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Electronic surface barrier properties of boron-doped diamond oxidized by plasma treatment
Diamond & Related Materials 19 (2010) 213–216
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d
Electronic surface barrier properties of boron-doped diamond oxidized by plasma treatment C. Pietzka a, A. Denisenko a,⁎, A. Romanyuk b, P.J. Schäfer c, L.A. Kibler c, J. Scharpf a, E. Kohn a a b c
Inst. of Electron Devices and Circuits, University of Ulm, Ulm, Germany Inst. of Physics, University of Basel, Basel, Switzerland Inst. of Electrochemistry, University of Ulm, Ulm, Germany
a r t i c l e
i n f o
Available online 3 September 2009 Keywords: Single-crystal diamond Oxygen termination Surface barrier
a b s t r a c t The electronic surface barrier characteristics of single-crystal and nanocrystalline boron-doped diamond in electrolytes are evaluated. Two cases are compared: Oxidation by RF oxygen plasma treatment and oxidation by anodic polarization in alkaline electrolyte. It is shown that the plasma treatment reduces the surface barrier to about 1.0 eV compared to 1.7 eV when subjected to anodic oxidation. For single-crystalline diamond, the oxygen evolution reaction in 0.1 M H2SO4 electrolyte is almost insensitive to the oxidation method while the plasma-treated nanocrystalline diamond electrode shows an enhanced activity of grain boundary defects at anodic potentials. X-ray photoemission spectroscopy measurements reveal that the plasma oxidation induces a higher content of carbonyl surface groups than anodic oxidation as well as a small amount of non-sp3 contents. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Due to its chemical stability, wide potential window of water dissociation and low background current, oxygen-terminated diamond is widely used for electrochemical applications like waste water treatment [1–3], pH sensing [4], and electroanalysis [1,5]. However, the electronic and electrochemical properties like the response to redox or the flatband potentials in electrolyte are strongly dependent on the oxidation treatment [1,5,6], which could be wet-chemical, anodic, plasma or thermal oxidation. One possible origin of the different electrochemcial characteristics might be the different carbon–oxygen functional groups on the surface induced by the oxidation treatments [1,5,6]. Different carbon–oxygen groups will in turn result in different bond strengths on the surface and therefore induce a different band bending at the surface. In this paper, we focused on the surface band bending and the oxygen evolution reaction characteristics of single- and nanocrystalline highly-doped diamond electrodes exposed to RF oxygen plasma treatment and compare them to the results for anodic oxidation. 2. Experimental The single-crystalline diamond layers used in these experiments were grown on homoepitaxial (100) HPHT substrates (Sumitomo) by microwave-assisted chemical vapor deposition (MWCVD) in a hydro⁎ Corresponding author. Tel.: +49 731 502 6177; fax: +49 731 502 6155. E-mail address: [email protected] (A. Denisenko). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.08.014
gen/methane atmosphere. For boron doping we used a solid doping source technique [7]. Such a process leads typically to a doping concentration in the range of 5 × 1020 cm− 3 [8], which was verified by the impedance measurements shown below. The nanocrystalline diamond films were grown on silicon substrates by hot-filament chemical vapor deposition (HFCVD) in a hydrogen/methane atmosphere with a trimethyl borate (TMB) gas source for boron doping. The doping concentration was approx. 3 × 1020 cm− 3. The oxygen plasma process was performed at 100 mTorr pressure at 100 W under ambient temperature in a RF barrel reactor for 2 min. The diamond samples were placed onto the ground electrode to reduce the plasma-induced damage to a minimum (no DC self-bias). The anodic oxidation was performed in 0.1 M KOH under a potential of +2.0 V versus SCE for 30 s, resulting in a transferred charge during oxidation of approx. 2 mC/cm2, corresponding to more than 1016 charges/cm2. Such process results usually in a fully oxidized surface and removes also all graphitic components from the surface [5], which was also verified by the XPS measurements shown below. For the electrode measurements and the anodic oxidation in electrolyte, the samples were mounted on a copper holder, contacted with silver paste and passivated using a Teflon-based adhesive tape with a perforated opening of 1 mm in diameter, which determined the active electrode area. The electrochemical measurements were performed in a three-electrode glass cell with a Pt counter and a saturated calomel reference electrode (SCE) in 0.1 M H2SO4 using a PARSTAT 273 potentiostat (Princeton Applied Research). The impedance spectroscopy measurements were performed in a frequency range from 0.1 Hz to 10 kHz.
