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The Diamond/Aqueous Electrolyte Interface: an Impedance Investigation Jose A. Garrido,* Stefan Nowy, Andreas Ha¨rtl, and Martin Stutzmann Walter Schottky Institut, Technische UniVersita¨t Mu¨nchen, Garching, Germany ReceiVed NoVember 1, 2007. In Final Form: January 12, 2008 We have investigated the electrochemical interface between diamond electrodes and aqueous electrolytes using electrochemical techniques such as cyclic voltammetry and ac impedance spectroscopy. High-quality CVDgrown boron-doped polycrystalline diamond electrodes and IIa single crystalline natural diamond electrodes have been used in this study. In the case of hydrogen-terminated diamond electrodes, the electrochemical interface is dominated by the electrochemical double layer. Frequency-dependent impedance spectroscopy reveals a potential regime in which the contribution of ion adsorption becomes relevant. We have conducted experiments to evaluate the effect of pH and ionic strength on the double layer. Our results suggest that only ions resulting from water auto-dissociation, i.e., hydroxide and hydronium ions, are responsible for ion adsorption and, thus, able to modify the charge at the double layer. In contrast, no effect of the adsorption of several dissolved ions (such as Na+, K+, Cl-) has been observed On the basis of the electrochemical characterization of H-terminated diamond surfaces, we also discuss the phenomenon of the surface conductivity in diamond, as well as the pH sensitivity of the diamond surface. The influence of the O2/OH- and H2/H3O+ redox couples on the origin of the surface conductivity is discussed.
Introduction Despite the widespread used of diamond electrodes in electrochemical applications, there is a remarkable lack of agreement about the characteristics of the diamond/liquid electrochemical interface. Partially, this is caused by the fact that many different types of diamond electrodes have been useds single crystalline, polycrystalline, and more recently nano- and ultrananocrystalline electrodes. As for the doping, semi-insulating and metallic-like boron-doped electrodes have most commonly been used. For all these diamond electrodes, the influence of the surface termination (H- or O-terminated) on the kinetics of electron transfer to different redox molecules has been generally described in terms of surface states. Less attention has been paid to the effect of the microscopic nature of the electrochemical double layer. Swain et al. have investigated the electrochemical activity of boron-doped polycrystalline diamond electrodes, suggesting that surface states were responsible for the unexpected high activity.1 The effect of boron doping on the kinetics of electron transfer in diamond electrodes has been discussed by Ferreira et al.2 The role of the surface termination for the electrontransfer reactions has been investigated by several authors.3-6 Recently, Mahe and co-workers have discussed the effect of graphitic microdomains on the electrochemical reactivity of boron-doped diamond electrodes.7 The differences between single-crystal and polycrystalline diamond electrodes have also been investigated by the group of Angus.8 * To whom correspondence should be addressed. E-mail: garrido@ wsi.tum.de. (1) Swain, G. M.; Ramesham, R. Anal. Chem. 1993, 65, 345. (2) Ferreira, N. G.; Silva, L. L. G.; Corat, E. J.; Trava.Airoldi, V. J. Diamond Relat. Mat. 2002, 11, 1523. (3) Ferro, S.; De Battisti, A. Electrochim. Acta 2002, 47, 1641. (4) Granger, M. C.; Swain, G. M. J. Electrochem. Soc. 1999, 146, 4551. (5) Pastor-Moreno, G.; Riley, D. J. Electrochim. Acta 2002, 47, 2589. (6) Yagi, I.; Notsu, H.; Kondo, T.; Tryk, D. A.; Fujishima, A. J. Electroanal. Chem. 1999, 473, 173. (7) Mahe´, E.; Devilliers, D.; Comninellis, C. Electrochim. Acta 2005, 50, 2263. (8) Martin, H. B.; Argoitia, A.; Angus, J. C.; Landau, U. J. Electrochem. Soc. 1999, 146, 2959.
More recently, the use of surface conductive diamond films for pH and ion sensors based on solution gate field effect transistors (SGFETs) has added new issues of disagreement, which is directly related to diverging descriptions of the diamond/ liquid interface. The surface conductivity of undoped Hterminated diamond films has been explained by a surface transfer doping model, in which atmospheric adsorbates are thought to provide an acceptor level close to the valence band of diamond.9 Thus, the transfer of electrons from the diamond to the acceptor level of the surface adsorbate induces a quasi two-dimensional channel of holes at the diamond surface. On the basis of this model, the surface conductivity of diamond is expected to be influenced by the immersion in an aqueous electrolyte, due to the action of water molecules, as well as electrolyte ions. Whereas some groups have reported a positive pH dependence (i.e., an increase of the conductivity with increasing pH),10,11 others have reported the opposite behavior.12 In a previous publication we have suggested that the electrostatic interaction of hydroxyl (OH-) and hydronium ions (H3O+) with the diamond surface can explain the increase of surface conductivity that we observed for increasing pH.10 However, Nebel et al. have reported the opposite pH dependence, which has been explained assuming an equilibrium between the Fermi level at the diamond surface and the electrochemical potential of an electrochemical reaction in the aqueous electrolyte.12 Initially, Maier et al. have suggested that this electrochemical reaction is related to the hydrogen couple H2/H3O+, H2 + 2H2O T 2H3O+ + e-.9 More recently, Angus and co-workers have claimed that interfacial energy considerations rather point toward the oxygen couple O2/OH-, O2 + 2H2O + 4e- T 4OH-.13 (9) Maier, F.; Riedel, M.; Mantel, B.; Ristein, J.; Ley, L. Phys. ReV. Lett. 2000, 85, 3472. (10) Garrido, J. A.; Ha¨rtl, A.; Kuch, S.; Stutzmann, M.; Williams, O.; Jackman, R. Appl. Phys. Lett. 2005, 86, 073504. (11) Songa, K.-S.; Nakamura, Y.; Sasaki, Y.; Degawa, M., J.-Yang, H.; Kawarada, H. Anal. Chim. Acta 2006, 573-574, 3. (12) Nebel, C. E.; Kato, H.; Rezek, B.; Shin, D.; Takeuchi, D.; Watanabe, H.; Yamamoto, T. Diamond Relat. Mater. 2006, 15, 264. (13) Chakrapani, V.; Eaton, S. C.; Anderson, Tabib-Azar, A. B. M.; Angus, J. C. Electrochem. Solid-State Lett. 2005, 8, E4.
