*2.4. Fabrication and Measurement of the Ammonia Sensor*

To fabricate ammonia sensors, 20 mg sample were mixed with 40 μL NMP (*N*-menthylpyrrolidinone 1-methyl-2-pyrrolidinone) and were grinded in a mortar for 20 min. Through a paint pen, the mixture was coated on sensors purchased from Winsen Electronic Technology Co., Ltd. (Zhengzhou, Henan Province, China). Finally, sensors were welded on substrates. The configuration of the as fabricated device is shown in Figure 1a. The as fabricated sensor devices were dried for 20 min at 50 ◦C in a vacuum oven. Then, ammonia gas sensing was tested by Navigation 4000 Series Smart Sensor Tester purchased from Beijing ZhongKe Micro-Nano Networking Science Technology Co., Ltd. (Beijing, China). The sensor response S was measured by systematically exposing sensors to different concentrations of ammonia (5 ppm to 200 ppm) at room temperature using the following equation S = (Gg − Ga)/Ga × 100, while Ga and Gg are the conductance of the sample in air and ammonia gas, respectively.

**Figure 1.** Configuration of the as fabricated devices and morphology of as prepared Sample 3. (**a**) The configuration of sensor devices. (**b**,**c**) TEM (transmission electron microscopy) image of Sample 3 at different magnifications. (**d**) TEM image of Sample 3 which expands the top-left section of (**b**). (**e**,**f**) HRTEM (high-resolution transmission electron microscopy) image of corresponding white solid frames marked in (**c**).

#### **3. Results and Discussion**

#### *3.1. Characteristic of SnO Nanoshell Material*

In this work, Sample 3, annealed and heated from 0 ◦C to 300 ◦C for 1 h and then kept under 300 ◦C for 1 h, possesses the nanoshell structure and exhibits the highest performance when utilized as the conductive material of ammonia sensors. The configuration of sensor devices and morphology of the as-synthesized Sample 3 is shown in Figure 1. Figure 1a shows the configuration of the sensor devices. Figure 1b shows the TEM image of Sample 3 which exhibits the accumulation of SnO nanoshells. Figure 1c shows the higher magnification TEM image of the top-left section of Figure 1b. We clearly observed the shell structure in the top-left region of Figure 1c. Further expansion of the white solid frame in Figure 1c shows the nanoshell structure with a hollow core, presented in Figure 1d, marked with white dashed lines. The morphology of Sample 3 in Figure 1 confirmed the existence of the nanoshell structure. The HRTEM image of the regions within the white solid frames of Figure 1d were shown in Figure 1e,f, showing a lattice fringe spacing of 0.27 nm and 0.30 nm, which corresponds to the (110) plane and (101) plane of SnO, respectively [10].

The crystal structure of the as-prepared Sample 3 is shown in Figure 2. Figure 2a shows the X-ray diffraction of Sample 2. Peaks at 18.2◦, 29.8◦, 33.2◦, 37.1◦, and 47.7◦ correspond to the (001), (101), (110), (002), and (200) crystal planes of SnO, respectively. All the diffraction peaks can be assigned to SnO (JCPDS Card No. 06-0395) [10,13]. From the X-ray diffraction, the intensity of (110) and (101) peaks was much higher than the other which matched well with these two planes and could be found easily in TEM. In addition, the Raman spectrum of Sample 3 is shown in Figure 2b. The peaks at 112 cm−<sup>1</sup> and 210 cm−<sup>1</sup> correspond to the B1g and A1g vibration mode of SnO, respectively [26]. In A1g mode, Sn atoms vibrate towards or away from O atoms. B1g mode corresponds to the out-of-plane vibrations of O atoms [27]. These characterizations confirmed that this material is SnO. The morphology and Raman spectra of other samples (Sample 1, Sample 2, Sample 4, and Sample 5) are provided in Figures S2–S5. From the supplementary material, Sample 3 and sample 5 showed shell structure and the density of shell structure of Sample 3 is higher than Sample 5.

**Figure 2.** Characterization of the crystal structure of Sample 3. (**a**) X-ray diffraction of Sample 3; (**b**) Raman spectrum of Sample 3.

X-ray photoelectron spectroscopy shown in Figure 3 was used to analyze the surface chemical composition of Sample 3. As shown in Figure 3a, only peaks that correspond to Sn, O, and C were observed. Figure 3b shows the high resolution spectrum of Sn 3d. Peaks at 485.4 eV and 493.8 eV correspond to the energies of Sn 3d5/2 and Sn 3d3/2, respectively. The energy gap that reveals the amount of energy splitting between the two core levels is 8.38 eV, which corresponds to the energy splitting of Sn2+ [26]. Figure 3c shows the peak that corresponds to the Sn–O bond at 529.4 eV in O 1s spectroscopy. This result is consistent with previous work [28,29].

**Figure 3.** X-ray photoelectron spectroscopy of as-prepared Sample 3. (**a**) Sample 3 has Sn, O in the full spectrum; (**b**) High-resolution XPS of Sn 3d; (**c**) Sn–O bond in O 1s spectroscopy.

