Characterisation

The electrodes were characterised using the protocol described in Chapter 2.25

Micro-Raman spectroscopy (514.5 nm) data is presented in Figure 5.10 and

Figure 5.11, although measured for ring disc device number 1 (RD1) from Figure 5.7 this data is representative of all electrodes in this generation and is

used as a qualitative assessment of NDC content and boron concentration.26 Figure 5.10 presents data for an area of insulating diamond; the sharp peak at

1332 cm-1 _{is characteristic of sp}3 _{diamond carbon, indicating undoped}

polycrystalline diamond as expected. There is no evidence of the asymmetry (Fano resonance) usually present in a boron doped sample.7, 27, 28 _{This suggests}

that this method of growth has successfully avoided the issue observed in a previous method, where overgrowth of insulating diamond into machined

Page | 124 BDD results in diffusion of boron into the insulating layer due to etching and redeposition of the boron29 _{(unpublished work).}

Figure 5.10: Representative example of raman data on an insulating diamond area of a generation 1 all-diamond ring disc electrode using a 514.5 nm laser and 50× magnification.

On the contrary, in Figure 5.11, where Raman data for the BDD ring, (a) and (b), and disc, (c) and (d), are shown the sp3 _{peak, although still clearly visible,}

is shifted slightly and decreased in intensity due to the high boron concentration.7 _{The presence of a highly asymmetric Fano resonance, typically}

observed for boron dopant levels >1020 _{atoms cm}-3 _{with higher asymmetry for}

increased boron content, reflects the high levels of boron present in the lattice and indicates the electrodes are highly doped. This is corroborated by the presence of peaks at ~500 and 1230 cm-1_{, which although present to some}

extent in all four spectra are much larger in those of darker grains (b) and (d) suggesting higher boron uptake in these grains.7 _{NDC is indicated in raman}

spectra by the presence of broad peaks between 1400 and 1600 cm-1_,30 _no

Page | 125 Figure 5.11: Representative examples of Raman data for a BDD ring and disc in a generation 1 all-diamond ring disc for (a) Ring, light grain. (b) Ring, dark grain. (c) Disc, light grain. (d) Disc, dark grain. This data was collected from RD1 using a 514.5 nm laser and 50× magnification.

Overall this data points towards successful production of high quality, highly doped BDD ring and disc electrodes insulated in a high quality B free diamond substrate. Several of the electrodes (RD1 and RD20) were subsequently electrically contacted and assembled according to the method described in Section 5.4.1.

The most effective way to ascertain NDC surface presence in the BDD regions is to record solvent windows in electrolyte solutions.31, 32 _{This provides}

sensitive analysis of the entire surface area, whereas in Raman spectroscopy typically only individual spots are interrogated. Raman mapping, where the surface is analysed on a point by point basis is an extremely lengthy procedure. Both the capacitive contribution, and surface oxidation/reduction

Page | 126 processes affected by NDC presence can be observed in the background CV response. NDC is oxidized to an appreciable extent just prior to the onset of water oxidation, allowing NDC containing electrodes to be easily identified by the corresponding current peaks.1, 25 _{It has been commented previously that}

BDD does not catalyse oxygen reduction, however in the presence of NDC oxygen is sluggishly reduced, evidenced by a clear cathodic current at negative potentials.

Figure 5.12 presents a solvent window collected for the RD20 disc, the CV is

largely free of background processes with very little evidence of NDC oxidation in the anodic window for both this electrode and for RD1, suggesting there is no, or very small amounts of NDC present in these devices. Double layer capacitance measurements were also used to assess the quality of the electrodes; high capacitance values are indicative of low quality BDD,25, 33 _and

also poor sealing between insulating and doped diamond.23

Figure 5.12: Example solvent window, recorded in 0.1 M KNO3 at 100 mVs-1 for the disc of

Page | 127 Note, high capacitance can also be caused by poorly contacted electrodes. Capacitance data for both electrodes are presented in Table 5.2. A well contacted high quality BDD electrode is expected to have capacitance values <7 µF cm-2_{, values between this and 10 µF cm}-2 _{are generally acceptable for use,}

whereas for poor quality electrodes the capacitance increases well beyond this in some cases. In both cases the recorded capacitance values are high (>10 µF cm-2_{), with significantly larger values for the ring electrodes; this indicates}

either unsuccessful contacts or poor quality material. However, the Raman and solvent window data suggest the latter is unlikely and the electrical contact was thus investigated further (see below).

Page | 128 Table 5.2: Summary of electrochemical characterisation data for all-diamond devices RD1 and RD20.

Electrode # Ring Disc Capacitance (µF cm-2₎ Solvent Window (V) ΔEp (mV) Theoretical Ip (µA) Experimental Ip (µA) Capacitance (µF cm-2₎ Solvent Window (V) ΔEp (mV) Theoretical Ip (µA) Experimental Ip (µA) 1 13.73 1.56 71 0.16 0.2 26 2.6 85 0.14 0.15 20 10.34 3.35 63 0.3 0.4 19.35 3.25 76 0.52 0.52

Page | 129 To further assess the electrochemical characteristics of these devices CVs were performed at a range of scan rates (10 – 500 mV s-1_{) in solutions containing the}

redox mediator Ru(NH3)63+, and supporting electrolyte. The theoretical

separation between anodic and cathodic current peaks (ΔEp), in accordance

with the Nernst equation, is 59 mV for a 1 electron process. For the all- diamond devices investigated here ΔEp was found to be somewhat larger

generally ranging between 70 – 85 mV, although a value of 63 mV was recorded for the disc of RD20. The theoretical Ip values (at 100 mV s-1) for the

reduction of Ru(NH3)63+ are also presented in Table 5.2, alongside the

experimentally measured values, assuming a diffusion-controlled process. Generally, the experimental values although slightly higher, fit to theory within 0.1 µA. It is thought that these results add further evidence for electrical connection issues; wires were connected to these electrodes using silver paint. Replacing with conductive silver epoxy has since been found to improve the electrochemical response by lowering capacitance and decreasing ΔEp, as

shown in section 5.4.4.