Chapter 1



1.0       Outline


·          This chapter outlines the history, properties and uses of natural and synthetic diamond.


·          The use of synthetic diamond as an electrochemical electrode is considered in terms of the properties of the material.


·          The existing literature on the electrochemistry of diamond is summarised.


·          The chapter concludes with an brief summary of the work presented in this thesis.


1.1       The History of Diamond


Diamond has been valued for its rarity for thousands of years. In the late 15th Century, polishing techniques were developed to produce brilliant gems which utilised the unique optical properties of diamond. In recent centuries, global industries have developed which are involved with the mining, trading, polishing and retailing of diamond gems. 1


The mining and processing of natural diamond produces a large proportion of diamond that is insufficiently aesthetically pleasing to be incorporated in to jewellery. A range of applications have been developed that put this by-product to practical use. Applications include those that utilise its hardness (e.g. grinding, cutting and polishing), its thermal properties (e.g. heat sinks) and its optical properties.

Naturally occurring diamond of terrestrial origin is formed at high temperatures and pressures in the Earth’s mantle. Volcanic action is then responsible for carrying diamond into the Earth’s crust. 3 Early finds of diamond were confined to isolated crystals that had become exposed on the surface of the planet. However, since the discovery of significant quantities of diamonds in South Africa in the 1870s, industrial mining techniques have been developed. Diamond remains a rare mineral but deposits have now been located at various locations spread widely across the planet.


With the beginnings of modern chemical studies in the late 18th Century came the first indication of the composition of diamond. In 1796, Smithson Tennant studied the combustion of diamond and showed it to be of the same composition as other forms of carbon. 4


The confirmation of the structure of diamond came from X-ray crystallography studies in the early part of the twentieth century. 5 Figure 1.1 shows the structure and symmetry of diamond.


1.2       Allotropes of Carbon


There are two main allotropes of carbon: diamond and graphite.




·          sp3 bonding.


·          carbon atoms forming tetrahedral bonds with four nearest neighbours.


·          crystals with a cubic structure (see figure 1.1).




·          sp2 bonding.


·          carbon atoms forming three strong coplanar bonds with neighbouring atoms.


·          planes of carbon atoms in a hexagonal arrangement.


·          unhybridised 2p orbital electron associated with each carbon atom creates p bonding between the neighbouring carbon atoms in the plane (see figure 1.2).


·          only weak bonding between the planar layers of atoms.


·          graphite is the most common allotrope of carbon.


In 1985, a new class of stable carbon molecules, known as fullerenes, was discovered. 6 They are based upon sp2 bonding configurations where the introduction of larger or smaller rings of carbon atoms (heptagons and pentagons) into a layer of graphite hexagons causes the structure to bend out of plane. A vast number of interestingly shaped molecules are possible including the highly symmetrical C60, other ball-like structures 6,7 and even tube-like structures. 8 The structure of C60 is represented in figure 1.3.


The different bonding in diamond and graphite give the materials dramatically different properties. For example, the strong rigid bonding in diamond makes the material mechanically hard and electrically resistive. This contrasts with graphite, where the weak interactions between graphitic planes make the material soft and the delocalised p electrons make the material electrically conductive in directions parallel to the plane.


Figure 1.1

The structure of diamond


Figure 1.2

The structure of graphite
(reproduced from reference 9)


Figure 1.3

Buckminster Fullerene, C60


The very significant differences between the properties of diamond and graphite are an important consideration because small amounts of graphitic sp2 impurities in a diamond crystal can dramatically alter the properties of the material.


In addition to crystalline carbon, forms of amorphous carbon exist containing both sp2 and sp3 bonding in varying ratios. These materials often contain a significant proportion of hydrogen in their structures. 10,11


Graphite is the thermodynamically stable form of carbon at room temperature and pressure. Diamond is only more thermodynamically stable than graphite at temperatures greater than 1300 °C and pressures greater than 40 kilobar. There is only a small difference in the thermodynamic stability of the two allotropes. At a temperature of 298 K and a pressure of 1 standard atmosphere, the standard Gibbs free energy of formation of diamond 12 is 2.9 kJ mol –1. A phase diagram for carbon is shown in figure 1.4.


