These studies investigated the use of boron doped polycrystalline diamond as an electrode material. Diamond is a unique material with many extreme properties. Reliable, inexpensive methods of synthesis may be used to make diamond for a wide range of applications.
Pure diamond is a very good electrical insulator but the material may be doped with boron to produce electrodes with semiconducting or semimetallic properties. The use of diamond electrodes has been proposed for many electrochemical applications but the existing literature focuses on highly doped electrodes and does not fully explain the mechanism of the electron transfer across the diamond/electrolyte interface.
9.2 Growth and Characterisation
CVD apparatus has been assembled which can deposit boron doped polycrystalline diamond films with a wide range of doping levels from 130,000 p.p.m. to 6 p.p.m. (corresponding to doping levels from 2 ´ 1022 B atoms cm‑3 to 1 ´ 1018 B atoms cm‑3 †).
SIMS, SEM and optical microscopy have been used to show the films to be continuous and of high quality.
Diamond films have been deposited selectively onto patterned silicon wafers. The use of patterned silicon wafers with a partial titanium covering provided a novel method to fabricate boron doped diamond films with good Ohmic contacts.
9.3 Electrical Contacts
A variety of methods were used to make electrical contacts to the diamond films. For highly doped diamond films used over a limited potential range, simple silver dag contacts were sufficient to provide a linear response. Low doped films required more sophisticated contacts to avoid the formation of Schottky barriers.
Despite suggestions in the existing literature, evaporated gold contacts were found to be insufficient to provide Ohmic contacts. Three layer metal (3LM) contacts are a popular way of forming contacts to diamond. However, 3LM contacts were found to be difficult to fabricate and it proved impossible to prepare a good Ohmic contact in a standard laboratory environment. Titanium underlayer (TiUL) contacts provided a novel solution to the problem. They were relatively simple to fabricate and gave a linear response.
9.4 Standard Electrochemical Theory
Metal electrodes exhibit a symmetrical response to applied potential. Oxidation and reduction peaks can both be seen in cyclic voltammograms taken with metal electrodes.
p-type semiconductor electrodes are expected to show oxidative peaks, as there will be a forward bias at positive overpotential. The rate of reduction at negative overpotential should reach a limiting value where the total current density is equal to the equilibrium cathodic current density.
High doping levels can lead to degenerate doping conditions where the semiconductor will exhibit metallic behaviour.
An expression may be derived to relate the overpotential to the steady state current for surface-state mediated charge transfer at p-type semiconductors.
9.5 Electrochemistry of Highly and Moderately Doped Diamond Films
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 128 in single crystal studies.
Diamond samples were grown for a number of studies by French and German collaborators. An undergraduate project studied the possibility of detecting copper (Cu) underpotential deposition (UPD) of diamond in acidic environments.
9.6 Electrochemistry of Low Doped Diamond Films
At low doping levels, boron doped polycrystalline diamond films exhibited semiconductor behaviour and the electrochemistry was dependant on surface termination.
Titanium underlayer (TiUL) contacts did not require treatment of the sample after the diamond growth step. Ohmic contacts could therefore 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.
9.7 Electrochemical Theory for Boron Doped Diamond Films
The standard theories for metal or semiconductor electrochemistry, as outlined in chapter 4, are not sufficient to provide a model for the electrochemistry of boron doped polycrystalline diamond films.
The mechanism of charge transfer must be considered. The presence of surface states explains the difference in behaviour between hydrogen and oxygen terminated diamond films.
A relationship between the current density and applied potential has been derived. This agrees with the experimental results, which show that metallic behaviour predominates in highly doped diamond samples, while semiconductor behaviour is seen in low doped samples.
The surface state model may be applied to the AC impedance studies and an expression obtained that predicts the two time constants that have been seen in the experimental results.
9.8 Intensity Modulated Photocurrent Spectroscopy
IMPS results have been obtained for boron doped polycrystalline diamond electrodes. Values for wmax have been obtained for the Fe2+/3+ redox couple. The values of wmax are independent of DC potential and the choice of excitation wavelength (470 nm or 430 nm).
The surface state model developed for the AC impedance of the diamond electrodes has been extended to cover intensity modulated photocurrents. The results suggest that charge transfer occurs via a surface state.
9.9 Possible Future Work
This study has shown the electrochemical behaviour of boron doped diamond to be dependent on the boron doping level. At low doping levels, semiconductor behaviour has been detected.
Further investigations into the effect of concentration on the redox species could be performed to test the theory that has been developed (see section 6.6).
Future work on the electrochemistry of the films could look at a greater number of redox couples and electrolyte systems. In particular, the couples could be selected to cover a greater range of energy levels (as shown in figure 7.1).
In addition to the aqueous electrochemistry of diamond, the material should prove useful for non-aqueous electrochemical studies. Preliminary results show the diamond samples to perform well in organic electrolytes.
Electrochemical studies of highly oriented CVD diamond samples could provide an insight into the effect of the polycrystalline nature of the electrodes on the surface state mediated electron transfer.
Further study into the photoelectrochemistry of boron doped diamond could be performed. In particular more photocurrent spectroscopy could be performed to gain an insight into the surface states. Further IMPS work could be performed using a tuneable laser to replace the LEDs as the light source.
A number of applications for diamond electrodes have already been investigated, including: the detection of trace metals in various electrolytes; and the use of diamond in biological systems. The use of diamond electrodes for a wide range of electroanalytical and electrosynthetic reactions could be investigated.
The electrode geometry could be investigated. This study has only used macroscopically flat electrodes. CVD diamond may be deposited on a range of substrate geometries and so other electrode designs could be fabricated.
The diamond deposition process could be further studied. Surface analysis using techniques such as: SIMS profiling; Raman and photoluminescence spectroscopy at a range of different excitation wavelengths; and IR spectroscopy would provide a more detailed characterisation of the films.
In addition to boron doping recent work has shown that it may be possible to sulphur dope diamond. Studies could be performed to see if sulphur doped polycrystalline diamond acts as a n-type semiconductor at low doping levels. Interesting studies could also be performed by co-doping diamond with more than one dopant. Electrochemical studies could investigate diamond doped with varying quantities of boron, nitrogen and sulphur.
Studies could also investigate the effect of modifying the surface of the diamond with either halogens or organic species.
† assuming a proportional incorporation of boron and carbon atoms in the diamond film.