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3. Results and discussion 3.1. Surface analysis The roughness (rms-values) of the as-grown surfaces measured by atomic force microscopy (AFM) was 0.4 nm for the single-crystal and 20 nm for nanocrystalline diamond, respectively. No significant changes after the oxygen plasma treatment were detected for both surfaces. The oxidized surface of the diamond electrodes were analyzed by X-ray photoemission spectroscopy (XPS). Details on the measurement and evaluation procedure can be found elsewhere [9]. Fig. 1 shows the O1s core level spectra of the anodically and plasma-treated surfaces of the single-crystalline diamond surfaces. The XPS data revealed that the diamond surface exposed to oxygen plasma contained an additional peak at approx. 534 eV energy, which implies a higher content of carbonyl (C O) groups [9]. A spectral component related to carbonyl groups was also resolved in the C1s spectrum of the plasma-treated surface (data not shown), similar to data on plasma-exposed diamond reported in literature [5,10]. A small signal at approx. 283 eV binding energy was resolved also in this C1s XPS spectrum, which can be attributed to non-sp3 carbon phases due to plasma-induced defects [9]. Using the results of angle-resolved XPS, the amount of this defectrelated phase was estimated to approx. 1% of the total peak related to carbon–oxygen surface groups on the surface. The thickness of surface oxidized layer of single-crystal diamond was estimated to approx. 1 nm after the plasma treatment. This value was in the range of the surface roughness (peak-to-peak value) of this sample. 3.2. Single-crystal diamond The cyclic voltammetry measurements in 0.1 M H2SO4 (Fig. 2a) showed similar background currents below 1 µA/cm2 for both oxidation treatments and no difference in the onset potential for the oxygen evolution in the µA/cm2 range. Both electrodes showed an adsorption peak at about 1.5 V vs. SCE (Fig. 2a), similar to results reported in the literature [11,12]. This adsorption peak is sensitive to the oxidation treatment. The voltammetry characteristics were recorded for an oxygen-terminated surface after several previous cycles and were reproducible. Therefore we ascribe the observed peaks to adsorption processes and not to a modification of the surface termination, like the oxidation of C–H surface bonds of the as-grown surface, as shown in ref. [12].
Fig. 1. XPS spectra (O1s core level) for single-crytsal diamond oxidized by anodic (curve 1) and oxygen plasma (curve 2) treatment. The measurements reveal a higher amount of C O groups on the plasma-treated surface.
Fig. 2. a) Cyclic voltammetry measurements and b) Mott–Schottky plots for singlecrystalline diamond. Curve 1: anodic oxidation, curve 2: oxygen plasma. The Mott– Schottky plots were determined from impedance spectroscopy measurements at each potential point.