There is no easy rearrangement mechanism by which diamond can convert to graphite. The energetic activation barrier for conversion is very high and the conversion is therefore kinetically unfavourable. Hence, diamond will remain in a meta-stable state at room temperature and pressure without converting to graphite.



Figure 1.4

The phase diagram of carbon

(reproduced from reference 9)


1.3       Properties of Diamond


Diamond has a unique range of extreme properties which have been reviewed extensively elsewhere. 5,13,14 A summary of properties is shown in Table 1.1.


A number of these properties make diamond a promising choice of electrode material:


·          Diamond has limited reactivity and may be used in a wide range of harsh environments.


·          Diamond is chemically inert with respect to organic chemicals under normal conditions. Diamond is therefore biologically compatible and so diamond electrodes may be used as sensors in biological and medical experiments.


·          The transparency of diamond over a broad spectral range allows for electrochemical and spectroscopic studies to be combined. 15


·          The doping of diamond with various concentrations of boron allows the electrical properties of a device to be tailored.


·          CVD diamond techniques allow thin layers of diamond to be deposited on to other materials and therefore the material may be incorporated into electronic devices.


·          Diamond has a wide potential window which allows many electrochemical reactions to occur (see section 1.6).


·          The high thermal conductivity of diamond allows for greater temperature control at the surface of the electrode.


·          Diamond is radiation hard and devices have been fabricated that behave in a stable manner while being irradiated. 16


                            Property                                                      Value                    


                             Hardness                                             1
´ 10 4  kg mm -2
                        Tensile Strength                                              > 1.2  GPa                 
                   Compressive Strength                                        > 100  GPa

            Dynamic Coefficient of Friction                                       0.03
                        Sound Velocity                                          1.8
´ 10 4  m s-1
                              Density                                                   3.52  g cm-3

                      Young’s Modulus                                             1.22  GPa
                        Poisson’s Ratio                                                    0.2
             Thermal Expansion Coefficient                                 1
´ 10 -6  K-1
                    Thermal Conductivity                                     20.0  W cm-1 K-1
                Thermal Shock Parameter                                 3.0
´ 10 8  W m-1
                     Debye Temperature                                            2200  K
               Optical Index of Refraction                                          2.41
at 591 nm
                         Optical Transmissivity                                               225
from deep ultra-violet to far infra-red
                         Loss Tangent at 40 Hz                                         6.0 x 10 –4
                     Dielectric Constant                                                 5.7
                      Dielectric Strength                                      1.0
´ 10 7  V cm –1
                       Electron Mobility                                     2200  cm 2  V –1 s  1
                         Hole Mobility                                         1600  cm 2  V –1 s –1
               Electron Saturated Velocity                                2.7
´ 10 7  cm s-1
                  Hole Saturated Velocity                                   1.0
´ 10 7  cm s-1
           Work Function on [111] surface                                  “negative”
                             Bandgap                                                     5.45 eV
            Resistivity of undoped diamond                                ~ 10 16 
W cm

                         Heat Capacity                                          6.195  J mol-1 K-1
               Boron Diffusivity at 1073 K                              6.9
´ 10 –20  cm2 s-1



Table 1.1

Selected properties of diamond

(figures are for single crystal diamond)

(values measured or calculated at room temperature unless otherwise stated)


1.4       The Synthesis of Diamond


Early experiments to produce synthetic diamonds used a high pressure, high temperature (HPHT) process that produced diamond under conditions where it was the thermodynamically stable form of carbon. This technique was pioneered by the General Electric company of America in 1955. 13  HPHT grown diamond tends to consist of small crystals of a few millimetres in diameter.


Chemical vapour deposition (CVD) is a process that can be used to synthesise diamond at much lower temperatures and pressures under non-equilibrium conditions. The technique is now established and has been well described in a number of review articles. 11,17,18


Chapter 2 outlines the CVD process used in these studies.


1.5              Uses of Diamond


Many applications of diamond that make use of the extreme physical properties of the material are in use or have been proposed. 19 A number of these applications are outlined below:


1.5.1    Abrasives


Existing uses of natural and HPHT diamond include abrasion, cutting and polishing. These applications exploit a number of the properties of the material including: hardness, thermal conductivity, lack of chemical reactivity and the low coefficient of friction for hydrogen terminated diamond. The high cost of diamond restricts its use to heavy duty industrial tools, such as diamond saws and precision tools, such as scalpels and dental drills.


