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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: [email protected] 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
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 <20 Å at the plasma treatment time of 50 and 80 s. This suggests that only the volume of the micropores increased. It is believed that the increase in the micropore volume is the reason for the improvement in the specific surface area and the total pore volume, as shown in Table 1. Moreover, the increase in the micropore volume for the plasma treatment times between 50 and 80 s is believed the result of bombardment of the plasma particles. Plasma consists of active ions, electrons, photons, and molecules, which can cause etching33 and burning-up effects. It is believed that active plasma species may modify the surface roughness of the ACFs and reveal some isolated micropores existed in the ACFs, which led to the increase of the specific surface area as well as micropore volume. However, the bombardment effect is closely related to the treatment time. When the time was too short, the development of micropores was weak. However, if the bombardment time was too long, the surface of the ACFs was overetched and the crosslinking of molecular bonds led to the blockage of the micropores. Therefore, at a short treatment time
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TABLE 1: Effect of the Plasma Treatment Time on the Structural Properties of ACFsa treatment time (s)
TABLE 2: Effect of the Plasma Treatment Power on the Structural Properties of ACFsa treatment power (W)
specific surface area (m2/g)
external surface area (m2/g)
0 50 80 100 120 180
2120 2128 2156 2205 2317 2362
0.95 0.95 0.98 0.96 1.07 1.07
0.93 0.92 0.95 0.94 1.02 1.03
17.99 17.78 18.18 18.02 18.48 17.99
9.83 7.95 12.39 11.08 21.83 13.02
Plasma treatment time, 30 s; pressure, 250 mTorr.
of 15-30 s and long time of 120 s, the micropore volume was lower than that at a treatment time between 50-80 s. Figure 4 shows the nitrogen adsorption isotherms for the ACFs plasma-treated at various powers ranging from 50 to 180 W. Table 2 shows the changes in the microstructural properties, as listed in Table 1, for the ACFs treated for a fixed time of 30 s at different plasma treatment powers. The results showed that
all the microstructural properties, such as micropore volume and specific surface area generally increased with increasing plasma treatment power. In particular, at high plasma powers of 120 and 180 W, the specific surface area as well as the other microstructural properties increased by 10%. However, a low power treatment (<80 W) did not cause any appreciable changes. Parts a and b of Figure 5 show the increase in micropore volume for micropores with a diameter <20 Å when using a high-power treatment. The above results showed that the surface porosities were also closely dependent on the plasma treatment power. It is believed that an increase in plasma power resulted in an increase in the etching rate and plasma densities.34-35 As stated before, plasma treatment can etch the surface and reveal some isolated micropores in the ACFs. This eventually results in the increase of the specific surface area and micropore volume. The higher power of plasma treatment is thought to be able to more efficiently etch the surface and reveal the isolated micropores. However, it is also believed that the surface of the ACFs can be overetched and some micropores are possibly blocked, if the plasma treatment power is too high. By comparison of these results with those reported in the literature, novel effects of the plasma treatment on the microstructural properties of the ACFs were observed. In previous studies, the structural properties of the ACFs such as the specific surface area, micropore volume, and total pore volume always
Figure 3. Pore size distribution of the as-received and plasma-treated ACFs at different treatment times. (Plasma treatment power, 100 W; pressure, 250 mTorr).
Figure 4. BET N2 adsorption isotherms of the as-received and plasmatreated ACFs using different treatment power. (Plasma treatment time, 30 s; pressure, 250 mTorr).
Novel Effects of Surface Modification
Figure 5. Pore size distribution of as-received and plasma-treated ACFs using different treatment power. (Plasma treatment time, 30 s; pressure, 250 mTorr).
