Anal. Chem. 1966, 60, 2766-2769
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Uptake of Ions into Electrochemically Treated Glassy Carbon Tsutomu Nagaoka,* Toshikazu Fukunaga, and Takashi Yoshino Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, Ube 755, Japan
Iwao Watanabe and Toshiro Nakayama Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan
Satoshi Okazaki Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
The surface of glassy carbon electrodes was very porous after oxidative electrochemical treatment. The porous structure was confirmed by flow-through electrolysis with a bundle of treated carbon fibers in acetonitrile (AN). There was Ion uptake arlslng from the porous structure, which was detected as changes in the concentrations of ions. Alkalimetal kns were taken up into the pores at negative potentials and releaeed at positive potentials. Anlons (Br- and Ci-) also were taken up at positive potentials and released at negative potentials. The amount of the catlons taken up Increased In the sequence of Ba2+ < K+ = Na' < LI'. I n HCi0,-AN solutlons, the broad peaks were observed In the cyclic vottammograms of treated carbon and assigned to the uptake of protons into the carbon.
After the oxidative treatment of glassy carbon, broad peaks appear in its cyclic voltammograms, and the activities of the treated electrodes increase in many redox reactions (1-13). Furthermore, the charging current in the voltammograms increases greatly. In the surface chemistry of treated glassy carbon, some important problems related to the above phenomena are still unsolved (1)The broad peaks have been attributed to redox reactions of the oxygen surface sites introduced by treatment with little direct evidence. (2) The increase in the charging current can be explained in terms of surface roughing by surface oxidation, but there have been few discussions of this in detail (14,15). (3) The origin of the enhanced activities is not understood. The enhanced activities are often attributed to electrocatalysis by surface quinones, because quinones catalyze many redox reactions in which the treated electrodes have enhanced activities. However, we have reported than in oxygen reduction, the electrochemical behavior of the surface sites on treated glassy carbon is very different from that of quinones confined at the carbon surface (13). Kuwana e t al. have also reported that surface oxides are unnecessary for electrocatalytic oxidation of ascorbic acid (16).
To understand the surface chemistry of treated glassy carbon more clearly, we have studied the microporous structure of the surface from the results of proton and catechol adsorption (12). Picq et al. have also reported on the porous structure from the results of secondary ion mass spectrometry; after glassy carbon electrodes undergo hydrogen evolution, the hydrogen concentration (mass = 1) of the treated electrodes is significantly higher throughout the material than that of the electrodes not treated (17). Koresh and Soffer have studied the microporous structures of oxidized graphite surfaces; ions are taken up into the pores electrochemically, and molecular sieve effects are observed (18-21). Elsewhere, we have reported that the treated carbon surface can interact with metal ions in AN (22). Cyclic voltammograms in AN that 0003-2700/88/036O-2766$01.50/0
contain metal ions were similar to those in aqueous solutions. We thought that the interaction between the carbon surface and the metal ions occurs because of the uptake of the ions into the micropores, and we discuss here the validity of this interpretation more precisely.
EXPERIMENTAL SECTION The reference electrodes used in aqueous media were Ag/AgCl (saturated KCl), and in AN, Ag/lO mM AgNOS(AN). (i) Cyclic Voltammetry at Disk Electrodes. Cyclic voltammetry at the disk electrodeswas done with a laboratory-made potentiostat and a Toho Giken Model 2230 potential scanner at 25 f 0.5 OC. The working electrodes were short rods 3 mm in diameter (Grade GC-20, Tokai Mfg.) press-fitted coaxially into a Teflon housing. After being polished with fine alumina (0.06 pm) and sonicated for about 5 min, the electrode surface was oxidized at 1.8 V vs Ag/AgCl for 20 min in pH 7 phosphate buffer and then reduced at -1.5 V for about 1min. Cyclic voltammograms were recorded in AN containing 0.1 M metal perchlorate, after the treated electrodes were dried with hot air. (ii) X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were recorded with a Shimadzu HIPS-70 with Mg Ka X-radiation (210 W). Carbon samples (GC-20,rods 6 mm in diameter) were treated in aqueous solution and then in Li+-AN as in section i. The samples