CVD techniques can be used to coat diamond onto carbide forming metals but widespread use of diamond coated tools is prevented by the mismatch in the thermal expansion coefficients of diamond and the underlying metals. Recent studies have investigated the feasibility of reducing the temperature required for CVD diamond deposition. A reduction in the deposition temperature may widen the range of materials that may be coated and permit a greater number of diamond tools to be produced.


CVD diamond has been used to replace natural diamond in a number of applications where control of the geometry can improve performance of the device. For example, a range of scalpels specifically designed for eye surgery have been produced. 20


While the hardness of diamond has not yet been utilised in cutting tools produced for the mass market, hard amorphous carbon films, which can be deposited at room temperature have been developed for use on razor blades. 21


1.5.2    Thermal Management


Diamond has a very high thermal conductivity and it is an electrical insulator. This makes it a suitable material for thermal management applications. The use of diamond as a heat sink for integrated circuits has the potential to allow closer packing of features and lower operating temperatures. Diamond is currently used as a heat sink for specialist applications such as semiconductor laser diodes, microwave diodes and small microwave integrated circuits. 14,22


1.5.3    Electronic Devices


The extreme properties of diamond would be of great benefit if the material could be incorporated into electronic devices. Undoped diamond is an electrical insulator with a wide-bandgap (5.45 eV) but the electrical properties of diamond, like other group IV elements, may be altered by the addition of impurities.

Text Box: band gap = 5.45 eV



















B = boron

N = nitrogen

Li = lithium

V = vacancy


Figure 1.5

An energy diagram of selected states in the band gap of diamond

(one dimensional representation)


Boron doping can generate p-type behaviour in the material. The substitutional boron dopant atoms form an acceptor levels at ~ 0.35 eV above the valence band as shown if figure 1.5. 5 At low doping levels, the diamond acts as an extrinsic semiconductor. At high doping levels the material acts as a semi-metal. 23,24


Diamond has a close packed rigid structure which does not readily accept substitutional impurity atoms. Doping with phosphorous and arsenic has not successfully generated n-type dope diamond. Current research is investigating the electrical properties of sulphur doped diamond. 25,26


The use of CVD diamond films for semiconductor devices is hampered by the polycrystalline nature of the films. The numerous grain boundaries and crystal defects reduce electron and hole mobilities and generally degrade electronic performance. Therefore only limited success has been achieved the production of electronic devices. 27 Greater success has been achieved in fabricating sensors 16,28 and electrodes. 29


The low or negative electron affinity that is observed in diamond is a property that it may be possible to exploit in field emission displays. 30,31


1.5.4    Optical Windows


Diamond has a broad band transparency and is across the visible, infra-red and ultra-violet regions of the spectrum. In particular, diamond is transparent in the wavelength range from 8 mm to 12 mm. This corresponds to an atmospheric “window” where there is no significant absorption of infra-red radiation by molecules in the atmosphere.


Many applications, such as thermal imaging, require infra-red windows. Many common materials used for these windows have drawbacks to their use. For example, zinc sulphide (ZnS) is too soft to be used in hostile environments. CVD diamond can be used to coat infra-red windows to give then a hard, thermally conductive outer layer. Alternatively, free standing CVD grown diamond films could be used as a wide spectrum window giving transparency over a range of wavelengths.


There are currently a number of technical problems that must be overcome before the optical properties of diamond can be fully exploited:


·          Growth of CVD diamond on planar substrates is well established. Growth of diamond onto substrates with more interesting geometries, such as convex surfaces, is less common. Many optical applications require non-planar shapes.


·          CVD grown diamond has a rough top surface due to the polycrystalline nature of the films. Optical applications require smooth surfaces and the grinding of diamond to smooth its surface is problematic due to its extreme hardness.


·          Impurities and defects in the crystals alter the optical properties of diamond. The common classification of natural diamond as Class I or Class II was established in 1934 and was based upon transparency to 8 mm infra-red radiation. 5  It is now known that nitrogen impurities are responsible for making Class I diamond opaque to 8 mm radiation. Low concentrations of impurities can be tolerated but most applications require constant predicable performance. It is therefore important to ensure that any impurities are at low levels, evenly distributed and in known forms.


