# 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
#### 2.1.2 Bottom-Up Fabrication  
Formwork-assisted methods encompass hard formwork and sacrificial formwork strategies. The hard formwork technique facilitates the controlled deposition of a desired substance onto a substrate, thereby dictating the growth pattern of the 2D MOFs based on the substrate type. [PERSON] et al. [112] exemplify this approach by coating a polyacrylamideonitrile (PAN) substrate with poly(sodium-p-styrenesulfonate)-modified (PSS) halloysite nanotubes (HNTs), followed by the deposition of zeolitic imidazolate framework-L (ZIF-L) onto the support surface (Figure 3a). The consistent arrangement of HNTs crucially impacts the growth direction of ZIF-L, especially for substrates with uniform PSS-functionalized HNT loading, where the initial ZIF-L crystallization tends to occur horizontally on the surface (Figure 3b,c). On the other hand, [PERSON] et al. [113] utilized the surfactant cetrimonium bromide (CTAB) as a sacrificial template to synthesize mesoporous MOF nanosheets. Upon adding a Cu(CH\({}_{3}\)COO)\({}_{2}\) solution to a mixed solution of CTAB and 1,4-dicarboxybenzene (H\({}_{2}\)BDC) at room temperature, and subsequent removal of CTAB with hot ethanol, meso-CuBDC MOF nanosheets were obtained (Figure 3d). The SEM image (Figure 3e) displays the nanosheet morphology of the CuBDC, with its mesoscopic structure displaying uniform dispersion within 200-300 nm and a small square shape. The mesoscopic CuBDC nanosheets were  
Figure 3: **(a)** Schematic representation of the PAN-HNTs-ZIF-L film preparation process. **(b)** SEM image showcasing the top-surface morphology of the PAN-HNTs-ZIF-L film. **(c)** SEM image complemented by Energy Dispersive Spectrometry mapping, providing elemental analysis of the cross-sectional region of the PAN–HNTs–ZIF-L film. Adapted with permission from [112]. Copyright 2019, Royal Society of Chemistry. **(d)** A step-by-step diagram of the synthesis procedures resulting in meso-CuBDC nanosheets. **(e,f)** Detailed SEM and AFM images revealing the structural properties of meso-CuBDC nanosheets. **(g)** Histogram illustrating the thickness distribution of the meso-CuBDC nanosheets. Adapted with permission from [113]. Copyright 2020, Elsevier. meticulously characterized using AFM. These nanosheets are approximately 200 nm in size, possess a square shape (as depicted in Figure 3f), and maintain a thickness of less than 8 nm (refer to Figure 3g), thus underscoring the precision in our preparation method. In parallel, [PERSON] et al. [114] achieved the synthesis of ultrathin NiCo-layered double hydroxide@MOFs nanosheets (NiCo-LDH@MOFs) utilizing an in situ semi-sacrificial formwork-assisted approach. This procedure initiated the synthesis of NiCo-LDH nanosheets as precursors through a hydrothermal method. These nanosheets were subsequently deprotonated with H\({}_{2}\)BDC, allowing the metal within the NiCo-LDH to coordinate and form bimetal MOFs (BMOFs) nanocrystals in situ. The nanocrystals thus formed on the LDHs' surface and then served as secondary growth nucleation sites for BMOFs. Ultimately, the ultra-thin BMOF nanosheets were obtained through secondary growth on the NiCo-LDH surface. While the formwork-assisted synthesis method can engineer 2D MOFs with controlled morphology, it presents some drawbacks. Firstly, the synthesis procedure is rather intricate, which can limit its broader application. Additionally, the method yields a modest quantity of product. Thus, it is paramount for researchers to refine the synthesis method and route to augment the yield at each step of the formwork-assisted synthesis, thereby improving overall efficiency and scalability. Interface or Interlayer Growth Method  
Two-phase interface synthesis processes generally encompass methods involving liquid-gas, liquid-liquid, solid-liquid, and solid-gas interfaces. In these, organic linkers and metal ions are initially distributed in two phases, then diffused to the two-phase interface, and finally reacted at the interface to yield 2D MOF nanosheets.