10.1021/la703413y CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
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In this paper, we investigate the electrochemical double layer of high-quality metallic-like H-terminated B-doped polished polycrystalline diamond electrodes using cyclic voltammetry and frequency-dependent impedance spectroscopy. The results are further compared to the case of surface conductive IIa singlecrystalline diamond electrodes. We discuss the degree of reversibility of the above-mentioned redox reactions at the diamond surface and its implications on the origin of the surface conductivity of diamond films in aqueous solutions. Furthermore, frequency-dependent impedance spectroscopy reveals a potential regime in which ion adsorption dominates the electrochemical double layer of the H-terminated diamond/aqueous solution interface. We have conducted experiments to evaluate the effect of pH and ionic strength on the double layer. Our results point out the important role of hydroxide and hydronium ions, which are suggested to be specifically adsorbed at the diamond surface. Experimental Section Two types of diamond films have been used in our experiments: CVD-grown boron-doped polished polycrystalline diamond (BPCD) films (from Element Six, UK) and natural-type IIa single-crystal diamond (SCD) samples, both 100 and 111 oriented (from Element Six BV, The Netherlands). The SCD samples have a size of 3 × 3 mm2 and exhibit a low surface roughness, with an rms value of 0.2 nm, as determined by AFM measurements. The BPCD samples have a size of 4 × 4 mm2, with an average surface roughness below 0.5 nm. The free carrier concentration in the boron-doped material, as determined from Hall effect measurements, was 7 × 1020 cm-3, slightly above the metal-insulator transition for B-doped diamond. Before processing, the samples were cleaned in aqua regia (HCl/ HNO3 ) 3:1) at 150 °C for 1 h to remove any possible metal rest from the surface. Next, the samples were chemically oxidized via the following steps: first, a concentrated H2SO4 solution containing the diamond samples was heated to 225 °C, and then, a few mm3 of KNO3 salt were added. The samples were kept in this solution for 1 h, and afterward thoroughly rinsed with deionized water. Besides the surface oxygen termination, this chemical treatment is known to remove non-diamond carbon from the surface. The oxidation was confirmed using contact angle experiments. Finally, the samples were hydrogen-terminated using a hot-filament setup. The samples were heated in a vacuum chamber (base pressure 5 × 10-7 mbar) to a temperature of 700 °C. H2 (flux 150 sccm) was introduced and activated with two 2100 °C hot tungsten wires. The hot sample was exposed to hydrogen radicals for 30 min at a constant chamber pressure of 1.5 mbar. Finally, the samples were cooled down in hydrogen atmosphere. Contact angles of around 90° indicate hydrophobic surfaces. In the case of the SCD samples, the hydrogenation process induces a surface conductive channel.9 Typically, the value of the surface conductivity is of the order of 10-4 Ω-1. According to Hall effect measurements the corresponding hole densities were of the order of 1013 cm-2, and the mobilities around 100 cm2 V-1 s-1. Electrodes were fabricated by standard photolithography: two Ti/Au (20/200 nm) contacts were deposited by electron-beam evaporation. The active area of the devices was 1 × 1 mm2. The processed diamond samples were mounted onto a ceramic holder, and the two metal contacts were wire-bonded to Au contact pads on the holder. For the experiments reported in this paper, these two contacts were connected. A chemically resistant silicone glue was used to prevent direct contact between any metal and the electrolyte. Only the active diamond area was exposed to the aqueous solution. A glass cell with a three-electrode setup was used for the electrochemical measurements. The reference electrode consisted of an Ag/AgCl electrode, and the counter electrode was a Pt wire. All potentials used in the following are referred to the Ag/AgCl electrode. The electrochemical measurements were performed using a commercial potentiostat (Princeton Applied Research PARSTAT 2263). Cyclic voltammetry experiments were typically performed in a potential window between -0.7 and +0.7 V, with scan rates
Figure 1. CVs of H-terminated (a) CVD-grown boron-doped polished polycrystalline (BPCD) electrode and (b) natural singlecrystalline diamond (type IIa) electrode (SCD) measured in a 10 mM PBS/10 mM KCl solution at two different pH. The solid lines correspond to pH 7, whereas the dashed lines correspond to pH 4.3 and 3 for the BPCD and the SCD electrodes, respectively. The CVs have been recorded with a scan rate of 100 mV s-1. ranging from 5 to 200 mV s-1. The investigated electrochemical potential window is such that no irreversible modification of the diamond surface is observed. Frequency-dependent impedance spectroscopy was conducted using an excitation signal of 10 mV AC, at different DC bias (typically between -0.7 and 0.7 V). The investigated frequency region was from 100 mHz to 100 kHz. K-based PBS buffer, to which KCl, HCl, or KOH were added, was used to control the pH of the solution. The electrolyte was degassed using a membrane pump and then purged with dry nitrogen for 1 h prior to use. In order to reduce regassing, a flow of nitrogen was maintained above the electrolyte surface during measurements.
