Effect of target temperature during nitrogen ion implantation on electrochemical properties of ion-implanted glassy carbon

i~

.~,

1~

JOURN~. OF

I Journal of Electroanalytical Chemistry 396 (1995) 541-546

ELSEVIER

Effect of target temperature during nitrogen ion implantation on electrochemical properties of ion-implanted glassy carbon 1 Katsuo Takahashi a,*, Masaya Iwaki a, Hiroshi Watanabe b a The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama, 351-01, Japan b Gakushuin University, Mefiro, Toshima-ku, Tokyo 171, Japan Received 31 January 1995; in revised form 27 April 1995

Abstract

The electrochemical properties of glassy carbon (GC) modified by nitrogen ion implantation (a fluence of 1 × 1016 ions cm -2) at various target temperatures were studied by conventional electrochemical methods. The structure of ion-implanted surface layers of carbon materials has already been investigated by Raman spectroscopy, which has revealed that the implantations at high and low target temperatures produce a graphitic and an amorphous structure respectively. GC implanted at high temperatures (150, 300 and 400°C) showed electrochemical properties resembling those of non-implanted GC as a result of its graphitic structure. In contrast, GC implanted at low temperatures ( - 70 and 30°C) forms an electrode surface with an electrochemically inert nature and a low double-layer capacity, which is due to the formation of an amorphous surface layer. A structure of this type is commonly formed by ion implantation into diamond, highly ordered pyrolytic graphite and GC, which has high electrical resistivity and high wear resistance. It was found that the target temperature during ion implantation has a strong effect on the structure and properties of carbon materials. The electrochemically inert and highly wear-resistive carbon formed by low temperature ion implantation will be useful as a superior stable carbon material. Keywords: Temperature effects; Surface structure; Ion implantation; Glassy carbon

I. Introduction

Carbon materials such as graphite and. glassy carbon (GC) have important roles as electrode materials. Recently, fullerenes have been investigated as a unique Carbon material, and amorphous carbons, which are known as diamond-like carbon (DLC), are also attractive in the field of electronic materials science. In view of these applications it is important to elucidate the relationship between the structure and the properties of these carbon materials. The chemical bonding states and structure of various carbon materials have been analyzed by Raman spectroscopy and related to the electrochemical properties of the carbon electrodes. These investigations have been reviewed by McCreery [1] and by Leon y Leon and Radovic [2]. The effects of ion implantation on the structure of carbon surfaces have been studied by a number of workers

1 In honor of Professor Kenichi Honda, Professor Hiroaki Matsuda and Professor Reita Tamamushi on the occasion of their 70th birthdays. * Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

SSD10022-0728(95)04114-1

[3-6], and the structure change during ion implantation has been investigated by Nakamura and Kitajima [7]. We have reported the electrochemical properties of ion-implanted GC [8,9], which have the inactive nature and low residual current of this material to amorphization of the surface layer. Studies of the ion-implanted layer of diamond have shown a marked effect of the target temperature during implantation on the structure and electrical resistivity of the modified layer [10-13]. Raman spectroscopy data revealed that a similar structure is formed at the same target temperature for the surface layers of diamond, highly oriented pyrolytic graphite (HOPG) and GC. A higher resistivity for ion-implanted diamond and a more amorphous structure for diamond, HOPG and GC are produced when the surface layers are implanted at a lower target temperature [12-17]. It is suggested that the target temperature has a strong effect on the surface properties of the carbon materials. We have investigated effects of ion implantation on the electrochemical properties of GC electrodes. N ÷ ions were implanted into GC at various target temperatures ranging from - 7 0 to +400°C, and the electrical double-layer properties and some electrode reaction characteristics of

542

K. Takahashi et aL / Journal of Electroanalytical Chemistry396 (1995) 541-546

the ion-implanted electrodes were measured by conventional electrochemical methods. N ÷ ion implantation was carried out at a fluence of 1 × 1016 ions cm -2 at an energy of 150 keV, because studies have shown that the typical surface layer structures (amorphous and graphitic carbon) are produced under these conditions [12,15]. The electrochemical properties measured are correlated with the surface layer structures characterized by multicomponent analysis of Raman spectra. It has been found that the amorphous layer shows excellent wear-resistive features [14], and therefore it is important to elucidate the correlations of its electrochemical and other properties.

lOOj

i

,

I

i

!

