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Sonoelectrochemistry at tungsten-supported boron-doped CVD diamond electrodes
Diamond and Related Materials 8 (1999) 824–829
Sonoelectrochemistry at tungsten-supported boron-doped CVD diamond electrodes Christiaan H. Goeting, John S. Foord, Frank Marken *, Richard G. Compton Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, OX1 3QZ, UK Received 27 July 1998; accepted 14 September 1998
Abstract Highly boron-doped (atomic concentration 1020–1021 cm−3) conductive diamond films were deposited on tungsten substrates by hot-filament assisted chemical vapour deposition from a gaseous feed of methane and diborane in hydrogen. The boron-doped diamond film electrodes were characterised by Raman spectroscopy, scanning electron microscopy (SEM ), and both conventional and sonoelectrochemical methods. The one-electron reduction of Ru(NH )3+ was investigated in aqueous solutions under normal 36 and power ultrasound conditions. Well-defined voltammetric responses were observed from which the standard rate constant for electron transfer, k0=3×10−3 cm s−1, was estimated ignoring the effect of surface roughness. The electrode was used for electrochemical processes in the presence of 90 W cm−2 ultrasound without any significant deterioration of the properties. Although diamond is known to be exceptionally chemically inert, it was found that anodic polarisation of the boron-doped diamond electrode gave rise to changes in the surface properties. In order to rejuvenate the diamond electrode surface, a hydrogen plasma treatment was used. The two-electron reduction of dioxygen to give H O was studied in an aqueous 0.1 M phosphate buffer 2 2 solution (pH 2). This process was found to be strongly affected by the state of the electrode surface with an increase in the observed current after negative polarisation. Voltammograms obtained under ultrasound conditions suggest that a potential pretreatment can switch the process from being nearly mass-transport controlled to one where mass transport effects are virtually absent. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Oxygen reduction; Plasma treatment; Sonoelectrochemistry; Tungsten
1. Introduction Several electrochemical applications of conductive boron-doped diamond film electrodes have been studied intensively over recent years [1,2]. The properties of diamond, such as chemical inertness [3], dimensional stability [4] and extreme hardness, together with its wide potential window in aqueous media [5], can be of considerable importance. Many substrate materials for diamond film electrodes have been investigated, of which Si is used most extensively. Only a few reports [5,6 ] mention the use of tungsten as an electrode substrate. Free-standing diamond has also been reported to be very promising. Some metals, such as Cu [7], graphite [8] and glassy carbon [9] result in weak adhesion properties and the diamond films then become vulnerable to harsh conditions such as the application of ultrasound. In this * Corresponding author. Tel: +44 1865 275410; Fax: +44 1965 275410; e-mail: [email protected]
paper, stable and adhesive diamond electrodes grown on a tungsten substrate material have been employed. Power ultrasound has a potentially important role in electrochemistry to enhance the mass transport of reactants to the electrode surface and to keep the surface clean. However, in the presence of power ultrasound [10,11] the electrodes are exposed to extreme conditions with mechanical strain induced by pressure waves of up to 10 bar in amplitude and cavitation-induced jets strong enough to cause severe erosion [12]. Therefore, due to its extreme hardness and corrosion resistance, diamond could be a promising alternative to replace the conventionally used carbon-based materials graphite and glassy carbon. In this study the electrochemical properties of borondoped diamond films grown on tungsten substrates are reported, both in the presence and absence of power ultrasound, for a simple one-electron reduction process and for the complex two-electron reduction of dioxygen. The in situ generation of hydrogen peroxide, a chemical of considerable importance for processes such as disin-
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C.H. Goeting et al. / Diamond and Related Materials 8 (1999) 824–829
fection, peroxidation and dye bleaching [13], by electrochemical reduction directly from dioxygen, and is shown to be possible on boron-doped diamond electrodes after pretreatment of the electrode surface.
825
experiments, which were conducted at room temperature (T=20±2 °C ). After electrochemical experiments, the diamond–tungsten electrodes were treated for 2.5 h in an RF hydrogen plasma at 4 mbar pressure and a temperature of 25 °C using a dissipated power of 320 W.
