Carbon Activado y Plasma

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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 effectsof 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 andpower. The changes in the structural properties of the ACFs are discussed in detail with the respect of plasmaetching. X-ray photoelectron spectroscopy revealed new oxygen-containing groups, such as CsO, CdO, andOsCdO, had formed on the surface of the ACFs after plasma treatment. Plasma surface oxidative reactionssuch as the generation of radicals, the combination of the radicals and active oxygen species in the plasmachamber, and the generation of the various oxygen-containing groups are believed to have occurred. Theeffect 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 surfacefunctionalities of the ACFs were increased under moderate treatment conditions (50 s and 100 W).

1. Introduction

Activated carbon fibers (ACFs) are unique porous materialsthat contain slit-shaped pores and a large surface area. Thepeculiar porous structure and surface properties of ACFs playimportant roles in their applications to gas separation,1-5

polarizable electrodes,6 methane and hydrogen storage,7 adsorp-tion of SO2, NOx, VOCs, lead, and nickel,8-10 catalysis,11 theproduction of cigarette filters, and medical treatments.12 How-ever, 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 considerableeffort has been made to improve the surface properties of theACFs using different methods.13-16

Plasma is an efficient method in the field of surfacemodification. The surface of various materials can be readilymodified using plasma.17-24 Recently, some researchers haveapplied plasma techniques to the surface modification of carbon-based materials. Park et al.25 examined the surface and texturalproperties of ACFs with an atmospheric pressure plasmatreatment. They reported that a plasma treatment was an efficientmethod for generating new oxygen-containing functional groupson the surface of the ACFs. Orfanoudaki et al.26 studied themodification of ACFs using a plasma deposition technique with

the aim of forming pore constrictions by narrowing the surfacepore system of the ACFs. They used propylene/nitrogen andethylene/nitrogen as the plasma reaction gases and reported thatplasma deposition made an external film on the surface of theACFs and incorporated nitrogen groups into the surface. Boudouet al.27 investigated the surface modification of an isotropiccarbon fiber with microwave oxygen plasma. They suggestedthat the plasma treatment moderately increased the surfaceroughness of the carbon fibers and demonstrated that gentle

plasma exposure was sufficient to generate a large amount ofoxygen-containing functional groups on the surface. Theyattributed this to two competing effects, i.e., the removal ofsurface atoms or clusters of atoms by the etching reaction andadditional reactions between the reactive sites and the reactantoxygen species in the plasma. In addition, they showed that amore intense treatment had negative effects on the surfacefunctionality.

In previous studies, it was a simple process to significantlymodify the surface functionalities of the ACFs with a plasma

treatment by immobilizing polar components on the surface.However, the structural properties always decreased after theplasma treatment, i.e., the specific surface area and microporevolume always decreased. It was believed that the plasmatreatment might block the entrance of micropores on the surfaceby the plasma etching and prevent the formation of new oxygenfunctionalities.28,29 In this study, however, we apparentlyobserved different change in the microstructural properties ofthe ACFs, which resulted from the increase of the specificsurface area and micropore volume at a certain plasma treatmenttime and power. The new effect of the plasma treatment wasattributed to the low-pressure oxygen plasma system used inthis experiment, which has seldom been used on ACFsbefore.30,31 The plasma particles under low pressure are thoughtto possess higher kinetic energy and a lower plasma densitythan those in atmospheric pressure plasma systems. Throughthe analysis of the plasma-etched surface by scanning electronmicroscopy (SEM), it was believed that the low-pressure plasmacould develop the microstructural properties on some of thefibers by creating tiny voids and opening the isolated pores inthe ACFs. It was previously reported that, although the surfacefunctional groups of ACFs could be modified, the mechanismfor how the plasma treatment altered the surface functionalitywas not completely understood. In this paper, the authorspropose a plasma reaction mechanism based on an analysis of

* To whom correspondence should be addressed. E-mail: [email protected]. Phone: 82-42-821-5689. Fax: 82-42-822-8995.

