Chapter 6
The Electrochemistry of Low Doped Diamond Films
6.0 Outline
·
The
chapter describes the electrochemistry of low doped diamond films. Cyclic
Voltammogams were recorded with a number of well known redox couples to show
the behaviour of the films.
·
The
effect of surface termination of the electrode was investigated.
·
The
effect of the concentration of the electroactive species was studied.
·
The
results of the electrochemical experiments are explained in terms of the theory
outlined in chapters 4 and 7.
6.1 Experimental Set-up
The apparatus and procedures
used were as outlined in section 5.1.
6.2 Surface Termination
To obtain Ohmic contacts to
the diamond working electrodes, three layer metal contacts (3LM) and titanium
underlayer contacts (TiUL) were used.
The fabrication process for the formation of the 3LM contacts necessitated that
the surface of the diamond was oxidised. TiUL
did not require any special treatment of the diamond samples after they had
been grown, so experiments could be performed on both native, hydrogen
terminated specimens and oxidised specimens.
6.3 Cyclic
Voltammetry of oxygen-terminated low-doped diamond
6.3.1 Cyclic Voltammetry of
Ferrous Sulphate and Ferric Sulphate
Sample B131a was grown in a
hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 37 p.p.m. in the
gas phase (corresponding to a boron doping level of 7 ´ 1018 cm-1 if it is
assumed that carbon and boron are incorporated into the diamond film in
quantities proportional to their gas phase concentrations).
A 3LM contact was then
applied to the surface of the diamond sample following the procedure outlined
in table 3.1.
Figures 6.1 and 6.2 show
cyclic voltammograms of sample B131a recorded in a aqueous solution containing
0.5 mol dm-3 H2SO4,
0.01 mol dm-1 FeSO4 and 0.005 mol dm-1
Fe2(SO4)3.
The oxygen-terminated
low-doped diamond sample exhibited irreversible behaviour with the Fe2+/3+
redox couple. A distinct forward, oxidative peak can be seen but there is no
clear reverse, reductive peak. These experimental results fit with the theory
presented in the next chapter. At low doping levels, semiconductor behaviour is
to be expected.
Figure 6.3 shows a cyclic
voltammogram recorded with a platinum (Pt) wire working electrode in the same
solution. Reversible electrochemistry is exhibited by the Fe2+/3+
redox couple at this metal electrode. Figure 6.4 shows a plot for diamond
sample B131a superimposed on the scan for the platinum working electrode.
6.3.2 Cyclic
Voltammetry of Potassium Ferrocyanide
and Potassium Ferricyanide
Figures 6.5 and 6.6 show
cyclic voltammograms of sample B131a recorded in a aqueous solution containing
1 mol dm-3 KCl, 3 ´ 10-4 mol dm-1
K3Fe(CN)6 and 3 ´ 10-4 mol dm-1
K4Fe(CN)6.
The oxygen-terminated
low-doped diamond sample exhibited irreversible behaviour with the Fe(CN)63-/4-
redox couple. A distinct forward, oxidative peak can be seen but there is no
clear reverse, reductive peak.
Figure 6.7 shows a cyclic
voltammogram recorded with a platinum (Pt) wire working electrode in the same
solution. Reversible electrochemistry is exhibited by the Fe(CN)63-/4-
redox couple at this metal electrode. Figure 6.8 shows a plot for diamond
sample B131a superimposed on the scan for the platinum working electrode.
Sample B129b was grown in a
hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 50 p.p.m. in the
gas phase (corresponding to a boron doping level of 9 ´ 1018 cm-1 assuming the
solid phase concentration is proportional to the concentration in the gas
phase).
A 3LM contact was then
applied to the surface of the diamond sample following the procedure outlined
in table 3.1.
Figures 6.9 and 6.10 show
cyclic voltammograms of sample B129b recorded in a aqueous solution containing
1 mol dm-3 KCl, 3 ´ 10-4 mol dm-1
K3Fe(CN)6 and 3 ´ 10-4 mol dm-1
K4Fe(CN)6.
The oxygen-terminated
low-doped diamond sample exhibited irreversible behaviour with the Fe(CN)63-/4-
redox couple. A distinct forward, oxidative peak can be seen but there is no
clear reverse, reductive peak.