2. Experimental The gases for the CVD experiments were obtained from BOC Gases and were used without further purification. The cylindrical tungsten substrates (5 mm diameter and 5 mm in height) were purchased from Goodfellow. The tungsten substrates were polished using abrasive paper and cleaned with acetone prior to deposition. A thin (~30 mm thick), highly conductive borondoped diamond film was deposited using a hot-filament assisted CVD reactor from a gaseous feed of diborane and methane in hydrogen. The filaments consisted of 11 turns of 0.25 mm tantalum wire (3.5 mm coil diameter), and were positioned at a distance of 2 mm from the sample. The substrate surface temperature was measured via a chromel–alumel thermocouple. The gas flows were controlled by mass flow controllers and the temperature was kept stable (±10 °C ) using a photocell-controlled feedback loop to the power supply. The resistance from the top diamond surface to the underlying substrate was measured by DVM to be 0.2 V, which is indicative of a high boron doping level. The growth parameters were as follows: deposition time 10 h, sample temperature 850±10 °C, pressure 40±0.1 mbar, total gas flow 178.4 sccm min−1 (±1%), gas feed composition 0.002% diborane/1.0% methane/hydrogen. Scanning electron microscopy (SEM ) was performed on a JEOL JSM-5200 system. A Dilor Labram spectrometer with a He–Ne 20 mW laser (l=632.817 nm) was employed for Raman analysis. The metal complex Ru(NH ) Cl was obtained com36 3 mercially from Aldrich and was used without further purification. Electrolyte solutions were prepared from KCl, orthophosphoric acid, NaOH (all BDH ) and ultrahigh quality water of a resistivity not less than 18 MV · cm produced with an Elgastat system (High Wycombe, Bucks, UK ). The boron-doped diamond electrodes were mounted in Teflon holders by a heat-shrink procedure. The electrical connection was made by spot-welding a nickel contact to the reverse side of the tungsten substrate. For the electrochemical measurements an Autolab PGSTAT 20 potentiostat system ( ECO Chemie, The Netherlands) was used. The cell and methodology for sonoelectrochemical experiments have been described elsewhere [14]. A thick gold wire served as the counterelectrode and all measurements were performed versus a saturated calomel electrode (SCE ) (Radiometer Kopenhagen). The aqueous solutions were degassed with argon gas for approximately 10 min prior to the
3. Results and discussion 3.1. Characterisation of the boron-doped diamond films by microscopy and spectroscopy An earlier report on the electrochemistry of highly conducting boron-doped diamond films grown on carbon substrates concluded that the mechanical strength and adhesion of these films on graphite were not satisfactory [8]. Therefore, in this study tungsten substrates were used following successful work by Martin et al [5]. An SEM image of the surface of the as-grown diamond film on tungsten is shown in Fig. 1a. The approximately 30 mm thick and continuous polycrystalline diamond film consists of randomly orientated crystallites of a nominal size of 1–15 mm size exposing predominately triangular {111} faces. A Raman spectrum of the as-grown boron-doped
(a)
(b) Fig. 1. (a) SEM image of the highly boron-doped diamond film on a tungsten substrate. (b) Raman spectrum for the boron-doped diamond film on tungsten (30 s integration time).
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diamond film on tungsten ( Fig. 1b) shows a strong characteristic diamond signal at 1331 cm−1 (FWHM=9 cm−1). A broad non-diamond carbon sp2 peak centred around 1500 cm−1 is virtually absent, which is indicative of good quality diamond. The broad band at 1200 cm−1 has been attributed to disorder in the diamond structure [15] of the highly boron-doped diamond, or may be associated with photoluminescence phenomena. After use in electrochemical experiments, especially after applying positive potentials in the oxygen evolution region, it was noted that the state of the electrode surface changed, which, on a macroscopic level, caused the surface to become more hydrophilic. The change from a hydrophobic to a hydrophilic diamond surface has been proposed to be indicative of an oxygen functionalisation and partial loss of the hydrogen termination [16 ]. Therefore, a hydrogen plasma treatment was used to ‘‘recondition’’ the surface of the boron-doped diamond film. This treatment reproducibly gave a hydrophobic surface with defined electrochemical characteristics. However, the properties detected for the ‘‘asgrown’’ diamond film detected by SEM and Raman techniques did not change and were also detected for the hydrogen-plasma treated sample. 3.2. Electrochemical characteristics of the boron-doped diamond film: the reduction of Ru(NH )3+ 3 6 The electrochemical properties of sufficiently highly doped diamond electrodes resemble in some aspects those of glassy carbon type materials. For example, simple outer-sphere redox reactions have been shown to proceed without complications and can be used to characterise the quality and properties of the diamond film. The reduction of Ru(NH )3+ is commonly used 36 for this purpose and allows comparison with data obtained in electrochemical experiments with other types of diamond and carbon electrode materials. The reduction of Ru(NH )3+ in aqueous 0.1 M KCl follows the 36 one-electron pathway given in Eq. (1): Ru(NH )3++e−PRu(NH )2+ (1) 36 36 Cyclic voltammograms for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl have been obtained 36 with various scan rates ( Fig. 2a and b), and electrochemical data are summarised in Table 1. The data analysis has been based on the model of a quasi-reversible electron transfer at a flat electrode surface [17], and the standard rate constants for the heterogeneous electron transfer calculated from the peak-to-peak separation in the voltammetric responses may therefore be regarded as estimates and upper limits. However, the results suggest good electrochemical properties. The standard rate constant reported here is closely related to the corresponding value reported recently for a polished
Table 1 Electrochemical data obtained from cyclic voltammograms for the reduction of 1 mM Ru(NH )3+ in 0.1 M KCl at a boron-doped 36 diamond film electrode (A=as grown sample, B=hydrogen plasma treated sample) at T=20±2 °C Scan rate ( V s−1) A 0.01 0.02 0.05 0.10 B 0.01 0.02 0.05 0.10
Ea 1/2 ( V versus SCE)
DE p (mV )
I p,cath (×10−6 A)
k0b (cm s−1)
−0.18 −0.18 −0.18 −0.19
75 80 95 109
−9 −13 −22 −31
3.1×10−3 3.6×10−3 3.0×10−3 2.2×10−3
−0.20 −0.20 −0.20 −0.21
72 81 84 96
−8 −11 −17 −22
0.7×10−3 0.6×10−3 0.8×10−3 0.8×10−3
aObtained as the mid-potential E =0.5(E +E ). 1/2 p,cath. p,anod. bStandard rate constant for heterogeneous electron transfer calculated from DE [17]. p
boron-doped diamond electrode under the same experimental conditions [18]. Further, the standard rate constant for heterogeneous electron transfer observed for the Ru(NH )3+/2+ redox couple at other types of dia36 mond and glassy carbon electrodes has a similar magnitude [19,20]. As-grown boron-doped diamond films are initially found to be hydrophobic towards aqueous electrolyte solutions. This observation has been reported previously
(a)
(b) Fig. 2. Cyclic voltammograms (a, b, d, e) and sonovoltammograms (c, f ) for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 36 boron-doped diamond film electrode (area=9.8×10−2 cm2; (a)–(c) as grown, (d)–(f ) hydrogen plasma treated) obtained at a scan rate of (a, d) 10 mV s−1, (b, c, e, f ) 100 mV s−1 (argon, T=20±2 °C ).
