The Electrochemistry of Highly Doped Diamond Films
ˇ This chapter contains details of the apparatus used for the electrochemical experiments in these studies.
ˇ The chapter describes the electrochemistry of highly doped diamond films. Cyclic Voltammogams were recorded with a number of well known redox couples to show the behaviour of the films.
ˇ The effect of surface termination of the electrode was investigated.
ˇ AC Impedance and Mott-Schottky plots are presented.
ˇ The results of the electrochemical experiments are explained in terms of the theory outlined in chapters 4 and 7.
5.1 Experimental Set-up
Electrochemical experiments were performed using a three electrode system. This section describes the apparatus used.
5.1.1 Electrolyte Solutions
Aqueous solutions were made up with 18.3 MW cm ultrapure deionised water (Millipore).
Standard laboratory grade reagents were used to make up the background electrolytes. Appendix A contains details of the stated maximum levels of impurities.
Glassware was cleaned with a four step process:
in a base bath
[saturated sodium hydroxide (NaOH) in ethanol(C2H5OH)];
2. immersing and rinsing in deionised water;
in an acid bath
[2.5 dm-3 nitric acid (HNO3) to 10 dm-3 water (H2O)].
4. immersing and rinsing in deionised water.
Dissolved oxygen (O2) was removed from the electrolyte solutions by purging the liquid with standard laboratory grade oxygen-free nitrogen (N2) for at least 30 minutes.
5.1.2 Electrochemical Cells
The main type of cell used in these studies was a single chamber design. A schematic diagram is shown in figure 5.1. This cell design proved ideal for performing electrochemistry with silver dag, TLM and TiUL contacts.
The electrochemical cells were fabricated from white cylinders of PTFE (50 mm in diameter). A central bore was removed from the main segment of the cell to provide a reservoir for the electrolyte. The diamond sample that was to be used as a working electrode (WE) was laid horizontally and sandwiched between two PTFE cylinders. A 3mm hole in the base of the main piece and an O ring (3 mm internal diameter) exposed a selected area of the electrode surface to the electrolyte. Therefore, the approximate electrode area exposed the electrolyte was 0.07 cm2 (7 mm2).
Figure 5.1 A schematic diagram of an electrochemistry cell
The base of the cell was tapered to allow small samples to be used. All samples had to be flat and the minimum sample size was 15 mm ´ 5 mm. Typical sample dimensions were 15 mm ´ 10 mm.
The bottom piece of PTFE was attached to a brass plate to provide a stable base.
A transparent top piece was fitted over the cell. The reference electrode (RE) and the counter electrode (CE) were placed through holes in the top. They were held in place by rubber rings (not illustrated in figure 5.1). Two additional holes allowed for deoxygenating to be performed. Nitrogen could be bubbled through the electrolyte via a needle that was passed down into the solution.
Control experiments could be performed with platinum (Pt) working electrodes. Platinum wires could be passed down through one of the gas holes.
Optical experiments were performed by removing the lid of the vessel. The reference and counter electrodes were then clamped in place and the sample was illuminated from above, through a minimum depth of electrolyte.
Early experiments performed with indium/gallium (In/Ga) eutectic contacts to the silicon substrate were performed in a three compartment glass cell. The sample was attached to a steel strip and covered with adhesive PTFE tape. This provided a temporary waterproof cover over the electrode. A circular hole with a diameter of 5 mm was punched into the PTFE tape so as to expose an area of approximately 80 mm2 of the electrode surface. This working electrode was placed in the central chamber. A reference electrode was placed in a side compartment which was connected to the central chamber through a Luggin capillary tip. A counter electrode was placed in the other side compartment which was connected to the main chamber through a glass frit.
5.1.3 Reference Electrodes
Silver | silver chloride (Ag | AgCl) reference electrodes were used (BAS [Bioanalytical Systems, Inc.] and CH Instruments, Inc.). These electrodes comprised of a silver chloride coated silver wire in a glass tube with a porous polymer tip (Vycor frit). The glass tube formed an electrode compartment which was filled with 3 mol dm-1 potassium chloride (KCl) or sodium chloride (NaCl). A diagram of a reference electrode is shown in figure 5.2.