1820 J. Phys. Chem. C2007, 111,1820-1829

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the XPS results, which shows how oxygen-containing groups,e.g., CsO, CdO, OsCdO, and peroxides, are generated onthe surface of ACFs.

2. Experimental Section

Materials and Apparatus. (1) Materials. Commerciallyavailable cellulose-based activated carbon fibers (KF-2000,Toyobo, Japan) were used in this study. Ultrapure O2(PraxairKorea Co. LTD) was used to generate the plasma.

(2) Plasma Treatment.The ACFs were treated with oxygenplasma at 250 mTorr using a radio frequency of 13.56 MHz(Model EPPs 2000, PLASMART Inc., Korea), as shown inFigure 1. The flow rate of the O2gas was controlled using amass flow controller (MFC; Model 5850E, Brooks, Japan). Theeffects of the other plasma treatment parameters such as theplasma treatment time (15, 30, 50, 80, and 120 s) and plasmatreatment power (50, 80, 120, and 150 W) were examined. Afterthe plasma treatment, Ar gas was introduced into the plasmatreatment chamber at a high flow rate (3-4 liter per min) toremove the small particles sputtered by the plasma from thesurface of the ACFs.

BET Measurement. The porosities of the ACFs werecharacterized 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 porediameter (Ap), and external surface area of ACFs were obtainedusing the nitrogen-BET equation. The pore size distributionof 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.SEMstudies were carried out using an SM-500 (ETPSEMRA,Sydney, Australia) with an operational working distance of 5mm and a voltage of 10 kV. (2) XPS.The surface compositionof the ACFs before and after the plasma treatment wasinvestigated using an ESCA 2000 (VG Micro Tech Co.). Thepressure 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 XPSPEAK4.1 software. The binding energies (BEs) were determined byreference to the BE of the C1s peak at 284.6 eV prior to peakfitting.

3. Results and Discussion

Structural Properties of ACFs by BET Treatment.Figure2 shows the nitrogen adsorption isotherms measured at 77Kfor the as-received and plasma-treated ACFs at various plasmatreatment times. Table 1 shows the changes in the microstruc-tural properties of the ACFs such as the specific surface areaand micropore volume as a function of the plasma treatmenttime.32 It was found that, compared with the as-received ACFs,

the specific surface area of the ACFs increased by 10% whenthe plasma treatment time was between 50 and 80s. On the otherhand, the specific surface area and the micropore volume ofthe ACFs were lower when the plasma treatment time wasoutside this range. The other microstructural properties exceptfor the external surface area also showed a similar trend to thatobserved with the specific surface area. Parts a and b of Figure3 show the pore size distribution of the as-received and plasma-treated ACFs. The figure shows that the pore volume mainlyincreases for pores with a diameter


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of 15-30 s and long time of 120 s, the micropore volume waslower 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 180W. Table 2 shows the changes in the microstructural properties,as listed in Table 1, for the ACFs treated for a fixed time of 30s at different plasma treatment powers. The results showed that

all the microstructural properties, such as micropore volumeand specific surface area generally increased with increasingplasma treatment power. In particular, at high plasma powers

of 120 and 180 W, the specific surface area as well as the othermicrostructural properties increased by 10%. However, a lowpower treatment (


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decreased after the plasma treatment, which was attributed tothe destruction of the micropores by plasma etching andoxidation. Park and Kim25 once obtained similar results on theACFs (AW2001) using Ar/O2 atmospheric pressure plasma.They reported that the properties of the micropores decreaseddue to blockage of the micropores as a result of plasma etching.Domingo-Garcia et al.33 also reported the decrease in the surfacearea of glassy carbon after O2plasma treatment and attributedthis to the oxygen functionalities formed by plasma oxidation,which occupied the entrance of the micropores and blocked themicropores. However, in this study, a large improvement in themicrostructural properties of ACFs was obtained using low-pressure oxygen plasma, e.g., the specific surface area and

