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J. Phys. Chem. C 2007, 111, 1820-1829
Novel Effects of Surface Modification on Activated Carbon Fibers Using a Low Pressure Plasma Treatment Shen Tang, Na Lu, Ji Ku Wang, Seung-Kon Ryu, and Ho-Suk Choi* School of Applied Chemistry and Biological Engineering, Chungnam National UniVersity, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, Korea ReceiVed: September 11, 2006; In Final Form: NoVember 3, 2006
Activated carbon fibers (ACFs) were surface modified with oxygen plasma at low pressure. The novel effects of the plasma treatment on the microstructural properties of the ACFs were characterized using the Brunauer, Emmett, and Teller method and scanning electron microscopy. Micropores developed on the ACFs. Moreover, the specific surface area and micropore volume increased by 10% at a certain plasma treatment time and power. The changes in the structural properties of the ACFs are discussed in detail with the respect of plasma etching. X-ray photoelectron spectroscopy revealed new oxygen-containing groups, such as CsO, CdO, and OsCdO, had formed on the surface of the ACFs after plasma treatment. Plasma surface oxidative reactions such as the generation of radicals, the combination of the radicals and active oxygen species in the plasma chamber, and the generation of the various oxygen-containing groups are believed to have occurred. The effect of the plasma treatment parameters such as plasma treatment time and power was examined from the perspective of both surface structure and chemistry. It was observed that the micropores and surface functionalities of the ACFs were increased under moderate treatment conditions (50 s and 100 W).
1. Introduction Activated carbon fibers (ACFs) are unique porous materials that contain slit-shaped pores and a large surface area. The peculiar porous structure and surface properties of ACFs play important roles in their applications to gas separation,1-5 polarizable electrodes,6 methane and hydrogen storage,7 adsorption of SO2, NOx, VOCs, lead, and nickel,8-10 catalysis,11 the production of cigarette filters, and medical treatments.12 However, the lack of polar groups in the structure makes the surface of ACFs quite hydrophobic, which limits their applications. Therefore, surface modification is essential, and considerable effort has been made to improve the surface properties of the ACFs using different methods.13-16 Plasma is an efficient method in the field of surface modification. The surface of various materials can be readily modified using plasma.17-24 Recently, some researchers have applied plasma techniques to the surface modification of carbonbased materials. Park et al.25 examined the surface and textural properties of ACFs with an atmospheric pressure plasma treatment. They reported that a plasma treatment was an efficient method for generating new oxygen-containing functional groups on the surface of the ACFs. Orfanoudaki et al.26 studied the modification of ACFs using a plasma deposition technique with the aim of forming pore constrictions by narrowing the surface pore system of the ACFs. They used propylene/nitrogen and ethylene/nitrogen as the plasma reaction gases and reported that plasma deposition made an external film on the surface of the ACFs and incorporated nitrogen groups into the surface. Boudou et al.27 investigated the surface modification of an isotropic carbon fiber with microwave oxygen plasma. They suggested that the plasma treatment moderately increased the surface roughness of the carbon fibers and demonstrated that gentle * To whom correspondence should be addressed. E-mail: hchoi@ cnu.ac.kr. Phone: 82-42-821-5689. Fax: 82-42-822-8995.
