Uso de suelo

It has already been mentioned that initial studies of pBDD revealed its potential uses and significant advantages over other electrodes, however, the electrochemistry of pBDD is complex and subject to a number of factors. Growth conditions are undeniably important when attempting to produce films of high quality and purity; nitrogen is abundant in the atmosphere and is therefore hard to exclude from the growth chamber. The concentration of dopants such as boron has a pronounced effect on the electrical properties of diamond, in turn affecting its electrochemical performance. A perfect diamond crystal should be comprised of entirely sp3 hybridised carbon, though other forms of carbon bonding (sp2_{) can occur which changes the}

electrochemical response. The surface termination of pBDD may also affect electrochemical response, different chemical terminations may be realised by specific treatments with oxygen and hydrogen termination being the most common.

It was quickly discovered that as-grown (hydrogen terminated) CVD diamond possessed an unusually wide potential window,126 _{greater than that of HOPG and}

platinum. BDD therefore permits the study of processes that occur outside the potential windows of other electrodes, for example the Ce3+/Ce4+ couple.127 When poor quality pBDD was tested however, water electrolysis was observed to occur much faster, resulting in a narrower potential window as shown in Figure 1.11. This lower grade of pBDD contained appreciable amounts of sp2 _{carbon, as evidenced by Raman}

26

Figure 1.11 – Potential windows of a) high quality pBDD, b) low quality pBDD, c) platinum and d)

HOPG in 0.5 M H2SO4.128

A correlation exists between sp2 carbon content and overpotential for some electrochemical reactions, Bennett et al129 _{found that the presence of this non-diamond}

impurity reduced the kinetic overpotential for water electrolysis but had little effect on reversible outer-sphere mediators.

It was thought that the hydrogen terminated surface was responsible for the slow kinetics of water electrolysis, as the hydrophobic nature of this surface renders reactions that require adsorption unfavourable.128 _{However, oxygen terminated pBDD}

was found to give only a slightly better result in aqueous media than hydrogen terminated pBDD,130 indicating that the wide solvent window of pBDD was not due to the hydrogen terminated surface present on as-grown films. Slow reaction kinetics for the hydrogen and oxygen evolution reactions (OER, HER) are responsible for the wide potential window of BDD, requiring large overpotentials to drive them. Water reduction and oxidation are inner sphere processes, adsorption to the electrode is therefore required and the diamond surface structure is important. It can be concluded

27

then that neither the oxygen nor hydrogen terminated diamond surface is favourable for water adsorption in comparison to Pt or GC. Suffredini et al131 _{investigated the}

decomposition of water at both extremes of the potential window of BDD, calculating activation energies for the OER and HER as 106 and 150 KJmol-1 respectively.

As previously mentioned, hydrogen termination increases the surface conductivity of diamond by introducing charge carriers and surface electronic states. This added conductivity is superfluous when the boron concentration is sufficiently high (> 2 × 1020 cm-3), but may allow better than expected electrochemistry (faster electron transfer) for semiconducting diamond with lower doping levels. At lower boron concentrations for oxygen-terminated diamond, resistance effects hinder the kinetics of water electrolysis widening the potential window.

Another impressive quality of BDD that may be exploited electrochemically is its lack of reactivity to many different species, rendering diamond very chemically stable in a wide variety of media. For example, for the oxidation of cyanide ions, electrodes such as RuO2 and IrO2 are not stable and show low current efficiencies, however this is not

the case using pBDD. 132 Li et al133 proved BDD to be suitable for the deposition of lithium from a polymeric electrolyte; during deposition no alloys were formed on the BDD surface indicating that it does not promote underpotential deposition. Various studies in corrosive (acidic and alkaline) media have been performed demonstrating the ability of BDD to withstand harsh conditions. In 1989 Natishan and Morrish134 showed that undoped diamond grown on a molybdenum film protected the metal from corrosion and oxidation, the diamond was found not to degrade during anodic polarisation. Chen et al135 compared thin film BDD electrodes against traditional carbon electrodes for the generation of chlorine in solution of 1 M HNO3 and 2 M

28

respectively. No significant changes in morphology of the sp3 bonded carbon were observed in the BDD films, although surface oxidation and etching of non-diamond carbon was observed. Swain136 performed a series of analyses of carbon electrodes after potential cycling for 2 h in a solution of 1 M HNO3 and 0.1 M NaF at 50 oC,

observing again that the morphological structure of BDD is relatively unchanged whereas HOPG and GC suffered corrosion from pitting, oxidation and cavitation. A subsequent study by DeClements and Swain137 _{comparing both high and low quality}

BDD showed that preferential etching of non-diamond carbon at the grain boundaries occurred upon anodic polarisation in KOH for the low quality films. Conversely, no structural damage was seen for the high quality films, the authors ascribe this to the lack of non-diamond carbon present in these films.

Of particular importance in analytical electrochemistry are low background currents,122 which are manifested due to capacitance of the electrode/electrolyte interface and reduction/oxidation of the electrode itself. BDD possesses very low background currents, approximately one order of magnitude lower than that of GC.119,

138 _{This property allows improvement of signal-to-noise}139 _{and hence lower (better)}

detection limits by utilising BDD as an analytical tool.140, 141 _{The low background}

currents of BDD may be attributed to its low double layer capacitance. Highly ordered pyrolytic graphite (HOPG) is another form of carbon consisting of stacked layers of graphite (sp2 bonded carbon atoms), which possesses an even lower capacitance of approximately 2 µF cm-2,142 provided that no electrode degradation occurs.

It must not be forgotten that BDD is an extrinsic semiconductor, and at doping levels beneath that of metallic conductivity there is a low concentration of electronic states at the Fermi level. A space charge layer exists at the electrode/solution interface due to depletion of charge carriers at potentials less positive than Efb for p-type

29

semiconductors. Any potential drop occurs over the space charge layer resulting in a small potential gradient and hence small double layer capacitance. Even at BDD doped above the metallic threshold, the number of charge carriers is approximately 3 orders of magnitude less than that at a metal electrode.

30