micropore volume increased by 10%. These changes wereattributed to the low-pressure plasma systems used in thisexperiment. The energy and density of the plasma species wereonce specified in O2rf plasma at 13.56 MHz.36 It is believedthat the kinetic energy of the plasma particles at low pressureis higher than that in atmospheric pressure plasma systems atsimilar treatment conditions due to the longer mean free pathof the plasma particles without losing their energy, whichimproved the etching efficiency of these particles. It is alsobelieved that at similar treatment conditions the dischargedensity of the low-pressure plasma is lower than that in theatmospheric pressure plasma systems due to dispersion ofplasma particles in the vacuum chamber. This characteristic canweaken the burn-up effects during the plasma treatment.

Therefore, the microstructural properties were improved atmoderate 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 plasma-treated at different treatment times. The surface of the as-received ACFs (Figure 6a) was relatively smooth. When theplasma treatment time was 15 and 30 s (parts b and c of Figure6, respectively), the surface became rougher and some tiny voidswere produced. This phenomenon was due to plasma etching,

which has been explained in previous research.27 During thisperiod, there were no significant changes in the surface porositiesof the ACFs because the plasma treatment time was short. Withincreasing in plasma treatment time to 50 s, the surface becameetched even further and more tiny voids were created, as shownin Figure 6d. When the plasma treatment time was 80 s, thesurface roughness increased and the number of plasma generatedvoids decreased, as shown in Figure 6e. This phenomenon wasattributed to the long plasma etching time. A long plasmatreatment time can cause over etching and burn-up effects onthe surface, which increase the surface roughness and destroythe surface morphology. Figure 6f shows the overetching effectsat long time plasma treatment, in which the tiny voids haddisappeared 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 treatmentat moderate treatment time such as 50 and 80 s. It is believedthat highly energetic plasma species can etch the external surfaceof ACFs and reveal some internal micropores. Thus, themicropore volume and specific surface area were increased inthis period. After this period, further extending treatment timeto 120 s, plasma overetching effect were more and more seriousand the burning-up and molecular crosslinking may lead to theblockage of some micropores. Therefore, the micropore volumeand specific surface area decreased at this period. The plasmaeffect should be relatively weak at a short treatment time of15-30 s. At short time treatment such as 15-30 s, thebombardment of plasma particles was relatively weak. So thatthe increase of the micropore volume and specific surface areawas less apparent than those treated at 50 s. The SEM resultsshow agreement with the BET results, in which the microstruc-tural properties such as the specific surface area, microporevolume, and total pore volume had developed at a treatmenttime of 50-80 s. At other treatment times, such as 15-30 or120 s, the microstructural properties changed slightly or declineddue to over etching, respectively.

On the basis of the results of Figure 6 and Table 1, it is alsoconsidered that not all the micropores were modified andrevealed by plasma treatment. Some of the micropores, which

are hidden inside the fibers, may remain unchanged, since theplasma species can lose their energy after impinging into newpores and cannot smartly get into the irregularly distributed poresin the ACFs. Therefore, the specific surface area and microporevolume were increased up to a maximum of 10% as shown inTable 1.

Figure 7 shows SEM images of the ACFs treated withdifferent plasma powers. Compared with the as-received sampleshown in Figure 6a, the sample treated at 50 W showed a smallincrease in surface roughness (Figure 7a). When the power was80 and 100 W, the surface roughness increased markedly andplasma generated voids could be clearly seen, shown in parts band c of Figure 7. These phenomena were also attributed to thebombardment of plasma particles, which caused plasma etching.

Figure 5. Pore size distribution of as-received and plasma-treated ACFsusing different treatment power. (Plasma treatment time, 30 s; pressure,250 mTorr).