#### *3.2. Test of Gas Sensor Device*

In the experiment, we prepared 15 samples (three samples for each kind) for ammonia sensing tests from 0 to 200 ppm. A sketch added in supplementary material Figure S6 was shown to describe the mechanism. The response of the fabricated ammonia gas sensors, shown in Figure 4, reveals the huge difference between Sample 3 and other samples. As shown in Figure 4a,b, the response of Sample 3 is 313%, 874%, 2757%, 3116%, and 3757% under gas concentration of 5 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm, respectively, which is much higher than the other four samples. This result demonstrates that the large surface area of the nanoshell structure is able to absorb more ammonia. The response to different ammonia concentrations is shown in Figure 4c,d. From Figure 4c,d, all sensors fabricated had approximate linear response to ammonia below 20 ppm. Due to saturation of absorbance to ammonia, curves have a lower slope after 50 ppm and only the response of Sample 3 and Sample 2 maintain an increasing trend. The accurate response of Sample 3 from 0 to 40 ppm was shown in Figure S7. This test aimed to reveal that the sensor fabricated with Sample 3 is sufficiently sensitive to work under lower concentrations. In supplementary material, Table S1 was also used to compare the differences of each sample. From the table, we concluded that the main factor that contributed to the highest sensitivity of Sample 3 was its highest surface to volume ratio resulting from its highest density of shell structure.

**Figure 4.** Response of ammonia gas sensors using different samples. (**a**) Response-recovery curves of the sensors up to 0–200 ppm NH3; (**b**) Magnification of the black dashed pane in (**a**); (**c**) Response towards five (5–200 ppm) different concentrations of NH3 in air; (**d**) Magnification of the black dashed pane in (**c**).

Response of as fabricated sensors compared with other work mentioned in this paper was shown in Figure 5a. Results in this figure were normalized as S = (Ra − Rg)/Ra × 100 where Ra and Rg are the resistance of the sample in air and ammonia gas, respectively. From the histogram, the response of our sensor is 97%, which is much higher than the other sensing materials reported in the literature [11–13,15,16]. The mechanism of ammonia sensing is related to the redox reactions. 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. During the sensing process, these reactions would take place:

$$2\text{ O}\_2\text{ (gas)} \to 2\text{O}\text{ (adsorbed)}\tag{3}$$

$$\text{O (adsorbed)} + \text{e}^- \text{ (from SrO)} \to \text{O} \tag{4}$$

$$2\text{ NH}\_3\text{ (adsorbed)} + 3\text{ O}^- \rightarrow \text{N}\_2 + 3\text{ H}\_2\text{O} + 3\text{e}^-\tag{5}$$

**Figure 5.** Response of as fabricated sensors compared with other work and in different gases. (**a**) Response of ammonia sensors compared with other work based on metal oxides. (The result was normalized as S = (Ra − Rg)/Ra × 100) (**b**) Response of as fabricated sensors in different gas environments.

The oxygen adsorption relies on the oxygen vacancy of the material [21]. As prepared SnO is an unsaturated metal oxide that tends to absorb oxygen and be further oxidized to SnO2. From our previous work, the photoluminescence of oxygen vacancy on SnO nanoshell was studied which shows the high oxygen vacancy density of our SnO material [30]. In summary, the high response of our sensors depends on the high oxygen adsorption with high surface-to-volume ratio and high oxygen vacancy density of the as-prepared SnO nanoshell. Additionally, we investigated the test of selectivity of our sensors. Since there are many papers about volatile organic compound sensors based on metal oxide, we used different volatile organic compounds for comparison [31–34]. In Figure 5b, the response of our sensors in ammonia is also much higher than that of dry atmosphere or other organic gases. In the experiment, we used single gas for each test. This confirms the outstanding selectivity of our sensors. These results show the high performance of our samples. In the supplementary materials, the response (98 s) and recovery time (30 s) are shown in Figure S8. Comparison of response time and recovery time with previous work is shown in Table S2. These results show the high performance of our samples. Furthermore, we investigated the repeatability of the SnO nanoshell materials in a certain amount of 20 ppm NH3 and found that the SnO nanoshell material possesses good repeatability for at least one week.

#### **4. Conclusions**

In conclusion, we prepared SnO nanoshell through a solution method and annealing. SEM, TEM, XRD, XPS, and Raman measurements were used to characterize the present samples. The SnO nanoshell exhibited high responses of 313%, 874%, 2757%, 3116%, and 3757% under gas concentrations of 5 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm, respectively. The mechanism of ammonia sensing is related to the redox reactions. From the mechanism, we realized high sensitivity was due to a large surface area and higher oxygen vacancy of the SnO nanoshell. This material also showed good selectivity and repeatability in ammonia sensing. This work can potentially aid in the study of similar structures and applications of IV–VI metal monoxides for real field applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/3/388/s1, Figure S1: schematic figure about the synthesis process of tin mono oxide, Figure S2: characteristic of sample 1, Figure S3: characteristic of sample 2, Figure S4: characteristic of sample 4, Figure S5: characteristic of sample 5, Figure S6: schematic figure for the ammonia sensing mechanism of as-prepared sensors, Figure S7: Accurate response of ammonia gas sensor of sample 3, Figure S8: Response and recovery time of sample 3 (Tested under 20 ppm.), Table S1: Difference of samples in the work, Table S2: Comparison of response and recovery time with previous work.

**Author Contributions:** The experiments and characterizations were carried out by H.W., Z.M., with the assistence of Z.L. and H.S., under the guidance of S.Y. and Y.S., H.W. and S.Y. wrote the manuscript and prepared all figures. Y.S. and S.Y. supervised and coordinated all the work.

**Funding:** This research was funded by the National Basic Research Program of China (973 Program: 2018YFA0209100), the National Science Foundations of China (No. 61205057, No. 11574136), Qing Lan Project, the "1311 Talent Plan" Foundation of Nanjing University of Posts and Telecommunications, and Six talent peaks project in Jiangsu Province (JY-014).

**Conflicts of Interest:** The authors declare no competing financial interest.