C.H. Goeting et al. / Diamond and Related Materials 8 (1999) 824–829
[5], and was ascribed to the hydrogen termination of the freshly grown diamond surface. The surface of the boron-doped diamond film electrodes becomes more hydrophilic in nature after scanning the potential positive into the oxygen evolution region. In order to reverse any chemical effects caused by electrochemical polarisation of the surface of the diamond electrodes, a treatment in an RF hydrogen plasma for a period of 2.5 h was used. The hydrogen-plasma treatment turned the hydrophilic electrode back into a hydrophobic form. This plasma-treated form of the boron-doped diamond electrode surface, which is believed to be chemically related to the surface of the ‘‘as-grown’’ diamond, has also been characterised for the reduction of Ru(NH )3+ . In Fig. 2d–f the corresponding voltammo36 grams are shown, and the electrochemical data are included in Table 1. Comparison of the shape of the voltammetric responses and the standard rate constants suggests that the activity of the electrode surface is reduced after the 2.5 h hydrogen plasma treatment. However, chemically the ‘‘as-grown’’ surface and the hydrogen-plasma treated surfaces are similar, and the electrode may therefore be regarded as ‘‘reconditioned’’. One area of electrochemistry in which the use of materials of extreme hardness is required and in which diamond electrodes could be very useful is the application of intense ultrasound in sonoelectrochemistry [10,11]. In the presence of power ultrasound the electrode is exposed to cavitation processes and microjets, which are known to cause substantial erosion on the surface of less hard materials [12] and strong agitation in the solution phase. The effect of 90 W cm−2 ultrasound on the current observed for the reduction of 1 mM Ru(NH )3+ is shown in Fig. 2c and f. The current 36 is enhanced by about one order of magnitude and the shape of the response has changed significantly. The characteristic peak-shaped current response usually observed in cyclic voltammetry disappears and a masstransport enhanced quasi steady-state response can be seen. The magnitude of the limiting current can be varied by changing the ultrasound intensity or the separation between the ultrasonic horn probe and the electrode, and in the simplest case the electrochemical response may be expected to be independent of time. That is, the observed current should be independent of the scan rate and the scan direction. Changes in the current response observed upon scanning the potential into the negative direction and then reversing the potential scan direction must be attributed directly to changes in the surface properties of the diamond (vide infra). 3.3. Electrochemical characteristics of the boron-doped diamond film: the reduction of dioxygen Complex electrode processes involving adsorption and multi-electron transfer processes have been suggested to
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proceed only with relatively slow kinetics on borondoped diamond electrodes [21]. In the cases of the reduction of chlorine, the reduction of quinone [1] and gas evolution processes such as hydrogen evolution and oxygen evolution [5], large overpotentials have been reported. In the latter two cases a considerable advantage of the use of diamond electrodes arises from the wide potential window in aqueous media. However, in other cases it is more desirable to achieve a fast rate for the multi-electron transfer process. The reduction of dioxygen is an example of a multi-electron transfer process of considerable technological importance, and this process is therefore analysed in this study. A voltammogram obtained for a cyclic potential scan applied to a boron-doped diamond film electrode immersed in an acidic aqueous 0.1 M phosphate buffer solution (pH 2) in the absence of dioxygen is shown in Fig. 3a. A wide potential window from −1.1 V versus SCE to +2.2 V versus SCE is detected, and a characteristic anodic current response occurs at +2.0 V versus SCE. This type of response, which appears to be chemically irreversible and which remains over the course of many potential cycles, has been reported in the literature [5] and is believed to be associated with the oxidation of non-diamond sp2-type carbon at the electrode surface. The treatment of the surface of the boron-doped diamond electrode for 2.5 h in a hydrogen plasma further improved the electrochemical characteristics in the sense of further widening the potential window (−1.4 to +2.5 V versus SCE ) and removing the spurious anodic response at +2.0 V versus SCE (see Fig. 3b).
Fig. 3. Cyclic voltammograms (a, b) and sonovoltammograms (c, d ) obtained for a boron-doped diamond film electrode (area=9.8×10−2 cm2; (a) as grown, (b)–(d) after treatment in a hydrogen plasma for 2.5 h in aqueous 0.1 M phosphate buffer solution (pH 2) at a scan rate of 100 mV s−1. Experiments (a, b) under an inert atmosphere of argon and (c, d) under an atmosphere of dioxygen corresponding to a concentration of 0.87 mM dioxygen (ultrasound 90 W cm−2, T=20±2 °C ).
