McLaughlin, Maeve Helen Sorcha; (2021) Diamond electrodes for sensing in extreme environments. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Heavily boron doped diamond (BDD), which displays quasi-metallic properties, is the ultimate electrode material for high sensitivity detection in extreme environments. BDD has a unique combination of chemical, mechanical, and electrical properties. These are manipulated in this thesis to optimise the sensing capabilities of the BDD electrodes used. One exceptional property of BDD is its electrochemical window, which is the widest of any known material. This means that with a BDD sensor it is possible to detect the broadest range of chemical reactions. This parameter is well understood at room temperature. However, until the work presented in this thesis there was no systematic study investigating how it changes as the temperature of the electro chemical reaction is increased above 100 °C. Through comparison of polished and unpolished BDD electrodes (with hydrogen and oxygen surface terminations) it is determined that the electrochemical window of BDD electrodes narrows as temperatures increase. The corresponding activation energies are reported. Three methods of determining the precise size of the electrochemical window from experimental results are critically compared. The addition of catalytically active gold nanoparticles (AuNPs) have previously been shown to improve the sensitivity of BDD electrodes for mercury detection in aqueous environments. In this work, two AuNP sizes were deposited onto as grown and mechanically polished BDD electrodes, in pursuit of defining the optimum combination of these parameters to achieve the highest sensitivity for mercury detection. A novel method of producing small AuNPs (10 nm diameter) on the BDD surfaces was developed, using a TEM grid as a shadow mask. Exceptional sensitivity (pM) is achieved with the AuNP decorated polished BDD electrodes. Electrochemical impedance spectroscopy (EIS) is used to investigate the mechanism by which the AuNPs improve the sensitivity of the BDD electrodes. AuNP decorated BDD electrodes are directly compared to bare BDD electrodes (as grown and mechanically polished) under the same conditions as in the first electrochemical study. The performance of each electrode is assessed by their electron transfer rate (k0), alongside the capacitance and electron transfer resistance at their surface, recorded during mercury detection measurements.

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