I

0

l -so I I ?

i,oo G

2. Experimental The substrates were GC sheets (Tokai Carbon type GC-20) of dimensions 1 cm × 1 cm × 1 mm thick, They were mirror-polished mechanically by a buffing wheel using diamond slurry and then cleaned ultrasonically in trichloroethylene. N ÷ ions were implanted into the substrates at a fluence of 1 × 1016 ions cm -2 and an energy of 150 keV using a R/KEN 200 keV low current ion implanter. The ion beam current density was kept below 1 txA cm -2 to prevent ion irradiation heating of the specimen. The target temperatures during ion implantation were measured with an alumel-chromel thermocouple. They were maintained constant at - 7 0 ° C and room temperature (approx. 30°C) using target holder with circulating liquid nitrogen and water respectively, and at 150°C, 300°C and 400°C using a holder with a tantalum heater [11]. The pressure in the target chamber during ion implantation was kept at about 1 × 10 -4 Pa by a turbo molecular pump. No surface contamination of the specimens under vacuum in the target chamber could be detected. A conventional three-electrode cell was used to measure the double-layer capacitance and the voltammetric behavior. The working electrodes were ion-implanted GC covered with Scotch Kapton Tape (no. 5413) except for the surface area of 0.29 cm 2 (6 mm diameter). A mirrorpolished GC electrode and a HOPG electrode with a basal plane surface were also used as controls. Conventional potential sweep voltammetric methods and a small-amplitude triangular-wave method [18] were used to determine the voltammograms and the double-layer capacitance respectively.

3. Results

3.1. Cyclic voltammetry Conventional cyclic voltammograms (CVs) with a wide potential domain were measured to determine the general electrode characteristics for specimens in a 0.5 mol dm -3

lOO E / Vvs. SCE

Fig. 1. Typical CVs for non-implanted GC (G) and GC implanted with ions at the target temperatures400°C and -70°C (14o0 and I_ 70 respectively) measured over a wide potential domain in 0.5 mol dm--~ Na2SO4 at a potential sweep rate of 0.1 V s- ~.

Na2SO 4 aqueous solution at 25°C. The specimens are non-implanted GC, basal-plane HOPG and GCs implanted with ions at target temperatures of - 7 0 ° C , 30°C, 150°C, 300°C and 400°C. Typical voltammograms are shown in Fig. 1. The voltammetric background current (residual current) for GC implanted at - 70°C is significantly lower than that for GC implanted 400°C which is similar to the CV for non-implanted GC. CVs for GCs implanted at 300°C and 30°C (not shown) were similar to those at 400°C and - 7 0 ° C respectively, and the CV for HOPG showed a significantly lower background current. At higher potential regions ( [ E[ > 1 V) the anodic and cathodic currents increased substantially for GC implanted at 400°C and for non-implanted GC.

3.2. Double-layer capacity CVs were recorded over a narrow potential domain as a function of the potential sweep rate vs to characterize the polarized electrode property. Typical CVs are shown as a function of vs in Fig. 2. The current density at each potential is approximately proportional to vs, which means that the current is capacitive. The capacity C, normalized by the geometric area of the specimen electrode A, is given by

C = 1/( a d E / d t ) = j / ( d E / d t )

(1)

where I and j are the current and the current density, E is the electrode potential and t is the time. We determined the capacity C~I, which is a slow response capacity obtained from the narrow-potential-do-

K. Takahashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 541-546 '

10

I

I

'

d

I

543

i

I

I

I

i 400 200

f"

i b

!