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For a treatment at 120 W, the plasma-generated voids increased,as shown in Figure 7b, which is similar with the image shownin Figure 6d. When the visible power reached 180 W, asignificant number of plasma-generated voids were visible, asshown in Figure 7e. The increase in the plasma treatment powermeans an increase in the plasma kinetic energy and number ofplasma particles with a stronger energy. Therefore, the surfacewas rougher at the higher plasma treatment power and therewere more plasma generated voids. However, as shown inFigure 7e, it is also found that the surface of the ACFs wasetched most severely compared with the samples treated at lowerpower. This suggests that the plasma etching effect becamestronger when the plasma treatment power was higher. It is

believed that the surface of the ACFs could be overetched withcontinuous increases in the plasma treatment power, whichwould result in the disappearance of the plasma generated voidsin a similar manner to that observed with increases in thetreatment time.

The changes in the surface morphology of the ACFs withthe plasma treatment power in parts a-e of Figure 7 can beexplained in the same way as the plasma treatment time. Atmoderate treatment power such as 120 W, the microstructuralproperties of the ACFs could be improved due to the bombard-ment of the plasma particles. It is also believed that the isolatedpores in the ACFs are more efficiently revealed at highertreatment power and the energetic plasma particles can possiblypenetrate into the plasma generated voids. Therefore, the SEM

results in parts a-e of Figure 7 also show agreement with theBET results in Table 2. At low plasma treatment power suchas 50-80 W, the increase of the micropore volume and specificsurface area was less apparent than those treated at higher powerof 100-180 W.

In combination of the SEM and BET results, the surfaceporosities of the ACFs were closely related to the plasmatreatment conditions including the treatment time and power.The surface morphology observed from SEM images canpossibly be affected by the plasma-generated surface function-alities, which decorated the surface to make it look differentfrom that of initial fibers.

Surface Compositions of the ACFs with Plasma Treat-

ment.The effect of oxidation is another important characteristicof a plasma treatment, which can add surface functionality tothe treated substrate. This section discusses the effects of theplasma treatment time on the surface functionality. Figure 8shows the XPS results of the as-received and plasma-treatedACFs as a function of the plasma treatment time, in whichcurves A, B, C, and D represent the samples plasma-treated at0, 15, 50, and 80 s, respectively. It was found that the surfaceconcentration of oxygen-containing groups increased withincreasing treatment time while the concentration of carbon-containing groups decreased, indicating a plasma oxidationeffect. Table 3 shows the relative peak area of C1s and O1s onthe as-received and plasma treated ACFs. The data showscontinuous decreases in the C/O ratios indicating the extent of

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) 50s; (e) 80 s; (f) 120 s. (Plasma treatment power, 100 W; pressure, 250 mTorr).

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surface oxidation with increasing plasma treatment time. Theother peaks such as S2p3/2, Cl2p3/2, and Au4p3/2shown in Figure8 were thought as impurities during the preparation of thesamples and were neglected in this part.

The C1s peak was deconvolved into a number of componentGaussian peaks, including CsC (284.6 eV), CsO (286.1 eV),

CdO (287.6 eV), OsCdO (289.6 eV), and -* shake upbonds in order to further examine the process of plasmaoxidation. Parts a-d of Figure 9 show the deconvolutions of

the C1s corresponding to the curves A, B, C, and D in Figure8. Table 4 shows the area ratio of the five deconvolvedGaussians at different treatment times, in which the decreasein the concentration of CsC groups and -* shake up bondsand the increase in the concentration of CsOxgroups can beobserved. These changes suggest that the CsC bonds areoxidized and new CsOxgroups were generated on the surfaceof the ACFs through plasma treatment. The surface becamemore severely oxidized at longer plasma treatment times. Thechanges in the CsOxgroups including CsO, CdO, and OsCdO bonds were not as regular as those observed with the C-Cbonds. It is believed that various oxidative reactions occurredduring plasma treatment. With oxygen plasma treatment, freeradicals can be created on the treated surface, which can then

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) 180W. (Plasma treatment time, 30 s; pressure, 250 mTorr).

Figure 8. XPS spectra of the as-received and plasma-treated ACFs atdifferent plasma treatment times. (Plasma treatment power, 100 W;pressure, 250 mTorr).