Results and Discussion Figure 1 shows typical cyclic voltammograms (CVs) obtained using H-terminated B-doped polycrystalline diamond samples (Figure 1a) and H-terminated undoped single crystalline surface conductive diamond samples (Figure 1b). Clear maxima in the cathodic and anodic currents are observed for both SCD- and BPCD-type samples, with a pH-dependent position of the peak. In addition, a steady increase of the anodic current is observed at potentials above +0.5 V vs Ag/AgCl. The exact potential at which the anodic current increase occurs is also pH dependent. On the other hand, a cathodic current increase is observed in Figure 1a at potentials more negative than -0.6 V vs Ag/AgCl. This potential is independent of the solution pH. Different experiments have been performed in order to elucidate the origin of the different features observed in the CVs of Figure 1. A series of experiments (results not shown) have been done in which the ionic strength of the solution was modified by increasing the PBS concentration between 10 and 200 mM, as well as for variations of the KCl concentration between 10 and 200 mM. We have observed that the CVs at different salt concentrations are almost identical, with only clear variations at potentials more negative than -0.5V, where the observed increase of the cathodic current sets in. Therefore, we can conclude that neither the local maxima of the cathodic and anodic current nor the steady increase of the anodic current at very positive potentials are influenced by the ionic strength of the solution. However, the pH dependence of some of the features in the CVs can help to reveal its origin. Let us first discuss these initial results in the context of the band diagrams shown in Figure 2. Figure 2a represents a H-terminated metallic-like B-doped diamond in
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Figure 2. Schematic representation of the energy band diagram of the interface between an aqueous electrolyte and H-terminated (a) BPCD and (b) SCD. Both cases show negative electron affinity as a result of the C-H surface dipole. In the electrolyte region, the level of the reference electrode (EREF) is shown, as all applied voltages are referred to its energy. In addition, the electrochemical potential for electrons corresponding to the redox reactions of the O2/OH- and H2/H3O+ couples are shown in (a) in the pH range 0-14. In the case of the SCD (part b), a positive applied potential (UG) with respect to the reference electrode induces the hole accumulation at the diamond surface.
contact with an aqueous solution. The H-termination is known to induce a negative electron affinity, with a value of -1.3 eV measured in vacuum.14 The presence of water molecules screening the C-H surface dipole is expected to reduce the value of the electron affinity (χ) toward less negative values. Thus, we have chosen a value of ∼ -1.0 eV; this assumption will be discussed later on. The metallic nature of the B-doped diamond film is represented by the position of the Fermi energy, which is below the valence band maximum (at 4.45 eV assuming χ ) -1.0 eV). Under these conditions, the B-doped diamond electrode can be described as a normal metal electrode: the applied potential with respect to the reference electrode drops entirely at the diamond/liquid interface, and the potential drop within the diamond electrode is negligible. In the schematic of Figure 2a, we have also included the energy positions corresponding to the electrochemical potential of the reactions
O2 + 2H2O + 4e- T 4OH-
(1)
H2 + H2O T H3O+ + e-
(2)
and
at different pH values. These two reactions are important to be considered, as several authors have suggested that, when immersed in an aqueous solution, the Fermi level of H-terminated diamond films is in thermodynamic equilibrium with the electrochemical potential determined by these redox reactions.9,13 The position of the electrochemical potential for the H2/H3O+ couple (see eq 2) with respect to the Ag/AgCl reference electrode is -0.6 and -0.4 V at pH 7 and 4.3, respectively, assuming a partial pressure of H2 of 10-3 mbar (Figure 2a). Should the electron-transfer rate for the H2/H3O+ couple at the diamond surface be high enough, an increase of the cathodic current will be observed at those potentials. In the CV of Figure 1a a steady current increase is observed at potentials more negative than -0.6V, but the position of the current increase does not change with pH. In addition, we have previously discussed that the cathodic current in that potential regime depends on the ionic strength of the solution. Therefore, we conclude that diamond shows a high overpotential for the H2/H3O+ couple, i.e., larger (14) Maier, F.; Ristein, J.; Ley, L. Phys. ReV. B 2001, 64, 165411.