The standard potential, E0, for the
Ag | AgCl redox couple at 25 ēC is
+ 0.22239 ą 0.00005 V relative to the normal hydrogen electrode (NHE). 127-128 The redox potential for the reference electrode, E, is dependant on the concentration of chloride ions in the electrode compartment. The Nernst equation for the Ag|AgCl electrode can be represented as:
where g = activity coefficient for the solution
[Cl-] = concentration of Cl- ions
This gave a redox potential for the reference electrode of + 0.194 V vs. NHE at 25 ēC.
Figure 5.2 (top) A schematic diagram of a Ag | AgCl reference electrode
(adapted from BAS sales literature)
Figure 5.3 (bottom) A schematic diagram of a platinum counter electrode
5.1.4 Counter Electrodes
Platinum (Pt) counter electrodes were made from a curled square of platinum mesh attached to a platinum wire. This provided a suitably large surface area. A glass tube was used to hold the electrode. This was designed with the same diameter as the reference electrode. The tube was sealed to the platinum wire at the bottom of the electrode. The top of the tube, which was never submersed into the electrolyte, was left open. This allowed for repairs to be made to the electrode if the seals or joins became damaged. Nickel wire was used inside the glass tube because it was less expensive than the platinum wire. The wire was wound round a notch at the top of the electrode, so that the weight of the attached crocodile clip did not apply a force on to the platinum-nickel join or the platinum-glass seal. A quick fit stopper was fitted to the top of the electrode provide compatibility with a number of other designs of electrochemical cell. A diagram of a counter electrode is shown in figure 5.3.
Cyclic voltammograms were recorded with a computer controlled E-Chemie m‑Autolab potentiostat.
A Solartron 1286 potentiostat and a Solartron 1250 Frequency Response Analyser (FRA) were used to take AC impedance measurements. The experiments were controlled and the results captured with the ZPlot/ZView and CorrWare/CorrView software packages.
IMPS studies were performed with the Solartron FRA and a potentiostat that was built in-house by the electronics workshop. The in-house potentiostat was designed to prevent phase shifting of the signal over a wide range of frequencies.
5.1.6 Faraday Cages
Experiments were performed in Faraday Cages. The cages were fabricated in-house by the mechanical workshop. They were welded together from sheet steel and painted black. The boxes measured approximately 1 m (height) ´ 1 m (width) ´ 1 m (depth). The front panel incorporated a large hinged door.
The Faraday Cages isolated the experiments from electromagnetic interference and external sources of light.
5.2 The Cyclic Voltammetry Technique
Cyclic voltammetry is a technique which can be used to study the electrode kinetics of a redox couple in a three electrode system.
The potential of the working electrode is swept between two set potentials. The scan rate (the magnitude of the rate of change of the applied potential with time) is kept constant.
As the applied potential is increased, oxidation will occur and a positive current will flow (electron transfer from the species being oxidised to the electrode). Conversely, as the potential is decreased, reduction will occur and a negative current will flow (electron transfer form the electrode to the species being reduced).
5.2.1 Reversible Electrode Dynamics
Reversible electrode dynamics are exhibited in systems where the heterogeneous rate constants are large. Reactions will start to occur as soon as the potential is such that they are thermodynamically viable. As both the forward and back reactions are rapid, the concentrations of the reduced and oxidised species will be effectively maintained at the ratio predicted by the Nernst equation. The resulting cyclic voltammogram will exhibit the following features. 120
ˇ The scans will be symmetrical.
ˇ The potentials at which the peaks occur will be independent of scan rate.
ˇ The peak separation will be approximately equal to (59 ´ n) mV at 25 °C where n equals the number of electrons transferred in the reaction.
ˇ The peak height will vary as the square root of the scan rate.
5.2.2 Irreversible Electrode Dynamics
A system which exhibits irreversible electrode dynamics will produce a cyclic voltammogram with different characteristics to those produced by a reversible system.
The onset of the reaction will not occur at potentials immediately beyond the equilibrium potential. An appreciable overpotential will be required to induce the reaction as it is not kinetically favourable.
As the overpotential is increased, the homogeneous rate constant increases and the rate of reaction will rise. This will be countered by the depletion of the reactant at the surface of the electrode. The position of the peak maximum reflects the balance between the increasing heterogeneous rate equation and the decrease in the surface concentration of the reactant. The current is controlled by the electrode kinetics up to the peak maximum. After the peak maximum, the current simply reflects the rate at which the reactant diffuses though the solution to the surface of the electrode.
Irreversible systems are not kept at equilibrium and the back reaction is negligible.
