Chapter 5
The Electrochemistry of Highly Doped Diamond Films
5.0 Outline
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:
1.
soaking
in a base bath
[saturated sodium hydroxide (NaOH) in ethanol(C2H5OH)];
2.
immersing
and rinsing in deionised water;
3.
soaking
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.
5.1.5 Potentiostats
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.
Figure
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.
Figure
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.
Figure
5.15 CV of a platinum wire in 10
mM FeSO4, 5 mM Fe2(SO4)3 & 0.5
M H2SO4
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
Figure
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.
Figure
5.18
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+).
Figure
5.23
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
Figure
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
Figure
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
5.10 Summary
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.