*2.2. Synthesis Methods*

In this experiment, 45 mL of deionized water and 33.75 mg of thioacetamide were introduced into a 100 mL bottle. After stirring for 1 min, 30 mg of NaOH was added to the bottle. While the solution was stirred, 101.54 mg of SnCl2·2H2O was added and the color of the solution became milky white. The reaction of

$$\text{SnCl}\_2\cdot2\text{H}\_2\text{O} + \text{H}\_2\text{O} \rightarrow \text{Sn(OH)Cl} + \text{HCl} \tag{1}$$

took place [25]. Afterwards, the solution was stirred for another 20 min. The suspension was centrifuged for 20 min at 10,000 rpm and the precipitate was dried at 80 ◦C for 12 h in a vacuum oven. From our previous study, dried precipitate was Sn6O4(OH)4 which was a precursor of SnO nanoshell. The precursor showed shell structure and this structure could be kept only annealed under suitable conditions. In fact, we found only Sample 3 and 5 kept shell structure in subsequent tests and Sample 3 had higher density of shell than Sample 5. The dried precipitate was then annealed under different conditions. Under specific annealing conditions, the decomposition reaction of precursor

$$\text{Sn}\_6\text{O}\_4\text{(OH)}\_4 \rightarrow \text{6SnO} + \text{H}\_2\text{O} \tag{2}$$

took place. In this work, Sample 1, 2, 3, and 4 were obtained after heating from 0 ◦C to 200 ◦C, 250 ◦C, 300 ◦C, and 350 ◦C for 1 h and kept at 200 ◦C, 250 ◦C, 300 ◦C, and 350 ◦C for 1 h, respectively. Sample 5 was obtained after heating from 0 ◦C to 350 ◦C for 30 min and kept at 350 ◦C for 30 min. In Supplementary Materials, we have added a flow chart to demonstrate the synthesis process in Figure S1. To demonstrate the difference between each sample from preparing to investigating, we concluded Table S1 in Support Information.
