# Two-Dimensional Metal-Organic Frameworks and Their Derivative Electrocatalysts for Water Splitting
## 2 Fabrication Strategy of 2D MOF and Their Derivatives
### Fabrication Strategy of 2D MOF
#### 2.1.1 Top-Down Synthesis  
Mechanical Exfoliation  
The constituent planes of numerous MOF crystals [98,99] are interconnected through weak mutual interactions manifested as hydrogen bonds or van der Waals (vdW) forces. These weak interactions can generally be disrupted by external physical forces [91,92,100,101,102], thus separating the layers from one another. In a seminal study conducted by [PERSON] and team in 2010 [103], a 2D hybrid valence copper ligand polymer [Cu\({}_{2}\)Br(IN)\({}_{2}\)]\({}_{2}\) (IN = isoniccitinato) was subjected to ultrasonic treatment, leading to the generation of 2D MOF nanosheets. The underlying geometric configuration of this compound constitutes a four-ligand unit, in which the ligands along the bc plane direction bridge the [Cu\({}_{2}\)Br(IN)]\({}_{4}\) units into a 2D wavy structure. The bromine ligands reside on both sides of this plane, resembling the structure of the iodine complex. Given the structural similarities of [Cu\({}_{2}\)Br(IN)\({}_{2}\)]\({}_{2}\)n with graphite, the team employed ultrasound as a physical means to attenuate the interlayer forces in the coordination polymer, analogous to the method used to extract graphene from graphite. As a result, the 2D nanosheets, exhibiting a uniform height of 5 \(\pm\) 0.15 A, were successfully distributed in layered formation on substrates of highly oriented pyrolytic graphite, as revealed by atomic force microscopy (AFM). Apart from the ultrasonic exfoliation technique, micromechanical exfoliation methods also exist. These methods generally utilize physical forces applied by grinding [104] or sphere milling [105] to disrupt the interlayer forces in the bulk material, subsequently separating the layers to yield nanosheets. In one instance, [PERSON] et al. [106] employed a combination of wet sphere milling and ultrasonication to exfoliate ZSB-1 (Zn\({}_{2}\)(SBA)\({}_{2}\)(BPTP), SBA = 4,4\({}^{\prime}\)-sulfonyl)bibenzoic acid, BPTP = 3,5-bis(5-(pyridin-4-yl) thiophen-2-yl)pyridine)  
Figure 1: A conceptual representation of the synthesis strategies employed for fabricating 2D MOF materials. into ultra-thin, rectangular nanosheets. Figure 1a illustrates the process of synthesizing 2D ZSB-1 nanosheets. The thickness distribution of the 2D sheet was assessed using AFM images, with a flat, smooth platform of 11.8 \(\pm\) 2.3 nm observed for the ZSB-1 nanosheet in the height profile (Figure 1b). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figure 1c,d) indicated that the structural integrity of the ZSB-1 nanosheets was maintained following wet ball milling and ultrasonic treatment. These images clearly depicted the undamaged rectangular structure of the ZSB-1 nanosheets, confirming the successful preservation of their morphology. Importantly, the crystalline structure of the nanosheets remained intact after treatment, underlining the high stability of the material. In another example, [PERSON] et al. [107] employed transparent adhesive strips to mechanically peel-off vdW MOF-2, a method known as micro-mechanical exfoliation. Figure 1e shows the characteristic lamellar architecture of the deposited MOF-2 crystals, with SEM images illustrating the sheet-like slices (\(\approx\)10 nm) of vdW crystals (Figure 1e,f). The monolayers in these crystals were separated using transparent tape and subsequently deposited on a substrate (Figure 1g,h). Sphere milling serves as another viable approach to induce physical forces, demonstrated by the suspension of Zn\({}_{2}\)(bim)\({}_{3}\) (bim = benzimidazole) nano-flakes via sphere milling. Despite the relative simplicity of mechanical exfoliation, it suffers from certain limitations, such as issues with reproducibility and challenges in obtaining uniform nanosheets. For instance, [PERSON] et al. [108] combined wet sphere milling with ultrasonic treatment to exfoliate 2D Zn\({}_{2}\)(bim)\({}_{3}\) MOFs, resulting in Zn\({}_{2}\)(bim)\({}_{3}\) nanosheets with a thickness of 1.6 nm.