The Mott–Schottky plots (Fig. 2b) showed that the projected flatband potential was shifted towards lower anodic potentials for the plasma-treated electrode compared to the case of anodic oxidation. This could be explained either by the formation of an additional dielectric layer at the surface, or by a change of the electronic surface barrier potential (i.e., the surface band bending at equilibrium). A dielectric layer could be expected for the oxygen plasma process due to surface damage. Such a dielectric layer would decrease the overall capacitance and therefore shift the flatband potential for the case of plasma oxidation to higher anodic potentials. However, the opposite tendency was observed (see Fig. 2b). Therefore we assume a lowering of the surface barrier for the plasma-treated surface. The surface barrier potential φB related to the surface band bending is shown schematically in the band diagram of the diamond– electrolyte interface in Fig. 3a. The difference between the Fermi level and the reference potential at equilibrium is given by the open circuit potential VOCP, which was approx. + 0.45 V vs. SCE for anodic oxidation and +0.35 V vs. SCE for oxygen plasma, respectively. The surface barrier at equilibrium can be calculated as the difference between the part of the flatband potential dropping across the depletion layer (measured vs. SCE) and the value of VOCP. The values for the flatband potentials were extracted from the Mott–Schottky plots shown in Fig. 2b. Each data point was obtained from the analysis of the electrochemical impedance spectroscopy measurement. The data were fitted using one RC parallel circuit, where the capacitance was replaced by a constant phase element Q with an admittance YCPE = Q2⋅(2πf)n (where f is the frequency) for a better fitting. The n-factor was typically approx. 0.97, indicating a non-ideal capacitance. The deviation from an ideal capacitance (n = 1) can be attributed to slow processes in the depletion layer [13]. In general, the diamond–electrolyte interface in electrolytes without redox couples can be described by two RC circuits, representing the
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Fig. 2a. For both oxidation treatments, the onset of oxygen evolution in the µA/cm2-range is approx. 2.1 V vs. SCE, which is close to or above the projected flatband potentials for the two cases. This indicates that the electronic surface barrier is not the limiting parameter which determines the oxygen evolution reaction at the diamond surface. A possible reason could be that the oxygen evolution reaction is a multistep process, which might involve adsorption of intermediate reaction products. 3.3. Nanocrystalline diamond
Fig. 3. a) Schematical band diagram of the diamond surface in contact to the electrolyte at equilibrium. b) General equivalent circuit of the diamond–electrolyte interface. c) Simplified equivalent circuit valid in the inactive region.
depletion layer in diamond and the electrochemical double layer in electrolyte, as shown in Fig. 3b. Here one should note that the capacitance of the depletion layer dominates the impedance characteristics even for highly-doped (often called “semimetallic”) diamond electrodes [4,6,14,15]. Within the inactive region of the potential window, the parallel resistive elements in Fig. 3b can be omitted, leading to the simplified equivalent circuit with the depletion layer capacitance CSC and the double-layer capacitance CDL in series, as shown in Fig. 3c. The double-layer capacitance can be taken as a constant value. Therefore, the overall capacitance shown in the Mott–Schottky plot can be fitted using a model similar to a semiconductor–oxide–metal (MOS) junction [16]. In this case, the Mott–Schottky plot still exhibits a linear characteristic with a slope corresponding to the doping concentration [17]. To derive the surface barrier potential from these measurements, the value of CDL has to be known. It can be determined from the saturation part of the plot, which is found near the onset of oxygen evolution for oxygen-terminated diamond (Fig. 2a/b), since the depletion layer capacitance is short-circuited in this region. The fit for the two electrodes (shown in Fig. 2b by the dashed curves) revealed an identical doping concentration of 5 × 1020 cm− 3 for both electrodes, which is consistent with the value expected from the growth process. Within this potential range of no activity, the value of the parallel resistance was in the range of 10 MΩ×cm2 and could therefore be neglected, as described above. The double-layer capacitance extracted from the saturation value at approx. +2.0 V vs. SCE was 10 to 12 µF/cm2 for both anodic and plasma oxidation, which is similar to data reported in literature [13,18]. The corresponding parallel resistance at +2.0 V vs. SCE was approx. 50 kΩ×cm2, which was large enough for an accurate extraction of the double-layer capacitance. Using the data of the open circuit potentials and the double-layer capacitance, the MOS model yielded a surface barrier of approx. 