# Graphene-Based Materials for the Separator Functionalization of Lithium-Ion/Metal/Sulfur Batteries
## 2 Preparation Methods of Graphene-Based Materials
### Others
#### 2.3.5 Vacuum Filtration and Liquid-Air Interface Self-Assembly  
In order to fabricate graphene-based devices with tailored structures to meet specific application requirements, vacuum filtration, and liquid-air interface self-assembly methods are indispensable. These methods have been widely utilized for the post-treatment of graphene, particularly in the preparation of graphene membranes and composite membranes. Vacuum filtration, for instance, is a prevalent approach for the fabrication of such membranes. Under the effect of vacuum attraction, the flow of graphene-based dispersion generates an orientation force vertical to the filter membrane, leading to the interlocking and parallel assembly of graphene sheets. The density and thickness of the as-filtered membrane can be controlled simply by adjusting the filtration time, concentration, and/or volume of the graphene dispersion [90]. In 2007, [PERSON] et al. reported a free-standing GO paper via vacuum filtration of GO colloidal dispersion, of which the thickness could be controlled by adjusting the volume of the dispersion solution [91]. [PERSON] et al. reported a graphene oxide-polyethyleneimine hybrid (GPH) film. The graphene oxide-polyethyleneimine (GO-PEI) solution obtained by ultrasound-assisted LPE was filtrated with an acetylcholinesterase membrane filter under a vacuum condition to prepare a drug-loaded GPH film (Figure 4E) [92]. [PERSON] et al. introduced polystyrene microsphere templates into vacuum filtration for porous graphene preparation [93]. [PERSON] et al. reported a graphene/carbon nanotube (Gr-CNT) composite film via vacuum filtration and successive calcination [94]. On the other hand, the liquid-air interface self-assembly method relies on the orientation force generated during the volatilization process of the dispersion solvent, resulting in the assembly of graphene at the liquid-air interface to form the desired structures (Figure 4F) [95]. [PERSON] et al. reported a uniform wrinkled graphene (WG) grown on the interlayer of an air-ethanol/deionized water solution [96]. [PERSON] et al. reported a free-standing graphene-based film assembled via solvent evaporation of a PVP-stabilized graphene suspension at the liquid-air interface [97]. Figure 4: Other methods for graphene preparation. **(A) (i)** Schematic view of mechanical exfoliation device; **(ii)** Scanning electron microscope image of one of our experimental devices prepared from FLG [11]. Reproduced with permission: Copyright 2004, The American Association for the Advancement of Science. **(B)** Mechanisms of the LPE process for the production of graphene [68]. Reproduced with permission: Copyright 2020, American Chemical Society. **(C)** Mechanisms of reduction of GO for graphene preparation [79]. Reproduced with permission: Copyright 2012, Elsevier. **(D) (i)** Schematic diagram of graphene preparation by arc discharge; **(ii)** TEM images of carbon nanospheres (CNSs) produced in He atmosphere at 73 kPa [89]. Reproduced with permission: Copyright 2016, Elsevier. **(E)** Vacuum filtration process for GPH film preparation [92]. Reproduced with permission: Copyright 2015, Royal Society of Chemistry. **(F) (i)** Illustration for GO membrane preparation by liquid-air interface self-assembly; **(ii,iii)** SEM images of GO [95]. Reproduced with permission: Copyright 2009, John Wiley and Sons.