were then sonicated in acetone or AN, and photoelectron spectra were recorded. The C 1s binding energy of the graphitic carbon was taken to be 284.6 eV for calibration purposes. (iii) Flow-Through Electrolysis. The main body of the flow-through electrodeconsisted of Pyrex glass tubing. The deaign of the flow-throughelectrode was essentially the same as reported elsewhere (23).However, instead of Nafion tubing, a Vycor glass tube was used as the separator between the working- and counter-electrode compartments because of the high ohmic resistance of Nafion in AN. The working electrode waa a bundle of glassy carbon fibers (Grade GC-20, 10 pm in diameter) with a high total surface area. Alumina polishing of the fibers was difficult,80 we merely washed them with NJV-dimethylformamide, HCl, and distilled water before oxidative treatment. A dropping mercury electrode (DME) was placed at the end of the flow line to detect the concentration of the ions taken up into the carbon fibers. The fibers were treated like the disk electrode in section i, being anodized at 1.8 V in the pH 7 buffer for 20 min at the flow rate of about 1cm3m i d and then cathodized at -1.5 V for about 1 min. Carrier solutions were 0.1 M tetraethylammonium perchlorate (TEAP)-AN with 5 mM objective ions unless otherwise noted. The flow rate of the system was regulated to 1.0 f 0.1 cms min-l with an Atto Model SJ-1211 pump unless otherwise noted. Experiments in flow-through electrolysis were done at ambient temperatures (19 & 1 "C). (iv) Chemicals. AN was treated with CaHzand distilled twice over P206.Other chemicals used were of analyticalreagent grade. All metal ions were their perchlorates, and bromide and chloride were tetraethylammonium salts of polarographic pure grade. RESULTS (i) Cyclic Voltammetry at Disk Electrodes. Figure 1A shows cyclic voltammetric responses of the treated carbon disk 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
Table I. Comparison of Redox Potentials of Treated Glassy Carbon w i t h Those of Typical Quinones in Acetonitrile
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a t 25 "Ca
Ellz vs AgllO mM Ag+(AN)/Vc
compounds
TEA+^
E J V ~ (PH 7.0)
GC-20 disk GC-20 fiber 1,2-naphthoquinone 1,4-naphthoquinone 9,lO-anthraquinone 9-fluorenone
-0.05
(-0.2)
-0.02
N.D.f -0.84 -0.97
Na+ -0.10
Li+ -0.02
-0.10
-0.05
-0.43
-1.22
-0.61 -0.85 -1.15
-1.61
-1.54
H+e
Ba2+
Mg2+
0.50
0.16
0.19
-0.34 -0.61 4.90 -1.26
-0.43
0.52
-0.71
-0.98 -1.33
-0.02 -0.73 -1.14
"The concentrations of the metal ions were 0.1 M. bMidpoint potentials between the cathodic and anodic peaks of glassy carbon in aqueous pH 7.0 phosphate buffer; the potential was referred to Ag/AgCl. cFirst half-wave potentials for quinones and the midpoint potentials for glassy carbon; estimated accuracy is &30 mV for carbon and *lo mV for quinones. dTetraethylammonium ion. eConcentration, 0.5 mM. fNot detected. t
200 s
0.1mA
-40 a
I
-30
I ; -;- 2 0 -10
J
-
t b
0 -
Figure 3. Profile of Li+ uptake and exclusion at glassy carbon fibers. The potential of the flow-through electrode was stepped from 1.0 to -1.6 V (a) and from -1.6 to 1.0 V (b); ID,DME current at -2.5 V.
5I 1
"
0.6
'
0.2
'
'
~
-0.2
I
-1.0
-0.6
E /V
Flgure 1. Cyclic voltammograms of the treated carbon disk (A) and fiber (B) in 0.1 M TEAP-AN (---) and In 0.1 M LiCIO,-AN (-).
I
1 -5
10 count/s
. 1
0.0
I
1
-1.6
-0.8
-2.4
E, / V
..:
I
I 60
58
*....*
.
56 54 Binding energy/eV
.*I
....
. .... 52
50
Flgure 2. XPS spectrum (Li 1s) of carbon treated in Li+-AN.
eledrode. In the presence of a metal ion, large peaks appeared, and the peak potential increased with a decreasing ionic radius of the cation. The peak potentials of treated carbon in metal perchlorate solutions are summarized in Table I. (ii) Photoelectron Spectroscopy. Figure 2 shows the Li IS XPS spectrum of a treated carbon disk, which was observed after the potential returned to the initial value, 0.5 V. The Li IS peaks were detected a t 55.4 eV but were weak because of the small photoionization cross section of Li. We observed another peak at about 52 eV with almost the same intensity, which was a C 1s ghost peak arising from Al X-rays from the Al window in the anode. To eliminate thispeak, the spectrum of carbon treated with Li+ was subtracted from the spectrum of the carbon treated in the aqueous solution only. The C 1s
Flgure 4. Changes in the Li+ concentration (AC) as a function of the stepped potential (E,) in AN: the negative AC values were for uptake (the potential was stepped from 1.O V to E,) and the positive values for exclusion (the potential was stepped from to 1.0 V); carbon Rbers wlth treatment p),and wRhout treatment (A).
