Surface modification and characterization of carbon black with oxygen plasma

CarbonVol. 34, No. 9, pp. 1087-1091,1996 Copyright0 1996ElsevierScienceLtd

Printed& Great Britain. All rights reserved OCO8-6223/96 %15.00+ 0.00 SOOO8-6223(%)000541

SURFACE MODIFICATION AND CHARACTERIZATION CARBON BLACK WITH OXYGEN PLASMA

OF

T. TAKADA,~ M. NAKAHARA, b H. KUMAGAI,’ and Y. SANADA’ “Process Research and Product Evaluation Department, Japan Energy Corporation, 2-l Ushiodori, Kurashiki, Okayama 712, Japan bFaculty of Engineering, Gunma University, Kiryu, Gunma 376, Japan ‘Center for Advanced Research of Energy Technology, Hokkaido University, N-13 W-8, Sapporo 060, Japan (Received 20 March 1995; accepted in revised form 7 March 1996)

Abstract-The surface of carbon black was modified with an oxygen plasma treatment. The initial increase and following reduction of carbon black weight due to the treatment were observed as the treatment time increased. The chemical structure of carbon black surfaces formed by the oxidation process has been characterized by X-ray photoelectron spectroscopy (XPS). The XPS O/C atomic ratio of the surfaces showed a plateau after an initial increase and saturated at about 0.42 after 120 minutes regardless of the plasma input power. The introduction rate of oxygen-containing functional groups onto the carbon black surfaces increased by increasing the plasma input power. The oxygen plasma was found to cause changes in surface C=O or O-C-O and O-C=0 functionalities, and their concentrations were proved to increase with the treatment time up to about 120 minutes irrespective of the input power. Copyright 0 1996 Elsevier Science Ltd Key Words-Surface tional groups, XPS.

modification, plasma treatment, oxidation, carbon black, oxygen-containing

func-

2. EXPERIMENTAL

1. INTRODUCTION The surface properties of carbon materials are influenced to a large extent by foreign elements, such as fluorine, introduced onto the surfaces [ 11. The surface structure plays an important role in a large number of practical applications such as reinforcement and adsorption materials [ 2-53. For carbon blacks, a model surface with ordered zones of small graphitic crystallites and functional groups has been proposed by Donnet et al. [6]. In order to modify the surface structure, carbon blacks have been submitted to various surface treatments [7]. The present authors [ 81 have found that microwave oxygen plasma treatment mainly introduces keto-enol groups onto the edge surfaces of pyrolytic graphite, which is nearly parallel to the c-axes of constituent graphitic crystallites. For pyrolytic graphite, the surface functionality can be identified by X-ray photoelectron spectroscopy (XPS) and FTIRattenuated total reflection (ATR) method [1,8-111. In addition, a study of carbon blacks using a different technique has been reported for the purpose of characterizing the surface functionalities [ 121. Characterization of surface functionalities for carbon blacks is important in understanding and controlling the surface properties [ 131. In the present paper, we present the use of a microwave plasma to treat the surfaces of a carbon black. After the plasma treatment, the structural changes of carbon black surfaces have been monitored by XPS. Some relationships between the surface chemistry and the plasma treatment conditions are discussed.

The carbon black used in this study was #SO (ASTM grade N762) supplied from Asahi Carbon, whose average spherical particle size was about 65 nm. The as-received carbon black was pulverized under 200 mesh in advance of plasma treatment. The elemental analysis of the as-received carbon black was 99.26% of C, 0.29% of N, 0.00% of H and 0.45% of others. The SAMCO BP-l RF plasma kit (13.56 MHz) with planar electrodes in diode arrangement was used to treat the carbon black surface (Fig. 1). The distance between electrodes was 25 mm. The weight of the

Electrodes Bell-jar Petri dish (sample) Vibrator

Fig. 1. Schematic diagram of microwave plasma treatment apparatus.