·          The reactivity of diamond in hot, oxygen containing environments limits the use of diamond in the aerospace industry.


·          Diamond films can too be brittle for practical use.


1.6       The Electrochemistry of Boron Doped Diamond


Carbon based electrode materials are well established and have been used in varied electrochemical technologies from electroanalysis to energy storage. 23 The structure of existing electrode materials varies but they are all based upon graphitic (sp2) bonding schemes, for example: glassy carbon, graphite, carbon fibres.


As summarised in section 1.3, diamond possesses a range of properties that may make it useful for electrochemical applications. The advent of relatively inexpensive CVD diamond techniques has generated great interest in the electrochemical applications of diamond. 29


The first electrochemical experiments on CVD diamond were published in 1987 32 but work was hampered by poor understanding and characterisation of the non-diamond carbon and dopant atom content of the films. Early experiments performed with boron doped diamond electrodes studied heavily doped samples that gave promising results. Xu et al. (1997) 23 published a review of the early results which is summarised below:


·          low background currents
(an order of magnitude less than for glassy carbon)


·          low double layer capacitance
(an order of magnitude less than for glassy carbon)


·          featureless response in 0.1 M KCl from -1000 mV to 1000 mV
(current density < 50 mV/cm2)


·          wide potential window 33
(large overpotential for the evolution of chorine, oxygen and hydrogen)


Three possible factors that may explain the behaviour are given:


1.      an absence of electroactive carbon-oxygen functionalities on the hydrogen-terminated surface.


2.      a low density of surface electronic states near the Fermi level caused by the semimetal-semiconductor nature of boron doped diamond.


3.      the diamond surface acts as an array of microelectrodes with many separate “electrochemically active” regions separated by more insulating regions.


The possibility of an inert working electrode with a wide potential range with a low background current has generated a number of papers outlining possible uses for diamond electrodes. A selection of the available literature is listed in table 1.2.


First Named

Author & Year



Tenne, R.



Efficient electrochemical reduction of nitrate to ammonia using conductive diamond film electrodes

Katsuki, N.



Electrolysis by using diamond thin film electrodes

Beck, F.



Boron doped diamond/titanium composite electrodes for electrochemical gas generation from aqueous electrolytes

Bouamrane, F.



Underpotential deposition of Cu on boron-doped diamond thin films

Compton, R. G.



Sonoelectrochemical production of hydrogen peroxide at polished boron-doped diamond electrodes

Katsuki, N.



Water electrolysis using diamond thin-film electrodes

Yano, T.



Electrochemical behavior of highly conductive boron-doped diamond electrodes for oxygen reduction in alkaline solution

Agra-Gutiérrez, C.



Anodic stripping voltammetry of copper at insonated glassy carbon-based electrodes: application to the determination of copper in beer


Table 1.2a

A sample of the literature on the applications of diamond electrochemistry


First Named

Author & Year



Fujishima, A.



Electroanalysis of dopamine and NADH at conductive diamond electrodes

Granger, M. C.



Polycrystalline diamond electrodes: basic properties and applications as amperometric detectors in flow injection analysis and liquid chromatography

Manivannan, A.



Detection of trace lead at boron-doped diamond electrodes by anodic stripping analysis

Nakabayashi, S.



Dye sensitization of synthetic p-type diamond electrode

Okino, F.



Electrochemical fluorination of 1,4-difluorobenzene using boron-doped diamond thin-film electrodes

Popa, E



Selective electrochemical detection of dopamine in the presence of ascorbic acid at anodized diamond thin film electrodes

Saterlay, A. J.



Sono-cathodic stripping voltammetry of manganese at a polished boron-doped diamond electrode: application to the determination of manganese in instant tea

Vinokur, N.



Cathoidic and anodic deposition of mercury and silver at boron-doped diamond electrodes

Yano, T.



Electrochemical behavior of highly conductive boron-doped diamond electrodes for oxygen reduction in acid solution

Yoshihara, S.



Photoelectrodeposition of copper on boron-doped diamond films: application to conductive pattern formation on diamond. The photographic diamond surface phenomenon

Beck, F.



Boron doped diamond (BDD)-layers on titanium substrates as electrodes in applied electrochemistry

(paper briefly reviews waste water treatment and electro-synthesis)

Fujishima, A.