decreased after the plasma treatment, which was attributed to the destruction of the micropores by plasma etching and oxidation. Park and Kim25 once obtained similar results on the ACFs (AW2001) using Ar/O2 atmospheric pressure plasma. They reported that the properties of the micropores decreased due to blockage of the micropores as a result of plasma etching. Domingo-Garcia et al.33 also reported the decrease in the surface area of glassy carbon after O2 plasma treatment and attributed this to the oxygen functionalities formed by plasma oxidation, which occupied the entrance of the micropores and blocked the micropores. However, in this study, a large improvement in the microstructural properties of ACFs was obtained using lowpressure oxygen plasma, e.g., the specific surface area and micropore volume increased by 10%. These changes were attributed to the low-pressure plasma systems used in this experiment. The energy and density of the plasma species were once specified in O2 rf plasma at 13.56 MHz.36 It is believed that the kinetic energy of the plasma particles at low pressure is higher than that in atmospheric pressure plasma systems at similar treatment conditions due to the longer mean free path of the plasma particles without losing their energy, which improved the etching efficiency of these particles. It is also believed that at similar treatment conditions the discharge density of the low-pressure plasma is lower than that in the atmospheric pressure plasma systems due to dispersion of plasma particles in the vacuum chamber. This characteristic can weaken the burn-up effects during the plasma treatment.
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1823 Therefore, the microstructural properties were improved at moderate conditions by the low-pressure plasma in this study. SEM Images of ACFs Plasma-Treated under Different Conditions. Figure 6 shows SEM images of the ACFs plasmatreated at different treatment times. The surface of the asreceived ACFs (Figure 6a) was relatively smooth. When the plasma treatment time was 15 and 30 s (parts b and c of Figure 6, respectively), the surface became rougher and some tiny voids were produced. This phenomenon was due to plasma etching, which has been explained in previous research.27 During this period, there were no significant changes in the surface porosities of the ACFs because the plasma treatment time was short. With increasing in plasma treatment time to 50 s, the surface became etched even further and more tiny voids were created, as shown in Figure 6d. When the plasma treatment time was 80 s, the surface roughness increased and the number of plasma generated voids decreased, as shown in Figure 6e. This phenomenon was attributed to the long plasma etching time. A long plasma treatment time can cause over etching and burn-up effects on the surface, which increase the surface roughness and destroy the surface morphology. Figure 6f shows the overetching effects at long time plasma treatment, in which the tiny voids had disappeared and only an over etched surface could be observed when the plasma treatment time is 120 s. SEM did not reveal any visible micropores, and only the surface of the ACFs could be seen. The above SEM pictures, however, show many tiny voids formed after plasma treatment at moderate treatment time such as 50 and 80 s. It is believed that highly energetic plasma species can etch the external surface of ACFs and reveal some internal micropores. Thus, the micropore volume and specific surface area were increased in this period. After this period, further extending treatment time to 120 s, plasma overetching effect were more and more serious and the burning-up and molecular crosslinking may lead to the blockage of some micropores. Therefore, the micropore volume and specific surface area decreased at this period. The plasma effect should be relatively weak at a short treatment time of 15-30 s. At short time treatment such as 15-30 s, the bombardment of plasma particles was relatively weak. So that the increase of the micropore volume and specific surface area was less apparent than those treated at 50 s. The SEM results show agreement with the BET results, in which the microstructural properties such as the specific surface area, micropore volume, and total pore volume had developed at a treatment time of 50-80 s. At other treatment times, such as 15-30 or 120 s, the microstructural properties changed slightly or declined due to over etching, respectively. On the basis of the results of Figure 6 and Table 1, it is also considered that not all the micropores were modified and revealed by plasma treatment. Some of the micropores, which are hidden inside the fibers, may remain unchanged, since the plasma species can lose their energy after impinging into new pores and cannot smartly get into the irregularly distributed pores in the ACFs. Therefore, the specific surface area and micropore volume were increased up to a maximum of 10% as shown in Table 1. Figure 7 shows SEM images of the ACFs treated with different plasma powers. Compared with the as-received sample shown in Figure 6a, the sample treated at 50 W showed a small increase in surface roughness (Figure 7a). When the power was 80 and 100 W, the surface roughness increased markedly and plasma generated voids could be clearly seen, shown in parts b and c of Figure 7. These phenomena were also attributed to the bombardment of plasma particles, which caused plasma etching.