1.0 eV for the diamond electrode oxidized by plasma treatment, compared to 1.7 eV for anodic oxidation. The different surface barriers correlated with the results from the XPS measurements and could be either ascribed to the higher amount of C O bonds or to the sp2-like defects. Here one should note that a deviation from the MOS model was observed in the range corresponding to the adsorption peak at anodic potentials (see Fig. 2a), which has been reported for oxygen-terminated diamond electrodes [4,14,15]. The reason is most probably that the reactive part of the impedance includes also capacitive components related to the adsorption reaction. However, such elements could not be clearly resolved by the impedance measurements. At the same time, the different electronic surface barrier potentials had a limited impact on the oxygen evolution reaction, as seen in
A similar analysis was applied to the nanocrystalline diamond samples exposed to the identical anodic oxidation and oxygen plasma processes. The cyclic voltammetry plots of the two NCD electrodes are shown in Fig. 4a. For anodic oxidation, the onset of oxygen evolution in the µA/cm2 range was observed at approx. 2.1 V vs. SCE, which is almost identical as for the single-crystal electrodes in Fig. 2a. But for the NCD electrode exposed to oxygen plasma, the apparent onset of the anodic current in the same current range appeared at a lower potential of 1.7 V vs. SCE. The origin of this effect can be clarified by expanding the measurements to higher anodic currents, as shown in Fig. 5 in semilogarithmic scale. It can be observed that the low onset potential for the anodic current on the plasma-exposed surface was related to a process which saturates at approx. 100 µA/cm2 current (curve 2 in Fig. 5). At higher currents, the characteristics of both the oxidized NCD electrodes were almost identical (curves 1 and 2 in Fig. 5) and similar to the voltammograms of the single-crystalline diamond electrodes. The origin of the anodic current at 1.7 V vs. SCE may therefore be ascribed to defect-like grain boundaries activated by the plasma treatment. Similar saturation characteristics have been reported for single-crystal diamond electrodes with the surface modified by nanostructures [8,19]. Such structure represents a heterogeneous
Fig. 4. a) Cyclic voltammetry measurements and b) Mott–Schottky plots for the NCD electrodes. Curve 1: anodic oxidation, curve 2: oxygen plasma.
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Fig. 5. Cyclic voltammetry measurements of the NCD electrodes at higher anodic potentials (only anodic region displayed): Curve 1: anodic oxidation, curve 2: oxygen plasma.
0.1 M H2SO4 by cyclic voltammetry and impedance spectroscopy techniques and compared to the case of anodic oxidation. For single-crystal and nanocrystalline diamond, the oxygen plasma induced a significantly lower electronic surface barrier potential of approx. 1.0 eV compared to anodic oxidation case with the surface barrier potential of approx. 1.7 eV. The differences in the electronic surface barrier between the two oxidation methods might be ascribed to differences in the surface carbon–oxygen groups, or to the contribution of surface defects induced by the plasma exposure. In the cyclic voltammetry, the plasma treatment of the single-crystal surface induced a slight modification of the adsorption characteristics prior to the onset of oxygen evolution, but did not affect the oxygen evolution reaction itself. For nanocrystalline diamond, the oxygen plasma induced a noticeable component of the anodic current, which is suggested to grain boundary defects activated by the plasma treatment.
References surface with chemically active nano-areas and a surrounding inactive material matrix. In case of the NCD surface exposed to oxygen plasma, the grain boundary areas might represent a preferential path for the anodic current transport, while the surface of the diamond grains remain inactive until the onset of oxygen evolution reaction at higher potentials. The saturation of the current at approx. 100 µA/cm2 might be explained by current crowding around the limited area of the grain boundaries. The evaluated surface barrier potential extracted from the MS plots (Fig. 4b) was approx. 1.0 eV for the NCD surface after the oxygen plasma treatment (with VOCP ≈ +0.4 V vs. SCE), similar to the corresponding single-crystal electrode. The slope of the MS plot yielded a doping concentration of approx. 3 × 1020 cm− 3. However, the capacitance data for the anodically oxidized electrode deviated from the results obtained for single-crystalline diamond at positive potentials. The capacitance plot for the NCD electrode prepared by anodic oxidation showed a different slope than the plot for the NCD electrode by plasma treatment although the doping concentration is the same, since both samples were adjacent pieces from the same wafer. This deviation might be related to surface states distributed in energy [20] or to a contribution of additional reactance elements ascribed to the activity of grain boundary defects. Nevertheless, when using the capacitance data at negative potentials, the fitting curve Fig. 4b, curve 1 yielded a surface barrier of approx. +1.6 V (with VOCP = +0.45 V vs. SCE), which was similar to the singlecrystalline diamond electrode oxidized by anodic treatment. 4. Conclusion Single-crystal and nanocrystalline boron-doped diamond electrodes exposed to oxygen plasma treatment have been characterized in