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In the presence of dioxygen, the electrode process was found to be strongly affected by the surface state of the boron-doped diamond electrode, in agreement with a recent study by Yano et al. [22]. In fact, the reduction of dioxygen may be virtually switched off and switched on by an appropriate pretreatment of the electrode surface with potential steps. This effect can be demonstrated most clearly in ‘‘quasi steady-state’’ sonovoltammograms. In Fig. 3c a cyclic sonovoltammogram for the reduction of 0.87 mM dioxygen in 0.1 M phosphate buffer (pH 2) is shown after a 1 s second pretreatment at +10 V versus SCE was applied to the electrode. On the forward scan towards negative potentials the trace follows a straight line similar to that observed in the absence of dioxygen (Fig. 3b), until at sufficiently negative potentials hydrogen evolution begins. However, on the reverse scan in the positive direction the traces do not superimpose. A substantial cathodic current with a current plateau of ~0.25 mA and a half wave potential of −0. V versus SCE suggest that the reduction of dioxygen is possible. The change of the nature of the surface of the boron-doped diamond electrode must have occurred in the negative potential region in which hydrogen evolution also occurs. In contrast, a pretreatment of the electrode surface with 1 s potential step to −10 V versus SCE resulted in a smaller change of the surface reactivity, as shown in Fig. 3d. On the forward scan towards negative potential, the response associated with the reduction of dioxygen is immediately detected as a distinct shoulder. After hydrogen evolution is detected on the reverse scan, a well-defined response with a limiting current of the order of 0.45 mA and a half wave potential of −0.7 V versus SCE are detected. The limiting current for a sonovoltammetric response can in a first approximation be expressed by I =nFDAc/d [14]. In this expression, a flat electrode lim surface is assumed and n (the number of electrons transferred per molecule) can be estimated from the limiting current I for known F (the Faraday constant), lim D (the diffusion coefficient [18]), A (the electrode area), c (the concentration) and d (the diffusion layer thickness). Only the diffusion layer thickness is unknown, and can be obtained from a calibration procedure employing the reduction of Ru(NH )3+ [14]. 36 For the experiments shown in Fig. 3c and d, the diffusion layer thickness is typically d=3 mm, and the value calculated for the number of electrons transferred (n) is found to be between 1 and 2. This value is consistent with a limiting current for a two-electron reduction process when the process is close to becoming mass-transport controlled. Therefore, under the experimental conditions used here, the overall process which is operative is proposed to be the reduction of dioxygen to hydrogen peroxide ( Eq. (2)): O +2e−+2H+H O 2 2 2
(2)
A very similar effect of a potential pretreatment on the course of the dioxygen reduction process has been reported recently for free-standing and polished borondoped diamond film electrodes [18]. However, the details of the surface chemistry of boron-doped diamond responsible for the dramatic change in reactivity is not well understood, and will be a topic of further work.
4. Conclusions The use of boron-doped diamond electrodes in electrochemistry and especially in sonoelectrochemistry promises a considerable extension of the range of processes which can be studied and/or applied at the high rates of mass transport found under ultrasound conditions. The resistance of diamond towards mechanical erosion, its known chemical inertness and dimensional stability combined offer major advantages over conventional carbon-based materials. One very important aspect of the electrochemical characteristics of boron-doped diamond films is the change in the surface chemistry induced by polarisation pretreatment. This is important for surface-sensitive electrochemical processes, as demonstrated for the reduction of dioxygen, and is also important for the reproducibility of results. The relatively simple treatment of electrodes in a hydrogen plasma for a period of time sufficient to recondition the electrode surface appears to be a good alternative to the commonly procedure of surface renewal by polishing, which is inapplicable to diamond.
Acknowledgement F.M. thanks the Royal Society for the award of a University Research Fellowship and New College, Oxford, for a Stipendiary Lectureship.
References [1] J.S. Xu, M.C. Granger, Q.Y. Chen, J.W. Strojek, T.E. Lister, G.M. Swain, Anal. Chem. 69 (1997) A591. [2] J.L. Davidson, W.D. Brown, A. Gicquel, B.V. Spitsyn, J.C. Angus (Eds.), Proceedings of the Fifth International Symposium on Diamond Materials, The Electrochemical Society, Pennington, 1998. [3] G.M. Swain, J. Electrochem. Soc. 141 (1994) 3382. [4] Q.Y. Chen, M.C. Granger, T.E. Lister, G.M. Swain, J. Electrochem. Soc. 144 (1997) 3806. [5] H.B. Martin, A. Argoitia, U. Landau, A.B. Anderson, J.C. Angus, J. Electrochem. Soc. 143 (1996) L133. [6 ] A.Ya. Sakharova, Yu.V. Pleskov, F. Di Quarto, S. Piazza, C. Sunseri, I.G. Tereemetskaya, V.P. Varnin, J. Electrochem. Soc. 142 (1995) 2704. [7] M.L. Hartsell, L.S. Plano, J. Mater. Res. 9 (1994) 921. [8] C.H. Goeting, F. Jones, J.S. Foord, J.C. Eklund, F. Marken, R.G.
