*3.3.2. Ultrathin nickel nanosheets through topotactic reduction transformation*

Except for topotactic oxidation, toporeduction is also frequently used to prepare nonlayered materials. Sun [41] used this method and synthesized ultrathin nickel nanosheets array recently. Ni(OH)2 nanosheet array on a metal substrate is prepared first. And then, ethylene glycol is used as the reduction agent to get Ni nanosheets. The slow conversion kinetics keeps the ultrathin nanosheet morphology and atomic thickness.

**Figure 9** reveals the morphology inherence between Ni and Ni(OH)2 nanosheet array. **Figure 9a**, **b** shows SEM and HRTEM of Ni(OH)2 sheet array. It is seen that the thickness of Ni(OH)2 is about 5.8 nm (10–12 layers), whereas the thickness of Ni nanosheet shown in

**Figure 9.** (a and c) SEM images of the as-synthesized Ni(OH)<sup>2</sup> nanowall array and Ni-NSA. (b and d) HRTEM image of a scrolled Ni(OH)2 nanowall and a vertically laid Ni nanosheet. The inset shows the thickness of Ni(OH)<sup>2</sup> nanowall and Ni nanosheet [41].

**Figure 9c**, **d** is about 2.2 nm (10 atomic layers). This provides strong evidence that Ni nano‐ sheets are obtained by toporeduction transformation.

### **3.4. Chemical vapor deposition (CVD)**

closer to the thickness of (111)-oriented Co3O4 nanosheets. More specifically, the discrepancies between α-Co(OH)2 {100} (*d*=2.76 Å) and Co3O4 {220} (*d*=2.84 Å) are calculated to be below 3%. Moreover, Co2+ atomic arrangement in α-Co(OH)2 (001) plane and Co3O4 (111) plane is very closer (**Figure 8b**). These features enable atomically thick Co3O4 nanosheets to preserve the

Atomically thick Co3O4 nanosheets were prepared by this method, as confirmed by TEM, AMF and XRD. From TEM image, one could clearly see ultrathin nanosheets with lateral size of about 400 nm with the near transparency of the sheets (**Figure 8c**). HRTEM image, and the corresponding fast Fourier transform (FFT) pattern in **Figure 8d** and **e** for the as-obtained product depicts its [111] preferential orientation. The thickness of the as-synthesized Co3O4 nanosheets is also evaluated by tapping-mode atomic force microscopy (AFM). AFM image and the corresponding height distribution (**Figure 8f, g**) show that the measured height is about 1.5 nm. Microscopically, the colloidal suspension of the product displays the Tyndall phenomenon (**Figure 8h**), so that the formation of homogeneous ultrathin nanosheets in ethanol can be inferred. As shown in **Figure 8i**, XRD pattern of the atomically thick nanosheetbased films can be readily indexed to be pure Co3O4 (JCPDS no. 42-1467) and also displays the highly preferred [111] orientation, which is corresponding to the result of HRTEM. All of the results undoubtedly confirm that Co3O4 nanosheets with ultrathin thickness have been successfully synthesized. The oriented growth of ATCNs is attributed to α-Co(OH)2-to-Co3O4

topochemical conversion with the relationship of [001] α-Co(OH)2 and [111] Co3O4.

Except for topotactic oxidation, toporeduction is also frequently used to prepare nonlayered materials. Sun [41] used this method and synthesized ultrathin nickel nanosheets array recently. Ni(OH)2 nanosheet array on a metal substrate is prepared first. And then, ethylene glycol is used as the reduction agent to get Ni nanosheets. The slow conversion kinetics keeps

**Figure 9** reveals the morphology inherence between Ni and Ni(OH)2 nanosheet array. **Figure 9a**, **b** shows SEM and HRTEM of Ni(OH)2 sheet array. It is seen that the thickness of Ni(OH)2 is about 5.8 nm (10–12 layers), whereas the thickness of Ni nanosheet shown in

**Figure 9.** (a and c) SEM images of the as-synthesized Ni(OH)<sup>2</sup> nanowall array and Ni-NSA. (b and d) HRTEM image of a scrolled Ni(OH)2 nanowall and a vertically laid Ni nanosheet. The inset shows the thickness of Ni(OH)<sup>2</sup> nanowall

*3.3.2. Ultrathin nickel nanosheets through topotactic reduction transformation*

the ultrathin nanosheet morphology and atomic thickness.

and Ni nanosheet [41].

thickness of the precursor possible.