TABLE 3: Relative Peak Area of C1s and O1s on theAs-Received ACFs and Plasma-Treated ACFsa

C1s% O1s% C1s/O1s

as received 88.4 11.6 7.6220s 62.5 37.5 1.7450s 25.2 74.8 0.3480s 23.5 76.5 0.31

a Plasma treatment time, 50 s; plasma treatment power, 120 W;pressure, 250 mTorr.

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couplewithactivespeciesfromtheoxygenplasmaenvironment. 37-40

The following structure shows the molecule configuration ofthe ACFs.39

On the basis of this structure the possible reaction mechanismsthat occur during the plasma treatment include the generationof the CsO, CdO, and OsCdO bonds, as shown in Scheme1. Since the bonds in CdC are the most susceptible to plasmaattack, it is believed that radicals were first generated on thedissociated bonds, which then further reacted with activeoxygen atoms. This explains the decrease in the concentrationof-* shake up bonds with increasing treatment time in Table4. This process might result in the formation of C-O bonds,

and C-OH bonds can be formed through the stabilization byproton transfer from the same or neighboring chain. Oxygenradicals were also generated on the surface, as shown in Scheme1a. The new CdO bonds are believed to have formed fromthese oxygen radicals through intramolecular reorganization on

the CsC bonds, as shown in Scheme 1b. New OsCdO bondswere believed to have formed on the CdO bonds through thecombination of the plasma generated radicals on the CdO bondsand the active oxygen atoms. After stabilization with protontransfer, HOsCdO could be formed, as shown in Scheme 1c.

In combination of the results shown in Table 4 with thesuggested reaction mechanisms shown in parts a-c of Scheme1, it is believed that when the plasma treatment time was asshort as 15 s, large amount of bonds were dissociated andprotoxides such as CsO and CdO groups were generated whilefew of the new OsCdO groups were formed. When the plasmatreatment time reached 50 s, the CdO bonds were oxidized andtransferred into OsCdO bonds. This can explain the changes

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, -* 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

CsC (284.6 eV) CsO (286.1 eV) CdO (287.6 eV) OsCdO and peroxide (289.6 eV) sp* shakeup (291.4-294.1 eV)

as received 52.59% 17.04% 10.37% 7.41% 12.59%20s 40.85% 24.01% 22.01% 8.51% 4.62%50s 35.92% 25.30% 11.30% 24.28% 3.20%80s 34.03% 26.68% 11.45% 24.75% 3.09%

a Plasma treatment time, 50 s; plasma treatment power, 100 W; pressure, 250 mTorr.

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shown in Table 4, where the percentage of the -* shake upbonds decrease and the CsO and CdO bonds increase whilethere are almost no obvious changes in the percentage of OsCdO bonds at a treatment time of 15 s. This can explain whythe percentage of the CdO bonds decreased with the concomi-tant increase in OsCdO bonds at the treatment time of 50 s.Table 4 shows that there were almost no changes in the

percentages of Cs

O, Cd

O, and Os

Cd

O groups when theplasma treatment time was 80 s comparing with the sampleplasma-treated at 50 s with only a slight decrease in the CsCand the -* shake up being observed. This suggests thatalthough a small amount of CsC bonds had been oxidized, thefurther oxidation effect of the plasma was quite weak in thisperiod and a few new oxygen groups were generated on thesurface.