potentials than the expected from thermodynamics have to be applied. This high overpotential can be either the result of very slow kinetics at the interface or a weak interaction between H3O+ and the hydrogenated diamond surface. Thus, the thermodynamic equilibrium between the Fermi level at the diamond and the electrochemical potential related to this redox couple is not favored. On the other hand, the position of the electrochemical potential for the O2/OH- couple (see eq 1) with respect to the Ag/AgCl reference electrode is +0.5 and +0.7 V at pH 7 and 4.3, respectively, assuming a typical partial pressure of O2 of 0.21 bar. In Figure 1a, a steady increase of the anodic current is indeed observed at those potentials for both pH 7 and 4.3. This suggests that the increase of the anodic current observed at the more positive potentials could be related to the O2/OH- couple, which thus would show fast enough kinetics at the diamond surface. Similar results are observed in the case of a SCD H-terminated surface conductive electrode, as shown in Figure 1b. However, the band diagram presented in Figure 2b has several differences as compared to the band diagram corresponding to the BPCD electrode (Figure 2a). The H-termination of the diamond surface induces the same negative electron affinity, χ ) -1.0 eV, which fixes the value of the valence band maximum at the same position, 4.45 eV. In the case of the metallic-like BPCD, the Fermi level lies below the valence band maximum as a result of the very high doping level. However, in the case of the undoped H-terminated SCD electrode in an aqueous electrolyte, the position of the Fermi level at the surface can be determined by (a) the electrochemical potential energy of a redox reaction occurring in the solution, assuming thermodynamic equilibrium, and (b) the electrochemical potential determined by the reference electrode. In our case, in which the reference electrode and a potentiostat control the potential, the system cannot be considered to be in thermodynamic equilibrium. When a potential of 0 V is imposed by the potentiostat between the diamond electrode and the reference electrode, the position of the Fermi level at the diamond surface lies about 0.3 V below the valence band maximum (EF - EVBM ) µREF - EVBM ≈ 4.7 - 4.4 ≈ 0.3 V). Thus, an accumulation of holes is induced at the diamond surface. Increasing the applied potential (UG) toward more positive values between the diamond and the reference electrode, the Fermi level is driven further into the valence band of diamond, increasing
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the density of accumulated holes. Under this condition, with the Fermi level within the valence band, the surface conductive diamond electrode behaves like a metal electrode. However, if the applied potential is reversed and more negative potentials than -0.3 V are applied, the Fermi level position at the diamond surface is above the valence band maximum and the surface conductive channel at the diamond surface will eventually disappear. We have measured the surface conductivity as a function of the applied potential between the diamond electrode and the reference electrode, and the obtained results follow very closely the previous description:15 below -0.3 to -0.4 V of applied potential between diamond and the Ag/AgCl reference electrode, no surface conductivity was observed. Between -0.3 and 0 V, a surface conductivity was measured, increasing nonlinearly with the applied potential. For potentials above 0 V, the surface conductivity increases linearly with the applied potential. The reasonable agreement between these results and the band diagram of Figure 2b indicates that the chosen value of the electron affinity for the H-terminated diamond immersed in an aqueous solution, χ ) -1.0 eV, is correct. This description of the H-terminated diamond surface/aqueous interface has important implications for the understanding of the pH sensitivity of surface conductive diamond solution-gate field effect transistors.15 Nebel and co-workers suggested that the thermodynamic equilibrium between the Fermi level at the diamond surface and the electrochemical potential of the H2/H3O+ or the O2/OHcouple could explain the dependence of the surface conductivity with the pH that they observed in their solution-gate field effect transistors.12 However, we have recently demonstrated16 that the pH sensitivity reported by Nebel and co-workers12 is masked by the pH response of the reference electrode they used in their experiments. The correct account of the pH effect on the potential of the reference electrodes resolves the discrepancy. Thus, if the measurement setup is controlled by a potentiostat and the potential is referred to a reference electrode, as done in our experiments, the system is not in thermodynamic equilibrium, and therefore the pH sensitivity cannot result from the variation of the electrochemical potential of the redox couple with the pH. Moreover, the previously discussed description of the operation of the diamond solution gate field effect transistor suggests that the transfer doping model used to explain the surface conductivity of H-terminated diamond in air does not apply in this case.15 Instead, we have recently propose that the almost ideally polarizable diamond/aqueous electrolyte interface allows for the capacitive charging of the surface, and no charge transfer across the diamond/liquid interface is required.15 In this sense, the diamond SGFET works like any field effect device in which a carrier accumulation can be induced by the action of a gate potential.15 So far, we have suggested that for positive applied potentials above 0 V the metallic H-terminated BPCD and the undoped H-terminated SCD electrodes should behave similarly, as observed in Figure 1a and b. After the basic discussion of the band diagrams for the different diamond electrodes used in this work, we can go back to examine the origin of the different features observed in the CV of Figure 1. Whereas the increase of the anodic current at very positive potentials has been tentatively assigned to the O2/OH- couple, we have not yet discussed the pH-dependent local maxima of the cathodic and anodic current. In order to obtain more information, cyclic voltammetry experiments have been conducted at different scan rates. Figure (15) Garrido, J. A.; Ha¨rtl, A.; Dankerl, M.; Reitinger, A.; Eickhoff, M.; Helwig, A.; Mu¨ller, G.; Stutzmann, M. J. Am. Chem. Soc., in press. (16) Dankerl, M.; Reitinger, A.; Stutzmann, M.; Garrido, J. A. Phys. Stat. Sol. (RRL) 2008, 2, 31.
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Figure 3. CVs of a H-terminated BPCD electrode measured in a 10 mM PBS, 10 mM KCl solution at pH 7. The CVs have been recorded using scan rates of 5 (d), 10 (c), 50 (b), and 100 mV s-1 (a).