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[9] [10] [11] [12] [13] [14] [15]
Compton, P.R. Chalker, C. Johnston, J. Electroanal. Chem. 442 (1998) 207. R. DeClements, B.L. Hirsche, M.C. Granger, J. Xu, G.M. Swain, J. Electrochem. Soc. 143 (1996) L150. D.J. Walton, S.S. Phull, Adv. Sonochem. 4 (1996) 205. R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis 9 (1997) 509. N.A. Madigan, C.R.S. Hagan, H. Zhang, L.A. Coury Jr., Ultrasonics Sonochem. 3 (1996) S239. S. Wilson, Chem. Ind. 4 (1994) 255. H.A.O. Hill, Y. Nakagawa, F. Marken, R.G. Compton, J. Phys. Chem. 100 (1996) 17395. J.W. Ager III, W. Walukiewicz, M. McCluskey, M.A. Plano, M.I. Landstrass, Appl. Phys. Lett. 66 (1995) 616.
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[16 ] P.E. Pehrsson, J.P. Long, M.J. Marchywka, J.E. Butler, Appl. Phys. Lett. 67 (1995) 3414. [17] R.S. Nicholson, Anal. Chem. 37 (1965) 1351. [18] R.G. Compton, F. Marken, C.H. Goeting, R.A.J. McKeown, J.S. Foord, G. Scarsbrook, R.S. Sussmann, A.J. Whitehead, J. Chem. Soc., Chem. Commun. (1998) 1961. [19] R. DeClements, G.M. Swain, T. Dallas, M.W. Holtz, R.D. Herrick II, J.L. Stickney, Langmuir 12 (1996) 6578. [20] M.R. Deakin, K.J. Stutts, R.M. Wightman, J. Electroanal. Chem. 182 (1985) 113. [21] N. Vinokur, B. Miller, Y. Avyigal, R. Kalish, J. Electrochem. Soc. 143 (1996) L238. [22] T. Yano, D.A. Tryk, K. Hashimoto, A. Fujishima, J. Electrochem. Soc. 145 (1998) 1870.
Sonoelectrochemistry at tungsten-supported boron-doped CVD diamond electrodes Christiaan H. Goeting, John S. Foord, Frank Marken *, Richard G. Compton Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, OX1 3QZ, UK Received 27 July 1998; accepted 14 September 1998
Abstract Highly boron-doped (atomic concentration 1020–1021 cm−3) conductive diamond films were deposited on tungsten substrates by hot-filament assisted chemical vapour deposition from a gaseous feed of methane and diborane in hydrogen. The boron-doped diamond film electrodes were characterised by Raman spectroscopy, scanning electron microscopy (SEM ), and both conventional and sonoelectrochemical methods. The one-electron reduction of Ru(NH )3+ was investigated in aqueous solutions under normal 36 and power ultrasound conditions. Well-defined voltammetric responses were observed from which the standard rate constant for electron transfer, k0=3×10−3 cm s−1, was estimated ignoring the effect of surface roughness. The electrode was used for electrochemical processes in the presence of 90 W cm−2 ultrasound without any significant deterioration of the properties. Although diamond is known to be exceptionally chemically inert, it was found that anodic polarisation of the boron-doped diamond electrode gave rise to changes in the surface properties. In order to rejuvenate the diamond electrode surface, a hydrogen plasma treatment was used. The two-electron reduction of dioxygen to give H O was studied in an aqueous 0.1 M phosphate buffer 2 2 solution (pH 2). This process was found to be strongly affected by the state of the electrode surface with an increase in the observed current after negative polarisation. Voltammograms obtained under ultrasound conditions suggest that a potential pretreatment can switch the process from being nearly mass-transport controlled to one where mass transport effects are virtually absent. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond; Oxygen reduction; Plasma treatment; Sonoelectrochemistry; Tungsten
1. Introduction Several electrochemical applications of conductive boron-doped diamond film electrodes have been studied intensively over recent years [1,2]. The properties of diamond, such as chemical inertness [3], dimensional stability [4] and extreme hardness, together with its wide potential window in aqueous media [5], can be of considerable importance. Many substrate materials for diamond film electrodes have been investigated, of which Si is used most extensively. Only a few reports [5,6 ] mention the use of tungsten as an electrode substrate. Free-standing diamond has also been reported to be very promising. Some metals, such as Cu [7], graphite [8] and glassy carbon [9] result in weak adhesion properties and the diamond films then become vulnerable to harsh conditions such as the application of ultrasound. In this * Corresponding author. Tel: +44 1865 275410; Fax: +44 1965 275410; e-mail: [email protected]
paper, stable and adhesive diamond electrodes grown on a tungsten substrate material have been employed. Power ultrasound has a potentially important role in electrochemistry to enhance the mass transport of reactants to the electrode surface and to keep the surface clean. However, in the presence of power ultrasound [10,11] the electrodes are exposed to extreme conditions with mechanical strain induced by pressure waves of up to 10 bar in amplitude and cavitation-induced jets strong enough to cause severe erosion [12]. Therefore, due to its extreme hardness and corrosion resistance, diamond could be a promising alternative to replace the conventionally used carbon-based materials graphite and glassy carbon. In this study the electrochemical properties of borondoped diamond films grown on tungsten substrates are reported, both in the presence and absence of power ultrasound, for a simple one-electron reduction process and for the complex two-electron reduction of dioxygen. The in situ generation of hydrogen peroxide, a chemical of considerable importance for processes such as disin-
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 27 8 - 7
C.H. Goeting et al. / Diamond and Related Materials 8 (1999) 824–829
fection, peroxidation and dye bleaching [13], by electrochemical reduction directly from dioxygen, and is shown to be possible on boron-doped diamond electrodes after pretreatment of the electrode surface.