TiO2 photocatalysts and diamond electrodes

(paper describes the detection of histamine, the detection of the coenzyme NADH and the trace analysis of lead)

Michaud, P. -A.



Preparation of peroxodisulfuric acid using boron-doped diamond thin film electrodes

Rao, T. N.



Recent advances in electrochemistry of diamond

(paper describes the detection of histamine and sulfadiazine)

Wang, J.



Incorporation of Pt particles in boron-doped diamond thin films: Applications in electrocatalysis

(considers the underpotential deposition of hydrogen and oxidation of methanol)


Table 1.2b

A sample of the literature on the applications of diamond electrochemistry

Despite the widespread attempts to utilise diamond for electrochemical applications, the mechanism of the electron transfer across the diamond/electrolyte interface still needs to be determined.


Early experiments investigating the cyclic voltammetry and AC impedance analysis of boron doped diamond used samples with uncontrolled or undisclosed levels of boron doping and focused on the effects of non-diamond content in the films. 58,59


AC impedance studies of highly doped single crystal diamond (homoepitaxial layers grown on natural diamond) have shown that the charge transfer was mediated by undetermined surface states. 60


1.6              Summary


Diamond is a unique material with many extreme properties. The advent of reliable, inexpensive methods of synthesis could lead to the use of diamond in 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. The existing literature focuses on highly doped electrodes and does not fully explain the mechanism of the electron transfer across the diamond/electrolyte interface.


1.7       Outline of the Thesis


Table 1.3 summarises the contents of the chapters in this thesis.


Chapter 1



·    This chapter outlines the history, properties and uses of natural and synthetic diamond.

·    The use of synthetic diamond as an electrochemical electrode is considered in terms of the properties of the material.

·    The existing literature on the electrochemistry of diamond is summarised.

·    The chapter concludes with an brief summary of the work presented in this thesis.

Chapter 2



·    This chapter describes the diamond growth method used in this study and explains why it was chosen.

·    The main components of the diamond growth apparatus are described.

·    The benefits of four different types of electrical contact are compared.

·    Scanning electron microscopy, optical microscopy and Raman spectroscopy were used to characterise the films and the results are presented here.

Chapter 3


Electrical Contacts

·    This chapter describes the electrical contacts between the diamond electrodes and the copper connecting wires.

·    The requirement for Ohmic contacts is discussed.

·    Details are given for the fabrication processes for two types of Ohmic contact: 3LM contacts and TiUL contacts. The practicality of the two processes for use in the laboratory are compared.

·    Current-voltage plots are presented for the four types of contacts described in chapter 2. A comparison is made between the performance of the four types of contact.

Chapter 4


Standard Electrochemical


·    This chapter outlines the standard electrochemical theory on which this study was based.

·    A comparison is made between the behaviour of metal and semiconductor electrodes.

·    An expression is derived relating the steady state current to the overpotential.

Chapter 5



Electrochemistry of Highly Doped Diamond Films

·    This chapter contains details of the apparatus used for the electrochemical experiments in these studies.

·    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.

·    The effect of surface termination was investigated.

·    AC Impedance and Mott-Schottky plots are presented.

Chapter 6


The Electrochemistry of Low Doped Diamond Films


·    The chapter describes the electrochemistry of low doped diamond films. Cyclic Voltammogams are presented for a number of well known redox couples.

·    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.

Chapter 7

Theoretical Model for the

Electrochemistry of Boron Doped Diamond

·    This chapter develops a theoretical model for the electrochemistry of boron doped diamond.

·    The relationship between current density and applied potential is investigated

·    The AC impedance results are considered in terms of the surface state model.

Chapter 8

Intensity Modulated Photocurrent Spectroscopy

·    This chapter presents the results of Intensity Modulated Photocurrent Spectroscopy (IMPS) experiments on boron doped diamond films.

·    A theory is presented to explain the results.


Chapter 9




·    The main results of the research are summarised.

·    Suggestions for possible future work are presented.

Table 1.3  -  outline of the chapters in this thesis



The proportion of mined diamond that is put to non-gem use is estimated to be 80%. 2

This amorphous carbon is currently marketed as “a diamond” by a well known razor blade manufacturer

see also reference 39