1087

1088

T.TAKADA

carbon black sample in the Petri dish was about 200 mg. The reactor was first evacuated to a pressure of about 2 Pa before the plasma gas was introduced. A vibratory fixture was mounted in contact with the electrodes to carry out homogeneous plasma treatment of the carbon black surfaces. Before oxygen plasma treatment, contamination on the carbon black surfaces was remved by argon plasma treatment for 30 minutes under the condition of a pressure of about 10’ Pa and a flow rate of about 100ml min-’ with a constant 50 W of radio frequency input. Exchange of argon gas for oxygen gas was performed without taking the sample out of the reactor chamber. The input plasma power for oxygen plasma treatment varied between 1 and 20 W, the treatment time between 5 and 480 minutes, under a pressure of about 10’ Pa and a flow rate of about 100ml min-‘. Before and after the plasma treatment, the weight of the sample was measured. The X-ray photoelectron spectroscopy (XPS) measurements were carried out by a Vacuum Generators ESCALAB MK II spectrometer using an Mg Ka X-ray source (1253.6 eV, 300 W, 15 kV) to obtain surface information on the O/C atomic ratio and oxygen-containing functional groups. The carbon black samples were placed on double-sided adhesive carbon tape mounted on a sample holder. The vacuum in the XPS chamber was maintained at 6.5 x 10m7 Pa or less during analysis. All measurements were carried out at the 75” take-off angle. The relative sensitivity factor was determined to be 2.76 by using anthraquinone (C14Hs02) for the estimation of the O/C atomic ratio from the Ols/Cls peak area ratio. Curve fitting of the Cls spectra was conducted using a Gaussian/Lorentzian product function with fixed full width at half maximum [ 111.

3. RESULTS

AND DISCUSSION

etal.

the etching effect of surface carbon structures. The etching effect would become dominant as the input power increases. An almost linear weight reduction for 20 W in Fig. 2 indicates that the elimination reaction of the oxides is predominant in the treatment condition. As the input power increases, the elimination reaction is superior to the formation of surface oxides. When the straight lines of weight reduction are extended to a treatment time of 0 minutes as shown in Fig. 2, the value of intercept shows a constant value, about 2.5%, at all the plasma input powers except for 1 W. This value would be the maximum amount of oxygen which can be introduced to the carbon black surfaces if the elimination reaction does not occur.

3.2 XPS analysis 3.2.1 O/C atomic

ratio It was observed that oxygen plasma treatment significantly increases XPS 01s peak intensity as the treament time increases in the region of l-20 W. Surface O/C atomic ratio is plotted against treatment time as shown in Fig. 3. It saturates around 0.42 irrespective of the input power after an initial increase with all the plasma input powers. Treatment time at which O/C atomic ratio reaches a plateau is about 120 minutes regardless of the value of plasma input power investigated in this study. In the region of a shorter treatment time of less than 60 minutes, an increment ratio of the O/C atomic ratio appears to increase as the input power increases. The tendency is consistent with the data concerning weight gain described above. Ar plasma etched carbon black surfaces would be exposed by many active sites for the formation of oxides. Since the oxygen introduced has been determined by XPS measurements, the oxygen would be covalently bonded to the carbon black surfaces. Therefore, the oxygen introduction process is irreversible. The oxi-

3.1 Weight change A change in weight of carbon black samples by oxygen plasma treatment is shown in Fig. 2. The weight of the plasma treated samples increases initially at all the plasma input powers and the ratio of the initial weight increment tends to increase with the input power. A following decrease in weight of the samples is apparently recognized for the prolonged treatment at higher input powers. For 1 W, a decrease in weight of the sample could be observed at 480 minutes treatment. Therefore, increasing the exposure time to the oxygen plasma leads to a reduction in weight of the carbon black. The authors assume that this is because the longer exposure time causes surface heating with a weight loss as evolved gases such as CO, and CO. The degree of the weight reduction increases with the input power. Treatment time exhibiting the maximum value of sample weight change tends to reduce as the input power increases. The increase in the weight of sample is due to the introduction of oxygen onto the carbon black surfaces and the following reduction is due to

Treatment Fig. 2. Effect of plasma

time (min)

treatment on the weight carbon black.

change

of

Surface modification

and characterization

1089

o.o~....:~...:.~‘.:~~.,:..““..’ 0 10 20 30

Treatment Fig. 3. Effect of plasma

time (min)

treatment on O/C atomic XPS analysis.

Treatment ratio

by

40

50

60

time (min)

Fig. 4. Effect of plasma treatment on the ratio of oxygen introduction onto the carbon black surface.

dation process is supposed to be due to a first-order irreversible reaction. The concentration of the surface active site and the concentration of oxygen introduced by the plasma treatment are denoted by [AS] and [OZ], respectively. Since [AS] varies in inverse proportion to [Or], the rate of change of [AS] can be expressed as: -d[AS]/dt=d[OZ]/dt=k([AS],-[OZ]) - ln([AS& - [OZ]) = kt + C

(1) (2)

where [AS], is the value of [AS] for the untreated surface, k is the kinetic constant for the introduction of oxygen onto the surface active sites, and t is the oxygen plasma treatment time. Here the relative concentrations for untreated (0 minutes) are defined by the following equations. [AS&, = 1

(3a)

[orlo=

(3b)