825
experiments, which were conducted at room temperature (T=20±2 °C ). After electrochemical experiments, the diamond–tungsten electrodes were treated for 2.5 h in an RF hydrogen plasma at 4 mbar pressure and a temperature of 25 °C using a dissipated power of 320 W.
2. Experimental The gases for the CVD experiments were obtained from BOC Gases and were used without further purification. The cylindrical tungsten substrates (5 mm diameter and 5 mm in height) were purchased from Goodfellow. The tungsten substrates were polished using abrasive paper and cleaned with acetone prior to deposition. A thin (~30 mm thick), highly conductive borondoped diamond film was deposited using a hot-filament assisted CVD reactor from a gaseous feed of diborane and methane in hydrogen. The filaments consisted of 11 turns of 0.25 mm tantalum wire (3.5 mm coil diameter), and were positioned at a distance of 2 mm from the sample. The substrate surface temperature was measured via a chromel–alumel thermocouple. The gas flows were controlled by mass flow controllers and the temperature was kept stable (±10 °C ) using a photocell-controlled feedback loop to the power supply. The resistance from the top diamond surface to the underlying substrate was measured by DVM to be 0.2 V, which is indicative of a high boron doping level. The growth parameters were as follows: deposition time 10 h, sample temperature 850±10 °C, pressure 40±0.1 mbar, total gas flow 178.4 sccm min−1 (±1%), gas feed composition 0.002% diborane/1.0% methane/hydrogen. Scanning electron microscopy (SEM ) was performed on a JEOL JSM-5200 system. A Dilor Labram spectrometer with a He–Ne 20 mW laser (l=632.817 nm) was employed for Raman analysis. The metal complex Ru(NH ) Cl was obtained com36 3 mercially from Aldrich and was used without further purification. Electrolyte solutions were prepared from KCl, orthophosphoric acid, NaOH (all BDH ) and ultrahigh quality water of a resistivity not less than 18 MV · cm produced with an Elgastat system (High Wycombe, Bucks, UK ). The boron-doped diamond electrodes were mounted in Teflon holders by a heat-shrink procedure. The electrical connection was made by spot-welding a nickel contact to the reverse side of the tungsten substrate. For the electrochemical measurements an Autolab PGSTAT 20 potentiostat system ( ECO Chemie, The Netherlands) was used. The cell and methodology for sonoelectrochemical experiments have been described elsewhere [14]. A thick gold wire served as the counterelectrode and all measurements were performed versus a saturated calomel electrode (SCE ) (Radiometer Kopenhagen). The aqueous solutions were degassed with argon gas for approximately 10 min prior to the
3. Results and discussion 3.1. Characterisation of the boron-doped diamond films by microscopy and spectroscopy An earlier report on the electrochemistry of highly conducting boron-doped diamond films grown on carbon substrates concluded that the mechanical strength and adhesion of these films on graphite were not satisfactory [8]. Therefore, in this study tungsten substrates were used following successful work by Martin et al [5]. An SEM image of the surface of the as-grown diamond film on tungsten is shown in Fig. 1a. The approximately 30 mm thick and continuous polycrystalline diamond film consists of randomly orientated crystallites of a nominal size of 1–15 mm size exposing predominately triangular {111} faces. A Raman spectrum of the as-grown boron-doped
(a)
(b) Fig. 1. (a) SEM image of the highly boron-doped diamond film on a tungsten substrate. (b) Raman spectrum for the boron-doped diamond film on tungsten (30 s integration time).
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diamond film on tungsten ( Fig. 1b) shows a strong characteristic diamond signal at 1331 cm−1 (FWHM=9 cm−1). A broad non-diamond carbon sp2 peak centred around 1500 cm−1 is virtually absent, which is indicative of good quality diamond. The broad band at 1200 cm−1 has been attributed to disorder in the diamond structure [15] of the highly boron-doped diamond, or may be associated with photoluminescence phenomena. After use in electrochemical experiments, especially after applying positive potentials in the oxygen evolution region, it was noted that the state of the electrode surface changed, which, on a macroscopic level, caused the surface to become more hydrophilic. The change from a hydrophobic to a hydrophilic diamond surface has been proposed to be indicative of an oxygen functionalisation and partial loss of the hydrogen termination [16 ]. Therefore, a hydrogen plasma treatment was used to ‘‘recondition’’ the surface of the boron-doped diamond film. This treatment reproducibly gave a hydrophobic surface with defined electrochemical characteristics. However, the properties detected for the ‘‘asgrown’’ diamond film detected by SEM and Raman techniques did not change and were also detected for the hydrogen-plasma treated sample. 3.2. Electrochemical characteristics of the boron-doped diamond film: the reduction of Ru(NH )3+ 3 6 The electrochemical properties of sufficiently highly doped diamond electrodes resemble in some aspects those of glassy carbon type materials. For example, simple outer-sphere redox reactions have been shown to proceed without complications and can be used to characterise the quality and properties of the diamond film. The reduction of Ru(NH )3+ is commonly used 36 for this purpose and allows comparison with data obtained in electrochemical experiments with other types of diamond and carbon electrode materials. The reduction of Ru(NH )3+ in aqueous 0.1 M KCl follows the 36 one-electron pathway given in Eq. (1): Ru(NH )3++e−PRu(NH )2+ (1) 36 36 Cyclic voltammograms for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl have been obtained 36 with various scan rates ( Fig. 2a and b), and electrochemical data are summarised in Table 1. The data analysis has been based on the model of a quasi-reversible electron transfer at a flat electrode surface [17], and the standard rate constants for the heterogeneous electron transfer calculated from the peak-to-peak separation in the voltammetric responses may therefore be regarded as estimates and upper limits. However, the results suggest good electrochemical properties. The standard rate constant reported here is closely related to the corresponding value reported recently for a polished