H

"--4

100 e~

N

Et3

o

• -.,,

-S

E

-1 O0

-10 --200 t -0.1

-15 -0.2

I 0

I 0.1

I -0.8

I 0.2

Fig. 2. Typical CVs measured over a narrow potential domain at various potential sweep rates for ion-implanted GC (Tt = 4000C) in 0.5 mol dm -3 Na 2 SO 4. Sweep rates: (a) 0.02 V s - 1; (b) 0.05 V s - 1; (c) 0.1 V s - m; (d) 0.2 V s -] .

main CV, from Eq. (1). Fig. 3(a) shows C~l at E = 0 plotted against the target temperature Tt during the ion implantation; The C,~ values for the non-implanted GC and HOPG are also plotted as controls. A small amplitude triangular wave (SATW) method was used to determine the double-layer capacity with a fast response (fast potential sweep). A + 5 0 mV alternating triangular wave with a sweep rate of 10 V s -t was

J

I

I

I

I

0

I

80

(b) 60

EtO

(a)

it

o 0

40

zx

Z~

(..) A

20

O A A

o - 1 oo

2 I

I

0

100

i

i

ZOO 300

= 400

[ 0

I 0.4

I 0.8

I 1 .z

E / Vvs. SCE

E / Vvs. SCE

100

I -0.4

G

H

Tt 1 % Fig. 3. (a) Double-layer capacity in 0.5 mol dm -3 Na2SO 4 for GCs implanted with ions at various target temperatures Tt, non-implanted GC (G), and HOPG (H); O and zx are obtained by the narrow potential domain CV and SATW methods respectively. (b) Current signal in SATW method; the charging current AI is obtained at At = 4 ms.

Fig. 4. Typical CV for the Fe(II)/Fe(lll) redox reaction for GCs implanted at 400 and -70"C (I4o0 and I_ 70), non-implantedGC (G) and HOPG (H) in 2.5 mmol dm-3 K4[Fe(CN)6]+0.5 mol dm-3 Na2SO4 solution: vs =0.1 V s -~. supplied at E = 0. The current wave height A1 was measured as the output of a potentiostat (as shown in Fig. 3(b)) using a digital transient oscilloscope. If we substitute I=AI/2A in Eq. (1), the capacity cfl, which is the double-layer capacity with a fast response, can be obtained from the observed current A I, The value of Cf] at E = 0 is also plotted in Fig. 3(a). 3.3. Fe(H / II1) redox reaction

The [Fe(CN)6]4-/[Fe(CN)6] 3- redox system is a typical charge transfer electrode reaction, which is useful for characterizing the electrode kinetic property of the specimen electrodes by cyclic voltammmetry. Typical CVs for GCs with the implantations at high and low target temperatures are shown in Fig. 4. They clearly show the target temperature effect on the electrode kinetic features. The potentials Ep+ and E_- for the anodic and cathodic current peaks and El~2( = (Ep+- Ep-)/2) for the specimen electrodes obtained from the CVs are plotted against Tt in Fig. 5. The potentials for the non-implanted GC and HOPG are also plotted in the figure. The solutions used were 2.5 mmol dm -3 K4[Fe(CN) 6] and K3[Fe(CN) 6] containing 0.5 mol dm -3 Na2SO 4. The results show a significantly larger peak separation Ep÷-Ep- on CVs for GCs implanted with ions at low target temperatures (GC-LT) and a relatively small peak separation for GCs implanted with ions at high temperatures (GC-HT). The former is similar to that for HOPG, which represents a very slow charge transfer reaction or an irreversible nature for the redox reaction on GC-LT. The values of El~ 2 obtained in solutions of both [Fe(CN)6]- 4 and [Fe(CN)6]- 3 is almost constant for all the GC electrodes, as shown in Fig. 5.

544

K. Takahashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 541-546 I

I

I

I

I

I

I

~.o

o

•

O.S

Ep +

0

,o0

6

>

•

~ E1/z g

0 % ~

A Ep- •

E / V vs. SCE -1

,..--.