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Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d
Electronic surface barrier properties of boron-doped diamond oxidized by plasma treatment C. Pietzka a, A. Denisenko a,⁎, A. Romanyuk b, P.J. Schäfer c, L.A. Kibler c, J. Scharpf a, E. Kohn a a b c
Inst. of Electron Devices and Circuits, University of Ulm, Ulm, Germany Inst. of Physics, University of Basel, Basel, Switzerland Inst. of Electrochemistry, University of Ulm, Ulm, Germany
a r t i c l e
i n f o
Available online 3 September 2009 Keywords: Single-crystal diamond Oxygen termination Surface barrier
a b s t r a c t The electronic surface barrier characteristics of single-crystal and nanocrystalline boron-doped diamond in electrolytes are evaluated. Two cases are compared: Oxidation by RF oxygen plasma treatment and oxidation by anodic polarization in alkaline electrolyte. It is shown that the plasma treatment reduces the surface barrier to about 1.0 eV compared to 1.7 eV when subjected to anodic oxidation. For single-crystalline diamond, the oxygen evolution reaction in 0.1 M H2SO4 electrolyte is almost insensitive to the oxidation method while the plasma-treated nanocrystalline diamond electrode shows an enhanced activity of grain boundary defects at anodic potentials. X-ray photoemission spectroscopy measurements reveal that the plasma oxidation induces a higher content of carbonyl surface groups than anodic oxidation as well as a small amount of non-sp3 contents. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Due to its chemical stability, wide potential window of water dissociation and low background current, oxygen-terminated diamond is widely used for electrochemical applications like waste water treatment [1–3], pH sensing [4], and electroanalysis [1,5]. However, the electronic and electrochemical properties like the response to redox or the flatband potentials in electrolyte are strongly dependent on the oxidation treatment [1,5,6], which could be wet-chemical, anodic, plasma or thermal oxidation. One possible origin of the different electrochemcial characteristics might be the different carbon–oxygen functional groups on the surface induced by the oxidation treatments [1,5,6]. Different carbon–oxygen groups will in turn result in different bond strengths on the surface and therefore induce a different band bending at the surface. In this paper, we focused on the surface band bending and the oxygen evolution reaction characteristics of single- and nanocrystalline highly-doped diamond electrodes exposed to RF oxygen plasma treatment and compare them to the results for anodic oxidation. 2. Experimental The single-crystalline diamond layers used in these experiments were grown on homoepitaxial (100) HPHT substrates (Sumitomo) by microwave-assisted chemical vapor deposition (MWCVD) in a hydro⁎ Corresponding author. Tel.: +49 731 502 6177; fax: +49 731 502 6155. E-mail address: [email protected] (A. Denisenko). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.08.014
gen/methane atmosphere. For boron doping we used a solid doping source technique [7]. Such a process leads typically to a doping concentration in the range of 5 × 1020 cm− 3 [8], which was verified by the impedance measurements shown below. The nanocrystalline diamond films were grown on silicon substrates by hot-filament chemical vapor deposition (HFCVD) in a hydrogen/methane atmosphere with a trimethyl borate (TMB) gas source for boron doping. The doping concentration was approx. 3 × 1020 cm− 3. The oxygen plasma process was performed at 100 mTorr pressure at 100 W under ambient temperature in a RF barrel reactor for 2 min. The diamond samples were placed onto the ground electrode to reduce the plasma-induced damage to a minimum (no DC self-bias). The anodic oxidation was performed in 0.1 M KOH under a potential of +2.0 V versus SCE for 30 s, resulting in a transferred charge during oxidation of approx. 2 mC/cm2, corresponding to more than 1016 charges/cm2. Such process results usually in a fully oxidized surface and removes also all graphitic components from the surface [5], which was also verified by the XPS measurements shown below. For the electrode measurements and the anodic oxidation in electrolyte, the samples were mounted on a copper holder, contacted with silver paste and passivated using a Teflon-based adhesive tape with a perforated opening of 1 mm in diameter, which determined the active electrode area. The electrochemical measurements were performed in a three-electrode glass cell with a Pt counter and a saturated calomel reference electrode (SCE) in 0.1 M H2SO4 using a PARSTAT 273 potentiostat (Princeton Applied Research). The impedance spectroscopy measurements were performed in a frequency range from 0.1 Hz to 10 kHz.