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

Recently, CVD technique has shown promise to generate high-quality TMD layers with a scalable size, controllable thickness and excellent electronic properties. CVD is a hightemperature chemical synthesis process by which a desired material is deposited on substrates. CVD processes have been extensively studied for synthesizing thin film coatings of a wide range of materials, including metals, semiconductors, and insulators. The scheme of CVD method is shown in **Figure 10a** [1]. The precursor vapor is introduced from outside or generated inside the tube furnace. The main advantages of CVD synthesis process are represented by the accesses to high quality, high purity 2D nanomaterials with controlled properties, which allow one to control the morphology, crystallinity and defects of 2D nanostructures by tuning the process parameters [42]. Due to these advantages, some 2D nanomaterals have been prepared by this way conveniently, such as graphene, MoS2, h-BN [43–49]. CVD method is also another annealing strategy for the growth of high-quality singlecrystalline 2D sheet on substrates. Even so, CVD suffers from the requirement of high tem‐ perature, high vacuum and specific substrates. In the following, we will take h-BN and MoS2 as an example to elaborate this strategy.

**Figure 10.** (a) Schematic illustration of experimental setup for the 2D materials synthesis by a noncatalytic vapor depo‐ sition process. The precursor vapor can be introduced from outside or generated inside the tube furnace [1]. (b) AFM images of the initial flakes of h-BN. White arrows point to the sharp edges of these flakes. Inset: TEM image of the folded edge of a monolayer h-BN [43]. (c) AFM image of a monolayer MoS2 film on a SiO2/Si substrate. Inset: corre‐ sponding height profile of as-obtained nanosheets [46].

Hexagonal BN is called "white graphite" due to its similar lamellar structure to graphite. h-BN possesses many unique properties such as chemical stability, strong mechanical strength, high-thermal conductivity and low dielectric constant. What is more, the band gap of h-BN can be tuned with thickness. To produce BN precursor, the first heating zone is ramped up to *T*1=60∼90°C with a heating belt. Turn on the on-off valve between the first and second heating zone only during CVD process, one can prevent the undesired growth. The second heating zone is heated up to *T*<sup>2</sup> = 1000°C for typically 10∼90 min under 10 sccm hydrogen atmosphere at a pressure of 350 mTorr after mounting the copper foil. Copper foil (25 μm, Alfa Aesar) is used as the metal catalyst substrate. Before the growth of h-BN, copper foil is annealed at 1000°C for 20 min under 10 sccm H2 to grow the copper grain and to obtain a smooth surface. AFM of as-obtained sample provides direct evidence for single-layer h-BN synthesized by chemical vapor deposition method (**Figure 10b**). AFM image and the corresponding height distribution inset in **Figure 10b** show that the measured height is about 0.42 nm, consistent with monolayer thickness (c-axis spacing for h-BN is ∼0.32 nm). In addition, TEM image of monolayer h-BN clearly demonstrates that the number of layer of h-BN is one (**Figure 10c**). Moreover, for a complete h-BN layer (**Figure 10d**), the wrinkles of h-BN film (indicated by a yellow arrow) under scanning electron microscopy (SEM) can be clearly seen. These wrinkles are characteristic of h-BN and graphene films due to the negative thermal expansion coeffi‐ cients of h-BN and graphene. All of the results undoubtedly confirm that h-BN with monolayer thickness have been successfully synthesized.

Synthesis of transition metal dichalcogenides (TMDs) using CVD is the cutting edge research area in recent years. Lee [46] synthesized single-layer MoS2 by this method recently. As shown in **Figure 10c**, ultrathin MoS2 nanosheets with smooth surface are observed with AFM. The cross-sectional height in inset of **Figure 10c** reveals that the thickness of MoS2 film is ∼0.72 nm, which corresponds to a monolayer MoS2.