plasma exposure was sufficient to generate a large amount of oxygen-containing functional groups on the surface. They attributed this to two competing effects, i.e., the removal of surface atoms or clusters of atoms by the etching reaction and additional reactions between the reactive sites and the reactant oxygen species in the plasma. In addition, they showed that a more intense treatment had negative effects on the surface functionality. In previous studies, it was a simple process to significantly modify the surface functionalities of the ACFs with a plasma treatment by immobilizing polar components on the surface. However, the structural properties always decreased after the plasma treatment, i.e., the specific surface area and micropore volume always decreased. It was believed that the plasma treatment might block the entrance of micropores on the surface by the plasma etching and prevent the formation of new oxygen functionalities.28,29 In this study, however, we apparently observed different change in the microstructural properties of the ACFs, which resulted from the increase of the specific surface area and micropore volume at a certain plasma treatment time and power. The new effect of the plasma treatment was attributed to the low-pressure oxygen plasma system used in this experiment, which has seldom been used on ACFs before.30,31 The plasma particles under low pressure are thought to possess higher kinetic energy and a lower plasma density than those in atmospheric pressure plasma systems. Through the analysis of the plasma-etched surface by scanning electron microscopy (SEM), it was believed that the low-pressure plasma could develop the microstructural properties on some of the fibers by creating tiny voids and opening the isolated pores in the ACFs. It was previously reported that, although the surface functional groups of ACFs could be modified, the mechanism for how the plasma treatment altered the surface functionality was not completely understood. In this paper, the authors propose a plasma reaction mechanism based on an analysis of
10.1021/jp065907j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
Novel Effects of Surface Modification
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1821
Figure 2. BET N2 adsorption isotherms of the as-received and plasmatreated ACFs at different treatment times. (Plasma treatment power, 100 W; pressure, 250 mTorr).
Figure 1. Schematic diagram of the oxygen plasma system used in this study.
the XPS results, which shows how oxygen-containing groups, e.g., CsO, CdO, OsCdO, and peroxides, are generated on the surface of ACFs. 2. Experimental Section Materials and Apparatus. (1) Materials. Commercially available cellulose-based activated carbon fibers (KF-2000, Toyobo, Japan) were used in this study. Ultrapure O2 (Praxair Korea Co. LTD) was used to generate the plasma. (2) Plasma Treatment. The ACFs were treated with oxygen plasma at 250 mTorr using a radio frequency of 13.56 MHz (Model EPPs 2000, PLASMART Inc., Korea), as shown in Figure 1. The flow rate of the O2 gas was controlled using a mass flow controller (MFC; Model 5850E, Brooks, Japan). The effects of the other plasma treatment parameters such as the plasma treatment time (15, 30, 50, 80, and 120 s) and plasma treatment power (50, 80, 120, and 150 W) were examined. After the plasma treatment, Ar gas was introduced into the plasma treatment chamber at a high flow rate (3-4 liter per min) to remove the small particles sputtered by the plasma from the surface of the ACFs. BET Measurement. The porosities of the ACFs were characterized using N2 adsorption at 77 K, from which the Brunauer-Emmett-Teller (BET) isotherm was obtained.28 Total pore volume (Vt), micropore volume (Vmi), average pore diameter (Ap), and external surface area of ACFs were obtained using the nitrogen-BET equation. The pore size distribution of the ACFs was calculated using the Barret-Joyner-Halenda (BJH) adsorption model. Characterization by Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). (1) SEM. SEM studies were carried out using an SM-500 (ETPSEMRA, Sydney, Australia) with an operational working distance of 5 mm and a voltage of 10 kV. (2) XPS. The surface composition of the ACFs before and after the plasma treatment was investigated using an ESCA 2000 (VG Micro Tech Co.). The pressure inside the chamber was held at below 1 × 10-9 Torr
during analysis. The analyzed surface area was 1 mm × 1 mm, and the photoelectron takeoff angle was 45°. Preliminary data analysis and quantification were performed using XPSPEAK 4.1 software. The binding energies (BEs) were determined by reference to the BE of the C1s peak at 284.6 eV prior to peak fitting. 3. Results and Discussion Structural Properties of ACFs by BET Treatment. Figure 2 shows the nitrogen adsorption isotherms measured at 77K for the as-received and plasma-treated ACFs at various plasma treatment times. Table 1 shows the changes in the microstructural properties of the ACFs such as the specific surface area and micropore volume as a function of the plasma treatment time.32 It was found that, compared with the as-received ACFs, the specific surface area of the ACFs increased by 10% when the plasma treatment time was between 50 and 80s. On the other hand, the specific surface area and the micropore volume of the ACFs were lower when the plasma treatment time was outside this range. The other microstructural properties except for the external surface area also showed a similar trend to that observed with the specific surface area. Parts a and b of Figure 3 show the pore size distribution of the as-received and plasmatreated ACFs. The figure shows that the pore volume mainly increases for pores with a diameter