Table 1 Electrochemical data obtained from cyclic voltammograms for the reduction of 1 mM Ru(NH )3+ in 0.1 M KCl at a boron-doped 36 diamond film electrode (A=as grown sample, B=hydrogen plasma treated sample) at T=20±2 °C Scan rate ( V s−1) A 0.01 0.02 0.05 0.10 B 0.01 0.02 0.05 0.10
Ea 1/2 ( V versus SCE)
DE p (mV )
I p,cath (×10−6 A)
k0b (cm s−1)
−0.18 −0.18 −0.18 −0.19
75 80 95 109
−9 −13 −22 −31
3.1×10−3 3.6×10−3 3.0×10−3 2.2×10−3
−0.20 −0.20 −0.20 −0.21
72 81 84 96
−8 −11 −17 −22
0.7×10−3 0.6×10−3 0.8×10−3 0.8×10−3
aObtained as the mid-potential E =0.5(E +E ). 1/2 p,cath. p,anod. bStandard rate constant for heterogeneous electron transfer calculated from DE [17]. p
boron-doped diamond electrode under the same experimental conditions [18]. Further, the standard rate constant for heterogeneous electron transfer observed for the Ru(NH )3+/2+ redox couple at other types of dia36 mond and glassy carbon electrodes has a similar magnitude [19,20]. As-grown boron-doped diamond films are initially found to be hydrophobic towards aqueous electrolyte solutions. This observation has been reported previously
(a)
(b) Fig. 2. Cyclic voltammograms (a, b, d, e) and sonovoltammograms (c, f ) for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 36 boron-doped diamond film electrode (area=9.8×10−2 cm2; (a)–(c) as grown, (d)–(f ) hydrogen plasma treated) obtained at a scan rate of (a, d) 10 mV s−1, (b, c, e, f ) 100 mV s−1 (argon, T=20±2 °C ).
C.H. Goeting et al. / Diamond and Related Materials 8 (1999) 824–829
[5], and was ascribed to the hydrogen termination of the freshly grown diamond surface. The surface of the boron-doped diamond film electrodes becomes more hydrophilic in nature after scanning the potential positive into the oxygen evolution region. In order to reverse any chemical effects caused by electrochemical polarisation of the surface of the diamond electrodes, a treatment in an RF hydrogen plasma for a period of 2.5 h was used. The hydrogen-plasma treatment turned the hydrophilic electrode back into a hydrophobic form. This plasma-treated form of the boron-doped diamond electrode surface, which is believed to be chemically related to the surface of the ‘‘as-grown’’ diamond, has also been characterised for the reduction of Ru(NH )3+ . In Fig. 2d–f the corresponding voltammo36 grams are shown, and the electrochemical data are included in Table 1. Comparison of the shape of the voltammetric responses and the standard rate constants suggests that the activity of the electrode surface is reduced after the 2.5 h hydrogen plasma treatment. However, chemically the ‘‘as-grown’’ surface and the hydrogen-plasma treated surfaces are similar, and the electrode may therefore be regarded as ‘‘reconditioned’’. One area of electrochemistry in which the use of materials of extreme hardness is required and in which diamond electrodes could be very useful is the application of intense ultrasound in sonoelectrochemistry [10,11]. In the presence of power ultrasound the electrode is exposed to cavitation processes and microjets, which are known to cause substantial erosion on the surface of less hard materials [12] and strong agitation in the solution phase. The effect of 90 W cm−2 ultrasound on the current observed for the reduction of 1 mM Ru(NH )3+ is shown in Fig. 2c and f. The current 36 is enhanced by about one order of magnitude and the shape of the response has changed significantly. The characteristic peak-shaped current response usually observed in cyclic voltammetry disappears and a masstransport enhanced quasi steady-state response can be seen. The magnitude of the limiting current can be varied by changing the ultrasound intensity or the separation between the ultrasonic horn probe and the electrode, and in the simplest case the electrochemical response may be expected to be independent of time. That is, the observed current should be independent of the scan rate and the scan direction. Changes in the current response observed upon scanning the potential into the negative direction and then reversing the potential scan direction must be attributed directly to changes in the surface properties of the diamond (vide infra). 3.3. Electrochemical characteristics of the boron-doped diamond film: the reduction of dioxygen Complex electrode processes involving adsorption and multi-electron transfer processes have been suggested to
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proceed only with relatively slow kinetics on borondoped diamond electrodes [21]. In the cases of the reduction of chlorine, the reduction of quinone [1] and gas evolution processes such as hydrogen evolution and oxygen evolution [5], large overpotentials have been reported. In the latter two cases a considerable advantage of the use of diamond electrodes arises from the wide potential window in aqueous media. However, in other cases it is more desirable to achieve a fast rate for the multi-electron transfer process. The reduction of dioxygen is an example of a multi-electron transfer process of considerable technological importance, and this process is therefore analysed in this study. A voltammogram obtained for a cyclic potential scan applied to a boron-doped diamond film electrode immersed in an acidic aqueous 0.1 M phosphate buffer solution (pH 2) in the absence of dioxygen is shown in Fig. 3a. A wide potential window from −1.1 V versus SCE to +2.2 V versus SCE is detected, and a characteristic anodic current response occurs at +2.0 V versus SCE. This type of response, which appears to be chemically irreversible and which remains over the course of many potential cycles, has been reported in the literature [5] and is believed to be associated with the oxidation of non-diamond sp2-type carbon at the electrode surface. The treatment of the surface of the boron-doped diamond electrode for 2.5 h in a hydrogen plasma further improved the electrochemical characteristics in the sense of further widening the potential window (−1.4 to +2.5 V versus SCE ) and removing the spurious anodic response at +2.0 V versus SCE (see Fig. 3b).