3 shows different CVs measured with a H-terminated BPCD in a 10 mM KCl/10 mM PBS solution at pH 7, using scan rates (υ) of 5, 10, 50, and 100 mV s-1. From the dependence of the anodic (ip,a) and cathodic (ip,c) peaks with the scan rate, important information concerning the redox process can be derived. Figure S1 in the Supporting Information shows this dependence, in addition to the current value at +0.6 V vs Ag/AgCl, as a function of the scan rate. The anodic and cathodic peak currents, occurring at potentials close to +0.2 and -0.3 V, respectively, follow a linear dependence with the scan rate. The exponents of the curve i ∝ υR are 1.05 and 0.97 for the anodic and the cathodic currents, respectively (see Figure S1 in Supporting Information). On the other hand, for the current value at +0.6 V, the exponent is clearly lower than 1, R ) 0.7. For diffusion-limited electrontransfer processes at metallic electrodes, the current-scan rate relationship is expected to be ip ∝ υ0.5. The current at +0.6 V is strongly influenced by the anodic process at about +0.2 V which is the reason why the exponent R is not closer to 0.5. Above, we have tentatively assigned the ET process at potentials above +0.5 V to the O2/OH- couple. Therefore, the analysis of scan rate dependence of the current suggests that the ET to the O2/OH- couple at the diamond surface is limited by diffusion. On the other hand, there are several possibilities to account for the linear dependence observed for ip,a and ip,c. A non-diffusionlimited electron-transfer process, for instance a kinetically limited ET or ET involving surface redox groups, could result in such a linear dependence. Another option is that the observed current is related to a non Faradaic process, i.e., it has a capacitive origin. If the electrode/electrolyte interface can be described by a capacitance (as is the case for an ideally polarizable electrode), the time-varying potential applied during the cyclic voltammetry experiments induces a charging (non-Faradaic) current, which is proportional to the interfacial capacitance
inF ) Cintυ
(3)
where Cint is the interfacial capacitance and υ is the scan rate. To investigate the capacitive origin of the features observed in the CVs, the capacitance of the BPCD and SCD electrodes have been measured as a function of the applied potential. Typical measurements conducted at 11 Hz in a 10 mM PBS/10 mM KCl solution at pH 3.5 are shown in Figure 4. Both H-terminated BPCD and SCD electrodes show a similar behavior for potentials above 0 V vs Ag/AgCl: a peak in the capacitance-potential curve is observed at about +0.3 V and an increase of the capacitance above +0.5 V. However, marked differences are observed for potentials below 0 V, whereas the BPCD shows a
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Figure 4. Potential dependence of the capacitance of the Hterminated BPCD electrode (open symbols) and the SCD electrode (solid symbols). The measurements were performed at a fixed frequency of 11 Hz in a 10 mM KCl/10 mM PBS solution at pH 3.5.
clear minimum in the capacitance, the SCD electrode exhibits a continuous decrease. As discussed above, the SCD electrode is expected to have a metallic-like behavior similar to the BPCD for potentials above 0 V, as confirmed by the results of Figure 4. However, for potentials below 0 V the density of accumulated holes at the diamond surface is expected to be too low to induce metallic-like conductivity. In this situation the interfacial capacitance is not dominated anymore by the electrochemical double layer but by the capacitance of the diamond electrode (Cdia). To avoid any influence of Cdia, the following experiments have been conducted using BPCD electrodes, in which the double layer capacitance (Cdl) dominates in the whole potential range. For comparison, the capacitance of an O-terminated B-doped PCD sample has also been measured. Figure S2 in the Supporting Information shows typical Mott-Schottky plots (C-2 versus potential) recorded at different AC excitation frequencies. In the case of the O-terminated diamond electrode, a linear dependence between C-2 and the potential is observed, as expected for the capacitance of a semiconductor/electrolyte interface dominated by the depletion region of the semiconductor, which follows
2 1 ) (U - Ufb) 2 q Cdepl 0rN
(4)
where N is the volume density of boron impurities (estimated about N ≈ 5 × 1020 cm-3) and Ufb is the flat band potential (close to 2 V). The frequency dependence of the capacitance which is observed in Figure S2 is a common characteristic of O-terminated diamond electrodes.17 On the other hand, the capacitance-potential dependence of the H-terminated diamond electrode shown in Figure 4 does not correspond to the expected capacitance introduced by the depletion region of a semiconductor (eq 4). Therefore, the features in Figure 4 are tentatively attributed to the interfacial impedance of the electrochemical double layer. Further discussion about this assumption is presented below. Figure 5 compares the measured capacitance (symbols) to the value of the capacitance derived from the CV (solid and dashed lines) using eq 3 for H-terminated BPCD at two different pH values. The general shape is quite similar, suggesting that the observed peak in the CV is related to the peak observed in the capacitance. However, the absolute value of both capacitances differs by almost 1 order of magnitude, which is currently not understood. (17) Pleskov, Y. V. Russ. J. Electrochem. 2002, 38(12), 1275.
Figure 5. Comparison between the measured capacitance (symbols) of a BPCD electrode and the capacitance value derived from the cyclic voltammetry experiment (lines) at two different pH values (7 and 4.3). The capacitance experiments were performed at a fixed frequency of 11 Hz in a 10 mM KCl/10 mM PBS solution.