I','Y

-,00 _

.... - ' ~ / ~ /

_.oo 2,.o ,:oo41ii ,/

-o.s - t -101

0

3oo

A

l.aj

& I 0

: ~.~.,,.~ _ - -

!, L/-,,oo i:~1

-

la.j

-2

I , , l 100 ZOO 300 400 Tt /°(2

I/i / I/! I //."/i

-,oo-

.oo ,oo

11 G H

Fig. 5. The peak potentials Ep+ and Ep- and Ell 2 obtained from the CVs for GCs implanted at various target temperatures Tt, non-implanted GC (G) and HOPG (H): ( O , zx, D) and ( 0 , i , l l ) were measured using solutions of 2.5 mmol dm -3 K4[Fe(CN) 6] and K3[Fe(CN)6] containing 0.5 mol dm -~ Na2SO 4 respectively.

o

1

2

o

E / Vvs. SCE Fig. 6. Voltammograms for various specimen electrodes in 0.5 tool dm -a Na2SO4 showing the oxidation and reduction of water: H, HOPG; G, non-implanted GC; I, ion-implanted GCs (the subscripts indicate the target temperature Tt). The potential sweep rate was 20 mV s -I .

3.4. Potential window If an inert electrolytic solution (0.5 tool dm -3 Na2SO 4) is used, the oxidation and reduction of water can be observed as anodic and cathodic current increases on the voltammograms. Some voltamograms with a slow potential sweep are shown in Fig. 6. Both anodic and cathodic currents increase from low potentials for GC implanted at 400°C and non-implanted

GC, and from high potentials for GC implanted at - 7 0 ° C implantation and HOPG. These results suggest that ion implantation at a low target temperature makes the GC surface inert. The potentials E a and E c giving current densities of + 7 0 ixA cm -2 and - 7 0 I~A cm -2 respectively on the voltammograms are plotted against Tt in Fig. 7. The

Table 1. Correlations between ion implantation conditions and the properties of the ion-implanted glassy carbon a

Ion implantation conditions Fluence Acceleration energy (30-150 keV) Ion mass Target temperature

L o w ( < 1 X 1016) High

H i g h ( > 1 × 1016 ions cm -2) Low

Light High (150-400°C)

Heavy Low ( - 70 to 30°C)

Structural properties characterized by Raman spectroscopy [14,15,17] Structure Shape of the spectrum

Graphitic Two peaks at 1360 and 1590 cm - l

Major chemical bonding state

sp 2

Amorphous Single broad peak (1500 cm -I ) incorporating peaks at 1100, 1360, 1500, and 1590 c m - i peaks sp 3 and sp 2

High High Quasi-reversible (fast) Active Narrow

Low Low Irreversible (slow) Inert Wide

Low Low (102 O / [ ] )

High High 0 0 5 ,0/12)

Electrochemical properties Cdj Background current Redox reaction kinetics Water electrolysis Potential window

Other properties Wear resistance [ ! 4] Sheet resistivity l 10-13] (diamond surface layer)

a The results obtained in this study are presented in bold type.

K. Takahashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 541-546 I 2.0

'

'

'

'

I

•

8

I

OO

0

8

o

8

8

> 1.0 i

o

I

I

,

,

J

-1 O0

0

oo

zoo

aoo

40O

11 G

H

Tt /°C Fig. 7. The potentials E a ( 0 ) and E c ( O ) giving + 7 0 p,A cm -2 and - 7 0 p~A crn -2 for water oxidation and reduction in 0.5 mol dm -3 Na2 SO4 aqueous solution for GC implanted at various target temperatures Tt, nonqmplanted GC(G) and HOPG (H).

potential domain between E a and E c is the potential window for the specimen electrodes. GC-LT and HOPG showed a wide potential window, and GC-HT has a narrower potential window than non-implanted GC, which means that low temperature implantation produces an inert electrode surface, whereas high temperature implantation produces an active surface. 3.5. Electron microscope examination

Surface roughness is an important factor in the evaluation of the current density and double-layer capacity. Conventional electron microscope examination at a magnification of 2000 X reveals no significant difference in the surface roughnesses of the specimen GCs. Similar current densities for the Fe(II)/Fe(III) redox reaction were obtained for GC-LT and HOPG (Fig. 4), which suggests that the surface roughness of GC-LT is similar to that of basal-plane HOPG. The marked characteristic differences between the GCs cannot be due to differences in their surface roughness.