Fig. 3. Cyclic voltammograms (a, b) and sonovoltammograms (c, d ) obtained for a boron-doped diamond film electrode (area=9.8×10−2 cm2; (a) as grown, (b)–(d) after treatment in a hydrogen plasma for 2.5 h in aqueous 0.1 M phosphate buffer solution (pH 2) at a scan rate of 100 mV s−1. Experiments (a, b) under an inert atmosphere of argon and (c, d) under an atmosphere of dioxygen corresponding to a concentration of 0.87 mM dioxygen (ultrasound 90 W cm−2, T=20±2 °C ).
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In the presence of dioxygen, the electrode process was found to be strongly affected by the surface state of the boron-doped diamond electrode, in agreement with a recent study by Yano et al. [22]. In fact, the reduction of dioxygen may be virtually switched off and switched on by an appropriate pretreatment of the electrode surface with potential steps. This effect can be demonstrated most clearly in ‘‘quasi steady-state’’ sonovoltammograms. In Fig. 3c a cyclic sonovoltammogram for the reduction of 0.87 mM dioxygen in 0.1 M phosphate buffer (pH 2) is shown after a 1 s second pretreatment at +10 V versus SCE was applied to the electrode. On the forward scan towards negative potentials the trace follows a straight line similar to that observed in the absence of dioxygen (Fig. 3b), until at sufficiently negative potentials hydrogen evolution begins. However, on the reverse scan in the positive direction the traces do not superimpose. A substantial cathodic current with a current plateau of ~0.25 mA and a half wave potential of −0. V versus SCE suggest that the reduction of dioxygen is possible. The change of the nature of the surface of the boron-doped diamond electrode must have occurred in the negative potential region in which hydrogen evolution also occurs. In contrast, a pretreatment of the electrode surface with 1 s potential step to −10 V versus SCE resulted in a smaller change of the surface reactivity, as shown in Fig. 3d. On the forward scan towards negative potential, the response associated with the reduction of dioxygen is immediately detected as a distinct shoulder. After hydrogen evolution is detected on the reverse scan, a well-defined response with a limiting current of the order of 0.45 mA and a half wave potential of −0.7 V versus SCE are detected. The limiting current for a sonovoltammetric response can in a first approximation be expressed by I =nFDAc/d [14]. In this expression, a flat electrode lim surface is assumed and n (the number of electrons transferred per molecule) can be estimated from the limiting current I for known F (the Faraday constant), lim D (the diffusion coefficient [18]), A (the electrode area), c (the concentration) and d (the diffusion layer thickness). Only the diffusion layer thickness is unknown, and can be obtained from a calibration procedure employing the reduction of Ru(NH )3+ [14]. 36 For the experiments shown in Fig. 3c and d, the diffusion layer thickness is typically d=3 mm, and the value calculated for the number of electrons transferred (n) is found to be between 1 and 2. This value is consistent with a limiting current for a two-electron reduction process when the process is close to becoming mass-transport controlled. Therefore, under the experimental conditions used here, the overall process which is operative is proposed to be the reduction of dioxygen to hydrogen peroxide ( Eq. (2)): O +2e−+2H+H O 2 2 2
(2)
A very similar effect of a potential pretreatment on the course of the dioxygen reduction process has been reported recently for free-standing and polished borondoped diamond film electrodes [18]. However, the details of the surface chemistry of boron-doped diamond responsible for the dramatic change in reactivity is not well understood, and will be a topic of further work.
4. Conclusions The use of boron-doped diamond electrodes in electrochemistry and especially in sonoelectrochemistry promises a considerable extension of the range of processes which can be studied and/or applied at the high rates of mass transport found under ultrasound conditions. The resistance of diamond towards mechanical erosion, its known chemical inertness and dimensional stability combined offer major advantages over conventional carbon-based materials. One very important aspect of the electrochemical characteristics of boron-doped diamond films is the change in the surface chemistry induced by polarisation pretreatment. This is important for surface-sensitive electrochemical processes, as demonstrated for the reduction of dioxygen, and is also important for the reproducibility of results. The relatively simple treatment of electrodes in a hydrogen plasma for a period of time sufficient to recondition the electrode surface appears to be a good alternative to the commonly procedure of surface renewal by polishing, which is inapplicable to diamond.
Acknowledgement F.M. thanks the Royal Society for the award of a University Research Fellowship and New College, Oxford, for a Stipendiary Lectureship.
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