Surface modification of activated carbon fiber by low-temperature oxygen plasma: Textural property, surface chemistry, and the effect of water vapor adsorption

The effects of adsorbent characteristics on its water adsorption performance were investigated using low-temperature oxygen plasma technology (LTOP) as a surface modification method. A series of activated carbon fiber (ACF)-based adsorbents was prepared to get further insight into the functions of water vapor adsorption behavior with textural property and surface chemistry. The result shows that LTOP treatment does not alter drastically the physical surface properties and simultaneously functionalize oxygen-containing functional groups (OFGs) especially carboxyl groups on the ACF surface when the treatment time and effective output voltage are 15 min and 8–9 kV, respectively. Meanwhile, the carbon atoms with unpaired electrons are found to be increased during the process of plasma treatment and used as active sites for chemical adsorption. Both OFGs and carbon atoms with unpaired electrons on the carbon surface play a vitally important role for water vapor adsorption. The surface functionalities and chemical adsorption may govern the adsorption–desorption behavior of water vapor at low relative humidity (RH less than 40%). Adsorption mechanism of water vapor on the carbon surface before and after LTOP treatment is proposed. After several adsorption and desorption cycles, the adsorption capacity of LTOP-treated ACFs is still higher than that of ACF-raw. Therefore, this technology may shed light on the potential application of LTOP-treated ACFs in harvesting water from air at low relative humidity.