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3. Results and discussion 3.1. Surface analysis The roughness (rms-values) of the as-grown surfaces measured by atomic force microscopy (AFM) was 0.4 nm for the single-crystal and 20 nm for nanocrystalline diamond, respectively. No significant changes after the oxygen plasma treatment were detected for both surfaces. The oxidized surface of the diamond electrodes were analyzed by X-ray photoemission spectroscopy (XPS). Details on the measurement and evaluation procedure can be found elsewhere [9]. Fig. 1 shows the O1s core level spectra of the anodically and plasma-treated surfaces of the single-crystalline diamond surfaces. The XPS data revealed that the diamond surface exposed to oxygen plasma contained an additional peak at approx. 534 eV energy, which implies a higher content of carbonyl (C O) groups [9]. A spectral component related to carbonyl groups was also resolved in the C1s spectrum of the plasma-treated surface (data not shown), similar to data on plasma-exposed diamond reported in literature [5,10]. A small signal at approx. 283 eV binding energy was resolved also in this C1s XPS spectrum, which can be attributed to non-sp3 carbon phases due to plasma-induced defects [9]. Using the results of angle-resolved XPS, the amount of this defectrelated phase was estimated to approx. 1% of the total peak related to carbon–oxygen surface groups on the surface. The thickness of surface oxidized layer of single-crystal diamond was estimated to approx. 1 nm after the plasma treatment. This value was in the range of the surface roughness (peak-to-peak value) of this sample. 3.2. Single-crystal diamond The cyclic voltammetry measurements in 0.1 M H2SO4 (Fig. 2a) showed similar background currents below 1 µA/cm2 for both oxidation treatments and no difference in the onset potential for the oxygen evolution in the µA/cm2 range. Both electrodes showed an adsorption peak at about 1.5 V vs. SCE (Fig. 2a), similar to results reported in the literature [11,12]. This adsorption peak is sensitive to the oxidation treatment. The voltammetry characteristics were recorded for an oxygen-terminated surface after several previous cycles and were reproducible. Therefore we ascribe the observed peaks to adsorption processes and not to a modification of the surface termination, like the oxidation of C–H surface bonds of the as-grown surface, as shown in ref. [12].
Fig. 1. XPS spectra (O1s core level) for single-crytsal diamond oxidized by anodic (curve 1) and oxygen plasma (curve 2) treatment. The measurements reveal a higher amount of C O groups on the plasma-treated surface.
Fig. 2. a) Cyclic voltammetry measurements and b) Mott–Schottky plots for singlecrystalline diamond. Curve 1: anodic oxidation, curve 2: oxygen plasma. The Mott– Schottky plots were determined from impedance spectroscopy measurements at each potential point.
