensitivity and selectivity under ethanol vapor to be used as the active layer for an ethanol gas sensor in a given application. Such sensors could be used, for example, in food quality checks, or in the bioenergy industry in ethanol production. Ethanol is also considered as a renewable fuel, which can be produced from various materials and plants, collectively known as \"biomass\". In this context, Co\({}_{2}\)SnO\({}_{4}\) could be introduced as a promising new candidate for the ethanol gas sensor industries. The morphological characterizations of our prepared Co\({}_{2}\)SnO\({}_{4}\) film, as well as its ethanol sensing properties and sensing mechanism, were investigated according to the annealing temperature.