6.3.3 Cyclic
Voltammetry of Other Redox Couples
Attempts were made to study
other well know redox couples. Experiments were performed with europium
sulphate ( Eu2(SO4)3 ) and cobalt
sulphate (CoSO4). Neither of these species were detected at the
diamond electrode.
Figures 6.11 and 6.12 show
cyclic voltammograms of sample B134b recorded in a aqueous solution containing
0.5 mol dm-3 H2SO4, 1 ´ 10-5 mol dm-1
Eu2(SO4)3. The scans show a small forward peak
which was due to contamination by Fe(CN)64- ions. There is no evidence of
europium electrochemistry at the electrode.
Contamination peaks could be
removed by thorough cleaning of the apparatus.
No hypothesis has yet formed
to explain the lack of response of Eu3+ and Co2+ ions at
diamond electrodes.
6.4 Cyclic Voltammetry of hydrogen-terminated
low-doped diamond
6.4.1 Cyclic
Voltammetry of Potassium Ferrocyanide
and Potassium Ferricyanide
Sample B144a was grown in a
hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 50 p.p.m. in the
gas phase (corresponding to a boron doping level of 9 ´ 1018 cm-1).
A TiUL contact was formed as outlined in table 3.2.
Figures 6.13 and 6.14 show
cyclic voltammograms of sample B144a recorded in a aqueous solution containing
1 mol dm-3 KCl, 0.01 mol dm-1 K3Fe(CN)6
and 0.01 mol dm-1 K4Fe(CN)6.
The hydrogen-terminated
low-doped diamond sample exhibited considerably more reversible behaviour than
was seen for the oxygen terminated samples. The scans are approximately
symmetrical with clearly defined forward and reverse peaks.
If the electrode was not
exposed to oxidising environments then the reversible behaviour could be
maintained. Figures 6.15 and 6.16 show
a repeat experiment with sample B144a. The scans were performed two days after
those shown in figures 6.13 and 6.14.
To maintain the unoxidised surface, high potentials and basic
environments were avoided.
It should be noted that iR compensation was not used in these
experiments. There was a potential drop across the electrolyte and so
reversible voltammograms exhibited a peak separation of greater than 59 mV.
6.5 Cyclic Voltammetry of low-doped diamond with indeterminate surface termination
The results presented in section
6.3 showed electrochemistry at a well oxidised surface (diamond samples B129b
and B131a).
Section 6.4 showed the
results for a hydrogen terminated diamond surface (sample B144a).
In this section, results are
presented for a series of experiments which were performed on a sample that had
not been fully oxidised.
Sample B134b was grown in a
hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 37 p.p.m. in the
gas phase (corresponding to a boron doping level of 7 ´ 1018 cm-1).
A 3LM contact was applied to
the surface of the diamond film. Only
the surface of the film that was to be placed under the metal contact was
exposed to the full chromic acid treatment.
This left the surface of the diamond that was to be used as electrode
unoxidised. However, it should be noted that the other post-treatments required
to fabricate a 3LM contact may have altered the surface termination of the
diamond.
6.5.1 Cyclic Voltammetry of
Ferrous Sulphate and Ferric Sulphate
Figures 6.17 and 6.18 show
cyclic voltammograms of sample B131a recorded in a aqueous solution containing
0.5 mol dm-3 H2SO4,
0.01 mol dm-1 FeSO4 and 0.005 mol dm-1
Fe2(SO4)3. A forward peak can be seen but no
reverse peak can be seen.
6.5.2 Cyclic
Voltammetry of Potassium Ferrocyanide
and Potassium Ferricyanide in an Aqueous Solution of Potassium Chloride
Figures 6.19 and 6.20 show
cyclic voltammograms of sample B134b recorded in a aqueous solution containing
1 mol dm-3 KCl, 0.01 mol dm-1 K3Fe(CN)6
and 0.01 mol dm-1 K4Fe(CN)6. The
scans showed considerable peak separation.
Both forward and reverse peaks could be seen.
The electrochemical response
changed with time and figure 6.21 compares two cyclic voltammograms recorded at
a scan rate 200 mV/s. The first plot shows an early scan where forward and
reverse peaks can both be seen. The second plot shows a later scan where the
surface has become oxidised and the reverse peak is no longer visible. This
change may be due to the oxidation of the diamond surface at high potentials (up
to 2 V vs. Ag|AgCl (3 M Cl-) ).