The interpretation of capacitance-voltage curves measured with metal electrodes in aqueous electrolytes has been a very active discussion topic since the first interest in electric double layers.18 The interaction of water dipoles with the metal surfaces was suggested to be the origin of the features of the differential capacitance-potential curves measured with mercury electrodes (for a review see ref 19). Molecular Dynamic (MD) and Monte Carlo (MC) simulations predict that at zero charge the water dipoles in contact with the electrode surface are oriented almost parallel to the surface, favoring H-bond formation between them. At negatively charged surfaces, the contacting water molecules turn their oxygen atom away from the metal, and the opposite occurs at positively charged metal surfaces. This behavior results in a maximum of the capacitance in the vicinity of the point of zero charge,20 with a capacitance magnitude of a few µF/cm2,21 very similar to the results shown in Figure 4. On the other hand, the adsorption of ions onto metal surfaces has also been investigated for many years and it is well-known that it can produce maxima in the capacitance-potential curves.22 Pajkossy and co-workers have studied the effect of ion adsorption on the double-layer capacitance of different metal electrodes in aqueous solutions.23,24 In the presence of specifically adsorbed ions, frequency dispersion of the capacitance is typically found, which has been attributed to the slowness of the adsorption process and/or the diffusion of the surface-active species.23 We have measured the capacitance of the H-terminated BPCD electrodes at frequencies between 5 and 120 Hz, and the results are shown in Figure 6. Clear evidence of the frequency dispersion is observed in the potential range -0.2 to +0.3, where the capacitance peak is observed, as well as for potentials above +0.5 V. In the other regions, the frequency dispersion is noticeably (18) Grahame, D. C. Chem ReV. 1947, 41, 441. (19) Brockis, J. O’M.; Conway, B. E.; Yeager, E. ComprehensiVe Treatise of Electrochemistry. Vol I: The Electrical Double Layer; Plenum Press: New York, 1980. (20) Guidelli, R.; Schmickler, W. Electrochim. Acta 2000, 45, 2317. (21) Aloisi, G.; Foresti, M. L.; Guidelli, R.; Barnes, P. J. Chem. Phys. 1989, 91, 5592. (22) O’M Brockis, J.; Khan, S. U. M. Surface Electrochemistry. A molecular LeVel Approach; Plenum Press: New York, 1993 (23) Pajkossy, T.; Wandlowski, T.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 209. (24) Pajkossy, T.; Kolb, D. M. Electrochim. Acta 2001, 46, 3063.
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Figure 6. Frequency dispersion of the measured capacitance of a H-terminated BPCD electrode. The experiments have been carried out at 5, 11, 50, and 120 Hz. Frequency dispersion is observed in the potential range of the capacitance peak and for potentials more positive than +0.4 V.
Figure 7. Equivalent circuits used to simulate the frequencydependent interfacial electrochemical impedance. Circuit (a) is very similar to the classical Randles circuit, in which the pure capacitance element has been substituted by a constant phase element, accounting for the electrochemical double layer (CPEdl). RS represents the resistance of the electrolyte solution, and RF a Faradaic resistance. Circuit (b) is used to take into account an adsorption process: RADS represents the adsorption resistance and WADS is the diffusion-related (infinite-length) Warburg impedance.
weaker. It has been shown that in the case of ion adsorption, a more detailed analysis using frequency-dependent impedance spectroscopy is necessary to investigate the interfacial capacitance.23 A typical impedance measurement (solid symbols), performed at +0.1 V DC between 0.1 Hz and 30 kHz, is shown in the Supporting Information, Figure S3, in which the impedance is represented by its module |Z| and its phase. Physical information from the impedance spectra can be derived using a proper equivalent circuit to model the interfacial impedance of the H-terminated diamond/aqueous interface. Figure 7 shows two equivalent circuits which have been used to simulate the experimental results. The circuit of Figure 7a is very similar to the classical Randles circuit, in which the pure capacitance element has been substituted by a constant phase element (CPEdl) to account for the frequency dispersion of the double layer capacitance.25 The admittance of a CPE element is given by YCPE ) Y0CPE(jω)φ, in which Y0CPE has units of S‚sφ/cm2. In the case of an ideal capacitive interface, the exponent φ ) 1 and the CPE element is purely capacitive. RS represents the resistance of the electrolyte solution and RF the Faradaic resistance, which is used (25) Barsoukov, E.; MacDonald, J. R. Impedance Spectroscopy; Theory, Experiments, and Applications, 2nd ed.; John Wiley: New York, 2005.
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Figure 8. Impedance spectra (100 kHz to 0.1 Hz clockwise) of the H-terminated BPCD measured at (a) -0.5 V vs Ag/AgCl and (b) +0.1 V vs Ag/AgCl. C′′ and C′ corresponds to the imaginary and real part of the complex parameter C(ω) ) Y(ω)/(jω), respectively. Solid symbols corresponds to the experiment results, which have been simulated with the equivalent circuits of Figure 7. In (a) the experiments have been successfully simulated using the circuit in Figure 7a. In (b) the simulations using the circuits in Figure 7a (dashed line) and b (solid line) are shown for comparison, demonstrating the need for considering the diffusion Warburg impedance.
