*3.1.1. Hydro/solvothermal synthesis*

Hydro/solvothermal method is a common strategy used for the synthesis of inorganic materials. Advantages of hydro/solvothermal method over other types of crystal growth include low temperature (generally in a temperature range of 100–240°C) and convenience of adjusting reaction conditions. Many factors play the key roles in the synthesis of ultrathin nanostructure, which include temperature, reaction time, reactant ratio and so on. Many 2D materials have been prepared by this method, just like MoS2, TiO2, ZnO, Co3O4, MnO2 and Rh [26–29]. In the following, we will take Li's work as an example to explain this strategy.

Ultrathin Rh nanosheets were synthesized successfully through solvothermal method by Li et al. recently [29]. For detailed steps, Rh(acac)3 and PVP are dissolved in solvent mixed by benzyl alcohol and formaldehyde. The mixture is stirred vigorously for 1 h and then transferred to a Teflon-lined stainless steel autoclave. The autoclave is sealed and maintained at 180°C for 8 h and then cooled to room temperature. The resulted black product is precipitated with acetone (10 mL), separated in a centrifuge and washed three times with ethanol (10 mL) and finally dried under vacuum.

**Figure 5.** (a) Low-magnification TEM image of PVP-capped Rh nanosheet, (b) high-magnification TEM image of PVPcapped Rh NSs, (c) aberration-corrected microscopy image of PVP-capped Rh NS (inset, the corresponding filtered im‐ age using crystallographic average method to improve signal-to-noise ratio), (d) AFM image and the corresponding height profiles of a bare Rh NS [29].

Detailed structure information for the synthesized atomically thick Rh nanosheets is indicated by TEM, HRTEM and AFM. TEM image clearly shows that ultrathin nanosheet has a dimen‐ sion lateral size of about 500 nm, while the near transparency of the sheets indicates the ultrathin thickness (**Figure 5a, b**). As shown in **Figure 5c**, Rh nanosheets have a single crystalline nature of Rh nanosheets, which possesses a hexagonal structure, with a lattice parameter of ca. 2.6 Å, closer to the atomic distance of Rh (2.69 Å) in the (111) plane of the bulk Rh phase. AFM image of Rh NS shows that the sheet-like features are consistent with TEM images (see **Figure 5a**, **b**) Section analysis and height profile (**Figure 5d**) reveal that the height of Rh NS is about 4 Å. This is closer to the diameter of Rh atoms (the radius of Rh atomic is about 1.73 Å). All of these results undoubtedly confirm the successful synthesis of Rh nano‐ sheets with an atomic thickness.