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Figure 6. SEM images of the as-received and plasma-treated ACFs at different plasma treatment times: (a) as received; (b) 15 s; (c) 30 s; (d) 50 s; (e) 80 s; (f) 120 s. (Plasma treatment power, 100 W; pressure, 250 mTorr).
For a treatment at 120 W, the plasma-generated voids increased, as shown in Figure 7b, which is similar with the image shown in Figure 6d. When the visible power reached 180 W, a significant number of plasma-generated voids were visible, as shown in Figure 7e. The increase in the plasma treatment power means an increase in the plasma kinetic energy and number of plasma particles with a stronger energy. Therefore, the surface was rougher at the higher plasma treatment power and there were more plasma generated voids. However, as shown in Figure 7e, it is also found that the surface of the ACFs was etched most severely compared with the samples treated at lower power. This suggests that the plasma etching effect became stronger when the plasma treatment power was higher. It is believed that the surface of the ACFs could be overetched with continuous increases in the plasma treatment power, which would result in the disappearance of the plasma generated voids in a similar manner to that observed with increases in the treatment time. The changes in the surface morphology of the ACFs with the plasma treatment power in parts a-e of Figure 7 can be explained in the same way as the plasma treatment time. At moderate treatment power such as 120 W, the microstructural properties of the ACFs could be improved due to the bombardment of the plasma particles. It is also believed that the isolated pores in the ACFs are more efficiently revealed at higher treatment power and the energetic plasma particles can possibly penetrate into the plasma generated voids. Therefore, the SEM
results in parts a-e of Figure 7 also show agreement with the BET results in Table 2. At low plasma treatment power such as 50-80 W, the increase of the micropore volume and specific surface area was less apparent than those treated at higher power of 100-180 W. In combination of the SEM and BET results, the surface porosities of the ACFs were closely related to the plasma treatment conditions including the treatment time and power. The surface morphology observed from SEM images can possibly be affected by the plasma-generated surface functionalities, which decorated the surface to make it look different from that of initial fibers. Surface Compositions of the ACFs with Plasma Treatment. The effect of oxidation is another important characteristic of a plasma treatment, which can add surface functionality to the treated substrate. This section discusses the effects of the plasma treatment time on the surface functionality. Figure 8 shows the XPS results of the as-received and plasma-treated ACFs as a function of the plasma treatment time, in which curves A, B, C, and D represent the samples plasma-treated at 0, 15, 50, and 80 s, respectively. It was found that the surface concentration of oxygen-containing groups increased with increasing treatment time while the concentration of carboncontaining groups decreased, indicating a plasma oxidation effect. Table 3 shows the relative peak area of C1s and O1s on the as-received and plasma treated ACFs. The data shows continuous decreases in the C/O ratios indicating the extent of
Novel Effects of Surface Modification
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1825
Figure 7. SEM images of the plasma-treated ACFs using different plasma treatment power: (a) 50 W; (b) 80 W; (c) 100 W; (d) 120 W; (e) 180 W. (Plasma treatment time, 30 s; pressure, 250 mTorr).
TABLE 3: Relative Peak Area of C1s and O1s on the As-Received ACFs and Plasma-Treated ACFsa as received 20s 50s 80s
88.4 62.5 25.2 23.5
11.6 37.5 74.8 76.5
7.62 1.74 0.34 0.31
a Plasma treatment time, 50 s; plasma treatment power, 120 W; pressure, 250 mTorr.
Figure 8. XPS spectra of the as-received and plasma-treated ACFs at different plasma treatment times. (Plasma treatment power, 100 W; pressure, 250 mTorr).
