**3.3. Topochemical transformation**

is beneficial for the synthesis of ultrathin nanosheets. When CuO template is etched out, a heat treatment is carried out for the dehydrogenation of Fe hydroxide nanosheets, which leads to

AFM and TEM image provide direct evidence for successful exfoliation of the sample. As shown in **Figure 6b**, **c**, the lateral size of α-Fe2O3 nanosheets is up to about 1 μm, and thickness is about 0.55–0.59 nm. The XRD pattern of the α-Fe2O3 shown in **Figure 6d** exhibits only a broad and weak diffraction peak corresponding to the (110) orientation plane of α-Fe2O3. It is interesting to note that ultrathin α-Fe2O3 nanosheet shows an intrinsic ferromagnetism of 0.6

Except for hard template, soft colloidal templated synthesis is used to prepare two-dimen‐ sional2D materials frequently. The long-chain oleylamine and/or oleic acid surfactants are often used as the soft colloidal templates for directing the crystal growth. Cheon et al. used this method to synthesize ultrathin ZrS2 nanodiscs [36]. Intermediate lamellar complexes composed of 2D arrays of ZrCl4 and alkyl amine are first obtained, in which alkyl amine serves as the soft colloidal template. Then, CS2 is injected into the mixture aforementioned to form

As indicated in **Figure 6e**–**h**, the resultant ultrathin ZrS2 nanodiscs possess radius of ∼15 nm and thickness of 0.5 nm. The spacing between the discs is ∼1.5 nm (**Figure 6h**), which corre‐ sponds to the length of the oleylamine surfactant layers. When compared to bulk ZrS2, ultrathin ZrS2 discs show the unique nanoscale size effects, enhanced discharge capacity by 230% and

In recent years, microwave-assisted chemical synthesis strategy has become a well-established technique to promote and enhance chemical reactions. The main advantages of this method are represented by much shorter reaction time (generally in only a few minutes) and higher energy efficiency when comparing to other conventional strategies. Due to these advantages, some 2D nanomaterials can be prepared by this way conveniently, such as SnO2, α-Ni(OH)2,

To understand it more clearly, let us take α-Ni(OH)2 as an example (**Figure 7**) [36]: firstly, precursors were prepared by starting materials of Ni(NO3)2·6H2O, urea, deionized water and ethylene glycol at given proportions. Then, the resulting solution is transferred into a home‐ made round-bottomed flask and treated under microwave irradiation in a microwave reactor at 700 W for several minutes. Finally, the green powder is obtained by centrifugation and washed several times with distilled water and absolute ethanol. After that, the powder was dried in vacuum at 80°C for 12 h. Detailed structure information for the synthesized Ni(OH)<sup>2</sup> nanosheets is unraveled by FESEM and TEM images in **Figure 7b**–**d**. In comparison with traditional wet-chemical syntheses, the microwave-assisted liquid-phase growth can

the formation of the stable and free-standing α-Fe2O3 nanosheets.

12 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

μB/atom at 100 K and remains ferromagnetism at room temperature.

*3.1.2.2. Ultrathin zirconium disulfide nanodiscs*

ultrathin ZrS2 nanodiscs dispersed in solution.

greatly improved stability.

K0.17MnO2 and CuSe [33–37].

**3.2. Microwave-assisted method**

Topological conversion is a strategy in which the product's morphology is inherited from their precursor through nucleation and growth inside the precursors. The key to success is the degree of lattice match between precursor and their product. From the viewpoint of anisotropy of layered compounds, it is easier to obtain 2D nanostructure of hydroxide rather than oxides. This method is applicable of preparing nonlayer 2D nanostructure oxides, such as Co3O4, CeO2 and δ-FeOOH [38–40]. In the following, we will take Co3O4 and Ni as examples to illuminate this method [38, 41].
