Graphene is a two-dimensional material composed of a single layer of carbon atoms arranged in a hexagonal lattice, and its unique structure endows it with a series of groundbreaking physical and chemical properties. Although graphene itself has a relatively excellent structure, its surface has almost no hydrophilic groups, and its density is extremely low with a large specific surface area, resulting in poor hydrophilicity and water dispersibility of graphene. To improve the dispersion state of graphene in liquid, it is necessary to modify its surface. At present, the main methods for modifying graphene are plasma modification and chemical reagent modification, which change the physical structure of graphene or achieve element doping, and regulate its related properties to meet the requirements of engineering applications.

Plasma Treatment

Plasma is a state of matter, similar to solids, liquids, and gases, also known as the fourth state of matter. When a gas is subjected to sufficient energy, it will transform into a plasma state. The active components in plasma include high-speed moving electrons, activated neutrons, as well as free radicals, ionized atoms and molecules. Unreacted atomic molecules exhibit completely neutral electrical characteristics. Due to the highly active charged particles in plasma, plasma treatment has become an effective method for altering the surface characteristics of materials. In addition, this processing method has relatively simple process control and is environmentally friendly and economical. New functional groups can be introduced through plasma treatment, resulting in better surface activity of the material.

Oxygen plasma treatment can functionalize materials chemically by introducing new oxygen-containing functional groups. Previous studies by many scholars have shown that oxygen plasma treatment can improve the dispersibility and electrocatalytic activity of various carbon materials, such as SWCNT, MWCNT, GNPs, etc. After the reaction between oxygen atoms and carbon atoms, new oxygen-containing functional groups are usually introduced, and the outermost carbon atoms of the material are removed through etching reaction.

Through low-temperature plasma treatment, three types of functional groups can be formed on the surface of objects: C=O carbonyl, - COOH carboxyl, and - OH hydroxyl. These special compounds can provide good hydrophilicity and play an important role in adhesion and coating. The schematic diagram of plasma treatment is shown in Figure 1.

Figure 1 Plasma treatment process

Comparison of dispersibility of graphene in water before and after plasma treatment

To characterize the surface activity of GNPs, GNPs before and after plasma treatment were separately added to water to observe their dispersion. The dispersibility of GNPs in water before and after plasma treatment is shown in Figure 2. Figure 2 (a) shows the dispersibility of untreated GNPs in water. It can be seen from the figure that GNPs float on the water surface and are not easily dispersed in water. Figure 2 (b) shows that the dispersion of GNPs in water has been greatly improved after plasma treatment. The improvement in dispersibility of plasma treated GNPs can be attributed to the breaking of molecular bonds in GNPs by O2 ion bombardment, and the introduction of rich oxygen-containing functional groups such as hydroxyl (- OH), carboxyl (- COOH), carbonyl (C=O), and epoxy (C-O-C) through functional modification. These functional groups altered the hybridization state of carbon atoms, slightly distorted the lattice structure of graphene, causing distortion and significant changes in its properties. Hydroxyl and carboxyl groups and other oxygen-containing groups can form hydrogen bonds with water molecules, which enables graphene to exhibit good dispersibility in aqueous media and form long-term stable colloidal solutions.