4. Discussion

On the basis of our previous studies of ion implantation effects on carbon structure, we can summarize the correlation between the carbon structure and the ion implantation conditions as shown in Table 1. The structure was analyzed by multicomponent analysis of the Raman spectrum [14,15,17]. The electrical properties of the ion-implanted diamond and the wear properties of the GC are also summarized. The electrochemical properties depending on the target temperature during ion implantation obtained in this work are shown in the table. In this study, we chose ion implantation conditions to make amorphous and graphitic carbon surface layers depending on the target temperature. When the fluence is at

545

1 × 1016 ions cm -2, the acceleration energy is 150 keV and the target temperature is below room temperature, the structure becomes amorphous as shown by laser Raman spectroscopy [ 15]. The properties of GC-HT as shown in Table 1 are generally similar to those of non-implanted GC. Since the shape of the Raman spectrum of GC-HT shows a graphitic structure including a graphite peak (1360 cm -1) and a disordered graphite peak (1590 cm -1 ) [14,15], the similarity between the electrochemical properties of GC-HT and non-implanted GC can be attributed to their similar graphitic structure. In contrast, the implantation at lower temperatures produces an amorphous structure (diamondlike carbon) which has the following electrochemically unique properties. (1) Low double-layer capacity. The value of (:el for GC-LT ( < 10 I~F cm -2) is about a factor of 5 smaller than that for GC-HT and slightly larger than that for basal-plane HOPG. The structures and the electronic states or GC-LT and HOPG are completely different; however, the capacities are not significantly different. (2) Low background current for CV. The major component of the background current for CV is a double-layer charging current, and therefore the low background current for GC-LT is due to its low double-layer capacity. Since there is a slight difference between Cfl and C~l, the background current for slow potential sweep CV involves some surface faradaic reactions such as redox reactions of a variety of quinones [1]. The low background current for GC-LT may be due to both the low Cdl and the low density of the reactive groups on the electrode surface. (3) Low reactivity for a redox system. The anodic peak potential for GC implanted at - 7 0 ° C is higher than that for HOPG, and the cathodic peak potential is similar to that for HOPG, as shown in Fig. 4. Such a large peak potential separation ( E p + - E p - ) for GC-LT demonstrates the very low rate constant (or the irreversible nature) for the charge transfer reaction on GC-LT electrode. (4) Wide potential window. The potentials E a and E c for the oxidation and reduction of water are plotted in Fig. 7. The values of E a and E c for GC-LT are ca. + 2 V, which are about + 0.5 V larger than those of GC-HT or non-implanted GC. This means that GC-LT has a very wide potential window in an aqueous electrolytic solution. From these results, it is clear that GC-LT has a low Cdl, a low background current, a low reactivity for the Fe(II)/Fe(III) redox couple, a wide potential window and a low reactivity for water electrolysis. These properties are characteristic of a low reactivity of the electrode surface or inertness of the electrons of the carbon atoms at the surface layer. Despite the significant difference between GC-LT and the basal plane of HOPG in the crystalline and electronic structure, their macroscopic and electrochemical properties are similar. The low reactivity and the low double-layer capacity for GC-LT and HOPG are probably due to the inertness of the electrons at the electrode surface

546

K. Takahashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 541-546

layer: the surface of GC-LT consists of carbon atoms with an sp a electronic orbital, and the surface of the basal-plane HOPG consists of single-crystal carbons with "tr (or sp 2) electrons. The low reactivity of GC-LT can also be correlated with a carbon structure with high electrical resistivity. The Raman spectrum for GC-LT is similar to that of the diamond surface layer implanted with ions at low temperatures (Dia-LT) [12,15]. The sheet resistivity of Dia-LT was about a factor of 103 higher than that of diamond implanted at high temperature (Dia-HT); this has been attributed to the amorphization of the diamond surface layer by low temperature ion implantation [12,15]. It is believed that the similarities between the electronic structures of the amorphous carbon layers of GC-LT and Dia-LT result in the low electrical conductivity and the electrochemical inertness of the electrode surface. However, the resistance of the surface layer of GC-LT itself (approx. 0.4 Ixm thick), which is ca. l0 -5 /2 cm -2 connected in series with the double layer is negligibly small for the electrode reaction kinetics and IR drop, compared with a conventional solution resistance. There is no peak separation on CV with fast and slow oxidations of Fe(II) (or reductions of Fe(III)) for GC-LT, which means that the inert characteristics of GC-LT are due to a homogenous electrode surface composed of sp 3 and sp 2 carbons. However, GC-HT has an electrochemically active nature and a strong double-layer capacity, which may be because the sp 2 carbon atoms at the surface have a similar nature to the edge plane of HOPG.