A cyclic voltammogram for an irreversible system will exhibit the following features. 120
ˇ The peak separation will be wider than the (59 ´ n) mV at 25 °C seen for reversible systems.
ˇ The peak separation will increase with scan rate.
ˇ Irrespective of the electrode dynamics, the peak height will vary as the square root of the scan rate.
5.2.3 Quasi-Reversible Electrode Dynamics
Reversible electrode behaviour is the limiting case when the electrode kinetics are fast relative to the mass transport conditions. Conversely, irreversible electrode behaviour is the limiting case when the electrode kinetics are slow relative to the mass transport conditions. Intermediate cases exist where the surface concentrations of the reduced and oxidised species depend on both the electron transfer rates (forward and reverse) and the rate of mass transport. These cases are said to exhibit quasi-reversible behaviour.
5.3 Cyclic Voltammetry in Dilute Nitric Acid
Cyclic voltammograms were recorded in an aqueous solution containing 1 mol dm-3 nitric acid (HNO3) and 2 mol dm-3 sodium chloride (NaCl).
A highly doped diamond film used as a working electrode (sample B67, doping level 5 ´ 1021 cm-3, silver dag contact). Before use, the diamond film was refluxed in concentrated nitric acid in order to reduce the presence of any non-diamond carbon. Electrical contacts were then made to the top of the film using silver dag.
Figure 5.4 shows a typical cyclic voltammogram recorded at a scan rate of 50 mVs-1. The scan shows that the electrode has a wide potential window, with no significant solvent breakdown occurring in a range from 0.7 V to 1.1 V with respect to the Ag | AgCl (3 M Cl-) reference electrode. This window was significantly wider than those reported for many commonly used electrode materials.
5.4 CV of sample B67 in 1 mol dm-3
HNO3 & 2 mol dm-3 NaCl
Scan rate 50 mV/s, Ag dag contact, geometric area of working electrode = 20 mm2
5.4 Cyclic Voltammetry of 4-Aminophenol
Figures 5.5 and 5.6 show cyclic voltammograms recorded using a highly doped diamond electrode (sample B67, doping level 5 ´ 1021 cm‑3, silver dag contact) in an aqueous solution containing 3 ´ 10-3 mol dm‑3 4-aminophenol ( C6H4(OH)(NH2) ), and 0.5 mol dm‑3 sulphuric acid (H2SO4).
4-aminophenol is an aromatic organic molecule, which may undergo a two step oxidation.
The cyclic voltammograms show two oxidation peaks and two reduction peaks per scan.
5.5 Cyclic Voltammetry of Potassium Ferrocyanide
Figures 5.7 and 5.8 show cyclic voltammograms recorded using a highly doped diamond electrode (sample B69, doping level 2 ´ 1021 cm-3, silver dag contact) in an aqueous solution containing 3 x 10-3 mol dm-3 potassium ferrocyanide (K4[Fe(CN)6), and 1 mol dm-3 potassium chloride (KCl).
Ferrocycanide ( Fe(CN)64+ ) and ferricyanide ( Fe(CN)63+ ) are a classic redox couple. The cyclic voltammograms show a reversible reaction.
The cyclic voltammograms shown in figures 5.7 and 5.8 were performed after the diamond sample (B69) had been refluxed in concentrated nitric acid (HNO3). This treatment was primarily intended to remove non-diamond impurities on the surface of the specimen (such as graphitic carbon and tantalum). The process may also have caused oxidation of the surface.
Figures 5.9 to 5.12 show cyclic voltammograms of another diamond electrode (sample B107, doping level 5 ´ 1020 cm-3, silver dag contact). The solution used contained 3 ´ 10-3 mol dm-3 potassium ferrocyanide ( K4[Fe(CN)6] ), 3 ´ 10‑3 mol dm-3 potassium ferricyanide ( K3[Fe(CN)6] ) and 1 mol dm‑3 potassium chloride (KCl).
The hydrogen atmosphere used in the diamond growth process should have ensured that samples are hydrogen terminated when they are removed from the CVD reaction chamber.
The diamond sample was visually inspected and appeared to be uniform over the entire substrate. The sample was taken directly from the growth chamber and immediately used for the cyclic voltammograms presented in figure 5.9. The scans show the behaviour to be irreversible.
