Surface chemical and particle modification of diamond electrodes

Boron-doped diamond electrodes (BDDs) offer a range of advantageous properties for electrochemical sensing. They are normally used with a chemical surface termination comprised of oxygen species, but hydrogen terminations are also stable. A hydrogen surface termination offers different hydrophobicity, band alignments and surface conductivity of the BDDelectrolyte interface. The broad aim of this thesis is therefore to establish the influence of surface termination on the electrochemical sensing properties of BDDs. The work also encompasses the properties of BDD electrodes decorated with metal, metal hydroxide and two types of carbon black particles that introduce additional chemical functionality. The structural and chemical properties of the prepared BDD interfaces are examined with a range of experimental techniques including SEM, AFM and XPS. This information is then related to electrochemical performance as measured using cyclic voltammetry and other dynamic electrochemical methods.
The first study explores the use of gold nanoparticles on BDD for mercury sensing. BDD decoration was achieved by either electrochemical deposition or by drop coating preformed nanoparticles onto hydrogen- and oxygen-terminated BDDs. Although the deposition methods affected characteristics such as morphology and dispersion of the gold nanoparticles on the BDDs, the BDD surface termination was of greater importance than the deposition method for the purposes of mercury sensing. In particular, the gold nanoparticles from either deposition method were in the best electrical communication with the underlying electrode when the BDD had a hydrogen-terminated surface, on which the gold nanoparticles were also notably more stable, making the hydrogen-terminated, gold-decorated BDD the electrode of choice for mercury sensing.
The second study examines commercial BDD electrodes functionalised with electrodeposited nickel hydroxide for the electrochemical oxidation of glucose. The results were significantly affected by variability in the BDDs used. In general, nickel hydroxide on hydrogenterminated BDDs had consistently larger glucose oxidation currents in accordance with the increased surface conductivity and the better electrical contact of hydrogen-terminated BDDs. However, the nickel hydroxide-decorated, oxygen-terminated BDD had more stable and reproducible oxidation currents in the glucose concentration range of 0.1-1.3 mM.
The third and fourth studies investigate the role of surface termination in the modification of BDDs by carbon black. Acetylene carbon black and Monarch 430 carbon black, which differ in their chemical composition and physical structure, were used to investigate the electro-oxidation of acetaminophen and hydroquinone. The electrochemical properties of the electrodes depended on both the BDD surface termination and the carbon black. The detection limit for acetaminophen was 100 nM for hydrogen-terminated BDDs and dropped to a state-of-the-art 10 nM when modified by carbon black. A similar performance was not obtained with oxygen-terminated BDDs even in the presence of carbon black. With regards to hydroquinone, hydrogen-terminated BDDs displayed a competitive detection limit of 0.5 μM that was not improved by carbon black decoration. In contrast, oxygen-terminated BDDs were largely inactive to the electro-oxidation of hydroquinone, although some activity could be observed with carbon black modification.
The thesis concludes with a discussion of key results and suggests future research topics.

Advisor: Foord, J