The electrochemical cell was
adjusted to expose a fresh area of the diamond sample to the electrolyte
solution. The reverse peak could again be seen, as shown in figures 6.22 and
6.23.
Figure 6.22 – CV of sample
B134b in 1 mol dm-3 KCl, 0.01 mol dm-1
K3Fe(CN)6 and 0.01 mol dm-1 K4Fe(CN)6, various scan rates, 3LM contact geometric
area of working electrode = 7 mm2
6.5.3 Cyclic
Voltammetry of Potassium Ferrocyanide
and Potassium Ferricyanide in an Aqueous Solution of Potassium Hydroxide
Figures 6.24 and 6.25 show
cyclic voltammograms of sample B134b recorded in a aqueous solution containing
1 mol dm-3 KOH, 0.01 mol dm-1 K3Fe(CN)6
and 0.01 mol dm-1 K4Fe(CN)6.
The presence of hydroxide
ions in the electrolyte increased the rate of oxidation of the films and the
electrochemical response rapidly became irreversible. Figures 6.26 and 6.27
show cyclic voltammograms recorded in a aqueous solution containing
1 mol dm-3 KCl, 0.01 mol dm-1 K3Fe(CN)6
and 0.01 mol dm-1 K4Fe(CN)6. The two figures compare the response before
and after exposure to the basic solution. A reverse, reductive peak can be seen
prior to the exposure to a basic medium. In the cyclic votammograms recorded
after exposure, this reverse peak is not well defined.
Figures 6.28 to 6.32 show
cyclic voltammograms recorded with a platinum (Pt) wire working electrode.
6.6 Concentration
Effects
The theory presented in chapter 7 suggests that at low concentrations of electrocactive species, there should be a switch from semiconductor behaviour to metal behaviour as the rate determining step changes from the electron transfer across the space charge region to the electron transfer across the Helmholtz layer.
Diamond sample B146b was
grown in a hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 50
p.p.m. in the gas phase (corresponding to a boron doping level of 9 ´ 1018 cm-1).
A TiUL contact was formed as outlined in table 3.2.
The sample was given to an
undergraduate project student who performed a series of experiments to
investigate the effects of concentration on the electrochemistry of the film.
Figure 6.33 shows the
results of the experiments.
The inset graph shows a
cyclic voltammogram of sample B146b recorded in a aqueous solution containing
0.1 mol dm-3 KCl, 0.01 mol dm-1 K3Fe(CN)6
and 0.01 mol dm-1 K4Fe(CN)6 with a
scan rate of 100 mV/s. The scan showed
a similar reversible response to that shown in section 6.4.
After the sample had been
oxidised by exposure to various electrolytes, the reverse peak became less well
defined. A series of different solutions were then used. The concentration of
the electroactive species was varied from 1.5 ´ 10-3 mol dm-3
to 0.094 ´ 10-3 mol dm-3.
No clear switch to
reversible behaviour was seen. However, the presence of impurities in the
solution became significant at low concentrations. This may have masked any
effect.
6.7 Mott-Schottky Plots
Diamond sample B134b was
grown in a hydrogen/methane/diborane atmosphere with a [B]/[C] ratio of 50
p.p.m. in the gas phase (corresponding to a boron doping level of 9 ´ 1018 cm-1).
A 3LM contact was then
applied to the surface of the diamond sample following the procedure outlined
in table 3.1.
Figure 6.34 shows
Mott-Schottky plots, recorded at different frequencies, for sample B134b
immersed in an indifferent electrolyte (1 mol dm-3 KCl). 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. These results are comparable to those for B107 as presented in section 5.7.
6.8 Summary
At low doping levels, boron
doped polycrystalline diamond films exhibited semiconductor behaviour.
At low doping levels, the
electrochemistry of boron doped polycrystalline diamond films was dependant on
surface termination.
Titanium underlayer (TiUL) contacts allowed Ohmic contacts to be
made to hydrogen terminated diamond electrodes.
Oxygen terminated diamond
samples exhibited irreversible behaviour with an absence of a reverse reductive
peak.
Hydrogen terminated samples
were more reversible. Both forward and reverse peaks were visible in cyclic
voltammograms.
Mott-Schottky plots for
oxidised diamond samples showed a flatband potential corresponding to a surface
state at an energy level at Ev
= 1.7 eV.