The Mott–Schottky plots (Fig. 2b) showed that the projected flatband potential was shifted towards lower anodic potentials for the plasma-treated electrode compared to the case of anodic oxidation. This could be explained either by the formation of an additional dielectric layer at the surface, or by a change of the electronic surface barrier potential (i.e., the surface band bending at equilibrium). A dielectric layer could be expected for the oxygen plasma process due to surface damage. Such a dielectric layer would decrease the overall capacitance and therefore shift the flatband potential for the case of plasma oxidation to higher anodic potentials. However, the opposite tendency was observed (see Fig. 2b). Therefore we assume a lowering of the surface barrier for the plasma-treated surface. The surface barrier potential φB related to the surface band bending is shown schematically in the band diagram of the diamond– electrolyte interface in Fig. 3a. The difference between the Fermi level and the reference potential at equilibrium is given by the open circuit potential VOCP, which was approx. + 0.45 V vs. SCE for anodic oxidation and +0.35 V vs. SCE for oxygen plasma, respectively. The surface barrier at equilibrium can be calculated as the difference between the part of the flatband potential dropping across the depletion layer (measured vs. SCE) and the value of VOCP. The values for the flatband potentials were extracted from the Mott–Schottky plots shown in Fig. 2b. Each data point was obtained from the analysis of the electrochemical impedance spectroscopy measurement. The data were fitted using one RC parallel circuit, where the capacitance was replaced by a constant phase element Q with an admittance YCPE = Q2⋅(2πf)n (where f is the frequency) for a better fitting. The n-factor was typically approx. 0.97, indicating a non-ideal capacitance. The deviation from an ideal capacitance (n = 1) can be attributed to slow processes in the depletion layer [13]. In general, the diamond–electrolyte interface in electrolytes without redox couples can be described by two RC circuits, representing the
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Fig. 2a. For both oxidation treatments, the onset of oxygen evolution in the µA/cm2-range is approx. 2.1 V vs. SCE, which is close to or above the projected flatband potentials for the two cases. This indicates that the electronic surface barrier is not the limiting parameter which determines the oxygen evolution reaction at the diamond surface. A possible reason could be that the oxygen evolution reaction is a multistep process, which might involve adsorption of intermediate reaction products. 3.3. Nanocrystalline diamond
Fig. 3. a) Schematical band diagram of the diamond surface in contact to the electrolyte at equilibrium. b) General equivalent circuit of the diamond–electrolyte interface. c) Simplified equivalent circuit valid in the inactive region.
depletion layer in diamond and the electrochemical double layer in electrolyte, as shown in Fig. 3b. Here one should note that the capacitance of the depletion layer dominates the impedance characteristics even for highly-doped (often called “semimetallic”) diamond electrodes [4,6,14,15]. Within the inactive region of the potential window, the parallel resistive elements in Fig. 3b can be omitted, leading to the simplified equivalent circuit with the depletion layer capacitance CSC and the double-layer capacitance CDL in series, as shown in Fig. 3c. The double-layer capacitance can be taken as a constant value. Therefore, the overall capacitance shown in the Mott–Schottky plot can be fitted using a model similar to a semiconductor–oxide–metal (MOS) junction [16]. In this case, the Mott–Schottky plot still exhibits a linear characteristic with a slope corresponding to the doping concentration [17]. To derive the surface barrier potential from these measurements, the value of CDL has to be known. It can be determined from the saturation part of the plot, which is found near the onset of oxygen evolution for oxygen-terminated diamond (Fig. 2a/b), since the depletion layer capacitance is short-circuited in this region. The fit for the two electrodes (shown in Fig. 2b by the dashed curves) revealed an identical doping concentration of 5 × 1020 cm− 3 for both electrodes, which is consistent with the value expected from the growth process. Within this potential range of no activity, the value of the parallel resistance was in the range of 10 MΩ×cm2 and could therefore be neglected, as described above. The double-layer capacitance extracted from the saturation value at approx. +2.0 V vs. SCE was 10 to 12 µF/cm2 for both anodic and plasma oxidation, which is similar to data reported in literature [13,18]. The corresponding parallel resistance at +2.0 V vs. SCE was approx. 50 kΩ×cm2, which was large enough for an accurate extraction of the double-layer capacitance. Using the data of the open circuit potentials and the double-layer capacitance, the MOS model yielded a surface barrier of approx. 1.0 eV for the diamond electrode oxidized by plasma treatment, compared to 1.7 eV for anodic oxidation. The different surface barriers correlated with the results from the XPS measurements and could be either ascribed to the higher amount of C O bonds or to the sp2-like defects. Here one should note that a deviation from the MOS model was observed in the range corresponding to the adsorption peak at anodic potentials (see Fig. 2a), which has been reported for oxygen-terminated diamond electrodes [4,14,15]. The reason is most probably that the reactive part of the impedance includes also capacitive components related to the adsorption reaction. However, such elements could not be clearly resolved by the impedance measurements. At the same time, the different electronic surface barrier potentials had a limited impact on the oxygen evolution reaction, as seen in