to model nonideally polarizable electrodes. However, as we will show later on, this simple circuit is not able to simulate the frequency dispersion in the whole potential range, especially in the region where the capacitance peak is observed. It has been suggested that in the presence of a slow ion adsorption process, the equivalent circuit shown in Figure 7b is more appropriated.23 This circuit includes a contribution from the adsorption resistance, RADS, and if a diffusion process is to be taken into account, a serial diffusion impedance must be included. In our case, this diffusion-related impedance element will be represented by the Warburg element (in the infinite-length approximation), with an admittance YW ) Y0W(jω)0.5.25 For instance, the solid line in Figure S3 is the result of fitting a measured impedance spectrum using the equivalent circuit of Figure 7b. Figure 8 shows the comparison between the impedance measurements (symbols) and the fittings (lines) using different equivalent circuits, at two different applied potentials between the reference and the diamond electrode: -0.5 and +0.1 V. In this figure, we have chosen to represent the impedance by C(ω) ) Y(ω)/(jω), which is a complex parameter and can be represented in a Nyquist plot, i.e., the imaginary part (C′′(ω)) versus the real part (C′(ω)). This representation helps to visualize the low-frequency region of the spectra. For an applied potential of -0.5 V vs Ag/AgCl, the simulating circuit corresponds to the one in Figure 7a, in which no adsorption elements are included. The same analysis is valid in the potential range between -0.6 and -0.25 V. The impedance analysis for potentials more positive than -0.25 V have been performed using the circuit in Figure 7b. As observed in Figure 8b the use of a Warburg impedance element is necessary to fit the impedance spectra in the low-frequency regime. As mentioned above, the need of Warburg impedance elements in the equivalent circuit suggests that a slow adsorption/desorption process dominates at low frequency in this potential range. Figure 9 compares the AC capacitance, measured at 11 Hz, and the values of the CPEdl element derived from the fitting of the impedance spectra at different potentials. The value of the exponent φ is about 0.97 in the whole potential range. The peak observed in the AC capacitance is not present in the CPEdl, as the contribution of the ion adsorption is being transferred to the adsorption
The Diamond/Aqueous Electrolyte Interface
Figure 9. Comparison between AC capacitance (solid symbols), measured at 11 Hz, and the values of the CPEdl element (open symbols) derived from the fitting of the impedance spectra at different potentials. The value of the exponent φ is 0.96 ( 0.01 in the whole potential range.
Figure 10. Adsorption (RADS) and Faradaic (RF) resistances obtained by fitting the impedance spectra measured with different buffer concentrations (10, 20, and 50 mM PBS). For potentials above -0.3 V, we have used the circuit in Figure 7b, and therefore, the symbols correspond to RADS. For potentials more negative than -0.3 V, the circuit used was the one in Figure 7a, and therefore, the symbols correspond to RF. Both solid and dashed lines are guides to the eye.
resistance and the Warburg impedance. Figure 10 shows the corresponding values of RADS obtained by fitting the impedance spectra measured at different concentrations of the buffer (10, 20, and 50 mM PBS) using the circuit of Figure 7b. As observed, RADS does not dependent on the concentration of PBS buffer. A similar insensitivity of RADS has been found (results not shown) for KCl salt in the same concentration range, from 10 to 50 mM. In the potential range where the peak of the AC capacitance is observed, the adsorption resistance shows a minimum, indicating that an adsorption process occurs at those potentials. In addition, for potentials more positive than +0.4 V, the adsorption resistance decreases with increasing positive potential. From the insensitivity of RADS to the ions of the dissolved salts in the solution (K+, Cl-, and phosphate anions H2PO4- and HPO42-), we can conclude that these ions do not show adsorption on H-terminated diamond surfaces in the investigated potential range (we have recently reached the same conclusions using a different experimental approach26). As mentioned before, the circuit of Figure 7a has been used for fitting the impedance at potentials more negative than -0.3 V, which indicates that a Faradaic process (i.e., charge transfer across the interface) governs the impedance behavior at (26) Ha¨rtl, A.; Garrido, J. A.; Nowy, S.; Zimmermann, R.; Werner, C.; Horinek, D.; Netz, R.; Stutzmann, M. J. Am. Chem. Soc. 2007, 129, 1282.
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Figure 11. Effect of pH (3.5, 7, and 10) on the adsorption resistance (RADS) derived by fitting the impedance spectra measured at different voltages. In this experiment, a 10 mM PBS/10 mM KCl solution was used. Arrows indicate the potential of the minimum of RADS, which shifts about 60 mV per pH unit.