The experiment was repeated three days later (figure 5.10) and seven days later (figure 5.11). On each of these subsequent occasions a fresh area of the diamond was exposed to the electrolyte.
A non-evacuated vacuum desiccator was used to store the electrode between experiments, so the surface was exposed to dried air. Both the electrodes and the aqueous solutions were stored in the dark between experiments.
Figures 5.10 and 5.11 show that the period of exposure to air corresponded to a change in the properties of the electrode. The cyclic voltammograms became much more reversible. Figure 5.12 compares the results for the 5 mVs-1 scans.
The change in the behaviour of the electrode over time is likely to be due to partial oxidation of the surface.
This explanation was supported by a simple test on the diamond. On removal from the CVD deposition chamber, a drop of deionised water was rapidly shed by the specimen. After three days, the sample had become significantly more hydrophilic and a drop of water would adhere to the surface more readily. This change from hydrophobic to hydrophilic behaviour suggests that the surface had become more polar.
5.10 CV of sample B107 in 3
mM K4[Fe(CN)6], 3 mM K4[Fe(CN)6]
& 1 M dm-3 KCl
Various scan rates, Ag dag contact, day 4, geometric area of working electrode = 7 mm2
5.6 Cyclic Voltammetry of Ferrous Sulphate and Ferric Sulphate
Figures 5.13 and 5.14 show cyclic voltammograms recorded using a highly doped diamond electrode (sample B107, doping level 5 ´ 1020 cm-3, silver dag contact) in an aqueous solution of 0.01 mol dm-3 iron (II) sulphate ( Fe(SO4) ), 0.005 mol dm-3 iron (III) sulphate ( Fe2(SO4)3 ) and 0.5 mol dm-3 sulphuric acid (H2SO4).
Prior to these experiments the diamond electrode was immersed in a chromic acid solution (sat. soln. of K2Cr2O7 in conc. H2SO4) in order to obtain an oxygen terminated surface.
The scans show that the kinetics appeared to be irreversible for this classic outer-sphere electron transfer system.
Figure 5.15 shows the results of the same experiment, repeated with a platinum working electrode. The reaction kinetics were reversible.
Figure 5.16 compares the diamond and platinum behaviour. Note that the diamond electrode possessed a much greater potential window that that seen for the platinum electrode.
5.15 CV of a platinum wire in 10
mM FeSO4, 5 mM Fe2(SO4)3 & 0.5
Various scan rates
Mott-Schottky plots show the reciprocal of the square of the apparent capacitance, 1/Cp2, against the applied potential, V. For a semiconductor under depletion conditions the plot will be linear. A positive gradient is characteristic of a n‑type semiconductor and a negative gradient is characteristic of a p‑type semiconductor. The donor density can, in principal, be calculated from the slope of the line. The flat band potential can be determined by extrapolating the line to the intercept of the V axis. 143
Figure 5.17 shows Mott-Schottky plots, recorded at different frequencies, for an as-prepared diamond electrode [B107] immersed in an indifferent electrolyte. The amplitude of the AC potential modulation was 5 mV.
The plots are linear over a potential range of ‑0.2 V to 0.2 V and have a common intercept at 1.7 V. This implies a more positive flatband potential for an oxygen terminated surface than that reported for hydrogen terminated diamond, a result consistent with the change in polarisation of the surface bond. It is noted that, although Mott-Schottky theory suggests that a plot of 1/C2 against applied potential should be independent of the measuring frequency, such results are infrequently observed for semiconductor electrodes. Surface roughness, dielectric relaxation and impedance across the sample surface have all been advanced as possible explanations of frequency dispersion in Mott-Schottky plots. 129
Prior to these experiments the diamond electrode was immersed in a chromic acid solution in order to obtain an oxygen terminated surface. It was noted that this procedure resulted in a considerable reduction in the surface conductivity of the film. It has been postulated that the change in conductivity with surface termination results from a change in band bending, with a pinned surface level at Ev = 1.7 eV in oxygenated samples. 130
5.17 Mott-Schottky plots recorded at various frequencies
in an aqueous solution of 0.5 mol dm-3 H2SO4
Straight line fits are shown for the region from -0.2 V to 0.3 V
5.8 AC Impedance