This phenomenon suggests that there is a saturation state forsurface oxidation, at which the surface could not accept newoxygen species from the plasma environment due to theoxidation level and molecular steric hindrance. At a treatmenttime of 50 s, it is believed that the level of the oxidation on thesurface approached or reached this state. Therefore, further

increases in the treatment time would not remarkably alter thesurface functionality. This is also consistent with the resultsshown in Table 3, where the carbon to oxygen ratio at treatmenttimes of 50 and 80 s does not show any significant change. Inaddition to the above reactions shown in parts a-c of Scheme1, plasma can also induce inner chemical reactions on the surfaceof the ACFs, e.g., a HOsCdO bond and a neighboring CdO

bond can form lactone groups through plasma treatment asshown in Scheme 1d.39 New peroxides might be formed whenthe plasma-treated surface is exposed to air.41 It is also believedthat new peroxides of sCsOsOsH groups were generatedon the surface of the ACFs in this experiment. It is believedthat the new peroxides could be formed in various ways on thecarbon atoms, as shown in parts a-c of Scheme 2. The chemicalbinding energy of the peroxides in the XPS spectrum wasbelieved to be near the OsCdO bonds and peak 4 is believedto be an aggregate of the carboxyl and peroxide groups in Figure9 and Table 4.

With the exception of the above-mentioned reaction mech-anisms in Scheme 1 and 2, other reaction mechanisms mighthave been in play during the plasma treatment because the active

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 lactonegroups.



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plasma particles may induce radicals on different chemical bondson the surface of the ACFs. However, it is believed that mostof the chemical reactions occurred between the plasma-generatedradicals and the active species in the plasma atmosphere. Here,the authors took the radicals generated on the bonds as anexample and illustrated the possible reactions on the surface ofthe ACFs with plasma treatment.

In this paper, it is also believed that the plasma generatedfunctionalities may not block the micropores. As shown inFigures 3 and 5, the diameter of the micropores was mostly inthe range of 7-20 after the plasma treatment. The bond lengthof CsC, CsO, and CdO bonds are between 1.3 and 1.5 .42

The chemical composition of the surface of the ACFs wasmainly composed of these groups. Therefore, it is reasonableto assume that the micropores may not be blocked when newoxygen containing functionalities are immobilized on thesurface. The plasma treatment may generate tiny voids andincrease the surface roughness of the micropores, which cancorrespondingly increase the specific surface area and microporevolume on the ACFs. In addition, the increased roughness andvoids on the surface may offer space to accommodate the newlygenerated oxygen functional groups.

4. Conclusions

Surface modification on the ACFs was performed using alow-pressure oxygen plasma, and novel results were obtainedin the microstructural properties of the ACFs based on the BETand SEM results. The specific surface area, micropore volumeand total pore volume were increased by 10% at certain plasmatreatment times and powers. The bombardment of the plasmaparticles was able to etch the surface and generate tiny voidson the surface of some of the micropores, which correspondingly

brought the development in the microstructural properties onthe ACFs. It was also found that the surface of the ACFs couldbe overetched and some of the micropores might be blocked atthe long plasma treatment time or high plasma treatment power.

New functional groups, i.e., Cs

O, Cd

O, Os

Cd

O, andperoxides, were generated on the surface of the ACFs. Anoxidative reaction mechanism was proposed, in which theformation of oxygen containing functional groups was deduced.The reactions between the plasma generated radicals and activespecies in the plasma atmosphere were found to be the essentialfor the changes in the surface functionalities. It was alsodemonstrated that there was a saturation state for surfaceoxidation, at which the surface could not continue acceptingnew oxygen species from the plasma environment. Moderateplasma treatment parameters such as 50 s and 100 W werefound, at which the microstructural properties and surfacefunctionalities could be both improved. The novel effects ofthe plasma treatment on the structural properties of the ACFs

were attributed to the low-pressure plasma used in this experi-ment. It is believed that the high kinetic energy and low plasmadensity of the low-pressure plasma developed some of themicropores and weakened the burn-up effects than those inatmospheric pressure plasmas. The development of the mi-cropores, i.e., the increased roughness and generated tiny voidsmight offer the space to accommodate the new functionalgroups, and the newly generated oxygen containing groupsmight not block the micropores.

Acknowledgment. This research was a project for result ofGraduate Research Resource Development for the SpecialRegional Industries supported by the Korean Ministry of

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.



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Commerce, Industry and Energy. The authors would like tothank the various people involved in this project for theirsupport.

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