4. Conclusions (1) The effects of the target temperature during ion implantation on the electrochemical properties of ion-implanted glassy carbon were characterized by conventional electrochemical methods. (2) The dependence of properties such as the electrochemical reactivity and the double-layer capacity on the target temperature during ion implantation are attributed to modification of the carbon structure by the ion implantation. (3) Ion implantation at low temperatures (30°C and -70°C) produces an electrochemically inert surface layer due to the formation of an amorphized surface layer involving carbon atoms with an sp 3 electronic orbital.

(4) The amorphous state formed by the low temperature implantation is one of the metastable states of carbon which is commonly formed at the surface layers of a variety of carbon materials by ion implantation. This surface layer has excellent wear resistance and an electrochemically stable nature, which suggests that carbon modified by ion implantation will be useful as a superior carbon material.

Acknowledgement The authors acknowledge Dr. Y. Suzuki for performing the electron microscope examination.

References [i] R.L. McCreery in, A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 17, Dekker, New York, 1991, p. 221. [2] C.A. Leon y Leon and L.R. Radovic in, P.A. Therower (Ed.), Chemistry and Physics of Carbon, Vol. 24, Dekker, New York, 1994, p. 213. [3] B.S. Elrnan, M.S. Dresselhaus, G. Dresselhans, E.W. Maby and H. Mazurek, Phys. Rev. B, 24 (1981) 1027. [4] M. Endo, T.C. Chieu, G. Timp, M.S. Dresselhaus and B.S. Elman, Phys. Rev. B, 28 (1983) 6982. [5] M. lwaki, S. Sato, K. Takahashi and H. Sakairi, Nucl. Instmm. Methods, 209-210 (1983) 1129. [6] M.J. Kenny, J.T.A. Pollock and L.S. Wielunski, Nucl. lnstrum. Methods B, 39 (1989) 704. [7] K. Nakamura and M. Kitajima, Appl. Phys. Len., 59 (1991) 1550; Phys. Rev. B 45 (1992) 78; Surf. Sci., 283 (1993) 255. [8] K. Takahashi, K. Yoshida and M. lwaki, Nucl. lnstrum. Methods B, 7-8 (1985) 526. [9] K. Takahashi, K. Yoshida and M. lwaki, Electrochim. Acta, 35 (1990) 1279. [10] S. Sato, M. lwaki and H. Sakairi, Nucl. Instrum. Methods B, 19-20 (1987) 822. [I 1] S. Sato and M. lwaki, Nucl. lnstrum. Methods B, 32 (1988) 145. [12] S. Sato, H. Watanabe, K. Takahashi, Y. Abe and M. lwaki, Nucl. lnstrum. Methods B, 59-60 (1991). [13] S. Sato, H. Watanabe, K. Takahashi and M. lwaki, Rad. Effects Defects Solids, 124 (1992) 43. [14] M. lwaki, K. Takahashi and A. Sekiguchi, J. Mater. Res., 5 (1990) 2562; Diamonds Relat. Mater., 3 (1993) 47. [15] H. Watanabe, K. Takahashi and M. lwaki, Nucl. Instrum. Methods B, 80-81 (1993) 1489. [16] H. Watanabe, K. Awazul H. Yoshida, K. Takahashi and M. Iwaki, Diamonds Relat. Mater., 3 (1994) 1117. [17] H. Watanabe, K. Takahashi and M. lwaki, lonics, 20 (1994) 43. [18] R. Rice and R.L. McCreery, Anal. Chem., 61 (1989) 1637.