and 0 1s spectra were very similar to those reported by several authors (24, 25). (iii) Flow-Through Electrolysis. Figure 1B shows cyclic voltammograms of the treated carbon fibers in AN, which were similar to those of the disk electrodes (A). The peak potentials of the fiber electrode agreed with those of the disk electrode (Table I). Figure 3 shows the results of potential step experiments done with the flow system. Changes in the Li+ concentration were monitored at DME. The potential of DME was -2.5 V, at which all the metal ions studied showed limiting currents. First, the potential of the flow-through electrode was held at 1.0 V. After the potential was stepped to -1.6 V (a), a dip in the DME current was observed because of Li+ uptake into the surface pores of the carbon. After the DME current reached a steady state, the potential was returned to the initial value (b). There was a peak arising from the exclusion of Li+ from the pores. Figure 4 shows the changes in
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
2768
*/'
'
-4.0
I
0.2
-0.6
-2.2
-1.4
v Flgure 5. Changes in the concentrations of metal ions as a function of E,: Li+ (O),Na+ (m), K+ (A),Ba2+ (0). 0 -
ioos
20
-
240
160
320
t
400
_All"
a
B 10
'
-6
E
c _
-
80
0
ES i
4
-1.0
Flgure 7. AC of Br- and CI- as a function of E,: Br- (O), CI- p).
1.0
. a.
0.0
Eslv
I
-5.0
b
'
1.0
d
I
618 a40
- Jt a
680
600 Time 1s
520
440
b
1 Lt
-35
= 0.2 V; the potential was stepped from -1.0 V to E, (a) and from E, to -1.0 V (b); DME potential, 0.0 V.
600
520
440
160
240
320
400
-30.
4 . -
-25.
0
-20
Li+ concentrations (AC) at DME as a function of the stepped potential (E8).The AC values were calculated from the dip and peak currents. The uptake of Li+ was observed even for the untreated carbon fibers probably resulting from oxidized and porous surface structures formed during the heat treatment of the carbon (6). Such structures can be removed by alumina polishing for the disk electrodes but not for the fibers. However, the effect of surface oxidation is clear by comparing the AC values of the treated and untreated fibers. Figure 5 shows the same plots for other metal ions. Uptake was also observed for anions, although the behavior was studied only for Br- and C1-. Figure 6 shows the DME current in the potential step experiments for Br-. The base anodic current arose from Hg2Br2formation. Br- and C1- were oxidized at potentials more positive than 0.4 V at the flowthrough electrode. At 3 A) with an increase in the oxidation time up to 120 min. The amounts of Li+ uptake were evaluated from the peak areas in the potential step experiments. The fibers treated for 120 min took up Li+ of 2.2 X 10"' mol g-' at -1.6 V. If monolayer adsorption occurs on the outer surface, the separation of each Li+ is calculated to be 0.28 A, which is too small to explain adsorption, because the separation is much smaller than the ionic radius. If we use the BET area instead of the outer surface area, which assumes that adsorption takes place in the pores, the separation of Li+ in the BET surface is calculated to be 6.5 A, which is a much more likely value if we consider the mutual repulsion of adsorbed Li+. Trapped Li+ was also confirmed by XPS. From the ratios of the C 1s and Li 1s peak heights and the photoionization cross sections of C and Li, the atomic ratio of C and Li was estimated to be ca. 1 0 1 at 0.5 V. The separation of each Li+ is estimated to be 4-5 A in the layer 10-30 A below the outer surface, from which XPS signals arise. At this potential the net amount of uptake was slight, so Li+ may be concentrated more in the layer close to the outer surface. However, we cannot compae the electrochemical results with the XPS results directly, because the degree of oxidation of the fiber is not necessarily the same as that of the disk even if the oxidation time is the same. From Figure 5, uptake seems to increase with a decrease in an ionic radius. Both the peak heights and areas were larger for Li+ than for Na+. Several workers have observed ion uptake into graphite electrodes; the oxidation of graphite introduces a pore system, which has molecular sieve properties (18-21,27). However, observed Li+ selectivity may arise from selective adsorption to the BET surface rather than the molecular sieve properties of the pores, since the average pore radius of treated glassy carbon was even larger than the ionic radius of the tetraethylammonium ion. The peak areas were almost the same as the dip areas for all the ions studied, so the cations taken up would be completely released from the pores if a sufficiently positive potential is applied to the carbon. Therefore, we think that the interactions between the pore surfaces and the ions are essentially electrostatic, which is also supported by the fact that the surface pores also took up the anions at positive potentials. (ii) Assignment of Voltammetric Peaks. The broad voltammetric peaks observed a t the treated glassy carbon electrodes in aqueous media are often attributed to the redox reactions of the surface quinones, because quinonefhydroquinone is a stable redox couple and can explain the stability of the voltammetric peaks. Other oxygen surface sites, such as phenols and carbonyls, are unlikely candidates for the redox sites, because molecules with corresponding functional groups are usually followed by irreversible chemical reactions after an electron is transferred in aqueous media (28). Here, stable voltammetric peaks cannot be expected. We think that the redox peaks of treated glassy carbon cannot be assigned to the surface quinones for the following reasons: (1)Quinones should be reduced with two one-electron steps in aprotic
2769
media, but there was only one cathodic peak for treated carbon in AN. (2) The peak potentials of the treated carbon were very different from those of quinones (Table I). (3) The treated carbon electrodes and the electrodes modified with quinone behave differently against oxygen reduction (13).It seems to be necessary to consider another mechanism in which the surface sites are not reduced or oxidized. We wish to propose such a mechanism for proton uptake. The voltammetric peaks would result by the following mechanism:
H++ Cq(C0) + e-
F?