those potentials. In the CVs of Figure 1a this Faradaic process is observed as an increase of the cathodic current. At this point, we do not have an explanation for this Faradaic process. Finally, we have investigated the effect of the pH on the adsorption process; the values of the calculated adsorption resistance for pH 10, 7, and 3.5 are shown in Figure 11. The minimum of the RADS, which we have previously related to the AC capacitance maximum, shifts toward more negative values as the pH increases. The evaluation of the minimum potential versus the pH results in a shift of about 60 mV per pH unit. As shown before, the potential of RADS minimum does not depended on the concentration of salt in the solution, which, together with the pH shift, indicates that the adsorption process is related to species originating from the solvent dissociation, i.e., OH- and/ or H3O+ ions. OH- adsorption on metal electrodes, such as Au,27 Ag,28 and Pt29 has been investigated using cyclic voltammetry, impedance spectroscopy, and Raman and IR spectroscopy. The adsorption process can be generally represented by the following reaction
OH- f OHads + e-
(5)
The pH dependence of the adsorption process can be therefore understood from eq 5. Recently, on the basis of electrokinetic experiments, we have suggested that OH-, as well as H3O+, shows unsymmetrical adsorption on H-terminated diamond surfaces even when there is no potential applied between the diamond surface and the electrolyte bulk.26 It has been suggested that hydroxide ions from the aqueous solution can be adsorbed on the hydrophobic surfaces, as is the case of the H-terminated diamond surface, due to the partial orientation of water molecules at the interface.30 Therefore, we suggest that the origin of the adsorption features observed as a (broad) anodic peak in the CVs (Figure 1), a peak in the AC capacitance (Figure 4), and a minimum of the adsorption resistance derived from the impedance spectra (Figure 11), is related to the adsorption of hydroxide ions on the H-terminated diamond surface. At pH 7, the adsorption process occurs at potentials of +0.15 ( 0.05 V vs Ag/AgCl. Another adsorption process is observed for potentials more positive than +0.4 V vs Ag/AgCl at pH 7. As well, the onset (27) Chen, A.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682. (28) Savinova, E. R.; Kraft, P.; Pettinger, B.; Doblhofer, K. J. Electroanal. Chem. 1997, 430, 47. (29) Jaff-Golze, K. A.; Kolb, D. M.; Scherson, D. J. Electroanal. Chem. 1986, 200, 353. (30) Zangi, R.; Engberts, J. B. F. N. J. Am. Chem. Soc. 2005, 127, 2272.
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of this adsorption process shows a pH dependence, also very close to the expected value of 60 mV/pH. Therefore, it should also be related to OH- or H3O+ ions. During the discussion of the CVs, we have suggested that the increase of the anodic current observed at high positive overpotentials could be related to the O2/OH- couple (eq 1). Thus, a slow desorption/adsorption process of the species O2 and OH- could explain the need of a Warburg (diffusional) element in the simulations of the impedance spectra. However, another possibility should be considered to explain the adsorption process at high positive overpotentials. Similarly to what has been suggested in the case of the hydroxide adsorption on Au electrodes, adsorbed OH- ions can induce the formation of oxygen adsorbed on the surface, as described by the following reaction
OHads + OH- f Oads + H2O + e-
(6)
Currently, there is not enough information to exclude any of the different explanations discussed above. The adsorption of OH- on the surface of H-terminated diamond electrodes is an intriguing question, and more detailed surface spectroscopy (such as infrared spectroscopy,27 sum frequency generation spectroscopy,31 and in situ Raman spectroscopy28) should be applied in order to confirm the hypothesis presented in this paper. The values of the Warburg impedance resulting from the same simulations as in Figure 11 are shown in the Supporting Information, Figure S4. A clear dependence on pH is observeds the higher the pH the larger the Warburg admittance (YW), as expected due to the increase of the concentration of the OH- at higher pH. However, the absolute values and the voltage dependence of the Warburg impedance are currently not well understood.
Conclusions We have investigated the interface between the H-terminated diamond surface and aqueous electrolytes using cyclic voltammetry and frequency-dependent impedance spectroscopy. In this study we have employed B-doped polycrystalline diamond electrodes and undoped single crystalline diamond films. The role of the O2/OH- and H2/H3O+ redox couples for the cyclic (31) Vidal, F.; Tadjeddine, A. Rep. Prog. Phys. 2005, 68, 1095.
voltammetry experiments has been discussed in order to elucidate their influence on the origin of the surface conductivity of H-terminated diamond. We have discussed that the transfer doping model, which is normally used to explain the surface conductivity of H-terminated diamond in air, does not apply in the case of the diamond/liquid interface if a potentiostat is used to control the position of the electrochemical potential. We suggest that in this situation the surface conductivity is simply the result of a gate-induced field effect.15 The interfacial impedance of metallic-like B-doped and undoped surface conductive films has been compared. We have observed that both types of electrodes show almost identical metallic behavior in a certain potential range: once the surface conductivity of the undoped film is fully developed upon the action of the applied potential, the electrochemical properties of this film resembles closely the properties of the metallic B-doped film. Our results indicate that in the case of the H-terminated diamond surface, the electrical double layer is governed by ion adsorption at the Helmholtz planes, in contrast to the case of the O-terminated diamond surfaces. By investigating the dependence of the interfacial impedance with the ionic strength and pH of the aqueous electrolyte, we have concluded that the adsorption of OH- ions is the dominant process, whereas no effect of the adsorption of other dissolved ions (such as Na+, K+, and Cl-) has been observed. We suggest that the water ordering at the hydrophobic H-terminated diamond/aqueous interface strongly influences the properties of the electrical double layer. Thus, the adsorption of OH- is the main factor affecting the surface charge. Acknowledgment. This work has been partially funded by the EU Marie Curie Research Training Network DRIVE (Diamond Research on Interfaces and Versatile Electronics), and the EU project DREAMS (Diamond to Retina Artificial Micro-Interfaces Structures). Supporting Information Available: Scan rate dependence of the cathodic and anodic peaks shown in Figure 3; Mott-Schottky representation of the interfacial capacitance measured in an O-terminated B-doped diamond electrode; typical frequency-dependent impedance spectrum (modulus and phase versus frequency) of an H-terminated diamond electrode; and pH dependence of the diffusion-related Warburg impedance. This material is available free of charge via the Internet at http://pubs.acs.org. LA703413Y