surface oxidation with increasing plasma treatment time. The other peaks such as S2p3/2, Cl2p3/2, and Au4p3/2 shown in Figure 8 were thought as impurities during the preparation of the samples and were neglected in this part. The C1s peak was deconvolved into a number of component Gaussian peaks, including CsC (284.6 eV), CsO (286.1 eV),
CdO (287.6 eV), OsCdO (289.6 eV), and π-π* shake up bonds in order to further examine the process of plasma oxidation. Parts a-d of Figure 9 show the deconvolutions of the C1s corresponding to the curves A, B, C, and D in Figure 8. Table 4 shows the area ratio of the five deconvolved Gaussians at different treatment times, in which the decrease in the concentration of CsC groups and π-π* shake up bonds and the increase in the concentration of CsOx groups can be observed. These changes suggest that the CsC bonds are oxidized and new CsOx groups were generated on the surface of the ACFs through plasma treatment. The surface became more severely oxidized at longer plasma treatment times. The changes in the CsOx groups including CsO, CdO, and Os CdO bonds were not as regular as those observed with the C-C bonds. It is believed that various oxidative reactions occurred during plasma treatment. With oxygen plasma treatment, free radicals can be created on the treated surface, which can then
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Figure 9. XPS spectra of C1s of the as-received and plasma-treated ACFs at different plasma treatment times. Peak 1, CsC 284.6 eV; peak 2, CsO 286.1 eV; peak 3, CdO 287.6 eV; peak 4, OsCdO and peroxide 289.6 eV; peak 5, π-π* shakeup. (a) As received; (b) 15 s; (c) 50 s; (d) 80 s. (Plasma treatment power, 100 W; pressure, 250 mTorr).
TABLE 4: Relative Subpeak Area of C1s of the As-Received ACFs and Plasma-Treated ACFsa as received 20s 50s 80s a
couplewithactivespeciesfromtheoxygenplasmaenvironment.37-40 The following structure shows the molecule configuration of the ACFs.39
On the basis of this structure the possible reaction mechanisms that occur during the plasma treatment include the generation of the CsO, CdO, and OsCdO bonds, as shown in Scheme 1. Since the π bonds in CdC are the most susceptible to plasma attack, it is believed that radicals were first generated on the dissociated π bonds, which then further reacted with active oxygen atoms. This explains the decrease in the concentration of π-π* shake up bonds with increasing treatment time in Table 4. This process might result in the formation of C-O bonds,
and C-OH bonds can be formed through the stabilization by proton transfer from the same or neighboring chain. Oxygen radicals were also generated on the surface, as shown in Scheme 1a. The new CdO bonds are believed to have formed from these oxygen radicals through intramolecular reorganization on the CsC bonds, as shown in Scheme 1b. New OsCdO bonds were believed to have formed on the CdO bonds through the combination of the plasma generated radicals on the CdO bonds and the active oxygen atoms. After stabilization with proton transfer, HOsCdO could be formed, as shown in Scheme 1c. In combination of the results shown in Table 4 with the suggested reaction mechanisms shown in parts a-c of Scheme 1, it is believed that when the plasma treatment time was as short as 15 s, large amount of π bonds were dissociated and protoxides such as CsO and CdO groups were generated while few of the new OsCdO groups were formed. When the plasma treatment time reached 50 s, the CdO bonds were oxidized and transferred into OsCdO bonds. This can explain the changes
Novel Effects of Surface Modification
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1827
SCHEME 1: Possible Reaction Mechanisms on the Plasma Treated ACFs
(a) Generation of CsO bonds; (b) generation of CdO bonds; (c) generation of OsCdO bonds; (d) transfer between carboxyl and lactone groups.
