**1. Introduction**

Recently, ammonia (NH3) sensors have been widely studied by researchers and widely used in the high volume control of combustibles in the chemical industry, the control of emission of vehicles, and the monitoring of dairy products in the food industry. Currently, many nanomaterials have been utilized in ammonia sensors. These include two-dimension materials [1–5], IV–VI metal chalcogenides, conductive polymers, and alkali metal materials. Among these materials, owing to the low cost, high sensitivity, and environmentally-friendly nature, tin monoxide and tin dioxide have been used to fabricate ammonia gas sensors [6–16]. Compared with tin dioxide, tin monoxide synthesized under lower temperature is more stable and absorbs ammonia more easily when utilized as gas sensors [17,18].

As one of the common pollutants and toxic gases, ammonia (NH3) can cause several effects on the human body like irritation of the eyes, skin, throat, and respiratory system. According to the US Occupational Safety and Health Administration (OSHA), the exposure of under 35 ppm of ammonia by volume in environmental air for 15 min or under 25 ppm of volume for 8 h potentially harms people's health [12,13]. However, it is impossible for humans to detect ammonia below 50 ppm, which reflects the importance of ammonia sensing. Hence, a highly sensitive and selective room temperature NH3 gas sensor is highly desirable in today s world [19,20]. In previous work, the effect of material structure on the sensitivity of indium oxide-based ammonia sensors has been discussed [21]. Deren Yang et al. demonstrated that broken indium oxide nanotube structure with ultrahigh surface-to-volume ratio exhibited higher performance than regular nanotube, nanowire, and nanoparticle [21]. This is because the ultrahigh surface-to-volume ratio material can potentially provide larger interface to absorb gas [22]. Shell structure is another structure with ultrahigh surface-to-volume ratio that is suitable for gas sensing [23]. After referring to this work and the fabrication of CdS nanoshell structure [24], we synthesized a SnO nanoshell structure that also possesses high surface-to-volume ratio with the aim of improving the sensitivity of our ammonia sensors. SnO is a kind of monoxide and the mechanism of ammonia sensing is related to the redox reactions. As is similar to other metal oxide sensors, when the SnO is exposed to air, oxygen will be adsorbed on its surface, and oxygen molecules attract electrons. As a result, the conductivity of the SnO decreases. Then, when the sensor is exposed to a reducing gas such as NH3, the reducing gas may react with the adsorbed oxygen molecules and release electrons into the SnO, thereby increasing the conductivity. From this mechanism, the oxygen adsorption in the primary step is very important for the performance of the sensor [14,21]. The oxygen adsorption relies on the oxygen vacancy of the material and high oxygen vacancy density of SnO nanoshell also contributes to its high sensitivity.

In this paper, we prepared Sn6O4(OH)4 as precursors through a facile solution method and further prepared SnO nanoshell through different annealing conditions. The morphology, structure, and chemical composition of our samples were investigated by instruments. Among all samples annealed under different conditions, Sample 3 showed shell structure and the highest response in ammonia sensing. Compared with reported works, gas sensors prepared by Sample 3 showed a much higher response. These prepared sensors also showed outstanding selectivity and stability. The mechanism of response was revealed, and two factors that contributed to the high sensitivity of the as prepared sensor were ultrahigh surface-to-volume ratio and high oxygen vacancy density. This work provided novel structure for conductive materials which were suitable for a high performance gas sensor. We hope this work will provide new ideas for applications of IV–VI metal monoxides in the gas sensor field.

#### **2. Experimental Details**

#### *2.1. Materials*

In this experiment, all chemicals used were of analytical grade and were applied as-received, without further purification. Thioacetamide and NaOH powder were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Stannous chloride (SnCl2·2H2O) was purchased from Aladdin Industrial Corporation (Shanghai, China). Ultrapure water that was used in the experiment was purified using the Millipore water purification system (Millipore Corporation, Burlington, MA, USA). *N*-menthylpyrrolidinone (1-methyl-2-pyrrolidinone) (NMP) was purchased from Sigma-Aldrich (St. Louis, MO, USA).