-B.Ot.,... 0.0

Using eqns (l)-(3b), the relation between [OZ] or [AS] and t is given by the following equations: -kt

ln(l-[Or])= ln[AS]=

-kt

(44 (4b)

The relation between [OZ] and t is shown in Fig. 4. The data obtained in this study are found to follow a straight line for each input power. The following empirical formula concerning the relationship between the slope of the straight line (k) and plasma input power (P) can be obtained by taking into account a good linearity in the region of O-O.5 W -I, as shown in Fig. 5: k+exp{ -(7.2+2.5/P)}

(5)

When [AS]/2 is substituted in [AS] of eqn (4b), the following equation is obtained: ln2 = kz

(6)

0.2

0.4

0.6

l/p (W Fig. 5. Plot of In k versus

0.6 1 1

1.0

1.2

l/P for the carbon black surface.

where z is the half-life period, or the time required for the surface active sites to be reduced to one-half of its initial value. From eqns (5) and (6) the following equation can be obtained: t(s) + 0.7exp{ 7.2 + 2.5/P(W)}

(7)

Consequently, it was found that increasing the input power leads to an increase in the formation rate of oxides. 3.2.2 Carbon functionalities Further information about chemical functions for the plasma-treated carbon black surfaces are obtained by peak fitting the Cls XPS spectra [ll]. The Cls spectra may be deconvoluted into five components [14]. The main component (component (1)) corresponds to sp’ and sp3 non-functionalized carbons (C-C) of polyaro-

T. TAKADA et al.

1090

(4 60 :% .

0 L!'

0 a l n

2w SW low 2ow

I_A

a 0

n l

, 8

q

8 0

a

3

F

20

20

40

60

60

100120

1

A

25t

f

l

a

8

D

Treatment time (min)

Treatment time (min)

(4 ::iy 0 2

B n

0 -

n

0

c u

6

0

6

1w

Ii 2w c 5w 0 n 40

60

60

100

n

4

low 2ow 120

0 .

I 2

20

A

140

Treatment time (min)

0

".

20

*I*

40

60

80

100

120

140

Treatment time (min)

Fig. 6. Effects of plasma treatment on functionalities on the carbon black surface. (a) C-C, (b) C-O, (c) C=O or O-C-O, and (d) 0-C=O.

matic ring structures on the carbon black surface. The outer four components were shifted by about 1.3, 2.9, 4.5 and 6.0 eV from the main peak, respectively. Component (2) corresponds to carbon singlybonded to one oxygen atom (C-O). Component (3) originates from carbon doubly-bonded to one oxygen or singly-bonded to two oxygen atoms (C=O, or O-C-O). Component (4) is related to carboxylate groups (0-C=O). Component (5) is probably attributed to carbonate carbons (O-CO-O) or the rt+rr* shake-up satellite [ 151. The percentages of component (5) for the all plasma-treated carbon blacks are less than 10% and the component appears to be unaffected by the plasma treatment. The behavior of each component proportion, except for component (5), with treatment time and input power are shown in Fig. 6. For C-C,

the percentage tends to decrease after the plasma treatment. For C-O, the percentage tends to increase in the region of O-30 minutes and thereafter a decrease in the percentage is recognized for all the input powers. This suggests that a part of component (2) changed into component (3) and/or component (4) by longer treatment. Both of the C=O or O-C-O and O-C=0 functionalities tend to increase with the treatment time and almost saturate at 120 minutes for higher input powers (5-20 W). The behavior of their functionalities are similar to those for the O/C atomic ratio. This indicates that the oxygen plasma treatment mainly introduces C=O or O-C-O and O-C=0 functionalities onto the carbon black surface. For pyrolytic graphite (PG), the O-C=0 functionality was not introduced onto the edge surface for

Surface modification and characterization 50 W oxygen plasma treatment [ 81. The introduction of the O-C=0 functionality was not also recognized in the region l-20 W. Apparently, the result for PG is found to be different from that for carbon blacks. The authors already demonstrated that O-C=0 functionality was formed on the disordered carbon skeleton structure [3]. Therefore, the O-C=0 functionality formation is assumed to be introduced to a partially disordered structure on the carbon black surface.

4. CONCLUSIONS Microwave oxygen plasma treatment can change the carbon black surface chemistry considerably and raise surface oxygen functionality. The O/C atomic ratio increases with the treatment time and almost saturates at 120 minutes in the region of plasma input power of l-20 W. The saturated value of the O/C atomic ratio is about 0.42 regardless of the input power. The introduction rate of oxygen onto the carbon black surface can be raised by increasing the input power. The main oxidation products on the surface are C=O or O-C-O and O-C=0 type groups, unlike the case of pyrolytic graphite.

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and