[Cq(CO)]-H+
(1)
where Cq(C0) and [Cq(CO)]- are the pore surface of the oxidized carbon and ita charged form. When H+ associates with the pore surface, electrons are injected into the carbon to compensate for the cationic charge, not to reduce the surface sites. Depsite the monotonous increase of AC with the potential, voltammetric peaks would appear by the mechanism of eq 1when the potential scan is fast; the cathodic peaks can result if the rate of uptake increases with a decrease in the potential and then decreases because of the saturation of protons near the entrance of the pores. It is clear from Figure 8 that the voltammetric peaks of treated carbon should be assigned to the uptake and release of H+. The voltammetric peaks in aqueous media would also be assigned to the uptake and release of H+.
LITERATURE CITED Engstrom. R. C. Anat. Chem. 1982, 5 4 , 2310. Engstrom. R. C.: Strasser, V. A. Anal. Chem. 1984,56. 136. Taylor, R. J.; Humffray, A. A. J . Electrdnal. Chem. 1973, 42, 374. Evans, J. F.; Kuwana, T. Anal. Chem. 197P,49, 1632. Hu. LF.; Kuwana, T. Anal. Chem. 1968, 58, 3235. Bjellca, L.; Parsons, R.; Reeves, R. M. C a t . Chern. Acta 1980, 53, 211. Wang, J.; Hutchlns, L. D. Anal. Chkn. Acta 1985, 167, 325. Cenas, N.; Rozgake, J.; Pocius. A.; Kulys, J. J . Elechoanal. Chem. 1983, 154, 121. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974,46, 1952. Gunashgham. H.; Fleet, B. Analyst ( L W n ) 1982, 107, 896. Molroux, J.; EMng, P. J. Anal. Chem. 1978. 5 0 , 1056. Nagaoka, T.; Yoshlno, 1.Anal. Chem. 1988, 5 8 , 1037. Nagaoka. 1.;Sakal, T.; Ogura, K.;Yoshlno, 1.Anal. Chem. 1986, 58, 1953. Hollax, E.; Cheng, D. S. Carbon 1985, 2 3 , 655. Randin, J.-P.; Yeager. E. J . €lechoanal. Chem. 1975,58, 313. Fagan, D. T.; Hu. I.-F.; Kuwana, 1. Anel. Chem. 1985, 5 7 , 2759. Plcq. 0.; Reeves, R.; Ribourg, P.; Vennereau, P. J . Electroanal. Chem. 1984, 162, 225. Koresh, J.; Soffer, A. J . Chem. SOC., Faraday Trans 1 1980. 76, 2457. Koresh, J.; Soffer, A. J . Chem. Soc.,Faraday Trans. 1 1980, 76, 2472. Koresh, J.; Soffer. A. J . Ektr&. Soc. 1977, 124, 1379. Koresh, J.; Soffer, A. J . Electroanel. Chem. 1983. 147, 223. Nagaoka, T.; Fukunaga, T.; Yoshino. T. J . Elechoanal. Chem. 1987, 217, 453. Nagaoka, 1.;Sakal, 1.; Ogura, K.; Yoshino, 1.J . Chem. Soc., faraday Trans. 1 1987,83, 1823. Cabaniss. G. E.; Damantis, A. A.; Murphy, W. R., Jr.; Linton, R. W.; Meyer, T. J. J . Am. Chem. Soc. 1985, 107, 1845. Kozlowski, C.; Shrwood, P. M. A. J . Chem. Soc.,faraday Trans. 1 1985,8 1 , 2745. Kawamura, K.; Kimura, S. Bull. Chem. Soc. Jpn. 1988, 5 9 , 2991. Kastening, B.; Spinzig. S. J . Electroanal. Chem. 1988. 214, 295. Organic Electrochemktry; Baker, M. M., Ed.; Marcel Dekker: New York, 1973.
RECEIVED for review February 2,1988. Resubmitted July 19, 1988. Accepted September 22, 1988.