shown in Table 4, where the percentage of the π-π* shake up bonds decrease and the CsO and CdO bonds increase while there are almost no obvious changes in the percentage of Os CdO bonds at a treatment time of 15 s. This can explain why the percentage of the CdO bonds decreased with the concomitant increase in OsCdO bonds at the treatment time of 50 s. Table 4 shows that there were almost no changes in the percentages of CsO, CdO, and OsCdO groups when the plasma treatment time was 80 s comparing with the sample plasma-treated at 50 s with only a slight decrease in the CsC and the π-π* shake up being observed. This suggests that although a small amount of CsC bonds had been oxidized, the further oxidation effect of the plasma was quite weak in this period and a few new oxygen groups were generated on the surface. This phenomenon suggests that there is a saturation state for surface oxidation, at which the surface could not accept new oxygen species from the plasma environment due to the oxidation level and molecular steric hindrance. At a treatment time of 50 s, it is believed that the level of the oxidation on the surface approached or reached this state. Therefore, further
increases in the treatment time would not remarkably alter the surface functionality. This is also consistent with the results shown in Table 3, where the carbon to oxygen ratio at treatment times of 50 and 80 s does not show any significant change. In addition to the above reactions shown in parts a-c of Scheme 1, plasma can also induce inner chemical reactions on the surface of the ACFs, e.g., a HOsCdO bond and a neighboring CdO bond can form lactone groups through plasma treatment as shown in Scheme 1d.39 New peroxides might be formed when the plasma-treated surface is exposed to air.41 It is also believed that new peroxides of sCsOsOsH groups were generated on the surface of the ACFs in this experiment. It is believed that the new peroxides could be formed in various ways on the carbon atoms, as shown in parts a-c of Scheme 2. The chemical binding energy of the peroxides in the XPS spectrum was believed to be near the OsCdO bonds and peak 4 is believed to be an aggregate of the carboxyl and peroxide groups in Figure 9 and Table 4. With the exception of the above-mentioned reaction mechanisms in Scheme 1 and 2, other reaction mechanisms might have been in play during the plasma treatment because the active
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SCHEME 2: Possible Reaction Mechanisms for the Generation of New Peroxides on the ACFs
(a) Generation of peroxides on sCHs; (b) generation of peroxides on dCHsOH; (c) generation of peroxides on sCHdO.
plasma particles may induce radicals on different chemical bonds on the surface of the ACFs. However, it is believed that most of the chemical reactions occurred between the plasma-generated radicals and the active species in the plasma atmosphere. Here, the authors took the radicals generated on the π bonds as an example and illustrated the possible reactions on the surface of the ACFs with plasma treatment. In this paper, it is also believed that the plasma generated functionalities may not block the micropores. As shown in Figures 3 and 5, the diameter of the micropores was mostly in the range of 7-20 Å after the plasma treatment. The bond length of CsC, CsO, and CdO bonds are between 1.3 and 1.5 Å.42 The chemical composition of the surface of the ACFs was mainly composed of these groups. Therefore, it is reasonable to assume that the micropores may not be blocked when new oxygen containing functionalities are immobilized on the surface. The plasma treatment may generate tiny voids and increase the surface roughness of the micropores, which can correspondingly increase the specific surface area and micropore volume on the ACFs. In addition, the increased roughness and voids on the surface may offer space to accommodate the newly generated oxygen functional groups. 4. Conclusions Surface modification on the ACFs was performed using a low-pressure oxygen plasma, and novel results were obtained in the microstructural properties of the ACFs based on the BET and SEM results. The specific surface area, micropore volume and total pore volume were increased by 10% at certain plasma treatment times and powers. The bombardment of the plasma particles was able to etch the surface and generate tiny voids on the surface of some of the micropores, which correspondingly
brought the development in the microstructural properties on the ACFs. It was also found that the surface of the ACFs could be overetched and some of the micropores might be blocked at the long plasma treatment time or high plasma treatment power. New functional groups, i.e., CsO, CdO, OsCdO, and peroxides, were generated on the surface of the ACFs. An oxidative reaction mechanism was proposed, in which the formation of oxygen containing functional groups was deduced. The reactions between the plasma generated radicals and active species in the plasma atmosphere were found to be the essential for the changes in the surface functionalities. It was also demonstrated that there was a saturation state for surface oxidation, at which the surface could not continue accepting new oxygen species from the plasma environment. Moderate plasma treatment parameters such as 50 s and 100 W were found, at which the microstructural properties and surface functionalities could be both improved. The novel effects of the plasma treatment on the structural properties of the ACFs were attributed to the low-pressure plasma used in this experiment. It is believed that the high kinetic energy and low plasma density of the low-pressure plasma developed some of the micropores and weakened the burn-up effects than those in atmospheric pressure plasmas. The development of the micropores, i.e., the increased roughness and generated tiny voids might offer the space to accommodate the new functional groups, and the newly generated oxygen containing groups might not block the micropores. Acknowledgment. This research was a project for result of Graduate Research Resource Development for the Special Regional Industries supported by the Korean Ministry of
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