To gain further information on the mechanism of charge transfer at the electrode/electrolyte interface the impedance was recorded at a range of potentials. In figure 5.18, the plots of the electrochemical impedance at 0.4 V, near the open circuit potential are displayed. The amplitude of the AC potential modulation was 5 mV. At high over-potential the response shows a RC peak at high frequencies and a diffusional Warburg element at low frequencies, indeed the data may be fitted using the classic Randles equivalent circuit. 138 More interestingly, at the potential shown, two time constants are observed for the redox reaction, suggesting that the electron transfer does not occur via direct injection but in a two step process. The results point to a surface state mediated electron transfer process. Such a model has been proposed by van de Lagemaat for the electron transfer process at single crystal boron doped diamond electrodes. 123,124,131
The results of capacitance-voltage and current-voltage studies of Metal-Insulator-Semiconductor (MIS) diodes formed on oxygenated diamond indicate that at an oxygen terminated diamond surface the Fermi level is pinned at approximately 1.7 eV above the valence band. 132,133 The flat-band potential of 1.7 V obtained in these studies for the electrode immersed in 0.5 mol dm-3 H2SO4 would suggest that the surface states lie at approximately -0.5 V on the silver / silver chloride potential scale. This corresponds closely to the region in which electron transfer to the redox couple is observed and supports the surface state mediated electron transfer model. Further evidence for a surface state mediated process is the fact that in cyclic voltammetry studies of ferri/ferro-cyanide redox couple at oxygen and hydrogen terminated diamond indicate that the reaction is more facile at the oxygenated surface.
Plot of the modulus of the electrochemical impedance against frequency in an aqueous
solution of 0.5 mol dm-3 H2SO4, 0.01 mol dm-3 Fe(SO4) and 0.005 mol dm-3 Fe2(SO4)3
5.9 Moderately Doped Films
Cyclic voltammograms of the iron(II)/iron(III) redox couple and AC impedance measurements were repeated with diamond sample B102. B102 was grown for 6Ŋ hours in a chamber with no added diborane. However, the CVD chamber had previously been used to deposit diamond films with a boron concentration of 5 ´ 1021 cm-3 for a period of 11Ŋ hours.
Figures 5.20 and 5.21 show the cyclic voltammograms, which are similar to those obtained with an actively doped diamond film (c.f. figures 5.13 and 5.14).
Figures 5.22 to 5.23 show AC impedance results for sample B112. These are similar to those obtained for the actively doped film (c.f. figures 5.18 and 5.19).
A Bode plot is presented in figure 5.24. It clearly shows two time constants for the reaction. Two semicircles can be seen. The high frequency curve is independent of frequency while the low frequency curve varies with the frequency of the modulation on the applied potential.
Figure 5.25 shows that the behaviour is independent of concentration of electroactive species (Fe2+ and Fe3+).
Plot of the phase angle of the electrochemical impedance against frequency in an aqueous
solution of 0.5 mol dm-3 H2SO4, 0.01 mol dm-3 Fe(SO4) and 0.005 mol dm-3 Fe2(SO4)3
5.24 - Bode plot for sample B102 in an aqueous solution of
0.5 mol dm-3 H2SO4, 0.01 mol dm-3 Fe(SO4) and 0.005 mol dm-3 Fe2(SO4)3
5.25 - Bode plot for sample B102 in an aqueous solution of
0.5 mol dm-3 H2SO4, 0.01 mol dm-3 Fe(SO4) and 0.005 mol dm-3 Fe2(SO4)3
Diamond films with high to moderate doping levels (over 1020 cm-3) exhibited metallic behaviour. Oxidation and reduction peaks could be seen in cyclic voltammograms of several well known redox couples.
The surface termination of boron doped diamond electrodes was an important factor in the electrode kinetics. For oxygenated polycrystalline diamond electrodes two time constants were observed in impedance studies of a simple redox reaction. The result suggests that the electron transfer process was mediated by surface states, in agreement with a model proposed by van de Lagemaat 131 in single crystal studies.
PTFE is a polymer, polytetrafluoroethene ( -(CF2)n- ), that is commonly known by the trade name, Teflon. The properties of PTFE make it a suitable choice of material as it should remain inert in the presence of a wide range of electrolytes.
The BAS electrodes were filled with NaCl. The CH Instruments electrodes were filled with KCl. The choice of cation should not have effected the performance of the reference electrode in aqueous solutions. 126
A full list of growth conditions for each diamond specimen is provided in Appendix B.
Iron (II) sulphate is also known as ferrous sulphate.
Iron (III) sulphate is also known as ferric sulphate.