A similar analysis was applied to the nanocrystalline diamond samples exposed to the identical anodic oxidation and oxygen plasma processes. The cyclic voltammetry plots of the two NCD electrodes are shown in Fig. 4a. For anodic oxidation, the onset of oxygen evolution in the µA/cm2 range was observed at approx. 2.1 V vs. SCE, which is almost identical as for the single-crystal electrodes in Fig. 2a. But for the NCD electrode exposed to oxygen plasma, the apparent onset of the anodic current in the same current range appeared at a lower potential of 1.7 V vs. SCE. The origin of this effect can be clarified by expanding the measurements to higher anodic currents, as shown in Fig. 5 in semilogarithmic scale. It can be observed that the low onset potential for the anodic current on the plasma-exposed surface was related to a process which saturates at approx. 100 µA/cm2 current (curve 2 in Fig. 5). At higher currents, the characteristics of both the oxidized NCD electrodes were almost identical (curves 1 and 2 in Fig. 5) and similar to the voltammograms of the single-crystalline diamond electrodes. The origin of the anodic current at 1.7 V vs. SCE may therefore be ascribed to defect-like grain boundaries activated by the plasma treatment. Similar saturation characteristics have been reported for single-crystal diamond electrodes with the surface modified by nanostructures [8,19]. Such structure represents a heterogeneous
Fig. 4. a) Cyclic voltammetry measurements and b) Mott–Schottky plots for the NCD electrodes. Curve 1: anodic oxidation, curve 2: oxygen plasma.
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Fig. 5. Cyclic voltammetry measurements of the NCD electrodes at higher anodic potentials (only anodic region displayed): Curve 1: anodic oxidation, curve 2: oxygen plasma.
0.1 M H2SO4 by cyclic voltammetry and impedance spectroscopy techniques and compared to the case of anodic oxidation. For single-crystal and nanocrystalline diamond, the oxygen plasma induced a significantly lower electronic surface barrier potential of approx. 1.0 eV compared to anodic oxidation case with the surface barrier potential of approx. 1.7 eV. The differences in the electronic surface barrier between the two oxidation methods might be ascribed to differences in the surface carbon–oxygen groups, or to the contribution of surface defects induced by the plasma exposure. In the cyclic voltammetry, the plasma treatment of the single-crystal surface induced a slight modification of the adsorption characteristics prior to the onset of oxygen evolution, but did not affect the oxygen evolution reaction itself. For nanocrystalline diamond, the oxygen plasma induced a noticeable component of the anodic current, which is suggested to grain boundary defects activated by the plasma treatment.
References surface with chemically active nano-areas and a surrounding inactive material matrix. In case of the NCD surface exposed to oxygen plasma, the grain boundary areas might represent a preferential path for the anodic current transport, while the surface of the diamond grains remain inactive until the onset of oxygen evolution reaction at higher potentials. The saturation of the current at approx. 100 µA/cm2 might be explained by current crowding around the limited area of the grain boundaries. The evaluated surface barrier potential extracted from the MS plots (Fig. 4b) was approx. 1.0 eV for the NCD surface after the oxygen plasma treatment (with VOCP ≈ +0.4 V vs. SCE), similar to the corresponding single-crystal electrode. The slope of the MS plot yielded a doping concentration of approx. 3 × 1020 cm− 3. However, the capacitance data for the anodically oxidized electrode deviated from the results obtained for single-crystalline diamond at positive potentials. The capacitance plot for the NCD electrode prepared by anodic oxidation showed a different slope than the plot for the NCD electrode by plasma treatment although the doping concentration is the same, since both samples were adjacent pieces from the same wafer. This deviation might be related to surface states distributed in energy [20] or to a contribution of additional reactance elements ascribed to the activity of grain boundary defects. Nevertheless, when using the capacitance data at negative potentials, the fitting curve Fig. 4b, curve 1 yielded a surface barrier of approx. +1.6 V (with VOCP = +0.45 V vs. SCE), which was similar to the singlecrystalline diamond electrode oxidized by anodic treatment. 4. Conclusion Single-crystal and nanocrystalline boron-doped diamond electrodes exposed to oxygen plasma treatment have been characterized in
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